Oxygen as a Crucial Comonomer in Alkyne-Based Polymerization toward Functional Poly(tetrasubstituted furan)s

Article abstract of DOI:10.1021/acs.macromol.8b01293

Oxygen (O₂), one of the most essential gases, is abundant, cheap, nontoxic, and environmentally friendly. However, O₂ has not been used as a monomer to synthesize polymers yet, probably because of the difficulty in chain growth. In this paper, we develop a new alkyne-based polymerization in which O₂ serves as a comonomer under mild reaction conditions. Functionalized diynes could react with O₂ in the presence of Pd(OAc)₂/ZnCl₂ in N,N-dimethylacetamide (DMAc) and perfluorodecalin under atmospheric oxygen. Soluble poly(tetrasubstituted furan)s with high weight-average molecular weights (M_w up to 14 900) were obtained in high yields (up to 86%). The ratio of isomeric repeating units of P1(m) can be identified by analyzing the components of model reaction products. Moreover, P1(6) could function as a fluorescent sensor for explosive detection with superamplification quenching effect. P2-P5 with conjugated structures possess large two-photon absorption cross sections up to 1570 GM, showing great potential as two-photon absorption materials. Thus, this work not only establishes a powerful O₂-involved alkyne-based polymerization but also provides poly(tetrasubstituted furan)s with versatile properties, which are potentially applicable in optoelectronic and biological fields.

Full text of DOI:10.1021/acs.macromol.8b01293

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Oxygen as a Crucial Comonomer in Alkyne-Based Polymerization
toward Functional Poly(tetrasubstituted furan)s
†
†
,†
,†,‡
Bo Song, Kun Hu, Anjun Qin,* and Ben Zhong Tang*
†State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, Guangzhou International
Campus, South China University of Technology, Guangzhou 510640, China
‡
Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and
Reconstruction, Institute for Advanced Study, and Department of Chemical and Biological Engineering, The Hong Kong University
of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
*
S Supporting Information
ABSTRACT: Oxygen (O ), one of the most essential gases, is abundant,
2
cheap, nontoxic, and environmentally friendly. However, O has not been
2
used as a monomer to synthesize polymers yet, probably because of the
difficulty in chain growth. In this paper, we develop a new alkyne-based
polymerization in which O serves as a comonomer under mild reaction
2
conditions. Functionalized diynes could react with O in the presence of
2
Pd(OAc) /ZnCl in N,N-dimethylacetamide (DMAc) and perfluorodecalin
2
2
under atmospheric oxygen. Soluble poly(tetrasubstituted furan)s with high
weight-average molecular weights (M up to 14 900) were obtained in high
w
yields (up to 86%). The ratio of isomeric repeating units of P1(m) can be
identified by analyzing the components of model reaction products. Moreover, P1(6) could function as a fluorescent sensor for
explosive detection with superamplification quenching effect. P2−P5 with conjugated structures possess large two-photon
absorption cross sections up to 1570 GM, showing great potential as two-photon absorption materials. Thus, this work not only
establishes a powerful O -involved alkyne-based polymerization but also provides poly(tetrasubstituted furan)s with versatile
2
properties, which are potentially applicable in optoelectronic and biological fields.
INTRODUCTION
searching for a great deal of organic reactions, we found that
■
2
the answer is “yes”. Many alkyne/O reactions have been
2
Oxygen (O ), one of most essential gases, constitutes about
2
13−16
reported in the past few years.
However, most of them are
1% (by volume) of the earth’s atmosphere, playing an
important role in burning, respiration, energy metabolism of
living organisms, etc. Nowadays, O is widely used in many
fields, including production of steel, plastics, and textiles;
oxygen therapy; life support systems in aircraft, submarines,
spacecraft, and diving; and so on. In contrast, O can also cause
damage, like spoilage of food, corrosion of metals, aging of
polymer materials, etc. Thus, utilizing O correctly to bring
difficult to develop into a polymerization because the reactions
between O and alkynes will prohibit the chain growth, which
2
2
is crucial for polymer synthesis. Delightfully, in 2009, Jiang and
co-workers reported a Pd(OAc) /Zn(OTf) catalyzed oxida-
2
2
tion and cyclization of aromatic alkynes with O under high
2
2
17
oxygen pressure. This reaction can furnish tetrasubstituted
furans, which are one of the most prominent classes of five-
membered heterocycles and have great significance in
2
benefits instead of damage is significant.
18−21
medicinal, agricultural, and material chemistry.
In 2011,
Generally, oxygen has to be completely excluded during
reactions because of its oxidation properties. Meanwhile,
oxygen is also widely used as a green oxidant in organic
reactions, because it is abundant, cheap, nontoxic, and
environmentally friendly. Therefore, O has attracted more
attention, and a great diversity of new O -involved oxidation
reactions have been developed in recent years.
Although there is growing interest in using O as a green
oxidant to perform oxidation reactions, O has not been used
as a green monomer to synthesize polymers yet. Attracted by
the authors simplified this reaction by using fluorous solvents
to maximize the solubility of O in the reaction system, and the
2
22
plausible mechanism is shown Scheme S1. Because of the
used fluorous solvents, alkynes can be efficiently oxidized by
and cyclized with O2 simultaneously under atmospheric
pressure. Thus, this reaction became suitable for development
1
,2
2
2
3
−9
23,24
into a new polymerization.
2
After systematic investigation of the reaction conditions, a
new polymerization of diynes and oxygen was successfully
established for the first time. The polymerization can be
2
the advantages of O , we took this challenge to use O as a
2
2
comonomer in the development of new polymerizations based
1
0−12
on triple-bond building blocks.
Alkyne is one of the most
Received: June 20, 2018
Revised: August 14, 2018
vital triple-bonds and can be oxidized by many oxidants like
KMnO . We wondered whether it can be oxidized by O . After
4
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.8b01293
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Scheme 1. Polymerization of Diynes and O2
Scheme 2. Model Reaction of Monoyne 3 and O2
carried out in the presence of Pd(OAc) /ZnCl in N,N-
shifts. Fortunately, they could be separated by recycling
preparative high-performance liquid chromatography (HPLC)
using dichloromethane (DCM) as eluent after 6 cycles (Figure
2
2
dimethylacetamide (DMAc) and perfluorodecalin under
atmospheric oxygen at 70 °C, and soluble poly(tetrasubstituted
furan)s with high molecular weights were obtained in high
yields after 8 h (Scheme 1). The ratio of isomeric repeating
units in P1(m) can be identified via analyzing the products of
the model reaction. Moreover, P1(6) could be used as a
fluorescent sensor for explosive detection with superamplifica-
tion quenching effect. Because of the conjugation structures of
P2−P5, they possess large two-photon absorption cross
sections up to 1570 GM.
1
S1). Eventually, the isomeric structures were confirmed by H
NMR (Figure S2) and high-resolution mass spectroscopy
(Figure S3). Moreover, from HPLC spectra of 4−6, we could
calculate their molar ratios, which are shown in Scheme 2.
Polymerization. With the successful experience of the
highly efficient model reaction of monoyne 3 and O , we
2
optimized the reaction conditions of polymerization of diynes
and O (Scheme 1). First, we used 1(6) and O as monomers
2
2
to investigate the time course of the polymerization. Table S1
summarizes the polymerization results. The polymerization
RESULTS AND DISCUSSION
■
carried out in the presence of Pd(OAc) /ZnCl in DMAc and
perfluorodecalin under atmospheric oxygen for 8 h could
readily furnish poly(tetrasubstituted furan)s with high weight-
Model Reaction. Before exploring this O -involved alkyne-
based polymerization, we examine whether the internal
2
2
2
monoyne 3 could react with O (Scheme 2). The reaction
was catalyzed by Pd(OAc) in the presence of ZnCl in DMAc
2
average molecular weight (M
4). Further prolonging the reaction time to 10 h led to a slight
increase in M and yield of the product. In consideration of
) in high yield (Table S1, entry
2
2
w
and perfluorodecalin under atmospheric oxygen, which
generated three tetrasubstituted furan isomers 4−6 in a total
yield of 85% after purification. Because 4−6 possess similar
physical properties, it is hard to purify them by column
chromatography and characterize them by H and C NMR
spectroscopy as they also resonate at very similar chemical
w
reaction efficiency, we chose 8 h as the preferable reaction
time.
Second, the effect of temperature on the polymerization was
investigated. We set 5 parallel experiments with different
1
13
B
DOI: 10.1021/acs.macromol.8b01293
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temperatures from 60 to 100 °C by 10 °C interval. The
polymerization results showed that the reaction at 70 °C
(Table S2, entry 2) could generate poly(tetrasubstituted
spectrum of P1(6) is shown in Figure 1. For comparison, the
spectra of monomer 1(6) and model compounds 4−6 are also
furan)s with excellent molecular weight (Mw = 11 500) in
high yield (82%). When the temperatures were elevated to 90
and 100 °C, the produced poly(tetrasubstituted furan)s are
partially soluble, probably because higher reaction temper-
atures activate the reactivity of monomers and promote the
generation of products with much higher M that are poorly
w
soluble in the reaction solvent. Hence, 70 °C was considered as
the optimal temperature.
Third, the effect of the concentration of Pd(OAc) on the
2
polymerization was studied. The experimental results showed
that the M values of the products decreased from 11 500 to
w
5
000 and the yields reduced from 82 to 53% when the
concentration of Pd(OAc) lowered from 20 to 5 mmol %
2
(
Table S3). Thus, we keep the original concentration of the
catalyst for further polymerization.
Fourth, we checked the effect of the molar ratio of
Pd(OAc) and ZnCl on the polymerization. As indicated in
2
2
Figure 1. FT-IR spectra of (A) monomer 1(6), (B) its model
compounds 4−6, and (C) polymer P1(6).
Table S4, the M values of the products increased from 6300
w
to 11 900 and the yields enhanced from 72 to 86% with the
molar ratio of Pd(OAc) and ZnCl changing from 1:1 to 1:4.
2
2
given. The spectrum of P1(6) shows absorption peaks
inherited from monomer 1(6) and model compounds 4−6
However, when the molar ratio is 1:4, the polydispersity index
of the polymer broadened to 2.32. Therefore, we still keep
their molar ratio at 1:3.
Finally, the effect of monomer concentration on the
polymerization was investigated. When the concentrations of
−
1
excepting that at 2218 cm , which is readily assignable to the
CC group. These results indicate that the CC group of
1
(6) has been almost consumed by the O -involved alkyne-
2
based polymerization.
1
(6) increased progressively from 0.05 to 0.20 M, both the
1
Their H NMR spectra are also compared in Figure 2. The
yields and Mw values of the polymers were enhanced
accordingly (Table S5, entries 1−4). Hence, the concentration
of 0.20 M is the best one.
peaks at δ 7.63−5.93 in P1(6) were associated with the
With these optimized polymerization conditions in hand,
other diynes were subjected to polymerization with O to test
2
its universality. Table 1 summarizes the polymerization of
Table 1. Polymerization of 1(m), 2−5, and O under
2
a
Optimized Reaction Conditions
b
c
entry
polymer
yield (%)
Mw
Đ
1
2
3
4
5
6
7
P1(4)
P1(6)
P1(8)
P2
P3
P4
81
85
76
74
83
76
63
12 200
14 900
14 000
11 600
11 400
13 100
7 000
1.98
2.63
2.02
2.29
2.08
2.56
1.58
P5
a
Carried out in DMAc and perfluorodecalin (v/v = 1:1) at 70 °C
under O (balloon) atmosphere for 8 h in the presence of Pd(OAc)
2
2
and ZnCl . [M] = 0.2 M, [Pd(OAc) ] = 0.04 M, and [ZnCl ] = 0.12
2
2
2
b
M. Estimated by advanced polymer chromatography (APC) in
tetrahydrofuran (THF) on the basis of a polystyrene calibration. Đ,
c
polydispersity index (M /M ).
w
n
other diynes and O under optimal reaction conditions. The
2
results show that all the polymerizations proceeded smoothly,
furnishing their corresponding polymers P1(m) and P2−P5
with high M values (up to 14 900) in satisfactory yields.
w
Structural Characterization. All the polymers were fully
characterized by Fourier transform infrared (FT-IR) and NMR
spectroscopies, and satisfactory results corresponding to their
molecular structures were achieved. Herein, the FT-IR and
NMR spectra of P1(6) are given as examples. The FT-IR
Figure 2. 1H NMR spectra of (A) monomer 1(6), (B) model
compounds 4−6, and (C) polymer P1(6) in DCM-d . The solvent
2
peaks are marked with asterisks.
C
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aromatic proton resonance, which are similar to that in model
compounds 4−6, confirming the occurrence of the polymer-
ization and the expected polymer structure. The resonant
peaks of P1(6) are much broader than those of compounds 4−
Solubility and Thermal Stability. Because of the
irregular distribution of the repeating tetrasubstituted furan
units, the polymers exhibit good solubility in common organic
solvents, such as DCM, chloroform, THF, and toluene, and
could be fabricated into high-quality films by spin-coating or
static-casting processes. In addition, the decomposition
temperatures of P1(m) and P2−P5 at 5% weight loss are in
the range of 253−379 °C under nitrogen, making them
suitable for general applications (Figure S22).
1
3
6
, indicative of its polymeric nature. The C NMR
spectroscopy could also offer some informative analysis data
Figure 3). For example, the 13C NMR spectrum of P1(6)
(
Photophysical Properties. After their structures were
confirmed, the ultraviolet−visible absorption and photo-
luminescence (PL) of all resultant poly(tetrasubstituted
furan)s were investigated (Figure S23 and Table S6). The
maximum absorption and emission of P1(4), P1(6), and
P1(8) are located at 329 and 396 nm, respectively, which are
2
5
similar to those of tetraphenylfuran. The absolute PL
quantum yield measurement showed that P1(6) possesses
the highest value of 35.4%. P2−P5 are conjugated polymers, so
their THF solutions exhibit redder absorption and emission.
P2 shows green emission (517 nm), and P3 and P4 exhibit
deep blue emission (444 and 449 nm). Because of the
incorporated electron-accepting cyano and pyrazine groups of
P5, its emission is red-shifted with a maximum peak at 564 nm,
which is much redder than other polymers.
Explosive Detection. Sensors based on fluorescent
polymers have attracted much attention because of their
amplified response and superior sensitivity to analytes
compared with their low molar mass congeners. The
conjugated structure and high Φ value of P1(6) in THF
F
solution enable us to explore its potential application as a
chemosensor for explosive detection in antiterrorism implica-
tions. Picric acid (PA) was used as a model explosive because
of its commercial availability. The quenching behavior of THF
solution of P1(6) was studied by monitoring their PL changes
with addition of PA (Figure 4). The PL quenching can be
clearly identified at a PA concentration as low as 0.1 μg/mL.
The PL of P1(6) dissolved in THF was weakened gradually
with increasing the amount of explosives (Figure 4A). At a PA
concentration of 0.65 mM, no emitting curve could be
recorded by the spectrofluorometer. The Stern−Volmer plot of
Figure 3. 13C NMR spectra of (A) monomer 1(6), (B) model
compounds 4−6, and (C) polymer P1(6) in DCM-d . The solvent
2
peaks are marked with asterisks.
exhibits no resonance peaks of acetylene carbon atoms of 1(6)
at δ 89.1 and 90.6. Furthermore, its spectral pattern largely
resembles that of 4−6, confirming the structure of P1(6) again.
The FT-IR and NMR spectroscopical data of P1(4), P1(8),
and P2−P5 are summarized in the Supporting Information and
all solidly confirm their respective structures (Figures S4−
S21).
relative PL intensity (I /I − 1) versus the PA concentration
0
gave an upward bending curve instead of a linear one (Figure
26
4B), indicative of a superamplification quenching effect. It is
Figure 4. (A) PL spectra of P1(6) in THF containing different amounts of PA. (B) Stern−Volmer plot of (I /I − 1) values versus PA
0
concentrations in THF of P1(6). I , peak intensity at [PA] = 0 M; concentration, 10 μM; excitation wavelength, 329 nm.
0
D
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Figure 5. (A) Open-aperture Z-scan trace of P2 (scattered squares, experimental data; straight line, theoretical fitted data) in DCM at 600 nm
excitation wavelength. (B) Two-photon absorption spectra of P2−P5 in DCM (concentration: ∼1 mM).
worth noting that the quenching behaviors of the resultant
polyfurans are superior to that of polysiloles and poly-
germoles.
To explain such quenching phenomena, we studied the
absorption spectrum of PA and the PL spectrum of P1(6)
through HPLC, we unambiguously identified the ratios of
isomeric repeating units of P1(m). Moreover, P1(6) can
function as a fluorescent chemosensor for sensitive detection of
explosive with superamplification quenching effect. Because of
the conjugated structures of P2−P5, their TPA cross sections
were measured to be as high as 1570, 1208, 1371, and 868
GM, respectively, showing great potential as two-photon
2
7
(Figure S24). The obvious spectral overlap of the absorption of
PA and emission of P1(6) in the range of 330−465 nm can be
observed, greatly facilitating the energy transfer from the
excited state of P1(6) to the ground state of PA. On the other
hand, the electron-deficient PA is supposed to exert the
strongest electric interaction with P1(6) because the furan
absorption materials. Thus, this work not only first uses O as a
2
monomer in the alkyne-base polymerization but also furnishes
poly(tetrasubstituted furan)s with versatile properties, which
could be potentially applied in optoelectronic and biological
fields, etc. We also believe that this polymerization will inspire
polymerization with other green monomers.
28
moieties are electron-rich.
Two-Photon Absorption Properties. Organic materials
exhibiting large two-photon absorption (TPA) cross sections
(
σ) are of interest in applications in the areas including
ASSOCIATED CONTENT
■
photodynamic therapy, optical pulse suppression, two-photon
fluorescence microscopy, 3D microfabrication, 3D optical data
storage, etc. A variety of organic materials with the structure
*
S
Supporting Information
2
9
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b01293.
of donor−π−acceptor (D−π−A) and donor−π−donor
(
D−π−D) have been developed to realize large TPA cross
30−33
sections.
Because P2−P5 contain D−π−D or D−π−A
Experimental details, reaction conditions optimization
parameters, characterization data (TGA, FT-IR, NMR,
UV, PL, PLQY) (PDF)
conjugated structural motifs, we are curious to know whether
they also possess large TPA cross sections. The TPA cross
sections of P2−P5 were thus measured by the open-aperture
femtosecond Z-scan technique in DCM solutions with laser
excitation in the range of 530−800 nm. Figure 5A shows the
fitting curve of the open-aperture Z-scan of P2 at 600 nm
excitation wavelength as an example. The TPA cross section
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: msqinaj@scut.edu.cn.
*E-mail: tangbenz@ust.hk.
(σ) can be calculated by the equation σ = hνβ/N , in which N
0 0
=
N C is the number density of the absorption centers, N the
A A
3
4−36
Avogadro constant, and C the solute molar concentration.
The largest TPA cross sections of P2−P5 at different laser
wavelength excitation (Figure 5B) were recorded to be 1570,
ORCID
Anjun Qin: 0000-0001-7158-1808
Ben Zhong Tang: 0000-0002-0293-964X
Notes
1
208, 1371, and 868 GM, respectively, which are greater than
those of conjugated polymers previously reported by our
37−39
groups and others.
The authors declare no competing financial interest.
CONCLUSIONS
ACKNOWLEDGMENTS
■
■
In this work, we have developed a new facile O -involved
This work was financially supported by the National Natural
Science Foundation of China (21788102, 21525417, and
21490571), the National Program for Support of Top-Notch
Young Professionals, the Natural Science Foundation of
Guangdong Province (2016A030312002), the Fundamental
Research Funds for the Central Universities (2015ZY013), and
the Innovation and Technology Commission of Hong Kong
(ITC-CNERC14S01).
2
alkyne-based polymerization under atmospheric oxygen. The
polymerization could be performed in the presence of
Pd(OAc) /ZnCl in DMAc and perfluorodecalin at 70 °C,
2
2
and poly(tetrasubstituted furan)s with high molecular weights
were readily produced in high yields after 8 h. The resultant
polymers are soluble and could be fabricated into high-quality
films. By analyzing the components of model reaction products
E
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Synthesis of Annulated Furans and Cyclopentenones. J. Org. Chem.
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DOI: 10.1021/acs.macromol.8b01293
Macromolecules XXXX, XXX, XXX−XXX