Isomerization Reactions
The isomerization of cis-polybutadiene can be carried out with the aid of ultraviolet light or with gamma radiation. When light is used, the free-radical reaction requires photoinitiators [102, 103]. The mechanism involves freely rotating transitory free-radicals on the polymer backbone. These form from additions of photoinitiator fragments to the double bonds. The adducts break up again, releasing the attached initiator fragments and reestablishing the double bonds. The new configurations are trans because they are more thermodynamically stable [102, 103]. This can be shown as follows:
where, X• represents a free-radical fragment from a photoinitiator. With gamma radiation, there is no need for any additives [104, 105]. Here, the mechanism of isomerization is believed to involve direct excitation of the p-electrons of the double bonds to anti bonding orbitals where free and geometric interconversions are possible. In benzene solutions, energy transfers take place from excited benzene molecules to the polymer double bonds. When 1,4-polyisoprene films are irradiated with light [122], cis–trans isomerizations occur. In the process, the quantities of 1,4-unsaturations decrease. Also, vinyl and vinylidene double bonds and cyclopropyl groups form. This can be illustrated as follows:
Polybutadiene can also isomerize cis–trans and lose unsaturation when irradiated in the solid state. This must be done in vacuum with ultraviolet light of 1,236 or 2,537 A ˚ [106, 107]. Both free-radical and ionic mechanisms are suspected to operate simultaneously.
