0.1-0.8 part of molybdenum or vanadium(IV) oxide acetyl-acetonate is then added as a catalyst, or a V4A wire gauze is wound around the stirrer. A stream of air, preheated if appropriate, of about 15 liters per hour is then introduced through an immersed tube at a temperature of 103° C. for about 8-9 hours. The stream of waste gas is passed over a separator for the water of reaction, over a cooler and over a washing column, for example with polyethylene glycol dimethyl ether, for absorption of xylene emissions. The current concentration of the oxygen is measured in the stream of waste gas. It is about 15% by volume on average. In this apparatus arrangement, the oxygen content of the air is utilized to the extent of about 25%. Samples are taken hourly for liquid chromatography (HPLC), and a conversion curve, expressed in percent by weight, of starting material and product is thus plotted. The 1st derivation (angle of the tangents) should decrease constantly from about 50% of the conversion. About 20% of the xylene (149 parts with 24 parts of propionic acid) is then distilled off. The remaining majority of the xylene is then distilled off by steam distillation, an aqueous suspension of 2,5-di-p-toluidino-terephthalic acid dimethyl ester finally being obtained. The xylene removed by steam distillation is purified and used for the preparation of the SucEst. The 2,5-di-p-toluidino-terephthalic acid dimethyl ester is isolated by filtration over a heated pressure suction filter (preferably fitted with a stirrer). The crystal-line crude product (226 parts by weight, melting point 205°-207° C.) on the suction filter is blown out with steam until it is odorless. It is then washed with methanol. 195 parts of methanol-moist 2,5-di-p-toluidino-terephthalic acid dimethyl ester (=97% of theory, based on the SucEst,) of melting point 215°-220° C. are obtained.
