Linear polyesters based on heterocyclic dicarboxylic acids

A linear thermoplastic polyester with a relative viscosity of 1.2 to 3, measured at 30.degree. C in a 1% solution in a solvent consisting of equal parts of phenol and tetrachloroethane, and having a structural formula consisting of 50 molar percent of radicals derived from the dicarboxylic acids of components (a) plus (b) and 50 molar percent of radicals derived from the diols of component (c), which consists essentially of

• a. 5 to 50 molar percent of dicarboxylic acid radicals of formula I ##STR8## wherein R represents alkylene of 1 to 2 carbon atoms;
• alkylene of 1 to 2 carbon atoms substituted with alkyl of 1 to 6 carbon atoms;
• cyclohexylene, phenylene or benzylene, the methylene group of which is attached to the nitrogen atom;
• and
• each of R.sup.1 and R.sup.2 represents hydrogen, alkyl of 1 to 18 carbon atoms, cycloalkyl of 5 to 5 ring members or phenyl;
• or
• R.sup.1 and R.sup.2 together represent --(CH.sub.2).sub.n --, in which n is 4, 5, 6 or 7, and if R is phenylene, R.sup.1 represents hydrogen and R.sup.2 represents alkyl of 1 to 18 carbon atoms or cycloalkyl of 5 to 8 ring members;
• b. 0 to 45 molar percent of dicarboxylic acid radicals derived from the acids selected from the group consisting of terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenylsulfone-dicarboxylic acid and alkanedioic acids of 6 to 12 carbon atoms;
• and
• c. 50 molar percent diol radicals of at least one diol derived from the diols selected from the group consisting of the alkylene diols of 2 to 10 carbon atoms;
• 1,4-cyclohexanediol, 1,4-bis-(hydroxymethyl)cyclohexane, 1,1'-methylene-bis [3-(2-hydroxyethyl)-5,5-dimethylhydantoin] and a diol of the formula II ##STR9## wherein X' and X" represent hydrogen, chlorine or bromine, or X' represents hydrogen and X" represents chlorine or bromine.