STUDY OF THE PROPAGATION CENTER IN THE ANIONIC-POLYMERIZATION OF (METH)ACRYLIC MONOMERS .4. NMR AND QUANTUM-CHEMICAL STUDY OF THE INITIATION SYSTEMS CONTAINING TERT-BUTYL 2-LITHIOISOBUTYRATE AND LITHIUM TERT-BUTOXIDE OR LITHIUM 3-METHYLPENTAN-3-OLATE

In contrast to methyl 2-lithioisobutyrate (I), tert-butyl 2-lithioisob ulyrate (A) in tetrahydrofuran (THF) appears to have one form only irr espective of concentration (0,1-0,8 mol/L) or temperature (203-273 K). According to its relaxation characteristics, this form probably is a tetramer. The results indicate, however, that about 20% of A do not pr ovide well-developed NMR signals and may be present as a partly organi zed pseudophase. A is shown to form 2:2 and 1:3 complexes with lithium tert-butoxide (B) and lithium 3-methylpentan-3-olate (C) in THF in de pendence on their mole ratio and partly on storage time. The Li-7, H-1 and C-13 NMR relaxation behaviour of these complexes support their es sentially tetrameric nature. Although intramolecular exchange of Li at oms in a complex appears to be rapid, intermolecular exchange of any o f the components is vanishingly slow on the NMR time scale. The comple xes of A with either B or C exhibit remarkable stability above 273 K, in contrast to that of A itself. The quantum chemical analysis of the complexes shows that they are held together by a cooperative system of polycentric bonds and electrostatic interactions. Thus they are not e asily destroyed by thermal activation but they react readily with mono mer. Implications of the results for anionic polymerization of (meth)a crylates are discussed.