Organo-Catalyzed Cationic Ring-Opening Copolymerization of Cyclic Anhydrides with Oxolanes: Access to Structurally Diverse Polyesters

The cyclic ether candidates for ring-opening copolymerization (ROCOP) are mainly three-membered oxiranes with high ring tension, while ROCOP of cyclic anhydrides with low ring strain oxolanes was seldom thoroughly investigated. Herein, we report rapid cationic copolymerizations of tetrahydrofuran (THF) with anhydrides to facilely synthesize alternating polyesters. These processes are realized by a highly active organic Lewis acid trityl tetrakis(pentafluorophenyl)borate ([Ph3C](+)[B(C6F5)(4)](-)) that is almost inactive for THF homopolymerization. Poly(ester-co-ether)s with AB, ABB, and ABBB-type repeating units (A = anhydride, B = THF) are formed during copolymerization. The alternating enchainment of anhydride and THF can produce AB-type units, while the coupling reactions between trityl ether and oxonium chain ends will generate ABB- and ABBB- type units. The model reactions suggests that AB-, ABB-, and ABBB-type repeating units are interconvertible, and their relative content is determined by reaction time, temperature, and monomer feed ratio. Alternating polyesters are finally formed by feeding equimolar THF and cyclic anhydride. [Ph3C](+)[B(C6F5)(4)](-)-catalyzed ROCOP is further extended to other biomass-derived oxolanes and 14 commercially available cyclic anhydrides, thus allowing for the fabrication of polyesters with structural and functional diversity. This methodology provides an alternative approach to some "AB"-type alternating polyesters that are usually obtained by traditional step-growth polycondensation.