The synthesis of a three-armed polymer with an isocyanurate-thiourethane core structure is described. Monofunctional reversible addition-fragmentation chain transfer (RAFT) agent 2 and trifunctional RAFT agent 5 were prepared from mercapto-thiourethane and tris(mereapto-thiourethane), which were obtained from the aminolysis of mono- and trifunctional five-membered cyclic dithiocarbonates, respectively. The radical polymerization of styrene in the presence of 2,2'-azobis(isobutyronitrile) and RAFT agent 2 in bulk at 60 degrees C proceeded in a controlled fashion to afford the corresponding polystyrene with desired molecular weights (number-average molecular weight = 3000-10,100) and narrow molecular weight distributions (weight-average molecular weight/number-average molecular weight < 1.13). On the basis of the successful results with the monofunctional RAFT agents, three-armed polystyrene with thiourethane-isocyanurate as the core structure could be obtained with trifunctional RAFT agent 5 in a similar manner. (c) 2005 Wiley Periodicals, Inc.