Study of the propagation centre in the anionic polymerization of (meth)acrylic monomers, 7 . Study of the interaction of the living dimer with lithium tert ‐butoxide in tetrahydrofuran

Using 1 H, 6 I i, 7 Li and 13 C NMR, di‐ tert ‐butyl 2‐lithio‐2,4,4‐trimethylglutarate ( A ) as a dimeric model of the living poly( tert ‐butyl methacrylate) is shown to form well‐defined aggregates with lithium tert ‐butoxide ( B ) in tetrahydrofuran. The prevailing aggregates with the composition A 2 B 2 and AB 3 are controlled mainly by the ratio A : B . The initial mixture of aggregates is formed very fast but is metastable. Using 1D and 2D spectra, 1 H‐ 1 H and 1 H‐ 6 Li nuclear Overhauser effect and relaxations of 1 H, 7 Li and 13 C, both aggregates are shown to exist mainly in three conformational forms ( b, c, d ) of the A ‐part which are very similar in both aggregate types. In agreement with the previously published infrared spectra, form d has the γ‐ester group of A coordinated to Li. d does not exchange with the ‘open’ forms b or c with perceptible rate on the NMR time scale. Its population is promoted by a larger content of B in the aggregate. By m odified n eglect of d ifferential o verlap (MNDO) quantum calculations, the aggregates A 2 B 2 and AB 3 are predicted to have a roughly cubical core (OLi) 4 which is distorted in the d form where one ( AB 3 ) or two ( A 2 B 2 ) Li atoms are pulled out of the cube by coordination to the γ‐ester group. The stabilization of the supramolecule A 2 B 2 or AB 3 and the promoting effect of B on intramolecular coordination to the γ‐ester group in A is explained in terms of a cooperative system of polycentric bonds and electrostatic interactions.