Synthesis and characterization of highly fluorinated diamines and benzoxazines derived therefrom

Article abstract of DOI:10.1016/j.jfluchem.2009.04.002

A novel method for the synthesis of highly fluorinated benzoxazines in a high yield derived form a α,ω-diamine-polyfluoroalkane and α,ω-dianiline-polyfluoroalkane is described. The synthetic method increases the yield by 20% and reduces the reaction time

Full text of DOI:10.1016/j.jfluchem.2009.04.002

Journal of Fluorine Chemistry 130 (2009) 573–580
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis and characterization of highly fluorinated diamines and benzoxazines
derived therefrom
*
Pedro Velez-Herrera, Hatsuo Ishida
Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel method for the synthesis of highly fluorinated benzoxazines in a high yield derived form a a,v-
Received 27 January 2009
Received in revised form 25 March 2009
Accepted 2 April 2009
diamine-polyfluoroalkane and
a,v-dianiline-polyfluoroalkane is described. The synthetic method
increases the yield by 20% and reduces the reaction time by 90% in comparison to the currently known
method, allowing synthesis of large quantity of highly fluorinated diamines. The diamines are used as the
precursors for benzoxazine compounds. The diamines and benzoxazines are obtained in high yield and
purity. The structures are characterized by nuclear resonance spectroscopy (NMR) and Fourier transform
infrared spectroscopy (FT-IR).
Available online 14 April 2009
Keywords:
Aliphatic amine-based benzoxazine
Fluorination
ß 2009 Elsevier B.V. All rights reserved.
Fluorinated benzoxazine
1. Introduction
Fluorinated benzoxazine monomers derived form pentafluor-
aniline and bisphenol-A had been obtained in high yield in an acid
Fluorine-containing polymers have a number of unique proper-
ties [1]. The effect of the incorporation of fluorine into the polymer
backbone for improving the thermal stability has been investigated
[2–4]. The enhancement of thermal stability is a result of the
strength of the C–F bond [5]. Other properties such as chemical
stability, oxidative stability and melting point can also be
improved by fluorine substitution [6,7]. The decrease in dielectric
constant makes fluorocompounds very attractive to the electronics
industry. The low moisture absorption due to the hydrophobicity
of fluorine-contained polymers has also been reported [8,9]. The
low moisture content of fluoropolymers is also favorable to
dielectic constant reduction. In addition; fluorine incorporation
can influence flammability, refractive index, friction coefficient,
adhesion and crystallinity. A large variety of high performance
fluorinated-polymers such as polyimides, polyether and poly-
benzoxazoles have been synthesized and characterized [10].
Benzoxazine chemistry of small compound has been known for
more than 50 years [11]. However, the usefulness of benzoxazines
as precursors for a class of thermosetting phenolic resins with
excellent mechanical and thermal properties has not been
recognized until recently [12]. Polybenzoxazines possess several
outstanding properties such as near-zero shrinkage after curing,
high thermal stability and low water absorption. Additionally,
benzoxazines have high glass transition temperature [13].
medium. It was found that the pH value of the reaction medium is
the controlling factor in the yield for the compounds from very
weak amines [14]. Another fluorinated benzoxazine was synthe-
sized using bisphenol-AF and trifluoromethyl aniline (F-1). A
copolybenzoxazine of F-1 and bisphenol-A/aniline (BA-a) showed
a dielectric constant of 2.36 on 50/50% (w/w) blend [15]. The
thermal properties of BA-a and bisphenol AF aniline (BAF-a) type
benzoxazine were studied. It was found that, under the same
curing conditions, the Tg of the polybenzoxazine derived from BAF-
a was about 40–50 8C higher than the BA-a based polybenzoxazine.
Thermogravimetric analysis showed an increase from 290 8C to
330 8C in the 1% weight loss temperature. The char yield, as defined
by the residual weight obtained at 800 8C under inert environment,
of the fluorinated benzoxazine was about 30% higher than the non-
fluorinated counterpart [16]. Lin et al. showed that fluorinated
aromatic amine component leads to higher thermal stability as
well as lower dielectric constant in polybenzoxazine [17].
Commercial availability of highly fluorinated diamines is poor
while their dialcohol counterparts that can be the precursor for the
diamines are readily available. Scale-up of highly fluorinated
diamines cannot be easily achieved using the known methods. We
studied a novel approach to synthesize these diamines with high
yield and ease of scale-up. Several methods have been used to
synthesized amines from alcohol. The Mitsunobu reaction is one of
the most common methods to obtain amines with the dehydration
of alcohols when treated with diethyl azodicarboxylate and
triphenylposphine on a mild to neutral condition in an anhydrous
aprotic solvent at room temperature [18]. The most notable
* Corresponding author.
E-mail address: hxi3@cwru.edu (H. Ishida).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.04.002

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P. Velez-Herrera, H. Ishida / Journal of Fluorine Chemistry 130 (2009) 573–580
disadvantage of the Mitsunobu reaction is the difficulty in isolating
the product from the unreactive materials and from the oxidized/
reduced compounds. The purification method usually includes a
chromatographic column, which makes it unsuitable for large
quantity synthesis. In the preparation of N-alkylimides, general
yield is close to 85%, whereas 40–60% is obtained when azides are
used instead of imides. Scheme 1 shows the general procedure for
the Mitsunobu reaction as well as the main products from the
reaction.
with metal azide to give an alkyl azide. This is reduced to the amine
by one of a variety of reagents. In this case, the intermediates have
to be isolated and problems associated with handling toxic and
potentially explosive (alkyl azide) are encountered [19]. This
method has been used to synthesize symmetric diamines in
hexamethylphosphoramide (HMPA); however, this method
requires isolation of the azide and the use of hydrogen, which
are not the first choice for industrial process [20,21]. The method
used in this paper is a novel combination of both. We replaced the
sodium azide with a phthalamide potassium salt, which is more
reactive than the phthalamide used in the Mitsunobu reaction, to
Another popular method, shown in Scheme 1, first converts the
alcohol into the alkyl halide or sulfonate, which is then reacted
Scheme 1. Synthesis of the aliphatic amine from an alcohol. On the left side the synthesis with sodium azide; on the right Mitsunobu reaction.

P. Velez-Herrera, H. Ishida / Journal of Fluorine Chemistry 130 (2009) 573–580
575
polymerization of benzoxazines is beyond the scope of this paper.
However, application examples of using highly fluorinated diamines
forpreparationofpolymerizablebenzoxazinemonomersareshown.
Using these newly developed diamines, polybenzoxazines with
oxazine rings in the polymer main chain have been developed and
reported elsewhere [23]. The crosslinked polybenzoxazines with
dielectric constant as low as 2.2 has been prepared.
2. Experimental
2.1. Materials
Reagents used for syntheses are commercially available.
2,2,3,3,4,4,5,5-Octafluro-1-6-hexanodiol (96%) and 1,6-diiodoper-
fluorohexane were obtained from Oakwood Products Inc. p-
toluenesulfonyl chloride (98%), phthalimide potassium salt
(98%), paraformaldehyde (95%), 4-iodoaniline (97%), 4-fluorophe-
nol (99%), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone
(DMPU) (98%), pyridine (98%), and hydrazine hydrate (50–60%)
were purchased from Aldrich. Except for the 4-iodoaniline all of
these were used without further purification. 4-Iodoaniline was
purified by dissolving it in ethyl ether and stirring with activated
charcoal for 8 h, filtered out and evaporated under vacuum.
2.2. Equipment
Thestructureofthecompoundwasverifiedbyproton(1H),carbon
(
13C) and fluorine (19F) nuclear magnetic resonance spectroscopy
(NMR) using Varian Inova NMR spectrometer at proton frequency of
600 MHz as well as the corresponding carbon and fluorine
frequencies at room temperature using deuterated chloroform as
solvent. Signals were averaged from 256 transients for 1H NMR and
19F NMR, and 1024 transients for 13C NMR to yield spectra with
sufficient signal-to-noise ratio. A relaxation time of 10 s was used for
the integrated intensity determination. 19F-decoupled NMR spectra
were recorded using the waltz16 composite pulse decoupling.
Infrared spectra were recorded using a Bomem Michelson
MB100 Fourier transform infrared (FT-IR) spectrometer with
deuterated triglycine sulfate (DTGS) detector under dry air purge.
Co-addition of 32 scans at a 4 cmꢀ1 resolution was used.
2.3. Synthesis
The synthetic routes are shown is Schemes 2 and 3.
2.3.1. Synthesis of 2,2,3,3,4,4,5,5-octafluorohexane-1,6-ditosyl (3)
A
solution of 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (1)
Scheme 2. Synthesis of the fluorinated aliphatic amine-based benzoxazine
monomer proposed in this work.
(25.0 g, 95.4 mmol) in pyridine (20 ml) was added to a solution
of p-toluenesulfonyl chloride (2) (43.0 g, 225 mmol) in pyridine
(30 ml) and stirred overnight at 50 8C. Cold water was added and
the precipitates were filtered out and recrystallized twice from
methanol. White crystals were obtained in 87% yield (44.2 g,
82 mmol).
obtain the same product with high purity and without chromato-
graphic column separation, as shown in Scheme 2. We also have
replaced the HMPA that is a carcinogenic solvent with 1,3-
dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU).
The Ullmann coupling has been used to produce fluroalkyl
substituted aromatic compound. This reaction has been previously
1H NMR (Chloroform-d):
4.41 (t, S–CH2–CF2, 4H), 2.45 (s, CH3–Ar, 6H). 19F NMR (Chloro-
d = 7.78 (d, Ar, 4H), 7.36 (d, Ar, 4H);
applied to the synthesis of
diiodofluroalkanes and iodoaromatic compound with copper in
polar aprotic solvent [22].
a
,
v
-diarylperfluroalkenes from
a
,
v
-
form-d)
CF2, 4F).
d
= ꢀ119.79 (s, CH2–CF2–CF2, 4F), ꢀ123.65 (s, CH2–CF2–
In this paper, two highly fluorinated diamines have been
synthesized in high yield and purity. Since highly fluorinated
diamines are not commercially available, the emphasis of this paper
is placed on the easier preparation method than the existing
methods of such amines as precursors to many compounds. The
newly adopted approach will allow easy scale-up in production. The
diamines were used to obtain novel benzoxazine monomers with 4-
flurophenol and paraformaldehyde in chloroform. Thus, discussing
2.3.2. Synthesis of 2,20-(2,2,3,3,4,4,5,5-octafluorohexane-1,6-
diyl)diisoindoline-1,3-dione (5)
Phthalimide potassium salt (4) (20.0 g, 108 mmol) was added to
a solution of 3 (25 g, 46.5 mmol) in 25 ml of DMPU and the
temperature was increased from room temperature to 140–150 8C,
the mixture was stirred for 3–4 h under nitrogen. The reaction was
characterized by the formation of yellow foam. The reaction was
stopped when no more foam was produced. The reaction products

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P. Velez-Herrera, H. Ishida / Journal of Fluorine Chemistry 130 (2009) 573–580
evaporated under vacuum. The pure product was obtained as a
clear liquid by vacuum distillation. After several days at room
temperature, crystals are formed. White crystals; 72% yield (9.0 g,
34.5 mmol).
1H NMR (Chloroform-d):
d
= 3.26 (t, N–CH2–CF2, 4H). 19F NMR
(Chloroform-d)
CF2–CF2, 4F).
d
= ꢀ122.29 (s, CH2–CF2–CF2, 4F), ꢀ124.39 (s, CH2–
2.3.4. Synthesis of 3,30-(2,2,3,3,4,4,5,5-octafluorohexane-1,6-
diyl)bis(6-fluoro-3,4-dihydro-2H-benzooxazine) (ALF8M)
Paraformaldehyde (2.3 g, 77 mmol) and 4-fluorophenol (4.3 g,
38.5 mmol) were added to a solution of 6 (5 g, 19.2 mmol) in
chloroform (50 ml) and catalytic amount of triethylamine. The
mixture was stirred for 8 h at reflux temperature. The solvent was
evaporatedundervacuum.Thesolidswereredissolvedinethylether
and washed with an aqueous solution of 0.1N NaOH, dried over
sodium sulfate, filtered out, and evaporated under vacuum, and
recrystallized twice from 1-butanol. 80% yield (5.72 g, 10.7 mmol).
Anal. Calcd: C, 49.63; H, 3.41; N, 5.26. Found: C, 49.13; H, 3.70; N,
5.21.
1H NMR (Chloroform-d):
6.69 (dd, Ar, 2H), 4.81 (s, O–CH2–N<, 4H), 4.09 (s, >N–CH2–Ar, 4H),
3.38 (t, >N–CH2–CF2, 4H). 19F NMR (Chloroform-d)
d = 6.86 (td, Ar, 2H), 6.77 (dd, Ar, 2H),
d
= ꢀ118.59 (s,
CH2–CF2–CF2–, 4F), ꢀ122.64 (s, Ar–F, 2F), ꢀ123.67 (s, CH2–CF2–
CF2–, 4F),
2.3.5. Synthesis of 4,40-(perfluorohexane-1,6-diyl)
dibenzeneamine (8)
Scheme 3. Synthesis of the fluorinated aromatic-amine based benzoxazine
monomer proposed in this work.
Copper–bronze was activated as reported elsewhere [24]. 4-
Iodoaniline (7.5 g, 34.3 mmol) and copper–bronze (10 g) were
stirred in 20 ml of degasified DMSO at about 110 8C under argon. A
solution of diiodoperfluorohexane (10 g, 18.0 mmol) in 10 ml of
degassed DMSO was added slowly below the surface of the
solution and the mixture was stirred for 12 h. The product was
isolated by treating the mixture with water and ethyl ether, and
filtered out to remove the cuprous salts. The organic phase was
washed with brine until free of DMSO. The solution was dried over
sodium sulfate with activated charcoal, filtered out, and evapo-
rated under vacuum. Recrystallization from hexane yielded
yellowish crystals in 91% yield (7.5 g, 15.6 mmol).
were cooled to room temperature and brine (300 ml) was added.
The solids were filtered out. A light brown powder was obtained at
a yield of 90%; however, according to the 1H NMR spectra, the
compound was obtained in 90–95% purity for the repeat trials as
shown in Fig. 1. The compound was used on the next step without
any further purification (21.7 g, 41.7 mmol).
1H NMR (Chloroform-d):
d = 7.91 (s, Ar, 4H), 7.24 (s, Ar, 4H); 4.37
(t, S–CH2–CF2, 4H). 19F NMR (Chloroform-d)
d
= ꢀ116.14 (s, CH2–
CF2–CF2, 4F), ꢀ123.44 (s, CH2–CF2–CF2, 4F).
2.3.3. Synthesis of 2,2,3,3,4,4,5,5-octafluorohexane-1,6-diamine (6)
5 (25 g, 48.0 mmol) was treated with hydrazine hydrate in
50 ml of pure ethanol under reflux for 4 h. The solvent was
1H NMR (Chloroform-d):
(s, –NH2, 4H). 19F NMR (Chloroform-d)
d = 7.33 (d, Ar, 4H), 6.69 (d, Ar, 4H), 3.95
d
= ꢀ109.85 (s, Ar–CF2–CF2,
4F), ꢀ121.85 (s, Ar–CF2–CF2–, 4F), ꢀ122.54 (s, Ar–(CF2)2–CF2–, 4F).
2.3.6. Synthesis of 3,30-(4,40-(perfluorohexane-1,6-diyl)bis(4,1-
phenylene))bis(6-fluoro-3,4-dihydro-2H-benzoxazine) (ARF12M)
A 20% (w/w) solution of paraformaldehyde (1.24 g, 41.2 mmol),
4-fluorophenol (2.31 g, 20.6 mmol), and 8 (5 g, 10.3 mmol) in
chloroform was prepared. The mixture was stirred for 72 h at reflux
temperature. The solvent was evaporated under vacuum. The solids
were redissolved in ethyl ether and washed with an aqueous
solution of 0.1N of NaOH, dried over sodium sulfate, filtered out and
evaporated under vacuum. The solids were washed with 1-butanol
and dried. A powder in 79% yield was obtained (6 g, 8.0 mmol).
Anal. Calcd: C, 53.98; H, 2.93; N, 3.70. Found: C, 52.95; H, 3.05; N,
3.53.
1H NMR (Chloroform-d):
(td, Ar, 2H), 6.77 (dd, Ar, 2H), 6.69 (dd, Ar, 2H), 5.34 (s, O–CH2–N<,
4H), 4.65(s, >N–CH2–Ar, 4H). 19F NMR (Chloroform-d)
d = 7.48 (d, Ar, 4H), 7.11 (d, Ar, 4H), 6.86
d
= ꢀ110.34
(s, Ar–CF2–CF2, 4F), ꢀ121.81 (s, Ar–CF2–CF2–, 4F), ꢀ122.35 (s, Ar–
(CF2)2–CF2–, 4F), ꢀ122.65 (s, Ar–F, 2F).
3. Results
Fig. 1. 1H NMR spectra in CDCl3 of the fingerprint region for the methylene group of
the compounds 1, 3, 5 and 6.
The synthesis of two new highly fluorinated benzoxazine
monomers has been performed using 6 and 8 as flexible diamines.

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577
Fig. 4. Degree of conversion of 1-6-diiodoperflurohexane in DMSO at 120 8C with
copper–bronze.
Fig. 2. 1H NMR spectra in CDCl3 of the compounds 1, 3, 5 and 6.
3.1. Synthesis and characterization of the fluorinated diamines
of the foam stops, the substitution seems to slow down drastically.
The integrated intensity ratio of the 1H NMR spectra of the
methylene group at 4.41 ppm for compounds 3 and 4.37 ppm for
compound 5 indicated that most of the product is formed during
the first 3 h. The use of longer reaction times, 12–36 h produced a
black insoluble compound with a decrease in the overall yield. A
black insoluble compound is also produced if the temperature of
the reaction is higher than 160 8C. Once the reaction products were
cooled to room temperature, brine was added and brown solids
were precipitated out. The solids were mostly 1,6-di-imide-
polyfluorohexane (5) and small amount of the unreacted 1,6-di-
tosyl-polyfluorohexane (3). This can be seen in Fig. 1 where most of
the triplet at 4.41 ppm had disappeared and a new triplet at
4.37 ppm appeared, indicating that the reaction was successful.
Additionally, Fig. 2 shows two doublets at 7.91 ppm and 7.24 ppm
in the aromatic region, indicating the absence of unreacted
phthalamide. The 1H NMR spectrum of compound 5 in Fig. 2
shows two small resonances around 2 ppm and 3 ppm; these
frequencies come from the remaining DMPU. Unlike the Mitsu-
nobu reaction, only the water-soluble phthalamide salt was used
as a reactive material having a yield higher than 90%. No further
purification was needed.
For the synthesis of the 1,6-di-aniline-polyflurohexane,
Scheme 2, a novel synthetic method was developed in this paper.
The method presents several advantages over the previously
reported methods. The first step consists of the substitution of the
OH group for the tosyl leaving group that will be used for the
nucleophilic substitution. This method has been performed before
and the results obtain in this paper were similar to those reported
[21,23]. The key element of our new method comes from the
second step that involves the substitution of the tosyl group with
the imide group. The use of phthalamide potassium salt instead of
sodium azide eliminated the problems associated with the
handling of toxic and potentially explosive azide compounds
and produced the
a,v-di-imide-polyfluoroalkane that is the same
compound obtained with the Mitsunobu reaction. By exchanging
these two nucleophiles, the reaction time decreased into one-tenth
from 48 h to 3–4 h while the yield increased by 13% from 77% to
90% [21]. This reaction was characterized by the formation of a
yellow foam that started producing at 140 8C. Once the production
Fig. 3. 19F NMR spectra in DMSO of the reaction of the 1-6-diiodoperflurohexane
and 4-iodoaniline in DMSO at 120 8C with copper–bronze at different times. (a) At
0 min; (b) at 2.5 h and (c) 12 h.
Fig. 5. 1H NMR spectra in CDCl3 of the benzoxazine monomers derived from
fluorinated diamines.

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Fig. 7. 13C NMR spectra in CDCl3 of the benzoxazine monomers derived from
Fig. 6. 19F NMR spectra in CDCl3 of the benzoxazine monomers derived from
fluorinated diamines.
fluorinated diamines.
Once the 1,6-di-imide-polyfluorohexane was obtained, it was
hydrolyzed with hydrazine hydrate in ethanol to obtain the 1,6-di-
The synthesis of 1,6-di-amine-polyfluorohexane (8), Scheme 3,
was followed by 19F NMR spectroscopy and the results indicate
that, when the reaction temperature is below 120 8C, the structure
of the fluoroalkyl group remains unchanged and biaryls are absent.
But, if the temperature is higher than 120 8C the biaryls are formed
and the dehalogenation of the fluoroalkyl occurs with the
formation of allyl compounds. The reaction was followed by the
amine-polyfluorohexane on
a quantitative yield, but other
hydrolysis methods can also be used [25]. With the hydrolysis
process used, there was no need for the reduction with hydrogen
and palladium. The 1H NMR spectra of the compounds obtained in
each step of Scheme 2 are shown in Fig. 2.
Fig. 8. Chemical shift for the 1H, 13C and 19F analysis of the benzoxazine monomer in CDCl3 at room temperature.

P. Velez-Herrera, H. Ishida / Journal of Fluorine Chemistry 130 (2009) 573–580
579
Fig. 9. Chemical Shifts for the 1H, 13C and 19F analysis of the benzoxazine monomer in CDCl3 at room temperature.
decrease of the intensity and frequency change of the terminal –
CF2– resonance. This resonance is observed at ꢀ69.03 ppm when it
is located next to an iodine group and ꢀ108.33 ppm when aniline is
the contiguous group, as shown in Fig. 3. After 45 min, the
conversion to 8 is 22% and 10% of unreacted diiodofluoroalkyl had
shifted its resonance from ꢀ69 ppm to ꢀ67 ppm. This is due to the
formation of the fluoroalkylcoper solvated complex that is an
intermediate. This complex then coordinates with the aromatic
halide followed by an exchange of ligands at copper [22]. At 2.5 h,
almost all unreacted diiodofluoroalky is on the form of the
metallocomplex, and the degree of conversion is 75%. After 12 h,
there is no evidence of any unreacted or solvated complex left in
the mixture. The degree of conversion is shown in Fig. 4. Although
19F NMR shows 100% conversion within the detection limit of the
spectrum obtained, the overall yield was close to 90% due to the
fact that it was not possible to recover the entire product with
solvent extraction.
The synthesis of the ALF8M was carried out in chloroform.
During this step the concentration was an important factor. If the
concentration was higher than 20% (w/w) an insoluble gel was
formed after 30 min of reaction. This gel, characterized by a 1H
NMR resonance at 4.46 ppm, is very likely to be an infinite network
of 1,3,5-triaza compound that is an intermediate in the benzox-
3.2. Synthesis and characterization of the fluorinated diamine-based
benzoxazine monomers
3.2.1. Nuclear magnetic resonance spectroscopy
All benzoxazine monomers were obtained in satisfactory
yields and high purity. The 1H NMR, 13C NMR and 19F NMR
spectra are shown in Figs. 5–7. The assigned resonances are listed
in Figs. 8 and 9.
Fig. 10. Fourier transform infrared spectra of the benzoxazine monomers.

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P. Velez-Herrera, H. Ishida / Journal of Fluorine Chemistry 130 (2009) 573–580
azine synthesis [26]. Once this gel is formed, it is not possible to
break the triaza ring and the formation of the oxazine ring does not
proceed. On the other hand, if the concentration is lower than 10%,
only a small amount of solids are observed and the reaction is
almost complete after 48 h.
of asymmetric trisubstituted benzene. The bands between 1200
and 1100 are assigned to the CF2 stretching [30]. The absence of
hydroxyl groups is confirmed by the lack of intermolecular
hydrogen bonded group, which appears between 3600 cmꢀ1 and
3200 cmꢀ1 [31]. This further confirms that the purification has
successfully eliminated any unreacted phenol and oligomers.
Compound 9 shows a strong band at 1615 cmꢀ1 which is related to
the –C C– ‘‘quadrant stretching’’ of the para-substitued benzene
ring.
The synthesis of ARF12M was carried out in chloroform. During
this step the concentration was again an important factor. After 2 h
of reaction a swollen-insoluble compound was formed and a 1H
NMR resonance at 4.91 ppm was found. This resonance value
corresponds to the 1,3,5-triaza structure that is an intermediate on
the benzoxazine synthesis [26]. Unlike the triaza compound
formed with the aliphatic amine (6), this can be broken by
decreasing the pH of the solution to 1 or 2 or by increasing the
concentration of the solution [14]. If the concentration is 30% (w/
w) or higher, the resonance at 4.91 ppm decreases while, at the
same time, two new resonances at 5.3 ppm and 4.6 ppm start to
appear. After 48 h of reaction, most of the resonance at 4.91 ppm
has despaired.
The difference in the stability of the 1,3,5-triazas formed from
compound 6 and compound 8 is because 1,3,5-triaza compound
from aliphatic amines are extremely weak base and only very small
quantities of monoprotonated cations are sufficient to inhibit
further base hydrolysis. They are, therefore, exceptionally stable
and can be hydrolyzed only after several days under very acidic
conditions using an aqueous solution of HCl. On the other hand
1,3,5-triaza compound from aromatic amines can be hydrolyzed
due to the fact that the aromatic ring allows the stabilization of the
amino-carbonium ion with its resonance structure the iminium
ion [26,27].
The 1H NMR spectra, Fig. 5, suggest that the hydrogenated
group next to the amine, either methylene or phenyl, has a greater
influence in the chemical shift than the fluorinated portion of the
amine. The typical characteristic chemical shift for the oxazine
methylene groups in ALF8M are 4.09 ppm and 4.81 ppm that are
very close to the aliphatic-diamine based benzoxazine, whose two
peaks appear at approximately 4.0 ppm and 4.9 ppm [28]. For
compound ARF12M, the typical peaks are located at 4.65 ppm and
5.34 ppm and are similar to BA-a benzoxazine, which is
characterized for the peaks at 4.6 ppm and 5.3 ppm [26]. The
absence of any proton resonances between the two frequencies
indicates the absence of a 1,3,5-triaza type compound. This fact
indicates that the purification was successful at separating any
intermediate compounds. Furthermore the absence of any
Mannich bridge protons of open oxazine ring around 3.7 ppm
indicates the absence of oligomeric species. The integration
analysis of the proton resonances shows the closed-ring content
of each compound better than 99%.
4. Conclusions
Two novel, highly fluorinated benzoxazine monomers have
been successfully synthesized and characterized. A new synthetic
method that leads to high yield with shorter reaction time and
allows synthesis of large quantity of highly fluorinated diamines
has been proposed. These diamines were used as the precursors for
the newly developed benzoxazine compounds. The structure of
these diamine-based benzoxazine monomers has been verified by
1H, 13C and 19F NMR and infrared spectroscopy.
Acknowledgment
The authors gratefully acknowledge the financial support of
Sekisui Integrated Research.
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3.2.2. Infrared spectroscopy
The infrared spectra of the benzoxazine monomers are
presented in Fig. 10. There are a number of infrared bands that
are common to both spectra. The presence of the benzoxazine ring
aromatic ether is confirmed by the absorbance peak at 1223 cmꢀ1
and 1038 cmꢀ1, due to the C–O–C antisymmetric and symmetric
stretching modes, respectively [29]. The absorbance at 932 cmꢀ1
and 926 cmꢀ1 are characteristic modes of benzene with an
attached oxazine ring. The band at 1501 cmꢀ1 is characteristic
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