Substantially enhancing the catalytic performance of bis(imino)pyridylcobaltous chloride pre‐catalysts adorned with benzhydryl and nitro groups for ethylene polymerization

Abstract

Variations in the ligand structure of homogeneous late transition metal catalysts through judicious choice and location of substituent is the foremost strategy in improving their catalytic performance for ethylene polymerization. In this contribution, symmetrical and unsymmetrical bis(imino)pyridylcobaltous chloride complexes adorned with nitro and benzhydryl groups {2‐[1‐(2,6‐dibenzhydryl‐4‐nitrophenylimino)ethyl]‐6‐[1‐(alkylphenylimino)ethyl]pyridylcobaltous chloride (alkyl: R1 = Me and R2 = H, Co1; R1 = Et and R2 = H, Co2; R1 = iPr and R2 = H, Co3; R1 and R2 = Me, Co4; R1 = Et and R2 = Me, Co5; R1 = benzhydryl and R2 = NO2, Co6)} have been prepared and applied as catalysts for ethylene polymerization. The molecular structure of Co1 and Co2 revealed the unequal steric protection of the cobalt center induced by bis(imino)pyridine chelate. In the presence of methylaluminoxane (MAO) or modified methylaluminoxane (MMAO) activators at different ethylene feeding rates (1 and 10 atm), catalysts Co1–Co5 displayed high activities at 10 atm ethylene and produced strictly linear polyethylene (PE) with high molecular weight, Co2/MMAO being the most highly active catalytic system showing the highest activity of 9.41 × 106 g of PE (mol of Co)−1 h−1 which is three times higher than that of prototypal cobalt catalyst (Co0) under identical conditions. Moreover, high melt temperature and unimodal molecular weight distribution are the characteristics of the resulting polyethylene.