Synthesis of Sulfonated Poly(1,4-diphenoxybenzene) for Proton ...

Sulfonated Poly(1,4-diphenoxybenzene) for Proton Exchange MembraneSulfonated Poly (1,4-diphenoxyphenylene) (4)To a solution of polymer 2 (0.13 g, 0.5 mmol) in dichloromethane(2 mL), a solution of chlorosulfonicacid (0.13 mL, 2 mmol) in dichloromethane (0.8 mL)was slowly added with stirring under nitrogen. Themixture was stirred at room temperature for 24 h,and then poured into hexane. The precipitate was collectedby filtration and stirred in hot toluene. The precipitatewas collected and washed with toluene andhexane. It was dried at 100 C for 8 h under the reducedpressure to give the fully sulfonated polymer4a. The yield was 0.19 g (90%). IR (KBr, ); 1184(-SO 3 H), 1223 (C-O-C), 690, 1493, 1601 cm 1 (Ph).The partially sulfonated polymers 4b and 4c were preparedby using 1.00 (0.07 mL) and 0.5 (0.03 mL)mmol of chlorosulfonic acid, respectively in the similarconditions described above. The numbers of sulfonicacid groups in the repeating unit of 4b and4c were 0.74 and 0.38, respectively. Ion exchangecapacity (IEC) was determined by titration with0.02 M NaOH aq.MeasurementsThe infrared spectra were recorded on a Horiba FT-720 spectrophotometer. The 1 H and 13 C NMR spectrawere obtained on a JEOL JNM-AL400 spectrometerat 1 H, 400 MHz and 13 C, 100 MHz. Number- andweight-average molecular weights (M n and M w ) weredetermined by a gel permeation chromatograph (GPC)on a Hitachi LC-7000 system equipped with a polystyrenegel column (TSKgel GMHHR-M) eluted withtetrahydrofuran (THF) at a flow rate of 1.0 mL min 1calibrated by standard polystyrene samples. The cyclicvoltammogram was measured at room temperaturein a typical three electrode with a working (Ptwire), a reference (Ag/AgCl), and a counter electrode(Pt spiral) under a nitrogen atmosphere at a sweepingrate of 10 mV/s (Hokuto Denko HSV-100). A 0.1 Msolution of tetrabutylammonium perchlorate in anhydrousacetonitrile was used as an electrolyte. Thermalanalysis was performed on a Seiko EXSTAR 6000TG/DTA 6300 thermal analyzer at a heating rate of10 C/min for thermogravimetry (TG), differentialthermal analysis (DTA), and a Seiko EXSTAR 6000DSC 6200 at a heating rate of 10 C/min in the temperaturerange from 25 to 300 C for differential scanningcalorimetry (DSC) under nitrogen. Disk-likesamples with thickness of 600 mm were prepared bycompression molding. Proton conductivity measurementswere performed on the acid form of the disklikesamples using a Hioki 3532-80 Chemical impedancemeter. An impedance spectrum was recordedfrom 5 Hz to 1 MHz. The resistance of the samplewas taken at the frequency that produced the minimumimaginary response. All the impedance measurementswere performed at 80 C above water with 95%or 50% relative humidity (RH).CalculationThe density functional theory (DFT) with the threeparameterBecke-style hybrid functionals (B3LYP)was used for calculation of the Mulliken populationcharges for neutral compounds and the local spin densitiesfor radical cations. Molecular geometries in theneutral sates were optimized under no constraints, andthe Fukui functions were calculated as differences inMulliken charges between the neutral and the radicalcation states having the same molecular geometries.The 6-311G(d) basis set was used for the geometryoptimization and the calculations of Mulliken chargesand spin densities. Radical cations were always treatedas open shell systems (uB3LYP). The moleculargeometries of radical cations were optimized independentlyfrom the neutral molecules prior to the calculationsof local spin densities. In addition, the UV/Vis optical absorption spectra were calculated formonomer (1) and model compounds using the timedependentDFT methods with the B3LYP functionaland the 6-311 þþ G(d,p) basis set. The details for thetheoretical prediction of absorption spectra have beenreported elsewhere. 8 All the calculations were performedusing the software of Gaussian-03 (RevisionC02). 11RESULTS AND DISCUSSIONCyclic Voltammogram of Monomer 1It is important to measure the oxidation-reductionpotentials of 1 for the selection of oxidants. The cyclicvoltammogram of 1 was measured in acetonitrile solutioncontaining 0.1 M tetrabutylammonium perchloratein a cell equipped with a reference electrode(Ag/AgCl), a working electrode (Pt), and an auxiliaryelectrode (Pt). The redox process was observed at theoxidation potential value of 1.6 V vs SCE (Figure 1).Thus, FeCl 3 was selected as the oxidant for oxidativecoupling polymerization of 1.-2 -1 0 1 1.63 2V (vs Ag/AgCl)Figure 1. Cyclic voltammogram of monomer 1.Polym. J., Vol. 39, No. 8, 2007 883

Sulfonated Poly(1,4-diphenoxybenzene) for Proton Exchange MembraneSO 3 HOOOOFeCl 3ClSO 3 HNitrobenzene, 24hn CH 2 Cl 2nOO1 2 4aHO 3 SScheme 1.Oxidative coupling polymerization of monomer 1 and sulfonation of the resulting polymer.Figure 2.DMSO-d 6 .Expanded 13 C NMR spectra of 1 and polymer 2 inFigure 3.DMSO-d 6 .Expanded 13 C NMR spectra of 3 and polymer 4a in2 are shown in Figure 2. The 13 C NMR signal of polymer2 is very similar to that of monomer 1, but it isdifficult to determine the coupling positions because13 C peaks of coupling positions are too weak to identify.Thus, to estimate the structure of polymer 2,a model compound, p-phenylenedioxybis(4-benzenesulfonicacid) (3) was prepared by sulfonation of 1because the para position (1) of the phenoxy groupin 1 would be the other probable coupling position.The structure of 3 was confirmed by IR and NMRspectroscopy and elemental analysis. However, thecoupling reaction of position (1) of the phenoxy groupin 1 cannot be completely excluded because of week13 C peaks of coupling positions.Synthesis of Polymer 4aSulfonation of polymer 2 was also carried out toshow that the para position (1) of the phenoxy groupwas not involved in the coupling (Scheme 1).Figure 3 shows the 13 C NMR spectra of 3 and polymer4a. The C-1 resonance of 122.96 ppm in 1 shiftsto 141.57 ppm in 3, and the C-1’ resonance of 123.11ppm in polymer 2 also shifts to 141.31 ppm in polymer4a. All other peaks are well assigned to the proposedstructure of 3 and polymer 4a. These findingsindicate the C-1 position in 1 is almost intact duringthe polymerization and is selectively sulfonated bythe subsequent treatment with chlorosulfonic acid.These reacted positions are in very good agreementwith those predicted by the DFT calculations.The UV/Vis absorption spectra of monomer 1 andpolymer 2 in chloroform with the concentrations of5 10 5 mol/L was measured. The absorption edge( e )of2 appears at 550 nm, which is much longerwavelengths than 1, suggesting the delocalization of-electrons (-conjugation) along the polymer mainchain (Figure 4). Polymer 2 was soluble in tetrahydrofuran,chloroform and nitrobenzene at room temperature.On the other hand, polymer 4a was soluble inwater and polar aprotic solvents, such as N,N-dimethylformamide,N,N-dimethylacetamide, and dimethylsulfoxide at room temperature. The thermal propertiesof polymer 2 were examined by thermogravimetry(TG) and differential scanning calorimetry (DSC).Polym. J., Vol. 39, No. 8, 2007 885

Sulfonated Poly(1,4-diphenoxybenzene) for Proton Exchange MembraneAdamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O.Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D.Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q.Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B.Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B.Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A.Pople, Gaussian, Inc., Wallingford CT, 2004.Polym. J., Vol. 39, No. 8, 2007 887