ABSTRACT:An NMR study was conducted on the mechanisms of side reactions in anionic polymerization of acrylonitrile (AN) with di-n-hexylmagnesium as an initiator in mixed-xylene at 25°C. In the initiation step the metalation of AN monomer leads to deactivation of the initiator. In the propagation stage both chain transfer to an a-proton in propagating chain and metalation of monomer proceed in parallel with addition reaction of the AN monomer to the propagating chain. The former two reactions result in I) formation of branching structures and 2) deactivation of active site, respectively. Some new NMR peaks resulting from above side reactions, which did not appear in 13 C NMR spectra of radically prepared PAN, were assigned. According to the assignments in this work, the number-average molecular weight, M. (330---5600) and branching density, which means the number of branching per 100 repeating units, were estimated for acetone-soluble or acetone-insoluble products.KEY WORDS Anionic Polymerization / Acrylonitrile / Side Reaction / NMR / Two-Dimensional NMR / Metalation / Intramolecular Chain Transfer / Branching Structure / Acrylonitrile (AN) can be polymerized to give polyacrylonitrile (PAN) not only by radical initiators but also by anionic initiators. 1 In general, the advantages of anionic polymerization are easiness of controling the molecular weight distribution by means of the "living" nature of its propagating chain and route that they provide for the syntheses of new functional polymers such as block copolymer and star-like polymer. Anionic polymerization also affords a possible route for the production of stereoregular polymers by nature of counter ion structure forming in its propagating end group.In the case of polymerization of monomer with polar group such as AN, side reactions based on the attack of propagating carbanion on the polar group result in some loss of the "living" nature of the system. 2 • 3 In fact, the "living" nature has never been found in the * To whom all correspondences should be addressed.anionic polymerization of AN because of the direct attack of the propagating chain on the cyano group 4 or an attack against electrophilic a-proton activated by the cyano group. 5 In order to obtain high molecular weight PAN (ca. 5 x 10 5 ) with high yield, it is necessary to polymerize under strictly limited conditions, for example, at very low temperature. 5 This may be a major reason why the anionic polymerization of AN has not been utilized commercially, though only short time is needed for completion of the polymerization owing to low activation energy.As mentioned above, side reactions cannot be ignored in anionic polymerization of AN. Many authors have proposed mechanisms for the polymerization 4 -8 : Kawabata and Tsuruta6 indicated that the metalation reaction of monomer occurs at an early stage with butyllithium as an initiator. Ottolenghi and 245
