Styrene emulsion polymerization. The effects of initiator charge

Abstract

The results are reported of a series of kinetic studies of the seeded emulsion polymerization of styrene, designed to investigate the effect of different initiator species on the rate coefficient for the entry of free radicals into the latex particles. The initiators employed were persulphate anions (which give negatively charge free-radical species), Fe²⁺/H₂O₂(giving rise to neutral free-radical species) and 2,2′-azobis-(2-amidinopropane) hydrochloride (generating positively charged free-radical species). Entry rate coefficients were determined from the observed rates of approach to the steady state, making due allowance for the processes of free-radical exit, re-entry and aqueous-phase heterotermination. It was found that the efficiency of capture of aqueous-phase free radicals by the latex particles was relatively low (the rate coefficient for entry being proportional to [initiator]^0.5–0.9), at least for charged initiator species; the dependence on initiating species concentration for the Fe²⁺/H₂O₂ system could not be directly determined. The dependence of the entry rate coefficient (p_A) on particle number density (N_c) and initiator concentration ([I]) was fitted by a mechanism incorporating the conversion of initiating free-radical species into oligomeric free radicals, mutual aqueous-phase termination of these oligomers and entry of these oligomers into latex particles. This gives an expression for p_A(N_c, [I]) containing only two rate parameters. It was found that data over wide ranges of [I] and N_c could be accurately fitted with this expression. It was thus found that the charge on the initiating species has only a weak effect on entry and mutual termination of oligomers, indicating that both processes are dominated by the hydrophobic tail of the species involved.