Polymerization chromium

Many polymeric chromium(III) complexes in this general class exist. For the general formula [Cr(L)(0PRR 0)2] , compounds have been prepared involving hydroxide, perfluorocarboxy-late, alkoxide, aryloxide and carboxylates.849,850... 

It seems likely that the chromate species can exist on the silica surface and acts as parent for an active site. Thus, pairing of chromium atoms is not a requirement for polymerization. Chromium trioxide (Cr03) probably binds to the silica as chromate initially, at least at the ordinary 1 % loading. But some rearrangement to dischromate at high temperatures may occur. If so, it could account for the change in color from yellow to orange, and even to red in some modified catalysts. 

Many of the organochromium compounds exist as dimers, e.g. diallyl-Cr(II), and one exists as a tetramer i.e., Cr(II)4tmsm8 (82,83). This is interesting in view of the assertion by some (2) that paired chromium is necessary for polymerization. In fact, neither species is very active for ethylene polymerization until it has been supported on a carrier. The monomeric organochromium compounds behave in about the same way. It seems likely that these polymeric chromium compounds react with the support to form isolated monomeric surface species, thus becoming coordin-atively unsaturated. When monomeric compounds react with the surface the loss of a ring or other multicoordinate ligand probably also leaves vacancies in the coordination sphere. 

Polymeric Chromium(III)-bis(pkosphinates) 91 TABLE I Preparation of [Cr(H20KOHXOPRR 0)2] ... 

Triisobutylaluminum olefin mfg., straight-chain Trimethylaluminum olefins, polymerization Chromium phosphate oleic acid mfg. 

Jabri A, Mason CB, GambarottaS, BurcheUTJ, Duchateau R Isolation of single-component trimerization and polymerization chromium catalysts the role of the metal oxidation state, Angew Chem Int Ed 47(50) 9717-9721, 2008. 

A quaternary ammonium trifluoroacetochromate(VI) polymer (26), prepared from Amberlyst A-26 with Cr03 and trifluoroacetic acid, showed greater activity than (25) for oxidation of secondary alcohols. Reaction of 2-octanol in cyclohexane at 70 °C with 3.8 molar equivalents of (26) gave 82% yield of 2-octanone in 4 h as shown in equation (11). A major advantage of the polymeric chromium reagents is the ease of isolation of the oxidation product from chromium salts. The major drawbacks are the initial expense of the polymer support and the relatively large amounts of polymer that must be used. 

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. 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

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. 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

These siUca-supported catalysts demonstrate the close connections between catalysis in solutions and catalysis on surfaces, but they are not industrial catalysts. However, siUca is used as a support for chromium complexes, formed either from chromocene or chromium salts, that are industrial catalysts for polymerization of a-olefins (64,65). Supported chromium complex catalysts are used on an enormous scale in the manufacture of linear polyethylene in the Unipol and Phillips processes (see Olefin polymers). The exact stmctures of the surface species are still not known, but it is evident that there is a close analogy linking soluble and supported metal complex catalysts for olefin polymerization. 

Polyethylene. Low pressure polymerization of ethylene produced in the Phillips process utilizes a catalyst comprised of 0.5—1.0 wt % chromium (VI) on siUca or siUca-alumina with pore diameter in the range 5—20 nanometers. In a typical catalyst preparation, the support in powder form is impregnated with an aqueous solution of a chromium salt and dried, after which it is heated at 500—600°C in fluid-bed-type operation driven with dry air. The activated catalyst is moisture sensitive and usually is stored under dry nitrogen (85). 

Although equation 9 is written as a total oxidation of sugar, this outcome is never realized. There are many iatermediate oxidation products possible. Also, the actual form of chromium produced is not as simple as that shown because of hydrolysis, polymerization, and anion penetration. Other reduciag agents are chosen to enhance the performance of the product. 

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. 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

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