Diaziridyl Ether of Bisphenol A

Article abstract of DOI:10.1021/acs.macromol.8b00550

Increased complexities in applications involving curable materials virtually need new materials that can overcome the limitations of existing ones. Resins, the structure of which is based on bisphenol A backbone terminated with three membered N-heterocycles - aziridines - have been synthesized, and their thermal-curing performance in solution and solid state was evaluated by NMR and FT-IR spectroscopies, differential scanning calorimetry, and single lap shear strength test and compared with that of analogous epoxy resin (diglycidyl ether of bisphenol A; DGEBA). Results reveal that the chemical reactivity of the aziridine-based resins is fine-tunable by controlling the N-substituent of aziridine. These resins can undergo ring-opening polymerization in the presence of various curing agents under unprecedentedly mild conditions and show remarkably rapid curing rate, wide substrate scope, and excellent chemoselectivity as compared to the analogous epoxy resin. Our results demonstrate superb curing ability of aziridine, making it promising for applications in materials and polymer sciences.

Full text of DOI:10.1021/acs.macromol.8b00550

Article
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Diaziridyl Ether of Bisphenol A
Seohyun Kang, Hyun Kyung Moon, and Hyo Jae Yoon*
Department of Chemistry, Korea University, Seoul 02841, Korea
S
* Supporting Information
ABSTRACT: Increased complexities in applications involving
curable materials virtually need new materials that can overcome
the limitations of existing ones. Resins, the structure of which is
based on bisphenol A backbone terminated with three
membered N-heterocyclesaziridineshave been synthesized,
and their thermal-curing performance in solution and solid state
was evaluated by NMR and FT-IR spectroscopies, differential
scanning calorimetry, and single lap shear strength test and
compared with that of analogous epoxy resin (diglycidyl ether of
bisphenol A; DGEBA). Results reveal that the chemical
reactivity of the aziridine-based resins is fine-tunable by
controlling the N-substituent of aziridine. These resins can
undergo ring-opening polymerization in the presence of various
curing agents under unprecedentedly mild conditions and show remarkably rapid curing rate, wide substrate scope, and excellent
chemoselectivity as compared to the analogous epoxy resin. Our results demonstrate superb curing ability of aziridine, making it
promising for applications in materials and polymer sciences.
tosyl and mesyl for the N-substituent undergoes ring-opening
reaction under basic conditions.22,23 On the other hand, ring-
opening reaction of aziridine with an electron-donating group
(EDG) such as the alkyl moiety for N-substituent barely
proceeds until it is activated by a Lewis acid.24,25 Aziridines
have been utilized mainly for synthesis of small molecules with
complex structures through regio-26,27 and enantioselective28,29
ring-opening reaction of aziridine, and click chemistry was
achieved by conjugate reaction of aziridine.30,31 A limited
number of literatures have also reported ring-opening polymer-
ization (ROP) of aziridine monomers, and these often relied on
N-EWG-aziridine derivatives.32−34 Despite a fairly long history
of aziridine in organic synthesis, the ability of aziridine for
curable materials has been rarely established.
Although there are a few aziridine-based cross-linkers
available in the market (see the Supporting Information for
their structures), they are often limited to carboxylic acid
substrates,35 and rather surprisingly, little has been investigated
regarding whether aziridine can indeed offer the ability as
curing resin for surpassing conventional epoxy. In this work, we
wish to provide detailed information on properties of aziridine-
based thermal cure resins in a chemistry point of view and
thereby gauge the potential of aziridine for the development of
resin by replacing epoxide in a prevalent epoxy material with
aziridine and comparing each other in the context of curing
performance. A representative epoxy resin in polymer industry
and materials science is diglycidyl ether of bisphenol A
INTRODUCTION
■
The field of curable materials is highly multidisciplinary and
competitive given a wide range of applications ranging from
surface modification1 and adhesive2 to dielectric films3 and
nanomaterials.4 From a chemistry perspective, epoxy is arguably
one of the successful curable materials and has been the focus
of the majority of research in the field.5−7 Figure 1a summarizes
some examples for utility of epoxy.8−11 Epoxy materials,
however, do face certain limitations, and intense research is
taking place to develop new curable materials with improved
properties.12,13 In recent years, structurally modified epoxy
resins have been developed to enhance curing rate,14 reduce
reaction temperature,15 and improve chemoselectivity16 and
substrate scope.17 All of these properties are desirable for
resolving the challenges in epoxy-related materials, and
although these studies are early and limited in number, they
highlight the opportunities that arise when the chemical
structure of epoxide, a core functional group in epoxy, is
further modified.
Many problematic features in epoxy materials rely on
intrinsic chemical properties of epoxide. Engineering chemical
properties of epoxide is limited because of divalent bonding
nature of oxygen. A neighbor compound of epoxide, aziridine
(Figure 1b)three-membered N-heterocycleshould prove
even more useful for improving and tailoring chemical
properties of curable materials and perhaps leads to curing
ability that cannot be achieved by epoxy. Specifically, aziridine
is based on trivalent nitrogen and can offer a unique
opportunity to tune the chemical property through rational
design of the N-substituent.18−21 For example, aziridine
possessing an electron-withdrawing group (EWG) such as
Received: March 14, 2018
Revised: May 11, 2018
© XXXX American Chemical Society
A
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Figure 1. (a) Examples of epoxy resins for curable materials where epoxide plays a key role in curing performance (AGE: allyl glycidyl ether; mPDA:
m-phenylenediamine; DGEBA: diglycidyl ether of bisphenol A; HMPA: hexahydro-4-methylphthalic anhydride). (b) Two types of diaziridyl ether of
bisphenol A (denoted as DzEBA): Bn-DzEBA and Ts-DzEBA including EDG and EWG substituents, respectively, at the nitrogen of aziridine. These
are the analogues of diglycidyl ether of bisphenol A (DGEBA). (c) 1H NMR spectra of Ts- and Bn-DzEBAs. Insets are photos of purified and dried
DzEBAs. (d) Traces of thermogravimetric analysis (TGA).
(DGEBA; Figure 1a) containing the backbone of bisphenol A,
which is inexpensive and easily accessible as well as mechanical-
strengthening- and thermal-resistive.36−39 If one could create
analogous curing resins with unexplored or new functional
moiety over the same backbone, one could straightforwardly
compare their curing performances to unveil the potential of
the tested moiety for a new class of curing resin. Herein we
describe synthesis of aziridine-based curing resins, diaziridyl
ether of bisphenol A (denoted as DzEBA; Figure 1b), and a
study that determines their performance for thermal curing,
which underlies ROP of two terminal aziridines. Analysis of the
thermal-curing process of DzEBA resins over various curing
agents with NMR and FT-IR spectroscopies, differential
scanning calorimetry (DSC), and single lap shear strength
test in solution or solid state indicates that the curing ability of
DzEBAs largely depends on the electronic structure of the N-
substituent of aziridine, proceeds unprecedentedly fast under
mild conditions, and exhibits excellent chemoselectivity and
wide substrate scope. These attractive features provide
molecular insight into the potential utility of aziridines for
developing novel curing resins and for overcoming the
limitations of conventional epoxy. Furthermore, we envision
that the chemoselectivity of resins demonstrated here would
bring benefit to the design of heterogeneous resin scaffold for
advanced polymeric systems.40
RESULTS AND DISCUSSION
■
We prepared DzEBAs having N-Bn and -Ts as N-EDG and
-EWG aziridines, respectively; the Supporting Information
contains detailed synthetic procedures and characterization data
for them. Briefly, Bn-DzEBA was synthesized by coupling
reaction between bisphenol A and N-benzylaziridine having
methyl tosylate at the C-2. For Ts-DzEBA, DGEBA was ring-
opened with p-toluenesulfonamide followed by mesylation and
ring-closure reactions. All analytical data, including H and 13C
1
NMR spectroscopies, and electrospray ionization mass
spectrometry (ESI-MS) were in full agreement with the
1
proposed structures. For example, the H NMR spectra of
DzEBAs in Figure 1c exhibit highly diagnostic resonances at
2.78 (JH−H = 7 Hz) and 2.33 ppm (JH−H = 4.6 Hz) as doublets
for Ts-DzEBA and at 1.88 ppm (JH−H = 3.5 Hz) and 1.61 ppm
(JH−H = 6.7 Hz) as doublets for Bn-DzEBA, indicative of the
two inequivalent hydrogen atoms in the aziridine rings. The
carbons at C-2 for the aziridines in Bn- and Ts-DzEBAs were
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Figure 2. (a) 2D heatmaps of conversion ratio for ROP of DzEBAs and DGEBA resins in the presence of various curing agents at 80 °C monitored
by 1H NMR spectroscopy. Heatmaps for 6, 24, and 48 h are exemplary plots to show chemoselectivity at different reaction times. Curing agents used
for ROP test were dodecanedioic acid (DDDA), terephthalic acid (TPA), ethylenediamine (EDA), p-phenylenediamine (PDA), ethylenediol
(EDO), hydroquinone (HQ), 1,3-propanedithiol (PDT), and 1,4-benzenedithiol (BDT). (b) Comparison of reaction kinetics for Ts-DzEBA and
DGEBA in the presence of alkylamine (ethylenediamine; EDA). (c) Chemoselectivity for the DzEBAs over a curing agent, EDA. (d)
Chemoselectivity for DzEBAs over a curing agent, alkyl diacid (dodecanedioic acid; DDDA). (e) Dependence of ROP of Ts-DzEBA by EDA on
reaction temperature.
stereocenters. For the stereochemistry of products, thin layer
chromatography (TLC) monitoring during synthesis showed
new single spot indicating one product, and H NMR spectra
determine the number of stereoisomers, we conducted chiral
HPLC analysis and found that there were three peaks with
1:2:1 integration ratio for the products. This finding could be
attributed to the presence of two meso compounds and two
1
also showed the presence of one type of aziridine. To
C
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diastereomers. See the Supporting Information for detailed
chiral HPLC data and the stereoisomer structures. The thermal
and optical properties of DzEBAs were investigated. Figure 1d
shows the thermogravimetric analysis (TGA) profile over the
temperature range from 30 to 600 °C for the DzEBAs. Thermal
degradation beginning at ∼322 and ∼348 °C for Bn-DzEBA
and Ts-DzEBA, respectively, was observed. For comparison,
DGEBA begins to decompose thermally at ∼370 °C.41 Ts- and
Bn-DzEBAs were white powder and light-yellowish viscous
liquids in ambient condition, respectively (the insets in Figure
1c; see the Supporting Information for UV−vis absorption
spectra).
temperature increased from 25 to 80 °C, and also the ROP was
completed even at 40 °C.
We conducted DSC studies to investigate the cure process of
resin-curing agent formulations under solvent-free conditions in
a nonisothermal manner.43−45 For nonisothermal DSC study,
samples containing 1:1 mole ratio of individual resins and
curing agents (shown in Figure 2a) were cured at various
heating rates from 2 to 20 °C/min. Note that some of curing
and unsuitable for DSC experiment; hence, the alkylamine and
alcohol were replaced with 1,12-diaminododecane (DAD) and
1,12-dodecanediol (DDD) (shown in Table S1). Alkyl thiol
suitable to DSC study was not accessible. Under the same
curing condition, the peak temperature (Tp) was measured and
taken as an indicator to compare the reactivity of curing process
for various formulations.46
To determine the substrate scope and chemoselectivity for
the ROP of DzEBAs, compared to that of DGEBA as a control,
1
we carried out H NMR spectroscopic analysis for ROP over
Some of results of DSC experiments are summarized in
various curing agents (structurally simple alkyl/aryl amines,
acids, alcohols, and thiols) in solution under identical reaction
conditions (80 °C in DMSO-d6 containing 1:1 mole ratio of
resin (Ts-DzEBA, Bn-DzEBA, or DGEBA) and curing agent, in
air; see the Supporting Information for details). For
investigating curing performance, we set the temperature of
80 °C as the highest; in preliminary experiments, we have
surveyed the performance of thermal curing of DzEBAs over a
wide range of temperature and found that the curing for the Ts-
DzEBA and ethylenediamine combination was completed in
few minutes at 80 °C. This temperature was sufficient to
determine whether DzEBAs function under mild conditions as
compared to DGEBA. Figure 2 shows chemical structures of
curing agents we tested and summarizes the results. We
observed four significant features. (i) Overall, DzEBAs showed
a pronouncedly wide substrate scope: alkyl and aryl acids,
alcohols, amines, and thiols acted as efficient curing agents.
However, the ROP of DGEBA was triggered limitedly by
amines, aryl acid, and alkyl thiol; DGEBA remained completely
intact or showed very sluggish reaction over alkyl acid, alkyl/
aryl alcohols, and aryl thiol (Figure 2a). (ii) The ROP for
DzEBAs proceeded remarkably rapidly under mild conditions
as compared to that of DGEBA. For example, the ROP of Ts-
DzEBA in the presence of alkylamine (ethylenediamine; EDA)
was completed within 10 min at 80 °C; the analogous reaction
with DGEBA showed ∼10% conversion (the ratio of product to
the initial reactant, determined by 1H NMR analysis) in 10 min
under the identical condition. (Figure 2b). (iii) The ROP of
DzEBAs exhibited excellent chemoselectivity: alkyl acid
(dodecanedioic acid; DDDA) was relevant to Bn-DzEBA and
not Ts-DzEBA, whereas alkylamine (EDA) chemoselectively
opened only Ts-DzEBA (Figure 2c,d). These findings led to the
conclusion that, unlike DGEBA, the curing performance of
DzEBAs is highly efficient and chemoselective over different
curing agents. The chemoselectivity of DzEBAs was attributed
to the controlled electronic structure on the N-substituent in
aziridine.19,42
Table 1 (see the Supporting Information for the other data).
Table 1. Curing Characteristics of Various Resin-Curing
a
Agent Formulations at 20 K/min
b
curing agent
curing resin
Tp [°C]
Ea [kJ/mol]
ΔH [J/g]
PDA
DGEBA
135
96
78
113
121
58
386
20
PDA
Ts-DzEBA
Bn-DzEBA
DGEBA
DDDA
DAD
DAD
113
133
21
425
c
c
c
Ts-DzEBA
76
−
−
a
Out of 21 resin-curing agent formulations, five formulations showed
b
single exothermic peaks corresponding to thermal curing. Values of
Ea were determined with DSC curves measured at different heating
c
rates and eq 1. An exothermic peak was overlapped with the melting
point of DAD, and we were unable to estimate meaningful values of Ea
and ΔH.
Out of 21 formulations, seven (PDA/DGEBA, PDA/Ts-
DzEBA, DDDA/Bn-DzEBA, DAD/DGEBA, DAD/Ts-
DzEBA, BDT/Bn-DzEBA, and BDT/Ts-DzEBA) were relevant
to DSC studies. The rest of formulations did not show peaks
corresponding to the curing process; only endothermic peaks
corresponding to melting process of resins and curing agents
were observed (see the Supporting Information for DSC
curves). Results in DSC experiments could be summarized as
follows. (i) Each of the formulations relevant to DSC exhibited
single exothermic peaks, indicative of thermal curing. (ii) The
tested formulations showed that Tp increased with increasing
heating rate (see exemplary plots in Figure 3a−d). (iii) Overall,
the DzEBAs showed lower Tp values than DGEBA. For
example, Tp (∼96 °C) for Ts-DzEBA/PDA was lower than the
analogous system of DGEBA (∼135 °C) (Table 1).
While DGEBA was not cured by DDDA, the Bn-DzEBA/
DDDA system showed Tp of ∼113 °C (Figure 3c). (iv)
Chemoselectivity was observed: PDA was relevant to Ts-
DzEBA and not Bn-DzEBA. On the other hand, Bn-DzEBA,
not Ts-DzEBA, was cured well by DDDA. The excellent curing
performance and chemoselectivity of DzEBAs were consistent
We further investigated the dependence of rate of DzEBAs’
ROP on reaction temperature. Figure 2e shows exemplary plots
of conversion ratio for a mixture of Ts-DzEBA and EDA at
variable temperatures. The rate of ROP increased as the
temperature increased from 25 to 80 °C; rather surprisingly, the
reaction was completed in 10 min at 80 °C whereas the
analogous reaction of DGEBA proceeded slowly in the same
reaction time. The ROP reaction of Ts-DzEBA was completed
even at 40 °C. Bn-DzEBA exhibited the similar correlation with
reaction temperature: as shown in Figure S26 of the Supporting
Information, the rate of Bn-DzEBA’s ROP increased as the
1
with the results of H NMR study shown in Figure 2. (v)
Thermal curing of Ts-DzEBA in the presence of DAD was
much faster than that of DGEBA (Figure 3d,e), and the
exothermic peak of curing appeared at the low-temperature
region where the endothermic peak corresponding to the
melting point of DAD (67−69 °C) appeared. Although
estimating Tp for this system was not accurate, it was no
doubt that the thermal curing was remarkably rapid.
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Figure 3. DSC thermograms of (a) Ts-DzEBA/p-phenylenediamine (PDA), (b) DGEBA/PDA, (c) Bn-DzEBA/dodecanedioic acid (DDDA), (d)
DGEBA/1,12-diaminododecane (DAD), and (e) Ts-DzEBA/DAD at different heating rates (2, 5, 10, 15, and 20 °C/min). (f) Linear plots of ln(β/
2
Tp ) versus 1/Tp for two resins (DGEBA and Ts-DzEBA) cured by PDA based on Kissinger’s equation (eq 1). Circles in the plots indicate peak
temperature (Tp).
Figure 4. Cured film of Bn-DzEBA/DDDA formed by a conventional solvent-cast method. (a) Photos of the film after thermal curing at 80 °C for 3
h. (b) ATR FT-IR spectra of the film and intact Bn-DzEBA resin.
The DSC curves of DzEBAs cured with aryl acid
(terephthalic acid; TPA) and aryl alcohol (hydroquinone;
HQ) did not display exothermic peak, indicating no cure
occurred (Figures S29 and S31). This finding was inconsistent
with that of H NMR study and could be explained with the
high melting point of TPA (mp = 300 °C) and HQ (mp = 172
°C). These curing agents were not melt in the temperature
range we examined and thus did not mix with curing resins for
DSC experiments.
According to Kissinger’s method,43 the activation energy (Ea)
for curing process could be estimated in nonisothermal DSC
study using eq 1. Here, Tp is the peak temperature, β is heating
rate, A is pre-exponential factor, and R is the universal gas
constant.
1
E
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Figure 5. (a) Scheme of the single lap shear test and the lap shear strength. Shear strength measured (b, c) at different curing temperatures and (d,
e) different curing times. PA and TETA are poly(acrylic acid) and triethylenetetramine, respectively, as curing agent (n.a. = no adhesion).
were similar (Δk = ∼2 × 10−4 s−1), and at 500 K the k value of
Ts-DzEBA/PDA was ∼400 times larger than that of DGEBA/
PDA. Overall, these findings indicate the curing rate for DzEBA
is dominated by the A factor rather than the Ea, which accounts
for the great curing performance of DzEBAs as compared to
DGEBA.
The feasibility of DzEBAs as curing resin in solid state was
further demonstrated by thermally curing a millimeter thick
film comprising resin and curing agent. A film of N-Bn-DzEBA
and DDDA mixture at 1:1 molar ratio was formed through a
simple solvent-cast process (see the Supporting Information for
details). Upon thermal curing at 80 °C for 3 h the liquid film
was converted into the corresponding solid form (Figure 4a).
The completion of ROP could be determined by ATR FT-IR
spectroscopic analysis (Figure 4b): comparison of FT-IR
spectra before and after thermal curing exhibited the
appearance of new transmittance peak at ∼3400 cm−1, which
corresponded to secondary amine and confirmed the formation
of the desired ring-opened product.
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞
⎟
⎠
Ea
β
p
AR
⎜
ln
⎜
⎟
⎟
= ln
−
T 2
E
RT
a
p
(1)
2
Figure 3f shows the plots of ln(β/Tp ) versus 1/Tp for DGEBA
and Ts-DzEBA cured by PDA, and the slope of linear fitting
plots corresponds to −Ea/R. The Ea value (∼78 kJ/mol) of the
DGEBA/PDA system is lower than that (∼113 kJ/mol) of Ts-
DzEBA/PDA. This difference in Ea was attributed to the
different nature of ROP for aziridine and epoxide.47 This was
also reflected into the difference in ΔH: as shown in Table 1,
there was approximately 1 order of magnitude difference in ΔH
between DzEBAs and DGEBA.
According to Arrhenius equation (k = A exp(−Ea/RT) where
k is the rate constant, A is the pre-exponential factor, R is the
universal gas constant, and T is the reaction temperature), the
rate constant k is associated with not only activation energy Ea
but also pre-exponential factor A which is intrinsic to chemical
reaction. In Kissinger’s method, we were able to estimate the A
factor: log(A) values for thermal curing of Ts-DzEBA and
DGEBA with PDA were ∼13.4 and ∼7.1, respectively. While
the difference in Ea for Ts-DzEBA and DGEBA was little
(×∼1.5), the difference in A was significant (×∼106). The
Arrhenius equation led us to estimate the k values over the
temperature range (from ∼300 to ∼500 K) tested in this work:
at 300 K, the k values for Ts-DzEBA/PDA and DGEBA/PDA
One of applications for epoxy materials is to adhere materials
together via surface bonding.48,49 Thus, we further compared
the adhesion performance for the resins through single lap
shear strength test based on ASTM D3163 (Figure 5a). We
used poly(acrylic acid) (PA)50 and triethylenetetramine
(TETA)51 as curing agents; these have been previously tested
for DGEBA resin. In a typical experiment, a 1:1 mixture of resin
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crude was extracted with DCM. The organic phase was dried with
magnesium sulfate, and the solvent was removed under reduced
pressure at room temperature. The resulting crude was purified by
flash column chromatography to obtain the corresponding product in
92% yield (10.91 g) as a white solid. The analytical data for this
compound were in excellent agreement with the reported data.53
Synthesis of (1-Benzylaziridin-2-yl)methanol (S5). Lithium
aluminum hydride (1.849 g, 48.72 mmol) was slowly added to a
solution of ethyl 1-benzylaziridine-2-carboxylate (5 g, 24.36 mmol) in
diethyl ether (130 mL). The mixture was stirred at room temperature
for 3 h. The reaction mixture was quenched with water (1.9 mL) and
15% NaOH aqueous solution (1.9 mL) and then water (5.6 mL) After
the filteration of the mixture, the crude compound was extracted with
ethyl acetate. The organic phase was dried with magnesium sulfate,
and the solvent was removed under reduced pressure. The product
was obtained in 87% yield (4.31 g) as a yellowish powder. The
analytical data for this compound were in excellent agreement with the
reported data.54
(0.11 M; Ts- or Bn-DzEBA, or DGEBA) and curing agent
(0.11 M) in THF was homogeneously placed by spin-coating
between prerinsed aluminum substrates, and shear strength was
measured by a universal testing machine (UTM). Figure 5b,c
shows the results. Overall, the adhesives including DzEBAs
(∼25 and ∼50 kgf/cm2 for Bn-DzEBA/PA and Ts-DzEBA/
TETA) showed higher shear strength than those including
DGEBA (0 and ∼44 kgf/cm2). DzEBAs displayed adhesion
performance when curing temperature was lowered from 80 to
40 °C. As other experiments above, chemoselectivity was
observed for DzEBAs. The use of PA resulted in adhesion for
only Bn-DzEBA while TETA was relevant to Ts-DzEBA and
not Bn-DGEBA. In the plots of shear strength as a function of
curing time (Figure 5d,e) we were able to confirm that DzEBAs
were cured rapidly as compared to DGEBA.
CONCLUSIONS
■
Synthesis of (1-Benzylaziridin-2-yl)methyl 4-Methylbenze-
nesulfonate (S6). A solution of (1-benzylaziridin-2-yl)methanol (1 g,
6.127 mmol) in triethylamine (3.0 mL, 21.64 mmol) was added to a
solution of p-toluenesulfonyl chloride (1.402 g, 7.352 mmol) in DCM
(15 mL) at 0 °C under a N2 atmosphere. The mixture was stirred at 0
°C for 2 h. The reaction crude was extracted with DCM. The organic
phase was dried with magnesium sulfate, and the solvent was removed
under reduced pressure at room temperature. The resulting crude was
purified by flash column chromatography to obtain the corresponding
product in 50% yield (973 mg) as a white solid. The analytical data for
this compound were in excellent agreement with the reported data.55
Synthesis of 2,2′-(((Propane-2,2-diylbis(4,1-phenylene))bis-
(oxy))bis(methylene))bis(1-benzylaziridine) (S1). (1-Benzylazir-
idin-2-yl)methyl 4-methylbenzenesulfonate (277 mg, 0.872 mmol) was
added to a solution of bisphenol A (108 mg, 0.436 mmol), 18-crown-
ether (15.5 mg, 0.0586 mmol), and KOH (135 mg, 2.398 mmol) in
THF (15 mL). The mixture was refluxed for 17 h. The reaction
solution was cooled to room temperature and extracted with DCM.
The combined organic layer was dried over anhydrous MgSO4, filtered
off, and concentrated in vacuo. The crude product was purified by flash
column chromatography (EA:hexane = 2:1) to obtain the correspond-
ing product in 44% yield (100 mg) as a yellowish oil. 1H NMR
(CDCl3): δ 7.24−7.52 (m, 5H) 7.14 (d, J = 9.00 Hz, 2H) 6.82 (d, J =
8.60 Hz, 2H) 3.89−4.05 (m, 2H) 3.54 (td, J = 13.50, 8.60 Hz, 1H)
1.97−2.06 (m, 1H) 1.88 (d, J = 3.52 Hz, 1H) 1.66 (s, 3H) 1.60 (d, J =
6.65 Hz, 1H). 13C{1H} NMR (CDCl3): δ 156.4, 143.2, 138.8, 128.2,
127.9, 127.6, 127.0, 113.82, 70.0, 64.2, 41.6, 37.9, 32.0, 31.0. MS (ESI)
m/z: [M + H]+ calcd for C35H39N2O2: 519.3012; found: 519.3009.
Synthesis of N,N′-(((Propane-2,2-diylbis(4,1-phenylene))bis-
(oxy))bis(2-hydroxypropane-3,1-diyl))bis(4-methylbenzene-
sulfonamide) (S7). A mixture of potassium carbonate (6.09 g, 44.06
mmol), p-toluenesulfonamide (5.03 g, 29.37 mmol), diglycidyl ether of
bisphenol A (5 g, 14.69 mmol), and 18-crown-6 (233 mg, 0.8814
mmol) in DMSO was heated at 100 °C with strring for 2 h. After
cooling to room temperature, the reaction mixture was diluted with
ethyl acetate and washed with distilled water. The organic layer was
dried over anhydrous Na2SO4 and concentrated under reduced
pressure. The crude product was purified by flash column
chromatography (EA:hexane = 3:1) to obtain the corresponding
product in 47% yield as white powder. 1H NMR (CDCl3): δ 7.75 (d, J
= 8.24 Hz, 2 H) 7.31 (d, J = 8.55 Hz, 2 H) 7.11 (d, J = 8.85 Hz, 2 H)
6.75 (d, J = 8.85 Hz, 2 H) 4.82 (d, J = 5.19 Hz, 1 H) 4.08 (dd, J =
10.53, 6.26 Hz, 1 H) 3.89−3.96 (m, 2 H) 3.21−3.27 (m, 1 H) 3.04−
3.13 (m, 1 H) 2.43 (s, 3 H) 1.62 (s, 3 H). 13C{1H} NMR (CDCl3): δ
155.93, 143.63, 143.54, 136.36, 129.76, 127.68, 127.03, 113.83, 69.20,
68.64, 45.51, 41.61, 30.90, 21.47. MS (ESI) m/z: [M + Na]+ calcd for
C35H42N2NaO8S2: 705.2280; found: 705.2276.
In conclusion, the experiments reported in this work were
designed particularly to compare systematically the perform-
ance of aziridine-containing curing resins with the analogous
epoxy-based curing resin. We replaced the epoxide terminal
units in DGEBA with the N-EDG or -EWG aziridines while
keeping other structures and reaction conditions constant and
found that the chemistry of aziridines could be exploited for
achieving efficient curing resin and overcoming critical
limitations observed in epoxy-based resin: sluggish curing
rate, harsh reaction conditions, narrow substrate scope, and no
or little chemoselectivity. Although it should be further
improved for satisfying industrial needs such as cost-
effectiveness and large-scale synthesis, this work demonstrates,
as a proof-of-concept, the utility of aziridines as a new
functional moiety for curing resin.
EXPERIMENTAL SECTION
■
Materials. All reagents were used as supplied unless otherwise
specified. All organic solvents were purchased from Daejung while
water was purified using an Aqua MAX-Basic System (deionized water,
electrical resistivity of which is ∼18.2 MΩ·cm).
1
Characterization. H and 13C NMR spectra were recorded on a
Bruker FT-NMR Advance-500 using CDCl3 and DMSO as solvent
and residual solvents as an internal standard. Chemical shifts are
expressed in parts per million (ppm) related to internal TMS, and
coupling constants (J) are in hertz. MS (ESI-QTOF) measurements
were recorded on a Bruker compat Q-TOF MS. UV−vis spectra were
measured using Agilent Technologies 8453 UV−vis spectrometer.
Measurements of FT-IR were carried out using a Bruker ALPHA FT-
IR spectrometer. Adhesive lap joint shear strength tests were
conducted using a Withlab-WL2100C universal testing machine
(UTM). Differential scanning calorimetry analysis for curing samples
was tested using a Scinco-DSC N-650.
Synthesis of Ethyl 2,3-Dibromopropanoate (S3). To a
solution of ethyl acrylate (10.63 mL, 99.88 mmol) in DCM (75
mL), bromine (5.14 mL, 99.88 mmol) was added dropwise at 0 °C for
20 min under an inert atmosphere. The reaction mixture was allowed
to stir at 0 °C for 1 h and then at rt for 3 h. After completion of the
reaction, the mixture was quenched with saturated Na2S2O3 aqueous
solution followed by extraction of the mixture with DCM and water,
and then separation of the organic layer gave the dibromo compound
in 95% yield (24.62 g). The analytical data for this compound were in
excellent agreement with the reported data.52
Synthesis of Ethyl 1-Benzylaziridine-2-carboxylate (S4).
Ethyl 2,3-dibromopropanoate (15 g, 57.71 mmol) and trimethylamine
(16 mL, 69.6 mmol) were sequentially added to a solution of
benzylamine (6.184 mL, 57.71 mmol) in anhydrous ethanol (120 mL)
at 0 °C under a N2 atmosphere. The mixture was stirred at 60 °C for 1
h. The reaction solution was concentrated in vacuo, and the reaction
Synthesis of 2,2′-(((Propane-2,2-diylbis(4,1-phenylene))bis-
(oxy))bis(methylene))bis(1-tosylaziridine) (S2). To a solution of
N,N′-(((propane-2,2-diylbis(4,1-phenylene))bis(oxy))bis(2-hydroxy-
propane-3,1-diyl))bis(4-methylbenzenesulfonamide) and triethyl-
amine (120.9 mg, 1.195 mmol) in THF (5 mL) was added
G
DOI: 10.1021/acs.macromol.8b00550
Macromolecules XXXX, XXX, XXX−XXX

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methanesulfonyl chloride (40.3 mg, 0.3515 mmol) at 0 °C under a N2
atmosphere. The mixture was heated at room temperature and stirred
for 5 h. The reaction mixture was quenched with distilled water, and
extractive work-up was performed with ethyl acetate. The combined
organics were then washed with brine and dried over Na2SO4 and
concentrated under reduced pressure. Without any purification, the
mixture of crude product and potassium carbonate in acetonitrile was
stirred at room temperature for 16 h. The reaction mixture was diluted
with ethyl acetate and poured into distilled water, and extractive work-
up was performed with ethyl acetate. The crude product was purified
by flash column chromatography (EA:hexane = 1:2) to obtain the
corresponding product in 72% yield as a white powder. 1H NMR
(CDCl3): 7.83 (d, J = 8.24 Hz, 2 H) 7.32 (d, J = 7.93 Hz, 2 H) 7.06 (d,
J = 8.85 Hz, 2 H) 6.63 (d, J = 8.85 Hz, 2 H) 3.84−4.07 (m, 2 H)
3.07−3.21 (m, 1 H) 2.78 (d, J = 7.02 Hz, 1 H) 2.44 (s, 3 H) 2.33 (d, J
= 4.58 Hz, 1 H) 1.60 (s, 3 H). 13C{1H} NMR (CDCl3): δ 155.80,
144.60, 143.54, 134.57, 129.59, 127.99, 127.58, 113.71, 66.87, 41.57,
38.07, 31.03, 30.88, 21.59. MS (ESI) m/z: [M + Na]+ calcd for
C35H38N2NaO6S2: 669.2069; found: 669.2067.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the NRF of Korea (NRF-
2016R1D1A1A02937504; NRF-2017M3A7B8064518;
NRF20100020209) and the Future Research Grant of Korea
University. We thank Mr. Do Young Park and Prof. Cheol-
Hong Cheon for kindly helping us to perform chiral HPLC
analysis.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b00550.
Additional experimental images (Figures S1−S39), tables
(Tables S1 and S2), and schemes (Schemes S1 and S2)
(PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: hyoon@korea.ac.kr (H.J.Y.).
■
ORCID
Hyo Jae Yoon: 0000-0002-2501-0251
H
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