Template synthesis of functionalized polystyrene in ordered silicate channels

Article abstract of DOI:10.1039/b402794a

A mesostructured nanocomposite was fabricated by using a novel electroactive, polymerizable surfactant as a template in a sol-gel process, and the first example of well-resolved polystyrene with redoxactive functional group synthesized in silicate matrices was provided.

Full text of DOI:10.1039/b402794a

Template synthesis of functionalized polystyrene in ordered silicate
channels†
Guangtao Li,*ab Sheshanath Bhosale,b Sidhanath Bhosale,b Fengting Li,a Yihe Zhang,a Ruirong Guo,a
Hesun Zhua and Jurgen-Hinrich Fuhrhop*b
a Department of Chemistry, Tsinghua University, 100084 Beijing, P.R. China.
E-mail: LGT@mail.tsinghua.edu.cn; Fax: +86 10 62785967; Tel: +86 10 62785369
b Freie Universitaet Berlin, Institut fuer Organische Chemie, Takustr. 3, D-14195 Berlin, Germany.
E-mail: fuhrhop@chemie.fu-berlin.de; Fax: +49 30 83855589; Tel: +49 30 83855394
Received (in Cambridge, UK) 24th February 2004, Accepted 27th May 2004
First published as an Advance Article on the web 24th June 2004
A mesostructured nanocomposite was fabricated by using a
novel electroactive, polymerizable surfactant as a template in a
sol–gel process, and the first example of well-resolved polysty-
rene with redoxactive functional group synthesized in silicate
matrices was provided.
A highly ordered hexagonal channel structure and uniform tunable
pore size make the mesoporous silicate MCM-41 very attractive as
an ideal host for rational nanomanufacturing,1–5 especially as a
synthetic scaffold for controlled polymerization.6–8 Inspired by the
pioneering work of Bein and coworkers,9 the polymerization of
various monomers, such as styrene, alkynes, ethylene and metha-
crylates, have been reported within MCM-41.6–8,10
Recently, following the in-situ polymerization strategy devel-
oped by Brinker and Aida groups,11–12 we employed a template
with a terminal thiophene group, which functions both as structure-
directing agent and monomer, for MCM-41 framework synthesis
using tetraethoxysilane (TEOS) in the sol–gel process. After the
MCM-41 formation the densely packed template monomers were
directly polymerized in a controllable fashion, and submicrometer
long, separated polythiophene molecular wires were fabricated in
gram scale.13 In this study, we report two new surfactants bearing
polymerizable styrene units. Besides the quaternary ammonium
group, an electroactive viologen moiety was induced as polar head
group in surfactant synthesis. It is found that, although viologen has
a large volume, the synthesized surfactant is still suitable as a
template for the construction of a well-defined hexagonal channel
structure under certain conditions. Using the same strategy
described above (see scheme), the water-soluble polystyrene
molecular wire with electroactive functional group was produced
for the first time in ordered silicate channels.
The synthesis of surfactant monomers 2 and 3 started with a
Li2CuCl4 mediated Grignard coupling of 1-chloromethyl-4-vi-
nylbenzene with 1,10-dibromodecane. The obtained bromide was
subsequently transformed into the desired products by the reaction
with trimethylamine and 1-methyl-4,4A-bipyridinyl-1-ium, respec-
tively. Hexagonal mesoporous silica powders were prepared by
hydrolyzing TEOS in the presence of polymerizable surfactant 2 or
3 and NH4OH (28%). In a typical synthesis, 1.0 g of tetra-
ethoxysilane (TEOS) was added to an aqueous solution of NH4OH
containing styrene derivative 2 or 3 under stirring. The molar ratio
of 1.0 Si : 114 H2O : 8.0 NH4OH : 0.12 styrene 2 was established
in the reaction mixture. In the case of styrene 3, a lower molar ratio
(0.10) was used. After 60 min stirring at room temperature, the
resulting viscous gels were left to condense at 90 °C for 5 h in a
closed polyethylene bottle. A white powder 2a (in the case of 2) or
a yellow powder 3a (in the case of 3) was collected by filtration, and
washed thoroughly with water and acetone. The samples were dried
in air at room temperature.
(110), (200) and (210) reflections of the hexagonal structure.
Unexpectedly, surfactant 3 with a large polar head group is also
very suitable for forming a well-ordered hexagonal mesoporous
solid. Three typical diffraction peaks indexed as (100), (110), and
(200) were detected for the sample 3a. The d spacing of 2a and 3a,
calculated from the position of the intense peak, is 2.4 nm and 3.1
nm respectively, which is near to the value expected for the
diameter of rod micelles from 2 and 3. Complementary to the XRD
data, the images of transmission electron microscopy (TEM) of
both samples 2a and 3a display a hexagonal array of mesopores
throughout the sample. A typical TEM image of the mesoporous
structure formed from 3 as monomer template was depicted in Fig.
1.
The polymerization of the densely filled styrene monomers in
silicate channels was performed in a sealed Schlenck tube under
nitrogen atmosphere. 10 ml of a toluene dispersion containing 2 g
of synthesized silica powder and 2 ml of 0.05 M AIBN in toluene
were mixed and stirred at 60 °C for 72 h. After the removal of the
solvent and washing with dichloromethane, the residue was then
dispersed in 20 ml of 0.2 M HF aqueous solution. In both cases of
silicate 2a and 3a, an orange precipitate was formed, collected by
filtration, washed carefully with water, and dried in air. Generally,
we obtained about 500 mg polymer from 2 g of silicate powder.
As expected, both obtained polymers 2b and 3b were very
soluble in water. TEM provided direct visualization of the formed
polystyrene wires. A representative TEM image of sample 3b
stained by 1% phosphotungstate is shown in Fig. 1b. It is clear that
massive, coiled, individual polymer wires or strands are distributed
over large areas (Fig. 1, also in supporting information†). These
wires or strands have a tendency to entangle with each other.
Attempts to obtain separated single wire or strands by dilution and
to detect clear-cut ending and thus determine the length of
individual wire failed. These wires or strands, however, have a
uniform width of about 6 nm, as deduced from the measurement of
X-ray diffraction (XRD) of the sample from 2 shows the
characteristic well-resolved pattern with an intense (100) reflection
at low angle and three more weak intensities corresponding to
† Electronic supplementary information (ESI) available: TEM image of
sample 3b. See http://www.rsc.org/suppdata/cc/b4/b402794a/
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C h e m . C o m m u n . , 2 0 0 4 , 1 7 6 0 – 1 7 6 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

dashed line in Fig. 2]. The peak currents scale linearly with the scan
rate, showing that the electroactive polymer is in contact with the
electrode and the charge transfer is controlled by a diffusion
process. Cathodic–anodic peak potential separation for this wave is
ca. 230 mV at 100 mV s21. In accordance with the general
observations in redox polymer-coated electrodes, these values are
significantly larger than those found for the polymer in solution,
indicating also kinetic limitation in charge transport through this
type of redox films.
Charge transport rate limitations in redox polymer film can in
principle be minimized by admixture of the redox polymer with
new film components which process high electron mobility.14 In
our case, small gold particles (13 nm) were incorporated in the
polymer film using layer-by-layer approach (alternative deposition
of positively-charged polymer and negatively-charged gold parti-
cles on the electrode surface). Indeed, the electrochemical behavior
of synthesized polystyrene is qualitatively improved. The redox
process characteristic of viologen with a moderate peak potential
splitting as well as the large peak currents is clearly detected for
electrode film consisting of small gold particles embedded in the
viologen functionalized polystyrene 3b (solid line in Fig. 2). By
integrating the area under the first viologen reduction peak in Fig.
2, we obtained a surface coverage of viologen (9.6 3 1028 mol
cm22). This experiment further confirms that after sol–gel, in-situ
polymerization, and etching processes the functional viologen
groups are correctly retained
Fig. 1 TEM images of synthesized mesoporous silicate using surfactant 3 as
structure-directing agent (top) and the viologen functionalized polystyrene
molecular wires from the polymerisation of 3 in silicate channels
(bottom).
In conclusion, we succeeded in the fabrication of a mesos-
tructured nanocomposite using a novel electroactive, polymer-
izable surfactant as template in a sol–gel process, and provided, to
the best of our knowledge, the first example of well-resolved,
polymer wires with redoxactive functional groups synthesized in
silicate matrices. Since the quaternary ammonium bromide can be
replaced by other polar head groups such as PEG and phosphonate
for MCM-41 synthesis, different functionalized polystyrene molec-
ular wires should also be accessible using our method. This related
work is currently being undertaken in our lab.
the width of ten distinguishable strands. This value corresponds to
the width of styrene repeating units in the all-trans conformation in
a micellar double strand. Similar results were also observed in the
case of sample 2b and in our previous work.13 These observations
suggest that the in-situ polymerization in mesoporous silicate
allows the fabrication of separated molecular wires.
Attempts to determine the molecule weight of synthesized
polystyrene by size-exclusion chromatography experiments are
unsuccessful, presumably because of the strong electrostatic
interaction of the positive charged polystyrene with columnar
statuary phase. MALDI-TOF spectroscopic measurements also
gave no positive results.
The Deutsche Forschungsgemeinschaft (SFB 448 “Mesoscopic
Systems”) and the FNK of the Free University of Berlin provided
generous financial support.
Fig. 2 shows the cyclic voltammograms of polymer 2b and 3b
modified ITO electrodes, prepared by a spin-coating approach from
aqueous solution, in dichloromethane solution containing 0.1 M
TBNPF6. As expected, polystyrene 2b with the usual quaternary
ammonium groups did not display any electroactivity, and only a
typical background wave was observed (dotted line in Fig. 2). In
contrast, in the case of polymer 3b, a much broader redox wave,
producing presumably the viologen radical cation, was detected at
a potential of E1/2 = 20.82 V [E1/2 = (Epa + Epc)/2 vs. Ag/AgCl,
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Fig. 2 Cyclic voltammograms of the polymer-coated ITO-electrodes in 0.1
M TBAPF6/CH2Cl2 at 100 mV s21, curve a: polymer 2b; curve b: polymer
3b; curve c: polymer 3b with embedded gold nanoparticles.
C h e m . C o m m u n . , 2 0 0 4 , 1 7 6 0 – 1 7 6 1
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