Highly branched unsaturated polyethylenes achievable using strained imino-cyclopenta[b]pyridyl-nickel precatalysts

Abstract

A new family of strained imino-cyclopenta[b]pyridines, 7-(ArN)-6-Me₂C₈H₅N (Ar = 2,6-Me₂Ph (L1), 2,6-Et₂Ph (L2), 2,4,6-Me₃Ph (L3), 2,6-Et₂-4-MePh (L4), 2,6-i-Pr₂Ph (L5)), have been synthesized in reasonable yield by a sequence of reactions from 2-chloro-cyclopenta[b]pyridin-7-one. Treatment of L1 and L3 with NiCl₂·6H₂O generates mononuclear bis-ligated [7-(ArN)-6-Me₂C₈H₅N]₂NiCl₂ (Ar = 2,6-Me₂Ph (Ni1), 2,4,6-Me₃Ph (Ni3)), while with L2 and L4, the chloride-bridged binuclear complexes [7-(ArN)-6-Me₂C₈H₅N]₂Ni₂(μ-Cl)₂Cl₂ (Ar = 2,6-Et₂Ph (Ni2), 2,6-Et₂-4-MePh (Ni4)), have been isolated; no apparent reaction occurred with L5. On activation with either MAO or MMAO, Ni1–Ni4 exhibited high activities towards ethylene polymerization with Ni3 the most active (5.02 × 10⁶ g PE per mol Ni per h at 20 °C); rapid regeneration of the active species (3096–5478 h⁻¹ at 20 °C) is a feature of their catalytic performance. A detailed microstructural analysis of the polyethylenes reveals the presence of vinyl and higher levels of internal vinylene groups indicative of high rates of chain isomerization, e.g., the ratio of (–CHCH–) to (H₂CCH–) groups is 2.2 : 1 using Ni3/MAO at 60 °C. Agostic interactions involving γ-, δ- and higher-hydrogens are inferred in addition to β-hydrogen elimination to account for the vinylene groups and the longer chain alkyl branches. The molecular structures of Ni1 and Ni2·2MeOH are also reported.