An experimental study on the kinetics and mechanisms of styrene polymerization in oil-in-water microemulsion initiated by oil-soluble initiators

Abstract

In order to clarify the kinetic role of oil-soluble initiators in microemulsion polymerization, the oil-in-water (O/W) microemulsion polymerizations of styrene are carried out using four kinds of azo-type oil-soluble initiators with widely different water-solubility. The results are compared with those observed when a water-soluble initiator, potassium persulfate (KPS) is used. For all the oil-soluble initiators used, the molecular weight of polymers and the average size of polymer particles do not change with the monomer conversion and the initial initiator concentration. The monomer conversion is expressed as a function of ri0.5t, where ri is the rate of radical generation in the whole reaction system and t is the reaction time. These characteristics are quite the same as those observed when KPS is used as an initiator. When the polymerizations are carried out with the rate of radical generation in the whole reaction system fixed at the same value, the rates of polymerization are almost the same for all the oil-soluble initiators employed, irrespective of their water-solubility, but are significantly lower (ca. 1/3) than that with KPS. Then, the following conclusions are given: (1) The radicals generated not only in the aqueous phase, but also in the micelle and polymer particle phase are almost equally effective for the polymerization. However, (2) only a small portion (ca. 1/9) of the radicals generated in both phases participate in the polymerization. (3) Bimolecular termination of a growing radical in the polymer particle with an entering radical and with a pair of radicals generated in the polymer particles is negligible, and hence, the molecular weight of polymers is determined only by chain transfer to monomer.