FIELD: chemical technology. SUBSTANCE: invention describes method for preparing graft copolymer that involves stages under conditions of complete absence of oxidation: (a) treatment of polymeric material with organic compound initiating agent of free- radical polymerization; (b) treatment of propylene polymeric material simultaneously with stage (a) or after stage (a) with overlapping temporary periods of both treatments or without that with amounts of at least one grafting monomer able to free-radical polymerization with values about from 5 to about 240 parts per 100 parts of propylene polymeric material, and (c) removal unreacted grafting monomer from prepared grafting polymeric material, decomposition of unreacted initiating agent and deactivation of residual free radicals in material being (a) and (b) stages are carried out in the presence of nitrogen oxide that is added with inert gas in the amount that is equal to values from about 0.05 part to about 50 parts of nitrogen oxide per million parts of inert gas for reducing reactor stoppage being grafting monomer is taken among group consisting of vinyl-substituted aromatic, heterocyclic and alicyclic compounds; unsaturated aliphatic carboxylic acids and their derivatives; unsaturated aliphatic nitriles; vinyl esters of aromatic and saturated aliphatic carboxylic acids and their mixtures. Invention allows reducing reactor stoppage in continuous feeding nitrogen oxide in preparing polypropylene graft copolymers. EFFECT: enhanced effectiveness of technology. 5 cl, 3 tbl, 16 ex
