Homogeneous anionic polymerization. V. Association phenomena in organolithium polymerization

Abstract

AbstractA study has been carried out of the association phenomena which occur during the polymerization of styrene, isoprene, and butadiene by organolithium initiation. This has been done by means of viscosity measurements of these systems after the conclusion of the polymerization reaction. It has been found that, in general, the polymerlithium species, in hydrocarbon systems, are associated in pairs. This applies to the styrene system in benzene and to the isoprene and butadiene systems in n‐hexane. In tetrahydrofuran solution, none of these systems show any polymer association. For the polyisoprenyl lithium in n‐hexane, the equilibrium constant for the dissociation of the associated pairs at 30°C. is of the order of 10−6, and the heat of dissociation is about 37 kcal./mole. By means of this viscosity technique, it has also been found possible to show that tetrahydrofuran forms a monosolvate with the polyisoprenyl lithium (RMiLi · THF) and that the heat of solvation is about 18 kcal./mole. These findings are in accord with the proposed mechanism in which the active species, in hexane, is represented by the unassociated polymer‐lithium, RMi,Li, while, in THF, it is the solvated species, RMiLi · THF. On this basis the following propagation rate constants have been deduced for isoprene polymerization: kp(hexane) = 3.4 × 103 exp {−4100/RT} = 8.9 at 60°C.; kp(THF) = 1.5 × 104 exp{ −6800/RT} = 0.4 at 60°C.