Ultraviolet radiation, photooxidation

In the presence of oxygen or ozone, as soon as free radicals form, oxygenation of the radicals gives rise to peroxy radicals, which through a complex series of reactions result in polymer degradation. Oxidative degradation may occur at moderate temperature (thermal oxidation) or under the influence of ultraviolet radiation (photooxidation). Unsaturated polyolefins are particularly susceptible to attack by oxygen or ozone (Equation 9.6). 

Shick, J.M., M.P. Lesser, and W.R. Stochaj. 1991. Ultraviolet radiation and photooxidative stress in zooxan-thellate anthozoa the sea anemone Phyllodiscus semoni and the octocoral Clavularia sp. Symbiosis 10 145-173. 

Aguilera J, Dummermuth A, Karsten U, Schriek R, Wiencke C (2002) Enzymatic defenses against photooxidative stress induced by ultraviolet radiation in Arctic marine macroalgae. Polar Biol 25 432—441... 

Kawaguchi, H., Oxidation efficiency of hydroxyl radical in the photooxidation of 2-chlorophenol using ultraviolet radiation and hydrogen peroxide, Environ. Technol., 14, 289-293, 1993. 

Henriksen, A. 1970. Determination of total nitrogen, phosphorus and iron in fresh water by photooxidation with ultraviolet radiation. Analyst 95 601-608. 

De AK, Bhattacharjee S, Dutta BK. Kinetics of phenol photooxidation by hydrogen peroxide and ultraviolet radiation. Ind Eng Chem Res 1997 36 3607-3612. 

However, these polymers do degrade when subjected to terrestrial ultraviolet radiation, and this has been attributed to the presence of small amounts of impurities which absorb light and initiate oxidative chain reactions within the polymer. In the oxidation process, compounds containing peroxy and keto groups are formed, and these absorb in the long wavelength region and accelerate the breakdown of the polymer chains. Obviously a detailed study of such reactions in polymers will be difficult because of the frequently unknown nature of the trace impurities which initiate the reactions and of the multiplicity of products formed in the photooxidation. 

Thermal oxidation is also autocatalytic and considered as metal-catalyzed because it is very difficult to eliminate trace metals (from fats and oils or food) that act as catalysts and may occur as proposed in Equation 4. Redox metals of variable valency may also catalyze decomposition of hydroperoxides (Scheme 2, Equations [6] and [7]). Direct photooxidation is caused by free radicals produced by ultraviolet radiation that catalyzes the decomposition of hydroperoxides and peroxides. This oxidation proceeds as a free radical chain reaction. Although there should be direct irradiation from ultraviolet light for the hpid substrate, which is usually uncommon under normal practices, the presence of metals and metal complexes of oxygen can become activated and generate free radicals or singlet oxygen. 

Phosgene may be formed when chlorinated hydrocarbons are exposed to ultraviolet radiation in the presence of air [627]. Dichloromethane was found to contain trace quantities of phosgene after storage for a few days in clear glass flasks, despite being protected from exposure to direct sunlight [855]. Trichloromethane decomposes photooxidatively in a similar way to that described for its thermal oxidation (Section 3.3.3), the process differing in the... 

Volume 14 deals with all aspects of polymer degradation, classified on the basis of the method of initiation for the process. Thus, Chapter 1 covers thermal degradation, Chapter 2 radiolysis initiated by high-energy radiation such as X- and 7-radiation and electrons, Chapter 3 photodegradation arising from exposure of polymers to visible or ultraviolet radiation and Chapter 4 discusses oxidative degradation, oxidation and photooxidation. 

A promising recent technique that could possibly be applied to the determination of soluble oxidized nitrogen is the photooxidation of organic matter by ultraviolet radiation (3). 

Armstrong, F.A.J., Williams, P.N. and Strickland, J.D.FI. (1966) Photooxidation of organic matter in seawater by ultraviolet radiation, analytical and other applications. Nature2i 1, 481. 

To relate the wettability changes more firmly to the photooxidation processes and products, a detailed study was carried out with polystyrene. This polymer was selected because the formation of oxidation products in the hydrocarbon surface gave rise to large changes in wettability and because these products would be readily accessible to optical methods of analysis. The ultraviolet absorption spectrum of polystyrene shows a sharp cut-off, and the extinction coefficients for the radiation absorbed are sufficiently high that almost all of the photochemical reaction should be confined to the surface layers. 

Photochemical reaction This term is generally used to describe a chemical reaction caused by absorption of ultraviolet, visible, or infrared radiation. There are many ground state reactions which have photochemical counterparts. Among these are photoadditions, photocycloadditions, photoeliminations, photoenolizations, photo-Fries rearrangements, photoisomerizations, photooxidations, photoreductions, photosubstitutions, etc. 

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