Poly radiolysis yields

Hydrogen, which is a major product in the radiolysis of most hydrocarbon polymers, is only a minor product in the radiolysis of poly(olefin sulfone)s, although it is of the largest yield between the minor products. Hydrogen is formed by H atoms combination or by an H atom abstracting hydrogen... 

Maleimides Alkyl and aryl maleimides in small concentrations, e.g., 5-10 wt% significantly enhance yield of cross-link for y-irradiated (in vacuo) NR, cw-l,4-polyisoprene, poly(styrene-co-butadiene) rubber, and polychloroprene rubber. A-phenyhnaleimide and m-phenylene dimaleimide have been found to be most effective. The solubihty of the maleimides in the polymer matrix, reactivity of the double bond and the influence of substituent groups also affect the cross-fink promoting ability of these promoters [82]. The mechanism for the cross-link promotion of maleimides is considered to be the copolymerization of the rubber via its unsaturations with the maleimide molecules initiated by radicals and, in particular, by allyfic radicals produced during the radiolysis of the elastomer. Maleimides have also been found to increase the rate of cross-linking in saturated polymers like PE and poly vinylacetate [33]. 

As with the aliphatic carboxylic acid model compounds, the major volatile product observed on gamma radiolysis of the poly acids is carbon dioxide. However, the carbon dioxide yields are somewhat larger than those observed for the model compounds. 

Two stable radical intermediates are observed following gamma radiolysis at 303 K. The alpha carbon radical VIII and the side chain radical IX are formed in approximately equal yields, with the total G-value for radical production equal to 3.2. This value is similar to that observed for the poly acids. The observed radicals are those which would be expected on the basis of the aliphatic carboxylic acids and previous studies of the poly amino acids with aliphatic side chains. 

In the poly carboxylic acids, carbon dioxide is the major product of radiolysis, but the carbon monoxide yields are greater than they are for the aliphatic carboxylic acids. However, the radical yields are not greater than expected on the basis of the model compounds, which suggests that excited states play an important role in the degradation of these poly acids. 

Table II. Yields of Butene Isomers from the Radiolysis of Poly (1-Butene Sulfone) at 30 °C...

We have studied the alkane and alkene yields from the radiolysis of copolymers of ethylene with small amounts of propylene, butene and hexene. These are examples of linear low density polyethenes (LLDPE) and models for LDPE. Alkanes from Ct to C6 are readily observed after irradiation of all the polymers in vacuum. The distribution of alkanes shows a maximum corresponding to elimination of the short-chain branch. This is illustrated in Figure 8 for the irradiation of poly (ethylene-co-1-butene) containing 0.5 branches per 1,000 carbon atoms at 20 C. 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

Table 11.3. Kinetics and yields (G unit 10 7 mol J ) of Ura release in the y-radiolysis of poly(U) in N20-saturated agueous solution. (Deebleet al. 1986) ...

In poly(A), the strand breakage yield is only a quarter of that observed with poly(U) or poly(C) (Table 11.2). Pulse radiolysis experiments using laser lightscattering for detection revealed two processes with ty2 -120 ps and -500 ms (Washino and Schnabel 1982 Washino et al. 1984). The slower one was quenched by cysteamine, wherefrom the rate of strand break has been calculated at 1.7 s 1 and that of the reaction of cysteamine with the precursor radicals at 3.4 x 106 dm3 mol-1 s 1. The low yields of strand breaks is also reflected in the comparatively lowyields ofbase release G(Ade)immediate = 0.55 x 10 7 mol J (Hildenbrand et al. 1993), 0.28 X 10 7 mol J 1 (Fuciarelli et al. 1987), G(Ade) after heating = L0 x 10 7 mol... 

Infrared studies of the radiation-induced degradation of PMPS by Bowden et al. (19) supports the oligomerization process and also shows that the oligomers can be removed by post-exposure baking. These effects have not been seen for other poly (olefin sulfone)s (2.3). Figure 8 and Figure 9 show the yield versus dose curves for irradiation of poly(l-butene sulfone) and poly (cyclohexene sulfone) respectively (20). No comparable shift of the S02/olefin ratio towards unity is observed in the radiolysis of these polymers. 

Flash photolysis studies can also be used to look for electron transfer products from either methylene blue or thionine with guanine-containing DNA. Our initial studies showed that the yield was small with either DNA or poly[d(G-Q] [7,13]. More recently Dunn et al [8] have observed a low yield of the electron transfer products upon the flash photolysis of methylene blue in the presence of GMP. Analogous studies that we have carried out with thionine and dGMP at pH 7 reveal only very low concentrations of long-lived products - which we are unable to assign unambiguously to the electron transfer products. Pulse radiolysis experiments have shown that... 

Thermal degradation in the presence of a catalyst seems to be an interesting alternative to simple pyrolysis. For example, both polyisobutylene and poly(isobutylene-co-isoprene) were degraded at low temperatures in the presence of both a Bronsted acid and a Lewis acid to yield a,co-dialkenic telechelics (equation 54). Radiolysis of polyisobutylene also yields a,o)-dialkenic telechelics (equation 55). " ... 

---