Efficient Synthesis of Low-Polydispersity UHMWPE by Elevating Active Sites on Anchored POSS Molecules

Abstract

Ultrahigh-molecular-weight polyethylene (UHMWPE) with a low polydispersity exhibits excellently physical and mechanical properties. In this work, a polyhedral oligomeric silsesquioxane (POSS)-modified Ziegler–Natta catalyst has been explored to synthesize the UHMWPE with a narrow molecular weight distribution. Diffuse reflection infrared spectroscopy combined with CO as the molecular probe, X-ray photoelectron spectroscopy, and diffuse reflectance UV–vis spectroscopy are employed to trace the evolution of the catalyst structure based on the investigation of vibrational and electronic properties of molecules. The results demonstrate that the chemically grafted POSS molecules are distributed evenly on the silica surface at the molecular level. The POSS can capture the MgTi compounds through protonation with the residual silanols, acting as a vertical support for active sites. The resulting active sites grafted on the POSS present much stronger Lewis acidity and were less heterogeneous (only tetra-coordinated Mg2+ species) in comparison to the active sites directly grafted or absorbed on the silica surface. This less heterogeneity of active sites enables to synthesize the UHMWPE with a much narrow distribution, reducing the molecular weight distribution from 6.7 to 3.9.