Nuclear magnetic resonance studies of the propagating chain end in the organolithium polymerization of dienes. I. Butadiene in hydrocarbon media
High-resolution nmr studies have been carried out on the organolithium polymerization of butadiene in hydrocarbon solvents. As a measure of any possible complexation between the monomer and the active chain end, the proton resonance spectrum of butadiene was determined in the presence and absence of short-chain poly(perdeuteriobutadienyl)lithum (xn ∼ 20) at high concentrations (0.2 M) of both, in benzene-d6 or toluene-d8. No effect was observed on the proton resonances of the monomer, indicating that, if such complexation does occur, the concentration of the complex must be very low (<1% based on a slow exchange rate). Using ethyllithium-d5 to initiate short-chain poly(butadienyl)lithium and the technique of "pseudotermination" by butadiene-d6, it has been found possible to assign the three resonance peaks corresponding with protons on the terminal carbon (α), as well as the penultimate (β) and antepenultimate (γ) carbon atoms in the chain. These peaks indicate that the terminal chain unit has a 1,4 structure (no detectable 1,2), whereas the in-chain units show about 9% 1,2 structure. This has been taken to indicate the existence of an equilibrium between a localized (covalent) and a delocalized (π-allyl) chain-end unit, the latter being in too low a concentration to be detected, but leading to the inclusion of a 1,2 in-chain unit by subsequent monomer addition.