Bifunctional benzoxazines: Synthesis and polymerization of resorcinol based single isomers

Article abstract of DOI:10.1002/pola.27966

Aside from their outstanding properties such as thermal and chemical stability and excellent mechanical performance, benzoxazines suffer from high polymerization temperatures. Isomeric mixtures of bifunctional benzoxazines based on resorcinol proved already to be highly reactive monomers enabling polymerizations at lower temperatures. This contribution describes the polymerization behavior of single benzoxazine isomers and furthermore the influence of different substituents at the aniline moiety on the curing temperature. Single isomers of bifunctional benzoxazines are now accessible in a straightforward one-pot synthesis starting from resorcinol and the appropriate N-phenyl functionalized aniline component. The asymmetric benzoxazine monomers bearing no (R-a: T_peak = 179 °C) or electron-donating substituents in meta position to N (R-3,5dma: T_peak = 183 °C) succeed in lowering the polymerization temperature. Additionally, the impact of several initiating systems was studied resulting in a decrease of the polymerization temperature for all studied resorcinol derived benzoxazine isomers (down to 144 °C).

Full text of DOI:10.1002/pola.27966

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ARTICLE
Bifunctional Benzoxazines: Synthesis and Polymerization of Resorcinol
Based Single Isomers
1
1
2
3
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Hannes Schafer, Andre Arnebold, Johannes Stelten, Jordi Marquet,
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Rosa Marıa Sebastian, Andreas Hartwig, Katharina Koschek
1Adhesive Bonding Technology and Surfaces, Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM,
Wiener Strasse 12, Bremen, D-28359, Germany
2Institute for Organic and Analytical Chemistry, University of Bremen, Leobener Strasse NW2C, Bremen, D-28359, Germany
3
ꢁ
Department of Chemistry, Universitat Autonoma De Barcelona, 08193 Bellaterra (Cerdanyola Del Valle`s), Barcelona, Spain
Correspondence to: K. Koschek (E-mail: katharina.koschek@ifam.fraunhofer.de)
Received 4 August 2015; accepted 22 October 2015; published online 12 November 2015
DOI: 10.1002/pola.27966
ABSTRACT: Aside from their outstanding properties such as
thermal and chemical stability and excellent mechanical per-
formance, benzoxazines suffer from high polymerization tem-
peratures. Isomeric mixtures of bifunctional benzoxazines
based on resorcinol proved already to be highly reactive
monomers enabling polymerizations at lower temperatures.
This contribution describes the polymerization behavior of sin-
gle benzoxazine isomers and furthermore the influence of dif-
ferent substituents at the aniline moiety on the curing
temperature. Single isomers of bifunctional benzoxazines are
now accessible in a straightforward one-pot synthesis starting
from resorcinol and the appropriate N-phenyl functionalized
aniline component. The asymmetric benzoxazine monomers
bearing no (R-a: Tpeak 5 179 8C) or electron-donating substitu-
ents in meta position to N (R-3,5dma: Tpeak 5 183 8C) succeed
in lowering the polymerization temperature. Additionally, the
impact of several initiating systems was studied resulting in a
decrease of the polymerization temperature for all studied res-
C
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orcinol derived benzoxazine isomers (down to 144 8C). 2015
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2016, 54, 1243–1251
KEYWORDS: benzoxazine isomers; bifunctional benzoxazines;
high performance polymers; initiators; thermosets; resorcinol
tent initiators, generating a superacid or a reactive carboca-
tion, have been successfully used in the polymerization
process of benzoxazines.6 Phenolic components were
described as possible initiators, due to their catalytic
effect.7–9 Furthermore, Lewis acids as aluminum chloride or
titanium (IV) chloride,10 metal complexes containing manga-
nese, iron and cobalt11 or salts of the trifluoromethanesul-
INTRODUCTION Polybenzoxazines belong to the group of
phenolic type resins and were first mentioned by W. Holly
and A. C. Cope in 1944.1 They exhibit high functionality and
molecular design variability with unique properties ranging
from low water absorption, near-zero shrinkage to high ther-
mal and mechanical stability. Due to these facts, they keep
up with todays challenging demands and are therefore of
great interest for future applications in the field of high per-
formance or flame-resistant materials.2
12
fonic acid as Zn(OTf)2 or Al(OTf)3 were investigated as
potential initiating systems among others.
Despite their superior and versatile properties, benzoxazines
have drawbacks like high curing temperatures, toxic starting
materials and the brittleness of polybenzoxazines.3–5
Variously modified benzoxazines have been synthesized with
the aim of enhancing the reactivity. Andreu et al. for exam-
ple, studied the influence of electron-withdrawing and
electron-donating substituents on the curing behavior, start-
ing with a phenol and aniline based 1,3-benzoxazine.13
Current research objectives deal with the reduction of proc-
essing temperatures during the polymerization of benzoxa-
zine monomers. There are two main approaches to affect
polymerization temperatures: The use of (i) initiators and
catalysts and (ii) highly reactive benzoxazine monomers.
Recently, Arnebold et al. have described the use of resorcinol
as phenolic component for the preparation of bifunctional
benzoxazines.6 The obtained isomeric mixtures proved to be
very reactive. Accordingly, resorcinol seems to be a promis-
ing reactant for the synthesis of new benzoxazines as highly
Several different catalysts and initiators are known to
enhance the reactivity of benzoxazines. Recently, thermola-
Additional Supporting Information may be found in the online version of this article.
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2015 Wiley Periodicals, Inc.
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reactive bifunctional monomers. It is essential, however, to
study the polymerization of the pure isomers in order to
gain further insight into the relations of chemical structure
and reactivity of resorcinol derived benzoxazines.
Thermogravimetric Analysis (TGA)
Thermogravimetric measurements were carried out on a
Q5000 from TA Instruments with a temperature range from
0 8C to 800 8C and a heating rate of 10 8C/min under nitrogen.
Accordingly, the present work addresses the polymerization
behavior of single benzoxazine isomers based on resorcinol
and aniline derivatives. With the aim of further lowering the
curing temperature, the polymerizations were studied in
dependence on the substitution pattern of the aniline com-
ponent and the presence of different initiators, respectively.
Monomer Synthesis
3,9-Diphenyl-3,4,9,10-Tetrahydro-2H,8H-[1,3]
Oxazine-[6,5-F][1,3]-Benzoxazine (R-a (1)) and 3,9-Bis
(3,5-Dimethylphenyl)-3,4,-9,10-tetrahydro-2H,8H-[1,3]
Oxazino-[6,5-F][1,3] Benzoxazine (R-3,5dma (2))
40 mmol of the appropriate aniline derivative (R-a: aniline; R-
3,5dma: 3,5-dimethylaniline) were added to a solution of
formaldehyde (6.0 mL, 80 mmol) in ethyl acetate (10 mL).
The solution was stirred for 30 min (R-a: 20 8C; R-3,5dma:
0 8C) and then heated to reflux. A solution of resorcinol (2.2 g,
20 mmol) in ethyl acetate (5 mL) was added slowly (5 minute
period) and the mixture was refluxed for additional 5 h. The
mixture was subsequently washed three times with distilled
water (30 mL each time) and the organic phase was evapo-
rated slowly under ambient conditions for 24 h. The precipi-
tated colorless solid was filtered, washed with a small amount
of cold ethyl acetate and dried under reduced pressure. In
case of R-a (1) a white solid was obtained (2.75 g, 40%).
This is the first study examining the impact of isomerically
pure resorcinol based benzoxazines and its derivates on the
ring opening reaction of benzoxazines.
EXPERIMENTAL
Materials
All chemicals were used as received from commercial suppli-
ers. Aniline (99%), formaldehyde solution (37% in H2O, sta-
bilized with 10–15% methanol), resorcinol (99%), p-
anisidine (99%), 4-carbomethoxyaniline (98%), 3,5-dimethy-
laniline (98%), and 1,4-dioxane (99%) were obtained from
Sigma-Aldrich (Steinheim, Germany). Ethyl acetate (95%)
and petroleum ether (p.a., 40–60 8C) were purchased from
Roth (Karlsruhe, Germany). Toluene (p.a.) was obtained from
Merck (Darmstadt, Germany). The latent initiators p-methoxy-
benzyl tetrahydrothio-phenium hexafluoroantimonate (p-MOB-
THT-SbF6) (5) and benzyl tetrahydrothiophenium hexafluor-
oantimonate (B-THT-SbF6) (6) were prepared according to the
literature.14,15
R-a (1): NMR (1H 360 MHz/13C 90 MHz, CDCl3): see Table 1;
HRMS (EI, m/z): calcd. for C22H20N2O2, 344.15248; found,
344.15150; IR (ATR): m 5 1599 (m), 1590 (m), 1497 (m) and
1486 (m) (CAr5CAr), 1249 (s) and 1039 (s) (mC-O-C; CAr-O-C),
939 (s) (out-of-plane-vibration, oxazine ring), 795 (m) (dC-H
;
1,2,3,4-tetra-substituted benzene), 753 (s) and 693 cm21 (s)
(dC-H; monosubstituted benzene) (Fig. 2).
The product R-3,5dma (2) was also obtained as a white solid
(1.38 g, 17%).
Physicochemical Characterization
R-3,5dma (2): NMR (1H 360 MHz/13C 90 MHz, CDCl3): see
Table 1; HRMS (EI, m/z): calcd. for C26H28N2O2, 400.215088;
found, 400.21428; IR (ATR): m 5 1599 (s) and 1488 (m)
(CAr5CAr), 1249 (m) and 1053 (s) (mC-O-C; CAr-O-C), 965 (w)
(out-of-plane-vibration; oxazine ring), 850 (m), 834 (s) and
694 (m) (dC-H; 1,3,5-trisubstituted benzene), 802 cm21 (m)
(dC-H; 1,2,3,4-tetrasubstituted benzene) (Fig. 2).
1D and 2D NMR Spectroscopy
NMR spectra (1H, 13C, HSQC, HH-COSY, HMBC, NOESY) of the
bifunctional monomers were recorded on a Bruker AVANCE
DPX-200 and on a Bruker AVANCE NB-360 spectrometer at
22 6 1 8C in deuterated chloroform. Tetramethylsilane was
used as external standard.
Infrared (IR) Spectroscopy
3,7-Di(4-Carbomethoxyphenyl)-3,4,6,7-Tetrahydro-2H,
8H-[1,3]Oxazine-[5,6-G] [1,3]Benzoxazine (R-4cmoa (3))
4-Carbomethoxyaniline (6.05 g, 40 mmol) was dissolved in
ethyl acetate (35 mL) at 70 8C. Afterwards, formaldehyde
(6.0 mL, 80 mmol) was added and the mixture was stirred at
80 8C for 30 min. Resorcinol (2.2 g, 20 mmol) was dissolved in
ethyl acetate (11 mL), added slowly (5 minute period) and the
reaction mixture was refluxed for 22 h. After cooling to room
temperature the precipitated solid was filtered and dried under
reduced pressure. The crude product was dissolved in 100 mL
chloroform by stirring it for 1 h at room temperature. The solu-
tion was filtered, the solvent was evaporated and the remaining
white solid was dried under reduced pressure (1.84 g, 20%).
The measurements were performed on a Bruker Equinonox
55 FT-IR spectrometer in attenuated total reflection (ATR)
with a Golden Gate cell at 22 6 1 8C with a resolution of
4 cm21 (32 scans).
High-Resolution Mass Spectrometry (HRMS)
Mass spectra were recorded on a Finnigan MAT 95 mass
spectrometer using electron ionization (70 eV) and a direct
sample inlet.
Differential Scanning Calorimetry (DSC)
DSC measurements were performed on a DSC 2920 Modu-
lated calorimeter from TA Instruments with a sealed pan in
a temperature range from 20 8C to 250 8C and a heating rate
of 10 8C/min under air.
NMR (1H 360 MHz/13C 90 MHz, CDCl3): see Table 2; HRMS
(EI, m/z): calcd. for C26H24N2O6, 460.16344; found,
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TABLE 1 1H and 13C NMR Signals of the Bifunctional and Asymmetric Benzoxazine Monomers R-a (1) and R-3,5dma (2),
Respectively
Asymmetric R-a (1)
13C, d(ppm)
Asymmetric R-3,5dma (2)
Number
Structure
1H, d(ppm)
Number
Structure
13C, d(ppm)
1H, d(ppm)
2
O-CH2-N
N-CH2
@CAr
79.6
5.37
4.56
–
2
O-CH2-N
N-CH2
@CAr
79.6
5.37
4.55
–
4
49.9
4
49.8
4a
112.3
125.1
109.2
153.7
78.9
4a
112.4
124.9
109.2
153.7
78.8
5
@CH
6.78
6.41
–
5
@CH
6.80
6.44
–
6
@CH
6
@CH
6a
@CAr-O
O-CH2-N
6a
@CAr-O
O-CH2-N
8
5.30
4.53
–
8
5.30
4.54
–
10
N-CH
46.1
10
N-CH
46.2
2
2
10a
10b
11
@CAr
@CAr-O
N-CAr
@CH
109.4
151.0
148.3
118.0/118.2
129.2
121.2/121.4
148.4
10a
10b
11
@CAr
109.5
151.0
148.2
115.9
138.8
21.5
–
@CAr-O
N-CAr
@CH
–
–
–
12/18
13/19
14/20
17
7.11
7.28
6.95
–
12
6.75
–
@CH
13/19
13a/19a
14
@CAr(-CH3)
-CH3
@CH
2.31
6.63
–
N-CAr
@CH
123.2
148.4
115.6
122.9
17
N-CAr
@CH
18
6.77
6.61
20
@CH
Assignment was confirmed by 2D NMR measurements (360 MHz). Numbering refers to the structures in Figure 1.
TABLE 2 1H and 13C NMR Signals of the Symmetric Benzoxazine R-4cmoa (3) and Asymmetric R-pmoa (4), Respectively
Symmetric R-4cmoa (3) Asymmetric R-pmoa (4)
Number
Structure
13C, d(ppm)
1H, d(ppm)
Number
Structure
13C, d(ppm)
1H, d(ppm)
2
O-CH2-N
N-CH2
@CAr
77.7
5.34
4.61
–
2
O-CH2-N
N-CH2
@CAr
80.8
5.28
4.47
–
4
49.2
4
50.6
4a
5
113.5
124.4
105.0
153.5
151.4
115.9
131.2
121.9
166.6
51.6
4a
112.2
125.0
109.0
153.6
80.2
@CH
6.69
6.32
–
5
@CH
6.76
6.40
–
10
10a
11
12
13
14
15
16
@CH
6
@CH
@CAr-O
N-CAr
@CH
6a
@CAr-O
O-CH2-N
–
8
5.22
4.45
–
7.04
7.93
–
10
N-CH
46.7
2
@CH
10a
10b
11
@CAr
@CAr-O
N-CAr
@CH
109.2
150.9
142.2
120.6
114.4
154.7/154.8
55.4
@CAr
–
C@O
–
–
O-CH3
3.86
12
7.06
6.82
–
13/19
14/20
14a/20a
17
@CH
@CAr
O-CH3
N-CAr
@CH
3.76
–
142.4
120.4
18
7.07
Assignment was confirmed by 2D NMR measurements (360 MHz). Numbering refers to the structures in Figure 2.
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FIGURE 1 Synthesized asymmetric benzoxazine monomers R-a (1) and R-3,5dma (2).
460.16300; IR (ATR): m 5 1710 (s) (mC@O; CAr-CO-O-CH2),
1606 (s) and 1509 (m) (CAr5CAr), 1267 (s) and 1106 (s)
(mC-O-C; CAr-CO-O-CH2), 1237 (w) and 1074 (s) (mC-O-C; CAr-O-
C), 987 (m) (out-of-plane-vibration, oxazine ring), 876 (w)
(dC-H; 1,2,4,5-tetrasubstituted benzene), 825 (w) and 695
NMR (1H 360 MHz/13C 90 MHz, CDCl3): see Table 2; HRMS
(EI, m/z): calcd. for C24H24N2O4, 404.17361; found,
404.17240; IR (ATR): m 5 1240 (s) and 1031 (s) (mC-O-C; CAr-
O-CH3), 1044 (s) (mC-O-C; CAr-O-CH2), 939 (m) (out-of-plane-
vibration, oxazine ring), 822 (s) (dC-H; 1,4-bi-substituted ben-
zene), 807 cm21 (s) (dC-H; 1,2,3,4 tetra-substituted benzene)
(Fig. 2).
(w) (dC-H; 1,4-disubstituted benzene), 762 cm21 (m) (dC-O-C
;
CAr-CO-O-CH2) (Fig. 2).
Polymerization of Benzoxazine Monomers
To achieve a complete polymerization of the monomers, the
heating profile according to Huntsman Corporation was
used.16 Briefly, the samples were placed in a preheated con-
vection oven for two hours at 180 8C following 2 h at
200 8C. In case of applying catalysts, the appropriate initiat-
ing agent (5 wt %) and benzoxazine monomer were mixed
firstly with a spatula and the polymerizations of those mix-
tures were then conducted as described above. In all cases a
dark-brown, opaque solid was obtained.
3,9-Di(4-methoxy-phenyl)-3,4,9,10-tetrahydro-2H,8H-[1,3]
oxazine-[6,5-F][1,3]benzoxazine (R-Pmoa (4))
p-Anisidine (49.3 g, 400 mmol) was dissolved in 1,4-dioxane
(100 mL) by stirring for 20 min. Within 5 min formaldehyde
(60 mL, 0.80 mol) was added at room temperature. An
ice bath was placed under the flask and the solution was
stirred for 30 min. Subsequently, the solution was heated to
reflux.
Resorcinol (22 g, 0.20 mol) dissolved in 1,4-dioxane (50 mL)
was added slowly and the mixture was refluxed for further
50 min. After 50 h at room temperature without stirring a
slightly yellow solid crystallized at the bottom of the flask.
This raw product was recrystallized from toluene and the
colorless pure product was dried under reduced pressure.
Dichloromethane (100 mL) was added to the residual solu-
tion, the mixture was washed two times with distilled water
(300 mL each time) and the organic solvent was evaporated.
A mixture of petroleum ether and toluene (5:1) was added
to the dark residue and heated to reflux for 30 min. The
clear liquid was decanted and cooled. From this solution fur-
ther product precipitated and was dried under reduced pres-
sure. The dark residue was treated this way several times.
Related product fractions were combined (10.6 g, 13%).
RESULTS AND DISCUSSION
Synthesis and Characterization of Bifunctional
Benzoxazines
The recently published study on the polymerization behavior
of the bifunctional benzoxazine R-a showed promising
results, namely a significant lowering of the polymerization
temperature to Tpeak 5 179 8C.6 The applied benzoxazine
monomer, however, was an isomeric mixture of the symmet-
ric and asymmetric bifunctional compound (isomeric ratio:
asymmetric/symmetric 5 7/1). To gain insight into the influ-
ence of the resorcinol component on the polymerization tem-
perature, isomerically pure resorcinol based benzoxazine
monomers were synthesized. Additionally, the influence of
the substitution of the aniline component on the curing
FIGURE 2 Synthesized symmetric benzoxazine monomer R-4cmoa (3) and asymmetric monomer R-pmoa (4).
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