Homogeneous anionic polymerization. V. Association phenomena in organolithium polymerization

Abstract

A 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.

Association Phenomena in Organolithium Polymerization

“…The case of polybutadiene deserves special attention, since it shows an Ν value above 2. Other measurements 2,3 also corroborated that this polymer showed Ν values in hexane ranging between 2 and 3, indicating states of association higher than 2. This apparently anomalous result can, however, be * Ν should, of course, actually be obtained from the ratio of number-average molecular weights.…”

“…Such measurements were carried out using the same viscometric method as described previously for the case of polystyrene 7 and polyisoprene. 3 Some of the data are reproduced in Table 5.7 for polystyrene and in Table 5.8 for polyisoprene. It can be seen that the H 4furan is much more effective in disrupting the association of the polystyryl lithium than the polyisoprenyllithium, only trace amounts ([C 4 H 8 0]/[RLi] ~ 10) being sufficient in the former case, while much larger concentrations are needed in the latter case.…”

Model for the aggregation state of living anionic polymers

“…The Li-Li distance on an edge between two occupied faces is somewhat shorter than the Li-Li distance between one occupied and one unoccupied face. 58 The energy calculated by DGAUSS was also slightly higher than that of the highest-level calculation in Ref.…”

Natural rubber (NR) biosynthesis: perspectives from polymer chemistry

“…Morton et al studied the effect of addition of tetrahydrofuran (thF) into the anionic polymerization of iP (Morton et al, 1963). the authors found that 3,4-enchainment dramatically increased in this system (Morton et al, 1963;Morton and Fetters, 1964). Worsfold and Bywater continued this study using cyclohexane/thF solvent systems and found that the polarity of the solvent affected the effective counter cation size (Worsfold and Bywater, 1964).…”