Demystifying the two-sided role of inorganic halides in the structure and performance of Ziegler–Natta catalysts

Abstract

In this paper, the results of adding SiCl₄ (as a model inorganic halide) to the formulation of an industrially well-known Ziegler–Natta catalyst (ZNC) have been investigated to shed light on the hidden sides of the catalysts. Four different ZNCs, which differ only in the addition steps, were synthesized and fully characterized using numerous techniques. Furthermore, the catalysts were used for the production of a set of monomodal and bimodal ethylene/1-butene copolymers. Finally, differential scanning calorimetry (DSC), successive self-nucleation annealing (SSA), and some rheological tests were carried out on the synthesized copolymers to reveal the roles of SiCl₄ in the molecular architecture of polyolefins made using the ZNCs. The results showed that inorganic halides play two different roles depending on the stage of their addition during the ZNC synthetic procedure. Firstly, adding the inorganic halide before the main chlorination phase can increase the active centers' binding energy (BE) by 0.4–0.8 eV, which can be interpreted by considering an electron-poorer environment around the centers. In other words, in this case, SiCl₄ can be doped into the structure of MgCl₂, and it led to a 30% to 60% reduction in the amount of hexane-soluble fractions and fine particles produced during copolymerization reactions. On the other hand, the addition of SiCl₄ after the chlorination phase resulted in a 0.4 eV reduction in the BE of titanium species, which means that, in this case, the inorganic halide can enrich the electronic states of active centers without doping into the structure of MgCl₂. By doing so, a dramatic change occurred in the stereospecificity of the produced copolymeric chains, leading to a jump of about 10–30% in the strain hardening modulus of the final copolymers.