P-scission fragmentation

The reactivity of the monomer and the reaction conditions determine the relative importance of P-scission. Fragmentation reactions are generally favored by low monomer concentrations, high temperatures and low pressures. Their significance is greater at high conversion. They may also be influenced by the nature of the reaction medium. 

Many radicals undergo fragmentation or rearrangement in competition with reaction with monomer. For example, f-butoxy radicals undergo p-scission to form methyl radicals and acetone (Scheme 3.6). 

Stansbury and Bailey. A review by Colombam on addition-fragmentation processes is also relevant. Monomers used in ring-opening are typically vinyl (e.g. vinylcyclopropane - Scheme 4.20 Section 4.4.2.1) or methylene substituted cyclic compounds (e.g. ketene acetals - Section 4.4.2.2) where addition to the double bond is followed by p-scission. 

The p-scission of a phosphoniumyl radical yields a cation and a phosphonyl radical, while its reaction with a nucleophile generates a phosphoranyl radical which can undergo SET reactions and a- or p-fragmentations (Scheme 14). 

In the presence of the DNA -Lo-P/P-Lo-DNA combination, DNA S5 is first hydrolyzed by Ce(iv)/EDTA (lane 2), and the scission fragments are purified by PAGE. To this reaction mixture, an oligonucleotide having a monophosphate at the 5 -end (DNA< igated) arbitrary sequence and length) and DNA(template22) template are added, and the mixture... 

The P-scission process with peroxy radicals that are highly resonance stabilized, such as with triphenyl methyl peroxyls may be observed even at a markedly lower temperature. The regeneration of alkyl radicals in a reaction system may however proceed through numerous fragmentation and transfer reactions which makes the correct determination of ceiling temperature of peroxy radicals backward decomposition more complicated. 

Since larger free radicals are more stable than those with small molecules, the fragmentation in the middle of the polymeric chain is favored thermodynamically compared to the formation of small molecules. However, kinetic factors also may play a role in determining the abundance of a specific compound. The formation of small radicals from the end of a polymeric chain can be kinetically favored, and, as a result, formation of small radicals in the initiation step is more common than expected based on the thermodynamic criteria. Taking as an example polystyrene, an end chain p-scissions to the aromatic ring can be written as follows ... 

It is interesting that cracking splits the cyclohexane into C2 and C4 fragments rather than into two C3 fragments. If the adsorbed species were singly bonded cyclohexyl, then P scission should produce a secondary alkyl, and a second P scission would produce propylene and leave adsorbed isopropyl which should yield a second propylene molecule. 

The formation of ethylene and butadiene seems to argue that the intermediate which leads to these products is a di-o adsorbed cyclohexane fragment, from which two P scission events would liberate ethylene and leave adsorbed C4 which coulp desorb as butadiene. 

Radicals also undergo fragmentation reactions. Most of these are p>-scission reactions, such as illustrated by decarboxylation and fragmentation of alkoxy radicals, but decar-bonylation, an a-cleavage, is also facile. 

The only path for this bicyclic oxy radical is fragmentation via favorable P-scission reactions. For the above radical, such scission would produce... 

Experimental support for direct fragmentation of 60 was found in studies of pyrazoline oxide 65, which cannot cyclize to 67 without engendering about 22 kcal/mol of ring strain. The fact that 65 gives a 34% yield of ethylene shows that it fragments directly to 66, which implies that 60 follows the same mechanism. Isomeric radical 68 gave no ethylene because P-scission would produce 69, which is far less stabilized than 66 (see Table I and its associated discussion below). 

Another important pathway by which alkoxyl radicals form secondary prodncts is via P-scission reactions. Here, cleavage of the carbon-carbon bond on the side opposite from the conjugated double bonds results in an alkanyl radical fragment... 

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