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Abstract: Cp2Zr(CH3)R and its derivatives can serve as powerful olefin polymerization catalysts after activation by a Lewis acid A to form the ion-pair [Cp2ZrR]+[CH3A]– (I) which is held together by a Zr-μ-CH3A– bridge. It is generally assumed that the cation Cp2ZrR+ (II) of I is the active species whereas the influence of the anion CH3A– (III) is less well understood. We have conducted an extensive study based on density functional theory (DFT) of ethylene polymerization catalyzed by both I and II in order to probe the influence of the anion CH3A– for the case where A=B(C6F5)3. The reaction between ethylene and the cation II leads in the first place to a π−complex in a highly exothermic and exogonic reaction without any (uptake) barrier. Even the subsequent insertion process has a transition state that is lower in energy than II and C2H4 at full separation. The only (internal) barrier is the modest energy required to proceed from the π-complex to the insertion transition state. For the reaction between the ion-pair and ethylene the monomer can approach cis or trans to the Zr-μ-CH3A– bridge. In addition with R = Pr, cis and trans approaches are possible for 4 different orientations of the propyl chain. We find for all of these approaches that the rate determining step is the (partial) displacement of the anion CH3A–. For the first insertion (R=Me) the total insertion barrier is 13.0 kcal/mol for the most favorable (trans) approach. The second (R=Pr) insertion (which likely also is a good model for subsequent propagation steps) prefer a cis-approach in which the ethylene uptake barrier of 9.5 kcal/mol is rate determining whereas the barrier for the subsequent insertion process only is 6.8 kcal/mol. Displacement of the anion was found to be more pronounced for R=Pr than R=Me and larger for the insertion transition state than the uptake transition state. Solvation effects were seen to stabilize anion displacement and thus reduce especially insertion barriers. Thus for the favored cis-path of the second propagation, the insertion was the rate determining step in the gas-phase with a barrier of 10.0 kcal/mol whereas the corresponding uptake process became the rate determining step in solution with a barrier of 8.6 kcal/mol. After the insertion, the ion-pair was found to recombine completely so that ethylene will have to displace the anion in the next propagation step. Considerations were also given to the chain termination step by transfer of hydrogen to the monomer. Here the rate determining step is again cis-uptake off ethylene with a barrier of 12.5 kcal/mol whereas the subsequent hydrogen transfer barrier only is 10.6 kcal/mol. To cite this article: T. Ziegler, C. R. Chimie 8 (2005) . [Copyright &y& Elsevier]