Bis(amido)titanium complexes having chelating diaryloxo ligands bridged by sulfur or methylene and their catalytic behaviors for ring-opening polymerization of cyclic esters

A series of bis(dialkylamido) titanium complexes coordinated by O–E–O (E=S, CH2) chelating bis(aryloxo) ligands, Ti[E(4-Me-6-tBuC6H2O)2](NR2)2 (1: E=S, R=Me; 2: E=S, R=Et; 3: E=CH2, R=Me; 4: E=CH2, R=Et), were synthesized by the reaction of Ti(NR2)4 (R=Me, Et) with 2,2′-E(4-Me-6-tBuC6H2OH)2 (E=S, CH2). The crystal structures of complexes 2 and 4 were determined by X-ray diffraction study. Complex 2 has a trigonal bipyramidal structure with the sulfur and nitrogen atom in trans positions and complex 4 has a tetrahedral structure. The complex 2 was found to initiate the controlled polymerization of ε-caprolactone in toluene at 100 °C affording poly(ε-caprolactone) and poly(l-lactide) with relatively narrow molecular weight distributions. The complex 4 showed higher activity for the polymerization of ε-caprolactone to give a polymer with a broader molecular weight distribution in comparison with complex 2.