Thermoresponsive helical poly(phenylacetylene)s

Article abstract of DOI:10.1021/ma5003529

Poly(phenylacetylene) (PPA) bearing dendritic oligo(ethylene glycol) (OEG) as pendants was synthesized, and its thermoresponsiveness and helical conformation were investigated. Despite the steric hindrance of the bulky pendants in the homopolymer PPA-OEG,

Full text of DOI:10.1021/ma5003529

Article
pubs.acs.org/Macromolecules
Thermoresponsive Helical Poly(phenylacetylene)s
Shu Li, Kun Liu, Guichao Kuang,* Toshio Masuda, and Afang Zhang*
Laboratory of Polymer Chemistry, Department of Polymer Materials, College of Materials Science and Engineering, Shanghai
University, Nanchen Street 333, Materials Building Room 447, Shanghai 200444, China
S
* Supporting Information
ABSTRACT: Poly(phenylacetylene) (PPA) bearing dendritic
oligo(ethylene glycol) (OEG) as pendants was synthesized, and
its thermoresponsiveness and helical conformation were inves-
tigated. Despite the steric hindrance of the bulky pendants in the
homopolymer PPA-OEG, the chirality could be efficiently
transferred from pendant alanine moieties to PPA main chain
through ester linkage. In order to examine the steric effect of
pendants on chiral transformation, a model PPA homopolymer
PPA-Boc which carries less bulky moieties was prepared for
comparison. The chiroptical properties of these thermoresponsive
PPAs were further investigated by varying temperature to examine
the effects of their thermoresponsiveness. In addition, PPA
copolymers PPA-BDY bearing OEG dendron and fluorescent boradiazaindacene (BDY) chromophore showed excellent
thermoresponsive properties and interesting fluorescence enhancement at elevated temperatures. To investigate the rigidity
effects of polymer backbone on the thermally induced fluorescence enhancement, a nonchiral polymer carrying the same
pendants but with polymethacrylate as the backbone (PMA-OEG) was prepared. It was found that the chiroptical and
fluorescence properties of these PPAs are dependent not only on their chemical structures but also on the thermoresponsiveness.
functionalities but also pave a new avenue to broaden their
applications; thus, helical polymers capable of responding to
external stimuli have received considerable attention. For
example, functional PPAs which are responsive to chirality
were proposed by Yashima to tune the preferential helical
conformation and to switch the helical sense through host−
guest6,10 or acid−base interactions.11 PPAs bearing riboflavin
pendants were found to show switchable helicity triggered by
reduction−oxidation.12 PPAs carrying azobenzene pendants
were reported by Tang to show photoinduced isomerization.13
PPAs bearing urea acceptors are responsive to anions and,
therefore, can be used for colorimetric detection of anions
depending on their sizes. The PPAs display different colors
related to the compactness of their helical conformation, which
was utilized successfully by Kakuchi to identify different anions
in organic14 or aqueous solutions.15 PPAs carrying free amino
groups were found by Hu to be responsive to benzoic acid, and
their helical conformation can be mediated accordingly.16
Recently, attention has been paid to thermoresponsive helical
polymers because temperature is one of the most convenient
parameters to change. Attaching oligoethylene glycol (OEG)
pendants to helical polymers, we demonstrated that chiral
polyisocyanides can be afforded thermoresponsiveness with
tunable cloud point temperatures (Tcp).17 Thermoresponsive
polyisocyanides carrying OEGs were also reported by Nolte
INTRODUCTION
■
The increased understanding of the π-conjugated polymers has
stimulated advances in cross-disciplinary areas of materials
sciences, biological chemistry, and biomedical therapy.1 As a
class of widely studied π-conjugated polymers, poly-
(phenylacetylene) (PPA) and its derivatives exhibit interesting
properties such as liquid crystallinity, electrical conductivity,
catalysis, fluorescence sensing, and chiral recognition.2 One
intriguing property for PPAs is that they can be mediated to
adopt helical conformation with preferred handedness, which
have attracted great interest in the past decades.3 In principle,
PPA derivatives adopt a dynamic conformation whose helical
sense is governed by hydrogen bonds between pendant amide
groups or by steric hindrance from bulky substituents.4 Various
helical PPAs have been reported, and chirality transmission
from pendant chiral groups to the polymer main chain can be
significantly amplified through covalent bonds5 or by non-
covalent interactions.6 The magnitude of Cotton effects from
circular dichroism (CD) spectra depends greatly on the
distance between chiral center and the backbone.3b Although
PPAs display increased molar ellipticity with increasing
substituent bulkiness, most dendronized PPAs show decreased
CD intensity with increase of dendritic generations due to the
overcrowded pendants. It has been documented that the
handedness and compactness of helical PPAs depend on the
chirality of the pendants,3−6 metal coordination,7 polar or
donor effects of solvents,8 and temperature.8d,e,9
Received: February 18, 2014
Revised: April 11, 2014
Published: May 5, 2014
Combination of stimuli responsiveness with conjugated
polymers will not only afford these polymers novel
© 2014 American Chemical Society
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Chart 1. Chemical Structures of the Polymers Discussed in the Present Study
and found to form thermally induced hydrogels.18 Attaching
EXPERIMENTAL SECTION
■
OEG moieties of different chain lengths to polyisocyanates
(PICs), Kakuchi reported that functional PICs can be
thermoresponsive with Tcp dependent on their molar masses.19
We initiated a research program recently to address the
versatile thermoresponsive polymers with three-folded OEG-
based dendrons through covalent linkage20 or supramolecular
interaction21 and found that these polymers showed fast and
sharp phase transitions without obvious hysteresis. In
implementing our previous studies about thermoresponsive
dendronized polymers, a series of thermoresponsive helical
polyisocyanides carrying dipeptide pendants and OEG
terminals were prepared.17 Tcps of these polymers are not
only dependent on their overall hydrophilicity and molar
masses but also predominately related to their ordered
secondary structures. Considering the high rigidity of
polyisocyanides, it was found that the thermally induced
aggregation did not show an obvious influence on the helical
conformation. Inspired by this progress on the helical and
thermoresponsive dendronized polymers, we here report on the
first example of thermoresponsive PPAs. These polymers carry
the first-generation three-folded dendritic OEG units through
alanine linkage (Chart 1). They were synthesized through
macromonomer route with a rhodium(l) catalyst and
characterized with NMR spectroscopy and atomic force
microscopy (AFM). CD measurements were performed to
investigate chirality transformation from stereogenic center of
pendants to the polymer main chain through an ester linkage.
In order to compare the possible steric hindrance aroused from
the bulky dendron in biasing helical sense of the polymers,
model polymer PPA-Boc bearing a much smaller pendant Boc-
protected alanine was prepared. The thermoresponsiveness of
these chiral PPAs was investigated by turbidity measurements
with UV/vis spectroscopy. In view of promising applications of
these thermoresponsive PPAs as fluorescent thermometer,
thermoresponsive helical copolymer PPA-BDY, which contains
a small amount of boradiazaindacene (BDY) chromophore, was
prepared, and its fluorescence properties during thermally
induced phase transition processes were followed with UV/vis
and fluorescence spectroscopies. In order to compare the
effects of polymer backbone rigidity and helicity on thermally
induced fluorescence properties, a much more flexible and
achiral copolymer PMA-BDY, which contains a polymethacry-
late backbone and a small amount of BDY chromophores as
pendants, was synthesized.
Materials. Compound OEG-OH was prepared according to our
previous report.21a Azobis(isobutyronitrile) (AIBN) was recrystallized
twice from methanol. Dichloromethane (DCM) was dried over CaH2.
Tetrahydrofuran (THF) was predried over sodium and refluxed over
LiAlH4 before use. Triethylamine (TEA) was dried over sodium
hydroxide (NaOH) pellets. Other reagents and solvents were
purchased at reagent grade and used without further purification. All
synthetic steps for monomer synthesis were run under a nitrogen
atmosphere. Macherey-Nagel precoated TLC plates (silica gel 60 G/
UV254, 0.25 mm) were used for thin-layer chromatography (TLC)
analysis. Silica gel 60 M (Macherey-Nagel, 0.040−0.063 mm, 200−300
mesh) was used as the stationary phase for column chromatography.
All samples were kept under high vacuum prior to analytical
measurements to remove strongly adhering solvent molecules.
Instrumentation and Measurements. 1H and 13C NMR spectra
were recorded on a Bruker AV 500 (1H: 500 MHz; 13C: 125 MHz)
spectrometer, and chemical shifts are reported as δ (ppm) relative to
internal tetramethylsilane. High-resolution MALDI-TOF-MS analyses
were performed on Ionspec Ultra instruments. Gel permeation
chromatography (GPC) measurements were carried out on a Waters
GPC e2695 instrument with a three-column set (Styragel HR3 + HR4
+ HR5) equipped with refractive index detector (Waters 2414) and
DMF (containing 1 g L−1 LiBr) as eluent at 45 °C. Multiangle light
scattering detector (Wyatt Technology Corporation, Down EOS 243-
E) was used for some of the measurements. The calibration was
performed with poly(methyl methacrylate) standards (from Polymer
Standards Service-USA Inc.) with molar masses (Mp) in the range of
2580−981 000. Light scattering (LS) measurements were performed
on an ALV/CGS-3 compact goniometer system with an ALV/LSE-
5004 correlator and a Uniphase HeNe laser (22 mW output power at
λ = 632.8 nm wavelength). CD measurements were performed on a
JASCO J-815 spectropolarimeter with a thermo-controlled 1 mm
quartz cell (three accumulations, “continue scanning” mode, scanning
speed: 100 nm min−1; data pitch: 0.5 nm; response: 1 s; bandwidth:
2.0 nm). UV/vis turbidity measurements were carried out on a PE
UV/vis spectrophotometer (Lambda 35) equipped with a thermo-
controlled bath. Aqueous polymer solutions were placed in the
spectrophotometer (path length 1 cm) and heated or cooled at a rate
of 0.2 K min−1. The absorptions of the solutions at λ = 600 nm were
recorded per 5 s. The cloud point temperature (Tcp) was determined
as the one at which the transmittance at λ = 600 nm reached 50% of its
initial value. AFM measurements were performed on a Bruker
Nanoscope VIII Multi-Mode microscope with an “E” scanner
(scanning range 10 μm × 10 μm) and operated in peak force mode
at room temperature in air. Bruker silicon tip was attached on nitride
lever cantilevers (T: 0.65 μm; L: 115 μm; W: 25 μm; f 0: 70 μm; k: 0.4
N/m). Samples were prepared from chloroform solutions by spin-
coating with a speed of 2000 rpm. The scanning rate was at line
frequency of 1.0 Hz. The images analyses were performed by using
Nanoscopy image processing software. The fluorescence spectra were
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a
Scheme 1. Synthetic Procedures for the Monomers and Polymers
a
Reagents and conditions: (a) 1, LiAlH4, THF, 0 °C, 4 h (83%); (b) 2, L-Boc-Ala-OH, EDC·HCl, DMAP, DCM, 0 °C−rt, 29 h (89%); (c) 3, HCl,
ethyl acetate, 0 °C−rt, 8.5 h (98%); (d) 4, OEG-OH, EDC·HCl, HOBt, DCM, 0 °C−rt, 22 h (58%); (e) PA-OEG, PA-BDY or PA-Boc,
[(nbd)RhCl]2, TEA, THF, 25 °C, 22 h for PPA-OEG (43%); 17 h for PPA-BDY (60%); 19 h for PPA-Boc (80%). (f) 3, TBAF, CH3OH, rt, 24 h for
PA-BDY (79%), 16 h for PA-Boc (80%). Abbreviations: DCM = dichloromethane, DMAP = 4-(dimethylamino)pyridine, EDC·HCl = 1-(3-
dimethyl-aminopropyl)-3-ethylcarbodiimide hydrochloride, HOBt = 1-hydroxy-1H-benzotriazole, TBAF = tetrabutylammonium fluoride, THF =
tetrahydrofuran, [(nbd)RhCl]2 = bicyclo[2.2.1]hepta-2,5-diene−rhodium(l) chloride dimer.
recorded on a spectrophotometer (Horiba Jobin Yvon Fluorolog-3)
equipped with a Peltier temperature controller and heated or cooled at
a rate of 0.2 K min−1.
solution (10 mL) of OEG-OH (0.34 g, 0.52 mmol) and 1-hydroxy-
1H-benzotriazole (HOBt; 0.09 g, 0.65 mmol). EDC·HCl (0.12 g, 0.60
mmol) was added slowly to the mixture solution at 0 °C. The mixture
solution was stirred at room temperature for 24 h before partitioned
between brine and DCM. The organic phase was dried over MgSO4.
After filtration, the solvent was evaporated in vacuo. The residue was
purified with chromatograph by using DCM/MeOH (60:1) as eluent
Synthesis of Monomer and Polymers. Compound 2.
Compound 1 (1.00 g, 4.90 mmol) in THF (60 mL) was added
dropwise to the THF solution (10 mL) of LiAlH4 (0.37 g, 9.80 mmol)
at 0 °C. The reaction mixture was stirred for another 4 h. Then, H2O
was added dropwise at 0 °C until no bubble was observed. The
generated precipitate was dissolved by adding hydrochloric acid
solution (5 mL, 10 wt %). After evaporation of THF, the residue was
dissolved with DCM and washed with brine. The organic phase was
dried over MgSO4. Purification by chromatography using DCM as
1
to afford monomer PA-OEG as white solids (0.25 g, 58%). H NMR
(CDCl3): δ = 1.16−1.20 (m, 9H, CH3), 1.51 (d, J = 7.1 Hz, 3H, CH3),
3.10 (s, 1H, CH), 3.48−3.84 (m, 38H, CH2), 4.18−4.21 (m, 6H,
CH2), 4.76−4.79 (m, 1H, CH), 5.18 (dd, J = 12.6, 25.8 Hz, 2H, CH2),
6.75 (d, J = 7.2 Hz, NH), 7.09 (s, 2H, Ar−H), 7.30 (d, J = 8.2 Hz,2H,
Ar−H), 7.46 (d, J = 8.2 Hz, 2H, Ar−H). 13C NMR (CDCl3): δ =
14.96, 18.02, 26.56, 31.31, 36.51, 48.49, 66.29, 68.95, 69.60, 70.51,
72.24, 77.80, 82.88, 107.27, 127.73, 128.45, 128.69, 132.08, 135.93,
152.21, 162.38, 166.33, 172.66. HRMS (ESI): m/z calcd for
C43H65NO15 [M + Na]+: 858.4266, found: 858.4246.
1
eluent afforded compound 2 as white solids (0.83 g, 83%). H NMR
(CDCl3): δ = 0.25 (s, 9H, CH3), 1.82 (br, 1H, OH), 4.68 (d, J = 4.1
Hz, 2H, CH2), 7.28 (d, J = 7.9 Hz, 2H, Ar−H), 7.46 (d, J = 7.9 Hz,
2H, Ar−H).
Compound 3. Compound 2 (0.50 g, 2.45 mmol) in DCM (2 mL)
was added to a DCM solution (20 mL) of L-Boc-Ala-OH (0.56 g, 2.90
mmol) and DMAP (0.15 g, 0.49 mmol). EDC·HCl (0.70 g, 3.68
mmol) was added slowly to the solution at 0 °C. The mixture was
stirred at room temperature for 29 h before partitioned between brine
and DCM. The organic phase was dried over MgSO4. After filtration,
the solvent was evaporated in vacuo. The residue was purified with a
chromatograph using DCM/MeOH (50:1) as eluent to afford
Monomer PA-Boc. Tetrabutylammonium fluoride (TBAF) (0.54 g,
1.70 mmol) was added to the methanol solution of compound 3 (0.20
g, 0.53 mmol) at room temperature. After being stirred for 16 h,
methanol was evaporated. The residue was partitioned between brine
and DCM. The organic phase was dried over MgSO4. After filtration,
the solvent was evaporated in vacuo. The residue was purified with
chromatograph using hexane/EtOAc (5:1) as eluent to afford
1
1
compound 3 as white solids (0.82 g, 89%). H NMR (CDCl3): δ =
monomer PA-Boc as a white oil (0.12 g, 80%). H NMR (CDCl3):
0.25 (s, 9H, CH3), 1.39 (d, J = 7.2 Hz, 3H, CH3), 1.43 (s, 9H, CH3),
4.34−4.36 (m, J = 7.1 Hz, 1H, CH), 5.16 (dd, J = 12.8, 36.6 Hz, 2H,
CH2), 7.27 (d, J = 8.0 Hz, 2H, Ar−H), 7.45 (d, J = 8.2 Hz, 2H, Ar−
H).
δ = 1.38 (d, J = 7.2 Hz, 3H, CH3), 1.43 (s, 9H, CH3), 3.09 (s, 1H,
CCH), 4.34−4.37 (m, 1H, CH), 5.02 (br, 1H, NH), 5.07−5.19 (m,
2H, CH2), 7.29 (d, J = 8.1 Hz, 2H, Ar−H), 7.47−7.49 (d, J = 8.2 Hz,
2H, Ar−H).
Compound 4. Ethyl acetate solution of HCl (4.02 g, 110 mmol)
was added dropwise to the ethyl acetate solution (10 mL) of
compound 3 (0.96 g, 2.57 mmol) at 0 °C. The mixture was stirred at
room temperature for 6 h. Evaporation of the solvent afforded
compound 4 as white solids (0.80 g, 98%). 1H NMR (D2O): δ = 1.54
(d, J = 7.3 Hz, 3H, CH3), 3.52 (s, 1H, CH), 4.21 (dd, J = 7.4, 14.6 Hz,
1H, CH), 5.28 (dd, J = 12.4, 17.7 Hz, 2H, CH2), 7.42 (d, J = 8.1 Hz,
2H, Ar−H), 7.57 (d, J = 8.2 Hz, 2H, Ar−H).
Monomer PA-BDY was prepared according to previous report22
1
with a yield of 80%. H NMR (CDCl3): δ = 3.26 (s, 1H, CCH), 6.56
(d, J = 3.8 Hz, 2H, pyrrole-H), 6.91 (d, J = 4.1 Hz, 2H, pyrrole-H),
7.54 (d, J = 8.4 Hz, 2H, Ar−H), 7.65 (d, J = 8.2 Hz, 2H, Ar−H), 7.95
(s, 2H, pyrrole-H). 13C NMR (CDCl3): δ = 80.09, 82.61, 118.86,
124.94, 130.50, 131.48, 132.20, 134.07, 134.75, 144.55, 146.22. HRMS
(ESI): m/z calcd for C17H11BF2N2 [M + H]+: 291.1021, found:
291.1042.
Monomer PA-OEG. DCM solution (5 mL) of compound 4 (0.20 g,
0.52 mmol) and DIPEA (0.13 g, 1.00 mmol) was added to the DCM
Polymer PPA-OEG. THF solution (0.5 mL) of [(nbd)RhCl]2 (1.1
mg, 0.002 mmol) and TEA (2 mg, 0.02 mmol) were added to the
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Table 1. Polymerization Conditions and Characterization Results of the Polymers
b
polymerization conditions
GPC results
a
c
entries
[M]/[I] [M] (mol/L) time (h) solvent temp (°C) yield (%) [BDY] (mol %) Mw (×10−4
)
DP
PDI
Tcp (°C)
PPA-OEG
PPA-BDY
PMA-BDY
PPA-Boc
50:1
50:1
23:1
50:1
0.096
0.096
2.80
22
17
25
19
THF
THF
DMF
THF
25
25
70
25
42
60
55
67
0
67.5
49.4
30.1
34.2
311
307
175
369
2.59
1.93
2.39
3.03
36.0
36.2
35.0
NA
1/957
1/1018
0
0.096
a
Determined by comparison of the absorbance (A500 nm) for PPA-BDY with PA-BDY (ε500 nm = 63 700 M−1 cm−1) in methanol or the absorbance
(A498 nm) for PMA-BDY with MA-BDY (ε498 nm = 81 300 M−1 cm−1) in DCM at 25 °C, respectively (Figure S3 in Supporting Information).
b
c
Determined by GPC with DMF as eluent. The apparent Tcp of the polymers was determined as the temperature at 50% of the initial transmittance
at λ = 600 nm. NA = not available.
THF solution (0.5 mL) of PA-OEG (80 mg, 0.096 mmol). The
mixture was stirred at 25 °C for 22 h before concentrated on a rotary
evaporator. The residue was purified with chromatograph by using
DCM as eluent to afford polymer PPA-OEG as yellow solids (34 mg,
43%). 1H NMR (CDCl3, 50 °C): δ = 1.11−1.18 (m, 9H, CH3), 1.24−
1.37 (br, 3H, CH3), 3.41−3.73 (m, 37H, CH2 + CH2), 4.05 (d, J =
31.8 Hz, 6H, CH2), 4.65 (br, 1H, CH), 4.83 (br, 1H, CH2), 4.95 (br,
1H, CH2), 5.61 (br, 1H, CCH), 6.62 (br, 2H, Ar−H), 6.93 (br, 2H,
Ar−H), 7.05 (br, 2H, Ar−H), 7.72 (br, H, NH).
The polymerizations of phenylacetylene monomers were
carried out by using [(nbd)RhCl]2 catalyst with a constant
monomer/catalyst ratio of 50/1 in polar solvent THF at 25 °C.
TEA was added as a base, which could greatly accelerate the
reaction and facilitate to obtain high molecular weight
polymers.23 All polymerizations proceeded homogeneously
without obvious viscosity changes. A typical free radical
copolymerization was conducted for the methacrylate mono-
mers by using AIBN as initiator (see Supporting Information
for details). All polymers are totally soluble in common organic
solvents or in water. They were purified through silica-gel
column chromatography with DCM as eluent, and the total
monomer conversions were calculated gravimetrically. The
molar masses of obtained polymers measured by GPC with
DMF as eluent were in the range of millions g·mol−1. Detailed
polymerization conditions and characterization results for the
polymers/copolymers are summarized in Table 1. The weight-
average molecular weights (Mw) of PPA-OEG and PPA-BDY
determined by GPC analyses are 6.75 × 105 and 4.94 × 105,
respectively. However, the corresponding Mws were determined
by static laser scattering to be 3.01 × 106 and 2.30 × 106,
respectively, indicating that GPC measurements underestimate
significantly the molar masses of these polymers comprising
repeat units of high molar mass.17
Polymer PPA-BDY. THF stock solution (0.5 mL) of PA-BDY (0.10
mg, 0.34 mol), [(nbd)RhCl]2 (1.1 mg, 0.002 mmol), and TEA (2.0
mg, 0.02 mmol) were added to the stock solution (0.5 mL) of PA-
OEG (150 mg, 0.18 mmol). The mixture solution was stirred at 25 °C
for 17 h before concentrated on a rotary evaporator. The residue was
purified with a chromatograph by using DCM as eluent to afford
1
polymer PPA-BDY as yellow solids (90 mg, 60%). H NMR (CDCl3,
50 °C): δ = 1.11−1.18 (m, 9H, CH3), 1.24−1.37 (br, 3H, CH3),
3.41−3.73 (m, 37H, CH2 + CH2), 4.05 (d, J = 31.8 Hz, 6H, CH2),
4.65 (br, 1H, CH), 4.83 (br, 1H, CH2), 4.95 (br, 1H, CH2), 5.61 (br,
1H, CCH), 6.63 (br, 2H, Ar−H), 6.94 (br, 2H, Ar−H), 7.06 (br, 2H,
Ar−H), 7.80 (br, H, NH).
Polymer PPA-Boc. THF solution (0.5 mL) of [(nbd)RhCl]2 (1.1
mg, 0.002 mmol) and TEA (2.0 mg, 0.020 mmol) were added to the
THF solution (0.5 mL) of PA-Boc (100 mg, 0.33 mmol). The mixture
solution was stirred at 25 °C for 19 h before concentrated on a rotary
evaporator. The residue was purified with chromatograph by using
DCM as eluent to afford polymer PPA-Boc as yellow solids (80 mg,
All polymer structures were characterized by 1H NMR
spectroscopy (for spectra, see Supporting Information). As an
1
80%). H NMR (DMSO-d6, 80 °C): δ = 1.19 (d, J = 7.2 Hz, 3H,
CH3), 1.33 (s, 9H, CH3), 4.02−4.05 (m, 1H, CH), 4.87 (dd, J = 12.8,
34.6 Hz, 2H, CH2), 5.70 (s, 1H, CCH), 6.60 (d, J = 6.7 Hz, 2H, Ar−
H), 6.94 (d, J = 7.5 Hz, 2H, Ar−H).
1
example, the H NMR spectra of monomer PA-OEG and its
corresponding homopolymer PPA-OEG, together with the
model polymer PPA-Boc, are combined in Figure 1. The
proton signal at δ = 3.09 ppm ascribed to the proton of ethynyl
group from the monomer PA-OEG disappeared for polymer
PPA-OEG; instead, a new peak at δ = 5.68 ppm appeared
which is ascribed to the olefinic proton from the conjugated
main chain.24 Besides, the proton signals from PPA-OEG
broadened when compared to these from the corresponding
monomer. These results strongly demonstrate successful
transformation from monomer PA-OEG to polymer PPA-
OEG. Highly stereoregular cis-transoidal conformation for the
PPA main chain is expected since a rhodium catalyst was
used.25 According to the calculation methods proposed by
Percec and co-workers,26 the cis-content was determined to be
92% for PPA-OEG. In contrast to the sharp and well-resolved
proton signals from PPA-Boc, the broadened proton signals
from PPA-OEG (Figure 1) indicate that the polymer is more
rigid than PPA-Boc in CDCl3 solution. Attention was paid to
the splitting of the proton signals (f) of methyl group from
alanine moiety in the monomer and polymers. Both the
monomer PA-OEG and the model polymer PPA-Boc showed
the expected symmetric doublet. The chemical shift difference
between the split peaks (Δδ) was about 0.014 ppm. While for
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization of Homo-
and Copolymers. The synthesis of phenylacetylene mono-
mers PA-OEG and PA-Boc started from the same commercially
available aldehyde 1 as depicted in Scheme 1. Reduction of 1
with LiAlH4 gave corresponding alcohol 2, which was treated
with N-(tert-butoxycarbonyl)-L-alanine (L-Boc-Ala-OH) to give
compound 3 with protected alkyne and amine. Monodepro-
tection of trimethylsilyl (TMS) group with TBAF provided
monomer PA-Boc. Double deprotection of both TMS and Boc
groups with HCl yielded 4. Its amidation with OEG-OH
yielded monomer PA-OEG by applying a typical peptide
coupling method in the presence of EDC and HOBt. The
monomer PA-BDY was prepared according to the literature.22
The synthesis of aliphatic monomer MA-BDY was similar to
that for PA-BDY with the exception of final methacrylation
reaction (refer to Scheme S1 in the Supporting Information).
1
All new compounds were identified by H NMR spectroscopy,
and new monomers were further characterized by 13C NMR
spectroscopy as well as high-resolution mass spectrometry.
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1
Figure 1. H NMR spectra of monomer PA-OEG (CDCl3, 25 °C), polymer PPA-OEG (CDCl3, 50 °C), and polymer PPA-Boc (CDCl3, 50 °C).
The solvent peaks are marked with asterisks.
PPA-OEG, the proton signals (f) split into two asymmetric
peaks with intensity ratio around 2:1 at 50 °C, and Δδ
increased into 0.11 ppm (inset in Figure 1). The asymmetric
splitting of proton signals of methyl group from alanine may
indicate that PPA-OEG provides a restrained environment,
which prevents the methyl group from free rotation. In order to
the peak at upfield, suggesting that the restrained environment
from PPA-OEG mediated the pendants to adopt two possible
stable spatial arrangements: one is preferred at low temper-
ature, while the other is favorable at the higher temperature.
Similar to its homopolymer PPA-OEG, the copolymer PPA-
1
BDY also showed broad signals in its H NMR spectrum (see
1
confirm this hypothesis, H NMR spectra of PPA-OEG in
Figure S2 in the Supporting Information). Because of the signal
broadness, it is difficult to estimate the content of BDY
moieties from their 1H NMR spectra. Instead, the molar
contents of BDY units in copolymers were determined by
comparison of UV absorbance intensities for the copolymer
DMSO-d6 were recorded at different temperatures (for spectra,
see Figure S1 in the Supporting Information). The proton
signals from f split asymmetrically into two peaks at 40, 60, and
80 °C. The intensity for the peak at downfield increased with
solution temperature, accompanied by diminishing intensity for
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with those from the corresponding monomer and were found
in the range of 1/1000.
conformation do not show obvious influence on the phase
transition temperatures.
AFM measurements were conducted to directly visualize the
morphologies of the helical polymers on substrates. Because of
the repulsive effects from bulky dendritic side chain, an
extended conformation was expected for these helical polymers,
which allows easy visualization of single molecules by AFM.
Typical AFM images for PPA-OEG, PPA-BDY, and PPA-Boc
are shown in Figure 2. Individual polymer chains can be
To further elucidate the thermally induced dehydration
1
processes of these copolymers, temperature-varied H NMR
1
spectroscopy was utilized. The H NMR spectra from PPA-
OEG in the temperature range from 24 to 45 °C were
assembled in Figure 4. Below its Tcp, proton signals ascribed to
Figure 2. AFM images of (a) PPA-OEG (3.0 mg/L), (b) PPA-BDY
(2.6 mg/L), and (c) PPA-Boc (3.2 mg/L) on mica. Samples were
prepared from chloroform solutions by spin-coating (2000 rpm).
Apparent height ∼0.5 nm and width ∼7 nm for PPA-BDY.
identified with length from very short ones to these in the range
of a few hundred nanometers for both PPA-OEG and PPA-
BDY (Figure 2a,b). In contrast, the model polymer PPA-Boc is
much more flexible due to absence of the bulky OEG pendants,
and the polymer chains tend to adopt coiled conformation and
to aggregate together (Figure 2c).
Figure 4. Temperature-varied 1H NMR spectra of PPA-OEG (0.40 wt
%) in D2O (Tcp = 33.9 °C).
OEG moieties (δ ∼ 1.0 and 3.5 ppm) were quite resolved (the
Tcp of PPA-OEG determined at this concentration is 33.9 °C;
for turbidity curve see Figure S4a). When the temperature rose
to 33 °C, which is around its Tcp, the proton signals ascribed to
OEG dendritic moieties became broader, indicating their
dehydration and collapse. The broadening of OEG signals
was further enhanced with further temperature increase to
above the Tcp of PPA-OEG, caused by polymer aggregation.
Consistent results were obtained for copolymer PPA-BDY (for
1H NMR spectra, see Figure S2).
Chiroptical Properties. CD measurements were carried
out to investigate whether the chirality could transfer from
alanine moieties to the PPA main chain through the ester
linkage for PPA-OEG. In addition, its chiroptical properties
were monitored during the thermally induced aggregation
processes in aqueous solutions to check the possible effects of
thermoresponsiveness on the helical conformation. For
comparison, chiroptical property of the model polymer PPA-
Boc was also examined to check the bulkiness effect from the
Thermoresponsive Properties. Except for PPA-Boc, both
polymers with rigid poly(phenylacetylene) backbone (PPA-
OEG) or flexible polymethacrylate backbone (PMA-OEG)
showed good solubility in water at low temperature. Upon
heating to elevated temperatures, the slightly yellow but
transparent solutions turned opaque due to the thermally
induced aggregation of the polymers (see Figure 3a).
Therefore, turbidity measurements by using UV/vis spectros-
copy were performed to investigate the thermoresponsive
behavior of PPA-OEG in detail. Its typical turbidity curves are
shown in Figure 3b. Quite fast phase transitions and very small
hysteresis were observed. In fact, the two copolymers PPA-BDY
and PMA-BDY also show thermoresponsive behavior (for
turbidity curves, see Figure S4c,d). Tcps for PPA-OEG, PPA-
BDY, and PMA-BDY were determined to be 36.0, 36.2, and
35.0 °C, respectively. These Tcps are very close to one another,
suggesting that a trace amount of hydrophobic BDY moiety in
the copolymer will not contribute much to the overall
hydrophilicity, and both backbone rigidity and helical
Figure 3. (a) Photographs of PPA-OEG aqueous solutions (0.025 wt %) under visible light at temperature below and above its Tcp, respectively. (b)
Turbidity curves for PPA-OEG aqueous solution (0.025 wt %). Heating and cooling rate 0.2 K min−1.
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Figure 5. CD and UV/vis spectra for (a) PPA-OEG (0.025 wt %) in aqueous solutions at 25 and 40 °C and(b) PPA-Boc (0.030 wt %) in DCM at
25 °C.
Figure 6. Fluorescence spectra (λex = 490 nm) of aqueous solution of (a) PPA-BDY (0.025 wt %) and (b) PMA-BDY (0.10 wt %) at different
temperatures. The fluorescence intensities at their maximum emission (λem = 522 nm for PPA-BDY and λem = 524 nm for PMA-BDY) at 48 °C are
set to 1. The inset in (a) shows photographs of aqueous solutions of PPA-BDY (0.025 wt %) under UV irradiation (365 nm) at 30 and 48 °C.
OEG pendants. The CD spectra are shown in Figure 5. From
Figure 5a, a strong Cotton effect was observed for PPA-OEG in
aqueous solution at 25 °C, indicating that the PPA backbone
takes a preferential helical conformation with a large excess in
one handedness. Interestingly, after the temperature increased
to 40 °C, which is above the phase transition temperature of
PPA-OEG (Tcp = 36.0 °C at this concentration), the CD
spectrum red-shifted more than 10 nm. As the temperature
decreased to 25 °C, the original spectrum was almost restored.
This red-shift is likely associated with solvophobic collapse of
the OEG dendritic pendants.27 In contrast, negligible change
was detected in the CD spectrum of PPA-OEG solution in
methanol at different temperatures (Figure S5). We believe this
red-shift is caused by enhanced steric repulsion from the
collapsed OEG dendrons above phase transition temperature,
resulting in a stretch of PPA backbone to increase the helical
pitch. To the best of our knowledge, this is the first example to
report the helical structures of PPAs can be mediated by their
thermoresponsiveness in aqueous solutions. However, the
Cotton effect of PPA-OEG decreased by about 40% after
storage for 3 days (Figure S6), indicating structural isomer-
ization of the PPA main chain as reported previously.28 The
model compound PPA-Boc showed even higher molar
ellipticity value in DCM than PPA-OEG (Figure 5b), indicating
that the bulky OEG dendritic pendant in PPA-OEG is not
favorable for the formation of the helical conformation.
widely in various polymers as dye probe.29 Therefore, helical
copolymer PPA-BDY containing a trace amount of BDY
chromophore was synthesized and expected to behave as a
thermoresponsive fluorescent sensor. As described above, PPA-
BDY showed excellent thermoresponsive properties with Tcp at
36.2 °C. Thus, fluorescence measurements were performed to
monitor its thermoresponsive processes, and the fluorescence
spectra are shown in Figure 6a. Significant fluorescence
enhancement (Figure S7) was observed when the temperature
increased from 30 °C (below Tcp) to 48 °C (above Tcp). We
believe this enhancement is due to confinement environment
caused by thermally induced chain collapse and aggregation. At
low temperature, the BDY chromophores attached to the rigid
backbone through a single bond have the freedom to rotate,
which would weaken the fluorescence emission through
nonradiative pathway.30 However, when the temperature
increased to above Tcp, OEG dehydration and collapse,
followed by polymer chain aggregation, will provide a confined
environment around the BDY chromophores to greatly depress
the intramolecular rotation and, in turn, restore the
fluorescence intensity. In contrast, the copolymer PMA-BDY
with flexible polymethacrylate backbone showed negligible
fluorescence enhancement during the thermally induced
aggregation processes (temperature from 30 to 48 °C, Figure
6b). UV absorption spectra probably could give some clues for
this phenomenon. The aromatic copolymer PPA-BDY and
monomer PA-BDY displayed the same absorptions with
maximum centered at 500 nm, while aliphatic copolymer
PMA-BDY exhibited absorption with maximum centered at 511
Fluorescence Properties. BDY has been proved to be an
efficient fluorescent dye which tends to emit relatively sharp
fluorescence peaks with high quantum yields, and incorporated
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nm in DCM, 13 nm red-shifting to that of corresponding
aliphatic monomer (Figure S3). This red-shift indicates
enhancement of supramolecular interaction between OEG
moieties and BDY chromophores within the flexible copolymer.
The enhanced interaction might suppress the intramolecular
rotation of BDY chromophores even below the phase transition
temperature; thus, no obvious fluorescent enhancement was
detected during the thermally induced aggregation process.
Overall, comparison of fluorescence properties between PPA-
BDY and PMA-BDY leads to the conjecture that the
supramolecular interactions between OEG moieties and BDY
chromophores should be enhanced for the flexible polymers.31
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CONCLUSIONS
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A novel category of poly(phenylacetylene) homo- and
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*Ph +86-21-66138053; Fax +86-21-66131720; e-mail
gckuang@shu.edu.cn (G.K.).
*Ph +86-21-66138053; Fax +86-21-66131720; e-mail azhang@
shu.edu.cn (A.Z.).
Notes
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ACKNOWLEDGMENTS
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We thank Dr. Hongmei Deng from the Instrumental Analysis
Division of Research Center (Shanghai University) for her
assistance in NMR measurements. This work is financially
supported by National Natural Science Foundation of China
(Nos. 21034004, 21204047, and 21374058) and the Ph.D.
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