Mechanistic pathways for the polymerization of methylol-functional benzoxazine monomers

Article abstract of DOI:10.1021/ma301963d

The polymerization mechanism of methylol-functional benzoxazine monomers is reported using a series of monofunctional benzoxazine monomers synthesized via a condensation reaction of ortho-, meta-, or para-methylol-phenol, aniline, and paraformaldehyde fol

Full text of DOI:10.1021/ma301963d

Article
pubs.acs.org/Macromolecules
Mechanistic Pathways for the Polymerization of Methylol-Functional
Benzoxazine Monomers
Mohamed Baqar,†,§ Tarek Agag,‡,⊥ Rongzhi Huang,‡ Joao Maia,‡ Syed Qutubuddin,*,†,‡
̃
and Hatsuo Ishida‡
†Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
‡Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States
ABSTRACT: The polymerization mechanism of methylol-
functional benzoxazine monomers is reported using a series of
monofunctional benzoxazine monomers synthesized via a
condensation reaction of ortho-, meta-, or para-methylol−
phenol, aniline, and paraformaldehyde following the traditional
route of benzoxazine synthesis. A phenol/aniline-type
monofunctional benzoxazine monomer has been synthesized
as a control. The structures of the synthesized monomers have
1
been confirmed by H NMR and FT-IR. The polymerization
behavior of methylol monomers is studied by DSC and shows
an exothermic peak associated with condensation reaction of methylol groups and ring-opening polymerization of benzoxazine at
a lower temperature range than the control monomer. The presence of methylol group accelerates the ring-opening
polymerization to give the ascending order of para-, meta-, and ortho-positions in comparison to the unfunctionalized monomer.
Furthermore, rheological measurements show that the position of methylol group relative to benzoxazine structure plays a
significant role in accelerating the polymerization.
more phenolic groups are formed, which promote an
autocatalytic polymerization process.6a,b
INTRODUCTION
■
Benzoxazine resins are a new class of thermosetting polymers
that are characterized by several unique properties such as near-
zero shrinkage or volumetric expansion upon polymerization,
considerable molecule-design flexibility, low water absorption
and flammability, low surface energy, and high glass transition
temperature.1 Additionally, the polymerization of benzoxazine
monomers takes place through thermally accelerated ring-
opening mechanism without any added initiator or catalyst.2
These numerous unusual properties of benzoxazine resins are
associated with the existence of inter- and intramolecular
hydrogen bonds in the network structure.3a−d Therefore, in
order to understand the structure−property relationships of the
polymer, it is of great importance to know the mechanistic
pathways of polymerization.
Although the polymerization chemistry of benzoxazine
monomers still remains rather poorly understood, the high
basicity of benzoxazine monomers, which is attributed to both
the oxygen and the nitrogen of the oxazine ring by Lewis
definition, suggests that the ring-opening polymerization of
benzoxazine monomers proceeds through a cationic mecha-
nism.4a−c During the polymerization, the monomer comes to
equilibrium with the corresponding zwitterionic intermediate
which eventually forms the polymer as depicted in Scheme 1.
Several fundamental studies for polymerization mechanisms
were conducted to understand the polymerization pathways of
benzoxazine monomers.5a−c These investigations suggest that,
as the polymerization of benzoxazine monomers progresses,
Wang and Ishida7 showed that monomers synthesized using
unsubstituted phenol exhibited variations of the ring-opening
polymerization temperature. A more detailed study by Andreu
et al.,8 who synthesized several substituted 3-phenyl-3,4-
dihydro-2H-1,3-benzoxazine monomers using substituent
phenol and aromatic amine, showed that increasing the
electron-withdrawing in the para-position of phenol exhibited
a noticeable decrease in the polymerization temperature. This is
attributed to the acidic phenol species that are formed due to
the electron-withdrawing groups. On the other hand, increasing
the electron-withdrawing in the para-position of the phenyl
substituent monomers showed an increase of the polymer-
ization temperature due to the destabilization of the
propagating iminium intermediates as proposed by McDonagh
and Smith.9a,b However, for the electron-donating substituents
no notable effect on the polymerization was observed regardless
of the position of the substituent.
The effect of functionalization and copolymerization of
carboxylic group into benzoxazine structure has been
studied10a−c and a significant decrease in polymerization
temperature was observed. Polybenzoxazine chains preferen-
tially form intramolecular 6-membered hydrogen bond,11a−i
which tends to slow the propagation reaction.12a,b This
Received: September 18, 2012
Revised: September 21, 2012
Published: October 1, 2012
© 2012 American Chemical Society
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Scheme 1. Mechanism of Ring-Opening Polymerization of
Benzoxazine Monomers
RESULTS AND DISCUSSION
Preparation of Monofunctional Benzoxazine Mono-
■
mers. A successful synthesis of methylol monofunctional
benzoxazine monomers has been achieved using aniline,
paraformaldehyde, and hydroxybenzyl alcohols (ortho-, meta-
and para-isomers) as well as phenol (P) for comparison as
indicated in Scheme 2. The monomers are designated as
Scheme 2. Preparation of Monofunctional Benzoxazine
Monomers
hydrogen bonding by COOH group to the newly formed
phenolic OH groups competes with the intramolecular 6-
membered ring formation between the phenolic and amine
groups,13 hence the retarding effect on chain propagation by
the intramolecular hydrogen bonding.
(oHBA-a, mHBA-a, and pHBA-a) in which the smaller italic
letters represent the position of methylol group into
benzoxazine structure, either in ortho, meta or para,
respectively. P-a represents a phenol/aniline type monofunc-
tional benzoxazine monomer synthesized as a control with no
methylol group in the structure.
Kiskan et al.14a,b synthesized hydroxyethyl terminated ether
chain-functional benzoxazine monomers and reported the
reduced polymerization temperature of these monomers in
comparison to ordinary unfunctional benzoxazine monomers.
Kudoh et al.15 subsequently studied the mechanistic aspects of
the polymerization and the role of hydroxylethyl group in
activating the ring-opening of hydroxyethyl functional benzox-
azine monomers. They found that the hydroxyethyl monomer
polymerized much faster than a similar structure of other N-
alkylbenzoxazines, which is attributed to the intramolecular
reaction of hydroxyl group with cationic moieties of the
zwitterionic intermediates formed by the ring-opening reaction
of benzoxazine to afford a 5-membered cyclic N,O-acetal.
Sudo et al.16 showed that increasing the bulkiness of the
substituent N-alkyl group on to benzoxazine monomers leads
to a decrease in the polymerization rate due to the release of a
larger amount of N-alkylimine as a volatile compound. Oie et
al.17 reported that N-allylbenzoxazine monomers exhibit a faster
rate of polymerization than N-(n-propyl)benzoxazines due to
the presence of allyl group on nitrogen atom. Consequently,
the intramolecular interaction between the cationic moieties
and electron-rich carbon−carbon double bond of this group
prompted the formation of the zwitterionic intermediates.
In continuation of our observed reduction of the ring-
opening polymerization temperature of methylol benzoxazine
monomers compared to the unfunctionalized monomers,18a−c
this paper is aimed at studying the role of incorporating
methylol groups into benzoxazine structure. Furthermore, the
polymerization mechanism of benzoxazine monomers with
various isomeric methylol positions is proposed.
1
The structures of the monomers were confirmed using H
1
NMR and FT-IR spectra. In the H NMR spectra, as depicted
in Figure 1, the typical resonances attributed to the benzoxazine
structure, Ar−CH2−N− and −O−CH2−N−, for pHBA-a,
mHBA-a, oHBA-a and P-a are observed at 4.62, 5.36; 4.64, 5.37;
4.66, 5.41; and 4.74, 5.48 ppm, respectively. Also, the 1H NMR
spectra confirm the presence of methylol group (−CH2OH)
from the resonance of −CH2−O of methylol group at 4.55,
4.56, and 4.64 ppm for pHBA-a, mHBA-a, and oHBA-a,
respectively.
There are a number of infrared absorption peaks, highlighted
in Figure 2, that are used to verify the formation of oxazine
rings in each monomer. For example, the characteristic out-of-
plane absorption modes of benzene with an attached oxazine
ring are located at 947, 950, 945, and 946 cm−1 for the pHBA-a,
mHBA-a, oHBA-a, and P-a, respectively.19 Furthermore, the
presence of the benzoxazine ring aromatic ether in the
monomers is indicated by an absorbance peak centered in
the range of 1035−1032 cm−1 due to the C−O−C symmetric
stretching mode. Also, the peak between 1242 and 1226 cm−1
for the asymmetric stretching modes confirms the presence of
the oxazine ring in the monomer structure.20a,b Peaks
characteristic of asymmetric trisubstituted benzene that appear
in the methylol monomers between 1505 and 1500 cm−1
confirm the incorporation of methylol group into benzoxazine
monomers. Furthermore, the presence of methylol group in the
synthesized monomers is confirmed through the very broad
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1
Figure 1. H NMR spectra of benzoxazine monomers.
Figure 2. FT-IR spectra of benzoxazine monomers.
absorption peak between 3340 and 3320 cm−1 due to the OH
stretching mode.21a,b
Figure 3. DSC thermograms of benzoxazine monomers.
Effect of Methylol Group on the Polymerization
Behavior of Benzoxazine Monomers. The polymerization
profiles of methylol benzoxazine monomers were studied by
DSC as depicted in Figure 3 and the results are summarized in
Table 1. The thermograms show that the onset of the ring-
opening polymerization of unfunctionalized benzoxazine
monomer started at 237 °C with its maximum centered at
255 °C. However, the onset of polymerization is shifted in the
presence of methylol group to as low as 151, 183, and 205 °C
for oHBA-a, mHBA-a, and pHBA-a, respectively. In addition,
the peaks are significantly broadened and the maxima are
shifted to temperatures of 196, 214, and 231 °C for oHBA-a,
mHBA-a, and pHBA-a, respectively. Moreover, the methylol-
functional benzoxazine monomers show lower values for the
heat of polymerization compared to the control. For example,
the heat of polymerization values were 201, 203, and 241 J/g
for oHBA-a, mHBA-a, and pHBA-a, respectively. However, P-a,
which has no methylol groups, exhibits the higher exotherm of
336 J/g.
Table 1. Results of the DSC Analysis of Benzoxazine
Monomers
heat of polymerization
(J/g)
monomer onset temp (°C) max temp (°C)
P-a
237
151
183
205
255
196
214
231
336
201
203
241
oHBA-a
mHBA-a
pHBA-a
From the DSC results, the values of the heat of polymer-
ization for the functionalized monomers were lower than the
control, showing clearly that another thermal event is involved
in the ring-opening reaction of methylol benzoxazines. Since
the presence of methylol groups is the main feature of these
monomers, one possible thermal event may be attributed to the
release of water as a byproduct from the polymerization of
methylol groups similar to traditional phenolics.22a,b The
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occurrence of methylol condensation reaction together with
ring-opening polymerization of methylol benzoxazine mono-
mers has been confirmed elsewhere.18a
Chemorheological Studies of Monomers. Viscoelastic
properties during the polymerization of each monomer were
evaluated during isothermal polymerization at 140 °C. A
qualitative comparison of the rates of polymerization for the
monomers is determined by the increase of the viscoelastic
moduli as shown in Figure 4. The figure shows that methylol-
closed molecular structure in comparison to open structure that
tends to extend.
Since both the storage and loss moduli eventually attain
maximum values and remain at the plateaus, the following
rheological fractional conversion is defined assuming reaction is
complete:23
G′(t)
α =
G′(α)
(1)
where G′(t) is the storage modulus at time t and G′(α) is the
maximum modulus at the end of polymerization. The resultant
time conversion plots are shown in Figure 5. The fractional
conversion increases rapidly immediately after the reaction is
prompted, due to the double catalytic effects of methylol and
phenol as stated above.
Figure 4. Storage (G′) and loss (G″) modulus as a function of time at
◊
□
○
△ ▽
, ), and P-
140 °C for oHBA-a ( , ),mHBA-a ( , ◁), pHBA-a (
○
a (▷, ) monomers.
functional benzoxazine monomers react faster than the control
(P-a) because of the lack of initiating species other than the
impurity level phenolic structures. The concentration of the
phenolic structures builds gradually and autocatalytic process
starts, accelerating the rate of polymerization. In the polymer-
ization of P-a, G′ did not increase for 45 min, and then
increased suddenly. However, in the case of methylol
monomers, the onset of polymerization occurred at much
shorter times: oHBA-a exhibits the quickest onset of
approximately 6 min, followed by mHBA-a at 20 min and
pHBA-a at 32 min. This is because of the catalytic effect of the
methylol groups, in addition to the aforementioned autocata-
lytic effect of the phenolic structure produced by the ring-
opening reaction of the oxazine rings.
The DSC technique is often used to study the polymer-
ization behavior of benzoxazine monomers through ring-
opening polymerization. The observed values of the curing
peak of the DSC thermogram indicate the local reactivity of the
monomers toward ring-opening. The chemorheological results
are interpreted with emphasis on the onset of the increase in
storage modulus which also represents the reactivity of the
monomer. Therefore, the DSC peaks and G′ results are
consistent in their trend: (ortho, meta, para). However, the G′
and G″ crossover represents not the reactivity but the
formation of the global infinite network. Hence the network
formation does not have to follow the trend observed in
exothermic peak. In other words, the local reactions detected by
DSC do not necessarily have to agree with the global network
formation. This is the case if the local structure tends to form
Figure 5. Isothermal fractional conversion as a function of time for
methylol monomers.
Figure 6 shows a number of highlighted infrared bands,
which can be used to verify the ring-opening reaction and the
subsequent polymerization of oxazine rings in each monomer.
For example, the monomers, pHBA-a, mHBA-a, oHBA-a, and
P-a show a decrease in the characteristic absorption bands of
the benzene ring to which oxazine is attached that are located at
947, 950, 945, and 946 cm−1, respectively. In addition, the
absorption bands located in the range of 1242−1226 cm−1 due
to the symmetric stretching of C−O−C are decreased
confirming the ring-opening polymerization. Furthermore,
methylol monomers show a decrease in the peaks characteristic
of asymmetric trisubstituted benzene between 1505 and 1500
cm−1 while the appearance of new peaks around 1490 cm−1 due
to the formation of tetra-substituted benzene confirms the
polymerization of methylol benzoxazine monomers.
Both DSC and rheological analysis show that the methylol
group accelerates the autocatalytic ring-opening polymerization
of benzoxazine. The remarkable difference in reactivity of
methylol monomers over the unfunctionalized benzoxazine
monomer suggests a mechanism where the methylol group has
an important role to promote the ring-opening polymerization.
In Scheme 3, only the o-methylol mechanisms for the cross-
linking reaction are shown as an example. In this proposal, the
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moiety.4b,9a,14−16 Another type of zwitterionic intermediate (B)
is a tautomer of the intermediate (A) which is considered as a
carbocationic. Since the methylol monomers have a higher
possibility for hydrogen bonding, and the intermediate (A + B)
would be unstable because of the high concentration of OH
ions. Therefore, by the ionization of phenol and as nucleophilic
phenolic nuclei are strengthened, the methylene bridge
formation is favored by means of the formation of phenoxide
ions. The zwitterionic intermediate (A and B) then proceed
through different pathways where water is a byproduct of the
methylol condensation toward the formation of methylene
linkages. In addition, the resulting structure has an iminium and
phenoxide moieties, of which reactions can contribute to
increase number of cross-linking points and eventually a
network structure of polybenzoxazine. The proposed mecha-
nims in the current study is fundamentally different from the
mechanisms proposed by Kudoh et al. for hydroxyethyl
functional benzoxazine since methylol group is incapable of
forming the intermediate structure proposed by those
authors.15
Moreover, the DSC thermograms show that the ring-opening
polymerization is shifted to the lower curing temperature as the
methylol group is closer to the oxygen atom in the cyclic
benzoxazine. The results support the proposed mechanism
where, in the case of o-methylol, the resonance of the
benzoxazine ring is affected by the methylol to form the
hydrogen bonding. This ring activates the oxazine ring to open
at lower temperature. However, the mechanisms propose that
Figure 6. FT-IR spectra of unpolymerized and 140 °C polymerized
monomers.
monomer reaches equilibrium with the corresponding zwitter-
ionic intermediate (A) which has a phenoxide and an iminium
Scheme 3. Proposed Polymerization Mechanism of Methylol Benzoxazine Monomers
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stretching of Ar−O−C), 1033 (symmetric stretching of C−O−C),
950 (out-of-plane vibration, benzene ring to which oxazine is
attached).
benzoxazine ring is less affected by methylol in the case of meta-
and para-monomers as supported by the DSC thermograms.
1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.56 (s,
−CH2−O), 4.64 (s, C−CH2−N), 5.37 (s, N−CH2−O−), 6.79−7.30
(m, Ar).
CONCLUSIONS
■
A series of methylol functional benzoxazine monomers with
different hydroxybenzyl alcohol isomers were successfully
synthesized. The DSC results show that exothermic peaks
due to condensation reaction of methylol groups and ring-
opening polymerization of benzoxazine are 231, 214, and 196
°C for monomers that methylol group placed on para-, meta-,
and ortho-position, respectively. However, the exothermic peak
of unfunctionalized monomer shows a higher value of 255 °C.
The rheological study indicates that the onset of polymerization
occurs at much shorter times of 6, 20, and 32 min for oHBA-a,
mHBA-a, and pHBA-a compared to the unfunctionalized
monomer that takes 45 min. The highest reactivity of the
methylol monomers is attributed to the catalytic effect of the
methylol group on the ring-opening due to intramolecular
hydrogen bonding between the methylol and the oxygen in the
benzoxazine ring as proposed in the polymerization mecha-
nism.
Preparation of (3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazin-6-yl)methanol [abbreviated as pHBA-a]. In a 500 mL
bottom-rounded flask, pHBA (14.95 g, 120 mmol), aniline (11.18 g,
120 mmol), and paraformaldehyde (7.52 g, 250 mmol) were mixed
together and refluxed for 6 h in toluene (135 mL). The product was
concentrated using a vacuum evaporator and redissolved in chloroform
followed by base-washed then once with water. The product was then
dried over anhydrous sodium sulfate and recrystallized from
chloroform to yield a white product (yield: 21.15 g, 73%).
IR spectra (KBr, cm−1): 3340 (stretching of OH of methylol), 1500
(stretching of trisubstituted benzene ring), 1228 (asymmetric
stretching of Ar−O−C), 1032 (asymmetric stretching of C−O−C),
947 (out-of-plane vibration, benzene ring to which oxazine is
attached).
1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.55 (s,
−CH2−OH), 4.62 (s, C−CH2−N), 5.36 (s, N−CH2−O−), 6.80−
7.28 (m, Ar).
Characterization and Measurements. Proton nuclear magnetic
resonance (1H NMR) spectra were acquired on a Varian Oxford
AS300 at a proton frequency of 300 MHz using an average number of
transients of 64. A relaxation time of 10 s was used for the integrated
EXPERIMENTAL SECTION
■
1
Materials. 2-Hydroxybenzyl alcohol (oHBA) (99%), 3-hydrox-
ybenzyl alcohol (mHBA) (99%), 4-hydroxybenzyl alcohol (pHBA)
(98%), phenol (98%), and aniline (99%) were obtained from Sigma−
Aldrich and used as-received. Paraformaldehyde (96%) was obtained
from Acros Organics, USA. Toluene, ethyl acetate, chloroform,
hexanes (a mixture of isomers), and 1,4 dioxane were obtained from
Fisher and used as received.
intensity determination of H NMR spectra. Deuterated chloroform
(CDCl3) was used to obtain the spectra with tetramethylsilane (TMS)
as an internal standard. Fourier transform infrared (FT-IR) spectra
were acquired on a Bomem Michelson MB100 which was equipped
with a deuterated triglycine sulfate (DTGS) detector at a resolution of
4 cm−1 with 32 coadditions. The spectra were taken by casting a thin
film onto a KBr plate. Thermal analysis of the samples was performed
via differential scanning calorimetry (DSC) using TA Instruments
DSC model 2920 with temperature ramped at 10 °C/min and a
nitrogen flow rate of 65 mL/min. All samples were crimped in
hermetic aluminum pans with lids. The evolution and comparison of
rheological properties during polymerization for each of the different
benzoxazine monomers were performed utilizing an Anton Paar
Rheometer (Model Physica MCR 501) with disposable parallel upper
and lower plates, measuring 25 mm and 50 mm in diameter,
respectively. Small amplitude oscillatory shear time sweep experiments
over temperatures at 140 °C were performed using a constant
frequency of 10 rad/s for all experiments, but continuously increasing
stress between a minimum of 1 Pa and a maximum of 400 Pa.
Although the actual stress ramp varied per experiment, it was always
set to a range of values that kept the fluids’ response in the linear
viscoelastic regime, while providing a high enough strain to provide
reproducible results.
Preparation of 3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazine [abbreviated as P-a]. P-a was prepared from phenol,
aniline and paraformaldehyde following the reported method.8 IR
spectra (KBr, cm−1): 1230 (asymmetric stretching of Ar−O−C), 1035
(symmetric stretching of C−O−C), 946 (out-of-plane vibration,
benzene ring to which oxazine is attached).
1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.74 (s, C−
CH2−N), 5.48 (s, N−CH2−O−). 6.99−7.44 (m, Ar).
Preparation of (3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazin-8-yl)methanol [abbreviated as oHBA-a]. Into a 100 mL
bottom-rounded flask were mixed oHBA (3.73 g, 30 mmol), aniline
(2.8 g, 30 mmol), and paraformaldehyde (1.9 g, 60 mmol) and the
mixture refluxed in 1,4-dioxane (35 mL) for 4 days. The product was
filtered and then concentrated using a rotary evaporator. The product
was then redissolved in ethyl acetate and base-washed followed by
once with water. The organic layer was then dried over sodium sulfate
anhydrous, followed by vacuum evaporation to afford viscous oily
product (yield: 3.71 g, 51%).
IR spectra (KBr, cm−1): 3320 (stretching of OH of methylol), 1505
(stretching of trisubstituted benzene ring), 1226 (asymmetric
stretching of Ar−O−C), 1032 (symmetric stretching of C−O−C),
945 (out-of-plane vibration, benzene ring to which oxazine is
attached).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sxq@case.edu.
Notes
1H NMR spectra, δH (300 MHz, CDCl3, TMS, ppm): 4.64 (s,
−CH2−OH), 4.66 (s, C−CH2−N), 5.41 (s, N−CH2−O−), 6.90−
7.29 (m, Ar).
The authors declare no competing financial interest.
§
On leave from Azzaytuna University, Libya.
On leave from Tanta University, Tanta, Egypt.
⊥
Preparation of (3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazin-7-yl)methanol [abbreviated as mHBA-a]. In a 100 mL
bottom-rounded flask, mHBA (7.52 g, 60 mmol), aniline (5.64 g, 60
mmol), and paraformaldehyde (3.61 g, 120 mmol) were mixed
together and refluxed for 48 h in 1,4 dioxane (65 mL). The product
was concentrated using a vacuum evaporator and redissolved in ethyl
acetate followed by base-washed then once with water. The product
was then dried over sodium sulfate anhydrous, followed by vacuum
evaporation to afford a viscous product (yield: 9.12 g, 63%).
IR spectra (KBr, cm−1): 3325 (stretching of OH of methylol), 1500
(stretching of trisubstituted benzene ring), 1242 (asymmetric
ACKNOWLEDGMENTS
■
M. Baqar acknowledges the Ministry of Higher Education and
Scientific Research of Libya and Azzaytuna University-Libya for
a scholarship.
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