Intermediates of radiation-induced polymerisation of maleimides studied by ESR

Radicals produced by radiolysis and photolysis of maleimides (maleimide, N-methylmaleimide, N-ethylmaleimide, N-phenyl-maleimide) in bulk have been identified by ESR spectroscopy. γ-Irradiation of maleimides yields up to five different radicals depending on the type of maleimide. Because of the great linewidth of powder ESR spectra, the hyperfine splittings are determined by means of computer simulation. For N-alkyl maleimides the starting species of polymerisation are formed by abstraction of a H-atom from the alkyl group in radiolysis as well as in photolysis. The structure of the propagating radical shows that polymerisation proceeds via the ethylenic double bond. The detection of maleimide ions makes participation of these ions in polymerisation conceivable. Radiolysis of maleimide and N-phenylmaleimide yields the same type of radicals whereas only ions could be detected after u.v.-irradiation.