Effects of steric factors on the electrosynthesis and properties of conducting poly(3-alkylthiophenes)

Two series of thiophene monomers 3-substituted with linear and branched alkyl chains have been synthesized, and their electropolymerization has been investigated together with the properties of the resulting polymers. An analysis of the effects of the electrosynthesis conditions by visible absorption spectroscopy shows that the mean conjugation length presents an increasing dependence on the applied electrical conditions as the length of the alkyl chain increases. In the case of linear alkyl substituents (R3 = CH₃ to C₁₈H₃₇), the length of the alkyl chain affects slightly the polymerization reaction but produces important modifications on the structure and on the properties of the polymers. Thus, the mean conjugation length and the electrochemical reversibility admit an optimum for the polymers containing 7-9 carbons in the alkyl chain while the polymer films remain highly conductive up to R3 = C₁₀H₂₁. In contrast, the steric hindrance to conjugation caused by branched alkyl substituents totally inhibits the polymerization (isopropyl) or leads to weakly conjugated polymers (isobutyl). It is shown that the distance between the thiophene ring and the branched alkyl chain is the determining parameter which governs the aptitude for electropolymerization and the properties of the resulting polymer.