Oxidative addition, photo

An additional oxidation pathway is observed for 13 with a methyl group at the C-atom of the silene moiety. Here, the primary adduct of 13 and 302 under photo lytic conditions, i.e. triplet diradical 17, can either ring-close to give the corresponding dioxetane (the dioxetane is not stable under the photolytic conditions required to induce the photooxidation, and therefore cannot be observed) leading to products derived from it, or produce dimethylvinylsilyl hydroperoxide (18) via H-abstraction from a methyl group (equation 6). 

Histidine may undergo either photo-catalyzed or metal-catalyzed oxidation. Photolytic oxidation results in the formation of 2-oxohistidine, also known as 2-oxoimidazoline, as shown in Scheme 3. Because His is an effective chelating agent, it is highly sensitive to metal-catalyzed oxidation. Additional oxidation products may be observed through metal-catalyzed oxidation of His, including the formation of Asp or Asn, but the mechanism for the formation of these products is not understood. ... 

In the cell suspension culture treated with the processed water, CLA chemiluminescence (Fig. 2) and aequorin luminescence (Fig. 3) reflecting the generation of superoxide and increase in cytosolic calcium ion concentration ([Ca2+]c), respectively, were measured. We observed the spikes of CLA chemiluminescence in the cell suspension culture after addition of photo-catalytically processed waters, suggesting that photo-catalytic process generated or conditioned the waters enabling the stimulation of plant cells with oxidative stress. 

An alternative mechanism by which additives may protect polymers from photo-oxidation is radical trapping. Additives which operate by this mechanism are strictly light stabilizers rather than antioxidants. The most common materials in this class are the hindered amines, which are the usual additives for the protection of poly (ethylene) and poly (propylene). The action of these stabilisers is outlined in Reactions 8.3-8.5. 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

The hindered secondary amines can be highly effective photostabilizers for various polymers (]+.,5.,.6) Various hindered amines have been shown to retard oxidation, but most share the common feature of being secondary or tertiary amines with the a-carbons fully substituted. The most widely exploited representatives of this class are based on 2,2,6,6-tetramethylpiperidine either in the form of relatively simple low molecular weight compounds, or more recently as backbone or pendant groups on quite high molecular weight additives ( i.,5.,6). The more successful commercial hindered amines contain two or more piperidine groups per molecule. Photo-protection by tetra-methylpiperidines (near UV transparent) must result from the interruption of one or more of the reactions 1 to 3. Relatively recent results from our own laboratories, and in the open literature will be outlined in this context. 

Most kinetic treatments of the photo-oxidation of solid polymers and their stabilization are based on the tacit assumption that the system behaves in the same way as a fluid liquid. Inherent in this approach is the assumption of a completely random distribution of all species such as free radicals, additives and oxidation products. In all cases this assumption may be erroneous and has important consequences which can explain inhibition by the relatively slow radical scavenging processes (reactions 7 and 9) discussed in the previous section. 

The photo-oxidation of a solid branched alkane can be expected to proceed in localized domains, new oxidation chains being generated from the photo-cleavage of -00H products, and chain propagation (reactions 2 and 3) being concentrated close to each initial site in a given domain to produce a zone of high -00H concentration. Thus the distribution of an additive in and around these domains is of special importance. 

Several carbonyl-containing peroxide additives have been shown to increase the initial rate of the nonoxidative photo-dehydrochlorination of PVC (54). In studies with polymeric ketones unrelated structurally to PVC, the excited singlet and triplet states of the carbonyl groups in these polymers were found to sensitize 0-0 homolysis at rates approaching diffusion control (55). Similar reactions may well occur in oxidized vinyl chloride polymers. 

As discussed earlier under Section 2.3, Carbonyl index, in one relatively recent comparison of the photo-oxidative and thermal (oven-aged) degradation behaviour of different polyethylenes, additive free grades of a metallocene (mPE), an HDPE and a linear low-density PE (LLDPE) were analysed by a combination of mid-IR spectroscopy, TGA and CL [13]. The mid-IR... 

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