Study of the propagation center in the anionic polymerization of (meth)acrylic monomers: NMR study of di-tert-butyl 2-lithio-2,4,4 '-trimethylglutarate and living poly(tert-butyl methacrylate) oligomers in THF/toluene mixtures

Solvation influences both the stereoregularity and molecular weight distrib ution (MWD) of poly(tert-butyl methacrylate) (PtBMA) prepared by anionic po lymerization of tBMA with a lithium counterion in THF/toluene mixed solvent s at -78 degrees C. As a probe of the structural changes of the propagating species, multinuclear Li-7, H-1, and C-13 NMR analysis of di-tert-butyl 2- lithio-2,4,4'-trimethylglutarate (a model dimer, A) and of short-length liv ing PtBMA chains is reported and discussed. Whatever the system, the main s tructure of the lithium ester enolate in THF/toluene mixtures is similar to that one reported in THF. In the case of A and for toluene contents exceed ing 50% v/v, partly organized pseudophases can additionally be detected in the Li-7 NMR spectra, but they do not provide well-developed H-1 and C-13 N MR signals. The chemical shifts of H-1 and C-13 signals of A support the di meric form of this model. An equilibrium between dimeric and nonaggregated living PtBMA chains has been previously proposed to dominate in pure THF. H owever, marked self-termination of low-molecular weight living oligomers pr epared in THF/toluene mixtures can be observed at 263 K, in contrast to the stability observed in pure THF up to 298 K. Another contrasting feature is the broadening of the molecular weight distribution which can be explained by an aggregation process that stabilizes oligomers at the early stage of the anionic polymerization as supported by SEC-RI and SEC-UV eluograms.