[en] Although the anionic polymerization of tert-butyl methacrylate (tBMA) induces a narrow and unimodal molecular weight distribution (MWD) when performed in either pure THF or pure toluene at -78 degrees C in the presence of a Lithium counterion, that in THF/toluene mixtures, keeping other conditions unchanged, surprisingly gives rise to broad bi- and even multimodal MWD's. Moreover, with increasing polymerization temperature from -78 to 0 degrees C, that multimodal MWD gets narrower, atypical of termination reactions, and indeed at 0 degrees C, a very narrow unimodal MWD is again obtained. These results have been satisfactorily interpreted in terms of a multiple solvation equilibria mechanism: there coexist several types of THF-solvated and nonsolvated active species and the exchange among them is slow at -78 degrees C but fast at 0 degrees C compared to the monomer propagation rate, thus resulting in a multimodal MWD in the former case and a narrow one in the latter. Furthermore, this solvation mechanism also dominates the stereoregulation of the tBuMA polymerization in mixed solvent. It has indeed been demonstrated that addition of various types of ligands, i.e., lithium chloride (LiCl) and lithium tert-butoxide (LiOtBu) (both mu-ligands), 12-crown-4 (12CE4) and cryptand 211 (K211) (both sigma-chelating ligands), and Lithium 2-(2-methoxyethoxy)ethoxide (LiOE(2)M) (a mu/sigma dual type of ligand), affects the tBMA anionic polymerization in a way which can be accounted for by these so-called solvation/ligation multiple equilibria among THF-nonsolvated, THF-solvated, and Ligated ion pairs. Once again, the fact that, in comparison with 12CE4 and LiOtBu, such ligands as LiOE(2)M, K211, and LiCl can effectively promote a much narrower MWD is nicely related to their high propensity to coordinate with a lithium ester enolate and at the same time to either promote a fast-exchanging ligation equilibrium or form a single type of active complex, in good agreement with the NMR investigation of corresponding model systems as previously reported by us.
Center for Education and Research on Macromolecules (CERM)
Elf Atochem ; Services de Programmation de la Politique Scientifique, Brussels, Belgium