Intrinsic self-initiating thermal ring-opening polymerization of 1,3-benzoxazines without the influence of impurities using very high purity crystals

Article abstract of DOI:10.1002/pola.28723

A phenol/aniline type monofunctional benzoxazine monomer, PH-a, is synthesized and highly purified to study the intrinsic thermal ring-opening polymerization of benzoxazines without the influence of any impurity. The successful synthesis of the monomer and its corresponding chemical structure are confirmed by Fourier transform infrared spectroscopy (FTIR) and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy. Purity of the compound is evaluated through differential scanning calorimetry (DSC) as well as elemental analysis (EA). Moreover, the thermal behavior of benzoxazine monomer toward polymerization is also studied by DSC, indicating that the highly purified benzoxazine monomer actually polymerize upon heating. The results present evidence of an intrinsic tendency for 1,3-benzoxazines to undergo thermally induced ring-opening polymerization upon heating only without any impurity participating during the reaction. This reveals that polybenzoxazines can be obtained by both the traditional thermally accelerated (or activated) polymerization, where impurities or purposefully added initiators are involved in the reaction; or, by the classic thermal polymerization, where only heat is enough to initiate the reaction.

Full text of DOI:10.1002/pola.28723

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Intrinsic Self-Initiating Thermal Ring-Opening Polymerization of
1,3-Benzoxazines Without the Influence of Impurities Using Very High
Purity Crystals
1
1,2
1
Lu Han, Marıa Laura Salum, Kan Zhang,3 Pablo Froimowicz,1,4 Hatsuo Ishida
ꢀ
1Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202
2
ꢀ
ꢀ
CIHIDECAR-CONICET y Departamento de Quımica Organica, Facultad de Ciencias Exactas y Naturales Institution Universidad de
Buenos Aires, Pabellon II, 3er piso, Ciudad Universitaria, Buenos Aires 1428, Argentina
3School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
4Design and Chemistry of Macromolecules Group, Institute of Technology in Polymers and Nanotechnology (ITPN),
UBA-CONICET, School of Engineering, University of Buenos Aires, Buenos Aires CP C1127AAR, Argentina
Correspondence to: H. Ishida (E-mail: hxi3@cwru.edu) or P. Froimowicz (E-mail: pxf106@case.edu)
Received 21 April 2017; accepted 5 July 2017; published online 00 Month 2017
DOI: 10.1002/pola.28723
ABSTRACT: A phenol/aniline type monofunctional benzoxazine
monomer, PH-a, is synthesized and highly purified to study the
intrinsic thermal ring-opening polymerization of benzoxazines
without the influence of any impurity. The successful synthesis
of the monomer and its corresponding chemical structure are
confirmed by Fourier transform infrared spectroscopy (FTIR)
and 1H nuclear magnetic resonance (1H NMR) spectroscopy.
Purity of the compound is evaluated through differential scan-
ning calorimetry (DSC) as well as elemental analysis (EA).
Moreover, the thermal behavior of benzoxazine monomer
toward polymerization is also studied by DSC, indicating that
the highly purified benzoxazine monomer actually polymerize
upon heating. The results present evidence of an intrinsic
tendency for 1,3-benzoxazines to undergo thermally induced
ring-opening polymerization upon heating only without any
impurity participating during the reaction. This reveals that pol-
ybenzoxazines can be obtained by both the traditional ther-
mally accelerated (or activated) polymerization, where
impurities or purposefully added initiators are involved in the
reaction; or, by the classic thermal polymerization, where only
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heat is enough to initiate the reaction.
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cals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 00, 000–
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KEYWORDS: benzoxazines; mechanisms; polybenzoxazines;
structure–activity relationships; thermal polymerizations
desire lower polymerization temperatures than this typical
range. Many compounds have been reported to influence the
polymerization temperature of benzoxazines, including (1)
strong and weak carboxylic acids and even phenols;9 (2)
basic compounds such as amines and imidazoles;10 and (3)
metal-containing compounds such as metal halides.11
INTRODUCTION Benzoxazine is a newly developed thermo-
setting resin that can be synthesized by the Mannich conden-
sation from phenol, amine, and formaldehyde. Upon
polymerization, these resins form polybenzoxazines, which
have been extensively reviewed due to their outstanding fea-
tures.1 Among the major advantages of polybenzoxazines are
the low flammability,2 high thermal resistance3 even for fully
bio-based polybenzoxazines,4 near-zero shrinkage upon poly-
merization,5 low surface free energy,6 and excellent chemical
stability.7 One of the most interesting characteristics of this
class of polymer is the extraordinarily rich molecular design
flexibility that allows designing a vast variety of molecular
structures to tailor the desired properties.8
Although various promoting effects caused by catalysts on
the ring-opening polymerization have been studied, the
intramolecular catalyzing effect based on the units or within
the structures of benzoxazine molecule itself has rarely been
reported. In this regard, Andreu et al. evaluated the influence
of the electronic effects caused by different substituent
groups in the benzoxazine nuclei on the polymerization tem-
perature.12 The study particularly focused on several 3-
phenyl-3,4-dihydro-2-H-1,3-benzoxazine monomers with
electron-withdrawing or electron-donating groups in the 6
and 40 positions only. Complementing the prior study, Wang
Polybenzoxazines are in general produced via cationic ring-
opening polymerization by heating benzoxazine monomers
without added initiators and/or catalysts in the typical tem-
perature range of 150–230 8C; however, some applications
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et al. reported the influence of electronic effects but in this
case from the bridging groups of bisphenols on the ring-
opening polymerization of benzoxazines.13 They found that
electron-withdrawing groups promoted the thermally acti-
vated polymerization, as detected from a polymerization
temperature decrease by increasing the bond length and
lowering the bond energy of CAO in the oxazine rings.
polybenzoxazines. Since then, although still with comprehen-
sible minute differences, high reproducibility and consistency
were experimentally observed not only in the polymerization
itself but also in the polymerizations’ parameters for similar
systems carried out by different researchers at different pla-
ces. However, if impurities were indeed the promoters for
these polymerizations to happen, it would be extremely diffi-
cult to expect such a high and consistent reproducibility. The
vast variety of procedures reported in the literature to
obtain those resins used as raw materials, the actual differ-
ent background and skills of every researcher performing
the syntheses, and the unlikely possibility of having equal
quality for every single chemical compound and solvents uti-
lized, make impossible to originate the exact same impurity
at the exact same ratio in every synthesis carried out in all
laboratories. Thus, the existence of an uncatalyzed, common,
and intrinsic mechanism for the polymerization of benzoxa-
zine resins induced only by heat seems to be possible. It
must be emphasized at this stage, that we do not disagree in
any manner with the well-studied, supported, and estab-
lished self-catalyzed mechanisms accepted hitherto. For
example, it is well known that the polymerization of a given
monomer could be achieved by different mechanisms. It is
also possible for a polymerization to follow the same general
mechanism, after having undergone different initiation pro-
cesses. A good example here is styrene, which can undergo
anionic and radical polymerization, two different mecha-
nisms. Furthermore, the radical polymerization of styrene
has also been reported by different mechanisms, such as
ATRP,17 and RAFT polymerization18 in addition to the uncon-
trolled free radical polymerization.19 Even this last uncon-
trolled free radical polymerization, which follows one
mechanism, can still be initiated in different ways, thus
establishing thermally accelerated (or activated) polymeriza-
tions when radical initiators are used, or thermal polymeriza-
tions when no radicals are added to the system. The latter
has also been called spontaneous, self-initiated, or simply
thermal polymerization, and has been extensively studied.20
Recently, polybenzoxazines derived from benzoxazines con-
taining ortho-functional phenolic components have been
shown to possess significant advantages over their para-phe-
nolic counterparts.14 Among these ortho-functional benzoxa-
zines, ortho-amide benzoxazine, which can act as a precursor
for polybenzoxazoles,15 has been shown to polymerize at
much lower temperatures than any other known pure ben-
zoxazine without added initiators and/or catalysts. In fact, it
has long been hypothesized16 and recently demonstrated,4
the existence of an intramolecular 5-membered ring hydro-
gen bonding between the NH of the amide and the oxygen in
the oxazine ring acting as an internal incentive to stimulate
the ring-opening polymerization in a smart way, like a self-
complementary initiator.
It is also known that existing impurities generated during
the synthesis of the benzoxazine monomers can act as very
efficient initiators and/or catalysts in the polymerization. For
instance, free phenolic AOH groups in the resin, coming
from unreacted phenols or small oligomers formed during
the synthesis of the monomers, have been reported to effec-
tively lower the polymerization temperature.9 These impuri-
ties have played contrasting roles during the development of
the benzoxazine field. While they were helpful for lowering
the polymerization temperatures and broadening the range
of applications, they interfered with undertaking reliable
mechanistic studies strictly on pure benzoxazines. As a con-
sequence, the most accepted mechanisms assume the partici-
pation of a very small amount of a cationic initiator or
initiators. This is the main reason for which the use of the
term, thermal polymerization, has been discouraged for ben-
zoxazine polymerizations since the term implies a thermal
event like a bond cleavage, molecular rearrangement, or
spontaneous reaction induced only by heat in the absence of
any other participating substance. Hence, as the mechanisms
always assumed not only the existence but also the involve-
ment of those cationic species intermolecularly, the more
appropriate denomination is thermally accelerated (or acti-
vated) polymerizations.16(b) However, the close terminology
which fundamentally differs very much in meaning has
unfortunately continued to be misused without evidence of
its existence in the literature introducing a conceptual error.
To date, no paper that specifically aims at studying the possi-
bility of ring opening via thermally induced mechanism has
been reported.
Thus, based on these facts, we have developed a particular
interest in studying the possible intrinsic self-initiating ring-
opening polymerization of 1,3-benzoxazines induced only by
heat and without the influence of impurities. The results will
then reveal that polybenzoxazines might be obtained by ther-
mally accelerated (or activated) polymerizations when impu-
rities, or any added catalyst or initiator, are indeed
participating in the process; or, by thermal polymerizations
when only heat promotes the reaction in the absence of the
initiating impurities or added initiators.
It is, therefore, one of the purposes of this paper to obtain
strong experimental evidence of benzoxazine polymerization
by an intrinsic-ring opening mechanism, rather than the initi-
ation by the external initiators or impurities. To achieve this
goal, meticulous effort for monomer purification was carried
out. Thus, only single crystals of the monomers that were
prepared under carefully controlled environment were used
in this work. The detailed synthetic strategy, purifications,
and polymerization mechanisms are discussed in this article.
In the last few years, as a consequence of the increasing
demands for this exciting and growing new polymer field,
much more emphasis has been put on the purity of the
monomeric benzoxazines used to produce the resulting
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EXPERIMENTAL
Third Recrystallization of PH-a
PH-a (5 g) obtained from the second recrystallization was
dissolved in anhydrous n-hexane (100 mL). The solution was
placed in a 250 mL flask and covered with aluminum foil
with small holes. This flask was covered by an upside-down
1 L flask to reduce the rate of solvent evaporation. After 10
days, transparent square-rectangular cross-section columnar
crystals formed. The crystal was dried in a vacuum oven
overnight at 30 8C. Thermogram obtained for these crystals
is designated as sample 3.
Materials
Phenol (98%) and paraformaldehyde (96%) were used as
received from Sigma-Aldrich. Aniline (99%) was purchased
from Sigma-Aldrich and purified by distillation. Ethyl acetate
(ꢀ99.9%), toluene (ꢀ99.5%), hexane (ꢀ98.5%), chloroform
(ꢀ99.8%),
1,4-dioxane
(ꢀ99%),
sodium
hydroxide
(ꢀ97.0%), and sodium sulfate (ꢀ99%) were obtained from
Fisher Scientific and used as received. 4-Methoxyphenol
(99%) from Sigma-Aldrich was recrystallized several times
in ethyl acetate before use. Anhydrous n-hexane (ꢀ99.9%)
was purchased from Fisher Scientific, and kept over molecu-
lar sieves and packaged under argon in resealable Chem-
SealTM bottles.
Fourth Recrystallization of PH-a
This fourth recrystallization was carried out in a glove box.
PH-a (2 g) that were recrystallized three times as before
described were dissolved in anhydrous n-hexanes (8 mL) in
a 20 mL glass vial. Additional small amounts of anhydrous
n-hexanes (2 mL) were added every hour. After the fourth
time, all PH-a totally dissolved. The vials were covered with
aluminum foil with small holes. The vials were then covered
by an upside-down flask to ensure slow evaporation of the
solvent. The dryness of the glove box was monitored by
placing a piece of Na metal and checking the metallic shiny
surface of the metal. After 8 days, transparent square-
rectangular cross-section columnar crystals formed. The
crystals were dried in a vacuum oven overnight at 30 8C.
DSC thermogram carried out for of these samples are desig-
nated as sample 4.
Preparation of 3-Phenyl-3,4-Hidhydro-2H-Benzo[e][1,3]-
Oxazine (Abbreviated as PH-a)
Phenol (50 g, 0.53 mol), aniline (47.12 g, 0.51 mol), and
paraformaldehyde (37.99 g, 1.265 mol) were added to a
250 mL round bottom flask. The solvent-less synthesis
method was adopted by magnetically stirring the mixture at
90 8C for 2 h to minimize the presence of the solvent from
the final product. The product was then dissolved in toluene
and washed by NaOH (1M) three times, followed by distilled
water three times. The organic layer was collected after
washing. Toluene was eliminated by a rotary evaporator. The
product was dissolved in methylene chloride for column
chromatographic purification using hexane as the eluent. The
differential scanning calorimetric (DSC) thermogram carried
out for this product is designated as sample 0. Further puri-
fication was carried out and the results of these processes
are presented in the section. 1H NMR (DMSO-d6), d (ppm):
4.63 (s, 2H, ArACH2AN, oxazine), 5.42 (s, 2H, OACH2AN,
oxazine), 6.69–7.22 (mult., 9H, Ar). FTIR (KBr), cm21: 1225
(CAOAC asymmetric stretching), 937 (benzoxazine-related
band). Elemental analysis: detailed data and discussion about
the elemental analysis results will be shown in the Results
and Discussion sections.
Fifth Recrystallization of PH-a
Sample 4 was used to carry out the fifth recrystallization.
The procedure followed at this instance was a repetition of
the fourth recrystallization of PH-a. The obtained sample
was named sample 5.
Sample Preparation by the Addition of a Known
Concentration of 4-Methoxyphenol
Phenol is widely used as an initiator and catalyst for benzox-
azine polymerization, yet not easy to work with especially at
low and quantitative conditions. Given the close chemical
structure, 4-methoxyphenol was chosen as a model com-
pound for this purpose, in additions to the advantage of
being crystalline and having the melting temperature of
57 8C, which is very close to the melting temperature of PH-a
(59 8C). As a result, it makes the mixing easier and even more
as the two-phase will melt together at the same time. Homo-
geneity of the two compounds is essential to evaluate the
effect of the added initiator and/or catalyst (4-methoxyphe-
nol) to the monomer (PH-a). The PH-a and 4-
methoxyphenol were weighed using a quartz balance with a
sensitivity of 6 1 lg. The two compounds were mixed in a
glass vial and heated to 65 8C, followed by keeping it at the
melt state for 2 min. The mixture was fully mixed by shaking
in the glass vial. After cooling to room temperature, the mix-
ture was kept at room temperature overnight to recrystallize.
The sample was grounded by a mortar and pestle to fully
and evenly mix before recording the DSC thermograms. The
programed ramp rate was set as 10 8C/min until 65 8C,
keeping at 65 8C for 2 min to fully melt the crystals and
Recrystallization of PH-a
First Recrystallization of PH-a
Column chromatographically purified PH-a (20 g) was mixed
with normal hexane (80 mL) and the system was slowly
heated to 40 8C until all PH-a was dissolved. After 10 min at
room temperature, needle-like white crystals formed. The
crystals were dried in a vacuum oven overnight at 30 8C.
The DSC thermogram obtained for this sample is designated
as sample 1.
Second Recrystallization of PH-a
PH-a (16 g) obtained from the first recrystallization was dis-
solved in normal hexane (100 mL) with stirring. After 30
min at room temperature, needle-like white crystals formed.
The crystals were dried in a vacuum oven overnight at
30 8C. DSC thermogram obtained for this sample is desig-
nated as sample 2.
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homogenize and stabilize the system, and further heated to
300 8C at a ramp rate of 10 8C/min.
Preparation of Poly (PH-a)
Polymerization of PH-a was done by heating at a rate of 10
8C/min until onset temperature (260 8C), isothermal heating
for 1 h at 260 8C. Next, PH-a was heated at a rate of 10 8C/
min until exothermic peak temperature, isothermal heating
for another 1 h. Poly (PH-a) was obtained.
Characterization
1H and 13C nuclear magnetic resonance (NMR) spectra were
acquired on a Varian Oxford AS600 at a proton frequency of
600 MHz. The average number of transients for 1H and 13C
NMR measurement was 64 and 1024, respectively. A relaxa-
tion time of 10 s was used for the integrated intensity deter-
mination of 1H NMR spectra. Fourier transform infrared
(FTIR) spectra were obtained using a Bomem Michelson
MB100 FTIR spectrometer, which was equipped with a deu-
terated triglycine sulfate (DTGS) detector and a dry air purge
unit. Coaddition of 64 scans were recorded at a resolution of
4 cm21. A TA Instruments Differential Scanning Calorimeter
(DSC) Model 2920 was used with a heating rate of 10 8C/
min and a nitrogen flow rate of 60 mL/min. In the analysis
to determine the activation energy of benzoxazine polymeri-
zation, the samples (2.0 6 0.2 mg) were scanned at the dif-
ferent heating rates of 2, 5, 10, 15, 20 8C/min. All samples
were sealed in hermetic aluminum pans. Elemental analysis
(carbon, hydrogen, and nitrogen) was performed at Galbraith
Laboratories, Inc. The samples were shipped under argon.
PerkinElmer 2400 Series II CHNS/O Analyzer was used for
the elemental analysis. Samples were dried before the mea-
surement. Elemental analysis was performed under argon
atmosphere. Thermogravimetric analysis (TGA) was carried
out on a TA Instruments Q500 TGA with a heating rate of 10
8C/min under nitrogen at a flow rate of 60 mL/min.
FIGURE 1 FTIR spectrum of benzoxazine monomer, PH-a.
[Color figure can be viewed at wileyonlinelibrary.com]
chromatography and recrystallization, are followed. However,
the nature of impurities and purity of the benzoxazine used
in this work are essential parameters for this study. There-
fore, the full detailed description of the purification proce-
dure, followed by the characterization of the materials
obtained after each step to evaluate its successfulness
throughout the entire purification process is reported in the
Experimental section. Briefly, the product was first re-
dissolved in methylene chloride and then purified by column
chromatography using hexane as eluent. An aliquot of this
sample was kept apart as well as studied by DSC. Next, the
column chromatographed product was further purified by
successive recrystallization steps following different condi-
tions, every time more exhaustive than before, as fully
described in the Experimental section. After each recrystalli-
zation performed, aliquots were separated and evaluated by
DSC. All results will be discussed in the following section.
RESULTS AND DISCUSSION
Synthesis and Exhaustive Purification of PH-a
Synthesis of PH-a was carried out in a solvent-less method
following a modified reported procedure,21 as shown in
Scheme 1.
However, before going forward, a deeper spectroscopic char-
acterization of the obtained PH-a was necessary. In this
regard, FTIR allowed us to monitor the success of the syn-
theses. The FTIR spectra of the benzoxazine monomer, PH-a,
using the KBr pellet method is shown in Figure 1. The pres-
ence of the aromatic oxazine ring in the PH-a monomer is
indicated by bands centered at 1225 cm21, which is due to
the CAOAC antisymmetric stretching modes, and the charac-
teristic oxazine mode is observed at 937 cm21 for PH-a.22
The workout of the reaction crude was performed as usual.
The crude was dissolved in toluene, washed three times
with NaOH (1 M), and then three more times with distilled
water. Toluene was evaporated from the organic layer, thus
generating the crude product.
Figure 2 shows the 1H NMR spectra of ultrapure PH-a. The
1H NMR spectrum is consistent with the ultra-high purity of
the compound.
Purification of a synthesized compound is typically a routine
operation, which often does not require fully detailed
descriptions if known procedures, such as column
It shown in Figure 2 the characteristic spectrum of PH-a
where no signals other than those belonging to PH-a are
observed. It is worth noticing the two groups of signals sym-
metrically located at each side of the peaks associated to H2
and H4 from the oxazine ring of PH-a. These signals are
SCHEME 1 Solvent-less PH-a synthesis.
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FIGURE 2 1H NMR spectrum of PH-a. Satellites of each ACH2A
peak corresponding to H2 and H4 can be better observed in the
inset of the figure, which shows a close look near the baseline
within the region between 4 and 6 ppm. Spectrum recorded at
25 8C, using DMSO-d6 as solvent. [Color figure can be viewed
at wileyonlinelibrary.com]
called satellites and are important because often they are an
observable characteristic on the spectra of highly pure sam-
ples even though they are in no way proof of purity.
Evaluation of Purity
From the 1H NMR spectra, one can clearly see that no other sig-
nals other than those strictly belonging to the compounds are
observed in the spectra, except for the solvent peak (DMSO-d6)
and H2O contained in that solvent. However, NMR spectroscopy
is not sensitive enough to be considered as a purity criterion.
Therefore, it was important to use complementary techniques
to evaluate the purity of the resin synthesized in this work.
Thus, elemental analysis and DSC studies were carried out for
each crystalline benzoxazine resin system, and the results are
presented in Table 1 and Figure 4, respectively.
FIGURE 3 (a) DSC thermograms showing the change in the poly-
merization temperature of PH-a after successive recrystallization
processes. (b) Crystals of PH-a after all recrystallization pro-
cesses. [Color figure can be viewed at wileyonlinelibrary.com]
the purity of PH-a increased with each recrystallization step.
The polymerization temperature 272 8C of the final sample
was the highest since further purifications did not increase
the polymerization temperature (sample 5).
After the first two recrystallizations, needle-like crystals of
PH-a were obtained due possibly to the small amount of
impurities influencing the molecular packing. The third and
fourth time, the recrystallizations were carried out with
anhydrous n-hexane. As the purity of the crystals improved,
the form of the crystals changed from needle-like to a
square-rectangular cross-section column nature [Fig. 3(b)].
The fourth recrystallization was carried out in a glove box
under dried argon atmosphere, with copper catalyst to cap-
ture oxygen, which was a more stable environment. In those
conditions, the crystals of PH-a grew without the influence
of oxygen and moisture. As shown in Figure 3(a), the poly-
merization temperature of PH-a increased with the increas-
ing number of recrystallizations performed, indicating that
TABLE 1 Elemental Analysis Results of PH-a
Element
Calculated
Found
C
79.59
6.63
79.46
6.64
N
H
O
6.20
7.58a
6.34
7,56a
a
These values are obtained by difference.
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FIGURE 4 DSC thermograms, region between 240 and 190 8C showing the exothermic polymerization peaks of different mixture
samples with increasing percentages of 4-methoxyphenol: (a) 0.00% of 4-methoxyphenol (pure PH-a), (b) 0.23 mol% of 4-methoxy-
phenol, (c) 0.44 mol% of 4-methoxyphenol, (d) 0.56 mol% of 4-methoxyphenol, (e) 0.88 mol% of 4-methoxyphenol, and (f) 1.05
mol% of 4-methoxyphenol. [Color figure can be viewed at wileyonlinelibrary.com]
Table 1 shows the elemental analysis results. Samples were
dried before analysis for 24 h at 30 8C under vacuum; how-
ever, tiny amounts of water, moisture, or even humidity will
influence the results. Furthermore, the precaution of working
under argon as inert atmosphere instead of molecular nitro-
gen (N2) was taken into consideration thus minimizing the
influence of atmospheric N2 on the nitrogen content of our
samples. The results are summarized as follows.
substance. If in fact the external phenolic compound (in
many cases impurities from the synthesis) is required, the
polymerization exotherm peak temperature should increase
as the amount of the added phenolic compound approaches
zero mole %. However, if the intramolecular mechanism
based on an intrinsic ring-opening dominates without the
effect of the external initiator, the exotherm peak tempera-
ture should reach an asymptotic value.
To maintain the homogeneity of the samples and reproducibil-
ity of the DSC analyses results, two strategies were adopted:
The elemental analysis results summarized in Table 1 show
the excellent agreement between the calculated and found
percentage values for each element, thus evidencing that the
targeted compound, PH-a, was obtained in high purity.
1. 4-methoxyphenol was chosen for this experiment because it
has a melting point of 57 8C, which is very close to PH-a
(59 8C), shows similar solubility as well as good miscibility
with PH-a, and does not bear any polymerizable group.
2. The different phenolic content mixture sample was pre-
pared as shown in Table 2. The mixture samples were all
weighed at similar weights, approximately 1.5 6 0.2 mg.
The specific weight of each sample is shown in Table 3.
Controlling the weight will maintain similar mass/heat
transfer balance, thus minimizing the influence on the
polymerization temperature triggered by any small differ-
ence in the amount of material in each specific sample.
3. The temperature program was set as follow: ramp rate at 10
8C/min to 65 8C, kept at 65 8C for 2 min, and new ramp rate
Polymerization Study of PH-a with Known Amounts of
an Added Initiator/Catalyst
Working with a benzoxazine monomer which have demon-
strated to have a very high purity, allows us now to examine
the effect of a phenol-containing compound at precisely
known amounts on the polymerization temperature. By sys-
tematically varying the amount of a phenol-containing com-
pound, one might examine the mechanism of oxazine ring
opening, whether or not the ring opening is intermolecularly
induced by the presence of such a phenolic compound or
intramolecularly even in the absence of an external
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TABLE 4 DSC Exotherm Peaks for the Polymerization Process
TABLE 2 Weight of 4-Methoxyphenol and PH-a of Different
of Mixture Samples
Phenolic Content Mixture Sample
Phenol
content
Phenol content (mol%)
4-Methoxyphenol (mg)
PH-a (mg)
0.23
0.44
0.56
0.88
1.05
0.045
0.057
0.079
0.088
0.071
32.745
21.770
23.843
16.865
11.400
(mole
%)
1
2
3
Average
Standard
deviation
(8C)
(8C)
(8C)
(8C)
0
272
272
272
272.00
0.00
0.58
1.00
1.15
1.53
1.15
0.23
0.44
0.56
0.88
1.05
272
272
269
266
268
271
270
271
269
266
271
271
271
268
266
271.33
271.00
270.33
267.67
266.67
at 10 8C/min to 300 8C. The stabilization temperature (65
8C) was chosen safely right above the melting temperature
of both compounds and way below the onset of the exother-
mic peak, thus avoiding premature polymerization.
used in such elegant publications. They have demonstrated
in their articles such good purity through melting point and
elemental analysis. What needs to be mentioned is that they
have achieved such a high purity of the monomers, and yet
these benzoxazine monomers polymerized upon heating. The
controversy arises in the fact that if we take for granted the
accepted mechanism of benzoxazine polymerization where
only the impurities are the promoters for initiating the poly-
merization, these polymerizations should not have occurred
using monomers with such a purity. These results are in
accordance with our interest in elucidating a possible intrin-
sic ring-opening polymerization of benzoxazines. In other
words, the capacity of benzoxazine to polymerize only upon
heating without any other substance participating in the
reaction. This would scientifically establish for the first time
the concept of thermal polymerization of benzoxazines, in
addition-but not instead-to the general accepted thermally
accelerated (or activated) polymerization of benzoxazines
mechanism. Thus, the polymerization studies of the high
purity PH-a is presented next.
Figure 3 presents the results obtained by DSC showing the
exotherm peaks corresponding to the polymerization process
of all studied mixture samples. To evaluate the reproducibil-
ity of the results, each system was measured three times.
The average polymerization temperature and standard devia-
tion were calculated and are shown in Table 4 and Figure 4.
Figure 5 shows the change in the polymerization tempera-
ture of PH-a as a function of the increasing molar concentra-
tions of 4-methoxypheonol. The fitted data is represented by
the polynomial equation described by eq 1.
2
Tp5271:96–0:78½Cꢁ–4:21½Cꢁ
(1)
where [C] is the molar concentration of 4-methoxyphenol.
This correlation indicates an extrapolated polymerization
temperature of 271.96 8C at zero concentration of the pheno-
lic compound. This temperature is therefore considered as
the intrinsic ring-opening polymerization temperature of the
oxazine ring of PH-a without the influence of any residual
impurity behaving as initiator or catalyst.
The thermal behavior of PH-a toward polymerization was
then studied by DSC as depicted in Figure 6.
Thermal Behavior of PH-a
As briefly mentioned in the Introduction section, Andreu
et al. as well as Wang et al. presented extensive studies on
how the nature and strength of different substituents in the
benzoxazine monomers affect the polymerization tempera-
ture.12,13 To enhance the reliability of the influence of those
substituents on the polymerization mechanism, which should
not be affected by impurities, they have particularly paid
great attention to the purity of each one of the monomers
TABLE 3 Phenolic Content in Each PH-a/4-Metoxyphenol Mix-
ture and the Specific Weight of Every Sample Studied by DSC
Phenol content (mol%)
1 (mg)
2 (mg)
3 (mg)
0
1.529
1.621
1.574
1.365
1.508
1.364
1.398
1.559
1.580
1.300
1.371
1.376
1.420
1.505
1.360
1.577
1.717
1.359
0.23
0.44
0.56
0.88
1.05
FIGURE 5 DSC exotherm maximum corresponding to the poly-
merization process as a function of the concentration of a
phenol-containing compound (4-methoxypheonol). [Color fig-
ure can be viewed at wileyonlinelibrary.com]
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mechanism, the polymerization of benzoxazine proceeds
through the initiation, propagation, and finally termination
steps.
For many years, polymerization of benzoxazine resins have
been thought to be initiated without added initiators because
of the presence of cationic impurities remaining from the
monomer synthesis. Nevertheless, there was no detailed
study reported on this subject. From a different perspective
and despite achieving high purity and using only benzoxa-
zine single crystals in this study, benzoxazine reins still poly-
merized with similar rates, albeit somewhat at higher
temperatures, as shown in Figure 6.
It is known that for a reaction between two species to happen
both species should be sufficiently activated. The cases pre-
sented in this study evaluate a single compound reacting with
itself. Under these conditions, the compound should then pre-
sent at least two different positions appropriately and suffi-
ciently activated to be able to react between them. In this
regard, compound PH-a can be analyzed in detail as follows.
FIGURE 6 DSC thermogram of PH-a from 30 to 300 8C. The
endothermic peak at 59 8C is assigned to the melting while the
exothermic one at 272 8C to the polymerization of PH-a.
The thermogram of PH-a shows that this compound does
polymerize despite its high purity. The exothermic peak
assigned to polymerization has its maximum centered at 272
8C. It must be highlighted, however, that PH-a in this condi-
tion has polymerized at a noticeably much higher tempera-
ture than the usual temperatures reported for this
compound, usually around 260 8C.12,23 What is shown in this
study, however, is that even though the polymerization tem-
peratures is indeed affected, increased by the absence of
impurities, there is real polymerization of PH-a.
Figure 7 shows the positions in the benzoxazines that are
actually rich in a negative density, induced by electronic
delocalization along the structure. It can be clearly seen that
all these positions are not only activated but also reactive,
although perhaps at different and specific temperatures in
each case, to some chemical reactions. Thus, while a given
position may be responsible for the initiation process,
another one less favored might be the cause for crosslinking
and even side reactions.
As has been mentioned earlier, the main interest in this
study is in evaluating the possibility of an intrinsic ring-
opening polymerization of benzoxazines. This will then set
the bases for establishing a complementary form of initiation
for polymerizing benzoxazines. It is important at this point
to remember that the accepted mechanism is a thermally
accelerated (or activated) polymerization since there must
exist an impurity able to react with the benzoxazine to only
Polymerizations
In recent years, extensive works on polybenzoxazine chemis-
try have been reported by benzoxazine researchers.1(a) After
intensive studies of various cationic, anionic and radical ini-
tiators, and mechanistic understanding, it is accepted that
the ring-opening polymerization of benzoxazines proceeds
through a cationic mechanism. As any other polymerization
FIGURE 7 Resonance structures of the unsubstituted benzoxazine PH-a. Structures in involving the aromatic ring that was adjacent
to the oxazine ring (a) and the aniline moiety (b).
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FIGURE 8 Simplified mechanisms for the polymerization of benzoxazines. (a) The classically accepted thermally accelerated (or
activated) polymerization mechanism, where in addition to heat impurities must participate in the initiation step; and (b), thermal
polymerization mechanism, induced only by heat.
then be capable of undergoing propagation. A simplified rep-
resentation of this mechanism is depicted in Figure 8(a),
where the H1 provided by the impurity promotes the poly-
merization at a certain temperature. It is worth remembering
that benzoxazines polymerize via a cationic polymerization,
where the cation in the chain-like tautomer is attacked by a
nucleophile present in the reaction medium. Nevertheless,
and in agreement with reported data,12,13 the benzoxazine
single crystals synthesized in this work does undergo poly-
merization induced only by heat without any impurity par-
ticipating in or during the reaction. Therefore, under this
condition, benzoxazines are polymerized through a strictly
thermal polymerization. This conceptually new, but somehow
expected, mechanism is illustrated in Figure 8(b), where no
impurities are present and the polymerization occurs at a
certain temperature, which is understandably higher than
the temperature needed for the previous mechanism includ-
ing an impurity. It must be highlighted that both initiation
processes are, in a way, similar since both proceed establish-
ing the ring-chain tautomeric equilibrium induced by ther-
mal treatment. One important difference between these two
mechanisms is that in Mechanism A the presence of impuri-
ties will cause a significant shifting in the equilibrium
toward the chain-like tautomers as well as induce this equi-
librium to happen at lower temperature, thus promoting
eventually a lower polymerization temperature. A similar
equilibrium is present in Mechanism B, although this time
the equilibrium is much less favored than in the previous
case, thus needing higher temperatures to happen. Thus, the
absence of such impurities might be the simple reason for
the resins to need a higher temperature to yield the same
ring-chain tautomerism. Once the tautomeric pair is formed,
the carbocation will react through an electrophilic substitu-
tion on any activated position (preferably on the ortho-posi-
tion since is the most activated one) of other benzoxazine
monomer, thus starting and establishing the propagation
step. From this stage forward both mechanisms are very
likely to proceed in the same manner. The proposed mecha-
nisms are consistent with the previous concentration depen-
dent experiments where not only the presence of the
impurity influences the polymerization temperature but also
permits to see that increasing the concentration of the impu-
rity shifts the ring-chain tautomeric equilibrium to the chain-
like form thus inducing the polymerization to happen at
lower temperatures.
The ring-chain tautomeric equilibrium of benzoxazine resins
is where the cation responsible for the cationic polymeriza-
tion of benzoxazines is formed. This equilibrium is often sim-
plified as depicted in Figure 9.
However, a closer look to the chain-like tautomer shows that
not only the two zwitterionic structures often described are
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FIGURE 9 Ring-chain tautomeric equilibrium of the unsubsti-
tuted benzoxazine PH-a.
possible, but instead there are at least six (6) different struc-
tures resonating, as shown in Figure 10.
Resonance structures of the chain-like tautomer of PH-a
involving the charge delocalization in the aniline moiety are
much less favored and therefore unlikely to form. One reason
for this less favored charge delocalization on the aniline moi-
ety is that two positive charges will have to be far too close,
in fact right next to each other and competing for the same
nitrogen atom, thus making the system very unstable. It
must be emphasized, however, that the aniline moiety does
have the ability to delocalize those charges, although not in
the chain-like tautomer form.
FIGURE 11 Plots to graphically calculate the activation energy
(Ea) of the polymerization reaction of PH-a applying the Kissinger
and Ozawa methods. In the plots, b is the constant heating rate
and Tp is the maximum value of the exothermic polymerization
peak. Linear regression obtained for each method is: R2 5 0.996
for the Kissinger method and R2 5 0.997 for the Ozawa method.
[Color figure can be viewed at wileyonlinelibrary.com]
All these frequently overlooked chemical structures, most likely
present in very little amounts, bear their own reactivity and
are reactants for side reactions to happen during the entire
polymerization process, including the initiation, propagation,
and termination steps. Thus, the combination of all these sys-
tems reacting simultaneously makes difficult a fair comparison
on how an “initiator” acts in a conventional controlled/living
(radical or ionic) polymerization or in benzoxazine polymeriza-
tion. This is, for instance, why there is not clear relationship
between the impurity concentration and the molecular weight
reached after polymerization of benzoxazines.
can be assumed to happen if the plots of ln (b/Tp2) and ln b
as a function of 1/Tp are linear (being b the constant heating
rate and Tp the maximum value of the exothermic polymeri-
zation peak).24 Therefore, based on this concept, we graphi-
cally calculated the Ea of the polymerization reaction of PH-a
applying the Kissinger and Ozawa methods, and the results
are presented in Figure 11.
As shown in Figure 11, the Ea is easily calculated from the
slope of the resulting straight lines for the polymerization
reaction of PH-a, being 92.9 and 96.7 kJ/mol for the Kissin-
ger and Ozawa methods, respectively.
As the herein uncovered self-initiation process occurs with-
out disrupting the rest of the mechanism for this resin, the
entire polymerization should proceed through a single mech-
anism. Therefore, the graphical manner to calculate the acti-
vation energy (Ea) of this system can easily be applied to
corroborate this behavior. Specifically, a single mechanism
The most meaningful result at this point, however, is the fact
that in both cases the plots resulted in perfect straight lines.
This result indicates that the polymerization of the highly puri-
fied PH-a follows a single mechanism, same as the accepted
mechanisms where initiation is promoted by impurities.
Having observed an efficient self-initiated polymerization
occurring through a single mechanism for this benzoxazine
monomer, we developed an interest in comparing the thermal
properties of the resulting polymer against the ones commonly
reported in the literature which polymerizations are based on
impurities acting as initiators. This comparison serves as an
indication on the similarity between the possible chemical
structures of the polybenzoxazines obtained in each process.
In this regard, thermogravimetric analysis (TGA) is a very use-
ful tool to evaluate, although indirectly, the possible similarities
between those polymeric structures. This might be achieved
by studying the degradation profile and measuring the typical
thermal properties of the corresponding polymers.
FIGURE 10 Resonance structures of the chain-like tautomer of
the unsubstituted benzoxazine PH-a involving the aromatic ring
that was adjacent to the oxazine ring.
Thermal analysis and degradation studies of PH-a have been
reported and are often used for comparison, although the
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synthesized using different PH-a resins, thus suggesting that
polymeric structure should in consequence be reasonably
similar.
This last result complements our hypothesis where the initi-
ation step of the intrinsically self-initiated thermal polymeri-
zation does not affect in any way the propagation and
termination steps since the polymer structure should, in
principle, be the same.
Combining the previous results, it can be demonstrated that
benzoxazines are capable of undergoing self-initiation upon
heating and without the influence of any impurity, thus initi-
ating a strictly and intrinsic thermal polymerization. Then,
this self-initiation step is followed by the same propagation
and termination steps as the already proposed and accepted
thermally accelerated (or activated) polymerization promoted
by impurities. In other words, this self-initiating thermal
polymerization of benzoxazines is in full agreement with the
well-studied and established accelerated (or activated) poly-
merization mechanisms initiated/catalyzed by impurities
accepted hitherto.
FIGURE 12 TGA thermogram of the highly purified poly(PH-a).
[Color figure can be viewed at wileyonlinelibrary.com]
resin never presented the level of purity reported herein. As
most polybenzoxazines, poly(PH-a) presents a degradation
profile consisting of three main steps, being the evaporation
of amine moieties at the chain ends and branches the first
one, followed by loss of amines from the main chain, to
finally undergo simultaneous breaking of phenolic bonds and
decomposition of remaining Mannich bases.25 Similar results
have been obtained by TGA and pyrolysis when comparing
the respective first derivative mass loss with the total ion
current, TIC, curves.26
CONCLUSIONS
A phenol-aniline based benzoxazine single crystal was suc-
cessfully prepared and highly purified. A novel initiation step
for an intrinsic ring-opening polymerization of benzoxazines
is herein proposed. We have demonstrated in this study that
temperature alone is sufficient to initiate the polymerization
of benzoxazines.
The first step of this study was the actual polymerization of
the highly purified PH-a sample, which was carried out
reproducing a reported method to enhance the validity of
the comparison.27 Then, TGA was performed as usual and
the results obtained for the highly purified poly(PH-a) are
shown in Figure 12.
The results presented in this work demonstrating the intrin-
sic thermal polymerization of benzoxazines are in no way in
disagreement with any of the thermally accelerated (or acti-
vated) polymerization studies well-known and accepted in
the past. Instead, they are complementary. The relevance of
the herein presented results is that it has now been shown
that impurities are not necessarily needed to initiate the
polymerization. However, they do help to lower the polymer-
ization temperature. The implications of this result are not
only in proving the possibility of this inherent new property
of benzoxazines, namely “intrinsic ring-opening polymer-
ization,” but also in the purity in which new benzoxazines
and polybenzoxazines might be generated.
As shown from Figure 12, the TGA thermogram of the pol-
y(PH-a) synthesized in this work presents the typical three-
step degradation profile. Specific data and thermal properties
are summarized in Table 5.
The data shown in Table 5 shows good agreement for each
property between the different poly(PH-a) polymers
TABLE 5 Summary of Thermal Properties for the Highly Purified Poly(PH-a) Compared with Others from the Literature
Degradation step (8C)
Tg (8C)
Td5 (8C)
Td10 (8C)
Char yield (%)
From – to
From – to
From – to
References
Poly(PH-a)
Poly(PH-a)
Poly(PH-a)a
Poly(PH-a)b
120
163
n.r.c
n.r.c
259
288
326
260d
341
343
353
340d
44
180–300
190–315
190–330
190–350
300-450
315-460
330-440
350-460
450–650
460–650
440–550
460–650
This work
27(b)
40
35
n.s.e
25(a), 26
26
a
d
e
Data obtained by TGA.
Data obtained by pyrolysis.
n.r.: not reported.
Data estimated from the figure shown in original paper.
n.s.: not shown.
b
c
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