PLP-SEC Investigation of the Influence of Electrostatic Interactions on the Radical Propagation Rate Coefficients of Cationic Monomers TMAEMC and MAPTAC

The decisive role of counterions on the propagation mechanism in the radical polymerization of ionized monomers is comprehensively studied. Propagation rate coefficients, k p, were obtained for cationic monomers [2-(methacryloyloxyethyl)]­trimethylammonium chloride (TMAEMC) and [3-(methacryloylaminopropyl)]­trimethylammonium chloride (MAPTAC) in aqueous solution by pulsed-laser polymerization combined with size-exclusion chromatography over broad ranges of monomer concentration (1–40 wt %), temperature (5–90 °C), pH (2–12), and salts selected to provide a molar counterion concentration, c counterion, between 0.05 and 3 mol·L–1. Both monomers behave similarly under conditions at which repulsive electrostatic interactions between a polymer chain and a reacting monomer dominate (c counterion < 0.2 mol·L–1). The k p values increase linearly with c counterion above this value due to screening of the repulsive interactions and follow the family behavior of nonionized methacrylate and methacrylamide monomers manifested by a higher k p for TMAEMC than for MAPTAC. The hydration of the polyelectrolyte chain is considered as an additional factor contributing to higher k p values for cationic TMAEMC at high c counterion values compared to its nonionized analogue 2-dimethylaminoethyl methacrylate. The extensive set of k p data can be represented using average activation energies of 14.4 ± 2.7 and 14.7 ± 2.0 kJ·mol–1 for TMAEMC and MAPTAC, respectively, and a preexponential factor that increases with c counterion. The batch polymerization of MAPTAC compared to that of TMAEMC reflects the differences in k p behavior for these monomers.