Binuclear And Multinuclear Titanocenes: Synthesis And Their Use In Ethylene Homopolymerization

Metallocene catalysts are excellent single site catalysts for olefin polymerization combination of high catalytic activity and ability to produce the polymers with controllable molecular weight, molecular weight distributions and microstructures. However, the narrow molecular weight distribution of the polymer produced bymetallocene catalyst causes difficulties in polymer processing and become one of the obstacles in industrialization. Recently, binuclear metallocenes became very attractive in the development of new metallocene catalysts for olefin polymerization. The two metals in the same catalyst molecule may create more than one active sits in olefin polymerization and produce the polyolefins with broad molecular weight distributions. In addition, the catalytic properties may be improved by designing the structures of catalyst. Lots of binuclear metallocene with a flexible bridge(such as polymethylene, polysiloxane, silane) or rigid bridge (such as phenylene, biphenylene, double bridges) or single atom bridge (such as carbon, silicon) have been prepared and used in olefin polymerization. In general, binuclear metallocenes with a rigid bridge show higher activity and thermal stability. Asymmetry binuclear metallocenes should be able to produce polymer with broader or bimodal molecular weight distribution. Till now, there are few multinuclear metallocenes and their use as catalysts for olefin polymerization have been reported.In this paper, a series of novel binuclear titanocenes {(C₅H₅TiCl₂)₂[(C₅H₄)CH₂-(p-C₆H₄)O(CH₂)_nO(C₆H₄-p)CH₂(C₅H₄)] (n=2-6) (Cat.1-Cat.5) and p-[(C₅H₅TiCl₂)-C₅H₄CH₂]C₆H₄O(CH₂)_n[C₅H₄(TiCl₂C₅H₅)] (n=2-5) (Cat.6-Cat.9)} with the bridge combination of rigid and flexible segment were synthesized to investigate the influence of structure and length of bridging ligand on catalytic properties of binuclear metallocene. In addition, trinuclear titanocenes {C₆H₃[CH₂C₅H₄Ti(C₅H₅)Cl₂]₃-1, 3, 5(Cat.10), {C₆H₃[CH₂-C₅H₄Ti(C₅H₄CH₃)Cl₂]₃-1, 3, 5(Cat.11)} and tetranuclear titanocene {C₆H₂[CH₂C₅H₄Ti-(C₅H₅)Cl₂]₄-1, 2, 4, 5(Cat.12)} were prepared to investigate their catalytic properties for ethylene polymerization. All these complexes were characterized by elemental analysis and NMR spectra. Their catalytic activity for ethylene polymerization was investigated in the presence of aluminoxane (MAO). The effects of the polymerization conditions such as catalyst concentration, mole ratio of MAO/Ti, polymerization temperature, polymerization time and ethylene pressure on catalytic behaviors of some of the complexes were investigated in detail. The results showed as follow:1. The catalytic activity and molecular weights of polyethylene (PE) decrease in order Cat.1-Cat.9 (binuclear)＞Cp₂TiCl₂ (mononuclear)＞Cat.10-Cat.12 (multinuclear); Cat.10＞Cat.11-Cat.12. Which demonstrate that the steric effect of multinuclear titanocene is unfavorable for coordination and insertion of ethylene, so it is unlikely, all three or four metal centers in multinuclear metallocene would convert into active sites.2. The catalytic activity decrease in order Cat.5＞Cat.4＞Cat.3＞Cat.2＞Cat.1; Cat.9＞Cat.8＞Cat.7＞Cat.6; However, the molecular weight of PE increase in order: Cat.5＜Cat.4＜Cat.3＜Cat.2＜Cat.1; Cat.9＜Cat.8＜Cat.7＜Cat.6. It demonstrates that the catalytic behaviors of binuclear metallocenes are highly dependent on the length of bridge. With the length of bridge becoming longer, the catalytic activity increases, Whereas the molecular weight of the produced PE decreases.3. The molecular weight distributions of PE decrease in order Cat.12(tetranuclear)＞Cat.11 (trinuclear)＞Cat.10 (trinuclear)＞Cat.6-Cat.9 (asymmetry binuclear)＞Cat.1-Cat.5 (symmetry binuclear)＞Cp₂TiCl₂ (mononuclear). Which indicate that multinuclear and asymmetry binuclear metallocene are favorable for the formation of multi-active species and producing PE with broader molecular weight distribution.4. The melting points of PE produced by complexes Cat.1-Cat.12 were all higher than 130℃and the ¹³C NMR spectra (methylene and methyl region: 20-60ppm)of PE produced by Cat.10-Cat.12/MAO system show only one strong signal at 29-30ppm for methylene which indicates that the PE are highly linear and highly crystalline.5. Each complex has a proper catalyst concentration and mole ratio of(nMAO/nTi) which is favorable for reaching high catalytic activity and producing PE with high molecular weight; Both the catalytic activity and molecular weight of PE decrease rapidly with the rise of temperature; The catalytic activity decreases with the prolongation of time, however the molecular weight of PE is still increasing; High ethylene pressure is effective for both increasing of catalytic activity and molecular weight of the produced PE.