Summary: Propagation rate coefficients, kp, for acrylic acid (AA) polymerization at 6 °C in aqueous solution were measured via pulsed laser polymerization (PLP) with the degree of ionization, α, varied over the entire range between 0 and 1. These measurements were carried out in conjunction with aqueous‐phase size‐exclusion chromatography (SEC). Strictly speaking, the reported kp's are “apparent” propagation rate coefficients deduced from the PLP‐SEC data under the assumption that the local monomer concentration at the radical site is identical to overall monomer concentration. At an AA concentration of 0.69 mol · L?1, the apparent kp decreases from 111 000 L · mol?1 · s?1 at α = 0 to 13 000 L · mol?1 · s?1 at α = 1.0. The significant lowering of kp with higher α is attributed to the repulsion between both monomer molecules and macroradicals becoming negatively charged. Addition of up to 10 mol‐% (with respect to AA) sodium hydroxide to the fully ionized aqueous AA solution leads to an enhancement of kp up to 57 000 L · mol?1 · s?1.
Dependence of apparent kp values on the degree of ionization of acrylic acid (a) and on pH (b) for aqueous polymerizations of acrylic acid.
