Highly Active Chiral Oxazolinyl Aminophenolate Magnesium Initiators for Isoselective Ring-Opening Polymerization of rac-Lactide: Dinuclearity Induced Enantiomorphic Site Control

The most important challenge in ring-opening polymerization of rac-lactide today is to obtain isotactic PLAs with high molecular weights. The goal of our work is to address this important challenge by developing novel catalysts that enable precise control over stereoselectivity and produce high molecular weight PLAs at the same time. Here we report some rare examples of isoselective magnesium complexes toward the ROP of rac-LA, which are supported by chiral oxazolinyl aminophenolate ligands. Preliminary kinetic investigations confirmed that isotactic PLAs could be achieved via an enantiomorphic site control mechanism by using these chiral magnesium complexes. A pair of racemic magnesium initiators proved to integrate hyperactivity (TOF up to 54 000 h^–1 at 25 °C) with sufficient isoselectivity (P_m = 0.80) toward the ROP of rac-LA, leading to the formation of isotactic stereoblock PLA with high molecular weight (M_n = 461 kg mol^–1) and a semicrystalline property (T_m = 164 °C). Detailed structural investigation on the magnesium lactate model complexes suggested that the high degree of enantiomorphic site control is derived from both the ligand chirality and the dinuclear feature of the magnesium active species.