By PMR, three dioxane–Li+ complexes have been detected for the oligopolyisoprenyl–lithium–dioxane mixtures in benzene solution. The (4‐1) living end isomerizes into the cis configuration when the sample temperature or the dioxane concentration is increased, but the isomerization phenomenon is reversible only if dioxane/Li+ ratios are smaller than unity. The specificity of the vinyl addition (86%), mainly (4‐3) (70%), due to the stronger negative charge at the Cγ carbon atom is effective only at low temperature (+15°C). At higher temperature, because of the steric hindrance of the methyl group, this stereospecificity decreases.

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.

Analysis by 250‐MHz proton magnetic resonance (PMR) allows more precise examination of the microstructure of anionic polyisoprenes and polybutadienes obtained in the presence of organo‐alkali metals. Besides vinyl addition, the propagation of contact ion pairs in dioxane solvent gives a mixture of cis and trans products with isoprene, whereas only trans product is obtained with butadiene. The presence of 1,4‐trans‐polyisoprene appears to be inconsistent with the cis configuration of the polyisoprenyllithium living end in dioxane or tetrahydrofurane media. In taking into account the existence of complexation phenomena, it was suggested that propagation mechanisms should involve a transition state which should begin with the trans configuration before isomerization into the more stable cis configuration. For the free ions, the living end of the carbanion should be entirely trans.

SYNOPSISIn the present work, we use the binary insertion compound LiClz to polymerize styrene, methyl methacrylate, butadiene, isoprene, and to copolymerize isoprene and styrene in various hydrocarbon solvents (aromatics and aliphatic) and etheral solvents. We show that the styrene polymerization in aromatic solvents gives better yields than in the etheral solvent, the polymer being atactic. Methyl methacrylate does not polymerize in toluene but does so completely in DME. More generally, the yields of polymerization are better with KC37 than with LiClz because of the different capacities of the monomer to get into the carbon layers. The polymerization of dienes with LiClz shows that the microstructures of the polymer obtained in ?r-or n-donor solvents are similar to the ones obtained by homogenous polymerization with Li cation in such solvents. However, for isoprene in cyclohexane, the results are different. The isoprene styrene copolymers are statistical ones and the mean length of styrene blocks is less than 5. The monomer interaction with the insertion compound and the growing chain geometry between the carbon layers are the facts which control either the stereospecificity of the polymerization or the selectivity of the copolymerization.