Structural studies of polyalkylthiophenes with alternating sidechain positioning

Polyalkylthiophenes with alternating head-to-head and tail-to-tail couplings (poly(3,3′-dialkyl-2,2′-bithiophene)s) have been synthesized with methyl, butyl, hexyl and octyl sidechains. The materials have been studied by X-ray diffraction, and structural models have been proposed. The structure of the methyl-substituted polymer is found to be similar to that of poly(3-methylthiophene), with planar polymer chains, whereas the polymers with longer sidechains are found to have twisted main chains. The material with octyl sidechains has been studied during heating and during doping with iodine. This polymer is found to possess a negative thermal expansion coefficient. Iodine-doping leads to a transition to a phase with planar backbones, accompanied by an observed increase in conductivity.