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

Abstract

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.