Organic/inorganic Composite Support For Immobilizing Catalyst Using In Ethylene Polymerization

Tailored polyethylene chain structure is one of the most important issues to obtain high performance polyethylene in order to fit for the end-use. The intra and inter chain structures, such as molecular weight and molecular weight distribution, branch chain structure and distribution, can be well tailored according to the combination of polymerization process and catalyst. Bimodal polyethylene is composed of two parts. One is the low molecular part which can improve the processability of the resin; the other is the high molecular weight part using to increase the operation performance.Using hybrid catalyst is one of the most important methods to prepare bimodal polyethylene. In this thesis, a novel composite microsphere support was prepared using in the immobilization of hybrid catalyst. The immobilized hybrid catalyst was separated by polymer support and the suitable supported environment for each catalyst was built-up. The thesis focused on the following parts:Firstly, styrene and acrylic copolymer (PSA) was coated on the silica following the phase-inversion method. Emission scan electron microscope (SEM), laser granulometer, Fourier transform infrared spectroscopy (FTIR), thermogravimetic analysis (TGA) and Brunauer-Emmett-Teller (BET) methods were used to investigate the phase structure and morphology of the composite support. It was shown that the PSA was compactly coated on the surface of silica according to the control of non-solvent evaporation rate. Different preparation conditions for achieving the support were well studied. Finally, the stability of composite support in the toluene was studied as well.Secondly, this organic/inorganic composite support for immobilizing hybrid catalyst was devised. The composition of hybrid catalyst was as following:PSA was the organic part for binding (n-BuCp)2ZrCl2, and SiO2/MgCl2 was the inorganic part to immobilize TiCl3. As an alternative to a tandem or cascade type process, a two-step polymerization method was carried out in a single reactor on a laboratory scale. Utilizing this two-step slurry polymerization process, reactor blends of ultra high molecular weight copolymer and low molecular weight homo-polymer were produced. The results showed that the hybrid catalyst had long and stable catalyst activity (almost 8 h). The obtained polymer blends had bimodal molecular weight distribution and broad melt flow rate (MI21.6/MI2.16=79). The property of the PSA layer during the whole polymerization was well studied. It was found that the barrier property of PSA layer to cocatalyst MMAO was observed by ethylene polymerization. This outstanding barrier property made MMAO activating (n-BuCp)2ZrCl2 prior to M-1 in the hybrid catalyst. In addition, the morphology, branch degree, molecular weight, molecular weight distribution and crystalline property of polymers obtained at different polymerization time were investigated by SEM, FTIR, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC), respectively. It was shown that polymer properties showed little variance in the initial polymerization stage. However, obvious change of the properties was found after the polymerization time was longer than 40 min. It indicated that M-1 catalyst was gradually activated by MMAO after the polymerization was conducted for 40 min. Furthermore, the mechanism of polyethylene particle growth during the polymerization was discussed. Finally, the miscibility and thermal behaviors of blends were also investigated according to different DSC techniques.The performance of hybrid catalyst (n-BuCp)2ZrCl2/PSA/TiCl3 was investigated by gas-phase ethylene polymerization in order to study the suitable polymerization process of composite support based catalyst. PSA was firstly modified with n-BuSnCl3 to achieve high activity. The results of gas phase ethylene polymerization showed that the modified catalyst had higher activity, reaching to 2.56×106 gPE·(molZr)-1·h-1·bar-1. Furthermore, the polymer obtained at different polymerization time was studied in order to observe the fragmentation behavior of PSA support during the gas phase polymerization. Finally, the results of gas phase polymerization were compared with that of slurry phase polymerization. It was showed that slurry polymerization was the most suitable process for the composite support based catalyst.Fourthly, the composite support was used in the immobilization of late transition iron catalyst. Iron acetylacetonate and bis(imino)pyridyl ligands homogeneous catalyst system was bound on the PSA. Both high crystallinity (72%) polyethylene and a-olefin were produced by the catalyst at the same time. The different membrane materials and polymerization conditions (cocatalyst, pressure, temperature, Al/Fe ratio) to the polymerization activity and properties of polymers were studied. It was found that the temperature had the greatest impact on the weight radio of oligomer to PE. The iron catalyst system binding on the PSA after the treatment of magnesium chloride produced the polyethylene of lower molecular weight (Mw=11.9×104g-mol-1) and higher crystallinity (72%). The polyethylene can be used as the low molecular weight part of bimodal polyethylene.Finally, styrene and acrylic copolymer was the organic part for binding TiCl4, and SiO2 was the inorganic part to immobilize Cp2ZrCl2. The styrene and acrylic copolymer was coated on the surface of SiO2, compactly. The barrier property of organic layer to the cocatalyst was characterized by ethylene polymerization. The results implied that an induction period presented in the initial polymerization due to the introduction of the styrene and acrylic copolymer. It indicated that the diffusion effect of the triethylaluminium (TEA) was not ignored during the chain propagation. Furthermore, ethylene polymerization was also performed by hybrid catalyst. Bimodal polyethylene was achieved when TEA was used as the cocatalyst in the ethylene/1-hexene copolymerization.