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A Well-Defined Ladder Polyphenylsilsesquioxane (Ph-LPSQ) Synthesized via a New Three-Step Approach: Monomer Self-Organization-Lyophilization—Surface-Confined Polycondensation Zhong-Xing Zhang, Jinkun Hao, Ping Xie,* Xiaojing Zhang, Charles C. Han,* and Rongben Zhang* Beijing National Laboratory for Molecular Sciences (BNLMS), Joint Laboratory of Polymer Sciences and Materials, State Key Laboratory of Polymer Physics and Chemistry, The Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed June 16, 2007. ReVised Manuscript ReceiVed NoVember 16, 2007
A high-molecular-weight and well-defined ladder polyphenylsilsesquioxane (Ph-LPSQ) was synthesized via a new three-step approach: monomer self-organization in solution, lyophilization, and surface-confined polycondensation. A ladder superstructure, which served as a template to direct the polycondensation, was self-assembled from the 1,3-diphenyl-tetrahydroxy-disiloxane monomer (M) in acetonitrile solution. Following that, it was lyophilized to form a thin layer on the inner surface of a flask. Subsequently, polycondensation of the ordered monomeric thin layer was performed under a triethylamine (TEA) atmosphere. This strategy increased the ladder regularity of the Ph-LPSQ by preventing common complications faced in solution polycondensation of silanol-containing monomers, such as cyclization and gelation side reactions. 29Si NMR analysis showed a very narrow peak (peak width at half-height, w1/2 ) 2.5 ppm) at δ ) –78.5 (corresponding to a Ph-SiO3/2unit), indicating a high degree of regularity of the polymer structure.
Introduction Ladder polymers are of particular interest in the field of advanced materials in light of their superior chemical and physical properties.1 Recently, silicon-based ladder polymers have been widely investigated. These ladder polysiloxanes (LPSs), such as oxygen-bridged ladder polysilsesquioxanes (LPSQs) and organo-bridged ladder polysiloxanes (OLPSs) (Chart 1), possess excellent thermal stability, radiationresistance, and good mechanical properties due to the special double-chained molecular structure and high bond energy of the Si-O linkage.2 These polymers are potential candidates for the fabrication of hybrid organic–inorganic nanocomposites. The development of these nanocomposites possessing unique and valuable properties via soft chemical processes has been a popular area of research in recent years.3 Synthesis of LPSQs with perfect double-chained skeletons, (RSiO3/2)n, from multifunctional silicon-containing monomers has been the subject of intense research by polymer scientists * To whom correspondence should be addressed. Tel.: 86-10-62565612(P.X.); 86-10-62618089(C.C.H.); 86-10-62565612(R.Z.). Fax:86-10-62559373 (P.X.); 86-10-62559373 (C.C.H.); 86-10-62559373 (R.Z.). E-mail:
[email protected] (R.B.);
[email protected] (C.C.H.).
(1) Bailey, W. J. In Concise Encyclopedia of Polymer Science & Engineering; Kroschwitz, J. I., Eds.; John Willey & Sons: New York, 1990; pp 516–528. (2) (a) Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry, Willey-Interscience: New York, 2000, pp 309–331. (b) Zhang, R. B.; Dai, D. R.; Cui, L.; Xu, H.; Liu, C. Q.; Xie, P. Mater. Sci. Eng., C 1999, 3, 13–18. (3) (a) Sanchez, C.; Soler-Illia, G. J. de. A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061–3083. (b) Sanchez, C.; Arribart, H.; Guille, M. M. G. Nat. Mater. 2005, 4, 277–288.
for the past four decades.4 However, this target has not been easily achieved because of the fact that some uncontrolled side reactions were encountered in solution polycondensation of silanol-containing monomers, such as cyclization and gelation reactions. This resulted in complicated products with cyclic, ladder, cage, cube, and cross-linked structures.5 As Brook pointed out, the regularity of the structures of the LPSQs depended greatly on the methods used for their preparation. In particular, he concluded that high-molecularweight LPSQs were generally random networks.2a These irregular polysilsesquioxane have poor mechanical properties and would be ill-suited for the production of highperformance materials. To synthesize regular LPSs, Zhang et al. proposed a supramolecular template strategy named “supramolecular architecture-directed stepwise coupling/polymerization”,2b by which a series of ordered OLPSs6 and LPSQs7 have been prepared. The crux of this strategy focuses on the templatedirected confined synthesis (by H-bonding, π-π stacking, etc.) via formation of ladder superstructure. The regularity (4) (a) Brown, F., Jr.; Vogt, L. H., Jr.; Katchman, A.; Eustance, J. W.; Kiser, K. M.; Krantz, K. W. J. Am. Chem. Soc. 1960, 82, 6194–6195. (b) Frey, C. L.; Klosowski, J. M. J. Am. Chem. Soc. 1971, 93, 4599– 4601. (c) Unno, M.; Suto, A.; Matsumoto, H. J. Am. Chem. Soc. 2002, 124, 1574–1575. (5) Abe, Y. Gunji T. Prog. Polym. Sci. 2004, 29, 149–182. (6) (a) Zhang, T.; Deng, K.; Zhang, P.; Xie, P.; Zhang, R. B. Chem.sEur. J. 2006, 12, 3630–3635. (b) Wan, Y.; Yang, L.; Zhang, J.; Zhang, P.; Fu, P.-F.; Zhang, T.; Xie, P.; Zhang, R. B. Macromolecules 2006, 39, 541–547. (c) Tang, H.; Sun, J.; Jiang, J.; Zhou, X.; Hu, T.; Xie, P.; Zhang, R. B. J. Am. Chem. Soc. 2002, 124, 10482–10488.
10.1021/cm071602l CCC: $40.75 2008 American Chemical Society Published on Web 01/10/2008
Ph-LPSQ Via New Three-Step Approach
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Chart 1. Schematic Structures of Two Kinds of Ladder Polysiloxanes (LPSs): (a) Oxygen-Bridged Ladder Polysilsesquioxane (LPSQ) and (b) Organo-Bridged Ladder Polysiloxane (OLPS)
of the resultant LPSs depended greatly on strength and stability of the template. On the basis of this strategy, we recently proposed a new approach based on the self-organization of R,ω-ditriphenylene tetrahydroxy-disiloxane monomer by concerted π-π stacking and H-bonding to yield a highly regular ladder superstructure. A soluble, high-molecular-weight and perfect ladder polysilsesquioxane with triphenylene side group was prepared by dehydration condensation.7a In this case, introduction of bulky electrorich triphenylene groups notably enhanced the template effects and resulted in a supramolecular channel for confined polycondensation. However, this approach is not universally suited for the synthesis of common LPSQs with phenyl or methyl side groups. Here, we report a new approach to the synthesis of PhLPSQ as shown in Figure 1. On the basis of this approach, the obtained product has a high molecular weight and a highly regular structure. Phenyltrimethoxysilane was subjected to controlled hydrolysis and condensed to yield 1,3diphenyl-tetrahydroxy-disiloxane monomer (M). Based on X-ray diffraction (XRD) analysis of the lyophilized samples of M on glass slides, we found that ordered ladder superstructures were formed from the self-assembly of monomers via silanol’s square-planar (parallelogram-type) multi-Hbonding interactions in acetonitrile or acetonitrile/toluene solutions. However, solution polycondensation of this monomer resulted only in the formation of ladder polymer with low structure regularity and low molecular weight. In addition, high fractions of side-products, such as cyclooligomers, cages, and random gels, were found in the final products (Table 1). The reason could be attributed to the lower template effect of ladder superstructure in solution polycondensation and strong cyclization tendency of the siloxane monomers. Hereby combining lyophilization and (7) (a) Zhang, X.; Xie, P.; Shen, Z.; Jiang, J.; Zhu, C.; Li, H.; Zhang, T.; Han, C. C.; Wan, L.; Yan, S.; Zhang, R. B. Angew. Chem., Int. Ed. 2006, 45, 3112–3116. (b) Deng, K.; Zhang, T.; Zhang, X.; Zhang, A.; Xie, P.; Zhang, R. B. Macromol. Chem. Phys. 2006, 207, 404– 411. (c) Liu, C.; Liu, Y.; Shen, Z.; Xie, P.; Zhang, R. B.; Yang, J.; Bai, F. Macromol. Chem. Phys. 2001, 202, 1581–1585.
surface confined synthesis,8 we propose a new three-step approach: “monomer self-organization-lyophilization-surfaceconfined polycondensation” for the synthesis of well-defined highly regular Ph-LPSQ. In the first step, ladder superstructure was formed by self-assembly of Ms in an acetonitrile solution. In the second step, lyophilization was realized by rotating a flask containing monomer/acetonitrile solution, while the flask was immersed in liquid nitrogen. This results in the formation of a continuous thin layer on the inner surface of the rotating flask (Figure 1). In the third step, the self-assembled ladder superstructure immobilized in the solid thin layer was further converted into covalent ladder polymer by dehydrating polycondensation under TEA atmosphere. The rotation of the flask induced the orientation of ladder superstructures. Lyophilization fixed their orientation and structure. These factors promoted the confined polycondensation and prevented the cyclization and gelation side reactions, resulting in the formation of a soluble, highmolecular-weight, and highly regular Ph-LPSQ. Experimental Section Materials. All reagents are commercially available. Toluene, 1,4dioxane (DOX) were distilled over sodium and benzophenone. Triethylamine (TEA) was distilled over potassium hydroxide. Acetonitrile was dried with CaH2 and distilled over P2O5. Phenyltrimethoxylsilane was obtained by methanolysis of the commercially available phenyltrichlorosilane. Instrumentation. Fourier transform infrared (FTIR) spectra were recorded using a Bruker Tensor 27 spectrophotometer in the range of 400–4000 cm-1 (KBr pellets) at 25 °C. 1H and 29Si nuclear magnetic resonance (1H NMR and 29Si NMR) spectra were obtained with a Bruker DX300 spectrometer at 300 and 59.6 MHz, respectively, at room temperature. For 29Si NMR measurements, acetonitrile and toluene were used as the solvents for monomer and polymer, respectively. For 1H NMR measurements, tetramethylsilane (TMS) was used as the reference (0 ppm) to obtain all other chemical shifts. Matrix assisted laser desorption/ionizationtime-of-flight (MALDI-TOF) and electron spray ionization (ESI) (8) (a) Keisuke, T.; Takuzo, A. Chem. Commun. 2000, 24, 2399–2412. (b) Schmidt, I.; Madsen, C.; Jacobsen, C. J. H. Inorg. Chem. 2000, 39, 2279–2283. (c) Lin, S.; McCarley, R. L. Langmuir 1999, 15, 151– 159. (d) Gopireddy, D.; Husson, S. M. Macromolecules 2002, 35, 4218–4221.
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Figure 1. (a) Synthetic route to 1,3-diphenyl-tetrahydroxy-disiloxane monomer. (b) Schematic representation of the preparation of Ph-LPSQ via monomer self-organization-lyophilization-surface-confined polycondensation. Table 1. Selected Comparison Data for Preparation of Ph-LPSQ under Different Reaction Conditions reaction system synthesis route approach (1)
approach (2)
solvent CH3CN dioxane CH3CN:toluene (1:2) CH3CN: dioxane (1:2) CH3CN:toluene:dioxane (2:4:1) CH3CN:toluene (1:2)c,d CH3CN c
fractions of the products (%)
Ph-LPSQ
CMa (mol/L)
Ph-LPSQ
cages, cyclo-oligomers
gel
w1/2b (ppm)
0.35e 0.35 0.20 e 0.35 0.35 0.20e 0.35e
37.5 29.6 55.2 39.7 52.4 90.4 >99
34.1 35.2 25.1 29.8 23.2 9.6
