Polymer peroxides were synthesized by copolymerizing tert-butyl-3-isopropenylcumylperoxide (D-120) with styrene (St). Exothermic peak at 192.7 degrees C in DSC thermogram indicated that peroxy bonds in D-120 remained intact during the copolymerizing process. The polymeric peroxide was used to initiate polymerization of St. GPC results showed that polystyrene (PS) initiated by the polymeric peroxides was composed of both linear and branched molecules. In addition, the rheology test showed that PS samples initiated by polymeric peroxide contained branched structure and had lower shear viscosities. (c) 2006 Wiley Periodicals, Inc.