Synthesis and structures of tridentate ketoiminate zinc complexes that act as l-lactide ring-opening polymerization catalysts

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 ¹H and ¹³C 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.