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Bottom-Up Hybridization: A Strategy for the Preparation of a Thermostable Polyoxometalate-Polymer Hybrid with Hierarchical Hybrid Structures [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) A bottom-up strategy for the preparation of hierarchical hybrid materials with good thermostability is reported. The hybrid molecule was constructed from a Wells-Dawson-type polyoxometalate (POM) cluster and a poly(ethylene glycol) (PEG) chain through covalent-bond formation. The large distinction between the POM cluster and PEG chain results in a microphase separation to form POM and PEG layers that further alternatively arrange into hybrid lamellae with a sub-20 nm thickness. Then the hybrid lamellae could simultaneously organize into spherulitic superstructures. Because of this hierarchical structuring, strong electrostatic interactions between POM clusters are maximized within the POM layers. This gives rise to thermostability. Structurally, the hybrid lamellae and the superstructures are unchanged, even at 160 °C, and indeed the shear storage modulus of the hybrid material remains nearly constant within this same temperature range. This study demonstrates the concept of bottom-up hybridization, in which rationally selecting building blocks, designing the hybrid molecule, and then manipulating hierarchical structures can generate thermostable hybrid materials. Keywords : organic-inorganic hybrid composites · polymers · polyoxometalates · synthesis design · thermochemistry Introduction It is currently known that new materials are crucial to advance science and technology. Organic-inorganic hybrid materials, in particular, play a key role, and thus, have attracted considera- ble attention.[1-4] In recent decades, many investigations have focused on natural materials, such as bone and nacre, which are typical biological-inorganic hybrid materials. These findings provide inspiration for designing and fabricating novel man- made hybrid materials,[5, 6] because the properties of these nat- ural materials far exceed what could be expected from a simple mixture of the two building blocks, owing to their highly sophisticated structures with complex hierarchy. There- fore, it is widely recognized that successful manmade hybrid materials are not a simple mixture of the respective character- istics of organic and inorganic building blocks. Rather, they ex- hibit exceptional properties or functions attributed to the syn- ergy of organic and inorganic building blocks that are highly dependent on the nature of the organic/inorganic building blocks, the interactions between them, and their structural hi- erarchy.[1-4] Thus, both the careful design and synthesis of hy- brids at the molecular level through a rational selection of or- ganic and inorganic building blocks with desired functions and an accurate manipulation and/or control of hierarchical hybrid structures at the supramolecular level are pivotal steps to create novel hybrid materials. Herein, we report a bottom-up strategy for the preparation of hierarchical hybrid materials with good thermostability. We selected inorganic polyoxometalates (POMs) and organic poly- mers as building blocks to construct POM-polymer hybrid ma- terials. The POM clusters were selected as inorganic building blocks because of their polyanionic and nanometer-scale char- acteristics.[7, 8] Organic polymers were selected for their ease of processability and attractive mechanical properties.[3] The former means that strong electrostatic interactions offer ther- mostability to the hybrid materials, and the latter offers ease of formation of ordered structures in hybrid materials. Because great distinctions between POM clusters and polymers in phys- ical and chemical properties may result in a macrophase sepa- ration, we use a covalent bond to link the POM cluster and the polymer.[9-28] In the specific design of a POM-polymer hybrid molecule, a Wells-Dawson-type POM cluster was used as the inorganic building block because it could be covalently modi- fied,[29] and the organic block was commercially available poly(- ethylene glycol) (PEG). The hybrid molecule is thus denoted as POM-PEG hybrid hereafter. Results and Discussion The molecular structure of the POM-PEG hybrid is shown in Scheme 1 A. The POM cluster is a trivanadium-substituted de- rivative of a Wells-Dawson-type polyoxotungstate with two phosphorus heteroatoms in the center.[30] Its chemical structure is (Bu4N + )6H3(P2W15V3O62)9^ with a calculated molecular weight of 5422 Da. This POM cluster is encapsulated by six cationic tetrabutylammonium (Bu4N + ) ions to facilitate its solubility in organic solvents such as N,N-dimethylformamide (DMF). The covalent modification on the three vanadium atoms with Tris was achieved in DMF.[29] The 5000 Da PEG (denoted as 5K- PEG) has a methyl ether at one terminus and a hydroxyl at the other. The synthetic steps for preparation of the POM-PEG hybrid are outlined in Scheme 1 B. The hydroxyl termi- nus was reacted with succinic anhydride to give the carboxyl- terminated PEG (1; PEG-COOH). Then, amidation of 1 and Tris was performed to produce the trihydroxyl-terminated PEG (2 ; PEG-Tris) by using EEDQ as an activator in acetonitrile.[31, 32] Fi- nally, the POM-PEG hybrid was prepared by coupling POM with 2 through esterification in dry DMF at 80 8C for 7 days. Detailed synthesis and characterization procedures are given in the Ex- perimental Section. In the 1H NMR spectra for 2 and the POM-PEG hybrid in Fig- ure 1 A, the disappearance of the hydroxyl proton resonance signal at d = 4.65 ppm and the appearance of the signal of the methylene proton resonance ad- jacent at d = 5.52 ppm suggest esterification between the three hydroxyl groups in 2 and three vanadium atoms in the POM.[29] The 31P NMR spectra of the POM cluster and the POM- PEG hybrid in Figure 1 B show two 31P NMR spectroscopy sig- nals shifting from d =^7.05 to ^7.36 ppm after esterification of the POM cluster with 2. This result and the FTIR spectrum given in Figure S1 in the Supporting Information indicate the structural integrity of the POM cluster in the hybrid.[29] GPC characterization results in Figure 1 C show molecular weights and polydispersity indices (PDI) of M^ n = 5500 Da and PDI=1.08 for 5K-PEG, M^n=2400 Da for the cluster, and M^n= 8400 Da and PDI = 1.07 for the POM-PEG hybrid. Notably, this cluster molecular weight is smaller than its absolute molecular weight (5422 Da) because the GPC molecular weight measure- ments are relative to PEG standard samples. The molecular weight of the hybrid is the sum of the molecular weights of the 5K-PEG and POM precursors. This indicates that the POM cluster has been coupled with 2. The TGA data in Figure 1 D show the thermal degradation of the three samples. The ther- mal degradation of the POM-PEG hybrid starts at about 180 8C (Td^ 180 8C). Thus, all subsequent characterizations were per- formed below 180 8C. The observed residue in the hybrid was 38.7 wt %, in correspondence with a calculated value of 39.1 wt %. These results convincingly verified the synthesis of the purified POM-PEG hybrid with a thermal degradation simi- lar to that of the building blocks. It is problematic to prepare the hybrids with PEGs with molecular weights higher than 5000 Da because of measurement accuracy and purification of the crude product mixtures. The as-prepared POM-PEG hybrid (AP-hybrid) is a tough solid over a wide temperature range, even higher than the melting point of 5K-PEG. In contrast to the parent POM cluster, the hybrid is solution processable. A film sample was prepared from its solution for investigation of the mechanical properties of the hybrid as a function of temperature. We performed dy- namic rheology to characterize its dynamic shear storage mod- ulus (G') as a function of temperature.[33] To better understand the hybrid, we also measured the shear storage moduli of the 5K-PEG sample and a 100K-PEG sample (a 100k Da PEG) under the same conditions. Plots of shear storage modulus, G', versus temperature, T,in Figure 2 show clear differences in the temperature depend- ence of the shear storage moduli for the three samples. Clearly, the values of G' decrease near the melting point of PEG (ca. 608C). Herein, we selected G'(50) and G'(70), the storage moduli at T = 50 and 708C, to compare the changes of the shear storage moduli owing to melting of the PEG. Table 1 provides a summary of the values of G' at T= 50 and 70 8C. At T = 508C, G'(50)=2.61^106 Pa for the 5K-PEG sample, G'(50)= 1.53^106Paforthe100K-PEG sampleandG'(50)=1.04^106Pa for the POM-PEG sample. These values mean that 5K-PEG is a hard plastic, and 100K-PEG and POM-PEG hybrid are tough plastics or solids. If the temperature is higher than the melting point of PEG, the difference becomes much greater. At T= 708C, for in- stance, G'(70) = 6.28 ^ 10^2 Pa for the 5K-PEG sample, G'(70) = 2.52^104 Pa for the 100K-PEG sample, and G'(70)=5.90^ 105 Pa for the POM-PEG sample. These values mean that 5K- PEG becomes a liquid, 100K-PEG is a typical rubber, and the POM-PEG hybrid is still a tough solid. For the viewpoint of polymer physics, it is easy to under- stand these differences between the two PEG homopolymers. It is known that chain entanglements, which increase with mo- lecular weights, can hinder polymer crystallization, but increase the contribution to rubber elasticity.[33, 34] The degrees of crys- tallinity, calculated from enthalpy experimentally measured by differential scanning calorimetry (DSC), are 0.94 for 5K-PEG, 0.82 for 100K-PEG, and 0.55 of the PEG phase in as-prepared POM-PEG hybrid (Figure S2 and Table S1). The values of G'(50)=2.61^106 Pa and G'(70)=6.28^10^2Pa for 5K-PEG present a tremendous change from a crystalline plastic with 0.94 crystallinity degree in T< 608C to a liquid without chain entanglements in T>608C, and the values of G'(50)=1.53^106 and G'(70) = 2.52 ^ 104 Pa for 100K-PEG corresponds a transition at its melting point from a tough plastic with a 0.82 crystallini- ty degree to a high-elasticity rubber with a large number of chain entanglements. For the POM-PEG hybrid, G'(50) =1.04 ^ 106 Pa is smallest value and G'(70) = 5.90 ^ 105 Pa is largest of all three. The huge difference in the G'(70) value between the POM-PEG hybrid and 5K-PEG implies that the hybrid is more thermally stable. This property cannot be explained by the mo- lecular weight of the two building blocks. Thus, further investi- gations into the hierarchical structures of the hybrid at both the molecular and supramolecular levels and on their thermo- stability were performed. The hierarchical structures at the micrometer level and their thermostability were investigated by using polarizing light mi- croscopy (PLM). Figure 3 shows six PLM micrographs of a thin- film sample prepared by using the solution-casting method and taken under crossed polarizers at different temperatures. We noted large spherulitic superstructures with the Maltese cross pattern and an average diameter of 30 mm (Figure 3 A). Because the PEG blocks crystallized, the birefringence of these superstructures stemmed at least partially from crystalline PEG. The formation of the superstructures means that there are some impurities remaining in the center of the individual spherulites, which act as nucleation seeds to induce crystalliza- tion of the whole hybrid molecules from the concentrated so- lution during solvent evaporation.[35] As this sample was heated at 5 8Cmin^1 rate, no perceptible change in the optical pattern of the spherulitic superstructures was found in T < Tm (Tm is melting point of the PEG blocks). In T > Tm, for instance, 80 8C or even 160 8C, their birefringent nature remained (Fig- ure 3 B and C). Compared with Figure 3 A, however, we can see a decrease in brightness owing to melting of the PEG crystal. It is rare that the birefringent pattern of the spherulitic super- structures persists in samples of crystalline homopolymers and their blends, composites, and block copolymers when the tem- perature is beyond the melting point of crystalline homopoly- mers. When the sample was cooled to 80 8C, no change in the op- tical features of these spherulitic superstructures was observed (Figure 3D). When the temperature dropped to 40 8C, however, a very bright area gradually grew from the upper right corner (Figure 3E) and finally covered the whole area (Figure 3 F) with optical features identical to those observed in Figure 3 A. The uneven boundary between the bright and dark areas in Fig- ure 3 E is the boundary between the spherulitic superstruc- tures, which indicates that the PEG blocks recrystallized, but not from the individual centers of the previous spherulitic su- perstructures. This suggests that those nucleation seeds died, and thus, could not induce recrystallization of the melted PEG blocks. These observations show that recrystallization of the melted PEG blocks in the hybrid is totally different from crys- tallization of conventional homopolymers and their blends, composites, and block copolymers. The hierarchical structures at the nanometer level and their thermostability were further investigated through TEM, wide- angle X-ray diffraction (WAXD), and in situ small-angle X-ray scattering (SAXS). The TEM micrograph in Figure 4 A displays a typical lamellar structure within a part of the spherulite found in an ultrathin film sample of an AP-hybrid. An identical lamellar structure was also found in Tr-hybrid, treated at 160 8C for 1 h and then subsequently cooled to room temperature (Figure S3). At first glance, this lamellar morphology is identical to those found in spherulites of crystalline polymers.[33, 35] Energy-dispersive X-ray (EDX) analysis of these lamellae shows signals of phosphorus, tungsten, and vanadium elements (Fig- ure S4). This confirms the existence of POM clusters. Direct imaging of the lamellae under bright-field conditions without staining is due to the large electron density differences be- tween the POM cluster and the PEG chain. The POM cluster is rich in the dark layer and the PEG chain is rich in the bright layer because the POM cluster scatters more electrons than the PEG chain. This observation confirms that the hybrid lamel- lae are composed of alternatively arranged POM and PEG layers that are distributed throughout the superstructure and even the bulk sample. Figure 4 B shows our WAXD measurements of 5K-PEG, AP- hybrid, Tr-hybrid, and POM samples. The reflection peaks are at 2q =19.1 and 23.38 for the PEG crystal[36] and at 2q = 6.4 and 7.38 for the POM crystal.[37] The reflection peaks of the AP- hybrid and Tr-hybrid are at 2q = 6.1 (or 6.6), 19.1, and 23.38. The one-to-one correspondence of the reflection peaks be- tween the hybrid samples and the building blocks also indi- cates that the PEG block and the POM cluster, respectively, formed their own structures. The weak and broad reflection peaks found in the hybrid samples mean that the degree of crystallinity of the PEG phase is low and POM packing in the POM layer is poor (see Figure S2 and Table S1). Heat treatment caused a reflection peak position that shifted from 2q = 6.1 to 6.68 and a narrowing of peak width. Thus, the d spac- ing decreases from 1.45 to 1.34 nm and the POM packing improves. Figure 4 C shows the SAXS profiles of an AP-hybrid sample heated from 20 to 160 8C and then subse- quently cooled to 20 8C. These SAXS profiles display two wide scattering peaks at q1 = 0.30-0.45 nm^1 and q2 = 0.60-0.90 nm^1, respectively, depending on the temperature. The ratio of q1/q2= 1/2 indicates a lamel- lar microdomain with the d spacing changing in a range from d ^ 14 to 20 nm. As the temperature in- creases, the scattering peaks shift to the large q regime (indicated by arrows in Figure 4 C) and the peak height increases and the peak width narrows. These evidently reflect an ordering process of the lamellar microdomain. When the sample was cooled to 20 8C, the scattering peak position was identical to that at 160 8C; this indicates preservation of the or- dered microdomain. Quantitative investigation of the ordering process was conducted by plot- ting the d spacing and FWHM of the first-order scattering peak versus the temperature (Fig- ure 4 D). The solid curves are sig- moid fits. The d spacing decreas- es from d = 19.8 nm at T =20 8C to d = 13.9 nm at T= 160 8C. Thus, Dd^ 6 nm is a sizeable change. This change starts at T ^ 60 8 C, just above the melting point of the PEG crystals, and ends at T ^ 1408C. The FWHM decreases from 0. 0.23 nm^1 at T = 20 8C to 0.08 nm^1 at T = 160 8C; this indicates an im- provement in the distribution of the lamellar alignment. To the best of our knowledge, the thermostability of this hybrid is rarely found in homopolymers and their blends, composites, and block copolymers. Charac- terization and analyses clearly show that the hierarchical hybrid structures play a pivotal role in creating the high thermostability of this hybrid. Scheme 2 shows the bottom-up hybridization of the hierarchical hybrid structures from a single molecule to lamellae and finally to spherulitic su- perstructures. From the viewpoint of the bottom-up approach, a covalent bond between the PEG chain and the POM cluster in the hybrid molecule is the first crucial step in preventing macrophase separation owing to the large Flory-Huggins in- teraction parameters between the two building blocks. Indeed, microphase separation takes place at the molecular scale, and hence, results in the formation of hybrid lamellae consisting of alternatively arranged POM and PEG layers. The hybrid lamellae further organized into superstructures. These processes should simultaneously occur when the sample was prepared by slow evaporation of the solvent from a dilute solution of the hybrid. We note that fullerene (C60) or polyhedral oligomeric silses- quioxane (POSS) and PEG or poly(e-caprolactone) (PCL) chains have been used to synthesize hybrid molecules with identical molecular architectures, including C60-PEG,[38-40] POSS- PEG,[39-41] and POSS-PCL.[42] However, the spherulitic super- structures or ordered lamellae found in these hybrids disap- peared when the temperature was above the melting point of the PEG or PCL crystals. In contrast to C60 and POSS, the POM clusters are polyanionic, and thus, have electrostatic interac- tions between them. These interactions are stronger than those between C60 or POSS clusters. In the hybrid, the POM clusters within the POM layers retain strong interactions, and thus, the POM layers are thermostable at temperatures well above the melting point of the PEG crystal. As shown in Scheme 2, the POM and PEG layers may form an interpenetrating network within superstructures. This net- work can uniformly distribute in the whole sample, and thus, will control the properties of the POM-PEG hybrid. In the temper- ature regime below the PEG melting point, the PEG phase in the hybrid is semicrystalline and its degree of crystallinity is 0.55, so it is soft in comparison with the two PEG homopolymers with degrees of crystallinity of 0.94 or 0.82. In this way, the shear storage modulus of the POM-PEG hybrid should be lower than those of the two PEG homopolymers. In the tempera- ture regime above the PEG melt- ing point, the interpenetrating network of POM layers results in thermostable hybrid lamellae and superstructures, even the PEG block melted, so the shear storage modulus of the POM-PEG hybrid is higher than those of the two PEG homopolymers and remains almost unchanged within the temperature range from 60 to 160 8C, that is, the hybrid has a thermostability much better than that of the two PEG homopolymers. Meanwhile, the birefringent pattern of the spherulitic superstructures can remain after melting of the PEG block. It is worth noting that the appearance of the birefrin- gent pattern is due to structural birefringence associated with structural anisotropy of the hybrid lamellae. It is very important to have a further discussion on the hier- archical hybrid structures and their thermostability. As shown before, the hybrid molecule is constructed from the POM clus- ter and PEG chains, which have large distinctions in size, shape, and property. Notably, the formation of hybrid lamellae composed of a POM layer and a PEG layer means a balance be- tween the volumes occupied by the POM clusters and PEG blocks. In other words, the volume occupied by the POM clus- ters should be identical to that occupied by the PEG blocks, particularly at high temperatures, for instance, T > 140 8C. The POM cluster used has a fixed size and shape : an ellipsoidal shape with a long axis of 1.2 nm and a short axis of 1.0 nm.[37] Because the d spacing between adjacent POM clusters is dPOM = 1.34 nm, as determined by our WAXD measurements (Figure 4 B), the estimated cross-sectional area of the POM clus- ter encapsulated by six Bu4N + is about 1.41 nm2. In the molten state, the PEG chains must have the same cross-sectional area in the hybrid lamellae. Thus, they are required to adopt an ap- propriate conformation to fit the 1.41 nm2 tubes, as depicted in Scheme 3 A. Because within the tube the estimated length of each PEG chain with a molecular weight of 5500 Da is 5.8 nm at 140 8C based on the density of amorphous PEG of 1.01 g cm^3 at the same temperature,[43] the estimated total length of a POM-PEG hybrid molecule is about 7 nm. There- fore, we suggest a hybrid lamella model featuring a phase-sep- arated structure of a cluster-to-cluster bilayer and a chain-to- chain bilayer. Thus, the estimated thicknesses of the hybrid la- mellae are about 14 nm, as depicted in Scheme 3 B; this is in good agreement with the 13.9 nm thickness of the hybrid la- mellae measured at T > 140 8C. In this ideal model, the space left for the PEG blocks is about 11.4 nm. Taking into account that the crystallinity of the Tr-hybrid sample is about 0.5, the estimated thickness of the PEG crystal is about 5.7 nm. Most likely, the PEG chains, linked on the two sides of the POM layers, interdigitate to form the lamellar crystals. Based on SAXS characterization, the hybrid lamellae become more ordered with increasing temperature. As we stated before, the crystallization of the PEG blocks and phase separa- tion of the PEG and POM blocks might simultaneously occur during the concentration process of the hybrid solution. It is understandable that the hybrid lamellar structure in the AP- hybrid sample is imperfect, mainly because of an imperfect ar- rangement of POM clusters, as shown in Scheme 3C a. This point was confirmed by our WAXD and SAXS characterization. The AP-hybrid sample shows a wide diffraction peak at 2q = 6.18 (Figure 4 B) and a wide and weaker SAXS scattering peak at q = 0.30 nm^1 (Figure 4 C). The Tr-hybrid sample shows a narrow diffraction peak at 2 q = 6.6 8 . Both narrowing of the diffraction peak width and shifting of the peak position from 2q = 6.1 to 6.68 mean that treating the AP-hybrid sample at a temperature higher than the melting point of the crystallized PEG blocks results in an ordering process of the POM cluster arrangement. Concurrently, the hybrid lamellar structure became more ordered. The representations in Scheme 3 C show the ordering process from less-ordered hybrid lamellae with crystallized PEG layers (Scheme 3 C a) or melted PEG layers (Scheme 3 Cb) to well-arranged hybrid lamellae with melted PEG layers (Scheme 3 C c) with increasing temperature because thermal energy may drive the POM clusters to rearrange in an orderly manner. This ordering process continued to about 140 8C and resulted in thinning of the layers. The well-ordered hybrid lamellae were retained when the PEG layers recrystal- lized (see Scheme 3 C d). Conclusion We selected a POM cluster and a PEG chain as the building blocks for a hybrid molecule. A microphase separation of the two blocks resulted in hybrid lamellae that consisted of alter- natively arranged POM and PEG layers. Then the hybrid lamel- lae further organized into spherulitic superstructures. Thus, these findings demonstrated a bottom-up strategy for the con- struction and manipulation of hierarchical hybrid structures. Importantly, strong electrostatic interactions of the POM cluster were retained within the POM layer, and thus, yielded thermo- stability to many mechanical properties (shear storage modu- lus) of the hybrid materials through the hierarchical hybrid structures. The results indicate that a strategy whereby an or- ganic-inorganic hybrid with exceptional properties or func- tions can be created through the rational selection of inorgan- ic and organic building blocks, the precise construction of hybrid molecules, and careful manipulation of the hierarchical structures. This concept can also be used to design and pre- pare new hybrid materials with other properties or functions for advanced applications. Experimental Section Reagents The (Bu4N + )6H3(P2W15V3O62)9^ cluster was synthesized according to the literature.[30] PEG with a number-average molecular weight of 5000 Da, succinic anhydride (99%), Tris (99.8 %), and EEDQ (99%) were purchased from Aldrich. DMAP (99 %) was purchased from Alfa Aesar. Other reagents were purchased from major chemical supplies and used as received unless otherwise noted. All solvents were dried and distilled prior to use. Characterization The 1H and 31P NMR spectroscopy experiments were performed on a Varian Unity Plus-400 NMR spectrometer. GPC was performed with Waters 515 HPLC pumps, a Waters 2414 refractive index de- tector, and Waters Styragel columns. DMF with LiBr (0.01 mol L^1) was used as the eluent at a flow rate of 1.0 mL min^1 at 50 8C. PEG standards were used for calibration. TGA data were collected on a Netzsch TGA 209 instrument under nitrogen carrier gas with a rate of 10 8 C min ^1 in the range of 25-800 8 C. FTIR spectra were collected by using a Bio-Rad FTS 6000 spectrometer. DSC measure- ments were performed on a DSC thermal analysis system (Dia- mond from PerkinElmer). The samples were heated from room temperature to 80 8C at a heating rate of 5 8C min^1 under a nitro- gen atmosphere. The spherulitic superstructures of the POM-PEG hybrid were observed by using an Olympus BX51 polarizing light microscope, equipped with a Linkam THMSE 600 hot stage. WAXD studies involved a Rigaku D/Max-2500 X-ray diffractometer equipped with a CuKa radiation (l = 0.154 nm) source operating at 40 kV/100 mA along a range of 0.68 < 2q < 408. In situ SAXS meas- urements were performed by using a Bruker Nanostar SAXS instru- ment equipped with a CuKa radiation source operating at 40 kV/ 650 mA. The SAXS measurements were taken during heating at 2 8C min^1. Data was collected for 3600 s per frame. TEM and EDX spectroscopy were performed by using a field-emission transmis- sion electron microscope (FEI Tecnai G2 F20) operating at an accel- eration voltage of 200 kV. The dynamic shear storage modulus (G') of the samples was obtained by using an Anton Paar MCR302 rhe- ometer. The parallel-plate geometry with plate diameters of 25 mm was used with sample thicknesses of 1.0 mm. For temperature-de- pendent modulus measurements, the samples were heated from 308C to above the melting point of PEG at a rate of 2 8C min^1, while measuring the modulus at 0.1 % strain and an angular fre- quency of 10 rad s^1. Synthesis of carboxyl-terminated PEG (1) PEG (5 g, 1.0 mmol), succinic anhydride (0.12 g, 1.2 mmol), and DMAP (0.13 g, 1.1 mmol) were dissolved in anhydrous toluene (50 mL), and the reaction was stirred at 80 8C for 12 h. The solvent was evaporated completely with a rotary evaporator. The residue was dissolved in dichloromethane and the polymer was precipitat- ed in diethyl ether. The purified product was dried under vacuum until a constant weight was observed. The yield was 94.3 %. 1H NMR data (see Figure S5; 400 MHz, CDCl3,): d=4.26 (t, 2 H; -COOCH2-), 3.46-3.82 (m, 514 H; -OCH2CH2O-), 3.38 (s, 3 H; CH3O-), 2.65 ppm (m, 4 H; -OCOCH2CH2OCO-). Syntheses of three-hydroxymethyl-terminated PEG (2) Compound 1 (0.4 g, 0.08 mmol) and EEDQ (29 mg, 0.12 mmol) were dissolved in CH3CN (30 mL), and the reaction was kept at 80 8C for 30 min. Then, Tris (12 mg, 0.10 mmol) was added to the solution and the mixture was heated at reflux for 24 h. The solu- tion was cooled to room temperature and evaporated completely by using a rotary evaporator. The residue was dissolved in di- chloromethane and washed with distilled water three times. The organic layer was concentrated to minimum volume. The concen- trated liquid was then added dropwise to diethyl ether to yield a white solid. The purified product was dried under vacuum until constant weight was reached at a yield of 92.7 %. 1H NMR (see Fig- ure S6; 400 MHz, [D6]DMSO): d=7.20 (s, 1 H; -NH-), 4.65 (t, 3H; -OH), 4.11 (t, 2 H; -COOCH2-), 3.42-3.70 (m, 520 H; -C(CH2OH)3-, -OCH2CH2O-), 3.24 ppm (s, 3 H; CH3O-). Synthesis of the POM-PEG hybrid The (Bu4N +)6H3(P2W15V3O62)9^ (0.23 g, 0.04 mmol) and 2 (0.21 g, 0.04 mmol) were dissolved in DMF (40 mL). The reaction was kept at 80 8C for 7 days under argon and then cooled and concentrated to a minimum volume. The solution was then precipitated in dieth- yl ether. The yellow solid was redissolved in the minimum volume of CH3CN and then reprecipitated in diethyl ether. The purified product was dried under vacuum until constant weight was reached. The yield of the product was 88.4 %. 1H NMR (see Fig- ure S7; 400 MHz, [D6]DMSO): d= 7.60 (s, 1H; -NH-), 5.52 (s, 6H; -C(CH2O)3-), 4.11 (t, 2H; -COOCH2-), 3.42-3.70 (m, 514 H; -OCH2CH2O-), 3.24 (s, 3H; CH3O-), 3.17 (t, 48 H; NCH2CH2-), 1.58 (m, 48H; NCH2CH2-), 1.32 (m, 48H; NCH2CH2CH2-), 0.94 ppm (t, 72H; NCH2CH2CH2CH3). Acknowledgements We appreciate financial support from the National Natural Sci- ence Foundation of China (Grant NSFC 21274069, 21334003, and 51203171), the Program for Changjiang Scholars and Innovative Research Team in Nankai University (PCSIRT-IRT1257), and the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry. We also thank Ellen Gao and Eric Zhang for their assistance with the rheometrical experiments carried out at the Anton Paar Experi- mental Station (Beijing). [1] Special issue on nanostructured and functional hybrid organic - inor- ganic materials, Chem. Mater. 2001, 13, 3059 - 3809. [2] G. Kickelbick, Prog. Polym. Sci. 2003, 28, 83 -114. [3] G. Kickelbick, Hybrid Materials : Synthesis Characterization, and Applica- tions, Wiley-VCH, Weinheim, 2007. [4] Special thematic issue on hybrid materials, Chem. Soc. Rev. 2011 , 40, 471-1150. [5] M. A. Meyers, P. Y. Chen, A. Y. M. Lin, Y. Seki, Prog. Mater. 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Mather, Macromolecules 2006, 39 , 9253 - 9260. [42] B. Alvarado-Tenorio, A. Romo-Uribe, P. T. Mather, Macromolecules 2011, 44, 5682 -5692. [43] L. Zhu, S. Z. D. Cheng, B. H. Calhoun, Q. Ge, R. P. Quirk, E. L. Thomas, B. S. Hsiao, F. Yeh, B. Lotz, Polymer 2001, 42, 5829 - 5839. Received : April 2, 2014 Published online on July 15, 2014 Jing Tang,[a] Wei Yu,[a] Min-Biao Hu,[a] Yu Xiao,[a] Xiao-Gang Wang,[a] Li-Jun Ren,[a] Ping Zheng,[a] Wen Zhu,[b] Yongming Chen,[b] and Wei Wang*[a] [a] J. Tang, Dr. W. Yu, Dr. M.-B. Hu, Y. Xiao, X.-G. Wang, L.-J. Ren, P. Zheng, Prof. Dr. W. Wang Centre for Synthetic Soft Materials, Key Laboratory of Functional Polymer Materials of Ministry of Education and Institute of Polymer Chemistry, Nankai University and Collaborative Innovation Centre of Chemical Science and Engineering (Tianjin), Tianjin 300071 (P. R. China) E-mail : [email protected] [b] Dr. W. Zhu, Prof. Dr. Y. Chen Laboratory of Polymer Physics and Chemistry Institute of Chemistry, The Chinese Academy of Sciences Beijing 100190 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402092. (c) 2014 Blackwell Publishing Ltd. |