Hydroperoxides photodecomposition

The photodecomposition of the intermediate hydroperoxide results in cleavage of the polymer chain and formation of a ketone. 

Kavun and Buchachenko (247) have found that the "primary" amino radicals derived from thioamines in the presence of hydroperoxide are very stable and are not readily converted into "secondary" nitroxide radicals. Kommandeur and Wiersma (107) studied the photodecomposition of tetraphenylhydrazine in rigid solution at 77°K, and the observed ESR spectrum was assigned to the dimer of the diphenyl amino radical (248). Shida et al. (249) have also studied the photodimer of tetraphenylhydrazine and the photochromic dimer of triphenylimidazolyl at low temperature by optical and ESR techniques. These authors noted that photolysis of these compounds induces the homolysis while y-irradiation leads to heterolytic dissociation. Blinder et al. (250) reported the ESR spectrum of the monomer tetraphenylpyrryl radical in... 

The conversion of hydroxamic acids into the corresponding amides is believed to involve initial formation of amidyl radicals by nitrogen-oxygen bond homolysis. Photodecomposition of hydroperoxides, peroxides and ozonides has also been examined. 

Table II. Quantum Yield of Photodecomposition of cis-1,4-Polyisoprene Hydroperoxide at 313 nm Light Intensity = 7.2 X 10 7 Einstein/min.

In a series of further experiments, the photolysis of tert-butyl hydroperoxide and also the ds-polyisoprene hydroperoxide was studied in the presence of a variety of ketones. The results of the photodecomposition of tert-butyl hydroperoxide in hexane in the presence of methyl isobutyl-... 

Table IV. Efficiency of Polymer Chain Scission Relative to Photodecomposition of Hydroperoxide at 313 nm...

UV wavelengths which damage certain polymers. The absorbed photons raise electrons to an excited state and cause bond dissociation reactions. An example is the photodecomposition of hydroperoxide groups introduced during melt processing. 

Radiolytically, RO can be produced via the electron reaction with hydroperoxides or quinones [131, 132] or via the one electron oxidation of phenolic compounds [133]. RO radicals can also be generated photolytically via photochemical reduction of quinones, photodecomposition of peroxides or via direct photolysis of phenols [134—136]. 

Active hydroperoxides reach a constant concentration (in which they are steadily destroyed and formed) after a short induction period (Fig. 2.3). 2. Inactive hydroperoxides resulting from the propagation of polymer peroxy radicals outside a sphere, which avoids photodecomposition. 

Photodecomposition of polymer hydroperoxides yields polymer oxy radicals (PO ), and finally carbonyl and hydroxide groups ... 

Acetophenone end-groups, which develop as a result of thermal and/or photodecomposition of hydroperoxide groups (cf. section 3.11.1.6) ... 

Photodecomposition of hydroperoxide (OOH) groups subsequently gives end-hydroxyl (OH) and end-formyl(aldehyde) (—C ) chains ... 

Photodecomposition of the polymeric hydroperoxide (4.12) leads to formation of the polymer side oxy radical 4.13) ... 

The chain scission reaction may occur by thermal and/or photodecomposition of the hydroperoxide (OOH) group [785] ... 

Cleavage of 0-0 bonds from hydroperoxides further assure photodecomposition together with aUcoxy radicals P-scissions afterwards. Another route to polymer photodegradation resides in the further reactions of the hydroperoxides to generate other chromophores and/or new functional entities, such as carbonyls, carboxylic acids, alcohols and olefins, which further contribute to main chain scissions [6] (Scheme 1). 

On the basis of the data published for thermal (7) and photochemical (57) decomposition of low-molecular-weight and polymeric hydroperoxides, Ershov, Lukovnikov and Baturina reported the following mechanism to explain their results for the kinetics of photodecomposition of atactic polypropylene h3droperoxides in the absence of oxyg ... 

In order for Eq. (33) to be in agreement with the experimental result, it must be admitted that the second term in the right member is negligible with respect to the first therefore, under the experimental conditions adopted, hydroperoxides disappear almost exclusively due to photodecomposition (26). The decrease in decomposition rate in the presence of oxygen is obviously due to the well-known reaction of alkyl radicals with oxygen, which leads to the additional formation of hydroperoxide groups [reactions (3) and (4)] hence the rate of photochemical decomposition apparently decreases. 

Whilst Schemes 3-5 above are quite well documented, more recent studies on both polyalkenes and polyamides clearly indicate that titanium dioxide and zinc oxide pigments owe their photocatalytic activity to catalysis of the photodecomposition of hydroperoxides in amorphous regions of the polymers. 

We review in this chapter the nature of the photodecomposition pathways of several of the major products of atmospheric oxidation of the hydrocarbons, namely, the acyclic aldehydes in section IX-B the aldehydes containing additional functional groups in section IX-C the acyclic ketones in section IX-D the cyclic ketones in section IX-E the ketones containing additional functional groups in section IX-F the acyl halides and carbonyl halides in section IX-G nitrous acid, the alkyl nitrites, the nitroalkanes, and the nitroso-compounds in section IX-H nitric acid and the alkyl nitrates in section IX-I the peroxyacyl nitrates in section IX-J the alkyl hydroperoxides in section IX-K and some oxygenates derived from the aromatic hydrocarbons in section IX-L. 

The absorption spectrum of hydroxymethyl hydroperoxide is very similar to that of the related compound, methyl hydroperoxide (Bauerle and Moortgat, 1999) see figures IX-K-1 and IX-K-3. It is probable that its photodecomposition follows the pathway demonstrated for methyl hydroperoxide ... 

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