Cobalt-60 initiation

Since the partial insolubility of the starch-ceric ammonium nitrate reaction product complicates the interpretation of solubility data, we ran a second series of graft polymerizations using cobalt-60 as an initiator in an attempt to remove this variable (Table II). If combination of PAN macroradicals is occuring during graft polymerization, it should occur during cobalt-60 initiated polymerizations as well as in those initiated by ceric ammonium nitrate. In the first four reactions of Table II, starch was irradiated as a water slurry under graft polymerization conditions, but in the absence of acrylonitrile, to determine the influence of different doses of irradiation on starch solubility. 

Both starch-g-PAN and starch-g-polyacrylamide were prepared by cobalt-60 initiation, since acrylamide does not graft efficiently in the presence of ceric ammonium nitrate (16). Both graft copolymers were irradiated with ultrasound to give totally soluble products the ultrasonic treatment of starch-g-PAN was carried out in DMSO, while that of starch-g-polyacrylamide was run in water. 

Graft Polymerization. Cobalt-60 Initiation. The cobalt-60 source was a Gammacel 200 unit from Atomic Energy of Canada, Ltd. Dose rate at the center of the chamber was 0.44-0.42 Mrad/hr, as calculated from the original dosimetry data provided by the manufacturer and the decay rate of cobalt-60. 

Preparation of DMS0-Soluble Starch-g-PAN Samples and Water-Soluble Starch-g-Polyacrylamide. Five grams of starch- -PAN (prepared by either ceric- or cobalt-60 initiation) was dispersed in 333 g of DMSO, and the dispersion was heated for 2 hr at 115°C. The cooled dispersion was then treated with ultrasound for 10 min to mechanically rupture gel particles and to render the polymer soluble (Branson Sonifier, Model S 125 ... 

Titanates trigger peroxide-initiated curing of unsaturated polyesters to give products of superior color, compared to conventional cobalt-initiated... 

It corresponds to the cobalt initially exchanged into the HMOR porosity. Nevertheless, a fraction of cobalt oxide - Co304 - is produced after calcination, as previously seen in the case of Cat I, on the surface of zeolite grains. 

Christoph Hengst was born in Giefien in 1978. He studied chemistry at the Philipps-University in Marburg and is currently a Ph.D. student working in the group of Prof. Hilt in the field of cobalt-initialized multicomponent reactions. 

Again, cobalt hydrotricarbonyl probably adds initially. The addition could be either a 1,4-addition or a 1,2-addition in which cobalt initially adds to the second carbon atom of the diene, and then undergoes an allylic rearrangement. The 3-pentenoylcobalt tricarbonyl triphenylphosphine prepared in this manner contains considerably more of the cis isomer than the same compound prepared from trans crotyl bromide and sodium cobalt tetracarbonyl. This, indicates that the hydrocarbonyl prefers cis addition (47). 

Nikishin and co-workers have carried out extensive studies of the reactions of aliphatic aldehydes with Mn(III) and Co(III) acetates in acetic acid in the presence of olefins. Depending on the reaction conditions, a variety of interesting products are formed. In the presence of catalytic amounts of cobalt(II) acetate and a limited oxygen supply, ketones are formed via the cobalt-initiated addition of acyl radicals to the olefin,324 326b e.g.,... 

The ratio of jy -epoxide (shown above) to anti-epoxide is 10—25 1 with TYZORTPT catalysis, whereas vanadylacetylacetonate is less selective and / -chloroperoxybenzoic acid gives the reverse 1 25 ratio. It is supposed that TYZOR TPT esterifies the free hydroxyl, then coordinates with the peroxide to favor syn-epoxidation (135). This procedure is related to that for enantioselective epoxidation of other allylic alcohols in 9—95% enantiomeric excess (135). Titanates trigger peroxide-initiated curing of unsaturated polyesters to give products of superior color, compared to conventional cobalt-initiated... 

One strategy to ehminate side-reactions in NCA polymerizations is the use of transition metal complexes as active species to control addition of NCA monomers to polymer chain-ends. The use of transition metals to control reactivity has been proven in organic and polymer synthesis as a means to increase both reaction selectivity and efficiency [21]. Using this approach, substantial advances in controlled NCA polymerization have been realized in recent years. Highly effective zerovalent nickel and cobalt initiators (i.e. bpyNi(COD) [22,23] and (PMc3)4Co [24]) were developed by Deming that... 

As described earlier carbon-centered radicals can be efficiently generated by homolysis of an alkylcobalt(III) species. This species can be synthesized by a reductive process from an alkyl halide and a nucleophilic Co(I) reagent [1-6). This chapter describes the recent advances in cobalt-initiated carbon-centered free radicals (generated via a reductive process) in organic synthesis. The cobalt-mediated free-radical reactions generated via this protocol can be broadly divided into the following two categories. 

Only two of the cobalt atoms restore activity but, in the case of cobalt, it is the last 2 gram atoms of metal bound which result in catalytic function (Figure 3). The first two cobalt atoms bound aflFect activity but little. Apparently, zinc first occupies the catalytic sites and the remaining structure-stabilizing sites only thereafter. In contrast, cobalt initially occupies only structural sites and thereafter the catalytic sites. Presumably, such a difference in site selection might arise from the known differences in preferred ligand configuration for each metal (9). 

Ma, C., Givens, R.S., Busch, D.H., and Subramaniam, B. (2007) Xylene uses a cobalt/initiator catalyst in CO expanded solvents (CXLs). Abstract of Papers, 233rd ACS National Meeting, Chicago, IL, March 25-29,... 

Ketone Peroxides. These materials are mixtures of compounds with hydroperoxy groups and are composed primarily of the two stmctures shown in Table 2. Ketone peroxides are marketed as solutions in inert solvents such as dimethyl phthalate. They are primarily employed in room-temperature-initiated curing of unsaturated polyester resin compositions (usually containing styrene monomer) using transition-metal promoters such as cobalt naphthenate. Ketone peroxides contain the hydroperoxy (—OOH) group and thus are susceptible to the same ha2ards as hydroperoxides. 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

Others. Oxahc acid is used for the production of cobalt, as a raw material of various agrochemicals and pharmaceuticals, for the manufacture of electronic materials (76—83), for the extraction of tungsten from ore (84), for the production of metal catalysts (85,86), as a polymerization initiator (87—89), and for the manufacture of zirconium (90) and beryhium oxide (91). 

Basic oxides of metals such as Co, Mn, Fe, and Cu catalyze the decomposition of chlorate by lowering the decomposition temperature. Consequendy, less fuel is needed and the reaction continues at a lower temperature. Cobalt metal, which forms the basic oxide in situ, lowers the decomposition of pure sodium chlorate from 478 to 280°C while serving as fuel (6,7). Composition of a cobalt-fueled system, compared with an iron-fueled system, is 90 wt % NaClO, 4 wt % Co, and 6 wt % glass fiber vs 86% NaClO, 4% Fe, 6% glass fiber, and 4% BaO. Initiation of the former is at 270°C, compared to 370°C for the iron-fueled candle. Cobalt hydroxide produces a more pronounced lowering of the decomposition temperature than the metal alone, although the water produced by decomposition of the hydroxide to form the oxide is thought to increase chlorine contaminate levels. Alkaline earths and transition-metal ferrates also have catalytic activity and improve chlorine retention (8). 

Catalyst Selection. The low resin viscosity and ambient temperature cure systems developed from peroxides have faciUtated the expansion of polyester resins on a commercial scale, using relatively simple fabrication techniques in open molds at ambient temperatures. The dominant catalyst systems used for ambient fabrication processes are based on metal (redox) promoters used in combination with hydroperoxides and peroxides commonly found in commercial MEKP and related perketones (13). Promoters such as styrene-soluble cobalt octoate undergo controlled reduction—oxidation (redox) reactions with MEKP that generate peroxy free radicals to initiate a controlled cross-linking reaction. 

V-methyl aniline formed in the initial step to give the leuco base of methyl violet. Treatment with aqueous acid produces the dye. Because Michler s hydrol may also react with dimethyl aniline instead of the /V-methyl aniline to give crystal violet, commercial-grade methyl violet is usually a mixture. A cobalt complex has converted 4,T-dimethylaminodipheny1methane and dimethyl aniline in the presence of atmospheric oxygen to crystal violet in one step (50). 

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerisation is usually initiated by alkah hydroxides, especially potassium hydroxide. In the base-catalysed polymerisation of propylene oxide, some rearrangement occurs to give aHyl alcohol. Further reaction of aHyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly diftmctional. By using sine hexacyano cobaltate as catalyst, a more diftmctional polyol is obtained (20). Olin has introduced the diftmctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalysed homopolymerisation of tetrahydrofuran. Copolymers derived from tetrahydrofuran and ethylene oxide are also produced. 

Stacking faults thereby providing barriers to sHp. If carbides are allowed to precipitate to the point of becoming continuous along the grain boundaries, they often initiate fracture (see Fracture mechanics). A thorough discussion of the mechanical properties of cobalt alloys is given in References 29 and 30 (see also Refractories). 

Cobalt is one of twenty-seven known elements essential to humans (28) (see Mineral NUTRIENTS). It is an integral part of the cyanocobalamin [68-19-9] molecule, ie, vitamin B 2> only documented biochemically active cobalt component in humans (29,30) (see Vitamins, VITAMIN Vitamin B 2 is not synthesized by animals or higher plants, rather the primary source is bacterial flora in the digestive system of sheep and cattle (8). Except for humans, nonmminants do not appear to requite cobalt. Humans have between 2 and 5 mg of vitamin B22, and deficiency results in the development of pernicious anemia. The wasting disease in sheep and cattle is known as bush sickness in New Zealand, salt sickness in Florida, pine sickness in Scotland, and coast disease in AustraUa. These are essentially the same symptomatically, and are caused by cobalt deficiency. Symptoms include initial lack of appetite followed by scaliness of skin, lack of coordination, loss of flesh, pale mucous membranes, and retarded growth. The total laboratory synthesis of vitamin B 2 was completed in 65—70 steps over a period of eleven years (31). The complex stmcture was reported by Dorothy Crowfoot-Hodgkin in 1961 (32) for which she was awarded a Nobel prize in 1964. 

Application. Polyesters are cured by free radicals, most commonly produced by the use of peroxides. A wide range of peroxide initiators (qv) are available for use in curing polyesters. Most peroxide initiators are thermally decomposed into free radicals, and the common initiators used at room temperature requke the use of a promoter such as dimethylaniline or cobalt octoate. 

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