Photooxidation photo-initiators

The photoimaging process occurs via a photooxidation process photo-initiated by residual transition metal impurities in the presence of oxygen and terminated by coupling of polymer-bound radicals. Photoinduced cross-linking thus requires generation of a critical, and large, concentration of free radicals. 

Most of the photo-initiated processes in the atmosphere are radical reactions. However, the hydroxyl radical ( OH) is of special significance for the chemistry of the atmosphere (Ehhalt, 1999). This reactive species is mainly responsible for the photooxidation of trace organic chemicals in the troposphere and hence for the oxidative cleansing of the atmosphere (Fabian, 1989). It is nature s atmospheric detergent (Comes, 1994, Ravishankara, et al. 1998). Furthermore, several years ago it was well established that the interaction of UV/VIS radiation and environmental pollution seems to be responsible for the dramatic forest decline that has been observed, for example in the higher areas of the Black Forest or the Ore Mountains in Germany (Schenck, 1985). 

Tab. 5-2 presents several examples of recent research concerning photo-initiated AOPs that are related to the photooxidation or photominerahzation of specific compounds or to the diminution of global water parameters. It is a long way from being a comprehensive list (this would inevitably go beyond the scope of this book), and it was randomly selected from the current Hterature. Nevertheless, it demonstrates the enormous amount of research activity in the field of photo-initiated AOPs in aqueous systems that elucidate reaction mechanisms and that establish the broad application potential of these processes. Examples of O3-UV AOPs are not included. They were reviewed by GoUschalk et al. (2000). 

In the present ehapter we consider the inter- or intramolecular photoinduced electron transfer phenomenon. We mainly focus on photoinduced electron transfer processes that lead to the photoinitiation of polymerization, and on processes initiated by photoredueed or photooxidized excited states. We concentrate especially on a description of the kinetic schemes, a description of the reactions that follow the primary proeess of eleetron transfer, and the characteristics of intermediates formed after electron transfer. Understanding the complexity of the processes of photo-initiated polymerization requires a thorough analysis of the examples illustrating the meehanistie aspects of the formation of free radicals with the ability to start polymerization. 

Figure 16. Effect of photo-initiators in the photooxidation of 1,1-diamylethylene. (a) No additive (b) tert-butyl hydroperoxide (2 X 10"4 mol g 1) (c) diamyl ketone (2 X 10 4 mot g"1).

As well as influencing chromophore types and levels, variations in production conditions result in fibers of varying morphology (orientation, crystallinity, and supramolecular order) (3). Because morphology itself may markedly effect photooxidative sensitivity, the extent of this effect was explored by first studying the susceptibility of the various filaments to y-initiated oxidation. The y-initiation is believed to be completely random and it is not dependent on trace quantities of chromo-phores or the build-up of oxidation products, in contrast to photooxidation, yet it is followed by the same propagation and termination steps as photo-initiated oxidations (2,4). 

Photolysis reactions often are associated with oxidation because the latter category of reactions frequently can be initiated by light. The photooxidation of phenothiazines with the formation of N- and S-oxides is typical. But photolysis reactions are not restricted to oxidation. In the case of sodium nitroprusside, it is believed that degradation results from loss of the nitro-ligand from the molecule, followed by electronic rearrangement and hydration. Photo-induced reactions are common in steroids [36] an example is the formation of 2-benzoylcholestan-3-one following irradiation of cholest-2-en-3-ol benzoate. Photoadditions of water and of alcohols to the electronically excited state of steroids have also been observed [37],... 

Natural sunlight induced photooxidation of naphthalene in aqueous solution has also been reported by McConkey et al. to produce six major products including 1-naphthol, coumarin, and two hydroxyquinone [9]. The authors proposed that the initially formed 2 + 2 and 2 + 4 photo cyclo addition products undergo subsequent oxidation and/or rearrangement to form the observed products [9]. Grabner et al. have studied solvent effects on the photophysics of naphthalene and report that fluorescence lifetime decreases by a factor of 2.5 in aqueous solution compared to organic solvents (e.g. ethanol, hexane, acetonitrile) [10]. Based on the observed differences in naphthalene excited triplet state properties in aqueous and organic media, the decrease... 

Ranby et have also used model compounds such as 3-phenylpentane for investigating the mechanisms of photo-oxidation of polystyrene. The photooxidation of polystyrene has been investigated in solution by a number of workers. Woolinski has observed the development of unsaturation and implicated the involvement of singlet oxygen in the mechanism of photo-oxidation, whereas Easton and MacCallum " invoke the involvement of polymer-solvent complexes in initiation. The photo-oxidation of polystyrene has been found to have a kinetic chain length of 10 —... 

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