WHEREIN R represents saturated, open-chain alkylene having from 1 to 15 carbon atoms, phenylene, biphenylene or phenylalkylene, R1 and R2 represent alkyl or cycloalkyl groups, as well as phenyl or benzyl and R3 represents saturated, open-chain alkylene and n is from 1 to 30, are described as well as suitable processes for the preparation of the di-phosphinic acid ester named and their use for flame-proofing polyesters.

Di-phosphinic acid esters of the formula