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

Abstract

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.

Effect of the Degree of Cluster TiCl4 on the Polymerization of Isoprene over Ziegler–Natta Catalysts: A DFT Study

“…Conventional TMC consists of support MgCl 2 , main catalyst TiCl 4 , cocatalyst alkyl aluminum R n AlX 3‑ n (R is alkyl, X is halogen, n = 1, 2, and 3), and electron donor . To explore the adsorption mechanism of active centers on the MgCl 2 support as well as the mechanism by which the active centers interact with Ti compounds, a large number of researchers have conducted extensive experiments − and theoretical calculations, − but no consensus has yet been formed for their divergent opinions regarding the distribution and precise structure of active Ti sites on the surface of the MgCl 2 support that the valence state of active center Ti affects the performance of TMC for olefin polymerization that could be inferred by many research studies. Seth examined the effects of the external electron donor tetrahydrofuran on the ethylene polymerization properties of the TiCl 4 /MgCl 2 heterogeneous TMC using density functional methods and found the donor bound most strongly to the active sites formed from TiCl 3 among all the proposed ones.…”