The present work reports some new results on the dependence between the stereospecificity of the anionic propagation of isoprene and the nature of the active centers. The stereospecificity of the propagation of macroheterobicylce separated ion pairs does not depend on the nature of the alkali counterions, and the microstructure of the polyisoprenes obtained does not differ from that of those produced by free ions. Variations in the microstructure were observed in the propagation of contact ion pairs, depending on the nature of the cation and the solvent used. These variations are likely to be related to the degree of intimacy of the contact ion pairs. There are two main factors which affect the stereospecificity of the propagations in different ways: the size of the cation and the donating power of the solvent. Finally, the external solvation of the propagation ions pairs was confirmed by the microstructure of polyisoprenes synthesized in mixed (inert + donor) solvents.

By taking into account different possible interactions between the living end, the counterion and the nature of the solvent used on the one hand, and the influence of the temperature on the kinetics and the microstructures of polydienes on the other hand, it has been possible to suggest some new explanations concerning the mechanisms of the anionic propagation of butadiene and isoprene. In hydrocarbon media, the stereospecificity of the 1,4 propagation initiated by lithium should be considered as the consequence of the coordination of the counterion by both of the two bonds of the diene molécule. The stereospecificity of the vinyl propagation by the same counterion in dioxane solvent should be the consequence of the competition between the (Li+, dioxane) and (Li+, diene) coordination complexations. In this case, the Li+ counterion should only be coordinated by only one of the two double bonds of the diene molecule. With isoprene, the π‐electron donation should originate mainly from the C3C4 double bond. The decrease of the stereospecificity is due to the increasing size of the alkali counterion and the separation or the dissociation of the growing ion‐pairs.

With accumulated HR‐NMR spectra of anionic polyisoprenes, it has been possible to study the influence of the nature of the propagating species on the microstructure of the obtained polymers If free ions are responsible for the propagation, the microstructure (1,4‐: 25%, 1,2‐: 33%, 3,4‐: 42%) does not depend on the nature of the cations. But with contact ion pairs, the different addition modes are governed by the size of the alkali metal counterions. Mechanisms of anionic propagations via diene–cation coordination are proposed.