Lactide Polymerization with Chiral β-Diketiminate Zinc Complexes

Abstract

N,N′-Di(S-phenylethyl)-2-amino-4-iminopent-2-ene, S,S-nacnacCH(Me)PhH, 1a, and N,N′-dibenzyl-2-amino-4-iminopent-2-ene, nacnacBnH, 1b, react with ZnEt2 to form the corresponding nacnacZnEt complexes 2a and 2b. Neither complex is reactive with 2-propanol or methyl lactate to produce the corresponding alkoxide complexes. In reactions with 2b, ligand redistribution occurs and nacnacBn2Zn was obtained. Reaction of 1a and 1b with Zn(N(SiMe3)2)2 yielded nacnacZnN(SiMe3)2, 6a and 6b. From further reactions with 2-propanol nacnacZnOiPr, 7a and 7b, were obtained. Both complexes were catalytically active for polymerization of rac-lactide with apparent first-order rate constants of kapp = 0.013−0.019 min−1 and 0.019−0.038 min−1 for 7a and 7b, respectively. Obtained polymers were highly heterotactic, with Pr = 0.84−0.87 (7a) and 0.65−0.71 (7b). Analysis of remaining monomer after 75% conversion showed negligible ee and indicates that chiral 7a does not show enantioselectivity in rac-lactide polymerization. Complex 7a, but not 7b, catalyzes unselective transesterification of the polymer during and after polymerization. PLA microstructures in polymerizations with 7b were independent of temperature (23 or 0 °C) or monomer/catalyst ratio (100:1 to 400:1). However, slightly higher Pr values were obtained in the presence of 10−100 equiv of MeCN or pyridine. Complexes nacnacBn2Zn, 2b, 6b, and 7a were characterized by an X-ray diffraction study.