On the anionic polymerization of dialkylaminoisoprenes, 3. A nuclear magnetic resonance study of the structure of oligo[5‐(N,N‐diisopropylamino)isoprenyllithium]

The structure of oligo[(5‐ N,N ‐diisopropylamino)isoprenyllithium] (number‐average degree of polymerization P n = 1‐2) prepared by reaction of alkyllithium with 5‐( N,N ‐diisopropylamino)‐isoprene System. name: N,N ‐diisopropyl‐2‐vinylallylamine. in nonpolar (benzene) and polar (tetrahydrofuran) solvents has been studied by 1 H and 13 C NMR spectroscopy. In tetrahydrofuran, as expected, the negative charge at the terminal chain unit is more delocalized than in benzene and, consequently, the carbon‐lithium bond has a more ionic character. In both solvents the living chain end shows a cisoid ‐4, 1‐structure. No cistrans isomerization could be detected. This independence of the carbanion structure of the solvent polarity is explained by the complexation of the Li + counter ion to the amino group of the terminal monomer unit and by the bulkiness of the isopropyl side groups, which hinder isomerization and favour the cisoid conformation of the living chain end.