Enhanced thermostability of C2-symmetrical bis(imino)pyridine-iron precatalysts for ethylene polymerisation via a hybrid steric strategy†

The poor performance of bis(arylimino)pyridyl iron precatalysts in ethylene polymerisation at high-temperature makes them less attractive as drop-in catalysts for existing ethylene polymerisation technologies. In this study, employing a one-pot template approach, a series of C₂-symmetric bis(imino)pyridine-iron precatalysts were prepared from 2,6-diacetylpyridine, ferrous chloride, and aniline derivatives (eight distinct anilines bearing various steric and electronic substituents) and were characterized by FT-IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. These precatalysts showed high catalytic activity at industrially relevant temperatures. In situ activation with either MAO or MMAO, these complexes exhibited high catalytic activities (on the order of 10⁶–10⁷ g_PE mol_Fe⁻¹ h⁻¹) over a wide temperature range (30–100 °C) and produced high-molecular-weight polyethylene (M_w up to 433.1 kg mol⁻¹) with unimodal to bimodal molecular weight distributions. The polymerisation activity, polymer molecular weights, and dispersity are significantly dependent on the ortho-substituents of the N-bound phenyl groups. Less sterically hindered substituents favored higher catalytic activities, while more hindered substituents facilitated the formation of higher molecular weight polyethylene. Moreover, these C₂-symmetric precatalysts with hybrid steric hindrance exhibited exceptional activity for producing high-molecular-weight polyethylene, outperforming previously reported symmetrical analogues that showed little to no activity. DSC and NMR analyses confirmed a highly linear polyethylene microstructure with predominantly methyl end groups.