Synthesis of a multifunctional alkoxysiloxane oligomer

Article abstract of DOI:10.1039/c4nj00204k

An alkoxysiloxane oligomer (1, SiMe[OSi(CHCH₂)(OMe)₂][OSi(CH₂)₃Cl(OMe)₂]₂), containing vinyl and chloropropyl groups, was synthesized as a precursor for sol-gel synthesis. Di-tert-butoxymethylhydroxysilane (t-BuO)₂MeSiOH was reacted with (MeO)₂(CH₂CH)SiCl to form (t-BuO)₂MeSiOSi(CHCH₂)(OMe)₂ which was further alkoxysilylated with Cl(CH₂)₃SiCl(OMe)₂ to form 1. The ¹H, ¹³C, ²⁹Si NMR and HR-MS data confirmed the formation of 1, indicating the successful synthesis of an alkoxysiloxane oligomer possessing different kinds of functional groups by a chemoselective route. Hydrolysis of 1 under acidic conditions was completed in a few hours. The solution state ²⁹Si NMR spectra of samples hydrolyzed and condensed at various reaction times show no signals due to species generated by the cleavage of the siloxane bonds in 1, indicating the validity of the synthesized substance as a precursor for the formation of hybrids with homogeneously distributed functional groups. Intramolecular condensation of 1 to form cyclic trisiloxane units proceeds more preferentially than intermolecular condensation.

Full text of DOI:10.1039/c4nj00204k

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Synthesis of a multifunctional alkoxysiloxane
oligomer†
Cite this:DOI: 10.1039/c4nj00204k
Masashi Yoshikawa,a Ryutaro Wakabayashi,ab Misa Tamaia and Kazuyuki Kuroda*ab
An alkoxysiloxane oligomer (1, SiMe[OSi(CHQCH2)(OMe)2][OSi(CH2)3Cl(OMe)2]2), containing vinyl and chloro-
propyl groups, was synthesized as a precursor for sol–gel synthesis. Di-tert-butoxymethylhydroxysilane
(t-BuO)2MeSiOH was reacted with (MeO)2(CH2QCH)SiCl to form (t-BuO)2MeSiOSi(CHQCH2)(OMe)2
which was further alkoxysilylated with Cl(CH2)3SiCl(OMe)2 to form 1. The 1H, 13C, 29Si NMR and HR-MS
data confirmed the formation of 1, indicating the successful synthesis of an alkoxysiloxane oligomer
possessing different kinds of functional groups by a chemoselective route. Hydrolysis of 1 under acidic
conditions was completed in a few hours. The solution state 29Si NMR spectra of samples hydrolyzed
and condensed at various reaction times show no signals due to species generated by the cleavage
of the siloxane bonds in 1, indicating the validity of the synthesized substance as a precursor for the
formation of hybrids with homogeneously distributed functional groups. Intramolecular condensation of
1 to form cyclic trisiloxane units proceeds more preferentially than intermolecular condensation.
Received (in Montpellier, France)
10th February 2014,
Accepted 26th August 2014
DOI: 10.1039/c4nj00204k
www.rsc.org/njc
distribution and/or separation between functional groups in
hybrids by using oligomers as single precursors.7 Alkoxysiloxane
1. Introduction
Silica-based inorganic–organic hybrid materials have been studied oligomers are suitable as building blocks of silica-based inorganic–
for applications in many fields.1 Their properties are affected by organic hybrid materials because they have hydrolyzable alkoxy
functional group diversity, distribution, and the separation or groups. However, examples of alkoxysiloxane oligomers with
distance between individual functional groups. For example, defined structures are quite limited.7–10 Furthermore, there are few
bifunctional catalysts like acid–base catalysts show higher catalytic examples that demonstrate the successful synthesis of compounds
activity than monofunctional catalysts because of cooperativity possessing both plural functional groups and alkoxysilyl groups.11
between acidic and basic sites.2 In addition, the catalytic activity In fact, a previous report11 used the following two methods. One
is generally affected by the change in the distribution and separa- method uses stepwise synthesis of those compounds and the other
tion between two kinds of functional groups.3,4 However, it is involves silylation of silanol-containing compounds with chloro-
difficult to control the distribution and/or separation between silanes possessing two kinds of functional groups. The former
different kinds of functional groups by conventional methods for consists of hydrolysis of alkoxy groups and silylation of silanol
preparing hybrids, such as post-grafting5 and co-condensation groups. In this method, it is necessary to use a precursor that is
using sol–gel methods.6 Inhomogeneous distribution of functional resistant to hydrolysis, such as polyhedral silsesquioxanes (POSSs),
groups may be induced by post-grafting. Such heterogeneity to introduce plural functional groups in regioselective ways while
also results from different rates of reaction between precursors retaining the oligosiloxane structure of the precursor. Thus, this
(mainly mixtures of organoalkoxysilanes and tetraalkoxysilanes) approach is not versatile because of the limitation of available
by co-condensation methods.
precursors. The latter uses chlorosilanes possessing two kinds of
To overcome this problem, it is reasonable to use a well- functional groups for silylation of silanols allowing the synthesis
designed oligomer as a hybrid precursor because the separation of an alkoxysiloxane oligomer with different functional groups.
and number of functional groups can be fixed at the molecular However, the ratio and separation of the different functional
level for such oligomers. Therefore, one expects to control the groups is fixed, and it is difficult to control these factors by this
method. Different from those reports, the present study shows
a Department of Applied Chemistry, Faculty of Science and Engineering,
Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo, 169-8555, Japan.
E-mail: kuroda@waseda.jp; Fax: +81 3 5286 3199; Tel: +81 3 5286 3199
b Kagami Memorial Research Institute for Materials Science and Technology,
Waseda University, Nishiwaseda 2-8-26, Shinjuku-ku, Tokyo, 169-0051, Japan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj00204k
stepwise introduction of different plural functional groups under
non-hydrolytic conditions, and this method can control the ratio
and distance of functional groups. Even though different func-
tional groups can be bonded to polyhedral silsesquioxanes
(POSSs),12 such modified POSSs are not normally constructed
with alkoxysilyl functional groups.
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The following sections from 2.2 to 2.5 provide a brief
summary of the synthesis of compounds a, b, c, and d. The
experimental details and full characterization data are shown
in the ESI† (Fig. S1–S6 for a, Fig. S7–S9 for b, Fig. S10–S12 for c,
and Fig. S13–S15 for d.).
2.2 Synthesis of (t-BuO)2MeSiOH (a)
Compound a ((t-BuO)2MeSiOH) was synthesized from trichloro-
methylsilane. At first, trichloromethylsilane was alkoxylated with
potassium tert-butoxide at room temperature, and it formed
(t-BuO)2MeSiCl. The remaining Cl did not react with the butoxide
because of the steric hindrance of two t-BuO groups. Next,
(t-BuO)2MeSiCl was hydrolyzed with H2O to synthesize a.
Scheme 1 Synthesis of alkoxysiloxane oligomers.
Q
2.3 Synthesis of (MeO)2(CH2 CH)SiCl (b)
(MeO)2(CH2QCH)SiCl (b) was synthesized by the alkoxylation
of trichlorovinylsilane with tert-butyl methyl ether as we pre-
viously reported.14
Recently, we reported non-hydrolytic synthesis of branched
alkoxysiloxane oligomers (Scheme S1, please see the ESI†).13
Siloxane bonds can form by this method from alkoxysilyl groups
which release stable carbocations, such as t-Bu+ and Ph2HC+.
Therefore, this approach offers the potential to control the structure
and functional groups on the alkoxysiloxane oligomers by designing
a precursor and selecting silylating agents. However, the synthesis
of an oligomer with several kinds of functional groups has not yet
been achieved.
Q
2.4 Synthesis of (t-BuO)2MeSiOSi(CH CH2)(OMe)2 (c)
(t-BuO)2MeSiOSi(CHQCH2)(OMe)2 (c) was synthesized by alkoxy-
silylation of a with b.
2.5 Synthesis of Cl(CH2)3SiCl(OMe)2 (d)
Here, we report the synthesis of a typical oligomer having
different functional groups (Scheme 1). In this work, we succeeded
in designing and thereafter constructing an oligosiloxane structure
with selected placement of functional groups. Chloropropyl and
vinyl groups were chosen as functional groups because these
groups can be converted to catalytically active groups, including
amino, thiol, and carboxy groups. In addition, the hydrolysis and
condensation process was investigated by liquid state NMR to
confirm the absence of siloxane bond cleavage of the oligomer.
Because compound 1 (Scheme 1) retains the original oligosiloxane
structure after condensation, the distribution and separation of
functional groups in the resulting hybrids can be controlled by
using 1 as a precursor.
Cl(CH2)3SiCl(OMe)2 (d) was synthesized by the same way as b.
(3-Chloropropyl)trichlorosilane was used instead of trichloro-
vinylsilane.
2.6 Synthesis of
Q
SiMe[OSi(CH CH2)(OMe)2][OSi(CH2)3Cl(OMe)2]2 (1)
A solution of c (11.1 g, 32.2 mmol) in hexane (10 mL) was added
to a solution of d and BiCl3 in a 100 mL Schlenk flask. The
molar ratio of c/d/BiCl3 was 1/3/0.033. The mixture was stirred
at room temperature for 24 h under a N2 atmosphere. Pyridine
and methanol were added into the mixture and the mixture was
stirred for 30 min for methoxylation of SiCl groups formed by a
functional group exchange reaction between d and 1. After the
completion of the reaction, confirmed by 1H and 13C NMR,
the solvents, Cl(CH2)3Si(OMe)3, tert-butyl chloride, remaining
pyridine, and methanol, were removed under reduced pressure.
Then, colorless clear liquid 1 (4.1 g, 7.2 mmol, yield 23%) was
isolated by vacuum distillation after a solid residue was removed
by filtration.
2. Experimental
2.1 Materials
Acetonitrile (Wako Pure Chemical Co. Ltd, 499.0%, dehydrated),
bismuth chloride (Kanto Chemical Co. Ltd, 498.0%), tert-butyl
methyl ether (Wako Pure Chemical Co. Ltd, 499.0%, dehy-
drated), (3-chloropropyl)trichlorosilane (Tokyo Kasei Co. Ltd,
497.0%), hexane (Wako Pure Chemical Co. Ltd, 495%), 0.01
mol Lꢀ1 hydrochloric acid (Wako Pure Chemical Co. Ltd),
methanol (Wako Pure Chemical Co. Ltd, 499.8%, dehydrated),
potassium tert-butoxide (Tokyo Kasei Co. Ltd, 497.0%), pyridine
(Wako Pure Chemical Co. Ltd, 499.5%, dehydrated), sodium
sulfate (Wako Pure Chemical Co. Ltd, 499.0%, dehydrated),
tetrahydrofuran (THF) (Wako Pure Chemical Co. Ltd, 499.5%,
dehydrated, stabilizer free), trichloromethylsilane (Tokyo Kasei
Co. Ltd, 498.0%), and trichlorovinylsilane (Tokyo Kasei Co. Ltd,
498.0%) were used without further purification.
1
Compound 1. H NMR (500 MHz; CDCl3; 25 1C; TMS): d =
0.27 (s, 3H), 0.75–0.78 (m, 4H), 1.86–1.91 (m, 4H), 3.50–3.53
(t, 4H), 3.55 (s, 12H), 3.56 (s, 6H), 5.85–5.92 (dd, 1H), 6.00–6.05
(dd, 1H), 6.12–6.16 ppm (dd, 1H); 13C NMR (126 MHz; CDCl3;
25 1C; TMS) d = ꢀ3.2, 8.3, 26.4, 47.3, 50.30, 50.33, 128.9,
137.1 ppm; 29Si (99 MHz; CDCl3; 25 1C; TMS) d = ꢀ51.4 (2Si,
T1), ꢀ63.9 (1Si, T1), ꢀ66.8 ppm (1Si, T3);‡ HRMS (Electrospray
Ionization, 2 kV) calcd for C15H36O9Cl2Si4Na+ [M + Na]+:
565.0707; found: 565.0706.
‡ Symbols of Tn denote bonding states of the Si atom; Tn: R1Si(OSi)n(OR0, OH,
or Oꢀ)3ꢀn
.
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2.7 Hydrolysis and condensation of compound 1
methyl groups appears at around ꢀ65 ppm.16 These assign-
ments are also supported by a computational analysis. The
Compound 1, THF containing 10 vol% of THF-d8, H2O and HCl
(0.01 mol Lꢀ1) were mixed in a Teflon vessel and the mixture
was stirred at room temperature. THF was used as the solvent
because it is a compatible solvent for both compound 1 and H2O.
The molar ratio of compound 1/THF/H2O/HCl was 1/25/6/0.0005.
Hydrolysis and condensation behaviors of the oligomer were
investigated by constantly following the evolution of the relevant
13C and 29Si NMR signals. The details of the measurements are
described in the characterization section.
details of the method are shown in ref. 17. The results show
3
that the signal due to Si of the SiT -methyl group appears
at higher magnetic field by 4.02 ppm than that due to Si of the
1
SiT -vinyl group. As mentioned above, the measured signal due
3
to Si of SiT -methyl appeared at higher magnetic field by 2.9 ppm
1
than that due to Si of SiT -vinyl. The high-resolution ESI-MS
spectrum shows a peak at m/z = 565.0706 corresponding to
[M + Na+] (calcd mass: 565.0707). These results confirmed the
synthesis of compound 1.
Therefore, it is successful to synthesize the siloxane oligomer
which has alkoxy groups and plural functional groups. It is reason-
ably expected to synthesize siloxane oligomers which possess both
acidic and basic functional groups by transforming the vinyl and
chloropropyl groups of 1, respectively. In addition, alkoxysiloxane
oligomers which have other functional groups can be synthesized
by changing the kind of alkoxychlorosilanes under the same
reaction processes. Furthermore, the ratio of functional groups
can be controlled by changing the order of the alkoxysilylation.
Because alkoxychlorosilanes are easily synthesized from chloro-
silanes, the synthetic method presented here is very versatile.
2.8 Characterization
Solution 1H, 13C and 29Si NMR spectra were recorded on a
Bruker AVANCE 500 spectrometer with resonance frequencies
of 500.0 MHz, 125.7 MHz, and 99.3 MHz, respectively, at ambient
temperature. Sample solutions were put in 5 mm glass tubes. The
chemical shifts were referenced to internal tetramethylsilane (TMS)
at 0 ppm. CDCl3 and THF containing 10% of THF-d8 were used
to obtain lock signals. A small amount of Cr(acac)3 (acac = acetyl-
acetonate) was also added as a relaxation agent for 29Si nuclei. 13
C
NMR was measured with a recycle delay of 2 s and accumulations
of 64 FIDs. 29Si NMR was measured with a recycle delay of 10 s
and accumulations of 64 FIDs for quantitative analyses. HR-ESI
mass analysis was carried out using a JEOL JMS-T100 CS instru-
ment. Samples were dissolved in methanol.
3.2 Hydrolysis and condensation of compound 1
The solution 13C NMR spectra of compound 1 hydrolyzed for
different reaction times (Fig. 2) indicate complete hydrolysis
between 2–3 h after the beginning of the reaction. Please note
that the chemical shifts of 13C NMR signals in Fig. 2 (0 h) were
slightly different from those in Fig. 1(b) because THF-d8 was
used to obtain the spectra of Fig. 2 (0 h) while CDCl3 was used
3. Results and discussion
3.1 Characterization of compound 1
1
The H NMR spectrum of compound 1 (Fig. 1(a)) shows nine for the spectrum of Fig. 1(b). THF-d8 was chosen because THF is
signals at 0.27 (–SiMe), 0.75–0.78 (–Si–CH2–CH2–CH2–Cl), 1.86– compatible with both 1 and H2O.
1.91 (–Si–CH2–CH2–CH2–Cl), 3.50–3.53 (–Si–CH2–CH2–CH2–Cl),
The signal intensity assigned to the methoxy group (50.4 ppm)
3.55 (–Si–(CH2)3Cl(OMe)2), 3.56 (–Si–(CH2QCH)(OMe)2), 5.85– decreased steadily, and disappeared after 3 h. A new signal
5.92 (–CHQCH2), 6.00–6.05 (–CHQCH2 (cis position to Si)), and assigned to methanol simultaneously appeared at 49.7–49.8 ppm.
6.12–6.16 ppm (–CHQCH2 (trans position to Si)). The integration In addition, all the signal intensities due to methyl (ꢀ3.1 ppm),
ratio of these signals is 1.0 : 1.0 : 1.0 : 20.7(containing all the signals vinyl (130.2 ppm and 136.4 ppm) and 3-chloropropyl groups
at 3.50–3.56 ppm because of the overlapping) : 3.8 : 3.8 : 3.0 and (9.0 ppm, 27.4 ppm, and 47.8 ppm) of compound 1 are weakened
these correspond to the calculated ratio (1:1:1:22:4:4:3). On the with the time course of hydrolysis and condensation, while many
other hand, there are no signals assigned to t-BuO groups of the new signals for the same functional groups appeared at chemical
compound c (t-BuO)2MeSiOSi(CHQCH2)(OMe)2.
shifts similar to those for compound 1 (The magnified spectra
These results indicate the alkoxysilylation accompanied by are shown in Fig. S16, ESI†). These results are interpreted to
complete elimination of t-BuO groups. Furthermore, vinyl and mean that the environments of the carbon atoms of those
chloropropyl groups are retained without any side reactions. groups change as hydrolysis and condensation progresses. The
The 13C NMR spectrum (Fig. 1(b)) also shows the presence of diversification of the environments is caused by the following
chloropropyl and vinyl groups and the absence of t-BuO groups, factors. (i) Because compound 1 contains hydrolysable six
which strongly supports this conclusion.
–SiOMe groups, very many kinds of molecules will be generated
The 29Si NMR spectrum of compound 1 (Fig. 1(c)) shows three by hydrolysis of 1. (ii) In the condensation process, two kinds of
signals at ꢀ51.4, ꢀ63.9 and ꢀ66.8 ppm and their intensity ratio cyclic siloxane will be formed by intramolecular condensation
is 1.9 : 1.1 : 1.0. The singlet signal at ꢀ51.4 ppm is assigned to (Scheme 2), and the stereoisomers will be generated when these
–Si(CH2)3Cl, because the number of –Si(CH2)3Cl groups is twice molecules are formed. Detailed interpretation and assignments
that of –Si(CHQCH2) or –SiMe groups. The other two signals of 13C NMR signals are shown in the ESI†.
1
at ꢀ63.9 and ꢀ66.8 ppm are assigned to –SiT (CHQCH2) and
Multiple signals are observed in the 29Si NMR spectra (Fig. 3)
3
–SiT Me, respectively, because of the following two reasons. (i) of hydrolyzed solutions of compound 1, and assignments of
The signal due to T1 Si atoms possessing vinyl groups appears at these signals are shown in Table 1. (Please note that the
around ꢀ62 ppm.15 (ii) The signal due to T3 Si atoms possessing chemical shifts of 29Si NMR signals in Fig. 3 (0 h) were slightly
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Fig. 1 Liquid state (a) 1H, (b) 13C and (c) 29Si NMR spectra of 1 in CDCl3.
different from those in Fig. 1(c) because of the difference in complete within 2–3 h. Therefore, these signals are assigned to
deuterated solvent.) It is quite difficult to assign these signals SiOH groups generated by hydrolysis (Scheme 2). These three
precisely because their chemical shifts are very close together. signals disappear steadily after 2 h, and new signals appear
Three signals appear at ꢀ49.9 ppm, ꢀ62.7 ppm, and ꢀ66.5 ppm at ꢀ49.7 ppm to ꢀ50.5 ppm, ꢀ56.9 ppm to ꢀ57.4 ppm, and
in 0.5 h. As confirmed by the 13C NMR (Fig. 2), the hydrolysis is ꢀ63.1 ppm to ꢀ63.9 ppm. We will discuss below the assignments
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Fig. 2 13C NMR spectra (THF-d8) of 1 hydrolyzed for 0 h, 0.5 h, 1 h, 1.5 h, 2 h, and 3 h.
conditions,15,18 it is quite reasonable to conclude that the
oligosiloxane structure of compound 1 is not degraded under
the conditions used here.
Next, intramolecular condensation of compound 1 is discussed.
The overlapped signals of the sample after the reaction for 2 h,
appearing at ꢀ49.7 ppm to ꢀ50.5 ppm, ꢀ56.9 ppm to ꢀ57.4 ppm,
and ꢀ63.1 ppm to ꢀ63.9 ppm, are assignable to –Si(CH2)3Cl,
–Si(CHQCH2), and –SiMe of cyclotrisiloxanes, respectively
(Table 1). Intramolecular condensation naturally forms cyclic
siloxane structures and the terminal T1 silicon atoms of
compound 1 become T2 silicon atoms. Signals corresponding
to T2 silicon atoms of cyclotrisiloxanes appear downfield from
T2 silicon atoms in non-cyclic compounds because of ring
strain.19 The presence of too many signals can be explained
by the formation of two types of cyclic siloxanes (Scheme 2) and
their geometric isomers.20
Finally, intermolecular condensation of compound 1 is dis-
cussed. The signals in the ranges from ꢀ56.9 ppm to ꢀ57.4 ppm
and from ꢀ62.8 ppm to ꢀ63.9 ppm might be assigned to inter-
molecularly condensed species of compound 1, in which the
hydroxy groups of –Si(OH)2(CH2)3Cl are used for condensation
(Scheme 2), because it is known that signals corresponding to
T2 and T3 silicon atoms of intermolecularly condensed species
Scheme 2 Proposed reaction processes for hydrolysis and condensation
of compound 1.
of these signals to two types of molecules; (i) molecules generated appear upfield than T1 silicon atoms.19 However, there are no
by cleavage of siloxane bonds of compound 1 or (ii) molecules signals corresponding to intermolecularly condensed species
generated by intra- and inter-molecular condensation.
derived by condensation of hydroxy groups of –Si(OH)2(CHQCH2)
At first, the cleavage of siloxane bonds of compound 1 at around –80 ppm. The absence of species resulting from
was examined. If the siloxane bonds of compound 1 are intermolecular condensation of the –Si(OH)2(CHQCH2) group
cleaved, Cl(CH2)3Si(OMe)n(OH)3ꢀn, MeSi(OMe)n(OH)3ꢀn and can be explained by the following two reasons. At first, the
(CH2QCH)Si(OMe)n(OH)3ꢀn (n = 0–3) would be generated intensity of the signals due to – Si(OH)2(CHQCH2) is lower than
(Scheme 2). However, there are no signals in the T0 region that of –Si(OH)2(CH2)3Cl because the number of –Si(CHQCH2)
corresponding to these molecules (ꢀ36 ppm to ꢀ44 ppm, groups is half of –Si(CH2)3Cl groups. Second, the signal intensity
ꢀ52 ppm to ꢀ56 ppm, and ꢀ38 ppm to ꢀ40 ppm,15,18 respectively). of each silicon atom is weakened because the environment
Because these molecules are easily observed under acidic of silicon atoms is diversified and the molecular mobility is
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Fig. 3 29Si NMR spectra (THF-d8) of 1 hydrolyzed for different reaction times: 0 h, 0.5 h, 1 h, 1.5 h, 2 h, 3 h, 5 h, 10 h, and 24 h.
Table 1 Assignments for 29Si NMR signals of hydrolyzed 1a
Time/h
–Si(CH2)3Cl
–Si(CHQCH2)
–SiMe
T1ðOMeÞ : ꢀ 51:4 ppm
T1ðOMeÞ : ꢀ 64:3 ppm
T3ðOSiÞ : ꢀ 66:7 ppm
0
2
2
2
T1ðOHÞ : ꢀ 49:9 ppm
T1ðOHÞ : ꢀ 62:7 ppm
T3ðOSiÞ : ꢀ 66:5 ppm to ꢀ 66:7 ppm
0.5–1.5
2–10
2
2
2
T2OH;OSi; Tð3OSiÞ : ꢀ 49:7 ppm to ꢀ 50:5 ppm
T2OH;OSi; Tð3OSiÞ : ꢀ 56:9 ppm to ꢀ 57:4 ppm
T3ðOSiÞ : ꢀ 63:1 ppm to ꢀ 63:9 ppm
2
2
2
a
Symbol T of Tnx denotes three oxygen atoms on Si. The superscript n of Tnx means the number of Si bonded to the central Si atom. The subscript x
of Tx denotes the kind of functional groups linked to the Si atom.
decreased by intermolecular condensation. The same reasons condensation is competed with intermolecular condensation
can be applied to explain the absence of the signals assigned to except for in a dilute solution.21 In comparison with a linear
–SiMe groups after 3 h. For these reasons, signals due to oligosiloxane, silanol groups generated by hydrolysis of alkoxy
intermolecularly condensed species are normally observed groups of compound 1 are sterically close to one another.
to much lesser degrees. Therefore, we can not completely Therefore, it is quite reasonable that intramolecular condensa-
exclude the possibility of intermolecular condensation. How- tion proceeds preferentially because silanol groups generated
ever, the observed signals are clearly assigned as the progress by hydrolysis of alkoxy groups are close to each other and
of intramolecular condensation, which leads us to conclude condense easily. Cyclic siloxanes generated by intramolecular
that the last one proceeds preferentially rather than inter- condensation possess four silanol groups which are not used
molecular condensation.
for intramolecular condensation. After the formation of cyclic
As shown above, the siloxane linkages are not broken, which siloxanes, Si–O–Si networks can be constructed by intermole-
indicates that the incorporated functional groups are located at cular condensation among these silanols. The formation of
the original positions. This means that the method presented Si–O–Si networks by intermolecular condensation should result
here is quite useful for the design of precursors for the prepara- in the broadening of NMR signals and the decrease in their
tion of hybrid materials as well as mesostructured materials intensities because the environment of Si atoms is diversified
possessing functional groups distributed homogeneously with and the molecular mobility is decreased by the Si–O–Si network
some fixed distance. This is superior to the grafting method3 formation. In fact, the signals in the spectra of the products
using two kinds of reagents, which provide hybrids with only reacted for 10 h and 24 h after the beginning of hydrolysis were
locally distributed functional groups.
broadened and also the intensity of signals in those spectra
The preference of cyclization should be attributed to decreased (Fig. 3). Hybrid materials prepared using branched
the branched oligosiloxane structure of compound 1. In the oligomers are expected to possess unique features in terms of
case of linear oligosiloxane, it is reported that intramolecular porosity and density because the position of silanol groups is so
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6 (a) S. Huh, H.-T. Chen, J. W. Wiench, M. Pruski and
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silanol groups. The study on the relationship between the proper-
ties of hybrids and siloxane structures in various length scales is
challenging for future design of siloxane-based materials.
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4. Conclusions
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Alkoxysiloxane oligomer 1 possessing different plural func-
tional groups has been successfully synthesized by combining
a conventional silylation with non-hydrolytic silylation which
does not involve the formation of the silanol group during the
reaction. The siloxane bonds are not cleaved during hydrolysis
and condensation of compound 1, as judged by 29Si NMR data.
It is indicated that intramolecular condensation proceeds more
preferentially than intermolecular condensation. Therefore,
compound 1 can be used as a building block to prepare hybrid
materials via the sol–gel process. The distribution and separa-
tion between individual functional groups can be controlled by
using this type of oligomer. Reactive functional groups are
appended to vinyl and 3-chloropropyl groups in this study.
The extension of the kinds of functional groups that could be 12 D. B. Cordes, P. D. Lickiss and F. Rataboul, Chem. Rev.,
used to prepare hybrid materials with controlled functionalities
will be a fruitful area of future research in sol–gel chemistry.
2010, 110, 2081.
13 (a) R. Wakabayashi, K. Kawahara and K. Kuroda, Angew.
Chem., Int. Ed., 2010, 49, 5273; (b) R. Wakabayashi,
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17 Structural optimization was performed at the B3LYP/
6-31G(d) level using Gaussian09 Rev.D/Linux. Calculation
of nuclear shielding of NMR was performed at the B3LYP/
6-311G+(2d.p) level by using Gaussian09 Rev.D/Linux.
Because the absolute values by calculation are not so consis-
tent with the actual ones for Si atoms, we used the relative
difference for the discussion.
Acknowledgements
We thank Prof. Richard Laine (University of Michigan) for his
comments which have greatly improved the manuscript. This
work was supported by Grant-in-Aid for Scientific Research (No.
23245044) and in part by Elements Science and Technology
Project, ‘‘Functional Designs of Silicon-Oxygen-Based Compounds
by Precise Synthetic Strategies’’. We thank Dr T. Shibue and
Mr N. Sugimura (Materials Characterization Central Lab., Waseda
University) for their help with NMR and MS measurements as well
as computational analysis.
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