Lithium polyisoprenyl

M 1/2s 1, both at 30 °C. The kinetic work was supplemented by a conductance study yielding Kdiss = 5 10 9 M. This value might be too high because some conducting products could be formed by the side reactions. If accepted, it leads to = 3 104 M 1s 1. The remarkably low Kdiss, about 300 times smaller than that of lithium polystyrene, implies a very tight structure of lithium polyisoprenyl in that solvent. 

The cis-form of lithium polyisoprenyl is also preferred in diethyl ether solution401. The spectroscopic and NMR studies were supplemented by a chemical fixation of the structure of a model compound addition of trimethyl silyl chloride yields adducts (presumably without isomerizing the reacting salt) which were eventually separated by a chromatographic technique into cis- and trans-isomers. The results confirmed the previous NMR analysis. 

The electronic spectrum of lithium polyisoprenyl in diethyl ether changes at higher concentration of the salt, > 10-3 M. Apparently the ion-pairs associate in this medium into higher aggregates and the concomital decrease of their reactivity implies a lower reactivity of the aggregates, a conclusion made previously by Sinn4065. 

Thus, the lA order observed for the lithium polystyrene propagation might change into A at some sufficiently high concentrations of living polystyrene, or the A order observed for lithium polyisoprenyl should become Yi on their appropriate dilution. However, there are limitations and technical difficulties which could make such a test at least difficult if not... 

Spectrophotometric determination of the dissociation of an aggregated lithium polyisoprenyl in benzene and in n-octane led to the values of the dissociation constant given below433) ... 

The problem of the nature of the dissociation process needs clarification. The authors believe that lithium polyisoprenyl is aggregated into tetramer and dissociates into dimers, and the extremely low dissociation of the latter yields the propagating monomers. 

The fact that the difunctional dimethyldichlorosilane had no effect on the flow time of the polyisoprenyl lithium offers perhaps the most convincing evidence that these chain ends must have been associated in pairs prior to the linking reaction. The corresponding values provide the necessary confirmation that the linking reaction has actually taken place. It should also be noted that flow times of the terminated linked polymers can also be used to detect the presence of some unlinked, but associated, chains. 

One additional item of experimental evidence for the dimeric association of polyisoprenyl lithium was provided by a light scattering study (21), in n-hexane at 25°C., where it was found that the molecular weight of the terminated polymer was very close to one-half that of the active polymer. All of these data seem to leave no doubt that the active chain ends in the organo-lithium polymerization of styrene, isoprene and butadiene, in non-polar solvents, are associated as pairs, at least at chain-end concentrations of 10 2 M or less. This conclusion has also been supported by data obtained in four other laboratories (22,... 

Another complication introduced by the associative properties of organolithium solutions in non-polar solvents is the fact that the alkyllithium initiators are themselves associated and can be expected to "cross associate" with the active polymer chain ends. Thus some of our studies (26) on the effect of added ethyl lithium on the viscosity o -polyisoprenyl lithium solutions in n-hexane support the following association equilibrium... 

Butadiene and isoprene have also been studied in tetrahydrofuran (72). At 0° the rates are close to first order in polyisoprenyl or poly-butadienyllithium concentration which indicates that the rate constant for the ion-pair is being measured in the concentration range studied (> 10-s molar). The rate constants at 30°, k9 (butadiene) =1.8 litre/mole sec, kj, (isoprene) = 0.13 litre/mole sec, are appreciably lower than for styrene. For butadiene at —39° a kp value of 6 X 10 2 litre/mole sec can be derived from the results of Spirin (98). This value checks well with that extrapolated from Morton s data. The observed propagation constant for isoprene is rather low and is in fact equal to that of the mono-etherate in solvent mixtures of appreciably lower dielectric constant. At room temperature there is evidence for isomerization of polyisoprenyl-lithium in tetrahydrofuran which becomes particularly marked as the... 

Figure 3. Variation of the propagation rate with concentration of polyisoprenyl-lithium at 30° C., o—Worsfold and Bywater (29) A—Sinn and co-workers (19) +—Spirin and co-workers (23) x—Morton and co-workers (15).

Fig. 5. A typical plot of In a against t—t, for the reaction between isoprene and polyisoprenyl lithium when the concentration of the latter is held constant. The numerical data for this plot are given in Table 4.

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