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, [TiX 2 {2,2‘-Te(4-Me-6- t Bu-C 6 H 2 O) 2 }] 2 ( 5: X = Cl; 6: X = O i Pr), catalyzed the ring-opening polymerization of cyclic esters such as ε-caprolactone, δ-valerolactone, and l -lactide. The strong dependence of polymerizations on the solvent was observed in this catalytic system. When the polymerizations of ε-caprolactone and l -lactide were carried out in toluene at 100 °C, 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 ε-caprolactone and l -lactide was carried out in anisole or in dioxane at 100 °C, 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 ε-caprolactone were also obtained with the catalyst system 5 in anisole. The diblock copolymers showed two melting endothermic at 44.7−53.5 °C derived from the ploy(ε-caprolactone) block and at 155.2−156.8 °C derived from the ploy( l -lactide) block.