Polymerization of acrylonitrile with magnesiumdialkyls. Reaction conditions and stereospecificity

The stereospecificity of acrylonitrile polymerization initiated by organometallic compounds of alkali and alkaline earth metals was investigated. The triad tacticities of the polymers obtained was analyzed by 13C NMR and the molecular weights and their distributions were measured by size exclusion chromatography. A decisive influence of temperature on acrylonitrile polymerization initiated by dialkylmagnesium (di‐n‐hexyl‐ and di‐n‐butylmagnesium) in mesitylene was found. Products polymerized at temperatures above 100°C contain high molecular weight fractions with predominantly isotactic configuration (isotactic triad content 50—55%). The stereospecificity of polymerization was observed only in a medium which is a non‐solvent for the product. All the other investigated metalorganics do not initiate a stereospecific polymerization on these conditions. In order to elucidate the stereospecific polymerization mechanism we examined NMR data by model calculations. The triad tacticity of some predominant isotactic polyacrylonitrile samples is described by the enantiomorphic sites (e.m.s.) model. This means that the propagation takes place at enantiomeric catalyst sites which determine the configuration of the incoming monomer unit. Summarizing the experimental observations and the interpretation of the polymer structure, a model of the active centers involving associated dialkylmagnesium structures is proposed.