Zirconium and hafnium complexes bearing pyrrolidine derived salalen-type {ONNO} ligands and their application for ring-opening polymerization of lactides.

The reactions of pyrrolidine derived salalen-type {ONNO} ligands (S)-L-H with 1 equiv. M(OPr)(HOPr) (M = Zr or Hf) in diethyl ether yielded complexes [LM(OPr)] (L = L, M = Zr (1); L = L, M = Zr (2), Hf (3); L = L, M = Zr (4), Hf (5)). All of these complexes were well characterized by NMR spectroscopy, elemental analyses and single-crystal X-ray analysis in the case of complexes 1 and 3-5. X-ray structural determination revealed that these complexes were analogous mononuclear species and had a similar structure in which the metal centers were six-coordinated to two oxygen atoms and two nitrogen atoms of one ligand and two oxygen atoms of two isopropoxy groups. All of these complexes efficiently initialized the ring-opening polymerization of lactides to afford polymers with controlled molecular weight and narrow polydispersity. Furthermore, the ring-opening polymerization of rac-lactide catalyzed by complexes 1-5 afforded isotactic-enriched polymers in solution (P = 0.74-0.80) and under melt conditions (P = 0.63-0.72).