In this study, we investigated the synthesis of high-impact polystyrene through a series of (batch and bulk) polymerizations of styrene in the presence of 6 wt % polybutadiene. The reactions with rubber involved three stages: dissolution, prepolymerization, and finishing. The prepolymerizations were isothermal (at 70degreesC) and nonisothermal (with an initial heating period from 70 to 120degreesC). Purely thermal reactions were compared with chemically initiated polymerizations involving tert-butyl peroctoate (TBPO), 2,2'-azobisisobutyronitrile (AIBN), and their mixtures. Most prepolymerizations involved a stirring rate of 125 rpm. The particle morphology was mainly determined by the evolution of the grafting efficiency, stirring rate, and system viscosity during prepolymerization. In the nonisothermal prepolymerizations with pure AIBN, the grafting efficiency was lower than in equivalent prepolymerizations without an initiator, yielding unstable emulsions and a poor final morphology. In the nonisothermal prepolymerizations with pure TBPO, a large amount of graft copolymer was produced early in the prepolymerization, yielding a submicrometer core-shell morphology. The particle size and morphology could be controlled by the appropriate adjustment of the recipe and the prepolymerization conditions. (C) 2004 Wiley Periodicals, Inc.