Proposed photodegradation pathway

Photolysis of Model Compound I. The bis-n-propyl ether of bisphenol-A (I) is a colorless liquid and easily miscible in 1,4-dioxane. Figure 3 shows the GC as drawn by the computer and Table I shows the various significant peaks that appeared in the GC spectrum and their corresponding m/e ratios for all three photolysis runs at various intervals of time. The proposed photodegradation pathways are shown in Scheme 1. 

Figure 4. Proposed photodegradation pathway of 3,5,6-trichloro-2-pyridinol. (Reproduced with permission from reference 18, Copyright 1998 Society of Environmental Toxicology and Chemistry.)...

Photo-oxidation is the main photofading pathway for the majority of dyes.18 Oxygen, particularly singlet oxygen, and radicals such as hydroxyl and hydroperoxyl, are believed to be the major electrophilic species responsible for photodegradation. Attack normally occurs at or around the azo (or hydrazone) group and plausible mechanistic pathways for the photo-oxidation of hydro-xyazo dyes have been proposed.18,19... 

In summary, we note that almost all of the products observed by investigators for the photodegradation of PET and PECT are nicely accounted for in the summary schemes we have proposed. Additionally, there are several products that should be sought to prove or disprove additions to the previously reported pathways proposed by us and by others. It should be noted, however, that until more proof of these various schemes is obtained, then the mechanisms are still speculative even though they are very consistent with all of the available data. 

Photocatalysis is mostly thought of in terms of the photodegradation of molecules initiated either by oxidative or by reductive processes. However, photosynthesis may also result from a photocatalytic process, as observed for the reduction of C02 to formic acid by natural minerals. This reaction is the first step in an abiotic pathway for the synthesis of organic molecules and has been proposed to have played a role in the origin of life on earth [9]. 

Two distinct mechanistic pathways have been proposed on the basis of the results obtained and they are shown in Schemes 13 and 14 for short and long wavelength irradiation conditions. All the products shown were identifiable. At high humidity the kinetics of the photooxidation of polycarbonate is influenced to some extent in that on short-wavelength irradiation only products which are soluble in water appear to be produced. Furthermore, the presence of the moisture has an accelerating effect on photodegradation of the polymer. The life expectancy of stabilised polycarbonate is expected to be at least... 

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