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Bridged polyhedral oligomeric silsesquioxane (POSS): A potential member of silsesquioxanes
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Chinese Chemical Letters 23 (2012) 181–184 www.elsevier.com/locate/cclet
Bridged polyhedral oligomeric silsesquioxane (POSS): A potential member of silsesquioxanes Jian Kun Hu, Qun Chao Zhang, Shu Ling Gong * College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China Received 29 August 2011 Available online 22 December 2011
Abstract Two novel and well-defined polyhedral oligomeric silsesquioxanes (POSS) with two same Si8O12 cores and a reactive NH group, namely bridged-POSS (2a and 2b), have been prepared by the traditional ‘corner-capping’ reaction. X-ray diffraction demonstrates that those two POSS have the similar T8 structure. From the thermo-gravimetric analysis, bridged-POSS shows the better thermal degradation stability than the contrastive POSS. # 2011 Published by Elsevier B.V. on behalf of Chinese Chemical Society. Keywords: Polyhedral oligomeric silsesquioxane; Bridged POSS; The corner-capping reaction; Organic–inorganic hybrid
Polyhedral oligomeric silsesquioxanes (POSS) are a special class of nanosized cage-type structures, derived from hydrolysis and condensation of trifunctional organosilanes with a formula of [RSiO3/2]n (n: even integer from 6 to 12), and where R are various types of organic groups [1]. Since their first synthesis in 1946, these materials have received increasing concerns due to their fascinating structure and unique properties, including high thermal stability, oxygen permeability, low thermal conductivity and thermal expansion, dielectric permittivity, and surface energy that make them suitable for extensive applications such as excellent viscoelastic and good mechanical properties of the copolymers [2–5]. Preparation of POSS, especially the functional POSS, prove to be a great challenge in silicon chemistry [6]: that without some sort of templating or directing agent, there is nothing to limit how many different isomers are going to form, and it may be very difficult to separate those that do. Further complicating the matter is that there is no way to determine beforehand either what mixture of products of varying substitution will form or whether they will be separable [7–13]. However, one of the most successful ways to prepare POSS derivatives with different types of substituents is the corner-capping method, in which the functional trialkoxysilane is added to the trisilanol–POSS solution and the desired POSS is obtained by removing solution or precipitated in the indiscerptible solvent [14]. These trialkoxysilanes that have been employed in the preparation of functional POSS are amine, alkyl halide, epoxy, methacrylate, and vinyl compounds [15]. In comparison with these functional trialkoxysilanes, another special functional alkoxysilane-the bridged alkoxysilane such as bis(trimethoxysilylpropyl)amine HN[(CH2)3Si(OCH3)3]2, not obtain enough attention. As known, bridged alkoxysilane can easily be varied in length, rigidity, geometry of
* Corresponding author. E-mail address: [email protected] (S.L. Gong). 1001-8417/$ – see front matter # 2011 Published by Elsevier B.V. on behalf of Chinese Chemical Society. doi:10.1016/j.cclet.2011.11.006
182
J.K. Hu et al. / Chinese Chemical Letters 23 (2012) 181–184
substitution, and functionality [16]. Variation of its chemical nature is a powerful tool of the molecular design [17]. For the purpose of obtaining this variability, designing appropriate bridged alkoxysilane, devoted to the reaction of bridged alkoxysilane with partially condensed trisilanol–POSS in order to get novel bridged polyhedral oligosilsesquioxanes (bridged-POSS), may be an excellent candidates for broadening the scope of POSS. Bridged-POSS are a novel family of hybrid organic–inorganic materials, which possessing two Si8O12 cores communicated by a special organic bridging group just like a ‘bridge’. It was first synthesized in reasonable yield in 2004 by addition of an appropriate hexachlorodisiloxane Cl3SiOSiCl3 to 2 eq. of partially condensed trisilanol– cyclohexyl–POSS [18]. Unluckily, since the first preparation of the bridged-POSS, no novel or well-defined bridgedPOSS structures with organic groups were reported and bridged-POSS was almost neglected currently. Probably there are three reasons: (i) the difficulty of synthesis is further than that of common POSS, although which is already hard to prepare [6]; (ii) the starting material is not easy to obtain; a lot of solvent, two weeks reaction time and complex steps only get the desired product with low yield [19] and (iii) some potential bridged-POSS may be limited for its applications because all groups are inert [20]. We report here, two novel bridged POSS 2a and 2b (Scheme 1) with two same Si8O12 cores and a reactive NH group were prepared in a high yield. Through the ‘corner-capping’ reaction, it can be readily prepared from a commercially available organosilicon precursor trisilanolisobutyl–POSS (1a), trisilanolphenyl–POSS (1b) and bis(trimethoxysilylpropyl)amine, to get the desirable compound. Meanwhile, another well-defined POSS (3a) with only one Si8O12 core and a reactive NH group, was also prepared for contrast. Initially, we tried to obtain bridged POSS 2a in dry tetrahydrofuran with a temperature at 20 8C without any catalyst, as the hydrolysis and condensation of silanes having amino functions are autocatalyzed. It is a standard milder reaction, but takes a long reaction time (more than two days) with a yield of 32%. The subsequent reaction step is carried in the presence of tetraethyl ammonium hydroxide (Et4NOH), which provides convenient access to a wide variety of the single functional POSS [15]. But in this case the products are complicated mixtures of isomers detected by ESI–MS. Reducing the concentration of catalyst is useless. The possible reason is the hydrolytic condensation of methoxy group in the presence of strong base is so fast that leads to a variety of mixtures. And then, di-n-butyltin dilaurate was chosen as a catalyst considering that this catalyst is a well-known catalyst in silicone hydrolysis and condensation reactions. Most important, dibutyltin dilaurate catalyst has been recently to prepare octakis(3chloropropyl)-octasilsesquioxane conveniently [21]. After 24 h, bridged-POSS 2a was obtained with 51% yield. Another contrastive POSS (n-Bu-3-aminopropyl-POSS) (3a) was also prepared by the similar way. Bridged-POSS 2a has been confirmed by 1H, 13C, and 29Si NMR and ESI–MS. The results of ESI–MS provide the direct evidence that the target compound was formed (m/z = 1732 [M+H]+, 100%). Meanwhile, analysis by 1H and 13C NMR displayed peaks consistent with the desired product. Moreover, 29Si NMR analysis displayed three peaks at d
Scheme 1. The synthetic route of POSS.
J.K. Hu et al. / Chinese Chemical Letters 23 (2012) 181–184
Fig. 1.
183
29
Si NMR of 2a and 2b.
66.50, 66.86, 67.06, originating in a T structure was present in a ratio of 1:3:4 (Fig. 1). The presence of three singlets in the 29Si NMR confirms that bridged-POSS 2a has already been formed (see Supporting information). However, compound 2b could not be obtained satisfactorily by the above-mentioned solution system for the yield was too low (only 5%). In an effort to obtain good yield, dry toluene was used as a solvent instead of dry tetrahydrofuran because toluene show better solubility than THF for phenyl-POSS. Then alkoxysilane was added droply to the solution at 8 8C, the mixture was heated up to room temperature overnight to finish the reaction. Analysis by 1H, 13C NMR and 29Si NMR (Fig. 1) and ESI–MS displayed peaks consistent with the desired product. Fig. 2a shows the XRD pattern of three POSS. The X-ray pattern obtained for 2a has well-defined reflections found at 2u = 8.18, 10.88, 12.18, and 18.88, similar to those reported for others isobutyl-POSS [22]. According to the classical works on POSS structure, the crystallographic structure is described with a rhomboedral or hexagonal cell [23]. For a hexagonal cell, interplanar distances are related to Miller indices by
dh
k l
4 l2 2 2 ¼ ðh þ k þ hkÞ þ 3a2 c2
1=2 (1)
After refinement analysis, the peaks fitting according to Eq. (1) gave the other two fundamental hexagonal cell parameters, calculated as a = 1.59 nm and c = 1.69 nm. Such results are in quite good with the structure of isobutylPOSS reported by Turri and Levi [24]. Interestingly, the reflections of other two POSS (2b and 3a) completely overlapped with 2a, which indicates that these three POSS have same crystallographic structure.
Fig. 2. XRD (a) and TG (b) curves of three POSS.
J.K. Hu et al. / Chinese Chemical Letters 23 (2012) 181–184
184
Fig. 2b shows the thermogravimetric analysis (TA) of three POSS in nitrogen (10 8C/min). The initial thermal decomposition temperature (Td) was defined as the temperature at which 5% mass loss occurs, while the special values of 2a and 3a are 291 8C and 231 8C, respectively. It was worth noticing that the 50% mass loss of 2a with the thermal decomposition temperature was 388 8C, which was higher about 100 8C than 3a (284 8C). Moreover, the degradation remaining residues of the 2a and 3a at higher temperature were much different. The 3a sample was almost completely decomposed above 400 8C in nitrogen, but bridged-POSS 2a retained 20% of the initial mass at 800 8C. This result demonstrates that bridged-POSS has the more excellent thermal stability. From Fig. 2b, it is clearly shown that the thermal stability of 2b is higher than 2a, mainly due to the thermal stability of phenyl groups prior to isobutyl groups. Most important, the TG curves of 2a and 2b are somewhat distinctive, which indicated that the peripheral groups may affect the degradation mechanism of bridged-POSS under a nitrogen atmosphere. For 2a, the first loss of 51% beginning at 250 8C was attributed to cleavage of the peripheral arms attached to the POSS core, whereas the second weight loss beginning at 407 8C can be attributed to breaking down of the core Si–O structure [25]. For 2b, the mass loss of 59% from 470 8C up to 660 8C was attributed to cleavage of the pendant groups, and leaving 41% residues. In conclusion, we have prepared two novel bridged POSS (2a and 2b) with a single functional group by the traditional ‘corner-capping’ reaction. XRD proved that 2a and 2b have the same rhomboedral or hexagonal cell, whereas substitutional groups are much different. The results from TA indicated that thermal stability of 2a and 2b were higher than the contrastive POSS. What is more, the reactivity of its secondary amine (NH) group makes it an attractive synthetic platform for modifying hybrid polymers or other materials. Acknowledgments We are grateful to the National Natural Science Foundation of China (No. 20772092) and the Hubei Province Natural Science Fund for Distinguished Young Scholars (No. 2007ABB021) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2011.11.006. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Y.H. Liu, K. Zeng, S.X. Zheng, React. Funct. Polym. 67 (2007) 627. D.W. Scott, J. Am. Chem. Soc. 68 (1946) 356. S.A. Madbouly, J.U. Otaigbe, Prog. Polym. Sci. 34 (2009) 1283. R. Baney, M. Itoh, A. Sakakibara, et al. Chem. Rev. 95 (1995) 1409. J.M. Huang, H.J. Huang, Y.X. Wang, J. Polym. Sci. Part A: Polym. Phys. 47 (2009) 1927. D.B. Cordes, P.D. Lickiss, F. Rataboul, Chem. Rev. 110 (2010) 2081. J.D. Lichtenhan, J.J. Schwab, Y.Z. An, US 6972312B1, 2005. F.J. Feher, R. Terroba, J.W. Ziller, Chem. Commun. (1999) 2309. H. Liu, S.I. Kondo, N. Takeda, J. Am. Chem. Soc. 130 (2008) 10074. V. Ervithayasuporn, X. Wang, Y. Kawakami, Chem. Commun. (2009) 5130. H. Oikawa, K. Yoshida, K. Iwatani, et al., WO02/094839, 2002. Y. Morimoto, K. Ito, H. Oikawa, et al., US 2004030084, 2004. S.T. Iacono, A. Vij, W. Grabow, et al. Chem. Commun. (2007) 4992. J.J. Schwab, Y.Z. An, US 20070232823A1, 2007. J.D. Lichtenhan, J.J. Schwab, Y.Z. An, US 2003055193A1, 2003. P. Dieudonne, M.W. Man, B.P. Pichon, Small 5 (2009) 503. K.J. Shea, D.A. Loy, Chem. Mater. 13 (2001) 3306. S. Giessmann, A. Fischer, F.T. Edelmann, Z. Anorg. Allg. Chem. 630 (2004) 1982. F.J. Feher, T.A. Budzichowski, R.L. Blanski, Organometallics 10 (1991) 2526. J.T. Flodin, J.D. Lichtenhan, Y.Z. An, et al., US 2009176006A1, 2007. B. Marciniec, M. Dutkiewicz, H. Maciejewski, et al. Organometallics 27 (2008) 793. A.K. Nanda, D.A. Wicks, S.A. Madbouly, et al. Macromolecules 39 (2006) 7037. A.J. Waddon, E.B. Coughlin, Chem. Mater. 15 (2003) 4555. S. Turri, M. Levi, Macromolecules 38 (2005) 5569. D. Neumann, M. Fisher, L. Tran, et al. J. Am. Chem. Soc. 124 (2002) 13998.
Chinese Chemical Letters 23 (2012) 181–184 www.elsevier.com/locate/cclet
Bridged polyhedral oligomeric silsesquioxane (POSS): A potential member of silsesquioxanes Jian Kun Hu, Qun Chao Zhang, Shu Ling Gong * College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China Received 29 August 2011 Available online 22 December 2011
Abstract Two novel and well-defined polyhedral oligomeric silsesquioxanes (POSS) with two same Si8O12 cores and a reactive NH group, namely bridged-POSS (2a and 2b), have been prepared by the traditional ‘corner-capping’ reaction. X-ray diffraction demonstrates that those two POSS have the similar T8 structure. From the thermo-gravimetric analysis, bridged-POSS shows the better thermal degradation stability than the contrastive POSS. # 2011 Published by Elsevier B.V. on behalf of Chinese Chemical Society. Keywords: Polyhedral oligomeric silsesquioxane; Bridged POSS; The corner-capping reaction; Organic–inorganic hybrid
Polyhedral oligomeric silsesquioxanes (POSS) are a special class of nanosized cage-type structures, derived from hydrolysis and condensation of trifunctional organosilanes with a formula of [RSiO3/2]n (n: even integer from 6 to 12), and where R are various types of organic groups [1]. Since their first synthesis in 1946, these materials have received increasing concerns due to their fascinating structure and unique properties, including high thermal stability, oxygen permeability, low thermal conductivity and thermal expansion, dielectric permittivity, and surface energy that make them suitable for extensive applications such as excellent viscoelastic and good mechanical properties of the copolymers [2–5]. Preparation of POSS, especially the functional POSS, prove to be a great challenge in silicon chemistry [6]: that without some sort of templating or directing agent, there is nothing to limit how many different isomers are going to form, and it may be very difficult to separate those that do. Further complicating the matter is that there is no way to determine beforehand either what mixture of products of varying substitution will form or whether they will be separable [7–13]. However, one of the most successful ways to prepare POSS derivatives with different types of substituents is the corner-capping method, in which the functional trialkoxysilane is added to the trisilanol–POSS solution and the desired POSS is obtained by removing solution or precipitated in the indiscerptible solvent [14]. These trialkoxysilanes that have been employed in the preparation of functional POSS are amine, alkyl halide, epoxy, methacrylate, and vinyl compounds [15]. In comparison with these functional trialkoxysilanes, another special functional alkoxysilane-the bridged alkoxysilane such as bis(trimethoxysilylpropyl)amine HN[(CH2)3Si(OCH3)3]2, not obtain enough attention. As known, bridged alkoxysilane can easily be varied in length, rigidity, geometry of
* Corresponding author. E-mail address: [email protected] (S.L. Gong). 1001-8417/$ – see front matter # 2011 Published by Elsevier B.V. on behalf of Chinese Chemical Society. doi:10.1016/j.cclet.2011.11.006
182
J.K. Hu et al. / Chinese Chemical Letters 23 (2012) 181–184
substitution, and functionality [16]. Variation of its chemical nature is a powerful tool of the molecular design [17]. For the purpose of obtaining this variability, designing appropriate bridged alkoxysilane, devoted to the reaction of bridged alkoxysilane with partially condensed trisilanol–POSS in order to get novel bridged polyhedral oligosilsesquioxanes (bridged-POSS), may be an excellent candidates for broadening the scope of POSS. Bridged-POSS are a novel family of hybrid organic–inorganic materials, which possessing two Si8O12 cores communicated by a special organic bridging group just like a ‘bridge’. It was first synthesized in reasonable yield in 2004 by addition of an appropriate hexachlorodisiloxane Cl3SiOSiCl3 to 2 eq. of partially condensed trisilanol– cyclohexyl–POSS [18]. Unluckily, since the first preparation of the bridged-POSS, no novel or well-defined bridgedPOSS structures with organic groups were reported and bridged-POSS was almost neglected currently. Probably there are three reasons: (i) the difficulty of synthesis is further than that of common POSS, although which is already hard to prepare [6]; (ii) the starting material is not easy to obtain; a lot of solvent, two weeks reaction time and complex steps only get the desired product with low yield [19] and (iii) some potential bridged-POSS may be limited for its applications because all groups are inert [20]. We report here, two novel bridged POSS 2a and 2b (Scheme 1) with two same Si8O12 cores and a reactive NH group were prepared in a high yield. Through the ‘corner-capping’ reaction, it can be readily prepared from a commercially available organosilicon precursor trisilanolisobutyl–POSS (1a), trisilanolphenyl–POSS (1b) and bis(trimethoxysilylpropyl)amine, to get the desirable compound. Meanwhile, another well-defined POSS (3a) with only one Si8O12 core and a reactive NH group, was also prepared for contrast. Initially, we tried to obtain bridged POSS 2a in dry tetrahydrofuran with a temperature at 20 8C without any catalyst, as the hydrolysis and condensation of silanes having amino functions are autocatalyzed. It is a standard milder reaction, but takes a long reaction time (more than two days) with a yield of 32%. The subsequent reaction step is carried in the presence of tetraethyl ammonium hydroxide (Et4NOH), which provides convenient access to a wide variety of the single functional POSS [15]. But in this case the products are complicated mixtures of isomers detected by ESI–MS. Reducing the concentration of catalyst is useless. The possible reason is the hydrolytic condensation of methoxy group in the presence of strong base is so fast that leads to a variety of mixtures. And then, di-n-butyltin dilaurate was chosen as a catalyst considering that this catalyst is a well-known catalyst in silicone hydrolysis and condensation reactions. Most important, dibutyltin dilaurate catalyst has been recently to prepare octakis(3chloropropyl)-octasilsesquioxane conveniently [21]. After 24 h, bridged-POSS 2a was obtained with 51% yield. Another contrastive POSS (n-Bu-3-aminopropyl-POSS) (3a) was also prepared by the similar way. Bridged-POSS 2a has been confirmed by 1H, 13C, and 29Si NMR and ESI–MS. The results of ESI–MS provide the direct evidence that the target compound was formed (m/z = 1732 [M+H]+, 100%). Meanwhile, analysis by 1H and 13C NMR displayed peaks consistent with the desired product. Moreover, 29Si NMR analysis displayed three peaks at d
Scheme 1. The synthetic route of POSS.
J.K. Hu et al. / Chinese Chemical Letters 23 (2012) 181–184
Fig. 1.
183
29
Si NMR of 2a and 2b.
66.50, 66.86, 67.06, originating in a T structure was present in a ratio of 1:3:4 (Fig. 1). The presence of three singlets in the 29Si NMR confirms that bridged-POSS 2a has already been formed (see Supporting information). However, compound 2b could not be obtained satisfactorily by the above-mentioned solution system for the yield was too low (only 5%). In an effort to obtain good yield, dry toluene was used as a solvent instead of dry tetrahydrofuran because toluene show better solubility than THF for phenyl-POSS. Then alkoxysilane was added droply to the solution at 8 8C, the mixture was heated up to room temperature overnight to finish the reaction. Analysis by 1H, 13C NMR and 29Si NMR (Fig. 1) and ESI–MS displayed peaks consistent with the desired product. Fig. 2a shows the XRD pattern of three POSS. The X-ray pattern obtained for 2a has well-defined reflections found at 2u = 8.18, 10.88, 12.18, and 18.88, similar to those reported for others isobutyl-POSS [22]. According to the classical works on POSS structure, the crystallographic structure is described with a rhomboedral or hexagonal cell [23]. For a hexagonal cell, interplanar distances are related to Miller indices by
dh
k l
4 l2 2 2 ¼ ðh þ k þ hkÞ þ 3a2 c2
1=2 (1)
After refinement analysis, the peaks fitting according to Eq. (1) gave the other two fundamental hexagonal cell parameters, calculated as a = 1.59 nm and c = 1.69 nm. Such results are in quite good with the structure of isobutylPOSS reported by Turri and Levi [24]. Interestingly, the reflections of other two POSS (2b and 3a) completely overlapped with 2a, which indicates that these three POSS have same crystallographic structure.
Fig. 2. XRD (a) and TG (b) curves of three POSS.
J.K. Hu et al. / Chinese Chemical Letters 23 (2012) 181–184
184
Fig. 2b shows the thermogravimetric analysis (TA) of three POSS in nitrogen (10 8C/min). The initial thermal decomposition temperature (Td) was defined as the temperature at which 5% mass loss occurs, while the special values of 2a and 3a are 291 8C and 231 8C, respectively. It was worth noticing that the 50% mass loss of 2a with the thermal decomposition temperature was 388 8C, which was higher about 100 8C than 3a (284 8C). Moreover, the degradation remaining residues of the 2a and 3a at higher temperature were much different. The 3a sample was almost completely decomposed above 400 8C in nitrogen, but bridged-POSS 2a retained 20% of the initial mass at 800 8C. This result demonstrates that bridged-POSS has the more excellent thermal stability. From Fig. 2b, it is clearly shown that the thermal stability of 2b is higher than 2a, mainly due to the thermal stability of phenyl groups prior to isobutyl groups. Most important, the TG curves of 2a and 2b are somewhat distinctive, which indicated that the peripheral groups may affect the degradation mechanism of bridged-POSS under a nitrogen atmosphere. For 2a, the first loss of 51% beginning at 250 8C was attributed to cleavage of the peripheral arms attached to the POSS core, whereas the second weight loss beginning at 407 8C can be attributed to breaking down of the core Si–O structure [25]. For 2b, the mass loss of 59% from 470 8C up to 660 8C was attributed to cleavage of the pendant groups, and leaving 41% residues. In conclusion, we have prepared two novel bridged POSS (2a and 2b) with a single functional group by the traditional ‘corner-capping’ reaction. XRD proved that 2a and 2b have the same rhomboedral or hexagonal cell, whereas substitutional groups are much different. The results from TA indicated that thermal stability of 2a and 2b were higher than the contrastive POSS. What is more, the reactivity of its secondary amine (NH) group makes it an attractive synthetic platform for modifying hybrid polymers or other materials. Acknowledgments We are grateful to the National Natural Science Foundation of China (No. 20772092) and the Hubei Province Natural Science Fund for Distinguished Young Scholars (No. 2007ABB021) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2011.11.006. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Y.H. Liu, K. Zeng, S.X. Zheng, React. Funct. Polym. 67 (2007) 627. D.W. Scott, J. Am. Chem. Soc. 68 (1946) 356. S.A. Madbouly, J.U. Otaigbe, Prog. Polym. Sci. 34 (2009) 1283. R. Baney, M. Itoh, A. Sakakibara, et al. Chem. Rev. 95 (1995) 1409. J.M. Huang, H.J. Huang, Y.X. Wang, J. Polym. Sci. Part A: Polym. Phys. 47 (2009) 1927. D.B. Cordes, P.D. Lickiss, F. Rataboul, Chem. Rev. 110 (2010) 2081. J.D. Lichtenhan, J.J. Schwab, Y.Z. An, US 6972312B1, 2005. F.J. Feher, R. Terroba, J.W. Ziller, Chem. Commun. (1999) 2309. H. Liu, S.I. Kondo, N. Takeda, J. Am. Chem. Soc. 130 (2008) 10074. V. Ervithayasuporn, X. Wang, Y. Kawakami, Chem. Commun. (2009) 5130. H. Oikawa, K. Yoshida, K. Iwatani, et al., WO02/094839, 2002. Y. Morimoto, K. Ito, H. Oikawa, et al., US 2004030084, 2004. S.T. Iacono, A. Vij, W. Grabow, et al. Chem. Commun. (2007) 4992. J.J. Schwab, Y.Z. An, US 20070232823A1, 2007. J.D. Lichtenhan, J.J. Schwab, Y.Z. An, US 2003055193A1, 2003. P. Dieudonne, M.W. Man, B.P. Pichon, Small 5 (2009) 503. K.J. Shea, D.A. Loy, Chem. Mater. 13 (2001) 3306. S. Giessmann, A. Fischer, F.T. Edelmann, Z. Anorg. Allg. Chem. 630 (2004) 1982. F.J. Feher, T.A. Budzichowski, R.L. Blanski, Organometallics 10 (1991) 2526. J.T. Flodin, J.D. Lichtenhan, Y.Z. An, et al., US 2009176006A1, 2007. B. Marciniec, M. Dutkiewicz, H. Maciejewski, et al. Organometallics 27 (2008) 793. A.K. Nanda, D.A. Wicks, S.A. Madbouly, et al. Macromolecules 39 (2006) 7037. A.J. Waddon, E.B. Coughlin, Chem. Mater. 15 (2003) 4555. S. Turri, M. Levi, Macromolecules 38 (2005) 5569. D. Neumann, M. Fisher, L. Tran, et al. J. Am. Chem. Soc. 124 (2002) 13998.