A process that can be used for direct esterification of a dicarboxylic acid such
as terephthalic acid with a glycol such as 1,3-propanediol. The process comprises
(1) contacting, at an elevated temperature, optionally in the presence of a preformed
oligomer, the acid with the glycol to produce a product mixture comprising (i)
a water-glycol vapor mixture, which or a portion of which exits the product mixture
at the temperature to form a water-glycol vapor mixture and (ii) a liquid product
mixture comprising an oligomer having a degree of polymerization of from about
1.9 to about 3.5 and comprising repeat units derived from the acid; (2) separating
the glycol from the water-glycol mixture to produce a recovered glycol; and (3)
returning the recovered glycol to the product mixture such that the liquid product
mixture comprises an excess free glycol.
