AbstractThiol/dimethyl sulfoxide systems are good initiators for the polymerization of methyl methacrylate (MMA) and some other acrylate monomers in benzene medium, but they fail to initiate the polymerization of styrene, vinyl acetate and acrylonitrile. The rate of polymerization of MMA is dependent on the chain length of the thiol used and dodecanethiol is found to be the most reactive one. The presence of traces of oxygen increases the rate of polymerization probably by forming a co‐redox system with the thiol. The polymerization is completely inhibited by 2,2‐diphenyl‐1‐picrylhydrazyl and benzoquinone indicating the free radical nature of the reaction.The effect of butanethiol as well as that of dimethyl sulfoxide (DMSO) individually on the AIBN‐catalysed polymerization of MMA in benzene medium was examined. Butanethiol does not affect the rate of the radical polymerization, but reduces the molecular weight of the polymers formed due to a chain transfer. DMSO has no effect on the radical polymerization up to a concentration of 6,42.10−2mol 1−1, above which it reduces the rate and also the molecular weight probably by forming a growing radical‐DMSO adduct of a reactivity lower than that of the original g