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

Area of Science:

• Polymer Chemistry
• Organometallic Chemistry

Background:

• Ethylene polymerization demands catalysts with high activity and polymer molecular weight, especially at elevated temperatures.
• Iron complexes are explored as alternatives to traditional catalysts for olefin polymerization.

Purpose of the Study:

• To synthesize and evaluate novel pyridyliron precatalysts bearing hydroxyl functionality for solution ethylene polymerization.
• To investigate the impact of hydroxyl groups and activators (MAO vs. MMAO) on catalytic performance and polyethylene properties.

Main Methods:

• Synthesis of a series of 2-(1-(2,6-dibenzhydryl-4-hydroxyphenylimino)-ethyl)-6-(1-(arylimino)-ethyl)-pyridyliron precatalysts.
• Activation of iron complexes with methylaluminoxane (MAO) and modified methylaluminoxane (MMAO).
• Ethylene polymerization experiments at various temperatures (30-110 °C).
• Characterization of polyethylene using gel permeation chromatography (GPC) and differential scanning calorimetry (DSC).

Main Results:

• MAO activation yielded higher polymerization activities (up to 23.18 × 10^6 g mol^-1 h^-1 at 70 °C) and superior thermal stability (2.53 × 10^6 g mol^-1 h^-1 at 110 °C) compared to MMAO.
• Polyethylene with higher molecular weights (up to 557.75 kg mol^-1) and controlled unimodal molecular weight distributions was obtained with MAO activation.
• The hydroxyl functionality in the iron precatalysts significantly improved activity, molecular weight, and dispersity across a broad temperature range (30-70 °C).
• High-temperature polymerization facilitated β-H elimination, producing linear polyethylene with terminal unsaturation, confirmed by DSC and NMR.

Conclusions:

• The developed hydroxyl-functionalized pyridyliron precatalysts demonstrate excellent performance in solution ethylene polymerization, outperforming previous analogues.
• The OH functionality plays a crucial role in enhancing catalyst activity, thermal stability, and polymer properties, likely through interaction with the cocatalyst.
• These findings provide valuable insights for designing efficient catalysts for solution ethylene polymerization and controlling polyethylene microstructure.