Radiation initiation mechanism

This kind of reaction can proceed in solution by a carbocation mechanism, but in the radiation-induced case, it proceeds almost exclusively by a radical mechanism. In most cases, the radiation initiates reactions that are of chain character. 

The 7-radiation-initiated oxidation of PE at 295 K was accompanied by the formation of both hydroperoxy (1.4 x 10-2 mol kg ) and carbonyl (1.6 x 10-2 mol kg-1) groups [95], The most likely mechanism of their formation is through the consecutive intramolecular isomerization of secondary peroxyl radicals (see Chapter 2). 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

Therapeutic irradiation is known to have multiple interactions with the vasculature of the irradiated tissue (12). Radiation has direct cytotoxic effects on the vascular endothelium, likely due to induction of oxidative injury. Radiation-induced injury stimulates inflammation and influx of inflammatory cells in addition to creating aprocoagulant state in the vascular space by the transcriptional induction of tissue factor with the subsequent activation of coagulation factors as well as von Willebrand factor and platelets. Experimental evidence suggests that the mechanism by which radiation initiates these responses is in part through the induction of cell-adhesion molecules including ICAM-1, E-selectin, and P-selectin and in part through local cytokine production and release (13). 

The copolymerization of carbonyl monomes with alkenes has been even less studied than that between different carbonhyl monomers. The radiation-initiated copolymerization of styrene with formaldehyde proceeds by a cationic mechanism with a trend toward ideal behavior, r = 52 and r2 = 0 at —78°C [Castille and Stannett, 1966]. Hexafluoroacetone undergoes radiation-initiated copolymerization with ethylene, propene, and other a-olefins [Watanabe et al., 1979]. Anionic copolymerizations of aldehydes with isocyanates have also been reported [Odian and Hiraoka, 1972]. 

Si(lll) plane [300]. Because the redox potential of the Cd +/Cd couple is much lower than the flat band potential of Si substrate, the surface electron concentration is sufficiently high. Thus the process occurs similarly as on a metal surface at relatively low cathodic overpotentials. The initial stages of Cd deposition were explained by progressive nucleation and cluster growth controlled by hemispherical diffusion. CdTe deposition on Si was also studied due to interest in application in IR radiation detectors. Mechanisms of this process on different planes of n-Si(lOO) was also discussed ([203, 301, 302] and references given therein). 

The basic mechanisms of radiation-initiated transformations in polymers are far from being completely understood. Most reactions are commonly interpreted on the basis of free radical processes, but other species—e.g., ions, and other reactive intermediates—may play a significant role. A better understanding of the basic reactions occurring in irradiated polymers is required, and this would undoubtedly spur further developments and industrial applications. This paper is devoted to a survey of the different species found in irradiated polymers. Atten-... 

Pulse radiolysis studies concerning the polymerization as well as the degradation, crosslinking and radiation resistance of polymers are surveyed. Initiation mechanisms of the radiation-induced polymerization of styrene and other monomers are discussed on the basis of the direct measurements of the reaction intermediates. Optical and kinetic data on the short-lived chemical intermediates produced in the solution of polymers and in the rigid polymers are surveyed and discussed with special reference to the degradation mechanism of polymers. 

Application of pulse radiolysis to polymers and polymerization was motivated at first by the success of radiation-induced polymerization as a novel technique for polymer synthesis. It turned out that a variety of monomers could be polymerized by means of radiolysis, but only a little was known about the reaction mechanisms. Early studies were, therefore, devoted to searching for initiators of radiation-induced polymerization such as radicals, anions and cations derived from monomers or solvents. Transient absorption spectra of those reactive intermediates were assigned with the aid of matrix isolation technique. Thus the initiation mechanisms were successfully elucidated by this method. Propagating species also were searched for enthusiastically in some polymerization systems, but the results were rather negative, because of the low steady state concentration of the species of interest. 

It is from these perspectives that we have reviewed the pulse radiolysis experiments on polymers and polymerization in this article. The examples chosen for discussion have wide spread interest not only in polymer science but also in chemistry in general. This review is presented in six sections. Section 2 interprets the experimental techniques as well as the principle of pulse radiolysis the description is confined to the systems using optical detection methods. However, the purpose of this section is not to survey detail techniques of pulse radiolysis but to outline them concisely. In Sect. 3, the pulse radiolysis studies of radiation-induced polymerizations are discussed with special reference to the initiation mechanisms. Section 4 deals with applications of pulse radiolysis to the polymer reactions in solution including the systems related to biology. In Sect. 5 reaction intermediates produced in irradiated solid and molten polymers are discussed. Most studies are aimed at elucidating the mechanism of radiation-induced degradation, but, in some cases, polymers are used just as a medium for short-lived species of chemical interest We conclude, in Sect. 6, by summarizing the contribution of pulse radiolysis experiments to the field of polymer science. 

Under irradiation with polychromatic light at X > 300 nm and 60 °C, representative of outdoor exposures, polystyrene (PS) homopolymer, copolymers and blends do not directly absorb the incident radiation. It is well known that the photooxidation of these polymers results from light absorption by chromopho-ric impurities [1,2]. Photooxidation generates modifications of the chemical structure of the material, which results in the formation of oxidized groups, the development of discoloration and the loss of the initial mechanical properties. 

The principal light-absorbing groups of lipids are double bonds, peroxide 0—0 bonds, and carbonyls the last two are most important. The primary mechanism by which ultraviolet radiation initiates lipid oxidation is actually indirect, mediated through homolytic scission of any preformed hydroperoxides to generate the true initiators— LO, HO, and RO —that abstract hydrogens from lipid molecules and form the ab initio L. ... 

There are three initiation mechanisms for the free radical lipid peroxidation in the living cells. At the first lipoperoxidation in the body can be induced by non-enzymatic mechanism. In this processes different physical factors such as ionizating irradiation or UV radiation as well as action of some chemical toxicants including air pollutants, pesticides and herbicides from food and drinking water may act as a initiating factors. 

The absorption of the energy associated with ionizing radiation by a medium results, initially, in ionization and electronic excitation. These processes occur regardless of the nature of the radiation. The mechanism of excitation and ionization by charged particles is different, however, from that effected by high-energy photons. 

Electron Beam-Cured Inks. Electron beam-cured inks are similar in principle to ultraviolet light-cured inks except that no photoinitiator is needed. Vinyl polymerizations may be initiated Iqr any form of ionizing radiation, e. g., neutrons, a-particles, y-rays, and x-rays, as well as by high-energy electrons ( 5-rays). The mechanism of initiation is more complex than that of photochemical initiation in that radiation of vinyl monomers gives cations and anions as well as free radicals however, most radiation-initiated polymerizations are radical-initiated because the cations and anions formed are not stable at the temperature of polymerization and therefore dissociate to form radicals. 

The radiation initiated polymerization of vinyl monomers in pure liquid or in solution was initially thought to be exclusively free radical in nature. In 1957 Davison et al. (7) demonstrated that isobutylene could be polymerized both v/ith high energy electrons and with cobalt 60 radiation. Because this monomer pciynerizes only by a cationic mechanism it was clear that radiation could, indeed, initiate ionic polymerization in the liquid state. Since then there has been considerable research with a number of monomers. The field has, however, continued to be dominated by studies of free radical processes. Almost all the ionic studies have been concerned with cationic processes although a few anionic polymerizations have been reported, notably with nitrcethylene (8). 

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