Thermoplastic block copolyesters, a process for their production and their use for the production of mouldings

Block copolyesters having a relative viscosity (as measured in phenol/o-dichlorobenzene, ratio by weight 1:1--at 25.degree. C.) of from 1.15 to 3.0 and, disregarding the terminal groups, consisting of:

• (A) from 1 to 60% by weight, based on the block copolyester, a long-chain units corresponding to the following formula ##STR7## and/or ##STR8## and/or ##STR9## and (B) from 40 to 99% by weight, based on the block copolyester, of short-chain units corresponding to the following formula ##STR10## characterised in that M is a difunctional polymethylene radical containing terminal bonding sites and having an average molecular weight M.sub.n of from 400 to 5000;
• any side chains present must be methyl and/or ethyl groups and the polydiene radical on which the polymethylene residue is based contains at least 50 mole percent of 1,4-bonds,
• G is a difunctional aliphatic radical containing from 1 to 12 carbon atoms and/or a difunctional cycloaliphatic radical containing from 6 to 12 carbon atoms and having a molecular weight below 300, at least 70 mole percent of G consisting of one and the same radical,
• X is a direct bond or a difunctional aliphatic, cycloaliphatic and/or aromatic radical containing from 1 to 14 carbon atoms and having a molecular weight below 300, at least 70 mole percent of X consisting of 1,4-phenylene radicals.