Functionalized Cyclic Poly(α‐Hydroxy Acids) via Controlled Ring‐Opening Polymerization of O‐Carboxyanhydrides

Linear poly(α-hydroxy acids) are important degradable polymers, and they can be efficiently prepared by ring-opening polymerization of O-carboxyanhydrides with pendant functional groups. However, attempts to prepare cyclic poly(α-hydroxy acids) have been plagued by side reactions, including epimerization and uncontrolled intramolecular chain transfers or termination, that prevent the synthesis of high-molecular-weight stereoregular cyclic polyesters. Herein, we report a scalable method for the synthesis of high-molecular-weight (>100 kDa) stereoregular functionalized cyclic poly(α-hydroxy acids) by means of controlled polymerization of O-carboxyanhydrides using a catalytic system consisting of a lanthanum complex with a sterically bulky ligand and a manganese silylamide. Additionally, using this system, we could readily prepare cyclic block poly(α-hydroxy acids) by means of sequential addition of O-carboxyanhydrides. The obtained cyclic polyesters and their cyclic block copolyesters exhibit distinctive physicochemical properties-including elevated phase transition temperature, improved toughness, and reduced viscosity-compared to their linear counterparts.