Benzoxazole resin: A novel class of thermoset polymer via smart benzoxazine resin

Article abstract of DOI:10.1021/ma300924s

Among the wide list of known high performance polymers, polybenzoxazoles (PBOs) have gained a prominent position as the most heat-resistant polyheterocyclic polymer. Nonetheless, PBOs have found applications in a rather restricted variety of technologies,

Full text of DOI:10.1021/ma300924s

Article
pubs.acs.org/Macromolecules
Benzoxazole Resin: A Novel Class of Thermoset Polymer via Smart
Benzoxazine Resin
Tarek Agag,*,† Jia Liu,† Robert Graf,‡ Hans W. Spiess,‡ and Hatsuo Ishida*,†
†Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, United
States
‡Max-Planck-Institut fur Polymerforschung, Postfach 3148, D-55021 Mainz, Germany
̈
ABSTRACT: Among the wide list of known high performance
polymers, polybenzoxazoles (PBOs) have gained a prominent
position as the most heat-resistant polyheterocyclic polymer.
Nonetheless, PBOs have found applications in a rather restricted
variety of technologies, mainly in the form of fibers. Herein, we
report our pioneering work for producing cross-linked
polybenzoxazole via a novel route using recently developed
smart class of benzoxazine resins as precursors. This class of
benzoxazines incorporates multiple smart features all in one
molecule. The most attractive feature is its structural trans-
formation into a more thermally stable cross-linked polybenzox-
azole without the harmful consequences of traditional poly-
benzoxazole synthesis, such as the use of poly(phosphoric acid)
as solvent. By this smart conversion, the flame-retardant oxazole moieties are successfully incorporated into the network
structure. Further advantages of this new route for cross-linked polybenzoxazoles include outstanding flexibility in molecular
design, cost effectiveness, and easy processability.
polyethylene fibers (Spectra). The major disadvantage of this
method is the long-term instability of PBO fibers due to the
residual acid when exposed to moisture at moderate to elevated
temperatures, causing a hydrolytic degradation of the
benzoxazole ring.6 As a result, a premature failure in ballistic
performance of body armor containing the PBO fiber has been
reported.7
The second method is two-step condensation reaction of an
aromatic diacid chloride with a bis(o-aminophenol)s in solution
at room temperature to form poly(o-hydroxyamide), followed
by sequential thermal intramolecular cyclodehydration reaction
in the bulk state or in solution with loss of water to yield the
final aromatic polybenzoxazole.8−11 A chlorine-free synthetic
method is also applied to prepare poly(o-hydroxyamide)s using
aromatic dicarboxylic active diester and bis(o-aminophenol).
Recently, Mathias and others reported the possible thermal
conversion of α-hydroxypolyimides into polybenzoxazoles with
loss of carbon dioxide at temperatures over 400 °C.12−17 More
recently, however, Hodgkin et al. questioned this proposed
mechanism and reported that it actually yields cross-linked
polyimide containing biphenylene structure.18
1. INTRODUCTION
There is a growing interest for the next generation of materials
to meet recent demands by the space and electronics
technologies. Polymers of outstanding performance are a
crucial requirement for such applications. Aromatic poly-
benzoxazoles (PBOs) as a class of heterocyclic polymers are
ranked as materials of superb thermal stability, extremely high
tensile strength, and good chemical resistance. The excellent
performance of PBOs makes them attractive for many
applications; in particular, PBOs are suitable for ballistic
applications.1 The commercial applications of PBO fibers
include protective garments such as firefighter clothing, ballistic
vests and helmets, and reinforcing fibers for heat-resistant
conventional and molecular composites. Also, photosensitive
PBOs are attractive materials for microelectronics technology
because of their remarkable thermal and mechanical properties
and lower dielectric constants and loss factors than photo-
sensitive polyimides.2 There are two traditional methods for the
synthesis of aromatic PBOs. The first method is one-step
synthesis in which a direct polycondensation reaction takes
place between bis(o-aminophenol)s and aromatic diacids using
poly(phosphoric acid) (PPA),3 phosphorus pentoxide/meth-
anesulfonic acid (PPMA),4 or trimethylsilyl polyphosphate
(PPSE)/o-dichlorobenzene5 as the reaction medium. The only
successful commercial application of PBO produced by this
method is ultrahigh strength PBO fibers that possess superior
properties to other ultrahigh performance fibers, such as
polyaramids (e.g., Kevlar, Twaron) and ultrahigh modulus
While the very high rigidity of PBO molecules is responsible
for the outstanding performance, it also leads to difficulties in
synthesis and fabrication. The wholly aromatic PBOs suffer
Received: May 9, 2012
Revised: October 31, 2012
© XXXX American Chemical Society
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Scheme 1. Direct Polycondensation Synthesis of Polybenzoxazol
Scheme 2. Two-Step Condensation Synthesis of Polybenzoxazole via Intermediate Poly(o-hydroxyamine)
from various associated difficulties: (i) they are soluble only in
harsh acids, such as sulfuric, methanesulfonic, triflic, and
polyphosphoric acids, and complex-mediated Lewis acid/
solvent systems, such as AlCl3/nitromethane; (ii) the
decomposition temperature of PBOs is below their melting
points, which impedes the melt processing of PBOs; and (iii)
bis(o-aminophenol)s used as the main starting materials for
most synthesis approaches of PBOs are extremely expen-
sive.19,20 All of these shortcomings limit the application of this
highly useful class of high performance polymers. The
drawbacks of the aforementioned methods for polybenzoxazole
synthesis motivated us to develop a new route to solve, if not
all, many of the disadvantages of the traditional approaches.
Polybenzoxazines are a new class of high performance
polymers that have boundless potential for growth in research
and industrial applications.21 Compared with other classes of
polymers, polybenzoxazines exhibit high chemical and thermal
stability as well as low flammability. In addition, polybenzox-
azines possess low dielectric constants and dissipation factors,
low water absorption, and unusually high glass transition
temperatures. Their polymerization through a thermally
accelerated ring-opening mechanism generates no reaction
byproducts; furthermore, the presence of very stable intra-
molecular 6-membered-ring hydrogen bonding in the main
chain of polybenzoxazine leads to near-zero shrinkage upon
polymerization. The most interesting and unique characteristic
of this class of polymers is their extraordinarily rich molecular
design flexibility that allows designing a variety of molecular
structures of any desired properties.
Scheme 3. Preparation of N1,N3-Bis(2-
hydroxyphenyl)isophthalamide (Compound 1)
of DMAc. Triethylamine was added into the system afterward. The
solution was then cooled to 0 °C, and isophthaloyl dichloride (10.08 g,
49.7 mmol) was added dropwisely with stirring. The solution was kept
at 0 °C for 4 h at room temperature. After confirming the structure of
1
the product by H NMR, the reaction mixture was poured into cold
water to eliminate extra DMAc. The precipitate was filtered and dried
afterward under vacuum. Light pink crystal was obtained (yield: 85%);
mp 175 °C. 1H NMR (DMSO-d6), ppm: δ = 5.35 (s, OH), 6.80−8.50
(12H, Ar), 9.15 (s, NH). 13C NMR (DMSO-d6), ppm: δ = 125.8 (s,
Ar−C−NH−), 148.4 (s, Ar−C−OH) and 164.7 (s, CO).
2.3. Preparation of Difunctional o-Amide Benzoxazine.
Difunctional o-amide benzoxazines were synthesized via a solvent
synthesis method (as shown in Scheme 4). The as-synthesized amide
containing biphenol, Compound 1, aniline, and paraformaldehyde
were added to a round-bottom flask in stoichiometric amounts (1:2:4).
The reactants were mixed with 1,4-dioxane and further refluxed for 5
h. After cooling, the reaction mixture was poured into 100 mL of cold
water to give a powder-like precipitate. The product was further
purified by washing with 3 N NaOH solution and water to eliminate
the residual starting materials. The purified products were dried over
sodium sulfate, followed by filtering and drying under vacuum to
obtain a white fine powder (yield 82%). 1H NMR (DMSO-d6), ppm: δ
= 4.61 (s, Ar−CH2−N), 5.54 (s, −O−CH2−N), 6.62−8.57 (20H, Ar),
and 9.15 (s, NH). 13C NMR (DMSO-d6), ppm: δ = 47.84 (Ar−C−N),
77.68 (O−C−N), and 124.31 (C−NH) and 164.74 (Ar−CO−
NH). FTIR (KBr), cm−1: 3320 (N−H stretching), 1664 (carbonyl
stretching), 1466 (stretching of trisubstituted benzene ring), 1226
(asymmetric stretching of C−O−C), 1164 (asymmetric stretching of
C−N−C), 952 (out-of-plane C−H of benzene ring to which oxazine
ring is attached).
2. EXPERIMENTAL SECTION
2.1. Materials. o-Aminophenol (>98%), p-aminophenol (98%),
isophthaloyl dichloride (98%), triethylamine (99%), paraformaldehyde
(99%), sodium borohydride (>98%), sodium carbonate (99.8%), and
trifluoroacetic anhydride (>99%) were used as received from Sigma-
Aldrich. Aniline (98%), sodium sulfate anhydrous (99%), sodium
chloride (>99%), ethyl acetate, 1,4-dioxane, chloroform, tetrahydrofur-
an, dimethylacetamide (DMAc) hexane, 2-propanol, methanol,
xylenes, and sodium sulfate were obtained from Fisher Scientific.
2.2. Preparation of N1,N3-Bis(2-hydroxyphenyl)-
isophthalamide (Compound 1). As shown in Scheme 3, N1,N3-
bis(2-hydroxyphenyl)isophthalamide (1) was synthesized by reacting
o-aminophenol with isophthaloyl dichloride and triethylamine in
DMAc. o-Aminophenol (10.84 g, 99.3 mmol) was dissolved in 100 mL
2.4. Preparation of 3-Phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazin-6-amine (P-a-NH2) by Deprotection of 2,2,2-Trifluoro-
N-(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)-
acetamide (Compound 2). P-a-NH2 was prepared according to
methods reported elsewhere.22 Into a 50 mL round flask was dissolved
compound 1 (2.034 g, 9 mmol) in a mixture of ethyl acetate and
methanol at a ratio of 100:1 (100 mL:1 mL), followed by addition of
NaBH4 (1.7024 g, 45 mmol). The resulting mixture was stirred for 6 h
at room temperature under a nitrogen atmosphere. Then the reaction
mixture was washed carefully four times with brine and one time with
B
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Scheme 4. Preparation of Difunctional o-Amide Benzoxazine
Scheme 5. Preparation of Difunctional p-Amide Benzoxazine
Figure 1. Smart features for easy synthesis of benzoxazole resin.
deionized water. The ethyl acetate solution was dried over sodium
sulfate anhydrous. After filtering and removal of the excess solvent by a
rotary evaporator under reduced pressure afforded a colorless viscous
(N−H stretching), 1646 (carbonyl stretching), 1496 (stretching of
trisubstituted benzene ring), 1226 (asymmetric stretching of C−O−
C), 1186 (asymmetric stretching of C−N−C), and 954 (out-of-plane
C−H of benzene ring to which oxazine ring is attached).
1
material (yield 85%). H NMR (DMSO-d6), ppm: δ = 4.50 (s, CH2
oxazine), 4.59 (s, NH2), 5.29 (s, CH2 oxazine), 6.32−7.26 (8H Ar).
13C NMR (DMSO-d6), ppm δ = 48.93 (Ar−C−N, oxazine), 78.18
(O−C−N, oxazine). FT-IR (KBr), cm−1: 3353 (N−H stretching),
1500 (stretching of trisubstituted benzene ring), 1224 (asymmetric
stretching of C−O−C), 1180 (asymmetric stretching of C−N−C),
950 (out-of-plane C−H of benzene ring to which oxazine ring is
attached).
2.6. Measurements. 1H and 13C NMR spectra were acquired on a
Varian Oxford AS600 at a proton frequency of 600 MHz and its
corresponding carbon frequency of 150.9 MHz. The average number
1
of transients for H and 13C MNR measurement was 64 and 1024,
respectively. A relaxation time of 10 s was used for the integrated
intensity determination of 1H NMR spectra. Fourier transform
infrared (FTIR) spectra were obtained using a Bomem Michelson
MB100 FTIR spectrometer, which was equipped with a deuterated
triglycine sulfate (DTGS) detector and a dry air purge unit.
Coaddition of 32 scans was recorded at a resolution of 4 cm−1.
Transmission spectra were obtained by casting a thin film on a KBr
plate for partially cured samples. Elemental analysis was performed by
MHW Laboratories. A TA Instruments DSC model 2920 was used
with a heating rate of 10 °C/min and a nitrogen flow rate of 60 mL/
min for all tests. All samples were crimped in hermetic aluminum pans
with lids.
Dynamic mechanical analyses were done on a TA Instruments
Q800 DMA applying controlled strain tension mode with amplitude of
10 μm and a ramp rate of 3 °C/min. Thermogravimetric analyses
(TGA) were performed on a TA Instruments Q500 TGA with a
heating rate of 10 °C/min in a nitrogen atmosphere at a flow rate of 40
mL/min.
2.5. Preparation of Difuncitonal p-Amide Benzoxazine. As
shown in Scheme 5, difunctional para-amide benzoxazine was prepared
by a reaction between P-a-NH2 and isophthaloyl dichloride in the
presence of triethylamine as reported in our previous work.1 Into a 25
mL flask were dissolved P-a-NH2 (2.26 g, 10 mmol) and triethylamine
(4.8 mL) in 50 mL of chloroform and cooled in an ice bath. The
solution was stirred for 15 min, followed by dropwise addition of the
50 mL of chloroform solution of isophthaloyl chloride (1.015 g, 5
mmol). The mixture was stirred for 1 h in the ice bath and another 5 h
at room temperature. The solution was washed with 0.5 N Na2CO3
aqueous solution and then with deionized water. The solution was
dried over sodium sulfate anhydrous, followed by concentration and
precipitation in 200 mL of hexane. The precipitate was removed by
1
filtration and dried to give a white powder (yield: 90%). H NMR
(DMSO-d6), ppm: δ = 4.67 (s, AR-CH2−N), 5.44 (s, −O−CH2−N),
6.72−8.47 (20H, Ar), and 9.35 (s, NH). 13C NMR (DMSO-d6), ppm:
δ = 48.98 (Ar−C−N, oxazine), 78.71 (O−C−N, oxazine), 130.31
(C−NH), and 164.58 (Ar−CdO−NH). FTIR (KBr), cm−1: 3288
C
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of amide-functional polybenzoxazines is produced that can also
be coined as poly(o-hydroxyamide)s or poly(o-amidephenol)s,
owing to the presence of amide linkages next to the triggered
phenolic moieties upon ring-opening polymerization.
3. RESULTS AND DISCUSSION
An attractive class of multiple smart polybenzoxazine
precursors, known as o-amide functional benzoxazines, has
been developed and has potential for use as precursors for
benzoxazole resin. Figure 1 summarizes the exceptional features
of this class of polymers that is regarded as a unique class, not
only among traditional polybenzoxazines but also among all
commonly known classes of high performance polymers. The
precursors can be easily synthesized in various molecular
architecture forms, including monomers, oligomers, and main-
chain-type as well as side-chain-type polymers that allow
performance and processability control.
Interestingly, this class of benzoxazines is shown to
polymerize via cationic ring-opening polymerization at much
lower temperatures than any other known class of benzoxazines
without added initiators and/or catalysts to form amide-
containing polybenzoxazines. The presence of intramolecular
hydrogen bonding between an amide linkage and the adjacent
oxazine ring acts as an internal incentive to stimulate the ring-
opening polymerization in a smart way, like a self-
complementary initiator (Figure 2). As a result, a novel class
Furthermore, when this smart class of polybenzoxazines is
exposed to higher temperatures via postcuring above 200 °C or
catching fire, it generates water through intramolecular
cyclization between the neighboring hydroxyl and amide
groups to form oxazole rings as a repeating units in the new
network structure. As a result, the new polymeric network
structure becomes even more thermally stable by converting
into a new class of polymer thermosets. Such materials have
never been reported and are now collectively termed
benzoxazole resins, representing a new class of thermosetting
resins.
The concept has been initially demonstrated by designing
two examples of monomeric amide-functional benzoxazine
isomers (ortho and para) based on isophthaloyl chloride, as
shown in Scheme 6. Upon polymerization at 200 °C, the ortho-
isomer is expected to give cross-linked polybenzoxazine of
poly(o-hydroxyamide) structure, which is very similar to
poly(o-hydroxyamide)s as a typical polybenzoxazole precursor.
Thus, it is believed that such precursors can be structurally
rearranged into a thermosetting polybenzoxazole. On the other
hand, the para-isomer gives a polybenzoxazine of poly(p-
hydroxyamide) structure that has no possibility for structural
rearrangement to oxazole structure before degradation.
The thermal stability of the o- and p-benzoxazine isomers has
been studied by thermogravimetric analysis (TGA) (Figure 3)
after thermal treatment at 200 °C. It is well-established that the
polymerization of benzoxazines occurs through oxazine ring-
opening without producing any byproduct and the thermal
degradation of polybenzoxazines starts over 300 °C. p-
Benzoxazine isomer (line B, Figure 3) shows degradation
similar to typical polybenzoxazine; however, o-benzoxazine
isomer shows a bimodal degradation profile (line A, Figure 3)
in which the first step of weight loss occurs at lower
temperature than polybenzoxazine degradation and in the
same temperature range of the typical cyclodehydration of
poly(o-hydroxyamide)s to give polybenzoxazole.
Using the advantage of tremendous flexibility in molecular
design of this novel class of polybenzoxazines, another series of
o-amide functional benzoxazine monomers of different
Figure 2. DSC polymerization behavior of monomers.
Scheme 6. Polymerization of Amide-Functional Benzoxazine Isomers
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of both isomers shown in Figure 6, which indicates the
presence of weight loss at earlier temperature from the ortho-
isomer within the same temperature range.
The ring-opening polymerization of benzoxazine structure
and the thermal cyclodehydration of o-amide polybenzoxazine
have been monitored by Fourier transform infrared spectros-
copy (FT-IR). For consistency, the sample has been prepared
by casting a thin film on a KBr plate from a chloroform solution
of o-amide benzoxazine main-chain polymer followed by
stepwise heating process up to 300 °C. Arrows shown in
Figure 7 indicate the characteristic vibrational modes which are
useful for this study. The typical absorption band at 925 cm−1
which can be used to study the ring-opening reaction of
benzoxazine structure gradually decreases and almost dis-
appears by 200 °C, suggesting the formation of cross-linked o-
amide polybenzoxazine. The cyclization to poly(o-hydroxya-
mide) was supported by the disappearance of the amide I mode
at 1646 cm−1. The formation of benzoxazole structure gives rise
to a band at 1595 cm−1.24,25 The FTIR spectra of this novel o-
amide-functional polybenzoxazine, cross-linked poly(o-hydrox-
yamide), is practically identical to those reported for
polybenzoxazole synthesized through the traditional approach
of linear poly(o-hydroxyamide) method, indicating that thermal
cyclization has been completed.
Dynamic mechanical analysis (DMA) has been performed to
further understand the effect of cyclization and polybenzoxazole
thermoset formation on the thermomechanical properties with
respect to the o-amide polybenzoxazine as a smart precursor.
The spectrum in Figure 8 for the o-amide-functional
polybenzoxazine which is heat-treated at 200 °C shows clearly
that the storage modulus maintained a stable value and started
decreasing in the glass transition region. Interestingly, the
modulus tends to increase in the rubbery plateau as shown in
Figure 8, implying the presence of restriction in the segmental
mobility and, perhaps, the further structural rearrangement to
further increase the rigidity of the molecules. The E″ of o-amide
polybenzoxazine which is heat-treated at 200 °C, reveals one
relaxation peak at 212 °C which is attributed to the Tg and less
defined peak above 300 °C. Two clearly resolved relaxations
have been observed from the tan δ peak, reflecting the
cyclodehydration and formation of polybenzoxazole of rigid
molecular structure of lower segmental mobility. The relatively
low Tg of the o-amide polybenzoxazine allows a greater mobility
of chain segments and thus assisted the cyclodehydration and
the easy formation of polybenzoxazole. The DMA spectrum of
the o-amide polybenzoxazine postcured at 300 °C for 1 h shows
a stable storage modulus up to 300 °C due to the formation of
polybenzoxazole of a stiffer molecular structure.
Figure 3. Thermogravimetric analysis of amide-functional benzoxazine
monomers after thermal polymerization at 250 °C.
molecular structure (Figure 4, series X1) have been synthesized,
using various aromatic diamines and N-(2-hydroxyphenyl)-
benzamide. The polymerization behavior of this series has been
compared with various traditional aromatic amine based-
benzoxazine monomers (series X2, Figure 4). Detailed
information about the synthesis of these monomers will be
published elsewhere. Expectedly, the TGA thermograms of
phenol/diamine based benzoxazine monomers treated at 200
°C shows typical features for polybenzoxazine degradation in
which the degradation starts above 300 °C. On the contrary, o-
amide benzoxazine monomers, treated also at 200 °C, show an
early stage of weight loss, which is attributed to the loss of
water and a formation of polybenzoxazole.
A detailed example to support the conversion of this class of
polybenzoxazines into polybenzoxazole resin has been studied
using main chain-type benzoxazine polymers containing amide
functionalities (Figure 5). The polymerization behavior of the
benzoxazine resins has been studied by differential scanning
calorimetry (DSC), and the results showed a typical polymer-
ization exotherm at 212 and 259 °C for ortho and para
polymers, respectively. The benzoxazine with o-amide has a
lower polymerization temperature than the para-isomer and
much lower than the traditional benzoxazine monomer as
described earlier. For the ortho-isomer polybenzoxazine, the
exotherm is followed by an endotherm, which is attributed to
the dehydration and formation of polybenzoxazole as shown
earlier in Scheme 1, in a way similar to poly(o-hydroxyamide)s.
This phenomenon has been supported by TGA thermograms
Figure 4. Structure of bifunctional benzoxazines.
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Figure 5. Structure of ortho and para main-chain type.
Figure 8. Dynamic mechanical spectra of o-amide benzoxazine with
(solid line) and without (broken line) heat treatment at 200 °C for 1
h.
Figure 6. TGA and DSC thermograms of ortho and para main-chain-
type benzoxazine polymers.
and magic-angle spinning (MAS) was also used as shown in
Figures 9 and 10. After 1 h of heating at 220 °C, both orth- and
Figure 9. 13C CP MAS spectrum of o-amide benzoxazine monomer at
nascent and 250 °C for 1 h.
para-model compounds showed similar spectra, though with
expected minor differences due to the difference in substitution
position. The similarity is due to the similar structural changes
caused by oxazine ring-opening polymerization. However, when
the heating condition was changed to 250 °C for 1 h, the
spectra for the ortho- and para-model compounds showed
drastic differences. The 13C resonances at 163, ∼148, and ∼142
ppm, which can be assigned to the benzoxazole group, are well
resolved. However, for the p-amide polybenzoxazine, at the
same conditions, none of the above benzoxazole signals are
observed, although broad resonances heavily overlap at 163 and
148 ppm. In fact, little changes in spectral feature were
observed after this thermal treatment for p-amide polybenzox-
Figure 7. FT-IR spectra of ortho main-chain-type benzoxazine
polymers after various thermal treatments.
The maximum amplitude of the α-relaxation peak decreases
with the rearrangement of o-amide polybenzoxazine resin into
polybenzoxazole, suggesting a more severe restriction of the
segmental mobility, due to the formation of rigid benzoxazole
units in the polymer. This behavior implies that the
cyclodehydration of the polybenzoxazine is successfully taking
place.
In order to further confirm the occurrence of benzoxazole
cyclization, solid state 13C NMR with cross-polarization (CP)
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to Dr. Richard E. Lyon of Federal
Aviation Administration (FAA) for the evaluation of HRC of
the benzoxazole resin.
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Heat release rate (HRR) is governed by the heat release
capacity (HRC) which is the maximum amount of heat released
by combustion per degree of temperature rise per unit mass of
polymer in the mesophase. HRC is considered as a semi-
quantitative predictor of fire performance and flammability of
materials which is related to flame resistance. HRR and HRC
for a set of polybenzoxazine derived from amide-functional
benzoxazines have been evaluated to be ∼92 J/(g K). In
general, the lower the values of both HRC and HRR, the higher
the flame resistance. The HRC value of polybenzoxazole
thermoset was one of the lowest among 47 polymers studied by
Lyon et al.26 Furthermore, this new class of thermoset polymers
is better than poly(ether imide) which shows HRC of 121 J/(g
K) and UL-94 rating of V0. It has been concluded that this
novel class of polymers has excellent flame resistance; a detailed
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4. CONCLUSION
A new class of cross-linkable polybenzoxazole (PBO), namely
benzoxazole resin, has been successfully developed. The in-situ
thermal conversion of amide-functional benzoxazines as smart
class of polybenzoxazine precursors has been proposed as an
alternative approach to the traditional PBO formation. Using
the tremendous molecular design flexibility of these novel
polybenzoxazole precursors, which cannot be found in the
traditional concept for polybenzoxazole formation, it has
become possible to produce polybenzoxazoles through easily
processable precursors. The water is produced due to the
endothermic nature of cyclodehydration reaction of this class of
smart polybenzoxazines when producing polybenzoxazole,
which provides an additional advantage for fire retardancy.
The greatest advantage of this concept is that the benzoxazole
resin is obtained from a low molecular weight compound
(resin) unlike the high molecular weight poly(o-hydroyamide)s.
AUTHOR INFORMATION
Corresponding Author
*E-mail: taa16@case.edu (T.A.); hxi3@cwru.edu (H.I.).
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dx.doi.org/10.1021/ma300924s | Macromolecules XXXX, XXX, XXX−XXX