Zwitterionic group 4 aminophenolate catalysts for the polymerization of lactides and ethylene

Abstract

A series of group 4 metal chloride complexes based on aminophenol ligands were synthesized and characterized by conventional spectroscopic techniques, elemental analysis and X-ray crystallography. All the complexes are zwitterionic in nature wherein the protonated nitrogen atom of the aminophenolate backbone is charge neutralised by a negative charge on the metal center. The molecular structures show that moderate electrostatic hydrogen bonding interactions prevail between the NH⁺ donor moieties and acceptor chlorine atoms, which stabilise these complexes. These complexes show precise control towards the ring opening polymerization (ROP) of lactides (rac-LA, L-LA). Heterotactic enriched poly(lactic acids) (PLA) with good number average molecular weights (M_n) and narrow molecular weight distributions (MWDs) were obtained. The MALDI-TOF and ¹H NMR spectra of low molecular weight oligomers were analysed to understand the microstructure of the polymers and this revealed that the aminophenolate ligand initiates the ROP. The complexes, activated by methylaluminoxane (MAO), were also found to be moderately active for ethylene polymerization.