Polymerization of ethene catalyzed by nine different zirconocenes with the general formula (eta (5)-C5H4R)(2) ZrCl2 (R = H, alkyl) activated by methylaluminoxane (MAO) has been performed. Kinetic data and mechanistic interpretations in terms of propagation, termination and isomerization are presented. At T = 50 degreesC, the time to reach maximum activity increases with the length of R, from 1 min with R = H to about 20 min with the longest alkyl substituents. The propagation rate is analyzed by kinetic modeling. At T = 50 degreesC, both the highest average activity and propagation rate are achieved with R = n-propyl. At higher temperatures, no systematic differences can be observed between the different catalysts. Polymer unsaturation is studied by infrared spectroscopy. Alkyl substitution on the cyclopentadienyl ligand shifts the distribution from vinyl toward trans-vinylene. We rationalize this by a competition between termination and isomerization reactions where kinetics is the decisive factor. At T = 50 degreesC, the highest trans-vinylene content is found for R = n-propyl. Density functional calculations show that for R = ethyl and longer an additional agostic interaction between the metal center and a hydrogen on the alkyl substituent may be important. We suggest that this has an impact on the kinetics and is responsible for several experimental observations.