AbstractHigh molecular weight thermoplastic copolyesters from bisphenol A, neopentyl glycol, and terephthaloyl chloride have been prepared by a melt‐polymerization technique carried out in a twin‐screw, vacuum‐vented extruder. Low molecular weight polymer, polymerized in a conventional melt reactor, was further polymerized to copolyesters with intrinsic viscosities, under the conditions investigated, of up to 0.76 dl/g. The optimum extruder reaction conditions with respect to temperature profile, vent configuration, and polymer throughput are described. A variety of properties for two representative copolyester compositions have been determined. Particular attention was given to melt viscosity and impact strength, two properties which depend markedly on whether the polymer is prepared by the subject melt process or, alternately, by a low temperature interfacial polycondensation. It is concluded, based on a study of deliberately branched, interfacially prepared copolyesters, that the property differences which depend on the polymerization method are attributable to the formation of branches in the copolyesters prepared by the high‐temperature melt
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