A series of Salen-Co(Ⅱ) complexes were synthesized to study the effect of O2 on the catalytic performance of Salen-Co complexes for the copolymerization of PO/CO2.The Salen-Co(Ⅱ) complexes showed low activity on the cyclo-addition of CO2 to PO with the aid of a cocatalyst such as PPNCl.Unexpectedly,with the addition of O2,the activity of Salen-Co(Ⅱ) complexes was obviously increased and 100％ cyclic carbonate was obtained.As the pressure of O2 increased,the activity of the complex also increased.With the existence of O2,the activity of the complexes was influenced by their structures and the pressure of O2,and the complexes with the conjugated structure showed higher activity.The structures of cocatalyst also played a crucial role as for the change of the activity.By altering the electrophilicity of Salen-Co(Ⅲ),O2 can also be used as cocatalyst for the copolymerization of PO/CO2.