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

Lithium and sodium compounds supported by tetradentate amino-bis(phenolato) ligands, [Li-2(N2O2BuBuPip)] (1), [Na-2(N2O2BuBuPip)] (2) (where [N2O2BuBuPip] = 2,2'-N,N'-homopiperazinyl-bis(2-methylene-4,6-tert-butylphenol), and [Li-2(N2O2BuMePip)] (3), [Na-2(N2O2BuMePip)] (4) (where [N2O2BuMePip] = 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, Li-7 NMR experiments show growth of a new resonance when 1 is mixed with 1 equiv. LA and H-1 NMR data suggests formation of a Li-alkoxide species upon reaction of 1 with BnOH.