Effects of doping LiCl into MgCl₂-supported ziegler-natta catalyst on the molecular weight distribution and isotacticity of polypropylene

A series of MgCl₂-supported Ziegler-Natta catalysts was prepared by supporting TiCl₄ on LiCl-doped MgCl₂, and these catalysts were used to polymerize propylene, using triethylaluminum as a co-catalyst. LiCl and MgCl₂ were coprecipitated from ethanol to prepare LiCl-doped MgCl₂·nEtOH adducts. The LiCl-doped catalysts were prepared by dipping the adducts into a TiCl₄ solution. The crystalline and morphological structures of the adducts and catalysts were characterized by wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). Two types of mixed crystals-Li₂MgCl₄ and LiMgCl₃-were found in the catalyst that contained 18 mol% LiCl (LMSC18). The molecular weight distribution (MWD) and isotacticity of polypropylene (PP) varied with the dosage of LiCl in the catalyst. PP with a broader MWD was synthesized using LiCl-doped catalysts that contained 11-18 mol% LiCl. Deconvolution of the MWD profiles with multiple Flory "most-probable" functions were applied to analyze the distribution of active centers. There were six types of active centers in LMSC18: A, B, B′, C, D, and E. The combined productivity of (E + D)-type active centers in catalyst LMSC18 was much larger than that of the other catalysts. All the results showed that the properties and distribution of active centers of the catalysts supported on LiCl-doped MgCl₂ were markedly changed at a certain range of LiCl dosage.