Adjustment of Electronic (OMe and NO2) and Steric (CHPh2) Effects in Bis(imino)pyridinyliron Precatalysts for Producing High Molecular Weight Linear PE

Based on variations in steric and electronic substituents in the N-aryl unit, a set of five 2-[1-(arylimino)ethyl]-6-[1-(2-benzhydryl-4-nitro-6-methoxyphenylimino)ethyl]pyridyliron dichloride complexes (where aryl=2,6-dimethylphenyl (Fe-Me₂), 2,6-diethylphenyl (Fe-Et₂), 2,6-diisopropyl (Fe-iPr₂), 2,4,6-trimethylphenyl (Fe-Me₃), 2,6-diethyl-4-methylphenyl (Fe-Et₂Me)) were synthesized and investigated for ethylene polymerization. Their structures and compositions were confirmed by FTIR, elemental analysis, and X-ray diffraction analysis (Fe-Et₂ and Fe-iPr₂), revealing a distorted square pyramidal geometry around the iron center. When activated in situ with MAO or MMAO, these iron complexes acted as highly active precatalysts, achieving activity levels up to 29.0×10⁶ g_PE mol_Fe⁻¹ h⁻¹ for a 5-min reaction at 60 °C. A notable characteristic of these precatalysts is their high thermal stability, maintaining significant activity (2.0×10⁶ g_PE mol_Fe⁻¹ h⁻¹) at temperatures up to 100 °C. Importantly, the incorporation of strong electron-withdrawing groups in the ligand structure favored the production of polyethylene with high molecular weights (up to 407.5 kg mol⁻¹) across various reaction conditions, surpassing those of most previously reported iron complexes. Moreover, less hindered iron precatalysts showed higher activity compared to more hindered ones. However, the trend reversed when comparing the molecular weight of obtained polyethylene: more sterically hindered precatalysts yielded higher molecular weight polymers. The molecular weight distributions ranged from monomodal to bimodal with broad dispersity. DSC thermograms and ¹H/¹³C NMR spectra of the polyethylene confirmed its highly linear microstructure, evidenced by sharp endothermic peaks and pronounced singlets for the −(CH₂)_n− repeating units.