Peroxides, as radical initiators

Aramid particles that contain a peroxide as radical initiator can be used for the vulcanization of an elastomer [77]. Rubber with excellent hysteresis and heat buildup without other adverse effects are obtained. 

Low-density polyethylene (LDPE) can be made by radical polymerization of ethylene using peroxides as radical initiators. LDPE is soft and transparent and is made into films using a blow-molding technique. 

Zhao and Du s group [63] developed a metal-free cross-dehydrogenative coupling (CDC) of various 2-(A -arylamino)aldehydes 79 for direct aryl-aldehyde C (sp )-C(sp ) bond formation to provide a convenient approach for the synthesis of biologically important acridone derivatives 80 (Scheme 19). PIDA was used in combination with a substoichiometric amount of benzoyl peroxide as radical initiator for this oxidative intramolecular annulation reaction which presumably proceeds via the intermediacy of acyl radicals 81. 

The addition of carbon tetrachloride to olefin double bonds catalyzed by peroxides as radical initiator or by transition metal complexes is known as Kharasch reaction [59]. The reaction proceeds in a... 

Zard and Quiclet-Sire described the use of 2-propanol as solvent and hydride donor in conjunction with lauroyl peroxide as radical initiator for xanthate deoxygenation. Collidine is a necessary additive to prevent hydrolysis of acid sensitive functionalities by the lauric acid by-product. 

Polymerization of styrene is carried out under free radical conditions often with benzoyl peroxide as the initiator Figure 1111 illustrates a step m the growth of a poly styrene chain by a mechanism analogous to that of the polymerization of ethylene (Sec tion 6 21)... 

Work should be on a scale of <0.5 g for novel but potentially explosive material until the hazards have been fully evaluated and <5 g for established, commercially available, substances such as peroxide free-radical initiators. 

MMA onto cellulose was carried out by Hecker de Carvalho and Alfred using ammonium and potassium persulfates as radical initiators [30]. Radical initiators such as H2O2, BPO dicumylperoxide, TBHP, etc. have also been used successfully for grafting vinyl monomers onto hydrocarbon backbones, such as polypropylene and polyethylene. The general mechanism seems to be that when the polymer is exposed to vinyl monomers in the presence of peroxide under conditions that permit decomposition of the peroxide to free radicals, the monomer becomes attached to the backbone of the polymer and pendant chains of vinyl monomers are grown on the active sites. The basic mechanism involves abstraction of a hydrogen from the polymer to form a free radical to which monomer adds ... 

Interaction of ethylene and carbon tetrachloride at elevated temperatures and pressures, initiated with benzoyl peroxide as radical source, caused violent explosions on several occasions. Recommended precautions include use of minimum pressure and quantity of initiator, maximum agitation, and presence of water as an inert moderator of high specific heat. 

A discussion of some of the radical sources used for mechanistic studies was given in Section 12.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as reaction initiators, are also important in synthetic chemistry. 

Perfluoro-M-alkyl diacyl peroxides decompose homolytically to give perfluoroalkyl radicals under mild conditions, and radicals formed in such a manner have been used synthetically or as radical initiators for polymerizations [66,67]. 

Radical cyclization with iodine atom transfer of a highly functionalized propiolic ester 103 using dibenzoyl peroxide as an initiator gave the a-methylene-y-butyrolactone 104 in good yield [95T11257]. The relative stereochemistry at carbon atoms 4 and 5 are established during the reaction. The intermediate 104 has been converted to the anti-tumor agent (-)-methylenolactocin 105. 

The formation of oxygen- and carbon-centered radicals by the thermolysis of peroxides or azo compounds is well known. Today, these compounds have been also used as radical initiators. For example, treatment of a CC14 solution of toluene and Af-bromosuccinimide (NBS) in the presence of a catalytic amount of benzoyl peroxide in refluxing conditions gives benzyl bromide in good yield as shown in Scheme 1.5. This is called the Wohl-Ziegler reaction. 

Heat resistant IPN systems were obtained by simultaneous radical polymerization of divinylbenzene with benzoyl peroxide as an initiator and Zn acetate as cyclotri-merization catalyst [122], Hot-curing composition contains BPA/DC, BMI, epoxide resin, Zn acetate and divinylbenzene [123]. Crosslinked compositions consisting of BPA/DC and BPA bis(vinylbenzyl) ether show Tg values above 240 °C [124]. 

In 1967, Heiba and Dessau reported perhaps one of the earliest examples of a radical cychzation cascade that is initiated by intermolecular addition of C-centered radicals to alkynes. Reaction of carbon tetrachloride with 1-heptynes 1 in the presence of benzoyl peroxide (BPO) as radical initiator resulted, among other products, in the formation of 1,1-dichlorovinylcyclopentane derivatives 2 in moderate yields (Scheme 2.1). °... 

The reaction course of hydrogenation and the composition of hydrogenated products strongly depend on polarity and donor strength of the used solvent. No hydrogenation reaction was observed in nonpolar or less polar solvents, such as hydrocarbons and aromatics. In these solvents the reaction starts by the addition of radical initiators, e.g., AIBN or dibenzoyl peroxide. The radical initiated reactions result in the formation of SiHCl3. A similar course was observed in less polar ethereal solvents, such as 1,4-dioxane, anisole, and dibenzyl ether. Radical steps seem to play an important role for the hydrogenation in these solvents. 

A number of radical reactions involving substitution at an allyUc position, for example with TV-bromosucdnimide (NBS), are useful synthetic methods. NBS, in the presence of dibenzoyl peroxide as an initiator, reacts more rapidly at a secondary rather than a primary allylic position. 

Peroxide (Section 15.2) A reactive organic compound with the general structure RO—OR. Peroxides are used as radical initiators by homolysis of the weak 0—0 bond. 

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