We present a density functional theoretical study of neutral and negatively charged complexes of acrylonitrile (AN) with several clusters representing the Cu(100) surface. An external electric field is used to induce electron transfer onto the adsorbed molecule, in order to model the initial stage of electrochemical polymerization of AN on a copper electrode. We compare different possibilities of AN interaction with the metal and determine the most favorable configurations of the adsorbates in the absence and in the presence of the electric field. We discuss the influence of the electrochemical polarization on the type of AN adsorption, in relation with the experimental data on the adsorption of AN and electrochemical grafting of polyacrylonitrile.