Evidence of gel effect and living free radicals in particles: Soap-free emulsion polymerization of styrene by using 2,2'-azobis(isobutyronitrile)

Batch, in situ seeded and stepwise dosing monomer, and normal seeded soap-free emulsion polymerizations (SFEP) of styrene were carried out by using the oil-soluble 2,2 '-azobis(isobutyronitrile) as initiator. Gel permeation chromatography (GPC) and scanning electron microscopy were employed to characterize the experimental results. Typical auto-accelerative effect in the bulk free-radical polymerization was also observed in the macro-kinetics of batch SFEP at 60 and 65 degrees C. Flat-roof GPC curves appeared at the post-auto-acceleration. In addition, M(n )in particles exhibited a positive relationship with the size of particles. The bigger size the larger M-n. In the in situ seeded polymerization, the flat-roof GPC curves of seed turned into the asymmetric Gaussian type exhibiting the great increase of the fraction of larger M(n )with the sequential addition of styrene. In addition, the number of seeds was kept constant throughout the polymerization. It indicated that the free radicals were generated and confined in the growing particles. In the normal seeded polymerization, showed a strong dependence on the average assigned amount of St per seed particle. The more monomers per seed particle the larger M(n )was gained. Based on these results, it was postulated that the free radicals were living due to the gel effect occurring in particles, namely that the free radicals were segregated by the dry polymer chains. The kinetics model was also given based on the microdroplets theory.