Photoisomerization of polybutadiene

Polybutadiene, in the solid state, and in the absence of added sensitizer, undergoes photochemical cis-trans isomerization and loss of unsaturation when irradiated in vacua with krypton 1236 A. or mercury 2537 A. radiation. The estimated quantum yields for these two reactions are 0.25 and 1.53, respectively, for the krypton irradiations, and 0.09 and 0.07 for the mercury irradiations. The G-values (or 100-e.v. yields) for the vacuum photochemical isomerization and consumption of double bonds indicated by the krypton data (2.5 and 15.3) are comparable to the G-values obtained previously for the corresponding radiation-induced reactions in polybutadiene (7.2 and 13.6–7.9, respectively). This suggests that the mechanisms in these two systems are similar, involving both ionized and excited species; the high yield for loss of double bonds with krypton is attributed to a chain cyclization analogous to that postulated in the radiation chemistry of diene polymers. The photoisomerization with mercury is assumed to be sensitized by adventitious impurities present in the polymer which absorb in the ordinary ultraviolet (where the polymer is transparent) and which transfer energy to the double bonds, causing their excitation and eventual isomerization.