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

In order to clarify the kinetic role of oil-soluble initiators in microemul sion polymerization, the oil-in-water (O/W) microemulsion polymerizations o f styrene are carried out using four kinds of ate-type oil-soluble initiato rs with widely different water-solubility. The results are compared with th ose observed when a water-soluble initiator, potassium persulfate (KPS) is used. For all the oil-soluble initiators used, the molecular weight of poly mers and the average size of polymer particles do not change with the monom er conversion and the initial initiator concentration. The monomer conversi on is expressed as a function of r(i)(0.5)t, where r(i) is the rate of radi cal generation in the whole reaction system and t is the reaction time. The se 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 ra dical generation in the whole reaction system fixed at the same value, the rates of polymerization are almost the same for all the oil-soluble initiat ors employed, irrespective of their water-solubility, but are significantly lower(ca. 1/3) than that with KPS. then, the following conclusions are giv en: (1) The radicals generated not only in the aqueous phase, but also in t he 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) Bimolecula r 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.