Stepwise Detitanation of Ziegler–Natta Catalysts: Unraveling the Role of Dormant Active Species in Ethylene Polymerization

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.