Thermal decomposition benzoyl peroxide

Free-radical carboxymethylation of several aromatic compounds has been reported, " the -CHaCOOH radical being produced by the thermal decomposition of benzoyl peroxide in acetic acid. More recently the carboxymethylation of dibenzofuran brought about by the thermal decomposition of chloroacetylpolyglycolic acid (41) has... 

Diacyl peroxides undergo thermal and photochemical decomposition to give radical intermediates (for a recent review, see Hiatt, 1971). Mechanistically the reactions are well understood as a result of the many investigations of products and kinetics of thermal decomposition (reviewed by DeTar, 1967 Cubbon, 1970). Not surprisingly, therefore, one of the earliest reports of CIDNP concerned the thermal decomposition of benzoyl peroxide (Bargon et al., 1967 Bargon and Fischer, 1967) and peroxide decompositions have been used more widely than any other class of reaction in testing theories of the phenomenon. 

CIDNP studies of the decomposition have centred mainly on thermal decompositions photochemical decomposition has generally been less intensively investigated. While most reports of polarization refer to n.m.r. spectra, a number of papers have described polarization of other nuclei, (Kaptein, 1971b Kaptein et al., 1972), (Lippmaa el al., 1970a, b, 1971 Kaptem, 1971b Kaptein et al., 1972 Kessenikh et al., 1971), and F (Kobrina et al., 1972) contained in the peroxide reactant. Additionally, polarization of P has been reported in the products of decomposition of benzoyl peroxide in phosphorus-containing solvents (Levin et al., 1970). 

Since the benzene emission in the thermal decomposition of benzoyl peroxide results from radical transfer by the phenyl component of a benzoyloxy-phenyl radical pair, phenyl benzoate produced by radical combination within the same pair should appear in absorption. A weak transient absorption has been tentatively ascribed to the ester (Lehnig and Fischer, 1970) but the complexity of the spectrum and short relaxation time (Fischer, personal communication) makes unambiguous assignment difficult. Using 4-chlorobenzoyl peroxide in hexachloro-acetone as solvent, however, the simpler spectrum of 4-chlorophenyl 4-chlorobenzoate is clearly seen as enhanced absorption, together with... 

In the decomposition of benzoyl peroxide, the fate of benzoyloxy radicals escaping from polarizing primary pairs remains something of a mystery. Benzoic acid is formed but shows no polarization in and C-spectra, and the carboxylic acid produced in other peroxide decompositions behaves similarly (Kaptein, 1971b Kaptein et al., 1972). Some light is shed on the problem by studies of the thermal decomposition of 4-chlorobenzoyl peroxide in hexachloroacetone containing iodine as... 

Literature data for the suspension polymerization of styrene was selected for the analysi. The data, shown in Table I, Includes conversion, number and weight average molecular weights and initiator loadings (14). The empirical models selected to describe the rate and the instantaneous properties are summarized in Table II. In every case the models were shown to be adequate within the limits of the reported experimental error. The experimental and calculated Instantaneous values are summarized in Figures (1) and (2). The rate constant for the thermal decomposition of benzoyl peroxide was taken as In kd 36.68 137.48/RT kJ/(gmol) (11). 

The first example we address is taken from a paper by Bawn and Mellish, published some 50 years ago [323]. It reports kinetic studies of the thermal decomposition of benzoyl peroxide in several solvents (reaction 15.5), over the temperature range of 49-76 °C. Here, we analyze the data obtained in toluene over the temperature range of 49.0-70.3 °C. 

Thermal decomposition of peroxides generates a variety of radical groups which can covalently react with CNTs. This strategy has been employed in particular by Peng and co-workers who functionalized sidewalls with benzoyl, lauroyl and carboxyalkyl derivatives [38]. 

Perfluoroalkyl radicals were generated according to a procedure developed by this research group, involving iodine abstraction from perfluoroalkyl iodides by phenyl radical (Equation 14.13) this latter derives from thermal decomposition of benzoyl peroxide (Equation 14.14). 

The use of chemical sensitizers such as benzoyl peroxide, cumene hydroperoxide, or azo-bis-isobutyronitrile, which decompose thermally to give free radicals in a convenient temperature range (i.e., 60 C to 150 C), makes it possible to study polymerizations over an extended temperature range. The form of the rate law with chemical initiations would be given by setting III = 2k (ln)< >i in Eq. (XVI.10.4). Here (In) is the initiator concentration, k I its specific rate constant of decomposition which can usually be measured independently, and is the efficiency with which its radicals initiate chains. The measure of t is subject to the difficulties already indicated in connection with the photolysis systems. ... 

The free radical arylation of thiazole (391) has been performed either by the Gomberg-Bachmann (392) decomposition of aryldiazonium chlorides (119,393), by the thermal decomposition of benzoyl peroxide (394-397) or N-nitrosoacetanilide (398), or by the photolysis of benzoyl peroxide or iodobenzene (398). The three monophenylthiazoles are obtained in the practically constant proportions 2-phenyl, 60% 5-phenyl, 30% 4-phenyl, 10%, giving the order, 2>5>4, of decreasing reactivity of the three positions of thiazole toward phenyl radicals (398). Competition reactions with nitrobenzene (397) gave an estimation of the globed reactivity of thiazole relative to benzene of 0.75 with the partial rate factors f2 = 2.2, /s=1.9, /4 = 0.5. When the thermolysis of benzoyl peroxide is performed in acetic acid solution, the substrate in reaction is the conjugate acid of thiazole the global reactivity is enhanced to 1.25,... 

Kinetic data for the thermal decomposition of benzoyl peroxide in various solvents are given in Table 70 according to rate law (1). Barnett and Vaughan assumed that, by employing low concentrations of peroxide, the observed rate coefficient was equivalent to the first order coefficient, k. In other instances styrene or iodine was used to trap radicals so that induced decomposition would be suppressed and thus the observed rate coefficient would correspond to kf <... 

The effect of substituents upon the rate of thermal decomposition of thenoyl peroxides was similar to that reported for benzoyl peroxide. Hammett constants were used for the 5-substituted-3-thenoyl peroxides (I) and 4-substituted-2-thenoyl peroxides (II), while constants were used for the 5-substituted-2-thenoyl peroxides (III). A p-value of about —0.4 was obtained for the thenoyl peroxides using... 

Kinetic data for thermal decomposition of the related dialkyl peroxydicarbonates are given in Table 112. Variation of the substituent groups R has little effect on the rate coefficients or the activation energy. In addition the activation energies are in the range of those reported for benzoyl peroxides. This suggests a one-bond homolysis reaction. Activation energies for dialkyl peroxydicarbonates with... 

The thermal decomposition of dimethyl trisulfide at 80 °C also results in the formation of di- and tetrasulfides . It was further established that the decomposition proceeds via a radical mechanism, probably homolytic S-S cleavage, since in a weakly polar medium such as benzoyl peroxide and triethylamine, the rate of decomposition is greatly accelerated. 

In the derivation of the kinetic relations it was assumed that free radicals enter the particles one by one the initiation process just described satisfies this condition. This is not the case when radicals are formed by thermal decomposition of an oil-soluble initiator. Such decomposition produces pairs of radicals in the hydrocarbon phase. One would expect a pair of radicals, confined to the extremely small volume of a latex particle, to recombine rapidly. The kinetics of this type of polymerization have been described above. It is recalled here that the subdivision factor, z, and hence rate and degree of polymerization are smaller than 1 and decrease with a. These predictions from kinetic theory are in contradiction to experimental observations. Although some oil-soluble initiators, which are good catalysts in solution systems, are poor initiators in emulsion polymerizations—e.g., benzoyl peroxide—other thermally decomposing peroxides and azo compounds produce polymer in emulsion at rates comparable to those observed in polymerization initiated by water-soluble catalysts, where the radicals enter the particles one by one. Such is the case for cumene hydroperoxide, which at low concentrations yields a rate of polymerization per particle equal to that of a persulfate-initiated reaction. It must therefore be concluded that, although oil-soluble initiators may decompose into radical pairs within the particles, polymer radicals are formed one by one. The following mechanisms are consistent with formation of polymer radicals singly. 

The thermal decomposition rate of benzoyl peroxide at mouth or ambient temperature is much too slow to cure acrylic monomers. At such temperatures initiator accelerator systems are commonly employed. 

An examination of the behavior of peroxides in the course of their rapid thermal decanposition, provides some insight into the nature of the decomposition intermediates and/or products. NMR spectra taken during the thermolysis of various peroxides and per-oxyesters in solution at temperatures where they have short half lives, e.g. benzoyl peroxide and t-butyl peroxypivalate at 110 C, contain emission peaks and/or enhanced absorption peaks. These peaks are presumed to be those of the reaction products resulting from interactions involving the transient decomposition products, e.g. interactions between pairs of caged radicals or from radical pair encounters. The NMR spectra are indicative of the occurrence of chemically induced dynamic nuclear polarization (CIDNP). 

Although NMR spectra without emission peaks are obtained during thermal decomposition of benzoyl peroxide in CCl. at 87 C,... 

The implied relationship between the rapid thermal decomposition of peroxides and photochemical reactions is supported by the generation of CIDNP spectra when benzoyl peroxide is exposed to ultraviolet light in CCl at 25 C. CIDNP spectra are also obtained when di(4-chlorobenzoyl) peroxide and other peroxidic compounds either undergo rapid decomposition at temperatures where they have short half lives or are exposed to ultraviolet light at ambient temperatures. 

Chain growth polymerization has the characteristic of having an intermediate within the process that cannot be isolated [5], The intermediate can be a metal complex, a free radical, or an ion. These intermediates are transient to the process. The terms vinyl, olefin, and addition polymerization have been associated with this process [13], Monomer units add to a chain very rapidly once it has been initiated. Initiation is the creation of an active center such as a free radical or carbanion [13], An example is the thermal decomposition of benzoyl peroxide shown in Figure 3.4. To propagate the chain, an additional monomer is added at a very rapid rate as monomer concentration is reduced. Figure 3.5 shows the propagation of polystyrene. 

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