Halobenzenes as sensitizers for the radiation‐induced polymerization of styrene

Abstract

Halogenated benzenes, carbon tetrachloride, and other compounds have been used to sensitize the gamma‐ray‐induced polymerization of styrene. Benzene and fluorobenzenes are about equally effective, while the data from solutions of mono‐ and dihalogenated benzenes indicate that the order of increasing sensitivity to radiation is: fluorobenzenes < chlorobenzenes < bromobenzenes < iodobenzenes. One halogen is less effective than two, and the ortho isomers are more effective than the meta and para. The relations between the G values for radical production in the solutions and the electron fractions of solvent are linear only for benzene and the monohalogenated aromatics. With very highly halogenated benzenes and carbon tetrachloride, the departure from linearity is very marked. Significant sensitization is observed at very low electron fractions with the highly halogenated materials, except with hexafluorobenzene, which appears to be very stable to radiation. The generalizations concerning the effects of the kind and number of substituents in mono‐ and dihalogenated benzenes are not valid for more highly halogenated benzenes. Much of the curvature observed at high concentrations of carbon tetrachloride is apparently due to changes in the rate constants as the monomer concentration is changed. At low concentrations this is not the case with carbon tetrachloride and presumably also with the other highly halogenated materials. Various reaction mechanisms are considered, and one is advanced that reproduces many of the important features of the observed data.