A theoretical analysis of coordination and insertion of the ethylene molecule on different metallocenic active sites was performed. The sites were modelled as ionic-pairs where the counterion is close to the cation. The steps going from the isolated species ionic-pairs and the monomer to the new propyl group linked to the ionic-pair were calculated and analyzed. The influence of the counterion on the energy barrier that the olefin must overcome for its coordination and insertion into the Zr-C-CH3 bond was considered. The energy barrier is decreased by the inclusion of an AlCl3 additive. As an additional study, the presence of a second ethylene molecule in the neighborhood of the active site was evaluated. This situation reduces the maximum energy and improves the stability of the final propyl ionic-pair. (c) 2006 Elsevier B.V. All rights reserved.