Titanium Alkoxide Complexes of Phenoxy-Azo and Phenoxy-Imine Ligands for Cyclic Ester (Co)Polymerization Studies: Tuning Polymerization Activity through Ligand Design

Two series of titanium complexes supported by monoanionic phenoxy-azo (1-7) and phenoxy-imine (8-12) ligands were successfully synthesized and fully characterized. X-ray structure analysis of the bis(phenoxy-azo)titanium complexes 1 and 4 revealed that the titanium atom was in a distorted octahedral geometry, with the trans-O, cis-N, cis-OiPr configuration. All complexes were active for the ring-opening polymerizations of rac-lactide, ε-caprolactone, and ε-decalactone. For rac-LA polymerization, the bis(phenoxy-azo)titanium complex bearing alkyl phenoxy substituents exhibited superior catalytic activity compared to its bis(phenoxy-imine)titanium analogue with identical substituents. In contrast, a reverse trend was observed upon introducing halogenated phenoxy substituents. Kinetic investigations revealed the origin of the diminished catalytic activity of the halogenated bis(phenoxy-azo)titanium complexes. Perfect diblock copolymers, poly(CL-b-rac-LA) and poly(CL-b-l-LA), were successfully synthesized via a sequential-feed copolymerization, while a single-feed copolymerization of rac-LA/l-LA and ε-CL afforded practically random copolymers. This study demonstrated that the catalytic activity of bis(phenoxy-imine)titanium complexes was enhanced by substituting the imine unit with an azo moiety, highlighting that catalytic activity can be fine-tuned through ligand design.