The transition from a generation III Ziegler-Natta catalyst with a monodentate Lewis base to a more modern generation IV/V system, containing a tetrahydrofuran derivative, the tetrahydrofurfuryloxide C4H7O-CH2O- bidentate ligand (THFFO), was studied by means of molecular modeling and DFT calculations. This particular ligand was carefully chosen so that it remained in the titanium coordination sphere in the model active site. With such a constraint, the dual role of tetrahydrofurfuryloxide was identified: it was demonstrated how the presence of this ligand limits the number of isomeric active sites as well as enhances the selectivity of the species that can still exist. The results indicate that the catalyst with a bidentate ligand attains good selectivities, albeit at a cost of activity.