Record-High-Molecular-Weight Polyesters from Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides Catalyzed by Hydrogen-Bond-Functionalized Imidazoles

Abstract

Polyesters, with potential for degradability and sustainability, are some of the most versatile polymer materials. However, the limitation of molecular weight (MW) presents a barrier to their applications. The synthesis of polyesters with high MW by the ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is promising but rare and challenging. Herein, we report a series of air-stable, hydrogen-bond-functionalized imidazole catalysts for the copolymerization. These catalysts can produce polyesters (4 examples) using cyclohexane oxide (CHO), propylene oxide (PO), phenyl glycidyl ether (PGE), 4-vinyl-1-cyclohexene 1,2-epoxide (VCHO), and phthalic anhydride (PA) with record-high MW: M_n = 171.2 kDa for poly(CHO-alt-PA), M_n = 518.5 kDa for poly(PO-alt-PA), M_n = 100.5 kDa for poly(PGE-alt-PA), and M_n = 236.4 kDa for poly(VCHO-alt-PA). Furthermore, it can achieve an unprecedented efficiency of 15.6 kg of polyester/g of catalyst at a molar ratio of catalyst/PA/PO = 1:40000:60000. The record-high MW achieved can be attributed to the unique anionic-cationic coexisting ROCOP mechanism, which can reduce transesterification, chain transfer, and annulation side reactions. All high M_n polyesters showed excellent thermal stability, high tensile strength, and a Young’s modulus comparable to some commodity thermoplastics like polystyrene and polylactic acid.