AbstractRadical polymerization of acrylonitrile in the presence of N-methylmorpholine N-oxide has been performed for the first time. It has been shown that the presence of N-methylmorpholine N-oxide leads to the decrease in the molecular mass of the polymer and induces its partial cyclization. These features have been preserved when N-methylmorpholine N-oxide has been introduced in the polymerization simultaneously with the monomer and the initiator as well as when it has been added at high conversion of the monomer. The introduction of N-methylmorpholine N-oxide into polyacrylonitrile has led to the cyclization of the nitrile units in two stages, the low-temperature one being initiated by N-methylmorpholine N-oxide and the high-temperature one, typical of pure polyacrylonitrile. The ratio between the intensity of these processes has been governed by the content of N-methylmorpholine N-oxide.