Thermally stable Diels–Alder polymer using an azodicarbonyl compound as the dienophile

Article abstract of DOI:10.1016/j.polymer.2019.01.071

A novel Diels–Alder polymerization system was developed, using an azodicarbonyl compound as the dienophile. A bis(azodicarbonyl) monomer was prepared by quantitative oxidation of bis(diacylhydrazine) with ^tBuOCl in the presence of pyridine. Diels–Alder polymerization was carried out with a bisdiene monomer, using AgOTf as the catalyst. High thermal stability of the polymer is characteristic for the products of this Diels–Alder polymerization system, in which the Diels–Alder reaction is irreversible. The C=C bond in the Diels–Alder polymer can be cleaved by ozonolysis to give a polyketone.

Full text of DOI:10.1016/j.polymer.2019.01.071

Polymer167(2019)60–66
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Thermally stable Diels–Alder polymer using an azodicarbonyl compound as
the dienophile
Masayoshi Sakurai, Nobuhiro Kihara∗
Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, 259-1293, Japan
H I G H L I G H T S
Diels–Alder polymerization using an azodicarbonyl compound as the dienophile is effectively catalyzed by silver triflate.
Diels–Alder polymer obtained from the azodicarbonyl monomer is thermally stable.
C=C double bond in the Diels–Alder polymer can be cleaved without decrease of molecular weigh of the polymer.
•
•
•
A R T I C L E I N F O
A B S T R A C T
Keywords:
A novel Diels–Alder polymerization system was developed, using an azodicarbonyl compound as the dienophile.
A bis(azodicarbonyl) monomer was prepared by quantitative oxidation of bis(diacylhydrazine) with BuOCl in
the presence of pyridine. Diels–Alder polymerization was carried out with a bisdiene monomer, using AgOTf as
the catalyst. High thermal stability of the polymer is characteristic for the products of this Diels–Alder poly-
merization system, in which the Diels–Alder reaction is irreversible. The C=C bond in the Diels–Alder polymer
can be cleaved by ozonolysis to give a polyketone.
t
Diels–Alder polymerization
Azodicarbonyl compound
Ozonolysis
1. Introduction
responsive polymers, in which the response is reversible, have been
developed by using Diels–Alder polymerization [22–29]. A thermoset-
The Diels–Alder reaction is one of the most important addition re-
actions [1–6], and it is basically reversible. To shift the equilibrium in
favor of the reaction progressing, the Diels–Alder reaction is generally
carried out with an excess of either the diene or the dienophile. A de-
crease in the reaction temperature is also effective, although addition of
a Lewis acid catalyst is essential in this case. Diels–Alder polymerization
has also been studied [7–18]. However, as a result of the equilibrium
nature of the Diels–Alder reaction, Diels–Alder polymerization has at-
tracted less attention than other addition polymerizations, such as
polyurethane formation and the curing of epoxide resin. To attain
quantitative addition with stoichiometric amounts of the diene and
dienophile, electron-donating and electron-withdrawing groups are
introduced into the diene and dienophile, respectively, and the poly-
merization is carried out at a low temperature in the presence of a Lewis
acid catalyst. Combination with an irreversible reaction, such as ar-
omatization, is another method that has been effectively employed to
shift the equilibrium to the product side [19–21]. Due to the retro-
Diels–Alder reaction, Diels–Alder polymers without aromatization have
been recognized as thermally unstable polymers. Various thermo-
ting polymer crosslinked by the Diels–Alder reaction can change its
shape when heated [30]. Self-healing polymers can be obtained by
using Diels–Alder crosslinking [31,32].
Recently, we developed a quantitative Diels–Alder reaction by using
an azodicarbonyl compound as the dienophile (Scheme 1) [33]. Be-
cause the azodicarbonyl compound was an excellent dienophile, the
Diels–Alder reaction with a stoichiometric amount of 2,3-dimethylbu-
tadiene afforded the corresponding Diels–Alder product quantitatively,
in the presence of Hf(OTf)4 or AgOTf catalyst. Because of the high ef-
ficiency of this reaction, it is expected that this Diels–Alder reaction
system can be applied to Diels–Alder polymerization without the in-
troduction of any irreversible process. Furthermore, the N–N and C=C
bonds in the Diels–Alder product could be cleaved by SmI2 reduction
and ozonolysis, respectively [33]. Thus, the polymer obtained from the
Diels–Alder polymerization using an azodicarbonyl compound as the
dienophile could be converted into another functionalized polymer.
In this paper, we report a novel Diels–Alder polymerization system
using an azodicarbonyl compound as the dienophile moiety. It was
found that the resulting polymers exhibited high thermal stability, even
∗ Corresponding author.
E-mail address: kihara@kanagawa-u.ac.jp (N. Kihara).
https://doi.org/10.1016/j.polymer.2019.01.071
Received 30 November 2018; Received in revised form 21 January 2019; Accepted 27 January 2019
Availableonline28January2019
0032-3861/©2019ElsevierLtd.Allrightsreserved.

M. Sakurai, N. Kihara
Polymer167(2019)60–66
Scheme 1. Quantitative Diels–Alder reac-
tion using an azodicarbonyl compound as
the dienophile.
though they were obtained by a simple Diels–Alder polymerization.
Furthermore, C=C bond-cleavage of the obtained polymer afforded a
novel polymer that had carbonyl groups on the backbone.
evaporated, and the atmosphere was replaced with argon.
Dichloromethane was added to dissolve the azodicarbonyl compound 1,
and diene 6 (1.00 equiv) and AgOTf (0.20 equiv, 40 mol%) were added
to the resultant red solution. After being stirred for 1 d, the reaction
mixture was diluted with dichloromethane. The insoluble fraction was
removed by filtration, and the filtrate was concentrated in vacuo. The
residue was dissolved in tetrahydrofuran, and the solution was poured
into methanol. The precipitate was collected by filtration, washed with
methanol, and dried in vacuo to obtain polymer 11.
2. Experimental
2.1. Preparation of the bis(diacylhydrazine)
2.1.1. Dihydrazide 4
A solution of 3 (15.57 g, 55.9 mmol) and hydrazine monohydrate
(15.0 mL, 300.0 mmol) in ethanol (100 mL) was stirred for 1 d. After
removal of the volatiles in vacuo, the residue was recrystallized from
ethanol to obtain 4 (8.78 g, 35.1 mmol, 64%) as colorless crystals.
2.4. Oxidative scission of C=C bonds by ozone: general procedure
An oxygen stream containing ozone was passed into a solution of 11
in dichloromethane-methanol (4:1, v/v) at −98 °C for 15 min. After the
nitrogen stream had passed through for 5 min, tributylphosphane was
added to reduce the ozonide. After the mixture had been stirred for 1 h,
the solvent was evaporated; the residue was dissolved in tetra-
hydrofuran and poured into hexane. The precipitate was collected by
filtration, washed with hexane, and dried in vacuo to obtain polymer
12.
2.1.2. Bis(diacylhydrazine) 2
Compound 5 (7.1 mL, 39.1 mmol) was added dropwise to a solution
of 4 (4.14 mL, 16.5 mmol) and pyridine (3.0 mL, 37.0 mmol) in N,N-
dimethylformamide (DMF; 20 mL) at 0 °C. After being stirred for 1 d,
the reaction mixture was poured into diethyl ether. The precipitate was
collected by filtration, washed with diethyl ether, and dried in vacuo.
The crude product was recrystallized from DMF–ethanol to obtain 2
(7.41 g, 13.0 mmol, 78%) as colorless crystals.
3. Results and discussion
2.2. Preparation of the bisdiene
3.1. Monomers
2.2.1. Monobrominated sulfolene 8
Bis(azodicarbonyl) monomer 1 was synthesized by oxidation of the
corresponding bis(diacylhydrazine) 2, which was prepared by hy-
drazination of diester 3, followed by acylation with acid chloride 5, as
shown in Scheme 2. The introduction of the three tert-butyl groups in
the structures of 1 and 2 was necessary to make the monomer and
precursor soluble in organic solvents. Azodicarbonyl compounds are
not only thermally unstable but also highly reactive toward nucleo-
philes such as water. Therefore, bis(diacylhydrazine) 2 was oxidized
with excess tert-butyl hypochlorite in the presence of a catalytic amount
of pyridine, and bis(azodicarbonyl) 1 was used for polymerization, after
evaporation of the volatiles, without further purification [33].
Bisdiene monomer 6 was synthesized by using sulfolene 7 as a
protected butadiene, as shown in Scheme 3 [34,35]. Radical bromina-
tion of 7 afforded monobrominated sulfolene 8 in 82% yield. Alkylation
of dicarboxylic acids 9 with 8, followed by thermal decomposition of
the sulfolene group, afforded the bisdiene monomers in good yields.
Aromatic dicarboxylic acids, as well as an aliphatic one, were used as
the dicarboxylic acids.
A solution of 7 (2.8000 g, 19.15 mmol) and N-bromosuccinimide
(3.4829 g, 19.56 mmol) in chloroform (60 mL) was heated to reflux
under argon for 10 h. After evaporation of the volatiles, the residue was
dissolved in diethyl ether and the insoluble materials were filtered out.
The filtrate was concentrated in vacuo to give a brown oil (4.8827 g),
which was subjected to chromatography over silica gel (eluent: ether-
hexane-chloroform, 1/2/5) to furnish 8 (3.1540 g, 14.01 mmol, 74%) as
a white solid.
2.2.2. Bis(sulfolene) 10: typical procedure
A solution of 9a (0.89 g, 4.00 mmol), potassium carbonate (1.19 g,
8.61 mmol), and 8 (1.81 g, 8.02 mmol) in DMF (5.0 mL) was stirred for
1 d. After addition of water, the reaction mixture was extracted three
times with ethyl acetate. The organic layer was dried over magnesium
sulfate, filtered, and evaporated in vacuo. The residue was subjected to
chromatography over silica gel (eluent: ethyl acetate-hexane, 1/1) to
obtain 10a (1.40 g, 2.74 mmol, 68%) as a white solid.
2.2.3. Bis(diene) 6: typical procedure
3.2. Diels–Alder polymerization
A solution of 10a (0.32 g, 0.70 mmol) and hydroquinone (0.012 g,
0.11 mmol) in xylene (12.0 mL) was heated to reflux under argon for
15 h. After removal of the solvent, the residue was subjected to chro-
matography over silica gel (eluent: ether-hexane, 1/6) to obtain 6a
(0.21 g, 0.65 mmol, 93%) as a white solid.
Diels–Alder polymerization was first examined with bis(azodi-
carbonyl) 1 and bisdiene 6a as the monomers in dichloromethane.
When 2,3-dimethylbutadiene was used as the diene for the Diels–Alder
reaction with azodicarbonyl compounds, Hf(OTf)4 showed the best
catalytic activity, and the AgOTf catalyst also gave the Diels–Alder
product almost quantitatively [33]. However, when Hf(OTf)4 was used
as the Lewis acid catalyst for the Diels–Alder polymerization, no me-
thanol-insoluble part was obtained. It was supposed that the ester
groups coordinated with Hf(OTf)4 to deactivate it. Therefore, AgOTf
was used as the Lewis acid catalyst, and the polymer was obtained as a
2.3. Diels–Alder polymerization: general procedure
Tert-Butyl hypochlorite (1.5 equiv, 300 mol%) and pyridine (0.07
equiv, 0.14 mol%) were added to a suspension of 2 in dichloromethane
at 0 °C. After the mixture had been stirred for 1 h, the volatiles were
61

M. Sakurai, N. Kihara
Polymer167(2019)60–66
Scheme 2. Synthesis of the azodicarbonyl monomer.
methanol-insoluble part (Scheme 4). The polymerization conditions
were examined. The results are summarized in Table 1.
to the product side. When the polymerization was carried out from 0 °C
to room temperature, the polydispersity index (PDI) of polymer 11a
ranged from 1.72 to 2.54, the expected values for polyaddition.
It was found that the signals in the 1H NMR spectrum of 11a were
very broad (Fig. S19), and no structural information was obtained from
them. It has been reported that the 1,2-diacyl-1,2,3,6-tetra-
hydropyridazine derivative exhibits very broad 1H NMR signals [33].
The structure of 11 was therefore elucidated by using the 13C NMR
spectrum. Fig. 1 shows the 13C NMR spectra of 2, 6a, and 11a. Because
the 13C NMR spectrum of 11a contained very broad peaks at room
temperature, the spectrum measured at 80 °C is shown in Fig. 1.
The characteristic carbon signals (I, J, K, and L) of the butadiene
moiety, observed in the spectrum of 6a, disappeared in the spectrum of
11a. Instead, very broad signals that can be assigned to methylene
groups (J′ and L′) appeared at around 50 ppm. At the same time, the
signals for the aliphatic carbon atoms in 6a, attached to the butadiene
system (H and M), shifted to the higher field (H′ and M′), and those for
the carbonyl carbon atoms of the diacylhydrazine moiety shifted to the
lower field (g′ and h′) in the spectrum of 11a. These changes indicate
the progress of the Diels–Alder reaction to form the 1,2-diacyl-1,2,3,6-
tetrahydropyridazine moiety. The 13C NMR spectra of 1,2-bis(4-tert-
butylbenzoyl)-1,2,3,6-tetrahydropyridazine at different temperatures
First, the Diels–Alder polymerization was carried out in di-
chloromethane in the presence of 40 mol% AgOTf. The characteristic
red color of the azodicarbonyl group disappeared after 24 h. When the
polymerization was carried out for 24 h or 48 h, no change was ob-
served in the Mn value. Because of the low thermal stability of the
azodicarbonyl group, decomposition of the azodicarbonyl group ac-
companies the polymerization, and the polymerization stopped within
24 h. Therefore, further polymerizations were carried out for 24 h, and
the effect of the polymerization temperature was examined. In general,
the yield and Mn value of the polymer increased when the reaction
temperature was increased. However, when polymerization was carried
out under reflux, the yield and Mn value decreased, probably because of
thermal decomposition of azodicarbonyl monomer 1. The best result
was obtained when the polymerization was initiated at 0 °C and the
flask was gradually warmed to room temperature. When the amount of
AgOTf was reduced to 20 mol%, no methanol-insoluble part was ob-
tained because of slow polymerization. However, an increase in the
amount of AgOTf to 100% did not considerably affect the result. The
concentration of the monomers also had little effect, which indicates
that the equilibrium of the Diels–Alder reaction was completely shifted
Scheme 3. Synthesis of the bisdiene monomers.
62

M. Sakurai, N. Kihara
Polymer167(2019)60–66
Scheme 4. Diels–Alder polymerization.
Table 1
Diels–Alder polymerization of 1 and 6a.
Yielda (%)
Mn (Pn)b
Mw/Mn
b
Entry
Lewis acid (mol%)
Conc. (M)
Temperature (°C)
1
2
3
4
5
6
7
8
Hf(OTf)4 (40)
AgOTf (40)
AgOTf (40)
AgOTf (40)
AgOTf (40)
AgOTf (100)
AgOTf (20)
AgOTf (40)
0.3
0.3
0.3
0.3
0.3
0.3
0.3
1
0
−20
0
0 → r. t.
reflux
0 → r. t.
0 → r. t.
0 → r. t.
0
59
63
80
40
73
0
7700 (16.2)
8000 (16.9)
9900 (20.9)
6600 (13.9)
12000 (25.3)
1.43
1.46
2.54
1.30
1.72
80
9800 (20.6)
2.20
a
MeOH-insoluble part.
Estimated by GPC (THF, PSt standards).
b
are shown in Figs. S1, S2, and S3. All signals were observed as a pair of
peaks at room temperature. At higher temperature, the peaks became
broad and coalesced. At 120 °C, most signals were observed as single
peaks. However, the signal for J′ (L′) was still broad, even at 120 °C.
Such characteristic behavior in the 13C NMR spectrum can be explained
by slow conversion between s-cis/s-trans conformations because the
rotation of two vicinal acyl groups on a rigid cyclic structure requires
high activation energy. The signals of J′ and L′ are observed as very
broad separated peaks, even at 80 °C, which indicates the formation of
the 1,2-diacyl-1,2,3,6-tetrahydropyridazine structure. Furthermore,
some signals were observed as multiple peaks, indicating the presence
of 1,3- and 1,4-addition structures.
The Diels–Alder polymerization of 1 with various diene monomers
was examined using AgOTf as the catalyst (Scheme 5). The results are
Fig. 1. 13C NMR spectra of (a) 2 (150 MHz, DMSO‑d6), (b) 6a (150 MHz, DMSO‑d6), and (c) 11a (150 MHz, DMSO‑d6 at 80 °C).
63

M. Sakurai, N. Kihara
Polymer167(2019)60–66
Scheme 5. Diels–Alder polymerization using various bisdiene monomers.
Table 2
Table 4
Diels–Alder polymerization of 1 and 6.a
Ozonolysis of polymer 11.a
c
6
Yieldb (%)
Mn (Pn)c
Mw/Mn
11
Mn of 11b
12
b
b
A
B
C
D
E
80
84
86
65
78
82
65
9900 (20.9)
8300 (18.6)
10000 (20.6)
10000 (20.6)
5600 (11.4)
8800 (17.9)
14000 (32.1)
2.54
1.75
2.54
1.80
1.57
1.81
1.67
Yield (%)
Mn
Mw/Mn
a
b
c
d
e
f
9900
8300
10000
10000
8800
100
100
100
100
100
100
12000
9000
9900
9800
7500
12000
1.77
1.65
2.23
1.71
1.82
1.56
ed
F
14000
a
Polymerization was carried out in dichloromethane from 0 °C to r.t. for 24 h
a
Ozonolysis was carried out in dichloromethane-methanol at −98 °C for
15 min. After a nitrogen stream had been passed through the mixture for 5 min,
tributylphosphine was added.
in the presence of 40 mol% AgOTf. Monomer 1 was prepared from 2 by oxi-
dation with excess tert-butyl hypochlorite in the presence of pyridine and was
used after evaporation of the volatiles, without further purification.
b
b
Estimated by GPC (THF, PSt standards).
MeOH-insoluble part.
Estimated by GPC (THF, PSt standards).
In the presence of 100 mol% AgOTf.
c
d
3.3. Thermal properties
Table 3
Because the N=N double bond gives rise to high reactivity and
thermal instability of the azodicarbonyl compound, the 4,5-diazacy-
clohexene moiety produced by the Diels–Alder reaction is expected to
be stable enough for the thermal retro-Diels–Alder reaction to occur.
When NMP solution of 1,2-bis(4-tert-butylbenzoyl)-1,2,3,6-tetra-
hydropyridazine (Figs. S1–S3) was allowed to stand at 200 °C for 5 h, no
decomposition was observed. Therefore, the thermal properties of
polymer 11 were studied using TGA and DSC. A clear glass-transition
temperature was not observed in every case. The 5%-weight-loss tem-
peratures (Td5) of 11 are summarized in Table 3.
The Td5 values of polymer 11 were observed to be above 300 °C in a
nitrogen atmosphere, and above 200 °C even in air. The thermal sta-
bility of 11 is much higher than that observed for other Diels–Alder
polymers, which were characterized by thermal retro-Diels–Alder de-
polymerization. It was supposed that the retro-Diels–Alder reaction is
negligible for 11 because of the instability of the azodicarbonyl moiety.
5%-Weight-loss temperatures (Td5) of Diels–Alder polymer 11.a
11
Td5 (°C)
Residue at 500 °C
in N2
in N2
in air
in air
a
b
c
d
e
f
311
308
308
313
305
297
277
276
209
283
229
196
30
29
31
25
27
26
16
17
25
20
18
24
a
Measurements were carried out by using TGA (10 °C/min).
summarized in Table 2. Polymer 11, with Mn = 8000–10000 was ob-
tained in 70–85% yields. The PDI ranged from 1.54 to 2.54, indicating
that a typical polyaddition reaction had occurred. When 6e was used as
the monomer, the yield and Mn value increased when the amount of
AgOTf was increased.
3.4. Polymer reaction
It has been reported that 4,5-diazacyclohexene can be easily con-
verted into an α-aminoketone after cleavage of the N–N and C=C
Scheme 6. Ozonolysis of Diels–Alder polymer.
64

M. Sakurai, N. Kihara
Polymer167(2019)60–66
Fig. 2. IR spectra of 11a and 12a.
bonds by SmI2 reduction and O3 oxidation, respectively [33]. Because
of the 4,5-diazacyclohexene moiety introduced during the Diels–Alder
polymerization, polymer 11 can be converted into another functional
polymer. Because SmI2 reduction may cleave not only the N–N bond but
also the allyl ester moiety [36], the ozonolysis of 11 was examined.
Even though C=C bonds are present on the polymer backbone, it is
expected that the polymer structure will remain after C=C bond clea-
vage, because the C=C bonds are included in the cyclic structure.
Polymer 11 was treated with O3 at −98 °C in methanol-CH2Cl2; this
was followed by addition of Me2S to reduce the ozonide. No decrease in
the Mn value was observed, and the polymer was recovered almost
quantitatively in every case. These results proved that the C=C bond is
installed in the cyclic structure and that the polymerization occurred by
the Diels–Alder reaction. In the 13C NMR spectrum of the product, a
signal appeared at around 200 ppm, which is characteristic of a ketone
carbonyl carbon atom. However, some increase in the Mn and Mw/Mn
values was observed, indicating that a side reaction of the carbonyl
group, such as an aldol reaction and/or acetalization, occurred to some
extent during the reduction of the ozonide. A change of the reducing
agent was examined to suppress the side reaction, and it was found that
the increase in the Mn and Mw/Mn values could be suppressed when
nBu3P was used as the reducing agent. Therefore, ozonolysis of 11 was
carried out with nBu3P as the reducing agent (Scheme 6). The results are
summarized in Table 4.
catalyst for the Diels–Alder polymerization. High thermal stability of
the polymer is characteristic of this Diels–Alder polymerization system,
in which the Diels–Alder reaction is actually irreversible. The C=C
bond in the Diels–Alder polymer can be cleaved by ozonolysis to form a
polyketone.
Acknowledgment
We acknowledge financial support from the MEXT-Supported
Program for the Strategic Research Foundation at Private Universities,
2013–2017 (Creation of new fusion materials by integration of highly
ordered nano-inorganic materials and ultra-precisely controlled organic
polymers).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.polymer.2019.01.071.
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