AbstractAcrylonitrile was polymerized in liquid sulfur dioxide at a temperature of 50°C., using azo‐bis‐isobutyronitrile as initiator. Elemental analyses of the polymer obtained showed that the product was homopolymer acrylonitrile irrespective of the conditions and the conversion. Radical polymerization of vinyl compounds in liquid sulfur dioxide was thought to give a polysulfone and the present result can be explained by considering a repulsion between sulfur dioxide and acrylonitrile. Polymerization generally proceeds in heterogeneous system and the overall rate of the polymerization (Rp) was found to be represented by the equation: equation image The activation energy for the reaction was calculated as 22.4 Kcal/Mole. The decomposition rate constant of the initiator and its chain transfer constant to the acrylonitrile monomer in liquid sulfur dioxide were calculated.