Continuous manufacture of polybutylene terephthalates

A process for the continuous manufacture of linear, high molecular weight polybutylene terephthalates, wherein

• a. dimethyl terephthalate is reacted with 1,4-butanediol, if desired in the presence of up to 40 mole% of further starting materials which form linear polyesters, in the molar ratio of from 1:1.2 to 1:1.5 in a plurality of successive stages at temperatures rising from 160.degree. to 230.degree. C in the presence of trans-esterification catalysts, and methanol is distilled off continuously.
• b. the resulting mixture of dihydroxybutyl terephthalate and its oligomers is subsequently passed, at from 230.degree. to 270.degree. C and from 20 to 2 mm Hg, upward through a bundle of stationary heated tubes, and the resulting mixture of vapor and liquid is passed through a hold tank located immediately above the tube ends, the volume of the hold tank being at least 1/4 and at most 2.5 times the volume of the tube bundle and the mean residence time in the tube bundle and hold tank together being from 10 to 60 minutes, most of the amount of heat required to heat the reaction mixture and vaporize the volatile constituents of the reaction mixture being supplied to the reaction mixture in the tubes, and the mixing of the reaction mixture being effected predominantly by the vapor bubbles rising from the tubes, and
• c. the precondensate thus obtained is polycondensed at from 240.degree. to 260.degree. C and from 0.1 to 2 mm Hg, with continuous formation of thin films.