Effect of polar materials on chain propagation

Lithium butoxides increase the rate of reaction of lithium alkyls with olefins in cyclohexane or hexane but decrease it in benzene. The propagation rate is, however, decreased in both types of solvent [77, 78] according to information presently available. In fact, as far as is known, butoxides reduce rates where the mechanism has been suggested to be dissociative and increase them in the other cases. More data are still required to confirm that this always happens. The experiments with polystyryllithium [77] show that the polymer dimers in solution are not dissociated by lithium fert.-butoxide as would be expected if mixed aggregates of the type (PstLi. BuOLi ) were formed. In this case, at least, the rate effect appears to be caused by addition of butoxide to the polystyryllithium dimers. The reaction still shows half order characteristics, and the rate depression is almost complete at a 1 1 ratio of butoxide to polymer chains. The major species present in solution would seem to be (PstLi. BuOLi)2 at this point. Similar results have been obtained with polyisoprenyllithium in cyclohexane [78]. The nature of 

The effect of small amounts of cyclic ethers on the rates of lithium catalyzed reactions can be more pronounced, especially in styrene polymerization (Fig. 12). The observed propagation rate first increases rapidly, passes through a maximum, and then decreases for both dioxane 

With isoprene, the effect of tetrahydrofuran is much smaller and no maximum is observed at low tetrahydrofuran concentrations [33, 68]. With O.OIM THE in cyclohexane (which corresponds to a maximum rate for styrene) the rate of polymerization is barely changed. At O.IM THE the order in polyisoprenyllithium has increased from one quarter to one half and complete aggregate dissociation has probably not been achieved. Evidence for specific solvation is thus difficult to obtain since so much THE would have to be added that the nature of the medium is drastically changed. This suggests that the self-association of polyisoprenyllithium must be much stronger than for polystyryllithium, for there is no reason to doubt that THE does in fact strongly solvate all systems having a lithium counter-ion. 

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