Bimolecular termination events in the seeded emulsion polymerization of styrene

Bimolecular termination processes in the seeded emulsion polymerization of styrene have been studied by using large seed particles and γ-ray initiation. The observed relaxation kinetics differed significantly from those previously found for methyl methacrylate in that the results could not be explained simply in terms of propagation-driven ("residual" or "reaction-diffusion") termination. The data have been successfully interpreted, however, by a "short-long" model that differentiates between free radicals attached to short chains, whose mobility is not significantly diminished by entanglements, and those attached to long, entangled chains. The latter are formed by the rapid growth to entanglement length of the former, whereas the short chains arise both from free-radical entry into the particles and from chain transfer to monomer by long chains. Bimolecular termination between short and long chains is the predominant radical-annihilation event. The dependence upon weight fraction of polymer of the rate coefficient obtained experimentally for short-long termination is in good agreement with that calculated from the rate of diffusion of short species through the latex particles. The relaxation-derived termination rate coefficients were also found to predict the experimentally observed Trommsdorff-Norrish gel effect in chemically initiated seeded emulsion polymerization systems.