The relation between the structural features of C-2-symmetric zirconocenes and their performance in ethene/propene (E/P) copolymerization has been investigated using a combined experimental and quantum mechanical approach. The following ligands have been studied: (CH3)(2)Si(Indenyl)(2); (CH3)(2)Si(benz[e]indenyi)(2); (CH3)(2)Si(4-phenyl-indenyl)(2) and their 2-methyl substituted variants. Describing trends in molecular weights for ethene/propene copolymerization, using calculated relative free energies of activation for monomer insertion and chain transfer to monomer, does not work. The results suggest that this may be due to ethene propagation being Limited by a step different from the insertion itself. Besides other possible hypotheses, in particular counterion effects, we have shown that a larger energy barrier may be associated with chain rotation. Combination of experimental r(2) values with calculated barriers for propene propagation and chain transfer to both monomers works much better, presumably because the anomalies associated with ethene insertion are concentrated in this experimental r(2) value. The fair agreement achieved for this mixed description method indicates that for the other reactions the rate-limiting steps are "normal". For propene homopolymerization, our results indicate that slowing down the propagation after 2,1-insertion can be important, and show that studies of copolymerization can yield valuable information about homopolymerization. Preparing high molecular weight copolymers appears to require catalyst modifications displaying a more balanced ratio of propagation and termination. (c) 2005 Elsevier B.V. All rights reserved.