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

Three new dimethyl aluminium complexes 1-3 containing aminophenol ligands (LAlMe<inf>2</inf>, 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 (M<inf>n</inf>) 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.