A π–π interaction strategy for targeting highly thermostable bis(arylimino)pyridyliron precatalysts in ethylene polymerization

Abstract

The suboptimal catalytic performance of bis(arylimino)pyridyliron precatalysts in ethylene polymerization at elevated temperatures remains a significant challenge. In this study, a series of six bis(arylimino)pyridyliron chloride precatalysts with the incorporation of the benzosuberyl steric bulk were prepared and investigated for ethylene polymerization. Capping the axial sites of the iron center with the benzosuberyl steric bulk resulted in π–π interactions between the phenyl group and the chelate backbone, with a distance of ca. 3.293 Å for Fe3^iPr, shorter than the sum of the van der Waals radii of carbon atoms (3.4 Å), as confirmed by single-crystal X-ray diffraction. These non-covalent interactions enhanced the thermal stability, catalytic activity, and polymer molecular weights. On activation with MAO or MMAO cocatalysts, these precatalysts exhibited the maximum activity of 2.53 × 10⁷ g_PE mol⁻¹_Fe h⁻¹ at 80 °C and 1.88 × 10⁷ g_PE mol⁻¹_Fe h⁻¹ at 100 °C, demonstrating their unprecedented thermal stability. The molecular weight of the produced polyethylene remained high, even at elevated temperatures. Moreover, ligand modifications had a pronounced impact on polymerization outcomes: the least sterically hindered complex, while producing comparatively lower molecular weight polyethylene, displayed higher activity and the most sterically hindered one showed the opposite tendency. The molecular weight dispersity of polyethylene showed a strong correlation with the precatalyst structure and reaction conditions. High melting points confirmed the presence of strictly linear structures with high vinyl end groups (up to 74%), as verified using ¹H/¹³C NMR spectra. A comparison of structurally related iron precatalysts revealed significant improvements in both thermal stability and catalytic activity, attributed to the π–π interactions present in the current iron precatalysts.