Process for the purification of thiophenes

Abstract

The invention relates to a process for the purification of thiophenes by means of precipitation. The purified thiophenes are liquid at room temperature, have a purity of at least 99.50 wt. %, and are represented by the following general formula (I), wherein R1 and R2 independently of each other are, for example, a linear or branched C1-C20-alkyl group, or together form a fused C1-C20-dioxyalylene ring. The process involves: (I) precipitating the thiophene by cooling a solution of the thiophene and at least one solvent; or (II) precipitating the thiophene by adding the thiophene to a cooled solution of solvent and optionally the thiophene. The solutions are cooled to a temperature below the melting point of the thiophene.

Description

FIELD OF THE INVENTION The invention relates to a process for the purification of thiophenes which are liquid at room temperature, the thiophenes purified by this process and their use. BACKGROUND OF THE INVENTION Thiophenes are used, for example, for the preparation of conductive polymers. Poly(3,4-alkylenedioxythiophenes) such as are described, for example, in EP-A 339 340, are of particular interest in this context. These compounds are distinguished by particular properties, such as high conductivity, high transparency and outstanding long-term stability. They have therefore found increasing use in industry as organic conductive polymers. Thus e.g. through-plating of printed circuit boards, antistatic treatment of photographic films and use as an electrode or solid electrolyte in solid electrolyte capacitors are described as important fields of use. An important prerequisite in the preparation of organic conductive polymers is high purity of the starting substances needed for ...

AI Technical Summary

Benefits of technology

There was therefore still a need for a process for the purification of thiophenes which are liquid at room temperature in which

Problems solved by technology

Impurities contained in the starting substance can adversely influence the polymerization in that the polymerization does not take place, or takes-place only very slowly or incompletely, or is accelerated to an uncontrolled extent.

The processing time of these monomers can consequently drop drastically, so that these can no longer be employed in the processing processes.

In addition, the properties of the resulting polymers may also be adversely influenced in that the impurities, for example, adversely change the intrinsic colour of the resulting polymer and as a result the transparency, which is essential for the use of the polymers e.g. as transparent conductive or antistatic coatings, is impaired.

Impurities which are also capable of polymerization can be co-incorporated into the polymer and thereby significantly lower the conductivity thereof.

Further adverse effects of impurities can be that the order of the conductive layers may be lowered by impurities, whereby poorer conductivities result, that impurities become concentrated on the surface of the polymer after the polymerization and undesirable transition resistances thereby result, so that the function of the conductive layer is restricted, or that the long-term stability of the conductive polymers is adversely influenced in that the impurities, for example, initiate reaction of the conductive polymer with oxygen and thus significantly impair the properties of the polymer.

Furthermore, separating off of compounds by distillation is only possible if the components to be separated differ significantly, i.e. by more than 1° C., in their boiling points.

The less the boiling points differ, the greater the expenditure on apparatus for the separation, so that such separations are no longer to be carried out economically.

Since substituted thiophenes, such as, for example, alkylenedioxythiophenes, are preferably distilled under reduced pressure, the difference in the boiling points is reduced further, which further increases the expenditure on separation.

The purification of 3,4-alkylenedioxythiophenes, in particular of 3,4-ethylenedioxythiophene, which are contaminated with 3,4-dimethoxythiophene represents a particular difficulty.

Thus, for example, 3,4-dimethoxythiophene produced during the synthesis of 3,4-ethylenedioxythiophene can be separated off only with a high expenditure because of the molecular weight differing by only two units and the very similar structure, which makes purification via distillation no longer economical beyond a certain degree of purity.

3,4-Dimethoxythiophene as an impurity has the disadvantage, however, that it is co-incorporated into the polymer during polymerization and can thus adversely influence properties of the polymer, such as, for example, the conductivity.

However, chromatographic separation also has disadvantages.

Furthermore, the chromatographic separation cannot be operated continuously with the aid of simple apparatuses, so that in each case only small amounts of the desired purified thiophene are obtained.

A continuous separation of large amounts would therefore be associated with an extremely high expenditure on apparatus, so that such a purification of thiophenes can no longer be carried out economically.

However, this process is limited to substances or substance mixtures which contain relatively large amounts of impurities which can be separated off in liquid form.

Small amounts of impurities can be removed only uneconomically via this process, since large amounts of the desired compound have to be separated off at the same time in order to wash out the small amount of impurity.

Moreover, melt crystallization is critical in respect of the temperature programme and therefore expensive on apparatus.

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