Effect of ionization by proton transfer on propagation rate coefficients: a PLP-SEC study of methacrylic acid-amine monomers

Abstract

Previously, we reported the dependence of the propagation rate coefficient (k_p) for methacrylic acid (MAA) and sodium methacrylate (MAANa) on monomer concentration, degree of ionization, and temperature (I. Lacík, L. Učňová, S. Kukučková, M. Buback, P. Hesse and S. Beuermann, Macromolecules, 2009, 42, 7753–7761). In this study, we extend this work to investigate the ionization of MAA via a proton transfer mechanism in the presence of primary isobutylamine (IBA) and tertiary triethylamine (TEA), which differ in their affinity for the carboxylic proton. An advantage of these systems lies in their solubility in both water and non-polar solvents due to the presence of a hydrophobic group in the cation moiety. NMR and FTIR spectroscopy showed that complete proton transfer occurs for both monomers in water, and for MAA-IBA in DMSO. In contrast, MAA-TEA forms a hydrogen-bonded molecular complex in DMSO. The Kamlet–Taft α parameter was determined as a measure of the hydrogen bond donor ability of these systems. The k_p values for these MAA-amine monomers were determined in water and DMSO over a monomer concentration range of 0.45–1.82 mol L⁻¹ and a temperature range of 20–60 °C, using pulsed laser polymerization coupled with size-exclusion chromatography. In both solvents, the k_p values for MAA-IBA are lower than those for MAA-TEA, with the difference being modest in water (up to a factor of 2) and more pronounced in DMSO (up to a factor of 4). The influence of monomer concentration on k_p is less significant than for MAANa. Activation energies, E_A(k_p), increase from 19.3 ± 1.5 and 17.8 ± 0.3 kJ mol⁻¹ in water to 28.2 ± 1.6 and 26.9 ± 1.7 kJ mol⁻¹ in DMSO for MAA-IBA and MAA-TEA, respectively. The pre-exponential factor ∼0.9 × 10⁶ L mol⁻¹ s⁻¹ is similar for both monomers in water and is increased by an order of magnitude in DMSO. These results demonstrate that k_p depends on monomer speciation and a complex interplay between electrostatic, hydrogen bonding, and hydrophobic interactions.