ISSN 0012�5008, Doklady Chemistry, 2010, Vol. 432, Part 2, pp. 168–174. © Pleiades Publishing, Ltd., 2010.Original Russian Text © Iv.I. Ponomarev, I.I. Ponomarev, P.V. Petrovskii, Yu.A. Volkova, D.Yu. Razorenov, I.B. Goryunova, Z.A. Starikova, A.I. Fomenkov, A.R. Khokhlov, 2010,published in Doklady Akademii Nauk, 2010, Vol. 432, No. 5, pp. 632–638.

168

Poly(benzimidazoles) (PBIs) are one of the most

promising and well�studied classes of polymers uti�

lized as membranes in hydrogen�based high�tempera�

ture fuel cells (HTFC) as polymer–electrolyte com�

plexes with orthophosphoric acid [1, 2].

A polymer based on 3,3',4,4'�tetraaminodiphenyloxide and 3,3�bis(p�carboxyphenyl)phthalide (PBI�O�PH) was prepared for the first time at the Nesmey�anov Institute of Organoelement Compounds, Rus�sian Academy of Sciences, and was successfully usedin the design of membrane–electrode assembly(MEA) of hydrogen�based HTFC [3, 4].

Scheme 1.

O

O

HO

O

OH

O

OH2N

H2N

NH2

NH2

O

O

N

N

H

O

N

N

H+

PBI�О�PH

n

The current–voltage characteristics of MEA withPBI�O�PH membrane are on a level with the bestworld’s samples, however, require further elaboration [4].

Previously [5], we prepared PBIs with rather highmolecular weights from monomers containing fiveP–OH groups per repeating unit of the polymer.However, the films made of these PBIs proved to haveinsufficient strength properties and could not be used

for making MEA. Therefore, we used an approach ofpolymer�analogous transformation of high�molecu�lar�weight film�forming PBI�O�PH into its phospho�rylated analogue. For this purpose, in this work wehave studied previously unknown reaction of 2�aryl�substituted benzimidazoles with O,O�diethylvinylphosphonate (DEVP) using 2�phenylbenzimida�zole (2�PhBI) as an example and have carried outpolymer�analogous transformation of PBI�O�PH intophosphonoethylated polymer (PEPBI�O�PH) usedfor the preparation of proton�conducting membranessuccessfully tested in a model of HTFC MEA.

CHEMISTRY

Synthesis of N�Phosphonoethylated Cardo Poly(benzimidazole) and Testing of Proton�Conducting Membranes Made of ItIv. I. Ponomarev, I. I. Ponomarev, P. V. Petrovskii, Yu. A. Volkova, D. Yu. Razorenov,

I. B. Goryunova, Z. A. Starikova, A. I. Fomenkov, and Academician A. R. Khokhlov

Received January 19, 2010

DOI: 10.1134/S0012500810060042

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia

DOKLADY CHEMISTRY Vol. 432 Part 2 2010

SYNTHESIS OF N�PHOSPHONOETHYLATED CARDO POLY(BENZIMIDAZOLE) 169

EXPERIMENTAL

The methods for studying individual compoundsand polymers have been described in detail in ourrecent works [3–5].

Polybenzimidazole PBI�O�PH was obtained byour procedures [3]. Phosphonoethylation reaction wascarried out with a polymer sample having ηr =2.1 dL/g (0.5% solution in N�MP at 25°С), whichcorresponds to Mw/Mn = 155000/64400 = 2.4 accord�ing to GPC [3].

Synthesis and X�ray diffraction study of O,O�diethyl2�(2�phenyl�1H�benzimidazol�1�yl)ethylphosphonate (I).A mixture of 1.94 g (0.01 mol) of 2�phenylbenzimida�zole, 10 mL of N�MP, and 1 mL of a solution of 0.02 g(0.5 mmol) of NaOH and 0.028 g (0.5 mmol) of KOHin MeOH was stirred at 60°С in a dry argon flow for5 h; then, 3.28 g (0.02 mol) of DEVP was added andthe mixture was stirred for another 10 h (Scheme 2).The reaction course was monitored by TLC (toluene–ethanol, 5 : 1 (v/v) as eluent) and 31Р{1H} NMR spec�troscopy. The reaction mixture was evaporated in avacuum at 90–100°С (0.133 Pa), and the oily matterwas flooded with 10 mL of water and allowed to standfor 72 h in a refrigerator. The resultant colorless crys�tals were separated by filtration and dried in a vacuumat 60°C to give 3.47 g (97%) of the product that was amixture of two kinds of crystals with Tm = 67 and82.8°C (DSC).

For C19H23N2O3P anal. calcd. (%): C, 63.68;H, 6.47; N, 7.82; P, 8.64. M = 358.38. Found (%):C, 63.59; H, 6.57; N, 7.79; P, 8.68.

1H NMR (DMSO�d6, δ, ppm, J, Hz): 1.11 (t, 6H,СН3,

3JHH = 7.0), 2.27 (dt, 2H, СН2Р, 3JHH = 7.7,2JHР = 18.5), 4.47 (dt, 2H, NСН2,

3JHH = 9.4, 3JHР =6.7), 3.88 (dq, 4H, ОСН2,

3JHH ≈ 3JHР = 7.2), 7.23–7.39 (m, 2H, СH�5, 6), 7.55–7.65 (m, 3H, m,p�C6H5), 7.65 (d, 1H, СН�4, 3JHH = 7.7), 7.71 (d, 1H,СН�7, 3JHH = 7.6), 7.75–7.85 (d, 2H, o�C6H5).

31Р{1H} NMR (DMSO�d6, δ, ppm): 26.63 s.

Colorless platelet crystals of compound I wereobtained from water as monohydrate, С19H23N2О3P ·H2O (M = 376.38).

Atomic coordinates and thermal parameters, bondlengths, and bond angles are deposited with the Cam�bridge Crystallographic Data Center (CCDCno. 747698) and available free of charge athttp://www.ccdc.cam.uk/conts/retrieving.html (orfrom CCDC, 12 Union Road, Cambridge CB2 1EZ;fax: +44 1223 335 033; or deposit@ccdc.cam.ac.uk).

Synthesis of 2�(2�phenyl�1H�benzimidazol�1�yl)ethylphosphonic acid (II). A suspension of 1.79 g(5 mmol) of compound I in 20 mL of 18% HCl washeated under reflux for 2 h and cooled; the resultant

white crystalline precipitate was separated by filtra�tion, washed with water, and dried in a vacuum at100°C (Scheme 3) to give 1.50 g (99%) of compound II.Tm = 265°C (DSC).

For C15H15N2O3P anal. calcd. (%): C, 59.60;H, 5.00; N, 9.27; P, 10.25. M = 302.27. Found (%):C, 59.59; H, 5.27; N, 9.14; P, 10.18.

1H NMR (DMSO�d6, δ, ppm): 2.14–2.40 (m, 2H,СН2Р), 4.49–4.74 (m, 2H, NСН2), 7.60–8.10 (m,9H, arom.).

31Р{1H} NMR (DMSO�d6, δ, ppm): 19.57 s.

Phosphonoethylation of PBI�O�PH with O,O�diethylvinylphosphonate. PBI�O�PH (0.532 g, 1 mmol) wasdissolved in 10 mL of N�MP, and 1 mL of a catalystsolution (10 mol % as calculated for reacting groups)in methanol (for example, a mixture of NaOH (4 mg,0.1 mmol) and KOH (5.6 mg, 0.1 mmol)) was added.The solution was stirred at 60°С for 8 h in a dry argonflow, and then DEVP (0.656 g, 4 mmol, a twofoldmolar excess) was added (Scheme 4). The reaction ofDEVP addition was studied at 20–90°С. The reactioncourse was monitored by 31Р{1H} NMR spectroscopyby sampling the reaction solution. The sample wasdiluted with DMSO�d6, the extent of phosphonoethy�lation was calculated from the ratio of the integratedintensities of the 31P NMR signals at δ = 16.95 ppm forDEVP and δ = 26.50 ppm for PEPBI�O�PH, takinginto account the partial polymerization of DEVP (31PNMR signal at δ = 28.56 ppm) and alkylation (31PNMR signal at δ = 14.15 ppm).

The samples were simultaneously analyzed byGPC.

To obtain PEPBI�O�PH membranes, the reactionsolution was poured onto a glass support and the sol�vent was evaporated at 60–80°С on a hot bench. Then,the films were removed from the glass and heated for30 min up to 350°С in air to form spatially crosslinkedstructure of the polymer. The doping of the mem�branes for MEA and the study of discharge character�istics were carried out similarly to the works [4, 5].

RESULTS AND DISCUSSION

In this work, we have studied in detail for the firsttime the model reaction of 2�PhBI with DEVP. Theprocesses that occur in N�MP (or DMSO) in the pres�ence of basic catalysts is shown in Scheme 2.

170

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PONOMAREV et al.

Scheme 2.

Depending on reaction temperature and the natureof the solvent and catalyst used, the formation ofdesired addition product I was found to be accompa�nied by the side reactions of polymerization anddealkylation (hydrolysis) of DEVP and ethylation of2�PhBI to give 1�ethyl�2�phenylbenzimidazole thatproceed intensely above 80°С. Under optimal condi�tions (60°С, N�MP, 10 mol % of a mixture of NaOH +KOH (1 : 1)), the reaction affords compound I in vir�tually quantitative yield.

The structure of this compound was confirmed by1H and 31Р{1Н} NMR spectral data and by the X�ray

analysis of its monohydrate (Fig. 1). The moleculeof compound I has standard geometrical parame�ters. The phenyl substituent is tilted toward the ben�zimidazole plane by 41.5°. The molecules arehydrogen bonded in crystal via N(2)⋅ ⋅ ⋅H(Ow)(N⋅ ⋅ ⋅H 2.11 Å) and O(1)⋅ ⋅ ⋅H(Ow) (О⋅ ⋅ ⋅Н 2.07 Å)interactions to form layers parallel to the diagonalplane of the ab cell.

When crystals of I are heated at reflux in 18% HClfor 2 h, a complete hydrolysis takes place to formphosphonic acid II (Scheme 3).

Scheme 3.

The IR spectrum of N�phosphonoethylated 2�PhBI (I) shows the lack of a wide band of stretchingvibrations of NH group of benzimidazole in the region3450 cm–1 and the presence of a strong band of P=Ostretching vibrations at 1272 cm–1. The spectrum ofacid II shows a wide absorption band in the region936–1150 cm–1 that includes a series of vibrationalbands of phosphonic groups partially dissociated dueto protonation of nitrogen atoms of benzimidazole

and a band at 2300–3100 cm–1 typical for the vibra�

tions of P–O–H and –H groups.

The data obtained in the study of model reac�tions were further used for the examination of poly�mer�analogous transformation of PBI�O�PH. Thegeneral scheme for the synthesis of PEPBI�O�PH(Scheme 4) and its repeating unit irregularity [6] areshown below.

N

N

H

P(O)(OEt)2CHCH2

N

N

CH2 CH3

N

N

CH2 CH2 P(O)(OEt)2

CH2 CH

PEtO O

OEt

CH2 CH

PEtO O

OH

CH2 CH P(O)(OEt)(OH)

nn

31P, δ 26.63 ppm

I

Hydrolysis ofpolyDEVP

Hydrolysis ofDEVP

Dealkylation

31P, δ 28.56 ppm

31P, δ 16.95 ppm

(DEVP)

N�МP (DMSО), Cat.20–100°C

(Cat. = NaOH, KOH, NaOH + KOH, ТEBAH)

2�PhBI

)ОН

N

N

CH2 CH2 P(O)(OEt)2

N

N

CH2 CH2 P(O)(OH)2

I II

N+

DOKLADY CHEMISTRY Vol. 432 Part 2 2010

SYNTHESIS OF N�PHOSPHONOETHYLATED CARDO POLY(BENZIMIDAZOLE) 171

Scheme 4.

The optimization of the process of modification ofPBI�O�PH was carried out taking into account dataobtained previously in the work [7]. Some results are

given in the table. This table shows that the use ofpotassium tert�butoxide as a catalyst of PBI�O�PHphosphonoethylation at 20–45°С leads to the lack of

O

O

N

N

H

O

N

N

H

n

CH2=CH–P(O)(OEt)2 (DEVP)

N�МP, NaOH + KOH

10 mol %, 60–90°C

n

mk l

PEPBI�O�PH

O

O

N

N

Et

O

N

NN

NH

(EtO)2P(O)

PBI�О�PH

k, l = 0 – 2 (k + l = 2)

С(12)

С(13)С(11)

С(10)

С(9)

С(8) С(1)

N(2)

С(7)

С(6)

С(5)

С(4)

С(3)

С(2)N(1)

С(14)

С(16)P(1)

O(3)

C(18)

C(19)

O(1)

O(2)

С(17)

С(15)

Fig. 1. Crystal structure of O,O�diethyl 2�(2�phenyl�1H�benzimidazol�1�yl)ethylphosphonate (I).

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DOKLADY CHEMISTRY Vol. 432 Part 2 2010

PONOMAREV et al.

the target addition product (the yield is not higherthan 12–13%). At 60°С, the polymerization of DEVP(Scheme 2) proceeds much faster than its addition tothe benzimidazole ring. Triethylbenzylammoniumhydroxide (TEBAH) is also a good catalyst for DEVPpolymerization: the yield of the polymer at 80°С is vir�tually 100% over 42 h. However, if the catalyst is addedin four portions within 50–80°С, PEPBI�O�PH con�taining up to 63–67% of diethoxyphosphoryl groupscan be obtained within 88 h. The content of polymer�ized DEVP in this case is not higher than 14%.

The best results were obtained when the reactionwas carried out in an N�MP medium at 70–90°С inthe presence of an equimolar mixture of NaOH andKOH as a catalyst. Under these conditions, virtuallyno polymerization of DEVP is observed (1.5%), whilethe conversion of PBI�O�PH reaches 80–81%.

Figure 2 shows kinetic curves for the phosphonoet�hylation of PBI�O�PH and the side reactions of poly�merization of DEVP and its dealkylation resulting inthe ethylation of PBI�O�PH according to Scheme 4.

The process of the polymer�analogous transforma�tion of PBI�O�PH was also monitored by GPC. Fig�

Study of the reaction of phosphonoethylation of PBI�O�PH

Polymer T, °C Reaction time, h Content of phospho�noethylation product*

Content of poly(DEVP)* P/PE** (%)

DMF, t�BuOK

1 40 4.5 0 0 –

60 10 8 36 –

60 24 11.5 48 0.96/13.4

2 20 56 <1 0 –

40 96 ~3.5 0 –

45 144 12 0 –

DMF, TEBAH

3 45 4 <1 – –

80 42 <1 100 –

DMF, TEBAH (in 4 portions)

4 50 8 0 0 –

80 10 2 0 –

80 52 25 0 –

80 56 42 5 –

80 64 53 12 –

80 72 60 14 –

80 80 63 14 –

80 88 63 14 4.81/67

N�MP, NaOH + KOH

5 55 24 6 0 –

70 32 24 0 –

80 48 48 0 –

85 56 76 0 –

90 80 84 1 4.52/63

6*** 80 18 13 0 –

80 96 55 0 –

80 192 76 1 –

80 216 80 ~1.5 5.83/81

* According to 31P{1H} NMR data.** The ratio of phosphorus content to the content of diethoxyphosphoryl groups in PEPBI�O�PH according to elemental analysis data (the

calculated content of P is 7.2/100) for the polymer precipitated with alcohol.*** According to elemental analysis data for the polymer precipitated with water.

DOKLADY CHEMISTRY Vol. 432 Part 2 2010

SYNTHESIS OF N�PHOSPHONOETHYLATED CARDO POLY(BENZIMIDAZOLE) 173

ure 3 shows the chromatograms of the initial polymerand its phosphonoethylation product (yield 80–81%)obtained for their solutions in DMF with LiCl addi�tives (the table, polymer 6) [3]. It follows from the fig�ure that the content in PEPBI�O�PH of the microgelthat is present in the initial PBI�O�PH considerablydecreases (from 33% to 8%) [3], and the molecularweight distribution of the main fraction becomesmuch wider (Mw/Mn increases from 1.7 to 9.90).

Strong and elastic films based on PEPBI�O�PHwere cast onto glass supports from reaction solutions

of the polymer in N�MP and, after thermal treatmentat 160°С, had good mechanical properties (tensilestrength was 120–140 MPa, ultimate elongation was12–18%, and Young’s modulus was 2500–3000 MPa).

According to the data of dynamic TGA in air, theobtained polymers show high thermal stability compa�rable to that of the known polymers of the PBI family[1–3]. The temperatures of their fast degradation arehigher than 580°С, while degradation onset tempera�ture is 360°С for the diethoxyphosphoryl derivative ofPEPBI�O�PH and ~400°С for its Р(О)(ОН)2 deriva�tive.

With the aim to transform the diethoxyphosphorylgroups of the polymers into acid ones, the films wererefluxed for 5 h in 18% HCl, which resulted in phos�phonic acids with the 31P NMR chemical shift δ ~19.5 ppm (HCOOD).

Measurements of the proton conductivity of thefilm samples of PEPBI�O�PH showed that its value iswithin 10–3–10–4 S/cm, whereas the conductivity ofknown PBIs doped with orthophosphoric acid (PA)reaches 0.15 S/cm [8–10]. Such a large difference inproton conductivity seems to result from the fact thatthe doped systems involve up to 5–6 Н3РО4 moleculesper polymer repeating unit, which is equivalent to 15–18 P–OH groups, whereas the obtained PEPBIs con�tain only 3–3.5 P–OH groups per unit. For the prac�tical application of membranes in HTFC, the requiredproton conductivity values are ≥5 mS/cm at 150–200°С, better ≥50 mS/cm; therefore, the applicationof the films of undoped PEPBI�O�PH appear to beinappropriate. However, the preparation of complexesof this polymer with PA may decrease the total amountof free PA necessary to maintain the optimal perfor�

00 50 100 150 200 250

τ, h

20

40

60

80

β, с, %

1

23

Fig. 2. Kinetics of phosphonoethylation of PBI�O�PH(80°С, N�MP, 10 mol % of NaOH + KOH (1 : 1, mol)):1, extent of phosphonoethylation (β); 2, content ofdealkylated DEVP (content of N�ethyl groups in PEPBI�O�PH); 3, content of poly(DEVP) in reaction solution.

500

400

300

200

100

0

0 5 10 15 20

7

6

5

4

3

log MU, mV

1

2

3

Time, min

Fig. 3. Chromatograms of initial PBI�O�PH (1), its phosphonoethylation product (2), and calibration curve (3) relative to poly�styrene.

174

DOKLADY CHEMISTRY Vol. 432 Part 2 2010

PONOMAREV et al.

mance of a fuel cell under different operating condi�tions. Since excess PA is rather aggressive at 160–200°С and causes the degradation of the active layersof a fuel cell [2], one can expect that PEPBI�O�PHmembranes doped with a smaller amount of PA willshow longer service life.

A seven�layer MEA was assembled on the basis of afilm of polymer 6 (table) doped with 77% PA at 50°Сfor 72 h to 3–3.5�fold weight increase as described in[4], with the use of BASF Celtec® P1000 commercialelectrodes (Germany). Figure 4 shows the dischargecharacteristics of this MEA at 160, 180, and 200°С.

It is obvious that the MEA on the basis of PEPBI�O�PH has higher current�voltage characteristics thanan analog assembled with the use of non�phosphory�lated PBI�O�PH membrane and at current densityabove 0.2 A/cm2 is on a level of BASF Celtec® P com�mercial reference sample.

We explain the slightly lowered values of open cir�cuit voltage (OCV) of the tested MEA (0.760 V) by thefeatures of the chemical structure of PEPBI�O�PH;the polymer seems to have enhanced gas permeabilityand correspondingly crossover current on account of alarge number of diethoxyphosphoryl groups [1, 2].

The MEA designed in this paper has worked in agalvanostatic mode (current density j = 0.4 A/cm2) for100 h at 160°С and 300 h at 180°С, showing a stablepotential difference ΔU = 0.595 and 0.615 V, respec�tively.

ACKNOWLEDGMENTS

We are grateful to Z.S. Klemenkova for recordingIR spectra and E.I. Goryunov for discussion of theresults.

This work was supported in part by the RussianAcademy of Sciences (Complex Program no. 7 of theDivision of Chemistry and Materials Science “Devel�opment of Scientific Principles of Novel ChemicalTechnologies and Preparation of ExperimentalBatches of Compounds and Materials”).

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10. Savinell, R.F., Yeager, E., Tryk, D., Landau, U., Wain�right, Weng, D., Lux, K., Litt, M., and Rogers, C.,J. Electrochem. Soc., 1994, vol. 141, pp. L46–L48.

450

0 0.2 0.4 0.6 0.8i, A/cm2

550

650

750

850

950

ΔU, mV

123

4

Fig. 4. Temperature dependences of discharge characteris�tics of seven�layer hydrogen–air MEA with membranes onthe basis of: PEPBI�O�PH (1–3), PBI�O�PH (4), andCeltec®�P (160°С, at i = 0.4 A/cm2, star), PEMEAS P�1000electrodes. Temperature, °C, for PEPBI�O�PH: 160 (1),180 (2), 200 (3); for PBI�O�PH: 160.