On the mechanism of emulsion polymerization of styrene. III. Polymerization initiated by oil‐soluble compounds

Abstract

AbstractThis report is concerned with the kinetics of the emulsion polymerization of styrene initiated by the thermal decomposition of oil‐soluble initiators particularly cumene hydroperoxide (CHP). Only those systems are considered in which the latex particles are so small that two radicals cannot coexit for a significant length of time. Polymerization rate, particle size, and molecular weight of the polymer have been determined at 40, 50, and 70°C. as a function of initiator concentration. At low CHP concentrations the polymerization rate per particle conforms to Smith‐Ewart kinetics, i.e., on the average one polymer radical propagates during half the time in each particle. Therefore it is concluded that, although CHP may decompose into radical pairs, polymer radicals are formed one by one. It is shown that even very low rates of transfer of single radicals from the particles to the aqueous phase, or vice versa, suffice to account for the observed polymerization rates per particle. An alternative mechanism is considered in which CHP decomposes on the surface of the particle, the organic radical enters the particle, and the hydroxyl radical remains in the aqueous phase. The rate of polymerization per particle is observed to decrease with increasing CHP concentration. The fact that the transfer coefficient of this initiator is several orders of magnitude higher than that of cumene sustains the supposition that the polystyryl radical abstracts the periodic hydrogen from the peroxide. The decrease in polymerization‐rate is attributed to the inactivity of the resulting ROO·radical. The Mayo formula for the reciprocal degree of polymerization is reformulated to incorporate this kinetic feature. The dependence of polymerization rate on CHP concentration, derived from the new formula, generally agrees with experimental results. Transfer coefficients of the polystyryl radical to CHP and other organic peroxides are given. From the data it can also be concluded that the decomposition rate of CHP in emulsion polymerization is an order of magnitude larger than in styrene solution.