The Radiation-Induced cis-trans Isomerization of Polybutadiene. IV

Abstract

The cis double bonds in a high cis-polybutadiene in solution are readily convented into trans double bonds on exposure to gamma -rays in the presence of a suitable sensitizer such as an organic bromide. The isomerization proceeds in the direction of attaining an equllibrium cis/trans ratio of approximately 5/95. This reaction is analogous to the recently reported photosensitized cis-trans isomerization of polybutadiene. In addition, a very much slower ''unsensitized'' radiation induced isomerization of the double bonds was also observed in solution. However, no change in cis/trans ratio could be obtained on gamma -irradiation of the solid polymer. The kinetics of the isomerization of polybutadiene in benzene were studied using allyl bromide, thyl bromide and ethylene bromide as sensitizers, and found to be first order with respect to the cis content in the polymer and somewhat less than half-order in organic bromide. The mechanism is considered to involve pi complexing between the double bonds and free bromine atoms formed in the radiolysis of the bromide. An interesting application of this isomerization is its possible use as a dosimeter in radiation chemistry. (auth)