The aim of our Ph.D. work consisted in analyzing the stereo-electronic properties of various external donors (EDs) considered alone and within a realistic model of complete catalyst to understand better the role played by these organic molecules in the activity and stereo specificity of the catalyst. At first, we compared the stereo-electronic properties of different EDs between them as well as with those of the di-isobutyl phthalate used as internal donor. This study allowed us to bring to light several characteristics the ED should have to be competitive. Then, we optimized the geometry of a monometallic active site’s “3 sites” model alone and with the various EDs by the QM/MM method. We showed clearly that ED modifies the geometry of the active site to allow the monomer complexation and that the choice of the ED influences the number of potential active configurations for the isotactic propylene polymerisation.