The thermal polymerization of styrene (St) in the presence of cyclic 1,2-disulfides at 120 degreesC was investigated. In the polymerization of St in the presence of 1,2-dithiane (DT), that is, six-member cyclic 1,2-disulfide, the polymer yields and molecular weights increased with the reaction time. The linear relation between the polymer yields and molecular weights was observed, and the line passed through an original point. The molecular weight distributions of the polymers remained almost constant but were not narrow. For this polymerization with a living nature, we pro posed the following mechanism: the propagating St radical reacted with thiyl radicals derived from DT, leading to the formation of dormant species, and the formed CS bond of the dormant was dissociated again to give the propagating polystyryl radical and thiyl radical. Similar results were obtained in the thermal polymerization of St at 120 degreesC in the presence of 1,2-dithiacycloheptane, that is, seven-member cyclic 1,2-disulfide. (C) 2000 John Wiley & Sons, Inc.