Patent ID: 7473794

Claim:
A process for preparing cyclic phosphonic anhydride of the formula (III) by a) reaction of a phosphonic acid derivative of the formula (I) with acetic anhydride at a temperature in the range from 30 to 150° C. and simultaneous distillative removal of a mixture of acetic acid and acetic anhydride, b) subsequent reactive distillation of the oligomeric phosphonic anhydride of the formula (II) obtained in step a) and conversion to the corresponding cyclic trimeric phosphonic anhydride of the formula (III) where n is an integer from 0 to 300 and R are allyl, aryl or open-chain cyclic or branched C 1 to C 8 -alkyl radicals, aryloxy, allyloxy or alkoxy having open-chain cyclic or branched C 1 to C 8 -alkyl radicals.