Double-stranded helical polymers consisting of complementary homopolymers

Article abstract of DOI:10.1021/ja711447s

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.

Full text of DOI:10.1021/ja711447s

NOTE
pubs.acs.org/Macromolecules
Synthesis and Visualization of a Core Cross-Linked Star Polymer
Carrying Optically Active Rigid-Rod Helical Polyisocyanide Arms and
Its Chiral Recognition Ability
Toshitaka Miyabe, Hiroki Iida, Motonori Banno, Tomoko Yamaguchi, and Eiji Yashima*
Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku,
Nagoya 464-8603, Japan
S Supporting Information
b
’ INTRODUCTION
the neighboring amide NÀH groups.12 Because of the anom-
alous stiff backbones of the helical polyisocyanides, we antici-
pated that the resulting star polymer would form a unique
star-shaped “spiny” architecture and their topologically interest-
ing individual molecular structures including the shape, the
molecular lengths and numbers of arms, and the helical structures
of the arms (helical pitch, helical sense (P or M), and helical sense
excess as well) might be directly visualized by AFM. Although
AFM has become a powerful tool to study a variety of polymer
materials, such as star and branched polymers,13 the visualization
of the branching topology of star polymers is still challenging and
their real images of topological structures remain difficult to
observe,14 except for star polymers carrying three or four
polymer brushes as the arms or those prepared by uniform-
arm macroinitiators14b,15 mainly because many flexible arms
attached to a central core fold into condensed random coils in
an overlapping interdigitated manner, leading to complicated
images.
Star polymers, a class of branched polymers, have attracted
great attention over the past few decades because of their greater
accessibility by well-established living polymerization techni-
ques, structural diversity in three-dimensional (3D) coreÀshell
architectures, and unique properties and functions being differ-
ent from those of the corresponding linear counterparts.1 Living
ionic and radical polymerizations of monomers followed by
cross-linking with
a difunctional comonomer (“arm-first”
approach)2 have frequently been employed for the design and
synthesis of star polymers consisting of a densely cross-linked
core and linear radiating multiple arms, which enables us to
produce star polymers bearing specific functions,1b such as a
catalytic activity3 and molecular recognition ability,4 by introdu-
cing the desired functional groups at the core or the arm
segments. Although a variety of star polymers with a varying
number of arms, chemical compositions, and functional groups
have been prepared, most of the star polymers preparedtodate are
optically inactive.1 Optically active star polymers, in particular
those carrying rodlike one-handed helical arms,4b,5 are quite
rare in spite of their potential in practical applications as novel
chiral materials for enantioselective catalysis and adsorbents6
because of their unique helix-bundle structures generated by self-
assembled helical arms, which may provide confined chiral
environments for the efficient chiral recognition of enantiomers
and chiral polymers.
’ RESULTS AND DISCUSSION
The optically active star polymer (M-poly-L-1(À)-b-3) was
prepared by cross-linking of the living left-handed helical M-poly-
L-1(À) (the number-average molecular weight (Mn) and its dis-
tribution were Mn = 6.00 Â 104 and Mw/Mn = 1.06, respectively)8,11
with a cross-linker 3 (0.1 equiv based on the monomer units of
M-poly-L-1(À)) in tetrahydrofuran (THF) at 55 °C. The cross-
linking reaction, the progress of which was monitored by IR, was
terminated after 3 had been completely consumed. Because the
obtained core cross-linked star polymer carrying the rigid-rod
helical arms contained a small amount of the unreacted macro-
initiator (M-poly-L-1(À), ca. 11%), the polymer was purified by
the recycling preparative size exclusion chromatography (SEC),
yielding the higher MW star polymer (M-poly-L-1(À)-b-3); the
composition (m and n in Figure 1) was calculated to be m:n = 1:9
based on the conversion of 3 and M-poly-L-1(À), since the NMR
of the star polymer was too broad to determine the composition.
The Mn and the absolute weight-average molecular weight (Mw)
of the star polymer were determined to be 5.79 Â 105 and 1.11 Â
106, respectively, by multiangle light scattering (MALS) coupled
To this end, we have synthesized a novel optically active star
polymer (M-poly-L-1(À)-b-3) bearing rodlike helical polyiso-
cyanide arms based on the “arm-first” approach (Figure 1). Our
design approach builds upon our previous findings that both
right (P)- and left (M)-handed helical polyisocyanides with a
different molecular weight (MW) and a narrow molecular weight
distribution (MWD) (P- and M-poly-L-1s, respectively) can be
obtained by the living polymerization of an enantiomerically pure
phenyl isocyanide bearing an L-alanine pendant with a long
n-decyl chain (L-1) using the μ-ethynediyl PtÀPd catalyst (2),7
followed by the facile fractionation with acetone (Figure 1).8,9
The fractionated single-handed helical polyisocyanides (P- and
M-poly-L-1s)10 as revealed by high-resolution atomic force
microscopy (AFM)8 maintained their living feature and can
be used as an initiator (macroinitiator) for the further block
copolymerizations of isocyanides.11 The polyisocyanides
possess an unprecedented long persistence length of 220 nm
stabilized by intramolecular hydrogen-bonding networks through
Received: September 2, 2011
Revised:
September 27, 2011
Published: October 07, 2011
r
2011 American Chemical Society
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|

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NOTE
Figure 1. Schematic illustration of the synthesis of a star polymer M-poly-L-1(À)-b-3. The helix-sense-selective living polymerization of L-1 with the
μ-ethynediyl PtÀPd catalyst (2) gives a mixture of diastereomeric, right-handed, and left-handed helical poly-L-1s with different MWs and a narrow
MWD, which can be further separated into the right- and left-handed helical poly-L-1s by fractionation with acetone. Left-handed helical M-poly-L-1(À)
was used as the macroinitiator for further cross-linking reaction with the cross-linker 3, yielding a star polymer M-poly-L-1(À)-b-3.
assemble to form a helix-bundle structure on a hydrophilic mica
substrate (Figure 3B,C); thereby the number of arms may be
underestimated (Figure 3C).
Based on an evaluation of more than 100 star polymers
including Figure 3B,C, the number-average diameter (Dn), the
weight-average diameter (Dw), and its distribution (Dw/Dn)
were estimated to be 39 ( 10 nm, 42 ( 10 nm, and 1.07,
respectively (Figure S4A). These values were measured without
compensation for the tip radius of curvature (ca. 10 nm), and the
measured values might be overestimated by around 10 nm
because of the broadening effect of the tip. The number-average
molecular height (Hn = 2.5 nm), the weight-average molecular
height (Hw = 3.0 nm), and the height distribution (Hw/Hn = 1.17)
Figure 2. CD and absorption spectra of M-poly-L-1(À) (a) and M-
of the star polymers were also estimated (Figure S4B).
Previously, we found that rigid-rod helical polyisocyanides8,11
including P- and M-poly-L-1s self-assembled to form regular two-
dimensional (2D) helix bundles on a highly oriented pyrolytic
graphite (HOPG) substrate after organic solvent vapor expo-
sures, which allows the visualization of the helical structures. We
then applied this procedure to the star polymer consisting of the
identical M-poly-L-1(À) as the arms.17
poly-L-1(À)-b-3 (b) in chloroform (0.2 mg/mL) at 25 °C.
with SEC, and thus the number of arms (f) was calculated to
be 16.16
Figure 2 shows the CD and absorption spectra of the macro-
initiator (M-poly-L-1(À)) and star polymer (M-poly-L-1(À)-b-3)
in chloroform at 25 °C. The first Cotton effect intensity of M-
poly-L-1(À)-b-3 (Δε364 = À17.7) at 364 nm, which reflects a
helical sense excess of the polymer backbone,8,11 slightly de-
creased (ca. 14%) after the cross-linking reaction of M-poly-L-
1(À) (Δε364 = À20.6) with achiral 3, suggesting that the core
units derived from 3 may not have a preferred-handed helical
conformation, but the left-handed helical conformation of M-
poly-L-1(À) was almost completely retained after the block
copolymerization.
Figure 3 shows the AFM images of the isolated M-poly-L-1(À)
and M-poly-L-1(À)-b-3 molecules prepared by casting a dilute
chloroform solution on mica. Individual rodlike linear macro-
initiator chains with a number-average height (Hn) of ca. 1.2 nm
were clearly observed (Figure 3A), while the star polymers
showed a unique “spiny”-shape topology with varying rodlike
arms attached to a cross-linked central core and the rodlike arms
of the number-average molecular length of ca. 14 nm tend to
Figure 4A shows the typical AFM images of the star polymer,
M-poly-L-1(À)-b-3, deposited on HOPG from a dilute chloro-
form solution followed by chloroform vapor exposure at ambient
temperature for 12 h. Interestingly, the star polymer also self-
assembled to form regular 2D monolayers in a hexagonal-like
arrangement reflecting the crystallographic structure of the
HOPG substrate, in which individual molecules are clearly resolved
as densely packed stars. Since the rodlike arms with a constant
length form regular helix-bundle structures within a star and
between the stars as seen in Figure 4B, the high-resolution AFM
measurements allowed the number of arms in the individual star
polymers (f = 8.9 ( 3.3) (Figure 4C) and its distribution
(Figure 4F) to be obtained. In addition, the left-handed helical
structure of the arms with a helical pitch of 1.33 ( 0.10 nm
(Figure 4D,E) was clearly observed. The observed helical sense
and helical pitch of the arms fairly agreed with those previously
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NOTE
Figure 3. AFM phase (a, b, d, e) and height (c, f) images of M-poly-L-1(À) (A) and M-poly-L-1(À)-b-3 (B) prepared by casting a dilute solution in
chloroform (0.001 mg/mL) on mica. The cross-sectional profile denoted by a white dashed line is also shown in (c, f). (C) AFM phase images (scale
80 nm  80 nm) of M-poly-L-1(À)-b-3 carrying various arms on mica. The number of arms is indicated in each image.
determined for its homopolymer (M-poly-L-1(À)),8 whereas the
average number of arms of the star polymers estimated by AFM on
HOPG was lower than that obtained by the SEC-MALS measure-
ment results (f = 16). This disagreement may be ascribed to
overlapping of the arms on the HOPG, in particular, for star
polymers carrying a higher number of arms. Nevertheless, these
AFM results emphasize a novel core cross-linked star polymer
bearing stiffhelical arms with a controlled helical sense and a constant
length that provides almost the entire structural and topological
information on the star polymers that involves the molecular shape
and branching arm topology of individual star polymers, such as the
number and length of the arms and its handedness (helicity) by high-
resolution AFM. Such a molecular level visualization except for the
helicity is possible only for densely grafted brushlike star polymers
prepared by uniform-arm macroinitiators.14b,15
The chiral recognition ability of the star polymer, M-poly-L-
1(À)-b-3, was then investigated for the racemates (4À6) and a
racemic helical polyisocyanide bearing no chiral pendants using
the enantioselective adsorption technique (Table 1 and Support-
ing Information).18 The star polymer preferentially adsorbed one
of the enantiomers of 4À6, furnishing 23, 6.1, and 11% en-
antiomeric excess (ee), respectively. Based on the amounts and
ee values of the analytes remaining in the supernatants during the
enantioselective adsorption with the star polymer, the separation
factor (α), which is a useful measure to evaluate the chiral
recognition ability of chiral hosts toward racemic analytes in
chiral HPLC, was calculated to be 1.81, 1.20, and 1.45 for 4, 5,
and 6, respectively (Table 1). These values are high enough for
the almost complete separation of enantiomers when used as a
CSP for HPLC, indicating that the star polymer possesses a chiral
recognition ability for small racemic molecules. It should be
noted that the compounds 4 and 5 were enantioselectively
adsorbed better on the star polymer than on the macroinitiator
(M-poly-L-1(À)), although the compound 6 was resolved
slightly better on M-poly-L-1(À) (Table 1).10
Racemic helical polyisocyanides (M-poly-7(À) and P-poly-
7(+)), which had been prepared by a previously reported method
based on the “helicity induction and memory strategy”19 followed
by modification of the side groups (Supporting Information),19c,e,f
was then employed as an analyte. Interestingly, P-poly-7(+) was
helix-sense selectively adsorbed on the star polymer carrying the
opposite left-handed helical arms with a moderate enantioselec-
tivity (α = 1.14), whereas the corresponding macroinitiator
showed a poor chiral recognition (α = 1.03) (Table 1, entry 4,
and Supporting Information). This helix-sense selective adsorp-
tion of racemic helices may have taken place as a result of a
confined chiral space created by the intermolecularly self-as-
sembled bundled helical arms of the star polymers, within which
a large macromolecule with a preferred-handed helicity could be
selectively encapsulated.
’ CONCLUSIONS
In conclusion, we have prepared a novel optically active star
polymer bearing rigid-rod one-handed helical polyisocyanide
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Macromolecules
NOTE
directly visualized by high-resolution AFM. The chiral adsorption
experiments revealed the potential of this topologically unique
star polymer for separating not only small molecules with a point
chirality but also large molecules with a macromolecular helicity.
These findings will be useful for developing further topologically
unique optical active star polymers with specific functions as
chiral materials.
’ MATERIALS AND METHODS
Materials. 4-Isocyanobenzoyl-L-alanine decyl ester (L-1),8,12,17b the
μ-ethynediyl PtÀPd complex (2),7 and optically active right- and left-
handed helical poly(phenyl isocyanide)s with macromolecular helicity
memory (P-poly-7(+) and M-poly-7(À), respectively)19 were prepared
according to the reported methods. The solvents used in the chromato-
graphic experiments were of HPLC grade. The racemates were com-
mercially available or were prepared by the usual methods.20
Synthesis of Star Polymers. The right- and left-handed helical
poly(phenyl isocyanide)s (P-poly-L-1(+) and M-poly-L-1(À)) were
prepared in a similar way to that previously reported (Figure 1).8 The
polymerization of L-1 (2.00 g, 5.60 mmol) was carried out in a dry glass
ampule under a dry nitrogen atmosphere in THF using 2 ([1]/[2] =
100) as the catalyst at 55 °C for 20 h. The obtained poly-L-1 (1.86 g, 93%
yield) was then fractionated with acetone into high-MW left-handed
helical M-poly-L-1(À) and low-MW right-handed helical P-poly-L-1(+)
in 62 and 13% yield, respectively (Figure 1). The Mn and Mw/Mn were
1.18 Â 105 and 1.09 (M-poly-L-1(À)) and 6.74 Â 104 and 1.08 (P-poly-
L-1(+)), respectively, determined by SEC with polystyrene standards
and THF containing 0.1 wt % tetra-n-butylammonium bromide (TBAB)
was used as the eluent at the flow rate of 1.0 mL/min. The resulting M-
poly-L-1(À) was further used as the macroinitiator. The Mn and Mw of
M-poly-L-1(À) as determined by SEC-MALS measurements were 6.00
 104 and 6.38  104, respectively.
A solution of the cross-linking monomer 3 (12.6 mg, 0.0336 mmol) in
dimethylformamide (DMF) (0.8 mL) was added to a solution of M-
poly-L-1(À) (120 mg, 0.335 mmol) in THF (2.6 mL), and the mixture
was then stirred under a dry nitrogen atmosphere and heated to 55 °C.
After 20 h, the resulting star polymer (M-poly-L-1(À)-b-3) was pre-
cipitated into a large amount of methanol (50 mL), collected by
centrifugation, and dried in vacuo at room temperature overnight.
The star polymer was further fractionated by the recycling preparative
SEC to remove the unreacted macroinitiator, yielding the higher MW
star polymer (23.8 mg, 18% yield) (Figure 1). The SEC measurement
revealed that the purified star polymer contained the macroinitiator of
less than 1 mol % after the recycling preparative SEC. The Mn and Mw/
Mn of the fractionated star polymer were 9.40 Â 105 and 1.10,
respectively, as determined by SEC with polystyrene standards and
THF containing 0.1 wt % TBAB was used as the eluent at the flow rate of
1.0 mL/min. The Mn and Mw of the star polymer were also measured by
SEC-MALS and were 5.79 Â 105 and 1.11 Â 106, respectively. The
number of arms per molecule (f) was then calculated to be 16 according
to the equation f = (weight fraction of arm) Â Mw(star)/Mw(arm).16
Spectroscopic data of M-poly-L-1(À)-b-3: IR (film, cmÀ1): 3278
(νNÀH), 1747 (νCdO ester), 1633 (amide I), 1536 (amide II). 1H NMR
(CDCl3, 55 °C, 500 MHz): δ 0.87 (broad, CH3, 3H), 1.25 (broad, CH2,
14.4H), 1.53 (broad, CH3 and CH2, 5.4H), 3.30À4.30 (broad, CH2,
2.4H), 4.30À4.80 (broad, CH, 1H), 4.80À7.80 (broad, aromatic, 4.9H),
Figure 4. (A) AFM height (left) and phase (right) images (scale
250 nm  250 nm) of 2D self-assembled M-poly-L-1(À)-b-3 on HOPG.
The samples were prepared by depositing a dilute chloroform solution
(0.02 mg/mL), followed by exposure to chloroform vapor for 12 h at
ambient temperature (ca. 25 °C). The cross-sectional profile denoted by
a white dashed line is also shown. (B) AFM phase image (scale 162 nm
 162 nm) of 2D self-assembled M-poly-L-1(À)-b-3 on HOPG. Zoomed
AFM image corresponding to the area indicated by a white square is
shown in (C). Schematic representations of possible helix-bundle
arrangements are also shown (bottom). Each molecule is indicated by
different colors. (D) AFM phase image (scale 210 nm  210 nm) of 2D
self-assembled M-poly-L-1(À)-b-3 on HOPG. Zoomed AFM image
corresponding to the area indicated by a white square is shown in (E).
The yellow oblique lines indicate the left-handed helical array of the
pendants. (F) Histogram of distribution of the number of arms of M-
poly-L-1(À)-b-3 obtained from AFM images taken from more than 100
molecules.
20
8.00À9.00 (broad, NH, 1.2H). [α]D À1553 (c 0.1, chloroform). Anal.
Calcd (%) for (C21H30N2O3)9(C22H22N4O2)1(H2O)3.2: C, 69.27; H,
8.22; N, 8.42. Found: C, 69.22; H, 8.10; N, 8.42.
arms by the copolymerization of the rodlike polyisocyanide
macroinitiator with a bifunctional cross-linking agent. The star-
shaped “spiny” structure with individual helical arms including
the number and length of the arms and its handedness has been
Typical Procedure for Enantioselective Adsorption of
Small Chiral Compounds. A typical experimental procedure is
described below. The star polymer, M-poly-L-1(À)-b-3 (16.0 mg), was
thoroughly washed with a hexaneÀ2-propanol mixture (90:10, v/v)
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Table 1. Enantioselective Adsorption of Racemic Compounds (4À6) and Helical Polymer (Poly-7) on M-Poly-L-1(À)-b-3a
yield (ee) of adsorbed
ee of free analyte
in solutionb (%)
separation
factorc (α)
separation factor
(α) of M-poly-L-1(À)d
entry
analyte
analyteb (%)
1e
4
5
6
22.0 ( 0.4 (23 ( 1 (À))
34.6 ( 1.9 (6.1 ( 0.7(+))
42.4 ( 2.2 (11 ( 1 (À))
6.91i (6.0 (+))i,j
6.5 ( 0.3 (+)
3.2 ( 0.3 (À)
7.9 ( 0.5 (+)
0.45 (À)j
1.81 ( 0.05 (+)
1.20 ( 0.02 (À)
1.45 ( 0.02 (+)
1.14 (À)
1.70 (+)
1.10 (À)
1.53 (+)
1.03 (À)k
2e,f
3e,g
4h
M-poly-7(À)/P-poly-7(+)
a Experimental conditions: M-poly-L-1(À)-b-3 16.0 mg; analyte 0.005 mg (200 μL portion from a 0.025 mg/mL solution in hexaneÀ2-propanol (98:2,
v/v)). b Determined by HPLC analysis of the supernatant solution using a Chiralcel OD-H column; eluent, hexaneÀ2-propanol (90:10, v/v); flow-rate,
0.5 mL/min. c Calculated according to the equation α = (Fmajor (%)/Fminor (%))/(Amajor (%)/Aminor (%)), where Fmajor and Fminor are the percentages of
major and minor enantiomers of free analyte in the supernatant solutions, respectively, and Amajor and Aminor are those of major and minor enantiomers
of adsorbed analyte, respectively. d Cited from ref 10. These values were determined by chromatographic separation using M-poly-L-1(À) chemically
bonded to silica gel as the CSP. e Average values of four runs. f M-poly-L-1(À)-b-3 8.0 mg. g M-poly-L-1(À)-b-3 4.0 mg. h Experimental conditions: M-
poly-L-1(À)-b-3 film 0.5 mg; analyte 0.41 mg (7.5 mL portion from a 0.055 mg/mL solution in toluene). i Determined by absorption spectra of the
supernatant solution. j Determined by CD spectra of the supernatant solution (see Figure S2). k Experimental conditions: M-poly-L-1(À) chemically
bonded to silica gel 13 mg; analyte 1.1 mg (20 mL portion from a 0.055 mg/mL solution in toluene).
prior to the enantioselective adsorption experiments. A solution of racemic
trans-stilbene oxide (4) in a hexaneÀ2-propanol mixture (98:2, v/v)
(0.025 mg/mL, 200 μL) was added to a screw-capped sample bottle
containing the star polymer. A small amount of the supernatant
solution was withdrawn after stirring for 2 h and analyzed by a chiral
HPLC using a Daicel Chiralcel OD-H column (25 cm  0.46 cm
(i.d.)) equipped with UV and polarimetric detectors to determine
the concentration and ee of the analyte in the supernatant. The
results are summarized in Table 1. Separation factor (α) was
calculated according to α = (Fmajor (%)/Fminor (%))/(Amajor (%)/
Aminor (%)), where Fmajor and Fminor are the percentages of major and
minor enantiomers of free analyte in the supernatant solution,
respectively, and Amajor and Aminor are those of major and minor
enantiomers of adsorbed analyte, respectively.18
Typical Procedure for Helix-Sense Selective Adsorption of
Helical Poly(phenyl isocyanide)s with Macromolecular He-
licity Memory. A solution of M-poly-L-1(À)-b-3 in chloroform (500
μL, 1.0 mg/mL) was placed in a sample bottle with a screw cap.
Chloroform was removed by evaporation to produce the M-poly-L-
1(À)-b-3 film, and the resulting film was dried in vacuo. A toluene
solution containing an equal amount of P-poly-7(+) and M-poly-
7(À) (7.5 mL, 0.055 mg/mL) was added to a screw-capped sample
bottle containing the star polymer film. A small amount of the
solution was withdrawn at appropriate time intervals, and the
UVÀvis and CD spectra of the supernatant solution were measured
at 25 °C to estimate the amount and helical sense excess of the
polymer analytes adsorbed on the star polymer film (Figure S2). The
helical sense (P or M) and helical sense excess of the free analytes in the
supernatant solution were determined by comparison of the sign and
anisotropy factor, g value (Δε/ε), of the first Cotton effect to those of M-
poly-L-1(À), whose helical sense and helical sense excess had been directly
determined by high-resolution AFM observations.8,11 The results are sum-
marized in Table 1 (run 4).
of 0.8À1.3 V, and a scan rate of 1.5 Hz. The Nanoscope image
processing software was used for the image analysis. Polymer
diameters were measured using the Image J program, developed at
the National Institutes of Health.
AFM Measurements of 2D Assembled M-Poly-L-1(À)-b-3
on HOPG. A stock solution of M-poly-L-1(À)-b-3 in chloroform (0.02
mg/mL) was prepared. Samples for the AFM measurements of M-poly-
L-1(À)-b-3 were prepared by casting 15 μL aliquots of the stock solution
on a freshly cleaved HOPG at room temperature (ca. 25 °C), and the
solvents were then slowly evaporated under a chloroform vapor atmo-
sphere. After the polymer had been deposited on the HOPG, the HOPG
substrates were exposed to chloroform vapors at room temperature for
12 h, and the substrates were then dried under vacuum for 2 h. The
organic solvent vapors were prepared by putting 1 mL of chloroform
into a 2 mL flask that was inside a 50 mL flask, and the HOPG substrates
were then placed in the 50 mL flask. The typical settings of the AFM for
the high-magnification observations were as follows: a free amplitude of
the oscillation frequency of ca. 1.0À1.5 V, a set-point amplitude of
0.9À1.4 V, and a scan rate of 2.5 Hz.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details in the
b
synthesis and characterization of 3 and P- and M-poly-7, CD
and absorption spectral changes of P/M-poly-7 in the presence of
star polymers, SEC-MALS and DLS measurement results, and
histograms of the size and height distributions of star polymers
obtained from the AFM images. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: yashima@apchem.nagoya-u.ac.jp.
AFM Measurements of Individual Polymers and Star
Polymers on Mica. Stock solutions of M-poly-L-1(À)-b-3 and M-
poly-L-1(À) in chloroform (0.001 mg/mL) were prepared. Samples for
the AFM measurements were prepared by casting 7 μL aliquots of
the stock solutions of the polymers on a freshly cleaved mica, the
solution was blown off simultaneously with a stream of nitrogen,
and then the mica substrate was dried in vacuo overnight to measure
the AFM images. The AFM measurements were performed using
a Nanoscope IIIa microscope in air at ambient temperature with
standard silicon cantilevers in the tapping mode. The typical settings
of the AFM observations were as follows: a free amplitude of
the oscillation frequency of ca. 1.0À1.5 V, a set-point amplitude
’ ACKNOWLEDGMENT
We are deeply grateful to Professor K. Onitsuka (Osaka
University) for his generous supply of the PtÀPd catalyst (2).
This work was supported in part by Grant-in-Aid for Scientific
Research (S) (E.Y.) and Grant-in-Aid for Young Scientists (B)
(H.I.) from the Japan Society for the Promotion of Science
(JSPS) and the Global COE Program “Elucidation and Design of
Materials and Molecular Functions” of the Ministry of
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Macromolecules
NOTE
Education, Culture, Sports, Science, and Technology, Japan. T.
M. and M.B. express their thanks for JSPS Research Fellowships
for Young Scientists (no. 3247 and no. 9164, respectively).
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