Process for manufacture of high molecular weight polyester resins from 2,6-naphthalene dicarboxylic acid

High quality linear polyester resin is prepared by reacting in a first step or zone 2,6-naphthalene dicarboxylic acid with a molten prepolymer derived from a glycol and 2,6-naphthalene dicarboxylic acid to form a low molecular weight product of the first step or zone; reacting this resultant product with a glycol in at least two successive steps or zones of glycol addition to form prepolymer; and polymerizing at least a portion of the resulting prepolymer to form polyester resin which exhibits midpoint glass transition temperatures above about 123 DEG C. The resulting prepolymer is particularly suitable for reaction with the 2,6-naphthalene dicarboxylic acid in the first step or zone. Advantageously, at least a portion of the prepolymer is, therefore, returned to the first step or zone.