Quantum chemical calculations on small idealized systems of poly(3-thiophen-3-yl-acrylic acid methyl ester), a new polythiophene derivative soluble in polar solvents, have been performed to propose a model for this polymer. The model, which was derived from calculations at the HF/6-31G(d) and B3PW91/6-31+G(d,p) levels is defined by both the head-to-tail polymer linkages and the conformation of the acrylic acid methyl ester side groups. The pi-pi* lowest transition energy predicted for the polymer is in excellent agreement with the experimental values, which were determined in this work using UV-visible experiments. Finally, we investigated the variation of the electronic properties produced by small chemical modifications in the side group of poly(3-thiophen-3-yl-acrylic acid methyl ester), such changes being designed to enhance the solubility of the polymer in water. (c) 2007 Elsevier Ltd. All rights reserved.