Emulsion and miniemulsion polymerizations with an oil-soluble initiator in the presence and absence of an aqueous-phase radical scavenger

Butyl acrylate conventional emulsion (macroemulsion) and miniemulsion polymerizations were carried out with an oil-soluble initiator (azobisisobutyronitrile) in the presence or absence of an aqueous-phase radical scavenger. For macroemulsion polymerization, in the presence of an aqueous-phase radical scavenger, no particle nucleation occurred, whereas in the absence of an aqueous-phase radical scavenger, particle nucleation proceeded as expected. For miniemulsion polymerization, the rate of polymerization was much higher in the absence of an aqueous-phase radical scavenger than in its presence. Furthermore, in the absence of an aqueous-phase radical scavenger, the miniemulsion polymerization rate increased with reduced droplet size, whereas in the presence of an aqueous-phase radical scavenger, the trend was reversed. It is concluded that (1) for macroemulsion polymerization, the contribution from free radicals originating in the aqueous-phase is predominant in the micellar nucleation of particles; (2) free radicals originating in the particle phase contribute to the rate of polymerization and the contribution increases with an increase in the particle size; and (3) for polymer particles with diameters of up to approximately 100 nm, polymerization is initiated from free radicals originating in the aqueous phase. (C) 2002 Wiley Periodicals, Inc.