Accurate density-functional-theory (DFT) calculations on oligothiophenes containing up to 6 units have been performed in order to understand the various possible mechanisms of growth. There are two possible mechanisms for this process which involve the coupling of cationic radicals with either another cationic radical or with a neutral oligomer. Most of the electronic properties of these oligomers are well understood, but there is very little known about the nature of the intermediates and the transition states leading to polymerization. The initial oxidation, forming stable intermediates, releasing protons and/or further oxidations are studied in terms of the energetics, changes in the geometry, charge distributions and possible signatures in the vibrational spectra. The radical-radical pathway is found to be the more probable one between two mechanisms. Also the attack of monomers/or shorter oligomers on the longer ones is found to be faster than the coupling of equal size chains. (C) 2004 Elsevier Ltd. All rights reserved.