The new fully planar cyclopentadithiophene, 4-n-dodecylidene-4H-cyclopenta(2,1-b;3,4-b′)dithiophene, shows extensive π-stacking in the solid state with short intermolecular distances (ca. 3.5 Å) between adjacent molecules. Polymerisation of this monomer by two different protocols gave solution processable alkenyl-bridged cyclopentadithiophene polymers with extended π-conjugation in the main chain.