Living polymers

Abinitio molecular dynamics study of the initiation step in the polymerization of isoprene induced by ethyl lithium

Abstract

The polymerization of isoprene induced by alkyl lithium compounds 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 abinitio molecular dynamics simulations based on density functional theory. Specifically, we have investigated the reaction of isoprene with ethyl 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 favourable. 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 determining and, although the addition reaction has a low barrier, it constitutes the stereoselective step. Our calculations 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 agostic interactions.