The cationic ring-opening polymerization of a five-membered thiourethane [3-benzyl-1,3-oxazolidine-2-thione (BOT)] with boron trifluoride etherate afforded the corresponding polythiourethane with a narrow molecular weight distribution in an excellent yield. The molecular weight of the polymers could be controlled by the feed ratio of the monomer to the initiator. A kinetic study of the polymerization revealed that the polymerization rate of BOT (1.3 x 10(-2) L mol(-1) min(-1)) was two times larger than that of the six-membered thiourethane [3-benzyltetrahydro-1,3-oxazolidine-2-thione (BTOT); 6.8 x 10(-3) L mol(-1) min(-1)], and the monomer conversion obeyed the first-order kinetic equation. These observations, along with the successful results in the two-stage polymerization, supported the idea that this polymerization proceeded in a controlled manner. Block copolymerizations of BOT with BTOT were also carried out to afford the corresponding di- and triblock copolymers with narrow molecular weight distributions. The order of the 5% weight loss temperatures was as follows: poly(3-benzyltetrahydro1,3-oxazolidine-2-thione) [poly(BTOT)] > poly(BTOT54-b-BOT46) > poly(3-benzyl-1,3-oxazolidine-2-thione) [poly(BOT)]. This indicated that an increase in the BTOT unit content raised the decomposition temperature. The order of the refractive indices was poly(BOT) > poly(BTOT54-b-BOT46) > poly(BTOT54-b-BOT46-b-BTOT50) > poly(BTOT); this was in accord with the order of the sulfur content in the polymer chain. (c) 2006 Wiley Periodicals, Inc.