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.     

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.     

Supported bis(indenyl)– and bis(fluorenyl)–chromium catalysts for ethylene polymerization

Silica-supported bis(indenyl)– and bis(fluorenyl)–chromium catalysts show good activity in ethylene polymerization. For maximum productivity with the indenyl chromium catalyst, the silica must be dried, with higher dehydration temperatures giving a significant increase in polymerization activity. Less deactivation on thermal aging of the supported bis(indenyl)–chromium catalyst allows ethylene polymerization to proceed for many hours, which provides polyethylenes of low residual chromium content. In contrast to the behavior of supported chromocene catalysts, the indenyl–and fluorenyl–chromium catalysts require a higher hydrogen/ethylene ratio to achieve a specific polymer melt index. Nevertheless, highly saturated polyethylenes are produced with these new catalysts. This result indicates that chain transfer to hydrogen remains the major chain transfer reaction. Addition of cyclopentadiene to a supported indenyl–chromium catalyst provided a catalyst with a much higher transfer response to hydrogen. This result suggests that ligand exchange occurred, producing a supported chromocene catalyst. These overall results are consistent with an active-site model which comprises a supported divalent chromium center attached to an indenyl or fluorenyl ligand during the polymerization process. Polymerization is believed to occur by a coordinated anionic mechanism of the type previously discussed for a supported chromocene catalyst.     

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)).     

Polymerization of 2,2,2-trifluoroethyl vinyl ether

The polymerization of 2,2,2-trifluoroethyl vinyl ether by six different catalyst systems was examined. Low-temperature studies (?78°C) with boron trifluoride etherate catalyst in hydrocarbon and chlorinated solvents slowly yielded low molecular weight polymers which were amorphous and noncrystallizable upon cold drawing. Under similar conditions, polymerizations with boron trifluoride gas were spontaneous, quantitative, and gave relatively high molecular weight, form-stable, amorphous polymer. Heterogeneous polymerizations with chromium trioxide crystals in toluene at 68°C and bulk reactions with ethylmagnesium bromide–carbon tetrachloride catalyst at 40°C failed to produce polymer. Room temperature runs with triisobutylaluminum–titanium tetrachloride catalyst gave amorphous, tacky material. Aluminum hexahydrosulfate heptahydrate (AHS) initiated polymerizations conducted at 25 and 60°C gave low yields of mixtures of amorphous and crystalline polymers, the ratio depending upon the polymerization solvent employed. The infrared spectra and x-ray diffraction intensity curves of crystalline and amorphous poly(trifluoroethyl vinyl ether) are reported and compared herein for the first time.     

The CrOx/SiO2/Si(100) catalyst – a surface science approach to supported olefin polymerization catalysis

Depositing catalytically active particles onto flat, thin and oxidic support forms an attractive way to make supported catalyst suitable for surface science characterization. Here we show how this approach has been applied to the Phillips (CrO_x/SiO₂) ethylene polymerization catalyst. The model catalyst shows a respectable polymerization activity after thermal activation in dry air (calcination). Combining the molecular information from X‐ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) we can draw a molecular level of the activated catalyst that features exclusively monochromate species, which are anchored to the silica support via ester bonds with the surface silanol groups. These surface chromates form the active polymerization site upon contact with ethylene. Upon increasing calcination temperature we observe a decrease in chromium coverage as some of the surface chromate desorbs from the silica surface. Nevertheless, we also find an increasing polymerization activity of the model catalyst. We attribute this increase in catalytic activity to the isolation of the supported chromium, which prevents dimerization of the coordinatively unsaturated active site. Diluting the amount of chromium to 200 Cr‐atoms/nm² of silica surface enables the visualisation of polyethylene produced by a single active site.     

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-based catalysts for ethylene polymerization: Thermal removal of the cyclopentadienyl ligand

Thermal aging of a chromocene catalyst, (C₅H₅)₂Cr/SiO₂, in an inert atmosphere leads to a modified catalyst which shows poor response to hydrogen as a transfer agent. Polyethylenes prepared at a polymerization temperature of 90°C with this modified catalyst have a low melt index and high vinyl unsaturation level. By thermogravimetry the weight loss of the catalyst, relative to dehydrated silica, was equivalent to loss of one cyclopentadienyl ligand per chromium site. Pyrolytic gas chromatography showed cyclopentadiene was liberated in the thermal process. These overall studies provide strong evidence that loss of a cyclopentadienyl ligand in supported chromium catalysts has a profound effect on overall polymerization behavior.     

High activity Ziegler–Natta catalysts for the preparation of ethylene copolymers

High activity ethylene polymerization catalysts have been prepared by the interaction of ethylmagnesium chloride in tetrahydrofuran with high surface area silica, followed by reaction with excess titanium tetrachloride in heptane. The catalysts were tested in ethylene—hexene copolymerization reactions in the presence of AlEt₃ at 80°C. For comparison purposes, the copolymerization properties of a similar catalyst prepared without silica were also evaluated. Preparative conditions were identified which provide catalysts that possess high reactivity towards 1-hexane. The silica and the amount of magnesium used in catalyst preparation strongly affect the copolymerization properties of the catalysts. Generally, catalysts prepared with silica showed much higher sensitivity to 1-hexene (effective reactivity ratio r₁ = 25–60) while a similar catalyst prepared without silica exhibited an r₁ value of 125. Fractionation of the copolymer with a series of boiling solvents showed that all the catalysts exhibit a wide distribution of active centers with respect to reactivity ratios, with the r₁ values varying from 5–7 to ca. 200. The width of a the center distribution depends on catalyst composition—it is the narrowest for the catalyst prepared without silica and is the widest for the catalysts with intermediate Ti : Mg ratios.     

Chemistry of olefin polymerization reactions with chromium‐based catalysts

Detailed GC analysis of oligomers formed in ethylene homopolymerization reactions, ethylene/1‐hexene copolymerization reactions, and homo‐oligomerization reactions of 1‐hexene and 1‐octene in the presence of a chromium oxide and an organochromium catalyst is carried out. A combination of these data with the analysis of ¹³C NMR and IR spectra of the respective high molecular weight polymerization products indicates that the standard olefin polymerization mechanism, according to which the starting chain end of each polymer molecule is saturated and the terminal chain end is a C?C bond (in the absence of hydrogen in the polymerization reactions), is also applicable to olefin polymerization reactions with both types of chromium‐based catalysts. The mechanism of active center formation and polymerization is proposed for the reactions. Two additional features of the polymerization reactions, co‐trimerization of olefins over chromium oxide catalysts and formation of methyl branches in polyethylene chains in the presence of organochromium catalysts, also find confirmation in the GC analysis. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5330–5347, 2008     

Ring‐opening polymerization of the cyclic dimer of poly(trimethylene terephthalate)

Poly(trimethylene terephthalate) (PTT) was prepared by the ring‐opening polymerization of its cyclic dimer. Antimony(III) oxide, titanium(IV) butoxide, dibutyltin oxide, and titanium(IV) isopropoxide were used as catalysts. Among the catalysts, titanium(IV) butoxide was the most effective for the same reaction conditions. A weight‐average molecular weight of 63,500 g/mol was obtained from ring‐opening poly merization at 265 °C for 2 h in the presence of 0.5 mol % titanium(IV) butoxide. The PTTs obtained from the polymerization catalyzed with increasing amounts of antimony(III) oxide showed increasing weight‐average molecular weights and reaction conversions. When 1 mol % antimony(III) oxide was used, the weight‐average molecular weight was 32,000 g/mol and the conversion was 82% after 1 h of polymerization at 265 °C. In the case of the polymer catalyzed by titanium(IV) butoxide under the same conditions, the weight‐average molecular weight and conversion were 40,000 g/mol and 77% when 0.25 mol % was used, whereas 0.5 mol % catalyst produced a weight‐average molecular weight of 27,000 g/mol and a conversion of 95%. To get an acceptable molecular weight and relatively high reaction conversion, a catalyst concentration of at least 0.5 mol % was found to be necessary, in contrast to conventional condensation polymerizations, which require only about one‐tenth of this amount of the catalyst. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6801–6809, 2006     

New phenoxyketimine titanium complexes: combining isotacticity and living behavior in propylene polymerization

A series of titanium bis(phenoxyketimine) olefin polymerization catalysts were synthesized and screened for propylene polymerization. The phenoxyketimine ligands contain pentafluorophenyl N-aryl groups and ortho-phenol substituents of varying size. Catalysts with ortho-phenol substituents of intermediate size produce living, substantially isotactic polypropylene. The living nature of these catalyst systems is demonstrated through the synthesis of block copolymers with narrow molecular weight distributions.     

Titanium alkoxides as initiators for the controlled polymerization of lactide

Fourteen titanium alkoxides were synthesized for comparison of their catalytic properties in the bulk and solution polymerization of lactide (LA). In bulk polymerizations, they are effective catalysts in terms of polymer yield and molecular weight. Titanatranes gave polylactides with significantly increased molecular weight over more extended polymerization times, and those with five-membered rings afforded polymers in higher yields and with larger molecular weights than their six-membered ring counterparts. Steric hindrance of the rings was found to significantly affect polymer yields. Increased heterotactic-biased poly(rac-LA) was formed as the number of chlorine atoms increased in TiCl(x)(O-i-Pr)(4)(-)(x). In solution polymerizations, titanium alkoxides catalyzed controlled polymerizations of LA, and end group analysis demonstrated that an alkoxide substituent on the titanium atom acted as the initiator. That polymerization is controlled under our conditions was shown by the linearity of molecular weight versus conversion. A tendency toward formation of heterotactic-biased poly(rac-LA) was observed in the solution polymerizations. The rate of ring-opening polymerization (ROP) and the molecular weight of the polymers are greatly influenced by the substituents on the catalyst, as well as by factors such as the polymerization temperature, polymerization time, and concentration of monomer and catalyst.     

Studies on Ziegler-Natta catalysts. Part IV. Chemical nature of the active site

The determination of the number of sites active in the polymerization of ethylene on the surface of α-TiCl₃–Al(CH₃)₃ dry catalysts leads to the conclusion that this number is small in comparison to the total surface of the catalyst. Qualitatively this conclusion is also reached by two other independent methods. Infrared spectra of the catalyst before and after polymerization do not show a change in the type of bonds present in the surface. Electron microscopy proves that no active sites are formed on the basal plane of the α-TiCl₃ which constitutes 95% of the total surface. The results strongly favor the lateral faces of α-TiCl₃ as the preferred location of active centers. The lateral faces contain chlorine vacancies and incompletely coordinated titanium atoms. This must then be the essential conditions for the formation of active centers. The propagation of the polymer chain has been repeatedly shown to follow an insertion mechanism. The active site, therefore, necessarily contains a metal–carbon bond. The study of catalysts derived from TiCl₃CH₃ leads to the conclusion that a Ti? C bond on titanium of incomplete coordination is the active species in these cases. The alkylation of surface titanium atoms was proven to be an intermediate step in the catalyst formation from TiCl₃ and AlR₃. Survival of titanium–alkyl bonds on the lateral faces, where titanium atoms are incompletely coordinated explains best, in the light of our data, the activity of Ziegler-Natta catalysts. Coordination of aluminum alkyl compounds in or around the active center probably complicates the structure of the active centers.     

Bimetallic Chromium Catalysts with Chain Transfer Agents: A Route to Isotactic Poly(propylene oxide)s with Narrow Dispersities

Bimetallic chromium catalysts are investigated for the enantioselective polymerization of propylene oxide. The catalyst is composed of two salen chromium species linked by an alkyl chain, the length of which significantly impacts the rate of polymerization. While the use of a chloride initiator on the catalyst resulted in bimodal molecular weight distributions, switching to a trifluoroacetate initiating group and adding a diol chain transfer agent afforded polymers of controllable molecular weight with low, unimodal dispersities.     

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.     

Heterogenization of titanium(IV) complexes with amine bis(phenolate) ligands onto SBA‐15: exploring their catalytic epoxidation and electrochemical behaviour

This paper describes the synthesis and characterization of titanium catalysts supported onto SBA‐15 via chemical bonding. This was done by first modifying the support with amine bis(phenol) groups as functional linkers and hexamethyldisilizane as capping agent to mask the remaining silanol groups on the silica surface. Finally, titanium tetraisopropoxide was immobilized by reaction with the modified SBA‐15. All the materials were characterized using X‐ray diffraction, X‐ray fluorescence, nitrogen adsorption‐desorption, Fourier transform infrared, UV‐visible diffuse reflectance and solid‐state nuclear magnetic resonance spectroscopies, and solid‐state electrochemical techniques. The titanium materials were tested as cyclohexene epoxidation catalysts. The stability and reusability of the catalysts were also examined using voltammetry measurements. Copyright © 2016 John Wiley & Sons, Ltd.     

One-phase supported titanium-based catalysts for polymerization of ethylene. II. Effect of hydrogen

The relation between composition of the one-phase titanium-based silica supported catalysts for gas-phase ethylene polymerization, and the ability of these catalysts to control the molecular weight of polymer using hydrogen has been studied. Halogen containing alkylaluminium compounds and alkoxy groups on titanium promote the chain transfer process. A significant polymerization rate lowering effect is caused by hydrogen. However, catalyst activity fully revives after hydrogen removal from the polymerization system. The proportion of active titanium was found to be 18±4% in the presence of hydrogen, and the value of propagation rate constant (k_p) was calculated to be 190±45 L/mol.s. © 1993 John Wiley & Sons, Inc.     

Broad Molecular Weight Polypropylene Synthesis using Mixed Internal Donor Incorporated Magnesium Dichloride Supported Titanium Catalyst

Magnesium dichloride supported titanium tetrachloride catalyst with mixed internal donor, comprised of ethylbenzoate and sulfolane, were synthesized. The composition characteristics of catalysts indicate incorporation of sulfolane to max 8.2 wt%, in addition to ethylbenzoate. The amount of incorporated donors in solid catalyst is dependent on the sulfolane concentration and reaction temperature of the process. A physical characteristics study of catalyst indicated that morphology, crystallite size, surface area, pore volume and titanium distribution have not been influenced much by incorporation of the second internal donor (sulfolane). The catalysts, in combination with triethyl aluminum as cocatalyst and p-ethoxyethylbenzoate as external donor, show good activity for propylene polymerization. The polypropylene synthesized with an optimized catalyst system shows broad molecular weight distribution as compared to ethylbenzoate based catalyst.     

聚丙烯接枝马来酸酐/SiO_2复合载体型钛催化剂及其聚乙烯的形态研究

扫描电子显微镜(SEM)是研究材料形态、形貌的强有力工具,并广泛用于载体催化剂的研究[1～4].在工业化的烯烃聚合催化剂中,通常需要首先进行负载化,由于无机和有机载体各自具有特殊的优点并存在一些不利因素,如何利用不同类型载体进行复配获得性能优良的复合载体,已成为烯烃聚合催化剂载体化研究的一个重要方向.本工作合成了聚丙烯接枝马来酸酐(PMA)/SiO2复合载体,负载了TiCl4作为烯烃聚合催化剂,既可以避免由纯无机物为载体而引入过多的无机灰分,又能克服仅用聚合物为载体而造成的负载率不高的缺点,同时保持了无机载体固有的刚性和聚合物载体官能团多分布性的特点.该类复合载体制备工艺简单,原料易得,成本较低.为获得复合载体化催化剂形态及其对聚合产物形态影响的信息,我们采用扫描电子显微镜(SEM)跟踪催化剂和聚合物的形态,本文将报道该研究的初步结果.