Effects of Doping LiCl into MgCl2-Supported Ziegler−Natta Catalyst on the Molecular Weight Distribution and Isotacticity of Polypropylene

Abstract

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.