Vinyl radicals electron spin resonance

The term free radical is often used in the context of a reactive intermediate, as in the case of polymerization of vinyl monomers, but the same structure (unpaired electron) can and does exist in a kind of immobilized environment. For example, a bulk-polymerized (monomer and initiator only in the polymerization system) poly(methyl methacrylate) (PMMA) contains an appreciable number of free radicals that can be detected by electron spin resonance (ESR) [1]. When the polymerization system becomes highly viscous toward the end of the bulk polymerization, gel formation occurs and immobilizes the growing end of free radical chain growth polymerization, preventing recombination of two free radical ends of growing chains. 

Finally, free radicals formed during vinyl polymerization can be observed directly by electron spin resonance measurements. 

Zarycz N, Botek E, Champagne B, SciannameaV, Jerome C, Detrembleur C. Joint theoretical experimental investigation of the electron spin resonance spectra of nitroxyl radicals application to intermediates in in situ nitroxide mediated polymerization (in situ NMP) of vinyl monomers. J Phys Chem B 2008 112 10432-10442. 

Three different types of radicals were identified by electron spin resonance (ESR) spectroscopy. In the first case, isolated poly(maleic anhydride) contained one radical identifiable as the MA propagating radical. This same radical was shown to be capable of polymerizing styrene and copolymerizing styrene-methyl methacrylate mixtures. It was also shown that the two other radicals were transformed into active species capable of the cationic polymerization of isobutyl vinyl ether or vinyl ethers in the presence of monomeric... 

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