Homolytic degradation

A number of papers have been dedicated to homolytic degradation of TN under the action of ultraviolet or 7-rays. The reaction of TNM with bases (e. benzidine) beings with the formation of CT complexes [183] yielding radical anions which in turn are split into radicals (NO ) and anions (e.g. nitroformanion Irradiation with 7-rays at 77K yielded radicals N02 and C(N02)3 (184). As most nifro compounds TNT inhibits polymerization induced by radiati-[185, 186] and free radical polymerization [I8O- 191]. This is rationalized f the fact that TNM is a radicals acceptor. The higher the number of nitro groui in nitro alkanes the stronger the inhibition of polymerization (189). 

On the other hand, the pyrograms observed in flash Py-GC at 720°C almost entirely consisted of the fragments formed via homolytic degradations, although many were identical with those observed by Py-FMS. In addition, the difference between Nomex and Kevlar with Py-GC was much less than those observed in Py-FIMS. Moreover, the formation of secondary products, such as biphenyl derivatives in Py-GC, was much less than that in Py-FIMS. The differences between the results by Py-GC and by Py-FIMS could be attributed to the difference in the flnal pyrolysis temperature and the heating rate. 

Chain scission is the ultimate fate of a stressed bond. At some value below the critical stress for chain rupture, bond angle deformation may result in an increase in reactivity. As stated in Sect. 3.1, mechanically activated hydrolysis of polymers containing ester groups can lead to the scission of the bond this concurrent reaction should be differentiated from homolytic chain scission, for example by looking at any pH-dependence as was found to be the case during shear degradation of DNA [84]. 

The only author to postulate a homolytic mechanism in the last few decades was Deng (1989). His arguments are based on the formation of small amounts of fluorinated bi- and polyphenyls in thermal fluoro-de-diazoniations and in mass-spec-trometric degradations of benzenediazonium tetrafluoroborate and its substituted derivatives. However, he does not include a critical discussion of his work. 

The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

It was proposed that an initially generated silyl radical 3, by reaction of i-BuO radical and polysilane 2, attacks another silicon atom in the same backbone to give a cyclic polysilane that contains an acyclic chain and another silyl radical (Scheme 8.1) [12]. The last silyl radical can either cyclize or abstract a hydrogen atom from another macromolecule, thus propagating the chain degradation. The reaction in Scheme 8.1 is an example of intramolecular homolytic substitution (ShO, a class of reactions discussed in Chapter 6. 

The proposed mechanism for the degradation involves SET to the peroxide resulting in homolytic cleavage of the 0—0 bond. An Ol-centred radical led to the formation of the enone, while an 02-centred radical afforded the diol. 

The final mechanism of stress relief is thermomechanically activated chain scission. Primary bond breakage can be homolytic, ionic or by a degrading chemical reaction. It is worthwhile to note that the relative slippage of chains, microfibrils and fibrils reduces or prevents the mechanical scission of chains in quasi-isotropic polymeric solids. In other words, chain scission is an important mode of fracture only in highly oriented thermoplastic fibers or in thermosets. 

The degradation can be photochemically induced (a) homolytic or (b) heterolytic cleavage at the weaker bonds. The photolysis of the type (a) may lead to elimination reactions and the type (b) may lead to free radical formation. The point of bond cleavage may not be the seat for light absorption. The energy can migrate from unit to unit until it finds itself at the seat of reaction. 

Aryl halides are often susceptible to photochemical degradation. As described in Chapter 10 later in this book, cleavage of the C-X bond occurs with low quantum yield for aryl chlorides (132), higher quantum yields for aryl bromides and iodides (133), and high quantum yields for some aryl fluorides (e.g., fluoroquinolones) (134). Aryl chlorides are photolabile to homolytic and/or heterolytic dechlorination (43). For sertraline hydrochloride, decomposition of the aryl dichloride moiety occurs in solution when exposed to light (ultraviolet and fluorescent conditions). As shown in the following proposed mechanism, the major photochemical decomposition products include mono-chloro- and des-chloro-sertraline via homolytic cleavage (Fig. 91) (86). 

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