High molecular weight linear polyesters and method for their preparation

The molecular weight of linear polyesters such as poly(butylene terephthalate) and poly(ethylene terephthalate), especially the former, is increased by contacting a polyester having a maximum particle diameter of 1.0 and preferably 0.5 mm., with a mixture of an inert gas and an aliphatic diol such as 1,4-butanediol in the gaseous state at a temperature about 5 DEG -15 DEG C. below its equilibrium melting temperature, at a diol partial pressure of about 0.1-10 torr, to reduce the proportion of acid end groups; and subjecting the product thus obtained to solid state polymerization under vacuum. By this method, polyesters having a number average molecular weight of at least 100,000, useful for blow molding and the like, can be obtained.