Homogeneous anionic polymerization. IV. Kinetics of butadiene and isoprene polymerization with butyllithium

Abstract

AbstractThe kinetics of polymerization of butadiene and isoprene with butyllithium initiator have been studied both in n‐hexane and tetrahydrofuran solution. The overall polymerization rates were much faster in the THF solutions than in n‐hexane, and the latter also exhibited relatively slow initiation rates compared to propagation. By use of a preinitiation technique, where necessary, it was possible to study the propagation kinetics without any interference by initiation. The propagation reaction was found to be first‐order with respect to monomer in both solvents, but much faster in THF. Furthermore, it was first‐order with respect to growing chain concentration in the THF system, but only half‐order in the n‐hexane system. This was taken to indicate the existence, in n‐hexane solution, of an association equilibrium between active single chains and inactive chain pairs. The activation energy for propagation was found to be 6–7 kg. cal. in THF and about 22 kg. cal. in n‐hexane for both monomers, but the latter value obviously includes the heat of dissociation of the associated chain pairs. The Arrhenius expression for the propagation rate in THF contains a very low frequency factor (∼︁104) and leads to a propagation rate constant at least two orders of magnitude less than the corresponding value for free radical propagation.