In‐situ gas phase poly(propylene) (PP) formation over a high activity TiCl4‐MgCl2‐supported Ziegler‐Natta catalyst has been studied by video microscopy combined with ex‐situ light microscopy, SEM, high‐resolution TEM, and STEM/PEELS/EDX for the first time. In‐situ observation revealed rapid formation of poly(propylene) beads 9–10 μm in size (< 1/30 s) as well as growth of significant amounts of polymer within local regions. Catalyst particles containing 2–5 nm‐sized MgCl2 crystalline domains are subjected to transformations during catalysis that form PP/catalyst aggregated structures of 30–50 μm in size. 相似文献
Quenched‐flow studies of MgCl2‐supported Ziegler–Natta catalysts were combined for the first time with 13C NMR fingerprinting of the nascent polymer and conclusively proved that, depending on the catalyst formulation, propene polymerization can be slowed down significantly by the occurrence of the few regiodefects (2,1 monomer insertions), changing active sites into dormant sites. Catalysts modified with ethylbenzoate show little dormancy. The more industrially relevant phthalate based catalysts, instead, are highly dormant and require the presence of H2 to counteract the deleterious effect of this self‐poisoning on productivity and stereoselectivity. 相似文献
Superactive Ziegler–Natta catalysts have been prepared from a soluble MgCl2·2-ethyl hexanol adduct in the presence of organic esters through reactions with TiCl4 and activated with AlEt3/phenyltriethoxy-silane. Electron paramagnetic spectra (EPR) were used to elucidate the nature and amount of those Ti+3 ions not bridged to another Ti+3 ion; the chlorine bridged Ti+3 ions are EPR silent. The EPR spectra were attributed to two rhombic Ti+3 sites with principal values for the g-tensors (1.967, 1.949, 1.915; and 1.979, 1.935, 1.887). The total amount of the EPR species, obtained by double integration of the EPR spectra, is in close agreement with the concentration of isospecific catalytic sites determined by radiotagging. This suggests that the nonspecific sites are EPR silent. When o-phthalic ester was present during the catalyst synthesis, there appears an EPR signal at the free electron g-value. This signal was attributed to a Ti+3 phthalate species with resonance stabilization and spin delocalization; it is absent in the catalysts made in the presence of monoesters such as ethyl benzoate. The effects of monomer, O2, H2O, and I2 on the EPR spectra were investigated. The changes in the EPR spectral intensity and the total Ti+3 ions, the latter determined by redox titrations during a polymerization or catalyst aging, are described. The results were extensively compared with those observed for supported Ziegler–Natta catalyst prepared with crystalline MgCl2. 相似文献
The successive self-nucleation/annealing technique (SSA) by differential scanning calorimetry has been applied to study the heterogeneity of ethylene–hexene-1 copolymers produced with supported catalytic systems of different compositions: highly active supported Ziegler–Natta (Z–N) catalysts—a titanium–magnesium catalyst TiCl4/MgCl2 (TMC) and a vanadium–magnesium catalyst VCl4/MgCl2 (VMC), a supported zirconocene catalyst Me2Si(Ind)2ZrCl2/SiO2 (MAO), and a chromium-oxide catalyst CrO3/SiO2. Comparative data by SSA technique with the same temperature program were obtained for copolymers differed by MWD from narrow to very broad (Mw/Mn = 2.4–54) and short chain branching distribution from narrow (zirconocene catalyst) to very broad (TMC and chromium oxide catalysts). It is demonstrated that copolymers produced with the zirconocene catalyst have the narrowest melting range and do not contain thick lamellae. The widest lamella thickness distribution has been found for a copolymer produced with the chromium-oxide catalyst. Copolymers produced with the supported Z–N catalysts are ranked in the middle with a more narrow lamella thickness distribution for copolymer prepared with VMC as compared with the one produced with TMC. The SSA results are compared with the data on copolymer fractionation by TREF. It is shown that these methods give a good correlation for copolymers with narrow short-chain branching distribution produced with the supported zirconocene catalyst. In the case of copolymers produced with TMC, TREF yields a higher content of the high-branched fractions. 相似文献
The MCM‐41 and SiO2 supported TiCl4 and TiCl4/MgCl2 catalysts with different molar ratios of Mg/Ti were synthesized and used for ethylene polymerization under atmospheric pressure. The nanochannels of MCM‐41 serve as nanoscale polymerization reactor and the polyethylene nanofibers were extruded during the reaction. The nanofibers were observed in SEM micrographs of resulting polyethylene. The effect of MgCl2 on catalytic activity and thermal properties of resulting polyethylene is investigated too. In the presence of MgCl2, the catalytic activity increased and more crystalline polyethylene with higher melting points were formed. However, no fibers could be observed in the polyethylene prepared by SiO2 supported catalysts. 相似文献
The results of studies of the synthesis and properties of supported titanium-magnesium catalysts for propylene polymerization performed at the Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, are considered. The composition of the catalysts is TiCl4/D1/MgCl2-AlEt3/D2, where D1 and D2 are stereoregulating donors. With the use of the procedure proposed for the synthesis of titanium-magnesium catalysts, the morphology of catalyst particles depends on the stage of the preparation of a Mg-containing support. The titanium-magnesium catalysts developed afforded polypropylene (PP) in a high yield; this PP was characterized by high isotacticity and excellent morphology. The controllable fragmentation of the catalyst by the polymer is of crucial importance for the retention of the morphology of titanium-magnesium catalyst particles in PP. The fragmentation of catalyst particles to microparticles occurred in the formation of more than 100 g of PP per gram of the catalyst. The surface complexes were studied by DRIFT and MAS NMR spectroscopy and chemical analysis. It was shown that the role of internal donors is to regulate the distribution of TiCl4 on different MgCl2 faces and, thereby, to influence the properties of PP. It was found that chlorine-containing complexes of aluminum compounds were formed on the catalyst surface by the interaction of the catalyst with AlEt3; these complexes can block the major portion of titanium chloride. Data on the number of active sites and the rate constants of polymer chain propagation (kp) at various sites indicate that donor D1 increases the stereospecificity of the catalyst because of an increase in the fraction of highly stereospecific active sites, at which kp is much higher than that at low-stereospecificity active sites. Donor D2 enhances the role of D1. Similar values of kp at sites with the same stereospecificity in titanium-magnesium catalysts and TiCl3 suggest that the role of the support mainly consists in an increase in the dispersity of titanium chloride. 相似文献
Monocyclopentadienyl compounds, CpMCl3 (M = Ti, Zr) supported on activated MgCl2 were used for the polymerizations of ethylene in the presence of methylaluminoxane (MAO) or a common alkylaluminium as a cocatalyst. By supporting CpMCl3 on MgCl2, the catalyst activity was increased drastically to show high activity similar to MgCl2‐supported TiCl4 catalysts. The activity of the CpZrCl3 /MgCl2 catalyst was higher than that of the CpTiCl3/MgCl2 one. Both catalysts gave polymers with high molecular weight (Mw) and broad molecular weight distribution (Mw/Mn) in comparison with the corresponding soluble half‐metallocene catalysts. 相似文献
A series of (SiO2/MgO/MgCl2)?TiClx Ziegler‐Natta polyethylene catalysts have been synthesized using water‐soluble Mg‐compounds such as magnesium acetate as Mg‐source at different TiCl4 treating temperatures. The catalyst shows the highest activity of homopolymerization when preparation temperature is 120 °C with only 1.25 of optimal Al/Ti molar ratio, which is much lower than industrial value, resulting in much lower catalyst preparation and polyethylene production cost. The operation condition is relatively moderate and the synthesized catalysts exhibit rather high activity, good hydrogen response, and copolymerization ability with high 1‐hexene incorporation. The polymers obtained from these catalysts have high molecular weight and medium molecular weight distribution. Compared with the conventional industrial Ti/Mg Ziegler‐Natta catalysts using the relatively expensive, anhydrous and moisture‐sensitive Mg‐sources, the most unique feature of our novel catalysts is the capability of utilization of any soluble Mg‐compounds under mild conditions and can achieve rather high activity with only a small amount of cocatalyst, hence show great potential for application in polyethylene industry.
This study focuses on gas-phase polymerization of ethylene using the titanium-based Ziegler–Natta catalysts prepared from different magnesium sources including MgCl2 (Cat A), magnesium powder (Cat B), and Mg(OEt)2 (Cat C). During polymerization, different cocatalysts were also used. It was found that Cat C with triethylaluminum as a cocatalyst exhibited the highest activity. This was likely attributed to optimal distribution of active sites on the catalyst surface. It can be observed by increased temperature in the reactor due to highly exothermic reaction during polymerization. By the way, the morphologies of the polymer obtained from this catalyst were spherical, which is more preferable. Besides the catalytic activity, crystallinity and morphology were also affected by the different magnesium sources used to prepare the catalysts. 相似文献
The olefin polymerizations were carried out by using silica supported metallocene/MAO catalysts and MgCl2 supported Ziegler-Natta catalysts under mild reaction conditions and stopped at very low yield. The surface and cross sectional morphology of the polymer particles were characterized by using scanning electron microscopy (SEM). A homogeneous distribution of (co)catalyst on the support material is a prerequisite condition to get a homogeneous fragmentation and uniform polymer particle morphology. In the present work the catalysts show two different fragmentation behaviors. They can gradually fragment from the outer to the inner surface of the catalyst particle, or instantaneously break up into a large amount of small sub-particles at the beginning of the polymerization. The incorporation of comonomer does not change the general catalyst fragmentation scheme but delays the catalysts break-up progress. 相似文献
A method for the preparation of well‐defined crystallites of MgCl2‐supported Ziegler‐Natta catalysts on Si wafers has been developed. This has been achieved by the spin‐coating of a MgCl2 solution onto a flat Si wafer, followed by controlled crystal growth to give well‐defined MgCl2 · nEtOH crystallites. The growth of the crystallites on the flat silica facilitates their characterization using electron and scanning probe microscopy. The relative proportions of 120° and 90° edge angles indicate the preference for the formation of a particular crystallite face for the MgCl2. Polyethylene has been identified to be formed on the lateral faces of the crystallite.