Alternating Radical Ring-Opening Polymerization of Cyclic Ketene Acetals: Access to Tunable and Functional Polyester Copolymers

Radical ring-opening polymerization of cyclic ketene acetals (CKAs) provides a route to synthesize degradable polyesters or to impart degradability onto traditionally nondegradable vinyl polymers under the mild conditions afforded by radical polymerization. Copolymerization of CKAs with vinyl monomers is a promising strategy to prepare functional degradable polymers. However, such copolymerizations often result in poor incorporation of the ester moiety due to low relative reactivity of CKAs and the propensity to polymerize through the exomethylene bond via 1,2-addition, without ring-opening to the ester moiety. We demonstrate that the copolymerization of 5,6-benzo-2-methylene-1,3-dioxepane (BMDO) with a variety of maleimides proceeds in a highly alternating fashion with quantitative ring-opening of BMDO to the ester, producing alternating polyester copolymers. Modifying the N-substituent of the maleimide provided a straightforward route to tuning the thermal properties of the polymers and introducing functionality for postpolymerization modification. Additionally, a hydrophilic macro-chain transfer agent was chain extended via alternating copolymerization of BMDO and a maleimide to yield amphiphilic block copolymers that self-assembled into degradable nanoparticles.