AbstractWe present the synthesis, characterization (NMR, mass spectroscopy, IR) and polymerization of 3‐cyclohexylthiophene. Although anodic polymerization is ineffective, it was possible to polymerize 3‐cyclohexylthiophene with FeCI3as the oxidizing reagent, the resulting poly(3‐cyclohexylthiophene) (PC6T) having a peak molar mass of 73 000 g · mol−1. NMR‐Investigations reveal that the coupling ocurred mainly via the 2,5‐positions, and the band gap, as determined from UV‐vis spectroscopy and cyclic voltammetry, was 2,3–2,4 eV. The nonlinear susceptibility χ(3)(−3ω; ω ω ω) of the neutral polymer and the electrical conductivity of the CIO4−and PF−6salts were determined. The effect of the steric requirements of the cyclohexyl substituent was estimated by comparing the polymer to a poly(3‐hexylthiophene) (PN6T) of the same molar mass and with a comparable number of defects (mainly head‐to‐head and tail‐to‐tail coupling of the subunits). In the case of PC6T the band gap of the neutral polymer is larger by 0,3 to 0,4 eV, the nonlinear susceptibility has approximately half the value, and the electrical conductivity of the oxidized polymer is two orders of magnitude lower than the corresponding values of PN6T. These effects are explained by the increased sterical requirements of the cyclohexyl substituent, which forces the thiophene rings out of a coplanar conformation an