Process for producing polytrimethylene terephthalate

This invention is an improvement upon the known process for polymerization of polytrimethylene terephthalate by esterification of terephthalic acid or dimethyl terephthalate with 1,3-propane diol, precondensation of the esterification product to produce a precondensation product, and polycondensation of the precondensation product to produce polytrimethylene terephthalate and, optionally, solid stating of this product to produce higher intrinsic viscosity polytrimethylene terephthalate. The cyclic dimer of terephthalic acid and 1,3-propane diol is produced as a byproduct of the above process and the improvement comprises(a) recovering the cyclic dimer from the polymerization,(b) reacting from 0.5 to 25 percent by weight of the cyclic dimer in 1,3-propane diol in the presence of from 0.1 to 10 percent by weight of the cyclic dimer of a basic catalyst for five minutes to 24 hours at 50 to 210° C.,(c) optionally neutralizing the basic catalyst in the reaction product of (b), and(d) recycling the reaction product into the polymerization process.