Preparation of polysilsesquioxane (CH3SiO1.5) 8 crystals from swollen PHEMA

Article abstract of DOI:10.1246/cl.2006.278

Polysilsesquioxane (CH₃SiO_1.5)₈ crystals were prepared from CH₃Si(OCH₂CH₃) ₃-trapped poly(2-hydroxyethylmethacrylate) (PHEMA) swollen in methanol in 23% yield under certain acidi

Full text of DOI:10.1246/cl.2006.278

278
Chemistry Letters Vol.35, No.3 (2006)
Preparation of Polysilsesquioxane (CH3SiO1:5)8 Crystals from Swollen PHEMA
Qun Luo, Daihua Tang, Xiaodong Li, Qi Wang, Zhipeng Wang, Zhen Zhen, and Xinhou Liuꢀ
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, P. R. China
(Received November 29, 2005; CL-051478; E-mail: xhliu@mail.ipc.ac.cn)
Polysilsesquioxane (CH3SiO1:5)8
crystals were prepared
condition adjusted with hydrochloric acid (37 wt % in water)
or aqueous ammonia (25 wt % in water), such as hydrochloric
acid/methanol (v/v ¼ 1=100) or aqueous ammonia/methanol
(v/v ¼ 1=20).
from CH3Si(OCH2CH3)3-trapped poly(2-hydroxyethylmeth-
acrylate) (PHEMA) swollen in methanol in 23% yield under
certain acidic or alkaline condition.
For example, a 16.44 g amount of platelets was immerged
in aqueous ammonia/methanol (v/v ¼ 1=20) at 40 ꢁC for 3 d.
Upon completion of swelling, the platelets were taken out and
dried in air. A large number of cubic crystals were observed,
and the platelets became translucent or even transparent.
0.12 g of (CH3SiO1:5)8 crystals was obtained from the platelet
surface in 22.94% yield.
There has been a growing interest in the preparation of var-
ious functionalized silsesquioxanes (RSiO1:5)n, such as ladder,
cage, and partial cage frameworks.1–3 Especially, spherosilicate
silsesquioxane cages with octasilsesquioxanes (T8) structures
have been studied extensively for their promising applications
in liquid crystals, biocompatible materials, catalysts, and so
on.4–6 However, in the preparation of these frameworks, most
synthetic methods encountered the insurmountable disadvantage
that alkyltriethoxysilane vigorously hydrolyzed and condensed
to form undesirable crosslinked resins or gels by the facilitation
of acidic or alkaline catalysts so that silsesquioxane cage crystals
occurred as trace amount of byproducts because of silsesquiox-
ane cages’ stringent geometrical requirements, and more, it was
difficult to isolate and purify these hydrolytic mixtures owing to
their poor solubility in solvents.2,7–9 Although Taylor recently
introduced a new synthetic scheme for the preparation of T8
using tetra-n-butylammonium fluoride as catalyst with the best
yield of 95%, in this case, it was proven unsatisfactory that the
Two key steps to prepare these crystals were as follows:
1. Precursors MTES and HEMA were mixed at a ratio of
HEMA:MTES = 9:1 (v/v). 2. Hydrolytic condensation reaction
of MTES took place when polymer was swelling in hydrochloric
acid/methanol = 1/100 (v/v) or in aqueous ammonia/
methanol = 1/20 (v/v).
As shown in Figure 1a, MTES was dispersed homogeneous-
ly into the polymer networks when HEMA bulk polymerized in-
to PHEMA, and the polymer chains separated MTES into micro-
droplets (<200 nm) within the polymer matrix. During PHEMA
was swelling, methanol molecules could permeate into the
polymer matrix, and MTES would diffuse outward in opposite
directions. Physical blocking actions and molecular interactions
coexisted within PHEMA hydrogels, and the diffusion coeffi-
cient of MTES would decrease with the decrease of its fraction
in the gels.13 The network of polymer chains was perceived as a
‘‘controller’’ for the diffusion of MTES through matrix in swol-
len state. As a result, MTES could be controlled to move calmly
and slowly. It was convenient to keep the concentration of
yield of (CH3SiO1:5 8 from methyltriethoxysilane (MTES,
)
CH3Si(OCH2CH3)3) was still less than 1%.10 Therefore, a feasi-
ble route needs to be developed for the preparation of this spe-
cies in high yield under mild synthetic conditions.
Here, we report a novel approach concerning the precursors
predispersed in polymer matrix, and then the (RSiO1:5)8 crystal
product obtained outside the matrix. Namely, (CH3SiO1:5)8 crys-
tals can be prepared using swollen poly(2-hydroxyethylmeth-
acrylate) (PHEMA) as a ‘‘controller.’’11 Habsuda reported that,
in poly(silicic acid)s (PSA)/HEMA, MPTS/HEMA and PSA/
MPTS/HEMA systems, HEMA components did not chemically
link with alkyltriethoxysilanes because of the substitution reac-
tion of alkyloxy (OR) groups with HEMA.12 MTES trapped in
PHEMA in advance could diffuse gradually and release slowly
through swollen PHEMA matrix immerged in alcohol under cer-
tain pH condition. In this way, the concentration of MTES was
controlled at a low level outside the polymer matrix over a 3
day period, where hydrolysis–condensation reaction proceeded
completely to yield discrete, morphological octasilsesquioxanes
(CH3SiO1:5)8 (‘‘Me-T8’’) crystals.
To a glass tube were added 2 mL of methyltriethoxysilane
(MTES) (d ¼ 0:895 g/cm3, Aldrich) and 18 mL of 2-hydroxy-
ethylmethacrylate (HEMA, 0.1 wt % initiator AIBN) (d ¼
1:073 g/cm3, Aldrich). After ultrasonic shake, bulk-polymeriza-
tion took place at certain temperature (50 ꢁC for 24 h, 75 ꢁC for
24 h, and 95 ꢁC for 4 h) to produce a white opaque polymer rod.
The rod was cut into several platelets and the platelets were
immerged in methanol (anhydrous, 99.5%) under certain pH
Figure 1. SEM of the interior of PHEMA before swelling (a)
and after swelling (b), and SEM of (CH3SiO1:5)8 crystals on
the surface of swollen PHEMA (c) and (d).
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.3 (2006)
279
Figure 3. Structure and infrared spectra of the (CH3SiO1:5)8
crystal.
Figure 2. Electronic diffraction and powder X-ray diffraction
image of the (CH3SiO1:5)8 crystal.
provide an important clue to prepare other organosilsesquioxane
(RSiO1:5)n crystals. The further work is in progress.
MTES outside the matrix at a low level. In addition, there was
only a little water in this system so that hydrolysis–condensation
of MTES could proceed gradually instead of vigorously. Under
This work is financially supported by the National Nature
Science Foundation of China (No. 20374058).
this condition, a large number of (CH3SiO1:5)8 crystals were
obtained. Moreover, neither gels nor resins were observed.
Cleanness and transparency of the solution could sustain
throughout. Although the same hydrolysis–condensation reac-
tion also occurred within the polymer matrix, it could be control-
led to proceed mainly outside the matrix during PHEMA was
swelling continuously. Just as seen in Figure 1b, after the
polymer was completely swollen, it was difficult to find MTES
or (CH3SiO1:5)8 crystals. The matrix became translucent or even
transparent.
References
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Figures 1c and 1d show that the crystals have cubic shape,
clean surface. As seen in Figure 1c, some crystals are with
dimensions of 20–30 um. The electron diffraction pattern and
powder X-ray diffraction pattern were presented in Figure 2,
and the XRD data were identical with the previous result.1 The
elemental analysis results (for CH3SiO1:5, calcd C, 17.91; H,
4.51%. found C, 17.98; H, 4.52; C/H molar ratio = 1/3.00)
suggested that the crystal composition was in accordance with
the formula (CH3SiO1:5)n. The GC-MS data indicated that the
molecular weight was 536 and that this species was with high
purity. Infrared spectra of the crystals was given in Figure 3,
clearly indicating two distinctive features of the molecule:
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Si–CH3 group (2972, 1270, and 774 cmꢂ1 14 and Si–O–Si silses-
)
quioxane skeletal absorption (1117, 515, and 463 cmꢂ1).15 In
addition, there is almost no absorption at 3600 cmꢂ1, which is
ascribed to Si–OH.16,17 The crystals would sublime without
melting when heating at 231–258 ꢁC under atmospheric pressure
(see Supporting Information). All these results highly prove that
the product is (CH3SiO1:5)8 crystal. This is in accordance with
Barry’s conclusion1 that the crystal belongs to the cage octamer
(CH3SiO1:5)8 whose structure is shown in Figure 3.
In conclusion, we have developed a novel and efficient
approach to the preparation of discrete and morphological poly-
silsesquioxane (CH3SiO1:5)8 crystals from MTES trapped in
swollen polymer systems (such as PHEMA). It is noteworthy
that swelling-based methods are being explored for preparing
functional species.18,19 We expect that the present findings will
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19 G. O. Brown, C. Bergquist, P. Ferm, K. L. Wooley, J. Am.
Chem. Soc. 2005, 127, 11238.
Published on the web (Advance View) February 11, 2006; DOI 10.1246/cl.2006.278