Unprecedented activity (TOP > 270 000 h(-1)), high polymer selectivity (>99%), and excellent durability (TON > 600 000) were observed in the copolymerization of carbonyl sulfide (COS) and epoxides mediated by the single-site bifunctional chromium catalyst containing a Lewis acidic metal center and a sterically hindered organic base in a molecule, selectively producing the corresponding poly(thiocarbonate)s with completely alternating structure, high molecular weight, and narrow monodispersity. No oxygen sulfur exchange reaction occurred even at an elevated temperature of 80 degrees C. Nevertheless, this catalyst was not efficient for CO2/epoxides copolymerization. The presence of CO2 completely inhibits the reactivity of COS. Contrarily, the corresponding Co(III) complex with the same ligand showed very low activity for COS/epoxides copolymerization but excellent activity (TOP > 13 000 h(-1)) for CO2/epoxides copolymerization at both ambient and high temperatures, affording the polycarbonates with >99% carbonate linkages and high molecular weight up to 467.0 kg/mol.