Flame retarded polymers

Use of phosphorus compounds of the formulae (I) or (II)

• 4
• where
• X is chloro or bromo,
• R1 and R2 independently of one another are hydrogen, C1-18 alkyl, C6-18 aryl, C7-24 alkylaryl, or R1 and R2 together form a cyclohexyl or cyclohexenyl radical, and
• R is chloro, bromo or a group of the formula (III)
• 5
• and
• n is a number between 0 and 20,
• as flame retardants for polyacrylonitrile, its copolymers or polymer blends thereof, especially fibers or films of these materials.