Electron paramagnetic resonance spectroscopic study of the photoinitiated polymerization of alkyl methacrylates
Within a continuous-flow system, dilute oxygen-free solutions of 2,2′-azobis[isobutyronitrile], RN:NR, have been irradiated with near-ultraviolet mercury radiation, this occurring as the liquid passed through the cavity of an X-band electron paramagnetic resonance, epr, spectrometer. By this means transient radicals have been observed. The solvents used were toluene, methyl methacrylate, ethyl methacrylate, isobutyl methacrylate, n-butyl methacrylate, and separate mixtures of toluene with each of these esters. The flow rate was varied 30-fold and the temperature from 25 to 39° with little appreciable effect, except as noted. With toluene as the solvent, the only epr signal observed was that from the R· radical. As the toluene was replaced by an increasing proportion of methyl methacrylate keeping the other reaction conditions unchanged, the signal from the R· radical fell in intensity and a new absorption took its place, growing progressively more intense so that when the solvent was undiluted methyl methacrylate there was only a small signal from the R· radical. The new absorption is consistent with addition radicals of structure R-(-CH2-C(COOCH3)-CH3-)n-CH 2-Ċ(COOCH3)CH3 with the size of n unknown. Kinetic arguments are given which suggest that the average of n may be at least about ten. The results obtained with the use of the other alkyl methacrylates were very similar to those found for methyl methacrylate and are interpretable in a like fashion.
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