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 °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 ring-expansion 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.