Possible pathways for the reaction of ethylene with Cp(2)Zr(C2H5)(+) are studied with the Car-Parrinello Projector Augmented Wave (CP-PAW) technique, which is based on density functional theory (DFT). "Slow growth" constrained first-principles molecular dynamics at 300 K were used to sample the phase space in the vicinity of the saddle points of the reactions. The simulations considered the growing chain in a resting state between insertions where a hydrogen on the beta-carbon is attached to the metal. We have investigated three mechanisms, namely frontside (FS) insertion with ethylene approaching toward the beta-agostic Zr-H bond, backside (BS) insertion with ethylene approaching toward the Zr-C-alpha bond, and hydrogen transfer (HT) chain termination with transfer of the beta-agostic hydride to the incoming ethylene. Which pathway is followed is determined by the out-of-plane rotation of the ethyl group, which in turn is determined by the agostic interactions of the ethyl group. Agostic interactions are found to shift rapidly, with typical lifetimes of 0.5 ps for the FS insertion and 2 ps for the FS insertion. The activation barriers increase in the order front-side insertion < back-side insertion < hydrogen transfer. The shape of the potential surface obtained from previous static calculations was confirmed. The ethylene molecule is mobile enough to partake in fast exchange equilibria with surrounding substrate in solution.