Enhanced Activity and Thermal Stability of Hydroxyl-Modified Bis(imino)pyridylcobalt Catalysts for Ethylene Polymerization

Bulky N-aryl substituents in bis(imino)pyridylmetal catalysts enhance the ethylene polymerization, though maintaining activity at higher temperature remains a challenging point. Herein, the newly synthesized symmetrical and unsymmetrical 2-(1-(2,6-dibenzhydryl-4-hydroxyphenylimino)ethyl)-6-(1-(arylimino)ethyl)pyridylcobalt chloride complexes have been systematically and sterically tailored and further explored for their performances in ethylene polymerization. Employing the bulky N-aryl unit modified through dibenzhydryl and hydroxyl substituents, the cobalt complexes show a significant improvement in both activity and thermal stability. Activated with methylaluminoxane in the temperature range (30–100 °C), the resulting polyethylenes are characterized as highly linear and high molecular weights (M_w = 10⁵ g mol⁻¹) along with their dispersity controlled; the activities are exceptionally high with a maximum of 14.29 × 10⁶ g mol⁻¹ h⁻¹ at 70 °C and the unprecedented 1.96 × 10⁶ g mol⁻¹ h⁻¹ at 100 °C, indicating the robust thermal stability that is best catalytic performance in comparison with all previous bis(imino)pyridylcobalt systems reported. Among unsymmetrical complexes, ligands with higher steric hindrance result in lower activities of complex catalysts along with the polyethylenes with higher molecular weights; meanwhile, its symmetrical complex is far less active. In all cases, the polyethylenes are highly linear with terminal vinyl groups, being verified by DSC and high-temperature NMR measurements.