The ring-expansion reaction of equimolar cyclic dithioester 1 with 3-phenoxypropyrene sulfide (PPS) (cyclic dithioester 1/PPS = 1/2) was examined in the presence of TBAC as a catalyst in NMP for 24 h. It was found that intermolecular ester-exchange reaction occurred during the insertion reaction of PPS into cyclic dithioesters, and cyclic polymers with M(n)s = 3600-8900 were obtained in 80-96% yields. The structures of the obtained cyclic polymers were confirmed by the H-1 NMR, C-13 NMR IR, and MALDI TOF-MS spectroscopy. The continuous insertion reaction of excess PPS into cyclic dithioester (cyclic dithioester 1/PPS = 1/50) was also examined, and it was found that the insertion reaction proceeded quantitatively to afford the corresponding cyclic polysulfide with M. = 42 000 and M-w/M-n = 4.50 in a 96% yield.