Optically active amino acid-based polyacetylenes: Effect of tunable helical conformation on infrared emissivity property

Article abstract of DOI:10.1016/j.reactfunctpolym.2014.05.006

Chiral amino acid-based monosubstituted acetylene monomers [N-tert-butoxy-carbonyl-l-phenylalanine-N′-propargylamide (LP), and N-tert-butoxycarbonyl-l-serine-N′-propargylamide (LS)] were (co)polymerized with rhodium zwitterion catalyst in THF to afford he

Full text of DOI:10.1016/j.reactfunctpolym.2014.05.006

Reactive & Functional Polymers 82 (2014) 17–24
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Optically active amino acid-based polyacetylenes: Effect of tunable
helical conformation on infrared emissivity property
⇑
Xiaohai Bu, Yuming Zhou , Zhenjie Chen, Man He, Tao Zhang
School of Chemistry and Chemical Engineering, Southeast University, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral amino acid-based monosubstituted acetylene monomers [N-tert-butoxy-carbonyl-L-phenylalanine-
N -propargylamide (LP), and N-tert-butoxycarbonyl-L-serine-N -propargylamide (LS)] were (co)polymer-
Received 22 September 2013
Received in revised form 10 January 2014
Accepted 15 May 2014
0
0
ized with rhodium zwitterion catalyst in THF to afford helical polyacetylenes (PAs) with moderate
molecular weights (4400–14,800) in good yields. The optically active PA copolymers were soluble in com-
mon organic solvents and proven to adopt predominately single-handed helical conformations according to
their intense Cotton effect and large specific rotations. Various contents of amino acid units in side chains
facilitated controllable helical secondary structure while the helix was primarily stabilized by hydrogen
bonding and steric repulsion between the substituents. Intramolecular hydrogen bonds constructed
between hydroxyl and amide groups, in particular, played a significant role in adjusting the helicity and
Available online 27 May 2014
Keywords:
Amino acid
Polyacetylenes
Helical conformation
Intramolecular hydrogen bonds
Infrared emissivity
orderliness of the helix. The infrared emissivity values of the PAs at wavelength of 8–14 lm were measured
and the correlation between helical conformations and their effect on infrared emission were systemati-
cally investigated. The results showed that more well-ordered and compact screw-sense could enhance
the performance of organic polymers in lowering their infrared emissivity. Among all the as-prepared
PAs, poly(LP50-co-LS50) exhibited the lowest infrared emissivity value (e25 = 0.632) and possessed excellent
resistance against heat.
Ó 2014 Elsevier Ltd. All rights reserved.
1
. Introduction
maintaining their usual properties. Helix is one of the most
common structures in naturally occurring biomacromolecules as
well as in synthetic polymers. Helices are primarily constructed
through intra- and/or intermolecular associations by noncovalent
forces, such as hydrogen bonding, hydrophobic-stacking, and elec-
trostatic interactions [10]. Helix means molecularly asymmetric;
hence, the polymer chain will prefer predominantly a single-
handed screw sense and exert optical activity [11]. For numerous
synthetic helical polymers, conformational transition can lead to
interesting changes in their physical and chemical properties, thus
providing opportunities to develop smart and intelligent macro-
molecules [12–14]. Recently, in the case of organic polymers with
tunable infrared emissivity, considerable effort has been devoted
to applying helical polymers in this field owing to their adjustable
secondary structure. For example, Wang et al. [15] firstly reported
that optically active polyurethane-urea had a lower infrared emis-
sivity value compared with racemic polymers owing to its ordered
single-handed conformation. Yang et al. [16,17] prepared optically
active polyurethanes with enhanced performance in lowering
emissivity values and further studied the correlation between the
helical structure and its effect on infrared emission. According to
the previous works, well-ordered structure seems to be efficient
in decreasing the infrared emissivity, in spite of the presence of
Materials with tunable infrared emissivity have attracted wide-
spread attention due to their importance in both thermal insula-
tion and military stealthy [1–3]. Various inorganic or metallic
materials have been widely used to achieve low emissivity because
of their high reflectance in infrared waveband [4,5]. Organic poly-
mers containing highly absorptive chemical bonds, e.g. CAH, C@O,
NAH, etc., result in relatively high emissivity (commonly larger
than 0.90) [6–9], which limit their applications in low-emissivity
field. However, organic polymers possess unique properties includ-
ing low density, tractability, anti-corrosion and the most value,
their adjustable composition and structure, which can provide
the possibility of adjusting their emission performance effica-
ciously according to practical applications. Thus, it has still
remained a meaningful research objective to develop novel classes
of organic polymers with tunable infrared emissivity.
Special interest in helical polymers arises from the realization
that their high-ordered stereoregular structure is essential in
⇑
Corresponding author. Tel./fax: +86 25 52090617.
E-mail address: ymzhou@seu.edu.cn (Y. Zhou).
http://dx.doi.org/10.1016/j.reactfunctpolym.2014.05.006
381-5148/Ó 2014 Elsevier Ltd. All rights reserved.
1

1
8
X. Bu et al. / Reactive & Functional Polymers 82 (2014) 17–24
unfavorable absorptive groups in synthetic polymers. As we know,
the vibration of unsaturated functional groups in synthetic poly-
mers mainly brings about high infrared radiation. Theoretically,
more compact accumulation of fragments and orderly inter- and
intramolecular interactions will accelerate the changing of vibra-
tional state of unsaturated bonds and thermal restraining in back-
bones, which can largely influence the infrared emission
performance [15,16,18]. Namely, a high-ordered secondary struc-
ture in the helical polymer, to some extent, can change the vibra-
tion mode in the whole macromolecular to reduce the index of
hydrogen deficiency and the unsaturated degree, thus eventually
facilitating the remarkable decrease in infrared emissivity values.
2. Experimental section
2.1. Materials
Propargylamine, N-tert-butoxycarbonyl-L-phenylalanine and
N-tert-butoxycarbonyl-L-serine were purchased from Aladdin. Iso-
butyl chloroformate and 4-methymorpholine were purchased from
Sinopharm Chemical Reagent Co., Ltd. (nbd)Rh [
(nbd = 2,5-norbornadiene) was prepared by the reaction of
+
6
ꢀ
5
6 6 5 3
g -C H B (C H ) ]
[(nbd)RhCl] with NaB(C H ) as reported [37]. THF used for poly-
2
6
5 4
merization was distilled over CaH2 prior to use. All other regents
were used as received without further purification.
Substituted polyacetylenes are prototypical p-conjugated poly-
mers and exhibit interesting properties and functions, including
liquid crystallinity [19], photoconductivity [20], luminescence
2.2. Measurements
[
21], gas permeability [22], and asymmetric catalysis [23]. They
Melting point (mp) was measured by an X-4 micro-melting
possess alternating double bonds along the main chain, in which
the conjugation significantly depends on the type, number, and
bulkiness of the pedants [24–26]. To date, monosubstituted PAs
bearing amino acid moieties in the side chains has been the subject
of study, because amino acid endows them with new features orig-
inating from their inherent chirality, diverse groups and ability to
form hydrogen bonds. The introduction of appropriate amino
acid-based moieties to the side chains easily induces them to take
regulated higher order structure. A wide variety of helical PAs
derived from chiral amino acid have been designed and synthe-
sized. Masuda et al. [27–29] synthesized PAs derived from various
amino acids and discussed their conformational transition by
changing temperature and solvent. Tang and co-workers focused
on helical poly(phenylacetylenes) carrying amino acid moieties,
some of which exhibit attractive properties including forming heli-
cal nanofibers [30,31], self-assembling [32], and tuning the helicity
by pH change [33]. These PAs form helices stabilized by hydrogen
bonding along with steric repulsion, while whose sense, tightness,
and stability depend on the functional groups in the pendants such
as amide, hydroxyl, and aromatic groups. Especially, hydroxyl
group forms hydrogen bonding in a different way which can not
only assist the formation of hydrogen bonds but also adjust the
helicity and stability of the helix [34]. In fact, participation of
hydroxyl group in intramolecular hydrogen bonding changes the
torsional angles in the main chain, which largely affects the confor-
mation and property of the PAs [35,36]. This finding is highly sig-
nificant for further designing amino acid-based PAs with tunable
helical conformation for special applications, for instance, helical
PAs for low infrared emissivity application.
point apparatus. Solid and solution state FT-IR spectra were carried
out on a Bruker Tensor 27 FT-IR spectrometer at room temperature
using KBr pellets and a KBr liquid cell, respectively. The spectra
ꢀ1
were obtained at a 4 cm resolution and recorded in the region
ꢀ
1
1
13
of 4000–400 cm
. H and C NMR spectra measurements were
recorded on a Bruker AVANCE 300 NMR spectrometer. Chemical
shifts were reported in ppm. The molecular weights and molecular
weight polydispersities were estimated by gel permeation chroma-
tography (Shodex KF-850 column, THF as the eluent, polystyrene
calibration). UV–vis spectra were measured on a Shimadzu UV-
3
600 spectrometer. CD spectra were determined with a Jasco
J-810 spectropolarimeter using a 10 mm quartz cell at room tem-
perature. Specific rotations ([ ) were measured in a WZZ-2S
2SS) digital automatic polarimeter at room temperature. Thermal
a]
D
(
analysis experiments were performed using a TGA apparatus oper-
ated in the conventional TGA mode (TA Q-600, TA Instruments) at a
ꢀ1
heating rate of 10 K min in a nitrogen atmosphere, and the sam-
ple size was about 5 mg. The infrared emissivity values ( ) of poly-
mers at wavelength of 8–14 m were investigated on the IRE-2
e
l
Infrared Emissometer of Shanghai Institute of Technology and
Physics, China. Infrared emissivity values of the samples at
3
0–160 °C were recorded with a temperature control instrument
ꢀ1
at a heating rate of 5 K min
.
2.3. Monomer synthesis
0
2.3.1. N-tert-butoxycarbonyl-
L-phenylalanine-N -propargylamide (LP)
N-tert-butoxycarbonyl- -phenylalanine (1.46 g, 5.5 mmol), iso-
L
To the best of our knowledge, there are few reports dealing with
the copolymerization of hydroxyl group containing amino acid PAs
and the analyses about their conformation are rare. Besides, there
are also few reports concerning the application of helical polymers
to achieve tunable infrared emissivity, let alone helical PAs. Herein,
butyl chloroformate (0.72 mL, 5.5 mmol) and 4-methymorpholine
(0.6 mL, 5.5 mmol) were sequentially added into 35 mL THF. The
solution was stirred at room temperature for about 15 min and
then propargylamine (0.303 g, 5.5 mmol) was added into the solu-
tion. The resulting mixture was stirred at room temperature for
24 h. The white precipitate was filtered off, and then the filtrate
was collected, to which AcOEt (200 mL) was added to extract the
desired product. The combined solution was subsequently washed
a series of PAs derived from L-phenylalanine and L-serine was
designed and synthesized. Phenyl rings in LP are incorporated into
the PA backbone with the hope that the bulky moieties exert asym-
metric forces to induce the chain to take a compact helical confor-
mation, while hydroxyl groups in LS are incorporated for the
helicity and stability tuning. As the structurally composition of
the chiral monomers varies, the change of the degree of hydrogen
bonding and steric interactions in the PAs can generate differing
quantities of steric configuration in macromolecules. Thence, it
can be expected that the optimization of the helicity and orderli-
ness of the polymer chains can afford novel PAs with enhanced
performance in controlling infrared emission. Furthermore, after
examining the infrared emissivity values of the PAs, we try to cap-
ture the relationship of the secondary structure, hydrogen bonding
and the infrared emissivity of the PAs in an effort to study their
potential applications.
3
with 1 M HCl (three times), saturated aq. NaHCO , saturated aq.
NaCl, dried over anhydrous MgSO and concentrated by rotary
4
evaporation. The residue was purified by flash column chromatog-
raphy on silica gel eluted with n-hexane/AcOEt (2/1, v/v) to obtain
solid monomer. Yield = 84%, white solid powder, mp: 102 °C.
ꢀ
1
ꢀ1
[a]
D
= ꢀ3.5° (c = 0.1 g dL , THF, rt). FT-IR (cm , KBr): 3326,
1690, 1655, 1529, 1446, 1390, 1367, 1271, 1091, 1049, 1027,
1
855, 758, 702, 668, 577, 513. H NMR (300 MHz, CDCl
[s, 9H, (CH ], 2.21 (s, 1H, C„CH), 3.09 (s, 2H, CHCH
2H, CH NH), 4.36 (s, 1H, H CCHNH), 5.05 (s, 1H, NHCOO), 6.17
(1H, NHCO), 7.22–7.31 (m, 5H, CH
CDCl ): d 28.30, 29.03, 38.73, 55.64, 71.58, 79.21, 80.17, 126.86,
128.56, 129.38, 136.36, 155.60, 171.42.
3
): d 1.42
3
)
3
2
), 4.00 (s,
2
3
1
3
2 6
C H
5
).
C NMR (75 MHz,
3

X. Bu et al. / Reactive & Functional Polymers 82 (2014) 17–24
19
0
2
.3.2. N-tert-butoxycarbonyl-
The title compound was synthesized from N-tert-butoxycar-
bonyl- -serine in manner similar to the synthesis of LP.
Yield = 62%, colorless crystal, mp: 156–158 °C. = 5.9°
c = 0.1 g dL , THF, rt). FT-IR (cm , KBr): 3454, 3330, 3303,
L
-serine-N -propargylamide (LS)
3. Results and discussion
L
a
3.1. Polymer synthesis and characterization
[a]
D
ꢀ1
ꢀ1
(
Table 1 summarizes the conditions and results of the polymer-
3
1
1
096, 1710, 1662, 1570, 1536, 1410, 1365, 1283, 1249, 1174,
ization of
LS catalyzed with (nbd)Rh [
In the polymerization, (nbd)Rh [
L
-phenylalanine- and
L
-serine-derived monomers LP and
1
+
6
ꢀ
058, 1046, 930, 863, 785, 659, 526. H NMR (300 MHz, CDCl
.49 [s, 9H, (CH ], 2.20 (s, 1H, C„CH), 2.91 (s, 1H, CH OH), 3.69
CHNH), 4.09 (s, 2H, CHCH OH), 4.16 (m, 2H, CH NH),
C NMR (75 MHz,
3
): d
g
-C H B (C H ) ] in THF (Scheme 1).
6
+
5
6
5 3
6
ꢀ
3
)
3
2
g
-C H B (C H ) ] successfully
6
5
6
5 3
(
s, 1H, CH
2
2
2
afforded the polymers with moderate molecular weights
13
5
.60(s, 1H, CHNHCO), 7.02 (s, CH
2
NHCO).
(M = 4400–14,800) in good yields. The copolymers and poly(LS)
n
CDCl
3
): d 28.30, 29.35, 56.50, 62.89, 71.74, 79.03, 80.99, 157.82,
were soluble in common organic solvents including MeOH, CHCl3,
1
71.28.
AcOEt, DMF, DMSO and THF, while poly(LP) was partly soluble in
these solvents. LP did not polymerize to achieve a homopolymer
n
with high M and high solubility under this condition, which
2.4. Polymerization
should be due to strong steric hindrance and repulsion caused by
the excess bulky phenyl rings in the neighboring pedants. The spe-
cific rotations of the PAs measured at room temperature ranged
Polymerizations were carried out in a Y-shaped glass tube
equipped with a three-way stopcock under nitrogen. Monomers
+
6
ꢀ
from ꢀ120° to ꢀ386° in CHCl
tion of the PAs. The absolute values of [a]
0–96 times as large as that of monomers at the maximum, indi-
3
, which depended on the composi-
and (nbd)Rh [
catalyst] = 5 mM) were respectively dissolved in THF under N
g -C H B (C H ) ] ([initial monomers] = 0.5 M,
6 5 6 5 3
of polymers was about
[
2
D
4
atmosphere. Then the monomers and catalyst were mixed and
stirred at 30 °C for 12 h. After polymerization, the resultant
solution was added dropwise into a large amount of hexane/AcOEt
cating that chiral amplification occurred. The large specific rota-
tions of the copolymers demonstrated that the PAs might take
preferred single-handed helical conformations. Furthermore, as
(
10/1, v/v) to precipitate the formed polymers. The precipitate
n
far as the yields and M of the polymers concern, it could be
was collected by filtration and dried under reduced pressure to
obtain the PAs product. Polymers synthesized by proportional
feed ratio of LP and LS are listed in Table 1. The spectroscopic
data of poly(LP), poly(LP50-co-LS50) and poly(LS) are shown as
follows:
inferred that hydroxyl group in LS did not hamper the polymeriza-
tion which was consistent with previous reports [34–36], but pro-
moted some of the PAs to form more stable and orderly helical
structures.
1
Poly(LP) FT-IR (cmꢀ1, KBr): 3347, 3260, 3086, 2978, 2929, 1692,
The structures of the PAs were examined by H NMR spectros-
copy. The disappearance of the chemical shift assigned to the acet-
ylene proton at about 2.2 ppm of all the PAs indicated that the
monomers had been consumed by the polymerization reaction.
However, the peaks corresponding to the resonances of the protons
of the polyene backbone were hardly recognized for the broad pro-
ton signals around 4–6 ppm. Since Rh zwitterion complex com-
monly afford monosubstituted PAs with cis–transoidal structure
1
7
5
1
653,1578, 1519, 1457, 1396, 1368, 1247, 1171, 1050, 1017, 858,
1
50, 700; H NMR (300 MHz, CDCl
.0 (m, CHCH , CH NH, H CCHNH), 5.05 (s, 1H, NHCOO), 5.53 (s,
H, C@CH), 6.58 (1H, NHCO), 7.29 (m, 5H, CH ); poly(LP50-
3 3 3
): d 1.44 [br, 9H, (CH ) ], 2.6–
2
2
3
2 6
C H
5
ꢀ1
co-LS50) FT-IR (cm , KBr): 3423, 3324, 3084, 2978, 2930, 1699,
1
7
2
662, 1518, 1456, 1395, 1367, 1250, 1165, 1061, 1021, 853, 787,
1
51, 701; H NMR (300 MHz, CDCl
.3–2.70 (br, 1H, CH OH), 3.0–5.0 (m, 10H, CH
NH), 5.44–6.90 (m, 4H, CHNHCO, C@CH), 7.30 (m, 5H, CH
3
): d 1.44 [br, 18H, (CH
3
)
3
],
[
38], we assume that the steric structures of the present polymers
2
2
CHNH, CHCH
2
OH,
are also this case.
CH
2
2
C
6-
ꢀ1
The thermal stability of the PAs was investigated by TGA tech-
niques under a nitrogen atmosphere from 50 to 600 °C. As shown
in Fig. 1, the TGA curves for dry PAs exhibit a smooth, stepwise
manner, suggesting a two-step thermal degradation. The onset
H
3
1
5
), 8.04 (br, 2H, CH NHCO); poly(LS) FT-IR (cm , KBr): 3600–
2
200, 3006, 2980, 2930, 2884, 1698, 1658, 1523, 1459, 1399,
367, 1256, 1169, 1065, 918, 858, 782, 591; 1H NMR (300 MHz,
CDCl
m, 5H, CH
C@CH), 8.07 (br, 1H, CH
3
): d 1.45 [br, 9H, (CH
CHNH, CHCH OH, CH
NHCO).
3
)
3
], 2.5–3.0 (br, 1H, CH
2
OH), 3.1–5.0
temperatures (T
00 °C, indicating considerably high thermal stability of the mono-
substituted PAs. The temperatures of the onset and 5% weight loss
, T ) of the PAs have also been calculated by means of thermo-
0
) of weight loss of the polymers were all above
(
2
2
2
NH), 6.10 (br, 2H, CHNHCO,
2
2
1
The FT-IR and H NMR data of other copolymers are similar to
(T
0
5
poly(LP50-co-LS50) and shown in Figs. S1 and S2.
grams and used as criterion for evaluation of thermal stability of
these PAs (Table 2). The decomposition temperatures for the poly-
mers containing more phenylalanine moieties are higher than
those containing more serine moieties, revealing that the incorpo-
ration of phenyl groups in the PAs can improve their thermal
stability.
Table 1
a
Copolymerization of LP with LS .
Monomer feed ratio (LP:LS) Yieldb (%)
M
n
c
M
d
w
/M
n
D
[a] (°)
CHCl
3
MeOH
e
e
1
7
6
5
3
2
0
00:0
5:25
2.5:37.5
0:50
7.5:62.5
5:75
88
94
98
96
95
90
80
4400
1.3
3.2. Hydrogen bonding of the PAs
10,500 1.9
14,800 1.6
14,200 2.0
11,100 1.9
10,700 1.8
ꢀ120 ꢀ36
ꢀ229 ꢀ60
ꢀ386 ꢀ93
ꢀ285 ꢀ220
ꢀ204 ꢀ281
ꢀ152 ꢀ456
Hydrogen bonds are an essential and fundamental interaction
to form and maintain the helical structure of the amino acid-based
PAs. Participation in hydrogen bonding decreases the frequency of
free NAH and C@O vibrations but increases the intensities, making
the absorption features useful in investigating hydrogen bonds
:100
9900
2.3
a
Conditions: catalyst (nbd)Rh+[
6-C
ꢀ
5
g
6 6 5 3
H B (C H ) ], [monomer]/[catalyst] = 100,
at 30 °C for 24 h under N
2
.
[
12,15]. The solution state FT-IR spectra of the monomer and poly-
b
Insoluble part in n-hexane/AcOEt (10/1, v/v).
Determined by GPC eluted with THF based on polystyrene standards.
c
mer samples are examined to determine the existence of hydrogen
bonding in the PAs. Fig. 2A shows the FT-IR spectra of LP, LS, and
d
Measured in CHCl
3
and MeOH by polarimetry at room temperature,
c = 0.05 g dL 1.
Not determined for poor solubility.
ꢀ
poly(LP50-co-LS50) (other copolymers see in Fig. S3). In CHCl
3
, LP
e
and LS showed the amide and carbamate absorption at 1705 and

2
0
X. Bu et al. / Reactive & Functional Polymers 82 (2014) 17–24
Scheme 1. Synthesis of the PAs.
significant difference at the absorption bands of carbonyl groups.
However, but even at such low concentrations, still high degree
of hydrogen bonding derived from the NAH and C@O in amide
groups existed, indicating the existence of intramolecular hydro-
gen bonding. Summarizing the observations in the FT-IR spectra
(Table S1), it can be inferred that most of carbamate groups are
probably participated in the formation of intermolecular hydrogen
bonds, while the inner amide groups prefer to form intramolecular
hydrogen bonds in the PAs.
3.3. Helical conformation of the PAs
Fig. 3 shows the CD and UV–vis spectra of the PAs measured at
room temperature in CHCl . All the PAs obviously exhibited Cotton
3
effect in the corresponding UV–vis absorption wavelength except
poly(LP), which was hard to measure for its poor solubility.
Poly(LP75-co-LS25) exhibited a CD signal at ꢁ295 nm with negative
sign which confirmed the existence of single-handed helical con-
Fig. 1. TGA curves of the PAs.
Table 2
Thermal properties of the PAs.
formation. Poly(LP62.5-co-LS37.5
)
and poly(LP50-co-LS50
)
also
showed Cotton effects around 300 nm with negative signs simi-
larly to poly(LP75-co-LS25), but intensified gradually, manifesting
the formation of more stable but slightly looser helical structures.
In the case of poly(LP37.5-co-LS62.5) to poly(LS), positive signals
appeared and shifted to lower wavelength, while the negative sig-
nals shifted to higher wavelength and the intensities gradually
decreased. The positive and negative CD signals of poly(LS) cen-
tered at ꢁ280 nm and ꢁ345 nm, respectively. The change of the
CD signs with the content of LS from 50% to 100% represented
unstable and much looser helices in the PAs and we could further
infer that the helical structure of these PAs might be formed in dif-
ferent ways. From the fact of CD spectra, poly(LP50-co-LS50) was
examined to have the most stable and compact helical secondary
structure among all the polymers. Additionally, during the course
of CD variation phenomenon, the PAs all exhibited an absorption
peak around 320 nm which was assigned to the main chain con-
junction. Compared with reported amino acid-based PAs [27,28],
it is further confirmed that hydroxyl group containing PAs have
relatively more compact helical structures. No obvious difference
in the absorption spectra was observed in the absorption spectra
of PAs with different composition. Consequently, it can be con-
cluded that the main chain conjunction in the PA backbone
changes slightly with changing composition, but its two-
dimensional spatial structure, especially the torsional angle of
the alternating double bonds dramatically changes.
Monomer feed
ratio (LP:LS)
Onset decomposition
Decomposition
temperature (°C) T
5
temperature (°C) T
0
1
7
6
5
3
2
0
00:0
5:25
2.5:37.5
0:50
7.5:62.5
5:75
255
236
235
220
219
215
201
275
264
261
253
250
248
238
:100
1
678 (1675) cmꢀ1, while poly(LP50-co-LS50) exhibited two corre-
ꢀ1
sponding absorption peaks at 1698 and 1658 cm . From the fre-
quency decease of both the carbonyl groups, it can be concluded
the PA partly forms inter- and/or intramolecular hydrogen bonds
at amide and carbamate groups. As previously described, the intra-
molecular hydrogen bonding in the acid amino-based PAs domi-
nantly stabilize the helical structure, while intermolecular
hydrogen bonding may have the opposite effect. To further validate
the kinds of the formed hydrogen bonds, the solution FT-IR spectra
of poly(LP50-co-LS50) at different concentrations is also conducted
3
in CHCl as an example. As can be seen in Fig. 2B, with the decrease
of the concentrations from 5 to 0.5 mM, the absorptions of hydro-
gen bonded NAH groups decreased together with the increasing
amount of free NAH groups, indicating the disappearance of
formed intermolecular hydrogen bonding. Besides, the absorption
bands of carbonyl groups also became narrower. Interestingly, an
obvious red-shift from 1698 to 1705 cm of the band correspond-
ing to carbamate groups was observed upon the decreasing con-
centration, while no frequency shift occurred at the band of
amide. When the concentration was below 0.5 mM, there was no
Since the helical PAs belong to dynamic helical polymers, it is
likely that external stimuli such as heat and solvents can influence
the formation and stabilization of their helical structure. The helix
sense can be manipulated continuously by different solvents and
this can help to understand the effect of phenyl and hydroxyl
groups in controlling the helix of the PAs. As shown in Fig. 4, the
ꢀ1

X. Bu et al. / Reactive & Functional Polymers 82 (2014) 17–24
21
Fig. 2. (A) Solution FT-IR spectra of LP, LS, and poly(LP50-co-LS50) in CHCl
3
(c = 1 mM). (B) Solution FT-IR spectra of poly(LP50-co-LS50) in CHCl
3
at the concentrations of (a)
5
mM, (b) 2.5 mM, (c) 1 mM, (d) 0.5 mM, (e) 0.25 mM, (f) 0.1 mM, and (g) 0.05 mM.
3
Fig. 3. CD and UV–vis spectra of the PAs measured in CHCl at room temperature (c = (0.73–1.01) mM).
Fig. 4. Solvent effect on the CD spectra of the PAs measured in THF and MeOH (c = (0.73–1.01) mM).

2
2
X. Bu et al. / Reactive & Functional Polymers 82 (2014) 17–24
Fig. 5. Helical conformation and hydrogen bonding of typical pitches in poly(LP50-co-LS50) chains.
Table 3
Infrared emissivity values of the PAs at room temperature.
generated by amide-amide intramolecular hydrogen bonds and
strong hydrophobic interactions between the neighboring phenyl
groups in side chains [40], the incorporation of LS can increase
the spacer length of the neighboring phenyl groups to relieve the
hydrophobic interactions and provide PAs with moderate molecu-
lar weights. Besides, the hydroxyl group in the side chain is prefer
to form inner amide–hydroxyl intramolecular hydrogen bonds to
release the helix and increase its stability [35,36]. As LS increases
from 0% to 50% in poly(LP-co-LS)s, hydrophobic interactions
between the neighboring phenyl groups gradually weaken which
in turn result in less compact helical structure. Meanwhile, part
of the amide–amide intramolecular hydrogen bonds firmly con-
verts to amide–hydroxyl hydrogen bonds which also have some
effect in the decrease of helicity. Nevertheless, we find that the sta-
bility of helix increases largely which may be due to the weakened
stereo-hindrance effect of bulky substituents and more convenient
formation of hydrogen bonds. When the content of LS is raised fur-
ther from 50% to 100%, steric effects between the substituents
weaken rapidly because of the decreasing amount of phenyl groups
and more easy formation of intramolecular amide–hydroxyl
hydrogen bonds. This eventually makes the helix much looser.
Owing to more short side chains, intermolecular hydrogen bonds
constructed between hydroxyl, carbamate, and other groups are
widely formed which results in unpromising decrease in the helical
stability [41]. In general, poly(LP50-co-LS50) has taken the most sta-
ble helical conformation with a relatively high helicity and it can
be assumed that the competition effect on conformation between
the steric factors and hydrogen bonding is controlled by the ratio
of the two monomers and it will reach an optimal balance state
of stability when the ratio comes to 1:1. Additionally, the structure
Monomer feed ratio (LP:LS)
Infrared emissivity e25 (8–14 lm)
1
7
6
5
3
2
0
00:0
5:25
2.5:37.5
0:50
7.5:62.5
5:75
0.964
0.751
0.697
0.632
0.705
0.793
0.825
:100
solvent effect on the helical secondary structure of the PAs was
estimated by CD spectra in THF and MeOH. In THF, the CD signals
of the all PAs appeared at ꢁ325 nm and similar tendency of the sta-
3
bility was observed as in CHCl , indicating that the helical structure
formed in a similar way. On the other hand, the PAs in MeOH
shows quite different Cotton effect. In MeOH, the positive and neg-
ative CD signals around 270 and 310 nm suggest that the PAs also
take helical structure. However, with the increasing proportion of
LS, the intensities of both the negative and positive signals under-
went a corresponding increase, which confirmed that the hydroxyl
group played an important role in the forming the secondary struc-
ture of the macromolecules. Since the helical conformation is likely
to be stabilized by intramolecular hydrogen bonding along with
steric repulsion, strong polar or protic solvents may change its
helicity and stability by influencing the formation of hydrogen
bonds to change asymmetric forces between substitutes. For THF,
the strong polarity can affect the hydrophobic interaction of the
bulky phenyl groups in the side chains to decrease the asymmetric
forces of the main chain to loose the helix and provide more pos-
sibility to form hydrogen bonds [27,28,39]. In addition, MeOH
can shield the inner amide groups by preventing them to form
intramolecular hydrogen bonds with hydroxyl groups [34–36].
Thence, it is not hard to assume that hydroxyl groups participate
in forming intramolecular hydrogen bonds with inner amide
groups and the ratio of LP and LS is important in controlling the
helix. Furthermore, the CD intensities of PAs in different solvents
varied greatly, although the concentration of each PA was approx-
information and hydrogen bonding formation of poly(LP50-co-LS50
)
is also in good agreement with the MMFF94 simulation (Fig. S5).
The typical pitches in poly(LP50-co-LS50) chains is also schemati-
cally illustrated in Fig. 5, although exact structure information still
remains not so clear.
3.4. Infrared emissivity analysis of the PAs
imately equivalent. The [
different from those in CHCl
spectra. For instance, the [
93°, which was much smaller than the value in CHCl
in Table 1). These observations also agree with the fact that the
hydroxyl group has significant effect in controlling and stabilizing
the helix of the amino acid-based PAs.
By combining previous reports [31,35,36,40], information about
the formation and stabilization of the helix in the PA can be inter-
preted as follows: Since the extremely compact helix in poly(LP) is
a
]
D
values measured in MeOH were also
which were consistent with the CD
of poly(LP50-co-LS50) in MeOH is
The infrared emissivity values of the as-prepared PAs at wave-
length of 8–14 lm measured at room temperature are shown in
3
a
]
D
Table 3. The infrared emissivity values of the PAs vary from
0.632 to 0.964. As mentioned in the Introduction, the vibration of
unsaturated groups in PAs in the polymer chain brings about high
infrared emissivity values. Closely packed pitches in the polymers
are favored over the decrease the index of hydrogen deficiency and
the unsaturated degree, thus reducing the infrared emissivity. As
stable and compact helical structure improves the orderliness of
molecules, the effective optimization of orderliness of the helix
ꢀ
3
(ꢀ386°,

X. Bu et al. / Reactive & Functional Polymers 82 (2014) 17–24
23
Fig. 6. Dependence of the infrared emissivity and Cotton effect intensity on the feed ratio of monomers.
proves that the importance of the participation of intramolecular
hydrogen bonds in maintaining the helix of the PAs.
4
. Conclusion
In summary, we have designed and synthesized a series novel
optically active PAs derived from chiral phenylalanine and serine.
The (co)polymers carrying proportional phenyl and hydroxyl
groups in the side chains have shown unique features in their opti-
cal and thermal properties. Systematic experimental investigations
have been carried out to determine the roles of hydrogen bonding
and steric factors in stabilizing the helical structure of the PAs. The
infrared emissivity values of the PAs have been measured and the
correlation between the infrared emissivity and secondary struc-
ture has been captured. The results show that the improvement
of the stability and helicity of the helical structure in organic poly-
mers significantly contributes to the decrease in infrared emissiv-
ity. Among these polymers, poly(LP50-co-LS50) shows the lowest
infrared emissivity (e25 = 0.632) value and excellent resistance
against heat (e105 = 0.670), which have met the requirements of
common use. Therefore, the incorporation of the chiral amino acid
in the resulting PAs provides a great promise in the development of
polymers with tunable infrared emissivity. Overall, the application
of PAs in reducing infrared emission in this paper is important not
only because they exemplify extend application of helical poly-
mers, but also because they provide a novel method to develop
materials with low-emissivity.
Fig. 7. Infrared emissivity values of poly(LP50-co-LS50) at 30–160 °C.
in PAs significantly contributes to the decrease in infrared emissiv-
ity. By combining data of the infrared emissivity values and their
optical activities of the PAs in Fig. 6, empirical evidence has shown
positive correlation between well-ordered and compact helical
secondary structures and lower infrared emissivity values.
Poly(LP50-co-LS50) which possesses the most orderly and compact
helix has the lowest infrared emissivity. This has confirmed our
assumption on the correlation between the secondary structure
of helical polymer and their infrared emissivity in previous reports.
Furthermore, to further validate the effect of temperature on
infrared emission, the infrared emissivity values of poly(LP50-co-
LS50) measured from 30 to 160 °C were investigated and presented
in Fig. 7. From 30 to 105 °C, the infrared emissivity value of
poly(LP50-co-LS50) has increased slowly to 0.670; however, above
Acknowledgements
1
05 °C, the value is increased much more rapidly. There is an obvi-
The authors are supported by National Nature Science Founda-
tion of China (51077013), Key Program for the Scientific Research
Guiding Found of Basic Scientific Research Operation Expenditure
of Southeast University (Grant No. 3207043101) and Instrumental
Analysis Fund of Southeast University, Fund Project for Transfor-
mation of Scientific and Technological Achievements of Jiangsu
Province of China (No. BA2011086), the Innovation Research Foun-
dation of College Graduate in Jiangsu Province (CXLX12-0107), and
ous inflectional point at about 105 °C and the infrared emissivity
value is as high as 0.828 at 150 °C. These observations indicate that
a large number of intramolecular hydrogen bonds have gradually
disappeared above 105 °C and this may force the PA chain to adopt
a randomly coiled conformation. As the orderliness and helicity of
poly(LP50-co-LS50) decrease rapidly, the infrared emissivity dra-
matically increases at a high speed. Moreover, this result also

2
4
X. Bu et al. / Reactive & Functional Polymers 82 (2014) 17–24
the Scientific Research Foundation of Graduate School of Southeast
University (YBJJ1417).
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