Investigating the Effects of Para-methoxy Substitution in Sterically Enhanced Unsymmetrical Bis(arylimino)pyridine-cobalt Ethylene Polymerization Catalysts

A group of five bis(arylimino)pyridine-cobalt(II) chloride complexes, [2-{(2,6-(Ph2CH)2-4-MeOC6H2)N = CMe}-6-(ArN = CMe)C5H3N] CoCl2 (Ar = 2,6-Me2C6H3Co1, 2,6-Et2C6H3Co2, 2,6-iPr2C6H3Co3, 2,4,6-Me3C6H2Co4, 2,6-Et2-4-MeC6H2Co5), each containing one N-4-methoxy-2,6-dibenzhydrylphenyl group and one smaller sterically/electronically variable N-aryl group, have been synthesized in good yield (>71%) from the corresponding neutral terdentate nitrogen-donor precursor, L1–L5. All complexes have been characterized by 1H-NMR and FTIR spectroscopy with the former highlighting the paramagnetic nature of these cobaltous species and the unsymmetrical nature of the chelating ligand. The molecular structures of Co3 and Co4 emphasize the steric differences of the two inequivalent N-aryl groups and the distorted square pyramidal geometry about the metal centers. In the presence of MAO or MMAO, Co1–Co5 collectively displayed high activities for ethylene polymerization producing high molecular weight polyethylenes that, in general, exhibited narrow dispersities (Mw/Mn values: 2.12–4.07). Notably, the least sterically hindered Co1 when activated with MAO was the most productive (6.92×106 gPE·mol−1(Co)·h−1) at an operating temperature of 60 °C. Conversely, the most sterically hindered Co3/MMAO produced the highest molecular weight polyethylene (Mw=6.29×105 g·mol−1). All the polymers displayed high linearity as demonstrated by their melting temperatures (>130 °C) and their 1H- and 13C-NMR spectra. By comparison of Co1 with its para-methyl, -chloro and -nitro counterparts, the presence of the para-methoxy substituent showed the most noticeable effect of enhancing the thermal stability of the catalyst.