Alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO2) was realized under mild conditions with a moderate turnover frequency (TOF), employing sole bifunctional cobalt salen complexes containing Lewis acid metal center and covalent bonded Lewis base on the ligand. Variation of the covalent bonded Lewis base substituents on the salen ligands could tailor the catalytic activity with TOF changing from 19.3 to 34.9 h(-1), polymeric/cyclic carbonate selectivity from 95.3 to 72.8%, and the head-to-tail structure in the polymer from 72.2 to 86.0%. The IR analysis confirmed that the Lewis base moiety on one molecule could coordinate with cobalt center of adjacent molecule, playing similar role to the Salen metal complex/Lewis base binary catalytic system. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:359-365,2010