METHOD FOR THE PRODUCTION OF POLYBENZIMIDAZOLES

Description

[0001] The present invention relates to a new process for the production of polybenzimidazoles. STATE OF THE ART

[0002] Aromatic polybenzimidazoles, i.e., polymers in which two benzimidazole units are linked via an aromatic linker to form a through-conjugated system, represent valuable high-performance polymers due to their special properties (high melting point, hardness, compressive strength, light absorption). They are used, for example, in fire protection, for high-temperature membranes in polymer electrolyte fuel cells, or in photovoltaics. Their synthesis is generally achieved by the polycondensation of aromatic tetraamines, more precisely bis(o-diamines), with aromatic di- or higher-grade carboxylic acids—or their esters, anhydrides, or aldehydes—while heating the reactants to temperatures well above 100 degrees Celsius (when using high-boiling solvents) up to several hundred degrees Celsius (in the solid state).When using divalent carboxylic acids or aldehydes, polymers are obtained in which the residue X of the respective carboxylic acid or aldehyde molecule forms a bond between each pair of imidazole rings of the polybenzimidazoles, as shown below.

[0003] The benzene rings of the polybenzimidazoles are connected—depending on the choice of tetraamine—via a (preferably aromatic) linker Y, which can also be a direct chemical bond, as occurs, for example, when using tetraaminobiphenyl (diaminobenzidine, DAB) as the tetraamine. A special case occurs when using tetraaminobenzene, since here the two imidazole rings are bonded to the same benzene ring, as shown below.

[0004] The synthesis of such polymers using aromatic dialdehydes is described in detail, particularly in a comprehensive review by Eberhard Neuse (Adv. Polym. Sci. 47, 1-42 (1982)) and, specifically for diaminobenzidine as a tetraamine, in another article by the same author (Neuse and Loonat, Macromolecules 16(1), 128-136 (1983)). According to this review, the mere mixing of tetraamine and dialdehyde leads to the formation of a polymer described as a Schiff base, as illustrated below for the reaction of terephthalic acid dialdehyde and diaminobenzidine: which is subsequently cyclized to polybenzimidazole. Both articles explicitly emphasize the necessity of excluding oxygen when mixing the reactants and its presence in the subsequent cyclization step. Otherwise, i.e., in the presence of O₂, undesirable oxidation reactions are said to occur in the first polycondensation step, while the cyclization in the absence of O₂ is described as "highly inefficient" because it proceeds very slowly.

[0005] As can be seen from Neuse's review, as well as from the literature cited therein, such as Vogel and Marvel, J. Polym. Sci. Pol. Chem. 50, 511 (1961), or Iwakura et al., J. Polym. Sci. A 2, 2605 (1964), a variety of classical synthetic methods for polybenzimidazoles are known. The two most common strategies are: (i) the "Marvel method," i.e., solvent-free melt polycondensation of an aromatic tetraamine with a diphenyl ester of an aromatic dicarboxylic acid at temperatures up to 400 °C; and (ii) the "Iwakura method": solution polycondensation of the hydrochloride of an aromatic tetraamine with a free aromatic dicarboxylic acid in polyphosphoric acid at temperatures of about 200 °C. Both techniques are technically demanding, complex, not energy-efficient, and require a carefully selected heating protocol as well as pressure control or the absence of air.

[0006] Literature on the synthesis of monomeric benzimidazoles is of course much more common, including hydrothermal syntheses, which have become increasingly popular in recent years, i.e., reactions in water as the sole or at least main solvent at temperatures above 100 °C, since no sometimes highly toxic solvents need to be disposed of. See, for example, Dudd et al., Green Chem. 5, 187-192 (2003), for the synthesis of 2-phenylbenzimidazole from diaminobenzene and benzoic acid in water at temperatures up to 400 °C, although yields above 50% could not be achieved below 350 °C and a yield above 90% was only observed after 14 hours, as well as Nagao et al., Green Chem. 18, 3494-3498 (2016), for the synthesis of 1,2-diphenylbenzimidazole from 2-aminodiphenylamine and benzoic anhydride at temperatures between 400 and 445 °C.

[0007] When using tetravalent carboxyl or carbonyl compounds, e.g., tetracarboxylic acids or their anhydrides, a further cyclization step occurs in addition to the ring closure of the imidazole ring. For example, when using benzenetetracarboxylic acid with tetraaminodiphenyl ether, as disclosed by von Bell and Pezdirtz, J. Polym. Sci. Pol. Lett. 3(12), 977-984 (1965), the first polycondensation is an amidation reaction involving the reaction of one amino acid functionality with one carboxylic acid functionality, resulting in a so-called poly(amino acid amide), also known as poly(AAA).In the second condensation step, the first ring closures then occur: either through attack of the free amino functionalities on the previously amidated carboxyl carbons with simultaneous elimination of water to form imidazoles fused to the benzene rings of the tetraamine while retaining the free carboxyl groups, or through attack of the amide nitrogens on the free carboxyl carbons with elimination of water to form two imide groups on the aromatic ring of the tetracarboxylic acid while retaining the free amino groups. In the third and final condensation step, the ring closures then take place, forming the second 5-membered rings, as shown below. Strictly speaking, however, the intermediate product shown on the right with free amino groups actually forms a different condensation product, namely the condensation product mirrored about a horizontal axis, since the units in the polymer are not freely rotatable.This is also noted by Bell and Pezdirtz ("designation of the positions is arbitrary").

[0008] According to Bell and Pezdirtz, the polymeric AAA intermediates are isolated and used as a solution for coating surfaces, after which the fully condensed polybenzimidazoles are formed only by heating to 325 °C.

[0009] These polybenzimidazoles are also referred to in the literature as polyimidazopyrrolones or simply polypyrrones. See, for example, Dawans and Marvel, J. Polym. Sci., Part A: Polym. Chem. 3, 3549-3571 (1965), Bell and Jewell, J. Polym. Sci., Part A: Polym. Chem. 5, 3043-3060 (1967), and Johnston and Epps, J. Polym. Sci., Part A: Polym. Chem. 10, 2751-2765 (1972), where syntheses starting from pyromellitic dianhydride are consistently disclosed, either in solid form at temperatures between 200 and 300 °C or as a solution in aprotic, high-boiling solvents (e.g., dimethylacetamide, DMAc).

[0010] Interestingly, VL Bell, who in 1965 together with GF Pezdirtz had still considered both variants shown above to be possible for the formation of intermediates from the initial polycondensate "Poly(AAA)", revealed two years later in Bell and Jewell (1967, so) only the intermediate on the right, which includes free amino groups and cyclic imides, but no free carboxylic acid groups.

[0011] Using naphthalenetetracarboxylic acid instead of pyromellitic acid yields corresponding polybenzimidazoles with a saturated 6-membered ring. Due to their structural identity with the organic pigment and semiconductor perinone, these have recently been referred to as "polyperinones." For synthesis, see, for example, Van Deusen, J. Polym. Sci. Pol. Lett. 4, 211-214 (1966), and Zhou and Lu, J. Appl. Polym. Sci. 58, 1561-1565 (1995). The latter literature also describes a reaction in solution with DMAc, in which, however, only the non-ring-closed intermediate was initially prepared and isolated, and after subsequent film-drawing of a DMAc solution, the ring was closed by heating to 300 °C. Other authors also describe the precipitation and isolation of the intermediate, or the filtration or centrifugation of the solution before the subsequent ring closure. Van Deusen (so) reveals, however, the direct polycondensation to the completely ring-closed polyperinone in polyphosphoric acid at temperatures of up to 220 °C, after which a sometimes complex purification of the polymers must take place.

[0012] Morgan and Scott, J. Appl. Polym. Sci. 16, 2029-2050 (1972), disclose the preparation of stoichiometric mixtures and salts of tetracarboxylic acid and tetraamine using N₂ as a protective gas against oxygen ingress, followed by polycondensation of the mixtures or salts with simultaneous shaping of the resulting polymer by hot pressing under temperatures of 450 °C. However, this process yields products with highly variable quality and stability, with consistently higher stability observed under nitrogen than in air.

[0013] All these polybenzimidazoles with an fused 5- or 6-membered ring have in common that cis- and trans-isomers exist with respect to the free carboxyl or amino groups of the intermediates and thus also of the carbonyl groups of the fully condensed polymers, as Beil and Pezdirtz (so) and Van Deusen (so) indicate and have been confirmed by the inventors of the present application, which will be discussed in more detail later.

[0014] The inventors' research group has previously conducted extensive research on hydrothermal syntheses for the production of polyimides; see, for example, PCT / AT2016 / 050140 and PCT / AT2017 / 000058. Specifically, research has also been carried out on the aforementioned polybenzimidazoles with an fused 6-membered ring ("polyperinones"); see, for example, Michael Taubländer, "Development of Novel Synthetic Routes Towards Polyimides and Poly(perinone)s," Diploma Thesis, Vienna University of Technology, 2017. This research revealed that, in principle, the aforementioned 6-membered polyperinones can be produced via hydrothermal synthesis starting from naphthalenetetracarboxylic acid dianhydride (NTCADA) and diaminobenzidine (DAB). However, the products exhibited very low molecular weights (determined by IR analysis), and the aqueous phases were heavily contaminated and dark purple in color.This was attributed to a significant proportion of oxidative polymerization with the formation of various byproducts, which disrupted the stoichiometry, preventing the achievement of a high molecular weight of the desired polyperinone. Furthermore, these byproducts were difficult to separate from the target polymer, as both aqueous and ethanolic washing solutions remained intensely colored even after numerous extraction cycles. EP 1 441 015 A1 describes a synthesis method for polybenzimidazoles. A polyimine intermediate of terephthaldehyde and tetraaminobiphenyl (diaminobenzidine, DAB) in dimethylacetamide (DMAc) is prepared, which is then cyclized to the polybenzimidazole by heating.Against this background, the aim of the present invention was to develop an improved process for the production of polybenzimidazoles, in particular the above polybenzimidazoles without fused 5- or 6-membered rings of divalent carboxyl or carbonyl compounds and tetraamines, by which high molecular weight polymers can be obtained in a relatively simple manner and without the formation of large quantities of difficult-to-separate byproducts. REVELATION OF THE INVENTION

[0015] The present invention achieves this objective by providing a process for the preparation of polybenzimidazoles of the following formula (1) or (2), wherein n ≥ 2: by polycondensation of corresponding dialdehydes and tetraamines by joint heating of the reactants, characterized in that: the preparation of the polybenzimidazoles of formula (1) or (2) is carried out starting from the dialdehydes and essentially without any formation of by-products, by a) first, the dialdehyde and the tetraamine are mixed together in water at room temperature, forming a polyimine intermediate of formula (3) or (4), wherein n and m are each ≥ 1: where b) the polycondensation is carried out under hydrothermal conditions by heating in water as a solvent under pressure to temperatures above 100 °C.

[0016] The inventors surprisingly discovered that a synthesis of polybenzimidazoles of formulas (1) and (2) from the corresponding dialdehydes and tetraamines under hydrothermal conditions: requires no cooling or exclusion of oxygen during the mixing of the reactants; is fully feasible after a short reaction time at relatively low temperatures; generates no byproducts; and does not require the presence of oxygen; the latter circumstance was particularly surprising in light of Neuse's revelations.

[0017] In preferred embodiments of the invention, a polybenzimidazole of formula (1) is prepared according to the reaction scheme below by mixing terephthalic dialdehyde, TDA, and diaminobenzidine, DAB, in water in step a) and polycondensing them under hydrothermal conditions to form the polybenzimidazole of formula (1) in step b):

[0018] In alternative preferred embodiments, a polybenzimidazole of formula (2) is prepared according to the reaction scheme below by mixing isophthalic dialdehyde, IDA, and diaminobenzidine, DAB, in water in step a) and polycondensing them under hydrothermal conditions to form the polybenzimidazole of formula (2) in step b):

[0019] In both cases, the reactants can be mixed at room temperature without the use of an inert gas, which significantly simplifies the reaction compared to the procedure described by Neuse (so). Furthermore, the polycondensation step does not require the presence of oxygen, as clearly demonstrated by Example 5, in which the reaction mixture was degassed with argon before heating to hydrothermal conditions, but nevertheless reacted in a relatively short time in an identical manner to form the desired polybenzimidazole.

[0020] The polycondensation in step b) is preferably carried out at a temperature of at least 180 °C for a duration of at least 30 min, more preferably for at least 2 h, or, which is particularly preferred according to the present invention, at a temperature of at least 250 °C for a duration of at least 30 min, in order to shorten the reaction time. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will now be described in more detail with reference to non-limiting examples and the accompanying drawings, of which: Fig. 1 the FTIR-ATR spectrum of the polyimine intermediate obtained in Example 1 shows; Fig. 2 the FTIR-ATR spectrum of the polybenzimidazole obtained in Example 1 shows; and Fig. 3 The FTIR-ATR spectrum of the polybenzimidazole obtained in Example 5 is shown. EXAMPLES Example 1

[0022] Preparation of a polybenzimidazole of formula (1) at 250 °C 64 mg of terephthalaldehyde (TDA) (0.48 mmol, 1 equiv) were suspended in a glass liner in 40 ml of distilled water with stirring at room temperature. 103 mg of 3,3'-diaminobenzidine DAB (0.48 mmol, 1 equiv) were added and stirred for a further 10 min at room temperature, forming a red solid. The resulting red suspension was used directly for hydrothermal synthesis without isolating the solid. To characterize this intermediate, the solid was filtered off in a separate batch, washed, dried, and analyzed by IR spectroscopy. The FTIR-ATR spectrum is shown in Fig. 1 This was illustrated and led to the identification of the red solid as a polyimine intermediate, similar to that described in the introduction. In fact, it corresponds to the following formula (3):

[0023] Similar to the polybenzimidazoles with fused 5- or 6-membered rings mentioned earlier, the polyimine intermediates of formula (3) – as well as those of formula (4) – exhibit cis and trans isomers with respect to the free amino groups due to the restricted rotation of the conjugated and therefore planar molecules. Consequently, these isomers can point in opposite directions or in the same direction, as illustrated here in formulas (3) and (4) by the different numbers of units present, n and m respectively. Strictly speaking, a third variant of the units also exists, namely that in which both amino groups point downwards instead of upwards. For the sake of clarity, this variant is not explicitly shown here.The ratio between these units cannot be definitively determined and, due to their identical chemical properties, is not significant for the further course of the hydrothermal condensation, in which the isomerism is also eliminated. Therefore, it has no influence whatsoever on the properties of the polybenzimidazoles obtained, which is why it will not be discussed further here.

[0024] The characteristic bands of this polyimine are: v(CH lmin ) = 2875 cm -1< , v(C=N) =1595 cm -1< and v(CN) = 1205 cm -1< .

[0025] For the hydrothermal polymerization to the desired polybenzimidazole, the previously obtained red suspension was transferred to a microwave autoclave (120 ml).

[0026] The reaction mixture was then heated to 250 °C for 15 minutes with stirring, and this temperature was maintained for another 15 minutes. The reaction vessel was then cooled by a stream of compressed air for 30 minutes. The orange suspension obtained after opening the autoclave was filtered, yielding an orange solid and a clear liquid. The resulting solid was thoroughly washed with distilled water and then with ethanol, and dried at 80 °C in a vacuum drying oven. The FTIR-ATR spectrum is shown in Fig. 2 The results show and confirm that this is the desired polybenzimidazole. The bands characteristic of polybenzimidazoles are: v(C=N / C=C) = 1615 cm⁻¹, v(ring vibration) = 1585 cm⁻¹ (characteristic of the conjugation between the benzene and imidazole rings), v(benzimidazole; "in-plane" deformation vibration) = 1445 cm⁻¹, and v(benzimidazole, ring breathing) = 1285 cm⁻¹.

[0027] Extraction of samples of the polybenzimidazole of formula (1) with various organic solvents (MeOH, EtOH, iPrOH, phenol, PE, EE, CDCl₂, CDCl₃, acetone, acetonitrile) yielded clear filtrates free of impurities. Further examination of the aqueous phase after hydrothermal polymerization revealed that it also contained no byproducts of the hydrothermal polymerization. Example 2 Preparation of a polybenzimidazole of formula (1) at 180 °C

[0028] The reaction procedure was essentially the same as in Example 1, except that the reaction mixture was heated to only 180 °C in the autoclave within 10 minutes but held at this temperature for 2 hours, and cooling by means of a compressed air stream took 20 minutes. Again, in addition to a clear aqueous phase, an orange-colored solid was obtained, whose FTIR-ATR spectrum was practically identical to that of the reaction with the other reaction. Fig. 2 was.

[0029] In this case too, extraction attempts of the polybenzimidazole of formula (1) and investigations of the aqueous phase after hydrothermal polymerization yielded no results. Example 3 Preparation of a polybenzimidazole of formula (1) at 180 °C without stirring

[0030] The reaction procedure was essentially the same as in Example 2, except that the red suspension formed by mixing the reactants was transferred to an unstirred batch autoclave, which was placed in an oven preheated to 180 °C, where the reaction mixture was allowed to react for 4 h. The autoclave was then cooled by quenching with cold tap water. Again, a clear aqueous phase and an orange solid were obtained, the FTIR-ATR spectrum of which was also practically identical to that obtained in Example 2. Fig. 2 was.

[0031] Again, extraction attempts of the polybenzimidazole of formula (1) and investigations of the aqueous phase after hydrothermal polymerization yielded no results. Example 4 Preparation of a polybenzimidazole of formula (1) at 250 °C under argon

[0032] The reaction procedure was initially essentially the same as in Example 1, except that the reaction mixture was degassed by introducing argon before heating to remove oxygen from both the aqueous phase and the gas space above it. It was then heated to 250 °C within 60 minutes and held at this temperature for another 60 minutes (total reaction time: 2 hours). The reactor was subsequently cooled not by a stream of compressed air, but by quenching with cold tap water. Again, in addition to a clear aqueous phase, an orange solid was obtained, whose FTIR-ATR spectrum was also practically identical to that of the reaction in Example 1. Fig. 2 was, which proves that - contrary to the teaching of the prior art - the presence of oxygen is not required for the cyclizing condensation of the intermediate polyimine to the desired polybenzimidazole under hydrothermal conditions in order to complete it in a relatively short time.

[0033] Extraction attempts of the polybenzimidazole of formula (1) and investigations of the aqueous phase after hydrothermal polymerization also yielded no results in this case. Example 5 Preparation of a polybenzimidazole of formula (2) at 250 °C

[0034]

[0035] The reaction procedure was essentially the same as in Example 1, except that the same amount of isophthalic dianhydride (IDA) was used instead of TDA. In this case as well, an orange solid was obtained (and purified analogously) in addition to a clear aqueous phase, the FTIR-ATR spectrum of which was shown in Fig. 3 is shown. Based on this, the solid was identified as the desired polybenzimidazole of formula (2). The characteristic bands are: v(C=N / C=C) = 1625 cm⁻¹, v(ring vibration) = 1585 cm⁻¹, v(benzimidazole; "in plane" deformation vibration) = 1440 cm⁻¹, and v(benzimidazole, ring breathing) = 1285 cm⁻¹.

[0036] Analogous extraction experiments with the polybenzimidazole of formula (2) as for that of formula (1) as well as investigations of the aqueous phase after hydrothermal polymerization yielded no results, so that in this case too no by-products had been formed.

Claims

1. A method for preparing polybenzimidazoles of formula (1) or (2) below, wherein n is ≥ 2: by polycondensation of corresponding 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 formation of any by-products, wherein 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 n and m are each ≥ 1: whereafter b) polycondensation is carried out under hydrothermal conditions by heating, in water as a solvent and under 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 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):

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 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):

4. The method according to any one of claims 1 to 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.

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 any one of the preceding claims, 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.