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.