Photo-oxidative mechanism, photodegradation

Scoponi M, PradeUa F, Carassiti V. Photodegradable polyolefins. Photo-oxidation mechanisms of innovative polyolefin copolymers containing double bonds. Coord Chem Rev 1993 125 219-230. 

Treatment of these samples with SF4 gas to convert the carboxylic acids produced in the weathering process into carbonyl fluorides showed [2, 11] that the acids are actually a mixture of aliphatic and aromatic acids (Figure 18.12). Aromatic acid species are by far the predominant ones, however. The origin of these acids will be discussed below in conjunction with the overall mechanisms of photodegradation. Aliphatic acid species were detected by GC/MS in the artificial device exposure of PECT [11], Note that the PECT copolymer produced more aromatic acids with the same exposure as PET but that the aliphatic acid production was several times higher for the PECT copolymer. The photo-oxidation of the co-glycol must be the reason for this difference. 

The mechanism is essentially different from the autocatalytic oxidation of the polymers solely with respect to initiation, decomposition of hydroperoxides, and photochemical reactions of the carbonyl and aldehyde groups. The course of all other reactions is the same. A number of reviews [Refs. 149, 182, 217, 228, 232, 258, 280, 311, 492, 506, 678, 679] discuss the photodegradation and photo-oxidation of polymers. 

Photoactive Additives.—Ferric compounds, in particular, the chloride, continue to attract much interest as photosensitizers for thermoplastics. " From e.s.r. work the mechanism appears to involve a redox reaction resulting in the formation of active hydroxy-radicals. Photodegradable polyethylene film has been developed by doping it with radiation-modified atactic polypropylene and hydroxyethyl-ferrocene. Several workers have studied the dye-sensitized photo-oxidation of polyisoprene and di-n-butyl sulphide embedded in Augustyniak and... 

The aims of the research project have been to obtain information on the rates and mechanisms for a range of VOC photo-oxidation reactions under simulated atmospheric conditions. By deriving detailed quantitative kinetic data on the reactions involved in the complex chemistry of the tropospheric photodegradations of VOC, we seek to provide key parameters required in tropospheric modelling. Such mathematical modelling studies will ultimately form the basis for drawing up scientifically sound control strategies for the release of VOC into the atmosphere. 

Photodegradation and photo-oxidative degradation and the Norrish Type II mechanisms ... 

Ozone accelerates the photodegradation of polyacrylamide in water solution [726, 727]. Photo-oxidative degradation of polyamides causes a decrease of their mechanical properties [1395, 1396, 2305]. 

In inert atmosphere there may occur specific degradative processes (macromolecules scissions and crosslinking). In air atmosphere oxygen also influences photodegradation through photo-oxidative processes occurring via radical intermediates as chained mechanisms. 

The mechanisms of photochemical degradation depend peculiarly on the type of polymer, as well as on the specificity of environmental factors. Usually, whereas the applications of polymaic materials run in the presence of air and of atmospheric oxygen, the polymo- degradation lead to a variety of physical and chemical effects. This damaging process is practically an oxidative photodegradation (photo-oxidation). 

Whenever free radicals are produced in the presence of oxygen by a primary photolysis process then the alternative reaction of photo-oxidation usually becomes the predominant mechanism for photodegradation. The subjects of stabilization and prevention of oxidation induced by photolysis are discussed in Volume 6, Chapter 19. 

It is apparent that photo-oxidation of polymers is a complex and very important component of photodegradation of such materials. Detailed discussion of mechanism and the effects of these reactions is not part of this review. However, prevention of breakdown has a very obvious economic impact and a few generalizations on stabilization of polymers are relevant before the detailed discussion which follows in Volume 6, Chapter 19. Given that radiation produces free radicals, which become involved in a chain reaction, then any substance which preferentially absorbs the harmful radiation (a screen) or any substance which can efficiently remove radicals via non-reactive products (an antioxidant) will operate as a stabilizer. However, the planning and chemical design of stabilizers is not easy and Volume 6, Chapter 19 gives an excellent survey of just how complicated the process of stabilization can be. 

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