Cyclotrimerization of diisocyanates toward high-performance networked polymers with rigid isocyanurate structure: Combination of aromatic and aliphatic diisocyanates for tunable flexibility

Article abstract of DOI:10.1002/pola.26651

A series of networked polymers bearing isocyanurate moiety was synthesized by cyclotrimerization of diisocyanates, with employing methylenediphenyl 4,4′-diisocyanate and 1,6-hexamethylenediisocyanate (HMDI) in several feed ratios. In spite of the large difference in intrinsic reactivity between these two diisocyanates, their coannulation proceeded efficiently by using sodium p-toluenesulfinate (pTolSO₂Na) and 1,3-dimethyl-2-imidazolidinone as a catalyst and a solvent, respectively. The resulting networked polymers were transparent and exhibited excellent thermal stability. In addition, HMDI-rich feed ratios allowed for the formation of networked polymers with increased flexibility.

Full text of DOI:10.1002/pola.26651

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Cyclotrimerization of Diisocyanates Toward High-Performance Networked
Polymers with Rigid Isocyanurate Structure: Combination of Aromatic
and Aliphatic Diisocyanates for Tunable Flexibility
Masaki Moritsugu, Atsushi Sudo, Takeshi Endo
Molecular Engineering Institute, Kinki University, Iizuka, Fukuoka, 820-8555, Japan
Correspondence to: T. Endo (E-mail: tendo@moleng.fuk.kindai.ac.jp)
Received 10 November 2012; accepted 22 February 2013; published online 26 March 2013
DOI: 10.1002/pola.26651
ABSTRACT: A series of networked polymers bearing isocyanu-
rate moiety was synthesized by cyclotrimerization of diisocya-
nates, with employing methylenediphenyl 4,40-diisocyanate
and 1,6-hexamethylenediisocyanate (HMDI) in several feed
ratios. In spite of the large difference in intrinsic reactivity
between these two diisocyanates, their coannulation pro-
ceeded efficiently by using sodium p-toluenesulfinate (pTol-
SO2Na) and 1,3-dimethyl-2-imidazolidinone as a catalyst and a
solvent, respectively. The resulting networked polymers were
transparent and exhibited excellent thermal stability. In addi-
tion, HMDI-rich feed ratios allowed for the formation of net-
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worked polymers with increased flexibility.
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KEYWORDS: cyclotrimerization; films; high performance poly-
mers; isocyanurate; networks; thermal properties
various monoisocyanate to the reaction system.1,2 However,
the addition of a large amount of monoisocyanate leads to
decrease in thermal stability and tensile strength.
INTRODUCTION The development of transparent and heat re-
sistant polymer materials is attracting much attention from
aerospace and electronics fields. Flexibility is also an impor-
tant property for polymer materials for next generation dis-
plays, solar cells, and e-paper. From these backgrounds,
recently, we have focused on the development of high-
performance networked polymers bearing isocyanurate moi-
eties.1,2 This cyclic structure has some advantageous features
involving its excellent thermal stability3–5 and its facile syn-
thesis by the cyclotrimetization of a wide range of isocya-
nates.6 So far, isocyanurate linkage has been frequently used
in polyurethane chemistry to enhance heat resistance, flame
retardation, and chemical resistance of polyurethanes.7 In
addition, there have been a few reports on cyclotrimetization
of diisocyanates that afforded the corresponding networked
polymers bearing isocyanurate groups.8–15 We reinvestigated
the cyclotrimerization conditions, leading to the achievement
of rapid and quantitative formation of isocyanurate by
employing sodium p-toluenesulfinate (pTolSO2Na) and 1,3-di-
methyl-2-imidazolidinone (DMI) as the optimized catalyst
and the solvent, respectively. Under the conditions, methyle-
nediphenyl 4,40-diisocyanate (MDI) underwent cyclotrimeri-
zation to afford the corresponding networked polymer that
was consisted of isocyanurate moieties with minimized
chemical defect,ꢀand thus was colorless, transparent, and sta-
ble up to >400 C. Furthermore, flexibility of the film-shaped
networked polymers was successfully attained by addition of
Herein, we report a dual-diisocyanate system for synthesiz-
ing networked polymers that exhibit excellent thermal stabil-
ity, transparency, and flexibility. The feature of this system is
the combination of an aromatic diisocyanate and an aliphatic
diisocyanate, and to the best of our knowledge, such a com-
bination is the first example in this area. The diisocyanates
employed herein were MDI and 1,6-hexamethylenediisocya-
nate (HMDI) which bear a rigid aromatic structure and a
flexible linear alkyl spacer, respectively.
EXPERIMENTAL
Materials and Instruments
p-Tolylisocyanate (TI) and propyl isocyante (PI) were pur-
chased from Tokyo Chemical Industry (Tokyo, Japan) and
distilled under reduced pressure. MDI and 1,6-hexamethyle-
nediisocyante (HMDI) were purchased from Wako Pure
Chemical Industry (Osaka, Japan) and distilled under
reduced pressure. Sodium p-toluenesulfinate (pTolSO2Na)
was purchased from Tokyo Chemical Industry (Tokyo,
Japan) and dried under reduced pressure prior to use. DMI
was purchased from Wako Pure Chemical Industry (Osaka,
Japan) and distilled under reduced pressure over calcium
hydride.
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Proton NMR spectra were recorded on Varian UNITY INOVA
400 in CDCl3 with TMS as an internal standard. Gas chroma-
tography–mass spectrometry (GC–MS) analysis was carried
out on a Shimadzu GC–MS QP5050 machine with helium as
the carrier gas. The GC column was a RTX-5SilMS (30 m 3
0.25 mm ID with a coated film thickness of 0.25 mm). The
Synthesis of Networked Polymers Based on
Cyclotrimerization of MDI and HMDI
A Typical Procedure
In a dry test tube equipped with a three-way stopcock, MDI
(3.0 mmol, 0.75 g), HMDI (3.0 mmol, 0.48 mL), DMI
(0.25 mL), and a solution of pTolSO2Na (11 mg, 0.060 mmol
[0.5 mol % to NCO groups]) in DMI (0.75 mL) were mixed.
Within 1 min, the resulting solution was put into a mold
(60 mm 3 40 mm 3 0.10 mm), which was fabricated with
two pieces of PTFE-tape-coated slide glass and PTFE spacer
(thickness, 100 mm), placed in a glove box at 25 ꢀC under
nitrogen for 2 h. After 2 h, the mixture in a mold was contin-
uously heated at 100 ꢀC for 1 h and at 150 ꢀC for 1 h. The
cured film was taken out from the mold, immersed in water
at 25 ꢀC for 12 h, and dried under vacuum at 150 ꢀC for
ꢀ
GC furnace temperature initially was held at 60 C for 1 min
and next was programmed to 300 ꢀC and held for 15 min;
then, it was programmed to 300 ꢀC at 20 ꢀC/min. The GC/
MS interface was set at 280 ꢀC. All samples were dissolved
in dichloromethane before injection. IR spectra were
recorded on a Thermo Scientific NICOLET iS10 FTIR spec-
trometer equipped with a SMART iTR ATR sampling acces-
sory. The features of the cross-section of the films were
investigated by a scanning electron microscope (SEM, JEOL
JSM-6010LA). Differential scanning calorimetry (DSC) was
carried out with a DSC-6200 (Seiko Instrument) using alumi-
num pan under an N2 flow of 20 mL/min at the heating rate
of 5 ꢀC/min. Thermal gravimetric analysis (TGA) was per-
formed with a TG-DTA 6200 (Seiko Instrument) using alu-
mina pan under an N2 flow of 50 mL/min at a heating rate
overnight: FTIR (ATR): 1696, 1506, 1391, 1018, 754 cm21
.
Synthesis of Networked Polymers Based on
Cyclotrimerization of HMDI
In
a 25-mL flask equipped with a condenser, HMDI
(3.0 mmol, 0.48 mL), pTolSO2Na (5.3 mg, 0.030 mmol
[0.5 mol % to NCO groups]), and DMI (1.0 mL) were added
under dry conditions, and the mixture was heated and
stirred at 150 ꢀC for 2 h. After reaction, the mixture was
cooled to room temperature, and the conversion of the iso-
cyanate groups was determined by FTIR analysis. The result-
ing solid was grinded and washed with chloroformꢀ in a
Soxhlet’s extractor, and dried in a vacuum oven at 150 C for
overnight to yield the networked polymer 5 (0.48 g, 94%):
ꢀ
of 10 C/min.
Cyclotrimerization of TI
To a solution of pTolSO2Na (0.03 mmol, 0.053 g) in DMI
(1.0 mL), TI (6.0 mmol, 0.76 ml) was added under dry con-
dition. After stirring the solution at 25 ꢀC for 2 h, purifica-
tion of the resulting precipitate by column chromatography
(silica gel, eluted with dichloromethane) gave N,N0,N00-tris-
(p-tolyl)isocyanurate (0.77 g, 5.8 mmol, 97%) as a white
FTIR (ATR): 2933, 2860, 1674, 1454, 1373, 1327, 761 cm21
.
ꢀ
powder: Mp 5 270 C.
RESULTS AND DISCUSSION
1H NMR (CDCl3): d 5 7.23 (s, 12H), 2.32 ppm (s, 9H); FTIR
(ATR): 1694, 1511, 1401, 806, 748 cm21
.
Cyclotrimerization of TI and n-PI as a Model Reaction
Prior to performing cyclotrimerization of diisocyanate, we
investigated cyclotrimerization of TI and PI as a model reac-
tion (Scheme 1). Recently, we have reported a highly efficient
cyclotrimerization of TI with employing sodium p-toluenesul-
finate (pTolSO2Na) and DMI as the optimized catalyst and
the solvent, respectively.1 With 0.5 mol % of pTolSO2Na, the
cyclotrimerization completed at room temperature within
2 h to give the corresponding isocyanurate was isolated in
97% yield. In contrast to this successful cyclotrimerization
of an aromatic isocyanate, it was much less efficient in ali-
phatic. The cyclotrimerization of PI under the optimum con-
ditions for that of TI did not proceed at room temperature,
Cyclotrimerization of PI
To a solution of pTolSO2Na (0.015 mmol, 1.8 mg) in DMI
(0.3 mL), PI (3.0 mmol, 0.28 mL) was added under dry con-
dition. After stirring the solution at 80 C for 24 h, the mix-
ture was cooled to room temperature. Purification of the
mixture by column chromatography (silica gel, eluted with
ꢀ
dichloromethane)
gave
N,N0,N00-tris-(propyl)isocyanurate
(0.20 g, 2.3 mmol, 78%) as colorless liquid.
1H NMR (CDCl3): d 5 3.85 ppm (t, 6H), 1.66 ppm (m, 6H),
0.94 ppm (t, 9H); FTIR (ATR): 2963, 1678, 1452, 1381,
761 cm21
.
Cyclotrimerization of TI and PI
To a solution of pTolSO2Na (0.060 mmol, 11 mg) in DMI
(0.50 mL), TI (3.0 mmol, 0.38 mL) and PI (3.0 mmol,
0.28 mL) were added under dry condition. The mixture
was stirred at 80 ꢀC for 5 h. After stirring the solution
at 80 ꢀC for 5 h, the mixture was cooled to room tem-
perature. Purification of the mixture by column chroma-
tography (silica gel, eluted with dichloromethane) gave
the mixture of four isocyanurate compounds of quantita-
tive yield.
SCHEME 1 Cyclotrimerization of isocyanates.
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promoting effect became much more remarkable, leading to
the quantitative consumption of PI within 5 h. It is worth
emphasizing again that it took 24 h for 90% conversion of
PI in the absence of TI. As shown in Figure 2, it is apparent
that TI acts as a role of accelerator for the cyclotrimerization
of PI. In the dual system of TI and PI, the sulfinate anions as
a catalyst preferentially attack to the more electrophilic TI in
the initial state, leading to the formation of intermediates
such as 1:1 or 1:2 adducts of sulfinate with TI. It is seen
that these intermediates having the higher nucleophilicity
can assist progress of the cyclotrimerization of PI.
For getting more insight into the system, the reaction prod-
ucts were analyzed by GC/MS. Figure 3 shows the obtained
total ion count chart. The peaks a and d, whose correspond-
ing m/z values were 255 and 399, were attributable to the
cyclotrimerization product of PI and that of TI, respectively.
The other two peaks, whose corresponding m/z values were
303 and 351, were assigned to the isocyanurates formed by
the coannulation of PI and TI at ratios of 1:2 and 2:1,
respectively.
FIGURE 1 Time dependence of conversion for the cyclotrimeri-
zation of PI at 80 ꢀC. The conversions were determined by
1H-NMR.
and for the eꢀfficient progress of the reaction, a higher tem-
perature (80 C) and a prolonged reaction time (24 h) were
required (Figure 1). This significant difference in reactivity
between aromatic and aliphatic isocyanates is attributable to
the difference in their intrinsic electrophilicity, that is, the
more electrophilic aromatic isocyanate can accept the nucle-
ophilic attack of the sulfinate catalyst more easily than the
aliphatic isocyanate.
In summary, the model system investigated herein clarified
that aromatic and aliphatic isocyanates undergo the cyclotri-
merization not in an orthogonal manner in spite of the sig-
nificant difference in reactivity between them. Their ability
to undergo coannulation in a statistic manner allowed us to
expect that the dual system composed of aromatic and ali-
phatic diisocyanates described below can potentially give
networked polymers bearing isocyanurate cores formed by
coannulation of them, but not an interpenetrated network
formed by parallel progress of two independent cyclotrimeri-
zations of them.
Upon confirming the large difference between the efficiency
in cyclotrimerization between TI and PI, a cyclotrimetization
system where these two isocyanates coexist in a molar ratio
of 1:1 became our interest (Scheme 2). The system was
investigated at room temperature and 80 ꢀC, and the corre-
sponding time dependencies of conversions of the two iso-
cyanates are shown in Figure 2(a,b). The reaction behaviors
thus clarified involve several characteristic features as fol-
lows. (1) At room temperature, TI was smoothly consumed;
however, the consumption became much slower than that in
the absence of PI, presumably owing to the dilution with PI.
(2) Even at room temperature, a noticeable amount of PI
was consumed, to imply that the presence of TI caused a
certain promoting effect Figure 2(a). (3) At 80 ꢀC, this
Synthesis of Networked Polymers by a Dual System
Composed of Aromatic and Aliphatic Diisocyanates
With employing the combination of pTolSO2Na and DMI as a
catalyst and a solvent, respectively, we next investigated syn-
thesis of networked polymers with using MDI and HMDI as
an aromatic and an aliphatic diisocyanates, respectively
(Scheme 3). To obtain the networked polymers as films, the
reactions were performed in a thin mold. Mixtures of the iso-
cyanates in several feed ratios, catalyst, and solvent were
prepared and each of them was poured into the mold, left to
SCHEME 2 Coannulation of TI and PI.
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FIGURE 2 Time dependencies of conversion for the coannulation of TI (circle) and PI (square) at (a) room temperature and
(b) 80 ꢀC. The conversions were determined by 1H-NMR.
FIGURE 3 GC profile of crude product obtained by coannulation of TI and PI at 80 ꢀC.
SCHEME 3 Synthesis of the networked polymer by coannulation of MDI and HMDI.
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TABLE 1 Thermal Properties of the Networked Polymers 1–5
Obtained by Coannulation of MDI and HMDI
[MDI]0/
a
a
[HMDI]0
Product (mol ratio) (ꢀC)
Td5
Td10
(ꢀC)
Char
yieldb (%)
c
Tg (ꢀC)
1
1e
3.0/0
455
447
452
446
440
404
475
79
75
62
48
40
<5
N.D.d (50–300)
N.D.d (50–300)
N.D.d (50–300)
N.D.d (50–300)
N.D.d (50–300)
115
3.0/0
467
466
460
455
441
2
3.0/1.0
3.0/2.0
3.0/3.0
0/3.0
3
4
5
a
Determined by TGA under N2.
Chair yield at 500 ꢀC under N2.
Determined by DSC under N2.
Not detected.
b
c
d
e
The cyclotrimerization was carried out at room temperature (see ref. 1).
stand at room temperature for 2 h, heated at 100 ꢀC for
1 h, and finally heated at 150 ꢀC for 2 h to obtain the cor-
responding networked polymers as films (Table 1). The
experiments involve the cyclotrimerization of MDI and that
of HMDI, which gave the corresponding networked poly-
mers as comparatives. In the case of HMDI, the homogene-
ous networked polymer film could not be prepared because
of the formation of foam owing to a drastic exotherm
occurred during reaction. Instead, the cyclotrimerization of
HMDI was performed in a test tube. Figure 4 shows IR
spectra of MDI, HMDI, and the films fabricated by coannu-
lation of MDI and HMDI in three different feed ratios and
just taken from the mold. As shown in Figure 4(a,b), the
strong absorption attributable to isocyanate was observed
at 2250 cm21. After the reaction, this absorption became
negligible [Fig. 4(c–g)] to imply that the isocyanates were
consumed completely. Surprisingly, the cyclotrimerization of
HMDI proceeded more rapidly and quantitatively compared
with the model reaction. This is based on that fact that the
local concentration of isocyanate groups increased during
the formation of the network structure.
FIGURE 4 FTIR spectra of (a) MDI, (b) HMDI, the networked
polymers (c) 1, (d) 2, (e) 3, (f) 4, and (g) 5 by coannulation of
MDI and HMDI.
After the IR analysis, the products were immersed in water
to remove water-soluble pTolSO2Na and DMI and dried at
ꢀ
150 C in a vacuum oven. Figure 5(a) shows the photographs
of the film of networked polymer 2 and thus obtained as a
representative example. All the films obtained in this study
were colorless and transparent. The film of the networked
polymer 1 obtained by the cyclotrimetization of MDI was
rather brittle presumably owing to its relatively rigid struc-
ture. On the other hand, the networked polymer films 2–4
obtained by the combination of MDI and HMDI were flexible
owing to the presence of alkyl chains. In addition, the frac-
ture of the films was not observed even under bending load
FIGURE 5 Photographs of (a) the networked polymer film 2 and (b) film 2 under a bending load.
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FIGURE 8 DSC curves of the networked polymers 1–5.
FIGURE 6 SEM image of cross-section of networked polymer
film 2.
as shown in Figure 5(b). Figure 6 shows the SEM image of
the cross-section of the networked polymer 2, which did not
indicate any recognizable phase separation.
The Tg of the networked polymer 1 was not observed in a
range from 50 to 300 ꢀC, implying that the motion of the
polymer chains was strictly restricted owing to the rigid
structure of these networked polymers. On the other hand,
the networked polymer 5 bearing flexible alkyl chains exhib-
ited grass transition at 115 ꢀC. For the networked polymers
2–4, Tgs were not observed by DSC in a temperature range
from 50 to 300 ꢀC, suggesting that the present MDI–HMDI
dual system involved statistic coannulation of MDI and
HMDI as observed in the model system with the use of cor-
responding monofunctional isocyanates, TI and PI.
The networked polymers were analyzed by TGA and DSC
to investigate their thermal stability. Figures 7 and 8 show
the resulting thermograms. The corresponding Td5 and Td10
(5 and 10% weight loss temperature, respectively), char
yield at 500 ꢀC, and glass transition temperature (Tg) are
listed in Table 1. Previously, we have reported the Td5 and
Td10 values of a networked polymer obtained by cyclotri-
merization of MDI at room temperature, which were 447
and 467 ꢀC, respectively.1 In this case, the conversion of
isocyanate determined by IR analysis was 97%. In compari-
son with this, the thermal stability of the networked poly-
CONCLUSIONS
Coannulation of TI and PI proceeded efficiently by using
sodium pTolSO2Na and DMI as a catalyst and a solvent,
respectively. Based on this finding, networked polymers
were synthesized by a MDI–HMDI dual-diisocyanate system.
The resulting products were selectively consisted of isocya-
nurate moiety and thus exhibited transparency and excellent
thermal stability. Utilization of HMDI with varying feed ratio
allowed successful control of flexibility of the networked
polymer films, without serious deterioration of thermal sta-
bility. These networked polymers consisting of isocyanurate
moieties are expected to be used as a high-performance ma-
terial in a wide range of applications.
mer
1 obtained in this study was slightly improved,
presumably because of the quantitative conversion of iso-
cyanate attained by the higher reaction temperature. The
Td5 values of the networked polymers 2–4 obtained by the
MDI–HMDI dual system were ꢁ440 ꢀC although those
became lower accordingly to the increase in the initial con-
tent of HMDI.
REFERENCES AND NOTES
1 M. Moritsugu, A. Sudo, T. Endo, J. Polym. Sci. Part A: Polym.
Chem. 2011, 49, 5186–5191.
2 M. Moritsugu, A. Sudo, T. Endo, J. Polym. Sci. Part A: Polym.
Chem. 2012, 50, 4365–4367.
3 H. E. J. Reymore, P. S. Carleton, R. A. Kolakowski, A. A. R.
Sayigh, J. Cell. Plast. 1975, 11, 328–344.
4 C. L. Wang, D. Klempner, K. C. Frisch, J. Appl. Polym. Sci.
1985, 30, 4337–4344.
FIGURE 7 TGA curves of the networked polymers 1–5.
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5 P. I. Kordomenos, J. E. Kresta, Macromolecules 1981, 14,
1434–1437.
10 D. W. Duff, G. E. Maciel, Macromolecules 1991, 24, 651–658.
11 S. Dabi, A. Zilkha, Eur. Polym. J. 1980, 16, 831–833.
12 S. Dabi, A. Zilkha, Eur. Polym. J. 1981, 17, 35–40.
13 S. Dabi, A. Zilkha, Eur. Polym. J. 1982, 18, 549–553.
6 I. C. Kogon, J. Am. Chem. Soc. 1956, 78, 4911–4914.
7 R. Samborska-Skowron, A. Balas, Polym. Adv. Technol. 2002,
13, 653–662.
ꢀ
ꢁ
14 V. Y. Kramarenko, T. A. Ezquerra, I. Sics, F. J. Balta-Calleja,
8 P. I. Kordomenos, J. E. Kresta, K. C. Frisch, Macromolecules
1987, 20, 2077–2083.
V. P. Privalko, J. Chem. Phys. 2000, 113, 447–452.
9 D. W. Duff, G. E. Maciel, Macromolecules 1990, 23, 3069–
3079.
15 V.-Y. Kramarenko, I. Alig, V. P. Privalko, J. Macromol. Sci.
Part B: Physics 2005, 44, 697–709.
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