Ethylene polymerization studies with supported cyclopentadienyl,arene, and allyl chromium catalysts

Highly active catalysts for low pressure ethylene polymerization are formed when chromocene, bis (benzene)- or bis (cumene)-chromium or tris- or bis (allyl)-chromium compounds are deposited on high surface area silica-alumina or silica supports. Each catalyst type shows its own unique behavior in preparation, polymerization, activity, isomerization, and response to hydrogen as a chain transfer agent. The arene chromium compounds require an acidic support (silicaalumina) or thermal aging with silica to form a highly active catalyst. At 90°C polymerization temperature arene chromium catalysts produced high molecular weight polyethylene and showed, in contrast to supported chromocene catalysts, a much lower response to hydrogen as a chain transfer agent. An increase in polymerization temperature caused a significant decrease in polymer molecular weight. Addition of cyclopentadiene to supported bis (cumene)-chromium catalyst led to a new catalyst which showed a chain transfer response to hydrogen typical of a supported chromocene catalyst. Polymerization activity with tris- or bis (allyl)-chromium appears to depend on the divalent chromium content in the catalyst. Changes in the silica dehydration temperature of supported allyl chromium catalyst have a significant effect on the resulting polymer molecular weight. High molecular weight polymers were formed with catalysts that were prepared using silica dehydration temperatures below about 400°C. Dimers, trimers, and oligomers of ethylene were usually formed with catalysts that were prepared on silica dehydrated much above 400°C. The order of activity of the different types of catalysts was chromocene/silica > chromocene/silica-alumina > bis (arene)-chromium/silica-alumina ? allyl chromium/silica.     

Performance of the Cr[CH(SiMe3)2]3/SiO2 catalyst for ethylene polymerization compared with the performance of the Phillips catalyst

The catalysis of a silica‐supported chromium system {Cr[CH(SiMe₃)₂]₃/SiO₂} was compared with a silica‐supported chromium oxide catalyst, the Phillips catalyst (CrO₃/SiO₂). This catalyst was prepared by the calcining of the typical silica support used for the Phillips catalyst at 600 °C and by the support of tris[bis(trimethylsilyl)methyl]chromium(III) {Cr[CH(SiMe₃)₂]₃} on the silica. In the slurry‐phase polymerization, this catalyst conducted the polymerization of ethylene at a high activity without organoaluminum compounds as cocatalysts or scavengers. The activity per Cr was about 6–7 times higher than that of the Phillips catalyst. Upon the introduction of hydrogen to the system, the molecular weight of polyethylene did not change with the Phillips catalyst, but it decreased with the Cr[CH(SiMe₃)₂]₃/SiO₂ catalyst. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 413–419, 2003     

Modeling of Ethylene Polymerization Kinetics over Supported Chromium Oxide Catalysts

Summary: Silica supported chromium oxide catalysts have been used for many years to manufacture polyethylene and they still account for more than 50% of world production of high‐density polyethylene. Along with its commercial success, the catalytic mechanism and polymerization kinetics of silica supported chromium oxide catalysts have been the subject of intense research. However, there is a lack of modeling effort for the quantitative prediction of polymerization rate and polymer molecular weight properties. The chromium oxide catalyzed ethylene polymerization is often characterized by the presence of an induction period followed by a steady increase in polymerization rate. The molecular weight distribution is also quite broad. In this paper, a two‐site kinetic model is developed for the modeling of ethylene polymerization over supported chromium oxide catalyst. To model the induction period, it is proposed that divalent chromium sites are deactivated by catalyst poison and the reactivation of the deactivated chromium sites is slow and rate controlling. To model the molecular weight distribution broadening, each active chromium site is assumed to have different monomer chain transfer ability. The experimental data of semibatch liquid slurry polymerization of ethylene is compared with the model simulations and a quite satisfactory agreement has been obtained for the polymerization conditions employed.

Polymerization rates at different reaction temperatures: symbols – data, lines – model simulations.     

Ethylene polymerization with supported bis(triphenylsilyl) chromate catalysts

Bis(triphenylsilyl) chromate is an active catalyst for ethylene polymerization without further treatment or additives. Catalytic activity is markedly increased when the compound is deposited on silica–alumina and is further increased if it is deposited on silica and then treated with an aluminum alkyl. Polymer molecular weight can be controlled by reaction temperature, hydrogen addition, support type, and reducing agent structure to give polymers ranging in melt index from essentially zero to > 100. In the supported catalysts the bis(triphenylsilyl) chromate appears to be bound to the support and to undergo a reduction step either by reaction with ethylene or with aluminum alkyl prior to polymerization. The active site is envisioned as chromium alkyl, bound to the support, with propagation occurring by insertion of the monomer into a Cr? C bond. Chain termination is by chain transfer to monomer.     

Chromocene catalysts for ethylene polymerization: Scope of the polymerization

Chromocene deposited on silica supports of high surface area forms a highly active catalyst for polymerization of ethylene. Polymerization is believed to occur by a coordinated anionic mechanism previously outlined. The catalyst formation step liberates cyclopentadiene and leads to a new divalent chromium species containing a cyclopentadienyl ligand. The catalyst has a very high chain-transfer response to hydrogen which permits facile preparation of a full range of molecular weights. Catalyst activity increases with an increase in silica dehydration temperature, chromium content on silica, and ethylene reaction pressure. The temperature-activity profile is characterized by a maximum near 60°C, presumably caused by a deactivation mechanism involving silica hydroxyl groups. A value of 72 was estimated for the ethylene–propylene reactivity ratio (r₁). Linear, highly saturated polymers are normally prepared below 100°C. By contrast with other commercial polyethylenes, the chromocene catalyst produces polyethylenes of relatively narrow molecular weight distribution. Above 100°C, unsaturated, branched polymers or oligomers are formed by a simultaneous polymerization–isomerization process.     

Silica‐Supported Pentamethylcyclopentadienyl Ytterbium(II) and Samarium(II) Sites: Ultrahigh Molecular Weight Polyethylene without Co‐Catalyst

Designing highly active supported ethylene polymerization catalysts that do not require a co‐catalyst to generate electrophilic metal alkyl species is still a challenge despite its industrial relevance. Described herein is the synthesis and characterization of well‐defined silica‐supported cyclopentadienyl Ln^II sites (Ln=Yb and Sm) of general formula [(≡SiO)LnCp*]. These well‐defined surface species are highly activite towards ethylene polymerization in the absence of added co‐catalyst. Initiation is proposed to occur by single electron transfer.     

New Chromium on Silica Gel Catalysts with very high activity for the polymerization of ethylene

Together with the known chromium (II)/silica gel catalyst (Phillips catalyst) for the polymerization of ethylene, two new ones have been investigated. It was found that a chromium(II)-“repoly” catalyst (prepared by short reaction of the chromium(II)/silica gel with ethylene at temperatures between 100 and 225°C) and a chromium(III)/silica gel catalyst have up to hundred times higher activity than the chromium(II) one. Activation energies were calculated as 54.6, 49.6 and 43.8 kJ per mol, respectively. The number of active sites was determined by measuring the integrated absorbance of the C? H and C?O stretching vibrations of the polymer. At low chromium concentration (0.056%) roughly 50% of all chromium was catalytically active in the case of chromium(II) and chromium(III) on silica gel. For the chromium(II)-“repoly” catalyst all chromium atoms can be active. The turnover numbers for the polymerization at 20°C were calculated as 0.1 (chromium(II)), 7.5 (chromium(II)-“repoly”) and 20 (sec^?1 atm^?1) (chromium(III)).     

Synthesis of LPPE by gas-phase copolymerization of ethylene with 1-butene: A highly efficient chromium oxide supported catalyst

A modified chromium oxide supported catalyst has been developed and applied in industry for the manufacture of LPPE via the gas-phase (co)polymerization of ethylene. The catalyst contains surface chromium oxide in the oxidation number Cr²⁺, two modifiers (aluminum oxide and fluorine surface compounds), and silicon dioxide as a support. The activity of the new chromium oxide catalyst in the gas-phase copolymerization of ethylene with 1-butene is higher by a factor of 4–5 than that of the traditional commercial catalytic system based on the supported bis(triphenylsilylchromate) catalyst. An increased reactivity of 1-butene in its copolymerization with ethylene in the presence of the chromium oxide catalyst makes it possible to reduce the consumption of 1-butene in the synthesis of a linear medium-density PE (0.937–0.938 g/cm³). Gas pipes made of PE prepared with the new catalyst are characterized by improved resistance to crack propagation.     

The “comonomer effect” in ethylene/α‐olefin copolymerization using homogeneous and silica‐supported Cp2ZrCl2/MAO catalyst systems: Some insights from the kinetics of polymerization,active center studies,and polymerization temperature

Homogeneous and silica‐supported Cp₂ZrCl₂/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 °C 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 °C. Active center studies during ethylene/4‐MP‐1 copolymerization indicated that the rate depression during copolymerization using the homogeneous catalyst system at 70 °C 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. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 267–277, 2008     

Ethylene polymerization and copolymerization with 10‐undecen‐1‐ol using the catalyst system DADNi(NCS)2/MAO

The catalyst DADNi(NCS)₂ (DAD = (ArN?C(Me)? C(Me)?ArN); Ar = 2,6‐C₆H₃), activated by methylaluminoxane, was tested in ethylene polymerization at temperatures above 25 °C and variable Al/Ni ratio. The system was shown to be active even at 80 °C and when supported on silica. However, catalyst activity decreased. The catalyst system was also tested in ethylene and 10‐undecen‐1‐ol copolymerization at different ethylene pressures. The best activities were obtained at low polar monomer concentration (0.017 mol/L), using triisopropylaluminum (Al‐i‐Pr₃) to protect the polar monomer. The incorporation of the comonomer increased with the increase of polar monomer concentration. According to ¹³C NMR analyses, all the resulting polyethylenes were highly branched and the polar monomer incorporation decreased as ethylene pressure increased. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5199–5208, 2007     

Ethylene polymerization over supported MAO/(nBuCp)2ZrCl2 catalysts: Influence of support properties

The catalytic system methylaluminoxane (MAO) and bis(n-butylcyclopentadienyl)zirconium dichloride ((nBuCp)₂ZrCl₂) was immobilized on commercial silica, silica-alumina and aluminophosphate calcined at different temperatures. The properties of the supports were determined by using N₂ adsorption-desorption isotherms at 77 K, FT-IR spectroscopy and SEM. After aluminium and zirconium impregnation, the catalysts were analyzed by ICP-AES, FT-IR and UV-vis spectroscopy. Ethylene polymerizations were carried out in a Schlenk tube at 70 °C and 1.2 bar of ethylene pressure. The polyethylene obtained was characterized by GPC, DSC and SEM.Catalysts supported on silica-alumina exhibited higher polymerization activity than those supported on silica and aluminophosphate. Besides, the activity of MAO/(nBuCp)₂ZrCl₂ catalytic system supported on silica-alumina and aluminophosphate decreased strongly with support calcination temperature, while remained almost constant when silica was employed as support. All these experimental features suggest a role of the support acid properties and hydroxyl group population in the generation of active polymerization species.     

On the polymerization mechanism of the Chromium(III) and Chromium(II)-B Silica Gel Catalysts

FTIR spectra of the chromium(III)/silica gel catalyst after short polymerization with ethylene show weak bands at 2 685 and 1 447 cm^?1 from the stretching and the deformation vibration of the methylene group, which binds the growing polymer to the active chromium(III) site. The band at 2 685 cm^?1 is reversibly removed by CO adsorption at low temperatures (?145°C). This adsorbed CO shows a broad band at 2 184 cm^?1. From the intensity of CO IR bands on unchanged chromium(III) before and after polymerization it was calculated that 12% of the chromium(III) is catalytically active, which is in good agreement with previous measurements by an entirely different determination method (11%). The chromium(II)-B catalyst showed also a weak band at 2 685 cm^?1 and it is therefore concluded that in this case the chromium(II) is oxidised by the ligating surface silanol group (in cooperation with an ethylene molecule). The band at 2 685 cm^?1 is discussed in relation to that from normal methylene groups at 2 925 and 2 855 cm^?1 and to that from the chromium(II)-A catalyst at 2 750 cm^?1. Evidence for the existence of a mononuclear chromium(II)-A species is found. This one is, in contrast to the dinuclear chromium(II)-A species, not polymerization active.     

Preparation and application of a novel core–shell‐particle‐supported zirconocene catalyst

The use of functional groups bearing silica/poly(styrene‐co‐4‐vinylpyridine) core–shell particles as a support for a zirconocene catalyst in ethylene polymerization was studied. Several factors affecting the behavior of the supported catalyst and the properties of the resulting polymer, such as time, temperature, Al/N (molar ratio), and Al/Zr (molar ratio), were examined. The conditions of the supported catalyst preparation were more important than those of the ethylene polymerization. The state of the supported catalyst itself played a decisive role in both the catalytic behavior of the supported catalyst and the properties of polyethylene (PE). IR and X‐ray photoelectron spectroscopy were used to follow the formation of the supports. The formation of cationic active species is hypothesized, and the performance of the core–shell‐particle‐supported zirconocene catalyst is discussed as well. The bulk density of the PE formed was higher than that of the polymer obtained from homogeneous and polymer‐supported Cp₂ZrCl₂/methylaluminoxane catalyst systems. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2085–2092, 2001     

Effect of Ga- and BCl3-modified silica-supported [t-BuNSiMe2(2,7-t-Bu2Flu)]TiMe2/MAO catalyst on ethylene/1-hexene copolymerization

In this present study, the copolymerization of ethylene and 1-hexene was conducted over the [t-BuNSiMe₂(2,7-t-Bu₂Flu)]TiMe₂ (CGC)/MAO catalyst immobilized on different supports. The effects of Ga and the Lewis acid BCl₃ modification of the silica support on the copolymerization behavior were investigated based on catalytic activity and polymer properties. It was found that the silica support modified with BCl₃ exhibited the highest activity. However, both Ga and BCl₃ modifiers are capable of enhancing the catalytic activity, probably attributed to stronger interaction between the MAO and support together with the acidic sites exerted by the modification which could assist MAO to activate the catalyst during polymerization. Besides, a role of BCl₃ as a spacer to keep apart the catalyst on the silica surface was proposed as another probable reason for activity increased. This led to the more homogeneous-like behavior with less effect of support and also caused the higher comonomer incorporation content. Moreover, the results revealed that narrow polymer molecular weight distribution can be achieved by the supported CGC catalyst, especially for acidic modified supports. On the other hand, there was no noticeable effect with regards to the melting temperature and copolymer microstructure of the Ga and BCl₃ modification. Therefore, based on the study it may be regarded that the efficient supported CGC catalyst can be accomplished through the acidic modification namely Ga and BCl₃.     

Homogeneous tandem catalysis of the bis‐(diphenylphosphino)‐amine/chromium tetramerization catalyst with metallocene catalysts

Homogeneous tandem catalysis of the bis(diphenylphoshino)amine‐chromium oligomerization catalyst with the metallocenes Ph₂C(Cp)(9‐Flu)ZrCl₂ and rac‐EtIn₂ZrCl₂, is discussed. GC, CRYSTAF, and ¹³C NMR analysis of the products obtained from reactions at constant temperatures show that during tandem catalysis, α‐olefins, mainly 1‐hexene and 1‐octene, are produced from ethylene by the oligomerization catalyst and subsequently built into the polyethylene chain. At 40 °C the Cr/PNP catalyst acts as a tetramerization catalyst while the polymerization catalyst activity is low. Copolymerization of ethylene and the in situ produced α‐olefins have also been carried out by increasing the temperature from 40 °C, where primarily oligomerization takes place, to above 100 °C, where polymerization becomes dominant. The melting temperature of the polymer is dependent on the catalyst and cocatalyst ratios as well as on the temperature gradient followed during the reaction, while the presence of the oligomerization catalyst reduces the activity of the polymerization catalyst. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6847–6856, 2006     

Mixed Organo/Oxide Chromium Polymerization Catalysts

Supported chromium oxide catalysts are activated by calcination in dry air at 400–900°C, which dehydrates the carrier and binds chromium(VI) to the surface. A reduction by ethylene then provides the active species. Alternatively, the carrier can be calcined alone and afterward impregnated with an organochromium compound. Both procedures can produce an active catalyst for ethylene polymerization, but the two types differ considerably in their behavior. A third type is made by reacting an organochromium compound with the activated chromium oxide catalyst. This “mixed” catalyst displays some of the characteristics of both parents, but is not a simple combination of the two.     

Kinetic features of ethylene polymerization over supported catalysts [2,6‐bis(imino)pyridyl iron dichloride/magnesium dichloride] with AlR3 as an activator

The effects of polymerization temperature, polymerization time, ethylene and hydrogen concentration, and effect of comonomers (hexene‐1, propylene) on the activity of supported catalyst of composition LFeCl₂/MgCl₂‐Al(i‐Bu)₃ (L = 2,6‐bis[1‐(2,6‐dimethylphenylimino)ethyl] pyridyl) and polymer characteristics (molecular weight (MW), molecular‐weight distribution (MWD), molecular structure) have been studied. Effective activation energy of ethylene polymerization over LFeCl₂/MgCl₂‐Al(i‐Bu)₃ has a value typical of supported Ziegler–Natta catalysts (11.9 kcal/mol). The polymerization reaction is of the first order with respect to monomer at the ethylene concentration >0.2 mol/L. Addition of small amounts of hydrogen (9–17%) significantly increases the activity; however, further increase in hydrogen concentration decreases the activity. The IRS and DSC analysis of PE indicates that catalyst LFeCl₂/MgCl₂‐Al(i‐Bu)₃ has a very low copolymerizing ability toward propylene and hexene‐1. MW and MWD of PE produced over these catalysts depend on the polymerization time, ethylene and hexene‐1 concentration. The activation effect of hydrogen and other kinetic features of ethylene polymerization over supported catalysts based on the Fe (II) complexes are discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5057–5066, 2007     

New architecture of supported metallocene catalysts for alkene polymerization

We report the synthesis of a supported metallocene catalyst that exhibits the same activity as a homogeneous catalyst for ethylene polymerization reactions. The key to this new catalytic system is a hybrid organic–inorganic polymer obtained by the cocondensation of an organotrialkoxysilane (OTAS; 40 mol %) with tetraethoxysilane (TEOS; 60 mol %). The particular organic group of OTAS enabled us to avoid gelation when the hydrolytic condensation was performed with a thermal cycle attaining 150 °C. The resulting product [soluble functionalized silica (SFS)] was a glass at room temperature that was soluble in several organic solvents such as tetrahydrofuran and toluene. The ²⁹Si NMR spectrum of SFS showed that the OTAS units were fully condensed (T₃ species), whereas the TEOS units were mainly present as tricondensed (Q₃) and tetracondensed (Q₄) units. SFS was grafted onto activated silica through a reaction of silanol groups. The metallocene [(nBuCp)₂ZrCl₂] was covalently bonded to the SFS‐modified support. The polymerization of ethylene was carried out in toluene in the presence of methylaluminoxane. The activity of the supported catalyst was similar to that of the metallocene catalyst in solution. The simplest explanation accounting for this fact is that most of the metallocene was grafted to SFS species issuing from the surface of the support through a reaction with their silanol groups. This improved the accessibility of the monomer to the reaction sites. Specific interactions of the metallocene species with neighboring organic branches of SFS might also affect the catalytic activity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5480–5486, 2007     

SIMULATION MODELING ON THE COORDINATION MECHANISM OF ETHYLENE MONOMER ON VARIOUS PREREDUCED Cr(II)Ox/SiO2 PHILLIPS POLYETHYLENE MODEL CATALYSTS

As one of the most important catalysts in polyethylene industry,Phillips catalyst(CrOx/SiO2) was quite unique for its activation by ethylene monomer without using any activator like alkyl-aluminium or MAO.In this Work.the density functional theory (DFT) calculation combined with paired interacting orbitals(PIO) method was applied for the theoretical studies on coordination reaction mechanism between ethylene monomer and two model catalysts namely Cr(II)(OH)2(M1) and silsesquioxane-supported Cr(II)(M2) as surface Cr(II) active site precursors on Phillips catalyst at the early stage of ethylene polymerization.Unexpected multiplicity of the coordination states of ethylene monomer on both M1 and M2 model catalysts had been first reported on a molecular level.In general,increasing the coordination numbers of ethylene.the corresponding binding energy per ethylene for all the complexes was decreased.The supporting eflfect of chromium oxide onto silica gel surface was found to be destabilizing the corresponding complexes and decreasing the multiplicity of the coordination states as well due to both electronic and steric effect.Moreover.tri-and tetra-or higher ethylene coordination states could not be possibly formed on the supported catalyst as on the Cr(II)(OH)2.The optimized complex geometries were adopted for determining the intermolecular orbital interactions.In-phase overlap orbiral interaction for all the molecular complexes indicated favorable coordination between ethylene and Cr(II)sites.The molecular orbital origin of the π-bonded Cr(II),and mono-and di-C2H4 M1 complexes had been elucidated by PIO method showing high possibility of the formation of metallacyclopropane or metallacyclopentane active sites in the subsequent initiation of polymerization stage.     

Chlorotitanium (IV) tetradentate Schiff‐base complex immobilized on inorganic supports: Support type and other factors having effect on ethylene polymerization activity

A titanium complex with [O,N,N,O]‐type tetradentate Schiff base (LTiCl₂), never used before in polymerization of olefins, was immobilized on silica‐ and magnesium‐type carriers, and it was used in ethylene polymerization. The conducted research revealed that the catalytic properties of the complex LTiCl₂ supported on those carriers were different for both the catalytic systems studied, and simultaneously they turned out different from those of the unsupported system. The supported catalysts require the use of Me₃Al, Et₃Al, or MAO as the activator to be able to offer high catalytic activities, whereas Et₂AlCl is needed for the nonsupported catalyst. This finding, together with considerable changes in polymerization yields and in properties of polymers versus composition of the catalytic system, suggest that there are different types of active sites in the studied catalysts. The catalyst anchored on the carrier produced in the reaction of MgCl₂·3.4EtOH with Et₂AlCl is definitely the most active one within the support systems tested. Its activity remarkably increases with the increasing reaction temperature. Moreover, that catalyst does not undergo deactivation over the studied period of time, irrespective of the type of the activator used and of the process temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4811–4821, 2009