Various olefins that are activated towards free radical addition and contain a homolytic leaving group in the allylic position are effective chain transfer agents in free radical polymerizations of methyl methacrylate and other monomers. These allylic compounds include bromides, phosphonates, stannanes, thioethers, sulfoxides, and sulfones. Allylic silanes and chlorides, however, do not posess significant chain transfer activity. Suitable activating substituents towards radical addition are phenyl, ethoxycarbonyl, cyano, and acetoxy. Several of the compounds have an advantage over thiols in that they do not contain sulfur.