Effect of monomer diffusion in the polymerization of olefins over Ziegler-Natta catalysts (original) (raw)
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Surface and Interface Analysis, 2001
Modern surface science techniques have been applied to the heterogeneous Ziegler-Natta catalysis system, polymerizing a-olefins to produce polyolefins, and revealed the correlation between the catalyst surface structure and polymer properties. Two types of thin films -TiCl x /MgCl 2 and TiCl y /Au -were fabricated on an inert gold substrate, using chemical vapor deposition methods, to mimic the high-yield catalysts of MgCl 2 -supported TiCl 4 and TiCl 3 -based catalysts, respectively. Once activated with triethylaluminum (AlEt 3 ) vapor, both catalysts were active for polymerization of ethylene and propylene in the absence of excess AlEt 3 . The model catalyst films were as active as the high-surface-area industrial catalysts. Both catalysts were terminated with chlorine at the surface but had different surface structures. The TiCl x /MgCl 2 film had a distribution of two structures: the dominant sites had the (001) basal plane of these halide crystallites and the minority sites had a non-basal plane structure. The surface of the TiCl y /Au film assumed only the non-basal plane structure. These structural differences resulted in different tacticity of the polypropylene produced with these catalysts. The TiCl x /MgCl 2 catalyst produced both atactic and isotactic polypropylene, whereas the TiCl y catalyst without the MgCl 2 support produced exclusively isotactic polypropylene. The titanium oxidation states did not appear to be an important factor in determining the tacticity of the polypropylene.
Ziegler-Natta catalysts for olefin polymerizations
Progress in Polymer Science, 1997
The current development of the metallocene-based Ziegler-Natta catalysts has been reviewed. The discovery of these catalysts has offered the opportunity to obtain a deeper insight into the mechanism of Ziegler-Natta polymerizations. In this review, some mechanistic models for polymerization and stereoregulation, as well as the factors which affect the activity and stereospecificity of the catalysts, have been discussed. The technology of olefin polymerization with the metallocene-based catalysts is in the early stage of commercialization. Using these catalysts, a large number of novel polymers with special properties have been obtained.
Iranian Polymer Journal, 2019
A fundamental understanding of the heterogeneous olefin polymerization process is critical for imparting desired properties to the final polymer product. In this work, we have developed a comprehensive model integrating the meso-scale level intraparticle resistances to mass and heat transfer as well as the micro-scale level kinetics. The model formulation is based on the combination of the polymer flow model with the intrinsic kinetic model derived using the method of moments approach. The model is employed to study the effect of varying the mass transfer and kinetic parameters on the monomer concentration and temperature profiles inside the growing polymer macro-particle and the subsequent implications on the catalyst activity, polymer molecular weights and the polydispersity index (PDI). The simulation results showed that the steeper monomer concentration gradients in the polymer macro-particle arose on decrease of the bulk diffusivity (D b) and increase of the number of active sites. The model also predicted the interdependence between the radial monomer concentration and temperature profiles. Further, with appropriate choice of D b , the number of active catalyst sites, initial catalyst active site concentration and kinetic rate constants, the model predicted the catalyst activity exceeding 100 kg/g cat.hr and PDI values higher than 2. We showed that the model is capable of predicting the experimental reported polymer product properties for Ziegler-Natta, hybrid Ziegler-Natta/metallocene and supported metallocene catalyst systems.
Theoretical Foundations of Chemical Engineering, 2011
The process of the growth of polymer particles in olefin polymerization is studied. The regions of the possible overheating of particles due to the polymerization reaction in the gas and liquid phases of a monomer and in a solvent are estimated. It is shown how the rates of external mass transfer, internal diffu sion, and reaction in the polymer particle vary with its growth. The cause of unusual transition of the reaction from the diffusion region to the kinetic one with the growth of particles is clearly explained. A model for the dynamics of the polymerization process is proposed that takes into account the diffusion of active component ions of a catalyst (chlorine in Ti x Cl y and oxygen in Cr x O y ). The results of the study are used in the application of high activity catalysts to the slurry process of propylene polymerization.
Journal of Polymer Science Part A: Polymer Chemistry, 2008
Homogeneous and silica-supported Cp 2 ZrCl 2 /methylaluminoxane (MAO) catalyst systems have been used for the copolymerization of ethylene with 1-butene, 1-hexene, 4-methylpentene-1 (4-MP-1), and 1-octene in order to compare the ''comonomer effect'' obtained with a homogeneous metallocene-based catalyst system with that obtained using a heterogenized form of the same metallocene-based catalyst system. The results obtained indicated that at 70 8C there was general rate depression with the homogeneous catalyst system whereas rate enhancement occurred in all copolymerizations carried out with the silica-supported catalyst system. Rate enhancement was observed for both the homogeneous and the silica-supported catalyst systems when ethylene/4-MP-1 copolymerization was carried out at 50 8C. Active center studies during ethylene/4-MP-1 copolymerization indicated that the rate depression during copolymerization using the homogeneous catalyst system at 70 8C was due to a reduction in the active center concentration. However, the increase in polymerization rate when the silica-supported catalyst system was used at the same temperature resulted from an increase in the propagation rate coefficient. V
Polymer Engineering & Science, 1999
A Ziegler-Natta catalyst was modified with a metallocene catalyst and its polymerization behavior was examined. In the modification of the TiCl, catalyst supported on MgCb (MgCb-Ti) with a rac-ethylenebis(indeny1)zirconium dichloride (rac-Et(Ind)&Cb. EIZ) catalyst, the obtained catalyst showed relatively low activity but produced high isotactic polypropylene. These results suggest that the EIZ catalyst might block a non-isospecific site and mod@ a Ti-active site to form highly isospecific sites. To combine two catalysts in olefin polymerization by catalyst transitioning methods, the sequential addition of catalysts and a co-catalyst was tried. It was found that an alkylaluminum like triethylaluminum (TEA) can act as a deactivation agent for a metallocene catalyst. In ethylene polymerization. catalyst transitioning was accomplished with the sequential addition of bis(cyclopentadieny1)zirconium dichloride (Cp&rClJ/methylaluminoxane (MAO), TEA, and a titanium tetrachloride/vanadium oxytrichloride (TiC14/VOCl,, Ti-V) catalyst. Using this method, it was possible to control the molecular weight distribution (MWD) of polyethylene in a bimodal pattern. In the presence of hydrogen, polyethylene with a very broad MWD was obtained due to a different hydrogen effect on the Cp,ZrCl, and Ti-V catalyst. The obtained polyethylene with a broader MWD exhibited more apparent shear thinning.
2006
KEYWORDS ABSTRACT Various parameters including temperature, pressure, and Al/Ti molar ratio were used to evaluate the polymerization of ethylene using an L 27 taguchi experimental design. Response surface method was employed to analyze the catalyst activity and polymerization yield. The results reveal that poly-merization yield and catalyst activity are increased by pressure and Al/Ti molar ratio. However, raising temperature results in a decrease in both above-mentioned responses. According to results obtained in this work, pressure and temperature have synergistic effects on both polymerization yield and catalyst activity. A synthesized and two commercial catalysts were used to consider the effect of different catalysts on the polymer properties. The downward trend of catalyst activity in the presence of hydrogen can be easily seen for all the catalysts. Despite a decrease in weight-and number average molecular weight in the presence of hydrogen, the polydipersity index remains ap...