With the aim of a possible rationalization of the probability distributions of stereochemical configurations of the regioirregular units in isotactic polymer samples (prepared in the presence of rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride or rac-ethylenebis(l-indenyl)zirconium dichloride and methylaluminoxane), a detailed molecular mechanics analysis on previously proposed model catalytic sites is reported. In particular, catalytic intermediates suitable for primary and secondary insertions of propene on primary and secondary polypropene chains, for both systems with rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl) and rac-ethylenebis(1-indenyl) ligands, are compared. The models are able to rationalize the observed enantioselectivities, not only for the regioregular placements but also for the regioirregular placements (secondary insertion on a primary chain and primary insertion on a secondary chain). Moreover, the nonbonded interactions in the models give a contribution in favor of the monomer coordination suitable for the primary insertion, even after an occasional secondary insertion, and are able to account for the higher regiospecificity observed for titanocene-based, with respect to zirconocene-based, catalytic systems.