A theoretical investigation of the electronic structures of the oligomers of thiophene (T) and their derivatives, namely, 2-methylthiophene (2MT), 3-methylthiophene (3MT), 2-cyanothiophene (2CT) and 3-cyanothiophene (3CT), are presented. The most stable forms of the monomer, dimer, trimer and tetramers are obtained by the semi-empirical quantum mechanical methodology using AM1 parametrization. All possible binding sites are investigated in order to understand the bonding in polythiophenes. The 3-substituted molecules form dimers which lie nearly at trans-planar positions but 2CT and 2MT favor cis conformations. The activation energies between these structures and local minima change from 0.2 to 1.8 kcal/mol. Methyl- and cyano-substituted polythiophenes favor linear growth mechanisms. The branching requires 3-6 kcal/mol per substitution. Polythiophenes seem to form fairly flexible chains as understood from the relatively low rotational barriers.