A First Principle Study On The Mechanism Of Ziegler-natta Catalyst

This mechanism of Ziegler-Natta catalytic system was focused in order to design novel catalyst for polyolefin. The interplay between the TiCl4-ID, MgCl₂-TiCl4, and MgCl₂-ID interactions can significantly modulate the performance of MgCl₂-supported Ziegler-Natta catalysis. The mechanism of interaction between donor and MgCl₂ was explored to explain the role of donor in Ziegler-Natta catalyst. Moreover, the novel catalysts were designed, which were TiCl₃（OAr） for the polymerization of ethylene and TiCln（OR）_4-n for the polymerization of propene. In order to verify reliability for theoretical simulation of the polymerization mechanism, the novel catalysts were synthesized for the ethylene and propene polymer ization. Finally, the space steric effect of alkyl on cyclopentadienyl in Cp2TiCl2 was studied in process of ethylene polymerization.Firstly, the stabilized effect of surface coverage on the coordination properties of methanol on the（110）,（100）, and（104） surfaces of MgCl₂ by periodic DFT was examined. The adsorption energy of different MgCl₂ surfaces, which showed the dominance of the（110） surface over the（100） and（104） surfaces, was investigated to prove that methanol could increase surface stability. It demonstrated that the complete coverage of the Mg vacancies on the surface by coordinating methanol was hampered by steric repulsion between the vicinally coordinated donor molecules. The results indicated that the surface of MgCl₂ was controlled by choosing an appropriate choice of electron donor.Next, the interactions of donors with MgCl₂ surfaces were evaluated using periodic DFT. The selected donors included 1,3-dimethoxy-2,2-dimethylpropane, dimethoxydimethylsilane, and dimethyl phthalate. The absorption energies and distances of Mg-O were obtained from α-MgCl₂（110）,（100）, and β-MgCl₂（110）,（100）, and（104） surfaces. The donors on α and β-MgCl₂（ 110） surfaces reflected the largest absorption energies among the complexes examined. The results indicated that the surface of MgCl₂ was controlled by choosing an appropriate choice of electron donor. The surface of MgCl₂ affected the activity and stereoselectivity of the catalyst. The result explained the relation between MgCl₂ and the property of catalyst in theory.Thirdly, the TiCl₃（OAr） catalysts were designed for the polymerization of ethylene, and the mechanism of the active site by the insertion of ethylene into Ti-C bond in [TiClOAr Et]⁺ was evaluated when the TiCl₃（OAr） had different structure ligands. The selected Ar- groups included C₆H_5-, 2,6-Me₂C₆H_3- and 2,6-iPr₂C₆H_3-. Meanwhile, the C2H5-, Cp- and Cl- groups were also insteaded of the Ar- groups to explore the mechanism. The mechanism of the aryloxy effect on the polymer ization behaviors was that their chemical environment on the sound of titanium cation was slightly changed. The calculated result showed the activation energy and reaction heat of insertion of ethylene into Ti-C bond in [TiClOAr Et]⁺ were lied between that of [TiClCp Et]⁺ and [TiCl2Et]⁺. The characteristics of ethylene polymerization were like metallocenes catalyst. Polyethylene had higher molecular weight and relatively narrow molecular weight distribution. Moreover, the polymer ization of ethylene was studied using TiCl₃（OAr） catalysts, the results were consist with calculation results by DFT, indicating that the control of polyethylene is attainable by appropriate selection of Ar- in TiCl₃（OAr）.Fourthly, TiCln（OR）_4-n catalysts were designed for the polymerization of propene. The activity and stereoselectivity of the catalyst w ere tuned by alkoxy because of the steric effect and the electronic effect of alkoxy. The mechanisms in the polymerization of propene were studied when the active center were [TiCl2Et]⁺、[TiCl（OEt）Et]⁺、[TiCl（OPh）Et]⁺ and [Ti（OEt）2Et]⁺. In the traditional Ziegler-Natta catalyst such as [TiCl2Et]⁺, the insertion of propene into active center in 2, 1-mode and producing random polypropylene. To improve the stereoselectivity of the catalyst, the alkoxy groups were introduced in the active center. Then, the active center has asymmetric structure. The polymerization of propene by [TiCl（OEt）Et]⁺ and [TiCl（OPh）Et]⁺ were in 1,2-mode from the re-face. Moreover, [Ti（OEt）2Et]⁺ with symmetric structure was study, the polymerization of propene was favorable in 1,2-mode, but the stereoselectivity didn’t exist in the process of polymer ization. Moreover, the polymerization of propene was studied using TiCln（OR）_4-n catalysts, and the results were consist with calculation results by DFT.Finally, the space steric effect of alkyl on cyclopentadienyl in Cp2TiCl2 was studied in process of ethylene polymerization, the active center included [Cp2（R）TiCH3]⁺ and [NCp2（R）TiCH3]⁺（R=H, Me, iPr）. The mechanism in the polymerization of ethylene was studied when the active centers were [Cp2（R）TiCH3]⁺ and [NCp2（R）TiCH3]⁺. The steric hindrance of the alkyl is bigger, so the activation energy is higher, and the ethylene polymerization is more difficult.