The semicrystalline poly(monothiocarbonate)s were prepared by the copolymerization of carbonyl sulfide (COS) and ethylene oxide, an achiral epoxide, using a bifunctional chromium(III) complex as catalyst. The resultant copolymer, possessing perfectly alternating structure, high molecular weight, and narrow polydispersity, has a melting temperature of 128.2 degrees C, with a melting enthalpy up to 75.44 J/g. Moreover, an ABA triblock copolymer containing the "hard" semicrystalline poly(ethylene monothiocarbonate) (A) and the "soft" amorphous poly(propylene monothiocarbonate) (B) is synthesized by stepwise addition of epoxides. The tensile testing demonstrates the triblock copolymer may have the potential as a thermoplastic elastomer.