Novel one-pot syntheses of sulfur-containing polymers from a bifunctional five-membered cyclic dithiocarbonate (1a) were carried out. Polythiourethanes were obtained by the polyaddition of 4,4＇-methylenebis(phenyl isocyanate), tolylene 2,6-diisocyanate, and hexamethylene diisocyanate with a dithiol(2a) obtained by the reaction of 1a and benzylamine under mild conditions. Polythioesters were also obtained by the polycondensation of terephthaloyl and succinyl chlorides with 2a. Further, polythioether was obtained by the polycondensation of alpha,alpha＇-dibromo-p-xylene with 2a.