Group 4 Complexes with Aminebisphenolate Ligands and Their Application for the Ring Opening Polymerization of Cyclic Esters

A series of tetradentate amine(bisphenolate) ligands have been synthesized and complexed to Ti(IV), Zr(IV), and Hf(IV) centers. Two distinct ligand sets have been synthesized both exhibiting ONNO coordination. The steric bulk of the ligands was varied to assess the impact of sterics on the catalytic performance in the ring-opening-polymerization (ROP) of cyclic esters. In all cases the metal centers are pseudo octahedral in geometry. Several group 4 amine(bisphenolate) complexes have been characterized by single-crystal X-ray diffraction. The complexes were tested for the ROP of ε-caprolactone and L/rac-lactide, with significant activity seen for complexes of all group 4 metals. For Ti(IV), complexes of sterically demanding ligands were more effective initiators, whereas sterically less demanding ligands were more successful for Zr(IV) complexes. Copolymers of poly(ε-caprolactone) and poly-l-lactide were produced utilizing the Zr(IV) initiators. The polymerization of rac-lactide was also studied allowing stereoselectivity of a range of metal−ligand combinations to be assessed. Isotactic polymer was produced with both Zr(IV) and Hf(IV) centers whereas Ti(IV) produced an atactic polymer.