Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides

Poly(α-hydroxy acids) are important biodegradable polymers with wide applications. Attempts to synthesize them from O-carboxyanhydrides with pendant functional groups by various methods, including methods involving organocatalysts or organometallics, have been plagued by uncontrolled polymerization, including epimerization for some monomers, which hampers the preparation of stereoregular high-molecular-weight polymers. Herein we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, allowing for the synthesis of isotactic polyesters with expected molecular weights (>140 kDa) and narrow molecular weight distributions (M_w/M_n < 1.1). Mechanistic studies indicate that the use of a low temperature (-20 °C) and photoredox Ni/Ir catalysis synergistically accelerates ring-opening and decarboxylation of the monomer for chain propagation while avoiding the formation of the undesired Ni-carbonyl complex.