Fluoro- & trifluoromethoxy-modified bis(imino)pyridyliron chloride enhancing the activity and stability of vinyl polyethylenes

The bis(imino)pyridylferrous chloride complexes bearing <i>para</i>-trifluoromethoxy and <i>ortho</i>-difluorobenzhydryl substituents, namely, 2-(1-{2,6-bis[bis(4-fluorophenyl)methyl]-4-trifluoromethoxyphenyl-imino}ethyl)-6-[1-(2,6-di(R¹)-4-R²-phenylimino) ethyl]pyridyl iron(II) chloride (where R¹ = Me, R² = H, Fe1; R¹ = Et, R² = H, Fe2; R¹ = i-Pr, R² = H, Fe3; R¹ = Me, R² = Me, Fe4; R¹ = Et, R² = Me, Fe5; and R¹ = (4-FC₆H₄)₂CH, R² = OCF₃, Fe6) were successfully synthesized and thoroughly characterized. The X-ray crystal structures of Fe4 and Fe6 were also determined. The catalytic performance of all the new iron complexes was systematically evaluated for ethylene polymerization, with a detailed comparative analysis conducted against previously published studies. Notably, iron complexes Fe1-Fe5 demonstrated exceptional catalytic activity and thermal stability upon activation with MAO, while Fe6 showed inferior performance due to its significant steric hindrance. Among them, Fe1 demonstrated the highest activity, achieving a peak of 17.28 × 10⁶ g (PE) mol^-1 (Fe) h^-1 at 70 °C in <i>n</i>-hexane, which markedly surpassed the activity levels of other structurally related iron complexes, indicating the beneficial impact of incorporating multi-fluoro groups. Remarkably, the current iron pre-catalysts generally revealed superior performance in hexane compared to toluene, even when using a reduced quantity of co-catalyst, thereby underscoring the pivotal role of solvent polarity in governing the polymerization process. Furthermore, the analysis of the generated polyethylenes (PEs) revealed a high degree of linearity in the polymers, featuring vinyl end groups and exhibiting a broad spectrum of molecular weights (<i>M</i>_w: 4.5-567.9 kg mol^-1).