Synthesis of Functionalized Sustainable Polyesters via Controlled Ring-opening Polymerization of O-carboxyanhydrides

Abstract

Despite the degradability and biocompatibility of poly(α-hydroxy acids), their utility remains limited because their thermal and mechanical properties are inferior to those of commodity polyolefins, which can be attributed to the lack of side-chain functionality on the polyester backbone. Attempts to synthesize high-molecular-weight functionalized poly(α-hydroxy acids) from O-carboxyanhydrides have been hampered by scalability problems arising from the need for an external energy source such as light or electricity. Herein, an operationally simple, scalable method for synthesizing stereoregular, high-molecular-weight (>200 kDa) functionalized polyesters have been developed by means of controlled ring-opening polymerization of O-carboxyanhydrides mediated by a highly redox reactive manganese complex and a zinc-alkoxide. Mechanistic studies indicated that the ring-opening process proceeded via the Mn-mediated decarboxylation with alkoxy radical formation (Chapter 2). In addition to the polymerization, a two-step facile chemical recycling strategy for poly(α-hydroxy acids) was developed to achieve closed-loop life cycles (Chapter 3). Moreover, this synthetic strategy is not limited to preparing homopolymers and block copolymers but also to producing stereoblock and gradient copolymers (Chapter 4). In particular, the gradient copolymers exhibited better ductility and toughness than their corresponding homopolymers and block copolymers, highlighting the potential feasibility of functionalized polyesters as strong and resilient polymeric materials (Chapter 5). Next, an atom-economical, scalable method for block copolymerization of O-carboxyanhydrides and epoxides to prepare functionalized poly(ester-b-carbonates) with high molecular weights (>200 kDa) was identified, that uses a single Lewis acidic zinc complex at room temperature in the absence of pressurized CO2 (Chapter 6). Kinetic studies showed that the first stage of the process, ring-opening polymerization of the O-carboxyanhydrides, exhibited zero-order kinetics, suggesting that the polymerization rate was independent of monomer concentration, thus allowing for a sharp switch in mechanism without a tapering effect (Chapter 7). The obtained poly(ester-b-carbonates) showed better toughness than their corresponding homopolymers and outperformed some commodity polyolefins (Chapter 8). Exploring this new chemical space of poly(ester-b-carbonates) via stereosequence-controlled synthetic methods would be a critical step toward improving this promising class of functionalized sustainable polymers (Chapter 9).