Synergistic Regulation of Hydrogen Partial Pressure and Ligand Effects on the Metallocene-Catalyzed Synthesis of Polyethylene Wax

Abstract

Polyethylene wax (PEW) produced by metallocene catalysts has attracted increasing interest due to its excellent physicochemical properties. Research on using constrained geometry catalysts (CGCs), a class of metallocene catalysts, to control the molecular weight of PEW by modifying ligand steric and electronic effects has rarely been mentioned. In this work, five titanium-based CGC-type metallocene catalysts with systematically tuned ligand architectures were synthesized and evaluated for ethylene polymerization toward PEW. The combined influences of ligand steric/electronic properties and H₂ partial pressure on catalytic behavior and PEW characteristics were investigated. The results reveal that steric and electronic effects synergistically regulate chain-transfer and chain-growth processes. CGCs with reduced steric hindrance and stronger electron-donating ligands promote chain transfer, affording low-molecular-weight PEW with broader molecular weight distributions. In contrast, increased steric bulk and/or weakened electron donation suppress chain transfer and delay chain termination, producing higher-molecular-weight PEW with narrower dispersity. Notably, the [t-BuNSiMe₂(Me₄Cp)]TiMe₂/MAO system at an H₂ partial pressure of 0.9 MPa delivered PEW with M_n = 0.24×10⁴ g·mol⁻¹, PDI = 2.28, and a melt flow index of 13.83 g·10 min⁻¹. This work demonstrates that CGC ligand engineering combined with H₂ regulation provides an effective strategy for the directional synthesis of PEW with tailored properties.