Thermal-Robust Phenoxyimine Titanium Catalysts Bearing Bulky Sidearms for High Temperature Ethylene Homo-/Co-...

A family of titanium complexes (Ti1-Ti7) with the general formula LTiCl_3, supported by tridentate phenoxyimine [O^-NO] ligands (L1-L7) bearing bulky sidearms, were synthesized by treating the corresponding ligands with stoichiometric amount of TiCl₄. All the ligands and complexes were well characterized by ¹H and ¹³C NMR spectroscopies, in which ortho- methoxyl groups on N-aryl moieties shifted to downfield, corroborating the successful coordination reaction. Structural optimization by DFT calculations revealed that one of the phenyl groups on dibenzhydryl moiety could form π-π stacking interaction with the salicylaldimine plane, because of which the obtained titanium complexes revealed good thermal stabilities for high-temperature polymerization of ethylene. The thermal robustness of the complexes was closely related to the strength of π-π stacking interactions, which were mainly influenced by the substituents on the dibenzhydryl moieties; Ti1, Ti4 and Ti5 emerged as the three best-performing complexes at 110 °C. With the aid of such π-π stacking interactions, the complexes were also found to be active at >150 °C, although decreased activities were witnessed. Besides homopolymerizations, complexes Ti1-Ti7 were also found to be active for the high-temperature copolymerization of ethylene and 1-octene, but with medium incorporation percentage, demonstrating their medium copolymerization capabilities.