Alkyl peroxide initiators, decomposition rates

Azonitriles are not susceptible to radical-induced decompositions (56) and their decomposition rates are not usually affected by other components of the environment. Cage recombination of the alkyl radicals occurs when azo initiators are used, and results in the formation of toxic tetrasubstituted succinonitrile derivatives (56). This can be a significant drawback to the use of azo initiators. In contrast to some organic peroxides, azonitrile decomposition rates show only minor solvent effects (54—56) and are not affected by transition metals, acids, bases, and many other contaminants. Thus azonitrile decomposition rates are predictable. Azonitriles can be used as thermal initiators for curing resins that contain a variety of extraneous materials since cure rates are not affected. In addition to curing of resins, azonitriles are used for polymerization of commercial vinyl monomers. 

A kinetic study has been carried out in order to elucidate the mechanism by which the cr-complex becomes dehydrogenated to the alkyl heteroaromatic derivative for the alkylation of quinoline by decanoyl peroxide in acetic acid. The decomposition rates in the presence of increasing amounts of quinoline were determined. At low quinoline concentrations the kinetic course is shown in Fig. 1. The first-order rate constants were calculated from the initial slopes of the graphs and refer to reaction with a quinoline molecule still possessing free 2- and 4-positions. At high quinoline concentration a great increase of reaction rate occurs and both the kinetic course and the composition of the products are simplified. The decomposition rate is first order in peroxide and the nonyl radicals are almost completely trapped by quinoline. The proportion of the nonyl radicals which dimerize to octadecane falls rapidly with increase in quinoline concentration. The decomposition rate in nonprotonated quinoline is much lower than that observed in quinoline in acetic acid. 

The differences in the rates of decomposition of the various initiators are related to differences in the structures of the initiators and of the radicals produced. The effects of structure on initiator reactivity have been discussed elsewhere [Bamford, 1988 Eastmond, 1976a,b,c Sheppard, 1985, 1988]. For example, k,i is larger for acyl peroxides than for alkyl peroxides since the RCOO- radical is more stable than the RO radical and for R—N=N—R, kd increases in the order R = allyl, benzyl > tertiary > secondary > primary [Koenig, 1973]. 

By comparison to peroxides, the azo compounds are generally not susceptible to chemically induced decompositions. It was shown,however, that it is possible to accelerate the decomposition of a,a -azobisisobutyronitrile by reacting it with bis(-)-ephedrine-copper(II) chelate. The mechanism was postulated to involve reductive decyanation of azobisisobutyronitrile through coordination to the chelate. Initiations of polymerizations of vinyl chloride and styrene with a,a -azobisisobutyronitrile coupled to aluminum alkyls were investigated. Gas evolution measurements indicated some accelerated decomposition. Also, additions of large amounts of tin tetrachloride to either a,a -azobisisobutyronitrile or to dimethyl-a,a -azobisis-obutyrate increase the decomposition rates. Molar ratios of [SnCl4]/[AIBN]= 21.65 and [SnCl4]/[MAIB] = 19.53 increase the rates by factors of 4.5 and 17, respectively. Decomposition rates are also enhanced by donor solvents, like ethyl acetate or propionitrile in the presence of tin tetrachloride. ... 

Most frequently the polymerization process is initiated by free radicals obtained through the decomposition of hydroperoxides, alkyl peroxides, dialkyl peroxides, acyl peroxides, carboxylic ester peracids, salts of (tetraoxo)sulphuric acid, hydrogen peroxide, aliphatic azo compounds and bifunctional azobenzoin initiators. The rate of decomposition of different initiators into free radicals depends on their stmcture and on temperature. A measure of the efficiency of the initiator in the pol5mierization process is the half-decomposition period. 

Oxidation reactions of hydrocarbons have a typical course. From the low rates, the reaction accelerates successively due to the consecutive formation of another source of free radicals which increases the rate of the primary initiation reaction. The amplification of the number of reactive free radicals is caused mainly by the decomposition of alkyl hydroperoxides, dialkyl and diacyl peroxides and peracids which are formed as intermediates in the oxidation reaction. 

Several investigations have been concerned with the effect of solvent upon the rate of perester decomposition. These data also shed some light on the importance of the ionic structure (II) in the transition state. Some data on the effect of solvent may be obtained from previous tables. For the decomposition of peresters where R is a primary alkyl group in RCO3C4H9-/ and one-bond homolysis is the mechanism of choice, changes in solvent polarity have little effect on the rate of decomposition. The rate coefficients for the decomposition of r-butyl percaprate at 110 °C in chlorobenzene, nitrobenzene and diphenyl ether are 8.30 x 10 , 6.58 x 10" and 6.39 X 10" sec"S respectively" . The rates are also independent of initial concentration of the peroxide this may indicate that induced decomposition is unimportant cf. sub-section 13.4.1). 

The formation of PP macroradicals is an easy process initiated by more or less any radical initiator. It occurs spontaneously in oxidative processes. Alkyl radiccils (except for methyl) are usually not reactive enough to initiate an efficient macroradical formation in PP. Oxyl radicals, formed by a thermal decomposition of peroxides, are the most convenient species for crosslinking initiation. The transfer of the radical centre to PP is selective to a certain extent. At temperatures usual for peroxide decomposition, the ratio of the rate constants of the abstraction of hydrogen from primary, secondary, and tertiary carbon by the oxyl radical is approximately 1 3 10 [2]. 

The oxidation of alkanes by r-butyl hydroperoxide (TBHP) has been catalysed by titanium alkoxides, producing the corresponding alcohols and ketones. A radical mechanism is proposed in which r-butoxyl radical formed from TBHP and titanium alkoxide initiates the reaction. The evolution of oxygen (from the decomposition of peroxide) and the abstraction of hydrogen from alkane to form alkyl radical occur competitively. A method for the determination of both the primary and secondary KIEs at a reactive centre based on starting-material reactivities allows the determination of the separate KIEs in reactions for which neither product analysis nor absolute rate measurements are applicable. It has been applied to the FeCls-catalysed oxidation of ethylbenzene with TBHP, which exhibits both a primary KIE and a substantial secondary KIE the findings are in accordance with previous mechanistic studies of this reaction. The oxidation of two l-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy-acids and TBHP catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin [Fe(ni)P] is a two-step process. In single turnover reactions, dye and Fe(in)P compete for the initially formed OFe(IV)P+ in a fast reaction and OFe(IV)P is produced the peroxy acid dye stoichiometry is 1 1. This is followed by a slow phase with 2 1 peroxy acid dye stoichiometry [equivalent to a... 

Unlike the schemes considered previously, in this scheme we introduced the states of pairs differed by mutual orientation and is the rate constant of mutual turn of the radicals. For the dimer obtained by the recombination of two a-phenylethyl radicals formed in the decomposition of optically active azo-a-phenylethane (benzene with the acceptor of alkyl radicals), the following composition is observed 31% DD(-) (configurations of both radicals remained unchanged), 48% meso (one of the radicals had time to turn), and 21% LL(+) (both radicals turned). The ratio = 15, i.e., the rotation of radicals in the cage occurs very promptly. The close results were obtained for the recombination of the a-phenylethyl—benzyl radical pair k k = 16 (benzene, 383 K). The decomposition of optically active peroxides in the cage affords predominantly recombination products of radicals, which did not retain the initial mutual orientatioa... 

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