ANIONIC-POLYMERIZATION OF ACRYLIC-MONOMERS .11. NMR INVESTIGATION OF THE MIXED COMPLEXATION OF METHYL ALPHA-LITHIOISOBUTYRATE AND LITHIUM-CHLORIDE

Solutions of methyl alpha-lithioisobutyrate (MIBLi) in THF added with various amounts of lithium chloride (LiCl) have been analyzed by Li-7 NMR spectroscopy in a wide range of temperatures. Although below ca. 2 05 K tetramers of MIBLi have been reported to be in equilibrium with d imers, 1/1 and 2/1 LiCl/MIBLi mixed complexes instantaneously form upo n the addition of 1 and 2 mol equiv of LiCl, respectively. Further add ition of LiCl more likely promotes the formation of a less stable 3/1 complex. Both 1/1 and 2/1 adducts are stable between -80-degrees-C and at least +12-degrees-C. From C-13 NMR spectroscopy, it appears that t he addition of an equimolar amount of LiCl to the MIBLi solution in TH F is at the origin of a ca. 7.0 ppm upfield shift of the metalated alp ha-carbon compared to the tetrameric MIBLi species and a ca. 1.0 ppm d ownfield shift with respect to the dimeric MIBLi. However, the charge distribution over the carbanion does not change anymore when the amoun t of LiCl exceeds 1 mol equiv. In contrast to nonligated MEBLi, the OR group has a considerable rotational freedom in LiCl-modified MIBLi. A comparison of the structure and dynamics of the LiCl-modified MIBLi s pecies in THF at 197 K with the characteristic features of the anionic polymerization of MMA under the same conditions suggests that the for mation of one type of LiCl-complexed species has a decisive effect on the control of the living polymerization of MMA. This conclusion is in agreement with a kinetic study carried out by Muller for the same sys tem.