Semiconducting poly(monocyanoacetylenes)

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Abstract
Poly(monocyanoacetylenes) (PMCA) were synthesized by anionic, Ziegler–Natta, metathesis, and photo initiations. The Ziegler–Natta‐catalyzed polymers probably have highly stereoregular cis‐transoid structure that contains very few defects and the nitrile groups are difficultly cyclized. It has M̄n = 1100. PMCA obtained by anionic polymerization at −78°C has M̄n ∼ 4800; it is rich in trans‐transoid structures but probably contains other isomeric units as well. The unpaired spin concentrations in these polymers are very high, comparable to that in trans‐polyacetylene (PA) isomerized above 150°C. UV irradiation initiated rapid polymerization of cyanoacetylene in solid state at low temperature but the products were bleached in color after long irradiation. The unpaired spins in PMCA are immobile; nitrile cyclization causes some decrease in EPR linewidth and increase in room‐temperature conductivity (σRT). There was also a large increase in unpaired spin concentrations to about 200 monomer units/spin. Iodine doping increases σRT to about 10−3 (ω cm)−1 but the dopant is readily removed by evacuation and the polymer returns to its original insulating state. The properties of pristine and doped PMCA, such as EPR g‐value, ΔHpp, T1, T2, and σRT are very similar. The similarities persist after cyclization and doping for this pair of polymers. These properties are also compared with those of poly(methylacetylene), poly(phenylacetylene), poly(dicyanoacetylene) and PA, and the significance is discussed.

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