Study of the propagation centre 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-lithioisobutyrate ( A ) in tetrahydrofuran (THF) appears to have one form only, irrespective 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 provide well-developed NMR signals and may be present as a partly organized 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 dependence on their mole ratio and partly on storage time. The ⁷Li, ¹H and ¹³C NMR relaxation behaviour of these complexes support their essentially tetrameric nature. Although intramolecular exchange of Li atoms in a complex appears to be rapid, intermolecular exchange of any of the components is vanishingly slow on the NMR time scale. The complexes 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 easily destroyed by thermal activation but they react readily with monomer. Implications of the results for anionic polymerization of (meth)acrylates are discussed.