The mechanism of butadiene and isoprene polymerization in the presence of CpTiCl3-MAO initiator has been investigated by means of EHT and ab-initio computations, with the purpose of understanding the factors responsible for the very different homopolymerization rates of the two monomers. The catalytically active species is assumed to be the highly unsaturated [CpTi-R](+) organotitanium cation (R = growing polymer chain). It is shown that coordination of an incoming monomer to the active species forces the ending unit of R to rearrange from the initial eta(3) allylic coordination to the a one. This step is energetically much easier when the ending unit is butenyl rather than 2-methylbutenyl. No significant differences between butadiene and isoprene have been found for the insertion step; therefore, the large difference in the homopolymerization rates of butadiene and isoprene appears to be due to the highest reactivity of the growing chain ending with a butenyl unit.