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Inorganica Chimica Acta 361 (2008) 1385–1394

Formation of inorganic protonic-acid polymer via inorganic–organichybridization: Synthesis and characterization

of polymerizable olefinic organosilyl derivatives of mono-lacunaryDawson polyoxometalate

Takeshi Hasegawa a,b, Hideyuki Murakami a, Kaori Shimizu a, Yuhki Kasahara a,Shoko Yoshida a, Takayuki Kurashina a, Hideaki Seki a, Kenji Nomiya a,*

a Department of Materials Science, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japanb R&D Center, KIMOTO Co., Ltd, Suzuya Chuou-ku, Saitama 338-0013, Japan

Received 14 April 2007; received in revised form 28 August 2007; accepted 4 September 2007Available online 12 September 2007

Abstract

A novel polymerizable organosilyl-modified Dawson-type polyoxometalate (POM) [a2-P2W17O61{CH2@C(CH3)COO(CH2)3Si}2O]6�

(1) was synthesized as both Me2NH2þ salt (Me2NH2-1) and H+ form (H-1). They were characterized with complete elemental analysis,

thermogravimetric and differential thermal analysis (TG/DTA), FTIR, (1H, 13C, 29Si, 31P and 183W) NMR and n-butylamine titrationmethod. H-1 was immobilized to a polymer network through free radical copolymerization with methyl methacrylate (MMA). The acid-ities of H-1 and hybrid copolymer (H-1-co-MMA) were evaluated using the Hammett indicators (dicinnamalacetone and benzalacetoph-enone; pKa values of the protonated indicators are �3.0 and �5.6, respectively). The pKa value of H-1 was estimated as that between�3.0 and �5.6 in CH3CN solution and H-1 was immobilized in H-1-co-MMA with the original acidity being retained. Glass transitionpoint (Tg) and molecular weight distribution of H-1-co-MMA were affected by the used amount of H-1 because of the cross-linking effectof H-1.� 2007 Elsevier B.V. All rights reserved.

Keywords: Dawson-type polyoxometalate; Organosilyl; Free-acid form; Acidity; Radical polymerization; Immobilization

1. Introduction

Polyoxometalates (POMs) are molecular metal–oxideclusters, which are of current interest as soluble metal oxi-des and for their application to catalysis, medicine andmaterials science [1–12]. POMs have been practicallyapplied to several industrial processes [13–16]. In particu-lar, free-acid forms of POMs such as H3[PW12O40] Æ nH2Oand H4[SiW12O40] Æ nH2O showing stronger Brønsted acid-ity than sulfuric acid have been practically used as acid-cat-

0020-1693/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2007.09.002

* Corresponding author. Tel.: +81 463 59 4111; fax: +81 463 58 9684.E-mail address: nomiya@kanagawa-u.ac.jp (K. Nomiya).

alysts for olefin hydration, trioxane production fromformaldehyde, diphenylmethane production from benzenewith formaldehyde and ring-opening polymerization of tet-rahydrofuran [15,16]. In general, heterogenization orimmobilization of such catalysts has been required becauseof ease of recycling and ease of separation of the catalystsfrom the reaction products. Many POM-based heteroge-neous catalysts, some of which have shown higher activitiesand more effective selectivities than the POM-basedhomogenous catalyst systems, have been so far accom-plished by grafting POM, through relatively weak interac-tions such as electrostatic interaction, coordinationinteraction and physical mixing on the inert supports suchas SiO2, Al2O3 and organic polymers [13,17–26]. Immobi-

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lized POM-based protonic acids have been expected aswidely useful acid-catalysts and as proton conductivitymembranes for polymer-electrolyte fuel-cell (PEFC). How-ever, immobilization of the free-acid form without decreaseof the acidity has not been reported.

Many organically modified POMs, e.g. organosilyl deriv-atives [27–44], organostannyl derivatives [32,47–52],organophosphonyl derivatives [53–58], and organogermylderivatives [51] have been prepared by reaction of lacunaryPOMs with organometallics. Some of the modified POMshaving reactive functional groups such as thiol, vinyl andmethacryl can be used as precursors for immobilization ofPOMs [28,31,34,35,44]. For example, several polymerizableorganosilyl-modified Keggin POMs have been reported, e.g.[SiW11O39(CH@CH2Si)2O]4� [31], [c-SiW10O36(H2C@CHSi)2O]4� [35], [c-SiW10O36{H2C@C(CH3)COO(CH2)3-Si}2O]4� [28,35] and [c-SiW10O36{H2C@C(CH3)COO-(CH2)3Si}4O]4� [30,35]. These POMs have beenimmobilized to polymer materials through free-radical poly-merization [28,30]. Copolymerization of these modifiedPOMs with acrylic monomers has also been investigated[28,30]. Most studies of organosilyl derivatives reported sofar are concerned with the Keggin POMs. On the otherhand, there have been reported several examples of theDawson POMs, such as [a2-P2W17O61(RSi)2O]6� (R =C6H5—, 4-((C2H5O)2P(@O)CH2)-C6H4—) [33], [a2-P2W17O61-(RSi)2O]6� (R = –(CH2)3SH, –(CH2)3SCN, –Ph) [43], [a2-P2W17O61{SH(CH2)3Si}2O]6� [44] and [a2-P2W17O61{(CH3)3-N(CH2)3Si}2O]4� [45].

In this work, we prepared novel polymerizable organo-silyl-modified Dawson POMs, i.e. [a2-P2W17O61{CH2=C(CH3)COO(CH2)3Si}2O]6�: 1 as Me2NH2

þ salt (Me2-NH2-1), by the reaction of the mono-lacunary DawsonPOM K10[a2-P2W17O61] with c-methacryloxypropyltri-methoxysilane (CH2@C(CH3)COO(CH2)3Si(OMe)3:MAS).1 Also, we were successful in changing Me2NH2-1to the free-acid form (H-1) by passing it through a cat-ion-exchange resin column. Immobilization of the free-acidform (H-1) to an organic polymer was achieved by copoly-merization with methyl methacrylate (MMA). Molecularstructure [46] and polyhedral representation of polyoxoan-ion 1 are shown in Fig. 1.

Herein, we report full details of the unequivocal charac-terization of H-1 and Me2NH2-1 with complete elementalanalysis, thermogravimetric and differential thermal analy-sis (TG/DTA), FTIR, and solution (1H, 13C, 29Si, 31P and183W) NMR spectroscopy. The immobilization of H-1through free radical polymerization to H-1-co-MMA isreported. The cross-linking effect of H-1 is confirmed bythe gel permeation chromatography (GPC) and dynamicmechanical measurements. Also, reported is the evaluationof Brønsted acidity of H-1 and H-1-co-MMA using theHammett indicators. This is the first example of the cova-

1 After submission of this article, we determined molecular structure ofMe2NH2-1 and reported the results elsewhere [46].

lently-immobilized free-acid form POM in the organicpolymer network.

2. Experimental

2.1. General

The following were used as received: (CH3)2NH2Cl,CH3OH, EtOH, Et2O, CH3CN, tetrahydrofuran, 1 Maqueous HCl, Amberlite IR120B NA, c-methacryloxypro-pyltrimethoxysilane, n-butylamine, methyl methacrylate,a,a 0-azobisisobutyronitrile, Hammett indicators [1,9-diphe-nyl-1,3,6,8-nonatetraen-5-one (dicinnamalacetone), 1,3-diphenyl-2-propen-1-one (benzalacetophenone)], H3[PW12

O40] Æ 12H2O (all from Wako); D2O, DMSO-d6, CD3CN(Isotec). The precursor K10[a2-P2W17O61] Æ 19H2O [59]was prepared according to the literature and identified.H6[a-P2W18O62] Æ 21H2O was derived from K6[a-P2W18

O62] Æ 14H2O [59] by passing through the cation-exchangeresin column (Amberlite IR120B NA).

Complete elemental analyses were carried out by Mikro-analytisches Labor Pasher (Remagen, Germany). The sam-ples were dried at room temperature under 10�3–10�4 Torrovernight before analysis. Infrared spectra were recordedon a Jasco 4100 FTIR spectrometer in KBr disks and werealso recorded on Perkin–Elmer Spectrum 100 equippedwith DuraScope� in the ATR method at room tempera-ture. Thermogravimetric (TG) and differential thermalanalyses (DTA) were acquired using a Rigaku Thermo Plus2 series TG/DTA TG 8120 instrument. TG/DTA measure-ments were run under air with a temperature ramp of 4 �Cper min between 20 and 500 �C.

1H (399.65 MHz), 13C{1H} (100.40 MHz) and 31P(161.70 MHz) NMR spectra in (D2O, DMSO-d6 andCD3CN) solutions were recorded in 5-mm outer diametertubes on a JEOL JNM-EX 400 FT-NMR spectrometerand a JEOL EX-400 NMR data-processing system. 31PNMR spectra in solution were referenced to an externalstandard of 25% H3PO4 in H2O in a sealed capillary. The31P NMR signals were shifted to �0.101 ppm by using85% H3PO4 as a reference instead of 25% H3PO4. 1H and13C{1H} NMR spectra were measured in D2O and organicsolutions with reference to internal DSS or TMS. 29SiNMR (79.30 MHz) spectra in (D2O and DMSO-d6) solu-tions were recorded in 5-mm outer diameter quartz tubeson a JEOL JNM-EX 400 FT-NMR spectrometer. Thesespectra in D2O were referenced to an external standardof DSS and those in a DMSO-d6 solution were referencedto an external standard of TMS, both measured by the sub-stitution method. 183W NMR (16.50 MHz) spectra wererecorded in 10-mm outer diameter tubes on a JEOLJNM-EX 400 FT-NMR spectrometer equipped with aJEOL NM-40T10L low-frequency tunable probe and aJEOL EX-400 NMR data-processing system. 183W NMRspectra in DMSO-d6 were referenced to an external stan-dard of saturated Na2WO4–D2O solution. The 183WNMR signals were shifted to �0.787 ppm by using a 2 M

Fig. 1. (a) Polyhedral representation of organosilyl-modified Dawson polyoxotungstate anion [a2-P2W17O61{CH2C(CH3)COO(CH2)3Si}2O]6� 1. TheWO6 octahedra are depicted with the white octahedra and the central PO4 groups are depicted with the internal yellow tetrahedra. The organosilyl groupsare shown with silicon (blue), oxygen (red) and carbon atoms (black). (b) Molecular structure of Me2NH2-1.

T. Hasegawa et al. / Inorganica Chimica Acta 361 (2008) 1385–1394 1387

Na2WO4 solution as a reference instead of saturatedNa2WO4 solution.

Gel permeation chromatography (GPC) was performedby using GPC-101 (SHOKO, Japan) equipped with threecolumns of two KF-806M and one KF-802, and THF asa solvent with a flow rate of 1.0 mL. Monodispersedpolystylenes were used as standards.

Dynamic mechanical measurements were obtained usinga Rheogel E4000 (UBM, Japan). The data were obtained atfrequencies of 10.0 Hz, using a heating rate of 3 �C min�1.

The Brønsted acidity of H-1, H3[PW12O40] Æ 12H2O,H6[P2W18O62] Æ 21H2O and 95% H2SO4 in CH3CN was

evaluated using the Hammett indicators (dicinnamalace-tone and benzalacetophenone; pKa values of the proton-ated indicators are �3.0 and �5.6, respectively). Theconcentrations of the indicator and H+ were adjusted to4.0 · 10�5 and 2.6 · 10�2 mol/L, respectively.

2.1.1. Synthesis of (Me2NH2)6[a2-P2W17O61{CH2C-

(CH3)COO(CH2)3Si}2O] Æ 6H2O (Me2NH2-1)

c-Methacryloxypropyltrimethoxysilane (CH2@C(CH3)-COO(CH2)3Si(OCH3)3) (475 lL, 2.0 mmol) was addedto 200 mL of a MeOH/H2O mixture (150/50 v/v). Afterstirring for 5 min, to the suspension was added 5.0 g

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(1.0 mmol) of solid K10[a2-P2W17O61] Æ 19H2O followedby stirring for 5 min. The dispersed solution was adjustedto pH 1.8 with 1 M aqueous HCl solution. The solutiongradually changed to pale yellow. After stirring for30 min, the white powder in the suspension was removedwith a membrane filter (JG 0.2 lm). The pale yellowfiltrate was concentrated to ca. 70 mL with a rotaryevaporator at 30 �C. To it was added 5.5 g (37 mmol)of Me2NH2Cl. The white powder formed was collectedon a glass filter (G3.5), washed with ethanol(50 mL · 3) and Et2O (50 mL · 2) and dried in vacuofor 2 h.

The white powder obtained in 81.6% (3.98 g scale) yield[based on the 6H2O hydrated] was soluble in hot water, andDMSO, slightly soluble in water, sparingly soluble in ace-tone, methanol, ethanol, and acetonitrile, and insolublein ethylacetate, dichloromethane, and chloroform. Anal.

Calc. for {(CH3)2NH2}6[P2W17O61{CH2C(CH3)COO-(CH2)3Si}2O] Æ H2O or H72C26N6O67Si2P2W17 (FW =4784.43): H, 1.52; C, 6.53; N, 1.76; O, 22.41; Si, 1.17; P,1.29; W, 65.33. Found: H, 1.52; C, 6.47; N, 1.80; O, 21.6;Si, 1.25; P, 1.24; W, 65.8%; total 99.68%. A weight lossof 1.43% was observed during the course of drying at roomtemperature at 10�3–10�4 Torr overnight before the ele-mental analysis, suggesting the presence of 3–4 water mol-ecules weakly solvated or adsorbed.

TG/DTA under atmospheric conditions: a weight lossof 2.22% was observed at below 186.8 �C based on dehy-dration; calc. 2.22% for x = 6 in {(CH3)2NH2}6[P2W17-O61{CH2C(CH3)COO(CH2)3Si}2O] Æ xH2O and a weightloss of 6.72% was observed between 186.8 and 501.5 �Cwith an exothermic point at 385.0 �C based on a decom-position of Me2NH2

þ cation and organosilyl group.FTIR (KBr): 1702m, 1629m(br), 1466m, 1412w, 1327w,1304w, 1186m, 1088s, 1039m, 954s, 922w, 793 vs,566w, 527m cm�1. 31P NMR (22.6 �C, D2O): d �10.3,�13.3. 31P NMR (23.5 �C, DMSO-d6): d �10.3, �13.2.29Si NMR (25.4 �C, DMSO-d6): d �54.1. 183W NMR(22.2 �C, DMSO-d6): d �116.3 (2W), �118.8(1W),�162.4(2W), �163.4(2W), �166.7 (2W), �173.8(2W),�179.6(2W), �199.9(2W), �320.0 (2W). 1H NMR(21.3 �C, D2O): d 0.93–0.94 (br, 2H, –CH2–Si), 1.93 (s,3H, –CH3), 2.01–2.05 (br, 2H, –CH2–), 2.74 (–CH3 ofMe2NH2

þ), 4.28–4.29 (br, 2H, –O–CH2–), 5.69 and6.15 (s, 1H, CH2@). 1H NMR (23.7 �C, DMSO-d6): d0.53–0.56 (br, 2H, –CH2–Si), 1.80–1.83 (m, 2H, –CH2–), 1.89 (s, 3H, –CH3), 2.59 (–CH3 of Me2NH2

þ), 4.11–4.14 (t, 2H, –O–CH2–), 5.64 and 6.05 (s, 1H, CH2@).13C NMR (26.3 �C, D2O): d 19.3 (–CH2–), 23.2(–CH3), 36.5 (–CH3 of Me2NH2

þ), 69.0 (–O–CH2–),128.4 and 138.0 (C@C) [The 13C NMR signals due tocarbonyl and CH2–Si– group were not found becauseof its slow relaxation rate and short relaxation time,respectively]. 13C NMR (24.8 �C, DMSO-d6): d 8.95(CH2–Si), 17.9 (–CH2–), 22.1 (–CH3), 34.5 (–CH3 ofMe2NH2

þ), 66.3 (–O–CH2–), 125.3 and 136.0 (C@C),166.5 (C@O).

2.1.2. Synthesis of H6[a2-P2W17O61{CH2C(CH3)COO-

(CH2)3Si}2O] Æ 14H2O (H-1)

The precursor (Me2NH2-1) (0.4 g (0.082 mmol)) was dis-solved in 60 mL of water on a water bath at 90 �C. Afterbeing cooled to room temperature, the colorless solutionwas passed through a cation-exchange resin column inH+ form [Amberlite IR 120B NA, 100 mL] at a rate ofone drop per second. Further, 100 mL of water was passedthrough the column (It was confirmed that the pH of theeluent became the same as that of water). The aqueoussolution was evaporated to dryness by a rotary evaporatorat 40 �C. The color of the solution changed to yellow-orange. The yellow-orange powder obtained was dried invacuo for 2 h. Hygroscopic yellow-orange powder obtainedin 80.0% (0.301 g scale) yield was highly soluble in water,soluble in DMSO, methanol, ethanol, acetonitrile, and ace-tone, and insoluble in ethylacetate, dichloromethane, andchloroform. This compound was very sensitive to metal-ware, i.e. it showed a color change to blue by contact withthe metalware. Anal. Calc. for H6[P2W17O61{CH2C(CH3)-COO(CH2)3Si}2O] Æ 6H2O or H40C14O72Si2P2W17 (FW =4604.00): H, 0.88; C, 3.65; N, 0.00; O, 25.02; Si, 1.22,; P,1.35; W, 67.89. Found: H, 0.80; C, 3.73; N, <0.1; O,23.6; Si, 1.30; P, 1.27; W, 67.9% total 98.6%. A weight lossof 3.25% was observed during the course of drying at roomtemperature at 10�3–10�4 Torr overnight before the ele-mental analysis, suggesting the presence of 8–9 water mol-ecules weakly solvated or adsorbed. TG/DTA underatmospheric conditions: a weight loss of 5.52% wasobserved at below 226.1 �C with endothermic points at77.8 and 175.7 �C based on dehydration; calc. 5.31% forx = 14 in H6[P2W17O61{CH2C(CH3)COO(CH2)3Si}2O] ÆxH2O and a weight loss of 3.00% was observed between226.1 and 501.5 �C based on a decomposition of theorganosilyl group. IR (KBr): 1686m, 1629w(br), 1419w,1368w, 1306w, 1238w, 1189w(br), 1090s, 1039m, 957s,915m, 805vs(br), 597w, 566w, 531m cm�1. 31P NMR(23.1 �C, D2O): d �10.3, �13.3. 29Si NMR (22.6 �C,D2O): d �51.6. 1H NMR (21.4 �C, D2O): d 0.89–0.92 (t,2H, –CH2–Si), 1.91 (s, 3H, –CH3), 1.97–2.04 (m, 2H,–CH2–), 4.25–4.26 (t, 2H, –O–CH2–), 5.67 and 6.13 (s,1H, CH2@). 13C NMR (26.1 �C, D2O): d 7.41 (–CH2–Si),17.0 (–CH2–), 21.0 (–CH3), 66.7 (–O–CH2–), 126.2, 135.8(C@C), 169.7 (C@O).

2.1.3. Copolymerization of H-1 with MMAA typical procedure for synthesis of the copolymer H-1-

co-MMA is as follows.The free-acid form of H-1 (2 g, 0.43 mmol) and methyl

methacrylate (MMA: 18 g, 180 mmol) was dissolved in30 mL of CH3CN in a three-necked flask equipped with amagnetic stirring bar, condenser tube and thermometer.The system was purged with nitrogen gas and the yellow-orange solution was heated in an oil bath at 75 �C. To itwas added a,a 0-azobisisobutyronitrile (AIBN: 0.04 g,0.28 mmol). The reaction was exothermic and the temper-ature of the system reached max. 75.8 �C. The system

T. Hasegawa et al. / Inorganica Chimica Acta 361 (2008) 1385–1394 1389

was kept at 75 �C for 4 h. The viscosity of the solutionincreased. The viscous solution, after being cooled to roomtemperature, was evaporated at 30 �C. To the resultinghighly viscous polymer solution was added 500 mL ofmethanol. Yellow-white precipitate was formed. After stir-ring overnight, the white precipitate was collected on amembrane filter (JG 0.45 lm) and dried in vacuo for 2days. White powder was obtained in 34.5% (6.90 g scale)yield. FTIR (ATR): 1724s, 1480w, 1447m, 1435m, 1387w,1268m, 1240m, 1191m, 1445s, 1094w, 1065w, 983m,962m, 912w, 839m, 825m, 810m, 750m cm�1. 31P NMR(23.1 �C, CD3CN): d �9.14, �12.3 ppm.

3. Results and discussion

3.1. Synthesis and compositional characterization of

Me2NH2-1 and H-1

The novel polymerizable organosilyl-modified DawsonPOM was prepared as Me2NH2

þ salt (Me2NH2-1) by thereaction of the organosilyl precursor, CH2@C(CH3)-COO(CH2)3Si(OCH3)3, with the mono-lacunary DawsonPOM, K10[a2-P2W17O61] Æ 19H2O, in an HCl–acidicwater/methanol mixed solution at room temperature.Me2NH2-1 was isolated as analytically pure, white powderin 81.6% (3.98 g scale) yield. In the complete elementalanalysis performed on compound Me2NH2-1 that wasdried at room temperature under 10�3–10�4 Torr overnightbefore analysis, all elements including oxygen, total 99.68%for Me2NH2-1 were observed, the data of which were con-sistent with the composition of {(CH3)2NH2}6[a2-P2W17O61{CH2C(CH3)COO(CH2)3Si}2O] Æ H2O. Theweight loss observed during the course of drying beforeanalysis was 1.43%, which corresponded to 3–4 water mol-ecules weakly solvated or adsorbed for Me2NH2-1. On theother hand, TG/DTA measurement performed underatmospheric conditions showed a weight loss of 2.22%under 186.8 �C due to dehydration of 6–7 water molecules.The number of water molecules (6–7) by TG/DTA wasalmost consistent with the number of the water molecules(1) found in the elemental analysis plus the water molecules(3–4) estimated from the weight loss before analysis. Thus,the number of water molecules in the formula was deter-mined as the six hydrated form. The solid FTIR spectrumof Me2NH2-1, measured in a KBr disk, showed character-istic bands of the Dawson a2-isomer (1088, 954, 922,793 cm�1) (Fig. 2a) [60]. Additional bands were observedat 1702 and 1629 cm�1, which were assigned to the C@Oand C@C bands in the methacryl group, respectively[23,25]. The 31P NMR spectrum of Me2NH2-1 in DMSO-d6 showed a two-line spectrum at �10.3 and �13.2 ppm,confirming its purity and single product nature (Fig. 3a).The downfield resonance was assigned to the phosphorousclosest to the organosilyl site and the upfield resonance wasassigned to the phosphorous closer to the W3 cap. The 29SiNMR spectrum of Me2NH2-1 in DMSO-d6 showed a clearsingle peak at �54.1 ppm. This spectrum indicates that two

silicon atoms incorporated in the POM framework areequivalent, thus, showing Me2NH2-1 with Cs symmetry insolution. The 183W NMR spectrum of Me2NH2-1 inDMSO-d6 showed a nine-line spectrum of �116.3,�118.8, �162.4, �163.4, �166.7, �173.8, �179.6,�199.9, and �320.0 ppm with the integrated intensity ratioof 2:1:2:2:2:2:2:2:2. This NMR spectrum is also consistentwith the Cs symmetry compound (Fig. 4). The 1H and13C NMR spectra of Me2NH2-1 in DMSO-d6 are summa-rized in Table 1. All peaks of the methacryloxypropylgroup and the Me2NH2

þ counterion are found.The free-acid form H-1 was obtained as analytically

pure, hygroscopic yellow powder in 80.0% (0.301 g scale)yield. In the complete elemental analysis performed oncompound H-1 that was dried at room temperature under10�3–10�4 Torr overnight before analysis, total 98.6% forH-1 was observed, the data of which were consistent withthe composition of H6[a2-P2W17O61{CH2C(CH3)-COO(CH2)3Si}2O] Æ 6H2O. The observed data of O analy-sis contain a larger range of error. However, since thedifference in the found and calc. values of O analysis isclose to the difference (ca. 1.4%) in the sum of the observedvalues from total 100%, the difference is attributable to theexperiments of O analysis. The weight loss observed duringthe course of drying before analysis was 3.25%, which cor-responded to 8–9 water molecules weakly solvated oradsorbed for H-1. On the other hand, TG/DTA measure-ment performed under atmospheric conditions showed aweight loss of 5.52% at under 226.1 �C due to dehydrationof 14–15 water molecules. Thus, the number of water mol-ecules in the formula was determined as 14 hydrated form.The solid FTIR spectrum of H-1, measured in a KBr disk,showed the characteristic bands of the Dawson a2-isomer(1090, 957, 915, and 805 cm�1) (Fig. 2b) [60]. Additionalbands were observed at 1686 and 1629 cm�1 which wereassigned to the C@O and C@C bands in the methacrylgroup, respectively. The band based on Me2NH2

þ

(1466 cm�1) of the precursor disappeared. The 31P NMRspectrum of H-1 in D2O showed a two-line spectrum at�10.3 and �13.3 ppm (Fig. 3b). The 29Si NMR spectrumof H-1 in D2O showed a single peak at �51.6 ppm, indicat-ing the two silicon atoms incorporated to a POM frame-work equivalent and, therefore, H-1 with Cs symmetry insolution. The 1H and 13C NMR spectra of H-1 measuredin D2O are also summarized in Table 1. All peaks basedon the methacryloxypropyl group are measured, but thepeak of Me2NH2

þ in the precursor disappeared.

3.2. Synthesis and characterization of the hybrid copolymer

(H-1-co-MMA)

The organic–inorganic hybrid copolymers (B–E; H-1-co-MMA) containing the immobilized H-1 and the homo-polymer without H-1 (A; PMMA) were synthesizedthrough free radical polymerization in CH3CN. The usedamount of H-1 and yields of the polymers are summarizedin Table 2. The yield of the polymers (A–E) was A; 10.2 g,

Fig. 2. FTIR spectra in the polyoxoanion of (a) Me2NH2-1 and (b) H-1.

Fig. 3. 31P NMR spectra of (a) Me2NH2-1 in DMSO-d6, (b) H-1 in D2Oand (c) H-1-co-MMA in CD3CN.

Fig. 4. 183W NMR spectrum o

1390 T. Hasegawa et al. / Inorganica Chimica Acta 361 (2008) 1385–1394

B; 4.9 g, C; 6.9 g, D; 6.0 g and E; 6.6 g, respectively. In themeasurement of GPC, as the amount of H-1 increased, dis-tribution of molecular weight (Mw/Mn) increased becauseH-1 acted as a cross-linking reagent (Table 3).

In the hybrid copolymer E, the measurement of GPCwas unsuccessful because the homogeneous solution wasnot obtained owing to high cross-linking density. TheFTIR of these copolymers showed the absence of the poly-merizable double bond, i.e. the bands due to C@C ofMMA and H-1 at 1638 and 1629 cm�1 disappeared. Asthe used amount of H-1 increased, the notable band(1090 cm�1) based on P@O in H-1 increased (Fig. 5).

The 1H NMR of H-1-co-MMA in CD3CN showed thatthe vinylic protons of the organically functionalized POM(H-1) as well as those of the methyl methacrylate (MMA)were consumed. FTIR and 1H NMR showed that the unre-acted species of H-1 and MMA were not present. The 31P

f Me2NH2-1 in DMSO-d6.

Table 1Solution 1H and 13C NMR spectral data for compounds Me2NH2-1 and H-1

1H 13C

Me2NH2-1 (in DMSO-d6) 0.53–0.56 (a) 8.95 (a)1.80–1.83 (b) 17.9 (b)1.89 (g) 22.1 (g)2.59 (Me2NH2

þ) 34.5 (Me2NH2þ)

4.11–4.14 (c) 66.3 (c)5.64, 6.05 (e) 125.3, 136.0 (e, f)

166.5 (d)

H-1 (in D2O) 0.89–0.92 (a) 7.41 (a)1.91 (g) 17.0 (b)1.97–2.04 (b) 21.0 (g)4.25–4.26 (c) 66.7 (c)5.67, 6.13 (e) 126.2, 135.8 (e, f)

169.7 (d)

Table 2The used amounts of H-1, MMA and AIBN and yields (g) of the hybridcopolymers H-1-co-MMA (B–E) by radical polymerization

(g) Aa B C D E

H-1 0 1 2 3 4MMA 20 19 18 17 16AIBN 0.04 0.04 0.04 0.04 0.04

Yields 10.2 4.9 6.9 6.0 6.6

a The homopolymer without H-1.

Table 3GPC measurements of the hybrid copolymers (A–E)

A B C D E

Mna 50648 37570 21068 13779 —

Mwa 89397 66450 38845 27857 —

Mw/Mna 1.76 1.77 1.84 2.02 —

a Mn and Mw are based on the polystyrene standard.

T. Hasegawa et al. / Inorganica Chimica Acta 361 (2008) 1385–1394 1391

NMR of H-1-co-MMA in CD3CN showed the two broaderpeaks, assigned to the two phosphorus atoms in H-1(Fig. 3c).

Fig. 5. FTIR spectra of the hybrid copolymers H-1-co-MMA (A–E). Theincorporated POMs increased.

The amount of H-1 contained in H-1-co-MMA wasevaluated using an absorbance at 308 nm due to H-1 inthe homogeneous CH3CN solution (Table 4). The initialmolar ratio [H-1]0/[MMA]0 was compared with the evalu-ated molar ratio in the hybrid copolymer [H-1]/[MMA].As the initial content of H-1 increased, the amount ofH-1 in H-1-co-MMA increased. These results are consistentwith FTIR data. UV–Vis spectrum of the hybrid copoly-mer (E) was not measured, because E was insoluble inCH3CN.

The dynamic mechanical measurements results areshown in Fig. 6. As the amount of H-1 increased, the glasstransition point (Tg) of the polymers (A; 142, B; 148, C; 160and D; 166 �C) moved to higher temperature range and thetand peak based on Tg became broad. The storage elasticmodulus (E 0) also moved to higher temperature range.These results are attributable to the cross-linking effect ofH-1 [62–64].

The immobilization of H-1 was successful by free radicalpolymerization with MMA and the amount of H-1 in H-1-co-MMA was changed by the initial amount of H-1 before

band at around 1090 cm�1 based on the P@O group increased as the

Table 4The initial molar ratios ([H-1]0/[MMA]0) and the evaluated molar ratios([H-1]/[MMA]) of H-1 in the hybrid copolymers H-1-co-MMA (B–E)

B C D E

[H-1]/[MMA] 5.6 · 10�4 3.0 · 10�3 4.7 · 10�3 —[H-1]0/[MMA]0 1.1 · 10�3 2.3 · 10�3 3.7 · 10�3 —

Fig. 7. Titration with n-butylamine. Typical procedure is as follows;10 lmol of the heteropolyacid was dissolved in 50 mL of CH3CN, andthen the solution was stirred for 3 h. To it was added 5 lmol of n-butylamine and the mV value was measured. The abscissa exhibits thenumber of protons consumed with n-butylamine, which was convertedfrom the titration amount of n-butylamine. (a) H6[P2W17O61{CH2C(CH3)-COO(CH2)3Si}2O] Æ 14H2O H-1, (b) H3[PW12O40] Æ 12H2O and (c)H6[P2W18O62] Æ 21H2O.

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reactions. The cross-linking effect of H-1 was also con-firmed by the change in solubility and the thermal proper-ties of H-1-co-MMA.

3.3. Acidity of H-1 and H-1-co-MMA

The acid property of the novel free-acid POM H-1 inCH3CN was evaluated by n-butylamine titration [61], andthose of H3[PW12O40] Æ 12H2O (PW12) and H6[P2W18O62] Æ21H2O (P2W18) as a comparison were also evaluated(Fig. 7). The mV value in H-1 became zero when sixprotons were consumed, indicating that H-1 had six pro-tons. These data are consistent with the full elementalanalysis. The n-butylamine titration also showed thatPW12 and P2W18 had three and six acidic protons,respectively.

Acidity measurements using the two Hammett indica-tors with the different pKa values of �3.0 and �5.6 inCH3CN solution, where pKa values are based on proton-ated indicators, are summarized in Table 5. The 95%H2SO4 did not show the acidic color for both indicators.On the other hand, PW12, P2W18 and H-1 showed theacidic color (red) due to protonation of the indicator ordicinnamalacetone (pKa = �3.0) but they did not showthe acidic color (yellow) of the other indicator or benza-lacetophenone (pKa = �5.6). Thus, these POMs act asstronger acid than 95% H2SO4, and their pKa values inCH3CN solution are evaluated as those between �3.0and �5.6.

Fig. 6. Comparison of the dynamic mechanical measurements results at 10 Hshown in the left axis. The tand was shown in the right axis. The glass transitio

The acidity of the hybrid copolymer (H-1-co-MMA)was also evaluated in CH3CN solution (homogeneousconditions) and in CH3OH (heterogeneous conditions)using the same Hammett indicators. Under homogeneousconditions (in CH3CN), the hybrid copolymer showedthe acidic color (red) of the indicator (pKa = �3.0), butdid not show the acidic color (yellow) of the indicator(pKa = �5.6). Under heterogeneous conditions (inCH3OH), H-1-co-MMA showed the acidic color of theindicator (pKa = �3.0) on the surface of the copolymer,and the supernatant solution did not show acidic color(Fig. 8). The experiments under the heterogeneous condi-tions revealed that H-1-co-MMA showed the acidic colordue to the immobilized H-1 in the polymer and, more-over, the effluent H-1 from the polymer was not present

z for H-1-co-MMA samples (A–D). The storage elastic modulus (E 0) wasn point (Tg) was observed for the peak of tand around 140–170 �C (A–D).

Table 5Acidity measurement of H-1 and H-1-co-MMA using the two Hammett indicators

Dicinnamalacetone pKa; �3.0 Benzalacetophenone pKa; �5.6

95% H2SO4 � �H3[PW12O40] Æ 12H2Oa + �H6[P2W18O62] Æ 21H2Oa + �H-1a + �Hybrid Copolymera + �Hybrid Copolymerb +c �+: acidic color (red) and �: basic color (yellow) of dicinnamalacetone.+: acidic color (yellow) and �: basic color (colorless) of benzalacetophenone.

a In acetonitrile solution (homogenous conditions). (See Section 2).b In methanol solution (heterogeneous conditions).c Only the surface of the polymer showed the acidic color, but the supernatant solution did not (see Fig. 8).

Fig. 8. The hybrid copolymer (C) in CH3OH (heterogeneous condition) inthe presence of the Hammett indicator (dicinnamalacetone), showing theacidic color only on the surface of the copolymer. The supernatantsolutions did not show the acidic color.

T. Hasegawa et al. / Inorganica Chimica Acta 361 (2008) 1385–1394 1393

because the supernatant solution did not show the acidiccolor.

4. Conclusion

A novel organosilyl-modified Dawson-type POM wassynthesized as Me2NH2

þ salt and H+ form, which werefully characterized. H-1 was immobilized in the hybridcopolymer (H-1-co-MMA) through free radical polymeri-zation. The pKa values of H-1 and H-1-co-MMA were eval-uated as those between �3.0 and �5.6. The cross-linkingeffect of H-1 was confirmed by the change in Tg and solu-bility. Thus, H-1 was successfully immobilized in thecopolymer through free radical polymerization with theoriginal acidity of H-1 being retained. H-1-co-MMA canbe used as a useful solid acid catalyst.

Acknowledgements

This work was supported by Grant-in-Aid for ScientificResearch (C) No. 18550062 and a High-Tech ResearchCenter Project, both from the Ministry of Education, Cul-ture, Sports, Science and Technology, Japan.

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