DOI: 10.1002/chem.201303759
Tailor-Made Hybrid Organic–Inorganic Porous Materials Based on Polyhedral Oligomeric Silsesquioxanes (POSS) by the Step-Growth Mechanism of Thiol-Ene “Click” Chemistry Filipa Alves and Ivo Nischang*[a] The term “click” chemistry originates with Barry Sharpless who codified its philosophy.[1] Click reactions have since been associated with particular characteristics, such as high yields, functional group, solvent and oxygen tolerance, as well as absence of byproducts and the associated easy purification.[2] The thiol-ene addition reaction may also fit within this concept.[3] Radical-mediated polymerizations with this reaction may proceed exclusively through a step-growth pathway or through a combination of a step- and chain-growth pathway. This depends on the nature of the thiol and alkene involved, and, as a result on the nature of the thyil radical, the intermediate carbon-centered radical and their stabilities.[4] Polyhedral oligomeric silsesquioxanes (POSS) have the basic structure (RSiO = )n, (n = 8, 10, 12). These are organic–in3
2
[a] F. Alves, Dr. I. Nischang Institute of Polymer Chemistry Johannes Kepler University Linz Welser Strasse 42, 4060 Leonding (Austria) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ chem.201303759. 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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Scheme 1. a) vinylPOSS (RSiO = )n shown for n = 8, 10, 12; b) variety of thiol compounds explored in this study; the nomenclature attributed to the linker is identical to the respective polymer; and c) hybrid polymer 1 prepared with vinylPOSS and linker 1. 3
2
2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 17310 – 17313
COMMUNICATION organic hybrid, cage-like nanostructures (e.g., Scheme 1 a) that, in their chemical diversity, have already been used as building blocks for the generation of porous materials through thermolysis[5] and hydrosylation,[6] as well as other coupling reactions.[7] The preparation of monolithic porous materials involving POSS has often been addressed with amines,[8] and through uncontrolled, free-radical copolymerization with other monomers.[9] Recently, porous monolithic adsorbents prepared by simple free-radical polymerization of vinylPOSS have been reported. These studies indicated low reactivity of the tightly tethered vinyl group in free radical polymerization.[10] The exploitation of novel chemical cross-linking routes capable of connecting POSS precursors efficiently and in a controlled and versatile manner, which allows the derivation of porous materials uniformly organized at a molecular level and simultaneously macroscopically shaped, is still an arena with room for innovation. This is interesting primarily from a fundamental point of view, but also to pave the way for technological advances toward materials in widely defined areas such as catalysis, chromatography, membrane filtration, nanomedicine, solid-phase extraction, etc.[11] The initial screening experiments of the reaction described herein utilized vinylPOSS precursors (Scheme 1 a) in the presence of thiol and initiator. In situ reaction monitoring with Raman spectroscopy showed nearly equal consumption of the thiol and vinyl functional groups, strongly indicating that a step-growth pathway is favored (Figure S1).[4] This allowed for a simple and versatile approach to mold nanostructured materials based on vinylPOSS and suitable thiol-containing entities serving as linkers (Scheme 1 a, b). Experiments carried out with linker 1 with an inert diluent mixture of tetrahydrofuran (THF) and polyethylene glycol 200 (PEG200) in the presence of initiator and triggered either by heat (Figure 1 a) or UV light (Figure 1 b) led to the formation of rigid materials adapting to the shape of their given mold. Whereas thermal initiation for polymer 1 a resulted in a continuous skeleton intertwined by large pores, the morphology of the photo-polymerized material polymer 1 b is entirely different and shows particle-shaped, covalently attached features. Such differences in morphology may be associated with the kinetics of material formation including phase separation. Nonetheless, both macroscopically shaped materials showed a permanent macroporous structure with pores on the order of micrometers. Furthermore, the powder X-ray diffraction pattern of polymer 1 a (Figure S2) exhibits a typical broad amorphous silica halo around 218 2q as well as an additional peak around 88 2q indicating a reoccurring interatomic distance. This suggests some structural ordering in the amorphous polymeric material.[12] Also, the features of the macroporous materials are apparently nonporous in the dry state, indicated by nitrogen adsorption/desorption measurements showing negligible dry-state Brunauer–Emmett–Teller surface areas. The change in the inert diluent composition revealed differences in their porous structure, with a lower weight percentage of PEG200 in the
Figure 1. Optical photographs (top row) and scanning electron microscopy images at different magnifications as indicated (middle and bottom rows) for a) polymer 1 a prepared in situ in sealed 4 mL glass vials through thermal initiation, and b) polymer 1 b prepared in quartz glass tubes by photoinitiation. Both polymers had identical polymerization mixture compositions (mixture composition: 40 % monomers with a ratio of “ene” to thiol functional groups of 1:1.5, 30 % THF and 30 % PEG200, all w/w).
polymerization mixture leading to smaller pores in both cases (Figure S3 in the Supporting Information). The rigid materials in Figure 1 prepared by a thermallyor UV-light-triggered reaction were stable in air up to 300 8C (Figure S4 a). Due to the same basic material composition, and linking chemistry, a very similar thermal degradation pattern with the same ceramic yields was observed. In instances, this was also observed for materials prepared with linker 3 (Figure S4 b), as well as with linker 4 (Figure S4 c). Hybrid polymer 1 a was taken as an example and the corresponding 29Si and 13C NMR spectra (Figure 2), Fourier transform infrared (FTIR), and Raman spectra (Figure 3), revealed the almost complete disappearance of the signals associated with the presence of vinyl groups of the hybrid vinylPOSS monomer and the appearance of new bands ACHTUNGREassociated with the covalent linkage between the vinylPOSS and the thiols. Typical disulfide bands (at 500 cm 1 in Raman) are not observed in the respective Raman spectrum (Figure 3 b). The above conclusions could also be drawn from the photochemical nature of the polymerization process (polymer 1b, Figure S5) and from the polymerization processes involving other linkers, such as linkers 2 and 3 (polymer 2a, Figure S6 and polymer 3a, Figures S7 and S8), as well as linker 4 (polymer 4a, Figure S9). Whereas linkers 1, 2, and 3 enabled the preparation of rigid materials under any solvent composition, linker 4 led to monolithic gels showing more flexibility and hydrophilicity. Such gel properties were greatly enhanced with linker 5
Chem. Eur. J. 2013, 19, 17310 – 17313 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org
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Figure 2. a) Solid-state 29Si CP-MAS NMR spectra, and b) solid-state 13 C CP-MAS NMR spectra of vinylPOSS monomers (black lines) and polymer 1 a (red lines). The vinyl associated signals present in the 29Si ( 87 to 75 ppm) and 13C NMR (130 to 150 ppm) spectra of the vinylPOSS monomer are almost absent for the polymer. In the 29Si spectrum of the polymer an almost quantitative shift of the neighboring 29Si downfield is observed and in the respective 13C spectrum new signals emerge upfield at 10–50 ppm associated with the new saturated bonds formed.
(Figure S10). The resulting hydrogel showed a water uptake that reached 300 % of its initial weight, which is in the same order of magnitude of state of the art related hydrogel materials (Figure S11).[13] A desirable side effect of the preparation described herein is that the scaffolds formed inherently possess pendant thiol groups on their internal structure (Scheme 1), which are clearly discernible in the Raman spectra (e.g., Figure 3 b). This functionality can further be utilized through post-polymerization modification steps,[3] allowing versatile interface decoration of the generic scaffolds by thiol-ene chemistry even in the absence of initiator. Figure 3 also shows FTIR (a) and Raman (b) spectra of polymer 1 a after grafting with acrylic acid. The appearance of a new carbonyl band together with a broad O H stretch in the FTIR spectrum, originating from the grafted acrylic acid, is seen in Figure 3 a. This was in concert with a reduction in intensity of the S H stretching signal shown in the Raman spectrum (Figure 3 b). Thermogravimetric analysis data (shown in Figure S12 a) showed the substantial grafting load of acrylic acid on poly-
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Figure 3. a) FTIR and b) Raman spectra of vinylPOSS precursor mixture (black lines), polymer 1 a (red lines), and polymer 1 a grafted with acrylic acid (blue lines). Either in the FTIR, or in the Raman spectrum the peaks associated with the monomer stretching (str.) vibrations of the vinyl groups at 3000–3150 cm 1 and at 1620–1680 cm 1 disappeared, whereas vibrations related to alkane at 2915–2940 cm 1 and carbon sulfur bonds at 630–720 cm 1 emerged. In addition, an S H str. band at 2570 cm 1 can be seen in the Raman spectrum of polymer 1 a, whereas the cage Si O Si str. at 1010–1090 cm 1, very intense in FTIR, is maintained throughout formation of polymer and subsequent grafting. Grafting with acrylic acid shows an additional C=O str. band at 1705 cm 1 and a broad O H str. band at 2500–3500 cm 1 in FTIR. A reduction of the SH str. band at 2570 cm 1 in Raman corroborates the success of the modification.
mer 1 a. The same procedure worked equally well with other scaffolds derived, for example, from linker 3 (Figure S12 b). In initial attempts to use these polymers and test their properties, we prepared samples in 100 mm ID fused silica capillaries, the internal walls of which were previously functionalized with methacrylate moieties.[14] This allowed covalent anchorage of the in situ formed porous polymers (Figure S13). Although further optimization is necessary, it was possible to confirm the existence of an interconnected pore structure along the capillary for polymers 1 a and 3 a. The materials possessed pressure stability at increased flow rates (Figure S14). Example in situ modification of the hydrophobic polymer 1 a with acrylic acid resulted in a polymer with much reduced hydrophobicity (Figure S12 a, Figure S15 a). In turn, grafting less hydrophobic polymer 2 a with butyl methacrylate resulted in a polymer with enhanced hydrophobic properties (Figure S12 b, Figure S15 b). Through this simple approach, internal surfaces of tailored pristine scaffolds can be rendered hydrophilic or hydrophobic and are, therefore, more chemically diverse.
www.chemeurj.org 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 17310 – 17313
POSS Hybrid Organic–Inorganic Porous Materials
In summary, we have demonstrated the preparation of hybrid, organic–inorganic monolithic materials employing vinylPOSS and multi-functional thiols by a robust singlestep, radical-mediated step-growth process. The versatile linking chemistry available allows the preparation of hybrid materials, showing the widest variety of chemical nature (from hydrophobic to hydrophilic) and physical behavior (from gels to rigid materials) dependent only upon choosing the proper linking unit. The linking unit was observed to not affect the principal mechanism of formation over a wide range of precursors, but leads to its desirable physical properties. Initial results indicate tailorability of the macroporous properties through proper adjustment of the porogenic solvent system. The tailorability of morphology, pore size, and functional versatility of these materials including preparation of gels and rigid materials targeting specific applications in a variety of formats are all factors that are currently under investigation.
Experimental Section In an example preparation, vinylPOSS cage mixture was first dissolved in THF, followed by addition of the desired amount of porogenic solvent component, for example, PEG200. Then, the respective thiol was added, with the molar ratio of the “ene” to thiol functional groups maintained at a ratio of 1:1.5. The ratio of overall reactive monomers to porogenic solvent comprising THF only or THF and for example, PEG200 was 40/60 (%, w/w). The single phase homogeneous polymerization mixture additionally contained azobisisobutyronitrile or 2,2-dimethoxy-2-phenylacetophenone (1 wt % with respect to the thiol) for initiation either thermally at 60 8C for 24 h in a water bath, or alternatively by irradiation with UV light at 22 8C for 30 min. UV light triggered reactions for monolith formation or surface grafting were performed in a Rayonet–Chamber reactor with an illumination wavelength of 253 nm and a fixed illumination time of 30 min. The bulk samples were prepared in sealed 4 mL glass vials for thermal initiation processes and in sealed quartz glass tubes for photoinitiation. Following polymerization, the bulk polymers were cut into smaller pieces, soxhlet-extracted with THF for 16 h, and dried in a vacuum oven at 40 8C overnight. For bulk sample modifications, the polymer sample (typically 100 mg) was suspended in a solution containing 0.2 mL of “ene” functional molecules in chloroform (3 mL). Samples were then irradiated with UV light for 30 min at a reactor temperature of 22 8C under stirring. The resulting materials were then washed repeatedly with chloroform, followed by THF before drying and further analysis. Further experimental details and results are provided in the Supporting Information.
Acknowledgements This work was supported by the Austrian Science Fund (FWF) under project number P24557-N19. The authors acknowledge Wolfgang Schçfberger for the solid-state NMR measurements at the Austro–Czech RERI-uasb NMR center, established with financial support from the EU through the EFRE INTERREG IV ETC-AT-CZ programme (project M00146, RERI-uasb). Gnter Hesser at the ZONA is acknowledged for support with the SEM measurements and Cezarina C. Mardare at the ICTAS for the X-ray diffraction measurement. We acknowledge Krisztina Vincze-Minya and Prof. Sabine Hild at the IPS for support with the Raman spectra acquisitions. Finally, we thank RECENDT GmbH for supporting us with in situ Raman measurements.
COMMUNICATION Keywords: click chemistry · grafting · hybrid monolith · polyhedral oligomeric silsesquioxanes · polymerization
[1] H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. 2001, 113, 2056 – 2075; Angew. Chem. Int. Ed. 2001, 40, 2004 – 2021. [2] C. Barner-Kowollik, F. E. Du Prez, P. Espeel, C. J. Hawker, T. Junkers, H. Schlaad, W. Van Camp, Angew. Chem. 2011, 123, 61 – 64; Angew. Chem. Int. Ed. 2011, 50, 60 – 62. [3] C. E. Hoyle, C. N. Bowman, Angew. Chem. 2010, 122, 1584 – 1617; Angew. Chem. Int. Ed. 2010, 49, 1540 – 1573. [4] N. B. Cramer, S. K. Reddy, A. K. OBrien, C. N. Bowman, Macromolecules 2003, 36, 7964 – 7969; B. H. Northrop, R. N. Coffey, J. Am. Chem. Soc. 2012, 134, 13804 – 13817; Q. Li, H. Zhou, C. E. Hoyle, Polymer 2009, 50, 2237 – 2245. [5] M. F. Roll, J. W. Kampf, Y. Kim, E. Yi, R. M. Laine, J. Am. Chem. Soc. 2010, 132, 10171 – 10183. [6] J. J. Morrison, C. J. Love, B. W. Manson, I. J. Shannon, R. E. Morris, J. Mater. Chem. 2002, 12, 3208 – 3212; L. Zhang, Q. Yang, H. Yang, J. Liu, H. Xin, B. Mezari, P. C. M. M. Magusin, H. C. L. Abbenhuis, R. A. v. Santen, C. Li, J. Mater. Chem. 2008, 18, 450 – 457; P. G. Harrison, R. Kannengiesser, Chem. Commun. 1996, 415 – 416; R. M. Laine, J. Mater. Chem. 2005, 15, 3725 – 3744; D. Hoebbel, K. Endres, T. Reinert, I. Pitsch, J. Non-Cryst. Solids 1994, 176, 179 – 188. [7] D. Wang, L. Xue, L. Li, B. Deng, S. Feng, H. Liu, X. Zhao, Macromol. Rapid Commun. 2013, 34, 861 – 866; W. Chaikittisilp, M. Kubo, T. Moteki, A. Sugawara-Narutaki, A. Shimojima, T. Okubo, J. Am. Chem. Soc. 2011, 133, 13832 – 13835; Y. Kim, K. Koh, M. F. Roll, R. M. Laine, A. J. Matzger, Macromolecules 2010, 43, 6995 – 7000; W. Chaikittisilp, A. Sugawara, A. Shimojima, T. Okubo, Chem. Eur. J. 2010, 16, 6006 – 6014; Y. Wada, K. Iyoki, A. Sugawara-Narutaki, T. Okubo, A. Shimojima, Chem. Eur. J. 2013, 19, 1700 – 1705; Y. Peng, T. Ben, J. Xu, M. Xue, X. Jing, F. Deng, S. Qiu, G. Zhu, Dalton Trans. 2011, 40, 2720 – 2724. [8] H. Lin, J. Ou, Z. Zhang, J. Dong, H. Zou, Chem. Commun. 2013, 49, 231 – 233; H. Guo, M. A. B. Meador, L. McCorkle, D. J. Quade, J. Guo, B. Hamilton, M. Cakmak, G. Sprowl, ACS Appl. Mater. Interfaces 2011, 3, 546 – 552. [9] J. J. Ou, Z. B. Zhang, H. Lin, J. Dong, H. F. Zou, Anal. Chim. Acta 2013, 761, 209 – 216; M. Wu, R. a. Wu, R. Li, H. Qin, J. Dong, Z. Zhang, H. Zou, Anal. Chem. 2010, 82, 5447 – 5454. [10] I. Nischang, O. Brggemann, I. Teasdale, Angew. Chem. 2011, 123, 4688 – 4692; Angew. Chem. Int. Ed. 2011, 50, 4592 – 4596; F. Alves, P. Scholder, I. Nischang, ACS Appl. Mater. Interfaces 2013, 5, 2517 – 2526. [11] M. Dalwani, J. Zheng, M. Hempenius, M. J. T. Raaijmakers, C. M. Doherty, A. J. Hill, M. Wessling, N. E. Benes, J. Mater. Chem. 2012, 22, 14835 – 14838; C.-H. Lu, F.-C. Chang, ACS Catal. 2011, 1, 481 – 488; H. Ghanbari, B. G. Cousins, A. M. Seifalian, Macromol. Rapid Commun. 2011, 32, 1032 – 1046; H. Lin, J. Ou, S. Tang, Z. Zhang, J. Dong, Z. Liu, H. Zou, J. Chromatogr. A 2013, 1301, 131 – 138; D.-G. Kim, H. Kang, S. Han, J.-C. Lee, ACS Appl. Mater. Interfaces 2012, 4, 5898 – 5906. [12] R. Tamaki, J. Choi, R. M. Laine, Chem. Mater. 2003, 15, 793 – 797; C. Zhang, F. Babonneau, C. Bonhomme, R. M. Laine, C. L. Soles, H. A. Hristov, A. F. Yee, J. Am. Chem. Soc. 1998, 120, 8380 – 8391; N. Takamura, L. Viculis, C. Zhang, R. M. Laine, Polym. Int. 2007, 56, 1378 – 1391. [13] M. Malkoch, R. Vestberg, N. Gupta, L. Mespouille, P. Dubois, A. F. Mason, J. L. Hedrick, Q. Liao, C. W. Frank, K. Kingsbury, C. J. Hawker, Chem. Commun. 2006, 2774 – 2776; L. Wang, K. Zeng, S. Zheng, ACS Appl. Mater. Interfaces 2011, 3, 898 – 909; Y.-H. La, R. Sooriyakumaran, B. D. McCloskey, R. D. Allen, B. D. Freeman, R. Al-Rasheed, J. Membr. Sci. 2012, 401 – 402, 306 – 312. [14] I. Nischang, F. Svec, J. M. J. Frchet, Anal. Chem. 2009, 81, 7390 – 7396. Received: September 25, 2013 Published online: November 15, 2013
Chem. Eur. J. 2013, 19, 17310 – 17313 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org
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Tailor-Made Hybrid Organic–Inorganic Porous Materials Based on Polyhedral Oligomeric Silsesquioxanes (POSS) by the Step-Growth Mechanism of Thiol-Ene “Click” Chemistry Filipa Alves and Ivo Nischang*[a] The term “click” chemistry originates with Barry Sharpless who codified its philosophy.[1] Click reactions have since been associated with particular characteristics, such as high yields, functional group, solvent and oxygen tolerance, as well as absence of byproducts and the associated easy purification.[2] The thiol-ene addition reaction may also fit within this concept.[3] Radical-mediated polymerizations with this reaction may proceed exclusively through a step-growth pathway or through a combination of a step- and chain-growth pathway. This depends on the nature of the thiol and alkene involved, and, as a result on the nature of the thyil radical, the intermediate carbon-centered radical and their stabilities.[4] Polyhedral oligomeric silsesquioxanes (POSS) have the basic structure (RSiO = )n, (n = 8, 10, 12). These are organic–in3
2
[a] F. Alves, Dr. I. Nischang Institute of Polymer Chemistry Johannes Kepler University Linz Welser Strasse 42, 4060 Leonding (Austria) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ chem.201303759. 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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Scheme 1. a) vinylPOSS (RSiO = )n shown for n = 8, 10, 12; b) variety of thiol compounds explored in this study; the nomenclature attributed to the linker is identical to the respective polymer; and c) hybrid polymer 1 prepared with vinylPOSS and linker 1. 3
2
2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 17310 – 17313
COMMUNICATION organic hybrid, cage-like nanostructures (e.g., Scheme 1 a) that, in their chemical diversity, have already been used as building blocks for the generation of porous materials through thermolysis[5] and hydrosylation,[6] as well as other coupling reactions.[7] The preparation of monolithic porous materials involving POSS has often been addressed with amines,[8] and through uncontrolled, free-radical copolymerization with other monomers.[9] Recently, porous monolithic adsorbents prepared by simple free-radical polymerization of vinylPOSS have been reported. These studies indicated low reactivity of the tightly tethered vinyl group in free radical polymerization.[10] The exploitation of novel chemical cross-linking routes capable of connecting POSS precursors efficiently and in a controlled and versatile manner, which allows the derivation of porous materials uniformly organized at a molecular level and simultaneously macroscopically shaped, is still an arena with room for innovation. This is interesting primarily from a fundamental point of view, but also to pave the way for technological advances toward materials in widely defined areas such as catalysis, chromatography, membrane filtration, nanomedicine, solid-phase extraction, etc.[11] The initial screening experiments of the reaction described herein utilized vinylPOSS precursors (Scheme 1 a) in the presence of thiol and initiator. In situ reaction monitoring with Raman spectroscopy showed nearly equal consumption of the thiol and vinyl functional groups, strongly indicating that a step-growth pathway is favored (Figure S1).[4] This allowed for a simple and versatile approach to mold nanostructured materials based on vinylPOSS and suitable thiol-containing entities serving as linkers (Scheme 1 a, b). Experiments carried out with linker 1 with an inert diluent mixture of tetrahydrofuran (THF) and polyethylene glycol 200 (PEG200) in the presence of initiator and triggered either by heat (Figure 1 a) or UV light (Figure 1 b) led to the formation of rigid materials adapting to the shape of their given mold. Whereas thermal initiation for polymer 1 a resulted in a continuous skeleton intertwined by large pores, the morphology of the photo-polymerized material polymer 1 b is entirely different and shows particle-shaped, covalently attached features. Such differences in morphology may be associated with the kinetics of material formation including phase separation. Nonetheless, both macroscopically shaped materials showed a permanent macroporous structure with pores on the order of micrometers. Furthermore, the powder X-ray diffraction pattern of polymer 1 a (Figure S2) exhibits a typical broad amorphous silica halo around 218 2q as well as an additional peak around 88 2q indicating a reoccurring interatomic distance. This suggests some structural ordering in the amorphous polymeric material.[12] Also, the features of the macroporous materials are apparently nonporous in the dry state, indicated by nitrogen adsorption/desorption measurements showing negligible dry-state Brunauer–Emmett–Teller surface areas. The change in the inert diluent composition revealed differences in their porous structure, with a lower weight percentage of PEG200 in the
Figure 1. Optical photographs (top row) and scanning electron microscopy images at different magnifications as indicated (middle and bottom rows) for a) polymer 1 a prepared in situ in sealed 4 mL glass vials through thermal initiation, and b) polymer 1 b prepared in quartz glass tubes by photoinitiation. Both polymers had identical polymerization mixture compositions (mixture composition: 40 % monomers with a ratio of “ene” to thiol functional groups of 1:1.5, 30 % THF and 30 % PEG200, all w/w).
polymerization mixture leading to smaller pores in both cases (Figure S3 in the Supporting Information). The rigid materials in Figure 1 prepared by a thermallyor UV-light-triggered reaction were stable in air up to 300 8C (Figure S4 a). Due to the same basic material composition, and linking chemistry, a very similar thermal degradation pattern with the same ceramic yields was observed. In instances, this was also observed for materials prepared with linker 3 (Figure S4 b), as well as with linker 4 (Figure S4 c). Hybrid polymer 1 a was taken as an example and the corresponding 29Si and 13C NMR spectra (Figure 2), Fourier transform infrared (FTIR), and Raman spectra (Figure 3), revealed the almost complete disappearance of the signals associated with the presence of vinyl groups of the hybrid vinylPOSS monomer and the appearance of new bands ACHTUNGREassociated with the covalent linkage between the vinylPOSS and the thiols. Typical disulfide bands (at 500 cm 1 in Raman) are not observed in the respective Raman spectrum (Figure 3 b). The above conclusions could also be drawn from the photochemical nature of the polymerization process (polymer 1b, Figure S5) and from the polymerization processes involving other linkers, such as linkers 2 and 3 (polymer 2a, Figure S6 and polymer 3a, Figures S7 and S8), as well as linker 4 (polymer 4a, Figure S9). Whereas linkers 1, 2, and 3 enabled the preparation of rigid materials under any solvent composition, linker 4 led to monolithic gels showing more flexibility and hydrophilicity. Such gel properties were greatly enhanced with linker 5
Chem. Eur. J. 2013, 19, 17310 – 17313 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org
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Figure 2. a) Solid-state 29Si CP-MAS NMR spectra, and b) solid-state 13 C CP-MAS NMR spectra of vinylPOSS monomers (black lines) and polymer 1 a (red lines). The vinyl associated signals present in the 29Si ( 87 to 75 ppm) and 13C NMR (130 to 150 ppm) spectra of the vinylPOSS monomer are almost absent for the polymer. In the 29Si spectrum of the polymer an almost quantitative shift of the neighboring 29Si downfield is observed and in the respective 13C spectrum new signals emerge upfield at 10–50 ppm associated with the new saturated bonds formed.
(Figure S10). The resulting hydrogel showed a water uptake that reached 300 % of its initial weight, which is in the same order of magnitude of state of the art related hydrogel materials (Figure S11).[13] A desirable side effect of the preparation described herein is that the scaffolds formed inherently possess pendant thiol groups on their internal structure (Scheme 1), which are clearly discernible in the Raman spectra (e.g., Figure 3 b). This functionality can further be utilized through post-polymerization modification steps,[3] allowing versatile interface decoration of the generic scaffolds by thiol-ene chemistry even in the absence of initiator. Figure 3 also shows FTIR (a) and Raman (b) spectra of polymer 1 a after grafting with acrylic acid. The appearance of a new carbonyl band together with a broad O H stretch in the FTIR spectrum, originating from the grafted acrylic acid, is seen in Figure 3 a. This was in concert with a reduction in intensity of the S H stretching signal shown in the Raman spectrum (Figure 3 b). Thermogravimetric analysis data (shown in Figure S12 a) showed the substantial grafting load of acrylic acid on poly-
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Figure 3. a) FTIR and b) Raman spectra of vinylPOSS precursor mixture (black lines), polymer 1 a (red lines), and polymer 1 a grafted with acrylic acid (blue lines). Either in the FTIR, or in the Raman spectrum the peaks associated with the monomer stretching (str.) vibrations of the vinyl groups at 3000–3150 cm 1 and at 1620–1680 cm 1 disappeared, whereas vibrations related to alkane at 2915–2940 cm 1 and carbon sulfur bonds at 630–720 cm 1 emerged. In addition, an S H str. band at 2570 cm 1 can be seen in the Raman spectrum of polymer 1 a, whereas the cage Si O Si str. at 1010–1090 cm 1, very intense in FTIR, is maintained throughout formation of polymer and subsequent grafting. Grafting with acrylic acid shows an additional C=O str. band at 1705 cm 1 and a broad O H str. band at 2500–3500 cm 1 in FTIR. A reduction of the SH str. band at 2570 cm 1 in Raman corroborates the success of the modification.
mer 1 a. The same procedure worked equally well with other scaffolds derived, for example, from linker 3 (Figure S12 b). In initial attempts to use these polymers and test their properties, we prepared samples in 100 mm ID fused silica capillaries, the internal walls of which were previously functionalized with methacrylate moieties.[14] This allowed covalent anchorage of the in situ formed porous polymers (Figure S13). Although further optimization is necessary, it was possible to confirm the existence of an interconnected pore structure along the capillary for polymers 1 a and 3 a. The materials possessed pressure stability at increased flow rates (Figure S14). Example in situ modification of the hydrophobic polymer 1 a with acrylic acid resulted in a polymer with much reduced hydrophobicity (Figure S12 a, Figure S15 a). In turn, grafting less hydrophobic polymer 2 a with butyl methacrylate resulted in a polymer with enhanced hydrophobic properties (Figure S12 b, Figure S15 b). Through this simple approach, internal surfaces of tailored pristine scaffolds can be rendered hydrophilic or hydrophobic and are, therefore, more chemically diverse.
www.chemeurj.org 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 17310 – 17313
POSS Hybrid Organic–Inorganic Porous Materials
In summary, we have demonstrated the preparation of hybrid, organic–inorganic monolithic materials employing vinylPOSS and multi-functional thiols by a robust singlestep, radical-mediated step-growth process. The versatile linking chemistry available allows the preparation of hybrid materials, showing the widest variety of chemical nature (from hydrophobic to hydrophilic) and physical behavior (from gels to rigid materials) dependent only upon choosing the proper linking unit. The linking unit was observed to not affect the principal mechanism of formation over a wide range of precursors, but leads to its desirable physical properties. Initial results indicate tailorability of the macroporous properties through proper adjustment of the porogenic solvent system. The tailorability of morphology, pore size, and functional versatility of these materials including preparation of gels and rigid materials targeting specific applications in a variety of formats are all factors that are currently under investigation.
Experimental Section In an example preparation, vinylPOSS cage mixture was first dissolved in THF, followed by addition of the desired amount of porogenic solvent component, for example, PEG200. Then, the respective thiol was added, with the molar ratio of the “ene” to thiol functional groups maintained at a ratio of 1:1.5. The ratio of overall reactive monomers to porogenic solvent comprising THF only or THF and for example, PEG200 was 40/60 (%, w/w). The single phase homogeneous polymerization mixture additionally contained azobisisobutyronitrile or 2,2-dimethoxy-2-phenylacetophenone (1 wt % with respect to the thiol) for initiation either thermally at 60 8C for 24 h in a water bath, or alternatively by irradiation with UV light at 22 8C for 30 min. UV light triggered reactions for monolith formation or surface grafting were performed in a Rayonet–Chamber reactor with an illumination wavelength of 253 nm and a fixed illumination time of 30 min. The bulk samples were prepared in sealed 4 mL glass vials for thermal initiation processes and in sealed quartz glass tubes for photoinitiation. Following polymerization, the bulk polymers were cut into smaller pieces, soxhlet-extracted with THF for 16 h, and dried in a vacuum oven at 40 8C overnight. For bulk sample modifications, the polymer sample (typically 100 mg) was suspended in a solution containing 0.2 mL of “ene” functional molecules in chloroform (3 mL). Samples were then irradiated with UV light for 30 min at a reactor temperature of 22 8C under stirring. The resulting materials were then washed repeatedly with chloroform, followed by THF before drying and further analysis. Further experimental details and results are provided in the Supporting Information.
Acknowledgements This work was supported by the Austrian Science Fund (FWF) under project number P24557-N19. The authors acknowledge Wolfgang Schçfberger for the solid-state NMR measurements at the Austro–Czech RERI-uasb NMR center, established with financial support from the EU through the EFRE INTERREG IV ETC-AT-CZ programme (project M00146, RERI-uasb). Gnter Hesser at the ZONA is acknowledged for support with the SEM measurements and Cezarina C. Mardare at the ICTAS for the X-ray diffraction measurement. We acknowledge Krisztina Vincze-Minya and Prof. Sabine Hild at the IPS for support with the Raman spectra acquisitions. Finally, we thank RECENDT GmbH for supporting us with in situ Raman measurements.
COMMUNICATION Keywords: click chemistry · grafting · hybrid monolith · polyhedral oligomeric silsesquioxanes · polymerization
[1] H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. 2001, 113, 2056 – 2075; Angew. Chem. Int. Ed. 2001, 40, 2004 – 2021. [2] C. Barner-Kowollik, F. E. Du Prez, P. Espeel, C. J. Hawker, T. Junkers, H. Schlaad, W. Van Camp, Angew. Chem. 2011, 123, 61 – 64; Angew. Chem. Int. Ed. 2011, 50, 60 – 62. [3] C. E. Hoyle, C. N. Bowman, Angew. Chem. 2010, 122, 1584 – 1617; Angew. Chem. Int. Ed. 2010, 49, 1540 – 1573. [4] N. B. Cramer, S. K. Reddy, A. K. OBrien, C. N. Bowman, Macromolecules 2003, 36, 7964 – 7969; B. H. Northrop, R. N. Coffey, J. Am. Chem. Soc. 2012, 134, 13804 – 13817; Q. Li, H. Zhou, C. E. Hoyle, Polymer 2009, 50, 2237 – 2245. [5] M. F. Roll, J. W. Kampf, Y. Kim, E. Yi, R. M. Laine, J. Am. Chem. Soc. 2010, 132, 10171 – 10183. [6] J. J. Morrison, C. J. Love, B. W. Manson, I. J. Shannon, R. E. Morris, J. Mater. Chem. 2002, 12, 3208 – 3212; L. Zhang, Q. Yang, H. Yang, J. Liu, H. Xin, B. Mezari, P. C. M. M. Magusin, H. C. L. Abbenhuis, R. A. v. Santen, C. Li, J. Mater. Chem. 2008, 18, 450 – 457; P. G. Harrison, R. Kannengiesser, Chem. Commun. 1996, 415 – 416; R. M. Laine, J. Mater. Chem. 2005, 15, 3725 – 3744; D. Hoebbel, K. Endres, T. Reinert, I. Pitsch, J. Non-Cryst. Solids 1994, 176, 179 – 188. [7] D. Wang, L. Xue, L. Li, B. Deng, S. Feng, H. Liu, X. Zhao, Macromol. Rapid Commun. 2013, 34, 861 – 866; W. Chaikittisilp, M. Kubo, T. Moteki, A. Sugawara-Narutaki, A. Shimojima, T. Okubo, J. Am. Chem. Soc. 2011, 133, 13832 – 13835; Y. Kim, K. Koh, M. F. Roll, R. M. Laine, A. J. Matzger, Macromolecules 2010, 43, 6995 – 7000; W. Chaikittisilp, A. Sugawara, A. Shimojima, T. Okubo, Chem. Eur. J. 2010, 16, 6006 – 6014; Y. Wada, K. Iyoki, A. Sugawara-Narutaki, T. Okubo, A. Shimojima, Chem. Eur. J. 2013, 19, 1700 – 1705; Y. Peng, T. Ben, J. Xu, M. Xue, X. Jing, F. Deng, S. Qiu, G. Zhu, Dalton Trans. 2011, 40, 2720 – 2724. [8] H. Lin, J. Ou, Z. Zhang, J. Dong, H. Zou, Chem. Commun. 2013, 49, 231 – 233; H. Guo, M. A. B. Meador, L. McCorkle, D. J. Quade, J. Guo, B. Hamilton, M. Cakmak, G. Sprowl, ACS Appl. Mater. Interfaces 2011, 3, 546 – 552. [9] J. J. Ou, Z. B. Zhang, H. Lin, J. Dong, H. F. Zou, Anal. Chim. Acta 2013, 761, 209 – 216; M. Wu, R. a. Wu, R. Li, H. Qin, J. Dong, Z. Zhang, H. Zou, Anal. Chem. 2010, 82, 5447 – 5454. [10] I. Nischang, O. Brggemann, I. Teasdale, Angew. Chem. 2011, 123, 4688 – 4692; Angew. Chem. Int. Ed. 2011, 50, 4592 – 4596; F. Alves, P. Scholder, I. Nischang, ACS Appl. Mater. Interfaces 2013, 5, 2517 – 2526. [11] M. Dalwani, J. Zheng, M. Hempenius, M. J. T. Raaijmakers, C. M. Doherty, A. J. Hill, M. Wessling, N. E. Benes, J. Mater. Chem. 2012, 22, 14835 – 14838; C.-H. Lu, F.-C. Chang, ACS Catal. 2011, 1, 481 – 488; H. Ghanbari, B. G. Cousins, A. M. Seifalian, Macromol. Rapid Commun. 2011, 32, 1032 – 1046; H. Lin, J. Ou, S. Tang, Z. Zhang, J. Dong, Z. Liu, H. Zou, J. Chromatogr. A 2013, 1301, 131 – 138; D.-G. Kim, H. Kang, S. Han, J.-C. Lee, ACS Appl. Mater. Interfaces 2012, 4, 5898 – 5906. [12] R. Tamaki, J. Choi, R. M. Laine, Chem. Mater. 2003, 15, 793 – 797; C. Zhang, F. Babonneau, C. Bonhomme, R. M. Laine, C. L. Soles, H. A. Hristov, A. F. Yee, J. Am. Chem. Soc. 1998, 120, 8380 – 8391; N. Takamura, L. Viculis, C. Zhang, R. M. Laine, Polym. Int. 2007, 56, 1378 – 1391. [13] M. Malkoch, R. Vestberg, N. Gupta, L. Mespouille, P. Dubois, A. F. Mason, J. L. Hedrick, Q. Liao, C. W. Frank, K. Kingsbury, C. J. Hawker, Chem. Commun. 2006, 2774 – 2776; L. Wang, K. Zeng, S. Zheng, ACS Appl. Mater. Interfaces 2011, 3, 898 – 909; Y.-H. La, R. Sooriyakumaran, B. D. McCloskey, R. D. Allen, B. D. Freeman, R. Al-Rasheed, J. Membr. Sci. 2012, 401 – 402, 306 – 312. [14] I. Nischang, F. Svec, J. M. J. Frchet, Anal. Chem. 2009, 81, 7390 – 7396. Received: September 25, 2013 Published online: November 15, 2013
Chem. Eur. J. 2013, 19, 17310 – 17313 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org
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