Nonisocyanate route to 2,5-bis(hydroxymethyl)furan-based polyurethanes crosslinked by reversible diels–alder reactions

Full text of DOI:10.1002/pola.29418

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
RAPID COMMUNICATION
Nonisocyanate Route to 2,5-Bis(hydroxymethyl)furan-based Polyurethanes
Crosslinked by Reversible Diels–Alder Reactions
2
Lu Zhang ,1 Frederick C. Michel Jr ,1 Anne C. Co
1Department of Food, Agricultural and Biological Engineering, Ohio Agricultural Research and Development Center, The Ohio State
University, 1680 Madison Avenue, Wooster, Ohio 44691
2Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210
Correspondence to: A. C. Co (E-mail: co.5@osu.edu)
Received 10 April 2019; Revised 16 May 2019; accepted 24 May 2019
DOI: 10.1002/pola.29418
KEYWORDS: 2,5-bis(hydroxymethyl)furan; Diels–Alder reaction; nonisocyanate polyurethane; renewable polymers; transurethanization
On the other hand, significant amounts of BD could also be
removed by the N2 flow. To address this issue, BD was reacted with
dicarbamates to obtain hydroxyl-terminated dicarbamates as less
volatile diols before being used to react with the methoxycarbonyl-
terminated prepolymers. Finally, a series of BHMF-based NIPUs
were successfully synthesized from the transurethanization bet-
ween the previously synthesized methoxycarbonyl-terminated
prepolymers and the hydroxyl-terminated dicarbamates. After the
furan-bearing NIPUs were obtained, a bismaleimide was used as a
crosslinker to produce crosslinked NIPU networks by means of
reversible Diels–Alder reaction, providing the NIPU materials with
great potentials for developing eco-friendly PU products with recy-
clable, mendable, and self-healing properties. The overall procedure
for the synthesis of the BHMF-based NIPUs is outlined in Figure 1.
INTRODUCTION Polyurethane (PU) is a group of polymers that
have applications in a diverse range of industries. Current commer-
cial PU products are primarily produced from diisocyanates, which
are typically synthesized from highly toxic phosgene. In order to
avoid the safety risk associated with the use of phosgene, alternative
isocyanate-free routes have been put forward for the production of
nonisocyanate PUs (NIPUs) as replacement for conventional PUs.1
2,5-Bis(hydroxymethyl)furan (BHMF) is considered to be a promis-
ing biobased building block resembling aromatic monomers for
polymer productions, and there have been studies using BHMF as a
diol for the synthesis of conventional PUs.2–5 In this work, we dem-
onstrated the feasibility of using BHMF for the production of NIPUs
through an isocyanate-free route. A series of NIPUs were obtained
from the transurethanization between BHMF and dicarbamates,
which were synthesized from the methoxycarbonylation of
diamines with dimethyl carbonate (DMC). In addition to BHMF,
1,4-butanediol (BD) was also used to provide the NIPUs with good
balance of hardness and flexibility.
Three dicarbamates (Dicarbamate−C2, Dicarbamate−C6, and
Dicarbamate−C8) were obtained by reacting three diamines (eth-
ylenediamine, 1,6-hexanediamine, and 1,8-diaminooctane), respec-
tively, with an excess of DMC. Then, BHMF was reacted with the
dicarbamates at molar ratios of 1:2 and 1:4, respectively, to prepare
a series of methoxycarbonyl-terminated prepolymers. It was found
that Dicarbamate−C2 had no reactivity toward BHMF, probably
because the low chain flexibility of the two-carbon alkylated
dicarbamate gave rise to high viscosity, and thus increased its diffi-
culty to react with BHMF. On the other hand, Dicarbamate−C6 and
Dicarbamate−C8 showed similar reactivity toward BHMF, and over
90% of the alcohol groups of BHMF were substituted by the end of
the reactions, according to 1H NMR analysis. In the next step,
Dicarbamate−C6 and Dicarbamate−C8 were reacted with an excess
of BD, respectively, to obtain corresponding hydroxyl-terminated
dicarbamates as less volatile diols, and over 99% product purities
were obtained after purification according to the 1H NMR analysis.
One major challenge of using BHMF for NIPU synthesis is that
BHMF tends to undergo side reactions at high temperatures
(> 140 ꢀC). In our preliminary experiments, the NIPUs synthesized
ꢀ
from a one-step heating procedure (at 140 C for 3 h) were not
totally soluble in dimethyl sulfoxide (DMSO) due to the insoluble
byproducts generated from BHMF. To solve this problem, trans-
urethanization reactions were firstly carried out at low tempera-
tures (100–130 ꢀC) with excess of dicarbamates being introduced
in relation to BHMF, to obtain methoxycarbonyl-terminated
prepolymers. The prepolymers were then further reacted with
diols at higher temperatures (140–150 ꢀC) to produce NIPUs.
Another challenge lies in the volatility of BD, making it difficult to
accurately control the BD loadings at stoichiometric proportions.
Because of the reversibility of the transurethanization process, it is
necessary to remove the methanol generated during the reaction
by N2 flow continuously, in order to allow the reactions to proceed.
A series of BHMF-based NIPUs were synthesized from the
transurethanization between the previously synthesized
© 2019 Wiley Periodicals, Inc.
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2019
1

JOURNAL OF
POLYMER SCIENCE
RAPID COMMUNICATION
WWW.POLYMERCHEMISTRY.ORG
H
H
H
H
O
N
N
O
O
O
N
N
O
HO
R
R
HO
OH
OH
O
O
O
O
BD
Dicarbamate
BHMF
Dicarbamate
O
O
O
O
H
H
O
O
N
N
O
R
R
HO
R
OH
O
N
N
O
O
N
N
O
H
H
H
H
O
O
Hydroxyl-terminated dicarbamate
Methoxycarbonyl-terminated prepolymer
O
O
O
O
O
N
O
O
O
R
R
R
O
N
N
O
O
N
H
N
N
N
y
x
z
H
H
H
H
H
NIPU
O
O
O
N
N
N
O
O
O
O
Bismaleimide
Bismaleimide
O
O
O
O
O
O
N
N
O
O
O
O
Crosslinked NIPU
FIGURE 1 Procedure for the synthesis of BHMF-based NIPUs.
methoxycarbonyl-terminated prepolymers (prepared from
Dicarbamate−C6 and Dicarbamate−C8) and the hydroxyl-
terminated dicarbamates (prepared from Dicarbamate−C6 and
Dicarbamate−C8), respectively. In contrast, a series of aliphatic
NIPUs were also synthesized from the transurethanization
between the methoxycarbonyl- and hydroxyl-terminated dic-
arbamates (Dicarbamate−C6 and Dicarbamate−C8) without
using BHMF. The NIPUs with furan moieties were obtained as
brown solids, and those without furan moieties were obtained as
white solids at room temperature. All the NIPUs were insoluble
in water, chloroform, acetone, acetonitrile, and tetrahydrofuran
(THF), slightly soluble in hot methanol, and totally soluble in
DMSO and dimethylformamide (DMF).
groups at the end of the reactions might be attributed to the more
or less volatility of the hydroxyl-terminated dicarbamates. Thus,
the signal of the remaining terminal methoxycarbonyl groups were
used to estimate the NIPU yields. In general, higher BHMF to BD
molar ratios led to lower product yields, as a result of the lower
reactivity of BHMF toward the methoxycarbonyl groups (Table 1).
This lower reactivity could be explained the lower nucleophilicity
of the alcohol groups of BHMF, induced by the electron withdraw-
ing effect exerted on its carbonyl group. Besides product yields, the
molar contents of urethane, urea, BHMF, and BD repeating units in
1
the polymer chains could also be estimated according to the H
NMR spectra. It was worth noting that the NIPUs prepared from
Dicarbamate−C6 seemed to have higher urea contents, compared
to those from Dicarbamate−C8, especially without using BHMF,
presumably because the nitrogen of Dicarbamate–C6 had greater
nucleophilicity to attack on the carbonyl group of another
dicarbamate molecule (Table 1). It was also found that at a 1:6
molar ratio of BHMF to BD, the contents of BHMF incorporated
into the polymer chains were consistent with the BHMF loadings
in the feed, while at a 1:2 molar ratio, the BHMF contents in the
polymer chains were obviously lower than those in the feed
(Table 1). The 1H NMR results indicated that significant amounts
of furan moieties in the prepolymers were converted back to the
The NIPUs were characterized by 1H NMR analysis (Fig. 2). The 1H
NMR spectra confirmed the occurrence of the transurethanization
between the methoxycarbonyl and hydroxyl groups. In addition,
urea linkages were also observed presumably due to the trans-
urethanization between two methoxycarbonyl groups.6 The signals
corresponding to the terminal BHMF and BD groups were smaller
compared to those of the terminal methoxycarbonyl groups. The
inequivalent ratios of the terminal methoxycarbonyl and hydroxyl
2
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2019

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
RAPID COMMUNICATION
FIGURE 2 1H NMR spectrum of the NIPU synthesized from Dicarbamate−C6, BHMF, and BD at a molar ratio of 7:1:6.
TABLE 1 Chemical Composition of NIPUs
BHMF:BD
(molar ratio)
Samplea
Dicarbamate
Yieldb (%)
Xurethanec (%)
Xureac (%)
XBHMFc (%)
XBDc (%)
NIPU–C6–1/2
NIPU–C6–1/6
NIPU–C6–0/1
NIPU–C8–1/2
NIPU–C8–1/6
NIPU–C8–0/1
Dicarbamate−C6
Dicarbamate−C6
Dicarbamate−C6
Dicarbamate−C8
Dicarbamate−C8
Dicarbamate−C8
1:2
1:6
0:1
1:2
1:6
0:1
85
87
92
71
83
86
72
76
74
64
82
88
16
14
24
18
8
22
14
0
42
67
78
68
59
76
14
13
0
6
a Sample was denoted as NIPU–Cx–y, where x is the number of carbons
in the alkene chain of the dicarbamate that used, and y is the molar
ratio of BHMF to BD in the feed.
c Xurethane, Xurea, XBHMF, and XBD represented the molar contents of the
urethane, urea, BHMF, and BD repeating units in the NIPU chains calcu-
lated based on their characteristic signals in the 1H NMR spectra.
b Yield is determined based on signal of the methine protons of the ter-
minal methoxycarbonyl groups in the 1H NMR spectra.
starting BHMF monomer during the polymerization processes at
a high BHMF to BD ratio (1:2). Therefore, it was advised to
avoid high BHMF to BD ratios for the synthesis of BHMF-
based NIPUs using this procedure. The limited solubility of
the NIPUs in most solvents, such as THF, makes them difficult
to analyze by GPC. However, the 1H NMR results indicated
that the number average molecular weight (Mn) might be
around 2000–3000 g/mol for the NIPUs prepared at low
BHMF to BD ratios (1:6 and 0:1).
DSC analysis was performed to understand the thermal prop-
erties of the NIPUs (NIPU–C6–1/6, NIPU–C6–0/1, NIPU–
C8–1/6, and NIPU–C8–0/1). In their DSC heating curves, no
glass transition temperatures (Tg) were observed, while sharp
melting peaks were exhibited, indicating their semicrystalline
structures. The melting ꢀpoints (Tm) of these NIPUs appeared
to be similar (148–157 C) in spite of their different chemical
structures, whereas their melting enthalpy (ΔHm) seemed to
be related to the number of carbons in the alkene chains in
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2019
3

JOURNAL OF
POLYMER SCIENCE
RAPID COMMUNICATION
WWW.POLYMERCHEMISTRY.ORG
their monomer units. The NIPUs prepared from Dicarbamate−C8
generally showed higher ΔHm (51.7–57.3 J/g), compared to those
prepared from Dicarbamate−C6 (39.0–42.2 J/g). This could possi-
bly be explained by their higher crystallinity due to the higher
flexibility and lower free volume induced by their longer hydro-
carbon chains.7 Next, the thermal stability of these NIPUs was
assessed by TGA, and the results showed that all these NIPUs
were thermally stable above 200 ꢀC, with decomposition temper-
Linear
st
1
heating cycle
Linear
T
m
nd
2
heating cycle
ꢀ
atures at 5% weight losses (T5%) ranging from 214 to 224 C.
These T5% results were similar to those NIPU or conventional PU
analogues reported in the previous studies.8–10 Generally, the
BHMF-based NIPUs had relatively lower T5% (214–215 ꢀC),
compared to those prepared without BHMF (220–224 ꢀC).
Because of the low reactivity of BHMF toward the met-
hoxycarbonyl groups, it was reasonable for the urethane link-
ages formed with BHMF to exhibit relatively poor thermal
stability.11 Evenꢀso, the T5% of the BHMF-based NIPUs were
still at least 50 C higher than their Tm, demonstrating their
suitability for thermal processing.
Crosslinked
T
m
st
1
heating cycle
Retro Diels–Alder
Crosslinked
T
m
nd
2
heating cycle
The furan moieties within the polymer chains can react with a
bismaleimide by the Diels–Alder reaction to give reversible
crosslinked NIPUs, and the crosslinking and de-crosslinking of
the NIPUs can be seen by their solubility behavior. To observe
the change of solubility, NIPU–C6–1/6 and NIPU–C8–1/6 were
dissolved in DMSO, respectively, followed by the addition of
the bismaleimide. It was observed that the solutions started
to gel slowly at room temperature, indicating the formation of
crosslinks between their linear NIPU chains. After being
heated to 130 ꢀC, the gelled samples went back to solutions
immediately, because the Diels–Alder bonds started to open at
high temperatures. After being placed at room temperature
for a while, the solutions became gels again. In addition,
FT − IR analysis also confirmed the occurrence of the Diels
−Alder reaction, as well as the presence of some unreacted
maleimide groups.
T
m
50
100
Temperature (°C)
150
FIGURE 3 DSC thermograms of linear and crosslinked NIPUs
(NIPU–C6–1/6) in the first and second heating cycles. [Color
figure can be viewed at wileyonlinelibrary.com]
In conclusion, BHMF has been recognized as a promising bio-
based building block for the syntheses of various renewable
polymers. In this study, a series of NIPUs were synthesized using
BHMF, in combination with BD, through transurethanization
reactions with dicarbamates. The chemical structures and ther-
mal properties of these BHMF-based NIPUs were characterized
by a number of characterization techniques. These furan-bearing
NIPUs were then crosslinked using a bismaleimide by means of
reversible Diels–Alder reaction, and the thermo-reversibility of
these crosslinked NIPUs was demonstrated by their solubility,
FT − IR, and DSC analyses. In sum, this study provided an insight
into the production of PUs from a biobased furanic platform
chemical though a nonisocyanate route, coupled with the benefits
of reversible crosslinking. Future application of the BHMF-based
NIPUs could be related to the development of eco-friendly PU
products with recyclable, mendable, and self-healing properties.
DSC analysis was also performed on the crosslinked NIPUs to
further confirm the reversibility of the Diels–Alder reaction
ꢀ
(Fig. 3). Sharp endothermic peaks ascribed to the Tm (159 C)
of the crosslinked NIPUs were observed in the first and sec-
ond heating cycles in the DSC curves. In addition, a broad
endothermic peak ranging from 110 to 130 ꢀC was observed
in the first heating cycle, but absent in the second one. This
broad peak was likely to correspond to the cleavage of the
crosslinked bonds through the retro Diels–Alder reaction at
high temperatures, and this temperature range was in accor-
dance to those observed in previous literatures.12–16 The
enthalpy of the retro Diels−Alder reaction was measured to
be approximately 8–16 J/g with NIPU−C8–1/6 appearing to
have a stronger ability to form Diels−Alder crosslinks. Next,
TGA was used to investigate the thermal stability of the NIPUs
after the formation of crosslinks. The crosslinked NIPUs
showed slightly improved T5% (~8 ꢀC higher) compared to
their noncrosslinked counterparts. The final residue contents
of the crosslinked NIPUs stabilized at approximately 15%,
which was slightly higher than those of their noncrosslinked
counterparts (11%), presumably because the bismaleimide
generated more char residues.
EXPERIMENTAL
Three dicarbamates (Dicarbamate−C2, Dicarbamate−C6, and
Dicarbamate−C8) were obtained by reacting three diamines
(ethylenediamine, 1,6-hexanediamine, and 1,8-diaminooctane)
with an excess of DMC catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-
5-ene (TBD), respectively, according to the work of other
researchers.17,18 Typically, DMC (200 mmol), diamine (20 mmol),
and TBD (2 mmol) were loaded into a 250 mL flask and kept
stirring at 80 ꢀC for 6 h. After the reaction, the excess of DMC
was evaporated to obtain the crude dicarbamate product.
Dicarbamate−C6 and Dicarbamate−C8 were purified by washing
with water and filtration for several times followed by recrystalli-
zation from hot methanol. Dicarbamate−C2 was purified by
4
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2019

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
RAPID COMMUNICATION
washing with cold DMC for several times. The chemical struc-
tures and high purities (>99%) of the dicarbamates were con-
firmed by 1H NMR analysis. The yields of Dicarbamate−C2,
Dicarbamate−C6, and Dicarbamate−C8 were 50%, 50%, and
53%, respectively.
All the 1H NMR spectra were recorded with a Bruker ARX
−300 spectrometer (Bruker Corporation, Billerica, MA) oper-
ating at a frequency of 400 MHz. DMSO-d6 was used as a solvent
for 1H NMR analysis. All the FT−IR spectra were recorded with a
PerkinElmer Spectrum Two IR spectrometer (PerkinElmer, Wal-
tham, MA). DSC analysis was performed using a TA Q20 differen-
tial scanning calorimeter (TA Instruments, New Castle, DE). The
samples (~5 mg) were heated from room temperature to 180 ꢀC
(first heating scans), and then cooled to −50 ꢀC at a rate of
BHMF was reacted with the dicarbamates at molar ratios of 1:2
and 1:4, respectively, to prepare a series of methoxycarbonyl-
terminated prepolymers. Typically, BHMF (0.25 or 0.5 mmol)
and a dicarbamate (1.0 mmol) were introduced into a 50 mL
round-bottom flask. K2CO3 (0.1 mmol) was added as a catalyst,
and a small amount of DMSO (0.4 mL) was added as a solvent
to reduce the high viscosity of the reaction mixture. The flask
ꢀ
10 C/min, followed by second heating scans over the tempera-
ture range from −50 to 180 ꢀC at the same rate. Tm and ΔHm
were determined from the DSC thermograms obtained in the
second heating scans. TGA was performed using a TA Q50
thermogravimetric analyzer (TA Instruments). The samples
ꢀ
was progressively heated to 100 C and kept for 3 h, followed
by another heating process at 130 ꢀC for 3–6 h. During these
two heating processes, the reaction mixture was kept stirring
under a N2 atmosphere.
ꢀ
(~10 mg) wꢀere heated from room temperature to 550 C at
a rate of 10 C/min in a N2 atmosphere.
Dicarbamate−C6 and Dicarbamate−C8 were reacted with an
excess of BD, respectively, to obtain hydroxyl-terminated dic-
arbamates (bis(4-hydroxybutyl) hexane-1,6-diyldicarbamate and
bis(4-hydroxybutyl) octane-1,8-diyldicarbamate). Typically, BD
(50 mmol), a dicarbamate (5 mmol), and K2CO3 (1 mmol) were
introduced into a 50 mL round-bottom flask. The flask was pro-
gressively heated to 120 ꢀC and kept for 1.5 h. During the
heating process, the reaction mixture was kept stirring under a
N2 atmosphere. After the reaction, the hydroxyl-terminated
dicarbamate was precipitated in water and then collected via fil-
tration for several times. The chemical structures and high
purities (>99%) of the hydroxyl-terminated dicarbamates were
confirmed by 1H NMR. The yields of bis(4-hydroxybutyl)
hexane-1,6-diyldicarbamate, and bis(4-hydroxybutyl) octane-1,-
8-diyldicarbamate were 80% and 83%, respectively.
ACKNOWLEDGMENTS
This work was supported by the Department of Chemistry
and Biochemistry and the Department of Food, Agricultural
and Biological Engineering at The Ohio State University.
REFERENCES AND NOTES
1 O. Kreye, H. Mutlu, M. A. R. Meier, Green Chem. 2013, 15,
1431.
2 T. T. Truong, H. T. Nguyen, M. N. Phan, L. T. T. Nguyen,
J. Polym. Sci. Part A Polym. Chem. 2018, 56, 1806.
3 P. M. Visakh, S. Thomas, A. K. Chandra, A. P. Mathew,
Advances in Elastomers II: Composites and Nanocomposites;
Springer: Berlin, Germany, 2013, p. 171.
A series of BHMF-based NIPUs were synthesized through the
transurethanization between the methoxycarbonyl-terminated
prepolymers and hydroxyl-terminated dicarbamates at an equiva-
lent molar ratio of methoxycarbonyl to hydroxyl groups. Typically,
a methoxycarbonyl-terminated prepolymer and a hydroxyl-
terminated dicarbamate were introduced into a 50 mL round-
bottom flask. The flask was progressively heated to 150 ꢀC, and
kept for 3 h. During the heating process, the reaction mixture
was kept stirring, and N2 was continuously purged into the reac-
tor to simultaneously remove the methanol generated from the
transurethanization reaction. After heating, the NIPU was dried
in a vacuum oven at 80 ꢀC for 12 h to remove DMSO. In contrast,
a series of aliphatic NIPUs were also synthesized from the trans-
urethanization between the methoxycarbonyl- and hydroxyl-
terminated dicarbamates at a molar ratio of 1:1 using the same
procedure. The yields of the NIPUs were between 71% and 92%,
according to 1H NMR analysis.
4 C. Moreau, M. Naceur, A. Gandini, Top. Catal. 2004, 27, 11.
5 S. Boufi, A. Gandini, M. N. Belgacem, Polymer 1995, 36, 1689.
6 N. Kébir, M. Benoit, C. Legrand, F. Burel, Eur. Polym. J. 2017,
96, 87.
7 A. S. More, L. Maisonneuve, T. Lebarbé, B. Gadenne, C. Alfos,
H. Cramail, Eur. J. Lipid Sci. Technol. 2013, 115, 61.
8 Z. Mou, S. Feng, E. Y. X. Chen, Polym. Chem. 2016, 7, 1593.
9 Z. Mou, E. Y. X. Chen, ACS Sustain. Chem. Eng. 2016, 4, 7118.
10 C. Duval, N. Kébir, A. Charvet, A. Martin, F. J. Burel, Polym.
Sci. Part A Polym. Chem. 2015, 53, 1351.
11 Z. Shen, J. Zhang, W. Zhu, L. Zheng, C. Li, Y. Xiao, J. Liu,
S. Wu, B. Zhang, Eur. Polym. J. 2018, 107, 258.
12 K. Roos, E. Dolci, S. Carlotti, S. Caillol, Polym. Chem. 2016, 7,
1612.
13 Z. Karami, M. J. Zohuriaan-Mehr, A. Rostami, J. CO2 Util.
2017, 18, 294.
14 J. Li, G. Zhang, L. Deng, S. Zhao, Y. Gao, K. Jiang, R. Sun,
C. Wong, J. Mater. Chem. A 2014, 2, 20642.
The BHMF-based NIPUs (~0.5 g) were dissolved in DMSO
(1 mL), respectively, followed by the addition of a crosslinking
agent (1,10-(methylenedi-4,1-phenylene)bismaleimide) at an
equivalent molar ratio ꢀof furan to maleimide groups. The solu-
tions were kept at 25 C for 4 h, followed by a Soxhlet extrac-
tion in acetone at 60 ꢀC to eliminate DMSO.
15 L. Gu, Q. Y. Wu, J. Appl. Polym. Sci. 2018, 135, 46272.
16 X. Shen, X. Liu, J. Wang, J. Dai, J. Zhu, Ind. Eng. Chem. Res.
2017, 56, 8508.
17 P. Deepa, M. Jayakannan, J. Polym. Sci. Part A Polym. Chem.
2008, 46, 2445.
18 B. Liu, H. Tian, L. Zhu, J. Appl. Polym. Sci. 2015, 132, 1.
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2019
5