New Coordination Modes for Modified Schiff Base Ti(IV) Complexes and Their Control over Lactone Ring-Opening Polymerization Activity.

A series of eight new bis(alkoxy)bis(phenoxy-imine)titanium(IV) catalysts, coordinated by Schiff base ligands derived from o-vanillin (2-hydroxy-3-methoxybenzaldehyde), show good activity and control for the ring-opening polymerization of ε-caprolactone and ω-pentadecalactone. The new complexes are easily prepared in two high-yield steps from commercial reagents. The new ligands can all adopt two different coordination modes, depending on the steric bulk on the imine: a six-membered N-O chelate and/or a five-membered O-O chelate. The complexes show three different structures, depending on the ligand coordination mode: type A (N-O:N-O), type B (N-O:O-O), and type C (O-O:O-O). In all cases, the structures were confirmed in solution using variable temperature NMR spectroscopy and in the solid state using X-ray crystallography. The complex structure influences the polymerization rate, with the catalytic activities decreasing in the order: type C > type B > type A for both monomers. Overall, the work demonstrates potential to use these new ligands to access particular coordination modes, which allows enhancement of catalytic activity.