High impact polystyrene (HIPS) was prepared by bulk graft copolymerization with four tetrafunctional peroxide JWEB50 as initiator. Phase inversion at constant and staged stirring conditions was analyzed through study of particle structure and viscosity of the polymerization system by transmission electron microscope and change in relative torque. Similar results were obtained from both analyses. For constant stirring

conversion at phase inversion decreased with the increase of stirring speed. For staged stirring

conversion at phase inversion increased when stirring speed was reduced before phase inversion and no effect on conversion at phase inversion was observed if stirring speed was reduced after phase inversion. Therefore

only “effective stirring speed” corresponding to the stirring speed at phase inversion process actually influenced phase inversion. These results could be very important for performance control of HIPS.