Synergistic Steric and Electronic Ligand Engineering Unlocks High-Performance Schiff Base for Lactide Ring-Opening Polymerization

Abstract

Advancing the catalytic precision for the ring-opening polymerization (ROP) of lactide is essential for the scalable and sustainable production of polylactic acid (PLA). Here, we report a ligand engineering strategy for Schiff base salen catalysts, where the synergistic combination of a π-extended naphthalene spacer and an electron-deficient benzoic acid moiety is harnessed to tailor the metal center's activity and selectivity. A library of ligands and their Cu (II), Mn (II), and Ni (II) complexes was synthesized and structurally authenticated. Screening identified the Cu (II) complex bearing the naphthalene-benzoic acid hybrid ligand (HNA-DABA@Cu) as a standout performer, delivering PLA in 49.4% isolated yield with > 94% monomer conversion under optimized conditions (150°C, 12 h, catalyst/monomer = 1:2000) and affording polymer with molecular weight (Mw) of ~180 kDa. Comprehensive characterization (XRD, FT-IR, SEM, EA and HR-ESI-MS) reveals that the synergistic ligand framework promotes the formation of a well-defined Cu (II) active site embedded in a highly crystalline molecular assembly, as evidenced by the sharp XRD reflections and the needle-like single-domain morphology. This work establishes that the concerted steric and electronic modulation offered by judiciously paired functional groups is a potent design principle for developing high-performance catalysts in controlled polymerization.