Cationic ring-opening polymerization of a cyclic carbonate carrying an exomethylene moiety, 5-methylene-1,3-dioxan-2-one (ExTMC), was carried out, and its behavior was compared with a cyclic carbonate having no exomethylene group, 1,3-dioxan-2-one (TMC). ExTMC afforded the polymers with larger molecular weights than TMC under any conditions. The observed molecular weights of the polymers agreed well with the calculated ones in the polymerization of ExTMC with TfOH at 0 degrees C. A linear relationship was observed between the conversion and number-average molecular weight of the polymer obtained in the polymerization of ExTMC, which could not be observed in the polymerization of TMC. After ExTMC was completely converted, the polymerization took place again when the same amount of ExTMC was introduced into the polymerization mixture, The effect of the exomethylene group on the polymerization behavior was analyzed by an ab initio molecular orbital calculation to determine the possibility of the presence of an allylic cationic intermediate. It was suggested that the stabilization of the propagating cation by the exomethylene group caused the living character of the cationic polymerization of ExTMC. Furthermore, a block copolymer of ExTMC with a vinyl ether was synthesized as an application of the pseudo-living polymerization of ExTMC.