An Air Stable Cationic Indium Catalyst for Formation of High-Molecular-Weight Cyclic Poly(lactic acid)

Overcoming entropic constraints to selectively produce high-molecular-weight cyclic poly(lactide) (c-PLA) is a challenge. In this work, we use an air-stable cationic alkyl indium complex featuring a hemilabile pyridine pedant arm to reproducibly synthesize c-PLA exclusively with low dispersity (Đ ∼ 1.30) and high molecular weights (up to 416,000 g mol–1). The complex remains active after extended periods of exposure to high-humidity air. The process allows the formation of high-molecular-weight c-PLLA, c-PDLA, and their stereocomplex with no epimerization. We propose that polymerization occurs through a cooperative Lewis-pair-based coordination–insertion mechanism where the monomer coordination to the cationic indium center and ring-opening of lactide by the hemilabile pyridine donor group are essential steps.