Synthesis and characterization of diethyl-p-vinylbenzyl phosphonate monomer: precursor of ion exchange polymers for fuel cells

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1 Superficies y Vacío () -,septiembre de Sociedad Mexicana de iencia y Tecnología de Superficies y Materiales Synthesis and characterization of diethyl-p-vinylbenzyl phosphonate monomer: precursor of ion exchange polymers for fuel cells M. campo-fernández osgrado en iencia de Materiales, Universidad Autónoma del Estado de idalgo arretera achuca-tulancingo Km..5,, Mineral de la Reforma, go., México. Ana M. errera, T. Méndez-Bautista, J. García-Serrano * entro de Investigaciones en Materiales y Metalurgia, Universidad Autónoma del Estado de idalgo arretera achuca-tulancingo Km..5,, Mineral de la Reforma, go., México. (Recibido: de mayo de ; Aceptado: de junio de ) Synthesis of diethyl-p-vynilbenzyl phosphonate (DEpVB) monomer and its homopolymer, precursors of ion exchange materials, are reported. DEpVB monomer was synthesized by means of Michaelis-Arbusov reaction of p-vinylbenzyl chloride with tryethyl phosphite in the presence of cupric chloride using chlorebenzene as solvent with a yield of 75%. omopolymer was synthesized by free radical polymerization reaction using AIBN as a free radical initiator at 7 º in DMF. The structures of the compounds were determined from NMR, FT-IR and Raman spectroscopy. Good thermal stability of homopolymer (up to º) indicates that theses compounds are excellent candidates to be used in the development of new proton exchange membranes for fuel cells. Keywords: Ion exchange polymers; hosphonated polymers; Metallorganic compounds. Introduction An ion exchange material is a solid that exchanges one ion for another ion. The process of ion exchange involves the change of atoms but with conservation of charge. The ion-containing polymers, such as ion-exchange resins, polyelectrolytes, and ionomers, are an important class of materials which have found use in many areas, such as fuel cells [], water splitting [], electro-organic synthesis [], catalysis [], and nanoparticles synthesis [5]. To date proton-exchange membrane fuel cells (EMF) are the most versatile method for power generation []. The proton conducting membranes employed in EMFs are today typically based on polymers with hydrated sulfonic acid units. erfluorinated sulfonic acid membranes have been successfully employed as proton conductors in fuel cells based mainly on the catalytic oxidation of hydrogen [7]. The acid units of the polymer works as proton source and are dissociated by water which acts as the proton solvent and thus facilities the proton transport [7]. Acid group-containing polymers have attracted much attention as solid electrolyte membranes in batteries and fuel cells. In particular, the polymers functionalized with phosphonic acid have recently attracted attention as an alternative material for proton-exchange membrane fuel cells to sulfonic acid copolymers. There have been several reports of phosphonic acid-containing polymers as membranes for fuel-cell applications []. They have lower acidity than sulfonic acids; however, their better chemical and thermal stability with respect to corresponding sulfonic acid-functionalized polymers offers potential advantages. owever, phosphonic acid groups grafted onto the polymer * jserrano@uaeh.edu.mx main chains used as alternative ion exchange group have not been widely investigated due to the difficulty to produce - bonds [, ]. Also, most of these syntheses involve a large number of reactions steps []. A simple way to prepare phosphonated materials is the modification of available molecules with phosphorylating agents using the Arbuzov or Michaelis-Becker reactions, in witch alkyl halides react with trialkyl phosphites to form dialkyl alkylphosphonates [, ]. Diethyl-vinylbenzyl phosphonate is a very interesting monomer that has been synthesized by abbaso and co-workers [, ] from the reaction of vinylbenzyl chloride (a mixture of meta and para isomers 7:) with triethyl phosphite in presence of - ter-butyl-,-dimethylphenol, with a % yield. In the present work, we report the synthesis, characterization, and polymerization of diethyl-pvinylbenzyl phosphonate (DEpVB) monomer. It was prepared by means of Michaelis-Arbuzov reaction using a modified abbaso method, where the main differences were the use of ul and chlorobenzene as inhibitor agent of the monomer polymerization and solvent, respectively, in air atmosphere. Also, we use only the para isomer of vinylbenzyl chloride compound (with a purity of %) as precursor of the DEpVB monomer. It was purified by flash chromatography (silica gel, chloroform) obtaining a yellow liquid with a yield of 75%. The polymerization of DEpVB was carried out via free radicals, using, azoisobutyronitrile (AIBN) as a free radical initiator and

2 Superficies y Vacío () -,septiembre de Sociedad Mexicana de iencia y Tecnología de Superficies y Materiales l p-vinylbenzyl chloride l ul / º l chloroethane triethyl phosphite diethyl-p-vinylbenzyl phosphonate Scheme. Michaelis-Arbuzov reaction of diethyl-p-vynilbenzyl phosphonate. AIBN /7º DMF/h Scheme. Free Radicals polymerization of the diethyl-p-vinylbenzyl phosphonate ` ppm Figure. -NMRspectrum of the DEpVB monomer * n * dimethylformamide as solvent at 7º with hours reaction time in argon atmosphere. Formation and molecular structures of the synthesized compounds were confirmed from their infrared (IR), and Raman spectra as well as from their elemental analysis. Thermal stability of homopolymer was studied by thermogravimetric analysis (TGA).. Experimental Section.. Materials Triethyl phosphate (% purity), p-vinylbenzyl chloride, ul and AIBN, each with % purity, were purchased from Aldrich. Initiator AIBN was recrystallized using methanol. All solvents were used as delivered... Synthesis of diethyl-p-vynilbenzyl phosphonate (DEpVB) monomer DEpVB monomer was prepared by means of Michaelis-Arbuzov reaction (scheme ) according to modified method from the reference. A 5 ml threenecked flask equipped with a mechanical stirring device, reflux condenser and thermometer was charged with. ml (7. mmol) of triethyl phosphite, ml (. mmol) of p-vinylbenzyl chloride, g (7. mmol) of ul as inhibitor of the polymerization of p-vinylbenzyl chloride and ml of chlorobenzene as solvent. The mixture was heated to º for h. After this period, the reaction mixture was cooled at room temperature and the unreacted triethyl phosphite was removed at room temperature in vacuum. The crude product was purified by flash chromatography on silica gel using chloroform, and dried at room temperature in vacuum for h, obtaining.5 g (yield 75%) of DEpVB monomer as a yellow liquid. Elemental analysis found: % and 7.%; calculated for :. % and 7.%. NMR ( Mz, d -chloroform): δ 7. (d,. z,, Ar-), 7. (d,. z,, Ar-),.7 (m,, =), 5.7 (d, 7.57 z,, = ), 5.7 (d,. z,, = ),. (m,, - ),. (s,, -),. (t,. z,, - ). AT NMR ( Mz, d -chloroform): δ. ( ),. ( ),. ( ),.75 ( =),.5 (h),. (h),.5 (=),.5 (h), and 7.75 ppm (h). FT-IR (KBr pellets):, 7,,,,,,, (vinyl =), (Ar = stretch), 5, 5, 7,,, 5, 75, (= stretch),,, (-- stretch), (-- stretch),,,, 7,,, 5, and cm ppm Figure. AT NMR spectrum and molecular structure of the DEpVB monomer. 7

3 Superficies y Vacío () -,septiembre de Sociedad Mexicana de iencia y Tecnología de Superficies y Materiales Transmittance (a.u) DEpVB poly(depvb) = vinyl Wavelength (cm - ) Figure. IR spectra of the DEpVB and poly(depvb). Intensity (a.u) DEpVB poly(depvb) = vinyl Raman shift / cm - = Aromatic = Aromatic Figure. Raman spectra of the DEpVB and poly(depvb)... Synthesis of homopolymer poly(diethyl-p-vinylbenzyl posphonate) DEpVB monomer was polymerized by free radicals reaction in DMF using AIBN as a free radical initiator (scheme )..5 g of monomer and. mg ( wt % of the monomer) of initiator were charged in a glass test tube containing ml of DMF, which was bubbled with argon gas for min. The test tube was then sealed under argon and placed in a constant temperature (7 º) water bath for h. After this period, the reaction mixture was cooled to room temperature, and the polymer was isolated by precipitation of the product in cold water ( times the volume of the reaction mixture) and with subsequent drying at º in vacuum for h. The homopolymer in form of a light brown solid was obtained with a % yield (. g)... haracterization NMR and AT NMR spectra of the samples were recorded on JEL ( Mz/ Mz) NMR spectrometer, using deuterated chloroform and tetramethyl silane (TMS) as the internal reference. IR spectra were recorded on a erkin-elmer Spectrum GX FT-IR spectrophotometer, with a spectral resolution of cm -. For FTIR studies, mg of solid sample was mixed with mg of dry KBr homogeneously to make pellets of 7 mm diameter and.5 mm thick. Raman spectra were recorded on a Raman erkin-elmer system spectrophotometer. Elemental analysis of the samples was carried out on a erkin-elmer,, N analyzer (model ). Thermogravimetric measurements of the compounds were carried out with a Mettler-Toledo thermal analyzer TGA/SDTA 5. The measurements were performed from room temperature to 5 º, at a heating rate of /min under nitrogen atmosphere. Weight(%) º º, loss.% 5º, loss.% º, loss.% Temperarure (º ) Figure 5. TGA curve of poly (DEpVB). º, loss.7%. Results and discussion.. Monomer characterization Formation and molecular structure of the DEpVB was confirmed from their NMR, AT NMR, infrared, and Raman spectra. Figure shows the NMR spectrum of the DEpVB monomer and the results are summarized in the Table. The most important signals for the compound, which indicate its formation were the multiplet at.7 ppm assigned to the proton on the = group, the doublets at 5.7 and 5.7 ppm assigned to the protons on the =, the multiplet at. ppm assigned to the protons on -, the singlet localized at. ppm corresponding to the protons of -, and the triplet at. ppm for the protons of - group. The AT NMR spectrum of the DEpVB (Figure ) revealed nine signals. The main signals that confirming the formation of compounds were localized at.7 and

4 Superficies y Vacío () -,septiembre de Sociedad Mexicana de iencia y Tecnología de Superficies y Materiales.5 ppm corresponding to the atoms of the vinyl group; the signals at. and. ppm due to the atoms on the and of the diethyl-phosphonate group, respectively. Formation of the DEpVB is also evident from their FTIR and Raman spectra (see Fig. and Fig. ). IR spectrum revealed strong absorption bands due to the = stretching vibration mode at cm -, --Et stretching at - cm -, -- stretching at cm -, all corresponding to the ( ) group; = stretching vibration mode at cm - of vinyl group; and = stretching at cm - of phenyl group. These assignments were based on literature vibrational studies of organic and phosphonate materials [-7]. n the other side, in the Raman spectrum the signals corresponding to the = stretching modes of the vinyl and phenyl groups appeared at cm - and 7 cm -, respectively... olymer characterization The poly(depvb) homopolymer was found to be soluble in several polar solvents like ethanol, methanol, DMF, and DMS; but insoluble in toluene, ciclohexane, and water. Formation of the homopolymer was followed by Raman and IR spectroscopies. Fig. and Fig. show the IR (- cm - ) and Raman (in the spectral range of 5-5 cm - ) of the monomer and polymer, indicating the region correspond to the = stretch. Is clear from Raman spectra that the principal evidence of polymer formation came from the disappearance of the signal due to = stretching vibration mode of vinyl group of the monomer. In the IR spectrum of the polymer (Figure ) also the absorption band due to = stretching of the vinyl group is absent. Thermal stability of the poly(depvb) was studied through thermogravimetric measurements realized at a heating rate of /min under nitrogen atmosphere. Figure 5 Termogravimetric curve of the polymer shows distinct regions of weight loss with the increase of temperature. The first, a gradual weight loss of about. % in the temperature range - was attributed to the evaporation of water and DMF. In the second region (- 5 ), about. % of the weight loss was associated to the partial thermal degradation of poly(depvb) due to the loss of the ethyl ester group of the vinylbenzyl phosphate. Thermal degradation of VB due to the loss of ester groups has been reported at -5 []. rogressive thermal degradation of the polymer observed between 5 and 5, is probably due to the degradation of the main chain of polymer and the degradation of - bonds of the pendant group of polymer. These results indicate that the poly(depvb) synthesized is stable until. Above this temperature, the degradation of polymer occurs in several steps, resulting in the loss of about 7 % of the weight of polymer.. onclusions DEpVB monomer was synthesized by means of Michaelis-Arbusov reaction using ul as inhibitor agent and chlorebenzene as solvent, with a yield of 75%. Synthesis process permits to improve the yield on reported (%). oly(depvb) synthesized by free radicals polymerization shows a good thermal stability until º. Molecular structures of the synthesized compounds were confirmed by NMR, AT NMR, IR, and Raman spectroscopies. wing to the ion exchange capacity conferred by the phosphonate group and the good thermal Table. Assignment of -NMR peak positions of the DEpVB monomer. hemical Shift δ (ppm) Integration Multiplicity J - (z) Assignment 7 7' 7. d d..7 m d d. 7. m ---. s ---. t. s = singlet, d = doublet, t = triplet and m = multiplet

5 Superficies y Vacío () -,septiembre de Sociedad Mexicana de iencia y Tecnología de Superficies y Materiales stability of polymer, the DEpVB and poly(depvbd) is under study by our research group for the synthesis of copolymers useful as polymer exchange membranes for fuel cells. Acknowledgments M. campo-fernandez acknowledges to NAyT, Mexico for the graduate fellowship. The work was partially supported by RME, Mexico. References []. Itoh T.; amaguchi Y.; Uno, T.; Kubo, M.; Aihara, Y.; Sonai, A. Solid State Ionics, 77, 5 (). []. Linkous,. A.; Anderson,.R.; Kopitzke, R.W.; Nelson, G.L. Int. J. ydrogen Energy,, 55 (). []. Madden, T..; Stuve, E. M.; J. Electrochem. Soc., 5, E57 (). []. Bryant, D. E.; Kilner, A. J. Mol. atal. A: hem.,, (). [5]. García-Serrano, J.; al, U.; errera, Ana M.; Salas,.; Angeles-havez,. hem. Mater., 5 (). []. Gudrun Schmidt-Naake, hem. Eng. Technol,, (5). [7]. Donald J. Burton, Journal of Fluorine hemistry,, (7). []. arvole, J. Macromolecules,, (). []. Kotov, S.; Burton, D. J. Fluorine hem., (7). []. Jakoby, K.; Nunes, S.. Macromol.hem.hys.,, (). []. Bhattacharya, A.; Thyagarajan, G. hem. Rev., 5 (). []. abasso, I.; Yu, Z. J. olym. Sci. art A, 7 (). []. Tamber,.;Smid J.; abasso I. hem. Mater, 5 (7). []. Nakanishi, K. Infrared absorption spectroscopy (olden- Day, San Francisco, ). [5]. Danilich, M.J.; Burton, D.J.; Marchant, R.E. Vib. Spectroscopy, (5) []. opa, A., Davidescu,.M.; Negrea,.; Ilia, G.; Katsaros, A.; Demadis, K.D. Ind. Eng. hem. Res. 7, (). [7]. Yamabe, M.; Akiyama, K.; Akatsuka, Y.; Kato, M. European olymer Journal, 5 ().