Analytical strategy for the molecular weight determination of random copolymers of poly(methyl methacrylate) and poly(methacrylic acid)

Abstract

Molecular weight characterization of random amphiphilic copolymers currently represents an analytical challenge. In particular, molecules composed of methacrylic acid (MAA) and methyl methacrylate (MMA) as the repeat units raise issues in commonly used techniques. The present study shows that when random copolymers cannot be properly ionized by MALDI, and hence detected and measured in MS, one possible analytical strategy is to transform them into homopolymers, which are more amenable to this ionization technique. Then, by combining the molecular weight of the so-obtained homopolymers, as measured by MS, with the relative molar proportion of the MMA and MMA units, as given by 1 H NMR spectrum, one can straightforwardly estimate the molecular weight of the initial copolymer. A methylation reaction was performed to transform MAA-MMA copolymer samples into PMMA homopolymers, using trimethylsilyldiazomethane as a derivatization agent. Weight average molecular weight (M w ) parameters of the MAA-MMA copolymers could then be derived from M w values obtained for the methylated MAA-MMA molecules by MALDI, which were also validated by pulsed gradient spin echo (PGSE) NMR. An alkene function in one of the studied copolymer end-groups was also shown to react with the methylation agent, giving rise to MMA-like polymeric by-products characterized by tandem mass spectrometry and which could be avoided by adjusting the amount of the trimethylsilyldiazomethane in the reaction medium. (J Am Soc Mass Spectrom 2010, 21, 1075-1085) © 2010 American Society for Mass Spectrometry P olymers based on weak acids such as poly-(methacrylic acid) (PMAA) have attracted considerable attention because of the ability of the system to change strongly upon variations in the pH and ionic strength of the solution [1]. For example, amphiphilic PMAA-based block copolymers were reported to efficiently adsorb uranyl ions [2] or to selfassemble to produce dynamic micelles sensitive to different stimuli [3][4][5][6]. In particular, block copolymers containing PMAA and poly(methyl methacrylate) (PMMA) segments were the subject of intensive research activities [7][8][9][10]. However, interesting properties were also demonstrated for copolymers containing random MAA-MMA segments [11][12][13][14]. Performance of such materials highly depends on structurally-related parameters but also on molecular weight distribution.Determination of molecular weight parameters is not a trivial task for copolymers. Liquid state nuclear magnetic resonance (NMR) is the most commonly used technique for this purpose but only allows the number average molecular weight (M n ) parameter to be obtained. Size exclusion chromatography (SEC) requires calibration with narrow standards of the same chemical nature and structure as the analyte. Alternatively, one can use calibration which relies on Mark-HouwinkKuhn-Sakurada (MHKS) parameters being known for the standards and the analyte [15], since these parameters depend on both the nature of co-monomers and their relative proportion with...

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