The highly active ring-opening copolymerization of epoxides with γ-thiobutyrolactone (TBL) was achieved by using a three-component catalytic system consisting of chromium complex, bis(triphenylphosphine)iminium chloride and lithium tert-butoxie (_t^BuOLi). In the polymerization process, _t^{BuOLi can} effectively stabilize the negative sulfur anions at the chain ends and suppress backbiting reaction to ensure the reaction activity and molecular weight of the polymer. The copolymer structure were characterized by ₁^H NMR, ₁₃^{C NMR}, ESI-MS and FTIR. The thermal properties of the resulting poly(ester-thioether)s were investigated by DSC and TGA methods. The resultant copolymers display perfect alternating structures and amorphous forms, with glass transtion temperatures (T_g) ranging from ?53 ℃ to ?18 ℃ and the decomposition temperatures at 5% weight loss between 258 and 300 ℃, showing good thermal stability. Further oxidation of thioether units resulted in the formation of the polymers containing sulfoxide and sulfone structures, which show enhanced T_gs. Taking the copolymer of tert-butyl glycidyl ether and TBL as an example, the thioether was oxidized to sulfoxide and sulfone structure, and the T_g was increased by 49 and 60 ℃, respectively.