Liquid Chromatography Ftir Microspectroscopy, Microwave Assisted Synthesis

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Adv Polym Sci (2003) 163: 1–60 DOI 10.1007/b11050

Recent Advances in Liquid Chromatography Analysis of Synthetic Polymers Taihyun Chang Department of Chemistry and Center for Integrated Molecular Systems, Pohang University of Science and Technology, Pohang, 790-784 Korea. E-mail: [email protected]

Abstract Synthetic polymers are rarely homogeneous chemical species but have multivariate distributions in molecular weight, chemical composition, chain architecture, functionality, and so on. For a precise characterization of synthetic polymers, all the distributions need to be determined, which is a difficult task, if not impossible. Fortunately in most of the cases it is sufficient to analyze a limited number of molecular characteristics in order to obtain the information required for a given purpose. Nonetheless, it is still nontrivial if there exists distributions for more than one molecular characteristic. There have been continuing efforts to solve the problem. One approach is to find chromatographic methods sensitive to one molecular characteristic only. In favorable cases, the effect of all but one molecular characteristic can be suppressed to a negligible level. Various interaction chromatographic techniques as well as size exclusion chromatography are employed for the purpose. Also the multiple detection methods each sensitive to a specific molecular characteristic can provide additional information. Various detection methods developed recently such as FT-IR, FT-NMR, and mass spectrometry brought about significant progress in the characterization of complex polymers. This review presents the recent developments in the analysis of various heterogeneities in synthetic polymers by a variety of liquid chromatographic separation as well as detection methods. Keywords Chromatography · Polymer characterization · Molecular weight · Chain architecture · Chemical composition · Functionality · Stereoregularity

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Chromatographic Separation Principles . . . . . . . . . . . . . . .

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3

Molecular Weight Distribution of Linear Polymers . . . . . . . . .

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Chain Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4.1 4.2

Branched Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ring Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18 22

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Chemical Composition Distribution . . . . . . . . . . . . . . . . .

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5.1 5.2

Mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Block Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25 28

© Springer-Verlag Berlin Heidelberg 2003

Taihyun Chang

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5.3 5.4

Random Copolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37 46

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Microstructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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List of Symbols and Abbreviations 2D 2D-LC -bBA C18 C18AB CCD ci cm cp cs DNA EA ELSD EPDM ESI FAD FT-IR FT-MS FT-NMR -gHEMA HPLC iIC IR K KIC KSEC k¢ LC

Two dimension(al) Two-dimensional liquid chromatography -blockButyl acrylate Octadecyl Polymeric phase octadecyl Chemical composition distribution Concentration of a solute in the interstitial space Concentration of a solute in the mobile phase Concentration of a solute in the pore space Concentration of a solute in the stationary phase Deoxyribonucleic acid Ethyl acrylate Evaporative light scattering detector Ethylene-propylene-diene rubber Electron spray ionization Full adsorption-desorption Fourier transform-infrared spectroscopy Fourier transform-mass spectrometry Fourier transform-nuclear magnetic resonance spectroscopy -graft2-Hydroxyethyl methacrylate High performance liquid chromatography Isotactic Interaction chromatography Infrared spectroscopy Equilibrium constant Equilibrium constant at interaction chromatography Equilibrium constant at size exclusion chromatography Capacity factor Liquid chromatography

Recent Advances in Liquid Chromatography Analysis of Synthetic Polymers

LC-CAP LCCC LC-LCA

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Liquid chromatography at the critical adsorption point Liquid chromatography at the critical condition Liquid chromatography under limiting conditions of adsorption LC-LCD Liquid chromatography under limiting conditions of desorption LC-PEAT Liquid chromatography at the point of exclusion-adsorption transition LC-TEA Liquid chromatography at the theta exclusion-adsorption condition M Molecular weight MA Methyl acrylate MALDI-TOF MS Matrix assisted laser desorption/ionization time of flight mass spectrometry MALS Multi-angle light scattering MAn Maleic anhydride MMA Methyl methacrylate Number average molecular weight Mn Mw Weight average molecular weight MWD Molecular weight distribution NMR Nuclear magnetic resonance spectroscopy NP Normal phase NPLC Normal phase liquid chromatography PB Polybutadiene PBA Poly(butyl acrylate) PDMA Poly(decyl methacrylate) PDMS Poly(dimethylsiloxane) PE Polyethylene PEMA Poly(ethyl methacrylate) PEO Poly(ethylene oxide) PI Polyisoprene PLMA Poly(lauryl methacrylate) PMMA Poly(methylmethacrylate) PnBMA Poly(n-butyl methacrylate) PPO Poly(propylene oxide) PS Polystyrene PtBMA Poly(tert-butyl methacrylate) R Gas constant RP Reversed phase RPLC Reversed phase liquid chromatography S Styrene sSyndiotactic

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SEC SI T TGIC THF TLC v/v Vi Vm Vp Vr Vs DGo DGoIC DGoSEC DHo DHoIC DHoSEC DSo DSoIC DSoSEC n

Taihyun Chang Size exclusion chromatography Polystyrene-block-polyisoprene copolymer Temperature Temperature gradient interaction chromatography Tetrahydrofuran Thin layer chromatography Volume/volume Interstitial volume Mobile phase volume Pore volume Retention volume Stationary phase volume Standard Gibbs free energy change Standard Gibbs free energy change in interaction chromatography Standard Gibbs free energy change in size exclusion chromatography Standard enthalpy change Standard enthalpy change in interaction chromatography Standard enthalpy change in size exclusion chromatography Standard entropy change Standard entropy change in interaction chromatography Standard entropy change in size exclusion chromatography Number average degree of polymerization

1

Introduction Liquid chromatography (LC) is a powerful tool for the characterization of natural and synthetic macromolecules that are often heterogeneous in molecular weight, chain architecture, chemical composition, microstructure, and so on. Among numerous variations of LC methods, size exclusion chromatography (SEC) has dominated the area of the molecular weight distribution (MWD) analysis of synthetic polymers [1–3]. SEC has many advantages over other classical techniques in the characterization of molecular weight distribution of polydisperse polymers in speed, effort, required amount of sample, etc. SEC fractionates polymer molecules by partition equilibrium of polymer chains between common solvent phases located at the interstitial space and the pores of the column packing materials, typically in the form of uniform size porous beads. Therefore the partition equilibrium is mainly governed by the conformational entropy difference of the polymer chains located in two different physical environ-

Recent Advances in Liquid Chromatography Analysis of Synthetic Polymers

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ments. The eluent is commonly chosen to minimize the enthalpic interaction between the polymeric solutes and the stationary phase. In results, SEC separates the polymer molecules in terms of the size of a polymer chain in the elution solvent. If there exists a simple relationship between the polymer chain size and its molecular weight such as for linear and chemically homogeneous polymers, SEC retention volume is well correlated with the molecular weight of polymers. However, the same cannot be said for nonlinear chain polymers or copolymers, in which a simple relationship between the molecular weight and the molecular size does not exist and SEC is not able to separate them according to their molecular weight. For the same reason, SEC is not an efficient tool to separate the molecules in terms of chemical heterogeneity, such as chemical composition differences of copolymers, tacticity, and end-group difference. Interaction chromatography (IC) is suitable for the purpose since its separation mechanism is sensitive to the chemical nature of the molecules. In contrast to SEC, IC utilizes the enthalpic interaction, adsorption or partition of solute molecules to the stationary phase and IC has been employed frequently to separate polymers in terms of their chemical composition distribution (CCD) or functionality [4]. IC has also been used for the separation of polymers in terms of molecular weight [5, 6]. In general IC exhibits higher molecular weight resolution than SEC, but SEC is more universal in a sense that most of thermodynamically good solvents can be used for the SEC separation unless there exists specific interaction between the polymer of interest and the porous packing materials. On the other hand, the enthalpic interaction strength in IC has to be controlled precisely to achieve a reproducible and high-resolution separation. Therefore the mobile and stationary phases have to be chosen for individual polymer system of interest and the gradient elution is often required [7]. Recent development in liquid chromatography analysis of complex polymers shows a clear trend to combine more than one LC separation mechanism/technique together with multiple detection techniques. It is a quite natural direction for the analysis of complex polymers with multivariate distributions in molecular characteristics. The coupled LC techniques have gained wide attention recently in the characterization of complex polymers and there are a number of monographs and reviews on this topic [4, 8–13]. In this chapter, recent advances in LC separation of polymers are reviewed. References are generally restricted to the works published after 1995 since most of the works prior to the mid-1990s have been well summarized already [8, 10, 11, 13]. 2

Chromatographic Separation Principles The retention in SEC, in which the solute partition takes place between the common eluent phases in two different environments, interstitial and pore space, is expressed as follows:

Taihyun Chang

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Vr = Vi + K SEC V p , K SEC =

cp

(1)

ci

where the subscripts p and i refer to the pore and interstitial spaces, respectively. The distribution constant KSEC, the ratio of the solute concentration in pore space (cp) to interstitial space (ci), has a value between 0 (total exclusion) and 1 (total permeation). Like other equilibrium constants, KSEC is related to the standard Gibbs free energy change (DGo) of the partition process and DGo can be further divided into the enthalpic and entropic contribution of the process: Ê DG o SEC K SEC = expÁ – RT Ë

o ˆ ˆ Ê DS o DH SEC SEC exp – = ˜ ˜ Á RT ¯ ¯ Ë R

(2)

In the SEC separation process, the separation condition is usually chosen to minimize the enthalpic interaction of polymer solutes with the packing materials. Therefore in the ideal SEC condition (DH0=0) KSEC is a function of the conformational entropy loss (DSo