Benzoyl peroxide, decomposition

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... 

These substitution reactions were discovered by Nozaki and Bartlett [57,58] in their study of benzoyl peroxide decomposition in different solvents. When benzoyl peroxide is decomposed, the formed benzoyloxyl radical attacks the solvent (RH), and the formed alkyl radical (R ) induces the chain decomposition of the peroxide (see Chapter 3). 

Bartlett and Nozaki [65] discovered the chain reaction of benzoyl peroxide decomposition in the presence of alcohols where the chain propagating step is the reaction of the ketyl radical with peroxide. 

As an example the carrier addition method 2 is described in more detail in the appendix since this is important for establishing the radiochemical purity of labelled compounds. The other two procedures are similar in principle and are described elsewhere (29). The technique has been used to measure the products of initiator side reactions in azoiso-butyronitrile initiated polymerizations (30—33) and in benzoyl peroxide decomposition (34) and is capable of much wider application than it has so far received. 

Although the kinetics of benzoyl peroxide decomposition were studied prior to 1946330 the rate law was not established until this date by Nozaki and Bartlett . A review of the kinetics of benzoyl peroxide decomposition was given in 1954. The presently accepted rate law is... 

Inii/ioHon-. Any material which decomposes spontaneously or under external stimidus into free radicals may be used as an initiator for polymerization at the double bond A variety of peroxides satisfy this condition, such as, for instance, benzoyl peroxide and tertiary butylhydroper-oxide in systems where the initiator is dissolved in the monomer itself or in a monomer solution also hydrogen peroxide and potassium persulfate in emulsion polymerizations where the initiator is dissolved in an aqueous medium. Benzoyl peroxide decomposition occurs by Unimolecular reao tion ... 

The investigation of benzoyl peroxide decomposition in the presence of quaternary ammonium halides has shown that the nature of anion as well as cation of the onium salt influenced the peroxide substrate decomposition. Authors [19] have proposed the association model of diacylperoxides decomposition activation by onium salts (Fig. 4). 

Investigation of benzoyl peroxide decomposition activated by onium halides do not let exclude the cation effect on the kinetics of peroxide decomposition (Fig. 6). Two possible ways of association between tetraethylammonium - cation and peroxide along symmetry axis have been considered (Fig.5). 

The investigations of benzoyl peroxide decomposition kinetics with the tetraethylammonium perchlorate assistance have shown the salt to be kinetically inert [8]. Taking into accoimt that this salt is completly dissosiated in acetonitrile we can conclude that the cation does not participate in the reaction. Thus the only cation action could not lead to peroxide activation. 

Figure 6. The effect of cation on the activation parameters of benzoyl peroxide decomposition reaction in acetonitrile.

In a broad sense radical-induced decomposition may be considered as the reaction of any substrate promoted by a free radical, but the more common usage is to restrict consideration to those cases in which the molecule undergoing reaction is a free radical initiator (1, ). Thus radical-induced decomposition may be de-fTned as reaction of a free radical with the primary source of the radical. This phenomenon is particularly noticeable when a free radical initiator does not react with first-order kinetics, but is consumed by the radicals it generates in chain processes. This case was originally elucidated for the kinetics of benzoyl peroxide decomposition in solvents such as ethyl ether ( - ), and was found to occur with radical displacement (the SH2 reaction) (S) by solvent derived radicals on the peroxidic oxygen of benzoyl peroxide as shown in Fig. 1. 

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