AbstractA precise description of the active site for heterogeneous stereospecific catalysts for propene polymerization has not been achieved, taking into account the effect of all the components of the catalytic system. A new model is proposed based on the following facts: the active Ti is directly bonded to the support MgCl2; it is bridged with an Al atom of the alkylaluminium; the latter is complexed with the external Lewis base; the more efficient Lewis bases are known to be dialkoxy dialkyl (or aryl) silane containing at least one bulky alkyl group; it is suggested that the two alkoxy groups are complexed, one with the aluminium atom, the second with the titanium atom. The structure of such a site has been built using a Chem 3D software and starting from an octahedral Ti and a bipyramidal Al. Both are bridged with the polymer chain. The silane is located so that each oxygen atom is just above each of the two metal atoms; the bulky alkyl group covers the vacancy of the Ti octahedre being in agostic interaction with the Ti. Such a tight geometry imposes the isospecific coordination of the monomers before its insertion in the polymer chain. The model is flexible enough to allow for the transition state, the insertion of the incoming monomer and the rotation of the new bridge, the driving force being the agostic interaction. The successful replacement of the silane with a diether with 3 carbons atoms in between the oxygen atoms is easily explained by the model, as well as the effect of the structure of the alkoxy and of the alkyl groups of the silane.