Hydrogen-bonding characteristics and unique ring-opening polymerization behavior of Ortho-methylol functional benzoxazine

Article abstract of DOI:10.1002/pola.28253

Monofunctional benzoxazine with ortho-methylol functionality has been synthesized and highly purified. The chemical structure of the synthesized monomer has been confirmed by ¹H and ¹³C nuclear magnetic resonance spectroscopy (NMR),

Full text of DOI:10.1002/pola.28253

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Hydrogen-Bonding Characteristics and Unique Ring-Opening
Polymerization Behavior of Ortho-Methylol Functional Benzoxazine
Kan Zhang,1,2 Pablo Froimowicz,2,3 Lu Han,2 Hatsuo Ishida2
1School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
2Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106
3Design and Chemistry of Macromolecules Group, Institute of Technology in Polymers and Nanotechnology (ITPN), School of
Engineering, University of Buenos Aires, Buenos Aires, CP 1127AAR, Argentina
Correspondence to: H. Ishida (E-mail: hxi3@cwru.edu) or P. Froimowicz (E-mail: pxf106@case.edu)
Received 7 July 2016; accepted 29 July 2016; published online 00 Month 2016
DOI: 10.1002/pola.28253
ABSTRACT: Monofunctional benzoxazine with ortho-methylol
functionality has been synthesized and highly purified. The
chemical structure of the synthesized monomer has been con-
firmed by 1H and 13C nuclear magnetic resonance spectroscopy
(NMR), Fourier transform infrared spectroscopy (FT-IR) and ele-
mental analysis. One-dimensional (1D) 1H NMR is used with
respect to varied concentration of benzoxazines to study the
specific nature of hydrogen bonding in both ortho-methylol
functional benzoxazine and its para counterpart. The
polymerization behavior of benzoxazine monomer has been
also studied by in situ FT-IR and differential scanning calorime-
try, experimentally supporting the polymerization mechanism
of ortho-methylol functional benzoxazine we proposed before.
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KEYWORDS: benzoxazine; hydrogen-bonding; ring-opening poly-
merization; thermosets; functionalization of polymers
described convenient use of hydroxyl group for polyconden-
sation and initiation of ring-opening of lactone. Thermally
induced polymerizations of benzoxazines containing a 2-(2-
hydroxyethoxy)ethyl group have also been studied.19 The
polymerizations of these monomers proceeded smoothly at
relatively low temperature may be due to the presence of
hydrogen bonding offered by OH group. In addition, Endo
and coworkers reported polymerization behavior of 3-(2-
hydroxyethyl)21,3-benzoxazine. They proposed that the
nucleophilic hydroxyl group would highly activate the ring-
opening of benzoxazine moiety in an intramolecular man-
ner.20 Recently, Baqar et al. synthesized a series of methylol
functional benzoxazine monomers with different hydroxy-
benzyl alcohol isomers. They proposed that the highest cata-
lytic effect observed was in fact caused by the presence of
an intramolecular hydrogen bond between the hydroxyl
group placed on ortho position and the oxygen in the oxa-
zine ring, which led to acceleration of the polymerization.21
As can be seen from the previous examples, there are vari-
ous effects which have been hypothesized to be the driving
force and/or the cause for the lower polymerization temper-
ature of benzoxazines having hydroxyl group.
INTRODUCTION Benzoxazine resin is an important thermo-
setting resin widely used in industrial applications, such as
in electronic packaging materials, materials for aerospace,
adhesives, and other transportation applications, as well as
ballistics.1,2 It has been extensively reviewed due to its out-
standing advantages.3–6 The most interesting and unique
characteristic of this class of polymers is their extraordinari-
ly rich molecular design flexibility that allows designing a
variety of molecular structures to tailor the desired proper-
ties. Additionally, benzoxazine monomers can be easily syn-
thesized from various phenols, amines, and formaldehyde.
This versatile synthetic method has allowed development of
various benzoxazines having functional groups such as
amide,7–9 imide,9–11 allyl,12,13 nitrile,14 furyl,15 and benzoxa-
zole,16 which were effectively used for various chemical
modifications of the benzoxazine monomers and the corre-
sponding polybenzoxazines including cross-linking reactions.
Polybenzoxazines can typically be produced via cationic ring-
opening polymerization by heating benzoxazine monomers
without added initiator and/or catalyst in the temperature
range of 150–220 8C. However, some applications require
lower polymerization temperature than this typical range.
Several benzoxazines and naphthoxazines containing hydrox-
yl group have been developed.17,18 These literatures
The intramolecular hydrogen bonding hypothesis can explain
the experiment results and let propose polymerization mech-
anisms. However, the intramolecular hydrogen-bonding
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system in benzoxazines has rarely been studied and the
intramolecular hydrogen-bonding in hydroxyl functional ben-
zoxazines still has not been experimentally demonstrated to
exist.22 Moreover, it is known that impurities that exist dur-
ing the synthesis of benzoxazine monomer acts as an effi-
cient initiator and/or catalyst. Therefore, it is essential that a
highly purified monomer must be used to study mechanistic
aspects of the ring-opening polymerization of benzoxazine.
For this reason, we used only single-crystal samples of ben-
zoxazine monomers that exhibit sharp differential scanning
calorimetry (DSC) melting endothermic peaks in this study.
FIGURE 1 Chemical Structures and full numbering of oHBA-a
and pHBA-a for complete identification of each position in the
benzoxazine nuclei and of every position of each substituent
present in the molecule. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
The main purpose of this work is to experimentally study
the intramolecular hydrogen-bonding interactions that are
possible for the hydroxyl-containing benzoxazine compound
oHBA-a (Fig. 1). In this compound, the methylol group is
placed at the 8-position (ortho with respect to the oxygen in
the oxazine ring) in the benzoxazine nucleus. For compari-
son, pHBA-a (Fig. 1), the methylol groups is placed at the 6-
position (para with respect to the oxygen in the oxazine
ring) was also investigated. The results of this study will
provide for the first time experimental data demonstrating
the existence of such intramolecular hydrogen bonding only
in ortho-methylol functional benzoxazine. At the same time,
our discovery of the unique polymerization behavior of
oHBA-a is described to further support the polymerization
mechanism of ortho-methylol functional benzoxazine we pro-
posed before, supplementing our earlier report on the syn-
thesis and polymerization mechanism of methylol-functional
benzoxazine monomers.21
Model 2920 was used with a heating rate of 10 8C/min and
a nitrogen flow rate of 60 mL/min. In the analyses to deter-
mine the activation energy of benzoxazine polymerization,
the samples (2.0 6 0.5 mg) were scanned at different heating
rates, specifically 2, 5, 10, 15, and 20 8C/min. All samples
were sealed in hermetic aluminum pans with lids.
Synthesis of (3-Phenyl-3,4-dihydro-2H-
benzo[E][1,3]oxazin-8-Yl)Methanol (oHBA-a)
Into a 50 mL round flask were added 1,4-dioxane (20 mL),
aniline (1.4 g, 15 mmol), oHBA (1.86 g, 1.5 mmol), and para-
formaldehyde (0.96 g, 30 mmol). The mixture was stirred at
100 8C for 48 h, and subsequently cooled to room tempera-
ture. Then the product was concentrated using a rotary
evaporator. The product was then re-dissolved in chloroform
and washed by water for three times. The chloroform solu-
tion was dried over anhydrous sodium sulfate to obtain the
crude product. Crude product was fractionated by the col-
umn chromatography (eluent: hexanes and ethyl acetate, vol-
ume ratio 5 4:1). After solvent removal, the resulting white
crystals were purified by recrystallizing in chloroform (yield
EXPERIMENTAL
Materials
2-Hydoxybenzyl alcohol (oHBA) (99%), 4-Hydoxybenzyl alco-
hol (pHBA) (98%), aniline, and paraformaldehyde (96%)
were used as received from Sigma-Aldrich. Ethyl acetate, tol-
uene, hexane, chloroform, 1,4-dioxane, sodium hydroxide
(NaOH), and sodium sulfate were obtained from Fisher sci-
entific and used as received. 3-phenyl-3,4-dihydro-2H-ben-
zo[e][1,3]oxazine (PH-a) and (3-phenly-3,4-2H-benzo[e][1,3]-
oxazin-6-yl)-methanol (pHBA-a) were prepared following the
methods developed in our laboratory, which were published
elsewhere.21
1
58%). H NMR (DMSO-d6), ppm: d 5 4.40 (d, –CH2-OH), 4.62
(s, Ar-CH2-N, oxazine), 4.93 (t, –OH), 5.41 (s, O-CH2-N, oxa-
zine), 6.90–7.24 (m, Ar). IR spectra (KBr), cm21: 3306 and
3212 (stretching of OH of methylol), 1498 (stretching of tri-
substituted benzene ring), 1221 (Ar-O-C asymmetric stretch-
ing), 929 (out-of-plane C-H of benzene ring to which oxazine
ring is attached).
Anal. calcd. for C15H15NO2: C, 74.69%; H, 6.22%; N, 5.84%.
Found: 74.29%; H, 6.30%; N, 5.71%.
Characterization
RESULTS AND DISCUSSION
1H and 13C nuclear magnetic resonance (NMR) spectra were
acquired on a Varian Oxford AS600 at a proton frequency of
600 MHz and its corresponding carbon frequency of 150.864
Synthesis of Ortho-Methylol Functional Benzoxazine
Monomer
MHz. The average number of transients for H and 13C NMR
1
Prior to this study, a series of methylol functional benzoxa-
zine monomers with different hydoxylbenzyl alcohol isomers
have been reported aiming to understand the isomeric effect
on ring-opening polymerization of benzoxazine.21 However,
the purity of oHBA-a monomers used in the previous work
was not sufficient due to the existence of the intramolecular
hydrogen bonding in oHBA, leading the difficulties of ring-
closing of oxazine ring. The potential impurities, such as phe-
nols, amines, and benzoxazine oligomers all show the
measurement was 64 and 1024, respectively. A relaxation
time of 10 s was used for the integrated intensity determina-
tion of 1H NMR spectra. Fourier transform infrared (FT-IR)
spectra were obtained using a Bomem Michelson MB100 FT-
IR spectrometer, which was equipped with a deuterated tri-
glycine sulfate (DTGS) detector and a dry air purge unit.
Coaddition of 64 scans was recorded at a resolution of
4 cm21. A TA Instruments Differential Scanning Calorimeter
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FIGURE 2 1H NMR spectra of ortho-methylol functional ben-
zoxazine (oHBA-a) in DMSO-d6. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 3 13C NMR spectrum of oHBA-a in DMSO-d6. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
promoting effect on the ring-opening polymerization of ben-
zoxazines.23,24 Therefore, column chromatograph followed by
exhaustive recrystallization were used to separate the impu-
rities in order to obtain high purity benzoxazine monomers
in this study.
of-plane absorption mode of benzene with an attached oxa-
zine ring is located at 929 cm21 26
The excellent agreement
.
between the calculated and observed data from the elemen-
tal analysis of the purified sample, it shows that the targeted
compound oHBA-a was obtained in high purity.
1H NMR Studies in Solution
In order to better understand the hydrogen-bonding interac-
tions in oHBA-a, and for comparison, pHBA-a was also
studied.
Formation of the structures of oHBA-a was confirmed using
1H NMR, 13C NMR, FT-IR, and elemental analysis. Figure 2
shows the 1H NMR spectrum of oHBA-a. Typically, benzoxa-
zine monomers have two singlet resonances with equal inte-
gration values in 1H NMR spectra due to the two –CH2– in
the oxazine ring. The characteristic resonances attributed to
the benzoxazine structure, Ar-CH2-N– and –O-CH2-N– are
observed at 4.62 and 5.41 ppm, respectively. Also, the 1H
NMR spectra confirm the presence of methylol group (-
CH2OH) from the resonances of –CH2-O and hydroxyl of
methylol group at 4.40 and 4.93 ppm, respectively.
1
In H NMR, the quality of alcohol signals usually depends on
the solvent used. In general, and for the very same OH
group, non-protic solvents provide better resolution than sol-
vents presenting the ability to carry out intermolecular
proton-deuterium exchange with the substrate. Clearly, more
acidic solvents produce higher intermolecular proton-
deuterium exchange rates. For example, when D2O and
13C NMR analysis was also performed to further confirm the
structures of oHBA-a as shown in Figure 3. The characteris-
tic carbon resonances of oxazine ring appear at 49.49 and
79.40 ppm for Ar-CH2-N– and –O-CH2-N-, respectively. The
resonance at 58.06 ppm is assigned to the –CH2OH of meth-
ylol group.
The FT-IR spectrum of oHBA-a -a using the KBr pellet meth-
od is shown in Figure 4. The hydroxyl stretching vibration
region in the FT-IR spectrum of oHBA-a shows bands at
3306 and 3212 cm21, corresponding to the stretch vibra-
tions of the free –OH groups and hydrogen-bonded –OH
groups, respectively. In addition, band characteristic of anti-
symmetric trisubstituted benzene that appear in oHBA-a at
1498 cm21 confirm the incorporation of methylol group into
benzoxazine monomer. Also, the presence of the benzoxazine
ring aromatic ether in the monomers is indicated by bands
centered at 1221 cm21, which is due to the C-O-C asymmet-
ric stretching modes.25 Furthermore, the characteristic out-
FIGURE 4 FT-IR spectrum of oHBA-a.
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FIGURE 5 Representative 1H NMR spectra of both compounds oHBA-a and pHBA-a. Spectra were recorded using CDCl3 as solvent
and at 25 8C. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
deuterated alcohols are used as solvents for alcohol-
containing compounds the OH groups are usually not
detected in the 1H NMR spectrum. This “disappearance” of
the OH signal in some solvents is often smartly exploited to
detect not only the presence of alcohols, but in occasions
also to distinguish between primary, secondary and tertiary
alcohols. However, if a solvent that is less acidic than D2O
but that still performs this intermolecular proton-deuterium
exchange, such as CDCl3, is used instead, broad signals nor-
mally not integrable for the OH groups are seen in the spec-
trum. This broadening is caused by the rate of the proton-
deuterium exchange, which is much lower in CDCl3 than in
D2O or deuterated alcohols, due to its lower acidity. This is,
however, applicable to free alcohol groups exhibiting high
mobility given by their good solvation in the solvent. Never-
theless, the opposite situation happens when the OH groups
are somehow involved in different interactions or stable
molecular motifs. This causes a diminution of their mobility
and/or accessibility, thus impeding their rate of intermolecu-
lar proton-deuterium exchange, which in turn favors a better
resolution of the OH signals. Sometimes, it is even possible
to measure the coupling constant for the OH signal with the
corresponding neighboring protons; however, this is certainly
not necessarily the case for all systems. For a single system,
for instance, it can be seen on the one hand that protons
inside of aggregates, nanoparticles or micelles,27 are mostly
not detected in the spectrum since their mobility are highly
decreased causing the broadening of their signals on the
order of hundreds of Hz thus making them imperceptible.28
The reactivity of OH and NH groups is also affected depend-
ing on their placement in these systems, low exposure to the
solvent gives rise to lees accessibility, and thus their reactivi-
ty is highly reduced.29–31 And, on the other hand, protons
that are on the surface and exposed to the solvent are easily
observed since they are well solvated.32
with the presence of hydrogen-bonding systems in each sam-
ple. Nevertheless, no further information about the nature of
these hydrogen-bonding can be concluded without further
and complementary analysis. In this regard, it has been
reported that hydrogen-bonding has also the effect of shift-
ing the OH signals downfield (high frequency, higher chemi-
cal shift).33 The magnitude of this shifting has also been
exploited as a manner to distinguish between inter- and
intramolecular hydrogen-bonding systems.34 Moreover, inter-
and intramolecular hydrogen-bonds can be further studied
and better identified using simple 1H NMR experiments, by
evaluating the corresponding chemical shifts for each OH sig-
nal as a function of the concentration.35,36
Figure 6 shows a summary of the chemical shifts for the OH
signals belonging to oHBA-a and pHBA-a as a function of
their concentrations, obtained from the 1H NMR spectra
using CDCl3 as a solvent. It can be seen that the OH signal
belonging to pHBA-a experienced an important chemical
FIGURE 6 Chemical shift summary of the OH signal belonging
to oHBA-a (᭿) and pHBA-a (•) as a function of the concentra-
tion. Spectra recorded using CDCl3 as solvent at 25 8C. Inset:
magnification of the plot at concentration between 0.00 and
0.60 mM, where only oHBA-a signals were detected. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
As shown in Figure 5, however, well defined and resolved
signals of the OH groups using CDCl3 as the solvent for the
simple 1H NMR spectra of the herein studied compounds
oHBA-a and pHBA-a were observed. This indicates that the
OH groups are involved in some interactions or forming sta-
ble structural motifs, and yet solvated. This is consistent
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participate in intermolecular hydrogen-bonding since they
are already forming intramolecular ones. This observation of
complex behavior is, at a first glance, counterintuitive as
ordinary strong intramolecular hydrogen bonds shows inde-
pendence of OH chemical shift as a function of concentration.
It is possible that the six-membered intramolecular
hydrogen-bond observed in this molecule is not that strong.
Thus, the nitrogen atom in the oxazine ring can disturb the
preexisting hydrogen-bonding and break up a small fraction
of the intramolecular hydrogen-bonds forming intermolecu-
lar hydrogen-bonds when the intermolecular distance
becomes shorter at high concentrations.
FIGURE 7 Proposed hydrogen-bonding interactions in pHBA-a
and oHBA-a. Long green dotted lines depict possible intermo-
lecular hydrogen-bonding in both pHBA-a and oHBA-a, while
red dotted lines represent the intramolecular hydrogen-
bonding only present in oHBA-a. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
Figure 8 shows a typical 1H NMR spectrum of oHBA-a that
is highly diluted using CDCl3 as solvent. Given the very low
concentration, two regions of interest are expanded for clari-
ty and presented next. The first region from 4.5 to 5.5 ppm
shows the two –CH2– belonging to the oxazine ring of the
benzoxazine, as well as the –CH2–OH in ortho position
respect to the oxygen of the oxazine ring. This last signal is
overlapped with the one from Ph–CH2–N5 present at 4.65
ppm. The second region from 2.0 to 2.3 ppm allows us to
see a very nicely resolved triplet belonging to the OH signal.
shift toward greater d (downfield) upon increasing the con-
centration, while the OH signal of oHBA-a shifted only slight-
ly for the same range of concentrations. A closer look at
concentrations below 0.60 mM (inset in Fig. 6) shows no
changes in the chemical shifts of the OH belonging to oHBA-
a, which remains constant even at very high dilutions. In the
case of pHBA-a, however, the OH signal is no longer seen at
these high dilutions.
A second piece of supporting evidence for the intramolecular
hydrogen-bonding in oHBA-a is obtained from the 1H NMR
spectra at very low concentrations where the dependence
upon concentration is no longer observed. As mentioned ear-
lier, using CDCl3 as solvent and at very low concentration
the OH signals are typically seen as broad signals usually not
integrable. Occasionally, OH signals might not be seen due to
a high rate of intermolecular proton-deuterium exchange
between the substrate and the solvent. However, in the case
of compound oHBA-a at low concentrations, discrete triplets
for the OH signals are observed, indicating that those OH
These results strongly suggest the existence of hydrogen-
bonding in both molecules (oHBA-a and pHBA-a), although
different in nature (Fig. 7).
The clear dependence of the OH chemical shift for pHBA-a
upon concentration is indication of intermolecular hydrogen-
bonding. The higher the concentration, the higher the down-
field chemical shift, due to the decreased in-average hydro-
gen-bonding distance between different molecules of pHBA-
a, strengthening the interaction. At concentrations below
0.60 mM, however, the OH signal is no longer seen, probably
because of the high rate of proton-deuterium exchange that
could take place at that diluted conditions by the loss of
intermolecular interactions among the benzoxazine mole-
cules. At these low concentrations, only the intermolecular
interaction between the benzoxazine and solvent molecules
exist.
In the case of oHBA-a, however, there seems to be a more
complex situation than for pHBA-a depending on the con-
centration range. First, and in contrast to what happens with
the OH signal in pHBA-a, in highly diluted solution the OH
signal is clearly seen and is independent of the concentra-
tion. This is a strong indication that the hydrogen-bond in
oHBA-a is intramolecular. Nevertheless, intermolecular
hydrogen-bond interactions start to be noticeable at concen-
tration higher than 2.50 mM since upon this concentration
slight concentration dependence is perceptible. This weak
dependence of the chemical shifts of the OH belonging to
oHBA-a compared to those from pHBA-a is explained by the
lower availability of the OH groups from oHBA-a to
FIGURE 8 1H NMR spectrum of oHBA-a, using CDCl3 as solvent
at a concentration of 0.16 mM. The expanded region between
4.5 and 5.5 ppm shows the two –CH2– belonging to the oxazine
ring and the –CH2–OH, which is overlapped with the signal Ph–
CH2–N5, while the expanded region between 2.0 and 2.3 ppm
shows the OH signal.
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TABLE 1 Results From the DSC Analysis and Literature of Ben-
zoxazine Monomers
Onset
Temp
(8C)
Max
Temp
(8C)
Heat of
Polymerization
(J/g)
Monomer
Ref.
oHBA-a
pHBA-a
PH-a
185
205
237
220
231
255
195
241
336
This work
21
21
The thermogram in Figure 9 shows a very sharp endother-
mic peak at 88 8C, indicating the excellent purity of oHBA-a.
Moreover, oHBA-a shows the onset of the ring-opening poly-
merization at 185 8C with its maximum centered at 220 8C.
Comparing to the unsubstituted PH-a, benzoxazines with the
methylol substituent have significantly reduced the onset
and maxima of polymerization temperature. The remarkably
high reactivity of methylol functional benzoxazine could be
caused by participation of polar and potentially nucleophilic
hydroxyl group from the view of electronic effects by the
substituents on the polymerization mechanism. Additionally,
oHBA-a is much more reactive than pHBA-a to imply that
the ortho position of the hydroxyl group accelerating the
polymerization more effectively compared with the para
position. Combining with the 1H NMR analysis of hydrogen
bonds in solution, the presence of intramolecular hydrogen
bonding between an methylol linkage and the adjacent oxa-
zine ring of oHBA-a would act as an incentive to further
stimulate the ring-opening polymerization. Also, the DSC
results in this study further support our previous proposed
mechanism where, the intramolecular hydrogen bonding ring
activates the oxazine ring to open at lower temperature.21
FIGURE 9 DSC polymerization behavior of monofunctional
benzoxazine monomers.
groups are not free but involved in hydrogen-bonded sys-
tems. The facts that these discrete and well resolved triplets
for the OH groups are seen even at high dilution (Fig. 8) as
well as their chemical shifts do not move upon increasing
the concentration (Fig. 6) are strong evidence supporting the
existence of intramolecular hydrogen-bonding in oHBA-a.
A third piece of supporting evidence for the intramolecular
hydrogen-bonding in oHBA-a is obtained after performing
straightforward recognition of exchangeable protons studies
by 1H NMR at very low concentrations as control experi-
ments. The protocol is as follows: a simple 1H NMR spec-
trum is recorded and fully assigned using CDCl3 as solvent.
Then, two drops of D2O are added to the same sample,
which is then shaken and allowed to separate into two
phases (the organic phase remains at the bottom as CDCl3 is
denser than water). Finally, the second spectrum is recorded
and fully assigned. The possible consequences of this control
experiment are (a) labile acidic protons belonging to the
substrate will be exchanged and no longer seen on the sec-
ond spectrum; (b) no signals are vanished on the second
spectrum, demonstrating that no labile acidic protons are
present; or (c) known labile acidic protons present in the
substrate are strongly participating in hydrogen-bonding,
and therefore are unchangeable due to their involvement in
that interaction. The last situation was observed when a
highly diluted sample of oHBA-a was studied. This result is
supporting once again the intramolecular hydrogen-bonding
system present only in oHBA-a.
Interestingly, the exothermic peak of oHBA-a is significant
broader and the value for the polymerization heat is much
lower compared with other traditional benzoxazine resins.
The occurrence of methylol condensation reaction together
Polymerization Behavior of oHBA-a
The polymerization behavior of ortho-methylol functional
benzoxazine monomer was studied by DSC as depicted in
Figure 9 and the results are summarized in Table 1. In addi-
tion, the DSC thermograms for the unsubstituted PH-a as
well as the para-methylol functional benzoxazine pHBA-a,
which have been reported elsewhere,21 are also summarized
in Table 1 for comparison.
FIGURE 10 DSC curves of oHBA-a at different heating rates.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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Therefore, the above experimental results further and
strongly support the ortho-methylol mechanisms for the
crosslinking reaction which we have been reported
elsewhere.21
CONCLUSIONS
The ortho-methylol functional benzoxazine monomer was
successfully synthesized and highly purified. The concentra-
tion dependent 1D 1H NMR results indicate the existence of
the intramolecular hydrogen bonding in ortho-methylol func-
tional benzoxazine. The DSC and FT-IR results show that the
occurrence of both condensation reaction and ring-opening
polymerization during thermal treatment of ortho-methylol
functional benzoxazine. Both experimental studies, hydrogen-
bonding interactions and polymerization, further and strong-
ly support the ortho-methylol mechanisms for the crosslink-
ing reaction we proposed.
FIGURE 11 FT-IR spectra of oHBA-a after various thermal treat-
ment. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
with ring-opening polymerization of ortho-methylol bifunc-
REFERENCES AND NOTES
tional benzoxazine has been confirmed elsewhere.37
1 F. Holly, A. C. Cope, J. Am. Chem. Soc. 1944, 66, 1875–1879.
2 X. Ning, H. Ishida, J. Polym. Sci., Part A: Polym. Chem. 1994,
32, 1121–1129.
Figure 10 shows the DSC thermograms of oHBA-a at differ-
ent heating rates. Figure 10 also shows the initial polymeri-
zation temperature (Ti) and peak temperature (Tp) at
different heating rates. Normally, the endothermic and exo-
thermic peaks would shift to a higher temperature with the
increase in heating rate for benzoxazine resins. However, the
exothermic peak of oHBA-a does not follow the regulation
as the normal traditional benzoxazine resins, showing clearly
that another thermal event is involved in the ring-opening
reaction.
3 H. Ishida, In Handbook of Benzoxazine Resins; H. Ishida, T.
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methylene bridge at 1468 cm21 38
As shown in Figure 11,
.
additional characteristic signals of methylene bridge C-H
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1032 cm21 due to the stretching of C-O (methylol) confirms
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