A new regioregular polythiophene derivative bearing side chains with exactly six oxyethylene units has been synthesized. From the thermochromic and ionochromic behavior of this polymer, it is concluded that the hexa(oxyethylene) side chains induce a more planar conformation of the polythiophene backbone than random-length oxyethylene side chains with on average seven oxyethylene units. It is further observed that the ionochromic transition from the planar to the nonplanar conformation is [K+](2) dependent, suggesting that solvation of Kions by adjacent side chains is required. The polymer is ionically as well as electronically conductive. The ionic conductivity is only measurable at 80 degrees C, i.e., above T-g and the order-disorder temperature of the polymer.