A Perfect Couple: PLP/SEC/ESI-MS for the Accurate Determination of Propagation Rate Coefficients in Free Radical Polymerization

摘要

Pulsed laser polymerization (PLP) in conjunction with online size exclusion chromatography – electrospray ionization mass spectrometry (SEC/ESI-MS) has successfully been applied for the determination of accurate propagation rate coefficients, k_p, in free radical polymerization. SEC/ESI-MS is used in this context to precisely determine the molecular weight distribution of the polymer produced by the PLP-experiment. The novel approach allows for a highly accurate internal calibration of the size exclusion system to be obtained from first principles, as the molecular weight of the polymer is determined with ppm accuracy by the mass spectrometer and allows corrections for SEC band broadening effects to be made. Molecular weight measurements can furthermore be performed regardless of the monomer class, as long as the macromolecule is amenable to electrospray ionization. The proposed approach successfully eliminates work-steps and problems incurred in the analysis of new and poorly characterized monomers, where universal calibration or multi-detector SEC had to be applied until now and provides an alternative also to PLP/MALDI-MS. Applicability of the method is demonstrated for the homologous series of methyl-, ethyl- and butyl methacrylate. The experimentally determined k_p values were 5–10% higher than the currently available benchmark data. The experimental k_p is in agreement with the results of kinetic simulations, assuming a short chain length dependency of k_p, in accordance with the empirical model of Smith, Heuts and Russell.