PROCESS FOR PREPARING POLYBENZIMIDAZOLES

Abstract

The invention relates to a method for preparing polybenzimidazoles of formula (1) or (2) below, wherein n and m are each 1:

##STR00001##

by polycondensation of corresponding tetracarboxylic acids or dianhydrides and tetraamines by jointly heating the reactants, characterized in that
the preparation of polybenzimidazoles of formula (1) or (2) is carried out by using tetracarboxylic acids as starting material and substantially without the formation of any by-products, wherein a) first, a stoichiometric salt is formed from the tetracarboxylic acid and the tetraamine; b) polycondensation is carried out under hydrothermal conditions by heating the stoichiometric salt obtained in step a), in water as a solvent and under pressure, to temperatures above 100 C., wherein the values of n and m and, thus, the molecular weight and/or the extent of cyclization in the polycondensate obtained, are/is regulated by means of the temperature and/or the duration of the polycondensation; and c) optionally, a solvent-free thermal treatment of the polycondensate is carried out in order to achieve complete cyclization.

Claims

1. A method for preparing polybenzimidazoles of formula (1) or (2) below, wherein n and m are each 1: ##STR00016## by polycondensation of corresponding tetracarboxylic acids or dianhydrides and tetraamines by jointly heating the reactants, characterized in that the preparation of polybenzimidazoles of formula (1) or (2) is carried out by using tetracarboxylic acids as starting material and substantially without the formation of any by-products, wherein a) first, a stoichiometric salt is formed from the tetracarboxylic acid and the tetraamine; b) polycondensation is carried out under hydrothermal conditions by heating the stoichiometric salt obtained in step a), in water as a solvent and under pressure, to temperatures above 100 C., wherein the values of n and m and, thus, the molecular weight and/or the extent of cyclization in the polycondensate obtained, are/is regulated by means of the temperature and/or the duration of the polycondensation; and c) optionally, a solvent-free thermal treatment of the polycondensate is carried out in order to achieve complete cyclization.

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), naphthalene tetracarboxylic acid (NTCA) is reacted with diaminobenzidine (DAB) to form the stoichiometric salt NTCA.DAB which in step b), is polycondensed under hydrothermal conditions to form the polybenzimidazole of formula (1): ##STR00017##

3. The method according to claim 2, characterized in that the polycondensation in step b) is carried out at a temperature of not more than 250 C. and/or for a duration of not more than 1 h in order to produce a polybenzimidazole (1) having a relatively low molecular weight.

4. The method according to claim 2, characterized in that the polycondensation in step b) is carried out at a temperature of not more than 300 C. and/or for a duration of not more than 2 h in order to produce a polybenzimidazole (1) having a medium molecular weight.

5. The method according to claim 2, characterized in that the polycondensation in step b) is carried out at a temperature above 300 C. and/or for a duration of at least 2 h in order to produce a polybenzimidazole (1) having a relatively high molecular weight.

6. 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), pyromellitic acid (PMA) is reacted with diaminobenzidine (DAB) to form the stoichiometric salt PMA.DAB which in step b), is polycondensed under hydrothermal conditions to form a non-cyclized or partially cyclized intermediate (3) and/or the polybenzimidazole of formula (2): ##STR00018## whereafter, optionally, step c) is carried out in order to convert the intermediate (3) completely into the polybenzimidazole of formula (2).

7. The method according to claim 6, characterized in that the polycondensation in step b) is carried out at a temperature of not more than 250 C. and/or for a duration of not more than 1 h in order to substantially produce only the intermediate (3) which, in the subsequent step c), is cyclized to form the polybenzimidazole (2).

8. The method according to claim 6, characterized in that the polycondensation in step b) is carried out at a temperature of not more than 275 C. and/or for a duration of not more than 2 h in order to produce a mixture of the polybenzimidazole (2) and the intermediate (3) which, in the subsequent step c), is fully cyclized to form the polybenzimidazole (2).

9. The method according to claim 6, characterized in that the polycondensation in step b) is carried out at a temperature of at least 350 C. and/or for a duration of at least 2 h in order to substantially produce only the polybenzimidazole (2).

10. The method according to claim 6, characterized in that the solvent-free thermal treatment in step c) is carried out at a temperature of at least 200 C., preferably at least 300 C., more preferred about 400 C.

11. The method according to claim 7, characterized in that the solvent-free thermal treatment in step c) is carried out at a temperature of at least 200 C., preferably at least 300 C., more preferred about 400 C.

12. The method according to claim 8, characterized in that the solvent-free thermal treatment in step c) is carried out at a temperature of at least 200 C., preferably at least 300 C., more preferred about 400 C.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0039] In the following, the present invention will be described in more detail by means of nonlimiting examples and with reference to the appended drawings, which show the following:

[0040] FIG. 1 is an enlarged view of the reaction scheme for the preparation of polybenzimidazoles of formula (1);

[0041] FIG. 2 is an enlarged view of the reaction scheme for the preparation of polybenzimidazoles of formula (2);

[0042] FIGS. 3 and 4 show the IR and .sup.1H NMR spectra of the stoichiometric salt prepared in Example 1;

[0043] FIGS. 5 to 7 show the IR spectra of the polybenzimidazoles of formula (1) prepared in Examples 2, 4 and 5;

[0044] FIGS. 8 and 9 show the IR and .sup.1H NMR spectra of the stoichiometric salt prepared in Example 7; and

[0045] FIGS. 10 and 11 show the IR spectra of the polybenzimidazoles of formula (2) prepared in Examples 8, 4 and 9.

EXAMPLES

Synthesis Example 1

Preparation of 1,4,5,8-naphthalene tetracarboxylic acid (NTCA)

[0046] ##STR00009##

[0047] For hydrolyzing of the commercially purchased anhydride to give the acid, 1.0057 g of naphthalene tetracarboxylic acid dianhydride (NTCADA) (3.75 mmol, 1 eq.) were dissolved in 50 ml 1 M NaOH while stirring at room temperature. The clear solution was then cooled in an ice bath and concentrated HCl was added dropwise until reaching pH 1. During acidification, naphthalene tetracarboxylic acid (NTCA) precipitated as a white solid which was isolated by centrifugation, but not dried in order to avoid another cyclization to give the anhydride.

Example 1Step a) of the Method According to the Invention

Preparation of a monomer salt from NTCA and 3,3-diaminobenzidine

[0048] ##STR00010##

[0049] The NTCA (3.75 mmol, 1 eq.) obtained in Synthesis Example 1 was suspended in 750 ml of distilled H.sub.2O and ice-cooled while the suspension was degassed with Ar for 10 min. 0.8035 g of 3,3-diaminobenzidine (DAB) (3.75 mmol, 1 eq.) were added to the cold degassed suspension and stirred overnight, while temperature was slowly increased to room temperature. This yielded a light brown suspension. The solid formed was filtered off, thoroughly washed with H.sub.2O and then EtOH and dried in an exsiccator. FIGS. 3 and 4 show the FTIR-ATR and .sup.1H NMR spectra of the obtained stoichiometric salt, NTCA.DAB. The IR spectrum differs greatly from the two educts NTCA and DAB and additionally contains the following characteristic bands: v(NH.sub.2)=3375 cm.sup.1 and 3220 cm.sup.1, v(NH.sub.3.sup.+)=2885 cm.sup.1 and 2585 cm.sup.1, v(CO, carboxylate)=1505 cm.sup.1 and v(CO, carboxylic acid)=1710 cm.sup.1. In the .sup.1H NMR spectrum, a DAB:NTCA molar ratio of 1:1 is clearly visible from the integral.

Example 2Step b) of the Method According to the Invention

Hydrothermal Polymerization to Give a Polybenzimidazole of Formula (1)

[0050] ##STR00011##

[0051] In a glass liner, 150 mg of NTCA.DAB were thoroughly suspended in 40 ml of distilled H.sub.2O. The liner was transferred into a microwave autoclave (120 ml). Next, the reaction mixture was heated to 250 C. within 15 min while stirring and this temperature was maintained for another 15 min. Subsequent cooling was carried out by means of a stream of pressurized air and took about 30 min. The deep black suspension obtained after opening of the autoclave was filtrated, and a black solid and a clear liquid were obtained. The solid was thoroughly washed using distilled H.sub.2O and then EtOH and dried at 80 C. in a vacuum drying oven. The FTIR-ATR spectrum of the solid is shown in FIG. 5 and confirms that this is the desired polybenzimidazole (1). The characteristic bands are: v(CO)=1700 cm.sup.1, v(CN)=1615 cm.sup.1 and v(benzimidazole)=1450 cm.sup.1. Furthermore, additional intensive bands at 1780 cm.sup.1 and 1740 cm.sup.1 are present, which, according to literature, indicates the presence of reactive terminal anhydride groups. The distinct visibility of these terminal group bands is indicative of a relatively low molecular weight of the polybenzimidazole (1).

[0052] 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 3Step b) of the Method According to the Invention

Hydrothermal Polymerization to Give a Polybenzimidazole of Formula (1)

[0053] In a glass liner, 130 mg of NTCA.DAB were thoroughly suspended in 25 ml of distilled H.sub.2O. The liner was transferred into a non-stirred batch steel autoclave (80 ml). Next, the reaction mixture was heated as quickly as possible to 250 C. without stirring in an external heating furnace (duration: about 45 min) and this temperature was maintained for another 15 min (total reaction time: 60 min). Then, the steel autoclave was cooled off by quenching using cold tap water. The deep black suspension obtained after opening of the autoclave was filtrated, and a black solid and a clear liquid were obtained. The solid was thoroughly washed using distilled H.sub.2O and then EtOH and dried at 80 C. in a vacuum drying oven. In the FTIR-ATR spectrum of the solid, once again there were intensive bands of reactive terminal anhydride groups at 1780 cm.sup.1 and 1740 cm.sup.1 besides the characteristic polybenzimidazole bands at 1700 cm.sup.1, 1615 cm.sup.1 and 1450 cm.sup.1, which implies that, again, only a relatively low molecular weight was achieved despite the longer reaction time.

[0054] Again, extraction attempts of the polybenzimidazole of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 4Step b) of the Method According to the Invention

Hydrothermal Polymerization to Give a Polybenzimidazole of Formula (1)

[0055] In a glass liner, 130 mg of NTCA.DAB were thoroughly suspended in 25 ml of distilled H.sub.2O. The liner was transferred into a non-stirred batch steel autoclave (80 ml), which was pressurized at a pressure of 10 bar using argon. Next, the reaction mixture was heated as quickly as possible to 300 C. without stirring in an external heating furnace (duration: about 60 min) and this temperature was maintained for another 60 min (total reaction time: 120 min). Then the steel autoclave was cooled off by quenching using cold tap water. The deep black suspension obtained after opening of the autoclave was filtrated, and a black solid and a clear liquid were obtained. The solid formed was thoroughly washed using distilled H.sub.2O and then EtOH and dried at 80 C. in a vacuum drying oven. The FTIR-ATR spectrum of the solid is shown in FIG. 6. Only very faint bands of reactive terminal anhydride groups were detectable besides the characteristic polybenzimidazole bands. The significantly reduced intensity of these bands as compared to Examples 2 and 3 is indicative of a significantly higher molecular weight.

[0056] Again, extraction attempts of the polybenzimidazole of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 5Step b) of the Method According to the Invention

Hydrothermal Polymerization to Give a Polybenzimidazole of Formula (1)

[0057] Example 4 was substantially repeated, with the exception that the reaction mixture was heated to a temperature of 350 C. within 90 min and the temperature was maintained for another 30 min. Like in Example 4, a black solid was obtained, the FTIR-ATR spectrum of which is shown in FIG. 7. In this case, no bands of reactive terminal anhydride groups at all were detectable besides the characteristic polybenzimidazole bands, which suggests a relatively high molecular weight of the polybenzimidazole of formula (1) obtained.

[0058] Extraction attempts of the polybenzimidazole of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 6Step b) of the Method According to the Invention

Hydrothermal Polymerization to Give a Polybenzimidazole of Formula (1)

[0059] Example 4 was substantially repeated, with the exception that the temperature of the reaction mixture was maintained at 300 C., not for 1 h, but for 11 h, so that the total reaction time amounted to 12 h. Again, the FTIR-ATR spectrum of the isolated black solid showed no bands of reactive terminal anhydride groups besides the characteristic polybenzimidazole bands, which is again indicative of a relatively high molecular weight of the polybenzimidazole of formula (1) obtained.

[0060] Extraction attempts of the polybenzimidazole of formula (1) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 7Step a) of the Method According to the Invention

Preparation of a monomer salt from pyromellitic acid and DAB

[0061] ##STR00012##

[0062] For the preparation of a stoichiometric salt, 0.9531 g of pyromellitic acid (PMA) (3.75 mmol, 1 eq.) were dissolved in 750 ml of distilled H.sub.2O and ice-cooled while the solution was degassed using Ar for 10 min. 0.8035 g of 3,3-diaminobenzidine (DAB) (3.75 mmol, 1 eq.) were added to the cold degassed suspension and stirred overnight, while the temperature was slowly increased to room temperature. This yielded a red suspension. The solid formed was filtered off, thoroughly washed using H.sub.2O and then EtOH and dried in an exsiccator. FIGS. 8 and 9 show the FTIR-ATR and .sup.1H NMR spectra, respectively, of the stoichiometric salt PMA.DAB obtained. The IR spectrum differs greatly from the two educts PMA and DAB and additionally contains the following characteristic bands: v(NH.sub.2)=3435 cm.sup.1 and 3355 cm.sup.1, v(NH.sub.3.sup.+)=2885 cm.sup.1 and 2600 cm.sup.1, v(CO, carboxylate)=1500 cm.sup.1 and v(CO, carboxylic acid)=1685 cm.sup.1. In the .sup.1H NMR spectrum, a DAB:PMA molar ratio of 1:1 is clearly visible from the integral.

Example 8Step b) of the Method According to the Invention

Hydrothermal Polymerization to Give a Mixture of a Polybenzimidazole of Formula (2) and an Intermediate of Formula (3)

[0063] ##STR00013##

[0064] As in the reaction of Example 2, 150 mg of PMA.DAB were thoroughly suspended in 40 ml of distilled H.sub.2O in a glass liner. The liner was transferred into a microwave autoclave (120 ml). Next, the reaction mixture was heated to 250 C. within 15 min while stirring and this temperature was maintained for another 15 min. Subsequent cooling was carried out by means of a stream of pressurized air and took about 30 min. The orange suspension obtained after opening the autoclave was filtrated, and an orange solid and a clear liquid were obtained. The solid was thoroughly washed using distilled H.sub.2O and then EtOH and dried at 80 C. in a vacuum drying oven. The FTIR-ATR spectrum of the solid confirms that this is a mixture of the desired polybenzimidazole (2) and an intermediate of formula (3) with free carboxyl groups. The characteristic bands are: v(CN/CC)=1630 cm.sup.1, v(ring vibration)=1585 cm.sup.1 (characteristic for the conjugation between the benzene and the imidazole ring), and v(benzimidazole)=1445 cm.sup.1. However, a band v(CO)=1760 cm.sup.1 characteristic for the entirely cyclized product is barely detectable, which suggests that mainly the intermediate of formula (3) was formed.

Example 9Step c) of the Method According to the Invention

Solvent-Free Thermal Treatment

[0065] ##STR00014##

[0066] The orange solid isolated in Example 8 was subjected to a solvent-free thermal treatment at 400 C. (30 min holding period) in order to effectuate entire cyclization and convert the intermediate of formula (3) into the polybenzimidazole of formula (2). FIG. 11 shows the FTIR-ATR spectrum of the dark brown solid thus obtained, which shows that no free carboxyl groups were present anymore, but that the CO bands at 1760 cm.sup.1 were now especially pronounced: v(CO)=1760 cm.sup.1, v(CN)=1620 cm.sup.1 and v(benzimidazole)=1440 cm.sup.1.

[0067] Extraction attempts with 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, so in this case, no by-products were formed either.

Example 10Step b) of the Method According to the Invention

Hydrothermal Polymerization to Give a Mixture of a Polybenzimidazole of Formula (2) and an Intermediate of Formula (3)

[0068] In a glass liner, 130 mg of PMA.DAB were thoroughly suspended in 25 ml of distilled H.sub.2O. The liner was transferred into a non-stirred batch steel autoclave (80 ml), which was pressurized at a pressure of 10 bar using argon. Then, the reaction mixture was heated as quickly as possible to 275 C. without stirring in an external heating furnace (duration: about 60 min) and this temperature was maintained for another 60 min (total reaction time: 120 min). Then the steel autoclave was cooled off by quenching using cold tap water. The brown suspension obtained after opening of the autoclave was filtrated, and a brown solid and a clear liquid were obtained. The solid was thoroughly washed using distilled H.sub.2O and then EtOH and dried at 80 C. in a vacuum drying oven. The FTIR-ATR spectrum of this solid was almost identical to the one in FIG. 10, but with a somewhat stronger CO band at 1760 cm.sup.1, which suggests a higher proportion of already cyclized polybenzimidazole of formula (2).

Example 11Step c) of the Method According to the Invention

Solvent-Free Thermal Treatment

[0069] Thermal treatment of the brown solid in an analogous manner to Example 9 yielded a dark brown solid, the FTIR-ATR spectrum of which was substantially identical to the one in FIG. 11 and which shows entire cyclization to a polybenzimidazole of formula (2).

[0070] Extraction attempts of the polybenzimidazole of formula (2) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.

Example 12Step b) of the Method According to the Invention

Direct Polycondensation of PMA.DAB to Give a Polybenzimidazole of Formula (2)

[0071] ##STR00015##

[0072] Example 10 was substantially repeated, using 130 mg of PMA.DAB in 25 ml of distilled H.sub.2O, with the exception that the reaction mixture was heated to a temperature of 350 C. within 90 min and the temperature was maintained for another 90 min. The brown suspension obtained after opening of the autoclave was filtrated, and a dark brown solid and a clear liquid were obtained. The solid was thoroughly washed using distilled H.sub.2O and then EtOH and dried at 80 C. in a vacuum drying oven. The FTIR-ATR spectrum of this solid was practically identical to the one in FIG. 11, which suggests that, in this case, only an entirely cyclized polybenzimidazole of formula (2) and substantially no intermediate of formula (3) was formed.

[0073] Once more, extraction attempts of the polybenzimidazole of formula (2) and examinations of the aqueous phase after hydrothermal polymerization did not yield any results.