T-shaped and H-shaped polymers constructed from UV-induced strain promoted azide-alkyne cycloaddition reaction

Article abstract of DOI:10.1016/j.polymer.2016.01.027

Topological polymers with T-shaped and H-shaped molecular architecture were built on the combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted azide-alkyne cycloaddition (SPAAC) reaction. In the presence of a cycloprope

Full text of DOI:10.1016/j.polymer.2016.01.027

Accepted Manuscript
T-shaped and H-shaped polymers constructed from UV-induced strain promoted
azide-alkyne cycloaddition reaction
Wen Zhu, Zi Li, Peng Sun, Lixia Ren, Ke Zhang
PII:
S0032-3861(16)30027-1
10.1016/j.polymer.2016.01.027
JPOL 18379
DOI:
Reference:
To appear in: Polymer
Received Date: 17 November 2015
Revised Date: 31 December 2015
Accepted Date: 12 January 2016
Please cite this article as: Zhu W, Li Z, Sun P, Ren L, Zhang K, T-shaped and H-shaped polymers
constructed from UV-induced strain promoted azide-alkyne cycloaddition reaction, Polymer (2016), doi:
10.1016/j.polymer.2016.01.027.
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Graphical Abstract
ACCEPTED MANUSCRIPT
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T-shaped and H-shaped polymers constructed from UV-induced strain promoted
azide-alkyne cycloaddition reaction
Wen Zhu a, Zi Li a, Peng Sun a, Lixia Ren b, Ke Zhang a,*
a Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of
Sciences, Beijing 100190, China
b School of Materials Science and Engineering, Tianjin University, No. 92 Weijin Road, Tianjin 300072,
China
*Corresponding author: K. Zhang (E-mail: kzhang@iccas.ac.cn)
Abstract
Topological polymers with T-shaped and H-shaped molecular architecture were built on the
combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted
azide-alkyne cycloaddition (SPAAC) reaction. In the presence of a cyclopropenone-masked
dibenzocyclooctyne functionalized dibromo ATRP initiator, reactive polystyrene (PS) was synthesized
to have a cyclopropenone-masked dibenzocyclooctyne group in the middle of polymer chain. After
releasing dibenzocyclooctyne from deprotecting cyclopropenone-masked dibenzocyclooctyne under UV
irradiation, T-shaped and H-shaped topological polymers were constructed by reacting PS with
mono-end and di-end azide functionalized poly(ethylene oxide) (PEO), respectively, based on the
SPAAC reaction.
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Keywords: Atom transfer radical polymerization; H-shaped polymer; T-shaped polymer; Topological
polymer; UV-induced strain promoted azide-alkyne cycloaddition
1. Introduction
Polymer topology plays an important role in determining the properties and functionalities of polymer
materials.1 The synthesis of topological polymers has been always a core direction in the field of
polymer chemistry. Thanks to the development of modern living/quasiliving polymerization2 techniques
(LP) and the wide application of click chemistry in polymer science, a variety of polymer topology has
been fabricated based on their combination including starlike,3 brushlike,4 grafted,5 hyperbranched,6
cyclic macromolecular7 architectures and polymer networks.8 In this strategy, LP was used to prepare
varied polymer building blocks functionalized with clickable groups. The click reactions were then used
to couple the related building blocks to form the topological polymers. As the simplest branched
polymers, synthesis of T-shaped (3-arm star) and H-shaped polymers could be viewed as the golden
candidate to evaluate the combination of LP and click chemistry for fabricating polymer topology.
To date, a series of LP and click chemistry combined methods have been developed for the formation
of T-shaped and H-shaped polymers. Based on the LP, a series of click reactions have been used to
fabricate the T-shaped and H-shaped polymers, including copper-catalyzed azide-alkyne cycloaddition
(CuAAC),3,9-13 Diels-Alder reaction,14,15 and thiol-ene reaction.16,17 In these techniques, CuAAC was
one of most important click reactions due to its high efficiency and small sized reaction groups. In
addition, the azide group could be easily introduced at the end of polymers based on atom transfer
radical polymerization (ATRP) and substitution of bromide end group by azide. To achieve high
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reaction efficiency, however, a large amount of copper catalysts was required by CuAAC. This may
cause the contamination of the resultant topological polymers and restrict their application in the field of
advanced technologies such as biology and photoelectronics.
Recently, the strain promoted azide-alkyne cycloaddition (SPAAC) reaction was developed as a
bio-orthogonal reaction to circumvent the awkward situation of CuAAC in biology.18 By confining an
alkyne in a small ring molecule, the strain activated alkyne group could efficiently react with azide
group at room temperature requiring no any catalyst. Since its birth, SPAAC has been widely used in the
formation of bio-conjugates in vitro and in vivo.19 In addition, SPAAC has also been applied to prepare
polymeric materials including polymer surface brush20 and networks.21 Considering the importance of
SPAAC click reaction, we synthesized a dibenzocyclooctyne functionalized ATRP initiator and
systematically studied its ATRP behavior.22 For polymerizing styrenic and methacrylate monomers, the
polymerization held living characteristics. For polymerizing acrylate monomers, however, the
polymerization lost living behavior. Based on the combination of ATRP and SPAAC, we successfully
prepared the block, star and brush topological polymers.22,23 Furthermore, by designing the ATRP
initiator containing a cyclopropenone-masked dibenzocyclooctyne, the well-defined polystyrenics,
polymethacrylates and polyacrylates could be produced by a standard ATRP protocol and the
polymerizations all behaved living characteristics.24 Under UV irradiation, cyclopropenone-masked
dibenzocyclooctyne quantitatively released the reactive dibenzocyclooctyne group, which in-situ reacted
with azide by the SPAAC mechanism. By the combinations of ATRP/UV-induced SPAAC25 and
macromolecular design via the interchange of xanthate (MADIX)/UV-induced SPAAC,26 the cyclic
polymer topology has been successfully prepared.
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As a continuous contribution in this research topic herein, T-shaped and H-shaped polymers were
prepared by the combination of ATRP and UV-induced SPAAC click reaction. As shown in Scheme 1,
by a difunctional ATRP initiator, the well-defined linear PS building blocks were synthesized from
ATRP to have a cyclopropenone-masked dibenzocyclooctyne in the middle of the polymer chain. After
quantitatively releasing dibenzocyclooctyne group under UV irradiation, T-shaped and H-shaped hybrid
copolymers were prepared by reacting linear PS building blocks with mono- and di-end azide
functionalized poly(ethylene oxide) (PEO), respectively.
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O
O
O
O
O
O
Br
Br
O
O
Br
Br
O
O
17
17
Initiator
PS
O
N3
(ii)
N
(ii)
N3
(i) UV
(i) UV
3
O
O
23
23
Br
Br
O
O
O
O
O
O
Br
Br
17
17
O
O
O
N
N
N
N
23
N
N
N
N
O
N
O
23
O
O
O
O
Br
Br
T-shaped polymer
H-shaped polymer
Scheme 1. Synthetic route of T-shaped and H-shaped polymers.
2. Experimental section
2.1. Materials
Styrene (St, 99%, Aldrich) was dried over CaH2 and distilled under reduced pressure. Copper (I)
bromide (CuBr, 98%, J&K) was washed by acetic acid and ethanol. Tetrahydrofuran (THF, AR, Beijing
Chemical Reagent Co.) was distilled from sodium/benzophenone. Cyclopropenone-masked
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dibenzocyclooctyne functionalized ATRP initiator,24 mono- and di-end azide functionalized PEO22
(PEO23-N3
and
N3-PEO23-N3)
were
synthesized
according
to
the
literatures.
Pentamethyldiethylenetriamine (PMDETA, 99%, Energy Chemical), anisole (AR, Beijing Chemical
Reagent Co.) and other reagents were purchased from commercial retailers and used as received. A low
pressure mercury lamp (120 W) (CEL-LPH120-254, Beijing China Education Au-light co.Ltd) was used
as the UV light source.
2.2. Measurements
1H NMR spectra were obtained on a Bruker DMX400 spectrometer. CDCl3 was used as solvent and
TMS as internal standard. Apparent molecular weights (Mn,GPC) and molecular weight distributions
(Mw/Mn) of the polymers were determined by gel permeation chromatography (GPC) equipped with a
Waters 1515 isocratic HPLC pump, a Waters 2414 refractive index detector, and a combination of
column Styragel HT-2, HT-3, HT-4 and HT-5. Linear polystyrene standards were applied for calibration.
The eluent was THF at a flow rate of 1 mL/min at 35 °C. The preparative GPC (SHIMADZU LC-20AR)
equipped with a LC-20AR pump, a RID-20A refractive index detector, and an Agilent Plgel Mixed-D
column. The eluent was THF at a flow rate of 10 mL/min at room temperature. UV spectra were
recorded using a TU-1901 UV-Visible Spectrophotometer. IR spectra were recorded using a Nicolet
AVATAR 330 FT-IR Spectrophotometer.
2.3. Synthesis of linear PS
St (1.56 g, 15 mmol), ATRP initiator (28 mg, 0.05 mmol), PMDETA (9 mg, 0.05 mmol) and anisole
(1.5 g as a solvent) were mixed into a 25 mL Schlenk tube. After degassing through three
freeze-pump-thaw cycles, CuBr (7.2 mg, 0.05 mmol) was added to the mixture in frozen state under N2.
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After another three freeze-pump-thaw cycles, the reaction was stirred at 80 °C for 40 min, and then
quenched by exposing to air. The mixture was diluted with 150 mL CH2Cl2 and passed through a short
basic alumina column to remove the catalyst. The solution was concentrated and precipitated three times
in methanol. The polymer was dried under vacuum.
2.4. Synthesis of T-shaped polymer
Linear PS (40 mg, 0.01 mmol) in 100 mL THF was stirred under UV irradiation for 3 h at room
temperature. The solution was then concentrated to 2 mL. After adding PEO23-N3 (20 mg, 0.02 mmol) in
situ, the mixture was stirred at room temperature in air for another 12 h. The solution was precipitated in
methanol to remove excess PEO23-N3. The polymer was dried under vacuum.
2.5. Synthesis of H-shaped polymer
Linear PS (80 mg, 0.02 mmol) in 200 mL THF was stirred under UV irradiation for 3 h at room
temperature. The solution was then concentrated to 2 mL. After adding N3-PEO23-N3 (5 mg, 0.005 mmol)
in situ, the mixture was stirred at room temperature in air for another 36 h. The product (85 mg, 42.5
mg/mL in THF) was purified directly by GPC fractionation to remove excess PS. The polymer was
dried under vacuum.
3. Results and discussion
3.1. Synthesis and characterization of linear PS
Cyclopropenone-masked dibenzocyclooctyne functionalized PS was synthesized via ATRP, using the
functional ATRP initiator. Fig. S1 (Supporting Information) shows the synthetic route of
cyclopropenone-masked dibenzocyclooctyne functionalized ATRP initiator, and the experimental details
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1
were previously reported.24 The H NMR spectrum and the related peak assignments are shown in Fig.
S2, which clearly indicated the successful preparation of the initiator. Then it was applied to synthesize
PS via ATRP. Using a polymerization condition of [St]0:[initiator]0:[CuBr]0:[PMDETA]0 = 300:1:1:1
and [St]0 = 10 mol/L in anisole, the linear PS was obtained after 40 min polymerization at 80 °C. Fig. S3
1
shows the H NMR spectrum of the raw polymerization solution without purification, in which the
signals of Ha1 and (Ha2 + Hb2) were assigned to the protons of St monomer (2H) and the resultant PS (3H)
respectively. By virtue of the ratio of Area(Ha2+Hb2)/( ꢀꢁAreaHa1+ Area(Ha2+Hb2)), the St conversion was
calculated as 11.2%. This produced a degree of polymerization (DP) of 34 for the resultant
1
cyclopropenone-masked dibenzocyclooctyne functionalized PS. The H NMR spectrum of purified
linear PS is shown in Fig. 1 including the peak assignments. The characteristic signals of Ha, Hd and Hd’
were preserved in PS from the ATRP initiator. The signals of Hb and Hc of the initiator were overlapped
with the H proton signals of phenyl groups in PS. Peak Hk (2H) at 4.41 ppm represented the H of
terminal St unit close to Br. A peak area ratio of 1 was obtained between Ha (2H) and Hd (2H), which
indicated that the cyclopropenone-masked dibenzocyclooctyne group was successfully introduced into
each PS chain. In addition, the peak area ratio between Hh (10nH) and Hk (2H) was 87, and DP was
further calculated as 34, which was consistent with that calculated by monomer conversion. Thus the
Mn,NMR of PS was 4100.
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Fig. 1. 1H NMR spectrum of cyclopropenone-masked dibenzocyclooctyne functionalized PS in CDCl3.
Fig. 2A (black) shows the GPC curve of linear PS, in which a well-defined monomodal and
symmetric peak was observed. The Mn,GPC and Mw/Mn were integrated as 4880 and 1.04, respectively. It
is reported that cyclopropenone-masked dibenzocyclooctyne ATRP initiator possessed a characteristic
peak at 314 nm in the UV-Vis spectrum.24 As shown in Fig. 3A (black), linear PS inherited the peak of
314 nm from the ATRP initiator. These results clearly indicated the successful preparation of linear PS
with the cyclopropenone-masked dibenzocyclooctyne group in the middle of polymer chain.
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PS
PS
N3-PEO23-N3
PEO23-N3
A
B
H-shaped polymer (crude)
H-shaped polymer (purified)
T-shaped polymer (crude)
T-shaped polymer (purified)
32
33
34
35
36
37
30
31
32
33
34
35
36
37
Elution time (min)
Elution time (min)
Fig. 2. (A) GPC curves of PS (black), PEO23-N3 (red), crude (blue) and purified (olive) T-shaped
polymers. (B) GPC curves of PS (black), N3-PEO23-N3 (red), crude (blue) and purified (olive) H-shaped
polymers. THF was used as the eluent and polystyrene standards were used for the calibration.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PS
PEO23-N3
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PS
A
N3-PEO23-N3
B
T-shaped polymer (crude)
T-shaped polymer (purified)
H-shaped polymer (crude)
H-shaped polymer (purified)
260 280
300
320
340 360
380
260 280 300 320 340 360 380
Wavelength (nm)
Wavelength (nm)
Fig. 3. (A) UV-Vis spectra of PS (black), PEO23-N3 (red), crude (blue) and purified (olive) T-shaped
polymers in THF. (B) UV-Vis spectra of PS (black), N3-PEO23-N3 (red), crude (blue) and purified (olive)
H-shaped polymers in THF.
3.2. Synthesis and characterization of T-shaped polymer
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The cyclopropenone-masked dibenzocyclooctyne functionalized PS and PEO23-N3 were selected to
prepare T-shaped polymers via the UV-induced SPAAC. PEO23-N3 was synthesized according to the
route of Fig. S4, and the details were previously reported.22 1H NMR spectrum of PEO23-N3 is shown in
Fig. S5, and the peak area of Ha (94H) and Hb (2H) was 47.25 (theoretical value is 47), indicating the
hydroxyl end group has completely transformed to azide group. The Mn,NMR of PEO23-N3 was 1100. Fig.
4A (red) shows the IR spectrum of PEO23-N3, in which the peak at 2100 cm-1 was ascribed to azide
group. Fig. 2A (red) shows the GPC curve of PEO23-N3, and the Mn,GPC and Mw/Mn were integrated as
2140 and 1.03, respectively.
A low pressure mercury lamp was used as the UV light source to deprotect the
cyclopropenone-masked dibenzocyclooctyne group. A THF dilute solution (1
×
10-4 mol/L) of PS was
irradiated by UV light for 3 h. Then the solution was concentrated to 2 mL and 2 equivalent of PEO23-N3
was added in situ. After 12 h reaction at room temperature in air, T-shaped PS-PEO copolymer was
conveniently obtained. The crude product was characterized by GPC, UV-Vis and IR. As shown in Fig.
2A (blue), the GPC curve of the newly formed polymer moved to a higher molecular weight direction,
but remained the mono-modal and symmetric peak shape compared to that of PS (black). Reasonably,
there was a surplus of excess PEO23-N3 in the GPC curve. This clearly indicated the successful
preparation of the T-shaped polymer via the UV-induced SPAAC reaction. In the UV-Vis spectrum of
Fig. 3A (blue), the absorption peak at 314 nm was disappeared completely, indicating the complete
consumption of the cyclopropenone-masked dibenzocyclooctyne groups in linear PS.
The crude product was purified by precipitating in methanol to remove excess PEO23-N3. GPC curve
of the purified T-shaped polymer shows no residual PEO23-N3, and the Mn,GPC and Mw/Mn were
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integrated as 5620 and 1.03, respectively (Fig. 2A, olive). The IR spectrum of the purified T-shaped
polymer (Fig. 4A, olive) shows no vibration peak at 2100 cm-1 (characteristic peak of azide group),
again confirmed the excess PEO23-N3 was completely removed. The UV-Vis curve of the purified
1
T-shaped polymer (Fig. 3A, olive) shows a similar pattern as the crude product. Fig. 5A shows the H
NMR spectrum of the T-shaped polymer. The peak area ratio of Hb (170H of PS) and Hc (2H) was
approximately 85, which was close to that of Hh and Hk (AreaHh/AreaHk = 87) in Fig. 1. It indicated the
linear PS was completely consumed for the construction of T-shaped polymers. It is notable that the
peak area of Ha (82.56) was slightly smaller than the theoretical value (94), indicating a shorter PEO
chain connected onto linear PS. It is becuase the shorter PEO chians had relatively higher reacivity than
the longer ones, due to the smaller steric effects.
PS
PS
A
B
PEO23-N3
N3-PEO23-N3
T-shaped polymer (crude)
H-shaped polymer (crude)
T-shaped polymer (purified)
H-shaped polymer (purified)
3500
3000
2500
2000
1500
1000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 4. (A) IR spectra of PS (black), PEO23-N3 (red), crude (blue) and purified (olive) T-shaped
polymers. (B) IR spectra of PS (black), N3-PEO23-N3 (red), crude (blue) and purified (olive) H-shaped
polymers.
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Fig. 5. 1H NMR spectra of (A) T-shaped and (B) H-shaped polymers in CDCl3.
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3.3. Synthesis and characterization of H-shaped polymer
H-shaped polymer was composed of two molecules of linear PS linked by a PEO chain via the
UV-induced SPAAC. The PEO chain, N3-PEO23-N3, has azide groups on both ends, which was
1
synthesized according to the route of Fig. S4. Fig. S6 shows the H NMR spectrum of N3-PEO23-N3.
According to the peak area ratio of Ha (96H) and Hb (4H), the hydroxy groups on both ends of PEO had
completely transformed to azide groups. The Mn,NMR of N3-PEO23-N3 was 1170. Fig. 4B (red) shows the
IR spectrum of N3-PEO23-N3, and the characteristic peak at 2100 cm-1 again confirmed the successful
introduction of azide groups. Fig. 2B (red) shows the GPC curve of N3-PEO23-N3, and the Mn,GPC and
Mw/Mn were integrated as 2100 and 1.03, respectively.
The UV irradiation condition was the same as above to deprotect the cyclopropenone-masked
dibenzocyclooctyne group. To ensure the product was H-shaped polymer, 0.5 equivalent (azide/
dibenzocyclooctyne = 1/2) of N3-PEO23-N3 was added to the dibenzocyclooctyne functionalized PS
solution. After 36 h reaction at room temperature in air, H-shaped PS-PEO copolymer was obtained. The
crude product was characterized by GPC, UV-Vis and IR. The GPC curve (Fig. 2B, blue) of the crude
product of H-shaped polymer shows no peak of N3-PEO23-N3, but a peak of residual PS. The newly
generated peak was moved to a higher molecular weight direction, but remained the mono-modal and
symmetric peak shape compared to that of PS (black). This clearly indicated the successful preparation
of the H-shaped polymer via the UV-induced SPAAC reaction. The IR spectrum of the crude product
(Fig. 4B, blue) shows no characteristic peak of azide group at 2100 cm-1, indicating N3-PEO23-N3 was
completely consumed. In the UV-Vis spectrum of Fig. 3B (blue), a small peak at 314 nm was observed,
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indicating a residual of dibenzocyclooctyne functionalized PS.
The crude product was purified by GPC fractionation to remove excess PS. GPC curve of the purified
H-shaped polymer was shown in Fig. 2B (olive), in which a trace amount of unreacted PS could not be
removed completely although it slightly affected the related analysis. The Mn,GPC and Mw/Mn were
integrated as 9600 and 1.02, respectively. The UV-Vis curve of the purified H-shaped polymer was
shown in Fig. 3B (olive), in which the dibenzocyclooctyne absorption peak was hardly observed at 314
1
nm. Fig. 5B shows the H NMR spectrum of the H-shaped polymer. The peak area ratio of Ha (96H of
PEO) and Hc (4H) was 23.7, which was close to the theoretical value of 24. However, the peak area
(241.28) of Hb was significantly less than its theoretical value (340), indicating shorter PS chains
connected in the H-shaped polymers. It is becuase the shorter PS chians had relatively higher reacivity
than the longer ones, due to the smaller steric effects. As shown in Fig. 2B (blue curve), the residual PS
possessed a shorter elution time than the reactant PS (black curve), which indicated that the residual PS
did have a larger Mn,GPC and the shorter PS chains were consumed preferentially.
4. Conclusions
UV-induced SPAAC reaction has been demonstrated as a powerful tool for the construction of
topological polymers, since the reaction between cyclopropenone-masked dibenzocyclooctyne and azide
could be carried out under mild conditions without any catalyst but UV irradiation. Using a
cyclopropenone-masked dibenzocyclooctyne functionalized dibromo ATRP initiator, linear PS was
synthesized with a cyclopropenone-masked dibenzocyclooctyne in the middle of polymer chain.
Well-defined T-shaped and H-shaped polymers were then produced by coupling PS with mono- and
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di-end azide functionalized linear PEO, respectively. Due to its clean and efficient reaction
characteristics, UV-induced SPAAC reaction is a golden candidate for preparing varied topological
polymers required in photoelectronic materials, biomedical materials, and many other related
applications.
Acknowledgments
Generous supports were primarily provided by Ministry of Science and Technology of China
(2014CB932200), National Science Foundation of China (21374122 and 51203171), Beijing National
Laboratory for Molecular Sciences, and Bairen project from The Chinese Academy of Sciences.
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T-shaped and H-shaped polymers constructed from UV-induced strain promoted
azide-alkyne cycloaddition reaction
Wen Zhu, Zi Li, Peng Sun, Lixia Ren, Ke Zhang*
Highlights of POLYMER
ꢀ Construction of T-shaped and H-shaped topological polymers
ꢀ Catalyst free UV-induced strain promoted azide-alkyne cycloaddition reaction
ꢀ Reactive polymer with a cyclopropenone-masked dibenzocyclooctyne group in the middle of
polymer chain.