A series of NNO tridentate Schiff base ligands were used to prepare zinc amide and zinc phenoxide complexes that were shown to be efficient l -lactide ring-opening polymerization (ROP) catalysts. The complexes were prepared from ketoimines bearing a pendant quinoline donor, zinc bis(trimethylsilyl)amide, and 2,6-di- tert -butylphenol. They were characterized with 1 H and 13 C NMR, absorbance spectroscopy, microanalysis, and X-ray crystallography. The zinc amide and zinc phenoxide structures showed mononuclear complexes with tridentate coordination by the ketoiminate ligands. ROP of l -lactide with the zinc amides and phenoxide complexes gave isotactic poly- l -lactide with generally low molecular weight distributions. As compared to their amide counterparts, the zinc phenoxide complexes showed superior lactide ROP behavior in terms of percent conversion as a function of time, measured molecular weights closer to the predicted values, and lower polydispersity index values. Increasing size of the substituent at the 2-position on quinoline (H, Me, Ph) improved the synthesis of the complexes but adversely affected the ROP.
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Roberts et al. (2012) studied this question.