Synthesis of sodium-polystyrenesulfonate-grafted nanoparticles by core-cross-linking of block copolymer micelles

Article abstract of DOI:10.1016/j.tet.2004.05.057

Sodium poly(styrenesulfonate)(polySSNa)-grafted polymer nanoparticles were synthesized by core-cross-linking of block copolymer micelles and subsequent chemical transformation. Block copolymers, poly(p-((1-methyl)silacyclobutyl) styrene-block-poly(neopent

Full text of DOI:10.1016/j.tet.2004.05.057

Tetrahedron 60 (2004) 7197–7204
Synthesis of sodium-polystyrenesulfonate-grafted nanoparticles
by core-cross-linking of block copolymer micelles
*
Kozo Matsumoto, Hirohiko Hasegawa and Hideki Matsuoka
Department of Polymer Chemistry, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Received 6 March 2004; revised 21 May 2004; accepted 21 May 2004
Available online 11 June 2004
This paper is dedicated to Professor Robert H. Grubbs on the occasion of his receipt of the 2003 Tetrahedron Prize
Abstract—Sodium poly(styrenesulfonate)(polySSNa)-grafted polymer nanoparticles were synthesized by core-cross-linking of block
copolymer micelles and subsequent chemical transformation. Block copolymers, poly(p-((1-methyl)silacyclobutyl)styrene-block-
poly(neopentyl p-styrenesulfonate)s, polySBS-b-polySSPen, were synthesized by nitroxy-mediated living radical polymerization. The
block copolymers formed micelles (R ¼15–23 nm, where R represents the hydrodynamic radius) with a polySBS core and polySSPen shell
h
h
in acetone. The micelle core was cross-linked by ring-opening polymerization of silacyclobutyl groups in polySBS. Hydrolysis of the
h
neopentyl groups provided polySSNa-grafted nanoparticles. The R of the particles before the hydrolysis ranged from 12 to 21 nm in acetone,
while they varied to the range from 50 to 110 nm in water after the hydrolysis.
q 2004 Elsevier Ltd. All rights reserved.
1
. Introduction
mediated living polymerization, and its micelle was formed in
acetone. Then, the core of the micelle was cross-linked by
ring-opening polymerization of silacyclobutyl groups, and
finally the ester groups on the grafting polymers were
hydrolyzed to obtain sodium poly(styrene sulfonate)-grafted
nanoparticles. The outline of this study is shown in Figure 1.
The particles synthesized here are expected as new materials
in wide variety of applications from industrial surfactants to
nano-capsules for biomedical usage. They may also attract
fundamental scientific attention as academic samples, since
the particle has a well-defined strong polyelectrolyte brush
layer on the surface with high grafting density.
Block copolymers self-assemble to form micelles in
solvents selective for one of the blocks. The core of the
micelle consists of an insoluble block, and the shell, which
is often called corona, consists of both a soluble block and
the solvent. Such micelle formation has attracted much
attention for industrial and biomedical applications. Conse-
quently, many block copolymer micelles have been studied
not only for use as surfactants such as lubricant, dispersant,
and emulsifier, but also as additives in cosmetics or drug
carriers for drug delivery systems. Compared with the
micelles of low molecular weight surfactants, the block
copolymer micelles are generally larger, ranging from
several nanometers to tens of nanometers. Additionally,
they are more stable due to their slow exchange between
associated and non-associated molecules (micelle-unimer
exchange). These properties are quite advantageous in
constructing nano-scale materials. By cross-linking the core
of the micelle, we can obtain nanoparticles with shell-
1
,2
3
,4
2. Experimental
2.1. Materials
2
1
Neopentyl p-styrenesulfonate (SSPen) and N-t-butyl-1-
2
2
diethylphosphono-2,2-dimethylnitroxyl radical (DEPN)
were prepared as reported. Benzoyl peroxide (BPO) was
5
–20
forming polymers grafted from the particle surface.
In
0
this study, we synthesized strong-ionic-polymer-grafted
nanoparticles, using a styrene-based block copolymer, in
which one segment has cross-linkable silacyclobutyl groups
and the other has potentially ionic surfonate ester groups. The
precursor block copolymer was prepared by nitroxy-radical-
purchased from Nacalai Tesque (Kyoto, Japan), 2,2 -
azobisisobutyronitrile (AIBN) and chloroplatinic acid
hexahydrate (H PtCl ·6H O) were purchased from Wako
2
6
2
Pure Chemicals (Osaka, Japan), iodotrimethylsilane was
purchased from Tokyo Chemical Industries (Tokyo, Japan),
and they were used as delivered. Tetrahydrofuran (THF) and
benzene was distilled over sodium benzophenone ketyl
under an argon atmosphere. Acetone was purified by
distillation over MS-4A. Carbon tetrachloride was distilled
Keywords: Living radical polymerization; Polymer micelle; Cross-linking;
Nanoparticle; Surface-graft; Sodium polystyrenesulfonate.
*
Corresponding author. Tel.: þ81-75-383-2597; fax: þ81-75-383-2599;
e-mail address: matsumo@star.polym.kyoto-u.ac.jp
over CaH . Water used for dialysis and polymer sample
2
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.05.057

7198
K. Matsumoto et al. / Tetrahedron 60 (2004) 7197–7204
Figure 1. Schematic explanation of synthesis of sodium-polystyrenesulfonate-grafted nanoparticle.
preparation was obtained by a Milli-Q system (Millipore,
Pittsburgh, PA) whose resistance was more than 18 MV cm.
THF solution of 1-chloro-1-methylsilacyclobutane was
prepared from 22.3 mL of 1-chloro-1-methylsilacyclo-
butane (140 mmol) and magnesium (4.13 g, 170 mmol) as
described above. Then the solution was cooled to 0 8C and a
phenylmagnesium bromide (1.0 mol/L THF solution,
200 mmol, 200 mL) was slowly added. The mixture was
stirred at room temperature over night. The resulting
solution was poured into 1 M aq. HCl, and the product
was extracted with hexane. The organic layer was washed
twice with water, dried with anhydrous Na SO , and
2
.1.1. Synthesis of p-(1-methylsilacyclobutyl)styrene
SBS). A THF solution of 1-chloro-1-methylsilacyclo-
butane was prepared as follows. First, 1,2-dibromoethane
0.3 mL) was added to a suspension of magnesium (1.9 g,
0 mmol) in THF (5 mL) to activate the magnesium. Then a
(
(
8
THF solution (45 mL) of 3-chloropropylmethyldichloro-
silane (9.5 mL, 60 mmol) was added dropwise over a period
of 10 min. Then, the reaction mixture was heated at 50 8C
for 2 h. A THF solution of p-styrylmagnesium bromide was
prepared as follows. First, 1,2-dibromoethane (0.3 mL) was
added to a suspension of magnesium (1.5 g, 60 mmol) in
THF (5 mL) to activate the magnesium. Then a THF
2
4
concentrated. Distillation of the residual oil over calcium
hydride under reduced pressure gave the title compound
(15.2 g, 94 mmol) in 67% yield. Bp, 87–89 8C/15 Torr; IR
(neat) 3067, 3050, 2927, 2855, 1589, 1487, 1428, 1396,
2
1
1300, 1249, 1184, 1112, 998, 925, 899, 866, 772, 732 cm
1
;
(
50 mL) solution of p-bromostyrene (6.5 mL, 50 mmol) was
H NMR (CDCl ) d 0.55 (s, 3 H), 1.16 (dt, J¼8.4, 8.4 Hz,
3
added dropwise over a period of 10 min, and the solution
was stirred for 2 more hours. The p-styrylmagnesium
bromide solution thus prepared was slowly added to the
2H), 1.30 (dt, J¼8.4 Hz, 2H), 2.19 (tt J¼8.4 Hz, 2H), 7.33–
1
3
7.43 (m, 3H), 7.57–7.66 (m, 2H); C NMR (CDCl ) d
3
21.69, 14.42, 18.31, 127.83, 129.32, 133.39, 138.57.
Found: C, 73.90; H, 8.91%. Calcd for C H Si: C, 73.99;
H, 8.71%.
1
-chloro-1-methylsilacyclobutane solution prepared above
1
0 14
at an ambient temperature over a period of 10 min. Then the
mixture was heated at 40 8C over night. The resulting
solution was poured into 1 M aq. HCl, and the product was
extracted with hexane. The organic layer was washed twice
with water, dried with anhydrous Na SO , and concentrated.
2.2. Polymerization
DEPN-capped SBS113 was prepared as follows. A mixture
of SBS (3.80 g, 20.1 mmol), BPO (13.6 mg, 0.04 mmol),
DEPN (30.1 mg, 0.10 mmol), and benzene (3 mL) was
charged in a glass tube equipped with a Teflon screw cock,
degassed, and sealed under argon. The Teflon screw cock
was used to seal the tube easily and surely. The mixture was
kept at 115 8C for 2.5 h. Reprecipitation with a toluene/
methanol system, followed by vacuum drying gave DEPN-
capped polySBS (2.96 g, 78% SBS conversion) in
quantitative yield. M ¼21,700, M /M ¼1.31.
2
4
The residual oil was purified by silica-gel column
chromatography to give the title compound (29 mmol,
5
1
7
8
8
1
2
.4 g) in 57% yield. IR (neat) 3062, 2927, 2855, 1629, 1597,
543, 1389, 1249, 1105, 1028, 989, 907, 866, 828, 770,
2
1 1
20 cm ; H NMR (CDCl ) d 0.58 (s, 3H), 1.21 (dt, J¼8.4,
3
.4 Hz, 2H), 1.33 (dt, J¼8.4, 8.4 Hz, 2H), 2.22 (tt, J¼8.4,
.4 Hz, 2H), 5.30 (d, J¼10.8 Hz, 1H), 5.83 (d, J¼17.6 Hz,
H), 6.75 (dd, J¼10.8, 17.6 Hz, 1H), 7.46 (d, J¼12.0 Hz,
1
3
H), 7.62 (d, J¼12.0 Hz, 2H); C NMR (CDCl ) d 21.68,
3
n
w
n
1
1
4.46, 18.31, 114.38, 125.61, 133.67, 136.70, 138.16,
38.45. Found: C, 76.74; H, 8.72%. Calcd for C H Si:
C, 76.51; H, 8.58%.
The block copolymer, SBS113-b-SSPen208, was prepared as
follows. A mixture of SSPen (2.21 g, 8.70 mmol), DEPN-
1
2 16
capped SBS113 (M ¼21,700, 0.65 g) prepared above,
n
2
.1.2. Synthesis of 1-methyl-1-phenylsilacyclobutane. A
1.5 mg of DEPN, and benzene (2.5 mL) was charged in a

K. Matsumoto et al. / Tetrahedron 60 (2004) 7197–7204
7199
glass tube equipped with a Teflon cock, degassed, and
sealed under argon. The mixture was kept at 115 8C for
scattering angle of 908. AFM measurements were performed
by SPI3800 probe station and SPA300 unit system of
Scanning Probe Microscopy System SPI3800 series (Seiko
Instruments, Tokyo, Japan). The cantilever was made of
silicon (Olympus, Tokyo, Japan) and its spring constant was
2 N/m. The measurements were performed in Dynamic
Force Mode (non-contact mode). For sample preparation, a
THF or aqueous solution (ca.0.1 wt%) of the polymer was
dropped on a microslide glass (IWAKI, Japan) and air-
dried.
2
.0 h. Reprecipitation with a chloroform/hexane system,
followed by vacuum drying gave SBS113-b-SSPen208
1.08 g, 62% SSPen conversion) in 54 wt% yield.
M ¼50,500, M /M ¼1.59.
(
n
w
n
2
.3. Core-cross-linking of the block copolymer micelle
A core-cross-linked (CCL) block copolymer, CCL-(SBS113
b-SSPen208), was synthesized. About 20 mg of H PtCl ·6-
-
2
6
H O was placed in a two-necked 500 mL-round bottomed
2
flask equipped with a reflux condenser, and the flask was
filled with argon. Then an acetone (100 mL) solution of
SBS113-b-SSPen208 (1.0 g) was added, and the mixture was
heated at 55 8C for 3 h. The resulting solution was filtrated
and concentrated to about 1/5 in volume by evaporation.
Then 1,4-dioxane (200 mL) was added and the solution was
freeze-dried to give a powdered CCL polymer CCL-
3. Results and discussion
3.1. Block copolymer synthesis
Controlled radical polymerization of vinyl compounds has
resulted in a wide variety of well-defined block
2
3
24–26
(
SBS113-b-SSPen208) (1.0 g) in quantitative yield.
copolymers.
Okamura and co-workers reported a
quite sophisticated method of synthesizing polystyrene-
block-polySSNa by a nitroxy-radial-mediated living
polymerization of styrene with styrene having neopentyl
surfonate group at the para-position (SSPen) and a
2
.4. Hydrolysis of neopentyl sulfonate
A CCL SBS113-b-(sodium styrenesurfonate)208 (CCL-
SBS113-b-SSNa208)) was synthesized. Trimethylsilyl
iodide (1.5 mL, 10 mmol) was added to a solution of
2
1
(
successive hydrolysis of neopentyl ester. On the other
hand, it has been well-known for a long time that
silacyclobutanes can be readily polymerized in the presence
CCL-(SBS113-b-SSPen208) (900 mg) in carbon tetrachloride
(
2
7
30 mL), and the mixture was stirred at 50 8C for 15 h. The
mixture was concentrated, and the neopentyl and the
resulting residue was dissolved in 200 mL of methanol/HCl
of platinum catalyst, while the four-membered ring is
tolerant under radical conditions as shown in Scheme 1. We
confirmed that no consumption of 1-methyl-1-phenyl-
silacyclobutane occurs in heating a bulk 1-methyl-1-
phenylsilacyclobutane in the presence of AIBN or BPO at
120 8C. This suggests that styrene derivatives having
silacyclobutanes might be radically polymerized without
affecting the cyclic group in one step, whereas the resulting
styrenic polymer can be cross-linked by ring-opening
polymerization in the later step.
(
1 mol/L) mixture, then aqueous NaOH (1 mol/L) was
added to neutralize the solution. The solution was filtrated
and dialyzed against deionized water for a week. The
dialysate was diluted with water to afford 0.1 wt% aqueous
solution of CCL-(SBS113-b-SSNa208) (780 mL).
2
.5. Measurements
Gel permeation chromatography (GPC) was carried out in
THF on a JASCO PU-980 chlomatograph (JASCO
Engineering, Tokyo, Japan) equipped with two polystyrene
gel columns (Shodex KF804L; separation range in molecu-
In this study, we used styrenes having a silacyclobutyl group
at the para-position (SBS) and SSPen as monomers for a
5
lar weight of polystyrene: 100 to 4£10 ) and JASCO RI-930
refractive index detector. The averaged polymerization
degree of polySBS and molecular weight distributions of
both polySBS and polySBS-b-polySSPen were determined
relative to polystyrene standards. Proton NMR spectra were
recorded on a JEOL AL-400 spectrometer in CDCl3,
acetone-d , and D O. The averaged polymerization degree
6
2
of polySSPen segment in polySBS-b-polySSPen was
calculated from the polymerization degree of SBS pre-
polymer (by GPC) and composition of the block copolymer
Scheme 1.
1
estimated from H NMR measurement. IR spectra were
measured on a Shimadzu FTIR-8400 spectrometer. The
elemental analyses were carried out at the Elemental
Analysis Center of Kyoto University. The dynamic light
scattering (DLS) measurements were performed on a DLS
apparatus of Photal SLS-6000HL (Otsuka Electrics, Osaka,
Japan) equipped with a correlator (Photal GC-1000). He–
Ne laser (the wavelength of 632.8 nm) was used for the
measurements. Sample solutions (1–0.1 wt%) were filtrated
through a membrane (Millex-HN, Millipore, pore-size of
0
.45 mm). The measurements were performed at 20 8C at
Scheme 2.

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K. Matsumoto et al. / Tetrahedron 60 (2004) 7197–7204
The desired block copolymer, polySBS-b-polySSPen, was
synthesized by nitroxy-radical-mediated living polymeriz-
ation as shown in Scheme 3. In the first step, we prepared a
nitroxy-capped polySBS with narrow molecular weight
distributions by polymerization of SBS using benzoyl
2
2
peroxide (BPO) as a radial initiator and DEPN as a
radical mediator. Then SSPen was polymerized using
1
polySBS as a macro initiator. A representative H NMR
spectrum and GPC charts of the obtained polymer are given
1
in Figures 2 and 3. The H NMR spectrum indicates the
existence of both polySBS and polySSPen. The peak on the
GPC charts shifted to the higher molecular weight region
after polymerization of SSPen, which indicates formation of
a block copolymer. In the first polymerization step, three
polySBS samples having different polymerization degrees
were synthesized by tuning the molar ratio of BPO and SBS,
while the molar ratio of the radical mediator to the radical
initiator was kept constant ([DEPN]/[BPO]¼2.5) in all
cases. In the second polymerization step, five different
samples were prepared by tuning the molar ratio of SBS
prepolymer and SSPen. The polymerization results are
summarized in Table 1. The polymerization degree of the
polySBS (m) was estimated by GPC measurements relative
to polystyrene standard, and the polymerization degree (n)
Scheme 3.
1
of polySSPen was estimated by H NMR spectra of the
block copolymers, thus the m and n are not absolute values.
It is also true that the products were contaminated with a
trace amount of ‘dead’ polySBS prepolymer as detected by
GPC, which broadened the molecular weight distribution,
although most of the products were block copolymers.
However, we used the crude products in the later
experiments without further purification, because the
homopolymer was solubilized in the core of the block
copolymer micelle and did not pose a crucial problem in the
cross-linking step.
1
3
Figure 2. H NMR spectrum of SBS113-b-SSPen208 in CDCl .
3.2. Micelle formation
PolySBS is nonpolar and soluble in nonpolar solvent such as
hexane, toluene, chloroform, and THF, while it is insoluble
in polar solvent such as acetone, dimethylsulfoxide
(DMSO), and N, N-dimethylformamide (DMF). On the
other hand, polySSPen is moderately polar and insoluble in
hexane but soluble in all the other solvents described above.
Thus the block copolymers were soluble in chloroform,
1
Figure 3. GPC charts for (A) DEPN end-capped SBS113 and (B) SBS113-b-
SSPen208
which is a good solvent for both blocks, and we could see H
NMR signals of both polySBS and polySSPen in CDCl as
.
3
shown in Figure 2. In the meanwhile, the block copolymers
were also soluble in acetone, which is a selective solvent for
polySSPen. In this case, however, the signals only for
polySSPen were observed but the signals for polySBS were
block copolymer synthesis. The SBS monomer was readily
prepared by treatment of 1-chloro-1-methylsilacyclobutane
with p-styrylmagnesium bromide as shown in Scheme 2.
Table 1. Polymerization results of SBS and SSPen
[
BPO]
0
/[SBS]
0
Conv. (%) of SBS
m
M /M
w n
of polySBS
[polySBS] /[SSPen]
0
0
Conv. (%) of SSPen
n
M
w
/M
n
of block copolymer
1
1
/256
/348
—
/480
—
75
82
—
78
—
64
89
89
113
113
1.36
1.23
1.23
1.31
1.31
1/122
1/137
1/251
1/194
1/290
50
57
60
64
62
70
97
163
134
208
1.38
1.62
1.56
1.52
1.59
1
[BPO]
to polystyrene standard. n was determined by H NMR using the value of m. M
0
/[SBS]
0
: initial molar ratio of BPO and SBS. [polySBS]
1
0
/[SSPen]
0
: initial molar ratio of SBS prepolymer and SSPen. m was determined by GPC relative
/M was determined by GPC relative to polystyrene standard.
w
n

K. Matsumoto et al. / Tetrahedron 60 (2004) 7197–7204
7201
acetone-soluble polySSPen and the solvent. To examine
more about the micelle formation, we measured the DLS of
the block copolymer solutions. In all cases, strong light
scattering intensity was observed, which suggested the
existence of block copolymer micelles. Hydrodynamic
radius (R ) of the block copolymer micelles in acetone for
h
all the samples examined here are summarized in Table 2.
The R ranged from 15 to 23 nm, all of which are reasonable
h
values for the block copolymer micelles, and they increased
as the total chain length of the block copolymer increased.
3.3. Micelle core cross-linking
It is well-known that silacyclobutanes can be readily
2
1
6
Figure 4. H NMR spectrum of SBS113-b-SSPen208 in acetone-d .
7
polymerized by a hexachloroplatinic acid (H PtCl ). We
2
6
Table 2. Hydrodynamic radius of SBS -b-SSPen
m
n
micelle and CCL-(SBS
Hydrodynamic radius, R
CCL-(SBS -b-SSPen
m
-b-SSPen
n
) evaluated by DLS measurement
(nm)
CCL-(SBS
n
m
h
SBS -b-SSPen
m
n
CCL-(SBS
m
-b-SSPen
n
)
m
n
)
m
-b-SSNa
n
)
m n
CCL-(SBS -b-SSNa )
in acetone
in acetone
in THF
in water
in 0.3 M NaCl aq.
6
8
8
1
1
4
9
9
13
13
70
97
163
134
208
15
17
19
17
23
12
15
19
21
19
13
13
19
20
19
50
82
87
90
40
66
66
63
70
110
utilized this reaction to a CCL of the block copolymer
micelle. The micelle solution in acetone was heated at 55 8C
in the presence of catalytic amount of H PtCl . Figure 5
2
6
1
shows H NMR spectrum of the products obtained by
treatment with Pt catalyst in acetone and redissolved in
CDCl . The four-membered ring methylene signals at
3
2
.2 ppm (observed in Figure 2) completely disappeared
and signals of the methyl group on the silicon atom at
.4 ppm (observed in Figure 2) were detected as signifi-
0
cantly broad signals in Figure 5, indicating the occurrence of
ring-opening reactions (Scheme 4). Particle size in the
acetone solution was examined by DLS measurement. The
Rh values for the samples evaluated here are summarized in
Table 2. R of the obtained particles ranges from 12 to
h
21 nm in acetone, which are in good agreement with those
of the block copolymer micelles before the ring opening
reaction. The DLS measurement was also carried out in the
THF solution. Light scattering were observed even in THF,
which is a good solvent for both polySBS and polySSPen,
evidenced the formation of CCL micelles. In addition, the
hydrodynamic sizes of the particles evaluated in THF were
analogous to those in acetone (Table 2), suggesting that the
1
Figure 5. H NMR spectrum of CCL-(SBS113-b-SSPen208) in CDCl
3
.
not observed as shown in the spectrum of the block
copolymer in acetone-d6 (Fig. 4). This suggested that
micelles were formed in acetone, whose core consists of
acetone-insoluble polySBS and the shell consists of
Scheme 4.

7202
K. Matsumoto et al. / Tetrahedron 60 (2004) 7197–7204
Scheme 5.
1
9
in the case of CCL-(polySBS-b-polySSPen), completely
disappeared in the spectra for products after hydrolysis.
50 cm2 typical for sulfonate ester, which were observed
The hydrolyzed products were soluble in water. DLS
analysis of the particles was performed in aqueous solutions.
The size distribution of CCL-(SBS113-b-SSNa134) in water
is given in Figure 7, which shows narrow size distribution.
The R values for all the samples are summarized in Table 2.
h
The R of the hydrolyzed CCL-micelles ranged from 50 to
h
110 nm. The size of these particles was about four times
larger than that of the esterified particles in organic media.
This increase in Rh is probably caused by the drastic
conformation change of the grafted polymer chains. The
non-ionic polySSPen chain took a corona conformation in
the organic solvent, while the ionic polySSNa chains
adopted a stretched conformation due to the electrostatic
repulsion between the ionic groups. DLS measurement was
carried out in salt-added aqueous solutions to confirm this
Figure 6. IR-spectra of CCL-(SBS113-b-SSPen134) and CCL-(SBS113-b-
SSNa134).
cross-linking proceeded so enough that the core was not
swollen even by good solvent. These results clearly indicate
the formation of the CCL micelles.
effect. The R values for the hydrolyzed particles in 0.3 M
h
NaCl solutions are given in Table 2. The particle sizes were
reduced by salt addition in all cases. We consider this to be
because the electrostatic interaction in the polySSNa chains
is screened by the added ions. Another significant
characteristics of this material is that the CCL-(polySBS-
b-polySSNa) aqueous solution was very stable even in a
salted water, that is no flocculation or precipitation of the
polymer particles was observed in 0.5 mol/L NaCl aqueous
solution at room temperature even after one week. We
consider that a high steric stabilization effect of grafting
polySSNa chains might play an important role in this
phenomenon.
3
.4. Synthesis of polySSNa-grafted particle
The neopentyl sulfonates of the grafting chains were
transformed into trimethylsilyl sulfonates by treatment
with trimethylsilyl iodide in carbon tetrachloride, and the
silyl sulfonates were further transformed to sodium
sulfonates by sequential exposure to HClaq and NaOHaq
(Scheme 5). Completion of the hydrolysis was confirmed by
IR measurements (Fig. 6). Absorption at 1350 and
The obtained polymer nanoparticles before and after the
hydrolysis were further investigated by AFM. Figure 8
shows typical AFM images for CCL-(SBS113-b-SSPen134
)
and CCL-(SBS113-b-SSNa134). Many protubances of
ca.70 nm in diameter were observed for CCL micelles
both before and after hydrolysis, suggesting the existence of
spherical particles. The height profiles for the images were
also given in the figure. The height of the protubances is
ca.6 nm for a particle before hydrolysis, while that for a
particle after hydrolysis is ca.30 nm. Because of the
convolution effect of a silicon probe used in the AFM
measurement, horizontal size is always larger than the real
one. Therefore, we estimated the particle size from the
height profiles. By AFM analysis, the diameters of the
particles were determined at 6 and 30 nm before and after
hydrolysis, respectively. These results also suggest that the
polySSNa chains in the hydrolyzed particles have a more
Figure 7. Distribution of hydrodynamic radius (R
SSNa134) in water evaluated by DLS CONTIN analysis. Scattering angle:
08.
h
) of CCL-(SBS113-b-
9

stretched conformation than polySSPen in the particles
before hydrolysis. However, it should be noted that the
particles evaluated here are significantly smaller than those
obtained by DLS. This is because the structures of the
particles observed by AFM are in a dry state and different
from those in a wet state evaluated by DLS. Nevertheless,
the AFM observation clearly revealed that spherical
nanoparticles could be synthesized by core-cross-linking
of block copolymer micelles.
of Education, Culture, Sports, Science, and Technology,
Japan.
References and notes
1
. Hamley, I. W.; 3rd ed. Encyclopedia of Polymer Science and
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4
. Conclusions
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A well-defined styrene-based block copolymer having the
cross-linkable silacyclobutane and sulfonate groups,
polySBS-b-polySSPen, was synthesized by nitroxy-
mediated ‘living’ radical polymerization. The block
copolymer formed a micelle whose R ranged from 15 to
h
3 nm in acetone, and the core of the micelle was cross-
linked to provide polySBS nanoparticles with polySSPen
grafting from the surface. Hydrolysis of the neopentyl ester
in polySSPen gave ionic poly(styrene sulfonate)-grafted
5
. Proch a´ zka, K.; Baloch, M. K.; Tuzar, Z. Makromol. Chem.
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. Wilson, D. J.; Riess, G. Eur. Polym. J. 1988, 24, 617.
6
7
2
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8
. Saito, R.; Ishizu, K.; Fukutomi, T. Polymer 1990, 31, 679.
. Saito, R.; Ishizu, K.; Fukutomi, T. Polymer 1991, 32, 531.
9
polySBS nanoparticles. The R of the particle with ester-
h
1
1
1
1
1
1
1
1
1
0. Saito, R.; Ishizu, K.; Fukutomi, T. Polymer 1991, 32, 2258.
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protected graft chains was 12–21 nm in acetone, while that
of the particle with ionic polySSNa chains was 50–110 nm
in water. This size difference is induced by electrostatic
repulsion between ions on the polymer chains. The
electrostatic interaction in the grafting polySSNa chains is
currently being investigated in detail.
5. Wang, G.; Henselwood, F.; Liu, G. Langmuir 1998, 14, 1554.
6. Henselwood, F.; Liu, G. Macromolecules 1998, 31, 4213.
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Acknowledgements
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9. Won, Y.; Davis, H. T.; Bates, F. S. Science 1999, 283, 960.
This work was financially supported by a Grant-in-aid
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(A15205017) and 21st century COE program, COE for a
United Approach to New Materials Science, of the Ministry