New Insights on the Kinetics of Persulfate-Initiated Itaconic Acid Free-Radical Polymerization

A mathematical model is proposed that couples the decomposition kinetics of persulfate (S₂O₈²⁻) and the free radical polymerization kinetics of itaconic acid (IA); the results are compared with experimental data reported in the literature. It is found that the former is highly affected by the acidification of the aqueous medium which is caused by the equilibrium dissociation of IA but mainly by HSO₄⁻ produced by side reactions of the S₂O₈²⁻ decomposition. This qualitatively explains the dependence of d[S₂O₈²⁻]/dt with [IA] and the initial concentration of persulfate ([S₂O₈²⁻]₀), reported in the literature. According to the model results, temperature overshoots are very likely to occur in the experiments so there is doubt whether the reaction order >1 with respect to [IA] that is sometimes reported in the literature for the rate of polymerization (Rp) is an artifact related to an imprecise temperature (T) control or is due to a more complex mechanism. Due to the higher activation energy of the persulfate decomposition compared to the propagation reaction, small variations of T can lead to significant variations of d[S₂O₈²⁻]/dt but to an almost imperceptible effect on Rp. Recommendations for future experimental work and refinement of the kinetic model are provided.