Abstract: In this work, the soluble Ziegler-type catalyst systems prepared from VO(OR)₃ Cl-Al (i-Bu)₃ for the alternating copolymerization of butadiene and propylene have been studied.The valence of the vanadium ion in the center of the V-A1 complex was measured by means of the potentiometric titration method. From comparing the data of potentiometric analysis with polymerization results, it was concluded that the active species is V⁺³ ion. After investigating the reaction process between the two components of catalyst, an octa-hadrel complex model for the active center was suggested. In addition, the mechanisms of initiation oriented propagation and termination of chain for this catalytic system have been probed.By analyzing the NMR and IR spectrum, it was demonstrated that the termination of chain takes place mostly at the end of propylene unit.