Hydroperoxide radicals, bond

With the formation of free radicals having been initiated, these radicals react with oxygen (Reaction 3) to begin the propagation of the radical chains in forming a peroxy radical. The peroxy radical then attacks the 10-carbon-hydrogen bond to form the hydroperoxide radical (Reaction 4). [The possibility of such an intramolecular attack has been demonstrated in an aliphatic system where two reactive hydrogen atoms are located in the favorable 1,4-positions (9)]. 

Products of degradation hydrogen, water, carbon dioxide, ketone, unsaturations, hydroperoxides, radicals, chain scissions, crosslinks, quinomethane structures, benzene, acetophenone, benzaldehyde, benzene, formic acid, acetic acid, benzoic acid, conjugated double bonds ... 

Products of degradation hydroperoxides, radicals, ketones, carboxyl groups, hydroxyls, peresters, esters, lactones, chain scission, crosslinking, hydroxyls, double bonds ... 

The first type of hydrocarbon membrane for fuel cell applications was the sulfonated polystyrene-divinylbenzene co-polymer membranes equipped for the power source in NASA s Gemini space flights, but the sulfonated polystyrene had low chemical stability for long-term applications, because the proton on the tertiary carbons and benzylic bonds are easily dissociated in an oxygen environment forming hydroperoxide radicals. Since a styrene monomer is easily co-polymerized with other vinyl monomers via radical polymerization methods, various styrenic polymers were researched intensively. Two commercial polystyrene-based/related membranes are available BAM (Ballard), and Dais Analytic s sulfonated styrene-ethylene-butylene-styrene (SEBS) membrane. Dais membranes are produced using... 

Depending on the peroxide class, the rates of decomposition of organic peroxides can be enhanced by specific promoters or activators, which significantly decrease the energy necessary to break the oxygen—oxygen bond. Such accelerated decompositions occur well below the peroxides normal appHcation temperatures and usually result in generation of only one usehil radical, instead of two. An example is the decomposition of hydroperoxides with multivalent metals (M), commonly iron, cobalt, or vanadium ... 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133—172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utiUty as thermal initiators. The oxygen—hydrogen bond in hydroperoxides is weak (368-377 kJ/mol (88.0-90.1 kcal/mol) BDE) andis susceptible to attack by higher energy radicals ... 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

It is virtually impossible to manufacture commercial polymers that do not contain traces of hydroperoxides. The peroxide bond is relatively weak and cleaves homolyticaHy to yield radicals (eqs. 2 and 3). Once oxidation has started, the concentration of hydroperoxides becomes appreciable. The decomposition of hydroperoxides becomes the main source of radical initiators. 

The peioxy free radicals can abstract hydrogens from other activated methylene groups between double bonds to form additional hydroperoxides and generate additional free radicals like (1). Thus a chain reaction is estabhshed resulting in autoxidation. The free radicals participate in these reactions, and also react with each other resulting in cross-linking by combination. 

Treatment of 2-methylthiirane with t-butyl hydroperoxide at 150 °C in a sealed vessel gave very low yields of allyl disulfide, 2-propenethiol and thioacetone. The allyl derivatives may be derived from abstraction of a hydrogen atom from the methyl group followed by ring opening to the allylthio radical. Percarbonate derivatives of 2-hydroxymethylthiirane decompose via a free radical pathway to tar. Acrylate esters of 2-hydroxymethylthiirane undergo free radical polymerization through the double bond. 

Free radical initiators can polymerize olefmic compounds. These chemical compounds have a weak covalent bond that breaks easily into two free radicals when subjected to heat. Peroxides, hydroperoxides and azo compounds are commonly used. For example, heating peroxybenzoic acid forms two free radicals, which can initiate the polymerization reaction ... 

The ROO-H bond of hydroperoxides is weak compared to most other X-H bonds. Thus, abstraction of the hydroperoxidic hydrogen by radicals is usually an... 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

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