Nuclear Magnetic Resonance Studies of the Propagating Chain End in the Organolithium Polymerization of Dienes. II. Isoprene and 2,3-Dimethylbutadiene in Hydrocarbon Solvents

Abstract

High-resolution studies have been carried out on the organolithium polymerization of isoprene and 2,3-dimethylbutadiene in hydrocarbon solvents.As in the case of the previously reported butadiene polymeriza- tion, no evidence could be found of any measurable amount of complexation of the isoprene with the organol- ithium chain end.The proton resonance spectrum of the chain-end unit in the case of isoprene indicates an ex- clusively 4,1 structure, rather than a 1,4 (no ß hydrogen), showing no 4,3 (or 1,2) structures, despite the fact that the in-chain units contain approximately 10% 3,4 structures.Similarly, the spectrum of the poly(2,3-dimeth- ylbutadiene) chain end confirms the peak assignments based on the butadiene and isoprene studies, and shows an exclusively 1,4 structure, despite the presence of 15-20% 1,2 in-chain units.Hence, as in the case of the buta- diene, it is suggested that these 1,2 or 3,4 units result from the presence of an undetectably small proportion of -allyl chain ends in equilibrium with the main component of -bonded (localized) chain ends.The isoprene chain-end spectrum permits the resolution of cis-and trans-4,1 chain-end units, which are present in approxi- mately the same ratio as their in-chain counterparts (cis/trans = 2).The observed differential rates of decompo- sition of these two isomeric forms of the chain end is taken as evidence that the 2,3 bond is not labile, i. e., no 4,1-4,3 tautomerism.This further strengthens the hypothesis of a localized-delocalized equilibrium ( -allyl-Tr- allyl) to account for the 3,4 unit content of the polymer, since such an equilibrium would not affect the cis-trans isomerism.