Propagation Kinetics of Acrylic and Methacrylic Acid in Water and Organic Solvents Studied by Pulsed-Laser Polymerization

Abstract

Pulsed laser-induced polymerization (PLP) in combination with analysis of formed polymer by size exclusion chromatography (SEC) has been successfully applied in determining propagation rate coefficient, kp, of acrylic and methacrylic acid in organic solvents and water. For methacrylic acid solution polymerization in methanol and in dimethyl sulfoxide (DMSO), minor but not negligible variations of kp with solvent have been observed. In contrast, kp values for polymerizations in water are significantly influenced by the solvent and furthermore by monomer concentration. The kp values obtained from polymerization experiments in water are significantly larger than the corresponding values obtained in methanol or DMSO. Weighted nonlinear least-squares fitting (NLLS) has been applied to calculate frequency factors, A, and activation energies, Ea, from the temperature dependence of kp for methacrylic acid in methanol, DMSO, and water in order to underline the reliability of the data. For acrylic acid it turns out that optimized experimental conditions have to be chosen in order to determine reliable kp values at ambient temperature. Laser pulse repetition rates of at least 90 Hz are necessary to ensure that “termination by the laser pulse” is the main chain stopping event. Smaller values of the laser pulse repetition rate will not yield reliable kp data. Furthermore, evidence of at least one first overtone inflection point at L2 ≈ 2L1 is not only recommended, it is necessary to ensure a reliable measure of kp. This is the most important consistency criterion showing that experimental conditions are suitable.