Dibenzoyl peroxide induced decomposition

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

With sensitizers, initiation stops when the source of radiation is turned off, which is followed by a rapid decay of the polymerization process. When a conventional initiator, such as dibenzoyl peroxide, is also present, the process is more rapid than when the sensitizer is used by itself. It also seems to continue after the radiation source has been discontinued. It is presumed that ultraviolet (UV)-induced decomposition of the peroxide becomes involved in the process. By this method, polymerizations may be carried out at temperatures well below those normally used with thermal initiators such as organic peroxides. 

This solvent effect is caused by what is known as induced decomposition. For example, in the explosive decomposition of dibenzoyl peroxide in... 

The formation of a-butoxybutyl free radicals is believed to occur in the induced decomposition of dibenzoyl peroxide by butyl ether ... 

The free radical yield/for AIBN in styrene and various solvents at 50°C is /= 0.5. Because of induced decomposition,/varies strongly with solvent in the case of BPO. If, for steric reasons, the primary free radicals cannot recombine, then the free radical yield can, according to conditions, increase up to/= 1. Thus, the start reaction is rarely a simple function of added initiator concentration, since it depends on free radical yield and may also depend on induced decomposition. Because of this, faster initiator decomposition need not necessarily produce faster polymerization. For example, dibenzoyl peroxide decomposes a 1000 times faster in benzene than cyclohexyl hydroperoxide, but only polymerizes styrene five times as fast. 

Under certain conditions, no free radical polymerization at all occurs with certain free radical initiators. For example, azobisisobutyronitrile polymerizes vinyl mercaptals, CH2=CH—S—CH2—S—R, to high-molar-mass compounds. But no polymer at all is produced under the same conditions by dibenzoyl peroxide the mercaptal groups induce dibenzoyl peroxide decomposition, producing benzoic acid and an unstable ester, CH2=CH—S— CH(00CC6H5)—S—R. Thus, the initiator is completely consumed by this side reaction. 

Nitrobenzene initially behaves as a hindering agent and then as a retarding agent. Many substances play such double roles. For example, tetraphenyl hydrazone inhibits the polymerization of methyl methacrylate but induces the free radical decomposition of dibenzoyl peroxide. 

Although in this system radicals are generated and the free valence is conserved in the act of interaction of PhCOO- with RH, no chain reaction appears because the cyclic sequence of steps with conservation of the free valence is not realized. Therefore, second, the fulfillment of one more condition is very in ortant principle of cyclicity of radical steps with conservation of free valence. The chain reaction to occur requires such a combination of the reactants that the cycle of transformations with conservation of fiee valence and reproduction of the initial radical (atom) takes place. In the example presented above, it is enough to replace hydrocarbon by secondary alcohol to induce the chain reaction of dibenzoyl peroxide decomposition with the following steps of chain propagation ... 

The decomposition of dibenzoyl peroxide in dimethylaniline is extremely fast, which is also due to induced decomposition. The reaction mechanism involves the formation of radical cation and a subsequent transformation into radicals and stable species. In polymer synthesis, small quantities of dimethylaniline are sometimes added to BPO to promote radical generation ... 

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