Polymerizations of cyclic esters catalyzed by titanium complexes having chalcogen-bridged chelating diaryloxo ligands

A series of titanium complexes having tellurium-bridged chelating bis(aryloxo) ligands, [TiX2-{2,2'-Te(4-Me-6-Bu-t-C6H2O)(2)}](2) (5: X = Cl; 6: X = (OPr)-Pr-i), catalyzed the ring-opening polymerization of cyclic esters such as E-caprolactone, delta-valerolactone, and L-lactide. The strong dependence of polymerizations on the solvent was observed in this catalytic system, When the polymerizations of epsilon-caprolactone and L-lactide were carried out in toluene at 100 degreesC, tellurium-bridged bis(aryloxo)titanium complex 5 was found to give polymers with rather broad molecular weight distribution due to back-biting. When the polymerizations of c-caprolactone and L-lactide was carried out in anisole or in dioxane at 100 degreesC, complex 5 was found to initiate the controlled polymerization, to result in quantitative polymer yields and narrow molecular weight distributions (living nature). The diblock copolymers of L-lactide and c-caprolactone were also obtained with the catalyst system 5 in anisole, The diblock copolymers showed two melting endothermic at 44.7-53.5 degreesC derived from the ploy(E-caprolactone) block and at 155.2-156.8 degreesC derived from the ploy(L-lactide) block.