Effect Of Internal Donors On Performances Of TiCl₄/MgCl₂ Catalysts For Propylene Polymerization

?Internal donors have much important impact on TiCl₄/MgCl₂ Ziegler-Natta catalysts for propylene polymerization. In this work, a series of catalysts with different compounds as internal donor and different internal donor content were prepared according to the synthesis method of N-catalysts. The reactions which occur in catalyst preparation systems, the interaction between internal donors and the other components in catalysts and the effect of internal donor on performance of catalysts were investigated systematically.1．The reaction that di-n-butyl phthalate is formed in the catalyst preparation systems was studied in detail. The effect of di-n-butyl phthalate formed in-situ on propylene polymerization performances was investigated.By IR, UV-Vis, GC-MS and GC analyses, it was found that an electron donor compound di-n-butyl phthalate was formed by the reaction between phthalic anhydride and tributyl phosphate. However, the reaction was rather weak, and raising temperature was unfavorable for it. Only TiCl₄ promoted the reaction in the catalyst preparation systems, however, the reaction was still very weak even with the presence of TiCl₄ resulting in rather little amount of di-n-butyl phthalate.X-ray photoelectron spectroscopic analysis indicated that the little amount of di-n-butyl phthalate formed in-situ had no effect on electronic states of Ti species in catalysts.The results of Propylene polymerization showed that the little amount of di-n-butyl phthalate formed in-situ had some effect on propylene polymerization performances. It restrained formation of atactc active centers resulting in decrease of room temperature decane soluble fraction, as a result increasing stereospecificity.2．The interaction between internal donors and the other components in catalysts and the effect of internal donors on electronic states of Ti species were investigated.By IR and TG analyses, it was found that no matter what kind of internal donors were used in catalysts, such as aromatic monoester, aromatic diester, diether and dibenzoate, they all interacted directly with MgCl₂ only and there was no direct interaction between internal donors and TiCl₄ in catalysts.X-ray photoelectron spectroscopic analysis results indicated that all internal donors had significant influence on electronic states of Ti species in TiCl/MgCl/ID catalysts and TiCl₄·ID complexes. They made the binding energy of Ti_2p3/2 peak shift to a low energy region by providing electrons to Ti species. The electron donation ability of different internal donors was different because of their different molecular structures. The electron donation ability decreased in the following order: 2, 4-pentadiol dibenzoate > di-n-butyl phthalate > ethyl benzoate≈2-isoamyl-2-isopropyl-1,3-di-methoxypropane. Electronic states of Ti species varied when internal donor content in catalysts changed. However, they became uniform gradually with the increase of internal donor content.3．The effect of internal donors on propylene polymerization performance and on distribution of isospecific active center was elucidated.Internal donors lowered room temperature decane soluble fraction, increased boiling heptane insoluble fraction, as a result, enhanced stereospecificity. Stereospecificity increased with the increase of internal donor content, however, it did not increase when internal donor content is higher than a certain value.Propylene polymerization results showed that internal donors enhanced the activity for propylene polymerization. Internal donors increased not only the activity for isospecific polypropylene which is insoluble in boiling heptane but also the activity for atactic polypropylene which is soluble in boiling heptane. The propylene polymerization activity increased with the increase of internal donor content. However, the activity decreased because of much low specific surface areas when internal donor content was higher than a certain value.By deconvolving the molecular weight distribution curves of isospecific polypropylene, it was found that different internal donors significantly influenced distribution of isospecific active centers and molecular weight of polypropylene corresponding to the active centers. However, internal donor content had no much effect on distribution of isospecific active center and the molecular weight of popypropylene corresponding to the active centers. It indicated that the nature of internal donors is the key factor that determined the nature of isospecific centers.