Tuning the solubility of polymerized ionic liquids by simple

RAPID COMMUNICATION

Tuning the Solubility of Polymerized Ionic Liquids by Simple

Anion-Exchange Reactions

REBECA MARCILLA,1J.ALBERTO BLAZQUEZ,2JAVIER RODRIGUEZ,1JOSE A.POMPOSO,1DAVID MECERREYES1 1Centre for Electrochemical Technologies(CIDETEC),Paseo Miramon196,Donostia-San Sebastian20009,Spain

2Polymer Science and Technology Department and Institute for Polymer Materials“POLYMAT”,University of the Basque Country,9bced33c01f69e3142329413rdizabal,3,20018Donostia-San Sebastian,Spain

Received4August2003;accepted9October2003

Keywords:polycations;ionic liquids;high performance polymers;polyelectrolytes;

high performance polymers;hydrophilic polymers

Ionic liquids are organic salts with a low melting point (?100°C)that have been the focus of many investiga-tions because of their chemical stability,low?amma-bility,negligible vapor pressure,high ionic conductiv-ity,and wide electrochemical window.1Today,ionic liquids?nd industrial applications as green solvent systems for organic synthesis and in liquid–liquid ex-tractions substituting more-toxic chlorinated solvents.2 Other applications involve its use as high-performance liquid electrolytes in solar cells,3lithium batteries,con-ductive-polymers-based supercapacitors,4arti?cial muscles,and electrochromic devices.5This wide scope of applications offers good opportunities for the devel-opment of ionic liquid-based polymers or gels.Never-theless,the interest of ionic liquids in polymer chemis-try is very recent.Typical research has reported its use as green solvents for the synthesis of homopolymers6 and block copolymers.7Also,its original and pioneering use as an organic catalyst for the living ring-opening polymerization of lactones and lactides has been de-scribed.8–10Interestingly,functionalization of poly-mers having some of the characteristics of ionic liquids has been pursued as a way of developing high-perfor-mance polymer electrolytes.11,12Yoshizawa and co-workers11,12have polymerized novel ionic liquid mono-mers.Alternatively,in this work we describe the direct polymerization of imidazolium ionic liquid monomers and the subsequent modi?cation by carrying out the anion-exchange reaction directly into the polymers. The goal of this communication was to formulate this easy,original method as a way of tuning the solubility of the obtained polycations.

Polymerizable vinyl imidazolium halides were pre-pared following classical literature procedures13for the synthesis of alkyl imidazolium ionic liquids.In our case,we used1-vinyl imidazole as a starting material. As a result,we obtained two,different,new monomers [Scheme1(a)].1-Vinyl-3-ethyl-imidazolium bromide (ViEtIm?Br?)was synthesized when ethyl bromide was used as the starting material,and1-vinyl-3-butyl-imidazolium chloride(ViBuIm?Cl?)was synthesized in the case of1-butyl chloride.The synthesis of the monomers was simple,and high-purity materials were obtained without time-consuming puri?cation steps. Next,polymerizations were carried out in chloroform at 60°C for3h under inert atmosphere with azobis(2-methylpropionitrile)as the thermal radical initiator [Scheme1(a)].Therefore,poly(1-vinyl-3-ethyl imidazo-lium)bromide[poly(ViEtIm?Br?)]and poly(1-vinyl-3-butyl imidazolium)chloride[poly(ViBuIm?Cl?)]pre-cipitated directly into the reaction vessel.After puri?-cation with chloroform,the product was obtained quantitatively.Figure1shows the1H NMR spectra of

Correspondence to:D.Mecerreyes,Centre for Electrochem-ical Technologies(CIDETEC)-New Materials Department, Paseo Miramon,Donostia-San Sebastian20009,Spain (E-mail:dmecerreyes@cidetec.es)

Journal of Polymer Science:Part A:Polymer Chemistry,Vol.42,208–212(2004)?2003Wiley Periodicals,Inc.

208

the ViEtIm ?Br ?monomer and the polymerized form poly(ViEtIm ?Br ?).Disappearance of the vinyl signals at 7.53,6.17,and 5.47ppm is observed with the ap-pearance of new signals at 4.7and 2.7ppm attributed to the protons of the polymeric backbone.Polymers exhibiting a broadening of the signals were observed,which agreed with the classical spectrum.Very similar results were obtained in the case of the polymerization of ViBuIm ?Cl ?.Thus,new poly(1-vinyl-3-alkyl imida-zolium)halides were synthesized by radical polymer-ization of ionic liquid monomers,leading to the new polycations bearing a positively charged backbone.A commonly used reaction in the synthesis of ionic liquids involves the anion exchange of an ionic liquid with a salt,which is usually accompanied by phase separation because of the hydrophobicity of the newly

formed ionic liquid.14As outlined in Scheme 1(b),we carried out this original approach directly into our poly-mers.For example,poly(ViBuIm ?Cl ?)(X ?,A Cl ?)was dissolved in water and mixed with an excess of an aqueous solution of CF 3SO 3Li (Y ?Z ?).Immediately after mixing,a new polymer precipitated quantita-tively as a result of the anion-exchange reaction.A similar procedure was repeated with other salts bear-ing large hydrophobic anions such as LiClO 4,NaBF 4,NaPF 6,(CF 3SO 2)2NLi,and (CF 3CF 2SO 2)2NLi.In all these cases,precipitation occurred indicating the an-ion-exchange reaction and the insolubility of the formed polycation in water.Negative results were ob-tained in the case of KCN probably because of the similarities in size and polarity between the cyanide and the chloride anions.The obtained polymers were characterized by IR and 1H NMR.For example,the

IR

Figure 2.From bottom to top:infrared spectra of poly(ViBuIm ?X ?)where X ?is (a)Cl ?,(b)

(CF 3SO 2)2N ?,(c)CF 3SO 3?,and (d)PF 6?

.

Scheme

1

Figure 1.250-MHz 1H NMR spectra of 1-vinyl-3-ethyl-imidazolium bromide and poly(1-vinyl-3-ethyl-imidazolium)bromide.

RAPID COMMUNICATION 209

spectra of the initial poly(ViBuIm ?Cl ?)and the ob-tained poly(ViBuIm ?X ?)[where X ?is (CF 3SO 2)2N ?,

CF 3SO 3?,and PF 6?

]are depicted in Figure 2.In all the IR spectra,the bands were attributed to the polyviny-butyllimidazolium cation between 2800and 3300and 1550cm ?1.However,a complete disappearance of the band corresponding to the imidazolium chloride (3390cm ?1)was observed with the appearance of new bands attributed to the hexa?uorophosphate anion (835.5cm ?1),tri?ate anion (1251,1162,and 1034cm ?1),and amidotri?ate anion (1354,1194,1136,and 1053cm ?1).Similar indications of the quantitative anion exchange were observed by 1H NMR.For example,Figure 3shows the 1H NMR spectra of poly(ViEtIm ?Br ?),poly-(ViEtIm ?CF 3SO 3?

),and poly[ViEtIm ?(CF 3SO 2)2N ?].The shape and chemical shifts of the signals were at-tributed to the imidazolium ring protons at 7.5–7.7

ppm (3H)change completely from the starting material

to the newly formed poly(ViEtIm ?CF 3SO 3?

)signals that appeared as two new peaks at 7.5–8.0(2H)and 8.9–9.1ppm (1H).A similar result was observed in the 1

H NMR spectrum of poly[ViEtIm ?(CF 3SO 2)2N ?],which exhibited three different peaks at 7.0–7.5(1H),7.7–8.0(1H),and 8.5–9.0(1H)ppm for the imidazolium protons.IR and 1H NMR corroborated our synthetic approach supporting the quantitative anion-exchange reaction into our polymers.

In the case of ionic liquids,anion exchange leads to hydrophobic liquids immiscible with water and misci-ble with organic solvents such as acetone,tetrahydro-furan (THF),and dichloromethane.15Interestingly,the anion-exchange reaction also affected the solubility of the polymers described here.Table 1summarizes the solubility of the polycations in different solvents.For instance,hydrophilic poly(ViBuIm ?Cl ?)soluble only in water and methanol (MeOH)was transformed into hy-drophobic poly[ViBuIm ?(CF 3SO 2)2N ?]insoluble in MeOH and water but soluble in acetone,THF,and ethyl acetate.According to Table 1,the anion structure in?uenced the solubility of the polymer,and large hy-drophobic counterions such as (CF 3SO 2)2N ?and (CF 3CF 2SO 2)2N ?were useful to make our polymers soluble in solvents like THF and ethyl acetate.It is worth to remark the effectiveness of this reaction and the potential application of organically soluble polyca-tions in many different ?elds.

Furthermore,these polymers are expected to be sol-uble in equivalent ionic liquids because of their similar chemical structure.For example,Table 1shows the solubility of poly(ViEtIm ?X ?)in the ionic liquid ViEtIm ?(CF 3CF 2SO 2)2N ?.Polymers bearing Br ?,BF 4?,PF 6?were insoluble in the ionic liquid,whereas anions such as (CF 3CF 2SO 2)2N ?,(CF 3SO 2)2N ?,and

CF 3SO 3?

led to soluble polymers.As another example,

poly(ViBuIm ?BF 4?

)was soluble in the

commercially

Figure 3.From top to bottom:250-MHz 1H NMR spectra of poly(ViEtIm ?X ?)where X ?is (a)Br ?,(b)

CF 3SO 3?

,and (c)(CF 3SO 2)2N ?.

Table 1.Solubility of Poly(ViEtIm ?X ?)and Poly(ViBuIm ?X ?)Polycations in Different Solvents

H 2O

MeOH Acetone THF EtAc ViEtIm ?

Tf 2N ?

Poly(ViEtIm ?X ?)

Br ???????BF 4???????PF 6?

??????CF 3SO 3?

??????(CF 3SO 2)2N ?

??????(CF 3CF 2SO 2)2N ???????

Poly(ViBuIm ?X ?)

CI ??????BF 4??????PF 6?

?????CF 3SO 3?

?????(CF 3SO 2)2N ?

?????(CF 3CF 2SO 2)2N ??????CIO 4?

?

?

?

?

?

210J.POLYM.SCI.PART A:POLYM.CHEM.:VOL.42(2004)

available ionic liquid1-butyl-3-methyl-imidazolium tetra?uoroborate.Therefore,these new polymers could be the matrix of choice for obtaining new gels composed of an ionic liquid and a polymer that are relatively scarce and potentially useful in a number of applica-tions.

We have demonstrated an easy method of tuning the solubility of the poly(1-vinyl-3-alkyl-imidazolium)ha-lides by a simple anion-exchange reaction.The effect of the different counteranions and the molecular weight in the polymer properties such as glass-transition tem-perature,thermal degradation,and ionic conductivity is under investigation.

EXPERIMENTAL

Materials

Bromoethane(98%),1-vinylimidazole(99?%),1-chlo-robutane(99%),azobis(2-methylpropionitrile)(99%), sodium tetra?uoroborate(98%),sodium hexa?uoro-phosphate(98%),and lithium perchlorate trihydrate (98%)were obtained from Aldrich and used as received. Ethyl acetate(99.5%)was obtained from Merck.Lith-ium tri?ate(95%),bistri?uoromethanesulfonimide lithium salt(99%),and bispenta?uoroethanesulfonim-ide lithium salt(99%)were a gift from3M.All other solvents were obtained from Panreac Quimica S.A. Synthesis of1-Vinyl-3-ethylimidazolium Bromide (ViEtIm?Br?)

The product,1-vinyl-3-ethylimidazolium bromide,was prepared following the procedures described in the lit-erature.Under vigorous stirring,40g(0.367mol)of bromoethane were added dropwise to20g(0.2125mol) of1-vinylimidazole in a250-mL,one-necked,round-bottom?ask.The mixture was re?uxed for16h.The resulting white-yellow solid was allowed to cool to room temperature,and then it was washed several times with ethyl acetate.The product was?ltered and dried in a vacuum oven until constant weight(32.75g of ViEtIm?Br?,yield:76%).

1H NMR[acetone-d

6with a drop of methanol-d

3

,

?/ppm relative to tetramethylsilane(TMS)]:10.20(s, 1H),8.25(s,1H),8.02(s,1H),7.53(dd,1H),6.17(dd, 1H),5.47(dd,1H),4.53(q,2H),1.64(t,3H).13C NMR (250MHz,acetone-d

6

)?:206.68,128.77,122.98, 119.48,108.65,45.23,14.44ppm.

Synthesis of1-Vinyl-3-butylimidazolium Chloride (ViBuIm?Cl?)

A very similar procedure was used as for ViEtIm?Br?. Thus,1-chlorobutane(37g,0.4mol)was added drop-wise to20g(0.2125mol)of1-vinylimidazole.The solu-

tion was heated under stirring for50h to70°C.Phase separation occurred and the product was obtained as a viscous liquid that was washed several times with ethyl acetate.ViBuIm?Cl?was?ltered and dried in a vac-uum oven until constant weight.ViBuIm?Cl?was ob-tained as a white solid(11.40g,yield:28.74%).

1H NMR(acetone-d

6

with a drop of methanol-d

3

,?/ppm relative to TMS):10.46(s,1H),8.295(s,1H), 8.289(s,1H),8.01(dd,1H),6.19(dd,1H),5.41(dd,1H), 4.48(t,2H),1.99(m,2H),1.40(m,2H),0.96(t,3H).13C

NMR(250MHz,acetone-d

6

)?:206.59,128.81,123.28, 119.46,108.54,49.64,31.63,19.08,12.67ppm.

General Procedure for Polymerization

Three grams of previously synthesized monomer ViEtIm?Br?were diluted in30mL of chloroform in a 250-mL,Schlenk round-bottom?ask.Then,0.06g of azobis(2-methylpropionitrile)(radical initiator)(2%) was poured in the?ask and added to the mixture.The

solution was heated under N

2

atmosphere for3h to70°C.Afterward,white-yellow solid precipitates appeared that were washed several times with chloroform.After drying in a vacuum oven,2.974g of poly(ViEtIm?Br?) were obtained(yield:99%).

General Procedure for Anion-Exchange Method

To prepare poly[1-vinyl-3-ethylimidazolium bis(tri?u-oromethylsulfonyl)amide],a solution of6g of lithium bis(tri?uoromethylsulfonyl)amide in12mL of distilled water and a solution of1g of poly(ViEtIm?Br?)in20 mL of distilled water was mixed in a50-mL,round-bottom?ask.After stirring for30min at room temper-ature,the resulting white-yellow solid was?ltered and dried in a vacuum oven until constant weight(1.63g, yield:85%).This simple procedure was used with other salts and obtained satisfactory results in each case.

Characterization

1H NMR and13C NMR spectra were recorded with a Bruker AM250(250-MHz)spectrometer.Infrared spectroscopy was performed in Avatar360FT-IR Thermo Nicolet equipment.

R.Marcilla acknowledges the Basque Government and Cegasa for a fellowship.The authors thank V.Alvaro for the technical assistance.The Diputacion de Gipuz-koa and The Basque Government are thanked for their ?nancial support.

RAPID COMMUNICATION211

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