Initiator of free radical

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

The addition of halogenated aliphatics to carbon-carbon double bonds is the most useful type of carbon-carbon bond forming synthetic method for highly halogenated substrates Numerous synthetic procedures have been developed for these types of reactions, particularly for the addition of perfluoroalkyl iodides to alkenes using thermal or photolytic initiators of free radical reactions such as organic peroxides and azo compounds [/]... 

Addition of phosphonyl radicals onto alkenes or alkynes has been known since the sixties [14]. Nevertheless, because of the interest in organic synthesis and in the initiation of free radical polymerizations [15], the modes of generation of phosphonyl radicals [16] and their addition rate constants onto alkenes [9,12,17] has continued to be intensively studied over the last decade. Narasaka et al. [18] and Romakhin et al. [19] showed that phosphonyl radicals, generated either in the presence of manganese salts or anodically, add to alkenes with good yields. 

The decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

The initiation of free radical reactions by ozone in the gas phase at elevated temperatures occurs due to ozone monomolecular decomposition [131,132] ... 

The first term characterizes the initiation of free radicals by the reaction of ozone with cyclohexane and the second chain reaction with chain propagation... 

Metals and metal oxides, as a rule, accelerate the liquid-phase oxidation of hydrocarbons. This acceleration is produced by the initiation of free radicals via catalytic decomposition of hydroperoxides or catalysis of the reaction of RH with dioxygen (see Chapter 10). In addition to the catalytic action, a solid powder of different compounds gives evidence of the inhibiting action [1-3]. Here are a few examples. The following metals in the form of a powder retard the autoxidation of a hydrocarbon mixture (fuel T-6, at T= 398 K) Mg, Mo, Ni, Nb V, W, and Zn [4,5]. The retarding action of the following compounds was described in the literature. 

There are numerous in vitro and in vivo studies, in which the damaging free radical-mediated effects of iron have been demonstrated. Many such examples are cited in the following chapters. However, recent studies [170,171] showed that not only iron excess but also iron deficiency may induce free radical-mediated damage. It has been shown that iron deficiency causes the uncoupling of mitochondria that can be the origin of an increase in mitochondria superoxide release. Furthermore, a decrease in iron apparently results in the reduction of the activity of iron-containing enzymes. Thus, any disturbance in iron metabolism may lead to the initiation of free radical overproduction. 

AIBN is a very common initiator of free radical reactions. The radical derived from its fragmentation abstracts H from Ph3SnH to give Ph3Sn. ... 

Rasmussen and co-workers. Chapter 10, have shown that many free-radical polymerizations can be conducted in two-phase systems using potassium persulfate and either crown ethers or quaternary ammonium salts as initiators. When transferred to the organic phase persulfate performs far more efficiently as an initiator than conventional materials such as azobisisobutyronitrile or benzoyl peroxide. In vinyl polymerizations using PTC-persulfate initiation one can exercise precise control over reaction rates, even at low temperatures. Mechanistic aspects of these complicated systems have been worked out for this highly useful and economical method of initiation of free-radical polymerizations. 

Diaroyl peroxides and diacyl peroxides without a-branches are significantly more thermally stable than those with mono- or di-cr-substituents. Tlie primary use of most commercial diacyl peroxides (16. R1 = R2 = alkyl or aryl) is initiation of free-radical reactions. 

The main industrial use of rert-alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. 

The vitamin can be carried by the high-affinity receptor for low-density lipoprotein in fibroblasts [251]. It has been suggested to act in vivo as a specific enzyme inhibitor (of lipoxygenase). A vitamin-E-lipoxygenase complex in vivo could terminate the initiation of free radicals and other oxidized products. The binding of the vitamin to the enzyme is probably through the hydrophobic chain and involves one peptide [135]. 

Diazo compounds (diazenes) are an important class of compounds typically used as initiators of free-radical reactions. Upon heating, diazenes undergo decomposition as illustrated in Scheme 4.3. The concerted pathway is important for symmetrical diazenes (R = R ), while unsymmetrical diazenes (R f R) decompose via the stepwise pathway, especially when R is more stable than R . 

Diacyl peroxides are another important source of free-radicals and, consequently, are also commonly used as initiators of free-radical reactions. There is a vast amount of data pertaining to the kinetics and mechanism of decomposition of these compounds in conventional solvents there are a number of side reactions, both radical and ionic in nature, that complicate the kinetics of their decomposition. Generally, these compounds decompose by initial 0-0 bond cleavage that generates carboxyl radicals (RC02 ), which subsequently decarboxylate yielding R (Scheme 4.7)... 

V Organometallic transition metal complexes as initiators of free radical polymerization 8... 

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