Radical scission

Phosphoms-containing additives can act in some cases by catalyzing thermal breakdown of the polymer melt, reducing viscosity and favoring the flow or drip of molten polymer from the combustion zone (25). On the other hand, red phosphoms [7723-14-0] has been shown to retard the nonoxidative pyrolysis of polyethylene (a radical scission). For that reason, the scavenging of radicals in the condensed phase has been proposed as one of several modes of action of red phosphoms (26). 

The main route for the photodegradation of the polymers 10 and 11 in the solid films in air consists of radical scission of the silicon-silicon bonds, analogous to that observed in solution, but in the films, the resulting silyl radicals react with oxygen to give... 

Deoxygenation of alcohols by means of radical scission of their corresponding thiocarbonyl derivatives. 

Table III. Calculations of Radical, Scission, and Crosslink Concentrations in Irradiated Polystyrene...

Selective bond rupture at entanglement points, or other such sites of stress concentration, could magnify the effect of a chain scission in the presence of an external stress, but it seems unlikely that this is occurring since the sol-gel data actually indicated a (slightly) lower ratio of scissions to crosslinks with an imposed stress. It also is difficult to visualize how the formation of free radicals, scissions, and crosslinks could directly cause the radiation expansion noted under no stress. Therefore, the mechanism of accelerated creep is probably not caused by the formation and reaction of macromolecular free radicals in the polymer specimens. 

In view of the possibility of random free-radical scission of some covalent bonds, despite a great deal of dissipation of energy in the same sample, this repolymerization may be the likely interpretation, as the effect could result from recombination (or rather free-radical condensation) in the compressed material. 

Radical scission of an 5-benzyl derivative with tributylstannane initiated by azo-bis(isobutyronitrile) (AIBN) in refluxing benzene generates a tributylstannyl thiolate that can be used directly in thioglycosidation reactions as shown in Scheme 5.11.2812[Pg.367]

The formation of p-hydroxyphenylacetic add in the foregoing reaction is noteworthy. Clearly the standard radical scission mechanism depicted in Scheme 1.21 cannot apply (see section 1.2.12). Instead, the mechanism shown in Scheme 7.32 has been proposed 65... 

In some cases, the mechanism for the formation of at least some of the product(s) of reaction appears to be radical scission of the anion produced initially. 

Other reactions that may involve radical scission of anion intermediates include the rearrangement of anions adjacent to either trivalent or tetrava-lent nitrogen. 

In this example, an anion formed initially undergoes radical scission, recombination, and then anionic rearrangement to the final product. 

It is important to recognize that scission does not necessarily stop after reaction of initial alkoxyl radicals. Scissions of secondary products generated during lipid oxidation also contribute to propagation and to the ultimate product mix (346). Malonaldehyde is perhaps the best known example of this, as will be discussed further in Section 4.2. 

Despite that it is named as such quite often in the literature, the radical -scission or -cleavage is not a fragmentation in the sense considered in this chapter. The flrst example of a macrolide synthesis through such a reaction type was achieved by Ohloff ° and resulted in an industrial synthesis of exal-tolide (134 Scheme 70). Schteiber has described iron-copper-promoted fragmentation reactions of a-al-koxy hydroperoxides as a route to macrolides. Two recent informative publications which cover the prior literature have been written by Suginome and Macdonald. ... 

Free Radical Scission of HA Chains Contribute to HA Turnover... 

Scission of HA can occur by free radicals under oxidative conditions, and is promoted by transition metal ions [179-181]. The exact proportion of total HA turnover contributed by free-radical scission in the vertebrate organism is not known. 

There is a vast literature describing the scission of HA by free radicals. A major challenge to the field would be studies that integrate free radical scission of HA with the physiological situations, using cultured cells or cell-free extracts. A more extensive description of HA scission by free radicals is presented elsewhere in this review. 

Reactions of Diazonium Salts. Many of the reactions of diazonium salts appear to proceed by free radical mechanisms. Thus, depending upon the environment of the diazonium salt, it is possible that cleavage might occur by free radical scission (A) or by ionization (B) ... 

The proposal of Clarke et al eliminates the requirement of strongly basic sites for proton removal from C-H, a major drawback of most schemes. Assuming that radicals can be generated by thermolysis of surface methoxyls, and that the radical scission of Clarke et al occurs, the following initiation mechanism would be plausible ... 

Studies carried out by Gardette and co-workers [26] provided further data on the reactions undergone by PET under UV irradiation. They confirmed the probable initial radical scission reactions undergone by the PET chain, where the three possible pairs of radicals formed from the initial structure 0(C=0)Ph(C=0)0CH2CH2 were ... 

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