Aluminium complexes bearing N,O-aminophenol ligands as efficient catalysts for the ring opening polymerization of lactide

Abstract

Three new dimethyl aluminium complexes 1–3 containing aminophenol ligands (LAlMe2, where L1 = diethylaminophenoxide, L2 = diisopropylaminophenoxide and L3 = piperidinophenoxide) were synthesized and characterized by different spectroscopic techniques and elemental analysis. The solid state structure of 1 was determined by single crystal X-ray diffraction studies. The data revealed that 1 possesses as a distorted tetrahedral geometry around the aluminium centre. The catalytic activities of these complexes towards the ring opening polymerization (ROP) of rac-lactide (rac-LA) and L-lactide (L-LA) were examined and good efficiency with controlled number average molecular weight (Mn) and narrow molecular weight distributions (MWDs) were achieved. Homonuclear decoupled 1H NMR spectrum confirms the heterotactic enrichment in the PLA obtained from rac-LA. The MALDI-TOF mass and 1H NMR spectra of low molecular weight oligomers revealed that the aminophenoxide ligand initiates the ROP. The geometries of all the complexes were optimized at B3LYP/LANL2DZ level and DFT calculations were carried out in order to establish the relation between the reactivity and the HOMO–LUMO energy gap.