Poly(ester-alt-sulfide) (polymer 1) was synthesized by the alternating copolymerization of glycidyl phenyl ether (GPE) with gamma-thiobutyrolactone (TBL) catalyzed by either quaternary onium salts or crown ether complexes. The copolymerization proceeded to produce polymer 1 with good yields in neat or in various organic solvents at 30-120 degrees C, in which quaternary onium salts having Cl- as a counteranion such as tetrabutylammonium chloride (TBAC) had higher activity than quaternary onium salts such as tetrabutylammonium bromide having Br- as a counteranion. It was also found that the alternate copolymer (polymer 1) of GPE with TBL was obtained selectively under different feed ratios of GPE and TBL, although ring-opening homopolymerizations of GPE and TBL did not proceed. Copolymerizations of various oxiranes such as butyl glycidyl ether, styrene oxide, and 1,2-hexene oxide with TBL catalyzed by TBAC also proceeded, and the corresponding poly(ester-alt-sulfide)s (polymers 2, 3, and 4) were obtained under the same conditions as for the synthesis of polymer 1.