Thermally resilient cobalt ethylene polymerization catalysts under the joint influence of co-catalyst, gem-dimethyl substitution and ortho-cycloalkyl ring size

The N,N,N′-chelated 2-(1-arylimino)ethyl-8-arylimino-7,7-dimethyl-5,6- dihydroquinoline-cobalt (II) chloride complexes, Co1 (aryl = 2-(C5H9)-6-MeC6H3), Co2 (aryl = 2-(C6H11)-6-MeC6H3), Co3 (aryl = 2-(C8H15)-6-MeC6H3), Co4 (aryl = 2-(C5H9)-4,6-Me2C6H2), Co5 (aryl = 2-(C6H11)-4,6-Me2C6H2) and Co6 (aryl = 2-(C8H15)-4,6-Me2C6H3), have been isolated in reasonable yield using a simple one-pot method. The molecular structures of Co1 and Co5 emphasize not only the gem-dimethyl-substitution present in the carbocyclic-fused tridentate ligand but also the substantial steric properties exerted by the ortho-cycloalkyl groups. Complexes Co1 – Co6, on interaction with MAO, were highly active catalysts for ethylene polymerization reaching a maximum level of 9.21 × 106 g PE mol−1 (Co) h−1 at 70 °C for cyclopentyl-containing Co1 and Co4. Strictly linear polyethylenes with low molecular weights (Mw range: 9.25–38.5 kg mol−1) and narrow dispersities were generated with selectivities for vinyl-terminated polymers approaching quantitative levels. By comparison, polymerizations conducted using Co1 – Co6 with MMAO as co-catalyst proved less thermally stable and less active, with vinyl chain-end selectivity proving temperature dependent. Common to both MAO and MMAO, the most sterically hindered precatalyst Co6 incorporating an ortho-cyclooctyl group gave the highest molecular weight polymer of the series.