The polymn. of isoprene induced by alkyl lithium compds. in hydrocarbon solvents is known to proceed with a high stereoselectivity to (Z)-1,4-polyisoprene. We have studied the initiation step of this reaction with the help of ab initio mol. dynamics simulations based on d. functional theory. Specifically, we have investigated the reaction of isoprene with Et lithium and, by applying simulated annealing techniques, we have characterized the products of the initiation step and found a cis-isomer to be energetically most favorable. Several alternative reaction pathways have been characterized in detail in terms of their corresponding transition states and reaction products. For our model system, the cis-trans isomerization turns out to be rate detg. and, although the addn. reaction has a low barrier, it constitutes the stereoselective step. Our calcns. allow us to discriminate between different possible reaction mechanisms and give a rationalization for the high stereoselectivity. In this regard, lithium plays a crucial role by preferentially stabilizing specific structures through the formation of 'chelated' complexes and agnostic interactions. [on SciFinder (R)]