The completely alternating copolymerization of carbonyl sulfide(COS) and styrene oxide was found to occur under mild reaction conditions (0?30 °C and 1.5 MPa) in the presence of (salen)CrCl/onium salt catalyst systems toafford high molecular weight poly(monothiocarbonates) with narrow molecularweight distributions. Ring-opening of styrene oxide was shown to be 88% selectiveat the methylene carbon. That is the reaction is driven by steric hindrance, wherering-opening occurs preferentially at the less congested carbon center. Similarresults were found upon utilizing the tetramethyltetraazaannulene (tmtaa)CrCl/onium salt catalyst. On the other hand, upon employing a zinc?cobalt doublemetal cyanide (Zn?Co DMCC) catalyst, where the ligands around the active zincsite are not sterically encumbering, ring-opening of styrene oxide occurspredominantly at the methine carbon site; i.e., the reaction is electronically driven.