Propagation Kinetics of Free-Radical Methacrylic Acid Polymerization in Aqueous Solution. The Effect of Concentration and Degree of Ionization

Propagation rate coefficients, k_p, of free-radical methacrylic acid (MAA) polymerization in aqueous solution are presented and discussed. The data has been obtained via the pulsed laser polymerization – size-exclusion chromatography (PLP-SEC) technique within extended ranges of both monomer concentration, from dilute solution up to bulk MAA polymerization, and of degree of ionic dissociation, from non-ionized to fully ionized MAA. A significant decrease of k_p, by about one order of magnitude, has been observed upon increasing monomer concentration in the polymerization of non-ionized MAA. Approximately the same decrease of k_p occurs upon varying the degree of MAA ionization, α, at low MAA concentration from α = 0 to α = 1. With partially ionized MAA, the decrease of k_p upon increasing MAA concentration is distinctly weaker. For fully ionized MAA, the propagation rate coefficient even increases toward higher MAA concentration. The changes of k_p measured as a function of monomer concentration and degree of ionization may be consistently interpreted via transition state theory. The effects on k_p are essentially changes of the Arrhenius pre-exponential factor, which reflects internal rotational mobility of the transition state (TS) structure for propagation. Friction of internal rotation of the TS structure is induced by ionic and/or hydrogen-bonded intermolecular interaction of the activated state with the molecular environment.