Photo-oxidation mechanism

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

We note also that the schemes discussed until now only show the oxidation of the ethylene glycol moiety. In the PECT copolymer, the 1,4-cyclohexylenedi-methylene moiety is also available for oxidation. Indeed, given that the oxidizable hydrogens are tertiary, one reasonably expects a greater ease of production of a radical from that center. Grossetete et al. [11] reported such to be the case with the observation that photo-oxidation reactions occurred much faster with the PECT copolymer than with PET itself. The aliphatic acids that they reported, as identified by the SF4 treatment, could also account for previous aliphatic acid reports [25], This is also additional support that the photo-oxidation mechanism is operating as proposed (Scheme 18.4). 

The oxidation mechanisms need much more research to be well defined at present only the photo-oxidation mechanism has been studied in any detail. 

Scheme 3. Photo-oxidation mechanism of polymers RH is most labile hydrogen.

For instance, the bulk photo-oxidation mechanism is diffusion controlled, while the surface is continually exposed to an abundant... 

Radicals NC H2 are formed intermediately and are oxidized via hydroperoxide 121 into the formate salt 122. The ROOH and radical attacks are more prone to thermal processes taking part in heat stabilizing activity of HAS. Mechanism involving 102 and excited CTC fits participation of NCH3 in the photo-oxidation mechanism [43,111]. 

The development of a blue/green fluorescence in aged human lenses [129-130] is well known. This rise in fluorescence was reproduced in vitro by incubating HK with crystallins under aerobic conditions [116]. Subsequent protein digestion and aminoacid analysis confirmed the covalent modification. The extent and pattern of protein modification of these experiments are quite similar to those obtained with cataractous lenses. A photo-oxidative mechanism operating in age-related cataract development [123, 131-134] is therefore confirmed [116]. Further ex-... 

Polyolefins.—Polyolefin oxidation continues to be a subject of considerable controversy. Wiles and co-workers have written a comprehensive review of polyolefin photo-oxidation mechanisms with particular emphasis on the role of... 

The photostabilization of polymers continues to be a rapidly advancing area of scientific and technological interest. Carlsson and Wileshave written several reviews on photostabilizing mechanisms in polymers, while Swasey has given an updated guide to stabilization, and Reid has discussed the effects of stabilizers in vinyl polymers. Nemzek and Mayo have predicted the service life of polypropylene, and Bredereck has reviewed the photostabilization of PVC. Several comprehensive review articles have appeared. Pospisil has reviewed in considerable depth the photo-oxidation mechanisms of phenolic anti-oxidants, Shlyapintokh has reviewed the kinetics of stabilizer distribution, Vink has... 

A. Carton, D.). Carlsson, D.M. Wiles, Photo-oxidation Mechanisms in Commercial Polyolefins, in N.S. AUen (ed.). Developments in Polymer Photochemistry - I, Applied Science, London (1980), Chapter 4, p. 93. 

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. 

Dyes comprise a large fraction of visible light photoinitiators because their excited electronic states are more easily attained. Co-initiators, such as tertiary amines, iodonium salts, triazines, or hexaarylbisimidazoles, are required since dye photochemistry entails either a photoreduction or photo-oxidation mechanism. Numerous dye families are available for selection of an appropriate visible initiation wavelength an example of a thiazine dye (with an absorption peak aroimd 675 nm) is methylene blue (11). 

In the absence of an intentionally added chromophore, most polymers photochemically degrade by the so-called photo-oxidative mechanism. " Practically... 

Mineral filler influence on the photo-oxidation mechanism degradation of high density polyethylene. Part 11 Natural exposure test. Polymer Degradation and Stability, 83 (2004), p. 237-240... 

A Garton. In NS Allen, ed. Photo-Oxidation Mechanisms of Commercial Polyolefins in Developments in Polymer Photochemistry, Vol. 1. London Applied Science, 1980, chap 4. 

Garton, A., Carlsson, D.J., and Wiles, D. M. (1980) Photo-oxidation mechanisms in commercial polyolefins, in Developments in Polymer Photochemistry, 1 (ed. N.S. Allen), Applied Science,... 

Photo-oxidation mechanism in luminescent conjugated polymers. Reprinted with permission from J.H. Park eta/., Chem. Mat 16,688 (2004). 

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