Free radicals in diene polymers induced by ultraviolet irradiation. I. An ESR study of <i>cis</i>‐1.4‐poly(isoprene)

Abstract

Abstract Two different samples of purified cis ‐1.4‐poly(isoprene) were irradiated in vacuo at 77°K with ultraviolet light from a medium pressure mercury lamp. The free radicals formed by this irradiation were studied by electron spin resonance (ESR). The spectra obtained were interpreted as due to radicals formed by scission of the CC bonds midway between the double bonds. However, no radicals formed by hydrogen abstraction were observed. The radicals which contributed to the ESR spectra were: magnified image In addition the spectra contained a weak singlet interpreted as due to polyenyl radicals: magnified image with n equal to 3. The radical concentration was found to be dependent on the temperature. At liquid nitrogen temperatures all radicals were stable, but near the glass transition temperature the radical concentration decreased sharply. A sample of purified trans ‐1.4‐poly(isoprene) showed after UV irradiation the same ESR spectra as cis ‐1.4‐poly(isoprene), and was accordingly interpreted in the same way. The microstructural changes of poly(isoprene) caused by UV irradiation are discussed in relation to the identified radicals.