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.     

Characterization of vanadium-based olefin polymerization catalyst precursors by solid-state 51v-nmr

Several VOCL₃-based ethylene polymerization catalyst precursors were prepared on silica and studied by solid-state ⁵¹V-NMR. The structure of the vanadium species in these samples, as determined by ⁵¹V-NMR, did not have any significant effect on the resultant polyethylene MI or MWD. This result is significant since conventional wisdom says the attachment of the transition metal to the silica plays a key role in polymer properties. VOCl₃ reacted with hexamethyldisilazane-treated silica and with 250°C dried silica results in double attachment of the vanadium to the silica, yet the catalysts which formed had different reactivities and produced polyethylene with different HLMIs. On the other hand, VOCl₃ reacted with 600°C dried silica results in single attachment of the vanadium to the silica, yet this catalyst had a similar reactivity and produced polymer properties similar to the doubly attached vanadium on 250°C dried silica. Two theories are offered to explain the lack of correlation between catalyst precursor structure and catalyst performance. © 1995 John Wiley & Sons, Inc.     

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.     

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.     

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.     

A Flat Model Approach to Tethered Bis(imino)pyridyl Iron Ethylene Polymerization Catalysts

A surface science model for a silica supported bis(imino)pyridyl iron complexes is applied to reveal the surface chemistry of these heterogeneous polymerization catalysts. The polymerization activity of these models and the molecular weight distribution of the resulting polymer are comparable to similar catalysts supported on amorphous silica. The catalyst deactivates partially during the first hour of ethylene polymerization. Based on photoelectron spectroscopy (XPS) we attribute this deactivation to iron extrusion by the aluminium alkyl activator.     

One-phase supported titanium-based catalysts for polymerization of ethylene. III. ESR spectra of the ternary system Sio2—R3Al—TiCl4

The ternary catalyst systems based on activated silica, aluminum alkyl, and titanium tetrachloride were polymerized in an ethylene gas-phase polymerization process, and further studied using ESR spectroscopy. Two types of titanium (III) ESR-active centers were observed and a linear dependence between the concentration of that characterized by a rhombic anisotropic signal with g₁ = 1.962, g₂ = 1.945, and g₃ = 1.913 values and catalyst productivity was found. © 1993 John Wiley & Sons, Inc.     

Synthesis and characterization of polylactide‐block‐polyisoprene‐block‐polylactide triblock copolymers: new thermoplastic elastomers containing biodegradable segments

Anionic polymerization of isoprene initiated by an alkyl lithium containing a silyl ether protected hydroxyl functionality followed by termination with ethylene oxide gave α,ω‐functionalized polyisoprene with narrow molecular weight distribution and prescribed molecular weight in high yield. Deprotection resulted in α,ω‐hydroxyl polyisoprene (HO‐PI‐OH) that was reacted with triethylaluminium to form the corresponding aluminium alkoxide macroinitiator. The macroinitiator was used for the controlled polymerization of lactide to yield polylactide‐block‐polyisoprene‐block‐polylactide triblock copolymers with narrow molecular weight distributions and free of homopolymer (HO‐PI‐OH) contamination. Microphase separation in these novel triblock copolymers was confirmed by SAXS and DSC.     

双(三苯基甲硅烷)铬酸酯催化剂及其配制过程中表面化学反应的研究

本文采用红外光谱分析等方法,研究了双(三苯基甲硅烷)铬酸酯的加热、光照和水解反应的一些性质;探讨了双(三苯基甲硅烷)铬酸酯催化剂配制过程的化学变化。当其沉淀在硅胶上时,与硅胶表面羟基反应,生成表面有机铬化合物,铬的价态不变。加入烷基铝后,表面铬化合物还原生成含有Cr-C或Cr-O键的低价表面化合物,其对乙烯聚合具有催化活性。并指出了本催化剂活性组份可能的负载机理。     

Synthesis of environmental benign biolubricant from wild castor seed by reactive extraction and optimization

Castor oil alkyl esters are a possible biolubricant since they contain 90% hydroxyl fatty acid, which improves the oil's lubricity. Due to the limitations of the conventional approach, castor oil propyl ester (COPE) from wild castor seed was synthesized by reactive extraction. The factors influencing yield of reaction was optimised by response surface methodology to obtain a high yield. The influence of amount of catalyst, propanol to oil proportion, temperature, and rotating speed on castor oil propyl ester yield was investigated using a central composite design.The optimised reaction condition is propanol to oil molar proportion of 275: 1 with 1.5 wt% of catalyst loading at 90 °C and rotating speed of 450 rpm with COPE yield of 78.6% in 3hrs. Physico-chemical properties of alkyl esters were determined. COPE can be employed as a bioadditive to ultra-low sulphur diesel fuels due to its high lubricity.     

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     

SiO2气凝胶负载无水四氯化锡催化顺酐与正丁醇酯化反应的研究

通过表面反应制备了气凝胶SiO2负载无水四氯化锡催化剂并应用于顺酐与正丁醇的酯化反应.结果表明,酯化产物主要为马来酸二丁酯(DBM)与富马酸二丁酯(DBF),其优化条件为:载体SiO2气凝胶焙烧温度500℃,物料摩尔比顺酐∶正丁醇:催化剂=1∶2∶0.0028,反应时间6 h.并且通过XRD,FTIR,DRUV-vis,N2物理吸附以及元素分析技术对所合成催化剂进行表征.结果表明:无水四氯化锡成功负载于SiO2气凝胶表面,在催化酯化反应当中实现了86.1%的顺酐转化率,此结果接近于国内外所报道的均相体系成果,同时在催化回收利用六次后,其活性虽逐步下降,但仍是空白的2倍.分析测试表明,反应过程中Cl的流失量较大、反应物及产物在催化剂表面产生了吸附,这些可能是导致催化活性下降的主要原因.     

Efficient conversion of fructose to 5‐hydroxymethylfurfural by functionalized γ‐Al2O3 beads

Millimeter size γ‐Al₂O₃ beads were prepared by alginate assisted sol–gel method and grafting organic groups with propyl sulfonic acid and alkyl groups as functionalized γ‐Al₂O₃ bead catalysts for fructose dehydration to 5‐hydroxymethylfurfural (5‐HMF). Experiment results showed that the porous structure of γ‐Al₂O₃ beads was favorable to the loading and dispersion of active components, and had an obvious effect on the properties of the catalyst. The lower calcination temperature of γ‐Al₂O₃ beads increased the specific surface area, the hydrophobicity and the activity of catalysts. Competition between the reaction of alkyl groups and ‐SH groups with surface hydroxyl during the preparation process of the catalyst influenced greatly the acid site densities, hydrophobic properties and activity of the catalyst. With an increase in the alkyl group chain, the hydrophobicity of catalysts increased obviously and the activity of the catalyst was enhanced. The most hydrophobic catalyst C₁₆‐SO₃H‐γ‐Al₂O₃–650°C exhibited the highest yield of 5‐HMF (84%) under the following reaction conditions: reaction medium of dimethylsulfoxide/H₂O (V/V, 4:1), catalyst amount of 30 mg, temperature of 110°C and reaction time of 4 hr.     

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.     

Linear Alpha-Olefins by Catalytic Oligomerization of Ethylene

This paper describes a new catalytic process for linear alpha-olefins by oligomerization of ethylene using a soluble alkyl aluminum chloride-titanium tetrachloride catalyst. Important catalytic and polymerization variables, such as catalyst activity, selectivity to linear products, and molecular weight, are discussed. An ionic mechanism is proposed to explain the unusual features of this new process.     

改性固体表面的吸附作用——甲基化硅胶的热稳定性和吸附性能

本文利用三甲基氯硅烷与硅胶表面羟基反应的方法制备了甲基化硅胶。测定了亲水硅胶和甲基化硅胶的热处理对其自四氯化碳中吸附乙酸的等温线的影响,并配合有热重分析(TG)和红外光谱(IR)的测定。结果表明:(1)甲基化后的硅胶对乙酸的吸附能力大大下降;(2)甲基化硅胶的热处理温度达500℃时吸附能力完全恢复到甲基化前的硅胶的水平,甲基化层明显开始破坏的温度为450℃;(3)甲基化硅胶高温处理后吸附能力得以恢复的主要原因是重新形成表面自由羟基。     

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)).     

超支化铬系催化剂的合成及乙烯齐聚性能研究

以不同端基烷基链长度的1.0G超支化大分子为桥联基,通过对其端基氨基进行催化功能改性,合成了系列具有不同桥联基长度的超支化PNP铬系催化剂。采用红外光谱(IR)、核磁共振氢谱(1H-NMR)、核磁共振磷谱(31P-NMR)、紫外光谱(UV)和质谱(MS)等表征方法证明合成催化剂的结构与理论结构相符。详细考察了溶剂种类、反应温度、Al/Cr摩尔比、反应压力、催化剂用量和催化剂结构对催化剂乙烯齐聚性能的影响。实验结果表明,当以甲苯为溶剂,甲基铝氧烷(MAO)为助催化剂时,超支化PNP铬系催化剂表现出良好的催化乙烯齐聚性能,产物以低碳烯烃为主。最佳条件下,催化活性最高可达到1.69×105g/mol Cr·h,己烯和辛烯的选择性为43.3%以上。相同聚合条件下,其催化活性随着端基烷基链长度的增加而下降。     

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