A computational study of the ethylene-styrene copolymerization with rac-ethylenebis(tetrahydroindenyl)MCl2 [M = Ti, Zr] systems using DFT methods is presented. The complexation, coordination and insertion energies for ethylene and styrene monomers as well as for the styrene-ethylene copolymerization steps into the catalytic active site models [Et-2(IndH(4))(2)]MCH3+ [M = Ti, Zr] were calculated. The goal of this study is to examine the influence of the metal atom [Ti, Zr] on the copolymerization activity. It could be concluded that zirconocene catalyst is much more active than the titanocene based catalyst. This could be explained by the higher steric congestion around the Ti as compared to the Zr complex. Furthermore, it was found that the primary styrene insertion gives rise to complexes in which the active sites are blocked by the phenyl ring in both metal atoms, so that only the secondary insertion of the styrene is possible. These facts might help to clarify the already published experimental results.