Kinetic relaxation studies (using γ-rays from a ⁶⁰Co source as initiator) of the seeded emulsion polymerization of styrene have shown that the presence of hydrocarbon diluents, such as ethylbenzene, toluene and cyclohexane, can significantly increase the rate coefficient for the exit of free radicals from the latex particles. This increase was evident even when the chain-transfer constant for the diluent was less than that for styrene. Such an increase may arise from the low reactivity of the diluent free radicals with the monomer molecules. This allows the diluent free radicals additional time, compared with the monomer free radical, to undergo diffusional exit from the particles before reaction with the monomer confines the species to the latex particles. The free-radical exit rate coefficient is thus determined inter alia by both the chain-transfer constant for the diluent and the reactivity of the diluent free radicals, as demanded theoretically. Large increases in the exit rate coefficient are predicted to occur with certain diluents at high replacements of the styrene by diluent.

Seeded emulsion polymerizations of styrene initiated by potassium peroxydisulphate confirmed that certain diluents may increase the exit rate coefficient significantly. Surprisingly, it was found that the entry rate coefficient may also be increased significantly. This was attributed to the presence of the diluent promoting the stabilization of those free-radical species that are capable of entering the latex particles at lower degrees of aggregation. This reduces the rate of bimolecular termination of the free-radical species located outside the latex particles. At higher monomer replacement levels, however, it is possible that the rate of entry of free radicals into the latex particles may be reduced as a consequence of the depletion of the monomer concentration in the aqueous phase. Thus the entry rate coefficient in the presence of diluents is determined inter alia by two processes whose effects tend to be opposed: on the one hand, enhanced stabilization of the coagulating colloidal free-radical species and, on the other, depletion of the monomer concentration in the aqueous phase.