Influence of electronic effects from bridging groups on synthetic reaction and thermally activated polymerization of bisphenol-based benzoxazines

Article abstract of DOI:10.1002/pola.24566

Six bis-benzoxazines based on bisphenols with different bridging groups, -C(CH₃)₂-, -CH₂-, -O-, -CO-, -SO₂-, and single bond, were synthesized in toluene. The influence of electronic effects from bridging groups on ring-forming reaction and thermal ring-opening polymerization were relatively discussed in detail. Their structures were characterized by high-performance liquid chromatography, Fourier transform infrared, ¹H NMR, differential scanning calorimetry, and elementary analysis. The quantum chemistry parameters of the bisphenols and bis-benzoxazines were calculated by molecular simulation. The results indicated that the electron-withdrawing groups inhibited the synthetic reaction by decreasing the charge density of α-Cs of bisphenols and increasing energy barriers of the synthetic reactions. However, the electron-withdrawing groups promoted the thermally activated polymerization, which resulted from their activation energy and curing temperature decrease by increasing the bond length and lowering the bond energy of C-O on oxazine rings. Besides, because of stronger electron-withdrawing sulfone group, there were more arylamine methylene Mannich bridge structure in the polybenzoxazine.

Full text of DOI:10.1002/pola.24566

Influence of Electronic Effects from Bridging Groups on
Synthetic Reaction and Thermally Activated Polymerization
of Bisphenol-Based Benzoxazines
XIAOYING WANG, FENG CHEN, YI GU
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering,
Sichuan University, Chengdu 610065, China
Received 24 November 2010; accepted 31 December 2010
DOI: 10.1002/pola.24566
Published online 24 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Six bis-benzoxazines based on bisphenols with dif-
ferent bridging groups, AC(CH3)2A, ACH2A, AOA, ACOA,
ASO2A, and single bond, were synthesized in toluene. The
influence of electronic effects from bridging groups on ring-
forming reaction and thermal ring-opening polymerization
were relatively discussed in detail. Their structures were char-
acterized by high-performance liquid chromatography, Fourier
transform infrared, 1H NMR, differential scanning calorimetry,
and elementary analysis. The quantum chemistry parameters
of the bisphenols and bis-benzoxazines were calculated by
molecular simulation. The results indicated that the electron-
withdrawing groups inhibited the synthetic reaction by
decreasing the charge density of a-Cs of bisphenols and
increasing energy barriers of the synthetic reactions. However,
the electron-withdrawing groups promoted the thermally acti-
vated polymerization, which resulted from their activation
energy and curing temperature decrease by increasing
the bond length and lowering the bond energy of CAO on
oxazine rings. Besides, because of stronger electron-withdraw-
ing sulfone group, there were more arylamine methylene Man-
C
V
nich bridge structure in the polybenzoxazine.
2011 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1443–
1452, 2011
KEYWORDS: barrier; benzoxazine; bridging group; electronic
effect; molecular modeling; quantum chemistry
INTRODUCTION Polybenzoxazines, as an alternative to tradi-
tional phenolic resins, are widely used in the areas of electri-
cal insulation, electrical encapsulation, aeronautical technol-
ogy, and astronautical technology. They exhibit excellent heat
resistance, mechanical properties, electric insulating proper-
ties, good dimension stability, low water absorption, and low
dielectric constant.1–6
such as 1,3,5-triphenylhexahydro-1,3,5-triazine or 2-hydroxy-
benzaldehyde.19 At the same time, the thermally activated
polymerization of benzoxazine precursors20–23 and their cata-
lytical polymerization from carboxylic acids, Lewis acids, imi-
dazoles, and other kinds of compounds with labile hydro-
gen24–26 were also investigated widely. The synthetic reaction
and thermally activated polymerization behaviors of some
monofunctional benzoxazines with different substituting
groups were also studied.27
Benzoxazine is a novel thermosetting resin, which is easily
derived from phenol, formaldehyde, and primary amine via a
Mannich condensation and can be polymerized thermally or
catalytically. It was first synthesized in solvent by Holly and
Cope in 1944.7 Later, Burke et al.8–11 systemically studied the
Mannich reaction and found that the reaction mechanism of
the benzoxazine synthesis proceeded by two steps. The first
step was adding amine to formaldehyde to form an N,N-dihy-
droxymethylamine. The second step was the N,N-dihydroxyme-
thylamine reacting with ortho position of the phenol and the
labile hydrogen of the phenolic hydroxyl group, respectively, to
form the benzoxazine ring. Since 1990 in the world, Ishida
and coworkers12–18 have synthesized numerous benzoxazines
based on different phenolic compounds as well as aromatic or
aliphatic primary amines, and then discussed the properties of
their correspondent polybenzoxazines. Ishida and Lin devel-
oped new synthetic routes of benzoxazines from the reactants
However, there were some insufficiencies in the previous
researches. First, the systematic and comparative investiga-
tion of different monomers from the view of structural fac-
tors is needed. Then, the existed researches were mainly
relied on experimental data and lack of detail analysis at mo-
lecular and electronic level. The relationship of ring-forming
reaction and thermal ring-opening polymerization is rarely
reported. Besides, monofunctional benzoxazines, as model
compounds, are hard to produce casting samples to test
their physical and mechanical properties, so that it is neces-
sary to study the difunctional benzoxazines combinating syn-
thetic reaction, thermally activated polymerization, structure,
and property together.
To systemically investigate the influence of electronic effects
from bridging groups of bisphenols on synthetic reaction,
Correspondence to: Y. Gu (E-mail: guyi@scu.edu.cn)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 1443–1452 (2011) 2011 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of BX-as.
thermally activated polymerization and the properties of the
polymers, six difunctional benzoxazines based on bisphenols
with different bridging groups, AC(CH3)2A (BA-a), ACH2A
(BF-a), AOA (BO-a), ACOA (BZ-a), ASO2A (BS-a), and single
bond (BP-a), as the reference, were prepared via Scheme 1.
The influence of electronic effects on synthetic reaction and
thermally activated polymerization are systemically dis-
cussed by experimental data and molecular simulation analy-
sis in this article, and the influence of electronic effects on
the properties of polybenzoxazines will be reported later.
mol) were dissolved in toluene (50 mL) in a 250-mL three-
necked flask. The mixture was stirred and refluxed at 80 C
for 5 h. Then, the product was washed with 1 N solution of
sodium hydroxide and cooled overnight. The product was fil-
trated, washed with ethyl alcohol, and dried in vacuum dry-
ing oven. A white crystal was obtained finally.
ꢁ
BA-a. Yield: 86%. Melting point: 123 ꢁC (DSC). Purity: 99%
(HPLC). 1H NMR (CDCl3, ppm): 5.30 (NACH2AO), 4.55
(OACH2AAr), 1.57 (ACH3). FT-IR (cmꢀ1): 1231, 1028
(CAOAC), 952 (benzoxazine ring).
BF-a. Yield: 83%. Melting point: 122 ꢁC (DSC). Purity: 99%
(HPLC). 1H NMR (CDCl3, ppm): 5.35 (NACH2AO), 4.60
(OACH2AAr), 3.79 (ArACH2AAr). FT-IR (cmꢀ1): 1227, 1027
(CAOAC), 948 (benzoxazine ring).
EXPERIMENTAL
Materials
The following chemicals: phenol, phosphoric acid (14.7 mol
L
ꢀ1), aqueous formaldehyde solution (13.4 mol Lꢀ1), aniline,
BO-a. Yield: 75%. Melting point: 122 ꢁC (DSC). Purity: 99%
(HPLC). 1H NMR (CDCl3, ppm): 5.35 (NACH2AO), 4.59
(OACH2AAr). FT-IR (cmꢀ1): 1221, 1029 (CAOAC), 937 (ben-
zoxazine ring).
toluene, sodium bicarbonate, sodium hydroxide, ethyl alcohol,
and HPLC-grade THF were purchased from the Chengdu
Kelong Chemical Reagents Corp. (China). The following
bisphenols: 4,40-dihydroxydiphenylpropane (BA), 4,40-dihy-
droxydiphenylether (BO), 4,40-dihydroxybenzophenone (BZ),
4,40-Dihydroxydiphenylsulfon (BS), and 4,40-dihydroxybi-
phenyl (BP) were bought from J&K scientific (China). All
reagents were used as received.
Method 2
To a 250-mL three-necked flask toluene (50 mL) and stoi-
chiometric amounts of aniline (0.2 mol, 18.6 g), aqueous
formaldehyde solution (0.4 mol, 32.5 g) and bisphenols
(0.1 mol) were charged. The mixture was stirred and
refluxed for 5 h. Powders were precipitated gradually. The
product was filtrated, washed with ethyl alcohol and dried
in vacuum drying oven. A crystal product was obtained
finally.
Synthesis of 4,40-Dihydroxydiphenylmethane (BF)
BF was synthesized by mixing phenol (0.3 mol, 28.2 g), phos-
phoric acid (0.2 mol, 23.0 g), and aqueous formaldehyde solu-
ꢁ
tion (0.1 mol, 8.1 g) and stirring at 45 C for 4 h. The product
was filtrated, and then washed with 1 N NaHCO3 solution. A
white crystal product was recrystallized from ethyl alcohol.
Yield: 23%. Melting point: 167 ꢁC (DSC). 1H NMR (CDCl3,
ppm): 3.68 (ArACH2AAr), 9.13 (AOH).
Synthesis of Benzoxazines
Method 1
Stoichiometric amounts of aniline (0.2 mol, 18.6 g), aqueous
formaldehyde solution (0.4 mol, 32.5 g), and bisphenol (0.1
TABLE 1 Yields and Melting Points of Boz-BXs
Yield (%)
Melting Point (ꢁC)
BA-a
BF-a
BO-a
BP-a
BZ-a
BS-a
86
83
75
65
63
47
123
122
122
205/216
206
183
FIGURE 1 HPLC spectrum of BX-as.
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FIGURE 2 FT-IR spectra of BX-as.
BP-a. Yield: 65%. Melting point: 205/216 ꢁC (DSC). 1H
NMR (CDCl3, ppm): 5.39 (NACH2AO), 4.63 (OACH2AAr).
FT-IR (cmꢀ1): 1232, 1026 (CAOAC), 919 (benzoxazine
ring).
performed using the Dmol3 program embedded in Material
Studio 4.0 package. All transition state searches were con-
ducted using the Dmol3 program with the linear synchro-
nous transit/quadratic synchronous transit method.
BZ-a. Yield: 63%. Melting point: 206 ꢁC (DSC). 1H NMR
(CDCl3, ppm): 5.43 (NACH2AO), 4.67 (OACH2AAr). FT-IR
(cmꢀ1): 1206, 1029 (CAOAC), 917 (benzoxazine ring).
Characterizations and Measurements
FT-IR spectra were obtained with a Nicolet Magna 650 spec-
trometer at a resolution of 2 cmꢀ1 using the KBr pellet tech-
nique. 1H NMR spectroscopies were conducted on a Bruker
TD-65536 NMR (400 MHz) in deuterated chloroform as sol-
vent with tetramethylsilane as the internal reference. HPLC
analyses were conducted with a HPLC system consisting of a
Shimadzu-CBM20A equipped with a VP-ODS column (5 lm,
250 ꢂ 4.6 mm2, Shimpack, Japan) set at 30 ꢁC. THF/water
(v/v ¼ 1:1) was used as the eluant at a flowrate of 0.5 mL
minꢀ1. The eluate was monitored at 254 nm. DSC was per-
formed using a TA Instruments DSC Q20 at a variety heating
Method 3
Synthesis of BS-a. 1,4-Dioxane (50 mL) and 4,40-Dihydroxy-
diphenylsulfon (BS, 0.1 mol, 25.0 g) were added into a 250-
mL three-necked flask. The mixture was stirred at 70 ꢁC
until it became transparent. Then, aqueous formaldehyde so-
lution (0.4 mol, 32.5 g) was added. Aniline (0.2 mol, 18.6 g)
was charged by a drop funnel for 1 h. The mixture was
stirred and refluxed for 4 h. Then, the mixture was washed
with 1 N solution of sodium hydroxide, and cooled overnight.
Faint yellow crystal was obtained.
Yield: 47%. Melting point: 183 ꢁC (DSC). Purity: 99%
(HPLC). 1H NMR (CDCl3, ppm): 5.45 (NACH2AO), 4.70
(OACH2AAr). FT-IR (cmꢀ1): 1239, 1028 (CAOAC), 915 (ben-
zoxazine ring).
Preparation of Polybenzoxazines
To remove entrapped air, the benzoxazine crystals were
melted under vacuum for 10 min. The appropriate cure cꢁon-
ditions were listed as follows: BA-a, BO-a, and BF-a: 140 C/
ꢁ
ꢁ
10 min (under vacuum), 180 C/5 h. BZ-a and BP-a: 210 C/
10 min (under vacuum), 180 ꢁC/5 h. BS-a: 190 ꢁC/10 min
(under vacuum), 180 ꢁC/5 h. After melting and prepolyme-
rizing under vacuum for 10 min, BZ-a, BP-a, and BS-a did
ꢁ
not recrystallize and kept the states of fusant at 180 C.
Computation
Molecular simulation was determined with the software Ma-
terial Studio 4.0 (Accelrys, USA). Density functional theory
method with local density approximation calculation was
FIGURE 3 1H NMR spectra of BX-as.
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SCHEME 2 Steps in the synthesis of the benzoxazine ring.
rates of 5, 10, 15, and 20 ꢁC minꢀ1 under nitrogen atmos-
phere. Elementary analysis was tested by a Elementar Vario
Micro Organic Elemental Analyzer.
in the figure, there is only one peak in each plot and the in-
tegral area percentage of each peak is over 99%. The results
indicated that each monomer is composed of only one main
compound with high purity. The HPLC spectra of BP-a and
BZ-a are not obtained because they are not dissolvable in
eluant, tetrahydrofuran (THF).
RESULTS AND DISCUSSION
All monomers were recrystallized to remove the by-products.
Yields of the bisphenol-based benzoxazine monomers (BX-as)
were listed in Table 1. The electronic effect of bridging
groups in bisphenol influences the synthetic reaction. The
yield gets higher with the increase of the electron-donating
efficiency of bridging groups. BA-a exhibits the highest yield
of 86%. On the contrary, with the increase of the electron-
withdrawing character, the yield is decreased. BS-a with
strong electron-withdrawing bridging group shows the low-
est yield of 47%.
The chemical structures of BX-as were confirmed by Fourier
transform infrared (FT-IR) and 1H NMR spectra. The FT-IR
spectra (Fig. 2) show the typical absorption bands because
of out-of-plane bending vibration of the benzoxazine rings at
BA-a: 952 cmꢀ1, BF-a: 948 cmꢀ1, BO-a: 937 cmꢀ1, BP-a: 919
cmꢀ1, BZ-a: 917 cmꢀ1, and BS-a: 915 cmꢀ1, respectively. The
bands moved to high wavenumber by the effects of electron-
donating groups. The results indicated that electron-donating
groups increased the energy of the bending vibration of the
1
benzoxazine ring. In the H NMR spectroscopies (Fig. 3), the
Melting points of BX-as obtained from differential scanning
calorimetry (DSC) are also listed in Table 1. The bridging
groups of BA-a, BF-a, and BO-a are flexible, so the melting
points of three monomers are lower. The molecular struc-
tures of BP-a, BZ-a, and BZ-a are rigid, so their melting
points are higher.
characteristic resonances observed at BA-a: 5.30 ppm, BF-a:
5.35 ppm, BO-a: 5.35 ppm, BP-a: 5.39 ppm, BZ-a: 5.43 ppm,
and BS-a: 5.45 ppm, respectively, are assigned to the struc-
ture of NACH2AO on benzoxazine rings. The typical
Figure 1 shows the high-performance liquid chromatography
(HPLC) spectra of BA-a, BF-a, BO-a, and BS-a. As can be seen
TABLE 2 Charge of a-C, Bond Length of OAH and Energy
Barrier of Reaction Obtained from MS
Energy Barrier of
Bond
Reaction (kJ molꢀ1
)
Charge
Length
˚
of a-C
of OAH (A)
Step 2
Step 3
BA
BF
BO
BP
BZ
BS
ꢀ0.0735
ꢀ0.0721
ꢀ0.0686
ꢀ0.0715
ꢀ0.0672
ꢀ0.0644
0.9740
0.9740
0.9740
0.9742
0.9749
0.9749
223.9
234.3
211.5
332.0
364.1
418.9
173.9
152.7
135.0
134.6
100.0
118.5
FIGURE 4 Calculated relative energies of synthetic reaction via
Step 2 and Step 3.
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FIGURE 6 Conversion curves of BX-as at 210 ꢁC.
FIGURE 5 Nonisothermal DSC thermograms of BX-as.
rings. The electron-withdrawing groups are opposite. BA has
the maximum net charge of a-C, ꢀ0.0735, which is higher
than that of BS, ꢀ0.0644. It is in accordance with the phe-
nomenon that yield of BA-a is 39% higher than that of BS-a.
resonances at BA-a: 4.55 ppm, BF-a: 4.60 ppm, BO-a: 4.59
ppm, BP-a: 4.63 ppm, BZ-a: 4.67 ppm, and BS-a: 4.70 ppm,
respectively, are assigned to the structure of ArACH2AN on
benzoxazine rings. The resonances shifted to high field by
the influence of electron-donating groups. The results of FT-
The energies of reactants, transition states, and products
were calculated by the transition state searches. The Step 1
in Scheme 2 of each system is the same. Therefore, only
Steps 2 and 3 were calculated. The energies of the reactants
of Step 2 (Rꢀ2) are considered as benchmarks, that is, calcu-
late the energies of transition states and products using the
formula 1:
1
IR and H NMR indicated that the characteristics of oxizaine
rings are changed because of the electronic effect of bridging
groups, which may influence the ring-open reaction.
The reaction pathway of the Mannich condensation synthe-
sizing benzoxazine in solution by traditional one-pot
method is separated into three steps, which is illustrated in
Scheme 2. It proceeds by adding aniline to formaldehyde to
form N,N-dihydroxymethylamine, which then reacts with
ortho position of the bisphenol and the phenolic hydroxyl
group, respectively, to form the benzoxazine ring.9 Therefore,
the charge density of the a-C on ortho position of bisphenol
and the ability of removing the labile hydrogen of the pheno-
lic hydroxyl group become the main factors affecting the
ring-forming reaction.
Erelative ¼ Ecalculated ꢀ ERꢀ2
(1)
The calculated relative energies of synthetic reaction via
Steps 2 and 3 are shown in Figure 4, and the calculated
energy barrier of each step is listed in Table 2. When com-
pared with BP system, the energy barriers of Step 2 of BA,
BF, and BO systems with electron-donating groups are much
The net charges of a-Cs by Hirshfeld method and the OAH
bond lengths of bisphenols calculated using the Dmol3 pro-
gram embedded in Material Studio 4.0 package are listed in
Table 2. The bond lengths of OAH are slightly changed. The
electron-donating groups increase the net charges of a-Cs,
which enhance the activities of bisphenol to form oxazine
TABLE 3 Summary of the DSC Results
Tonset (ꢁC)
Tpeak (ꢁC)
Toffset (ꢁC)
DH (kJ molꢀ1
)
BA-a
BF-a
BO-a
BP-a
BZ-a
BS-a
261
257
254
251
234
206
267
264
260
257
239
215
270
266
263
260
247
228
144.0
140.2
144.8
143.9
160.7
157.3
FIGURE 7 Relationship between activation energy and conver-
sion of benzoxazines.
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SCHEME 3 Thermal ring opening polymerization mechanism of benzoxazines.
lower, whereas those of BZ and BS systems with electron-
withdrawing groups are much higher. The trend of Step 3 is
opposite, but at a moderate range. Thus, Step 2 is the con-
trolling approach of the synthetic reaction. Step 2 of BA
system proceeded via the transition state TS-2 with an
energy barrier of 223.9 kJ molꢀ1. The energy barrier of Step
2 of BS system is 418.9 kJ molꢀ1, nearly twice as much as
that of the BA system. In this case, N,N-dihydroxymethylamine
can condense to form oligomers without reacting with bisphe-
nols. Thus, it was difficult to synthesize BS-a via the tradi-
tional one-pot method. We add aniline with a drop funnel to
control the reaction of aniline with formaldehyde to make
more BS participate in the ring-forming reaction. In a word,
the electron-withdrawing groups can increase the energy
barriers of the synthetic reactions and restrain the reactions.
TABLE 4 Bond Length of CAO and Charge of C1 and C2
Obtained from MS
Charge Partitioning by
Hirshfeld Method
Bond Length
˚
of CAO (A)
C1 C2
All above is consistent with the result that electron-donating
bridging groups of bisphenol are benefitial for synthesis of
benzoxazine monomers. Electronic effects also have a great
effect on the stability of oxizaine ring, which can be seen
from the typical bands of benzoxazine rings in FT-IR and the
characteristic resonances of protons on oxazine rings in 1H
NMR. Thus, the thermal open-ring polymerization may
change.
BA-a
BF-a
BO-a
BP-a
BZ-a
BS-a
1.4303
1.4308
1.4311
1.4310
1.4401
1.4436
ꢀ0.0666
ꢀ0.0653
ꢀ0.0618
ꢀ0.0651
ꢀ0.0601
ꢀ0.0570
ꢀ0.0644
ꢀ0.0634
ꢀ0.0608
ꢀ0.0635
ꢀ0.0616
ꢀ0.0666
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TABLE 5 Weight Loss of Benzoxazines During Thermal
Polymerizationa
Weight Loss at
180 ꢁC for 5 h (%)
Weight Loss
at 200 ꢁC for 2 hb (%)
BA-a
BF-a
BO-a
BP-a
BZ-a
BS-a
a
1.73
1.89
2.10
2.26
2.35
3.21
1.77
1.96
2.13
2.34
2.42
3.41
Monomer of 2.5 g was heated in a weighing bottle under nitrogen
atmosphere.
b
The sample was tested after heated at 180 ꢁC for 5 h.
the optimal kinetic parameter of Ea by multiple linear
regression.
FIGURE 8 FT-IR spectrum of poly(BX-a)s.
The general Ozawa’s iso-conversion method:
Ea
lnðbÞ ¼ C ꢀ 1:052
RTa
Figure 5 shows the nonisothermal DSC thermograms (10 ꢁC
minꢀ1) of the benzoxazine monomers, and the summary of
the results is listed in Table 3. BA-a shows an endothermic
peak at 123 ꢁC attached to melting and an exothermic peak
at 267 ꢁC because of thermally activated polymerization.
When compared with BP-a, the exothermic peak tempera-
tures of BA-a, BF-a, and BO-a with electron-donating groups
are higher, whereas those of BZ-a and BS-a with electron-
withdrawing groups are lower. The exothermic peak temper-
ature of BS-a with strong electron-withdrawing group is at
215 ꢁC, which is 52 ꢁC lower than that of BA-a with strong
electron-donating group. Besides, the exothermic enthalpies
on curing of BZ-a and BS-a are ꢃ160 kJ molꢀ1, of which the
other four monomers are close to 140 kJ molꢀ1. The results
indicated that the polymerization mechanisms of BZ-a and
BS-a, affected by electron-withdrawing groups, may be differ-
ent from the other four monomers.
where b is heating rate in K minꢀ1; Ea is activation energy of
conversion at a in kJ molꢀ1; and Ta is temperature of conver-
sion at a in K.
The dependence of the apparent activation energies of both
benzoxazine resins as a function of degree of curing deter-
mined by the general Ozawa’s iso-conversion method is
shown in Figure 7. For all monomers, the Ea values
decrease at early stage of curing because of the catalysis of
phenolic hydroxyl groups derived from polymerization.
Then, the Ea values increase owing to the elevation of vis-
cosity and crosslink density. The trend of BA-a we obtained
is different from the regularity of BA-a reported before.30 It
is reported that the apparent activation energy at initial
stage is less than 80 kJ molꢀ1 and the apparent activation
energy increases with the conversion increasing. It is
because that their sample is impure and the impurity can
catalyze the reaction. Contrasted with BP-a, the Ea values of
BA-a, BF-a, and BO-a are a little higher, and those of BZ-a
and BS-a are obviously lower. Hence, the conversion of BS-
A at 210 ꢁC reaches high level in a short time. The results
indicated that the electron-donating group restrained the
polymerization, whereas the electron-withdrawing group
facilitated it. In addition, the Ea values of BZ-a increases
rapidly just at a conversion of 40% and those of BS-a rises
fast even at a conversion of 30%. The Ea values of other
four systems increase quickly at a conversion of 70%. The
To further study the polymerization behavior, isothermal
ꢁ
DSC scanning of benzoxazine monomers at 210 C was done,
and the conversion versus time curve is shown in Figure 6.
It was obvious that benzoxazines with electron-withdrawing
groups reacted much faster than those with electron-donat-
ing groups. The conversion of BS-a reached to 90% only in
10 min, but the conversion of BA-a is below 20% even after
reacting for 30 min.
The Ozawa method28,29 is frequently used to evaluate
dynamic DSC data to study reactions. This linear equation
with the form y ¼ a þ bx (x ¼ 1/T), can be used to calculate
SCHEME 4 Possible degradation
pathway involved in the thermally
activated polymerization of benz-
oxazine.
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TABLE 6 Results from Elementary Analysis of Two Benzoxazines and Their Polymers
N Content (%) C Content (%) H Content (%)
Theoretical Measured Theoretical Measured Theoretical Measured
Value
Value
Value
Value
Value
Value
BA-a
6.06
5.83
5.79
5.36
6.04
5.79
5.75
5.40
80.52
79.14
69.42
66.85
80.50
79.11
69.22
66.90
6.49
6.08
4.96
4.74
6.46
6.22
4.90
4.87
Poly(BA-a)
BS-a
Poly(BS-a)
results revealed that polymerization of BZ-a and BS-a
became diffusion controlling at initial stage of curing
because of the different polymerization mechanism which
had been discussed in detail below.
most unstable. This trend is in accordance with the results
that BS-a with strong electron-withdrawing react faster
and initial Ea values is much lower. When increasing the
reactivity in the benzoxazine ring, the electron-withdrawing
groups also lead to a decrease in the yield simply because of
the fast oligomerization processes during the synthesis. As
the oligomers were removed during the work up, the yield
decreased. This also is the reason that the yield of BS-a is
lower.
Scheme 3 shows the mechanism of thermally activated poly-
merization of benzoxazines.20,31,32 The ring-opening initiation
of benzoxazine results in the cleavage of CAO bond and the
formation of a carbocation and an oxygen anion. Then, the
active carbon atoms marked C1 and C2 suffer electrophilic
attack from the carbocation and the protons transfer to the
oxygen anions to form hydroxyl groups. Two types of cross-
link structures signed Type 1 and Type 2 are formed. Thus,
the stability of CAO bond and the activity of C1 and C2 are
crucial to the thermally activated polymerization. Theoreti-
cally, electron-withdrawing group in benzoxazine monomers
can decrease the charge density of oxygen on oxazine ring
and destabilize CAO bond, which becomes easier to cleave
and initiate polymerization. As a result, the bond length of
CAO will increase. Electron-donating groups play an oppo-
site role.
In the systems of BA-a, BF-a, BO-a, and BP-a, charges of C1
are higher than those of C2. But in the systems of BZ-a and
BS-a, charges of C2 are higher. Especially in BS-a, charge of
C2 (ꢀ0.0666) is evidently higher than that of C2 (ꢀ0.0570).
Therefore, it is easier for benzoxazines with electron-with-
drawing groups to form the Type 2 structure.
To prove the conclusion above, FT-IR of polybenzoxazines is
conducted (Fig. 8). The typical absorption bands of benzoxa-
zine rings at 917–952 cmꢀ1 nearly disappear. The absorption
bands at 690 and 750 cmꢀ1 associated to monosubstituted
benzene in poly(BZ-a) and poly(BS-a) are weaker than those
of the other four polymers. In addition, the absorption bands
of 1,4-substitued benzene at 820 cmꢀ1 in poly(BZ-a) and
poly(BS-a), especially the later one, are much stronger than
those in the others. The results indicated that BZ-a and BS-a
tended to form the Type 2 structure and the other four
monomers inclined to generate the Type 1 structure.
To analyze the influence of electronic effects on thermally
activated polymerization quantitatively, bond lengths of
CAOs and charge densities of C1 and C2 are calculated by
Material Studio. The results are listed in Table 4. The elec-
tron-donating groups shorten the bond lengths of CAOs.
However, the electron-withdrawing groups extend them. The-
oretically, bond length is related to bond energy, that is, the
longer the bond length is, the lower the bond energy is.
Thus, the CAO bond of BA-a with the strongest electron-
donating group is the most stable and the CAO bond of BS-a
with the strongest electron-withdrawing group is the
It is reported that one of the defects of benzoxazine is the
out-gassing issue during the polymerization process because
of the dissociation of the zwitter-ionic intermediate accom-
panied by release of imine.33 The volatile organic compounds
cause foaming or formation of pore. The weight loss during
TABLE 7 CAN1 Bond Length of the Zwitter-Ionic Intermediate Obtained from MS
BA-a
1.4506
BF-a
1.4454
BO-a
1.4469
BP-a
1.4522
BZ-a
1.4530
BS-a
Bond length of CANþ (A)
1.4837
˚
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ARTICLE
polymerization influenced by electronic effect is also investi-
gated in our work, and the results are listed in Table 5. The
weight loss of BS-a is 3.21%, and the weight losses of ben-
zoxazines with electron-donating groups are all close to 2%.
It is reported that the weight loss at the same condition for
the bisphenol A/aniline based monomer is 8.7%,33 whereas
the current result is 1.77%. This significant discrepancy
must be because of their sample used, which is impure. Dur-
ing the thermally activated polymerization process of bisphe-
nol-based benzoxazine, the possible degradation pathway is
shown in Scheme 4. To confirm the structure of fugitive con-
stituent, the contents of C, N, and H of BA-a, BS-a, and their
polymers are measured by elementary analysis. We postulate
that the releaser is structure 1 and calculate the theoretical
values of the contents of C, H, and N of the cured resins
using the formulas 2, 3, and 4:
ther, electronic effect changed the activity of ortho-C of ben-
zoxazine rings and para-C of aniline, which altered the poly-
merization mechanism and the structures of the polymers.
Owing to the effect of electron-withdrawing sulfone group,
there were more arylamine methylene Mannich bridge type
polybenzoxazine formed and more weight loss during poly-
merization produced in BS-a system.
The authors would like to acknowledge the financial support
from National Science Foundation of China (Project No.
50873062 & 20774060).
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