Enhancing thermostability of iron ethylene polymerization catalysts through N,N,N-chelation of doubly fused α,α′-bis(arylimino)-2,3:5,6-bis(hexamethylene)pyridines

Abstract

The ferrous chloride complexes, [2,3:5,6-{C₅H₁₀C(NAr)}₂C₅HN]FeCl₂ (Ar = 2,6-Me₂Ph Fe1, 2,6-Et₂Ph Fe2, 2,6-i-Pr₂Ph Fe3, 2,4,6-Me₃Ph Fe4, and 2,6-Et₂-4-MePh Fe5), each bearing a N,N,N-ligand incorporating two partially saturated fused eight-membered rings, have been synthesized by the one-pot template reaction of α,α′-dioxo-2,3:5,6-bis(hexamethylene)pyridine, iron(II) chloride tetrahydrate and the corresponding aniline in acetic acid. The structures of Fe3 and its oxidized diferric derivative, [2,3:5,6-{C₅H₁₀C(N(2,6-i-Pr₂Ph))}₂C₅HN]FeCl(μ-O)FeCl₃ (Fe3′), revealed square pyramidal geometries with either a chloride or an oxo ligand filling the apical sites, respectively. On treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all precatalysts displayed good thermostability (optimal operating temperatures: 50–80 °C) and moreover delivered exceptionally high activities for ethylene polymerization [up to 12.23 × 10⁶ g (PE) mol⁻¹ (Fe) h⁻¹] producing highly linear polyethylene of high molecular weight (M_w up to 62.5 kg mol⁻¹ even at 80 °C). The catalytic activities fall in the order, Fe1 > Fe4 > Fe2 > Fe5 > Fe3 (MMAO or MAO), with both steric and electronic factors influential; iron(III)-containing Fe3′ was less active. Distinct chain termination processes for the polymerizations have been identified through end-group analysis with both β-H elimination and chain transfer to aluminum operative with MMAO, while only transfer to aluminum has been detectable with MAO. Notably with MMAO, the different rates of these termination processes manifests itself in bimodal molecular weight distributions for the polyethylenes.