Side-chain scission

UV Etching. A typical mass spectrum of the vaporized UV etching products is shown in Figure 4, together with a background spectrum obtained without UV irradiation. The comparison clearly shows that UV irradiation causes an increase in intensity for various mass peaks. For example, the intensity of the peaks of m/e=15, 31, 59 and m/e=41, 69 increased drastically by UV irradiation. The former three are due to side-chain scission caused by UV absorption at the C=0 unit, while the latter two are due to main-chain scission initiated by side-chain scission (11). The structure and mass numbers of typical vaporized species are shown in Table I. From here on, we use the spectral intensity after the background is subtracted. 

Poly (4-methyl-1-pentene). Poly(4-methyl-l-pentene) has not yet drawn much attention in radiation chemistry. As far as we know, only one study on high energy-irradiated poly(4-methyl-l-pentene) has been published (25), and this was in the form of a short communication. The ESR spectrum at liquid nitrogen temperature was a sextet with a hyper-fine splitting constant of 23 gauss. The radicals producing this spectrum were supposed to have structure XXI—i.e., radicals formed by side-chain scission. 

The mechanism of pyrolysis of polystyrene was discussed in Section 2.1 in relation with the description of general mechanisms encountered in pyrolysis. The formation of traces of benzene, toluene, etc in the pyrolysate is also discussed in literature [15, 40, 61, 62]. Although they account for only a very small proportion of the molecules formed during pyrolysis, some side chain scissions may occur as shown below ... 

Stepwise decay was also observed when PMMA was irradiated in the presence of ethyl mercaptan (EtSH) [245]. The initial decay rate of the radicals measured at 150°K is proportional to the concentration of EtSH, indicating that the decaying pairs are mixed pairs formed by a radical from PMMA and a radical from EtSH. In fact, radiolysis of pure PMMA results in the formation of pairs of macroradicals. Some are due to main-chain scission, others to hydrogen abstraction from the polymer by CH 3 or CH30 radicals produced by side-chain scission. At 150°K, in pure irradiated polymethylmethacrylate, the mobility of the macroradicals is limited and their rate of decay comparatively low. In the presence of ethyl... 

A different mechanism can be proposed in agreement with the recent experimental results. Side- and main-chain scission are clearly related since a one-to-one ratio is observed at room temperature and at 77°K. Main-chain scission, however, is inhibited in the presence of ethyl mercaptan whereas volatile formation is unchanged. This means that side-chain scission precedes and induces main-chain scission. Negative and positive charges are involved since anions and cations of acridine [395,... 

Other radiolytic processes can occur in such proteins that lead to side chain scission and the formation of volatiles from specific amino acid moieties [17],... 

Higher energy radiation such as from gamma ray or electron beam sources results in the scission of both main and side chains (60). The ratio of backbone to side-chain scission is determined by the nature of the side chain (61,62). 

Pyrolysis of poly(methyl methacrylate) at low temperature produces monomer, whereas other acrylics fragment with loss of side chains, scission of the chain backbone, elimination or rearrangement of the products. Knowledge of the degradation pathways for particular polymer sequences is required to interpret the fragmentation patterns obtained from pyrolysis (65-70). 

By contrast with PMMA, poly(methyl acrylate), PMA, and several other aliphatic polyacrylates were found by Shultz and Bovey (240) to imdergo cross-linking and gel formation on irradiation with 1-MeV electron beams. They reported G(S) = 0.15 and G(X) = 0.52 for PMA. Graham (241,242) reported that the phenyl, benzyl, and 2-phenyl ethyl acrylate polymers also undergo cross-linking on y radiolysis imder vacuum. There was evidence of side-chain scission also, and according to Fox and co-workers (243,244) the major volatile products were similar to those observed for PMMA. 

It has been proposed that the loss of the ester side group is the precursor for main-chain scission in PMMA, and so the lower G-values for scission below the /3-transition temperature should be associated with lower G-values of volatile side-chain products. Kudoh and co-workers (250) foimd that the yield for hydrogen for PMMA was not sensitive to the radiolysis temperature. However, the yields of the products of ester side-chain scission and decomposition, principally CO and CO2, were depressed by a factor of about 5 to 10 times when compared with those found for room temperature radiolysis. Kudoh and co-workers (260) also found a large difference in the tensile properties consistent with less chain scission at 77 K. 

FIGURE4.10 Side-chain scission mechanism. (Redrawn from Chen, C.andFuller.T.F. 2009. Po/ymerDegradation... 

Aromatics also undergo side-chain scission ... 

Ester side chain scission which forms small radicals COOCH3 (which give a single line spectrum), CHO (which gives a doublet spectrum) and CH3 (a quartet spectrum). 

Methyl side chain scission, which produce CH3 radicals. 

Of incidental interest is a finding by Werbin et al. (1960) who has shown that j8-sitosterol-H (Fig. 8) is transformed to cortisol by guinea pig in vivo. Cholesterol-H was also isolated, and a comparison of its specific activity with that of cortisol-H led to the conclusion that the former could not have been an intermediate in the conversion. Thus it would appear that the additional ethyl group in the side chain does not hinder the action at C-20, 22 of the hydroxylases and desmolase in their preparation and execution of the side chain scission. Pregnenolone-H and possibly a Cg-branched aldehyde or acid moiety are the products at this stage, the steroid being rapidly converted to cortisol-H through the action of adrenal enzymes. The treatment of j8-sitosterol by the liver on the other hand was such that cholesterol-H was resultant. 

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