CO2(CO)(8) catalyzes the ring-opening copolymerization of propylene oxide with CO to afford the polyester in the presence of various amine cocatalysts. The H-1 and C-13 {H-1} NMR spectra of the polyester, obtained by the Co-2(Co)(8)-3-hydroxypyridine catalyst, show the following structure -[CH2-CH(CH3)-O-CO](n)-. The CO2(CO)(8)-phenol catalyst gives the polyester, which contains the partial structural unit formed through the ring-opening copolymerization of tetrahydrofuran with CO. The bidentate amines, such as bipyridine and N,N,N',N'-tetramethylethylenediamine, enhance the Co complex-catalyzed copolymerization, which produces the polyester with a regulated structure. Acylcobalt complexes, (RCO)Co(CO)(n) (R = Me or CH2Ph), prepared in situ, do not catalyze the copolymerization even in the presence of pyridine. This suggests that the chain growth involves the intermolecular nucleophilic addition of the OH group of the intermediate complex to the acyl-cobalt bond, forming an ester bond rather than the insertion of propylene oxide into the acyl-cobalt bond. Co-2(Co)(8)-Ri(3)(CO)(12) mixtures also bring about the copolymerization of propylene oxide with CO. The molar ratio of Ru to Co affects the yield, molecular weight, and structure of the produced copolymer. The catalysis is ascribed to the Ru-Co mixed-metal cluster formed in the reaction mixture.