Exploring ortho‐(4,4′‐dimethoxybenzhydryl) substitution in iron ethylene polymerization catalysts: Co‐catalyst effects, thermal stability, and polymer molecular weight variations

Abstract

Six different types of 4,4′‐dimethoxybenzhydryl‐substituted bis(arylimino)pyridine‐iron(II) chloride complex—[2‐{{2,6‐((p‐MeOPh)2CH)2‐4‐MeC6H2}N=CMe}‐6‐(ArN=CMe)C5H3N]FeCl2 (Ar = 2,6‐Me2C6H3 (Fe1), 2,6‐Et2C6H3 (Fe2), 2,6‐iPr2C6H3 (Fe3), 2,4,6‐Me3C6H2 (Fe4), 2,6‐Et2‐4‐MeC6H2 (Fe5), 2,6‐((p‐MeOPh)2CH)2‐4‐MeC6H2 (Fe6)—are reported. The molecular structures of Fe2 and Fe3 emphasize the unsymmetrical nature of the N,N,N‐chelating ligand and the steric protection exerted by the ortho‐(4,4′‐dimethyoxybenzhydryl) groups. A range in catalytic activities for ethylene polymerization was observed when Fe1–Fe6 were treated with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO). In particular, Fe1/MAO bearing the least bulky N‐2,6‐dimethylphenyl group exhibited a very high activity of 1.11 × 107 g (PE) mol−1 (Fe) h−1 at 70°C/10 atm over a 30‐min run time that remained high even after 60 min, an observation highlighting its appreciable catalyst lifetime and thermal stability. In addition, the polyethylene formed by this catalyst class displayed desirable characteristics such as high linearity and high molecular weight (Mw range: 5.17–7.62 × 105 g mol−1). Indeed, the molecular weight of the polymer produced using Fe1 (with MAO or MMAO) exceeds that obtained using related benzhydryl‐containing bis(imino)pyridine‐iron catalysts. As a general feature, activation of Fe1–Fe6 with MMAO led to lower activity and lower molecular weight polyethylene, especially for runs performed at 1 atm ethylene pressure.