Organic Lewis pairs for selective copolymerization of epoxides with anhydrides to access sequence-controlled block copolymers†
Poly(ester-b-ether)s, a kind of meaningful and potentially bio-based block copolymer, are typically synthesized via ring-opening polymerization (ROP) of cyclic esters initiated by the pre-prepared polyether macroinitiator. However, a multi-step procedure is required in this traditional method. Herein, we detailed a one-step selective copolymerization process of mixed monomers to precisely prepare poly(ester-b-ether) block copolymers, which could minimize the additional catalytic processes and organic reagents of the multi-step procedure. The strategy enables ring-opening alternating copolymerization (ROAC) of anhydrides with epoxides and ROP of epoxides sequentially in a closed system. Organic Lewis pairs were utilized to catalyze the selective copolymerization and were easily removed from the reaction system which avoided the metal residues in the copolymers. Regioselective and living copolymerization is realized by using cooperative organocatalysts, yielding the poly(ester-b-ether) with a controlled monomer sequence, high isotacticity and predictable molecular weight (MW). The optimized organocatalysis exhibits broad monomer adaptability and allows fabricating structurally diverse poly(ester-b-ether)s. Epimerization was suppressed even when tricyclic and bicyclic anhydrides were used, which enables the synthesis of stereoregular block copolymers. Significantly, well-defined block copolymers based on renewable succinic anhydride were achieved. A tandem copolymerization is also performed by using lactide as an external trigger which could switch the selective copolymerization to lactide polymerization. The study provides a new route to the facile synthesis of poly(ester-b-ether) block copolymers using a metal-free catalytic system.