Narrow dispersed linear polyethylene using cobalt catalysts bearing cycloheptyl-fused bis(imino)pyridines; probing the effects of ortho-benzhydryl substitution

The cycloheptyl-fused cobalt(II) chloride complexes, [2,3:5,6-{C4H8C(N(2-R1-4-R2-6-(CHPh2)C6H2))}2C5HN]CoCl2 (R1 = R2 = Me Co1, R1 = Me, R2 = CHPh2 Co2, R1 = Et, R2 = CHPh2 Co3, R1 = i-Pr, R2 = CHPh2 Co4, R1 = Cl, R2 = CHPh2 Co5, R1 = CHPh2, R2 = i-Pr Co6, R1 = CHPh2, R2 = t-Bu Co7, R1 = CHPh2, R2 = F Co8), have been prepared by a one-pot template approach. The molecular structures of Co1, Co6 and Co8 highlight the varying degrees of steric protection of the metal center imposed by the ortho-benzhydryl/-R1 group pairing. On activation with either MAO or MMAO, Co1–Co5 collectively displayed high activity (up to 2.87 × 106 g (PE) mol−1 (Co) h−1 at 40 °C) affording highly linear low molecular weight polyethylene (Mw range: 12.8–59.4 kg mol−1) with narrow polydispersities (Mw/Mn range: 1.4–2.7). End-group analysis of the polymers highlights the importance of both β-H elimination and chain transfer to aluminum as termination pathways. In relative terms, MMAO proved a marginally more effective activator than MAO with the catalytic activity, in either case, falling in the order: Co1 > Co5 > Co2 > Co3 > Co4. In marked contrast, the 2,6-(CHPh2)2-substituted precatalysts, Co6, Co7 and Co8 gave, in the presence of the same aluminoxane activators, only trace amounts of polyethylene.