Synthetic Biodegradable and Biobased Polymers: Industrial Aspects and Technical Products (Advances in Polymer Science, 293) 3031458613, 9783031458613

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Advances in Polymer Science  293

Andreas Künkel · Glauco Battagliarin · Malte Winnacker · Bernhard Rieger · Geoffrey Coates   Editors

Synthetic Biodegradable and Biobased Polymers Industrial Aspects and Technical Products

Advances in Polymer Science Volume 293

Editorial Board Members Akihiro Abe, Tokyo Polytechnic University, Yokohama, Japan Ann-Christine Albertsson, KTH Royal Institute of Technology, Stockholm, Sweden Geoffrey W. Coates, Cornell University, Ithaca, NY, USA Jan Genzer, North Carolina State University, Raleigh, NC, USA Shiro Kobayashi, Kyoto Institute of Technology, Kyoto Sakyo-ku, Japan Kwang-Sup Lee, Hannam University, Daejeon, Korea (Republic of) Ludwik Leibler, Ecole Supe`rieure de Physique et Chimie Industrielles (ESPCI), Paris, France Timothy E. Long, Virginia Tech, Blacksburg, VA, USA Martin Mo¨ller, RWTH Aachen DWI, Aachen, Germany Oguz Okay, Istanbul Technical University, Istanbul, Tu¨rkiye Virgil Percec, University of Pennsylvania, Philadelphia, PA, USA Ben Zhong Tang, The Chinese University of Hong Kong, Shenzhen, Shenzhen, China Eugene M. Terentjev, University of Cambridge, Cambridge, UK Patrick Theato, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany Brigitte Voit, Leibniz Institute of Polymer Research Dresden (IPF), Dresden, Germany Ulrich Wiesner, Cornell University, Ithaca, NY, USA Xi Zhang, Tsinghua University, Beijing, China

Aims and Scope The series Advances in Polymer Science presents critical reviews of the present and future trends in polymer and biopolymer science. It covers all areas of research in polymer and biopolymer science including chemistry, physical chemistry, physics, and material science. The thematic volumes are addressed to scientists, whether at universities or in industry, who wish to keep abreast of the important advances in the covered topics. Advances in Polymer Science enjoys a longstanding tradition and good reputation in its community. Each volume is dedicated to a current topic, and each review critically surveys one aspect of that topic, to place it within the context of the volume. The volumes typically summarize the significant developments of the last 5 to 10 years and discuss them critically, presenting selected examples, explaining and illustrating the important principles, and bringing together many important references of primary literature. On that basis, future research directions in the area can be discussed. Advances in Polymer Science volumes thus are important references for every polymer scientist, as well as for other scientists interested in polymer science - as an introduction to a neighboring field, or as a compilation of detailed information for the specialist. Review articles for the individual volumes are invited by the volume editors. Single contributions can be specially commissioned. Readership: Polymer scientists, or scientists in related fields interested in polymer and biopolymer science, at universities or in industry, graduate students.

Andreas Ku¨nkel • Glauco Battagliarin • Malte Winnacker • Bernhard Rieger • Geoffrey Coates Editors

Synthetic Biodegradable and Biobased Polymers Industrial Aspects and Technical Products With contributions by F. Adams
M. Agari
L. Al-Shok
T. Aoshima
J. Auffermann
M. P. Barth
G. Battagliarin
A. Denk
D. M. Haddleton
S. Kato
A. Kindler
N. Kosaka
A. Künkel
F. Lauer
J. Lohmann
C. Lott
A. Nabifar
S. Nitta
B. Rieger
M. Sander
K. O. Siegenthaler
X. Tang
T. Ueda
M. Ullrich
M. Weber
F. Weinelt
M. Winnacker
M. Yamamoto
O. Zelder
M. Zumstein

Editors Andreas Ku¨nkel BASF SE Ludwigshafen am Rhein, Germany

Glauco Battagliarin BASF SE Ludwigshafen am Rhein, Germany

Malte Winnacker WACKER-Lehrstuhl fu¨r Makromolekulare Technische Universita¨t Mu¨nchen Garching bei Mu¨nchen, Germany

Bernhard Rieger WACKER-Lehrstuhl fur Makromolekular Technische Universitat Munchen Garching b. Mu¨nchen, Bayern, Germany

Geoffrey Coates Department of Chemistry and Chemical Biology Cornell University Ithaca, NY, USA

ISSN 0065-3195 ISSN 1436-5030 (electronic) Advances in Polymer Science ISBN 978-3-031-45861-3 ISBN 978-3-031-45862-0 (eBook) https://doi.org/10.1007/978-3-031-45862-0 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 Chapters “BioPBSTM (Polybutylene succinate)” and “Polymer biodegradability 2.0: A holistic view on polymer biodegradation in natural and engineered environments” are licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/). For further details see license information in the chapter. This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Preface

Polymers are an essential part of our modern life and fulfill multiple functions. Structural polymers are used for different consumer goods like shoes and cloth, in the transportation industry, for electronic goods or hygienic packaging of our food, just to mention some of the major applications. Functional polymers are used in the home and personal care industry or to apply agricultural ingredients in an efficient way. In both areas, polymeric materials have essential functions and enable improvements for sustainability (polymers are essential parts of wind turbines, photovoltaic panels, new batteries for transportation or polymers that enable the use of less energy and reduce water consumption for washing cloth or dishes). Despite these advantages, the polymer community (academic and industry) faces two major challenges and tasks, respectively: enable a circular flow of polymers and become carbon neutral. The actual polymer solutions need to be improved significantly and new approaches as part of a multiple “solution toolbox” have to be applied (Ellen McArthur 2014; SYSTEMIQ 2022; Von Vacano et al. 2023). This book concentrates on structural polymers and will give a deep insight, how synthetic biodegradable and biobased polymers contribute to a circular economy and improve the carbon footprint as part of a “solution toolbox” for all types of existing structural polymers. To support the development of appropriate end-of-life management and improved carbon footprint, different legislative measurements have been implemented in the EU and first measurements are taken as well in Asia (e.g., China). To understand the overall “solution toolbox,” as a first step, a categorization of all structural polymeric materials is helpful. With this approach, structural polymers can be divided into four major categories (see Fig. 1). (1) Durable and fossil polymers. Many of the traditional polyolefins or polyamides belong to this category. These polymers are durable (=non-biodegradable) and are derived from fossil feedstocks. (2) Durable and biobased polymers (Siegenthaler et al. 2022). These are 1:1 biobased replacements of the fossil counterparts (e.g., PE, PP) or modified variants (e.g., polyamide 5.10). The major property of durability or non-biodegradability remains and is not influenced by the biobased content, because v

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Durable polymers (Category 1,2)

Biodegradable polymers (Category 3,4)

Bio PE

PLA Polyamide 5.10

PHA

Starch

Biobased raw materials

Compounds (e.g. ecovio®, Mater-Bi®)

Biodegradable polyester

PE Polyamide 6.6

(e.g. PBS, Origo-Bi® , ecoflex®)

Fossil raw materials

Fig. 1 Biodegradable & biobased polymers and durable & biobased polymers. Biobased (renewable) or recycled refers to the origin of the carbon atoms in the polymers. Biodegradation ability (catalyzed by micro-organisms) is a matter of polymer structure, not of carbon origin

biodegradability/non-biodegradability is a matter of the polymer structure and not of the carbon origin. (3) Biodegradable and fossil polymers (Ku¨nkel et al. 2016). These polymers (e.g., PBAT) are biodegradable and the fossil variants have been the starting point of the development of biodegradable synthetic polymers simply based on monomer availability at the time of development in the 1990s. With the increasing availability of biobased monomers like 1,4 Bio-butanediol and biobased dicarboxylic acids, a shift to biodegradable and biobased variants has been started. (4.) Biodegradable and biobased polymers (Ku¨nkel et al. 2016). A significant part of the respective biodegradable market products and applications is biobased as well (e.g., certified compostable organic waste bags and fruit & vegetables bags). The biobased content is depending on the raw material cost position and the legislative requirements (e.g., Italy, France). Overall, this category is getting higher relevance for biodegradable polymers in the future. Besides the biobased property, biodegradability is the second core property of this material class and is dependent on different material properties, environmental and biological factors. Significant progress has been made in the recent years to understand this complex interaction, whereas biodegradation is a process catalyzed by microbes which converts polymeric materials into energy, CO2, water, and biomass (Zumstein et al. 2018; Nelson et al. 2022). As a second step toward an overall “solution toolbox,” the life cycle of polymeric materials needs to be described (see Fig. 2). There are 2 major cycles for polymeric materials: the first, “technical cycle” targets the reuse of polymeric materials and is based on appropriate collection systems of the materials and different technologies to include polymeric materials back into the loop e.g., mechanical- or chemical recycling, gasification (Mangold and von Vacano 2022). The “biological cycle” targets applications to enable the appropriate recycling of organic waste (or “organic value matter”) for the generation of biogas and valuable

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Technical cycle

Biological cycle

(durable & biobased materials)

(biodegradable & biobased materials)

Collection 3

of „biowaste“

Collection C 2

3

2

Compost

Consumer

Recycling

Product 4 Recycled and renewable raw materials

Agriculture

Product

1

4

1

Production

H2

Renewable energy and hydrogen

Production

Renewable raw materials

Fig. 2 Circular economy approach: the future

compost and thus closing the nutrient loop. This cycle includes applications where collection and recycling are difficult to realize like mulch films and therefore the biodegradable counterpart is the superior solution due to cost/performance when sustainability requirements are included. In both cycles, the collection of the materials is a decisive point to prevent leakage of the material streams. Therefore, effective collection systems for both cycles are an important task of the next years (Ellen Mc Arthur 2014; SYSTEMIQ 2022). In line with an increased use of recycled and renewable raw materials for the two cycles, the availability of renewable energy and hydrogen will be part of the future development to increase the sustainability of structural polymer use (see Fig. 2). The four categories of polymers can be connected to the two cycles (technical and biological): Durable and fossil (category 1), as well as durable and biobased polymers (category 2) target the technical cycle. Appropriate collection of these two categories is an essential requirement to prevent negative effects on the environment (Ellen Mc Arthur 2014; SYSTEMIQ 2022). The biodegradable polymers (category 3,4) mainly target the “biological cycle” (Ku¨nkel et al. 2016; Bauchmu¨ller et al. 2021). Whereas, technically, they can be applied as well in a “technical loop.” Recycling of polyesters is a standard process which can be applied to different polyesters like PET, but is also possible for, e.g., PLA. ecovio® coated paper board can enter the “technical loop” of paper recycling as well as having the composting option. Therefore, these biodegradable and biobased polymers (category 4) have in principle the option to enter both cycles, in contrast to the categories of the durable materials (category 1 and 2) which are restricted to the “technical cycle.” The chapters in the book will give an insight in the recent developments for synthetic biodegradable and biobased polymers and their contribution to the end-oflife management in the two cycles, enabling a circular economy for structural polymeric materials. Starting with recent availability of biobased building blocks (Zelder et al. (BASF SE)), new terpenes (Winnacker (TU Munich)), and the significant progress in understanding the principles of biodegradation (Sander et al. (ETH Zu¨rich)), the fundament for the material classes of biodegradable and

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biobased materials is described in detail. The progress for biodegradable and biobased materials is described in the following chapters. Siegenthaler et al. (BASF SE) provide a chapter about the important polyesters ecovio® and ecoflex®. Rieger et al. (TU Munich) summarize new developments of the synthesis and biodegradability of CO2-based polycarbonates. Closely related is an article of F. Adams et al. (Universita¨t Stuttgart) about recent developments of ring-opening polymerizations of different lactones to (bio)polyesters. Kato et al. (Mitsubishi Chemical Corporation) present the recent developments of (bio)polybutylene succinate (PBS). Tang (Peking University) presents the latest advances of catalytic pathways toward poly(hydroxyalkanoates) (PHAs), giving an overview on three possibilities to design materials with defined properties. Information on durable and biobased polymers will conclude the chapters in this book. Weinelt and Ullrich (both Evonik Industries) together with Winnacker (TU Munich) provide an overview about biobased polyamides and their academic and industrial aspects for their development and respective applications. Synthetic biodegradable and biobased polymers are part of an overall approach of the polymer community to enable a circular flow of polymers, thus preventing negative effects toward the environment and become carbon neutral. This book is a contribution to a constructive, fact-based discussion and to encourage all stakeholders to combine the positive contributions of polymeric materials for our life today with a circular and carbon neutral economy based on the application and realization of the significant potentials of polymer science and technology. Ludwigshafen, Germany Ludwigshafen, Germany Mu¨nchen, Germany Mu¨nchen, Germany Ithaca, NY, USA

Andreas Ku¨nkel Glauco Battagliarin Malte Winnacker Bernhard Rieger Geoffrey Coates

References Bauchmu¨ller V, Carus M, Chinthapalli R, Dammer L, Hark N, Partanen A, Ruiz P, Lajewski S (2021) BioSinn, products for which biodegradation makes sense. nova-Institut fu¨r politische und € okologische Innovation GmbH. Accessible at www.renewable-carbon.eu/publications Ku¨nkel A, Becker J, B€orger L, Hamprecht J, Koltzenburg S, Loos R, Schick MB, Schlegel K, Sinkel C, Skupin G, Yamamoto Y (2016) Polymers, biodegradable. Ullmann’s encyclopedia of industrial chemistry Mc Arthur E (2014) The new plastic economy. Rethinking the future of plastics Nelson TF, Baumgartner R, Jaggi M, Bernasconi SM, Battagliarin G, Sinkel C, Ku¨nkel A, Kohler HP, McNeill K, Sander M (2022) Biodegradation of poly(butylene succinate) in soil laboratory incubations assessed by stable carbon isotope labelling. Nat Commun 13:5691 Mangold H, von Vacano B (2022) The frontiers of plastic recycling: rethinking waste as a resource for high value applications. Macromol Chem Phys 223(1-17):2100488

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Siegenthaler KO, Jung P, Ku¨nkel A, Lohmann J, Loos R, Neumann P, Parlings M, Porc O, Schr€ oder H, Van Berkel J, Xi L, Yamamoto M (2022) Polymers, biobased. Ullmann’s encyclopedia of industrial chemistry SYTEMIQ (2022) Reshaping plastics (published 4th of April 2022) Von Vaccano B, Mangold H, Vandermeulen GWM, Battagliarin G, Hofmann M, Bean J, Ku¨nkel A (2023) Angew Chem Int Ed 62: e202210823 Zumstein MT, Schintlmeister A, Nelson TF, Baumgartner R, Woebken D, Wagner M, Kohler HPE, McNeill K, Sander M (2018) Biodegradation of synthetic polymers in soils: Tracking carbon into CO2 and microbial biomass. Sci Adv 4(7): eaas9024

Contents

Biotechnological and Chemical Production of Monomers from Renewable Raw Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alois Kindler and Oskar Zelder The Terpenes Limonene, Pinene(s), and Related Compounds: Advances in Their Utilization for Sustainable Polymers and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Malte Winnacker Polymer Biodegradability 2.0: A Holistic View on Polymer Biodegradation in Natural and Engineered Environments . . . . . . . . . . . Michael Sander, Miriam Weber, Christian Lott, Michael Zumstein, Andreas Ku¨nkel, and Glauco Battagliarin

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65

ecoflex® and ecovio®: Biodegradable, Performance-Enabling Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 K. O. Siegenthaler, M. Agari, J. Auffermann, M. P. Barth, G. Battagliarin, A. Ku¨nkel, F. Lauer, J. Lohmann, A. Nabifar, and M. Yamamoto Biobased Synthesis and Biodegradability of CO2-Based Polycarbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Alina Denk and Bernhard Rieger Progress in Catalytic Ring-Opening Polymerization of Biobased Lactones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 Lucas Al-Shok, David M. Haddleton, and Friederike Adams BioPBS™ (Polybutylene Succinate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 Satoshi Kato, Tadashi Ueda, Takayuki Aoshima, Naoyuki Kosaka, and Shigeki Nitta

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Chemical Synthesis of Polyhydroxyalkanoates via Metal-Catalyzed Ring-Opening Polymerization of Cyclic Esters . . . . . . . . . . . . . . . . . . . . 305 Xiaoyan Tang Biobased Polyamides: Academic and Industrial Aspects for Their Development and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327 Matthias Ullrich, Frank Weinelt, and Malte Winnacker Correction to: The Terpenes Limonene, Pinene(s), and Related Compounds: Advances in Their Utilization for Sustainable Polymers and Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 Malte Winnacker

Adv Polym Sci (2024) 293: 1–34 https://doi.org/10.1007/12_2022_138 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 Published online: 11 October 2022

Biotechnological and Chemical Production of Monomers from Renewable Raw Materials Alois Kindler and Oskar Zelder

Contents 1 The Chemical Value Chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Demand for Chemicals Produced with Renewable Energy and Renewable Raw Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Renewable Raw Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Introducing RRM into Established Chemical Value Chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 The “Efficiency Trap” for RRM Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Substitution of Fossil Naphtha by RRM in Existing Chemical Value Chains: The Biomass Balance (BMB) Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Direct Chemical or Biotechnological Conversion of Specified First Gen RRM toward Dedicated Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Direct Conversion of RRM to Chemical Products by Fermentation . . . . . . . . . . . . . . . . . . 7.2 Conversion of RRM to Chemical Products Using Chemical Synthesis . . . . . . . . . . . . . . 7.3 Examples for the Direct Conversion of RRM to Chemical Products by Biotechnology or Chemical Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 4 4 5 7 8 9 9 10 12 29 29

Abstract The demand for sustainable chemical products that exhibit a low product carbon footprint and that are biobased and/or biodegradable has tremendously increased. This can be observed in many applications like home and personal care, packaging, or engineering plastics. Many customer industries of the chemical industry want to replace their petroleum-based monomers, chemical building blocks,

A. Kindler Biobased Chemistry (RGU/AI), Ludwigshafen am Rhein, Germany e-mail: [email protected] O. Zelder (*) Industrial Biotechnology I (RGD/BD), Ludwigshafen am Rhein, Germany e-mail: [email protected]

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and ingredients by renewables-based ones. Furthermore, there is the need to reduce microplastics in the environment, which can be achieved by substitution of conventional polymers by biodegradable polymers that are – in the best case – even biobased. Moreover, there is substantial political pressure to reduce green-housegas (GHG) emissions as stipulated by the green deal of the European Union. All these developments enforce a fundamental change of energy supply, raw material base, and production technologies within the chemical industry. The supply of customer industries with biobased products forces the chemical industry to include renewable raw materials to a large extent. This requires new chemical or biotechnological conversion strategies. First generation renewables like sugar and plant oil are existing commodities and are already used within the chemical industry. Advanced renewable raw materials also known as second generation renewables comprise lignocellulose, agricultural residues, food wastes, other organic waste, or seaweed. They have a great potential as future raw material as they are considered even more sustainable. The utilization of renewable raw material is based on value chains completely different from the current petrochemical value chain. Firstly, the structurally complex raw biomass is refined to specified raw material like carbohydrates or plant oil by separation and purification technologies. Secondly, this specified raw material is converted into chemical products using chemical synthesis or biotechnology. An example that is already used at a commercial scale is the biomass balance approach, the feeding of “bio-naphtha” into the steam-cracker. It makes use of existing assets and can quickly deliver drop-in products declared as biobased. However, it cannot deliver products with traceable 14C-carbon. In contrast to biomass balance, many chemical end products containing traceable 14C-carbon are obtained through direct conversion of specified renewable raw material by biotechnology or chemistry. Several examples for alcohols, diols, organic acids, and amines are discussed in this chapter. Keywords Chemical synthesis · Fermentation · Metabolic engineering · Renewable raw materials

1 The Chemical Value Chains The chemical industry is an important part of the global economy and a key supplier to many other industries, e.g. health and nutrition, agriculture, personal care, cleaning, automotive, and construction. Currently, chemical production uses almost exclusively fossil-based energy sources like natural gas, coal, or mineral oil. In addition, the vast majority of chemical products are based on value chains that have their roots in petrochemical raw materials (Fig. 1). In large oil refineries gases and crude oil are fractionated into methane, ethane, liquid petroleum gas (LPG) (C3-C4), naphtha (C5-C12), kerosene (C9-C13), and gas oil. Steam-crackers convert LPG and naphtha into chemical precursors that are the

Biotechnological and Chemical Production of Monomers from Renewable. . .

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Fig. 1 The “Chemis-Tree”: The chemical value chains can be regarded as a tree that starts with very few raw materials and branches out toward thousands of different products

Fig. 2 First steps of the petrochemical value chain starting with crude oil and natural gas. After refinery the steam-cracker is regarded as the starting point of the chemical value chain

feedstock for further chemical conversions (Fig. 2). These alkanes, alkenes, and aromatics are the basis for the resulting chemical value chains, which are named according to the number of C-atoms of the respective educt, e.g. the C3-value chain starting from propylene or the C4-value chain starting from butene or butadiene. Almost all other chemicals ranging from complex pharmaceutical actives to simple monomers can be attributed to these chemical value chains. For instance, the important monomers methylene diphenyl diisocyanate (MDI), toluene-diisocyanate (TDI), terephthalic acid, styrene, adipic acid, hexamethylenediamine, and caprolactam are derived from the C6- or aromatics value chain. Acrylic acid – the monomer for polyacrylates – is produced from propylene and as such part of the C3-value chain. Other large volume monomers like 1,4-butandiol can be produced either from butane via maleic anhydride or from acetylene and formaldehyde.

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2 Demand for Chemicals Produced with Renewable Energy and Renewable Raw Materials Customer demand for sustainable chemical products that exhibit a low product carbon footprint (PCF) and that are biobased and/or biodegradable has tremendously increased. This can be observed for many product classes ranging from cosmetic ingredients to engineering plastics. Large corporations from different industries want to replace their petroleum-based chemical building blocks and ingredients by renewables-based ones. Furthermore, there is the need to reduce microplastics in the environment, which can be achieved by substitution of conventional polymers by biodegradable polymers that are – in the best case – also biobased. Moreover, there is substantial political pressure to reduce green-house-gas (GHG) emissions as stipulated by the green deal of the European Union [1]. All these developments enforce a fundamental change of energy supply, raw material base, and production technologies within the chemical industry. Driven by climate change policies many chemical companies launched strategies to reduce their GHG-emissions by switching towards renewable energy sources. For instance, BASF has entered into partnerships for offshore wind-farms [2] to get access to renewable electricity. Furthermore, many chemical companies explore the increased use of renewable raw materials (RRM) replacing petrochemistry. With respect to PCF, the exploitation of RRM is extraordinarily beneficial, as they come with a carbon credit: RRM are derived from plants that use photosynthesis for energy generation and fix CO2 from the environment. Hence, their biomass can be regarded as a CO2-trap. The PCF of glucose isolated from starchy crops, for example, is in the range of -1.5 t CO2/t glucose, if only the generation of biomass via photosynthesis is considered. After subtraction of all CO2 generated by agriculture, processing and transport a PCF of still –0.5 t CO2/t glucose results [3]. Consequently, the use of RRM reflects the consumption of atmospheric carbon, while the use of petrochemical feedstock generates a surplus of atmospheric carbon. In 2020, 4.2 million tons of biobased polymers were produced worldwide [4]. This amounts to only 1% of the global production volume of fossil-based polymers. However, the annual growth rate of 8% is significantly above the 3–4% growth assumed for the entity of all polymers. This trend is expected to continue.

3 Renewable Raw Materials Well-known RRMs, carbohydrates, or plant oils that are established commodities are referred to as first generation (first gen) RRMs (Fig. 3). Within the carbohydrates, these are starch-based glucose [5] obtained from cereals and sucrose [6] produced from sugarcane or sugar beets. Typical first gen plant oils [7] are soy-bean oil, rapeseed oil, wheat oil, sunflower oil, and palm oil. Besides their utilization in food and animal feed production, large quantities of these first gen RRMs are converted

Biotechnological and Chemical Production of Monomers from Renewable. . .

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Fig. 3 Schematic drawing of the value chains for RRM. First gen RRM are derived from edible crops and converted to specified raw materials such as glucose, sucrose, or vegetable oil. These established commodities are widely used for chemical or biotechnological conversion into renewable chemicals. Advanced RRM, also known as second gen RRM, do not compete with the foodchain and are extensively explored as future RRM for use in the chemical industry

into biofuels like bioethanol and biodiesel, and to a minor extent, they are even used as raw material in technical applications for the chemical industry. The ongoing land-use debate deals with a potential competition of first gen RRMs used for biofuels and chemistry against the global food production. However, the land-use for biobased polymers is estimated to be only 0.006% of the global agricultural land [4]. Nevertheless, research institutions and companies work on the development of novel technologies that make advanced RRMs (Fig. 3), also known as second generation (second gen) or even third generation (third gen) RRMs, available for chemicals and fuels manufacturing. Advanced RRMs do not compete with human nutrition as they are not immediately edible for humans. Often, they are by-products of food production or they require arable land of inferior soil quality for their growth that is not used for cultivation of edible crops. The most prominent example is lignocellulose, e.g. woody matter or agricultural residues like straw, consisting of indigestible plant polymers such as celluloses, hemicelluloses, and lignin. Further approaches deal with the utilization of other renewable raw materials like food wastes, other organic waste, or seaweed [8]. Advanced RRMs are not yet fully established sales products. Their use is still a matter of intense scientific investigation. Application on a larger scale is limited to some selected industrial demonstration plants.

4 Introducing RRM into Established Chemical Value Chains To enable RRM as feedstock for the traditional chemical industry on a large scale requires two major transformation operations:

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1. From “biomass” to “specified raw material”: Development of separation and purification technologies to transform structurally complex biomasses like woody matter or food waste into standardized biogenic raw materials like carbohydrates, ethanol, or fatty acids. 2. From “specified raw material” to “basic chemical”: Development of efficient chemical “deoxygenation technologies” or “biotechnologies” to transform biogenic, oxidized molecules, like syngas, ethanol, lignin, or sugars, into alkanes, alkenes, alcohols, dicarboxylic acids, or other molecules of interest. For first gen RRM, these transformations are industrial standard operations in sugar mills, corn wet mills, and vegetable oil refineries. The resulting specified raw materials plant oils, glucose, and sucrose are established commodities. In the case of advanced RRM, however, both transformation types from “biomass” to “specified raw material” and from “specified raw material” to “basic chemicals” are challenging and encompass a variety of technological options. Most are under intensive research, usually by start-up companies backed by venture capital or other financing schemes. Some technologies are already offered by machine and technology suppliers as process packages to provide the refining and chemical industry with corresponding investment opportunities. Mature processes in providing “specified raw materials” today (2021) are • Fatty acid methyl ester (FAME) production via transesterification from fats and oils as fuels • Hydrogenated vegetable oils (HVO) production via hydrotreatment and hydrocracking from fats and oils • Bio-oils production via pyrolysis of plant matter for fuels • Bio-naphtha, a mixture of C5 to C12 aliphatics, obtained as side product from HVO-processes • Sugar production via specific depolymerization of, e.g., woody matter, agricultural residues in “biorefineries” • Ethanol (EtOH) production via fermentation from “sugars.” The distinction between first gen EtOH and “advanced or lignocellulosic EtOH” has been described above. • Syngas production via gasification of various biogenic raw materials like sawmill dust, food waste, wastewater treatment sludges, agricultural residues • Bio-methane via anaerobic fermentation of biogenic waste Mature processes providing “basic chemicals” from “specified raw materials” today (2021) are • Ethylene production via dehydroxylation of ethanol • Mixed Olefins production via syngas-based chemistry (usually via methanol or dimethyl ether) • Mixed olefins production via bionaphtha-based steam-cracking

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5 The “Efficiency Trap” for RRM Utilization Even though the range of basic chemical products accessible via ethylene, bio-naphtha, and syngas is impressive, raw material availability and process costs are still a serious hurdle for commercial use. Process costs are expected to decrease over time via optimization of operations. Raw material costs, however, are likely to stay “above” current (2021) petro-based raw material costs due to the unfavorable oxidation state of biomass – usually sugars – relative to fossil naphtha or coal. This simple but profound stoichiometric effect can be referred to as “efficiency trap” and is captured in Fig. 4. From a chemical or engineering perspective, both effects (H2-in and CO2-out) are valid for both fossil and renewable resources. The fossilized resources have had millions of years for this transformation process of plant matter, etc. to form coal, oil, and gas. For “contemporary” renewables, this reaction sequence requires additional process steps with additional costs. In today’s discussion on greenhouse gas emissions, it is conceivable that introducing carbon taxes on fossil raw material might compensate for this specific cost disadvantage of renewable raw material. It is expected that this kind of tax will speed up development efforts to substitute fossil raw material by renewables. Two options are available for an “interim period” to satisfy market demand for a limited number and amount of biobased chemical products: 1. Substitution of fossil naphtha by RRM in existing chemical value chains: The Biomass Balance (BMB) approach [9] 2. Direct chemical or biotechnological conversion of specified first gen RRM toward dedicated chemicals

Fig. 4 The transformation of biobased raw materials (polymeric sugars and lignin) to petrochemicals requires oxygen removal. This can be achieved by either reduction of the carbon chain via hydrogen (“Hydrogen-in” strategy) or by removal of CO2 (Carbon-rejection” strategy). Both routes require costly energy in the form of hydrogen or heat and reduce the molecular weight of the incoming biomass. Both facts lead to a significant increase in “effective” raw material cost

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6 Substitution of Fossil Naphtha by RRM in Existing Chemical Value Chains: The Biomass Balance (BMB) Approach For BMB, the basic feedstock for the chemical value chains naphtha and natural gas are substituted or replenished by bio-naphtha and biogas [10]. Bio-naphtha is usually hydrotreated or hydrogenated vegetable oil (HVO) similar to biodiesel. Full substitution of naphtha by RRM would generate a fully biobased value chain with fully biobased chemical products. However, due to high costs of bio-naphtha, usually only a small amount of bio-naphtha is subjected to the steam-cracker in addition to conventional naphtha. Thus, the resulting products are a mixture of fossil- and bio-based products which are not distinguishable based on their composition or technical characteristics (Fig. 5). To allocate the specific characteristics of fossil or bio-feedstock to the final product, the producers declare a certain share of their production to stem from renewable resources in proportion to the share of bio-feedstock used as an input [11]. Several chemical producers have established a tracking system for the renewable feedstock they use down to the final product. An independent certification confirms that the respective producer has replaced the required quantities of fossil resources for the biomass balanced product with renewable feedstock. BASF has transferred the certification of all its biomass balanced products to the new European REDcert2 scheme for the chemical industry in 2019. It needs to be recognized that the chemicals and monomers originating from BMB do not contain 14C-traceble carbon as the method only allocates biobased carbon by calculation, similar to the way it is done for renewable electricity. The big advantage of BMB is that the production technology remains the same and existing production facilities can be used completely. Furthermore, the BMB method leads to chemicals that are drop-ins, meaning that they are identical to their petroleum-based counterpart.

Fig. 5 Biomass balance, as operated by BASF. Bio-naphtha is blended into the feed of the steamcracker yielding cracker products with a certain portion of renewable carbon. Finally, a share of the resulting products is declared to be biobased depending on the amount of bio-naphtha converted

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7 Direct Chemical or Biotechnological Conversion of Specified First Gen RRM toward Dedicated Chemicals The direct conversion of polysaccharides or lignin to the molecule or polymer of choice avoids the “hydrogen-In” or “carbon rejection” efficiency trap. Thus, several commercial processes using either fermentation or direct chemical transformation of sugars to intermediates have been established. Consequently, intensive research and development is under way to transform “specified raw material” to chemicals.

7.1

Direct Conversion of RRM to Chemical Products by Fermentation

In contrast to direct chemical conversion of RRM, fermentation-based intermediates dominate the products directly gained from RRM at a commercial scale today (Fig. 6). Target products are often acids or alcohols. Aldehydes are not available in useful quantities by fermentation routes as the reactive carbonyl-function is usually toxic to organisms or enzymes. Olefins or alkanes are not part of the “natural” fermentation product portfolios. Driven by increased customer demand for biobased and biodegradable chemicals biotechnological production is gaining importance as reflected by annual growth rates of up to 7% [12]. Fermentation has a long history for chemicals manufacturing, both for specialties and for bulk products. Classical fermentation products are molecules that are naturally generated in large amounts by microorganisms. Examples are ethanol [13], citric acid [14], and lactic acid [15, 16]. These products do not require extensive microbial strain-engineering. Nevertheless, industrial production strains for these chemicals are usually selected for improved productivity by mutation and selection [17] over decades. Furthermore, development of robust and reliable fermentation and downstream processes is essential to secure competitiveness and low production costs [18]. A powerful lever for novel fermentation routes is metabolic engineering [19]. Over the past 10 years,

Fig. 6 Fermentation products sorted according to the number of carbon atoms and their application

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metabolic or strain-engineering has achieved spectecular technological progress. Novel methods for genetic engineering, e.g. CRISPR/Cas9, have been developed [20] and have led to higher throughput and shorter experimental development times in the laboratory. Even automated workflows in so-called bio-foundries are used to accelerate microbial strain development [21–23]. In the past, gene technology was focused on a small number of model organisms. Meanwhile, a large number of microbes are accessible for genetic engineering [20]. Along with rapid progress in DNA and RNA sequencing technologies, computational methods and bioinformatics revolutionize the insight into microbial cells and the understanding of biochemical pathways [24] and enzyme-function. As a result, the science of strain development is moving from an empirical trial-and-error approach toward rational design of novel production organisms [25]. Consequently, a wide range of chemicals can be produced using biotechnology [26, 27]. Even first non-natural products or metabolic pathways are accessible [28].

7.2

Conversion of RRM to Chemical Products Using Chemical Synthesis

A small number of intermediates made from sugars today (2021) are available via classic chemistry approaches and can be purchased in technical quantities. The number of examples is limited, as the existing chemical transformation methods lack the intrinsic selectivity of bio-catalysts with respect to the multitude of C-OH bonds present in sugars [29]. For technically relevant chemical reactions, either oxidation to acids, dewatering to aldehydes like furfural or 5-Hydroxmethylfurfural, or just plain hydrogenations to molecules like sorbitol or ethylene-glycol are relevant examples (Fig. 7). A short summary of major sugar-based chemicals obtained by purely chemical transformations is listed in Fig. 8. Additional chemical conversion technologies are under development, usually representing variants of the above cited examples or routes following deoxydehydration pathways (DODH) leading to product mixtures. The major hurdle to selectively obtain chemicals from sugars is the intrinsic thermal instability of sugars at temperatures higher than 80°C. Above this threshold, most sugars start to caramelize – “dewatering” by polymerization and/or aldolreactions to oligomeric or polymeric material, colloquially referred to as humin. These undefined substances are strongly colored and based on alpha-beta unsaturated carbonyl-moieties [30]. There are three strategies to circumvent this hurdle: 1. Combining chemistry and biotechnology: Fermentative transformation of sugar to an intermediate with less “free OH-groups and aldehydes” followed by chemical reaction (Fig. 9)

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Fig. 7 General chemical transformation routes from sugars to the specific target molecules gluconic acid, glucaric acid, 5-hydroxymethylfurfural, sorbitol, isosorbide, and glycols. Oxidation, reduction by hydrogenation, or “dewatering” by acid catalysis are the general chemistry options

Fig. 8 Chemical products generated by chemical synthesis from C6/C5 sugars and their application

2. Using “Stabilizing solvents” like ionic liquids 3. Development of low temperature catalytic transformations The combination of biotechnology and chemistry is a successful strategy to gain access to basic chemicals and intermediates. It is expected that more “combination type” processes will be reported in the coming years. Use of ionic liquids (ILs) as solvent systems facilitates phase separation, enables reactions under reduced temperature stress, and enhances solubility of reagents. However, commonly employed ILs must be recycled due to their high price. As most ILs are difficult to distill, high boilers and salts tend to accumulate in the

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Fig. 9 Chemical products generated by chemical synthesis from fermentation products generated from sugar and their application

reaction system compromising the above-cited advantages in the long run. So far, no technically viable IL-based process has been established in commercial scale. Low temperature catalytic transformations are still in a very early stage of development. The high activity of the metal catalyst required for reactions below 100°C makes the catalyst systems prone to deactivation by coagulation of nanoparticles on the catalyst surface or deactivation by trace-impurities in the raw material or combinations thereof. It remains a topic of debate if this strategy will become viable in competition with the abovementioned two other strategies.

7.3 7.3.1

Examples for the Direct Conversion of RRM to Chemical Products by Biotechnology or Chemical Synthesis C2: Ethylene from Bioethanol

Ethylene is the monomer for the polyethylene (PE), the most common plastic nowadays with an annual production volume of around 100 mio tons/year accounting for more than 30% of the entire plastics market. Ethylene is directly generated from the steam-cracking process without subsequent chemical process steps. An alternative production method is via dewatering of Ethanol (EtOH). Since EtOH is usually Bio-EtOH produced from fermentation, the resulting ethylene and PE are biobased and contain 14C-tracable carbon. Bio-ethylene is chemically identical to petroleum-based ethylene. Therefore, no new technology is required for conversion into downstream products. Currently, the annual production volume of Bio-ethylene is around 200,000 t/year and thus still very small [31]. Life-cycle-assessments show that production of bio-ethylene is beneficial in terms of PCF and energy consumptions in comparison with petro-based ethylene [32]. However, extensive production of bio-ethylene can compete with food and feed production for the availability of arable land. In addition, if pristine land is converted into arable land for EtOH feedstock production, CO2 emissions increase which can offset the environmental benefit [33].

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Bio-EtOH is converted to bio-ethylene using alumina or silica-alumina catalysts with excellent yields of 99%. Bio-EtOH is produced via fermentation with Saccharomyces cerevisiae, the bakers-yeast, as production organism. The microorganism has an extremely high natural capability to generate ethanol from sucrose generated from sugarcane in South America or glucose from grain in North America and Europe. It is extraordinarily robust and can tolerate high EtOH concentrations of up to 15%. The product is purified by distillation. Bio-EtOH is the biggest fermentation product by volume and the production process is a robust standard procedure that is widespread over all regions of the world. The major use is as biofuel, as in many countries, Bio-EtOH is blended into gasoline. To counteract competition with food production, large efforts have been undertaken to switch to alternative raw materials, such as lignocellulosic sugars, and to produce Bio-EtOH from advanced RRM. Naturally, S. cerevisiae is not able to utilize cellulose and hemicellulose which contains the C5 sugars xylose and arabinose, as well as the C6 sugars mannose, galactose, and rhamnose. Consequently, many research groups and companies have developed metabolically engineered yeast strains that can grow and produce ethanol on these carbohydrates [34]. Production of lignocellulosic ethanol has been scaled up into pilot and largescale demonstration plants [35, 36]. A completely new approach for ethanol production has been developed by Lanzatech using waste gases as raw material. Their technology recycles carbon from industrial off-gases, e.g. syngas generated from any biomass resource (municipal waste, organic industrial waste, or agricultural waste) and reformed biogas in an anaerobic fermentation. This reduces emissions, while new products are generated for a circular carbon economy. The company has developed a fermentation process using the anaerobic strain Clostridium autoethanogenum [37] that belongs to the group of acetogenic bacteria. They use the Wood-Ljungdahl-pathway, also known as the reductive acetyl-CoA-pathway, to enable growth on C1 compounds like CO2 or CO. In 2018 Lanzatech, and its joint venture partner, Shougang Group, a leading Chinese iron and steel producer, have started up the first commercial facility (46,000 t ethanol/year) using this technology converting industrial emissions to sustainable ethanol in Caofeidian, China [38].

7.3.2

C3: Lactic Acid (LA)

Lactic acid was among the very first commercial fermentation products. The first lactic acid factory started operations in Littleton, USA in 1883 [15]. The process invented by Charles Avery used corn meal that was hydrolyzed with sulfuric acid as raw material. The fermentation was carried out at rather high temperatures of up to 45°C to avoid contamination in wooden barrels of 4m3. The pH was slightly acidic and regulated by addition of CaCO3. Importantly, pure cultures of the lactic acid bacterium Lactobacillus delbrueckii were used instead of inoculum stemming from rotten cheese. The purification was very basic, as the first products were quite dilute aqueous solutions of 10% lactic acid [15]. Initially, lactic acid was used as a preservative for food. Later, many diverse uses, e.g. in detergents and various

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Fig. 10 Metabolic pathway of homofermentative lactic acid bacteria. Glucose is degraded via the glycolytic pathway to pyruvate. Regeneration of reducing equivalents (NADH + H+) is accomplished by lactate dehydrogenase – the last step in the reaction sequence. Lactate formation is extremely efficient with a yield of 90 g lactate/g glucose

technical and pharmaceutical applications requiring acidity were established. Since the 1990s, lactic acid has played an increasingly important role as a monomer building block for polylactic acid, a biodegradable polyester that is even biobased. Besides a larger number of small producers, there are three big companies that dominate the lactic acid markets: Corbion, formerly known as Purac, Galactic and Nature Works, a subsidiary of Cargill. Since Avery’s first commercial lactic acid process, many improvements have been implemented, e.g. the switch to steel vessels for the production process. Most industrial processes still use homofermentative lactic acid bacteria. They exhibit an anaerobic lifestyle and generate only 2 mol of ATP from the conversion of 1 mol glucose to 2 mol lactate. For comparison: aerobic respiratory metabolism generates 36 mol ATP from oxidation of 1 mol glucose to CO2. Consequently, lactic acid bacteria have a low energy yield. They need to metabolize glucose and generate lactate at very high rates to fuel their life. As the space-time-yield (STY = gProduct/ l*h) is high and product yield (YS/P = g lactate/g glucose) in lactic acid fermentations is close to the theoretical maximum of around 90 g lactate/g glucose, these processes are extremely efficient and lead to low production costs (Fig. 10). Lactic acid bacteria can tolerate slightly acidic pH values. Nevertheless, pH regulation during the fermentation is necessary to keep the microbes vital. This is achieved by addition of lime or chalk to the fermentation broth. After the fermentation, the product is recovered by addition of sulfuric acid to convert the lactate to lactic acid which is then further purified by crystallization (Fig. 11). Gypsum is a typical by-product. Even though gypsum has many applications, it is of low value and can pile up to large amounts. In order to avoid gypsum formation Cargill has developed a yeast-strain producing lactic acid at pH of 3, which generates free lactic acid that can directly be purified after cell removal [39]. The basic microorganism was identified in extensive screening and subsequently developed toward an efficient industrial strain using metabolic engineering. This strain is now used for large-scale manufacturing of lactic acid at the NatureWorks site in Blair, Nebraska (USA).

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Fig. 11 Simplified process scheme for a lactic acid production process. The fermentation is done by lactic acid bacteria using sugar as a raw material and lime to adjust the pH. The resulting fermentation product is calcium lactate. Through acidification with sulfuric acid lactic acid and the by-product gypsum (CaSO4) are generated. Further concentration and purification steps result in the final product

7.3.3

C3: 3-Hydroxypropionate (3-HP)

Another C3 organic acid that is of utmost importance in many applications is acrylic acid. Mainly, it is used as a monomer ending up in plastics, coatings, adhesives, elastomers, paints, and as superabsorber in hygienic and medical products. Traditionally, most of the acrylic acid is synthesized by oxidation of propylene. Switching from propylene to a renewable raw material could reduce the PCF and deliver a biobased product. Consequently, there are large research efforts underway to discover a synthesis route for “bio-acrylic acid.” An obvious solution seems to be the dehydration of lactic acid (2-hydroxypropionic acid), as it is an established bulkfermentation product that is available in large amounts and at low costs. However, the dehydration step represents an extremely challenging chemical reaction as the hydroxyl group is in the a-position. Instead, the dehydration of 3-hydroxypropionic acid (3-HP) with the hydroxyl group in the b-position appears more promising. The chemical dehydration of 3-HP in aqueous solution mimicking a fermentation product is described in literature [40] and should work at a large scale. However, 3-HP is no metabolic end-product like lactic acid that can be produced at high concentrations using natural microbes. Consequently, the metabolic engineering of synthetic metabolic pathways for production of 3-HP is a competitive research field. Several metabolic routes (Fig. 12) have been proposed and investigated [41]. Most of the pathways proved to be thermodynamically unfavorable and/or did not yield sufficient ATP for cell growth. The most promising route seems to be via pyruvate, ß-alanine, and 3-oxopropanoate. However, there is no production strain yet that is

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Fig. 12 Various metabolic pathways from glucose to 3-HP have been investigated, via propionylCoA, lactate, aspartate, alanine, or malonyl-CoA. The most promising route seems to be via pyruvate, ß-alanine, and 3-oxopropanoate

capable of making 3-HP at competitive costs. The product yields reported in literature do not significantly exceed 0.5 g3-HP/ gglucose which is far below the yield (0.9 glactate/gglucose) that can be achieved for lactic acid.

7.3.4

C3: 1,3-Propanediol (1,3-PDO)

In 2018, DuPont Tate & Lyle Bio Products, LLC, a joint venture (JV) between DuPont and Tate & Lyle, announced an expansion of their manufacturing facilities for fermentation-based 1,3-propanediol in Loudon, Tennessee, to increase annual production to 70,000 t/year [42]. The JV had already started operations in 2004 with the scale-up of a fermentation process for 1,3-PDO. Together with terephtalic acid the diol forms the polyester polytrimethylene terephthalate (PTT) which is used in fiber applications similar to Nylon, e.g. in the carpet industries. 1,3-PDO is known as a fermentation product of some anaerobic microorganisms from the feedstock glycerol. As the availability of glycerol in very large amounts (> 100,000 t/year) is limited, the Du Pont scientists decided to engineer a production organism that can convert glucose – which is available in very large amounts – to the desired product. They chose Escherichia coli as the production organism. This organism has the advantage that it is relatively easy to engineer genetically, as E. coli is a very well-studied model organism. Furthermore, it can not only grow under aerobic conditions, but also anaerobically and microaerobic. However, it is not a natural 1,3-PDO producer. Consequently, the scientists had to engineer the 1,3-PDO biosynthesis pathway into their production host. Thus, a large number of

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Fig. 13 Engineered metabolic pathway for production of PDO with E. coli. Gene names: blue; metabolites: black. Pathway from DHAP to 1,3-propanediol is non-native to E. coli and was introduced. More details in the text

genes had to be introduced into the strain or deleted. The newly designed pathway (Fig. 13) originates from dihydroxyacetone phosphate (DHAP), that is a metabolite in glycolysis, a standard pathway for glucose catabolism. Genes encoding the following enzymes were introduced: glycerol-3-phosphate dehydrogenase (DAR1), glycerol-3-phosphate-phosphatase (GPP2), from S. cerevisiae, glycerol dehydratase (dhaB1, dhaB2, dhaB3), and the reactivation enzyme for glycerol dehydratase (dhaBX) from Klebsiella pneumoniae, and the native E. coli oxidoreductase (yqhD). To avoid the formation of by-products, genes encoding the following enzymes were deleted: glycerol kinase (glpK) and glycerol dehydrogenase (gldA), as well as glycerol aldehyde dehydrogenase activity (gap). To secure the availability of phosphoenolpyruvate the phosphoenolpyruvate transferase system (PTS) for glucose uptake was replaced by a galactose permease (galP).

7.3.5

C4: 1,4-Butanediol (BDO)

BDO is a major intermediate with an annual production of several million tons. It is used as a monomer building block for polyesters and can be converted to other important C4 compounds like tetrahydrofuran (THF). THF is precursor for polyTHF, a polymer with various applications like elastic fibers required for sportswear and swimwear, thermoplastic polyurethanes, thermoplastic polyetheresters, and polyetheramides. In BDO production, there are different chemical processes in operation using acetylene or butane as raw materials. In addition, the Italian

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Fig. 14 BDO biosynthetic pathway as developed by Genomatica. Glucose is channeled to succinyl-CoA via the TCA cycle. Gene names in blue. Succinate Semialdehyde is produced through the conversion of succinyl-CoA via CoA-dependent succinate semialdehyde dehydrogenase (SucD) from Porphyromonas gingivalis. 4-hydroxybutyrate dehydrogenase (4HBd) from P. gingivalis converts succinate semialdehyde to 4-hydroxybutyrate (4HB) which is then converted to 4-hydroxybutyryl CoA (4HB-CoA) and acetate by 4-hydroxybutyryl-CoA transferase (Cat2) from P. gingivalis. Extensive screening was required to identify an aldehyde dehydrogenase (ALD) from Clostridium beijerinckii that converts 4HB-CoA to 4-hydroxybutyraldehyde which is then converted to BDO by a native alcohol dehydrogenase (ADH). Figure adapted from [3]

company Novamont employs a fermentation process which directly produces BDO from glucose. The technology was developed by Genomatica, a San Diego-based biotechnology company. Genomatica scientists engineered an Escherichia coli strain to make BDO – a remarkable effort, as BDO is not a natural compound. A synthetic metabolic pathway that converts succinyl-Coenzyme A (succinyl-CoA) to BDO in five steps was compiled with enzymes from Porphyromonas gingivalis and Clostridium beijerinckii (Fig. 14): • from succinate semialdehyde to 4-hydroxybutyrate (4HB) by a 4-hydroxybutyrate dehydrogenase (4HBd) • from 4HB to 4-hydroxybutyryl CoA (4HB-CoA) and acetate by 4-hydroxybutyryl-CoA transferase (Cat2) • from 4HB-CoA into 4-hydroxybutyraldehyde catalyzed by an aldehyde dehydrogenase (ALD) • and from 4-hydroxybutyraldehyde to BDO by a native alcohol dehydrogenase (ADH) To optimize BDO production, pathways leading to undesired by-products had to be modified. This required, for example, the knock-out of thioesterases and the expression of a lactonase to avoid γ-butyrolactone (GBL) formation [43], as well as optimization of the acetate metabolism. Further unwanted side products were eliminated and reducing equivalents were conserved for BDO production through

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deletions of alcohol dehydrogenase (adhE), pyruvate formate lyase (pfl), lactate dehydrogenase (ldh), and malate dehydrogenase (mdh) genes. Enzyme activity at high NADH levels was enhanced by • substitution of E. coli lipoamide dehydrogenase LpdA (a component of pyruvate and α-ketoglutarate dehydrogenases) with the enzyme from Klebsiella pneumonia carrying the mutation D345K • introducing an R163L mutation to the citrate lyase GltA • deleting arcA, a transcriptional repressor of many TCA genes. Furthermore, the design of a fermentation process including purification of the final product resulted in an industrial process with excellent properties: a titer of more than 120 g/L, productivity exceeding 3 g/L/h, and a product yield in the range of 0.4 gBDO/gglucose [44]. The technology was licensed by Genomatica to BASF [45], to Novamont that is running a 30 kt bio-BDO plant in Adria, Italy [46] and to Cargill/Helm [47]. A technoeconomical analysis revealed that the process may have lower cash costs of 15–30% relative to fossil-derived BDO and is reducing greenhouse gas emissions by up to 60% [48]. For further sustainability improvements of Genomatica’s BDO process, a production strain was developed that can simultaneously utilize glucose, xylose, and arabinose with enhanced energetics of C5 utilization, and improved inhibitor tolerance [49]. However, until now, this lignocellulose fermentation toward BDO has not yet been realized at a commercial scale.

7.3.6

C4: Succinic Acid (SA)

Succinic acid is chemically synthesized from maleic anhydride. It is a specialty product used only in small volumes. Nevertheless, succinic acid is considered a monomer with great potential for biodegradable polyesters. As it is a natural fermentation product, it can be produced by many microbes [50–52]. Great efforts have been undertaken to develop high-end production strains by metabolic engineering [53–57]. Most important strategies to improve succinic acid formation comprise increasing the carbon-flux toward the reductive tricarboxylic acid (TCA) cycle by enhancing anaplerotic reactions, as well as the elimination of undesired by-products, e.g. lactic acid and acetic acid by deleting the corresponding biosynthesis genes (Fig. 15). Furthermore, adaptive evolution was applied to further increase productivities and enhance the robustness of the microorganisms toward high-osmolarity and high product concentrations. Finally, strains were generated that produce far above 100 g/L of succinic acid with excellent yields of around 1 gsuccinic acid/gsugar. The purification of succinic acid from the fermentation broth is a challenge and several options have been investigated [58]. A standard strategy is via acidification of the fermentation broth to generate the protonated free acid followed by crystallization. As discussed for lactic acid, this may generate large amounts of salt as by-product. Another strategy is the use of yeast strains that tolerate low pH values as production organisms. This requires less acidification of the fermentation

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Fig. 15 Metabolic network for succinate production. Succinate is derived from the TCA cycle. For engineering a high-performing succinate producing strain pathways toward by-products, e.g. lactate, formate, acetate, and ethanol have to be knocked out or downregulated. The anaplreotic reactions that fill up the TCA cycle need to be enhanced

broth and generates less salt as by-product [59]. Although there were several bioprocesses developed for succinic acid production and even transferred into industrial scale [58], the demand for this product still remains at a relatively low level and volumes increase only slowly. However, the current awareness for biobased and biodegradable products may boost succinic acid utilization in the future.

7.3.7

C5: 1,5-Diaminopentane (DAP) or Pentamethylenediamine (PMDA)

A smart strategy to develop new biobased monomer building blocks is to modify production strains for existing bioproducts. This has been successfully achieved for

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1,5-diaminopentane (DAP), also known as pentamethylenediamine (PMDA), which was developed on the basis of fermentative lysine production. Lysine is one of the largest fermentation products and is manufactured in worldscale plants from sugar (sucrose) or glucose. Annual production is estimated in the range of 2 mio t. As lysine is an essential amino acid, it is applied in human and animal nutrition and in pharmaceutical applications. Production organisms for lysine are Corynebacterium glutamicum or E. coli. Wild-type microorganisms only produce as much lysine as they need for their own growth. Consequently, strain development was required to generate overproducing strains. Early production strains were generated via mutation and selection. Cultures were mutagenized using radiation or mutagenic chemicals and the resulting mutants were screened in large shake-flask assays for overproducing strains [61]. The rational development of lysine production strains using metabolic engineering is described for C. glutamicum in detail [62]: A series of 12 rational genetic changes were introduced into a wildtype C. glutamicum strain to yield a powerful lysine overproducer with excellent yields (YS/P = g lysine/g glucose = 0.55 g/g, Titer = 120 g/L, STY = 4 gProduct/L h). The genetic changes comprise mutations to remove allosteric product inhibition of lysine biosynthesis, overexpression of lysine biosynthesis genes, improvement of the anaplerotic reactions at the pyruvate node, minimization of carbon loss by reducing flux through the TCA cycle, and optimization of NADH supply by increased flux through the pentose phosphate pathway. This powerful lysine overproducing strain was converted into a DAP overproducing strain by introducing a lysine decarboxylase and by removing a DAP-acetylation enzyme and the lysine exporter [63–65] (Fig. 16). Using this strain a production method consisting of fermentation and downstream processing for DAP was demonstrated and the resulting product was polymerized with sebacic acid to polyamide 5.10 (PA 5.10) [60]. In the meantime, Cathay Biotech has scaled fermentative production of DAP into a commercial plant [66].

7.3.8

C6: Isosorbide

Access to isosorbide is achieved via a chemically simple hydrogenation of glucose to sorbitol, followed by acid-catalyzed cyclization (dehydroxylation) to a stiff, bicycling ring system (Fig. 17). Technical hydrogenation to sorbitol is done using nickel-based heterogeneous catalysts in batch mode. Nobel metal-based heterogeneous catalysts have been developed to allow continuously operated processes at lower hydrogenation temperatures [67]. Sorbitol has its current main applications in the food and pharma industries requiring careful purification and de-colorization workup procedures. The same holds true for the chemically simple acid-catalyzed dewatering under ringclosure to isosorbide. Both, insomerization of the sugar-backbone and formation of colored side products take place as acid-catalyzed processes. The corresponding purification requirements for low color, uniform isosorbide are high. Isosorbide is used as “stiff” diol component in polyesters and – due to the lack of double bonds –

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Fig. 16 Metabolic map of DAP synthesis in Corynebacterium glutamicum. Sixteen genetic modifications were engineered into C. glutamicum. Green color indicates increased activity by overexpression. Red color, dotted line, and “X” reflects gene deletion. Abbreviations: zwf, glucose 6-phosphate dehydrogenase; pgl, 6-phosphogluconolactonase; tkt, transketolase; tal, transaldolase; fbp, fructose 1,6-bisphosphatase; pck, phosphoenolpyruvate carboxykinase; pycA, pyruvate carboxylase; icd, isocitrate dehydrogenase; lysC, aspartokinase; hom, homoserine dehydrogenase; dapB dihydrodipicolinate reductase; ddh, diaminopimelate dehydrogenase; lysA, diaminopimelate decarboxylase; lysE, lysine exporter; and ldcC, lysine decarboxylase from E. coli, NCgl1469, N-acetyl transferase; and cg2893, major facilitator permease. Overexpression of the tkt operon (Psod tkt), overexpression of fructose1,6-bisphosphatase (Ptuf fbp), modification and amplification

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Fig. 17 The simple two-step synthesis of isosorbide from glucose. Challenge of the technical process is the purification of the products from high boiling isomers and colored impurities

offers a high UV stability [68]. Esters of Isosorbide are used as plasticisers [69]. Due to the “roof-like” secondary structure of isosorbide one of the free OH-groups points “under the roof” and consequently, the two free OH-groups have a markedly different reactivity. This makes uniform chemical modification beyond esterification difficult. For example, amination to the bis-amino derivative of isosorbide is difficult to obtain and so far, no corresponding applications at a technical scale have been reported [70, 71].

7.3.9

C6: Adipic Acid (ADA)

Adipic acid is the dicarboxylic acid with the biggest economic impact with an annual production of more than 3 mio t. It is used as monomer for polyamides, polyesters, and polyurethanes. As a C6 molecule adipic acid is produced from KA oil (for ketone-alcohol, a mixture of cyclohexanone and cyclohexanol) that is derived from cyclohexane, phenol, or cyclohexene based on the aromatic feedstock benzene (Fig. 18). After about 60 years of continuous optimization, the level of overall chemical yield from benzene to adipic acid is >80%, even though this is a three-step reaction sequence. Moreover, the final purification of adipic acid to deliver fiber grade material of purity higher than 99.99% is a multi-step crystallization sequence leading to one of the purest products in chemical industry. Due to the low production costs of this classical petrochemical production route, there is not yet a commercialized bio-process available that is competitive and based on renewable feedstock. If desired, technical quantities of adipic acid can be provided employing biomass balanced (BMB-based) benzene. Technical sources of 14C-tracable biobased benzene and phenol do not yet exist. However, there is a technically mature option to provide bio-benzene: the transformation of bio-naphtha via aromatization, e.g. using  ⁄ Fig. 16 (continued) of pycA (Psod pycAP458S), deletion of pck (Δpck), attenuation nof icd (icdatt), modification, and amplification of lysC (Psod lysCT311I), attenuation of hom (homV59A), amplification of dapB (Psod dapB), duplication of ddh (2xddh), duplication of lysA (2xlysA), overexpression of codon-optimized ldcC (Ptuf ldcCopt) from E. coli, deletion of NCgl1469 (ΔNCgl1469), deletion of lysE (ΔlysE), and overexpression of cg2893 (Psod cg2893). Figure adapted from [60]

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Fig. 18 Potential access routes toward adipic acid (ADA). The classical technical solutions, starting from petro-based raw material, are listed above; the biobased alternative routes start from glucose. Today biobased routes are not as carbon efficient as the petro-based ones. Abbreviations: Red. = reduction; Ox. = oxidation; Dew. = dewatering; Fer. = fermentation

the established AROMAX ™ process. This option is not used today due to its high cost (approx. > 2x current market price). 14C traceable ADA is not yet available on a technical scale. Therefore, there are massive ongoing research efforts to develop processes for “bio-adipic acid” production [72]. The biobased options to directly synthesize ADA have not yet reached technical maturity. Moreover, due to the necessity for both oxidation and reduction during the synthesis pathway, the biobased options are not as cost efficient as the petrochemical routes. If biomass is considered a poly-sugar with the general formula of C6H12O6, the transformation to adipic acid (C6H10O4) does not require too much “hydrogen-in” or “carbon-rejection” (Fig. 4). In that respect, it serves as a good example to illustrate the challenges associated with the use of biomass as feedstock for basic chemicals. There are a couple of glucose-based processes under development. An overview scheme of some selected routes based on glucose as raw material is given in Fig. 19. Omitted are those routes that need more than three major transformation steps. It is assumed that yield loss and capital expenditure for these multi-step-routes will not be able to deliver a cost-efficient access to ADA as basic chemical. Routes via chemical oxidation of glucose to glucaric acid are attractive in the first place as the oxidation of primary OH-groups and aldehydes can be achieved with sufficient selectivity relative to the other secondary OH-group in glucose [73]. Moreover, formation of glucaric acid bis-lactone as key intermediate provides abstraction of 2 OH-groups by simple “dewatering” instead of “de-hydrogenation.” The key to technical success, however, is the subsequent reduction of the glucaric acid bis-lactone to ADA. The method suggested by Rennovia in 2010 makes use of hydrogen bromide as halogen-intermediate to facilitate the OH-abstraction during “de-hydrogenation” of the inner OH-groups. Even though attractive yields could be demonstrated, a technical process has not yet been established [74]. Another chemical route comprises “Diketo-Adipic” acid as an intermediate, possibly available in due time. So far, the combination of sugar-oxidation and modification of the “inner OH-groups” coupled with the high reactivity of these

Fig. 19 Different routes, both fermentation and chemistry, towards adipic acid investigated by industrial players

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electron-poor compounds defeated both fermentative and chemical access beyond orienting lab scale experiments. In the recent decade, the transformation of glucose via fructose to 5-Hydroxymethylfurfural has attracted a lot of attention. The formally simple dewatering reaction delivers a highly reactive building block with a high density of reactive groups. Formally, 5-HMF recommends itself as raw material for diols or diacids for polyesters, diamines for polyamides, as solvent precursor or even – via Diels-Alder reactions toward the synthesis of biobased aromatics. On top of these chemical conversions, several biotechnological approaches are being closely investigated. As ADA is neither a standard fermentation product nor a central metabolite, metabolic engineering has to be employed to generate microorganisms that produce ADA with high STY and mass yields from sugars or lignin derivatives. Three principal approaches for metabolic pathways toward adipic acid are outlined in literature (Fig. 20): 1. Employing the metabolism of aromatic compounds with muconic acid as intermediate: using aromatic feedstock coming from lignin [75] or combining aromatics biosynthesis from sugar/glucose with elements of aromatics degradation with glucose/sugar as feedstock [76, 77] 2. Reverse β-oxidation: starting from glucose/sugar and building up hexanoic acid with subsequent terminal oxidation [78] 3. Reverse adipate-degradation pathway: from glucose/sugar combining acetylCoA (C2) and succinyl-CoA (C4) to 3-oxo-adipyl-CoA (C6) with subsequent reduction [79] All these routes have in common that the respective cost of production is in the range of >1.5× of the current petrochemical cost level. The PCF of the biobased routes discussed here are in general significantly lower than the PCF of benzenebased chemically synthesized ADA (about 4t CO2 per t adipic acid). However, energy demand of production drives the PCF of the potential bio-ADA acid routes shown here up to about 2t CO2/t adipic acid. So far, these differences between petrochemical and biobased routes have not been considered sufficient to compensate for the much higher difference in production cost.

7.3.10

C6: Caprolactam (CPL) and Hexamethylenediamin (HMD)

CPL and HMD are the most important monomers for polyamides used in textiles, automotive industry, carpets, kitchen utensils, and sportswear due to their high durability and strength. The transportation manufacturing industry is the major consumer, accounting for 35% of polyamide (PA) consumption. Polyamide 6 (PA 6) is generated by ring-opening-polymerization from CPL while generation of Polyamide 6.6 (PA 6.6) requires polymerization of HMD with a dicarboxylic acid – usually adipic acid – as a second monomer. Aside from polyamide production, HMD is used to create coatings. The HMD market alone comprised two million tons in 2021, nearly all of which is currently produced from fossil raw materials. HMD is

Fig. 20 Engineering metabolic pathways toward ADA has been intensively investigated by a large number of academic and industrial players. This summary of different metabolically engineered pathways toward ADA highlights the main routes. The metabolism of aromatics can be employed starting from sugars (glucose, sucrose, cellulose) or lignin and yields cis,cis-muconic acid that needs to be chemically converted to ADA. Other pathways starting from sugars go via hexanoic acid or acetyl-CoA and succinyl-CoA

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Fig. 21 Scheme of a potential fermentation-based C6 value chain. Starting from sugars or lignin ADA, Hexanediol, HMD and CPL can be approached. Processes for production of CPL and HMD have been announced by Genomatica

a key ingredient in the USD 6.4 billion market for PA 6.6. The automotive market uses 500,000 tons of polyurethane coating based on HMD. Both, CPL and HMD are derived from petrochemical aromatic precursors. As there is a demand for more sustainable products with improved PCF, there are projects underway to develop biobased solutions for these monomers. In literature, different biosynthetic approaches toward CPL and HMD are described. However, until now, only the routes developed by Genomatica seem promising [80] (Fig. 21). As in the case of BDO, they have developed E. coli production strains for both compounds. For CPL, Genomatica announced a partnership with the Italian company Aquafil, a producer of textile fibers with special focus on plastics recycling. The aim of the partnership is to build a demo plant to test the technical feasibility of producing bio-caprolactam on a pre-industrial scale. The start-up of operations is scheduled for H1 2022 [81]. More recently, Genomatica and Covestro announced a collaboration on fermentation-based HMD [82].

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8 Concluding Remarks Climate change, increasing pollution of the environment, customer demand, and political pressure are powerful drivers for a raw material change within the chemical industry from petrochemical toward RRM. Currently, the share of RRM in chemicals production is still very small but is expected to increase steeply. Biobased monomers are already available via the biomass balance approach delivering drop-in solutions for existing monomers. However, biomass balance does not result in products with traceable renewable 14C-carbon. Due to high production costs, it is not expected that the demand for biobased monomers can be fully served via the biomass balance approach. Competitive direct chemical or biotechnological conversion of RRM is already possible for a growing number of monomers: ethylene from bioethanol, lactic acid, 1,3-propanediol, 1,4-butanediol, succinic acid, and isosorbide are successfully commercialized in monomer applications. Fermentation derived caprolactam and hexamethylenediamin are under development. Market introduction can be expected within the next few years. Extensive research for development of fermentation-based acrylic acid and adipic acid has not yet resulted in commercial products but is expected sooner or later. New products like pentamethylenediamine that are easy to produce using biotechnology but do not exactly match the existing market standard (in this case hexamethylenediamin) have been successfully developed and currently find their way into the markets. Taken together, all these developments give reason to expect that biobased monomers will exhibit significant growth within the near future. Due to the diversity of renewable feedstock and conversion technologies we must accept that there will be no “onefits-all”-solution like it is the case in petrochemical value chains. Moreover, several different decentralized value chains that fit to each region with its specific RRMs can be expected.

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Adv Polym Sci (2024) 293: 35–64 https://doi.org/10.1007/12_2022_123 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 Published online: 1 June 2022

The Terpenes Limonene, Pinene(s), and Related Compounds: Advances in Their Utilization for Sustainable Polymers and Materials Malte Winnacker

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Polymerization of Acyclic Terpenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Polymerization of Cyclic Terpenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Cycloolefin (Hydrocarbon) Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Polycarbonates and Polyesters from Terpenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Polyamides, Polyurethanes, and Others from Terpenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

36 39 41 42 43 48 52 52 53

Abstract The preparation of sustainable polymers from renewable resources is a main task in modern polymer and materials chemistry. For this, terpenes, that have many functions in nature, are very important building blocks, e.g., due to their abundance, availability, structural diversity, and functionalities. Accordingly, many approaches have been and are currently developed for the utilization of many terpenes for the preparation of a variety of sustainable polymers and materials. These approaches are elucidated herein with a focus on different polymers The original version of this chapter was revised: reference citation and figure citation updated. The correction to this chapter can be found at https://doi.org/10.1007/12_2022_133 M. Winnacker (*) WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universität München, Garching bei München, Germany Catalysis Research Center (CRC), Technische Universität München, Garching bei München, Germany e-mail: [email protected]

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(polyolefins, polyesters, polyamides, etc.) and their recent advances in synthesis and properties. Keywords Polyamides · Polycarbonates · Polyesters · Polyurethanes · Sustainable polymers · Terpenes

1 Introduction Sustainable polymers from renewable resources are a main research topic in modern polymer and materials chemistry. They are interesting with regard to an independence from fossil resources, but also with regard to possible new structures which are difficult to reach via fossil-based chemistry and that can result in materials with enhanced performances. The modern chemical industry relies nowadays mainly on fossil resources, but due to ecological and also economical reasons, the research on biobased and/or biodegradable polymers as alternatives to petroleum-based polymers is getting more and more important in both industry and academia [1, 2]. This is also based on the fact that modern society has severe environmental problems due to the often uncontrolled disposal of single-used plastic waste, which can impact marine life and potentially cause health hazards due to the formation of microplastic particles [3–6]. Accordingly, there is a growing awareness for environmental responsibility with regard to climate, production processes, etc., which has a strong impact also on materials and their (circular) economies [7–12]. For instance, (non-) biodegradable bags and their constituents as well as the use of certain substances are – depending on the context and on the regions – strongly regulated [13, 14]. In this regard, biomass is a desired and abundant carbon-neutral renewable resource for the production of polymers and materials. Natural polymers, e.g. cellulose [15] and polyhydroxyalkanoates (PHAs) [16–22], are well-known and have been used for a long time for materials applications [23]. In addition to the utilization of suchlike polymers directly or upon modification, an important example is also the production of (dimeric) L-Lactide from starch and its polymerization to poly(lactic) acid with high industrial relevance [24]. Furthermore, there are numerous studies focusing on the preparation of renewable monomers from starch [25] and from sugars that have a very high structural diversity and modifiability based on OH-groups, etc. [26]. Also other compounds from vegetable oils [27], cellulose [28], and lignin [29] are very important in this context. The utilization of CO2 as building block is also an important and sustainable approach for the synthesis of sustainable polymers by means of different catalysts, which has been (and is) investigated in many studies [30, 31]. Especially the synthesis of polyurethanes is important in this context [32]. Furthermore, Nature can serve as a source for other types of biobased raw materials, also various a priori monomeric building blocks, for the preparation of many sustainable polymers [33–37]. Here, a classification can be made into oxygen-rich biomass (e.g., carboxylic acid, polyols(glycerol), etc.), hydrocarbon-rich biomass (e.g., vegetable oils, fatty acids, terpenes, etc.), hydrocarbon-biomass (e.g., ethane, propene, etc.), and non-hydrocarbon biomass

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(e.g., carbon dioxide and carbon monoxide, etc.) [37]. This results in quests for sustainable polymers and materials (polyolefins, polyesters, polyamides, etc.), which are also expected to be much further extended in the future [38–45]. Terpenes are a class of natural products and a natural source of unsaturated hydrocarbons with diverse functions in nature, e.g. growth regulators, defense signals for the repellation of insects, and also as activators of symbiotic mechanisms (e.g., attracting insects to stimulate cross-pollination processes) [46, 47]. There are more than 80,000 terpenes and terpenoids (the latter are mainly oxygenated compounds) with many different structures, where a classification into linear (acyclic) and cyclic terpenes can be made, whereby the latter can be monocyclic, bicyclic, and polycyclic, and into the number of isoprene-units (C5) that these compounds contain [48]. They have been known for centuries as components of essential oils extracted from the leaves, flowers, fruits, and spices, and also, e.g., insects, marine microorganisms, and fungi can produce them. The bio-genic isoprene rule was formulated, suggesting that the carbon skeleton of these compounds is composed of isoprene units (C5) linked in a regular (head-to-tail) or irregular head-to-head arrangement. Accordingly, they can be defined as monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), sesterpenes (C25), and triterpenes (C30), and so on. Most of the terpenes are abundant/ubiquitous, and they are synthesized in nature in many organisms mainly starting from the compound isoprene, mainly via the so-called Mevalonate pathway (MEP) [49, 50]. This starts from three Acetyl-CoA units leading to mevalonic acid, the precursor for isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP). IPP and DMAPP react to geranyl pyrophosphate (GPP), which reacts with another DMAPP to form farnesyl pyrophosphate (FPP). All terpenes can be stepwise generated by this synthetic sequence through enzymatic catalysis (terpene synthases) and optionally further modifications (Scheme 1). Terpenes can be catalytically upgraded to a wide variety of related chemical compounds and are thus important parts of many value chains [51]. In addition to their industrial utilization as e.g. aroma ingredients and additives, terpenes have been becoming more and more important for the synthesis of sustainable polymers and materials, promoted by their high structural diversity and modifiability, for which also their double bonds with different reactivities play important roles [52–56]. This field is very extensive, and a complete covering would be beyond the scope of this article – many aspects have also been reviewed before in detail [57–62]. Instead, selected aspects are covered herein with a focus on recent examples and developments. Generally, this terpene utilization strategies thus fulfill an important criterion of green chemistry that claims – amongst others – for the utilization of sustainable starting materials, e.g., for chemical transformations [63–66]. Generally, end-of-life disposal and the reuse of polymers are also important factors to sustainable polymer development [67, 68].

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Scheme 1 Mevalonate pathway and biosynthesis of terpenes. Reprinted with permission from Ref. [52]. © 2020 The authors. ChemPlusChem published by Wiley-VCH GmbH

Turpentine is a main fraction of oleoresins obtained from conifers that can be divided into sulfate turpentine, gum turpentine, and wood turpentine [69–71]. Its global production is estimated to be >330 kton per year and it is obtained during wood processing and – relating thereto – as by-product in the pulp industry during Kraft process [72, 73]. The major components of turpentine oil are unsaturated monoterpenes, namely α-pinene (45–97%) and β-pinene (0.5–28%) and smaller amounts of other terpenes such as 3-carene, limonene, camphene, β-phellandrene, and myrcene, and others (Fig. 1).

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Fig. 1 Selected terpenes: The main cyclic components of turpentine oil (above; most of them have stereocenters (here marked with *) and thus stereoisomers), acyclic terpenes (middle), and some polycyclic terpenes (below). Box: The “Basic C5 unit” of terpenes, Isoprene

2 Polymerization of Acyclic Terpenes There are a variety of methods for the often stereoregular polymerization of acyclic terpenes, which have in particular also been reviewed elsewhere in detail (Fig. 2) [52]. Due to their linear or branched structure comprising double bonds, they are mainly used for the preparation of elastomers that are meanwhile also very interesting for industrial applications. Generally, Polyisoprene, a major constituent of natural rubber, has been known for more than a century. It has still an important status in rubber technology and it is an important elastomer with regard to different applications [74–76]. A classification can be made, e.g., into the applied catalysts which are mainly based on titanium, Group 3 and rare-earth metals, iron or cobalt [52]. High molar masses can be achieved here. In the following section, this article focuses on selected examples of the polymerization of selected terpenes. For instance, several polymerizations of Myrcene have been investigated, where also different procedures have been described [77–84]. Highly-ordered structures are possible in this way, which is important for the tuning of different properties. ABA triblock Copolymers with α-Methyl-p-methyl-Styrene (AMMS) have also been investigated and shown to be thermoplastic elastomers with an upper service temperature about 70
C and remarkably low energy loss attributes [85]. Similar procedures have also been described [86, 87]. The polymerization of Ocimene has also been investigated [88]. Capacchione et al. reported the stereoselective polymerization of β-Myrcene and β-Ocimene, where the stereoselectivity was shown to depend on the titanium catalysts and on the different reaction conditions [89]. Furthermore, the binary copolymers β-myrcene/styrene and β-ocimene/styrene were obtained, and a complete characterization of the copolymers indicates a multiblock

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Fig. 2 Stereoregular polymerization of the acyclic terpenes Myrcene, Ocimene, and Farnesene, affording polymers with different unit-connections (schematic). © 2021 The Authors. ChemPlusChem published by Wiley-VCH GmbH. Reprinted with permission [52]

microstructure. Allocimene can also be polymerized [90], and also the copolymerization of, e.g., Farnesene with butadiene by means of Ti-based catalysts activated by modified methylalumoxane (mMAO) under mild conditions was investigated [91].

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3 Polymerization of Cyclic Terpenes Many cyclic terpenes can be transformed to various interesting polymers. Here, the utilization of the pinenes for the synthesis of sustainable polymers has been described in many studies (Scheme 2) [92]. For instance, pinene-based polyolefins have been investigated in detail by cationic polymerization of α-pinene with the binary catalyst AlCl3/SbCl3 [93, 94]. Suchlike investigations are important for further developments in this direction [95]. Furthermore, similar direct pinene polymerizations are possible in different ways, and one of the most successful systems is that based on RCl/EtAlCl2/Et2O which leads to high-molecular-weights poly-(β-Pinene) with high molecular weights (Mw > 100,000) and – after hydrogenation – the respective cycloolefin polymer with excellent thermal and mechanical properties [96]. Further approaches for pinene-derived polymers can be found in the related literature [92]. Another example are polyketones from α-pinene [97]. For this, α-pinene was converted under visible light into pinocarvone, which possesses a reactive exo methylene group. This vinyl ketone was then polymerized in fluoroalcohols by selective ring-opening radical polymerization of the four-membered ring with 99% selectivity. The resulting polymers containing chiral six-membered rings with conjugated ketone units in the main chain display good thermal properties and optical activities. By using appropriate trithiocarbonate reversible addition fragmentation chain transfer (RAFT) agents, RAFT polymerization was successfully accomplished, which thus enabled the synthesis of biobased ketones as thermoplastic elastomers that are based on controlled macromolecular architectures. Different block-copolymers with polyacrylate were also synthesized. Furthermore, (+)-β-Pinene can be oxidized to (+)-Nopinone, which is an important and intensively investigated reaction in terpene chemistry in different contexts [98]. This compound can then, e.g., be transformed to

Scheme 2 Different polymers derived from α-Pinene or β-Pinene. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Adapted with permission from Ref. [92]

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4-isopropycaprolactone and then to polyesters, which can be accomplished by means of different Zr-based catalysts with yields up to >99% [99]. This makes the pinenes to be very useful platform chemicals for polymer chemistry (Scheme 2). In this context, also limonene from citrus fruits is one of the most important terpenes [100]. It is mainly isolated from their oils and peels, and it is thus extensively studied with regard to different applications, e.g., in perfumery, cosmetics, as substitutes for other different petroleum-based chemicals and even in medicine [101, 102]. It has a cyclic structure and two double bonds (one exocyclic and one endocyclic), which are suitable for further modification, and it occurs as different stereoisomers (Fig. 1). These features together with its availability have enabled accesses to diverse structurally significant materials and thus also make limonene to be a platform compound also for polymer chemistry. The functionalization and modification of limonene has also been described in a number of ways, e.g., oxidation and hydroformylation [103, 104]. This can result in aldehyde Lim-CHO in high quantities. This can be used for hydroboration-oxidation, and the resulting diols can be transformed to the corresponding diamines, for which quantitative yields as well as reducing of solvent waste and by-product formation have been described. This enables its versatile utilization as building block especially for polymer science, and there is thus a high polymer diversity derived from limonene.

3.1

Cycloolefin (Hydrocarbon) Polymers

Due to the two double bonds, a direct polymerization of limonene to polylimonene via cationic or radical mechanisms is obvious and has been investigated in several studies (Scheme 3). Indeed, polylimonene can be formed, e.g., using benzoyl peroxide as an initiator in xylene as the solvent [105]. The system follows non-ideal kinetics due to both primary radical termination and degradative chain transfer reaction. The glass transition temperature (Tg) of this polylimonene was shown to be 116
C. Polylimonene is applied, e.g., as additive to adhesives, sealants, and coatings. In further studies similar procedures have been described, also with regard to the preparation of different copolymers [106–108].

Scheme 3 General radical polymerization of limonene to polylimonene

Radical polymerization with initiator. *

*

C H2C n

Limonene

Polylimonene .

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Direct pinene polymerization has also been investigated and has already been mentioned in detail above. There are similar methods that have been described elsewhere more detailed, and thus different polyolefins polymers can be produced from many terpenes [92]. The different reactivities of the double bonds are an important factor here.

3.2

Polycarbonates and Polyesters from Terpenes

Polycarbonates (PCs) are important polymers that can be divided into aromatic and aliphatic PCs based on the structures of their repeating units. Commercial PC that is based on bisphenol-A (BPA) has very useful optical, thermal, and mechanical properties and finds applications as engineering plastics, e.g., in the automotive industry, in construction, and in electronics [109]. Due to some issues concerning the production safety of aromatic PCs – as e.g. toxic phosgene is often required and polymer degradation can occur and result in to some critical metabolites – alternatives for them are desirable [110, 111]. The role of these metabolites is important and should be considered in this context. For instance, bisphenol A (BPA) can be a disruptor by interacting with receptors, as shown also by simulations [112]. On the contrary, aliphatic PCs are biocompatible and biodegradable and they have emerged as possible alternatives for BPA-based PC [113, 114]. However, they are still limited due to a narrow window of mechanical and thermal properties. There is, e.g., aliphatic polypropylene carbonate (PPC), whose property profiles – however – still need some improvements with regard to some performance applications [115]. Their application as oligomeric polyols in polyurethane production is useful, e.g., with regard to chain extension [116, 117]. Therefore, new, structurally rigid monomers derived from renewable and non-toxic resources are very important to further increase the application potential of these polymers. In this whole context, ROCOP of epoxides with CO2 has been shown to be a valuable strategy for the preparation of different polycarbonates by means of various catalysts [118–122]. Accordingly, also various terpene-based epoxides, obtained via oxidation of a double bond, can be transformed to sustainable PC, analogous to the “classical” cyclohexanone oxide (CHO). Such cyclic epoxides are generally more prone for the production of PCs by reaction with CO2 [123]. α-pinene oxide (APO) and limonene oxide (LO) are attractive compounds for suchlike procedures to obtain aliphatic PCs (Scheme 4). APO makes the already mentioned turpentine oil to be a very valuable source also for this approach [124, 125]. Accordingly, a synthesis of Poly(α-pinene carbonate) has been described based on ROCOP of APO and CO2 promoted by a binary catalyst consisting of a CrIII(salen) complex and bis(triphenylphosphine)iminium chloride ([PPN]Cl) [126]. Furthermore, one of the most promising and investigated Limonene-derived polymers is Poly(limonene carbonate) (PLC), which can be synthesized via copolymerization of limonene oxide and CO2, where also its stereocomplex and crystal

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R2

O R2

R1 Epoxide

O

O

CO2 Catalyst

O

Ring-opening copolymerization (ROCOP).

H 3C

O

R1

H3C

Aliphatic PC

CH3 APO

n

O

H 3C CHO

O

LO

CH3

Scheme 4 Ring-opening copolymerization of epoxides and CO2 for the preparation of aliphatic polycarbonates, and structures of cyclohexanone oxide (CHO), α-pinene oxide (APO), and limonene oxide (LO). The additional labeling of the different stereocenters is omitted for simplification Scheme 5 The copolymerization of limonene oxide and CO2 in the presence of different catalysts can yield highmolecular weight PLC. Below: characteristic catalyst according to Ref. [133]

structures have been described [127, 128]. These polymers were then further investigated with regard to enhanced synthetic approaches [129, 130]. Polylimonene carbonate oxide (PLCO) was prepared by regioselective copolymerization of limonene dioxide (LDO) with CO2 promoted by a Zn-based catalyst [131]. The gas permeability of PLC was also investigated in terms of membrane and energy saving applications [132]. This polymer has one double bond per repeating unit, and different catalysts can be used for this sustainable polymerization (Scheme 5) 0.33. For instance, the alternating copolymerization of (R)- or (S)-Limonene oxide using b-diiminate zinc acetate catalysts was reported already in 2004 [134]. At 100 psi CO2 and 25
C, the catalyst was shown to have a high selectivity for the trans isomer and produces regioregular polycarbonate. This copolymer contains >99% carbonate linkages, a narrow molecular weight distribution and an Mn value that is consistent with the [epoxide]/[Zinc] ratio. Kleji et al. reported on a

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similar procedure with a nucleophilic cocatalyst and on corresponding DFT calculations [133]. The preparation of LO/CHO/CO2 terpolymers was later also described by means of an AlIII-based complex [135]. Gradient copolymers were obtained with Mn values between 3.6 and 8.2 kDa. Significant improvements of this and related procedures could then be achieved [136–139]. Later, also the already mentioned cocrystallization of amorphous enantiomeric polylimonene carbonate was described, which resulted in crystalline stereocomplexed polymers [127, 128]. A packaging model in which sheets of enantiopure polymer chains interdigitate with layers of the opposite enantiomer, thus forming a” steric zipper,” was proposed by means of X-ray powder diffraction data. Furthermore, a synthetic toolbox for PLimC was developed that includes a variety of chemical modification strategies. These allow for the tuning of the properties of these aliphatic polycarbonates in nearly any direction to obtain many functional materials via this green platform. Examples are the transformation from an engineering thermoplast into a rubber, the addition of permanent antibacterial activity, hydrophilization, and even a pH-dependent water solubility of the polycarbonate. The completely saturated counterpart could also be yielded, which exhibits improved heat processability due to lower reactivity. Low molecular weight PLC was also investigated for coating applications [140, 141], and similar highperformance thermosets were studied [142]. Furthermore, it was reported that PLC can be crosslinked through photo-initiated thiol-ene reactions. These crosslinked PLCs showed improved pencil-hardness (2H) and also solvent resistance when compared against non-crosslinked PLCs [143]. There are further different approaches to improve and further unlock the processability and the recyclability of PLC [144]. Due to its relatively low decomposition temperature, it is difficult to process PLC successfully from the melt state. Indeed, melt-processed PCL samples are often brittle and colorized. This can, e.g., be improved by compounding with (biobased) ethyl oleate (EtOL). Via this EtOL content, the glass transition temperature (Tg) and also the melt viscosity of these compounds can be readily controlled. The compounds show improved mechanical properties with similar optical properties than neat PCL (which is important for engineering applications), and they can be melt-processed a second-time without significant loss of these properties, which also marks an important feature for recyclability (Fig. 3). Furthermore, a controlled depolymerization and also different further factors can set the basis for an efficient recycling of PLC, which also make this polymer suitable for sustainable materials solutions [145, 146]. Polyesters are very important and useful sustainable polymers due to their hydrolytic degradation and their biocompatibility, but also due to some prosperous mechanical properties [147, 148]. They are frequently used for consumables and commodities, but also for different special applications. Generally, they can be synthesized by polycondensation of dicarboxylic acids or esters with diols (Scheme 6a), by ring-opening polymerization (ROP) of cyclic esters (lactones) (Scheme 6b) and by ring-opening copolymerization (ROCOP) of epoxides with cyclic anhydride (Scheme 6c) [149–151]. The latter has meanwhile attracted much attention as this

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Fig. 3 Optical appearance of hot-pressed PLimC/EtOL compounds with different EtOL contents. The respective EtOL contents are given below the images. Reprinted with permission from Ref. [144]. Copyright 2020 American Chemical Society

Scheme 6 Syntheses of aliphatic polyesters by (a) polycondensation, (b) ring-opening polymerization (ROP), and (c) ring-opening copolymerization (ROCOP)

method allows for regulating the materials properties combining a much larger number of suitable epoxides and anhydride combinations. This is useful for facilitating also the incorporation of functional groups that are useful for post-synthetic modifications [152, 153]. This method enables the regulation of the material properties very efficiently, as a very large number of epoxides and anhydrides are available for suchlike combinations. Furthermore, the incorporation of functional groups is thus facilitated, that can be useful for further tuning or for post-synthetic modifications. In this context, limonene-based diols and also hydroxy-acids were reported that were transformed to polyesters via polycondensation reactions (Scheme 7a). Furthermore, several oxidation methods were described for this conversion [154]. A two-step, chemoselective oxidation of the primary alcohol to the carboxylic acid was the most effective method here. Polycondensations of the diols with succinic acid were performed, where several catalysts were applied. For instance, with [Ti (nOBu)]4, Mn values of up to 30 kDa and Tg values between 7 and 23
C were

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Scheme 7 Summary of different Terpene-derived polyesters based on (a) Limonene; (b) ()Borneol, (c) Camphor, and (d) ()-Menthol (explanations see main text)

described. For the polycondensation of the limonene-based hydroxyl carboxylic acid, Mn values of up to 2.6 kDa were reached (with [Sn(Oct)2] as the catalyst) after very long reaction times. The preparation of polyesters based on borneol was described in 2017 (Scheme 7b) [155]. This compound is a natural terpenoid that can also be obtained from turpentine oil [156, 157]. The process comprises a conversion of in 5-exo-hydroxyborneol using Pseudomonas putida KT2440 in a whole-cell biocatalytic process. The terpene-based diol was then successfully copolymerized with succinic acid dimethyl ester, and an aliphatic polyester was thus obtained with Mn values in the range 2-4 kDa as well as a modest Tg of 70
C. The applied Sn-based catalyst is very convenient, but alternatives could be of interest here. Further very interesting syntheses of polyesters have also been described, where different catalysts have been applied [158, 159]. Camphor is an interesting bicyclic terpene that can be isolated, e.g., from camphor trees or also by transformation of α-Pinene [160, 161]. It is thus also a very valuable building block that can, for instance, be transformed to polyesters (e.g., via camphoric acid and its polycondensation with different diols) with interesting thermal behavior (e.g., poly(ethylene camphorate) (PEC)) (Scheme 7c) [162]. Generally, seven-membered lactones from terpenes represent useful alternatives or supplements to, e.g., petrochemical ε-caprolactone (CL), which is one of the most frequent monomers for polyester and also copolymer synthesis. For instance, the properties of PLA can be modulated to some extent by copolymerization of LA with CL [163, 164]. This fact makes especially modified biobased ketones that are often terpenoid structures, very interesting for the synthesis of polyesters with adjusted and improved properties suitable for different applications [165]. ROP is usually promoted by metal complexes and is one of the most effective strategies for the preparation of PEs. The synthesis of a new polyester by ROP of the lactone ()-Menthide, which can be obtained from menthol by means of a BaeyerVilliger-oxidation, was already described in 2005 by Hillmyer and Tolman (Schemes 3d and 7d). This ROP is promoted by a phenoxy-amine-Zn complex and can result in poly(menthides) (PMs) with Mn values in the range 3-91 kDa by variation of the monomer-catalyst ratio [166]. Menthol is an important cyclic terpene

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used in the fragrance and flavor industry which is produced with several thousands of tons every year [167]. Copolymers with other monomers could also be obtained [168, 169]. These contain very good tensile and elastic properties, which renders them to be useful candidates for thermoplastic elastomers. Furthermore, dihydrocarvide, dihydrocarvide oxide, and carvomenthide, that can be obtained from carvone, can be transformed to the corresponding polyesters via ROP by means of ZnEt2 and benzyl alcohol [170–172]. Copolymerization of dihydrocarvide oxide with CL can yield crosslinked networks with good shape-memory properties. Ketones from pinene have also been synthesized by means of chemo-enzymatic synthesis [173]. Different from other caprolactone-like polyesters, the resulted Poly (verbanone lactone) (PVL) showed a relatively high Tg of 26
C likely being the result of the rigid cyclobutane ring incorporated into the polymer chain. Terpenes can be used for the preparation of polyesters also by means of, e.g., tandem catalysis, where the cyclization of dicarboxylic acids followed by alternating copolymerization of the resulting anhydrides with epoxides is performed [174]. This was thus shown to be an operationally simple method for the production of different new biodegradable polyesters. Later, Thomas et al. employed this tandem methodology for the preparation of APEs with pendant double bonds by the reaction of CA and functional epoxides [175]. Williams and Stößer communicated the efficient preparation of ABA triblock polyesters from decalactone/cyclohexene oxide/cyclic anhydride monomer mixtures using Cr-based catalysts [176]. Further similar examples and approaches have also been described, e.g., regarding the use of tricyclic anhydrides and of phthalic anhydride [177, 178]. Phellandrene was also described for the synthesis of polyesters [179]. Here, the melt polymerization of diglycerol with bicyclic anhydride monomers (BCA 1 and BCA2), that are derived from this monoterpene, provides an interesting avenue for these sustainable polymers (Scheme 8). A solvent-free Diels-Alder reaction of α-Phellandrene with maleic anhydride at ambient temperature results in hydrophobic anhydrides that can effectively undergo the melt polymerizations with tetrafunctional diglycerol under different [diglycerol]/[anhydridie] ratios. The hydrophilicity of α-phellandrene directly impacts the swelling behavior of the polyesters. The low E factors (75% as well as polymer degradability are key factors of these polyesters.

3.3

Polyamides, Polyurethanes, and Others from Terpenes

There are a number of methods for the preparation of N-containing polymers such as polyamides and polyurethanes from terpenes. Generally, polyamides are very important polymers that are mainly used for the production of fibers, engineering plastics for transportation, electronics, packaging and customer product, as well as for medical applications and others. They are mainly synthesized via polycondensation

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Scheme 8 Utilization of Phellandrene for the preparation of polyesters as described in Ref. [179]

Scheme 9 Different Nylons (Polyamides) (a) Nylon 6,6; (b) Nylon 6 (Polyamide 6, Perlon©) via ROP of CLa

of dicarboxylic acids and diamines or via ROP of cyclic amides (lactams), as shown in Scheme 9a, b for Nylon 6,6 and Nylon 6, respectively (but also other methods exist) [180–183]. Although most polyamides are nowadays derived from fossil resources, biobased polyamides are becoming increasingly important with regard to sustainability, for the different reasons mentioned above [184]. With regard to terpene-based polyamides, terpene-based lactams for ROP are very interesting building blocks [185, 186]. For instance, polyamides derived from L-Menthone and from β-Pinene have been described (Scheme 10) [187–193]. Due to the significance of these building blocks (see above), these transformations enable accesses to very interesting polyamides. For this, the terpenoid ketones are transformed into oximes and then into lactams via Beckmann rearrangement, that are then polymerized via anionic or cationic (or other) ROP to the polyamides. Thus the chirality and the side groups of the terpenes are also transferred to the polymers, which results in interesting thermal and also mechanical properties of these polyamides. In some of these approaches, regioisomeric lactams and thus different polyamides are formed. For instance, the arrangement of the polymer chains can result in high melting temperatures, which makes these polyamides interesting for high-performance applications.

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Scheme 10 Polyamides from the terpenes L-Menthone (a, above) and β-Pinene (b, below) which are synthesized via the corresponding oximes and lactams by means of ROP.

Carane-based polyamides have also been described [194–197]. These PAs are also chiral and – dependent on the diastereomeric configuration of the lactam monomers – semicrystalline or amorphous. The obtained polyamides can compete with commercial high-performance polyamides regarding their thermal properties. Compared to the homopolyamides, copolyamides with ε-caprolactam and laurolactam exhibit an increased glass transition and amorphicity. A one-vessel four-step monomer synthesis, applying chemo-enzymatic catalysis for the initial oxidation step, was also established. These approaches further emphasize the sustainable approach of this biobased polyamide synthesis. A corresponding approach to Limonene-derived lactams and polyamides is under intensive investigation [198]. Copolymers of some of these lactams with lactones (e.g., ε-caprolactone) have also been investigated [199, 200]. Thus, polyesteramides are prepared that can combine the properties of polyamides and polyesters; they have thus also tunable biodegradability and adjustable mechanical properties depending on their compositions [201]. This makes them very interesting for, e.g., medical applications [202]. Sustainable polyamides based on limonene have also been described, where, e.g., polyaddition reactions of the terpene-based monomers can be applied [203, 204]. For this, Thiol-ene additions were applied by the addition of Cysteamine hydrochloride to (R)-(+)- and (S)-()-Limonene. Through different combinations, limonene, fatty acids as well as Nylon 6,6 copolymers were prepared and studied by, e.g., GPC and DSC, showing that these polymers have good and adjustable thermal

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Scheme 11 (a) General structure of a polyurethane (PU) for a variety of R’s; (b) Limonene-based PU obtained via thiol-ene additions [203, 204]. The filled circle in the polymer stands for several possible moieties

properties. Similarly, also a number of linear polyurethanes, from amorphous to semicrystalline, were synthesized via an isocyanate-free route and characterized. Polyurethanes find different applications such as foams, insulating materials, coatings, paints, sealings, fibers, elastomers in, e.g., shoes and others (Scheme 11a) [205]. The commercial PUs are mainly synthesized via polyaddition reactions between isocyanates and polyols [206]. Exposure to isocyanates can negatively affect human health, which results in many efforts to improve PU production [207]. Therefore, also many examples have been described for the preparation of sustainable PUs, with regard to the applied monomers and the synthetic procedures [208, 209]. Polyaddition reactions as described above for PA synthesis can also be applied for PU synthesis, as shown for different limonene-based monomers (Scheme 11b). Another very interesting approach is the utilization of cyclic limonene dicarbonate as a new terpene-based monomer for non-isocyanate oligo- and polyurethanes (NIPU). Here, a catalytic carbonation was performed with epoxidized limonene with CO2 to obtain CL. Curing CL with tri- and polyfunctional amines such as citric acid aminoamides and others afforded novel NIPU thermoset resins. With increasing amine functionality of the curing agent, both stiffness (Young’s modulus of 4,100 MPa) and glass transition temperature (62
C) could be increased [210]. Later, the same research group described an improved procedure for the preparation of trans-LDC using consecutive crystallization, where also the stereochemical assignment was corroborated by X-ray crystallography [211]. Also other NIPUs are important contributions to sustainable polymer technologies, where also poly(hydroxyl urethanes) are applied [212, 213]. Especially the improvements in the production of cyclic carbonates from CO2 and epoxides further drive suchlike procedures and enable enhanced accesses to this important polymer and materials class [214–221].

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4 General Remarks There has been a lot of effort for the utilization of renewable feedstock for the development of sustainable polymers from different renewable resources. In this context, the use of terpenes for different polymers and materials is still relatively new, but has an immense potential. Many Terpene-based polyolefins can be considered to be established polymers, and other types of terpene-based materials are getting more and more important. Generally, the structures and the availability of terpenes with regard to, e.g., occurrence and prices are crucial factors for their further utilization. There is still a need for a wide benchmarking of the performance of many terpene-based polymers and also for their comparisons against petroleum-based materials. Furthermore, special processing methods, e.g. fiber spinning, will have to be further investigated, and future research work is going to further address these important topics. The processes and also the energy considerations are very important in this context for the evaluation of the sustainability of the strategies and approaches. For industrial scale, also the CO2 footprints and their comparisons with possible alternatives are very important parameters that have to be evaluated. It will also be interesting to observe these considerations in dependence on the political and economic environment in the different countries and regions. An effective linkage of fundamental and applied research is also very important for further successes in these fields.

5 Conclusion These studies and developments show that the terpenes are – due to, e.g., their high abundance and their interesting structures – meanwhile very important building blocks for polymer science. Especially the abundant compounds Limonene and also Pinenes are very important for these developments, but also many other (also minor available) terpenes are worth to be investigated. For instance, in the case of polyesters and polyamides, the identification of alternatives for PCL and Nylon-6 highlights the role and the potential of terpene-based lactones and, respectively, lactams. Based on their functionalities, these can also result in advanced materials with improved properties that cannot be obtained so easily via classical procedures. As their availability can be improved via different methods, these approaches are becoming even more important. Furthermore, there are different strategies for an enhanced functionalization of some of these polymers, and especially postmodifications allow for further improvements. Terpenes offer structural modularity as a big advantage, as in nature they can be linear, cyclic, or polycyclic and equipped with different functional groups. This gives thus potential to utilize them for the production of new materials with an extended window of properties than it can be realized with many of the conventional monomers that are currently used in the chemical industry, which is interesting, e.g., with regard to different high-

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performance polymers. Furthermore, applications as different biomaterials in medicine for the directed interactions with living cells are interesting in this context: this is due to the fact that many factors that are important at the cell–materials interface, e.g. nanotopography, chemical functionality, etc., are representative and tunable especially in terpene-based polymers [222]. Additives and compounding can also be important. Different comparisons of, e.g., the thermal and mechanical data of the terpene-based materials with those of established petroleum-based polymers prove their advantages and show their good performance and their high potential also for further and improved applications [223, 224]. A further elucidation of these connections and comparisons would be beyond the scope of this chapter, details hereto can be found in the further literature [225–228]. In the course of the growing interest in green chemistry and various sustainable procedures especially for materials synthesis and their production processes, the utilization of terpenes for different applications is anticipated to further increase remarkably in the future. The information collocated herein will thus contribute to utilize much more – maybe also yet unidentified – potential of this fascinating class of renewable and sustainable raw materials. Acknowledgments The original research of M. W. about different terpene-based polymers has been (and is) funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under the Project Number 445011287, which is gratefully acknowledged. M. W. is also thankful to the TU München for general support.

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Adv Polym Sci (2024) 293: 65–110 https://doi.org/10.1007/12_2023_163 © The Author(s) 2023 Published online: 21 October 2023

Polymer Biodegradability 2.0: A Holistic View on Polymer Biodegradation in Natural and Engineered Environments Michael Sander, Miriam Weber, Christian Lott, Michael Zumstein, Andreas Künkel, and Glauco Battagliarin

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Definition of Polymer Biodegradability and Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Process Elucidation of Polymer Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Steps in Polymer Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Factors Controlling Polymer Biodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Variation in Polymer Biodegradation Between Receiving Environments . . . . . . . . . . . 4 Setups and Analytical Methods to Study Biodegradation and to Test Biodegradability of Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Laboratory Incubations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Mesocosm and Field Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Recent Analytical Advancements in the Assessment of Polymer Biodegradation . . 5 Standard Tests and Certifications for Polymer Biodegradability and Biodegradation . . . . . 5.1 Motivation for Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Levels of Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Status of Standard Test Methods and Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Certifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Research Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Variations in Polymer Biodegradation in and Across Different Receiving Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Assessing Uncertainties Associated with the Transferability of Results of Polymer Biodegradability from Laboratory Tests to the Real in-situ Biodegradation in the Receiving Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Identification of Key Degrading Enzymes and Microbial Degraders . . . . . . . . . . . . . . . . 6.4 Microbial Metabolic Utilization of Plastic and Polymer Carbon . . . . . . . . . . . . . . . . . . . . 6.5 Slowly Biodegrading Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Sander (✉) Institute of Biogeochemistry and Pollutant Dynamics, Swiss Federal Institute of Technology (ETH) Zurich, Zurich, Switzerland e-mail: [email protected]

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Abstract Biodegradable polymers are an important part of the solution toolbox to achieve circularity in the plastic economy and overcome negative impacts of a linear plastic economy. Biodegradable polymers need to excel not only on a mechanical performance level in the application to fulfill their function during the use phase but also on a biodegradation performance level after use. The biodegradation performance is tailored to the application and the receiving environment of the polymer product after use, which can be both engineered systems (e.g., compost, anaerobic digestors, wastewater treatment plants) and natural systems (e.g., soils, freshwater, or marine environments). This chapter addresses key aspects of polymer biodegradability and biodegradation in both natural and engineered systems with the goal to advance a more holistic view on the topic and, thereby, provide guidance for all stakeholders working on developing, testing, and regulating biodegradable polymers. These aspects include definitions of biodegradability and biodegradation, elucidating polymer- and environmental factors that control the biodegradation process, a discussion of the analytical chemistry of polymer biodegradation, polymer biodegradability testing and certification, as well as a brief overview of research needs. In accordance with the diverse backgrounds of the authors of the chapter, this chapter targets all stakeholder groups from academics to industry and regulators. Keywords Polymer biodegradation · Biodegradability · Mineralization · Biodegradable plastics · Bioplastic · Compostability

1 Introduction Polymers – including structural polymers, on which this chapter focuses, and watersoluble polymers – play essential roles in our modern life and fulfill many important functions in diverse applications. For instance, structural polymers are used in consumer goods such as shoes and garments, in the transportation industry, in electronic goods and for hygienic packaging of our food. Water-soluble polymers are used widely in home and personal care products and in agricultural formulations. In all the above-mentioned applications, the polymers deliver a desired functionality at high efficiency and thereby contribute to the sustainability of the application. The many unique benefits offered by using polymers are undisputed. Yet, academic and industrial researchers working on developing and applying polymers (and plastics composed thereof) face two major challenges: to establish circularity in the use of polymers (including preventing the accumulation of persistent (micro- and nano) plastics in the environment) and to become carbon neutral. Addressing these challenges requires research and innovation on polymers as well as new approaches in their use, all as part of a multiple “solution toolbox” [1–3]. The use of biodegradable and biobased polymers (both being part of the larger class of the so-called biopolymers) in specific applications is considered an integral part of this overall “solution toolbox” [4] (see also introduction of book).

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There are numerous applications in which the use of biodegradable instead of non-biodegradable polymers offers benefits toward attaining circularity and preventing environmental plastic pollution [5, 6]. These applications include, but are not limited to, compostable plastics (e.g., compostable bags for collection of domestic biowaste) and agricultural plastics (e.g., soil-biodegradable mulch films) [4]. In the first case, compostable (and possibly also biobased) plastics enable organic waste recycling and closing the nutrient loop [7]. Austria and Italy are good examples for countries which successfully implemented waste management structures that include compostable bags (e.g., “Biosackerl” in Austria) to collect organic waste. Using compostable plastic bags is essential to allow collecting biowaste separately from the normal household waste and, at the same time, to reduce or even avoid contamination of the compost by conventional plastics that persist in the compost. The use of compostable polymers (and plastics) thus is a prerequisite to ensure a high quality of the final compost [8]. Among the agricultural applications using biodegradable polymers are mulch films that are placed onto soils to increase crop yields [9, 10]. Conventional mulch films are composed of non-biodegradable polyethylene (PE) and require a minimum thickness of at least 25 μm to ensure that they can be completely recollectable from the field after use. Thinner films would suffice to fulfill the needed mechanical performance of the films during the use phase but would impair complete recollection of the film after its use phase, resulting in soil contamination by residual PE film fragments. Following use in the field, the recollected PE films often contain crop and soil attachments which render recycling and reuse of these films difficult if not impossible, leaving only undesirable incineration or landfilling as disposal options. As compared to conventional PE films, soil-biodegradable mulch films composed of biodegradable polymers can be thinner, such as 15 μm, as they do not need to be recollected after use but instead are plowed into the soil to then undergo biodegradation. This practice also substantially lowers the end-of-life costs because biodegradable films – as opposed to conventional PE-based films – do not need to be recollected from the field, transported, and disposed of. The possibility to use thin biodegradable films (instead of thicker conventional films) comes with the additional benefit that less polymer material is needed for the mulch film application. In specific applications (such as thin mulch films), the use of polymers that are biodegradable in the open environment is beneficial over the use of conventional, non-biodegradable polymers in that it helps to overcome environmental plastic pollution [4, 6]. Biodegradation as an end-of-life treatment is particularly warranted for applications in which complete recollection from the environment after use and/or reuse and recycling of the collected polymer material are not feasible. It was recently proposed to delineate three categories for such applications: (1) applications in which polymers (and plastics) are intentionally left in the environment after the application (i.e., seed coatings), (2) applications in which polymers (and plastics) are lost to the open environment through abrasion (e.g., paints and geotextiles), and (3) applications which have a high potential that the deployed polymer (and plastic) items are lost during (or after) use (e.g., certain fishing gear) [6].

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Compostable bags and soil-biodegradable mulch films are only two of a larger number of applications [4, 6] in which biodegradable polymers are recognized as an important component of the overall “solution toolbox” by academics, industry, NGOs, and decision makers. These applications have in common that biodegradation is the targeted end-of-life scenario or the preferred scenario for polymers that unintentionally leak into the environment from specific applications [11, 12]. Consequently, decision makers and regulators rightly demand that biodegradation standard- and legislative support frameworks are based on stringent scientific evidence demonstrating that the polymers indeed undergo biodegradation. In essence, biodegradable polymers therefore must perform on two levels: on a “mechanical performance level” during processing and during the application (e.g., processability and tensile strength of the polymer) and on a “end-of-life biodegradation performance level,” following the use phase. Providing scientific proof for “end-of-life biodegradation performance” – and, thereby, to ensure the acceptance of biodegradable polymers by different stakeholder groups – requires a revised and extended approach of understanding and assessing polymer biodegradability and biodegradation in receiving environments. The latter include both engineered systems (e.g., anaerobic digestors, industrial and home composting, wastewater treatment plants) and natural environments (i.e., soils, marine, and freshwater systems). This revised and extended approach of polymer biodegradability and biodegradation is critical to achieve three specific targets. Target 1: Establish reliable and science-based laboratory testing systems to provide a clear framework for the development of biodegradable polymers. The first target is to demonstrate, by experimental incubation tests, that a specific polymer material (or item made thereof) indeed biodegrades in the specific targeted receiving environment to a defined extent over a given time. This target is challenging to achieve given that biodegradation in almost all cases needs to be followed over incubation periods of several months and up to a few years, thereby requiring both extensive testing time and robust testing systems. Ideally, these test systems are designed to have incubation conditions that are representative of the conditions in the actual targeted receiving environment of the polymer. At the same time, the handling and control of the test systems need to be feasible. Furthermore, the system needs to allow for a continuous and direct monitoring of polymer biodegradation during the test period, as discussed in more detail below. For the certification of biodegradable polymers (and items made thereof), biodegradation standards need to define sufficiently stringent criteria that polymers passing the tests indeed biodegrade at desired rates in the actual receiving environment [13]. These criteria are also critical to prevent misuse of labels by helping to identify polymers or polymer additive technologies (e.g., pro-oxidant additives for conventional, non-biodegradable polymers) that do not fulfill the biodegradation criteria and therefore falsely claim polymer biodegradability [14–16]. Finally, standards with clear criteria set the boundaries for research on and innovation of future biodegradable polymers and provide guidance and boundaries for academic laboratories and industrial research and development.

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Target 2: Align laboratory testing systems with real (in situ) environmental conditions. The design of laboratory testing methods to determine polymer biodegradability (see Target 1) must be well-aligned with the conditions that prevail in the targeted receiving environment (e.g., engineered systems such as industrial composting or natural systems such as soils). This alignment is critical to ensure that results from laboratory tests adequately capture the “biodegradation performance” of the polymer also under real in situ conditions. The transferability of biodegradability results obtained in laboratory incubations to real in situ conditions in the actual receiving environment needs to be fulfilled for laboratory testing systems to be acceptable in standards and for certification in the long-term. This second target requests establishing separate laboratory testing systems for the different targeted receiving environments. The tests need to be adaptable to the specifics of the receiving environment in terms of temperature, humidity, and anticipated residence time of the polymer in that system. Demonstrating transferability of biodegradation results from the laboratory to real in situ field systems currently is a central request by different stakeholders, including political decision makers. Target 3: Ensure complete biodegradation of the polymer in the receiving environment. Stringent and elaborate testing of polymer biodegradation in the laboratory needs to be complemented by efforts to demonstrate complete biodegradation of the polymer also in the receiving environment over a time frame that is acceptable by different stakeholders. This target is needed to ensure no formation of residual persistent polymer particles that accumulate in the receiving environment. Achieving this target includes the development of analytical methods to track biodegradation and the formation dynamics and continuous biodegradation of potentially forming micro- and nanometer-sized plastic fragments as intermediates in the biodegradation process. To fulfill all the three above-defined targets, an extended approach (i.e., “Polymer biodegradability 2.0”) is needed. This approach needs to be based on a fundamental understanding of polymer biodegradation in the targeted receiving environment. In this chapter, we abide to the recently proposed concept that polymer biodegradability is a system property in that biodegradability depends on both the polymer material properties and the characteristics of the receiving environment [6]. This concept expands from the traditional view in which polymer biodegradability is considered primarily (and sometimes exclusively) a polymer material property. We also explicitly emphasize that the physicochemical properties of a biodegradable polymer that render it biodegradable are desired: intentional “designed to biodegrade” (i.e., biodegradation performance) fundamentally differs from “biodeterioration,” a term that has often been used to describe undesired deterioration of a polymer by processes involving microorganisms. Because of the multifactorial material and environmental effects on polymer biodegradation rates, elucidating this process and developing new materials requires expertise from a multitude of research disciplines, including – but not limited to – polymer chemistry, environmental chemistry, and microbiology. This chapter summarizes fundamentals and

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key concepts of polymer biodegradation in engineered and natural systems. This chapter also briefly highlights recent advances in analytical approaches, testing approaches, and certification standards for polymer biodegradation. The goal is to help advance a more holistic view on polymer biodegradability in both natural and engineered systems and, thereby, provide guidance for all stakeholders of developing, testing, and regulating biodegradable polymers.

2 Definition of Polymer Biodegradability and Biodegradation Stringent and clear definitions of polymer biodegradability and biodegradation are critical to advance our understanding of polymer biodegradation, testing and certifying polymer biodegradability and to clearly communicate these aspects. Conversely, the absence of concise definitions will result in misunderstandings and, more problematically, can even lead to false claims of polymer biodegradability and biodegradation. We herein define “polymer biodegradation” as follows [6]: Polymer biodegradation is the process by which microorganisms completely metabolize the organic carbon in the polymer under formation of carbon dioxide and microbial biomass, under oxic conditions, or carbon dioxide, methane, and microbial biomass, under anoxic conditions. This definition holds true for polymer biodegradation both in engineered and natural systems. In fact, it applies not only to structural polymers (which are used to make plastics and which are at the focus of the discussion in this chapter) but also to non-structural, water-soluble polymers. For polymers containing organically bound heteroatoms (i.e., N, P, and S), these heteroatoms need to be converted to the respective inorganic salts or also be incorporated into microbial biomass during the biodegradation process. However, the major biodegradable polymers currently marketed are composed solely of C, H, and O (see below). Based on the above definition, we can express polymer biodegradation as a simple reaction [17, 18]: Cpolymer → CCO2 ðþCCH4 Þ þ Cmicrobial biomass

ð1Þ

where Cpolymer is the organic carbon of the polymer (or polymers, in case of plastics composed of more than one), CCO2 and CCH4 is the polymer-derived carbon that has been metabolically converted by microorganisms to carbon dioxide and methane, respectively, and Cmicrobial biomass is the polymer-derived carbon incorporated into microbial biomass (i.e., intracellular bio(macro)molecules such as building blocks of cells). Conversion of Cpolymer to CCO2 is commonly referred to as “mineralization.” The evolved CO2 may either be formed directly from polymer carbon by respiration of

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polymer-derived substrates or it may form from the mineralization of polymer carbon first incorporated into microbial biomass. The latter formation pathway reflects that microbial biomass containing polymer-derived carbon is itself a transient carbon pool: Cmicrobial biomass in both alive and dead microbial cells can be reworked to CCO2. Microbial necromass may also be reworked into the (non-living) natural organic matter pool in the specific receiving environment (e.g., soil organic matter in soils), from which mineralization may be slow. Polymer carbon conversion to CO2 (and CH4) and to microbial biomass is a desirable and acceptable biodegradation endpoint. While Eq. 1 describes the “educts” and “products” of polymer biodegradation, it does not capture that polymer biodegradation is a time-dependent process. The latter can be accounted for by balancing polymer carbon at any time during the biodegradation according to Eq. 2: Cpolymer ðtÞ = Cpolymer ðt 0 Þ½CCO2 ðtÞ þ Cmicrobial biomass ðtÞ

ð2Þ

where Cpolymer (t) and Cpolymer (t0) are the amounts of polymer carbon at time point t during the biodegradation process and at the onset of the biodegradation process t0, respectively, and CCO2 (t) and Cmicrobial biomass (t) are amounts of polymer-derived carbon mineralized to CO2 and transferred into microbial biomass at time t, respectively. For simplicity, we here consider biodegradation only under oxic conditions and, therefore, omit CH4 as a possible biodegradation product under anoxic conditions from the equations. Furthermore, we make the simplification in Eq. 2 that Cmicrobial biomass does not undergo reprocessing over time (which is not the case). Polymer biodegradation can be directly followed in laboratory incubations in which the conversion of Cpolymer to CCO2 is followed over time through respirometric analyses. Respirometric analyses are critical to verify biodegradation given that CO2 is the final product of the biodegradation process under toxic conditions. We can therefore rearrange Eq. 2 for CCO2(t) to obtain Eq. 3: CCO2 ðtÞ = Cpolymer ðt 0 ÞCpolymer ðtÞCmicrobial biomass ðtÞ

ð3Þ

While respirometric analyses provide proof for polymer biodegradation, Eq. 3 highlights that these analyses provide no information on the relative importance of Cmicrobial biomass (t) and residual Cpolymer (t) to the non-mineralized polymer carbon. The nature of the non-mineralized polymer carbon thus remains unknown when only using respirometric analyses. Significant formation of Cmicrobial biomass would imply that respirometric analyses of CCO2 underestimate the actual extent of polymer biodegradation. One common approach in standard test methods to account for the fact that polymer carbon may be incorporated into biomass (i.e., the formation of Cmicrobial biomass) is to express CCO2 formed in polymer biodegradation not in absolute terms (i.e., % of polymer carbon converted to CO2) but instead relative to the CCO2 formed from a positive control – a known biodegradable reference (bio) polymer, such as

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cellulose or polyhydroxyalkanoates (PHA) – run in parallel incubations in the same matrix (e.g., soil or sediment). Biodegradation of these positive control materials often shows plateauing mineralization extents at values smaller than 100%, with the “missing” non-mineralized carbon being ascribed to Cmicrobial biomass formation. Expressing the amounts of CCO2 formed from polymers relative to the amounts of CCO2 formed from positive control reference polymers during incubation assumes that microorganisms in the tested medium have a similar metabolic utilization pattern for substrates derived from the polymers of interest and from the positive control reference polymer. More specifically, it is assumed that microorganisms in the tested medium have comparable carbon use efficiencies (CUEs) (i.e., the ratio of substrate carbon incorporated into microbial biomass relative to the total substrate carbon taken up) for the substrate molecules from the polymer of interest and from the positive control. While this assumption may be valid, information on CUEs for polymer-derived substrates is currently missing. The CUEs of polymers are likely dependent not only on the chemistry of the polymer-derived molecules and their specific intracellular metabolic processing, but also on the rate at which polymer-derived substrates become available to microbial cells during the biodegradation process: low CUEs are expected when rates at which polymer-derived substrates become available to microbial cell are low given that the cell metabolism is then likely directed toward energy generation rather than biomass formation. Furthermore, extents of incorporation of polymer carbon into microbial biomass may strongly depend also on the availability of nitrogen and phosphorus as these are required for biomass buildup, as previously shown for natural substrates [19]. We note that new solvent extraction-based analytical approaches to quantify Cpolymer (t) over the course of incubations promise to help identify the nature of non-mineralized polymer carbon and, therefore, the extent to which polymer carbon is incorporated into microbial biomass (Sect. 4 below provides more information). Equation 3 forms the basis for elucidating polymer biodegradation and testing polymer biodegradability, as detailed in Sects. 3–5 in this chapter. Biodegradation standards typically stipulate a high extent of mineralization of polymer carbon (commonly 90% of the carbon, either in absolute terms or relative to the extent of mineralization of a reference polymer, as discussed above) over a pre-defined incubation period. These incubation periods are specific to the product application of the tested biodegradable polymer and typically vary between weeks to months (e.g., for compostable polymers and plastics made thereof) and up to 2 years (e.g., for polymer biodegrading in natural systems, such as mulch films in agricultural soils). Given the specific definition of “biodegradation,” this term ought not to be used interchangeably with other terminology that describes alterations in the physicochemical properties of a polymer (or plastic) item, including “degradation,” “disintegration,” “breakdown,” “biodeterioration,” “biotransformation,” and “fragmentation.” While the latter processes may (all) be involved in polymer biodegradation (e.g., a biodegradable polymer or plastic may also undergo fragmentation into smaller particles during its biodegradation process), the term

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“biodegradation” is distinct from all others in that it stipulates that the entire polymer carbon is microbially converted to CO2 (and CH4) and microbial biomass. In addition to the term “polymer biodegradation” that describes a process, the term “polymer biodegradability” is also commonly used. The latter term describes the propensity of a polymer (or, for a plastic, of the polymer(s) contained in the plastic) to undergo biodegradation in a specific receiving environment. As stated above, we herein adopt the view that biodegradability is a system property in that it depends both on the physicochemical properties of the polymer and on the abiotic and biotic conditions of the receiving environment in which polymer biodegradation is tested or occurs. Because conditions vary substantially between different engineered and natural systems (e.g., industrial compost vs. soils vs. freshwater systems vs. marine systems), certifications and labels of polymers being “biodegradable” ought to specify in and for which engineered or natural environment biodegradation was tested and certified (i.e., “industrially compostable,” “soil biodegradable,” “freshwater biodegradable,” or “marine biodegradable”) (see also Sect. 5 of this chapter). Defining the receiving environment for which polymer biodegradation has been certified is critical to overcome the common misconception of biodegradable polymers being “universally” biodegradable across all receiving environments (which may, however, be indeed the case for a few biodegradable polymers). Finally, complementing certification labels with information on the receiving environment does not imply that this polymer shows the same biodegradation rates within different types (e.g., types of soils or types of marine sediments) and habitats of the same receiving environment (e.g., beach, seafloor, and water column habitats in marine environments). Instead, a certified biodegradable polymer may show variations in biodegradation rates between different types of the same receiving environment or between habitats of the same receiving environment for which the certificate was issued. We conclude the discussion of terminology with the term “intrinsically biodegradable.” This attribute has been proposed for polymers that have demonstrated extensive biodegradation in test systems that favor the biodegradation (e.g., elevated temperatures, controlled humidity, presence of nutrients). As such, “intrinsic biodegradability” may be viewed as a material attribute and as a proof that the polymer can, in principle, be metabolically utilized by microorganisms. However, the term “intrinsically biodegradable” should not be misinterpreted to imply that the polymer is universally biodegradable across receiving environments – in fact, it may not biodegrade in environments with conditions disfavoring its biodegradation. Section 3 will summarize key polymer properties and environmental conditions that govern polymer biodegradability.

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3 Process Elucidation of Polymer Biodegradation 3.1

Steps in Polymer Biodegradation

As defined in the previous subsection, polymer biodegradation refers to the process in which polymer carbon is metabolized by microorganisms for energy production under the formation of CO2 (and CH4) (catabolism) and for the formation of microbial biomass (anabolism). Therefore, the presence of microbial degraders in the polymer-receiving environment is central to biodegradation. The uptake of polymer-derived molecules into microbial cells and the intracellular processing of these molecules may, however, not be the rate-limiting steps of the overall biodegradation. Instead, the biodegradation rate in many engineered and natural systems is controlled by the rate at which the polymer breaks down into molecules sufficiently small for microbes to take the molecules up into their cells and metabolically use them as substrates [20, 21]. Molecules that can readily be taken up into microbial cells are expected to have an upper size limit in the range of 1,000 Da, a molecular weight that is substantially smaller than the typical molecular weights of polymers. Efficient breakdown of the polymer into small, microbially utilizable molecules separates biodegradable polymers from conventional polymers: while the latter may fragment into small particles – micro- and nanoplastics – these fragments persist and are not readily converted to organic molecules sufficiently small to be taken up by microorganisms. Consequently, degradation of conventional plastics commonly results in the formation of micro- and nanoplastics that accumulate in the environment. While biodegradation of biodegradable polymers may also involve fragmentation of the polymer into small polymer particles, these particles are only intermediates and continue to undergo biodegradation (possibly even at increasing rates given that the small polymer particles have higher surface-to-volume ratios than the original polymer specimen). We herein describe polymer biodegradation as a two-step process (Fig. 1). The first step is the extensive breakdown of the macromolecular chains of the polymer into small (i.e., low-molecular-weight) organic molecules. For most commercial biodegradable polymers, this breakdown occurs by hydrolysis of hydrolyzable bonds in the polymer backbone. For some polymers, such as polylactic acid (PLA), hydrolysis is primarily abiotic and increases in rate with increasing temperature. Abiotic hydrolysis may not only be constrained to the polymer surface but also occur in the bulk phase of the polymer if water molecules diffuse into the polymer matrix. These polymers then undergo “bulk erosion” [22, 23]. Yet, for many biodegradable polymers, abiotic breakdown is slow. Instead, breakdown of these polymers is catalyzed by extracellular enzymes that are secreted by microorganisms. Given that these enzymes have dimensions of a few nanometers, they cannot diffuse into the bulk phase of the polymers. Enzymatically mediated breakdown is therefore commonly restricted to the polymer surface, resulting in polymer “surface erosion” [22, 23].

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Fig. 1 Schematic for the two central steps in the process of polymer biodegradation: breakdown of the polymer into low molecular weight products (step 1) and subsequent uptake and metabolic utilization of these breakdown products by microorganisms (step 2). Polymer breakdown in step 1 may occur abiotically and/or be mediated by extracellular microbial enzymes. The breakdown needs to result in molecules of sufficiently small size to be taken up into microbial cells. Inside the microbial cell, these molecules are metabolically utilized resulting in the formation of CO2 and microbial biomass under aerobic conditions or CO2, CH4 and microbial biomass under anaerobic, methanogenic conditions

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For polymers requiring enzymatic catalysis in the breakdown, biodegradation rates and extents are controlled by the amount and activity of the respective enzymes in the specific environment in which biodegradation occurs. These enzymes are present in the receiving environment not because of an evolutionary response to the presence of biodegradable polymers but instead because the enzymes have evolved to catalyze the breakdown of natural biopolymers that have functional groups which are present also in the biodegradable polymer. For instance, cutinases have evolved to hydrolyze ester bonds in cutin, a wax-like polyester that forms a protective cover on the surfaces of plant leaves. These cutinases also hydrolyze ester bonds in the backbone of many synthetic polyesters [24]. Because the polymer-degrading enzymes are of microbial origin, enzymatic breakdown is expected to increase with increasing colonization of the polymer surface by microorganisms that secrete the respective enzymes. It is because of the importance of enzymatic breakdown and microbial colonization that polymer biodegradation is oftentimes also referred to as a three-step process: microbial surface colonization is viewed as an additional step that is followed by the breakdown of the polymer chains and microbial utilization of breakdown products. Herein, we instead advocate viewing biodegradation as a two-step process in which the first step, polymer breakdown, may (or may not) involve microbial colonization of the polymer and enzymatic polymer breakdown. We favor this view of a two-step process because it is more universal. For example, microbial colonization is not a necessity if breakdown occurs abiotically or if the enzymes actively breaking down the polymer are secreted by microorganisms that have not colonized the polymer surface. Furthermore, the conceptual framework of a two-step process also applies to water-soluble polymers in that they may undergo enzymatically mediated breakdown but lack a rigid surface that can be colonized by microbial cells. Most commercially important biodegradable polymers contain hydrolyzable bonds in their backbone. Polyesters dominate among the synthetic polymers, but hydrolytic breakdown and biodegradation have also been described for other polymers, including polyurethanes [25–27], polyamides [28, 29], and polycarbonates [30]. The hydrolyzable bonds in the polymer backbone can be considered “intended breaking points” to allow for step 1 of biodegradation. However, the mere presence of hydrolyzable bonds (e.g., ester bonds) in a polymer does not imply that it is biodegradable. This is showcased by polyethylene terephthalate (PET), a synthetic polyester. Enzymes that hydrolyze ester bonds in amorphous regions of PET have recently been identified [31–33] and are currently investigated for their potential use

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in PET recycling [34]. Yet, enzymatic hydrolysis of crystalline domains in PET remains slow [35, 36]. Consequently, PET remains a non-biodegradable polymer. Besides hydrolytic reactions, oxidations constitute a second class of (enzymatically mediated) reactions that may result in the breakdown of synthetic polymers. Oxidases occur naturally and are key in the breakdown of specific biopolymers in the natural environment: for instance, the breakdown of lignin is catalyzed by manganese peroxidases, hydroperoxidases, and laccases secreted by white-rot fungi [37– 39]. However, compared to hydrolytic breakdown, oxidative breakdown of synthetic polymers is expected to have two principal limitations. The first limitation is that oxidases require dioxygen or activated oxygen species (such as hydrogen peroxide) as co-substrates. While these can be present under oxic conditions, they are absent in anoxic natural systems such as many aquatic sediments. Consequently, polymers relying on oxidative breakdown are expected to be stable in sub-oxic and anoxic systems. Such stability in anoxic systems is well known for the natural biopolymer lignin which undergoes oxidative breakdown: wooden Viking ships have been preserved for centuries in anoxic marine sediments. As compared to oxygen, water as the reaction agent in hydrolytic polymer breakdown is present in almost all natural environments (except for extremely dry (micro-) environments in which hydrolytic breakdown of a biodegradable polymer may be impaired). The second limitation of oxidative breakdown is that the oxidants in enzymatic oxidations, particularly highly reactive oxidants, react in a less directed manner as compared to water that selectively hydrolyzes specific bonds. Consequently, not all oxidations on a polymer structure are expected to result in backbone cleavage and polymer breakdown. Instead, additional follow-up reactions may be required to result in cleavage of the polymer backbone. These two principal challenges of oxidative breakdown likely explain why most synthetic biodegradable polymers rely on hydrolytic and not oxidative breakdown in step 1. A critical assessment of oxidative polymer breakdown is particularly compulsory for the so-called pro-oxidant additive technologies (i.e., oxo-additives) that claim to render polyolefins biodegradable. Polyolefins persist in the environment because of the high stability of the carbon–carbon bonds in their backbone. The pro-oxidant additives contain complexed transition metals (i.e., typically stearates of Co, Fe, and Mn) [40, 41] and are added to polyolefins at a few weight percent. These transition metals are activated either thermally or photochemically by UV light and, in the presence of O2, trigger the formation of reactive oxygen species that attack the backbone carbon in polyolefins, ultimately leading to polyolefin fragmentation, scissions of their C-C bonds, introduction of oxygen functionality, and a decrease in the polymer molecular weight. However, for several reasons, there is broad consensus that such pro-oxidant additives fail to render polyolefins biodegradable in natural systems. First, a fundamental shortcoming of this technology is its dependence on the presence of O2, rendering this technology ineffective under anoxic conditions that are present or even prevail in many natural environments (e.g., soils, water bodies and sediments). Second, (UV) light is absent from many natural systems, implying that there is no UV light activation of pro-oxidant additives in such systems. Furthermore, temperatures in the environment are much

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lower than those typically used in laboratory settings to demonstrate activation of the pro-oxidant additives. Therefore, pro-oxidant activation and polymer breakdown demonstrated under enhanced UV light irradiation and/or at elevated temperatures, as performed by companies marketing pro-oxidants, fall short of demonstrating that activation also adequately occurs in the natural environment. Activation under natural conditions has not been demonstrated, even if claimed in recent studies: activation of polyolefins containing metal additives under natural Florida irradiation conditions failed to address that these irradiance conditions are not globally representative nor disclosed that polymer specimen were mounted in a manner expected to result in artificially high weathering temperatures [42]. Thirdly, direct experimental evidence for biodegradation of pro-oxidant-activated polyolefins in any relevant environment and without artificial pre-treatment remains missing from the literature. In fact, several studies have shown that pro-oxidant containing polyolefins do not biodegrade even after extensive activation [15]. Based on these considerations, the scientific community and regulators in many countries agree that pro-oxidant additive technology do not render polyolefins biodegradable [16, 43]. This view has led to a ban of pro-oxidant technology in the European Union [44].

3.2

Factors Controlling Polymer Biodegradation

Polymer biodegradation is dependent on both the physicochemical properties of the biodegradable polymer (e.g., the presence of hydrolyzable bonds in the backbone of the polymer, the crystallinity of a polymer) and the abiotic and biotic conditions in the receiving environment (e.g., temperature, presence of specific microorganisms that secrete hydrolases that break down the polymer, presence of water, etc.) [6, 23]. These conditions are thus decisive in determining the extent to which the biodegradation potential of the polymer is leveraged. In the following, we will provide a brief overview of key polymer-dependent and environment-dependent factors that control polymer biodegradation in natural and engineered systems. These factors provide the basis on which biodegradation in different receiving environments can be compared and tested. We refer to reviews that also summarize the factors controlling polymer biodegradation [23, 45].

3.2.1

Polymer-Dependent Factors that Control Polymer Biodegradation

Polymer backbone chemistry. The backbone chemistry of the polymer is a key criterion determining its biodegradability. As alluded to above, most commercially important biodegradable polymers contain hydrolyzable bonds in their backbone. Figure 2 shows the structures of selected commercially important biodegradable polyesters as well as the hydrolytic reaction that leads to bond breaking in polyester backbones. These polymers include aliphatic and aliphatic-aromatic (co)polyesters (e.g., PLA, PBAT, PBA, PBS, PHA, PBSA, PCL) that undergo hydrolysis of the

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Fig. 2 (a) Chemical structures of commercially important biodegradable polyesters (including aliphatic and aliphatic-aromatic co-polyesters) and of cellulose, which often is used as a positive control in polymer biodegradation tests. (b) Hydrolysis of an ester bond into a carboxylic acid and an alcohol during the initial step (i.e., breakdown) of polyester biodegradation. Hydrolysis can occur abiotically but is enzymatically mediated for many polyesters

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ester bonds to form a carboxylic acid and an alcohol. Figure 2 also shows the chemical structure of cellulose, a natural biopolymer with glycosidic bonds that is commonly used as positive control in biodegradation tests. While polyesters dominate the market of biodegradable polymers, biodegradation has also been demonstrated for other hydrolyzable polymers, such as polyamides (i.e., Nylons) [28, 29, 46]. Polymer form and morphology. The rate of polymer breakdown in step 1 of biodegradation increases as the surface-to-volume ratio of the polymer increases, particularly when the breakdown is enzymatically catalyzed and thus constrained to the polymer surface [45, 47]. As a consequence, larger plastic particles or items composed of one or more biodegradable polymers may require extensive time periods to biodegrade due to their small surface-to-volume ratio. Fragmentation of these items into micro-and nanometer-sized particles during biodegradation may largely enhance biodegradation rates as the surface-to-volume ratio increases. This point is noteworthy given that the formation of micrometer-sized polymers and plastics (i.e., microplastics) is often considered a concern per se without considering the chemistry of the polymer [48]. However, while conventional non-biodegradable polymers form nano- and microplastic particles that are persistent and accumulate in the environment, micrometer-sized particles composed of biodegradable polymers are only of transient nature as the polymers continue to biodegrade to CO2 (and CH4) and microbial biomass. For non-crystalline and semicrystalline polyesters, enzymatically mediated ester hydrolysis was reported to increase with decreasing glass transition temperature, Tg [49, 50]. Similarly, for semicrystalline polyesters, enzymatic hydrolyzability was found to be inversely correlated to the melting temperature, Tm [21, 51, 52]. Both dependencies reflect constrained mobility of polyester chains in glassy domains and microcrystalline lamellae, respectively, which impairs the formation of enzymesubstrate complexes and hence hydrolytic cleavage of the ester bond. The volume occupied by glassy domains in amorphous polymers and of crystallites in semicrystalline polymers depends not only on the polymer chemistry, the tacticity, and the molecular weight distribution of polymer chains, but also on the processing history of the polymer. The latter implies that the morphology of a polymer in a plastic product may differ from the morphology of the same polymer before it was processed. In such cases, it is mandatory to test the biodegradation of the actual plastic product and not (only) of the pure polymer(s) that are present therein. Testing the biodegradability and assessing the biodegradation rates of actual plastic products instead of only the constituting polymer(s) is also warranted for plastics that contain significant amounts of additives or that contain more than one polymer (i.e., polymer blends). Both the presence of additives and polymer blending have been reported to affect plastic biodegradability [53, 54]. Blending may lead to enhanced biodegradation of the polymers in the blend as compared to only the individual polymers [54].

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Environment-Dependent Factors that Control Polymer Biodegradation

Several abiotic and biotic environmental factors affect polymer biodegradation, including temperature, pH, humidity, oxygen and nutrient availability, and UV light irradiance as well as the abundance and activity of specific microbial degraders that secrete extracellular enzymes that catalyze polymer breakdown. These factors are often interdependent (e.g., microbial activity depends on temperature, pH, and humidity), resulting in multifactorial effects on polymer biodegradation. This interplay challenges any assessment of the relative importance of individual factors for polymer biodegradation, particularly when polymer biodegradation is determined in natural environments characterized by simultaneous spatiotemporal variations of several factors. Laboratory incubations under defined and controllable conditions provide a viable means to separately assess the importance of individual environmental factors (in addition to assessing the dependence of biodegradability on material properties, which is of key interest to the development of novel biodegradable polymers). Presence and activity of microbial degraders and their extracellular enzymes. Microorganisms are critical to polymer biodegradation as they take up the small organic molecules released during polymer breakdown and metabolically convert them to CO2 (and CH4) and microbial biomass. Furthermore, for biodegradable polymers that break down enzymatically, the presence of specific microorganisms is required that secrete these active enzymes. The abundance of microorganisms expressing and secreting competent enzymes is strongly polymer-dependent. Microorganisms secreting extracellular enzymes to break down naturally occurring biopolymers, such as cellulose, are abundant across environments [55]. By comparison, microorganisms secreting enzymes active on synthetic biodegradable polymers may be much less abundant and typically require that the synthetic polymers have sufficient structural resemblance to the biopolymer that is the natural substrate for the secreted enzyme (see above example for cutinases). However, the environmental occurrence of microorganisms that secrete enzymes active on synthetic polymers does not imply that the polymer also biodegrades in the environment. An illustrative example is PET: while some microorganisms have been identified that secrete enzymes hydrolyzing the amorphous regions (but not the crystalline domains) in PET [31, 56], PET remains non-biodegradable in natural environments. Temperature. Among the abiotic factors affecting polymer biodegradation, temperature is of key importance [57]. Over a wide range of temperatures in both natural and engineered system, increases in temperature will result in increase in the rates of biotic and abiotic chemical reactions that are involved in polymer biodegradation. However, polymer biodegradation rates are expected to increase only up to a temperature optimum in manipulated test systems. At temperatures above this optimum, biodegradation rates likely decrease, reflecting thermal inactivation of extracellular enzymes catalyzing polymer break down. This is because the microbial

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community and their secreted enzymes in a given polymer-receiving environment are typically well adapted to the prevailing temperatures in that environment, including thermophiles (>45°C, e.g., found in industrial compost), mesophiles (i.e., 20–45°C; e.g., found in tropical soils and waters), and psychrotrophs and psychrophiles (90% conversion to carbon dioxide after 80 days according to ISO 14855

The biodegradation of ecoflex® F has also been investigated under controlled composting conditions according to the standards ISO 14855 “Determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions – Method by analysis of evolved carbon dioxide.” In Fig. 3 the mineralization curve of ecoflex® F is depicted together with the positive control cellulose. Already after 80 days 90% of the theoretical carbon dioxide evolution is reached. Thus, ecoflex® F is ultimately biodegradable according to the ISO standard for compostable polymers – ISO 17088 – which requires 90% of the theoretical carbon dioxide evolution within 180 days. ecoflex® F has also been certified in the recent years for home-compostable applications. Home and industrial compostability have also been proven for the partially renewable grade ecoflex® FS. In addition, a large variety of ecovio® grades fulfill the requirements for home and industrial composting and have been certified by external certification bodies, such as TÜV Austria, DIN CERTCO and BPI (more details can be found on the websites of the certification bodies).

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Biodegradation and Environmental Fate of ecovio® M2351: Biodegradability Research Beyond Certification

ecovio® M2351 is the certified ecovio® grade for soil-biodegradable mulch film applications. The material is a compound based on ecoflex® F, PLA, and inorganic fillers. The material is certified “OK soil” from TÜV Austria and by DIN CERTCO according to the standard EN 17033 “Biodegradable mulch films for use in agriculture and horticulture – Requirements and test methods.” Both certifications require proof for full biodegradability, no adverse effects on plant germination and low heavy metal content. DIN CERTCO certification additionally requires – in compliance with the standard EN 17033 – control of constituents (which excludes the use of, e.g., substance of very high concern) and no negative effects of the biodegradation products on earthworms and nitrifying bacteria. Figure 4 shows the mineralization curve of ecovio® M2351 in a soil, expressed relative to cellulose which is typically used as positive reference in biodegradation studies. In the early 2010s, a cooperation between BASF research and the Environmental Chemistry group at ETH Zürich (Prof. M. Sander and Prof. K. McNeill) was started to elucidate the environmental fate of ecovio® M2351 and of other biodegradable polymers in soil. The need for such detailed investigations was recognized in the dialogue with customers and stakeholders, wanting to know how certified soilbiodegradable plastics undergo biodegradation. In the cooperation project, a set of analytical approaches has been developed, covering all relevant aspects of the end of life of soil-biodegradable mulch films. These range from microscopic techniques to study microbial colonization of the polymer surfaces, assays to investigate the hydrolytic breakdown of the polymers by enzymes, methods to elucidate the interactions of the polymers with the soil as well as the microbial metabolic utilization of

Fig. 4 Biodegradation of ecovio® M2351 based on CO2 evolution relative to cellulose control according to ISO 17556. Absolute biodegradation of 94.4% (±1.7%)

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Fig. 5 Schematic representation of the most relevant steps and processes during the biodegradation of an ecovio® M2351 soil-biodegradable mulch film

the polymer carbon. For these investigations commercially available materials have been used, as well as 13C-labeled polymers enabling a precise tracking of the polymeric carbon during the experiments. Figure 5 shows a schematic representation of the underlying steps and processes involved in (bio)degradation. For polymer carbon to be metabolized by microbes, the polymer chains first need to undergo “depolymerization” into sufficiently small molecules to be taken up into the microbial cells. In the case of ecoflex®-type polyesters, this breakdown occurs mainly through the action of exocellular esterases. The effect of variations in terephthalic acid contents in a series of PBATx was investigated by Zumstein et al. using six different grades. Here, x refers to the aromatic diacid to total diacid content of the PBATx (i.e., x = T/(T + A))
100): PBAT0, PBAT10, PBAT20, PBAT29, PBAT42, and PBAT50) [7]. In the study, the polymers were exposed to two different environmentally relevant esterases, Rhizopus oryzae lipase (RoL) and Fusarium solani cutinase (FsC). Using quartz-crystal microbalance with dissipation monitoring (QCM-D), the depolymerization step of PBATx thin films in solution was followed over time. For both enzymes, the rate of enzymatic hydrolysis decreased with increasing T content in the PBATx. More precisely, while for RoL depolymerization was measured for all samples with a T content up to 29% and partially also for PBAT42, with FsC all polymer films underwent full hydrolysis, including PBAT50. The T content of PBAT50 exceeds the one usually used for the commercial ecoflex® F grade, demonstrating the ability of some naturally abundant enzymes to depolymerize the base polymer of ecovio® M2351. After the depolymerization step, monomers and oligomers need to be taken up by microbes to be metabolized into carbon dioxide and biomass. While carbon dioxide can be tracked with the numerous respirometric methods, measuring biomass formation remains a challenge. The existing standard specifications for the certification of biodegradable mulch films require 90% mineralization of the carbon of the material in absolute terms or expressed relatively to the mineralization of a positive control, typically cellulose. However, until 2019, to the best of our knowledge, no

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direct evidence was reported for the formation of biomass during the biodegradation of a synthetic polymer. In 2019, Zumstein et al. investigated biodegradation of films composed of PBAT which was 13C-labeled in different monomers and at different carbon positions [8]. Films were incubated in soil and retrieved from the soil after initial biodegradation (reaching between 5 and 10% of mineralization). After fixation, the incubated films were imaged using nanoscale secondary ion mass spectrometry (Nano-SIMS), a microscopy technique capable of analyzing the isotopic composition of secondary ions formed by focusing an ion beam on the surface. Through this method, detailed “chemical” images of the surface were constructed, not only highlighting the colonization of the PBAT film by fungi and bacteria, but also demonstrating the presence of film-colonizing microorganisms enriched in PBAT-derived 13C. The 13C concentrations in the biomass exceeded not only those of naturally occurring soil organic matter, but also the concentrations of 13C in the polymer film itself. Uptake of PBAT carbon into microbial biomass could be shown from all monomeric units within PBAT, but at different extents. This finding can be rationalized based on the higher concentrations of terephthalic acid than adipic acid units in crystalline domains of the PBAT which undergo slower enzymatic hydrolysis. Despite the fact that the experiments do not quantitatively prove the extent of biomass formation at the end of biodegradation or full biodegradability of PBAT in soil, they provided for the first time undisputable evidence for the formation of microbial biomass utilizing carbon from a synthetic biodegradable polymer as food source. One of the main goals of the cooperation has been the development of methods to close the mass balance on polymeric carbon after incubation of the biodegradable polymers in soils. Closing the mass balance involves not only quantifying the polymer carbon converted into carbon dioxide, but also incorporated into microbial biomass. Methods to directly demonstrate and quantify biomass formation at concentrations as low as those expected during polymer biodegradation (usually assumed to be 10%, but typically varying in a range between 40% and 0%) need to rely on carbon isotopic labeling. Alternatively, the extent of carbon incorporation into microbial biomass can be determined indirectly by quantifying both polymer carbon mineralized to CO2 and carbon present as residual polymer at a given time point in the incubation. The difference between the sum of these two pools and the total amount of polymer carbon added to the incubation (e.g., soil) then is an estimate of the amount of carbon incorporated into microbial biomass and the larger natural organic matter pool. This approach, however, requires an analytical method for quantitative retrieval of polymer and metabolites from soil. Nelson et al. reported a Soxhlet-based method based on solvent extraction to retrieve polymer from soil samples [9]. The method was validated using different polymer and plastic samples, including ecovio® M2351, and for different model matrices and soils. Through baseline and spiking experiments, necessary to remove background signals and calibrate retrieval, practically quantitative retrieval could be achieved in all experiments. This method creates the basis for experiments under environmental conditions to investigate biodegradation relationships between laboratory and field conditions and, thereby, to identify decisive system factors (i.e., temperature,

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humidity, . . .) that control biodegradation in soils in the field. In addition, such methods open up numerous possibilities to perform experiments under real-life conditions, with much higher reliability than burial experiments combined with mass loss quantification. Quantification of residual polymer in soil through Soxhlet extraction and NMR analysis was also applied in combination with other techniques by Nelson et al. to close the mass balance for 13C-labeled poly(butylene succinate) (PBS), labeled at different monomers and carbon atoms [10]. Mineralization in soil was tracked using a cavity ring-down spectroscopy (CRDS) for more than 400 days. Successively, the soil samples were analyzed using a combination of an elemental analyzer coupled to a continuous flow interface and isotope-ratio mass spectrometer (EA-IRMS). EA-IRMS allowed the quantification of the residual 13C-carbon in soil, consisting of residual polymer and carbon incorporated into microbial biomass during the biodegradation process. Successive application of the above-mentioned Soxhlet extractions for the determination of the residual polymer in soil, it was possible to determine the exact quantities of residual polymer and indirectly of biomass formed during and at the end of the experiments and finally to close the mass balance.

2.2.3

Biodegradation in the Marine Environment of ecovio®

The biodegradation of ecoflex® and ecovio® by marine microbes has been investigated by Meyer-Cifuentes et al. An enriched marine microbial consortium was used to biodegrade ecovio® FT2341, a partly bio-based blend of ecoflex® and PLA used for thin film applications like bags [11]. Using metagenomics, metatranscriptomics, and metaproteomics, the authors elucidated the key enzymes and microbes involved in the biodegradation of PBAT. The microbial consortium, formed mainly by Alphaproteobacteria, Gammaproteobacteria, and Flavobacteria, as well as Actinobacteria, could use the plastic film as only carbon source. It was found that while the polyester film degradation was performed by the community attached to the surface of the material, other free-living organisms were involved in utilization of oligomers and monomers in a synergistic way. The depolymerization of the material occurred in two steps: the first, extracellularly, was performed by PETase-like enzymes (Ples), to give butanediol, adipic acid, and the monoester of terephthalic acid (BT); the second occurred intracellularly through MHETase-like enzymes (Mles) and lead to the cleavage of the BT unit. This type of mechanism was previously reported for soil microorganisms and in the degradation of PET under laboratory conditions. The investigations reported in the paper prove for the first time the presence of marine microbes able to completely biodegrade an ecovio® blend containing PBAT.

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Toxicological Assessment of Biodegradable Polymers

Apart from the pure process-driven requirements of the disintegration within a defined time span and the proof of ultimate biodegradability, it needs to be made sure that there is no negative impact of degradation products or intermediates of the degradation process on the environment. Adequate information for the comprehensive assessment of the environmental and toxicological safety has to be provided. ecoflex® F was tested according to the tests described in the following chapters and showed no adverse effects. These tests cover the assessment of eco-toxicological effects of the degradation intermediates and products (see Sects. 2.3.1–2.3.3) as well as the product safety during the use phase (see Sects. 2.3.4–2.3.7). The results of these laboratory tests are given in Table 2 [12]. Biodegradation of ecoflex® F causes no accumulation of environmentally dangerous compounds, neither in organisms nor in the ecosystem.

2.3.1

Water-Soluble Intermediates: Daphnia Test

In the toxicity tests, the toxicity of the water-soluble intermediates is particularly important because they can easily enter groundwater or be more readily absorbed by organisms. Testing of ecoflex® was carried out in accordance with DIN 38412 Part 30. In this test, the pollutant-dependent immobilization of the daphnia in solution of different concentrations (series of dilutions) is used. The control solution contains microorganisms that are known to biodegrade the test polymer. The stock solution at the end of the test also contains the degradation intermediates of ecoflex®. The polymer was successively diluted with water (pH 7.0) containing microorganisms and for each distinct concentration 10 daphnia were added to the test solutions. The solutions were kept at 20°C and after 24 h the number of daphnia Table 2 Toxicological tests performed with ecoflex® Test Acute toxicity to daphnia DIN 38412 part 30, fishes Terrestrial plant toxicity OECD 208 Earthworm toxicity OECD 207 Primary skin irritation rabbit OECD 404 Primary irritations of the mucus membrane rabbit OECD 405 Guinea pig OECD 406 (modified BUEHLER test) LD50 rat (oral) OECD 423 Ames test OECD 471

Result Passed No effects at the highest concentration No effects at the highest concentration Non-irritant Non-irritant Non-sensitizing >4,000 mg/kg, virtually non-toxic after a single ingestion Substance was not mutagenic

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still swimming was counted. Even with a low dilution (stage 2) as in the control solution, there were still nine daphnia swimming – the test was passed.

2.3.2

Plant Growth Test

The eco-toxicity of composted ecoflex® was studied in a plant growth test following the European standard for compostable plastics EN 13432, Annex E, which is based on the OECD guideline 208. In this test, effects on seedling emergence and early plant growth are investigated with different higher plant species exposed to treated compost. Seeds are planted into soil with compost in which ecoflex® was biodegraded before and into control soil with untreated compost. Four different plant species covering the three categories outlined in OECD guideline 208 were tested accordingly: wheat (Triticum sativum), summer barley (Hordeum vulgare), mustard (Sinapis alba/Brassica alba), and mung bean (Phaseolus aureus). The following samples were prepared and used for testing: Mixture of reference soil and 25% compost with addition of ecoflex® after 12 weeks composting. Mixture of reference soil and 50% compost with addition of ecoflex® after 12 weeks composting. The test results on seedling emergence and biomass showed no significant effects when treated soil was compared to the control (Fig. 6). With all four plant species both parameters reached at least 90% of the control level regardless of the test concentration.

2.3.3

Earthworm Acute Toxicity Test

The eco-toxicity of composted ecoflex® was investigated in an earthworm acute toxicity test following the OECD guideline 207 (reference). In this test, earthworms (Eisenia fetida) are exposed to control soil and treated soil. Earthworm mortality and Fig. 6 Result of the plant growth test (summer barley) according to OECD 208

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the biomass (body weight) of the surviving animals are recorded and results from treated soil are compared to the control soil. The test sample consisted of treated compost mixed with standard soil containing a 25% test concentration. There was no mortality in any of the treatments after 7 and 14 days. In the control and treated soil, a significant increase of biomass compared to the start of the test was observed. In conclusion, soil mixtures containing compost exposed to ecoflex® had no adverse effects on earthworm survival and biomass development.

2.3.4

Assessment of Skin and Eye Irritation

The potential of ecoflex® (powder) to cause acute dermal irritation or corrosion was assessed by a single topical application of the test substance to the intact skin of rabbits according to OECD guideline 404. After removal of the patch the application area was washed off and cutaneous reactions were assessed for 72 h. No cutaneous reactions were observed. Hence, ecoflex® (powder) is not irritating to skin. The potential of ecoflex® (powder) to cause eye irritation was assessed in rabbits, subjected to a single ocular application of the test substance for about 24 h according to OECD guideline 405. The ocular reactions were assessed for 72 h after application. According to this test, ecoflex® (powder) is not irritating to the eye.

2.3.5

Assessment of Sensitization

ecoflex® (powder) was tested for its sensitizing effect on the skin of the guinea pig in the Modified BUEHLER Test according to OECD guideline 406. Skin-sensitizing effects were not observed in these animal studies.

2.3.6

Acute Oral Toxicity

A study was performed to assess the acute toxicity following oral application of ecoflex® (powder) in rats according to OECD guideline 423. Single doses of 4,000 mg/kg body weight of test material preparations were given to the animals. None of the animals tested died. Neither clinical signs nor findings of macroscopic pathologic abnormalities were observed. The median lethal dose (LD50) of the test substance after oral administration was found to be greater than 4,000 mg/kg indicating that ecoflex® shows virtually no acute toxicity after a single ingestion.

2.3.7

Assessment of Mutagenicity

ecoflex® (powder) was tested for its mutagenic potential based on the ability to induce point mutations in several bacterial strains (Salmonella typhimurium and

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Escherichia coli) in a reverse mutation assay (Ames test) according to OECD guideline 471. Results revealed that the polyester is not mutagenic to bacteria.

3 Certified Compostable and Soil-Biodegradable Plastics: Their Role in a Circular Economy and Greenhouse Gas Emission Reductions Certified compostable and soil-biodegradable plastics are developed for special applications where there is a benefit in using them. For example, compostable plastics are preferred when they help to divert organic wastes from landfill or incineration, or when the application is significantly responsible for plastic contamination in the organic waste stream. On the other hand, soil-biodegradable plastics are suitable materials of choice for applications that are intentionally added into the environment, and their complete collection after usage is either not feasible or not economically and/or ecologically beneficial, e.g., thin mulch films (200 m/min.

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Lamination

Other possibilities to form structures combining properties of different materials are extrusion lamination and adhesive lamination. In these cases, a pre-extruded film (mono- or multilayer) is laminated on another substrate by an adhesion promoter. This offers also the option to include materials which are not co-extrudable, e.g., inorganic barrier layers that were deposited onto a carrier film. To improve the bond strength of the laminate, surface treatments can help to enable or improve adhesion. Flame, corona, plasma, and ozone treatments remove impurities and/or change the surface chemistry, chemical priming (e.g., with polyethylene imide) introduces another layer of adhesion promotion [66].

Extrusion Lamination Extrusion lamination can be done with the same equipment as shown in Fig. 22, i.e., the extruder can be used to extrude a polymer providing adhesion (e.g., ecoflex®). This thin layer is then inserted between the pre-extruded film and the substrate. In the lamination nip (Fig. 22 (4)), the laminate is pressed together at adjustable pressure and roller hardness and subsequently coiled at the winder (Fig. 22 (6)). Pressure, roller hardness, and web speed influence the effective time within the nip region. A longer time typically leads to better adhesion between the adhesion promoter, film, and substrate [66]. The nip roll is typically cooled by chilled water to solidify the polymer.

Adhesive Lamination Another option is to apply an adhesive as adhesion promoter between the films or between the film and the paper substrate [66]. There are several types of adhesives in use in flexible packaging: solvent-less, solvent-based, and water-based adhesives (e.g., Epotal® ECO 3702). Solvent-less adhesives consist of reactive components that are mixed right before coating them onto the film. Solvent-based adhesives have to be dried after coating (by IR or hot air) before entering the lamination nip, while water-based adhesives can be applied both for dry-lamination and wet-lamination (Fig. 23). Especially for paper-based laminates, the latter one can help to improve the bond strength on paper surfaces. The adhesive is typically applied by gravure coating. A more uniform coating is achieved by running the gravure cylinder in opposite direction (“reverse”) to the web direction. In contrast to extrusion lamination, the nip roll is heated to enable a good wetting of the substrate. Again, pressure, roller hardness, and web speed are influencing the peel strength result. Additional influencing factors are coating weight (i.e., the solid content deposited per area of substrate) and wetting behavior of the adhesive on the substrate.

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Drying Tunnel

Wind-up

Lacquer Application

Treater

Unwind

Lamination

Adhesive Application Treater Adhesive Bath

Unwind

Wet Lamination Process

Drying Tunnel

Windup

Lamination

Treater

Unwind Adhesive Application

Treater

Unwind Dry Lamination Process

Fig. 23 Schematic description of wet- and dry-lamination process [66]

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In order to ensure a proper drying of the adhesive, the drying system must be capable of evaporating enough solvent or water carried by the adhesive layer at the given web speed, while not exceeding the temperature limits of the involved films and substrates which would lead to distortion or shrinking. Some adhesives need a curing step with UV light or E-beam instead of drying. Care must be taken for the tension control of both film and substrate, otherwise the final laminate might curl or telescope.

5.2 5.2.1

Injection Molding Properties of ecovio® Injection Molding Grades

The main features of the ecovio® grades for injection molding (ecovio® I) are the following: • • • • •

Mainly bio-based Can be used on conventional injection molding machines Usable for single and multi-cavity tools Suitable for thin-wall applications Comply with food contact regulations

Depending on what is required, ecovio® I grades with different filler contents are available. When processed into molded parts, these ecovio® grades are noted for their excellent balance between high rigidity and good elasticity. ecovio® 60 IA1552 is similar in terms of mechanics to polypropylene (PP) whereas ecovio IS1335 is closer to high-impact polystyrene (HiPS). When compared with pure PLA, ecovio® IS1335 has an almost identical softening point of 55°C, but provides at the same time a higher impact strength, particularly at low temperatures. This is advantageous primarily for cold food packaging, which is packed, transported, and stored in the cold. With ecovio® IS and IA grades, it is possible not only to fill filigree thin-walled molds but also to achieve cycle times comparable to standard materials in the packaging industry. Furthermore, they exhibit a noticeably increased flowability relative to comparable biodegradable injection-molding grades. Apart from the product-specific thermal properties, the behavior of ecovio® IS components when heated is also dependent on the duration and type of heat and the stress. The component design is also crucial. In contrast to ecovio® IS, the injectionmolding grades labeled IA can be used with temperatures of up to around 100°C. It is therefore not necessary to adhere to a suitable geometry with the component design, as with ecovio® IS. This property makes the ecovio® IA grades a good choice for thin-walled packaging items with a hot filling or for use at elevated temperatures.

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Processing of ecovio® by Injection Molding

A single-flight, normal-cut, three-zone screw is suitable for processing ecovio® injection-molding grades. Shallow-cut screws can sometimes be advantageous. With the same screw diameter, the latter provide a shorter residence time of the material in the cylinder and a more even temperature distribution in the melt, but also sometimes the application of too great a shear. For processing ecovio® injection-molding grades, only wear-resistant steels should be used for the cylinder, screw, and non-return valve. Hot runner systems are recommended for ecovio® injection-molding grades. With hot runner systems and heated nozzles, externally heated systems offer greater operating safety because of the more homogeneous melt temperature and a reliable purging effect. Temperatures of between 10 and 40°C should be selected for the mold temperature. Processing with a mold temperature of 25°C is to be preferred. The mold temperature control should be so effective that even over long production times the desired temperatures are reached in all molding areas or specific temperature changes are created at particular points through independent heat circuits. The recommended material temperature range for the different ecovio® injectionmolding grades is from 180 to 210°C. Experience has shown that the optimum machine setting should start with a temperature of 195°C. A level or slightly rising temperature profile is advisable (Fig. 24). The choice of material temperature is Fig. 24 Example of temperature profile in the barrel

Heater zones 6 5

4

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dependent on the length of flow path and wall thickness as well as the residence time of the melt in the cylinder. Unnecessarily high material temperatures and excessively long residence times of the material in the cylinder can cause decomposition of the mold material. Experience has shown that the mold surface temperatures should be in the range of 10–40°C. Due to its heat dimensional stability of 55°C, parts of ecovio® IA grades are demoldable without deformations even after a short cooling time. This does not apply to ecovio® IS 1335. However, it is also advisable to use low mold temperatures in order to achieve a quick production cycle. The residence time of the plastic in the plasticizing cylinder has quite a crucial effect on the quality of the molded part. Extremely short residence times can lead to thermal inhomogeneity in the melt, whereas those which are too long (>5 min) often result in thermal defects (discoloration of the molded parts from dark stripes or burnt product particles on the molded part). During brief production stoppages the screw should be brought to the front position, and with longer stoppage times the cylinder temperature should also be reduced. Before restarting, even after brief stoppages, thorough purging of the plasticizing unit is required. A change of material requires cleaning of the screw and cylinder. For this case, high-molecular-weight as well as glass-fiber-reinforced polyethylene or polypropylene has been shown to have a good cleaning action. ecovio® IA and IS grades are suitable for co-injection. In this case a core and a skin layer will be injected in a same mold (Fig. 25). This is especially used to improve the performance (e.g., barrier properties) or the cost (e.g., re-grinded material) of the final part. Reprocessing of re-grinded parts and sprues is generally possible. If the material was processed sparingly in the first run, then generally up to 25% of the regranulate can be mixed with virgin granulate, without any appreciable decline in material characteristics occurring. In-mold labeling (IML) is one of the surface decoration processes. This involves a film being inserted into the open mold for surface decoration. In the subsequent mold filling process, the film is back-molded. In the process it melts onto the surface of the impacting hot melt and thus combines with the molded part. Very good results can be achieved with both ecovio® IS and IA grades. In the processing of ecovio® IS in the injection-molding process, the addition of a chemical blowing agent enables microcellular component structures to be produced. Tests showed that a weight reduction of up to 20% was possible. This way, besides the pure weight reduction, also the speed of industrial composting can be accelerated to some degree by generating larger surface areas during the degradation process.

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Fig. 25 Co-injection nozzle for a 3-layer part (Figure: Moldmasters)

5.3 5.3.1

Thermoforming Properties and Processing of ecovio® by Thermoforming

The products of the ecovio T series (ecovio® T and TA) were especially developed for cast/flat film extrusion (see Sect. 5.1.1.2) on conventional cast film systems in thicknesses between 200 μm and 1.5 mm and subsequent thermoforming. ecovio® T is a stiff and less heat resistant grade (HDT B 55°C), comparable to HiPS or ABS whereas ecovio® TA has a higher heat resistance (HDT B 95°C) and PP-like mechanical behavior. With ecovio® T and TA, processing on conventional sheeting equipment is possible with and without calendars. While standard equipment can be used for the processing of both ecovio® types, precise new calibration of the extrusion tools at the respective operational spots is required due to a different flow behavior. The result: A stiff yet very tough sheet which wraps extremely well – ideal prerequisite for the thermoforming of demanding components.

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Fig. 26 Schematic description of thermoforming process

Conventional slot nozzles, as used in the processing of PP or PS, are also suitable for ecovio® T and TA. It is recommended to heat the nozzle to the same temperature (or 5–10°C higher) than the last zone of the extruder. The polishing stacks should always be evenly cooled. In order to prevent adhesion to the polishing stacks, the temperature should be less than 40°C. It was determined that 20°C is a good starting temperature. ecovio® T and TA can be also combined with other ecovio® types as well as, e.g., barrier materials in more sophisticated multilayer systems. The films can subsequently be rolled up. Depending on the thickness of the film, a suitable core diameter should be selected. The film or flat film material can also be cut and stacked. Further processes such as stamping, folding, welding or thermoforming are possible. Flat films made of ecovio® T types possess excellent properties for thermoforming. This can be accomplished in a separate processing step or in line. The advantages of the thermoforming of ecovio® include (depending on machine and geometry): • • • • •

fast cycle times precise surface molding high stretching properties extremely wide processing window good stamping and stacking characteristics

The different thermoforming steps are described in Fig. 26. Preheating to 40°C can be applied if necessary. A short preheating zone, as is used for amorphous thermoplastics such as PS, is sufficient. The ideal processing window for thermoforming of the ecovio® types T and TA is at a surface temperature of 100–140°C. Depending on the design of the machine and

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Fig. 27 Example of a thermoformed ecovio with IML decoration

parts geometry, also lower temperatures down to 80°C can be applied. This can have a positive effect on sustainability analyses. Both process variations - positive and negative thermoforming - are possible. All existing parts of conventional systems (pre-stretching, cutting, stamping, and stacking device) can be used; this means, the cycle times are comparable to those of conventional thermoplastics. Compared to PLA, the cutting edges of thermoformed ecovio® parts are much cleaner and exhibit a low breaking tendency. Thermoformed parts of ecovio® T are generally characterized by surfaces with high image precision. In addition, the surfaces of building components made of ecovio® T have an interesting, mother-of-pearl surface sheen with the respective finish. The mechanical properties of ecovio® TA can be compared with stiff polypropylene (PP). The main advantage of ecovio® TA compared to ecovio® T is its distinctly higher service temperature of more than 90°C. During processing to thermoformed parts, ecovio® T and TA types are characterized by their good balance between great stiffness and good expansion, excellent capacity for sealing and printing, high molding precision, and easy de-stacking. By adding a chemical blowing agent during the processing of ecovio® T and TA, it is possible to produce microcellular foam. Orientational tests showed that a weight reduction of up to 25% can be achieved. ecovio® thermoforming grades are also nicely compatible with in-mold labeling (IML). IMLs can comprise the features of oxygen barrier and innovative decoration (see Fig. 27) – of course, for an overall compostable packaging, the IMLs need to be compostable themselves. This combination can be a nice add-on depending on the needs.

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No special measures such as purging are required for short-term interruptions of production. Prior to lengthy downtimes, e. g. overnight or over the weekend, the cylinder should be purged thoroughly with lightly flowing PE-LD (MVR ~ 4), as polyethylene is significantly less susceptible to thermal loads upon reheating. At the restart, the polyethylene should be rinsed out of the system with pure ecovio®.

5.4 5.4.1

Additives and Post Treatment Additives

To improve processing and performance of films made of ecoflex® or ecovio®, film qualities can be modified using additives in the form of masterbatches or by direct dosing. Additives used for compostable applications are required to fulfill the different composting norms and standards (EN 13432, EN 17033, NF T51-800, etc.) [67]. Usually, one has to match the heavy metal limit or to demonstrate non-ecotoxicity. The ultimate biodegradation could be needed as well if the additive shall be used at significant concentrations (more than 1% of mass). • Slip agents are used to reduce the coefficient of friction of the final film as well as the adhesion of the film to metal parts or to itself during processing. Biodegradable amides of fatty acids, e.g., oleamide, erucamide, ethylene-bis-stearamide, fatty acid esters like glycerol oleates or glycerol stearates as well as saponified fatty acids, e.g., stearates are typically used as slip agents for biodegradable polyesters. Such masterbatches for the processing of ecoflex® and ecovio® are offered by BASF (e.g., ecoflex® Batch SL05, ecoflex® Batch SL10B and ecoflex® Batch SL10C). The different batches provide different properties in terms of migration speed of the slip agent to the surface and the final coefficient of friction. • In order to improve the water vapor barrier of ecoflex® or ecovio® waxes can be used. Depending on the concentration, the water vapor transmission rates of ecoflex® films can be reduced by up to 75%. But the level of PE-LD has not been achieved yet [61]. • Anti-bloc agents like talc, chalk, or silica can be used in form of masterbatches [57, 61]. The anti-bloc batches for processing ecoflex® and ecovio® are sold by BASF under the name ecoflex® Batch AB 1. • Pigment masterbatches can be used to achieve specific colors. Examples for heavy metal free pigments are carbon black and titanium dioxide. Carbon black pigments are used to make black films which are applied, e.g., as mulch film to increase the soil temperature in spring. Coated titanium dioxide pigments allow to achieve white films, e.g., for carrier bags or for white mulch film reflecting infrared radiation to reduce the soil temperature. The maximum loading with titanium dioxide is limited by the standards that have to be met. A black

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masterbatch based on ecoflex® is also in the BASF portfolio of biodegradable plastics (ecoflex® Batch Black). • Anti-fog agents like fatty acid esters are used to avoid the formation of water droplets on the film under condensation conditions, e.g., in cling film applications at the transition from room temperature to a refrigerated warehouse. The anti-fog agent is hydrophilic. It reduces the surface tension of droplets so that a uniform water film does not impair the clarity of the film.

5.4.2

Post Treatment

Printing. In general, ecoflex® and ecovio® can be printed and welded on standard equipment for PE-LD. Both alcohol-based and water-based inks can be used after testing. Prior to printing the material has to be corona-treated if the surface tension is below 38 mN/m. In most cases, printing of ecoflex® and ecovio® using alcoholbased inks does not require corona treatment. The drying temperatures should be kept below PE-LD conditions. As drying conditions depend very much on the machine design, they need to be determined during a trial [61]. Metallization. Biodegradable polymer films show low barriers against oxygen and water vapor due to their chemical nature. Deposition of a thin metal or oxide layer under a high vacuum using a thermal or plasma process is one of the most efficient production processes for high barrier films. Mineral fillers, slip, and antibloc agents in the film have to be avoided, because surface defects reduce the barrier properties of the coating. Both ecoflex® and ecovio® can be metallized on standard equipment [68].

6 Applications The following paragraphs describe the major applications today and to come (loose fill applications excluded). Each paragraph is divided into a part for the application in general and a specific ecoflex®/ecovio® part. An overview of applications and estimated volumes for 2007 and 2022 is shown in Table 10. An overview of the material properties of ecovio® with respect to the applications and a comparison to other biodegradable polymers and compounds is provided in Table 11.

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Table 10 Development of applications of biodegradable polymers Application Organic waste bags, Fruit & Vegetable bags, and shopping/carrier bags Packaging including foam Mulch film and horticulture Sum

6.1 6.1.1

Volume 2007 [kt] 16

Volume 2022 [kt] 300

42

180

7 65

45 525

Comment Most established segment Food and non-food packaging Sum

Bag Applications Organic Waste Bags

Across the European Union, separate organic waste collection will become mandatory by the end of 2023. In Europe, only one third of organic waste is collected separately. Even in European countries with a well-developed composting infrastructure like Germany or the Netherlands, the organic fraction remains the largest part of municipal residual waste. In these two countries 95% and 60%, respectively, of all households have access to industrial composting plants. In the EU organic matter accounts for 30–40% of total domestic refuse. With the expectation of continuous expansion of the composting infrastructure, a significant growth of the market for organic waste bags is expected. In France unsorted household waste still contains approximately one third organic fraction, mostly food waste which amounts to 6.3 million tons per year. By now, only 36% of the French population has access to the separate food waste collection system. It is estimated that 60% of the food waste could be diverted from landfills and incineration to organic recycling. For this it would be necessary that most people have access to separate food waste collection. In several studies it was shown that the availability and the use of compostable organic waste bags can help to separate organic waste. By offering an alternative for clean waste collection, compostable organic waste bags can reduce the share of non-degradable materials in the organic waste stream which, in the end, enables the production of a cleaner, higher value compost (see also Sect. 3). In order to offer citizens a cleaner, safer, and easier collection of organic food waste, BASF developed the biopolymer ecovio®. It is certified for industrial composting according to EN 13432 which is the prerequisite to be allowed for food and organic waste collection in many countries, e.g., in Italy. Key technical benefits of ecovio® for organic waste bags are • • • •

High wet resistance Good tear resistance Good wet strength Fast biodegradation in industrial composting facilities

Table 11 Material properties of different biodegradable polymers with respect to applications

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Using compostable organic waste bags can contribute to sustainability benefits: • Separate collection and recycling of organic waste is more resource efficient than having it landfilled or incinerated. Using compostable organic waste bags, more organic waste can be separated from the residual waste stream (see Sect. 3). • Thus, ecovio® enables bigger amounts of food waste to be organically recycled and turned into compost. Composting helps to circulate valuable nutrients like phosphate back to agricultural production. This way, the use of fertilizers can be reduced. • ecovio® can support the production of renewable energy: With compostable bags, citizens collect more organic waste which can be used for biogas production. • The high content of renewable raw materials offers a reduced material carbon footprint, which can contribute to savings of greenhouse gas emissions along the value chain. From a technical point of view, compost bags have to be biodegradable according to international norms, e.g., EN 13432 (see also Sect. 2.2). Beyond the mechanical properties it is necessary to down gauge the bags to 15–30 μm to achieve a good LCA. At these low thicknesses it is required that the compost bag can be used in the collection phase at room temperature for minimum of 3–4 days without forming holes – due to an already beginning biodegradation of the bag. The temperature resistance should allow transport and storage at 60°C. Breathability, for gases and high wet resistance to avoid moisture accumulation in the bin are an advantage. ecoflex® and ecovio® fulfill all these basic requirements for compost bags. Even at low moisture levels of well below 50% r.h. the mechanical stability of ecoflex® and ecovio® stays intact. ecoflex®/starch compounds are very versatile in their biodegradation behavior: They fulfill the composting standard EN 13432 both for industrial and home compostability. ecoflex® acts as enabler for starch providing good processing properties on blown film lines to thicknesses below 20 μm and sufficient mechanical properties (e.g., toughness, impact resistance) for compost bags. However, because of the starch content and the strong tendency of absorbing moisture the wet resistance of the bags during the use face and the shelf life of stored bags are limited.

6.1.2

Shopping (Carrier) Bags

Changing consumer behavior based on higher sensitivity for environmental issues and an increased interest in environmentally friendly products accompanied by an increased interest of retailers to differentiate in the market are the major drivers for the market growth of biodegradable shopping bags. After shopping, these bags can be applied for the disposal of organic waste, resulting in a double use of the material. Thin biodegradable carrier bags have a property profile which is similar to organic waste bags:

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Good mechanical properties for loads of about 1,000 times of their own weight Good puncture and tear resistance, e.g., for carrying liquid beverage cartons Down gauging to 17–20 μm Good printability (8-color flexoprinting) for superior presentation Good welding performance for high speed bag-making Usefulness as compost bags after several services as a carrier bag

Except for the optical presentation and the resulting need for printability, biodegradable carrier bags have to fulfill very similar requirements as biodegradable compost bags. The specific material properties (e.g., high puncture resistance) of ecovio® are very suitable for applications like loop handle and T-Shirt shopping bags. Because of the higher prices of high-end carrier and shopping bags, paper bags are more often used as shopping than as compost bags: but paper bags with highquality printing and coatings are generally not biodegradable because of the coating materials applied.

6.1.3

Fruit and Vegetable Bags

The European Union sets targets for reducing plastic bag consumption. In the special focus are single use bags like the very thin fruit and vegetable bags. Nevertheless, it also recognizes the benefits of compostable bags for organic waste diversion. Like for shopping and carrier bags, also fruit and vegetable bags made from compostable plastics have a double function and unfold the benefits described in Sect. 6.1.1. Thus, the European Union allows its member states to exempt compostable bags from the reduction targets. Therefore, e.g., France has decided to exempt home-compostable bio-based fruit and vegetable bags from a ban of very lightweight plastic bags. ecovio® as certified home or industrial compostable and partly bio-based material meets the legal requirements, e.g., of Italy and France for bio-based fruit and vegetable bags. ecovio® is a finished product that can be used as a drop-in solution on standard plastic production machinery. The blown films can be processed to bags with or without handles. Fruit and vegetable bags made of ecovio® are just as highperforming and strong in use as those made of conventional plastics. Fruit and vegetable bags made of ecovio® are more than simple carrier bags. Due to the good breathability of the film, fruit and vegetables can stay fresh for a longer time. Films made of ecovio® offer high water vapor and oxygen transmission rates. With the right bag volume, optimal humidity and oxygen concentration for different fruit and vegetables in bags can be achieved. This contributes to shelf-life extension.

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Agricultural and Horticultural Applications Mulch Film

Mulch films are a well-established application in the EU as well as in Asia in countries like Japan and Korea. In the EU, this application is very cost sensitive because of the subsidy structure of the agricultural sector. But the regulations for waste disposal of mulch film require either recycling or adequate treatment (e.g., incineration). If thin mulch films are concerned, the recovery of the film from the field is crop-dependent, difficult, and often not possible. Cleaning of the soil residues is cost intensive and challenging to achieve completely. In this case it can be more cost effective to use soilbiodegradable mulch film, which is adapted to the climate and the fruit application. For example, cucumber is harvested up to 25 times per growing season using heavy machinery. At the end of the season the mulch film is difficult to recover. For this application the use of soil-biodegradable mulch film is of interest. For agricultural use, BASF offers the certified soil-biodegradable ecovio® M 2351 for mulch films. Mulch films made of ecovio® M 2351 can remain in the soil and ploughed in after mechanical harvest: Farmers do not have to laboriously remove and recycle them. Naturally occurring soil microbes like bacteria or fungi recognize the structure of the film as food they can metabolize. The remaining end products after biodegradation are CO2, water, and biomass (see Sect. 2). With the registration number 9X0001, ecovio® M 2351 has been the first plastic material certified for soil-biodegradable mulch films in accordance with the European standard EN 17033. The following are key technical benefits of ecovio® for mulch film applications: • Due to its very good mechanical properties, ecovio® M 2351 can be used to make mulch films with layer thicknesses of 12, 10, and even 8 μm. • ecovio® M 2351 is a ready-to-use compound that can be processed on conventional machines used for the extrusion of polyethylene films without any additional lubricants or antiblock agents. • With ecovio® M 2351, black mulch films can be manufactured. Compatible masterbatches are available.

6.2.2

ecovio® for Horticulture

Different horticultural items made of biodegradable polymers such as plant pots, seed/fertilizer tape and binding materials, foams and nets for erosion control offer reduction of system complexity by reducing the number of work steps like the removal of non-biodegradable items before or after use. ecovio® as a modular system allows an adaptation of the material properties to the requirements of the application like tapes or nets. Thus, a lot of applications are under development:

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• Seed tapes have to provide sufficient strength in the sowing/drilling process – without damaging seed or germination. Fertilizer tapes have to release the fertilizer slowly to the roots of the plants. In both cases, ecoflex® tapes can provide good strength and slow release performance. • Binding tapes from ecovio® or ecoflex® exhibit a high flexibility and mechanical strength. Because of the limited UV stability binding tapes in green houses provide a higher shelf life than in outdoor applications. • Ground nets for erosion control in constructions below ground level can be produced using a dryblend of ecovio® and ecoflex® F Blend to adjust the orientability and stiffness/toughness ratio of the nets. Carbon black can increase the UV resistance. The advantage of this ecovio® net is that it will degrade in the course of time. While plants and grass are growing, the roots reinforce the soil structure preventing a washing-out by the rain. In this critical phase a strong ground net is needed. After the flora has been grown the ground net is of no use anymore and can degrade over time.

6.3

Packaging

The packaging market, especially the food packaging market, offers large opportunities for biodegradable polymers. In the following section, the focus will be on describing flexible food packaging, rigid food packaging, and paper coating applications.

6.3.1

Flexible Food Packaging

Shrink films. Shrink films are used to combine several sales items in one packaging – e.g., six bottles in a six pack. The bottles are packed in a piece of film which is heated and shrunk in a heating tunnel above the melting point of the film for a short time. The shrink forces after relaxation have to stay high enough to store and carry the packaged goods along the logistic chain: Thus, the requirements are: • • • • •

High shrink values in extrusion direction MD > 60%, Low shrink values in cross direction CD < 20%, High shrink rate during heating in the oven, Welding of the loose end during shrink process, High shrink forces after relaxation.

For this large market the biodegradable ecovio® FS Shrink Film has been developed. ecovio® FS Shrink Film shows a good strength and an adequate heat resistance. Films thereof exhibit the necessary shrink values. A 20 μm ecovio® FS Shrink Film shows an equivalent shrink performance and functionality compared to a 50 μm PE film leading to materials saving and with this a by far better eco-efficiency.

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Fig. 28 Overview of different types of pouches

Cling films. Fresh vegetables and fruits are often packed on a tray covered with cling films, i.e., stretchable elastic films that adhere to themselves when wrapped around the package. After removing the cling film from the food items, it often carries organic residues rendering a mechanical recycling of the film difficult or even impossible. Cling films made from ecovio® can be composted after use. Furthermore, the higher water vapor permeability of ecovio® cling film grades compared to non-compostable cling films based, e.g., on PVC or LLDPE can contribute to a longer shelf life of the packaged food. Pouches. Pouches of various shapes and features (Fig. 28) are predominantly manufactured from laminates (see Sect. 5.1.4). Often, depending on the properties of the packaged good and on shelf-life requirements, both mechanical stability to avoid damages during transportation and excellent barrier characteristics are needed. These barrier properties involve oxygen, aroma, water vapor but sometimes also chemical resistance against acids and oils. Compostable liquid food packaging is still very challenging due to required high barrier against water and water vapor. Pouches can either be directly filled with the food product or serve as a secondary packaging for smaller sub-units, e.g., single-serve portions. In addition to the film applications described above, many laminates include a printed film or paper.

6.3.2

Rigid Food Packaging

Rigid packaging containers. These comprise trays, cups, clamshells, and many more shapes. They are used for a wide range of packaging goods, often combined with a lid sealed, e.g., onto the tray or cup. Rigid containers are typically produced in an

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extrusion/thermoforming, blow molding, or injection blow molding process (see Sect. 5). Among the compostable plastic materials, blends of PLA, PBS, and/or ecoflex® (e.g., ecovio® T and I grades) are used to manufacture rigid containers, as pure materials are often too soft or do not offer the thermal behavior required for processing. To ensure processability and mechanical properties matching the requirements of the application, the blends are designed to be comparable to PS or PP and thus can be processed on standard equipment. Coffee capsules. The bottom part of biodegradable coffee capsules is typically produced by injection molding (e.g., ecovio® I grades) while for the lid coated films or laminates with a high stiffness are used. The mechanical requirements arise from the coffee brewing process: First, holes are punched into the capsule at the bottom and the lid, then hot water flows through the capsule. Throughout the brewing process and the subsequent discarding, body and lid have to ensure the integrity of the capsule. Extruded foam trays. For loose fill applications foamed biodegradable polymers can be molded into trays. Various foamable grades based on PLA are offered on the market. Paper-based trays. Another option to manufacture trays is to combine a wet- or dry-molded tray body with a liner providing the necessary barrier properties and/or sealability, if the tray is to be closed by a lid. This liner can be thermoformed into the tray body by pressure and heat, or the tray is produced from a laminate of thermoformable paper and barrier film. Other options are to produce the tray from an extrusion-coated board or to apply a compostable barrier coating onto the tray body [69]. Paper-based cups. For cups replacing single use plastic cups, extrusion-coated paperboard is utilized. Here, biodegradable polymers can replace the predominant LDPE coating to enable a compostable solution. Cups that are meant to be filled with chilled or frozen food (e.g., beverages and ice cream) also need to have a good moisture stability on the outside to ensure mechanical stability against wetting by condensation. The sealing of tube and bottom must withstand the temperature of the fill goods.

7 Market Overview and Growth Drivers The market for biodegradable/compostable polymers (global production capacities amount to 1.1 million tons in 2022) is still a small niche segment in the global plastics applications market representing less than 1% of overall market [70]. However, contrary to a slight decrease in the overall global plastic production, the market for biodegradable/compostable polymers has continuously grown over the last decade. In the EU, the growth was fueled by new policies and legislations pushing for a more circular economy approach vs. a linear alternative. With the possibility to close

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the circle by organic recycling biopolymers are an enabler for a circular economy. For example, compostable organic waste bags assist the separate collection of valuable organic waste and its composting vs. incineration. The competitive advantages and market drivers of biodegradable/compostable polymers in specific applications are based on: • Changing consumer behavior based on higher sensitivity for environmental issues and an increased interest in environmentally friendly products • Increased interest of retailers and brand owners to differentiate in the market • Support by municipal authorities (providing a composting infrastructure) • Legislative frameworks to enhance the use of biodegradable products • EPR schemes on conventional plastic materials (e.g., PE mulch film) • Technology progress incl. access to new applications • Larger production plants and increasing production capacities The authors expect that based on those drivers the market for biodegradable/ compostable polymers will almost double in the next 5 years. Whereas the global growth over the past decade has been mostly fueled by the region EU, a high share of future growth will be contributed by other regions. As this forecast takes into account not only the potential but also the risks of new technologies/markets, it represents a conservative assessment of the overall market size. Especially for China the potential has been considered on the lower end as dynamics are still very high.

8 Outlook General outlook. Overall, biodegradable/compostable polymers combine different chemical and biotechnological steps in their synthesis and degradation and thus are an excellent example of a symbiosis between these disciplines. Already decades ago the development of this new polymer class was fueled by the increasing need for more sustainable and circular products and today this need is stronger than ever before. Although the value proposition of biodegradable/compostable polymers is acknowledged for specific applications the overall market dynamics remain very high. The expected growth rate of approx. 13% p.a. can still fluctuate significantly based on the drivers listed in Sect. 7. Outlook for BASF’s biodegradable/compostable polymers ecoflex® and ecovio®. BASF has been a pioneer in the field of biodegradable/compostable polymers starting R&D activities in 1993. ecoflex® has been developed as “the” enabling polyester for renewable raw materials like starch and PLA. The biodegradable/ compostable compound ecovio® offers specific material properties that makes it very suitable for application in bags, mulch film, or packaging. BASF’s activities grew strongly over the past decade. With upcoming new laws and regulations directing the use of biodegradable/compostable materials, the positive market development is expected to continue. With the significant efforts for the

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fundamental understanding of the biodegradation process in close cooperation with academic partners and the increase of production capacity for polyesters in AsiaPacific BASF shows a clear commitment to continue to drive the growth of biodegradable/compostable polymers.

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Adv Polym Sci (2024) 293: 177–196 https://doi.org/10.1007/12_2022_116 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 Published online: 2 March 2022

Biobased Synthesis and Biodegradability of CO2-Based Polycarbonates Alina Denk and Bernhard Rieger

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Monomer Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Propylene Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Cyclohexene Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Limonene Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Vegetable Oil-Based Epoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Thermal and Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

178 180 180 181 182 183 184 184 188 190 192 193 193

Abstract Stimulated by the growing interest in sustainable polymers, the past few decades have seen intensive research in the field of CO2-based polycarbonates. Within this research, the catalysts for the ring-opening copolymerization of epoxides and CO2 developed from early heterogeneous complexes to a wide variety of homogeneous catalysts. These complexes are continuously improved to meet growing expectations regarding activity and selectivity. Simultaneously, the range of epoxide monomers has steadily expanded due to the growing interest in biobased starting materials for polymers. The potential of the resulting aliphatic polycarbonates is evident in their thermal and mechanical properties, where they exhibit a wide range from soft to brittle and from low to high glass transition temperatures. Although the biodegradability of CO2-based polycarbonates has not yet been extensively researched, it has already been possible to gain knowledge A. Denk and B. Rieger (*) Technical University of Munich, Garching, Germany e-mail: [email protected]; [email protected]

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about poly(propylene carbonate) and its compostability. The entire potential of aliphatic polycarbonates is still far from being fully discovered, and with constant development of new catalysts and monomer feedstocks, the chance of a CO2- and biobased polymer achieving greater industrial relevance is increasing. Keywords Biobased · Carbon dioxide · Degradation · Epoxides · Polycarbonates · Ring-opening copolymerization · Sustainability

Abbreviations BDI BPA-PC cPC Đ E HPPO LO mCPBA Mn MTP Mw PC PCHC PLC PO PPC PPNCl ROCOP Salen TGA TOF TPP ZnGA εb σm

β-Diiminate Bisphenol A polycarbonate Cyclic propylene carbonate Polydispersity index (Mn/Mw) E-modulus (or Young’s modulus) Hydrogen peroxide propylene oxide Limonene oxide meta-Chloroperoxybenzoic acid Number-average molecular weight Methanol-to-propylene Weight-average molecular weight Polycarbonate Poly(cyclohexene carbonate) Poly(limonene carbonate) Propylene oxide Poly(propylene carbonate) Bis(triphenylphosphine)iminium chloride Ring-opening copolymerization Salicylaldimine Thermogravimetric analysis Turnover frequency Tetraphenylporphyrin Zinc glutarate Elongation at break Tensile strength

1 Introduction The successful synthesis of polycarbonates (PCs) from bisphenol A and phosgene was a milestone for the polymer industry in Germany, where large-scale production of aromatic PCs with the brand name Makrolon® was launched in 1958 by the Bayer

Biobased Synthesis and Biodegradability of CO2-Based Polycarbonates

HO

OH

O (b)

R1

+ R2

O

C

O

NaOH

O

+

(a)

Cl

[cat.]

179

O ∗

- NaCl

Cl

R2 ∗ O

O

∗

1

O O

R1

O

∗

1

Scheme 1 Synthesis of (a) conventional polycarbonate from bisphenol A and phosgene and (b) aliphatic polycarbonates from epoxides and CO2 [1, 4]

AG (see Scheme 1a) [1]. Properties like tensile strength and toughness of these thermoplastics combined with heat-resistance, flame-retardancy, transparency, and good resistance toward environmental impacts made PCs an attractive product for application in the automotive and electronics industry [1]. Since both phosgene and bisphenol A are health hazards [2, 3], the synthesis of PCs avoiding these educts is aspired to. Furthermore, the development of novel polymers from sustainable resources is of high interest in today’s research. A promising C1 building block can be found in CO2, a non-toxic and abundant greenhouse gas. Making use of it as a monomer, the first metal-catalyzed copolymerization of CO2 and epoxides was reported by Inoue and co-workers in 1969 [4]. The alternating incorporation of CO2 and an epoxide generates carbonate linkages, which leads to aliphatic PCs as pictured in Scheme 1b. By using this building block not only can an environmentally harmful gas emitted in large quantities be used, but also alternatives to petrochemical products are investigated. The first heterogeneous catalysts were further improved [5], and over the years the focus of related research shifted toward homogenous catalysis systems. Herein, the most important complexes are based on porphyrin, salen, phenoxide or β-diiminate ligands [6–10]. Tuning the selectivity and activity of the complexes, a wide range of catalytic systems are nowadays available for the coupling of CO2 and epoxides. Propylene oxide and cyclohexene oxide are the standard epoxy monomers for the synthesis of aliphatic PCs; however, new starting materials are constantly under research. The impact that polymer production has on the environment is not only of interest regarding the utilization of CO2, but also in the choice of the epoxide monomer and its origin. In this chapter, we will discuss the advances that research in the fields of aliphatic PCs made in terms of monomer scope, as well as catalyst development. The focus will lie on the most prominent epoxides and their copolymerization catalysts, as well as on some less popular examples for recent developments. Furthermore, the thermal, mechanical, and degradation properties of the resulting aliphatic PCs will be discussed. The described developments will also be repeatedly analyzed regarding their sustainability.

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2 Monomer Synthesis 2.1

Propylene Oxide

In regard to the copolymerization of epoxides and CO2, a lot of research focuses on propylene oxide (PO) as an epoxide monomer. It has a long history as a monomer in the epoxide/CO2 copolymerization and an even longer one in general use as a reactive chemical. The processes for PO synthesis have therefore developed over the years and have constantly been optimized, even with the growing interest in green chemistry. Two important processes for the synthesis of PO are the more traditional chlorohydrin process and the hydrogen peroxide propylene oxide (HPPO) process. In 1859, Wurtz reported the synthesis of PO from propylene chlorohydrin, setting the foundation for the chlorine route toward the epoxide [11]. Over the years, the process developed into industrial relevance using propylene and chlorine as initial reactants. The two steps in this synthesis are shown in Scheme 3a. First, the chlorohydrination converts propylene into propylene chlorohydrin, which is then epoxidized using sodium hydroxide or calcium hydroxide in a saponification reaction [12]. Another pathway toward PO is the chlorine-free HPPO process. With the use of the heterogeneous titanium silicalite catalyst TS-1, propylene can be epoxidized quickly and with high selectivity (97%) toward PO (see Scheme 3b). In this case, the olefin is oxidized with hydrogen peroxide in a mixture of methanol and water as solvents. Since the main by-product in this case is water, the synthesis is a more sustainable approach than the chlorohydrin process mentioned previously [13]. Since both routes depend on propylene, which is usually derived from mineral oil, as a starting material, it is therefore necessary that we investigate a potential biobased synthesis of propylene. In the process toward synthesizing the olefin, most routes proceed via methanol as an intermediate. On this pathway, the hydrogenation of carbon monoxide is a well-known method for the synthesis of methanol (Scheme 2 (1a)). Several catalytic systems have been developed for this reaction, with the most common being copper and zinc oxides supported on alumina [14]. The utilization of CO2 instead of CO in the synthesis of methanol is even more attractive, since it offers an opportunity to take advantage of an abundant greenhouse gas (Scheme 2 (1b)).

(1a)

CO

(1b)

CO2

(2)

+

+

2 CH3OH

[Cu/ZnO] 2 H2

CH3OH [Cu/ZnO]

3 H2

CH3OH

+

H2O

- H2O

CH3OH

[ZSM-5 cat.]

[ZSM-5 cat.]

H2O

O

- 2 H2O

Scheme 2 Conversion of (1a) CO [14] or (1b) CO2 [16] to methanol and (2) the methanol-topropylene (MTP) process [18]

Biobased Synthesis and Biodegradability of CO2-Based Polycarbonates

181

(a) Chlorohydrin process Cl2, H2O

Cl OH

- HCl

OH

+

Ca(OH)2 Cl

O

- CaCl2 - H2 O

(b) HPPO process H2O2, [TS-1]

O

- H 2O (c) glycerol route OH HO

OH

[alkali-loaded silica]

H2 - H2O

HO

OH

O

- H2O

Scheme 3 Synthetic routes toward propylene oxide: (a) the chlorohydrin process [12] (b) propylene oxidation with hydrogen peroxide [13] and (c) hydration of glycerol [21]

Studies also focus on the utilization of CO2 directly from industrial waste gas, which would allow a direct utilization instead of exhausting the pollutant [15]. With optimization of the Cu/ZnO-based catalytic system, selectivity of more than 99% toward methanol can be reached [16]. Even though the reaction with CO2 proceeds slower than with the conventional syngas, industrial application of this pathway is possible due to the selectivity and sustainability of the reaction [17]. With the methanol building block in hand, the synthesis of propylene can proceed via methanol-to-propylene (MTP, see Scheme 2 (2)) conversion with zeolites (e.g., ZSM-5) [18]. With optimization of already existing zeolite catalysts, high activity as well as selectivity toward propylene formation can be achieved. Since this method is widely applied in industry, it will not be discussed in more detail in this work [19, 20]. In contrast to the first two processes, Liu et al. published a propylene-free approach by synthesizing PO from glycerol. In the first step, glycerol is dehydrated and hydrogenated using catalysts known in the literature, resulting in propylene glycol. With alkali-loaded silica beds as active catalysts, the group was able to convert the intermediate propylene glycol into PO (Scheme 3c). However, the yields of around 30% PO and selectivities of 70% remained moderate [21]. This makes this synthesis pathway an interesting approach in terms of sustainability, yet not industrially competitive so far.

2.2

Cyclohexene Oxide

An intensively studied monomer for copolymerization with CO2 is cyclohexene oxide (CHO), which is usually generated via the epoxidation of cyclohexene. However, its most common synthesis route is based on petrochemical resources,

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H2, [cat.]

peroxide

O

(a)

triglycerides (b) from plant oils

[cat.]

mCPBA

O

H2, [cat.]

O

self-metathesis

Scheme 4 Synthesis of cyclohexene oxide from (a) benzene [22, 23] and (b) triglycerides [26, 27]

as shown in Scheme 4a, where benzene, derived from crude oil, is hydrogenated toward cyclohexene [22, 23]. The follow-up epoxidation of cyclohexene to form CHO can be performed with various peroxides, for example hydrogen peroxide or tert-butyl hydroperoxide [24, 25]. From this petroleum-based route, CHO may not be the ideal “green” building block, however, another synthesis approach based on renewable resources is shown in Scheme 4b [26]. Starting from plant oils without further purification, the triglycerides undergo self-metathesis to form 1,4-cyclohexadiene [27]. This intermediate can then be epoxidized, resulting in 1,2-epoxy-4-cyclohexene. The desired product CHO is then formed via hydrogenation of the remaining double bond [26].

2.3

Limonene Oxide

When talking about plant-based feedstocks for chemicals, terpenes are probably one of the first groups of compounds to come to mind. Herein (+)-limonene is an example of an abundant monoterpene, which can be found in citrus fruits [28]. The fact that (+)-limonene can be extracted out of their peels makes it even more attractive, since those are normally considered to be waste after juice production. This makes (+)-limonene not only biobased, but also a non-food feedstock in sustainable chemistry. It is therefore only logical that this molecule is also of special interest as a potential feedstock for monomers. A 1,2-epoxidation of the double bond in the ring structure of limonene can be performed with the help of peracetic acid, resulting in a mixture of the cis- and trans-product with a ratio of 46:54 (cis:trans) (Scheme 5a) [29]. Commercially available limonene oxide (LO) usually shows this ratio in the mixture of its isomers. As will be discussed later in this chapter, the most active catalysts in the copolymerization of LO and CO2 selectively polymerize the trans-isomer. The groups of Greiner and Rieger therefore reported a synthesis pathway toward 1,2-LO with an increased selectivity toward the trans-isomer (Scheme 5b) [30]. Other epoxidation methods, as for example titanium catalysts and hydrogen peroxide, are often less selective toward the 1,2-LO, and therefore produce diepoxide and other alcohol or ketone products [31, 32]. In contrast to propylene as a starting material, limonene is obviously more sterically hindered and its two double bonds are making selective epoxidation crucial. An epoxide monomer

Biobased Synthesis and Biodegradability of CO2-Based Polycarbonates O

183

O

peracetic acid

(a)

+

cis-LO 46%

trans-LO 54%

OH

O

Br

NBS

O

NaOH +

(b)

cis-LO 5%

trans-LO 95%

Scheme 5 Synthesis pathways of limonene oxide from (+)-limonene [29, 30] H2SO4, MeOH

O (a)

HO

mCPBA

O O

7

O (b)

soybean oil

[cat.]

O

R = R1, R2, R3 R1=

6

6

R2 =

6

8

R 3=

6

O

O

Cl

R

NaO

7

7

O

saponification

O O

R

O

4

Scheme 6 Generation of epoxides (a) from 10-undecenoic acid [33] and (b) from soybean oil [34, 35]

with two epoxide functionalities prone to ring opening would lead to branching in the ring-opening copolymerization instead of the aspired linear polymer.

2.4

Vegetable Oil-Based Epoxides

The presence of double bonds in unsaturated vegetable oils draws attention to them as potential starting compounds for the synthesis of biobased epoxides. In 2014, Zhang et al. reported copolymerization of CO2 with an epoxide they derived from 10-undecenoic acid. After methylation of the fatty acid, they were able to epoxidize the double bond with meta-chloroperoxybenzoic acid (mCPBA) (see Scheme 6a). The resulting epoxy methyl 10-undecenoate could then successfully be copolymerized with CO2. The production of 10-undecenoic acid from castor oil is

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also interesting, since it is a non-edible oil that can be derived from the castor bean plant [33]. Another plant-based oil that is under closer examination is soybean oil, which contains multiple double bonds in its fatty acid chains. For the synthesis of epoxides from this triglyceride, two possible routes can be conceived: (1) the epoxidation of double bonds in the fatty acid chains, or (2) the addition of a terminal epoxide function via reaction with epichlorohydrin as shown in Scheme 6 (b). The first pathway results in epoxides that show a high steric bulk at the oxirane function due to the long chain ends connected to it. With this steric hindrance, the activity of these compounds might be lowered, which makes them less attractive for industrial polymer synthesis. A terminal epoxide functionality as obtained via the second route could reduce this problem. Li and co-workers therefore saponified the soybean oil triglycerides followed by epoxide functionalization with epichlorohydrin [34, 35]. However, in this case the plant oil is extracted from soybeans, which means that the production of the resulting epoxide would be in competition with food cultivation [36].

3 Polymerization 3.1

Catalysts

Copolymerization of epoxides and CO2 was a milestone in the synthesis of PCs. In 1969, Inoue et al. applied a catalytic system based on diethyl zinc and water to PO and CO2, resulting in successful copolymerization of the two compounds [4]. Based on this work, interest in catalysis of ring-opening copolymerization grew in the scientific community. The group of Soga tested several aliphatic dicarboxylic acids in combination with Zn(OH)2 toward their activity in the polymerization of PO and CO2. The system of Zn(OH)2 with glutaric acid turned out to be the most promising [37]. The mechanism behind this heterogenous and air-stable catalyst was investigated years later by Rieger et al. It could be shown that zinc dicarboxylates work via a bimetallic pathway. The interaction of two adjacent metal centers on the surface of the catalyst allows the alternating insertion of PO and CO2 into the polymer chain. Furthermore, this work showed the importance of the distance between the interacting zinc centers, which directly influences the activity of the catalyst in the polymerization reaction [5]. While the heterogenous catalyst zinc glutarate (ZnGA) is a standard system applied in industry for the copolymerization of PO and CO2, the focus of research in the field of copolymerization catalysis progressed toward homogeneous catalysts. These offer the opportunity to vary the catalyst structure in different areas. With specific structure variations, the characteristics of a catalyst can be tuned, for example in terms of activity and selectivity. The aluminum porphyrin complex ((TPP)AlCl/EtPh3PBr, see Fig. 1) synthesized by the group of Inoue in 1985 allowed them to copolymerize CO2 with multiple epoxides toward copolymers with a low

Biobased Synthesis and Biodegradability of CO2-Based Polycarbonates Fig. 1 Examples of catalysts based on a porphyrin (left) [6] or a phenoxide (right) [7] ligand system

N

N Al

N Cl N

185

L O Zn L O

L = OEt2

polydispersity ( 90%). Transferring the system to CHO as epoxide monomer the highly active triethylborane/PPNCl-system (TOF ¼ 600 h
1) produces polymers of moderate to high molecular weights (28.3–76.4 g mol
1) [58]. A bifunctional catalyst carrying both the Lewis acidic organoboron site and the quaternary ammonium salt in one structure was published by Wu and co-workers in 2020. This catalyst exhibits a TOF of up to 4,900 h
1 in the copolymerization of CHO at a CO2 pressure of 15 bar [59].

3.2

Mechanism

The metal-catalyzed ring-opening copolymerization (ROCOP) of epoxides and CO2 can be divided into three basic steps: initiation (1), propagation (2), and termination via chain transfer (3), as shown in Scheme 7 [60, 61]. Depending on the nature of the initiating group, either an epoxide or a CO2 molecule is inserted as the first monomer. After this step, the initiating group is attached to the end of the growing chain, while the oxygen of the monomer is coordinated to the metal center. In the propagation process, the next monomer is inserted into this metal-oxygen bond; an alternating incorporation of the two monomers is intended herein to generate the characteristic carbonate linkages. However, a possible side reaction is the consecutive incorporation of epoxide into the growing polymer chain leading to ether instead of carbonate linkages. This can often be avoided by adjusting the reaction temperature and CO2 pressure, but also by tuning the catalyst system toward an unfavored incorporation of two epoxides in a row. The related sequential incorporation of two CO2 molecules, however, does not take place due to enthalpic barriers hindering the

Biobased Synthesis and Biodegradability of CO2-Based Polycarbonates Initiation

O

R

R [M]

R

O

R

O

CO2

X

189

[M]

O

X

O R

R [M] X CO2

[M]

O

R

O

R

R

X

O

O

O

[M]

X

O R

Propagation

O

R

R

R [M]

O

[M]

O O

O O

O

n

R

Termination

CO2

O R

O

O

X

n

H - [M]-OR

n

R

R

O

O

R

R R

O

R R-OH

X

O

R O

R

O

R

O

R

O

R [M]

n

R O

O

polyether formation

R

[M]

O

X

O

R

R O

n = n+1

R

O O

[M]

R

no dicarbonate bond formation R

O

O

R X

R

O CO2

R

O

O

O O

R

R

O O

X

O

n

R

O O

X

n

R

Scheme 7 General mechanism of the ROCOP of epoxides and CO2 [60, 61]

generation of a resulting dicarbonate linkage in the polymer [60–62]. Termination of a growing polymer chain is usually induced by the hydrolysis of the growing chain via the addition of alcohols or water to the reaction mixture. With chain transfer agents present in the reaction mixture, the growing chain can also be separated from the active center of the catalyst via chain transfer. This can be an intended reaction to introduce functional end groups to the polymer chain, but also an unwanted side reaction caused by impurities in the reactants [61]. The generation of a cyclic carbonate by-product via nucleophilic backbiting is in competition with the formation of PC. In the PO/CO2 coupling, the activation energy of the cyclic carbonate product 101 kJ mol
1) is only 33 kJ mol
1 higher than for the polymer product (68 kJ mol
1). This is also reflected in the PPC synthesis, where the tendency of cyclic propylene carbonate (cPC) formation via backbiting is a major challenge [63].

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4 Thermal and Mechanical Properties To be able to compete against the conventional bisphenol-A polycarbonate (BPA-PC), the thermal and mechanical properties of the aliphatic PCs must be studied. For an easy comparison of the different PCs, we will focus on particular characteristics that are most commonly of interest. The glass transition temperature (Tg) and the degradation temperature (Td) are used to characterize a polymer in view of its thermal properties. To report the degradation, it is common to use the temperature at which the sample weight is reduced by 5% (Td,5%). The mechanical properties can be analyzed via tensile testing, measuring the elongation of a specimen in relation to the force applied to the sample. The resulting values for the E-modulus (or Young’s modulus, E), tensile strength (σ m), and elongation at break (εb) will be compared in this chapter. However, as always, in the discussion of thermal and mechanical properties it is essential to be careful when comparing values from different research groups. The groups may have different measurement conditions, starting with sample preparation and continuing with applied rates or used solvents. We therefore tried to provide this data as an overview from the results of different groups, so that different impressions of the performing ranges of the polymers are accessible. Table 1 shows the thermal and mechanical properties of BPA-PC, PPC, PCHC, and PLC from different publications. The aliphatic PCs show decomposition temperatures in a similar range, while the decomposition of PCHC into volatile compounds requires the highest temperatures of approx. 280 C [68]. A significant difference between the PCs can be found in their Tgs. The absence of melting points in DSC measurements indicates a solely amorphous nature of the aliphatic PCs. Of all four PCs, PPC shows the lowest Tg of under 50 C due to its flexible polymer chain [65, 67]. A decomposition temperature of PPC at approx. 240 C, however, should be interpreted with caution. The degradation of the polymer can proceed via two mechanisms (see Scheme 8): (i, ii) via backbiting at lower temperatures (150–180 C) and (iii) via chain scission, usually at higher temperatures (>200 C). During the backbiting degradation of PPC, cPC is generated. This molecule has a boiling point at 240 C. Observations of mass losses above this temperature could therefore originate from the evaporation of this decomposition product. With standard thermogravimetric analysis (TGA) methods, decomposition of PPC toward cPC at lower temperatures cannot be detected, since the weight is only changing when the volatile cPC is evaporated [66]. In tempering Table 1 Mechanical and thermal properties of selected aliphatic polycarbonates Polymer BPA-PC [64] PPC [65–67] PCHC [64, 68] PLC [30]

Mn (Đ) (kg mol
1) 20–25 50.1–217 42–275

Tg ( C) 149 44–47 115

Td,5% ( C) – 254 280

E (MPa) 2,400  400 831  23 3,600  100

σm (MPa) 47  4 22–27 42  2

εb (%) 40  35 330  9 1.7  0.6

53.4

130

225

950

55

15

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(i) alkoxide backbiting O H

O

O

150-180 °C

O

O

polymer

+

O

HO

polymer

O (ii) carbonate backbiting O

O H

O

150-180 °C

O

O

polymer

O

+

O

HO O

O

(iii) chain scission R

O

O

O O

polymer

O

O

O

O

n

O

O

m

R

O O

O

> 200 °C R

O

O

O

OH n

+

CO2

+

O

O

O m

R

Scheme 8 Thermal decomposition of PPC via (i) alkoxide backbiting and (ii) carbonate backbiting or (iii) chain scission [69, 70]

experiments under air, a stability of PCHC up to 150 C and only minor degradation at 180 C could be determined. This allows an estimation of the processing window between 115 and 180 C if decomposition of the polymer should be avoided. In this case, the degradation has been analyzed via gel permeation chromatography (GPC) [68]. This method would also be interesting for the analysis of PPC degradation temperatures. The low thermal stability of PPC limits industrial application of the polymer [67]. With its low Tg and E-modulus, it is furthermore not a suitable substitute for BPA-PC. The thermally more stable PCHC shows a Tg of around 115 C and a substantially higher E-modulus (>3,500 MPa) than PPC. However, the brittleness of PCHC with an elongation at break of only a few percent limits its application as well. The promising biobased alternative is poly(limonene carbonate) (PLC), which has the highest Tg (130 C) of all three aliphatic PCs that is also close to the value for BPA-PC (149 C). In tensile testing, PLC shows a moderate elongation at break of 15%, higher than the values for PCHC. But in turn, the E-modulus (950 MPa) of PLC is relatively low compared to BPA-PC (>2,000 MPa), which results in a relatively brittle polymer as well [30]. To address the problem of brittleness, Thorat et al. synthesized a terpolymer of CO2, PO, and CHO with a PPC content of 20%, expecting a combination of the mechanical properties of the two PCs. Unfortunately, the incorporation of PPC did not have a positive effect on the brittleness of PCHC. The resulting terpolymer showed an elongation at break of 0.7%, which is within the range of pure PCHC [71].

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The polymers based on fatty acids mentioned previously show completely different properties, not only in thermal, but also in mechanical analysis. The Tgs of the 10-undecenoic acid-based PCs synthesized by Zhang et al. are around
40 C, which does not allow their application as thermoplastic materials, but could make them attractive polyol precursors in polymer synthesis [33]. The soybean oil-based PCs from Li and co-workers showed slightly higher Tgs of around 12 C. Via terpolymerization with PO, they were able to produce materials with a low to moderate tensile strength (5–17 MPa) and a high elongation at break (250–430%). Even though the properties of these biobased PCs do not seem to be competitive with the examples from Table 1, the double bonds make them interesting for postpolymerization functionalization [34].

5 Degradation When addressing the life cycle of polymeric materials, it is not sufficient to focus on the monomer origin. We must be aware of what happens to the product at the end of its use. With the development of CO2-based PCs, their potential biodegradable character drew further attention. The initial conviction that all aliphatic PCs are generally biodegradable has to be clearly contradicted based on the current state of scientific research. The degradation of polymers can be roughly divided into two categories: the biological and the physical or chemical degradation. Several studies reported biological degradation experiments with PPC with the help of microorganisms or enzymes. A detailed overview of studies concerning degradation of PPC was presented by Luinstra and Borchardt [66]. While PPC is stable in water [72], its decomposition with the help of lipases and fungi in water has been reported [66]. In another study, PPC films with molecular weights of 36–121 kg mol
1 were buried in soil. After 6 months, only minor changes in the surfaces of the film could be detected. Furthermore, the observed weight loss reached only 3% in this study [73]. With the change of conditions toward standard composting at 60 C, a visible degradation of PPC films after 3 months was found by Luinstra [70]. The thermal degradation of aliphatic PCs was already briefly addressed in the previous section. In addition, Lee et al. showed that PPC is thermally stable up to 170 C. The group also investigated the stability of the polymer in a weathering chamber. At 63 C, with a humidity of 50%, and irradiated with light (250–800 nm, 550 W/m2), the molecular weight of the polymer decreased over 3 months. During this process, no cPC formation could be detected. The authors propose therefore that the degradation of PPC happened via chain scission by water due to the humidity in the weathering chamber [72]. The pH dependency of the physical degradation of PPC was addressed by Ree and Kim et al., who showed that the decomposition is faster in a basic environment (pH 13) [74]. With the help of diethyl zinc, which is also a catalyst for the PPC formation, the polymer could be degraded toward cPC at 30 C [75]. This is also of interest, considering the catalyst residuals in the polymer product. To increase the

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lifetime and stability of the polymer, it might be useful to remove the catalyst residuals from the polymer product. Besides the biodegradation properties of PPC, which was intensively studied recently, other aliphatic PCs, such as PCHC or PLC, were not thoroughly investigated in this aspect. Greiner and co-workers reported that pure PLC showed no degradation after 60 days at 60 C in active compost. They propose the hydrophobic nature of PLC and the steric shielding of the carbonate units as explanation for its high biostability [76]. However, this feature is not necessary a disadvantage if the fields of application of these PCs request a certain stability against environmental impacts. We must be aware that when the carbonate bond is the point of degradation in the PCs, CO2 can be set free again. This means that CO2-based PCs are not a terminal CO2-storage option. However, the potential beneficial uses, especially as a substitute for petroleum-based plastics, underline the importance of these polymers.

6 Conclusions The class of aliphatic PCs that can be synthesized from epoxides and CO2 has become increasingly important in recent years. For this reason, a wide variety of catalysts have been developed to increase the selectivity and activity in polymer synthesis. At the same time, the monomer scope has been expanded, driven by the growing interest in biobased building blocks. The various thermal and mechanical properties exhibited by the resulting polymers can be tailored to meet specific requirements. In the future, questions will arise as to whether the PCs developed will only be used for niche products or if they are suitable for widespread use, possibly leading to less frequent demand for petroleum-based polymers. Acknowledgement The authors would like to thank Magdalena Kleybolte for the valuable discussions on this manuscript.

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Adv Polym Sci (2024) 293: 197–268 https://doi.org/10.1007/12_2021_111 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 Published online: 2 March 2022

Progress in Catalytic Ring-Opening Polymerization of Biobased Lactones Lucas Al-Shok, David M. Haddleton, and Friederike Adams

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Catalytic Processes for ROP of Lactones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Coordination Ring-Opening Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Ring-Opening Polymerization Using Organocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Enzymatic Ring-Opening Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Lactones from Renewable Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Terpene-Derived Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Sugar-Derived Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Amino Acid-Derived β-Lactones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Fatty Acid-Derived Monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Naturally Occurring Macrolactones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

198 199 199 207 212 215 215 225 234 242 244 246 247

Abstract Polymers that can be extracted or manufactured from renewable resources such as sugars, natural acids, or terpenes are increasingly relevant. In this context, biobased lactones are a group of monomers that can be synthesized from renewable feedstocks and combine versatility with the ability to yield high-performing materials including both thermoplastics and thermosets. In this chapter, an overview of different kinds of catalytic ring-opening polymerization (ROP) mechanisms including coordination, organocatalyzed, and enzymatic

L. Al-Shok and D. M. Haddleton Department of Chemistry, University of Warwick, Coventry, UK F. Adams (*) Chair of Macromolecular Materials and Fiber Chemistry, Institute of Polymer Chemistry, University of Stuttgart, Stuttgart, Germany Faculty of Science, University of Tübingen, Tübingen, Germany e-mail: [email protected]

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reactions is discussed. ROP of renewable lactones to yield polyesters are classified by their origin. In particular, we report on recent advances in ROP of terpene-based precursors such as limonene and pinene that lead to a variety of caprolactone derivatives which are available for polymerization. Moreover, lactones that can be converted from sugars and natural acids (e.g., fatty acids, amino acids) will be discussed. Keywords Amino acids · Coordination polymerization · Enzymatic polymerization · Fatty acids · Green chemistry · Lactones · Macrolactones · Organocatalysis · Polyesters · Renewable resources · Ring-opening polymerization · Sugars · Terpenes

1 Introduction The worldwide production of the most commonly used polymers or “commodity plastics” continues to grow rapidly and was estimated to about 367 metric tons for 2020 (EU plastics production and demand first estimates for 2020, 2021) [1]. However, while the majority of these materials are still derived from fossil resources, the increasing demand for polymeric materials cannot be satisfied by continuously exploiting this limited feedstock or without damaging our planet unsustainably. Bioderived polyesters represent the fourth largest group of biomacromolecules, following the three major groups: Nucleic acids, proteins, and polysaccharides [2]. The group of polyesters also provides society with essential industrial, fossilbased polymers such as poly(ethylene terephthalate). Thermoplastic polymers, including poly(ethylene terephthalate), poly(ethylene), poly(carbonate), or poly (vinyl chloride), find application in many, if not all sectors, including household, automotive, electronics, and the health care industry. However, the increasing limitation of raw materials requires an economical management of fossil resources, nevertheless, the demand and production of plastics which originate from petrolbased chemicals is still high. The substitution of petrochemical-based materials with polymers that are based on renewable resources is a requirement to preserve fossil resources and to move toward a more sustainable future [3–5]. In order to efficiently produce polymers with a predictable molar mass, narrow molar mass distributions, and a high end-group fidelity, to obtain materials with tunable thermal and mechanical properties, catalytic polymerization approaches can be elegant techniques. Highly efficient synthesis catalysts and processes are necessary to produce cost-efficient tons of polymers as bulk raw materials. Besides commodity polymers, specific materials for more challenging and specific applications are in demand. A significant ratio of these engineering polymers is already produced as eco-friendly specialty materials via ring-opening polymerization (ROP) [6].

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The ROP of ɛ-caprolactam to polyamide-6 (Nylon-6, Perlon) is a prominent example. Besides lactams, lactones can also be converted via ROP. Examples of lactones are ɛ-caprolactone (ɛCL), racemic β-butyrolactone (βBL), meso or racemic lactide (LA), glycolide, δ-valerolactone (δVL), and β-propiolactone (βPL). The resulting polyesters were first synthesized by polycondensation reaction, but to reach high molecular weight polymers, ring-opening polymerization of the respective lactones is a very attractive and simple route [7]. The number of processes reported for the ROP of lactones is extremely high. Most prominent are reactions via coordination, organocatalytic or enzymatic polymerization techniques [6]. In this chapter, the progress in catalytic ROP of biobased lactones is summarized with regard to different feedstocks. Lactones derived from terpenes, carbohydrates, amino acids, or vegetable oils were already used for catalytic ROP using metal-based, organo- or enzymatic catalysts.

2 Catalytic Processes for ROP of Lactones 2.1

Coordination Ring-Opening Polymerization

Coordination ROP is often considered to be the classic or conventional way for lactone polymerization. Studies have led to a variety of metal-based catalysts bearing, often, complex ligands, which are still unmatched in terms of controlled polymerization conditions. Especially, the possibility to tailor catalysts provides a toolbox for different thermodynamic circumstances or the stereoselective synthesis of functional materials. Jedliński et al. demonstrated that alkali metal-based catalysts (e.g., potassium alkoxides with crown ether as initiator) are able to initiate anionic ROP of β-lactones (βL), i.e., propiolactone, α-methyl α-ethyl propiolactone, and β-butyrolactone [8]. The polymerization of β-lactones can be considered to be different from ROP of other lactones as it can take place via two different mechanisms depending on the catalyst/initiator system leading to different configuration of stereocenters in the β-lactone (Scheme 1). If the lactone reacts with a negatively charged initiator (nucleophilic metal alkoxide) at the carbonyl group, an acyl-oxygen cleavage

Scheme 1 Mechanisms of ring-opening of β-lactones using nucleophiles

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Scheme 2 Side reactions that can occur during anionic/nucleophilic ROP of lactones including intramolecular/intermolecular transesterification and transfer to monomer. As an example, a substituted β-lactone is shown in this scheme

takes place, resulting in an alkoxy-terminated species. During acyl-bond cleavage, a retention of configuration of the stereocenter takes place. This mechanism is also observed for other lactones. Solely for β-lactones, a carboxyl-terminated species can be generated in case the monomer is attacked at the alkyl position (e.g., by a carboxylate salt) facilitating cleavage of the alkyl-oxygen bond [6, 9–11]. Alkylbond cleavage leads to an inversion of the stereocenter [12]. However, different conflicting opinions exist about the accurate mechanisms of the reaction of different nucleophilic initiators to β-lactones [6, 10, 13]. In terms of studies from Jedliński et al., βBL, which cannot be polymerized with common anionic initiators, was transferred to poly(3-hydroxybutyrate) (PHB) in presence of the potassium catalyst solution, however, with the slowest reaction rate among the three tested lactones. Mechanistic investigations showed that the polymerization proceeds via cleavage of the alkyl-oxygen bond resulting in polymers with low polydispersities [8]. Since then, further studies with alkali alkoxides showed that the mechanism of initiation and propagation highly depends on the substitution of the β-lactone, as contradictory to propiolactone and β-butyrolactone, the propagation using pivalolactone as monomer proceeded via acyl-oxygen ring cleavage with formation of cyclic oligomers [14]. To efficiently produce polymers with very narrow molar mass distributions (dispersity), a predictable molar mass, several functionalities, and a specific stereochemistry (tacticity), coordination-polymerization techniques are an implemented method in current research. In contrast to common anionic polymerization, which only proceeds in a living-type mechanism if protic species are absent,

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Fig. 1 Bimetallic μ-oxo metal alkoxides for livingtype coordinative ringopening polymerization

transesterification, side reactions, and generation of cyclic oligomers do not take place in an ideal coordination polymerization mechanism (Scheme 2) [6, 15]. Coordination-polymerization of olefins was developed by Karl Ziegler and Giulio Natta who afterwards in 1963 shared the Nobel Prize in chemistry. They were able to polymerize non-polar monomers (1-alkenes, dienes, etc.), i.e., with TiCl4 in combination with aluminum alkyls or alkyl halides, under mild conditions [16, 17]. Since then, olefin-polymerization with advanced metal-organic homogeneous catalysts has grown to be one of the largest processes in polymer production and indeed by far the highest volume chemicals produced on a global scale. To synthesize high molecular weight polyesters using lactones, coordinative ROP is an attractive and simple route. With this polymerization type, tailor-made polymers with narrow molar mass distributions and a predictable molar mass are accessible [7]. Since the first investigations on anionic metal-catalyzed ROP using transition and alkali metals, an enormous number of initiators have been reported in the literature for living-type coordination polymerization, in which lanthanides, aluminum, tin, and zinc complexes showed high activity while maintaining a high control over the polymerization [18]. One of the first examples of anionic living-type coordination-insertion polymerization using metal complexes was reported by Teyssié and coworkers in 1976 using bimetallic Al2/Zn(II) and Al2/Co(II) μ-oxo alkoxides with unsubstituted lactones such as ɛCL, βPL, and δVL (Fig. 1). No transfer or termination reactions were observed, shown by a linear relationship between monomer conversion and the number average molar masses as well as by obtaining defined polymers with low dispersities. The investigation of the mechanism revealed a coordination of the monomer before insertion and seems to be the rate-determining step in this polymerization mechanism (Scheme 3). Afterwards, monomer insertion mechanism via acyl-oxygen cleavage into the aluminum-alkoxide bonds of the catalyst was observed. Additionally, a flip-flop mechanism was considered in which two aluminum centers of different catalysts were involved in the chain growing process. Polymerization of γ-butyrolactone was not possible [19, 20].

2.1.1

Tin Octanoate

Several research groups investigated the polymerization of cyclic esters using tin octanoate (tin(II) bis(2-ethylhexanoate or Sn(O(O)CCH(C2H5)C4H9)2), abbreviated as Sn(Oct)2, as catalyst [21–27]. Since the first investigations, this catalyst is one of

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Scheme 3 Coordinationinsertion mechanism for anionic/nucleophilic ROP of lactones using metal-based catalysts

Scheme 4 Mechanism of polymerization using Sn(Oct)2 and an alcohol as initiator including formation of a tin alkoxide as the active species

the most frequently used initiators in the polymerization of lactones and cyclic esters especially for lactides and ε-caprolactone, but also for other lactones. However, proposed mechanisms during polymerization have been controversial in the initially performed studies and several mechanisms have been published so far. In an activated monomer mechanism, the monomer and the initiating alcohol are both coordinated to the tin metal center followed by a nucleophilic addition of the alcohol to the carbonyl. Another mechanism that was discussed in several studies included the formation of a tin alkoxide species prior to polymerization takes place (Scheme 4). An equilibrium with the initiator Sn(Oct)2 species and two alkoxide species was proposed. After activation, the tin alkoxide is the active species for polymerization of lactones in a coordination-insertion mechanism [21, 22].

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Aluminum-Based Catalysts

After initial investigations on polymerization using different aluminum alkoxides [28, 29] and the above-mentioned mixed systems containing aluminum and zinc, more complex aluminum- and zinc-based systems were developed. Spassky et al. were able to isospecifically polymerize racemic βBL with organometallic species (ZnEt2, CdMe2, AlEt3) and chiral (R)-3,3-dimethylbutane-1,2-diol as initiator via acyl-oxygen cleavage of the monomer (Fig. 2). The catalyst had different active sites leading to crystalline polymer of only moderate enantiomeric enrichment and amorphous polymer which had to be separated via precipitation [30]. Similar observations were made by polymerization of rac-βBL with aluminoxane-catalysts in which isotactic PHB was separated from amorphous polymer by fractionation [31–33]. In the early 1990s, Spassky and coworkers introduced a group of Schiff base aluminum complexes [34, 35], which were highly effective for the stereo-controlled synthesis of lactide [36]. The thereof derived salicylaldiminate (SALEN) aluminum catalysts (Fig. 2) were further investigated by Nomura et al. who were able to polymerize ɛCL in the presence of BnOH to yield polymers with molar masses up to 56 kg/mol and very small dispersities (1.06–1.27) [37, 38]. By conducting kinetic studies of the ROP of lactide and ω-pentadecalactone (ωPDL), it was shown that an increase in steric hindrance of the catalyst resulted in a decreased rate in lactidepolymerization, but barely affected the polymerization of ωPDL. In contrast, when increasing the size of the diimine bridge, thus generating a more spacious catalyst

Fig. 2 Examples of efficient metal-based catalysts for coordinative polymerization of lactones

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conformation, a significant increase in polymerization rate for small-ring lactones was observed. The polymerization rate for the macrolactones was mostly unaffected by the less rigid catalyst. In addition, the successful synthesis of random ɛCL-coωPDL polymers has been achieved. However, block copolymers synthesized by sequential addition of the monomers showed subsequent transesterification reactions with a polymerization rate similar to the one of ωPDL [38]. In 2019, Huang and coworkers investigated the ROP of lactide and ɛCL mediated by a salen-based aluminum catalyst bearing a β-ketoiminato ligand [39]. By systematically altering the bridge as well as the size of the benzocyclane ligand, they reported on an optimized structure bearing a dimethyl propylene bridge and a fivemembered cyclane structure. This catalyst showed high activities toward ROP of ωPDL with molar masses up to 164.5 kg/mol and moderate dispersities (1.46–1.95), as well as for small ring-sized lactones such as lactide (Mn up to 1.6 kg/mol, Ɖ < 1.23) and ɛCL (Mn up to 4.6 kg/mol, Ɖ < 1.29) [39]. Close proximity of metal centers as well as the use of bulky substituents like Ph2CH groups were beneficial toward achieving polymerizations of lactide and ɛCL. Thus, recently reported bimetallic salen and bidentate amido-phosphine aluminum complexes induced controlled polymerizations with immortal features [40, 41].

2.1.3

Zinc-Based Systems

The first highly active zinc-based catalysts for coordination polymerization were reported by Coates et al. and the groups of Hillmyer and Tolman. Hillmyer and Tolman investigated dizinc- or zinc-alkoxides (Fig. 2) and zinc-bis(phenolate)s for ɛCl, (-)-menthide and lactide-polymerization [42–45]. Coates et al. developed zincsingle-site β-diiminates for lactide, racemic βBL, and racemic βVL polymerization (Fig. 2) [12, 46]. Coordination-insertion polymerizations were performed in a livingfashion with up to 2000 equivalents of monomer under mild conditions resulting in polymers with high molar masses (up to 140 kg/mol) and narrow molar mass distributions [12]. Similar β-diiminato-complexes with bistrimethylsilylamide (BTSA ¼ N(SiMe3)2) as initiator and up to 10 eq. of alcohol were published by Carpentier et al. [47]. This study showed successful, and immortal, ROP of a β-lactone with zinc complexes being very robust against protic species [48]. Related systems were also highly efficient for immortal ROP of racemic lactide to produce PLA with molar masses up to 67 kg/mol and polydispersities below 1.30 [49]. During this immortal polymerization, a second nucleophile acts as chain transfer agent (CTA) and initiator. Each additionally added molecule of this substance leads to initiation of a polymerization chain so that the number of polymer chains exceeds the number of active catalyst molecules. After in situ generation of the alkoxide species of the catalyst, coordination-insertion polymerization with different exchange/transfer reactions between dormant and active polymerization chains and CTAs takes place (Scheme 5) [48]. Rieger and coworkers published electron-deficient β-diiminato-zinc-ethyl complexes that bear trifluoromethane-groups in the ligand backbone, enabling increased activities in lactide and βBL polymerization without

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Scheme 5 Distinction between living-type and immortal ROP of lactones and mechanism for immortal ROP if a catalyst bearing a nucleophilic initiator ([M]-Nuc) is used in combination with a CTA (ROH) exemplary using a substituted β-lactone [48]

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using a chain transfer agent due to higher Lewis-acidity of the catalysts [50]. Recently, the groups of Schacher and Kretschmer reported on novel binuclear bis(β-diketiminate) zinc complexes with flexible alkylene bridging groups for polymerization of substituted caprolactones with and without benzyl alcohol as initiator. By using benzyl alcohol, more control over the polymerization was observed in comparison with the mononuclear counterpart, in absence of benzyl alcohol broadening of the polydispersity occurred [51].

2.1.4

Lanthanide Complexes

The first examples of lanthanide mediated polymerizations were published in 1992 by McLain and Drysdale, who showed that lanthanide-isopropoxides (Ln ¼ Y, Er, Dm, Dy, La), which occur as an oxo-alkoxide cluster, are able to polymerize ɛCL in a controlled and fast fashion [6]. The catalyst and monomer scopes were broadened by Yasuda and coworkers, who used lanthanide-alkyl (SmMe(C5Me5)2(THF) and [SmH(C5M5)2]2) for the living-type polymerization of ɛCL and δVL and lanthanidealkoxides (SmOEt(C5Me5)2(OEt2), [YOMe(C5H5)2]2, and YOMe(C5Me5)2(THF)) for these two monomers and βPL [52]. Subsequently, yttrium, neodymium, and lanthanum iso-propoxides (e.g., by in situ reaction of iso-propanol with Y[N(SiMe3)2]3, Nd[N(SiMe3)2]3 or yttrium/lanthanum tris(2,6-di-tert-butylphenolate)s) were used for polymerization of L-lactide and meso-lactide, ɛCL, δ-hexalactone, and δVL [53– 58]. Mechanistic studies with La(OiPr)3 showed that the polymerization proceeds in an anionic-coordination mechanism via acyl-oxygen cleavage. βBL was also mentioned as monomer, but the polymerization remained less controlled due to observed side- and termination reactions [58]. Solely, Spassky et al. performed partly livingtype and fast reactions with racemic βBL using yttrium 2-methoxyethoxide, verified by narrow molar mass distributions and the possibility to perform copolymerizations with L-lactide [59]. Further extensive research on lanthanides was carried out as previous studies showed the high potential of these metals to perform controllable reactions. Carpentier and coworkers developed aminoalkoxybis(phenolate) yttrium and lanthanum complexes bearing amido initiators and a tert-butyl substituted aminomethoxy bis(phenolate) ligand ([ONOO]tBu) (Fig. 2). These complexes showed very high activity in ROP of lactide [60, 61]. In addition, further ligands with more sterically demanding substituents (i.e., adamantyl, CMe2Ph) and neodymium as metal center were tested in this polymerization. Insitu replacement of the initiator with 1–50 equivalents of 2-propanol was performed to obtain very efficient initiators for the synthesis of heterotactic poly(lactide) from rac-lactide in “classical-living” or immortal ROP with controlled molar weights (up to 160 kg/mol) and very narrow molecular weight distributions (1.06–1.40). The sterically demanding ligands played an important role in achieving high heteroselectivity in this chain-end controlled polymerization [62, 63]. These aminoalkoxy-bis(phenolate) ligands [ONOO]R are superior to others as they can stabilize the highly oxophilic and electrophilic metal center in presence of large amounts of alcohol [48]. The monomer scope was

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broadened when the same group published the syndiospecific βBL-polymerization. Especially, in situ generated iso-propoxide complexes were highly active with low polydispersities even when 2000 equivalents of racemic βBL were used [64, 65]. Immortal ROP with rac-βBL was only slightly possible with up to 3 equivalents of iso-propanol leading to polymer chains with narrow dispersities. A use of more than 5 eq. of alcohol was not possible and led to a drastic decrease in activity [48, 63]. Over time, a multitude of different initiators [66–69], ligand substituents [70, 71], rare-earth metals [67–69, 71–75], chain transfer agents [63], and side arms/backbone [67–69, 71, 72, 74–77] were introduced to bis(phenolate) catalysts [78]. Rieger et al. investigated in detail the influence of different metal centers on the activity and stereoselectivity of tert-butyl-substituted amido-bis (phenolate) complexes. They found an increase of activity and stereoselectivity with decrease in metal-radius (Sm < Tb < Y < Lu). The highest turnover frequency (TOF) of 6,900 h1 for an amido-complex was obtained for the Lu[ONOO]tBu(N(SiHMe2)2)(thf) complex [73]. Furthermore, other ligand structures were introduced to non-metallocene lanthanides. Salalen- [79], phenoxy-thioether- [80], salen-like fluorous dialkoxy-diimino[81], mixed fluorous alkoxy-diimino- [82], enediamido- [83], phenoxy-amidinate[78], bis(guanidinate)- [84], bis(naphtholate)- [85], and bis(amide) [83]-complexes and a vast number of other catalysts showed good results in ROP of different lactones. For a detailed review on rare-earth metal complexes for ring-opening polymerization, we recommend reference [86].

2.2

Ring-Opening Polymerization Using Organocatalysts

Although metal-organic catalysts have been established for controlled mediation of ring-opening polymerization, the possible compromising effect of metal impurities on biomedical [18] or electronic [87] applications has motivated the design of metalfree catalysts. Consequently, over the past two decades a variety of organocatalysts has been developed and is highlighted due to the catalysts’ higher tolerance toward aqueous or oxygen-containing impurities which resulted in comparably low toxicity, commercial availability, and easy storage [88–92]. As opposing to metal-based catalysts which usually mediate the ROP via a coordination-insertion mechanism, organocatalysts perform polymerization either by activating the monomer, activating the initiating (propagating) alcohol or both activations simultaneously [90].

2.2.1

Tertiary Amines and Phosphines

Using tertiary amines as organocatalysts, 4-dimethyaminopyridine (DMAP) and 4-pyrrolidinopyridine (PPY) were among the first substances to catalyze living ROP of lactide (molar mass up to 17 kg/mol, Ɖ > 1.19) [93]. The proposed mechanism involved the activation of the monomer by a nucleophilic attack of the

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Scheme 6 Pathways of the organocatalyzed ROP via (a) activated monomer (illustrated for DMAP, but analogous for PR3 as catalyst) or (b) activated alcohol mechanism

pyridine derivative to generate a reactive intermediate, which further reacts with a nucleophilic initiating or propagating species (ROH) that results in the separation of DMAP and propagating chain (Scheme 6a). However, computational studies conducted in 2008 suggested that the DMAP catalyzed ROP of lactide might take place via an alcohol activated mechanism (Scheme 6b) [94]. In contrast to the monomer activation mechanism, the catalyst firstly activates the nucleophilic initiator or propagating species via H-bonding. Afterwards, the resulting reactive nucleophile attacks the monomer and releases the propagating chain [94]. Phosphine-based catalysts like PPhMe2, PPh2Me, PPh3, P(nBu)3, or P(tBu)3 have been investigated by Hedrick and coworkers and had comparable activities for catalyzing ROP of lactide to DMAP. Moreover, the activity was described to increase with increased basicity of the phosphine and similar to DMAP, a monomer activated mechanism has been suggested (Scheme 6a) [95].

2.2.2

N-Heterocyclic Carbenes

Since the introduction of N-Heterocyclic Carbenes (NHCs) by Arduengo et al. [96, 97], these systems have been extensively investigated [98–100]. About a decade after their first successful isolation, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2ylidene (IMes) has been described to be a potent organocatalyst for the ROP of lactide, ɛCL and βBL producing polymers with predictable molecular weights (Mn up to 17 kg/mol) and low to medium dispersities (Ɖ < 1.33) [101]. In the same study, an activated monomer mechanism was suggested mirroring the pathway of ROP mediated by DMAP and phosphine bases (Scheme 7). In this mechanism, the

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Scheme 7 Monomer activation mechanism for the ROP of lactide mediated by 1,3-bis-(2,4,6trimethylphenyl)imidazole2-ylidene (IMes) [102]

unpaired electron pair acts as a nucleophile and activates the monomer by attacking the carbonyl carbon. Opening of the lactone created a zwitterionic intermediate. Subsequently, the carbene-monomer adduct is initiated or propagated via the incoming alcohol or propagating chain, respectively [101–103]. However, following reports of a proposed ROP mechanism via alcohol activation [104–106], further studies were conducted in order to test the viability of the activated monomer mechanism [107, 108]. It was claimed that even in the absence of the initiating alcohol NHCs were able to mediate the ROP of the monomer and interestingly produce cyclic polylactide [107, 108]. NHCs have also been used to produce cyclic poly(propiolactone) and poly(3-hydroxybutyrate) [109]. The mechanism by which the NHCs-mediated ROP takes place depends on several factors including structure of carbene and type of lactone [88, 90, 103, 110]. However, the ability to selectively produce cyclic polyesters makes them particularly useful as organocatalysts for various polymer topologies. For instance, cyclic polyesters exhibited different properties than linear polyesters in their thermal behavior, biodistribution, or degradation [111–113].

2.2.3

Guanidine/Amidine/Thiourea Cocatalysts

A further class of organocatalysts is represented by amidine- and guanidine-based superbases, namely 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene (MTBD) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which have, since the introduction for ROP by Pratt and Lohmeijer, been established as highly selective and active mediators [114, 115]. While MTBD and DBU actively catalyze ROP of lactide via an alcohol activated mechanism, they are insufficiently active for polymerization of other lactones such as ɛCL, δVL, or βBL [114]. (Thio)urea (TU) species are known to function as carbonyl activators and have been proven to catalyze ROP of lactide [116, 117]. The combination of both catalytic systems, TU, and amidine/guanidine base, gave rise to active and remarkably selective organocatalysts with minimal transesterification activity. A bifunctional mechanism was suggested at which thiourea activates the lactone, while MTBD or DBU function as the alcohol activator (Scheme 8) [114].

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Scheme 8 Proposed mechanism for the bifunctional activation of monomer (representatively shown for δVL) and alcohol [118]

Scheme 9 TBD-catalyzed ROP of lactones (representatively illustrated with δVL) [115]

TBD showed one of the highest activities as an organocatalyst, but also led to more transesterification reactions [114, 115]. Mechanistic studies suggested that the high activity could arise from the bifunctional character, which involves the simultaneous mediation of lactone and alcohol/propagating chain activation (Scheme 9) [115]. Due to its high activation rates, commercial availability, and easy handling, TBD is conveniently used for a variety of monomers such as ω-pentadecalactone [119], derivatives of δVL [120, 121], or highly strained rings like β-butyrolactone or β-malolactonate [122–124].

2.2.4

Phosphazene Bases

Similar to the amidine and guanidine-based TBD, DBU, and MTBD, the phosphazene bases N0 -tert-butyl-N,N,N0 ,N0 ,N00 ,N00 -hexamethylphosphorimidic triamide (P1-t-Bu), 1-tert-butyl-2,2,4,4,4-pentakis- (dimethylamino)-2λ5,4λ5catenadi(phosphazene) (P2-t-Bu), 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis [tris(dimethylamino)phosphoranyl-idenamino]-2λ5,4λ5-catenadi(phosphazene) (tBuP4) and 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2diazaphosphorine (BEMP) [125, 126] are known to be effective and highly active organocatalysts for ring-opening polymerization [127]. In 2007, Zhang and

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Scheme 10 Mechanism for the ROP of lactones using BEMP as catalyst (representatively illustrated with δVL) [128]

coworkers investigated the ROP of lactide, δVL, and ɛCL catalyzed by P1-t-Bu and BEMP and postulated a mechanism which involved the activation of the initiating alcohol or propagating chain by the phosphazene base which facilitated nucleophilic attack to the lactone (Scheme 10) [128]. The authors showed a living character of the lactide-polymerization catalyzed BEMP, which only showed transesterification activity at high monomer conversion [128]. When comparing the different phosphazene bases, the activity was highly dependent on the monomer structure, as for instance P2-t-Bu catalyzed the ROP of lactones but was insufficient for mediating polymerization of epoxides [128]. t-BuP4 was able to successfully catalyze the ROP of epoxides [129, 130], but displayed transesterification activity at high conversion for epoxides and lactones [131, 132]. Zhao et al. presented a novel cyclic trimeric phosphazene base (CTPB) to overcome these restrictions. The CTPB was synthesized from tris (dimethylamino)iminophosphorane and hexachlorocyclotriphosphazene and showed high activity and low transesterification for the catalyzed ROP of γBL (Mn up to 22.9 kg/mol, Ɖ < 1.72) [133]. In accordance with other phosphazene-based organocatalysts, the mechanism of ROP by CTPB was described to have an alcohol activation pathway (Scheme 11).

2.2.5

Organic Acids

A simple way to mediate the ROP of cyclic esters is the utilization of strong organic acids such as HCl Et2O [134–136], p-toluenesulfonic acid ( pTsOH) [135], and trifluoroacetic acid [134] as well as trifluoromethanesulfonic acid (triflic acid) [137]. Even weaker acids like lactic acid, tartaric acid, citric acid, and fumaric acid catalyzed the ROP of δVL and ɛCL [138–140]. In contrast to the aforementioned nucleophilic organocatalysts, organic acids were described to follow an electrophilic pathway of a monomer activation mechanism, in which the monomer is activated via protonation of the carbonyl oxygen, before the nucleophilic attack of the initiating alcohol or propagating chain takes place (Scheme 12) [90, 137].

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Scheme 11 Simplified illustration for the CTPB catalyzed ROP of γBL [133]

Scheme 12 Representation of the cationic ROP mediated by organic acids (EX) [90]

2.3

Enzymatic Ring-Opening Polymerization

Enzymes are highly selective for the mediation of crucial biochemical processes in all organisms. It was not until the mid-90s when the first studies regarding the use of enzymes to accelerate polymerization reactions in vitro were published [141– 143]. Shortly after, Uyama et al. highlighted the applicability of enzymes, in particular lipases, for ring-opening polymerizations of large lactones or macrolides (ring size 11–17) [144–147]. Compared to conventional non-enzymatic methods, enzyme-catalyzed ROP (eROP) bears certain advantages such as a naturally high degree of chemo-, stereo-, and regioselectivity. Moreover, enzymes can be used under mild condition regarding pH, pressure or temperature and are non-toxic. Therefore, there is no apparent need to specifically purify the product. However, immobilizing the enzyme, on e.g., polymer- or silica-based beads, provided the possibility of easy removal and reuse of the catalyst [148–150]. Nowadays, immobilized Candida antarctica Lipase B (CALB) which is commercialized under the name Novozym 435 [150], is the gold standard for eROP as it has proven

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Scheme 13 Simplified representation of the enzyme-catalyzed ROP of lactones (illustrated using δVL) [89]

to be versatile and highly active for ROP of various-sized lactones [151–153]. Bisht et al. directly compared the influence of immobilizing CALB and pointed out that the immobilized enzyme had an increasing effect on polymerization rate and molar mass of the polymers (62 kg/mol, Ɖ > 1.9) as compared to the free equivalent which produced polymers with molecular weights between 15 and 34 kg/mol [151]. Besides the immobilization of lipase enzymes, other parameters like temperature and water content have proven to be critical in determining the outcome of the conducted polymerization reactions [89, 151, 154, 155]. An optimal amount of water had to be applied in order to provide sufficient water for the enzyme in order to fold appropriately, however, oversupplied contents resulted in a decrease in polymerization rate [89]. The ROP mechanism proposed by Uyama and coworkers involved the activation of the lactone ring by the catalytic site consisting of aspartic acid, histidine, and serine amino acids (Scheme 13) [146]. The combination of these three amino acid residues, also known as a catalytic triad [156], acts as the active site of the enzyme at which an interacting pair of an acidic (aspartate) and basic (histidine) moiety is activating a nucleophilic (serine) residue that forms a covalent bond with the substrate (monomer). In the initial step, the alcoholic serine residue attacks the

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Fig. 3 Examples of lactones with cisoid and transoid conformations. Bold bonds represent conformationally locked bonds [162]

carbonyl carbon (I) of the lactone resulting in an enzyme intermediate (II) which converts to the alkoxide species that represents the so-called enzyme-activated monomer (EAM) (III) [89, 157]. Subsequently, a nucleophile (initiating or propagating alcohol) can attack the EAM leading to a second intermediate (IV) which in the following releases the propagating polymer chain. Since the generation of the EAM is widely accepted to be the rate-determining step, lipase catalyzed ROP can be considered to proceed via an activated monomer mechanism [151, 158, 159]. Enzymatic ROP is especially efficient for polymerization of macrolactones. Due to their low ring strain, larger lactones are not enthalpically favored to undergo ROP. Enzymes are indiscriminate between the ester bonds that are cleaved [160]. Moreover, the increased efficiency of lipases toward the ROP of macrolactones has been attributed to several parameters such as the increased hydrophobicity and therefore improved convergence of monomer and the active site [161]. Interestingly, it was observed that lactones change their conformation with increased ring size (Fig. 3). While lactones with smaller ring sizes (7) form transoid rings. The stereoselectivity of the lipase results in the preferred coordination of transoid compared to cisoid lactones [162].

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3 Lactones from Renewable Resources 3.1

Terpene-Derived Monomers

Different natural products can be used for the preparation of biobased polymers. As the largest class of natural products, terpenes are an important class as building blocks for polymers [163]. Terpenes derive from isoprene, i.e., their carbon skeleton consists of C5-units, and thus more than 25,000 of different structures are discovered until now [163–165]. Monoterpenes are a class of C10 isoprenoids occurring in oils and resins of many plants fulfilling a variety of tasks [166, 167]. In the case of the mint plant, production of oils, mostly consisting of p-menthane monoterpenes (product of the hydrogenation or hydrogenolysis of various terpenoids) occurs in glandular trichomes [166, 167]. Mint is produced in the scale of thousands of tons per year and is widely used in the pharmaceutical, fragrance, and food industry [168]. In general, monoterpene biosynthesis routes in mint (spearmint and peppermint) start with a condensation from isopentenyl diphosphate and dimethylallyl diphosphate to build the C10 building block geranyl pyrophosphate via geranyl pyrophosphate synthase (GPPS) enzyme (Scheme 14) [166, 167]. This precursor is the starting point for ()-limonene synthesis via ring-closure reactions. Afterwards, several reductase, isomerase, hydroxylase, dehydrogenase, and synthase enzymes catalyze functionalization of limonene, whereas ()-limonene-6-hydroxylase and ()-trans-carveol dehydrogenase catalyze reactions for ()-carvone synthesis in spearmint (Mentha spicata). Limonene-3-hydroxylase and isopiperitenol dehydrogenase are essential for pulegone synthesis in peppermint (Mentha piperita). Pulegone reductase can then catalyze reactions to ()-menthone, which is transferred to ()-menthol via ()-menthol reductase [166, 167, 169]. Similar biosynthetic pathways for (+)-limonene and (+)-carvone from geranyl pyrophosphate were discovered in the pericarp of fruits of caraway (Carum carvi L.) involving (+)limonene synthase, (+)-limonene-6-hydroxylase, and (+)-trans-carveol dehydrogenase [170].

3.1.1

Limonene-Based Monomers

Due to their low cost and simplicity of isolation the monoterpenes pinene, limonene, and myrcene have been studied as building blocks and monomers for polymer synthesis, however, are not directly suitable for ROP due to the lack of carbonyl groups [171]. Cyclic terpenoids (terpenes that contain functional groups) such as ()-menthone or ()-carvone can be converted to the corresponding lactones via Baeyer-Villiger oxidation, which can then be used as monomers for ring-opening polymerization (Fig. 4). In 1958, Hall and Schneider started research on terpene-derived monomer feedstocks for ring-opening polymerization by using ()-menthide and sodium [172]. Subsequently, Tolman and Hillmyer implemented menthide as a conventional

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Scheme 14 Biosynthesis route of () carvone in spearmint and ()-menthol in peppermint starting from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) to build geranyl pyrophosphate (GPP) via geranyl pyrophosphate synthase (GPPS) enzyme. LS ¼ limonene synthase, L6OH ¼ ()-limonene-6-hydroxylase, CD ¼ ()-trans-carveol dehydrogenase, L3OH ¼ ()-limonene-3-hydroxylase, IPD ¼ isopiperitenol dehydrogenase, PR ¼ pulegone reductase, MR ¼ ()-menthol reductase [81]

monomer in their research starting with zinc-alkoxide mediated ROP in 2005 [45]. ()-Menthide is a seven-membered lactone, derived from the natural, cyclic monoterpene ()-menthol. Starting from the ketone derivative ()-menthone [173], ()-menthide can be synthesized using Baeyer-Villiger oxidation. For their first production of ()-menthide, Tolman and Hillmyer used meta-chloroperoxybenzoic acid (mCPBA) in a multigram scale reaction. Afterwards, polymerization of the obtained ()-menthide with a phenoxyamino-zinc alkoxide catalyst induced a living and controlled polymerization under mild conditions and poly(()-menthide) with molar masses up to 90 kg/mol with narrow to slightly broader molar mass distributions (Ð ¼ 1.1–1.6) was obtained. However, after full monomer conversion was reached during polymerization, high molecular weight polymers with a broadening of the dispersity were observed caused by transesterification. Epimerization of the chiral centers was not observed [45].

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Fig. 4 Lactones from renewable terpenoid feedstocks derived from terpenes ()-limonene and pinenes

Since then, poly(()-menthide) has been the subject of more recent research, especially to produce thermoplastic elastomers (TPEs). In particular, the use of block copolymers combining amorphous poly(()-menthide) with other polymers exhibiting a high melting or glass transition temperature can lead to phase separation of the two blocks causing a multiphase morphology. Poly(()-menthide) acts as a soft elastomer while the other block ensures that the material can return to its original shape [174]. Using ZnEt2 as catalyst, Hillmyer and Tolman synthesized an α,ωhydroxy poly(()-menthide) via ring-opening polymerization of ()-menthide with diethylene glycol as a bifunctional initiator and quenching with water (Scheme 15). Molar masses were tuned by altering the monomer-to-initiator ratio and polymers with dispersities of approximately 1.3 were obtained. Reaction of these polymers with triethylaluminum formed an alkoxide macroinitiator. Controlled polymerization of lactide initiated by this poly(()-menthide) initiator resulted in poly(lactide)b-poly(()-menthide)-b-poly(lactide) (PLA-b-PM-b-PLA) triblock copolymers with 20 to 50 mass percent lactide and molar mass distributions between 1.2 and 1.4. Molar masses and ratios between PLA and PM were easily adjusted by the ratios of monomer and initiator. Microphase separation of these copolymers and formation of thermoplastic elastomers were confirmed using small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). In dependence of molar mass and ratio between PLA and PM, different morphologies of the PLA-b-PM-bPLA TPEs (i.e., lamellar, cylindrical, micellar) were obtained. Tensile measurements

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Scheme 15 Renewable thermoplastic elastomers from PLA-b-PM-b-PLA triblock polymers via two-step ring-opening polymerization. Adapted from Ref. [175]. With permission from American Chemical Society

verified elongations at break of up to 1,000%, relatively low stiffness and elastomeric properties using cycles pulled to 300% [174]. Further investigation into the degradability of PLA-b-PM-b-PLA triblock polymers was performed in a hydrolysis setup at 37 C in phosphate buffer solution (pH 7.4) for 45 weeks. Two different triblock copolymers (PLA-PM-PLA (7.6–337.6) and PLA-PM-PLA (12–3212), with the number values corresponding to the molar mass of the block in kg/mol) were compared to the respective homopolymers (PM(29) and PLA(11)) in terms of molecular weight, polymer composition, thermal properties, and water uptake. During these experiments, PLA rectangles showed autocatalyzed degradation characteristics whereas PM did not show any changes in appearance. Experiments with copolymer dog-bone shaped specimen turned cloudy and specimen increased in size after 29 weeks (Fig. 5a). In terms of mass loss, the rate of degradation was influenced by the chain length of the PLA block (Fig. 5b). Only little degradation was observed for all samples in the first 11 weeks. Afterwards, PLA reached a maximum mass loss of about 95%. Final mass loss of the two copolymers was 27% (PLA-PM-PLA (7.6–337.6)) and 37% for PLA-PM-PLA (12–3212), which is in accordance with the overall PLA content in the block copolymers. Only 3% and 14% of PLA were still incorporated in the two copolymers after 45 weeks (Fig. 5c). For a poly (()-menthide) homopolymer, only 8% mass loss was obtained indicating that PM does not show autocatalyzed hydrolytic degradation. These observations were substantiated by decreased molar masses of PLA and triblock polymers determined by SEC measurements [176]. These groups also synthesized triblock polymers with different types of polylactide using the same synthesis and analysis strategies. Amorphous poly(D, L-lactide), semicrystalline enantiopure poly(L-lactide), or poly(D-lactide) end segments were used, obtaining lactide mass percentages of up to 48%. The Young’s moduli and ultimate tensile strengths of the semicrystalline copolymers were up to three times higher than for the amorphous polymers. Additionally, the group mixed different triblock copolymers to further tune the stiffness, strength, and elastic

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Fig. 5 Degradation experiments of PM, PLA, and PLA-PM-PLA polymers depicted by (a) visual, (b) mass loss and (c) PLA content experiments. Adapted from Ref. [176]. With permission from American Chemical Society

properties [177]. The same polymers (PDLA-b-PM-b-PDLA) were also used to investigate their ability to successfully function as a crystal nucleating agent for poly(L-lactide) [178]. Over the years, different methods to synthesize PLA-b-PM-b-PLA have been established. A sophisticated method was the use of tin(II) ethylhexanoate as a catalyst, using diethyl glycol as a bifunctional initiator, in a one-pot reaction via sequential ring-opening polymerization. Furthermore, ()-menthide was synthesized in a Baeyer-Villiger oxidation using Oxone® (potassium peroxymonosulfate) instead of mCPBA as an eco-friendly alternative [179]. Transesterification reactions and polylactide homopolymer formation were minimized, and triblock copolymers with narrow molecular weight distributions were isolated. A different application was addressed with these triblock polymers. A pressure-sensitive adhesive (PSA) was designed by the formulation of the polymers with a renewable rosin ester tackifier [179].

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In a further variation of the system, poly(lactide) was exchanged by Tulipalin A (α-methylene-γ-butyrolactone (MBL)) [180]. MBL is a natural substance and the simplest member of the sesquiterpene lactone family found in the common tulip Tulipa gesneriana L [181], resulting in fully renewable PMBL-b-PM-b-PMBL triblock polymers [180]. Synthesis was performed using Sn(Oct)2 as a catalyst and diethyl glycol as initiator for ()-menthide polymerization followed by functionalization of the hydroxy end-groups with 2-bromoisobutyryl bromide to obtain Br-PM-Br. This macroinitiator was used for synthesis of well-defined PMBL-PM-PMBL triblock copolymers using atom transfer radical polymerization (ATRP) of MBL. Phase-separation of the triblock polymers was elucidated using DSC, AFM, and SAXS measurements. Tensile measurements showed high elongation and elastic recovery properties making these materials suitable candidates for high-performance and engineering thermoplastic elastomer materials [180]. MBL itself was polymerized via ring-opening polymerization for the first time by Chen et al. in 2016 using three different lanthanide non-metallocene complexes. Exclusively unsaturated polyester P(MBL)ROP with molar masses up to 21.0 kg/mol was produced. Additionally, a crossover propagation between vinyl-addition polymerization (VAP) and ROP was revealed leading to cross-linked polymer P(MBL)CLP when La[N(SiMe3)2]3 as a catalyst with or without benzyl alcohol was used. By adjusting the ratio between catalyst and initiator, ratio between VAP and ROP was tuned, because of different chemoselectivities of the respective metal-X bond. While La-alkoxide species led to ROP pathway, La-NR2 facilitated VAP via enolate species. Cross-propagation was proposed to happen between the double bonds in the repeating units of the polyester and the enolate species after VAP leading to a cross-linked polymer having predominately VAP chains [181]. For more information on MBL, we refer to the sugar-derived monomer section of this book chapter. In 2020, Rieger et al. combined racemic β-butyrolactone with ()-menthide in a novel approach using yttrium non-metallocene catalysts [182]. As βBL is synthetically available from carbon dioxide, it is a promising monomer for sustainable PHB production using metal-catalysis [183–186]. Poly(hydroxyalkanoates) (PHA) are a promising class of biocompatible polymers, while poly(3-hydroxybutyrate) represents one of the most used PHA. However, despite the high internal strain of the four-membered ring, βBL is a difficult monomer to polymerize being less reactive than lactide or ε-caprolactone. The first syndiotactic PHB was produced with tin catalysts (e.g., Bu3SnOMe, distannoxanes) [187–190]. Slightly syndiotactic PHB was obtained in a slow reaction which led to insufficient mechanical properties of the polymer to use them as engineering plastics [9]. Further research on lanthanides was carried out to perform controllable and especially stereospecific reactions of racemic βBL. Carpentier and coworkers synthesized aminoalkoxybis(phenolate) yttrium complexes with different nucleophilic initiators or via in-situ activation with 2-propanol for fast and syndioselective polymerization of βBL with a tacticity of PHB of up to 94% (see section on coordinative polymerization) [48, 60, 61, 63]. Similar catalysts with heteroaromatic pyridine-based initiators attached to bis (phenolate)yttrium complexes were developed by Rieger et al. and were fast and efficient catalysts for syndiospecific βBL polymerization [182, 191, 192]. Especially,

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Scheme 16 Sequential synthesis of PM-PHB and PHB-PM-PHB block copolymers using monoor bimetallic bis(phenolate) yttrium catalysts

a bimetallic aminoalkoxy-bis(phenolate)yttrium complex with a tetramethylpyrazine initiator enabled bifunctional initiation without in situ activation to synthesize BAB triblock copolymers. These block copolymers were used to modulate the stressstrain properties of syndiotactic PHB, which is often brittle and hard to process, by combining it with a soft amorphous polyester. Therefore, the group reported on the first lanthanide-based catalysts for ()-menthide, polymerization under mild conditions with very low molar mass distributions (Ɖ ¼ 1.00–1.16) [182]. Afterwards, AB and BAB block copolymers with poly(()-menthide) as the first and syndiotactic PHB as the second block were synthesized via sequential addition in a one-pot reaction (Scheme 16). DSC, XRD, and stress-strain measurements were used to study the influence of different composition and molar masses on the mechanical and thermal properties of the block copolymers showing that syndiotactic PHB is less brittle than naturally occurring isotactic PHB. By incorporation of PM, the elastic modulus is further lowered while increasing the elongation at break [182]. For more detailed information on stereoselective PHB production, we refer to the amino acid section of this book chapter.

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In an approach to exchange metal-based catalysts with organocatalysts, Li and coworkers used cyclic trimeric phosphazene base (CTPB) as the organocatalysts or a CTPB/urea binary system for ROP of ()-menthide to obtain PM with molar masses between 8.2 and 100.7 kg/mol, however with slightly increased dispersity between 1.32 and 1.94. When using glycerol as the initiating system and end-capping with acrylate groups, UV-curing led to cross-linked polyester elastomers with thermal stabilities similar to non-crosslinked poly(()-menthide) and good elastic properties [193]. A terpenoid that has attracted interest in the groups of Hillmyer and Tolman was ()-carvone. After hydrogenation, dihydrocarvone or carvomenthone can be obtained in dependence of the reaction conditions (Fig. 4). Baeyer-Villiger oxidation of dihydrocarvone using the eco-friendly method with Oxone® led to dihydrocarvide [194]. Due to preservation of the double bond, polyesters from dihydrocarvide are susceptible toward further post-polymerization functionalization [194]. Oxidation of carvomenthone with mCPBA yielded carvomenthide but can also be oxidized using Oxone® [194, 195]. Both oxidations were performed on a multigram scale. Polymerizations of carvomenthide and dihydrocarvide were performed using wellstudied ZnEt2 that was already used for ()-menthide polymerization. All polymers showed narrow molar mass distributions (Ð between 1.08 and 1.42) and molar masses up to 62.3 kg/mol. Additionally, copolymerization of both monomers and post-polymerization functionalization of dihydrocarvide using epoxidation and cross-linking with dithiols were investigated [194]. A further modification gave copolymers reacted with mercaptoethanol in a thiol-ene reaction. The resulting hydroxy-containing polymers were used as polyols in polyurethane synthesis [196]. When performing Baeyer-Villiger oxidation of dihydrocarvone using mCPBA, an oxidized dihydrocarvone (dihydrocarvone oxide) is obtained bearing an epoxy-group instead of the double bond (Fig. 4). When using either diethylzinc or tin(II)-ethylhexanoate as catalyst for polymerization, only oligomers in which both epoxide and lactone reacted were built since both catalysts facilitate epoxide and lactone polymerization. Copolymerization with ε-caprolactone with 0.3–50% of the epoxy lactone gave versatile cross-linked materials when using ZnEt2 at 60 C in bulk or solution with varying thermal properties and crystallinity in dependence of epoxy lactone percentage. These materials showed excellent shape memory properties for use in biomedical devices with melting temperatures close to body temperature [197]. Copolymerization of carvomenthide with other lactones was studied by the group of Shin. In 2014, they developed a pathway toward thermoset elastomers from carvomenthide by synthesizing a carboxy-functionalized poly(carvomenthide) (PCM). Using Sn(Oct)2 as catalyst and glycol as initiator, in analogy to triblock polymer synthesis using ()-menthide, HO-PCM-OH was isolated with very narrow molar mass distributions (Ɖ ¼ 1.05–1.09). Esterification with succinic anhydride enabled carboxy-functionalization of both hydroxy end-groups (Scheme 17). Crosslinked polymers were obtained by thermal curing with a trifunctional aziridine showing no crystallinity, high viscosity, elastic properties, strong thermal resistance, and low glass transition temperatures [195]. A few years later, similar thermoset

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Scheme 17 Synthesis of carboxy-functionalized poly(carvomenthide) and crosslinking with a trifunctional aziridine. Reproduced from Ref. [195]. With permission from American Chemical Society

elastomers were synthesized by implementation of an additional polymerization step. After polymerization of PCM, subsequent polymerization of D,L-lactide led to HO-PLA-PCM-PLA-OH. Functionalization with succinic anhydride and a trifunctional aziridine was performed in analogy to the previously described study. All polymers showed high thermal stability, viscoelastic properties, and phase separated PLA domains. Additionally, hydrolytic degradation experiments were performed via size-exclusion chromatography analysis showing degradation of the PLA block [198]. In an approach to use a biocatalytic route to polyesters from terpenoids, Scrutton et al. used BaeyerVilliger monooxygenase (BVMO) enzymes from Pseudomonas sp. HI-70 (CPDMO ¼ cyclopentadecanone monooxygenase) and Rhodococcus sp. Phi1 (cyclohexanone monooxygenase (CHMOPhi1)) to produce monomers ()menthide and dihydrocarvide [199]. CPDMO was used for ()-menthone conversion and regioselective oxidation of dihydrocarvide was enabled by site-directed mutagenesis of CHMOPhi1. Ring-opening polymerization was afterwards successfully performed via metal-catalysis using magnesium 2,6-di-tert-butyl-4methylphenoxide [199].

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Pinene-Based Monomers

Converting naturally occurring (+)-β-pinene into 4-isopropyl-caprolactone was facilitated by a four-step synthesis route developed by Jones et al. in 2017 [200]. Starting from geranyl pyrophosphate, α- and β-pinenes can be produced via (+)-3S-or ()-3R-linalyl diphosphate in pine trees (i.e., Pinus taeda) from turpentine [201]. Since turpentine production surpasses that of menthol by far, using pinenes as raw materials is a promising strategy toward renewable polyesters [200]. Additionally, pinenes are waste materials from the pulp, paper, and wood processing industries, as about 2 kg of turpentine is produced per ton of bleached sulfate Kraft pulp [202]. Synthesis of 4-isopropyl-caprolactone was either performed directly from 4-isopropylhexanone or from (+)-β-pinene via (+)-nopinone and ()-cryptone intermediates in an overall yield of 64%. The monomer was then converted to the respective polymer via ring-opening polymerization catalyzed by two different non-metallocene zirconium catalysts (zirconium bipyrrolidine salan (Zr(bis)(OiPr)2) and zirconium amine(trisphenolate) (Zr(tris)(OiPr)) complexes). In addition, commonly used catalysts ZnEt2/BnOH and Sn(Oct)2 were tested. All polymers showed narrow to moderate molar mass distributions (Ð ¼ 1.19–1.57) and molar masses up to 32 kg/mol. High dispersities were explained by transesterification reactions. Polymers from (+)-β-pinene-based monomer showed lower molar masses up to 4,900 g/mol and broader molar mass distributions likely due to contaminations in the monomer. All polymers were amorphous and showed low glass transition temperatures between 52 C and 47 C. Copolymerization with racemic and L-lactide using Zr(bis)(OiPr)2 and Zr(tris)(OiPr) was tested. Only low molecular weight copolymers were obtained, in which attempted block copolymers showed to be a mixture of the respective homopolymers [200]. Using α-pinene as a renewable feedstock, a verbenone-based lactone was synthesized via a chemoenzymatic route. Starting from α-pinene, (1S)-()-verbenone was isolated via electrochemical synthesis. After reduction, ()-cis-verbanone was obtained. A more stable mutant of cyclohexanone monooxygenase enzyme from Acinetobacter calcoaceticus (CHMOAcineto) was used for enzymatic oxidation of ()-cis-verbanone to ()-cis-verbanone lactone. Additionally, Baeyer-Villiger oxidation of ()-cis-verbanone to ()-cis-verbanone lactone using mCPBA was successfully performed. Cationic homo- and copolymerization of ()-cis-verbanone lactone with ε-decalactone using methane sulfonic acid as catalyst and benzyl alcohol as initiator gave polyesters with cyclobutane ring structures in the repeating unit. Low molecular weight polymers with molar masses around 3,000 g/mol and dispersities of approximately 1.1 were obtained. In comparison with pure poly(ε-decalactone), the increased glass transition and decomposition temperatures of verbanone lactone homo- and copolymers were attributed to the rigid ring structure of thermally-stable and biobased poly(verbanone lactone) [202].

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Sugar-Derived Monomers

Monomers generated from cellulose and fermentation of biomass, especially from glucose, are extremely versatile precursors for a variety of different polyesters. Most monomers are formed from carboxylic acids that originate from fermentation or glycolysis (Scheme 18) [203]. Hence, we further refer to them as monomers derived from sugars.

3.2.1

γ-Butyrolactone

One compound that derives from biotechnological fermentation processes is succinic acid, which is considered to be a promising C4 platform for producing 1,4-butanediol, tetrahydrofuran or maleic acid and anhydride [204, 205]. Moreover, succinic acid can be converted to γ-butyrolactone (γBL) by metal-catalyzed hydrogenation (Scheme 18) [206–210]. Poly(γ-butyrolactone) (PγBL) is a structural analogue of microbial poly(4-hydroxybutyrate) (P4HB), which is known to exhibit desirable degradation properties [211]. However, for a long time γBL has been known to be a “non-polymerizable” lactone due to its low ring strain energy which thermodynamically disfavors polymerization [212, 213]. Until recently, only low molecular weight PγBL was synthesized under high pressure [214, 215]. In 2016, Chen and coworkers demonstrated the successful polymerization of γBL by using rare-earth metal catalysts under ambient pressure and low temperatures (Fig. 6a) [216]. In their study, they systematically improved the polymerization

Scheme 18 Illustration of sugar-derived lactones with their corresponding acid precursors

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Fig. 6 Illustration showing (a) general conditions for the ROP of γBL, (b) metal-based catalysts and (c) organocatalysts used in the studies of Hong and Zhao [133, 216, 217]

conditions using Lanthanum (La)-based catalysts, revealing that low temperatures are crucial in order to promote the reaction. Further, the authors improved the activity by introducing bulkier alkoxides, i.e., 2,2-diphenylethanol (Ph2CHCH2OH) as CTA, inhibiting the formation of La-clusters. Moreover, changing the alkoxide moiety and its ratio relative to the La-catalyst, the authors switched between predominantly cyclic or linear PγBL, which was characterized by MALDI-ToF MS. By using a discrete single-site yttrium alkyl catalyst (Fig. 6b), the authors optimized the polymerization toward 90% yield on a multigram scale. Further, both species had different properties in terms of their thermal stability and intrinsic viscosity and can therefore be distinguished by thermogravimetric analysis (TGA) and SEC. When conducting mechanical testing via dynamic mechanical analysis (DMA), the linear PγBL showed more elastic properties than its cyclic counterpart [216]. Moreover, the authors demonstrated quantitative recyclability of the linear and cyclic PγBL by either thermal treatment over 24 h or in the presence of an organo- or metal-catalyst which resulted in decompositions at room temperature with a half-life of 6.9 min or 0.82 min, respectively [216]. While common organocatalysts such as TBD or DBU were described as inferior for the polymerization of γBL compared to metal-based catalysts, the same group achieved promising results by utilizing a phosphazene-based (t-BuP4) superbase for the synthesis of high molecular weight PγBL (Fig. 6c) [217]. In a systematic study, the polymerization was optimized by employing BnOH, Ph2CHCH2OH or iPrOH in addition to the phosphazene catalyst to generate PγBL with molar masses up to 27 kg/mol. Three different synthesis protocols which led to different results and

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three different mechanistic pathway proposals were described. If the superbase without alcohol was reacted with γBL, a relatively slow polymerization with low conversions was observed, whereas much more rapid reactions were achieved by premixing t-BuP4 and an alcoholic species. If premixing of alcohol and monomer before adding the catalyst took place, the reaction times were much slower, but yielded polymers with narrower molecular weight distributions. Consequently, the group postulated that depending on the protocol, different end-groups and topologies were achieved. In comparison, polymerization of γBL was also conducted by using conventional inorganic bases such as KH, tert-BuOK, and NaOMe, which resulted in comparable monomer conversions and molecular weights [217]. In accordance with their previous study [216], the authors pointed out that the successful polymerization required low temperatures and that the polymer was completely convertible to the monomer under thermal treatment (260 C for 1 h) [217]. This study was continued by Li et al. who reported on cyclic trimeric phosphazene base (CTPB) as a new organocatalysts for ROP of γBL (Fig. 6c). It was able to catalyze the polymerization of γBL and selectively produced linear PγBL with high molecular weight and low polydispersity (Mn up to 22.9 kg/mol, Ɖ < 1.72). The selectivity toward linear polymers was explained by the formation of a CTPBH+ cation which stabilized the growing chains and prevent backbiting [133].

3.2.2

α-Methylene-γ-Butyrolactone

Derivatives of γBL can be obtained from fermentation products of glucose, such as α-methylene-γ-butyrolactone (MBL) which is obtained from either itaconic acid or can be found in tulips, giving the monomer its name Tulipalin A [218–220]. Due to its structure, MBL can be seen as a cyclic analogue to petroleum-based methyl methacrylate and is therefore of interest as a feedstock for sustainable acrylic chemicals and materials [221, 222]. Consequently, as the ring-opening polymerization of five-membered rings is still remaining a challenge, MBL was polymerized via radical [223, 224], anionic [225], group transfer [226], or coordination [227] polymerization in the past utilizing the Michael-type system rather than the lactone moiety. Introducing ɛCL as a comonomer for the polymerization of MBL allowed Zhou et al. to change the thermodynamic parameters in order to favor ring-opening of the five-membered lactone using bismuth(III) trifluoromethanesulfonate as a catalyst [228]. Hong and coworkers used the same catalytic system and claimed that a mixture of ring-opened polymer and PMBL homopolymer is produced. By testing the copolymerization of MBL and ɛCL using a lanthanide-based catalyst, they demonstrated the synthesis of exclusively ring-opened MBL-ɛCL copolymer without any vinyl-addition product (Scheme 19) [229]. By combination of MBL and a lactone with a high-ring strain such as ɛCL, the net negative change in enthalpy compensates the positive change in entropy and therefore enables statistical ring-opening copolymerization. Moreover, the authors

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Scheme 19 Vinyl addition and ROP pathways in homo- and copolymerization of MBL [229]

showed that by decreasing the reaction temperature and using non-polar solvents, these conditions are beneficial for opening the γ-lactone without favoring homopolymerization of MBL via vinyl addition. In terms of mechanical properties of the copolymers, an increase of the MBL ratio relative to ɛCL resulted in a disruption of crystallinity and decreased the melting temperature of the polymers [229]. Two years later, the same group presented the first successful synthesis of purely (unsaturated) MBL homopolymer via ROP using La(N(SiMe3)2)3 as catalyst. By conducting reactions at low temperatures with high monomer concentration, the ring-opening of the γ-lactone structure was thermodynamically favored [181]. Similar to their work on PγBL [216, 217], the authors demonstrated the full recyclability of linear PMBL by thermal treatment (100–130 C for 1 h or 60 C for 24 h). Moreover, by changing the ratio of La(N(SiMe3)2)3 and alcohol initiator, the polymerization path can be tuned between ROP or VAP, also enabling a third crosslinking polymer (CLP) in which they combined the VAP and ROP pathways (Scheme 20). In detail, if the catalyst was premixed with the alcohol, an alkoxide species was in situ generated which facilitated ring-opening of MBL via coordination-insertion mechanism (Scheme 20a.I). However, if the polymerization was conducted in the absence of alcohol, the catalyst coordinated to the carbonyl oxygen followed by insertion of the N(SiMe3)2 species into the vinyl bond of the monomer (Scheme 20a.II). Subsequently, a second MBL monomer coordinates to the generated La-O species, triggering the homopropagation via vinyl addition (Scheme 20b). If applying an equimolar ratio of alcohol and catalyst, both, the propagation via ROP and VAP, can happen simultaneously. This resulted in a cross propagation (Scheme 20c), in which the active ester enolate chain end of La-P(MBL)VAP reacted with the external double bond of the alkoxy chain end of La-P(MBL)ROP via Michael addition (Scheme 20c.III). This led to an intermediate species containing both ROP and VAP chains with the respective propagating alkoxy (ROP) and enolate (VAP) chain ends (Scheme 20c.IV). In conclusion, the VAP, ROP, or CLP product can be achieved by Cat:ROH ratios of 1:0, 1:3 or 1:1, respectively. This was a result of either the formation of La-NR2 (in absence of

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Scheme 20 Proposed mechanism for the formation of cross-linked polymer P(MBL)CLP by La[N (SiMe3)2]3/BnOH with a molar ratio of 1:1 at 60 C. Reproduced from Ref. [181]. With permission from American Chemical Society

alcohol) which favors VAP or La-OR (in case of excess of alcohol) in which the unsaturated linear ROP polymer is favored [181]. Danko and coworkers presented alternative routes for the synthesis of MBL-ɛCL copolymers [230]. By introducing the commercially available catalyst aluminum triisopropylate (Al(OiPr)3) at low temperatures (between 0 C and room temperature), they described the selective production of ROP-copolymers without generating VAP side products. The copolymers synthesized exhibited molar masses up to 87 kg/mol with an MBL content between 1 and 25 mol%. A metal-free route using Brønsted acid catalysts such as diphenyl phosphate was also reported in which the cationic ROP was initiated by isopropyl alcohol. In comparison with the metal-based ROP, the organocatalysis yielded copolymers with significantly lower molar masses (up to 17 kg/mol) and MBL contents of max 11 mol%. Di-and multifunctional alcohols led to telechelic copolyester structures with 4.3–8.8 mol% MBL. They proved the accessibility of the MBL double bonds by post-polymerization modification, clicking benzothioxanthene fluorophore (BTXI-SH) or N-acetylcysteine via thiol-ene chemistry [230]. More recently, the selective ROP of MBL was demonstrated by using a binary CTPB/urea catalyst system at low temperatures (Scheme 21), producing either homo- or copolyesters (MBL-δVL or MBL-ɛCL) without production of the competing VAP product. High MBL ratios up to 90 mol% were achieved in the polymer by adjusting the respective feed ratio of monomers. Thermal and mechanical studies

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Scheme 21 Proposed mechanism for the ROCOP of MBL with ɛCL by CTPB/urea binary catalyst. Reproduced from Ref. [231]. With permission of American Chemical Society

using DSC revealed that copolyesters with high ratios of MBL incorporation showed decreased crystallinity suggesting that MBL disrupts the ordered state of the polymer therefore being potential candidates for a new class of thermoplastic elastomers [231, 232].

3.2.3

γ-Methyl-α-Methylene-γ-Butyrolactone and α-Angelica Lactone

Levulinic acid is derived from biomass and is used to yield two lactone monomers which are structurally very close to MBL, namely γ-methyl-α-methylene-γ-butyrolactone (MMBL) and α-angelica lactone (αAL) [233–235]. The MMBL is mainly interesting as a sustainable alternative for acrylic monomers due to its reactive exo-vinylidene group [221]. In similarity to MBL, the polymerization of MMBL was studied mainly concerning its vinyl addition products [236–239]. α-Angelica lactone has an internal double bond and hence is argued to have higher ring strain which enthalpically favors the ring-opening process [240]. Consequently, polymerization can either take place via ROP or polymerization of the internal double bond producing a polyfuranone species. While the latter path has only led to oligomeric species [241], producing poly(α-angelica lactone) (PAL) via ROP has been studied with various catalysts. Kaygorodov and Tarabanko studied the cationic and anionic ROP of αAL with either boron trifluoride etherate or sodium butoxide leading to oligomeric species with molar masses up to 2000 g/mol [242, 243]. Furthermore, by employing different alkoxide initiators, the produced PAL can reach up to 19.5 kg/mol [243]. Molar mass distributions were not reported in this context. In addition, both studies showed the biodegradability of PAL by microorganism or in soil, which led to a subsequent study of copolymerization with styrene, methyl methacrylate, ethylene terephthalate, and caprolactam. The copolymers showed enhanced biodegradability in aerobic composting conditions compared to homopolymers without any αAL incorporated [244]. Hou et al. conducted the polymerization of αAL with conventional catalyst tin(II)-octanoate at 130 C producing PAL with molar masses up to 29 kg/mol and dispersities 1.4), whereas the zinc β-diketiminate catalyst with 2-propanol as initiator solely produced copolymers

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when MLABz was added after βBL polymerization (Mn > 4,200 g/mol, Ɖ > 1.22) in a two-step sequential polymerization. If both monomers were added simultaneously, the Zn-based catalyst solely synthesized MLABz homopolymers (Mn ¼ 3,500 g/ mol, Ɖ > 1.24), leaving βBL unreacted in the reaction mixture. Thermal analysis conducted via DSC revealed that random copolymers produced via Nd(OTf)3 showed one singular glass translation temperature of 10 C, therefore representing an intermediate value of atactic PHB (3 C) [299] and PMLABz (30 C) [300]. Similar results were obtained for TBD and DBU, in case of simultaneous addition of MLABz and βBL [122]. Remarkably, BEMP produced selectively block copolymers whether the monomers were added simultaneously or in sequential order. The atactic block copolymers were synthesized with molecular weights up to 17.7 kg/mol and narrow polydispersity below 1.65. Similarly, catalyzing the sequential polymerization by TBD and DBU yielded block copolymers with (Mn > 55.2 kg/mol, Ɖ > 1.46), in which the second monomer added always showed a slower rate of polymerization compared to the rate of its corresponding homopolymerization [122]. Block copolymers produced either via metal- or organocatalysis revealed two distinct glass transition temperatures varying between 1.6 and 2 C for the PHB block and 38–46 C for the PMLABz block. SEC analysis showed monomodal traces therefore supporting the successful synthesis of copolymers rather than mixed homopolymers. Based on the performed studies regarding the homo- and copolymerization of MLABz and subsequent hydrogenolysis, Marion and Guillaume et al. investigated amphiphilic PMLA-b-PHB block copolymers [301]. Poly(malic acid) is more hydrophilic than other hydrophobic PHAs due to the carboxylic acid pendant groups giving access to amphiphilic polymers. Poly(malic acid) naturally occurs in many fruits such as apples and grapes and is considered as biocompatible and non-toxic as well as being biodegradable to malic acid (which further degrades to carbon dioxide and water) [266]. Their synthetic strategy involved sequential polymerization of βBL in bulk and MLABz in toluene at 60 C using TBD as organocatalysts (Scheme 29). After isolating the TBD-PHB-crotonate macroinitiator, the second step involved the ROP of MLABz in toluene as solvent rather than in bulk, as reported earlier, due to improved efficacy and control over the large-scale polymerization, when conducted in solution. The isolated PMLABz-b-PHB copolymer was further treated with Pd/charcoal under H2 pressure to deprotect MLABz via hydrogenolysis to obtain the corresponding amphiphilic PMLA-b-PHB block copolymers. Copolymers with different hydrophilic weight fractions were further investigated toward their self-assembly behavior in PBS buffer solution, revealing that copolymers with PMLA weight fractions between 52 and 65% exhibited relatively small hydrodynamic diameter (Dh: 17–50 nm) and narrow size distributions (PDI: 0.24–0.30). Additionally, in vitro cell viability analyzed by MTT assays conducted with HepaRG hepatoma and SK.MEL-28 melanoma cells showed acceptable toxicity at concentrations of about 180 μg/mL. Moreover, HepaG cell uptake was quantified via flow cytometry and fluorescence microscopy by using a lipophilic fluorescent dye. Copolymers with higher hydrophobicity showed improved uptake compared to

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Scheme 29 Synthesis route for PMLA-b-PHB copolymers showing the sequential polymerization of PHB and PMLABz, as well as subsequent hydrogenolysis of the side and end-groups [301]

those with higher PMLA ratios. Both mild cytotoxicity and efficient cell uptake proved PMLA-b-PHB as an interesting candidate for drug-delivery systems [301]. One of the first studies on a controlled and fast synthesis of amphiphilic graft copolymers based on different alkyl-β-malolactonate monomers was conducted by Dubois et al. [302]. The study included the anionic copolymerization of hydrophobic hexyl-β-malolactonate (MLAHex) and benzyloxypropyl-β-malolactonate (MLABP) initiated by tetraethyl ammonium benzoate which induced the O-alkyl cleavage of the lactone. Subsequent treatment of the statistical copolymer with hydrogen and Pd/C led to the deprotection of MLABP, hence providing functional hydroxypropyl β-malolactonate (MLAHP) units. In a second step, ROP of PMLABz was grafted from the hydroxyl groups within poly(MLAHex-co-MLAHP) using Sn(Oct)2 as catalyst (Scheme 30). SEC measurements revealed a dispersity of 1.44 and a molecular weight of 5,300 g/mol. In similarity to MLABP, MLABz was deprotected under hydrogenolysis conditions leading to poly(MLAHex-g-MLA) amphiphiles with an MLA weight fraction of 0.11. Amphiphilic properties were tested via interfacial tension measurements which revealed the decrease in interfacial tension (29 mN/m to 12 mN/m) of water when dosed with polymer solutions in chloroform, hence confirming the amphiphilic character of the polymer. In addition, a critical micellar concentration (CMC) was determined as 0.26 g/L [302]. After initial studies using enantiomeric pure β-malolactonates to obtain different microstructures [292], the first stereoselective controlled ROP of substituted racemic β-lactones (4-alkoxymethylene-β-propiolactones (BPLOR)) and β-malolactonates (BMLR)) was reported by Carpentier et al. [303, 304]. Highly active bis(phenolate) yttrium catalysts with the structure [(ONOO)RY(N(SiHMe2)2(thf)] with several substituted ligands and one equivalent of iso-propanol were used [303, 304]. In terms of propiolactones, altering the sterical crowding or the electronic properties of

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Scheme 30 Pathway for the synthesis of P(MLAHex-g-MLA) graft polymers involving the copolymerization of MLAHex and MLABP, a hydrogenation step, grafting MLABz from the deprotected MLAHP side chains and deprotection of the polymerized MLABz by hydrogenation [302]

the ligand had a drastic influence on the resulting microstructure of the polymer in case of all racemic BPLOR (R ¼ Methyl, Allyl or Benzyl) monomers. Chlorosubstituents promoted highly iso-selective polymerization, bulky substituents led to syndio-propagation. DFT calculations demonstrated that Cl-H interactions between the ligand structure and a proton of the methylene group of a side group of the growing polymer chain played a substantial role for iso-propagation of BPLOR as the energetic profile was lowered by this interaction [303]. Similar effects were

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Scheme 31 Summary of catalysts and monomers used for the syndiospecific copolymerization of MLA derivatives [285]

obtained by using bromo- and fluoro-substituted catalysts [305]. The use of the racemic β-malolactonates BMLR (with R ¼ Allyl or Benzyl) as monomers with all catalysts tested produced syndiotactic polymers with various degrees of syndiotactic diads (chloro/fluoro/methyl substituents: Pr up to >0.95) (Scheme 31). This is again in accordance with DFT calculations, since the exocyclic-methylene group of the lactone is exchanged with a carbonyl-group disabling Cl-H interactions in the propagation step [304]. However, as methyl substituents seemed to be just as selective for syndiotactic diads as F and Cl-substituents and as Br-substituents exhibited slightly lower selectivity (Pr 0.91–0.92), the authors concluded that electronic effects for the ROP of racemic β-malolactonates might not be the crucial factor for stereocontrol [305]. In terms of steric effect, Carpentier et al. confirmed in 2014 that catalysts with less bulky ligands had superior properties in terms of generating highly syndiotactic PMLABz (35.2–45.3 kg/mol, Ɖ < 1.27) or PMLAAll (20.3–29.3 kg/mol, Ɖ < 1.64) homopolymers so that steric effects provide major contribution to stereocontrol [285]. In previous studies, the steric properties of the ligand were described to have the opposite effect on the tacticity when conducted with βBL, because more bulky ligands induced higher and chloro-substituents drastically lowered the syndiotactic fractions in PHB [64, 70]. In addition, the syndiotacticity of the PMLAR copolymers had a distinct effect on crystallinity of the polymers determined via DSC experiments. Whereas atactic PMLABz and PMLAAll did not show a detectable melting temperature, polymers synthesized via

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the most selective chloro-substituted ligand revealed melting temperatures of 117 C and 112 C, respectively. Homopolymers from polymerizations catalyzed with more bulky ligands showed a decrease in their melting temperature (Pr: 0.79–0.85 and Tm: 77 C–87 C for PMLABz; Pr: 0.82–0.87 and Tm: 51–80 C for PMLAAll) attributed to lower amounts of syndiotactic fractions in the polymer. The glass transition temperature remained constant at about 12 C (PMLAAll) and 33 C (PMLABz) for all cases. Furthermore, the copolymerization of (R)-MLABz and (S)-MLAAll was conducted, yielding copolymers (33.0–38.7 kg/mol, Ɖ < 1.54) with different syndiotactic (alternating) MLABz – MLAAll microstructures (Scheme 31). A similar trend to the previously described homopolymers was observed in which bulky catalysts resulted in merely syndio-biased copolymers with a percentage of alternation (alt%) of about 52–65%, in which crystalline regions were absent. Polymerizations, catalyzed with the chloro-substituted complex, produced highly syndiotactic (alt% > 95) copolymers with a melting temperature of 63 C. After the copolymer was converted using hydrogenation treatment, the generation of an MLA-altMLAnPr copolymer was described in which the hydrophobic MLABz segments were converted to the corresponding hydrophilic MLA [285]. In conclusion, biobased, renewable racemic β-malolactonates are until now not suitable to produce isotactic or isotactic enriched poly(hydroxyalkanoates).

3.4

Fatty Acid-Derived Monomers

Plant oils are an important class of raw materials for the chemical industry. Vegetable oils consist mainly of five different fatty acids: Stearic acid, palmitic acid, oleic acid, linoleic acid, and linolenic acid. Ricinoleic acid (cis-12-hydroxy-octadeca-9enoic acid) is a fatty acid which is the main ingredient in castor oil (Oleum Palmae Christi) and contains a hydroxyl group β to the unsaturated alkene [306, 307]. Polymers from fatty acids are often considered to be biocompatible and often contain hydrolysable ester bonds [306]. Polyesters from hydroxy-containing fatty acids such as ricinoleic acid can either be produced via polycondensation or via lactone formation and subsequent ring-opening polymerization [307]. In case of ROP of small lactones, the driving force of the chemical polymerization is the reduction of the ring strain of the lactone. Therefore four-, six-, and sevenmembered lactones can be converted relatively easily to the respective polymers [6]. However, macrolactones, as produced from fatty acids show no ring strain and thus the driving force is the entropic gain due to a less hindered chain rotation. Since the entropic term becomes more favorable with increased temperatures, higher temperatures than for small lactones are required. An absence of ring strain also leads to similar reaction rates of polymerization and transesterification, drastically lowering the control over the reaction leading to cyclic oligomers and broad molar mass distributions [162]. By using enzymatic catalysts, the conversion rate is already increased at room temperature with larger ring sizes (and hence increased hydrophobicity) due to a faster formation of the enzyme-lactone complex (see section on

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Scheme 32 Synthesis of lactones and cyclic oligomers from ricinoleic acid and copolymerization with lactide using Sn(Oct)2

enzymatic ROP) [161]. Thus, the amount of reported enzymatic polymerizations of macrolactones is much higher than for small lactones. Besides polyanhydrides, from ricinoleic acid maleate [308–310], polyesters from ricinoleic acid can be synthesized via the ricinoleic acid lactone. This molecule can be obtained from reaction of ricinoleic acid with dicyclohexylcarbodiimide and dimethylaminopyridine to facilitate ester formation (Scheme 32). A mixture of the monolactone and cyclic oligomers (dimers to hexamers) was isolated in which the ratio between lactones was influenced by the ricinoleic acid concentration in the reaction mixture. Lactones were separated via gel chromatography. Polymerization studies included a catalyst screening involving tin(II) 2-ethylhexanoate, yttrium isopropoxide, and a zinc catalyst either with a lactone mixture or with the pure mono- or dilactone. Using the lactone mixture, all catalysts produced either only oligomers or were completely unsuccessful. Solely the use of the ricinoleic acid lactone dimer and Sn(Oct)2 led to the formation of slightly bigger oligomers with molar masses up to 4,400 g/mol. To increase molar masses, copolymerization of ricinoleic acid lactone and lactide was tested (Scheme 32). Weight ratios of lactide/ ricinoleic acid lactone were set to be between 9:1 and 5:5. Molecular weights were drastically increased with molar masses up to 16 kg/mol, but the ricinoleic acid content in the polymer was lower than the weight percentage of the monomer feed caused by termination reactions of low reactive ricinoleic acid chain ends. In addition, molar masses decreased with higher ricinoleic acid content and dispersities ranged between 1.11 and 2.22. With higher amorphous ricinoleic acid content, melting temperatures of the polymers were decreased [311]. Further studies on lactide-ricinoleic acid lactone copolymers included hydrolytic degradation and drug release from those polymers. Besides ROP, also melt condensation and transesterification were tested to produce copolymers. Polymers synthesized by ROP and transesterification showed higher crystallinity than polymers synthesized

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by polycondensation due to a random copolymer structure of polycondensates in which LA blocks are not able to form crystalline domains in the amorphous ricinoleic acid matrix [312]. Hydrolysis using a polymer with a weight ratio of lactide/ricinoleic acid lactone of 60:40 was monitored with regard to changes in molecular weight of the specimen and loss of lactide content. Lactide was found in the degradation solution for all tested polymers which showed partial degradation with about 20% of weight loss after 60 days without a clear relation between structure, molar mass, and crystallinity. Drug release was then tested with triamcinolone (hydrophobic) and 5-fluorouracil (hydrophilic). Release of water-soluble 5-fluorouracil was much faster than triamcinolone release, with the slowest release of both drugs from polymers prepared via ROP due to higher crystallinity which is inhibiting fast water penetration [312].

3.5

Naturally Occurring Macrolactones

In contrast to converting hydroxy-containing fatty acids to lactones, exaltolide (ω-pentadecalactone) is a naturally occurring cyclic ω-hydroxy fatty acid present in macrocyclic musk. It can be extracted from angelica plant root (Angelica archangelica) or musk deer (Moschus) and is FDA approved. Due to its commercial availability, the number of processes reported for the homo- and copolymerization of ω-pentadecalactone is extremely high and general studies for ROP of macrolactones were conducted with this monomer using enzymatic, organocatalytic, or coordinative routes [38, 39, 119, 313–319]. Depending on its molecular weight, polymers from ω-pentadecalactone have similar properties to low-density or high-density polyethylene [162]. Enzymatic routes were able to produce high molecular weight polymers, whereas chemical routes produced low to medium molecular weight polymers when using, e.g., Y(OiPr)3 or other rare-earth complexes. High molecular weight species were synthesized by using catalysts such as aluminum salen complexes with moderate to high polydispersities [315, 320, 321]. Decalactones are a class of naturally occurring cyclic ω-hydroxy fatty acids from macrocyclic musk that can occur as five (γ-decalactone), six (δ-decalactone) or seven-membered rings (ε-decalactone) [322]. δ-Decalactone was polymerized by using TBD with or without lactide comonomer in bulk leading to polymers with small to moderate dispersities (1.1–1.4) and molar masses up to 35 kg/mol (homopolymers) and 84 kg/mol (copolymers) [121]. In add-on studies, different functional or hydrophilic initiators were used to synthesize functional amphiphilic copolymers from δ-decalactone as drug-delivery systems [323–325]. A similar synthesis strategy was used for homo- and copolymerization of ε-decalactone with lactide in which TBD, Sn(Oct)2 or lanthanum complexes were chosen as catalyst systems [322, 326, 327]. ε-Decalactone induced flexibility to poly(lactide-co-ε-decalactone) materials synthesized with Sn(Oct)2 that also showed immiscibility of the two blocks indicating the formation of thermoplastic elastomers [322, 327]. For more detailed studies

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Scheme 33 Summary of different approaches for the homopolymerization of poly(ethylene brassylate), using organo- [328] or metal-based [329] catalysts

on these naturally occurring macrolactones, we refer to reference [162], a comprehensive review on polymers from macrolactones by Dove et al. Polymerization of an inexpensive and renewable macro(di)lactone called ethylene brassylate was reported in 2014 by Mecerreyes et al. (Scheme 33) [328]. Ethylene brassylate can also be obtained from castor oil and is less expensive than commonly used lactones such as ε-caprolactone or ω-pentadecalactone [330]. Different organocatalysts (DPP, DBSA, PTSA, TCHG, TIPG, and TBD) in combination with benzyl alcohol were tested (Scheme 33). All catalysts were active and generated a controlled reaction, but TBD-catalysis produced polymers with the highest molar masses (up to 13.2 kg/mol), a molar mass distribution of 1.9 and caused the highest polymerization rate. The obtained poly(ethylene brassylate) showed good thermal stability and poly(ε-caprolactone)-like properties as it is a semicrystalline polymer with a slightly higher melting temperature (Tm ¼ 69 C) than poly(ε-caprolactone) (Tm ¼ 56–65 C) [328, 331–334].

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Poly(ethylene brassylate)-co-(ε-caprolactone) random copolymers containing different monomer ratios were synthesized using low TBD concentrations to enable initiation solely by benzyl alcohol to obtain higher molar masses. Polymers with molecular weights between 5 and 10 kg/mol and dispersities between 1.6 and 1.8 were obtained and polymers were analyzed regarding their thermal properties using DSC. Melting temperatures were decreased with higher amounts of caprolactone and were between 39 and 69 C. High enthalpies during melt process indicated a co-crystallization of both monomers [330]. Further studies on ethylene brassylate included increasing the molar mass of poly(ethylene brassylate) homopolymers and copolymerization with δ-hexalactone and D,L-lactide using triphenyl bismuth (Ph3Bi) as catalyst [329, 335, 336].

4 Conclusion The increased interest in bioderived polyesters shows that the academia and the economical world are actively tackling the problem of long-lasting exhaustion of our planet from fossil resources. Within this chapter, we have given a broad overview regarding lactones derived from renewable feedstocks. Lactones are a promising group of monomers due to their abundance, versatility, and broadly distributed feedstock. Terpene-based structures such as limonene and pinenes can be converted to a range of substituted caprolactone derivatives. Limonene has been successfully converted to ()-menthide or dihydrocarvide which both were copolymerized with various lactones using different metal-based or organocatalysts. Block copolymers with rigid lactide blocks functioned as thermoplastic elastomers whereas other copolymers showed promising potential for pressure-sensitive adhesives or biomedical applications. In contrast, the exploration of pinene-derived lactones can still be considered to be in its beginnings. However, the abundance of these feedstocks holds the potential for a large group of materials to be investigated. Sugar-based lactones represent another significant feedstock for future renewable materials. Monomers based on β-methyl-δ-valerolactone and its derivatives which can be converted from glucose-based mevalonic acid are versatile starting materials that can produce a variety of polyesters with different properties depending on the valerolactone substituent. Moreover, advances in sugar-based five-membered lactones such as γ-BL and MBL which were known for being thermodynamically disfavored to polymerize were particularly promising. Very recently, multiple techniques for polymerizing and recycling these polyesters showed the great potential of γ-BL based monomers. With that, another group of abundant monomers was made accessible for polymer and material scientists. However, the gained knowledge about polymerizing five-membered lactones is yet to be fully exploited for, e.g., levulinic acid-based angelica lactones which is still insufficiently investigated. Acid-based monomers like β-malolactonates have been extensively investigated regarding their substitution with a variety of alkyl moieties leading to homo- and copolymers with a broad range of properties. In particular, derivatization to PMLA

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resulted in hydrophilic polymers and is therefore complementing the majority of hydrophobic polyesters produced by ROP. Amphiphilic PMLA copolymers represent a promising class of supramolecular materials. However, MLA-based polymers are yet to be optimized toward isotactic microstructures. Sustainable lactones with larger ring sizes can be derived from fatty acids but also naturally occurring macrolactones such as ethylene brassylate are supplementing the range of renewable starting materials. In recent years, a lot of studies focused on the homo- and copolymerization of, e.g., ricinoleic acid, ωPDL, γ-, δ-, and ε-decalactone. Besides commonly used enzymatic catalysts, synthetic strategies focused on polymerization conditions using metal-based or organocatalysts. Giving their flexible properties, macrolactones hold the potential for elastomeric materials. Considering the continuously growing number of monomers and in particular lactones that can be derived from renewable resources, enormous resources and effort have been put into the investigation of alternative materials. The continuously growing expertise in this field of polymers is going to facilitate the transition to a world economy less dependent on fossil-based resources. Acknowledgement F.A. is funded by the Federal Ministry of Education and Research (BMBF) and the Baden-Württemberg Ministry of Science as part of the Excellence Strategy of the German Federal and State Governments. L.A. thanks the Centre of Doctoral Training in Analytical Science (AS CDT) for a PhD studentship. Additional funding granted by Syngenta. The authors thank Thomas Pehl for the help with this article.

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Adv Polym Sci (2024) 293: 269–304 https://doi.org/10.1007/12_2023_159 © The Author(s) 2023 Published online: 22 September 2023

BioPBS™ (Polybutylene Succinate) Satoshi Kato, Tadashi Ueda, Takayuki Aoshima, Naoyuki Kosaka, and Shigeki Nitta

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 History of Biodegradable Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Why PBS among Aliphatic Polyesters? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Environmental Policy in Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 BioPBS™ and FORZEAS™ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Development of Bio-Based PBS by MCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Comparison of Biodegradable Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Biomass Conversion of Raw Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Development of Biodegradable Polymer Compound FORZEAS™ . . . . . . . . . . . . . . . . . 3 Characteristic Feature of BioPBS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 BioPBS™ (Outline of Bio-Based PBS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Manufacturing Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Basic Characteristics of PBS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Biodegradability and Certification in Various Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Certification of Bio-Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Food Contact Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Molding Processability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Storage Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Examples of Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Related Technological Developments and Topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

271 271 272 273 274 274 275 276 278 279 279 279 282 283 285 285 288 288 290 297 303 304

Abstract PBS (polybutylene succinate) is a biodegradable polymer that is spontaneously degraded into water and carbon dioxide due to the power of microorganisms under soil in the natural world. PBS has high heat resistance and good mechanical properties among general biodegradable polymers, is moldable into the molded

S. Kato (✉), T. Ueda, T. Aoshima, N. Kosaka, and S. Nitta Mitsubishi Chemical Corporation, Tokyo, Japan e-mail: [email protected]

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articles such as films, sheets, and fibers, and has a characteristic feature such that compatibility with other biodegradable polymers is high. Mitsubishi Chemical Corporation (MCC) has successfully achieved commercialization of PBS without using a chain extender in a trademark name of “GS Pla™” and commenced the market development since 2003. “GS Pla™” is completely a biodegradable polymer made of fossil-based succinic acid and 1,4-butanediol as the main raw materials. Since 2017, PTTMCC Biochem, a joint venture established by Mitsubishi Chemical and PTT Global Chemical Public Company Limited in Thailand, has performed continuous commercial production of bio-based PBS (trademark: BioPBS™) made of bio-based succinic acid and fossil-based 1,4-butanediol. Mitsubishi Chemical has also been developing, manufacturing, and selling FORZEAS™ that is a compounding material giving new functions to BioPBS™ utilizing the excellent compatibility with various biodegradable materials and biodegradability of BioPBS™. In this article, basic physical properties, biodegradability, moldability, certification acquisition, and characteristic features of BioPBS™ and FORZEAS™ were reviewed. Keywords 1,4-Butanediol · Bio-based · Biodegradable polymer · Succinic acid

Abbreviations LLDPE PBAT PBS PBSA PBT PCL PE PES PET PHB PHB/V PLA PP PS PVA PVC

Linear low density polyethylene Poly(butylene adipate-co-butylene terephthalate) Polybutylene succinate Poly(butylene succinate-co-butylene adipate) Poly butylene terephthalate Poly(caprolactone) Polyethylene Polyethylene succinate Polyethylene terephthalate Poly(3-hydroxybutylate) Poly(3-hydroxybutylate-co-3-hydroxyvalerate) Poly(lactic acid) Polypropylene Polystyrene Polyvinyl alcohol Polyvinyl chloride

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1 Introduction 1.1

History of Biodegradable Polymer

While natural polymers such as cellulose, proteins, polysaccharide and natural rubber have been utilized from of old by humankind, synthetic polymers were proposed by H. Staudinger about hundred years ago in 1920. Since then, various polymer materials have been invented and put into practical use in the twentieth century, and almost all of them were non-degradable polymers made of raw materials derived from petroleum (Fig. 1). In 1932, DuPont commercialized synthetic rubbers and in 1938, nylon. Regarding the aromatic polyesters, ICI commercialized PET in 1949, and Celanese industrialized PBT in 1971. And DuPont commercialized polyacetal in 1956. Research regarding the aliphatic polyester polymers has a long history. Before the development of PET and PBT, Carothers of DuPont who is famous as the inventor of nylon had performed the research of aliphatic polyesters and reported polycondensation of the aliphatic polyester in 1929 [1]. Later, P.J. Floly successfully increased the molecular weight using an acid chloride as the raw material [2]. However, the aliphatic polyesters were not put into commercial use as the material over a long time because of low heat stability in comparison with aromatic polyesters such as PET and PBT as well as polyamides such as nylon. In points of mechanical properties, the aliphatic polyesters have lower strength than the aromatic polyesters, it was needed to increase a degree of polymerization in order to put them into practical use. Meanwhile, the low heat stability of the aliphatic

1900 Natural Polymers

1950

2000

2050

Petroleum-based synthetic polymers (Polymer Chemistry + Organic Chemistry)

Biomass Polymers (Polymer Chemistry + Organic CHemistry Biotechnology, Fermentation) Application of petroleum-based polymers

Application of petroleum-based polymers

Invention of polymers

P H A (Pastuer Lab.) Aliphatic Polyester Lit.(Carothers) PHB (ICI) PLA Lit. (Carothers) PBS (Showa Denko) Synthetic rubber (Dupont) PLA (Shimazu) Nylon (Dupont) PLA (Mitsui Chemical) P E T (Dupont) PLA (Cargill Dow) P O M (Dupont) PBS (Mitsubishi Chemical) P E (Ziegler,Natta) P I (Dupont) ISBPC (Mitsubishi Chemical) PPO (GE) BioPBS (Mitsubishi Chemical) P B T ( Celanese ) PEEK (ICI)

Fig. 1 History of Polymer Industry

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polyesters made it difficult to increase the molecular weight by means of melt polycondensation reaction at high temperatures. For these reasons, it was forced to adopt a manufacturing method by obtaining a polyester having a relatively low molecular weight to some extent, and then connecting the terminals with each other using a chain extender to increase the molecular weight.

1.2

Why PBS among Aliphatic Polyesters?

Because the aliphatic polyesters have biodegradability and many of them are crystalline, it has been proposed that they will possibly have a potential capable of being processed into the molded articles such as films, sheets, and fibers, if it could be achieved to increase the molecular weight. There are three methods for the synthesis of aliphatic polyester: the polycondensation of an aliphatic dicarboxylic acid and an aliphatic diol, the homopolycondensation of an aliphatic oxycarboxylic acid, and the ring-opening polymerization of an aliphatic cyclic ester. The melting points of various polyesters made of an aliphatic linear dicarboxylic acid and an aliphatic linear diol were summarized in Table 1 [3]. As compared to the melting points of aromatic polyesters, those of aliphatic polyesters are low, and only four types of polyesters having a melting point of higher than 100°C could be made by a combination of oxalic acid or succinic acid with ethylene glycol or 1,4-butanediol. Because the polycondensation using oxalic acid as the raw material is technically difficult for industrial production, polybutylene succinate made of succinic acid and 1,4-butanediol as raw materials, which has highest heat resistance and is advantageous for industrial production, was considered to be the most promising biodegradable polymer. Further, Mitsubishi Chemical has hitherto had manufacturing technologies, technical know-how, and manufacturing equipment for polyesters of an aromatic dicarboxylic acid and ethylene glycol or 1,4-butanediol, such as PET and PBT and thus Table 1 Melting point(°C) of aliphatic polyesters polymerized from linear aliphatic dicarboxylic acids and linear aliphatic diols (Source: Saturated polyester handbook, K. Yumoto, The Nikkan Kogyo Shimbun, LTD)

C number of aliphatic diol

2 3 4 5 6 10 20

C number of aliphatic dicarboxylic acid 0 1 2 3 4 5 159 -22 102 -19 47 25 66 -25 43 35 36 41 103 -24 113 36 58 38 49 -26 32 22 37 39 70 -48 52 28 55 52 76 29 71 55 70 63 88 67 86 77 85 82

Note: C number is “n” in HOOC-(CH2)n-COOH or HO-(CH2)n-OH

6 63 47 43 61 70 86

7 44 46 49 46 52 67 34

8 22 49 64 53 65 71 87

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aimed for and succeeded in industrial production of PBS without using chain extenders.

1.3

Environmental Policy in Japan

As the countermeasure regarding climate change, the movement toward carbon neutrality has been activated. Responsive to this, high goals have been created all over the world, and policies thereof have been publicly announced. In Europe, in order to achieve the goals of the Paris Agreement toward realization of the carbonneutral society, goals were set to make the amount of carbon dioxide emissions substantially zero in 2050, and it has been decided to achieve 55% reduction of carbon dioxide emissions in 2030 from the 1990 level. Also in the USA, after change of government in 2020, goals were created so as to make the amount of carbon dioxide emissions substantially zero in 2050, and in China, goals were declared so as to make the amount of carbon dioxide emissions substantially zero in 2060. Meanwhile, in Japan, immediately after inauguration of the Prime Minister Suga in October 2020, he made the declaration of carbon neutrality and declared that in place of the previous goals for 80% reduction in greenhouse gases, it was aimed to achieve 100% reduction, namely to make the amount of carbon dioxide emissions substantially zero. In receiving the declaration of the Prime Minister Suga, in April 2021, Japan raised the goals for CO2 reduction amount in 2030 from 26% to 46% in comparison with the 2013 year. Further, the Japanese Government formulated the policy and the road map for achieving high goals toward the carbon-neutral society. Concretely, in May 2019, the Japanese Government formulated the circulation strategy for polymers (Fig. 2) and established milestones for reduce, reuse, recycle, and biomass plastics, respectively. In July 2020, a mandatory law for charging for plastic shopping bags that had

Fig. 2 Plastics Recycling Resource Strategy and Milestones

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been free of charge so far in supermarkets and convenience stores was enforced. In this law, marine biodegradable polymers or biomass polymers having a degree of biomass of 25% or more were excluded. As for the conversion of biomass into raw materials, the Japanese Government set highly challenged goals for introducing two million tons of bio-based plastics until 2030. Further, a bill regarding the “Plastic Resource Recycling Promotion Law” was passed in June 2021. According to this law, a guideline regarding the design for environment which manufactures should serve was formulated, and retailers and restaurants, etc. were required to reduce the provision of disposable plastic products. This law focused on the “material” as the plastic and was aimed to promote the resource recycling at each stage of a life cycle from the designing and manufacturing stages of plastics until disposal.

2 BioPBS™ and FORZEAS™ 2.1

Development of Bio-Based PBS by MCC

The progress of the development of PBS at Mitsubishi Chemical was summarized in Fig. 3. In 2003, Mitsubishi Chemical succeeded in the commercial production of the aliphatic polyester, PBS having an increased molecular weight in terms of a trademark “ GS Pla™” without using a chain extender for the first time in the world. The “GS Pla™” in those days was manufactured for biodegradable applications in batch production using fossil-based succinic acid and 1,4-butanediol as the main raw materials [4]. Meanwhile, as described below, since around 2000, Mitsubishi Chemical has attempted to replace chemical raw materials by non-fossil-based resources and started to grapple the development of a group of chemical products satisfying high functionalization simultaneously with CO2 reduction. In the “GS Pla™” business,

Fig. 3 History of PBS development at Mitsubishi Chemical Corporation

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Mitsubishi Chemical proceeded to perform research and development aiming to launch the bio-based PBS business in which the raw material succinic acid is replaced by succinic acid easily derived from the biomass. However, at the time of commencement of the development, the bio-based succinic acid derived from the biomass resource contained a large quantity of impurities inherent to the biomass resource as the polymer raw material, and thus, PBS having a high molecular weight that could be possible for molding and exhibit sufficient mechanical strength was not produced at all. Even when molded, the resulting molded articles were inferior in designability such as poor color tone and many foreign matters. In order to solve these problems, it was essential to combine biochemistry for biomass conversion and chemical engineering for purification technology of the resulting polyester raw materials, with polymer science for high-molecular-weight polymerization using these raw materials and molding of the resulting polyester. Mitsubishi Chemical accomplished conversion technology into a monomer for polyester with economic rationality and biodegradability control technology by biochemistry, efficient purification process with economic rationality by chemical engineering, and molecular weight-increasing technology using bio-based monomers containing specific impurities and molding technology by polymer science [5]. Afterward, Mitsubishi Chemical granted a license of technology to J/V established in 2011 together with Petroleum Authority of Thailand (PTT), designed continuous large-sized full-scale commercial plant (20k Ton/Y) for the first time in the world, and commenced the production of the bio-based PBS (trademark: BioPBS™) made of bio-based succinic acid and fossil-based 1,4-butanediol at PTTMCC Biochem Company Limited since 2017. At the time of 2021, the raw material 1,4-butanediol is derived from petroleum, but Mitsubishi Chemical plans to change it to a bio-based raw material in the near future. When this is realized, the business of all bio-based completely degradable polymers will be operated.

2.2

Comparison of Biodegradable Polymers

BioPBS™ is a polymer that is not only biodegradable but also bio-based. Polymers called as a biopolymer or green polymer are needed to be arranged in terms of two points including the viewpoint of biodegradation and whether the raw material is bio-based or fossil-based (Fig. 4). BioPBS™, PLA, and PHB are not only bio-based but also biodegradable, and bio polycarbonate (e.g., DURABIO™), bio polyethylene, bio nylon, and bioPET are bio-based but non-biodegradable polymers. Further, PCL and PBAT are fossil-based biodegradable polymers. Almost all of polymers which are currently put into practical use are fossil-based and non-biodegradable polymers. Table 2 listed basic physical properties such as thermal properties and mechanical properties of biodegradable polymers and biomass polymers which were put into practical use. PHB and PLA are bio-based polymers and hard-type polymers as in PET, polystyrene, etc., and PCL, PBS, and PBSA are soft-type polymers having

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Bio-based

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Fig. 4 Classification of biodegradable plastics and bio-based plastics

physical properties analogous to high density polyethylene (HDPE), low density polyethylene (LDPE), etc. PBAT is a soft biodegradable polymer having a lower elastic modulus. Because there are a few of different types of commercialized biodegradable polymers, for practical implementation of biodegradable polymers, in order to achieve necessary requirement characteristics, these polymers are often blended and used so as to have optimal properties and optimal biodegradation rate through complementary use or blending. PBS and PBSA are biodegradable polymers having heat resistance and mechanical properties close to LDPE. The grades and basic physical properties of BioPBS™ will be described in detail in another chapter.

2.3

Biomass Conversion of Raw Material

The chemical industry has grown so far in a way that provides safety and security to the society, with the objective of providing technologies for pursuing convenience and comfortability of people through high functionalization of products, as well as achieving energy conservation through ruggedization and weight reduction. Meanwhile, it has become an era such that countermeasures against the climate change with an increase in atmospheric CO2 emissions, and the depletion issue of fossil fuels, in recent years, as well as environmental loads on a global scale, such as a waste plastic issue and a recent marine plastic issue must be taken. However, since most of the current chemical products have largely depended on petroleum as a fossil resource, the chemical industry is required by society to create technologies that eliminate petroleum as a resource. Among such societal demands,

Glass transition temperature Melting point Flexural modulus Tensile strength Tensile elongation at break Izod impact strength

°C °C MPa MPa % kJ/m2

PHB 4 180 2,600 26 1 12 151 1,800 28 16 161

PHB/V

Table 2 Properties of biodegradable plastics and biomass plastics PLA 59 179 3,500 55 2 3

PVA 74 210 – 1 2 13

PCL -60 60 280 61 730 NB

PBS -22 115 640 36 210 10

PBSA -36 84 250 24 380 47

PES -11 100 750 25 500 186

PBAT -30 115 – 25 620 45

LLDPE Y  Sm was observed for rac-β-BL polymerization, which is opposite to the order of ionic radii, but both the ionic radii of lanthanide metals and the initiating RO-group had no obvious effect on the stereoselectivity. All the complexes 10a–c and 10e–f yielded P3HB with moderate syndiotacticity (Pr ¼ 0.82–0.83) and highest Mn can be up to 162 kDa with low dispersity (Ð ¼ 1.24). By changing the initiating groups from alkoxide group to guanidine group (11a–c) [66] or groups derived from p-benzenediol and 1,4-benzenedimethanol (12a–f) [67], it also seemed no obvious impact on the stereoselectivity, resulting in P3HB with Pr ¼ 0.80–0.82. In addition, a series of bis(guanidinate) alkoxide Group 3 metal complexes 13a–c reported by Carpentier and his coworkers were able to promote the stereoselective ROP of rac-βBL to afford syndiotactic P3HB with moderate Pr of 0.80–0.84 through a chain-end control mechanism [68].

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Fig. 8 The structures of complexes 10–13

In general, both the electronic and steric effects of substituents on the ligands play an extremely important role in determining their activities and stereoselectivities. The rare-earth complexes supported by tripodal tetradentate bis(phenolate) ligands are a series of unique catalysts for the synthesis of highly syndiotactic and crystalline P3HB in a controlled manner.

3 ROP of Eight-Membered Cyclic Diolides 3.1

Isotactic PHAs from rac-8DLR

As discussed above, no powerful and efficient catalysts were developed for the ROP of rac-β-BL to synthesize isotactic P3HB with Pm > 0.85 even after ~60-years’ extensive research efforts. Inspired by that the ROP of lactide, a cyclic dimer of lactic acid (LA), can produce high-molecular-weight and highly isotactic polylactide (PLA), Chen group recently developed a new chemical synthesis route via the ROP of a cyclic dimer of 3HB, namely eight-membered cyclic diolide (8DLMe, Fig. 9) to realize the synthesis of perfectly isotactic P3HB [24]. Moreover, the significantly increased sterics present in 8DLMe relative to β-BL may also contribute a higher degree of stereochemical control in the catalyzed ROP of 8DLMe, thereby potentially generating highly stereoregular P3HB materials. The ROP of rac-8DLMe was conducted with La[N(SiMe3)2]3 and discrete yttrium amido complexes 8a–b (a, R ¼ tBu; b, R ¼ CMe2Ph), which have been shown to be effective for the ring-opening (co)polymerization of γ-BL and α-methylene-γbutyrolactone (MBL) and/or the syndiospecific ROP of rac-β-BL [18, 26, 52, 53,

Chemical Synthesis of Polyhydroxyalkanoates via Metal-Catalyzed. . .

O R

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it-PHA tBu

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O (R)

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CPh3 N

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tBu

16

Fig. 9 The synthesis of isotactic PHAs via ROP of rac-8DLR by catalysts 14–16

69]. However, only iso-biased amorphous P3HB with isotacticities Pm ¼ 0.70–0.76 and low molecular weights (Mn ¼ 2.43–2.70 kDa, Ð ¼ 1.08–1.09) was obtained by these catalysts with moderate to low activities. Considering the steric hindrance of rac-8DLMe monomer with the eight-membered-ring framework and the low activity and isoselectivity of also sterically encumbered catalysts 8a–b, the sterically more open yttrium racemic salen complexes rac-14–16 were chosen as the catalysts. Excitingly, complex 14a with tert-butyl substituted group in the presence of BnOH as the initiator can convert rac-8DLMe completely into highly isotactic P3HB with Pm of 0.93–0.94 and [mm] (mm is isotactic triad made up of two adjacent meso diads) of 89% within minutes at room temperature in a controlled manner. The complex rac-14b with electron withdrawing fluoro group showed similar isoselectivity (Pm ¼ 0.95 and [mm] ¼ 88–89%) under similar conditions, indicating the electronic effect is negligible in this case. While the complex rac-14c with bulkier cumyl-substituted group produced P3HB with noticeably higher isotacticity (Pm ¼ 0.96 and [mm] ¼ 93–94%), which suggested that the sterics showed much more pronounced effects. So, the even bulkier trityl substituted complex rac-14d was synthesized. Remarkably, the perfectly isotactic P3HB with Pm > 0.99 and [mm] > 99% was obtained, and the Mn of the resulting P3HB can be up to 154 kDa with low dispersity index Ð ¼ 1.01 (vs. a typical Ð value of ~2.0 for bacterial P3HB), exhibiting a high Tm of 171
C. Additionally, the corresponding lanthanum salen complex rac-15 can also produce perfectly isotactic P3HB with Mn up to 198 kDa and Ð ¼ 1.03 [25]. However, the salph-based complex 16, which was used to probe the possible effects of the geometry of the backbone diimine linker, afforded P3HB with a considerably lower isotacticity (Pm ¼ 0.88 and [mm] ¼ 79%) than salcy-based analogue complex 14a. Additionally, kinetic resolution polymerizations of rac-8DLMe can be realized by an enantiomeric catalyst (R,R)-14d or (S,S)-14d. Moreover, the polymerization automatically stopped at 50% conversion and yielded enantiopure (R,R)-8DLMe or

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(S,S)-8DLMe with >99% e.e. and the corresponding poly[(S)-3HB] or poly[(R)3HB] with high Tm ¼ 175
C and crystallinity, which is essentially identical to that of the commercial natural P[(R)-3HB]. It also confirms this polymerization follows an enantiomorphic site-controlled coordination-insertion mechanism. Up to now, this is the only report to obtain poly[(S)-3HB], poly[(R)-3HB] or perfectly isotactic P3HB through chemical synthesis method, which represents a paradigm shift in the chemical synthesis of P3HB and opens up a plethora of opportunities for discovering new catalysts, materials, and processes in the ROP of rac-8DLMe and other diastereomers of 3HB cyclic dimers. To expand the monomer scope and thus modulate the properties of PHAs, the ROP of two longer alkyl substituted monomers, rac-8DLR (R ¼ Et, Bu), and benzyl substituted monomer, rac-8DLBn, was also realized readily accessible racemic metal-based catalysts, producing PHAs with high to quantitative isotacticity [25, 70, 71]. Notably, the sterically encumbered catalyst rac-14d, which exhibited extremely high activity and isoselectivity toward ROP of the smallest rac-DLMe, showed only marginal activity for the ROPs of rac-8DLEt and rac-8DLBu, whereas the less bulky cumyl-substituted rac-14c exhibited both high activity and isoselectivity for these two bulkier monomers, producing P3HV (R ¼ Et) with Pm ¼ 0.97 ([mm] ¼ 95%) and Tm ¼ 108
C, and P3HHp (R ¼ Bu) with Pm ¼ 0.97 ([mm] ¼ 94%) and Tm ¼ 50.8
C (Tg ¼ 34.3
C). Additionally, ROP of rac-8DLBn with rac-14c produced highly isotactic poly(3-hydroxy-4phenylbutyrate) (it-P3H4PhB) with [mm] > 99%, which is unnatural aromatic PHA, and displayed an endotherm peak at Tm ¼ 126
C on the DSC’s first heating scan when the sample was allowed to slowly crystallize from chloroform overnight [71]. All the results suggested that the catalyst/monomer steric matching is critical for achieving a highly active and stereoselective ROP of rac-8DLR. To synthesize crystalline isotactic PHA copolymers with potentially polyolefinlike thermal and mechanical properties, the copolymerization of rac-8DLMe with rac-8DLEt or rac-8DLBu was carried out using rac-14c under different conditions (Fig. 10) [70]. The high-molecular-weight (> 100 kDa) PHA random copolymers P3HBV or P3HBHp with various levels of rac-8DLEt or rac-8DLBu incorporation were produced. The Tm of P3HBV or P3HBHp decreases with the increase of rac8DLEt or rac-8DLBu incorporation, but they all exhibit similar thermal degradation O

O

O

it-PHA Copolymer

O +

O

O

O

O R

[S,S-Cat] O

O

R

[R,R-Cat]

O

+ O O

R rac-8DLR O

O

O O

O

R

O O f

O

rac-8DLMe O

O

O

O

R

O

R

O

O 1-f

R

O 1-f

O

O Of

R

P3HBV: R = Et; P3HBHp: R = Bu

Fig. 10 Stereoselective copolymerization of rac-8DLMe with rac-8DLR (R ¼ Et, Bu) to isotactic random copolymers P3HBV (R ¼ Et) and P3HBHp (R ¼ Bu)

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319

temperature of ca. 250
C. Notably, both P3HBV with rac-8DLEt incorporation of 19.8% and P3HBHp with rac-8DLBu incorporation of 19.6% are hard, ductile, and tough plastics having polyolefin-like thermal transition temperatures and mechanical properties, with an ultimate tensile strength (σ) of 25.0  0.2 MPa and 20.5  0.4 MPa, Young’s modulus (E) of 669  45 MPa and 226  9 MPa, and elongation at break (ε) of 374  19% and 578  15%, respectively.

3.2

Syndiotactic PHAs from meso-8DLR

Owing to the complete enantioselectivity of the (R,R)-catalyst for addition of (S,S)8DLMe and the (S,S)-catalyst for addition of (R,R)-8DLMe, these racemic catalysts should also be stereoselective toward meso-8DLMe, as the (R,R)-catalyst should selectively ring-open meso-8DLMe at the (S)-site while the (S,S)-catalyst should selectively cleave the ester bond of meso-8DLMe at the (R)-site, thus affording stP3HB [25]. Among these catalysts, the lanthanum complex rac-15 exhibited both good activity and high syndioselectivity toward the ROP of meso-8DLMe (Fig. 11), affording st-P3HB with a high Tm of 170
C and Pr~0.92. However, the activity of meso-8DLMe polymerization is still much lower than that of rac-8DLMe polymerization. Additionally, ROP of meso-8DLBn by yttrium catalyst rac-14c can afford syndiotactic P3H4PhB with a high syndiotacticity of [rr] (rr is syndiotactic triad made up of two adjacent rac diads) ¼ 92% and Mn up to 147 kDa [71].

3.3

Stereosequenced PHAs from Diastereomeric Monomer Mixtures 8DLR

Due to the much higher activities of rac-8DLMe polymerization than meso-8DLMe by rac-14d and rac-15, these two complexes should exhibit diastereoselectivity between rac-8DLMe and meso-8DLMe in this enantiomorphic site-controlled ROP. Moreover, they were to mediate living and stereoselective polymerization of racand meso-8DLMe to it- and st-P3HB, respectively, while exhibiting both high enantioselectivity between the rac-8DLMe stereoisomers. Therefore, ROP of the diastereomeric mixture (1/1 rac/meso 8DLMe) in a one-pot fashion was carried out by rac-14d and rac-15, in which system, the (R,R)-catalyst would selectively polymerize (S,S)-8DLMe, whereas the (S,S)-catalyst would polymerize (R,R)8DLMe [25]. When rac-8DLMe was almost consumed, chain propagation would Fig. 11 The synthesis of syndiotactic PHAs via ROP of meso-8DLR

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O O (R)

(R) O

O

O O O (S)

O (R)

Diastereomeric Mixture

+

O (R)

(S) O

O

(S) O

DiastereoSelective

O (R)

O (R)

O (S) O x

O

O

O (S) O y

O

Syndiotactic block

Isotactic block

[CAT]

EnantioSelective

Same or Different Diolides

O

O (R) O

Syndiotactic block

Isotactic block

O (S)

O (S) O

O (S) O

O (R) O x

O (S) O

O (R) O

O y

Fig. 12 The diastereo- and enantioselective ROP of a diastereomeric mixture of rac/meso-8DLMe into tapered it- and st-stereodiblock P3HB, it-sb-st-P3HB

O O (R)

(R) O

O +

O

O (S) O

rac-8DLEt

(S) O

O +

O (R)

(S) O

Syndiotactic-rich

Isotactic-rich

Syndiotactic-rich

Isotactic-rich

[cat]

O meso-8DLMe

Fig. 13 The synthesis of stereogradient block copolymer st-P3HB-sb-it-P3HBV from the copolymerization of meso-8DLMe and rac-8DLEt

cross over to addition of meso-8DLMe to form tapered stereodiblock P3HB, it-sb-stP3HB (Fig. 12). The tensile testing of dog-bone-shaped stereotapered it-sb-st-P3HB by rac-14d yielded σ ¼ 9.7  0.5 MPa, E ¼ 317  39 MPa, and ε ¼ 17  5%. Thus, the formation of it-P3HB-sb-st-P3HB enhanced the ductility by about a factor of 6 relative to it-P3HB (ε ¼ ~3%). To further enhance the ductility, and thus the toughness, of the stereoblock P3HB materials, copolymerization of a diastereomer of 8DLMe (rac or meso) with a diastereomer of ethyl-substituted 8DL (8DLEt) was investigated, with the aim of generating a PHA as 8DLMe/Et copolymer – namely, poly(3-hydroxybutyrate-co-3hydroxyvalerate) (P3HBV). It turned out that copolymerization of meso-8DLMe and rac-8DLEt with 1/1 by a selective catalyst rac-14c in THF led to gradient stereoblock copolymer st-P3HB-sb-it-P3HBV (consisting of the syndio-rich P3HB block with some rac-8DLEt incorporation and the iso-rich P3HBV block) according to the scenario outlined in Fig. 13. Additionally, the same copolymerization of meso8DLMe and rac-8DLEt in a 4/1 ratio produced a high-molecular-weight crystalline st-P3HB-sb-it-P3HBV (Mn ¼ 113 kDa, Đ ¼ 1.27, Tm ¼ 135
C, Tg ¼ 1.7
C, 18% 8DLEt content), which yielded impressive mechanical properties with σ ¼ 24.1  1.5 MPa, E ¼ 169  9 MPa, and ε ¼ 564  25%, thus revealing a strong, ductile, and tough material. On the basis of the established reactivity trend of rac-8DLMe > rac-8DLEt, meso8DLMe > meso-8DLEt, and the catalyst’s enantioselectivity toward the rac monomers and diastereoselectivity toward the rac/meso isomers, copolymerization of all

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Fig. 14 The synthesis of stereoblock copolymer it-P3HBV-sb-st-P3HBV from copolymerization of rac/meso-8DLMe/8DLEt

diastereomers of 8DLMe/Et (in a 1/1/1/1 ratio) together was performed to synthesize stereosequenced block copolymers of 8DLMe/Et, it-P3HBV-sb-st-P3HBV consisting of iso-rich P3HBV and syndio-rich P3HBV blocks (Fig. 14), which is the most convenient yet most challenging way. The catalyst rac-15 can afford semicrystalline it-P3HBV-sb-st-P3HBV with Mn ¼ 19.2 kDa (Đ ¼ 1.07), 8DLEt incorporation ¼ 20 mol%, and two Tm values of 125
and 110
C. Moreover, the n-butylsubstituted 8DL compounds rac-8DLBu and meso-8DLBu were copolymerized with rac-8DLMe and meso-8DLMe (1/1/1/1 ratio) by catalyst rac-15 (0.5 mol%, 1 equiv. BnOH) in CH2Cl2 at room temperature, affording semicrystalline PHA as block copolymer it-P3HBHp-sb-st-P3HBHp [P3HBHp ¼ poly(3-hydroxybutyrateco-3hydroxyheptanoate)] with Mn ¼ 20.4 kDa (Đ ¼ 1.10), 8DLBu incorporation ¼ 15 mol%, and two Tm values of 127
and 105
C. Generally, enantiomorphic site-controlled diastereoselective polymerization methodology was developed to synthesize stereosequenced crystalline PHAs with isotactic and syndiotactic stereodiblock or stereotapered block microstructures via direct polymerization of the mixtures of eight-membered diolide (8DL) monomers in one-pot fashion, which can avoid substantial material loss and added energy cost associated with the separation and purification process. The material properties can be tuned by varying the catalyst and monomer structures and the ratio of starting rac/ meso diastereomers. It represents a paradigm shift in the chemical synthesis of crystalline polymers.

4 Summary and Outlook Plastics are the most widely used man-made substances, due to their light weight, high strength, low toxicity, transparency, resistant to corrosion, and durability. However, their durability, one of plastic’s greatest assets, also resulted in the tremendous growth of disposed plastics waste, bringing about severe environmental consequences, especially the ocean pollution. PHAs, one type of the biodegradable and sustainable biopolymers, are an ideal alternative to petroleum-based plastics.

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The key to chemically synthesize PHAs with similar thermal and mechanical properties to polyolefins, the largest class of commodity thermoplastics, is stereospecific polymerization of cyclic esters. After over 60 years’ efforts made by scientists, two strategies have been developed. One strategy is ROP of four-membered β-lactone. The high ring strain of the fourmembered lactone makes the ROP of β-BL take place easily. The ROP of enantiopure monomers (R)-β-BL or (S)-β-BL can afford isotactic P3HB easily, but the control on the stereoselectivity of the ROP of much cheaper racemic monomer rac-β-BL is still a challenge. Whereas the isoselective ROP of rac-β-BL by various classes of chiral or achiral catalysts afforded isotactic P3HB with only modest isotacticity (up to 85%) and Tm values (~142
C), the syndioselective ROP of racβ-BL by discrete yttrium complexes supported by tetradentate, dianionic alkoxyamino-bis(phenolate) [O,N,O,O] ligands can produce highly syndiotactic P3HB with Pr up to 0.95. Although ROP of the racemic four-membered lactone can give highly crystalline syndiotactic P3HB with Tm~180
C, the other stereoregular PHAs have not been reported through this method. Therefore, more efficient catalysts for the ROP of four-membered lactones are highly demanded to synthesize PHAs with both high stereoregularity and high molecular weight. The other strategy is ROP of eight-membered diolides (8DL). The stereoselective ROP of rac-8DLMe produced P3HB with perfect isotacticity (>99%) and high Tm (171
C), which is the only chemical synthesis pathway to achieve perfect isotactic P3HB up to now. The kinetic resolution polymerizations of rac-8DLMe yielded enantiopure (R,R)-8DLMe or (S,S)-8DLMe with >99% e.e. and the corresponding poly[(S)-3HB] or poly[(R)-3HB] with high Tm ¼ 175
C and crystallinity, which matched naturally produced bacterial P3HB. The ROP of other racemic cyclic diolides with different substitutes (rac-8DLR) can produce isotactic P3HV (R ¼ Et), P3HHp (R ¼ Bu), and P3H4PhB (R ¼ Bn), implying ROP of 8DL may be widely applied in preparation of PHAs. The copolymerization of rac-8DLMe with rac-8DLEt or rac-8DLBu gave isotactic copolymers P3HBV or P3HBHp having polyolefin-like thermal transition temperatures and mechanical properties. Additionally, stereoselective ROP of meso-8DLMe produced highly syndiotactic P3HB with Pr~0.92 and Tm of 170
C, and meso-8DLBn polymerization can afford P3H4PhB with a high syndiotacticity of [rr] ¼ 92% and high molecular weight of Mn ¼ 147 kDa. Notably, diastereoselective polymerization methodology was developed to synthesize stereosequenced crystalline PHAs via direct polymerization of the mixtures of 8DL monomers in one-pot fashion. However, the atom economy of current 8DLR monomer synthesis route is low, and thus the development of economically more-viable preparations of the 8DLR monomers from inexpensive building blocks ideally from the chemicals resultant from deconstruction or degradation of PHAs toward a circular life cycle is desired. In general, catalyst/monomer steric and/or electronic matching is critical for achieving a highly active and stereoselective, and thus the key factor to chemically synthesize stereoregular PHAs with high molecular weight is the design and synthesis of the catalytic systems, which can be tuned by metal centers and the type of the ligands (including the steric and electronic effects of substituents on ligands).

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Therefore, novel catalysts should be developed to produce stereoregular PHAs with comparable thermal and mechanical properties to polyolefins. Additionally, the cost of the building blocks or monomers is another concern for industry, and future efforts should also be continuously devoted toward the development of more costeffective biomass conversion processes so that PHAs would become more economically competitive compared with the petroleum-based plastics.

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Adv Polym Sci (2024) 293: 327–396 https://doi.org/10.1007/12_2021_112 © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 Published online: 27 February 2022

Biobased Polyamides: Academic and Industrial Aspects for Their Development and Applications Matthias Ullrich, Frank Weinelt, and Malte Winnacker

Contents 1 Introduction: Structure of Polyamides and Their Possible Raw Material Resources . . . . . . 1.1 Polyamides: General Considerations (Why Polyamides?) and Basic Facts . . . . . . . . . 1.2 Why Biogenic Resources? Aspects of Sustainability and Structure Elements . . . . . . 1.3 Biogenic Raw Material Sources for Polyamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Examples from Nature (Polyamides Based on α-Amino Acids) . . . . . . . . . . . . . . . . . . . . . 1.5 Definitions of “Bio”-Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Bio-Based Building Blocks for the Synthesis of Polyamides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Structure Elements that are Accessible from Natural Compounds . . . . . . . . . . . . . . . . . . 2.2 Sources of Raw Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Target Compounds: General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 General Remarks on Reaction Parameters for the Polyamide Synthesis . . . . . . . . . . . . 3 Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Structure, Properties, and Applications: General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Stage of Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Biopolyamides via Polycondensation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Biopolyamides via Ring-Opening Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Challenges for the Production and the Application of New Biobased Polyamides . . . . . . . 7 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

M. Ullrich and F. Weinelt (*) Evonik Operations GmbH, Division Smart Materials, Business Line High Performance Polymers, Marl, Germany e-mail: [email protected]; [email protected] M. Winnacker (*) WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universität München, Garching bei München, Germany Catalysis Research Center (CRC), TU München, Garching bei München, Germany e-mail: [email protected]

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Abstract Polyamides are very important polymers for a wide range of applications. In the context of Green Chemistry and the development of sustainable polymers from renewable resources, many polyamides have meanwhile been developed that are derived from natural building blocks. In addition to sustainability, biobased polyamides can have special structures and properties that cannot be obtained so easily via fossil-based pathways. This article gives an overview over the recent developments in this field and elucidates the potential of these polymers for different applications. Keywords High-performance polymers · Polyamides · Polycondensation · Ringopening polymerization · Sustainable polymers

1 Introduction: Structure of Polyamides and Their Possible Raw Material Resources 1.1

Polyamides: General Considerations (Why Polyamides?) and Basic Facts

Polyamides (often abbreviated as PAs) constitute an established, wide class of petroas well as biobased polymers. The total production volume of biobased polymers was about 4.2 million tons in 2020. That is approximately 0.038% of the world’s biomass production. The arable land needed for this is 0.006% of the globally available. About 0.22 million tons are related to polyamides. A growth by about 36% is expected for polyamide until 2025 [1]. Technical polyamides are counted among the high-performance polymer materials. The regular repetition of the polar carboxylic acid amide group in the polymer chain is the reason of their outstanding material properties. Despite low average molecular weights of 20,000 to 30,000 g/mol for polyamides that have been hydrolytically polymerized, they exhibit excellent mechanical properties and chemical resistance, as well as a high dimensional stability under heat and good electrical properties. The main reasons for these properties are the formation of hydrogen bonds between adjacent polymer chains and their amorphous-crystalline character. While the crystalline parts increase the chemical resistance, amorphous regions allow the formation of a flexible tertiary structure that can absorb mechanical energy and repair microstructural defects after mechanical stress to some extent by the reformation of hydrogen bonds [2]. Polyamides (precisely, homopolyamides of the AB-type) can be prepared by ring-opening polymerization (ROP) of the corresponding lactams, or by polycondensation of amino acids (where in particular, w-amino acids are being used). Alternatively, the condensation of organic diamines with dicarboxylic acids or their reactive derivatives (e.g., methyl esters, acid chlorides, dinitriles, diisocyanates) leads to the structurally related AABB-type polyamides (copolyamides). Due to the

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Fig. 1 General polyamide structure of the AB- and the AABB-type

increasing importance of sustainable chemistry, also the research on biobased PAs is becoming more and more important (Fig. 1) [3]. However, there are also a series of examples in which a-amino acids and even not biobased b-amino acids or g-amino acids were used for the synthesis of polymers. In difference to the first group, these polymers are rather called peptides or polypeptides. They find many applications e.g., in biotechnology and medicine and in some cases, form interesting structures referred to as “foldamers.” However, going beyond the scope of this article, these important polymers and concepts have been reviewed in detail [4, 5], as have been the (biomimetic) polyamides that are used as artificial transcription factors as novel tools in molecular medicine [6]. Generally, the importance of e.g., vegetable proteins (soy or animal source) such as gelatin (of which in 2019, ca. 620 kt were sold) and the mechanical properties of spider silk (a natural polypeptide) demonstrate the significance of natural proteins also for technical applications. The wide range of different monomers, the possibility of variation of the monomers among themselves, including the synthesis of hybrids of the AB/AABB systems allows a wide variation of the property profiles of the polyamides to adapt them to the intended applications. In addition, polyamides have been developed that have additional structural elements, such as polyether segments or polyamide imide structures [7]. A key feature of polyamides is the formation of hydrogen bonds, which contribute chiefly to how the polymer chains arrange and how the material crystallizes, which influence (i.e., increase) the melting point, and which are the site where water equivalents bind i.e., to the carbonamide groups. Thus, it is not surprising that modifications have been synthesized in which polyamides were modified on their nitrogen atoms to enable specific applications [8, 9]. The investigation of the crystallinity by e.g., X-ray diffraction, is an important part of the analytical chemistry of polyamides [10, 11]. Polyamide 66 (also coined Nylon 6,6 or simply Nylon; Scheme 1a) was developed by W. Carothers and his team at DuPont [12–14], and polyamide 6 by Paul T. Schlack at IG Farben AG (polycaprolactam, Nylon-6 or Perlon; Scheme 1b) [15]. The molecular weight of typical commercial PA6.6 can be roughly specified – depending on the provider and further factors – in the range Mn ¼ 15,000–55,000 g/ mol [16]. As discussed also below, there are many similar, but also different polyamides that differ from the “classical” polyamides in terms of chain structure and/or

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Scheme 1 Synthesis of the “classical” polyamides with which it all once started: A) PA6.6 (Nylon6,6, Nylon) and B) PA6 (Nylon-6, Perlon)

Scheme 2 General structures and approaches for the synthesis of biobased polyamides. Reproduced with permission from reference [3]

segments and side groups. Furthermore e.g., many dendritic and hyperbranched polyamides have been described for a variety of applications [17]. Generally thus, polyamides have grown to become one of the most versatile and important polymer classes [18–21]. In general, also biobased polyamides can be synthesized via polycondensation of dicarboxylic acids and diamines or via ROP of lactams. As shown later, many such procedures have been described that are based on a variety of building blocks. Also, amino acids can be employed alike and furthermore, catalytic amidation of diamines with diols has been shown as a further synthetic alternative (Scheme 2).

1.2

Why Biogenic Resources? Aspects of Sustainability and Structure Elements

Recall that the first man-made polymer materials were all based on natural materials! These “old bioplastics” stemmed from e.g., casein, celluloid, cotton, or natural rubber, simply as the access to petroleum-based materials was not yet developed

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in those days. With the chemical industry strongly developing from the middle of the twentieth century i.e., scaling to large outputs and streamlining/standardizing its processes, raw material choice was then – with only a few exceptions – switched to petrochemical sources. Today however, bioplastics enjoy a renaissance of public and industrial attention for environmental and economic reasons, as described below. These “new” bioplastics can be classified into those with new chemical structures (e.g., polylactides or PLAs) on the one hand, and the so-called drop-ins i.e., polymers with established chemical structures, but now biobased (e.g., biobased polyethylene, bio-PET, etc.). The use of renewable resources for the manufacture of chemical products is highly significant for two reasons: (a) It ensures a long-term supply independent of fossil raw materials, and (b) the use of renewable resources is an essential prerequisite for a closed-carboncycle economy which, in conjunction with the use of renewable energies, could yield an overall environmentally benign carbon footprint. However, the use of biogenic raw material sources as the key toward a sustainable economy would not suffice – there are further challenges that need to be mastered. On the one hand, these include aspects of the basic principles of green chemistry [22–26]. On the other hand, with the special focus on polymers, the high-quality recycling of polymers will increasingly gain importance and likely may constitute a significant part of an overall closed-carbon-cycle economy. The composition of the respective biogenic raw material sources also opens up the challenge of utilizing all those components of specific sources as broadly as possible. In other words, changing the use of today’s coupled and/or even waste streams into becoming valuable sources for further material utilization is mandated. Current studies on the use of lignins or terpenes, which are described below, give an idea of where that journey may be taking the scientific and industrial community in the time ahead. The industrial interest of monomer synthesis, especially of the C6 building blocks adipic acid, caprolactam, 6-aminoadipic acid, based on renewable raw materials is increasing. Process developments include biotechnological processes, mixed processes of biotechnological and chemical steps, but also the design of suitable enzymes necessary for the realization of metabolic pathways to the desired end products. Examples include the former US startup Verdezyne [27] who described genetically engineered Candida yeasts that could produce adipic acid from vegetable oils, and the company Genomatica [28] which has developed proprietary processes to make 6-aminocaproic acid, caprolactam and adipic acid, respectively, via fermentative routes or mixed process steps. In a similar approach, Celexion [29] has claimed biotechnological process routes for C4 to C8 building blocks, in particular the C6 ones adipic acid, 6-aminocaproic acid, caprolactam, and 1,6-hexamethylene diamine. Though the focus is typically on an improved eco-balance [30, 31] compared to petrochemical raw material sources, there is another argument to consider renewable raw materials: It is the fact that nature can provide a variety of starting materials with

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certain chemical structural features e.g., side groups, carbocycles, stereocenters, etc. which cannot be obtained so easily via fossil-based pathways. Access to biopolymers is possible, on the one hand, by utilizing (and, optionally, modifying) natural polymers such as cellulose [32] or polyhydroxyalkanoates [33] (which can also be produced synthetically [34] by the way). On the other hand, many classes of monomers (and/or their precursors further upstream) based on renewable sources exist that can be polymerized in controlled fashion. One approach in recent literature to group the sheer plethora of all these monomers has been by looking at the ratio of carbon/hydrocarbon vs. heteroatom content of such biomass: This is handy as it also guides toward the associated chemical modification and/or polymerization processes possible per such group. In this sense, four groups of monomers/precursors have been put forth, being first oxygen-rich biomass (e.g., carboxylic acids, sugars, polyols) and the related nitrogen- and/or sulfur-rich [35], second hydrocarbon-rich biomass (e.g., vegetable oils, terpenes and terpenoids, lignin), third pure hydrocarbon biomass (e.g., ethene from bioethanol, propene), and fourth pure carbon i.e., non-hydrocarbon biomass (CO, CO2) [36, 37]. Within this overall scope of biopolymers – and for the sake of expanding its boundaries – fundamental research continuously explores the next possible, while applied research considers the applicability and economic use of the respective materials and approaches. From these joint efforts, examples have and will continue to emerge, where sustainable polymers exhibit superior performance compared to their petrochemical choices [38, 39]. The utilization of renewable platform chemicals for polymer synthesis is thus becoming more and more important even beyond ecological arguments i.e., chosen for economic reasons [40–42].

1.3

Biogenic Raw Material Sources for Polyamides

The manufacturing of chemical products from biomass is not new; today, it has two main objectives: (a) Improving the carbon footprint of these products, and (b) developing alternative raw material sources in order to mitigate or even eliminate the dependence on fossil raw materials. Furthermore, fundamental research on biobased products is driven by the interest in new structures and their effect on the resulting material property profiles, as well as by the motivation to explore new concepts and correlations. In turn, these can then “feed” the industry with ideas what could be worth to be further pursued and ultimately developed into future innovations. Due to the material diversity of natural raw material sources, the challenge to the industry at using these sources frugally is that of as many of the associated components as possible shall be utilized, ideally leaving no unused residual material stream as waste (examples: raw material source is a mixture of components whereof only one is the key raw material molecule; chemical processing of raw material produces

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Table 1 Selected, popular polyamides and some of their characteristic properties [3, 15] Polyamide PA6b PA6.6b PA6.10 PA10.10 PA11 PA12

Biosourcing [%] 0/100 0/100 63 100 100 Variablec

Tg [ C] 47 50 48 37 42 38

Tm ( C) 218 258 206 191 183 176

Modulusa [MPa] 3,000 2,500 2,100 1800 1,100 1,400

a

Moduli given as range averages of pertinent literature values, and for un-conditioned matter i.e., dry. Note that, with decreasing length of the average oligomethylene chain of a given PA (“less lipophilic”), water uptake (in wt.%) increases sharply, and short-chain PAs such as PA6 or PA6.6 show levels around 10% w/w water uptake possible, or even beyond. At such levels, wet-conditioned PAs show sharply decreased moduli – a “conditioning effect” much less visible with long-chain PAs such as PA11 or PA12, which represents one of their most prominent application advantages b “Classical” Route or biobased route. Also biobased routes are investigated c A bio-route has been developed by the industry but not yet commercialized

an “unwanted” coupled product). This challenge thus is associated with the development of case-specific, efficient process technologies for tapping these sources [43]. On the one hand, this scenario as pitched should trigger improved profitability of using these raw material sources. On the other hand, the generation of disproportionately large quantities of waste would be contrary to the principles of establishing a truly (at least largely) sustainable economy. And, to stay true to the goal of overall sustainability of one’s industry/economy, this also means that even biogenic waste materials from existing processes, such as pulp production, should be scrutinized as raw material sources. And thinking even a step further, one will quickly discover that in addition, biogenic raw material sources allow access to chemical structural elements of nature that are not readily obtainable from fossil raw materials. This opens up playroom for new materials not available before. Summing all of these statements up is how the approach toward the use of biomass for polyamides should be understood and engaged in. There are many different biogenic resources available for the preparation of biobased polyamides, which include fats and oils [44, 45], carbohydrates, terpenes, and lignins. While vegetable oils as well as sugars and starch are already used on an industrial scale to produce different monomers, terpenes and lignins are – with some exceptions – mainly still at the research and development stage as raw material bases for large-scale polyamide production. PAs that consist of several components can thus also be partially bio-sourced (see Table 1, together with selected pertinent properties) [15]. For some PAs, different production routes may exist (see later). The melting points and moduli correlate – as expected – with amide bond density (see table legend for further details). Interestingly as an exception, PA12 has a slightly higher modulus than PA11, which indicates that also other factors play a

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role in this context (e.g., van der Waals interactions, space group/intermolecular orientation and packing in the solid state, to name a few).

1.4

Examples from Nature (Polyamides Based on α-Amino Acids)

Naturally occurring polyamides are widespread throughout flora and fauna and are even found in microorganisms. With a few exceptions, they comprise of a-amino acids. They are called proteins if the polymer chains consist of more than 100 amino acid units. Short-chain derivatives are called peptides. A peptide bond is an amide bond of two amino acids. The characteristic chemical structural element of peptides and proteins is the backbone formed from levorotatory L-a-amino acids. The order of the amino acids and the chemical nature of the residues R define the wide variety of the proteins. In addition to some other amino acids, all 20 amino acids found in the genetic code can be involved in the construction of proteins (Fig. 2). Peptides and proteins play a major role in living nature, serving many vital functions [46]. Some of these proteins have been harnessed by humans in technical applications. Among the biologically active proteins, enzymes are being used, for example, in food production (cheese-making), in detergents, or – in specifically designed form (“metabolic engineering”) – in the fermentative production of chemicals [47]. Scleroproteins (also called scaffold proteins e.g., elastin and keratin) have a supporting and protective function in the animal organism. They are found as collagen in connective tissue, as keratins in hair, wool, horns and feathers, or as fibroins in silks. Gelatin is obtained from collagen, which is widely used in food and luxury food production but is also still used in exposure material for analog photography, photographic papers, or printing papers [48]. Wool has a cultural-historical significance for the production of textiles. The protein found in wool is the fibrous a-keratin. It is very elastic due to the disulfide bridges it contains and can absorb high amounts of moisture. Due to its airy structure, it is also in use as an insulating material [49]. Completing all this, silk proteins consist of a high proportion of b-keratin: A beta-sheet-block structure complemented by hydrogen bonding and hydrophobic interactions results in fibers that resist high tensile forces but have little

Fig. 2 General structure of proteins and peptides

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elasticity. These proteins are formed by butterfly caterpillars, spider [50], and mussels [51]. Silks, especially those from the cocoons of the mulberry silkworm, have a long tradition and are still of great economic importance for silk yarns and silk fabrics [52]. The top ten producers of natural silk manufactured more than 410 kt of silk in 2005 [53]. Beyond, spider silk keeps drawing in the particular fascination of technicians because of its high tensile strength and elasticity at very low weight [54]. Plant proteins, such as soy protein, have a great importance for food and feed production. In the 1940s, a composite material made from 50% soy protein, 50% glucose, and formaldehyde was used for automotive body parts and interiors. Today, these proteins have come back into technical interest. Soy proteins are used as binders for gluing wood and coating paper. The use for biodegradable packaging materials is being investigated [55].

1.5

Definitions of “Bio”-Terms

The description of plastics in terms of their material origin – in respect of necessary chemical modifications of primary starting materials via monomers to the polymers, and their behavior in the environment after finished use – has led to a terminology diversity which, although largely defined by standards [56], is still prone to produce misunderstandings. Thus here, to facilitate and to avoid unnecessary complexity that may lie in variances of existing standards, the most important terms have been defined by closely following the Oxford Dictionary of Biochemistry and Molecular Biology [57] as follows. • biobased: biogenic, renewable materials. For example: monomers are biobased raw materials when they are made of biogenic materials such as wood, sugarcane, or oil plants • biogenic: produced or brought about by living organisms without the contribution of technical procedures • biocompatible: no detrimental influence on living organisms • biodegradable: breakdown of materials by environmental conditions (such as air, light, water, and microorganisms) to carbon dioxide, water, methane, and biocompatible substances (of higher remaining chemical complexity) that can be incorporated back into biomass during metabolism; after 6 months, 90% of the organic material must have been converted to CO2 [58]; it does not matter whether the materials originated from renewable or fossil raw materials • biomass: natural substances formed by the growth of plants, animals, or microorganisms, and which can be utilized industrially • biopolymer: biomass consisting of a substance or mixture of substances that are macromolecules of repeating monomer units • biosynthesis: production of substances by living organisms including fermentative processes

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• biotransformation: synthesis of a chemical compound from any starting material by a living organism, including fermentative processes

2 Bio-Based Building Blocks for the Synthesis of Polyamides The chemical structure of industrially used polyamides has resulted, on the one hand, from the requirement profile of the application areas and, on the other hand, from the available raw material sources. Essential structural carbon units are aliphatic and cycloaliphatic carbon chains, which can be substituted by methyl side chains. In addition, aromatic repeating units are also used. Examples are shown in Fig. 3. A reconstruction of the same or similar structural elements from biogenic raw material sources is possible and is already used in parts today. Polyamide 11 is an example of an industrial polyamide that has been exclusively produced from the biogenic resource castor oil right from the start (see Sect. 5). In addition, due to the biosyntheses of primary and secondary metabolism established in nature, natural raw material sources offer structural elements that can only be obtained from petrochemical sources with considerable effort.

Fig. 3 Examples for industrially used polyamides and their building blocks

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Structure Elements that are Accessible from Natural Compounds

The metabolic pathways in nature have a decisive impact on the structure of the resulting and then utilized building blocks. Their elucidation would be beyond the scope of this article, but selected aspects will be mentioned in the following sections as required. In general, there is a wide variety of structural elements accessible in nature. This includes linear, branched, alicyclic and aromatic moieties, as well as stereocenters, and is enabled by the function and selectivity of many enzymes. While the synthesis of new stereocenters in organic chemistry is often laborious, the utilization of the natural pool can be very expedient in many cases.

2.2

Sources of Raw Materials

The use of biogenic raw material sources for the synthesis of monomer building blocks for the polyamide synthesis requires chemical modifications, to obtain bifunctional compounds preferred as dicarboxylic acids, diamines, or amino carboxylic acids. Several routes and processes have already been developed on a large scale and show the potential of renewable resources. Nevertheless, an overview of the basic biogenic material classes seems helpful at this point, because the variety of biobased structural elements not only makes it possible to replicate monomers from the petrochemical basis, but also enables the exploitation of interesting new structural elements. This could allow modifying the properties of base polymers to approach additional or new requirements of target applications. From today’s perspective, the most important biobased sources are: • • • •

Fatty acids and fatty alcohols from oils, fats, and waxes Monosaccharides from carbohydrates Terpenes from the class of isoprenoid substances Phenolic building blocks from lignin

Fats and oils provide an extensive feedstock. For instance, ricinoleic acid, available from castor oil after removing of glycerol, is a fundamental compound for two important pathways: One of them proceeds via the platform intermediate 10-undecenoic acid and then 11-aminoundecanoic acid to PA11, the other pathway runs via sebacic acid to the PAx.10 and PA10.x series. Carbohydrates are investigated as one main sources for biobased polyamides. For instance, biobased butadiene which is a starting material for e.g., adipic acid can be obtained from glucose, sorbitol, xylose, or arabinose. Glycopolymers are an established research field [59]. Terpenes and even lignin are meanwhile also highly interesting starting materials for different, structurally significant biobased PAs.

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From the perspective of sustainable management, the recycling of natural materials from by-product streams, such as cellulose production, is an important point to consider when using natural sources of raw materials.

2.2.1

Fats and Oils

The five most produced vegetable oils are palm oil, soybean oil, rapeseed oil, sunflower oil, and palm kernel oil [60]. Castor oil [61, 62] occupies a special position, as it is not used for nutrition due to its content of the toxic proteins ricin and ricinine. A variety of synthesis routes have been developed for the use of fats and oils as chemical raw materials [63]. In addition, several projects have and will continue to develop new sources for unsaturated oils based which do not compete with the food production [64]. The linear structure of fatty acids is ideally suited for the formation of monomer structures which are characterized by the chemical functionalization at the ends of an aliphatic main chain. In addition, the fatty acids already provide one functionalization by the carboxyl group which is suitable for the synthesis of polyamides. The synthesis strategies developed therefore aim to introduce either a second carboxyl function or an amino group. In this way, dicarboxylic acids and amino carboxylic acids can be prepared. The corresponding diamines can be obtained from the dicarboxylic acids via the nitriles. Some studies also describe the direct route to lactams as an alternative to the amino acids [65, 66]. Characteristic for all oils is the content of unsaturated fatty acids [60, 67–70]. The double bonds provide another functionality that offers further chemical modifications for the necessary bifunctionality of monomers. Among the unsaturated fatty acids ricinoleic acid already has a long tradition for obtaining important monomers for polyamides [71]. Depending on the nature of the oils, several process routes have been developed to synthesize monomers. In most cases the very first step is the hydrolysis or alcoholysis of the oils to the free acids or esters (see Fig. 3). The oxidative cleavage of the double bond is practiced in the form of ozonolysis. Brassylic acid [72] and azelaic acid [73] are produced by this route from erucic acid and oleic acid, respectively. Extensive knowledge has been developed using the olefine-metathesis reaction paths [74–76], since this reaction type opens the access to interesting starting materials for the polyamide synthesis, from unsaturated fatty acids or their derivatives with a shortened number of synthetic steps. A metathesis reaction is formally understood as an alternating transposition of substituents perpendicular to an olefinic double bond, preferably catalyzed by metal complexes of e.g., ruthenium, molybdenum, or tungsten (Scheme 3). The synthesis proceeds via intermediately metal-alkylidene compounds, which coordinate an entering olefin and induce a rearrangement through a metallacyclobutane intermediate. In the context of polyamide synthesis, the

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Scheme 3 Formal scheme of the alkene metathesis reaction COOR

H3C

CH3

H3C [catalyst]

+ COOR

H3C

+

COOR

H3C

+

(I) COOR

ROOC

COOR

H2C

+

[catalyst]

(II) CH2

H3C

H2C CH2

H3C H2C [catalyst] HN

H3C

CH2

+

(III) HN

O

O

Scheme 4 Three types of olefin metathesis: (I) intermolecular self-metathesis to dicarboxylic acids, (II) cross-metathesis to w-unsaturated carboxylic acids, and (III) intramolecular ring closing metathesis to lactams

Scheme 5 Formal metathesis reaction of linoleic acid

self-metathesis, the cross-metathesis, and the ring-closure metathesis are of particular interest here (Scheme 4). The synthesis of long-chain (that is, fatty) dicarboxylic acids from oleic and erucic acid is an example for the use of intermolecular self-metathesis. The longchain diacids 9-octadecenedioic acid [77] and 13-hexacosenedioic acid [78] and, after hydrogenation, their corresponding saturated fatty diacids are typically obtained in this way. Another example of intermolecular self-metathesis is the formation of 1,4-cyclohexadiene [79] and 9-octadecenedioic acid from linoleic acid (Scheme 5).

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Scheme 6 Ethenolysis of oleic acid

Cyclohexadiene has been used as the starting material for the C6-monomer e-caprolactam and substituted derivatives [80]. The cross-metathesis allows the synthesis of w-unsaturated fatty-acid derivatives when ethylene is used as reactant (Scheme 6). The implementation of large-scale production of 9-decenoate (9-deceneoic acid methyl ester, 9-DAME) was pursued by the companies Cargill and Materials [81], which founded the company Elevance Renewable Science Inc. for that matter. The metathesis catalyst utilized therein is based on the well-established Grubbs catalyst architecture [82]. The process is yet to become industrialized at larger scale. In related approach, very recently the company VERBIO (Vereinigte BioEnergie AG) announced the construction of an ethenolysis plant for the production of 9-DAME, 1-decene, and heptene from rapeseed oil methyl ester. With 9-DAME, an interesting raw material base appears within reach for polyamide monomers on an industrial scale. The synthesis route is also an olefin metathesis, for which cost-effective catalysts have been developed. Another interesting aspect is that oleic acid, a fatty acid found in many oilseeds, is used, thus offering the chance of ecologically acceptable use and socio-economic acceptance of renewable raw material sources for chemical products [83]. 9-DAME is an interesting intermediate for further different functionalization paths, such as hydroformylation [84, 85] or one more metathesis steps [86, 87] which can be further functionalized to the corresponding dicarboxylic acids or amino carboxylic acids. A special case of the olefin metathesis is the intramolecular conversion of bis-unsaturated fatty-acid amides as described by Mudiyanselage et al. [65] (see also Fig. 4, synthesis (III)). According to this proposal, lactams are accessible.) The thermal cleavages of castor oil and ricinoleic acid or its methyl esters to sebacic acid [88, 89], and 10-undecenoic acid [90], respectively, are long practiced industrial processes. While sebacic acid can be directly used as monomer for the polyamide synthesis the conversion of methyl 10-undecenoate to 11-aminoundecanoic acid [91, 92] is necessary beforehand. The latter aminocarboxylic acid is the basis for Polyamide 11 (see Sect. 5). Notwithstanding, it is also possible to functionalize saturated monocarboxylic acids microbiologically at the methyl chain end. Dicarboxylic acids and amino acids in particular are available via this route [93–97].

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Fig. 4 A selection of possible synthesis routes from vegetable oils to monomers for polyamides

2.2.2

Carbohydrates

Carbohydrates are main products of photosynthesis [98] and account for about 75% of the biomass [99] formed annually worldwide, which is approximately the weight equivalent of 180 billion tons. Of this, about 4% is used by man [99]. A classification of carbohydrates into monosaccharides, oligosaccharides, and polysaccharides has become established [100]. Monosaccharides are characterized in their chemical structure by carbon chain lengths of three (trioses) to six (hexoses). In the open-chain form, they can either have an aldehyde function at the end (Aldoses) or a keto function at the second carbon atom (Ketoses). The remaining carbon atoms of the carbon chain are connected with hydroxy groups. In general, monosaccharides can also contain heteroatoms, such as nitrogen in N-acetylglucosamine, which is the basic building block in the structure-forming chitin. Monosaccharides which are glycosidically bonded to each other to form chains of 2–10 sugar units are called oligosaccharides. Glycosidic bonds are formed when the hydroxyl group of a hemiacetal or hemiketal reacts with hydroxyl group of another monosaccharide to form the acetal or ketal [35]. Like monosaccharides, oligosaccharides are water-soluble. In the logic presented, polysaccharides are macromolecules composed of more than ten monosaccharides. The diversity of polysaccharides occurring in nature results mainly from the linkage of different hydroxy groups of a monosaccharide with a glycosidic carbon atom, but also from the formation of branches of the polysaccharide chains. Additional diversity can result from the combination of different monosaccharides in the macromolecules. The most common monosaccharide is glucose [98]. From these basic structural elements, first conclusions for a chemical-material utilization of the saccharides can be drawn. The length of the carbon skeleton of the monosaccharides is basically reflected in the derived basic chemicals. The hydroxyl groups and aldehyde and keto functions undergo many reactions typical of these

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functionalities, such as oxidation, reduction, dehydration, esterification, or acetal formation [101]. In addition to chemical synthesis, carbohydrates are also used in microbiological synthesis, such as alcoholic fermentation or fermentative production of amino acids [102]. From this variety of possible synthesis routes, an attempt has been made to define basic chemical entities that can be considered of high importance as biobased basic chemicals in the future [103]. A number of monomers can be derived which are of interest for polyamide synthesis. The following examples are intended to provide an exemplary overview.

Furfural Approximately 25,000 t/a of furfural are produced worldwide per year. The raw materials used are mainly agricultural and forestry wastes containing pentoses [104] (Scheme 7). In order to obtain suitable monomers for the production of polyamides, it is necessary to make furfural suitably bifunctional. A key role is played by 5-hydroxymethylfurfural (5-HMF). 5-HMF can be obtained from furfural by reaction with formaldehyde (Scheme 8). On the other hand, it is possible to produce 5-HMF directly from fructose [105, 106] or inoline hydrolysate by consecutive water removal [99]. It was shown that good results can be obtained with a high selectivity (91-92%) and a total conversion of 76% related to fructose [107]. The particular importance of 5-HMF arises from the possibility of producing base monomers such as adipic acid, hexamethylene diamine, and caprolactam [108], which today are mainly obtained from petrochemical feedstocks.

Scheme 7 Furfural synthesis

Scheme 8 Synthesis of 5-hydroxymethylfurfural from furfural

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Scheme 9 Monomer synthesis on the basis of 5-hydroxymethylfurfural (5-HMF)

Furthermore, 5-HMF opens the way to a number of other monomers that may be of interest for the synthesis of polyamides, such as 2,5-furandicarboxylic acid [107], 2,5-tetrahydrofurandicarboxylic acid, 2,5-bisaminomethylfuran [107], or 2,5-bisaminomethyltetrahydrofuran [108]. The synthesis of caprolactam via 5-HMF proposed by T. Buntara et al. [109] starts with the dehydration of D-fructose to 5-HMF, which is reductively converted to 1,6-hexanediol. Mild oxidation using H2O2 in the presence of polyacids then gives e-caprolactone in good yields (Scheme 10) [110, 111]. For the last step to e-caprolactam, a reference is made to the UCC procedure [112] using ammonia at 170 bar and a temperature of 300–400 C. 2,5-Furandicarboxylic acid (FDCA) is derived from 5-hydroxymethylfurfural (5-HMF) [113]. It can also be obtained by oxidative dehydration of glucose or galactose [114]. The U.S. Department of Energy has defined FDCA as a “Top Value Added Chemical from Biomass” [103]. It is classified as a platform chemical [115]. On the one hand, this arises from the substitution for the petrochemically produced terephthalic acid [99] in polyesters and, on the other hand, because it can be used not only as a monomer itself but also as the basis for the synthesis of other monomers (Scheme 9). Even though FDCA is seen as a substitute for terephthalic acid, the conversion with diamines to polyamides is associated with challenges and has not been satisfactorily mastered in practice to date. The reproducibility of the conversion to

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CH2 OH O H

HO OH

O CH2 OH

H HO

O

OH

HO

O

O

NH

O

H D-Fructose

5-Hydroxymethylfurfural

1,6-Hexanediol

-Caprolactone

- Caprolactam

Scheme 10 Synthesis of e-caprolactam from D-fructose

Scheme 11 Pure furan-based polyamides, Tm ¼ 325 C (decomposition)

polyamides from the free acid is difficult as Smith et.al summarized: “Various proposals have been made to synthesize polyamides made from FDCA in the literature. However, the inventors have discovered that none of the experiments and the products described could be satisfactorily reproduced.” [116]. Nevertheless, Mitiakoudis and Gandini succeeded in obtaining good polycondensation results in an example from 2,5-furandicarboxylic acid and p-phenylene diamine. The resulting polyamide showed a high glass transition temperature of Tg ¼ 325 C [117] and was thermally stable up to 350 C in TGA.

Studies by the authors on the preparation of polyamides that contained the structural element of a furan ring in both the dicarboxylic acid and the diamine included the condensation of 2,5-bis(aminomethyl)furan with 2,5-furandicarboxylic acid chloride (Scheme 11). However, the reactions led only to low degrees of polymerization. The authors attributed this behavior to insufficient purity of the diamine and side reaction due to high mobility of hydrogen in the methylene groups of the diamine. Instead of FDCA for the synthesis of polyamides, furan-2,5-dicarboxylic acid dimethyl ester was proposed and examples with aliphatic C6 to C12 diamines [115, 118] were presented. The use of 2,5-bisaminomethylfuran as monomer component in copolyamides was described in order to achieve further functionalization or selective crosslinking of the polymer chain through this unit [119]. Further examples based on furan-containing monomers were shown by Gandini and co-workers using bisfuran carboxylic acid derivatives and various diamines, and

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Scheme 12 A biobased binuclear bisfuran carboxylic acid derivative (Tg ¼ 70 C, Tm ¼ 200 C)

Scheme 13 An alternative route to a furan-based AB polymer (Tg ¼ 65–75 C)

by acid polycondensation of furanic acid amide with formaldehyde [120, 121] (Schemes 12 and 13). In addition to the unsaturated furan dicarboxylic acid, the saturated variant, tetrahydrofuran dicarboxylic acid, was also investigated for the synthesis of polyamides [122]. In contrast to the furandicarboxylic acid derivatives, semicrystalline polyamides could be prepared with the saturated dicarboxylic acid. Another advantage of using tetrahydrofuran dicarboxylic acid was described by the prevention of the formation of toxic furan during the synthesis. Future potential is also seen for 2,5-bisaminomethyltetrahydrofuran as a counterpart to the saturated carboxylic acid [103]. Overall, polymers derived from furan are considered to be of great importance for the future in the context of green chemistry [103, 107].

Hexoses Also of interest are developments in the biotechnological production of adipic acid. Due to the high importance of adipic acid as a chemical raw material, this topic is the focus of numerous scientific investigations. Based on the fundamental work of K.M. Draths and J.F. Frost [123, 124] c,c-muconic acid is formed fermentatively starting from D-glucose via dehydroshikimic acid and catechol in several steps. The hydrogenation gave adipic acid in 90% yield (Scheme 14). Meanwhile, the product titers were brought to about 60 g/L and the yields of c,cmuconic acid are 30% mol/mol [125] based on glucose. Using a recombinant Escherichia coli yeast, it was possible to synthesize adipic acid directly from glucose [126]. Matthiesen et.al [127]. synthesized t,t-muconic acid, t-hex-3-ene dicarboxylic acid, and adipic acid based on muconic acid base via electrochemical methods

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Scheme 14 Adipic acid synthesis on the basis of D-glucose by mainly biotechnological processes

Scheme 15 Synthesis of t-hex-3-enedioic acid and adipic acid by electrochemical methods OH H

air, O OH heterogeneous catalyst

H OH

H

H

OH

HO

H

D-Glucose

water, native pH

O

OH

air, heterogeneous catalyst OH HOOC solvent

OH

HO OH

OH

Glucaric acid

COOH

O

Adipic acid

Scheme 16 Adipic acid synthesis from D-glucose using chemical synthesis methods

under comparatively mild conditions. For hexene dicarboxylic acid in particular, this opens up a route that will make this monomer accessible for incorporation into polymers in a simplified manner in the future. This monomer component could be interesting for crosslinking reactions, for example (Scheme 15). A completely chemical-catalytic synthetic pathway to adipic acid from glucose has been worked out by the company Rennovia. It involves the catalytic air oxidation of glucose to glucaric acid, followed by hydrodeoxygenation to adipic acid. The process has a carbon footprint more than 80% better than the petrochemical route [128] (Scheme 16). The reaction of adipic acid with ammonia to give adiponitrile was for a long time the leading large-scale process on the way to 1,6-hexamethylene diamine, which is obtained by hydrogenation of the dinitrile [129]. Polyamide 6, polyamide 66, and copolyamides containing these monomers based on renewable raw materials can be made available via this route. Nevertheless, it has not yet been possible to develop these alternative process routes to the point where competitive alternative manufacturing processes can be used. From today’s point of view, the greatest potential is attributed to the route via cis,cis-muconic acid and via glucaric acid [130].

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Scheme 17 Biotechnological production of e-caprolactam and 1,6-hexamethylene diamine from sugars

Another way to obtain monomers for polyamides from hexoses is through amino acids. The amino acid L-lysine plays a special role. Several 100,000 metric tons of lysine are produced annually on a large scale by industrial fermentation [131]. Coryneform bacteria are used in the biosynthesis, among which a mutant of Corynebacterium glutamicum has proved to be well suited [132]. The production takes place in fermentation tanks, which can have a size of up to 500 m3. The process has been optimized to such an extent that titer concentrations of 120 g/L (calculated as hydrochloride) are possible today [102, 133]. e-Caprolactam can be prepared by deamination of L-lysine. For this purpose, the hydrochloride of lysine is boiled with NaOH [134]. In another route, cadaverine (1,5-pentamethylene diamine) can be obtained from lysine by means of a decarboxylase [135] (Scheme 17). In 2020, the Chinese company Cathay Biotech announced plans to build a plant to process 2.4 million tons per year of corn, to produce 500 kt of cadaverine, 900 kt of biobased polyamide, and 80 kt of long-chain biobased dicarboxylic acids [136, 137]. Cathay Biotech will offer polyamides with a content of 25% to 100% biobased monomers under the brand ECOPENT®. The product range includes hightemperature polyamides, such as PA5.2, PA5.T, and PA5.4, long-chain polyamides from PA5.10 to PA5.16 and PA5.18, transparent polyamides and general polyamides – however, the clear focus will be on PA5.6. In its own efforts, BASF [138] operates a fermentative manufacturing process for the production of 1,4-butanediol based on a patented technology from Genomatica [139–141] (GENO BDO™). The starting material is glucose [142], which is converted by fermentation. BASF uses 1,4-butanediol to produce polytetrahydrofuran of varying chain lengths (e.g., poly-THF 1000) which can be used, among others, as co-monomers in polyamide elastomers. Succinic acid is a renewable C4 building block for the synthesis of different polymers [143]. It can be obtained on a large scale from 1,4-butanediol by oxidation [144]. Work is also underway to produce succinic acid from carbohydrates or proteins by fermentation [145, 146] and large-scale processes are being worked on. Bio-based succinic acid based on glucose and hydrolyzed starch has already been produced in 4-digit ton amounts by several companies back in 2011 (BioAmber 3,000 t, BASF-Purac 500 t, Reverdia 300 t) [147]. Nevertheless, it cannot be taken

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for granted that the production of basic chemicals based on renewable resources is economically successful in an environment of low energy and raw material costs. In 2018 for example, BioAmber had to file for bankruptcy despite increased sales because the cost of its investments had narrowed its financial scope too much [148–150]. The monomer building block putrescine (1,4-tetramethylene diamine) for PA4.6 or PA4.T can be obtained from succinic acid [151].

Aldaric Acids (Oxidized Sugars) Glucaric acid, also known as sugar acid, is also one of the platform chemicals [103] due to its potential to serve as a base for large-volume downstream products. It is accessible via nitric acid oxidation of glucose, sucrose, or starch [152]. In the search for biodegradable polyamides, L. Chen and D.E. Kiely described the synthesis of hydrophilic polyamides based on glucaric acid. When sugar-based dicarboxylic acids are used as monomers for polymers, stereochemistry should be considered because the two carboxylic acid groups of a specific aldaric acid may not necessarily be symmetrical to each other. Polyamides can thus be oriented either stereoregularly or randomly, depending on how the synthesis strategy is laid out. This asymmetry also applies to glucaric acid. In addition to synthesizing a stereochemically random polyamide, the authors developed an efficient method for preparing stereoregular polyamides via differences in the reactivities of the carboxyl groups and by means of the selective preparation of a monomeric amide ester from diamines and the glucaric acid. The application of protecting groups was not necessary [153] (Scheme 18). Wroblewska et al. [154] investigated the formation and properties of polyamides and copolyamides from sebacic acid, dodecyl diamine, and dimethyl ketal or formal of mucic acid. They described a decrease in crystallinity and glass transition temperatures with increasing amounts of the mucic acid derivatives in the resulting polyamides. The polyamide of the pure dimethyl ketal of mucic acid and dodecyl diamine is amorphous (Tg ¼ 51 C). They attributed this behavior to a decrease in intermolecular hydrogen bonding between the amide functions.

Scheme 18 Glucaric acid-based polyamides, R ¼ -C2H4-, -C4H8-, -C10H20- and -C12H24-

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Lignin

Wood contains the three natural polymers cellulose, hemicellulose, and lignin. During the lignification process in plants, lignin is incorporated into cellulose and hemicellulose. This creates a natural composite material that, among other properties, gives plants their stability and elasticity. This composite material is called lignocellulose [155]. Lignin consists of phenylpropanoids and different functional groups of oxygen such as hydroxyl, methoxy, or alkoxy groups. The polymer structure is characterized by crosslinking of the phenolic units and propyl moieties with each other, forming carbon–carbon and ether linkages [156]. This network causes the high resistance to biological and chemical degradation. The phenolic units are the special feature of the chemical structure of lignin. It occupies a special position within the biomass and is therefore of particular interest for further material utilization [157]. Very early studies by Harris, D’Ianni, and Adkins showed that the hydrogenation of lignin from aspen wood using a Copper-Cobalt contact catalyst yields about 10% 4-propylcyclohexanol along with 30% 4-(3-hydroxypropyl)cyclohexanol and 1,2-dihydroxy-4-propylcyclohexane [158].

A classification of the various lignins was made on the basis of the contents of structural elements that can be deduced from the three phenolic building blocks:

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Coumaryl alcohol (4-hydroxycinnamyl alcohol), H-type

Coniferyl alcohol (4-hydroxy-3-methoxycinnamyl alcohol), G-type

Sinapyl alcohol (3,5-dimethoxy-4-hydroxycinnamyl alcohol), S-type

This allows plants to be characterized by their content of different lignin types, which provides initial indications of possible refining routes for the separated lignin: • Hardwood predominantly contains G- and S-lignin. • Softwood is characterized by a high content of G-lignin. • Grasses contain all three types of H-, G-, and S-lignins [159]. The lignin content varies depending on the plant source. Softwood contains the most lignin (27–33%) followed by hardwood (18–25%) and grasses (17–24%) [160]. In order to obtain lignin, the wood has to be digested first. The material recovery of monomers from lignin for polyamide production can be divided into three basic steps from the routes followed so far: 1. Separation of the lignin 2. Degradation of the polymer backbone to low molecular weight structural units 3. Further chemical modification of the low molecular weight substances to monomers A whole range of processes have been proposed for separating the lignins [161, 162]. Nevertheless, the two most important processes are still the sulfate process and sulfite process. The sulfate process [163] provides what is known as Kraft lignin (89% of the total production capacity in 2000 [160]) and the sulfite process [164] (3,7% of the total production capacity in 2000 [160]) results in lignosulfonate. The primary objective of both processes is to obtain cellulose. 98% of the 50 to 80 Mio tons of the by-product lignin is thermally utilized today [160, 165]. Since Kraft lignin is available at particularly low cost, it is not surprising that numerous considerations are aimed at obtaining aromatic building blocks from lignin. This has led to a research into ways and methods of making monomers accessible from this biomass source [166].

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Fig. 5 Selection of potential material utilization options for lignin

On the other hand, the Organosolv [167] process must be mentioned here as well. This process allows the production of comparatively pure lignin, what is important for further chemical modification. However, due to the use of organic solvents such as ethylene glycol, the subsequent precipitation of lignin with acidified water and more sophisticated process technology, the Organosolv processes are significantly more expensive than the sulfate or sulfite process. Whereas the lignins from the first two processes contain sulfur, the Organosolv lignins are sulfur-free. Another significant difference is the low average molecular weight of the separated Organosolv lignins [161, 168]: Lignin type Kraft lignin Lignosulfonate Organosolv lignin

Molecular weight 1,000–3,000 5,000–20,000