Abstract
Bulky N-aryl substituents in bis(imino)pyridylmetal catalysts enhance the ethylene polymerization, though maintaining activity at higher temperature remains as a challenging point. Herein, the newly synthesized symmetrical and unsymmetrical 2-(1-(2,6-dibenzhydryl-4-hydroxyphenylimino)ethyl)-6-(1-(arylimino)ethyl)pyridylcobalt complexes have been systematically and sterically tailored, and explored for their performances in ethylene polymerization. Employing bulky N-aryl unit modified through dibenzhydryl and hydroxyl substituents, the cobalt complexes perform the significant improvement in both activity and thermal stability. Activated with MAO in the temperature range (30-100 °C), the resulted polyethylenes are characterized as highly linear and high molecular weights (Mw = 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 the comparison with all previous bis(imino)pyridylcobalt systems reported. Among unsymmetrical complexes, ligands with higher steric hindrance resulted 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.
