Method for the preparation of oligomeric alkylene terephthalates

Process for production of oligomeric alkylene terephthalates having an average degree of condensation of 4 to 7, a CH.sub.3 O content of less than 0.05 wt.%, and a monomeric terephthalic acid dialkanediol ester content of up to 30 wt.%, by transesterification of dimethylterephthalate and diol of 3 to 8 carbon atoms and condensation of the transesterification product, which comprises:

• (a) continuously passing the dimethylterephthalate, the diol, and a catalyst for the transesterification through a series of superposed reaction chambers from the uppermost chamber serially to the lowermost chamber, for the transesterification, in liquid phase, methanol being formed in the transesterification reaction and being distilled in the reaction chambers from the liquid phase during the transesterification, and withdrawing the methanol distilled from each chamber without intermixing thereof with the liquid present in any of the other chambers,
• (b) the molar proportion of dimethylterephthalate to diol introduced into the reaction chambers being 1:1.1 to 1:1.5,
• (c) heating said materials in the reaction chambers so that the temperature in the reaction chambers is in the range of 130.degree.-245.degree. C. and increases stepwise from chamber to chamber, and maintaining the pressure at normal or above normal in said reaction chambers, and stirring the materials during the reaction thereof with stirrers,
• (d) passing the transesterification product of the reaction chambers through a vacuum zone maintained at a reduced pressure of 20-300 Torr and temperature of 180.degree.-250.degree. C. for expelling of diol and production of said oligomer and withdrawing the oligmer from the vacuum zone, the temperature in the vacuum zone being higher than the highest temperature in the reaction chambers.