Polyesters containing structural units derived from phosphonic acids or estere

A filament or fiber manufactured by spinning a snythetic linear modified polyester, drawing and after-treating, which comprises preparing said modified synthetic linear polyester by reacting

• a. at least one dicarboxylic acid as an acid or as an ester of a low molecular weight alcohol, said acid being suitable for the manufacture of a fiber-forming linear polyester
• b. at least one diol, said diol being suitable for the manufacture of a fiber-forming linear polyester said acid and diol being reacted by esterification or transesterification and polycondensation, while said acid and diol being reacted, adding prior to, during or shortly before termination of the polycondensation,
• c. about 5 to 15 mol percent based on the acid component, of a bifunctional alkylene-, arylene-, or aralkylene-diphosphinic acid, which may contain further hetero- and halogen atoms, and/or an ester of said diphosphonic acid of a low molecular weight alcohol or of a diol forming the diol component of said polyester, oligomer of said diphosphinic acid diol esters, or mixtures of oligomers of said diphosphinic acid diol esters with monomers of said ester of said diphosphinic acid, said bifunctional alkylene-, arylene, or aralkylene- diphosphinic acid, including said hetero- and halogen atoms, having the formula:
• ##EQU9## in which R represents a saturated open chain or cyclic alkylene, arylene or aralkylene radical, and where R may also have at least one hetero atom and/or at least one halogen atom selected from the group consisting of F, Cl, Br;
• R.sub.1 and R.sub.2 are identical or different and each represents an alkyl group having up to 6 C-atoms, an aryl group or an aralkyl group, which may contain at least one hetero atom and/or at least one halogen atom selected from the group consisting of F, Cl, and Br.