Silicon-based Inorganic Polymers [1 ed.] 9781608762569, 9781604563429

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SILICON-BASED INORGANIC POLYMERS

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No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.

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SILICON-BASED INORGANIC POLYMERS

ROGER DE JAEGER AND

MARIO GLERIA

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EDITORS

Nova Science Publishers, Inc. New York

Copyright © 2008 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS.

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LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Silicon-based inorganic polymer / Roger De Jaeger, Mario Gleria (editor). p. cm. ISBN 978-1-60876-256-9 (E-Book) 1. Silicon polymers. 2. Inorganic polymers. I. Jaeger, Roger De. II. Gleria, Mario. QD181.S6S4414 2008 547'.7--dc22 2007050781

Published by Nova Science Publishers, Inc.

New York

CONTENTS

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Preface

vii

Chapter 1

General Review on Polysiloxane Synthesis Marek Cypryk

Chapter 2

Silicones in Industrial Applications M. Andriot, S. H. Chao, A. Colas, S. Cray, F. de Buyl, J. V. DeGroot, A. Dupont, T. Easton, J. L. Garaud, E. Gerlach, F. Gubbels, M. Jungk, S. Leadley, J. P. Lecomte, B. Lenoble, R. Meeks, A. Mountney, G. Shearer, S. Stassen, C. Stevens, X. Thomas and A. T. Wolf

Chapter 3

Polysiloxanes as Templates and Building Blocks in Nanostructured Materials Guido Kickelbick

1 61

163

Chapter 4

Photochemistry of Polysiloxanes Frédéric Cazaux and Xavier Coqueret

191

Chapter 5

Polysilanes Julian Koe

217

Chapter 6

Polycarbosilanes Wolfram Uhlig

309

Chapter 7

Polysilazanes Markus Weinmann

371

Chapter 8

Polyferrocenylsilane-Based Polymer Systems Vasilios Bellas and Matthias Rehahn

415

Index

481

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PREFACE After the initial success of the first book we edited in 2004 titled: “Phosphazenes: A Worldwide Insight”, we devoted our interest to a second editorial venture mostly dedicated to inorganic macromolecules. The reason for this fact was dictated by the increasing scientific interest and the enhanced possible industrial and technological applications of these substrates. This book on “Inorganic Polymers” collects contributions coming from the most scientifically developed countries in the world (Europe, USA and Japan), written by eminent scientists on their own research topics. Thus, silicon-based inorganic polymers were treated by M Cypryk (Poland), G. Kickelbick (Austria), X. Coqueret (France), A. Colas (Belgium), J. Koe (Japan), W. Uhlig (Switzerland), and by M. Rehahn and M. Weinmann (Germany). Different aspects of phosphorus-containing macromolecules were described by F.F. Stewart (USA), R. De Jaeger and L. Montagne (France), and by M. Carenza, S. Lora, and M.Gleria (Italy). Tin- and germanium-based polymers were illustrated by M. Okano (Japan), while inorganic dendrimers were presented by A.M. Caminade and J.P. Majoral (France) and by V. Balzani (Italy). Miscellaneous topics covering the flame-retardant and the intumescent behavior of the inorganic macromolecules (S. Bourbigot, France), ionically-conductive inorganic macromolecules (E. Montoneri, Italy) and chiral inorganic polymers (G.A. Carriedo and J.F. Garcia-Alonso, Spain) were also addressed. This book does not pretend to be exhaustive, but we feel that it is wide enough to cover the most important and representative topics in the field of inorganic polymeric materials, providing a reasonable scientific view of this domain both to newcomers and to scientists already active in specific areas. We are deeply indebted to all the scientists that accepted to join us in this venture in an enthusiastic way and contributed to this book, making it possible. Roger De Jaeger and Mario Gleria

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In: Silicon-based Inorganic Polymers Editors: Roger De Jaeger and Mario Gleria

ISBN: 978-1-60456-342-9 © 2008 Nova Science Publishers, Inc.

Chapter 1

GENERAL REVIEW ON POLYSILOXANE SYNTHESIS Marek Cypryk Center of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Łódź, Poland

ABBREVIATIONS All silicones in the text are composed of, or contain some, or all, of the four basic units: M (monofunctional) ≡ R3Si(O)1/2 D (difunctional) ≡ R2Si(O2)1/2 T (trifunctional) ≡ RSi(O3)1/2 Q (tetrafunctional) ≡ Si(O4)1/2

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T and Q units constitute parts of many highly branched oligomers and polymers, or crosslinked networks. For the most common siloxanes, polydimethylsiloxanes, R = Me; Thus permethyl cyclic siloxanes would be referred to as Dn, e.g. D3, D4, D5, … Linear oligomers are often called MDnM, e.g. MM, MDM, MD2M, … DH = MeHSi(O)1/2; for example, DH4 ≡ tetramethylcyclotetrasiloxane (MeHSiO)4 V = MeViSi(O)1/2 P = PhMeSi(O)1/2 F = Me(CF3CH2CH2)Si(O)1/2 DDS = dimethyldichlorosilane List of abreviations used for polymer identification: PDMS = polydimethylsiloxane PMHS = polymethylhydridosiloxane, or polymethylsiloxane

2

Marek Cypryk PDHS = polydihydridosiloxane, or polysiloxane PMES = polymethylethylsiloxane PDES = polydiethylsiloxane

INTRODUCTION It was probably Albert Ladenburg who obtained silicone oil for the first time in 1872 [1]. However, the prospects for development of this area of chemistry seemed poor until the 1940’s, when the commercial process for the preparation of silicone precursors was invented independently by Rochow in the US and Muller in Germany. Since that time, thousands of papers and patents on the synthesis of polysiloxanes have appeared. Among them there are excellent books and reviews, to which the reader is referred for more comprehensive information [2-18]. The commercial success of polysiloxanes is due to their unique properties, which cannot be easily matched by organic replacements. The constantly expanding production and applications of polysiloxanes, in particular, their growing use for the design and synthesis of various well defined macromolecular architectures, call for a deeper knowledge of the polysiloxane chemistry. The purpose of this review is to give a general summary of the state of art in the polysiloxane synthesis. An emphasis has been put on the synthesis of linear polysiloxanes, although three-dimensional (3D) branched and dendritic macromolecular structures are also briefly reviewed.

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THE ELECTRONIC STRUCTURE OF THE SILOXANE BOND Electronic character of the siloxane bond has a great impact on the reactivity and physical properties of polysiloxanes. The siloxane bond, Si-O, is considerably shorter than what would be expected from simple additivity of the atomic radii of silicon (1.17 Å) and oxygen (0.66 Å). Hence, this linkage is more complex than a regular σ bond. A large difference in electronegativities of the constitutive silicon and oxygen atoms, which, according to Pauling scale, are equal to 1.8 and 3.5, respectively, results in partially ionic character, estimated to 40%-50% on the basis of empirical calculations [19].

O

Si

2 .5 k J

O

Si

1 .2 5 k J

O

Si 1 .6 5 A

145o

O 110o

Si O

Scheme 1.

General Review on Polysiloxane Synthesis

3

The Si-O bond distance shortening suggests a partial double bond character of Si-O. In earlier literature, it was interpreted in terms of the [p(O)→d(Si)]π interaction [3]. Indeed, there are several arguments supporting this hypothesis. Siloxanes are much weaker bases than ethers, which is in accord with the concept of delocalization of the lone pair of electrons from oxygen to the empty d orbitals of silicon. The SiOSi chain is extremely flexible. The SiOSi angle, ranging usually from 140° to 180°, is much wider than the tetrahedral angle [3]. The barrier of rotation around the Si-O bond of 2.5 kJ/mol in (Me3Si)2O, as well as the barrier of linearization of the SiOSi angle, ca. 1.3 kJ/mol, are very low [20]. This again is in agreement with the spatial arrangement of the d orbitals. Recent theoretical calculations show, however, that the contribution of the d orbitals at Si to the total electron distribution is small [21]. Many authors prefer to interpret these features as a consequence of the strongly ionic character of the Si-O bond and of the negative hyperconjugation [p(O)→σ*(Si-X)]π (also referred to as the anomeric effect, Scheme 2) [15,21]. The interaction is particularly strong when X is an electronegative atom, e.g., N, O, F. As the other authors indicate, the (p-d)π concept cannot be totally abandoned, because the theoretical calculations did not provide unequivocal information on the character of the Si-O bond [22]. Calculations suggest that the (p-d)π interaction plays an important role at least in the siloxane species having enhanced electron density on oxygen, such as silanolate anions and silanols hydrogen-bonded to a base [23].

p(O1) O

p(O2)

1

O

2

Si1

Si σ*(Si-O2)

p1(O)

O

σ*(Si-O1)

O

σ*(Si1-O)

p2(O)

O

Si2 σ*(Si2-O)

Scheme 2.

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REACTIVITY OF THE SILOXANE BOND General reactivity of the siloxane bond is largely a function of its electronic properties. Thus, in view of theoretical results, low basicity of siloxane oxygen is mainly due to the electron delocalization (by negative hyperconjugation or p-d back donation) along the OSiO (silaacetal) linkage. Electron delocalization and contribution from ionic structures are also responsible for an extra strengthening of the siloxane bond [15,21,24]. High energy of the siloxane bond, (estimated to fall in the range of 450 kJ/mol [25] to 570 kJ/mol [26]), makes it very resistant to homolytic cleavage. On the other hand, the siloxane bond is strongly polarized. Moreover, the silicon atom has a relatively large size and the substituents appear only at every second atom in the chain. In connection with the high flexibility of the chain, these features account for the relatively low steric hindrance and high accessibility of silicon to a nucleophilic attack. This results in relatively high reactivity of the siloxane bond toward heterolytic cleavage [3].

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Marek Cypryk

PROPERTIES AND APPLICATIONS There is a huge bibliography concerning properties and applications of polysiloxanes and siloxane-derived materials [2-14,14-16,16-18,27-30]. Detailed presentation of this area is not a subject of this review. The main points are only shortly marked to show the relationship between the electronic structure of the siloxane and silaacetal bonds and the macroscopic

Thermal and Chemical Stability As expected from the high values of bond energies, silicones, in the absence of acids or bases, are exceptionally thermally stable. Degradation of polydimethylsiloxane begins at about 350 °C. In inert atmosphere, the main products are low molecular weight (MW) cyclosiloxanes. Phenyl groups improve thermal stability of silicones [6,7,31,32]. Silicones are also resistant to oxidation and exhibit good flame retardancy [7,13,32] They are stable toward hydrolysis (except extreme pH conditions) and toward typical organic solvents [2,3].

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Elastomeric Properties Unusual mobility of the siloxane chain is a consequence of the electronic structure of SiOSi linkage and of the small steric hindrance of side groups due to the relatively long distance between them, as they occur at only every second atom in the backbone. On the other hand, nonpolar side groups like methyls effectively shield the polar SiO backbone, which results in the weak intra- and intermolecular forces in polysiloxanes. These features are manifested in low viscosity of polysiloxanes up to high molecular weight and in pronounced elasticity of polymers at low temperatures. Glass transition points for PMES, PMHS and PDES are the lowest of all common polymers [32]. Weak intermolecular forces are responsible for poor mechanical properties of polysiloxanes. However, by cross-linking of the chains as well as by reinforcement of silicone materials with resin or a filler, their mechanical properties can be significantly improved, making them applicable as elastomers in the exceptionally wide temperature range. Silicone rubbers can remain flexible at temperatures as low as -100 °C yet perform satisfactorily at temperatures over 200 °C [33-35].

Surface Properties Polar inorganic siloxane backbone surrounded by nonpolar pendant organic groups gives amphiphilic character to polysiloxane. High mobility allows the polysiloxane chain to easily adapt the most thermodynamically favored conformation at the interface. This feature interplaying with the exceptionally weak interactions between side groups give them extremely low surface tension [25,36,37]. Silicone fluids spread easily over the surfaces of various substances, and are thus widely used as water repellents, foam controllers, lubricants, surfactants and ingredients for cosmetics. By combination with hydrophilic fragments, the

General Review on Polysiloxane Synthesis

5

surface activity of silicones may be modified in a wide range making them applicable in even seemingly contradictory roles, from release coatings to excellent adhesives (such as pressuresensitive ones), from anti-foaming agents to foam stabilizers, etc. [7,13,18,25,37]. An important consequence of the low intermolecular forces in polysiloxanes is also the exceptional gas permeability of silicones [25].

Dielectric Properties Silicones exceed all comparable materials in their insulating properties as well as in the flexibility in electrical applications. They are non-conductive because of their chemical nature and maintain dielectric strength in temperature extremes far higher or lower than conventional materials can handle. Exposure to heat, cold, moisture, oil, and ozone does not significantly change their electrical properties. Thus, when compounded with the proper fillers and additives, are used to produce rubber for a wide range of electrical insulating applications. Very low dielectric permittivity in the range of 2.4-2.75 as well as high dielectric strength (13.7-15.8 kV/mm) and volume resistivity (about 1×1015 ohm·cm at 20 °C) of methylsilicones make them excellent materials for insulating purposes [38].

Biocompatibility Very important feature of silicones is their biological inertness, due to their hydrophobicity and chemical stability. They are odorless and tasteless, do not support bacteria growth, and will not stain or corrode other materials. Most importantly, silicone rubbers exhibit superior compatibility with human tissue and body fluids. They are essentially nontoxic and therefore are widely used for cosmetics and medical applications [18,39].

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High Technology Applications Silicon-based nanomaterials have potential as the “material of choice” for numerous applications in photonics, electronics, energy, and biology. For instance, siliconepoly(oxyethylene) copolymers form vesicles, which can function not only as encapsulants and delivery agents of active ingredients but also as nanoreactors. Microemulsions and liquid crystals formed from silicone surfactants have been used to enhance delivery of personal care products and to direct synthesis through templating of mesostructured materials. Siliconeorganic copolymers self-assemble to ordered or disordered phases, which control the physical and mechanical properties of the polymeric system and facilitate the stabilization of multiphase-component emulsions. The rapid development of thermotropic oligosiloxane liquid crystalline (LC) materials is associated with their promising potential. Oligosiloxane functionalization can be used to tune the properties of mesogens. The oligosiloxane component tends to induce microphase segregation, thus ordered phases (such as lamellar structures) are readily observed. This

6

Marek Cypryk

technology finds applications in the liquid crystal display market and in other electro-optics opportunities [40-42].

Functional Silanes - Precursors to Polysiloxanes Functional silanes of the general formula R4-nSiXn, where X is Cl, OR, OC(O)R, NR2 or other hydrolysable group, are the precursors for the polysiloxane synthesis. The most common precursor of polydimethylsiloxane is Me2SiCl2 (DDS). Since the organosilicon compounds do not appear in nature, all the polysiloxane monomers must be obtained synthetically. The methods of their preparation have not changed significantly since the beginning of production. The natural source of silicon is silica (SiO2). Silica is reduced to elemental silicon with carbon in the high temperature electro-thermic reduction [4,43]. Silicon is then transformed into organosilicon species, most often by one of the following methods:

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1. Reaction of organic compounds with silicon at elevated temperatures. This is the industrial route to methylchlorosilanes, called the “Direct Process” [15,43,44]. 2. Chlorination of silicon and a subsequent substitution of some chlorine atoms by organic groups with organometallic reagents, such as organolithium compounds, Grignard reagents, organic zinc compounds and others [45]. 3. Transformation of silicon into silyl hydrides and a subsequent addition to multiple bonds in a hydrosilylation process [46-48]. Dimethyldichlorosilane (DDS), the most important substrate for the silicone industry, is synthesized in the so-called Direct Process, which involves the reaction of gaseous methyl chloride with a contact mass of silicon containing copper as catalyst in continuously operating fluidized or stirred bed reactors at 250-300 ºC [44]. The yield of DDS can reach more than 90%. The other products are methylchlorosilanes, MenSiCl4-n, n = 0-4, in addition to a few percent of disilanes. The way of preparation of contact mass, the purity of the silicon and the conditions of the process strongly affect the product composition. Addition of Cl2 to the feed increases the proportion of MeSiCl3 and SiCl4, while introduction of HCl or H2 gives rise to the Si-H containing products. The use of other alkyl or aryl halides (ethyl chloride, vinyl chloride, allyl chloride, chlorobenzene) leads to the corresponding organohalosilanes, RnSiX4-n.

Si + MeCl

Cu Δ

MenSiCl4-n

(1)

An effort continues to improve an overall yield of the desired products of the direct process through the understanding of the mechanism of component reactions, of the effect of promoters and additives and of the silicon morphology [49]. The reactions of silicon with alcohols, leading to methoxysilanes are also exploited [50,51].

General Review on Polysiloxane Synthesis

7

The literature on the synthesis of polysiloxane precursors containing hydrolysable functions other than halogen at silicon is very rich, including numerous important monographs and reviews [2,3,15,17].

Industrial Synthesis of Polysiloxanes The general route to linear polysiloxanes from silicone monomers used in the industry is a two step process. The first step is a hydrolytic polycondensation of the bifunctional silane precursor (most often DDS). This process leads to a mixture of linear and cyclic oligosiloxanes, frequently referred to as the hydrolyzate. With DDS, the reaction occurs according to general equation 2. HO(Me2SiO)mH + (Me2SiO)n + 2(m+n) HCl

(m+n) Me2SiCl2 + (m+n+1) H2O

(2) The ratio of cyclic to linear oligomers can be varied by changing the reaction conditions (water/silane ratio, reaction time, temperature, the presence of additives).2,5,10,16 For example, rapid removal of HCl from the reaction mixture leads almost exclusively to short-chain siloxanediols. Cyclic oligomers comprising up to 2/3 of total product can be obtained after prolonged contact with HCl. Further increase of the yield of cyclics can be achieved when they are separated from the reaction mixture by vacuum distillation. The second step is a transformation of the oligomers into a high molecular weight polymer. The synthesis of the high polymer is performed either by a polycondensation of the hydroxy-terminated, low MW polysiloxanes (α,ω-polysiloxanediols) or by a ring opening polymerization of cyclic oligomers. The methanolysis process is a potentially interesting alternative to hydrolytic polycondensation. This process allows for a direct recovery of chloromethane from methylchlorosilanes, according to equation 3 [10,16]. (m+n) Me2SiCl2 + 2(m+n) MeOH

HO(Me2SiO)mH + (Me2SiO)n + 2(m+n) MeCl + (m+n-1) H2O

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(3)

POLYSILOXANES BY POLYCONDENSATION ROUTES Hydrolytic Polycondensation of Chlorosilanes The hydrolytic polycondensation may be presented as a set of reversible reactions (equations 4-7). ≡SiCl + H2O ⇌ ≡SiOH + HCl

(4)

≡SiCl + HOSi≡ ⇌ ≡SiOSi≡ + HCl

(5)

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8

Marek Cypryk 2 ≡SiOH ⇌ ≡SiOSi≡ + H2O

(6)

2 ≡SiCl + H2O ⇌ ≡SiOSi≡ + 2 HCl

(7)

Thermodynamics of the overall reaction (equation 7) is very favorable for siloxane formation. The enthalpy of hydrolysis/condensation of trimethylchlorosilane in the gas phase, deduced from bond dissociation enthalpies (BDE), is ca. -21 kJ/mol. (BDE’s for silicon compounds were taken from ref. [26,52,53]; BDE’s for H2O and HCl were taken from ref. [54]). The enthalpy of this reaction in water was measured to be as large as -46 kJ/mol to -52 kJ/mol. The hydrolysis/condensation of Me2SiCl2 is even more exothermic, ΔH298 = -134.3 kJ/mol [55,56]. Comparison of the enthalpy values for the reaction in gas and condensed phases indicates, that a large amount of heat produced in hydrolysis is due to the HCl hydration. Equilibrium constant for aqueous hydrolysis of Me3SiCl in constant volume conditions was estimated to about 9×1012 [57]. These data suggest that the reaction should proceed to virtually full conversion of chlorosilane. However, it was shown that some amount of SiCl may remain unhydrolyzed when saturated aqueous HCl is in equilibrium with the final hydrolysis products, due to the fact that the thermodynamic activity coefficient for HCl in concentrated HCl solution is very high, while that for H2O is low [58]. Thus, the overall energy and equilibrium position of this process strongly depends on the water/silane proportion, on the polarity of the medium and its ability to hydrogen bonding. The studies on hydrolytic condensation in the gas phase have also shown incomplete conversion of alkylchlorosilanes [59]. Kinetic data for the hydrolytic polycondensation of DDS in solution indicate that fast hydrolysis is followed by the slower, rate-controlling condensation. The overall reaction rate decreases strongly in order SiCl4 > RSiCl3 > R2SiCl2 > R3SiCl. [60]. The intermediate hydroxychlorosilanes are very unstable. Nevertheless, they have recently been observed by NMR in hydrolytic polycondensation of SiCl4 [61]. Products of the hydrolytic polycondensation process depend on the SiCl/H2O proportion. If water is used in excess, the products are hydroxy-terminated short chain polysiloxanes and cyclic siloxanes. In deficiency of water, the main products are chloro-terminated oligosiloxanes. However, formation of cyclics cannot be completely eliminated due to the fast functional group exchange (equation 4) [62]. Hydrolytic polycondensation, when carried out in a large excess of water, probably occurs via homofunctional silanol condensation (equation 6), which is effectively catalyzed by HCl protonating the SiOH group. On the other hand, the heterofunctional polycondensation (equation 5) may become important in the hydrolytic polycondensation of DDS carried out with concentrated HCl solution in a heterogeneous system, where the thermodynamic activity of H2O is low and that of HCl is very high. The heterofunctional reaction dominates when hydrolytic polycondensation is performed in the presence of bases [63].

General Review on Polysiloxane Synthesis

9

Hydrolytic Polycondensation of Alkoxysilanes Alkoxysilanes are the useful alternative to chlorosilanes in hydrolytic polycondensation processes. Their advantage is that they are more chemically stable than chlorosilanes and the by-product of hydrolysis, alcohol, is low toxic, non-aggressive and non-corrosive, compared to HCl. The reactions of alkoxysilanes Si(OR)4 and R’Si(OR)3 with hydroxy-terminated polysiloxanes are extremely important for the preparation of polysiloxane networks, particularly in sol-gel processes [8,15,64,65]. Reaction 8 was also used for the synthesis of linear polymers, in particular, block copolymers [66-69]. This reaction is useful for the synthesis of polysiloxanes containing basic side groups [69]. (8)

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≡SiOR + H2O ⇌ ≡SiOSi≡ + ROH

The process is very complex and sensitive to many factors: silane concentration, amount of water and alcohol, the type and concentration of catalyst, solvent and temperature. A variety of catalysts are used to promote this process: CF3COOH, [69,70] tin(II) carboxylates, [71-74] phosphazenium salts, [75] alkaline earth metal oxides, [76] and uncharged bases (amines) [66,67]. Heterofunctional condensation competes with the silanol homocondensation. Their relative rates vary with the reaction system. Polycondensation is accompanied by an SiOH-SiOR end group exchange resulting from fast hydrolysis and alcoholysis reactions [77,78]. The specific rates of these reactions are often much higher than those of polycondensation [70]. The SiOR + SiOH polycondensation is the equilibrium process. A reverse reaction, i.e., alcoholysis of various siloxanes (XMe2Si)2O, was studied in acidic systems [79]. Tin(II) carboxylates, e.g., stannous octoate, are very effective catalysts of the SiOR + SiOH polycondensation [71,74]. In some systems the process is very selective towards heterofunctional condensation, as it has been demonstrated for the reaction of α,ωdihydroxypolydimethylsiloxane with tetraethoxysilane [80]. In contrast, the condensation of α,ω-dihydroxypolydimethylsiloxane with methoxytrimethylsilane was accompanied by a significant chain extension [71]. A comparison of the activities of various catalysts in the condensation of silanols with alkoxysilanes led to the concept of bifunctional catalysis, in which the catalyst acts both as an acid and a base [81]. Possible structures of the transition states, when carboxylic acid and hydroxylamine are used as catalysts are shown as structures 1 and 2, respectively.

Si

O

Si

H O

C

OR

Si

O

H

H

O

Et2N O

R' 1

Si

2

OR H

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Marek Cypryk

The studies of the condensation of PhMe2SiOH with PhMe2SiOMe in the presence of amines have shown that the reaction rate is very sensitive to the amine structure [66]. For example, methylamine was almost inactive, while ethylamine appeared to be an efficient catalyst of the heterocondensation, which was 10 times faster than the silanol selfcondensation under these conditions. The use of (aminoalkyl)alkoxysilanes as the SiOR substrate allows to perform the reaction without additional catalyst. Thus, condensation of α,ω-dialkoxyoligomethyl(γ-aminopropyl)siloxane with α,ω-dihydroxyoligosiloxane leads to multiblock siloxane copolymers. Alcohol must be continuously removed from the reaction medium to avoid reverse and side reactions [67]. The acid-catalyzed controlled hydrolytic polycondensation of tetraethoxysilane (TEOS) led to polyethoxysiloxanes with weight-average molecular weights of 2300-11700, which depended on molar ratios of water, catalyst, and solvent to TEOS. The products were soluble in common organic solvents and stable to self-condensation [82]. Interesting synthesis of 1,3,5-trihydroxycyclotrisiloxanes, hardly available on different routes, was reported using the controlled hydrolysis and condensation of trialkoxysilanes carried out within the nanosized cavity of self-assembled coordination cage. This method is called by the authors the “ship in the bottle synthesis” [83]. There is a growing interest in enzyme-catalyzed hydrolysis/condensation of alkoxysilanes, following the biosilification process realized in nature by diatoms. The data suggested that homologous lipase and protease enzymes catalyze the formation of siloxane bonds under mild conditions (pH 7.0, 25 °C). In particular, the active site of trypsin, a proteolytic enzyme, was shown to selectively catalyze the in vitro condensation of silanols. In the presence of the lipid, the solubilized lipase was seen to catalyze the polycondensation of diethoxydimethylsilane over 20 h in isooctane containing 2 w% water at 40 °C. The low molecular weight oligomers (MW ≈ 1500) were monodisperse (polydispersity 1.06) [84-86]. In a series of works, Clarson et al. studied enzyme catalyzed hydrolytic polycondensation of TMOS to obtain well-defined silica [87-90].

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Homofunctional Polycondensation of Silanols General Considerations The product of hydrolytic polycondensation, a mixture of α,ω-polysiloxanediols, is a versatile starting material for the synthesis of high MW polysiloxanes [2,8,58,63]. Therefore, the chemistry of silanols has been a subject of extensive studies, both experimental and theoretical, summarized in several reviews [3,21,91-94]. The chemical behavior and physical properties of silanols depend much on their structure. Generally, they are more basic and more acidic than alcohols. Therefore, they form stronger hydrogen bonds and association phenomena play an important role in their chemistry. Their reactivity increases with the number of hydroxyl groups at the silicon atom in order R3SiOH < R2Si(OH)2 < RSi(OH)3 [93]. The enhanced reactivity of the monomeric polyols and the activating influence of the silanol group on the reactivity of other electronegative substituents at silicon are explained by the negative hyperconjugation (see Section 2) [91]. The influence of this effect is strongly enhanced by increasing the electron density on oxygen, for example, when the silanol group is hydrogen-bonded to a base [23].

General Review on Polysiloxane Synthesis

11

Scarce data on the thermodynamics of silanol condensation indicate that the reaction is weakly exothermic. The enthalpy of the condensation of trimethylsilanol in methylene chloride solution, ΔH= -21 kJ/mol agrees well with that in the gas phase (see the preceding section) [95]. Equilibrium position of reaction 5 depends strongly on the medium. Thus, equilibrium constant for condensation of Me3SiOH in CH2Cl2 is K308 = 860, [95] while in dioxane, where silanol is stabilized by hydrogen bonds, the equilibrium constant is somewhat lower, K298 = 130 [96]. The replacement of methyl by an electronegative substituent shifts the equilibrium toward silanol.[ [96-98]. The estimated values of thermodynamic data for the polycondensation of oligodimethylsiloxanediols in bulk at 25 °C were within the following ranges: ΔH= -16.3 kJ/mol to -20.6 kJ/mol, ΔS = -6.3 kJ/mol to -18.0 J/K mol, K = 340 to 450 [99]. Polycondensation of siloxanediols reveals a strong deviation from the Flory ideal polycondensation, which assumes an equal reactivity of all functional groups [100]. In contrast, the reactivity of α,ω-oligosiloxanediols decreases with the increase of the chain length. This feature was observed for various catalytic systems, such as stannous octoate, [73] solid CaH2 [101] and the strong protic acids [102,103]. In the acid-catalyzed polycondensation performed in an acid-base inert solvent or in bulk, the size dependence may be explained by intramolecular catalysis involving the silanol group at the other chain end, which acts as a proton acceptor (Scheme 3) [104]. Indeed, decamethylpentasiloxane-1,9-diol (HD5OH) in acid-base inert solvent reacts by two orders of magnitude faster than its analogue having only one hydroxyl group, undecamethylpentasiloxanol (MD4OH). Since the formation of the intramolecular hydrogen bond between silanol end groups becomes less probable with the elongation of the chain, the reactivity of a diol decreases in this order. Thus, the mechanism of chain growth involves a competitive reactions: (i) ordinary condensation of the two growing chains and (ii) addition of the more reactive smaller molecules to the growing chain end. An important consequence of this mechanism is the narrowing of the MW distribution as a result of addition mechanism [58]. Indeed, the molecular weight distribution as low as 1.25 was found, for example, for the product of polycondensation of α,ω-oligodimethylsiloxanediols catalyzed by stannous octoate [73]. Other explanation of this phenomenon was proposed on the ground of a chain conformation statistics [105].

Si

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Si

O H

O H H

H Si

Si

O Si H O

O Si

SiOH

H

H intermolecular condensation Scheme 3.

O H

cyclization

12

Marek Cypryk

Another important consequence of the mechanism assuming activation of silanols via the hydrogen bond (which is called the “intra-inter catalysis” [91]) is that the homofunctional polycondensation of short oligosiloxanediols in bulk or in acid-base inert solvents produces both linear polysiloxanes and small cyclics (D4-D6), at almost constant proportions, independent of the SiOH concentration [91,104].

Disproportionation of Polysiloxanols Disproportionation of polysiloxanols (exchange of the ultimate siloxane unit, equation 9) commonly accompanies the polysiloxanol condensation [106-109]. The reaction is particularly important in the presence of strong bases, where it is much faster than condensation [106]. The facile cleavage of the siloxane bond in the neighborhood of silanolate anion is explained by the negative hyperconjugation leading to a considerable contribution from the no bond-double bond mesomerism, equation 10 (see also Scheme 2) [91]. This explanation was supported by theoretical calculations, showing that the silanol group, particularly when partially ionized, weakens the adjacent siloxane bond by both the enhanced negative hyperconjugation and (p→d)π effect [23].

SiOSiOH + HOSiOSi

Si O Si O

SiOH + HOSiOSiOSi

Si O Si O

(9) (10)

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The disproportionation occurs also in the presence of acid catalysts [107,108]. It plays a minor role in the polycondensation catalyzed by strong protic acids, carried out in an acidbase inert solvent, but becomes more important when a weaker acid, such as CF3COOH, is used [108]. In the presence of water, the water-mediated disproportionation occurs [107]. Quantum mechanical calculations show that in acidic conditions the energy barrier for the ≡SiO-SiOH bond cleavage is lower than that for the ≡SiO-SiH3 bond [110]. Reaction 11 is responsible for the facile hydrolytic decomposition of hydroxy-terminated polysiloxanes, which is particularly important for polysiloxane degradation in biological systems and in the environment.

Si1OSiOH + H2O

HOSiOH +

Si2OH

acid or base

acid or base

Si1OH + HOSiOH

Si2OSiOH + H2O

(11) (12)

Kinetics, Mechanism and Catalysis Condensation of silanols is catalyzed by protic acids, Lewis acids, strong charged bases, weak uncharged bases, stannous salts, phosphonitrile chlorides, and some heterogeneous catalytic systems.

General Review on Polysiloxane Synthesis

13

Strong protic acids are very efficient catalysts of silanol condensation. The reaction is second order in silanol and first order in acid, which was interpreted, assuming the nucleophilic attack of silanol on its protonated form being the rate controlling step [111]. In bulk or the acid-base inert solvents, such as CH2Cl2, a reversible esterification of silanol is much faster than polycondensation [95,108]. Therefore, a mechanism involving heterofunctional condensation of the ester with silanol was also considered [102]. In such media, the strong protic acid is hydrated by water produced in condensation. Water often forms emulsion, in which part of the acid is trapped, which is the reason why the polycondensation, which occurs in the organic phase, slows down significantly [102]. Solid acid catalysts (acid clays, e.g, montmorillonite, Tonsil) and acidic resins (e.g., Amberlyst) are often used in commercial processes, since they allow to reduce the amount of cyclic siloxanes [112-114]. The catalysis of the condensation of monomeric dimethylsilanediol by strong bases in methanol is as effective as the catalysis by strong acids [111]. Typical catalysts are alkali metal hydroxides and alkoxides. Condensation of α,ω-siloxanediols proceeds mainly on a disproportionation route (see the preceding section) [106]. Some salts of divalent tin, such as stannous octoate, effectively promote the homofunctional SiOH polycondensation [72,73,80]. The reaction is second order in silanol and 0.5 order in the catalyst. These kinetics have been explained by assuming that the monomeric complex of the salt with silanol is a reactive intermediate, being at equilibrium with the inactive dimeric form, according to Scheme 4 [72]. Side reactions leading to tin oxidation suppress the catalytic activity of stannous salts. Phosphonitrile chlorides stand for a very important class of the silanol homofunctional polycondensation catalysts. Used in a small concentration, they are able to transform silanolended linear dimethylsiloxane oligomers into the polymer with molecular weight above 105 within minutes at ambient temperature. They also promote the redistribution of linear polysiloxanes without formation of a significant amount of cyclic species, thus making possible the synthesis of stable trimethylsiloxy-ended polymers. Therefore, they are readily used in silicone technology [115-118].

Si

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R C

O O

Sn

O O

Sn

O O

C R

R C

SiOH

R C

Si I +

O O I O O

SnOSi

SnOH

+

SiOSi

Scheme 4.

The first generation of catalysts of this type have been perchlorooligophosphazenium salts with complex anions ([Cl(Cl2P=N)nPCl3]+ MtXn- (3), where MtXn- = PCl6- or SbCl6-. [115-117]. More recently, some other phosphonitrile structures (Cl(Cl2P=N)nP(O)Cl2 (4), HO(Cl2P=N)nP(O)Cl2 (5), Me3SiO(Cl2P=N)nP(O)Cl2 (6), where n=1-5) have been found to be

14

Marek Cypryk

at least as effective in the catalysis of silanol polycondensation and siloxane rearrangement [118]. Catalysts 4, 5, 6 are uncharged, therefore they are better soluble in polysiloxane and contain no undesired metal complex anions. Cyclic phosphazenes are also very efficient as catalysts [119]. The kinetics of the condensation of pentamethyldisiloxanol was studied in n-heptane solution using catalysts: 3 (n=1, MtXn = SbCl6-) and 4 (n=1) [120]. For both catalysts the reaction showed the second order in silanol, while the order in the catalyst was 1.5. The rates and the activation parameters were similar for both catalysts. The simplified general mechanism was proposed, in which a proton is assumed to play an important role, generating the phosphonium cation center on which the condensation occurs [120,121]. Recently, the efficiency of the novel superacid and superbase catalysts has been tested in condensation of model oligosiloxanols (structures 7, 8) [122,123]. Both proved to be very active; because of efficient stabilization of the counter-ion, they exist in practically fully dissociated form, where the proton or hydroxide anion are extremely active. Moreover, these catalysts show excellent solubility in organic systems.

H+(H2O)n B(C6F5)4-

N

P N

P N

3

7

OH 3

8

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Like in the process catalyzed by classical acids or bases, a competition of condensation and disproportionation is observed. In the process catalyzed by the superacid, condensation dominates (like in reactions catalyzed by strong protic acids) whereas in the superbasecatalyzed process disproportionation is considerably faster than condensation (as in reactions catalyzed by strong bases). Kinetic results for these two processes provide evidence of strong solvation of the ions by silanols [122,123]. Amines seemed to be only weak catalysts of the silanol condensation. Recently, triethylenetetramine was shown to efficiently catalyze the conversion of Ph2Si(OH)2 to (Ph2SiO)4 [124].

Polycondensation in Aqueous Emulsion Hydroxyl-ended oligodimethylsiloxanes undergo condensation in aqueous emulsion stabilized by sulfonic acid surfactants, such as dodecylbenzenesulfonic acid (DBSA), which also serve as catalysts for polycondensation [63,125]. A high molecular weight polymer, up to Mn=106, may be obtained under mild conditions. The process follows second order rate law in silanol for the reversible reaction. The specific rate of polycondensation is proportional to the area of the oil-water interface, but it is a complex function of surfactant concentration. A proposed mechanism assumed that a complex, comprising silanol and two surfactant molecules, reacts bimolecularly at the oil-water interface. Condensation of α,ωpolysiloxanediols in aqueous emulsion is accompanied by redistribution reactions and by ring opening polymerization of cyclics (mainly D4) formed by back biting [126]. Polydimethylsiloxane microemulsion was also prepared by adding DDS with a controlled rate to water containing the electrosteric surfactant (sodium dodecylpolyoxyethylene(8)

General Review on Polysiloxane Synthesis

15

sulfate) which prevents coagulation. The linear PDMS of Mn ≈ 60000 (Mw⁄Mn ≈ 2) was obtained which contained a relatively small content of cyclosiloxanes (less than 5 w%) [127].

Heterofunctional Condensation Involving Silanol Groups Heterofunctional polycondensations of silanols are extensively used for the generation of polysiloxane networks and siloxane copolymers. The reaction proceeds according to general equation 13. The silanol group is reactive towards a wide range of functional groups bound to silicon [8,15]. There are some indications about the reactivity order of functional groups: Cl > AcO > R2C =NO > MeO > EtO [15]. Two types of the heterofunctional condensation of silanols are particularly important for the synthesis of linear polysiloxanes: the condensation with SiCl and with SiOR functions. Since the homofunctional silanol condensation may compete with these reactions, the choice of catalyst is essential to selectively perform the heterofunctional condensation. (13)

≡SiX + HOSi≡ ⇌ ≡SiOSi≡ + XH where X = H, Cl, OR, OC(O)R, NR2, N(R)C(O)R, ONR2, ON=CR2

SiCl + HOSi Condensation The SiCl + HOSi condensation process requires basic or nucleophilic catalysis. Acids protonate the SiOH group much more easily than the SiCl function, promoting the SiOH selfcondensation. In the presence of weak, uncharged bases, like amines or aromatic nitrogen heterocycles, such as pyridine, the heterofunctional condensation proceeds selectively [128]. Triethylamine was found to operate as a Brönsted general base forming a hydrogen bond complex with silanol, which reacts with chlorosilane in the rate-limiting step (equations 14, 15). SiOH + NR3

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SiO H NR3 + SiCl

δδ+ SiO H NR3

SiOSi

(14)

+ R3N.HCl

(15)

The amine plays also a role of the HCl acceptor, therefore it should be used in at least stoichiometric proportion with regard to the SiCl group. The condensation reaction is effectively catalyzed by strong, uncharged nucleophiles, such as 4-(N,Ndimethylamino)pyridine (DMAP) or N-methylimidazole (NMI) [128]. The catalysis by DMAP and other strong nucleophiles was explained by the formation of a strongly electrophilic complex of nucleophile with chlorosilane (equation 16) [128,129]. The catalytic system comprising DMAP and Et3N, as an HCl acceptor in heterofunctional SiCl + SiOH polycondensation, was used for the synthesis of alternating siloxane copolymers and block copolymers [130,131].

16

Marek Cypryk

Si+ Nu Cl-

SiCl + Nu

SiOH Et3N

SiOSi

+ Et3N.HCl + Nu (16)

SiOR + SiOH Condensation Condensation of SiOH with SiOR is usually performed as the hydrolytic polycondensation of alkoxysilanes and has been discussed in one of the preceding sections. Base-catalyzed hydrolysis/condensation of PhSi(OR)3 in the presence of transition metal salts leads to stereoregular phenylhydroxycyclosiloxanes [132]. Some other Heterofunctional Condensations of Silanols The Et3N/DMAP system is an efficient catalyst of the silanol-acetoxysilane condensation, although the effect of nucleophilic additives is not as profound as in the case of chlorosilanes [133,134]. This type of condensation is extensively exploited for the vulcanization of polysiloxane elastomers [8]. The coupling of silanol and of the silylamine-terminated oligosiloxanes offers a possibility to obtain siloxane copolymers with a controlled chain structure [135]. The polycondensation of hexamethyltrisiloxanediol with bis(dimethylamine)dimethylsilane has successfully been employed for the synthesis of high molecular copolymers with regularly arranged side-groups (equation 17) [136]. Me Et2N Si NEt2 + HO CH2CH2

N

Me SiO H Me

n

N

Me

Me

Si O

SiO n m

Me

CH2CH2

(17) Polycondensation reactions of optically active (S,S)-1,3-dimethyl-1,3diphenyldisiloxanediol with bis(dimethylamino)dimethylsilane and bis(dimethylamino)diphenylsilane gave stereoregular and optically active polysiloxanes (equation 18) [137]. Ph

Ph

*

*

Me

Me

R

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HO Si O Si OH + Me2N Si NMe2 R

Ph

Ph

R

Si* O Si* O Si O Me

Me

R

n

(18) Polycondensation of hydrogensiloxanes with silanols was applied for the crosslinking of polysiloxanes (equation 19) [4,11,15,16]. Hydrogen, generated as a by-product, can be used as the foam making agent [5]. The catalysts used for the coupling are tin salts [74] and transition metal complexes (rhodium, platinum, palladium) [15,138]. The process is highly chemoselective. Side reactions, like homocondensation of silanol groups and disproportionation of siloxane units, are not significant in this system. ≡SiH + HOSi≡ → ≡SiOSi≡ + H2

(19)

General Review on Polysiloxane Synthesis

17

Dehydrocoupling catalyzed by rhodium complexes was successfully applied for the synthesis of optically active stereoregular polysiloxanes (equation 20). Dehydrocoupling may also be performed by reaction of hydrosilanes with water in the presence of palladium catalysts [139]. Ph

Ph

*

HO Si

Me

Ph

*

O Si OH + H Si Me

*

Me

Ph O Si

*

H

[RhCl(cod)]2 -H2

Me

Ph

Ph

Ph

Ph

O Si O Si O Si O Si Me

Me

Me

Me

n

(20) Tris(pentafluorophenyl)borane, B(C6F5)3, was found to be an effective catalyst in the synthesis of optically pure and completely diisotactic phenyl- and naphthyl-substituted poly(siloxane)s under mild reaction conditions (equation 21). The optically active disiloxane units in the produced polymers had better controlled chemical and stereoregular structures than those obtained from the bis(silanol)s and bis(dimethylamino)dimethylsilane. Homopolymerization of the starting bis(silanol) could be almost completely suppressed [140]. R

R *

Me

Me

*

HO Si O Si OH + H Si X Si H Me

Me

Me

B(C6F5)3

Me

-H2

X=O, (CH2)4, p-C6H4

R

R

Me

*

*

Me

Me

Me

O Si O Si O Si X Si Me

Me

n

(21)

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Non-Hydrolytic Ways of Siloxane Synthesis The nonhydrolytic conversion of chlorosilanes into siloxanes may be performed, using dimethylsulfoxide (DMSO) as the oxygen source [141-143]. This reaction is particularly useful for the synthesis of cyclosiloxanes. The transformation was postulated to proceed through the transient formation of silanone [141,142]. However, it is more likely that sulfoxide intermediates, XMe2SiOS+Me2, are involved [143]. A similar method utilizes SeO2 to transform dichlorodimethylsilane into a mixture of oligomers [144,145]. This method was originally used for conversion of alcohols to the corresponding alkyl chlorides [144]. Another useful method of the siloxane bond formation is the reaction of diorganodichlorosilanes with some metal oxides, such as ZnO [146]. The reaction is particularly useful for the synthesis of cyclotrisiloxanes, which are rather difficult to prepare in other way. By this method they can be obtained with the yield 30%-60% [147,148]. The mechanism of this transformation was recently studied by theoretical methods [149]. n R2SiCl2 + n ZnO → (R2SiO)n + n ZnCl2

(22)

Cyclotrisiloxanes can also be obtained with good yields by the reaction with NaHCO3 in ethyl acetate (equation 23) [150,151]. The reaction was recently repeated for a series of dichlorosilanes. The content of cyclotrisiloxanes in the resulting mixtures of cyclosiloxanes was 30%-67%, depending on the organic substituents at silicon and on the solvent used [152].

18

Marek Cypryk n R2SiCl2 + 2n NaHCO3 → (R2SiO)n + 2n NaCl + 2 CO2

(23)

The chlorine exchange between silane and stannoxane was also recently reported to give cyclosiloxanes in two-step synthesis [153]. MeRSiCl2 + Bu3SnOSnBu3 → MeRSi(OSnBu3)2 + 2 Bu3SnCl

(24)

MeRSi(OSnBu3)2 + Cl(MeRSiO)n-2SiMeRCl → (R2SiO)n + 2 Bu3SnCl

(25)

where R=Me, CH2CH2CN; n=3,4,5 Fully deuterated PDMS was obtained by a hydrolytic polycondensation of Ph2Si(CD3)2 catalyzed by CF3SO3H (trifluoromethanesulfonic, triflic acid) using stoichiometric amount of water (equation 26). Triflic acid acts here as the dephenylating agent, the catalyst of condensation and subsequent equilibration reactions [154,155]. PDMS may also be obtained by condensation-redistribution of hexamethyldisiloxane in the presence of the strong Lewis acids, according to equation 27 [141]. Thus, even Si-Ph and Si-Me may serve as functional groups in the synthesis of polysiloxanes.

n Ph2Si(CD3)2 + n H2O

n Me3SiOSiMe3

GaX3

CF3SO3H

((CD3)2SiO)n + n C6H6

Me3Si(OSiMe2)pOSiMe3 + (Me2SiO)q + n SiMe4

(26) (27)

Reaction of Silyl Hydrides with Alkoxysilanes Very recently, the coupling of SiH with alkoxysilanes catalyzed by B(C6F5)3 was reported [156,157]. Reaction is highly exothermic, (ΔH ≈ 250 kJ/mol), proceeds rapidly and quantitatively. The by-product, alkane, is volatile and chemically inert.

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R3SiH + R'OSiR3

B(C6F5)3

R3SiOSiR3 + R'H

(28)

The reaction may find a potential application for polymer synthesis. It was used to prepare phenylene-siloxane copolymers [157]. However, studies on model systems revealed that it is often non-selective. Thus, when Me3SiO-n-Oct is reacted with Ph2MeSiH, a mixture of condensation products containing all possible combinations of silyl groups, is formed (Scheme 5) [158].

General Review on Polysiloxane Synthesis k1

Me3SiOn-Oct + Ph2MeSiH

19

C8H18 + Me3SiOSiMePh2

k2 k-2

Ph2MeSiOn-Oct + Me3SiH

+ Ph2MeSiH

k4

k5

Ph2MeSiOSiMePh2 + C8H18

k3

C8H18 + Me3SiOSiMePh2

Me3SiOn-Oct

Me3SiOSiMe3 + C8H18

Scheme 5.

Electrochemical Synthesis of Oligosiloxanes Cyclic and linear oligosiloxanes were obtained by electrochemical oxidation of bifunctional silanes. The mechanism is postulated to involve transient formation of silanones (equation 29) [159,160].

R2SiX2

O2,+2e

R [R2Si=O]

SiO R

n

(29)

where R = Me, Ph; X = Cl, OAlk

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Ring-Opening Polymerization of Cyclosiloxanes Ring-opening polymerization (ROP) of cyclic siloxanes is a process of the transformation of a cyclosiloxane monomer into a linear siloxane polymer as a result of the cleavage of the SiOSi bond in the monomer ring and the subsequent reformation of this bond in the polymer chain. The ring-opening polymerization (ROP) of cyclosiloxanes provides the possibility to synthesize high MW polysiloxanes with a better precision than the polycondensation methods. A great variety of known cyclic siloxane structures make this method fairly universal. Cyclic monomers containing other grouping in an addition to siloxane in their skeletons, such as carbosiloxanes, siloxazanes, oxysilylenes, arylenesiloxanes and others (Scheme 6), may also be considered cyclic siloxane monomers, since they undergo polymerization according to the same mechanism.

20

Marek Cypryk Me O

Me Me

Si

Si O

Me

Ph

O

O

Si

Me

Me

Me

Me

Si

Si O

Ph O Si

Me

Me

Me

Si O

O Si

Me Si O Me

A (D3) Me

O

B (D2P)

Me

Si Me Me Si Me Si Si Me O Me Me

E (2D2)

Me Me

Si

O

Si

Me

Si

O

Si

O

G

Me Si

Me Me Si O Me O Si Me Me

D (D4) Si N

Si

Me Si O

O

C (V3)

Me

F

Me

O Si

Si N

Si

H

Scheme 6. (references: E,[161] F,[162] G,[163] H[164]).

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There is a rich literature on the polymerization of cyclic siloxanes. The reader is referred to several reviews on this subject [2-7,10,11,16,58,63,165-167]. Two cyclic siloxane monomers, octamethylcyclotetrasiloxane, (Me2SiO)4 (D4) and hexamethylcyclotrisiloxane, (Me2SiO)3 (D3) are the most important in polymer synthesis as the source of PDMS and various copolymers. Larger cyclosiloxanes and mixtures of cyclics may be used as monomers, too. Other cyclosiloxane monomers obtained by substitution of methyls by various organic radicals, such as fluoroalkyl, vinyl, phenyl, are used to produce copolymers having more specialized properties. There are two general methods of ROP of cyclosiloxanes: equilibrium polymerization, however, its applicability is limited to those systems where the equilibrium polymer yield is relatively high (see next Section). The alternative route is non-equilibrium polymerization, quenched before the equilibrium is attained [6,7,63,165,168]. The ROP of cyclic siloxane is also classified as anionic or cationic, according to the structure of the active propagation center.

Equilibrium Polymerization Equilibrium polymerization of cyclic siloxanes (called also equilibration or thermodynamically controlled polymerization), is carried out to the equilibrium state of the process [58]. This state is, by definition, independent of the starting siloxane substrates and of the initiator used (anionic or cationic). The polymer yield and its characteristics are not related to the kinetics of the polymerization. Instead, the knowledge on the thermodynamics of the process is essential. The final state of reaction involves complex equilibria between the polymeric species of two homologue series, cyclic and linear polysiloxanes. The equilibrium state may be described by general equation 30. (R2SiO)n ⇌ (R2SiO)x + -(R2SiO)n-x-

(30)

Since the starting material is usually a mixture of unstrained cyclic siloxanes and the resulting products are also unstrained siloxanes, the net enthalpy effect is close to 0. The

General Review on Polysiloxane Synthesis

21

driving force for equilibration is the entropy change. The equilibrium position is thus the result of the balance between two tendencies: the formation of smaller molecules, which is favored because the greater number of molecules means the higher entropy, and the formation of the polymer chains showing higher conformational entropy. Obviously, the polymer yield depends on initial concentration of the siloxane units, as dilution promotes cyclization. For each type of polysiloxane, there is a critical concentration threshold below which only cyclics exist in equilibrium. The entropy gain upon polymerization decreases with the increase in size and polarity of the organic substituents at silicon. Consequently, the yield of polymer at equilibrium is strongly reduced. For example, the equilibrium amount of polymer in bulk dimethylsiloxanes is ca. 82 w%, in methylhydridosiloxanes ca. 88 w%, while in 3,3,3trifluoropropylmethylsiloxanes only 17 w% [169]. The cyclics/chains proportion is described by equation 31 [170,171].

K ≈ [( R 2 SiO ) n]eq ≈

[( R 2 SiO) n]eq [− R 2 SiO −]

(31)

Equilibration reaction is catalyzed by strong acids like CF3SO3H and bases like KOH, R4NOH and R4POH [15,18,58]. Phosphazenes are very efficient catalysts, too [118,172-174]. Phosphazene bases are able to equilibrate cyclosiloxanes within seconds at very low catalyst levels [175]. The choice of catalyst is related to the chemical sensitivity of the side groups at silicon. The reaction is usually carried out at elevated temperatures. The chain transfer agent (functional disiloxane or a mixture of α,ω-difunctional oligosiloxanes) is often applied in order to regulate the molecular weight and to introduce the functional groups to the chain ends (equation 32), where X may be H, Me, vinyl, OR, Cl, etc. (R2SiO)n + XMe2OSiMe2X ⇌ XMe2O(R2SiO)nSiMe2X

(32)

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The average molecular weight of the resulting polymer is defined by equation 33, where [R2SiO]eq is the equilibrium concentration of R2SiO units in linear chains, M is the molecular weight of R2SiO and [MX2] is the concentration of the terminating agent [15]. Since equilibration is a random process, the polymer usually has the normal Flory MW distribution of 2, [176,177] unless association phenomena occur which may result in broader distribution [178].

Mn =

[ R 2 SiO]eqM [ M 2X ]

(33)

Practical Considerations The equilibrium polymerization of cyclosiloxanes is often used for the synthesis of polysiloxanes both in the industrial processes and in research laboratories as it has many advantages shortly summarized below. 1. This method itself does not impose any restriction on the initiator. Both anionic and cationic initiators may be used. Thus, it is possible to find such initiator which would

22

Marek Cypryk

2. 3.

4.

5.

be tolerated by functional groups in the polymer, would allow to reach the equilibrium state sufficiently fast under mild conditions and may be easily removed from the polymer. There are no rigorous requirements for the moment of quenching the reaction. There are no restrictions concerning the size of the monomer ring. The same results are obtained using various monomers of the same homologous series. A mixture of cyclics or a mixture of cyclic and linear polysiloxanes may be used as well. Molecular weight may be controlled by using chain blockers which are disiloxanes or short chain oligosiloxanes. The initial concentration of the initiator, which is also a source of end groups, must be much lower than that of the blocker. Using functionalized blockers results in functionalization of both chain ends. If the blocker is an organic polymer containing the SiOSi group, an organic-siloxane-organic triblock copolymer may be obtained. The equilibrium ROP of cyclosiloxane is a convenient route to statistical siloxane copolymers. It is possible to introduce functional groups pendant to polysiloxane chain which are statistically spread along the polymer chain.

There are also some limitations of using the equilibrium ROP of cyclosiloxanes in synthesis [165]. The main restraint is concerned with the formation of cyclic oligomers. Since the polymer yield decreases dramatically with the increase of the size and polarity of substituents, the process is usually used for the synthesis of dimethylsiloxane and hydridomethylsiloxane polymers and their copolymers. The equilibrium ROP is carried out in bulk as dilution of the system favors formation of cyclics. The reaction cannot be used for the synthesis of polysiloxanes with a narrow molecular weight distribution and precisely functionalized at the single extremity of the polymer chain. It is not suitable for the synthesis of copolymers with specific distribution of siloxane units, such as alternate or gradient copolymers [166].

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Non-Equilibrium (Kinetically Controlled) ROP of Cyclosiloxanes The ROP of strained cyclotrisiloxanes and their unstrained homologues, for example, cyclotetrasiloxanes, both lead to the same equilibrium state, however, on different routes. Polymerization of unstrained cyclosiloxanes leads to simultaneous formation of polymer and cyclic oligomers. The polymer concentration increase monotonously to eventually achieve its equilibrium value. In contrast, strained cyclotrisiloxane is transformed in the first, rapid step of the process mostly into linear polymer, which is randomized and partly decomposed to cyclics in the second step. The system attains the equilibrium state according to general scheme (equation 34): monomer

polymerization (propagation)

linear polymer

equilibration (back biting chain scrambling)

equilibrium mixture of polymer & cyclics

(34)

General Review on Polysiloxane Synthesis

23

The separation of the two stages of polymerization is possible, because the propagation of strained monomers is much faster than the back biting and chain transfer. In contrast to the entropy driven polymerization of unstrained cyclosiloxanes, the driving force for the polymerization of cyclotrisiloxanes is the enthalpy of the ring strain release. If the polymerization is quenched at a suitable moment, a high yield of polymer may be obtained even in the case when the equilibrium concentration of linear polysiloxane is very low. There are some inconsistencies in thermodynamic estimation of the ring strain in cyclotrisiloxanes reported by various authors. For example, the ring strain in D3 was measured by different authors to be 13 kJ/mol - 17 kJ/mol and 23 kJ/mol, [3] 50 kJ/mol - 63 kJ/mol, [15] and as high as 80 kJ/mol [179]. According to ab initio calculations, the ring strain in (H2SiO)3 is 19 kJ/mol, which suggests that the lower values among those reported for D3 are more likely [180]. Slightly higher strain enthalpy values, 22 kJ/mol and 25 kJ/mol, were measured for (Me(CF3CH2CH2)SiO)3 and (MePhSiO)3, respectively [3]. The apparent activation energies of anionic polymerization of cyclotrisiloxanes are by about 13 kJ/mol - 17 kJ/mol lower than those for basically unstrained cyclotetrasiloxanes, which would suggest almost full strain release in the transition state [3]. This corresponds also with the results of quantum chemical calculations for the cationic polymerization of (H2SiO)3 [180]. The kinetically controlled ROP of cyclotrisiloxanes is more difficult to perform than the equilibrium process. It requires more expensive monomers, selected initiators and more stringent conditions. On the other hand, it allows to obtain polymers of controlled structures and is the method of choice for those polymers, which cannot be obtained with a reasonable yield by the equilibrium polymerization, due to the low equilibrium concentration of linear fraction.

Anionic Ring Opening Polymerization of Cyclosiloxanes

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General Mechanism The anionic polymerization of cyclic siloxanes is initiated by strong inorganic, organic or organometallic bases [6,7,13,15,18,58,63]. The initiation involves the formation of a silanolate anion (equation 35) which is the active propagation center, able to break the siloxane bond in the cyclic monomer. Then the monomer is added to the growing chain and the active center is regenerated (equation 36). Cat+ is usually an alkali metal, quarternary ammonium or phosphonium cation.

B- Cat+ +

B

Si

SiO- Cat+ +

O

ki

Si

Si

O

Si

B Si kp kdep

B

SiO- Cat+

SiOSi

SiO- Cat+

(35)

(36)

Reversibility of the propagation step is due to the reaction of the active propagation center with siloxane bond in the chain (back biting). The back biting process generates a series of the monomer homologues of various ring size. The silanolate center may attack another chain as well, leading to the chain transfer, according to equation 37, what results in

24

Marek Cypryk

chain randomization. In the absence of any acidic contaminations, the reaction proceeds without termination. Thus, the polymerization must be quenched to deactivate the silanolate center.

Si1O- Cat+ +

Si2OSi2

Si1OSi2

+ Cat+ -OSi2

(37)

Initiators may be mono- or bi-functional. A mixture of bifunctional oligodimethylsiloxanediolates may easily be prepared (equation 38), removing water from the reaction system [181]. For n > 2, the oligodimethylsiloxanediolates are soluble in polydimethylsiloxane and in typical solvents, in contrast to alkali metal hydroxides, which were often used as initiators in earlier works [58].

2 Cat+ OH- +

(Me2SiO)n

Cat+ -O(SiMe2O)nSiMe2O- Cat+ + H2O (38)

Kinetics of Polymerization The rate of polymerization depends on the initiator, medium and monomer. Although the reactive center has the anionic structure, the role of the cation is very important [2,58,182]. In most systems, free silanolate ion does not appear in a kinetically significant concentration and the ion pairs are the true active propagation species [58]. In the reaction medium they exist in equilibrium with higher aggregates [2,58,182-184]. Since these complexes are much less reactive in propagation (or even inactive), the association strongly reduces the polymerization rate and affects the kinetic law, leading to the fractional order in the silanolate (equation 39).

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−

1 d [Monomer ] 1 =( [SiO - Cat + ]) n (kp[Monomer ] − kdep) dt nKn

(39)

For typical polymerization systems, ~Me2SiOK in bulk PDMS and ~Me2SiOLi in THF, the multiplicity of the complex, n, is 2 [182] and 3 (or 4 if [SiOLi] > 10-2 mol/L), [184] respectively. Interestingly enough, the aggregation is almost unaffected by temperature, thus it is mostly controlled by entropy factors [184]. Silanolates with bulky cations with delocalized charge show little tendency to aggregation. Thus, the polymerization of D3 on trimethylammonium silanolate shows the first order in silanolate [185]. The rate of polymerization of cyclosiloxanes in bulk strongly increases in the series of silanolates SiOLi < SiONa < SiOK < SiORb < SiOCs ≈ SiONMe4 ≈ SiOPBu4 due to the loosening of the anion-cation interaction with increase of the cation size, which shifts the equilibrium toward non-aggregated ion pairs which are more reactive [3,182,184,186]. Phosphazene superbases (structures 8, 9 and 10) have recently been explored as an extremely effective initiators of the ROP of cyclosiloxanes [187-192]. Neutral phosphazene bases (9, 10) require a proton donor, such as methanol, to form the true initiator, phosphazenium alkoxide (equations 40, 41). Bulky phosphazenium cations have a positive charge effectively delocalized by the resonance effect [189]. An existence of the bare silanolate anion in such a system is highly probable. The advantage of these initiators is that they are well soluble in the polymerization system.

General Review on Polysiloxane Synthesis P(NMe2)3 N tBu N P N P(NMe2)3 + ROH N P(NMe2)3

25

P(NMe2)3 H N tBu N P N P(NMe2)3 + OR N P(NMe2)3

(40)

9 HNEt

NEt (Me2N)3P N

(Me2N)3P N

P(NMe2)2 + ROH

(41)

P(NMe2)2 + OR

10

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Phosphorus ylides show similar activity in polymerization of D4 (equation 42). They are thermolabile and can easily be removed from polymer [193]. H (42) (Me2N)3P CMe2 + ROH (Me2N)3P CMe2 OR Uncharged nucleophiles, such as hexamethylphosphorotriamide (HMPT), dimethylsulfoxide (DMSO), dimethylformamide (DMF), capable of interacting with metal cations, substantially reduce the strength of the anion-cation interaction and significantly increase the polymerization rate [3,183,184,194]. These additives are referred to as the polymerization promoters or activators. The supramolecular complexes of silanolates with crown ethers [195,196] and cryptands [197,198] show also very high reactivity in polymerization of cyclic siloxanes. The rate of polymerization depends on the ring size of the monomer and on the substituents at silicon. Cyclotrisiloxanes are particularly reactive, due to the ring strain. A significant increase in the reactivity toward the alkali metal silanolate centers was observed in the series of unstrained cyclodimethylsiloxanes D4 < D5 < D6 < D7 < D8, when the reaction was performed in bulk or in a non-polar acid-base inert solvent [195,199]. D7 and larger cycles were opened faster than D4 by the factor of 200 and twice as fast as the strained D3. Similar reactivity enhancement was observed for the back biting process. An analogous behavior was noted in the cleavage of linear siloxane series Me3Si(OSiMe2)nOSiMe3 by the oligosiloxanolates [195]. The reaction rate increased by more than three orders of magnitude, going from n = 1 to n = 10. That behavior was explained by the multidentate interaction of the siloxane chain with metal cation, resulting in the decrease of the energy barrier of reaction (equation 43) [58]. Si Si O Si

O

Si

O

Si O

Mt

O O

propagation

Si O

Si

Si

Si

back biting

O Si

O

O

Si O

Mt

O

Si O

Si

O

Si

(43)

26

Marek Cypryk

This mechanism does not operate when the interactions of the siloxane chain with the counter-ion are suppressed, as, for example, in the presence of a nucleophilic additive, basic solvent or promoter strongly interacting with cation, or when a large cation, with efficiently stabilized charge is used [195]. The elimination of the mechanism involving the cationsiloxane interaction significantly suppresses back biting and chain randomization during polymerization of D3 what allows to control the polymerization process better [168]. The multidentate interaction does not affect the rate of polymerization of D3, since stiff, almost flat six-membered ring is not able to interact multidentally with a cation, what has recently been confirmed by ab initio calculations [200,201]. The rate of the anionic ROP of cyclosiloxane depends also on the organic substituents at silicon. In general, the electron withdrawing substituents increase the reactivity, making the silicon atom more electrophilic. However, the reduced electron density on silicon lowers the nucleophilicity of silanolate ion and of the monomer making weaker its interactions with the counter-ion [183]. Therefore, the net effect of polar substituent may be small. The relative reactivity of cyclosiloxanes in copolymerization increases considerably with electronegativity of the substituents. When the mixture of perphenylcyclotetrasiloxane and permethylcyclotetrasiloxane is polymerized, the former is almost fully converted before the latter begins to polymerize [199]. The anionic ROP of cyclosiloxanes may be strongly accelerated by some nucleophilic functional groups on the polymer, such as (CH2)2CN [202] or (CH2)3P(O)Ph2, [203] which may directly interact with the counter-ion.

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Effect of Water, Alcohol and Silanol Anionic ROP systems often contain small amounts of water or of other protic contaminants such as alcohols. These contaminants do not suppress polymerization unless they are more acidic than silanols themselves, although they may affect the reaction rate. They easily enter the polymerization process, forming the end groups and thus reducing the molecular weight of polymer. Water produces silanol groups which are the dormant centers in propagation, undergoing fast interconversion with silanolate anions [204,205]. Silanol groups strongly accelerate the terminal siloxane unit exchange, leading to the broadening of the molecular weight distribution (equation 44) [205]. Silanol groups also undergo homofunctional polycondensation, although this reaction is much slower than the terminal unit exchange [106,205].

Si1O- Cat+ + HOSi2OSi2

Si1OSi2OH + Cat+ -OSi2 (44)

Water can participate in chain transfer, producing chains growing in two directions. In polymerization systems using monofunctional initiator, the competition of unidirectional and bidirectional chain growth leads to the broad, or bimodal molecular weight distribution of polymer [204].

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General Review on Polysiloxane Synthesis

27

Applications of Anionic ROP of Cyclosiloxanes The first patents on the anionic ROP of cyclosiloxanes appeared in the late 40-ties. Since that time this reaction has been broadly used in industrial synthesis of polysiloxanes and still the interest in the development of this process is vivid [28,42]. It may be applied for the synthesis of polysiloxanes with various substituents, provided they do not react with the basic initiator or the silanolate center. Thus, monomers having groups susceptible to the base cleavage from silicon, such as Si-CH2Cl, Si~COOH cannot be polymerized in this way. Some polymers bearing groups which had been believed to be unstable under basic conditions, were recently obtained by anionic non-equilibrium polymerization of cyclotrisiloxanes, for example, poly(3-chloropropyl)methylsiloxane [151] and polyhydridosiloxanes [206-208]. The industrial process of the anionic equilibration of cyclic siloxanes is usually carried out in bulk at elevated temperature. The choice of initiator is critical. Some contaminations, originating from the initiator, particularly those of acidic, basic or ionic nature, dramatically reduce thermal stability of polysiloxanes. Thermolabile silanolates, such as Me4N+ -OSi≡ or Bu4P+ -OSi≡, are often applied as the initiators, since they can be easily removed from polymer by thermal decomposition [209-211]. Silanolate centers must be neutralized to avoid decomposition of polymer. Me3SiCl is commonly used for this purpose, as it introduces the inert Me3Si groups to the chain ends, however, it may react not sufficiently fast with strongly aggregated silanolates. The non-equilibrium anionic ROP of cyclosiloxanes exploits the reactive strained cyclotrisiloxanes as monomers, which allows to obtain high MW polymer with a good yield (> 90%). The polymerization must be quenched before the monomer is totally consumed to minimize the role of back biting and chain randomization [168]. In contrast to the equilibrium polymerization, in this reaction, polymer may be obtained even in a dilute system. The kinetically controlled ROP is used when the linear polymer is thermodynamically disfavored, for example, in the case of poly(3,3,3-trifluoropropyl)methylsiloxane, [212] polyphenylmethylsiloxane [213] or polydiphenylsiloxane [144,214,215]. The AROP of cyclotrisiloxanes is also extensively used in research laboratories [216219]. The most important use of this process is the controlled synthesis of functionalized polysiloxane polymers and copolymers [166]. Elimination of the multidentate interaction of a counter-ion with the siloxane chain is crucial. Otherwise, as it was mentioned before, the equilibration reactions would make the precision polymerization impossible. Specific initiator-solvent systems used for this purpose may be divided into three groups: (1) basic solvent and a hard counter-ion, which interacts with solvent stronger than with siloxane, e.g., lithium/THF; (2) bulky and soft counter-ions, e.g. Me4N+, Bu4P+, and phosphazene base-conjugated acid, which weakly interact with nucleophiles; (3) basic additives strongly interacting with counter-ions, such as HMPT, DMSO, cryptands, crown ethers [58]. The non-equilibrium precision anionic ROP is commonly used for the synthesis of endfunctionalized polysiloxanes, in particular polysiloxane macromonomers [220,221] and macroinitiators [222]. It provides a high control of molecular weight, molecular weight distribution and functionalization [168]. Thus, a polydispersity index Mw/Mn < 1.1 may be achieved [223]. The functionalization of chain ends is accomplished by using a functionalized initiator, [224] a functionalized terminator [225] or both [226]. The functionalization of polysiloxane from both sides by the initiator method is also possible using the stoichiometric

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Marek Cypryk

amounts of bifunctional terminator, such as Me2SiCl2 [227]. Functional polysiloxanes are widely applied in macromolecular engineering for the synthesis of block copolymers, [198,228] graft copolymers, [220,229] regular star-branched polymers, [184,224,230] regular polymer networks, [225] and interpenetrating networks [18,227]. Quenching with the stoichiometric amount of a multifunctional terminator, such as MeSiCl3 or (Cl2MeSiCH2)2 gives a star-branched polymer [184,224,230,231].

Stereoselectivity and Regioselectivity in Non-Equilibrium Anionic Polymerization The unsymmetrically substituted polysiloxanes exhibit stereoisomerism. Subsequent cross-linking of polymers with enhanced stereoregularity gives siloxane elastomers which have properties superior to their atactic analogues [34]. The synthesis of stereoregular polysiloxanes was reviewed by Saam [232]. The cyclic trisiloxanes with different substituents at the same silicon atom (RR’SiO)3 exist as two isomers, cis and trans (structures 11 and 12).

X XO Si XO Si O Si

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11

X XO Si

Si O O

Si X

12

The stereoisomers were isolated for methylphenyl-, [233-236] methyl(3,3,3trifluoropropyl)-, [237] phenylethenyl- [238] and alkylhydroxy- [148,239] substituted cyclotrisiloxanes. 19F NMR studies of the microstructure of poly(3,3,3trifluoropropyl)methylsiloxanes, obtained by the non-equilibrium polymerization of cis and trans cyclotrisiloxanes in THF, initiated by Me3SiOLi, showed that the relative stereoconfiguration at silicon atoms in the monomer is preserved upon propagation [237]. There is an equal probability of the meso (m) and racemic (r) addition of the monomer, indicating that the attack of the silanolate on silicon in the ring is equally probable from both directions. Similar conclusions have been drawn for the reactions of phenylmethyl substituted isomers [236,240]. Polymerization of the trans isomer leads to polymer of a lesser degree of stereoregularity because the silicon atoms in the ring are not sterically identical. Thus, while the polymers obtained from the cis-trifluoropropyl-substituted cyclic are solid and crystalline at room temperature, the products obtained from the trans isomer and from the mixtures of isomers under these conditions, were liquid and amorphous [237]. The regioselective course of the polymerization of cyclosiloxanes containing different siloxane units would lead to regular alternative copolymers. The preparation of such copolymers by non-equilibrium polymerization of some cyclotrisiloxanes with mixed units was reported [213,241]. A detailed sequence analysis in copolymers obtained by anionic polymerization of 1,1-diphenyl-3,3,5,5-tetramethylcyclotrisiloxane has shown that the monomer ring was opened at both nonequivalent siloxane linkages, -Me2SiOSiMe2- and -Me2SiOSiPh2-.[242]. However, the proportion of both openings strongly depends on the counter-ion. On the other hand, polymerization of vinylpentamethylcyclotrisiloxane at -30 °C gave almost regular polymer with 90% of opening

General Review on Polysiloxane Synthesis

29

at vinyl-substituted silicon atom [243]. Regular polymers were also obtained by AROP of some hydridocyclotrisiloxanes [206].

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Cationic Ring Opening Polymerization of Cyclosiloxanes Cyclic siloxanes can be transformed into linear polymers, using both Brönsted and Lewis acid catalysts [6,7,13,58,63,165,166]. The cationic polymerization of cyclosiloxanes is a convenient route to the linear polysiloxanes, as it may be performed with a suitable rate at room temperature and the catalyst may be easily removed from the polymer. This is the method of choice for the synthesis of siloxane polymers and copolymers with substituents which are unstable in the presence of strong bases, such as SiH, SiCl, SiCH2Cl, Si(CH2)nCOOH. The main disadvantage of the cationic process is the formation of significant amounts of cyclic oligomers simultaneously with polymer. This general feature seriously limits the application of the process for the kinetically controlled synthesis of polysiloxanes. Strong protic acids, such as H2SO4, [244] sulfonic acids RSO3H, [112,244-246] HClO4, [247] and tetrakis(pentafluorophenyl)boric acid hydrate (7) [248] are the effective initiators of the cationic polymerization of cyclosiloxanes. Solid acids, such as ion exchange resins, e.g., sulfonated polystyrene, [249] acidic minerals and acid-activated clays [250-252] are often used, since they may be easily removed from polymer by filtration. Lewis acids, such as FeCl3 [253] and SnCl4, [254,255] are believed to initiate polymerization in cooperation with protic acids, resulting from the reaction of those species with traces of water or other acidic contaminations present in the system. Indeed, some Lewis acid-protic acid combinations are very effective initiators [253]. The studies of the polymerization of D3 in the presence of sterically hindered substituted pyridine used as a proton trap, proved that some nonprotic species, such as RC(O)Cl-SbCl5 complex [256] and ethylboron sesquitriflate (Et3B2(OTf)3) [257] are able to initiate polymerization. Silyloxonium ions are effective catalysts of the polymerization of cyclosiloxanes [258]. Trimethylsilyl triflate (TMSOTf) was considered to be inactive without addition of free trifluoromethanesulfonic (triflic) acid [259]. However, Jallouli and Saam reported that, after a long induction period, TMSOTf initiated the polymerization of some cyclotrisiloxanes, such as D3, even in the presence of 2,6-di-t-butylpyridine, used as the acid scavenger [260]. Various onium salts, such as oxonium, sulfonium, iodonium, acylium and others, having nonnucleophilic complexed counter-ions, e.g., Al(OTf)4, were also reported to initiate the cationic polymerization of siloxanes and other heterocyclic monomers [261,262]. Phosphonitrile halides have recently been proved to be very efficient catalysts [172,252]. In the industry, the cationic initiators are usually used for the equilibration of cyclosiloxanes in the presence of a chain stopper, which is used to control the molecular weight and to introduce the desired functional groups to the chain ends [58]. In this way, the telechelic polysiloxanes were prepared and further applied to the synthesis of siloxanesiloxane and siloxane-organic block copolymers. The cationic process appears to be more efficient than the anionic one for the polymerization of cyclotrisiloxanes substituted with large alkyl groups [263].

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Marek Cypryk

Mechanism of the Polymerization Initiated by Protic Acids Protic acids are the most common initiators, used in the cationic ring-opening polymerisation of siloxanes. Their initiating power increases with the acid strength. Thus, CF3SO3H and HClO4 are particularly effective, while CF3CO3H initiates only a slow polymerization of D3 [63]. The process is very sensitive to additives. Some of them, like the basic solvents, reduce the reaction rate, others, like weaker acids, accelerate the process. Water shows an ambivalent behavior, being able to act either as a promoter or as an inhibitor [7,165]. Sonification may effectively increase the polymerization rate and reduce the polydispersity of the polymer [264]. Most of kinetic investigations have been performed on the polymerization of D3 and D4, initiated by CF3SO3H (TfOH). Some studies on the polymerization of larger rings, D5, D6 and D7 have also been reported [265]. The results are comprehensively presented in earlier reviews [4,5,58]. A much higher reactivity of cyclotrisiloxanes, as compared to larger cyclics, is the reason for several important kinetic differences between these two classes of monomers. Some competitive reactions, like back-biting and chain scrambling, which are important in the polymerization of D4, are negligible in the polymerization of D3, as long as the monomer is present in the reaction medium. Nonetheless, large amounts of cyclic oligomers D3n are produced under these conditions [255]. The mechanism of the cationic polymerization of cyclosiloxanes is very complex and has been controversial for a long time. The most important features of the cationic polymerization of cyclosiloxanes are: [6,58,266,267]

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1. Considerable amounts of cyclic oligomers are formed simultaneously with polymer. 2. In the kinetically controlled (non-equilibrium) stage of polymerization of cyclotrisiloxanes cycles being the multiples of monomer, (R2SiO)3n, dominate. 3. The activity of initiator (protic acid) maintains throughout the entire process (continuous initiation). 4. Complex kinetic dependencies and complex influence of water indicate the importance of association phenomena. 5. Number average molecular weight of polymer increases proportionally with monomer conversion. 6. The polymer has a relatively broad MW distribution (1.6-2).

Initiation Initiation by protic acids involves cleavage of the siloxane bond in monomer with formation of the corresponding oligosiloxane, H(OSiR2)nA, terminated by hydroxyl group at one end and by the ester group at the other. High order of the reaction in acid was interpreted in terms of higher homocomplexes (or hydrates) of acid being the active species [266,268]. This assumption is supported by recent quantum chemical calculations [110].

General Review on Polysiloxane Synthesis

H

(HA)n-1 A

H

H

A O

H

(HA)n

O H O

H

H

O O

Si

13

31

Si

H O

H H

14

Silanol and ester end groups undergo fast exchange, resulting in the polymer chains growing in both directions. (45)

~R2SiA + HOSi1R2~ ⇌ ~R2SiOH + ASi1R2~

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Cyclization The excess of (R2SiO)3n cyclics in the kinetic stage of polymerization of cyclotrisiloxanes is due to the end-to-end coupling [255]. Cyclization, intermolecular condensation as well as the end group exchange lead to establishing of the stationary concentrations of water and acid, which is able to permanently initiate new polymer chains. Thus, the important condition of the living polymerization system, i.e., fast and quantitative initiation causing polymer chains to grow simultaneously, is not fulfilled. Propagation The detailed mechanism is still controversial. In analogy to the polymerization of cyclic ethers and acetals, it is usually assumed that the active propagation center is a tertiary siloxonium ion, resulting from the attack of monomer on the ester chain end. Kinetic data suggest that the ester group must be activated by acid, because siloxane monomer itself is too weakly basic to attack the inactivated ester [266] or it requires a very long time to proceed [260]. Alternatively, Sigwalt proposed a mechanism involving propagation on acid-activated ester groups and/or silyloxonium ions [259]. The concept of trisilyloxonium ion received strong support from Olah, who observed such ions directly by 29Si NMR. [269]. He also proved that such ions can induce polymerization of D3 and D4. Since the time when the tertiary siloxonium ions were detected by 29Si NMR in the presence of extremely low nucleophilic counter-ion (equation 46) and proved to initiate the polymerization of cyclic siloxanes, D3 and D4, their role as the active centers of propagation has become commonly accepted [258]. Me3SiH + Ph3C+B(C6F5)4- +

SiOSi

Si Me3Si O+ Si

B(C6F5)4- + Ph3CH

(46) However, kinetic studies led Sigwalt to conclusion that silyloxonium ions are not the dominating propagation species in the polymerization initiated by protic acids [270a,b].

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Recently, using Olah’s initiator and the more nucleophilic monomer, octamethyltetrasila-1,4dioxane (2D2, structure E in scheme 6), which forms more stable silyloxonium ions, it was possible to observe by 29Si NMR the transformation of the silyloxonium ions formed in reaction of monomer with initiator into silyloxonium ions at the end of the polymer chain (equations 47, 48) [271a]. Polymerization of cyclotrisiloxanes initiated by R3SiH/B(C6F5)3 system is also postulated to occur via silyloxonium active species [271b]. Si Si Et3SiO+ O B(C6F5)4Si Si

Si Si +

[Et3Si

B(C6F5)4-]

+ O

O Si Si

(47) Si Si

Si Si Et3SiO+ Si Si

O B(C6F5)4-

+ O

Si Si O

-

O B(C6F5)4

Et3SiOSiSiOSiSiO

Si Si

Si Si

(48) Studies of the microstructure of polymers obtained by cationic polymerization of 1,1diphenyltetramethylcyclotrisiloxane showed that all observed features of the cationic ring opening polymerization of cyclosiloxanes may be rationalized assuming silyloxonium active centers being in equilibrium with dormant ester end groups [272]. Due to continuous initiation, fast exchange of end groups and extensive chain transfer to the terminal trimethylsiloxy unit, cationic polymerization of cyclosiloxanes is not suitable for the precise synthesis of well-defined polymers. In a series of works, Weber et al. compared the regioselectivity of anionic and cationic ring opening polymerization of various functional cyclotrisiloxanes (15) [273,274]. In all cases anionic ROP appeared to be significantly more regioselective than the cationic process.

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R1 R2 Si O O Me Si Si Me O Me Me 15 (a) R1=R2=(CH2)2(CF2)5CF3; (b) R1=Ph, R2=m-C6H4CF3; (c) R1=Ph, R2=m-C6H4(CF3)2

Other Methods of Polymerization of Cyclosiloxanes Polymerization in Solid State Solid-state polymerization of cyclotrisiloxanes may be induced by both acidic and basic initiators. Thus, D3 polymerizes in the presence of gaseous HBr chemisorbed at the surface of the monomer crystal, giving polymer of molecular weight of 1.5×105 - 3×105 with up to 80%

General Review on Polysiloxane Synthesis

33

yield [275]. Anionic non-equilibrium polymerizations of cyclic siloxanes, initiated by KOH, crushed and mixed with the monomer s well as by potassium silanolates were reported to give high MW materials with a high yield [276,277]. Polydiphenylsiloxane obtained in reaction of the crystalline monomer with KOH showed a molecular weight of 4.4×104 and a very broad polydispersity, Mw/Mn = 21.7 [278]. Polymers having much higher molecular weights (Mn > 5×105) and polydisperities of about 2 were obtained when potassium oligomethylphenylsilanolate was used as the initiator. Polymerization proceeds inward from the surface of the monomer crystals, producing a highly crystalline material. Highly ordered crystalline state of hydroxycyclosiloxanes provides a possibility of solid-state synthesis of stereoregular polysiloxanes [279].

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Radiation Polymerization The radiation-induced polymerization of D3, D4 and D5 has been studied in both liquid and solid state [280,281]. The propagation occurs primarily via the cationic mechanism. In contrast to the polymerization initiated by acids, all the monomers show very similar reactivities. This was tentatively explained assuming that the mechanism involved the formation of highly reactive free silylium ions, which reacted in the same way with various monomers. The silylium ions are generated as a result of methide cleavage. The reaction proceeds at the surface. The polymer chains grow on the interface of formed polymer and the D3 crystal surface. The reaction requires the use of extremely pure and dry monomer. The impurities able to generate negative ions upon irradiation as well as crystal defects strongly decrease the reaction rate and molecular weight of the Polymerization in Emulsion Hyde and Wehrly demonstrated a possibility of performing the ROP of cyclosiloxanes in water emulsion [282]. High molecular weight polymer may be obtained, either in the presence of anionic initiator and cationic emulsifiers or using the cationic initiator and anionic emulsifier [283]. The use of emulsifier, capable of initiating the polymerization, such as dodecylbenzenesulfonic acid (DBSA) or dodecylbenzyldimethylammonium hydroxide, permits an effective stabilization of the siloxane emulsion and ensures a high rate of the polymerization under mild conditions (25 ºC - 80 ºC). The diameter of the particles obtained by cationic polymerization is 0.05 μm - 0.5 μm and the molecular weight is higher than 2×105. The polymer contains about 15% of cyclic oligomers D4 to D10 [284]. The anionic emulsion ROP of D4 gave the polymer of somewhat lower molecular weight Mn = 50000 with the particle diameter of 0.2 μm - 2 μm [285]. The proposed mechanisms of the polymerization are complex. The yield and characteristics of resulting polymers are controlled both by chemical reactions and by physico-chemical phenomena, such as the diffusion of monomer, phase equilibria and the nature of the interface. Since these factors depend on the composition of the reaction mixture, the mechanism may change with the extent of the reaction. Polymerization proceeds by the combination of the addition and condensation mechanism involving redistribution reactions [285,286]. First stage of the anionic polymerization process occurs in the siloxane-water interface or in the siloxane phase close to the surface. Once the chains reach a critical degree of polymerization corresponding to their loss of surface tension activity, they penetrate into the particles where side reactions such as redistribution and condensation hardly occur [286].

34

Marek Cypryk

Thus, the rate is strongly dependent on the size of the surface, which is the function of the concentration of emulsifier. Polycondensation is responsible for a rapid increase in molecular weight, observed at high monomer conversions. The condensation mechanism of the chain growth seems to be still more important in the cationic emulsion ROP of cyclosiloxanes. Although this process is very efficient for the synthesis of linear polymethylhydrogenosiloxanes (PMHS) from DH4, these conditions do not seem suitable for the polymerization of D4 [287-289]. Anionic miniemulsion process was also used for polymerization cyclosiloxanes with substituents other than methyl. Polymerization of 1,3,5-tris(trifluoropropyl)trimethyl cyclotrisiloxane (F3) produced well-defined α,ω-dihydroxy-terminated polymer in very high yields and with molar masses ranging from 2500 to 3500 [290]. A kinetic study showed that polymerization occurs in two stages. During the first stage, which corresponds to the anionic nonequilibrium ring-opening polymerization of F3, the maximum yield is close to 100% and the polydispersity remains at around 1.3. The second stage involves condensation and backbiting reactions. Molecular weight of polymer increases up to 30000-60000 and polydispersity approaches 2.0 [290] Polymerization of phenylmethylcyclosiloxanes, P3 and P4, was also reported. Narrow molecular weight of the obtained α,ωdihydroxypolymethylphenylsiloxanes and a dramatic reduction of back-biting reactions was observed [291]. Copolymerization of D4 with tetramethyltetravinylcyclotetrasiloxane (V4) gave a random copolymer with MW up to 50000. On the other hand, polymerization of V4 in the presence of linear PDMS led to multiblock copolymers [292].

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Siloxane Functional Polymers and Copolymers This section is dedicated to full-siloxane copolymers. Siloxane-organic copolymers constitute the separate very broad class of macromolecular structures, and the reader is referred to other reviews [10,16,18,166,293-295]. Side groups have a great impact on the physical properties and chemical reactivity of polysiloxanes, giving access to new applications. Copolymers often have properties which are superior to those exhibited by homopolymers. Thus, for example, introduction of methylphenyl- or diphenyl-siloxane units to PDMS significantly improves thermal stability of the polymer. Fluoroalkyl or cyanoalkyl groups in the siloxane chain increase the solvent resistance and reduce swelling [7,296]. Copolymers of diethylsiloxane with methylphenyl- or diphenyl-siloxane prevent crystallization and lower Tg of the copolymers down to -137 °C [297]. The character of side groups has a distinct impact on conformation and behavior of the polymer in solution [298]. Hydrophilic groups give copolymer excellent surfactant properties [37]. There are two general routes to siloxane copolymers: (1) synthesis of copolysiloxanes by polycondensation of bifunctional silane or siloxane monomers or by ring-opening polymerization of functional cyclosiloxanes and (2) modification of polysiloxanes by reactions on polymer [6,7,13,296,299].

Synthesis of Siloxane Copolymers Synthesis of siloxane copolymers may be accomplished by three methods: copolycondensation, sequential ROP and simultaneous ROP of the mixture of cyclics

General Review on Polysiloxane Synthesis

35

[7,13,296]. Copolycondensation of difunctional silanes (for example, hydrolytic polycondensation of two dichlorosilanes) usually leads to random copolymers [2]. Heterofunctional condensation of difunctional silanes or prepolymers terminated by the reactive groups makes possible the preparation of regular, alternate or multiblock copolymers, respectively [296]. Sequential ring opening copolymerization involves polymerization of a first cyclosiloxane monomer to the desired chain length, then the second monomer is introduced, which undergoes addition to growing chains. Diblock and triblock copolymers (depending on the initiatior: whether the chains grow in one or in both directions) are produced if the process is performed in a kinetically controlled way. Simultaneous polymerization of the mixture of cyclosiloxanes gives polymers which microstructure is not easily predictable. Equilibrium copolymerization of unstrained cyclosiloxanes leads to random copolymers [300,301]. Controlled anionic ROP of cyclotrisiloxanes is particularly important, as it allows to obtain well defined silicone structures. Thus, sequential copolymerization of two or more cyclotrisiloxanes is a good method for the precision synthesis of all-siloxane block copolymers [151,166,302]. Simultaneous copolymerization of two cyclotrisiloxanes with different reactivities gives gradient copolysiloxanes, which composition continuously changes along the chain [243,303]. Ring opening polymerization of a cyclotrisiloxane containing various substituents at silicon atoms in the ring is a special case, where homopolymerization of a single monomer results in the copolymer with different siloxane units uniformly distributed along the chain [131,206,207,219,242,272,273]. Cyclotrisiloxanes containing two different functional groups, Si-H and Si-Vi, such as 1-hydrido-1-vinyldimethylsiloxy-3,3,5,5tetramethylcyclotrisiloxane, may be “orthogonally” polymerized according to two different mechanistic pathways, which lead to different polymers (scheme 7) [208]. Obtained polymers may be easily crosslinked.

Si O

TfOH

Si

O Si

Li+-OSiPh

Si

H O Si

O "Pt"

- + 2O Li

Si

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

O O Si O Si O Si H

n

Si

O O

O Si

Si O n

Scheme 7.

Modification of Polysiloxanes It is the most extensively used method and it may be applied to homopolysiloxanes as well as to copolysiloxanes [299]. It is particularly useful when monomers bearing target functional groups are not easily obtainable or polymerizable. The alternative route to such materials is the synthesis of polymers with precursor groups which may be transformed to the desired functions by reaction on polymer. A wide range of reactions may be used to achieve

36

Marek Cypryk

these modifications, and among them, hydrosilylation is the most commonly used one. Hydrosilylation in general means the addition of Si-H to various kinds of multiple bonds, catalyzed by transition metal (mainly platinum) complexes [46-48]. Effectiveness of rhodium catalysts was also recently discussed [304]. This term is used both for the reaction of SiH groups on polymer with low molecular compounds containing a multiple bond and vice-versa (sometimes referred to as the reverse hydrosilylation). The literature up to 2000 is comprehensively covered in the review by Boutevin et al. [299]. This reaction is commonly used for synthesis of liquid crystalline (LC) polysiloxanes, very important class of silicone polymers [305-307]. Some examples of the recent applications of the hydrosilylation reaction for preparation of multifunctional polysiloxanes are presented below. In some cases the propagation is regioselective and leads to regular alternate order of units. This is the case shown in equations 49 and 50 where Si-H appears at every third silicon atom along the chain [206,207]. H O Si

Si

O

(LiO)2SiPh2/THF 2 Me3SiCl -78oC

Si

O

H Me3Si

H

OSi(OSi)2 OSiPh2O

(SiO)2SiO

n/2

SiMe3 n/2

I

(49) F

F

F

F

hydrosilylation

I +

(Pt)

H Me3Si

H

OSi(OSi)2 OSiPh2O

(SiO)2SiO

n/2

n/2

F F F

SiMe3

F

F

F

F

F F F

F

(50)

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Neighboring effect in hydrosilylation of allylglicidylether by MeHSiO-Me2SiO copolymer was studied [308]. Copolymers containing charge-transporting agents and electrooptical chromophores (scheme 8) were prepared by hydrosilylation involving hydridocopolysiloxanes [309]. CH3 Si

N

CH3 O

x

Si

O

y

R=

N

CH3

O N

R

N N N

N

N

N NO2

NO2

Scheme 8.

NO2

General Review on Polysiloxane Synthesis

37

Ungurenasu obtained the C60-substituted cyclotetrasiloxane by hydrosilylation of fullerene by DH4 on Pt/C and successfully polymerized this monomer in the presence of triflic acid to obtain the polysiloxane with MW = 95000 (equation 51) [310]. H Si O

C60H

C60, Pt/C 80o

4

Si O Me

Me

TfOH 4

C60H Si

O n

Me

(51)

Addition to a Vinyl Group The vinyl group at silicon is another commonly used function for various modifications. The modification is mainly achieved by free radical or transition metal catalyzed addition of an X-H reagent to the C-C double bond. Examples of such reactions are addition of Si-H (reverse hydrosilylation), [302,311-314] S-H (thiol-ene addition or hydrosulfidation) [315] and P-H (hydrophosphination) [203]. Vinyl groups may be transformed to epoxy functions by oxidation with peroxides [316,317]. Optically active all-siloxane polymers may be obtained by the AROP if the monomer bears an optically active substituent [217,315]. Nucleophilic Substitution of Halogen Another method to modify side chains in polysiloxanes is the nucleophilic substitution of the terminal halogen in a haloalkyl side chain [151,318]. The quaternization of amines by 3chloropropyl or 3-bromopropyl groups attached to polysiloxane chains. Highly hydrophilic linear all-siloxane copolymers bearing the quarternary ammonium groups are generated, which may find applications as powerful surfactants and biocidal additives or conducting materials (equation 52) [319-321]. They may also be obtained by quarternization of 3aminopropyl-substituted polysiloxane with methyl iodide [322]. +

Cl- NR3

Cl

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Bu

SiO

n

SiO

m

SiMe3

R3N

Bu

SiO

n

SiO

m

SiMe3

precursor polymer (52)

Silylative Coupling Vinyl-substituted cyclosiloxanes can be converted chemo- and regioselectively to styryland β-alkoxyvinyl-substituted derivatives via respective silylative coupling reactions with styrene and vinyl alkyl ethers catalyzed by ruthenium complexes. This method opens a new route to functionalization of siloxane monomers and polymers (equation 53) [238,323,324].

38

Marek Cypryk

R R

X Si X Si

Si

X Si X n

X = O, NH n = 0, 1

R [Ru-H], -CH2=CH2

X Si X Si

Si X Si X R

R

n

R = Ph, On-Bu, Ot-Bu, OTMS (53)

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Three-Dimensional Polysiloxane Architectures: Silsesquioxanes, Hyperbranched Polymers and Dendrimers There are several types of three-dimensional macromolecular architectures built of the siloxane bonds: hyperbranched and dendritic polymers, silsesquioxanes, polysiloxane networks (silicon rubbers, elastomers, resins), interpenetrating networks (IPN), and silica. In this review only silsesquioxanes and hyperbranched and dendritic siloxane polymers will be briefly discussed. The other structures, i.e., polysiloxane networks, [4,5,8,10,15,325,326,326] IPN [325,327,328] and silica [15,64,65,329-331] are extremely broad research areas, out of the scope of this review. Linear polysiloxane polymers and copolymers, functionalized even with large organic groups, or grafted with organic polymers, were arbitrarily discussed along with other linear polysiloxanes. When polysiloxane forms a branch on another polymer tree – organic or siloxane, such a structure would fall into the category of siloxane branched copolymers. Synthesis of various branched organic-siloxane and siloxane-siloxane copolymers is often accomplished using the AROP of cyclotrisiloxanes The reaction is exploited for the synthesis of macromonomers used further for copolymerization with vinyl monomers to obtain organic polymers with grafted polysiloxanes. The other approach is the grafting of an end-functionalized polysiloxane, generated by the AROP, on a side organic polymer with functionalized side groups [18,166]. Siloxane star-copolymers can be synthesized by the reaction of monofunctional siloxane capable of reacting with a multifunctional terminator [184,231,332] or by using polyfunctional initiators [333,334]. Dendritic branched polysiloxanes may be obtained by graft on graft or graft on star techniques [303]. It is not clear how can one classify poly(pentamethylcyclopentasiloxane) - a polymer with the lowest known Tg (-152 °C) recently studied by Kennedy and co-workers, which is apparently made of the randomly connected siloxane rings (scheme 9) [335,336].

General Review on Polysiloxane Synthesis O

CH3

CH3

Si O H

Si

O H3C

CH3

CH3 Si H

CH3

H Si

H

O

Si O

O

H

O

Si O

H CH3

O

CH3 Si

Si

H3C

O

H

Si

O

OH Si

H3C

-H2O

H3C

CH3

(intermediate, not isolated)

O

CH3

O

O

H H O

H3C O

Si

Si O

CH3

Si O

O

H

O Si O CH3 H3C O CH3 H C 3 CH3 Si O

O

O H3C

Si

O Si

O CH3

Si

O CH3 Si H H O H H3C O

Si

CH3

Si

Si H3C

Si O

Si

O

Si

O

H CH3

Si

O

H3C H3C

Si

O O

Si

H

CH3

O

Si H3C

H

H

O

Si

CH3

CH3

CH3

O

O

H3C

Si

Si

CH3

Si

O

Si

H3C

Si

Si

O

O

O

O

O

O

Si

H

CH3

O

O

HO

H3C

O

O

H3C O

H3C

Si

H

Si

H H

O

H

O

Si

CH3

CH3

H3C

Si

Si CH3 O

Si

Si

O Si O

O

O

O

O

Si

CH3

O

Si

H3C

CH3

DH5

Si

O

H

O

CH3

HO Si

Si

CH3

Pt H2O -H2

O

39

CH3

O

O

H3C H

H Si

H3C

Si

O Si

H3C O

Si

CH3

O

Si O

O

Scheme 9.

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Special types of supramolecular structures may be achieved by reversible association of telechelic polysiloxanes terminated by polar groups capable of forming strong hydrogen bonds [337-340].

Silsesquioxanes Silsesquioxane is a name given to silicon structures of the empirical formula RSiO3/2. Silsesquioxanes represent a wide class of more or less ordered three-dimensional structures, examples of which are shown in the scheme 10. They are usually generated by hydrolytic condensation of trialkoxy- or trichlorosilanes. Although the regular structures seem to be highly entropically unfavorable, they may be prepared with a reasonable yield by quasi-equilibration, taking advantage of their low solubility in certain solvent systems. However, thermodynamics of this process in acidic media must be more complex, since large amount of cage-like products were identified in solution before they start to crystallize [341]. There is a huge literature on their preparation, properties and applications, including a number of excellent reviews [341-350]. Silsesquioxanes, by their O/Si proportion take the intermediate position between siloxanes (O/Si = 1) and silica (O/Si = 2). They are often used as models of silica surfaces, silica-supported catalysts, etc. Heterosilsesquioxanes, in which one or more silicon atom is replaced by metal (aluminum, titanium, vanadium and so on) attract much attention as the potential catalysts.

40

Marek Cypryk R

R

Si O R O Si R SiO O O O OR Si O Si R O Si R

O R

Si

Si OO

R Si O O R

Si

O

R

O

R

Si

HO

O O Si R O Si R O O

R

Si

OO

Si O

T6

OH R R O O O Si Si Si Si R O HO R O O O O Si Si R O O R Si Si O Si R R R Si O OH O O O HO Si O R OH O Si O Si O Si O Si R R R R

Si

O

O O Si R O Si R O O Si

O

R

R T8

R

O

R Si O

Si

R

Si

partial cage structure

R

R

R Si

O

O

Si

O

Si

O

Si

O

Si

O

R

R

R

Si O

O Si

O

Si

O

R

R

R ladder

random

Scheme 10.

Functional poly- and oligo-silsesquioxanes are applied as resin additives and crosslinkers improving the properties of coatings. There is also a great number of silsesquioxane-organic copolymers of various architecture. Silsesquioxanes due to their multifunctionality are often used as the core for starburst copolymers [341,348,349]. They may also be copolymerized with cyclosiloxanes to get crosslinked polymers of various structures. For example, the crosslinked polysiloxanes were directly synthesized by anionic ring-opening copolymerization of Ph12-POSS as the multifunctional monomer with D4 and Ph8D4 using KOH or Me4NOH siloxanolate as initiators (equation 54). The DSC and TG results indicated that the crosslinked polysiloxanes exhibited distinct glass transition temperatures and excellent thermal stability [351]. R6Si6O9 (T6) cages having two incorporated cyclotrisiloxane rings are particularly interesting because they may serve as potential monomers for non-equilibrium polymerization [352]. Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Ph

Ph

Si O O

Si

Si

O O Si

+

O

Ph Si O

O Ph Si O OO O Ph Si O Si Ph O Ph O Si O Si Ph OO OO Si Si Ph Ph O O Si O Si Ph

Ph Si Ph + O

Ph Si O Si Ph Ph Ph

Ph

Si O Si

O

Si

Ph

base

O

Si

O n

Ph Si Ph

O n

Ph Si O

O n

Ph

(54) Ladder polysiloxanes were recently prepared by a sequential condensation of cyclic multihydroxysiloxanes with multichlorosiloxanes [353] or multiisocyanatosiloxanes [354] and by oxidation of the corresponding ladder polysilanes with m-chloroperbenzoic acid [355].

General Review on Polysiloxane Synthesis

41

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Complete oxidation of all Si-Si bonds requires a large excess of oxidizing agent and long reaction times.

Hyperbranched and Dendritic Polysiloxanes Dendrimers, hyperbranched and star polymers is the field itself, which exploded some 20 years ago. For a general background the reader is referred to the recent review with over 400 references by Inoue [356]. For a short review and a discussion of various types of hyperbranched and dendritic polymers the reader can also be referred to other important papers [357,358]. Dendrimers and hyperbranched polymers are characterized by a highly branched structure, having a multiplicity of reactive chain-ends. In contrast to dendrimers and star-shaped polymers, in which all bonds converge to a focal point or core, hyperbranched molecules may contain multiple focal points. The synthesis of dendrimers proceeds in a step-by-step sequences, by one of the two strategies: the divergent approach (from the inside out) and convergent approach (from the outside in), giving in both cases polymers with high degree of regularity and controlled molecular weights. Hyperbranched copolymers, although synthesized of the similar building blocks, do not have such a regular structure, so distinctive of dendrimers. Thus, correspondingly, they have properties intermediate between the branched or star polymers and the regular dendrimers. However, unlike dendrimers, which require tedious synthetic procedures, the hyperbranched polymers (sometimes called “poor man dendrimers”) are relatively easy to make in a one pot process. Most of the work in the field of organosilicon dendritic polymers is devoted to polycarbosilanes and carbosiloxanes [327,359-364]. In 1989 Muzafarov and co-workers reported the successful synthesis of a fourth generation siloxane dendrimer, which possesses SiO3 linkages. The preparation of these species, starting from MeSiCl3 as the initial core, includes nucleophilic substitution of the silicon-bound chlorides with Me(OEt)2SiONa and conversion of the silicon-bound ethoxy groups to chlorides with SOCl2, which allows the first step to be repeated (Scheme 11) [365]. A further possibility to synthesize dendritic polysiloxane polymers was reported by Kakimoto in 1991, using MeSi(OSiMe2OSiMe2Ph)3 as an initiator core and MeSi(OSiMe2Ph)3 as the building block [366]. Pyridyl end-capped staff-type dendrimers based on linear polymethylhydrosiloxane Me3SiO(MeHSiO)nSiMe3 (PMHS) as a core and using silyloxypropyl groups -(SiOCH2CH2CH2)- as a generating step were prepared [367]. The general strategies for the systematic synthesis of dendritic and hyperbranched silicone have been proposed [368,369]. For carbosiloxane dendrimers so-called “universal scheme” was recently developed, in which divergent and convergent approaches are combined [370].

42

Marek Cypryk Et O

Me Cl Si Cl Cl

3 NaOSi(Me)(OEt)2 - 3 NaCl

Et O Si O Me

Me

O Et

Si O

Si O Et

O

Me

Me

Cl SOCl2

Me Si O Et Et O

Cl Si O Me

Cl

Si O

Si Cl

O

Me

Me Si Cl Cl

OEt OEt Me OEt OEt EtO OEt EtO Si Me EtO EtO OEt Si Me Si OEt OEt Si Me O O Si EtO Me O Me Si OEtOEt Si O O Si Me Si O Me Si Me OEt OEt EtO O Me O Me Si O O Si OEt EtO Si O O O Si O Si Me Si Me Si OEt Me Me Me Me Me Me Me EtO Me Si O O O O Me Si OEt O Si O Si O Si EtO O Si Si O Si O Si O O Me Me O Si Me O Me Me Me Me O EtO Si OEt O Si OEt O Si Me EtO Me O Si Me Si O Si Si OEt EtO O O OEt Me O Me O Me EtO Me Si Si OEt Me Si O Si Me EtO Si OEt Me EtO Me O O O O O OEt EtO Si Me Me Si OEt Si OEt EtO EtO Si Me Si O OEt Me EtO Si O Me EtO Si EtO OEt Me OEt

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Scheme 11.

The hyperbranched carbosiloxanes were mostly made via hydrosilylation reaction of the appropriate di-and trifunctional silanes or siloxane oligomers [371,372]. Miravet and Frehet also studied the end-functionalization of such hyperbranched molecules with the aim of introducing other reactive end-groups, or to attach a preformed organic polymers to build a hyperbranched-linear star block copolymers [373]. One of the examples of the hyperbranched polysiloxanes includes polyethoxysiloxanes made by the condensation of triethoxysilanol, [374,375] used by the group of Muzafarov as a model in their studies of molecular silica [376]. The synthesis of a hyperbranched polysiloxane via the ring opening polymerization of 1-hydroxypentamethylcyclotrisiloxane and 1-(hydroxydimethylsiloxy)-pentamethylcyclotrisiloxane, regarded as the base-catalyzed proton-transfer polymerization, is also described (scheme 12) [377,378].

General Review on Polysiloxane Synthesis

43

OH

O

Si O Si

Si O

Si

O

O Si O Si

O Si O Si

Si

O O Si

O

Si

Si

Si O

HO

Si

O

Si

O

O Si

Si O Si O O

Si

O

Si

O

Si

O

Si

Si

O Si

O

O Si OH

O

O

Si

Si O Si

OH

Si

Si O Si OH O

Si

Si

O

O Si

Si

O

OH Si O Si

O Si

OH

O

O

O

Si

O Si O Si

Si O Si O Si O

Si

Si

Si O Si

O

O

O

O

HO

Si

O

O

O O

O

O

O

Si

O

Si

O

Si

O

Si

O

O

Si

Si

O

O

Si

Si

O

O

Si O Si OH OH

Si

Si

O

O

Si OH

Si

OH

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Scheme 12.

Functionalized branched polysiloxanes with star-branched, comb-branched and dendriticbranched topologies were discussed in a series of papers by Chojnowski and co-workers [303,332,379,380]. The branched macromolecules were generated by coupling of the reactive blocks, made via anionic polymerization of vinyl-substituted cyclotrisiloxanes and their copolymerization with D3, using a grafting technique. Gradient copolymers were also used to vary the density of the branches. Dendritic polysiloxanes of the first and second generation were obtained using starlike polysiloxane, functionalized with SiCl groups, as the core (scheme 13) [303,332,379,380]. Using the same technique, a series of linear and hyperbranched liquid crystalline polysiloxanes were prepared [381]. Solid catalyst supports were obtained by grafting the functionalized branched polysiloxanes of various structures on functionalized silica [380]. Acid catalyzed equilibration of octamethylcyclotetrasiloxane (D4) with the tetrafunctional chain stopper, tetrakis(dimethylsiloxy)silane, leads to irregular tetra-branched star polymers, tetrakis(ω-dimethylsiloxy)poly(dimethylsiloxy)silane.

44

Marek Cypryk

Cl Cl

Me

Me

Si

Si

Cl

Bu[SiO(SiO)2]n(SiO)3m- Li+

Cl

Bu[SiO(SiO)2]n

[(OSi)2OSi]nBu

Si

- 4 NaCl

Si [(OSi)2OSi]nBu

Bu[SiO(SiO)2]n (G=1) HMe2SiCl Pt(0)

SiCl

ClSi

SiCl

ClSi ClSi ClSi

Si

Si

SiCl

ClSi

Bu[SiO(SiO)2]n(SiO)3m- Li+

SiCl

- 4 NaCl

SiCl

(G=2)

ClSi

SiCl Scheme 13.

The terminal Si–H bonds have been modified by Pt-catalyzed hydrosilylation with 4vinylcyclohexane-1,2-epoxide to yield a tetra-branched star polydimethylsiloxanes (PDMS) with terminal epoxide groups (equation 55) [333]. Hexa-branched stars were obtained by the same method, using hexakis(dimethylsiloxymethyl)cyclohexasiloxane as the core. It is surprising that the cyclohexasiloxane ring does not open under these conditions [382]. O

Si(OMe2H)4

D4 CF3SO3H

Si[(OSiMe2)nOSiMe2H]4

"Pt"

Si(OSiMe2)nOSiMe2

O 4

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

(55) Hyperbranched polymers are very promising materials which have been utilized in various fields, ranging from additives to coatings to high technology applications such as sensors, photochemical devices, catalysts and nanoreactors [356].

CLOSING REMARKS The state of art and future predictions concerning the chemistry and applications of silicones were addressed in many chapters [18,28,29,40-42,383]. One conclusion, which can be learned from these impressive presentations, is that the organosilicon polymer chemistry in all its aspects is far from being satisfactorily explored. Probably, the main direction of future development of silicones is the material chemistry. There is a strong need in modern technology for the precisely designed and made, high

General Review on Polysiloxane Synthesis

45

performance materials of very specialized properties. Thus, the development in organosilicon polymer technology will be directed towards precision synthesis of well defined macromolecular structures, from linear to three-dimensional and supramolecularly organized, as well as towards synthesis of hybrid organic-inorganic materials. A big challenge is the optimization of the direct process [384]. It involves stripping off all oxygens from silica in an extremely energetic process just to regenerate half of the Si-O bonds in the subsequent step of siloxane production. A great portion of energy is lost. Thus, the hope for the future is to discover a process, in which the Si-O bonds would be replaced by the Si-C bonds selectively and the use of chlorine would be eliminated [28]. It seems appropriate to repeat after R. Corriu, that the constantly developing chemistry of organosilicon materials is the fascinating area, where the only limitation for the chemist is mainly his own creativity [385].

REFERENCES

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[1] [2]

Ladenburg, A. Liebigs Ann. Chem. 1872, 164, 300. Noll, W. The Chemistry and Technology of Silicones; Academic Press: New York, 1968. [3] Voronkov, M. G.; Mileshkevich, V. P.; Yuzhelevskii, Yu. A. The Siloxane Bond; Consultants Bureau: New York, 1978. [4] Stark, F. O.; Falender, J. R.; Wright, A. P. Silicones. In Comprehensive Organometallic Chemistry, Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; pp 305-363. [5] Hardman, B.; Torkelson, A. Silicones. In Encyclopedia of Polymer Science and Engineering, Frank, H. F., Bikales, N. M., Overberger, C. G., Menges, G., Eds.; WileyInterscience: New York, 1989; pp 204-308. [6] Kendrick, T. C.; Parbhoo, B. M.; White, J. W. Polymerization of Cyclosiloxanes. In Comprehensive Polymer Chemistry, Eastmond, G. C., Ledwith, A., Russo, S., Sigwalt, P., Eds.; Pergamon Press: Oxford, 1989; pp 459-523. [7] Kendrick, T. C.; Parbhoo, B. M.; White, J. W. Siloxane polymers and copolymers. In The Chemistry of Organic Silicon Compounds, Patai, S., Rappoport, Z., Eds.; Wiley and Sons: Chichester, 1989; pp 1289-1362. [8] Lebrun, J. J.; Porte, H. Polysiloxanes. In Comprehensive Polymer Chemistry, Eastmond, G. C., Ledwith, A., Russo, S., Sigwalt, P., Eds.; Pergamon Press: Oxford, 1989; pp 593-609. [9] Siloxane Polymers; Ellis Horwood/PTR Prentice Hall: Englewood Cliffs, NJ, 1993. [10] Moretto, H.-H.; Schulze, M.; Wagner, G. Silicones. In Uhlmann's Encyclopedia of Industrial Chemistry, VCH Publishers: Weinheim, 1993; pp 57-93. [11] Brown, S. S.; Kendrick, T. C.; McVie, J.; Thomas, D. R. Silicones. In Comprehensive Organometallic Chemistry II, Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; pp 111-135. [12] Rich, J.; Cella, J. A.; Lewis, L. N.; Stein, J.; Singh, N.; Rubinsztajn, S.; Wengrovius, J. Silicon Compounds: Silicones. In Kirk-Othmer Encyclopedia of Chemical Technology, 4 ed.; Wiley: New York, 1997; pp 82-142.

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46

Marek Cypryk

[13] Drake, R.; MacKinnon, I.; Taylor, R. Recent advances in the chemistry of siloxane polymers and copolymers. In The Chemistry of Organic Silicon Compounds, Rappoport, Z., Apeloig, Y., Eds.; Wiley-Interscience: New York, 1998; pp 2217-2244. [14] Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications; Kluwer Academic Publishers: Dordrecht, 2000. [15] Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry; Wiley and Sons: New York, 2000. [16] Moretto, H.-H.; Schulze, M.; Wagner, G. Silicones. In Industrial Polymer Handbook, Wilks, E. S., Ed.; Wiley-VCH: Weinheim, 2001; pp 1349-1408. [17] Butts, M.; Cella, J. A.; Wood, C. D.; Gillette, G.; Kerboua, R.; Leman, L.; Lewis, L. N.; Rubinsztajn, S.; Schattenmann, F.; Stein, J.; Wicht, D.; Rajaraman, S.; Wengrovius, J. Silicones. In Encyclopedia of Polymer Science and Technology, 3 ed.; Mark, H. F., Kroschwitz, J. I., Eds.; Wiley-Interscience: Hoboken, NJ, 2004; pp 765-841. [18] Mazurek, M. Silicones. In Comprehensive Organometallic Chemistry III, Crabtree, R. H., Mingos, M. P., Housecroft, C., Eds.; Elsevier Limited: Oxford, 2006; vol. 3, pp 651-697. [19] Pauling, L. The Nature of the Chemical Bond; Cornell University Press: Ithaca, 1960. [20] Grigoras, S. Modeling of polydimethylsiloxane. In Computational Modeling of Polymers, Bicerano, J., Ed.; Marcel Dekker: New York, 1993; p 161. [21] Apeloig, Y. Theoretical Aspects of Organosilicon Compounds. In The Chemistry of Organic Silicon Compounds, Patai, S., Rappoport, Z., Eds.; Wiley and Sons: Chichester, 1989; pp 57-227. [22] Gibbs, G. V.; Downs, J. W.; Boisen, M. B. Rev. Mineral. Geochem. 1994, 29, 331-368. [23] Cypryk, M.; Chojnowski, J. J. Organomet. Chem. 2002, 642, 163-170. [24] Cypryk, M. Macromol. Theory Simul. 2001, 10, 158-164. [25] Owen, M. J. Surface Chemistry and Applications. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; Ellis Horwood/PTR Prentice Hall: Englewood Cliffs, NJ, 1993; pp 309-372. [26] Walsh, R. Thermochemistry. In The Chemistry of Organic Silicon Compounds, Patai, S., Rappoport, Z., Eds.; Wiley and Sons: Chichester, 1989; pp 371-392. [27] Organosilicon Compounds for Industrial Applications. In Organosilicon Chemistry VI: From Molecules to Materials, Auner, N., Weis, J., Eds.; Wiley-VCH: Weinheim, 2005; pp 602-859. [28] Graiver, D.; Fearon, G. Polysiloxanes: Direction of Applications. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 233-243. [29] White, J. W. Recent Achievements in Industrial Silicon Polymer Science. In Progress in Organosilicon Chemistry, Marciniec, B., Chojnowski, J., Eds.; Gordon and Breach Publishers: Basel, 1995; pp 363-386. [30] Mark, J. E. Acc. Chem. Res. 2004, 37, 946-953. [31] Clarson, S. J. Depolymerization, Degradation and Thermal properties of Siloxane Polymers. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; Ellis Horwood/PTR Prentice Hall: Englewood Cliffs, NJ, 1993; pp 216-244.

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General Review on Polysiloxane Synthesis

47

[32] Dvornic, P. R. Thermal properties of polysiloxanes. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 185-212. [33] Clarson, S. J.; Mark, J. E. Siloxane Elastomers. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; Ellis Horwood/PTR Prentice Hall: Englewood Cliffs, NJ, 1993; pp 616-648. [34] Clarson, S. J. Preparation and Properties of Silicone Elastomers. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 139-155. [35] Folland, P. D.; Matisons, J. G. Siloxane Elastomers. In Polymeric Materials Encyclopedia, Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp 7759-7768. [36] Owen, M. J. Siloxane Surface Activity. In Silicon-Based Polymer Science: A Comprehensive Resource, Zeigler, J. M., Fearon, F. W. G., Eds.; Adv. Chem. Ser. 1990, 224, 705-739. [37] Owen, M. J. Surface Properties of Polysiloxanes. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 213-232. [38] Beevers, M. S. Electric Properties of Siloxanes. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; Ellis Horwood/PTR Prentice Hall: Englewood Cliffs, NJ, 1993; pp 415-477. [39] Van Dyke, M. E. Polym. Prepr. 2004, 45, 600. [40] De Poortere, M. The Markets for Silicones. In Organosilicon Chemistry VI: From Molecules to Materials, Auner, N., Weis, J., Eds.; Weinheim, 2005. [41] Owen, M. J. Polym. Prepr. 2004, 45, 560. [42] White, J. W. Silicon Science and Technology - an Industrialist's View of the Future. In Organosilicon Chemistry VI: From Molecules to Materials, Auner, N., Weis, J., Eds.; Wiley-VCH: Weinheim, 2005; pp 602-609. [43] Lewis, L. N. From sand to silicones: An overview of the chemistry of silicones. In Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds.; ACS Symp.Ser. 2000, 729, 11-19. [44] Lewis, L. N. Recent advances in the direct process. In The chemistry of organic silicon compounds, Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons: 1998; pp 15811597. [45] Birkofer, L.; Stuhl, O. General synthetic pathways to organosilicon compounds. In The Chemistry of Organic Silicon Compounds, Patai, S., Rappoport, Z., Eds.; Wiley and Sons: Chichester, 1989; pp 655-762. [46] Marciniec, B.; Guliński, J.; Urbaniak, W.; Kornetka, Z. Comprehensive Handbook on Hydrosilylation; Pergamon Press: Oxford, 1992. [47] Ojima I. The hydrosilylation reaction. In The Chemistry of Organic Silicon Compounds, Patai, S., Rappoport, Z., Eds.; Wiley and Sons: Chichester, 1989; pp 1479-1526. [48] Ojima I.; Li, Z.; Zhu, J. Recent advances in hydrosilylation and related reactions. In The Chemistry of Organic Silicon Compounds, Rappoport, Z., Apeloig, Y., Eds.; Wiley and Sons: Chichester, 1998; pp 1687-1792. [49] Bablin, J. M.; Lewis, L. N.; Bui, P.; Gardner, M. Ind. Eng. Chem. Res. 2003, 42, 35323543.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

48

Marek Cypryk

[50] Okamoto, M.; Abe, H.; Kusama, Y.; Suzuki, E.; Ono, Y. J. Organomet. Chem. 2000, 616, 74-79. [51] Okamoto, M.; Komai, J.; Uematsu, M.; Ono, Y. J. Organomet. Chem. 2001, 619, 235240. [52] Becerra, R.; Walsh, R. Thermochemistry. In The chemistry of organic silicon compounds, Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons: 1998; pp 153-180. [53] Walsh, R. Bond Dissociation Energies of Organosilicon Compounds. In Silanes and Silicones (Gelest Catalog 2000), Arkles, B., Larson, G., Eds.; Gelest, Inc. Morrisville, 2004; pp 187-194. [54] Handbook of Chemistry and Physics; CRC Press: Boca Raton, 1996. [55] Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds; Academic Press: London, 1970. [56] Ring, M. A.; O'Neal, H. E.; Kadhim, A. H.; Jappe, F. J. Organomet. Chem. 1966, 5, 124-129. [57] Hyde, J. F.; Brown, P. L.; Smith, A. L. J. Am. Chem. Soc. 1960, 82, 5854-5858. [58] Chojnowski, J.; Cypryk, M. Synthesis of Linear Polysiloxanes. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 3-41. [59] Sagitova, V. G.; Chernyak, V. I. Zh. Obshch. Khim. 1983, 53, 397-402. [60] Shaffer, L. H.; Flanigen, E. M. J. Phys. Chem. 1957, 61, 1595-1600. [61] Lehnert, R. Chlorosilanols - more stable than you think. In Organosilicon Chemistry II: From Molecules to Materials, Auner, N., Weis, J., Eds.; VCH: Weinheim, 1994; pp 7173. [62] Rühlmann, K. Phosphorus Sulfur 1986, 27, 139-152. [63] Chojnowski, J. Polymerization. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; PTR Prentice Hall: Englewood Cliffs, NJ, 1993; pp 1-71. [64] Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, 1990. [65] Corriu, R. J. P.; Douglas, W. E. Organosilicate oligomers and nanostructured materials. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 667-696. [66] Guibergia-Pierron, M.; Sauvet, G. Eur. Polym. J. 1992, 28, 29-36. [67] Helary, G.; Sauvet, G. Eur. Polym. J. 1992, 28, 37-41. [68] Mark, J. E. Adv. Polym. Sci. 1982, 44, 1-26. [69] Rubinsztajn, S.; Zeldin, M.; Fife, W. K. Macromolecules 1991, 24, 2682-2688. [70] Kaźmierski, K.; Chojnowski, J.; McVie, J. Eur. Polym. J. 1994, 30, 515-527. [71] He, X. W.; Widmaier, J. M.; Herz, J. E.; Meyer, G. C. Eur. Polym. J. 1988, 24, 11451148. [72] Martyakova, N. N.; Dolgoplosk, S. B.; Kagan, F. G.; Kostikyan, T. S.; Petruchno, L. A. Zh. Obshch. Khim. 1974, 44, 298-303. [73] Normand, F.; He, X. W.; Widmaier, J. M.; Meyer, S. C.; Herz, J. E. Eur. Polym. J. 1989, 25, 371-374. [74] Sharaf, M. A.; Mark, J. E. Makromol. Chem. 1989, 190, 495-503. [75] Chojnowski, J.; Cypryk, M.; Fortuniak, W.; Kaźmierski, K.; Taylor, R. G. J. Organomet. Chem. 1996, 526, 351-361.

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General Review on Polysiloxane Synthesis

49

[76] Hayes, S. E. U.S. Patent 4,395,563, 1983. [77] Rankin, S. E.; Macosko, C. W.; McCormick, A. V. J. Polym. Sci. A 1997, 35, 12931302. [78] Rankin, S. E.; Macosko, C. W.; McCormick, A. V. Aiche J. 1998, 44, 1141-1156. [79] Kelling, H.; Lange, D.; Surkus, A. In Organosilicon Chemistry II - From Molecules to Materials, Auner, N., Weis, J., Eds.; VCH: Weinheim, 1994; p 65. [80] Clarson, S. J.; Wang, Z.; Mark, J. E. Eur. Polym. J. 1990, 26, 621-622. [81] Chu, H.-K.; Cross, R. P.; Crossan, D. I. J. Organomet. Chem. 1992, 425, 9-17. [82] Abe, Y.; Shimano, R.; Arimitsu, K.; Gunji, T. J. Polym. Sci. A 2003, 41, 2250-2255. [83] Yoshizawa, M.; Kusukawa, T.; Fujita, M.; Yamaguchi, K. J. Am. Chem. Soc. 2000, 122, 6311-6312. [84] Bassindale, A. R.; Brandstadt, F. K.; Lane, T. H.; Taylor, P. G. J. Inorg. Biochem. 2003, 96, 401-406. [85] Bassindale, A. R.; Brandstadt, K. F.; Lane, T. H.; Taylor, P. G. Biocatalysis of siloxane bonds. In Polymer Biocatalysis and Biomaterials, 900 ed.; Gross, R. A., Cheng, H. N., Eds.; ACS Symp. Ser. 2005, 900, 164-181. [86] Brandstadt, F. K. Curr. Opin. Biotech. 2005, 16, 393-397. [87] Patwardhan, S. V.; Clarson, S. J. J. Inorg. Organomet. Polym. 2001, 11, 193-198. [88] Patwardhan, S. V.; Clarson, S. J. Silicon Chem. 2002, 1, 207-214. [89] Patwardhan, S. V.; Clarson, S. J. J. Inorg. Organomet. Polym. 2003, 13, 193-203. [90] Patwardhan, S. V.; Clarson, S. J. J. Inorg. Organomet. Polym. 2003, 13, 49-53. [91] Chojnowski, J. The Behaviour of the Silanol Function in Polycondensation Systems. In Silicon-Containing Polymers, Jones, R. G., Ed.; Royal Society of Chemistry: Cambridge, 1995; pp 59-72. [92] Lickiss, P. D. Adv. Organomet. Chem. 1995, 42, 147-262. [93] Lickiss, P. D. Polysilanols. In The Chemistry of Organic Silicon Compounds, Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons: Chichester, 2001; pp 695-744. [94] Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. Rev. 1994, 94, 2095-2160. [95] Wilczek, L.; Chojnowski, J. Makromol. Chem. 1983, 184, 77-90. [96] Bilda, S.; Lange, D.; Popowski, E.; Kelling, H. Z. Anorg. Allg. Chem. 1987, 550, 186. [97] Pohl, E. R.; Osterholz, F. D. In Silanes, Surfaces and Interfaces, Leyden, D. E., Ed.; Gordon and Breach: New York, 1986; p 481. [98] Pohl, E. R.; Osterholz, F. D. In Chemically Modified Surfaces in Science and Industry, Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach: New York, 1987; p 545. [99] Saam, J. C. Formation of linear siloxane polymers. In Silicon-Based Polymer Science, Zeigler, J. M., Fearon, F. W. G., Eds.; Adv. Chem. Ser. 1990, 224, 71-90. [100] Flory, P. Principles of Polymer Chemistry; Cornell University Press: Ithaca, N.Y., 1953. [101] Patel, S.; Buese, M. A. Macromolecules 1992, 25, 4821-4823. [102] Bischoff, R.; Sigwalt, P. Polym. Int. 1995, 36, 57-71. [103] Cypryk, M.; Sigwalt, P. Macromolecules 1994, 27, 6245-6253. [104] Chojnowski, J.; Rubinsztajn, S.; Wilczek, L. Macromolecules 1987, 20, 2345-2355. [105] He, X. W.; Herz, J. F. Eur. Polym. J. 1991, 27, 449-453. [106] Chojnowski, J.; Kaźmierski, K.; Rubinsztajn, S.; Stańczyk, W. Makromol. Chem. 1986, 187, 2039-2052. [107] Cypryk, M.; Rubinsztajn, S.; Chojnowski, J. J. Organomet. Chem. 1993, 446, 91-97.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

50

Marek Cypryk

[108] Rubinsztajn, S.; Cypryk, M.; Chojnowski, J. Macromolecules 1993, 26, 5389-5396. [109] Sarich, W.; Surkus, A.; Lange, D.; Popowski, E.; Kelling, H. Z. Anorg. Allg. Chem. 1990, 581, 199. [110] Cypryk, M.; Apeloig, Y. Organometallics 2002, 21, 2165-2175. [111] Lasocki, Z.; Chrzczonowicz, S. J. Polym. Sci. 1962, 59, 259-269. [112] Bordone, C.; Desmurs, J.-R.; Ghosez, L.; Martins, J.; Mignani, G. U.S. Patent 6,737,495, 2002. [113] Dromard, A.; Millet, C. U.S. Patent 4,426,508, 1984. [114] Takehara, D. K. U.S. Patent 5,357,016, 1994. [115] Burkhardt, J.; Wegehaupt, K. U.S. Patent 4,203,913, 1980. [116] Burkhardt, J.; Streckel, W.; Böck, A. E. Patent 258640, 1988. [117] Gilson, J. M.; Habimana, J. G.B. Patent 2,252,969, 1992. [118] Rubinsztajn, S. U.S. Patent 5,403,909, 1995. [119] Vaugeois, Y.; Mazzah, A.; de Jaeger, R.; Habimana, J. Phosphorus Sulfur 2004, 179, 1819-1840. [120] Chojnowski, J.; Fortuniak, W.; Habimana, J.; Taylor, R. G. J. Organomet. Chem. 1997, 534, 105-115. [121] Hager, R.; Weis, J. Z. Naturforsch. 1994, 49, 1774. [122] Grzelka, A.; Chojnowski, J.; Cypryk, M.; Fortuniak, W.; Hupfield, P. C.; Taylor, R. G. J. Organomet. Chem. 2004, 689, 705-713. [123] Grzelka, A.; Chojnowski, J.; Cypryk, M.; Fortuniak, W.; Hupfield, P. C.; Taylor, R. G. J. Organomet. Chem. 2002, 660, 14-26. [124] Deka, K.; Sarma, R. J.; Baruah, J. B. Inorg. Chem. Commun. 2005, 8, 1082-1084. [125] Saam, J. C.; Huebner, D. J. J. Polym. Sci. Polym. Chem. 1982, 20, 3351-3368. [126] Barrere, M.; Maitre, C.; Ganachaud, F.; Hémery, P. Makromol. Chem. Macromol. Symp. 2000, 151, 359-364. [127] Palaprat, G.; Ganachaud, F. C. R. Chimie 2003, 6, 1385-1392. [128] Cypryk, M.; Rubinsztajn, S.; Chojnowski, J. J. Organomet. Chem. 1989, 367, 27-37. [129] Chu, K.; Johnson, M. D.; Frye, C. L. J. Organomet. Chem. 1984, 271, 327-336. [130] Chojnowski, J.; Fortuniak, W.; Pluta, M.; Ścibiorek, M. Eur. Polym. J. 1998, 34, 931940. [131] Rózga-Wijas, K.; Chojnowski, J.; Zundel, T.; Boileau, S. Macromolecules 1996, 29, 2711-2720. [132] Shchegolikhina, O. I.; Pozdnyakova, Y. A.; Molodtsova, Y. A.; Korkin, S. D.; Bukalov, S. S.; Leites, L. A.; Lyssenko, K. A.; Peregudov, A. S.; Auner, N.; Katsoulis, D. E. Inorganic Chemistry 2002, 41, 6892-6904. [133] Boutevin, B.; Youssef, B. Makromol. Chem. 1989, 190, 277-285. [134] Rubinsztajn, S.; Chojnowski, J.; Cypryk, M. J. Organomet. Chem. 1989, 377, 197-204. [135] Pike, R. M. J. Polym. Sci. 1961, 50, 151-158. [136] Mazurek, M.; North, A. M.; Pethrick, R. A. Polymer 1980, 21, 369-371. [137] Oishi, M.; Kawakami, Y. Macromolecules 2000, 1960-1963. [138] Michalska, Z. M. Trans. Metal Chem. 1980, 5, 125-129. [139] Kawakami, Y.; Imae, I. Synthesis and Characterization of Siloxane-Containing Polymers Obtained by Dehydrocoupling Polymerization. In Synthesis and Properties of Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J.,

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

General Review on Polysiloxane Synthesis

51

Smith, S. D., Van Dyke, M. E., Eds.; Am. Chem. Soc.: Washington DC, 2003; pp 6171. [140] Zhou, D. Q.; Kawakami, Y. Macromolecules 2005, 38, 6902-6908. [141] Voronkov, M. G.; Basenko, S. V. J. Organomet. Chem. 1995, 500, 325-329. [142] Lu, P.; Paulasaari, J. K.; Weber, W. P. Organometallics 1996, 15, 4649-4652. [143] Le Roux, C.; Yang, H.; Wenzel, S.; Grigoras, S.; Brook, M. A. Organometallics 1998, 17, 556-564. [144] Lee, G. J.; Kang, K. K. J. Org. Chem. 1988, 53, 3634-3637. [145] Schulz, Jr. W. J.; Speier, J. L. U.S. Patent 5,126,470, 1992. [146] Takiguchi, T.; Sakurai, M.; Kishi, T.; Ichimura, J.; Izuka, Y. J. Org. Chem. 1960, 25, 310-311. [147] Shen, Q.; Interrante, L. V. Macromolecules 1997, 30, 5485-5489. [148] Unno, M.; Kishimoto, Y.; Matsumoto, H. Organometallics 2004, 23, 6221-6224. [149] Borisov, Y. A.; Papkov, V. S.; Rabkina, A. Y.; Zavin, B. G. J. Mol. Struct.-Theochem 2003, 664, 157-170. [150] G.B. Patent 683,182, 1952. [151] Fortuniak, W.; Chojnowski, J.; Sauvet, G. Macromol. Chem. Phys. 2001, 202, 23062313. [152] Zuev, V. V.; Kalinin, A. V. Phosphorus Sulfur 2003, 178, 1289-1294. [153] Zuev, V. V.; Kalinin, A. V. Phosphorus Sulfur 2003, 178, 595-601. [154] Dagger, A. C.; Semlyen, J. A. Polymer 1998, 39, 2621-2627. [155] Dagger, A. C.; Semlyen, J. A. Hydrogenated and Deuterated Cyclic Poly(dimethysiloxanes). In Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds.; ACS Symp. Ser. 2000, 729, 38-80. [156] Rubinsztajn, S.; Cella, J. A. Polym. Prepr. 2004, 45, 635-636. [157] Rubinsztajn, S.; Cella, J. A. Macromolecules 2005, 38, 1061-1063. [158] Chojnowski, J.; Rubinsztajn, S.; Cella, J. A.; Fortuniak, W.; Cypryk, M.; Kurjata, J.; Kaźmierski, K. Organometallics 2005, 24, 6077-6084. [159] Keyrouz, R.; Jouikov, V. New J. Chem. 2003, 27, 902-904. [160] Keyrouz, R.; Jouikov, V. Electrochemically Initiated "Silanone Route" for Functionalization of Siloxanes. In Organosilicon Chemistry VI: From Molecules to Materials, Auner, N., Weis, J., Eds.; Weinheim, 2005; pp 675-681. [161] Chojnowski, J.; Cypryk, M.; Kurjata, J. Prog. Polym. Sci. 2003, 28, 691-728. [162] Piccoli, W. A.; Haberland, G. G.; Merker, R. L. J. Am. Chem. Soc. 1960, 82, 18831885. [163] Dvornic, P. R.; Lenz, R. W. High Temperature Siloxane Elastomers; Huthig and Wepf: Basel, 1990. [164] Lasocki, Z.; Witekowa, M. J. Macromol. Sci. Chem. 1977, A11, 457-467. [165] Chojnowski, J.; Cypryk, M. Cyclic Siloxanes (Ring-Opening Polymerization). In Polymeric Materials Encyclopedia, Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp 1682-1696. [166] Chojnowski, J. Ring-opening polymerization of cyclosiloxanes. In Silanes and Silicones (Gelest Catalog 2004), Arkles, B., Larson, G., Eds.; Gelest, Inc.: Morrisville, 2004; pp 389-405. [167] Johannson, O. K.; Lee, C.-L. Cyclic Monomers; Wiley-Interscience: New York, 1972. [168] Chojnowski, J. J. Inorg. Organomet. Polym. 1991, 1, 299-325.

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52

Marek Cypryk

[169] Wright, P. V.; Semlyen, J. A. Polymer 1970, 11, 462-471. [170] Clarson, S. J. Cyclic Polysiloxanes. In Cyclic Polymers, 2 ed.; Clarson, S. J., Semlyen, J. A., Eds.; Kluwer Academic Publishers: New York, 2000; pp 161-183. [171] Semlyen, J. A. Cyclic Siloxane Polymers. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; PTR Prentice Hall: Englewood Cliffs, 1993; pp 135-215. [172] Meliani, A.; Vaugeois, Y.; Bali, H.; Mazzah, A.; de Jaeger, R.; Habimana, J. Phosphorus Sulfur 2000, 166, 283-301. [173] Schuster, J.; Piehler, M.; Muller, H.; Schneider, O. U.S. Patent 5,008,229, 1991. [174] Wegehaupt, K.; Braunsperger, K.; Patzke, J. U.S. Patent 4,975,510, 1990. [175] Hupfield, P. C.; Surgenor, A. E.; Taylor, R. G. A Facile Synthetic Route to Phosphazene Base Catalysts and Their Use in Siloxane Synthesis. In Organosilicon Chemistry VI: From Molecules to Materials, Auner, N., Weis, J., Eds.; Weinheim, 2005; pp 628-634. [176] Carmichael, B.; Winger, R. J. Polym. Sci. A 1965, 3, 971-984. [177] Scott, D. W. J. Am. Chem. Soc. 1946, 68, 2294-2298. [178] Mazurek, M.; Ścibiorek, M.; Chojnowski, J.; Zavin, B. G.; Zhdanov, A. A. Eur. Polym. J. 1980, 16, 57-69. [179] Voronkov, M. G.; Klyuchnikov, V. A.; Mironenko, E. V.; Shvets, G. N.; Danilova, T. F.; Khudobin, Y. I. J. Organomet. Chem. 1991, 406, 91-97. [180] Cypryk, M.; Apeloig, Y. Organometallics 1997, 16, 5938-5949. [181] Grubb, W. T.; Osthoff, R. C. J. Am. Chem. Soc. 1955, 77, 1405-1411. [182] Wright, P. V. In Ring Opening Polymerization, Ivin, K. J., Saegusa, T., Eds.; Elsevier: New York, 1984; p 1055. [183] Chojnowski, J.; Mazurek, M. Makromol. Chem. 1975, 176, 2999-3023. [184] Wilczek, L.; Kennedy, J. P. Polym. J. 1987, 19, 531-538. [185] Kopylov, V. M.; Prikhodko, P. L.; Koviazin, V. A. Vysokomol. Soed. 1982, A24, 17511756. [186] Hurd, D. T.; Osthoff, R. C.; Corrin, M. L. J. Am. Chem. Soc. 1954, 76, 249-252. [187] Grzelka, A.; Chojnowski, J.; Fortuniak, W.; Taylor, R. G.; Hupfield, P. C. J. Inorg. Organomet. Polym. 2004, 14, 85-99. [188] Hupfield, P. C.; Taylor, R. G. J. Inorg. Organomet. Polym. 1999, 9, 17-34. [189] Molenberg, A.; Möller, M. Macromol. Chem. Rapid Commun. 1995, 16, 449-453. [190] Teng, C. J.; Weber, W. P.; Cai, G. P. Macromolecules 2003, 36, 5126-5130. [191] Teng, C. J.; Weber, W. P.; Cai, G. P. Polymer 2003, 44, 4149-4155. [192] Van Dyke, M. E.; Clarson, S. J. J. Inorg. Organomet. Polym. 1998, 8, 111-117. [193] Bessmertynkh, A.; Ben, F.; Bacereido, A.; Mignani, G. J. Organomet. Chem. 2003, 686, 281-285. [194] Yuzhelevskii, Yu. A.; Kagan, E. G.; Fedosieeva, N. N. Vysokomol. Soed. 1970, A12, 1585-1592. [195] Mazurek, M.; Chojnowski, J. Makromol. Chem. 1977, 178, 1005-1017. [196] Terman, L. M.; Klapshina, L. G.; Kurskii, Yu. A.; Zislina, S. S. Vysokomol. Soed. 1988, A30, 2042-2045. [197] Hubert, S.; Hémery, P.; Boileau, S. Makromol. Chem. Macromol. Symp. 1986, 6, 247252. [198] Molenberg, A.; Siffrin, S.; Moller, M.; Boileau, S.; Teyssie, D. Makromol. Chem. Macromol. Symp. 1996, 102, 199-207.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

General Review on Polysiloxane Synthesis

53

[199] Laita, Z.; Jelinek, M. Vysokomol. Soed. 1962, 4, 1739. [200] Kress, J. D.; Leung, P. C.; Tawa, G. J.; Hay, P. J. J. Am. Chem. Soc. 1997, 119, 19541960. [201] Kress, J. D.; Leung, P. C.; Tawa, G. J.; Hay, P. J. Calculation of a Reaction Path for KOH-Catalyzed Ring-Opening Polymerization of Cyclic Siloxanes. In Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds.; ACS Symp. Ser. 2000, 729, 81-97. [202] Yuzhelevskii, Yu. A.; Dmokhovskaya, E. B.; Klebanskii, A. l.; Kozlova, N. V. Vysokomol. Soed. 1969, A11, 432-439. [203] Chojnowski, J.; Rózga, K. J. Inorg. Organomet. Polym. 1992, 2, 297-317. [204] Suryanarayanan, B.; Peace, B. W.; Mayhan, K. G. J. Polym. Sci. A 1974, 12, 10891107. [205] Suzuki, T. Polymer 1989, 30, 333-337. [206] Cai, G. P.; Weber, W. P. Macromolecules 2000, 33, 8976-8982. [207] Paulasaari, J. K.; Weber, W. P. Macromolecules 1999, 32, 6574-6577. [208] Paulasaari, J. K.; Weber, W. P. Macromolecules 1999, 32, 5217-5221. [209] Andrianov, K. A.; Nogaideli, A. I.; Khubulava, E. I.; Kopylov, V. M.; Khananashvili, L. M.; Kolchina, A. G.; Sycheva, N. G. Vysokomol. Soed. 1976, A18, 2619-2625. [210] Brandt, P. J. A.; Elsbernd, C. L. S.; Patel, N.; York, G.; McGrath, J. E. Polymer 1990, 31, 180-190. [211] Gilbert, A. R.; Kantor, S. W. J. Polym. Sci. 1959, 40, 35-58. [212] Veith, C. A.; Cohen, R. E. J. Polym. Sci. A 1989, 27, 1241-1258. [213] Clarson, S. J.; Stuart, J. O.; Selby, C. E.; Sabata, A.; Smith, S. D.; Ashraf, A. Macromolecules 1995, 28, 674-677. [214] Grigoras, S.; Qian, C.; Crowder, C.; Harkness, B. R.; Mita, I. Macromolecules 1995, 28, 7370-7375. [215] Harkness, B. R.; Tachikawa, M.; Mita, I. Macromolecules 1995, 28, 8136-8141. [216] Molenberg, A.; Klok, H. A.; Moller, M.; Boileau, S.; Teyssie, D. Macromolecules 1997, 30, 792-794. [217] Molenberg, A.; Michalke, D.; Moller, M.; Pieper, T. J. Polym. Sci. A 1997, 36, 169177. [218] Weber, W. P.; Cai, G. P. Macromolecules 2001, 34, 4355-4360. [219] Weber, W. P.; Paulasaari, J. K.; Cai, G. P. Synthesis of linear copolysiloxanes with regular microstructures and synthesis of soluble polysiloxanes with trifunctional RSiO3/2 (T) or tetrafunctional SiO4/2 (Q) units. In Synthesis and Properties of Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds.; ACS Symp. Ser. 2003, 838, 72-78. [220] Suzuki, T.; Lo, P. Y. Macromolecules 1991, 24, 460-463. [221] Suzuki, T.; Yamada, S.; Okawa, T. Polymer J. 1993, 25, 411-416. [222] Chang, T. C.; Chen, Y. C.; Ho, S. Y.; Chiu, Y. S. Polymer 1996, 37, 2963-2968. [223] Maschke, U.; Wegner, T.; Coqueret, X. Makromol. Chem. 1992, 193, 2453-2466. [224] Dickstein, W. H.; Lillya, P. C. Macromolecules 1989, 22, 3882-3885. [225] Kazama, H.; Tezuka, Y.; Imai, K. Macromolecules 1991, 24, 122-125. [226] Kumar, A.; Eichinger, B. E. Macromolecules 2005, 23, 5358. [227] Yin, R.; Hogen-Esch, T. E. Macromolecules 1993, 26, 6952-6957. [228] Tezuka, Y.; Nobe, S.; Shiomi, T. Macromolecules 1995, 28, 8251-8258.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

54

Marek Cypryk

[229] Aoyagi, T.; Takamura, Y.; Nakamura, T.; Nagase, Y. Polymer 1992, 33, 1530-1536. [230] Hammouch, S. O.; Beinert, G. J.; Ziliox, J. G.; Herz, J. E. Polymer 1995, 36, 421-426. [231] Wilczek, L.; Rubinsztajn, S.; Fortuniak, W. Bull. Pol. Acad. Sci. Chem. 2006, 37, 9197. [232] Saam, J. C. J. Inorg. Organomet. Polym. 1999, 9, 3-16. [233] Ahn, H. W.; Clarson, S. J. J. Inorg. Organomet. Polym. 2001, 11, 203-216. [234] Andrianov, K. A.; Yakushkina, S. E.; Svistunov, V. S.; Lavrukhin, B. A. Dokl. Akad. Nauk SSSR 1969, 188, 345-347. [235] Bresler, L. S.; Mileshkevich, V. P.; Yuzhelevskii, Yu. A.; Timofeeva, N. P. Zh. Strukt. Khim. 1978, 19, 453-459. [236] Momper, B.; Wagner, T.; Maschke, U.; Ballauff, M.; Fischer, E. W. Polym. Commun. 1990, 31, 186-189. [237] Kuo, C.-M.; Saam, J. C.; Taylor, R. B. Polym. Int. 1994, 33, 187-195. [238] Itami, Y.; Marciniec, B.; Kubicki, M. Organometallics 2003, 22, 3717-3722. [239] Ackerhans, C.; Roesky, H. W.; Labahn, T.; Magull, J. Organometallics 2002, 21, 36713674. [240] Ahn, H. W.; Clarson, S. J. Anionic ring-opening polymerization of cis-2,4,6-trimethyl2',4',6'-triphenyl cyclotrisiloxane (cis-P-3) in cyclohexane. In Synthesis and Properties of Silicones and Silicone-Modified Materials, Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds.; ACS Symp. Ser. 2003, 838, 40-49. [241] Baratova, T. N.; Mileshkevich, V. P.; Gurari, V. E. Vysokomol. Soed. 1983, A25, 24972505. [242] Cypryk, M.; Kaźmierski, K.; Fortuniak, W.; Chojnowski, J. Macromolecules 2000, 33, 1536-1545. [243] Chojnowski, J.; Cypryk, M.; Fortuniak, W.; Rózga-Wijas, K.; Ścibiorek, M. Polymer 2002, 43, 1993-2001. [244] Patnode, W.; Wilcock, D. F. J. Am. Chem. Soc. 1946, 68, 358-363. [245] Chojnowski, J.; Wilczek, L. Makromol. Chem. 1979, 180, 117-130. [246] Sauvet, G.; Lebrun, J. J.; Sigwalt, P. Cationic Polymerization of cyclosiloxanes: the various processes involved. In Cationic Polymerisation and Related Processes, Goethals, E., Ed.; Academic Press: London, 1984; pp 237-251. [247] Andrianov, K. A.; Shkolnik, M. I.; Kopylov, V. M.; Bravina, N. Vysokomol. Soed. 1974, B16, 893-899. [248] Grzelka, A.; Chojnowski, J.; Fortuniak, W.; Taylor, R. G.; Hupfield, P. H. J. Inorg. Organomet. Polym. 2004, 14, 101-116. [249] Schindler, S.; Rühlmann, K. Plaste Kautsch. 1978, 25, 384-385. [250] Elms, R. A. U.S. Patent 4,831,174, 1989. [251] Rashkov, I.; Gitsov, J. J. Polym. Sci. A 1986, 24, 155. [252] Razzano, J. S.; Anderson, P. P.; Perry, R. J. U.S. Patent 5,670,596, 1997. [253] Kendrick, T. C. J. Chem. Soc. 1965, 2027-2035. [254] Chernyshev, A. I.; Yastrebov, V. V. Vysokomol. Soed. 1969, A11, 525-531. [255] Chojnowski, J.; Ścibiorek, M.; Kowalski, J. Makromol. Chem. 1977, 178, 1351-1366. [256] Sigwalt, P.; Nicol, P.; Masure, M. Makromol. Chem. Suppl. 1989, 15, 15-30. [257] Jordan, E.; Lestel, L.; Boileau, S.; Cheradame, H.; Gandini, A. Makromol. Chem. 1989, 190, 267-276.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

General Review on Polysiloxane Synthesis

55

[258] Wang, Q.; Zhang, H.; Prakash, G. K. S.; Hogen-Esch, T. E.; Olah, G. A. Macromolecules 1996, 29, 6691-6694. [259] Toskas, G.; Besztercey, G.; Moreau, M.; Masure, M.; Sigwalt, P. Macromol. Chem. Phys. 1995, 196, 2715-2735. [260] Jallouli, A.; Saam, J. C. J. Inorg. Organomet. Polym. 1998, 8, 179-203. [261] Farooq, O. U.S. Patent 5,124,417, 1992. [262] Lamanna, W. M.; Palazzotto, M. C.; DeVoe, R. J.; McCormick, F. B.; Olofson, J. M.; Siedle, A. R.; Willett, P. S. U.S. Patent 5,514,728, 1996. [263] Out, G. J.; Klok, H.-A.; Möller, M.; Oelfin, D. Macromol. Chem. Phys. 1995, 196, 195210. [264] Price, G. J.; Hearn, M. P.; Wallace, E. N. K.; Patel, A. M. Polymer 1996, 37, 23032308. [265] Gobin, C.; Masure, M.; Sauvet, G.; Sigwalt, P. Makromol. Chem. Macromol. Symp. 1986, 6, 237-245. [266] Sigwalt, P. Polym. J. 1987, 19, 567-580. [267] Wilczek, L.; Rubinsztajn, S.; Chojnowski, J. Makromol. Chem. 1986, 187, 39-51. [268] Wilczek, L.; Chojnowski, J. Macromolecules 1981, 14, 9-17. [269] Olah, G. A.; Li, X.-Y.; Wang, Q.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc. 1995, 117, 8962-8966. [270] (a) Toskas, G.; Moreau, M.; Masure, M.; Sigwalt, P. Macromolecules 2001, 34, 47304736. (b) Toskas, G.; Moreau, M.; Sigwalt, P. Makromol. Chem. Macromol. Symp. 2006, 240, 68–77. [271] (a) Cypryk, M.; Kurjata, J.; Chojnowski, J. J. Organomet. Chem. 2003, 686, 373-378. (b) Chojnowski, J.; Rubinsztajn, S.; Fortuniak, W.; Kurjata J. J. Inorg. Organomet. Polym. Mater. 2007, 17, 173-187. [272] Chojnowski, J.; Cypryk, M.; Kaźmierski, K. Macromolecules 2002, 35, 9904-9912. [273] Gadda, T. M.; Nelson, A. K.; Weber, W. P. J. Polym. Sci. A 2004, 42, 5235-5243. [274] Paulasaari, J. K.; Weber, W. P. Polym. Prepr. 2000, 41, 133-134. [275] Chawla, A. S.; St-Pierre, L. E. J. Appl. Polym. Sci. 1975, 19, 353-359. [276] Andrianov, K. A.; Hananashvili, L. M.; Zavin, B. G.; Kuznetsova, A. G.; Golubkov, S. A.; Ivanov, V. I. Vysokomol. Soed. 1969, B11, 637. [277] Andrianov, K. A.; Godovskii, Yu. A.; Svistunov, V. S.; Papkov, V. S.; Zhdanov, A. A.; Slonimskii, G. L. Dokl. Akad. Nauk SSSR 1977, 234, 1326. [278] Buzin, M. I.; Gerasimov, M. V.; Obolonkova, E. S.; Papkov, V. S. J. Polym. Sci. A 1997, 35, 1973-1984. [279] Unno, M.; Takada, K.; Kawaguchi, Y.; Matsumoto, H. Mol. Cryst. Liq. Cryst. 2005, 440, 259-264. [280] Chawla, A. S.; St-Pierre, L. E. J. Polym. Sci. 1972, 10, 2691-2700. [281] Naylor, D. M.; Stannett, V. T.; Deffieux, A.; Sigwalt, P. Polymer 1994, 35, 1764-1768. [282] Hyde, J. F.; Wehrly, J. R. U.S. Patent 2,891,920, 1959. [283] De Gunzbourg, A. Ph.D. thesis, Pierre et Marie Curie University-Paris VI, Paris, FR, 1993. [284] Graiver, D.; Huebner, D. J.; Saam, J. C. Rubber Chem. Technol. 1983, 56, 918-926. [285] De Gunzbourg, A.; Favier, J. C.; Hémery, P. Polym. Int. 1994, 35, 179-188. [286] Barrere, M.; Ganachaud, F.; Bendejacq, D.; Dourges, M. A.; Maitre, C.; Hémery, P. Polymer 2001, 42, 7239-7246.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

56

Marek Cypryk

[287] De Gunzbourg, A.; Maisonnier, S.; Favier, J. C.; Maitre, C.; Masure, M.; Hémery, P. Makromol. Chem. Macromol. Symp. 1998, 132, 359-370. [288] Maisonnier, S.; Favier, J. C.; Masure, M.; Hémery, P. Polym. Int. 1999, 48, 159-164. [289] Palaprat, G.; Ganachaud, F.; Mauzac, M.; Hémery, P. Polymer 2005, 46, 11213-11218. [290] Barrere, M.; Maitre, C.; Dourges, M. A.; Hémery, P. Macromolecules 2001, 34, 72767280. [291] Caille, J. R.; Teyssie, D.; Bouteiller, L.; Bischoff, R.; Boileau, S. Makromol. Chem. Macromol. Symp. 2000, 153, 161-166. [292] Ivanenko, C.; Maitre, C.; Ganachaud, F.; Hémery, P. E-Polymers 2003, art-010. [293] Belorgey, G.; Sauvet, G. Organosiloxane block and graft copolymers. In SiliconContaining Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 43-78. [294] Simionescu, B. C.; Harabagiu, V.; Simionescu, C. I. Siloxane-Containing Polymers. In Polymeric Materials Encyclopedia, Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp 7751-7757. [295] Yilgör, I.; McGrath, J. E. Adv. Polym. Sci. 1988, 86, 1-86. [296] Kennan, J. J. Siloxane Copolymers. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; Ellis Horwood/PTR Prentice Hall: Englewood Cliffs, 1993; pp 72-134. [297] Liu, L. H.; Yang, S. Y.; Zhang, Z. J.; Wang, Q.; Xie, Z. M. J. Polym. Sci. A 2003, 41, 2722-2730. [298] Villegas, J. A.; Olayo, R.; Cervantes, J. J. Inorg. Organomet. Polym. 2003, 13, 205222. [299] Boutevin, B.; Guida-Pietrasanta, F.; Ratsimihety, A. Side group modified polysiloxanes. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 79-112. [300] Cancouët, P.; Daudet, E.; Hélary, G.; Moreau, M.; Sauvet, G. J. Polym. Sci. A 2000, 38, 826-836. [301] Ziemielis, M.; Saam, J. C. Macromolecules 1989, 22, 2111-2116. [302] Ścibiorek, M.; Gladkova, N. K.; Chojnowski, J. Polym. Bull. 2000, 44, 377-384. [303] Chojnowski, J.; Cypryk, M.; Fortuniak, W.; Ścibiorek, M.; Rózga-Wijas, K. Macromolecules 2003, 36, 3890-3897. [304] Ganicz, T.; Mizerska, U.; Moszner, M.; O'Brien, M.; Perry, R.; Stańczyk, W. A. Appl. Cat. A-Gen. 2004, 259, 49-55. [305] Ganicz, T.; Stańczyk, W. A. Prog. Polym. Sci. 2003, 28, 303-329. [306] Teyssie, D.; Boileau, S. Liquid-Crystalline Silicon-Containing Polymers. In SiliconContaining Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 593-614. [307] White, M. S. Liquid-Crystal Siloxanes. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; PTR Prentice Hall: Englewood Cliffs, NJ, 1993; pp 245-308. [308] Cancouët, P.; Pernin, S.; Hélary, G.; Sauvet, G. J. Polym. Sci. A 2000, 38, 837-845. [309] Li, J.; Qin, J. G.; Ye, C. Synthet. Metal. 2005, 152, 305-308. [310] Ungurenasu, C.; Pinteala, M. Macromol. Rapid Commun. 2005, 26, 707-709. [311] Bauer, J.; Husing, N.; Kickelbick, G. J. Polym. Sci. A 2002, 40, 1539-1551.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

General Review on Polysiloxane Synthesis

57

[312] Cai, G. P.; Weber, W. P. Polymer 2002, 43, 1753-1759. [313] Gupta, S. K.; Weber, W. P. Macromolecules 2002, 35, 3369-3373. [314] Hempenius, M. A.; Lammertink, R. G. H.; Vancso, G. J. Macromolecules 1997, 30, 266-272. [315] Rózga, K.; Chojnowski, J.; Boileau, S. J. Polym. Sci. A 1997, 35, 879-888. [316] Bauer, J.; Husing, N.; Kickelbick, G. J. Polym. Sci. A 2004, 42, 3975-3985. [317] Bauer, J.; Husing, N.; Kickelbick, G. Chem. Commun. 2001, 137-138. [318] Deschler, U.; Kleinschmit, P.; Panster, P. Angew. Chem. Int. Ed. Engl. 1986, 25, 236252. [319] Sauvet, G.; Dupond, S.; Kaźmierski, K.; Chojnowski, J. J. Appl. Polym. Sci. 2000, 75, 1005-1012. [320] Sauvet, G.; Fortuniak, W.; Kaźmierski, K.; Chojnowski, J. J. Polym. Sci. A 2003, 41, 2939-2948. [321] Fortuniak, W.; Rózga-Wijas, K.; Chojnowski, J.; Labadens, F.; Sauvet, G. React. Funct. Polym. 2005, 61, 315-323. [322] Kang, J.-J.; Li, W.-Y.; Lin, Y.; Li, X.-P.; Xiao, X.-R.; Fang, S.-B. Polym. Adv. Technol. 2004, 15, 61-64. [323] Marciniec, B. Coord. Chem. Rev. 2005, 249, 2374-2390. [324] Ganicz, T.; Kowalewska, A.; Stańczyk, W. A.; Butts, M.; Nye, S. A.; Rubinsztajn, S. J. Mater. Chem. 2005, 15, 611-619. [325] Mazurek, M. Silicone Copolymer Networks and Interpenetrating Networks. In SiliconContaining Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 113-138. [326] Thomas, D. R. Cross-Linking of Polydimethylsiloxanes. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; PTR Prentice Hall: Englewood Cliffs, NJ, 1993. [327] Bischoff, R.; Cray, S. E. Prog. Polym. Sci. 1999, 24, 185-219. [328] Frisch, H. L.; Huang, M. W. Interpenetrating Networks of Polydimethylsiloxane. In Siloxane Polymers, Clarson, S. J., Semlyen, J. A., Eds.; PTR Prentice Hall: Englewood Cliffs, NJ, 1993. [329] Avnir, D.; Klein, L. C.; Levy, D.; Schubert, U.; Wojcik, A. B. Organo-silica sol-gel materials. In The chemistry of organic silicon compounds, Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons: 1998; pp 2317-2362. [330] Florke, O. W.; Martin, B.; Blenda, L.; Paschen, S.; Bergna, H. E.; Roberts, W. O.; Welsh, W. A.; Ettinger, M.; Kerner, D.; Kleinschmit, P.; Meyer, J.; Gies, H.; Schiffman, D. Silica. In Uhlmann's Encyclopedia of Industrial Chemistry, VCH Publishers: Weinheim, 1993; pp 583-660. [331] Iler, R. K. The Chemistry of Silica; J. Wiley: New York, 1979. [332] Chojnowski, J.; Cypryk, M.; Fortuniak, W.; Kaźmierski, K.; Rózga-Wijas, K.; Ścibiorek, M. Controlled synthesis of all siloxane-functionalized architectures by ringopening polymerization. In Synthesis and Properties of Silicones and Silicone-Modified Materials, 838 ed.; Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds.; ACS: Washington, DC, 2003; pp 12-25. [333] Cai, G. P.; Weber, W. P. Polymer 2004, 45, 2941-2948. [334] Sargent, J. R.; Weber, W. P. Polym. Prepr. 2000, 41, 604-605.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

58

Marek Cypryk

[335] Kurian, P.; Kennedy, J. P.; Kisluik, A.; Sokolov, A. J. Polym. Sci. A 2006, 40, 12851292. [336] Nugay, N.; Erdodi, G.; Kennedy, J. P. J. Polym. Sci. A 2005, 43, 630-637. [337] Abed, S.; Boileau, S.; Bouteiller, L.; Lacoudre, N. Polym. Bull. 1997, 39, 317-324. [338] Abed, S.; Boileau, S.; Bouteiller, L. Macromolecules 2000, 33, 8479-8487. [339] Abed, S.; Boileau, S.; Bouteiller, L. Polymer 2001, 42, 8613-8619. [340] Ky Hirschberg, J. H. K.; Beijer, F. H.; van Aert, H. A.; Magusin, P. C. M. M.; Sijbesma, R. P.; Meijer, E. W. Macromolecules 1999, 32, 2696-2705. [341] Feher, F. Polyhedral Oligosilsesquioxanes and Heterosilsesquioxanes. In Silanes and Silicones (Gelest Catalog 2000), Arkles, B., Larson, G., Eds.; Gelest, Inc. Morrisville, 2004; pp 55-72. [342] Voronkov, M. G.; Lavrent'yev, V. I. Topics Curr. Chem. 1982, 102, 199-236. [343] Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. Rev. 1995, 95, 1409-1430. [344] Feher, F. J.; Budzichowski, T. A. Polyhedron 1995, 14, 3239-3253. [345] Loy, D. A.; Shea, K. J. Chem. Rev. 1995, 95, 1431-1442. [346] Lichtenhan, J. D. Silsesquioxane-based polymers. In Polymeric Materials Encyclopedia, Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; pp 7768-7778. [347] Provatas, A.; Matisons, J. G. Trends Polym. Sci. 1997, 5, 327-332. [348] Baney, R. H.; Cao, X. Polysilsesquioxanes. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 157-183. [349] Li, G.; Wang, L.; Ni, H.; Pittman Jr., C. U. J. Inorg. Organomet. Polym. 2002, 11, 123154. [350] Abe, Y.; Gunji, T. Prog. Polym. Sci. 2004, 29, 149-182. [351] Li, H. Y.; Yu, D. S.; Zhang, J. Y. Polymer 2005, 46, 5317-5323. [352] Harrison, P. G.; Kannengiesser, R.; Hall, C. J. Main Group Metal Chem. 1997, 20, 137141. [353] Unno, M.; Suto, A.; Matsumoto, H. J. Am. Chem. Soc. 2002, 124, 1574-1575. [354] Gunji, T.; Arimitsu, K.; Abe, Y. Polym. Prepr. 2004, 45, 624-625. [355] Unno, M.; Tanaka, R.; Tanaka, S.; Takeuchi, T.; Kyushin, S.; Matsumoto, H. Organometallics 2005, 24, 765-768. [356] Inoue, K. Prog. Polym. Sci. 2000, 25, 453-571. [357] Aulenta, F.; Hayes, W.; Rannard, S. Eur. Polym. J. 2003, 39, 1741-1771. [358] Yates, C. R.; Hayes, W. Eur. Polym. J. 2004, 40, 1257-1281. [359] Majoral, J. P.; Caminade, A.-M. Chem. Rev. 1999, 99, 845-880. [360] Frey, H.; Schlenk, C. Topics Curr. Chem. 2000, 210, 69-129. [361] Krska, S. W.; Son, D. Y.; Seyferth, D. Organosilicon dendrimers: Molecules with many possibilities. In Silicon-Containing Polymers, The Science and Technology of Their Synthesis and Applications, Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 615-642. [362] Lang, H.; Lühmann, B. Adv. Mater. 2001, 13, 1523-1540. [363] Son, D. Y. Silicon-based dendrimers and hyperbranched polymers. In The Chemistry of Organic Silicon Compounds, Rappoport, Z., Apeloig, Y., Eds.; John Wiley and Sons: Chichester, 2001; pp 745-803. [364] Lukevics, E.; Arsenyan, P.; Pudova, O. Main Group Metal Chem. 2002, 25, 135-154.

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General Review on Polysiloxane Synthesis

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[365] Rebrov, E. A.; Muzafarov, A. M.; Papkov, V. S.; Zhdanov, A. A. Dokl. Akad. Nauk. SSSR 1989, 309, 376. [366] Morikawa, A.; Kakimoto, M.; Imai, Y. Macromolecules 1991, 24, 3469-3474. [367] Kim, C.; Kwark, K. Main Group Metal Chem. 2002, 25, 475-478. [368] Dvornic, P. R.; Hu, J.; Meier, D. J.; Nowak, R. M. Polym. Prepr. 2004, 45, 585-586. [369] Uchida, H.; Kabe, Y.; Yoshino, K.; Kawamata, A.; Tsumuraya, T.; Masamune, S. J. Am. Chem. Soc. 1990, 112, 7077-7079. [370] Ignatieva, G. M.; Rebrov, E. A.; Myakushev, V. D.; Chenskaya, T. B.; Muzafarov, A. M. Polym. Sci. , Ser. A 1997, 39, 843-852. [371] Miravet, J. F.; Fréchet, J. M. J. Macromolecules 1998, 31, 3461-3468. [372] Rubinsztajn, S.; Stein, J. J. Inorg. Organomet. Polym. 1995, 5, 43-52. [373] Gong, C.; Fréchet, J. M. J. J. Polym. Sci. A 2000, 38, 2970-2978. [374] Kazakova, V. V.; Myakushev, V. B.; Strelkova, T. V.; Muzafarov, A. M. Polym. Sci. , Ser. A 1999, 41, 283-290. [375] Jaumann, M.; Rebrov, E. A.; Kazakova, V. V.; Muzafarov, A. M.; Goedel, W. A.; Moller, M. Macromol. Chem. Phys. 2003, 204, 1014-1026. [376] Kazakova, V. V.; Rebrov, E. A.; Myakushev, V. B.; Strelkova, T. V.; Ozerin, A. N.; Ozerina, L. A.; Chenskaya, T. B.; Sheiko, S. S.; Sharipov, E. Yu.; Muzafarov, A. M. From a Hyperbranched Polyethoxysiloxane Toward Molecular Forms of Silica: A Polymer-Based Approach to the Monitoring of Silica Properties. In Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds.; ACS Symp. Ser. 2000; 729, 503-515. [377] Paulasaari, J. K.; Weber, W. P. Macromol. Chem. Phys. 2000, 201, 1585-1592. [378] Paulasaari, J. K.; Weber, W. P. Macromolecules 2000, 33, 2005-2010. [379] Chojnowski, J. Synthesis of Organofunctional Polysiloxanes of Various Topologies. In Organosilicon Chemistry VI: From Molecules to Materials, Auner, N., Weis, J., Eds.; Wiley-VCH: Weinheim, 2005; pp 620-627. [380] Rózga-Wijas, K.; Chojnowski, J.; Fortuniak, W.; Ścibiorek, M.; Michalska, Z.; Rogalski, L. J. Mater. Chem. 2003, 13, 2301-2310. [381] Ganicz, T.; Pakula, T.; Fortuniak, W.; Białecka-Florjańczyk, E. Polymer 2005, 46, 11380-11388. [382] Weber, W. P.; Paulasaari, J. K.; Sargent, J. R. Polym. Prepr. 2000, 41, 562-563. [383] Clarson, S. J. Silicones and silicone-modified materials: A concise overview. In Synthesis and Properties of Silicones and Silicone-Modified Materials, Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds.; ACS Symp. Ser. 2003; 838, 1-10. [384] Rochow, E. G. Why Silicon? In Progress in Organosilicon Chemistry, Marciniec, B., Chojnowski, J., Eds.; Gordon and Breach: Basel, 1995; pp 3-15. [385] Corriu, R. J. Organomet. Chem. 2003, 686, 1.

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In: Silicon-based Inorganic Polymers Editors: Roger De Jaeger and Mario Gleria

ISBN: 978-1-60456-342-9 © 2008 Nova Science Publishers, Inc.

Chapter 2

SILICONES IN INDUSTRIAL APPLICATIONS M. Andriot, S. H. Chao, A. Colas, S. Cray, F. de Buyl, J. V. DeGroot, A. Dupont, T. Easton, J. L. Garaud, E. Gerlach, F. Gubbels, M. Jungk, S. Leadley, J. P. Lecomte, B. Lenoble, R. Meeks, A. Mountney, G. Shearer, S. Stassen, C. Stevens, X. Thomas and A. T. Wolf Dow Corning Corp., Midland MI, USA

ABSTRACT

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Silicones in industry usually refer to linear polydimethylsiloxanes. A combination of properties such as their backbone flexibility, low intermolecular interactions, low surface tension and thermal stability explain many of their applications. But the name silicone also is used for more complex structures, where some of the methyl groups have been replaced by other functional groups, from branched polymers to resinous materials and even cross-linked elastomers. This allows for modifying some of the silicones properties to specific needs. The objective of this chapter is to give the curious reader a short but scientific overview of the various applications where silicones are used, including their benefits as well as limitations.

1. INTRODUCTION By analogy with ketones, the name “silicone” was given in 1901 by Kipping to describe new compounds of the brut formula R2SiO. These were rapidly identified as being polymeric and actually corresponding to polydialkylsiloxanes. Among them, the most common are polydimethylsiloxanes (PDMS), trimethylsilyloxy terminated with the structure:

62

M. Andriot, S. H. Chao, A. Colas et al. Me

Me Me

Si

O

Si Me

Me

Me O

n

Si

Me

(1)

or Me3SiO(SiMe2O)nSiMe3

Me where n = 0, 1, .........

The methyl groups along the chain can be substituted by many other groups (e.g., phenyl, vinyl or trifluoropropyl). The simultaneous presence of “organic” groups attached to an “inorganic” backbone gives silicones a combination of unique properties and allows their use in fields as different as aerospace (low and high temperature performance), electronics (electrical insulation), health care (excellent biocompatibility) or in the building industries (resistance to weathering).

Nomenclature The main chain unit in PDMS, - (SiMe2O) -, is often shortened to the letter D because, as the silicon atom is connected with two oxygen atoms, this unit is capable of expanding within the polymer in two directions. In a similar way, M, T and Q units can be defined corresponding to: [1] Me

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Me

Si

Me O

O

Si

O O

O

Si

Me O

O

Si

O

Me

O

O

Me

M

T

Q

D

Me3SiO1/2

MeSiO3/2

SiO4/2

Me2SiO2/2

The above polymer (1) can also be described as MDnM. This allows simplifying the description of various structures like (Me3SiO)4Si or tetrakis(trimethylsilyloxy)silane, which becomes M4Q. Superscripts are sometimes used to indicate groups other than methyl (e.g., DH for HMeSiO2/2). The synthesis of siloxanes has been described elsewhere [1-3]. In summary, PDMS is obtained from the hydrolysis of dimethyldichlorosilane Me2SiCl2 , which leads to a mixture of linear and cyclic oligomers: x Me2SiCl2

+ H2O - HCl

x "Me2Si(OH)2" disilanol

- H 2O

y HO(Me2SiO)nH + z (Me2SiO)m cyclic linear m=3, 4, 5,.. mainly 4 n=20-50

Higher molecular weight PDMS is obtained after polymerisation, for example, of the above cyclics in the presence of an end-blocker such as hexamethyldisiloxane and catalysed by a strong acid or strong base according to:

Silicones in Industrial Applications

Me3SiOSiMe3 + x (Me2SiO)4

cat

63

Me3SiO(Me2SiO)nSiMe3

Using other chlorosilanes, different end-blockers and/or different cyclics leads to many structures including polymers with various functional groups grafted on the polymer chain and/or at the polymer ends (e.g., vinyl, hydrogeno, phenyl, amino alkyl). These can be formulated into solvent-based, emulsion or solventless products. Reactive polymers can be cross-linked into elastomers using: • • •

a peroxide-initiated reaction; in particular, if the silicone polymer carries some vinyl groups a condensation reaction; for example, between a hydroxy end-blocked PDMS and an alkoxysilane, in presence of tin salt or titanium alkoxide as catalyst an addition reaction; for example, between a vinyl-functional PDMS and an hydrogenomethyl dimethyl siloxane oligomer, in presence of a organic platinum complex

Such polymer, cross-linker and catalyst are formulated with various additives as one-part, ready-to-use products or two-part products to be mixed prior to use and to cure at room temperature or only at elevated temperatures.

Physicochemical Properties The position of silicon, just under carbon in the periodic table, led to a belief in the existence of analogue compounds where silicon would replace carbon. Most of these analogue compounds do not exist, or if they do, they behave very differently. There are few similarities between Si-X bonds in silicones and C-X bonds: [1-3]

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Element (X) Si C H O

Bond length (Å) Si-X C-X 2.34 1.88 1.88 1.54 1.47 1.07 1.63 1.42

Ionic character (%) Si-X C-X -12 12 -2 4 50 22

Between any given element and silicon, bond lengths are longer than for carbon with this element. The lower silicon electronegativity (1.8) vs. carbon (2.5) leads to a very polarised SiO bond, highly ionic and with a large bond energy, 452 kJ/mole (108 kcal/mol). The Si-C bond has a bond energy of ±318 kJ/mole (76 kcal/mol), slightly lower than a C-C bond, while the Si-Si bond is weak, 193 kJ/mole (46.4 kcal/mole). These values partially explain the stability of silicones; the Si-O bond is highly resistant to homolytic scission. On the other hand, heterolytic scissions are easy, as demonstrated by the re-equilibration reactions occurring during polymerisations catalysed by acids or bases. Silicon atoms do not form stable double or triple bonds of the type sp2 or sp with other elements, yet the proximity of the

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M. Andriot, S. H. Chao, A. Colas et al.

d orbitals allows dπ-pπ retro-coordination. Because of this retro-coordination, trialkylsilanols are more acid than the corresponding alcohols. Yet, the involvement of retro-coordination is challenged [4]. Another example of the difference between analogues is the tetravalent diphenyldisilanol, (C6H5)2Si(OH)2, which is stable, while its carbon equivalent, a gem-diol, dehydrates. The SiH bond is weakly polarised, but here in the direction of a hydride, and is more reactive than the C-H bond. Overall, there are few similarities between a silicone polymer and a hydrocarbon polymer. Silicones display the unusual combination of an inorganic chain similar to silicates and often associated with high surface energy but with side methyl groups that are, on the contrary, very organic and often associated with low surface energy [4]. The Si-O bonds are strongly polarised and without protection should lead to strong intermolecular interactions. However, the methyl groups, only weakly interacting with each other, shield the main chain. This is made easier by the high flexibility of the siloxane chain; rotation barriers are low, and the siloxane chain can adopt many conformations. Rotation energy around a CH2-CH2 bond in polyethylene is 13.8 kJ/mol but only 3.3 kJ/mol around a Me2Si-O bond, corresponding to a nearly free rotation. The siloxane chain adopts a configuration that can be idealised by saying that the chain exposes a maximum number of methyl groups to the outside, while in hydrocarbon polymers, the relative backbone rigidity does not allow “selective” exposure of the most organic or hydrophobic methyl groups. Chain-to-chain interactions are low, and the distance between adjacent chains is also higher in silicones. Despite a very polar chain, silicones can be compared to paraffin, with a low critical surface tension of wetting [4]. Yet because of their low intermolecular forces, PDMS materials remain liquid in a much wider range of molecular weights and viscosities than hydrocarbons. The surface activity of silicones is displayed in many circumstances: [4] •

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•

•

Polydimethylsiloxanes have a low surface tension (20.4 mN/m) and are capable of wetting most surfaces. With the methyl groups pointing to the outside, this gives very hydrophobic films and a surface with good release properties, particularly if the film is cured after application. Silicone surface tension is also in the most promising range considered for biocompatible elastomers (20 to 30 mN/m). Silicones have a critical surface tension of wetting (24 mN/m), which is higher than their own surface tension. This means that silicones are capable of wetting themselves, a property that promotes good film formation and good surface covering. Silicone organic copolymers can be prepared with surfactant properties, with the silicone as the hydrophobic part (e.g., in silicone polyether copolymers).

The low intermolecular interactions in silicones have other consequences: [4] •

•

Glass transition temperatures are very low (e.g., 146 K for a polydimethylsiloxane compared to 200 K for polyisobutylene, the analogue hydrocarbon); cross-linked PDMS will be elastomeric at RT in the absence of any plasticizers. The presence of a high free volume compared to hydrocarbons explains the high solubility and high diffusion coefficient of gas into silicones. Silicones have a high permeability to oxygen, nitrogen and water vapour, even if in this case liquid water is

Silicones in Industrial Applications

•

65

not capable of wetting a silicone surface. As expected, silicone compressibility is also high. In silicone, the activation energy to the viscous movement is very low, and viscosity is less dependent on temperature compared to hydrocarbon polymers. Moreover, chain entanglements are involved at higher temperature and contribute to limit the viscosity reduction [4].

The presence of groups other than methyl along the chain allows modification of some of the above properties: •

•

A small percentage of phenyl groups along the chain perturbs sufficiently to reduce crystallisation and allows the polymer to remain flexible at very low temperatures. The phenyl groups also increase the refractive index. Trifluoropropyl groups along the chain change the solubility parameter of the polymer from 7.5 to 9.5 (cal/cm3)1/2. These copolymers are used to prepare elastomers with little swelling in alkane or aromatic solvents.

Considering the above, many polymeric “architectures” can be prepared of different physical forms (volatile, liquid, viscoelastic, solid) with different functionalities, inert or capable of interacting or reacting with many other compounds. Formulation into convenient products leads to even more products. This explains the wide range of industries where silicones are used.

2. CHARACTERIZATION OF SILICONES

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Most analytical methods commonly used for organic materials also apply to silicones. Extensive reviews have been published about the different analytical techniques that are applicable for detecting and characterizing silicones [5]. The focus here will be on methods as they relate to typical application problems; some of these are commonly used methods. Others, such as elemental analysis and thermal analysis, are described in more detail.

Common Methods Applied to the Analysis of Silicones Infrared spectroscopy and, in particular, Fourier transform infrared spectroscopy (FTIR), is widely available and the easiest technique for detecting the presence of silicones and obtaining information about their structure. Silicones have strong absorption bands in the mid-infrared spectrum range, at 1260, 1100-1000 and 770 cm-1, meaning that levels as low as 1% can be detected. This method differentiates polydimethylsiloxane, trimethylsilyloxy groups, and copolymer-type materials. Quantification is possible using one of the strong silicone absorption peak signals. Corresponding height or area can then be correlated to a known standard and actual level calculated using Beer-Lambert’s law. Other infrared-based techniques like FTIR/ATR or FTIR/DRIFT are specifically used to detect silicones adsorbed on a substrate (see Figure 1). However, in many cases, the layer of

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M. Andriot, S. H. Chao, A. Colas et al.

silicone on the top of the sample surface is so thin that only the fingerprint of the bulk of the sample is seen. Better samples can be prepared through extraction using a good solvent: hexane (most alkanes are suitable), methylisobutylketone, toluene for siloxane or tetrahydrofuran for more polar copolymers like silicone polyethers. However, extraction recovery yields can be significantly lowered if the siloxane strongly bonds to the substrate. This issue is often encountered with amino-functional siloxanes.

Silicone on skin

Abs

Si-Me (Silicone)

Measured by Zn-Se ATR (7 reflections, 45 deg.)

1800

Amide I (Skin)

Amide II (Skin)

1649.12

1541.79

1700

1600

1259.82

1500

1400

1300

1200 cm-1

Figure 1. FTIR/ATR (attenuated total reflection) analysis of a silicone polymer applied on human skin (the amide peaks can be used as internal standards).

GC 1

GPC

2

2 3

8

8

1 3

4

5 6 7

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SFC 1

2 8 3 4

Figure 2. Comparison between different chromatographic techniques with a trimethylsilyloxy terminated polydimethylsiloxane before stripping. Peak 1 to 12 = cyclics (m = 4 to 10) and peak 8 = polymer.

Silicones in Industrial Applications

67

Gas chromatography coupled with mass spectroscopy detection (GC-MS) is another method used to detect silicones in a formulation, looking for the presence of siloxane cyclic oligomers, as such low molecular weight species are always associated with silicone polymers. The neat sample can be heated at a specific temperature (up to 250 ºC) in a headspace bottle and the generated volatiles injected. An alternative is to dissolve the sample, if feasible, and inject the solution. The most flexible injection mode is the use of a pyrolyser coupled to the GC-MS. This allows collecting and identifying volatiles within any selected temperature range. Yet CG-MS does not allow precise quantification. A precise quantification of those volatile cyclics is routinely done by coupling gas chromatography with flame ionization detection (GC-FID). In addition to GC, other techniques can be used to identify and/or quantify the lowest molecular weight species present in silicone polymers; for example, gel permeation chromatography (GPC) or supercritical fluid chromatography (SFC) (see Figure 2). Gel permeation chromatography (GPC) (also called size exclusion chromatography or SEC) using a refractive index detector allows one to obtain molecular weight averages and distribution information. Calibration is done with polystyrene standards, and Mark-Houwink constants are used to correlate results between the standards and siloxanes’ molecular weight. Adding a laser angle scattering detector provides information on the three-dimensional structure of the polymer in solution. In addition to infrared methods, nuclear magnetic resonance spectroscopy (NMR) can be used to obtain polymer structural details 1H and 13C NMR bring information about the type of organic substituents on the silicone backbone such as methyl, vinyl, phenyl or polyester groups, and identify the degree of substitution in these polymers. 1H NMR is also a technique used to measure the relative content of the SiH groups (proton chemical shift at 4.7 ppm) versus dimethylsilyloxy species (proton chemical shift close to 0 ppm). However, in some cases (e.g., as after a hydrosilylation reaction), the residual SiH levels are too low to allow for quantification. Here gas chromatography coupled with a thermal conductivity detector (GCTCD) is a more appropriate method. This method works in an indirect way, analyzing the hydrogen generated when the sample is hydrolyzed in presence of a strong base as a catalyst.

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Application Specific Methods for the Analysis of Silicones In addition to the methods described above, some more specific techniques are used to detect the presence of silicones as formulation ingredients or contaminants or to study their high/low temperature behavior. Atomic absorption spectroscopy (AAS) allows quantification of the silicone element in a given formulation. This approach is widely used to quantify the silicone content in materials made of, treated with or contaminated by silicones. If the formulation is known not to contain any other silicone element source than silicones, the presence of silicones can be easily detected by X-ray fluorescence (XRF), as the method does not require any particular sample preparation. XRF is capable of measuring silicone contents if standards can be prepared in the same matrix as the formulation. Surface tension measurement is another easy way to detect surface contamination by siloxanes, through comparison with a virgin reference. Contact angles of both suspected and

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M. Andriot, S. H. Chao, A. Colas et al.

clean surfaces are measured with a set of suitable liquids. Silicone contamination will be indicated by large contact angles resulting from a significant decrease in surface energy. X-ray photoelectron spectroscopy (XPS) or time of flight-secondary ion mass spectrometry (TOF-SIMS) are more sophisticated techniques that can also be applied to detect and characterize silicones within the 10-50 Å depth payers from the surface of materials. The average structure of silicone polymers is accessible through 29Si NMR thanks to the 29 Si isotope nuclear spin (I = ½) and its relative abundance (4.7%). However, the relative sensitivity of 29Si NMR is low versus 1H NMR (7.8 10-3 times lower), which implies long accumulation times for any measurements. Peak assignments are eased by large chemical shift differences and the use of decoupling (see Table 1). Yet silicone chain ends can not always easily be detected by 29Si NMR, especially in high molecular weight siloxane polymers. For structural purposes, a complementary technique to 29Si NMR has been developed by depolymerizing the siloxane backbone in the presence of an excess of an appropriate endblocker using a strong base or acid as catalyst. The recovered volatile oligomers are then quantified by GC-FID. This approach has proven applicable to quantifying traces of silicones on substrates like wool, paper or hair [6]. The performance of silicones versus organic polymers at high or low temperatures is verified when using thermal analysis methods such as thermogravimetric analysis (TGA) or differential scanning calorimetry (DSC), whether under air or inert atmosphere. In the latter conditions, the onset of polymer depolymerisation to cyclic species is usually found at temperatures higher than 350 ºC. However, traces of base or acid are sufficient to significantly decrease the temperature at which decomposition starts to occur by catalyzing the re-equilibration of the polymer into low molecular weight volatile species. TGA is the most appropriate technique for measuring the onset of weight loss (temperature ramp mode) or the amount of weight loss at a fixed temperature (isotherm mode).

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Table 1. Typical 29Si NMR Chemical Shifts Unit Structure Me3SiO1/2 Me2SiO2/2 MeSiO3/2 SiO4/2 HOMe2SiO1/2

Unit Type M D T Q MOH

Chemical Shift + 7 ppm - 22 ppm - 66 ppm - 110 ppm - 10 ppm

It is recommended to run TGA under inert atmosphere as the presence of oxygen will allow, at temperature higher than 300 ºC, the partial oxidation of the silicone chain groups into silica and the formation through methyl substituents oxidation into species like carbon monoxide, carbon dioxide, formaldehyde, hydrogen and water. However, unlike GC-MS techniques, TGA does not give information about the nature of the species volatilized (see Figure 3). On the other hand, DSC is particularly appropriate for analyzing the behavior of silicones at low temperatures. Due to the flexibility of the polysiloxane backbone, glass transition typically occurs below -120 ºC, a remarkably low temperature if compared to other polymers.

Silicones in Industrial Applications

69

Figure 3. TGA analysis under dry nitrogen of a blend of silicone volatile species and silicone elastomer (weight loss curve in green, first derivative curve in blue); precise content in volatile species (weight loss up to 150 ºC) and in elastomer (second weight loss step between 400-700 ºC and residue in 950ºC).

This is why polydimethylsiloxanes remain fluid and silicone elastomers remain flexible at low temperatures. Nevertheless, crystallization can be made to occur, at least for a fraction of the polymer, on slow cooling below -40 ºC. The cooling rate should be low enough to allow chains to form crystalline structures. Silicone polymers can also be supercooled to a glassy state without crystallization under fast cooling or quench. In this case, on reheating, a cold crystallization exotherm is observed followed by the usual endotherm(s) around -50 ºC (several “melting points” can be observed as multiple crystallization/melting events occur in the sample in the same temperature range) (see Figure 4). 1

Cold crystallization T = - 98 oC

2 Glass transition Tg = - 127 oC

Melting Tm = - 43 oC

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Crystallization

3

4 Melting Tm = - 43 oC

- 140

- 120

- 100

- 80

- 60

- 40

- 20

0

20

Figure 4. Typical low temperature DSC analysis of a silicone elastomer: the sample is super-cooled at 150 ºC, then heated from -150 ºC to 25 ºC (red curve). The following are detected: glass transition (Tg) at -127 ºC, cold crystallization at -98 ºC and melting (Tm) at -43 ºC. Afterwards, the sample is cooled down at a low cooling rate and reheated. A crystallization exothermal peak is observed during the cooling step (blue curve) and only a single melting endothermic peak during the second heating step (green curve).

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M. Andriot, S. H. Chao, A. Colas et al.

DSC is also a powerful technique for studying exothermic reactions such as the hydrosilylation reaction, which is associated with a strong exotherm. Dynamic or isotherm modes allow characterization of the cross-linking and optimization of formulations based on the temperature corresponding to the onset of cure of to the maximum cure rate (see Figure 5).

Tpeak = 147 oC Exothermal

90 oC

110 oC

130 oC

150 oC

Figure 5. Differential scanning calorimetry of the reaction between silicone polymers with SiVi groups and polymers with SiH groups in presence of a platinum catalyst.

3. SILICONE IN THE FOOD INDUSTRIES In food-related processes, silicones are very much associated with foam control agents because of the low surface tension displayed by polydimethylsiloxanes, and because this is a key property for formulating an effective antifoam. Foam control is critical here as in many other industries, as excessive foaming slows processes and can reduce volume efficiency.

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Polydimethylsiloxanes as Surface-Active Ingredients in Antifoams Silicone oils, or in particular polydimethylsiloxane (PDMS) materials, combine many unusual properties because of their molecular characteristics, such as the flexibility of the SiO-Si backbone and the very low cohesive energy existing between methyl groups. PDMS polymers have low surface tension, and most of them are nonvolatile and remain liquid even at quite high molecular weights. They are also highly insoluble in water. Because of the extreme flexibility of the siloxane backbone and ease with which various polymer configurations can be adopted, and despite the siloxane backbone’s considerable polarity, it is the polymer side groups that are the primary surface-active entities in the polymer structure (see Section 1). The pendant groups in PDMS are methyl groups, which show the weakest intermolecular interactions known: the London dispersion forces. The low surface tension, which is a direct manifestation of low intermolecular forces, confirms that the interactions between two PDMS chains occur only through their methyl groups. The polymer backbone controls the organization of the side groups at the surface, and its flexibility has a major effect on the ease with which the pendant groups can adopt preferred configurations.

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Thus, from a surface tension standpoint, the more flexible the backbone, the more readily will the lowest surface energy configurations be adopted. PDMS is a particularly favored case of very low intermolecular force pendant groups anchored along the most flexible backbone, thus allowing the methyl groups to be ideally presented to the external world. It is often observed that neat silicone oil shows low efficiency as antifoaming agent. But mixtures of such oils with hydrophobic particles such as treated silica or finely divided high melting point waxes are generally much more effective than the individual components [710]. In fact, the mixture performs well even if each component is ineffective when used alone. This synergy is observed for most combinations of oils and solids and in various types of foaming media. Effective foam control agents continue to be developed using such combinations to adjust for different types of foam problems. Many foam control agents are added to the foaming medium after being predispersed in water, either as self-dispersible neat materials, oil-in-water emulsions, or self-dispersible mixtures or compounds. This is because it is critical to have small droplets of antifoam in the liquid medium to have antifoaming activity. To rupture a foam film, an oil/hydrophobic particle droplet must in a first step emerge from the aqueous phase into the air-water interface during a process called entering. After this entering, some oil from the droplet can spread on the solution-air interface in a second step. Two coefficients measure the changes in the free energy of the system associated with these two steps. When an oil drop enters the air-water interface, the change is measured by the entering coefficient, E: E = σ AW + σ OW - σ OA A positive E means the surface tension of the antifoam liquid (σ OA) is lower than the sum of the surface tension of the foaming liquid (σ AW) and the interfacial tension between the antifoam and the foaming liquid (σ OW). This value is the opposite of the free energy associated with the entering step. When an oil drop spreads over the air-water surface, change is measured by a spreading coefficient, S:

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S = σ AW - σ OW - σ OA In this step, the water surface is replaced by an oil surface. A positive S means the surface tension of the foaming liquid (σ AW) is greater than the sum of the surface tension of the antifoam liquid (σ OA) and the interfacial tension between the antifoam and the foaming liquid (σ OW ). The free energy associated with this change is the difference between the energy of the end result (the sum of the oil-water interfacial tension and the oil surface tension) and the starting point (the water surface tension). The spreading coefficient is the opposite of the free energy change associated with the spreading step. Both the entering coefficient and the spreading coefficient must be positive for the corresponding processes to be energetically favorable.

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Entering is obviously essential to foam rupture, and it is generally agreed that the entering coefficient must be positive for a particle or droplet to cause rupture of a foam film. A recent study suggests that spreading of a layer of oil eases the foam breaking mechanism, suggesting that the ability to spread is an important property for an oil used in antifoam formulations [11]. For both the entering and spreading coefficient to be positive, it is important to have a liquid with a low surface tension, which is the case with silicone oils. It has been shown that hydrophobic particles ease entry of the antifoam droplet in the foam walls or film interfaces, explaining the benefit of their addition in antifoam formulation [8]. Once the silicone antifoam droplet has entered the two air-water interfaces (Figure 6), it forms an oil bridge between the two surfaces of the foam film. One of the mechanisms proposed in the literature involves at this stage the dewetting of the solution away from the antifoam droplets, because of the low surface tension of the oil, and leading eventually to the film rupture. Although this is a simplified view of the mechanism of antifoam action, it helps explain why silicones are very effective for rupturing foam.

A B C D

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Figure 6. Schematic presentation of the bridging of a foam film by a spherical antifoam droplet. In A, the antifoam droplet is entirely in the liquid film. In B, the antifoam droplet has entered the surface. In C, the antifoam droplet bridges the film. In D, the process of bridge dewetting occurs, leading to destabilization of the foam film and eventually to foam rupture.

Food and Beverage Foods are chemical mixtures consumed by humans for nourishment or pleasure. Most of the nutrients are provided by proteins, vitamins and minerals, whereas carbohydrates and fats provide energy. But any media containing such biomaterials and proteins with or without carbohydrates show high foaming tendencies. The proteins act as surfactants due to their amphiphilic structure. They can unfold and strongly adsorb at the interface, forming strong intermolecular interactions. This produces a viscoelastic, irreversibly adsorbed layer at the air-liquid surface, which stabilises the foam. These kinds of films are not easily broken. This explains why foam is often encountered during food handling, from production to end use. But uncontrolled foaming media are a source of severe loss of production capacity,

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including inefficient mixing or pumping, downtime from clogged lines, overflows, spillage hazards and product waste. Therefore, foam control technologies (either mechanical or based on chemical additives) have been developed to overcome these problems and reduce costs to a minimum [12-14]. Chemical additives designed to reduce such foaming problems are called antifoam agents, foam control agents or defoamers. But the choice of ingredients for applications in the food industries is limited because of regulations and the need to ensure that such ingredients do not to cause harmful effects. Silicone oils are effective ingredients in antifoaming agents. In food and beverage applications, only PDMS materials are used because of their low surface tension, water insolubility, thermal stability and chemical inertness. PDMS of sufficient molecular weight does not penetrate though biological membranes, and orally is not metabolised, but excreted unchanged (see Section 21). These PDMS materials are mixed with hydrophobic particles and formulated as powders, compounds or emulsions. The pathway followed by food materials starts from their production (e.g., plant growth), their processing and their uses. Foam can be produced in each of these steps. Silicone Antifoams in Food Production. Crop treatment often requires the spraying of various chemicals on plant leaves. Surface active materials, including silicones such as silicone polyethers, are often needed to help the wetting of the very hydrophobic plant leaves. This is often associated with foaming problems and antifoams are required; for example, during tank-filling operations. Silicone Antifoams in Food Processing. In the production of sugar from sugar beets, foaming is a serious problem, starting from the beet-washing stage to diffusion and evaporation stages. The foam is attributed to the numerous nonsugar materials present, such as cellulose, lignin, protein, vegetable bases (betaine and choline), and especially saponin [15]. Foam controllers employed in the beet-washing process are likely to appear in wastewaters. Therefore, their environmental profile is important to consider. Because sugar is intended for human consumption and trace amounts of antifoaming agents may be present in the finished product, various legal and health issues must also be considered. Furthermore, steam-volatile components must be avoided during the evaporation and boiling steps. Fermentation processes such as the production of drugs, yeasts or simply ethanol require antifoams to control the level of foam during the microorganisms’ growth and the endproduct formation. Biomaterials in the growing media often have a high foaming tendency, whether they are present in the blend of several carefully selected materials like protein extract, sugar, or as byproducts of other food production processes like sugar cane, sugar beet molasses or corn liquor production. On top of this, proteins are often produced by microorganisms and released during the fermentation process, making the foam harder to control. Apart from their essential antifoam properties, the ideal foam control agent for fermentation processes should not be metabolized by the microorganisms, be nontoxic to these microorganisms and to humans, should not cause problems in the extraction and purification of the final product, should not have detrimental effects on oxygen transfer and be heat sterilizable. Processing potatoes or vegetables requires a wash bath. Intensive foaming of potato juice, starch slurry and processing water is caused by proteins, other nitrogenous compounds and starch found in potatoes or vegetables. Starch foam is very stable and difficult to counter, and antifoam is the most practical and universal application solution.

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Beverages are also prone to foam problems during filling and bottling of alcohol beverages, coffee drinks, flavored water and fruit drinks, or when reconstituting powdered drinks like instant coffee or tea with water. Typical foam control agents recommended for use in food processing or packaging must: • •

have Kosher certification comply with FDA Regulation 21 CFR 173.340 (secondary direct additives).

Silicone Lubricating Oil in Food Processing Silicones suitable as food grade lubricating oils are generally straight-chain PDMS. They may be formulated with treated fume silica to obtain a grease and the right rheology profile, including a yield point (see Section 18). Silicone lubricating oil with incidental food contact must meet FDA regulation 21 CFR 178.3570. Immiscibility with many organic fluids, low temperature dependence of their physical characteristics, physiological inertness and high temperature stability are some of the key properties making silicone lubricating oils better than organic alternatives for these applications. Silicone lubricating oils are used in bearings, gears with rolling friction, on plastic surfaces and on rubber parts encountered on equipment used for food processing (see Section 18).

4. SILICONES IN THE PULP AND PAPER INDUSTRY Organosiloxane materials can be found throughout the processing of pulp and paper, from the digestion of wood chips to the finishing and recycling of papers. Some examples are: •

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• • •

As digester additives, silicones improve the impregnation of active alkali in the wood chips and improve the cooking As antifoams, silicones help de-airing or drainage in the pulp washing and papermaking processes As additives, silicones contribute in the finishing process of paper and tissues In the recycling of papers, silicones act as de-inking aids

Some specific examples are developed here to demonstrate how the properties of silicones can bring benefits as antifoams in the paper pulp-washing process and as softening agents in the treatment of tissue fibers.

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Antifoam in the Pulp-Washing Process or Brownstock Washing Kraft or sulfate pulping remains the most common chemical process used to produce bleached and unbleached pulp of high quality [16]. The wood chips are impregnated with an alkaline liquor containing NaOH and Na2S and digested at high temperatures. During this process, delignification and degradation of esters from fatty acids, resin acids and sterols occurs. This generates surface-active molecules that create excessive foam during the pulpwashing process. The presence of foam is a serious problem for the paper mill operator since it dramatically reduces washing efficiency, and in extreme conditions, can lead to an overflow from the filtrate vat spilling onto the washroom floor. In some cases, such an event can cause the shutdown of production. Both organic and silicone antifoams are used and subjected to harsh conditions of pH (11 to 12.5) and temperature (80 to 95°C). Antifoams are typically based on a combination of a hydrophobic and insoluble oil formulated with hydrophobic solid particles (see Section 3). These mixtures are generally called antifoam compounds [17]. Organic antifoam compounds are generally based on mineral, paraffin or vegetable oils and particles made of amide waxes like ethylene-bis-stearamide (EBS) or hydrophobized silica. Silicone antifoams are usually made of polydimethylsiloxane (PDMS) fluids compounded with hydrophobized silica. Silicone antifoam compounds are sometimes combined with more hydrophilic organic polyethers or silicone polyethers, which can help the emulsification of the silicone compound and act as co-antifoam agents if their cloud point is below the application temperatures. To control foaming over a long enough period of time, organic antifoam must generally be added at higher dosage levels (0.5 to 5 kg/t, expressed as kg of antifoam per ton of dried pulp) if compared to silicone-based antifoams (0.2 to 0.8 kg/t). In the most modern paper mills designed to run at high production rates but also with minimum water consumption, the washing of fiber stocks containing high soap levels as from Scandinavian softwoods or from birch is done under such harsh conditions that only silicone antifoams give the required level of performance. Silicone antifoams contribute to various effects in the process: as defoamers, they reduce the amount of foam immediately after their addition (this is called the “knock down” effect), but as antifoams they also prevent further foam formation and maintain their activity over a long period of time (this is called “persistency”). Silicone antifoams also help drainage and improve washing efficiency by reducing the level of entrapped air in the pulp mat [18]. Silicone antifoams for pulp and paper applications can be seen as a combination of polydimethylsiloxane (PDMS) chemistry, silica chemistry (as silica surface treatment is critical), and emulsion technology, as emulsions are sometimes the preferred route of delivery for the antifoam. Since its introduction in the pulp market in the early 90s, the technology of silicones as antifoams and drainage aids has dramatically evolved. Key improvements worth noticing in recent years are two-fold: • •

Improvement in persistency, which allowed a dramatic reduction in dosage level Optimization of the way the antifoam active is delivered and dispersed in the processing media

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Both are critical for reducing the risks of undesired antifoam hydrophobic deposits from these foam-control agents. Lower dosage reduces the amount of hydrophobic particle present, and improved delivery from specific emulsions (particle size, stability) reduces the risk of agglomeration of such insoluble components. The persistency of silicone-based antifoams has been improved using PDMS polymers of very high molecular weight that are more resistant towards deactivation, [19] less prone to emulsification upon use and have less tendency to liberate the hydrophobic silica particle if submitted to high shear. Careful selection of the silica used (structure, surface area, particle size, porosity) is key to achieving optimum performance of the silicone-based antifoam. Silicone-based antifoams are used at very low levels and generally formulated as selfdispersible concentrates or even more commonly as water-based emulsions. This allows a dramatic reduction in problems of pitch deposits that are commonly encountered with nonaqueous mineral oil/EBS-based antifoams [20]. Finally, over and above any technical requirements, antifoams for paper and pulp applications must meet acceptance under FDA Indirect Food Contact Guidelines 21 CFR 176.170, 180 or 210 and compliance with BGA Recommendations XV.1.A. and XXXVI.B.C1. These regulatory requirements are fulfilled by many silicone-based materials.

Silicone Finishes in Tissue Converting Silicone materials are used as a surface treatment for tissue softening to enhance the performance of bath, toilet and facial tissues; paper towels, napkins and tablecloths; wet and dry wipes; and other consumer and commercial paper products [21]. Similar to other applications such as textile finishing, fabric treatment or hair care, a wide range of performance results from the use of silicones. Most new products for tissue converting are water-based, solventless emulsions. Silicones provide softening by reducing the coefficient of friction without reducing wet or dry strengths, providing antistatic properties and reducing dust and lint during use. More hydrophilic silicone polyethers can also enhance water and liquid absorbency.

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5. SILICONES IN THE TEXTILE INDUSTRIES In the textile industries, silicones are used in all stages of the process, on the fiber during production, on the fabric and/or directly on the finished goods. Silicones are applied from different delivery systems to provide various benefits like lubrication, softening, foam control or hydrophobic coatings.

Silicones in Fiber Production Higher production rates oblige artificial fiber producers to continuously search for more efficient materials to lubricate fiber and spinneret and to avoid excessive overheating due to friction during high-speed manufacturing [22,23].

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Because of properties such as heat stability and good lubrication, silicones can provide a reduction of the dynamic coefficient of friction, reducing the risk of fiber melting and breakage during production (see Section 18). Low viscosity polydimethylsiloxane (PDMS) is generally used in combination with solid particles (e.g., those made of magnesium stearate), as this also reduces the static coefficient of friction. During the manufacturing of artificial fibers, PDMS can also be used as a lubricant to avoid adhesion of the thermoplastic fiber material to the spinneret, which would cause unstable production and cleaning issues. Silicones can also be used to achieve low coefficients of friction between the fibers themselves. Generally a silanol-functional silicone, a reactive cross-linker (e.g., a silane or an epoxy-functional silicone) and a condensation catalyst are formulated together into a coating to encapsulate the fiber. Such treated fibers will lead to high thermal insulating textiles and filling material for fiberfill systems as found in duvets or overalls. Cleaning silicones used during fiber production can sometimes be an issue. To minimize this, lubricant silicone polyethers have been developed with higher hydrophilicity and easier to clean (see Section 7).

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Silicone as Fabric Softeners Once produced, fibers can be treated with silicones to impart initial softness to the textiles made from these fibers. Softening is considered to come from the siloxane backbone flexibility and the freedom of rotation along the Si-O bonds. This allows exposure of the low interacting methyl groups, reducing fiber-to-fiber interactions. To enhance durability through multiple wash cycles, some methyl groups can be replaced on the silicone polymer by other functional groups to increase the silicone softener attraction to, and interaction with, the fibers to be treated. In this respect, amino-functional groups like -CH2-CH2-CH2-NH-CH2-CH2-NH2 are particularly popular for increasing physical adsorption and providing better softening properties. During the application generally done in acidic conditions, these amino groups are quaternized to cationic species (-NH3+), which have a stronger attraction for the negatively charged fabric. This is particularly true for cotton-based fabrics, which carry anionic charges on their surface. This improves deposition, performance and durability of the softener coating. These amino-functional silicones are best delivered to the textile surface under the form of a microemulsion. This offers a number of advantages if compared to macroemulsions. The quality of a microemulsion is easily controlled: visual appearance and good clarity ensures small particle size and long shelf life without the need of any sophisticated particle size testing equipment. Microemulsions have also excellent shelf stability and allow for higher dilutions with better shear stability. On the other side, microemulsions are often formulated with high levels of surfactants, and these can affect the softness normally provided by the silicones. Such surfactants must therefore be carefully selected. Amino-functional silicones can also yellow upon aging via chromophores generated on the amino group, in particular from linkages between amino groups. Modifying the amino

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groups with adequate blocking groups overcomes this problem, offering formulators nonyellowing fiber softeners [24]. Silicones will inherently increase the hydrophobic nature of any treated fabric, a feature not desired in some applications; for example, as it results in poor water absorbency on towels. Trends here are to design amino-functional silicone polymers with higher hydrophilicity [25].

Silicones as Process Aids As in many other processing industries, silicones are widely used in the textile industries as antifoams (see Section 3). Silicone antifoams can operate in a wide range of temperature and pH conditions and can manage highly foaming media. Their compositions can be complex, but there are some formulation rules well known to the silicone industry for producing highly efficient antifoams for many different applications and in various foaming media. Conditions are so diverse that a “universal” antifoam has not yet been formulated. In the textile industry, the main use of antifoams is during the scouring step, which is the cleaning of raw fibers before further processing or during the finishing step. Both of these are high foaming steps, as surfactants are extensively used to clean, or in the formulation of fabric softener emulsions. As the industry is also trying to minimize the amounts of water used in such process steps, this results in even higher surfactant concentrations. The greater use of high-shear jet machines requires antifoam emulsions that are stable under very high shear to avoid undesired localized deposition of silicone polymers. Such deposition can result in staining problems. Other process aids include: • •

Needle lubricants or PDMS fluids to avoid needle overheating during sewing Silicone polyethers to facilitate the wetting of difficult substrates that contain high levels of organic fats in their structures

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Silicones as Hydrophobic Agents Silicones provide very hydrophobic finishes on various fabrics. This treatment involves full fabric impregnation from silicone-in-water emulsions, usually via a padding process. The silicone phase of such emulsions contains SiH-functional polymers because of their reactivity towards the fabric, but also because these polymers can cross-link with each other into a hydrophobic and durable fabric treatment, particularly if formulated with a suitable catalyst [26].

Silicones in Fabric Coatings Silicones are not limited to fiber processing or finishing. Their use extends as coatings in diverse applications, from fashion wear such as women’s stockings to technically demanding

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air bags (see Section 14). Applications here call for substantially thicker coatings, with typical coating weights up to 10 to 800 g/m2. These applications are based on cross-linked silicone polymers or elastomers, which can be formulated into crystal-clear coatings that can be either soft and flexible or hard and rigid. All such coatings have very similar compositions and share common raw materials for up to 70% of their formulation. They perform well over a wide range of temperatures and with better thermal stability characteristics than organics. Apart from one-part RTV elastomer used in women’s stockings, liquid silicone rubbers (LSRs) are today the preferred material for such fabric coatings because of their ease of use and rapid cure when exposed to elevated temperatures. Cross-linking in these elastomers is achieved by the addition of SiH functional polymers to SiVi functional polymers using a platinum catalyst (see Section 9). These LSRs, as other silicone elastomers, contain fumed silica, as such fillers dramatically improve mechanical properties (see Section 14). However, compared to other silicone elastomers with high mechanical properties such as high consistency rubbers, LSRs can be metered/mixed with pumps and easily dispensed as coatings on various fabrics [27]. Silicone coatings remain flexible even at very low temperatures, typically -100 oC. Service life has been reported as 30,000 h at 150 oC and 10,000 h at 200 oC in air. When needed, additives such as cerium or iron oxides can used to further improve heat stability [28]. Compared to many organic elastomers, silicones do not contain organic plasticizers. They are therefore not prone to plasticizer migration problems or embrittlement due to plasticizer evaporation or degradation. Other properties make LSRs desirable as coating materials (see Table 2): •

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• • • •

Solventless compositions with long bath life at room temperature and low viscosity, (15,000 mPa.s) and therefore easy to process in coating operations using methods like “knife over roller” or “knife over air” Fast cure at elevated temperatures (e.g., 1 to 2 minutes at 160 oC) Good adhesion to various coated substrates like glass, polyamide or polyester fabrics Good visual appearance Adequate data to satisfy relevant regulatory requirements (e.g., food grade, skin contact). Table 2. Typical Properties of LSRs Used in Fabric-Coating Applications Mixed viscosity, mPa.s Tensile strength, MPa (psi) Elongation at break, % Tear strength, kN/m (ppi) Hardness, Durometer Shore A

15,000 - 200,000 3.5 - 9.0 (500 - 1300) 100 - 800 5 - 40 (28 - 230) 15 - 70

In many cases, the prime purpose of silicones in such fabric coatings is to provide some form of protection from exposure to high temperatures (as in conveyor belts), low temperatures (as with many outdoor goods) or exposure to stress over long periods of time

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(as in air bags or compensator bellows) (see Table 3). In such applications, silicones are more stable than other elastomers. Table 3. Typical Applications and Key Properties of Silicone Elastomer Fabric Coatings Coating type Soft coating

Application area Hold-up stockings (RTV)

Outdoor clothing and tents (LSR, RTV)

Air bags (LSR)

Hard coating

Conveyor belt coating (LSR)

Compensator bellow (LSR) Medical protective wear (LSR, RTV)

Key properties Ease to process Crystal clear Soft Non slip/high elongation Adhesion Flexible Thermal stability Colorless Hydrophobicity Strength Adhesion Slip Stability at elevated temp. Adhesion Non slip/abrasion resistance Thermal stability Food grade Adhesion Chemical/Thermal stability abrasion resistance Hydrophobic Autoclavable Adhesion

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6. SILICONES IN HOUSEHOLD CLEANING APPLICATIONS Silicones and household cleaning applications have been associated for more than 50 years, particularly in the laundry area, where the main use is foam control in consumer washing machines and fabric softening. But silicones are also used to provide extra benefits such as fabric dewatering, antiwrinkle characteristics, ease of ironing or improved water absorbency. Silicone additives have also been developed to reduce fabric mechanical losses over time or to improve perfume release.

Silicones as Foam Control Agents A wide range of foam control agents exists. These foam control agents prevent (antifoams), and knock down (defoamers) foam that occurs in both the manufacture of detergents and during their use by consumers at home, or by professionals. Antifoams are critical in many consumer applications. For example, in a washing machine, a steady control of foam is needed, rather than its complete elimination or

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prevention. Correct foam control is essential since consumers are very perceptive about foam levels. Moreover, incorrect foam levels can reduce the detergent’s cleaning efficiency. Silicone foam control agents are based on combinations of polydimethylsiloxane (PDMS) and finely divided silica particles. This particular combination seems most effective (see Section 3). To optimize antifoam performance, the foam control agent must be properly incorporated into the detergent product with an effective protection system so it will subsequently disperse into the wash liquor in the right form. Silicone antifoams have been developed either as compounds, ready-to-use emulsions for liquid detergents or solid powdered forms that are easy to incorporate in powder detergent formulations. Delivery form and stability of the silicone antifoams (i.e., protection from the detergent under various temperature or humidity conditions) are critical aspects. Silicone antifoams are also used in the manufacture of detergents to help processing of detergent liquids, to de-air the wet slurry and improve powder density in the spray-drying towers for powdered detergents, or to facilitate all types of bottle-filling operations.

Silicones for Delivering Fabric Care Benefits

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Within the textile industry, silicone products have been used for almost 50 years. The primary textile benefits and applications from silicones have been as antistatic agents, fibre and thread lubricants during fabric production, and antifoaming and fabric softening agents during the fabric-finishing step (see Section 5). It is known from both the textile and laundry industries that the laundry wash cycle process removes most of these fabric finishes. It is considered that about 10 wash cycles are sufficient to remove most of the garment’s initial fabric treatment. With ever-demanding consumers having less time for clothing care, wanting clothing to look better and as new as possible after repeated washings, and expecting clothes to be comfortable directly from the dryer, it became a market need to deliver the known textile industry technology benefits in consumer laundry products. Key technologies and silicone product parameters in fabric care are: • • • •

Polymer architecture and functionality Type of delivery vehicle Particle size properties of emulsion vehicles Surfactant systems used with emulsion vehicles

The association of a flexible backbone with low intermolecular interactions as limited to methyl groups explains the lubrication characteristics of PDMS (see Section 5). Some methyl groups can be substituted by other groups, such as hydroxy, amino, amido, polyether and longer alkyl, either along the chain (grafted) or at the ends of the chain (end-blocked). These functionalities allow adjustment of the architecture of the polymer to tailor the interactions between these polymers and the fabric during and after laundry. Some polymers can be cyclic, but the majority are linear with various molecular weights, from volatile fluids to high

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consistency gums. They can also be cross-linked to variable levels to provide higher substantivity, controlled spreading and elastomeric properties. Depending on the requirements of the application, the above silicone polymers can be delivered as self-emulsified in the formulation (polyethers), emulsified in situ (low molecular weight amino-functional polymers) or pre-emulsified (most polymers). Some formulations may contain volatile silicones as a secondary delivery system within the emulsion. Silicone emulsions are available either as microemulsions (100 nm), depending on the polymer architecture and functionality selected. The emulsion particle size is often related to the properties ultimately observed on a fabric. Microemulsions are able to penetrate into the yarns and deposit onto the fabric fibers, bringing a soft, dry feel to fabric. Studies suggest that the deposition of silicone is internal, which provides dry lubrication of individual fibers against each other with a very thin coating of silicone, probably reducing the static coefficient of friction. Macroemulsions deposit on the external surface of the fabric, causing superior lubrication through reduction of the dynamic coefficient of friction. They provide relatively good fabric softening performance. Silicone emulsions may be formulated with adequate anionic, nonionic or cationic surfactants. The choice is driven by the compatibility with the application formula and the mechanism of silicone action or deposition. To provide its benefits, the silicone must generally deposit on the fabric. This deposition is triggered either by the polymer functionality and/or by the emulsion surfactant system used in synergy with the application formulation. Fabric Softening. Numerous patents have been filed [29,30] on this application since 1976, and many commercial product implementations exist, [31] mainly in fabric softeners but also in liquid detergents. The fundamental properties of silicones behind this application benefit are their low surface tension, low intermolecular interactions, high spreading and nonadhesive characteristics. Some studies have demonstrated high levels of silicone deposition onto fabric when delivered from fabric softeners. Fabric Dewatering. There is an interest in terms of consumer convenience to accelerate fabric drying, matching wash and drying cycle times, and also to contribute to a reduction of electricity consumption when tumble dryers are used. Reducing the amount of water left in the fabric after the wash and spin cycle directly correlates with a reduction of drying time and energy. Studies have shown that when dosed at 1% active silicone in the softener, up to a 13% further reduction of water content can be achieved with the tested silicones over fabric rinsed with organic quaternary ammonium salts, which already reduce water content by 23% over the water-rinsed fabric. It is believed that the fundamental silicone properties behind this benefit are hydrophobisation of the fabric surface and its subsequent dewetting, as well as its low surface tension, which allows fast spreading. Several patents were issued and are commercially practiced [32,33]. Ease of Ironing. The ease of ironing benefit can be subjectively assessed through paired comparison panel tests but can also be objectively measured by coefficient of friction measurements. Several patents exist and are practiced in the fabric softener market [34,35]. The fundamental properties behind this application benefit are the same as softening, with the proviso of higher levels and external deposition of the silicone. Wrinkle-Related Benefits. Perhaps the most critical objective of fabric care is to reduce garment wrinkling after the wash cycle and during ironing, and also to improve wrinkle resistance during wear. This is a difficult technical challenge, because the mechanism of

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wrinkle formation is complex and not easy to access from a laundry application perspective. The textile industry has been able to meet this challenge to some extent through the application of “easy care” finishes. These treatments are based on a high-temperature cure of crease resist resins (dimethylol-dihydroxyethyleneurea or DMDHEU), organic quaternary ammonium salts and silicones. However, this is not compatible with consumer washing processes and safety. The current opportunity is great for technical improvements that would bring satisfactory performance. Many patents [36-39] have been published in this field for various product formats (e.g., sprays, fabric softeners and detergents), and they always combine silicone with other ingredients or polymers. It is suspected at this stage that fibre lubrication is most likely to be the added benefit in this process together with the silicone softening touch. Water absorbency. Fabric softeners have the down side effect of hydrophobizing fabric. This is a concern for consumers who want a soft, bulky towel with good absorbing properties. Surprisingly, in the middle 1980s, it was found that the addition of silicone polymer in a fabric conditioner composition actually improves the water absorbency of the fabric. Water absorbency is evaluated by a Drave’s wetting test, which measures the time required for a fabric sample to sink to the bottom of a 1 l beaker filled with demineralised water. Measurements are relative to a given organic quaternary ammonium salt type and are arbitrarily stopped after 300 s. Several patents [40] were issued from the mid 80s until recently for a variety of silicone structures and compositions, but so far no clear, convincing interpretation of this phenomenon has been proposed. Because low molecular weight PDMS is known to be the most effective in water absorbency studies, we suspect that the versatile orientation of the silicone molecule and its ability to modify surface hydrophobicity/hydrophilicity as well as its low viscosity and high spreading rate are involved. Modification of the Mechanical Properties of Fabric. Sophisticated investigation found that particular silicone products formulated in a fabric softener have a positive impact on the fabric’s mechanical strength compared to water-rinsed or pure softener-rinsed fabric. This was observed using the “tear crack propagation” method as described in the DIN ISO 139371:2000 standard. It is suspected that the elastomeric nature of the silicone polymer and its lubrication properties are involved in this phenomenon. It is also believed that treating fabric with silicone lubricants can reduce fabric wear abrasion and consequently improve color definition, reduce pilling and fuzziness, and help retain original fabric shape. Silicones as Perfume Release Modifiers. Perfumes are present in almost all consumer surfactant-based products in the household and cleaning segments. Perfumes are added to cover residual odours from raw materials but also for more subjective purposes. Both protection and controlled release of perfumes have been areas of development in recent years in household and cleaning products. Silicones are being considered here because of their high permeability to gas and low molecular weight organic molecules and also because this property can be reduced and adjusted (e.g., by using bulky alkyl side groups). Silicone can also easily be formulated under different product forms: from volatile dispersions, emulsions, or particles to devices made from a cured elastomer [41-45].

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7. SILICONES IN COATINGS Silicones are widely used in the coating industries as materials to protect and preserve but also to bring style to a wide variety of applications in our daily lives. The unique combination of properties of silicones is well suited to coating applications. Two families of products are used: silicone polymers as additives and silicone resins as the main component, or binder. At low levels, silicone polymers are used to ease application of paints. The surface properties of silicones enable a paint to wet a substrate easily and give it a smooth appearance once dry. Here silicones are behaving as performance enhancing additives during the coating application. They are effective at an addition level of a fraction of a percent (see Table 4). In contrast to the low-level use of silicone polymers as additives, silicone resins can be major components of the coating. Here they are used as binders or co-binders, imparting important benefits such as durability throughout the life of the coating. Silicone resins offer resistance to weathering in paints for exterior surfaces such as bridges and metal cladding on buildings. They also provide water repellence to masonry surfaces such as stone and brick. Silicone resins have greater resistance to high temperatures than organic resins and are used in paints for ovens, chimneys, car exhausts and barbecues. In these examples, the resilience of the silicone materials allows reduced frequency of maintenance painting and consequently reduced volumes of paint used over the lifetime of the coated item (see Table 4). Table 4. Silicones in Coatings and Associated Benefits Silicone as performance-enhancing additives (0.1 – 5.0 % w/w) Foam control Substrate wetting Leveling Adhesion Surface slip

Silicone resins and intermediates (30 – 100 % w/w) Weather resistance Heat resistance

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Silicones as Performance-Enhancing Additives Polydimethylsiloxane (PDMS) fluids of low-to-medium viscosity were the first silicone additives to be used in coatings. They readily dissolve in solvent-borne paints, reducing the surface tension of the liquid and enabling it to wet substrates, even if contaminated with dust, grease or oil. This reduces the appearance of film defects known commonly as “fisheyes” and “pinholes.” The silicone also reduces surface tension gradients across the coating film as it dries so a smooth surface is obtained rather than the undesirable “orange peel” effect. Silicone polymers can be modified by grafting polyether groups to give siliconepolyether copolymers. These behave as surfactants in aqueous media as they have both hydrophobic and hydrophilic components. Such surface active materials can perform many

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functions in inks, paints and coatings. The main uses of silicone surfactants are to provide defoaming, deaerating, improved substrate wetting, and enhanced slip properties [46]. Silicone-polyethers are usually obtained by a platinum-catalyzed addition reaction of an unsaturated polyether onto a SiH functional silicone polymer (see Section 1). Therefore very many structures are possible, altering the SiH functional silicone polymer (DP, % SiH) and/or the nature of the unsaturated polyether (DP, unit type) (see Figure 7).

a) end-blocked or ABA type Me R'O(CH2CHRO)n(CH2)3

Si Me

Me O

Si Me

Me O

x

Si (CH2)3

(OCH2CHR)nOR'

Me

where: R = H (polyethylene oxide) or CH3 (polypropylene oxide) R'= H, CH3, OAc (polyether end group)

b) raked/pendant or AxBy type: Me

Me

⏐

⏐

Me3Si – O – (Si – O)x – ( Si – O)y – SiMe3 ⏐

Me

⏐

(CH2)3 ⏐

(OCH2CHR)nOR’

where: R = H (polyethylene oxide) or CH3 (polypropylene oxide) R’= H, CH3, OAc (polyether end group)

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Figure 7. Molecular structures of silicone polyethers.

Much versatility is possible here. A copolymer with a high proportion of ethylene oxide in the polyether chains is far more miscible with water than a copolymer with predominantly propylene oxide units. The former will act as a wetting agent since it reduces the interfacial tensions at the liquid-air and air-substrate interfaces. The latter behaves as a foam control agent since it matches the criteria cited as necessary for antifoaming, particularly immiscibility with the foaming medium and a lower surface tension than the medium [6,47]. Limited miscibility with the coating film can also be used to design the silicone-polyether copolymer so it migrates towards the liquid-air surface during drying. This is beneficial in applications where surface slip is required. Cartons for food packaging often have a radiationcured overprint varnish to protect the printed text and graphics on the exterior surfaces from water and frost during storage. A silicone-polyether copolymer added at 0.5 to 2.0 % to the varnish reduces its coefficient of friction so the flat cartons slide easily from the stack in the production process rather than sticking together [48].

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Silicone Resins and Resin Intermediates in Weather-Resistant Paints Paints for exterior surfaces are exposed to sunlight in wet, dry, hot and cold conditions. The combination of UV radiation, variable temperature and humidity rapidly degrades organic polymers, roughening the coating surface and exposing the pigments. To the coating user, this is observed as loss of gloss and “chalking” (loose pigment on the paint surface). Since the 1940s, solvent-borne alkyds and acrylics have been blended with silicone resins to improve their weathering performance. In the 1950s, alkoxy- and silanol-functional silicone resin intermediates were developed which could be reacted with hydroxyl-functional organic resins to give even greater weather resistance. Chemically combining the silicone and organic resins gives a higher degree of compatibility, allowing a broader range of organic resins to be used. A comparison of the bond strengths between atoms that compose silicones and their organic counterparts gives some insight into why the silicone backbone is so robust when exposed to energetic conditions such as UV radiation or heat (see Table 5). Table 5. Bond Strengths for Some Common Combinations of Atoms in Coating Resins

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Bond Si – O C–C C–O Si – C

Bond strength (kJ/mol) 445 346 358 306

The Si-O bond has about 50% ionic character as calculated from Pauling’s electronegativity scale. In aqueous media, Si-O bonds are more susceptible to hydrolysis than C-C bonds, especially in the presence of an acid or base. This might suggest that silicones would be expected to show less resistance to weathering than organic resins. The reason that this is not so is because the products of hydrolysis, silanol groups, rapidly condense to reform the silicone linkage (see Section 8). Moreover, the silicone hydrophobicity limits wetting and surface contact with any waterbased media. However, water vapor can diffuse through most silicone polymer coatings, which is advantageous in some applications like masonry treatment. Typical silicone resin intermediates used in solvent-borne alkyd or acrylic resin paints are oligomeric materials including T units (see Section 1) with phenyl and propyl groups to improve their compatibility. They have some Si-OH or silanol groups that can be condensed with C-OH or carbinol groups of the alkyd or acrylic resin (see Figure 8). These silicone-organic copolymers are used in industrial maintenance paints to protect a variety of metal objects and structures, including railway carriages, chemical plants and bridges. The biggest application in the US is the painting of naval ships according to specifications set by the federal government. Periods between recoating were extended from a maximum of one year for straight alkyds to three years for the silicone-modified versions. A typical silicone-alkyd copolymer for this type of paint contains 30% silicone based on resin solids.

Silicones in Industrial Applications

Si

Si

Si

HO

Si

O

O O

HO

OH

O

O O

87

Si

O

Si

OH

Figure 8. A phenyl-, propyl-functional silicone intermediate used to modify organic resins (idealised structure).

The success of silicone-alkyds in naval applications led to the evaluation of siliconeorganic copolymers in coil coatings for residential and commercial aluminium sidings. As these coatings can be cured at elevated temperatures, silicone-polyesters without drying oils were found to be most appropriate. At first, a 50% silicone content was the standard based on accelerated weathering data, but as more field experience was gained it became apparent that 30% silicone is sufficient. Solvent-borne thermoplastic acrylic resins tend to have better chemical resistance than alkyds and can be cold blended with silicone resins to give weather-resistant paints for exterior applications. Addition of as little as 10% silicone can significantly increase the gloss retention and chalking resistance. The improvement that can be achieved in gloss retention of various organic coating resins through silicone modification is illustrated in Figure 9.

Acrylic-Siloxane

Epoxy-Siloxane

Siloxane-

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Alkyd

AcrylicUrethane

Siloxane

Alkyd Epoxy

Figure 9. Gloss retention of coatings made from organic resins and silicone-organic combinations; QUV-B accelerated weathering [49].

The modification of acrylic latexes (water-borne formulations) with silicones is proving to be an effective way to comply with regulatory restrictions on solvent use. Combinations of

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monomeric silicon intermediates with alkoxy functionality can be blended with hydroxyl functional acrylic latexes to give silicone-acrylic copolymers with excellent weather resistance. The ratio of alkyl- and aryl-bearing silicon monomers can be optimized to give the best balance of compatibility, film flexibility and durability. Gloss retention of paints formulated from acrylic latex with 10% modification is typically 50 to 70% after 30 months of south Florida exposure, compared to about 10% for the unmodified latex.

Silicone Resins in High Temperature Paints Silicone polymers or resins can be regarded as already partially oxidized as they consist partially of Si-O groups. This is one of the reasons for the high thermal stability of silicones compared to organic materials. The bond strengths in Table 5 provide additional explanation of the observed stability. Phenyl groups attached to silicon are far more resistant to thermal oxidation than methyl groups. So, most silicone resins for high temperature applications have a combination of methyl and phenyl substituents to achieve the required balance of heat stability, flexibility and compatibility with organic resins. Blends of silicone and organic resins are suitable for applications up to about 400°C. The proportion of silicone required increases vs. the expected upper operating temperature, as observed with the effect of adding a methyl/phenyl silicone resin into an alkyd paint exposed to various temperatures (see Figure 10). 60 deg. gloss 100 90 80 70 RT 177°C

60 50

190°C

40

204°C

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20 10 0 0

20

40

60

80

% silicone resin

Figure 10. 60° gloss vs. methyl/phenyl silicone resin content in an alkyd paint based on nondrying coconut oil after 16 hours exposure at different temperatures [50].

For temperatures above 400°C, silicone resins are used only as binders. These can be formulated with aluminum pigments to form a ceramic film as the silicone organic substituents are burned off to give a very durable fully oxidized siliceous layer.

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Silicones for Marine Fouling Release Coatings

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Solid surfaces immersed in seawater quickly become covered with algae, barnacles, tubeworms and other marine organisms. On ships this is referred to as fouling, which increases drag on the hull and raises fuel consumption by up to 40%. To prevent this, antifouling coatings are applied to the hulls. The most effective coatings were based on organo-tin compounds, and in the 1970s, 80% of the world’s shipping fleet had this type of coating. Environmental concerns have motivated many countries to ban organo-tin coatings. So considerable research and development is taking place in government agencies and paint companies to find alternatives. Silicones have been identified as critical materials. A typical silicone-based anti-fouling/release system consists of an epoxy or silane primer, an elastomeric silicone tie-coat and an elastomeric silicone top-coat that contains a release additive. The release additive must have limited compatibility with the coating so it will migrate to the surface. Organic oils and waxes have been shown to work as release additives, but the most effective materials are modified silicone polymers with a combination of methyl and phenyl substituents. The latter reduce the compatibility of the polymer with the predominantly PDMS network of the elastomeric coating. Figure 11 shows a system of this type applied to a test panel and immersed in the English Channel for two-and-a-half years. The panel is almost completely free of fouling organisms. Surprisingly, a comparison coating based on PTFE, which has also a very low surface energy, is completely covered. This indicates that a coating with a low surface energy is not a sufficient requirement for effective fouling release. The inclusion of a release additive, as in the silicone elastomeric system, has a dramatic and positive effect on performance.

Polyester Silicone elastomer with silicone release additive

PTFE

Figure 11. Extent of fouling of coated steel panels submerged for two-and-a-half years in the English Channel. Picture courtesy Dow Corning Ltd.

The ban on tin-containing anti-fouling coatings for marine applications has opened up an area that is surely a logical fit for silicone technology. This may well be the largest “release” application in the world, release being a function that silicones have provided for many years in bakeware, mold-making and adhesive label backing paper.

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8. SILICONES IN THE CONSTRUCTION INDUSTRY Silicone sealants and adhesives as used in the construction industry were introduced approximately forty years ago, and many of the silicones applied in the early days are still performing today. Products are available in a variety of forms, from paste-like materials to flowable adhesives. Both single- and multi-component versions are available, each with several different cure chemistries. The commercial importance of silicone sealants and adhesives is based on their unique combination of properties that permit them to satisfy important needs in a broad variety of markets. These properties include excellent weather and thermal stability, ozone and oxidation resistance, extreme low temperature flexibility, high gas permeability, good electrical properties, physiological inertness and curability by a variety of methods at both elevated and ambient temperatures. Because of their low surface energy, they wet most substrates, even under difficult conditions, and when formulated with suitable adhesion promoters, they exhibit very good adhesion. These unique characteristics are the result of a scientific endeavour to combine some of the most stable chemical and physical attributes of the inorganic world with the highly utilizable aspects of organic materials. A qualitative list of the features of siloxane polymers that contribute to the unique combination of properties of silicone sealants and adhesives relevant in construction applications is given in Table 6. Almost all these inherent attributes are a consequence of four fundamental aspects: the low intermolecular forces between dialkylsiloxane molecules, the dipolar nature and the strength of the siloxane bond and the flexibility of the siloxane backbone. Table 6. Silicone Attributes Contributing to Durability Sealant Property Excellent substrate wetting (adhesion) High water repellence

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Excellent flexibility

Small temperature variation of physical properties Low reactivity High gas permeability High thermal and oxidative stability Ultraviolet light resistance

Silicone Attribute Low surface tension Low surface tension Low glass transition temperature Large free volume Low apparent energy of activation for viscous flow Low activation energy of Si-O-Si bond rotation Configuration of siloxane polymer chain and small interaction between methyl groups Low activation energy of Si-O-Si bond rotation Configuration of siloxane polymer chain and small interaction between methyl groups Large free volume Low activation energy of Si-O-Si bond rotation High Si-methyl bond energy High Si-O bond energy

Probably the most important properties of silicone sealants for construction are durability and adhesion.

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Adhesion Although the primary function of sealants is to seal, in most applications they cannot provide this function without proper and durable adhesion to the substrate(s). Furthermore, in many applications, it is difficult to distinguish between an adhesive and a sealant. For example, structural silicone adhesives are used in the building construction industry owing to their sealing, adhesive and elastomeric properties, as well as their resistance to harsh environmental conditions. The type of application dictates the adhesion requirements. For instance, sealants and adhesives for general use are expected to achieve primerless adhesion to a broad variety of substrates. Siloxane polymers spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with sealant or adhesive, giving an interfacial phase that is continuous and without voids. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. However, these initial interactions are purely physical in nature. Theoretically, these physical intermolecular interactions would provide adhesion energy on the order of several mJ/m2. This would be sufficient to provide some basic adhesion between the adhesive and the substrate. However, the energy of adhesion required in many applications is on the order of kJ/m2. Therefore, physical intermolecular forces across the interphase are not sufficient to sustain a high stress under severe environmental conditions. However, chemisorption also plays an important role in the adhesion of reactive silicone sealants and adhesives; thus, physisorption and chemisorption both account for bond strength [51]. Obviously, the ideal silicone adhesive or sealant is one that is self-priming; that is, the adhesion promoter is included in the formulation and is generally part of the curing reaction system. This is the most common type of commercial silicone sealant or adhesive, as it often provides adhesion without the need of a complicated pretreatment procedure such as priming, corona- or plasma-treatment. However, even with self-priming systems, proper cleaning of the substrate prior to application is required to eliminate weak boundary layers and to achieve strong and durable adhesion.

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Durability Properly formulated silicone sealants and adhesives exhibit outstanding durability in a variety of environments. They are known for their high movement capability; their excellent resistance to ultraviolet light, high temperature and ozone; their low water absorption and low temperature flexibility, as well as their ability to form strong chemical bonds to the surface of typical construction and industrial substrates [52]. The outstanding UV stability of silicones is derived from the bond strength of the silicon-oxygen linkages in the polymer chain, as well as the absence of any double-bond or other ultraviolet (UV) light-absorbing groups. The principal environmental factors acting on a sealant or adhesive in outdoor exposures are:

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• • • • • •

Temperature extremes (high and low) Water Solar radiation (UV and IR) Oxygen/ozone Corrosive gases (sulphur dioxide, nitrogen oxides) Mechanical stress

For radiation energy to initiate chemical changes, the molecules of the material in question must absorb it. Silicones absorb very little ultraviolet radiation in the 300-400 nm region, which is the wavelength range that causes problems with most other polymers at, or near, ground level. When irradiated under conditions of natural photo-aging, silicones are slowly oxidised. The oxidation of the hydrocarbon side-groups results in the formation of carbonyl groups [53-56]. Since carbonyl groups do not interact strongly, the oxidation has little effect on the mechanical properties of the sealant or adhesive. This is consistent with the fact that, even after 20 years of outdoor weathering in sunny climates, silicone elastomers show comparatively little change in physical properties [57-58]. Under natural weathering conditions, the effects of oxygen and ozone are inextricably connected to those of elevated temperatures and sunlight. At room temperature, oxidation by oxygen is not noticeable. The excellent oxidation resistance of silicones is a consequence of the dipolar character of the siloxane backbone. The positively polarised silicon atom acts as an electron drain for the methyl group, rendering it less susceptible to oxidation [59]. Oxidation in air generally becomes noticeable above 200°C, resulting in cleavage of the Si-C bond. However, one can raise the upper service temperature by using suitable oxidation inhibitors. Changes in the physical properties of silicones under artificial or natural weathering conditions, involving alternating periods of wet and dry conditions, are mainly due to the physical effects of water [60]. Since the hydrolysis reaction is reversible, some of the siloxane bonds that were ruptured by hydrolysis are formed again by the condensation of silanol groups upon drying [61]. Thus, during alternating periods of wet and dry conditions, a relatively small number of siloxane bonds in the bulk of the sealant or adhesive are constantly broken and reformed. Silicone sealants and adhesives show excellent resistance to the combined effect of the key weathering factors: water, heat and ultraviolet light [63]. Compared to organic sealants and adhesives, silicones are more thermally stable, perform over a wider range of temperatures, have a higher movement capability and are less susceptible to fatigue resulting from cyclic mechanical strain. They are also more resistant to UV light as well as oxygen and ozone attack. They are also known for their low water absorption and the ability to form strong chemical bonds to typical construction substrates. A weaker aspect of the environmental stability of silicones is their susceptibility to hydrolysis reactions, particularly at the extremes of acidity or alkalinity and at elevated temperatures. Exposure to strong acids and bases as well as to super-heated steam are detrimental to the stability of silicone sealants. Under natural weathering conditions (involving small amounts of water incorporated in the bulk of the sealant or adhesive), mass action effects keep the hydrolysis reaction within limits, a condition much aided by the low water wettability of the siloxane polymer.

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Applications

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The construction industry represents the largest market segment for silicones. Silicone sealants, primarily as one-part room temperature vulcanisable (RTV) products, are widely used by the construction industry for applications such as sealing building and highway expansion joints, general weatherproofing of joints in porous and nonporous substrates, sanitary joints around bathroom and kitchen fixtures, as well as fire-rated joints around pipes, electrical conduits, ducts, and electrical wiring within building walls and ceilings. In a variety of applications, silicone sealants also perform the functions of an adhesive (i.e., they act as structural sealants). For example, silicones are used in structural glazing, where the cured sealant becomes part of the overall load-bearing design, or in insulating glass secondary seals, which structurally bond two panes of glass together. Structural glazing is the application that most importantly is enabled by the outstanding durability of silicone sealants. Structural silicone glazing (SSG) is the method of bonding glass, ceramic, metal, stone or composite panels to the frame of a building by using the bond strength, movement capability and durability of a silicone structural sealant. Figure 12 shows the Burj-Al-Arab hotel in Dubai as one example of the many exceptionally well-designed buildings sporting silicone structural glazing façades.

Figure 12. Burj-Al- Arab Hotel, Dubai; a tribute to the use of structural glazing silicone adhesive in a high-rise façade.

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Because of the elastomeric character and the chemical adhesion of silicone structural bonding seals, SSG design concepts offer a number of performance benefits: [64] • • • • •

Effective air- and weather-sealing of the façade Improved thermal and sound insulation Protection of the supporting structure from the elements by a durable glass skin Increased rigidity and stability of the façade, resulting in the ability to withstand higher wind-loads Ability to absorb differential movements between glass and building frame, resulting in superior performance of SSG façades during seismic events

For the façade designer, SSG provides the possibility to construct façades with freeflowing, uninterrupted bands of glass or smooth, uninterrupted total glass surfaces. The SSG technique uses both the adhesive and sealing properties of structural silicone sealants. Medium modulus, good elastomeric properties, and excellent, highly durable adhesion are important to support the weight of glazed panels and to resist wind load, while simultaneously being able to absorb differential movements between dissimilar materials induced by thermal fluctuations, seismic loading or other forces. It is essential for the success of SSG design to use a structural sealant and not a rigid adhesive because the structural seal needs to resist both loads and movements without creating unduly high stresses at the glass interface or failing cohesively [65]. Since the interface between structural seal and glass is directly exposed to sunlight, the sealant must develop extremely UV-stable bonds to the glass substrate to achieve an expected service life of 30 to 50 years. Because of this requirement, only silicone sealants are allowed for structural glazing applications.

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9. SILICONE RELEASE COATINGS FOR THE PRESSURE SENSITIVE ADHESIVE INDUSTRY In today’s modern environment there is a wide range of applications for silicone release liners with pressure sensitive adhesives, ranging from release labels to diaper closures, medical applications (e.g., wound dressings), building insulation and health and beauty products [66-68]. Release liners are part of a composite made of a label with its own adhesive on a release liner or carrier with its own release coating. The label comprises either a synthetic face stock such as polypropylene or paper. The adhesive is usually an organic material such as a polyacrylate or polyisoprene based rubber. The release liner can be made from various substrates treated with a suitable product, the release coating. Such release liners allow transporting labels with their adhesives from the point of manufacture to the point of label application (e.g., a filling station of some sort). The release coating allows easy delamination or easy label transfer from their liners onto the object to be labeled. The use of release liners began before World War II but really took off with the development of silicone release coatings in the early 1950s. There are several chemical types of release materials. However, many are migratory types; that is, significant amounts of the release material contaminate the surface of the released material. Those that do not migrate or

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transfer to the released material to any significant degree include polyacrylates, carbamates, polyolefins, fluorocarbons, chromium stearate complexes and silicones. Silicones enjoy a unique position because they can be applied and cured into a polydimethylsiloxane (PDMS) network on various backing substrates so limiting migration, but also because they allow substantially lower release forces than other materials. Silicone-coated substrates are sometimes referred to as siliconized release liners. The choice and combination of backing substrate, silicone release coating and adhesive needs to be carefully selected.

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Silicone Release Characteristics One of the key properties of silicone is its low surface tension, and in particular, its low critical surface tension of wetting or low surface energy. This is a consequence of low intermolecular forces and high chain flexibility (see Section 1) [69]. Unlike more rigid carbon-carbon backbones, PDMS polymers because of their backbone flexibility, and as they are at room temperature substantially above their Tg, can easily expose their low interacting/surface active methyl groups to provide low adhesion; or in other words, low release forces against adhesives they are exposed to. Organic adhesives as used on labels cannot easily wet such a low energy silicone surface as there are no groups to interact, which results in ease of delamination and ease of transfer of the label from the liner to its point of use. But low surface energy is not the only aspect to consider. Even fluorocarbons, despite a lower surface energy than silicones, do not match silicone release performance. Another key component is the rheological behaviour of the cured PDMS network applied onto the backing substrate [70]. Recent work has shown that interfacial slippage also plays a role in the low release values observed on the release of pressure sensitive adhesives from silicone release coated liners [71-73]. A mechanism has been proposed for cured PDMS network/release coatings in which interfacial slippage minimizes the bulk shear deformation experienced by the organic adhesive [73]. Commercially cured PDMS release coatings can exhibit significant interfacial slippage. Sometimes silicone resins known in the paper industry as release modifiers need to be added to a silicone formulation to increase release forces. This may be necessary for processing reasons to convert the laminate construction to the label, or it may depend on the release force required for particular dispensing application. It is believed that these release modifiers “freeze out” interfacial slippage, resulting in increased adhesive deformation upon delamination and higher release forces. The release modifier reduces the segmental mobility of the PDMS chains within the cured coating network. If the PDMS is constrained by a rigid backing, there is still slippage at the interface due to bending of the PDMS at the crack tip at finite peel angles. It has become clear that the great advantage of PDMS in release applications is its low coefficient of friction under shear, compared to lower surface energy but higher shear friction (more rigid) fluorocarbons.

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Factors to Consider for Silicone Release Coatings Many other factors influence the selection of coating technologies and materials for liners and laminates. These include end-user requirements like converting, die-cutting and printing requirements and environmental concerns. If using silicones, some factors are related to them, some not (see Table 7). The equipment used may drive the choice of release coating material. Most commonly used are either based on a three roll differential offset gravure or a five or six smooth roll coating head. Environmental and regulatory pressure may play a role as well, encouraging the selection of solventless or emulsion systems to deliver the required performance. Substrate type, cure temperature, dwell time and humidity can affect cure and anchorage of the silicone coating to the substrate. The selection of adhesive required for the application also has a major bearing on release and anchorage characteristics. In recent years, the use of plastic liners such as polyethylene, polypropylene or polyester films has increased. Siliconizing such substrates is a challenge because of their low resistance to high temperatures and their variability as they may contain additives such as antiblocking agents or stabilizers. Some of these are detrimental to the cross-linking of the silicone release coating. But overall these thermoplastic films are difficult substrates as they show poor adhesion and sometimes poor silicone cure to the applied coating. Special grades of film have been developed to improve adhesion, but they are more expensive. UV-cured silicone release coatings have been developed to avoid exposure to high curing temperatures, but overall the penetration of such UV-cured systems is low compared to heat-cured systems, which remain the preferred system. If using silicone release coating materials, there is an array of silicone chemistries to select from. UV cure is sometimes used when applying a release coating on a low melting temperature substrate such as low density polyethylene, [74] but the most widely used cured chemistry for silicone release liner preparation is thermal cure. To achieve a cured network, there are solvent-based, emulsion-based and solventless silicone systems [75]. Whereas the coating of the first two types is relatively straightforward, the coating of 100% solids materials is highly specialized and needs sophisticated coating equipment.

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Table 7. Factors to Consider When Using Silicone Release Coatings Non silicone related factors Equipment Substrate Cure temperature Dwell time Humidity Adhesive type

Silicone related factors Silicone cure chemistry Composition: - polymer architecture - modifier architecture - cross-linker architecture - additives

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Cure Chemistry To avoid migration, the PDMS release coating is applied and then heat cured onto the substrate to give a cross-linked silicone. To achieve cross-linking of the silicone release coating, the most predominantly used chemistry is cure via a hydrosilylation reaction. The composition of such silicone release coating consists of vinyl-functional PDMS, a hydrogenfunctional PDMS and a platinum catalyst (see Figure 13). a) hydrogen-functional siloxane Me ⏐

Me

Me

Me

⏐

⏐

⏐

Me – Si – O – (Si – O)x – (Si – O)y – Si – Me ⏐

Me

⏐

⏐

Me

H

⏐

Me

b) vinyl-functional siloxane

Me

Me H2C

CH

Si Me

O

(Si Me

Me

Me O)n

(Si

O)m Si

CH CH2

CH CH2 Me

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Figure 13. Structures of the silicone polymers used in release coatings.

These can be reacted together using a hydrosilylation reaction. Additives used include inhibitors to provide for long bath life at room temperature and release modifiers. These silicone coatings are formulated to achieve rapid cure when used and exposed to high temperature to achieve high coating speeds. The hydrosilylation reaction, especially of carbon-carbon multiple bonds, is one of the most important reactions in organosilicon chemistry and has been extensively studied for half a century [76-78]. This reaction is used to produce many organosilicon compounds. However, one of its primary uses is as a fast cross-linking or cure chemistry reaction as here to cure silicone release coatings. Hydrosilylation is the addition reaction of a silane group (SiH) on a vinyl group (SiCH=CH2) catalyzed by a noble metal such as rhodium or most often platinum. A general model has been proposed to explain how the platinum is involved in the reaction (see Figure 14) [79]. There are basically two different forms of this cure chemistry used industrially, both catalyzed by platinum. In one, a SiH-functional polymer reacts with a vinyl-functional polymer carrying Si-CH=CH2 groups. In the other, a SiH functional polymer reacts with a hexenyl functional polymer carrying Si-CH2-CH2-CH2-CH2-CH=CH2 groups (see Figure 14). This simplified proposed mechanism does not explain the difference between vinyl- and hexenyl-based systems. In the hexenyl-based system, the unsaturation has been distanced from the polymer backbone and is therefore less sterically hindered. This allows a release coating material with a slightly faster cure upon application.

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Associated with these two different forms of cure chemistry (vinyl or hexenyl), various inhibitors can be used to ensure sufficient bath life and prevent premature cure at room temperature of the coating mixture prior to use and curing. Inhibitors compete with the initial step of the hydrosilylation reaction and the addition of the unsaturated group from the polymer on the platinum catalyst (see Figure 14). So the selection of the platinum inhibitor has a major impact on cure speed [80-81]. An inhibitor strongly bound to the platinum catalyst forms essentially a very high barrier of access for the unsaturated group from the polymer to the active platinum catalyst center during stage one of the reaction mechanism above. Typical inhibitors employed here are acetylenic alcohols such as 1-ethynyl,1cyclohexanol or fumarate- or maleate-based inhibitors. Me Me O Me Me Si Si

Me Me Si O Me Si Me

Pt

Pt Si

Me Me Si O Me Si Me

Me Me O Me Me

Si Me2 O - ...

Activation

Si Me2 O - ...

Si

Si Me2 O - ... Pt

Oxidative addition ... O

Me Si O ...

H

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Me Si Me

O

Me Me Si O ... Si C H2

Pt

Me Si Me

... O

Me

Si O ...

Me Me Si

Pt

O

... O

Me

... O

Me Me Si

O Me Si Me

H2 C Si Me2 O - ...

H Si Me2 O - ...

Me Si

O ...

Pt

Si Me2 O - ... C C H2 H2

Reductive elimination Figure 14. Hydrosilylation/addition cross-linking and cure mechanism.

Insertion

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99

As the price of platinum increases (it has doubled in recent years), new polymer/crosslinker structures have been developed to reduce costs, in particular for solventless coating. Today rapid cure can be achieved with lower levels of platinum (i.e., 50 instead of 100 ppm). As line speeds increase, the silicone is submitted between applicators at the nip of the coating equipment and the substrate to shear rates of the order of 106 sec -1. At these high shear rates, the silicone behaves very differently than expectations based on rheological measurements, which are usually made under relatively low shear. Silicone misting is one such manifestation [82,83]. So additives have been developed that will greatly reduce the volume of mist produced, even at 1600 m/min [84].

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Greater Use of Plastic Release Liners In recent years, the use of plastic release liners such as polypropylene, polyethylene and polyester has increased, fueled in part by an increase in premium applications such as nolabel-look beverage labels for aesthetic appeal and brand enhancement. The siliconising of plastic films has a number of associated problems. These include the requirement of low curing temperature for polyolefin films, amongst others. Further, there is difficulty in adhering silicones to plastic films and maintaining anchorage of the silicone to the plastic film over time. This problem is particularly prevalent with polyester films. In addition, plastic films are quite variable. For example, several additives can be used in film production including antiblocking agents, heat stabilizers and plasticisers. These additives can affect cure and anchorage. This variability adds to the design complexity for a robust universal thermally-cured silicone system. Some of the cure and anchorage issues associated with some plastic films can be overcome by using special grades of film. For example, co-extruded or primed polyester is used to ensure that there is no adhesion failure of the silicone to the film over time. However, these special grades of film are generally more expensive than standard grades. Consequently, the widespread use of these ”treated” films has been limited. The challenges to the silicone supplier are to develop robust silicone release coating systems for general grades of film that overcome the problems discussed above. The first attempts to overcome some of the challenges of coating films were made with the introduction of UV-curing silicone systems. UV-curing silicone systems met the low temperature constraints for polyolefin films. However, due to customer preferences and allround release performance, thermally-curing silicone release coating systems that provide robust performance to films have been sought.

10. SILICONES AS MOLD-MAKING ELASTOMERS Long ago, people must have realised that clay could be used to take imprints of simple objects like a leaf, or later, a coin. Carefully withdrawing the clay gave a negative of the object or a mold in which other materials like plaster could be cast to reproduce the original. Clay is still used today to make molds, particularly to reproduce museum pieces like statues,

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not only because it is inexpensive, but also because clay is water washable and unlikely to contaminate or stain any valuable and unique original. Clay molds are made by applying a layer of clay a few centimetres thick, and not too wet, on the original coated with talc. The clay is covered with plaster to provide a rigid backing or countermold. A fine metallic wire can be laid onto the original surface before applying the clay to help dismantling. This allows the clay mold to be split neatly in smaller parts. If needed, the original is copied in many pieces. After re-assembling the mold and its backing, plaster can be used to fill the clay mold to make copies and disseminate an object that otherwise would be unique and could only be seen by a few. “Does it matter that a copy is being shown and exposed?” the question has been asked. Providing that the copy is properly finished, all the artist's original work will be visually as present in the copy as in the original. So much so, that copies have been stolen from museum displays! Yet the above technique suffers some shortcomings. Clay does not perfectly wet the original, and details are not perfectly captured. Seals between mold parts are difficult to make, leading to visible imperfections in the copies. The poor recovery after deformation or the clay’s plastic nature creates distortions upon demolding. And such clay molds may be good for making only one copy. A major improvement was found with the use of elastomers as mold-making materials. These mold-making elastomers are supplied as liquid compositions, usually two-part materials, and are easy to cast around the original. After hardening, they set into a flexible material that can be stretched to ease demolding, even around deep undercuts. However, because of their elastomeric nature, they return to their original shape to give a cavity containing in negative all details of the original surface. The first reference about the use of silicone molds appeared in the 50s. Mention is made of a composite made of mica or paper and a binder, and shaped around an electrical coil using a silicone mold [85]. The earliest true mold-making application with silicones, where the details of the original surface are being transferred via the mold, appears to be in dental molds, with commercial products available from 1955, and fast-curing compositions later [86]. Compared to molds made of metal and where a cavity must be created with all the details of the desired finished object in negative, elastomers used as mold-making material require little tooling, providing an original object exits and from which a mold can be made. Metal molds perform well when many copies are needed. Molds made from an elastomer are an interesting alternative for short-copy series.

Process Description Different elastomer products are available to prepare molds. Their common and key feature is that they are initially fluid compositions that can be poured around the original before hardening into a solid elastomer. This transformation is obtained by cooling for a thermoplastic elastomer, by water evaporation for a latex emulsion or by a chemical reaction for a two-part reactive system. The simplest mold is known as the one-piece block mold (see Figure 15):

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101

The original, master or model is fixed into a container or countermold with appropriate clearance left all around After processing of the mold-making elastomer material (melting or mixing), the liquid composition is poured in the space between the original and the countermold After hardening and disassembling, a one-piece elastomer block mold is obtained whose internal surface contains in negative all the details of the original.

Step 1: Fix the original in its counter-mould; apply a release agent; cast the two-part silicone elastomer after mixing and de-airing the twopart mixture.

Step 3: Dismantle to obtain the one-piece block mould.

Step 2: Allow the two-part silicone elastomer mixture to cure to an elastomer.

Step 4: Cast a suitable copying material (plaster or resin).

Step 5: Dismantle to obtain the copy.

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Figure 15. Process steps for making a one-piece silicone elastomer block mold (original in black, countermold in white; two-part silicone elastomer mixture and cured silicone mold in light gray; copy in dark gray).

This one-piece block mold can now be used to cast plaster, polyester or any other suitable material to obtain positive copies of the original (see Figure 16). But more complex molds are also used. A three-dimensional original (e.g., a statue) can be copied with a mold of two or more pieces (i.e., to render both front and back surfaces, which a simple block mold cannot do). In contrast, skin molds can be used for very large originals that are rather “flat” (e.g., a cathedral door). In this case, the object can be copied with only a thin layer of a thixotropic mold-making material to limit the amount of material used and reduce costs. Here thixotropy can be induced with additives like glycols or silicone polyethers capable of interacting with filler particles present as reinforcing fillers and by hydrogen bonding to give a nonflowing molding material. Such skin molds carry all the details of the original surface but are not self supporting like a block mold. A suitable

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countermold, usually made of fiberglass-reinforced polyester, must be built directly on top of the skin mold after the mold-making material has hardened.

Figure 16. Flexible one-piece block molds: a silicone mold being separated from a PU copy (left); a silicone mold, and two copies, one after and one before finishing (right). Pictures courtesy of Dow Corning.

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So, most complex molds are made of many pieces, each of the skin type. Less elastomer is used, but more time is spent preparing the original; that is, hiding or masking some parts of it with clay or plastiline to mold only part of the original surface at a time (see Figure 17).

Figure 17. A complex mold made of two skin pieces (in blue), each with its own supporting countermold (in white) around the original (in black). Protuberances are designed to ensure that the skins adjust properly to their respective countermold parts.

Mold-Making Elastomers Various elastomeric materials are used as mold-making material: • •

Thermoplastic, like plasticized PVC, is inexpensive, but the original must allow exposure to high temperatures from the hot melted mold-making material Latex-based emulsions of limited stability upon shelf aging or against the heat generated by some copying resins (see further)

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Two-part silicone elastomers

Two-part silicone elastomers have distinctive advantages: •

• • •

They are available as two-part materials; that is, as two components to be mixed just prior to use in a fixed ratio such as 10:1 or 1:1 to give a liquid mixture that can be poured or plastered around the original Their low surface tension allows them to pick up minute details from the original surface They harden, cure or cross-link into high-strength elastomers at room temperature without exotherm, and so do not expose the original to thermal stress Because of their low surface energy, various casting materials can be used to make copies without the risk of adhering to the silicone surface; because of the silicone heat stability, resins with strong exotherms can be used.

Two-Part Silicone Mold-Making Elastomers PDMS polymers are liquid at room temperature, even those of very high molecular weight. Their low Tg and the flexibility of their backbone make PDMS materials ideal candidates for formulating elastomers. A chemical reaction is yet required to attach or connect the free-flowing PDMS chains to form a solid, three-dimensional network or an elastomer capable of sustaining mechanical deformations. To allow for this, groups that can be reacted between polymer chains via a cross-linker in presence of a suitable catalyst must be present on the silicone polymer chains. Two different cross-linking systems have been developed, referred to as addition cure or condensation cure. The addition cure is based on vinyl end-blocked PDMS polymers that are cross-linked by a SiH functional PDMS oligomer using a platinum-based catalyst, according to: OMe2Si

CH CH2 + H Si

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where

Pt cat

OMe2Si

CH2

CH2

Si

represent the remaining part of the PDMS chain

If the SiH functional PDMS contains three or more SiH reactive groups, many PDMS chains can be linked together to form a three-dimensional network. This reaction is an addition cure reaction, and no byproducts are evolved. So molds made using this reaction do not show shrinkage (see further). A platinum-based catalyst is used here and is prone to inhibition problems. Platinum catalysts work because they can bind to the weak electrondonating vinyl groups of the polymer chains (see Section 9). But, if better electron-donating groups are available in the vicinity (e.g., amine or sulphide), these can permanently bind with the platinum catalyst and completely inhibit its activity. Such impurities may come from nearby tools such as sulphur-vulcanized rubber gloves or from the original surface. When encountered, inhibition keeps the two-part addition cure elastomer from cross-linking properly, and it may badly stain the original (a small trial in a nonconspicuous place is recommended).

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The condensation cure is based on hydroxy end-blocked PDMS polymers that are crosslinked by an alkoxy silane in presence of a tin catalyst according to

4

Me2Si where

OH + Si(OnPr)4

Sn Cat - 4 nPrOH

OSiMe2 Me2Si

O

Si

OSiMe2

OSiMe2

represents the remaining part of the PDMS chains

Alcohol is evolved during cure, resulting in some material loss and shrinkage (up to 2% linear shrinkage). Molds made using this reaction will not perfectly respect the dimensions of the original. But this reaction is not prone to inhibition except in very rare cases [87]. Advantages and limitations of both addition and condensation cure or cross-linking systems are summarized in Table 8. Table 8. Comparison of the Properties of Addition and Condensation Cure Silicone Two-Part Mold Making Elastomers

Inhibition Shrinkage (% linear) Heat stability

Addition cure Possible Low (< 0.1) Excellent

Condensation cure Very rare Medium – high (0.2 – 2) Limited

Two-part silicone elastomers are provided as two-component (Part A and Part B or base and curing agent, the latter sometimes improperly named catalyst) to separate polymer, crosslinker and catalyst from each other. These two components are mixed in a fixed ratio prior to use to allow cross-linking only after mixing. Various additives may be included like fillers (e.g., a high surface area fumed silica with levels up to 25 % w/w), as these dramatically improve mechanical properties, or cure rate control agents to allow for enough time after mixing to handle the mixed material and to have enough “pot life” while casting the two-part material around the original.

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The Art of Mold Making with Two-Part Silicone Elastomers Mold making with silicone elastomers is an art. Many aspects are to be considered to preserve the original as well as to create the best possible copies. Originals need little preparation, but they must tolerate the process; staining or removing lustre on an old artefact would be catastrophic. Countermolds are made from various materials, from simple cardboard to fiberglass-reinforced polyesters. Release agents are used to avoid adhesion from the two-part silicone elastomers onto the original and the countermold, or on any cured silicone surface when making a multipiece mold to avoid adhesion between mold pieces. Release agents are based on soaps in water, petrolatum in organic solvents, organic resins in water or fluoropolymers. The silicone elastomer is cast after adequate mixing and de-airing under vacuum to eliminate bubbles. Operations range from one casting for a simple block mold to many

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castings for a multiple-piece mold, with a strong release agent applied on any cured silicone surfaces to avoid subsequent adhesion. “Pegs” may be created to ensure that all mold pieces will adjust to each other properly later, and to minimise defects from seal lines. Various casting materials can be used to make copies, including plaster, peroxide-cured polyesters, or two-part organic resins like polyurethane or epoxy. Mold life is a critical aspect. Some casting resins can slowly swell the elastomer and cure within the silicone polymer network, actually forming an interpenetrating network (IPN). This can quickly lead to deformations in the copies with respect to the original or worse, adhesion of the copy to the mold. Among other casting materials are low melting point metallic alloys or waxes. Wax copies are used to make ceramic molds for high melting point metals. Finishing the copy is most important. Thick layers of pigmented coatings are inadequate and would remove all the surface detail transferred from the original to the copy thanks to the silicone elastomers. Silicone elastomers are capable of transferring submicron surface details and render appearance as detailed as velvet or wood structure [88]. So the finishing step is where mold makers can express all their art. Pigments can be included in the casting resin to provide for an adequate starting color, or fillers can be added to adjust density and render feel when handling the object. Lustre on artefacts is developed by very thin coating layers brushed and padded away.

Application Fields Much of the above is related to the use of two-part silicone elastomers for the reproduction of artefacts like museum pieces. The method has been used to reproduce very large objects like a horse with a man statue in France, [89] a Chinese dinosaur skeleton [90] or a pair of Easter Island Moai statues, [91] all full size! Yet such silicone elastomers are also used in our everyday life as dental impression materials (a challenging application, as molds are made on wet buccal surfaces and as moisture can interfere with the cross-linking reactions) or as intermediates to designing and preparing prototypes before market launch. More recently, bakery forms made from silicone elastomers were commercialised to cast and bake cakes.

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11. SILICONES IN THE ELECTRONICS INDUSTRIES Before 1943, planes could maintain high altitudes for only a few minutes before ignition losses due to moisture condensing in the engines. A simple thickened PDMS grease (see Section 18) was the solution and an early example of the excellent dielectric properties of silicones. This application also illustrates key properties of silicones in the electronic industries like hydrophobicity and high dielectric breakdown (keeping moisture away and avoiding loss of high voltage/low current signals), as well as their resistance to low or high temperatures, which allow use in harsh and critical environments [92].

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Table 9. Markets for Silicones and their Key Properties in Electronic Applications

(1) Low Tg and Tm impacts the influence of temperature on key properties like dielectric properties (2) Surface energy impacts the wetting behavior of the material (3) Relevant in optoelectronic applications (4) Tunable wetting properties (5) Degradation temperature impacts service temperature

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Today, despite a higher cost to acquire, the number of applications involving silicones continues to increase, in some instances driven by Moore’s law (chip complexity doubling approximately every two years), but also by tighter specifications. The presence of more and smaller components (e.g., sometimes thousands in today’s cars) requires resistance to higher temperatures to ensure reliability and to avoid increasing the probability of failure. Again, different silicones are used, and it is a combination of their properties that makes them perform well in various electronic and market applications (see Table 9).

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The Relationship of Structural Properties in Electronic Applications The Si-O-Si bond angle in a silicone polymer can vary between 105º and 180º, [93] and the rotation is essentially free [94] around these bonds. As a result, the chains are very flexible and occupy a rather large volume, resulting in a high free volume in the material. Consequently, silicones exhibit a very low glass transition temperature (Tg ≈ -125ºC). Low intermolecular interactions account for the low melting temperature (Tm ≈ -50ºC) of silicone materials. Once cross-linked, silicones are soft elastomers with hardness in the Shore A range if reinforced (see Section 14), or much softer in the absence of reinforcing filler and even gellike if only partially cross-linked. In many applications, this “softness” allows relief of stress induced by temperature changes as thermal dilatation mismatches. Silicone gels are compliant, self healing and outstanding for protecting thin wire-bonding from thermal shocks, vibration and corrosion. The response of their elastic and storage modulus is linear over a wide range of temperature and frequency. Dynamic mechanical analysis has been carried out at various temperatures on a standard PDMS gel. The reduced shear storage and loss modulus and tan δ are displayed against the reduced frequency for temperature ranging from -40ºC to 100ºC on Figure 18a and Figure 18b. The Arrhenius plot of the horizontal shift factor aT can be seen on Figure 18c, [95] showing a perfect fit with the Williams-Landel-Ferry (WLF) equation [96]. In that respect, silicone gels are exhibiting model behavior and could be used to study further fundamentals of cross-linked polymer mechanics. Other silicone materials also follow the WLF model as can be seen on the Arrhenius plot in Figure 18d, which compares the behavior of a silicone gel, a silicone elastomer and a silicone resin [95]. The service temperatures of silicones can be extended by replacing some methyl groups by phenyl groups on the siloxane backbone. The random inclusion of different groups along the chain hinders natural ordering and crystallization. As a result, the Tm is either lowered or eliminated. The presence of phenyl groups also improves high temperature stability.

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Figure 18. Dynamic mechanical analysis (DMA) shows: a) reduced shear storage and loss modulus of a standard silicone gel as a function of the reduced frequency at -40 ºC to 100ºC; b) tan δ of the gel as a function of the reduced frequency temperature range from -40 ºC to 100 ºC; c) Arrhenius plot of the horizontal shift factor for the gel; d) Arrhenius plot of the horizontal shift for a silicone gel, a silicone elastomer and a silicone resin.

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Electrical Properties of Silicones Despite strongly polarized Si-O bonds, silicone polymers are nonpolar, as the Me side groups prevent Si-O dipoles from approaching each other too closely. As a result, the intermolecular forces are weak and mainly composed of London-van der Waals interactions that decrease with the square of the distance between molecules. Due to this ambivalent character of the PDMS polymer chain, the polarizability of the molecule accounts for a relatively high dielectric constant of silicones in comparison to a nonpolar polymer like polyethylene (see Table 10). As expected, silicone copolymers in which Me groups have been substituted with more polarizable groups are not better either. The dielectric constant of PDMS increases with the degree of polymerization (DP) of the siloxane backbone before quickly reaching a plateau value (see Figure 19). This effect is related to the siloxane-to-methyl-groups ratio, which quickly increases, particularly in the shortest DP polymer. At higher DP, adding one more unit has little impact on the permittivity of the media, which explains the plateau region.

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Table 10. Dielectric Properties of Various Polymers [97]

Polymer High density polyethylene Cis-polyisoprene Poly methylmethacrylate Poly dimethyl siloxane (Me2SiO)n Poly diphenyl dimethyl siloxane (φ2SiO)5.5 (Me2SiO)94.6 Poly phenylmethyl dimethyl siloxane (φMeSiO)7.5 (Me2SiO)92.5 Poly phenyl methyl dimethyl siloxane (φMeSiO)30 (Me2SiO)70 Poly trifluoropropyl methyl siloxane [(CF3CH2CH2)MeSiO]n Viton® fluoroelastomer

Dielectric constant at 100 Hz 2.30 2.26 3.03 2.86

Dissipation factor at 100 Hz 0.00011 0.0094 0.057 0.00025

Dielectric strength at 60 Hz 811 577 608 552

Volume resistivity (Ohm.cm) 2.2 1016 7.1 1016 1.2 1016 5.3 1014

148 210 382 150

2.90

0.00041

661

9.8 1014

151

2.87

0.00010

661

3.0 1014

149

2.99

0.00024

720

4.4 1014

176

6.85

0.109

342

2.7 1011

199

8.55

0.0403

351

4.1 1011

255

Tg (K)

2.80 2.70

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Dielectric Constant

2.60 2.50 2.40 2.30 2.20 2.10 2.00 1.90 0

50

100

150

200

250

Degree of polymerization (DP)

Figure 19. Influence of degree of polymerization on the dielectric constant of polydimethylsiloxanes, measured at 1000 Hz at 23 ºC. [98]

In most organic polymers, the strong attractions between polymer chains diminish as the temperature increases, so many dependent properties change significantly. For silicones, the intermolecular forces are low and do not change much with temperature. Consequently, viscosity, mechanical properties, dielectric properties and many physical properties are little

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affected over a wide range of temperatures. Electrical properties like dielectric constant and the dissipation factor are also little affected over a wide range of frequencies [99-101]. The volume resistivity of silicones is marginally lower than organic materials; ion concentration tends to be low in PDMS and dependent only on the presence of impurities like residual traces of polymerization catalysts or other impurities due to presence of reinforcing fillers. However, ion mobility is favored because of the high free volume in PDMS [97]. Although not entirely related to volume resistivity, dielectric strength is also influenced by the presence of material impurities [97-102]. For a given matrix, the path for dielectric breakdown follows the weakest path as far as resistivity is concerned. Here again, the high free volume of silicone-based materials lowers their dielectric strength in comparison to organic materials. The dielectric properties of silicones are good, but not exceptional in comparison with organics. The success of silicone-based products is certainly related more to their stability over a wide range of temperature, humidity and frequency.

Water Absorption Although often overlooked, water absorption is a key property for products used in electronics, as absorbed water reduces dielectric properties and can contribute to corrosion. Yet, in many electronics applications where silicones are used, it is too simplistic to think that because of their hydrophobicity and water repellency, silicones provide better device protection against corrosion than organics. Corrosion occurs if reactants like oxygen, water and ions are simultaneously present at the interface. Silicone materials are very permeable to gases and therefore cannot limit oxygen or water vapor from reaching a metal interface.

% Water absorption

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1.E+00

1.E-01

Epoxy

Silicone

1.E-02

1.E-03 0

100

200

300

400

500

Time (min)

Figure 20. Water absorption of PDMS vs. epoxy in electronic products [104].

600

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111

However, ions surrounded by several layers of water molecules are poorly soluble in PDMS and, being large clusters, they have a very low coefficient of diffusion in any polymer matrix. Absorbed water molecules in the protective layer are like stones across the river for ions: they use these favorable “water” paths to migrate across the layer at a much greater rate of diffusion [103. Because of their high permeability, silicones will uptake water quickly compared to epoxies. Yet at saturation level, water content in silicones is ten times lower than in epoxies (see Figure 20). This is the main reason why silicones are so effective when used as corrosion protective materials. An additional benefit in electronics is that when temperature increases suddenly, water diffuses easily from silicones compared to organics and without local pressure build-up (popcorning issue).

Thermal Conductivity With trends toward miniaturization and higher power electronics, heat dissipation and protection at component interfaces is becoming more critical. Silicone thermal conductivity is rather low if compared to metals (see Table 11). However, it is about ten times better than air, which is most often responsible for poor heat conduction at metal-metal or metal-plastic interfaces. Because of low surface tension, [4] PDMS has an enormous advantage that is not demonstrated by its intrinsic heat conductivity if compared to organics. PDMS allows good surface wetting, and so displaces air at interfaces, reducing heat resistance between components. This in particular has driven the penetration of silicones in electronics thermal management applications.

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Table 11. Thermal Conductivities of Various Materials Material Silver Copper Silicone, thermally conductive (PDMS + silver filler) Silicone (PDMS) Epoxy Air

Thermal conductivity (W / m.K) 417.3 393.7 0.7 - 8.0 0.2 0.2 0.03

12. SILICONES FOR PHOTONICS As the photonics market develops, there is a continuing need to bring costs down. For opto-electronics, polymer-based components and structures are being considered because they are inherently easier to process than glass-based materials. Polymers can be batch processed by spin coating or stamping, but also processed continuously via printing or extrusion. In addition, this processing can take place at ambient temperatures and pressures. A variety of polymer systems have been investigated in photonics with varying degrees of success.

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Light transmission through polymers can be limited by electronic transitions in the UVvisible region or by vibrational absorptions in the near-IR or IR, including overtones. These sources of loss are specific to each polymer. Intrinsic light losses tend to be higher with polymers than with glass-based materials. Under high flux, polymer degradation can occur: organics will yellow because of the heat generated, and they can also degrade because of photo-initiated oxidation. Despite these shortcomings, polymers are considered to take advantages of their inherent benefits. These benefits include ease of processing (resulting in lower-cost production) and also some material-specific functionality like high thermo-optic coefficient (see discussion below). Optical applications are considered, despite the higher intrinsic light loss generally associated with polymers, because the loss is not as critical for short-length (e.g., less than one meter) applications such as passive waveguides for communications between circuit boards, between integrated chips or even for lenses for light emitting diodes (LED). In addition to their excellent thermal stability, mechanical properties and ease of processing, silicones are highly transparent to radiation in the visible all the way down to UV. Silicones also have good transmission at selected near-IR wavelengths [105]. Very low levels of Rayleigh scattering can be achieved with silicones. Therefore, silicone-based polymers possess a set of properties making them suitable for waveguide applications, as well as for lenses and encapsulants through which light must travel.

Necessary Properties for Photonics Applications For waveguide applications, critical material attributes are: [106] • • • •

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•

Low dielectric constant, and this usually also implies low refractive index Transparency with negligible light loss due to UV-visible electronic or IR vibration absorptions Homogeneity to minimize scattering Low intrinsic birefringence, and in most applications, low stress-induced birefringence Satisfactory thermophysical properties for the desired application

The most common silicone polymers are linear PDMS based on Me2SiO2/2 or D units with refractive indices approximately 1.40 - 1.42. More complex as well as more rigid structures can be engineered by including T or Q units (see Section 1). Some methyl groups along the chain can be substituted with phenyl groups to increase the refractive index to approx. 1.55 or with trifluoropropyl groups to reduce the refractive index below 1.40. In the following section, important characteristics including dielectric properties, thermophysical properties and absorption characteristics will be developed and highlighted with examples. Dielectric Properties. A low dielectric constant is desired because it minimizes light absorption by the material. The absorption and complex dielectric constants are linked through the Kramers-Kronig relations. Silicones in general have low dielectric constants when compared to other optically transparent plastics. The dielectric constant depends on the

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overall modulus of the system, state of cure, and overall system composition (i.e., type of polymers, cross-linkers and additives used). For PDMS with viscosities between 10 to 60,000 cSt, the dielectric constant ranges from 2.72 to 2.75 when measured from 100 to 10,000 Hz at 25 oC [107]. The dielectric constant for PDMS also varies with temperature: at 800 Hz, it measures 2.8 and 2.3 for 20oC and 200oC respectively [108]. For a polymethylphenyl siloxane, the dielectric constant has been measured at 2.98 at 25oC and independent of frequency from 100 to 1,000,000 Hz [109]. By comparison, polymethylmethacrylate (PMMA) has dielectric properties that range from 3.6 at 50 Hz, 3.0 at 1000 Hz, and 2.6 at 1,000,000 Hz when measured at 25oC, [110] and polycarbonate (PC) has a dielectric constant of 3.02 at 1000 Hz [111]. Because of the lower dielectric sensitivity to frequency, siloxane polymers in general have lower levels of dispersion than common organic polymers. Thermophysical Properties. Siloxane polymers are not prone to yellowing, and if care is taken to remove catalyst impurities, they have very good thermal stability (see Section 1). The inclusion of phenyl groups leads to polymers that are stable for short durations at 300oC under nitrogen or air (see Figure 21). Many siloxanes have continuous temperatures specified at 150 oC or above. For comparison, the continuous use temperature for PMMA is < 90oC and for PC is 121oC [111]. Waveguides made from silicones maintain their shape without cold flow because the materials are cross-linked. Because the Tg of PDMS is very low, stress birefringence remains low at most temperatures. The stress-optical coefficient for PDMS is 1.35 x 10-10 m2/N at 20oC and 632.8 nm [113]. For polymethylphenyl siloxane, the coefficient is reported to be slightly higher at 5.73 x 10-9 m2/N [114]. The thermo-optic coefficient (change of the refractive index vs. temperature), dn/dT, for siloxanes varies from -1.5 x 10-4 to -5 x 10-4, depending on composition and cross-linking density [106]. The capability to tune dn/dT can be of use in some applications like thermally controlled variable optical attenuators and athermalizing planar light circuit components. For PMMA, dn/dT = -1.1 x 10-4 below Tg, which is approximately 105oC [115].

Weight retention, %

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120 100 80 N2 Air

60 40 20 0 0

100

200

300

400

500

600

Temperature, oC Figure 21. Thermal gravimetric analysis of a poly methylphenyl dimethyl siloxane copolymer under nitrogen or air under a temperature ramp rate for the testing of 10 oC/min. [112].

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Phonon and Absorption Characteristics. Methyl siloxanes do not show characteristic absorption bands in the UV or visible spectrum, while methylphenyl copolymers have characteristic absorptions at 270, 264 and 250 nm. Both have many absorption bands in the NIR region (see Table 12) [105]. Table 12. Light Loss Characteristics of Silicone Polymers or Copolymers at Various Wavelengths Silicone Polymer or copolymer

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Dimethyl Dimethyl methylphenyl Methylphenyl Trifluoropropyl methyl 1 Trifluoropropyl methyl 2 Phenyl resin - 1 Phenyl resin - 2

Loss at specific wavelength dB/cm 1550 nm 0.67 0.66 0.62 0.54

1310 nm 0.14 0.28 0.35 0.16

850 nm < 0.01 0.03 < 0.01 < 0.01

633 nm 400 nm < 0.01 0.03 0.03 0.04 < 0.01 < 0.01 < 0.01 < 0.01

300 nm 0.09 0.24 0.55 < 0.01

0.35

0.07

0.12

0.22

0.64

1.36

0.49 0.39

0.41

0.01 0.03

0.02 0.05

0.06 0.11

2.39 2.94

Figure 22. NIR absorption spectra of a PDMS polymer (in teal) and shifts in absorption when different substituents are added. In general, decreased absorbance at 1160 nm and 1500 nm and increasing absorbance at ~ 1630 nm results from increasing the phenyl content of the system (in red-resin and in purple-polymer). The addition of trifluoropropyl groups reduces absorption around 1160 and 1500 nm (in blue).

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Phenyl groups or trifluoropropyl groups can be added to reduce the absorptions in the 1500-1560 nm wavelength regions. However, the phenyl can have a negative impact on the absorptions at 1100, 1280-1320 nm. Despite these trade-offs, silicones are capable of excellent loss properties in the datacom wavelengths and certainly are adequate for shortrange applications in the wavelength bands of interest for telecom (see Figure 22). In summary, silicones posses an interesting set of properties for photonic applications when compared to organic polymers. Silicones display high-temperature stability, which makes them compatible with solder reflow processing or in “under-the-hood” hightemperature applications, and they can be processed at room temperature. Silicones also have the optical characteristics necessary to enable them to function in waveguides with acceptable losses at telecom wavelengths and with very low losses over data-communications wavelengths.

13. SILICONE IN MEDIUM TO HIGH VOLTAGE ELECTRICAL APPLICATIONS The use of silicones in these applications is much related to cable end terminations or silicone rubber connections made at the end of underground high voltage cables insulated with polyethylene, as well as to silicone insulators for power lines. Key benefits from silicones are their high electrical resistivity, resistance to environmental degradations and to electrical aging as well as their hydrophobicity, which results in lower assembly and maintenance costs [116,117].

Silicone Cable End Terminations

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Modern materials allow pre-assembly and thus avoid problems associated with the use of molten casting material or mistakes made during manual assembly on the construction site. Today cable accessories are completely built at the supplier. Typically they consist of rubber terminations made of different insulating silicone rubbers. Silicones allows for two types of design: • •

Push-on technique where a PE ring acts as a space holder until placement, and using silicone rubbers with hardness from 35 to 50 Shore A Cold shrink technique using softer silicone rubbers with hardness from 25 to 35 Shore A

Insulation is made without chemical bonding between the termination and the cable, and it relies on the elastomeric characteristics of the silicone termination to exclude any entrapped air, particularly in areas of high electrical field and around the edges at the cable end. The high gas permeability of silicones allows any included air to diffuse out to leave an air-free joint.

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Such silicone rubber cable end terminations are produced by rubber injection molding using a silicone high consistency rubber (HCR) or by liquid injection molding using a twopart liquid silicone rubber (LSR). Silicones provide overall electrical insulation because of their high dielectric strength (see Section 11). In addition to their good resistance to high temperature, UV and ozone, they are hydrophobic and so do not promote surface insulation failures. But more important, specially formulated silicones have been developed to smooth the electrical fields within the connection end and to ensure long-term performance. This is achieved in composite cable terminations either using some electrically-conductive silicone rubbers or, in more modern and smaller accessories, shaped deflectors made from silicone rubbers with medium electrical permittivity (see Figure 23). Silicones are appreciated in cable end terminations because of their resistance to erosion caused by radiation. As silicones do not absorb UV-visible sunlight, they are not prone to chalking or cracking. Such phenomena are typical with organic-based materials and, associated with dirt pickup and humidity, can lead to a significant reduction of insulation properties. Silicone resistance to so-called “tracking” is also higher than with organic-based insulation materials. Tracking is the formation of electrically-conductive surface paths under intensive electrical surface leaks and discharges. In organic materials, this leads to the formation of carbon-based decomposition products that unfortunately show high conductivity. With silicones, even if poorly designed or not properly assembled, decomposition leads to nonconductive silica, and silicones will meet the highest class of electrical erosion resistance.

Figure 23. Field line density in a cable end termination at the cut of the screen without control (upper figure) or with a nonconductive/high permittivity field control silicone rubber (in green; lower figure).

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Silicone Insulators Another key property is hydrophobicity, particularly for electrical insulators, or devices installed between power lines and supporting structures. Water on an insulator made of a silicone elastomer remains as droplets and does not form a continuous film because of the low surface energy of the silicone elastomer surface [118-120]. This reduces surface currents on the insulator. Surface hydrophobicity is maintained even after surface discharges or deposition of airborne pollution because of the presence of low molecular weight, unreacted polydimethylsiloxane species in the composition of the silicone elastomers. These species can migrate to the external surface and maintain low surface energy or hydrophobicity [121]. Insulators made of silicone elastomer therefore need little cleaning or maintenance and perform over a long period of time (see Figure 24).

Figure 24. Comparison of an insulator after 23 years of use and exposure to pollution (left) vs. a retained sample kept at RT (right). Both still show excellent hydrophobicity as indicated by the high contact angle of the water droplets. (Picture courtesy of Lapp Insulators GmbH and Co. KG).

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14. SILICONES IN TRANSPORTATION: AUTOMOTIVE AND AVIATION Silicones, particularly silicone rubbers, have found use in a wide variety of transportation applications. Nonreinforced cross-linked silicone polymer networks are very weak. However, when filled with precipitated or fume silica reinforcing fillers and compounded into silicone elastomers or silicone rubbers, a tremendous improvement in mechanical properties is seen. Specific silicone rubbers have tear strengths of 60 kN/m and tensile strengths above 10 MPa, yet with low relative density, making them cost attractive on a volume basis (see Table 13). Adding high surface area fillers, such as silica, increases the viscosity of the blend and so requires the use of silica surface treatment agents to maintain enough ease of processing and prevent crepe hardening. Apart from reinforcing silica, other ingredients are included in the formulation, such as peroxide or cross-linkers and catalyst. These provide a “cure package” to cross-link the

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silicone polymer chains into a silicone rubber, as silicone rubbers are thermosets and are “cured” at elevated temperatures (see Section 1). Silicone rubber compounds are typically delivered as one-part materials to be crosslinked at elevated temperatures by either peroxide- or platinum-based catalysts. Where a onepart platinum catalyst based material is used, the activity of the platinum catalyst at room temperature has been reduced using appropriate inhibitors. These one-part products do not require mixing prior to use but have limited shelf life, typically ranging from three to six months. To ensure sufficient shelf life, a platinum catalyst encapsulated in a thermoplastic resin can be used, where upon heating, the capsule melts and liberates the platinum catalyst [123]. The cross-linking densities in silicone rubbers are low and as the cure package has no detrimental effects upon the polymers, silicone rubbers retain most of the key properties of the silicone polymers from which they are made. They offer resistance to weathering, ozone and UV radiation and aesthetically they are transparent and therefore easy to pigment. Glass transition temperature remains low, meaning that these silicone rubbers can be used in regions that encounter extremes of cold. Conversely, their stability at very high temperatures means they can survive the harshness of modern engine compartments, where rubbers are expected to coexist next to hot metal components and where upper service temperatures have been steadily increasing due to the higher running temperatures demanded by more efficient engines. Table 13. Typical Mechanical Properties of Selected Rubber Families [122] Material Tensile strength

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Elongation at break Hardness range Min. operating temperature Max. operating temperature (continuous) Relative density

Unit MPa psi % Shore A

Silicone 4 - 12 990 - 1265 570 – 1000 20–90

Natural Rubber 28 4000 700 30-90

EPDM 24 3500 550 25-85

Neoprene 28 4000 500 35-90

o

- 60 (*)

- 60

- 50

- 40

o

230

100

140

100

1.15

0.92

0.86

1.23

C C

(*) Special grades down to -116°C.

Silicone rubbers are easy to process and various types are available. Liquid silicone rubbers (LSRs) are paste-like materials and are widely used in injection molding for flashless parts, fabric coating, dipping and extrusion coating processes. High consistency rubbers (HCRs) and fluorosilicone rubbers (FSRs) are gum-like materials and can be calendered; injection, compression or transfer molded; or extruded. Grades of silicone rubbers can be formulated to resist attack from organic oils and greases. Where increased resistance to organic fuels is required, fluorosilicone rubbers (in which some of the -CH3 groups along the siloxane backbone have been replaced by -CH2CH2-CF3 groups) offer a step change in fluid resistance (see Table 14). This is a result of the slight polarity and the sheer size and bulkiness of the trifluoropropyl group, which imparts

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significant steric hindrance to the molecule and also reduces the free volume of the network. These factors combine to severely limit the penetration and swelling of the FSR by many solvents. Using silicones in the automotive industries is not without controversy. In the trade, there are many stories about paint shop managers banning silicones from their production areas. The issue here is surface contamination from either liquid silicones or from low molecular weight “airborne” volatile siloxanes liberated from other silicone-based compounds used in the vicinity. All are capable of binding to surfaces to be painted, leading to poor paint wetting and disastrous “orange peel” problems. This is linked to the low critical surface tension of wetting they induce after adsorption. This is a problem that can be prevented by using simple good working practices. Table 14. Fluid Resistance of Standard and Fluorosilicone Rubbers [122] Rubber Type MQ VMQ FVMQ

Water 3 days / 100°C Delta duro % swell -5 +5 -5 0 0 0

ASTM Oil #3 3 days / 150°C Delta duro % swell - 25 + 35 - 20 + 35 -5 +5

Toluene 7 days / 24°C Delta duro % swell na (*) na (*) na (*) + 205 -10 + 20

(*) na: not available. Note: MQ : dimethyl silicone based rubber VMQ: vinyl methyl silicone based rubber (HCR) FVMQ: fluoro vinyl methyl silicone based rubber (FSR)

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Another issue is headlight “fogging” linked to the degradation of low molecular weight volatile species and deposition on headlight lenses sealed to their frames with silicone sealants. These issues are real and need adequate management, but with appropriate precautions even silicone fluids are currently used in many automotive applications. For example, silicone polyethers are used as profoamers in the PU foams present in many cars, sometimes unknown to the production engineers, and silicone fluids are used in viscous couplings. In both these applications silicone use is without problems. On average, a car contains approximately 3 kg of silicones, mainly silicone rubbers, which are used to produce many parts. Body Components Heater hose Oil seal, water seal, air seal - filler cap O-ring seal Vibration and sound damping material; rubber exhaust/muffler hanger - Mirror mount adhesive Chassis - Heater hose – brake hose and clutch hose - Oil seal, water seal, air seal – dust cover seal, CVJ boot, and brake cap seal - Dynamic seal – power steering oil seal and booster piston seal Vibration/sound damping material – engine mount and suspension bushing

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M. Andriot, S. H. Chao, A. Colas et al. Electrical Components - Spark plug boot - Ignition cable - Lamp cap - headlamp and fog light - Weather pack connector seal Fuel Systems - Fuel seal – fuel filler seal, quick connector seal Diaphragm Power train - Turbocharger hose and heater hose - turbocharger hose, emission control hose, air duct hose, long life coolant hose (LLC) - Oil seal, water seal, air seal – gasket material for intake manifold gaskets, oil pan gaskets, rocker cover gaskets, front cover gaskets, radiator tank gaskets, oil filters, O-ring in long life coolant (LLC) Dynamic seal – crank shaft seal, camshaft seal, transmission oil seal

Safety -

Air bag coatings

Specific examples related to land transportation and aviation are described below.

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Land Transportation Turbocharger Hoses. Turbocharger hoses, also known as intercooler or crossover hoses, connect the turbocharger outlet to the air intake of the engine. These hoses are reinforced with fabrics such as knitted polyester, woven Nomex® or woven glass fibers to withstand high operating pressures during use. Stainless steel rings may also be used to limit the extent of hose expansion under pressure. In this application, a thin, single layer of FSR is typically used as the hose inner lining to prevent the leaking of engine lubricants, which condense onto the inside of the hose when the engine cools. The inner layer of FSR is covered with a number of plies of HCR to give added strength and increased heat resistance. Such FSR/HCR combinations are particularly suitable for turbocharger hoses. Water coolant Hoses. Silicone coolant hoses are used to carry water, air and oil, while resisting high temperatures and degradation. The increasing use of long-life coolants and aggressive rust inhibitors using organic acid technology (OAT) in combination with complex engine design makes hose replacement time consuming and expensive. Therefore, engine designers are looking for a material that is “fit and forget.” The lifetime cost of a silicone hose, when taken in conjunction with service intervals and replacements, often offers a cost saving over alternative materials that are perceived as lower cost. Air Bags. Rapid growth in the use of automotive air bags has resulted in a corresponding growth in the use of silicone for this application. Air bags are now commonplace in most cars, from luxury to entry models. The initial driver’s air bag also has been supplemented with passenger and side-curtain air bags to protect occupants in the event of a roll. Each bag has its specific requirements, whether initial impact softening through rapid inflation followed by controlled deflation, or sustained retention of pressure for protection when a car repeatedly rolls over. The excellent aging properties of silicone rubber means that an air bag that has

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remained folded into a small volume for many years functions perfectly when required, expanding to hold a high temperature gas as it explodes into action. Anti-Drain Back Valves. This application requires grades of silicone rubber that can resist degradation from engine oil. Such valves made of silicone prevent engine lubricant from draining into the bottom of the sump and ensure the engine is properly lubricated upon startup. Specific grades of silicone can resist the chemical attack of engine lubricants and remain flexible at extremely low temperatures, while at the same time offering extended product life. Flexible Connections in Trains and Buses. The flexible gangway connections between bus and train carriages have been made with a number of differing materials, but an ethylene acrylic elastomer was the most popular choice for a time, mainly based on cost. However, after a number of high profile fires and many fatalities, designers and specification writers reviewed the requirements for a material to fulfill this application. They considered features such as long service life, environmental resistance to cracking and fading, retained flexibility in regions with very cold winters, abrasion resistance, resistance to burning and, when fire does catch hold, low smoke and low toxicity (LSLT) properties, combined with ease of fabrication for companies already using the ethylene acrylic elastomer. Low smoke density and low smoke toxicity is particularly important in underground trains circulating in low diameter tunnels, as the only escape route in case of fire is through the carriage ends. Silicone became an obvious choice, offering a step change in LSLT performance and meeting the BS 6853:1999 category 1a standard.

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Aviation Many features that make silicone an ideal material for automotive applications also hold true for applications in aviation. The retained flexibility of silicone at the low temperatures found at high altitude, the resistance to burning and the subsequent LSLT properties are crucial. Combined with fabricators’ ability to construct complex parts with a material that is safe and easy to handle (thus contributing to cost effectiveness of the finished part), these characteristics make silicone a frequently used material in the world of aerospace. Applications include door and window seals, aileron flap seals and safety devices that require short term resistance to very high temperatures in the event of a fire. In areas that require resistance to jet fuel and lubricants, FSR can be used for hydraulic line and cable clamp blocks, fuel control diaphragms and fuel system O-rings. Silicone rubber products can withstand tremendous stresses and temperature extremes – whether in the air, the stratosphere or the frozen vacuum of space.

15. SILICONES IN THE PLASTICS INDUSTRY Silicones are used in the plastics industry as additives for improving the processing and surface properties of plastics, as well as the rubber phase in a novel family of thermoplastic vulcanizate (TPV) materials. As additives, silicones, and in particular polydimethylsiloxane (PDMS), are used to improve mold filling, surface appearance, mold release, surface lubricity and wear resistance. As the rubber portion of a TPV, the cross-linked silicone rubber imparts

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novel properties, such as lower hardness, reduced coefficient of friction and improved low and high temperature properties. Low molecular weight PDMS polymers, with viscosities less than 1000 cSt, are used extensively by the plastics industry as external release agents applied on the mold surface prior to injection molding. To eliminate an external application during processing, higher molecular weight PDMS materials, with viscosities ranging from 10,000 cSt to 60,000 cSt, have been used as internal additives in thermoplastic polymers to give processing advantages and surface property improvements [124,125]. Due to the incompatibility between dimethyl siloxanes and most thermoplastics, the PDMS is driven to the surface. For example, the solubility parameter for dimethyl siloxane is 14.9 MPa½ and the solubility parameter for nylon 6 is 27.8 MPa½ [126]. A concentration of the PDMS at the surface results in the observed processing and surface property benefits. A more recent advancement in the field of PDMS additives is the use of ultra high molecular weight (UHMW) PDMS, with viscosities ranging from 10 to 50 x 106 cSt. [127]. Additives are now available with 50 weight percent UHMW PDMS in various thermoplastic carriers and as pellets so as to allow easy addition of the additive directly to the thermoplastic during processing. An important improvement obtained using UHMW PDMS is that the loading of PDMS in the concentrated additive is increased from approximately 20 to 50 weight percent. As seen in Figure 25, the UHMW PDMS forms stable droplet domains in the thermoplastic carrier, with an average particle size of 2 microns.

Figure 25. Photomicrograph of a 50% UHMW PDMS dispersed in polypropylene and showing the fine dispersion of the silicone into the organic phase.

UHMW PDMS results in the same processing benefits such as improved mold release, easier mold filling, and lower extruder torque as compared to lower molecular weight PDMS, but it eliminates the “bleed-out” that can occur after processing. This benefit is clearly seen when comparing the print adhesion to polypropylene films containing various additives (see Figure 26). Low molecular weight PDMS (30,000 cSt) and common organic mold release additives significantly reduce the print adhesion due to their migration to the surface and eventual blooming or bleed-out from the plastic part. Conversely, the UHMW PDMS does not reduce the print adhesion because its high molecular weight reduces its mobility and effectively anchors the additive into the plastic.

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Print Remaining

UHMW PDMS additives are often used to improve the wear or abrasion resistance or to reduce noises generated by the motion of plastic parts. These benefits are reflected by the decrease in the coefficient of friction (see Figure 27). A rotating cylinder method was used to generate the coefficient of friction results, with a constant force of 2 kg and a varying velocity until sufficient heat generation occurred and the cylinder and barrel fused. The addition of 3 weight percent UHMW PDMS significantly reduced the coefficient of friction as well as delayed the fusing until much a higher velocity. 100 90 80 70 60 50 40 30 20 10 0

UHMW Silicone PDMS (30,000 cst.) Erucamide Commercial Organic Blend Additi

Note: Tested as molded Tape: 3M No. 250 0

0.2

1

3

5

% Additive

Coefficient of Friction

Figure 26. Print adhesion to polypropylene films containing various additives and tested with 3M tape no. 250 per ASTM D3359. 0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

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Velocity, (m/s) No Silicone

Silicone Masterbatch (3% Si)

Figure 27. Coefficient of friction measurements from pressure-velocity plots for polypropylene without or with UHMW PDMS additive.

Recently, a novel family of TPV products has been introduced and is based on crosslinked silicone rubber dispersed into various engineering thermoplastics [128]. The dispersion of the silicone internal phase is produced by dynamic “vulcanisation” or cross-linking of silicone polymers within the thermoplastic organic phase and results in a stable droplet type morphology (see Figure 28). Such a stable morphology is achieved only by using appropriate compatibilizers to ensure compatibility between the silicone and organic thermoplastic phases, which have very

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dissimilar solubility parameters. As for other TPVs, such thermoplastic compounds are melt processable and fully recyclable. Silicone TPVs have been commercialized using various engineering thermoplastics, but of greatest interest are polyamide and polyurethanes thermoplastics. Silicone polyamide TPV has found use as the jacketing material in automotive brake cables due to its excellent temperature and chemical resistance [128]. Silicone polyurethane TPV combines the benefits of excellent abrasion resistance from the polyurethanes as well as the lower coefficient of friction and improved temperature properties from the silicone rubber. The properties of silicone polyurethane TPV vs. a well known EPDM-PP TPV are compared in Table 15. In particular, the silicone polyurethane TPV outperforms the EPDM-PP TPV in oil resistance due to the miscibility of oils in polypropylene.

21 μ m

Figure 28. Transmission electron micrograph depicting the morphology of a silicone-based TPV (light gray areas are silicone rubber particles dispersed within the organic continuous phase).

Table 15. Comparison Between a Silicone Polyurethane TPV vs. a EPDM-PP TPV: Initial Properties and After Aging in Air or Oil at Elevated Temperatures

Property

Testing Method

Silicone polyurethane TPV

EPDM-PP TPV

Hardness, Shore A Tensile strength, MPa Elongation at Break, % 70 hours in air at 175°C: Change in hardness, Shore A Change in tensile strength, % (*) Change in elongation at break, % (*)

ASTM D2240 ASTM D412, Die D ASTM D412, Die D

71 16 600

66 6.5 457

ASTM D573-99 for heat aging. Same methods as above for testing

+7 +6.3 +12

-5 -32 -20

ASTM D471-98 for fluid immersion. Same methods as above for testing

-9 -23 -2.5 +23

-19 -29 -40 +80

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Initial:

70 hours in IRM 903 oil at 100°C: Change in hardness, Shore A Change in tensile strength, % (*) Change in elongation at break, % (*) Volume swell, %

(*) change expressed as percentage of initial value.

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Compared to thermoset silicone rubber, a silicone TPV offers the added benefit of bondability to various thermoplastics without the use of primers or adhesives via coextrusion and comolding/overmolding. Silicone polyurethane TPV was overmolded onto “cold” (i.e., room temperature) inserts of various thermoplastics and the bond strength was testing according to ASTM D1876. An example of the peel force is shown in Figure 29.

TPSiV Elongating

Peel starts

Load (lbf)

120 Peel ends

80 Peak Bond Strength

40 0 0

1

2 3 Extension (in)

4

5

Figure 29. Peel force of a silicone polyurethane TPV molded on polycarbonate substrate (6 repeats).

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The soft TPV first elongates until the bond begins to peel at the peak force. A bond strength of approximately 20 N/mm was observed on PC and ABS, while a bond strength of approximately 8 N/mm was observed on nylon. The bonding failure on PC/ABS is cohesive, while the bond failure on nylon is adhesive. This excellent bond strength has resulted in silicone TPV being an ideal material for applications that require the combination of soft and rigid plastics, such as overmolded electronic equipment (soft-touch grips and buttons) and overmolded seals. The applications of silicones in the plastics industry continue to grow as more benefits are identified by combining the unique properties of thermoplastics and silicone.

16. SILICONES IN PERSONAL CARE APPLICATIONS Silicones used in personal care applications are of diversified types, including cyclic, linear, or organo-functional polydimethylsiloxanes (PDMS), as well as silicone elastomer dispersions and resins. This wide range of molecules provides benefits that impact the performance of almost every type of beauty product, conferring attributes such as good spreading, film forming, wash-off resistance, skin feel, volatility and permeability. The first use of silicone in personal care applications dates back to the 1950s, when a PDMS was incorporated into a commercial formulation to provide skin protection [129]. Since then, the use of silicones has kept increasing, along with the evolution of the knowledge around those materials (see Figure 30). Further to their first success, silicones made another

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breakthrough in the antiperspirant segment during the 1970s. Low molecular weight cyclosiloxanes were used as volatile carriers for the antiperspirant active, enhancing consumer acceptance of products thanks to the pleasant skin feel they could confer as well as their nonstaining properties [130]. Silicones then made their entry into hair care products. Amino-functional polymers were incorporated into styling mousses and rinse-off conditioners, while fluid or emulsion forms of high molecular weight PDMS were formulated into two-in-one conditioning shampoos. More recently, silicone elastomer dispersions were introduced to the market and gave formulators access to a new sensory dimension in terms of silkiness. Today, silicones find a use in virtually all types of personal care products, in segments as diversified as hair care, hygiene, skin care, sun protection or color cosmetics.

Figure 30. History of silicone uses in personal care.

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Types of Silicones Used in Personal Care Applications The versatility of silicones accounts for their wide use in beauty care products. This diversity stems from the unique set of physicochemical properties of PDMS as well as the variety of polymer types that can be used. Silicones incorporated into personal care products vary in molecular weight, structure or substituents attached to silicon atoms. The most commonly used silicones are linear PDMS of various viscosities, ranging from the shortest possible chain, hexamethyldisiloxane with a viscosity of 0.65 cSt, to polymers with high degrees of polymerization and viscosities over 106 cSt, often called silicone gums. Cyclic PDMS with 4, 5 or 6 dimethylsiloxane units is also widely encountered in formulations. Because of their volatility, low molecular weight linear and cyclic PDMS materials are often referred to as volatile silicones. Changing the structure and going from linear species to network or cross-linked systems leads to silicone resins and silicone elastomer dispersions. Such resins contain a number of T

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or Q units in a three-dimensional structure resulting from the hydrolysis/condensation of the corresponding initial silane monomers. The preparation of those materials is described in Section 1. Silicone elastomer dispersions are cross-linked gels that can be prepared through a hydrosilylation reaction. The reaction involves low levels of catalyst, usually platinum derivatives, and is generally run into an adequate solvent. SiH-containing silicone polymers are reacted with di-vinylic materials to link independent silicone chains. If the reaction is carried out in cyclic PDMS as the solvent, it leads to the formation a swollen and looselyreticulated network or a silicone elastomer dispersion. Substitution of methyl with other groups allows significant modification of PDMS properties, accessing other benefits. Most common are linear alkyl, phenyl, polyether or aminoalkyl groups. This leads respectively to silicone waxes (if alkyl groups of sufficient length are grafted onto the backbone), water dispersible polymers or substantive polymers. All these materials can be prepared by hydrosilylation, through the addition of various molecules bearing a vinyl group on a SiH-containing silicone polymer. Another route to such polymers involves the manufacturing of specific chlorosilanes to generate functional polymers after hydrolysis.

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Silicone Benefits in Beauty Care Products In skin care, a fundamental aspect is the “feel” provided, or how the product is perceived on skin upon and after application. Silicones convey a very differentiated feel to cosmetics, described as smooth, velvety, non-greasy and non-tacky [131]. They can also help diminish the tackiness induced by other raw materials present in the formulation. They are appreciated by formulators because of their film-forming properties, providing substantivity, wash-off resistance and protection. PDMS materials have been found to be noncomedogenic and nonacnegenic, meaning they are not expected to encourage undesired skin pore clogging or acne [132]. Their antifoam characteristics also help reducing the so-called “soaping effect,” an undesired foaming phenomenon observed in skin creams formulated with soap-based emulsifiers (see Section 3). Sun care products are devoted to protecting and reducing damage to skin induced by UV radiation. Here, the formulator’s goal is to create on skin a film of UV-protective actives as homogeneous and as resistant to water removal as possible, even after a swim. Low molecular weight silicones like cyclics are included in sun care formulas to improve spreading [131,133]. Because of their hydrophobicity, PDMS and in particular high molecular weight polymers, have demonstrated substantivity. In such formulations, the active can be made more resistant to wash-off. This helps maintain the level of sun protection of the formulated product after application on skin. In addition to wash-off resistance, alkylmethylsiloxanes have also been shown to enhance the sun protection factor (SPF) of products containing either organic or inorganic sunscreens. In color cosmetics, silicones are used to confer either a matte or a shine effect [134] Phenyl silicones, because of their higher refractive index, help produce glossy films. This accounts for their use in products such as lipsticks or lip glosses, where shine is sought after. On the contrary, if a matte effect is desired, as in foundation applications, silicone elastomer dispersions can be used, possibly because of their effect on light scattering. Alkylmethylsiloxanes are also appreciated because of their ability to provide, together with a

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pleasant feel, a waxy consistency and an increased compatibility with organic ingredients commonly used in such formulations [135]. Low molecular weight silicones are used in facial cleansers because of their low surface tension, good wetting properties and ability to remove dirt or color cosmetic residues, while delivering a dry and nongreasy feel [136]. Hair conditioning relates to softness, volume, body, sheen, feel and fly-away control [135]. This also includes hair protection from daily aggressions such as chemical treatments, combing or drying. Silicones are most often used in hair care because they can provide these conditioning benefits, consequently becoming key ingredients in shampoo or after-shampoo products. High molecular weight PDMS as well as aminoalkyl copolymers (also called amodimethicones) can deposit on hair and are particularly efficient in making hair easier to comb [129]. In the case of PDMS, a thin film is formed, bringing gloss and soft feel to the hair shaft [137]. When amodimethicones are exposed to an aqueous environment, some nitrogen atoms will quaternize and bear a positive charge. Because of its keratinic nature, the hair shaft bears a global negative charge when wet, especially if it is damaged. This generates an electrostatic interaction thought to promote deposition and anchorage of the polymer, thus enhancing conditioning. Other types of silicones are used in hair care. Volatile silicones can be incorporated to reduce drying time in some rinse-off applications like shampoos, [138] thus limiting the need for hair dryers and the resulting heat damage to the hair shaft. Silicone resins have been proven to enhance hair volume, [139] while silicone polyethers are used in hair styling products to help confer optimized form to hair [140]. In antiperspirants, which typically contain aluminum salts as the active, low molecular weight cyclic silicones are used as carriers, thanks to their volatility and noncooling perception, which leads to a dry feel. They also help prevent salt transfer and cloth staining, a problem associated with mineral oil based products. These cyclic silicones have allowed the development of new product forms such as roll-ons, providing alternatives to CFC-based aerosol formulations [135]. In hygiene applications, the amount of foam is an important parameter, as a shower gel producing a generous foam will be better perceived by the consumer [136]. Due to their amphiphilic nature, silicone polyethers can impact the water-air interface of the foam structure, resulting in an increase in volume or a stabilization of the foam generated by the cleansing surfactants of the formulation. Some of those polymers also have been shown to reduce the eye irritancy that can be produced by such anionic surfactants.

Silicones and Skin Feel One of the main reasons skin care formulators incorporate silicones in their formulations is the unique skin feel that silicones confer to cosmetics, which is often described as smooth, silky, elegant or luxurious. Silicones combine an array of properties (low coefficient of friction, liquid at high molecular weight, low surface tension) that impart a perceptively positive feel on skin [141]. Skin feel is a complex phenomenon affected by many variables, so it is difficult to characterize theoretically. A common way to assess sensory properties for a product is to perform sensory panel tests, where a set of trained panelists assess and characterize sensory parameters. Such evaluations confirmed that parameters such as stickiness, gloss, residue,

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tackiness, oiliness, greasiness and waxiness were almost never cited by panelists evaluating low molecular weight polydimethylsiloxanes, while spreadability and smoothness were often mentioned [141]. Skin feel is impacted by silicone structure. Increasing the length of the chain leads to silicone gums, which have been characterized as giving a velvety feel. Cross-linked silicone elastomer dispersions exhibit a further differentiated feel, which can be described as silky or powdery.

Volatility of Silicones Low molecular weight silicones are characterized by their high volatility, which influences sensory properties. These materials leave no residue on skin, providing a light feel, which is dependant on the relative volatilities of the silicones considered. Because of their low heat of evaporation (when expressed per gram), they do not need significant heat from the skin to evaporate and consequently do not create the strong cooling effect experienced with water or ethanol-based formulations (see Table 16). This property is particularly sought after in many applications such as antiperspirants, where low molecular weight silicones provide a differentiated dry effect upon use. Table 16. Heat of Vaporization for Some Volatile Fluids Used in Cosmetics Fluid PDMS, cyclic (DP = 4) PDMS, cyclic (DP = 5) Hexamethyldisiloxane Ethanol Water

Heat of vaporization (kJ/kg) 172 157 192 840 2257

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Permeability of Silicones PDMS polymers exhibit high permeability to gases. A noteworthy particularity is that this permeability is rather independent of their degree of polymerization, contrary to hydrocarbons (mineral oil vs. petrolatum). Neither does structure type (e.g., linear polymers vs. three-dimensional networks) significantly impact permeability. Table 17 gives comparative data for different families of silicones. Permeability is linked to both solubility and diffusion coefficient. Silicones are permeable because they have a relatively high solubility for a number of gases and also exhibit high gas diffusion rates compared to other common polymers. This last characteristic stems from their low intermolecular forces [142]. This behavior is of particular interest for skin creams as it means a silicone film will let water vapor from the dermis and epidermis evaporate and so let the skin “breathe.” In personal care, this property is called “non-occlusivity” and is desirable for products such as body lotions, which are applied to large areas.

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M. Andriot, S. H. Chao, A. Colas et al. Table 17. Permeability Data for Some Volatile Fluids Used in Cosmetics Fluid PDMS, cyclic (DP = 5) PDMS, linear (12,500 cSt) Silicone gum Silicone resin Mineral oil Alkylmethylsiloxane (C30+) Petrolatum

Water vapor permeability (g/m2/h) 155.7 107.4 148.6 110.5 98.0 1.4 1.3

However, occlusivity can be increased by substituting methyl groups along the siloxane backbone by longer alkyl groups, thus retaining skin hydration and plasticisation. Surprisingly, aesthetic properties are retained to a great extent [135-142]. Controlled moisturization can be obtained by varying the grafted alkyl group length or the degree of substitution on the polysiloxane chain [142].

17. MEDICAL APPLICATIONS Silicone materials celebrate 60 years of use in medical applications. Quickly after their commercial availability in 1946, methylchlorosilanes were described to treat glassware to prevent blood from clotting [143]. At the same time, Dr. F. Lahey implanted a silicone elastomer tube for duct repair in biliary surgery [144]. Since these pioneers, the interest for silicones in medical applications has remained because of their recognized biocompatibility. Silicones are used today in many life-saving medical devices like pacemakers or hydrocephalic shunts [145]. Silicones are also used in many pharmaceutical applications from process aids like tubing used to manufacture pharmaceuticals, to excipients in topical formulations or adhesives to affix transdermal drug delivery systems [146]. They also have found use as active pharmaceutical ingredients in products such as antacid and antiflatulent formulations [147,148].

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Polydimethylsiloxanes and Biocompatibility In medical devices and pharmaceutical applications, silicones are used because of their biocompatibility in a wide variety of physical forms. These forms range from volatile and low oligomers to high molecular weight polymers with viscosities from 0.65 cSt to 20x106 cSt to viscoelastic compounds and cross-linked elastomers. Biocompatibility is defined as “the ability of a material to perform with an appropriate host response in a specific situation” [149,150]. The impact of the biomaterial on its host environment is assessed according to approved standards (e.g., ISO 10993, USP and European monographs) aligned with the performance requirements for the intended applications. Overall, medical grade silicones, and in particular PDMS fluids or PDMS-based elastomers, satisfy the criteria of the above standards, including nonirritating and nonsensitizing behaviors, which explain their wide use in personal care and skin topical

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applications. A long history of use in medical devices, including long term implants, has made silicones widely recognized as biocompatible. These standards are yet addressing the impact from the host on the foreign material to a lesser extent, as data on biodurability are difficult to acquire. But again, silicones perform well as demonstrated by studies on PDMSbased elastomers explanted and showing good biodurability (Table 1, page 704 in [145]). Silicones with side-chain groups other than methyl (Me) are less used; PDMS polymers are the “preferred material,” even if some unlisted non Compendia/non Pharmacopoeia materials are now also well established, (e.g., silicone pressure sensitive adhesives for transdermal systems). Potential improvements with new silicones are hindered by rigid regulatory requirements, and innovation is sometimes limited to the use of current materials in new applications. Linking physicochemical properties to biocompatibility is not yet fully understood for many materials. Various factors are involved to explain the successful use of PDMS-based materials in medical devices or pharmaceutical applications: •

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•

Because of their backbone flexibility, PDMS materials can preferably expose their low interacting Me group substituents at many interfaces, leading to low surface tension, low surface energy and low intermolecular interactions, resulting in a low overall level of interactions at their surfaces. Therefore PDMS materials are among the most favored polymers when considering biocompatibility [4]. Their composition is well established. PDMS polymers do not require stabilizers because of their intrinsic stability. PDMS elastomers do not require plasticizers because of their low Tg. Hemocompatibility studies have suggested that silicone tubing may be superior to PVC tubing [151]. Impurities are well characterized siloxane oligomers and the toxicology profile of these oligomers has been investigated recently in detail (see Section 21). Other impurities are catalyst traces, such as acids or bases used in polymerization, but these are easy to eliminate and usually not an issue. Similarly, traces of platinum catalyst used at very low levels in cross-linking reactions may be present (platinum content 5 ppm to 20 ppm), and again this is usually not an issue. Only some tin catalysts used in room temperature curing materials or byproducts of peroxides used as initiators in some high consistency rubbers (HCR) have raised concerns [145].

Medical Devices and Pharmaceuticals Apart from their prevailing biocompatibility, other properties contribute to the use of silicones in medical and pharmaceutical applications: •

•

Because of their low liquid surface tension around 20.4 mN/m and slightly higher critical surface tension of wetting of 24 mN/m, PDMS polymers spread easily to form films over substrates like skin but also spread over their own absorbed film. Because of their viscoelastic behavior, resin-reinforced silicones or partially crosslinked elastomers (e.g. gels) have pressure sensitive properties. Their soft, rubbery behavior makes such silicones very appropriate materials for contacting biological

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•

tissues by minimizing the risk of trauma at the interface (e.g., low skin stripping force, gentle removability, no adhesion to wound bed). This allows their use in transdermal drug delivery and wound management applications to secure patches or dressings to the skin with minimum impact on the contacting area [152]. Because of their high permeability, silicones allow the diffusion of many substances such as gases (i.e., oxygen, carbon dioxide, water vapor) but also the diffusion of various actives (i.e., plant extract, drug, or even protein). This explains their use in personal care, skin topical applications or wound dressings (nonocclusive properties, no maceration) [153]. It also explains their use as adhesives or elastomers in controlled drug delivery systems [154-157].

Another practical aspect should not be ignored. Because of their stability, silicones are easy to sterilize by steam or ETO. Gamma or beta radiation sterilization require more precautions as they can induce radical reactions [158]. Overall, it is often an association of properties that supports the use of silicones in medical applications (see Table 18) [159-160]. Table 18. Correlations between Silicone Materials, Performance and Applications Key Physical Characteristics and Performance

Medical and Pharmaceutical Applications

Fluids - Polydimethylsiloxane - Organofunctional siloxane - Silicone polyether - Silicone alkyl wax

- Spreadability, film-forming - Diluent, dispersing property - Substantivity - Controlled occlusivity - Hydrophobicity - Lubricant property - Emulsifying property

- Siliconization of needles and syringes - Medical device lubrication - Excipients for topical formulations - Skin protecting composition - Drug carrier

Compounds - Silica + polydimethylsiloxanes

- Antifoam - Diluent, dispersing property

- Antiflatulent (APIs)

Gels (unreinforced elastomers)

- Softness - Resilience - Tackiness - Transparency - Adjustable cure conditions: from ambient to elevated temperature - Foamable

- Cushioning material - Gentle adhesive for skin (soft skin adhesive) - Wound interface (nonadherent wound dressing, foam dressing) - Soft matrix for drug release

Elastomers - Cross-linked polydimethylsiloxanes - Reinforced with silica - Various cure system: radical, hydrosilylation, condensation

- Rubbery property - Mechanical resistance - Adjustable modulus - Adjustable cure conditions: from ambient to elevated temperature - Adjustable cross-linking conditions - Foamable - In-situ film-forming

- Soft and resilient material for medical device - Recognized biocompatibility for human implantation (e.g., pacemaker) - Medical adhesive (sealant) - Film-former

Pressure sensitive adhesives (PSAs) - Silicate resin in polydimethylsiloxanes

- Tacky material - Adhesion to skin and various substrates (e.g., plastic films) - Substantive film-forming

- Temporary fixation of devices on the skin (e.g., wig, catheter) - Film-former - Transdermal drug delivery system

Silicone Materials

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Cross-linked polydimethylsiloxanes

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Medical Devices. Contradictory to pharmaceuticals, medical devices are articles or associations of articles used in health care to support therapeutic treatments and assist patient life without pharmacological effects and interferences with biological processes. Silicones are used as components or fabricating materials in many such devices. Silicone fluids are used to lubricate or “siliconize” many medical surfaces like syringe pistons and barrels. The result is reduced “jerk” during injections or on needles, thus reducing pain [143-161]. Silicone polymers are easily converted into elastomers by creating covalent bonds between adjacent macromolecules to form three-dimensional networks [3]. Various chemical reactions are available to cross-link or cure silicone polymers (see Section 1): •

•

•

Condensation cure between hydroxy, alkoxy or acetoxy groups in presence of tin or titanium catalysts, with liberation of water, alcohol or acetic acid and formation of Si-O-Si bonds Radical initiated cure and reactions between alkyl and/or alkylene groups using peroxide to form Si-alkyl-Si bonds, but requiring post-cure to eliminate peroxide byproducts Addition cure or hydrosilylation of vinyl functional polymers by hydrogen functional siloxanes in the presence of platinum catalyst to form Si-CH2-CH2-Si bonds; in many applications, this reaction is preferred (addition reaction without byproducts; low level of Pt catalyst: 5 ppm to 20 ppm as Pt)

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Physical properties are adjusted by controlling the cross-linking density and using reinforcing fillers, usually fume silica. Barium sulfate is added when radiopacity is required. Because cross-linking points are far apart, dimethylsiloxane segments are most likely to be exposed on the surface of these elastomers, so elastomers display the good biocompatibility associated with PDMS fluids. Various methods are used like casting, molding and injection to produce parts, or extrusion to produce tubing. Applications range from short term, noninvasive devices to critical, long term devices and are as diverse as long term implants like mammary implants (not without controversy), pacemaker leads, peristaltic tubing in heart-bypass machines or hydrocephalus shunts for regulating cerebrospinal brain fluid (see Figure 31) [145,162-164].

Figure 31. Long term implant: silicone elastomer valve and tubing of a hydrocephalic shunt (Picture courtesy of Medos, a Johnson and Johnson company).

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Silicone coatings (solvent-based elastomer dispersions) are also used over other materials like natural latex to reduce adverse effects (see Figure 32).

Figure 32. Short term implant: comparison of the incrustation on a silicone-coated latex catheter (top) vs. a latex catheter (bottom) (Picture courtesy Dow Corning).

Silicone gels, adhesives and foams are part of various wound dressings used to reduce nursing costs, but also to improve comfort and therapy [165]. Silicone gels in particular have been successful in this case. These soft, relatively cohesive and tacky gels are platinum-cured elastomers without reinforcing fillers and are used: -

-

-

As filling materials in cushions to prevent pressure sores In wound dressings because of their permeability to oxygen and water vapor (no maceration dressing), with gentle adhesion to skin around a wound, but with nonadherence to damaged tissues and the healing wound bed In scar treatment where a dressing such as silicone gel sheeting has demonstrated its efficacy for the treatment of keloid and hypertrophic scars, as confirmed by various studies including a meta-study [166,167] More recently, the release of actives from adhesives has been investigated with enzymes for the debridement of necrotic tissues [153].

Pharmaceutical Process Aids. Silicones are commonly used as process aids in the production of pharmaceuticals like:

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•

• •

Silanes as temporary protective agents in the synthesis of complex molecules such as antibiotics (e.g., penicillin or cephalosporin). Specific groups are protected by silylation (e.g., carbinols reacted with trimethylcholorosilanes to form a Si-O-C bond later easily hydrolyzed to recover the active molecule) [168,169] Antifoams in fermentation process (see Section 3) Silicone tubing used to prepare drugs or vaccines in various fluid transfer operations, peristaltic pumping and filling operations. This tubing helps reduce investment costs in fixed stainless steel lines and, particularly for single-use applications, to eliminate costs associated with validation of cleaning-in-place (CIP) or sterilization-in-place (SIP) and disposal of contaminated waste waters [170-172]. Innovative biotech processes take advantage of silicone elastomer properties such as their gas permeability in fermentation cell systems, in which the oxygenation is directly achieved via gas permeation through the silicone tubing wall [160].

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Pharmaceutical Ingredients. Silicones are present in many pharmaceutical finished drug products, and more than 350 products containing silicones are listed in various compendia [173]. Cyclics (Cyclomethicone NF) are used in topical products because of their good spreading and volatility, with low heats of evaporation per gram of formulation (resulting in no cooling effect on the skin) [173]. In the US, silicone fluids (Dimethicone NF) around 1,000 cSt are recognized as skin protectants for use in over-the-counter products [174]. This benefit is exploited in creams and ointments, and is most likely due to the high spreadability and high hydrophobicity of PDMS. Increasing the molecular weight, using PDMS gums (fluids with viscosity around or higher than 600,000 cSt), leads to interesting film-forming materials that are transparent, long-lasting on the skin and capable of improving the substantivity of personal care ingredients as sunscreens or active pharmaceutical ingredients (APIs) (e.g., ketoprofen) [175]. Polydimethylsiloxanes alone (dimethicone) or compounded with silica (simethicone) are used in gastroenterology for their antifoam properties. They reduce foaming in the stomach without modifying the gastric pH, and are thus used in many antiflatulent/antacid products, in particular in countries using hot spices. They are considered an API, but their mode of action is physical; they are not metabolized but excreted as such [176]. Silicone pressure sensitive adhesives (PSAs), which are PDMS/silicone resin networks, are used in numerous transdermal drug delivery systems (TDDS) to fix the drug device onto the skin (see Figure 3) [177]. These are viscoelastic compounds in which the PDMS fluid contributes to the wetting and spreadability of the adhesive and the resin, acting as the reinforcing agent, to the elastic rheological component. Because of the PDMS permeability, these PSAs allow the slow and controlled diffusion of various actives for various treatments: nitroglycerin (angina pectoris), estradiol (hormone replacement), fentanyl (pain management) and others. Both reservoir and matrix systems are known, the latter often considered because of its greater construction simplicity [177]. Silicone elastomers are used in drug-loaded pharmaceutical devices for the release of various APIs such as levonorgestrel in a subcutaneous contraceptive implant or 17 betaestradiol in a vaginal ring for the treatment of urinary problems associated with menopause. In these reservoir devices, the release of the API is controlled by the permeability of the PDMS cross-linked network [176].

Figure 33. Transdermal drug delivery system or patch with a silicone pressure sensitive adhesive (Picture courtesy Dow Corning).

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In all the above applications, silicones have been considered because of their contribution to biocompatibility (medical devices), ease of use (pharmaceutical process aids) or improvement of comfort and/or treatment, allowing lower and local dosage forms with fewer side effects or making wound dressings easier to apply for potentially better compliance [177].

18. SILICONE LUBRICANTS IN INDUSTRIAL ASSEMBLY AND MAINTENANCE In this industrial segment, silicones are used as sealants (see Section 8) or lubricants. Lubricants will therefore be the focus of this section. Silicone lubricants are not limited to assembly and maintenance, as they are also used in other industries like the automotive, chemical or food industries. Historically, silicones have been used as lubricants right from the start of the industry in the form of a silica-thickened PDMS compound, which is sometimes referred to as a “noncuring sealant,” and well known to chemists as “vacuum grease” for lubricating glassware joints [178]. This application highlights the properties of silicones that make them superior to other lubricating liquids in some applications; for example, their possible use over a wide range of temperatures from synthesis at low temperatures as with liquid ammonia to distillation under vacuum at high temperatures.

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Tribology and Lubrication Mechanism Tribology is the engineering discipline that studies the friction and wear phenomena occurring between two moving surfaces in contact with each other as well as the mechanism of lubrication. Friction is physically characterized by the coefficient of friction and as the ratio of the force required for moving two surfaces to the applied force perpendicular to the moving direction. Friction consists of an adhesive and a destructive component. The later results in wear of various forms [179]. Coefficients of friction range from 0.0001 to 0.0005 for air bearings as used in dental drills up to 0.3 to 0.5 for automotive brake systems. Lubrication is basically a reduction of both wear and friction by generating a lubricating film between the moving surfaces. There are three modes of lubrication characterized by the ratio of lubricating film thickness to the sum of both surfaces’ roughness: • • •

Boundary lubrication, when the ratio is smaller or equal to 1, and when the surface asperities interfere with each other resulting in a high coefficient of friction Mixed lubrication, when the ratio is between 1 to 5, and when the surface asperities occasionally interfere with each other due to load variation Fluid film lubrication, when the ratio is larger than 5, and when a complete separation of the two moving surfaces is achieved, resulting in a low coefficient of friction

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The generation of a fluid lubricating film can be achieved by pressurizing the lubricating fluid via an external pump as done for turbine start-ups, but this is an exception. In the majority of fluid film lubrication, the geometries are designed in such a way that the necessary pressure is built internally within the fluid itself by the velocity of the surfaces in movement. The velocity profiles in the lubricating contact zone are a combination of Couette’s flow with linear velocity distribution and Poiseuille flow with a parabolic velocity distribution. The fluid flow is forced through a wedge that generates a pressure profile according to Bernoulli’s law. The pressure generation is similar to that of aircraft wings. In this analogy, one can compare the change from mixed to fluid film lubrication with the takeoff of an airplane, which is the minimum of the curve in Figure 34 (see further). So, the concept of lubrication is to separate two moving surfaces with a “softer and easier-to-shear” liquid material or lubricant located between the surfaces, and to build up enough pressure in the liquid to separate the two moving surfaces and reduce the coefficient of friction or the force needed to move them against each other under an applied load. As seen in Figure 34, at low speeds the lubricant does not have sufficient internal pressure to separate the two slow-moving surfaces, resulting in high friction. As speed increases, the internal pressure induced by shear in the lubricant separates the two moving surfaces and brings the coefficient of friction to a minimum. At higher speeds, the coefficient of friction increases again due to the work required to shear the lubricant. Figure 34 also shows that for lubricants of similar composition but of different viscosities, the higher the lubricant viscosity, the earlier surface separation or lubrication occurs, and also that optimum performance is therefore a function of the applied shear, or a corresponding rotational speed in many cases. This shows that successful lubrication depends on the selection of the most suitable fluid vs. the particular application conditions like speed as well as load, environmental aggressions and temperature.

High Coefficient of friction, μ

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Average

Viscosity

Low

Rotational speed, rpm

Figure 34. Coefficient of friction vs. rotational speed in a journal bearing, a plain bearing without rolling parts. Picture courtesy Dow Corning GmbH [180]

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Polymeric Lubricant Composition Among various liquids, polymeric materials have come out as the best option for lubrication. If considering the Mendeleev table, only two elements are liquid at room temperature, bromine and mercury, but neither is suitable as a lubricant (due to reactivity and toxicity). As liquid, water also comes to mind. But even though water has good lubricating properties as demonstrated by floating movements of boats and various ice compressing forms of movement, the limitation is that water is only available in one single and low viscosity, and it is liquid only in a narrow range of temperatures; not to mention its corrosiveness that further precludes its use as a lubricant. Actually, the same holds true for many other low molecular weight chemical species. So, this explains why lubricating fluids are mostly polymeric in nature; for example, organics such as mineral oil based fluids with various degrees of paraffinic, naphtenic or aromatic content. Or, they may be synthetic fluids like poly alpha olefins, neopolyol esters, polyalkylene glycols, dibasic esters, phosphate esters, polybutenes, dialkylbenzenes and perfluorinated polyethers, as well as silicones like PDMS [181]. Because of their low Me-to-Me intermolecular interactions and high backbone flexibility, PDMS materials have a low Tg and are liquid at room temperature, even if of high molecular weight. PDMS materials have high boiling points, and their viscosity is less affected by temperature changes than organics. These properties make PDMS polymers interesting as possible lubricants. Yet as their surface tension is low, they tend to spread on surfaces more than organic lubricants. High spreading and high compressibility limit the internal pressures than can build within PDMS materials when used as lubricants and limit their load-carrying capacity if compared to organic lubricants of the same initial viscosity. Today, three types of silicones are used as lubricants in industrial assembly and maintenance applications: • •

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•

Dimethyl siloxane polymers (PDMS, known as dimethyl silicone) Phenylmethyl dimethyl siloxane copolymers with phenyl substitution from 10 to 90% (known as phenyl silicone) Trifluoropropylmethyl dimethylsiloxane copolymers (known as fluorosilicone)

Silicones, like mineral oils and most synthetic lubricant fluids, are also compounded with thickeners such as metal fatty acids to give lubricating greases capable of keeping the lubricating fluid in close contact with the surfaces in movement. The thickener can be pictured as a sponge that holds the lubricating fluid in place, and such greases are used when total sealed enclosure is not possible. The fluids are further formulated with additives to improve the physical properties of the fluid itself or to add capabilities for mixed and boundary lubrication. Such formulations still represent a challenge for silicones beyond fluid film applications; the range of available additives is limited because these additives were tailored for organic-based materials.

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Examples of Silicone Lubricant Applications Each lubricant application is characterized by its specific operating conditions, which are load, environmental aggressions, temperature and speed. Load is a limiting factor for silicone lubricants, particularly in metal-to-metal lubrication; so when other conditions require a silicone lubricant, the dimensions of the lubricating contact surfaces may need to be increased. Fluorosilicone lubricants have higher loadcarrying capacity due to their higher adhesion to metal substrates. However, for all metal-toplastic or plastic-to-plastic combinations, silicone lubricants have sufficient load-carrying capacity. Environment aggressions have less effect on silicones if compared to organic lubricants. The oxidation resistance of silicones makes them suitable for long-life applications. Because of their inertness to most chemicals, silicone lubricants are widely used in the chemical industry, and also in food and beverage processing. Though the load-carrying capacity makes silicones a candidate for plastic lubrication, it is their inertness with almost all plastics or elastomer materials that makes them ideal in these applications. Poor compatibility is experienced only when silicones have to lubricate silicone elastomer surfaces because of the swelling they induce in the silicone elastomers. Temperature capability of silicone-based lubricants is unsurpassed as covering the widest range. Speed or better “high shear by design” is required for silicone lubricants in metal-tometal applications so as to generate enough internal pressure and load-carrying capacity. For plastic lubrication and when using a fluorosilicone lubricant, lower speeds are possible. Table 19 compares the three types of silicones used as lubricants vs. organics. Table 19. Silicone Lubricant Properties vs. Those of Organics [182] Lubricant

DP (*)

MW

Viscosity at 40 oC

Pour point o

o

Da

cSt

20 – 60

150 - 450

5 – 50

-63 to -57

165 - 258

10 – 180

1100 – 13,000

4 – 500

-90 to -30

n.a.

Dimethyl silicone

20 – 1,300

1,500 – 100,000

15 - 45,000

-60 to -41

230 - 316

Phenylmethyl

70 – 500

5,600 – 40,000

40 - 700

-73 to -13

275

40 – 100

5,000 – 10,000

150 - 5,300

-47 to -32

260 - 316

Poly alpha olefine

C

Flash point C

(PAO) Perfluorinated

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polyether (PFPE)

silicone Fluorosilicone

* DP: degree of polymerisation

Practical examples are given in Figures 35 through 38 [183].

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Figure 35. Clutch release bearing with a phenyl silicone grease, which has wide temperature capabilities. Picture courtesy of Dow Corning GmbH.

Figure 36. High power alternator and bearings lubricated with a fluorosilicone grease, which offers resistance to high temperatures. Picture courtesy of Dow Corning GmbH.

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Figure 37. Starter motor with silicone grease lubricant, which provides wide temperature capabilities and a high coefficient of friction to allow decoupling and prevent slippage. Picture courtesy of Dow Corning GmbH.

Figure 38. Brake systems (right to left, calliper guides and brake booster) with silicone lubricants, which give wide temperature capabilities and compatibility with plastics and elastomers. Pictures courtesy of Dow Corning GmbH.

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19. ORGANO-FUNCTIONAL SILANES

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FLEXURAL STRENGTH, MPa

The synergy between organic and silicon chemistries has been investigated for more than 50 years, and has lead to the development of many organo-functional silanes that are essential today in many applications [184-186]. Monomeric silicon chemicals are known as silanes. A silane that contains at least one silicon-carbon bond (e.g., Si-CH3) is an organosilane. The carbon-silicon bond is very stable and nonpolar, and in the presence of an alkyl group it gives rise to low surface energy and hydrophobic effects. Organo-functional silanes are molecules carrying two different reactive groups on their silicon atom so that they can react and couple with very different materials (e.g., inorganic surfaces and organic resins via covalent bonds and often via a polymeric “transition” layer between these different materials). The value of organo-functional silanes as coupling agents was discovered in the 1940s, during the development of fiberglass-reinforced composites [184]. When initially fabricated, these new composites were very strong, but their strength declined rapidly during aging underwater. This weakening was caused by a loss of bond strength between the glass fibers and the resin. Researchers found that certain organofunctional silanes prevented ingress of water and bond displacements at the fiber/resin interface but also significantly increased the composite initial strength (see Figure 39).

DRY STRENGTH

WET STRENGTH 72 hours boiling water

Figure 39. Effect of silane coupling agents on the strength of glass-reinforced epoxy.

Other applications were later discovered for such silanes, like the treatment of fillers to increase reinforcement, as additives in inks, coatings and sealants to improve adhesion or in plastics and rubbers to allow for cross-linking.

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Chemistry of Coupling with Organo-Functional Silanes Organo-functional silanes have the following typical molecular structure: X-CH2CH2CH2Si(OR)3-nR’n where n = 0, 1, 2 Many combinations are possible, but these are characterized by the fact that they contain two different types of reactive groups. The OR groups are hydrolyzable groups such as methoxy, ethoxy or acetoxy groups. The group X is an organo-functional group, such as epoxy, amino, methacryloxy, or sulfido. The presence of some Si-alkyl groups ensures low surface tension and good wetting properties (see Figure 40). CH2

OEt

MeO

NH2

EtO Si

MeO

CH Si

EtO OMe

Amino-silane γ-aminopropyltriethoxysilane

Vinyl-silane Vinyltrimethoxysilane

O H2C

CH3

O

MeO MeO Si

O

O

MeO

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OMe

Methacryloxy-silane Methacryloxypropyltrimethoxysilane

Si

OMe

MeO

Epoxy-silane γ-Glycidoxypropyltrimethoxysilane

Figure 40. Examples of organo-functional silanes showing the two different functionalities available for reaction on the Si atom: hydrolyzable alkoxy groups and organic-functional group.

The Si-OR bonds hydrolyze readily with water, even if only with moisture adsorbed on the surface, to form silanols Si-OH groups. These silanol groups can then condense with each other to form polymeric structures with very stable siloxane Si-O-Si bonds. They can also condense with metal hydroxyl groups on the surface of glass, minerals or metals to form stable Si-O-M bonds (M = Si, Al, Fe, etc…). This allows surface treatment, coupling and assembling of very dissimilar surfaces chemically, as between inorganic and organic materials (see Figure 41).

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Figure 41. Organo-functional silane hydrolysis, condensation and covalent bonding to an inorganic substrate.

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The organo-functional silane concentrations used here are such that more than a monolayer is being built at the interface. A tight polymeric siloxane network is created on the inorganic filler or metal surface, which becomes more diffuse into the adjacent organic resin. The properties of the organo-functional silane should match the reactivity of the resin with appropriate groups on the silane to react with the resin (e.g., epoxy or amino groups to react with epoxy resins, amino groups to react with phenolic resins or a methacrylate group to react with styrene in unsaturated polyester resins). But also the organo-functional silane should match the solubility parameter of the adjacent resin to ensure a smooth transition at the interphase. The formation of an interpenetrating network (IPN) at the boundary interphase appears essential and probably also explains the improved adhesion observed with thermoplastic polymers (see Figure 42) [187,188]. Organo-functional silanes have shown greatest benefits in three areas: mineral filler treatment, cross-linking and as adhesion promoters.

Figure 42. IPN structure created by an organo-functional silane at the interphase between an inorganic glass, mineral, metal substrate (M = Si, Al, Fe, ...) and an organic polymer.

Mineral Filler Treatment. Mineral fillers have become increasingly important modifiers for reinforcing organic polymers, thermoplastics or thermosets. Yet, the metal hydroxyl groups on the mineral filler surface are hydrophilic, and this translates to incompatibility with organic polymers. Organosilanes are ideal for treating the filler surface, making the filler

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more compatible and easier to disperse in the polymer. Any minerals with silicon or aluminum hydroxyl groups on their surfaces (e.g., silica, glass bead, quartz, sand, talc, mica, clay or wollastonite) can be treated with organo-functional silanes. These will ease dispersion of the fillers and improve wetting by, and adhesion to, the polymer. This results in lower filler/polymer mix viscosities and improved mechanical properties [189]. A typical example is the sulfido-silanes: (OR)3Si-(CH2)3-Sx-(CH2)3Si(OR)3

where x = 2 to 8

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Selecting the adequate sulfido-silane enables surface treatment of the silica used in green tires and bonding to organic rubber, which was proven extremely effective for optimizing the viscoelastic and mechanical properties of the silica-rubber composite for “more miles per gallon.” Cross-Linking. Polymers and polymeric composites are becoming increasingly attractive as engineering materials. They are highly competitive compared to metal or metal alloys due to their low cost and low density, ease of compounding using extrusion or injection molding processes, and inherent lack of corrosion-related problems. One way to improve performance of such plastics is to cross-link them to some degree. One well-known example using organo-functional silanes is the cross-linking of polyethylene to give partially cross-linked polyethylene or PEX [190-191]. This is achieved by grafting vinyl-functional alkoxy silanes on the PE chains using peroxide as an initiator. The vinyl groups allow for grafting on the PE backbone, and the alkoxy groups allow for subsequent cross-linking between the PE chains upon exposure to heat and moisture. The main applications are for piping of various kinds (e.g., under floor heating, drinking water) and wire and cable insulation. Similarly, cross-linking is used to enhance mechanical properties in thermoplastic vulcanisates (TPVs), through dynamic vulcanization process and where the silanes play many roles: cross-linker, adhesion promoter and even intermediate to generate in situ filler. Adhesion Promoter. Organo-functional silanes are known for surface modification. So as additives, they can enhance adhesion between dissimilar materials because of their low surface tension (which ensures good surface wetting), their reactivity to different surfaces and their ability to create interactions and make an adequate transition interphase between the adhesive layer and the substrate to bond [186,192,193].

Trends and Perspectives Today, there are two major trends: • •

The optimization of organo-functional silane molecules and conditions for processing them, aiming to reduce emissions of volatile organic compounds (VOCs) The design of new and sustainable composite materials with improved end-user benefits, taking advantage of the wide variety that commercially available silanes or those under development offer in terms of functionalities, reactivities and processing flexibility at relatively mild conditions.

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Low VOC Silanes and Processes. Conventional organo-functional silanes rely on the hydrolysis of their Si-OR groups and subsequent condensation for their coupling with inorganic surfaces or cross-linking within plastic matrices. Human health and environmental concerns are leading to the development of new products with less hydrolysis/condensation byproducts such as hydrolyzed, lower alkoxy-containing intermediates or solventless products. Prehydrolyzed silanes under well controlled conditions, [194,195] water-based silane solutions, or solid carrier supported silanes that could be added during plastic extrusion, and plasma surface treatment in presence of silanes are among the approaches currently investigated to address VOC issues. Design of New Materials. Sustainable composites that exploit the reinforcement properties of natural fillers like cellulose are being developed, in which silanes are considered to “manage” the highly hydrophilic nature of the surfaces of such fillers to improve compounding and load transfer to the surrounding plastic matrix [196-197]. Sol-gel processes refer to the polymerization in aqueous or organic medium of metal alkoxides into a monolithic gel via the formation and growth and/or network extension of discreet nanoparticles [198]. As such, traditional materials generated via sol-gel process include stable silica sol and colloids [199], thin films and coatings, composites such as ceramics generated by specific drying conditions of aerogels or xerogels [200], fibers, porous gels and membranes [201]. The potential added properties sol-gel materials bring to plastics in general therefore encompasses a wide variety of properties, including antigraffiti, antimicrobial, antifouling, anticorrosion [202], optical, protective, adhesive or anti-adhesive, mechanical, dielectric [203], and reinforcing.

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20. PLASMA AND SILICONES The use of plasma in conjunction with silicones is a new application field that allows interesting surface modifications. The term “plasma” covers a broad range of systems whose density and temperature vary by many orders of magnitude. Some plasmas, particularly those at low pressure (e.g., 100 Pa) where collisions are relatively infrequent, have their constituent species at widely different temperatures and are called “nonthermal equilibrium” plasmas. In these nonthermal plasmas the free electrons are very hot with temperatures of many thousands of Kelvin (K), whilst the neutral and ionic species remain cold. Because the free electrons have almost negligible mass, the total system heat content is low and the plasma operates close to room temperature, allowing the processing of temperature-sensitive materials, such as plastics or polymers, without imposing a damaging thermal burden onto the sample. However, the hot electrons create, through high energy collisions, a rich source of radicals and excited species with a high chemical potential energy capable of profound chemical and physical reactivity. It is this combination of low temperature operation plus high reactivity that makes nonthermal plasma technologically important and a very powerful tool for manufacturing and material processing. These properties provide a strong motivation for industry to adopt plasma-based processing, and this move has been led since the 1960s by the microelectronics community, which has developed “low pressure glow discharge plasma” into a high technology

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engineering tool for semiconductor, metal and dielectric processing. The use of plasma to deposit thin dielectric films is often referred to as plasma-enhanced chemical vapour deposition (PECVD) processing. Various precursors are available, specifically designed for the deposition of thin film dielectrics via PECVD and compatible with copper dual damascene and aluminum interconnect processes. These precursors are: •

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•

Gases like Me3SiH, which can be used with processing technology developed for silane-based dielectric film deposition Liquids like Me4Si, (SiHMeO)4 and SiMe2(OMe)2, which can be used with processing technologies developed for TEOS-based dielectric film deposition

Typical thin-film dielectrics formed by these precursors include silicon-carbide (aSiC:H), silicon-oxycarbide (a-SiOC:H) and silicon-nitride (a-SiCN:H). Typical applications include interlevel dielectric, copper diffusion barrier, etch stop, hard mask, low-k interlevel dielectric, gap fill, and passivation. Vacuum or low-pressure plasma has increasingly penetrated other industrial sectors since the 1980s, offering processes such as polymer surface activation for increased adhesion/bond strength, high quality degreasing/cleaning and the deposition of high performance coatings. However, due to operation at reduced pressure, processing is restricted to batch wise or at best is pseudo-continuous and thus not applicable to in-line production. Therefore, newly developed atmospheric pressure plasmas offer industry open port or perimeter systems providing free ingress into and exit from the plasma region by work-pieces/webs. Hence, atmospheric pressure plasma offers new continuous, on-line processing capability for many industrial sectors, such as textiles, packaging, paper, medical, automotive and aerospace. The work of Okazaki et al. in the 1980s showed that a stable glow discharge could be readily formed at atmospheric pressure, [204,205] which ignited a volume of research and a wide variety of plasma systems that now operate at atmospheric pressure. The early work by Okazaki focused on generating plasmas using helium as the process gas. Later this was extended to include argon and nitrogen. Further developments have produced atmospheric pressure plasmas in a wide variety of gases, including air [206]. The exact conditions employed vary depending upon the gas, electrode geometry and other factors. Typically these ambient temperature atmospheric pressure plasmas are referred to as diffuse dielectric barrier discharge, [207] a term generally used to cover both glow discharges and dielectric barrier discharges that are homogeneous plasmas across the width and length of a plasma chamber [208]. Technology is now available to combine unique precursors and their delivery into an atmospheric pressure plasma operating at ambient temperature to achieve deposition. This process is known as atmospheric pressure plasma liquid deposition (APPLD). Such APPLD equipment comes in two configurations: • •

Large-area plasma for processing flexible webs such as textiles, nonwovens, paper, films and foils, fibres, thread, yarn or filament Jet plasma for processing three-dimensional, rigid sheet materials or material in fibre/filament form

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By directly injecting an aerosol of liquid precursor into a homogeneous atmospheric pressure plasma, a thin conformal layer of polymerised coating can be deposited onto a substrate surface that is in contact with the plasma. Typically, these coatings are some tens of nanometres thick. The combination of liquid precursor and diffuse atmospheric pressure plasma ensures that this process retains all the original functional properties of the liquid precursor – even for large, complex molecules. This is a property unique to APPLD, as almost all other atmospheric pressure plasma processes destroy complex precursors (see Figure 43).

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Figure 43. XPS (ESCA) spectra of a polyester film after plasma treatment with a polyhydrogenmethyl siloxane polymer precursor, M(DH)nM, using the APPLD technology. The presence of peaks corresponding to T and Q units indicate that some modification of the original polymer has occurred, leading to cross-linking. But most of the polymer’s original functionality remains as indicated by the strong peak corresponding to DH units.

This enables tailoring of the surface chemistry with a specific chemical functionality and/or a specific surface response. This surface engineering can be applied to a variety of different substrate classes for a wide range of applications. Thus, advanced surface properties that include biofunctionality, oil repellency and adhesion promotion are now available from APPLD technology, offering the prospect of plasma processing penetrating a wide range of new, high-value industrial applications. Due to the unique nature of the APPLD process, a wide variety of silicones can be utilised as precursors to provide specific surface properties. For example, polydimethylsiloxane (PDMS) polymer coatings are widely used [209] for their excellent hydrophobic properties, which increase water repellency, release and “handle.” Tetramethylcyclotetrasiloxane and octamethylcyclotetrasiloxane have been successfully used as precursors to produce polysiloxane coatings, which have been shown to provide a water

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contact angle of 140° on a cotton substrate, whereas a water droplet applied to nontreated cotton wets out immediately (see Figure 44).

Figure 44. A water droplet on a silicone plasma treated cotton fabric. Picture courtesy of Dow Corning Plasma Solutions.

In oxidising plasma conditions, low molecular weight PDMS precursors are converted to silica-like (SiOx) coatings. The APPLD process is an alternative route to depositing organosilane molecules, without the requirement of using water or organic solvents.

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21. SILICONES AND TOXICOLOGY As described in the previous sections, silicones are used in a wide variety of applications. These silicones include low molecular weight linear and cyclic volatile oligomers or volatile methyl siloxanes as well as polydimethylsiloxane (PDMS) polymers with viscosities ranging from 10 to 100,000 cSt or higher. Volatile methyl siloxanes (VMS) like cyclic siloxanes, (SiMe2O)n, are widely used in skin care products, in particular the four (n = 4) and five (n = 5) member cyclics referred to as D4 and D5, respectively [210]. Extensive safety studies conducted on D4 and D5 have indicated effects that appear to be rat specific and, therefore, pose little or no risk to human health [211,212]. The effects observed with D4 include a reduction in litter size and in the number of implantation sites in the uterus and an increase in uterine endometrial hyperplasia and adenomas [213,214,215]. The fertility effects and uterine adenomas occur at the highest vapor exposure concentration achievable without formation of an aerosol (i.e., 700 ppm) and by modes of action that appear to be rat-specific [211,213,216,217]. Exposure to D5 at the highest achievable vapor concentration of 160 ppm caused an increase in uterine endometrial adenocarcinomas that is presumed to occur by a rat-specific mode of action like D4 [212,216,218]. Both D4 and D5 cause a non-adverse, adaptive increase in liver weight that is considered phenobarbital-like [219,220]. Neither of these materials are mutagenic or genotoxic nor are they immunotoxic [211]. Typically, D4 and D5 show around 0.5% and 0.05% dermal absorption, respectively [221,222,223]. Following dermal absorption, >80% of D4 and >90% of D5 is eliminated in expired air within 24 hours of exposure [224].

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The lowest molecular weight linear material is the highly volatile hexamethyldisiloxane, Me3SiOSiMe3 (HMDS). HMDS has generally shown no significant toxicity. However, recent data have indicated an earlier incidence of testicular tumors in male rats exposed to high levels of material via inhalation [225]. In this same study, there was also an increase in the incidence of kidney tumors in male rats, which have been shown to be mediated through a protein, α-2u-globulin, which is specific to male rats [225]. Other linear molecules of three, four, or five siloxane units have not exhibited hazards in studies to date, though the data are limited for long-term exposure [226]. The materials have very limited absorption via typical exposure routes. Like the higher molecular weight polymers, the low molecular weight linear PDMS materials are not mutagenic, irritating or acutely toxic [226]. The most widely used silicones are the trimethylsilyloxy end-blocked PDMS polymers, Me3SiO (SiMe2O)n SiMe3, with viscosities between 10 to 100,000 cSt. These materials have shown no toxicity during administration via typical exposure routes, which are either oral or dermal [227]. Due to their high molecular weight, they are neither absorbed from the gastrointestinal tract nor through the skin [228,229]. Following oral ingestion, PDMS is excreted in the feces without modification. In vitro studies have not indicated mutagenic or genotoxic effects. Repeated oral or dermal dosages of different viscosities demonstrated no adverse effects to a variety of mammalian species. Inhalation of aerosols of oily or fatty-type materials, including some kinds of silicones, into alveolar regions of the lung may result in acute toxicity that is likely related to physical disturbances of the lining of the lung with associated effects. There is no evidence of reproductive or teratogenic effects of PDMS from studies conducted with rats or rabbits. Overall, these data show no hazard of PDMS to humans [227].

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22. SILICONES AND THEIR IMPACT ON THE ENVIRONMENT A large number of studies have been conducted to evaluate the fate and effects of silicones in the environment throughout their life cycle [227]. Releases to the environment from the manufacture of polydimethylsiloxane (PDMS) are strictly controlled and must comply with emission limits specified by regulatory authorities. Subsequently, the environmental fate of silicones depends to a large extent on the nature of the application, the physical form of the material and the method of disposal. Low molecular weight PDMS polymers (< 1000 Da) are primarily used in personal and household care products. High molecular weight PDMS polymers are important as antifoams and lubricants for domestic and industrial use. However, a more important application is as a “solid” silicone such as PDMSbased rubbers or sealants, both of which may be used either in the home (e.g., bath sealants, bake-ware or baby teats) or diverse industrial applications such as textile coatings, electronics, silicone mouldings and rubber gaskets. “Solid” silicones enter the environment as a component of domestic or industrial waste and will be either land filled or incinerated. In the latter case, they are converted back to inorganic ingredients, amorphous silica, carbon dioxide and water vapour. “Liquid” silicones, both high and low molecular weights, which are used in rinse-off products such as shampoos, hair conditioners or silicone antifoams in detergents, become part of municipal wastewater. The same is true for PDMS used as antiflatulents in pharmaceuticals. High molecular weight

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silicones, are virtually insoluble in water, thus, as a consequence of their high binding potential for organic matter, they are effectively removed from municipal wastewater onto the sludge during wastewater treatment. Extensive studies show that more than 95% of silicones are removed from effluents in this way, and that the concentration in discharged effluents borders the level of detection (5 μg/l). [230,231]. The subsequent fate of silicones depends on the fate of the sludge. If incinerated, silicones degrade as indicated above. The other principal outlet for sludge is use as a soil conditioner or amendment. In small-scale field studies, the application of sewage sludgebound PDMS to soil caused no observed adverse effects on crop growth or soil organisms [232]. Little or no uptake into the plants was observed, which is consistent with animal studies showing that high molecular weight PDMS is too large to pass through biological membranes of either plants or animals. Extensive studies ranging from small-scale laboratory tests to field studies show that sewage-sludge bound PDMS degrades in soils as a result of contact with clay minerals [233-238]. The clay acts as a catalyst to depolymerise the siloxane backbone [238,239]. The primary degradation product, regardless of the PDMS molecular weight, is dimethylsilanediol, Me2Si(OH)2 [234]. Depending on the soil type, this undergoes further degradation either in the soil via biodegradation [239-240] or evaporates into the atmosphere, where it degrades oxidatively via reaction with hydroxyl radicals [241]. Whether degradation occurs in the soil or in the air, there is conversion to inorganic constituents, amorphous silica, carbon dioxide and water.

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23. CONCLUSION From the above, it can be seen that it is often an association of properties that has led to the successful industrial application of silicones. In PDMS, an unexpected, highly flexible backbone made of strong and very polar Si-O bonds, but shielded by low interacting methyl groups, leads to low intermolecular forces and properties such as low surface tension, high permeability and low viscosity, together with good chemical and thermal stability. Some other characteristics contribute to the use of silicones across many industries. The synthesis of PDMS materials does not require heavy metal catalysts or organic solvents. They are made from distilled intermediates and their impurity profile is easy to assess using recent toxicological and environmental studies. Silicone properties can be tailored to applications. The siloxane backbone is easily modified from linear to branched or cross-linked structures or functionalised with groups other than methyl to provide for specific properties.

NOTES •

The introduction section was adapted from an original paper published in “Chimie Nouvelle,” the journal of the “Société Royale de Chimie” (Belgium) and reproduced here with the permission of the editor.

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•

Portions of Section 12 are adapted from a SPIE paper originally presented in 2004 by J. V. DeGroot, A. M. Norris, S. O. Glover and T. V. Clapp from Dow Corning Corporation. This derivative work is permitted under the copyright agreement. The following are registered trade marks: Nomex® (E.I. du Pont de Nemours and Company), Viton® (DuPont Performance Elastomers).

REFERENCES [1] [2] [3] [4]

[7]

[8] [9]

[10]

[11] [12] [13]

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

[14] [15] [16] [17] [18] [19] [20] [21]

Hardman, B. Encycl. Polym. Sci. Eng. 1989, 15, 204. Rochow, E. G. Silicon and Silicones, Springer-Verlag: Berlin, Heidelberg, New York, 1987. Noll, W. Chemistry and Technology of Silicones; Academic Press: New York, 1968. Owen, M. J. Chemtech. 1981 11, 288; this article has been updated and republished in Chimie Nouvelle 2004, 85, 27.Smith, A. L. The Analytical Chemistry of Silicones, John Wiley and Sons, Inc.: New York, 1991; 210-211. Garret, P. R. The Mode of Action of Antifoams. In Defoaming: Theory and Industrial Applications; P. R. Garrett, Ed.; Surfactant Science Series; Marcel Dekker: New York, 1993, Vol. 45, Chapter 1. Denkov, N. D. Langmuir 2004, 20(22), 9463-9505. Wasan, D. T.; Christiano, S. P. Foams and Antifoams: a Thin Film Approach. In Handbook of Surface and Colloid Chemistry; Birdi. K. S., Ed.; CRC Press: New York, 1997; Chapter 6. Kulkarni, R. D.; Goddard, E. D.; Chandar,P. Science and Technology of Silicone Antifoam. In Foams: Theory, Measurements and Applications; Prud’homme, R. K., Khan, A., Editors; Marcel Dekker: New York, 1996; Chapter 14. Denkov, N. D.; Tcholakova, S.; Marinova, K.; Hadjiiski, A. Langmuir 2002, 18, 58105817. Byron, K. J. Crit. Rep. Appl. Chem. 1990, 30, 133-161. Höfer, R. et al. Foams and Foams control. In Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag, 2002; DOI: 10.1002/14356007.all_465. Lichtman, I.; Gammon, T. Defoamers. In Kirk Othmer Encyclopedia, Wiley-VCH Verlag, 2002; DOI: 10.1002/0471238961.0405061515230514.a01.pub2. Morgan, B. P.; Moynihan, M. S. Steroids. In Kirk Othmer Encyclopedia, Wiley-VCH Verlag, 2002; DOI: 10.1002/0471238961.1920051813151807.a01. Smook, G. A. Handbook for Pulp and Paper Technologist, Angus Wilde Publications: Vancouver Bellingham, 1992, 74-83. Garrett, P.R. The Mode of Action of Antifoams. In Defoaming Theory and Industrial applications, Garrett, P. R., Ed., Marcel Dekker: New York, 1993; 66-82. McGee, J. Silicones: The Environmentally Friendly Drainage Aids for Brown stock Washing, APPITA Conference; Melbourne, Australia, May 2, 1991. Marinova, K. G.; Tcholakova, S.; Denkov, N. D.; Roussev, S.; Deruelle, M. Langmuir 2003, 19 (7), 3084-3089. Habermehl, J. Pulp and Paper Technology 2005, (Summer), 59-62. Wilson, D.J. Pulp and Paper Technology 2005, (Summer), 37-41.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

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[22] Needles, H. L. Textile Fibers, Dyes, Finishes and Processes: a Concise Guide. [23] Joseph, M. L.; Hudson, P. B.; Clapp, A. C.; Kness, D. Joseph’s Introductory Textile Science, 6th edition; Harcourt Brace College Publishers: Orlando, Florida, 1992; 322335. [24] Skinner, M. W.; Qian, C.; Grigoras, S.; Halloran, D. J.; Zimmerman, B. Textile Research Journal 1999, 69 (12), 935-943. [25] Vazquez, F. Textile Technology International, 2004, 58. [26] Vazquez, F. Silicones: Beyond Softening in Garment Finishing, book of papers, AATCC Garment Finishing Symposium, 1999; addendum. [27] Blackwood, W. R. Achieving Functional Excellence with Silicone Coatings, Techtextile China Symposium in Shanghai, (September 2004). [28] Budden, G. “Some Like It Hot”; Journal of Coated Fabrics, 27, April 1998; 44-59. [29] Madore, L. M.; Zombeck, A., US Patent 4,846,982, July 11 1989. [30] York, D. W., EP0163352, December 4 1985. [31] Henault, B. et al. Soap Cosmet. 2001, 77 (6), 34-36, 38, 40. [32] Kasprzak, K.; Blizzard, J., EP 0224839, June 10 1987. [33] McHattie, G.S; Salmon, T.M. ; Small., S., US patent US20030220217, Nov 27 2003. [34] Clarcke, E. D.; Creutz, S.; Henault, B; Small, S., EP1187951, March 20 2002. [35] Burmeister, D.; Marzinkowski, J., EP 0230565, August 5 1987. [36] Coffindaffer,T.; Wong, L., EP 0300525, January 25 1989. [37] Murphy, D; Fox, D.; Meyer, F., EP 1124925, August 22 2001. [38] Kvita, P.; Otto, P.; Dubini, M.; Chrobaczek, H.; Geubtner, M.; Goretzki, R.; Weber, B.; Martin, E., EP99810897, July 3 2002. [39] Barnabas, M; Trinh, T.; Barnabas, F.; Showell, M.; Sine, M.; Smets, J.; Wernickle, T., EP 1123374, August 16 2001. [40] Blumenkopf, N.; Grandmaire, J.P.; Jacques, A.; Tack, V., EP 0354856, February 14, 1990. [41] Anderson, D.; Frater, G. US Patent 6,262,287, July 17, 2001. [42] Aguadisch, L; Bigot, E; Colas, A.; Delpech, G.; Fonta, F.; Mane, J., EP 1060751, December 20, 2000. [43] Krzysik, D.G.; LeGrow, G.E. US Patent 5,160,494, , November 3, 1992. [44] Aguadisch, L.; Berhod, D.; Van Buuren, G.; Donker, C.; Lenoble, B.; Renauld, F., EP 1218482, July 3, 2002. [45] Onishi, K. ; Fujiwara, N., JP6219932, August 9, 1994. [46] Silicone Surfactants; R. M. Hill, Ed.; Marcel Dekker, Inc.: New York; 1999. [47] Easton, T. and Stones, M. Surface Coatings International, Oil and Colour Chemists’ Association, Wembley, 1999, 82 (11), 549. [48] Cackovich, A.; Easton, T. European Coatings Journal 1999, 10, 26. [49] Mowrer, N. R. Polysiloxane Coatings Innovations, Ameron Int., Brea CA . Technology presentation on www.ameronpsx.com; 2003. [50] Brown, H. L. Silicones in Protective Coatings, in Treatise on Coatings, Volume 1, Part III, Film Forming Compositions, Marcel Dekker Inc.: New York, 1972; 545. [51] Parbhoo, B.; O'Hare, L. -A.; Leadley, S. R. Fundamental aspects of adhesion technology in silicones (Chapter 14). In Adhesion Science and Engineering, Volume II, Surfaces, Chemistry and Applications, M. Chaudhury and A.V. Pocius, Eds., Elsevier: Amsterdam, 2002; 677-711.

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154

M. Andriot, S. H. Chao, A. Colas et al.

[52] Wolf, A. T. Durability of silicone sealants, In Durability of Building Sealants, RILEM State-of-the-Art Report; A.T. Wolf, Ed., RILEM Publications: 92220 Bagneux, 2000; 253-273. [53] Israeli, Y.; Phillipart, J. L.; Cavezzan, J.; Lacoste, J.; Lemaire, J. Polymer Degradation and Stability 1992, 36 (2), 179-185. [54] Israeli, Y.; Cavezzan, J.; Lacoste, J. Polymer Degradation and Stability 1992, 37, 201208. [55] Israeli, Y.; Cavezzan, J.; Lacoste, J.; Lemaire, J. Polymer Degradation and Stability 1993, 42, 267-279. [56] Israeli, Y.; Lacoste, J.; Cavezzan, J.; Lemaire, J. Polymer Degradation and Stability 1995, 47(3), 357-362. [57] Gorman, P. D. Weathering of Various Sealants in the Field - a Comparison. In Science and Technology of Building Seals, Sealants, Glazing and Waterproofing, Fourth Volume, ASTM STP 1243, D. H. Nicastro, Ed., ASTM International: West Conshohocken, USA, 1995; 3-28. [58] Oldfield, D.; Symes, T. Polymer Testing 1996 15, 115-128. [59] Owen, M. J.; Klosowski, J. M. Durability of Silicone Sealants. In Adhesives, Sealants and Coatings for Space and Harsh Environments, L. -H. Lee, Ed., Plenum Publishing Corp.: New York, 1998; 281-291. [60] Bridgewater, T. J.; Carbary, L. D. Accelerated Weathering and Heat stability of Various Perimeter Sealants. In Science and Technology of Building Seals, Sealants and Waterproofing, Second Volume, ASTM STP 1200, J. M. Klosowski, Ed., ASTM International: West Conshohocken, USA, 1992; 45-63. [61] Vondracek, P.; Gent, A. N. Journal of Applied Polymer Science 1982, 27, 4517-4523. [62] Fedor, G.; Brennan, P. Adhesives Age 1990, 33:5, 22-27. [63] Chew, M.Y.L.; Yi, L.D. Building and Environment 1997, 32:3, 187-193. [64] Renckens, J., Ed., Facades and Architecture, FAECF Federation of European Window and Curtain Wall Manufacturers: Walter-Kolb-Strasse 1-7, 60594 Frankfurt/Main, Germany, ISBN 3-00-002321-6; 1998. [65] Iker, J.; Wolf, A. T. Materials and Structures 1992, 25, 137-144. [66] Handbook of Pressure Sensitive Adhesive Technology, 2nd Ed., Satas, D., Ed. Satas and Associates: New York, 1989; 1-37. [67] Benedek, I. Pressure Sensitive Adhesives and Applications, 2nd Ed., Marcel Dekker: New York, 2004; 320-352. [68] Jones D. Tappi Recycl. Symp., Volume 1, TAPPI Press: Atlanta, GA., 2000; 207-8. [69] Owen, M. J. Chimie Nouvelle 2004, 22 (85), 27-33. [70] Gordon, G. V.; Tabler, R. L.; Perz, S. V.; Stasser, J. L.; Owen, M. J.; Tonge, J. S. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), 1998; 39 (1), 537-538. [71] Vorvolakos, K.; Chaudhury, M. K.; ACS Symp. Ser., 741(Microstructure and Microtribology of Polymer Surfaces), 2000; 83-90. [72] Zhang Newby, B. Chaudhury, M. K. Langmuir 1997, 13 (6), 1805-9; (CJACS). [73] Chaudhury, M.K.; Zhang Newby, B. Polym. Mater. Sci. Eng. 1996, 75, 153-154. [74] (74). Frances, J. M.; Kerr, S., III; Pinto, O. Adhesives Age 2002, 45 (1), 30-34. [75] Fraser, W. A. Pap. Synth. Conf., (Proc), 1978; 81-86. [76] Speier, J.L. Adv. Organometal. Chem. 1979, 17, 407. [77] Amako, M; Schinkel, J.; Freiburger, L; Brook, M. Dalton Transactions 2005, 1, 74-81.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Silicones in Industrial Applications

155

[78] Gomez-Ruiz, S.; Prashar, S.; Fajardo, M.; Antinolo, A.; Otero, A.; Maestro, M. A.; Volkis, V; Eisen, M. S.; Pastor, C. J. Polyhedron 2005, 24 (11), 1298-1313. [79] Chalk, A.; Harrod, J. J. Am. Chem. Soc. 1965, 87 (1), 16. [80] Lewis, L. N.; Stein, J.; Colborn, R. E.; Gao, Y.; Dong, J. J. Organomet. Chem. 1996, 521 (1-2), 221-227. [81] Lewis, L. N.; Stein, J.; Gao, Y.; Colborn, R. E.; Hutchins, G. Platinum Met. Rev. 1997, 41 (2), 66-75. [82] Kilgour, J. A.; Cua, E. C.; Cummings, J. A. US Patent 6,727,338, April 27, 2004. [83] Herzig, C.; Lutenschlager, H.; Weizhofer C. European Patent 1,481,030, Dec 1, 2004. [84] www.dowcorning.com/paper (follow links to Product Lines, then to Release Coatings). [85] Botts, J.; Philofsky, H. US Patent 2,601,243, June 24, 1952. [86] Berridge, C. US Patent 2,843,555, July 15, 1958. [87] Stalet, G. private communication, 2005. [88] Yammanishi, A.; Sato, K. US Patent 4,518,031, May 21, 1985. [89] Rhone Poulenc, Guide de l'utilisateur, data sheet ML09-7j. [90] Dow Corning World, 1983, (January-February), 4. [91] Haenig, U. and Sauer D., 1500 Jahre Kultur der Osterinsel, 152, Philip von Zabern: Mainz, Germany 1989. [92] Mollie, J. P. Silicone Materials for Electronic Components and Circuit Protection. In Plastics for Electronics, 2nd Ed., M. Goosey, Ed.; Kluwer Academic Publishers: “Silicone materials for electronic components and circuit protection”, 1999; 25-48. [93] Voronkov, M. G.; Mileshkevich, V.P.; Yuzhelevskii, Y.A. In The Siloxane Bond; Livak, J., Trasnl; Consultants Bureau: New York, 1978; 12. [94] Tobolsky, A. V. In Properties and Structure of Polymers; John Wiley and Sons: New York, 1960; 67. [95] Dent, S. J.; Langley, N. R. Dynamic Mechanical Analysis of Electronics Materials using Time Temperature Superposition; Dow Corning internal report, 1993. [96] Ferry, J. D. Viscoelastic Properties of Polymers, 3rd Ed., John Wiley and Sons, Inc.: New York, 1980; Chapter 11; 264-320. [97] Swanson, J. W.; Dall, F. C. On the Dielectric Strength of Synthetic Electrical Insulating Materials; IEEE International Symposium on Electrical Insulation, 1976. [98] Baker, E. B.; Barry, A. J.; Hunter, M. J. Ind. and Eng. Chem. Nov 1946, 38; 1117-1120. [99] Von Hippel, A. Dielectric Materials and Applications; Technology Press of MIT and John Wiley and Sons Inc.: New York, 1954. [100] Bartnikas, R. IEEE Transactions on Electrical Insulations. April 1967; 2(1); 33-54. [101] Rolain, Y.; Barel, A.; Gubbels, F.; van Moer, W. Proceedings of the 21st IEEE Instrumentation and Measurement Techn. Conf., IMTC/2004; Como, Italy, May 18-20, 2004, 333-338. [102] Ku, C.C.; Liepins, R. Electrical Properties of Polymers: Chemical Principles; Hanser Publishers: New York, 1987; 141-179. [103] Teverovski, A. Chlorine Contamination Diffusion in Silicones; publication at NASA Electronics Parts and Packaging Program (NEPP), 2002. [104] Dow Corning internal test measurements. [105] Lipp, E. D.; Smith, A. L. Infrared, Raman, Near-Infrared, and Ultraviolet Spectroscopy. In The Analytical Chemistry of Silicones, Smith, A. L., Ed. Volume 112 in Chemical Analysis, Wiley-Interscience: New York, 1991; 305-345.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

156

M. Andriot, S. H. Chao, A. Colas et al.

[106] Norris, A. et al. High Reliability of Silicone Materials for Use as Polymer Waveguides, Proc. SPIE Linear and Nonlinear Optics of Organic Materials III, Kuzyk, M.; Eich, M.; Norwood, R., Eds., Vol. 5212, SPIE, 2003; 76-82. [107] Dow Corning 200 Fluid, Information about Dow Corning 200 Fluid, Form No.22931A-90, 22-926D-93, 22-927B-90, 22-928E-94, 22-929A-90, 22-930A-90. [108] Noll, W. Chemistry and Technology of Silicone, Chapter 6, Academic Press: New York, 1968. [109] Table of Dielectric Materials, Laboratory for Insulation Research, MIT, Cambridge, Massachusetts, 1953; 67. [110] Wood, L. A. Physical Constants of Different Rubbers. Polymer Handbook, 3rd ed., Brandrup, J.; Immergut, E. H., Eds., Wiley: New York, 1989; V7-V13. [111] Rodriquez, F. Principles of Polymer Systems, 3rd ed., Hemisphere Publishing Corp.: New York, 1989; 589-594. [112] DeGroot, J. V. et al. Highly transparent silicone materials, Proc. SPIE Linear and Nonlinear Optics of Organic Materials IV, Norwood, R.; Eich, M., Kuzyk, M., Eds. Vol. 5517, SPIE, 2004; 116-123. [113] van Krevelen, D.W. Properties of Polymers, 3rd ed., Elsevier: Amsterdam, 1997; 304. [114] Lorente, M. A. et al., Macromolecules, 1985, 18, 2663. [115] Wunderlich, W. Physical Constants for Poly(methyl methacrylate). Polymer Handbook, 3rd ed., Brandrup, J. and Immergut, E. H., Eds., Wiley: New York, 1989; pp V77-V80. [116] Gorur, R.S.; Cherney, E.A. ; Burnham, J.T., Outdoor Insulators; Ed. Ravi S. Gorur May 1999 Phoenix, Ariz,; 185-193. [117] Oesterheld, J. Dielektrisches Ver-halten von Siliconelastomer-Iso-lierungen (Dielectric Behaviour of Silicone Insulations), Dissertation TU Dresden, 1996, Fortschrittsberichte Reihe 21 Nr. 178 VDI-Verlag: Düsseldorf, 1995. [118] Bärsch, R.; Lambrecht, J.; Winter, H. J. On the Evaluation of Influences on the Hydrophobicity of Silicone Rubber Surfaces, 10th International Symposium on High Voltage Engineering, Montreal 1997; 13-16. [119] Lambrecht, J. Über Verfahren zur Bewertung der Hydrophobieeigen-schaften von Siliconelastomer-Formstoffen, Dissertation, TU Dresden, 2001. [120] Strassberger, W.; Winter, H. J. Silikonelastomere in der Mittel- und Hochspannungstechnik, (Silicone Elastomerics in Medium and High Voltage Technology), ETG Fachbe-richt Nr. 68 VDE-Verlag: Berlin (1997) [121] Kindersberger, J.; Kuhl, M.; Bärsch, R. Evaluation of the Conditions of Non-Ceramic Insulators after Long-Term Operation under Service Conditions, 9th International Symposium on High Voltage Engineering, No.3193, Graz, 1995. [122] Dow Corning web site at www.silastic.com. [123] Lee, C. L.; Juen, D. R.; Saam, J. C.; Willis, R. L. Jr. US patent 4,766,176, August 23, 1988. [124] Clark, P. J. Modification of Polymer Surfaces by Silicone Technology, In Polymer Surfaces, Clark, D. T., Feast, W. J., Eds., John Wiley: New York, 1978; 235-47. [125] Smith, R. F. Friction and Wear Characteristics of Silicone-Modified Thermoplastics, Paper 760371, Soc. Auto. Engineers, 1976; (Feb). [126] Van Krevelen, D. W. Properties of Polymers, 2nd Ed., Elsevier: Amsterdam, 1976; 143.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Silicones in Industrial Applications

157

[127] Ryan, K. J.; Lupton, K. E.; Pape, P. G.; John, V. B. Ultra High Molecular Weight Siloxane Additives in Polymer-Effects on Processing and Properties, Paper 1185, ANTEC, May 1999. [128] Shearer, G. and Liao, J. Silicone-Based TPV Offers High Performance Elastomer Solutions, Paper in TOPCON, Sept 2003. [129] DiSapio, A.; Fridd, P. International Journal of Cosmetic Science, 1998, 10, 75-89. [130] Abrutyn, E,; Bahr, B. Cosmetics and Toiletries, 1993, 108 (6), 51-54. [131] De Baecker, G.; Ghirardi, D. Perfumes and Cosmetics, June 1993. [132] Lanwet, M. Cosmetics and Toiletries, 1986, 101(2), 63-72. [133] DiSapio, A. Cosmetics and Toiletries, 1987, 102 (3), 102-6. [134] Van Reeth, I.; Dahman, F.; Hannington, J. Alkylmethylsiloxanes as SPF Enhancers – Relationship Between Effects and Physico-Chemical Properties, 19th IFSCC Congress, Sydney, 22-25 October, 1996. [135] Wilkinson, J. B.; Moore, R. J. Harry’s Cosmeticology; 7th Edition; Chemical Publishing Company: New York, 1983, 63, 135-138, 323. [136] Blakely, J. The Benefits of Silicones in Facial and Body Cleansing Products, Dow Corning Europe, Form 22-1549-01 (1996). [137] Roidl, J. Silicones: Transient Conditioners for Hair Care, Symposium on “The Future of Hair Care Technology,” (November 1990). [138] Hill, M. P. L.; Fridd, P. F. GB2 074 184, October 28, 1981. [139] Thomson, B.; Halloran, D.; Vincent, J. Use of Aqueous Silsesquioxanes for Providing Body and Volume Effects from Hair Conditioners, Dow Corning Corporation, Form 25271-92 (1992). [140] Knowlton, J., Pearce, S. Handbook of Cosmetic Science and Technology, 1st Edition; Elsevier Advanced Technology: Oxford, 1993, 278. [141] DiSapio, A. DCI, (May 1994), 29-36. [142] Van Reeth, I.; Wilson, A. Cosmetics and Toiletries, 1994, 109 (7), 87-92. [143] Jaques, L. B.; Fidlar, E.; Feldsted, E. T.; MacDonald, A. G. Can. Med. Assoc. J. 1946, 55, 26. [144] Lahey, F. H. Comments made following the speech “Results from using Vitallium tubes in biliary surgery,” read by Pearse, H.E. before the American Surgical Association, Hot Springs, VA. Ann. Surg. 1946, 124, 1027. [145] Curtis, J. M.; Colas, A. Dow Corning® Silicone Biomaterials: History, Chemistry and Medical Applications of Silicones, In Biomaterials Science, 2nd Edition; Ratner, B. D., Ed.; Elsevier: London, UK, 2004, 80. [146] Leeper, H. M.; Wright, R. M. Rubber Chem. Technol. 1983, 56 (3), 523. [147] Nickerson, M.; Curry, Coat over ulcers – new protection method; C. Sci. News Lett. 1953. [148] Rider, J.; Moeller, H. JAMA. 1960, 174, 2052. [149] Black, J. Biological Performance of Materials: Fundamentals of Biocompatibility, Marcel Dekker: New York, 1992; 3-28. [150] Remes, A.; Williams, D.F. Biomaterials 1992, 13 (11), 731. [151] Harmand, M. F.; and Briquet, F. Biomaterials 1999, 20 (17), 1561. [152] Silicone Adhesives in Healthcare Applications, Dow Corning technical paper, Form No. 52-1057-01.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

158

M. Andriot, S. H. Chao, A. Colas et al.

[153] Patent US20030180281. Preparation for topical use and treatment, Bott R.R., Gebert M.S., Klykken P.C., Mazeaud I., Thomas, X, Sep 25, 2003. [154] Liu, J. C.; Tan, E. L.; Chiang, C. C.; Tojo, K.; Chien, Y. W. Drug Dev. Ind. Pharm. 1985, 11 (6-7),1373. [155] Etienne, A. S.T.P. Pharma. 1990, 6 (1), 33. [156] Pfister, William R.; Sweet, Randall P.; Walters, Patrick A. Silicone-Based Sustainedand Controlled-Release Drug Delivery Systems, Natl. SAMPE Symp. Exhib., (Proc.), 30 (Adv. Technol. Mater. Processes), 1985, 490-8. [157] Moreau, J. C.; Mazan, J.; Leclerc, B.; Avril, J. L.; Couarraze, G. Optimization of a Silicone Polymer for Drug Controlled Release. Congr. Int. Technol. Pharm., 6th, Volume 3, Assoc. Pharm. Galenique Ind.: Chatenay Malabry, Fr. 1992; 323-32. [158] Rogers, W. Sterilisation of Polymer Healthcare Products, Rapra Tech. Ltd., 2005. [159] Dow Corning® Healthcare Selector Guide Form No. 51-988C-01. [160] Colas, A. Silicones in Medical Applications, Medical Plastics 2005, Hexagon Holding ApS.: Copenhagen, Denmark - 15/17; Nov. 2005. [161] Margulies, H.; Barker, N. W. Am. J. Med. Sci. 1973, 218, 42. [162] LaFay, H. A Father’s Last-Chance Invention Saves His Son. Reader’s Digest, January 1957; 29. [163] Baru, J. S.; Bloom, D. A.; Muraszko, K.; Koop, C. E. J. Am. Coll. Surgeons 2001, 192, 79. [164] Aschoff, A.; Kremer, P.; Hashemi, B.; Kunze, S. Neurosurg. Rev. 1999, 22, 67. [165] Dow Corning® Wound Management Solutions; CD ROM. Form No. 52-1071-01. [166] Quinn, K. J.; Courtney, J. H.; Gaylor, J. D. S. Burns 1985, 12, 102. [167] Solutions for Scar Care - Skin Care Expertise for Wound Care Applications; Dow Corning Technical Brochure. Form No. 52-1049-01. [168] Cooper, B. Chemistry and Industry Journal 1978, 20, 794. [169] Treadgold, R. Process Biochemistry, 1983, (Jan/Feb), 18(1), 30-3. [170] Colas, A.; Malczewski, R.; Ulman, K. PharmaChem 2004 (Mar), 30-36. [171] Schoenherr, B.; Séné, C. Manufacturing Chemist 2003 (Nov) 36. [172] Aranha, H.; Haughney, H. Contract Pharma 2003 (Jun), 68. [173] Colas, A.; Rafidison, P. PharmaChem 2005 (Oct), 46. [174] United States Food and Drug Administration, Skin Protectant Drug Products for Overthe-Counter Human Use; Final Monograph, 21 CFR 347. [175] Patent EP 0966972. Aguadisch, L, Stalet, G, Mallard, C, Chavel, A.L., Caylus, G, June 18, 1999. [176] Colas, A.; Aguadisch, L. Chimie Nouvelle 1997, 15(58), 1779. [177] Ulman, K.; Thomas, X. Silicone Pressure Sensitive Adhesives, In Advances in Pressure Sensitive Adhesive Technology-2. Satas, D., Ed.; Satas and Ass.: Warwick, Rhode Island, 1995; p 133. [178] Rochow, E. G. Silicon and Silicones, Springer-Verlag: Berlin Heidelberg New York, 1987, 97-107. [179] Neale, M. J. Tribology Handbook, Butterworths: London, 1973; F1-F6. [180] Zechel, R.; Lonsky, P.; Trautmann H. et. al. Molykote, Dow Corning GmbH, 1991. [181] Shubkin, L. Synthetic Lubricants and High Performance Functional Fluids, Marcel Dekker Inc.: New York, 1993; 183-203. [182] Jungk, M. Eurogrease 2003, 5, 14.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Silicones in Industrial Applications

159

[183] Hesse, D. Proc. 13th International Colloquium Tribology, Wilfried J. Bartz, 2002; 1109. [184] Plueddemann, E. P. Silane Coupling Agents, 2nd Ed., Plenum Press: New York and London, 1991. [185] Witucki, G. L. J. Coat. Technol. 1993, 65 (822), 57-60. [186] Plueddemann, E. P. Reminiscing on Silane Coupling Agents. In Silanes and Other Coupling Agents, K. L. Mittal, Ed., VSP: Utrecht, Netherlands, 1992; pp 3-19. [187] Gellman, A. J.; Naasz, B. M.; Schmidt, R. G.; Chaudhury, M. K.; Gentle, T. M. J. Adhes. Sci. Technol. 1990, 4 (7), 597-601. [188] Stelandre-Ladouce, L.; Flandin, L.; Labarre, D.; Bomal, Y. Rubber Chemical and Technology 2003, (April), Vol. 76(1), 145-159. [189] Scott, H.; Humphries, J. Modern Plastic, 1973, 50 (3), 82. [190] Thomas, B; Bowery, M. Wire J., 1977, Vol. 10(5), 88. [191] Gutowski, W. S.; Li, S.; Filippou, C.; Hoobin, P.; Petinakis, S. Interface/Interphase Engineering of Polymers for Adhesion Enhancement: Part II. Theoretical and Technological Aspects of Surface - Engineered Interphase-Interface Systems for Adhesion Enhancement, The Journal of Adhesion, 2003 Vol.79, 483-519. [192] Gentle, T. E.; Schmidt, R. G.; Naasz, B. M.; Gelleman, A. J.; Gentle, T. M. Organofunctional Silanes as Adhesion Promoters: Direct Characterization of the Polymer/Silane Interphase. In Silanes and Other Coupling Agents, Mittal, K. L., Ed.; VSP: Utrecht, Netherlands, 1992; 295-304. [193] de Buyl, F.; Comyn, J.; Shephard, N. E.; Subramanian, N. P. Int. J. Adhesives and Adhesion 2002, 22, 385-393. [194] Arkles, B.; Steinmetz, J. R.; Zazyczny, J.; Mehta, P. Factors Contributing to the Stability of Alkoxysilanes in Aqueous Solution. In Silanes and Other Coupling Agents. Mittal, K. L., Ed.; VSP: Utrecht, Netherlands, 1992; 91-104. [195] Pohl, E. R.; Chaves, A.; Danahey, C. T.; Sussman, A.; Bennett, V. Sterically Hindered Silanes for Waterborne Systems: a Model Study of Silane Hydrolysis. In Silanes and Other Coupling Agents; Mittal, K. L., Ed., VSP: Utrecht, Netherlands, 2000; Vol 2, 1525. [196] Abdelmouleh, M.; Boufi, S.; Ben Salah, A.; Belgacem, M. N.; Gandini, A. Langmuir 2002, 18, 3203-3208. [197] Abdelmouleha, M.; Boufi, S.; Belgacem, M. N.; Duarte, A. P.; Ben Salah, A.; Gandini, A., International Journal of Adhesion and Adhesives 2004, 24 (1), 43–54. [198] Brinker C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press: San Diego, CA, 1990. [199] Chevalier, P. M.; Ou, D. L. J. Sol-Gel Sci. Technol. 2003, 26 (1-3), 597-603. [200] Boury, B.; Chevalier, P.; Corriu, R. J. P.; Delord, P.; Moreau, J. J. E.; Wong Chi Man, M. Chem. Mater. 1999, 11 (2), 281-91. [201] Noble, K.; Seddon, A. B.; Turner, M.; Chevalier, P. M.; MacKinnon, I. A. J. Sol-Gel Sci. Technol. 2000, 19 (1/2/3), 807-810. [202] Montemor, M. F; Simões, A. M.; Ferreira, M. G. S.; Williams, B.; Edwards, H. Progress in Organic Coatings 2000, 38 (1), 17-26. [203] Su, K.; Bujalski, D. R.; Eguchi, K.; Gordon, G. V.; Ou, D. L.; Chevalier, P.; Hu, S.; Boisvert, R. P. Chem. Mater. 2005, 17, 2520-29.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

160

M. Andriot, S. H. Chao, A. Colas et al.

[204] Kanazawa, S.; Kogoma, M.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1988, 21, 838-40. [205] Yokoyama, T.; Kogoma, M.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1990, 23, 1125–8. [206] Roth, J. R.; Rahel, J.; Dai, X.; Sherman, D. M. J. Phys. D: Appl. Phys. 2005, 38, 555567. [207] Sherman, D. M. et al. J. Phys. D.; Appl. Phys. 2005, 38, 547-554. [208] Kogelschatz, U. IEEE Trans. Plasma Sci. 2002, 30, 1400-8. [209] Owen M. J. In Siloxane Polymers, Clarson S. J., Semlyen J. A., Eds.; Prentice Hall: Englewood Cliffs, N.J., 1993, 309-372. [210] Ulman, K.; Neun, D.; and Tan-Sien-Hee, L. Pharmaceutical Formulation and Quality, 2005 (April/May), 36-42. [211] Scientific Committee on Consumer Products (SCCP). Opinion on octamethylcyclotetrasiloxane (D4), December 13, 2005, SCCP/089/05. [212] Environ International Corporation, Evaluation of Exposure to Decamethylcyclopentasiloxane (D5) for Consumers, Workers, and the General Public. Environ International, January 2006. [213] Meeks, R. G. Hazard assessment of octamethylcyclotetrasiloxane (D4) and lack of relevance to humans. European Chemicals Bureau (ECB) ECBI/37/98 Add. 16, January 2005. [214] Stump, D.G.; Holson, J.F.; Kirkpatrick, D.T.; Reynolds, V.R.; Siddiqui, W.H.; Meeks, R.G. Toxicologist 2000, 54 (1):370 (abstract 734). [215] Plotzke,K.P.; Jean, P.A.; Crissman, J.W.; Lee, K.M.; Meeks, R.G. Toxicologist 2005, 84(S-1): 307 (abstract 1507). [216] Jean, P.A.; McCracken, KA; Arthurton, J.A.; Plotzke, K.P. Toxicologist 2005, 84(S-1): 370 (abstract 1812). [217] Alison, R. H.; Capen, C. C.; Prentice, D. E. Toxicologic Pathology 1994, 22 (2), 179186. [218] Crofoot, S.D.; Crissman, J.W.; Jovanovic, M.L.; Smith, P.A.; Plotzke, K.P.; Meeks, R.G. Toxicologist 2005, 84(S-1): 308 (abstract 1509). [219] Klykken, PC; Galbraith, T.W.; Kolesar, G.B.; Jean, P.A.; Woolhiser, M.R.; Elwell, M.R.; Burns-Naas, L.A.; Mast, R.W.; McCay, J.A; White, K.L. Jr; Munson, A.E. Drug Chem Toxicol 1999, 22(4), 655-677. [220] McKim, J.M. Jr; Choudhuri, S.; Wilga, P.C.; Madan, A.; Burns-Naas, L.A.; Gallavan, R.H.; Mast, R.W. Mast; Naas, D.J.; , Parkinson, A; Meeks, R.G. Toxicological Sciences 1999, 50, 10-19. [221] Jovanovic, M; McMahon, J.; McNett, D.; Tobin, J.; Gallavan, R.; Plotzke, K. P. Toxicologist 2000, 54 (1):148 (abstract 700). [222] Jovanovic, M.; McMahon, J.; McNett, D.; Tobin, J.; Gallavan, R.; Plotzke, K.P. Toxicologist 2004, 78 (S-1):23 (abstract 114). [223] McMahon, J.M, Plotzke K.P., Jovanovic, M.L,. McNett, D.A, Galavan, R.H., and Meeks, R.G. Toxicologist 2000, 54(1):149 (abstract 701). [224] Reddy, M. B.; Looney, R. J. ; Utell, M. J.; Jovanovic, M. L.; McMahon, J. M.; McNett, D. A.; Plotzke, K. P. Submitted for publication, Toxicol. Sci. [225] Jovanovic, M.L.; Crofoot, S.D.; Crissman, J.W.; Smith, P.A.; Plotzke, K.P.; Meeks, R.G. Toxicologist 2005, 84(S-1): 308 (abstract 1508).

Silicones in Industrial Applications

161

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[226] Toxicity Profile, Polydimethylsiloxane, BIBRA working group, BIBRA Toxicology International, 1991. [227] European Centre for Ecotoxicology and Toxicology of Chemical, Linear Polydimethylsiloxanes (viscosity 10-100,000 centistokes), ECETOC Joint Assessment of Commodity Chemicals No. 26., September 1994. [228] Jovanovic, M.L.; Varaprath, S.; McNett, D.A.; Plotzke, K.P.; Malczewski R.M. Synthesis and Use of Radiolabeled Polymer for Understanding Fate and Distribution in the Body. 7th World Biomaterial Congress, Sidney, Australia. 2004. [229] Jovanovic, M.; McNett, D.; Regan, J.M.; Gallavan, R.; Plotzke, K.P. Toxicologist 2002, 66(1-S):137 (abstract 668). [230] Watts, R. J.; Kong, S.; Haling, C. S.; Gearhart, L.; Frye, C. L.; Vigon, B.W. Water Res. 1995, 29 (10), 2405. [231] Fendinger, N. J.; McAvoy, D. C.; Eckhoff, W. S.; Price, B. B. Environ. Sci. Technol. 1997, 3 (5), 1555. [232] Tolle, D. A.; Frye, C. L.; Lehmann, R. G.; Zwick, T. C. Sci. Total Environ. 1995, 162 (2,3), 193. [233] Lehmann, R. G.; Varaprath, S.; Annelin, R.B.; Arndt, J. L. Environ. Toxicol. Chem., 1995, 14 (8), 1299. [234] Lehmann, R. G.; Varaprath, S.; and Frye, C. L.; Environ. Toxicol. Chem. 1994, 13 (7), 1061. [235] Lehmann, R. G.; Frye, C. L.; Tolle, D. A.; Zwiek, T. C. Water Air Soil Pollut. 1996, 87 (1-4), 231. [236] Buch R.R.; Ingebrigtson, D.N. Environ. Sci. Technol., 1979, 13, 676-679. [237] Lehmann, R.G.; Miller, J.R.; Xu, S.; Singh, U.B.; Reece, C.F. Sci. Technol 1998, 32, 1199-1206. [238] Xu, S. Environ. Sci. Technol 1998, 32, 3162-3168. [239] Lehmann, R. G. et al. Water Air Soil Pollut., 1998, 106, 111-122. [240] Lehmann, R. G.; Miller, J. R. Environ. Toxicol. Chem. 1996, 15 (9), 1455. [241] Tuazon, E. C.; Aschmann, S. M.; Atkinson, R. Env. Sci. Technol. 2000, 34, 1970.

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In: Silicon-based Inorganic Polymers Editors: Roger De Jaeger and Mario Gleria

ISBN: 978-1-60456-342-9 © 2008 Nova Science Publishers, Inc.

Chapter 3

POLYSILOXANES AS TEMPLATES AND BUILDING BLOCKS IN NANOSTRUCTURED MATERIALS Guido Kickelbick∗ Vienna University of Technology, Institute of Materials Chemistry Getreidemarkt 9-165, A1060 Vienna, Austria

ABSTRACT

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Polysiloxanes are, due to their many extraordinary physical and chemical properties, ideal building blocks for a variety of nanostructured materials. This review shows the application of polysiloxanes in the preparation of inorganic-organic nanocomposites using clays, sol-gel based structures, nanoparticles or nanotubes as the inorganic components. In many of these cases, the polysiloxane acts as the (elastomeric) matrix for the embedment of inorganic structural units. In addition, the formation of specific nanostructures such as micelles or tubular type structures based on the unique solubility of polysiloxanes or polysiloxane segments in block copolymers is reviewed. Another field, which is shortly introduced, is the emerging field of surface nanostructuring via soft lithography where polysiloxanes play a major role as stamps due to their hydrophobicity and their elastomer properties.

Keywords: polysiloxanes, lithography.

nanocomposites,

sol-gel,

nanoparticles,

nanotubes,

soft

INTRODUCTION Polysiloxanes attracted much interest in the past as consumer polymers in many large scale applications due to their unique properties, such as their low glass transition temperature (Tg), their biocompatibility, high UV and thermal stability, their hydrophobicity, or their elastomeric properties in crosslinked systems [1]. While these bulk consumer products are of ∗

E-mail: [email protected]

164

Guido Kickelbick

high economic value, in recent years polysiloxanes started also a renaissance in the academic community in particular in the preparation of nanostructured materials. Polysiloxanes offer some properties that cannot easily be obtained with classical purely organic polymer systems or that have to be introduced into the organic polymers by synthetically complex reactions or by expensive monomers. Synthetic procedures to polysiloxanes are the hydrolysis and condensation of dichlorodiorganosilanes or dialkoxyldiorganosilanes, which results in a quite broad molecular weight distribution, or the highly controlled ionic ring opening polymerization (ROP). The latter can either be carried out under acidic or basic conditions, where basic reactions most often give the lower molecular weight distributions (Scheme 1). Low polydispersity indices (PDI) are in most cases only obtained if the cyclic monomers, such as hexamethyltrisiloxane D3, have a reasonable high ring strain which leads to a kinetic control of the polymerisation, while low ring strain, e.g. in the case of octamethylcyclotetrasiloxane D4, leads to thermodynamic control which usually gives a mixture of rings and chains as a final product. Very low polydispersities (PDI < 1.1) are typically obtained with anionic ring opening polymerizations. Hydrolysis of Dichlorosilanes R R2SiX2 + H2O

+ H2O - HX

R2SiX(OH)

R2Si(OH)2

- HX

R

n-times Si - n H2O

+

O

R

Si

O n

R

n

X = Cl or OR Ring Opening Polymerization Acidic Ring Opening of D4 CH3 H3C Si O CH3 H+ O Si CH 3 H3C Si O H3C O Si CH3 H3C H+ = e.g. H2SO4, HClO4, D4 F COSO H 3

CH3 Si

O

CH3

n

3

Basic Ring Opening of D3 H3C

CH3

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Si H3C

O

O

Si

Si

H3C

O D3

BCH3 CH3

B

O Si

Si

Si

O 1/3 n D 3

H3C CH3 H3C CH3 H3C CH 3

CH3

CH3

-

O

B

Si

O

Si

CH3

CH3 O

CH3

Si n

CH3 O

CH3

Si CH3

B- = e.g. Me3SiO-M+, R-Li R'R2SiCl CH3 B

Si CH3

Scheme 1.

CH3 O

Si n+3

CH3

R'

-

O

Polysiloxanes as Templates and Building Blocks in Nanostructured Materials

165

FUNCTIONALIZED POLYSILOXANES Functionalization, which is important for many polymer applications, is more difficult for polysiloxanes as in the case of purely organic polymers where in many cases simply monomers with functional groups are copolymerized: in this case, many desirable functional moieties cannot be introduced because they interact with the polymerization procedure. However, a few functionalization groups can be introduced during polysiloxane preparation (Scheme 2). Different positions in the polymer chain can be equipped with such functional groups. Beside the so-called telechelic terminal-functionalization at each chain-end, also monofunctional derivatives are available. The probably most important type of polysiloxanes of this type is OH-terminated polydimethylsiloxane (PDMS). In addition functional groups can also be introduced during the polymerization as pending groups, as for example in the cases of poly(vinylmethylsiloxane) (PVMS) or poly(hydromethylsiloxane). High control over the composition, the chain length, and the shape of the polysiloxanes is achieved by ROP, therefore this technique is often used in the synthesis of random or di- and multiblock copolymers, brush-systems or star-shaped polymers containing various functional groups [2]. Post-modification is applied if the desired functional group cannot be introduced during the polysiloxane synthesis, because it does not withstand the polymerization conditions, e.g. the initiator in basic anionic ring opening polymerization can react as a nucleophile with many groups. Typical examples for functionalization reactions are hydrosilations using Pt0 as catalyst [3, 4] or epoxidations of unsaturated bonds followed by a nucleophilic ring opening reaction (Scheme 3) [5, 6]. Particularly hydrosilation reactions of polyhydrosilanes are used as an important tool for the production of functional polysiloxanes. CH3 H

Si

CH3

CH3 O

CH3

Si

O

CH3

Si H n CH3

CH3

CH3 HO

Si

O

CH3

Si

Hydride-Terminated PDMS

H n

CH3

CH3

CH3 O

Si

O

CH3

Si n CH3

Vinyl-Terminated PDMS Hydroxy-Terminated PDMS

Functionalized Polysiloxanes CH3

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CH3 H3C

Si

O

R1

CH3

Si O

CH3

CH3 H3C

Si CH3 n CH3

R4

Si

O

O (CH2)3 Si CH3

H2N CH2 CH2 CH2Si CH3

O

Si O CH3

CH3 Si CH2 CH2 CH2 NH2 n CH3

Aminopropyl-Terminated PDMS

Scheme 2.

Si O

O

CH3 Si CH3 n CH3

Poly(vinylmethylsiloxane) PVMS

R3 n

CH3 CH3

CH3 O

CH3

R2

Poly(methylhydrosiloxane) PMHS

Si

CH3

CH3

CH3 O

Si O CH3

Si (CH2)3O n

CH3

Epoxypropoxypropyl-Terminated PDMS

O

166

Guido Kickelbick

Depending on the polysiloxane used either end-functionalized or pending functionalities are introduced by this technique. The presence of functional groups is very important for the preparation of nanostructured materials and the self-assembly of macromolecules as shown in the examples below. CH3

R

+

Si O H

CH3

Pt-Kat.

Si O

n

n

R CH3

Epoxidation

Nucleophilic Ring Opening

CH3

CH3

Si O

Si O

Si O n

n

n OH

O Nu

Scheme 3.

Some of the above mentioned functionalization reactions are also used for the crosslinking of single polymer chains to form polysiloxane elastomers. Si-H containing polysiloxanes can be crosslinked with vinyl-group containing polymers by hydrosilation reactions, or hydroxy-terminated polysiloxanes can react with tetraalkoxysilanes to give silicon crosslinked networks (Scheme 4).

H3C CH3 4

Si OH + Si(OR)4

- 4 ROH

CH3

CH3

O

CH3

Si O

Si O

Si

CH3

O

CH3

H3C

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Si CH3

Si CH3

Scheme 4.

NANOCOMPOSITES Nanocomposites consist of a matrix and structural elements, where at least one component is in the length scale of 1-100 nm [7, 8]. In inorganic-organic nanocomposites typically an inorganic nanometer-sized component is embedded in a polymer matrix. A critical point in the synthesis of homogeneous nanocomposites is to overcome the incompatibility between polymer and inorganic component exploiting attractive forces, such as hydrogen bonds, van-der-Waals or covalent interactions. For this purpose, generally, the

Polysiloxanes as Templates and Building Blocks in Nanostructured Materials

167

inorganic species is surface-functionalized to improve the interaction with the polymer matrix. A plethora of different inorganic components can be used as structural elements in nanocomposites and here we will give an overview of selected examples of inorganic components used in conjunction with polysiloxanes.

CLAY NANOCOMPOSITES Clays are inorganic minerals with a lamellar internal structure. In the natural materials metal cations are usually located between the layers which can be easily exchanged against organic molecules containing cationic groups and thus intercalation compounds, so-called organoclays, are formed [9]. The attractive interactions of the inorganic sheets can be completely overruled by the steric demand of polymers together with an interaction of functional groups on the surface of the inorganic sheets and thus the layered structure can exfoliate. In an ideal case, the single inorganic sheets are afterwards homogeneously distributed in the polymer matrix [10]. Generally, in a first step the clay mineral is either swollen in a specific solvent or the metal cations are exchanged with larger organic cations to allow a spreading and thus a better interpenetration of the macromolecules between the inorganic layers. Afterwards polysiloxanes can be intercalated between the galleries. Instead of a cation exchange of the metal cations in pure clays against organic cations, e.g. long alkyl chain ammonium ions, also cationic functionalized polysiloxanes can be applied to directly intercalate and potentially exfoliate the clay (Scheme 5) [11]. The thus modified clay slurry is mixed with a usual silicone rubber and exfoliation is achieved to improve the mechanical properties of the rubber. Poly(methylphenyl siloxane) was intercalated within dioctadecyldimethylammonium bromide modified layered silicates, such as hectorite and montmorillonite. The confined space between the clay galleries has an effect on the local reorientational dynamics of the polymers, which was detected by changes via dielectric relaxation spectroscopy. The relaxation mode of the polymers is much faster in the composite than the segmental αrelaxation of the bulk polymer [12, 13]. CH3 Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

H3C

Si

CH3

CH3 O

CH3

Si

O

(CH2)3 O

HO

CH3

CH3 CH2CH3

CH2 CH

Si

+

CH2

N

CH3

CH2CH3 Scheme 5.

Hydrox-terminated PDMS is often used in studies because of its commercial availability and based on its unusual structure revealing different polarities of the chain and the end-

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168

Guido Kickelbick

functionalities and thus allowing dissimilar interactions of the two positions with inorganic materials. This polymer type was used for the preparation of clay-nanocomposites with organically modified montmorillonite in concentrations of the organosilicate in the nanocomposites of 0.5-10% [14]. Swelling in the derived dispersed nanocomposites was dramatically decreased as compared to conventional composites. The swelling behaviour was correlated to the amount of bound polymer in the nanocomposites. Thermal analysis of the bound polymer chains showed an increase and broadening of the Tg and a loss of the crystallization transition. A systematic study of several factors, such as the alkyl chain length and the number of present alkylammonium-chain in modified layered-silicates as well as the number of silanol groups terminating the PDMS chains, demonstrated that both are important for the formation of exfoliated nanocomposites [15]. Further studies on synthetic clays, such as fluorohectorite, in which the cations have been replaced by hexadecyltrimethylammonium ions also intercalates hydroxy-terminated PDMS [16]. In these studies it was shown that the extent of intergallery swelling increased with increasing PDMS chain length, as expected. Little or no intercalation was observed for PDMS molecules terminated by methyl groups, indicating that terminal silanol interactions with the inorganic surfaces play an important role in the gallery swelling mechanism. Crosslinking reactions between PDMS and tetraethoxysilane (TEOS) in the presence of the organoclay afforded elastomeric nanocomposites in which the clay nanolayers were exfoliated. The nanolayer-reinforced polymer exhibited substantially improved tensile properties and resistance to swelling by an organic solvent in comparison to the original polymer. The oxygen permeability of the resulting material was not greatly influenced in the nanocomposites, which was ascribed to a more or less random orientation of the clay nanolayers in the polymer matrix. Clay minerals can contain free OH-groups, which were used as initiators for an in situ anionic ROP of cyclosiloxanes after deprotonation. This so-called in situ intercalative polymerization was for example carried out using (R4N)xH1-xTiNbO5 where exfoliation of the galleries resulted due to the polymerization process [17]. Beside ROP, hydrolysis of dimethyldichlorosilane between the layers of organicallymodified montmorillonite was carried out for in situ formation of polysiloxanes [18]. After separation of the free polymer chains from the derived material it was proven that residual PDMS chains are still present in the montmorillonite. The residual PDMS was grafted onto the clay layer surface via condensation of hydroxyl groups of PDMS and those that existed on the clay surface. These formed modified clays showed a different behaviour with regard to the compatibility towards various polymers. When a suspension of the treated layered silicate was blended with polar polymers, exfoliated nanocomposites were found, but when it was blended with nonpolar polymers, intercalated nanocomposites were obtained. This phenomenon was explained with regard to the different groups available on the surface of the montmorillonite. Not only standard clays were used for intercalation of polysiloxanes but also systems like layered perovskites such as (Rn(NH3))2MnCl4 (Rn = CnH2n+1; n = 2, 9) were dispersed in PDMS [19]. The resulting composites contained exfoliated MnCl4 layers as well as enhanced aspect ratio crystallites composed of bundles of a few MnCl4 sheets. The degree to which the two components interacted with each other at the interfacial region did not affect the static magnetic properties of the inorganic component.

Polysiloxanes as Templates and Building Blocks in Nanostructured Materials

169

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NANOPARTICLES An important class of nanometer sized-components for the production of nanocomposites are nanoparticles, which can introduce interesting physical properties into a nanocomposite [20]. Quantum size effects give these systems fascinating physical optical, electronic or magnetic properties and large surface to volume ratios offer exciting surface reactivities. Furthermore, their small size below the wavelength of the visible light makes them attractive candidates for optical devices. Filler-reinforced polymer composites are already in use since decades [21]. In the case of PDMS-elastomers filler particles, such as montmorillonite nanosize clay particles, were used, for example, in TEOS-crosslinked networks [22]. Contrary to the studies in the previous chapter these particles were not exfoliated. The comparison of hydroxy- and vinyl-terminated PDMS-chains showed that an anchoring of the hydroxyl end group to the silicate surface via hydrogen bonding binds pendent chain ends which reinforce the elastomers. In addition hydrogen bonding between surface hydroxyls and backbone siloxane species can occur which also took part in the reinforcement mechanism by a reduction of the PDMS chain mobility in the interfacial domain causing an effective "stiffening" of the polymer matrix [23, 24]. Polysiloxanes show a good permeability for various compounds, which is the reason why they are used as separation membranes. Systems containing well-dispersed silicalite particles of 50 nm diameter were successfully applied to the preferential pervaporation of acetic acid over water [25]. The nanocomposite membranes showed improvement on both seperation factor and permeation flux for the pervaporation process, as compared to plain PDMS membranes and composite membranes containing silicalite particles of 5 µm. This improvement was attributed to higher readily accessible separation surface area and higher sorption selectivity toward acetic acid of the nano-size silicalite particles. The membrane thermal stability was also improved by incorporation of the nano-size silicalite particles. Due to the high surface energy of nanoparticles it is often difficult to homogeneously distribute them into a polymer matrix avoiding agglomeration. Hence, surface-capping agents are used to reduce the surface energy and to compatibilize the particles with the solvent or matrix they should be introduced in. Typical examples are Au nanoparticles that are usually prepared in an aqueous phase and agglomerate as soon as they are transferred in an organic solvent or polymer. Applying tetradecanethiol-capped Au colloids changes the surface properties and thus allows for the incorporation of these colloids into PDMS matrixes [26]. Another approach leading to a similar result is provided by the use of silica-coated Au nanoparticles that are homogeneously distributed in a crosslinked PDMS matrix [27]. The homogeneous distribution of nanoparticles in casted nanocomposite films was demonstrated through the presence of well-defined plasmon absorbance bands in the visible, which clearly show that the composites retain the characteristic optical properties of single Au nanoparticles. The metal particle concentration can be tailored through either the amount or the concentration of the nanoparticle colloid in these materials. Ag nanoparticles can self-assemble in two-dimensional arrays on quartz and silicon surfaces. If these patterns are coated with PDMS, the resin films contained embedded nanoparticles in their original arrangement [28]. After etching away the embedded nanoparticles, spherical nanocavities in PDMS were produced, which are unattainable by using nanoimprint lithography. Such Ag nanoparticles arranged in 2-dimensional arrays also

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170

Guido Kickelbick

experience a quadrupolar coupling of plasmon resonances when irradiated with visible light [29]. This coupling gives the coherent plasmon mode characterized by an intense narrow resonance in the blue spectral range in the extinction spectrum. The coupling as well as the intensity of this mode is controlled by varying the distance between particles, which is possible if the PDMS films containing the particles are biaxial stretched. Ag-PDMS nanocomposites have also been demonstrated as promising substrates for the detection of model environmental pollutants via surface-enhanced Raman spectroscopy (SERS) [30]. Other nanoparticle types that were introduced into different polysiloxane matrices were magnesium ferrite, which was modified with the surface treating agent, 1,3divinyltetramethyldisilazane (VMS) [31], fumed silica [32], or Co3O4 nanoparticles surfacemodified with poly(butyl methacrylate) (PBMA) [33]. Other subnanometer building blocks for the synthesis of nanocomposites are fullerenes. Multihydroxylated [60]fullerene, so-called fullerenol, were used for the preparation of free standing nanocomposite films with bis(3-aminopropyl)-terminated PDMS [34]. Dynamic light scattering (DLS) measurements on THF solutions of these systems revealed that fullerenol-PDMS complexes were formed due to strong hydrogen bonding between the hydroxyl groups of fullerenol and the amino groups of amine-terminated PDMS. Small-angle X-ray scattering (SAXS) results indicated that nanodomains of fullerenol aggregates were confined homogeneously in the PDMS matrix. The resulting materials exhibit higher glass Tg’s, superior thermal and thermal mechanical stability, and greatly suppressed crystalline phase compared to the pure polymers applied. Moreover, the nanocomposites possess very attractive dielectric properties; i.e., high content of fullerenol increased permittivity while it severely decreased the loss factor of the nanocomposite materials. Recyclable highly active "Pd"-polysiloxane nanocomposites were generated by reduction of Pd(OAc)2 with poly(methylhydrosiloxane) [35]. The obtained materials act as catalysts after redispersion in common organic solvents for the chemoselective reduction of aryl α,β-unsaturated esters and enones, and vinylferrocene with alkenes, poly(methylhydrosiloxane) to yield arenes, esters, ketones and ethylferrocene in high yields. Furthermore, the nanocomposites were used as catalysts for macromolecular grafting via alcoholysis of polyhydrosiloxane under moderate reaction conditions with a variety of alcohols (primary, secondary, tertiary/sterically bulky and functional alcohols) [36]. Several spectroscopic investigations and catalyst poisoning studies demonstrated that polysiloxaneencapsulated palladium nanoclusters are the active catalytic species in the reactions. In addition to their role as reducing agents, in this catalyst the polysiloxane serves multiple functions: for example, it acts as a stabilizer for avoiding the aggregation of the nanoparticles; provides desired functional interfaces between the nanoparticles and the bulk of the materials; and facilitates the self-assembling and processing of the composites. Not only homopolymers are interesting for the formation of polysiloxane nanoparticle composites but also block copolymers and particularly aggregates of them were beneficially used in the formation of functional nanocomposites. Aggregates of tert-butylacrylate-bpolydimethylsiloxane triblock copolymer micelles stabilized with silica nanoparticles were used as nanocomposite building blocks for artifical superhydrophobic surfaces [37].

Polysiloxanes as Templates and Building Blocks in Nanostructured Materials

171

FERROFLUIDS Ferrofluids are a special class of nanocomposites. Usually these materials contain inorganic magnetic nanoparticles, most commonly magnetite or cobalt, surrounded by a stabilizing capping agent and these surface-modified nanoparticles are embedded in an oily phase [38, 39]. Based on the small diameter of the particles and the oily continuous phase, the macroscopic appearance of the systems is liquid-like and, due to the magnetic properties of the incorporated inorganic particles, the liquid is influenced by an external magnetic field. There is an increasing number of technological applications of such ferrofluids in particular in biomedical products [40]. H3C

(CH2) 3CN Si O

NC(H2C)3 H3C

O

Si

Si

O O

CH3 (CH2) 3CN

CH3

LiOH, 140°C Li+ O

-

Si

NC(H2C)3

CH3

Si

O

Si

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CN

CN

O

CH3

Si

Si O

H3C

n

CN

1. CH2Cl2/THF, 25-50°C

Si

CH3

2. (H3C)3SiCl

CH3

CH3

CH3

CH3 O

Si

O

Si

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CN

+ Cyclics

CH3

O

H3C Si O

-

O Li+

Si

CH2

Si

CH3

O

CH2

H3C

H3C

CH3

CH3

m

CN

n

CN

CH3 O

Si CH3 CH3

m

Co2(CO)8, Toluene, Δ

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Dispersed Stabilized Cobalt Nanoparticles in Toluene 1. PDMS 2. Evaporation of Toluene Dispersed Stabilized Cobalt Nanoparticles in PDMS

Scheme 6.

Stable suspensions of superparamagnetic cobalt nanoparticles were prepared in PDMS carrier fluids in the presence of poly[dimethylsiloxane-b-(3-cyanopropyl)methylsiloxane-bdimethylsiloxane] (PDMS-b-PCPMS-b-PDMS) triblock copolymers as steric stabilizers [41]. The copolymers formed micelles in toluene and served as 'nano-reactors' for thermal decomposition of the Co2(CO)8 precursor. The nitrile groups on the PCPMS central blocks

172

Guido Kickelbick

were thought to adsorb onto the particle surface, while the PDMS end-blocks protruded into the reaction medium to provide steric stability and minimize toxicity issues in biomedical applications. Applying these systems, the particle diameter was controlled by adjusting the cobalt to copolymer ratio. Non-aggregated cobalt nanoparticles with narrow size distributions, which are evenly surrounded with copolymer shells, were produced. Ordered self-assemblies of these cobalt particles formed spontaneously when the dispersions were cast from toluene. Magnetic measurements clearly showed the decrease in magnetic susceptibility over time for dispersions in contact with an air-containing atmosphere, which was attributed to surface oxidation. Thermal decomposition of Co2(CO)8 was also used in presence of other polysiloxane containing polymers to produce magnetic cobalt nanoparticles. Poly(dimethylsiloxane-bmethylvinylsiloxane) (PDMS-b-PMVS) diblock copolymers were synthesized via anionic living polymerization and the PMVS segments were subsequently functionalized with trimethoxysilethyl or triethoxysilethyl pendent groups to yield poly(dimethylsiloxane-b(methylvinyl-co-methyl(2-trimethoxysilethyl)siloxane) (PDMS-b-(PMVS-co-PMTMS)) or poly(dimethylsiloxane-b-(methylvinyl-co-methyl(2-triethoxysilethyl)siloxane) (PDMS-b(PMVS-co-PMTES)) copolymers, respectively [42]. Stable suspensions of mostly superparamagnetic cobalt nanoparticles with mean particle diameters ranging from approximately 10–15 nm were prepared in toluene in the presence of these block copolymers. The trialkoxysilane groups can be used to carry out the sol-gel process and to deliver an additional protection of the cobalt surfaces against oxidation [43]. Another typically used inorganic system, i.e. magnetite nanoparticles were also sterically stabilized in PDMS carrier fluids [44]. For this purpose trivinylsiloxy-terminated PDMS was functionalized with mercaptoacetic acid or mercaptosuccinic acid to produce PDMS stabilizers containing either three or six carboxylic acid groups, respectively, at one chainend. Magnetite nanoparticles were synthesized by a chemical co-precipitation reaction of FeCl2 and FeCl3 with hydroxide at pH 9-10. Subsequently, the PDMS stabilizers were adsorbed onto the magnetite nanoparticle surfaces via the carboxylate groups in an interfacial reaction at low pH values. The average particle diameter of the magnetite particles was around 7.5 nm. The polymer-magnetite nanoparticle complexes were dispersed in PDMS oligomers to afford polysiloxane ferrofluids.

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NANOTUBES In recent years nanotubes of different composition have been a focus of materials scientists due to their unique electronic and mechanical properties [45]. Hence, these nanoobjects also became of interest for the production of nanocomposites [46-48]. Carbon nanotubes (CNT) are usually grown from surfaces, such as organometallic micropatterns and these periodic nanotube arrays can be incorporated into a polysiloxane matrix resulting in free-standing nanotube/polymer composite films [49]. Because of the high flexibility of the polysiloxane films and the conductivity of the carbon nanotubes, these materials are appealing candidates for the development of new electronic devices where flexibility is required. The location and density of conducting channels within the composite can be easily controlled by soft lithography patterning (see below).

Polysiloxanes as Templates and Building Blocks in Nanostructured Materials

173

Oxidized multiwalled carbon nanotubes were functionalized with PDMS by epoxide opening of functionalized PDMS under ultrasonic irradiation and acidic conditions [50]. The resulting product is a uniform solution and allowed the wet-casting of thin films subsequently the films were cured using UV curing conditions. Partially conductive polymer composites were prepared using poly(urea urethane) (PUU) with a PDMS soft segment as matrix and conductive fillers such as carbon nanotubes, silvercoated carbon nanotube, and nickel-coated carbon nanotubes [51]. The results showed that highly conductive metals could improve the conduction of CNT nanocomposites significantly.

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SOL-GEL The previous examples of the formation of inorganic-organic nanocomposites have in common that the inorganic structural elements, e.g. nanoparticles, nanotubes, etc., are in most of the examples preformed well-defined building blocks. A different approach to polysiloxane nanocomposites is the in situ formation of the structural inorganic component in presence of the polysiloxane. One of the most prominent processes with regard to this goal is the sol-gel process, which is a mild route to the formation of silica or metal oxides using molecular precursors. Sol-gel chemistry is an important tool for the formation of hybrid materials that do show a distribution of inorganic and organic building blocks on the molecular scale [52]. Typical examples are highly crosslinked polysiloxanes that were prepared by sol-gel polymerization of 1,6-bis(diethoxymethylsilyl)hexane and 1,4-bis(diethoxymethylsilyl)benzene [53]. Hydrolysis and condensation of these compounds under acidic and basic conditions resulted in the formation of hexylene- and phenylene-bridged polysiloxane gels. The dry gels (xerogels) were intractable, insoluble materials that were noticeably hydrophobic, exhibiting no swelling in organic solvents or water. Hexylene-bridged polysiloxanes prepared under acidic conditions were always nonporous. Hexylene-bridged polysiloxanes prepared under basic conditions and all of the phenylene-bridged polysiloxanes revealed a mesoporous structure. Beside the dialkoxydiorganosilanes also other precursors can act as the polysiloxane precursors. In many cases OH-terminated PDMS was used and the oxide component was introduced via metal alkoxides, M(OR)n (M = Si, Ti, Zr, etc). The thus obtained materials are transparent, and can be flexible or brittle, and can be obtained over a large M/siloxane ratio [54]. Depending on the reactions conditions and the M/siloxane ratio, the structures of the obtained materials varied ranging from single phase systems with highly interconnected siloxane and oxide units, to nanocomposites made of polysiloxane chains crosslinked with oxide-based particles as investigated by 29Si CP MAS NMR spectroscopy [54, 55]. If a nonionic surfactant was present an ordering within the material with an initial d-spacing of 450 pm was observed which decreased to 350 pm after calcinations [56]. Polysiloxane-silica nanocomposites were also synthesized from single source precursors that have trialkoxysilane groups pending from a polysiloxane polymer. Acid and base catalyzed hydrolysis and condensation reactions were performed from solutions of the pure macromolecular precursor as well as from mixtures of this precursor with tetraalkoxysilanes

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followed by either supercritical drying with liquid CO2 or drying at ambient conditions. In both cases monolithic gels as well as powders were obtained [57]. The mechanical properties of hybrid materials obtained by mixing TEOS and PDMS were tailored by varying the ratio of the two components and the processing conditions [58]. Harder, stiffer and stronger hybrid materials were obtained when PDMS concentrations were low. Reaction conditions during hybrid formation are also important, e.g. harder materials are possible when ultrasonic irradiation is used during synthesis. When the PDMS concentration was increased, the hybrids took on a more flexible nature, and over a critical concentration they actually became rubbery. The properties of the resulting materials are most likely not only a consequence of mixing the two different materials but also a result of the multiple sizescale morphologies, ranging from Ångstroms to micrometers. The obtained so-called multiple-level hybrid materials showed improved structural integrity relative to pure sol-gel glasses (xerogels) as investigated by SAXS and SEM studies [59]. Interestingly, the reinforcement of PDMS elastomeric networks depends on the sizes and size distributions of in situ precipitated silica fillers [60]. Ultimate strength increases were obtained for networks filled with smaller filler particles; however, the extensibilities of the elastomers were generally improved by the larger filler particles. The reinforcement of PDMS filled with a bimodal distribution of filler particle size falls between the two unimodal distribution extremes, and bimodality provided no significant advantages. Both these studies show in conclusion that surface area and the interaction between the polymer chains and the filler surface play very important roles in the improvement of mechanical properties of filled polysiloxane elastomers. One of the main reasons to use PDMS as an additive to TEOS-based gels is the improvement of macroscopic properties for applications such as thermal insulators or mechanical properties [61]. Thermal treatment can also result in transformation of the nanocomposites into ceramic materials. Thus by heating PDMS-silica sol-gel materials under an inert gas atmosphere amorphous silicon oxycarbide glass fibers were obtained [62]. In many applications high homogeneity of the nanocomposites is crucial for their performance. One of the methods to increase this homogeneity as well as the density of the PDMS silica materials prepared by the sol-gel route is the use of ultrasound [63]. PDMS-silica materials were prepared by the sol–gel method on various substrates, such as Si wafers, Al and polystyrene. Therefore, the resulting hybrid materials have always one surface in contact with air and one with the underlying substrate. Both surfaces are homogenous and smooth but they can differ in their nanometer structures as observed with various analytical techniques [64]. The surface in contact with air had a silica-free PDMS top layer of approximately 2 nm thickness; while in the material in contact with the substrate SiO2 was located at or just beneath the outermost atomic layer, which is an important aspect for coating applications. Another application of PDMS-silica materials are fiber-optic pH sensors which were fabricated using fluorescein isothiocyanate immobilized in films resulting from copolymerization of TEOS, silanol-terminated PDMS, and 3-aminopropyltriethoxysilane and/or 3-glycidoxypropyltrimethoxysilane [65]. Mixed oxides can also be used for reinforcing the elastomeric networks. For example silica-titania mixed oxides were prepared in presence of PDMS by the sol-gel conversion of mixtures of TEOS and titanium n-butoxide [66]. The reinforced nanocomposites were found to have very good mechanical properties, and swelling equilibrium measurements indicate

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that there is good adhesion between the SiO2-TiO2 filler and the elastomeric matrix. Compared with networks containing silica alone, the networks with silica-titania mixed oxides had somewhat better combinations of high modulus and good extensibility. The oxide particle size in the PDMS was typically several hundred picometer in diameter but was found to be inversely dependent on the crosslink degree of the PDMS network. Thus, larger particle sizes can be obtained at lower crosslink densities (which give larger network cavity sizes). The distribution of particle sizes in such materials was relatively narrow, and there was very little particle aggregation. In the study of the hydrolysis and sol-gel transition reaction, it was found that co-condensation between TEOS and PDMS was the dominant reaction under the chosen conditions, and that most of the PDMS was thereby incorporated in the SiO2 network [67]. The presence of PDMS during the sol-gel reaction was found to greatly shorten the gelation time of the system. Further studies extended the obtained results for materials prepared by reactions of silanol-terminated PDMS and various metal alkoxides, e.g. Al(O-sec-C4H9)3, Ti(OC2H5)4, and Ta(OC2H5)5. It was proven that the molar ratio of M(OR)n/PDMS influence the structure and mechanical properties of the resulting materials. In such materials the interaction between the inorganic component and PDMS increased in the order Al-O-PDMS < Ta-O-PDMS < TiO-PDMS [68, 69]. Simultaneous formation of the polysiloxane and the metal oxide was achieved when diethoxydimethylsilane and the metal alkoxide, such as zirconium n-propoxide, are hydrolysed at the same time. Applying this method transparent, homogeneous and non-porous xerogels were obtained up to 70 mol% ZrO2 content [70, 71]. In these materials the interface between siloxane and zirconium oxo domains is composed primarily of Zr-O-Si(CH3)2 bonds and hydrogen bonds, their relative proportion being a function of the zirconium content. Pyrolysis under argon atmosphere resulted in the production of high surface area material at 600°C with pore sizes below 3 nm. Further heating provides amorphous samples up to 800°C, while at 1000°C the structural evolution of the silicon moiety produces an amorphous oxycarbide phase whereas the primary crystallization of tetragonal zirconia takes place, with crystallinity and crystallite sizes depending on the ZrO2 content. At 1400°C, the silicon oxycarbide phase generates a mixture of amorphous silica and crystalline silicon carbide polymorphs including tetragonal and monoclinic ZrO2 phases. PDMS-titania nanocomposites showed a similar behaviour with regard to high temperature treatment [72]. A further application of zirconium oxide/PDMS mixed hybrid materials are temperature tolerant proton conducting polymer electrolyte membranes [73]. They are formed by sol-gel processing of zirconium alkoxides and polysiloxanes in the presence of heteropoly acids such as 12-phosphotungstic acid (PWA). The hybrid membranes showed better thermal and mechanical properties than conventional membranes because of the temperature tolerant organic/inorganic moieties and the nanostructure of the hybrid matrix. The structure of PDMS-metal-oxo nanocomposites prepared from the hydrolysis dimethyl-diethoxy silane in presence of various metal alkoxides is based on amorphous metal-oxo nano domains embedded within a siloxane network. In these systems a strong influence of the crosslinking metal nature on the size of the metal-oxo nanoparticles and on the extent of the interface between inorganic domains and the siloxane component was found [74]. Spectroscopic measurements revealed a more important nanophase separation for the PDMS systems incorporating Al(III), Ti(IV), or Zr(IV) species as crosslinking agent than for the ones crosslinked by Nb(V), Ta(V), or Ge(IV) [75].

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Siloxane–metal oxide hybrid materials are very promising matrices for rare earth-doped photonic devices because of their transparency, their versatility in terms of structure and properties and their ability to encapsulate luminescent ions within the metal oxo domains. In addition, a thermal treatment of these matrices can be performed at moderate temperatures in order to remove the hydroxyl groups which limit the optical response of the rare earth ions [76]. Crosslinked PDMS was also used as a reaction matrix for the production of various silica, titania, and zirconia microstructures [77]. For this purpose PDMS was swollen applying various solvents and the voids created by solvent-swelling the polymer were used to template the products. The inorganic morphologies formed ranged from spheres to networks, depending upon the nature of the polymer, its degree of swelling, and the synthetic conditions. Organic solvents as well as pure metal alkoxide liquiods were used as swelling agents for the polymer. Once the alkoxide precursor was incorporated into the swollen polymers, water was introduced to start the hydrolysis and condensation reactions resulting in a textured metal oxide within a PDMS matrix.

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BIOACTIVITY Polysiloxanes are by sure one of the most thoroughly studied polymers for medical applications. One of the most interesting attributes of polysiloxanes, in particular PDMS, is their biocompatibility. Sometimes the term “bioinert” is used for the properties of silicones which imply that the body’s cellular and biomolecular systems cannot readily decompose a material. The bioinertness of silicones can be attributed to both its hydrophobicity and chemical inertness [78]. However, polysiloxanes can also be used as bioactive components if modified in a specific way. They can be used for the improvement hydroxyapatite growth, one of the inorganic components of bone, and are expected to be useful as a new kind of bone-repairing materials, because of their high bioactivity and unique mech. properties. Ca2+ ion modified TEOS/PDMS sol-gel material monoliths were examined concerning their bioactivity by application of a so called simulated body fluid (SBF), which is a mixture to evaluate bone-bonding ability of a material by examining the ability of apatite to form on its surface. This fluid has ion concentrations of Na+, K+, Mg+, Ca+, Cl-, HCO3-, HPO42-, and SO42- nearly equal to those of human blood plasma. The Ca2+ containing hybrid materials were bioactive in that way that they deposited apatite during soaking in SBF [79]. The addition of P2O5 in concentrations from 0 to 3 mol% decreased the induction period for apatite formation on these materials. A further increase of the P2O5 content, however, increased the induction period [80]. Dense and homogeneous monoliths of PDMS-modified CaO-SiO2-TiO2 hybrids were also synthesized by sol-gel process.[81] They were assumed to be composed of a silica and titania network incorporated with PDMS and the Ca2+ ionically bonded to the network. Depending on the concentrations of the various components in the formed materials and the processing of the material high apatite-forming ability, high extensibilities, and Young's moduli almost equal to those of the human cancellous bones was observed [82-84]. It was also observed that the apatite forming ability of the materials in SBF increased with decreasing PDMS content. The PDMS in the hybrid materials gave little effect on the

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mechanical properties in the examined compositional range, and all the examined products showed analogous mechanical properties [85, 86]. The addition of transition metal alkoxides, different from titania, for example Nb- or Ta-oxides was also investigated. With these materials it was shown that the releasing ability of calcium from the materials into simulated body fluid was different depending on the amount of transition metal incorporated [87]. PDMS-containing hybrid coatings with different inorganic additives were developed on various substrates with dip-coating technique. Blood compatibility of these coatings was evaluated in terms of blood clotting time and platelet adhesion [88]. The obtained hybrid coatings have high contact angles and low surface free energies compared to the coating-free metal surface. The blood clotting properties were not affected by the hybrid coatings but the platelet adhesion was depressed to less than 1/20 as much as that for the coating-free substrate. On the other hand some of the coatings showed a very good apatite formation on the surface, which makes them interesting materials in the orthopedic and dentistry industries [89]. Porous bioactive PDMS-TEOS structures were obtained using sucrose particles as porogenes. The resulting materials had a bimodal porous structure with pores of 300-500 µm and 10-50 µm in diameter [90]. In the hybrid scaffolds, human hepatocellular carcinoma cells (HepG2) proliferated actively and formed cell clusters more efficiently than they did in a polyvinyl-alcohol scaffold. When cultivated in PDMS-TEOS, HepG2 cells secreted an approximately three-fold greater amount of albumin than that secreted in a monolayer culture [91].

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SELF-ASSEMBLED MORPHOLOGIES Self-assembly of polymers is one of the most recognized processes in the formation of nanomaterials [92, 93]. In many cases block copolymers are used that have two or more incompatible segments that are connected by covalent bonds. The incompatibility leads to an internal phase separation between the segments where segments of different block copolymers agglomerate to three-dimensional super structures. Typical examples for such systems are amphiphilic block copolymers consisting of a hydrophobic and a hydrophilic segment. Polysiloxanes, in particular PDMS, are very interesting components for such block copolymers due to their hydrophobicity and their high chain flexibility which both improve phase segregation. Typical examples for PDMS containing amphiphilic block copolymers are PDMS-bpoly(ethylene oxide) diblock copolymers (PDMS-b-PEO). These systems self-assemble into different structures depending on the length of the two blocks [94-96]. Self-assembly of these amphiphilic diblock copolymers in water as a solvent selective for the PEO block leads to the formation of spherical and cylindrical micellar structures. The core of the resulting micelles is formed of the hydrophobic PDMS chains, whereas the corona is set up by the hydrophilic PEO blocks. α,ω-Heterotelechelic diblock copolymers were used to fix the core of the micelles by crosslinking through PDMS-sided (α) methacrylic end groups. The resulting nanoparticles possess the PEO-sided (ω) functional end groups introduced during the synthesis of the amphiphilic diblock copolymer. Various hydrophilic functionalities were thus

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introduced such as uncharged hydroxy end groups (-OH), hydrophilic, charged carboxylate (COOH) end groups, or hydrophobic benzylic end groups (-CH2C6H5) (Figure 1) [97]. Nanoparticles with an inorganic silica core were formed by reacting silica nanoparticles with monoglycidyl ether-terminated PDMS [98]. The PDMS arms of these star-like systems formed a domain to separate the silica particles and to prevent particle aggregation. The resulting systems exhibited good thermal stability and high activation energy of their degradation reaction, in comparison to linear PDMS polymers and PDMS/silica blended materials.

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Figure 1. Formation of polymer crosslinked nanoparticles from self-assembly of PDMS-PEO diblock copolymers.

A very interesting class of block copolymers containing PDMS segments are poly(ferrocenyldimethylsilane)-containing polymers because of their extraordinary properties achieved by their metal-containing ferrocenyl segment. Highly asymmetric block copolymers normally form spherical starlike micelles in a solvent selective for the longer block. Based on this observation poly(ferrocenyldimethylsilane)-b-PDMS (PFS-b-PDMS) diblock copolymers build long rodlike micelles in hexane solution (Scheme 7) [99, 100]. Several analytical techniques revealed that individual cylindrical micellar structures form with the iron-rich, organometallic PFS core surrounded by a PDMS corona. Time- and temperature-dependence studies revealed that a variety of morphologies are initially built depending on the conditions of sample preparation, but most of them eventually rearrange to form nanotubules. The block copolymer develops hexagonally packed cylinders in the bulk, and exposure to warm hexane causes the cylinders to disperse in the solvent. Light scattering measurements showed that the micelles are flexible rods which are stable in hexane even at 80°C. PFS can be oxidized to a semiconductive state therefore the rodlike micellar structures have the potential to function as nanoscale self-insulated wires because of the PDMS which acts as an insulator. Bu4Pb could be trapped in the cavity of the nanotubules [101].

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H3C CH3 H3C(H2C)3

Si

O

Fe

Si(CH 3)3

Si m

H3C CH3

n

PFS-b-PDMS Scheme 7.

The formed nanotubes undergo a remarkable morphological transition with a change in temperature in n-decane solution. While at 25 °C nanotubes are present, a rearrangement occurs at 50°C and short dense rods are formed [102]. When the solution is cooled to 25 °C, the system evolves back to nanotubes. Hydrosilylative crosslinking of the coronas of tubules of self-assembled PFS-b-PVMS in hexane result in shell-crosslinked organometallic nanotubes with reversible redox behavior and tunable swellability [103]. PFS-b-PDMS-b-PFS triblock copolymers form multiple micellar morphologies if dissolved in n-hexane solutions including spheres, cylinders, and flower-like supramolecular aggregates as observed by TEM after solvent evaporation [104].

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SOFT LITHOGRAPHY Soft lithography is a low-cost, non-photolithographic method for the fabrication of microand nanometer-sized structures. It uses organic materials to generate patterns on surfaces, without applying light or other high energy radiation. Therefore, it is a very convenient and economic way to produce sub-micrometer structures. A variety of surface-patterns with dimension ≥ 30 nm can be produced by techniques such as microcontact printing (µCP), replica molding, micromolding in capillaries (MIMIC), and microtransfer molding (µTM), have been demonstrated for the fabrication of patterns and structures of a variety of materials. The fundamental process in soft-lithography is the transfer of self-assembled monolayers (SAMs) by an elastomeric stamp of PDMS with an relief structure on its surface [105]. The advantage of using an elastomeric (rather than rigid) mold such as PDMS simplifies the separation between the replica and the mold, and greatly reduces the possible damage to the mold and the fragile structures on the surface of the replica. Replica molding against an elastomeric mold is an extended form of the conventional technique based on rigid molds. The use of elastomeric molds allows the sizes and shapes of the features on the final replicas to be controlled by using mechanical compression, bending, stretching, or a combination of these techniques, and thus, adds flexibility to the replica molding technique. Replica molding against a deformed elastomeric mold provides a unique new route to fabricate complex micro- and nano-structures with shapes, sizes, and periodicities that are significantly different from those on the original master. Replica molding against a PDMS mold is even capable to produce patterns on curved surfaces [106].

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In soft-lithography PDMS has several advantages over other potential polymers that can be used such as polyurethanes or polyimides [107]. PDMS as an elastomer conforms to the surface of the substrate over a relatively large area. Because of its deformable nature PDMS can achieve conformal contact even on surfaces that are nonplanar on the micrometer scale. The elastic characteristic of PDMS allows it to be released easily, even from complex and fragile structures. Furthermore, PDMS provides a surface that is low in interfacial free energy and it is chemically inert, which is a critical issue because the molecules that are molded should not adhere irreversibly to or react with the surface of the stamp. PDMS is homogeneous, isotropic, and optically transparent down to about 300 nm which makes it possible to cure prepolymers by UV cross-linking. In addition, PDMS is a durable elastomer and therefore the stamp can be used several dozen of times without noticeable degradation in performance.

Figure 2. Schematic illustration of the method of microcontact printing (µCP).

An often used system to obtain PDMS stamps is the commercially available Sylgard 184 from Dow Corning. Typically a hydrosilation reaction is used for crosslinking. Sylgard 184

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for example is composed of dimethylsiloxane oligomers with vinyl-terminated end groups, platinum catalyst, and silica filler (dimethylvinylated and trimethylated silica), while the curing agent contains a crosslinking agent (dimethylmethylhydrogen siloxane) and an inhibitor (tetramethyltetravinyl cyclotretrasiloxane). Crosslinking between the components occurs when vinyl and Si-H groups undergo a hydrosilation reaction in the presence of the catalyst (Scheme 8). Commercially available polysiloxane curing systems were often prepared for applications other than soft lithography therefore optimized formulations were prepared by various groups that overcame some of the drawbacks of commercial systems. Chemical modification of polysiloxanes meet the requirements of soft lithography much better, which include (i) the ability to photocure that avoids thermally induced stresses and shrinkage and enables photopatterning, (ii) medium to high modulus with the ability to tune this property, (iii) high physical toughness, (iv) low curing induced shrinkage, and (v) low thermal expansion coefficient. The optimization of the prepolymer molecular weight and the composition (vinyl-functionalized polymers to Si-H-functionalized polymers) already gives an improvement in the properties of the polysiloxanes for soft lithography applications [108]. In addition, a layered design of the stamps also improves their properties. So called composite stamps arranged in two polymer layers, a stiff layer supported by a flexible layer, extend the capabilities of soft lithography to the generation of 50-100 nm and even smaller features [109, 110]. This compares to usual pattern dimensions which are on the size scale of 500 nm and larger with conventional commercially available systems. The reasons for the range limit of the method are in part by the low elastic modulus of the PDMS that leads to a collapse of the very fine structures. For example, an elastomeric layer in between the backplate (support of the stamp) and a mechanically more rigid front layer show an improved performance to print in particular on uneven surfaces. A good surface hardness is important to reduce a collapse of the imprinted structures. Based on these studies novel systems were developed which consist of methacrylate-modified siloxane prepolymers synthesized from diepoxy- and bis(hydroxyalky)-terminated PDMS prepolymers through an “epoxy ring-opening reaction” in the presence of an amine catalyst [111]. Subsequently, the pendant hydroxyalkyl groups of the prepolymer were converted to urethane methacrylates by reaction with 2-isocyanatoethyl methacrylate. The resulting prepolymer is based on long, linear PDMS chains that are already polymerized but which remain in a viscous liquid phase. The prepolymerization minimizes the large shrinkage that can be associated with photocuring. The urethane methacrylate pendant groups serve as the rigid, photocurable cross-linkers. Therefore, the physical properties of the resulting cured systems are better in many respects than those of the commercially available materials. In addition, the physical toughness and modulus of this material system can be adjusted by controlling the cross-linking density. Its photocurability allows the elements to be patterned by masking the exposure light.

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Guido Kickelbick CH3 Si

CH3

CH3 O

Si

O

Si

O

CH3 CH3 Si

O

Si

O

Si H

O

+

Si CH3

O

CH3

Si H3C

H3C

O

m

CH3

Si

CH3 Si CH3

O

Si CH3

O m

CH3

[Pt]

m

H

CH3

CH3

CH3

O

Si

O

CH3

Scheme 8.

The surface properties of PDMS can be readily modified by various chemical treatments followed by the formation of SAMs to give appropriate interfacial interactions with materials that themselves have a wide range of interfacial free energies. The PDMS surface is usually hydrophobic and is changed to a hydrophilic surface by various techniques such as exposure to UV light or exposure to oxygen plasma (in microfluidics). Thus an OH-functionalized surface is produced which can be used for further chemical reactions (Scheme 9) [112].

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Scheme 9.

A different plasma process can also be used for the functionalization of PDMS surfaces with cyano groups. In this two-step procedure a mixed gas of argon and hydrogen was used to in a microwave plasma pretreatment to activate PDMS and subsequently acrylonitrile was grafted onto PDMS, generating a hydrophilic surface with cyano (-CN) groups. The PDMS hydrophilic surface exhibited high affinity for wetting acetonitrile, which is a conventional solvent for DNA synthesis, and the hydrophilicity was stable over a long time period [113]. Many different chemicals can be used as “ink” for soft lithography. For example uniform, close-packed monolayer and bilayer arrays of alkanethiol-coated gold nanoparticles have been used for microcontact printing [114]. During this process first a uniform monolayer of the nanoparticles is self-assembled on a water surface and is transferred intact to a patterned PDMS stamp pad. In the case of multilayer printing, this "inking" step is repeated as many times as desired. Because multilayer arrays are assembled on the stamp pad layer-by-layer, adjacent layers may be made up of the same or different particles. The nanoparticles were afterwards transferred to a substrate. This technique has been used to print patterned

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monolayer and bilayer arrays on both hydrophobic and hydrophilic substrates. For further examples the reader is referred to the extensive literature on soft lithography.

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TEMPLATING The self-assembly of block copolymers in solution and the subsequent formation of a material around the formed structures can also be used to direct the internal structure of various materials. In most cases the template is of organic and the material formed around it is of inorganic nature. Depending on the phases obtained by the self-assembly of the block copolymers lamellar, micellar or three dimensional cubic structure are obtained. Porous materials that can have an ordered pore structure are formed if the template is removed by washing procedures or, more often, by calcination. The thus obtained so called mesoporous materials show an increased scientific interest in recent years due to potential applications as membranes, catalyst supports or as a reaction media where chemical reactions are carried out in the confined space of the material. Lamellar surfactant/silica powders were obtained when surfactants containing a PDMS backbone and side chains of poly-(ethylene oxide)-b-poly(propylene oxide) were used a structure directing agents during the hydrolysis of TEOS in acidic media or at neutral pH. [115]. More complex silica mesostructures were formed if a mixture of the silicone surfactant and a commercial available PEO-PPO surfactant (P123) was used as the template. After calcination highly ordered lamellar hybrid oxides with large interlayer spacing were obtained using silicone surfactants as templates under a wide range of synthetic conditions. Lamellar structures were also formed when well-defined amphiphilic block PDMS-bPEO block copolymers were used as templates in a solvent-evaporation driven synthesis approach to self-assembled mesostructured silica films [116]. After calcinations at a certain temperature the organic segments in the template were removed, however the lamellar structure did not collapse. This unusual phenomenon was ascribed a kind of column formation between the lamellar layers that stabilizes the lamellar structure. Templating can also be used for structuring the surface of a material. Many natural products are structured on a micrometer and nanometer length scale resulting in extraordinary properties of the materials. Mimicking such natural systems has become one of the major sources for materials design in the last years. Again polysiloxanes are important tools for the production of novel systems due to their unmatched properties. Textured superhydrophobic surfaces were prepared by nanocasting with a real lotus leaf as template [117]. PDMS was cast on fresh lotus leafs and cured. The thus obtained complementary topological surface structure of the original template was used for a second replication process. In this way, the complex surface patterns of the lotus leaf are transferred onto the surface of solid-state PDMS with high fidelity. When viewed in an SEM, both the positive replica and the lotus leaf show the same surface morphology of small papillate hills. This results in similar properties like the surface-driven superhydrophobicity of the artificial leaf.

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ION-CONDUCTING AND MEMBRANE APPLICATIONS Polysiloxanes are ideal polymer backbones for ion conducting systems due to their extraordinary properties that are advantageous for these applications such as low Tg and low tendency to crystallize, simple side-chain functionalization and high thermal stability and chemical inertness. Typically the ion conductivity in these systems is provided by side chains pending from the siloxane backbone, many times ethylene glycol oligomers as side chains or in crosslinked systems are used [118, 119]. Usually the ion conductivity is tested for Li+, Na+ or K+ transport and is also strongly dependent on the counterions introduced in the material [120]. Strong ion pairing usually decreases the ion conductivity in these materials. Solvent-free polymeric alkali-metal ion conductors were obtained by polysiloxanes with oligo(oxyethylene) side chains and pendent sulfonate groups. These systems were synthesized by the hydrosilation of polyethylene glycol allyl methyl ether and allyl glycidyl ether, followed by sulfonation of the oxirane ring. However, due to the tightness of the alkali-metal sulfonate ion pair, conductivities were not improved and were temperature dependant. Additives such as linear and macrocyclic polyethers strongly enhance the conductivities by stretching the interionic SO3-M+ ion pair bonds and by lowering the glass transition temperature [121]. Other ion binding side groups such as crown ether modified azobenzenes were also used for this application [122]. These polymers exhibited a smectic liquid crystal phase. Polymer composite films containing polysiloxanes with these functional groups and an alkali-metal salt underwent remarkable photoinduced ionic-conductivity switching, which is ascribable to structural changes in its highly-oriented side chains, induced by isomerization of its azobenzene moiety. The ionic conductivity was drastically decreased by UV light and then restored to the initial value by visible light. Ion conductivity was also combined with other properties such as electrochromicity [123]. For this purpose, for example, poly(ethylene oxide)-polysiloxane hybrids were formed as a colourless and elastic membrane by condensation of a bis(silylpropyl) ether of oligo(ethylene oxide) and a polyalkoxysilane in the presence of LiClO4. The materials were swollen by propylene carbonate and a transparent solid-state electrochromic device composed of Prussian blue- and WO3-coated electrodes with PEO was prepared which could be colorswitched by applying a currency. Polysiloxanes also play a role in the formation of membranes due to their excellent permeability for different gases. This permeability is improved if composite materials are formed. A variety of fillers was already investigated. The probably most successful systems are zeolite fillers. The included zeolite fillers not only introduce additional porosity but also shapeselectivity into the filled elastomers. Such filled systems are excellent solvent-resistant nanofiltration membranes with enhanced fluxes and retentions compared to commonly used membranes, allowing use in non-polar solvents and at high temperatures [124]. The effect of zeolite particle size on the performance of silicalite-PDMS mixed matrix membranes was, for example, investigated at two different zeolite loadings [125]. The separation properties of the obtained membranes were characterized by permeability measurements for O2, N2 and CO2. The permeability of the membranes increased with

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increasing particle size and the variations occurring in the permeability values with changes made in the particle size are much more pronounced at the higher zeolite loading. Selectivity values corresponding to the mixed matrix membranes generally seem to be less affected by the changes made in the particle size. However, the permeability values corresponding to the mixed matrix membranes exceed those of the original polymer membrane only at relatively high zeolite loadings and/or for relatively larger particle sizes. Other mixed membranes were investigated in the separation of n-pentane from isopentane as a function of zeolite loading and various experimental conditions. Here, no improvement with respect to the n-pentane/iso-pentane ideal selectivity of the original polymeric membrane could be obtained [126]. ZSM-5 zeolite-incorporated PDMS membranes were investigated for the separation of iso-propyl alcohol/water mixtures at various temperatures. Both the permeation flux and selectivity increased simultaneously with increasing zeolite content in the membrane matrix. It was shown that a pure membrane exhibited higher activation energy values for permeability and diffusivity than zeolite-incorporated membranes [127]. Other rigid components that were incorporated into a PDMS matrix for the application as membranes were polyhedral oligomeric silsesquioxane (POSS) compounds. These molecular inorganic cubes were for example used for the synthesis of novel tricomponent amphiphilic membranes consisting of PDMS and PEO segments co-crosslinked and reinforced by octasilane polyhedral oligomeric silsesquioxane cages [128].

SUMMARY

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Polysiloxanes are extraordinary polymers which can be used in the development of many different materials. In particular, their properties that are not reached by organic polymers make them suitable building blocks for various applications in different fields. Major advantages of these polymers are their hydrophobicity, their chemical inertness and high thermal stability in combination with the optical transparency and UV stability. The elastomeric properties of crosslinked polysiloxanes are another advantage that makes them important materials in soft lithography. All these excellent properties lead to a transfer of the long time success of the polymers in consumer applications back to the fundamental scientific investigations and thus a renaissance of these materials in the academic community.

REFERENCES [1] [2]

[3]

J. E. Mark, H. R. Allcock, R. West, Inorganic Polymers, Prentice Hall, Englewood Cliffs, 1992. C. J. Embery, S. R. Clarke, J. G. Matisons, in Handbook of Organic-Inorganic Hybrid Materials and Nanocomposites, Vol. 2 (Ed.: H. S. Nalwa), American Scientific Publishers, Stevenson Ranch, CA, USA, 2003, pp. 331. J. W. Grate, S. N. Kaganove, Polym. News 1999, 24, 149.

186 [4]

[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

Guido Kickelbick B. Boutevin, F. Guida-Pietrasanta, A. Ratsimihety, in Silicon-Containing Polymers (Eds.: R. G. Jones, W. Ando, J. Chojnowski), Kluwer Academic Publishers, Dordrecht, Netherlands, 2000, pp. 79. G. Kickelbick, J. Bauer, N. Hüsing, in Silicon Chemistry (Eds.: P. Jutzi, U. Schubert), Wiley-VCH, Weinheim, Germany, 2003, pp. 439. J. Bauer, N. Hüsing, G. Kickelbick, J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3975. G. Kickelbick, Progr. Polym. Sci. 2003, 28, 83. C. Sanchez, B. Julian, P. Belleville, M. Popall, J. Mater. Chem. 2005, 15, 3559. E. Ruiz-Hitzky, P. Aranda, J. M. Serratosa, in Handbook of Layered Materials (Eds.: S. M. Auerbach, K. A. Carrado, P. K. Dutta), Marcel Dekker, New York, 2004, pp. 91. A. Usuki, N. Hasegawa, M. Kato, Adv. Polym. Sci. 2005, 179(Inorganic Polymeric Nanocomposites and Membranes), 135. J. Ma, Z.-Z. Yu, H.-C. Kuan, A. Dasari, Y.-W. Mai, Macromol. Rapid Commun. 2005, 26, 830. S. H. Anastasiadis, K. Karatasos, G. Vlachos, E. Manias, E. P. Giannelis, Mater. Res. Soc. Symp. Proc. 1999, 543(Dynamics in Small Confining Systems IV), 125. S. H. Anastasiadis, K. Karatasos, G. Vlachos, E. Manias, E. P. Giannelis, Phys. Rev. Lett. 2000, 84, 915. S. D. Burnside, E. P. Giannelis, Journal of Polymer Science, Part B: Polymer Physics 2000, 38, 1595. D. F. Schmidt, F. Clement, E. P. Giannelis, Adv. Funct. Mater. 2006, 16, 417. P. C. LeBaron, T. J. Pinnavaia, Chem. Mater. 2001, 13, 3760. S. Bruzaud, G. Levesque, Chem. Mater. 2002, 14, 2421. J. Ma, J. Xu, J.-H. Ren, Z.-Z. Yu, Y.-W. Mai, Polymer 2003, 44, 4619. A. Lappas, A. Zorko, E. Wortham, R. N. Das, E. P. Giannelis, P. Cevc, D. Arcon, Chem. Mater. 2005, 17, 1199. G. Schmid, Nanoparticles, Wiley-VCH, Weinheim, Germany, 2004. N. S. Enikolopyan, M. L. Fridman, I. O. Stal'nova, V. L. Popov, Adv. Polym. Sci. 1990, 96, 1. H. Takeuchi, C. Cohen, Macromolecules 1999, 32, 6792. R. H. Gee, R. S. Maxwell, L. N. Dinh, B. Balazs, Mater. Res. Soc. Symp. Proc. 2002, 710(Polymer Interfaces and Thin Films), 25. D. Fragiadakis, P. Pissis, L. Bokobza, Polymer 2005, 46, 6001. S.-Y. Lu, H.-Y. Huang, K.-H. Wu, J. Mater. Res. 2001, 16, 3053. M. K. Corbierre, N. S. Cameron, M. Sutton, S. G. J. Mochrie, L. B. Lurio, A. Ruehm, R. B. Lennox, J. Am. Chem. Soc. 2001, 123, 10411. I. Pastoriza-Santos, J. Perez-Juste, G. Kickelbick, L. M. Liz-Marzan, J. Nanosci. Nanotechn. 2006, 6, 453. S. Malynych, H. Robuck, G. Chumanov, Nano Lett. 2001, 1, 647. S. Malynych, G. Chumanov, J. Am. Chem. Soc. 2003, 125, 2896. M. A. de Jesus, K. S. Giesfeldt, M. J. Sepaniak, Appl. Spectrosc. 2004, 58, 1157. D. W. Kang, H. G. Yeo, K. S. Lee, J. Inorg. Organomet. Polym. 2004, 14, 73. T. Ogoshi, T. Fujiwara, M. Bertolucci, G. Galli, E. Chiellini, Y. Chujo, K. J. Wynne, J. Am. Chem. Soc. 2004, 126, 12284. J. T. Sun, Y. D. Huang, H. L. Cao, G. F. Gong, Polym. Polym. Compos. 2004, 12, 739.

Polysiloxanes as Templates and Building Blocks in Nanostructured Materials [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56]

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

[57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68]

187

J. Ouyang, S. Zhou, F. Wang, S. H. Goh, J. Phys. Chem. B 2004, 108, 5937. B. P. S. Chauhan, J. S. Rathore, T. Bandoo, J. Am. Chem. Soc. 2004, 126, 8493. B. P. S. Chauhan, J. S. Rathore, N. Glloxhani, Appl. Organomet. Chem. 2005, 19, 542. J. T. Han, X. Xu, K. Cho, Langmuir 2005, 21, 6662. N. Buske, H. Sonntag, T. Goetze, Colloids Surf. 1984, 12, 195. K. Raj, R. Moskowitz, J. Magn. Magn. Mater. 1990, 85, 233. S. Odenbach, J. Phys.: Condens. Matter 2004, 16, R1135. M. Rutnakornpituk, M. S. Thompson, L. A. Harris, K. E. Farmer, A. R. Esker, J. S. Riffle, J. Connolly, T. G. St. Pierre, Polymer 2002, 43, 2337. M. L. Vadala, M. Rutnakornpituk, M. A. Zalich, T. G. St. Pierre, J. S. Riffle, Polymer 2004, 45, 7449. J. Connolly, T. G. St. Pierre, M. Rutnakornpituk, J. S. Riffle, J. Phys. D: Appl. Phys. 2004, 37, 2475. K. S. Wilson, J. D. Goff, J. S. Riffle, L. A. Harris, T. G. St. Pierre, Polym. Adv. Technol. 2005, 16, 200. V. N. Popov, Mater. Sci. Eng., R 2004, R43, 61. G. Schmidt, M. M. Malwitz, Curr. Opin. Colloid Interface Sci. 2003, 8, 103. R. Andrews, M. C. Weisenberger, Curr. Opin. Solid State Mater. Sci. 2004, 8, 31. N. Grossiord, J. Loos, O. Regev, C. E. Koning, Chem. Mater. 2006, 18, 1089. E. Lahiff, C. Y. Ryu, S. Curran, A. I. B. Minett, Werner J., P. M. Ajayan, Nano Lett. 2003, 3, 1333. N. Zhang, J. Xie, M. Guers, V. K. Varadan, Smart Mater. Struct. 2004, 13, N1. C.-C. M. Ma, Y.-L. Huang, H.-C. Kuan, Y.-S. Chiu, J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 345. C. Sanchez, F. Ribot, B. Lebeau, J. Mater. Chem. 1999, 9, 35. D. A. Loy, G. M. Jamison, B. M. Baugher, S. A. Myers, R. A. Assink, K. J. Shea, Chem. Mater. 1996, 8, 656. F. Babonneau, L. Bois, J. Livage, S. Dire, Mater. Res. Soc. Symp. Proc. 1993, 286(Nanophase and Nanocomposite Materials), 289. F. Babonneau, New J. Chem. 1994, 18, 1065. K. Noble, A. B. Seddon, M. L. Turner, P. Chevalier, I. A. Mackinnon, J. Sol-Gel Sci. Technol. 2000, 19, 807. N. Hüsing, J. Bauer, G. Kalss, G. Garnweitner, G. Kickelbick, J. Sol-Gel Sci. Technol. 2003, 26, 73. J. D. Mackenzie, Q. Huang, T. Iwamoto, J. Sol-Gel Sci. Technol. 1996, 7, 151. L. Guo, J. Hyeon-Lee, G. Beaucage, J. Non-Cryst. Solids 1999, 243, 61. Q. W. Yuan, J. E. Mark, Macromol. Chem. Phys. 1999, 200, 206. J. Hyeon-Lee, L. Guo, G. Beaucage, M. A. Macip-Boulis, A. J. M. Yang, J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 3073. Y. Hu, J. Mater. Sci. 2000, 35, 3155. K. Morita, Y. Hu, J. D. Mackenzie, J. Sol-Gel Sci. Technol. 1994, 3, 109. J. H. Kim, B. W. Dudley, P. J. Moyer, J. Lightwave Technol. 2006, 24, 612. D. A. Nivens, Y. Zhang, S. M. Angel, Anal. Chim. Acta 1998, 376, 235. J. Wen, J. E. Mark, Rubber Chem. Technol. 1994, 67, 806. J. Wen, J. E. Mark, Polym. J. (Tokyo) 1995, 27, 492. N. Yamada, I. Yoshinaga, S. Katayama, J. Sol-Gel Sci. Technol. 1998, 13, 445.

188 [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90]

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

[91] [92] [93] [94] [95] [96] [97] [98] [99]

Guido Kickelbick N. Yamada, I. Yoshinaga, S. Katayama, J. Mater. Res. 1999, 14, 1720. S. Dire, R. Ceccato, S. Gialanella, F. Babonneau, J. Eur. Ceram. Soc. 1999, 19, 2849. C. Guermeur, J. Lambard, J.-F. Gerard, C. Sanchez, J. Mater. Chem. 1999, 9, 769. S. Dire, R. Ceccato, F. Babonneau, J. Sol-Gel Sci. Technol. 2005, 34, 53. J.-D. Kim, I. Honma, Electrochim. Acta 2003, 48, 3633. B. Julian, C. Gervais, E. Cordoncillo, P. Escribano, F. Babonneau, C. Sanchez, Chem. Mater. 2003, 15, 3026. B. Julian, C. Gervais, M.-N. Rager, J. Maquet, E. Cordoncillo, P. Escribano, F. Babonneau, C. Sanchez, Chem. Mater. 2004, 16, 521. B. Julian, R. Corberan, E. Cordoncillo, P. Escribano, B. Viana, C. Sanchez, J. Mater. Chem. 2004, 14, 3337. D. P. Brennan, A. Dobley, P. J. Sideris, S. R. J. Oliver, Langmuir 2005, 21, 11994. M. E. Van Dyke, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2004, 45(1), 600. K. Tsuru, C. Ohtsuki, A. Osaka, T. Iwamoto, J. D. Mackenzie, Mater. Res. Soc. Symp. Proc. 1996, 435, 403. C. M. Whang, D. W. Seo, E. O. Oh, Y. H. Kim, Glass Phys. Chem. 2005, 31, 396. Q. Chen, F. Miyaji, T. Kokubo, T. Nakamura, Biomaterials 1999, 20, 1127. Q. Chen, M. Kamitakahara, N. Miyata, T. Kokubo, T. Nakamura, J. Sol-Gel Sci. Technol. 2000, 19, 101. Q. Chen, N. Miyata, T. Kokubo, T. Nakamura, J. Mater. Sci. - Mater. Med. 2001, 12, 515. Q. Chen, N. Miyata, T. Kokubo, T. Nakamura, J. Am. Ceram. Soc. 2003, 86, 806. M. Kamitakahara, M. Kawashita, N. Miyata, T. Kokubo, T. Nakamura, J. Sol-Gel Sci. Technol. 2001, 21, 75. M. Kamitakahara, M. Kawashita, N. Miyata, T. Kokubo, T. Nakamura, J. Mater. Sci. Mater. Med. 2004, 14, 1067. M. Fukushima, E. Yasuda, H. Kita, M. Shimizu, Y. Hoshikawa, Y. Tanabe, Ceram. Eng. Sci. Proc. 2005, 26, 103. T. Yabuta, K. Tsuru, S. Hayakawa, A. Osaka, J. Sol-Gel Sci. Technol. 2004, 31, 273. N. Hijon, M. Manzano, A. J. Salinas, M. Vallet-Regi, Chem. Mater. 2005, 17, 1591. T. Yabuta, E. P. Bescher, J. D. Mackenzie, K. Tsuru, S. Hayakawa, A. Osaka, J. SolGel Sci. Technol. 2003, 26, 1219. K. Kataoka, Y. Nagao, T. Nukui, I. Akiyama, K. Tsuru, S. Hayakawa, A. Osaka, N.-h. Huh, Biomaterials 2005, 26, 2509. K. Ishizu, K. Tsubaki, A. Mori, S. Uchida, Progr. Polym. Sci. 2003, 28, 27. W. T. S. Huck, Int. J. Nanotechn. 2004, 1, 119. G. Kickelbick, J. Bauer, N. Huesing, M. Andersson, K. Holmberg, Langmuir 2003, 19, 10073. G. Kickelbick, J. Bauer, N. Huesing, M. Andersson, K. Holmberg, Colloids Surf. A 2005, 254, 37. G. Kickelbick, J. Bauer, N. Huesing, M. Andersson, A. Palmqvist, in Langmuir, Vol. 19, 2003, pp. 3198. O. Rheingans, N. Hugenberg, J. R. Harris, K. Fischer, M. Maskos, Macromolecules 2000, 33, 4780. Y.-L. Liu, S.-H. Li, Macromol. Rapid Commun. 2004, 25, 1392. J. Massey, K. N. Power, I. Manners, M. A. Winnik, J. Am. Chem. Soc. 1998, 120, 9533.

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Polysiloxanes as Templates and Building Blocks in Nanostructured Materials

189

[100] J. Raez, R. Barjovanu, J. A. Massey, M. A. Winnik, I. Manners, Angew. Chem., Intern. Ed. 2000, 39, 3862. [101] J. Raez, I. Manners, M. A. Winnik, J. Am. Chem. Soc. 2002, 124, 10381. [102] J. Raez, J. P. Tomba, I. Manners, M. A. Winnik, J. Am. Chem. Soc. 2003, 125, 9546. [103] X.-S. Wang, M. A. Winnik, I. Manners, Angew. Chem., Intern. Ed. 2004, 43, 3703. [104] R. Resendes, J. A. Massey, K. Temple, L. Cao, K. N. Power-Billard, M. A. Winnik, I. Manners, Chem. Eur. J. 2001, 7, 2414. [105] X.-M. Zhao, Y. Xia, G. M. Whitesides, J. Mater. Chem. 1997, 7, 1069. [106] Y. Xia, J. J. McClelland, R. Gupta, D. Qin, X.-M. Zhao, L. L. Sohn, R. J. Celotta, G. M. Whitesides, Adv. Mater. 1997, 9, 147. [107] Y. Xia, G. M. Whitesides, Angew. Chem., Intern. Ed. 1998, 37, 550. [108] H. Schmid, B. Michel, Macromolecules 2000, 33, 3042. [109] T. W. Odom, J. C. Love, D. B. Wolfe, K. E. Paul, G. M. Whitesides, Langmuir 2002, 18, 5314. [110] T. W. Odom, V. R. Thalladi, J. C. Love, G. M. Whitesides, J. Am. Chem. Soc. 2002, 124, 12112. [111] K. M. Choi, J. A. Rogers, J. Am. Chem. Soc. 2003, 125, 4060. [112] H. Ye, Z. Gu, D. H. Gracias, Langmuir 2006, 22, 1863. [113] Q. He, Z. Liu, P. Xiao, R. Liang, N. He, Z. Lu, Langmuir 2003, 19, 6982. [114] V. Santhanam, J. Liu, R. Agarwal, R. P. Andres, Langmuir 2003, 19, 7881. [115] A.-W. Xu, J. C. Yu, H.-X. Zhang, L.-Z. Zhang, D.-B. Kuang, Y.-P. Fang, Langmuir 2002, 18, 9570. [116] N. Hüsing, B. Launay, J. Bauer, G. Kickelbick, D. Doshi, J. Sol-Gel Sci. Technol. 2003, 26, 609. [117] M. Sun, C. Luo, L. Xu, H. Ji, Q. Ouyang, D. Yu, Y. Chen, Langmuir 2005, 21, 8978. [118] P. G. Hall, G. R. Davies, J. E. McIntyre, I. M. Ward, D. J. Bannister, K. M. F. Le Brocq, Polym. Commun. 1986, 27, 98. [119] D. P. Siska, D. F. Shriver, Chem. Mater. 2001, 13, 4698. [120] I. Albinsson, P. Jacobsson, B. E. Mellander, J. R. Stevens, Solid State Ionics 1992, 5356, 1044. [121] G. B. Zhou, I. M. Khan, J. Smid, Macromolecules 1993, 26, 2202. [122] H. Tokuhisa, M. Yokoyama, K. Kimura, Macromolecules 1994, 27, 1842. [123] K. Honda, M. Fujita, H. Ishida, R. Yamamoto, K. Ohgaki, J. Electrochem. Soc. 1988, 135, 3151. [124] L. E. M. Gevers, I. F. J. Vankelecom, P. A. Jacobs, Chem. Commun. 2005, 2500. [125] S. B. Tantekin-Ersolmaz, C. Atalay-Oral, M. Tatlier, A. Erdem-Senatalar, B. Schoeman, J. Sterte, J. Membr. Sci. 2000, 175, 285. [126] S. B. Tantekin-Ersolmaz, L. Senorkyan, N. T. Kalaonra, M., A. Erdem-Senatalar, J. Membr. Sci. 2001, 189, 59. [127] A. A. Kittur, M. Y. Kariduraganavar, S. S. Kulkarni, M. I. Aralaguppi, J. Appl. Polym. Sci. 2005, 96, 1377. [128] I. S. Isayeva, J. P. Kennedy, J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4337.

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In: Silicon-based Inorganic Polymers Editors: Roger De Jaeger and Mario Gleria

ISBN: 978-1-60456-342-9 © 2008 Nova Science Publishers, Inc.

Chapter 4

PHOTOCHEMISTRY OF POLYSILOXANES Frédéric Cazaux2 and Xavier Coqueret1,∗ 1

Laboratoire Réactions Sélectives et Applications - UMR CNRS 6519 Université de Reims Champagne Ardenne Bât. Europol’Agro - BP 1039 F-51687 Reims Cedex 2, France 2 Laboratoire de Chimie Organique et Macromoléculaire, UMR CNRS 8009 Université des Sciences et Technologies de Lille F-59655 Villeneuve d'Ascq Cedex, France

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INTRODUCTION This chapter on the photochemistry of polysiloxanes covers a broad range of reactions induced by UV-visible light. A classification of the various types of photochemical activation is proposed on the basis of the location and of the nature of the absorbing chromophore. These two parameters control in turn the nature of the initial chemistry and of the subsequent reactions. A variety of molecular transformations can be induced with all the specificities of radiation treatments that make it possible to exert a temporal and spatial control of the modifications with an additional degree of freedom in the independent adjustment of temperature. Though the siloxane bond or molecular skeleton –[Si-O]- is not directly involved in many of the photochemically induced reactions that will be presented and discussed in this chapter, the silicone nature of the reacting medium will generally have a strong influence on the observed reactivity, principally because of the very specific properties of this class of unique organic-inorganic polymers. The different types of reactions (Scheme 1) that will be dealt with are classified as follows: i. ii. ∗

Photofragmentation and oxidation of alkylsiloxane units, Photochemical aging of silicone polymers and materials,

E-mail: [email protected] Tel.-fax : + 33 (0) 3 26 91 33 38.

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Frédéric Cazaux and Xavier Coqueret iii. iv. v.

Photoreaction of chromophores bound to a polysiloxane carrier, Reactions induced by a photosensitive polysiloxane reagent in an organic medium, Reactions affecting siloxane compounds triggered by a photochemical process.

ii

i

hν

hν

CH3

CH3

CH3

Si O

Si O

Si O

n

p

CH3 q

Si O

r

CH3 reactive molecules

chromophore

+

monomer reactive group

+

photosensitive molecules

hν hν

iv

hν

iii

v

Scheme 1. Sketch of the different types of photochemical reactions involving polysiloxanes.

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The intrinsic chemical stability of simple alkyl or aryl substituted polysiloxanes together with the absence in their structure of strongly absorbing chromophores confer limited photochemical reactivity to silicones exposed to UV-visible light. Only high dose irradiation and / or the use of very short wavelength UV light yields strong chemical modification. We have been mainly involved for several years in the study of reactions of type (iii) to (v) where specific chromophores are activated onto polysiloxane derivatives or in their vicinity. Wherever appropriate, the tailored synthesis of functional siloxane-based compounds will be described. The influence of the silicone nature of the chemicals on their photo-induced reactivity as well as on the obtained properties will be emphasized.

1. PHOTOFRAGMENTATION AND OXIDATION OF ORGANOSUBSTITUTED SILOXANE UNITS Light absorption by polydialkylsiloxanes 1 is particular weak in the UV-A and UV-B range, compared to common organic polymers [1]. The conjunction of the absence of strongly absorbing chromophores with the inherent stability of Si-O, Si-C, C-C and C-H bonds exempt of unsaturation and of other heteroatomic function in their vicinity indeed provide non functional silicones with outstanding weathering properties under typical terrestrial conditions, as discussed in the next section.

Photochemistry of Polysiloxanes R n

R

Si O Ph

R = CnH2n+1

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1

2

Ph

CH3

CH3

Si O

193

n

Si O

Si O

n

CH3

p

Ph 3

The spectroscopic properties as well as the photophysics of various types of methylphenylsiloxane- and diphenylsiloxane-based oligomers and polymers have been studied in some details [2,3]. The presence of phenyl chromophores causes characteristic UV absorption in the UV-C region (λ < 280 nm) and gives rise to fluorescence bands based on the π–π* transition. When submitted to photochemical excitation with an excimer laser (KrF, 248 nm) or other appropriate sources, homopolymers such as poly(methylphenylsiloxane) 2 and copolymers such as poly(dimethylsiloxane-co-diphenylsiloxane) 3 exhibit monomer fluorescence at 290 nm in fluid solution at temperatures near RT, and monomer fluorescence together with phosphorescence in glassy matrices at 77 K. In addition to these emissions, excimer fluorescence at 330 nm is often observed depending on the molecular structure of the siloxanes [4]. The dependence of the luminescence efficiencies as a function of copolymer composition is rationalized by compositional factors. A low content in diphenylsiloxane units in random copolymers governs the mole fraction of the triads containing isolated diphenylsiloxane units neighbored by dimethylsiloxane units that constitute a photophysically inactive environment. At higher contents in aromatic units, the contribution of dyads and longer sequences of diphenylsiloxane units is revealed by an increasing contribution of excimer fluorescence [5]. Interestingly, the binding energy of the excimer does not seem to be enhanced by increasing the molar content in diphenylsiloxane units. A partial double bond character for the silicon-oxygen linkage in poly(phenylsiloxanes) has been proposed on the basis of UV and IR recorded spectra from a series of oligomers with different structures [6]. The observation of some changes in the emission spectra of phenylsiloxane polymers submitted to repeated 248 nm laser shots suggest the occurrence of chemical reactions modifying the structure of the luminescent chromophores [7]. Considerable chemical transformations can be induced by short wavelength irradiation applied to organic polymers, leading principally to photo-ablation [8]. By virtue of their inorganic skeleton silicones exhibit a distinct behavior. Linear PDMS, alkylsiloxane cage molecules as polymethylsilsesquioxanes, or macromolecular dimethylsiloxane networks are efficiently converted into SiOx materials. Because of the absence of aromatic chromophore, the molecular structure is activated by excitation of sigma or by non-bonding electrons that require absorption of high energy photons (E > 5 eV) [9]. The resulting chemistry involves photolytic cleavage of Si-methyl bond and oxidation of the fragments assisted by the photoactivation of ambient oxygen (Scheme 2). This reaction has been exploited for making waterproof coatings by photo-oxidation of a PDMS oil spin-coated onto the surfaces of various types of optical elements, such as a plastic lens, a laser mirror, and a nonlinear optical crystal [10]. Irradiation with a xenon excimer lamp in air successfully transforms the spread PDMS layer into a conformal coating with the properties of amorphous glass. An interesting set of useful properties are achieved including high transmission of ultraviolet rays of wavelengths even below 200 nm, high density, and

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resistance to environmental effects and to corrosion by water. The protection of polymers against photo-oxidation in outdoor conditions is another promising application.

O CH3

CH3

CH3

h ν (λ < 200 nm)

O Si O Si O Si O CH3

CH3

CH3

O O2

O

O Si O Si O O

O Si O Si O O

+

volatile oxidized organic compounds

O

Scheme 2. Simplified representation of fragmentation and oxidation reactions in PDMS exposed to UV radiation of short wavelength.

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The protective effect was anticipated from the well-known barrier property of SiOx with respect to the migration of oxygen and water [11]. Poly(2-bromoethylsilsesquioxane) was associated with PDMS in the formulation to form the photosensitive precursor for SiOx by treatment with UV in the presence of in-situ generated ozone. Formation of a protective layers of typical thickness in the range of 100 nm was found to be effective in protecting polyurethane automotive coatings, both alone and in conjunction with conventional photostabilizers [12]. Adhesive materials for assembling optical glass parts have been developed on the basis of the same photoreactions [13]. Other applications take advantage of the spatial control of photo-induced oxidative reactions for writing silica optical waveguides in silicone rubber [14]. The converted lines are patterned on the surface PDMS films exposed through a mask. The thin air layer above the substrate generates oxygen radicals that chemically assist in the silica transformation. Optimized laser processing leads to conditions generating crack-free waveguides with good optical transparency at red (635 nm) and infrared (1550 nm) wavelengths. The simple atomic composition and molecular structure of PDMS can therefore be activated under controlled short wavelength irradiation to afford high performance layer. Owing to the large absorption coefficient of the PDMS repeat unit to UV light of wavelength shorter than 220 nm, photon penetration in the depth of treated materials is limited to few micron and sub-micron layers. Surface treatments based on this type of localized surface chemistry that converts hydrophobic layers into hydrophilic and chemically functionalizable are believed to play an increasing role in fabrication of microfluidic devices [15].

2. PHOTOCHEMICAL AGING OF SILICONE MATERIALS Because of their unique properties, polysiloxanes are widely used as the main constituents or as a formulation ingredients in industrial fluids, rubbers, molded parts and coatings for a variety of specialized applications [16]. Compared with most organic molecules, the silicones are exceptionally stable in high temperature oxidative conditions and in aggressive radiation environments. Purely thermal degradation of PDMS occurs at high temperatures by two separate mechanisms that are determined by the temperature and the heating rate of the material. When PDMS is exposed to temperatures ranging from 450 to 650°C and a slow rate of heating, the degradation has been shown to proceed via

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Photochemistry of Polysiloxanes

195

depolymerization of the polysiloxane backbone leading to formation of cyclosiloxanes [1720]. The formation of the cyclic oligomers has been proposed to occur preferentially via an intramolecular, cyclic, four-centered transition state rather than by direct Si-O bond cleavage. The degradation of PDMS is affected by many factors, including end-group functionality, impurities or solvent, and oxidation, that all tend to enhance the rate of degradation. At higher temperatures (above 600°C) and a fast heating rate, the degradation of the PDMS occurs through a radical mechanism involving Si-CH3 homolytic bond cleavage followed by hydrogen abstraction to form methane [20,21]. The degradation of PDMS observed at high temperatures follows the thermodynamically favored pathway resulting from cleavage of the Si-C bond, which is less stable than the Si-O and C-H bonds in PDMS. Photodegradation studies devoted to silicones cover more particularly the various and generally undesirable photochemical reactions that take place within materials submitted to aging under the presence of UV-visible light [1]. Of particular interest is the aging under natural terrestrial conditions, with outdoor exposition to sunlight of wavelengths, ranging from ca. 285 nm to far infrared. The presence of minute amounts of ill-identified chromophores and of minor quantities of chemically reactive functions that are readily involved in the photo-induced processes has raised the need for basic studies addressing rationally the various aspects of these complex phenomena. The photo-oxidation of PDMS oils containing determined amounts of specific functions (Si-H, Si-CH=CH2, Si-Ph, Si-CH2-CH2-Si , Si-CH2-CH3) located in the main chain or as chain ends was studied by irradiation under controlled conditions, principally upon irradiation with UV-visible light of wavelength longer than 300 nm. In the case of Si-H containing PDMS, the generation of free radical from photo-activated chromophores (impurities, peroxides produced upon storage or thermal transformation) results principally in hydrogen abstraction from the hydrosilane bonds. Alternatively, the activated form of dioxygen generated by energy transfer can insert into the hydrogen-silicon bond. The unstable hydroperoxides formed by these various pathways are mostly transformed into silanols. Thus, the presence of hydrosilane functions strongly alters the photolytic behavior of PDMS. This effect was shown to depend weakly on the content in hydrosilane groups, and strongly on the location of the Si-H function. Terminal Si-H units undergo oxidation at a higher rate, probably because of the easy accessibility of the reactive groups [22]. In vinylsiloxane containing PDMS, the oxidation rates, assessed on the basis of photoproduct appearance and consumption, are lower than those observed in the case of hydrosiloxane containing PDMS. In the presence of oxygen, competition between scission and oxidation reactions occurs. Chain scission and direct oxidation of the vinyl group lead to a very complex mixture of photoproducts including silanols, carboxylic acids and esters, as revealed by gas chromatography and mass spectrometry [23]. PDMS containing bridging dimethylene and methine groups representing the bridging points in siloxane elastomers produced by Si-H Si-vinyl addition cross-linking reactions are subject to oxidation at similar oxidation rate to that of vinyl PDMS. The main photooxidation process is the hydroperoxidation of dimethylene groups and the photoscission of hydroperoxides group into β-silylated carboxylic acid (–SiCH2 –COOH) and silanol groups that can further condense together with dehydration [24]. Photo-oxidation studies at wavelengths longer than 300 nm have been conducted with dimethylsiloxane copolymers including various contents of MePhSi and Ph2Si groups to

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Frédéric Cazaux and Xavier Coqueret

examine the influence of the phenylsiloxane units. Compared with the dimethylsiloxane unit, the presence of an aromatic substituent is susceptible to increase light absorption but to reduce the reactivity, alkyl C-H groups being more sensitive to free radical attack than aromatic C-H in phenyl groups. Silicones were studied both in the form of linear and of crosslinked networks. FTIR, UV and fluorescence spectroscopies reveal a slow photo-oxidation involving both scission and oxidation of the phenyl ring. No significant yellowing can be associated with the presence of the aromatic substituents, except when photo-activation is induced by short wavelength irradiation [25]. The basic behavior of siloxane derivatives revealed by these studies makes it much easier to investigate the aging of more complex systems including formulated and filled silicone resins [26]. A simplified description of the overall oxidation process is summarized in Scheme 3.

HO Si

1/2

CH3

HOO

P

H3C Si O Si CH3

0.5 H2O

+

H Si CH3

hν (λ > 300 nm) O2 CH3 H Si O CH3

CH3

CH3

CH3

Ph

CH3

CH3

CH3

CH3

CH3

Si O

n Si O H

Si O

n Si O Ph

Si O

n Si O CH

Si O

n Si O

Si O

CH3

CH3

CH3

CH3

CH2

CH2

HOOC

P

PH or RH

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ROOC

CH2 O Si O

P° or R °

CH3

HO Si

hν

P

O2

(λ > 300 nm) P

HO Si

+

HOOC

CH2

CH3

CH3 n Si CH3 CH3

OOH

Si

Scheme 3. Main chemical reactions occurring during the photochemical aging of polysiloxanes.

Photochemistry of Polysiloxanes

197

3. PHOTOREACTION OF CHROMOPHORES BOUND TO A POLYSILOXANE CARRIER Two types of photosensitive silicone polymers have been synthesized and studied by our team since several years. The first group consists in photocross-linkable polysiloxanes modified with pendant photosensitive unsaturated moieties that can dimerize by a [2+2] cyclo-addition. The reactivity of such polymers in the condensed state reveals the strong influence of structural parameters on the intermolecular photochemical process. The functionalized silicones of the second type are copolymers containing aromatic ketones under the form of ester side-groups that can initiate free radical polymerization by photo-excitation in the presence of an amine co-reactant. The tailored synthesis of silicone copolymers with various combinations and amounts in functional side-groups allowed to evidence the main constitutional parameters that control the photo-initiation efficiency of this novel class of initiators designed for UV-curing applications. Both types were synthesized by a multi-step modification of a parent poly(3glycidoxypropyl methylsiloxane-co-dimethylsiloxane) 4 as depicted in Scheme 4. One or several carboxylic acids can be attached to the silicone backbone by a reaction under relatively smooth conditions. The formation of hydroxy-esters tends to provoke the formation of a physical gel by intermolecular hydrogen bonding. A final acetylation step was shown convenient to yield modified silicones as liquid resins (cinnamic derivatives, R = Ph-CH = CH-) or as meltable waxes (α-cyano-β-styrylacrylic derivatives, R = Ph-CH=CHCH=C(CN)-), depending on the type and amount of pendent photosensitive groups. CH3 H3C Si O CH3

CH3 Si O CH3

CH3 Si O CH2 CH2 CH2 O

m

O

CH2 CH CH2

n+p

CH3 Si CH3 CH3

1) RCOOH 2) acetic anhydride

CH3 H3C Si O CH3

CH3 Si O CH2 m CH2 CH2 O O CH2 H3C C O CH CH2 O C O

CH3 Si O CH3

n

CH3 CH Si 3 CH 3

R

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Scheme 4. Functionalization of polysiloxanes by the epoxy-carboxy coupling reaction.

3.1. Polysiloxanes with Photodimerizable Pendant Groups Solid-state photochemistry is a unique research area for exploring the control of chemical reactivity by geometrical and physical factors. The basic concepts of topochemistry [27] initially developed for interpreting the [2+2] photocycloaddition of mono-olefinic chromophores in the crystalline state have been successfully extended to crystalline compounds bearing two independent reactive groups, thus opening the route to solid state step-growth photopolymerization proceeding under strict control of the crystal lattice [28]. The original photodimerization reaction exhibited by various crystalline forms of cinnamic acid which is still a subject of academic interest [29,30] has been adapted some four decades ago for obtaining organic photo-imaging system based on photocross-linkable

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Frédéric Cazaux and Xavier Coqueret

polymers (Scheme 5) [31]. Polyvinylcinnamate (PV-Cin) is the forerunner of this group of photopolymers working as a negative resist, finding use in microlithography and related applications. The photoreaction proceeding in the polymeric system is conceptually similar to that of the parent acid, both yielding cyclobutane dimers in a single step [32].

O

O

H O

h ν (λ = 280 nm)

H

H C

H O

O

H C

C

C

H

H O

O

H O

Scheme 5. Crosslinking of poly(vinylcinnamate) by photocycloaddition [2+2]. CH3 H3C Si

O

CH3

CH3

CH3

Si O

Si

CH3

CH2

CH3 O

Si

n

CH2

O

O

CH2

CH2

CH3 O

CH2

O

O

C O

C O

H

CH3

CH3

CH3

CH3

Si O

Si

CH3

CH2

n

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O

H C C

C C H

O H C C H

CH3 O

CH2

Si

CH3 O

Si CH3

CH2

p

CH3

q

CH2

CH2

CH2

O

O

CH2

CH2

H3C C O CH

H

p

CH O C

CH2

C C

H3C Si

CH2 CH2

O H3C C O CH

H

Si CH3 CH3

CH2

CH2 m

CH3 O

O

CH O C

CH2

CH2

O

O

C O

C O

C

CH3

H C C

N

C

C C

H

H

N

Chart 1. General formulae of PS-Cin (top) and PS-CSA (bottom).

The geometrical requirements for the pair of reactants that must exhibit parallel olefinic bonds within a critical distance of 4.3 Å however expectedly lead to contrasting reactivity in the crystalline acid in comparison with the amorphous solid PV-Cin. The influence on the

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Photochemistry of Polysiloxanes

199

reactivity of the physical state of the photo-irradiated medium, diluted solution, amorphous glass or, eventually, liquid was quantified on the basis of the dimerization quantum yield and on the extent of the competing dimerization. The distribution of the produced isomeric cyclobutanes in photo-irradiated PV-Cin exposed to UV light at various temperatures afforded very convincing data for imaging the actual distribution of the reactive sites at the instant of the photoreaction. The cinnamoylated dimethylsiloxane copolymers (PS-Cin, see Chart 1) are viscous liquids [33]. The spacer linking the photosensitive moiety to the main siloxane chain is produced under the form of two isomeric structures as a consequence of the epoxy-carboxy reaction involved in the synthetic route [34]. The mole fraction of functional units having the secondary acetoxy group is typically 0.15, as indicated by NMR analysis of model polymers [35]. However, only the overall content in photosensitive units is considered for discussing polymer photoreactivity. The second series (PS-CSA) differs from the cinnamoylated one by the nature of the photodimerizable side-groups. The pendant α-cyano-β-styrylacrylic (CSA) ester is a strongly polarized phenylbutadiene chromophore with the absorption maximum located at longer wavelength than the analogous cinnamic esters (ca. 345 nm vs. 280 nm respectively). In addition to the differences in their spectroscopic features and in the expectable photochemistry, the polar nature of the CSA ester results in stronger molecular associations with consequences on the physical state of the functionalized polymers. At comparable degree of polymerization and molar content in photosensitive pendant groups, the cinnamoylated polymers PS-Cin are liquids of low to moderate viscosity, whereas their CSA analogues are soft waxes losing their opacity slightly above room temperature [36]. The photopolymers presented for this study were initially developed as photo-imagable materials with the remarkable chemical or physical properties of Si-containing molecular compounds. Most of the polymer samples whose characteristics are collected in Table 1 were however synthesized with the only aim of the photoreactivity study, thus neglecting in all regards the influence that molecular weight would exert on practical photosensitivity [37]. UV spectroscopy is easily applicable for following quantitatively simple reaction schemes where the final photoproduct does not absorb light in the reactant main absorption band. This is the case for cinnamic derivatives as a consequence of conjugation loss in the dimeric cyclobutanes. Initially, the cinnamic chromophores are in the E (trans) form exclusively. Upon photo-irradiation, dynamic photo-isomerization gives rise to E-Z interconversion that competes with dimerization, even at temperatures below the glass transition of the host polymer matrix. In dilute solution, PS-Cin give rise almost exclusively to dynamic E-Z isomerization, as does the low molecular weight model ethyl cinnamate. A typical series of UV spectra recorded for monitoring a PS-Cin film irradiated with 312 nm UV light is shown on Figure 1. Concentration profiles in E, Z and dimerized (D) chromophores as a function of exposure time can be determined by monitoring the reaction at two wavelengths. This treatment requires accurate spectroscopic data that can be determined from the study of a low molecular weight model that undergoes only isomerization in dilute solution [38].

200

Frédéric Cazaux and Xavier Coqueret 0.16

A 0.12

0.08

0.04

0 200

250

300

350

Wavelength (nm)

Figure 1. Changes in the UV-spectrum of a film of PS-Cin1 exposed to UV-light.

The variations of the mole ratio of E, Z and dimerized (D) cinnamoyl unit can be calculated by using equations (1)-(3) [16,34].

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[ E ]t [ E ]0

=

ε λE

E max

ε λE

E max

[ Z ]t [ E ]0

=

[ D]t [ E ]0

= 1−

− ε λZE

max

ε λE

E max

ε λE

E max

− ε λZE

Aλt iso Aλ0iso

max

⎡ Aλt E ε λZE ⋅ Aλt iso max ⎢ 0 − Emax 0 ⎢⎣ AλEmax ε λEmax ⋅ Aλ iso ⎡ Aλt E Aλt iso max ⎢ 0 − 0 ⎢⎣ AλEmax Aλ iso

⎤ ⎥ ⎥⎦

⎤ ⎥ ⎥⎦

(1)

(2)

(3)

When PS-Cin samples are irradiated in their original bulk liquid state, the photo-excited cinnamic chromophores initially in the E configuration give rise to extensive isomerization. In a representative example (PS-Cin1, aver. degree of polymerization = 120, chromophore content: 25 mol-%, 1.83 mmol.g-1, Tg = -45°C), the isomerization steady state is reached in the very early instants of irradiation (Figure 2). This behavior reveals that in the bulk liquid state the fraction of cinnamic groups ready for dimerization is extremely low and that the main route for activated chromophore relaxation is E-Z interconversion. From this point of view, the situation is not very different from that of irradiated solutions, and contrast from the photoreactivity of cinnamoylated polymers with the dimerizable units closely bound to a rigid polymer chain [39,40].

Photochemistry of Polysiloxanes

201

UV monitoring of PS-CSA samples photo-irradiation with 365 nm light reveals two clean-cut types of behaviors. A selected sample (PS-CSA1, aver. degree of polymerization = 129, chromophore content: 9.5 mol-%, 0.89 mmol.g-1, Tclarification = 25°C) gave the following representative results.

Figure 2. Progress of the competing isomerization and dimerization (E fraction ({), Z fraction (Δ), fraction of dimerized units (…)) in liquid films of neat PS-Cin1 (a), submitted to Xe lamp UV light (I0 = 45 mW.cm-2).

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In dilute solution, the spectra progressively change until a photostationary state is reached with a persistent isobestic point at 304 nm (Figure 3). These changes are assigned to the dynamic photo-isomerization equilibrium also observed with the low molecular weight CSA esters. The situation is more complex than with cinnamates, since in principle, four stereoisomers can exist for the substituted phenylbutadiene.

Figure 3. Changes in the UV spectrum of PS-CSA1 as a methanol solution exposed to 365 nm light (I0 = 0.45 mW.cm-2).

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Frédéric Cazaux and Xavier Coqueret

In the bulk state, the UV spectra of PS-CSA films treated in the same conditions show an isobestic point at a shorter wavelength than for the solutions, indicating that another but single process takes place (Figure 4).

Figure 4. Changes in the UV spectrum of PS-CSA1 as bulk film exposed to 365 nm light (left, I0 = 0.45 mW.cm-2) and then to 254 nm light (right), for inducing cyclobutane ring-opening.

The polymers become insoluble as a consequence of cross-linking. These changes are logically assigned to pendant group photodimerization. Moreover, irradiating with 254 nm light the photocross-linked films allows recovering most of the original absorbance at 345 nm. The reversible character of the photoreaction is a good indication that clean dimerization and not ill defined polymerization takes place. The almost complete reversibility observed when conversion of CSA is kept below 80 mol-% reinforces the view that the siloxane-bound CSA esters behave as CSA methyl ester does [41]. The spectra recorded after incremental exposure to the UV beam of thin films can be treated by means of Eq. 3 for the cinnamoylated polymers, or by direct measurement of the absorbance changes at 345 nm for the PS-CSA series, to examine the effect of polymer composition on the photodimerization rate R Dim . Since gelation is generally obtained at rather small dimerizable chromophore conversion, at comparable degree of polymerization

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DPw and entanglement density, [16] the initial kinetic behavior determines the relative merit of one photopolymer in a series. Under low absorbance conditions, the dimerization rate R Dim is related to Φ Dim by (Eq. 4) where C 0 is the initial concentration in chromophore, α the fraction of unreacted chromophore, I0 is the number of incident photons per surface and per time unit, and ε the molar absorption coefficient of the chromophore for the incident light beam. The initial quantum yield for dimerization (Φ Dim )0 is thus proportional to the ratio (R Dim )0 / C 0 , where

C 0 is the initial concentration in chromophore. R Dim = 2.303αεC 0 I 0 Φ Dim (α )

(4)

Photochemistry of Polysiloxanes

203

For the PS-Cin series, (RDim )0 / C 0 varies typically in the proportions 1.2 ± 0.2, 2.3 ± 0.3, and 5.6 ± 0.4 for an initial chromophore content increasing from 25, to 44 and to 100 mol-% of functional unit respectively in the series PS-Cin, confirming the expected dependence of dimerization efficiency with the probability of achieving bimolecular encounters. In the PS-CSA series, the initial values (Φ Dim )0 determined for the various samples were independent of their chromophore content. Such a leveled reactivity can be interpreted as the consequence of a phase separated morphology. Optical observation indeed indicates the biphasic structure of the waxy material, reminiscent of textures exhibited by thermotropic aromatic polyester with inherent chemical disorder [42]. Thermo-optical observation as well as X-ray diffraction and calorimetric analyses of PS-CSA materials support the existence of a phase-separated morphology. Photosensitive esters are gathered with part of the main chains in the ordered domains dispersed in a silicone-rich phase [43]. According to this representation, the results indicate almost unchanged composition of the segregated microphases (wt-fraction of chromophore y2) from one polymer to another. Several observations based on bulk state photochemistry indeed showed a leveled reactivity independent of the macroscopic wt-fraction X2 in photosensitive groups (Figure 5). The existence at the irradiation temperature of ordered layers of typical thickness d = 42 Å is intuitively consistent with the clean, reversible photoreaction and should also have some incidence on the observed kinetics.

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Φ0

0.5

0.5

0.4

0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 0.2

0.25

0.3

0.35

y2

0.4

X2 Figure 5. Plots of the initial dimerization quantum yield Φ0 and of the weight fraction of photosensitive pendant groups y2 as a function of their overall weight fraction X2 in the PS-CSA samples.

A kinetic approach based on precise spectroscopic measurements analyzed in combination with thermophysical characterization can be applied to various tailored photosensitive systems with dimerizable units which seem to exert a renewal of interest in polymer photochemistry. In this regard, the application of LC alignment layers, where the precise control of dimerization and isomerization seems important is exemplary [44].

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Frédéric Cazaux and Xavier Coqueret

3.2. Polysiloxanes with Arylketone Side-Groups as Free Radical PhotoInitiators The reactivity of functional groups connected to or inserted into a polymeric carrier is often found to be significantly different from the reactivity of the low molecular weight (LMW) and monofunctional analogue. This is frequently referred to as the polymer effect, the origin of which is complex since it results from the combination of constitutional and physico-chemical effects [45]. In an increasing number of applications, photoreactive polymers are tentatively used to improve the properties of conventional LMW systems. Restricting here our interest to those linear polymers that are modified with chemically functional side-groups, the primary differences are obviously the multifunctional and the macromolecular nature of the new carrier. Many advantages of practical concerns are expected with such systems, as exemplified by the current development of polymeric photoinitiators for UV-curable adhesives, coatings or inks [46]. Aside pigments, fillers and other unreactive additives, the commonly used formulations essentially include a functional prepolymer, a mixture of monomers as reactive diluents and a photo-initiating system which permits to start on-command the polymerization. Among the various problems or insufficiencies associated with the use of conventional photo-initiators, [47] several points can be circumvented or alleviated by using a polymeric photosensitive system. Increased compatibility with the monomer mixture as well as reduced amount of extractables and volatiles after curing are some of the claimed advantages [48]. Several authors have thus developped polymeric photo-initiators generating free radicals (Scheme 6) by monomolecular α-cleavage (Type I) [49] or by intermolecular hydrogen abstraction (Type II) [50]. H

H

hν

C C

C O

O OR

Type I

O C

O C

+ R2NCH3

+

C OR OH C

hν +

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R2NCH3

Type II

R2NCH2

exciplex

Scheme 6. Representative examples of the photogeneration of free radicals by a type I or type II process.

In this context, we have recently undertaken a detailed study of the photo-initiating efficiency of siloxane copolymers including benzophenone or thioxanthone chromophores as side-groups. The versatility of the epoxy-carboxy reaction applied to suitably modified polysiloxanes allows to play around with the substitution of the silicone backbone [51]. Alkyl esters of 2benzoylbenzoic (2-BBA) , 2-thioxanthonyloxyacetic (TXA) and 4-dimethylaminobenzoic (4-DMABA) acids are described in conventional photo-initiating systems. Well-defined polymers including one or more type of functionality, and the corresponding model molecules

Photochemistry of Polysiloxanes

205

(5-7) were thus prepared in such a way that the partners of the initiating reaction, the chromophore and the amine co-initiator, can be introduced in various forms, as LMW compounds or as polymeric reactants [52]. The model compounds containing the aromatic carbonyl chromophores were designed with a long and branched chain representing the spacer binding the functional groups to the polymers (Chart 2). O O C

C

O CH2 CH CH2 O CH2 CH CH2 CH2 CH2 CH3 O CH2 C O CH3 51 CH3

O

C

O CH2 CH3

2-BBA ester

7 3

O C

O CH2 CH CH2 O CH2 CH CH2 CH2 CH2 CH 3 O CH2 C O CH3 62 CH3

S

N H3 C

CH3 DMABA ester

TXA ester

Chart 2. Low molecular weight esters used as chromophore (5, 6) and amine co-initiator (7) in model photo-initiating systems.

The series of polymers which was prepared for the present study has a degree of polymerization DPn = 200 (Chart 3). The degree of functionalization expressed as the mole fraction of siloxane units binding a side-group is 0.25. The general formula of the modified polysiloxanes is depicted below and their constitutional characteristics, i.e. the nature and amount of functional side-groups, are collected in Table 1. In several polymers, both types of functional groups are connected to the same chain. CH3 H3C Si O

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CH3

Pi

CH3 Si O CH3 m CH2 CH2 CH2 O O CH2 H3C C O CH CH2 O C O CH O 2 O C CH3 Si O

S

CH3 Si O CH 2 n CH2 CH2 O O CH2 C O CH CH3 CH2 O O C O C

CH3 Si O CH 2 p CH2 CH2 O CH2 CH O CH2 O O C

q

O C CH3

CH3 CH3 Si CH3 Si O CH2 r CH3 CH2 CH2 O O CH2 CH O C CH3 CH2 O C O CH CH2

N CH3

CH3

Chart 3. General structure of functionalized polysiloxanes used as chromophore and/or amine co-initiator for photo-initiating acrylate polymerization.

206

Frédéric Cazaux and Xavier Coqueret Table 1. Constitutional characteristics of functionalized siloxane polymers P1-7

Polymer Sample P1 P2 P3 P4 P5 P6 P7

2-BBA 100 52 33 -

Nature and fraction (mole-%) of functional groups TXA 4-DMABA AA 100 48 32 35 100 85 15 56 44 -

The photo-initiating efficiency of the different type II systems including 2-BBA (or TXA, as indicated below) and 4-DMABA esters was evaluated with a polymerizable composition containing 2-ethylhexyl acrylate (EHA) and 1,6-hexanediol diacrylate (HDDA). The monomers were mixed in equimolar proportions to give an acrylate content of 7.1 - 7.3 mmol.g-1. To the acrylate formulation were added equimolar amounts of 2-BBA and 4DMABA esters to obtain a concentration of 0.62 mmol.g-1. The final mole concentration of 2BBA esters is equivalent to the chromophore concentration in a formulation with 1 wt-% of benzophenone. 1.0

τ lim (Rp )0

τ

A

0.5

0

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850

800

ν / cm -1

0

10

20

30 time / s

Figure 6. IR monitoring of the disappearance of acrylate functions upon UV-irradiation (left) and plot of progress of conversion (right).

The progress of the photo-initiated polymerization was followed by IR spectroscopy. Monitoring the decrease of the acrylate band at 810 cm-1 as a function of time of exposure (Figure 6) allows to compare the photo-initiating systems in terms of initial rate of polymerization (Rp)0 and of the limiting conversion τlim.(Table 2) [53]. Compared to the mixture of LMW reactants (system S1), polymer P1 containing 2-BBA chromophore associated with free amine co-initiator 7 was shown to induce initially a faster decrease of acrylate functions (Figure 7). Although system S5 seems at first glance very

Photochemistry of Polysiloxanes

207

unfavourable to the bimolecular process since it involves two independant polymeric reactants, the initial polymerization rate is still higher than with the reference composition S1. Table 2. Efficiency of the various combinations of aryl ketone - amine in the initiation of the polymerization of a mixture of acrylates (EHA-HDDA) upon UV irradiation Photo-initiating systema)

Source of chromophore

Source of amine

(Rp)0 b) mol.l-1.s-1

τlim c) in %

S1 S2 S3 S4 S5 S6 S7 S8 S9

5 P1 5 P3 P1 P4 6 P5 P7

7 7 P2 P3 P2 P4 7 7 P7

0.40 0.86 0.71 0.47 0.60 0.47 1.41 2.31 0.91

Æ100 90 82 80 90 75 Æ100 90 80

a) including equimolar amounts (0.62 mmol.g-1) of chromophore (2-BBA or TXA ester) and amine coinitiator (4-DMABA ester) and submitted to the polychromatic light of a 900 watts Xenon lamp. b) initial rate of polymerization. c) limiting conversion of acrylate monomers.

1

τ

lim

τ

0.5

0 Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

0

20

40

60

80

100

time / s Figure 7. Plot of the progress of acrylate conversion upon UV irradiation in the presence of LMW ( ‡ , system 1) or polymeric ( | , system 2 ) 2-BBA chromophores associated with co-initiator 7.

The comparison of the systems according the ultimate conversion obtained for a high dose of actinic radiation points out some differences of importance. LMW initiating systems 5 + 7, or 6 + 7 allow the polymerization to go to completion. Using polymeric reactants was always associated with the observation of remaining unpolymerized acrylate functions. The value of τlim was measured between 0.75 and 0.9, depending on the nature of the functional polymer.

208

Frédéric Cazaux and Xavier Coqueret 1

τ

0.5

0 0

20

40

60

80

100

time / s

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Figure 8. Plot of the progress of acrylate conversion initiated upon UV irradiation by polymer P1 associated with an equimolar amount of co-initiator 7: 0.31 (z), 0.62 (), and 0.88 (‡) mmol.g-1 of each reactant respectively.

Reactant isolation and trapping upon cross-linking polymerization is a rather trivial phenomenon. Upon polymerization, the liquid mixture of mono- and difunctional monomers is rapidly subject to gelation and to the vitrification. As expected, the mobility of the different partners of the process appears to be of prime importance in the late stage of the initiation. The presence of acrylic ester as a polymerizable pendant group (AA) in the photosensitive polymer (P4 in system S6) causes an additional decrease of the limiting conversion rate. Changing the concentration of initiating system S2 in the monomer mixture (Figure 8) causes the variation of the value of (Rp)0 as a simple consequence of the increase of light absorption. No effect on the value of τlim is observed. One must admit that at a given conversion of monomers, the cross-link density in the film does not allow sufficient diffusion of the two coreactants. The network develops in the vicinity of the initiating species and traps several unreacted functional groups for each efficient process. The second effect on the initial rate of polymerization is somewhat more unexpected. It requires further investigation and can be associated as a polymer effect. The early events of the initiation are photophysical and photochemical processes which have been studied in some details for a number of LMW molecules or systems by means of fluorescence and time resolved spectroscopy [53]. 2-BBA systems do not allow the selective photo-activation of the carbonyl chromophore. The strong absorption of 4-DMABA esters at λ = 310 nm overlaps the weak n-π* transition of the benzophenone chromophore. The strong band of TXA chromophore (Figure 9) with a maximum at λ = 398 nm was used to perform time-resolved spectroscopy (Figure 10) following 355 nm laser excitation. NR k

hυ Q TXA ⎯⎯→ TXA* ⎯⎯3⎯ → TXA° − NR3° + → free radicals

Scheme 7. Quenching of the triplet state of TXA derivatives by a tertiary amines NR3.

Photochemistry of Polysiloxanes

209

1

A

0 300

400 λ / nm

Figure 9. UV-visible absorption spectrum of polymer P6 in chloroform solution.

0.2 a

A 0.1

b

0 400

600

800

λ / nm

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Figure 10. Transient spectrum of absorption recorded after 355 nm laser excitation of a solution of ester 5 in toluene: triplet in the absence of amine at t = 0 µs (a), ketyl radical at 4 µs in the presence of amine 7 (b).

TXA triplet lifetime in toluene solution (τ0) as well as the rate of its quenching by amine 3 (Scheme 7) were determined for ester 5 and for polymers P5-7. As expected, the values of τ 0 collected in Table 3 decrease in the series P5 to P7. Intramolecular quenching is nevertheless not much enhanced compared to the mixtures of P1 and amine 7 introduced in the mole ratio corresponding to the composition of P6 and P7. [55]. Energy tranfers can be invoked to account for the longer lifetime of polymeric TXA triplet compared to free ester 7. However changes in the polarity of the environment might also cause measurable variations in the kinetics of the primary process. Intramolecular chromophore deactivation thus appears more efficient in polymers with dual functionalities irradiated in dilute toluene solution. This phenomenon should even be more pronounced in the monomer mixture, as consequence of the effect of viscosity on

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Frédéric Cazaux and Xavier Coqueret

diffusion. The photogeneration of free radicals would be more efficient with systems S4 and S9 compared to S2 and S8 respectively. Table 3. Lifetime of TXA triplet in toluene solution Source of TXA chromophore

7

P5

P6

P7

lifetime τ0 / µs

7

13

9

4

This is in apparent contradiction with the fact that (Rp)0 is lowered when the chromophore and the hydrogen donor are both connected to the same chain. For TXA derivatives, its value is even lower than that of model system S7. The recombination of primary radicals resulting from hydrogen abstraction probably competes with the bimolecular addition to the monomer as depicted in Scheme 8. Intramolecular effects may act again in this step and go against the efficiency of the initiation. + acrylate monomer

initiation

H R N

R'

O

CH2

*

R'

S

O

S

CH

R N

recombination

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Scheme 8. Competition between initiation and intramolecular combination of primary radicals photogenerated on a polymeric system including chromophore and amine co-initiator.

The determination of the quenching rate constant kQ was also performed for toluene solutions of TXA derivatives submitted to laser photo-excitation in the presence of amine 7. No significant difference was observed between model compound 6 (kQ = 1.9 108 l.mol-1.s-1) and polymer P5 (kQ = 1.8 108 l.mol-1.s-1). This result was confirmed by other experiments conducted with other amine quenchers and in other media [54]. Interestingly, polymerpolymer quenching of P5 by P2 proceeds also with comparable kinetics (kQ = 1.6 108 l.mol1 -1 .s ). One can conclude that the early photochemical processes are not responsible for the changes in polymerization kinetics. The results concerning cross-linking polymerization initiated by photosensitive polymers are from a phenomenological point of view in agreement with several recent studies on this topic. The general observation that polymeric chromophores induce an increase of the initial polymerization rate is confirmed for two series of type II systems including 2-BBA and TXA chromophores. Time-resolved spectroscopy performed with TXA functional systems indicates that the favourable effect on the apparent polymerization rate is not correlated with the rate of quenching of the triplet excited state by a tertiary amine. There is actually no indication that the polymer effect acts on the initiation step, although it is caused by the photoinitiator. The macromolecular nature of this initiator can play a determinant role on the rate of propagation. This quantity is of course directly dependant on the steady state concentration in free radicals. The microheterogeneity of the distribution of the partners involved in the complex initiation process should be evaluated more precisely [56]. Its influence on the termination process should be better understood as well. Kinetic chain length measurements

Photochemistry of Polysiloxanes

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can afford valuable information. The efficiency of this type of system also relies on the intrinsic photoreactivity of the 4-dialkylaminobenzoate moiety [57].

4 REACTIONS AFFECTING SILOXANE COMPOUNDS TRIGGERED BY A PHOTOCHEMICAL PROCESS 4.1. Photopolymerization of Silicone-Based Monomers

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Silicone-based monomers and oligomers fitted with acrylate, vinyl-ether, epoxy or oxetane functions have been synthesized and investigated for UV-curing applications (Chart 4) [58]. Photopolymerization of these multifunctional monomers in the presence of a convenient initiator results in the formation of coatings with low surface energy. A major industrial application of these systems is the cationic photocuring of silicone release coatings widely used by the pressure-sensitive adhesive industry for labels and various decorative materials [59]. The curing mechanism of silicone acrylates is very robust and unaffected by nucleophilic impurities in the substrates and by moisture. However, the free radical curing reaction is preferably conducted under nitrogen atmosphere to process the coating at high line speed with short UV exposure. The cationic curing mechanism uses an acidic initiator which can be poisoned, but the curing usually takes place without nitrogen inerting.

Chart 4. Examples of silicone oligomers fitted with monomer functionalities.

A specific feature of silicone acrylates reacted as bulk monomers is their high affinity for oxygen. The photo-induced radical cross-linking of silicones containing pendant acrylate and methacrylate groups has been investigated with calorimetric and ESR measurements. Oxygen exerts a very strong inhibiting effect on this process, which leads to a prolonged induction period and a pseudo first-order termination reaction between polymer radicals and oxygen [60]. Kinetically, such reaction steps are responsible for light intensity and monomer

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exponents, both of unity. In the absence of oxygen, second-order processes take place between polymer and primary radicals. The low Tg of the networks is indeed not a limiting factor for the progress of cross-linking. The results of conversion-time and reaction-rate time measurements using stationary irradiations and postpolymerization experiments agree with the corresponding kinetic expressions. In other types of applications, the reactive silicone oligomers are used as reactive additives for organic monomer blends. Telechelic polydimethylsiloxanes (PDMS) were modified by divinyl ethers to yield reactive surfactants that can be polymerized together with conventional monomers by a cationic mechanism [61,62]. Tailor-made functional polymers can be incorporated into epoxide / vinyl ether formulations. PDMS additives to effectively improve the wetting properties of these formulations coated onto various substrates. Contact angle measurements show a significant reduction of the surface energy for cured coatings containing the silicone additive. The presence of the vinyl ether-modified PDMS (0.25 – 5 wt-%) is also shown to impart stain-resistance properties. Long-term anti-graffiti effects are evidenced in comparison with reference formulations. The effect is associated to the very high silicone density revealed by analysis of the top layer of the coatings [63]. The limited compatibility with the organic formulation of reactive silicone oligomers in general, and acrylates particularly, [64] leads to complex effects affecting product distribution in the heterogeneous liquid, preferential sorption at interfaces, polymerization kinetics and final product morphology.

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4.2. Miscellaneous Various other photochemical processes take place on silicones or in their close environment. Organometallic complexes principally of Pt, Rh and Pd are effective for catalytic hydrosilylation reactions involving low molecular weight silanes as well as hydrosiloxane polymers. Most of the researches are concerned with the use of thermally activated catalysts. There exist few examples of photochemically activated platinum hydrosilylation catalysts. As an example, trimethyl(β-dicarbonyl) platinum(IV) complexes have been proved to be triggered photochemically to catalyze hydrosilylation of silicone polymers containing Si-H/Si-vinyl and Si-H/Si-epoxide moieties [65]. The photocatalytic effects of TiO2 have been exploited to achieve self-sterilization and self-cleaning of silicone-based medical devices [66]. Under UV illumination, the TiO2-coated silicone-catheters exhibits a bactericidal effect on Escherichia coli (E. coli), and can be useful in clinical uses by preventing catheter-related bacterial infections.

5. CONCLUSION Silicones exhibit a rich variety of behaviors under UV-visible radiation. One of their most useful properties is their high chemical resistance to thermal and photochemical degradation under terrestrial aging conditions and for many industrial uses. In spite of this outstanding stability, their unique aptitude to be converted into SiOx under short wavelength photochemical oxidation exemplifies another aspect of their richness of contradiction.

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Silicones also offer many possibilities of tailored functionalization that provides the chemist with a unique possibility to combine advanced organic photochemistry with inorganic media. A number of applications have already taken advantage of these features and future work and basic research in this area will continue to assist the development of fields of increasing interest as biomedical materials, microfluidics, self-cleaning materials, nanoelectronics and many others.

ACKNOWLEDGEMENTS The authors wish to express their gratitude to Prof. Loucheux (Lille) and Prof. J. P. Fouassier (Mulhouse) for many fruitful discussions during the course of this work.

REFERENCES [1] [2] [3] [4] [5] [6] [7]

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[8] [9] [10] [11] [12] [13] [14] [15] [16]

[17] [18] [19]

Handbook of polymer degradation, S. Halim Hamid Ed., Second edition, Marcel Dekker, New York, 2000. Salom, C.; Horta, A.; Hernandez-Fuentes, I.; Pierola, I. F. Macromolecules 1987, 20, 696. Itoh, T. Res. Chem. Intermediates 2001, 27, 669. Salom, C.; Gomez-Anton, M. R; Horta, A.; Hernandez-Fuentes, I.; Pierola, I. F. Macromolecules 1987, 20, 1627. Itoh, T.; Yang, M. H. J. Polym. Sci. Part B: Polym. Phys. 2002, 40, 854. Pankratova, L. N. ; Zhakhovskii, V. V. ; Kharitonova1, G. S. ; Rabkina, A. Y. ; Zavin, B. G. Russian Chem. Bull. 1988, 37, 2280. Suzuki, M. ; Nakata, Y. ; Nagai, H. ; Okutani, T. ; Kushibiki, N. ; Murakami, M. Mat. Sci. Engin: B 1997, 49, 172-174. Skurat, V.; Nucl. Inst. Meth. Phys. Res. B 2003, 208, 27-34. Skurat, V. E.; Dorofeev, Y. I. Angew. Makromol. Chem. 2003, 216, 205-224. Murahara, M.; Sato, N.; Ikadai A. Optics Letters 2005, 30, 3416-3418. Sato, N.; Sato, Y.; Okamoto, Y.; Murahara, M.; Okamoto, Y.; Murahara, M. Mat. Res. Soc. Symposium Proceedings 2006, 890, 217-222. Ouyang, M.; Klemchuk, P. P.; Koberstein, J. T.; Polym. Degrad. Stab. 2000, 70, 217228. Funatsu, T.; Kobayashi, M.; Okamoto, Y.; Murahara, M. Mat. Res. Soc. Symposium Proceedings 2005, 843, 221-226. Okoshi, M.; Li, J.; Herman, P. R. Optics Letters 2005, 30, 2730-2732. Kim, J.; Xu, X. J. Laser Appl. 2003, 15, 255-260. Rochow, E. G. in “Silicon and silicones: about stone-age tools, antique pottery, modern ceramics, computers, space materials, and how they all got that way” Springer Verlag, Berlin (1987). Thomas, T. H.; Kendrick T. C. J. Polym. Sci., Part A-2 1969, 7, 537. Thomas, T. H.; Kendrick T. C. J. Polym. Sci., Part A-2 1970, 8, 1823. Camino, G. ; Lomakin, S. M. ; Lazzari M. Polymer 2001, 42, 2395.

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Frédéric Cazaux and Xavier Coqueret

[20] Camino, G. ; Lomakin, S. M. ; Lageard, M. Polymer 2002, 43, 2011. [21] Manders, W. F. ; Bellama J. M. J. Polym. Sci., Part A: Polym. Chem. 1985, 23, 351. [22] Israeli, Y.; Philippart, J. L. ; Cavezzan, J. ; Lacoste, J. ; Lemaire J. Polym. Degrad. Stab. 1992, 36, 179-185. [23] Israeli, Y.; Cavezzan, J. ; Lacoste, J. Polym. Degrad. Stab. 1992, 37, 201-208. [24] Israeli, Y.; Cavezzan, J. ; Lacoste, J. ; Lemaire J. Polym. Degrad. Stab. 1993, 42, 267279. [25] Israeli, Y.; Cavezzan, J. ; Lacoste, J. ; Lemaire J. Polym. Degrad. Stab. 1995, 47, 357362. [26] Virlogeux, F. ; Bianchini, D. ; Delor-Jestin, F. ; Baba, M. ; Lacoste J. Polym. Int. 2004, 53, 163–168. [27] Schmidt, J. M. G. in Solid State Photochemistry, D. Ginsburg, Ed., Verlag Chemie, Weinheim 1978. [28] Hasegawa, M. Chem. Rev. 1983, 83, 507. [29] Cohen, M. D.; Schmidt, J. M. G. J. Chem. Soc., 1964, 1996. [30] Enkelmann, V.; Wegner, G.; Novak, K.; Wagener, K. B. J. Am. Chem. Soc. 1993, 115, 10390. [31] Minsk, L.; Smith, J. G.; Van Deusen, W. P.; Wright, J. F. J. Appl. Polym. Sci. 1959, 2, 302. [32] Williams, J. L. R.; Farid, S. Y.; Doty, J. C.; Daly, R. C.; Specht, D. P.; Searle, R.; Borden, D. G.; Chang, H. J.; Martic, P. A. Pure Appl. Chem. 1977, 49, 523. [33] Hajaiej, A. ; Coqueret, X. ; Lablache-Combier, A. ; Loucheux, C. Makromol. Chem. 1989, 190, 327. [34] Coqueret, X. ; El Achari, A. ; Hajaiej, A. ; Lablache-Combier, A. ; Loucheux, C. ; Randrianarisoa, L. Makromol. Chem. 1991, 192, 1517. [35] Coqueret, X. ; Lablache-Combier, A. ; Loucheux, C. Eur. Polym. J. 1988, 24, 1137. [36] Mercier, R. ; Coqueret, X., Lablache-Combier, A. ; Loucheux, C. Eur. Polym. J. 1988, 24, 639. [37] Tsuda, M. J. Polym. Sci.: Part A-2, 1964, 2907. [38] Coqueret, X. J. Photochem. Photobiol. 1998, 115, 1731. [39] Egerton, P. L. ; Pitts, E. ; Reiser, A. Macromolecules 1981, 14, 95. [40] El Achari, A. ; Coqueret, X. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 2513. [41] Swamy, H. R.; Ramamurthy, V.; Rao, C. N. R. Indian J. Chem. 1982, 21B, 79. [42] Martin, P. G.; Stupp, S. I. Macromolecules, 1988, 21, 1222. [43] Buisine, J. M. ; Coqueret, X. ; Lahmamssi, C. Mol. Cryst. Liq. Cryst. 1996, 281, 295. [44] Andrienko, D.; Dyadyusha, A.; Kurioz, Yu.; Reshetnyak, V.; Reznikov, Y. Proc. SPIE 1998, 3488, 175-179. [45] Sherrington, D.C. in Encyclopedia of Polymer Science and Engineering, Vol. 14, p. 101, Wiley, New York, 1988. [46] Li Bassi, G. J. Rad. Curing 1987, 14, 18. [47] Davidson, R.S. in Proceedings of Aspects of Photoinitiation, p.97, Egham, U.K., 1993 [48] Nahm, S. H. J. Coatings Technol. 1991, 63, 47. [49] Fouassier, J.P.; Ruhlmann, D.; Zahouily, K.; Angiolini, L.; Carlini, C. ; Lelli, N. Polymer 1992, 33, 3569. [50] Angiolini, L.; Caretti, D.; Carlini, C. J. Appl. Polym. Sc., 1994, 51, 133 [51] Coqueret, X.; Lablache-Combier, A.; Loucheux, C. Eur. Polym. J. 1988, 24, 1137

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Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

[52] Pouliquen, L.; Coqueret, X.; Lablache-Combier, A.; Loucheux, C. Makromol. Chem. 1992, 193, 1273 [53] Fouassier, J. P. J. Chim. Phys. 1980, 80, 339. [54] Pouliquen, L.; Coqueret, X. ; Morlet-Savary, F.; Fouassier J.P. Macromolecules 1995, 28, 8028. [55] Fox, M.A.; Chanon, M. in Photoinduced Electron Transfer, Elsevier, Amsterdam, 1989. [56] Leclercq, P.; Buisine, J. M.; Coqueret, X. Macromol. Chem. Phys. 1997, 198, 29772984. [57] Pouliquen, L.; Coqueret, Macromol. Chem. Phys. 1996, 197, 4045-4060. [58] Fouassier, J.P., Rabek, J.F., 1993. Radiation Curing in Polymer Science and Technology. Elsevier, Amsterdam, England, pp. 193–222. [59] Jang, M. ; Crivello, J. J Polym Sci Part A:Polym Chem 2003, 41, 3056. [60] Mueller, U.; Jockusch, S. and Timpe, H.-J. J. Polym. Sci.; Part A: Polym. Chem. 1992, 30, 2755-276 - Mueller, U.; Jockusch, S.; Haeusler, K.-G. and Timpe, H.-J. Angew. Makromol. Chem. 1992, 200, 61-76. [61] Cazaux, F. ; Coqueret X. Eur. Polym. J. 1995, 31, 521. [62] Cazaux, F. ; Coqueret X. Eur. Polym. J. 1995, 31, 527. [63] Cazaux, F.; Coqueret, X. Surface Coatings International, Part B: Coatings Transactions 2001, 84, 127-134. [64] Neo, W. K.; Chan-Park, M. B., Gao, J. X.; Dong L., Langmuir 2004, 20, 11073-11083. [65] Burget, D. ; Mayer, T. ; Mignani, G. ; Fouassier J. P. J. Photochem. Photobiol., A Chem. 1996, 97, 163-170. [66] Ohko, Y.; Utsumi, Y.; Niwa, C.; Tatsuma, T.; Kobayakawa, K.; Satoh, Y.; Kubota, Y.; Fujishima A. J. Biomed. Mater. Res. (Appl. Biomater.) 2001, 58, 97-101.

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In: Silicon-based Inorganic Polymers Editors: Roger De Jaeger and Mario Gleria

ISBN: 978-1-60456-342-9 © 2008 Nova Science Publishers, Inc.

Chapter 5

POLYSILANES Julian Koe∗ International Christian University, Mitaka, Tokyo,Japan 181-8585

1. INTRODUCTION Polysilanes are hybrid polymers comprising an inorganic helical main chain of catenating sp3 hybridized silicon atoms and (most commonly) organic side chains, and can be represented as in Figure 1 by their constitutional repeat unit (CRU). R1 Si n R2

R1, R2 = alkyl or aryl

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Figure 1. Representation of polysilane constitutional repeat unit (CRU).

Overlap of silicon σ orbitals along the silicon backbone results in σ electron delocalization with a HOMO-LUMO band gap of 3 to 4 eV. The electronically delocalized main chain is the distinguishing feature of the polysilanes (and also polygermanes and polystannanes) and gives rise to the unique optoelectronic properties of these Group 14 polymers. The backbone has both chromophoric and fluorophoric properties, with a UV absorption wavelength typically in the range of 300 to 400 nm, and relatively small Stokes shift emission. The HOMO-LUMO transition energy depends on the extent of electronic delocalization, which is determined by side chain-dependent structural, electronic and conformational factors and is affected by temperature, pressure, state, solvent and other ambient conditions. The polymer structures and properties can be probed by an extremely wide range of techniques, including optoelectronic, vibrational and magnetic resonance spectroscopies, X-ray diffraction and light-scattering, affording detailed information on structure-property relationships. ∗

The author gratefully acknowledges assistance from the Academic Frontier Project at ICU and from the Daiwa Foundation.

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Initial studies in the field were made in the 1920s, but due to the intractability of the products, a long period of dormancy followed. Interest was reawakened around 1980 with the realization that soluble high polymers, with an attractive range of potential applications for their unique photophysical properties, could be prepared simply by reducing the side chain crystallizability. This discovery precipitated a decade of almost explosive growth in polysilane knowledge, as can be seen in Figure 2, which has since matured as experimental and theoretical research stimulate and support each other in a continuing cycle of investigation and innovation.

Annual Publications

300 Journal (1838)

200

Patent (1257) Others (638)

100

0 1950

1960

1970

1980

1990

2000

Year

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Figure 2. Annual polysilane publications 1950-2004.

Current challenges in synthesis and molecular control are defined by the search for routes to designer polymers with particular properties and functions suitable for exploitation and application. Wide coverage of the area was given in a book published a few years ago with chapters contributed from many leading researchers [1], and advances in the field from 1993 to 2005 were recently reviewed [2a], updating earlier reviews in the same series [2b,2c]. Other reviews are noted in the text below. The present Chapter aims to review polysilanes generally, from their origins almost a century ago to the present, and is intended for new comers in the field of inorganic polymers, specialists in a specific class of inorganic macromolecules, desiring to extend their knowledge to other classes of these materials and those familiar with organic polymers requiring basic knowledge in inorganic polymers. Since space and time are not infinite (whatever the cosmologists may say), coverage is necessarily selective. After a historical introduction, synthetic routes to linear polysilanes including functionalization are treated, followed by a section on structure and property and finally branched systems.

2. NOMENCLATURE The class name "polysilane" is accepted by IUPAC, and will be used in this Chapter. Referring to specific polymers, structure-based names will be used since the polymer

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219

constitutional repeat unit (CRU) is different from the actual monomers in most cases, and a variety of synthetic routes starting from different monomers are known. Individual polysilane homopolymers will therefore be named as poly(CRU) and copolymers as poly(CRUAconnective-CRUB) (where the connective is co, ran, alt, block, graft, stat or per, indicating unspecified, random, alternating, block, graft, statistical or periodic copolymers, respectively, and the superscripts A and B refer to different CRUs). An example of the former is poly(methyl-n-pentylsilylene), 1, and of the latter is poly(dimethylsilylene-co-di-npentylsilylene), 2.

CH3 Si n CH2 CH2 CH2 CH2 CH3

CH3 Si x CH3

1

/

CH3 CH2 CH2 CH2 CH2 Si y CH2 n CH2 CH2 CH2 CH3 2

For more details, including the naming of higher dimensionality silicon polymers such as networks, ladders, cages and dendrimers, the reader is directed to IUPAC sources [3,4].

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3. HISTORICAL In the 1920s Kipping reacted neat dichlorodiphenylsilane with sodium [5,6], preparing the first polysilane, poly(diphenylsilylene) via a Wurtz-type coupling reaction. The product was highly crystalline and thus intractable and infusible, and with the limited analytical techniques of the time, therefore, very difficult to characterize, and of no apparent use. These reactions also generated the four-, five- and six-membered perphenylcyclosilane ring compounds, which were later structurally characterized by Gilman [7-9]. The second polysilane to be prepared was poly(dimethylsilylene), (PDMS) synthesized by Burkhard around 1950 via a similar route [10]. This compound was similarly crystalline, intractable and of little interest then. These choices of organochlorosilane were perhaps unfortunate, since they are among the very few to yield insoluble polymers. Had non-symmetrical monomers been chosen instead, the door to polysilane chemistry might have been opened much earlier, and the field might now look quite different. As it was, the door remained undiscovered for some while longer. Then around 1975, a crack in the wall of inscrutability of these polymers appeared when Yajima et al. reported that β-silicon carbide fibers could be prepared in a two-stage pyrolysis procedure from PDMS, as shown in Scheme 1 [11,12].

220

Julian Koe CH3 Si n CH3

450ºC Ar

H H Si CH2 Si CH2 n CH3 CH3

1. Δ 350ºC, air 2. Δ 1300ºC, N2

β-SiC

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Scheme 1. Preparation of β-SiC by pyrolysis of PDMS.

Migration of a methylene group from a side chain in PDMS to the main chain at relatively low temperature afforded a soluble, extrudable polycarbosilane preceramic. After processing to the desired form, such as a fiber, the polycarbosilane was surface oxidized in air to stabilize it dimensionally, and then pyrolyzed at high temperature to convert it to the high tensile strength ceramic fiber. Key to the utility of this method was the processability at the polycarbosilane stage, which derived, at least in part, from the greater assymmetry in the substituents at silicon. The Yajima process is the basis for the industrial production of Nicalon SiC fiber. It was then found that by choice of appropriate dichlorosilane monomers, a similar reduction of symmetry in polysilanes afforded polymers with lower crystallinity and consequently greater solubility – and finally the door to the modern era of polysilane chemistry was open. Wesson and Williams described soluble random copolymers prepared from dichlorodimethylsilane and either dichloroethylmethylsilane or dichloromethylpropylsilane. These still showed considerable crystallinity, however, and did not form good quality films [13]. Incorporation of phenyl and methyl groups was shown to be much more effective in reducing crystallinity and increasing solubility and processability: Trujillo described the preparation in refluxing dodecane of poly(methylphenylsilylene), PMPS [14], one of the most intensively studied polysilanes, and West described the formation of poly(dimethylsilylene-co-methylphenylsilylene) [15]. Worldwide interest was stimulated following these pioneering reports and the field expanded rapidly. Within a decade, many experimental and theoretical aspects of the chemistry and physics of a large number of alkyl and aryl substituted polysilanes had been investigated. The most notable discoveries concerned the unique electronically delocalized σ conjugated backbone: the UV absorption resulting from the silicon backbone σ-σ* (HOMO-LUMO) transition was found to be dependent on main chain length and affected by factors depending on side chain type and ambient conditions, such as conformation [16]. The photochemical and photophysical properties based on these, such as the photolability of the silicon main chain, forming radicals and/or extruding silylenes, were recognized early on and had led to consideration of their application in various technologies. These included use as photoinitiators in vinyl polymerizations [17], as electrical and photo- conduction [15,18] and charge transport [19] materials, as resists in microlithography [20], and in non-linear optics (NLO) applications, as described in the classic review of Miller and Michl [16]. Subsequent to these, numerous other potential applications have been considered, as can be seen from the extensive patent and research literature. However, currently the only industrial application of polysilanes is that of the Yajima process, and a break-through application of the unique delocalized σ electron system remains a challenge. To address this issue requires the synthesis of polysilanes with properties tailored to particular applications. This in turn necessitates polymers with particular functionalities and properties together with controlled microstructure and supramolecular structure. The classic Wurtz-type coupling with its typical conditions of molten sodium in hot toluene is, however,

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221

a notoriously unforgiving reaction, intolerant of many functionalities and permitting little stereo- or regioselectivity. A number of new routes to polysilanes which permit functional groups and allow structural control at micro- and macro- levels have therefore been devised. These have broadened the range of polysilane properties considerably and advances in these areas are described below.

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4. LINEAR POLYSILANES The term "polysilane" is generally taken to refer to essentially linear high molecular weight catenating silicon compounds. However, in its broadest sense, the term also includes lower molecular weight (oligomeric) compounds and those with branched, cyclic and network structures. The chain length beyond which oligomers are better called polymers is rather vague, although chains comprising fewer than about twenty catenating silicon atoms are usually considered oligomeric. It is found that for the lower molecular catenates the UV absorption maximum wavelength, which is a function of the degree of electronic delocalization in the chain, is molecular weight-dependent: the λmax moves to longer wavelength as the chain length increases, since the the delocalization-limiting factor is chain length (which in this regime is shorter than the conjugating segment length). For the high polymers, in contrast, there is only a slight dependency, as the delocalization-limiting factor becomes segment length rather than chain length [21]. Due to limitations of space, coverage concentrates more on the high polymers, although where relevant to this, oligomers are also discussed. In this section, linear polymers are considered (see Section 7 below for network and other structures). The uniqueness of the polysilanes rests on the above-mentioned σ electron delocalization and a major issue in polysilane chemistry is the control of the degree of delocalization, as this directly affects the optoelectronic properties. In addition to the effect of molecular weight noted above, there are also effects due to side chain electronic influence and backbone conformation, both of which are discussed in more detail below. However, it is useful to state here a few of the basic ideas concerning conformation, as it is a recurring theme in polysilane chemistry. The silicon backbone comprises Si-Si single bonds, about which rotation is relatively facile, depending on side chain steric interactions. In its most extended state (i.e. main chain dihedral angle, ω = 180°), the σ electron delocalization is maximized, the HOMOLUMO gap is minimized and the σ-σ* transition UV absorption maximum is at its lowest energy (most red-shifted). As the dihedral angle is decreased by rotation about the backbone Si-Si single bonds, affording a chain with a more coiled conformation, the degree of delocalization is reduced, leading to a spectral blue-shift. Certain dihedral angles result in conformations which are more stable than others, the terminology of which has recently been systematized into six categories by Michl and West: anti, A (ω ≈ 180°), transoid, T (ω ≈ 165°), deviant, D (ω ≈ 150°), ortho, O (ω ≈ 90°), gauche, G (ω ≈ 60°) and cisoid, C (ω ≈ 40°) [22].

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4.1. Synthesis This section covers progress in polysilane preparative methodology, aiming to expand the range of polymer properties and morphology accessible through the preparation of polymers with reactive functional groups, controlled stereochemistry and controlled higher order structure. Firstly covered are modifications of the Wurtz-type process through improved understanding of its mechanism; then dehydropolymerization, followed by polymerization of masked disilenes and ring-opening polymerization of cyclic monomers. In many cases these routes are quite structure-specific, so a generally applicable synthetic methodology allowing structural control with functionality tolerance is still a major research aim.

4.1.1. Wurtz-Type Reductive Coupling of Dihalosilanes The Kipping or Wurtz-type coupling is still the most general route to alkyl and aryl substituted polysilanes. The reaction proceeds by alkali metal-mediated reductive dechlorination of bis-halogenated silicon monomers, as in Scheme 2, and has been reviewed recently [23]. R1 Cl Si Cl R2

reducing metal, solvent controlled temperature

R1 Si n R2

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Scheme 2. Polysilane synthesis by Wurtz-type reductive coupling of dichlorosilanes.

Typically the dichlorosilane monomer is added to a flask containing molten sodium in refluxing toluene (a good solvent for polysilanes). A purple or blue color rapidly develops. After two to three hours (when an increase in viscosity may be noticed), the reaction is terminated, usually by addition of a chlorosilane such as trimethylsilane and stirring for a further 20 minutes. After filtration of the mixture, the polymer is fractionated by sequential precipitation of higher to lower molecular weight fractions using 2-propanol, ethanol and methanol, respectively. Depending on the substituents at silicon, however, modified conditions can improve yields and increase product molecular weights. At lower temperatures with solid sodium dispersed as a sand, the reaction essentially does not proceed for dialkylsubstituted monomers in toluene, although some success was reported using the additive 15crown-5 in diethyl ether [24] and either ethyl acetate or 15-crown-5 in toluene at 65°C [25]. Phenyl-substituted monomers react faster due to the stabilization of the silyl radical by the phenyl substituent [26,27], and reaction at temperatures below the melting point of sodium is possible. Recently, a study aimed at increasing the yields of poly(dialkylsilylene)s in the Wurtz-type synthesis, reported that synthesis at room temperature in THF afforded the polymers in crude yields of 50 to 80%, and that after isolation, the yield of high molecular weight fraction (Mn: 10,000 - 45,000) was around 50 – 60% [28]. However, the use of THF, while promoting yield, does appear to have a deleterious effect on molecular weight as has also been noted by other workers [27]. Alternative reductants are also in use. C8K [29-32] is appropriate for monomers with polar substituents [33,34], as described later in Section 4.2.2.1 (very low yields only are obtained with sodium), and in the preparation of poly(methylphenylsilylene) (PMPS), was found to afford polymers with slightly enhanced

Polysilanes

223

stereoregularity [31]. Using the more reactive K, reaction times are faster, but the products are of lower molecular weight [27]. Use of the room temperature liquid Na-K alloy, which has the dual advantages of continual fresh reductant surface exposure and low temperature, has also been reported [27]. Other metals including pyrophoric lead and yttrium, have been found to show activity, affording products with molecular weights of about 850 and 31,000, respectively after 30 days [35], but at levels too low to be practicable. The crown ethers assist in surface activating the alkali metals: use of 18-crown-6 with K and Na-K has been reported [36,37]. Heteroatom polysilanes with O or N attached directly to the silicon main chain usually cannot be prepared by Wurtz-type coupling of alkoxy- or aminochlorosilanes, since the alkoxy and amino functionalities compete with chlorine as a leaving group [38-41]. In an exception to this, however, polysilanes with pyrrolyl groups directly attached through the nitrogen atom to silicon have been synthesized [42]. Side chains with O- or N- groups remote from the main chain are more resistant towards the conditions in the Wurtz process and polar polysilanes bearing these groups with hydrophilic character have been prepared. These are described in the section below on functionalization. Random copolymers can be prepared by copolymerization of two different dichlorosilane comonomers. Regularly alternating copolymers are best prepared using alternative synthetic strategies (vide infra), although West and coworkers showed that sodium-mediated coupling of α,ω-dihaloooligosilanes of the type Br-SiR2-SiR'2-SiR2-Br led to the regularly alternating copolymers (SiR2-SiR'2-SiR2)n [43]. R R Br

Si

Si

R R Si

Br

CH3 CH3 CH3 CH3

Na toluene

Si

Si

Si

n

CH3 CH3 CH3 CH3 R = n-C6H13 or n-C4H9

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Scheme 3. Formation of alternating copolymers by Wurtz-type polymerization of oligosilanes.

The mechanism of the reaction, though rather obscured by its heterogeneity, was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures, with additives, and with different alkali metal reductants by several groups [23,44-47]. Silyl anions, silyl anion radicals and silyl radicals are believed to be involved [33,34,48,49], as shown in Scheme 4. Wurtz-type polymerizations usually afford products with polymodal molecular weight distributions [21]. The relative proportions of the fractions depends greatly on many factors such as the substituents on silicon, solvent, alkali metal and reaction temperature. The lowest molecular weight fraction (Fraction I) has a molecular weight of several hundred Daltons and is considered to arise from back-biting by a silyl anion chain end associated with the sodium surface forming the thermodynamically favorable 5- and 6-membered ring compounds.

224

Julian Koe Initiation

Propagation

R1 Cl Si Cl R2

+

Na

R1 Cl Si Cl R2

, Na+

R1 Si Cl R2

+

Na

R1 Si Cl R2

, Na+

R1 Si R2

R1 Si R2

+ Na

R1 Si R2

+

R1 Si R2

, Na+ +

R1 Cl Si R2

R1 Si R2

R1 Cl Si Cl R2 ,

+

NaCl

+

R1 R1 Si Si Cl R2 R2

Na+

NaCl

+ NaCl

Termination R1 Si R2

R1 R1 Si Si R2 R2 R1 Cl Si R2

R1 R1 Si Si R2 R2

+ NaCl

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Scheme 4. Wurtz-type polymerization mechanism.

The next fraction (Fraction II) has a molecular weight of about 4000 and was previously considered to arise from the forcing of some shorter, growing, chains off the sodium surface due to the decreasing sodium surface area as reductant is consumed the highest. However, a new rationale based on a correlation of the molecular weight of Fraction II with maximum electronic stabilization and polymer segment length has been proposed: the degree of σ-σ* conjugation in the chain increases as the chain length increases with the addition of repeat units in a regular conformation, i.e. transoid or deviant (in the recently proposed terminology of Michl and West [22]) up to about 35 or 40 backbone atoms in poly(methylphenylsilylene) (PMPS), after which there is no energetic advantage to maintaining the regular conformation, and the formation of a kink (conformational defect) becomes inevitable [47]. This chain length corresponds well with the molecular weight of Fraction II [21]. The conjugationsuppressing kink then diffuses from the free end along the chain towards the metal surface, where with maximum probability (due to the favorable conformation), back-biting may occur to clip out a cyclic, and the approximately 35 silicon atom chain disengages from the silicon surface affording Fraction II. This is shown schematically in Figure 3. Those chains which are not statistically eliminated by back-biting continue increasing in chain length and the probability of arrival of the kink at the sodium surface and back-biting/termination decreases. At double the segment length, a second kink is statistically expected to occur in the chain, and may interact with the first so as to annihilate them both. Thus, as the chain length increases, the probability of chain disengagement decreases, leading eventually to the highest molecular weight product, Fraction III. As the back-biting reaction is thermally activated, lower temperature polymerizations may result in greater predominance of Fraction III, although

Polysilanes

225

yields would be lower [21], and suitable anion-stabilizing solvent/additive systems must be used, as noted above.

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Figure 3. Defect diffucion-control in Wurtz-type polymerization [47]. Adapted with permission from Mcleish, T. C. B.; Jones, R. G.; Holder, S. J. Macromolecules 2002, 35, 548-554, Copyright 2002 American Chemical Society.

The mechanism is thus quite complex, despite the apparent simplicity of the reaction and discussions on the intermediates involved and end products continue. Chains were generally considered to be Cl-terminated, at least for phenyl-containing polymers [26,46], and there is also IR evidence for siloxy linkages [33]. Evidence for the intermediacy of silyl radicals was afforded by Si-H bands in the IR spectra of products generated in solvents susceptible to hydrogen atom abstraction by radicals [48-50]. Additionally, evidence for H-terminated symmetrically and asymmetrically substituted poly(dialkylsilylene)s was recently published [51]. The Si-H stretch intensity decreased with increasing molecular weight, suggesting their presence as end groups; 1H, 29Si and {1H-29Si} heteronuclear correlation spectroscopy showed only Si-H end groups, with no evidence for Si-Cl, Si-OR or Si-OH groups. These data strongly support the involvement of silyl radicals. Use of the Si-H group in further functionalization was also recognized. Various modifications of the Wurtz-type coupling, such as the use of various additives or different reductants as noted above, have been investigated in order to improve yields, to control the modality and to widen its applicability to functionalized monomers. Reproducibility is also an issue, since some of the reaction variables, such as the state of the sodium, are difficult to control. Ultrasound was found to provide a more reactive and homogeneous metal reductant surface. Initial investigations using ultrasonic activation in a sonochemical Wurtz reaction found reduced reaction times, improved yields and monomodal products [52-54]. More detailed studies compared the Wurtz reaction with and without

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ultrasonic activation, discussing the effects of reaction temperature, alkali metal, silicon substituents and solvent [27,55]. Both ultrasonic immersion probes and cleaning baths have been used as ultrasound sources, although the former was preferred due to the greater temperature control and reproducibility afforded [27]. Ultrasound acts so as to provide a homogeneous alkali metal dispersion with a continuously regenerated surface by cavitational erosion, which thus permits polymerization at lower temperatures (typically around 60°C; the very high local temperatures and pressures during sonication were found to have little if any effect on the polymerization reaction). Product monomodality stems from ultrasonically induced shear forces which selectively degrade higher molecular weight chains to a limiting value of about 50,000 and reduce the polydispersity index (PDI, equal to Mw/Mn) to ca.1.2. Optimum conditions for ultrasonically activated Wurtz-type synthesis using a probe-type sonicator in the cases of PDHS and PMPS, were found to be: initial metal:monomer ratio approx. 2.3; alkali metal reductant: K for PDHS and Na for PMPS; initial monomer concentration approx. 0.30 M; polymerization solvent: toluene; reaction temperature approx. 60°C. Quite recently, a lithium-mediated Wurtz-type synthesis was applied to the synthesis of a novel class of previously theoretically investigated [56] polysilanes: the poly(1,1-silole)s, exemplified by 3 [57-59]. These are polysilanes in which each CRU comprises a silole ring which catenates through the silicon atom. Methyl groups were necessary in ring positions 2 and 5 in order to alleviate steric congestion, and on the phenyls in silole positions 3 and 4, para-ethyl groups were substituted in order to increase solubility. Ar Me

Ar Si

Me n

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3

The UV absorption spectrum of 3 [λmax = 320 nm (sh); Mn = 6300, Mw/Mn = 1.14] was not significantly different to conventional polysilanes, though the fluorescence showed a large Stokes shift (excitation at 320 nm; emission λmax = 460 nm) [59], indicating significant differences between ground and excited states. 3 showed unusual chemical reactivity: in the presence of excess lithium complete degradation to the dilithiosilole occurred (conventional polysilanes degrade to oligomers).

4.1.2. Polymerization of Masked and in Situ Disilenes Searching for a synthetic route to polysilanes which avoided the limitations of the Wurtztype coupling, Sakurai and coworkers pioneered two polymerization routes, aiming at a silicon-based analog of a carbon-based vinyl polymerization. Unlike their carbon vinyl analogs, however, disilenes are not stable unless substituted with very bulky protecting groups, in which case polymerization is precluded by the same steric factors, and reactive Si=Si compounds cannot be prepared and stored for use as monomers in controlled polymerizations. In these elegant circumventive approaches, stable, yet nevertheless reactive, disilene precursors were therefore synthesized which underwent reaction to form polymers.

Polysilanes

227

Masked disilenes. The first route is from phenyldisilabicyclooctadienes, compounds such as 4 which could be considered as "masked disilenes" [60-63]. On a practical level, the synthesis of these disilene precursors is not trivial, but it affords polymers with a high degree of regioselectivity, and also tolerates Si-N-bound amino side groups. In the example given in Scheme 5 below, 2,2-dihexyl-3,3-dimethyl-1-phenyl-2,3-disila-bicyclo[2.2.2]octa-5,7-diene, 4, is anionically polymerized to give the regularly alternating H-T (head-to-tail) copolymer poly(dimethylsilylene-alt-di-n-hexylsilylene) copolymer, 5. Li Me Hex Cl Si Si Cl Me Hex

Me Me Si

Me Hex Si Si Cl Me Hex

THF, -78°C

Hex Si Hex Ph 4

Me Me Si

Hex Si Hex

n-BuLi (cat.) THF, -78°C

Ph 4

RT

Me Hex Bu Si Si Li n Me Hex

EtOH

Me Hex Bu Si Si H n Me Hex 6

5

Scheme 5. Synthesis of structurally controlled polysilanes from "masked disilenes".

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In this method, regioselective attack on the less hindered silicon of 4 by the anionic initiator (LiNaph, n-BuLi, potassium alkoxides/cryptand[2.2.2] [64] or silyl anions in benzene [65]) occurs to give an anionically terminated silanyllithium, 5. The latter then attacks another molecule of 4 at the less hindered silicon atom, extending the chain and the process continues rapidly in the manner of a "living" polymerization yielding 6 on work-up with alcohol. Using this approach, functionalized polysilanes can also be prepared. For amino-group containing side chains, direct N-functionalization is possible [62,63,65], as exemplified by 7, whereas for alkoxy groups, only remote O-functionalization [66], as exemplified by 8, is possible. Me Me Bu Si Si H n Me N 7

Me Me Bu Si Si H n Me 8

O

The di-n-butylamino polysilane 7 is soluble, though the diethyl and diisopropyl analogs are insoluble [65] (a hexyl(methyl)amino-substituted polymer was also reported though no data were given [67]). For 7 (Mn = 27,000), formed with 100% regioselectivity (an electronic effect due to the amino group was considered to augment the steric masked disilene regioselectivity effect), the silicon main chain UV λmax occurred at 363.4 nm (εSi-Si = 5,800),

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Julian Koe

strongly bathochromically shifted compared to dialkyl polysilanes. This red-shift is in agreement with theoretical predictions on the basis of a σ-n mixing interaction for polysilanes containing Si-bound heteroatoms with available non-bonding electrons [68,69,70a]. Such a shift also accords with experimental observations on alkoxy-substituted polysilanes (vide infra, Section 4.2.3.1). The UV absorption maximum of 7 showed a continuous small hypsochromic shift as the temperature was reduced. The labile amine group can be replaced by Cl using acetyl chloride to afford a strictly alternating Cl-substituted precursor polymer, which reacts with various nucleophiles affording a new class of strictly alternating functionalized copolysilanes [70b]. The 6-methoxyhexyl ether-substituted polymer, 8, formed good LB films. On different surfaces, these showed different UV absorption maxima due to the adoption of different surface-dependent global conformations: 306 nm on a hydrophobic quartz surface, due to a collapsed globular shape, with concomitantly reduced σ conjugation, and 322 nm on a hydrophilic quartz surface, due to an extended chain conformation, induced by hydrogen bonding with the hydrophilic surface. 8 also showed piezochromism, as described in Section 5.4 below [66]. The masked disilene approach also affords scope for further end-chain functionalization, since the technique results in an anionic terminus on the initally formed polymer chain. One possibility is the synthesis of polysilane block copolymers using the anionic chain end to polymerize alkenes. In one example, a polysilanyllithium, derived by masked disilene polymerization of 4, was used to initiate polymerization of triphenylmethyl methacrylate to form the block copolymer, poly(1,1-dimethyl-2,2-dihexyldisilene)-b-poly(triphenylmethyl methacrylate), PMHS-b-PTrMA, 9, as shown in Scheme 6 [71]. The reaction was performed in the presence of the chiral amine, (-)-sparteine, which led to the formation of the PTrMA part of 9 with a single helical screw sense, as shown by positive Cotton effects at 210 nm in the CD (circular dichroism) spectrum. At temperatures above -20°C, the polysilane part has a UV absorption at 310 nm and no band in the CD spectrum, consistent with the adoption of a random coil. Me Me Si

Hex Si Hex

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Ph

4

(i) n-BuLi (cat.) THF, -78°C (ii)

H2C C

Me Bu Si Me

Me

C O O Ph Ph Ph

Me Hex Si CH2 C m n C O Hex O Ph Ph Ph 9

/(-)-sparteine (iii) EtOH Scheme 6. Utility of masked disilenes in end chain functionalization.

Below -20°C, a new UV absorption grows in at 340 nm, coincident with a positive CD Cotton effect, indicating a new, more extended and presumably transoid conformation, with preferential screw sense (PSS) helicity. The transition is reversible: raising the temperature

Polysilanes

229

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results in the loss of the polysilane Cotton effect, but it is restored again on lowering the temperature, showing that chiral information stored in the PTrMA part can be transferred to the attached PMHS part, inducing preferential screw sense helical chirality, a phenomenon termed "helical sense programming" by the authors. In a related demonstration of the application of the masked disilene approach in end chain functionalization, use of the chiral anionic initiator (-) or (+) potassium menthoxide with the same masked disilene yielded a polysilane with a preferential screw sense (PSS) helical conformation [72]. Optically active polysilanes are discussed further in Section 6. The anionic polysilanyl chain ends generated in the masked disilene method can also be used to form end-graft polysilanes by reaction with substrate surfaces or functionalized substrate surfaces [73]. These systems are discussed in Section 4.2.3. The masked disilene approach offers significant advantages structural control during polymer synthesis, but has the disadvantages that the synthesis from commercially available reagents is quite involved and also that monomers bearing bulky groups (e.g. aryl or 1,1-di-iBu) or Si-OR substituents cannot be polymerized. In-situ disilenes. In the second of these two routes, polymerization of disilenes generated in situ from 1,2-dichlorodisilanes has been demonstrated [74]. Reaction of a number of dichlorodimethyldisilanes with the 3,3',5,5'-tetramethylbiphenyl anion radical at -78°C affords the corresponding polysilanes in moderate yields. The remaining disilane substituents can be alkyl or aryl, in geminal or vicinal positions. Polymerization proceeds in a random, non-regiospecific, manner affording products in yields of up to 60%, although rather low molecular weight (Mn up to ca. 7000, PDI=1.6. For successful reaction, the four methyl groups on the reductant are required to sterically and electronically prevent the formation of the masked disilene derivatives. The unique difference in reactivity between dichlorodisilanes and dichlorosilanes under the reaction conditions, together with a comparison of the different reductants used, is also described.

4.1.3. ROP (Ring-Opening Polymerization) Catalytically initiated ROP has been successfully applied to the synthesis of polysilanes with controlled microstructure and has been reviewed [63,75]. For ROP to occur, the negative entropy change resulting from the combination of many monomer molecules to form a single polymer molecule must be outweighed by the negative enthalpy change derived from the release of ring strain energy to give an overall negative free energy change. 6-membered and larger rings rarely undergo ROP as the ring strain is too low; 5-membered rings are borderline, but 3- and 4-membered rings are significantly strained and depending on the nature of the substituents, may undergo ROP. Low temperature favors the reaction, since the entropy term is negative. The ceiling temperature, Tc, monomer concentration and thermodynamic parameters are related as in Equation (1) [75].

Tc =

ΔH p o

o

ΔS p + Rln[M]0

(1)

Amongst the earliest examples of the preparation of polysilanes by ring-opening polymerization were the anionic ROP of the all-anti cyclotetrasilane (MePhSi)4, initiated by Me2PhSiK or n-BuLi [76,77] and the thermally initiated ROP of (MeHexSi)4 and (MePrSi)4

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Julian Koe

[78]. However, the stereoselectivity in these examples is not high, due to the very high reactivity of the initiator in the former case, or the radical nature in the latter case. In the case of anionically initiated ROP, it is important to terminate the reaction (e.g. by quenching with an alcohol), since otherwise rapid polymer degradation due to backbiting can occur [79]. Firstly conventional ROP of four-membered rings is treated separately, followed by photochemically induced ROP of a four-membered ring,and then ROP of five-membered rings.

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4.1.3.1. ROP of Cyclotetrasilanes More recent efforts in this area have employed less active, bulkier, silyl cuprate catalysts aiming at greater control of stereo-, chemo- and regioselectivities [79,80]. Use of the catalyst (PhMe2Si)2Cu(CN)Li led to ROP of the all-anti isomer of (MePhSi)4 with greater selectivity, affording polymers with controlled microstructure as a result of the preferential stereochemistry of the attack by the propagating anion on monomer rings with defined configurations. Additionally, macrocycle formation was reduced and no backbiting-induced depolymerization was observed, even after 2 hours. Data from 29Si NMR spectroscopy indicated that ring cleavage proceeded with inversion of configuration at both attacked and new chain end silicon atoms [75]. Use of n-BuLi/[2.1.1] cryptand as ROP initiator was considered to permit more facile pyramidal atomic inversion of the silyl end, and led to PMPS with a partially racemized chain end Si. 4.1.3.2. Photolytic ROP Recently, a novel route to polysilanes involving the photochemical topotactic ringopening polymerization of the bright yellow crystalline octachlorocyclotetrasilane, (SiCl2)4, 10, to the almost colorless, crystalline perchloropolysilane, (SiCl2)n, 11, and subsequent functionalization has been described. The single crystal X-ray structure of 10 shows planar, centrosymmetric (SiCl2)4 rings in a step-like array [81], as shown in Figure 4. The unit cell was monoclinic [P21/n with a = 4.0569(10), b = 6.783(2), c = 13.346(3) Å; β = 104.6800(10)º]. The Si4 ring is planar and almost square with Si-Si-Si bond angles of 90.12(5) and 89.88(5)º, Cl-Si-Cl bond angles of 112.17(7), and 111.84(7)º and Si-Si bond lengths of 2.372(2) Å [slightly longer than the typical 2.34 Å; Si-Cl bond lengths are 2.039(2) and 2.049(2) Å]. On photolysis of 10, the color appears to fade, although the sample remains crystalline. Single crystal X-ray analysis of the product, 11, showed infinite all-anti (SiCl2)n chains parallel-aligned in an orthorhombic unit cell [P212121 with a = 7.3255(2), b = 7.1815(2), c = 13.2844(4) Å] [82], with bond lengths for Si-Si of 2.414(8) Å (longer even than in the cyclic monomer) and for Si-Cl of 2.088(9) and 2.120(9) Å, and bond angles for Si-Si-Si of 114.4(6)° and for Cl-Si-Cl of 111.0(4)° as shown in Figure 5. The reaction occurs in the solid state within the crystals, and it is considered that the strained 4-membered ring undergoes photochemical cleavage forming radical species which recombine with radicals on the next step down in the step-like crystal lattice of Si4Cl8, forming the non-strained linear (SiCl2)n chain 11. This is the first example of a polysilane which has been analyzed by single crystal X-ray crystallography and a rare example of a truly all-anti conformation.

Polysilanes

231

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Figure 4. X-ray structure of 10, showing (a) arrangement of molecules within lattice and (b) single molecule [81]. (b) reprinted with permission from Koe, J. R.; Powell, D. R.; Buffy, J. J.; West, R. Polyhedron 1998, 17, 1791-1793, Copyright 1998 Elsevier.

Figure 5. Single crstal X-ray structure of 11, showing all-anti structure [82]. Reprinted with permission from Koe J. R.; Powell, D. R.; Hayase, S.; Buffy, J. J.; West, R. Angew. Chem. Int. Ed. 1998, 37, 14411442, Copyright 1998 Wiley-VCH.

The crystalline linear polymer is almost insoluble, although the Si-Cl bonds react with alcohols, amines and other nucleophilic functional groups affording the otherwise inaccessible heteroatom polysilanes, poly(bis-alkoxysilylene)s and poly(bis-aminosilylene)s, as described below in Section 4.2.2.1 [82-84]. These heteroatom polysilanes showed UV absorption maxima bathochromically shifted compared to polysilanes with carbon-attached substituents. Theoretical investigations of related silicon-oxygen [85,86], -halogen, [87,88], -

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sulfur [86,89], and –nitrogen [86] heteroatom polysilanes with lone pairs of electrons on the Si-bound heteroatom were consistent with the experimental results, indicating the origin to be an electronic effect due to the σ-n mixing interaction.

4.1.3.3. ROP of Cyclopentasilanes The lower ring strain in 5-membered rings reduces their utility in ROP applications. However, incorporation of the silyl anion-stabilizing phenyl group has been shown to increase reactivity [26,90], as well as structural order. Regioselective ROP of phenylnonamethylcyclopentasilane, 12, generates the sequence-ordered, atactic polysilane, 13, as indicated in Scheme 7 and evidenced by NMR spectroscopy. This methodology permitted the synthesis of a new class of polysilane incorporating the silole functionality [63,91,92] which had previously been investigated theoretically [93] due to the combination of high-lying electron-donor silicon chain HOMO, and low-lying electronacceptor silole LUMO orbitals. The synthesis from dichlorotetraphenylsilole, 14, via siloleincorporated monomer, 15 to regioselective silole-incorporated polysilane, 16, is shown in Scheme 8, and proceeds through a dimethylsilyl-terminated anion. Me

B Me Me Me

Me

Si Si Si Si Si

Me Me

Me Me

Ph

B- initiator THF or DME, -20 to -78°C

12

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Scheme 7. Regioselective ROP of cyclopentasilane, 12.

Scheme 8. Synthesis of silole-incorporated polysilanes by ROP.

B

Me Me Me Me Me Si Si Si Si Si n Me Me Me Me Ph 13

Polysilanes

233

This new class of polysilanes was characterized by NMR and optoelectronic spectroscopies. UV absorptions due to polysilane skeleton and silole ring are apparent at 320 and 360 nm, respectively, and a high fluorescence quantum yield (4.1×10-2) was found. The data indicated a large degree of energy transfer between polysilane chain and silole ring [93]. (The synthesis of silole-incorporated polymers by the Wurtz method is also possible [57-59], as described in Section 4.1.1 above). Other Group 14-substituted 5-membered rings have also been successfully subjected to ROP. For germanium, the germole homolog of 15 also underwent regioselective ROP, although by a different mechanism involving a germole-terminated anion, affording the sequence ordered Si/Ge copolymer, 17, in 60% yield (Mn = 11,000, Mw/Mn = 1.8; UV λmax = 320, 360 nm) [94]. For carbon, a 5-membered carbosilane ring was also successfully regioselectively ring-opened via a σ*-π-stabilized α-silyl carbanion intermediate affording the C/Si heterocopolymer, 18. Ph Ph

Ph Me Me Me Me CH2 Si Si Si Si n Me Me Me Me

Ph

Ge

SiMe2 17

4 n

18

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The delocalization of the σ electrons in the polysilanes and higher Group 14 polymers requires orbital overlap. The σ electrons in Si-O, Si-C and most other Si-heteroatom bonds do not overlap suitably and incorporation of these into the main chain thus results in segmentation of the conjugating, electronically delocalized backbone. However, UV spectroscopic studies on 18 indicated that the tetrasilanylene units are able to conjugate to a limited degree through the methylene unit: the UV absorption maximum of 18 occurs at 245 nm, whereas if no conjugation occurred, the absorption maximum should occur near that for Me(SiMe2)4Me], at 235 nm [93]. Thus, using a suitable catalyst, anionic ROP offers major advantages in the control of polymer microstructure, and the method is tolerant of functional groups (ROP of [(MeO)SiPh]4, [(NC)SiPh]4 and [(H3C≡C)SiPh]4, has been briefly reported, though with few analytical data or experimental details) [95].

4.1.4. Dehydrocoupling Polymerization Dehydrocoupling polymerization can be divided into two categories according to the type of catalyst employed: either early transition metals or late transition metals. The first of these is the more widely applicable and has been subjected to detailed investigation since the initial studies by the group of Harrod around 1980 [96], and reviewed several times [97-100], most thoroughly and recently by Corey [101]. The methods suffer from the drawback that only low to medium molecular weight products (typically up to 10,000) can be prepared and from a narrow range of monomer structural types (aryltrihydrosilanes for the early transition metals). The mechanism was not immediately apparent and proposals included those based on silylenes [102,103], radicals [104,105], and oxidative addition/reductive elimination [106].

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The currently accepted mechanism derived from kinetic and mechanistic investigations by the group of Tilley involves σ-bond metathesis [107]. Concerning the second, late transition metal, method by which generally only low molecular weight oligomeric materials are accessible, there are very few reports. In this section, a brief overview of the methods is given followed by recent developments.

4.1.4.1. Early Transition Metal-Catalyzed Typically, the neat liquid state primary silane is polymerized using a d0 Group 4 metallocene-based compound (may be a catalyst or a precatalyst) with release of hydrogen. The catalytically active species contains hydrogen bound to zirconium in a coordinatively unsaturated complex, and if not initally present, H is scavenged by the catalyst precursor (precatalyst)from the silane during an induction period [108]. The σ-bond metathesis catalytic cycle following the mechanism of Tilley [107] is shown in Scheme 9. Dehydropolymerization reactions are terminated by exposure of the reaction mixture to air which deactivates the catalyst by oxidation. H

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H

Ph Si H m+n H

LnZr H

H

Ph Si H n H

Zr Si

HH Zr Si Ph

H

H

Si

Ph Si H m H

LnZr

Ph Si H n H

H

H2

Scheme 9. Dehydropolymerization according to σ-bond metathesis mechanism.

A range of monomer and catalyst types has been investigated [99,100,109,110]. The presence of electron-withdrawing and/or less sterically demanding substituents on the monomer results in higher molecular weight polymers and it was also found that dehydropolymerization of monomers bearing the donor O-substituents pmethoxyphenylsilane and p-isopropoxyphenylsilane was successful, albeit with low molecular weight (Mn = 2000) [111]. Monomers bearing donor S, O and N groups on the phenyl ring have also been polymerized using an aryloxy-substituted catalyst 19 although the product molecular weights were low – possibly due to coordination of the donor monomer groups to the catalyst metal center [112]. The presence of sterically demanding (19a) or

Polysilanes

235

electron-withdrawing (19d) metal substituents results in higher molecular weights and shorter reaction times. R1

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Zr

R2 R3

O Cl

R1

R2

19a: R1 = tert-Bu, R2 = H, R3 = Me 19b: R1 = iso-Pr, R2 = R3 = H 19c: R1 = R2 = R3 = H 19d: R1 = R2 = R3 = F 19e: R1 = OMe, R2 = R3 = H

The effects of other electronegative substituents on the Cp2MY2 metal center, such as Y = NMe2, OPh and F have also been investigated, along with the effects of these catalysts on primary, secondary and tertiary silanes: with PhSiH3, polymerization and H/Y exchange occurrred; for PhMeSiH2, disilane and H/Y exchanged products were found, but no polymer; for Ph2MeSiH, no coupling occurred at all and only H/Y exchange was found [113].Various other modifications in the catalyst design have been investigated to enhance polymer yields, molecular weights and structural control, such as the use of zirconocene derivatives with chiral substituents targeting stereocontrolled coupling or arylsilanes [114,115]. No tacticity control was found in these cases, however, from which it was considered that radical reactions during synthesis could be implicated. Highest molecular weight products with linear chains of up to 100 catenating Si atoms are achieved using the monomer PhSiH3 with zirconium-based catalysts, the best result being that using the hydride [CpCp*ZrH2]2, as no induction period to form the M-H bond is necessary [for the product (PhSiH)n, Mw = 13,800 and Mn = 5660) [108]. However, the synthesis and manipulation of the hydrides requires more rigorous exclusion of air and moisture and the best compromise catalyst appears to be the "mixed ring" zirconium precatalyst, CpCp*Zr[Si(SiMe3)3]Me, which has good activity due to the good balance of steric properties. Ti- and Hf-based catalysts have also been investigated. The report [116] that a titanium(III) hydride was formed from Cp2TiF2 using PhSiH3 (driven by the formation of the very stable Si-F bond) prompted a comparison of the use of titanium and zirconium metallocene fluorides Cp2MF2 (M = Ti or Zr) [117]. Using the Ti derivative, initial vigorous evolution of hydrogen was observed, after which the reaction continued slowly for about a week. Product molecular weights were rather low, however, with a DP (degree of polymerizatiion) of approximately 20. Polymerization in the presence of diethylamine yielded a 50% diethylamino-substituted product – another useful route to heteroatom polysilanes. Experiments starting from the disilane H2MeSiSiMeH2 as monomer were not successful [117]. It has been found that Cp2TiMe2 and Cp2TiCl2/n-BuLi catalysts are deactivated by oxygen to a latent catalytic species, from which the active catalyst can be regenerated by heating [87]. This was developed and it was found that aryloxy-substituted metallocene catalysts are tunable, stable dehydrocoupling catalyst precursors, which formed the active TiH catalyst in the presence of monomer under mild conditions (50°C, 15 min.) [118]. the polymer resulting from the Poly(phenyl(H)silylene), (PhSiH)n, dehydropolymerization of PhSiH3, is useful as a precursor polymer for the preparation of functionalized polysilanes, using the reactivity of the Si-H functionality, as described below in Section 4.2.

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Julian Koe

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4.1.4.2. Late Transition Metal-Catalyzed The utility of the late transition metals as hydrogenation catalysts prompted their investigation as dehydropolymerization catalysts, though early reactions were not successful. In 1997, preparation of the polymers poly(hexylmethylsilylene) 14, and poly(diethylsilylene) 15, with reasonable molecular weights and good yields (for 14: Mn = 15,000, Mw = 25,000; 46%; for 15: Mn = 38,000, Mw = 39,000; 38%) by dehydropolymerization of the secondary silanes, hexylmethylsilane and diethylsilane using Pt(COD)2 was reported [119]. The UV absorption maxima of 290 nm for 14 and 268 nm for 15 however, are at considerably shorter wavelengths than expected, for the molecular weights given, possibly indicating crosslinked materials. Consistent with this possibility, phenyl-substituted monomers were observed to undergo redistribution reactions. Polymerization of silafluorene, however, was reported to afford poly(silafluorene) (Mn = 3,200, Mw = 8,100; 47%; UV/nm: 281, 290, 327). This apparently promising report was not followed up. A short while later, an indenyl Ni-based alkene polymerization catalyst, [(1-MeInd)Ni(PPh3)Cl], was reported to react with LiAlH4, AlCl3, AgBF4 or methylaluminoxane to from intermediates which catalyze the dehydropolymerization of PhSiH3 [120]. Dehydrocoupling polymerization of 1,2dimethyldisilane using Wilkinson's catalyst, [Pd(allyl)2Cl2], or [Pd(dba)2] affording polymers with Mw up to 19,000 (PDI = 1.5) has also been described in a book chapter [121]. In these reactions, a redistribution mechansim is considered to operate and is consistent with the observation of the redistribution product Ph2SiH2, and observations on the reaction of [Pt(PEt3)3] with the bulkier disilane 1,1,2,2-tetramethyldisilane in which redistribution occurred [122], although silylene participation was not established [121]. Thus, while oligomers can be synthesized by late transition metal-catalyzed dehydrocoupling [123], the application of this technique to the synthesis of high molecular weight polysilanes requires further development. 4.1.5. Electrochemical Polymerization Initially, this technique afforded only oligomeric products, although there has recently been considerable progress in the synthesis of higher molecular weight polysilanes and several reviews on the area have been published [124-126]. The reaction proceeds by electrochemical reductive coupling, as represented in Scheme 10 and is considered to involve a silyl anion mechanism [127], (although a radical-based mechanism has also been discussed [126]). The mild conditions under which it proceeds permit the synthesis of functionalized polymers such as the copolymer poly(methyl-4-methoxymethoxyphenylsilylene)-copoly(methylphenylsilylene), 20, Mw = 19,000 [128], with an alkoxy phenyl substituent, which can be deprotected to the phenolic polymer. Electrochemical reduction applied to the formation of Si-Si bonds was in use before the advent of high molecular weight polysilanes. Disilanes were produced using platinum electrodes in a divided cell in the 1970s [129-131], and related studies were published around 1980 [132-134]. From around 1990, experiments using an undivided cell and a low oxidation potential sacrificial anode of aluminium or magnesium were found to lead to greater Si catenation. Starting from dichlorosilane monomers, methyl- and phenyl-substituted oligomers [135] and poly(dimethylsilylene) [136] (insoluble) were synthesized using an Al anode, and PMPS with Mw = 9000 was synthesized using an Mg anode [137]. Other anodes used include Zn [124], Cu [127] and Ni [127]. In an undivided cell, the sacrificial anode is oxidized in

Polysilanes

237

preference to the chloride and silyl ions, necessitating choice of the electrode materials such that the reduction potential of the chlorosilane is more positive than that of the sacrificial anode of metal M, i.e. E°M > E°SiCl. Reaction at anode

2/n M

Reaction at cathode

R Cl Si Cl R

Reaction in solution

R Cl Si R

2/n Mn+ R Cl Si R

+ 2 eelectrolyte, solvent

R n Cl Si Cl R Cl

+

+

2 e-

Cl-

R Si Cl n R

Cl-

Overall reaction

n R2SiCl2

+

2n e-

(R2Si)n

+

2n Cl-

Scheme 10. Silyl anion mechanism of electropolymerization.

Si Si m n Me

O

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O 20

This order is always found when M is Mg, but to ensure a sufficiently negative reduction potential for other less electropositive metals, a complexing agent such as tris(3,6dioxaheptyl)amine (TDA-1) or hexamethylphosphoramide (HMPA) is added to the solution. Use of a complexing agent or co-solvent permits a much lower concentration (ca. 0.02 M) of supporting electrolyte than in a classical divided cell (typically equal to the monomer concentration i.e. ca. 0.3 M) [124]. The undivided cell system usually comprises sacrificial anode (bar of metal) and cathode (stainless steel mesh, or a bar of the same metal as the anode if an alternating polarity set-up is used), supporting electrolyte and solvent with complexing

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Julian Koe

co-solvent in a magnetically stirred glass cell, all deoxygenated and dried to avoid the formation of Si-O linkages in the polysilane main chain (removal of traces of water by addition of Me3SiCl and pre-electrolysis to remove resulting HCl is also effective). After addition of the monomer, the mixture is electrolyzed, often with ultrasound to ensure fresh electrode surfaces. Polymeric products can be isolated by removal of salts and reprecipitation (or distillation for short chain oligomers). The basic system has been modified to improve polymerization [124-126] through variation of solvent/co-solvent, electrolyte, sacrificial anode, cell polarity, use of ultrasound, temperature, total current passed and monomer concentration, with best results found using dimethyl ether or THF as solvent and a perchlorate supporting electrolyte. α,ωDichlorosilanes, germasilanes and trisilanes have also been successfully coupled. Primary and secondary silanes can be electrocoupled, although only to short chain oligomers. The route has the advantage, however, that a sacrificial anode is not required and so metal salts are not formed [138,139].

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4.1.6. Chemical Vapor Deposition (CVD) Polymerization Poly(di-n-propylsilylene) has been prepared from cyclo-Si4Pr8 by photoinduced CVD [140], relying on the fact that silylenes can be extruded upon photolysis of cyclosilanes, and also insert into Si-H bonds [2c]. The cyclic monomer was vaporised by heating under vacuum at 270°C and then irradiated using 254 nm UV light in the set-up schematically illustrated in Figure 6. The target surface was terminated with Si-H by treatment of a quartz surface with NaOH solution, followed by dichlorodimethylsilane and then LiAlH4. (SiPrn2)n (λmax = 345 nm) was deposited as a thin film (10-8 m) on the target. Trapping experiments confirmed the intermediacy of silylenes, suggesting formation of the polymer by insertion of silylenes into the surface Si-H bond. Poly(dipropylsilylene) is insoluble due to the highly ordered structure arising from short, identical side chains, which precluded solution state characterization. However, the film state optoelectronic spectra were consistent with a linear Si chain, and the UV λmax (345 nm) indicates catenation of least 20 silicon atoms.

Figure 6. Representation of photo-CVD apparatus for polysilane synthesis.

The photo-CVD approach may thus be useful where only small amounts of less soluble polymer or thin films are required.

Polysilanes

239

4.1.7. Others: Redistribution and Disproportionation Two other synthetic techniques, which give rise to network polymeric products of lower molecular weight (vide infra, Section 7.1) have also been described. Redistribution polymerization of the chlorodisilane fraction from the Rochow Direct Process Reaction is catalyzed by phosphonium salts [2c] and affords a material comprising multiple fused rings of probably variable and as yet incompletely determined structure [141]. Disproportionation polymerization (a similar process to the above), has been described for the ethoxide-catalyzed disproportionation of alkoxydisilanes via silyl anion intermediates to form branched network alkoxy polysilanes [142].

4.2. Functionalization

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Polysilanes with simple alkyl side chains are commonly soluble, light-sensitive materials which can be rendered semi-conducting by suitable dopants such as AsF5 or iodine [15]. Apart from their use as preceramics [11,16], and despite their attractive properties and potential, no widespread application has been devised to exploit these polymers. In order to expand the range of properties and thus also applications of these materials, modification of their properties is required, e.g. by functionalization with groups conferring greater stability or with groups which may be chemically interactive or physically responsive. The topic was reviewed in 2000 in Chapters 13 and 15 of the book Silicon-Containing Polymers [62,63]. Functionalization can be either pre-polymerization, in which case functionalization is carried out at the monomer stage, or post-polymerization, in which case polymers preformed by any synthetic method, termed here precursor polysilanes, are then further reacted. The former generally affords polymers with a greater degree of structural control, although the functional groups which can be incorporated are rather restricted by the conditions required for polymerization. Additionally, homopolymerization of the functionalized monomer may sometimes not be possible, in which case, copolymerization with conventional (Si-alkyl or Siaryl) monomers may succeed. In this section, a number of useful precursor poloysilanes are first noted. Thereafter, the synthesis and properties of some side chain-functional polysilanes, in which the functional group is either remote from the main chain (separated by an alkyl or aryl spacer) or directly attached to the main chain, are described. This is followed by end group-functionalized polysilanes, which can be attached to reaactive surfaces or used in the preparation of block copolymers. Chiral functionalization is treated in Section 6.

4.2.1. Precursor Polysilanes The most reactive typically contain ≡Si-Cl or ≡Si-OTf (OTf = triflate) bonds although those with ≡Si-H and ≡SiC6H4CH2-Cl are also used. Due to their greater reactivity, these polymers cannot be directly prepared by the Wurtz-type synthesis, and must be prepared by an alternative strategy, such as perchloropolysilane, 11, from ring-opening of octachlorocyclotetrasilane (see Section 4.2.3.1 below) [82], or by chemical modification of a Wurtz-coupled product, such as those with Si-aryl groups. Thus starting from poly(methylphenylsilylene) (PMPS), chloromethylation of phenyl groups leads to 21 [143,144], triflate substitution of Ph affords 22 [145,146], and chlorodephenylation (using HCl/AlCl3) leads to 23 with about 30% of the phenyl groups remaining [147]. Using AlX3 (X

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Julian Koe

= Cl, Br) and the corresponding acetyl halide, substitution essentially goes to completion (about 1% Ph groups remain), affording precursor polymers of type 24, although some chain scission occurs such that the products have a DP of about 25 [26]. Other precursor polysilanes with reactive Si-Cl or Si-H bonds include poly[methyl(H)silylene-co-methylphenylsilylene], 25, prepared by Wurtz-type coupling of the dichlorosilane comonomers [148], and poly[phenyl(H)silylene], 26, obtained by early transition metal-catalyzed dehydropolymerization of PhSiH3 [96,107]. Although there are few reports of polysilanes with alkene side chains, these can also be considered as precursor polymers, and addition of HCl or HBr to the double bonds has also been used to generate halogenated precursors [149]. CH3 Si

Cl Si n Cl

m

CH3 Si

CH3 Si

n-m

m

CH3 Si

CH3 Si

n-m

m

O

21

11

R Si

n

22

CH3 Si

w

X

CH3 Si x H

n-m

Cl

S O O CF3

CH2Cl

CH3 Si

23

n

H Si

n

R = Me, Pr, Hex, Oct X = Cl, Br 24

25

26

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4.2.2. Side Chain Functionalization Remote from Main Chain Pre-polymerization functionalization may be possible if the functional group is separated from the main chain by an alkyl or aryl spacer if the functional group is not sensitive to the conditions of the polymerization reaction. 4.2.2.1. Remote Alkoxy-Functionalization The Wurtz-type coupling using sodium can be applied to the synthesis of alkyl and aryl polysilanes with side chains containing one oxygen atom in a position remote from the main chain, such as 27 and 28. These polymers can undergo further reaction: 27 reacts with methyl triflate affording a new alkyl triflate precursor polysilane [145]; while 28 can be deprotected to the phenolic polysilane [150]. In addition to solubility in common nonpolar organic solvents, such alkoxy-functionalized polysilanes are often soluble in alcohols [151,152] and may form Langmuir-Blodgett films on water [153]. Multiply oligoether-substituted polysilanes such as 29 may even be soluble in water [154,155] due to their increased polarity which is also reflected in the very high surface tension found for the polymer, implying enhanced wetting and adhesion characteristics [154]. 29 also showed remarkable iono- [156] solvato- and thermochromism [157] (see Section 5.2 below).

Polysilanes Me Si

Me Si

n

n

O

CH3 Si

n

O

O O

27

241

28

O

O

SiMe3 29

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The sodium-mediated Wurtz coupling of monomers with two or more oxygen atoms in each side chain (e.g. in oligoether groups) proceeds, however, in low yield [154,155]. For such very high polarity monomers, the reductant C8K is much more effective (0°C, THF), probably due to intercalation of the polar oligoether groups between the C8K layers, although the product molecular weights are lower [155,158]. In copolymerizations of polar and apolar monomers, Na is the better reductant, since the yields do not depend on the feed ratio, in contrast to the use of C8K. Similar principles also apply to the synthesis of the pendant crown ether polysilanes, 30, 31 and 32 [158,159]. The water-soluble polymers were synthesized in yields from 6-28%, with molecular weights of ca. 104, and show thermochromism consistent with Schweizer's theory (see Section 5.2.3 below) [160].

The closely related crown ether-pendant copolymer 33 has been prepared by postpolymerization hydrosilation of precursor polysilane 25, poly[methyl(H)silylene-comethylphenylsilylene], although the low molecular weight of 25 limits that of the product 33 [161]. Polysilane copolymers with siloxane-containing side chains have also been prepared by sodium mediated coupling of dichloromethylphenylsilane and the siloxane-substituted comonomer, as in Scheme 11. It was considered that the partial tendency for phase separation of non-polar main chain and polar side chain would lead to air oxidation-resistant polysilanes, due to the resulting accumulation of siloxane groups at the surface [162,163].

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Julian Koe

Ph Cl Si Cl +

Me Me H Si O Si Me Me Me

Ph Cl Si Cl (CH2)3

Pt catalyst 50°C, toluene

Me Si Me O Me Si Me Me

Ph Cl Si Cl Me

Na, toluene 80°C, 2 h

Ph Si m Me

Ph Si n (CH2)3

Me Si Me O Me Si Me Me

Scheme 11. Preparation of siloxane-containing polysilanes.

Incorporation of the oxygen atom results in greater flexibility, as well as polarity [164]. This was neatly demonstrated in series of poly(oxaoctylmethylsilylene)s and poly(bisoxaoctylsilylene)s in which the oxygen atom was inserted in side chain positions 5, 6 and 7, which also demonstrates clearly the effect of side chain imbalance on polymer morphology [165]. O

O Si O

O Si

n

O

O

n

36

O

O

O

Si n Me 37

Si

35

34

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n

Si n Me 38

Si n Me 39

The balanced side chains in 34, 35 and 36 promote more ordered hexagonal columnar mesophase structures above their transition temperatures, and crystalline phases below; abrupt thermochromism is evident at the transition temperature. In contrast, 37, 38 and 39 are amorphous. Variable-temperature UV spectroscopy and X-ray diffraction indicate greater conformational flexibility as the oxygen is sited closer to the backbone, reflecting the increasing effect of the flexible oxygen linkage on the mobility of the side chain and main chain [165]. Alkoxyphenyl-substituted polysilanes are less well known, though there were several reports in the mid to late 1980s [16]. More recently, a number of poly(alkylalkoxyphenylsilylene)s, both with and without enantiopure chiral alkoxy groups have been synthesized (see Section 3.6 below for chirality in polysilanes) [166-169]. These polymers can be synthesized via the conventional sodium-mediated Wurtz-type procedure. However, if the alkyl substituent on the phenyl ring has more than one oxygen atom, modification of both monomer and polymer preparation is required. Concerning monomers synthesized by Grignard coupling, oligoethoxy substituents can coordinate strongly to the resulting magnesium salts, hindering distillation. To obviate this, a poorer solvent can be added to precipitate the salts [169,170]. Concerning polymer synthesis, it was found that the

Polysilanes

243

yields were very low using sodium. With C8K, however, although the product molecular weights were not so high, reasonable yields were obtained.

4.2.2.2. Remote Amino-Functionalization Polysilanes containing N-functional groups are of interest due to the electronic effects of N incorporation on the σ-σ* transition of the electronically delocalized silicon main chain [171], their thermoluminescence [172] and photoelectric properties [173], their charge carrier trapping [174] and charge-photogeneration abilities [175], and their relatively high electrical conductivity on doping with iodine [176]. Depending on the structure, these may be prepared by the Wurtz coupling technique. One of the first reported examples was the series of random copolymers 40 containing the electron-donating N,N-dimethylaminophenyl group [171]. Whereas the m = 100% homopolymer (PMPS) has a UV λmax at 345 nm, the n = 100% homopolymer has a UV λmax at 350 nm, the nitrogen substituent causing a red shift of ca. 5 nm. This was considered to be due to the possibility of the nitrogen atom being able to conjugate with the silicon atom, resulting in a direct electronic (rather than conformational) effect. Me

Si m Me

N

Me

Si n Me

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40

N-carbazolyl-substituted polysilanes have also been synthesized [175,176], as outlined in Scheme 12. In these polymers, the alkyl link between the silicon chain and the nitrogen isolates the carbazolyl group electronically from the delocalized main chain. High molecular weight products resulted in copolymerizations with dichloromethylphenylsilane, though homopolymerization of the sterically congested N-carbazolyl-substituted monomer, 41, resulted in low molecular weight materials (for a sample of Mn = 3200, UV λmax = 294 nm [175]: the absorption due to the Si σ-σ* transition is masked by the carbazolyl group absorption). The polymers 42 were obtained in yields between 20 and 75%. Less sterically hindered homopolymers and copolymers containing the N-pyrrolylalkyl and N,N-dimethylaminophenyl groups have also been reported [177]. For the N-pyrrolylalkylsubstituted homopolymers 43 - 45, the degree of polymerization (DP) increased as the alkyl spacer was lengthened. For the N,N-dimethylaminophenyl-substituted polymers of types 46 and 47, the DP increased as the fraction of methylphenylsilylene comonomer repeat unit increased. Ionic, quaternary ammonium halide polysilanes with Cn (n = 8, 12, 18) N-alkyl substituents have been prepared by amine quaternization of fully chloromethylated poly[methyl(β-phenethyl)silylene] [144,178,179]. The spectroscopic properties of their LB films, considered to comprise periodic double layers having a homogeneous and amorphous

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Julian Koe

character, were investigated. Polarized UV spectroscopy of deposited LB molecular films indicated that the Si backbone orients preferentially in the dipping direction [178,179]. These polymers, with long N-alkyl substituents, were not soluble in water. However, quaternization with trimethylamine yielded the first fully water-soluble polysilane, 48 [180,181]. Me Cl Si Cl N H Cl Si Cl Me

N

N H2PtCl6/n-BuOH toluene

Na, xylene 140°C, 4 h Si m Me

Cl Si Cl Me

Me Si

n

42

41

Scheme 12. Wurtz-type synthesis of N-carbazolyl-functionalized polysilanes 42.

N NMe2

N N

Si

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43

n

Si

44

n

Me2N

Si m Me

Si n Me

45

46

Si n Me

(CH2)x Si m Me

Si n Me

47

Me Me N Me Cl

Si n Me 48

48 is solvatochromic: in pure ethanol, the UV λmax is 280 nm and in pure water it is 293 nm. In a 1:1 ethanol-water mixture, the UV showest the greatest bathochromic shift to a λmax

Polysilanes

245

value of 304 nm. The emission spectra similarly show maximum intensity and red shift at this solvent composition [180]. As noted in Sections 1 and 4 above, one of the distinguishing features of the polysilanes is the sensitivity of the σ-σ* transition energy to various factors, including conformation: λmax red-shifts with increasing σ electron conjugation, which increases with extension of the chain to a maximum when the chain is in the fully extended all-anti conformation. It is thus considered that at this solvent composition, polymer main chain and side chains are maximally solvated. Similar spectroscopic effects observed on addition of the surfactant sodium dodecyl sulphate (SDS) to aqueous solutions of 48 were interpreted as indicating greater rod-like character, resulting from surfactant binding. In related studies on LB films of analogs of 48 where the length of alkyl spacer and Nalkyl chain were varied, intramolecular exciplex formation between a σ-conjugated main chain silicon atom and the attached ammonium center was proposed in the shortest side chain cases (no alkyl spacer) [182]. This was based on the observation of FL emission in the 400500 nm region and lack of solvatochromism. With methylene spacers present, the distance between the excited states was too great to allow orbital interaction, and no exciplex was observed. In cast (random) films, a further emission at ca. 560 nm was considered to be due to intermolecular N⋅⋅⋅⋅⋅Si exciplex formation, precluded by molecular ordering in the LB films but possible in the random films. Chiral amines have also been used to prepare the quaternary ammonium salts, forming optically active polysilanes (vide infra, Section 6.1.5) [183]. Hydrosilation of N-(allyl)cycloimmonium bromide using precursor polysilane 25 has also been employed as a route to this class of polysilanes, affording 49 as in Scheme 13 with properties consistent with the molecular dipole formation [184].

CH3 Si

w

CH3 Si x H

catalyst

CH3 Si

w

n

CH3 Si

x

Br N N

n

Br N N

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25

49 Scheme 13. Route to ionic polysilanes by hydrosilation.

4.2.2.3. Metal Functionalization Metal-functionalized polysilanes are relatively rare. In one of the first examples, Pannell and coworkers copolymerized dichloroferrocenylmethylsilane and dichloromethylphenylsilane to afford polymers with the chemically robust ferrocenyl group in side chain positions [185]. Despite the novel composition, the properties were comparable with those of PMPS except for some stabilization of the normally light-sensitive silicon

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Julian Koe

backbone. Manners et al. also prepared ferrocenylsilane polymers, with a backbone of alternating ferrocenyl and silylene units as a result of the ring-opening polymerization of strained silyl ferrocenophanes [186]. While acting as precursors for metal-doped ceramics and displaying novel magnetic properties [187], these latter polymers lack the σ conjugation which typifies polysilanes and are thus beyond the scope of this chapter. The metal functionalized polysilanes in the few more recent reports are essentially polymeric metal complexes: donor functionalized polysilane side chains coordinate to Lewis acid metal centers [188,189]. An acetylacetonate-substituted polysilane successfully coordinated Fe(III), though the product was intractable, and no metal-silicon interaction was evident [190]. There has been greater success with metals in low oxidation states. Hydrosilation of 1,3,5-hexatriene using the precursor polysilanes poly[methyl(H)silylene-comethylphenylsilylene], 25, led to the diene-substituted polymer 50 in 13% yield (with Mw = 12530 for w = 0.48, x = 0.52) which was metal-functionalized to give the iron tricarbonylpolysilane coordination complex, 51, by reaction with triiron dodecacarbonyl, as indicated in Scheme 14 [188]. Greater yields (ca. 80%) of the diene-substituted polymer were obtained using 1,3,5-heptatriene, since this has only one terminal double bond. Poly[methyl(H)silylene], 26, can be similarly functionalized. Me Si

w

Me Si x H

Me Si

cat. n

w

Me Si x H

Me Si

y

Me Si n

[Fe3(CO)12]

Me Si x H

w

Me Si

y

Me Si

z

n

Fe(CO)3

25

50

51

Scheme 14. Metal functionalization by coordination to diene substituent.

Pyrolysis of these polymers afforded α-Fe-SiC magnetic ceramic composites, as shown by XRD analysis. Polymeric piano stool-type complexes of PMPS with Mo [191] (52), Mn [192] (53) and Cr [189] and their model compounds have also been prepared, using the η6coordinating ability of the phenyl ring.

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Me Si

0.85

Me Si

0.15

n

Me Si

0.9

Mo(CO)3 52

Me Si

0.1

n

Mn(CO)3

BF4

53

The UV absorption maxima (224 and 337 nm) of 52 are almost the same as those of the parent PMPS (223 and 338 nm), possibly reflecting the low metal loading. In THF solution, the polymers are unstable towards air, and appear to cross-link, forming an intractable material [191].

Polysilanes

247

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4.2.3. Direct Functionalization at Silicon Main Chain Preparation of polysilanes with heteroatoms or other functionalities A attached directly to the silicon backbone typically requires alternative synthetic or post-polymerization methodologies due to the reactivity of both the Si-A and Si-Si linkages (vide supra, Sections 4.1 and subsequent). These polymers are of interest for the strong influence of the attached functionality on the electronic and conformational properties of the silicon main chain. 4.2.3.1. Heteroatom Functionalization Oxygen-substituted heteroatom polysilanes with the ≡Si-OR functionality are not accessible by Wurtz coupling since OR is a good leaving group [38-40]. The masked disilene approach, while useful for the preparation of aminopolysilanes (vide infra, and also Section 4.1.2 above), is not applicable to alkoxypolysilanes. Thus, ≡Si-OR substituted polymers are prepared from the precursor polymers 11 and 22 - 26 (see Section 4.2.1 above) by postpolymerization functionalization. Symmetrically substituted bis-alkoxy-substituted polysilanes can be prepared from octachlorocyclotetrasilane, 10, by two routes, as shown in Scheme 15. In one route, (SiCl2)4 is methoxylated to give octamethoxycyclotetrasilane, 54 which is then subjected to ring-opening polymerization (ROP) affording [Si(OMe)2]n, 55. The product was reported to have a UV σσ* transition at 332 nm and molecular weight of 45,000, although few details were given [95]. In the other route (as described in Section 4.1.3.2, above), crystallized 10 [81] undergoes topotactic photolytic ROP to give the almost insoluble crystalline [82] linear precursor polymer (SiCl2)n, 11, which reacts in toluene with alcohols or amines to give soluble bis(alkoxy)- and bis(amino)-substituted polysilanes (types 56 and 57, respectively) with a DP of about 25 in high yield (see also in Scheme 15) [83,84]. Dialkoxy-substituted polysilanes with short alkoxy side chains are hydrolytically sensitive, hydrolyzing to first Si-OH and then cross-linking [193], but polymers with long alkyl chains are stable, as the long alkyl chains presumably wrap around the core, protecting the sensitive Si-O linkage. Optically active poly(dialkoxysilylene)s were also prepared by reaction of 11 with enantiopure alcohols such as (S)-2-methylbutanol or (S)-2-methyloctanol (see Section 6 below for further discusssion on chirality [83,84]). The UV absorption maxima of poly(bis-alkoxysilylene)s exhibit bathochromic shifts about 22 nm relative to their dialkyl analogs [84], e.g. UV λmax for {Si[O-(S)-2-MeBu]2}n = 340 nm vs. λmax for {Si[(S)-2-MeBu]2}n = 323 nm, as can be seen from the spectra in Figure 7.

248

Julian Koe Cl Cl Cl Si Si Cl Cl Si Si Cl Cl Cl NaOMe MeO MeO Si MeO Si MeO

OMe Si OMe Si OMe OMe

10

hν

(solid state)

11

54

Cl Si n Cl

OMe Si n OMe

OR Si n OR

NR2 Si n NR2

55

56

57

Scheme 15. Synthesis of bis(heteroatom)-substituted polysilanes starting from 10, Si4Cl8.

Absorbance (arb. units)

[(S)-2-MeBu)2Si]n

[(S)-2-MeBuO)2Si]n

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250

300

350

400

450

500

Wavelength/nm Figure 7. Comparison of UV spectra of alkyl- and alkoxypolysilanes, showing red-shift due to σ-n mixing.

Precursor polymers 22 and 23 derived by triflate- or chloro-dephenylation of PMPS react with alcohols affording Si-OR substituted polysilanes [194,195]. Up to 80% of phenyl groups could be replaced by alkoxy groups. More effective dephenylation using an AlX3/RCOX system affording effectively fully Cl- or Br-substituted precursor polysilanes 24 has also been published [26]. A broad range of heteroatom-substituted polysilanes is accessible from 22 24, as shown in Scheme 16.

Polysilanes R1 Si

249 R1 Si

n

H R1 = Me, Pr, Hex, Oct

LiAlH4 Et2O

R1 Si

n

OR2

R2OH NEt3

R1 = Me, Pr, Hex, Oct R2 = Me, Et, n-Bu, i-Pr

n

X X = Cl, Br R1 = Me, Pr, Hex, Oct

R1 Si

R1 Si

24

n NR22

n

SR2

R1 = Me

R1 = Me

R2 = Et

R2 = Et, n-Bu

Scheme 16. Routes to Si-functionalized heteroatom polysilanes.

Free radical substitution of the hydrogen in precursor poly[phenyl(H)silylene] 26 has also been used to prepare a number of oxy-functionalized polysilanes, as shown in Scheme 17 [196,197]. R

O

H

AIBN, THF

H Si

CCl4 n

Cl Si

ROH n

OR Si

n

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26

Scheme 17. Radical-based routes to heteroatom polysilanes.

The Si main chain UV absorption in the (PhSiOR)n products was red-shifted 26 nm compared to (PhSiR)n alkyl analogs. In all of these alkoxy-substituted polysilanes, the UV absorption is bathochromically shifted. It is accepted that for Si-bound heteroatoms with available non-bonding electrons this results from an electronic σ-n mixing effect, narrowing the HOMO-LUMO band gap [198]. However, there is less consensus on the theoretical origin of this. It has been suggested that the mixing interaction results in destabilization of the HOMO, thus leading to a reduced band gap [85,86]. On the other hand, recently published experiments on poly(methylpentoxysilylene) and poly(methylpentoxysilylene-co-hexylmethylsilylene) and ab initio MO calculations on model silicon tetramers suggested that the oxygen orbital affects

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Julian Koe

more the excited state of the silicon backbone, reducing the HOMO-LUMO gap by stabilization of the LUMO [98,100]. Concerning the latter copolymers, increasing the alkoxy:alkyl side chain ratio resulted in progressively reduced thermochromism in non-polar solvents. This can be explained using the Schweizer thermochromism theory as resulting from the increasing electrostatic interactions between vicinal alkoxy groups leading to an increasing difference in energy between disordered and extended conformations [160]. The opposite trend was found in polar solvents. Whereas for alkoxy groups near side chain ends, solvatochromism can be considerable [157], in this case the effects were small due to the steric shielding of the oxygen atom from the solvent by the alkyl side chain [98]. As noted above, heteroatom polysilanes usually cannot be synthesized by the Wurtz-type coupling. In an exception to this however, pyrrolyl polysilanes of types 58 with the Si-N linkage have been prepared using sodium in toluene at 75°C although with lower molecular weights than conventional alkyl and aryl-substituted polsilanes [42]. From theoretical predictions [86] and spectroscopic data, σ-n mixing (dependent on the degree of rotation of the pyrrole group) in addition to σ-π mixing, was considered to occur.

N Si n R 58

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Application of the masked disilene strategy to the synthesis of dialkylamino-substituted polysilanes is described above in Section 4.1.2. It is of note that incorporation of the amino function leads to 100% regioselectivity and a regularly alternating copolymer. The amino group is easily replaced by halogen and thence further substituted. [70b].

4.2.3.2. Directly Attached Carbon-Functionalities Polysilanes containing carbon functional groups such as double or triple bonds directly attached to the silicon main chain are expected to show significant differences to conventional polysilanes in their properties due to the interaction of main chain σ electron and side chain π electron systems. However, reports on such materials are sparse due to difficulties in synthesis and characterization and product instability. An ethynyl-substituted polysilane prepared by reaction of triflate precursor polysilane 22 with lithium phenylacetylide was noted though full characterization is lacking [199]. Another report describes copolymers prepared via the Wurtz-type procedure from dichloro(phenylethynyl)methylsilane and dichloromethylphenylsilane [96]. However, the major products were cross-linked and cyclic materials. Incorporation of ethynyl groups in the minor soluble fraction is indicated by a C≡C absorption at 2160 cm-1 in the IR spectrum. The UV spectra are also atypical of conventional polysilanes. In contrast to expectations of a bathochromic shift due to additional electronic delocalization through the π as well as σ system, no shift is observed. Additionally, the intensity of the lowest energy transition (σ-σ*, modified by mixing with phenyl π orbitals in the copolymers with Si-Ph moieties) decreases

Polysilanes

251

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as the ratio of ethynyl-substituted monomer increases and the UV band tails into the visible. The emission spectra showed broad strong fluorescence at 480 nm upon excitation at 345 nm (σ-σ* UV absorption maximum), but no emission upon excitation at 280 nm (π-π* UV absorption), indicating the absence of energy transfer between the silicon backbone σ electron and side chain π systems. If the challenges presented by these materials can be overcome, novel applications can be expected. Further theoretical and experimental study of these systems is required. Silole-incorporated polysilanes were noted above in Sections 4.1.1 and Section 4.1.3.3 where poly(1,1-silole)s synthesized via Wurtz-type coupling [59] and silole polysilanes prepared by anionic ring-opening polymerization [91] were described. In these systems σ/π energy transfer is indicated [93]. Cyano-substituted polysilanes have been synthesized by ROP of a mixture of dicyanohexaphenylcyclotetrasilane isomers (obtained by passing a mixture of ditriflatohexaphenylcyclotetrasilane isomers through an cyano-containing ion-exchange resin. The resulting polymer had Mn = 6800 and a UV λmax at 342 nm, though few other details were given [95].

4.2.4. Surface Functionalization by End Group Tethering Surface-tethering of polysilanes requires the functionalization of one end (terminus) of the polymer main chain with a group which can be attached to a reactive surface or derivatized surface. In the section above on synthesis, the masked disilene approach was described (Section 4.1.2). Chain propagation occurs by attack of a silyl anion chain end on another masked disilene molecule [60]. It was noted above that the reactive polysilane chain end could be used in the formation of block copolymers and end-functionalized chains. Conveniently, this methodology also lends itself to end-grafting polysilanes onto substrate surfaces. Other methods also exist to generate reactive chain ends. Thus, the anionically terminated polymer chain, prepared by n-butyllithium-catalyzed polymerization of a masked disilene reacts with a siloxy alkylbromide reactive anchorderivatized quartz surface [73], giving end-graft polysilane 59, as shown in Scheme 18. The spectroscopic properties of the polysilane-functionalized substrates are consistent with the covalent linking of polysilane (Mn ≈ 17,000) and surface in a monodisperse yet solid state: UV λmax at 330 nm and absorptivity, ε ∝ reactive anchor density, undiminished after standing in toluene [200]. As shown in Figure 8, the variable temperature UV spectra of endgrafted 59 indicate a continuous disordering as the temperature increases, attributed to partially restricted molecular motion due to the tethering, in contrast to the spectra of the free polymer 60 in dilute solution, which on warming show an abrupt thermochromic transition at 260 K due to an ordered-to-random conformational transition. In solid films of 60, the rigid crystalline lattice prevents any conformational change, so no thermochromism is observed. A second technique described by the same authors, termed "cut and graft", involves the anionically induced scission of a high molecular weight polysilane to form a lithiumteminated chain [200,201], using a small amount of lithium 4,4'-di-t-butylbiphenylide. This route is useful for polymers which are inaccessible by the masked disilene technique e.g. those with long alkyl side chains, such as the Wurtz-coupled poly[n-decyl-(S)-2methylbutylsilylene] (PDMBS), for which, due to the longer side chains, a longer chain reactive anchor is required.

252

Julian Koe Pr Me Si

Me Si Me

Ph THF

Br

Si Me O Me

Pr Me Li Si Si Bu n Me Me (i) (ii)

SiO2

n-BuLi (cat.)

THF EtOH quench

Bu Me Si Me Me Si Pr n

Si Me O Me

Pr Me H Si Si Bu n Me Me

SiO2 59

60

Scheme 18. Surface-tethering of end-graft polysilanes obtained by anionic polymerization of masked disilene.

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End-graft PDMBS shows similar thermochromism to the solution state analog, indicating that both forms of PDMBS show behavior typical of isolated molecules with low intermolecular interaction, in contrast to the case of 59, above. Due to the possibility of controlling the reactive anchor surface density, and hence also that of the end-graft polymer, at low levels, individual end-graft polymer molecules could be imaged using atomic force microscopy (AFM) and were observed as isolated dots on flat quartz [200], sapphire and Si(1 1 1) surfaces [202]. PBPS [poly(bis-p-n-butylphenylsilylene] and PMPS were also similarly cut and grafted, AFM giving "worm" and dot images, respectively, the former due to the chain extension and rigidity induced by two phenyl rings [201]. Tris(thioether)silyl endfunctionalized polysilanes 61 and 62 have also been prepared and chemisorbed onto a gold surface [203].

Figure 8. UV thermochromism of (a) end-graft 59, (b) dilute solution 60 and (c) solid film 60 [200]. Reprinted with permission of the authors from Ebata, K.; Furukawa, K.; Matsumoto, N.; Fujiki, M. Polym. Prep. (ACS, Div. Polym. Chem.) 1999, 40, 157-158.

Polysilanes R1 Si (CH2)11 Si n R2

253

S Me S Me S Me

61 R1 = Me, R2 = n-Pr 62 R1 = n-octadecyl, R2 = i-Bu

Most recently, a mild, one-pot immobilization method for attaching polysilanes via a siloxy linkage to hydrophilic quartz or mica substrate surfaces together with AFM, UV and IR data was reported [204]. Triethylamine was used to catalyze the coupling surface –OH groups with the Si-OR and/or Si-H termini of the rigid rod-like helical poly(n-decyl-ibutylsilylene) generated during the course of Wurtz-type synthesis and work-up [52,204].

5. STRUCTURE, CONFORMATIONS AND PROPERTIES Perhaps the most important and interesting features of the polysilanes are their unique spectroscopic and electronic properties. Understanding the relationship between these and the polymer structure, state, morphology and environment is critical for the development and application of polysilanes. A number of reviews have been published [16,205,206].

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5.1. Electronic Structure With increase in the number of silicon atoms in the σ-electron-delocalized backbone, there is an increase in the number of HOMO and LUMO states, resulting in an electronic band structure for high molecular weight polysilanes. Absorption of a UV photon results in an electronic HOMO (σ) to LUMO (essentially σ*) transition giving rise to the characteristic UV spectral profile of polysilanes in the range 300 and 400 nm. The transition moment for this is parallel with the long molecular axis [207]. The great tunability of the absorption wavelength and intensity in this range, deriving from control of the sensitive interdependence of steric and electronic factors of both main chain and side chain, renders polysilanes very attractive candidates in various optoelectronic applications [16]. The lowest energy σ-σ* transition of a polysilane occurs when conjugation is maximized, which is when the chain is in the fully extended all anti conformation (i.e. dihedral angle = 180°). Additional bathochromic shifts occur when the silicon chain is directly connected to unsaturated π system side chains, due to σ - π mixing, or to heteroatoms with available lone pairs of electrons, due to σ - n mixing, both of which result in a smaller HOMO-LUMO gap. In the following sections, an overview of the background and recent developments in understanding polysilane optoelectronic properties is given.

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Julian Koe

5.1.1. Conformation and Phase Calculations on gas phase oligosilane molecules correlated with results from gas phase or matrix isolation experiments on small model compounds afford an understanding of the intramolecular interactions and conformational structure of an isolated polysilane molecule [208-210]. In the solid state, polymer-polymer intermolecular interactions and crystal packing effects increase the conformational complexity, and compound conformations (such as AD+AD-) are possible. Solid polymers can exist in amorphous, liquid crystalline or crystalline phases, which may undergo temperature-dependent phase transitions depending on substituents, repeat unit symmetry, temperature and also chain length [211]. In solution, additional interactions between polymer molecule and solvent (van der Waals, in the absence of any polar substituents on polymer and solvent) result in solvation of the polymer molecules, resulting in globule-, coil- or rod-like global conformations [212-216]. The molecular conformations in solution are dynamic, with mobile kinks and helical reversals [217] separating conformationally uniform segments. In the last two decades of polysilane investigations, three distinct conformational types could be distinguished in the solid state for symmetrically substituted polysilanes {Si[(CH2)m1CH3]2}n with linear alkyl C-x chains [2c]. These were: (i) 73 helix (dihedral angle of ca. 154°) with UV λmax in the range 310 – 320 nm and reported for m = 4, 5 [218]; (ii) AG+AG(or TGTG', as it was formerly termed) with alternating anti and gauche angles, proposed for m = 8, 9, 10, 12, 14 on the basis of molecular mechanics calculations and X-ray studies [219]. However, it has been suggested that a more reasonable conformation would be AD+AD- since the UV absorption for these polymers occurs at ca. 350 nm, around that of transoid polymers [205,206]. Newer correlations of UV, X-ray and DSC data suggest that a combination of D and T is possible (vide infra). (iii) all-anti (all-trans, in older terminology) (dihedral angle of 180°), with λmax in the range 330 – 370 nm (depends on side chain length), reported for m = 1 [i.e. (SiMe2)n] [220], 2 [221], 3 [221,222], 4 at high pressure [223,224], 6 and 7 [225], and the non-symmetrically substituted (MeSi-n-Pr)n (now considered to exist in a monoclinic unit cell [226,227]) and (EtSi-n-Pr)n [222]. Unsymmetrically substituted polymers pack less efficiently, resulting in lower transition temperatures. For small side chain differences, the allanti to hcm transition temperature decreases (e.g. (BuSiPent)n or (PentSiHex)n); for slightly greater differences, crystallization may not occur, e.g. (EtSiBu)n (PrSiPent)n and (BuSiHex)n showed liquid crystalline phases (hcm and/or nematic); for large differences between the side chains, such as PMPS, the polymers can be fully amorphous (elastomeric above, and glassy below Tg). The wealth of spectroscopic data now available on a large number of polysilane structural types suggest that more than three phases can exist. Solid state UV studies can show the number of phases, and a qualitative idea of conformation, though correlation with ionization potential, DSC, X-ray, Raman and theoretical data to elucidate the exact conformations is still lacking. 5.1.2. Small Molecules and Oligomers Useful conformational studies on short chain oligosilanes as model compounds of the high polymers have been carried out. Data from matrix isolation experiments were correlated with the results of ab initio calculations for tetrasilanes, from which five experimentally observed conformations could be assigned: anti, A, with dihedral angle ω ≈ 180°, transoid, T,

Polysilanes

255

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with ω ≈ 165°, deviant, D, with ω ≈ 150°), ortho, O, with ω ≈ 90°, gauche, G, with ω ≈ 60° and cisoid, C, with ω ≈ 40° (although C has not yet been experimentally observed) [22]. These conformations are expected to predominate in the high polysilanes. However, the A conformation is actually rather rare, occurring where side chain crystallization forces an allanti chain, as in poly(dihexylsilylene) [225], or where small substituent size permits it, as in poly(dimethylsilylene) [228] or perchloropolysilane [82] (vide supra, Section 4.1.3.2). As substituent size increases, interactions between side chains on silicons in positions 1 and 3 also increase, favoring a helical chain and disfavoring the A conformation [206]. Helix reversals raise the energy [229] such that at lower temperature, screw sense selectivity increases [217] and in solution the all-transoid pair of enantiomers is predicted to have the lowest enthalpy for all chain lengths [205]. In the solid state, combinations of these conformations, such as AD+AD- are possible, as noted above.

5.1.3. Electronic Structure of Polysilanes The rationalization and prediction of the optoelectronic properties of the polysilane silicon backbone in various electronic and steric environments, by ab initio and/or semiempirical computational methods addressing equilibrium structures, rotational barriers, vibrational frequencies and polarizabilities, is an essential part of the science of these materials. The accuracy of the calculations can be gauged by comparison of experimental ionization potentials with the predicted values (related by Koopman's theorem to orbital energies). Conformational energies have been calculated for the parent oligosilane, H(SiH2)nH [230], permethyl Sin oligomers including Si4 [231,323], Si5 [233], Si6 [234], Si7 and Si8 [235], the Si4 perethyl oligomer [236,237] and oligosilanes with phenyl [238], and chloro- [239] substituents, and have been recently reviewed [205]. Helical conformations were found to be general, with A favored only for polymers with the smallest substituents. In an interesting contrast to the neutral Si4 oligomers, cation and anion radicals showed only a stable all-anti conformation [240]. Calculations show an decrease in the σ-σ* transition energy as alkyl substituent size increases, in agreement with the experimentally observed red-shift in UV λmax [241]. The effects of introducing aryl side chains have also been described [242-244]. Similarly, substitution of heteroatoms with lone pairs directly onto the main chain results in σ-n mixing [68,69] observable as a bathochromic UV shift in λmax. Such effects have been the subject of theoretical investigation for halogens [87,88], oxygen [68,69,98], nitrogen [69] and sulfur [69,245]. Electronic delocalization in polysilanes and its dependence on conformation and side chain, was initially approximated using the Sandorfy C model [16], which did not include conformational terms [205]. The ground state (HOMO), has almost entirely 3pz character (assuming the long molecular axis to be in the z direction), and thus is sensitive to dihedral angle, whereas the LUMO, which has considerable 3s character, is much less sensitive [16]. The clear importance of conformation (from experimental data) led to the development of the "ladder C" and "ladder H" models which, in addition to the strongest, primary (βP) and geminal (βG), interactions, included also a vicinal term, βV, with dihedral angle dependence [230,246]. The ladder H model is the more comprehensive, including silicon main chain and

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Julian Koe

side chain orbitals and the orbital bonding with the silicon side chain orbital, as shown in Figure 9.

Figure 9. Ladder H model showing resonance integrals and orbitals (Si-Si in Plane, Si-C orbitals into paper, bold Si-C orbitals out of paper).

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The relationship between conformation and electronic structure has been investigated for Si chains up to 40 units long [235]. As the dihedral angle increases up to 180°, the HOMO destabilizes with greater chain lengths showing greater destabilization, as shown in Figure 10. Above about Si30, the HOMO energy changes little with further chain length increase. This computational result is in accordance with the experimentally observed bathochromic shift in UV λmax with increase in DP (degree of polymerization) [2c,21,47]. The sharp change in slope at ω ≈ 55° for longer chains was interpreted as a HOMO - HOMO-1 orbital crossing. This was also found in ladder H calculations for these orbitals and is in agreement with ab initio calculations [247].

Figure 10. Calculated ladder H HOMO energies for chains up to Si40 [235]. Adapted with permission from Schepers, T.; Michl, J. J. Phys. Org. Chem. 2002, 15, 490-498. Copyright 2002 John Wiley and Sons Ltd.

Polysilanes

257

Chain segmentation introduced by strong cisoid or gauche twists ("kinks") in an alltransoid or all-anti chain interrupts main chain electronic delocalization, localizing it in the longer segment [16,235], although for multiple gauche twists in a transoid chain (e.g. a repeating GT3GT sequence) the HOMO remains delocalized [235,248].

5.2. Polysilanes in Solution: Conformation, Thermo- and Solvatochromism

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The electronic and conformational properties of polysilanes in solution are strongly coupled, and several well-defined conformations are known and predicted. In dilute solution, interactions are limited to intrachain and polymer-solvent, since the polymer molecules are molecularly dispersed so that effects due to molecular packing which occur in solid phases do not arise (see next section for solid phases). In a poor solvent, intrachain interactions predominate, resulting in chain coiling which is spectroscopically evident. On further decrease of solvation (e.g. by addition of a non-solvent or reduction of temperature), polymerpolymer attractions take over, resulting in aggregation and then precipitation. In a good solvent, polymer-solvent interactions predominate to maximize solvent-polymer contact, resulting generally in extended chains. Reversible temperature-dependent UV spectroscopic changes are termed thermochromism, revealing the reversible sensitivity of the σ-σ* transition to substituent type, conformation, segment length and degree of polymerization. The origin of the phenomenon is thermally induced conformational and segment change, since no chemical modification occurs. Similarly, solvent-dependent UV spectroscopic changes are termed solvatochromism and the effects are generally small for polysilanes with typical (nonheteroatom) alkyl and aryl substituents, though they become significant in polymers functionalized with donor groups.

5.2.1. Solvation From studies of polysilanes in the unperturbed θ state in which polymer-solvent interactions equal solvent-solvent interactions, structure-property relationships of polymers and correlations between primary and higher-order structures can be established. Using sizeexclusion chromatography (SEC)-multiangle laser light scattering, SEC, and static light scattering (SLS), the θ solvents and characteristic ratios (C∞) of a number of polysilanes have been evaluated, with C∞ values typically in the range 20-40 for poly(dialkylsilylene)s, comparable with values from RIS calculations [212,213,229,249,250]. These values indicate that polysilane chains are stiffer and more extended than their carbon analogs (C∞ for polyethylene = 6.7) [251]. Combining these data with known UV/structure correlations, a master curve of average dihedral angle vs. UV λmax was constructed [229], as shown in Figure 11. Using this plot, the average dihedral angle for the 355 nm peak apparent in the low temperature UV spectra of PDHS and PDBS could be estimated as 175° [229]. This is rather greater than the value of 165° suggested in other reports [206], though comparable with the value measured by X-ray for a di-n-propylsilylene decamer [252].

258

Julian Koe 400 PDHS PDBS PMPrS

UV λmax /nm

350

300

250 130

140

150

160

170

180

Average dihedral angle /°

Figure 11. Correlation of average dihedral angle with UV absorption (λmax/nm) (replotted from data in Ref [229]).

5.2.2. Polysilane Global Conformation, Optical Characteristics and Chromisms The global conformation of polysilanes, dependent on side chain structure and degree of solvation, and categorized as globule, coil, stiff or rod-like according to their viscosity index α, have been correlated with their optoelectronic properties [214,253], due to the close relationship between the electronic structure of the chromophoric polysilane main chain and backbone conformation. Changes in the conformation as a result of some kind of environmental chemical or physical stimulus such as change in solvent or additive, temperature, pressure or electrical charge are consequently evident as a chromism (absorption wavelength shift) in the optoelectronic spectra. (S)

* (S)

*

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Si

63

*

(S)

(S) Si

n

Si n

64

65

n

* Si n Me

66

The UV absorption spectra (in THF at 30°C) of four poly(dialkylsilylene)s bearing bearing branched (chiral) side chains are shown in Figure 12: poly[n-hexyl-(S)-2-methylbutylsilylene], 63, poly[n-hexyl-(S)-3-methylpentylsilylene], 64, poly[n-hexyl-(S)-4methylpentylsilylene], 65 and poly[methyl-(S)-2-methylhexylsilylene], 66. The viscosity index, α, which is a measure of chain stiffness is used to characterize the global conformation and depends on the position of the side chain branch.

Polysilanes

259

The position of the side chain branch determines the sterics of interaction of the side chains with other side chains and the main chain and hence governs the backbone conformational mobility and stiffness [214,253,254] (see Section 6 below for chirality and optical activity in polysilanes). For polymers 63 – 66, α = 1.25, 0.92, 0.75 and 0.59, respectively.

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Figure 12. UV absorption spectra in THF at 300C of polysilanes 63-66 [253]. Reprinted with permission from Fujuki, M. Macromol. Rap. Commun., 2001, 22, 539-563. Copyright 2001 Wiley-VCH.

Figure 13. Correlation between viscosity indez, α, UV absorptivity, ε and full width at half maximum (fwhm) of polysilanes in THF at 300C [214]. Reprinted with permission from Fujuki, M. J. Am. Chem. Soc. 1996, 118, 7424-7425, Copyright 1996 American Chemical Society.

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As the value of α increases, the UV peak absorptivity increases, and the fwhm (full width at half maximum) decreases, indicative of the greater structural regularity induced by conformational locking in a preferential screw sense helix by the chiral branch. Combining data from these polymers and a number of others, a semi-empirical relationship between the global conformation in solution and main chain absorption characteristics was found: the viscosity index, α, increased linearly with exponential increase in the main chain UV peak absorptivity, ε (Si-repeat-unit)-1dm-3cm-1 and exponential decrease in fwhm, as shown in Figure 13 [214]. These differences are also evidenced in the 29Si NMR band shape, which is broader for more conformationally restricted backbones (see Section 6.2 below) [214,255]. The choice of side groups thus critically influences the global conformation, degree of σ conjugation and UV absorption characteristics. 63 adopts an extended rigid rod-like conformation due to the strong conformational locking of a side chain branch point close to the main chain and presence of another long, sterically balancing side chain. 66, in contrast, has the most globular shape despite the presence of the same chiral side chain since the other (methyl) side chain lacks the steric bulk required to force chain extension, and the molecule consequently coils in upon itself with the smaller methyl side chain on the inside and the bulkier chiral groups on the outside. In 64 and 65, the chiral side chain branching point moves further from the main chain, exerting progressively less steric control over conformational locking.

5.2.3. Thermochromism Five different types of solution state thermochromism are known, as shown in Table 1.

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Table 1. Different types of polysilane solution state thermochromism Type of Thermochromism

Polysilane types

Examples

Spectra

Ref

(i)

Abrupt transition to longer wavelengtha

(Si-n-Hex2)n

Figure 14(a)

[256]

(ii)

Smooth transition to longer wavelength

Identical or very similar long alkyl side chains Different side chains

[Si(n-Pr)(n-Hex)]n

Figure 14(b)

[256]

(iii)

Bathochromic drift to longer wavelength Hypsochromic drift or no thermochromism Abrupt transition to shorter wavelength

Very different side chains

[SiMe(n-Hex)]n

Figure 14(c)

[256]

Bulky side chains

[Si(4-t-BuPh)0.5(4n-BuPh)0.5]n

Figure 14(d)

[257]

Phenyl side chains, particularly those with alkoxy-substituents

[Si(4-PrOPh)2]n

Figure 14(e)

[257]

(iv)

(v)

a

Range of transition temperature usually very narrow.

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Figure 14. Five different examples of polysilane solution state thermochromism [in hexane for (a)-(c) [256] and toluene for (d) and (e) [257]. (a), (b) and (c) reprinted with permission from Sanji, T.; Sakamoto, K.; Sakurai, H.; Ono, K. Macromolecules 1999, 32, 3788-3794, Copyright 1999 American Chemical Society.

There have been many experimental theoretical investigations into the origins and driving force of solution state thermochromism. Calculations on oligosilanes [235] and polysilanes [214], and experiments on chiral polymers [258] (vide infra, Section 6) and polymers in the θ state polymers [229] indicate that in the absence of crystal packing and solvent effects, most polysilanes adopt helical main chain conformations. For polymers showing thermochromism in categories (i), (ii), and (iii) in Table 1, a bathochromic shift of UV with decrease in temperature [see Figure 14 (a), (b) and (c)] indicates a transition to a new phase with smaller HOMO-LUMO gap due to greater σ conjugation, implying that the new phase has a more extended structure with dihedral angles close to all-anti. For polymers with bulky side chains

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[16,259] such as [Si(4-t-BuPh)0.5(4-n-BuPh)0.5]n, even at elevated temperatures, the UV λmax is already at very long wavelength, due to the chain extension and electronic effects promoted by the aryl groups. At lower temperatures, a slight thermal Si-Si bond contraction may result in twisting of the backbone to relieve steric side chain interactions [259], thereby reducing σ conjugation and resulting in the observed blue shift. For [Si(4-PrOPh)2]n, the abrupt blue shift on reducing the temperature could indicate a transformation to a more twisted conformation to minimize unfavorable polar side chain dipole alignment [260]. An alternative, electronic, explanation involves twisting the phenyl ring π system out of maximum conjugation with the backbone σ system, thus annulling the red shift due to σ-π mixing which usually occurs in aryl-substituted polysilanes [259].

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Origins of Polysilane Thermochromism Three mechanisms have been suggested to account for the stabilization of the extended state in polymers with non-bulky groups. In Schweizer's "single chain" model [160], which is widely used [159,259,261,262], it is proposed that the driving force is the greater polarizability of the molecule in the extended state due to the increased σ conjugation. A conformation-dependent dispersion interaction between the polymer and the local medium (solvent, the polymer's own side chains, or those of an adjacent polymer molecule) determines whether abrupt thermochromism can occur – in solution or solid phases. In the "aggregation" model [219,263] (which does not apply to solution state thermochromism), it was considered that in the solid state, despite the lower calculated energy of the helical conformation, as a result of intra- or intermolecular interactions, the side chains in poly(dihexylsilylene) (PDHS) could pack more efficiently given an all-anti backbone conformation. Above the transition temperature of 42°C, the intermolecular interactions are disrupted, permitting adoption of the lower energy helical form. Finally, in the cooperative transition model [211,256], which has been successfully applied to the solution state thermochromism of PDHS, the temperature dependence of the conformational stabilization energies of extended and helical sequences determines the structure. Further discussion on the mechanisms and origins of thermochromism will be useful. 5.2.4. Solvatochromism Polysilanes do not normally exhibit strong solvatochromism unless functional (e.g. donor -OR) groups are incorporated into the side chains, allowing interaction with suitable solvents, such as alcohols. Strongly hydrogen bonding alcohols, such as hexafluoroisopropanol (HFIP), can induce particularly strong solvatochromic effects, as was found upon addition to a chloroform solution of {[n-PrO(CH2)4]2Si}n (poly[bis(4-propoxybutyl)silylene], 34 [264]. The UV absorption band at 325 nm shifts abruptly to 348 nm above an HFIP concentration of 4%, presumably due to the effective increase of side chain steric requirements and the attempt to maximize the strong coordination. In THF, the transition is not abrupt, since the stronger HFIP-THF interaction [265] reduces the HFIP-polymer interaction. The non-ionic water soluble polysilane 29 [157,215] [poly(4,7,10,13tetraoxatetradecylmethylsilylene); vide supra Section 4.2.2.1], and related polymers poly(4,7,10-trioxaundecylmethylsilylene), poly(4,7,10-trioxahexadecylmethylsilylene) and poly(4,7,10,13-tetraoxanonadecylmethylsilylene) (the last two of which contain a hexyl,

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263

instead of methyl, side chain end) [215], show considerable solvatochromism, as to be expected for polymers containing so many donor alkoxy functions.

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Figure 15. HFIP-induce abrupt solvatochromism of 34 [264]. Adapted with permission from Oka, K.; Fujiue, N.; Dohmaru, T.; Yuan, C.-H.; West, R. J. Am. Chem. Soc. 1997, 119, 4074-4075, Copyright 1997 American Chemical Society.

For 29, the longest wavelength UV absorption occurs at 322.4 nm in HFIP, the shortest at 299.6 nm in water and in THF, the value is intermediate at 310.4 nm (numerous other solvent systems were also investigated). Such solvatochromism is considered to derive from the different solubilities of the polar side chains and nonpolar main chain in the different solvents, and the degree of hydrogen bonding: in HFIP, due to the strong hydrogen bonding, main chain extension occurs; in water, the polar oligoethoxy side chain is well solvated, but the and methyl side chain and backbone are not, leading to chain coiling due to the strong asymmetry of the forces acting on the main chain and thence to a blue-shifted UV λmax; in THF, both side chain and main chain are well solvated, resulting in an extended chain and spectral red-shift (compared to water). The main chain extension results in an increase of the average backbone silicon segment length and thus also of the exciton coherence length, which is consistent with the observed increase in the quantum yield in going from polar aqueous to apolar organic solvents [215]. Recently, the Lewis acidic trifluoromethyl-containing polysilane 67 [216]. [poly(methyl3,3,3-trifluoropropylsilylene)] showed two different chromisms, though resulting in very similar UV spectral changes. Within the polymer, unusual, weak, non-covalent intramolecular Si...F-C interactions exist as depicted in Figure 16 which, in the absence of a stronger donor solvent, lock the main chain into a rigid helical conformation. In the presence of a stronger donor solvent, Si...solvent interactions predominate and the conformation-locking Si...F constraint is removed, releasing the chain to adopt the random coil conformation and UV spectrum expected for a poly(methyl-n-propylsilylene) analog (Figure 17 left). Additionally, the UV spectra of various molecular weight fractions of 67 (Figure17 right) are molecular weight-dependent: the isosbestic point suggests an equilibrium between globule- and rod-like conformations at room temperature [216]. The conformation of 67 is thus sensitively and uniquely controllable by solvent and molecular weight. The

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Julian Koe

photoluminescence intensity of solution and film state 67 is also sensitive to nitroaromatic compounds, leading to the possibility of their application as chemosensors for explosives [266]. F F F

DMF

F Si

Si

F

Si

Si

F

Si...F coordination in non-coordinating solvents ==> helical conformation

Si

DMF Si

Si

Si

DMF F F no locking in stronger donor solvents ==> globule-like / random coil morphology

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Figure 16. Solvent-dependent Si...F-mediated conformations of poly(methyl-3,3,3trifluoropropylsilylene), 67.

Figure 17. Solvent (left) and molecular weight (right)-dependent UV chromisms of 67 [216]. Reprinted with permission from Saxena, A.; Fujiki, M.; Naito, M.; Okoshi, K.; Kwak, G. Macromolecules 2004, 37, 5873-5879, Copyright 2004 American Chemical Society.

A molecular weight-dependency was also found for the thermochromic transitions of poly(dibutylsilylene) and poly(dihexylsilylene): in the low molecular weight range, the transition temperature is proportional to the inverse of the number average molecular weight [211].

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265

5.3. Polysilanes in the Solid State: Structure, Phase Transitions and Chromisms The solid state structure of polysilanes is considerably more complex than that in solution. Depending on substituent type, polysilanes in the solid state can adopt several different structures, comprising the molecular conformation (as for solution and gas phase samples), and additionally the lattice packing arrangement. These two structural aspects are interrelated and dependent also upon environmental factors such as temperature, pressure, electric field and chemical additives. However, in the solid state, X-ray diffraction, DSC (differential scanning calorimetry), AFM (atomic force microscopy) and solid state NMR spectroscopy are also additionally available as probes of structure and properties.

5.3.1. Single Crystal X-ray Diffraction There are few reports of single crystal X-ray analysis of polysilanes, although the data would be extremely useful in correlating optoelectronic and structural properties, since the growth of crystals of suitable quality and size is severely hampered by the structural variations in polymer samples. As a partial solution, single crystal studies have been performed on oligomers where the sample is molecularly homogeneous, yielding useful conformational data. 1,6-bis[(R)-2-phenylpropyl]dodecapropylhexasilane, 68(R) with chiral termini has M-screw sense all-transoid 157 helical conformation (backbone dihedral angles 172 to -177°) [252], as shown in Figure 18.

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Figure 18. 1,6-bis[(R)-2-phenylpropyl]dodecapropylhexasilane single crystal X-ray structure [252]. Reprinted with permission from Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997, 119, 1134511346. Copyright 1997 American Chemical Society.

The seven Si chain oligomer 1,7-dichlorotetradecaphenylheptasilane was also analyzed crystallographically, and significantly for a chain with the SiPh2 repeat unit, shows a helical conformation with SiSiSiSi torsion angles in the range of 154-162° [267]. Perchloropolysilane, (SiCl2)n (vide supra, Section 4.1.3.2) is the only polymer (as opposed to oligomer) to be successfully analyzed by single crystal X-ray diffraction. Crystals were prepared by a unique photochemical topotactic process from crystalline cyclo-Si4Cl8 and the main chain has an all-anti conformation [82].

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Julian Koe

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5.3.2. Powder XRD (X-ray Diffraction) and Correlation with UV and Raman Spectroscopy Most X-ray studies of polysilanes are obtained from powder samples. In combination with variable temperature UV data, these studies provide detailed insights into the correlation of conformational and electronic properties, since the UV is sensitive to the conformation and X-ray diffraction is sensitive to the inter- and intrachain packing and phase transitions. On the basis of the data collected, a number of dialkylpolysilanes are considered to have all-A conformations, viz. (SiMe2)n, (SiEt2)n, (SiPr2)n, (SiMePr)n [226,227,268] low temperature or high pressure forms of (SiBu2)n and (SiPent2)n and low temperature forms of (SiHex2)n, (SiOct2)n and (SiDec2)n. These polymers show UV λmax values at 342, 352, 355, 341, 350, 362 and around 375 nm for the last three, respectively. These widely varying UV maxima could be cause for doubt over the assignment of the backbone conformation. However, the samples also show pre-resonance Raman enhancement [269] indicative of maximum conjugation and an all-A conformation [270]. It is possible to rationalize these differences by considering that despite being essentially all-A, the segments in these polymers have different characteristic lengths which decrease as side chain length decreases, thus reducing the extent of electronic delocalization and leading to hypsochromic UV effects. The other accepted correlation of conformation and UV λmax is the X-ray-characterized 73 helix of crystalline poly(n-butylsilylene) [218,271]. The polymer has a dihedral angle, ω of ca. 154°, which categorizes the backbone as all-D, and a UV absorption at ca. 320 nm. Also absorbing at this wavelength are partially disordered hcm and fully disordered amorphous modifications. Raman spectroscopy can distinguish between all-A, all-D and AD+AD-, as shown in Figure 19, though the distinction between disordered and all-D phases is less clear. Indeed, the similarity of the Raman and UV data of these phases raises the possibility that the main chains in disordered phases may comprise essentially D conformations, though not in a regular lattice. 5.3.3. Atomic Force Microscopy (AFM) Using AFM, single polysilane molecules can be directly imaged. A high molecular weight (Mn = 4,110,000, PDI = 1.30) sample of poly[n-decyl-(S)-2-methylbutylsilylene] (PDMBS), 69, possessing rigid rod-like structure and preferential screw sense helicity was dispersed on an atomically flat sapphire (1012) surface. AFM images were taken in the noncontact mode in air at room temperature [253,272], and reveal the polymer chain as a yellow trace (the vertical patterning is due to the sapphire single atomic steps), as evident in Figure 20. The PDMBS chain is of total contour length 2000 nm, along which segments of length between 150 and 800 nm separated by kinks are evident in the image, corresponding to 800 to 4,000 Si repeat units. The great length and highly extended structure are consistent with the very intense, narrow 318 nm CD and UV bands. The average molecular height is 1.0 ± 0.2 nm, consistent with the molecular diameter estimate of 0.9 nm from a space-filling model of the polymer [273]. AFM has also been successfully applied to image a chemically tethered surface-grafted PDMBS chain and other polysilanes bonded to solid substrates [4,200,201,274,275].

Polysilanes

267

Figure 19. Raman patterns correlated with polysilane backbone conformation (excitation at 514.5 nm, room temperature) [270]. Reprinted with permission from Bukalov, S. S.; Leites, L. A.; Magdanurov, G. I.; West, R. J. Organomet. Chem. 2003, 685, 51-59. Copyright 2003 Elsevier. (S)

* Si

n

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1000 nm

69

Figure 20. PDMBS (69) (yellow trace) on sapphire surface, imaged by AFM [253]. Reprinted with permission from Fujiki, M. Macromol. Rap. Commun. 2001, 22, 539-563. Copyright 2001 Wiley-VCH.

5.3.4. NMR spectroscopy NMR spectroscopy is a powerful probe of polysilane structure, since in typical alkyl and aryl polymers, all nuclei (H, C and Si) have naturally occuring NMR-active isotopes. The 29Si chemical shift is sensitive to the Si environment, both in terms of the substituents at silicon and the main chain conformation. Additionally, in solution state NMR, narrower 29Si NMR linewidths are indicative of greater backbone conformational mobility. In the solid state, NMR is also useful in the conformational analysis of polysilanes, complementing studies by

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Julian Koe

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other techniques. For example, X-ray diffraction data for (SiMe2)n and (SiEt2)n indicate all-A or close to all-A conformations for the dimethyl- and diethyl-substituted polymers [220,221]. Vibrational spectrosocpy are consistent with this: mutual exclusion of IR and Raman bands indicate a centrosymmetric unit cell [222]. The conformations of these polymers together with (SiMe2-co-SiEt2)n and (SiMeEt)n were also investigated using a combination of computational and experimental NMR spectroscopy [276]. 29Si shielding constants were calculated (CNDO/MO) for the model compound Si11H24 at various backbone dihedral angles (ω), as shown in Figure 21, and compared with the experimental 29Si and 13C NMR spectra. As can be seen from the plot in Figure 21, for small deviations from planarity (i.e. 140° ≤ ω ≤ 180°), calculations indicate the chemical shift should move upfield from that of the all-anti conformation. A transoid conformation (ω ≈ 165°) should thus result in an upfield chemical shift in the solid state 29Si NMR spectrum, reaching a maximum at about 150°, i.e. an all-D (73 helix) conformation. For ω ≤ 140°, the shift moves rapidly downfield from that of the allanti conformation, as would thus be expected for ortho, gauche and cisoid twists. Figure 22 shows the experimental spectra for (SiMe2)n at selected temperatures. At higher temperature, a new resonance grows in slightly upfield from the low temperature crystalline peak, thus suggesting an increase in deviant turns within a largely anti conformation.

Figure 21. CNDO/MO-calculated 29Si nuclear shielding as a function of dihedral angle, ω for the central Si in Si11H24 [276]. Reprinted with permission fromTakayama, T. J. Mol. Struct. 1998, 441, 101117, Copyright 1998 Elsevier.

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269

Figure 22. Solid state 29Si NMR spectra and deconvolutions of (SiMe2)n, at selected temperature [276]. Adapted with permission from Takayama, T. J. Mol. Struct. 1998, 441, 101-117. Copyright 1998 Elsevier.

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For (SiEt2)n, a single peak is observed at low temperature, which drifts downfield (considered to indicate gauche enrichment) as the temperature increases and is accompanied by the growth of a second peak downfield from the first, considered to result from a phase transition from the ordered crystalline form to a disordered form [276]. Eventually at 25°C the peak originating from the low temperature form disappears (not evident from DSC). The new peak is downfield from the low temperature form, indicative of gauche or ortho conformations with ω ≤ 130°. For (SiMe2-co-SiEt2)n, at higher temperatures the SiEt2 peaks drift downfield, though the SiMe2 units remain virtually unchanged, consistent with greater flexibility in the ethyl side chain. For (SiMeEt)n, the data indicated an almost unchanged backbone conformation between -70 and 120°C, though above 80°C the side chains became more disordered. The analysis of the structure, phase transitions and properties of some representative alkyl and aryl polysilanes follows.

5.3.5 .Poly(dihexylsilylene), PDHS PDHS is one of the most intensively studied polysilanes, with multiple temperaturedependent forms showing UV maxima at 350, 365 or 377 nm [277]. Cast films of the same polymer prepared under different conditions of temperature and drying speed have absorption maxima at 357, 370 or 375 nm, with slow drying at warm temperatures (just below the orderdisorder transition temperature of 41°C) affording the most highly crystalline films and the longest wavelength. The UV spectra at various temperatures of the film obtained by slow drying from a toluene solution at 36°C are shown in Figure 23, with maxima evident at 322, 378 and 391 nm [278].

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Julian Koe

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Figure 23. UV spectra of PDHS film taken sequentially at 10-minute intervals after heating to 80 C for 10 minutes (spectrum 1 is as cast; 2 etc. after heating [278]. Reprinted with permission from Kyotani, H.; Shimomura, M.; Miyazaki, M.; Ueno, K. Polymer 1995, 36, 915-919. Copyright 1995 Elsevier.

These different absorptions arise from phases with different main chain conformation and packing, with greater main chain extension resulting in longer wavelength absorption. Careful examination of the spectra reveals five different phases, two evident as shoulders. UV and X-ray data (orthorhombic unit cell) [263,279,280] for PDHS have been correlated, confirming that the structure refinements below the crystalline to hcm (hexagonal columnar mesophase) transition temperature around 45°C are consistent with an all-anti backbone and pertain to the longest wavelength absorption maximum [280]. Moreover, preresonance Raman enhancement was found only for PDHS samples absorbing at 375 nm which should indicate that only this conformation is fully extended with dihedral angles of ≈ 180° [269,270]. The evidence available thus strongly suggests that PDHS and other dialkylpolysilanes absorbing in the UV at 375 nm have all-anti backbone conformations. It has also been found that the alkyl side chain structure strongly influences the main chain conformation: for PDHS analysis by X-ray and MM3 calculation indicates the side chains adopt an asymmetric cisoid-transoid conformation [281]. The UV bands of PDHS at 350 and 365 nm, however, have not yet been unambiguously assigned [similarly for (Si-n-Oct2)n and (Si-n-Dec2)n]. Since these bands fall between the 320 nm deviant and 375 nm anti, it is possible that they could be due to intermediate conformations such as transoid or compounds such as AD+AD-, TD+TD- or AT+ATSolid state 29Si and 2D NMR have also been applied to the investigation of the conformation and mobility of the backbone of PDHS and its deuterated analog, PDHS-d [282]. A discontinuous upfield shift was found in the 29Si magic angle spinning NMR spectra and taken to indicate an increasing proportion of gauche turns with increase in temperature, consistent with previous discussions [283]. However, considering the recent X-ray results above, these turns are possibly deviant. The CNDO/MO calculations on the dihedral angle dependence of chemical shift for silicon chains (see Figure 21 above) also appear to support

Polysilanes

271

this [276], along with X-ray and UV data [205]. Variable temperature 2H-NMR spectra for the PDHS-d indicate that at 200 K the polymer is immobile and rigid. A dramatic change in the spectra occur in the temperature range 310 – 320 K due to the transition to hcm, consistent with a flexible and highly mobile backbone in the hcm phase [282]. Finally, the thermochromism of PDHS in the solid state has been correlated with changes in the refractive index, consistent with the Schweizer theory [160] (vide supra) the thermochromism may derive from a change in molecular polarizability, i.e., the extent of σconjugation, and arises from a switching of the Si backbone between all-anti and helical conformations [284].

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5.3.6. Poly[(alkyl)(aryl)silylene]s Polyalkylarylsilanes such as PMPS (for which the UV bands have been assigned to electronic transitions [285]) have an absorption band around 345 nm due to the mixing of aryl π orbitals with the σ-delocalized Si backbone. Previously PMPS was thought to be amorphous and not to undergo thermochromic transitions. Recent investigations using variable temperature WAXS (wide-angle X-ray scattering) and optical polarizing microscopy [227,286] indicate clearly an ordered phase in the polymer, with ca. 10% crystallinity at ambient temperature. In the lower temperature region, the data fit a monoclinic crystal lattice of all-A atactic chains in near-hexagonal symmetry, with two types of disorder. At about 190°C, a transition to a hcm phase is observed: the chains rotate about their long axis, conformational locking by the phenyl rings maintaining the all-anti conformation. The polymer melts at ca. 260°C into an amorphous isotropic phase. Studies on PMPS substituted with unichiral groups (vide infra, Section 6.1.1), however, indicate that PMPS adopts a helical conformation with greater ordering than previously thought. 5.3.7. Poly(diarylsilylene)s For this class of polysilanes, the UV λmax is typically observed at around 395 nm due to a combination of steric and electronic factors resulting in a small HOMO-LUMO band gap [287]. Sterically, the bulky phenyl rings are best accommodated in an extended conformation. Electronically, σ-π mixing between Ph π and Si main chain σ systems results in a smaller HOMO-LUMO gap and thus lower energy absorption maximum to an even greater extent than for the alkylarylpolysilanes above [16,288]. Early discussions [16,289] on the conformation of diarylpolysilanes suggested that long all-A backbone segments could account for the long UV λmax. Unfortunately, X-ray investigations of diarylpolysilanes are hampered by the rotational disorder of the phenyl rings. The single crystal X-ray analysis of the model oligomer 1,7-dichlorotetradecaphenylheptasilane, however, was successful, revealing a helical conformation with main chain torsion angles between 154 and 162° [290]. Electron diffraction data for oriented thin films of (SiPh2)n are consistent with this, ruling out an all-A and also 73 helical conformation [291]. Solution state UV and CD spectroscopy of unichiralsubstituted diarylpolysilanes indicate an extended loose helical structure and viscometric studies show that the structure is rigid [292]. Data from force field calculations are consistent with a loose helix [259]. Diarylpolysilanes in the solid state show thermochromic properties similar to the solution state: poly(bis-4-butoxyphenylsilylene), 70, shows a UV λmax shift from 400 nm at higher temperature to 325 nm at lower temperature [260]. It was considered that the main chain

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undergoes a strong distortion at lower temperature to find a lower energy conformation avoiding the high energy C-O dipole alignment in an extended conformation. The lack of peaks in the DSC trace suggested that no phase change occurred. However, such thermochromism is not limited to diarylpolysilanes with alkoxy substituents: poly[bis(4-nbutylphenyl)silylene] (PBPS, 71) also shows hypsochromic thermochromism.

O

Si

O

Si

n

n

70

71

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However, in contrast to the case of 70, for 71 a structural phase transition evident by DSC occurs at about 355 K, accompanying the Si main chain conformational change evident from the variable temperature UV spectra shown in Figure 24. The phase change was suggested to arise due to a transition from the more conformationally extended higher temperature form with λmax = 390 nm to a disordered, lower temperature form with λmax = 315 nm [293]. It was proposed that the large 4-n-butylphenyl groups stabilize the extended conformation at higher temperatures by intramolecular steric interactions, but that at lower temperatures, packing forces favor a disordered form [293]. However, if molecular packing is a factor, a more regular structure such as a deviant helical conformation (rather than a disordered structure) might be more reasonable. Consistent with this suggestion, similar thermochromism is observed for poly{[4-n-butylphenyl][3-(S)-2methylbutylphenyl]silylene}: a pressed bulk sample (precipitated from toluene/isopropyl alcohol) showed a UV absorption with λmax at 397 nm and fwhm (full width at half maximum) of 25 nm, while a film sample showed a narrow UV absorption with λmax of 321 nm and fwhm of 11 nm while [257].

Figure 24. Thermochromism in solid film of PBPS, 71 [293]. Reprinted with permission from Bleyl, I.; Ebata, K.; Hoshino, S.; Furukawa, K.; Suzuki, H. Synth. Met. 1999, 105, 17-22. Copyright 1999 Elsevier.

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The extreme narrowness of the film UV profile is indicative of considerable structural regularity and consistent with an ordered helical phase. Presumably in such a conformation, the phenyl rings are also rotated out of conjugation with the backbone, since large hypsochromic shift would appear to indicate minimal σ-π mixing. Interestingly, these 320 nm diarylpolysilane UV absorptions occur at very similar wavelength to the high temperature hcm phase of PDHS and other dialkylpolysilanes, (see Figure 14 (a) and (b) above), though the absorptions for the dialkylpolysilanes are much broader. It is thus possible that in the diarylpolysilanes, the narrow absorption at 320 nm appears to indicate a regular deviant helical form, while in the dialkylpolysilanes, the hcm form is primarily a disordered deviant helix.

5.4. Other Chromic Responses to External Stimuli 5.4.1. Ionochromism Donor-functionalized polysilanes which coordinate to ions and undergo a change in UV λmax as a result are said to exhibit ionochromism. Compared with the enormous literature on thermochromism, there are relatively few reports on this topic. Films of poly(methyl4,7,10,13-tetraoxatetradecylsilylene), 29, show increasing hypsochromic UV shifts as LiClO4 content increases, indicating progressive restriction of the backbone [156]. At a Li:Si ratio of 1, four oxygens can efficiently intramolecularly coordinate a single Li cation, as shown in Scheme 19, although intermolecular coordination is also possible. In studies on a related polymer, poly(4-ethoxyethoxybutylmethylsilylene) [294], with only two oxygen atoms per side chain complexation of Li+ was much less efficient [295]. As might be expected considering the relative donor abilities of wtaer and oligoethers, aqueous solutions of 29 do not show ionochromism with Li+ [157]. CH3 Si

CH3 Si

LiClO4

n

O

O

O

O

n

THF

O

29 Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

O

Li+ O

O

Scheme 19. Coordination of Li+ by oligoether side chain of 29 resulting in ionochromism.

5.4.2. Piezochromism Reports on piezochromism are even more sparse. The phenomenon results when application of pressure to a sample results in a spectroscopically evident conformational change, and was documented early in the study of polysilanes in the pressure-induced helical to all-anti transitions of (SinBu2)n and (SinPent2)n bulk films at high pressure [224]. In such cases, phase transitions may also occur. In monolayers, however, 3-dimensional packing does not occur, so piezochromism in these samples reflects conformational changes. Polysilanes

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with purely hydrocarbon side chains do not form monolayers on water since they are hydrophobic; and polysilanes soluble in water will also not form monolayers. However, stable monolayers will form for polysilanes with affinity for water, such as the ethersubstituted 8 [66] (prepared by masked disilene polymerization, see Section 4.1.2 above) and Wurtz-coupled 72 – 77 [296]. 72: R = CH3, m = 6

Me Me Bu Si Si H n Me 8

R

Si n

O

H C OCH3 H m

73: R = CH3, m = 10 74: R = (CH2)6OCH3, m = 6 75: R = (CH2)10OCH3, m = 10 76: R = i-C4H9, m = 6 77: R = i-C4H9, m = 10

Reversible changes in the UV spectra during compression of 72 – 77 monolayers on a water surface indicate both compression-induced and flow-induced orientation of the molecules. The effects were greater for the polymers with shorter alkyl side chains, due to their more rigid backbones [66,296,297]. At lower surface pressures the UV λmax occurs at 334 nm, indicative of a 73 helical conformation; while above 17 mN m-1, an abrupt bathochromic shift to 347 nm is evident, suggested to be due to an all-anti conformation. Significant changes in the dichroic ratio of a monolayer of 8 coincident with the spectroscopic changes were considered to indictate realignment of the polysilane chain from parallel to the substrate drawing direction at lower pressure to perpendicular at higher pressure [66]. Other pressure- and surface-dependent chromisms have also been reported [179,298,299].

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5.4.3. Electrochromism Very few examples of electrochromism not involving an electrochemical reaction have been reported. Poly[(3,3,3-trifluoropropyl)methylsilylene0.45-co-methylpropylsilylene0.55] undergoes reversible solid-state electrochromism upon application of an electric field of 3.1×108 V m-1 to a thin film sample. The UV λmax shifts from 294 to 299 nm, a new absorption is evident at 314 nm and the absorptivity increases by approximately 50% in less than 100 ms. The strongly polar side chains were considered to orient under the influence of the electric field, thereby inducing silicon main chain conformational changes [207]. 5.4.4. Magnetic Field Effects on Spectroscopic Properties The effects of magnetic fields upon polysilane properties has also been investigated. The polar oligoethoxy side chains of 29 in THF solution were found to align perpendicular to a magnetic field applied colinearly with a UV beam. The Si backbones were reported to follow this alignment, also becoming preferentially oriented perpendicular to the magnetic field, thus orienting their transition moments for maximum absorption, since the UV σ-σ* transition moment is aligned along the long molecular axis [207]. The absorption intensity increased by about 5% in a 5 T field, although λmax was unshifted. Additionally, PMPS films prepared in a strong magnetic field show carrier transport properties markedly different from those prepared under no magnetic field [300].

Polysilanes

275

5.5. Photochemical and Chemical Reactions of Polysilanes 5.5.1. Degradation Polysilanes can undergo chemical reactions either involving the side chains, as in substitution or functionalization, or involving the main chain, as in chain scission to functionalize the terminal silicon atoms (e.g. preparation of end-graft or surface-graft polysilanes, as described above, or block copolymers) or in degradation (depolymerization). Except for degradation, which is the subject of this section, such chemical reactions have been described above. Degradation can result from both chemical and physical processes, caused by chemical reagents, light, heat, ionizing radiation, electrochemical means or ultrasound. Size exclusion chromatography (SEC, also known as gel permeation chromatography, GPC) is a primary technique used to analyze chain scission, and is supported by UV spectroscopy, since the σ-σ* UV transition shifts hypsochromically as molecular weight decreases. Detailed investigations of the degradation of both aryl - and alkyl-substituted polysilanes have been reported [301].

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Degradation by Chemical Agents Polysilanes are well known to degrade in the presence of bases, and form cyclic oligomers. With strong protic acids and halogens, α-ω difunctional oligosilanes are formed. Paradoxically, alkali metals, responsible for coupling dichlorosilanes to make polysilanes in the Wurtz-type reductive coupling procedure, are also responsible for the degradation of polysilanes, in the order of effect K > Na > Li. During both synthesis and degradation, cyclosilanes are produced, though different mechanisms are proposed on the evidence that different cyclics are formed: back-biting (degradation and reductive coupling; intermolecular favored over intramolecular) and end-biting (reductive coupling only) [301]. Alkali metalpromoted degradation depends also on polysilane substituents, solvent and temperature. In solvents which solvate the alkali metal cations, such as ethers or HMPA, degradation is promoted. In non-coordinating hydrocarbon solvents, the rate is reduced. Arylpolysilanes are more rapidly degraded than alkylpolysilanes, since the aryl ring stabilizes radical anions. Photolysis Under the influence of light of shorter wavelength than the σ-σ* λmax, the silicon chain is rapidly photolytically cleaved forming radical-terminated shorter chains which usually scavenge H or O (the latter resulting in the formation of siloxanes) from the surrounding medium. Longer chains are cleaved first, resulting in a lower PDI (polydispersity index) as photolysis progresses, while branched polysilanes [302] and those doped with fullerenes [303] or other electron-accepting groups [304] were found to be more resistant to degradation. The reaction is useful where reduction of polymer molecular weight is required and is the basis of the proposed use of polysilanes as chemical resists in microchip manufacture [305]. The degradation process also depends on solvent: irradiation of [(SiMePh)x-co-(SiPh2)y] in CH2Cl2 resulted in degradation, though in PhCl, none was observed [306]. The effects of photolysis on IR [307], UV [308], and fluorescence [309] properties have also been reported, together with a theoretical study of the effects of structure on photostability [310].

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Julian Koe

Thermolysis Thermal degradation leads to the formation of siloxanes and cyclosilanes and is accompanied by an increase of PDI. The process occurs more rapidly as the temperature increases, and is faster in the solid state than in solution [301]. Radiolysis Ionizing radiation such as ion beams and 60Co γ-ray radiation cause degradation [306,311-313]. Depending on the branching density, side chain type and whether or not radical initiators are added, both negative and positive resist characteristics are evident. Electrochemical Degradation Polysilanes have been investigated as electroluminescent (EL) materials [314-316], and poly(bis-n-butylphenylsilylene) (PBPS) has been shown to have the best lifetime and emission characteristics [317]. A major problem concerning polysilane EL is degradation, which is greater at higher current densities and limits the lifetime [314]. Strategies to greatly reduce the degradation occurring during electroluminescence are required if this area is to progress. Ultrasound Ultrasound causes homolysis of high molecular weight polysilanes due to the strong mechanical shear forces generated in cavitational processes (though in dilute solution chains are less likely to cleave). The fate of the radicals is not known, though no cyclic silanes are found in the products. Longer chains are cleaved more easily, molecular weights tending to a lower limit of ca. 50,000, thus leading to a decrease in PDI [301].

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6. HIGHER ORDER STRUCTURE - OPTICAL ACTIVITY Bulk properties arising from higher order structure include optical activity, magnetism, melting point, crystallinity (both liquid and solid). In this section, optical activity in polysilanes arising from intramolecular conformational higher order will be introduced, though for further detail, the reader is directed to more focussed reviews [2a,253,318-321] and the original literature. Polysilanes typically have conformationally mobile helical structures [206,318,320], with transoid (157) or deviant (73) conformations [22], as noted above. Kinks in the chains occur at points of significant change in main chain dihedral angle, and are mobile within the chain. If a kink is such that it effectively results in a twist of opposite sign but equal magnitude to its neighbors, it is known as a helix reversal [322]. Two such helix reversals arriving at the same point in a chain can can undergo mutual annihilation [47]. Polysilanes are generally not optically active, since they are internally racemized by the presence of equal proportions of opposite screw sense turns and helix reversals. However, chains with either a P (plus; righthanded) or M (minus; left-handed) preferential or unique screw sense, induced by application of a chiral field and restriction of main chain Si-Si bond rotation, are chiral, since a helix is a chiral motif. The molecular chirality of polysilanes is manifested very conveniently by optical activity in the circular dichroism, CD or circularly polarized luminescence, CPL

Polysilanes

277

optoelectronic spectra of the electronically delocalized σ-conjugating chromophoric and fluorophoric backbone.

6.1. Polysilane Preferential Screw Sense (PSS) Helicity and Chirality Preferential screw sense (PSS) helicity in polysilanes can be induced by an internal chiral field, such as substitution using enantiopure chiral (unichiral) groups in side chain or end chain positions, or an external chiral field, such as a unichiral solvent or additive.

6.1.1. PSS Induction by Unichiral Side Chains The first optically active polysilane copolymers [323] and homopolymers [273] were reported in 1994, prepared by Wurtz-type polymerization using unichiral monomers, as exemplified in Scheme 20. Since then, many unichiral-substituted dialkyl-, alkylaryl-, diaryl-, bis-alkoxy-polysilanes have been prepared. The branch of the unichiral side chain has two-fold importance: (i) provision of a chiral field (internal) by formation of a chiral center, and (ii) restriction of backbone conformational mobility by increase of side chain steric bulk. Energy minimization of the structure results in a rotationally restricted PSS helix. The position of the chiral branch is important [254,324]: too close to the main chain and the steric congestion inhibits synthesis; too far and the conformation-directing and -locking effect is too small to induce PSS helicity [253]. (S) Si (S)

*

n

(S)

*

*

Si Na

(S)

x

*

isooctane / toluene Si

Si

y n

n

78

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Scheme 20. Synthesis by Wurtz coupling of unichiral-substituted optically active polysilane copolymer.

Spectroscopic Characteristics of Polysilane PSS Helicity The helical main chain is chromophoric, and it is thus the delocalized σ electron system which exhibits optical activity: the circular dichroism spectrum shows a band (Cotton effect) coincident with the UV absorption. The helicity of the molecules (screw sense and pitch, and diastereomeric purity) can be characterized by the dimensionless Kuhn disymmetry ratio gabs, defined as Δε/ε, the ratio CD (polarized) to UV (unpolarized) molar absorptivity per repeat unit, and appears to be almost independent of both alkyl side group length and silicon main chain length [325]. However, helix reversals and segments of opposite screw sense, if absorbing at the same wavelength, will result in a decrease in gabs. Thus for a low gabs polysilane it is difficult to be ascertain whether the low value derives from low screw sense selectivity, or to smaller dihedral angles. Thus assessment should be made using a range of experimental probes including viscometric studies to indicate the global conformation, and the data compared with non-unichiral-substituted analogs. The greater structural regularity

278

Julian Koe

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and stiffness deriving from the conformationally locked PSS structure generally results in narrower, more intense UV absorptions, and a high value of the fluorescence anisotropy (FLA). Circularly polarized emission coincident with the fluorescence emission has also been observed [321,326], but due to the intense high energy exciting radiation required, special procedures need to be employed, as degradation during data collection is considerable. The restricted backbone mobility resulting from the conformational locking effect of the branched side chains is also evident in the 29Si NMR spectra: the increased relaxation time results in broader main chain 29Si resonances [253,255,327]. Such polysilanes are typified by 79, optoelectronic spectra for which are shown in Figure 25 [217]. In dilute ethanol at ca. 24°C, 79, precipitated from ethanol using water, with Mw = 7×106 and PDI = 2.40, shows UV, CD and FL spectral properties characteristic of a rigid rod-like chromophore and fluorophore with single-screw sense helical structure (see Section 5.2.2 above for a correlation of optoelectronic characteristics with global conformation). The polymer exhibits a very intense, narrow UV absorption at 324 nm (ε = 47,000 (Si-repeatunit)-1dm3cm-1, fwhm = 7.3 nm). This UV profile is similar to those of other PSS poly(dialkylsilylene)s, such as poly[n-decyl-(S)-2-methylbutylsilylene] [253,273], but considerably narrower and more intense than for conventional random coil poly(dialkylsilylene)s. Also, the CD band matches the UV band in wavelength and profile. The FL spectral profile at 328 nm is almost a mirror image of the UV and CD band profiles and the FL anisotropy around the 324-nm UV and CD bands almost reaches the theoretical limit of 0.4 expected for a random distribution of a rigid rod chromophores collinear with the fluorophore in a rigid medium. From the ε-α (α = viscosity index) and fwhm-α correlations [214], α was estimated to be 1.29 (also indicative of high rigidity). The Kuhn disymmetry ratio, gabs ≈ 2.1×10-4 for 79 was comparable to values of other poly(dialkysilylene)s, and taken to indicate a high degree of diasteromeric helical purity.

Figure 25. (a) UV, CD and FL spectra and (b) FL spectrum and FL anisotropy of poly(6,9,12trioxatetradecyl-(S)-2-methylbutylsilylene), 79, in ethanol at 23-250C 217]. Reprinted with permission from Fujiki, M.; Toyoda, S.; Yuan, C.-H.; Takigawa, H. Chirality 1998, 10, 667-675. Copyright 1998 Wiley-Liss, Inc.

For some polysilanes, segments of opposite screw sense and giving rise to different wavelength absorptions, due to side chain packing, helical pitch and thus dihedral angles. For such polymers, both positive and negative Cotton effects are observed, as shown for the example of poly[methyl-(S)-2-methylbutylsilylene], 66 in Figure 26 [328,329]. The positive

Polysilanes

279

and negative extrema in the spectra were shown by a "cut-and-paste" experiment to result from the existence of two opposite screw senses of differing screw pitch.

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Figure 26. UV and CD spectyra of poly[methyl-(S)-2-methylbutylsilylene] before (solid) and after (dotted) “cut-and0paste” experiment (isooctane at 250C) [253,328]. Reprinted with permission from Fujiki, M. Macromol. Rap. Commun. 2001, 22, 539-563. Copyright 2001 Wiley-VCH.

Irradiation of 66 in CCl4 at the longer wavelength (negative) Cotton effect caused this band to disappear and the UV absorption to undergo a hypsochromic shift to match the profile of the remaining (positive) Cotton effect, indicating selective photolytic degradation of the looser helical segments. After removal of the low molecular weight degradation products, the remaining Cl-terminated telomers were recombined using sodium in hot toluene, affording an almost single screw sense helical polymer showing the dotted line spectra in Figure 26. Chiroptic studies have also been applied to the investigation of diarylpolysilane conformations. As noted above, these polymers have the lowest energy HOMO-LUMO gap among the polysilanes due to both conformational and electronic effects [16,260]. It was considered that the highly extended, stiff backbones could actually exist in the all-anti conformation. To test this hypothesis, unichiral-substituted diarylpolysilanes were investigated [259,292,319,329,330]. Wurtz-coupled 80 shows UV λmax [see Figure 27(a)] at the same wavelength (ca. 395 nm) as its non-chiral-substituted analog, poly(bis-nbutylphenylsilylene), PBPS, indicating that the dihedral angles in these polymers are the same [292]. The observation of Cotton effects in the CD spectra is thus evidence that diarylpolysilanes adopt PSS helical (presumably transoid, rather than all-A) conformations. This was also indicated by force field calculations of potential energy vs. backbone dihedral angle for a 30-mer model of 80 [259], from which the very tentative suggestion of association of M screw sense with positive Cotton effect can be made (see Section 6.1.4 below for more on this issue). The helicity was corroborated by the results of a single crystal X-ray analysis of a poly(diphenylsilylene) heptamer, which showed dihedral angles in the range of 154-162° [267].

280

Julian Koe

Figure 27. (a) UV and CD spectra [292] for (ArAr*Si)n, 80 and (b) force field-calculated potential energy as a function of backbone dihedral angle [259] for syndiotactic 30-mer model of 80 (where Ar=4—n-butylphenyl, and Ar*=4-(S)-2-methylbutylphenyl. (a) Reprinted with permission from Koe, J. R.; Fujiki, M.; Nakashima, H. J. Am. Chem. Soc. 1999, 121, 9734-9735. Copyright 1999 American Chemical Society; (b) Reprinted with permission from Koe, J. R.; Fujiki, M.; Motonaga, M.; Nakashima, H. Macromolecules 2001, 34, 1082-1089. Copyright 2001 American Chemical Society.

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The chiroptic properties of unichiral-substituted poly[(alkyl)(aryl)silylene]s have also been investigated. Due to the greater difference between the side chain structures in poly[(alkyl)(aryl)silylene]s, the side chain-dependent conformational locking effects are not as strong as in the more symmetrical dialkyl- and diaryl- polysilanes, and optical activity due to an induced PSS helical backbone may or may not be observed [166,167,331]. UV, CD, FL and FLA studies of unichiral-substituted poly[(alkyl)(aryl)silylene]s and comparison with optically inactive PMPS indicated that such polymers should adopt similarly helical conformations, except that the latter is an internal racemate and thus not optically active. These studies indicate that poly[(alkyl)(aryl)silylene]s have more ordered conformations than previously supposed.

Copolymers and Cooperativity Cooperativity in copolymers refers to the degree of non-linearity between optical activity and mole fraction of chiral comonomer. Studies on polymers prepared from dichlorodi-npentylsilane and either dichloro-(S)-2-methylbutyl-n-pentylsilane or dichloro-bis-(S)-2methylbutylsilane (see polymer 78, above) revealed a linear dependency of optical activity on chiral comonomer mole fraction (i.e. zero cooperativity) [323]. Other studies on copolymers formed by the copolymerization of achiral (racemic) dichlorohexyl-2-methylbutylsilane and chiral dichlorohexyl-(S)-2-methylbutylsilane or dichlorohexyl-(R)-2-methylbutylsilane, such as 81, however, show strong positive cooperativity. The phenomenon, also termed the "sergeants and soldiers" effect [332], is shown by the induction of PSS helicity at 0.6 mol% of chiral comonomer, though the Kuhn dissymmetry ratio gabs indicates less screw sense selectivity than in the unichiral-substituted homopolymer. Above 5 mol%, though, the helicity is the same as that of the chiral homopolymer, as shown in Figure 28 [323,324]. Poly[(alkyl)(aryl)silylene] copolymer 82 also displays this type of cooperativity [331].

Polysilanes

281

*

(S)

(R) or (S)

(S) *

* x

Si

Si 1-x

n

x

Si

1-x n

Si

Si

x

1-x n

*

Si

(S)

81(R) or 81(S)

82

83

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In contrast, diarylpolysilane copolymer 83 shows unusual ring substitution positiondependent and chiral mole fraction-dependent cooperativity: positive cooperativity below 50% chiral content, but reversing above this and becoming negative for the 100% chiral polymer. Negative cooperativity is shown by the meta analog of 83, poly(bis-p-nbutylphenylsilylene)-co-(bis-m-(S)-2-methylbutylphenylsilylene) [333,319]. These cooperative effects were computationally confirmed using an Ising model modified to include bonding chiral and achiral repeat unit interactions and are represented in Figure 28 [334]. Another cooperativity effect, termed "majority rule" is exhibited by polyners in which opposite enantiomer chiral groups are both present in side chains, such as (S)- and (R)-2methylbutyl groups, whereby the optical activity is determined non-linearly by the enantiomeric excess only [332,335].

Figure 28. Calculated (solid line) and experimental (symbol) composition-dependence of enantiomeric excess 81 (R): { 81 (S): ~ 82, U and 83 z [334]. Reprinted with permission from Sato, T.; Terao, K.; Teramoto, A.; Fujiki, M. Macromolecules 2002, 35, 5355-5357. Copyright 2002 American Chemical Society.

282

Julian Koe

Solid State Chiroptic Studies The control of higher order structure in polysilanes has opened the door to a new range of potential applications, though in most of these, the polymer is required to be in the solid state, which presents challenges in the preparation and chiroptic analysis of polysilane films. Recently, however, solid state CD investigations of chiral polysilanes have been reported: a helix-coil transition in film samples of poly[(S)-3,7-dimethyloctyl-n-propylsilylene)] [336], and use of a chiral polysilane thin film as a helical command surface in chiral induction [337].

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6.1.2. PSS Induction by Unichiral Chiral End Groups Unichiral groups in end chain positions only can also exert a sufficient chiral field through cooperativity between the chiral termini and achiral side groups. Several examples have been reported. 1,6-bis[(R)-2-phenylpropyl]dodecapropylhexasilane, 68(R) with chiral termini has an Mscrew sense all-transoid 157 helical conformation (backbone dihedral angles -172 to -177°) [252], as shown in the X-ray structure in Figure 18 above. The decameric (R)-chiral analog 84(R) shows a strong positive Cotton effect below 153 K, also indicating PSS helicity induced by cooperativity. Subsequently, a PDHS chain was terminated with the same chiral group, 85, and showed bisignate [217,252] Cotton effects (resulting from the interaction between two chiral chromophores) below 233 K [338]. The work provides strong evidence for the existence of PDHS in a 157 helical conformation in solution at lower temperatures and adds to the debate on the conformation of PDHS (vide supra Sections 5.2 and 5.3). Chiral end chain functionalization is conveniently achieved via the anionic polymerization of masked disilenes (vide supra Section.4.1.2). Using the chiral potassium (+)- or (-)-menthoxide as anionic initiator, the respective (+) and (-) menthoxy-terminated poly(1,1-dimethyl-2,2-di-n-hexylsilylene)s, 86, are obtained, which show negative and positive bisigned Cotton effects in solution at -40°C, and positive and negative Cotton effects in the solid state, respectively [72] (by convention, the sign of a bisigned Cotton effect is assigned to that of the longer wavelength extremum – vide infra, Section 6.3 for further discussion). Finally, (S)-2-methylbutyl end-chain chiral functionalization of poly(n-hexyl-2methylpropylsilylene), 87, interestingly results in an induced PSS (gabs ≈ -1x10-5) of opposite sign to that when side chain-functionalized with the same chiral group [253,335].

Me Ph

H Si 10

H

84(R)

Me Ph

Me Ph

H Si

n

H

85(R)

Me Ph

H Si

Me O Si n Me

(1S,2R,5S)-(+)menthoxy-terminated 86

Si

n

*

Si *

(S)

(S)

87

6.1.3. PSS Induction by Chiral Solvents Chiral solvents can also provide the chiral field necessary for induction of PSS helicity. When the non-chiral-substituted polymers poly(methylphenylsilyene), PMPS, and poly(hexylmethylsilylene), PHMS, were dissolved in the optically active solvents (S)-(2-

Polysilanes

283

methylbutoxymethyl)benzene and (S)-2-methyl-1-propoxybutane, positive Cotton effects were observed [339].

6.1.4. PSS Induction by Helicity Transfer Recently, several other novel methods of induction of PSS helicity have been reported. These rely on the transfer of helicity from other helical species. In one case, termed "helical programming", the inherently helical poly(triphenylmethyl methacrylate) part of the block copolymer poly(1,1-dimethyl-2,2-di-n-hexylsilylene)-b-poly(triphenylmethyl methacrylate) induced helicity in the attached polysilane chain [71]. In the other cases, oligosilanes were wrapped in chiral sheaths, forming PSS helical oligosilane inclusion complexes with γcyclodextrin [340,341], as shown in Figure 29, M screw sense (i.e. left-handed helical) amylase [342] and P screw sense (i.e. right-handed) triple helix schizophyllan [342,343]. The oligosilane wrapped in M screw sense schizophyllan (SPG) exhibits a negative Cotton effect, while that in the P screw sense amylose exhibits a positive Cotton effect, in contrast to the suggestion above (Section 6.1.1) that a positive Cotton effect may be associated with M screw sense helicity. Further clarification in this area is desirable.

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Figure 29. PSS helicity in oligosilane induced by wrapping with γ-cyclodextrin [341]. Reprinted with permission from Sanji, T.; Kato, M.; Tanaka, M. Macromolecules 2005, 38, 4034-4037. Copyright 2005 American Chemical Society

6.1.5. PSS Induction by Post-Polymerization Functionalization: Quaternization As described in Section 4.2.2.2 above on functionalization, optically active amphiphilic polysilanes containing chiral ammonium substituents have been prepared by quaternization of chloromethylated poly(hexylphenylsilylene) (PHPS) using (+)-N,N-dimethyl-αmethylbenzylamine to give (+)-88, soluble in the polar solvents water, ethanol and acetonitrile [183]. Cl Ph Me H N

Si n

88

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Julian Koe

6.2. Temperature-Dependence of Helical Screw Sense and Selectivity

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It is apparent in Figure 27(a) that Cotton effect Δε magnitudes are temperaturedependent, generally decreasing with increase in temperature. This indicates greater helical order (i.e. PSS selectivity) at lower temperature, which decreases at higher temperatures due to thermal accessibility of the higher energy conformer. For some poly(dialkylsilylene)s [327,344,345] and poly(diarylsilylene)s [319,329], depending on side chain structure, an inversion of Cotton effect sign may occur at a particular temperature, Tc, indicating a reversal of the helical PSS, i.e. a helix-helix transition, which is of interest for potential application in molecular scale memory or switching devices. An entropic side chain order-disorder transition was suggested to be the driving force for the main chain helical inversion, whereby below Tc the polymer side chains pack in a very ordered state with a characteristic main chain screw sense, and above Tc, the side chains are disordered and the main chain adopts a new (opposite) characteristic screw sense, as indicated in Figure 30 [327]. For such transitions to occur, the energy barrier between the opposing rotameric states should not be too high [253,217,292]. This requires that the polymer backbone be flexible enough to allow limited conformational mobility, yet stiff enough to support a preferential screw sense. As noted above, the side chain branch position is critical for induction of PSS helicity: β-branching affords stiff, conformationally locked PSS polymers, while γ-branching affords less sterically hindered and thus more flexible chains, and hence the possibility of a helical inversion. Such polymers are also usually characterized by narrower 29Si NMR resonances and shorter persistence lengths [255,327,344]. 89, with γ-branched chiral and achiral side chains (viscosity index, α = 1.11), on warming between -40 and -5°C undergoes a helix-helix transition, evidenced by the negative-topositive change in the sign of the Cotton effect. In contrast, the stiffer (α = 1.29) and more sterically congested 90, with β-branched achiral and γ-branched chiral side chains, showed positive Cotton effects at all temperatures, demonstrating that no helix-helix transition took place.

Figure 30. Side chain-ordered and –disordered models of poly[(S)-3,7-dimethylctyl-2methylpropylsilylene], below and above Tc [327]. Reprinted with permission from Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336-3343. Copyright 2000 American Chemical Society.

Polysilanes

n

(S) Si

Si

n

*

(S)

89

(R)

90

* (S)

n

Si

x

* (S) or (R) Si

1-x

*

*

Si

*

(S)

285

91

92(S) or 92(R)

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Figure 31. Variable temperature CD and UV spectra for helix-helix transition polymer 91 [344]. Reprinted with permission from Fujiki, M.; Koe, J. R.; Motonaga, M.; Nakashima, H.; Terao, K.; Teramoto, A. J. Am. Chem. Soc. 2001, 123, 6253-6261. Copyright 2001 American Chemical Society.

Further studies directed at controlling the helix-helix transition phenomenon involved polymers such as 91 with centers of opposite chirality in two different structure side chains [344]. The side chains in such polymers have different entropies and packing preferences and hence exert opposing influences of different magnitude at different temperatures on preferential screw sense. Variable temperature CD and UV spectra for 91 are shown in Figure 31. Fine-tuning of Tc was accomplished in copolymer systems such as 92(S) or (R), comprising chiral repeat units of slightly different structure by varying the repeat unit ratio, and could be controlled between -64 and 79°C [327].

6.3. Polysilane Aggregate Chirality Aggregates, as micro-precipitates, are useful models for solid state polymers since investigation by solution state UV and CD spectroscopy is possible. Aggregates of PSS poly(dialkylsilylene)s [217,252,319] and poly(diarylsilylene)s [346] show a bisigned Cotton effect indicating a chiral arrangement of coupled excitonic interactions (Davidov coupling) between main chain chromophores. By convention, a bisigned Cotton effect takes the sign of the longer wavelength extremum. The UV λmax is usually very similar to that in solution, but due to light scattering by the micro particles, the absorption tails into the visible.

286

Julian Koe

On addition of mixtures of either (R) or (S) chiral alcohols with the non-solvent MeOH to THF solutions of achiral poly[(alkyl)alkoxyphenylsilylene]s [167,347] chiral aggregates form, as evidenced by bisigned Cotton effects indicating chirality transfer from the chiral alcohol to the polymer through hydrogen bonding between the alcohol and the side chain alkoxy group, and amplification by induction of a preferential screw sense and aggregation. The sign of the bisignate Cotton effect depends on the ratio of non-solvent to good solvent, solvent polarity and solvent addition order. A cholesteric-type hard core model was invoked to account for the observed effects. To probe further the chiroptical properties of poly[(alkyl)alkylphenylsilylene] aggregates and test the cholesteric hard core hypothesis concerning the origin of their sensitive switchable chirality, a series of poly[(alkyl)alkylphenylsilylene]s were designed such that the polymer chain diameter, d, and helical pitch, p, in the hard core model could be controlled by choice of the phenyl ring substituent position and chain length [348]. Two examples of the polymers, 93 and 94, demonstrate and confirm the validity of the hypothesis. Si

(S)

n

Si n

*

(S)

94

* 93

93 and 94 show almost identical UV and CD properties in THF solution, as expected from their structural similarity. However, on addition of the non-solvent MeOH to form aggregates, 93 shows a positive bisigned Cotton effect, while 94 shows almost the mirror image, as shown in Figure 32.

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*

THF/MeOH ratio 80:20

400 200

CD

0

20000

-200 10000

93

94 UV

0 250

300

350

-400 -600

400

450

Δε [(Si repeat unit)-1dm3cm-1]

ε [(Si repeat unit)-1dm3cm-1]

30000

-800 500

Wavelength / nm

Figure 32. CD and UV spectra of 93 and 94 aggregates in THF/MeOH (80:20) (* 94 CD intensity normalized to that of 93). Replotted from data in ref. [348].

This indicates that while the individual molecules of 93 and 94 are of the same screw sense, their aggregates are of opposite chirality. This is consistent with the cholesteric hard core model [in which the ratio of helical pitch, p, to diameter, d, is critical in the determination of the screw sense of the cholesteric superhelix: for p/d < π, right-handed screws generate a right-handed superhelix; for p/d > π, right-handed screws generate a lefthanded superhelix; see Figure 33] [349], since the n-butylphenyl-substituted polymer, 93,

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287

should have greater d than the ethylphenyl-substituted polymer. Computer modelling evaluation of p and d values supports the experimental results.

Figure 33. Cholesteric hard core model to rationalize aggregate chirality [348]. Adapted with permission from Peng, W.; Montonga, M.; Koe, J. R. J. Am. Chem. Soc. 2004, 126, 13822-13826. Copyright 2004 American Chemical Society.

6.4. Other Aspects of Polysilane Chirality Brief notes are given below on other aspects of polysilane chirality. Interested readers are directed to the source material for further information. (i) (ii)

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(iii)

Aggregation of optically active polysilanes confined inside poly(ureaurethane) microcapsules was achieved by addition to non-solvents [350]. The stiff β-branched poly(n-decyl-(S)-2-methylbutylsilylene) (PDMBS), 69, has been shown to exhibit both thermotropic cholesteric [351] and lyotropic liquid crystallinity [352]. Chiral polysilanes have been used to probe whether mirror-image molecules are energetically identical. Very high molecular weight polymers may amplify the very tiny, parity-violating weak neutral current at the atomic level, which may result in small chiroptical differences between mirror-image P and M single screw sense helical polymers [353]. Experimental data appeared consistent with the proposal, though further confirmation is required.

7. OVERVIEW OF HIGHER DIMENSIONAL SILICON POLYMERS As this Chapter focusses on essentially linear (one-dimensional) polysilanes, coverage of the higher dimensional polymeric organosilanes such as polysilynes, dendrimers, ladders and cages is beyond the scope of the discussion and the reader is referred to other reviews [2a]. However, a brief introduction to the area is given to illustrate the key distinctions between the linear polysilanes and their branched relatives. Polymerization of trifunctional monomers such as RSiCl3 leads to network polymers called polysilynes, (RSi)n, or in the smallest case, the cage molecule, octasilacubane. Due to

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their very high degrees of cross-linking, molecular weights and solubility are lower than for the linear polysilanes (R2Si)n prepared from difunctional monomers R2SiCl2. Incorporation of tri- and tetrafunctional units into linear polysilanes by copolymerization leads to branched polymers, which can be of regular structure, such as dendrimer or ladder polysilanes, or random. The increased density of silicon in the materials results in a strongly delocalized σelectron system. Due to their greater rigidity, the materials often crystallize and may be suitable for single crystal X-ray diffraction, providing important information in structureproperty correlations. Depending on the synthetic strategy, a broad range of silicon architectures can be generated, including network (irregular/random), ladder, cage and dendrimer structures. Many among the latter three types form crystals suitable for, and provide information valuable in correlations. As expected These are network polymers with a high degree of branching and cross-linking [354].

7.1.Polysilynes, (RSi)n Preparation Using trichlorosilane monomers with hexyl and longer side chains and just under 3 equivalents of alkali metal reductant, the Wurtz-type procedure affords soluble polysilynes in good yield [355,356], though with short side chains, products are intractable [357]. The synthesis has also been investigated theoretically [358]. Polysilynes have also been successfully prepared using electrochemical procedures [359362]. and by disproportionation of alkoxy-substituted disilanes [363] and disilane fractions from Direct Process residues [141]. Substituted and functionalized polysilynes have been prepared: poly(pentafluorophenylsilyne) was synthesized electrochemically [359,364]; amine-functionalized polysilanes were prepared by Wurtz-type coupling of N,Ndialkylamino-functionalized aryltrichlorosilanes [365,366]; and oligo(oxyethylene)substituted polysilynes were found to be water-soluble [367].

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R Si Cl Cl Cl

< 3 eq. Na/K (1:1) pentane, )))

RMgBr (to remove residual Si-Cl)

R Si

n

R = Me: insoluble R = Hex, Ph: soluble

Scheme 21. Preparation of polysilynes.

Properties Low crystallinity and structural regularity is indicated by DSC and X-ray. Studies on poly(dihexylsilylene) containing 5, 33 or 67% n-hexylsilyne branching units show that branching reduces crystallinity and charge carrier mobilities [368]. 29Si NMR shows a major, broad peak at -57 ppm in solution and -60 ppm in the solid state for poly(hexylsilyne) [c.f. 24.8 ppm for poly(dihexylsilylene)], with minor resonances between -10 and -30 ppm, due to SiHex2 edge groups [283]. UV spectra tail into the visible and the high molar absorptivity suggests extension of σelectron delocalization into three dimensions, as is evident in a comparison of the spectra of poly(phenylsilyne) (PPS, 95) and poly(methylphenylsilylene) (PMPS, 96) [354,369]; see

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289

Figure 34. ZINDO calculated spectra for higher dimensional silicon polymers were consistent with the experimental observations of red shifts of the UV absorptions tails of polysilynes [354].

Figure 34. (a) UV absorption and (b) FL emission spectra of PPS 95 and PMPS 96 films [369]. Adapted with permission from Watanabe, A.; Tsutsumi, Y.; Matsuda, M. Synth. Met. 1995, 74, 191-196. Copyright 1995 Elsevier.

Structure The data suggest a semi-rigid network of sp3 hybridized Si atoms bonded in sheets or open cages of fused rings connected by linear and branched Si chains [355,354,370,371], as represented in Figure 35.

Si

Si Si Si Si Si Si Si Si

Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si

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Figure 35. Polysilyne architecture incorporating branches, rings and chains.

Such an architecture would confer greater rigidity than in a polysilane and the variety of structural elements, and semi-linear nature would be consistent with the spectroscopic data and solubility.

7.2. Dendrimer Polysilanes These are more structurally defined than polysilynes and contain a core, from which multiple branches radiate [372].

Preparation Two main synthetic approaches have been used: divergent, which is appropriate for smaller dendrimers, and convergent, which is better for larger dendrimers. In the divergent

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synthesis, a core unit is functionalized and then substituted with a unit containing two or more functionalizable groups, as exemplified in Scheme 22 [373]. The second generation (2-G) dendrimer prepared by this route, 101, has a longest chain of 11 Si atoms and 31 in total. Ph Me Si Me Si H Si Me Me Ph Me

(i) t-Bu2Hg (ii) excess Li

97

Ph Me Si Me Si Li Si Me Me Ph Me

ClSiMe2Ph

98 (80%)

Ph Me Me Si Me Me Si Si Ph Me Si Me Ph Me 99 Core (100%) (i) CF3SO3H / CH2Cl2

Me Me Me Si Me Me Me Si MeSi MeMe Me Me Me Si Me Me Si Me Me Me Me Si Si Si Si Me Me Me Me Si Me MeSi Si Me Me Si Me Me Me Si Me Me Me Si

Si Si Me Me Si Me Me Me Me Me Si Me Si Si Me Me Si Me Me Me Si Si Si Si Me Me Me Me Si Me Me Si Me Me Me Me Me Me Si Si Me SiMe Me Me Me Me Me

(ii) 98

(i) CF3SO3H / CH2Cl2 (ii) 2-lithiotrisilane

Me Ph Me Me Me Si Ph Si Si Me MePh Me Me Si Me Si Me Me Si Si Si Me Me Si Me Me Me Si Me Ph Ph Si Si Me Me Me Si Me Me Ph 100 1-G (43%)

101 2-G (29%)

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Scheme 22. Divergent route to second generation (2-G) dendrimer polysilane.

In the convergent route, long, branched chain arms are constructed first and then attached to the core in the final step. By this method, a 31-Si dendrimer, [(Me3Si)3SiSiMe2SiMe2Si(SiMe3)2SiMe2]3SiMe, with longest chain of 13 Si atoms, was prepared [374,375]. Optically active dendrimers are also known: substitution of the chlorines in the first generation dendrimer (ClMe2Si)3SiMe with the optically active (R,R)-bis[methyl(1naphthyl)phenylsilyllithium affords a dendrimer with an optically active periphery [376].

Properties The UV λmax of dendrimers resemble more those of linear polysilanes than those of polysilynes, and shift bathochromically with increasing chain length. As shown in Figure 36, the 31-Si dendrimer [(Me3Si)3SiSiMe2SiMe2Si(SiMe3)2SiMe2]3SiMe shows absorption peaks at 260 nm (ε = 1.8×105 Si-unit-1dm3cm-1) and 283 nm (ε = 1.16×105 Si-unit-1dm3cm-1) [c.f. UV for an Si12 chain [377]: maxima at 264 nm (ε = 4.3×104 Si-unit-1dm3cm-1) and 285 nm, (ε = 4.35×104 Si-unit-1dm3cm-1)]. The dendrimer molar absorptivity, ε, is much greater than that of the linear polysilanes due to the multiplicity of electronically equivalent delocalized paths between peripheral silicon atoms on different branches [375].

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291

Figure 36. UV spectra of 31-Si dendrimer [(Me3Si)3SiSiMe2SiMe2Si(SiMe3)2SiMe2]3SiMe [375]. Reprinted with permission from Lambert, J. B.; Wu, H. Organometallics 1998, 17, 4904-4909. Copyright 1998 American Chemical Society.

Studies of dendrimer excited state dynamics suggest that a configuration coordinate model can be applied to the photophysics of branched silicon chains. Calculations show distortion of the excited state geometry localized at a branching point [354,378].

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Structure Dendrimer connectivity and structure can be distinguished by in the 29Si NMR chemical shifts [373] and very elegantly using double quantum coherence filtering 2-dimensional 29Si29 Si INADEQUATE NMR [379,380].

Figure 37. Single crystal X-ray structure of 31-Si dendrimer 101 [373]. Reprinted with permission from Sekiguchi, A.; Nanjo, M.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1995, 117, 4195-4196. Copyright 1995 American Chemical Society.

However, since dendrimers are crystallizable, structurally defined molecules, single crystal X-ray diffraction provides the clearest picture of structure [373,375], providing an excellent means to structure-property correlations: Figure 37 shows the single crystal X-ray structure of 101 (see Scheme 22 above for synthesis) [373].

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7.3. Ladder and Cage Polysilanes Ladder and cage polysilanes are multiply fused silicon ring systems [381], and constitute another set of more structurally defined macromolecular silicon materials.

7.3.1. Ladders Preparation Ladder polysilanes comprise linearly catenating cyclotetrasilane rings leading to a silicon double helix. Preparation is generally by a Wurtz-type co-condensation of 1,2dichlorodisilanes with 1,1,2,2-tetrachlorodisilanes, as shown for the i-Pr-substituted ladder in Scheme 23 and affords a mixture of air-stable crystallizable products with two, three, four and five fused rings, with syn and anti conformations, separable by recycling HPLC [382]. Pri Pri Si Cl Pri Si Cl Pri

n

Pri Cl Si Cl

Pri Cl Si Pri

Cl Si Cl Pri

Cl Si Pri Pri

Li THF

Pri Pri Si

Pri Si

Pri Si Pri

Pri Si Pri

Si Si Pri n Pri Pri 102

Scheme 23. Wurtz-type synthesis of ladder polysilanes.

Selective routes to specific structures have also been developed [381,383,384]. Very recently, ladder polysilanes with six, seven and eight ring chains have been described [385].

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Properties Ladder polysilane optoelectronic properties have also been investigated [385-387]. Increase in the number of catenating rings in all-anti ladders is accompanied by a bathochromic shift of the lowest energy UV λmax, indicating an increase in electronic delocalization, as for the linear polysilanes: for 2, 3, 4, 5, 6, 7 and 8 ring ladders, λmax was 310, 345, 380, 414, 440, 464 and 483 nm. Interestingly, syn conformer-containing ladders showed even longer wavelength absorptions due to the greater strain, causing destabilization of the HOMO. Structure Single crystal X-ray structures have been obtained for a number of ladder polysilanes [381], facilitated by the relative ease of their separation and crystallization due to their discrete molecular architectures. The Si4 rings are each non-planar, and in the all-anti forms, the ring twist induces single screw sense helicity in the ladders, as is apparent in Figure 38. Some ladders crystallize with both P and M helical forms in the same centrosymmetric unit cell, while in others, the two forms crystallize separately. Crystallization in a chiral field is expected to yield an excess of one screw sense.

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293

Figure 38. X-ray structure of all-anti ladder polysilane 103 [381]. Reprinted with permission from Kyushin, S.; Matsumoto, H. Adv. Organomet. Chem. 2003, 49, 133-166. Copyright 2003 Elsevier.

Reactions m-chloroperbenzoic acid (mCPBA) is known to insert oxygen efficiently into Si-Si bonds, and applied to the ladder polysilanes, and due to strain effects upon insertion, proceeds firstly along one side, then along the other and finally in the bridging bonds, leading under exhaustive conditions to the corresponding ladder polysiloxanes [388,389]. Polysilane/polysiloxane hybrids result if the process is carefully controlled. UV absorption bands of the O-insertion products are blue shifted compared to the parent ladders [381], consistent with the partially interrupted conjugation. With electrophiles, ring-opening occurs to cleave the bridgehead Si-Si bond [390]. On photolysis of the tricyclic ladder the transient intermediate four-membered cyclic disilene is extruded, which can be trapped using anthracene, 1,3-butadiene and methanol [381]. 7.3.2. Cages In contrast to the essentially two-dimensional Si ladders, the Si cages are threedimensional, whereby each core atom bonds to three other core atoms, in crystallographically determined cubic [391-394], trigonal prismatic [395] and tetrahedral [396] architectures. R

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Si R

R

Si Si

R Si

Si Si R Si R

104

R

Si R

R R Si

Si R Si

Si

R Si

R R

Si R

Si R 105

R

Si

Si

Si R

106

8. CONCLUDING REMARKS This Chapter has aimed to give an overview of the great variety of science to be found through a study of polysilanes. Research is leading to greater control of polysilane properties,

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challenging academe and industry together to exploit these fascinating, optoelectronically active inorganic/organic hybrid materials.

REFERENCES [1] [2]

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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[16] [17] [18] [19] [20] [21] [22] [23]

[24] [25] [26]

Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000. Koe, J. R. Organopolysilanes. In Comprehensive Organometallic Chemistry III; Crabtree, R. H.; Mingos, D. M. P., Eds.; Elsevier: Oxford, 2007; Vol. 3, Chapter 11, pp549-649. (2b) West, R. Organopolysilanes. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 2, Chapter 9, pp 365-397. (2c) West, R. Organopolysilanes. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon: Oxford, 1995; Vol. 2, Chapter 3, pp 77-110. Wilks, E. see also http://www.iupac.org/reports/IV/guide.html 2001, 1-17. Matyjaszewski, K.; Metanomski, W. V. Polym. Prepr. (ACS, Div. Polym. Chem.) 2002, 43, 755-757. Kipping, F. S. J. Chem. Soc. 1921, 119, 830. Kipping, F. S. J. Chem. Soc. 1924, 125, 2291. Gilman, H.; Schwebke, G. L.; J. Am. Chem. Soc. 1964, 86, 2693. Gilman, H.; Schwebke, G. L.; Adv. Organomet. Chem. 1964, 1, 132. Gilman, H.; Schwebke, G. L.; J. Organomet. Chem. 1965, 3, 382. Burkhard, C. J. Am. Chem. Soc. 1949, 71, 963. Yajima, S.; Hayashi, J.; Omori, M. Chem. Lett. 1975, 931. Yajima, S.; Okamura, K.; Hayashi, J.; Omori, M. J. Am. Ceram. Soc. 1976, 59, 324. Wesson, J. P.; Williams, T. C. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 959. Trujillo, R. E. J. Organomet. Chem. 1980, 198, C27. West R.; David, L. D.; Djurovich, P. I.; Stearley, K. L.; Srinivasan, K. S. V.; Yu. H. J. Am. Chem. Soc. 1981, 103, 7352. Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359-1410. Wolff, A.; West, R. Appl. Organomet. Chem. 1987, 1, 7. Kepler, R. G.; Zeigler, J. M.; Harrah, L. A.; Kurtz, S. R. Phys. Rev. B 1987, 35, 2818. Abkowitz, M.; Knier, F. E.; Yuh, H.-J.; Weagley, R. J.; Stolka, M. Solid State Commun. 1987, 62, 547. Miller, R. D.; MacDonald, S. A. J. Imaging Sci. 1987, 31, 43. Jones, R. G.; Wong, W. K. C.; Holder, S. J. Organometallics 1998, 17, 59-64. Michl, J.; West, R. Acc. Chem. Res. 2000, 33, 821-823. Jones, R. G.; Holder, S. J. In Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 353-373. Jones, R. G.; Benfield, R. E.; Cragg, R. H.; Swain, A. C.; Webb, S. J. Macromolecules 1993, 26, 4878-4887. Miller, R. D.; Jenkner, P. K. Macromolecules 1994, 27, 5921-5923. Herzog, U.; West, R. Macromolecules 1999, 32, 2210-2214.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polysilanes

295

[27] Matyjaszewski, K; Greszta, D.; Hrkach, J. S.; Kim, H. K. Macromolecules 1995, 28, 59-72. [28] Holder, S. J.; Achilleos, M.; Jones, R. G. Macromolecules 2005, 38, 1633-1639. [29] Jones, R. G.; Benfield, R. E.; Evans, P. J.; Swain, A. C. J. Chem. Soc., Chem. Commun. 1995, 1465-1466. [30] Lacave-Goffin, B.; Hevesi, L.; Devaux, J. J. Chem. Soc., Chem. Commun. 1995, 769770. [31] Benfield, R.E.; Evans, P.J.; Jones, R.G.; Swain, A.C. Polym. Prepr. (ACS, Div. Polym. Chem.) 1995, 36, 96-97. [32] Jones, R. G.; Benfield, R. E.; Evans, P. J.; Holder, S. J.; Locke, J. A. M. J. Organomet. Chem. 1996, 521, 171-176. [33] Cleij, T. J.; Jenneskens, L. W.; Kluijtmans, S. G. J. M. Adv. Mater. 1997, 9, 961-964. [34] Cleij, T. J.; Jenneskens, L. W. Macromol. Chem. Phys. 2000, 201, 1742-1747. [35] Benfield, R. E.; Cragg, R. H.; Jones, R. G.; Swain, A. C. Macromol. Rep. 1994, A31, 1271-1278. [36] Lacave-Goffin, B.; Hevesi, L.; Devaux, J. J. Chem. Soc., Chem. Commun. 1996, 765766. [37] Jedlinski, Z. J.; Kurcok, P.; Nozirow, F. Macromol. Rap. Commun. 1997, 18, 483-490. [38] Kipping, F. S.; Thompson, R. A. J. Chem. Soc 1928, 1989-1990. [39] Hengge, E.; Reuter, H. Naturwissenshaften 1963, 18b, 425. [40] Hengge, E. Preparations of Si-Si Linkages. In Inorganic and Organic Silicon Compounds, Topics in Current Chemistry, Vol. 51, 1974 p 83. [41] Koe, J. R.; Powell, D. R.; Buffy, J. J.; West, R. In Organosilicon Chemistry IV: From Molecules to Materials, [Lectures and Poster Contributions presented at the 4th Muechner Silicontage Muechen, Apr., 1998], 2000; pp 323-330. [42] Seki, S.; Kunimi, Y.; Nishida, K.; Aramaki, K.; Tagawa, S. J. Organomet. Chem. 2000, 611, 64-70. [43] Menescal, R.; West, R.; Blazso, M. J. Inorg. Organomet. Polym. 1995, 5, 217-236. [44] Gautier, S.; Worsfold, D. J. Macromolecules 1989, 22, 2213-2218. [45] Matyjaszewski, K. Polym. Prepr. (ACS, Div. Polym. Chem.) 1987, 28, 224-225. [46] Jones, R. G.; Budnik, U.; Holder, S. J.; Wong, W. K. C. Macromolecules 1996, 29, 8036-8046. [47] McLeish, T. C. B.; Jones, R. G.; Holder, S. J. Macromolecules 2002, 35, 548-554. [48] Ziegler, J. M.; Harrah, L. A.; Johnson, A. W. Polym. Prepr. (ACS, Div. Polym. Chem.) 1987, 28, 424-425. [49] Ziegler, J. M. Polym. Prepr. (ACS, Div. Polym. Chem.) 1986, 27, 109-110. [50] Fujino, M.; Hisaki, T.; Fujiki, M.; Matsumoto, N. Macromolecules 1992, 25, 10791083. [51] Saxena, A.; Okoshi, K.; Fujiki, M.; Naito, M.; Guo, G.; Hagihara, T.; Ishikawa, M. Macromolecules 2004, 37, 367-370. [52] Kim, H. K.; Matyjaszewski, K. J. Am. Chem. Soc. 1988, 110, 3321-3323. [53] Matyjaszewski, K. J. Inorg. Organomet. Polym. 1991, 1, 463-485. [54] Miller, R. D.; Ginsburg, E. J.; Thompson, D. Polymer J. 1993, 25, 807-823. [55] Price, G. J.; Wallace, E. N. K.; Patel, A. M. Special Publication - Royal Society of Chemistry 1995, 166, 147-155. [56] Yamaguchi, Y. Synth. Met. 1996, 82, 149-153.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

296

Julian Koe

[57] Yamaguchi, S.; Jin, R.-Z.; Tamao, K. J. Am. Chem. Soc. 1999, 121, 2937-2938. [58] Sohn, H.; Huddleston, R.; Powell, D. R.; West, R.; Oka, K.; Yonghua, X. J. Am. Chem. Soc. 1999, 121, 2935-2936. [59] Tamao, K.; Yamaguchi, S. J. Organomet. Chem. 2000, 611, 5-11. [60] Sakamoto, K.; Obata K.; Hirata, H.; Nakajima, M.; Sakurai, H. J. Am. Chem. Soc. 1989, 111, 7641-7643. [61] Sakamoto, K.; Yoshida, M.; Sakurai, H. Macromolecules 1990, 23, 4494-4496. [62] Went, M. J.; Sakurai, H.; Sanji, T. Modification and Functionalization of Polysilanes. In Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 419-437. [63] Sakurai, H.; Yoshida, M. Synthesis of Polysilanes by New Procedures: Part 1 Ringopening Polymerizations and the Polymerization of Masked Disilenes. In Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 375-399. [64] Sanji, T.; Kawabata, K.; Sakurai, H. J. Organomet. Chem. 2000, 611, 32-35. [65] Sakurai, H.; Sakamoto, K.; Funada, Y.; Yoshida, M. ACS Symposium Series: Inorg. Organomet. Polym. II 1994, 572, 8-17. [66] Yoshida, M.; Nakanishi, F.; Seki, T.; Sakamoto, K.; Sakurai, H. Macromolecules 1997, 30, 1860-1862. [67] Sakurai, H.; Sakamoto, K.; Funada, Y.; Yoshida, M. Polym. Prepr. (ACS, Div. Polym. Chem.) 1993, 34, 218-219. [68] Takeda, K.; Shiraishi, K. Solid State Commun. 1993, 85, 301-305. [69] Endo, T.; Sugimoto, Y.; Takeda, K.; Shiraishi, K. Synth. Met. 1998, 98, 161-172. [70] Motoyama, H.; Takeda, K.; Shiraishi, K. Mater. Res. Soc. Symp. Proc. 1998, 486, 385390. (b) Sanji, T.; Isozaki, S. Yoshida, M.; Sakamoto, K.; Sakurai, H. J. Organomet. Chem. 2003, 685, 65-69. [71] Sanji, T.; Takase, K.; Sakurai, H. J. Am. Chem. Soc. 2001, 123, 12690-12691. [72] Sanji, T.; Takase, K.; Sakurai, H. Bull. Chem. Soc. Jpn. 2004, 77, 1607-1611. [73] Ebata, K.; Furukawa, K.; Matsumoto, N. J. Am. Chem. Soc. 1998, 120, 7367-7368. [74] Sakurai, H.; Yoshida, M.; Sakamoto, K. J. Organomet. Chem. 1996, 521, 287-293. [75] Fossum, E.; Matyjaszewski, K. Special Publication - Royal Society of Chemistry 1995, 166, 115-131. [76] Matyjaszewski, K. Makromol. Chem., Macromol. Symp. 1991, 42/43, 269-280. [77] Kashimura, S.; Ishifune, M.; Bu, H.-B.; Takebayashi, M.; Kitajima, S.; Yoshihara, D.; Nishida, R.; Kawasaki, S.; Murase, S.; Shono, T. Tetrahedron Lett. 1997, 38, 46074610. [78] Gautier, S.; Worsfold, D. J. In Inorg. Organomet. Oligomers Polym., Harrod, J. F.; Laine, R. M., Eds., 1991; pp 37-47. [79] Chrusciel, J.; Fossum, E.; Matyjaszewski, K. Polym. Prepr. (ACS, Div. Polym. Chem.) 1993, 34, 221-222. [80] Cypryk, M.; Chrusciel, J.; Fossum, E.; Matyjaszewski, K. Makromol. Chem., Macromol. Symp. 1993, 73, 167-176. [81] Koe, J. R.; Powell, D. R.; Buffy, J. J.; West, R. Polyhedron 1998, 17, 1791-1793. [82] Koe, J. R.; Powell, D. R.; Hayase, S.; Buffy, J. J.; West, R. Angew. Chem. Int. Ed. 1998, 37, 1441-1442.

Polysilanes

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[83] [84] [85] [86] [87] [88]

297

Koe, J. R.; Motonaga, M. Fujiki, M. West, R. Macromolecules 2001, 34, 706-712. Koe, J. R.; Fujiki, M. Silicon Chemistry 2002, 1, 77-87. Takeda, K.; Shiraishi, K. Solid State Commun. 1993, 85, 301-305. Endo, T.; Sugimoto, Y.; Takeda, K.; Shiraishi, K. Synth. Met. 1998, 98, 161-172. Yamaguchi, Y. Synth. Met. 1992, 52, 51-56. Motoyama, H.; Takeda, K.; Shiraishi, K. Mater. Res. Soc. Symp. Proc. 1998, 486, 385390. [89] Yamaguchi, Y. Synth. Met. 1994, 63, 127-131. [90] Suzuki, M.; Kotani, J.; Gyobu, S.; Kaneko, T.; Saegusa, T. Macromolecules 1994, 27, 2360-2363. [91] Sakurai, H.; Sanji, T.; Sakai, T.; Hanao, H. Phosphorus, Sulfur and Silicon 1997, 124 and 125, 173-182. [92] Demoustier-Champagne, S.; Devaux, J. Recent Research Developments in Polymer Science 1998, 2, 3-8. [93] Yamaguchi, Y. Synth. Met. 1996, 82, 149-153. [94] Sanji, T.; Funaya, M.; Sakurai, H. Chem. Lett. 1999, 547-548. [95] Matyjaszewski, K. Makromol. Chem., Macromol. Symp. 1991, 42/43, 269-280. [96] Aitken, C.; Harrod, J. F.; Samuel, E. J. Organomet. Chem. 1985, 279, C11-C13. [97] Yamashita, H.; Tanaka, M. Bull. Chem. Soc. Jpn. 1995, 68, 403-419. [98] Gauvin, F.; Harrod, J. F.; Woo, H. G. Adv. Organomet. Chem. 1998, 42, 363-405. [99] Gray, G. M.; Corey, J. Y. Synthesis of Polysilanes by New Procedures: Part 2 Catalytic Dehydropolymerization of Hydrosilanes. In Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 401-418. [100] Tanaka, M.; Hatanaka, Y. Hydrosilation and Silylation in Organosilicon Polymer Synthesis. In Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 439-460. [101] Corey, J. Y. Adv. Organomet. Chem. 2004, 51, 17532. [102] Harrod, J. F.; Yun, S. S. Organometallics 1987, 6, 1381-1387. [103] Hengge, E.; Weinberger, M. J. Organomet. Chem. 1993, 443, 167-173. [104] Dioumaev, V. K.; Harrod, J. F. Organometallics 1994, 13, 1548-1550. [105] Dioumaev, V. K.; Harrod, J. F. J. Organomet. Chem. 1996, 521, 133-143. [106] Corey, J. Y.; Huhmann, J. L.; Zhu, X.-H.; Organometallics 1993, 12, 1121-1130. [107] Tilley, T. D. Acc. Chem. Res. 1993, 26, 22-29. [108] Imori, T.; Tilley, T. D. Polyhedron 1994, 13, 2231-2243. [109] Corey, J. Y.; Zhu, X.-H.; J. Organomet. Chem. 1992, 439, 1-17. [110] Li, H.; Gauvin, F.; Harrod, J. F. Organometallics 1993, 12, 575-577. [111] Hashimoto, H.; Obara, S.; Kira, M. Chem. Lett. 2000, 188-189. [112] Obora, Y.; Tanaka, M. J. Organomet. Chem. 2000, 595, 1-11. [113] Wang, Q.; Corey, J. Y. Can. J. Chem. 2000, 78, 1434-1440. [114] Dioumaev, V. K.; Rahimian, K.; Gauvin, F.; Harrod, J. F. Organometallics 1999, 18, 2249-2255. [115] Grimmond, B. J.; Corey, J. Y. Organometallics 2000, 19, 3776-3783. [116] Verdaguer, X.; Lange, U. E. W.; Reding, M. T.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 6784-6785.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

298

Julian Koe

[117] Lunzer, F.; Marschner, C.; Landgraf, S. J. Organomet. Chem. 1998, 568, 253-255. [118] Bourg, S.; Corriu, R. J. P.; Enders, M.; Moreau, J. J. E. Organometallics 1995, 14, 564566. [119] Chauhan, B. P. S.; Shimizu, T.; Tanaka, M. Chem. Lett. 1997, 785-786. [120] Fontaine, F.-G.; Kadkhodazadeh, T.; Zargarian, D. Chem. Commun. 1998, 1253-1254. [121] Mechtler, C.; Marschner, C. In Organosilicon Chemistry IV: From Molecules to Materials, [Lectures and Poster Contributions presented at the 4th Muenchner Silicontage, Muenchen, Apr., 1998], 2000; pp 554-557. [122] Yamashita, H.; Tanaka, M. Bull. Chem. Soc. Jpn. 1995, 68, 403-419. [123] Rosenberg, L. Macromol. Symp. 2003, 196, 347-353. [124] Bordeau, M.; Biran, C.; Leger-Lambert, M. P.; Spirau, F.; Deffieux, D. ACS Symposium Series 1994, 572, 18-31. [125] Kato, H.; Karatsu, T.; Kaito, A.; Kitamura, A. Polymer 2003, 44, 8005-8011. [126] Subramanian, K. J. Macromol. Sci. 1998, C38, 637-650. [127] Kashimura, S.; Ishifune, M.; Yamashita, N.; Bu, H.-B. Takebayashi, M.; Kitajima, S.; Yoshiwara, D; Kataoka, Y.; Nishida, R.; Kawasaki, S.; Murase, H.; Shono, T. J. Org. Chem. 1999, 64, 6615-6621. [128] Kashimura, S.; Ishifune, M.; Bu, H.-B.; Takebayashi, M.; Kitajima, S.; Yoshihara, D.; Nishida, R.; Kawasaki, S.; Murase, S.; Shono, T. Tetrahedron Lett. 1997, 38, 46074610. [129] Hengge, E.; Litscher, G. Angew. Chem. 1976, 88, 414-414. [130] Hengge, E.; Litscher, G. Monatsh. Chem. 1978, 109, 1217-1225. [131] Hengge, E.; Firgo, H. J. Organomet. Chem. 1981, 212, 155-161. [132] Corriu, R. J. P.; Dabosi, G.; Martineau, M. J. Chem. Soc., Chem. Commun. 1979, 457458. [133] Corriu, R. J. P.; Dabosi, G.; Martineau, M. J. Organomet. Chem. 1980, 188, 63-72. [134] Corriu, R. J. P.; Dabosi, G.; Martineau, M. J. Organomet. Chem. 1981, 222, 195-199. [135] Biran, C.; Bordeau, M.; Pons, P.; Leger, M.-P.; Dunogues, J. J. Organomet. Chem. 1990, 382, C17-C20. [136] Bordeau, M.; Biran, C.; Leger-Lambert, M. P.; Dunogues, J. J. Chem. Soc., Chem. Commun. 1991, 1476-1477. [137] Shono, T.; Kashimura, S.; Ishifune, M.; Nishida, R. Chem. Commun. 1990, 1160-1161. [138] Kimata, Y.; Suzuki, H.; Satoh, S.; Kuriyama, A. Chem. Lett. 1994, 1163-1164. [139] Kimata, Y.; Suzuki, H.; Satoh, S.; Kuriyama, A. Organometallics 1995, 14, 2506-2511. [140] Obata, K.; Sakamoto, K.; Kira, M. Macromolecules 2001, 34, 2739-2741. [141] Baney, R; Gaul, G. H. Jr.; Hilty, T. K. Organometallics 1983, 21, 859-864. [142] Kabeta, K.; Wakamatsu, S.; Imai, T. Chem. Lett. 1994, 835-838. [143] Ban, H.; Sukegawa, K.; Tagawa, S. Macromolecules 1987, 20, 1775-1778. [144] Seki, T.; Tamaki, T.; Ueno, K. Macromolecules 1992, 25, 3825-3826. [145] Matyjaszewski, K.; Hrkach, J.; Ruehl, K. Macromol. Rep. 1994, A31, 1029-1036. [146] Uhlig, W. J. Organomet. Chem. 1991, 402, C45-C49. [147] West, R. J. Organomet. Chem. 1986, 300, 327-346. [148] Sacarescu, G.; Sacarescu, L.; Ardeleanu, R.; Kurcok, P.; Jedlinski, Z. Macromol. Rapid Commun. 2001, 22, 405-408. [149] Stueger, H.; West, R. Macromolecules 1985, 18, 2349-2352. [150] Yoshida, H.; Kani, R.; Hayase, S.; Horie, K. J. Phys. Chem. 1993, 97, 5370-5374.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polysilanes

299

[151] Hrkach, J.; Matyjaszewski, K. Polym. Prepr. (ACS, Div. Polym. Chem.) 1992, 33, 156157. [152] Matyjaszewski, K.; Hrkach, J. J. Polym. Sci., Polym. Chem. 1994, 32, 1949-1956. [153] Embs, F. W.; Wegner, F. G.; Neher, D.; Albouy, P.; Miller, R. D.; Wilson, C. G.; Schrepp, W. Macromolecules 1991, 24, 5068-5075. [154] Oka, K.; Fujiue, N.; Nakanishi, S.; Takata, T.; Dohmaru, T.; Yuan, C.-H.; West, R. Chem. Lett. 1995, 875-876. [155] Walree, C. A. van; Cleij, T. J.; Zwikker, J. W.; Jenneskens, L. W. Macromolecules 1995, 28, 8696-8698. [156] Oka, K.; Fujiue, N.; Nakanishi, S.; Takata, T.; Dohmaru, T.; Yuan, C.-H.; West, R. Chem. Lett. 1997, 253-256. [157] Oka, K.; Fujiue, N.; Nakanishi, S.; Takata, T.; West, R.; Dohmaru, T. J. Organomet. Chem. 2000, 611, 45-51. [158] Cleij, T. J.; King, J. K.; Jenneskens, L. W. ACS Symposium Series 2001, 780, 238-254. [159] Jenneskens, L. W.; Cleij, T. J.; King, J. K. Polym. Prepr. (ACS, Div. Polym. Chem.) 2000, 41, 547-548. [160] Schweizer, K. S.; Harrah, L. A.; Zeigler, J. M. Silicon Based Polymer Science: A Comprehensive Resource, Zeigler, J. M.; Fearon, G. F., Eds.; Advances in Chemistry Series 224; American Chemical Society: Washington DC, 1990; p 379. [161] Sacarescu, L.; Ardeleanu, R.; Sacarescu, G.; Simionescu, M. Eur. Polym. J. 2004, 40, 57-62. [162] Matsukawa, K.; Ishikawa, M.; Inoue, H. Chem. Lett. 2000, 332-333. [163] Matsukawa, K.; Tamai, T.; Inoue, H. Appl. Phys. Lett.. 2000, 77, 675-677. [164] Yuan, C. H.; West, R. Macromolecules 1993, 26, 2645-2646. [165] Yuan, C.-H.; West, R. Macromolecules 1998, 31, 1087-1092. [166] Nakashima, H.; Fujiki, M.; Koe, J. R. Macromolecules 1999, 32, 7707-7709. [167] Nakashima, H.; Fujiki, M.; Koe, J. R.; Motonaga, M. J. Am. Chem. Soc. 2001, 123, 1963-1969. [168] Cleij, T. J.; King, J. K.; Jenneskens, L. W. Macromolecules 2000, 33, 89-96. [169] Peng, W.; Motonaga, M.; Koe, J. R. Book of Abstracts p. 45, presented at ISOS XIV Wuerzburg, 2005. [170] Peng, W.; Motonaga, M.; Koe, J. R. in preparation. [171] Smith, D. A. McL.; Williams, S. A.; Jenkner, P.; Miller, R. D.; Ginsburg, E. J.; Hochstrasser, R. M. J. Phys. Chem. 1994, 98, 7359-7365. [172] Glowacki, I.; Dobruchowska, E.; Ulanski, J. Synth. Met. 2000, 109, 139-142. [173] Mimura, S.; Naito, H.; Dohmaru, T.; Kanemitsu, Y.; Aramata, M. Appl. Phys. Lett.. 2000, 77, 2198-2200. [174] Glowacki, I.; Dobruchowska, E.; Luszczynska, B.; Ulanski, J. Macromol. Symp. 2004, 212, 491-496. [175] Lemmer, M.; Sepulchre, M.; Marc, N.; Moisan, J.-Y. Polym. Adv. Technol. 1997, 8, 116-124. [176] Tabei, E.; Fukushima, M.; Mori, S. Synth. Met. 1995, 73, 113-116. [177] Fukushima, M.; Tabei, E.; Aramata, M.; Hamada, Y.; Mori, S.; Yamamoto, Y. Synth. Met. 1998, 96, 239-244. [178] Seki, T.; Tamaki, T.; Ueno, K.; Tanaka, Y. Thin Solid Films 1994, 243, 625-629.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

300

Julian Koe

[179] Seki, T.; Tanigaki, N.; Yase, K.; Kaito, A.; Tamaki, T.; Ueno, K.; Tanaka, Y. Macromolecules 1995, 28, 5609-5617. [180] Seki, T.; Tohnai, A.; Tamaki, T.; Kaito, A. Chem. Lett. 1996, 361-362. [181] Seki, T.; Tohnai, A.; Tamaki, T.; Kaito, A. Macromolecules 1996, 29, 4813-4815. [182] Yamaki, T.; Nakashiba, Y.; Asai, K.; Ishigure, K.; Seki, S.; Tagawa, S.; Shibata, H. J. Nucl. Mater. 1997, 248, 369-373. [183] Terunuma, D.; Nagumo, K.; Kamata, N.; Matsuoka, K.; Kuzuhara, H. Chem. Lett. 1998, 681-682. [184] Ardeleanu, R.; Mangalagiu, I.; Sacarescu, G.; Simionescu, M.; Sacarescu, L. Macromol. Rap. Commun. 2004, 25, 1231-1235. [185] Pannell, K. H.; Rozell, J. M.; Ziegler, J. M. Macromolecules 1988, 21, 276-278. [186] Foucher, D.; Tang, B.; Manners, I. J. Am. Chem. Soc. 1992, 114, 6246-6248. [187] MacLachlan, M. J.; Aroca, P.; Coombs, N.; Manners, I.; Ozin, G. A. Adv. Mater. 1998, 6, 564-568. [188] Ungurenasu, C. Macromolecules 1996, 29, 7297-7298. [189] Wiseman, A. J.; Holder, S. J.; Went, M J.; Jones, R. G. Polymer Int. 1999, 48, 157-158. [190] Holder, S. J.; Swain, A. C.; Jones, R. G.; Went, M. J.; Benfield, R. E. Polym. Prepr. (ACS, Div. Polym. Chem.) 1995, 36, 312-313. [191] Wiseman, A. J.; Jones, R. G.; Swain, A. C.; Went, M. J. Polymer 1996, 37, 5727-5733. [192] Wiseman, A. J.; Jones, R. G.; Went, M. J. J. Organomet. Chem. 1997, 544, 129-132. [193] Koe, J. R.; Powell, D. R.; Buffy, J. J.; West, R. Octachlorocyclotetrasilane, Perchloropolysilane and New Dialkoxy- and Diaminopolysilanes. In Organosilicon Chemistry IV, Auner N.; Weis, J., Eds.; VCH-Wiley: Weinheim, 2000; pp 323-330. [194] Matyjaszewski, K.; Chen, Y. L.; Kim, M. K. Inorganic and Organometalic Polymers; ACS Symp. Ser.; American Chemical Society: Washington DC, 1988; Vol. 360, p 78. [195] Matyjaszewski, K.; Hrkach, J.; Kim, M. K.; Ruehl, K. Modifications of Well-Defined Polysilanes. In Silicon Based Polymer Science: A Comprehensive Resource, Zeigler, J. M.; Fearon, G. F., Eds.; Advances in Chemistry Series 224; American Chemical Society: Washington DC, 1990; p 285. [196] Banovetz, J. P.; Hsiao Y.-L.; Waymouth, R. M. J. Am. Chem. Soc. 1993, 115, 25402541. [197] Hsiao, Y.-L.; Waymouth, R. M. J. Am. Chem. Soc. 1994, 116, 9779-9780. [198] Kato, H.; Karatsu, T.; Kaito, A.; Matsuyama, S.; Kitamura, A. Polymer 2003, 44, 32693277. [199] Uhlig, W. J. Organomet. Chem. 1991, 402, C45-C49. [200] Ebata, K.; Furukawa, K.; Matsumoto, N.; Fujiki, M. Polym. Prepr. (ACS, Div. Polym. Chem.) 1999, 40, 157-158. [201] Furukawa, K.; Ebata, K. Appl. Phys. Lett.. 2000, 77, 4289-4291. [202] Furukawa, K.; Ebata, K.; Matsumoto, N. Appl. Phys. Lett. 1999, 75, 781-783. [203] Furukawa, K.; Ebata, K.; Nakashima, H.; Kashimura, Y.; Torimitsu, K. Macromolecules 2003, 36, 9-11. [204] Guo, G.; Naito, M.; Fujiki, M.; Saxena, A.; Okoshi, K.; Yang, Y.; Ishikawa, M.; Hagihara, T. Chem. Commun. 2004, 276-277. [205] Michl, J.; West, R. Electronic Structure and Spectroscopy of Polysilanes. In Siliconbased Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 499-529.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polysilanes

301

[206] West, R. in The Chemistry of Organic Silicon Compounds Vol. 3, Eds. Rappoport, Z. and Apeloig, Y.; 2001, John Wiley and Sons, Ltd.; pp 541-563. [207] Takeda, K.; Shiraishi, K.; Matsumoto, N. J. Am. Chem. Soc. 1990, 112, 5043-5052. [208] Neumann, F.; Teramae, H.; Downing, J. W.; Michl, J. J. Am. Chem. Soc. 1998, 120, 753-582. [209] Albinsson, B; Antic, D.; Neumann, F.; Michl, Josef. J. Phys. Chem. A 1999, 103, 21842196. [210] Fogarty, H. A.; Ottosson, C.-H.; Michl, J. Polymer Reprints 2000, 41, 546-546. [211] Sanji, T.; Sakamoto, K.; Sakurai, H. Bull. Chem. Soc. Jpn. 1995, 68, 1052-1055. [212] Cotts, P. M. J. Polym. Sci. Part B: Polym. Phys. 1994, 32, 771-778. [213] Cotts, P. M. Macromolecules 1994, 27, 2899-2903. [214] Fujiki, M. J. Am. Chem. Soc. 1996, 118, 7424-7425. [215] Cleij, T. J.; Jenneskens, L. W. J. Phys. Chem. B 2000, 104, 2237-2241. [216] Saxena, A.; Fujiki, M.; Naito, M.; Okoshi, K.; Kwak, G. Macromolecules 2004, 37, 5873-5879. [217] Fujiki, M.; Toyoda, S.; Yuan, C.-H.; Takigawa, H. Chirality 1998, 10, 667-675. [218] Schilling, F. C.; Lovinger, A. J.; Zeigler, J. M.; Davis, D. D.; Bovey, F. A. Macromolecules 1989, 22, 3055-3063. [219] KariKari, E. K.; Greso, A. J.; Farmer, B. L.; Miller, R. D.; Rabolt, J. F. Macromolecules 1993, 26, 3937-3945. [220] Lovinger, A. J.; Davis, D. D.; Schilling, F. C.; Padden, F. J. Jr.; Bovey, F. A.; Zeigler, J. M. Macromolecules 1991, 24, 132-139. [221] Lovinger, A. J.; Davis, D. D.; Schilling, F. C.; Bovey, F. A.; Zeigler, J. M. Polym. Commun. 1989, 30, 356-359. [222] Leites, L. A.; Bukhalov, S.; Yadritseva, T.; Menescal, R.; West, R. Macromolecules 1994, 27, 5885. [223] Walsh, C. A. Synth. Met. 1989, 28, C559. [224] Song, K.; Miller, R. D.; Wallraff, G. M.; Rabolt, J. F. Macromolecules 1992, 25, 36293632. [225] Kuzmany, H.; Rabolt, J. F.; Farmer, B. L.; Miller, R. D. J. Chem. Phys. 1986, 85, 74137422. [226] Furukawa, S.; Koga, T. J. Phys.: Condens. Matter 1997, 9, L99-L104. [227] Demoustier-Champagne, S.; Devaux, J. Thermal Properties and Phase Behaviour of Polysilanes. In Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 553-573. [228] Furukawa, S.; Takeuchi, K. Solid State Commun. 1993, 87, 931-934. [229] Sasanuma, Y.; Kato, H.; Kaito, A. J. Phys. Chem. B 2003, 107, 11852-11860. [230] Mintmire, J. W.; Ortiz, J. V. Macromolecules 1988, 21, 1189-1191. [231] Albinsson, B.; Teramae, H.; Plitt, H. S.; Goss, L. M.; Schmidbaur, H.; Michl, J. J. Phys. Chem. 1996, 100, 8681-8691. [232] Plitt, H. S.; Downing, J. W.; Raymond, M. K.; Balaji, V.; Michl, J. J. Chem. Soc., Faraday Trans. 1994, 90, 1653-1662. [233] Albinsson, B; Antic, D.; Neumann, F.; Michl, J. J. Phys. Chem. A 1999, 103, 21842196. [234] Ottosson, C. H.; Michl, J. J. Phys. Chem. A 2000, 104, 3367-3380.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

302

Julian Koe

[235] Schepers, T.; Michl, J. J. Phys. Org. Chem. 2002, 15, 490-498. [236] Fogarty, H. A.; Ottosson, C.-H.; Michl, J. J. Mol. Struct. 2000, 556, 105-121. [237] Fogarty, H. A.; Ottosson, C.-H.; Michl, J. Polymer Reprints 2000, 41, 546-546. [238] Ortiz, J. V.; McMichael Rohlfing, C. Chem. Phys. Lett. 1997, 280, 239. [239] Zink, R.; Magnera, T. F.; Michl, J. J. Phys. Chem. A 2000, 3829-3841. [240] Yamaguchi, Y. Int. J. Quantum Chem. 1997, 62, 393-401. [241] Crespo, R.; Piqueras, M. C.; Tomas, F. J. Chem. Phys. 1994, 100, 6953-6960. [242] Sun, H. Polym. Prepr. (ACS, Div. Polym. Chem.) 1995, 36, 669-670. [243] Sun, H. Macromolecules 1995, 28, 701-712. [244] Pelikan, P.; Kosuth, M.; Biskupic, S.; Noga, J.; Straka, M.; Zajac, A.; Banacky, P. Int. J. Quantum Chem. 2001, 84, 157-168. [245] Yamaguchi, Y. Synth. Met. 1994, 63, 127-131. [246] Plitt, H. S.; Michl, J. J. Chem. Phys. Lett. 1992, 198, 400-405. [247] Halgren, T. A. J. Comput. Chem. 1996, 17, 490-519. [248] Matsumoto, N.; Teramae, H. J. Am. Chem. Soc. 1991, 113, 4481-4483. [249] Kato, H.; Sasanuma, Y.; Kaito, A.; Tanigaki, N.; Tanabe, Y.; Kinugasa, S. Macromolecules 2001, 34, 262-268. [250] Cotts, P. M.; Ferline, S.; Dagli, G.; Pearson, D. S. Macromolecules 1991, 24, 67306735. [251] Flory, P. J. Statistical Mechanics of Chain Molecules; Interscience: New York, 1969. [252] Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997, 119, 11345-11346. [253] Fujiki, M. Macromol. Rap. Commun. 2001, 22, 539-563. [254] Terao, K.; Terao, Y.; Teramoto, A.; Nakamura, N.; Terakawa, I.; Sato, T.; Fujiki, M. Macromolecules 2001, 34, 2682-2685. [255] Fujiki, M. Polym. Prepr. Jpn. 1995, 44, 3557. [256] Sanji, T.; Sakamoto, K.; Sakurai, H.; Ono, K. Macromolecules 1999, 32, 3788-3794. [257] Koe, J. R. unpublished observations. [258] Fujiki, M.; Koe, J. R.; Terao, K.; Sato, T.; Teramoto, A.; Watanabe, J. Polym. J. 2003, 35, 297-344. [259] Koe, J. R.; Fujiki, M.; Motonaga, M.; Nakashima, H. Macromolecules 2001, 34, 10821089. [260] Miller, R. D.; Sooriyakumaran, R. Macromolecules 1988, 21, 3120-3122. [261] Gahimer, T.; Welsh, W. J. Polymer 1996, 37, 1815-1823. [262] Asuke, T.; West, R. J. Inorg. Organomet. Polym. 1994, 4, 45-59. [263] Patnaik, S. S.; Farmer, B. L. Polymer 1992, 33, 4443-4450. [264] Oka, K.; Fujiue, N.; Dohmaru, T.; Yuan, C.-H.; West, R. J. Am. Chem. Soc. 1997, 119, 4074-4075. [265] Eberson, L.; Hartshorn, M. P.; Persson, O.; Radner, F. J. Chem. Soc., Chem. Commun. 1996, 2105-2112. [266] Saxena, A.; Fujiki, M.; Rai, R.; Kwak, G. Chem. Mater. 2005, 17, 2181-2185. [267] Ovchinnikov, Y. E.; Dement'ev, V. V.; Shklover, V. E.; Frunze, T. M.; Struchkov, Y. T.; Antipova, B. A.; Igonin, V. A. Makromol. Chem. 1989, 190, 3195-3200. [268] Furukawa, S. J. Organomet. Chem. 2000, 611, 36-39. [269] Shorygin, P. P IVZ. AN SSR Ser. Fiz. 1948, 12, 576-581. [270] Bukalov, S. S.; Leites, L. A.; Magdanurov, G. I.; West, R. J. Organomet. Chem. 2003, 685, 51-59.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polysilanes

303

[271] Furukawa, S.; Takeuchi, K.; Shimana, M. J. Phys.: Condens. Matter 1994, 6, 1100711014. [272] Ebihara, K.; Koshihara, S.-Y.; Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Fujiki, M. Jpn. J. Appl. Phys. Part 2 1997, 36, L1211-L1213. [273] Fujiki, M. J. Am. Chem. Soc. 1994, 116, 6017-6018. [274] Furukawa, K.; Ebata, K.; Fujiki, M. Adv. Mater. 2000, 12, 1033. [275] Furukawa, K.; Nakashima, H.; Ajito, K.; Kashimura, Y.; Hu, W.; Torimitsu, K. Jpn. J. Appl. Phys. 2004, 43, 4521-4524. [276] Takayama, T. J. Mol. Struct. 1998, 441, 101-117. [277] Leites, L. A.; Bukhalov, S.; West, R. unpublished observations. [278] Kyotani, H.; Shimomura, M.; Miyazaki, M.; Ueno, K. Polymer 1995, 36, 915-919. [279] Furukawa, S. Thin Solid Films 1998, 331, 222-228. [280] Chunwachirasiri, W.; Kanaglekar, I.; Lee, G. H.; West, R.; Winokur, M. J. Synth. Met. 2001, 119, 31-34. [281] Winokur, M. J.; West, R. Macromolecules 2003, 36, 7338-7347. [282] Mueller, C.; Schmidt, C.; Frey, H. Macromolecules 1996, 29, 3320-3322. [283] Schilling, F. C.; Bovey, F. A.; Zeigler, J. M. Macromolecules 1986, 19, 2309-2312. [284] Sato, T.; Nagayama, N.; Yokoyama, M. J. Mater. Chem. 2004, 14, 287-289. [285] Phifer, C. C.; Thomes, W. J., Jr.; Simmons-Potter, K.; Potter, B. G., Jr. J. Chem. Phys. 2004, 120, 1613-1616. [286] Demoustier-Champagne, S.; Jonas, A.; Devaux, J. J. Polym. Sci. Part B: Polym. Phys. 1997, 35, 1727-1736. [287] Miller, R. D.; Sooriyakumaran, R. J. Polym. Sci., Polym. Lett. Ed. 1987, 25, 321-326. [288] Takeda, K.; Shiraishi, K. Comments Cond. Mat. Phys. 1997, 18, 91-133. [289] Cotts, P. M.; Miller, R. D.; Sooriyakumaran, R. Bull. Am. Phys. Soc. 1988, 33, 656. [290] Ovchinnikov, Y. E.; Dement'ev, V. V.; Shklover, V. E.; Frunze, T. M.; Struchkov, Y. T.; Antipova, B. A.; Igonin, V. A. Makromol. Chem. 1989, 190, 3195-3200. [291] Tanigaki, N.; Iwase, Y.; Kaito, A.; Okada, Y.; Yamahira, N.; Yase, K. Mol. Cryst. and Liq. Cryst. 2001, 370, 219-222. [292] Koe, J. R.; Fujiki, M.; Nakashima, H. J. Am. Chem. Soc. 1999, 121, 9734-9735. [293] Bleyl, I.; Ebata, K.; Hoshino, S.; Furukawa, K.; Suzuki, H. Synth. Met. 1999, 105, 1722. [294] Yuan, C.-H.; West, R. Chem. Commun. 1997, 1825-1826. [295] Cleij, T. J.; Jenneskens, L. W.; Wuebbenhorst, M.; van Turnhout, J. Macromolecules 1999, 32, 8663-8665. [296] Yoshida, M.; Mori, M.; Yokokawa, S.; Nakanishi, F.; Sakurai, H. Mol. Cryst. Liq. Cryst. 1998, 322, 135-140. [297] Yoshida, M.; Seki, T.; Nakanishi, F.; Sakamoto, K.; Sakurai, H. J. Chem. Soc., Chem. Commun. 1996, 1381-1382. [298] Nagano, S.; Seki, T. J. Am. Chem. Soc. 2002, 124, 2074-2075. [299] Nakano, Y.; Murai, S.; Kani, R.; Hayase, S. J. Polym. Sci. Part A: Polym. Chem. 1993, 31, 3361-3370. [300] Nakamura, T.; Nakagawa, H.; Oka, K.; Naito, H.; Mogi, I.; Watanabe, K.; Dohmaru, T. Solid State Commun. 1998, 106, 447-450. [301] Kim, H. K.; Matyjaszewski, K. J. Polym. Sci. Part A: Polym. Chem. 1993, 31, 299-307. [302] Kabeta, K.; Shuto, K.; Sugi, S.; Imai, T. Polymer 1996, 37, 4327-4331.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

304

Julian Koe

[303] Ninomiya, S.; Ashihara, Y.; Nakayama, Y.; Oka, K.; West, R. J. Appl. Phys. 1998, 83, 3652-3655. [304] Schauer, F.; Kuritka, I.; Nespurek, S. Polymer Degradation and Stability 2004, 84, 383391. [305] Miller, R. D.; Hofer, D.; McKean, D.; Willson, C. G.; West, R.; Trefonas, P. In Materials for Microlithography, Thompson, L. F.; Willson, C. G.; Frechet, J. M. J., Eds.; ACS Symposium Series 266, Ch. 14; American Chemical Society: Washington, DC 1984. [306] Seki, S.; Sakurai, Y.; Maeda, K.; Kunimi, Y.; Tagawa, S. Jpn. J. Appl. Phys. Part 1 2000, 39, 4225-4230. [307] Watanabe, A.; Ito, O.; Matsuda, M.; Suezawa, M.; Sumino, K. Jpn. J. Appl. Phys. Part 1 1994, 33, 4133-4134. [308] Villegas, A.; Olayo, R.; Cervantes, J. J. Inorg. Organomet. Polym. 1998, 8, 135-156. [309] Hu, H.; Chen, D. Eur. Polym. J. 1994, 30, 1243-1248. [310] Sharma, A.; Louderaj, U.; Deepak; Sathyamurthy, N. J. Phys. Chem. B 2005, 109, 15860-15867. [311] Herden, V.; Klaumunzer, S.; Schnabel, W. Nucl. Instrum. Meth. Phys. Res. 1998, 146, 491-495. [312] Seki, S.; Maeda, K.; Kunimi, Y.; Tagawa, S.; Yoshida, Y.; Kudoh, H.; Sugimoto, M.; Morita, Y.; Seguchi, T.; Iwai, T.; Shibata, H.; Asai, K.; Ishigure, K. J. Phys. Chem. B 1999, 103, 3043-3048. [313] Seki, S.; Kunimi, Y.; Sakurai, Y.; Tsuji, S.; Maeda, K.; Tagawa, S. J. Photopolym. Sci Technol. 2000, 13, 395-396. [314] Ebihara, K.; Koshihara, S.; Miyazawa, T.; Kira, M. Jpn. J. Appl. Phys. Part 2 1996, 35, L1278-L1280. [315] Hiraoka, T.; Majima, Y.; Murai, S.; Nakano, Y.; Hayase, S. Polym. Adv. Technol. 1997, 8, 465-470. [316] Adachi, A.; Yasuda, H.; Sanji, T.; Sakurai, H.; Okita, K. J. Lumin. 2000, 87-89, 11741176. [317] Yuan, C.-H.; Hoshino, S.; Toyoda, S; Suzuki, H.; Fujiki, M.; Matsumoto, N. Appl. Phys. Lett. 1997, 71, 3326. [318] Fujiki, M.; Koe, J. R. Optically Active Silicon-containing Polymers. In Silicon-based Polymers: The Science and Technology of their Synthesis and Application, Jones, R. G.; Ando, W.; Chojnowski, J., Eds.; Kluwer: Dordrecht, 2000; pp 643-662. [319] Koe, J. R.; Fujiki, M.; Nakashima, H.; Motonaga, M. Helical Poly(diarylsilylene)s: Effects of Higher Order Structure on Optical Activity. In Synthetic Macromolecules with Higher Order Structure, Khan, I. M. Ed.; ACS Symposium Series 812: Washington DC, 2002; pp 67-86. [320] Sato, T.; Terao, K.; Teramoto, A.; Fujiki, M. Polymer 2003, 44, 5477-5495. [321] Fujiki, M.; Nakashima, H.; Toyoda, S.; Koe, J. R. Chirality in the Polysilanes. In Materials Chirality: Topics in Stereochemistry 24, Green, M. M.; Nolte, R. J. M.; Meijer, E. W. Eds.; John Wiley and Sons, Inc.: New York, 2003; pp 209-280. [322] Terao, K.; Terao, Y.; Teramoto, A.; Nakamura, N.; Fujiki, M.; Sato, T. Macromolecules 2001, 34, 6519-6525. [323] Frey, H.; Moeller, M.; Matyjaszewski, K. Macromolecules 1994, 27, 1814-1818.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polysilanes

305

[324] Kim, S.-Y.; Saxena, A.; Kwak, G.; Fujiki, M.; Kawakami, Y. Chem. Commun. 2004, 538-539. [325] Fujiki, M. Appl. Phys. Lett. 1994, 65, 3251-3253. [326] Fujiki, M.; Koe, J. R.; Terao, K.; Sato, T.; Teramoto A.; Watanabe, J. Polym. J. 2003, 35, 297–344. [327] Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336-3343. [328] Fujiki, M. J. Am. Chem. Soc. 1994, 116, 11976-11981. [329] Koe, J. R.; Fujiki, M.; Nakashima, H.; Motonaga, M. Chem. Commun. 2000, 389-390. [330] Koe, J. R.; Fujiki, M.; Nakashima, H.; Motonaga, M. Polym. Prepr. (ACS, Div. Polym. Chem.) 2000, 41, 906-907. [331] Toyoda, S.; Fujiki, M. Macromolecules 2001, 34, 640-644. [332] Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860. [333] Koe, J. R.; Fujiki, M.; Motonaga, M.; Nakashima, H. Macromolecules 2001, 34, 10821089. [334] Sato, T.; Terao, K.; Teramoto, A.; Fujiki, M. Macromolecules 2002, 35, 5355-5357. [335] Fujiki, M. Polym. Prepr. (ACS, Div. Polym. Chem.) 1996, 37, 454-455. [336] Kim, S.-Y.; Fujiki, M.; Ohira, A.; Kwak, G.; Kawakami, Y. Macromolecules 2004, 37, 4321-4324. [337] Saxena, A.; Guo, G.; Fujiki, M.; Yang, Y.; Ohira, A.; Okoshi, K.; Naito, M. Macromolecules 2004, 37, 3081-3083. [338] Obata, K.; Kira, M. Macromolecules 1998, 31, 4666-4668. [339] Dellaportas, P.; Jones, R. G.; Holder, S. J. Macromol. Rap. Commun. 2002, 23, 99-103. [340] Sanji, T.; Yoshiwara, A.; Sakurai, H.; Tanaka, M. Chem. Commun. 2003, 1506-1507. [341] Sanji, T.; Kato, M.; Tanaka, M. Macromolecules 2005, 38, 4034-4037. [342] Sanji, T.; Kato, N.; Kato, M.; Tanaka, M. Angew. Chem. Int. Ed. 2005, in press. [343] Sanji, T.; Kato, N.; Tanaka, M. Chem. Lett. 2005, 34, 1144-1145. [344] Fujiki, M.; Koe, J. R.; Motonaga, M.; Nakashima, H.; Terao, K.; Teramoto, A. J. Am. Chem. Soc. 2001, 123, 6253-6261. [345] Fujiki, M.; Motonaga, M.; Tang, H.-Z.; Torimitsu, K.; Zhang, Z.-B.; Koe, J. R.; Watanabe, J.; Terao, K.; Sato, T.; Teramoto, A. Chem. Lett. 2001, 1218-1219. [346] Frey, H.; Moeller, M.; Turetskii, A.; Lotz, B.; Matyjaszewski, K. Macromolecules 1995, 28, 5498-5506. [347] Nakashima, H.; Koe, J. R.; Torimitsu, K.; Fujiki, M. J. Am. Chem. Soc. 2001, 123, 4847-4848. [348] Peng, W.; Montonga, M.; Koe, J. R. J. Am. Chem. Soc. 2004, 126, 13822-13826. [349] Nakanishi, K.; Berova, N. In Circular Dichroism: Principles and Applications, Nakanishi, K.; Berova, N.; Woody, R. W.; Eds.; VCH: New York, NY, 1994; chapters 5, 13 and 19. [350] Terao, K.; Mori, Y.; Dobashi, T.; Sato, T.; Teramoto, A.; Fujiki, M. Langmuir 2004, 20, 306-308. [351] Watanabe, J.; Kamee, H.; Fujiki, M. Polymer J. 2001, 33, 495-497. [352] Natsume, T.; Wu, L.; Sato, T.; Terao, K.; Teramoto, A.; Fujiki, M. Macromolecules 2001, 34, 7899. [353] Fujiki, M. Macromol. Rap. Commun. 2001, 22, 669-674. [354] Watanabe, A. J. Organomet. Chem. 2003, 685, 122-133.

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306

Julian Koe

[355] Bianconi, P. A.; Weidman, T. W. J. Am. Chem. Soc. 1988, 110, 2342-2344. [356] Bianconi, P. A.; Schilling, F. C.; Weidman, T. W. Macromolecules 1989, 22, 16971704. [357] West, R.; Indriksons, A. J. Am. Chem. Soc. 1972, 94, 6110. [358] Vink, R. L. C.; Barkema, G. T.; van Walree, C. A.; Jenneskens, L. W. J. Chem. Phys. 2002, 116, 854-859. [359] Watanabe, A.; Komatsubara, T.; Matsuda, M.; Yoshida, Y.; Tagawa, S. J. Photopolym. Sci. Technol. 1992, 5, 54. [360] Watanabe, A.; Komatsubara, T.; Matsuda, M.; Yoshida, Y.; Tagawa, S. Macromol. Chem. Phys. 1995, 196, 1229. [361] Vermeulen, L. A.; Huang, K. Polymer 1999, 41, 441-444. [362] Okano, M.; Fukai, H.; Arakawa, M.; Hamano, H. Electrochem. Commun. 1999, 1, 223226. [363] Kabeta, K.; Wakamatsu, S.; Imai, T. J. Polym. Sci. Part A: Polym. Chem. 1996, 34, 2991-2998. [364] Watanabe, A.; Ito, O.; Miwa, T. Jpn. J. Appl. Phys. Part 2 1995, 34, L1164-L1166. [365] Kobayashi, T.; Hatayama, K.; Suzuki, S.; Abe, M.; Watanabe, H. ; Kijima, M.; Shirakawa, H. Organometallics 1998, 17, 1646-1648. [366] Kobayashi, T.; Shimura, H.; Mitani, S.; Mashimo, S.; Amano, A.; Takano, T.; Abe, M.; Watanabe, H.; Kijima, M.; Shirakawa, H.; Yamaguchi, H. Angew. Chem. Int. Ed. 2000, 39, 3110-3114. [367] Cleij, T. J.; Tsang, S. K. Y.; Jenneskens, L. W. Macromolecules 1999, 32, 3286-3294. [368] van Walree, C. A.; Cleij, T. J.; Jenneskens, L. W.; Vlietstra, E. J.; van der Laan, G. P.; de Haas, M. P.; Lutz, E. T. G. Macromolecules 1996, 29, 7362-7373. [369] Watanabe, A.; Tsutsumi, Y.; Matsuda, M. Synth. Met. 1995, 74, 191-196. [370] Watanabe, A.; Matsuda, M.; Yoshida, Y.; Tagawa, S. ACS Symposium Series 1994, 579, Ch. 33. [371] Watanabe, A.; Fujitsuka, M.; Ito, O.; Miwa, T. Mol. Cryst. Liq. Cryst. 1998, 316, 363366. [372] Lambert, J. B.; Pflug, J. L.; Wu, H.; Liu, X. J. Organomet. Chem. 2003, 685, 113-121. [373] Sekiguchi, A.; Nanjo, M.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1995, 117, 41954196. [374] Apeloig, Y.; Yuzefovich, M.; Bendikov, M.; Bravo-Zhivotovskii, D.; Klinkhammer, K. Organometallics 1997, 16, 1265-1269. [375] Lambert, J. B.; Wu, H. Organometallics 1998, 17, 4904-4909. [376] Oh, H.-S.; Omote, M.; Suzuki, K.; Imae, I.; Kawakami, Y. Polym. Prepr. (ACS, Div. Polm. Chem.) 2001, 42, 194-195. [377] Boberski, W. G.; Allred, A. L. J. Organomet. Chem. 1975, 88, 65-72. [378] Watanabe, A.; Nanjo, M.; Sunaga, T.; Sekiguchi, A. J. Phys. Chem. A 2001, 105, 64366442. [379] Lambert, J. B.; Basso, E.; Qing, N.; Lim, S. H.; Pflug, J. L. J. Organomet. Chem. 1998, 554, 113-116. [380] Lambert, J. B.; Wu, H. Mag. Reson. Chem. 2000, 38, 388-389. [381] Kyushin, S.; Matsumoto, H. Adv. Organomet. Chem. 2003, 49, 133-166. [382] Matsumoto, H; Miyamoto, H.; Kojima, N.; Nagai, Y. J. Chem. Soc., Chem. Commun. 1987, 1316.

Polysilanes

307

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[383] Kyushin, S.; Yamaguchi, H.; Okayasu, T.; Yagihashi, Y.; Matsumoto, H.; Goto, M. Chem. Lett. 1994, 221. [384] Kumar, K.; Litt, M. H.; J. Polym. Sci., Polym. Lett. Ed. 1988, 26, 25. [385] Matsumoto, H.; Ueda, Y.; Tanaka, R.; Kyushin, S. 14th Intl. Symp. Organosilicon Chem., Wuerzburg, Germany July 31-Aug. 5, 2005; Book of Abstracts 114. [386] Tanaka, R.; Kyushin, S.; Unno, M.; Matsumoto, H. Enantiomer 2002, 7, 157-159. [387] Unno, M.; Tanaka, R.; Takeuchi, T.; Kyushin, S.; Matsumoto, H. Organometallics 2005, 24, 765-768. [388] Kyushin, S.; Sakurai, H.; Yamaguchi, H.; Goto, M.; Matsumoto, H. Chem. Lett. 1995, 815. [389] Kyushin, S.; Tanaka, R.; Arai, K.; Sakamoto, A.; Matsumoto, H. Chem. Lett. 1999, 1297. [390] Kyushin, S.; Sakurai, H.; Yamaguchi, H.; Matsumoto, H. Chem. Lett. 1996, 331-332. [391] Furukawa, K.; Fujino, M.; Matsumoto, N. Appl. Phys. Lett.. 1992, 60, 2744. [392] Matsumoto, H.; Higuchi, K.; Hoshino, Y.; Koike, H.; Naoi, Y.; Nagai, Y. J. Chem. Soc., Chem. Commun. 1988, 1083. [393] Matsumoto, H; Higuchi, K.; Kyushin, S.; Goto, M. Angew. Chem., Int. Ed. Engl. 1992, 31, 1354. [394] Sekiguchi, A.; Yatabe, T.; Kamatani, H.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1992, 114, 6260. [395] Sekiguchi, A.; Yatabe, T.; Kabuto, C.; Sakurai, H. J. Am. Chem. Soc. 1993, 115, 5853. [396] Wiberg, N.; Finger, C. M. M.; Polborn, K. Angew. Chem., Int. Ed. Engl. 1993, 32, 1054.

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In: Silicon-based Inorganic Polymers Editors: Roger De Jaeger and Mario Gleria

ISBN: 978-1-60456-342-9 © 2008 Nova Science Publishers, Inc.

Chapter 6

POLYCARBOSILANES Wolfram Uhlig∗ Laboratorium für Anorganische Chemie der Eidgenössischen Technischen Hochschule Zürich, ETH-Hönggerberg, Wolfgang-Pauli-Strasse 16, CH-8093 Zürich, Switzerland

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ABSTRACT The design and synthesis of new materials are two key steps in the advancement of technology. One of the most promising approaches of development of new materials that combine advantages of organic polymers with those of inorganic solids is to devise polymers that have a backbone of inorganic atoms to which are attached organic side groups. Among the best developed examples of "inorganic-organic polymers" are polycarbosilanes. Polycarbosilanes have been described as a broad class of polymers in which the polymer backbone contains silicon-carbon bonds. The main focus of this review is placed on linear polymers with regular −Six−Cy− backbone structures. However, there are also included sections on hyperbranched carbosilane polymers, as these hyperbranched carbosilanes are currently of strong interest as precusors to ceramic materials. The most useful synthetic methods for the preparation of polycarbosilanes can be devided into three classes: ring-opening polymerizations (ROP), condensations of small difunctional molecules (dehydration, reductive dehalogenation), and substitution reactions on preformed macromolecules. A major factor in the growth of interest in polycarbosilanes in recent years has been the potential for their use as precursors to silicon carbide, mainly as sources of continuous ceramic fibre. This has led directly or indirectly to the synthesis of many new polycarbosilanes. However, polycarbosilanes have also become a focus of research efforts in recent years that have little or nothing to do with the prospect of developing ceramic precursors. These efforts appear to be stimulated by a more general interest in polycarbosilanes as a novel class of polymers that have potential for use in a much wider range of applications, as well as from a more fundamental perspective.

Keywords: Polymers, Silicon, Silyl triflates, Poly(silylenemethylenes), Polycarbosilanes. ∗

E-mail: [email protected]

310

Wolfram Uhlig

1. INTRODUCTION The design and synthesis of new materials are two of the key steps in the advancement of technology. Significant advances in nearly all fields of technology depend almost entirely on the rate at which useful new materials can be devised and synthesized. Polymer chemistry is one of the main classical areas in the field of materials science. The others (metals, ceramics) are based on inorganic substances, whereas classical polymers are derived from organic starting materials (petroleum). Each of these three classes of materials has distinct property advantages and limitations. For example, organic polymers are lightweight, tough, and easy to fabricate, but in general they cannot withstand exposure to severe thermal stress, challenging oxidation conditions, or high energy radiation. On the other hand, most ceramics have the opposite set of characteristics. An answer to this dilemma is to develop and synthesize new materials that have properties combining those of different classical materials. One of the most promising approaches to the development of new materials that combine the advantages of organic polymers with those of inorganic solids is to devise polymers that have a backbone of inorganic atoms to which are attached organic side groups. Such polymers are known as "inorganic-organic" polymers [1-6]. The properties of such polymers that are controlled by the polymer backbone or side groups are listed in Table 1. Table 1. Properties of inorganic-organic polymers that are controlled by inorganic backbone and organic side groups

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Inorganic backbone Heat and fire resistance Radiation resistance Electrical conductivity Material flexibility

Organic side groups Solubility Liquid crystallinity Non-linear optical behavior Surface properties

Polymers of this type serve as an important bridge between the organic and inorganic materials areas. From the viewpoint of properties, they span the gap between classical biological and petrochemical polymers on the one hand, and mineralogical materials and synthesized ceramics on the other. Figure 1 illustrates the classification of different types of polymer systems, which was recommended by Allcock [6]. Among the best developed examples of "inorganic-organic" polymers are the polysiloxanes and polyphosphazenes. But in recent years much attention has been directed to other silicon containing polymers as sources of novel materials. These polymers show interesting physical properties such as photoluminescence [7], photoconductivity [8,9], and non-linear susceptibility [10]. Moreover, the polymers are of considerable interest because their thermal degradations lead to ceramics such as silicon carbide, silicon nitride and silicon carbonitride [11-19].

Polycarbosilanes Biopolymers

"Exotic-Organic" Polymere

Petrochemical Polymers

Proteins Polysaccharides Nucleic Acids

R

O C HN

311

C

C

C

C

n n

Polyamides

Polyacetylene

O R

C

R

O

N

n

Polyesters H

n

N

O

Polypyrrole

R

C

C

R

R

Si R

n

Silicones S n

n

S

R

Polythiazole

Polyolefins

N

P

N R

n

Polyphosphazenes N

n

R

Polyaniline

O

R O

Si

Si

R

R

Si O

C C

Polysilanes

n

Poly(phenylenevinylene)

H

R

C

Si

H

R

P O

n

BN

Polyphosphates Glass

Polycarbosilanes S

O O

n

N

n

Polysilicates

n

SiC

Quartz

n

Poly(sulfur nitride)

Si3N4

Asbestos

Inorganic-Organic Polymers

Inorganic Oxides

Non-Oxide Ceramics

Figure 1. Classification of different types of polymer systems [6]. R Si

R O

R

n

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1

Si

C

R

H

H

H

Si

C

C

R

R

Si

N n

Si

C

C

R

n

n 5

C

C

C

X

n

10

Figure 2. Basic structures of organosilicon polymers 1 – 10.

C n

8

Si n

Si

Si

R

Si

Si R

R

n

7

R

R

R

R

n

R

4

3 R

6

9

R

R

n

2

R

R

H

312

Wolfram Uhlig

The basic structures of polysiloxanes 1 [20], poly(silylenemethylenes) or polycarbosilanes 2 [13, 21-23], polysilazanes 3 [13, 24], polysilanes 4 [25-27], polysilynes 5 [28], poly(silyleneethenes) 6 [29], poly(silyleneethynes) 7 [30, 31], poly(silylenebutadiynes) 8 [32, 33], poly(silylenephenylenes) 9 [34], and poly(silyleneheteroarylenes) 10 (X = S [35], X = O [36]) are summarized in Figure 2. The most useful synthetic methods for the preparation of organosilicon polymers can be divided into the following three classes [13]: • •

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•

ring-opening polymerizations (ROP); condensations of small difunctional molecules dehalogenation); substitution reactions on preformed macromolecules.

(dehydration,

reductive

The early work in the area of organosilicon polymers was limited to polymers with methyl and phenyl groups on silicon because of the drastic synthetic methods (for instance high temperatures, use of molten alkali metals). Prior attempts to introduce chloro substituents into preformed polymers by the help of HCl / AlCl3 [37-39] have resulted in low molecular weight products that have received only little attention as starting materials for further substitution reactions. Reductive dehalogenation reactions of chlorosubstituted silicon containing molecules often lead to polymers, which does not possess a regular arrangement of silicon and carbon atoms in the backbone. This observation is caused by metal halogen exchange processes [40, 41]. Moreover, the utilization of organosilicon polymers as new materials requires high-yield synthetic routes to functionalized derivatives with defined structures. New synthetic methods are also needed for the preparation of potential preceramic polymers. The current problem in the synthesis of preceramic polymers was summarized by Jansen and Baldus [15] with the following proposition: "To date, the area of polymer synthesis has lacked sufficient control of molecular precursors and intermediates, that is 'molecular design'." Polycarbosilanes have been described, variously, as: a broad class of polymers in which the polymer backbone contains silicon-carbon bonds or in which pendant groups are bonded to the polymer chain by silicon-carbon bonds [42], organosilicon polymers which backbone is composed of silicon atoms, appropriately substituted, and difunctional organic groups which bridge the silicon atoms [43], and, for carbosilanes in general, compounds in which the elements carbon and silicon occupy alternate positions in the molecular framework [44]. For the purposes of this chapter, we will take polycarbosilane to mean, a polymer which backbone contains, as a major structural component, silicon-carbon bonds (polymers 2, 6-10 in Figure 2). As most of the well characterized examples of this type of polymer are linear polymers with regular −Six−Cy− backbone structures, the main focus of this review is placed on the polymers of this type. However, there are also included sections on hyperbranched carbosilane polymers , as these hyperbranched carbosilanes are currently of strong interest as precusors to ceramic materials. In addition to the reviews [42-44] on carbosilanes and polycarbosilanes noted above, there have been a number of recent reviews [13, 22, 23, 45] that have focused entirely or largely on polycarbosilanes as precursors to silicon carbide ceramics. These reviews provide a

Polycarbosilanes

313

good summary of a subject that currently constitutes the most important area of application for polycarbosilanes. The development of methodologies for forming Si-C bonds coincides with the beginning of organosilicon chemistry in the latter half of the last century. Alkylation of chlorosilanes by using organozinc and mercury reagents, or by Wurtz-Fittig coupling of chlorosilanes and alkyl halides with sodium, gave way to the more extensive use of Grignard and organolithium reagents for this purpose after the turn of the century, although Wurtz-Fittig coupling with sodium or potassium has continued to be widely used for the synthesis of carbosilanes to this date. After the development of the silicone industry in the 1940s, the direct synthesis of organosilanes by reactions of organic halides and elemental Si became the method of choice for the large-scale synthesis of simple organosilicon compounds. Along with these methods for alkylating silicon, either as the halide, hydride or in the elemental form, a wide range of new methods including the hydrosilylation of alkenes and alkynes, gas phase pyrolysis of organosilanes (including silacyclobutanes) and organochlorosilanes, redistribution of organosilanes and organohalosilanes (both thermally and with aluminum halides), ringopening of cyclic organosilanes, and electrolysis have become available in recent years for the synthesis of carbosilanes. As these methods are general to organosilicon chemistry and adequately summarized in the wide range of reference books on this subject, no attempt will be made here to provide a complete or even systematic description of their application to carbosilane formation. A major factor in the growth of interest in polycarbosilanes in recent years has been the potential for their use as precursors to silicon carbide, mainly as sources of continuous ceramic fibre. This has led directly or indirectly to the synthesis of many new polycarbosilanes. However, polycarbosilanes have also become a focus of research efforts in recent years that have little or nothing to do with the prospect of developing ceramic precursors. These efforts appear to be stimulated by a more general interest in polycarbosilanes as a novel class of polymers that have potential for use in a much wider range of applications, as well as from a more fundamental perspective. In this chapter, we have attempted to survey the current 'state-of-the-art' in polycarbosilane chemistry through the end of 2004; however, we do not claim to have been comprehensive in our coverage.

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2. POLYCARBOSILANES AS SIC PRECURSORS A major motivation for the synthesis and investigation of polycarbosilanes in recent years has been the prospect of using them to prepare silicon carbide ceramics, particularly in the form of continuous fibre. Much of this effort has been summarized in various reviews and book chapters [13,43,45]; however, due to its importance in the context of polycarbosilane synthesis, a brief survey of this subject will be presented here, with particular emphasis on the more recent developments. Before discussing the different types of polycarbosilanes that have been investigated as SiC precursors, it should be noted that SiC is a line compound in the SiC phase diagram and that its structures show little tolerance for 'off-stoichiometry' on either the C-rich or Si-rich side. Thus, on crystallisation of an amorphous SiC, solid, such as is generally obtained after pyrolysis of an organosilicon precursor to ca. 1000 °C, any excess C or Si will be rejected

314

Wolfram Uhlig

from the growing SiC crystallites and form a second phase, typically at the grain boundaries. The existence of such a second phase is often undesirable from the standpoint of oxidative stability and mechanical or thermal properties and its avoidance represents one of the criteria that may be imposed on the choice of a SiC precursor. However, criteria such as cost and the ability to use the precursor to fabricate particular forms for the SiC, such as continuous fibre or coatings, has often taken precedence over stoichiometry control in the search for suitable SiC precursors. In general, the applications for SiC precursors, which include matrixes for continuous SiC fibre-reinforced/SiC matrix composites, as well as the fibre itself and coatings or films, set the following criteria for the choice of SiC precursors: • • • • •

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•

low cost (inexpensive starting materials, high yield, facile, synthesis procedures, etc.) ability to fabricate required shapes or to infiltrate porous preforms (i.e. processability) high char yield (minimal loss of weight on pyrolysis) control of composition (generally at the 1 : l Si : C ratio, with minimal oxygen contamination) control of the ceramic microstructure (i.e. promote or retard crystallisation, control crystallite grain size, etc.) ability to handle the precursor in air (for ease/cost of processing)

In the initial work of Yajima, as is still mainly the case today, melt spinning was employed to spin the precursor fibre; therefore, an oxidative curing step was inserted after spinning to stabilise the fibre prior to pyrolysis. This curing step, along with the high excess carbon content of the precursor, actually led to the production of an amorphous silicon oxycarbide fibre which tends to degrade appreciably in strength above 1000 °C, or even lower in air, thus making it less useful for the many prospective applications for ceramic composites that require thermal stability, often in oxidizing environments, at these and even higher temperatures. Yajima chose polydimethylsilane 11 as the starting point for his development of a SiC precursor, which is obtained ultimately from dimethyldichlorosilane by Na coupling (Scheme 1). Although these polysilanes were not suitable for fibre spinning directly, Yajima found that on heating to 350 - 450 °C, they were converted to an intermediate form that was meltprocessable to give SiC fibre. This intermediate form is a largely Si−C- bonded, highly branched and partially crosslinked polymer 12, and to this date many individuals in the ceramics community associate the name “polycarbosilane” with this particular intermediate that Yajima developed in his search for a suitable precursor for SiC fibre production. Many studies have been subsequently carried out to identify the structure of this “Yajima polycarbosilane” and determine the chemistry involved in its preparation from the starting polydimethylsilane, as well as in its pyrolysis to SiCx (x > 1). It is generally agreed that a major portion of its structure, as well as its overall composition, can be represented by the formula [MeSiH−CH2]n and that the main reaction involved in its preparation from polydimethylsilane is a structural rearrangement which has become known as the Kumada rearrangement (Scheme 1) [46,47].

Polycarbosilanes Me

Me 2 Na, 110ºC

Cl

315

Cl

Si

Me

H

Si

C

H

H

400ºC, Ar

Si

- 2 NaCl

Me

Me

n

11

n

12

Me Structure of 12:

Me

H Me Si

Me

Me H

Si

Si

Si

H Me Si

Me H Si

Me Si

Me Me

H Me Si

H Me

Si

Si

Si

Si

Me

Me

Si

Me H

Me Me

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Scheme 1.

This reaction is apparently a radical process that is initiated by the cleavage of the relatively weak Si−Si bond. This silyl radical then presumably abstracts a hydrogen from a nearby methyl group which inserts into another Si−Si bond to form a Si−C linkage. However, it is clear that the actual structure of this polymer is much more complex than is represented by the simple [MeSiH−CH2]n formula, with some residual Si−Si bonding and extensive crosslinking and ring formation through additional Si−CHx−Si bridges. This crosslinking is apparently important to the physical properties of this polycarbosilane, which is generally obtained as a solid that can be melt-spun to produce fibre and which may or may not be soluble, depending on the degree of crosslinking. Two related, but structurally quite different, versions of Yajima's polymer have been reported in recent years. Frohling [48] and Neckers [49] have independently reported the preparation of a highly branched version of this polymer by Grignard coupling of chloromethylmethyldichlorosilane in THF, followed by LiAIH4 reduction of the resulting hyperbranched poly(chlorocarbosi1ane) 14 (Scheme 2, eq 1). Due to the presence of two chlorine atoms on each silicon, the −CH2MgCl Grignard reagent can couple with either one or both Si−Cl groups, leading, after reduction, to the formation of −(Me)Si≡, −(Me)SiH−, and −(Me)SiH2− units. The molecular weight (Mn) of this branched polymer was about 1700 (PD is about 30) and it undergoes pyrolysis to form a SiCx residue in about 50% yield. Pillot, Dunoges [38,50] and Interrante [51] have reported the preparation of the linear version of this [Si(H)(Me)CH2]n polymer, poly(methylsilylenemethylene) 19. The procedure employed by Pillot involved chlorination of 16 with AlCl3 and Me3SiCl, followed by a LiAlH4 reduction of the obtained poly[(methylchlorosilylene)methylene] 17 (Scheme 2, eq 2) [38]. The reduced linear polymer is a liquid that is soluble in most organic solvents, but its molecular weight (Mn) is relatively low (ca. 2000) due to cleavage of main-chain Si−C bonds by the chlorination procedure. The ring-opening polymerisation (ROP) procedure employed

316

Wolfram Uhlig

by Interrante and co-workers, on the other hand, yields a high molecular weight polymer 19 which was found to be atactic by NMR studies (Scheme 2, eq 3) [51,52]. Me

Me Cl

Si

Mg

CH2Cl

Si

THF

C H2

Cl

Cl

Me LiAlH4

Me Si

Si

Me

Me H2PtCl6

Me 15

Me Si

H

Si

C

Me

H

Si

Me

Cl

Me H2PtCl6

Si Cl

cis/trans mixture

Me3SiCl / AlCl3 - Me4Si

n

EtOH

Me Si

C

Cl

H

H LiAlH4

C H

n

n

H

Si

C

OEt H

H

Si

C

(2)

Cl

H

n

Me

H

Si

C

(3)

H

H

n

19 Mw ~ 47 000 BuMgCl

NEt3

Me

Me

17 Mw ~ 2 000

18

BuLi

n

14

16 Mw ~ 200 000

Cl

(1)

C H2

H

n

13

Me

Si

THF

{[MeBuSiCH2]0.7[MeClSiCH2]0.3} n

20 Mw ~ 185 000 crosslinked product 21

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Scheme 2.

Further information regarding polymer 18 and its use as a starting material for the preparation of other substituted poly(silylenemethy1enes) 20, 21 is included in the section on linear poly(silylenemethylenes) (chapter 3.1). The above versions of polycarbosilanes 14, 18 as is the case with Yajima's original polymer 12, all have a 2:1 ratio of C to Si, which usually results in excess carbon in the final SiC ceramic product. This has been found to be undesirable for many applications due to the higher oxidative instability, low degree of cystallinity and relatively poor creep resistance of such materials. In an effort to obtain precursors which would produce stoichiometric SiC free of both excess carbon and any oxygen impurity, polycarbosilanes with 1: 1 ratio of Si:C were prepared by Interrante and coworkers via two different approaches. A hyperbranched polycarbosilane 23 with an approximate [SiH2−CH2]n formula was made by Grignard coupling of chloromethyltrichlorosilane, followed by reduction with LiAlH4 (Scheme 3, eq 4) [41,53].

Polycarbosilanes

317

It was found by trapping experiments that the initial step in the polymerisation is a nearly quantitative formation of the Grignard compound Cl3SiCH2MgCl. This Grignard reagent then undergoes apparently exclusively head-to-tail (Si−C) coupling to yield a hyperbranched polycarbosilane polymer 22 with a [SiCl2−CH2] formula. After reduction polymer 23 contains the units SiH3−CH2−, −SiH2−CH2−, =SiH−CH2−, ≡Si−CH2−. A proposed structure for this hyperbranched polycarbosilane, which has been called hyperbranched polysilaethylene or hydridopolycarbosilane, is shown in Scheme 3. Cl

Cl Cl

Si

CH2Cl

Mg

H LiAlH4

Si

THF

Cl

Cl

Si

(4)

THF

C H2

C H2

H

n

22

n

23

proposed structure of 23: H3Si H3Si

C H2

H

CH2 Si H

C H2

Si H

C H2

H2C

Si

Si

C H2 Si

H2C

Si

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Scheme 3.

Preparation of a linear polycarbosilane with hydrogen as the only main chain substituent with a true [SiH2−CH2]n formula, has also been achieved by the ROP of 1,1,3,3-tetrachlorol,3-disilacyclobutane, followed by reduction with LiAlH4 (Scheme 4) [48,49]. Like its hyperbranched analogue 23, this linear polymer 24, [SiH2−CH2]n, can also be used to generate SiC by pyrolysis in high yield (ca. 85-90%); moreover, it gives mainly (>90%) H2 as the volatile by-product and is converted to stoichiometric, nanocrystalline SiC at 1000 °C [54,55]. Unfortunately, its high cost and difficult synthesis currently precludes its effective use as a SiC precursor. However, the simple structure of this polymer has made it an ideal system for the study of the pyrolytic conversion of a polycarbosilane to SiC [50,56,57]. A second method [23, 58] involves the ROP of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane, followed by a protodesilylation reaction with triflic acid and reduction with LiAlH4. Despite the low solubility of the [SiPh2−CH2]n intermediate, the product [SiH2−CH2]n polymer 24 was obtained in high yield (88% from [SiPh2−CH2]n) as a relatively high molecular weight (Mw = 37000, Mw/Mn = 2.8), organic soluble, viscous oil. A regular [SiH2−CH2]n structure was verified for this product by NMR spectroscopy and elemental analyses were also consistent with this formula. The thermal degradation of some poly(silylenemethylenes) was investigated by thermographimetric analysis (TGA) in an argon atmosphere. The results for the hydrogen derivative [H2Si–CH2]n 24, for the cross-linked polymer [Me(–CH2)Si–CH2]n 25 and for the linear [Me2Si–CH2]n 26 are compared in Figure 3 [59]. The highest ceramic yield (85%) is found for pyrolysis of 24. These result corresponds with the pyrolytic behavior of other polycarbosilanes containing silicon-hydrogen bonds [14, 51, 60, 61].

318

Wolfram Uhlig Ph 2 Cl

CH2Cl

Si

2 Mg

Ph

Ph Si

Si

Ph

Ph

Cu(acac)2

Ph

H

H

Si

C

Cl

H

LiAlH4

n

[54]

C

H

H

H2PtCl6

Cl

Cl Si

Cl

Si Cl

H

Si

H

Si

C

Ph

H

n

2 TfOH

"Polysilaethylene" Cl

Ph

OTf H LiAlH4

n

Si

OTf H

[58]

(5)

C n

24

4 MeCOCl

(i-Pr)O

O(i-Pr) Si

(i-Pr)O

Si O(i-Pr)

O(i-Pr) 2 Mg

2 Cl

CH2 Cl O(i-Pr) Si

Scheme 4.

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The cross-linking step (elimination of hydrogen) is not separated from the pyrolysis. The cross-linking occurs during the mineralization step. Decomposition of 25 started at about 300°C and the maximum rate of decomposition was found between 450 and 500°C. A char yield of 19% was observed. The thermal degradation of the purely linear, unfunctionalized poly(dimethylsilylenemethylene) 26 gave no ceramic residue. This result was also verified by investigations of Seiferth [39]. Mechanistic investigations of the pyrolytic process of organosilicon polymers are reviewed by Dunoguès [13] and Corriu [62].

Figure 3. Comparative TGA data [58] of poly(silylenemethylenes) [Me2Si−CH2−]n 26 (⎯⎯), [H2Si−CH2−]n 24 (----) and [Me(-CH2)Si−CH2−]n 25 (− − −).

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Polycarbosilanes

319

Various other polycarbosilanes have also been employd as SiC precursors. These are described in detail in the reviews on ceramic precursors [13, 45]. Only a few of notable examples will be mentioned here. Laine and Harrod [63] have reported the dehydrocoupling of methylsilane with Cp2TiMe2 (Cp = cyclopentadienyl) as the catalyst to obtain a polymethylsilane having a Mn = 1200 which gave a near stoichiometric SiC [SiC0.9H0.2O0.1] in relatively high yield (70-75%) on pyrolysis. Mixtures of polymethylsilanes produced by either route with a C-rich SiC precursor, such as the Yajima polycarbosilane, have also been used to obtain essentially stoichiometric SiC [64]. Although polymethylsilane is not a polycarbosilane, its inclusion here is partially justified by the observation, by solid state NMR spectroscopy [45], that it undergoes rearrangement on pyrolysis, in a similar manner as was described for the conversion of polydimethylsilane to Yajima's polycarbosilane 12, to give intermediates which have a polycarbosilane structure. Among the polycarbosilanes that have been employed as SiC precursors, the poly(vinylsi1anes) of the type [−RR’Si−CH2−CH2−]n produced by hydrosilylation / polymerisation of Si-H-containing vinylsilanes [14, 61] and the silyl-olefin and silyl- and disilylacetylene polymers of the type [−R2Si−CH=CH−]n, [−R2Si−C≡C−]n, and [−R2Si−C≡C−R2Si−]n [30-33, 65-69] are particularly noteworthy. Barton has reported the use of various silyl-olefine and silyl- and disilylacetylene polymers as a source of SiC fibre, films, and also as a component of a mixture with Si-Al powder that was used to join SiC [70-72]. These vinylsilane, silylolefin and silylacetylene polymers are described in more detail later (chapter 3.2). In recent years, multinary ceramics containing Si, B, N and / or C have found increasing interest [15-18,73]. Multinary systems are of special interest because they represent a combination of properties of the constituting binary phases. The stability of these materials results from a homogeneous distribution of the elements at atomic scale. The extremely low self-diffusion coefficients of silicon or boron in their respective nitrides and carbides inhibit crystallization and oxidation at temperatures up to more than 2000°C. A detailed overview on phase equlibria and material thermodynamics of the quaternary Si–B–N–C system and its binary and ternary subsystems is given by Aldinger [74]. Three examples for the synthesis of Si–B–N–C systems from organosilicon polymer precursors will be mentioned here. Riedel, Aldinger and Weinmann[17,75,76] described the synthesis of a polymer from the boron compound 27 and ammonia (Scheme 5, eq 6). The pyrolysis of this polymer gave a Si–B–N– C nanocomposite. Jansen [15] reported on the synthesis of a precursor polymer from 28 and methylamine (Scheme 5, eq 7). The pyrolysis at 1500°C leads to amorphous SiBN3C. Haberecht, Grützmacher and Nesper [77, 78] have described two different strategies for the synthesis of Si–B–N–C ceramic material with β-silyl substituted vinyl borazines as precursors. In the first approach, the trichlorosilylvinyl substituted borazine derivative [B3N3H3(CH=CHSiCl3)3] 29 [79] was transformed into a soluble prepolymer 30 (Scheme 6, eq 8) before conversion to a Si–B–N–C ceramic 31a. In the second approach, [B3N3H3(CH=CHSiCl3)3] was reacted to the tris(trihydrosilylvinyl)borazine [B3N3H3(CH=CHSiH3)3] 32 (Scheme 6, eq 9) which served as molecular precursor for ceramic synthesis [80, 81]. With this method, shrinking effects should be minimized and more compact materials 31b produced. In either way, a high degree of cross-

320

Wolfram Uhlig

linking is expected because of the high degree of unsaturation in the borazine precursors to which the Si–H bonds may add in hydrosilylation reactions under thermal activation. Me Cl

SiMeCl2

H3B-SR2

Si

B

CH 3

CH3

Cl

27 (6)

NH3 1200°C

Si/B/C/N (nanocomposite)

polymer H

H N Me3Si

N

SiCl4

SiMe3

H

Cl3Si

- Me3SiCl

SiMe3

N

BCl3 - Me3SiCl

Cl3Si

BCl2 28 (7)

MeNH2 1500°C

SiBN3C (amorphous)

polymer

Scheme 5. H N B

B

N H

MeNH2 (excess)

N B

H

[B3N3H3(CH=CHSi(NMe)1.5-x(NHMe)x) 30 3] D , 65% (8) SiBN1+xC2 31a: x = 1.5 31b: x = 0

HSiCl3 / Pt

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Cl3Si

D , 94%

H

H

SiCl3

H

H3Si

H

H

B

N H

B

N B

H

LiAlH4 (9)

N B

H

H

H H

SiCl3

SiH3

29

32

H

B

N H

H

Scheme 6.

SiH

3

N H

B

N H

H H

Polycarbosilanes

321

3. LINEAR POLYCARBOSILANES

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3.1. Poly(silylenemethylenes) Linear polycarbosilanes with a regular (Si–C)n main chain structure have been known, in the form of poly(sily1enemethylenes), since the 1950's. The first polymer of this type to be identified was apparently the dimethyl-derivative, which was obtained, in low molecular weight form, by the coupling of (chloromethy1)chlorodimethylsilane with sodium [82,83]. Later efforts by Weyenberg and Nelson [84], Nametkin [85,86] and Kriner [87-89] established the ring opening polymerisation (ROP) of disilacyclobutanes [90] as the method of choice for the preparation of high molecular weight polymers of this type. This ROP can be carried out thermally in some cases or, in general, by using transition metal catalysts. Poly(dimethylsilylenemethyene) 33 is easily obtained in high molecular weight, stereoregular form by either thermal or transition metal-catalysed ROP (Scheme 7, eq 11) and shows high thermal and chemical stability. The driving force for the polymerisation has been attributed to the strain energy of the four-membered disilacyclobutane ring (estimated as 17.2 kcal⋅mol-1 by calorimetry). The mechanism for the transition metal-catalysed ROP of this and other disilacyclobutanes has not been unambiguously determined; however, in 1964, Kriner proposed an ionic mechanism in which the ring opens to form a, presumably metal-stabilised, carbanion with the siliconium end of the ring-opened monomer serving as the reactive site for polymer propagation. In addition to its thermal stability under an inert atmosphere and in air [89], the physical properties of poly(dimethylsilylenemethy1ene) have been studied both in solution and in the neat form [91]. Its glass transition temperature (-87°C) was found to be intermediate between that of poly(isobuty1ene) and poly(dimethylsi1oxane), with no observed melting transition. It appears to be significantly more thermally stable than poly(dimethyl-si1oxane) in an inert atmosphere but less stable on heating in air, presumably due to oxidation of the bridging CH2 groups [89]. Its physical properties in solution and in the condensed state, as a γ-radiationinduced crosslinked solid, were studied in detail, employing carefully fractionated samples, and interpreted with the aid of theoretical studies of the polymer chain conformation [92]. The parent polymer poly(silylenemethy1ene) [SiH2–CH2]n 24 has already been mentioned in the context of SiC precursors (Scheme 4). This polymer has a unique structural relationship to polyethylene (PE) which has prompted a detailed study of its molecular structure by NMR methods as well as its thermal properties and crystalline form by DSC, Raman and IR spectroscopy and XRD [92-94]. The NMR results indicate a polymer with strictly alternating SiH2 and CH2 units [23,58]. The 29Si and 1H NMR spectra of 24 are shown in Figure4. In the course of efforts to prepare new poly(silylenemethylenes), various substituted 1,3disilacyclobutanes were prepared and polymerised directly to the corresponding poly(silylenemethylenes) by using Pt complex catalysts (Scheme 7, eq 11). Among the various polymers prepared was a series of asymmetrically and symmetrically alkylsubstituted poly(silylenemethylenes) 33. Both families of polymers were made via ROP of the corresponding 1,3-disilacyclobutanes and their phase transitions were studied by DSC and various other methods [52,95,96].

322

Wolfram Uhlig

Figure 4. 1H NMR (above) and 29Si NMR (below) spectra of [H2Si−CH2−]n 24 [58]. R Mg

(10)

Si R2

Si

R

2

Cl H2 C

R1

ClH2C

- MgCl2

1

R

1

R

2

H2PtCl6

Si C H2

R1

H

Si

C

R2

H

Me

H

Si

C

Ph

H

Ph

H

Si

C

Ph

H

(11) n

33

R1 = R2 = Me, Et, n-Pr, n-Bu, n-Pent, n-Hex 1

2

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R = Me; R = Et, n-Pr, n-Bu, n-Pent, n-Hex

Me

Me Si

Ph

Ph

Ph Si

Ph

Scheme 7.

Si

Ph

Si

Pt(PPh3)2Cl2 Me2HSiPh

Cu catalyst D

Ph

(12) n 34

(13) n 35

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Polycarbosilanes

323

In the case of the asymmetrically substituted poly(silyleneinethylenes), the polymers obtained thus far have all been atactic and show only glass transitions. In contrast to the asymmetrically substituted polymers, the corresponding symmetrically dialkyl substituted derivatives [Si(n-alkyl)2–CH2]n were found to be crystalline in the solid form, with a transition to a mesomorphic state apparently preceding melting to an isotropic liquid in the range 62 - 104°C. Efforts to prepare aryl-substituted poly(silylenemethylenes) apparently have been limited to the di-4-methylphenyl-, di-phenyl-, and the methylphenyl-derivatives, the latter two polymers having been reported quite early in the study of polycarbosilane chemistry [85,87] (Scheme 7, eqs 12,13). In contrast to the diphenyl-derivative 35, which was found to be completely insoluble in common organic solvents, poly(methylphenylsilylene-methylene) 34 is readily obtained in a high molecular weight, soluble, form by Pt-complex-catalysed ROP of the corresponding 1,3-dimethyl-1,3-diphenyl-l,3-disila-cyclobutane. Due to its ease of preparation, its chemical and thermal stability, and its relationship to poly(methylstyrene), it is among the best-studied and best characterized examples of the poly(silylenemethylenes). It is among the first polymers of this type to be prepared in the early 1960's by Namekin and coworkers [85] and later also by Kriner [87], although it was not characterized in detail until much later. In a preliminary communication by Hayakawa and Murakami [97], and later in more detail by Ogawa and Murakami [98], the isolation of the cis and trans isomers of 1,3dimethyl-l,3-diphenyl-disilacyclobutanes and their subsequent stereospecific polymerisation to yield 34 with varying tacticities was reported. The polymerisation was carried out both catalytically, by using transition metal catalysts in solution, and thermally at 170°C in the melt. The molecular weights of the resulting polymers varied from 104 to 2 · 106, with the bulk polymerization affording the highest molecular weight product when the trans isomer was used as the monomer. One of the catalyst systems employed was a Pt(PPh3)2Cl2 / Me2HSiPh mixture. In this case, polymerisation apparently proceeds through a hydrosilylation mechanism and the molecular weight of the polymer could be controlled through changes in the monomer / Me2HSiPh molar ratio (Scheme 7, eq 12). The stereochemical configuration of the monomer was found to be retained during the Ptcatalysed hydrosilylation. The resulting polymers showed various tacticities, ranging from predominantly isotactic to syndiotactic sequences, depending on the isomer and method of polymerisation employed; however, none of the polymers were fully isotactic or syndiotactic. Similar results were reported in an apparently parallel investigation of the polymerisation of cis / trans mixtures by Koopman, Frey and co-workers [99-101]. A detailed study of the structure and dynamics of the Si–C backbone and phenyl side groups of the 34 and 35 has been carried out by using solid state NMR methods [102]. Based on the results of 29Si and 13C NMR, it was concluded that 35 has two conformational isomers in the crystalline region and motion of the phenyl groups is restricted at room temperature. In contrast, 34 was found to have a distribution of conformations along the Si-C backbone and there is motion of the phenyl rings at room temperature. Poly(dimethylsilylenemethylene-co-diphenylsilylenemethylene), a copolymer having a [–SiPh2–CH2–SiMe2–CH2–] repeat unit, was reported by Koopmann and Frey [101]. This polymer was prepared by Pt - catalysed ROP of 1,1-dimethyl-3,3-diphenyl-1,3-disilacyclobutane, both in the bulk monomer at 100°C and in toluene solution at various temperatures. The resultant polymer ranged in Mn (Mw/Mn) from 6.5·103 (1.3-1.5) for the

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324

Wolfram Uhlig

solution polymerisations, which were carried out over a 5 day period (in 60-68% yield), to 3·105 (2.0) for the bulk polymerisation, which was completed in 3 min (42% yield). The availability of a monomer with two differently substituted Si atoms provided some insight into the nature of the Pt-catalyzed ROP polymerisation process which, as previously noted, is believed to proceed by oxidative addition of a Si–C bond of the disilacyclobutane monomer to the Pt complex catalyst to give a metallocycle intermediate. In this case there are two different types of –SiR2–CH2– bonds, which differ in the nature of the R group attached to Si. In fact, it is well known that the corresponding tetramethyl- and tetraphenyldisilacyclobutanes show quite different propensities for ROP, with the methyl-derivative undergoing polymerisation extremely rapidly in the presence of a Pt catalyst, even at room temperature or below, while the corresponding tetraphenyl-derivative is reportedly inactive toward ROP by Pt or Pd complexes and requires high temperatures to initiate polymerisation under any conditions. This is presumably associated, at least in part, with the steric bulk of the SiPh2 group. In the case of dimethyldiphenyl monomer, ROP obviously proceeds readily with H2PtCl6 as the catalyst, yielding a polymer which detailed structural analysis by NMR spectroscopy shows that there is a strong preference for opening the ring at a SiMe2–CH2 bond. However, this preference is not overwhelming, as the NMR spectra of the resultant polymers show that a minor fraction (estimated as ca. 13-25%) of the monomer units are incorporated irregularly to give sequences of SiR2–CH2–SiR2 units having the same R group. Although poly(diphenylsilylenemethylene) 35 was among the earliest polycarbosilanes to be prepared [85], apparently due to its intractability (it is insoluble in common organic solvents and melts at 340°C), it remained essentially uninvestigated for many years. More recently, due to its high thermal stability [103,104], and unique light emission behaviour [105], it has become the subject of renewed interest. Like the other polycarbosilanes, it is prepared by ROP polymerisation of the corresponding 1,1,3,3-tetraphenyl-l,3disilacyclobutane (Scheme 7, eq 13) monomer, either thermally in the melt at ca. 300°C, or catalytically in diphenylsulfone or xylene solution [103]. Two copper compounds, copper (II) chloride and copper bis-acetylacetonate, were successfully employed in this reaction. These Cu catalysts were also applied to the ROP of the 1,1,3,3-tetra-(m and p)-tolyl-l,3disilacyclobutanes [103] to yield the corresponding ditolyl-substituted poly(silylenemethylenes), which were similarly insoluble in most organic solvents. XRD and DSC studies showed that these symmetrically disubstituted polymers are highly crystalline solids with distinct melting transitions between 320 and 350°C and, except for the di-m-tolyl derivative, which was apparently too highly crystalline, glass transitions between ca. 130160°C. Copolymers of poly(diphenylsilylenemethylene) and poly(methylphenylsilylenemethylene) were prepared by both Cu-catalysed and uncatalysed co-polymerisation of the corresponding disilacyclobutane monomers [106]. The light emission properties of a series of phenyl-substituted poly(silylenemethylenes) were studied and compared with that of the corrsponding polysiloxanes [105]. All of the polymers having Si–Ph groups were found to exhibit emission in the range 300-400 nm (with a peak at ca. 350 nm) when irradiated with UV light. Eximer laser irradiation resulted in a similar emission; however, for the polymers having SiPh2 groups, after multiple laser pulses the emission decreased and shifted to longer wavelengths and longer lifetimes, presumably owing to some sort of decomposition process. Ogawa and Murakami [107] have also investigated the formation of polymer blends containing poly(diphenylsilylenemethylene) by the in situ polymerisation of 1,1,3,3-

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Polycarbosilanes

325

tetraphenyl-l,3-disilacyclobutane with various other organosilicon polymers. No chemical interaction between the component polymers was observed when the blend was prepared by thermal polymerisation of the disilacyclobutane in the presence of the other polymers, whereas when polymerisation was conducted by using a Cu(acac)2 catalyst, the resulting poly(diphenylsilylenemethylene) blends with poly(methylpheny1- and dimethylsilylenemethylene) and with poly(methylphenylsiloxane) appeared to be more homogeneous. Poly(diethoxysilylenemethy1ene) 36 was prepared from the corresponding disilacyclobutane monomer by ROP with chloroplatinic acid as the catalyst (Scheme 8, eq 15) [108]. This polymer is the first examples of stable dialkoxy substituted organosilicon polymer and have no analogues among the known polysiloxanes. In addition to the ROP approach for the preparation of poly(silylenemethylenes), reactions on preformed polymers have provided an important alternative route to functionalised poly(silylenemethylenes) which are difficult or impossible to prepare by direct polymerisation of the corresponding substituted disilacyclobutane. A prime example of this is provided by the synthesis of the parent poly(silylenemethylene) 24 [54,58] that were described in chapter 2 and its methylhydrido-relative 39 (Scheme 8, eq 20) [21]. In these cases, the Si–Cl group in the starting chloro-polymers is efficiently replaced by Si–H upon reaction with LiAlH4. In contrast to this method, which yields the [SiH2–CH2–]n polymer as a soluble melt with a regular H[SiH2–CH2–]n–H structure, attempts to carry out the direct polymerisation of [SiH2–CH2]n with Pt catalysts yielded an insoluble white solid which presumably is partially crosslinked by Si–O–Si or Si–Si bonding [109]. Another example of a successful application of this method is provided by the synthesis of the [SiF2–CH2–]n 37 polymer from the corresponding diethoxy-polymer 36 [110,111]. Poly(difluorosi1ylenemethylene) 37 is of particular interest as an analogue of an important organic polymer, poly(viny1idenefluoride). The extreme insolubility of the [SiF2–CH2–]n 37 polymer apparently leads to a low molecular weight product which shows a broad peak in the 29 Si NMR spectrum and which does not visibly melt when the direct ROP of the [SiF2CH2]2 monomer in pentane solution is attempted. In contrast, when neat tetraethoxydisilacyclobutane is first polymerised to 36 and then converted to 37 by reaction with F3B–OEt2, with appropriate care to avoid hydrolysis, the polymer product shows a relatively narrow peak in the 29Si NMR spectrum and melts reversibly at ca. 160°C (Scheme 8, eq 16). An additional advantage that was provided by using this method was the ability to characterize and purify the preformed polymer 36, because of its appreciable solubility. Moreover, the removal of the platinum ROP catalyst was easily accomplished by filtration, yielding a pure white solid for the final [SiF2–CH2–]n polymer 37, as compared to the gray material obtained from the [SiF2CH2]2 monomer. Similarly, the ethoxy substituted polymer [MeSi(OEt)–CH2–]n 40 can be converted to the corresponding fluoro polymer 42 by treatment with F3B–OEt2 (Scheme 8, eq 23) [112]. In contrast to the [SiF2–CH2–]n polymer 37, which was insoluble in all solvents tried and where the Si–F bond underwent hydrolysis fairly readily, the [MeSiF–CH2–]n polymer 42 was found to be readily soluble in a wide range of solvents and to be reasonably stable towards hydrolysis. This permitted a full characterization of its molecular structure by high-resolution solution NMR methods. As with the starting [MeSiCl–CH2–]n polymer 38 this one is completely atactic and shows splitting of the various 1H, 13C and 29Si NMR peaks due to the various possible diastereoisomeric sequences formed by the chiral silicons in the polymer chain.

326

Wolfram Uhlig

The known reactivity of the Si–Cl group toward ROH, R2NH, HM, RM (M = Group 1 and 2 metals) and various other reagents has prompted efforts to use Si–Cl substituted poly(silylenemethylenes) as substrates for various other types of substitution reactions. Except for the replacement of Si–Cl by Si–H (see Scheme 4), efforts to employ [SiCl2–CH2– ]n polymer in this manner have been frustrating by the relative insolubility, coupled with the high hydrolytic sensitivity of this polymer [54]. On the other hand, the corresponding [MeSiCl–CH2–]n polymer 38 was found to be quite soluble in ethers, benzene and various other solvents, permitting its use for various kinds of substitution reactions. The use of this polymer by Pilot [38,50] and later by Wu and Interrante [51] in their preparation of the [MeSiH–CH2–]n polymer 39 is shown in Scheme 8, eq 20. It was later used to prepare a series of OR (where R = Et, CH2CF3, Ac, Ph) derivatives 40 by reaction with a ROH/Et3N mixture in ether or THF solution (Scheme 8, eq 21) [108,112].

EtO

OEt Si

Si

EtO

OEt H2PtCl6

Si

OEt

Si

OEt

Si

Me Si

(20)

(18)

Si

Si OEt (17) OEt Si Me

Cl

(19) n 38

ROH/Et3N

Me

Me

Me

C H2

Cl

LiAlH4

Si Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Si

Cl

EtO

(21) Me

C H2

Si n

H

C H2 n

OR 40

39

R Pt

(22)

(23)

BF3OEt2

Me

Me

Si

Si

C H2 CH2CH2R

n 41

Scheme 8.

n 37

Me H2PtCl6

Si

Me

ClH2C

C H2

F

Cl

Mg

(16)

F

Cl

MeCOCl FeCl3

n 36

BF3OEt2

OEt ClH2C

(15)

OEt

(14)

Mg

C H2

C H2 n

F 42

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Polycarbosilanes

327

All of the polymers showed some hydrolytic sensitivity, which varied with the OR group in the following order: OPh < OCH2CF3 < OEt < OAc. As expected from the fact that the dichlorodimethyldisilacyclobutane used to prepare the starting polymer was a 50 : 50 mixture of the cis- and trans- isomers, this and its product polymer derivatives were all completely atactic with equal numbers of isotactic and syndiotactic sequences. Alkylation of the chloro polymer was also attempted by using Grignard regents. Allyl magnesium chloride gave the completely substituted poly(allylmethylsilylenemethylene), while butyl magnesium chloride resulted in only 77% substitution, presumably due to steric hindrance [112]. Hydrosilylation of olefins with poly(methylsilylenemethylene) 39 has been well studied and several different side chains [113], including liquid crystalline side groups [114-116], were successfully introduced onto the poly(methylsilylenemethylene) backbone 41 in this manner (Scheme 8, eq 22). It was found that hydrosilylation of poly(methylsilylenemethylenes) occurs with more difficulty than that of polysiloxanes, typically giving a somewhat lower degree of substitution than the corresponding polysiloxanes, perhaps due to the greater steric hindrance imposed by the two hydrogen atoms of the backbone methylene group as well as the less polar nature of the poly(methylsilylenemethylene) backbone. Habel and Sartori have reported the preparation of a series of chloropoly-carbosilanes of the type, [SiPh2-xClx–CH2–]n by cleavage of the phenyl groups with HCl/AlCl3 from the low molecular weight [SiPh2–CH2–]n product of a dibromomethane and dichloro-diphenylsilane Wurtz coupling reaction [117-119]. The fully cleaved [SiCl2–CH2–]n material was used to make [SiR2–CH2–]n polymers with various functional substituents via Grignard coupling reactions. The molecular weight of these substituted polymers ranged from several hundred to several thousand. Another route to functionalized poly(silylenemethylenes) involves the cleavage of alkyl or aryl side groups from the polymer chain by means of a strong protonic acid. Pilot [38,50] employed this method to prepare poly(chloromethylsilylenemethy1ene) from poly(dimethylsilylenemethylene) obtained by ROP ; however, the main chain cleavage in this case competes effectively enough with Si–CH3 cleavage to significantly degrade the polymer molecular weight. Based on the observation that the cleavage of the Si–Aryl bond is more selective than for the Si–alkyl bond, aryl groups in poly(silylenemethylene) polymers were reported to be selectively substituted by trifloromethanesulfate (triflate = OTf) by using triflic acid [120,121]. The cleavage of silicon–element bonds by triflic acid was found to be the most effective method for the preparation of silyl triflates. Systematic investigations of the reactions of compounds of the type R2SiXY (R = alkyl) with triflic acid have shown that the reaction is highly selective. This work was done by Bassindale [122], Matyjaszewski [123,124], Schmidbaur [125,126] and Uhlig [127-132]. The ease of replacement of Y by OTf decreases in the order NEt2 > allyl > para-anisyl > α-naphthyl > para-tolyl > phenyl > Cl > ethynyl > H >> alkyl. In no case were competitive cleavages of Si–X or Si–R observed. Silyl triflates are known to be strong silylating agents in organic chemistry. The extremly high reactivity of these compounds and of the less stable iodotrimethylsilane was confirmed by kinetic investigations [133]. Silyl triflates react rapidly and completely with acidic organic compounds [134] as well as active hydrogen compounds of group 14 to group 17 elements or with the corresponding lithium derivatives Exchange processes analogous to metal / halogen exchange are not observed. This unambigous reaction pathway prompted several groups to elaborate interesting methods for the formation of silicon-element bonds [135-145].

328

Wolfram Uhlig

Therefore, the interest was directed to the synthesis of highly reactive triflate-substituted poly(silylenemethylenes) and the investigation of their reactivity. As indicated in Scheme 9, poly(methylphenylsilylenemethylene) 34 is a suitable starting polymer for the preparation of numerous functional substituted poly(silylenemethylenes). The protodephenylation reaction with triflic acid (Scheme 9, eq 24) leads to the triflate derivative 43, which can be converted with nucleophiles to poly(silylenemethylenes) 44-49 [104,105]. Polymers 44-49 were characterized by 29Si, 13C, and 1H NMR spectroscopy and elemental analysis. The molecular weights, in the range of Mw ≈ 5·104, were similar to those of the starting polymer 34 (see Table 2) but were only determined in the case of polymers that were hydrolytically stable. Otherwise, measurement errors due to siloxane formation could not be excluded. From the similar molecular weights of 34 and the products can be concluded that no backbone cleavage occurs during the protodephenylation and substitution processes. H2 C

Me Si Ph

Me H

Me

[Pt]

Si

Si

Ph

C H2 cis/trans

Me H TfOH

C

Ph H

Si

Me H

Me H

Si

Si

C

C H

n

Me O

H

a f

b

Me H c

43

Si

C H

n e

49

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n 45

Me H C

d Me H

Si

C

Si

C

Me2 N

H

H

H

n

n 46

Me H

48

n

43

44

Si

(24)

OTf H

n

34

H

C

n 47

a: CH2=CH-CH2MgCl; b: CH2=CHMgCl; c: HC d: LiAlH4; e: Me2NH / Et3N; f: MeOH / Et3N

CMgCl

Scheme 9.

The polymers 12 and 47 can be described by the same empirical formula but their structures are strongly different. NMR spectroscopic investigations indicated a network structure for 12 (Scheme 1), while a purely linear structure was found for 47. Therefore, 47

Polycarbosilanes

329

can be named really as “polysilapropylene”. The atactic configuration of 47 is indicated by the 1:2:1 splitting of the methylsilyl signal in 1H NMR spectrum. The parent compound [H2Si–CH2–]n 24 was synthesized via silyl triflate intermediates analogously [58]. The reaction pathway was shown earlier (Scheme 4, eq 5). Uhlig [146] also attempted to synthesize poly(silylenemethylenes) with two different functional groups that are suitable for cross-linking reactions. The reaction of poly(methylphenylsilylenemethylene) 34 with triflic acid in the molar ratio PhSi : TfOH = 2:1 gives a triflate-substituted polymer {[PhMeSi–CH2–]0.5[(TfO)MeSi–CH2–]0.5}n, but broad NMR signals indicate a statistical distribution of the substituents in the resulting polymer. Therefore, a poly(silylenemethylene) 50 was synthesized containing two different leaving groups at the silicon atoms [121]. Schmidbaur found that para-anisyl-silicon bonds were cleaved by triflic acid much faster than phenyl-silicon bonds [125]. 1,3-dimethyl-1-paraanisyl-3-phenyl-1,3-disilacylobutane was prepared in a two-step reaction from an aminosubstituted parent compound. The ring-opening polymerization gives the corresponding poly(silylenemethylene) 50 and the protodephenylation with triflic acid leads to polymer 51, in which only the para-anisyl groups are replaced by triflate substituents (Scheme 10, eq 25). The signals of the 29Si NMR (Figure 5) are relatively narrow and indicate a high regularity of the structure, but it cannot be excluded that the structural modes shown in Scheme 10 exist side by side. Me 1. 2 TfOH

Me Si

Si

Me2N

2. p-AnisMgBr

Me 1. 2 TfOH

Me Si

Si

Me2N

NMe2

2. PhMgBr

p- Anis

Me

Me Si

Si

Ph

p- Anis H2PtCl6

Me

Me Si Ph

C H2

Si OTf

Me C H2

TfOH

Si Ph

n

51

(2 5)

Me C H2

Si p- Anis

C H2

n

50

Structural modes of 51: Me

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Me Si Ph

C H2

Si OTf

Me C H2

Si Ph

Me

Me C H2

Si OTf

C H2

Si Ph

Me C H2

Si Ph

Me C H2

Si OTf

Me C H2

Si OTf

C H2

Scheme 10.

Polymer 51 was reduced by LiAlH4 to give polymer 52 containing silicon-phenyl and silicon-hydrogen groups. Since the investigations of Bassindale [122], it is known that silicon-phenyl bonds are cleaved more easily by triflic acid than silicon-hydrogen bonds. Therefore, the reaction of 52 with triflic acid in the molar ratio PhSi:TfOH = 1:1 leads to the triflate-substituted polymer 53. This compound reacts with vinylmagnesium chloride to give poly(silylenemethylene) 54, in which one half of the silicon atoms are vinyl substituted.

330

Wolfram Uhlig

Figure 5. 29Si NMR (below) spectra of polymer 51 [MePhSi−CH2−Me(OTf)Si−CH2−]n [121]. Me Si Ph

Me C H2

Si OTf

Me

Me C H2

LiAlH4

Si Ph

n

51

C H2

Si H

C H2

52 TfOH

Me

Me Si

C H2

n

Si H

(CH2=CH)MgCl

C H2

n

54

Me

Me Si OTf

(26)

C H2

Si H

C H2

n

53 H2PtCl6 / C6H5Cl

{[Me(CH2CH2)SiCH2]x[Me((CH3)CH)SiCH2]y}n 55

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Scheme 11.

The hydrosilylation reaction was carried out in chlorobenzene, catalyzed by H2PtCl6 using the method of Corriu [60]. The branched polymer 55 was obtained with Mw = 41 000 and Mw:Mn = 5.7 [121]. The high polydispersity should be caused by the fact that the reaction can proceed intermolecularly or intramolecularly. The 1H NMR spectrum of 55 shows broad signals for CH3, CH2, and CH groups corresponding to α- and β-hydrosilylation modes. The reaction pathway is summarized in Scheme 11, eq 26. For better control, platinum catalyzed crosslinking was carried out between the vinylsubstituted poly(silylenemethylene) 45 and the derivative 47 containing silicon-hydrogen bonds (Scheme 12, eq 27). Now, the hydrosilylation reaction can proceed only intermolecularly and results in a polymer 25 (see chapter 2) with Mw = 65000 and Mw:Mn = 3.4. From the NMR spectra can be concluded that β-hydrosilylation is the dominating reaction mode.

Polycarbosilanes

331

Me H Si

Me H

C H

Si

n H2PtCl6

45

C6H5Cl

Me H Si

C

H

H

H2C

C H

Si

C

Me H n

(27)

CH2 H

n

25

47

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Scheme 12.

Recently, interest was directed to other types of intermolecular cross-linking reactions [121]. The triflate-substituted poly(silylenemethylene) 51 was used as the starting polymer. Its conversion with ethynylmagnesium chloride gave the polymer 56 with ethynyl side groups. 56 could be metallated using methylmagnesium chloride. In a last step the former magnesium compound was coupled with the triflate derivative 51 to give a poly(silylenemethylene) 57 containing ethynyl bridges between the silicon carbon chains (Scheme 13, eq 28). Some time ago, Uhlig found that triflate-substituted polysilanes and polysilynes can be reduced with potassium-graphite to give highly branched derivatives [147,148]. Potassiumgraphite C8K was found to be a very effective reducing agent for the formation of siliconsilicon bonds [149]. Consequently, the applicability of this cross-linking principle was tested for the synthesis of branched poly(silylenemethylenes). The reductive coupling of 51 with potassium-graphite proceeds at room temperature in high yields and short reaction times, leading to the polymeric network 58 shown in Scheme 13, eq 29 [121]. The reaction must be carried out using the “inverse reducing method” (addition of potassium-graphite to a solution of the triflate-substituted polymer). Otherwise (addition of the polymer solution to potassiumgraphite) the triflate anion can be reduced by an excess of C8K. It must be emphasized that the reduction can proceed either intramolecularely or intermolecularely. Therefore, relatively broad 29Si NMR signals and a broad molecular weight distribution were found for compound 58. The thermal degradation of polymers 57 and 58 were investigated by TGA and ceramic residues of 61% (57) and 52% (58) were found [121]. As expected, the final products are not pure silicon carbide. They contain elemental carbon but no oxygen. Combustion analyses gave the carbon content which was subtracted from the percentage silicon to reveal considerable and variable amounts of excess carbon in ceramic char: 57 (77.5% SiC in char) and 58 (68.2% SiC). Poly(silylenemethylenes) containing amino side groups (polycarbosilazanes) are interesting precursors for Si3N4 and Si/C/N ceramics [13,15,19,150]. Therefore, ammonia was tried as a branching agent. The polycarbosilazane 59 is formed from the triflate-substituted derivative 51 and a solution of ammonia and triethylamine (1:2) in ether according to Scheme 13, eq 30 [121]. The condensation can proceed intermolecularely or intramolecularely leading to an irregular structure. This is confirmed by the broad 29Si NMR signals.

332

Wolfram Uhlig Me

Me

Me

Me

MeMgCl

Si Ph

C H2

Si C

C H2

Si Ph

n

C H2

56 CH HC

Me Si Ph

Si OTf

51

(30)

Si

Me C H2

Si Ph

n

Si

Ph

Ph

C H2

Si Ph

NH

Me Si

Si

C H2 59

Si Me

Me

Me

Me C H2

Ph

C H2

Si

C H2

C H2

Si

C H2

Si C

C H2

C

Me

C8K

(29)

Me C H2

n

(28)

51

CMgCl

NH3 / 2 Et3N

Me

C

C H2

CMgCl

Me C H2

Si

C H2

Si Me

C H2

n

57

Me C H2

Si n

Ph

Me

n

58

Scheme 13.

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Finally, the results of ring-opening polymerization and of reductive polycondensation should be compared on the example of poly(silylenemethylenes) 60a and 60b. Both preparation methods of the polymer are shown in Scheme 14, and the 29Si NMR spectra are compared in Figure 6.

Figure 6. Comparative 29Si NMR spectra of polymers 60a (left) and 60b (right) [Cl(TfO)Si−CH2−]n [152].

Polycarbosilanes

333

Ph2SiCl2 + CH2Br2 4 Na

Ph

Ph

H 1. TfOH

Si

C

OTf H

H TfOH

Si

C

Cl

H

Si

C

Cl

H

(31)

2. LiCl

Ph

H

n

n

n

60a Cl

Tol Si

Si

Tol

Tol Cu(acac)2

Cl

OTf H

H TfOH

Si

C

Cl

H

n

Tol = p-Tolyl

Si

C

Cl

H

(32) n

60b

Scheme 14.

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The 29Si NMR spectrum of polymer 60a obtained by reductive coupling [40,151] (Scheme 14, eq 31) shows three main resonance groups, which can be assigned to the units Si–SiCl(OTf)–Si (a), Si–SiCl(OTf)–C (b), and C–SiCl(OTf)–C (c). The polymer contains oligosilane blocks and short alkyl chains in the backbone. Polymer 60b obtained by ROP with Cu(acac)2 as catalyst [103] is characterized by a regular alternating arrangement of silicon and carbon atoms in the polymer main chain [152]. Normally, the arylated and branched polymers formed yellow-brown solids. No clear melting points were observed. The arylated polymers 34, 50, and 52 change over to a highly viscous state with shrinkage at temperatures between 100 and 160°C. These conversions proceed in temperature intervalls of about 20°C. The branched derivatives decomposed without melting. The other polymers were obtained as yellow, high viscous oils. The molecular weights and polydispersities have been summarized in Table 2. All the polymers are entirely soluble in chloroform and THF, but insoluble in alcohols and aliphatic hydrocarbons. The triflate-substituted derivatives 43, 51, and 53 are hydrolytically sensitive. It must be emphasized that no substitution and reduction reactions should be carried out in THF, because silyl triflates can initiate the ring-opening polymerization of cyclic ethers [153]. Moreover, these syntheses proceed rapidly and completely in diethylether. Table 2. 29Si NMR data and molecular weights of poly(silylenemethylenes) space space No 24

Formula [SiH2−CH2]n

25

[Me(−CH2)Si−CH2]n

34

[MePhSi−CH2]n

43

[Me(OTf)Si−CH2]n

Mw Mw:Mn 37000 2.8 65000 3.4 82500 2.4 -

δ29Si ppm -35.1

Feature of polymers Yellow, highly viscous oil

1.5

Yellow solid

-4.7

Slightly yellow solid

39.8

Yellow, highly viscous oil; hydrolytically sensitive

334

Wolfram Uhlig Table 2. (Continued) δ29Si ppm 2.1

Feature of polymers Slightly yellow, viscous oil

1.9

Yellow, highly viscous oil

-17.9

Slightly brown, viscous oil

-13.5

Yellow, highly viscous oil

[Me(NMe2)Si−CH2]n

Mw Mw:Mn 65000 2.9 38900 3.2 35500 3.0 34600 3.4 -

7.4

49 50

[Me(MeO)Si−CH2]n [MePhSi−CH2−Me(pAnis)Si−CH2]n

88000 2.7

14.0 -6.2 br

Yellow, highly viscous oil; hydrolytically sensitive Yellow, highly viscous oil Slightly yellow solid

51

[MePhSi−CH2−Me(OTf)Si* −CH2]n

-

-3.8 38.6*

Yellow, highly viscous oil; hydrolytically sensitive

52

[MePhSi−CH2−MeHSi*− CH2]n

61400 3.2

-15.0* -5.8

Yellow solid

53

[Me(OTf)Si−CH2−MeHSi* −CH2]n

-

-15.9* 36.0

Yellow, highly viscous oil; hydrolytically sensitive

54

[Me(Vi)Si−CH2−MeHSi* −CH2]n

61400 3.2

-14.6* -0.1

Yellow, highly viscous oil

55

{[Me(CH2CH2)SiCH2]x [Me(CH3)CHSiCH2]y}n [MePhSi−CH2−Me(HC≡C) Si*−CH2]n [MePhSi−CH2−Me(≡C)Si* −CH2]n [MePhSi−CH2−Me(≡Si)Si* −CH2]n [MePhSi−CH2−Me(−NH)Si* −CH2]n

41000 5.7 53000 2.9 109000 3.1 48700 4.9 -

1.0 br

Yellow-brown solid

-16.5* -5.0 -18.0* -6.9 -21.0* -4.3 -6.3 br 3.3 br*

Slightly brown, highly viscous oil Slightly brown solid

No 44

Formula [Me(All)Si−CH2]nn

45

[Me(Vi)Si−CH2]n

46

[Me(HC≡C)Si−CH2]n

47

[MeHSi−CH2]n

48

56 57 58 59

Yellow solid Yellow-brown solid

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Vi = vinyl, All = allyl, p-Anis = para-anisyl

3.2. Poly(silyleneethylenes), Poly(silyleneethenylens) and Poly(silyleneethynylenes) Poly(silyleneethylenes) in which the polymer backbone is made up of a regular alternation of two carbon atoms and one silicon atom, have been made primarily by hydrosilylation polymerisation. For example, by using chloroplatinic acid as a catalyst, vinyldimethylsilane can be converted into poly(dimethylsilyleneethylene) 61 [14,60] (Scheme 15, eq 33). Actually, the hydrosilylation of vinyldimethylsilane proceeds in both α and β modes, thus the resultant polymer contains two kinds of units: Si(Me)2CH2CH2 (derived from β mode hydrosilylation) and Si(Me)2CH(CH3). NMR studies suggested that 30% of the α

Polycarbosilanes

335

units and 70% of the β units were formed. Likewise, vinyldichlorosilane was polymerised to form poly(dichlorosi1yleneethylene) 62 [66]. This dichloropolymer can be further reduced with lithium aluminum hydride to yield the parent poly(silyleneethylene) 63 (Scheme 15, eq 34). Me n H

Si

Me [Pt]

Me

(33)

Si Me

n 61

Cl n H

Si Cl

H

Cl [Pt]

LiAlH4

Si Cl

n 62

Si H 63

n (34)

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Scheme 15.

Polycarbosilanes with unsaturated carbon-carbon double or triple bonds and one silicon atom in the repeat unit have also been reported. Initial efforts to prepare poly(silylenevinylenes) employed hydrosilylation of separate dihydridosilane and diacetylene monomers [29] or catalytic redistribution of disiloxanes [154] and led to low molecular weight products. It was later found that hydrosilylation polymerisation of various H–Si(R)(Me)–C≡CH monomers with chloroplatinic acid catalyst gave high (>90%) yields of high molecular weight dimethyl- and methylphenylsilylene-E-1,2-vinylene polymers 64, in which silylene units regularly alternate with trans-carbon-carbon double bonds (Scheme 16, eq 35) [29]. These polymers are reported to melt reversibly, to be soluble in a wide range of solvents and to readily form fibres and films that crosslink under UV irradiation and pyrolyse in high yield to a C-rich SiC. The α,α-isomeric form of the dimethylsilylenevinylene polymer 65 has also been reported (Scheme 16, eq 36) [154]. The polymer containing only α,α-vinylene linkages was prepared by ROP of 1,1,3,3-tetramethyl-2,4-dimethylene-l,3-disilacyclobutane. This monomer was prepared by reaction of α-bromovinyldimethylchlorosilane with Mg and polymerized by anionic ROP with n-BuLi and HMPA in THF at -78°C. The corresponding silylenevinylene polymer was characterized by NMR spectroscopy. An alternative route to a polymer which contains –Si–C=C– linkages has been discovered which uses dimethyldivinylsilane as the starting monomer (Scheme 16, eq 37) [155]. The ruthenium complex, RuCl2(PPh3)3 has been found to be most effective as the catalyst for this reaction, which produces a copolymer 66 with both 1,2- and 1,1-divinyl-linkages between the silicon atoms. The resultant polymer has a relatively low molecular weight (Mw = 1510) but a reasonably low polydispersity (Mw/Mn = 1.19). Poly(silyleneethynylenes) were first reported in the early 1960s [156], as obtained by the reaction of the Grignard compound, BrMg–C≡C–MgBr, and Ph2SiCl2. The polymers, which were apparently oligomers, were described as black resins. An improved procedure, which

336

Wolfram Uhlig

employed dilithioacetylene (Scheme 16, eq 38,39), was later developed by Barton and coworkers which has been used to prepare a wide range of acetylene-bridged silylene and disilylene polymers by condensation polymerisation with various dichlorosilanes and dichlorodisilanes [30,32-34,157]. Me H

Me [Pt]

Si

(35)

Si

R

R

n 64

Me

Me Si

Si Me

Me [Pt]

(36)

Si

Me

Me

n 65 Me

Me [Ru]

Si Me

x

66 R

LiC

Si

Cl

Si

Si

y

Me

Me

R Cl

Si

R

CLi +

Me (37)

Me

Cl

Me Si

C

R

R

R

Si

Si

R

R

Cl

(38)

C n

67

R

R

Si

Si

R

R

C

(39)

C 68

n

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Scheme 16.

These polymers are obtained as the corresponding disubstituted poly(silyleneacetylenes) 67 or poly(disilyleneacetylenes) 68 in high yields as soluble white solids, most of which undergo reversible melting. They have been of interest primarily as SiC precursors. However, the polymers prepared by these methods always contain after hydrolysis small proportions of siloxy units in the polymer backbone, which would interrupt the electron delocalization. Moreover metal/halogen exchange processes during the condensation processes cannot be suppressed and polymer yields are often low while oligomers preponderate. Therefore, new synthetic routes have been developed in which no alkali-metal halide condensations are involved. Uhlig found an effective route for the preparation of such polymer chains using α,ωbis[(trifluoromethyl)sulfonyloxy]-substituted organosilicon compounds. These derivatives can be prepared from the corresponding amino-, allyl-, or phenylsilanes by relatively simple methods [120,126,127,132,158,159]. The syntheses of the α,ω-bis[(trifluoromethyl)-

Polycarbosilanes

337

sulfonyloxysilyl]ethynes 69,70 are shown as examples (Scheme 17, eqs 40,41). Moreover, it is remarkable that α,ω-bis(silyl triflates) are often easily formed when the synthesis of the corresponding chloro- or bromosilanes is difficult or has not been attempted. (TfO)Me2Si

C

C

x

2 TfOH

SiMe2(OTf)

PhMe2Si

C

C

x

SiMe2Ph

69: x = 1; 70: x = 2 H

Cl

Cl

Cl

(40)

2 Me2PhSi(OTf) BuLi

Cl

x=1

Li

Cl Cl

C

x

BuLi

(41)

MePhSi(OTf)2

Cl

Li

x=2

Cl Cl

Me

Me Si

C

C

C

TfOH

x

OTf

n

Si Ph

C

C

x n

71: x = 1; 72: x = 2

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Scheme 17.

The reaction of the α,ω-bis[(trifluoromethyl)sulfonyloxydimethylsilyl]ethyne 69 with dinucleophiles gave polymers in excellent yields (Scheme 18) [160]. The polymers 73-76 prepared by this method did not contain siloxy units after hydrolysis of the reaction mixture. A regular alternating arrangement of organosilicon and unsaturated organic groups was found, indicated by narrow signals in the 29Si NMR spectra. Special structured polysiloxanes 77, 78, e. g. silarylene-siloxane copolymers, which are known to be materials with an optimum combination of mechanical properties and thermal stability [161,162], could be also obtained. Moreover, the synthetic potential is broader, because other α,ω-bis(silyl triflates), e. g. butadiyne, phenylene or thienylene derivatives, can react with Li2C2 or Li2C4 to give numerous copolymers containing alkyne or dialkyne units. Weight-average molecular weights in the range Mw ≈ 4000 – 12000, relative to polystyrene standards, were found. The polydispersities were found in the range 1.6 – 2.6. Normally, the polymers are yellow-brown solids. Defined melting points are not observed. The solids became highly viscous fluids, with contraction of volume, at temperatures between 70 and 200°C. Some of the prepared poly(silylenealkynes) and poly(silylenebutadiynes) have been cast as thin films and conductivity have been determined [163]. All the films are insulators, with conductivity values from 10-12 to 10-15 Scm-1. The doping of these polymers has been carried out using an electron acceptor such as FeCl3 or SbF5. Upon doping the conductivity reaches 10-2 to 10-5 Scm-1. These relatively high values are comparable with those obtained for fully conjugated organic polymers [164]. The data [32,165-168] show that the intercalation of silicon atoms in a conjugated polymeric carbon backbone still allows the transfer of charge. The presence of two consecutive silicon atoms does not prevent conductivity in the polymer. The data also

338

Wolfram Uhlig

clearly outline the possibility of modulating the charge transport properties by electronic effects of substituents attached to silicon [32]. Me2 Si C

Me2 Si C

C

C

C

C

73

n

Li2C4

Me2 Si C

C

Me2 Si

Ph2 Si

5

Me2 Si C

(TfO)Me2Si

75

C

C

C

C

n

Li2C2

SiMe2(OTf)

69

BrMg-C6H4-MgBr

Me2 Si C

Me2 Si C

n

74

Li(SiPh2)5Li

C

Ph2Si(OH)2

Me2 Si

Me2 Si C

C

Me2 Si O

Ph2 Si O

n 76 Me2 Si C

C

77

HO-C6H4-OH

Me2 Si O

n

O

78 n

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Scheme 18.

Poly(silyleneethynes) and poly(silylenebutadiynes) also received interest as preceramic precursors. Barton [30,33] and Corriu[62,66] have almost simultaneously investigated the thermal behaviour. It was found that poly(silylenebutadiynes) undergo a low-temperature cross-linking essentially via 1,4-addition to the triple bonds [169]. Poly(silylenebutadiyne) – metal oxide composites were also pyrolyzed to various silicon carbide – metal carbide mixed ceramics [170]. The ceramic residues varied from 40 to 80%, with variable amounts of free carbon depending on the nature of the substituents at the silicon atoms. Uhlig found a useful method to exchange substituents attached to silicon and investigated the protodephenylation of phenylated poly(silyleneethyne) and poly(silylenebutadiyne) with triflic acid [120,132]. As expected, the dephenylation of the polymers in a 1:1 molar ratio proceeds quantitatively at room temperatures and short reaction times, leading to the triflate derivatives 71, 72 (Scheme 17, eq 41). Subsequent nucleophilic substitutions give the functional polymers 79-82 and 83-86 (Scheme 19, eq 42, Table 3). Similar molecular weights and polydispersities of the starting materials and of the resulting products indicate that no backbone cleavage occurs. Of course, it is possible to replace only a fraction of phenyl groups of poly(silyleneethyne) and poly(silylenebutadiyne) by triflic acid. However, the reactions of the polymers with CF3SO3H in a 2:1 molar ratio give products with statistically distributed triflate groups, and the regular structure of the polymer chains is lost during this process. The synthesis of

Polycarbosilanes

339

partly triflate-substituted poly(silyleneethyne) with an alternating arrangement of Ph–Si and TfO–Si groups in the backbone requires a starting polymer with different leaving groups. The stepwise preparation of poly(silyleneethyne) 87, which contains phenylsilyl and paratolylsilyl groups by turns, is shown in Scheme 19, eq 43. The reaction of 87 with triflic acid in a 2:1 molar ratio leads, with substitution of the para-tolyl groups, to a regular polymer 88 [120]. The conversion can be monitored by NMR spectroscopy. Figure 7 shows the 29Si NMR spectra of 87 and 88. The narrow signals confirm the regular structure. Additional signals of terminal groups are not observed if the stoichiometric ratios are strictly adhered to. This observation should be explained by the formation of large ring systems (Mw ≈ 10000). Polymer 88 can be cross-linked using difunctional nucleophilic reagents. The reaction with methylamine, which gives the nitrogen-branched poly(silyleneethyne) 89, illustrates this type of reaction. 89 is a potential organic precursor for Si/C/N-based materials. Me

Me Si

C

Si

C

OTf

C

X(Y)

n Et3N/HX

71

LiY

n

79: X = NMe2 80: X = OEt 81: Y = Allyl 82: Y = H

(42)

Me

Me Si

C

C

C

C

C

Si n

OTf

C

C

C

C n

X(Y) 83: X = PPh2 84: Y = SiPhMe2 85: Y = Vinyl 86: Y = H

72

Me

Me

Me 1. 2 LiBu

HC

C

Si

C

C

CH

p-Tol Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Me C

C

Si

C

Si Me

Scheme 19.

Si

C

C

C

C

C

C

Si OTf

Si Ph n

C

89

87

n (43)

Me

Me MeNH2 / 2 NH3

Si Ph

TfOH

Si Ph Me

C

p-Tol

Me

NMe C

C

2. MePhSi(OTf)2

C

C

Si Ph 88

n

340

Wolfram Uhlig

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Figure 7. 29Si NMR spectra of polymers [Me(Tol)Si−C≡C−Me(Ph)Si−C≡C−]n 87 (above) and [Me(TfO)Si−C≡C−Me(Ph)Si−C≡C−]n 88 (below) [120].

Figure 8. Comparative TGA data of polymers [H2Si−H2Si−C≡C−]n 91 (----) and of the methylated derivative [Me2Si−Me2Si−C≡C−]n (⎯⎯)[30,58].

As mentioned earlier in chapter 2, polymers containing Si–H bonds are suitable organic precursors for ceramics. They are cross-linked at temperatures below 200°C by hydrogen elimination. Uhlig tried to synthesize poly(silyleneethyne) 90 and poly(disilyleneethyne) 91 containing hydrogen substituents at the silicon atoms exclusively. Moreover, the Si:C ratio in polymer 91 averages 1:1. The starting materials are hydrogen-rich α,ωbis[(trifluoromethyl)sulfonyloxy]silane and –disilane respectively described by Schmidbaur [171,172]. The polycondensation with Li2C2 leads to polymers 90 and 91 (Scheme 20, eq 44)

Polycarbosilanes

341

[58,173,174]. In Figure 8 the pyrolysis behavior of [H2Si−H2Si−C≡C−]n 91 is compared with the behavior of the methylated derivative [Me2Si−Me2Si−C≡C−]n. .

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Table 3.29Si and 13C NMR data and molecular weights of poly(silylenealkynes) 69-91 No

Formula

Mw Mw:Mn

δ29Si ppm

69

[MePhSi−C≡C−]n

-45.7

MeSi 1.1

70

[MePhSi−C≡C−C≡C−]n

-43.6

0.8

82.6

90.6

126-141 (Ph)

71 72 73

[MeSi(OTf)−C≡C−]n [MeSi(OTf)−C≡C−C≡C−]n [Me2Si−C≡C*−Me2Si−(C≡C)2−]n

+4.0 +2.9 -19.3

5.4 4.0 -0.3

117.8 (CF3) 118.6 (CF3) -

[Me2Si−C≡C−Me2Si−(Si*Ph2)5−]n

-2.6

-

126-142 (Ph)

75

[Me2Si−C≡C−Me2Si−C≡C−]n

-38.6; -42.5* -37.3

110.5 81.1 82.1 110.5* 109.8

89.6 88.4

74

1.5

112.8

-

-

76

[Me2Si−C≡C−Me2Si−C6H4−]n

-20.9

-2.3

110.9

-

77

[Me2Si−C≡C−Me2Si−OSi*Ph2O−]n

2.9

111.9

-

[Me2Si−C≡C−Me2Si−OC6H4O−]n

-9.4; -44.2* -4.9

133.5; 139.0 (Ph) 126-139 (Ph)

78

-0.7

113.0

-

79 80

[MeSi(NMe2)−C≡C−]n [MeSi(OEt)−C≡C−]n

25000 2.2 10500 2.3 5100 1.9 10400 2.6 5400 1.7 7300 1.9 3800 2.0 5400 1.6 -

δ13C ppm SiC≡C SiC≡C Others 112.0 127-140 (Ph)

-33.9 -30.9

2.1 2.8

113.1 112.0

-

81

[MeSi(All)−C≡C−]n

18600 2.4

-41.2

-0.9

113.4

-

82

[MeSi(H)−C≡C−]n

-56.6

-1.7

111.5

-

83 84

[MeSi(PPh2)−C≡C−C≡C−]n [MeSi(Si*PhMe2)−C≡C−C≡C−]n

0.2 -2.4 -1.7 -1.0

90.6 91.7

126-137 (Ph) 127-140 Ph

[MeSi(Vi)−C≡C−C≡C−]n

-39.9 -59.9 -20.8* -43.0

83.0 84.5

85

84.7

91.0

86

[MeSi(H)−C≡C−C≡C−]n

-58.0

-2.2

83.5

90.6

129.5 (−CH=) 134.6 (=CH2) -

87

-46.6 -43.2 -44.0 +2.8* -42.5 -36.3* -80.1

-0.5 1.0 0.2 3.4 -0.3 2.3 -

111.8 112.9 109.9 112.5 113 br

-

90

[Me(Tol)Si−C≡C−Me(Ph)Si− C≡C−]n [Me(TfO)Si*−C≡C−Me(Ph)Si− C≡C−]n [Me(MeN)Si*−C≡C−Me(Ph)Si− C≡C−]n [H2Si−C≡C−]n

14000 2.5 13500 2.7 8600 2.4 5100 2.6 9700 2.3 -

122.2; 151.8 (Ph) 45.9 (NMe2) 15.0 (CH3) 56.4 (OCH2) 26.5 (SiCH2) 115.1 (=CH2) 132.5 (−CH=) -

111.2

-

23.4 (CH3) 128-141 (Ph) 118.5 (CF3) 127-140 (Ph) 42.8 (NMe) 125-141 (Ph) -

91

[SiH2−H2Si−C≡C−]n

-76.8

-

110.0

-

-

88 89

Vi = vinyl, All = allyl, Tol = para-tolyl.

9200 2.4 11500 2.6

-

342

Wolfram Uhlig

A weight loss of only 15% was found for 91, while a weight loss of 46% [30] was measured for the methylated polymer. XRD diffraction data were acquired for the pyrolytic residue of 91 obtained at 1000 and 1500°C. The product is X-ray amorphous at 1000°C. After pyrolysis to 1500°C the material exhibit diffraction patterns consistent with polymorphic SiC. The pyrolytic products were also investigated using 29Si MAS NMR spectroscopy [175]. At 1000°C a broad resonance at –30 ppm is observed. The 1500°C spectrum shows a relatively narrow peak at –20 ppm typical for SiC. H

H

H

2 TfOH

Tol

Si H x x = 1,2

Tol = H3C

Tol

TfO

Si

OTf

Li2C2

Si

x=1

H x

C

C

H Li2C2

n

90

(44)

x=2

H

H

Si

Si

H

H

C

C n

91

Scheme 20.

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3.3. Polycarbosilanes with (–Si–Cx–)n Units (x > 2) in the Main Chain Poly(silylenetrimethylenes) 92,93 were prepared in the early 1960s by thermally induced ROP polymerisation of the corresponding monosilacyclobutanes and have a linear chain structure with a regular alternation of the silylene and propylene units [176-178]. As with the disilacyclobutanes, the ROP of the monosilacyclobutanes is driven by the relief of ring strain. The thermal polymerisation can be influenced by the nature of the substituents attached to the silicon in the four-membered ring. Retardation of polymerisation can be caused by electronwithdrawing and possibly sterically inhibiting phenyl groups [178]. 1,1-Dimethyl-silacyclobutane can be polymerised at 80-85°C, whereas, 1-chloro-1methyl-silacyclobutane starts to polymerise only at 170°C (Scheme 21, eqs 45, 46). Electronegative substituents attached to the phenyl ring, especially halogens and the CF3 group, have a strongly retarding effect on the reactivity of the silicon-carbon bonds in this reaction, resulting in a much lower polymerisation rate [178]. The anionic ROP of 1,1-dihydrido-monosilacyclobutane was conducted by using a BuLi/HMPA catalyst in THF at low temperatures (Scheme 21, eq 47). Weber proposed that anionic ring opening polymerisation proceeds by nucleophilic attack at the silicon atom to form a hypervalent, pentacoordinate, siliconate intermediate that undergoes ring opening to form a primary carbanion, which then attacks another molecule of silacyclobutane [179]. The anionic polymerisation of 1,1-dimethyl, 1,1-diethyl, and 1-methyl-1-phenylsilacyclobutane was studied under different conditions by Matsumoto, and found to have a living nature in a THF-hexane mixed solvent system at -48°C [180]. The living polymer was

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Polycarbosilanes

343

successfully endcapped with electrophiles, such as chloromethyl-phenylsilane, and used in initial studies of block-copolymer formation with styrene. Block copolymers of poly(1,1-diethylsilylenetrimethylene) with styrene derivatives have been prepared by using the living polymer obtained by anionic ROP with PhLi as a macroinitiator for the polymerisation of the corresponding olefins [181]. This living polymer did not give poly(1,1-diethylsilabutane-co-methyl methacrylate) selectively with methyl methacrylate as the comonomer, presumably because of the high reactivity of the propagating center; however, after end-capping with 1,1-diphenylethylene, the resulting living polymer anion did yield the desired block copolymer with various methyl methacrylate derivatives. The synthesis and living anionic ROP of the corresponding 1,1-dipropylsilacyclobutane has also been reported [182]. As with the poly(silylenemethylenes), transition metal-catalysed ROP of silacyclobutanes constitutes an important approach for the preparation of high molecular weight poly(silylenetrimethylenes). A wide range of transition metal catalysts, have been used for this purpose [84,178]. A comparison of the catalytic activities of various compounds of Group VIll metals has indicated that the most effective catalysts were alkene chlorocomplexes of platinum such as di-μ−chlorodichlorobis(cyclohexene)diplatinum(II), which cause vigorous polymerisation of the four membered ring at 20 - 70°C. Insight into the mechanism for the Pt-catalysed ROP of mono- and disilacyclobutanes was provided by the isolation and structural characterisation of a 1-platina-2-silacyclopentane derivative from the reaction of Pt(PEt3)3 and 1,1-diphenyl-1-silacyclobutane [183]. Similar derivatives were identified for the 1,1-dichloro- and 1,1-dimethylmonosilacyclobutane, although pure samples were not isolated in these cases. In these reactions, the dimerisation of the monosilacyclobutanes to form 1,5-disilacyclooctane derivatives was also observed and was found to result from the reaction of the metallacycle with excess monosilacyclobutane. Thermal initiation or anionic catalysts did not work very well for the polymerisation of monosilacyclobutane monomers with bulky groups. The thermal initiation method requires a high temperature and often leads to partially crosslinked products. The anionic catalyst method is not applicable because of a sharp decrease in the polymerisation rate with increasing steric bulk of the substituents. Here, only the use of metal catalysts permits the generation of soluble, high molecular weight, polymers. On the other hand, transition metal catalysts may not be applicable to the polymerisation of monomers with Si–H groups, due to the possibility of metal-catalysed dehydrogenation of the Si–H groups, which could lead to a crosslinked polymer. The synthesis and polymerisation behaviour of monosilacyclobutane derivatives that contain a trimethyl- and triphenylsiloxy, as well as a siloxy group that contained a mesogenic side chain attached to the Si, has also been examined [184,185]. An alternative route to poly(silylenetrimethylenes) 95 with pendant side groups is provided by hydrosilylation of the preformed polymer 94 (Scheme 21, eq 48) [186]. This was found to proceed efficiently for a variety of terminal olefins, with only a slight amount of a side reaction, presumably involving accidental water, which results in hydrolysis of some of the Si–H bonds. Hydrosilylation of allylsilane derivatives with Si–H groups also generates polycarbosilanes with one silicon and three carbon atoms in the backbone. Chai reported that self-hydrosilylation of PhSi(allyl)(Cl)H with Karstedt's catalyst gave polymer 96, which led to the formation of non-stereoregular poly(1-phenyl-1-silabutane) 97 after reduction with

344

Wolfram Uhlig

LiAlH4 (Scheme 21, eq 49) [187]. Although not stereoregular in terms of the sequence of optical active Si isomers, the chain structure is quite regular in the sequence of silylene and trimethylene linkages, with no indication of 1,1-hydrosilylation of the vinyl groups. The selfhydrosilylation polymerisation method has also been used to prepare a poly(silylenetrimethylene) with a high degree of stereoregularity [188,189]. Another example of an optically active polycarbosilane, poly[(methylphenylsilylene)(2-propenylene)] 98 was prepared by Kawakami and coworkers by the hydrosilylation polymerisation of the optically active (R)-methylphenylpropargylsilane monomer (Scheme 21, eq 50) [190]. Organosilicon polymers containing a regular alternating arrangement of silylene and phenylene or thienylene units in the main chain have received increasing attention in the last few years [191,192]. Enhanced through-space interaction between the π-electron systems by the silicon bridge [193,194] and/or delocalization of π-electrons through the organosilicon unit by σ−π-conjugation in the backbone [195-203] provide the potential utilities of the polymers as organic semiconducters and hole transporting materials for electroluminescent devices. Several routes to poly(silylene-1,4-phenylenes) 99 [204-206] and poly(silylene-2,5thienylenes) 100, 101 [166, 214-216] have been found. Me

Me

Me

80°C

Si

(45)

Si Me

n 92 Me

Cl

170°C

Si

Cl (46)

Si

Me

n 93

H

R1

BuLi / HMPA

Si R 1 R = H, Me, Ph

(47)

Si

THF / -78°C

1

H

n 94 R1

BuLi / HMPA

94 +

2

R

R2

Si

THF / -78°C

(48)

n

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95 Cl Si

Ph H

[Pt]

Me Si

Si 96 Ph

[Pt]

n (49)

Me Si *

98

H Si

97

H

Scheme 21.

Ph LiAlH4

n

Ph Ph

Cl

(50) n

Polycarbosilanes

345

Wurtz-type reductive coupling reactions, polycondensation reactions of dilithio or dibromomagnesium compounds bearing π-electron systems with dichlorosilanes, and electrochemical syntheses have been described. Poly(silylene-1,3-phenylene) 102 was obtained by transition metal catalysed desilanative coupling reaction [217,218]. A starlike silicon compound containing dithienylene units was synthesized recently [214]. Some representative examples are summarized in Scheme 22, eqs 51-54. Me

Me BrMg

MgBr + Cl

Si

Cl

Si

Me

Me

(51) n

99 Me

Me Li

Li

Cl

+

S

Si

S

Me

Br

Br

+ R2SiCl2

S

n

2 F mol -1 R

R Si

Si S

R

Me

100

C-Kathode

R

S

(52)

Si

Cl

Si S

R

R

(53) S

101 [Ru]

H

- SiH4

H3Si

SiH3

(54)

Si n 102

H

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Scheme 22.

The functionalization of poly(silylene-1,4-phenylenes) and poly(silylene-2,5-thienylenes) is confronted by difficulties. Electrophiles cleave the polymeric backbone easily. Therefore, only a few functionalized poly(silylene-1,4-phenylenes) were known recently [204,215,216]. Uhlig investigated the protodephenylation of these polymers with triflic acid and found that the strong electrophile attacks both aromatic side-groups and polymer backbone [219, 220] (Scheme 23, eqs 55). Therefore, the functionalization of these polymers requires better leaving groups at the silicon atoms. Allyl [130], amino [131], and para-anisyl [125] groups attached to silicon are better leaving groups than phenyl units are. Poly(silylene-1,4phenylenes) 103,104 [219], poly(silylene-2,5-thienylenes) 105,106 [220], and poly[5,5´(silylene)-2,2´-dithienylenes] 107,108 [221] containing these side groups should be suitable starting materials for functionalized and branched derivatives.

346

Wolfram Uhlig Me

Me

Si

Si S

Ph

n

OTf + H S

Ph

TfOH

(55) Me

Me

Si

Si

Ph

Ph

n Me

Me Me2N

Si

OTf + H

4 TfOH

NMe2

TfO

Si

OTf

R

R

BrMg

MgBr

Li

Li S

Li

S

Li S

Me

Me

Si

Si

R

n

103:R = NMe2 104:R = p-Anis

R

Me Si S

105:R = NMe2 106:R = C3H5

TfOH

Me

Si

Si n

OTf

109

Si

Si

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n

112:X = H 113:X = SiPhMe2 114:X = vinyl 115:X = C6H13

X

n

108:R = C3H5 TfOH

Si S

n

OTf

S

S

n

111

MX

Me

(56)

107:R = NMe2

110

Me

S

S

Me

MX

X

R

TfOH

Me

OTf

n

MX

Me Si S

116:X = H 117:X = vinyl (57)

nn

X 118:X = H

S

S

119: X= C

n CH

(58)

Scheme 23.

Normally, the amino-substituted compounds are used as starting materials because the by-product dimethyl-ammonium triflate, which is insoluble in common organic solvents, separates as a solid. Progress and end of the reaction may, therefore, be observed by the increase of the salt phase. As indicated in Scheme 23, eqs 56-58, the conversion of 103-108 with triflic acid leads to the triflate substituted derivatives 109-111. The products can be isolated as red-brown, hydrolytically sensitive, and viscous oils. They show relatively narrow signals in 29Si NMR spectra. The regular alternating arrangement of silicon atoms and

Polycarbosilanes

347

aromatic units can be verified by these spectra. Novel functionalized poly(silylene-1,4phenylenes) 112-115, poly(silylene-2,5-thienylenes) 116,117, and poly[5,5´-(silylene)-2,2´dithienylenes] 118,119 could be synthesized in high yields from 109-111 and nucleophiles under mild conditions [222]. The compounds were characterized by 29Si, 13C, and 1H NMR spectroscopy (Table 4). The molecular weights were found to be similar to those of the parent compounds. From this observation can be concluded that no backbone cleavages occur during the protodephenylation and substitution processes. Platinum-catalyzed hydrosilylation reactions between silicon-vinyl- and siliconhydrogen-substituted polymers give intermolecular branched derivatives. Unfortunately, these products are insoluble in common organic solvents and could not be characterized by spectroscopic methods. This should be caused by the high degree of cross-linking. Therefore, Uhlig attempted to “dilute” the reactive groups in the starting materials. Poly[5,5´-(silylene)2,2´-dithienylene] 121 was prepared with an alternating arrangement of Me2Si and MeSiOTf units by a stepwise chain construction [221] (Scheme 24). Nucleophilic substitutions of 121 with vinylmagnesium chloride and LiAlH4 respectively gave the partially functionalized polymers 122,123, which are suitable for cross-linking reactions. The platinum-catalyzed hydrosilylation was carried out in chlorobenzene by the method of Corriu [60]. Me Si S

S

S

Me

S

1. 2 BuLi 2. (Me2N)MeSi(OTf)2

Me

Me

Si

Si NMe2

S

2

Me

Me 2 TfOH

S

Me

2

Si

Si

n

OTf

S

2

120

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2

S

2

Si

Si S

Me

2

n

S

H

2

122

Me

123 H2PtCl6

Me

Me

Si

Si H2 C

S

S

124

Me Si

Si

Scheme 24.

S

S

Me

CH2

Me

n

Me

Me

Si S

2

LiAlH4

Me

Si

S

121 MgCl

Me

Me

S

S

Me

S

S

n

n

348

Wolfram Uhlig

As indicated in Scheme 24, the branched polymer 124 was obtained, which is soluble in chlorinated hydrocarbons and toluene. The NMR spectra are consistent with the βhydrosilylation product. Recently, another convenient approach to functional substituted organosilicon polymers containing dithienylene units was described [221,222]. The polymer chain was prepared by polycondensation of 5,5´-dilithio-2,2´-dithiophene with α,ω-bis(triflate)-substituted trisilanes containing a functional group in 2-position. The trisilanes were obtained by protodephenylation of the corresponding α,ω-diphenyltrisilanes with triflic acid as shown in Scheme 25. The substituents in 2-position may be vinyl groups or hydrogen atoms. The hydrogen-substituted derivative 125 can be used as the starting material for further modification reactions. Treatment of 125 with a catalytic amount of PdCl2 in refluxing carbon tetrachloride afforded the chloro derivative 126. The amino compound 127 can be prepared from 126 with an excess of dimethylamine. The triflate-substituted polymer 128 was obtained by protodeamination of 127 with triflic acid. These reactions are summarized in Scheme 25. Poly(silylene-2,5-thienylenes) and poly[5,5´-(silylene)-2,2´-dithienylenes] show strong absorption bands in the UV region in THF solution (Table 4). The absorption maxima appear between 240 and 266 nm for poly(silylene-2,5-thienylenes) and between 328 and 346 nm for poly[5,5´-(silylene)-2,2´-dithienylenes]. The maxima are significantly red-shifted relative to those of thiophene (230 nm) and of dithiophene (304 nm), indicating the presence of σ−π conjugation. However, the influence of the substitution patterns of the silicon atoms on the UV maximum is relatively low. Me H

Me

Me H

Me

Si

Si

2 TfOH

Si

Si

Si

TfO

Me Me Me

Si

OTf

Me Me Me a

Me Cl Si

Me

Si

b

Si S

Me Me Me

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c

2

Si

Me

Si

Si

Si

Me Me Me

n

126

Si S

2

n

Si

Si

Scheme 25.

Li 2

n

2

n

b: CCl4 / PdCl2

Si

Me Me Me

127

S

2

Me OTf Me d

Me Me Me

a: Li

S 125

Me2 Me N Me Si

Me H

S 128

c: 2 Me2NH

d: 2 TfOH

Polycarbosilanes

349

Table 4.29Si, 13C NMR, UV data, and molecular weights of poly(silylene-1,4-phenylenes) 103,104,109,112-115 poly(silylene-2,5-thienylenes) 105,106,110,116,117, and poly[5,5´(silylene)-2,2´-dithienylenes] 107,108,111,118-128 No

Formula

103 104

[Me(NMe2)Si−(C6H4)]n [Me(Anis)Si−(C6H4)]n

105 106

[Me(NMe2)Si−(C4H4S)]n [Me(All)Si−(C4H4S)]n

107 108

[Me(NMe2)Si−(C4H4S)2]n [Me(All)Si−(C4H4S)2]n

109 110 111 112

[MeSi(OTf)−(C6H4)]n [MeSi(OTf)−(C4H4S)]n [MeSi(OTf)−(C4H4S)2]n [MeSiH−(C6H4)]n

113

[MeSi(Si*PhMe2)−(C6H4)]n

114

[MeViSi−(C6H4)]n

115

[Me(C6H13)Si−(C6H4)]n

116

[MeSiH−(C4H4S)]n

117

[MeSi(Vi)−(C4H4S)]n

118

[MeSiH−(C4H4S)2]n

119

[MeSi(C≡CH)−(C4H4S)2]n

120

[Me(NMe2)Si*−(C4H4S)2− Me2Si−(C4H4S)2]n [Me(OTf)Si*−(C4H4S)2− Me2Si−(C4H4S)2]n [MeViSi*−(C4H4S)2−Me2Si− (C4H4S)2]n [MeHSi*−(C4H4S)2−Me2Si− (C4H4S)2]n [Me(−CH2)Si−(C4H4S)2− Me2Si−(C4H4S)2]n [MeSi*H−Me2Si−(C4H4S)2− Me2Si]n [MeSi*Cl−Me2Si−(C4H4S)2− Me2Si]n [MeSi*(NMe2)−Me2Si− (C4H4S)2−Me2Si]n [MeSi*(OTf)−Me2Si− (C4H4S)2−Me2Si]n

121

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122 123 124 125 126 127 128

Mw Mw:Mn 24800 1.9 18000 2.9 -

δ29Si ppm -8.4 -15.1

16400 2.4 21900 2.3 17800 2.0 24100 2.2 11700 2.8 15000 3.1 10800 2.6 16600 3.5 -

32.9 30.1 33.0 -19.0

18700 2.3 15000 2.4 44000 5.0 27000 2.2 -

-13.6 -17.2 -12.8 -15.6

-24.8; -21.9* -14.8 -8.1 -32.0 -22.1 -31.8 -30.1 -10.6* -15.1 35.9* -12.8 -21.9* -15.0 -33.7* -14.7 -14.0 br -56.6* -22.0 2.5* -23.8 -35.5* -20.9 28.9* -23.8

δ13C ppm -1.6 (MeSi); 30.5 (NMe2); 134.4; 136.9 (Ph) -3.6 (MeSi); 53.0 (OMe); 115.1; 121-139; 160.0 (Ph) -1.0 (MeSi); 31.7 (NMe2); 137.0; 142.0 (Th) -1.9 (MeSi); 24.6 (SiCH2); 118.0 (CH2=); 133.6 (CH); 137.5; 140.5 (Th) -0.9 (MeSi); 31.7 (NMe2); 126.7-142.7 (Th) -2.0 (MeSi); 25.5 (SiCH2); 116.6 (CH2=); 133.0 (CH); 124.8-141.5 (Th) 6.3 (MeSi); 117.9 (CF3); 133.0; 135.8 (Ph) 5.9 (MeSi); 118.2 (CF3); 136.9; 143.0 (Th) 6.3 (MeSi); 118.5 (CF3); 125.7-142.9 (Th) -4.8 (MeSi); 134.9; 136.1 (Ph)

λmax nm -

-4.5 (MeSi); -2.6 (Me2Si); 126.5-139.0 (Ph)

-

243 337 -

-3.8 (MeSi); 135.1; 136.6 (CH2=CH); 133.9;136.8 (Ph) -3.1 (MeSi); 12.5-28.4 (C6H13); 134.2; 138.5 (Ph) -2.8 (MeSi); 137.8140.7 (Th) 240 245 -2.0 (MeSi); 135.0; 135.9 (CH2=CH); 137.9;141.0 (Th) -3.1 (MeSi); 124.9; 133.9; 138.6; 141.7 (Th) 338 -2.1 (MeSi); 81.0 (SiC≡); 94.8 (≡CH); 125.8-142.4(Th) -1.5 br (MeSi,Me2Si); 30.9 (NMe2); 123.9144.0 (Th) -1.4 (Me2Si); 5.6 (MeSi); 117.1 (CF3);122.9144.1(Th) -1.2 br (MeSi,Me2Si); 134.4; 136.1 (CH2=CH); 123.5-141.7 (Th) -0.9 br (MeSi,Me2Si); 124.5-142.0 (Th)

-

-1.7 br (MeSi,Me2Si); 9.3 (SiCH2); 122.6142.3 (Th) -2.5 br (MeSi,Me2Si); 125.5; 134.9; 138.0; 142.8 (Th) 0.5 br (MeSi,Me2Si); 125.3; 134.7; 138.2; 142.4 (Th) -2.2 br (MeSi,Me2Si); 32.6 (NMe2); 124.0141.3 (Th) -1.9 (Me2Si); 4.4 (MeSi); 118.0 (CF3);124.8143.1(Th)

346

Vi = vinyl, All = allyl, Anis = para-anisyl, Th = thienyl.

342 338

339 -

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350

Wolfram Uhlig

In general, polymers composed of alternating silylene units and π-electron systems are insulators. However, when the polymers are treated with an oxidizing agent, e.g. SbF5 [35,223], NOBF4 [224], FeCl3 or I2 vapors [195], they become conducting. Preliminary doping studies showed, when cast films of selected polymers were exposed to iodine vapor under reduced pressure, that the conductivity of the films increased and reached almost constant values after 10 h of doping. The conductivities at this point were found to be 5.2·10-5 (106), 8.1·10-5 (116), 4.8·10-5 S·cm-1 (117) for poly[(silylene)-2,5-thienylenes] and 2.5·10-5 (118), 3.5·10-5 (119), 5.9·10-5 (124), 8.6·10-5 S·cm-1 (125) for poly[5,5´-(silylene)-2,2´dithienylenes]. Anionic ROP of 1-silacyclobut-2-ene and 1-sila-2,3-benzocyclobutene gives rise to poly(1-sila-cis-butene) 129 (Scheme 26, eq 59,60) and poly(2,3-benzo-l-silabutene) 130, respectively , both of which contain unsaturated carbon-carbon double bonds [225,226]. Weber and co-workers have studied the anionic ROP of various 1-silacyclopent-3-ene derivatives with n-BuLi / HMPA as the catalyst (Scheme 26, eq 61). This anionic polymerisation proceeds in a stereospecific manner to yield poly( 1-sila-cis-pent-3-enes) 131 [227,228]. The molecular weight of the resulting polymer depends on the substituents on the silicon atom in the unsaturated five-membered ring. The methyl and phenyl substituted silacyclopentenes give high molecular weight polymers (Mn > 104) [229]. On the other hand, when hydrogen is bonded to the silicon atom, low molecular weight polymers are obtained from the corresponding monomers [230]. It is believed that the anionic ROP of 1silacyclopent-3-enes proceeds through hypervalent, pentacoordinate, negatively charged, siliconate species. A series of high molecular weight poly(1-sila-cis-pent-3-enes) with mesogenic pendant groups were prepared by anionic polymerisation of the corresponding 1-Me- and 1-Phsilacyclopent-3-enes with 1-(3'-ary1oxy)propyl side chains [231]. Polymers containing oligo(oxyethylene) groups as side chains were also prepared by anionic ROP of the corresponding substituted silacyclopentenes [232]. These polymers dissolve LiClO4 and were suggested for potential use as polymer electrolytes. Acyclic diene metathesis (ADMET) polymerisation of diallylsilanes with a tungsten carbene catalyst is another efficient way of synthesising poly(sila-pent-3-ene) (Scheme 26, eq 62) [233]. Ethylene gas is continuously released during this polymerisation reaction which involves the metathesis of the terminal Si−CH2−CH=CH2 bonds to form a Si−CH2−CH=CH−CH2−Si linkage and ethylene. This reaction has been carried out with a series of diallylsilanes of the type, CH2=CHCH2−X−CH2CH=CH2, where X = SiMe2, SiMe2CH2CH2SiMe2 and SiMe2-p-C6H4SiMe2 to yield polymers varying in Mn from 1300 to 50000. The polymer 132 with X = SiMe2 (Mn = 12000) differs from the mostly cis-polymer 129 obtained by Weber [227,228] by ROP in that it has a high trans-content (81%). The preparation of chloro-substituted polymers of this type has also been carried out by using ADMET polymerisation [234,235]. These polymers were reported to undergo reactions with various amine and alkoxide nucleophiles to produce rubbery or glassy materials. Poly(silapentenes) having a benzene ring incorporated into the main chain, instead of a carbon-carbon double bond, have also been prepared [236]. For example, with the catalysis of n-BuLi and HMPA, ring opening polymerisation of 3,4-benzo-1-silacyclopentane and its methyl or phenyl substituted derivatives generated the corresponding poly(3,4-benzo-1silapentenes) 133 at low temperature (Scheme 26, eq 63).

Polycarbosilanes

351 R

R

BuLi

Si

R Si

(59) n

R 129 R SiR2

BuLi

R Si

HMPA

(60) n

130 R BuLi

Si R

Si

R

(61) n

R R = Me, Ph, H

131 Me [W]

Si Me

Me

- C2H4

Me Si

(62) n 132

R BuLi

Si R

R

R = Me, Ph, H

Si

HMPA

(63) n

R 133

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Scheme 26.

A polymer 134 that contains alternate 1,4-dibenzo and ethylene linking groups has been prepared by a hydropolymerisation process starting from a mixture of the dihydrogen and divinyl monomers [237,238] (Scheme 27, eq 64). The reaction proceeded quantitatively and regioselectively at room temperature in the presence of platinum catalyst to give the corresponding β-addition product. In contrast, a prior attempt to make these polymers by selfhydrosilylation polymerisation of the hydridohinyl monomer gave only low yields (ca. 30%) of a low molecular weight product [239]. Polymers 135 in which phenylenevinylene groups alternate with silylene have also been prepared by hydrosilylation polymerisation [240-242]. These polymers have been obtained both by copolymerisation of disilanes with dialkynes or by the self-hydrosilylation polymerisation of a monomer with a terminal Si–H and acetylene group and are of interest as potential blue light-emitting materials as well as ceramic precursors. For example, Kim and Shim have used o-, m-, and p-(dimethylsily1)phenylacetylene as monomers in chloroplatinic acid catalysed hydropolymerisation reactions (Scheme 27, eq 65) [241]. A related series of polymers 136 were obtained by the hydrosilylation polymerisation of R1R2SiH2 with α,ωdialkynylarenes (Scheme 27, eq 66) [242].

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352

Wolfram Uhlig

These yellow solids were soluble in common organic solvents and were comprised mainly of the β-hydrosilylation product, with x ranging from 0.82 - 0.96. The UV visible absorption spectra of these polymers was quite similar to that of a model compound, (E,E')1,4-bis(dimethylphenylsilylethenyl)benzene, suggesting that the Si atom inhibits conjugation and that the conjugation length is restricted to a single repeat unit. The phenylene-containing polymers with R = Me were found to emit in the blue region of the spectrum, whereas the biphenyl systems emitted in the violet. The fluorescence emission intensities of the biphenyl derivatives were much stronger than those of the phenylene, reflecting high quantum efficiency resulting from the biphenyl groups. An attempt to copolymerise 1,3- and 1,4-(diethyny1)benzene with bis(dimethylsilyl)acetylene by Pt-catalysed hydrosilylation has also been reported. The corresponding polymer in the form of short chain oligomers (n = 5-7) were isolated as yellow powders; however, few details were given regarding the properties of these materials in this report [244]. Linear polymers 137 with both acetylene and phenylene units in the backbone, along with silylenes, have also been reported [245,246] (Scheme 27, eq 67). These polymers were obtained by condensation reactions using di-Grignard regents and, in the case of the PhHSi derivative, by dehydrogenative coupling of the diacetylene and PhSiH3 with MgO as a catalyst. They have been of interest mainly as precursors to SiCx ceramics and as heat resistant, non-flammable materials. In particular, they were found to have very high 1000 °C char yields (88-97%). Moreover, they are moldable, nonflammable, soluble in many low boiling solvents and show very little reduction in modulus or bend strengths after heating in air to 400°C. Oligomers and copolymers which contain “silole” (cyclo-sila-penta-2,3-4,5-diene) rings were originally prepared by Tamao [246-249] and are reported to exhibit novel properties such as high electrical conductivity on doping as well as nonlinear optical properties. A related series of polymers that have silole units regularly alternating with acetylene and diacetylene groups were later reported by Barton [250] (Scheme 27, eq 68). These novel polymers 138, 139 were obtained by reaction of the dibromosilole monomer with bis(tributylstannylacetylene) and a series of comonomers having bis(trimethylstannylethyny1)groups. A polymer which is thought to have silole units, along with four-membered ring, structures, in its complex structure has been prepared by MCl5 (M = Mo, W) catalysed polymerisation of diethynylsilanes [251]. This n-conjugated, deep violet-coloured, polymer is reported to have excellent nonlinear optical properties and is easily processed, being soluble in a variety of solvents and stable in air [252]. Recently, Interrante described carbosilane polymers 140 with imbedded disilacyclobutane rings in the main chain. These polymers were prepared by means of acylic diene metathesis (ADMET) polymerization of the corresponding 1,3-dibutenyl-1,3disilacyclo-butanes (Scheme 27, eq 69). Subsequent hydrogenation of the double bonds with p-toluenesulfonhydrazide (TSH) resulted in a saturated hydrocarbon structure in the main chain 141 without the silacyclobutane ring [253]. Solid state 29Si NMR spectra indicate that cross-linking occurs during heating to 250°C via opening of the imbedded four-membered rings. Various groups have reported the synthesis of hyperbranched polycarbosilanes by using hydrosilylation on AB2- and AB3-type monomer units. Mulhaupt and Frey have prepared

Polycarbosilanes

353

hyperbranched [254,255] and dendritic [256] polycarbosilanes with various types of functional groups and have investigated their structures, physical properties and reactions. R

R

Si

Si

Me

Me

R

R

R

R

H

H

Si

Si

Si

Si

C

C

Me

Me

Me

Me

H

H

[Pt] H

H +

(64) n

134

Me

Me

Me

Si

Si

Si

Me

Me

[Pt] H

x

(65) 1-x

Me

135 Ph

R Ph

Si

R Si

[Pt] PhRSiH2 +

(66) m

R = Me, Ph

m

m

x

m = 1,2

1-x

136 Ph MgO Si

PhSiH3 +

(67)

- H2

n

H 137 Hex

Hex Si

Br

Br

+ Me3Sn

SnMe3

[Pd] Ph

[Pd]

Bu3Sn

SnBu3 Si

Hex

(68)

Ph

Ph

Ph

Hex

n

Hex

Hex 138

Si

n

139

Ph

Ph

Me

[Ru] Si

Me

TPA Si

Si

Si

Si Me

Me 140

Me Si

n TSH

(69) Me

n

141

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Scheme 27.

The general synthetic route that they have employed for the preparation of the hyperbranched polycarbosilanes 142 is illustrated in Scheme 28, eq 70. Muzafarov and coworkers described syntheses and properties of carbosilane liquid crystalline dendrimeres [257-263]. The structure of such polymers were also studied by Interrante [264]. Investigations in the field of polycarbosilane-based liquid crystals were summarized by Ganicz and Stanczyk [265]. In addition to the AB3 monomer, triallylsilane, diallylpropylsilane and allylpropylsilane were also employed as AB2 monomers in this hydrosilylation reaction, which can either start with the monomer alone or with a multifunctional 'core' molecule, Bf ,where f is the functionality of the core. In a generalised computer simulation study of the effect of the core functionality on the molecular weight and polydispersity of the product, it was shown that this 'core-dilution / slow addition' technique is capable of preparing hyperbranched polymers in a

354

Wolfram Uhlig

controlled manner, providing the coupling reaction, Bf + ABm to Bf+m-1 (m = 2,3; f = 2-12) is rapid and quantitative [266]. Under these conditions, this synthetic strategy presumably allows one to control molecular weights, lower polydispersity, and enhance the degree of branching of the resultant hyperbranched polymers. A method for enhancing the degree of branching for hyperbranched polymers has also been advanced by this group [256]. The synthesis of hyperbranched poly(carbosilaneary1enes) 143 have been reported by Yoon and Son [267] by hydrosilylation of AB3 monomers with –SiMe2H (A) and – Si(CH=CH2)3 (B3) terminal groups (Scheme 28, eq 71). They have also reported the synthesis of a hyperbranched carbosilane that contains both double bonds and Si-H groups [268]. Reaction of propargyl chloride with methyldichlorosilane led to an mixture of the α- and βhydrosilylation products. Treatment of this monomer mixture with magnesium resulted in the formation of a hyperbranched oligomer mixture, which was then treated with LiAlH4 to give the final product that was reported to have an average degree of polymerisation of 8. This material gave a 24% char yield on pyrolysis but after crosslinking with a Pt catalyst this increased to 69%.

Si

Si Si

[Pt] Bf + x

H

Si

Si

Bf-1

Si

Si

Si

(70)

Si Si

142

Si

Me2Si Me

Me

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[Pt] H

Si

Si

H

Si

Si

(71)

Me

Me

Me2Si

Si

Me hyperbranched polymer 143

H

Si Me

Si Me2Si Si

Scheme 28.

Polycarbosilanes

355

Uhlig prepared organosilicon polymers containing SiH2 groups using α,ωsilylbis(triflates) as electrophilic starting materials. The dinucleophilic reactants were organometallic compounds. The reactions of the silyl triflates with 1,4-BrMg–C6H4–MgBr, Li2C2, Li2C4, and Ph2Si(OH)2 / NEt3 illustrate the potential of this method. Co-condensations of two electrophiles with four dinucleophiles gave 8 different structured polymers 144-151 [173,175]. The compounds were obtained at low temperatures, in short reaction times, and with high yields (Scheme 29). H Si

H

H Y

H

Si

C

Si

C

H

H Y

C

C

146,147 LiC

CLi

BrMg

Si

H Y

Y

Si

OTf

H

H

MgBr

Li2C4

H

H TfO

H

C n

144,145

Si

C

H

H

n

Si

Ph2Si(OH)2 / 2 NEt3

H

H

Si

Si

H

H

n

148,149

Ph

H Y

Si H

O

Si

O

Ph

n

150,151 Y=

C

C

;

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Scheme 29.

The structural characterization of 144-151 was mainly based on NMR spectroscopy. The narrow 29Si MNR signals indicate the regular alternating arrangement of the building blocks in the polymer backbone resulting from the fact that the condensation reactions are not accompanied by exchange processes analogous to metal halogen exchange. The 13C NMR data also support this view. Thus, only isolated ethynyl groups are detectable in the spectra of 144 and 145. Diethynyl units –C≡C–C≡C–, which are produced in variable yields (5-15%) by the reaction of Li2C2 with dihalogenosilanes, are not observed. Weight-average molecular weights in the range of Mw = 10000 - 20000, relative to polystyrene standards, were found (PD: 2.3-3.1). It must be emphasized, that the molecular weights are determined by the reaction conditions. Higher values of Mw were obtained using more concentrated solutions of the reactants. The exact compliance with the stoichiometric ratio of 1:1 is another important requirement. It is therefore necessary to determine the content of the organometallic compounds quantitatively before use. However, molecular weight can also fall below 5000 when diluted solutions are used.

356

Wolfram Uhlig

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3.4. Polycarbosilanes with (–Six–C–) Units (x > 1) in the Main Chain It is also possible to make polycarbosilanes in which two silicon atoms are alternately connected with one, or more carbons in the backbone. The simplest example of a polymer of this type is 152 which has a Si–Si–CH2 repeat unit and was prepared by the homopolycondensation of 2,4-dichloro-2,4-disilapentane (Scheme 30, eq 72). This material is also of interest as a SiC ceramic precursor. Direct pyrolysis of this oligomer gave a 14% ceramic yield. After heat treatment, the ceramic yield can be increased to 79% and the obtained SiC ceramic is reported to contain relatively little free carbon [269,270]. Polycarbosilanes having a monomeric unit that consists of two silicon and two carbon atoms, [Si–Si–C–C], have been known for a long time, as the products of ROP of 1,2disilacyclobutanes [271,272]. 1,2-disilacyclobutanes are so reactive that they undergo spontaneous ring opening polymerisation at room temperature via Si–Si bond cleavage, which results in high molecular weight polymers 153 and 154 (Scheme 30, eqs 73,74). A copolymer of 1,1,2,2-tetramethyl-1,2-disila-3,4-benzocyclobutane with styrene has also been obtained by warming to room temperature a frozen mixture of these two monomers [273]. Electronically, silicon-silicon single bonds have many properties that are analogous to those of carbon-carbon double bonds. Therefore, polymers containing a regular alternation of silicon-silicon and unsaturated units have also been of interest as conducting polymers. For example, Wurtz coupling of E-1,2-bis-(phenylmethyl-chlorosilyl)ethylene generates copoly(1,2-dimethyl-1,2-diphenyldisilyleneethenylene) 155 (Scheme 30, eq 75) [251], which has an electronically delocalised backbone that is analogous to that of poly(acetylene). Films of this copolymeric carbosilane can be doped with SbF5 to yield highly conducting materials. A linear polycarbosilane 156 with a [Si–Si–C–C–C–] backbone was obtained by ROP of 1,1,2,2-tetramethyl-1,2-disilacyclopentane (Scheme 31, eq 76). A palladium complex was found to be an effective catalyst for this polymerisation, yielding a relatively high molecular weight polymer [274]. Anionic ROP of 1,2,2-trimethyl-1-phenyl-1,2-disilacyclopentane was also achieved by using n-Bu4NF, PhMe2SiK, and (PhMe2Si)2Cu(CN)Li2 as catalysts [275]. The polymerisation initiated with (PhMe2Si)2Cu(CN)Li2 showed some evidence for a living character. Polycarbosilanes in which a disilene alternates with four or even more carbon atoms in the backbone have also prepared. For example, compound 157, a carbosilane electronically analogous to poly(1,4-phenylenevinylene) (Scheme 31, eq 77), can be prepared from 1,4bis(phenylethylchlorosilyl)benzene by means of a Wurtz coupling reaction [276]. A related series of poly[(disilanylene)phenylenes] that contain up to four sequential m-phenylene units in the polymer main chain were reported by Oshita [197,277]. These polymers included various combinations of Me, Et and Ph substituents on the silicons and 1,4-linked pphenylene groups. They have been examined with regard to their UV absorption spectra and photochemistry [197] as well as their thermal stability [277]. Polymers in which diacetylene units alternate with disilylenes were reported initially by Ishikawa [34] and Corriu [32,66,278]. In both cases the polymers 158 were produced by condensation of dithiobutadiyne with dichlorodisilanes. A more direct route to dilithiobutadiyne from the starting hexachlorobutadiene was subsequently reported by Barton [33]; this involves the use of 4 equivalents of BuLi in THF (Scheme 31, eq 78). Ishikawa prepared both the R = Et and R = Ph derivatives whereas Barton apparently obtained the

Polycarbosilanes

357

permethylated polymer. The R=Ph polymer was reported to become strongly conducting when spun films were exposed to SbF5 vapor [279]. Me Cl

Me

H

Si

C

Si

H

H

H

Cl

Na

Me

Me

H

Si

Si

C

H

H

H

(72) n

152

Me

Si

Si

Me

Me

Me

Me

Me

H

H

Si

Si

C

C

H

H

H

H

(73) n

153

Me

Si

Si

Me

Me

Me

Ph

Me Si

Cl

Ph

Me

Si

Si

Me

Me

(74) n

154

Cl Si

Me

Me

Na

Me

Me

Si

Si

Ph

Ph

(75) n

155

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Scheme 30.

Yuan and West have reported a series of polycarbosilanes 159 with disilylene units, carbon-carbon triple bonds, and phenylene units in the polymer's backbone [280]. These polymers were prepared by Pd catalysed coupling reaction of 1,4-diiodobenzene and 1,2diethynyldisilanes (Scheme 31, eq 79). As with the acetylene and diacetylene-bridged polysilylenes described above, the polymers of this type, with both acetylene and phenylene units in the main chain, have been of interest as electronically conductive materials. Thus, when doped with iodine or FeCl3 these polymers become semiconducting [280]. West has also reported the synthesis of a series of alkoxy-substituted phenylene-ethynylene bridged disilylene polymers analogous to those in eq 79 [281]. Oshita and Kunai have reported an anionic ring opening polymerisation (ROP) route to silole-containing polymers 160 that contain 3,4-linked diethynylenesilole groups alternating with disilylenes (Scheme 31, eq 80) (TBAF= tetrabutylammonium fluoride) [191]. These polymers were obtained with high molecular weights in high yields; however, they were observed to exhibit blue-shifted UV absorptions relative to those of poly[disilanylene(2,5-diethynylenesiloles)] prepared by Pd-catalysed dehydrobromination of 2,5-dibromotetraphenylsilole and 1,2-diethynyldisilanes [282]. This was attributed to the

358

Wolfram Uhlig

interruption of π-conjugation between carbons at 3,4-positions in the silole rings for the former polymers. Reaction of these poly[disilanylene(3,4-diethynylenesiloles)] with Fe(CO)5 under UV irradiation led to Fe(CO)3-coordinated silole units in the polymer backbone [283]. These polymers exhibit slightly red-shifted UV absorption bands relative to the parent uncoordinated polymers and, upon doping with FeCl3 show higher electrical conductivities.

Si

Me

[Pd]

Si

Me

Me

Me

Me

Me

H

H

H

Si

Si

C

C

C

Me

Me

H

H

H

(76) n

156

Cl

Me

Me

Si

Si

Ph

Ph

Cl

Na

Me

Me

Si

Si

Me

Me

(77) n

157

Cl

R

R

Si

Si

R

R

Li2C4

Cl

R

R

Si

Si

R

R

R = Me, Ph R

R

Si

Si

R

R

R Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Si

C

C

C n (78)

158

+

R = Me, Ph, Bu

R

C

R Si

I

I

R

R

Si

Si

R

R

C

[Pd], CuI, NEt3

C

C

C n (79)

R 159 TBAF

R

R

Si

Si

R

R

C

C

C

C n (80)

Me3Si

Si Me2

SiMe3

Me3Si

Si Me2 160

Scheme 31.

SiMe3

Polycarbosilanes

359

Uhlig reported on the synthesis of numerous new polycarbosilanes containing disilylene units. The starting α,ω-bis(trifluoromethylsulfonyloxy)-substituted derivatives of oligosilanes were obtained from the corresponding amino compounds. The organometallic dinucleophiles 1,2-dilithioethyne Li2C2, dilithiobutadiyne Li2C4 and 2,5-dilithiothienylene Li(C4H2S)Li are converted with the triflate derivatives of disilane (Scheme 32, eq 81) [173-175], tetrasilane (Scheme 32, eq 82) [284], 1,2-bis(disilyl)-ethyne (Scheme 33, eq 83) [285] and 1,4bis(disilyl)benzene (Scheme 33, eq 84) [286]. The reduction of silylbis(triflates) with potassium graphite C8K is also described. 29Si NMR data of the obtained polymers 161-171 are summarized in Table 5. Narrow NMR signals indicate the regular alternating arrangement of the building blocks in the polymer backbone.Weight-average molecular weights in the range of Mw = 8000-16000 were found. It must be emphasized that the molecular weights are strongly determined by the reaction conditions. Higher values of Mw were found using more concentrated solutions of the reactants. The exact compliance with the stoichiometric ratio of 1:1 is another important requirement. Obviously, the synthetic method via silyl triflates may currently appear to be too expensive for technical application. However, the essential advantage of silyl triflate chemistry consists in the possibility of preparing, with relatively little efforts, small amounts of numerous differently structured polycarbosilanes. H

H

Si

Si

H

H

C

C

n

H

H

Si

Si

H

H

C

C

C

C n

161

162

LiC

CLi

Li2C4 H

H

Si

Si

H

H

TfO MgBr

BrMg

(81)

OTf

Ph2Si(OH)2 / 2 NEt3 Ph

H

H

H

H

Si

Si

Si

Si

H

H

H

H

n

Si

O

Ph

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163

Ph

H

H

Ph

Si

Si

Si

Si

Ph

H

H

- 2 LiOTf

H

Ph

Si

Si

H

Ph

OTf

C

C

Ph

H

Si

Si

Ph

H

n (82)

165

Ph Li

S

- 2 LiOTf

Scheme 32.

n 164

Li2C2

TfO

O

Li

H

Ph

Si

Si

H

Ph

S 166

Ph

H

Si

Si

Ph

H

n

360

Wolfram Uhlig Ph

H

Si

Si

Ph

H

H

Ph

Si

Si

H

Ph

Li2C2

TfO

Ph

H

Si

Si

Ph

H

C

C

H

Ph

Si

Si

H

Ph

- 2 LiOTf OTf

2 C 8K - 2 KOTf - graphite

Li

S

Li

- 2 LiOTf

C

C

C

C n

167 Ph

H

Si

Si

Ph

H

C

C 168

Ph

H

Si

Si

Ph

H

C

C

H

Ph

Si

Si

H

Ph

(83)

H

Ph

Si

Si

H

Ph

n

S n

169

Li2C2

TfO

Ph

H

H

Ph

Si

Si

Si

Si

Ph

H

H

Ph

- 2 LiOTf OTf

Ph

H

H

Ph

Si

Si

Si

Si

Ph

H

H

Ph

C n

170

2 C8 K

- 2 KOTf - graphite

C

(84)

Ph

H

H

Ph

Si

Si

Si

Si

Ph

H

H

Ph

171

n

Scheme 33.

Table 5.29Si NMR data and molecular weights of polymers 161-171

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No 161 162 163 165 166 167 168 169 170 171

Formula [SiH2−SiH2−C≡C−]n [SiH2−SiH2−C≡C−C≡C−]n [SiH2−SiH2−(C6H4)−]n [SiH2−Si*Ph2−C≡C−Si*Ph2−SiH2−]n [SiH2−Si*Ph2−(C4H4S)−Si*Ph2−SiH2−]n [SiH2−Si*Ph2−C≡C−Si*Ph2−SiH2− C≡C−]n [Si*Ph2−SiH2−C≡C−SiH2−Si*Ph2−]n [Si*Ph2−SiH2−C≡C−SiH2−Si*Ph2−(C4H4S)−]n [SiH2−Si*Ph2−C≡C−Si*Ph2−SiH2−(C6H4)−]n [Si*Ph2−SiH2−(C6H4)−SiH2−Si*Ph2−]n

Mw 11500 14600 14300 8300 12400 15600 10400 13900 7500 8900

Mw:Mn 2.6 2.7 2.8 2.5 2.8 2.7 2.9 2.6 1.9 2.1

δ29Si (ppm) -76.8 -74.5 -61.5 -38.2*; -82.0 -21.3*; -82.4 -36.5*; -79.0 -37.2*; -81.5 -20.8*; -78.9 -35.9*, -64.3 -37.0*; -67.1

Linear polymers 172 having more than two silicon atoms in a chain interrupted by methylene units are also known [287] and have been of interest as heterostructured σconjugated systems. The structure and properties of these polymers are arguably closer to polysilanes than polycarbosilanes. Polymers that contain both –CmHn– and –O– bridging groups, and therefore can be considered as polycarbosiloxanes, are also well known [288293]. However, the prototypical copolymer, having a regular –Si–O–Si–C– main chain was unknown for a long time. Recently, Interrante was successful in the synthesis of this 1:1 alternating copolymer 173 by cationic ROP [294]. Polymers 174, 175 (Scheme 34) containing silicon and transition metals are attracting growing attention as a result of their interesting

Polycarbosilanes

361

physical and chemical properties [295-301]. But this types of polymers will not be discussed in detail here. Me

Me

Me

Me

H

Me

H

Me

Si

Si

Si

Si

C

Si

C

Si

Me

Me

Me

Me

H

Me

H

Me

n

172

Fe

O n

173 Me

R

Si

Si

Me

Cr n

174

R n

175

Scheme 34.

REFERENCES [1] [2] [3]

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[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

Heteroatom Ring Systems and Polymers; Allcock, H.R., Ed.; Academic Press: New York: 1967. Inorganic Polymers; Mark, J.E.; Allcock, H.R.; West, R., Eds.; Prentice Hall: Englewood Cliffs, NJ, 1992. Inorganic and Organometallic Polymers, ACS Symp. Ser. Vol. 360; Zeldin, M.; Waine, K.J.; Allcock, H.R., Eds.; American Chemical Society: Washington, DC, 1988. Peukert, M.; Vaahs, T.; Brück, M. Adv. Mater. 1990 ;2, 398. Contemporary Polymer Chemistry, 2nd ed.; Lampe, F.W., Eds.; Academic Press: San Diego, 1990, chapter 9. Allcock, H.R. Adv. Mater.; 1994, 6, 106. Kagawa, T.; Fujino, M.; Tageda K.; Matsumoto N. Solid State Commun. 1985, 5, 635. Kepler, R.G.; Zeigler, J.M.; Harrah, L.A.; Kultz, S. R. Phys. Rev. B 1985, 35, 2818. Fujino, M. Chem. Phys. Lett. 1987, 136, 451. Kajzar, F.; Messier, J.; Rosolio, C. J. Appl. Phys.; 1986, 60, 3040. West R. In Ultrastructure Processing of Ceramics, Glasses, and Composites, Vol. 1; Hench, L.L.; Ulrich D.R., Eds.;. Wiley Interscience: New York, 1984, pp 235. Sinclair, R.A. In Ultrastructure Processing of Ceramics, Glasses, and Composites, Vol. 1; Hench, L.L.; Ulrich D.R., Eds.;. Wiley Interscience: New York, 1984, pp 256 Birot, M.; Pillot, J.P.; Dunoguès, J. Chem. Rev. 1995 , 95, 1443. Corriu, R.J.P.; Leclercq, D.; Mutin, P.H.; Planeix, J.M.; Vioux, A. Organometallics 1993, 12, 454.

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[15] Jansen, M.; Baldus H.P. Angew. Chem. Int. Ed. Engl. 1997, 36, 328. [16] Gastreich, M.; Marian, C.M.; Jüngermann, H.; Jansen, M. Eur. J. Inorg. Chem. 1999, 75. [17] Riedel, R.; Kienzle, A.; Dressler, W.; Ruwisch, L.; Bill, J.; Aldinger, F. Nature 1996, 382, 796. [18] Riedel, R.; Kroll, P. Chem. Mater. 1998, 10, 2964. [19] Wideman, T.; Fazen, P.J.; Su, K.; Remsen, E.E.; Zank, G.A.; Sneddon, L.G. Appl. Organometal. Chem. 1998, 12, 681. [20] Kendrick, T.C.; Parbhoo, B.; White, J. In The Chemistry of Organic Silicon Compounds, Patai, S.; Rappoport, Z., Eds.; Wiley Interscience: New York, 1989, pp 1289. [21] Interrante, L.V.; Liu, Q.; Rushkin, I.R.; Shen, Q. J. Organometal. Chem. 1996, 521, 1. [22] Interrante, L.V.; Shen, Q. In Silicon-Containing Polymers, Jones, R.G.; Ando, W.; Chojnowski, J., Eds.; Kluwer Academic: Boston, 2000, pp 244. [23] Uhlig, W. Prog. Polym. Sci., 2002, 27, 255. [24] Dando, N.R.; Perrotta, A.J.; Strohmann, C.; Stewart, R.M.; Seyferth, D. Chem. Mater. 1993, 5, 1624. [25] Miller, R.D.; Michl, J. Chem. Rev. 1989, 89, 1359. [26] Richter, R.; Roewer, G.; Böhme, U.; Busch, K.; Babonneau, F.; Martin, H.P.; Müller, E. Appl. Organometal. Chem. 1997, 11, 71. [27] West, R. In The Chemistry of Organic Silicon Compounds, Patai, S.; Rappoport, Z., Eds.; Wiley Interscience: New York, 1989, pp 1207. [28] Bianconi, P.A.; Schilling, F.C.; Weidman, T.W. Macromolecules 1989, 22, 1697. [29] Ijadi-Maghsoodi, S.; Pang, Y.; Barton, T.J. Macromolecules 1993, 26, 5671. [30] Ijadi-Maghsoodi, S.; Pang, Y.; Barton, T.J. J. Polym. Sci. Part A: Polym. Chem. 1990, 28, 955. [31] Ishikawa, M.; Hasegawa, Y.; Kunai, A.; Yamanaka, T.; Organometallics 1989, 8, 2741. [32] Brefort, J.L.; Corriu, R.J.P.; Gerbier, P.; Guerin, C.; Henner, B.; Jean, A.; Kuhlmann, T.; Garnier, F.; Yasser, A. Organometallics 1992, 11, 2500. [33] Ijadi-Maghsoodi, S.; Barton, T.J. Macromolecules 1990, 23, 4485. [34] Nate, K.; Ishikawa, M.; Ni, H.; Watanabe, H.; Saheki, Y. Organometallics 1987, 6, 1673. [35] Ohshita, J.; Kanaya, D.; Ishikawa M. Appl. Organometal. Chem. 1993, 7, 269. [36] Hong, H.H.; Weber, W.P. Polym. Bull. 1989, 22, 363. [37] van Aefferden, B.; Habel, W.; Sartori, P. Chemiker-Ztg. 1989, 113, 169. [38] Bacque, E.; Pillot, J.P.; Birot, M.; Dunoguès J. Macromolecules 1988, 21, 30. [39] Seyferth, D.; Lang, H. Organometallics 1991, 10, 551. [40] van Aefferden, B.; Habel, W.; Sartori, P. Chemiker-Ztg. 1990, 114, 367. [41] Whitmarsh, C.K.; Interrante, L. Organometallics 1991, 10, 1336. [42] Weber, W.P. Trends Polym. Sci. 1993, 11, 356 [43] Seyferth, D. In Inorganic and Organometallic Polymers, ACS Symp. Ser. Vol. 360; Zeldin, M.; Waine, K.J.; Allcock, H.R., Eds.; American Chemical Society: Washington, DC, 1988, pp 21. [44] Fritz, G. Angew. Chem. Int. Ed. Engl. 1987, 26, 1111. [45] Laine, R.M.; Babonneau, F. Chem. Mater. 1993, 5, 260. [46] Yajima, S.; Omori, M.; Okamura,K. Nature 1976, 260. 683.

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[47] Yajima, S.; Okamura, K.; Hayashi, J.; Omori, M.; Hasegawa, Y.; Limura, M. J. Mater. Sci. 1978, 13, 2569. [48] Froehling, P. E. J. lnorg. Organomet. Polym. 1993, 3. 251. [49] Fry, B. E.; Guo, A.; Neckers, D. C. J Organomet. Chem. 1997, 538, 151. [50] Bacque, E.; Pillot, J.P.; Birot, M.; Dunoguès J. Macromolecules 1988, 21, 34. [51] Wu, H.J.; Interrante, L. V. Chem. Mater 1989, 1, 564. [52] Shen, Q.; Interante, L. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 3193. [53] Whitmarsh C. K.; Interrante L. V. US Patent, No 5,153.295, 1992. [54] Wu, H.J.; Interrante, L. V. Macromolecules 1992, 25, 1840. [55] Liu, Q.; Wu, H. J.; Lewis, R.; Maciel, G. E.; Interrante, L. V. Chem. Mater.1999, 11, 2038. [56] Interrante, L.V.; Wu, H. J.; Apple, T.; Shen, Q.; Smith, K. J. Am. Chem. Soc. 1994, 116, 12086. [57] Yuan, C. H.; Fujino, M.; Ebuta, K.; Furukawa, K. Macromolecus 1997, 30, 7618. [58] Uhlig, W. Z Naturforsch. B. Chem. So. 1999, 54(b), 270. [59] Uhlig, W. J. Prakt. Chem. 2000, 342, 132. [60] Boury, B.; Corriu, R. J. P.; Leclercq, D.; Mutin, P.H.; Planeix, J. M.; Vioux, A. Organometallics 1991, 10, 1457. [61] Boury, B.; Corriu, R. J. P.; Douglas, W. Chem Mater. 1991, 3, 487. [62] Corriu, R. J. P. Angew. Chem. Int. Ed. Eng. 2000, 39, 1376. [63] Harrod, J. F. In Inorganic and Organometallic Polymers, ACS Symp. Ser. Vol. 360; Zeldin, M.; Waine, K.J.; Allcock, H.R., Eds.; American Chemical Society: Washington, DC, 1988, pp 89. [64] Mu, Y.; Harrod, J. F. In Inorganic and Organometallic Oligomers and Polymers; Harrod, J. F.; Laine, R. M., Eds.; Kluwer, Netherlands, 1991, pp 23. [65] Boury, B.; Carpenter, L.; Corriu, R. J. P. Angew. Chem. Int. Ed. Eng. 1990, 29, 785. [66] Corriu, R. J. P.; Gerbier, P.; Guerin, C.; Henner, B. J. L.; Jean, A.; Mutin P. H. Organometallics 1992, 11, 2507. [67] Masnovi, J.; Bu, X. Y.; Conroy, P.; Andrist, H. A.; Hurwitz, F. I.; Miller, D. Mater. Res. Soc. Symp. Proc. 1990, 180, 779. [68] Masnovi, J.; Bu, X. Y.; Beyene, K.; Heimann, P.; Kacik, T.; Andrist, H. A.; Hurwitz, F. I. Mater. Res. Soc. Symp. Proc. 1992, 271, 771. [69] Hunvitz, F. I.; Kacik, T. A.; Bu, X. Y.; Masnovi, J.; Heimann, P. J.; Beyene, K. J. Mater. Sci. 1995, 30, 3130. [70] Unal, O.; Anderson, I. E.; Nosrati, M.; Maghsoodi, S. I.; Barton T. J.; Laabs, F. C. Ceram. Trans. 1997, 77, 185. [71] Barton, T. J. In Frontiers of Organosilicon Chemistry; Bassingdale, A. R.; Gaspar, P. P.; Eds.; Royal Soc. Chem., Cambridge, 1991, pp.3. [72] Kim, D. S.; Suh, M. C.; Shim, S. C. J. Polym. Sci. Part A: Polym. Chem. 1998, 36, 2275. [73] Jansen, M.; Jaeschke, T.; Jaeschke, B. In High Performance Non-Oxide Ceramics I; Jansen, M., Ed.; Springer-Verlag: Berlin, Heidelberg, 2002; Vol. 101, pp 137. [74] Seifert, H. J.; Aldinger, F. In High Performance Non-Oxide Ceramics I; Jansen, M., Ed.; Springer-Verlag: Berlin, Heidelberg, 2002; Vol. 101, pp 1. [75] Weinmann, M.; Schuhmacher, J.; Kummer, H.; Prinz, S.; Peng, J.; Seifert, H. J.; Christ, M.; Müller, K.; Bill, J.; Aldinger, F. Chem. Mater. 2000, 12, 632.

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364

Wolfram Uhlig

[76] Weinmann, M.; Hörz, M.; Müller, A.; Aldinger, F. In Organosilicon Chemistry VI; Auner, N.; Weis, J.; Eds.; WILEY-VCH Verlag, Weinheim, 2005, pp 987. [77] Haberecht, J.; Krumeich, F.; Grützmacher, H.; Nesper, R. Chem. Mater. 2004, 16, 418. [78] Haberecht, J.; Nesper, R.; Grützmacher, H. Chem. Mater. 2005, 17, 2340. [79] Haberecht, J.; Krummland, A.; Breher, F.; Gebhardt, B.; Rüegger, H.; Nesper, R.; Grützmacher, H. Dalton Trans. 2003, 11, 2126. [80] Nesper, R.; Haberecht, J.; Grützmacher, H., patent CH-0149/03, pending 2003. [81] Haberecht, J.; Krumeich, F.; Hameter, K.; Günther, D.; Nesper, R. In Organosilicon Chemistry VI; Auner, N.; Weis, J.; Eds.; WILEY-VCH Verlag, Weinheim, 2005, pp 981. [82] Goodwin, J.T. US. Patent, 2,607,791, 1952. [83] Knoth, W. H.; U.S. Patent, 2,850,514, 1958; Chem. Abstr. 1959, 53, 4166a. [84] Weyenberg, D. H.; Nelson, L. E. J. Org. Chem. 1965, 30, 2618. [85] Nametkin, N. S.; Vdovin, V. M.; Zav'yalov, V. I. Vysokomolekul. Soedin. 1965, 7, 757. [86] Nametkin, N. S.; Vdovin, V. M. Izv. Akad. Nauk SSSR, Ser. Khim. 1974, 1153. [87] Kriner, W. A. J. Polym. Sci. 1966, A-1, 4, 444. [88] Bamford, W. R.; Lovie, J. C.; Watt, J. A. J. Chem. Soc. (C) 1966, 1137. [89] Levin, G.; Carmichael, J. B. J. Polym. Sci. 1968, A-1 ,6, 1. [90] Auner, N.; Grobe, J. J. Organomet. Chem. 1980, 188, 151. [91] Ko, J. H.; Mark, I. E. Macromolecules 1975, 8, 869, 874. [92] Shen, Q.; Interrante, L. V. Macromolecules 1996, 29, 5788. [93] Tsao, M. W.; Pfeifer, K. H.; Rabolt, J. F.; Holt, D. B.; Farmer, B. L.; Interrante, L. V.; Shen, Q. Macromolecules 1996, 29,1130. [94] Interrante, L. V.; Rushkin, I.; Shen, Q. Appl. Organomet. Chem. 1988, 12, 695. [95] Koopmann, F.; Frey, H. Macromol. Rapid Commun. 1995, 16, 363. [96] Koopmann, F.; Frey, H. Macromolecules 1996, 29, 3701. [97] Hayakawa, K.; Tachikawa, M.; Suzuki, T.; Choi, N.; Murakami, M. Tetrahedron Lett. 1995, 36, 3181. [98] Ogawa, T.; Murakami, M. Chem. Mater. 1996, 8, 1260. [99] Koopmann, F.; Frey, H. Acta Polym. 1996, 47(9), 377. [100] Koopmann, F.; Frey, H. Macromolecules 1996, 23, 1490. [101] Koopmann, F.; Frey, H. Macromol. Chem. Phys. 1998, 199, 2119. [102] Kuroki, S.; Nanba, J.; Ando, I.; Ogawa, T.; Murakami, M. Polymer J. 1999, 31, 369. [103] Ogawa, T.; Tachikawa, M.; Kushibiki, N.; Murakami, M. J. Polym. Sci., Part A, Polym. Chem. 1995, 33, 2821. [104] Ogawa, T.; Murakami, M. J. Polym. Sci., Polym. Phys. 1996, 34, 1317. [105] Suzuki, M.; Nakata, Y.; Nagai, H.; Okutani, T.; Kushibiki, N.; Murakami, M. Mater. Sci. Eng. 1997, B49, 171. [106] Ogawa, T. Polymer, 1998, 39, 2715. [107] Ogawa, T.; Murakami, M. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 399. [108] Rushkin, I.; Interrante, L. V. Macromolecules 1995, 28, 5160. [109] I Smith, US Patent No 4,631,179, 1986. [110] Lienhard, M.; Rushkin, I.; Apple, T.; Interrante, L. V. J. Am. Chem. Soc. 1997, 119, 12020. [111] Lienhard, M.; Wiegand, C.; Apple, T.; Farmer, B.; Interrante, L. V. Polym. Prepr. 1998, 39(2), 808.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polycarbosilanes

365

[112] Rushkin, I.; Interrante, L. V. Macromolecules 1996, 29, 3123. [113] Rushkin, I.; Interrante, L. V. Macromolecules 1996, 29, 5784. [114] Mucha, M.; Ganicz, T. J. Thermal. Anal. 1996, 46, 795. [115] Ganicz, T.; Stanczyk, W.; Bialecka-Florjanczyk, E.; Sledzinska, I. Polymer 1996, 37, 4167. [116] Florjanczyk, E. B.; Ganicz, T.; Pluta M.; Stanczyk, W. J. Organomet. Chem. 1993, 444, C9. [117] Habel, W.; Oelschlager, A.; Sartori, P. J. Organomet. Chem. 1993, 463, 47. [118] Habel, W.; Mayer, L.; Sartori, P. J. Organomet. Chem. 1994, 474, 63. [119] Habel, W.; Oelschlager, A.; Sartori, P. J. Organomet. Chem. 1995, 486, 267. [120] Uhlig, W. J. Organomet. Chem. 1997, 545, 281. [121] Uhlig, W. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 725. [122] Bassindale, A. R.; Stout, T. J. Organometal. Chem. 1984, 271, C1. [123] Matyjaszewski, K.; Chen, Y. L. J. Organometal. Chem. 1988, 340, 7. [124] Ruehl, K. E.; Matyjaszewski, K. J. Organometal. Chem. 1991, 410, 1. [125] Rüdinger, C.; Beruda, H.; Schmidbaur, H. Chem. Ber. 1992, 125, 1401. [126] Rüdinger, C.; Beruda, H.; Schmidbaur, H. Z. Naturforsch. 1994, 49b, 1348. [127] Uhlig, W. Chem. Ber. 1996, 129, 733. [128] Uhlig, W.; Tzschach, A. J. Organometal. Chem. 1989, 378, C1. [129] Uhlig, W. Chem. Ber. 1992, 125, 47. [130] Uhlig, W. J. Organometal. Chem. 1994, 452, 29. [131] Uhlig, W.; Tretner, C. J. Organometal. Chem. 1994, 467, 31. [132] Tretner, C.; Zobel, B.; Hummeltenberg, R.; Uhlig, W. J. Organometal. Chem. 1994, 468, 94. [133] Hergott, H. H.; Simchen, G. Liebigs Ann. 1981, 1718. [134] Emde, H.; Domsch, D.; Feger, H.; Frick, U.; Götz, A.; Hergott, H. H.; Hofmann, K.; Kober, W.; Krägeloh, K.; Oesterle, T.; Steppan, W.; West, W.; Simchen, G. Synthesis 1982, 1. [135] Uhlig, W. J. Organometal. Chem. 1991, 409, 377. [136] Uhlig, W. J. Organometal. Chem. 1994, 421, 189. [137] Uhlig, W.; Tzschach, A. Z. Anorg. Allg. Chem. 1989, 576, 281. [138] Uhlig, W. Z. Anorg. Allg. Chem. 1991, 601, 125. [139] Uhlig, W.; Tzschach, A. Z. Chem. 1988, 28, 409. [140] Becker, G.; Eschbach, B.; Käshammer, D.; Mundt, O. Z. Anorg. Allg. Chem. 1994, 620, 29. [141] Trommer, M.; Sander, W.; Marquard, C. Angew. Chem. 1994, 106, 816. [142] Fronda, A.; Krebs, F.; Daucher, B.; Werle, T.; Maas G. J. Organometal. Chem. 1992, 424, 253. [143] Kleewein, A.; Stüger, H. Monatsh. Chem. 1999, 130, 69. [144] Uhlig, F.; Hermann, U.; Klinkhammer, K.; Hengge, E. In Organosilcon Chemistry III; Auner, N.; Weis, J.; Eds.; WILEY-VCH Verlag, Weinheim, 1998, pp 322. [145] Pöschl, U.; Siegl, H.; Hassler K. In Organosilcon Chemistry II; Auner, N.; Weis, J.; Eds.; WILEY-VCH Verlag, Weinheim, 1996, pp 113. [146] Uhlig, W. Polym. Adv. Techn. 1999, 10, 513. [147] Uhlig, W. Z. Naturforsch. 1995, 50b, 1674. [148] Uhlig, W. Z. Naturforsch. 1996, 51b, 703.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

366

Wolfram Uhlig

[149] Fürstner, A.; Weidmann, H. J. Organomet. Chem. 1988, 354, 15. [150] Seyferth, D.; Wiseman, G. H.; Schwark, J. M.; Wu, Y. F.; Poutasse, C. A. In Inorganic and Organometallic Polymers, ACS Symp. Ser. Vol. 360; Zeldin, M.; Waine, K.J.; Allcock, H.R., Eds.; American Chemical Society, Washington, DC, 1988, pp 143. [151] Uhlig, W. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 239. [152] Uhlig, W. Z. Naturforsch. 1997, 52b, 577. [153] Hrkach, J.S.; Matyjaszewski K. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 285. [154] Lin, J.; Maghsoodi, Si.; Barton, T. J. ACS Polym. Prepr. 1995, 36(1), 501. [155] Marciniec, B.; Lewandowski, M. J Polym. Sci., Part A: Polym. Chem. 1996, 34, 1443. [156] Korshak, V. V.; Sladkov, A. M.; Luneva, L. K. Izv. Akad. Nauk. SSSR, Otd. Khim Nauk 1962, 2251. [157] Lee, W. S.; Lyn, H. S. Polym. Bull. 1993, 30, 621. [158] Uhlig, W. Z. Naturforsch. 1994, 49b, 609. [159] Uhlig, W. Z. Anorg. Allg. Chem. 1994, 620, 939. [160] Uhlig, W. Organometallics 1994, 13, 2843. [161] Corriu, R. J. P.; Leclerq, D.; Mutin, H.; Samson, H.; Vioux, A. J. Polym. Sci. Part A: Polym. Chem. 1994, 32, 187. [162] Shea, K. J.; Loy, D. A.; Webster, O. J. Amer. Chem. Soc. 1992, 114, 6700. [163] Sze, M. E. In Physics of Semiconductive Devices. Wiley, New York, 1981, pp 31. [164] Diaz, A. F.; Bargon, J. In Handbook of Conducting Polymers, Vol 1; Skotheim, T. A., Ed.; Marcel Dekker, New York, 1985, pp 81. [165] Uhlig, W. Unpublished results. [166] Chicart, P.; Corriu, R. J. P.; Moreau, J. E.; Garnier, F.; Yasser, A. Chem. Mater. 1991, 3, 8. [167] Ishikawa, M. Pure Appl. Chem. 1991, 63, 851. [168] Wong, K. S.; Han, S. G.; Vardeny, Z. V.; Shinar, J.; Pang, Y.; Ijadi-Magshoodi, S.; Barton, T. J.; Grigoras, S.; Parbhoo, B. Appl. Phys. Lett. 1991, 58, 1695. [169] Corriu, R. J. P.; Gerbier, P.; Guerin, C.; Henner, B.; Foureade, R. J. Organomet. Chem. 1993, 449, 111. [170] Corriu, R. J. P.; Gerbier, P.; Guerin, C.; Henner, B. Angew. Chem.Int. Ed. Engl. 1992, 31, 1195. [171] Söldner, M.; Schier, A.; Schmidbaur, H. J. Organomet. Chem. 1996, 521, 295. [172] Söldner, M.; Schier, A.; Schmidbaur, H. Inorg. Chem. 1998, 37, 510. [173] Uhlig, W. In Organosilicon Chemistry V; Auner, N.; Weis, J.; Eds.; WILEY-VCH Verlag, Weinheim, 2003, pp 222. [174] Uhlig, W. ICMAT 2003, Conference Proceedings Symposium I (Advanced Polymers); Boey, F.Y.C.; Materials Research Society Singapore 2004 [175] Uhlig, W. Silicon Chemistry 2002, 1, 129 [176] Vdovin, V. M.; Pushchevaya, K. S.; Belivova, N. A.; Sultanov, R.; Plate, A. F.; Petrov, A. D. Dokl. Akad. Nauk SSSR 1961, 136, 96. [177] Nametkin, N. S.; Vdovin, V. M.; Poletaev, V. A.; Zav’yalov, V. I. Dokl. Akad. Nauk SSSR 1967, 175,1068. [178] Babich, E. D. In Polymeric Materials Encyclopedia; Salamone, I. C.; Ed., CRC Press, 1996, 10, 7621. [179] Liao, C. X.; Weber, W. P. Polym. Bull. 1992, 28, 281.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polycarbosilanes

367

[180] Matsumto, K.; Shiniazu, H.; Deguchi, M.; Yamaoka, H. J. Polym. Sci. Part A: Polym. Chem. 1997, 35, 3207. [181] Matsumto, K.; Nakano, M.; Deguchi, M.; Yamaoka, H. J. Polym. Sci. Part A: Polym. Chem. 1998, 36, 2699 [182] Knishka, R.; Frey, H.; Rapp, U.; Mayer-Posner, F. Makromol. Rapid Commun. 1998, 19, 455. [183] Yamashita, H.; Tanaka, M.; Honda, K. J. Am. Chem. Soc. 1995, 117, 8873. [184] Komuro, K.; Toyokawa, S.; Kawakami, Y. Polym. Bull. 1998, 40, 715. [185] Komuro, K.; Kawakami, Y. Polym. Bull. 1999, 42, 669. [186] Liao, C. X.; Weber, W. P. Macromolecules 1993, 26, 564. [187] Chai, M. H.; Saito, T.; Pi, Z. I.; Tessier, C.; Rinaldi, P. L. Macromolecules 1997, 30, 1240. [188] Kawakami, Y.; Takeyama, K.; Komuro, K.; Ooi, O. Macromolecules 1998, 31, 551. [189] Kawakami, Y.; Takahashi, T.; Yada, Y.; Imae, I. Polymer J. 1998, 30, 1001. [190] Kawakami, Y.; Nakao, K.; Shinke, S.; Imae, I. Macromolecules 1999, 32, 6874. [191] Ohshita, J.; Kunai, A. Acta Polym. 1998, 49, 379. [192] Ishikawa, M.; Ohshita, J. In Handbook of Organic Conductive Molecules and Polymers, Vol.2; Nalwa, H. S.; Ed.; Wiley, New York,. 1997, chapter 15. [193] Chen, R. M.; Chien, K M.; Wong, K. T.; Jin, B. Y.; Lu, T. Y. J. Am. Chem. Soc. 1997, 119, 11321. [194] van Walree, C. A.; Roest, M. R.; Schuddeboom, W.; Jenneskens, L. W.; Verhoeven, J. W.; Warman, J. M.; Kooijman, H.; Spek, A. L. J. Am. Chem. Soc. 1996, 118, 8395. [195] Kunai, A.; Ueda, T.; Horata, K.; Toyoda, E.; Ohshita, J.; Ishikawa, M.; Tanaka, K. Organometallics 1996, 15, 2000. [196] Tanaka, K.; Ago, H.; Yamabe, T.; Ishikawa, M.; Ueda, T. Organometallics 1994, 13, 5583. [197] Ohshita, J.; Watanabe, T.; Kanaja, D.; Ohsaki, H.; Ishikawa, M.; Ago, H.; Tanaka, K.; Yamabe, T. Organometallics 1994, 13, 5002. [198] Ohshita, J.; Takata, A.; Kai, H.; Kunai, A.; Komaguchi, K.; Shiotani, M.; Adachi, A.; Sakamaki, K.; Okita, K.; Harima, Y.; Kunugi, Y.; Yamashita, K.; Ishikawa, M. Organometallics 2000, 19, 4492. [199] Kunugi, Y.; Harima, Y.; Yamashita, K.; Ohshita, J.; Kunai, A.; Ishikawa, M. J. Electroanal. Chem. 1996, 414, 135. [200] Tang, H.; Zhu, L.; Harima, Y.; Yamashita, K.; Ohshita, J.; Kunai, A.; Ishikawa, M. Electrochim. Acta 1999, 44, 2579. [201] Fang, M. C.; Watanabe, A.; Matsuda, M. J. Organometal. Chem. 1995, 489, 15. [202] Fang, M. C.; Watanabe, A.; Matsuda, M. Macromolecules 1996, 29, 6807. [203] Fang, M. C.; Watanabe, A.; Matsuda, M. Polymer 1996, 37, 136. [204] Ohshita, J.; Yamashita, A.; Hiraoka, T.; Shinpo, A.; Kunai, A.; Ishikawa, M. Macromolecules 1997, 30, 1540. [205] Uhlig, W. Helv. Chim. Acta 1994, 77, 972. [206] Kira, M.; Tokura, S. Organometallics 1997, 16, 1100. [207] Ritter, S. K.; Noftle, R. E. Chem. Mater. 1992, 4, 872. [208] Ni, S. H.; Nagase, J.; Sato, H. Synth. Met. 1993, 58, 353. [209] Tanaka, K.; Ago, H.; Yamabe, T.; Ishikawa, M.; Uedo, T. Organometallics 1994, 13, 3497.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

368

Wolfram Uhlig

[210] Ohshita, J.; Kanaya, D.; Ishikawa, M. J. Organometal. Chem. 1994, 468, 55. [211] Adachi, A.; Ohshita, J.; Ohno, T.; Kunai, A.; Manhart, S. A.; Okita, K.; Kido, J. Appl. Organometal. Chem. 1999, 13, 859. [212] Moreau, C.; Serein-Spirau, F.; Bordeau, M.; Biran, C.; Dunogues, J. J. Organometal. Chem. 1996, 522, 213. [213] Moreau, C.; Serein-Spirau, F.; Biran, C.; Bordeau, M.; Gerval, P. Organometallics 1998, 17, 2797. [214] Ishikawa, M.; Lee, K. K.; Schneider, W.; Naka, A.; Yamabe, T.; Harima, Y.; Takeuchi, T. Organometallics 2000, 19, 2406. [215] Ohshita, J.; Ishii, M.; Ueno, Y.; Yamashita, A.; Ishikawa M. Macromolecules 1994, 21, 5583. [216] Kim, D. S.; Suh, M. C.; Shim, S. C. J Polym Sci. Part. A. Polym. Chem. 1998, 36, 2275. [217] Sakakura, T.; Kumberger, O.; Tan, R. P. T.; Authur, M. P.; Tanaka, M. J. Chem. Soc. Chem. Commun. 1995, 193. [218] Tanaka, M. Polym. Mater. Sci. Eng. 1999, 80, 426. [219] Uhlig, W. Monatsh. Chem. 1999, 130, 181. [220] Uhlig, W. J. Prakt. Chem. 1999, 341, 727. [221] Uhlig, W. Appl. Organometal. Chem. 1999, 13, 871. [222] Uhlig, W. Polym. Adv. Techn. 2001, 12, 607. [223] Ohshita, J.; Matsuguchi, A.; Furumori, K.; Hong, R.; Ishikawa, M.; Yamanaka, T.; Koike, T.; Shioya, J. Macromolecules 1992, 25, 2134. [224] Hockemeyer, J.; Castel, A.; Rivière, P.; Satgé, J.; Ryder, K. G.; Drury, A.; Ley, A. P.; Blau, W. J. Appl. Organometal. Chem. 1997, 11, 513. [225] Theurig, M.; Weber, W. P. Polym. Bull. 1992, 28, 17. [226] Theurig, M.; Sargeant, S. J.; Manuel, G.; Weber, W. P. Macromolecules 1992, 25, 3834. [227] Zhang, X.; Zhou, Q. S.; Weber, W. P.; Horvath, R.; Chan, T. H.; Manuel, G. Macromolecules 1988, 21, 1563. [228] Sargeant, S. J.; Tapsak, M. A.;Weber, W. P. Polym. Bull. 1993, 30, 127. [229] Zhou, Q. S.; Wang, L.; Liao, X.; Manuel, G.; Weber, W. P. J. lnorg. Organomet. Polym. 1991, 1, 199. [230] Zhou, Q. S.; Wang, L.; Weber, W. P.; Macromolecules 1990, 23, 1915. [231] Sargeant, S. J.;Weber, W. P. Macromolecules 1993, 26, 2400. [232] Wang, L.; Weber, W. P.; Macromolecules 1993, 26, 969. [233] Wagener, K. B.; Smith, D. W. Macromolecules 1991, 24, 6073. [234] Cummings, S.; Wagener, K. B.; Smith, D. W. Macromol. Rapid Commun. 1995, 16, 347. [235] Cummings, S.; Anderson, J.; Wagener, K. B. ACS Polym. Prepr. 1996, 37(2), 192. [236] Ko, H.; Weber, W. P. Polym. Bull. 1991, 26, 487. [237] Tsumura, M.; Iwahara, T.; Hirose, T. Polym. J. 1995, 27, 1048. [238] Tsumura, M.; Iwahara, T. Polym. J. 1999, 31, 452. [239] Znamenskaya, E. N.; Nametkin, N. S.; Pritula, N. A.; Oppengeim, V. D.; Chernysheva, T. I. Neftekhimiya 1964, 4, 487. [240] Rickle, G. K. J. Appl. Polym. Sci. 1994, 51, 605. [241] Kim, D. S.; Shim, S. C. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 2263. [242] Kim, D. S.; Shim, S. C. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 2933.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

Polycarbosilanes

369

[243] Son, D. Y.; Bucca, D.; Keller, T. M. Tetrahedron Lett. 1996, 37, 287. [244] Itoh, M.; Inoue, K.; Iwata, K.; Mitstizuka, M.; Kakigano, T. Macromolecules 1997, 30, 694. [245] Kuroki, S.; Okita, K.; Kakigano, T.; Ishikawa, J.; Itoh, M. Macromolecules 1998, 31, 2804. [246] Tamao, K.; Yamaguchi, S.; Shiozaki, M.; Nakagawa, Y.; Ito, Y. J. Am. Chem. Soc. 1992, 114, 5867. [247] Tamao, K.; Yamaguchi, S. Bull. Chem. Soc. Japan 1996, 69, 2327. [248] Tamao, K.; Yamaguchi, S.; Ito, Y.; Matsuzki, Y.; Yanabe, T.; Fuskushim, M.; Mori, S. Macromolecules 1995, 28, 8668. [249] Yamaguchi, S.; Iimura, K.; Tamao. K. Chem. Lett. 1998, 89. [250] Chen, W.; Maghsoodi, S. I.; Barton, T. J. ACS Polym Prepr. 1997, 38(1), 189. [251] Shinar, J.; Maghsoodi, S. I.; Ni, Q. X.; Pang, Y.; Barton, T. J. Synth. Met. 1989, 28, C593. [252] Wong, K. S.; Han, S. G.; Vardeny, Z. V.; Shinar, J.; Pang, Y.; Maghsoodi, S. I.; Barton, T. J.; Grigoras, S.; Parbhoo, B. Appl. Phys. Lett.1991, 58, 1695. [253] Wu, Z.; Papandrea, J. P.; Apple, T.; Interrante, L. V. Macromolecules 2004, 37, 5257. [254] Lach, C.; Muller, P.; Frey, H.; Mulhaupt, R. Macromol. Rapid Commun. 1997, 18, 253. [255] Lach, C.; Frey, H. Macromolecules 1998, 31, 2381. [256] Lorenz, K.; Frey, H.; Stuhn, B.; Mulhaupt, R. Macromolecules 1997, 30, 6860. [257] Ponomarenko, S. A.; Rebrov, E. A.; Bobrovsky, A. Yu.; Boiko, N. I.; Muzafarov, A. M.; Shibaev, V. P. Liquid Crystals 1996, 21, 1. [258] Vitukhnovsky, N. G.; Sluch, M. I.; Krasovskii, V. G.; Muzafarov, A. M. Synthetic Metals 1997, 91, 375. [259] Drohmann, C.; Moller, M.; Gorbatsevich, O. B.; Muzafarov, A. M. J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 485. [260] Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Richardson, R. M.; Whitehouse, I. J.; Rebrov, E. A.; Muzafarov, A. M. Macromolecules 2000, 33, 5549. [261] Sagidullin, A. L.; Muzafarov, A. M.; Krykin, M. A.; Ozerin, A. N.; Skirda, V. D.; Ignatewa, G. M. Macromolecules 2002, 35, 9472. [262] Lezov, A. V.; Polushina, G. E.; Mikhailova, M. E.; Rebrov, E. A.; Muzafarov, A. M.; Ryumtsev, E. I. Russian J. Phys. Chem. 2003, 77, 944. [263] Shumilkina, N. A.; Myakushev, V. D.; Tatarinova, E. A.; Gallyamov, M. O.; Khokhlov, A. R.; Buzin, M. I.; Muzafarov, M. A. Doklady Chemistry 2005, 402, 155. [264] Park, S. Y.; Zhang, T.; Interrante, L. V.; Farmer, B. L. Macromolecules 2002, 35, 2776. [265] Ganicz, T.; Stanczyk, W. A. Trends in Polym. Sci. 1997, 5, 305. [266] Hanselmann, R.; Holter, D.; Frey, H. Macromolecules 1998, 31, 3790. [267] Yoon, H.; Son, D. Y. Macromolecules 1999, 32, 5210. [268] Yao, J.; Son, D. Y. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 3778. [269] Pillot, J. P.; Birot, M.; Bacque, E.; Dunogues, J.; Biran, C.; Lapouyade, P.; Babort, O. Appl. Organomet. Chem. 1996, 10, 23. [270] Hemida, A. T.; Pillot, J. P.; Birot, M.; Dunogues, J.; Pailler, R. J. Chem. Soc., Chem. Commun. 1994, 2337. [271] Guselnikov, L. E.; Polyakov, Y. P.; Nametkin, N. S. Doklady Acad. Nauk SSSR 1980, 253(5), 1133. [272] Shina, K. J. Organomet. Chem. 1986, 310, C57.

Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

370

Wolfram Uhlig

[273] Guselnikov, L. E.; Volkova, V. V.; Volnina, E. A.; Gladkova, N. A.; Negrebetsky, V. V.; Blazso, M. J. Inorg. Organomet. Polym. 1998, 8, 89. [274] Ohshita, J.; Kanaya, O.; Ishikawa, M.; Yamanaka, T. J. Organomet. Chem. 1989, 369, C18. [275] Suzuki, M.; Kaneko, T.; Morishima, Y.; Obayashi, T.; Saegusa, T. Polym. J. 1996, 28, 16. [276] Uchimaru, Y.; Tanaka, Y.; Tanka, M. Chem. Lett. 1995, 164. [277] Oshita, J.; Sugimoto, K.; Watanabe, T.; Kunai, A.; Ishikawa, M.; Aoyama, S. J. Organomet. Chem. 1998, 564, 47. [278] Corriu, R.; Guerin, C.; Henner, B.; Kuhlmann, T; Jean, A. Chem. Mater 1990, 2, 351. [279] Ishakawa, M.; Hasegawa, Y.; Kunai, A. J. Organomet. Chem. 1990, 381, C57. [280] Yuan, C. H.; West, R. Appl. Organomet. Chem. 1994, 8, 423. [281] Li, H.; West, R. Macromolecules 1998, 31, 2866. [282] Oshita, J.; Mimura, N.; Arase, H.; Nodono, M.; Kunai, A.; Komaguchi, K.; Shiotani, M.; Ishikawa, M. Macromolecules 1998, 31, 7985. [283] Oshita, J.; Hamaguchi, T.; Toyoda, E.; Kunai, A.; Komaguchi, K.; Shiotani, M.; Ishikawa, M.; Naka, A. Organometallics 1999, 18, 1717. [284] Uhlig, W. In Organosilicon Chemistry VI; Auner, N.; Weis, J.; Eds.; WILEY-VCH Verlag, Weinheim, 2005, pp 330. [285] Uhlig, W. J. Organomet. Chem. 2003, 685, 183. [286] Uhlig, W. Z. Naturforsch. 2003, 58b, 183. [287] Isaka, H. Macromolecules 1997, 30, 344. [288] Zhu, H. D.; Kantor, S. K.; McKnight, W. J. Macromolecules 1998, 31, 850. [289] Andrianov, K. A.; Gavrikova, L. A.; Rodionova, Y. F. Polym. Chem. USSR 1971, A13, 1057. [290] Li, Y.; Kawakami, Y. Macromolecules 1998, 31, 5592. [291] Li, Y.; Kawakami, Y. Macromolecules 1999, 32, 8768. [292] Sargent, J. R.; Weber, W. P. Macromolecules 1999, 32, 2826. [293] Paulasaari, J. K.; Weber, W. P. Macromolecules 1999, 32, 5217. [294] Interrante L. V.; Shen, Q.; Li, J. Macromolecules 2001, 34, 1545. [295] Ni, Y.; Manners, I.; Sherian, J. B.; Oakley, R. T. J. Chem. Educ. 1998, 76, 766. [296] Ni, Y.; Rulkens, R.; Manners, I. J. Am. Chem. Soc. 1996, 118, 4102. [297] Manners, I. Science 2001, 294, 1664. [298] Temple, K.; Kulbaba, K.; Power-Billard, K. N.; Manners, I.; Leach, K. A., Xu, T.; Russell, T. P. Hawker, C. J. Adv. Mater. 2003, 15, 297 [299] Lastella, S.; Jung, Y. J.; Yang, H.; Vajtai, R.; Ajayan, P. M.; Ryu, C. Y.; Rider, D. A.; Manners, I. J. Mater. Chem. 2004, 14, 1791. [300] Wang, X.; Winnik, M. A.; Manners, I. Angew. Chem. Int. Ed. 2004, 43, 3703 [301] Berenbaum, A.; Manners, I. Dalton Trans. 2004, 14, 2057.

In: Silicon-based Inorganic Polymers Editors: Roger De Jaeger and Mario Gleria

ISBN: 978-1-60456-342-9 © 2008 Nova Science Publishers, Inc.

Chapter 7

POLYSILAZANES Markus Weinmann∗ Max-Planck-Institute for Solid State Research, Heisenbergstraße 1, D-70569 Stuttgart, Germany

1. INTRODUCTION The essential feature of a polysilazane is a structural framework or “backbone” made up entirely of alternate silicon and nitrogen atoms, with no carbon atoms or chains of carbon atoms in the “backbone” itself [1]. 1

R

2

R

Si N 3

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R

n

Though investigated thoroughly, there are unfortunately no detailed structural data available of silicon-nitrogen polymers. Single crystal X-ray diffraction analyses on cyclodisilazanes, [2,3] cyclotrisilazanes,[4,5] cyclotetrasilazanes [6,7] and a linear trisilazane [8] clearly show that silicon atoms in oligosilazanes possess a tetrahedral coordination sphere SiN2R1R2 which is usually slightly distorted. At least two of the nitrogen-bonded substituents are silicon atoms. The latter are essentially coordinated in a trigonal-planar fashion (NSi2R3) and the sum of angles Si1-N-Si2 + Si1-N-R3 + Si2-N-R3 usually approaches 360° [9]. Typical silicon nitrogen bond lengths are 1.70 – 1.75 Å. Si-N-Si bond angles strongly depend on the nature of the silicon and nitrogen-bonded substituents. An increasing steric demand of the nitrogen-bonded substituent R3 causes a decreasing Si-N-Si bond angle whereas increasing volume of silicon-bonded entities R1 and R2 results in a widening. For example, Si-N-Si angles in cyclotetrasilazanes (Me2SiNH)4, [6] (tBu2SiNH)4, [2] and (Me2SiNMe)4 [7] are

∗

E-mail: [email protected]

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372

Markus Weinmann

131°, 141°, and 123°, respectively. Because of significant ring tension, very small Si-N-Si bond angles of 92° - 96° appear in cyclodisilazanes [2,3]. Polysilazane structures can be derived from polysiloxanes by replacing oxygen atoms with isoelectronic NR building blocks. Nevertheless, silicon-nitrogen polymers have – from an industrial point of view – never achieved the same degree of importance as silicon-oxygen polymers. The main reason is their hydrolytic instability, i.e. their reactivity towards water and other protic solvents such as alcohols or acids. Both hydrolysis and alcoholysis result in the cleavage of Si-N bonds and the formation of thermodynamically favored Si-O bonds. Depending on the nitrogen-bonded substituents, ammonia or organic amines are the couple products in these reactions. As a consequence, synthesis and all manipulations of polysilazanes require for the application of expensive inert gas techniques [10]. Remarkably, polysilazanes, polycarbosilazanes or polysilazanes bearing hetero atoms such as boron or aluminum turned out to be promising candidates as precursors for refractory non-oxide ceramics such as silicon nitride, silicon carbide, silicon carbonitride etc. First investigations of the thermal transformation of these polymers into ceramics were published already in the 1960’s by Aylett et al., [11] Chantrell, [12] and Popper [13]. However, it was the pioneering work in the 1970’s of Yajima et al. [14-17] as well as Verbeek and Winter [1820] relating to the preparation of silicon carbide fibers and silicon carbonitrides, respectively, that motivated researchers to investigate this topic. In the mean time, various review articles appeared, dealing with the use of polysilazanes and related compounds as precursors for ceramics [21-29]. The general idea behind this process concept is to generate preferred structural features in the organometallic polymer and to subsequently transform the precursor into covalently bonded ceramics while retaining the especially designed building blocks (note that conventionally the low diffusion capability of non-oxide ceramics even at very high temperature is overrode by the use of sintering additives which otherwise degrade their unique properties such as thermal, chemical and mechanical stability). Therefore it is mandatory to also carefully consider individual condensation steps during the thermal conversion. In this regard latent reactivity, i.e. the ability of a polymer to undergo thermally induced cross-linking reactions is of major importance. For example, especially designed silazane copolymers [(SiH2NH)3(SiHCH3NH)]n or [(SiH2NH)3(SiH2NCH3)]n released phase pure silicon nitride / silicon carbide ceramics without “free” carbon (free carbon worsens thermal properties of silicon nitride-based ceramics significantly due to a carbothermal reaction of the latter [30]) in ca. 95% yield; the only condensation product was hydrogen whereas silicon, carbon, or nitrogen-containing species were released only in a negligible amount [31, 32]. The potential of polysilazanes as ceramic precursors is reflected in their increasing commercial availability. Commercially available polymers – usually copolymeric silazanes with properties tuned for special applications – are for example polyhydridomethylsilazane PHMS (Nichimen Corp. Japan), perhydridopolysilazane PHPS (Tonen Corp. Japan), [33] polyvinylsilazane PVS (VT50, Hoechst AG, Germany), [34] and polysilazane Ceraset 20 (various modifications, KION Corp. USA) [35].

Polysilazanes

373

2. SYNTHESIS OF POLYSILAZANES Several synthetic pathways have been described for the synthesis of polysilazanes. The desired molecular structure and chemical composition, the availability of the monomers which are used as starting compounds, preferred reaction conditions and applications as well as costs determine the synthetic methods applied. In general, one may distinguish three basically different approaches:

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i. ii. iii.

reactions occurring with formation of solid by-products reactions occurring with formation of volatile by-products reactions occurring without formation of by-products

Syntheses taking place with formation of solid by-products are for example ammonolysis or aminolysis reactions of halogenosilanes [36]. Ammonolysis and aminolysis are typical substitution reactions in which a silicon-bonded halide X- is replaced with an amide group NR2-. In such reactions one equivalent of base is required to bind hydrogen halide HX which forms upon this reaction. Moreover metathesis reactions of halogenosilanes with lithium or sodium amides MNR2 (M = Li, Na) also fall in this category. Dehydrocoupling reactions of hydridosilanes and ammonia or amines as well as aminolysis using hexamethyldisilazane (HMDS) deliver polysilazanes without formation of solid by-products. Both methods release volatile molecules, i.e. hydrogen or chlorotrimethylsilane, respectively. Such a processing has a significant advantage over the above-mentioned, since time-intensive filtration steps which are required to remove ammonium halide or amine hydrohalide that form during ammonolysis or aminolysis reactions of halogenosilanes are not necessary. However, the afore mentioned processes release silazane molecules with comparably low molecular weight, i.e. cyclomers with a low degree of polymerization. In contrast, ring-opening polymerization of cyclodisilazanes and cyclotrisilazanes using either acidic or basic catalysts delivers high-molecular weight polymers with narrow polydispersity. Moreover, the polymerization step occurs without formation of by-products. A disadvantage however is that prior to the polymerization cyclosilazanes have to be prepared in often complex and costly processes. In the following, a detailed presentation of the different synthetic procedures releasing oligo- and polysilazanes will be given starting with the most common and most frequently applied ammonolysis of chlorosilanes.

2.1. Salt Elimination Reactions 2.1.1. Ammonolysis Ammonolysis of chlorosilanes R3SiCl (or silicon halides in general) is a reaction in which upon a treatment with ammonia a silicon-bonded chlorine atom is replaced with an NH2 group, thereby forming a silylamine, R3SiNH2. Hydrogen chloride which forms as a couple product, adds to NH3 and causes precipitation of ammonium chloride which has to be removed by filtration from the product solution (presuming the reaction is performed in an

374

Markus Weinmann

organic solvent). Depending on the reaction conditions applied, i.e. the R3SiCl : NH3 stoichiometry, two principally different reactions may occur further (they mostly result in the same product). Using excess ammonia, for example by performing the reaction in liquid ammonia, causes the replacement of all chlorine atoms with NH2 groups. Subsequent transamination of R3SiNH2 by ammonia condensation results in the linking of silicon atoms by NH units. A different reaction pathway is observed, if performing the reaction with excess chlorosilane, e.g. by introducing gaseous ammonia into a chlorosilane solution. In this particular case the initially formed silylamine reacts with chlorosilane with HCl elimination and NH4Cl precipitation.

Si Cl

excess chlorosilane

- HCl

Si

Cl

2 NH3 - NH 4Cl

Si

H Si

NH2

N

Si

*2 - NH 3

excess ammonia

2.1.1.1. Ammonolysis of Chlorosilanes R3SiCl Ammonolysis of monochlorosilanes R3SiCl yields monomeric silylamines, disilazanes or trisilazanes: H N R3Si

x2 R3Si

Cl

2 NH3 - NH 4Cl

- NH 3 R3Si

SiR3

Disilazane

NH2

Silylamine

x2 SiR3

- 2 NH 3

N R3Si

SiR3

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Trisilazane

The reaction may be performed in common dry polar or unpolar solvents. Alcohols or other protic solvents are not suitable. Traces of water result in hydrolysis and formation of silanols and siloxanes. Monomeric silylamines may transfer into disilazanes (secondary silylamines), trisilazanes (tertiary silylamines), or mixtures thereof by ammonia condensation. Product formation depends on both the nature of the silicon-bonded substitutents R and the reaction conditions applied. Primary silylamines are obtained by ammonolysis of R3SiCl if R = Et, Pr, [38] Ph, [39, 40] and monochlorosilanes carrying other bulky substituents R. [41]. The sterically demanding groups inhibit ammonia condensation at ambient temperature and therefore efficiently hinder transformation into disilazanes. In general, the stability of aminosilane derivatives follows a consistent pattern: the greater the bulk of the organic groups attached to

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Polysilazanes

375

silicon, the more stable the aminosilane [42]. However, for the synthesis of sterically extremely hindered silylamines alternative synthetic pathways were chosen. For example, t Bu3Si-NH2 [43] was obtained by hydrogenation of tri-t-butylsilylazide, tBu3Si-N3, whereas (Me3Si)3Si-NH2 [44] was received from (Me3Si)3Si-OSO2CF3. Ammonolysis of chlorotrimethylsilane Me3SiCl delivers hexamethyldisilazane, HMDS [45]. In order to guarantee a high turn-over and to avoid formation of primary amines the reaction is performed at higher temperature in either non-protic solvents or in the product itself. Hexamethyldisilazane, HMDS, (in the older literature referred to as hexamethyldisilazine) [46] is technologically the by far the most important silazane; it is a universal silylating agent (e.g. for derivatization of alcohols, carboxylic acids, amines, amides, mercaptanes, etc.) and also for the production of silicon nitride-based ceramic coatings using chemical vapor deposition [47, 48] and related methods [49]. It is commercially available and produced on an industrial scale. [50]. In order to obtain disilazane derivatives carrying bulky substituents such as phenyl, different reaction pathways are required. For example, Ph3SiNH2 has to be deprotonated to form the respective amide Ph3SiNHM (M = Li, Na) which is subsequently treated with Ph3SiCl to yield hexaphenyldisilazane, (Ph3Si)2NH [52]. An unusual synthetic approach starting from Ph3SiN3 and Ph3SiH was reported by Tsai et al. [53]. Results of the X-ray structure analysis of Ph3SiNHSiPh3 indicate Si-N bond lengths of 1.72 Å and an Si-N-Si angle of 138° [54]. Disilazane, (H3Si)2NH, was obtained by reaction of excess ammonia with chlorosilane, ClSiH3 already in 1921 by Stock and Somieski [55]. This compound was thermally labile and decomposed at room temperature to SiH4, (H3Si)3N, and solid products (insoluble polymers, i.e. polysilazanes) which were not characterized in more detail. If ammonolysis of ClSiH3 was performed using an excess of the latter, trisilylamine formed directly. It is a colorless liquid which may be distilled (bp. 52°C) without decomposition. The molecular structure of (H3Si)3N was investigated by electron diffraction in the gas phase by Hedberg in 1955 [56]. The radial distribution function exhibited two strong peaks at 1.735 and 3.00 Å indicating SiN and Si-Si distances, respectively. The author concluded that such interaction corresponds to average Si-N-Si bond angles of 120° and thus a coplanar Si3N skeleton. This result was confirmed more recently by Beagly and Conrad [57] who suggested a considerable extend of π-bonding. This interaction and the planarity explain the chemical behavior of trisilylamine being a much weaker electron donor than N(CH3)3. Similar findings were obtained by Ebsworth et al. who refined the molecular structure of tris(methylsilyl)amine 25 years later also using electron diffraction of (MeH2Si)3N [58] in the gas phase [59]. They also observed a planar structure of the SiN3 skeleton as proposed by Hedberg and average Si-N bond lengths of 1.73 Å. The existence of (p→d) π-bonds i.e. the amount of delocalization of the N pz electrons into Si d-orbitals and the energy of the resulting (p→d) π-bonds in trisilylamine was calculated by Perkins [60]. Using the Pariser-Parr-Pople SCF method he calculated a mean energy of each Si-N bond of ca. 67 kcal/mol. The structural analog of triethylamine is tri(disil)azane (H3Si-SiH2)3N [61] which can be obtained by ammonolysis of chlorodisilane. If the steric demand of the silicon-bonded substituents i.e. the number of methyl groups increases, disilazanes are obtained rather than trisilazanes. However, metallation with formation of disilylamides and subsequent treatment with chlorosilanes delivers trisilazanes in quantitative yield [62, 63, 64]. Moreover this method allows for the production of trisilylamines carrying different SiR3 groups [65].

376

Markus Weinmann

A very unusual approach towards the synthesis of trisilazanes starting from chlorotrialkylsilanes R3SiCl (R = Me, Et) was reported by Shiina. Tris(trimethylsilyl)amine was obtained by transition metal-catalyzed reductive silylation of molecular nitrogen: [66]

6 Me 3SiCl + 6 Li + N 2

transition metal chloride

2 (Me 3Si)3N + 6 LiCl

The author by chance explored that molecular nitrogen used as inert gas was taken up at room temperature when Me3SiCl was reacted with lithium in the presence of chromium chloride and other transition metal chlorides. Table 1. Synthesis of silazanes R3SiNH2, disilazanes (R3Si)2NH, and trisilazanes (R3Si)3N Silazane Me3SiNH2 Et3SiNH2 Pr3SiNH2 Ph3SiNH2 t Bu3Si-NH2 (Me3Si)3Si-NH2 (H3Si)2NH (Me3Si)2NH (Et3Si)2NH (Ph3Si)2NH (H3Si)3N (MeH2Si)3N (Me2HSi)2(Me3Si)N (Me3Si)3N

(PhH2Si)3N (PhH2Si)2(Me3Si)N 1) Copyright © 2008. Nova Science Publishers, Incorporated. All rights reserved.

2)

Reaction Conditions Me3Si-NMe2 + exc. NH3, -78°C Et3SiCl + exc. NH3, -78°C Pr3SiCl + liq. NH3 Ph3SiCl + liq. NH3 t Bu3SiN3 + H2 + [Ra-Ni]1), 25°C (Me3Si)3Si-OSO2CF3 + NH3 H3SiCl + exc. NH3, 25°C 2) Me3SiCl + exc. NH3 (rflx. Et2O) Et3SiH + KNH2 (→ (Et3Si)2NK) + NH4Br (rflx. Et2O) Ph3SiNH2 + RLi (→ Ph3SiNHLi) + Ph3SiCl Ph3SiN3 + Ph3SiH (-N2) exc. H3SiCl + NH3, 25°C exc. MeH2SiCl + NH3, 25°C (Me2HSi)2NLi + Me3SiCl (Me3Si)2NM + Me3SiCl (M = Li, Na) Me3SiCl + Li + N2 Li3N + Me3SiCl PhH2SiBr + NH3, 25°C (PhH2Si)2NLi + Me3SiCl

Ref. [67] [37] [38] [39] [43] [44] [55] [46] [68] [52, 53] [55, 56] [58] [65] [62, 63, 66] [69] [71, 72] [9]

Raney Nickel-catalyzed hydrogenation. condenses to (H2SiNH)x.

2.1.1.2. Ammonolysis of Dichlorosilanes R2SiCl2 In contrast to chlorosilanes R3SiCl, ammonolysis of dichlorosilanes R2SiCl2 initially yields silyldiamines, R2Si(NH2)2 which by ammonia condensation or reaction with dichlorosilane can transform into higher molecular weight molecules. The condensation and thus the product formation mainly depends on the nature of the silicon-bonded substituents R. Thermally stable monomeric silyldiamines are obtained if the steric demand of the substituents R is sufficiently big to inhibit ammonia condensation. For example, ammonolysis of di-tert-butyldichlorosilane, (Me3C)2SiCl2 gave the respective diaminosilane (Me3C)2Si(NH2)2 in 50% yield [72]. Remarkably, the product could be distilled at atmospheric pressure (190°C) without decomposition, i.e. ammonia

Polysilazanes

377

condensation. Likewise, diaminosilanes (Me3C)RSi(NH2)2 (R = Me, Ph, C15H31) and (αnaphtyl)2Si(NH2)2 [73] were synthesized. In contrast, treatment of di-nbutyldichlorosilane with ammonia under similar reaction conditions gave as the main product a cyclotrimer, (nBu2Si-NH)3. [72]. n t

Bu

NH2

NH3

Si t

Bu

R = Bu

NH2

Bu

Si

NH3

R2SiCl2

t

n

Bu

HN

R = nBu

n

Bu n

NH

Si N H

Bu

n

Si

Bu

n

Bu

Accordingly, ammonolysis of dichlorosilanes R1R2SiCl2 bearing “small” substituents (R1, R2 = H, Me, Et, Pr, nBu, nHex, Ph etc.) with minor steric demand gives product mixtures. They consist mainly of cyclotrimers (R1R2SiNH)3 and cyclotetramers (R1R2SiNH)4 beside a small amount of linear oligomers (R1R2SiNH)n with terminal NH2-groups (against this, hydrolysis of dichlorosilanes, e.g. Me2SiCl2 produces equal amounts of cyclic and linear polymeric methysiloxanes [74]). Condensation of ammonia and transformation of the silyldiamines into cyclic or oligomeric silazanes may often be activated thermally. R1

R1 Cl

Si

Cl

NH3

R1

H2N Si NH2

- NH 4Cl

ΔT - NH 3

2

R

R

R

1

2

i

t

R , R = Pr, Bu, Ph, Mes ...

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Si

1/n

2

2

N H

n

low molecular weight cyclomers and oligomers

Members of both the cyclic and linear series with R1, R2 = Me, Et, were first mentioned by Brewer and Haber [75]. The six and eight-membered cyclosilazanes were separated and purified by (vacuum) distillation. [45]. Wannagat and coworkers reinvestigated the system in much detail by performing the reaction in either excess methyldichlorosilane (ammonia was introduced to a solution of the chlorosilane in petrol ether) or ammonia [76]. They concluded that beside substitution of chloride with amide and ammonia condensation, chain contraction occurs. This finding was proven by ammonolysis of 1,3-dichlorotetramethyldisilazane, ClMe2Si-NH-SiMe2Cl, an intermediate in the ammonolysis of Me2SiCl2, which yielded hexamethylcyclotrisilazane, (Me2Si-NH)3, along with the expected octamethylcyclotetrasilazane, (Me2Si-NH)4. H N +

H3 N

H

H

H 190ºC/4h

Si Br

-

- NH 3

N

+ Br

H

- NH 3

Si

Si N - HBr

Si

N

Si

condensation through transamination

Krüger and Rochow described the first catalytic process for the condensation (and thus polymerization) of ammonia from cyclosilazanes (Me2Si-NH)3 and (Me2Si-NH)4. High-

378

Markus Weinmann

molecular weight polysilazanes became accessible by a heat treatment of the cyclic species to ca. 190°C in the presence of ammonium halides [77]. The reaction was initiated by H-X (X = halide) addition to a silicon-nitrogen bond followed by Si-N bond cleavage thereby forming Si-X and Si-NH2 units. The first reacted with N-H bonds of the cyclomers with elimination of H-X and formation of a tertiary N atom whereas the latter underwent ammonia condensation through transamination. The two different reaction pathways resulted in both cross-linking and cycle-to-chain transformation. As-obtained polymers were colorless waxes which softened between 90 and 140°C; their molecular weight was > 10000 g/mol. Soon after Rochow’s discovery the potential of polysilazanes as precursors for silicon nitride-based high-temperature ceramics was recognized in the late 1960’s by Chantrell and Popper [12, 13]. It was the motivation to thoroughly investigate the synthesis of polymeric silazanes and resulted in the synthesis of a wide variety of structurally different especially functionalized molecules. The focus was on both precursors for silicon nitride as well as precursors for ternary materials (silicon carbonitrides). Depending on the chemical composition of the desired ceramic material, different approaches for the synthesis of the respective precursors were chosen. For example, a processable precursor for silicon nitride which could be synthesized by ammonolysis of dichlorosilane in polar solvents such as diethylether or dichloromethane is perhydridopolysilazane [78].

H Cl

Si H

H2 Si

H2 Si Cl

NH3 - NH 4Cl

HN H2Si

N H

N NH

Aging

SiH2

- H2

H2Si

NH H2 Si

SiH N H

N

N SiH

HSi N H

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schematical representation

The silazane obtained initially is composed of higher-molecular weight linear (not shown) and cyclic motifs. It rapidly ages by loosing hydrogen and further cross-linking thereby increasing its viscosity gradually from oily over waxy to glassy. The final polyperhydridosilazane can be transformed into a ceramic material in 70% yield by thermolysis to 1050°C. According to X-ray investigations the derived ceramic is composed of α-Si3N4, β-Si3N4, and elemental silicon. Silicon segregation worsens the unique hightemperature properties of silicon nitride but can be suppressed by performing the polymer-toceramic transformation in an ammonia atmosphere thereby converting excess silicon into silicon nitride. A critical issue however is to scale the synthesis into technical dimensions since H2SiCl2 is a highly flammable gas, which can disproportionate to SiH4 and SiCl4. A modified process, allowing for a safer handling of the highly reactive starting compound was developed at Tonen Company in Japan. Prior to ammonolysis, H2SiCl2 was modified to H2SiCl2*(NC5H5)2 [79] by reaction with pyridine [80]. While the structural diversity of polysilazanes obtained from H2SiCl2 is very limited ammonolysis of dichlorosilanes bearing aliphatic, aromatic, or olefinic groups or other

Polysilazanes

379

substituents R is much more multifaceted, since the silicon-bonded substituents determine physical-chemical properties and the chemistry i.e. the ceramization of the polymers. However, a characteristic feature of cyclosilazanes is their relatively low boiling point causing their volatilization during thermolysis. Consequently, ceramic yields are commonly unsatisfactory [81]. This fact confined the use of polysilazanes as precursors for ceramics until Seyferth and Wiseman discovered that the molecular weight of cyclic silazanes can be increased considerably by a catalytic process [82-86]. Small amounts of KH were added to a solution of cyclotetra(methylsilazane) and upon heat treatment (i.e. refluxing solvent) crosslinking by dehydrocoupling occurred. H Me

H Me

Si

H N

Si

H NH

Me

N H

Me

Si

HN H

H

Me

Si

[KH] - H2

Si

H N

Me Si

HN

Me

H

Si Me

H

H N

Me Si

H NH

HN Si

H

H

N H

Me

N N H

Si

Si

Si

Me

H

"Hydrosilylation"

Me

silylene-imine proposed intermediate

H Me Highly cross-linked polymethylsilazane

Si

HN - H2

H

Si Me

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Me H H Si Si NH Me HN Me Si N Si Me

H N

N H

Si H Me

HN H

Si

NH H Me

It was supposed that initially, a silylene-imine (an intermediate with Si=N double bond; it was neither isolated nor experimentally confirmed) forms by catalytic dehydrogenation of a NH-SiH unit. Intermolecular hydrosilylation, i.e. addition of a Si-H unit, results in the formation of a new Si-N bond connecting two former independent ring systems. Repeated dehydrogenation / hydrosilylation finally results in the formation of highly cross-linked polysilazanes. Macroscopically, the increased cross-linking density was reflected by an increased viscosity. Ceramic yields could be enhanced from below 30% to above 80%. Alternatively, higher molecular weights can be achieved by attaching reactive sites to the Si-N backbone which allow for cross-linking reactions. For example, vinyl-substituted oligosilazanes [(H2C=CH)SiR-NH]n have been obtained by ammonolysis of vinyldichlorosilanes (H2C=CH)SiRCl2 (R = H, Me). H C Cl Si R

H

CH2 Cl

C - NH 3 - NH 4Cl

1/n

CH2

Si

N

R

H

R = H, CH 3 n = 3, 4, ...

n

380

Markus Weinmann Vinylsilazanes may be cross-linked by various reaction pathways. • • • •

Olefin polymerization (R = H, CH3) Hydrosilylation (R = H) Dehydrocoupling (R = H) Transamination (ammonia condensation)

If R = CH3, only olefin polymerization, i.e. cross-linking by polyaddition of C=C units or transamination (not shown in the scheme) take place. Olefin polymerization may be activated thermally by a heat treatment to ca. 200°C [87, 88] or by treatment with conventional radical starters. The progress is best monitored by 1H or 13C NMR spectroscopy. During such a crosslinking, viscosity, solubility, or other physical-chemical properties can change dramatically. Olefin Polymerization CH

H

CH2

Si

N

R

H

C

n

R = H, CH 3

CH Si

N

R

H

+ n

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α-addition product

C R=H n

H

N

H

- H2

Si

CH2 N n

R=H

CH3

H

CH2

Si

Hydrosilylation

Dehydrocoupling

CH Si

CH2 N H

n

β-addition product

The major reaction pathway observed in the thermal cross-linking of oligovinylsilazane, [(H2C=CH)SiH-NH]n, is hydrosilylation, i.e. addition of a Si-H unit to a C=C double bond [89]. Transamination, olefin polymerization and dehydrocoupling occur to a minor extend. Hydrosilylation is not regioselective, rather two isomers may form, namely α- and βaddition products. Due to both electronic and steric effects, formation of the β- product is usually preferred. The addition reaction can be promoted radically, [90] by transition metals, [91] transition metal complexes such as Speyers catalyst, H2PtCl6 in i-PrOH [92] or other complexes of the late transition metals [93-96]. Addition of potassium hydride as catalyst in contrast results in cross-linking by dehydrocoupling according to the procedure developed by Seyferth et al. [82,84,85]. 2.1.1.3. Ammonolysis of Trichlorosilanes RSiCl3 According to the reactions described in the previous sections, ammonolysis of trichlorosilanes RSiCl3 using excess ammonia initially yields silyltriamines RSi(NH2)3. If ammonolysis of trichlorosilanes is performed not-stoichiometrically, i.e. with ammonia deficiency (for example by using calciumoctaammine dichloride, CaCl2(NH3)8 instead of “pure” ammonia), 1,1,3,3-tetrachlorodisilazanes RCl2Si-NH-SiRCl2 are accessible [108].

Polysilazanes

381

Table 2. Synthesis of silazanes from dichlorosilanes R2SiCl2 Dichlorosilane H2SiCl2 HMeSiCl2 HEtSiCl2 Me2SiCl2

MeEtSiCl2 MePhSiCl2 Et2SiCl2 i Pr2SiCl2 n Bu2SiCl2 t Bu2SiCl2 Ph2SiCl2 (H2C=CH)HSiCl2 (H2C=CH)MeSiCl2

Reactant / Conditions NH3 / Et2O solution NH3 / Et2O, T = 0°C NH3 / C6H6, T = 10 - 20°C NH3 / C6H6, T < 30°C NH3 / C6H6, T = 30 - 80°C NH3 / C6H6, T < 30°C liq. NH3

Main product (H2SiNH)n n ~ 8 1) (MeHSiNH)n oligomer mixture (EtHSiNH)n (n = 3: 18%; n = 4: 82%) 2) (Me2SiNH)n (n = 3: 46%; n = 4: 54%) 2) (Me2SiNH)n (n = 3: 47%; n = 4: 53%) 2) (Me2SiNH)n (n = 3: 52%; n = 4: 48%) 2) (Me2SiNH)n (n = 3: 62%; n = 4: 38%) 2)

(Me2SiNMe)3 / C6H6, T = 0°C NH3 / C6H6, T < 25°C NH3 / C6H6 liq. NH3 NH3 / Et2O, T = - 80°C n Bu2SiCl2 / liq. NH3 t Bu2SiCl2 in nPentane / liq. NH3 NH3 / Tol, T = 110°C NH3 / Et2O, T = - 80°C NH3 / Tol, T = 0°C NH3 / C6H6, T = 25°C

(Me2SiN(SiMe2Cl))2 (MeEtSiNH)3 (MePhSiNH)3 (Et2SiNH)n (n = 3, 4) 3) i Pr2Si(NH2)2 (nBu2SiNH)3 t Bu2Si(NH2)2 (Ph2SiNH)3 Ph2Si(NH2)2 [(H2C=CH)HSiNH]n n ~ 6.3 [(H2C=CH)MeSiNH]n (n = 3: 76%; n = 4: 24%) 2)

Ref. [87] [98] [99] [75] [1] [45] [75, [100] [101] [102] [103] [100] [104] [72] [72] [105] [104] [89] [106]

1)

stable at -30°C; ages at room temperature by cross-linking and increase in viscosity [107]. relative amount of cyclotrisilazane : cyclotetrasilazane. 3) relative amount of cyclotrisilazane : cyclotetrasilazane not given.

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2)

Silyltriamines are thermally extremely labile and undergo rapid ammonia condensation thereby releasing oligo- or polysilsesquiazanes [(R)Si(NH)1.5]n [109]. Monomeric species RSi(NH2)3 are for this reason difficult to obtain and there has only (TriphSi(NH2)3) (Triph = 2,4,6-Ph3C6H2) been isolated so far in which a group R is bonded via carbon to silicon [110]. Investigations by X-ray diffraction on TriphSi(NH2)3 clearly pointed to the fact that in contrast to other silylamines published in the literature the N atoms possess a pyramidal coordination sphere indicating only minor Si-N π-bonding. Silyltriamines with the general structure R-X-Si(NH2)3 (X = N, O) in which organic bulks are linked to silicon via nitrogen or oxygen atoms were prepared by ammonolysis of 2,6-iPr2C6H3N(SiMe3)SiCl3 and 2,4,6t Bu3C6H2OSiCl3, respectively, and also structurally investigated by means of X-ray diffraction [111]. Furthermore 2,6-iPr2C6H3N(iPrMe2Si)Si(NH2)3 could be synthesized and structurally identified by means of single crystal X-ray diffraction [112]. A decreasing steric demand of the silicon-bonded substituent R results in partial ammonia condensation. For example ammonolysis of PhSiCl3 in the presence of sodium yielded a cyclotrisilazane with exocyclic NH2 groups: [113]

382

Markus Weinmann

3 PhSiCl 3

Na, NH 3

H H2N Ph N Si NH2 Ph Si

- NaCl - H2

HN

Si

H 2N

NH Ph

Reaction of trichlorosilanes having smaller (organic) substituents such as hydrogen, methyl, ethyl etc. with excess ammonia releases oligomeric or high-polymer silazanes. First publications appeared already in 1905 by Ruff and Albert [114] who reported on silicohydrocyanic acid (HSiN)n (“Siliciumstickstoffhydruer”) the (polymeric) silicon analog of hydrocyanic acid. It was synthesized from trichlorosilane HSiCl3 and NH3. The “intermediate” polysilsesquiazane (HSi(NH)1.5)n was published almost 20 years later by Stock and Zeidler [115]. Polysilsesquiazanes bearing organic groups R were first published by Andrianov and Kotrelev who performed ammonolysis of methyltrichlorosilane: [116]

Me Cl Si Cl + 4.5 NH3

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Cl

1 /n

R

H

Si

N

(NH)0.5

+ 3 NH4Cl n

Burns et al. investigated both alkyl and aryl polysilsesquiazanes to study the effect of changing the functional group on properties of the polymer and ceramics derived thereof. They found that high molecular weight polymers (Mw > 10000) were obtained if the functional group was either propyl or allyl [117]. If R was a sterically more demanding function, molecular weights decreased significantly. Lower molecular weights were also observed for R = Me, Et, most probably due to the decreased solubility. In contrast to polysilsesquioxanes, (RSiO1.5)n, which can be obtained by hydrolysis of trichloro- or trialcoxysilanes under water-starved conditions and which are structurally well investigated, [118-120] not much was known for a long time about the molecular structure of silsesquiazanes. However it was expected that both discrete molecules with cage structures as well as oligomeric and highly branched polymeric products exist. Roesky et al. could isolate and structurally characterize the first example of a hexameric silsesquiazane (H3C)6Si6(NH)9 [121]. It was obtained from sodium, ammonia (→ sodium amide) and (H3C)SiCl3 in nhexane solution at -78°C and subsequent crystallization from diethyl ether. The authors preferred the use of sodium amide over ammonia thereby avoiding the formation of ammonium chloride and improving the crystallization of the product. Single crystal X-ray diffraction analysis displayed a cage-type structure of (H3C)6Si6(NH)9 in which two six-membered ring systems [(H3C)Si(NH)]3 with chair conformation are linked via the silicon centers with each one NH unit (nitrogen-bonded H atoms are omitted for clarity):

Polysilazanes

383 Me Si

6 MeSiCl 3 + 18 Na + 9 NH 3

- 18 NaCl - 9 H2

N

NN

NN Me

Si Si

Me

N

Me Si

N N

Si Me Si Me N

N-bonded H atoms are omitted

Interestingly, two sets of differently bonded nitrogen atoms (6 + 3) could be distinguished: those which are involved in the six-membered rings (dSi-N = 172.0(3) pm) and those which connect the Si3N3 rings (dSi-N = 173.6(3) pm). Following this procedure, the ethyl-substituted cage derivative Et6Si6(NH)9 which possessed similar structural features was synthesized [113].

2.1.1.4. Ammonolysis of Tetrachlorosilane SiCl4 Ammonolysis of tetrachlorosilane was performed using various modes, i.e. with or without solvent, in liquid ammonia, in the gas phase, and using different stoichiometries. Treatment of ammonia in diethyl ether solution with excess SiCl4 yields monomeric chlorine-containing silazanes such as hexachlorodisilazane, Cl3Si-NH-SiCl3 or hexachlorocyclotrisilazane, [SiCl2NH]3 [122-124]. However, the reaction has to be performed with care to avoid further condensation. The preparation of a polysilazane from SiCl4 using excess ammonia was reported already in 1885 by Schützenberger and Colson [125] and re-investigated by Glemser and Naumann in 1959 [126]: Cl n Cl Si

Cl + 6 n NH 3

[Si(NH)2]n + 4 n NH 4Cl

Cl

silicon diimide

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Silicon diimide, which is a colorless insoluble and infusible polymeric solid, can be understood as a condensation product of (unknown) Si(NH2)4. Upon a heat treatment, ammonia elimination occurs, thereby releasing amorphous silicon nitride, a-Si3N4. 3

/n [Si(NH) 2]n

800 ºC

a-Si3N4 + 2 NH 3

Remarkably, this process releases silicon nitride in higher purity compared with direct nitridation of elemental silicon and has become an established process for the large-scale synthesis of silicon nitride with high sinter activity.

2.1.2. Aminolysis of Chlorosilanes RxSiCl4-x Aminolysis of chlorosilanes, i.e. reaction of chlorosilanes with primary or secondary amines, H2NR or HNR2 respectively, proceeds similar to ammonolysis. By a nucleophilic substitution, a silicon-bonded chloride Cl- is replaced with an amide group HNR- or NR2-.

384

Markus Weinmann

The use of secondary amines HNR2 in the aminolysis of chlorosilanes yields silylamines RxSi(NR2)4-x. [127] A replacement of the nitrogen-bonded group R (alkyl or aryl) is in general not possible. Silylamines can for this reason not be polymerized by simple condensation [128]. Using excess chlorosilane (or amine deficiency) results in the formation of chlorosilylamines RxSiCly(NR2)4-x-y [129]. Even though neither oligomeric nor polymeric species form, it will be shown in the following sections that silylamines are suitable starting compounds for the synthesis of polysilazanes by transamination reactions. Aminolysis of chlorosilanes R2SiCl2 using primary amines RNH2 gives access to monomeric, cyclo-, and oligosilazanes (c.f. Table 3). The structure depends on reaction condition and silicon-bonded substituents. However, in contrast to silazanes obtained by ammonolysis, cross-linking through the nitrogen atoms by formation of NSi3 sites is not possible. Remarkably, the nitrogen-bonded substituents may also inhibit formation of highly cross-linked or cage-type silsesquiazanes when performing aminolysis of trichlorosilanes or even of tetrachlorosilane. For example, treatment of RSiCl3 (R = Cl, Me, Et, Pr) with methylamine at 90 – 120°C in a 1: 5 molar ratio yielded cyclotrisilazanes with exocyclic NHR groups. No (or only minor) transamination, i.e. methylamine condensation and crosslinking occurred under these conditions. Similarly, the reaction of one equivalent of trichlorosilane or tetrachlorosilane with three parts of methylamine released trichlorocyclotrisilazanes or hexachlorocyclotrisilazane, respectively [130]. R Me N

R

Si N Me

15 MeNH 2 R

- 9 MeNH 3Cl

3 RSiCl 3

N

9 MeNH 2 - 6 MeNH 3Cl

R = Cl, Me, Et, Pr

Me N

Si

Cl R

NHMe

Cl Si

Me

Me N

Si

MeHN

R

NHMe Si

Si N

R Cl

Me

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Table 3. Synthesis of silazanes by aminolysis of chlorosilanes RxSiCl4-x using primary amines RNH2

1)

Chlorosilane H2SiCl2 HMeSiCl2

Reactant / Conditions MeNH2 / Et2O, -78→0°C MeNH2 / C6H6, T = 15 – 20°C

HEtSiCl2

MeNH2 / C6H6, T = 15 – 20°C

HViSiCl2 Me2SiCl2

MeNH2 / Tol, T = 0°C MeNH2 / C6H6, T = 15 – 20°C

MeViSiCl2 Et2SiCl2 Ph2SiCl2

MeNH2 / C6H6, T = 0°C MeNH2 / PE, T = 20°C EtNH2 / neat 150°C, 2 weeks MeNH2 / exp. details not given MeNH2 / C6H6, T = 15 – 20°C RNH2 / Tol, T = 110°C

Main product (H2SiNMe)n (n ~ 10), (H2SiNMe)4 (HMeSiNMe)n (n = 3: 65%; n = 4: 35%) 1) + oligomer (HEtSiNHMe)2NMe, (HEtSiNMe)3, + oligomer (SiHViNMe)n (n ~ 4) (Me2SiNHMe)2NMe 23%, Me2Si(NHMe)2 39% + oligomer (Me2SiNMe)n (n = 3: 94%; n = 4: 6%) 1) (Me2SiNMe)n (n = 3: 69%; n = 4: 31%) 1) (Me2SiNEt)3 MeViSi(NHMe)2 Et2Si(NHMe)2 Ph2Si(NHR)2 (R = Me, iPr, nHex)

relative amount of cyclotrisilazane : cyclotetrasilazane.

Ref. [131] [132]

[132]

[133] [132]

[214] [135] [136] [137] [132] [105]

Polysilazanes

385

2.1.3. Reaction of Chlorosilanes RxSiCl4-x with hydrazine Reactions of chlorosilanes with hydrazine, H2N-NH2, or its derivatives proceed according to ammonolysis or aminolysis of chlorosilanes as described above. Depending on the molecular structure of the chlorosilane used, i.e. the silicon-bonded substituents, either monomeric or polymeric species form along with hydrazinium chloride (hydrazine hydrochloride), N2H5Cl. Special care has to be taken with respect to the thermal lability of hydrazine which requires for a careful temperature control during the exothermic reaction with chlorosilanes. Treatment of hydrazine with monochlorosilanes R3SiCl (R = Me, Et, iPr, Ph) results in N,N’-bis(trialkylsilyl)hydrazine, R3SiNH-NHSiR3 [138]. Later it was found that mixtures of N,N and N,N’ isomers form [139]. Monosubstituted R3SiNH-NH2 can be obtained if R is a sterically demanding substituent such as Ph [140]. Access to fully silylated hydrazinederivatives by hydrazinolysis of halogenosilanes is limited. One exception is (H3Si)2NN(SiH3)2 which was obtained by Aylett in a gas phase synthesis using H3SiI and hydrazine [141]. H 4 H

Si

SiH3

H3Si I

+ 5 N 2H4

N H3Si

H

+ 4 N 2H 5I

N SiH3

It is a colorless liquid which is stable in vacuum up to 90°C and which is explosively oxidized in air. Electron diffraction studies point to the fact that nitrogen atoms in (H3Si)4N2 have a planar co-ordination sphere with a dihedral angle of 82.5°. The N-N distance of 148.7 pm [142] is only slightly longer than in hydrazine (144.9 pm [143]). Alternatively, (MeH2Si)4N2 was received from MeH2SiBr and H4N2 in the presence of triethylamine as an auxiliary base [144]. To obtain tetrakis(trialkylsilyl)hydrazine derivatives other synthetic procedures than dehydrohalogenation are required. For example, deprotonation of (Me3Si)2N-NH(SiMe3) [145] and subsequent reaction with p-toluenesulfonyl azide, Tos-N3, yields bis(trimethylsilyl)diimine. The latter decomposes when exceeding -35°C to tetrakis(trimethylsilyl)hydrazine, (Me3Si)2N-N(SiMe3)2 and molecular nitrogen [146, 147]. Me 3Si

Li

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N Me 3Si

N

+ Tos-N 3 SiMe 3

-78ºC

Me 3Si

N

- N2 - Tos-NSiMe 3-Li+

SiMe 3

Me 3Si

-78ºC x2 - N2

N SiMe 3

N Me 3Si

N

.

SiMe 3

A very unusual approach towards tetrakissilylhydrazines was reported by Binnewies et al. Microwave plasma-assisted reaction of tetrachlorosilane and molecular nitrogen gave – though in small quantities – (Cl3Si)2N-N(SiCl3)2 which was assigned by GC-MS [148]. H

Me H Si

3 MeHSiCl 2 + 2 H 2NNH2

Et3N - Et 3NHCl

1/

Me Si

N n

HN Me

NH Si

N H

n

386

Markus Weinmann

Treatment of hydrazine with dichlorosilanes R2SiCl2 instead of chlorosilanes R3SiCl results in the formation of cyclic oligomers or polymers (polysiladiazanes). For example, hydrazinolysis of Me2SiCl2 yielded [SiMe2-NH-NH-]n, the average degree of polymerization depended on the reaction conditions applied [149, 150]. Using similar conditions, [SiViMeNH-NH-]n was obtained from ViMeSiCl2, H4N2, and Et3N. [151]. A polymer structure involving tetraazadisilacyclohexane rings bridged by MeHSi groups was suggested on the basis of 29Si NMR measurements for the product of the reaction of MeHSiCl2 and H4N2 [152].

2.2. Si-n Coupling without Salt Formation Due to the commercial availability of a variety of chlorosilanes, synthesis of polysilazanes is in general performed by ammonolysis and aminolysis of chlorosilanes as described above. However, a significant drawback of such processes is the formation of solid by-products during polymerization, i.e. ammonium chloride or amine hydrochlorides. The precipitates have to be separated subsequently from the polymers by filtration processes and must be disposed. The filtration step may be very time-intensive and in some cases difficult to perform. For example, highly cross-linked precursors which possess low solubility are generally difficult to separate from the insoluble salts that form. On the other hand, ammonium chloride and amine hydrochlorides are both partially soluble in highly concentrated silazane solutions causing chlorine impurities in the polymers. Another disadvantage is the low degree of polymerization, i.e. low molecular weights because of the formation of low molecular weight cyclomers. To avoid these drawbacks, alternative approaches to polysilazanes such as dehydrocoupling reactions of hydridosilanes and ammonia or amines, redistribution of silazanes and ring-opening polymerization of cyclosilazanes were developed. Such synthetic procedures can furthermore provide improved control of selectivity / purity over the conventional process and have thermodynamic advantages.

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2.2.1. Catalytic dehydrocoupling Cross-dehydrocoupling of ammonia or amines and hydridosilanes as molecular building blocks occurs with elimination of molecular hydrogen as the only by-product: Si

H +

H

N

[cat.] - H2

Si

N

It requires suitable catalysts and / or thermal activation. Efficient catalysts are either strong bases, transition metals or transition metal complexes. It was developed for both, the assembly of polysilazanes and the cross-linking of low-weight oligosilazanes to highly branched polymeric networks.

2.2.1.1. Dehydrocoupling Reactions Using Strong Bases as Catalysts The transformation of oligomeric (SiHMe-NH)n into a highly cross-linked polymer by potassium hydride-catalyzed dehydrocoupling was already argued in the previous section and

Polysilazanes

387

is therefore not discussed here again. Remarkably, dehydropolymerization of ammonia and hydridosilanes using KH as a catalyst has not been published so far.

2.2.1.2. Dehydrocoupling Reactions Using Transition Metal Catalysts The first publications on transition metal-catalyzed methods for the synthesis of oligosilazanes appeared in 1986. They were based on a procedure described by Sommer and Citron for the synthesis of monosilazanes such as R3Si-NHiBu from R3SiH and iBuNH2 using palladium on carbon or alumina as a catalyst [153]. Blum and Laine explored the catalytic activity of Ru3(CO)12 and Rh6(CO)16 for the oligomerization by dehydrocoupling of tetramethyldisilazane, HMe2Si-NH-SiMe2H, with ammonia [154]. Soon after, polymerization of dialkylsilanes and diarylsilanes with ammonia was reported [155]. R H

Si

R [Ru3(CO) 12] - H2

H + NH 3

1/n

R

Si

N

R

H

n

R = alkyl, aryl

The background was an investigation on the mechanism of catalytic ring-opening polymerization (ROP) of octamethylcyclotetrasilazane. The authors observed that even at low hydrogen pressure transition metal-catalyzed ROP was enhanced by two orders of magnitude compared with hydrogen-free environments. They found furthermore that metal hydrides such as H4Ru4(CO)12 are equally active [156] even in the absence of hydrogen and concluded that catalysis proceeds via hydrogenation of Si-N bonds (ring opening) followed by reaction of N-H and metal-activated Si-H bonds. Based on these observations, a mechanism including transition metal silyl hydrides was suggested [157-159]: Si

H2

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H

Si

NRH

H

M

M

Si

H

Si

M RNH

M

H

H

RNH2

H

M = Transition metal complex

Initially, Si-H adds oxidatively to the transition metal complex M thereby forming a metal silyl hydride Si-M-H. Ammonia or amine is coordinated and a silazane released subsequently. Finally, reductive elimination of molecular hydrogen from the metal dihydride recycles the catalyst. The suggested mechanism was confirmed by Wang and Eisenberg [160] who studied dehydrocoupling reactions of silanes and amines promoted by the binuclear rhodium complex Rh2H2(CO)2(dppm)2 (I, dppm = Ph2P-CH2-PPh2). They could isolate and structurally identify proposed reaction intermediates:

388

Markus Weinmann H

R Rh

OC P

Rh CO

H P P

RSiH3 - H2

P

NHMe

R

Si

H

Si

H H Rh Rh OC CO P P P P

MeNH 2 - H2

H H Rh Rh OC CO P P P P

P = PPh 2 I

II

IV

H Si R H Rh Rh OC CO P P P P H

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III

free coordination site

- activation of Si-H or - coordination of MeNH 2

Primary silanes react rapidly with I to Rh2(μ-SiHR)H2(CO)2(dppm)2 (II) whereas primary amines such as methyl amine do not. II is fluxional and intramolecular reductive Si-H elimination generates a coordinatively unsaturated intermediate III. The free coordination site may either enable activation of Si-H groups or coordination of methyl amine, launching the catalysis of dehydrogenative Si-H coupling. Whereas III was not yet isolated, the molecular structure of IV was confirmed spectroscopically. However, even though IV is an intermediate, it is uncertain whether it is part of the catalytic cycle or is simply connected to the cycle by a facile equilibrium. Liu and Harrod reported on dehydrogenative coupling reactions of ammonia and primary, secondary as well as tertiary hydridosilanes catalyzed by mononuclear dimethyltitanocene (DMT) [161]. Tertiary silanes, e.g. Ph2SiMeH or PhSiMe2H were transformed into disilazanes. In the reaction of secondary PhMeSiH2 it was observed that the silane was consumed very rapidly thereby forming the disilazane PhMeHSi-NH-SiHMePh. Long reaction times were required to obtain higher molecular weight oligomers. Even though the reaction between silanes and amines is very sensitive to steric factors, [162] the reactivity surprisingly decreased when changing to primary silanes RSiH3 (R = Me, Ph). The origin of this unexpected behavior was explained with a competition of homodehydrocoupling of RSiH3 which proceeded with Si-Si linking and cross-dehydrocoupling i.e. amination of silanes. Kinetic measurements pointed to the fact that homodehydrocoupling [163-165] was faster than the amination. Consequently, amination of simple Si-Si oligomers must have occurred for the incorporation of nitrogen into the oligomeric product. Thereby polyaminosilanes [SiR(NH2)]n are formed rather than polysilazanes [SiRH-NH]n: R Si H

R

NH3, [DMT] H

N H

n

- H2

Si H

"cross-dehydrocoupling"

NH3, [DMT]

R Si

H - H2

NH2 n

1. "homodehydrocoupling" 2. amination

R = Me, Ph; DMT = ( η 5-C5H5)2TiMe 2

Polysilazanes

389

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To improve selectivity, Liu and Harrod also examined copper(I)chloride as catalyst [166]. However, the catalytic activity was lower than that of DMT [167] and only low-weight oligomers were obtained. Harrod et al. furthermore investigated dehydrogenative Si-N coupling using hydrazine and various methylhydrazine derivatives [168, 169]. Dehydrocoupling of PhSiH3 with H2NNH2 yielded oligosilylhydrazines bearing -Si-NH-NH- building blocks whereas the use of Ph2SiH2 resulted in the formation of a cyclodimer (Ph2SiNHNH)2 which was characterized by single crystal X-ray analysis. The molecule has a chair-like conformation of the Si2N4 ring centered about an inversion center. Interestingly, two nitrogen atoms have a planar coordination sphere, whereas the other two have pyramidal geometry (sum of angles 335°). Remarkably, a mixture of a polysilazane and a polysilane, i. e. poly[(phenylsilyl)-N(methylamino)azane], [SiHPh-N(NHMe)] and poly[(phenyl)(methylhydrazyl)silane] [SiPh(NHNHMe)] was obtained when methylhydrazine was reacted, indicating that homodehydrocoupling with Si-Si bond formation competed with Si-N linking. Cross-dehydrocoupling of silanes and amines was also performed using the cationic actinide complex [(Et2N)3U][BPh4] [170]. This complex was proven successful in hydrosilylation of terminal alkynes with PhSiH3 involving uranium-hydride species [171]. The question that arose was whether the amido ligand Et2N could be replaced with RHN and whether the respective complexes catalyze dehydrocoupling. It turned out that [(RHN)3U][BPh4] indeed efficiently catalyses Si-N dehydrocoupling with very high selectivity i.e. without homodehydrocoupling, but oligomeric or polymeric species were not yet obtained.

2.2.2. Redistribution Reactions Alternative approaches which release poly- and oligosilazanes without formation of solid by-products are redistribution reactions. Using such polymerization techniques avoids timeintensive processing steps, i.e. the removal of ammonium chloride or amine hydrochlorides. Moreover, the contamination of the polymers with chloride (or other halides), which acts as a catalyst in the thermally induced cleavage of Si-N bonds is completely prevented. Two principally different reaction pathways have been published: (i) transamination and (ii) transsilylation i.e. amine or silane exchange, respectively (note: in the literature the terms trans-amination and trans-silylation also refer to trans-addition reactions to olefins or alkynes). Transamination reactions involving ammonia, primary amines, hydrazines, or diamines and either diaminosilanes, triaminosilanes, or tetraaminosilanes [172, 173] proceed according to 1

1

R R2N

Si 2

R

NR2

3 R NH2

- 2 HNR 2

R 1/n

Si 2

R

N 3

R

n

R = H, alkyl 1 2 R , R = H, alkyl, aryl, NR 2 3 R = H, CH 3, CH 2CH=CH2, Ph

Since chlorosilane intermediates are not required in this process, high purity oligosilazanes are obtained. In this context, a new synthetic pathway for

390

Markus Weinmann

dimethylaminosilanes, which has been developed by Kanner and Herdle, is worth being mentioned. Direct reaction of silicon metal with dimethylamine in a fluidized bed reactor gave (Me2N)3SiH in 90% yield [174]. However, catalytic amounts of strong organic acids such as p-toluenesulfonic acid or trifluoroacetic acid were required to produce non-volatile high-molecular weight polysilazanes from (Me2N)3SiH if performing either ammonolysis or aminolysis using MeNH2. Condensation polymerization of tetrakis(alkylamino)silanes, Si(NHR)4 [175-177] by simple heat treatment is a special transamination reaction which does not require an “external” amine source. The starting compounds in which silicon is only bonded to nitrogen are obtained readily from silicon tetrachloride and excess alkylamine. SiCl4

exc. RNH 2 - RNH 3Cl

Si(NHR)4

heat - RNH 2

Polysilazane

As proposed by Andrianov et al. subsequent heating causes polymerization by condensation of alkyl amine leading to complex mixtures of species containing four- and/or six-membered ring systems, depending on the amine group. For R = nPr or Ph polymers with Si2N2 rings having a spirocyclic structure at silicon (in the literature referred to as poly(spirocyclosilaorganoazanes)) were suggested:

Si(NHR)4

heat - RNH2

R N 1/2n

Si

Si N R

R N Si N R n

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n R = C3H7, Ph

On the basis of solid state 29Si NMR spectroscopy Narsavage et al. suggested a more complicated structure for polymers derived from Si(NHEt)4 in which Si2N2 rings are linked via NR bridges rather than a spirocyclic assessment. Transsilylation reactions using disilazanes such as commercially available hexamethyldisilazane Me3Si-NH-SiMe3 or heptamethyldisilazane Me3Si-NMe-SiMe3 and chlorosilanes R4-nSiCln (n = 2 – 4; R = Cl, alkyl, aryl …) were reported already in the early 1970’s [178, 179]. If performing such reaction with excess chlorosilane, the trimethylsilyl groups in Me3Si-N(R’)-SiMe3 (R’ = H, Me) are replaced stepwise thereby releasing R3-nClnSi-N(R’)-SiMe3 and R3-nClnSi-N(R’)-SiClnR3-n as well as Me3SiCl as the couple product. The replacement of the second Me3Si group usually requires the presence of suitable catalysts such as tetraalkyl ammonium fluorides, R4NF [180]. If changing the stoichiometry, i.e. to HMDS : R4-nSiCln > 3 polysilazanes are obtained. For example, treatment of HMDS with HSiCl3 in a 3:1 molar ratio released a melt-spinnable hydridopolysilazane in a strongly exothermic reaction. Me3SiCl and excess HMDS were removed by distillation while heating the polymer sample to 150°C [181]. Polymerization proceeds in analogy to ammonolysis of chlorosilanes. Initially, a SiMe3 group in HMDS is replaced by SiHCl2. Me3Si-NH-SiHCl2 that forms and excess HMDS react further to (Me3Si-NH)2SiHCl and finally to (Me3Si-NH)3SiH. Subsequent HMDS condensation delivers the hydridopolysilazane [HSi(NH)1.5]n [182, 183]:

Polysilazanes HSiCl3

391

HSiCl3

NH3

Me 3SiNHSiMe 3 - HCl * )

substitution

- Me 3SiCl

HSi(NH2)3

HSi(NHSiMe 3)3

- NH 3

- Me 3SiNHSiMe 3

[HSi(NH)1.5]n

condensation

[HSi(NH)1.5]n

*) as NH4Cl

2.2.3. Ring-Opening Polymerization Ring-opening polymerization (ROP) is a compartment of addition polymerization, in that an end of a growing (in the literature frequently also referred to as living) polymer chain acts as a reactive center that cleaves the cyclic structure of a monomer (which results in the loss of cyclic structural motifs) thereby propagating a linear polymer chain. ROP is typically initiated through cationic or anionic catalysts. Remarkably, this procedure allows for the synthesis of polymers such as polysiloxanes, [184, 185] polysilanes [186] as well as polysilazanes with narrow polydispersity.

X Y

X [cat]

X Y Initiation

Y

X [cat]

Y

X

Y

X

Y n

Propagation

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[cat]

ROP of cyclotrisiloxanes using strong nucleophilic reactants such as metal hydroxides (usually referred to as anionic ROP) proceeds via formation of metal silanolate. It is wellknown for more than 50 years and especially useful in silicone synthesis [187]. Ring-opening reactions of cyclotrisilazanes and cyclotetrasilazanes were published subsequently by Andrianov et al. [188]. It was found that the behavior of the latter was completely different from that of siloxanes. Product formation depended on silicon-bonded substituents, the electrophile or nucleophile used and reaction temperature. For example, treatment of [(CH3)2SiNH]3 with AlCl3 at 140°C gave a polysilazane in 16% yield. Increasing the temperature to 160°C increased the yield only by 1% whereas after heating to 240°C 70% of polymer could be isolated along with ca. 20% of [(CH3)2SiNH]4. Transition metal-catalyzed ROP of cyclotrisilazanes or cyclotetrasilazanes [189] (this reaction type was already mentioned in the previous section in connection with polymerization by dehydrocoupling) proceeds slowly and releases low molecular weight oligomers because of a competition of ROP with dehydrocoupling, resulting in a termination of the polymer chain. The low degree of polymerization is mainly a consequence of the thermodynamic stability of the monomers, i.e. low ring strain.

392

Markus Weinmann

To increase the reactivity it is therefore mandatory to reduce the number of ring atoms. The smallest ring systems with alternating Si-N sequences are cyclodisilazanes, which were first published by Fink, [190-194] Rochow et al., [195, 196] Bush et al., [197, 198] Wannagat, [101] and Breed et al. [199]. The four-membered Si-N-Si-N ring systems possess significantly higher ring strain compared to their six- or eight-membered analogs. A review on such silicon-nitrogen heterocycles providing synthetic approaches, molecular structures and selected properties is given in Ref. [200]. Stoichiometric treatment of cyclo-(Me2SiNR)2 (R = Me, Et, iPr) with organo lithium reagents R’Li results in the cleavage of the Si2N2 ring and formation of R’Me2Si-NR-SiMe2NRLi. The lithiated species are subsequently quenched with Me3SiCl, releasing R’Me2Si-NRSiMe2-NRSiMe3 [198, 201]. R SiMe 2 + R'Li

Me 2Si

Me

Et2O or THF solution

N

R'

N

Me

Si

N

Me R

Si

Me N

Me 3SiCl Li - LiCl

R'

Si

Me Si

N

N

SiMe 3

.

Me R

Me R

Me R

R R = Me, Et, iPr R' = Me, Bu

Catalytic ROP of cyclo-(Me2SiNMe)2 using RLi or RNa (R = Me, n-Bu) and subsequent quenching with Me3SiCl as reported by Seyferth et al. released linear polymers of type RMe2Si-NMe-(SiMe2-NMe)n-SiMe3 (n = 11-35). The required reaction temperatures reflected the different reactivity of the organoalkali reagents. Whereas RLi-initiated ROP was started at 0°C and finally heated to reflux (in THF), RNa-catalyzed ROP was started a -70°C and the reaction mixture was finally warmed to room temperature. Using RK instead it appeared that the propagation process was interrupted by “back-biting” of the reactive chain terminus, resulting in the extrusion of cyclo-(Me2SiNMe)3: Me Me Me

N Me 2Si

SiMe 2 + RK

RMe 2Si

N

N

Si Me

K N Me

Me

N

Me

Me2Si

SiMe2

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N Me

Me Me N Me 2Si MeN

RMe 2Si

N

Me Me Si Me N

Si

N

SiMe 2 NMe Si Me 2

- RMe 2SiNMeK

Si

K N Me

Me Me

Me Me

Me

Against this, ROP was not observed when reacting cyclo-(Me2SiNiPr)2 instead of cyclo(Me2SiNMe)2 under similar conditions, most probably due to steric hindrance of the initially formed RSiMe2NiPrSiMe2NiPrLi.

Polysilazanes

393

Very detailed investigations on both anionic and cationic ROP of cyclosilazanes were performed by Soum et al., [202-206] confirming the primarily results of Seyferth et al. The degree of polymerization, i. e. the molecular weight mainly depended on the molecular structure of the monomeric starting compound whereas the nature of the catalyst had only minor influence. For example cyclotrisilazanes and sterically crowded cyclodisilazanes of the general type cyclo-(Me2SiNR)2 (R = tBu, iPr) did not polymerize at all (or only very slowly). Cyclo-(Me2SiNEt)2 could be oligomerized in the presence of tBuLi or naph-Na. Treatment of (Me2SiNMe)2 with MeLi, PhLi, tBuLi gave polymers with M = 4000 within 8 min., the use of naph-Na released a polymer with M = 16000 within 2 min. Similar results were obtained when performing cationic ROP using methyl triflate, F3CSO3Me as a catalyst. Polymerization did not take place when using sterically crowded cyclodisilazanes. The tentative mechanism was investigated thoroughly [207]. It was proposed that in dichloromethane solution F3CSO3Me-catalyzed ROP proceeds via electrophilic attack of Me+ on one of the nitrogen atoms in cyclo-(Me2SiNMe)2 (“D2NMe”) with formation of an ammonium ion cyclo-(SiMe2NMeSiMe2NMe2)+. By ring opening the latter attacks another cyclodimer thereby releasing the modified ammonium ion A:

Me

Me 2 Si N N Si Me 2

Me

F3CSO 3Me Me - F 3CSO 3-

"D 2NMe"

Me

Me 2 Si + N N Si Me 2

Me 2N

Me 2N

Me 2 Si

- "D 4NMe"

Me 2 Me 2 Si + Si N N N Me Me Si Me 2 A "D 2NMe"

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"D 2NMe"

"D 2NMe" Me 2 Si

Me

Me

Me N

SiMe 2 NMe

MeN Me 2Si Me

(i)

Me 2 Me 2 Si + Si N N N Me 3 Me Si Me 2

(ii)

Me 2Si

+ N

SiMe 2 Me

"D 4NM eMe +"

Me

"D 2NMe" Polymer

The newly built species A has two options to react further: (i) intermolecular addition of “D2NMe” and chain propagation with formation of a linear polymer or (ii) intramolecular cyclization (indicated by the doted arrows) with formation of cyclo-(Me2SiNMe)4Me+ (“D4NMeMe+”). In terms of relative basicity, intramolecular cyclization should be preferred since basicity of organosilylamines increases in the row (Me3Si)2NR < Me3SiNHR