The ring-crossover polymerization of cyclic dithioester 1 was performed in the presence of quaternary onium salts as catalysts at 70-150 degrees C for 24 h in NMR It was found that predictable cyclic polymers with the same repeating structures as 1 were obtained with M(n)s in the range between 700 and 3,500, quantitatively. It was observed that intermolecular and intramolecular thioester-exchange reactions proceeded between cyclic monomer 1 and resulting cyclic polymers under thermodynamic control to give a lower-molecular-weight cyclic polymer with a lower polydispersity ratio (M-n = 2,400, M-w/M-n = 1.70). (c) 2006 Wiley Periodicals, Inc.