"Click polyester": Synthesis of polyesters containing triazole units in the main chain via safe and rapid "click" chemistry and their properties

Article abstract of DOI:10.1002/pola.24206

Click Cu(I)-catalyzed polymerizations of diynes that contained ester linkages and diazides were performed to produce polyesters (click polyesters) of large molecular weights [(～1.0-7.0) × 10⁴], that contained main-chain 1,4-disubstitued triazol

Full text of DOI:10.1002/pola.24206

‘‘Click Polyester’’: Synthesis of Polyesters Containing Triazole Units in the
Main Chain via Safe and Rapid ‘‘Click’’ Chemistry and Their Properties
YU NAGAO, AKINORI TAKASU
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Received 25 March 2010; accepted 28 June 2020
DOI: 10.1002/pola.24206
Published online in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Click Cu(I)-catalyzed polymerizations of diynes that
contained ester linkages and diazides were performed to pro-
duce polyesters (click polyesters) of large molecular weights
[(ꢀ1.0–7.0) ꢁ 104], that contained main-chain 1,4-disubstitued
triazoles in excellent yields. Incorporation of triazole improved
the thermal properties and magnified the even-odd effect of
the methylene chain length. We also found that, by changing
the positions of the triazole rings, the thermal properties of the
polyesters could be controlled. The use of in situ azidation was
a safe reaction, as explosive diazides are not used. In addition,
the microwave heating was found to accelerate the polymeriza-
tion rates. This is the first study that has applied click chemis-
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try for the synthesis of a series of polyesters.
2010 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4207–
4218, 2010
KEYWORDS: click reaction; functionalization of polymers; poly-
esters; step-growth polymerization; thermal properties
INTRODUCTION Even though many different kinds of poly-
mers have been synthesized, their properties characterized,
and used in commercial applications, polyesters still consti-
tute a useful class of polymers. Specifically, aliphatic poly-
esters are expected to soon replace conventional plastics,
because they are biodegradable and are mechanically
strong.1 However, the syntheses of high molecular weight
(Mn > 1.0 ꢁ 104) aliphatic polyesters obtained via polycon-
densations of dicarboxylic acids and diols require severe
conditions (>250 ꢂC and a greatly reduced pressure).2
Recently, we published reports concerning the ‘‘room-tem-
perature polycondensations’’ of dicarboxylic acids with diols
that were catalyzed by the Lewis acids, scandium trifluoro-
methanesulfonate [Sc(OTf)3],3(a) and scandium trifluorome-
thanesulfonimide [Sc(NTf2)3].3(b) These polycondensations
directly afforded aliphatic polyesters with Mns > 1.0 ꢁ 104.
Although identifying new Lewis acids has been a difficult
task,4 we focused on scandium catalysts, because they are
insensitive to protic compounds.5 After the polycondensa-
tions, the catalysts could be recovered and then subse-
quently reused.3(a,b,e) Another way to simplify the recovery
process, is to covalently couple the scandium catalyst
bromo substituents,3(c) hydroxyl groups (chemoselective
dehydration polycondensation),3(f,g) or chirality.3(e) Using
atom-transfer radical polymerization (ATRP) in combination
with poly(butylene bromoadipate) as the macroinitiator, it
was possible to synthesize polyesters that had poly(methyl
methacrylate) side chains.3(c)
It is still unusual to find examples of work that report new
synthetic strategies and catalysts3(h) for the synthesis of ali-
phatic polyesters under environmentally benign conditions.
Click chemistry, introduced by Sharpless and coworkers,6 is
used to efficiently prepare many different types of organic
compounds. The Cu(I)-catalyzed click 1,3-dipolar cycloaddi-
tions between azides and alkynes7(a) proceed readily under
mild conditions, without producing by-products, and with
great chemoselectivities. In addition, these Cu(I)-catalyzed
azide-alkyne cycloadditions (CuAAC) are usually highly regio-
selective, for example, the formation of 1,4-disubstituted
1,2,3-triazoles. Although there have been many reports con-
cerning the improvement of reaction conditions for click-
type syntheses,7(b) and click reactions have been used to
synthesize biologically active substances,8 its application to
polymer synthesis (click polymerization) has been limited.9
Polyesters are a major class of polymeric materials, never-
theless click chemistry has not been used to synthesize poly-
esters instead of condensation reactions.
to
a
polystyrene resin support.3(c) These so-called
‘‘room-temperature polycondensations’’ not only save energy
by not requiring high temperatures, but increase the types of
monomers that can be used. We have published reports con-
cerning room-temperature polycondensations of diols with
dicarboxylic acids that contained a C¼¼C double bond,3(c)
For industrial processes and applications, control of the ther-
mal properties of polymeric materials is necessary. Polymer
Additional Supporting Information may be found in the online version of this article. Correspondence to: A. Takasu (E-mail: takasu.akinori@nitech.
ac.jp)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 4207–4218 (2010) 2010 Wiley Periodicals, Inc.
CLICK POLYESTER, NAGAO AND TAKASU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthesis of click polyesters.
blends have been used to achieve the desired thermal pro-
perties, however, for the production of nanomaterials, the
microscale molecular design must be considered. Therefore,
we need to be able to control the physical and chemical
properties of the targeted polymer by changing the type(s)
of monomer(s) incorporated and/or the type(s) of synthetic
reaction(s). Click polymerization should enable the synthesis
of polyesters from diyne monomers containing ester linkages
and diazido monomers. Click polymerizations are polyaddi-
tion reactions, by which we can design a large number of
different molecular architectures by changing the type(s) of
monomer(s) incorporated. In addition, the polymerization
may be accompanied by the formation of main-chain
1,4-disubstituted triazoles. Therefore, production of main-
chain 1,4,-disubstitured triazole polyesters will substantially
increase the types of ‘‘next-generation’’ polyesters available
(Scheme 1).
CuAAC can be used to polymerize peptide-based polymers
from amino acids derivatives. To broaden the scope of micro-
wave-assisted CuAAc reactions, we report herein the applica-
tion of microwave heating to safely form polyesters via in situ
azidation. With the use of microwave irradiation and in situ azi-
dation, we reduced the reaction times needed and eliminated
the danger incurred when handling small azide compounds.
Herein, we report the use of click chemistry to synthesize
polyesters. These reactions were accompanied by the forma-
tion of main-chain 1,4-disubstituted triazoles. We found that
the triazole backbone improved the thermal properties of
the polyesters. We also found that, in conjunction with
in situ azidation, microwave irradiation accelerated the poly-
merization rates. To the best of our knowledge this is the
first time click chemistry has been used to synthesize a
series of polyesters.13
On the other hand, the small molecular weight, organic
azides that are used for CuAAC are dangerous compounds. A
small azide molecule is considered to have less than six
carbon and one oxygen atoms per azide functionality. This
definition is an empirically derived rule and does not guar-
antee that a particular azide-containing compound is stable.
Handling purified azides is not a safe practice, and they,
therefore, should not be purified. To improve the safety of
CuAACs for polyester, we developed a practical reaction that
uses azide reactants that are generated in situ from their
corresponding halides10 and that then react immediately
with Cu(I) acetylides, to form the corresponding 1,4-disubsti-
tuted-1,2,3-triazoles.
EXPERIMENTAL
Materials and Methods
Chemicals were obtained from commercial sources and were
used without further purification. A MWO-1000S synthesizer
(EYELA, Tokyo, Japan) was used for microwave irradiation.
1
ꢂ
H-NMR spectra were acquired at 27 C using Bruker Analy-
tik DPX200 and DPX600 spectrometers (200 and 600 MHz,
respectively). The chemical shift of tetramethylsilane (TMS)
was taken as zero ppm. Number average molecular weights
(Mn) and the polydispersity indexes (PDI; Mw/Mn) of the
polyesters were estimated using a size exclusion chromato-
graphy (SEC) system consisting of a Tosoh DP8020 pump
system, a Tosoh RI-8020 differential refractometer, and
Tosoh TSK-gel a-3000 and a-5000 columns (Tosoh, Tokyo,
Japan). The eluent was 0.05% LiBr, 100 mM of tetramethyle-
thylenediamine in DMF. The flow rate was 0.5 mL/min, and
Rapid syntheses induced by microwave irradiation to control
the temperature have attracted considerable attention recently,
because such procedures are more convenient and economical
than the corresponding traditional syntheses that use alterna-
tive heat sources, for example, an oil bath. Almost 4000 articles
have been published concerning microwave-assisted organic
synthesis since Gedye et al.11 first reported the use of micro-
wave heating to accelerate organic chemical transformations.
Microwave heating reduces reaction times from hours to
minutes, reduces the amounts of by-products produced,
increase yields, and improves reproducibility.11(c) Therefore,
many academic and industrial research groups are already
using microwave-assisted reactions for the rapid optimization
of reactions, for the efficient synthesis of new chemical entities,
and for the discovery and characterization of new chemis-
tries.11(c) Fokin and coworkers10(b) have developed a micro-
wave-enhanced, fast, completely regioselective, and efficient
three-component reaction that uses small compounds for the
generation of 1,4-disubstituted-1,2,3-triazoles. Furthermore, as
reported by Liskamp and coworkers,12 microwave-assisted
ꢂ
the temperature was 40 C. Differential scanning calorimetry
(DSC), using a DSC6220S calorimeter (Seiko Instruments
ꢂ
Inc., Chiba, Japan) was performed from ꢃ80 to 180 C, with
the temperature increased or decreased at a rate of 10 ꢂC/
min. The instrument was calibrated using indium and tin
samples. For all experimental samples, a complete tempera-
ꢂ
ꢂ
ture, heating cycle from ꢃ80 to 180 C and back to ꢃ80 C,
was obtained. Each sample weighted between 4 and 6 mg
and was placed into an aluminum pan that was covered with
a lid within the calorimeter. The glass transition temperature
(Tg) was taken as the inflection point of the corresponding
heat capacity jump of the DSC trace. The melting tempera-
ture (Tm) was defined as the minimum point of the endo-
thermic trough. Thermogravimetric analyses (TGA) were
ꢂ
performeꢂd under nitrogen, at rates of 10 C/min between 20
and 600 C (TG/DTA 220U; Seiko Instruments, Chiba, Japan).
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ARTICLE
1
Preparation of Diazido Monomers
bis(butynyl) succinate: H-NMR (CDCl3) d (ppm): 4.21 (t, 4H,
6.8 Hz), 2.67 (s, 4H), 2.54 (dt, 4H, 2.7 and 6.8 Hz), 2.01 (t,
2H, 2.7 Hz).
As an example, the preparation of 1,4-diazidobutane is
described. Into a 20-mL round-bottom flask were added 1,4-
dibromobutane (1.51 g, 7.0 mmol) and sodium azide (1.24 g,
19.0 mmol) both of which had been dissolved in 10 mL of
DMF. After stirring the mixture at room temperature for 24
h, 5 mL of water was added to quench the reaction. After
having been cooled to room temperature, the solution was
extracted with 30 mL of diethyl ether. The organic phases
were combined, washed twice with 10-mL volumes of 50%
NaHCO3(aq), and then dried over MgSO4. After filtration
and solvent evaporation, 1,4-diazidobutane was obtained in
90% yield (881 mg) as a white liquid. The other diazido-
alkanes, 1,5-diazidopentane (91% yield), 1,6-diazidohexane
(82% yield), 1,7-diazidoheptane (50% yield), and 1,3-diazido-
propane (32% yield), were prepared using similar procedures
and were isolated as syrups. m-Diazidoxylene (79% yield)
and p-diazidoxylene (64% yield), which were used as aro-
matic, monomeric diazido reactants, were prepared using
similar procedures and were obtained as white liquids. The
diazides used for this report were not purified and were
stored in ethyl acetate.
1
bis(butynyl) methylsuccinate: H-NMR (CDCl3) d (ppm): 4.19
(t, 4H, 6.7 Hz), 2.93 (quintet, 1H, 6.6 Hz), 2.78 (dd, 1H, 8.1
and 16 Hz), 2.57–2.50 (m, 4H), 2.45 (dd, 1H, 6.0 and 16 Hz),
2.00 (t, 2H, 2.5 Hz), 1.24 (d, 3H, 7.1 Hz).
bis(butynyl) adipate: 1H-NMR (CDCl3) d (ppm): 4.19 (t, 4H,
6.8 Hz), 2.57–2.49 (m, 4H), 2.36 (t, 4H, 6.4 Hz), 2.01 (t, 2H,
2.6 Hz), 1.71–1.65 (m, 4H).
bis(isobutynyl) succinate: 1H-NMR (CDCl3) d (ppm): 5.45
and 5.44 (2H, 2xq, 6.7 Hz), 2.67 (s, 4H), 2.45 and 2.44 (2xs,
2H), 1.51 (d, 6H, 2.9 Hz).
1
bis(isobutynyl) malonate: H-NMR (CDCl3) d (ppm): 5.50 and
5.49 (2H, 2xq, 6.7 Hz), 3.43 (s, 2H), 2.49 and 2.48 (2xs, 2H),
1.54 (d, 6H, 3.5 Hz).
bis(butynyl) phthalate: 1H-NMR (CDCl3) d (ppm): 7.78–7.71
(m, 2H), 7.60–7.53 (m, 2H), 4.43 (t, 4H, 6.8 Hz), 2.70–2.62
(m, 4H), 2.03 (t, 2H, 2.6 Hz).
bis(propynyl) methylsuccinate: 1H-NMR (CDCl3) d (ppm): 4.70
(s, 2H), 2.98 (m, 1H), 2.82 (dd, 1H, 4.0 and 16Hz), 2.48 (t,2H,
2.2 Hz), 2.49 (dd, 1H, 3.0 and 16 Hz), 1.26 (d, 3H, 3.6 Hz).
1,3-diazidopropane: 1H-NMR (CDCl3) d (ppm): 3.42 (t, 4H,
6.5 Hz), 1.84 (2H, quintet, 6.5 Hz).
1
1,4-diazidobutane: H-NMR (CDCl3) d (ppm): 3.33 (t, 4H, 6.2
Hz), 1.71–1.65 (m, 4H).
Model Reactions
The model compounds M1, M2, and M3 (Scheme 2) were pre-
pared by click reactions of 1,6-diazidohexane or p-diazidoxy-
lene, respectively, with 3-butyn-2-ol. As an example of the
procedure used, into a 10-mL round-bottom flask were added
1,6-diazidohexane (137 mg, 0.8 mmol) and three equivalents
of 3-butyn-2-ol both dissolved in 0.5 mL of DMF. The mixture
was stirred at room temperature for 1 h. The products were
1,5-diazidopentane: 1H-NMR (CDCl3) d (ppm): 3.39 (t, 4H,
6.6 Hz), 1.70–1.57 (m, 4H), 1.54–1.40 (m, 2H).
1
1,6-diazidohexane: H-NMR (CDCl3) d (ppm): 3.28 (t, 4H, 6.6
Hz), 1.69–1.58 (m, 4H), 1.48–1.35 (m, 4H).
1,7-diazidoheptane: 1H-NMR (CDCl3) d (ppm): 3.27 (t, 4H,
6.7 Hz), 1.68–1.58 (m, 4H,), 1.46–1.33 (br, 6H).
1
characterized using H-NMR spectroscopy and were not puri-
fied so that the extent of conversion could be determined.
m-diazidoxylene: 1H-NMR (CDCl3) d (ppm): 7.42 (t, 1H, 6.6
Hz), 7.30 (d, 2H, 3.2 Hz), 7.26 (s, 1H), 4.38 (s, 4H).
1
p-diazidoxylene: H-NMR (CDCl3) d (ppm): 7.34 (s, 4H), 4.36
(s, 4H).
Polymer Syntheses: Click Polyesters Synthesized Using
an Oil Bath for Temperature Control
The structures and chracterizations of all click polyesters
synthesized for this report are summarized in Figure 1 and
Table 1. As an example of the procedure used (run 4, Table
1), bis(butynyl) methylsuccinate, which served as the diyne
(236 mg, 1.0 mmol), and 1,4-diazidobutane (140 mg,
1.0 mmol), which served as the diazide were added into a
10-mL round-bottom flask that contained 0.5 mL DMF. Next,
with the contents of the flask contained under a nitrogen
atmosphere, 10 mg of CuBr was added. After stirring the
Preparation of Diyne Monomers
As an example, the preparation of bis(butynyl) methylsucci-
nate is described. Into a 50-mL round-bottom flask were
added methylsuccinic acid (633 mg, 4.80 mmol), Sc(OTf)3
(49 mg, 9.95 mmol), and 3-butyn-1-ol (1.36 g, 19.4 mmol).
The reaction mixture was stirred at 60 ꢂC for 6 h under
reduced pressure (25 mmHg). After the reaction mixture had
been diluted with 30 mL chloroform, the solution was
washed with 10 mL of water saturated with NaHCO3 and
then dried over MgSO4. After filtration and solvent evapora-
tion, bis(butynyl) methylsuccinate was obtained as an
auburn liquid in 71% yield (803 mg). Bis(butynyl) succinate
(62% yield, white solid), bis(butynyl) adipate (87% yield,
white solid), bis(isobutynyl) succinate (43% yield, auburn
liquid), bis(isobutynyl) malonate (15% yield, auburn liquid),
bis(butynyl) phthalate (10% yield, white solid), and bis
(propynyl) methylsuccinate (92% yield, auburn liquid) were
prepared using similar procedures.
ꢂ
mixture at 50 C for 30 h, it was diluted with 5-mL volumes
of DMF. A green solid was obtained in 99% yield after
twice being precipitated from the DMF solution with 90 mL
volumes of diethyl ether.
For runs 1 and 2 the yields could not be calculated, and
their 1H-NMR spectra could not be acquired, because the
products were insoluble in all organic solvents tested.
1
run 3 (yield: 99%): H-NMR (CDCl3) d (ppm): 7.93 (2H, brs),
4.34 (4H, t, 6.9Hz), 4.20 (4H, brs), 2.95–2.89 (5H, br), 2.74
CLICK POLYESTER, NAGAO AND TAKASU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 Model click reactions of
model reaction that used 1,4-diazido-
butane, 3-butyn-2-ol, and Cu(I)Br as
the catalyst in DMF at 50 ꢂC (M1). 1,6-
diazidohexane (M2) and p-diazidoxy-
lene (M3) with 3-butyn-2-ol. (solvent,
DMF; catalyst, Cu(I)Br, room tempera-
ture, 1 h reaction time).
(1H, dd, 6.6 and 15 Hz), 2.38 (1H, dd, 7.3 and 15 Hz), 2.36–
2.29 (2H, br), 1.02 (3H, d, 6.8 Hz).
run 12 (yield: 99%): 1H-NMR (CDCl3) d (ppm): 7.57 (2H,
brs), 6.05–6.00 (2H, br), 4.37 (4H, brs), 2.63 (4H, brs), 1.92
(4H, brs), 1.64 (6H, brs).
1
run 4 (yield: 89%): H-NMR (CDCl3) d (ppm): 7.50 (2H, brs),
4.37 (8H, br), 3.05 (4H, brs), 2.91–2.81 (1H, m), 2.68 (1H,
dd, 8.8 and 16 Hz), 2.38 (1H, dd, 5.1 and 16 Hz), 1.92 (4H,
brs), 1.17 (3H, d, 7.1 Hz).
run 5 (yield: 99%): 1H-NMR (CDCl3) d (ppm): 7.44 (1H, s),
7.43 (1H, s), 4.32 (4H, t, 6.4 Hz), 3.05 (4H, brs), 2.91–2.85
(1H, m), 2.69 (1H, dd, 8.6 and 16 Hz)), 2.40 (1H, dd, 5.4 and
16Hz), 1.94 (4H, brs), 1.34 (2H, brs), 1.17 (3H, d, 7.2 Hz).
run 6 (yield: 99%): 1H-NMR (DMSO-d6) d (ppm): 7.89 (2H,
s), 4.30–4.17 (8H, br), 2.94–2.87 (5H, br), 2.73 (1H, dd, 6.8
and 14 Hz), 2.39 (1H, dd, 6.0 and 17 Hz), 1.74 (4H, brs),
1.21 (4H, brs), 1.02 (3H, d, 7.0 Hz).
1
run 7 (yield: 84%): H-NMR (DMSO-d6) d (ppm): 7.87 (2H, s),
4.30–4.22 (4H, br), 4.22–4.17 (4H, br), 2.93–2.87 (5H, br), 2.74
(1H, dd, 7.1 and 14 Hz), 2.40 (1H, dd, 6.0 and 17 Hz), 1.81–1.67
(4H, br, 5.9 Hz), 1.29–1.13 (4H, br), 1.02 (3H, d, 7.0 Hz).
run 8 (yield: 64%): 1H-NMR (DMSO-d6) d (ppm): 7.93 (1H, s),
7.92 (1H, s) 7.25 (4H, s), 5.51 (4H, brs), 4.19 (4H, brt, 6.4 Hz),
2.89 (4H, brt, 6.9 Hz), 2.73 (1H, m), 2.70 (1H, dd, 6.7 and 13
Hz), 2.34 (1H, dd, 6.0 and 16 Hz), 0.96 (3H, brd, 7.0 Hz).
run 9 (yield: 70%): 1H-NMR (DMSO-d6) d (ppm): 7.94 (1H, s),
7.93 (1H, s), 7.35–7.16 (4H, br), 5.52 (4H, brs), 4.20 (4H, t, 6.4
Hz), 2.90 (4H, t, 6.4 Hz), 2.73 (1H, m), 2.70 (1H, dd, 6.7 and 16
Hz), 2.37 (1H, dd, 5.9 and 17 Hz), 0.98 (3H, brd, 7.1 Hz).
1
run 10 (yield: 53%): H-NMR (DMSO-d6) d (ppm): 7.96 (2H,
s), 7.34–7.17 (4H, br), 5.53 (4H, s), 4.21 (4H, t, 6.4 Hz),
2.94–2.90 (4H, br), 2.82 (4H, d, 32Hz).
1
run 11 (yield: 95%): H-NMR (CDCl3) d (ppm): 8.02 (2H, s),
7.66 (2H, brs), 7.52 (2H, brs), 4.51–4.40 (4H, br), 4.37–4.30
(4H, br), 3.18–3.10 (4H, br), 2.05–1.50 (6H, br).
FIGURE 1 The click polyesters synthesized in Table 1.
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ARTICLE
TABLE 1 Click Polymerizations of Dialkynes and Diazidesa
Yieldb (%)
Mn ꢁ 10ꢃ4
Mw/Mn
Tg (ꢂC)
Tm (ꢂC)
c
c
d
d
Run
1
Diyne
Diazide
e
e
e
–
Insoluble
Insoluble
–
–
e
e
e
2
3
–
Insoluble
0.88
Insoluble
2.08
–
–
99
17
76
4
89
99
99
84
64
70
53
95
99
85
99
73
1.37
1.36
5.08
1.13
5.93
0.84
6.69
0.19
0.89
0.59
1.33
1.14
2.49
2.85
2.46
2.90
2.26
1.70
2.53
1.40
3.24
5.83
2.72
2.52
14
ꢃ4
6
125
5
52
6
76
7
ꢃ3
51
29
50
72
8
137
9
Not detected
78
10
11
12
13
14
e
e
–
–
e
e
–
–
e
e
–
–
14
1
84
68
15
a
d
All runs were performed on a 1 mmol scale in 0.5 mL (DMF), at 50 ꢂC,
Tm was determined from the first heating DSC scan and Tg from the
for 30 h, with 10 mg CuBr as the catalyst.
b
second scan (heating and cooling rates: 10 ꢂC/min).
e
After reprecipitation from DMF with diethyl ether.
Determined by SEC with DMF as the eluent and relative to poly(sty-
Not measured.
c
lene) standards.
1
run 13 (yield: 85%): H-NMR (DMSO-d6) d (ppm): 8.10 (2H,
(1H, dd, 7.7 and 17 Hz), 2.44 (1H, dd, 4.6 and 16 Hz), 1.90
(4H, brs), 1.36 (4H, brs), 1.19 (3H, d, 6.5 Hz).
brs), 5.95 (2H, brs), 4.36 (4H, brs), 3.49 (2H, brs), 1.77 (4H,
brs), 1.51 (6H, brs).
1
run 15 (yield: 73%): H-NMR (CDCl3) d (ppm): 7.60 (1H, s),
run 14 (yield: 99%): 1H-NMR (CDCl3) d (ppm): 7.63 (2H,
brs), 5.19 (4H, brs), 4.35 (4H, brs), 2.93–2.79 (1H, m), 2.72
7.58 (1H. s), 5.19 (4H, d, 5.6 Hz), 4.34 (4H, t, 7.2 Hz), 2.98–
2.86 (1H, m), 2.73 (1H, dd, 8.4 and 17 Hz), 2.45 (1H, dd, 5.8
CLICK POLYESTER, NAGAO AND TAKASU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 3 One pot in-situ click
polymerization.
and 17 Hz), 1.89 (4H, brt, 6.4 Hz), 1.38–1.28 (6H, br), 1.30
(3H, d, 7.0 Hz).
and dried in vacuo. A brown solid was obtained in a 99%
yield.
runs 1–3: 1H-NMR (DMSO-d6) d (ppm): 7.88 (2H, brs), 4.32
(4H, brs), 4.21 (4H, brt, 6.4 Hz), 2.93–2.87 (5H, br), 2.72
(1H, dd, 7.1 and 15 Hz), 2.40 (1H, dd, 6.1 and 16 Hz), 1.74
(4H, brs), 1.02 (3H, brd, 7.0 Hz).
Safe Syntheses of Click Polyesters via In Situ Azidation
Using an Oil Bath for Temperature Control
The reactants and the properties of the products for the click
polyester syntheses via in situ azidation, using an oil bath
for temperature control, are given in Scheme 3 and Table 2.
As an example of the experimental procedure (run 2, Table
2), bis(butynyl) methyl succinate, which served as the diyne
(236 mg, 1.0 mmol), 1,4-dibromobutane (216 mg, 1.0 mmol),
and sodium azide (130 mg, 2.0 mmol) were added into a 10-
mL round-bottom flask that contained 1.0 mL of DMF. Then,
with the contents of the flask under nitrogen, 30 mg (0.20
mmol) of CuBr was added. After stirring at 50 ꢂC for 24 h,
the reaction mixture was diluted with 5-mL volumes of DMF
and the product was precipitated from the DMF solution
with two 90-mL volumes of diethyl ether, washed with water,
runs 4–5: 1H-NMR (DMSO-d6) d (ppm): 8.10 (1H, brs), 8.03
(1H, brs), 5.38–5.34 (1H, m), 5.09 (4H, brs), 4.39–4.31 (4H,
br), 3.82 (1H, brs), 2.88-2,68 (2H, m), 2.60–2.50 (1H, m),
2.16–1.78 (2H, m), 1.08 (3H, d, 6.9 Hz).
runs 6–7: 1H-NMR (DMSO-d6) d (ppm): 7.88 (1H, brs), 7.82
(1H, brs), 5.35–5.33 (1H, m), 4.44–4.19 (6H, m), 3.80 (brs,
1H), 2.94–2.85 (5H, m), 2.81–2.64 (1H, m), 2.55–2.36 (1H,
m), 1.92–1.74 (2H, m), 1.05 (3H, d, 6.8 Hz).
run 8: 1H-NMR (DMSO-d6) d (ppm): 8.12 (1H, s), 8.04 (1H,
s), 5.39 (1H, m), 5.10 (4H, brs), 4.48–4.31 (4H, m), 3.80 (brs,
TABLE 2 Safe Syntheses of Click Polyesters via In Situ Azidation that Used an Oil Bath for Temperature Controla
Dibromo or Diazide
Run
1
Diyne
Compound
DMF (mL)
0.5
Yieldb (%)
99
Mn ꢁ10ꢃ4
Mw/Mn
c
c
NaN3
NaN3
0.3
1.9
2
3
4
5
6
7
1.0
1.0
1.0
1.0
0.5
1.0
1.0
98
99
98
99
98
99
99
0.6
3.7
0.3
0.7
0.1
0.3
0.2
3.1
2.8
2.2
2.4
2.1
2.2
2.3
NaN3
NaN3
NaN3
NaN3
8
a
b
c
All runs are performed on a 1 mmol scale in DMF at 50 ꢂC, for 24 h,
with 30 mg of CuBr as the catalyst, and 1.0 equiv each of sodium azide
and the halide.
After reprecipitation from DMF using diethyl ether.
Determined by SEC measurement in DMF relative to poly(stylene)
standards.
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ARTICLE
TABLE 3 Safe Synthesis of Click Polyesters via In Situ Azidation that Used Microwave
Irradiation for Temperature Controla
Dibromo
Yieldb
(%)
Run
1
Diyne
Compound
Mn ꢁ 10ꢃ4
Mw/Mn
c
c
99
99
98
99
0.9
0.2
0.4
0.2
2.4
2
3
1.9
2.4
2.1
4
a
All runs are performed on a 1 mmol scale in 1 mL DMF, for 24 h, at 50 ꢂC that was maintained by modu-
lating the irradiation power between 0 and 50 W; with 30 mg CuBr as the catalyst; and 1.0 equiv of sodium
azide and 1.0 equiv. of the halide.
b
After reprecipitation from DMF using diethyl ether.
c
Determined by SEC with DMF as the eluent relative to poly(stylene) standards.
1H), 2.85–2.69 (2H, m), 2.61–2.56 (1H, m), 2.04–1.79 (2H,
Semiempirical Molecular Orbital Calculation
m), 1.05 (3H, d, 6.8 Hz).
The repeat units of the click polyesters, that is, Me-[OCOCH
(CH3)CH2COO(CH2)2-triazole-(CH2)4-trialzole-(CH2)2]2-Me and Me-
[OCOCH(CH3)CH2COO(CH2)2-triazole-(CH2)5-trialzole-(CH2)2]2-
Me using semiempirical molecular orbital calculations by
AM1 molecular orbital (MO) method14 in MOPAC 2000
(MOPAC 2000 version 1.0, Fujitsu, Tokyo, Japan, 1999) to
investigate the conformations of the corresponding click poly-
esters (runs 4 and 5 in Table 1), in which planar zigzag
structure of the polyester main chain is programmed for the
starting conformation.
Safe Syntheses of Click Polyesters via In Situ Azidation
Using Microwave Irradiation for Temperature Control
The reactants and properties of the polyester products are
given in Table 3. As an example of the experimental proce-
dure (run 1), bis(butynyl) methyl succinate as the diyne
(236 mg, 1.0 mmol), 1,4-dibromobutane (216 mg, 1.0 mmol),
and sodium azide (130 mg, 2.0 mmol) were added into a
test tube that contained 1.0 mL of DMF. Next, with the con-
tents of the tube under a nitrogen atmosphere 30 mg of
CuBr was added. After stirring for 24 h in the microwave
synthesizer (with the microwave power varied between 0
and 50 W, so that the temperature remained at 50 ꢂC), the
reaction mixture was diluted with 5-mL volumes of DMF,
precipitated from the DMF solution with two 90 mL volumes
of diethyl ether, washed with water, and dried in vacuo. A
brown solid was obtained in 99% yield.
Biodegradation Test
Using a biodegradation (BOD) tester (Model 200F; Taitec,
Koshigaya-shi, Japan) in accordance with the ISO standard
guidelines (ISO 14851), BOD was assessed as the amount of
ꢂ
oxygen consumed at 25 C during a 40-day period. The bio-
degradability was calculated as the percentage of the amount
of consumed oxygen and normalized to that of a blank test
sample to the theoretical amount of oxygen required for
complete oxidation of the sample. The BOD values are
reported as the means of three measurements. Activated
sludge was obtained from a sewage plant in Naka-ku,
Nagoya, Aichi Prefecture, Japan. After filtration of the sludge
samples, the supernatants (15 mL) were added to the incu-
bation medium (150 mL) that contained (mg/L): K2HPO4,
217; KH2PO4, 85; Na2HPO4, 447; NH4Cl, 5; CaCl2, 27;
MgSO4ꢄ7H2O, 23; and FeCl3ꢄ6H2O, 0.25. The sample (10 mg/
L) was coated onto the bottle, and NaOH(aq) (11 mol/L) was
placed within to absorb the CO2 produced.
Instant Click Polymerization via In Situ Azidation
The reactants and properties of the polyester products are
given in Table 4. As an example of the experimental proce-
dure (run 1), bis(butynyl) methylsuccinate as the diyne (236
mg, 1.0 mmol), 1,4-dibromobutane (216 mg, 1.0 mmol), and
sodium azide (130 mg, 2.0 mmol) were added into a test
tube containing 2.0 mL of DMF. Next, with the contents of
the tube under a nitrogen atmosphere, 30 mg of CuBr was
added. After stirring the reaction mixture for 30 min in the
microwave synthesizer (irradiation power at 50 W, 153 ꢂC,
which is the boiling point for DMF), the reaction mixture
was diluted with 5-mL volumes of DMF, precipitated from
the DMF solution with two 90-mL volumes of diethyl ether,
washed with water, and dried in vacuo. A brown solid was
obtained in 99% yield.
RESULTS AND DISCUSSION
Model Reactions
We first investigated a Cu(I)-catalyzed, Huigen-type, 1,3-dipo-
lar cycloaddition that used 1,4-diazidobutane as the azide
and 3-butyn-2-ol as the alkyne with Cu(I) (CuBr) as the
CLICK POLYESTER, NAGAO AND TAKASU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Instant ‘‘Click Polymerization’’ via In Situ Azidation Using Microwave Irradiation for Temperature Controla
Dibromo
DMF
(mL)
Time
(min)
Yieldb
(%)
Run
1
Diyne
Compound
Mn ꢁ 10ꢃ4
Mw/Mn
c
c
2.0
2.0
1.5
1.5
1.0
1.0
1.0
30
15
30
15
30
15
15
99
99
99
99
99
99
99
2.5
1.8
1.3
1.1
0.8
0.7
0.5
4.8
2
3
4
5
6
3.7
4.5
4.0
2.3
2.7
2.4
7
a
c
All runs are performed with 50 W irradiation power, with 30 mg CuBr
as the catalyst, 2.0 equiv of sodium azide, and 2.0 equiv of the halide.
Determined by SEC with DMF as the eluent and relative to poly(sty-
lene) standards.
b
After reprecipitation from DMF using diethyl ether.
catalyst. We heated a mixture of 1,4-diazidobutane (1.3
mmol) and 3-butyn-2-ol (3.9 mmol) to 50 C, then added 10
The click polymerizations were catalyzed by Cu(I) (as
CuBr) without an N-substituted Cu(I)-stabilizing ligand
that might have reduced the product yield by base cleav-
age of the ester bonds.15 The click polymerizations were
run in DMF at 50 ꢂC for 30 h, and the products were
obtained in excellent yields (85–99%, Table 1). The num-
ber averaged molecular weights (Mn) and the molecular
weight distributions (Mw/Mn) of the click polyesters were
determined using SEC. The measurements were per-
formed in the presence of 100 mM tetramethylethylenedi-
amine to eliminate the possibility that Cu would promote
the Cu(I)-mediated crosslinking of a newly formed tria-
zole pair.9(d)
ꢂ
mg CuBr in 1-mL volumes of DMF, and then stirred the mix-
ture at 50 ꢂC for 12 h (Scheme 2). The reaction proceeded
smoothly and gave the expected product (M1), the 1,4-disub-
stituted 1,2,3-triazole isomer, in high yield (ꢀ100%), and its
structure and yield were confirmed by characterization of
1
the H-NMR spectrum of the unpurified product.
The model reaction to synthesize M3 using the aromatic p-
ꢂ
diazidoxylene and 3-butyn-2-ol was also carried out at 25 C
for 1 h with a yield of 81% (Scheme 2). However, under the
same reaction conditions, with the substitution of 1,6-diazi-
dohexane for p-diazidoxylene, 91% conversion to M2 was
achieved in the same amount of time. Therefore, we could
not confirm the higher conversion based on the reactive na-
ture of benzyl azide and lower conversion compared to alkyl
Using bis(butynyl) methylsuccinate as the diyne monomer
and different alkane diazides (runs 3–7), the click polymer-
izations proceeded smoothly to give the corresponding poly-
esters with Mns between ꢀ1.0 ꢁ 104 and 5.0 ꢁ 104. For
each of their size exclusion chromatograms, only a single,
symmetrical peak was recorded. The click polymerizations
that used a diazide derivative of p-xylene in conjunction with
bis(butynyl) methylsuccinate also proceeded smoothly to
give the corresponding polyesters. When p-diazidoxylene
was used as the monomer, a high molecular weight polyester
(Mn ¼ 5.9 ꢁ 104) was obtained. It seems that the high mo-
lecular weight was ascribed to the conformation of the
obtain ‘‘click polyester’’ not to reactivity of benzylic azide
(see model reaction), as m-diazidoxylene as the monomer
did not afford much higher molecular weight polyester (Mn
¼ 0.8 ꢁ 104, run 9).
1
azide was obtained in the H-NMR spectrua. This is ascribed
to steric hindrance by the aromatic ring.
Polymer Syntheses
Because the model 1,3-dipolar cycloaddition reactions
proved to be regioselective, it appeared possible to use the
click reaction to synthesize polyesters (Scheme 1). The diyne
monomers, bis(butynyl) succinate, bis(butynyl) adipate, bis
(butynyl) methylsuccinate, bis(butynyl) phthalate, bis(isobu-
tynyl) succinate, bis(isobutynyl) malonate, bis(propynyl)
methylsuccinate, and the diazide monomers, 1,3-diazidopro-
pane, 1,4-diazidobutane, 1,5-diazidopentane, 1,6-diazidohex-
ane, 1,7-diazidoheptane, p-diazidoxylene, m-diazidoxylene,
were prepared according to published methods.13
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ARTICLE
wave irradiation did not improve the incorporation of mono-
mers that contained hydroxyl groups (runs 3 and 4). This
lack of improvement might be ascribed to absorbance of
microwave by the pendent hydroxyls during the polyaddtion.
Finally, the irradiation power was set to 50 W (Table 4).
Remarkable increases in the molecular weights of the poly-
esters were achieved to attain a more effective microwave-
assisted polymerization. For example, the polyester of run 1
(Mn ¼ 2.5 ꢁ 104) was prepared in only 30 min and that of
run 2 in 15 min (Mn ¼ 1.8 ꢁ 104). We also found that the
molecular weight of the polyester that incorporated 1,4-
dibromo-2-butanol increased (run 7). As expected, the mo-
lecular weights of the polyesters tended to decrease with
increasing monomer concentration, perhaps because transes-
terification was favored at the high temperature used, which
was the boiling point of DMF.
This microwave-assisted polymerization is a simple tech-
nique, requiring only the mixing of reagents and microwave
irradiation. It avoids the need to isolate and handle poten-
tially unstable small organic azides and shortens the reaction
time from hours to a minutes.
FIGURE 2 Plots of the Tm and Tg values of click polyesters as a
function of the main-chain number of methylene units.
Safe Synthesis of Click Polyesters via In Situ Azidation
We performed click polymerizations of diynes and dibromo
compounds in the presence of sodium azide to synthesize
click polyesters via in situ azidation. These reactions were
benign, as purified diazide compounds, which might explode,
were not involved (Scheme 3).
Themal Properties of ‘‘Click Polyester’’s
Polymerization shown in this study was accompanied by the
formation of main-chain 1,4-disubstituted triazoles. We
found that the presence of main-chain triazole rings
improved the thermal properties of the polyesters per se and
magnified the even-odd effect of the methylene chain length
We first investigated Cu(I)-mediated safe click reactions with
the reaction temperatures controlled by an oil-bath (Table
2). The reactions were performed at 50 ꢂC for 24 h. For all
synthesizes, the polyesters obtained via in situ azidation had
Mn values molecular weights (0.59 ꢁ 104, run 2; 0.33 ꢁ 104,
run 4; Table 2) that were smaller than the corresponding
click polyesters that had been synthesized using ioslated dia-
zido monomers (3.79 ꢁ 104, run 3; 0.68 ꢁ 104, run 5; Table
2). The high yield and 1H-NMR spectra indicated that the in
situ azidation proceeded completely and no side reaction
occurred under the employed conditions. We also performed
click polymerizations with the hydroxyldibromide, 1,4-
dibromo-2-butanol, in the presence of sodium azide, to syn-
thesize click polyesters containing pendent hydroxyl groups
(runs 6–8). However, only polyesters of relatively low molec-
ular weight were recovered (Mn ¼ (0.14–0.26) ꢁ 104).
Next, we attempted click polymerizations, with the tempera-
ture (50 ꢂC) controlled by modulating the power of the
microwave irradiation between 0 and 50 W and reaction
times of 24 h (Table 3). In contrast to the results obtained
when the temperature was controlled by an oil bath (Mn
¼
0.59 ꢁ 104, run 2; 0.33 ꢁ 104, run 4; Table 2), controlling
the temperature using microwave irradiation produced
higher molecular weight click polyesters (Mn ¼ 0.94 ꢁ 104,
run 1; 0.42 ꢁ 104, run 3; Table 3). The polymerizations
were completed in shorter periods of time when microwave
irradiation was used than when an oil bath was used and
that microwave radiation provides the effective driving force
for the click polymerizations.9(c),10(b),12,16 However, micro-
FIGURE 3 Semiempirical molecular orbital calculations to
model the conformations of the corresponding click polyesters
(runs 4 and 5 in Table 1).
CLICK POLYESTER, NAGAO AND TAKASU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 4 Changes in the positioning of
the triazole rings.
on the thermal properties. For example, the polyester synthe-
sized from bis(butynyl) methylsuccinate and 1,4-diazidobu-
tane (x ¼ 4, where x is the number of methylene groups)
COO(CH2)2-triazole-(CH2)5-trialzole-(CH2)2]2-Me using semi-
empirical molecular orbital calculations14 to model the con-
formations of the corresponding click polyesters (runs 4
and 5). The three-dimensional structures of the energy-mini-
mized models (Fig. 3), indicate that the value of the main-
chain torsion angle is larger for the click polyester synthe-
sized from 1,5-diazidopentane (run 5), which produces a
larger overall free volume and inhibits intramolecular
stacking.
4
ꢂ
had higher Tg and Tm values (Mn ¼ 1.37 ꢁ 10 , Tg ¼ 14 C,
Tm ¼ 125 ꢂC, run 4, in Table 1) compared with poly(octa-
methylene methylsuccinate), which did not have a main-
chain triazole ring (Mn ¼ 2.2 ꢁ 104, Tg ¼ ꢃ53 ꢂC, Tm was
not observable). In contrast, the polyester synthesized from
1,5-diazidopentane (x ¼ 5, run 5 in Table 1) had values for
its thermal properties that were much smaller (Tg ¼ ꢃ4 ꢂC,
ꢂ
Tm ¼ 52 C).
ꢂ
ꢂ
The differences for DTg (18 C) and DTm (73 C) (runs 4 and
5) were greater than those usually effected by the even-odd
effect for Tgs [e.g., for poly(nonamethylene methylsuccinate),
Tg ¼ ꢃ61 ꢂC, DTg (8 ꢂC)].17 The click polyester synthesized
from 1,6-diazidohexane (x ¼ 6, run 6) had larger Tg (6 ꢂC)
and Tm (76 ꢂC) values than did that synthesized from 1,5-
diazidopentane (x ¼ 5). Zigzag curves with zigzag shapes
were observed for plots of Tg and Tm versus the methylene
chain length of the diazides (Fig. 2). Although an even-odd
effect is usually observed for the thermal properties of poly-
esters,2,17 the height difference between the highest and low-
est points is usually small for the zigzag plots of aliphatic
polyesters. Therefore, the introduction of main-chain triazole
rings enhanced the even-odd effect.
Energy minimizations were performed for the repeat units of
the click polyesters, that is, Me-[OCOCH(CH3)CH2COO(CH2)2-
triazole-(CH2)4-trialzole-(CH2)2]2-Me and Me-[OCOCH(CH3)CH2-
FIGURE 5 Thermogravimetric analysis under nitrogen flow,
with a heating rate of 10 ꢂC minꢃ1
.
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ARTICLE
CONCLUSIONS
In this article, we used dialkynes containing ester linkages
and diazides to synthesize, via click polymerization catalyzed
by Cu(I), polyesters with high Mns [(1.0–7.0) ꢁ 104] in excel-
lent yields. These polymers contained main-chain 1,4-disubti-
tuted triazoles. We found that the presence of the main-chain
triazoles improved the thermal properties of the polyesters
and magnified the even-odd effect caused by the of methyl-
ene chain length on the thermal properties of the polyesters.
In addition, we could fine tune the thermal properties of the
polyesters by changing the positions of triazoles. We also
demonstrated that it is possible produce the click polymers
via microwave irradiation and that, when used in conjunc-
tion with in situ azidation, the synthesis of a click polyester
is a safe and effective procedure that should find industrial
use. Our work is the first time that click chemistry has been
used to synthesize polyesters and our results should provide
the foundation for the synthesis of novel polyester-based
materials.
FIGURE 6 BOD curves for of poly(nonamethylene methylsucci-
nate), and the polyesters of runs 4 and 5, each mixed with acti-
vated sludge (pH, 7.5, 25 ꢂC).
This article is dedicated to the 2 years memory of the late
Tadamichi Hirabayashi (Nagoya Institute of technology). We
acknowledge the Nagoya Institute of Technology’s Research
Promotion Program.
Interestingly, when the number of intervening methylenes
between the triazole rings was an even value, for example,
run 4 (number of intervening methylenes: 4) and 14 (num-
ber of intervening methylenes: 6) (Table 1), a decrease in
the Tm value was found (DTm(run 14-run 4) ¼ ꢃ41 ꢂC) with
increase of the methylene number; whereas, when the
number of intervening methylenes was an odd value, for
example, runs 5 (number of intervening methylenes: 5) and
15 (number of intervening methylenes: 7), an increase in the
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