Simple and rapid eco-fnendly synthesis of cubic octamethylsilsesquioxane using microwave irradiation

Article abstract of DOI:10.1246/cl.2010.354

Octamethylsilsesquioxane (Me₈Si₈O₁₂) having a cage structure was rapidly prepared by means of microwave-assisted sol-gel reaction of methyltrimethoxysilane as a trifunctional alkoxysilane with an aqueous basic solution in diglyme as a solvent. Moreover, octamethylsilsesquioxane was obtained as cubic particles in a good yield. Cubic octamethylsilsesquioxane could be formed under microwave irradiation owing to the formation of micelles which are organized by eight methyltrimethoxysilane molecules in diglyme.

Full text of DOI:10.1246/cl.2010.354

doi:10.1246/cl.2010.354
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Published on the web March 6, 2010
Simple and Rapid Eco-friendly Synthesis of Cubic Octamethylsilsesquioxane
Using Microwave Irradiation
Takeru Iwamura, Kaoru Adachi, and Yoshiki Chujo*2
1
2,³
1
Institute for Environmental Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526
2
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510
(
Received December 28, 2009; CL-091157; E-mail: chujo@chujo.synchem.kyoto-u.ac.jp)
H3C
CH3
Octamethylsilsesquioxane (Me8Si8O12) having a cage structure
O
CH3
CH3
Si
Si
H3C O O
O
was rapidly prepared by means of microwave-assisted sol­gel
reaction of methyltrimethoxysilane as a trifunctional alkoxysilane
with an aqueous basic solution in diglyme as a solvent. Moreover,
octamethylsilsesquioxane was obtained as cubic particles in a good
yield. Cubic octamethylsilsesquioxane could be formed under
microwave irradiation owing to the formation of micelles which are
organized by eight methyltrimethoxysilane molecules in diglyme.
Si
Si
OCH3
CH3 Si OCH3
OCH3
O
O
O
O
Diglyme
0 °C, 2 h
Microwave Irradiation
100 W, 3 min
H3C Si
Si
O
O
O
Si
Si
3
O
H3C
CH3
Yield 76 %
Scheme 1. Synthesis of octamethylsilsesquioxane using microwave
irradiation.
Table 1. Sol­gel reaction of MeTMOS under microwave irradiation
(100 W)
Silsesquioxane chemistry has been the subject of intense
interest for more than half a century.1 The term silsesquioxane
is the general name for organosiloxide species with an empirical
formula (RSiO1.5)n (R = H, hydrocarbon) and closely related
compounds. In particular, silsesquioxane structures, which are
a
­8
Catalyst
Run
Yield/%
Base
Conc./M
1
2
3
4
5
6
LiOH
NaOH
KOH
LiOH
NaOH
KOH
0.1
0.1
0.1
1.0
1.0
1.0
0
20
23
2
52
76
9
random, ladder, cage (cubic), or partial cage, are interesting.
Since the caged part of silsesquioxane has a structure similar to
silica gel, these compounds can be regarded as a model of
inorganic compounds. Silsesquioxane and its derivatives have a
wide range of potential applications. In recent years, the
polyhedral oligomeric silsesquioxanes (POSS) have been attract-
ing a great deal of attention. However, a long reaction time is
usually required to obtain silsesquioxanes. Moreover, the yields
of silsesquioxanes are not so high in general. Due to increasing
aConditions: MeTMOS 1.00 g, catalyst 0.5 mL, and diglyme 40 mL.
microwave oven. Into a 50-mL glass bottle, diglyme (40 mL) was
added followed by addition of MeTMOS (1.00 g) and sol­gel
reaction catalysts (0.5 mL). For the hydrolysis of MeTMOS, the
mixture was stirred at ambient temperature for 2 h. The solution
was then poured into a PTFE beaker. The beaker was placed at the
center of the microwave oven and the solution was irradiated at
100 W for 3 min. After microwave irradiation, the reaction
mixture changed to a suspension, except for the use of LiOH as a
catalyst (Table 1, Runs 1­3). To evaluate the effect of catalyst
concentration, the experiments were carried out in 1.0 M catalyst
solution. In the case of KOH (Run 6), the precipitate product was
obtained in a good yield. The yield of the product increased in
order of KOH > NaOH > LiOH (Table 1, Runs 4­6). In the case
of Run 6, the suspension was not obtained by the microwave
irradiation without stirring at room temperature for 2 h. Also the
suspension was not obtained by the sol­gel reaction without the
microwave irradiation.
environmental concern, green chemistry has been receiving
progressively more attention since the 1990s.1
0­12
Therefore,
there is a great necessity for the advancement of novel method-
ologies for chemical reactions using an environmentally eco-
friendly method. A promising approach from this standpoint
would be to search for new processes and technologies which
from the very beginning are aimed at sharply curtailing environ-
mental pollution by reducing the volumes of chemical production
wastes. Microwave irradiation has been applied to various
chemical reactions. Many review papers have been published
about microwave-assisted chemical reactions.1
3­16
The micro-
wave method provides fast and direct heating. In many cases, it
can dramatically reduce the reaction time from hours to minutes,
and would increase the product yields and enhance the product
purity. For example, the monodisperse submicrometer silica
spheres were prepared rapidly by the microwave-assisted sol­gel
In the case of Run 6, the precipitates obtained were
observed by scanning electron microscope (SEM) measure-
ments. Figure 1 shows the SEM image of the obtained
precipitates. The observed particles were sharply cubic particles.
The range of particle size was from 0.6 to 3.5 ¯m.
17
reaction of tetramethoxysilane. Silica mixed oxide such as
Mg2SiO4, and organic polymer­silica hybrid19 were also
synthesized using microwave-assisted sol­gel reaction. In this
study, we tried to overcome the difficulties in synthesis of
silsesquioxanes, using microwave irradiation (Scheme 1).
To find suitable reaction conditions, experiments were
carried out using different catalysts. As a preliminary experiment,
the reactions of methyltrimethoxysilane (MeTMOS) with various
sol­gel reaction catalysts (0.1 M LiOH aq., 0.1 M NaOH aq., or
18
2
4
The obtained particles were examined by XRD analysis.
The XRD patterns of the cubic particles agreed with the patterns
2
0
of the authentic sample and also the reported data. These
results suggest that the microwave irradiation gave a cubic
octamethylsilsesquioxane. As compared with an authentic
sample, the cubic particles had good crystallinity from the
0.1 M KOH aq.) were carried out in a PTFE beaker in a
Chem. Lett. 2010, 39, 354­355
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Na
O
OCH3
CH3 Si OCH3
OCH3
H2O
OH
CH3 Si OH
OH
NaOH
O
CH3 Si
O
ONa
Na
CH3 Si ONa
ONa
Na
1
2a
2b
2c
H3C
O
CH3
CH3
Si
CH3
Si
H3C
H3C
O
O
O
Si
Si
Microwave
O
O
O
O
H3C Si
Si
O
Si
O
O
Si
O
CH3
3
Scheme 2. Plausible mechanism for octamethylsilsesquioxane obtained
by microwave irradiation.
wave irradiation would result from the formation of micelles.
In summary, cubic octamethylsilsesquioxane (Me8Si8O12)
was rapidly prepared under microwave irradiation by the sol­gel
reaction of methyltrimethoxysilane with an aqueous basic
solution in diglyme as a solvent. Moreover, cubic octamethyl-
silsesquioxane was obtained as sharply cubic particles in a good
yield (76%). Under the microwave irradiation conditions, cubic
octamethylsilsesquioxane could be formed owing to the forma-
tion of micelles which are organized by eight methyltrimethoxy-
silane molecules in diglyme. Silsesquioxane and its derivatives
have a wide range of potential applications, for example, coating,
adhesion, catalysts, liquid crystalline polymers, hybrid materials,
and low dielectric constant materials. Since there is an expecta-
tion to use silsesquioxanes in various fields, our eco-friendly
method should make it simple to synthesize them.
Figure 1. SEM image of the particles obtained by microwave irradiation.
full-width at half maximum (FWHM) of XRD peak at about
10.6° (cubic particles: 0.119°, authentic sample: 0.152°). These
results may indicate that this compound has high symmetry.
29
Gorsh et al. reported that the Si NMR spectrum of
octamethylsilsesquioxane has two well-resolved lines with an
2
1
intensity ratio of 3:1 at ¹66.1 and ¹66.8 ppm, respectively. For
octamethylsilsesquioxane, the distortion of the silicon­oxygen
framework along the body diagonal is 0.06 ¡.22 This distortion
29
corresponds to a splitting of 0.7 ppm of the signal in the Si NMR
spectrum for octamethylsilsesquioxane. In the 29Si NMR spec-
trum of octamethylsilsesquioxane which was prepared by micro-
wave irradiation, the peak was observed at ¹66.2 ppm. Therefore,
2
9
the Si NMR spectrum suggests that the cage silsesquioxane
obtained by microwave irradiation has high symmetry. Moreover,
References and Notes
³
Present address: Department of Chemistry and Materials Technology, Kyoto
Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto
2
9
the Si NMR spectrum supported the result of XRD analysis.
The FT-IR spectrum of the obtained cubic particles was in
6
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1
2
3
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good agreement with the spectrum of octamethylsilsesquiox-
ane.24 The obtained cubic particles have IR absorptions at
1
997, 30, 2818.
¹
1
¹1
2972 cm
(asymmetric CH3 stretch), 1271 cm
(Si­CH3
4
5
6
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1
¹1
stretch), 1124 cm (Si­O­Si), and 773 cm (Si­CH3 due to
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absorption at around 3500 cm was observed. This absorption
could not be distinguished between the absorption of Si­OH
derived from the obtained cubic particles and the absorption of
23
¹
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7
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9
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2
particles was carried out in nujol, the absorption at around
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1
2
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which included H O. Consequently, it was clarified that the
2
obtained silsesquioxane has a complete cage structure.
Also, as a result of the elemental analysis for the obtained
silsesquioxane, the measured values of C, 17.84 and H, 4.44%
were obtained, wherein the evaluated values for the product
were C, 17.89 and H, 4.51%. Therefore, it was also clarified
from the elemental analysis that the obtained silsesquioxane has
a complete cage structure.
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We surmised the formation mechanism of octamethylsilses-
quioxane as depicted in Scheme 2. In the first step, silanol 2a
1
955, 77, 4248.
2
1
L. E. Gorsh, A. A. Shubin, V. M. Nekipelov, A. N. Kanev, Zh. Strukt. Khim.
1982, 23, 26.
might be generated from MeTMOS (1) by H O. In this system,
2
there might be equilibrium among 2a, 2b, and 2c. These silanol
derivatives could form the micelles which were organized by
eight silanol derivatives molecules (2a, 2b, and 2c) in diglyme.
Therefore, octamethylsilsesquioxane (3) generated under micro-
22 K. Larsson, Ark. Kemi 1960, 16, 209.
2
3
C. T. Chua, G. Sarkar, S. Y. M. Chooi, L. Chan, J. Mater. Sci. Lett. 1999, 18,
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3
2
4
Supporting Information is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2010, 39, 354­355
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/