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Synthesis and characterization of new nitrogen-rich
polymers as candidates for energetic applications
a
a
a
Mehrdad Mahkam , Mehdi Nabati , Abolfazl Latifpour & Javad Aboudi
a
a
Chemistry Department, Azarbaijan Shahid Madani University, Tabriz, Iran
Published online: 05 Dec 2013.
To cite this article: Mehrdad Mahkam, Mehdi Nabati, Abolfazl Latifpour & Javad Aboudi (2014) Synthesis and characterization
of new nitrogen-rich polymers as candidates for energetic applications, Designed Monomers and Polymers, 17:5, 453-457,
DOI: 10.1080/15685551.2013.867569
To link to this article: http://dx.doi.org/10.1080/15685551.2013.867569
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Designed Monomers and Polymers, 2014
Vol. 17, No. 5, 453–457, http://dx.doi.org/10.1080/15685551.2013.867569
Synthesis and characterization of new nitrogen-rich polymers as candidates for energetic
applications
Mehrdad Mahkam*, Mehdi Nabati, Abolfazl Latifpour and Javad Aboudi
Chemistry Department, Azarbaijan Shahid Madani University, Tabriz, Iran
Downloaded by [University of Chicago Library] at 08:42 09 October 2014
(Received 1 September 2013; accepted 17 November 2013)
The novel functionality of aromatic tetrazole derivatives with high nitrogen content predetermines a great interest to
tetrazole-containing polymers. Poly(5-vinyltetrazole) is one of the most attractive polymers containing tetrazoles. The
4-chloromethyl styrene (CMS) was copolymerized with acrylonitrile (in various mole ratios) by free radical
polymerization method at 70 °C using α,α-azobis(isobutyronitrile) as an initiator. The reaction azide ion with
copolymers, simultaneously with replacement of all the chlorine atoms in CMS units, causes the nitrile groups are
entirely converted to tetrazole in dimethylformamide at elevated temperatures. The polymers, obtained in quantitative
yields, were characterized by FT-IR and 1H NMR spectroscopy, differential scanning calorimetry, and gel permeation
chromatograph studies. Thermal properties nitrogen-rich polymers show that explosive thermal degradation takes place
at around 260 °C.
Keywords: poly(5-vinyltetrazole); acrylonitrile; 4-chloromethyl styrene; nitrogen-rich polymers; thermal properties
Introduction
The design of new energetic molecules is based on
compounds exhibiting a high density and an elevated heat
of formation. These fundamental properties, achieved
through the presence of numerous nitrogen atoms and/or
explosophoric groups, ensure high performance levels that
can be useful in target applications such as explosives,
propellants, or gas generators. The same basics also apply
when considering the use of polymers, instead of single
molecules, as energetic ingredients. Azaheterocycles are
obviously suitable scaffolds for achieving nitrogen-rich
polymers. There is a considerable interest in polyvinyltetrazoles (PVT) containing a large amount of nitrogen
because of their powerful energetics,[1–3] interpolymer
complexity,[4,5] biological activity, high thermostability,
[6–8] and good solubility in various solvents,[9] exhibiting
wide applications including dynamite, polyelectrolytes,
[10,11] distinctive complex,[12] biocompatible material of
different natures, and oxygen enriching membrane.[13–15]
The investigation on the radical (co)polymerization,[16]
radiation-induced bulk polymerization,[17] solution
interdiffusion,[18,19] solution properties and kinetics of
solution formation in various media,[20] rheological
properties,[21] swelling thermodynamics,[22] and thermal
degradation and its kinetics and mechanism of the PVT
has been reported. Well-known technique for the
preparation of the PVT is traditional radical polymerization
of 5-vinyltetrazole monomer,[23,24] Considering that the
5-vinyltetrazole monomer is not readily available at the
*Corresponding author. Email: [email protected]
© 2013 Taylor & Francis
present time because of the absence of commercial source
of 5-vinyltetrazole and also the difficulty in the synthesis of
the 5-vinyltetrazole,[25–27] a new and powerful technique
for the preparation of the PVT by tetrazole cyclization of
polyacrylonitrile (PAN) could be a real challenge.
This paper deals with an efficacious preparation and
characterization of the PVT by the tetrazolation of the
cyano groups in the PAN, that is, polymer-analogous
conversion. In this work, we first synthesized the copolymer of 4-chloromethyl styrene (CMS) with acrylonitrile by
radical polymerization. Reaction of azide ion with copolymers simultaneously with replacement of all the chlorine
atoms in CMS units causes the nitrile groups are entirely
converted to tetrazole in dimethylformamide at elevated
temperatures. Thermal properties nitrogen-rich polymers
were characterized differential scanning calorimetry (DSC).
Experimental
Materials
The CMS (Aldrich, 90%) and acrylonitrile (Merck) were
distilled under reduced pressure to remove inhibitors,
before use. The initiator α,α-azobis(isobutyronitrile)
(AIBN) (Merck) was purified by crystallization from
methanol.
Measurements
Infrared spectra were recorded with a 4600 Unicam
FT-IR spectrophotometer as KBr pellets. 1H NMR
454
M. Mahkam et al.
spectra were run on a Bruker 400 MHz spectrometer at
room temperature using CDCl3 as a solvent. The molecular weights (MW and Mn) were determined using a
Waters 501-gel permeation chromatograph (GPC) fitted
with 102 and 103 nm Waters styragel columns. THF was
used as an elution solvent at a flow rate of 1 mL/min,
and polystyrene standard was employed for calibration.
The DSC curves were obtained on a TGA/SDTA 851
calorimeter at heating and cooling rates of 10 °C/min
under N2.
Downloaded by [University of Chicago Library] at 08:42 09 October 2014
Copolymerization of 4-chloromethylstyrene with
acrylonitrile: PCSA
For preparing of copolymers (PCSA1 and PCSA2), a
mixture of 4-chloromethylstyrene with different amounts of
acrylonitrile with molar ratios of 1:1 and 1:2, respectively,
was dissolved in 15 mL of toluene and was mixed with
AIBN (1% molar) as a radical initiator, in a Pyrex glass
ampoule. The ampoule was degassed, sealed under
vacuum, and maintained at 75 ± 1 °C in a water bath, with
stirring for about 48 h. Then, the solutions were poured
from ampoules into cooled methanol. The precipitates were
collected and washed with methanol and dried under
vacuum to yield (approximately 85%) of copolymers
(Scheme 1). For PCSA1 and PCSA2: 1H NMR (DMSO-d6,
ppm) 0.88–2 (CH2–CH), 4.85 (CH2–Cl), 6.9–7.7 (Ar–H).
FT-IR (KBr, cm−1): 3085–3026 (aromatic C–H), 2926–
2860 (aliphatic C–H), 2239 (CN), 1600–1490 (aromatic
C=C).
Reaction of sodium azide with copolymers: PAST
The all of reaction with sodium azide and ammonium
chloride was carried out in a conical bottle equipped
with stirrer and reflux condenser. About 5 g of polymers
powder and 100 mL of DMF were added to a 250 mL
conical bottle with stirring at ambient temperature. Then
to the solution, 6.50 g of NaN3 and 5.35 g of NH4Cl
were added with stirring. The bottle was immediately
placed into an oil bath and heated to 100 °C and
maintained the temperature with stirring for 12 h. The
intermediate product was precipitated spontaneously and
gradually from the reaction mixture during the course of
reaction. The final reaction mixture was added into
Scheme 1. Copolymerization of 4-chloromethylstyrene with
acrylonitrile.
Scheme 2.
Synthesis nitrogen-rich copolymers.
distilled water for a complete precipitate and also an
elimination of DMF. The products obtained were treated
in 300 mL of 0.5 M HCl and repeatedly washed with
distilled water for a complete removal of Cl−, Na+, and
H+. The desired polymer was left to dry in air for several
days to constant weight (yield around 95%). 1H NMR
(DMSO-d6, ppm): 1–1.7 (CH2–CH), 4.4 (CH2–N3),
6.8–7.7 (Ar–H). FT-IR (KBr, cm−1): 3446, 3027
(aromatic C–H), 2926 (aliphatic C–H), 2099 (azide N3),
1490–1600 (aromatic C=C) (Scheme 2).
Results and discussion
The resulting copolymers are white solids and soluble in
THF, N,N-dimethylformamide, and dimethylsulfoxide
but insoluble in n-hexane, methanol, ethanol, and water.
Cycloaddition of C≡N bond of nitriles with sodium
azide in the presence of ammonium chloride occurs in
one step and results in tetrazole derivatives. To increase
the yield of reaction and complete conversion, excess
sodium azide was used in this work. In 1H NMR spectra,
with replacement of chlorine atoms with azide group, the
peak around 4.85 ppm corresponding to two methylene
protons of benzyl chloride completely disappeared, and
new peaks at 4.4 ppm corresponding to two methylene
protons attached to azide group appeared. Analysis of
the IR spectra shows that with reaction azide ion with
copolymers the band at 2239 cm−1 (the stretching
vibrations of the nitrile group) disappears, and in its
place, new absorption band appears at 2099 cm−1, which
are assigned to the stretching vibrations of the azide
(Figure 1).
The copolymer compositions were calculated from
the 1H NMR spectra data. In the past few decades,
1
H NMR spectroscopic analysis has been established as
a powerful tool for the determination of copolymer
compositions because of its simplicity, rapidity, and
sensitivity.[22] Spectrum of copolymer PCSA1 in
DMSO-d6 is shown in Figure 2. The molar compositions
of CMS and acrylonitrile in copolymer PCSA2 were
calculated from the ratio integrated intensities of the
peaks around 4.85 ppm, corresponding to two methylene
protons of benzyl chloride in CMS units to the total area
between 0.88 and 2 ppm, which were attributed to six
protons marked by (#) in CMS and (*) in acrylonitrile.
The molar compositions of CMS and acrylonitrile were
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Designed Monomers and Polymers
Figure 1.
Table 1.
455
Comparing FTIR spectra of copolymers PCSA with PAST.
Molar composition and GPC data of copolymers.
Copolymer
PCSA1 (CMS)m (acrylonitrile)n
PCSA2 (CMS)m (acrylonitrile)n
Molar composition
of monomers in
the feed (%)
Calculated
from the
1
H NMR
(% mole)
m:n
Mw
Mn
Mw/Mn
50:50
33:66
69.2:30.7
46.4:53.5
22,816
18,836
10,045
9125
2.27
2.06
calculated from Equations (1) and (2) where x and y
were the mole fractions of CMS and acrylonitrile,
respectively:
Area at 4:85
2m
¼
Area at 0:88 2 3m þ 3n
(1)
m þ n ¼ 100
(2)
A similar method was used to calculate the molar
compositions of monomers in copolymer PCSA1. The
compositions of copolymers are presented in Table 1.
Downloaded by [University of Chicago Library] at 08:42 09 October 2014
456
M. Mahkam et al.
Figure 2.
1
Figure 3.
Comparing thermal behaviour of copolymers.
H NMR spectrum of PCSA2 in DMSO-d6.
The study of composition of polymers shows that
monomers reactivity ratios are different and CMS is
more reactive than acrylonitrile toward propagating
species. Therefore, copolymers containing a large
proportion of the more reactive monomer (CMS) in
random placement.
Designed Monomers and Polymers
Thermal behavior
Downloaded by [University of Chicago Library] at 08:42 09 October 2014
Thermal properties of copolymer were evaluated using the
DSC technique. As can be seen from Figure 3, two copolymers (PCSA1 and PCSA2) are completely stable until
280 °C, while the decomposition nitrogen-rich copolymers
(PAST1 and PAST2) occurred about 260 °C, which that is
due to the fact that azide groups and tetrazole rings are both
destroyed in this stage. With comparing the two copolymers, PAST1 and PAST2, it is observed that in PAST1 with
increase in the azide percentage, increasing the amount of
energy released. So, the energy content of the azide group
is comparably higher than that of the tetrazole ring.
Conclusions
The copolymers were synthesized by free radical
solution polymerization. The molar compositions of the
obtained copolymers were calculated by the 1H NMR
spectral method. The nitrogen-rich copolymers show an
explosive thermal degradation together with a release of
huge heat and magnitude of heat increased by increases
of amount of azide groups in side chains of copolymer.
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