Carbocyclic-Fused Amino/iminopyridine-Iron Catalysts for Controlled Lactide Ring-Opening Polymerization

Pervasive plastic pollution poses a critical threat to ecosystems, public health, and global economies, highlighting the need for biodegradable polymers synthesized with 100% atom economy. In this study, a series of carbocyclic-fused amino/iminopyridine-iron dichloride complexes, varying in coordination mode (bidentate and tridentate) and steric/electronic properties, were evaluated for the ring-opening polymerization of lactide. In the presence of propylene oxide as an activator, iron complexes bearing bidentate ligands exhibited superior and more controlled catalytic performance than tridentate complexes, reaching turnover frequencies as high as 186 h^-1, whereas their tridentate counterparts produced polylactides with lower conversion and molecular weights. Modulation of the ligand structure exerted a pronounced influence on polymer characteristics, affording PLA with a number-average molecular mass of 4.8 × 10³ to 13.5 × 10³ g mol^-1 and narrow dispersities (Đ: 1.26-1.95). Especially, higher propylene oxide loadings enhanced catalytic activity at the expense of polymerization control, while a PO/Fe ratio of 300 resulted in slightly diminished conversion but improved molecular weight control. MALDI-TOF mass spectrometric analysis revealed polymer chains initiated through epoxide ring opening by chloride or trace moisture, producing end groups of -OCH-(Me)-CH₂Cl/-OH or -OCH-(Me)-CH₂OH/-OH.