Hydroxy-Functionalized Iron Catalysts for Producing Polyethylene with High Molecular Weight, Tunable Modes, and Vinyl...

Practical solution ethylene polymerization requires both high catalytic activities and high polymer molecular weights at high temperature. Herein, a series of 2-(1-(2,6-dibenzhydryl-4-hydroxyphenylimino)-ethyl)-6-(1-(arylimino)-ethyl)-pyridyliron precatalysts [aryl = 2,6-Me₂Ph (Fe1 ^Me ), 2,6-Et₂Ph (Fe2 ^Et ), 2,6-iPr₂Ph (Fe3 ^iPr ), 2,4,6-Me₃Ph (Fe4 ^Me ), 2,6-Et₂-4-MePh (Fe5 ^Et ), and 2,6-(Ph₂CH)₂ (Fe6 ^Bh )] were prepared and investigated for solution ethylene polymerization. These iron complexes were activated separately with MAO and MMAO and systematically examined for ethylene polymerization. Upon activation with MAO, the catalysts showed slightly higher polymerization activities (up to 23.18 × 10⁶ g mol^-1 h^-1 at 70 °C), superior thermal stability (2.53 × 10⁶ g mol^-1 h^-1 at 110 °C), relatively higher molecular weight polyethylene (up to 557.75 kg mol^-1), and most importantly controlled unimodal molecular weight distributions compared to what was observed with MMAO activation. Compared to the previous analogues as well as prototype iron complexes, the OH functionality in developed iron precatalysts resulted in simultaneously improved activities, molecular weights, and dispersity of polyethylene across a broad polymerization temperature range (30-70 °C). The performance likely originates from an OH functionality-cocatalyst interaction, which could offer important insights for controlling solution ethylene polymerization. Moreover, a high temperature facilitates β-H elimination, producing sterically linear polyethylene with unsaturation at terminal positions. This was confirmed by differential scanning calorimetry (T _m = 128.34-134.98 °C) as well as by high-temperature ¹H/¹³C NMR spectroscopic measurements.