Abstract: A process for producing a linear polyester comprising a main structural unit of an aromatic dicarboxylic acid ester of an alkylene glycol, particularly, comprising a polyethylene terephthalate or polytetramethylene terephthalate unit, is provided. In the process of the invention, the polycondensation reaction is conducted in the presence of a niobic acid catalyst that is obtained by heat-treating a hydrated niobium compound at the temperature range between 80.degree. C. and 400.degree. C. The process of the invention is also effecitvely applied to the manufacture of an elastomeric polyester comprising a polyalkylene terephthalate as a hard segment and a polyalkylene glycol as a soft segment.

Abstract: A process for the production of polyester comprising at least 80% of an ethylene terephthalate repeating unit using as a starting material an ester compound made mainly of terephthalic acid and ethylene glycol or an oligomer thereof is disclosed. This process is characterized in that a prepolymer having an intrinsic viscosity of not more than 0.4 dl/g and a carboxyl terminal group concentration [COOH] of not more than 90.times.[.eta.].sup.-0.4 .mu.eq/g is fed to a final polymerization unit and polymerized in the form of a molten thin film having a thickness of from 0.005 to 0.15 mm.