Bis(imino)-6,7-dihydro-5H-quinoline-cobalt complexes as highly active catalysts for the formation of vinyl-terminated PE waxes; steps towards inhibiting deactivation pathways through targeted ligand design

Abstract

A set of five related bis(imino)-6,7-dihydro-5H-quinoline-cobalt(II) complexes, [2-(ArN = CPh)-8-(NAr)-C₉H₈N]CoCl₂ (Ar = 2,6-Me₂C₆H₃ Co1, 2,6-Et₂C₆H₃ Co2, 2,6-i-Pr₂C₆H₃ Co3, 2,4,6-Me₃C₆H₂ Co4, 2,6-Et₂-4-MeC₆H₂ Co5), have been synthesized in reasonable yield by the template reaction of cobalt(II) chloride hexahydrate, 2-benzoyl-6,7-dihydro-5H-quinolin-8-one and the corresponding aniline. The molecular structures of Co1 and Co4 highlight both the differences in the two imino-carbon environments (phenyl-capped chain vs. cyclic) and also the steric properties exerted by the bulky N_imine-aryl groups. On pre-treatment with either modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), all complexes proved productive catalysts for the polymerization of ethylene. In particular, Co1/MAO was the most active reaching a very high level of 1.62 × 10⁷ g PE per mol (Co) per h over a 30 minute run time. Owing to the presence of the imino-phenyl substituent, Co1–Co5 were able to exhibit good thermal stability by displaying appreciable catalytic activity at temperatures between 50 and 80 °C, generating polyethylenes with narrow dispersities (M_w/M_n range: 1.66–3.28). In particular, the least sterically bulky precatalysts, Co1 and Co4 formed polyethylene waxes (M_w range: 1.94–5.69 kg per mol) with high levels of vinyl unsaturation as confirmed by high temperature ¹H/¹³C NMR spectroscopy and by IR spectroscopy.