Development of Highly Enantioselective Catalysts for Asymmetric Copolymerization of meso-Epoxides and Cyclic Anhydrides: Subtle Modification Resulting in Superior Enantioselectivity

The asymmetric alternating copolymerization of meso-epoxides with cyclic anhydrides promoted by chiral catalysts or reagents is a powerful strategy for the synthesis of optically active polyesters with main-chain chirality. Herein, we show that, in conjunction with a nucleophilic cocatalyst, enantiopure dinuclear Al(III) complexes efficiently catalyze this asymmetric copolymerization, exhibiting high activity and achieving enantioselectivities up to 99% ee under mild conditions. Copolymer enantioselectivity and catalytic activity are revealed to be strongly affected by the axial linker, chiral diamine structure, and phenolate ortho-substituents. Density functional theory calculations confirm that the reactions corresponding to ring opening at (R)-C-O and (S)-C-O bonds of the Al-coordinated meso-epoxide during copolymerization exhibit significantly different Gibbs free energies of activation (Delta(double dagger)G). Enantiopure dinuclear Al(III) complex 3 bearing a hydrogenated binaphthol linker with the matched configuration is demonstrated to be the most active and enantioselective catalyst, featuring a broad substrate scope and allowing one to obtain a wide range of isotactic polyesters with a completely alternating structure and low polydispersity. Notably, most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures between 120 and 240 degrees C. Additionally, the mixing of selected isotactic (R)- and (S)-polyesters in a 1:1 mass ratio afforded two crystalline stereocomplexes that exhibited enhanced thermal stability as well as new crystallization behavior and therefore significantly differed from the parent enantiopure polymers.