We synthesized two kinds of monocarbamothioates TZO(R) containing a thiazolidin-2-one (TZO) skeleton and TZD(R) containing a thiazolidine-2,4-dione (TZD) skeleton. In the reaction of TZO(R) and phenoxypropyrenesulfide (PPS) using tetrabutylammonium chloride (TBAC) as a catalyst in 1-methyl-2-pyrrolidinone at 60 degrees C, anionic ring-opening polymerization of PPS proceeded to give the corresponding linear polysulfides, polyPPS, with M-n = 75 860-206 700 (M-w/M-n = 1.54-1.73) in high yield. In contrast, in the reaction of TZD(R) and PPS, the continuous insertion reaction of PPS into TZD(R), i.e., living ringexpansion polymerization, occurred to afford cyclic polysulfides TZD(R)-c-poly(PPS)(n) with M-n = 1870-33 420 (M-w/M-n = 1.07-1.26) in high yield. The ring size could be controlled by the feed ratio of TZD(R)/PPS in the range of 1/5-1/100.