The bulk free-radical polymerization of styrene in the presence of a new cyclic trifunctional initiator, 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane, was studied. Full-conversion-range experiments were carried out to assess the effects of the temperature and initiator concentration on the polymerization kinetics, molecular weight, and polydispersity. Gel permeation chromatography was used to measure the molecular weight and the molecular weight distribution of polystyrene. When this multifunctional initiator was used for styrene polymerization at higher temperatures, it was possible to produce polymers with higher molecular weights and narrower molecular weight polydispersity at a higher rate. This showed that the molecular weight and polydispersity were influenced by the initiator concentration and the polymerization temperature in an unusual manner. Moreover, polystyrene, obtained with trifunctional peroxide, had O-O bonds in the molecular chains and was investigated with differential scanning calorimetry and gel permeation chromatography. (C) 2004 Wiley Periodicals, Inc.