Reactions Leading to Formation of Initiating Free Radicals

Reactions Leading to Formation of Initiating Free Radicals 

The initiating free radical can come from many sources. Thermal decompositions of compounds with azo and peroxy groups are common sources of such radicals. The radicals can also come from redox reactions or through various light-induced decompositions. Ionizing radiation can also be used to form initiating radicals. 

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Reactions Leading to Formation of Initiating Free Radicals Side reactions can occur here too, such as ... 

Oxidation. AH polyamides are susceptible to oxidation. This involves the initial formation of a free radical on the carbon alpha to the NH group, which reacts to form a peroxy radical with subsequent chain reactions leading to chain scission and yellowing. As soon as molten nylon is exposed to air it starts to discolor and continues to oxidize until it is cooled to below 60°C. It is important, therefore, to minimize the exposure of hot nylon to air to avoid discoloration or loss of molecular weight. Similarly, nylon parts exposed to high temperature in air lose their properties with time as a result of oxidation. This process can be minimized by using material containing stabilizer additives. 

This is still too high an activation energy to account for a simple one-step reaction, which has been reported. On the other hand, if the net increment in solvation energy is as large as 12 kcal., then Step 4 could conceivably be an initiation step in a reasonably long chain mechanism. If, however, a chain reaction is involved, the most likely next steps would be the attack of R radicals on either oxygen or on ozone. The oxygen reaction is likely to be diffusion controlled and would lead to peroxy-type products. However, attack on ozone is more likely to lead to formation of free acid and smaller alkyl radicals, and these are not observed as major products. 

In non-catalytic ethylbenzene oxidation at high temperatures the formation of active free radicals occurs in reaction of chain initiation (RH+O2—>) and under chain decomposition of PEH, the value Spjj to a significant extent should be determined by factor of instability of PEH P = WpEH / (WpEH sum rate of PEH decomposition (thermal (molecular) and chain), Wp j - rate of chain PEH formation). Actually, it turned out that value P in the course of non-catalyzed process of ethylbenzene oxidation is increased at the expense of rise of PEH chain decomposition rate that leads to reduction of Sp [20, 24],... 

Free radical chain polymerisation is the method used to prepare the most common polymers. A free radical is generated and reacts with one molecule of monomer (initiation). Then monomer molecules react with this first species, leading to formation of a long chain by successive additions of monomer (propagation). Finally, chains are terminated by reaction of two chains bearing radicals (termination). As radicals are very reactive species, side reactions are likely to occur and modify the simple process (transfer). 

Gold.— Full details are available of the reactions between benzenethiol and Au and Pt complexes [Au(Me)(L)] and m-[Pt(Me)(L)a] (L = PMea, PMegPh, PMePha, or PPhs) which give methane and [Au(SPh)(L)] and /ran.y-[Pt(Me)(SPh)(L)2] or trans-[Pt(SPh)a(L)2] respectively. Inhibition of these reactions by the radical scavenger galvinoxyl and rapid acceleration by the radical initiator azobis(isobutyronitrile) provide evidence that free-radical chain mechanisms are involved. In contrast, neither of these reagents affects the formation of cis-[Au(Me)2(SPh)(L)] from PhSH and the Au complexes [Au(Me)8(L)], so radical mechanisms do not seem to operate in these reactions. Two mechanisms, (4) and (5) are considered for the radical reactions and the fact that only (4) leads to formation of methyl radicals serves to dis-... 

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