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 1H 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. |