Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4 ESR SPECTROMETRY 31 1Some measure of the mobility of the radicals within the glass may be obtained,since elevation of the temperature from 77" K to 90" K led to the recombinationof the radicals. The variation in viscosity of substances which are glass-like at roomtemperature is less easily controlled, although some experiments have been described'67in which radicals are trapped in plastic films.Secondary radicals may be formed by reaction between primary radicals andsolvent. Thus hydroxyl radicals are produced by the photolysis of hydrogen peroxidein an isopropanol glass'68. At 110" K a seven-line spectrum, with intensitiesof 1 : 6 : 15 : 20 : 15 : 6 : 1, was observed. Such a structure is to be expected froman unpaired electron which interacts equally with six protons, and since it is knownthat there is no interaction with the proton of the -OH group in alcohol radicals,the spectrum suggests that the radical is (CH,),COH.Electron resonance spectroscopy has been used also in the investigation of vinylpolymerization in the solid phase. Thus Bamford et have studied theradicals produced during the photochemically initiated solid phase polymerizationof methacrylic acid, and the work demonstrates the scope of ESR methods. Theway in which the radical concentration varies with the temperature at which thereaction is carried out yields valuable information about the reaction mechanism.The spectrum normally consists of nine lines, and is believed to originate from themethacrylic acid radical. If, however, the polymerization is carried out at temperaturesbelow - 5" C, then the spectrum is replaced by a more complex one containingthirteen lines. The new spectrum is ascribed'69 to an abnormal conformation ofthe methacrylic acid radical.Polymerization studies have, in fact, been an important application of ESRspectroscopy. Although the stationary radical concentrations in radical polymerizationsare only just within the limit of sensitivity of ESR spectrometers, two typesof polymerization process are self-trapping, so that the radicals may be detected.A number of cross-linked vinyl polymers form a 'gel' during the course of polymerization,and the active radicals present at this stage of the reaction may be immobilisedwithin the gel. Fraenkel et ~ 1.'~' made the initial observations on systemsof this kind, using a number of monomers and initiators. Bamford et ~ 1.'~' showedthat similar steric trapping occurred when the polymer formed was insoluble inthe monomer. Thus polyacrylonitrile is insoluble in acrylonitrile, and is precipitatedout during the course of reaction. Quite high concentrations of radicals-upto 10'' spins per gram-become trapped in the dead polymer cage. Kinetic studies,such as those of Atherton et may be made by determination of the trappedradical concentrations after differing reaction times.Intermediate species in the gas phase may also be studied by electron resonancemethods, although the spectra obtained are usually more complex than those observedwith condensed phases. In small gaseous radicals and atoms with degenerateorbital states, the orbital angular momentum will not be quenched, and may makea contribution to the paramagnetism of the species. Even in orbitally non-degener-References pp. 336-342