Dynamic Network Formation of POSS-Pendanted Polymer via Cage

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Dynamic Network Formation of POSS-Pendanted Polymer via Cage Scrambling Mediated by Fluoride Ion Kousuke Tsuchiya, Hitoshi Arai, Yoshihito Ishida, and Atsushi Kameyama* Department of Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama-shi, Kanagawa 221-8686, Japan S Supporting Information *

ABSTRACT: Here we report the facile network formation of a methacrylate polymer containing polyhedral oligomeric silsesquioxane (POSS) in the side chain by a dynamic cage-scrambling reaction mediated by fluoride ion. With 1,4-bis(triethoxysilyl)benzene (BTSB) as a cross-linker in the presence of tetrabutylammonium fluoride (TBAF), a self-standing crosslinked film was obtained by a simple technique involving evaporation of the solvent from the mixture of the POSS polymer, TBAF, and BTSB under moderate conditions. The crosslinked POSS polymer shows high transparency and high thermal stability. It was revealed that treatment with excess triethoxyphenylsilane allowed the cross-linked POSS polymer to dissolve in tetrahydrofuran.

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INTRODUCTION Incorporation of inorganic materials into a polymer matrix is a powerful way to enhance the polymer physical properties, especially the thermal and mechanical stabilities. Transparent organic/inorganic hybrid materials with excellent properties are desired for various applications such as organic light-emitting diode (OLED) displays, hard coatings, touch screen displays, microlenses, and so on.1,2 As the inorganic part, metal oxide nanoparticles such as silica and titania3−5 and nanoclays6,7 are generally mixed with polymers to achieve the requirements for such applications. For example, a low coefficient of thermal expansion (CTE) was achieved by metal oxide nanoparticle/ polymer hybrids as transparent substrate materials in OLED displays in which a mismatch between the CTEs of the substrates and inorganic thin-film transistor (TFT) materials causes damage to the applications.4 On the other hand, the incorporation of titania nanoparticles into a polymer matrix provides a high refractive index for optical lenses.3 However, these materials usually suffer from aggregation of the inorganic additives without a dispersion technique, especially at high inorganic contents, resulting in the loss of the material properties described above. Polyhedral oligomeric silsesquioxanes (POSS) are nanosized clusters with a silica cage structure and show high chemical and thermal stabilities, mechanical strength, excellent dielectric properties, and so on. The vertices of a POSS cage can be substituted with various organic groups, which provide good compatibility with a polymeric matrix and fine-tuning of the physical properties. Therefore, numerous POSS derivatives have been designed and used for organic/inorganic hybrid materials, generally as inorganic nanofillers.8−11 Polymeric materials containing a POSS cage structure are also utilized in blend systems in which copolymers consisting of POSSpendanted units are mixed with organic polymers to give a phase-separated structure on the nanoscale.12,13 For hybrid © 2015 American Chemical Society

materials in which POSS is more dispersed at a molecular level, multifunctionalized POSS derivatives with various polymerizable groups are used for the formation of dense networks.14−19 These materials show unique properties derived from the POSS cage structure. As a unique POSS property, it has been reported that a fluoride ion can be encapsulated in the POSS cage by treating the POSS derivatives with tetraalkylammonium fluoride.20,21 The choice of the organic substituent is important because the interaction between the silicon atoms and fluoride ion is a driving force for the encapsulation. Electron-withdrawing groups, such as phenyl or vinyl groups, are suitable for the encapsulation, whereas no reaction occurs when POSS derivatives with electron-donating alkyl groups are used. During the formation of the fluoride-encapsulated POSS complex, scrambling of the POSS cage structure occurs via a reversible cleavage of the vertices of the POSS cage in a dynamic equilibrium.22 If two or more POSS cages with different organic substituents exist with the fluoride ion, the substituents are intermixed among the cages, resulting in mixtures of various POSS cages with multiple substituents. Laine and co-workers demonstrated that polysilsesquioxanes with random structures can be converted to octa- (T8), deca- (T10), or dodecasilsesquioxane (T12) cages via this structural scrambling using a tetraalkylammonium fluoride.23−25 These results suggest that the cage scrambling of POSS at the side chains of polymers enables dynamic chemical modification or network formation in hybrid polymeric materials. There is no report regarding cage scrambling using polymers containing discrete POSS cage structures. Received: January 20, 2015 Revised: March 3, 2015 Published: March 10, 2015 1636

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nylsilane (0.090 mL, 0.375 mmol), and THF (3.0 mL), and the mixture was stirred for 24 h. The resulting solution was poured into water, and the precipitate was collected by filtration. The polymer was dissolved in chloroform and reprecipitated in methanol. The precipitated polymer was filtered and dried under vacuum. The yield was 0.0284 g. Measurements. 1H and 29Si NMR spectra were obtained on a JEOL ECA600 instrument at 600 and 120 MHz, respectively. Deuterated chloroform or THF was used as a solvent with tetramethylsilane as an internal standard. Solid-state 29Si crosspolarization/magic angle spinning (CP/MAS) NMR spectra were obtained on a JEOL ECA400 instrument at 80 MHz. Samples were spun at a frequency of 5 kHz, and poly(dimethylsiloxane) was used as an external standard. Solid-state 19F MAS NMR spectra were obtained on a Bruker DSX-300 instrument at 282 MHz. Samples were spun at a frequency of 20 kHz. The infrared (IR) spectra were recorded on a JASCO FT/IR-4100 spectrometer. Number- and weight-average molecular weights (Mn and Mw, respectively) were determined by gel-permeation chromatography (GPC) with a Shodex GPC-101 column using refractive index and UV detectors, THF as the eluent at a flow rate of 0.5 mL min−1, and standard polystyrene samples for calibration. Thermogravimetric analysis (TGA) was performed on a Seiko EXSTAR 6000 TG/DTA 6200 thermal analyzer at a heating rate of 10 °C/min under a nitrogen atmosphere. Differential scanning calorimetry (DSC) was performed on a HITACHI X-DSC 7000 calorimeter at heating and cooling rates of 10 °C min−1 under a nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) measurements were performed using a RIGAKU RINT Ultima III X-ray diffractometer with Cu Kα radiation. UV−vis analysis was conducted on a JASCO V-670 spectrophotometer for the thin film of the polymer coated on a quartz plate. The film for UV measurement was prepared by spin-coating the solution of PMAPhPOSS (0.05 mmol of unit) with TBAF (0.05 mmol) and BTSB (0.025 mmol) in THF (11 wt %) at a rate of 1000 rpm for 60 s.

In this article, we report the reaction of POSS-containing polymers prepared by the polymerization of methacryloxypropyl-substituted POSS derivatives with fluoride ions and the fluoride-mediated cage-scrambling nature of the POSS moieties with a difunctional trialkoxysilane derivative as a cross-linker to induce the formation of a networked structure. Furthermore, it was found that this cross-linked POSS polymer is converted to a soluble polymer by the reversible cage-scrambling reaction.

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EXPERIMENTAL SECTION

Materials. Heptaisobutyl(3-methacryloxypropyl)-POSS (MethacrylIsobutyl POSS) and heptaphenyl(3-methacryloxypropyl)-POSS (MethacrylPhenyl POSS) were purchased from Hybrid Plastics Inc., and used as received. For tetrahydrofuran (THF), a stabilizer-free grade was used as received from Wako Pure Chemical Industries, Ltd. Tetrabutylammonium fluoride (TBAF) was used as a solution in THF (1 M) purchased from Tokyo Chemical Industry Co., Ltd. The other chemicals were used as received. Synthesis of PMABuPOSS. To a Schlenk tube equipped with a stirring bar were added MethacrylIsobutyl POSS (1.13 g, 1.2 mmol), 2,2′-azobis(isobutyronitrile) (0.0022 g, 0.013 mmol), and THF (1.8 mL). The mixture was subjected to freeze−pump−thaw cycles to eliminate the air and then stirred at 60 °C for 24 h under nitrogen. The reaction was quenched by liquid nitrogen, and the solution was poured into methanol. The white precipitate was filtered, dissolved in THF, and reprecipitated in methanol. The precipitate was filtered and dried to afford PMABuPOSS as a white powdery polymer. The yield was 0.95 g (83%). 1H NMR (CDCl3, ppm): δ 3.82 (br, 2H), 2.06− 1.48 (m, 2H), 1.86 (br, 7H), 1.70 (br, 2H), 1.10−0.78 (m, 45H), 0.60 (br, 16H). IR (KBr, cm−1): 2953 (C−H, aliphatic), 1733 (CO), 1232 (C−O), 1112 (Si−O−Si). Synthesis of PMAPhPOSS. To a Schlenk tube equipped with a stirring bar were added MethacrylPhenyl POSS (5.42 g, 5.0 mmol), 2,2′-azobis(isobutyronitrile) (0.018 g, 0.11 mmol), and THF (21 mL). The mixture was subjected to freeze−pump−thaw cycles to eliminate the air and then stirred at 60 °C for 40 h under nitrogen. The reaction was quenched by liquid nitrogen, and the solution was poured into methanol. The white precipitate was filtered, dissolved in THF, and reprecipitated in acetone/methanol (1/1 v/v). The precipitate was filtered and dried to afford PMAPhPOSS as a white powdery polymer. The yield was 3.72 g (69%). 1H NMR (CDCl3, ppm): δ 7.85−7.46 (m, 14H), 7.46−6.90 (m, 21H), 3.64 (br, 2H), 2.09−1.38 (m, 4H), 0.86 (br, 3H), 0.32 (br, 2H). IR (KBr, cm−1): 3074 (C−H, aromatic), 2946 (C−H, aliphatic), 1737 (CO), 1136 (Si−O−Si). Reactions of the Polymers with Tetrabutylammonium Fluoride. To a solution of PMABuPOSS (0.050 g, 0.050 mmol of POSS) or PMAPhPOSS (0.054 g, 0.050 mmol of POSS) in THF (0.45 mL) was added TBAF (1 M in THF; 1 equiv (0.050 mL, 0.050 mmol) or 0.1 equiv (5.0 μL, 0.0050 mmol)) in a vial equipped with a stirring bar. After 24 h of stirring at room temperature, the solution was poured into water. The precipitate was filtered, washed with water, and dried under vacuum. The resulting polymer was characterized by IR measurements. Cross-Linking Reaction of PMAPhPOSS. To a solution of PMAPhPOSS (0.054 g, 0.050 mmol of POSS) in THF (0.45 mL) was added TBAF (1 M in THF; 1 equiv (0.050 mL, 0.050 mmol) or 0.1 equiv (5.0 μL, 0.0050 mmol)) in a vial equipped with a stirring bar. After 1 h of stirring at room temperature, 1,4-bis(triethoxysilyl)benzene (BTSB) (0.010 mL, 0.025 mmol) was added to the solution. The resulting solution was stirred for 24 h at room temperature, and the THF was gradually evaporated at atmospheric pressure. The solidified product was dried under vacuum to give a transparent solid. The yield was 0.072 g. To remove TBAF and the residual leaving groups, the sample was washed with diethyl ether, methanol, acetone, or THF and dried under vacuum. In the case of film preparation, the solution was cast on the glass plate and dried at room temperature. De-Cross-Linking Reaction of Cross-Linked PMAPhPOSS. To a vial equipped with a stirring bar were added cross-linked PMAPhPOSS (0.081 g, 0.075 mmol of POSS unit), triethoxyphe-

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RESULTS AND DISCUSSION The POSS-pendanted methacrylate polymers with isobutyl (PMABuPOSS) or phenyl groups (PMAPhPOSS) substituted at the vertices of the POSS cage were prepared by free radical polymerization of the corresponding monomers. The values of M n and the molecular weight dispersity (Mw /M n) of PMABuPOSS determined by GPC were 10100 and 1.30, whereas those of PMAPhPOSS were 6000 and 1.16, respectively. Because of the rigid backbone of the POSScontaining polymers, the Mn values of PMABuPOSS and PMAPhPOSS are generally underestimated by GPC measurements, as reported in the literature.26 Indeed, the absolute Mw values of PMABuPOSS and PMAPhPOSS detected by multiangle laser light scattering (MALLS) were 42000 and 21000, respectively. In the literature, it was found that a fluoride-encapsulated complex was not obtained from alkylsubstituted POSS compounds.20 As a preliminary experiment, both polymers were mixed with TBAF in THF at room temperature. After 24 h of stirring, the polymers were retrieved by precipitation in water. The resulting polymers were characterized by IR spectroscopy, as shown in Figure 1. In the case of PMABuPOSS, the peaks derived from silanol groups were observed at 3234 and 892 cm−1 after treatment with TBAF (Figure 1a). This indicates that the fluoride ion partially cleaved the isobutyl-substituted POSS to form silanols. The resulting polymer from PMABuPOSS was unstable; condensation of the silanols immediately occurred in the solid form, and an insoluble polymer was obtained. On the other hand, the polymer retrieved after treatment of PMAPhPOSS with TBAF showed no significant change in the IR spectrum (Figure 1b). The peak of the ester groups appeared at 1736 cm−1, indicating 1637

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NMR spectra of PMAPhPOSS and the blend with TBAF, respectively. Three signals derived from the POSS cage were clearly observed (Figure 2d). One signal assignable to the silicon attached at the methylene spacer connected to the polymer main chain appeared at −65.07 ppm, and two signals assignable to the other silicons substituted by a phenyl group were observed at −78.06 and −78.41 ppm. The signals shifted upfield to −72.35, −79.94, and −80.57 ppm in the presence of equimolar TBAF (Figure 2e). In addition, new signals appeared at −128.1 and −128.8 ppm (Figure 2f). These new signals are probably due to the five-coordinate silicons coordinated to the fluoride ion.27,28 We assume that these signals are assignable to fluoride-coordinated silanols in the leaving group and the partially cleaved POSS cage. In the 19F NMR spectra, the signal of TBAF appeared at −115.5 and −120.8 ppm before and after mixing with PMAPhPOSS, respectively (Figure 3). In the case of octaphenyl-substituted POSS mixed with TBAF, the signals derived from the silicons of the POSS cage were shifted upfield by the inclusion of the fluoride ion.22 It was also reported that the signal assignable to the fluoride ion included in the octaphenyl-substituted POSS was drastically shifted downfield compared with that of TBAF in the 19F NMR spectrum. Therefore, the change in the chemical shift of PMAPhPOSS mixed with TBAF in the 29Si NMR spectra is due not to the inclusion of the fluoride ion but to silanols generated by the hydrolysis of the POSS cages with TBAF and a trace amount of water in THF. In the 1H NMR spectra, the signals assignable to the butyl groups of TBAF, which appeared at 3.47, 1.73, 1.44, and 0.99 ppm in THF-d8, were shifted upfield to 3.19, 1.52, 1.27, and 0.84 ppm, respectively, upon mixing with PMAPhPOSS (Figure S1 in the Supporting Information). Furthermore, the broad signals derived from the aromatic rings in PMAPhPOSS at 7.80−7.03 ppm were split into complex multiplets. Even with a smaller amount of TBAF (0.1 equiv),

Figure 1. IR spectra of (a) PMABuPOSS and (b) PMAPhPOSS before (dashed lines) and after (solid lines) treatment with TBAF in THF at room temperature for 24 h.

that the ester linkages were maintained in the presence of TBAF. The reaction of PMAPhPOSS with TBAF was investigated by NMR measurements in THF-d8. Figure 2a,b shows the 29Si

Figure 2. 29Si NMR spectra of (a, d) PMAPhPOSS, (b) PMAPhPOSS with TBAF (1 equiv), and (c, e, f) PMAPhPOSS with TBAF (1 equiv) and BTSB (0.5 equiv) in THF-d8. 1638

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fast reversible cage-scrambling reaction. This was also supported by the 1H NMR spectrum of the mixture, in which the signal at 7.63 ppm assignable to the aromatic protons of BTSB disappeared and the signals of ethanol appeared at 3.53 and 1.09 ppm (Figure S1 in the Supporting Information). Because BTSB is a bifunctional component, the scrambling reaction between the POSS cage and BTSB induces the crosslinking; however, no gelation or precipitation occurred at this concentration even after the solution of a mixture of PMAPhPOSS, TBAF, and BTSB was stirred for over 72 h. After THF was slowly evaporated from the solution of the three-component mixture under atmospheric pressure, a transparent solid was obtained. Once the solvent was removed, the resulting material became insoluble in any of the common solvents that are good solvents for PMAPhPOSS, such as THF and chloroform. The IR spectrum of the insoluble polymer showed no peaks of the silanols at 891 cm−1 (Figure S3 in the Supporting Information), which are generally observed in a partially cleaved POSS cage, indicating that all of the silanols generated by TBAF were converted to silsesquioxane structures. Because no gelation occurred in the solution, films could also be prepared by casting the solution on a glass substrate followed by drying of the solvent. The films were selfstanding and highly transparent below 300 nm, as shown in the UV−vis absorption spectra (Figure S4 in the Supporting Information). This phenomenon suggests that the condensation of the silanols generated by TBAF, which involves BTSB, gradually occurs during the process of solidification to give the cross-linked polymer. The obtained cross-linked polymer showed no glass transition temperature (Tg) below its decomposition temperature (Figure S5 in the Supporting Information). To verify the hypothesis that the cage-scrambling reaction of the POSS units induces the cross-linking of PMAPhPOSS, the reaction was performed with various cross-linkers, as listed in Table 1. When PMAPhPOSS was mixed with BTSB in the absence of TBAF, the cross-linked polymer was not formed after removal of the solvent (run 1). The cross-linking proceeded when the amount of either TBAF or BTSB was decreased (runs 2 and 3). In the case when tetraethoxysilane (TEOS) or triethoxyphenylsilane (TEPS) as a typical crosslinker was used instead of BTSB, the insolubilization was not observed (runs 5 and 6). Thus, random condensation of alkoxysilanes or silanols is not a major reason for the crosslinking. Mixing PMABuPOSS, the polymer containing isobutylsubstituted POSS cages, with TBAF and BTSB also resulted in no cross-linking. It was reported that the cage-scrambling

Figure 3. 19F NMR spectra of (a) TBAF and (b) PMAPhPOSS and TBAF (1 equiv) in THF-d8.

the aromatic signals of the POSS moiety gradually changed to complex multiplets after 24 h (Figure S2 in the Supporting Information). These facts indicate that the POSS cages were cleaved into silanols by TBAF and strongly interacted with fluoride ions to provide the five-coordinate silicons observed in the 29Si NMR spectra. Because the polymer retrieved from precipitation in water showed no significant peaks assignable to silanols (Figure 1b), the POSS moiety of PMAPhPOSS reversibly turned into partially cleaved silanols in the presence of TBAF. As described in the Introduction, the phenyl-substituted POSS compounds undergo cage scrambling under equilibrium conditions in the presence of a fluoride ion in THF.22−25 We applied this phenomenon to the dynamic covalent cross-linking between the PMAPhPOSS cages. To the solution of PMAPhPOSS and TBAF (1 equiv with respect to POSS) in THF (0.1 M monomer) was added BTSB (0.5 equiv with respect to POSS), which was expected to function as a crosslinker via migration of the vertices in the POSS cages in the presence of fluoride ion (Scheme 1). The addition of BTSB did not change the chemical shift of the mixture of PMAPhPOSS and TBAF in the 29Si NMR spectrum, whereas no signal derived from BTSB appearing at −59.0 ppm was observed, as shown in Figure 2c. In addition, the intensity of the signals at around −130 ppm attributed to the five-coordinate silicons increased. These facts indicate that BTSB was involved in the

Scheme 1. Cross-Linking of PMAPhPOSS by Cage-Scrambling Reaction Mediated by TBAF

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in the cross-linked polymer. This is probably due to the capture of fluoride ions in the POSS cage or the POSS network. These facts were also confirmed by the IR measurements. The peaks derived from TBAF at 2960 and 1475 cm−1 decreased after the cross-linked PMAPhPOSS was washed with methanol, and the spectrum almost corresponds to that of PMAPhPOSS except that the peak of Si−O−Si at 1130 cm−1 is broadened (Figure S6 in the Supporting Information). On the other hand, the peaks of TBAF were clearly observed in the spectrum for the cross-linked PMAPhPOSS washed with THF. Thus, it was found that the TBAF involved in the cross-linked PMAPhPOSS can be selectively removed by a certain solvent such as methanol. In order to investigate the chemical structure of the crosslinked PMAPhPOSS in the solid state, the 29Si CP/MAS NMR spectrum was obtained for the cross-linked PMAPhPOSS, as shown in Figure 5. Before cross-linking, PMAPhPOSS showed

Table 1. Cross-Linking of PMAPhPOSS with Various CrossLinkers run

TBAF (equiv)

cross-linker (equiv with respect to POSS unit)

property

1 2 3 4 5 6

0 0.1 1 1 1 1

BTSBa (0.5) BTSBa (0.5) BTSBa (0.25) BTSBa (0.5) TEOSb (1) TEPSc (1)

soluble insoluble insoluble insoluble soluble soluble

a 1,4-Bis(triethoxysilyl)benzene. bTetraethoxysilane. cTriethoxyphenylsilane.

reaction and the encapsulation of a fluoride ion do not occur for POSS compounds substituted by electron-donating alkyl groups, in contrast to electron-withdrawing groups such as the phenyl group.20,21 This fact indicates that using phenylsubstituted POSS cages in the cage-scrambling reaction with TBAF is essential for the formation of the network structure. TGA was performed on the cross-linked PMAPhPOSS in order to investigate the thermal properties. Figure 4 shows the

Figure 4. TGA profiles of PMAPhPOSS before and after cross-linking with TBAF (1 equiv) and BTSB (0.5 equiv).

TGA curves of PMAPhPOSS before and after the cross-linking using TBAF (1 equiv) and BTSB (0.5 equiv). PMAPhPOSS exhibited a high thermal stability with a 5% weight loss decomposition temperature (Td5) at 379 °C. The decomposition temperature of the as-made cross-linked PMAPhPOSS after the treatment with TBAF and BTSB decreased to around 150 °C as a result of the decomposition of the TBAF and the leaving groups. The weight loss in a plateau region at 300 °C was 35 wt %, which nearly corresponds to the sum of TBAF (16.5 wt %) and the leaving phenyltrihydroxysilane (15.2 wt %). Washing the cross-linked polymer with methanol could effectively remove these small molecular components (the yield was 74%). The resulting polymer showed high thermal stability with Td5 = 403.7 °C. The char yield at 600 °C for the crosslinked polymer washed with methanol increased to 75 wt % compared with PMAPhPOSS. This is assumed to result from the densely cross-linked structure based on PMAPhPOSS created by the cage-scrambling reaction. Meanwhile, when the cross-linked PMAPhPOSS was washed with a good solvent such as THF, the yield of the residual insoluble part was reduced to 52%. However, the TGA profile after washing with THF still showed a 15 wt % decomposition at around 150 °C, indicating the existence of TBAF (or other small component)

Figure 5. 29Si CP/MAS NMR spectra of (a) PMAPhPOSS, (b) crosslinked PMAPhPOSS, (c) cross-linked PMAPhPOSS washed with THF, and (d) cross-linked PMAPhPOSS washed with methanol.

sharp signals at −65.6 and −78.5 ppm corresponding to those in the 29Si NMR spectrum in THF solution (Figure 5a), whereas two broad signals were clearly observed at −70 and −80 ppm in the 29Si CP/MAS NMR spectrum of the crosslinked PMAPhPOSS treated with TBAF and BTSB (Figure 5b). Compared with PMAPhPOSS, the signal at −70 ppm was shifted upfield with an increase in the intensity. It was reported that the scrambling reaction generates a mixture of POSS compounds with different cage sizes, namely, T8, T10, and T12.23−25 On the basis of the fact that no silanol peaks were observed in the IR spectrum (Figures S3 and S6 in the Supporting Information), the change in the upfield shift and the intensity is mainly attributed to the incorporation of POSS moieties with different cage sizes or perhaps a random network structure,29 as shown in Chart 1. Washing the cross-linked PMAPhPOSS with THF or methanol produced no significant change in the spectrum, as shown in Figure 5c,d, indicating that the POSS structure was maintained after the washing process. 1640

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Macromolecules Chart 1. Possible Structures of the Cross-Linked PMAPhPOSS

Reducing the amount of TBAF (0.1 equiv with respect to POSS) decreased the intensity of the signal at −70 ppm (Figure S7 in the Supporting Information). This is probably due to suppression of the formation of the POSS moieties with larger sizes and a random network structure. The solid-state 19F MAS NMR spectra of the cross-linked PMAPhPOSS are shown in Figure 6. Two sharp signals appeared at −123.7 and −115.5

Figure 6. 19F MAS NMR spectra of cross-linked PMAPhPOSS (a) before and (b) after washing with THF.

ppm, corresponding to those of TBAF with and without PMAPhPOSS in THF solution. In addition, a broad signal ranging from −90 to −140 ppm overlapped them. After the cross-linked PMAPhPOSS was washed with THF, only the broad signal remained, with the top of the peak at −113.1 ppm (Figure 6b). On the basis of the fact that the fluoride ion encapsulated in POSS cage shows a signal around −26 ppm as reported in the literature,20 this result indicates that the fluoride ion is entrapped not in the POSS cage but in the POSS network. The possible structural variety in the POSS cages is assumed to broaden the signal of the entrapped fluoride ion. The structure of the cross-linked PMAPhPOSS was characterized by wide-angle X-ray diffraction. Figure 7 shows the diffraction patterns of the PMAPhPOSS before and after cross-linking with TBAF and BTSB. PMAPhPOSS shows two major amorphous halo peaks at 2θ = 8.3° (d = 10.6 Å) and 18.2° (d = 4.9 Å) corresponding to the main chains or POSS cages and phenyl substituents of the POSS cages, respectively, with a shoulder peak at 2θ = 24.5° (d = 3.6 Å) for a finer structure (Figure 7a).26,30,31 After the treatment with TBAF and BTSB, there are still two amorphous halo peaks at 2θ =

Figure 7. WAXD profiles of (a) PMAPhPOSS, (b) cross-linked PMAPhPOSS, and (c) cross-linked PMAPhPOSS washed with methanol.

7.1° (d = 12.4 Å) and 20.2° (d = 4.4 Å), as shown in Figure 7b. The first peak related to the length between the main chains or POSS cages is shifted to a lower angle, probably because of the incorporation of POSS cages with larger sizes such as T10 and also bulky tetrabutylammonium fluoride entrapped in the POSS network. On the other hand, the second peak derived from the distance of the phenyl rings is shifted to a higher angle, suggesting that the POSS cages become closer by crosslinking. It was reported that the amorphous halo peak at around 2θ = 7−8° diminished when the concentration of the POSS cages was diluted in the polymer matrix.30 Therefore, the POSS cages were densely maintained after the treatment with TBAF and BTSB, and the cross-linking is probably attributable not to the condensation between the silanols to give a random polysilsesquioxane network but to the incorporation of BTSB connecting the POSS cages via dynamic scrambling. The 1641

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resulting cross-linked polymer exhibited a WAXD profile similar to that before washing with methanol (Figure 7c), indicating that the network structure connecting the POSS cages with BTSB remains after the removal of TBAF. The BTSB moiety connecting the POSS cages in the PMAPhPOSS network is expected to be interchangeable in an equilibration process using fluoride ion, which means that a soluble polymer can be retrieved from the cross-linked polymer by utilizing the fluoride-mediated reversible reaction, as shown in Scheme 1. Excess triethoxyphenylsilane (5 equiv with respect to POSS) was added to the cross-linked polymer in THF, and the mixture was stirred for 24 h at room temperature. The polymer was completely dissolved. After precipitation in water, a white polymer was obtained. The 1H NMR spectrum of the resulting polymer showed broadened signals compared with those of the intact PMAPhPOSS, suggesting a change in the chemical structure (Figure S8 in the Supporting Information). Figure 8 shows that the GPC chromatogram of the retrieved

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ASSOCIATED CONTENT

S Supporting Information *

IR spectra, 1H and 29Si CP/MAS NMR spectra, UV−vis absorption spectrum, and DSC profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities (2013-2017). The authors acknowledge Prof. Shinji Ando and Prof. Shigeki Kuroki at Tokyo Institute of Technology for the measurement of solid-state 19F MAS NMR spectra of the cross-linked PMAPhPOSS.

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REFERENCES

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Figure 8. GPC profiles of PMAPhPOSS (solid line) and de-crosslinked polymer (dashed line) prepared from cross-linked PMAPhPOSS with triethoxyphenylsilane (5 equiv) eluted with THF.

polymer was broadened after the de-cross-linking compared with PMAPhPOSS, and the Mn and Mw/Mn of the polymer were 5100 and 1.47, respectively. The IR spectrum of the retrieved polymer was similar to that of PMAPhPOSS except for the novel weak peaks emerging at 962 and 787 cm−1 (Figure S9 in the Supporting Information), indicating that the obtained polymer involved partially cleaved POSS structures. Therefore, solubilization of the cross-linked PMAPhPOSS could be achieved by the scrambling reaction with the trialkoxysilane additive.

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CONCLUSION We have demonstrated that the fluoride-ion-mediated cagescrambling reaction of POSS cages pendant from the side chain of a methacrylate polymer can be utilized as a dynamic crosslinking system of POSS-containing polymers. The addition of a bifunctional trialkoxysilane derivative to the mixture of PMAPhPOSS and TBAF provided cross-linked materials by a simple drying method under moderate conditions. The resulting transparent material exhibited excellent thermal properties due to the dense network structure with a high POSS content. Moreover, the cross-linked PMAPhPOSS was converted to the soluble polymer by treatment with triethoxyphenylsilane and TBAF by a reversible cross-linking reaction through cage scrambling. This fluoride-mediated technique for the formation of cross-linked polymers can be applied to various POSS-containing (co)polymers for applications requiring high transparency and thermal stability. 1642

DOI: 10.1021/acs.macromol.5b00120 Macromolecules 2015, 48, 1636−1643

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DOI: 10.1021/acs.macromol.5b00120 Macromolecules 2015, 48, 1636−1643