NNR spectroscopy was used to study the mchanics of side reactions during anionic polymerization of acrylonitrile (AN) with di-n-hexylmagnesium as an initiator in mixed xylene at 25 degrees centigrade. The metalation of AN monomer in the initiaion step led to deactivation of the initiator. During propagation, both chain transfer to an alpha proton in the propagating chain and metalation of the monomer proceeded in parallel with the addition reaction of the AN monomer to the propagating chain. The reactions resulted in formation of branching structures and deactivation of the active site. New NMR weight and branching density of the acetone soluble or acetone-insoluble products were estimated 34 refs.