Optically active polyacrylamides bearing an oxazoline pendant: Influence of stereoregularity on both chiroptical properties and chiral recognition

Article abstract of DOI:10.1002/pola.24346

Enantiopure acrylamide derivatives, N-[o-(4-methyl-4,5-dihydro-1,3-oxazol- 2-yl) phenyl]acrylamide (MeOPAM), N-[o-(4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl) phenyl]acrylamide (PrⁱOPAM), and N-[o-(4-phenyl-4,5-dihydro-1,3- oxazol-2-yl)phenyl]acry

Full text of DOI:10.1002/pola.24346

Optically Active Polyacrylamides Bearing an Oxazoline Pendant: Influence
of Stereoregularity on Both Chiroptical Properties and Chiral Recognition
WEI LU, LIPING LOU, FANGYU HU, LIMING JIANG, ZHIQUAN SHEN
Department of Polymer Science and Engineering, MOE Key Laboratory of Macromolecular Synthesis and Functionalization,
Zhejiang University, Hangzhou 310027, China
Received 2 April 2010; accepted 12 August 2010
DOI: 10.1002/pola.24346
Published online 5 October 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Enantiopure acrylamide derivatives, N-[o-(4-methyl-
4,5-dihydro-1,3-oxazol-2-yl) phenyl]acrylamide (MeOPAM), N-
[o-(4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl)phenyl]acrylamide
(PriOPAM), and N-[o-(4-phenyl-4,5-dihydro-1,3-oxazol-2-yl)phe-
nyl]acrylamide (PhOPAM), were synthesized and radically
polymerized in the presence of rare earth metal trifluorometha-
nesulfonates (Ln(OTf)3, Ln ¼ La, Nd, Sm, and Y) to yield corre-
sponding optically active polymers. Among these Lewis acids,
Y(OTf)3 was found to be most effective for increasing the iso-
tactic specificity during the radical polymerizations when using
n-butanol as solvent. Also, the effect of the Lewis acids was
significantly influenced by the ratio of Ln(OTf)3 to monomer.
The relationship of both chiroptical property and the chiral rec-
ognition with the stereoregularity was then examined for the
resulting polymers having various tacticity by spectroscopic
techniques such as NMR, fluorescence, and circular dichroism.
The results indicated that the polymers rich in isotacticity
exhibited a favorable enantioselective discrimination ability to-
ward 1,10-bi-2-naphthol as evidenced by 1H NMR study, where
the characteristic hydroxyl proton signal was split into two
peaks that ascribed respectively to the levo- and dextro-isomer;
furthermore, the splitting magnitude was linearly correlated
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with the diad isotacticity of the polymers. 2010 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 48: 5411–5418, 2010
KEYWORDS: chiral; enantioselective recognition; radical poly-
merization; rare-earth trifluoromethanesulfonates; stereospe-
cific polymers
INTRODUCTION Stereochemistry is one of the main factors
that determine the physical and mechanical properties of a
polymeric material as well as its functions.1,2 Polymers that
have stereocenters in the repeat unit can exhibit two struc-
tures of maximum order, isotactic and syndiotactic. The ster-
eoregular polymers are typically crystalline and have found
use in many applications. Metal-based catalysts or initiators
have proven to be one of the most promising methodologies
for the synthesis of stereoregular polymers,3 in which the
stereospecific polymerization is usually attained via an ionic
or coordination mechanism. In contrast to these controlled
ionic and coordinative polymerizations, the stereocontrol in
radical polymerization is very difficult because of the lack of
efficient methods for providing an asymmetric coordination
environment around the propagating radical species.4 Never-
theless, this decade also witnessed great progress in terms
of stereocontrol during radical polymerization. For example,
Okamoto et al. first reported a general method applicable to
some polar monomers, such as (meth)acrylamides, a-(alkoxy-
methyl)acrylates, and methacrylates where the added lan-
thanide trifluoromethanesulfonates (Ln(OTf)3) catalytically
affect the polymerization stereochemistry to isotactic-selec-
tive manner because of their strong coordination ability to
the polar group.5–8 More recently, the Lewis acids were used
in conjunction with controlled/living radical polymerization
techniques for the simultaneous control of the molecular
weight and the tacticity of polyacrylamide derivatives.9,10
The efficient dual control was ascribed to the effective inter-
action of the Lewis acid with the amide moieties around the
polymer terminal and/or the incoming monomer.
For several years, we have been interested in the synthesis
of polymeric derivatives bearing side-chain optically active
oxazoline moieties, because the presence of these chromo-
phores is expected to establish the dissymmetric chain con-
formations with a prevailing handedness and the formation
of polymeric complexes. We found that maleimide- and
methacrylamide-based optically active polymers bearing the
oxazoline pendant show a moderate enantioselectivity to-
ward some substrates.11 As a part of our continuing research
to understand the fundamentals of chiral recognition, we
herein report an efficient Ln(OTf)3-mediated radical poly-
merization procedure for synthesizing optically active poly-
acrylamide derivatives with various tacticity (Scheme 1). In
addition to the polymerization features and the investigation
of chiroptical and fluorescent properties of the optically
active polymers, the morphology on solid substrates and the
enantioselective interaction with 1,10-bi-2-naphthol (BINOL)
were examined with atomic force microscopy (AFM) and 1H
Additional Supporting Information may be found in the online version of this article. Correspondence to: L. Jiang (E-mail: cejlm@zju.edu.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 5411–5418 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
AC6H5), 7.86–7.89 (d, 1H, AC6H5), 7.48–7.53 (t, 1H, AC6H5),
7.10–7.13 (t, 1H, AC6H5), 6.44–6.49 (d, 1H, ¼¼CH2), 6.29–
6.35 (m, 1H, ¼¼CHCO), 5.77–5.80 (d, 1H, ACH2), 4.41–4.46 (t,
1H, OCH2), 3.17–4.20 (m, 1H, ¼¼NCH), 4.07–4.11 (t, 1H,
OCH2), 1.83 (m, 1H, ACH(CH3)2), 1.00–1.09 (d, 6H,
ACH(CH3)2). Anal. calcd. for C15H18N2O2: C, 69.40; H, 7.48;
N, 10.79. Found: C, 69.47; H, 7.69; N, 10.61.
(R)-PhOPAM
SCHEME 1 Synthesis of poly(ROPAM)s.
Yield: 85%, colorless crystal; mp 127.5–128.0 ꢀC, [a]D25 ¼ þ
223.0ꢀ (c ¼ 1.00 g/dL, l ¼ 10 cm, THF). FT-IR (KBr, pellet,
cm-1): 3408 (ANHA); 1687, 1539 (ACONHA); 1635, 1065,
957 (oxazoline ring); 1589, 1493 (phenyl). 1H NMR (400
MHz, CDCl3, d, ppm): 12.56 (s, 1H, NH), 8.87 (d, 1H, AC6H5),
7.94 (d, 1H, AC6H5), 7.50–7.54 (t, 1H, AC6H5), 7.25–7.41 (m,
5H, AC6H5), 7.11–7.15 (t, 1H, AC6H5), 6.31–6.35 (d, 1H,
¼¼CH), 6.17–6.23 (m, 1H, ¼¼CHCO), 5.63–5.66 (d, 2H, ¼¼CH2),
5.50–5.55 (t, 1H, OCH2), 4.75–4.80 (t, 1H, ¼¼NCH), 4.18–4.22
(t, 1H, OCH2). Anal. calcd. for C18H16N2O2: C, 74.21; H, 5.21;
N, 9.62. Found: C, 74.31; H, 5.47; N, 9.64.
NMR spectroscopy, respectively. To the best our knowledge,
although many poly[(meth)acrylamide]s bearing optically
active side chains have been synthesized by a radical process
and used as chiral stationary phases for high-performance
liquid chromatography,12 there were relatively few reports
on the relationship of the stereoregularity with both chiropti-
cal properties and chiral discrimination with respect to these
optically active polymers.5(a),8
EXPERIMENTAL
Materials
Polymerization and Fractionation Experiment
a,a0-Azobisisobutyronitrile (AIBN, Shanghai Chemical Reagent
Co., China) was purified by recrystallization from methanol.
Solvents for the polymerizations were treated over a benzo-
phenone- sodium complex for three days and distilled before
use. 1,10-Bi-2-naphthol (BINOL, Aldrich) and other chemicals
were used as received. Rare earth metal trifluoromethanesul-
fonates (Ln(OTf)3, Ln ¼ La, Nd, Sm, and Y) were prepared
according to a reported method and used after drying in
vacuo.13
The radical polymerization was carried out unꢀder dry nitro-
gen using Schlenk techniques with AIBN at 60 C. After 24 h,
the reaction mixture was poured into a large amount of
mixed CH3OH/H2O (3:1, v/v) to precipitate the product. The
collected polymer was purified by reprecipitation from a
THF-CH3OH system in three cycles and dried under reduced
pressure at 40 ꢀC for 2 days. For AFM measurements, the
polymer samples with a narrow molecular weight distribu-
tion (MWD) (Mw/Mn ꢂ 1.2) were prepared by a successive
fractionation from THF solution with n-hexane as the nonsol-
vent according to a reported procedure.15
Monomer Synthesis
The chiral monomers (ROPAM, R ¼ Me, Pri, and Ph) were
synthesized by the reaction of enantiopure ortho-oxazolinyl-
substituted anilines and acryloyl chloride in the presence of
triethylamine in tetrahydrofuran (THF) according to our pre-
viously described method.14
Measurements
1H and 13C NMR spectra were recorded on a Bruker Avance
AMX-400 NMR instrument in CDCl3. D-Line specific optical
rotations ([a]2D5) were measured in THF at 25 ꢀC using a
Perkin Elmer polarimeter (model 341). Elemental analysis
was performed on a ThermoFinnigan Flash EA 1112 ana-
lyzer. The molecular weights of the polymers were deter-
mined by gel permeation chromatography (GPC) using a PL-
GPC 220 apparatus equipped with a PLgel 3 lm (300 mm ꢃ
7.5 mm) MIXED-B columns and a refractive index detector
(eluent, THF; a flow rate, 1.0 mL/min). The GPC chromato-
gram was calibrated against standard polystyrenes. ꢀCircular
dichroism (CD) spectra were obtained in THF at 25 C using
a quartz cell of 1 cm with a JASCO J-820 spectropolarimeter.
UV spectra were recorded on a JASCO V-530 spectrophoto-
meter. AFM measurements were carried out on a SPI3800N
AFM in the tapping mode. Height and phase images were
simultaneously measured at the resonance frequency of the
silicon tips with cantilevers 20 lm in length. Steady-state
fluorescence spectra were recorded on a PerkinElmer LS-55
fluorescence spectrometer in the right-angle geometry (90ꢀ
collecting optics). The excited wavelength for the measure-
ment was set at 310 nm.
(S)-MeOPAM
Yield: 66%, colorless crystal; mp: 56.5–57.5 ꢀC, [a]D25
¼
þ6.2ꢀ (c ¼ 1.00 g/dL, l ¼ 10 cm, THF). FT-IR (KBr, pellet,
cmꢁ1): 3420 (ANHA); 1682, 1544 (ACONHA); 1619, 1064,
960 (oxazoline ring); 1604, 1449 (phenyl). 1H NMR (400
MHz, CDCl3, d, ppm): 12.56 (s, 1H, NH), 8.83 (d, 1H, AC6H5),
7.85 (d, 1H, AC6H5), 7.45–7.49 (t, 1H, AC6H5), 7.07–7.10 (t,
1H, AC6H5), 6.45 (d, 1H, ¼¼CH2), 6.30 (m, 1H, ¼¼CHCO), 5.75
(d, 1H, ACH2), 4.48 (m, 2H, OCH2), 3.91 (m, 1H, ¼¼NCH),
1.37 (d, 3H, ACH3). Anal. calcd. for C13H14N2O2: C, 68.11; H,
5.72; N, 12.22. Found: C, 68.44; H, 5.99; N, 12.01. The (R)-
isomer was also synthesized by the same procedure for
comparison.
(S)-PriOPAM
Yield: 65%, colorless crystal; mp 28.0–28.5 ꢀC, [a]D25 ¼ þ
28.4ꢀ (c ¼ 1.00 g/dL, l ¼ 10 cm, THF). FT-IR (KBr, pellet,
cm-1): 3412 cmꢁ1 (ANHA); 1688, 1540 (ACONHA); 1635,
1065, 973 (oxazoline ring); 1587, 1449 (phenyl). 1H NMR
(400 MHz, CDCl3, d, ppm): 12.58 (s, 1H, NH), 8.86 (d, 1H,
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ARTICLE
TABLE 1 Radical Polymerization of (S)-MeOPAM in the Presence of Ln(OTf)3 at 60 8Ca
Tacticity
(m/r)d
[a]2D5
(deg)e
Entry
Ln (mol/L)
Solvent
Yield (%)b
Mn (103)c
Mw/Mn
c
1
None
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
n-BuOH
MeOH
MeOH
MeOH
EtOH
67.9
69.2
57.2
67.1
59.2
53.1
50.4
53.8
70.0
57.6
52.8
61.8
52.0
75.6
38.9
67.1
37.8
14.5
13.1
18.2
12.5
11.0
10.0
8.3
1.75
2.07
1.84
1.98
1.88
1.85
1.96
1.83
1.63
1.76
1.60
2.05
1.64
1.65
1.41
1.82
1.75
36/64
69/31
70/30
52/48
71/29
85/15
91/9
þ10.0
ꢁ9.3
ꢁ9.8
þ3.3
ꢁ16.7
ꢁ19.6
ꢁ28.5
ꢁ33.4
þ11.4
þ7.4
þ1.7
þ10.1
0
2
La(0.05)
Nd(0.05)
Sm(0.05)
Y(0.05)
Y(0.15)
Y(0.25)
Y(0.5)
3
4
5
6
7
8
8.1
95/5
9
None
34.8
15.0
12.2
10.1
13.6
11.2
10.0
2.87
4.46
15/85
43/57
48/52
35/65
52/48
40/60
50/50
45/55
45/55
10
11
12
13
14
15
16
17
a
Y(0.05)
Y(0.25)
None
Y(0.05)
None
EtOH
EGBEf
EGBEf
THF
þ7.7
þ4.7
þ6.0
þ6.5
Y(0.05)
None
Y(0.05)
THF
[Monomer]0 ¼ 0.5 mol/L, [AIBN]0 ¼ 0.01mol/L, time ¼ 24 h.
b
c
d
e
f
Methanol-insoluble part.
Determined by GPC in THF (calibrated against standard polystyrene samples).
Determined by 1H NMR measurement in CDCl3.
c ¼ 0.1 g/dL, THF, l ¼ 10 cm.
EGBE, ethylene glycol butyl ether.
RESULTS AND DISCUSSION
more favorable for the isotactic-specific polymerization of
(S)-MeOPAM. For example, the polymerization in n-butanol
with 10 mol % of Y(OTf)3 afforded the isotactic-rich polymer
Polymerization Characteristics
Table 1 summarizes the experimental conditions and results
on the radical polymerization of (S)-MeOPAM in the presence
of a catalytic amount of lanthanide trifluoromethanesulfo-
nates (Ln(OTf)3) in n-butanol, methanol, ethanol, ethylene
glycol butyl ether, and THF. The tacticity of the resultant
polymers could be estimated by 13C NMR spectroscopy on
the basis of the splitting of methine signals,16 and a repre-
sentative example is shown in Figure 1. The data from Table
1 indicate that the isotactic-rich polymers (meso diad m ¼
52–95%) were obtained in the Ln(OTf)3-mediated radical
polymerizations, whereas the conventional process without
Lewis acid produced polymers rich in syndiotacticity. Among
the Lewis acids, Y(OTf)3 was found to be most effective with
respect to the stereocontrol during the radical polymeriza-
tion. When the polymerization carried out in n-butanol, the
isotacticity of polymers increased with an increase in the
amount of added Y(OTf)3 and reached a maximum (meso
diad m ¼ 95%) at the ratio of [Y(OTf)3]/[(S)-MeOPAM] ¼
1.0. Further addition of Y(OTf)3 did not enhance the isotactic
selectivity but resulting in the decrease of polymer yield and
molecular weights.
FIGURE 1 13C NMR spectrum of poly[(S)-MeOPAM] prepared
by the radical polymerization with Y(OTf)3 in n-butanol (entry 5
in Table 1).
As presented in Table 1, solvents significantly influenced the
effect of Lewis acids on the radical polymerization stereo-
chemistry. In the present systems, n-butanol seemed to be
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
cific rotations increased in the negative direction with an
increase in the isotacticity (entries 1, 2, and 5–8). Interest-
ingly, the m content of 52% seemed to be a critical point, at
which a specific rotation of [a]2D5 ¼ 0 was observed (entry
13), indicating that this polymer sample was optically inac-
tive. Considering the tendency for the isotactic polymer
chains to form a helical conformation,17 we assume that the
Lewis acid-mediated polymerization likely afforded the heli-
cal polymers with a prevailing handedness; furthermore, the
higher ordered structure would exert an opposite effect on
the optical activity with respect to the chiral pendant groups.
In this hypothesis, the variation in optical rotation for the
polymers can be interpreted as being attributed to the result
of combining the configurational effect from lateral groups
with the helical conformational effect of the main chain.
Figure 2 depicts the change in specific optical rotation ([a]D)
of poly[(S)-MeOPAM]s with different tacticity upon the mea-
surement temperature in THF. Clearly, the specific rotations
of all the polymers decreased with increasing temperatures
and this variation was strongly dependent on their stereore-
gularity. In the cases of syndiotactic-rich polymer (m/r ¼
36/64) and isotactic-rich polymer (m/r ¼ 79/21), the values
FIGURE 2 Relationship between the specific rotation and the
measurement temperature in THF for poly[(S)-MeOPAM]s with
various tacticities: (n) m/r ¼ 36/64 (entry 1), (~) m/r ¼ 79/21
(entry 5), (l) m/r ¼ 95/5 (entry 8). Da ¼ [a]1D5 ꢁ [a]DT . (n, ~ and
l: heating; h, ~ and *: cooling). [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
(m/r ¼ 79/21, entry 5 in Table 1); but, poor stereocontrol
was observed in other solvents such as methanol, ethanol,
ethylene glycol butyl ether, and THF under the same condi-
tions (m ¼ 43–54%, see: entries 10, 13, 15, and 17). In fact,
such a solvent dependence has been reported by Okamoto
et al., in that case methanol was found to be the most suita-
ble solvent for the stereocontrolled polymerizations of
(meth)acrylamide.5(a),6 Apparently, this phenomenon may be
ascribed to the fact that the nature of solvents should affect
the coordination ability of the Lewis acid thus changing its
interaction with the propagating radicals and hence the
stereochemistry during the polymerization.
In addition, the radical polymerization of (S)-PriOPAM with
Lewis acids also exhibited a similar tendency as observed in
the (S)-MeOPAM polymerization. That is, a moderate stereo-
control was achieved in the presence of Y(OTf)3 using n-bu-
tanol as solvent to give isotactic-rich polymers with m values
ranging from 0.54 to 0.86. However, for the alcohol-insoluble
monomer PhOPAM, we only obtained atactic polymers in
THF or toluene medium. Representative data including NMR
spectra can be found in the Table S1 and Figure S1 in the
Supporting Information.
Chiroptical and Fluorescent Properties
As can be seen from Table 1, the optical activity of poly[(S)-
MeOPAM]s correlated clearly with their tacticity. The syndio-
tactic polymers, prepared by the polymerization with or
without Lewis acids, showed a specific rotation having the
same sign as the corresponding monomer (S)-MeOPAM
([a]2D5 ¼ þ6.2ꢀ). In contrast, the polymers with an isotactic-
rich structure were all levorotary, and the magnitude of spe-
FIGURE 3 CD and UV spectra of (a) poly[(S)-MeOPAM] (m/r ¼
91/9, [a]2D5 ¼ ꢁ28.5ꢀ, entry 7), (b) poly[(S)-MeOPAM] (m/r ¼ 36/
64, [a]2D5 ¼ þ10.0ꢀ, entry 1), (c) poly[(R)-MeOPAM] (m/r ¼ 85/15,
[a]2D5¼ þ24.4ꢀ), and (d) poly[(R)-MeOPAM] (m/r ¼ 34/66, [a]2D5
¼
ꢁ7.8ꢀ). [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
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ARTICLE
the pꢁp* transition region. In the case of poly[(S)-MeOPAM]s
(Curves a and b), the CD peak intensity at the first and sec-
ond Cotton effects for the isotactic polymer (m/r ¼ 91/9,
[a]2D5 ¼ ꢁ28.5ꢀ) was all smaller than that of the correspond-
ing syndiotactic-rich sample (m/r ¼ 36/64, [a]2D5 ¼ þ10.0ꢀ).
In other words, the CD peak intensity decreased with the
increasing isotacticity of the polymers. Furthermore, this CD
intensity difference at the first Cotton effect was larger than
that at the second Cotton effect. On the other hand, the sec-
ond Cotton effect of poly[(R)-MeOPAM] with m/r ¼ 85/15
([a]2D5 ¼ þ24.4ꢀ) was much weak compared with the syndio-
tactic polymer (m/r ¼ 34/66, [a]2D5 ¼ ꢁ7.8ꢀ) accompanying
a minor red-shift, whereas the difference in the first Cotton
effect was negligible (Curves c and d). These results sug-
gested that a small conformational difference based on the
stereoregularity may exist among these polymers.
FIGURE 4 Absorption and fluorescence spectra: (a) (S)-MeO-
PAM, (b) poly[(S)-MeOPAM] (m/r ¼ 95/5, entry 8 in Table 1),
and (c) poly[(S)-MeOPAM] (m/r ¼ 36/64, entry 1 in Table 1). c ¼
5 ꢃ 10ꢁ5 mol/L (monomeric unit); Excitation wavelength ¼ 310
nm, in THF, 25 ꢀC.
As well as the aforementioned poly(MeOPAM)s, poly[(S)-
PriOPAM] also showed a similar CD spectral pattern in the
pꢁp* transition region with a negative first Cotton effect
around 270 nm and a positive second Cotton effect around
235 nm (Supporting Information Fig. S2). In contrast to
what observed in the case of poly(MeOPAM)s, the CD peak
intensity seemed to be directly proportional to the m content
of the polymers. That is, the higher m content of the poly-
mers and more intensive the CD peak was observed at the
both Cotton effects. As for atactic poly[(R)-PhOPAM]s synthe-
sized by either the Lewis acid-mediated radical polymeriza-
tion or a conventional process, split-type CD peaks with
same sign were observed at 250 and 320 nm, and the pat-
tern is different from that for poly[(R)-MeOPAM] and
poly[(S)-PriOPAM]. In addition, the CD intensity of poly[(R)-
PhOPAM]s were almost same each other although they had
different values of specific rotation. These facts indicate that
the structure of substituents connecting to the lateral chiral
center had a significant influence on the chiropical property
of the polymers.
of temperature coefficient (D[a]D/DT) were ꢁ0.05 and
ꢁ0.19, respectively. The highest value of ꢁ0.36 was observed
for the polymer sample with m/r ¼ 95/5 (entry 8 in Table
1). The noticeable temperature dependence indicated that
the isotactic polymers might contain a conformational chiral-
ity in addition to a configurational one. However, the contri-
bution of conformational chirality to the optical activity
seems to be slight compared with one-handed helical poly-
methacrylates bearing a bulky pendant,17(c),18 because the
change of specific optical rotation is reversible and the tem-
perature coefficient is rather small.
The chiroptical properties of the optically active polymers
were investigated with UV–vis and CD spectroscopies. Figure
3 shows the representative spectra of poly(MeOPAM)s with
different tacticities in THF solution. Poly[(S)-MeOPAM] and
its (R)-analogue displayed split-type CDs of mirror images in
Because the presence of a chiral lateral chromophore con-
necting to the polymer backbone, by the fluorescence
FIGURE 5 Typical tapping-mode
AFM (left) height and (right)
phase images of the fractionated
poly[(S)-MeOPAM] sample (Mn
¼
1.3 ꢃ 104, MWD ¼ 1.19, m/r ¼ 95/
5, entry 8 in Table 1) on mica.
The sample concentration was
0.5 wt % in THF.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 6 The hydroxyl region in 1H NMR spectra of racemic BINOL (5 mmol/L) in the presence of poly(ROPAM)s (R ¼ Me and Pri)
prepared by the radical polymerization under various conditions; the polymer concentration ¼ ꢄ5 mmol/L (monomeric unit),
CDCl3, r.t.
measurement we were able to gain some additional insight
into the stereoregularity and its specific effects. As shown in
Figure 4, the polymers emitted excimer-based fluorescence
at 510 nm with more than eight times larger intensity than
the monomer did, which means the chromophore groups of
poly[(S)-MeOPAM]s took a position that was favorable to
form the excimer.19(a) Moreover, the isotactic-rich polymer
(m/r ¼ 95/5) exhibited a more intensive fluorescence com-
pared with the syndiotactic-rich one (m/r ¼ 36/64), their
fluorescence quantum yield (Uf) were 10.8% and 8.9%,
respectively. It has been reported that the emission spectra
of polymers associate largely with the secondary ordered
structure, and the helical structure is believed to be favor-
able to the formation of the excimer.19(b,c) Accordingly, the
fact that the poly[(S)-MeOPAM] rich in isotacticity had a
stronger fluorescence intensity may be another proof for the
existence of the prevailing one-handed helical chain sections.
could form aggregates with spiral structure through a self-
assembly process.
AFM may be one of the most powerful tools to investigate
the aggregative structure of the polymers in nanometer
scale. When adsorbed polymers were long enough, their heli-
cal conformations might be clearly visible in the high-resolu-
tion AFM images.8(b),20–22 In this study, we first fractionated
the original sample poly[(S)-MeOPAM] having the m content
of 0.95 (entry 8 in Table 1) by the usual method,15 and then
chose the fifth fraction (Mn ¼ 1.3 ꢃ 104, MWD ¼ 1.19) for
AFM measurements. Figure 5 shows the typical AFM images
of this polymer sample deposited onto the surface of freshly
cleaved mica at room temperature. In the AFM images, there
seemed to be some helical aggregates with an average height
of near 30 nm, although the visualization and discrimination
of the right- and left-handed helical structures were not
achieved. These helical shape images observed with AFM
possibly reflect the macromolecular helical conformation of
the polymer in solution.
AFM Observations of Isotactic-Rich Poly[(S)-MeOAM]
To some extent, the prevailing one-handed helical conforma-
tion was responsible for the observed difference in chiropti-
cal and fluorescent properties among the optically active
polymers having various stereoregularity. This excited us to
explore the possibility of whether the isotactic-rich polymers
NMR Study on the Interaction of
Poly(ROPAM)s with BINOL
The chiral recognition ability of the obtained optically active
polymers was examined by NMR technique using BINOL as
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ARTICLE
Chem Int Ed 2002, 41, 1614–1617; (c) Yamamoto, C.; Okamoto,
Y. Bull Chem Soc Jpn 2004, 77, 227–257.
a probe molecule. Because of the restricted rotation of the
two naphthyl rings around its 1,1-bond, BINOL exists in the
atropisomeric form; however, the racemic compound only
give single hydroxyl proton signal in an achiral NMR
environment.
3 Coates, G. W. Chem Rev 2000, 100, 1223–1252.
4 (a) Matsumoto, A. In Handbook of Radical Polymerization;
Matyjazsewski, K.; Davis, T. P., Eds.; Wiley: Hoboken, NJ,
2002; pp 691–773; (b) Nakano, T.; Okamoto, Y. Macromol
Rapid Commum 2000, 21, 603–612; (c) Nakano, T.; Okamoto,
Y. Chem Rev 2001, 101, 4013–4038; (d) Nagara, Y.; Nakano,
T.; Okamoto, Y.; Gotoh, Y.; Nagura, M. Polymer 2001, 42,
9679–9686.
For comparison, Figure 6 shows the hydroxyl region in the
1H NMR spectra of racemic BINOL in the presence of poly
(ROPAM)s (R ¼ Me and Pri) with a variety of tacticity. In all
cases, the hydroxyl proton signal of BINOL at d 5.05 was
shifted downfield on interaction with the polymers. With the
exception of the syndiotactic-rich poly[(S)-MeOPAM]s synthe-
sized by the radical polymerization in methanol (a–c), the
polymers having an isotactic-rich structure made the
hydroxyl signal split into two peaks that are ascribed respec-
tively to the levo- and dextro-isomer. The assignment of the
two split peaks was determined by NMR measurements with
a separate enantiomer under the same measurement condi-
tions. It is noteworthy that the magnitude of this signal split
was closely dependent on the stereoregularity of the poly-
mers that were obtained in the polymerization system using
n-butanol as solvent, as shown in Figure 6(d–f and g–i). It is
similar to that previously observed in the case of polymetha-
crylamide analogs.11(b) These results indicate that the stereo-
regularity of the obtained optically active poly(ROPAM)s sig-
nificantly affected the chiral recognition of the polymers, as
seen in other systems.23
5 (a) Okamoto, Y.; Habaue, S.; Isobe, Y.; Nakano, T. Macromol
Symp 2002, 183, 83–88; (b) Isobe, Y.; Fujioka, D.; Habaue, S.;
Okamoto, Y. J Am Chem Soc 2001, 123, 7180–7181.
6 (a) Suito, Y.; Isobe, Y.; Habaue, S.; Okamoto, Y. J Polym Sci
Part A: Polym Chem 2002, 40, 2496–2500; (b) Isobe, Y.; Suito,
Y.; Habaue, S.; Okamoto, Y. J Polym Sci Part A: Polym Chem
2003, 41, 1027–1033; (c) Habaue, S.; Isobe, Y.; Okamoto, Y. Tet-
rahedron 2002, 58, 8205–8209.
7 Baraki, H.; Habaue, S.; Okamoto, Y. Macromolecules 2001,
34, 4724–4729.
8 (a) Isobe, Y.; Nakano, T.; Okamoto, Y. J Polym Sci Part A:
Polym Chem 2001, 39, 1463–1471; (b) Morioka, K.; Suito, Y.;
Isobe, Y.; Habaue, S.; Okamoto, Y. J Polym Sci Part A: Polym
Chem 2003, 41, 3354–3360.
9 (a) Lutz, J.-F.; Neugebauer, D.; Matyjazsewski, K. J Am Chem
Soc 2003, 125, 6986–6993; (b) Lutz, J.-F.; Jakubowski, Matyjaz-
sewski, K. Macromol Rapid Commun 2004, 25, 486–492.
CONCLUSIONS
10 (a) Ray, B.; Isobe, Y.; Morioka, K.; Habaue, S.; Okamoto, Y.;
Kamigaito, M. Macromolecules 2003, 36, 543–545; (b) Kami-
gaito, M.; Satoh, K. Macromolecules 2008, 41, 269–276; (c)
Jiang, J.; Lu, X.; Lu, Y. Polymer 2008, 1770–1776.
The radical polymerization of the acrylamide derivatives
bearing a chiral oxazoline chromophore was carried out in
the presence of rare earth metal trifluoromethanesulfonates
(Ln(OTf)3, Ln ¼ La, Nd, Sm, and Y). It was found that
Y(OTf)3 could effectively increase the isotactic specificity
during the radical polymerizations when using n-butanol as
solvent. The chiroptical and fluorescence properties of the
optically active polymers were significantly dependent on
their stereoregularity. On the basis of the temperature de-
pendence of the specific rotation together with the
preliminary AFM observations, the polymers rich in isotactic-
ity might contain the prevailing one-handed helical chain
sections in solution. Furthermore, the enantioselective inter-
action of the isotactic-rich polymers with BINOL predicts
they have some potential uses in the area of chiral
separation.
11 (a) Xi, X. J.; Lou, L. P.; Jiang, L. M.; Sun, W. L.; Shen, Z. Q.
Polymer 2008, 49, 2065–2070; (b) Liu, G. X.; Lu, W.; Jiang, L.
M.; Sun, W. L.; Shen, Z. Q. Acta Polym Sinica 2009, 8, 775–780.
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The authors are indebted to the financial support of the
National Natural Science Foundation of China (Grant No.
21074107).
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