Poly(N-propargylamide)s bearing cholesteryl moieties: Preparation and optical activity

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.08.008

We synthesized a novel chiral cholesteryl-based N-propargylamide (M _ch, HCCCH₂NHCOCH₂CH₂COOch, ch = cholesteryl) from which homopolymers [P(M_ch)] with different molecular weights (number-average molecular weight: 8600, 14100 and 30000) were prepared. The polymers formed helical structures with a preferential helicity. The three polymers increased in both helix content and specific rotation as the molecular weight increased. P(M_ch)-8600 was studied in detail as the model polymer. P(M_ch)-8600 adopted helical conformations in toluene, THF, CHCl₃ and CH₂Cl₂, exhibited thermal stability with a decomposition temperature of 273°C and formed a lyotropic liquid crystal under the studied conditions. Copolymers of different compositions of M_ch and an achiral monomer (M_et) were prepared. The copolymers formed helices to different degrees depending on the specific composition, indicating an effective approach for controlling the formation of helices in synthetic helical polymers.

Full text of DOI:10.1016/j.reactfunctpolym.2012.08.008

Reactive & Functional Polymers 72 (2012) 832–838
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Poly(N-propargylamide)s bearing cholesteryl moieties: Preparation and
optical activity
⇑
Chaohong Zhang, Dong Liu, Bolin Zhou, Jianping Deng , Wantai Yang
State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2012
Received in revised form 6 August 2012
Accepted 7 August 2012
Available online 14 August 2012
We synthesized a novel chiral cholesteryl-based N-propargylamide (Mch, HC„CCH2NHCOCH2CH2COO
Ach, ch = cholesteryl) from which homopolymers [P(Mch)] with different molecular weights (number-
average molecular weight: 8600, 14100 and 30000) were prepared. The polymers formed helical struc-
tures with a preferential helicity. The three polymers increased in both helix content and specific rotation
as the molecular weight increased. P(Mch)-8600 was studied in detail as the model polymer. P(Mch)-8600
adopted helical conformations in toluene, THF, CHCl3 and CH2Cl2, exhibited thermal stability with a
decomposition temperature of 273 °C and formed a lyotropic liquid crystal under the studied conditions.
Copolymers of different compositions of Mch and an achiral monomer (Met) were prepared. The copoly-
mers formed helices to different degrees depending on the specific composition, indicating an effective
approach for controlling the formation of helices in synthetic helical polymers.
Keywords:
Helical polymer
Substituted polyacetylene
Optical activity
Cholesteryl group
Liquid crystalline polymer
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Cholesterol and its derivatives are important natural products
that are commercially available and inexpensive. In addition, the
Helical structures occur widely in natural biomacromolecules
and play essential roles in living organisms. Encouraged by the
presence of exceptional and powerful biomacromolecules in nat-
ure, a significant number of artificial helical polymers [1] have
been reported including polyisocyanides [2], polymethacrylates
[3], polyisocyanates [4], polysilanes [5] and substituted polyacet-
ylenes [6–11]. Substituted polyacetylenes are attractive due to
their unique properties, which have been reported by Yashima
et al. [9], Masuda [10], Tang et al. [11] and other groups. Substi-
tuted polyacetylenes with desirable properties can be prepared by
attaching diverse pendent groups with the appropriate length and
bulkiness.
We have synthesized a series of novel helical, substituted poly-
acetylenes in organic solvents, including poly(N-propargylamide)s
[12], poly(N-propargylsulfamide)s [13], poly(N-propargylurea)s
[14,15] and poly(N-propargylthiourea)s [16]. We have also devel-
oped methods for preparing helical polyacetylenes in aqueous
media [17] and by asymmetric emulsion polymerization [18,19].
Furthermore, chiral hybrid core/shell nanoparticles [20], hydrogels
[21], magnetic composite microspheres [22], enzyme-mimicking
polyacetylene catalysts [23] and amphiphilic polymer brushes
[24] have been prepared and successfully utilized for enantioselec-
tive crystallization, chiral recognition, chiral resolution and asym-
metric aldol reactions.
cholesteryl group has an interesting functional architecture for
the preparation of cholesteric liquid crystal (LC) polymers [25].
Cholesteric LCs possess excellent chiroptical properties that can
potentially be used in optical data storage systems and asymmetric
reactions [26]. In addition, helical polymers with biologic compat-
ibility are currently of interest for potential applications as biomi-
metic materials [7]. As the essential component in cell membranes,
polymers with cholesteryl side chains are expected to exhibit
desirable compatibility with natural biomacromolecules. In addi-
tion, the cholesteryl group possesses multiple chiral carbon atoms;
thus, the result polymers are chiral [4b].
The combination of a helical substituted polyacetylene back-
bone and cholesteryl side chains would provide innovative materi-
als. To date, reports of cholesteryl-based substituted polyacetylenes
are quite limited. Lam et al. [27,28] and Qu et al. [29,30] reported
disubstituted polyacetylenes and monosubstituted polyacetylenes
bearing pendent cholesteryl groups. However, novel optically ac-
tive cholesterol-based polyacetylenes are still necessary for the cre-
ation of novel functional materials. Herein, we designed and
synthesized a novel cholesteryl N-propargylamide (Mch), and the
prepared homopolymer formed one-handed helical conformations.
The optical activities, thermal stabilities and LC properties of the
homopolymers were examined. To strengthen the optical activity
of the homopolymers, an achiral monomer, whose homopolymer
showed a strong propensity to adopt a helical structure (i.e., with
equal right – and left-handed helices) [31], was introduced to copo-
lymerize with Mch. The variation in composition of the resulting
⇑
Corresponding author. Tel./fax: +86 10 6443 5128.
E-mail address: dengjp@mail.buct.edu.cn (J. Deng).
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.08.008

C. Zhang et al. / Reactive & Functional Polymers 72 (2012) 832–838
833
Scheme 1. Scheme for the synthesis of the monomer and (co)polymers.
Table 1
Polymerization of Mch
a
.
Yield (%)b
Mn
Mw/Mn
[a]D (deg)d
c
c
Run
Cat. (mol%)
P(Mch)-8600
P(Mch)-14100
P(Mch)-30000
7
1
0.1
93
90
94
8600
14100
30000
1.38
1.20
2.05
À40
À45
À83
a
b
c
With (nbd)Rh+BÀ (C6H5)4 catalyst in THF at 30 °C for 3 h, [M]0 = 0.2 M.
Hexane-insoluble part.
Measured by GPC (polystyrenes as the standards, THF as eluent).
Toluene as solvent (c = 0.08À0.10 g/dL).
d
copolymers led to changes not only in the maximum wavelength
and CD signal intensity but also in the sign of the CD signals. The
present results are of special interest for the further design of func-
tional substituted polyacetylenes with interesting properties.
2. Experimental section
2.1. Materials
Succinic anhydride (Aldrich, Shanghai China), cholesterol
(Aldrich, Shanghai China), isobutyl chloroformate (Alfa Aesar, Tian-
jin China), 4-methylmorpholine (Aldrich, Shanghai China), 2-ethyl-
n-butyric acid (Alfa Aesar, Tianjin China), and propargylamine
(Aldrich, Shanghai China) were used as received. The pyridine
was distilled over KOH under reduced pressure. Typical solvents
were used as received without further purification. The Rh catalyst
[(nbd)Rh+BÀ(C6H5)4] (nbd = norbornadiene) was prepared accord-
ing to a previously reported protocol [32].
2.2. Synthesis of monocholesterylsuccinate (ChMS)
ChMS was synthesized according to a previously reported proto-
col [33], which is shown in Scheme 1. The cholesterol (2.5 g) was
dissolved in 6 mL of pyridine, and succinic anhydride (1.8 g) was
added. The reaction was allowed to proceed at 90 °C for 24 h with
stirring under a nitrogen atmosphere. After completion of the
Fig. 1. Molecular weight dependence of the: (a) UV–vis and (b) CD spectra of
P(Mch); c = 0.1 mM, measured in THF at room temp. [insert in b: CD spectra of
P(Mch)-8600].

834
C. Zhang et al. / Reactive & Functional Polymers 72 (2012) 832–838
Table 2
obtain the desired product. The combined extracts were washed
Solubility of Mch and P(Mch)-8600.
with 2 N HCl three times and then washed with saturated aqueous
NaHCO3 to neutralize the solution. The solution was subsequently
dried over anhydrous MgSO4, filtered, concentrated, recrystallized
THF CHCl3 CH2Cl2 C7H8 DMSO Ethyl acetate DMF Acetone Ethanol
Mch
P(Mch) s
s
s
s
s
s
s
s
h
h
h
h
h
h
h
Â
Â
and dried to yield the target monomer Mch
.
Â
Monomer Mch: yield 72%, white solid, [a]D = À29°, melting point
s: Soluble, h: Partly soluble, Â: Insoluble. Measured at room temp.
199 °C (through DSC).
Mch IR (KBr, cmÀ1): 3336 (NAH), 3297 (HAC„C), 2980–2839
(CAH2, CAH), 2124 (C„C), 1227 (C=@O, ester), 1645(C@O, amide),
1553 (CANAH).
reaction, the dark brown colored reaction mixture was cooled to
room temperature and poured into distilled water. The resulting
precipitate was thoroughly washed with distilled water. The ob-
tained product was dried under reduced pressure and purified by
recrystallization from acetone. The recrystallized product was dried
under vacuum at room temperature (yield = 2.92 g; 93%). IR (KBr,
cmÀ1): 3200–2500 (ACOOH), 2980–2839 (CH2, CH), 1709 (C@O),
1181 (CAOAC).
Mch 1H NMR (CDCl3, 400 MHz, 20 °C, d ppm): d = 0.70–2.05
(41H), 2.25 (1H, HAC„C), 2.33–2.35 (2H), 2.51 (2H, CH2ACOO),
2.68 (2H, CH2AC@O), 4.07 (2H, CH2ANH), 4.64 (1H, CHAOOC),
5.39 (1H, CH@C), 5.88 (NAH).
Mch elemental analysis calcd (%): N 2.67, C 77.96, H 10.20;
found (%): N 2.47, C 77.76, H 10.29.
2.3. Synthesis of the monomers
2.4. Polymerization and copolymerization
Both monomers (Mch and Met) were synthesized according to a
previously reported protocol [31]. Monomer Mch is a new com-
pound, and the typical synthesis of Mch is described below. ChMS
(2 g, 4.1 mmol), which was synthesized above, isobutyl chlorofor-
mate (0.54 mL, 4.1 mmol), and 4-methylmorpholine (0.42 mL,
4.1 mmol) were sequentially added to THF (20 mL). The solution
was stirred at room temperature for 20 min, and then propargyl-
amine (0.30 mL, 4.1 mmol) was added to the solution. After 4 h,
the mixture was filter to remove the white precipitate that formed.
The filtrate was collected, and extracted with CHCl3 (ca. 30 mL) to
Based on earlier studies [13–16], polymerizations were per-
formed with the (nbd)Rh+BÀ (C6H5)4 catalyst in dry THF under
nitrogen at 30 °C for 3 h with a monomer concentration of 0.2 M
and the given Rh catalyst concentration (see Table 1). After poly-
merization, the solution containing the resulting polymer was
poured into a large amount of hexane to precipitate the prepared
polymer. The precipitate was filtered, washed, and dried under re-
duced pressure.
For the copolymerizations, the total monomer concentration
was maintained at 0.2 M, and the concentration of the Rh catalyst
Fig. 2. Solvent dependence of the polymers: (a) UV–vis and (b) CD spectra of P(Mch)-8600; (c) UV–vis and (d) CD spectra of P(Mch)-30000 (c = 0.1 mM, room temp.).

C. Zhang et al. / Reactive & Functional Polymers 72 (2012) 832–838
835
in the Section 2. The molecular structure of Mch is shown in Scheme
1.
Mch was subjected to homopolymerization in THF at 30 °C with
different Rh catalyst concentrations (see Table 1). The resulting
homopolymers were pale yellow solids, which was a preliminary
indication that the polymers contained helical structures [31,34].
The detailed homopolymerization results are presented in Table 1
and demonstrated that Mch was homopolymerized easily under
the investigated conditions, resulting in polymers with number
average molecular weights (Mn) ranging from 8600 to 30000. The
specific rotations ([a]D) of the homopolymers were measured by a
polarimeter at room temperature. The [a]D of the homopolymers
were larger than that of the monomer ([a]D of Mch: À29°). However,
even for a polymer [P(Mch)-30000] with Mn as high as 30000, the
[a]D of the polymer was less than 3 times the [a]D for the corre-
sponding monomer. Therefore, chiral amplification was not pro-
nounced in the homopolymers. However, we assume that the
homopolymers adopted helical structures based on the results from
the CD spectra measurements presented below.
3.2. Effects of molecular weights on the helical structure of the
polymers
The obtained homopolymers and monomer Mch were subjected
to UV–vis and CD spectroscopy, which have proved to be powerful
techniques to explore the secondary structures of substituted poly-
acetylenes [11–24]. Fig. 1 depicts the obtained UV–vis and CD spec-
tra of the monomer and the polymers. The monomer showed no
absorption and a CD effect in a wavelength range of 275–400 nm.
Fig. 3. Solvent dependence of the: (a) UV–vis and (b) CD spectra of P(Mch)-8600 in
solvent mixtures of C7H8 and CH2Cl2 at different ratios (c = 0.05 mM, room temp.).
was 2 mM. The other reaction conditions were maintained the
same as those employed in the homopolymerization.
2.5. Measurements
1H nuclear magnetic resonance (NMR) spectra were recorded on
a Bruker AV400 spectrometer with CDCl3 solvent. FT-IR (Fourier
transform infrared) spectra were recorded with a NICOLET NEXUS
670 infrared spectrometer (KBr tablet). Elemental analysis was per-
formed with an Elementar vario EL cube element analyzer. UV–vis
absorption and CD (circular dichroism) spectra were recorded on a
JASCO 810 spectropolarimeter. Specific rotations were measured on
a JASCO P-1020 digital polarimeter with a sodium lamp light source
at room temperature. The molecular weights and molecular weight
distributions of the polymers were determined by gel permeation
chromatography (GPC, Waters 515–2410 system) calibrated with
polystyrenes and THF as the eluent. The thermogravimetric analy-
ses were measured on a METTLER TGA STAR System. The melting
point of monomer Mch was measured on a METTLER DSC. The polar-
ized optical microscope images were captured with a CARL ZEISS
polarized optical microscope.
3. Results and discussion
3.1. Homopolymerization and homopolymers
In the current study, a novel cholesteryl N-propargylamide
monomer, Mch, wassynthesizedandstructurallyidentified,asstated
Fig. 4. TGA of Mch: (a) and P(Mch)-8600 (b) (recorded under N2 at a heating rate of
10 °C/min from 30 to 500 °C).

836
C. Zhang et al. / Reactive & Functional Polymers 72 (2012) 832–838
the helix contents and optical activities increased with increasing
polymer Mn. In other words, increasing Mn favors the formation of
helices by the polymer chains. This relationship agrees well with
observations from other helical polymers [35,36].
3.3. Effects of solvents on the helical structure of the polymers
To elucidate the effects of solvents on the obtained poly
(N-propargylamide)s, we used P(Mch)-8600 as the polymer model.
Four solvents, THF, C7H8 (toluene), CHCl3 and CH2Cl2 were chosen
because of the good solubility of the polymer in these solvents (see
Table 2). As shown in Fig. 2a and b, P(Mch)-8600 showed a UV–vis
absorption and CD signal in both THF and C7H8 at approximately
300 nm, indicating that P(Mch)-8600 preferentially formed a heli-
ces in these two solvents. The strongest CD signal was observed
in C7H8. P(Mch)-8600 exhibited no CD effect in CHCl3 and CH2Cl2
at a concentration of 0.1 mM, but UV–vis absorption could be
clearly observed at the same wavelength as those in THF and
C7H8 (see Fig. 2a). Actually, when the concentration of P(Mch)-
8600 was increased to 1 mM in CHCl3 and CH2Cl2, the intensity
of the corresponding UV–vis absorption increased approximately
10 times that at 0.1 mM. Therefore, we considered P(Mch)-8600
to have adopted helical conformations in CHCl3 and CH2Cl2. This
result greatly differed from that observed with the polymer with
pendent cholesteryl groups [29], where the polymer showed posi-
tive CD signals in THF, C7H8, and negative CD signal in CHCl3. Nev-
ertheless, our observation is similar to other helical polymers
[27,28] in that the helical polymers exhibited the strongest specific
rotation in C7H8, followed by THF, and the weakest rotation was
observed in CHCl3.
Regarding P(Mch)-14100 and P(Mch)-30000, phenomena similar
to P(Mch)-8600 were observed in the four solvents. P(Mch)-14100
(not shown) and P(Mch)-30000 (see Fig. 2c and d) showed UV–vis
Table 3
a
Copolymerization of monomers Mch and Met
.
Yieldb
(%)
Mch/Met
Mn
Mw/
Mn
[a]D
(deg)
c
d
e
Polymer
Mch/Met(feed)
(mol/mol)
d
(mol/mol)
P(Mch92
co-
-
-
90/10
80/20
70/30
60/40
50/50
30/70
20/80
10/90
0/100
90
88
81
83
89
87
90
86
85
92/8
13300 1.28 À84
12600 1.36 À75
13700 1.33 À60
27000 1.96 À51
14600 1.36 À150
31000 3.45 À128
31000 3.62 221
26500 2.33 383
Met8
P(Mch83
co-
)
83/17
72/28
64/46
47/53
33/67
25/75
15/85
–
Met17
P(Mch72
co-
Met28
P(Mch64
co-
Met46
P(Mch47
co-
Met53
P(Mch33
co-
Met67
P(Mch25
co-
Met75
P(Mch15
co-
)
-
Fig. 5. POM images of P(Mch)-8600 at various concentrations of toluene: (a)
15 wt%; (b) 10 wt%; (c) 5 wt%; room temp. Insert in a: aggregation of crystals.
)
-
)
-
The homopolymers demonstrated a pronounced UV absorption at
approximately 300 nm (Fig. 1a). In Fig. 1b, intense CD effects also
appeared at approximately 300 nm. According to our earlier studies
[12–24], a combination of the UV absorptions and CD effects to-
gether with the [a]D mentioned above demonstrated the formation
of a predominant screw sense and that the helical polymers were
chiral. The occurrence of UV–vis absorption and CD effects at a rel-
atively low wavelength (approximately 300 nm) implied that the
helical pitches of the homopolymers were short [16,29,30,34].
In Fig. 1a, homopolymers with different molecular weights dem-
onstrated different UV–vis absorbencies. The UV–vis absorptions
decreased in the order of P(Mch)-30000, P(Mch)-14100 and P(Mch)-
8600. In Fig. 1b, results similar to those in Fig. 1a are shown. The
absolute values of the CD signal intensities increased in the same se-
quence as that of the UV absorptions. Based on the observations dis-
cussed above, we conclude that all the three polymers with varied
Mn adopted helices with one predominant helicity. Furthermore,
)
-
)
-
)
-
Met85
)
P(Met
)
18200 3.12
–
a
With (nbd)Rh+BÀ (C6H5)4 catalyst in THF at 30 °C for 3 h, [M]0 = 0.2 M,
[M]0/[Rh] = 100 (mol/mol).
b
c
Hexane-insoluble part.
According to 1H NMR spectra.
d
e
Measured by GPC (polystyrenes as the standards, THF as eluent).
Toluene as solvent (c = 0.08–0.10 g/dL).

C. Zhang et al. / Reactive & Functional Polymers 72 (2012) 832–838
837
absorptions at approximately 300 nm in all of the solvents exam-
3.5. Liquid crystal properties of P(Mch)-8600
ined. CD signals could be clearly observed only in THF and C7H8,
and the strongest CD signal was observed in C7H8. All three poly-
mers, P(Mch)-8600, P(Mch)-14100 and P(Mch)-30000, behaved
similarly in the examined solvents.
Becausepolymers withcholesterylgroupscaneffectivelyformLC
structures, we performed a DSC measurement of P(Mch)-8600, but
could not observe a phase transition temperature in the range from
30 to 250 °C. In other words, P(Mch)-8600 was not a thermotropic li-
quid crystal, but it did develop a lyotropic LC property. Fig. 5 shows
the polarizedoptical microscopeimages of a P(Mch)-8600 solution in
toluene at room temperature. It exhibits birefringence patterns with
a weight concentration between 15% and 5%, clearly suggesting the
formation of lyotropic liquid crystal in the polymer. When the con-
centration reached 15%, abundant crystals could be observed in
the image (Fig. 5a), and some of them aggregated together to form
larger crystals (5a insert). As the concentration decreased to 10%,
thecrystals becamemoreregularly distributed (Fig. 5b). The number
of liquid crystals was remarkably reduced when the polymer con-
centration was 5% (Fig. 5c). No LC could be observed if the concentra-
tion of P(Mch)-8600 was further decreased to 1%.
To further understand the solvent dependence of the poly(N-
propargylamide)s, we examined P(Mch)-8600 in solvent mixtures
consisting of C7H8 and CH2Cl2 in different ratios. The experimental
results are presented in Fig. 3. P(Mch)-8600 exhibited a negative-
signed CD signal at 300 nm in C7H8. As the CH2Cl2 concentration
was increased to 40%, the CD signal intensity gradually decreased.
The CD signal completely disappeared when at 50% CH2Cl2. How-
ever, the intensity of the UV–vis absorptions only changed slightly
during the process. The helical structures formed with the present
polymers seem to be very sensitive to the solvent. Detailed effects
of the solvents on the secondary structures of the polymers require
more study.
3.4. Thermostabilities of the monomer and polymers
Thermogravimetric analysis (TGA) results indicated that both
Mch and P(Mch) were relatively thermally stable and did not lose
any weight even when heated to a temperature of 270 °C (see
3.6. Synthesis and optical activity of the copolymers
To further examinethe cholesteryl-based substituted polyacetyl-
enes and their secondary structures, copolymerizations of Mch and
Fig. 4). The decomposition temperature for both Mch and P(Mch
)
was approximately 273 °C (97%); and the fastest decomposition
temperatures for Mch and P(Mch) were 331 and 317 °C, respectively.
The higher thermal stability for the poly(N-propargylamide)s is
most likely due to the ‘‘jacket effect’’ [37]. The double bonds in
the polyacetylene backbone are surrounded by the cholesteryl rings
and shielded from external chemical and thermal reactions.
Met were accomplished. We also expected to take advantage of the
outstanding ability of P(Met) (the homopolymer from Met) to form
helical structures [31]. The copolymerizations of the two monomers
were carried out at various monomer feed ratios. In Table 3, result of
the copolymerizations and homopolymerizations of Met are listed.
The(co)polymerizationsproceededsmoothlywithcopolymeryields
Fig. 6. UV–vis (a) and CD (b, c) spectra of (co)polymers (in toluene, c = 0.1 mM, room temp.).

838
C. Zhang et al. / Reactive & Functional Polymers 72 (2012) 832–838
of 81–90%, and the polymers possessed moderate molecular
weights, from 12600 to 31000. The compositions of the copolymers
were determined by 1H NMR [24] using the signal (3.3–4.4 ppm)
attributed to AC@CACH2ANA in both the Mch and Met units and a
typical signal (5.2–5.4 ppm) from ACH@CA in the cholesteryl
groups of the Mch units. As shown in Table 3, the copolymers possess
compositions similar to the corresponding monomer feed ratio.
The obtained (co)polymers were analyzed by UV–vis and CD
spectroscopy, and the relevant results are shown in Fig. 6. P(Met)
exhibited an intense UV–vis absorption at approximately 390 nm
but lacked a CD signal [31]. The copolymers exhibited interesting
phenomena with the formation of helical structures. For
P(Mch92-co-Met8), P(Mch83-co-Met17), P(Mch72-co-Met28), and
P(Mch64-co-Met36), the Mch content was higher than 50 mol%, and
the UV–vis and CD spectra signals appeared at approximately
300 nm, which was similar to P(Mch) (Fig. 6a and b). In addition,
the CD intensities weakened as the Mch content of the copolymers
decreased, which is consistent with the ‘‘sergeants and soldiers ef-
fect’’ [4b]. For P(Mch47-co-Met53), P(Mch33-co-Met67), P(Mch25-co-
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50 mol%, and the UV–vis absorption and CD signal appeared at
approximately 390 nm (Fig. 6a and c), which is consistent with
the results from P(Met) [31]. The variation in the absorption maxi-
mum wavelength indicated a change in the helical pitch of the poly-
mers [34]. However, the changes in the helical pitch as a function of
the composition differed from earlier reports [34,38]. The changes
in both the maximum wavelength and the sign and intensity of
the CD signals indicated that the copolymers did not entirely follow
the ‘‘sergeants and soldiers effect’’. However, positive synergistic ef-
fects existed between the two types of monomer units, which re-
sulted in the formation of helical structures for the copolymers.
The specific rotations in Table 3 reflected the corresponding
changes. The presence of Met enhanced chiral amplification because
the specific rotations of the copolymers were higher than the spe-
cific rotations of P(Mch) with similar molecular weights. Therefore,
the copolymerization of Mch and Met effectively decreased the steric
repulsion between the crowded side chains and allowed the copoly-
mers to efficiently form helical structures and exhibit enhanced
optical activities. This observation indicates that an effective ap-
proach for controlling the helical structures in polymers via copoly-
merization has been achieved.
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Acknowledgement
This work was supported by the ‘‘National Natural Science
Foundation of China’’ (20974007, 21174010).