Diacyl peroxides, decomposition

Diacyl peroxide decompositions also are cataly2ed by the metal ions of copper, iron, cobalt, and manganese ... 

The CIDNP technique gives much useful information about diacyl peroxide decomposition. We shall discuss an example from the work of Ward, the decomposition of propionyl benzoyl peroxide (14), which in the presence of I2 yields the products in Equation 9.27.84 Figure 9.1 shows the proton magnetic resonance spectrum recorded while the reaction is occurring at 100°C. The first step... 

Diacyl peroxide decomposition can yield two types of radicals (RCOO, R ) ... 

M. A. Turovskyj, Yu, S. Tsehnskij Quantum chemical analysis of diacyl peroxides decomposition activated by quaternary ammonium chloride salts. Ukrainian Chem. Bull 1994, 60, 16... 

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 ... 

Aromatic diacyl peroxides such as dibenzoyl peroxide (BPO) [94-36-0] may be used with promoters to lower the usehil decomposition temperatures of the peroxides, although usually with some sacrifice to radical generation efficiency. The most widely used promoter is dimethylaniline (DMA). The BPO—DMA combination is used for hardening (curing) of unsaturated polyester resin compositions, eg, body putty in auto repair kits. Here, the aromatic amine promoter attacks the BPO to initially form W-benzoyloxydimethylanilinium benzoate (ion pair) which subsequentiy decomposes at room temperature to form a benzoate ion, a dimethylaniline radical cation, and a benzoyloxy radical that, in turn, initiates the curing reaction (33) ... 

Peroxyesters, particularly those with a-hydrogens or conjugated double bonds, are susceptible to iaduced decomposition under certain conditions, but they are generally less susceptible than diacyl peroxides. Lower molecular weight peroxyesters that have some water solubiUty can be hydroly2ed. 

This is basically the same type of induced decomposition that occurs with other peroxide classes, eg, the dialkyl peioxydicaibonates and diacyl peroxides. Table 8. Commercial rerf-Alkyl Hydroperoxides ... 

Chemical Properties. Diacyl peroxides (20) decompose when heated or photoly2ed (<300 mm). Although photolytic decompositions generally produce free radicals (198), thermal decompositions can produce nonradical and radical iatermediates, depending on diacyl peroxide stmcture. Symmetrical aUphatic diacyl peroxides of certain stmctures, ie, diacyl peroxides (20, = alkyl) without a-branches or with a mono-cx-methyl... 

Of these diacyl peroxides the ones that generate the most stable radicals (R ) are the most unstable diacyl peroxides. Most other diacyl peroxides decompose by competing free-radical and polar decomposition, ie, carboxy iaversion (188). Carboxy iaversion occurs to a much greater extent with certain diacyl peroxides having unsymmetrical diacyl peroxide stmctures (52,187,188,199) ... 

Diacyl peroxides (20, = alkyl oi aiyl) also undergo three types of radical induced decomposition (187) all of which produce the radical... 

As a consequence of their susceptibility to radical-induced decomposition, neat and concentrated solutions of diacyl peroxides undergo self-accelerating... 

Alkyl peroxyesters are much less sensitive to radical-induced decompositions than diacyl peroxides. Induced decomposition is only significant in peroxyesters containingnonhindered a-hydrogens or a, P-unsaturation (213,242). 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are intitially formed lose CO2 very rapidly. In the case of aroyl peroxides, products may be derived from the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

Aryl migrations are promoted by steric crowding in the initial radical site. This trend is illustrated by data from the thermal decomposition of a series of diacyl peroxides. The amount of product derived from rearrangement increases with the size and number of substituents ... 

The thermal decomposition of diacyl peroxides has been the most frequently employed process for the generation of alkyl radicals. The rate and products of the unimolecular decomposition of acetyl peroxide have been the subject of many studies. Acetyl peroxide decomposes at a convenient rate at 70-80°C both in the solution and in the gas... 

The decomposition of diacyl peroxides provides a fairly clean method for the production of alkyl radicals and, therefore, it has been used in most quantitative and preparative studies of the alkylation of heterocyclic compounds,... 

Goldschmidt and Minsinger " isolated some of the products formed during the decomposition of several diacyl peroxides in pyridine (Table IV). The yields obtained are so far the highest reported for any free-radical alkylation. 

Goldschmidt and Beer have examined the products formed during the thermal decomposition of diacyl peroxides of the type [COgMe —(CHziw—CHz—COO] 2, where n = 1 and 3, in the presence of a series of organic compounds including pyridine and acridine. The products and yields of the reaction with some aromatic and heterocyclic compounds are shown in Table VI. As expected, acridine and... 

Qiu et al. [11] reported that the aromatic tertiary amine with an electron-rich group on the N atom would favor nucleophilic displacement and thus increase the rate of decomposition of diacyl peroxide with the result of increasing the rate of polymerization (Table 1). They also pointed out that in the MMA polymerization using organic peroxide initiator alone the order of the rate of polymerization Rp is as follows ... 

Azo-compounds and peroxides undergo photodecomposition to radicals when irradiated with light of suitable wavelength. The mechanism appears similar to that of thermal decomposition to the extent that it involves cleavage of the same bonds. The photodecomposition of azo-compounds is discussed in Section 3.3.1.1.2 and peroxides in Sections 3.3.2.1.2 (diacyl peroxides) and 3.3.2.3.2 (peroxyesters). Specific photoinitiators are discussed in Section 3.3.4. It is also worth noting that certain monomers may undergo photochemistry and direct photoinitiation on irradiation of monomer is possible. 

Diacyl peroxides are particularly prone to induced decomposition (Scheme 3.16). Transfer to initiator is of greatest importance for polymerizations taken to high conversion or when the ratio of initiator to monomer is high. It has been shown that, during the polymerization of S initiated by BPO, transfer to initiator can be the major pathway for the termination of chains.7,41... 

The rates of thermal decomposition of diacyl peroxides (36) are dependent on the substituents R. The rates of decomposition increase in the series where R is aryl-primary alkyKsecondary alkyKtertiary alkyl. This order has been variously proposed to reflect the stability of the radical (R ) formed on (i-scission of the acyloxy radical, the nucleophilicity of R, or the steric bulk of R. For peroxides with non-concerted decomposition mechanisms, it seems unlikely that the stability of R should by itself be an important factor. 

Observed rates of disappearance for diacyl peroxides show marked dependence on solvent and concentration.146 In part, this is a reflection of their susceptibility to induced decomposition (see 3.3.2.1.4 and 3.3.2.1.5). However, the rate of disappearance is also a function of the viscosity of the reaction medium. This is evidence for cage return (see 3.3.2.1.3).143 The observation144 of slow scrambling of the label in be.x LQy -carbonyl- %0 peroxide between the carbonyl and the peroxidic linkage provides more direct evidence for this process. 

Diacyl peroxides may also undergo non-radical decomposition via the carboxy inversion process to form an acylcarbonate (Scheme 3.27).46 The reaction is of greatest importance for diaroyl peroxides with electron withdrawing substituents and for aliphatic diacyl peroxides (36) where R is secondary, tertiary or ben/,yl.157 The reaction is thought to involve ionic intermediates and is favored in polar solvents 57 and by Lewis acids.158 Other heterolytic pathways for peroxide decomposition have been described.150... 

Other radicals present in the reaction medium may also induce the decomposition of BPO and other diacyl peroxides. These include initiator-derived140 and stable radicals e.g. gal vinoxyl,132 triphcnylmcthyl164,165 and nitroxides166). 

The decomposition of diacyl peroxides (36) is catalyzed by various transition metal salts,46,167 for example, Cu+ (Scheme 3.28).168,169 A side reaction is oxidation of alkyl radicals by the oxidized fonn of the metal salt e.g. Cu2+). 

Nitro- and nitroso-compounds,170171 amines, and thiols induce the decomposition of diacyl peroxides in what may be written as an overall redox reaction. Certain monomers have been reported to cause induced decomposition of BPO. These include AN,172 A -vinylcarbazole,17,177 Ar-vinylimidazole178 and NVP.177... 

Laboratory studies have generally focused on the diisopropyl, dicyclohexyl and di-t-butyl derivatives. These and the. s-butyl and 2-cthylhcxyl derivatives arc commercially available.189 The rates of decomposition of the peroxydicarbonates show significant dependence on the reaction medium and their concentration. This dependence is, however, less marked than for the diacyl peroxides (36) (see 3.3.1.1.4). Induced decomposition may involve a mechanism analogous to that described for diacyl peroxides. However, a more important mechanism for primary and secondary peroxydicarbonates involves abstraction of an cx-hydrogen (Scheme 3.31).190... 

---