A (ONSO)CrCl complex-based binary catalyst for efficient ring-opening copolymerization (ROCOP) of epoxide and norbornene anhydride (NA) in a stereoselective manner to afford polymers with variant geometries is reported. The role of (ONSO)CrCl complex on the cis/trans selectivity of resulting polyester as well as enhancement of the reaction rate have been revealed. The formation of chelates between the endo-diester unit of the polyester chain and the metal center plays a determinate role in the stereoselectivity of the cis-(endo, endo)-polyester in solution copolymerization of endo-NA with epoxides in the presence of metal complex. Additionally, the geometric structure of resulting polyester is also dependent on the polymerization method, NA type, as well as the bulkiness of the substituent of the formed ester units in the polyester chain. Accordingly, a proposed mechanistic pathway for the stereostructure formation of polyesters is provided.