Photocleavable amphiphilic diblock copolymer with an azobenzene linkage

Article abstract of DOI:10.1039/c6ra12129b

Amphiphilic diblock copolymers with an azobenzene linkage were synthesized by a macromolecular azo coupling reaction between a PEG based macromolecular diazonium salt and a PS based polymeric block with a functional terminus. Colloidal aggregates could be

Full text of DOI:10.1039/c6ra12129b

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Photocleavable Amphiphilic Diblock Copolymer with an
Azobenzene Linkage
Received 00th January 20xx,
Accepted 00th January 20xx
Jilei Wang, Yiyun Ouyang, Shang Li, Xiaogong Wang and Yaning He*
Amphiphilic diblock copolymer with an azobenzene linkage was synthesized by a macromolecular azo coupling reaction
between the PEG based macromolecular diazonium salt and PS based polymeric block with functional terminal. Colloidal
aggregates could be directly prepared by selfꢀassembly of the prepared amphiphilic copolymer in selective solvents
(THF/H2O). By irradiation with UV light, the azobenzene linkage in the block copolymer can be efficiently photocleaved.
Thus, the colloidal aggregates of the amphiphilic diblock copolymer will be disrupted due to the cleavage of the azobenzene
junction.
DOI: 10.1039/x0xx00000x
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1. Introduction
the copolymer any more. Selfꢀassembly of this azobenzene linked
copolymer can result uniform colloidal aggregates. By irradiation
with ultraviolet light, the cleavage of the azobenzene junction in the
copolymer will disrupt the assembly aggregates (Scheme 1).
Block copolymers composed of two or more homopolymer chains
linked by covalent bond have been attracted much attention.1ꢀ6 Due
to their unique architecture and properties, block copolymers as one
of the most important classes of synthetic materials were often used
to create nanostructures by selfꢀassembly in bulk or in solution,
which can be used as nanoscale templating, nanoscale separations or
drug delivery systems and so on.7ꢀ12 Recently, researches have been
focused on the smart block copolymers responding to external
stimuli, such as temperature, pH, enzyme, gas, light and many others.
13ꢀ23
Particularly, photocleavable polymers have received much
attention in recent years.24ꢀ27 Light can accurately approach to
materials in nonꢀcontacting manners. In addition, various types of
parameters including light intensity, wavelength, polarization etc can
be simply tuned. Such photocleavable polymers can be exactly
degraded by photo irradiation, which can be potential used in drug
delivery, biomaterials, photodynamic therapeutics and many others.
Compared with side chain type photocleavable polymers, the main
chain type photocleavable polymers will generate fewer small
molecular byꢀproducts. Up to date, many efforts have been made to
synthesize such block polymers containing the cleavable covalent
junction.28ꢀ33 For example, the block polymers can be synthesized by
introduce the terminal segment bearing the cleavable covalent bond
to one polymer block then followed with various living/controlled
polymerization or click reaction with another polymer block. Very
recently, we reported a modular strategy for the efficient synthesis of
amphiphilic diblock copolymers by macromolecular azo coupling
reaction between two polymer blocks.34ꢀ37 It has been reported that
azobenzene groups can be photodegraded just by ultraviolet light
irradiation.38 Thus, in our strategy, an azobenzene linkage will
naturally generate after the macromolecular azo coupling reaction. It
does not need to purposely introduce the cleavable covalent bond to
Scheme 1. Schematic representation of selfꢀassembly aggregates formation
from azobenzene linked diblock copolymer, subsequent step of disruption of
these aggregates due to the degradation of the azobenzene linkage.
2. Experimental Section
2.1 Materials and characterization
2ꢀ(NꢀEthylanilino)ethanol ( > 97 %) and Poly(ethylene glycol)
methyl ether (Mn = 5000, 1900) were received from TCI and
Aldrich respectively. pꢀToluenesulfonyl chloride (99 %, TsCl), 2,2'ꢀ
Dipyridyl (99 %) and Cuprous Bromide were purchased from J&K
Chemical, especially the cuprous bromide were successively purified
with glacial acetic acid, ethyl alcohol absolute and anhydrous diethyl
ether. Styrene (Acros, 99 %) was distillated to remove the radical
inhibitor before utilization. Ultrapure water (resistivity > 18.0
Department of Chemical Engineering, Key Laboratory of Advanced Materials
(MOE) Tsinghua University, Beijing, 100084, P. R. China
*
Correspondence author. Eꢀmail: heyaning@mail.tsinghua.edu.cn
Electronic Supplementary Information (ESI) available: [Experimental details,
UV/Vis absorption spectrum, 1H NMR, GPC.]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Mꢁ.cm) was supplied by a MilliꢀQ water purification system and The amphiphilic diblock copolymer with an azobenzene linkage was
used for all experiments. All other solvents and reagents were synthesized by the macromolecular azo coupling reaction between
purchased commercially and used as received. JEOL JNMꢀECA 600 PEG based macromolecular diazonium salt and PS based polymeric
spectrometer was used to measure the 1HNMR spectra. A gel block with functional terminal. PSꢀNꢀPh (130 mg, 0.01 mmol) was
o
permeation chromatography (GPC) apparatus using THF as eluent at dissolved in 50 mL DMF at 0 C. The diazonium salt of PEGꢀNH2
a flow rate of 1.0 mL/min at 25 °C was used to measure the number was prepared by adding an aqueous solution of sodium nitrite (3 mg
average molecular weights (Mn) and molecular weight distributions in 0.3 mL of water) into a mixture of PEGꢀNH2 (102 mg, 0.02
o
(PDI), which was calibrated by using linear polystyrene standards. mmol), HCl (36%, 0.03 mL) and H2O (0.4mL) at 0 C. The mixture
Photodegradation process was carried out using a 1kW high pressure was stirred at 15 min at that temperature and then was added
mercury lamp as
a light source. An Agilent 8453 UVꢀVis dropwise into above DMF solution. Then the reaction was kept at 0
spectrophotometer was used to measure the UVꢀVis spectra. Hitachi oC for 72 h. Then the solution was precipitated with plenty of water.
Hꢀ7650B microscope with an accelerating voltage of 80 KV was The precipitate was collected by filtration and dried in a vacuum
used to perform the transmission electron microscopy (TEM) oven for 72 h. Mn (GPC) = 18000, and PDI = 1.22.
observation.
2.5 Synthesis of diblock copolymer (PEG-b-PS) without
2.2 Synthesis of tosylate ended poly (ethylene glycol) (PEG-Ts)
azobenzene linkage
Poly(ethylene glycol) monomethyl ether (PEG) (Mn = 5000, PDI =
1.10) (15.8 g, 3.16 mmol) and 3 mL triethylamine were dissolved in
50 ml CH2Cl2. This solution was cooled in an ice bath. Then 3 g of
4ꢀtoluenesulfonylchloride (15.7 mmol) was added. The reaction
mixture was kept at 0 °C for 2 h and stirred at room temperature for
12 h. The product was then obtained by precipitated in cold diethyl
ether and recrystallized from ethanol twice. PEGꢀTs: Mn (GPC) =
6300, PDI = 1.09.
Macromolecular ATRP initiator PEGꢀBr was prepared by the
esterification reaction between poly(ethylene glycol) methyl ether
(Mn = 1900) and 2ꢀbromoisobutyryl bromide in dry CH2Cl2.39 CuBr
(28.7 mg, 0.2 mmol) and 2,2ꢀdipyridyl (62.5 mg, 0.4 mmol) were
added to a Schlenk flask. Then it was degassed and backꢀfilled with
argon three times. Following this step, deoxygenated PEGꢀBr (206.5
mg, 0.1 mmol) and styrene (2.5 ml, 21.8 mmol) were added via gasꢀ
tight syringes which had been previously purged with argon. After
degassing by three freeze–pump–thaw cycles, the flask was
immersed in an oil bath preheated to 110 oC. After the
polymerization for 3.0 h, the reaction mixture was diluted with THF
and passed through an alumina column to remove catalyst. The
filtrate was concentrated and poured into an excess amount of
methanol. The precipitate was collected by filtration, washed with
methanol and then dried in a vacuum oven for 24 h. Mn (GPC) =
11600, and PDI = 1.14.
2.3 Synthesis of PEG-NH2
PEGꢀTs (15.3 g, 3 mmol) and pꢀaminobenzoic acid (1.7 g, 12 mmol)
were dissolved in 50 mL DMF. Potassium carbonate (2.1 g) was
added into above DMF solution. The reaction mixture was kept at
60 °C for 24 h with stirring. Most DMF was removed by vacuum
evaporation. Then 60 mL ice water was added to above mixture. The
solution was extracted with CH2Cl2 and washed with ice water. After
dried with anhydrous MgSO4 and vacuumed to remove most of the
solvent, the mixture was poured into an excessive amount of cold
diethyl ether. The final product was collected by filtration. Yield:
70%. Mn (GPC) = 6300, and PDI = 1.08.
2.6 Self-assembly of PEG-Azo-PS
PEGꢀAzoꢀPS (4.0 mg) was dissolved in anhydrous THF (1.0 mL),
which is a good solvent for both PEG and PS blocks. To prepare the
selfꢀassembly aggregates, the THF solution was added dropwise into
ultrapure water (4.0 mL), which is a good solvent for the PEG block
but not for the PS block. The polymer solutions were gently stirred
during the addition. The final solution was then dialyzed against
deionized water for 48 hours to remove THF.
2.4 Synthesis of PS with anilino functionality in the end of the
polymer chain (PS-N-Ph)
CuBr (35.9 mg, 0.25 mmol) and 2,2ꢀdipyridyl (78.1 mg, 0.5 mmol)
were added to a Schlenk flask. Then it was degassed and backꢀfilled
with argon three times. Following this step, deoxygenated ATRP
initiator (41 mg, 0.13 mmol) and styrene 5 mL were added via gasꢀ
tight syringes which had been previously purged with argon. After
degassing by three freeze–pump–thaw cycles, the flask was
immersed in an oil bath preheated to 110 oC. After the
polymerization for 4 h, the reaction mixture was diluted with THF
and passed through an alumina column to remove catalyst. The
filtrate was concentrated and poured into an excess amount of
methanol. The precipitate was collected by filtration, washed with
methanol and then dried in a vacuum oven for 24 h. Mn (GPC) =
13000, and PDI = 1.20.
2.7 Photodegradation of PEG-Azo-PS
Suitable amounts of PEGꢀAzoꢀPS samples were dissolved in THF to
yield homogeneous solutions with the concentration at 3.0×10ꢀ5
M
for UV analysis. Then the solutions were irradiated with high
pressure mercury lamp (1kW) every 150 seconds until the absorption
no longer changes. The colloidal aggregates of PEGꢀAzoꢀPS in
ultrapure water were irradiated with the high pressure mercury lamp
for about 20 min. The white solid precipitation was then isolated by
centrifugation and the corresponding supernatant was evaporated
and vacuum dried for further GPC and 1H NMR analyses.
3. Results and discussion
2.4 Synthesis of azobenzene linked diblock copolymer (PEG-Azo-
PS)
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The photo cleavage of the azobenzene linked amphiphilic
diblock copolymer was studied by using UVꢀVis spectra.
Figure 1 gives the UVꢀVis spectra of PEGꢀAzoꢀPS THF
solution upon irradiation with of UV light (1 kW high
pressure mercury lamp). Before irradiation of UV light, the
curve shows typical absorption behavior of the pseudoꢀ
stilbene type of azo chromophores, the maximum
absorbance of PEGꢀAzoꢀPS in THF appears at 427 nm,
which is related to the πꢀπ* transition of the trans isomers of
the azobenzene units. After irradiation with UV light, this
peak at 427 nm gradually decreased. The relationship
between the maximum absorbance of the azobenzene linked
diblock copolymer and the irradiation time is showed in
Figure 1(b). It can be found that the λmax decreased with the
irradiation time. After about 15 min irradiation with UV
light, the absorbance at 427 nm almost decreased to zero,
which indicated the photo cleavage of the azobenzene
linkage. From the photo image of the PEGꢀAzoꢀPS THF
solution, it can be found that the color of the solution has
changed from orange to colorless after the UV light
irradiation. Anyway, it should be pointed out that the wellꢀ
known photochromic azobenzene group will show obviously
photoisomerization behavior instead of photodegradation
upon irradiation with an appropriate wavelength matched the
absorption band, which has been shown in Figure S7.
Scheme 2 Synthetic route of amphiphilic diblock copolymer
PEGꢀAzoꢀPS through macromolecular azo coupling reaction.
The azobenzene linked amphiphilic diblock copolymer (PEGꢀ
AzoꢀPS) was synthesized in a manner similar to that reported
previously (Scheme 2).37 Anilineꢀfunctionalized PEG was
prepared by substitution reaction between the tosylate ended
PEG and
pꢀaminobenzoic acid. The hydrophilic block with
diazonium salt group was obtained by the diazotization of
above anilineꢀfunctionalized PEG. The hydrophobic block with
terminal suitable for azoꢀcoupling reaction was obtained by
ATRP method. Then the diblock copolymer was prepared
through macromolecular azo coupling reactions between the
above mentioned two polymeric blocks.
Figure 2. Typical TEM images of the selfꢀassembly aggregates of the
diblock copolymer PEGꢀAzoꢀPS before and after the UV light
irradiation.
Figure 3. 1H NMR spectra (600 MHz, CDCl3, 298 K) of PSꢀAzoꢀPEG, PEGꢀ
UV and PSꢀUV.
Figure 1. UV/Vis absorption spectra of amphiphilic diblock copolymer
(PEGꢀAzoꢀPS) in THF solution (3.0×10ꢀ5 M) upon irradiation with UV
light (a) and (b) shows the absorption intensity at 427 nm as a function
of irradiation time. The insert shows the photographs of polymer
solution before and after treating with UV light irradiation.
The selfꢀassembly aggregates of the PEGꢀAzoꢀPS were
prepared in selected solvents. 1000 µL PEGꢀAzoꢀPS THF
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solution (4.0 mg/mL) was added dropwise into 10 mL of
deionized water under stirring. The dispersions were then
dialyzed against deionized water for 48 hours to remove
THF before further measurements. TEM image of
aggregates of the azobenzene linked block copolymer is
showed in Figure 2a. It can be found that spherical
structures have been formed by hydrophobic aggregation of
the polymeric chains in a THFꢀH2O dispersion medium. The
size of the aggregates is about 60ꢀ80 nm. Treatment with the
UV light irradiation, the yellow polymer suspension turned
colorless and a white solid appeared (Inserts of Figure 2).
The aggregates could be photo triggered disruption, which
was confirmed by TEM (Figure 2b). The spherical selfꢀ
assembly aggregates could no longer be observed after the
UV light irradiation.
same UV irradiation. The only difference between the PEGꢀ
AzoꢀPS and PEGꢀ
found that the GPC trace of the PEGꢀ
b
ꢀPS is the azobenzene linkage. It can be
ꢀPS without azobenze
b
linkage is same before and after the UV irradiation (Figure 5).
The above GPC results also confirm the photo cleavage of the
azobenzene linkage.
The white solid precipitation was isolated by centrifugation
to give the PS part (PSꢀUV). After removal of the white
solid, the obtained aqueous solution was vacuum dried to
give the PEG part (PEGꢀUV). The resulted polymer
segments were examined with 1H NMR spectroscopy
(Figure 3). Compared with PSꢀAzoꢀPEG, the peaks in the
aromatic protons regions (7.20ꢀ6.86 ppm, 6.75ꢀ6.28 ppm)
and in high magnetic field (1.98ꢀ1.20 ppm) are totally
disappeared in PEG part. At the same time, the characteristic
peaks at 3.72ꢀ3.58 ppm are also disappeared in PS part,
illustrating the efficient degradation of the copolymer after
treating with UV light.
Figure 5. GPC traces of diblock copolymer PEGꢀbꢀPS without
azobenzene linkage before and after the UV light irradiation.
The photo controlled release experiments were carried on by
using pyrene as a model molecule (Figure S8). It can be seen
that the intense fluorescence was detected in the polymer
colloidal aggregates loading pyrene, while little fluorescence
was found in the absence of polymer aggregates since the
hydrophobic pyrene could hardly be dissolved in water
(Figure S8a). The fluorescence of the mixture decreased
with increasing UV irradiation time were shown in Figure
S8b. After UV irradiation for 24 min, most of the loaded
molecules could be released since the hydrophobic core of
the colloidal aggregates disappeared because of its photoꢀ
cleavage.
Conclusions
In conclusion, we have reported the photocleavage of
amphiphilic diblock copolymer with an azobenzene linkage,
which was synthesized by a macromolecular azo coupling
reaction. The azobenzene linkage will naturally generate after
the macromolecular azo coupling reaction. This kind of
amphiphilic diblock copolymer could selfꢀassemble into
colloidal sphere nanostructures. Treatment with UV light
irradiation, the selfꢀassembly aggregates of the amphiphilic
diblock copolymer will be disrupted due to the cleavage of the
azobenzene junction, which endowed the selfꢀassembly
Figure 4. GPC traces of aniline functionalized PEG (PEGꢀNH2),
terminalꢀfunctionalized PS (PSꢀNꢀPh), diblock copolymer PEGꢀAzoꢀPS
and the obtained PEGꢀUV and PSꢀUV after the UV light irradiation.
The GPC measurements were used to verify above photo
cleavable property of the azobenzene linkage in this diblock
copolymer. Figure 4 shows the traces of anilineꢀfunctionalized
PEG (PEGꢀNH2), functionalized PS block (PSꢀNꢀPh), the
coupled diblock copolymer (PEGꢀAzoꢀPS), photo cleaved PS
and PEG parts (PSꢀUV and PEGꢀUV). The calculated
molecular weights from GPC of PEGꢀNH2, PSꢀNꢀPh, PEGꢀ
AzoꢀPS, PSꢀUV and PEGꢀUV are 6300, 13000, 18000, 13000
and 6000, respectively. It can be seen that the photo cleaved
result polymers exhibit similar retention times as the initial
anilineꢀfunctionalized PEG (PEGꢀNH2) and functionalized PS
nanostructures with
molecules.
a controlled release of encapsulated
Acknowledgements
The authors gratefully acknowledge financial support from the
Natural Science Foundation of China (Grant No. 21474056).
block (PSꢀNꢀPh). For comparison, the PEGꢀbꢀPS prepared by
ATRP method was used to measure the GPC data after the
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Graphical Abstract
Photocleavable Amphiphilic Diblock Copolymer with an Azobenzene Linkage
Jilei Wang, Yiyun Ouyang, Shang Li, Xiaogong Wang and Yaning He*
Diblock copolymer with an azobenzene linkage synthesized by a macromolecular azo coupling
reaction can be efficiently photocleaved upon UV light irradiation. Thus, the colloidal aggregates of
this kind of amphiphilic diblock copolymer will be disrupted due to the cleavage of the azobenzene
junction.
1