Ring-opening polymerization of cyclic esters with lithium amine-bis(phenolate) complexes

Abstract

Lithium compounds of tetradentate amino-bis(phenolato)-tetrahydrofuranyl ligands, Li₂[L1] (1) and Li₂[L2] (2) (where [L1] = 2-tetrahydrofuranyl-N,N-bis(2-methylene-4-methyl-6-tert-butylphenolate), and [L2] = 2-tetrahydrofuranyl-N,N-bis(2-methylene-4,6-tert-butylphenolate)) were characterized by multinuclear solution NMR and solid-state ⁶Li and ⁷Li NMR spectroscopy. The proligands, n-propylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), (H₂[L3]) and benzylamino-N,N-bis(2-methylene-4,6-di-tert-amylphenol), H₂[L4] were reacted with n-butyllithium in THF to give the related dilithium compounds Li₂[L3] (4) and Li₂[L4] (5), respectively. The pyridine adduct of 1, (py)₂Li₂[L1] (3) and complexes 4 and 5 have been structurally characterized by single-crystal X-ray diffraction and NMR spectroscopy. The reactivity of these complexes for the ring-opening polymerization of rac-lactide, as well as the influences of monomer concentration, monomer/Li molar ratio, polymerization temperature and time, were studied.