ANIONIC-POLYMERIZATION OF ACRYLIC-MONOMERS .13. C-13 NMR CHARACTERIZATION OF THE MIXED COMPLEXATION OF METHYL ALPHA-LITHIOISOBUTYRATE BY LITHIUM TERT-BUTOXIDE IN TETRAHYDROFURAN

The mixed complexation of methyl alpha-lithioisobutyrate(MIBLi)(a mono meric model structurally corresponding to the living end of PMMA-Li+) with lithium tert-butoxide (LiOtBu) in THF has been studied for the fi rst time by using C-13 NMR spectroscopy. It is shown that LiOtBu, like LiCl, is very effective in coordinating with MIBLi tetramer in THF, i ncrementally replacing MIBLi with the formation of other tetrameric co mplexes, MIBxLi4(OtBu)4-x (x = 1-3). Complextion of MIBLi by LiOtBu in duces a shielding C-13 shift of the MIBLi C(alpha) resonance, which mi ght suggest that lithium cations associated with the electron-rich lig ands, i.e., LiOtBu, can also form weaker bonds with the carbonyl oxyge n of MIBLi (i.e., mu-type complexes), thereby decreasing the localizat ion of the negative charge on that latter atom. In comparison with LiC l, one of the most significant features is that only one single type o f species results from mixed complexation of MIBLi and LiCl at LiCl/MI BLi = 1, 2, and greater-than-or-equal-to 3, respectively, at low tempe rature, whereas several LiOtBu-complexed species may coexist, whatever the LiOtBu/MIBLi ratio; furthermore, their exchange is found to be ve ry slow at a temperature as high as 0-degrees-C. Tentatively, this str iking feature may explain nicely the large difference in molecular wei ght distribution of poly(meth)acrylates anionically prepared in the pr esence of LiOtBu and LiCl, respectively.