Role of electron donor in the polymerization of propylene with MgCl ₂-supported catalysts

MgCl₂-supported Ziegler-Natta (Z-N) catalysts containing three different Lewis bases (diisobutyl phthalate, 9,9-bis(methoxymethyl) fluorine and 1,1-bis(methoxymethyl) cyclobutane) as internal donors were prepared for propylene polymerization. The relationship between the structure of donors and the performance of catalysts were investigated. The Z-N catalyst containing 9,9-bis(methoxymethyl) fluorine as internal donor and the related compounds were characterized by FTIR spectroscopy. It was found that the internal donor in the MgCl₂/TiCl₄/Donor catalyst system was directly coordinated to MgCl₂. The introduction of internal donors decreased the Ti content in the catalyst system, increased the catalytic activity and narrowed the molecular mass distribution of polymers obtained. The stereospecificity of Z-N catalysts greatly depended on the structure of electron donors. A model of multi-active sites on heterogeneous Z-N catalyst was proposed to explain the experimental results.