Carbonyl sulfide (COS), an analogue of carbon dioxide (CO2), can be converted to CO2 via the carbonic anhydride enzymes widely existing in nature. COS is an ideal monomer for making poly(monothiocarbonate)s, which are difficult to synthesize by traditional methods. Herein, for the first time, we describe an anionic copolymerization of COS with epoxides using alkali metal alkoxides as the catalysts (initiators), affording poly(monothiocarbonate)s with 100% alternating degree, >99% tail-to-head (T-H) content, high number-average molecular weights (Mns, up to 90.3 kg/mol) with narrow molecular weight distributions (?＝Mw/Mn, 1.05—1.31 for COS/propylene oxide copolymers) under solvent-free and mild conditions. Oxygen-sulfur exchange reaction (O/S ER), which can result in the production of contaminated dithiocarbonate and carbonate units in the main chain, was nearly completely depressed at 0 oC. In addition, in contrast to previously reported salen chromium (iron) complexes that required multiple synthetic steps, this work provides simple, low-cost, and effective catalysts for making colorless sulfur-containing polymers.