Synthesis and properties of highly cross-linked thermosetting resins of benzocyclobutene-functionalized benzoxazine

Article abstract of DOI:10.1021/ma3004218

Benzocyclobutene- (BCB-) functionalized benzoxazine (BOZ) monomers and resins have been successfully prepared. BCB-functionalized BOZ monomers were synthesized by the reaction of 4-aminobenzocyclobutene (AMBCB), phenol and paraformaldehyde or by the reaction of 4-hydroxylbenzocyclobutene (OHBCB), arylamine, and paraformaldehyde in toluene under reflux. Fourier transform infrared spectroscopy (FTIR), ¹H and ¹³C nuclear magnetic resonance spectroscopy (NMR) are used to characterize the structure of the monomers. The monomers possess two kinds of ring-opening polymerizable functional groups of BCB and BOZ. So the monomers can be cured at proper temperature to prepare highly cross-linked resins. The polymerization behavior of the monomers is studied by differential scanning calorimetry (DSC) and FTIR. DMA study demonstrates that these polymers show high storage moduli which can be maintained in a wide temperature range up to 300°C and high T_g at ca. 350°C. The TGA study shows that the cured resins possess good thermal stability.

Full text of DOI:10.1021/ma3004218

Article
pubs.acs.org/Macromolecules
Synthesis and Properties of Highly Cross-Linked Thermosetting
Resins of Benzocyclobutene-Functionalized Benzoxazine
Yuanrong Cheng, Jun Yang, Yunxia Jin, Dunying Deng, and Fei Xiao*
Department of Materials Science, Fudan University, 220 Handan Road, Shanghai 200433, P. R. China
S
* Supporting Information
ABSTRACT: Benzocyclobutene- (BCB-) functionalized ben-
zoxazine (BOZ) monomers and resins have been successfully
prepared. BCB-functionalized BOZ monomers were synthe-
sized by the reaction of 4-aminobenzocyclobutene (AMBCB),
phenol and paraformaldehyde or by the reaction of 4-
hydroxylbenzocyclobutene (OHBCB), arylamine, and paraf-
ormaldehyde in toluene under reflux. Fourier transform
1
infrared spectroscopy (FTIR), H and 13C nuclear magnetic
resonance spectroscopy (NMR) are used to characterize the
structure of the monomers. The monomers possess two kinds of ring-opening polymerizable functional groups of BCB and BOZ.
So the monomers can be cured at proper temperature to prepare highly cross-linked resins. The polymerization behavior of the
monomers is studied by differential scanning calorimetry (DSC) and FTIR. DMA study demonstrates that these polymers show
high storage moduli which can be maintained in a wide temperature range up to 300 °C and high Tg at ca. 350 °C. The TGA
study shows that the cured resins possess good thermal stability.
linked to improve the heat resistance, toughness and
processability properties. Agag et al. has synthesized a series
of allyl containing BOZ, for which the cured polymer shows
glass transition temperature up to 322 °C. Liu et al.8
synthesized a phthalocyanine containing maleimide of BOZ
monomer, which has good processing (low melting point of
52−55 °C and good solubility in common solvents), and good
heat resistance for the cured monomers (glass transition
temperature of 204 °C, 10% weight loss temperature of 366
°C). Yagci et al. reported a new monomer possessing both
benzoxazine and coumarin rings.11 The monomer underwent
dimerization via the 2 + 2 cycloaddition reaction to form
cyclobutane rings upon photolysis around 300 nm. However,
thermogravimetric analysis showed unfavorable thermal
stability (T5% at 303.7 °C) of the cured bifunctional
benzoxazine monomer, which was caused by the poor thermal
stability of the cyclobutane rings.
Meanwhile, benzocyclobutene (BCB) based polymer materi-
als have drawn much attention for their outstanding properties
(such as high thermal stability, low dielectric constant and
dielectric loss, good flatness and mechanical properties).12−16
They were especially applied in the field of electrical and
electronic packaging. BCB ring can be converted to conjugated
diene structure under proper temperature and undergoes
radical polymerization or Diels−Alder reaction. Such polymers
have excellent electrical performance and have been applied in
integrated circuit packaging, MEMS packaging, etc.
INTRODUCTION
■
Benzoxazine (BOZ), a novel series of six-membered hetero-
cyclic ring monomers that contains oxygen and nitrogen, has
got many people’s attention.1−3 It can be prepared by
dehydration condensation reaction of primary amines, phenols
and aldehydes under certain conditions. The benzoxazine ring
is stable at room temperature for a long period and undergoes
ring-opening polymerization to generate a nitrogen-containing
phenolic resin similar to PF resins at proper temperature. The
low viscosity of the BOZ monomers facilitates for process of
manufacture. The polymerization of benzoxazine does not
generate any volatiles and near zero shrinkage occurs during the
period of polymerization of the monomers. The benzoxazine
resins also possess many good properties, such as good flame
retardancy, good chemical resistance, low water absorption,
excellent mechanical properties and good electrical properties,
etc. BOZ can be widely used as heat-resistant materials,
composite materials, electronic packaging materials and others.
Traditional BOZ also suffers some shortcomings such as
relatively low thermal stability, high curing temperature and
brittleness, etc.1 Modification or molecular engineering of BOZ
monomer is necessary to overcome these problems. In order to
improve the thermal stability and performance of BOZ resins,
bifunctional precursors were utilized or polymerizable func-
tional groups were introduced. BOZ prepared by bisphenol
compounds such as bisphenol A,2 4,4′-dihydroxy benzophe-
none,4 and 1,5-naphthalenediol5 has showed improved thermal
mechanical property than the ordinary epoxy resins. Allyl-,6
acetylene-,7 maleimide-,8−10 and norbornene-based10 precur-
sors can also be employed to prepare BOZ. Polymerization of
these monomers with multi-functional groups can be cross-
Received: February 29, 2012
Revised: April 18, 2012
© XXXX American Chemical Society
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down to room temperate, the solution was condensed on a rotary
evaporator and the crude product was obtained. The crude product
was then dissolved in 50 mL dichloromethane and washed with 20%
NaOH solution for 3 times and then with deionized water until
neutral. The dichloromethane solution was separated and dried over
anhydrous sodium sulfate for 12 h, and then filtered, concentrated and
In this paper, BCB ring as polymerizable group was
introduced in the BOZ monomers to realize the performance
enhancement of BOZ resins. We originally synthesized
monomers with multifunctional groups containing BCB and
BOZ. The monomers possess two kinds of ring-opening
polymerizable functional groups of BCB and BOZ. So the
monomers can be cured at proper temperature to prepared
highly cross-linked resins. These resins may demonstrate
improved heat resistance, thermal properties, and thermal
mechanical properties compared to the BOZ resin reported.
1
dried under vacuum to give 4.20 g of HB−A in 88% yield. H NMR
(500 MHz, CDCl3): 3.07 (s, 4H), 4.57 (s, 2H), 5.30 (s, 2H), 6.78−
7.10 (m, 7H).
3-(Benzocyclobutene-4-yl)-3,4-dihydro-2H-benzo[e][1,3]oxazine
(HB−AB). The preparation method is similar to that of HB−A except
that aniline was substituted by AMBCB. 4.80 g of the product was
1
obtained in 91% yield. H NMR (500 MHz, CDCl3): 3.08 (s, 8H),
EXPERIMENTAL SECTION
■
4.55 (s, 2H), 5.26 (s, 2H), 6.52−6.96 (m, 5H).
Materials. 4-Bromobenzocyclobutene (BrBCB, 97%) and 4-
hydroxylbenzocycolbutene (OHBCB) were purchased from Chem-
target Technologies Co., Ltd. Phenol, bisphenol A (BPA), 2-
allylphenol, toluene, dichloromethane, paraformaldehyde, anhydrous
sodium sulfate, and sodium hydroxide all were used as received.
Aniline was purified by vacuum distillation. 4-Aminobenzocyclobutene
(AMBCB) was prepared by aminating of 4BrBCB with ammonia−
water according to the literature else.17 6,6′-(Propane-2,2-diyl)bis(3-
phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine) (B-a) was synthesized
via a solvent-free method.18
Preparation of Thermosetting Resins. The prepared monomers
existed as powders or semifluid at room temperature were placed into
glass-plate molds (40 mm × 20 mm × 2 mm). After degassing in a
vacuum oven at 130 °C for an hour with an approximate pressure at
0.01 MPa, the molds were then heated at 150 °C for 2 h, then at 180,
200, 220, 240, and 260 °C for 1 h, respectively. After cooled to room
temperature, the thermosetting resins were polished as rectangular
bars with a size of 30 mm × 13 mm × 2 mm for DMA test.
RESULTS AND DISCUSSION
1. Preparation and Characterization of BCB-Function-
alized BOZ. The synthesis of BCB-functionalized BOZ
1
Characterization. H NMR and 13C NMR spectra were recorded
■
with a Bruker DMX-500 Spectrometer using CDCl3 as solvent. Fourier
transform infrared (FTIR) spectra were recorded on a Nicolet Magna-
IR 550 II FTIR spectrophotometer operating at a resolution of 4 cm−1.
Thermal gravimetric analysis (TGA) was performed on a Shimadzu
DTG-60H simultaneous DTA−TG apparatus with a heating rate of 10
°C·min−1 in nitrogen atmosphere. Differential scanning calorimetry
(DSC) was measured on a TA Q200 calorimeter. Dynamic mechanical
analysis (DMA) was carried out on TA Q800 instrument by the 3-
point bending method at a heating rate of 3 °C·min−1 with the test
frequency at 1 Hz.
Scheme 1. Preparation of AMBCB-Based Benzoxazine
Monomers
Preparation of Monomers. 3-(Benzocyclobutene-4-yl)-3,4-
dihydro-2H-benzo[e][1,3]oxazine (P−AB). AMBCB (1.19 g, 10
mmol), paraformaldehyde (0.60 g, equal to 20 mmol formaldehyde),
phenol (0.94 g 10 mmol) and toluene (20 mL) was added in a 50 mL
single necked round-bottomed flask equipped with a PTFE-coated
magnetic stir bar and a reflux condenser. The solution was stirred and
heated to reflux for 5 h, and the turbid solution became transparent
with light yellow. After cooling down to room temperate, the solution
was condensed on a rotary evaporator and the crude product was
obtained. The crude product was then dissolved in 50 mL
dichloromethane and washed with 20% NaOH solution for 3 times
and then with deionized water until neutral. The dichloromethane
solution was separated and dried over anhydrous sodium sulfate for 12
h, and then filtered, concentrated and dried under vacuum to give 2.01
g P−AB in 85% yield. 1H NMR (500 MHz, CDCl3): 3.07 (s, 4H), 4.57
(s, 2H), 5.30 (s, 2H), 6.78−7.10 (m, 7H).
Scheme 2. Preparation of OHBCB-Based Benzoxazine
Monomers
6,6′-(Propane-2,2-diyl)bis[3-(benzocyclobutene-4-yl)-3,4-dihy-
dro-2H-benzo[e][1,3]oxazine] (B−AB). The preparation method is
similar to that of P−AB except that phenol (0.94 g 10 mmol) was
substituted by BPA (1.14 g, 5 mmol). 2.10 g of the product was
1
obtained in 82% yield. H NMR (500 MHz, CDCl3): 1.57 (s, 6H),
3.09 (s, 8H), 4.53 (s, 4H), 5.28 (s, 4H), 6.68−6.95 (m, 12H).
8-Allyl-3-(benzocyclobutene-4-yl)-3,4-dihydro-2H-benzo[e][1,3]-
oxazine (ALP−AB). The preparation method is similar to that of P−
AB except that phenol (0.94 g, 10 mmol) was substituted by 2-
allylphenol (1.34 g, 10 mmol). 2.22 g of the product was obtained in
1
80% yield. H NMR (500 MHz, CDCl3): 3.07(s, 4H), 3.33 (d, 2H),
4.55 (s, 2H), 5.03 (m, 2H), 5.30 (s, 2H), 5.97 (m, 1H), 6.79−6.96 (m,
6H).
6,7-(Ethane-1,2-diyl)-3-phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazine (HB−A). OHBCB (2.40 g, 20 mmol), paraformaldehyde (1.20
g, equal to 40 mmol of formaldehyde), aniline (1.86 g, 20 mmol), and
toluene (20 mL) was added in a 50 mL single necked round-bottomed
flask equipped with a PTFE-coated magnetic stir bar and a reflux
condenser. The solution was stirred and heated to reflux for 5 h, and
the turbid solution became transparent with light yellow. After cooling
B
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Figure 4. FTIR spectra of BCB-functionalized benzoxazine monomers.
1
Figure 1. H NMR spectrum of AMBCB-prepared BOZ monomers.
Figure 5. DSC curves of AMBCB-based benzoxazine monomers.
Figure 2. 13C NMR spectrum of P−AB.
Figure 6. Vibrational analysis of the polymerization of oxazine ring
and BCB ring in P−AB at different cure stages.
1
Figure 3. H NMR spectrum of OHBCB-prepared BOZ monomers.
monomers were based on two routes. The first route is based
on the reaction of AMBCB and paraformaldehyde with
different phenols such as phenol, bisphenol A, and 2-
allylphenol, and the scheme is shown in Scheme 1. The
second route is based on the reaction of OHBCB and
paraformaldehyde with different aromatic amines such as
aniline and AMBCB, as shown in Scheme 2.
shown in Figure 1 established the structures of BCB-functional
BOZs prepared by AMBCB. As to P−AB, 3.07(s, 4H) is
attributed to the methylene protons of the four-membered ring
of BCB. The aromatic protons appeared as multiplet at 6.78−
7.10 ppm. Here, 4.57 (s, 2H), and 5.30 (s, 2H) are attributed to
−Ar−CH2−N− and −N−CH2−O−, respectively, which prove
the formation of benzoxazine ring. As to B−AB, the peaks at
1.57 ppm attributed to the C(CH3)2 with other peaks similar to
The chemical structure of the prepared BCB-functional
1
1
1
BOZs was characterized by H NMR. The H NMR spectra
that of P−AB. Besides, the H NMR spectrum of ALP−AB is
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Figure 10. FTIR of HB−AB at different curing stage.
Figure 7. Vibrational analysis of the polymerization of oxazine ring
and BCB ring in ALP−AB at different cure stages.
Figure 11. DMA curves of BCB-functionalized benzoxazine polymer
compared with B-a resin.
carbons in the BCB moiety. The characteristic carbon
resonances of the oxazine ring appear at 51.15 ppm for C−
C*H2−N− and at 80.64 ppm for N−C*H2−O−, respectively,
which support the formation of benzoxazine. Besides, all the
peaks refer to the corresponding carbon in the monomer,
which suggest the structure of P−AB. 13C NMR spectra for
other monomers were shown in Supporting Information.
As shown in Scheme 2, two structures may be generated as
OHBCB was used for preparing the BCB functionalized BOZ.
However, according to the 1H NMR shown in Figure 3, we can
determine that structure A is right and structure B is not
generated. As to HB−A, 3.07 (s, 4H) was ascribed to the
methylene protons of the four-membered ring of BCB. 4.57 (s,
2H) and 5.29 (s, 2H) was ascribed to the methylene protons of
benzoxazine, which indicates that the formation of benzoxazine
ring. The singlet peaks at 6.53 and 6.67 indicated that the two
aromatic protons on the BCB ring are not adjacent, so we can
confirm the structure of HB−A as structure A. The reason may
be that the hindrance of BCB on the phenol influences the ring
formation of BOZ. The 1H NMR spectrum of HB−AB was also
similar to that of HB−A, and we can confirm the structure of
HB−AB as structure C. The 13C NMR spectra of the
monomers were shown in Supporting Information.
Figure 8. DSC curves of OHBCB-prepared monomers.
Figure 9. FTIR of HB−A at different curing stage.
shown in Figure 1. The two multiples at 5.03 and 5.97 ppm are
typical for the protons of CH2 and CH− in the allyl
group, respectively. The protons of −CH2− of the allyl group
showed a doublet at 3.33 ppm. Other peaks are similar to that
of P−AB. The analysis of 1H NMR shows that we have
prepared BCB-functional BOZs successfully. 13C NMR also
proved the structures of the BCB-functional BOZs.
All the FTIR spectra of the monomers are shown in Figure 4.
The characteristic absorptions of benzoxazine structure for the
monomers were at 1230−1236 cm−1 (asymmetric stretching of
C−O−C of BOZ ring for AMBCB prepared monomers),
1028−1036 cm−1 (symmetric stretching of C−O−C of
benzoxazine ring), 1327−1340 cm−1 (CH2 wagging), and
Figure 2 shows the 13C NMR spectra for P−AB. The peaks
at 21.15 and 21.90 ppm are assigned to the −CH2−CH2−
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Table 1. Thermal Properties of Polybenzoxazines
1470−1600 cm−1 (benzene ring), which was in accordance with
that of B-a. The characteristic band at 940−950 cm−1 due to
the out-of-plane C−H vibration of the benzene ring to which
the oxazine ring is attached is observed.6 The characteristic
peak for BCB at 1475 cm−1 was overlapped by trisubstituted
benzene ring of BOZ. Another peak at 2832 cm−1 was assigned
to the vibration of −CH2− in BCB.19 As to ALP−AB, allyl
group appeared at 3084 cm−1 (stretching of C−H) and 1637
cm−1 (stretching of CC).
2. Curing Behavior of BCB-Functional BOZ Monomer.
The cure behavior of BCB-functional BOZ monomers was
studied by DSC from room temperature to 350 °C at a ramping
rate of 10 °C/min, and the DSC curves are list in Figure 5. For
P−AB and B−AB, the initial exothermal temperature begins at
170 °C, and two exothermal peaks at 215−220 °C (primary)
and 245 °C (secondary). For P−AB, the exotherm shows a
maximum at 219 °C with a moderate peak at 247 °C and the
total heat of polymerization is 641.2 J/g. For B−AB, the
exotherm shows a maximum at 217 °C with a shoulder at 242
°C and the total heat of polymerization is 485.2 J/g. We think
that the first sharp exothermal peak is ascribed to the ring-
opening polymerization of BOZ and the second moderate peak
is ascribed to the ring-opening polymerization of BCB. In other
words, ring-opening reaction of BOZ ring is easier than that of
BCB ring, which can be confirm by the following FTIR study.
The initial exothermal temperature of ALP−AB is 210 °C and
only an exothermal peak at 253 °C with a shoulder was found,
and the total heat of polymerization is 457.7 J/g. This may be
Figure 12. TGA curves of resins cured by BCB-functionalized
benzoxazine compared with B-a resin.
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Scheme 3. Possible Thermal Curing and Degradation Mechanism of BCB-Functionalized Benzoxazine
that the allyl group on the phenyl ring is a steric hindrance for
the formation of phenolic Mannich bridge structure via the
ortho position, and the polymerization is forced to proceed
through the less favorite para position.20 The ring-opening
temperature of BOZ enhanced to 250 °C, which overlap the
ring-opening reaction of BCB.
The cure behavior of P−AB and ALP−AB was studied by
FTIR. FTIR was tested after the samples were treated 1 h at
150, 180, 210, and 240 °C, respectively and the FTIR spectra
are shown in Figure 6 and Figure 7. The appearance of
absorption peak at 3380 cm−1 for the phenolic hydroxyl group
after the curing indicates the ring-opening of benzoxazine. The
characteristic absorption bands of benzoxazine structure at
932−942 cm−1 (vibrational mode of cyclic C−O−C), 1037
cm−1 (symmetric stretching of C−O−C), and 1348 cm−1 (CH2
wagging) gradually decreased, and disappeared by the end of
the 240 °C cure, suggesting the completion of ring-opening of
benzoxazine to afford polybenzoxazine.
hour, which indicates that oxazine ring is easier to open. This
may be that the AMBCB make the ring- tension of oxazine
increased and the thermal stability of oxazine decreased.
3. Property of Cured BCB−BOZ Resin. The DMA curves
of cured BCB−BOZ resins are shown in Figure 11. As shown in
Table 1, all the resins exhibit high storage modulus. The initial
storage modulus of P−AB is as high as 4.5 GPa at 40 °C. The
high initial storage modulus of the resins was caused by the
high cross-linking of the monomers by dual ring-opening
polymerization. Compared to traditional BOZ such as B-a, the
cured BCB−BOZ resins except from HB−A resins can
maintain high storage modulus in a wide temperature range
up to 300 °C. Except from cured HB−A, the tan δ curves of the
polymers exhibit that the resins shows low loss factor below
300 °C. Glass transformation by tan δ of the polymers is as high
as 366 °C for P−AB. The high Tg was also attributed to the
high cross-linking of the monomers. The cured HB−A shows a
relative low glass transition temperature below 300 °C. This
may be that the network of cured HB−A is thermal unstable
and thermal degradation happened.
TGA curves (Figure 12) shows that the resins have good
thermal stability as the temperature below 380 °C except from
HB−A, and the 5% and 10% weight loss temperatures (T5 and
T10) are list in Table 1. Compared with the traditional
benzoxazine such as B-a, the incorporation of BCB rings
improves the thermal stability of the polymers. Typically, the
cured P−AB and HB−AB show T5 at 396 and 397 °C,
respectively. The cured HB−A shows relatively low T5 at 348
°C. A possible thermal degradation mechanism of cured BCB-
functionalized benzoxazine is listed in Scheme 3. As aniline is
used for preparing HB−A, the Mannich bridge in the network
tend to break up and arylamine such as aniline, N-
methyleneaniline and N-methylaniline may be released.21,22
However, as AMBCB is used for preparing benzoxazine, no
small molecule is released as the Mannich bridge breaks in the
network, which can prevent the resin from thermal degradation.
From Figure 6 for P−AB, 942 cm−1 for BOZ ring
disappeared after the sample was treated at 180 °C for 1 h.
The BCB ring at 2829 cm−1 disappeared after the sample was
treated at 240 °C. For ALP−AB in Figure 7, 942 cm−1 for BOZ
ring still exist after the sample was treated at 180 °C for 1 h.
This indicates that the BOZ ring for ALP−AB shows less
reaction action than that of P−AB, which caused by the
hindrance of allyl on the BOZ ring in ALP−AB. This is in
accordance with the judgment of DSC. The absorption peak for
allyl group at 1637 cm−1 disappeared after treatment of 210 °C
for 1 h, which indicated that allyl group may undergo radical
polymerization or Diels−Alder reaction with BCB.
The cure behavior of HB−A prepared by OHBCB is quite
different from that of P−AB and B−AB. Because the BCB ring
and oxazine ring exits on the same benzene ring, the BCB ring
may lead to the hindrance for the ring-opening reaction of
oxazine, which may increase the temperature for the polymer-
ization of BOZ. HB−A shows a single exothermic peak at 255
°C based on DSC test (shown in Figure 8). According to FTIR
curves for different cure stage shown in Figure 9, after curing at
180 °C for an hour, 937 cm−1 for oxazine still exists apparently
and 2829 cm−1 for BCB also exists. Finally after cured at 240
°C for an hour, 937 cm−1 for BOZ and 2829 cm−1 for BCB
disappeared. Compared to HB−A, HB−AB exhibits an
exothermic peak at 227 °C and a shoulder at 240 °C. From
the FTIR curves for different cure stage in Figure 10, 937 cm−1
for oxazine became not obvious after curing at 180 °C for an
CONCLUSION
■
A series of novel benzoxazine monomers functionalized by
BCB groups have been successfully prepared. 1H NMR and 13
C
NMR spectroscopes were used to confirm the structures of the
monomers. The cure behavior of the monomers was tested by
DSC and FTIR. Because of the high cross-linking density arisen
from the dual ring-opening polymerization, the BCB-function-
alized benzoxazine resins show high Tg at ca. 350 °C and high
F
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storage moduli which can be maintained in a wide temperature
range up to 300 °C. The high thermal stability was confirmed
by TGA with the highest 5% weight loss temperature above 390
°C, which is attributed to the incorporation of BCB ring of the
monomers.
ASSOCIATED CONTENT
■
S
* Supporting Information
13C NMR data for the monomers of B−AB, ALP−AB, HB−A,
and HB−AB. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-21-65642110. Fax: +86-21-65103056. E-mail:
feixiao@fudan.edu.cn.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to acknowledge the financial support
from ther National Science and Technology Major Project with
Contract Nos. 2009ZX02038 and 2011ZX02602.
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