Stereospecificity of Alkaline Earth Metal Compounds in the Anionic Polymerization of N,N-Dimethylacrylamide

Abstract

N,N-Dimethyllacrylamide (DMA) was polymerized with several iniators containing alkaline earth metal compounds such as dibutylmagnesium, calcium alkoxyalkoxide, bis(triphenylmethyl)barium, and magnesium and barium alkoxyalanates. The organobarium initiators, like the organolithium ones, give mainly isotactic, THF-insolube and partially crystalline polymer, whereas the structure and properties of the polymer obatined with R 2 Mg are similar to those reported for the Cs-initated polymer. The isotactic triad content is determined ny the naure of the alkaline earth metal used, and is almost independent of the presence of aluminium in the active center, and of the replacement of THF solvent by toluene. Therefore, the stereopecificity of the Group II metal compounds, with respect to the formation od isotactic poly- (DMA), increase with increasing counterion radius which is opposite to the tendency found for the polymerization with alkali metals. The situation is similar to that in the anionic polymerization of butadiene, where the effect of the counterion size on polymer structure for alkaline earth initiators is also opposite to that found with alkali meatls.