**4.3. New advances in the copolymerization of CO2 and epoxides**

To enhance the catalytic activity and selectivity towards the copolymerization of CO2 and epoxides, several measures have been taken such as bimetallic catalyst systems, non-metallic catalysts, ionic liquids (ILs), and supported catalysts.

## *4.3.1. Bimetallic catalyst systems*

The idea behind the bimetallic pathway for the copolymerization of carbon dioxide and epoxides is to conduct the ring opening of epoxide and activation of carbon dioxide at the same time. In fact, one of the metal centres is in charge of the coordination and activation of the epoxide and the other metal centre associates with coordination and activation of CO2 [28]. It was found that the bimetallic pathway involved a second-order dependence on the metal concentration, whereas the monometallic pathway associated with a first order kinetic [30]. For instance, a double zinc centre complex showed high activity and selectivity in the conver‐ sion of cyclohexene oxide and CO2 to PCHC at very low CO2 pressure [83]. Particularly, the bimetallic pathway showed remarkable improvement in the catalyst activity of the salen catalysts. In addition, a piperidinium end-capped (salen) Co(III) complex copolymerized PO and CO2 without the generation of cyclic propylene carbonate [59].
