Elucidating the correlation between active species and branch distribution of polyethylene in Ziegler-Natta catalysis

Abstract

The ethylene/α-olefin copolymers synthesized by Ziegler-Natta catalysts generally exhibit much higher Short-Chain Branch (SCB) concentration in the low-molecular-weight fraction, which hampers the mechanical properties of the final products. Herein, we investigate the mechanism underlying the origin of SCB distribution at a molecular level. We design a spherical catalyst combining two alcohols with different mobility, after which a second titanation step with incremental TiCl₄ loading is applied to drive the migration of alcohol molecules. This migration induces not only the escape of alcohols from the catalyst surface but also the coordination of alcohols with Ti centers, which regulates the mobility of active species on the catalyst surface. We demonstrate that the aggregation behavior of Ti species is directed by their bonding to the MgCl₂ surface, where either TiCl₃-like clusters or isolated Ti³⁺ sites can be preferentially synthesized during triethylaluminium activation. Further polymerization results combined with electron paramagnetic resonance analysis show that TiCl₃-like clusters and the “dormant” active species contribute to the uniform SCB distribution over different molecular weights, whereas the predominated isolated Ti³⁺ sites on the MgCl₂ (110) plane concentrate the SCB in the low-molecular-weight fraction of the synthesized polyethylene. Taken together, we reveal a correlation between polymer branching distribution and catalytic structures, and show that the latter can be regulated by tailoring the migration of alcohols.