Acrylonitrile radical attack

For low radiation doses, peroxides accumulate almost linearly with dose. However, after a certain dose has been reached, their concentration tends to level off. This conclusion can be derived from the observed change in the rate of graft copolymerization initiated by polymers subjected to increasing doses of preirradiation in air. Figure 2 illustrates this effect in the case of grafting acrylonitrile onto polyethylene (2). The drop in the yield of peroxide production presumably results from the efficient radiation-induced decomposition of these peroxides. Peroxides are known to decompose under free radical attack, and selective destruction of peroxides under irradiation has been established experimentally (8). This decomposition can become autocatalytic, and sometimes the concentration of peroxides may reach a maximum at a certain dose and decrease on further irradiation. Such an effect was observed in the case of poly (vinyl chloride). Figure 3 shows the influence of preirradiation dose on the grafting ratio obtained with poly (vinyl chlo-... 

As previously discussed, the copolymers produced in the zinc chloride-free radical system are not necessarily random copolymers but are probably the result of the copolymerization of the acrylonitrile-complexed acrylonitrile complex with the olefin-complexed acrylonitrile complex. Further, the olefin-alkylaluminum halide complexed acrylonitrile complex only differs from the olefin—zinc chloride complexed acrylonitrile complex in degree rather than in kind—i.e., the former is an unstable charge transfer complex capable of spontaneous uncoupling of the diradical system followed by intermolecular diradical coupling, while the latter is a stable charge transfer complex requiring radical attack to uncouple the diradical system. 

The copolymerization of furan and 2-methylfuran with dienophiles such as maleic anhydride leads to polymer structures with furan pendent functionality. Furan, 2-methylfuran, and 2,5-dimethylfuran have been copolymerized with acrylic monomers (51,52) and acrylonitrile (52,53). The furan ring of furan, 2-methylfuran, and 2,5-dimethylfuran participates as a diene in a free radical copolymerization with acrylonitrile. The initial step for furan and for 2,5-dimethylfuran is the attachment of an acrylonitrile radical at the 2-position, but for 2-methylfuran, the attack is at the-5-position. Propagation proceeds by the attack of the furan radical on an acrylonitrile molecule, to leave one olefinic bond in the structure derived from the furan ring. If this bond is in the 4,5- or 2,3-position, it may be involved in a second additional reaction by the return of the propagating chain. 

The CTC and neutral-monomer reactivity ratios may change both by dilution and with the nature of the solvent. For example, in the p-dioxene-MA-acrylonitrile system in benzene or toluene, the acrylonitrile content in the polymer increased to a maximum and then decreased as the amount of solvent increased.The observed solvent effects support the concept that when a CTC becomes solvated with a 7r-electron-rich solvent the CTC becomes stable to free-radical attack. 

The dependence of relative rates in radical addition reactions on the nucleophilicity of the attacking radical has also been demonstrated by Minisci and coworkers (Table 7)17. The evaluation of relative rate constants was in this case based on the product analysis in reactions, in which substituted alkyl radicals were first generated by oxidative decomposition of diacyl peroxides, then added to a mixture of two alkenes, one of them the diene. The final products were obtained by oxidation of the intermediate allyl radicals to cations which were trapped with methanol. The data for the acrylonitrile-butadiene... 

Although the reactivity of 1,2-disubstituted ethylenes in copolymerization is low, it is still much greater than their reactivity in homopolymerization. It was observed in Sec. 3-9b-3 that the steric hinderance between a P-substituent on the attacking radical and a substituent on the monomer is responsible for the inability of 1,2-disubstituted ethylenes to homopolymerize. The reactivity of 1,2-disubstituted ethylenes toward copolymerization is due to the lack of P-substituents on the attacking radicals (e.g., the styrene, acrylonitrile, and vinyl acetate radicals). 

Intennoleciilar Reactions. The intermolecular version of free radical reactions of sugar-derived radicals consists mainly of addition onto suitably activated olefins, such as acrylonitrile, generally used in excess. This approach has been explored by Giese [102]. The stereochemical course of the reaction is dictated by steric effects of the vicinal substituents, as seen from the reaction of radical 71 where equatorial attack is favored over the axial with acrylonitrile (Scheme 28). Only equatorial attack is observed using... 

Intermolecular free radical reactions. Giese notes the diastereoselectivity of reactions of acrylonitrile with cyclic 5- and 6-membered ring radicals can be controlled by adjacent substituents. Thus an axial 3-substituent can favor axial attack, whereas an equatorial 3-substituent favors equatorial attack in the case of 6-membered cyclic radicals. Glucosyl radicals, regardless of the precursor, yield a-substi-tuted products (88 12). 

A drastic example of this phenomenon is encountered in the cathodic hydrodimerization 76>8°1 of acrylonitrile to adiponitrile. This can be accomplished in very high yield in a concentrated solution of a tetraalkylammonium tosylate in water. Practically no propionitrile, the product of hydrogen addition, is formed. The reaction is believed to occur via formation of the acrylonitrile anion radical (6), which then attacks a second molecule of acrylonitrile. Further reduction of the resulting anion radical (7) followed by protonation of the dianion gives adiponitrile (Eqs. (21), (22) and (23) ). 

An electrochemical procedure for acrylonitrile coupling has been industrially successful. Here a reductive acrylonitrile coupling is effected by the electrons coming from the cathode of an electrolytic cell. The first formed radical carbanion attacks the second acrylonitrile molecule. Uptake of a new electron from the cathode and of two protons gives adiponitrile in high yield [M. Baizer and D. E. Danly, Chem. Ind. (London), 1979, 435, 439]. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

Finally, a novel three-component radical cascade reaction involving isonitriles has just been published [6]. In this paper, aromatic disulfides, alkynes, and isonitriles have been reported to react under photolytic conditions to afford -arylthio-substituted acrylamides 49 or acrylonitriles 50 in fair yields as mixtures of the E and Z geometric isomers (Scheme 21). The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical on the isonitrile. A fast reaction, for example, scavenging by a nitro-derivative (route a) or f-fragmentation (route b), is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The reaction provides very easy access to potentially useful poly-functionalized alkenes through a very selective tandem addition sequence. 

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