Hemi-salen aluminum catalysts bearing N, N, O-tridentate type binaphthyl-Schiff-base ligands for the living ring-opening polymerisation of lactide

Abstract

Four hemi-salen aluminum complexes based on tridentate N, N, O-type binaphthyl-Schiff-base derivatives (1: R = ⁱPr; R₁ = H; 2: R = ⁱPr; R₁ = Cl; 3: R = 2-adamantyl; R₁ = H; 4: R = 2-adamantyl; R₁ = ^tBu) were prepared. These complexes were characterized by ¹H, ¹³C NMR spectroscopy and elemental analysis, these four complexes were employed for ring-opening polymerisation of L-lactide and rac-lactide. Complex 2, which was based on pro-ligand L₂ with smaller steric hindrance and electron-withdrawing substituents, displayed the highest activity for ROP of L-lactide among these complexes, and complex 4, which was supported by pro-ligand L₄ with the biggest steric hindrance, showed the highest stereoselectivity for the ROP of rac-lactide with partially isotactic polylactide with a P_m of 0.65. Kinetic studies revealed the ROP of L-lactide initiated by complex 3 had first-order dependency on [LA] as well as [Al].