Preparation of Weakly Entangled and Fine-sized Ultra-High-Molecular Weight Polyethylene by a MgCl2-Based Ziegler–Natta Catalyst

The preparation of ultra-high-molecular weight polyethylene (UHMWPE) with few entanglements and a fine particle size remains a significant problem for academia and industry. In this research, a novel MgCl2-based heterogeneous Ziegler–Natta catalyst modified with polyhedral oligomeric silsesquioxane (4-OH POSS) of fine particle size was successfully synthesized by screening solvents and Grignard reagents. Scanning electron microscopy, high-angle annular dark-field imaging, diffuse reflectance infrared spectroscopy, and X-ray photoemission spectroscopy tests were utilized to analyze the macroscopic morphology, microscopic composition, and active site distribution of the catalyst. Numerous POSS nanocrystals were stably embedded on the catalyst surface, and TiCl4 active sites are prone to immobilization on the POSS nanocrystals. However, TiCl4 loaded on the POSS nanocrystals exhibits nearly no activity, which acts as the isolator for separating the active sites loaded on the δ-MgCl2 surface. The isolation effect of POSS nanocrystals and the increased crystallization behavior of non-porous features for the catalyst collectively lead to the preparation of weakly entangled UHMWPE. The effects of sensitivity of cocatalyst reducibility, TiCl4 loading contents, and polymerization temperature on the catalytic activity and the UHMWPE performance were explored. Interestingly, the UHMWPE with less entanglements (initial elastic modulus = 0.23 MPa) and fine particle size (∼110 μm) was synthesized, exhibiting the overall reinforced strength/stiffness/toughness balance.