Anionic polymerization of acrylic monomers. 14. Carbon-13 and lithium-7 NMR characterization of the complexation of methyl α-lithioisobutyrate with various lithium cation-binding ligands in tetrahydrofuran

The aggregation equilibrium and electronic structure of methyl alpha-lithioisobutyrate (MIBLi), a monomeric model structurally corresponding to a living poly(methyl methacrylate) chain end associated with a lithium countercation, has been examined by Li-7 and/or C-13 NMR spectroscopy in tetrahydrofuran (THF) in the presence of various lithium cation-binding ligands. The addition of dimethoxyethane (DME), triglyme (glyme-3),12-crown-4 (12-CE-4), and hexamethylphosphoric triamide (HMPA) to coexisting tetrameric and dimeric MIBLi in THF induces an increase in dimeric population in the order DME < glyme-3 < 12-CE-4 <HMPA. That order corresponds to an increasing strength of complexation between MIBLi (lithium cation) and the (solvating) ligands For either tetrameric or dimeric species, moreover, the negative charge distribution around MIBLi does not seem to be significantly modified due to the presence of these complexing agents. In striking contrast, complexation of MIBLi by cryptand 211 (K211) does shift the equilibrium between tetramer and dimer toward the formation of a single monomeric K211-complexed species, which gives rise to ca. 13 and 5 ppm upfield shifts at C(alpha) in the C-13 NMR spectra, compared to tetrameric and dimeric species, respectively. The effect of 12-CE-4/K211 on the original aggretation equilbrium for MIBLi in THF nicely explains the significantly different MWD of PMMA anionically prepared in the presence of each of these ligands.