On the basis of quantum-chemical calculations, it was shown that among six types of active centers (ACs) that can form during the polymerization of butactiene with lanthanide-based catalytic systems, five types (containing electron-accepting chlorine atoms in the coordination sphere of a lanthanide) exhibit a pi-allyl binding of the terminal unit of a growing polymer chain to a lanthanide atom and function as cis-regulating. The sixth type of ACs is characterized by a sigma-alkyl structure and shows a trans-stereospecificity. This results was used to interpret the data on the microstructure of polybutadiene prepared using NdCl3 . 3TBP-Al(iso-C4H9)(3), NdCl3 . 3TBP-Mg(n-C4H9)(iso-C8H17) catalytic systems and their combinations (TBP is tributyl phosphate). (C) 2003 Wiley Periodicals, Inc.