Macromolecules
Article
confirmed our successful synthesis of dibenzo-crownether-
2NH2 in highly purity.
1347 cm−1 for stretching of the aromatic C−H and NO2
groups. Signals appeared at 3428 and 3356 cm−1 for N−H
stretching in dibenzo-crownether-2NH2 [Figure S5c]. The
typical absorption bands of dibenzo-crownether-salicylalde-
hyde [Figure S5d] were centered at 831, 1599, 1619, 3064,
1620, and 3446 cm−1, corresponding to stretching of the
aromatic rings, CN bonds, and the O−H groups of the
phenolic units. The spectrum of dibenzo-crownether-hydrox-
ybenzylamine [Figure S5e] featured absorption bands at 3383
and 2940 cm−1 for N−H and aliphatic C−H stretching. The
FTIR spectrum of crown-ether BZ [Figure S5f] featured
absorption bands at 1360, 1240, 1044, and 937 cm−1
representing vibrations of its tetrasubstituted benzene ring,
asymmetric C−O−C stretching, symmetric C−O−C stretch-
ing, and the oxazine ring. The purity of crown-ether BZ was
confirmed using high-resolution mass spectrometry; the signal
that appeared for the [M + Na]+ ion was consistent with the
molecular weight predicted for crown-ether BZ (Figure S6).
Thus, FTIR spectroscopy, NMR spectroscopy, and high-
resolution mass spectrometry confirmed our successful
preparation of crown-ether BZ with high purity.
Synthesis of Crown-Ether BZ. The one-pot Mannich
condensation of an aromatic phenol, an aliphatic/aromatic
amine, and paraformaldehyde does not always proceed well for
benzoxazine preparation because of low selectivity depending
on the position of the substituents. Indeed, in this study, we
could not synthesize our benzoxazine monomer through the
one-pot Mannich condensation presented in Scheme 1e. The
reaction conditions such as reaction time and solvent effect of
the crown ether-functionalized benzoxazine monomer using a
one-pot Mannich condensation approach are summarized in
Table S1. Ishida and Lin et al. provided another method for
the synthesis of benzoxazine monomers52−54 from o-hydrox-
ybenzylamine, which is readily prepared through the reduction
of a Schiff base; a ring-closing reaction of o-hydroxybenzyl-
amine with an aldehyde derivative then affords the benzoxazine
monomer without any complicated side reactions. As such, we
prepared the precursor of our difunctional benzoxazine
monomer through reduction of the Schiff base formed from
dibenzo-crownether-2NH2 and salicylaldehyde [Scheme 1f].
Subsequent reaction of the resulting o-hydroxybenzylamine
derivative [Scheme 1g] with paraformaldehyde in 1,4-dioxane
and absolute EtOH at 100 °C for 12 h provided the target
crown-ether BZ [Scheme 1h].
Thermal Curing Polymerization of Crown Ether-
Functionalized Benzoxazine Monomer. The DSC ther-
mogram of pure crown-ether BZ [Figure 2A] revealed two
1
Figure 1A displays the H NMR spectra, recorded at room
temperature, of dibenzo-crownether-salicylaldehyde, dibenzo-
crownether-hydroxybenzylamine, and crown-ether BZ in
DMSO-d6. The characteristic signals for the OH group of
the phenolic units and the NCH groups of dibenzo-
crownether-salicylaldehyde [Figure 1A-a] were centered at
13.34 and 8.96 ppm, respectively. The spectrum of dibenzo-
crownether-hydroxybenzylamine [Figure 1A-c] featured signals
at 7.12−6.03 ppm for the aromatic protons; the signal for the
NCH group at 8.96 ppm had disappeared, and a new peak
for NHCH2 group appeared at 3.94 ppm, whereas signals
appeared for the protons of the NH and OH groups that were
consistent with their intramolecular hydrogen bonding. The
spectrum of crown-ether BZ [Figure 1A-e] featured signals at
7.08−6.55 ppm for the aromatic protons and at 5.36
(OCH2N) and 4.56 (ArCH2N) ppm, at a 1:1 ratio, for the
protons of the oxazine ring. Furthermore, we recorded 13C
NMR spectra [Figure 1B] to confirm the chemical structures
of dibenzo-crownether-salicylaldehyde, dibenzo-crownether-
hydroxybenzylamine, and crown-ether BZ. The 13C NMR
spectrum of dibenzo-crownether-salicylaldehyde [Figure 1B-b]
featured signals at 161.28, 148.52, 68.85, and 67.86 ppm for
the COH, NCH, and O−CH2 carbon nuclei, respectively.
The spectrum of dibenzo-crownether-hydroxybenzylamine
[Figure 1B-d] featured characteristic signals for the carbon
nuclei at 158.42−99.03 (aromatic) and 42.69 (NH−CH2)
ppm. As displayed in Figure 1B-f, the characteristic signals for
the oxazine unit in crown-ether BZ appeared at 79.67
(OCH2N) and 49.56 (ArCH2N) ppm.
Figure 2. (A) DSC and (B) FTIR spectral analyses of crown-ether BZ
at various temperatures: (a) uncured; (b) 110, (c) 150, (d) 180, and
(e) 210 °C.
thermal events. The first was an endothermic process centered
at 152 °C, attributed to the melting temperature of the
benzoxazine monomer. The second thermal process was
exothermic with a curing temperature of 210 °C and a heat
of polymerization of 162 J g−1. The sharpness of these
endothermic and exothermic peaks confirmed the high purity
of crown-ether BZ. According to the DSC thermogram, the
polymerization exotherm maximum temperature for crown-
ether BZ (210 °C) was significantly lower than that of the
typical benzoxazine monomer 3-phenyl-3,4-dihydro-2H-ben-
zooxazine (Pa, 255−263 °C),55,56 presumably because the
flexibility of the crown ether moiety on the main chain
backbone structure catalyzed the ring opening polymerization
at a relatively lower temperature. As displayed in Figure 2A,
after the first polymerization step at 110 °C, the endothermic
Figure S5 presents the FTIR spectra of dibenzo[18]crown-6,
dibenzo-crownether-2NO2, dibenzo-crownether-2NH2, diben-
zo-crownether-salicylaldehyde, dibenzo-crownether- hydroxy-
benzylamine, and crown-ether BZ. The spectrum of
dibenzo[18]crown-6 [Figure S5a] featured absorption bands
for aromatic C−H stretching, aliphatic C−H stretching, and
C−O−C stretching at 3051, 2922, and 1255 cm−1
,
respectively. The spectrum of dibenzo-crownether-2NO2
[Figure S5b] featured absorption bands at 3083, 1517, and
D
Macromolecules XXXX, XXX, XXX−XXX
Mohamed, Mohamed Gamal
