Peroxide initiators are disclosed that can be used at lower temperatures during HIPS production. Enhanced formation of graft polymers between polystyrene and polybutadiene (rubber) can be accomplished by using these lower temperature peroxide initiators that have a 1 hour half-life period at one or more temperatures from 95° C. to less than 111° C. during the pre-inversion stage. “Higher” temperature initiators are used during and/or after inversion. The use of such “low” temperature initiators improves polymerization rates and grafting values, while reducing the swell index and the amount of rubber used, thereby producing improved HIPS more rapidly and at a lower cost. Also disclosed are HIPS compositions and products produced by such “low” temperature initiators as well as a method of producing improved HIPS.