The cyclic trithiocarbonate 1,5-dihydrobenzo[e][1,3]diehiepine-3-thione was synthesized by two different methods: dialkylation of trithiocarbonate anion with alkyl halides in a two-phase system using an onium salt as a phase-transfer agent and reaction between carbon disulfide and alkyl halides using a commercially available hydroxide form of an anion-exchange resin. Styrene polymerization was carried out in the presence of cyclic trithiocarbonate. The molecular weights of polymers before and after treatment with diamine were quite different, and they increased linearly with the polymerization time, while the molecular weight distribution of all polymers obtained with the cyclic trithiocarbonate were broad as compared with the current RAFT agent. It is proposed that polystyrenes prepared in the presence of the cyclic trithiocarbonate have multiblock structure with more than one trithiocarbonate function per chain, and the cyclic trithiocarbonate functions rather than comonomer since it keeps the merit of the RAFT agent.