PURPOSE: To provide the titled compsn. which can be easily molded, is not discolored during heating, gives moldings having excellent heat resistance, etc. and is suitable for use in the production of gas barrier film, etc., by blending a dimethylformamide/boron trifluoride complex with polyacrylonitrile.

CONSTITUTION: 5W50pts.wt. dimethylformamide/boron trifluoride complex is blended with 100pts.wt. polyacrylonitrile to obtain the desired molding polyacrylonitrile compsn. By this blending, the intermolecular cohesive force of polyacrylonitrile is relaxed to thereby facilitate molding and further polyacrylonitrile can be prevented from being discolored by an intramolecular cyclization reaction during heating. The dimethylformamide/boron trifluoride complex is present in the form of a white powder at room temp. and can be obtd. by adding an equimoler amount of boron trifluoride to dimethylformamide.