Photoinduced microstructural changes in 1,4‐polyisoprene

An infrared and NMR study was made of the microstructural changes produced in thin films of purified cis ‐ and trans ‐1,4‐polyisoprene when irradiated with ultraviolet light in vacuo at room temperature. The major photochemical processes observed were cis–trans isomerization and loss of 1,4 double bonds, the latter process being accompanied by the formation of vinylidene and vinyl double bonds as well as some endlinking. Very surprisingly, the loss of original double bonds contributed also to a novel photocyclization which gave rise to cyclopropyl groups in the polyisoprene chain. The isomerization and the formation of cycloprophyl groups are presumed to proceed through triplet and biradical states of the 1,4 double bonds, while the vinylidene and vinyl double bonds must result from chain repture at the carbon–carbon bond joining successive isoprene units. Hydrogen abstraction and double‐bond migration are of neglible importance in the overall photochemistry of polyisoprene.