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] (numbe r-average degree of polymerization P-n = 1-2) prepared by reaction of alkyllithium with 5-(N,N-diisopropylamino)isoprene in nonpolar (benzen e) and polar (tetrahydrofuran) solvents has been studied by H-1 and C- 13 NMR spectroscopy. In tetrahydrofuran, as expected, the negative cha rge 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 c is-trans isomerization could be detected. This independence of the car banion structure of the solvent polarity is explained by the complexat ion 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 i somerization and favour the cisoid conformation of the living chain en d.