Preparation and characterization of fully renewable polybenzoxazines from monomers containing multi-oxazine rings

Article abstract of DOI:10.1039/c5ra17164d

Two fully renewable benzoxazine monomers containing multi-oxazine rings were synthesized and used to prepare polybenzoxazines. The structures of the monomers, prepared from renewable feedstock (vanillin, guaiacol, octadecylamine and furfurylamine) were su

Full text of DOI:10.1039/c5ra17164d

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PAPER
Preparation and characterization of fully renewable
polybenzoxazines from monomers containing
multi-oxazine rings†
Cite this: RSC Adv., 2015, 5, 96879
Lei Zhang, Yejun Zhu, Dan Li, Min Wang, Haibin Chen and Jingshen Wu*ab
a
a
b
b
ab
Two fully renewable benzoxazine monomers containing multi-oxazine rings were synthesized and used to
prepare polybenzoxazines. The structures of the monomers, prepared from renewable feedstock (vanillin,
guaiacol, octadecylamine and furfurylamine) were supported by FT-IR and NMR spectral analyses. The
melting points and curing temperatures were characterized through DSC. Thermal initiated ring-opening
polymerization of the monomers was monitored by FT-IR. The solubility of the polybenzoxazines was
examined and cross-linking differences between polymers prepared from monomers with mono-
oxazine and multi-oxazine rings were clarified. Through energy calculations, substitution was found to
preferably take place at the furan ring. The thermal properties were investigated through TGA, which
Received 25th August 2015
Accepted 3rd November 2015
ꢀ
ꢀ
showed good thermal stability (Td5% at 284 C and 335 C) and high char yield (up to 59.6%). The
mechanical properties were characterized through DMA, which revealed a glass transition temperature at
DOI: 10.1039/c5ra17164d
ꢀ
1
55 C. Due to structure speciality, the polybenzoxazines also showed pH responsiveness and can
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reversibly change color under different conditions.
Generally, thecorrespondingmonomersofpoly-benzoxazines,
,3-benzoxazines, are prepared from phenols, amines and form-
Introduction
1
5
Polybenzoxazines are a new generation of phenolic type ther- aldehyde through Mannich condensation. Till now, efforts were
mosets, which inherit the excellent thermal properties of mainly contributed to exploring renewable phenols because
traditional phenolic resins and overcome their disadvantages, renewable amines are relatively rare in both species and
4
such as harsh synthesis conditions and the release of volatile amounts. Only a few papers based on renewable multi-amine,
6,7
8
molecules during curing. Polybenzoxazines also possess other chitosan, have been reported. Aer the rst trial on 1999,
striking features, including structure design exibility, low there was an 8 years' silence until researchers used cardanols,
melting viscosity, near-zero curing shrinkage, low water which can be easily isolated from cashew nut shell oil, for
1
9
absorption and high chemical resistance. Therefore, they preparing benzoxazine monomer in 2007. With the long exible
became some of the rare new polymers which were commer- alkyl chain, the fabricated monomers can be used as reactive
2
10
cialized over the past 30 years.
diluents for epoxy to enhance the toughness. Urushiol based
11
Due to environmental and sustainable economic concern, benzoxazine monomers achieves similar functions. Moreover, it
growing attempts have evolved from incorporating petroleum- haslowercuringtemperatureowingtotheextrahydroxylgroupon
based raw materials to natural renewable feedstock in benzene ring. Eugenol, a renewable mono-phenol which can be
3,4
12
synthesis of benzoxazine resin in the last few years. Poly- extracted from clove, was selected to synthesize benzoxazines.
benzoxazines cannot be an exception. In fact, not only The additional allyl moiety was considered to enhance the cross-
academics, but also industries have engaged in exploring linking density by participating the ring-opening polymeriza-
1
2,13
14
renewable polybenzoxazines, since trials on developing other tion,
renewable polymers were succeeded and available in market.
although opposite results had been reported as well.
4
Besides phenols extracted from specic plants, increasing
researchers have focused on lignin-derived phenols recently due
15
to the unique ultra-abundant stock in nature. In 2012, two fully
renewable bio-based benzoxazine monomers were prepared from
Department of Mechanical and Aerospace Engineering, The Hong Kong University of guaiacol, which is a lignin derivative. The thermal stability was
a
16
Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. E-mail:
enhanced by copolymerization of the two monomers. In 2013,
mejswu@ust.hk
“lignin-like” coumaric acid, ferulic acid and phloretic acid as well
b
Center for Engineering Materials and Reliability, Fok Ying Tung Research Institute,
The Hong Kong University of Science and Technology, Nansha, China
as the corresponding esters were used to prepare benzoxazine
1
monomers. Electron withdrawing effect of the carboxyl group
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra17164d
was found to deteriorate the thermal stability of the monomers
1
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 96879–96887 | 96879

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when unsaturated chain conjugates with benzene ring. Vanillin, from J&K Scientic. Paraformaldehyde (96%), sodium sulfate
which is another renewable phenol, has been widely used in food anhydrous (99%), potassium bromide (99.5%) and octadecyl-
industry as a avour. It can now be industrially manufactured amine (90%) were acquired from Sigma-Aldrich. Sodium
17
from lignin within the waste of paper industry. In 2014, vanillin hydroxide (99%), chloroform (99%) and sulfuric acid (95%)
based benzoxazine monomers were reported simultaneously by were purchased from VWR. Absolute ethanol (99.9%) was ob-
1
8,19
H. Ishida and I. K. Varma.
decrease the curing temperature as well as be further function- received.
alized into a reactive surfactant. But it will also generate CO
during polymerization, which may limit its application for
The aldehyde group can help tained from Merck. All the above chemicals were used as
2
Measurements
19
preparing bulk materials.
The FT-IR spectra of the compounds' structures and the curing
process were recorded by a Bruker FT-IR spectrometer. KBr
Although several renewable benzoxazine monomers had
been prepared so far, most of them have only one oxazine ring
in the monomer. However, rather than polymers with high
molecular weight, these monomers generate merely oligomers
ꢁ1
pellet technique was used with 16 scans at a 4 cm resolution.
The structures of the compounds were veried by nuclear
magnetic resonance spectroscopy ( H-NMR and
1
13
C-NMR)
2
0
of four to six repeating units. The ring-opening polymerization
using a Varian Mercury VX 300 NMR. The characterization
procedure was conducted under room temperature with
21,22
was obstructed by the intramolecular hydrogen bonding.
a consequence, the products prepared from monomers with
one oxazine ring may not suitable for structure applications.
Fortunately, monomers or pre-polymers with multi-oxazine
rings can overcome the trouble,
precursors have to contain multi-phenol, multi-amine or ami-
However, the monomers prepared
from multi-phenols so far, such as terpenediphenol and 4,4-bis
hydroxyphenyl)pentanoic acid, and more recently, cardbi-
sphenol, are just partially renewable.
rare amine variety in nature, past benzoxazine monomers with
multi-oxazine rings were synthesized from renewable mono-
phenol, but petroleum-derived multi-amines. In regard to
aminophenols, only one paper published till now due to the
species restriction. On the other hand, monomers with func-
tional groups can react with other moieties to achieve multi-
oxazine structures. However, it requires other renewable
precursors containing specic reactive groups, which compli-
cates the process and restricts feedstock selection. As a result,
the challenge remains.
This study focuses on the synthesis and characterization of
two polybenzoxazines cured from fully renewable monomers
with multi-oxazine rings. First, a fully renewable triphenol was
prepared from vanillin and guaiacol. Then, it reacted with
octadecylamine and furfurylamine, which are derived from
furfural and vegetable oil, to produce the benzoxazines. The
monomers' structures, polymerization process, thermal and
mechanical properties were supported by Fourier transform
infrared spectroscopy (FT-IR), nuclear magnetic resonance
spectroscopy (NMR), mass spectroscopy (MS), differential
As
deuterated chloroform (CDCl ) as solvent and tetramethylsilane
23
3
(TMS) as internal standard. The molecular weight was detected
by MALDI Micro MX mass spectroscopy by Waters
MICROMASS.
a
24,25
but the corresponding
The thermal stability and char yield of the monomers and
cured polymers were studied with a TA Instrument Q50 TGA
26,27
nophenol structures.
ꢁ1
which equipped with a nitrogen ow rate of 25 mL min . The
heating rate was 10 K min if not specically illustrated. The
masses of all samples were about 5 mg. The polymerization
(
ꢁ
1
8,28,29
Meanwhile, due to
process and the glass transition temperatures (T
g
) were moni-
tored by a DSC from TA Instrument, Q2000. The scan rates were
30
ꢁ
1
g
set at 10 K min under nitrogen atmosphere. The T was ob-
tained through two heating scans of the cured samples. The
27
middle point between the tangent lines of the steps was dened
as T . All samples with mass of 1–3 mg were sealed in lids
g
covered aluminum pans. DMA was performed using a TA
ꢁ
1
Instrument, Q800, at a heating rate of 5 K min from room
31
ꢀ
temperature to 30 C above T
g
. The samples were tested under
three-point bending mode with a frequency of 1 Hz and
displacement of 10 mm aer moulded into 70 mm ꢂ 10 mm ꢂ 2
mm bars. The solubility of the cured polymers were determined
by the weight change before and aer Soxhlet extraction for 24
hours using chloroform as solvent. The residues aer extraction
ꢀ
were dried under vacuum oven for 12 hours under 60 C before
weighing.
Methods
0
00
0
00
Synthesis of 4,4 ,4 -trihydroxy-3,3 ,3 -trimethoxy-triphenyl-
scanning calorimetry (DSC), thermogravimetric analysis (TGA) methane [TP]. Vanillin (7.5 g, 50 mmol) and guaiacol (12.4 g,
and dynamic mechanical analysis (DMA), respectively. Due to 100 mmol) were dissolved using 8 mL anhydrous ethanol in
the particular chemical structures, the polymers exhibited a 250 mL ask. 8 mL of sulfuric acid was mixed with 8 mL of
reversible color changes under different pH conditions.
anhydrous ethanol and kept in a charging hopper. The ask was
maintained at a temperature below 5 C before gradually
ꢀ
introducing the acidic ethanol solution. The mixture was gently
Experimental
Materials
ꢀ
stirred under N
2
atmosphere and a temperature below 10 C for
more than 12 hours to generate a dark red turbid solution. The
All reagents used for synthesis are commercially available. mixture was then washed by distilled water until it became
Vanillin (99%), guaiacol (99%) and CDCl (with 0.03 vol% of neutral. The solid was crystallized through chloroform and
3
0
00
TMS as reference) were purchased from Aladdin Reagent ltered, to obtain the pale pink needle-like crystals of [4,4 ,4 -
0 00
(
Shanghai) Co., Ltd., China. Furfurylamine (99%) was obtained trihydroxy-3,3 ,3 -trimethoxy-triphenyl-methane] [TP]. Yield:
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ꢀ
6
3
7%, mp 125–128 C. MALDI mass: m/z ¼ 381.2 (positive mode), evaporated to leave a pale yellow powder. It was puried by
ꢀ
80 (negative mode).
ethanol to obtain a white powder. Yield: 75%, mp 60–62 C.
ꢁ
1
FT-IR (KBr, cm ) 3400 (O–H), 3053 (C–H, Ar), 2940 and 2850 MALDI mass: m/z ¼ 418.4 (positive mode).
ꢁ
1
(
8
(
3
C–H), 1614 (C]C, Ar), 1508 (C–H, Ar), 1269 and 1033 (C–O–C),
FT-IR (KBr, cm ): 2920 and 2851 (C–H), 1583 (C]C, Ar),
, ppm) 6.81 (Ar-H, 3H), 6.62 1487 (C–H, Ar), 1317 (CH , wagging), 1224 and 1076 (C–O–C),
–CH, 1H), 1159 (C–N–C), 916 (oxazine ring), 734 (Ar) (Fig. S1†).
1
70 and 756 (Ar); H-NMR (CDCl
3
2
Ar-H, 3H), 6.57 (Ar-H, 3H), 5.50 (Ph–OH), 5.34 ((Ar)
3
1
3
1
.78 (O–CH
36.41, 121.98, 113.93, 111.87, 55.86 (O–CH
Synthesis of tris(8-methoxy-3-octadecyl-3,4-dihydro-2H-1,3- (O–CH
3
, 9H); C-NMR (CDCl
3
, ppm) 146.29, 143.91,
3
H-NMR (CDCl , ppm): 6.82 (Ar-H, 1H), 6.75 (Ar-H, 1H),
1
3
), 55.66 (Ar –CH).
3
6.59 (Ar-H, 1H), 4.96 (N–CH
2
–O, 2H), 4.00 (N–CH
2
–Ar, 2H), 3.87
1
3
3
, 3H), 2.74, 1.25, 0.88 (N–C18
H37); C-NMR (CDCl ,
3
benzoxazin-6-yl)methane [Bz_TPo]. In a 250 mL ask, TP (10 ppm): 147.59, 143.54, 120.80, 119.79, 119.37, 109.28, 82.83,
mmol), paraformaldehyde (2.0 g, excess) and n-octadecylamine 55.76, 51.43, 50.01, 15–35 (Fig. S2†).
(30 mmol) was added with 80 mL of chloroform. 20 mL of
Synthesis of 8-methoxy-3-furfuryl-3,4-dihydro-2H-1,3-ben-
anhydrous ethanol was introduced to benet the solubility of zoxazine [Bz_Gf]. The synthesis process of Bz_Gf was similar as
TP. The solution was reuxed for 20 hours resulted in an orange synthesis of Bz_Go except for adding furfurylamine (100 mmol)
transparent solution. The solution was washed by 1 N sodium instead of n-octadecylamine. It was recrystallized from ethanol
ꢀ
hydroxide solution for several times and followed by distilled to obtain a white crystal. Yield: 70%, mp 90–92 C. MALDI mass:
water until the solution became neutral. The solution was dried m/z ¼ 246.1 (positive mode).
ꢁ
1
with anhydrous sodium sulfate and ltered to obtain a trans-
FT-IR (KBr, cm ): 3153 and 3124 (C–H, furan ring), 3093
parent solution. The solvent was evaporated before further (Ar-H), 2958, 2941, 2913 and 2836 (C–H), 1583 (C]C, Ar), 1485
puried in anhydrous ethanol to acquire a white powder. Yield: (C–H, Ar), 1338 (CH , wagging), 1267 and 1074 (C–O–C), 1149
2
ꢀ
7
2%, mp 50 C. MALDI mass: m/z ¼ 1263.0 (positive mode).
(C–N–C), 924 (oxazine ring), 732 (Ar) (Fig. S3†).
ꢁ
1
1
FT-IR (KBr, cm ): 2920 and 2850 (C–H), 1589 (C]C, Ar),
497 (C–H, Ar), 1320 (CH wagging), 1226 and 1097 (C–O–C), 6.78 (Ar-H, 1H), 6.60 (Ar-H, 1H), 6.33 (C]CH–C, 1H), 6.26 (C]
147 (C–N–C), 925 (oxazine ring), 856 and 719 (Ar); H-NMR CH–C, 1H), 5.00 (N–CH
, ppm): 6.49 (Ar-H, 3H), 6.24 (Ar-H, 3H), 5.21 ((Ar) –CH, CH –C, 2H), 3.89 (O–CH
H), 4.93 (N–CH –O, 6H), 3.91 (N–CH –Ar, 6H), 3.76 (O–CH , 147.69, 143.25, 142.60, 120.20, 120.14, 119.44, 110.17, 109.52,
H), 2.74, 1.25, 0.88 (N–C H ); C-NMR (CDCl , ppm): 147.35, 109.00, 82.41, 55.82, 49.08, 48.29 (Fig. S4†).
41.88, 135.79, 120.31, 119.91, 110.64, 82.76, 55.80, 55.78,
1.57, 50.29, 15–35.
3
H-NMR (CDCl , ppm): 7.42 (C]CH–O, 1H), 6.85 (Ar-H, 1H),
1
1
2
1
2
–O, 2H), 4.02 (N–CH
2
–Ar, 2H), 3.96 (N–
1
3
(CDCl
3
3
2
3
, 3H); C-NMR (CDCl , ppm): 151.57,
3
1
9
1
5
2
2
3
1
3
1
8
37
3
Calculation
Synthesis of tris(8-methoxy-3-furfuryl-3,4-dihydro-2H-1,3-
The energy calculations were performed to study the competed
substitutions on furan ring and benzene ring of Bz_TPf. The
energy difference between the iminium intermediate and
different transition state of substitution was evaluated. The
calculations were made on a Gaussian 09 (ref. 32) using the
density functional theory (DFT) under B3LYP functional and 6-
31G+(d,p) basis sets. All of the structures were optimized before
frequency calculation. The transition states were checked with
unique virtual frequency. IRC analysis was applied to examine
the structures as well.
benzoxazin-6-yl)methane [Bz_TPf]. The synthesis process of
Bz_TPf monomer was similar as synthesis of Bz_TPo except for
adding furfurylamine (30 mmol) instead of n-octadecylamine.
White powder was obtained aer further puried in ethanol.
Yield: 65%, mp 165 ꢀC. MALDI mass: m/z ¼ 746.4 (positive
mode).
ꢁ
1
FT-IR (KBr, cm ): 3141 and 3114 (C–H, furan ring), 3062
C–H, Ar), 2931, 2904 and 2830 (C–H), 1591 (C]C, Ar), 1495
C–H, Ar), 1321 (CH
(
(
(
2
, wagging), 1222 and 1094 (C–O–C), 1148
C–N–C), 920 (oxazine ring), 850 and 735 (Ar).
1
H-NMR (CDCl
3
, ppm): 7.40 (C]CH–O, 3H), 6.53 (Ar-H, 3H),
6
.32 (C]CH–C, 3H), 6.25 (Ar-H, 3H), 6.24 (C]CH–C, 3H), 5.22
Results and discussion
((Ar) –CH, 1H), 4.96 (N–CH –O, 6H), 3.96 (N–CH –Ar, 6H) 3.95
3 2 2
1
3
(N–CH –C, 6H), 3.80 (O–CH , 9H); C-NMR (CDCl , ppm): Synthesis of TP was carried out through a phenol-aldehyde
2 3 3
1
1
51.53, 147.47, 142.57, 141.66, 136.05, 119.88, 119.67, 110.83, condensation reaction under acid catalysis (Scheme 1). The
10.18, 108.89, 82.25, 55.87, 55.85, 49.43, 48.38. chemical structures were conrmed by FT-IR and NMR spectral
Synthesis of 8-methoxy-3-octadecyl-3,4-dihydro-2H-1,3-ben- methods. Fig. 1(a) shows the FT-IR spectrum of TP. The
ꢁ
1
zoxazine [Bz_Go]. For comparison, two other monomers with absorbance peak near 3400 cm indicates the –OH group. The
mono-oxazine ring were synthesized. In order to avoid differ- peaks at 3053 and 3010 cm are the stretching of C–H on
ences caused by preparation method, solvent method was used benzene ring. The peaks around 2850–2980 cm are the C–H
instead of solventless method reported by X. D. Liu et al.
Specically, guaiacol (12.4 g, 100 mmol), paraformaldehyde (6.6 around 1672 cm represents the aldehyde group of vanillin.
g, excess) and n-octadecylamine (100 mmol) was dissolved and This peak is absence in FT-IR of TP, which means the partici-
added into a 250 mL ask. Aer reuxing for over 20 hours, pation of the aldehyde group in the reaction. The peak at 1614
a pale yellow solvent was obtained. By successively washing with cm is the stretching of the benzene ring. Comparing with
1
ꢁ1
ꢁ1
16
stretching of methyl group. As shown in Fig. 1(b), the peak
ꢁ
1
ꢁ
1
ꢁ
1
N sodium hydroxide solution and distilled water for several guaiacol and vanillin, the absorbance at 1508 cm conrms
times until the solution became neutral. The solvent was then the trisubstituted benzene ring. The peaks at 1269 and 1033
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Scheme 1 Synthesis of TP.
Fig. 2 FT-IR spectra of (a) Bz_TPo and (b) Bz_TPf.
1
All structures were further conrmed by the H-NMR char-
acterization (Fig. 3). The trisubstituted benzene rings in TP give
rise to the doublets at 6.81, 6.57 ppm and singlet at 6.62 ppm.
The singlet at 5.34 ppm is due to the triphenyl methyl group.
The integration value of the peak at 5.50 ppm is less than 3,
which is attributed to the –OH active protons. The existence of
–
3
OCH protons is evidenced by the singlet at 3.78 ppm. The
Fig. 1 FT-IR spectra of (a) TP and (b) comparison between TP,
guaiacol and vanillin.
ꢁ
1
cm are attributed to the asymmetric and symmetric stretch-
ing of the C–O–C. The preparation of the benzoxazine mono-
mers from TP was implemented through Mannich
condensation with paraformaldehyde, n-octadecylamine and
furfurylamine (Scheme 2). The high absorbance intensity at
ꢁ
1
2
920 and 2850 cm in Fig. 2(a) represents the long alkyl chain
ꢁ1
of Bz_TPo. New absorbance peaks at 1320 and 925 cm
conrms the formation of the oxazine ring. This is also proved
by the substitution change on benzene ring indicated by the
ꢁ
1
18
peaks shiing from 1508 to 1497 cm
. Similar results can be
found in Fig. 2(b), which shows the FT-IR spectrum of Bz_TPf.
ꢁ
1
The absorbance peaks at 1321 and 920 cm
indicate the
generation of the oxazine ring. The peak changes from 1508 to
ꢁ
1
1
495 cm can be attributed to the tetrasubstituted benzene
ꢁ1
ring. New peaks at 3141 cm represents the incorporation of
furan ring into Bz_TPf.
1
Scheme 2 Synthesis of Bz_TPo (upper) and Bz_TPf (lower).
Fig. 3 H-NMR spectra of (a) TP (b) Bz_TPo and (c) Bz_TPf.
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characteristic oxazine rings in Bz_TPo are proved by the two 110.18 and 108.89 ppm. The peak appears at 48.38 corre-
resonances at 4.93 and 3.91 ppm, which represent the protons sponding to the –CH –N of the furan ring.
2
of N–CH –O and Ar-CH –N, respectively (Fig. 3(b)). Another
Moreover, MALDI-TOF was applied to further determine the
2
2
evidence is the tetrasubstituted benzene rings indicated by the molecular weight. The results have been summarized in the
peaks at 6.49 (singlet) and 6.24 ppm (singlet). The incorporation Experimental section and in Fig. S5,† which are in good agree-
of octadecylamine in Bz_TPo is conrmed by the resonances at ment with theoretical calculation. It is worth to note that, the
2
.74, 1.25 and 0.88 ppm. The triphenyl methyl structure gives benzoxazine monomers can decompose during the measure-
1
+
the singlet at 5.21 ppm. Similar results can be found in the H- ment following the [M + H ꢁ n ꢂ 12] rule (Fig. S5†), which were
33,34
NMR spectrum of Bz_TPf monomer (Fig. 3(c)). Specically, the reported by P. Vainiotalo et al. and R. Quevedo et al. before.
generation of the oxazine rings are proved by the resonances at In sum, the above characterization results conrm the
.96 and 3.95 ppm which represent the protons of N–CH –O successful preparation of the molecules.
4
2
and Ar-CH –N, respectively. The protons of furan rings give rise
The polymerization behaviour of both TP based monomers
were monitored by DSC (Fig. 5). The results are summarized in
2
to the resonances at 7.40, 6.32 and 6.24 ppm, respectively.
1
3
ꢀ
ꢀ
Fig. 4 shows the C-NMR of TP, Bz_TPo and Bz_TPf. The Table 1. The endotherm peaks at 50 C and 165 C are the
resonance at 55.66 ppm indicates the trisubstituted methyl melting points of the monomers. The onset and maximum
ꢀ
group and the peak at 136.41 ppm represents the adjacent curing temperatures of Bz_TPo are around 216 and 240 C,
1
3
ꢀ
carbons on the benzene rings (Fig. 4(a)). The C-NMR spectra while lower temperatures at 209 and 234 C can be found when
in Fig. 4(b) and (c) conrm the formation of the Bz_TPo and cure Bz_TPf. The long alkyl chains can hinder the polymeriza-
Bz_TPf. The characteristic oxazine rings are proved by the tion process and cause higher curing temperature of Bz_TPo
carbon resonances at 82.76 (O–CH –N), 51.57 (Ph–CH –N) ppm than Bz_TPf. Meanwhile, the furan ring can participate into the
2
2
for Bz_TPo and 82.25 (O–CH
2
–N), 49.43 (Ph–CH
2
–N) ppm for polymerization while the alkyl chain is inert during the
35
Bz_TPf. The incorporation of long aliphatic chain in Bz_TPo can process. The thermal stability of the monomers were exam-
be proved by the peaks at 50.29 and 15–35 ppm. The existence of ined through TGA (Fig. 6), which show comparable results with
furan ring in Bz_TPf can be conrmed by the peaks at 142.57, most petroleum-based benzoxazine monomers reported by
36
others.
In order to clarify the curing process, FT-IR was applied to
monitor the changes in chemical structures. Fig. 7 shows the
ꢀ
ꢀ
ꢀ
FT-IR spectra of monomers cured at 150 C, 175 C, 200 C and
2
ꢀ
25 C in air for 1 hour, respectively. For both monomers,
dramatic decrease in peak intensity around 920 (oxazine ring),
Table 1 DSC data of Bz_TPo and Bz_TPf monomers at a heating rate
ꢁ1
of 10 K min
ꢀ
ꢀ
ꢀ
ꢁ1
Monomer
T
m
( C)
T
onset ( C)
T
max ( C)
DH (J g
)
Bz_TPo
Bz_TPf
50
165
216
209
240
234
92
104
ꢀ
Fig. 5 DSC curves of (a) Bz_TPo and (b) Bz_TPf at heating rate of 10 C
min , respectively.
13
ꢁ1
Fig. 4
C-NMR spectra of (a) TP (b) Bz_TPo and (c) Bz_TPf.
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comparison, polymers prepared from Bz_Go and Bz_Gf
(monomers with one oxazine-ring) were examined as well.
Insoluble solids can be found among all the polymers, but the
residue weight percentages are quite different. Comparing with
polymers prepared from guaiacol, polybenzoxazines using TP as
phenol source generate more insoluble residues aer test. The
results imply that multi-oxazine rings can signicantly facilitate
the formation of the cross-linked structures. The insoluble
PBz_TPo indicates the substitution can take place on the
benzene ring, as we mentioned before. Meanwhile, polymers
show enhancement in cross-linking when furan ring was
introduced, especially a fully insoluble PBz_TPf can be obtained
by such kind of incorporation. Based on the experimental
results and the iminium ion mechanism proposed by H. Ishida
Fig. 6 TGA curves of monomers (a) Bz_TPo, (b) Bz_TPf.
39
and J. Dunkers, while later on extended by T. Endo, A. Sudo
40
and their coworkers, the related polymerization process for
2
222 (asymmetric stretching of C–O–C) and 1320 (CH wagging Bz_TPo and Bz_TPf are sketched in Scheme 3.
1
of oxazine ring) indicates the oxazine ring-opening during
In fact, although compete substitutions on furan and
polymerization. Meanwhile, The peaks at 3100–3150 cm (C– benzene rings had been observed by many other researchers as
37
ꢁ1
ꢁ
1
16,18
H, furan ring) and 1495 cm (C–H, Ar) diminish at higher well,
temperature suggest the both furan ring and benzene ring kinetic analysis are still unknown so far. Moreover, the widely
the preference of the two pathways and the related
38
involve the polymerization process. As the process is similar to accepted ring-opening mechanism involved two main steps,
the Friedel–Cras reaction, and the unchanged of peaks at 2920 which are the carbocation/iminium generation and the elec-
ꢁ
1
41
and 2850 cm (C–H, alkyl chain) proved the non-reactivity of trophilic substitution. Thus, many convenient methods, such
the chain during polymerization. Therefore, only phenyl as DSC, may not be suitable because the obtained activation
substitution stimulated by the positioning effect of methoxy
16,18
group can take place when polymerize Bz_TPo,
while both
benzene ring and furan ring are reactive when cure Bz_TPf. In
order to study the inuence of multi-oxazine rings to the poly-
mer structure, solubility test of the cured polymers was imple-
mented and the results were summarized in Table 2. For
Table 2 Solubility test of the polymers: PBz_TPo, PBz_TPf, PBz_Go
and PBz_Gf, cured from the corresponding monomers
Sample
Insoluble weight (%)
PBz_TPo
PBz_TPf
PBz_Go
PBz_Gf
26.5
>99
<5
10.8
Scheme 4 Energy change during ring-opening polymerization of
Bz_TPf.
Scheme 3 Mechanism for the ring-opening polymerization of Bz_TPo and Bz_TPf.
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42
energy value is only apparent. For this reason, quantum
chemistry calculation based on transition state theory was
applied to estimate the energy changes during the two path-
ways. Since zwitter iminium intermediate generates within both
pathways, only the energy differences between the intermediate
and transition states of the electrophilic process need to be
considered. The calculated energy results were showed in
Scheme 4. The energy barrier between furan substituted tran-
ꢁ
1
sition state and iminium intermediate is 61 kJ mol , which is
ꢁ
1
Fig.
8 DSC curves of (a) PBz_TPo and (b) PBz_TPf for T
g
smaller than the benzene substituted structure (69 kJ mol ),
but not very signicant. Therefore, based on the transition state
theory understanding, substitution prefers to happen at the
furan ring. However, it cannot determine the whole polymeri-
zation kinetics because many other factors may inuence the
determination.
ꢀ
determined by loss modulus curve is 155 C, which is consistent
with the result obtained from DSC.
42
reaction kinetics as well. Aer all, the quantum chemistry
calculation results provide useful information based on energy
viewpoint for further analysing the sophisticated polymeriza-
tion kinetics, to be exact.
The thermal properties of PBz_TPo and PBz_TPf were
examined through TGA (Fig. 10). The results are concluded in
Table 3. It is observed that PBz_TPf has higher degradation
temperature than PBz_TPo. This can be understood through the
enhancement in cross-linked structure by furan ring, thus
eliminates the fragment decomposition. Meanwhile, high
char yield of PBz_TPf (60%) can be achieved at 800 C under
nitrogen atmosphere due to the incorporation of the furan ring
into the polymer. According to van Krevelan and Hofytzer's
equation, the value of char yield can be used to calculate the
limiting oxygen index (LOI), which is used to evaluate the ame
The polymers were rst characterized by DSC to determine
their T (Fig. 8). Generally, T is effected by both the rigidity of
g
g
38
the polymer chain and the cross-linking density. As mentioned
above, the PBz_TPf is more cross-linked than PBz_TPo, while
ꢀ
the exible long alkyl chain further lower the T
g
of PBz_TPo.
ꢀ
These reasons give rise to the T
g
at 145 C of PBz_TPf and much
ꢀ
lower T
g
at 58 C of PBz_TPo. The mechanical performance of
the PBz_TPf was checked through DMA (Fig. 9). The Tg
43
retardancy. The LOI value of PBz_TPf is 41.3, which indicates
the possible self-extinguishing performance. Meanwhile, it is
worth to note that, the triphenyl methyl structure in the poly-
benzoxazines can be modied by reacting with phosphorus
containing chemicals, such as DOPO, to further enhance their
44

ame resistance.
Interestingly, by overviewing the experiment results, it was
noticed that a carbonyl band gave rise to the high intensity at
Fig. 9 Storage and loss modulus of PBz_TPf measured by DMA.
Table 3 TGA data of PBz_TPo and PBz_TPf polymers
ꢀ
ꢀ
Polymer
PBz_TPo
T
d5% ( C)
T
d10% ( C)
Char yield (%)
LOI values
284
335
321
364
16.7
59.6
24.2
41.3
Fig. 7 FT-IR spectra of (a) Bz_TPo and (b) Bz_TPf cured at different PBz_TPf
temperatures for 1 hour.
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was purposed. Furthermore, through comparing the solubility
of different polymers, the multi-oxazine ring structure was
proved to enhance the cross-linking. Moreover, enhancement in
Tg, thermal stability and char yield can be found in the poly-
benzoxazines incorporated with furan rings. Besides, oxidation
at high curing temperature resulted in a conjugated structure,
which exhibits color change under different pH condition and
can be easily recovered. In conclusion, the work shows the
feasibility in preparing fully renewable polybenzoxazines based
on monomers containing multi-oxazine rings. These polymers
exhibit desirable properties as well as other smart functions.
Fig. 10 TGA curves of (a) PBz_TPo and (b) PBz_TPf.
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