A polyester is generally produced by direct esterification of an aromatic dicarboxylic acid and glycol, or by ester exchange reaction of an alkyl ester of aromatic dicarboxylic acid and glycol to give a glycol ester and/or an oligomer followed by heating them with stirring under high vacuum to effect polycondensation.
While, U.S. Pat. No. 2,597,643 discloses that polymerization of polyester can be carried out in a solvent. In such reaction, the polymerization is, however, carried out in a solution which can completely dissolve a resulting polyester, and hence, the resulting polyester has a very low intrinsic viscosity of 0.4-0.7.
There has been widely and industrially used a process wherein polycondensation is carried out by heating and stirring under a high vacuum. This process, however, requires certain vacuum apparatus to keep high vacuum and high power for stirring of material having high viscosity.
Further, since materials for industry use, such as a tire cord and the like, require higher physical properties, there is used polyester having a high molecular weight. However, it is difficult to stir such polyester having higher molecular weight, and hence, a polymer obtained by melt polycondensation is generally subjected to further polymerization in a solid phase under an inert gas flow for a long term of time.