Anionic polymerization of isoprene. Ion and ion-pair contributions to polymerization in tetrahydrofuran

Abstract

The kinetics of the propagation reaction in the polymerization of isoprene initiated by butyllithium in tetrahydrofuran solution have been studied. The kinetic behavior has indicated that the reactive species involves both the polyisoprenyllithium ion-pair and the free poly-isoprenyl carbanion. The conductances of solutions of polyisoprenyllithium have been measured and the ionic dissociation constant derived. From these the free ion rate constant has been evaluated. It has also been shown that the free ion reaction may be suppressed by the common ion effect when the salt lithium tetraphenylboron is added. The rate constants for the ion-pair and free ion are 0.20 M−1 s−1 and 2.8 × 103 M−l s−1 respectively; the ionic dissociation constant is 5.0 × 10−10. Nuclear magnetic resonance determinations of the structures of the polymers formed primarily by the free carbanion, and by the ion-pair only, show that the lithium ion has only a small directing effect.