Peroxides thermal cleavage

Initiation normally requires molecules with weak bonds to undergo homolytic cleavage to produce free radicals. Since bond homolysis even of weak bonds is endothermic, energy in the form of heat (A) or light (hv) is usually required in die initiation phase. However, some type of initiation is required to get any free-radical reaction to proceed. That is, you must first produce free radicals from closed-shell molecules in order to get free-radical reactions to occur. Benzoyl peroxide contains a weak 0-0 bond that undergoes thermal cleavage and decarboxylation (probably a concerted process) to produce phenyl radicals which can initiate free-radical chain reactions. 

Long, straight and branched chain perfluoroalkanes may also be prepared by thermal cleavage of peroxides,92 e.g. a mixture of perfluorotetradecane, C14F30 (19) (12%). i-C14F30 (45%), and i-C7F15-i-C7F15 (42%) was prepared from perfluorooctanoyl peroxide.92... 

Most of the living radical polymerizations using organic radicals as regulating persistent species involved nitroxides. Exceptions are triphenylmethyl and other carbon-centered radicals in the early work of Otsu and Braun.24,25 More recently, Chung showed that borinate radicals 10 formed by the thermal cleavage of in situ generated alkyl boryl peroxides (Scheme 31) can be employed to control methacrylate... 

The reachon with aryl, acyl, or aroyl radicals, generated from the respective peroxides by thermal cleavage, leads to an arylation or acylation of the diamond surface (Figure 6.40). Substitutions performed on the aromatic rings then ahow for further modifications of the surface and for consecutive reactions. 

Thermal cleavage of the above peroxides results in macromolecules with free-radicals sites. Hydroxy radicals also form and initiate formations of homopolymers. Decompositions of the peroxides by redox mechanisms, however, increase the yields of graft copolymers, but do not stop all formations of hydroxy radicals ... 

Fig. 4.7 Thermal cleavage of (supposed) rubrene oxide and of bis-trialkyl peroxide. The difference was thought to reside in the simultaneous breaking (ropture, ebranlement) of the two bonds in opposite direction in the first molecule. Reprinted with permission from [93]...

Not only persistent radicals can be reacted with low-valent metal complexes but also those that are generated in situ by the thermal cleavage of peroxides. To our best knowledge, so far, only samarium [41] and ytterbium [42] compounds were reported into which a tm-butoxide moiety was introduced by reduction of di-rcrr-butylperoxide by the metal center. Here, the first Cp-type titanium monoalkoxide complex synthesized by reacting a titanium(lll) species with di-tert-butylperoxide is reported. 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Almost all organic peroxides are thermally and photolyticaHy sensitive owing to the facile cleavage of the weak oxygen—oxygen bond, ie, the range of AHis about —84 to —184 kJ/mol (—20 to —44 kcal/mol) (9—11) ... 

Thermal decomposition of dihydroperoxides results in initial homolysis of an oxygen—oxygen bond foUowed by carbon—oxygen and carbon—carbon bond cleavages to yield mixtures of carbonyl compounds (ketones, aldehydes), esters, carboxyHc acids, hydrocarbons, and hydrogen peroxide. 

Azo-compounds and peroxides undergo photodecomposition to radicals when irradiated with light of suitable wavelength. The mechanism appears similar to that of thermal decomposition to the extent that it involves cleavage of the same bonds. The photodecomposition of azo-compounds is discussed in Section 3.3.1.1.2 and peroxides in Sections 3.3.2.1.2 (diacyl peroxides) and 3.3.2.3.2 (peroxyesters). Specific photoinitiators are discussed in Section 3.3.4. It is also worth noting that certain monomers may undergo photochemistry and direct photoinitiation on irradiation of monomer is possible. 

Various authors have studied the ageing of triterpenoid resins to understand and possibly slow their deterioration [3, 4, 12, 13, 17 36]. The main degradation pathway is autoxida-tion, an oxidative radical chain reaction [37, 38] after formation of radicals, oxygen from the air is inserted, leading to peroxides. The peroxides can be homolytically cleaved, resulting in new radicals that continue the chain reaction. The cleavage of peroxide bonds can be induced thermally or photochemically. 

The rest of the photo-oxidation on the glycol portion is continued in Scheme 18.3. The cleavage of the 0-0 bond should be very facile. Both photolytic and thermal decomposition of these peroxides are possible. 

Oxidation indices, 656-72 peroxide determination, 762-3 peroxide value, 656, 657-64 colorimetry, 658-61 definition, 657 direct titration, 657 electrochemical methods, 663-4 IR spectrophotometry, 661-3 NIR spectrophotometry, 663 UV-visible spectrophotometry, 658-61 secondary oxidation products, 656, 665-72 tests for stability on storage, 664-5, 672 thermal analysis, 672 Oxidative amperometiy, hydroperoxide determination, 686 Oxidative cleavage alkenes, 1094-5 double bonds, 525-7 Oxidative couphng, hydrogen peroxide determination, 630, 635 Oxidative damage... 

This chemoluminescence results from interaction of 156 (generated from 155 under thermal conditions) and 1,3-DIBF (formed in a minor amount from 156). The first step is the formation of an encounter complex. Electron transfer generates a peroxide radical anion of 156 and a radical cation of 1,3-DIBF. Cleavage of the 0-0 bond in the radical anion of 156 forms an o-dibenzoylbenzene radical anion. Annihilation of the oppositely charged ions gives an excited singlet of 1,3-DIBF (with subsequent fluorescence) (82JA1041). 

Whatever the exact nature of these sensitized decompositions, they can be useful in generation of radicals by cleavage of certain peroxides, such as 4-r-butylcyclohexanepercarboxylate esters, which rearrange by nonradical paths during thermal decomposition.489 They can also be a nuisance, as evidenced by the quenching of triplet triphenylene observed in plastics when peroxides were used to polymerize the samples.470... 

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