As part of a systematic study of the impact of donor-acceptor substitution on the structure and properties of pi-conjugated compounds, we present a theoretical investigation of the all-trans polyacetylene backbone, end-capped with moieties of different donor or acceptor natures and different strengths, focusing on the effects induced by these substituents on bond lengths and shape of the conjugated chain. Optimized geometries for polyacetylene containing 15 and 20 double bonds have been computed by means of density functional theory with the Coulomb-attenuating B3LYP (CAM-B3LYP) functional. We show that the simultaneous presence of two substituents has a cooperative effect on the lengths of single and double bonds. We also show that, depending on the substitution pattern, distortion toward bow- or S-shaped structures occurs. Two new geometric parameters are defined in order to evaluate the mode and intensity of this distortion. Cubic Bezier curves have been used as simple geometric models for the chain bending behavior.