Poly(styrene peroxide) (PSP), an alternating copolymer of oxygen and styrene, has been used as an initiator for the radical polymerizations of methyl methacrylate and styrene. Thermal decomposition of PSP follows first order kinetics with an activation energy of 160.0 kJ/mol. Polymerizations of MMA and styrene (60-90°C) follow the usual kinetics. The efficiency of the initiator, PSP, was found to be low (0.006-0.014), attributed to the competing unimolecular decomposition of macroalkoxy radicals and primary radical initiation. The presence of peroxy segments in the backbone of PMMA and polystyrene (PS) was confirmed by H 1 NMR and DSC studies.