Diene compounds propagating radicals

In an ESR study on the light-irradiated BPO-initiated polymerization of VAc as well as the diene compounds shown below in frozen benzonitrile at — 120 C, Kamachi et al. obtained well-resolved ESR spectra of the propagating radicals depicted in Fig. 26. 

In this article, I have summarized applications of ESR spectroscopy to the radical polymerization of vinyl and diene compounds, with particular emphasis put on the advantages of the special cavities designed for enhancing the sensitivity of ESR spectrometers. These cavities allow determinations of the conformations of propagating radicals as well as the propagation rate constants for several monomers under conditions similar to usual radical polymerizations. However, I am afraid that the reliability of the kinetic data obtained with them is not yet high enough for precise determination of the rate constants because of experimental errors involved in the measurement of radical concentrations and polymerization rates. It is desirable that the ESR spectrometer be made about 10 times more sensitive for really reliable data... 

The allylic cyclohex-2-enyl radical has its unpaired electron delocalized over two secondary carbon atoms, so it is even more stable than the unsubstituted allyl radical. The second propagation step may occur at either of the radical carbons, but in this symmetrical case, either position gives 3-bromocyclohexene as the product. Less symmetrical compounds often give mixtures of products resulting from an allylic shift In the product, the double bond can appear at either of the positions it occupies in the resonance forms of the allylic radical. An allylic shift in a radical reaction is similar to the 1,4-addition of an electrophilic reagent such as HBr to a diene (Section 15-5). 

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