polyester forming derivatives thereof and the corresponding diol mixture in the presence of catalysts sorenson and campbell preparative methods of polymer chemistry interscience publishers inc n y 1961 pages 111 to 127 kunststoff handbuch plastics handbook volume viii c hanser verlag munich 1973 and j polym sci part a 1 4 pages 1851 to 1859 1966 the reaction temperatures are in the range from 160 u00b0 to 350 u00b0 c preferably in the range from 170 u00b0 to 280 u00b0 c the process is carried out in the substantial absence of oxygen either under normal pressure or in vacuo if the reaction is performed under normal pressure it is carried out in an inert gas atmosphere suitable inert gases include noble gases nitrogen and carbon dioxide the polycondensation stage is preferably carried out in vacuo the process of the present invention is preferably used to prepare poly ethylene terephthalate pet or poly butylene terephthalate pbt a suitable aromatic dicarboxylic acid for use in the present invention is terephthalic acid however other aromatic dicarboxylic acids such as phthalic acid isophthalic acid or naphthalene dicarboxylic acid or mixtures thereof can also be employed up to 30 mol of the aromatic dicarboxylic acid in the polyesters can be replaced by other conventional c 2 c 36 dicarboxylic acids suitable dicarboxylic acids include isophthalic acid phthalic acid 1 4 cyclohexanedicarboxylic acid adipic acid sebacic acid azelaic acid decanedicarboxylic acid and dimeric fatty acid the diol component is a combination of c 2.