Ion-radical initiation

One principally different method of photoexcitation consists in the so-called two-color, two-laser flash photolysis. This method allows exciting ion-radicals already generated during the first laser (first color) flash photolysis. With a short-time delay after the first laser flash, the ion-radicals are selectively excited by the second laser (second color) flash. This flash is of the wavelength tuned to the absorption of the ion-radical initially formed. As a result, the ion-radical transforms into... 

Without ion-radical initiation, the yield of the resulted product reaches 50% for 24 h. Practically the same yield can be achieved for the same time in the presence of tris(4-bromophenyl)ammoniumyl hexachloroantimonate and for only 6 h on sonication (Nebois et al. 1996). Sonication accelerates the rate-determining formation of the diene cation-radical. Of course, hydroxynaphthoquinone is strong enough as an electron-acceptor with respect to 2-butenal Af,Af-dimethylhydrazone. Therefore, the question remains whether sonication is more or less the general method for the initiation of ion-radical cycloaddition. A possible role of sonication in optimization of ion-radical reactions was considered in Section 5.2.5. 

Without ion radical initiation, the yield of the resulting product reaches 50% for 24 hr. Practically the same yield can be achieved for the same time in the presence of tris(4-bromophenyl)ammoniumyl hexachloroantimonate and for only 6 hr upon sonication (Nebois and associates 1996). Sonication accelerates the rate-determining formation of the... 

Ion-Radical Initiated Polymerizations. Scott (60) added sodium to a solution of naphthalene in dimethoxyethane and obtained a green solution with which he had some success in polymerizing 1,2-dihydro-naphthalene. Scott thought that this green solution consisted of the naphthalene disodium adduct C10H8Na2, but the true nature of this solution was later shown by Paul, LiPKiNand Weissman (87) to be an ion-radical of the type... 

Another class of ion-radical initiators has been studied. Smith (62) studied the polymerization of styrene by sodium-benzophenone ketyl (2)... 

An interesting polymerization that may involve an ion-radical initiation is the y-ray initiation of acrylonitrile polymerization at — 78° (42). Polymerization did not occur in solvents capable of accepting electrons, but did occur in dimethylformamide and amines. When styrene and acrylonitrile mixture was used, the polymer was essentially polyacrylonitrile. 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

In the studies marked with an asterisk an initiator or accelerator of oxidation other than heat (e.g. air flow, metal ions, radical initiator) was used CAM = campesteryl femlate, CAF = cycloartenyl ferulate, 24-met-CAF = 24-methylenecycloartanyl ferulate, CF = cholesteryl ferulate,... 

Electron-transfer Initiation. Highly electron-rich monomers like A-vinylcarbazole can be polymerized readily by powerful electron acceptors such as chloranil, tetracyanoethylene, and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). Recently the killing of polymerizations at low conversion by addition of methanol has shown that the ion radicals initially formed combine to give a zwitterion in the case of DDQ with methyl vinyl ether (Scheme 6), although to what extent this structure is maintained throughout polymerization is not certain. 

The early studies demonstrated the possibility of initiating polymerization by intensive mechanical dispersion of certain inoiganic substances, including metals (Fe, Al, Mg, Cr, W) in vinyhc monomers. The degree of polymerization of styrene, vinyl acetate, acrylonitrile, or MMA depended on the dispersion intensity. The fresh metal surfaces play the role of catalyst and initiator. These surfaces are the sites of electron transfer from the surfaces metal atoms to the monomers to form ion-radical initiating particles. Colloidal particles of Au, Tl, and Pt were found to influence substantially the bulk and solution polymerization of styrene. - ... 

Hence, reactions considered for PVPR illustrate the mechanism connected with the conversions of primary radical cations generated by nitrosyl nitrate. Nevertheless, the direct detection of radical cations by ESR fails in this system apparently because of fast detachment of protons. For confirmation of the ion-radical initiation concept under the action of NO, triphenylamine (TPA) is a suitable model compound. TPA does not contain chemical bonds capable of reacting with monoradicals of NO. The formation of radical cations in TPA has been revealed in reactions with some Lewis... 

Thus, widely used aromatic polyamides with high mechanical characteristics and thermal stability [45] are rather reactive relative to NO. This is because of the specific ion-radical initiation. In this process, chemically inert aromatic rings are transformed into active cyclohexadiene structures, and in this way simple synthesis of spin-labeled... 

In Ibis book, novel results obtained by physicochemical methods especially electron spin resonarKo spectroscopy are considered for various polymers. The influence of different functional groups on the mechanism of interaction of nitrogen oxides with polymers are discussed. The features of ion-radical initiation of conversions of polymers by dimers ol nilmgen dioxide are also considered. Various other techniques for studying these inlerar lions are also discussed. 

An expanding development is the use of peroxodisulfates as oxidants in organic chemistry (80,81). These reactions are initiated by heat, light, gamma rays, or transition-metal ions. The primary oxidising species is usually the sulfate ion radical, P hskip -3pt peroxodisulfate anion... 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

Etherification and esterification of hydroxyl groups produce derivatives, some of which are produced commercially. Derivatives may also be obtained by graft polymerization wherein free radicals, initiated on the starch backbone by ceric ion or irradiation, react with monomers such as vinyl or acrylyl derivatives. A number of such copolymers have been prepared and evaluated in extmsion processing (49). A starch—acrylonitrile graft copolymer has been patented (50) which rapidly absorbs many hundred times its weight in water and has potential appHcations in disposable diapers and medical suppHes. 

The chlorination of toluene in the absence of catalysts that promote nuclear substitution occurs preferentially in the side chain. The reaction is promoted by free-radical initiators such as ultraviolet light or peroxides. Chlorination takes place in a stepwise manner and can be controlled to give good yields of the intermediate chlorination products. Small amounts of sequestering agents are sometimes used to remove trace amounts of heavy-metal ions that cause ring chlorination. 

Free-radical initiation of the chain reaction of oxidation can arise from the action of heat, light, metal ions and, sometimes, ozone on a variety of chemical moieties such as those indicated in Table 7.4. 

The preparation of the less stable isomer (53b) of the oxazolone 53a involves a rather tedious procedure. It has been reported that 53a is rapidly isomerized to 53b in 48% hydrobromic acid saturated with gaseous HBr. In this way four azlactones have been converted into their isomers.It has been established, moreover, that the isomerization is radical-initiated and does not involve a carbonium ion intermediate. The isomerization can be reversed by pyridine. ... 

Water-soluble free radical initiators (i.e., potassium persulfate, K2S2O8) are used in the emulsion polymerization process. Upon heating, the persulfate ion decomposes into two sulfate ion free radicals according to the following reaction ... 

Recently, in our laboratory the following novel Ce(IV) ion redox initiation systems have been investigated for vinyl radical polymerization. 

Water is extensively used to produce emulsion polymers with a sodium stearate emulsifrer. The emulsion concentration should allow micelles of large surface areas to form. The micelles absorb the monomer molecules activated by an initiator (such as a sulfate ion radical 80 4 ). X-ray and light scattering techniques show that the micelles start to increase in size by absorbing the macromolecules. For example, in the free radical polymerization of styrene, the micelles increased to 250 times their original size. 

The ceric ion also is also known to trap carbon-centered radicals (initiator-derived species, propagating chains) by single electron transfer (Scheme 3.60). 

Packer and Richardson (1975) and Packer et al. (1980) made use of the fact that electrons can be generated in water by y-radiation from a 60Co source (Scheme 8-29) to induce a free radical chain reaction between diazonium ions and alcohols, aldehydes, or formate ion. It has to be emphasized that the radiolytically formed solvated electron in Scheme 8-29 is only a part of the initiation steps (Scheme 8-30) by which an aryl radical is formed. The aryl radical initiates the propagation steps shown in Scheme 8-31. Here the alcohol, aldehyde, or formate ion (RH2) is the reducing agent (i.e., the electron donor) for the main reaction. The process is a hydro-de-diazoniation. 

Besides radical additions to unsaturated C—C bonds (Section III.B.l) and sulfene reactions (see above), sulfonyl halides are able to furnish sulfones by nucleophilic substitution of halide by appropriate C-nucleophiles. Undesired radical reactions are suppressed by avoiding heat, irradiation, radical initiators, transition-element ion catalysis, and unsuitable halogens. However, a second type of undesired reaction can occur by transfer of halogen instead of sulfonyl groups283-286 (which becomes the main reaction, e.g. with sulfuryl chloride). Normally, both types of undesired side-reaction can be avoided by utilizing sulfonyl fluorides. 

Although atherosclerosis and rheumatoid arthritis (RA) are distinct disease states, both disorders are chronic inflammatory conditions and may have common mechanisms of disease perpetuation. At sites of inflammation, such as the arterial intima undergoing atherogen-esis or the rheumatoid joint, oxygen radicals, in the presence of transition-metal ions, may initiate the peroxidation of low-density lipoprotein (LDL) to produce oxidatively modified LDL (ox-LDL). Ox-LDL has several pro-inflammatory properties and may contribute to the formation of arterial lesions (Steinberg et /., 1989). Increased levels of lipid peroxidation products have been detected in inflammatory synovial fluid (Rowley et /., 1984 Winyard et al., 1987a Merry et al., 1991 Selley et al., 1992 detailed below), but the potential pro-inflammatory role of ox-LDL in the rheumatoid joint has not been considered. We hypothesize that the oxidation of LDL within the inflamed rheumatoid joint plays a pro-inflammatory role just as ox-LDL has the identical capacity within the arterial intima in atherosclerosis. 

Aldehydes, and particularly aromatic ones, are highly susceptible to autoxidation thus benzaldehyde (97) is rapidly converted into benzoic acid (98) in air at room temperature. This reaction is catalysed by light and the usual radical initiators, but is also highly susceptible to the presence of traces of metal ions that can act as one-electron oxidising agents (cf. p. 306), e.g. Fe3 , Co3 , etc ... 

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