Chromium polymeric reactions

Most chromium-based catalysts are activated in the beginning of a polymerization reaction through exposure to ethylene at high temperature. The activation step can be accelerated with carbon monoxide. Phillips catalysts operate at 85—110°C (38,40), and exhibit very high activity, from 3 to 10 kg HDPE per g of catalyst (300—1000 kg HDPE/g Cr). Molecular weights and MWDs of the resins are controlled primarily by two factors, the reaction temperature and the composition and preparation procedure of the catalyst (38,39). Phillips catalysts produce HDPE with a MJM ratio of about 6—12 and MFR values of 90—120. 

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

Chromium hexacarbonyl is used as an additive to gasoline to increase the octane number as a catalyst in isomerization and polymerization reactions and in the preparation of chromium mirror or plate. 

Despite their importance in olefin polymerization reactions, little attention has been paid to the nature of the adsorbed oxygen species on supported chromium oxide systems. 

The more cationic halogen containing compounds produced other products. Cobalt bis-allyliodide produced cis-polybutadiene and the even more cationic chromium, produced cyclododecatriene. Only with the more cationic system which introduced trans-structures, was cyclization and reduction of the metal able to intercept the polymerization reaction. Cyclization was not possible in the less cationic cobalt which produces all cis-polybutadiene nor was the hydride transfer possible with the less anionic chromium tris-allyl compound. 

Studies (90, 91) with Cr02/Si02 catalyst have shown that formation of a surface chromate takes place by reaction of Cr02 and surface silanol groups on silica (Reaction 17). Reaction of this chemisorbed chromate with ethylene results in an oxidation-reduction reaction (90-95) with formation of a low-valent chromium center (Reaction 18). Proposals for Cr(II) as the active site are based on studies of the catalyst after reduction by ethylene, carbon monoxide, or hydrogen. One study (93. 94) showed that the polymerization rate increased with the fraction of Cr(II) in the catalyst. Another study (92) showed by polarography that the chromium is reduced to a divalent state by ethylene. 

For the monomer to be dispersible in the suspension system, it must be immiscible or fairly insoluble in the reaction medium. In some instances, partially polymerized monomers or prepolymers are used to decrease the solubility and also increase the particle size of the monomer. The initiators employed in the polymerization reaction are mainly of the peroxide type and, in some cases, are azo and ionic compounds. Examples include benzoyl, diacetyl, lauroyl, and /-butyl-peroxides. Azo-bis-isobutyronitrile (AIBN) is one of the most frequently used azo initiators, while aluminum and antimory alkyls, titanium chloride, and chromium oxides are typical ionic initiators. The amount of catalyst used depends on the reactivity of the monomer and the degree of polymerization, varying from 0.1 to 0.5% of the weight of the monomer. 

On the other hand, variation of pH in keratin solutions causes changes in amino-acid behavior related to the electronic charge response. Thus, if keratin solution has a pH below keratin s isoelectric point (reported at pH of 4), a cationic response can be expected, whereas higher values of pH produce an anionic performance. Therefore keratin has the capability of interact with different kind of species pollutants, including not only chromium but also other metals that could be removed. Besides the pH of pollutant solutions also plays an important role as was demonstrated ecently by Manrique-Juarez et al. In this study, the authors reported that pH has a strong influence on the morphology, mechanical properties and Cr(VI] removal efficiency, since if pH in keratin solution is 2.5, the protein separates from water solution, and therefore the polymerization reaction is affected producing a more closed cell in the membrane. The Cr(VI)... 

The termination of the polymerization reaction by BHE to the chromium center and subsequent dissociation of the resulting olefin is found to require about 25 kcal mol and to be thermodynamically much less feasible than the alternative termination process by BHT to a monomer. The latter process involves spin change two minimum-energy crossing points (MECPs) and further transition states and intermediates have been identified [39]. The termination reaction may also be controlled by the use of polymerization additives [40]. 

The reaction shown in Figure 3.18 also requires two adjacent surface Si-OH groups in order to proceed. This species does contain a Cr-H bond that can initiate the ethylene polymerization reaction by coordination of ethylene to the chromium center, followed by migratory insertion into the Cr-H bond, but such a species has not been identified. 

Theopold [26] has reported a cationic chromium (III) complex with structural similarities to the chromocene/sillca catalyst that polymerizes ethylene as a homogeneous catalyst in dichloromethane (CH cy. This complex, [Cp Cr(THF)2CH3] [BPhJ (where Cp is pentamethylcyclopen-tadienyl) is shown in Figme 3.26 undergoing an ethylene polymerization reaction [51]. 

Kissin YV, Brandolini AJ, Garlick JL Kinetics of ethylene polymerization reactions with chromium oxide catalysts,y Polym Sci A Polym Chem 46(16) 5315—5329, 2008. 

A new catalyst in which chromocene was supported on silica gel was developed by Union Carbide" for use in gas-phase polymerization reactions. A typical catalyst contained about 2.5% by weight of chromium, and was activated by the addition of small amounts of alkylaluminium alkoxides, such as diethyl-aluminium ethoxide. Use of such a catdyst formulation gave rise to a polymer with much narrower molecular weight distributions. The average molecular weight could be controlled by the addition of hydrogen, which caused termination of the growing chain. 

In the early 1950s transition metals such as titanium, vanadium, and chromium were shown to be especially effective for alkene polymerization reactions. Today most of the industrial production of HDPE is based on titanium or chromium catalysts. While studying reaction 6.2.1, Ziegler invented the Ti-based catalyst in 1953. He discovered that trace amounts of transition metal ions exercise dramatic effects on the polymerization reaction. 

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. 

Ca.ta.lysts, A more important minor use of chromium compounds is ia the manufacture of catalysts (Table 14). Chromium catalysts are used ia a great variety of reactions, including hydrogenations, oxidations, and polymerizations (229—231). Most of the details are proprietary and many patents are available. 

The change of shape of the kinetic curves with monomer and inhibitor concentration at ethylene polymerization by chromium oxide catalysts may be satisfactory described 115) by the kinetic model based on reactions (8)-(14). 

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