Polyhedral oligomeric silsesquioxane (POSS) nanoaggregates can serve as a modifier of the MgCl2-supported Z-N catalyst, thus controlling the polymerization behavior and modifying the microstructure of the synthesized polymer. In this study, a series of POSS-modified MgCl2-supported Z-N catalysts were explored to reveal the relationship between the entanglement formation of ultrahigh-molecular-weight polyethylene (UHMWPE) and the adhesive effect control of hydroxy groups in POSSs. X-ray photoelectron spectroscopy (XPS) and CO in situ FTIR low-temperature adsorption technology were employed to investigate the catalytic surface structure. It was evidenced that POSSs can coordinate with the MgCl2 (110) plane, acting as adhesive of a cage structure to be in situ embedded into the MgCl2-based support. Therefore, the loaded TiCl4 active sites were isolated in the molecular scale on the MgCl2-based support and the entanglements were greatly suppressed thereof. The ″adhesion effect″ of the MgCl2 support can be controlled by adjusting the type (i.e., hydroxyl content) of POSS introduced, thereby suppressing entanglements to a greater extent. The MgCl2 support modified with POSS with no hydroxy groups endowed it with the optimum entanglement reduction effect. As a consequence, the as-synthesized polyethylene exhibits an overall promotion of the impact resistance (+14.1%), tensile strength (+13.9%), Young’s modulus (+16.8%), and break elongation (+10.5%), compared to those of UHMWPE synthesized from the POSS-free catalyst. In addition, the POSS-modified MgCl2-based catalyst shows more stable catalytic activity and responsiveness of molecular weight with hydrogen modulation, which exhibits great prospects of industrial application in polyethylene production.
Wei Li,Tao Guo,Zaitian He,Jingdai Wang,Yongrong Yang
Investigating the correlation between the active center structure and chain structures of the synthesized polyethylene in Ziegler–Natta catalysts has remained a persistent challenge due to the diversity and complexity of their active sites, limiting the development of advanced catalysts for the synthesis of high-performance polyethylene. Here, we present a strategy for the stepwise removal of active species in classical ZN catalysts. The reaction product formed between disilanolisobutyl polyhedral oligomeric silsesquioxane (2OH-POSS) and TiCl4 is successfully separated from the original catalyst by exploiting the solubility of 2OH-POSS in n-hexane, achieving a significant reduction in titanium content from 10.2 wt % to just 0.77 wt %. Advanced spectroscopic characterizations (e.g., EPR, CO-FTIR, and UV–vis) reveal a synergistic interplay between MgCl2 lattice reorganization and titanium removal during this detitanation process. As detitanation progresses, residual active Ti species on the MgCl2 surfaces facilitate the formation of isolated Ti3+ sites. More significantly, this process dramatically enhances the formation of dormant active species, achieving the maximum content of 24.56 wt %. The catalysts enriched with dormant active species enhance the uniformity of the short-chain branching distribution in synthesized ethylene/1-hexene copolymers. Notably, upon hydrogen introduction during ethylene polymerization, these dormant-species-enriched catalysts simultaneously reduce the polyethylene molecular weight while unexpectedly enhancing the activity by 52.9%, representing the first report of such coupled effects in the ethylene polymerization of Ziegler–Natta catalysts.
危险品化学品经营许可证（带存储）