Process for preparing polybenzimidazoles

Abstract

The invention relates to a method for preparing polybenzimidazoles of formula (1) or (2) below, wherein n is ≥2: ##STR00001## by polycondensation of corresponding dicarboxylic acids or dialdehydes and tetraamines by jointly heating the reactants, characterized in that the preparation of polybenzimidazoles of formula (1) or (2) is carried out by using the dialdehydes as starting material and substantially without the formation of any intermediates, wherein a) first, the dialdehyde and the tetraamine are mixed in water at room temperature, which results in the formation of a polyimine by-product; whereafter b) polycondensation is carried out under hydrothermal conditions by heating, in water as a solvent and under pressure, to temperatures above 100° C.

Claims

1. A method for preparing polybenzimidazoles of formula (1) or (2) below, wherein n is ≥2: ##STR00013## by polycondensation of corresponding dialdehydes and tetraamines, characterized in that a) first, the dialdehyde and the tetraamine are mixed in water at room temperature, which results in the formation of a polyimine intermediate of formula (3) or (4), wherein p and m are each ≥1: ##STR00014## whereafter b) polycondensation is carried out under hydrothermal conditions by heating, in water as a solvent and under elevated pressure, to temperatures above 100° C.

2. The method according to claim 1, characterized in that a polybenzimidazole of formula (1) is prepared according to the reaction scheme below, wherein, in step a), terephthalic acid dialdehyde (TDA) and diaminobenzidine (DAB) are mixed in water and, in step b), they are polycondensed under hydrothermal conditions to form the polybenzimidazole of formula (1): ##STR00015##

3. The method according to claim 1, characterized in that a polybenzimidazole of formula (2) is prepared according to the reaction scheme below, wherein, in step a), isophthalic acid dialdehyde (IDA) and diaminobenzidine (DAB) are mixed in water and, in step b), they are polycondensed under hydrothermal conditions to form the polybenzimidazole of formula (2): ##STR00016##

4. The method according to claim 1, characterized in that the polycondensation in step b) is carried out at a temperature of at least 180° C. and for a duration of at least 30 min.

5. The method according to claim 4, characterized in that the polycondensation in step b) is carried out at a temperature of at least 180° C. and for a duration of at least 2 h.

6. The method according to claim 4, characterized in that the polycondensation in step b) is carried out at a temperature of at least 250° C. and for a duration of at least 30 min.

7. The method according to claim 1, characterized in that step a) is carried out in the presence of atmospheric oxygen and/or step b) is carried out in the absence of atmospheric oxygen.

8. The method according to claim 2, characterized in that the polycondensation in step b) is carried out at a temperature of at least 180° C. and for a duration of at least 30 min.

9. The method according to claim 8, characterized in that the polycondensation in step b) is carried out at a temperature of at least 180° C. and for a duration of at least 2 h.

10. The method according to claim 8, characterized in that the polycondensation in step b) is carried out at a temperature of at least 250° C. and for a duration of at least 30 min.

11. The method according to claim 8, characterized in that step a) is carried out in the presence of atmospheric oxygen and/or step b) is carried out in the absence of atmospheric oxygen.

12. The method according to claim 3, characterized in that the polycondensation in step b) is carried out at a temperature of at least 180° C. and for a duration of at least 30 min.

13. The method according to claim 12, characterized in that the polycondensation in step b) is carried out at a temperature of at least 180° C. and for a duration of at least 2 h.

14. The method according to claim 12, characterized in that the polycondensation in step b) is carried out at a temperature of at least 250° C. and for a duration of at least 30 min.

15. The method according to claim 12, characterized in that step a) is carried out in the presence of atmospheric oxygen and/or step b) is carried out in the absence of atmospheric oxygen.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) In the following, the present invention will be described in more detail by means of non-limiting examples and with reference to the appended drawings, wherein

(2) FIG. 1 illustrates the FTIR-ATR spectrum of the polyimine intermediate obtained in Example 1;

(3) FIG. 2 illustrates the FTIR-ATR spectrum of the polybenzimidazole obtained in Example 1; and

(4) FIG. 3 illustrates the FTIR-ATR spectrum of the polybenzimidazole obtained in Example 5.

EXAMPLES

Example 1

(5) Preparation of a polybenzimidazole of formula (1) at 250° C.

(6) ##STR00010##

(7) In a glass liner, 64 mg of terephthalic dialdehyde (TDA) (0.48 mmol, 1 eq.) were suspended in 40 ml of distilled H.sub.2O while stirring at room temperature. 103 mg of 3,3′-diaminobenzidine DAB (0.48 mmol, 1 eq.) were added and stirred for another 10 min at room temperature, yielding a red solid. The obtained red suspension was used directly, without isolating the solid, for hydrothermal synthesis. In order to characterize this intermediate product, in a separate batch, the solid was filtered off, washed, dried and analyzed by IR spectroscopy. The FTIR-ATR spectrum is shown in FIG. 1 and resulted in the identification of the red solid as a polyimine intermediate similar to that described in the introductory section. Actually, it corresponds to the following formula (3):

(8) ##STR00011##

(9) Similar to the polybenzimidazoles having a fused 5- or 6-membered ring that were mentioned in the introduction, the polyimine intermediates of formula (3)—and those of formula (4)—show cis- and trans-isomerisms with regard to the free amino groups due to the limited rotability of the conjugated, and thus planar, molecules. The latter may therefore point in opposing directions or in the same direction, as is illustrated in formulae (3) and (4) herein by the different moieties which are present n times or m times, respectively. Strictly speaking, there is a third alternative of the moieties in which both amino groups do not point up, but down. For the sake of clarity, this description refrains from an explicit illustration of this alternative. The proportion between these moieties cannot be determined explicitly and, due to their identical chemical characteristics, is not essential for the further course of the hydrothermal condensation, which removes the isomerism. Therefore, the latter does not affect the characteristics of the polybenzimidazoles obtained, which is why this will not be elaborated further herein.

(10) The characteristic bands of this polyimine are: v(C-H.sub.Imin)=2875 cm.sup.−1, v(C=N)=1595 cm.sup.−1 and v(C-N)=1205 cm.sup.−1.

(11) For hydrothermal polymerization giving the desired polybenzimidazole, the previously obtained red suspension was transferred into a microwave autoclave (120 ml). Next, the reaction mixture was heated at 250° C. within 15 min under stirring, which temperature was maintained for another 15 min. The reaction vessel was then cooled down within 30 min by means of a stream of pressurized air. The orange suspension obtained after opening the autoclave was filtrated, and a clear liquid was obtained in addition to an orange solid. The solid formed was thoroughly washed with distilled H.sub.2O and then EtOH and dried at 80° C. in a vacuum drying oven. The FTIR-ATR spectrum is shown in FIG. 2 and confirms that this is the desired polybenzimidazole. The bands characteristic for polybenzimidazoles are: v(C═N/C═C)=1615 cm.sup.−1, v(ring vibration)=1585 cm.sup.−1 (characteristic for conjugation between benzene and imidazole ring), v(benzimidazole; “in plane” deformation vibration)=1445 cm.sup.−1 and v(benzimidazole, ring breathing)=1285 cm.sup.−1.

(12) Extraction of samples of the polybenzimidazole of formula (1) using various organic solvents (MeOH, EtOH, iPrOH, phenol, PE, EE, CDCl.sub.2, CDCl.sub.3, acetone, acetonitrile) each yielded clear filtrates, not containing any impurities. Closer examination of the aqueous phase after hydrothermal polymerization showed that it did not contain any by-products of the hydrothermal polymerization, either.

Example 2

(13) Preparation of a polybenzimidazole of formula (1) at 180° C.

(14) The reaction was substantially the same as in Example 1, with the exception that the reaction mixture in the autoclave was heated at only 180° C. within 10 min, but maintained at this temperature for 2 h, and that cooling by means of a stream of pressurized air took 20 min. Again, in addition to a clear aqueous phase, an orange solid was obtained, the FTIR-ATR spectrum of which was substantially identical to the one shown in FIG. 2.

(15) Again, extraction attempts of the polybenzimidazole of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 3

(16) Preparation of a polybenzimidazole of formula (1) at 180° C. without stirring

(17) The reaction was substantially the same as in Example 2, with the exception that the red suspension formed by mixing the reactants was transferred into a non-stirring batch autoclave which was positioned into an oven preheated at 180° C., where the reaction mixture was left to react for 4 h. Next, the autoclave was cooled off by quenching with cold tap water. Again, a clear aqueous phase and an orange solid were obtained, the FTIR-ATR spectrum of which was substantially identical to the one shown in FIG. 2.

(18) Again, extraction attempts of the polybenzimidazole of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 4

(19) Preparation of a polybenzimidazole of formula (1) at 250° C. in argon

(20) Initially, the reactive process was substantially the same as in Example 1, although the reaction mixture was degassed by bubbling through argon before heating in order to remove oxygen both from the aqueous phase and from the head space above it. Then, it was heated at 250° C. within 60 min and maintained at this temperature for another 60 min (total reaction time: 2 h). Subsequent cooling of the reactor was not carried out by a stream of pressurized air, but by quenching with cold tap water. Again, in addition to a clear aqueous phase, an orange solid was obtained, the FTIR-ATR spectrum of which was also substantially identical to the one in FIG. 2, which shows that—as opposed to the teachings of the state of the art—the presence of oxygen is not required for completing cyclizing condensation of the polyimine intermediate to give the desired polybenzimidazole under hydrothermal conditions in a relatively short time.

(21) Once more, extraction attempts of the polybenzimidazole of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 5

(22) Preparation of a polybenzimidazole of formula (2) at 250° C.

(23) ##STR00012##

(24) The reactive process was substantially the same as in Example 1, although, instead of TDA, the same amount of isophthalic acid dianhydride (IDA) was used. Again, in addition to a clear aqueous phase, an orange solid was obtained (and cleaned in the same way), the FTIR-ATR spectrum of which is shown in FIG. 3. Using the latter, the solid was identified as the desired polybenzimidazole of formula (2). The characteristic bands are: v(C═N/C═C)=1625 cm.sup.−1, v(ring vibration)=1585 cm.sup.−1, v(benzimidazole; “in plane” deformation vibration)=1440 cm.sup.−1 and v(benzimidazole, ring breathing)=1285 cm.sup.−1.

(25) Extraction attempts of the polybenzimidazole of formula (2) analogous to those for the one of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results; thus, no by-products were formed in this case, either.