Effect of Electron Donor in Ziegler-Natta Catalysts on the Performance of Olefevs Polymerization
|School||Hebei University of Technology|
|Keywords||Ziegler-Natta catalyst electron donor carrier polyethylene poly(1-butene)|
In this dissertation, Ziegler-Natta catalysts containing different internal electron donors were prepared for olefin polymerization, combined with external donors. The effects of the catalyst systems on the polymerization were investigated. Based on the characterization of polymer, the relationships between structure of catalyst and performance of polymer were studied in detail. The main results were summarized as follows:Four bi-supported catalysts were prepared by the reaction of SiO2/MgCl2-butanol adduct with TiCl4, employing different types of SiO2. The type of SiO2led to a significant change of active site distribution, resulting in affecting both ethylene and ethyl ene/1-hexene polymerization behaviors. Cat-B prepared by employing757-SiO2led to the highest activity (26344g/gTi and40938kg/gTi) for ethylene polymerization and ethyl ene/1-hexene copolymerization. The effect of different SiO2on MW and MWD of polymers was more obvious. The deconvolution of GPC curves by multiple distribution functions was used to study the active site distribution of catalyst. The curves were deconvoluted into five Flory components. Thus, the MWD value of polymers were tunable to some degree simply by employing different type of SiO2, since it can modify the structure of the surface active sites, inducing multiple active sites.At the same time, the effects of type of cocatalyst, Al/Ti molar ratio and comonomer concentration on the performance of ethylene polymerization were investigated with bi-supported catalysts. The catalyst was characterized by means of Laser granulometer, SEM and XRD. Copolymers had been characterized by using DSC, FTIR and GPC. The results were indicated that the catalyst had good particle morphology and uniformity. Al(i-Bu)3as cocatalyst in the catalytic system showed higher activity and copolymerizability than that of AlEt3. The distributions of active centers in both the Al(i-Bu)3and AlEt3as cocatalyst were studied by deconvolution of molecular weight distribution curves of the polymer with multiple Schulz-Flory most probable distribution function. The active center distribution of Al(/-Bu)3and AlEt3as cocatalyst in the catalytic system had marked changes.Several MgCl2/SiO2supported Ziegler-Natta catalysts were prepared in the same way. The effect of the mass ratio of SiO2to MgCl2and the molar ratio of DIOP to MgCl2on the catalyst structure and its catalytic activity for ethylene polymerization with MgCl2/SiO2supported Ziegler-Natta catalyst was investigated in detail. The catalysts were characterized by BET, SEM and XRD. The results were indicated that the adding of SiO2and DIOP can improve particles morphology and uniformity of the catalysts. The activity of the catalyst was decreased along with the increasing of SiO2and DIOP dosage. Bulk density and melt index of polymer obtained were first increased and then almost unchanged.MgCl2/SiO2bisupported Ziegler-Natta catalysts containing different silane as internal donors (IDs) were prepared for ethylene polymerization with1-hexene. The relationship between the structure of donors and the performance of catalysts were investigated. It was found that the introduction of IDs decreased the Ti content in the catalyst system, but Ti content of the catalysts containing different IDs was different. MTAOS as the ID in the MgCl2/SiO2/ID/TiCl4catalyst system was directly coordinated to MgCl2. The different IDs changed the ratio of "Bronsted acid"(B acid) and "Lewis acid"(L acid) on the surface of the catalysts, and the catalytic activity increased with the increase of the ratio of peak area of L acid and B acid. The addition of IDs increased activity of catalyst. But IDs weakened the degree of activity of the "comonomer effect" except MEOS. MEOS as ID reduced the molecular weight of copolymer, but broadened the molecular weight distribution and improved the copolymerizability. However, the addition of other IDs in the catalysts helped to increase the molecular weight, to narrow the molecular weight distribution, to decrease the copolymerizability and enhanced the block of the chain structure of copolymer, especially IDs with electron-withdrawing groups. The Flory non-linear-fitting of copolymers molecular weight distribution curves showed that the addition of ID didn’t change the type of active centers, but change the active center distribution.Moreover, four aminosilane compounds as external electron donors were synthesized and employed for1-butene polymerization with MgCl2supported Ziegler-Natta catalyst (BQ catalyst). The effects of the structure of different external donors on the catalytic efficiency, isotacticity, melting temperature and molecular weight distribution of resulted poly(1-butene) were investigated in detail. Poly(1-butene)s have been characterized by using gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) analysis. The results indicated that the aminosilane compounds possessing dimethoxy groups show higher catalytic efficiency and isospecificity of polymer than those of compounds having diethoxy groups, whereas the aminosilane compounds containing diethoxy groups were benefit for obtaining broader molecular weight distribution polymer than those of compounds possessing dimethoxy groups. Furthermore, compared with DPPDMS or TEOS as external donor, DPPDMS/TEOS compound as complex external donor at a proper molar ratio not only increased the catalytic efficiency and isotacticity of polymers obtained, but also broadened the molecular weight distribution of the polymer. Moreover, the effects of different amount of hydrogen and concentration of external donor and cocatalyst on catalytic efficiency and isotacticity of polymers obtained were also studied.1-butene bulk polymerization catalyzed by BQ catalyst had higher catalytic activity and high isotacticity and wider molecular weight distribution of polymer obtained. However, the catalytic activity decreased significantly due to join β-nucleating agent, the isotacticity of polymer increased slightly, and narrowed molecular weight distribution. Poly(1-butene) was characterized by using POM, DSC, and XRD. The results indicated that the polymer’s "Black Cross" extinction phenomenon became weaker, grain was smaller and the grain size became more uniform with the increase of the amount of β-nucleating agent. The melting point and crystallinity of form II and form I crystal of poly(1-butene) also increased with the increase of the amount of β-nucleating agent. The poly(1-butene) produced by Ziegler-Natta catalyst contained polymorphs of form III and form I’, and the proportion of form III was increased by added the nucleating agent. Moreover, nucleating agent accelerated the change from form II to form I when the polymer melted.