Photodegradation upon exposure

This atom-abstraction route was also used for the preparation of polymeric networks from alkylated [3,3 ]bis-(trithia)ferrocenophanes. A bimodal molecular weight distribution, with maxima at 4/n 5,000 and 5 x 10, was obtained by GPC for the material from which a polymer fraction was isolated (4/ = 8.5 x 10 ). Low molecular weight poly(ferrocenylene perselenides) have also been prepared from the selenium analog of 114. These materials also undergo photodegradation upon exposure to UV light in air. ... 

As has already been shown, polyolefins are subjected to photodegradation upon exposure to sunlight. Additives and impurities eatalyze the breaking of the polymer chain by a series of UV-initiated free radical reactions. Some efforts foeus on the addition of photosensitive species, such as carbonyl groups or metal complexes, to accelerate these proeesses. 

The a- and [3-isomers of endosulfan undergo photolysis in laboratory tests after irradiation in polar solvents and upon exposure to sunlight on plant leaves. The a-isomer also undergoes isomerization to the P-isomer, which is relatively more stable (Dureja and Mukerjee 1982). A photolytic half-life of about 7 days was reported for endosulfan by EPA (1982c). The primary photolysis product is endosulfan diol, which is subsequently photodegraded to endosulfan a-hydroxyether. Endosulfan sulfate is stable to direct photolysis at light wavelengths of >300 nm however, the compound reacts with hydroxy radicals, with an estimated atmospheric half-life of 1.23 hours (HSDB 1999). 

The photodegradation of PVCa solutions has been studied to determine a mechanism for the photodegradation process and identify the excited states involved. The polymer was secondary grade (Aldrich Chemical Company) that was further purified by reprecipitation three times from methylene chloride solution by the dropwise addition into methly alcohol. Solutions in methylene chloride were found to yellow rapidly in air upon exposure to 360 nm light, whereas solutions degassed by the freeze-pump-thaw technique showed little yellowing. 

Figure 3. Photodegradation of 3,5,6-trichloro-2-pyridinol upon exposure to UV light. (Reproduced with permission from rrference 18. Copyright 1998 Society of Environmental Toxicology and Chemistry.)...

Photodegradation of polymers (photoageing), involving chain scission and/or cross-linking, occurs by exposure to solar or artificial radiation and causes structural modifications, usually accompanied by a dramatic deterioration of the physical and mechanical properties of the polymer.954 Typically, radical intermediates are formed upon excitation, which initiate subsequent (dark) degradation. The presence of other species, such as oxygen, water, organic solvents or additives, and also mechanical stress and heat, may enhance the efficiency of these processes. 

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