Peroxide linkage radicals

AH = 34.7+0.1 kcal.mole and AS = 10.5 0.2 eu, which are similar to homolysis of the peroxide bond in di-t-butyl peroxide (see the section on peroxides). Furthermore, pyruvic acid (CH3COCO2H) is a product of the reaction, which is an expected product from homolysis of the peroxide bond. Apparently decomposition via (X) is of minor importance compared to free radical decomposition of the peroxide linkage. 

The thermal scission of a compound is the most common means of generating radicals to initiate polymerization. The number of different types of compounds which can be used as thermal initiators is rather limited. Compounds with bond dissociation energies in the range 100-170 kJ/mol are usually suitable. (Others with higher or lower dissociation energies will dissociate too slowly or too rapidly to be useful.) The major class of compounds with bond dissociation in this range contain the 0-0 peroxide linkage. There are numerous varieties of compounds of this type and some are listed in Table 6.4. 

It decomposes by random cleavage to form shorter diradical species, which become incorporated into the parent polymer backbone. When fresh monomer is added and additional heating is supplied, the remaining peroxide linkages in the polymer backbone decompose to form terminal radicals and polymerization resumes leading to a block copolymer. 

The mechanism of transfer to BPO involves homolytic attack on one of the oxygen atoms of the peroxidic linkage (Scheme 3.16) with formation of an ester end group and expulsion of a benzoyloxy radical. The end group formed (a secondary ester) is distinct from that fonned in initiation. Such end groups may contribute to the reduced thermal stability of high conversion PS prepared with benzoyl peroxide (Section 8.2. l). In the case of VAc or VC polymerizations the chain end will be a hydrolytically unstable ketal or a-chloroester group respectively (Section 8.2.3). 

Encina MV, Lissi E, Sarasua M, Garagallo L, Radic D (1980) Ultrasonic degradation of polyvinylpyrrolidone effect of peroxide linkages. J Polym Sci Polym Lett Ed 18 757-760... 

PS Acetophenone, benzophenone, enones, diketones, phenylacetaldehyde, sucdnimides, benzoyl peroxide, in chain peroxide linkages, hydroperoxides, polycyclic aromatic hydrocarbons, Fe derivatives, Co salts of fatty acids, AICI3, silica-alumina catalyst Hydrogen, benzene, conjugated double bonds, methane, ethylene, radicals, crosslinks Water, CO2, ketones, unsaturations, hydrtperoxides, radicals, chain scissions, quinomethane structures... 

This stage is a stage of the forced chain branching (i.e. it leads to appearance on the new OH radical), which is an initiator of the chain process of carbohydrate oxidation. The macro-radical RO is usually less active and could cause the chain to break according to the reaction (6.12) with formation of the peroxide linkage ... 

Hydroperoxides are very rapidly reduced by both alkyl and aryl tertiary phosphines. The rate of reaction of BuOOH with a series of phosphines decreases in the order Bu3P > Et3P > Ph3P. The reaction is not inhibited by free radical traps and the mechanism involves nucleophilic attack at the peroxide linkage. 

Interest in the chemistry and biochemistry of peroxides has been recently stimulated by the fact that the important class of natural products, the prostaglandins, has two members (PGG and PGH) that contain the peroxide linkage. Further, the proposed mechanism for the biosynthesis of these peroxides involves a novel peroxy radical bicyclization. Thus, one mechanism for biosynthesis of PGG, proposed in 1967 ( 1,, involves an autoxidative-... 

An alternate pathway available to however, is intramolecular radical attack on the peroxide linkage (10,11) yielding ultimately the epoxy-alcohol product. We wished to investigate this Sfli pathway systematically and sought other, more controlled methods for the preparation of radicals like... 

As noted earlier, these are products expected from a 3-peroxy radical the cyclic peroxide resulting from H atom abstraction, the epoxy-alcohol from Sjji radical attack on the peroxide linkage. 

---