Ring-opening polymerization of rac-lactide mediated by tetrametallic lithium and sodium diamino-bis(phenolate) complexes

Abstract

Lithium and sodium compounds supported by tetradentate amino-bis(phenolato) ligands, [Li₂(N₂O₂^BuBuPip)] (1), [Na₂(N₂O₂^BuBuPip)] (2) (where [N₂O₂^BuBuPip] = 2,2′-N,N’-homopiperazinyl-bis(2-methylene-4,6-tert-butylphenol), and [Li₂(N₂O₂^BuMePip)] (3), [Na₂(N₂O₂^BuMePip)] (4) (where [N₂O₂^BuMePip] = 2,2′-N,N’-homopiperazinyl-bis(2-methylene-4-methyl-6-tert-butylphenol) were synthesized and characterized by NMR spectroscopy and MALDI-TOF mass spectrometry. Variable temperature NMR experiments were performed to understand solution-phase dynamics. The solid-state structures of 1 and 4 were determined by X-ray diffraction and reveal tetrametallic species. PGSE NMR spectroscopic data suggests that 1 maintains its aggregated structure in CD₂Cl₂. The complexes exhibit good activity for controlled ring-opening polymerization of rac-lactide (LA) both solvent free and in solution to yield PLA with low dispersities. Stoichiometric reactions suggest that the formation of PLA may proceed by the typical coordination–insertion mechanism. For example, ⁷Li NMR experiments show growth of a new resonance when 1 is mixed with 1 equiv. LA and ¹H NMR data suggests formation of a Li-alkoxide species upon reaction of 1 with BnOH.