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Catalytic Effect of Aluminum Acetylacetonate on Hydrolysis and Polymerization of Methyltrimethoxysilane Zuyi Zhang New Glass Research Center, Yamamura Glass Co., 2-1-18 Naruohama, Nishinomiya, Hyogo 663, Japan Received August 5, 1996. In Final Form: November 11, 1996X The hydrolysis of methyltrimethoxysilane (MTMS) and the polymerization of the silanols in acetone solutions have been investigated by infrared spectroscopy. The hydrolysis is monitored by using an absorption at 840 cm-1 due to methoxy groups and the polymerization by using one at 900 cm-1 due to silanol groups. When HNO3 is used as the hydrolysis catalyst, the rate of hydrolysis decreases upon adding aluminum acetylacetonate (Al(acac)3), and the polymerization proceeds to a high degree. In the absence of acid catalyst, Al(acac)3 plays a catalytic role in the hydrolysis. These catalytic effects are discussed by assuming a transfer of proton between Al(acac)3 and hydroxyl-containing species in the solutions.
Introduction
Table 1. Initial Molar Ratios of the Starting Materials in the Four Samples
In sol-gel processes from silicon alkoxides,1 an acid or base catalyst is usually used for the hydrolysis and polymerization. Iler described the acid-catalyzed and base-catalyzed mechanisms for the polymerization of Si(OH)4.2 These catalysts are also used for the polymerization of alkylsilanetriols3,4 and alkylsilanediols.5 In addition, organotin compounds are employed as the catalyst for the polymerization of siloxane.6 Recently, a catalytic effect of acetylacetonates was found in the gel formation of CH3SiO3/2 from methyltriethoxysilane.7 With the introduction of such complexes, cagelike polycondensation products disappeared at the early stage and the subsequent gelation behavior changed greatly compared with an undoped sample. These results indicate that, metal β-diketonates are very useful in controlling the polymerization of siloxane. The details of hydrolysis and polymerization in the presence of such complexes, however, have not been made clear yet. In this study, the hydrolysis of methyltrimethoxysilane and the polycondensation of silanol groups in acetone solutions were investigated by infrared spectroscopy with the intent of clarifying the catalytic effect of acetylacetonates. Acetone was used as the solvent instead of alcohol, because alcohol shows a strong absorption band near the asymmetric vibration of Si-O-Si8 and also causes difficulty in discriminating the alcohol arising from the hydrolysis.
purification was performed. Aluminum acetylacetonate (Al(acac)3) was from Dojindo Laboratories. These starting materials were examined in the form of their respective CS2 solutions by infrared spectroscopy, and no sign of OH bonds was observed. For N1 and N2 samples, an acetone solution of methyltrimethoxysilane and another one containing aluminum acetylacetonate (Al(acac)3) were prepared.10 Then, water was added into the two solutions in less than 5 s. The preparation procedures of H1 and H2 solutions were the same as those of N1 and N2, except for using an aqueous nitric acid in place of distilled water. Each infrared spectrum was measured on an FTIR spectrometer (Perkin-Elmer, Pragon 1000 PC) by scanning 10 times with a resolution of 2 cm-1. A liquid cell with RS-5 windows and a variable pathlength was used. The pathlength was 0.1 mm. No further dilution was performed for the measurements. All the samples were kept at 20 ( 2 °C under closed conditions except for the infrared spectral measurements.
Experimental Section
Results
Four solutions were used in the present study, and their initial compositions are shown in Table 1. Reagent grade acetone and distilled water were used. Methyltrimethoxysilane was from Shin-Etsu Chemical Co., and no further X Abstract published in Advance ACS Abstracts, January 15, 1997.
(1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990; Chapter 4. (2) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (3) Brown, J. F., Jr. J. Polym. Sci. 1962, C1, 83. (4) Pohl, E. R.; Osterholtz, F. D. In Molecular Characterization of Composite Interfaces; Ishida, H., Kumar, G., Eds.; Plenum: New York, 1985; p 157. (5) Lasocki, Z.; Chrzczonowicz, S. J. Polym. Sci. 1962, 59, 259. (6) Van der Weij, F. W. Makromol. Chem. 1980, 181, 2541. (7) Zhang, Z.; Tanigami, Y.; Terai, R. J. Non-Cryst. Solids 1995, 191, 304. (8) Ja¨glid, U; Lindqvist, O. Acta Chem. Scand. 1990, 44, 765. (9) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; Wiley: New York, 1981.
N1 N2 H1 H2 H3
MTMS
H2O
HNO3
Al(acac)3
CH3COCH3
1 1 1 1 1
3 3 3 3 3
0 0 0.001 0.001 0.001
0 0.005 0 0.005 0.0025
35 35 35 35 35
The spectra of the N2 sample at various times are shown in Figure 1 with that of a sample without water. Similar spectral changes were observed in H1 and H2 samples. Figures 2 and 3 show the spectra of H1 and H2 after reaction for 4 and 170 h, respectively. The spectrum of the N1 sample was essentially unchanged up to 100 h (not shown). To clarify the respective origins of the main peaks, the spectra of MTMS and acetone were measured under the same measurement conditions. A peak at 900 cm-1 appeared in acetone, but not in MTMS. MTMS showed a strong peak at 1090 cm-1, which was beyond the upper limit of the spectrometer. Acetone also showed a peak in the same region, which absorbance was about 1. At the (10) In applications as an optical material, a low concentration of Al(acac)3 is preferred, because of a strong absorption band at 280 nm. Although this complex appears very stable under a normal condition, it may be decomposed with exposure to a strong UV irradiation (Tohge, N.; et al. J. Sol-Gel Sci. Technol. 1994, 2, 581).
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Figure 4. Peak areas as a function of time. Open and closed circles indicate those of the N1 sample at 900 and 840 cm-1, respectively. Open and closed triangles show those of N2 sample at 900 and 840 cm-1, respectively. Open and closed squares indicate the intensities at 900 and 840 cm-1 prior to the reaction. The solvent, acetone contributes a peak area of ∼14 cm-1 to the absorption at 900 cm-1. Figure 1. Infrared spectra of the N2 sample at various times and a sample without H2O. The absorption scale refers to the sample without water.
Figure 5. Peak areas as a function of time. Open and closed circles indicate those of H1 sample at 900 and 840 cm-1, respectively. Open and closed triangles show those of H2 sample at 900 and 840 cm-1, respectively. The solvent, acetone, contributes a peak area of ∼14 cm-1 to the absorption at 900 cm-1. Figure 2. Infrared spectra of H1 and H2 samples after reaction for 4 h. The absorption scale refers to that of the H1 sample.
Figure 3. Infrared spectra of H1 and H2 samples after reaction for 170 h. The absorption scale refers to that of the H1 sample.
early stage, the strong peak at 1090 cm-1 in these solutions arises mainly from MTMS, and the peak at 900 cm-1 from acetone. Two peaks at 840 and 790 cm-1 are attributed to MTMS, because they were not observed in acetone. No appreciable spectral change was observed after addition of Al(acac)3 in the same molar ratio. The absorptions from Al(acac)3 are discarded in the present system. In Figure 1, a peak appears at 1030 cm-1 and increases in intensity with time. Correspondingly, the peak at 840 cm-1 decreases monotonically. The absorption at 1030 cm-1 is assigned to methanol,9 which arises from the hydrolysis. The peak at 900 cm-1 increases initially in intensity and decreases at the later stage. In addition to acetone, the stretching vibration of Si-OH contributes to this absorption.11 These spectral changes indicate that
Si-OH bonds in the solution increase with the hydrolysis and then decrease with their polymerization.12 It was reported that a broad peak from 1000 to 1200 cm-1 due to the asymmetric stretching vibration of Si-O-Si13,14 and a peak at 790 cm-1 appeared in CH3SiO3/2 gel.11 In Figure 1, an increase in the two peaks can be noticed in the spectrum of 48 h, indicating the formation of siloxane bonds. This tendency was observed in the H2 sample after more than 50 h. In the N1 sample, the hydrolysis occurred very slowly. Some precipitates, however were observed after 2 months. In the infrared spectrum of the upper solution, the peak at 1030 cm-1 became noticeable, but there were not remarkable changes in the other absorption bands. In order to evaluate the hydrolysis and the polymerization kinetics from the two peaks at 840 and 900 cm-1, four Gaussian peaks were used to fit the three absorption bands from 770 to 920 cm-1: two peaks for 900, one for 840, and one for 790 cm-1, respectively. The results are shown in Figures 4 and 5.15 In Figure 4, the difference between N1 and N2 samples indicates that Al(acac)3 promotes the hydrolysis of MTMS. In Figure 5, however, it is inferred that the addition of Al(acac)3 causes a decrease in the hydrolysis rate from the change in the decrease of (11) Zhang, Z.; Tanigami, Y.; Terai, R. J. Sol-Gel Sci. Technol. 1996, 6, 273. (12) Corresponding to these changes, there was a monotonic increase at 3500 cm-1 with the propagation of reactions. However, this O-H stretching vibration band appeared very broad (3000-3800 cm-1), and it was difficult to draw the direct information of SiOH, CH3OH, and H2O from this band. (13) Brown, J. F., Jr. J. Am. Chem. Soc. 1965, 87, 4317. (14) Voronkov, M. G.; Lavrent'yev, V. I. Top. Curr. Chem. 1982, 102, 199. (15) The error in the peak areas is expected to be less than (2, which arises from a change in the baseline.
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the peak area of 840 cm-1.16 Meantime, the content of Si-OH bonds is relatively low in the H2 sample, indicating the enhancement of the polymerization of silanols. Discussion The interactions between metal β-diketone complexes and other molecules have been reviewed by Siedle.17 Coordinatively unsaturated ones can react to form adducts with Lewis bases such as pyridine, H2O, etc. Hydrogen bonding between these complexes and donors such as methanol and water has been detected by proton NMR and infrared spectroscopy.18-20 In the present solutions, similar interactions are assumed to occur between hydroxyl-containing species and Al(acac)3.
Al(acac)3 + +H--O-[ ] f Al(acac)3 H+‚‚‚-O-[ ] (1) Namely, this complex tends to accept a proton from other molecule to form a weak complex with it.17 In fact, it was found that pH of an aqueous alcohol solution of nitric acid increased with an introduction of Al(acac)3.21 Sedler and Sayer reported a similar change in pH in solutions of metal alkoxides modified by acac- anions.22 This mechanism is also supported by the fact that the chelate rings could be deuterated on the central atom in an acidic D2O solution.23 Such acid-base processes associated with Al(acac)3 are proposed to be responsible for the changes in the hydrolysis and polymerization. 1. Hydrolysis of Methyltrimethoxysilane. Under acidic conditions, the hydrolysis of silicon alkoxides has been proposed to take place by protonation of alkoxide and a subsequent attack of molecular water on the silicon.4,24 This hypothesis is consistent with a general tendency that the hydrolysis kinetics increases with a decrease of pH under acidic conditions. Hence, the difference between H1 and H2 samples probably results from the decrease in the acidity of the solution due to the addition of Al(acac)3. To confirm this mechanism, the initial hydrolysis behavior was investigated on a sample (H3), in which Al(acac)3/MTMS was reduced to 0.0025. As expected, the peak at 900 cm-1 appeared large compared with the H2 sample after reaction for 4 h, but remained low compared with the H1 sample. The hydrolysis in the N2 sample is attributed to added Al(acac)3, because no acid is introduced in this sample. When hydrogen bonding occurs between Al(acac)3 and H2O as mentioned above, a intramolecular transfer of proton in the weak complex may take place occasionally.25 This mechanism increases the possibility of the electrolytic dissociation of H2O, which may be responsible for the catalytic effect of Al(acac)3 in (16) After reaction for more than 10 h, the peak area of 840 cm-1 of the H2 sample becomes a little small compared with that of the H1 sample. We believe that this change results from a higher rate of polymerization, which causes changes in the concentrations of silanols and water. (17) Siedle, A. R. In Comprehensive Coordination Chemistry; Wilkinson, S. G., Ed.; Pergamon: Oxford, 1987; Vol. 2, Chapter 15.4. (18) Davis, T. S.; Fackler, J. P., Jr. Inorg. Chem. 1966, 5, 242. (19) Vigee, G. S.; Watkins, C. L. Inorg. Chem. 1977, 16, 709. (20) Sievers, R. E. (Ed.) Nuclear Magnetic Shift Reagents; Academic: New York, 1973. (21) pH of an ethanol solution of HNO3 (5 × 10-4 M) was measured by a pH meter. It increased from 2.4 to 4.7 with increasing the concentration of Al(acac)3 from 0 to 1 × 10-3 M. (22) Sedlar, M.; Sayer, M. J. Sol-Gel Sci. Technol. 1995, 5, 27. (23) Collman, J. P. Angew. Chem., Int. Ed. Engl. 1965, 4, 132. (24) McNeill, K. J.; Dicaprio, J. A.; Walsh, D. A.; Pratt, R. F. J. Am. Chem. Soc. 1980, 102, 1859. (25) With an increase of the concentration of Al(acac)3 from 0 to 1 × 10-3 M in an aqueous ethanol solution (containing 2 vol % of distilled water), pH changed from 6.1 to 8, indicating the electrolytic dissociation of H2O. Also, such a complex is inferred to cause the deprotonation of silanols in the solutions.
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the N2 sample. An intramolecular transfer of proton is also considered to take place between two oxygens in the chelating agent, acetylacetone.17 No remarkable change, however, was found in the hydrolysis kinetics after adding acetylacetone into the N1 sample. Therefore, a transfer of protons from other molecules may be necessary for this catalytic effect. In addition, it should noted that pH decreases greatly with the formation of Si-OH bonds in the present system. After reaction for 4 h, the pH’s of N1 and N2 samples were 6.0 and 2.5, respectively. From the point of view of the effect of pH, a self-catalyzed mechanism may be possible, which influences the hydrolysis at the later stage. This seems to account for the fact that there was no remarkable difference in the hydrolysis kinetics between N2 and H2 samples. While, the difference between N1 and N2 samples indicates that the interaction expressed in eq 1 should play an important role in the hydrolysis at the early stage. 2. Polymerization of the Silanol Groups. According to Brown,13 alkylsilanetriol undergoes polycondensation in a fairly selective manner, and the cages are formed via an intermediate of cyclotetrasiloxanetetrol. Crystalline precipitates of octamethylsilsesquioxane tend to be formed in methyltrialkoxysilane-derived solutions with an acid or base catalyst.26,27 In the N1 sample, the same precipitates are formed in spite of the low rate of hydrolysis. In the N2 sample, the addition of Al(acac)3 depressed the formation of these precipitates, essentially similar to the previous report.7 Furthermore, some differences are seen in the infrared spectra. In Figure 3, the peak at 1030 cm-1 appears relatively large in the H1 sample. Since most of methoxy groups have been hydrolyzed in H1 and H2 samples at 170 h, there should be little change in the content of methanol between the two samples. Therefore, some siloxane bonds contribute to this absorption besides methanol in the H1 sample. Brown investigated the asymmetric vibration of various organosiloxanes and assigned the absorption near 1030 cm-1 to that of cyclotrisiloxane rings.13 This suggests that more small rings exist in the H1 sample. By contrast, a broad shoulder at 1120 cm-1 is notable in the H2 sample, which is a little high in frequency compared with unstrained siloxane. From the relationship between the vibration frequency and Si-O-Si bond angle,28 more stretched siloxane chains are inferred to be generated in the H2 sample. The same conclusion was reached from a change in the transparency of CH3SiO3/2 gel with adding this complex.7 Pohl and Osterholtz investigated the polymerization kinetics of alkylsilanetriols, and found that both deviations in pD from pD ) 4.5 promoted the polymerization.4 The change in the acidity of the present solutions due to the addition of Al(acac)3 is inferred to influence the polymerization. It is, however, difficult to explain the change in the polymerization products only based on the change of pH. A base-catalyzed mechanism has been proposed for the polymerization.2,29 A silanol is deprotonated and then attacks on a neutral silicate species forming a SiO-Si bond, followed by ejection of OH-. On the basis of the same deprotonation mechanism, the interaction between Al(acac)3 and Si-OH expressed in eq 1 is inferred to facilitate the deprotonation of silanols.25 This may lead to an increase in the rate of polymerization. Furthermore, (26) Barry, A. J.; Daudt, W. H.; Domicone, J. J.; Gilkery, J. W. J. Am. Chem. Soc. 1955, 77, 4248. (27) Minami, T.; Tohge, N. Hybrides 1991, 7, 15. Abe, Y.; Hatano, H.; Gunji, T. J. Polym. Sci. Part A 1995, 33, 751. (28) Devine, R. A. B. J. Vac. Sci. Technol. 1988, A6, 3154. (29) Swain, C. G.; Esteve, R. M.; Jones, R. H. J. Am. Chem. Soc. 1949, 11, 965.
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from a steric viewpoint, such a weak interaction should occur easily at the terminal silanols compared with the side ones. As a result, the growth of a linear chain is preferred during the polymerization rather than the formation of cagelike structures. This mechanism seems to be in good agreement with the spectral results.
solution, this complex catalyzes the hydrolysis. Furthermore, this complex enhances the polymerization, facilitating the formation of linear siloxanes during the polymerization. The transfer of proton between the complex and hydroxyl-containing molecules seems to be responsible for these catalytic effects.
Conclusion
Acknowledgment. The author thanks Mr. Y. Tanigami, Dr. R. Terai, and Dr. H. Wakabayashi for their helpful suggestions in this study. He also thanks Dr. K. Nakanishi of Kyoto University for his critical comments on the manuscript.
Aluminum acetylacetonate plays a catalytic role both in the hydrolysis of methyltrimethoxysilane and in the polymerization of hydrolyzed products. Under acidic conditions, it lowers the acidity of the solution, leading to the decrease of the hydrolysis rate. In the non-acid
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