Polymerization of acrylonitrile with magnesiumdialkyls. Reaction conditions and stereospecificity

Abstract

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 I3CNMRand the molecularweights 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.

Acrylonitrile Polymerization by Cy3PCuMe and (Bipy)2FeEt2

“…In addition to the main-chain - C H 2 C ( C N)H- resonances, several other peaks are present in the 13 C NMR spectra of PAN produced by 1 , which are labeled with lowercase letters in Figure or asterisks in Figure . These peaks are characteristic of branches formed in anionic polymerizations, 7b-d,32a,33a and their assignments by Kamide and co-workers 7c are shown in Figure . In contrast, signals for branched structures are absent from NMR spectra of PAN obtained by radical polymerization (Figures d and d). 4a-c,7c,32a, Approximately 30 branches per 500 monomer units (which includes monomer units in the branches) were found for AN:initiator ratios of ca.…”

“…The PANs produced by 1 between −37 and 23 °C are all slightly isotactic ( mm = 30−35%, Table ). PAN obtained by radical polymerization is usually atactic, while anionic polymerizations yield slightly to highly isotactic polymer. 32a, In previous work only small variations in tacticity were found in anionic polymerizations between −80 and 40 °C 32a4 Acrylonitrile Polymerizations with Different Initiators in Toluene Solution % Et ends a entryinitiator T (°C)[AN] (M)[AN]/[initiator]time (min)% conversionactivity b mm (%)branch number c observedtheory 1 1 80 2.3 440 120 1 0.3 2 1 23 2.3 440 355 5 0.5 3 1 23 6.5 4400 3 26 1700 32 16 0.03 0.1 4 d 1 0 3.9 1100 40 19 39 ± 18 e 34 34 0.1 0.5 5 1 0 f 3.4 1100 45 40 56 32 19 0.05 0.3 6 1 0 6.4 4100 60 20 60 32 26 0.04 0.1 7 1 −37 7.9 430 80 61 2 30 8 1 −37 6.6 4500 135 17 25 31 15 0.05 0.1 9 1 g 0 3.9 1000 40 11 15 35 44 0.2 1.0 <...…”