Chemical and photochemical degradation

Valerio, F., P. Bottino, D. Ugolini, M.R. Cimberle, G.A. Tozzi, and A. Frigerio. 1984. Chemical and photochemical degradation of polycyclic aromatic hydrocarbons in the atmosphere. Sci. Total Environ. 40 169-188. 

References [52-54] do not include any data directly comparing squaraine rotaxanes with common cyanine dyes such as Cy5 (GE Healthcare) and Alexa 647 (Life Technologies). Nevertheless, from the available data it can be concluded that squaraine rotaxanes are remarkably resistant to chemical and photochemical degradation, and likely to be very useful as a versatile fluorescent scaffold for constructing various types of highly stable, red and near infrared (NIR) imaging probes and labels. 

Rivas FJ, Beltran FJ, Acedo B. Chemical and photochemical degradation of acenaphthylene. Intermediate identification. J Hazard Mater B 2000 75 89-98. 

Holland and Cooper and Wells - comment on a side reaction which leads to the destruction of the photosensitizer and the formation of colored products at low concentrations of alcohol. This reaction may well be the same as that which was observed by Mooney and Stonehill and which was interpreted in terms of photosensitized decomposition of water, with subsequent hydroxylation of the anthraquinone salt. A similar type of reaction was noted by van Beek and Heertjes during their investigation into the chemical and photochemical degradation of anthraquinone mordant dyes in methanol. Irradiation of 3-nitroalizarin in methanol in Pyrex vessels, with visible or ultraviolet light from mercury-vapor lamps, leads to the formation of 4-methoxy-3-nitroalizarin. The methoxylation encountered in this instance would appear to be analogous to the hydroxylation of the sensitizer noted by Mooney and Stonehill, although, it must be pointed out, no such conclusion was drawn by the authors. 

Due to the ability of bR to convert light energy from sun into electrical eneigy, it has got immense applications in optical appliances, for therapeu-tic/medical use and in research. The application of this protein in different fields stems from the fact that it is stable towards thermal, chemical and photochemical degradation. Moreover, it has got good photoelectric and photochromic properties. 

Degradation of a herbicide by abiotic means may be divided into chemical and photochemical pathways. Herbicides are subject to a wide array of chemical hydrolysis reactions with sorption often playing a key role in the process. Chloro-j -triazines are readily degraded by hydrolysis (256). The degradation of many other herbicide classes has been reviewed (257,258). 

Compared with tar, which has a relatively short lifetime in the marine environment, the residence times of plastic, glass and non-corrodible metallic debris are indefinite. Most plastic articles are fabricated from polyethylene, polystyrene or polyvinyl chloride. With molecular weights ranging to over 500,000, the only chemical reactivity of these polymers is derived from any residual unsaturation and, therefore, they are essentially inert chemically and photochemically. Further, since indigenous microflora lack the enzyme systems necessary to degrade most of these polymers, articles manufactured from them are highly resistant or virtually immune to biodegradation. That is, the properties that render plastics so durable... 

Chemical and Photochemical Properties Degradation Chemical Reactions 610... 

Pospfsil J (1993) Chemical and photochemical behavior of phenolic antioxidants, a state of the art report, part I, Polym Degrad Stab 40 217-232. 

From the results of the various investigations on mobility of chemicals and their degradation, it can be stated that abiotic transformation in simulated conditions shed some light on the understanding of behaviour, mechanism, longivity, and reactivity of chemical substances in different environmental compartments. Therefore, the photochemical process has to be considered as an important pathway of either removal or to reduce the concentration of a chemical in the environment. 

The mechanisms of the thermal and photochemical degradation of poly(vinyl chloride) (PVC) continue to be active areas of research in polymer chemistry mainly because its high chemical resistance, comparatively low cost and wide variety of application m e PVC one of the most widely used thermoplastic materials. The wide variety of forms which the material can take includes pastes, lattices, solutions, films, boards and moulded and extruded pieces and depends to a very large extent on the good electrical and mechanical properties of the polymer. In spite of these advantages the even wider application of the material has been restricted by its low thermal and photochemical stability. Thermal instability is a problem since processing of the polymer is carried out at about 200 C and the photochemical instability places a limit on the extent of the outdoor applications which can be developed. 

J. Pospisil, Chemical and photochemical behaviour of phenolic antioxidants in polymer stabilization a state of the art report, Part II, Polym. Degrad. Stab. 1993, 39, 103-115. 

The functional transformation is an integral part of the stabilization capacity of antioxidants and has to be considered in the discussion of relationships between structure and efficiency. The analyses indicate that it is impossible to elucidate the complex mechanism of action of antioxidants without the knowledge of chemical and photochemical properties of products. The transformation products are cum-mulated in a polymer und play a specific role in various stages of its ageing. In this complex process, compounds with initiation, retardation, or antioxidant properties may be formed from original antioxidants. These compounds participate in the integral stabilization capacity of the additive used under real degradation conditions. This fact explains the differences in overall properties found under different conditions. 

As regards the relevant chemical processes, hydrolysis and oxidation of substances are of the greatest importance neutralization, precipitation and oxidation-reduction reactions and photochemical degradation can also take place. 

The degradation of chloral hydrate soils has been examined and the results suggest that microorganisms participate in the degradation in conjunction with chemical and photochemical processes. 

The objectives of this chapter are to describe bleach product compositions and their formulation and to review the chemistry of both chemical and photochemical bleaching (i.e., the oxidative degradation of hair pigments and the accompanying oxidative degradation of the proteins of human hair) and to describe the chemical nature of bleached hair. 

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