Iiodine-doping of 1,4-polybutadienes

Summary form only given. In 1988, Thakur reported that the conductivity of cis-1,4-polyisoprene can be increased by about 10 orders of magnitude by doping with iodine, and that the analogous insulator- semiconductor transition is also seen with trans-1,4-polyisoprene and trans-1,4-polybutadiene, but not with cis-1,4-polybutadiene)./sup 1/ This discovery attracted a great deal of interest, but on the other hand it generated some fundamental questions:/sup 2/ First, what is the mechanism from which the conductivity of doped rubbers arises? Second, why does cis- 1,4-polybutadiene not become electrically conductive by I/sub 2/-doping, but its trans-conterpart does? Previously, we have demonstrated that I.sub 2/-doping of cis1,4-polyisoprene produces conjugated sequences of unsaturated double bonds in the polymer backbone, which confer conductivity./sup 3,4/ Recently, we have found, using spectroscopic measurements and molecular orbital calculations, that I/sup 2/-doping of cis-1,4-polybutadiene does not lead to the formation of conjugated sequences at room temperature due to a molecularly unfavorable combination of electronic and steric interactions. In contrast, the trans-isomer / iodine can readily produces conjugated sequences. We have also studied the cis-trans isomerization on 1,4-polybutadiene in the solid state; the possibility of performing the isomerization in a patterned fashion opens up avenues for the fabrication of microelectronic devices using I/sup 2/-doped rubbers.