Study of the propagation center in the anionic polymerization of (meth)acrylic monomers: NMR study of Di-tert-butyl 2-Lithio-2.4'-trimethylglutarate and living poly(tert-butyl methacrylate) oligomers in THF/toluene mixtures

Solvation influences both the stereoregularity and molecular weight distribution (MWD) of poly(tert-butyl methacrylate) (PtBMA) prepared by anionic polymerization of tBMA with a lithium counterion in THF/toluene mixed solvents at −78 °C. As a probe of the structural changes of the propagating species, multinuclear 7Li, 1H, and 13C NMR analysis of di-tert-butyl 2-lithio-2,4,4‘-trimethylglutarate (a model dimer, A) and of short-length living PtBMA chains is reported and discussed. Whatever the system, the main structure 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 exceeding 50% v/v, partly organized pseudophases can additionally be detected in the 7Li NMR spectra, but they do not provide well-developed 1H and 13C NMR signals. The chemical shifts of 1H and 13C signals of A support the dimeric form of this model. An equilibrium between dimeric and nonaggregated living PtBMA chains has been previously proposed to dominate in pure THF. However, marked self-termination of low-molecular weight living oligomers prepared 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.