Design, Synthesis, Characterization, and Polymerization of Fused-Ring Naphthoxazine Resins

Article abstract of DOI:10.1021/acs.macromol.7b00932

Fused-ring naphthoxazine monomers are synthesized from 1-naphthols and cycloimines derived from piperidine and pyrrolidine. Their polymerization has been confirmed by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FT-IR) as well as by solution and solid-state ¹H and ¹³C nuclear magnetic resonance spectroscopy (NMR). A polymerization reaction similar to the traditional benzoxazine monomers is found with oxazine ring disappearing and hydroxyl group forming by the nonisothermal FT-IR experiment. Fused-ring monomers, studied in this work, polymerize at significantly lower temperatures compared to the ordinary benzoxazine resins.

Full text of DOI:10.1021/acs.macromol.7b00932

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX-XXX
Design, Synthesis, Characterization, and Polymerization of Fused-
Ring Naphthoxazine Resins
Carlos R. Arza,† Pablo Froimowicz,*,†,‡ Lu Han,† Robert Graf,§ and Hatsuo Ishida*,†
†Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United
States
‡Design and Chemistry of Macromolecules Group, Institute of Technology in Polymers and Nanotechnology (ITPN),
UBA-CONICET, School of Engineering, University of Buenos Aires, Av. Gral. Las Heras 2214 (PC C1127AAR), Buenos Aires,
Argentina
§Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
ABSTRACT: Fused-ring naphthoxazine monomers are synthesized from
1-naphthols and cycloimines derived from piperidine and pyrrolidine.
Their polymerization has been confirmed by differential scanning
calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier
transform infrared spectroscopy (FT-IR) as well as by solution and solid-
1
state H and 13C nuclear magnetic resonance spectroscopy (NMR). A
polymerization reaction similar to the traditional benzoxazine monomers
is found with oxazine ring disappearing and hydroxyl group forming by
the nonisothermal FT-IR experiment. Fused-ring monomers, studied in
this work, polymerize at significantly lower temperatures compared to the
ordinary benzoxazine resins.
have demonstrated the effectiveness of improving resin
performance by incorporating naphthalene groups into back-
bone of various polymers, such as epoxy,15 PEEK,16
polyimide,17 poly(ε-caprolactone),18 and bismelaimides.19
Compared to the wide range of studies in benzoxazine
chemistry, naphthoxazines have received much less attention
although some very interesting studies have been reported,
including synthesis and mechanical study on aniline-based
bifunctional naphthoxazines20 and their degradation mecha-
nism,21 thin film from telechelic naphthoxazine polymer,22 and
thermal decomposition studies of aliphatic amine-based
naphthoxazines and its polymer.23 These limited activities are
related in part to the easy evaporation of some naphthoxazine
monomers during polymerization; however, chemical reactivity
of naphthols is greater than that of phenols and thus could lead
to advantageous synthesis conditions, such as monomer
synthesis at room temperature.24
Fused-ring naphthoxazines typically have a cycloaliphatic ring
fused directly with oxazine ring, and they have been synthesized
using 1-naphthol25 or 2-naphthol26,27 as phenolic precursor
with many alternative pathways for different organic chemistry
studies. Despite the possibility of adding a great number of a
new class of benzoxazines that can potentially lead to new
polybenzoxazines, no study has yet been reported on the
successful polymerization of fused-ring benzoxazines or
INTRODUCTION
■
Polybenzoxazine is a new addition-type thermoset1 that has
aroused much scientific research interest2 and has relatively
recent been added to the list of commercially available
thermoset materials3 because of their outstanding chemical,
physical, and mechanical properties such as high thermal
resistance4 including fully bio-based polybenzoxazines,5 resist-
ance to flame,6 flexible molecular design,7 near-zero shrinkage,8
and very low surface free energy.9 The structural design
flexibility of benzoxazine monomers is advantageous since the
monomer synthesis utilizes widely commercially available
phenols, primary amines, and formaldehyde.10 Less common
methodologies have also been applied successfully for
architectures where typical benzoxazine formation strategy,
the Mannich condensation reaction, was incompatible. One of
the most used pathways for the synthesis of these resins is the
three-step strategy, which involves the formation of an open-
Mannich base through the reaction of a primary amine and a 2-
hydroxybenzaldehyde forming a Schiff’s base as the first step.
The second step involves the reduction of the Schiff’s base
usually with sodium borohydride and final closure of the ring
with the use of formaldehyde (Scheme 1).11 This strategy has
been employed, i.e., for the formation of hydroxyl function-
alized benzoxazine,12 aminostyrene based benzoxazines,13 and
cyanate ester containing benzoxazine.14
Polymers possessing naphthalene group are expected to
exhibit excellent mechanical property and thermal stability,
resulting from high backbone rigidity and aromatic content
provided by the multiaromatic-ring structure. Previous works
Received: May 5, 2017
Revised: November 7, 2017
© XXXX American Chemical Society
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Scheme 1. Synthesis of 1,3-Benzoxazines through (a) the One-Pot Modified Mannich Reaction of a Phenol, Primary Amine, and
Formaldehyde and (b) the Three-Step Strategy
15 h. After evaporation of the solvent, diethyl ether (20 mL) was
added; the organic phase was washed with a 0.5 N NaOH aqueous
solution (3 × 5 mL) as well as brine (2 × 5 mL) and dried over
MgSO4. The solvent was removed, and the resulting product was
dissolved in DCM (6 mL) and used without further purification, to
which formalin (37 wt % aqueous solution, 0.83 mL, 10.3 mmol, 3
equiv) was added dropwise. The reaction mixture was stirred at room
temperature overnight. After evaporation of the solvent, mixed hexane
isomers (hexanes) (30 mL) were added, and the resulting suspension
was filtered through a pad of alumina. Removal of the solvent afforded
a transparent oil which crystallized on standing at low temperature
naphthoxazines. Therefore, we developed an interest in
studying if such compounds might be considered and even
used as resins toward the generation of novel thermosets. Thus,
as a starting point, we tackled in this work one of the many
possible systems, specifically 1-naphthol-based fused-ring
naphthoxazines. This initial selection of the herein studied
resins was fundamentally based and supported by the reported
superior performance of 1-polynaphthoxazines over 2-poly-
naphthoxazines.20 To our knowledge, there are no previous
reports on the polymerization of this class of fused-ring
naphthoxazines, neither on the thermal stability of their
resulting polymers. Thus, it is the purpose of this paper to
study the feasibility of the polymerization of these fused-ring
naphthoxazines rather than showing excellent properties of the
resulting polymers. It is widely known that mono-oxazine
benzoxazines and naphthoxazines do not lead to mechanically
attractive properties due to the small oligomer formation.
However, development of novel materials exhibiting better
properties based on this new approach will be reported in time
elsewhere.
1
(yield: 55%). H NMR (600 MHz, CDCl3, 20 °C) δ, ppm: 8.15 (d,
1H, Ar), 7.74 (s, 1H, Ar), 7.47 (m, 2H, Ar), 7.10 (d, 1H, Ar), 5.12 (d,
1H, O−CH2−N), 5.07 (d, 1H, O−CH2−N), 4.76 (bb, 1H, Ar−CH−
(CH2−)N), 3.23−1.78 (m, 6H, −N−CH2−CH2−CH2−CH-
(CH2−)−). FT-IR ν (cm−1): 2968−2876 (C−H alkane str), 1325
(O−CH2−N wagg), 1240 (C−O antisym str), 1198 (C−N antisym
str), 1049 (C−O sym str), and 924 (oxazine-related band). HRMS
(EI) m/z, [M]+ calculated for C15H15NO+: 225.11536; found:
225.11548.
Synthesis of 9,10,11,11a-Tetrahydro-6H,8H-naphtho[2,1-e]-
pyrido[1,2-c][1,3]oxazine (Abbreviated as 1N-p6-f). A mixture of
1-naphthol (0.99 g, 6.87 mmol) and 1b (0.57 g, 6.87 mmol, 1 equiv)
in DCM (14 mL) was stirred at room temperature for 24 h. The
organic phase was washed with 0.5 N NaOH aqueous solution (3 × 15
mL) followed by water (2 × 15 mL). After removal of solvent, the
resulting mixture was dissolved in 20 mL DCM, and formalin (37 wt %
aqueous solution, 0.61 mL, 7.56 mmol, 1.1 equiv) was added. The
reaction mixture was stirred at room temperature in a 50 mL round-
bottom flask for 12 h. Then solvent was evaporated, and 50 mL of
hexanes was added to the mixture. The resulting suspension was
filtered through an alumina pad. The final product was obtained as a
EXPERIMENTAL SECTION
■
Materials. 1-Naphthol (for synthesis) was purchased from EMD
Millipore. N-Chlorosuccinimide (98%), piperidine (99%), pyrrolidine
(99%), sodium methoxide (25 wt % in methanol), sodium thiosulfate
(99%), and 1,2,3,4-tetrahydroisoquinoline (95%) were used as
received from Sigma-Aldrich. Acetonitrile, alumina, cyclohexane,
dichloromethane (DCM), diethyl ether, formaldehyde solution (37%
wt. in water), hexane, anhydrous magnesium sulfate (MgSO4), sodium
chloride (NaCl), sodium hydroxide (NaOH), and toluene were
obtained from Fisher Scientific. Ordinary compounds used were of
chemical reagent grades and utilized without further purification.
Synthesis of imines 1a−c were prepared following a reported
method.28
1
yellow solid (yield: 74.2%). H NMR (600 MHz, CDCl3, 20 °C) δ,
ppm: 8.15 (dd, 1H, Ar) 7.75 (dd, 1H, Ar) 7.45 (m, 2H, Ar), 7. 40 (d,
1H, Ar), 7.23 (d, 1H, Ar), 4.97 (d, 1H, O−CH2−N), 4.86 (d, 1H, O−
CH2−N), 4.13 (bb, 1H, Ar−CH(CH2−)−N), 3.02−1.56 (m, 8H,
−N−CH2−CH2−CH2−CH2−CH(CH2−)−). FT-IR ν (cm−1):
2939−2748 (C−H alkane str), 1333 (O−CH2−N wagg), 1242 (O−
C antisym str), 1196 (C−N antisym str), 1055 (C−O sym str), and
916 (oxazine-related band). HRMS (EI) m/z, [M]+ calculated for
C16H17NO+: 239.13101; found: 239.13130.
Synthesis of 8,9,10,10a-Tetrahydro-6H-naphtho[2,1-e]-
pyrrolo[1,2-c][1,3]oxazine (Abbreviated as 1N-p-f). Betti reac-
tion for the formation of aminocycloalkylnaphthol was conducted by
following the procedure as described.29 A solution of 1a (0.284 g, 4.11
mmol, 1.2 equiv) in DCM (3 mL) was added to a solution of 1-
naphthol (0.494 g, 3.43 mmol) in DCM (3 mL). The reaction mixture
was magnetically stirred in a sealed 15 mL vial at room temperature for
Synthesis of 9,13b-Dihydro-6H,8H-naphtho[2′,1′:5,6] [1,3]-
oxazino[4,3-a]isoquinoline (Abbreviated as 1N-dhiq-f). A
solution of 2c (0.14 g, 1.07 mmol 1.2 equiv) in DCM (2 mL) was
B
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Scheme 2. Synthesis of Fused-Ring Naphthoxazines
added to a solution of 1-naphthol (1.29 g, 0.89 mmol). The reaction
mixture was stirred for 24 h at room temperature in a sealed 15 mL
vial. After evaporation of the solvent, diethyl ether (10 mL) was added;
the organic phase was washed with a 0.5 N NaOH aqueous solution (3
× 5 mL) as well as brine (2 × 5 mL) and dried over MgSO4. Then, the
solvent was removed, and the resulting product was dissolved in DCM
and used without further purification. To the above mixture in a 15 mL
vial, formalin (37 wt % aqueous solution, 0.26 mL, 3.21 mmol, 3
equiv) was added dropwise. Then the vial was sealed, and the reaction
mixture was magnetically stirred at room temperature overnight. After
removal of the solvent, the crude product was washed with ethanol
and acetone. Recrystallization in acetone afforded light orange-white
angle spinning (MAS) NMR probe. The experiments were conducted
at ambient conditions and the MAS frequencies up to 35 kHz. 1H
MAS NMR spectra have been acquired with 16 transients at the 30
kHz MAS spinning frequency and 100 kHz radio frequency nutation
for direct excitation. 13C cross polarization (CP) experiments have
been performed at the MAS frequency of 25 kHz with a CP contact
time of 1 ms, 20000 transients, and swept frequency two-pulse phase
modulation (TPPM) high power 1H decoupling during acquisition.
RESULTS AND DISCUSSION
■
In this study, in addition to the aforementioned modified
Mannich reaction and three-step reactions in the Introduction
section, another stepwise reaction is employed which gives rise
to a novel type of benzoxazine-like resin namely fused-ring
naphthoxazines. This approach involves the formation of first
aminocycloalkylphenols by the Betti reaction of cyclic imines
and activated phenols, followed by closure of the ring with
formaldehyde.29
1
crystals (yield: 64%). H NMR (600 MHz, CDCl3, 20 °C) δ, ppm:
8.20 (dd, 1H, Ar), 7.71 (dd, 1H, Ar), 7.48−7.42 (m, 3H, Ar), 7.33 (t,
1H, Ar), 7.28 (dt, 1H, Ar), 7.26 (d, 1H, Ar), 7.21 (d, 1H, Ar), 7.20 (d,
1H, Ar), 5.60 (d, 1H, Ar−CH(Ar)−N), 5.50 (d, 1H, O−CH2−N),
5.22 (d, 1H, O−CH2−N), 3.34−2.76 (m, 4H, N−CH2−CH2−Ar).
FT-IR ν (cm−1): 2988−2833 (C−H alkane str), 1331 (O−CH2−N
wagg), 1246 (C−O antisym str), 1192 (C−N antisym str), 1065 (C−
O sym str), and 920 (oxazine-related band).
Characterization. Proton nuclear magnetic resonance (1H NMR)
spectra were acquired on a Varian Oxford AS600 at a proton frequency
of 600 MHz. The average number of transients for 1H NMR
measurement was 32. A relaxation time of 10 s was used for the
integrated intensity determination of 1H NMR spectra. Fourier
transform infrared (FT-IR) spectra were recorded using a Bomem
Michelson MB100 FT-IR spectrometer equipped with a deuterated
triglycine sulfate (DTGS) detector and a dry air purge unit. Co-
addition of 64 scans was recorded at a resolution of 4 cm−1. A TA
Instruments differential scanning calorimeter (DSC) Model 2920 was
used with a heating rate of 10 °C/min and a nitrogen flow rate of 60
mL/min. All samples were sealed in hermetic aluminum pans. Thermal
decomposition of the reacted materials was determined by
thermogravimetric analysis (TGA) using a TA Instruments Model
Q500 TGA. The TGA analysis was performed in a single heating run
from room temperature to 900 °C at a heating rate of 10 °C/min, with
a nitrogen flow rate of 60 mL/min. Size exclusion chromotography
(SEC) was performed on a TOSOH GPC instrument equipped with
Bryce-type double-path RI detector, UV-8320 detector, and
miniDAWN TREOS three-angle light scattering detector. The
stationary phase was TSKgel SuperMultiporeHZ-M HPLC column
maintained at 35 °C. The SEC mobile phase was HPLC grade THF at
a flow rate of 0.35 mL/min. The column set was calibrated against
polystyrene (TOSOH, Japan). High-resolution mass spectrometry
(HRMS) analyses were conducted on a KRATOS ms25 mass
spectrometer, using electron impact (EI) as ionization source at 140
°C, electron energy of 28 eV, and in positive mode. Solid State NMR
measurements have been performed at ambient conditions with a
Bruker Avance III console operating at the proton Larmor frequency
of 700.25 MHz using a commercial 2.5 mm double resonance magic
The fused-ring naphthoxazines synthesized in this study are
prepared by using 1-naphthol as activated phenols and
compounds 1a−c as the cyclic imines (Scheme 2). The
formation of aminocycloalkylphenols is carried out under mild
conditions and affords the desired product with good yields.
Closure of the ring is a straightforward reaction that is easily
accomplished with formaldehyde at room temperature. All
1
chemical structures were confirmed by H NMR. Figure 1
1
shows the H NMR spectrum and numbering of the positions
that are mentioned throughout this study using 1N-p-f as an
example. The numbering of the positions on the oxazine ring
are given by pure numbers respecting the benzoxazine
nomenclature, on the naphthalene portion of the molecule
following the naphthalene nomenclature using a number
accompanied by the character “n” for naphthalene, and finally
the fused ring using alphabetic letters (a, b, and c in the
example shown).
Unlike typical 1,3-oxazine protons in the oxazine ring which
usually appear as singlets, the proton signals of the oxazine
moiety in the fused-ring naphthoxazine ring at the 2-position
emerged as two different signals. This is caused by the more
rigid twisted-semichair conformation of the fused-ring naph-
thoxazine compared to ordinary benzoxazines and is attribut-
able to the two simultaneous rings attached to each side of the
oxazine ring, leading to an inhomogeneity of the magnetic
environment for each proton of the oxazine CH2 group.
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Figure 3. FT-IR spectra of (a) 1N-p-f, (b) 1N-p6-f, and (c) 1N-dihq-
f. Colored asterisks indicate characteristic C−H out-of-plane vibrations
◇
for substituted naphthalenes: two adjacent hydrogen atoms ( ), four
□
adjacent hydrogen atoms ( ), and four adjacent hydrogen atoms in
○
dihydroisoquinoline ( ) and out-of-plane ring deformation of
1
Figure 1. H NMR spectrum and molecular structure of 1N-p-f and
△
disubstituted naphthalenes ( ).
the numbering of its positions using pure numbers for the oxazine
positions (2 and 4), number accompanied by the character “n” for
naphthalene on the naphthalene moiety, and finally alphabetic letters
for the ring fused to the oxazine ring. The spectrum shown as an inset
is an expanded view of the aromatic proton region.
bending vibrations of substituted naphthalenes whose positions
have been correlated with the number of adjacent hydrogen
atoms in the aromatic rings. A common feature in all the fused-
ring naphthoxazines synthesized in this study is the presence of
two adjacent hydrogen atoms (positions 3 and 4) whose C−H
out-of-plane bending vibration can be clearly observed between
816 and 793 cm−1. Additionally, 1N-p-f, 1N-p6-f, and 1N-dhiq-
f show strong pair of bands between 762 and 735 cm−1 which
are associated with C−H out-of-plane bending of four adjacent
hydrogen atoms (positions 5, 6, 7, and 8). Furthermore, the
characteristic band due to out-of-plane ring deformation
vibration in disubstituted naphthalenes can be observed
centered at 653 cm−1 for 1N-p-f, 1N-p6-f, and 1N-dhiq-f.
The extra bands emerging in the shown region of the FT-IR
spectrum of 1N-dhiq-f are associated with the C−H out-of-
plane bending vibration of the four adjacent hydrogen atoms in
the dihydroisoquinoline moiety of the compound.
The structures of the synthesized fused-ring naphthoxazines
were also confirmed by FT-IR analysis. Figure 2 shows the FT-
Figure 2. FT-IR spectra of (a) 1N-p-f, (b) 1N-p6-f, and (c) 1N-dihq-
f. Colored asterisks indicate characteristic modes for oxazine ring:
−CH2 wagging ( ), C−O antisymmetric str ( ), C−N antisymmetric
str ( ), C−O symmetric str (⬢), and oxazine ring mode ( ).
Thermal Behavior of Fused-Ring Naphthoxazines. The
thermal behavior of the fused-ring naphthoxazines was studied
by differential scanning calorimetry. The thermograms of all the
compounds are shown in Figure 4 along with model
monofunctional benzoxazine, PH-a, that is synthesized from
phenol and aniline.
■
●
◆
▲
IR spectra of 1N-p-f, 1N-p6-f, and 1N-dhiq-f with character-
istic bands of the oxazine ring modes highlighted. Table 1
summarizes the characteristic values for CH2 wagging, Ar−O−
C symmetric and antisymmetric stretching, C−N antisym-
metric, and the oxazine ring mode.
In addition, Figure 3 shows the FT-IR spectra of fused-ring
naphthoxazines in the 1000−500 cm−l region. The bands in the
region of 900−650 cm−1 are associated with out-of-plane C−H
All thermograms show both an endothermic and an
exothermic event which are attributed to the melting and
ring-opening reaction of the different compounds, respectively.
Compared to the polymerization temperature of PH-a at 271
°C, the ring-opening reaction temperatures of all the fused-ring
naphthoxazines, 1N-p-f, 1N-p6-f, and 1N-dihq-f, are signifi-
cantly lower, specifically 189, 192, and 203 °C, respectively.
It is rather unusual to observe such low ring-opening reaction
temperatures in ordinary naphthoxazines and benzoxazines,
unless they are somehow influenced by some external or
internal driving forces, such as catalytic systems30,31 or
hydrogen-bond motives32,33 inducing such a property. Never-
theless, the reduction of the polymerization temperature might
be caused by multiple factors acting simultaneously. For
example, a strong basicity of the tertiary amine forming a
constitutive part of multicycle aliphatic structure, in addition to
an increased ring tension of the oxazine ring due to the
presence of the two fused-rings directly attached to it, the
Table 1. Characteristic Vibration Frequencies for Oxazine
Ring of Fused-Ring Naphthoxazines
vibration frequency (cm−1
)
CH2
wagg
C−O
C−N
C−O
oxazine-related
band
compound
antisym
antisym
sym
1N-p-f
1325
1333
1331
1240
1242
1246
1198
1196
1192
1049
1055
1065
924
916
920
1N-p6-f
1N-dhiq-f
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band of the open-ring structure could clearly be observed
emerging at 3329 cm−1. Interestingly, the region in the FT-IR
spectra between 900 and 600 cm−1, associated with the out-of-
plane C−H bending vibrations of substituted naphthalenes, was
also affected with increasing temperature. As the band due to
the two adjacent hydrogen atoms (out-of-plane C−H bend.) at
806 cm−1 decreased, the pair of bands centered at 755 cm−1,
associated with the four adjacent hydrogen atoms (out-of-plane
C−H bend.), experienced a clear change in shape. At the same
time, a new band of medium-weak nature appeared at 881
cm−1. This band falls into the region associated with the out-of-
plane C−H bending of an isolated hydrogen atom in a
substituted naphthalene, namely, between 900 and 855 cm−1.
Furthermore, the characteristic band due to the out-of-plane
ring deformation vibration in disubstituted naphthalenes at 653
cm−1, common in all the fused-ring benzoxazines, decreased as
temperature increased. What this seems to suggest is that
according to the proposed mechanism of ring-opening reaction
of benzoxazine,35 after the ring-opening step, the subsequent
electrophilic substitution occurs at the free para position (4n-
position) of the naphthol. However, the possibility of
substitutions in other position than 4n in the naphthalene
moiety should not be ignored. Thus, substitution in either the
6n- or 7n-position would also generate bands associated with
the out-of-plane C−H bending of both an isolated hydrogen
and two adjacent hydrogen atoms. A substitution in either the
5n- or 8n-position would, however, generate new bands in the
region 820−730 cm−1 due to the out-of-plane C−H bending of
three adjacent hydrogen atoms, which were not be observed.
Moreover, solid-state 1H and 13C NMR analyses were carried
out to further study the ring-opening polymerization reaction of
1N-p-f. Figure 6 shows solution and solid-state 1H NMR
spectra of 1N-p-f and 1N-p-f under MAS conditions after 5 h at
60 °C and solid-state 1H MAS NMR of poly(1N-p-f).
Although the spectrum of 1N-p-f in Figure 6b shows significant
line broadening, three regions can be clearly differentiated,
namely, 8.00−6.00, 4.50−3.50, and 2.50−0.50 ppm, whose
Figure 4. DSC thermograms of (a) 1N-p-f, (b) 1N-p6-f, (c)1N-dihq-
f, and (d) PH-a.
aromatic and the aliphatic ones, might act as internal driving
forces toward polymerization.
Ring-Opening Reaction of Fused-Ring Naphthoxa-
zines. FT-IR analysis was also used to study the ring-opening
reaction of these compounds. For instance, Figure 5 shows the
Figure 5. FT-IR spectra of 1N-p-f between the regions 4000−2000
and 1400−600 cm−1 at (a) rt, (b) 120 °C for 1 h, (c) 130 °C for 30
min, (d) 140 °C for 30 min, and (e) 150 °C for 30 min. Red and blue
dashed lines in the figure indicate where bands disappear and emerge,
respectively. Those processes are accompanied by the characteristic
splitting of the typical benzoxazine related band, reflecting the C−H
out-of-plane bending of the four vicinal aromatic hydrogens in the
naphthalene moiety combined with the O−C stretching mode from
the O bonded to the C at the 2-position, and other modes as minor
contributors.34
FT-IR spectra of a single sample of 1N-p-f at room
temperature, which then was consecutively heated at 120 °C
for 1 h, 130 °C for 30 min, 140 °C for 30 min, and finally at
150 °C for 30 min.
Characteristic bands associated with oxazine ring clearly
vanished as 1N-p-f was subjected to increasing temperatures,
namely, 1325, 1240, 1198, 1049, and 924 cm−1 (see Table 1 for
their assignment). Consequently, the O−H stretching vibration
Figure 6. 1H NMR spectra of (a) 1N-p-f (in solution using deuterated
chloroform), (b) 1N-p-f (under MAS at 30 kHz), (c) 1N-p-f after 5 h
at 60 °C (under MAS at 30 kHz), and (d) poly(1N-p-f) (under MAS
at 30 kHz).
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signals can be assigned to the aromatic region, the oxazine ring,
and the pyrrolidine moiety protons, respectively. As previously
reported,36 there is a constant frequency shift of about 0.9 ppm
between the proton signals between the solid and solution-state
1H NMR spectra. Figure 6c shows the spectrum of 1N-p-f
under MAS conditions, where the monomer starts to form
1
bigger structures. Because of their reduced mobility, the H
Figure 8. Idealized structure of the polynaphthoxazine, poly(1N-p-f),
obtained from heat-treated 1N-p-f.
signals broaden in the spectrum substantially, and the spectral
resolution of the different 1H sites is lost. In contrast to the fully
reacted polymer (Figure 6d), the new repeat unit shows some
resolution in the aromatic region and weak signals in the
naphtoxazine ring region between 4.50 and 3.50 ppm, which
are not observed in the poly(1N-p-f) sample due to ring-
opening polymerization reaction. Figure 7 shows the 13C NMR
was measured by SEC in tetrahydrofuran (THF) as the eluent
against polystyrene standards. Dynamic light scattering
information acquired from SEC presents molecular weight of
soluble part of 1N-p-f oligomer resulting from its partial
solubility in THF solvent. Results obtained using the ultraviolet
detector indicate number-average molecular weight (Mn) and
weight-average molecular weight (Mw) of 655 and 909 Da,
respectively, for the 1N-p-f oligomer. As can be estimated from
the aforementioned results, the degree of oligomerization of
1N-p-f oligomers is about 3 and 4 from the number-average
molecular weight and weight-average molecular weight,
respectively. This oligomerization behavior is very similar to
the well-known small oligomer formation of benzoxazine resins
containing only one oxazine ring.37,38 Additionally, the
complexity of degradation reactions proceed simultaneously
with the polymerization of naphthoxazines upon heat treatment
and may also contribute to the seemingly lower molecular
weight23 of the fused-ring oligomers. However, it should be
noted that the temperature applied to prepare 1N-p-f oligomers
was 150 °C, which is significantly lower than the exotherm peak
temperature, 189 °C, that was determined from DSC. It is
reasonable to extrapolate that larger molecular weight of
poly(1N-p-f) with higher heat treatment temperature could be
obtained. Because of the aforementioned solubility problem of
some fraction of the reacted resin, this low temperature was
chosen to unambiguously demonstrate the possibility of
polymerization, albeit forming small oligomers. SEC analysis,
together with the nonisothermal kinetics acquired by FT-IR, of
the 1N-p-f monomer suggests a similar polymerization pathway
as the typical benzoxazines. SEC data are summarized in Table
2.
Figure 7. 13C NMR spectra of (a) 1N-p-f (in solution using
deuterated chloroform), (b) 1N-p-f (after 5 h at 60 °C under MAS at
25 kHz), and (c) poly(1N-p-f) (under MAS at 25 kHz).
spectrum of 1N-p-f and 13C CP MAS spectra of both 1N-p-f
under MAS condition after 5 h at 60 °C and poly(1N-p-f). By
analogy to the solution-state 13C NMR spectrum, the carbon
signals of the oxazine ring in the 13C CP MAS spectrum can be
observed at 57 and 80 ppm. The signals between 20 and 49
ppm belong to the pyrrolidine moiety, while the rest of the
signals are from the aromatic carbons. Upon polymerization,
the carbon signal at 80 ppm, which relates to the 2-position of
the oxazine ring, vanished, whereas the carbon signal related to
the 4-position shifted from 57 to 62 ppm. Further indication of
polymerization reaction can be observed in the changes
occurred in the aromatic region, especially in the reduction of
the intensity of the signal at 119 ppm, which is assigned to the
para-activated carbon in the naphthalene moiety at the 4n-
position.
Although further detailed analysis will be required to
understand the exact structure of the polymer, based on the
FT-IR and solid state NMR analyses, the following most likely
structure of poly(1N-p-f) in idealized manner can be presented
in Figure 8.
Size Exclusion Chromatography (SEC) Analysis. The
1N-p-f model monomer was subjected to heat treatment at 150
°C for 2.5 h. Molecular weight of the heat treated monomer
Table 2. SEC Results of Oligomer from 1N-p-f Monomer
with Heat Treatment at 150 °C for 2.5 h
Thermal Stability. To study the thermal stability of the
materials generated upon polymerization of the fused-ring
naphthoxazines, the three resins were polymerized under the
same conditions. The polymerization cycle for every sample
consisted of four consecutive heating steps of 1 h each at the
following temperatures; 150, 180, 200, and 220 °C. The
resulting materials were studied by thermogravimetric analysis
(TGA), and the results are shown in Figure 9.
As can be observed from Figure 9, all polymers obtained
upon polymerization of the fused-ring naphthoxazines prepared
in this study showed a similar degradation profile. Two main
degradation steps are observed from the first-derivative curves
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cumulative heating experiments, indicating a similar ring-
opening reaction upon thermal polymerization. Together with
the thermal behavior investigation by DSC studies, a
significantly lower polymerization temperature than traditional
benzoxazine monomers was confirmed for this novel class of
resins, namely fused-ring naphthoxazines.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: hxi3@case.edu (H.I.).
*E-mail: pxf106@case.edu (P.F.).
ORCID
Hatsuo Ishida: 0000-0002-2590-2700
Notes
The authors declare no competing financial interest.
Figure 9. TGA thermograms of (a) poly(1N-p6-f), (b) poly(1N-p-f),
and (c) poly(1N-dhiq-f). Inset shows the derivative of the weight loss
of poly(1N-p6-f) (green line), poly(1N-p-f) (blue line), and
poly(1N-dhiq-f) (red line) as a function of the temperature.
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