Kinetic study for the decay rate of ethylene polymerization catalyzed over silica supported TiCl4/MgCl2 catalysts

The kinetics for decay rate of ethylene polymerization catalyzed with TiCl₄/MgCI₂/Si0₂ has been investigated in the range of temperatures between 40 and 90°C and in the range of ethylene pressures between 4 and 12.4 atm. The decay of polymerization rale was fitted well by the type of first order decay. The decay rale constant caused by monomer could be expressed by k _d ^/′ = C ·[M]^−1/2. Some plausible speculations have been proposed on the deactivation mechanism caused by monomer. The activation energy for the deactivation reaction is 9.8 kcal/mole.     

Activation of Phillips Cr/silica catalysts: IV. Mobility of Cr(VI)

The saturation coverage behavior of Cr(VI) on silica has been used to study the mobility of hexavalent chromium during commercial activation of the Phillips catalyst in a fluidized bed. Cr(VI) was found to migrate not only within each silica particle, but even between particles. The mechanism of transfer is thought to be particle-to-particle contact during the fluid bed calcining process, and Cr migration was observed at temperatures as low as 300–400 °C in these experiments. Transfer can take place even in the absence of oxygen, which suggests that redox cycling is probably not the main method of mobility. However, once the silica is calcined at high temperatures the migration is stopped at low temperatures, indicating that the thermal history of the surface is important in the transfer process. Migration of Cr on and between alumina particles behaved in much the same way, and Cr transfer was also observed between silica and alumina in both directions. Cr(III) on deactivated commercial Cr/silica catalysts could be reclaimed to Cr(VI) surprisingly easily by co-activation with virgin support. The spreading out of Cr(VI) onto a fresh unoccupied surface probably provides a powerful force to reoxidize the otherwise seemingly inert alpha-Cr₂O₃ crystallites. The mobility has important consequences for commercial manufacture and use of the catalyst.     

Characteristics of ethylene polymerization over Ziegler–Natta/metallocene catalysts Comparison between hybrid and mixed catalysts

Two types of inorganic supports, MgCl₂ and SiO₂, for the impregnation of catalysts were prepared by the recrystallization and the sol–gel method, respectively. The polyethylene produced by the Ziegler–Natta/metallocene hybrid and mixed catalysts showed two melting temperatures and a bimodal MWD corresponding to products arising from each of the individual catalysts. This suggests that these Ziegler–Natta/metallocene catalysts acted as individual active species, and as a result, produced a blend of polymers.     

Electron microscopic characterization of Cr/silica catalyst for ethylene polymerization

Ethylene polymerization was carried out over both porous and non-porous 5 wt% Cr/silica catalysts in a slurry reactor. The polymerization was stopped at selected times to obtain samples for SEM and TEM characterization. Despite the different physical characteristics of the two silica-supported catalysts and their different behavior in the early stages of reaction, high resolution SEM micrographs (taken after runs of longer duration) revealed similar, fibrous and very porous polymer layers on both. This accessibility of the ethylene enables transport of monomer to the active sites at the very high reaction rates.     

Novel nickel(II)-based catalysts for the polymerization of ethylene

A series of Ni(II)-based bidentate -diimine complexes bearing two alkyl (alkyl = methyl, ethyl and isopropyl) substituents on each imine aryl group were studied as precatalysts for the polymerization of ethylene. These new catalysts were observed to show high activity in combination with methyl aluminoxane (MAO) and produced high molecular weight polyethylenes. The effects of the steric bulk of ortho-aryl substituents of the ligand on the catalytic activity and the resulting polyethylene microstructure were investigated. Kinetics of polymerization was also studied by changing important parameters such as temperature and MAO concentration. The polymerization activity, polymer molecular weight and resulting polymer microstructure were drastically changed according to the catalyst structure modification and polymerization parameters.     

Hybrid poly(ethylene terephthalate)/silica nanocomposites prepared by in‐situ polymerization

The nanocomposites, based on hybrid poly(ethylene terephthalate) (PET)/silica nanoparticles, were prepared via in‐situ condensation polymerization of terephthalic acid and ethylene glycol in the presence of silica nanoparticles pretreated with a silane coupling agent. Such a polymerization process ensured that the silica nanoparticles were well dispersed in PET matrix with the size ranging from 40 to 60 nm, which was confirmed by transmission electron microscope (TEM) observation. Attributed to the unique bonding between SiO₂ nanoparticle and PET, the crystallization behavior of PET was improved significantly, at a low temperature in particular. To further explore the effects of silica nanoparticles on crystallization, extensive differential scanning calorimeter (DSC) measurements were performed in an attempt to reveal the impact of the morphology of the dispersed silica nanoparticle (i.e., sphere or gel‐like) on the peak temperature during melting as well as the amount of heat involved in crystallization. The influences of the structure of polyether glycol (PEG) used for PET preparation as well as the addition of glass fibres (GF) were also investigated using DSC. It was concluded that the synergy among silica nanoparticles, modified PEG, and GFs lowers both T_g and T_m of PET, thus facilitating the injection processes in application. POLYM. COMPOS. 28:42–46, 2007. © 2007 Society of Plastics Engineers     

Preparation,characterization and testing of Cr/AlSBA-15 ethylene polymerization catalysts

Ethylene polymerization catalysts have been prepared by grafting chromium(III) acetylacetonate onto AlSBA-15 (Si/Al = ∞, 156, 86 and 30) mesoporous materials. A combination of XRD, nitrogen adsorption, TEM, ICP-atomic emission spectroscopy, H₂-TPR, TGA, UV–vis and FT-IR spectroscopy, were used to characterize the prepared Cr–AlSBA-15 catalysts. By reducing the Si/Al ratio of the AlSBA-15 supports increases the amount of chromium anchored, promotes the stabilization of chromium species as chromate and decreases the reduction temperature of Cr⁶⁺ ions determined by H₂-TPR. Attachment of Cr species onto AlSBA-15 surface results from the interaction of hydroxyl groups with the acetylacetonate ligands through H-bonds. On the contrary, a ligand exchange reaction may occur over siliceous SBA-15.The polymerization activity of Cr–AlSBA-15 catalysts is significantly improved by increasing aluminium content of the AlSBA-15 supports. Particularly, the chromium catalyst prepared with AlSBA-15 (Si/Al = 30) support is almost four times more active than a conventional Cr/SiO₂ Phillips catalyst. Polymers obtained with all the catalysts showed melting temperatures, bulk densities and high load melt indexes indicating the formation of linear high-density polyethylene.     

Norbornene polymerization and ethylene/norbornene copolymerization catalyzed by constrained geometry cyclopentadienyl-phenoxytitanium catalysts

Norbornene polymerization and ethylene/norbornene copolymerization were studied using constrained geometry complexes 2-(tetramethylcyclopentadienyl)-4,6-di-tert-butylphenoxytitanium dichloride (1), 2-(tetramethylcyclopentadienyl)-6-tert-butylphenoxytitanium dichloride (2), and 2-(tetramethylcyclopentadienyl)-6-phenylphenoxytitanium dichloride (3) as catalysts with AlⁱBu₃ and Ph₃CB(C₆F₅)₄ as cocatalysts. Polymerization results indicate that these catalyst systems are highly active for both the homopolymerization of norbornene and the copolymerization of ethylene with norbornene. The norbornene homopolymerization is vinyl addition polymerization. Ethylene/norbornene copolymers with high norbornene incorporation (>50%) were easily obtained with these catalyst systems by increasing the norbornene feed concentration. The produced polymers were characterized by ¹³C NMR, IR, DSC and GPC.     

Novel functionalized bis(imino)pyridine cobalt(II) catalysts for ethylene polymerization

This report describes synthesis and ethylene polymerization in the various conditions by two novel 2,6-bis(imino)pyridine (BIMP) catalysts B and C based on cobalt activated by methylaluminoxane (MAO) in a slurry semi-batch reactor. The catalyst activities as well as polymer properties were affected dramatically by electronic effects of the attached substitutions on the para-position of the pyridine ring. Theoretical study exhibited more positive charge on the central metal of the catalyst B resulted in higher activity at the expense of lower thermal stability and lifetime. The polymer obtained using the catalysts exhibited high molecular weight and almost narrow molecular weight distribution (MWD) ranging from 2.35 to 4.10 at the employed polymerization conditions. The highest and lowest molecular weight of the obtained polymers were produced by the catalyst A and C respectively. Hydrogen could slightly increase the catalyst activities with the exception of the catalyst B. The catalyst C bearing electron-donor OMe substitution at the para-position of the pyridine ring, produced PE with narrower PDI relative to the polymer resulted by catalysts A and B.     

Morphology studies of poly(ethylene terephthalate)/silica nanocomposites

Poly(ethylene terephthalate)/silica nanocomposites have been prepared through in situ polymerization. The morphology was investigated by atomic force microscopy in the tapping mode and scanning electron microscope. The interface morphological structure of the poly(ethylene terephthalate)/silica nanocomposites strongly depends on the ratio of silica in the matrix. When silica weight fraction is lower than 3 wt%, the system consists of aggregated silica particles dispersed in the organic matrix; beyond this concentration, the structure is co-continuous with that of the organic matrix. Surface of poly(ethylene terephthalate) was smooth; while nanocomposites were rough, there are good interfacial adhesion and compatibility between the polymer matrix and the nanofillers.     

Interface structure of poly(ethylene terephthalate)/silica nanocomposites

The interface structure of the poly(ethylene terephthalate) (PET)/silica nanocomposites was characterized by Fourier transform infrared and solid-state nuclear magnetic resonance. Our study reveals that PET chains are grafted onto the surface of silica nanoparticles, and they form branched and lightly crosslinking structures during the polycondensation. Gel permeation chromatography measurements indicate that the grafted PET chains have a lower molecular weight and broader distribution. Furthermore, a model has been developed to elucidate the interaction of an entanglement network between silica and PET chains that lead to enhancements of G′, G″ and η* values of PET/2 wt% silica nanocomposites.     

Impact of silica pore structure on the performance of metallocene catalysts in ethylene gas-phase polymerization

Several commercial silicas were used to support metallocene active centres, and the resulting precatalysts were used to study the impact of the pore size and pore size distribution of the support on the polymerization kinetics and resulting polymer properties. Pore volume distribution played a major role in the fragmentation of silica-supported catalysts, where mesoporous silicas with a narrow distribution in the region obtained higher activities and faster fragmentation than silicas with a broad pore volume distribution. Therefore, it is shown that care must be taken when using standard information on particle porosity, as this quantity can be misleading. It appears that the minimum pore size, particularly on the particle surface, can be a very important parameter even if it does not impact the estimate of the porosity.     

Morphological influences in the gas phase polymerization of ethylene by silica supported chromium oxide catalysts

Dramatic changes occur during the initial stages of olefin polymerization over heterogenous catalysts. As polymer accumulates, the catalyst fragments and the void space within the growing particle becomes filled with polymer. The changing monomer transport rate to the active sites, dissipation of heat and stress with the particle, and eventually, dispersion of catalyst fragments within the growing particle can control the polymerization. We focus on the changes in, and influence of, polymer, catalyst and void morphology during the nascent polymerization for silica supported chromium oxide catalysts. We find that the course of the polymerization depends on the initial stages yet is seldom considered in polymerization models. We review several years of morphological characterization that identify the important physical phenomena which occur during nascent heterogeneous olefin polymerization.     

Kinetic study of ethylene polymerization by highly active silica supported TiCL4/MgCl2 catalysts

The kinetics of ethylene polymerization with TiCl₄/MgCl₂/SiO₂ has been investigated in the range of temperatures between 40 and 90°C and in the range of ethylene pressures between 4 and 12.4 kg/cm². The role of MgCl₂ was discussed from the dependence of the Mg/Ti ratio on the catalytic activity. The polymerzation rate was first order with respect to the monomer concentration and the dependence of the polymerization rate on the concentration of Al(C₂H₅)₃ could be described by the Langmuir–Hinshelwood mechanism. The dependence of initial rate and the time to reach the maximum polymerization rate on the concentration of Al(C₂H₅)₃ was also discussed. Polymerization rates as a function of the polymerization temperature showed a maximum and the activation energy was 11.8 kcal/mol between 50 and 80°C. The polymerization rate decreased with the increase of hydrogen partial pressure. The active site concentration (C^*) was 1.9 × 10^?2 mol/mol Ti by the inhibition method with carbon monoxide.     

Syndiospecific polymerization of styrene catalyzed by half-titanocene catalysts

Those effective catalyst precursors for syndiotactic styrene polymerization, Cp*Ti(OCH₂-CHCH₂)₃ (I), Cp*Ti(OCH₂-CHCHC₆H₄)₃ (II), Cp*Ti(OCH₂C₆H₅)₃ (III), Cp*Ti(OCH₂C₆H₄OCH₃)₃ (IV) were synthesized, and the influence of catalyst ligands on the catalytic activity and properties of polymer were investigated. The polymer thus obtained coupled with higher molecular weight and higher syndiotacticity determined by GPC and ¹³C NMR as well as solvent extraction manners, respectively. Those catalysts promoted by methyaluminoxane (MAO) as cocatalyst exhibited higher catalytic activity. Of all catalysts mentioned foregoing, Cp*Ti(OCH₂-CHCHC₆H₄)₃ (II), Cp*Ti(OCH₂C₆H₅)₃/MAO (III) and Cp*Ti(OCH₂C₆H₄OCH₃)₃ (IV) catalysts showed higher activity and stability even at fairly low Al/Ti ratio of 600, and possessed excellent control of the stereoregular insertion of monomer, exhibited a significant increase of the ratio of the propagation rates to chain transfer termination. The kinetic and titration results also indicated that those metallocene catalysts (II), (III), and (IV) showed higher catalytic activity and produced polymer with higher molecular weight, because of a great number of active species, and lower ratio of Kβtr/Kp, higher ratio of Kβtr/Ktrs which indicate that β-H elimination was predominant.     

Effects of diethylaluminum ethoxide pre-reduction and chromium loading over bistriphenylsilyl chromate/silica for ethylene polymerization

Bistriphenylsilyl chromate (BC)/silica (S-2 catalyst) is a commercially used high-density polyethylene (HDPE) catalyst with great importance. In this work, the catalytic performance of this catalyst was tried to be improved by increasing the loading amount of Cr and pre-reducing with diethylaluminum ethoxide (DEALE). It is found that, when the Cr loading amount was doubled, the productivity in gPE · g⁻¹Cat became almost doubled as well, indicating the maintenance of overall efficiency for Cr centres. Meanwhile, pre-reduction of BC/silica with a small amount of DEALE (the molar ratio of DEALE/Cr < 2.4) was adequate to promote its productivity greatly, while large dose of DEALE led to the activity decay fast, which might originate from the over-reduction of the Cr centre, as well as pore blockages caused by the added DEALE. Moreover, it is found that both the increased Cr loading and DEALE pre-reduction offered an alternative way to reduce the weight-average molecular weight (MW) and molecular weight distribution (MWD) of the polyethylene products mainly by shaving the high MW shoulder.     

Photocatalytical reduction of Cr(VI) over ZnO powder

The photocatalytical reduction of Cr(VI) in aqueous suspensions of ZnO under uv-illumination is studied. The amount of Cr(VI) photoreduced at different irradiation times, initial Cr(VI) concentrations, pH and temperatures have been determined. Different ZnO—doped samples have been used in order to improve the yield of the Cr(VI) photoreduction. Finally, some experiments under sunlight have been performed. The results obtained in the present work show that ZnO can be used as efficient catalyst for the photoreduction of Cr (VI) in aqueous solutions even under sunlight exposure.     

Low-pressure polymerization of ethylene. II

This paper presents the kinetic study of polymerization of ethylene with VOCl₃ and aluminum alkyls such as Et₃Al and Et₂AlCl. The effect of various parameters like the [Al]/[V] ratio, catalyst concentration, reaction time, temperature, solvents, and additives on rate of the reaction, yield, and molecular weight is reported. Each of these parameters has a remarkable effect on the yield and the rate of polymerization for both catalyst systems. Triethylamine is found to increase the catalyst efficiency and the rate. It is also observed that aliphatic hydrocarbons acted as a better polymerization medium than did the aromatic ones.