Synthesis and Thermal Behavior of Some
Anthracene-Based Copolymers Obtained by
Diels–Alder Cycloaddition Reactions
M. Grigoras, M. Sava, G. Colotin, C. I. Simionescu
‘‘P. Poni’’ Institute of Macromolecular Chemistry, Iasi 700487, Romania
Received 9 May 2006; accepted 20 November 2006
DOI 10.1002/app.26466
Published online 27 September 2007 in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: New polymer structures have been synthesized via Diels–Alder cycloaddition of bisdiene compounds bearing two anthracene groups and different bisdienophiles, all containing bismaleimide or biscitraconimide functions. The monomers and polymers were
characterized by FTIR, UV, and 1H NMR techniques and
compared with two models having a cycloadduct struc-
INTRODUCTION
Many reactions, very much known in organic chemistry, can be successfully extended in macromolecular chemistry to obtain new polymers with particular
architectures and properties. Such an example is the
Diels–Alder reaction or ‘‘diene synthesis,’’ where an
electrophilically activated double or triple bond adds
1,4 to a conjugated diene so that the product is
always a six-membered ring. This reaction is one of
the most used reaction in organic chemistry to
obtain mono-, bi-, and polycyclic compounds with
bridge, and in the last years, it was extended to synthesis of the polymers.1
The Diels–Alder reaction involves a 2 1 4 cycloaddition between a diene and a dienophile compound.
The application of Diels–Alder chemistry to polymer
synthesis,2–13 modification14–17 and (reversible) crosslinking,18–23 or polyphenylene and ribbon-like (twodimensional) conducting polymers24 has been
reported. However, the lack of success in using at
large scale of this method is limited by thermal reversibility of the cycloaddition and low molecular
weight of the products.
The synthesis of polymers by Diels–Alder cycloaddition can be performed by a self-polymerization reaction (when both complementary functions are situated
in the same molecule, an AB-type monomer) or by an
intermolecular reaction (when bisdienes and bisdienophiles are used as comonomers, A-A and B-B comonoCorrespondence to: M. Sava (msava@icmpp.ro).
Journal of Applied Polymer Science, Vol. 107, 846–853 (2008)
C 2007 Wiley Periodicals, Inc.
V
ture. The polymers were studied by thermogravimetric
analyses. Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 107:
846–853, 2008
Key words: bisanthracene compounds; bismaleimides; biscitraconimides; Diels–Alder copolymerization; thermal behavior
mers). Anthracene and its derivatives are well known
as fairly reactive dienes that easily participate to the
Diels–Alder cycloaddition reactions with different
dienophiles.25 The self-polymerization of anthracenebased monomers by Diels–Alder cycloadditions can
take place if the double or triple bond, electrophilically activated by presence of one or two carbonyl
groups, i.e., acrylate, methacrylate, maleimide, citraconimide, propiolate, etc, is bounded at anthracene
ring.26–33 In some cases, this reaction can be intramolecular one, depending on length of the spacer connecting the diene and dienophile, when a wide variety
of 9,12-bridged ethano- and etheno anthracenes have
been obtained by cyclization of 9-substituted anthracenes.26 The self-polymerization of A-B anthracene
monomers by Diels–Alder cycloadditions has been
reported for the first time by Simionescu and coworkers.27–32 The following A-B type monomers have
been studied in self-polymerization reactions: 1- and
2-anthryl maleimide,29,33 1-anthryl citraconimide,31
1-anthryl and 2-anthryl itaconimide,32 and 9-anthrylmethyl methacrylate, 9-anthrylmethyl acrylate, 9anthrylethyl acrylate and methacrylate,27,28 and 9anthrylmethyl propiolate.30
The first use of bisanthracene compounds in
Diels–Alder copolymerization was reported by Meek
et al.,34–36 using bismaleimide compounds. Meador
and coworkers,37–40 has used an effective bisdienophile diepoxyanthracene in reaction with anthracene-end capped polyimide oligomers to obtain
matrix resins for high-temperature applications. The
polymers obtained have high softening points and
good thermal oxidative stabilities, making them applicable for high-temperature composites.
ANTHRACENE-BASED COPOLYMERS
847
TABLE I
Some Characteristics of Obtained Polymers
No.
Polymer
Mn
(g mol21)
Polydispersity
Yield
(%)
IR spectra (cm21)
1
2
3
4
5
6
7
8
7
8
9
10
11
12
13
14
8260
10950
–
–
6390
7850
–
3480
1.17
1.32
–
–
1.21
1.24
–
1.46
65.0
61.0
67.0
63.0
58.1
61.1
48.5
69.0
3400, 1705, 1660, 1500, 1395, 1245, 1155, 1100, 775, 455
3400, 1710, 1660, 1500, 1395, 1245, 1160, 840, 700, 510
3450, 1705, 1660, 1500, 1250, 1050, 730, 500
3400, 1705, 1660, 1500, 1395, 1145, 1050, 840, 740, 550
3450–3300, 3050, 2950, 1705, 1660, 1500, 1395, 1195, 1050, 740, 500
3400, 3060, 2910, 1700, 1660, 1500, 1395, 1240, 1150, 740, 500
3450, 1705, 1660, 1500, 1195, 1050, 750, 500
3400, 1705, 1660, 1550, 1445, 1395, 1290, 1250, 1145, 1050, 740, 595, 550
The present work is an extension of our previous
interest on synthesis by Diels–Alder polycycloaddition reactions27–32 of new polymer architectures applicable as thermostable coatings and optoelectronic
materials using A-A and B-B monomer pairs, such
as anthracene-based bisdienes and bismaleimide or
biscitraconimide comonomers. A special attention is
focused on the thermal stability of the copolymers
due to the possibility of cycloadduct-type structures
to depolymerize by a retro Diels–Alder reaction.
EXPERIMENTAL
Materials
9-Anthrylcarboxaldehyde was synthesized by a
known method,41 while 1,2-diaminoethane, 1,3-diaminopropane, maleic anhydride, citraconic anhydride, and p-toluenesulfonic acid are commercially
products (Aldrich) and are used as received. The
synthesized monomers were purified by recrystallization. Commercially, solvents (benzene, toluene,
and N,N-dimethylformamide) (Fluka, Germany)
were dried and purified by standard methods.
Monomer (1)—yield: 85% and mp 5 170–1718C;
H NMR (CDCl3, ppm): 9.50 (s, 2H,
CH¼
¼N ),
8.60–7.35 (m, 18H, aromatic protons), 4.52 (s, 4H,
N CH2); UV (CHCl3, kmax): 262, 334, 352, 372,
388 nm. Monomer (2)—yield: 88% and mp 5 139–
1408C; 1H NMR (CDCl3, ppm): 9.56 (s, 2H,
CH¼
¼N ), 8.70–7.46 (m, 18H, aromatic protons),
4.23 (t, 4H, N CH2), 2.57 (q, 2H, (CH2) ); UV
(CHCl3, kmax): 262, 333, 353, 370, and 389 nm.
1
Synthesis of the bisdienophile monomers
Bismaleimide (3) and biscitraconimide (4) were synthesized by the reaction of maleic anhydride and
citraconic anhydride, respectively, with 4,40 -diaminodiphenylether, according to the method reported by
Dix et al.42
Bismaleimide (3) was recrystallized from toluene.
Yield 68%, mp 178–1838C. Anal. Calcd for
C20H12N2O5 (%): C, 66.66; H, 3.35; N, 7.77. Found: C,
65.93; H, 3.67; N, 7.38. IR (KBr), cm21: 1785, 1720
(C¼
¼O imide), 1250 ( O ). 1H NMR (DMSO-d6,
ppm): 7.50–7.33 (d, aromatic protons ortho to imide),
Synthesis of the monomers
Synthesis of the bisdiene monomers
Bis[(9-anthryl)-N,N-dimethylene]azometine monomer
(1) and bis[(9-anthryl)-N,N-three-methylene]azometine monomer (2) were synthesized using a reported
method11 by a coupling reaction between 9-anthrylcarboxaldehyde and 1,2-diaminoethane, or 1,3-diaminopropane, respectively. The monomers were twice
recrystallized from methanol as yellow needles.
Scheme 1
Scheme 2
Journal of Applied Polymer Science DOI 10.1002/app
848
GRIGORAS ET AL.
Scheme 3
7.22–7.08 (d, aromatic protons ortho to O ), 6.80–
6.72 (m, vinyl protons).
Biscitraconimide (4) was recrystallized from
dichlorethane/ethanol. Yield 67%; mp 175–1788C; IR
(KBr) cm21: 1780 and 1710 (C¼
¼O imide), 1250
( O ). Anal. Calcd for C22H16N2O5 (%): C, 68.03;
H, 4.15; N, 7.21. Found: C, 68.25; H, 4.61; N, 6.98. 1H
NMR (DMSO-d6, ppm): 7.50–7.33 (d, aromatic protons ortho to imide), 7.22–7.08 (d, aromatic protons
ortho to
O ), 6.80–6.72 (m, vinyl protons), 2.11–
2.01 (m, aliphatic protons).
Bismaleimide (5) and biscitraconimide (6) were
synthesized according to the method of Hoyt et al.43
Bismaleimide (5) was recrystallized from dichlorethane/ethanol. Yield 69%; mp 235–2408C. Anal.
Calcd for C34H20N2O10S (%): C, 62.96; H, 3.10; N,
4.31; S, 4.94. Found: C, 62.31; H, 2.92; N, 4.18; S, 5.21.
IR (KBr) cm21: 1785 and 1725 (C¼
¼O imide), 1755
(C¼
¼O ester), 1330 and 1160 ( SO2 ). 1H NMR
(DMSO-d6, ppm): 8.22–8.07 (d, aromatic protons,
ortho to COO), 8.11–8.00 (d, aromatic protons, ortho
to SO2 ), 7.63–7.48 (d, aromatic protons, ortho to
maleimide and ortho to O ), 7.16 (s, olefinic protons).
Biscitraconimide (6) was recrystallized from
dichlorethane/ethanol. Yield 72%; mp 1808C. Anal.
Scheme 4
Journal of Applied Polymer Science DOI 10.1002/app
Scheme 5
Calcd for C36H24N2O10S (%): C, 63.90; H, 3.57; N,
4.13; S, 4.73. Found: C, 63.57, H, 3.82; N, 3.92; S,
4.49..IR (KBr) cm21: 1790 and 1720 (C¼
¼O imide),
1755 (C¼
¼O ester), 1330 and 1165 ( SO2 ). 1H
NMR (DMSO-d6, ppm): 8.56–8.37 (m, aromatic protons, ortho to SO2 ), 8.26–8.15 (m, aromatic protons, ortho to COO), 8.00–7.65 (m, aromatic protons,
ortho to imide and ortho to
O ), 7.09–7.00 (m,
olefinic protons) and 2.25–2.00 (m, aliphatic protons).
Figure 1 The UV spectra of the model 15.
ANTHRACENE-BASED COPOLYMERS
Figure 2 The 1H NMR (CDCl3) spectrum of the model 15.
Diels–Alder polycycloaddition
An equimolar mixture containing 1 mmol bisdiene, 1
mmol bisdienophile, and 20 mL N,N-dimethylformamide was heated at 1208C under a nitrogen atmosphere for 20 h. After cooling at room temperature,
the homogeneous dark solution was precipitated in
water. The polymer was filtered, dried, and purified
twice by reprecipitating in methanol from DMF solution. The yields and some structural characteristics
of the synthesized polymers are presented in Table I.
Cycloadduct models synthesis
849
spectra were obtained on a Specord M42 Carl Zeiss
Jena spectrophotometer (Jena, Germany) in chloroform solutions using 10-mm quartz cells. Melting
points are uncorrected and were measured with a
Boetius microscope (Dresden, Germany). 1H NMR
spectra were recorded on a Bruker NMR spectrometer
Avance DRX 400 MHz (Rheinstetten, Germany),
using CDCl3 or DMSO-d6 solutions as solvent, at
room temperature (TMS as internal standard). The
relative molecular weights were determined by gel
permeation chromatography using a PL-EMD instrument (England), polystyrene standards for the calibration plot, and N,N0 -dimethylformamide as solvent.
Thermal gravimetric analysis (TGA) was performed
on MOM (Budapest, Hungary) thermobalance apparatus, in air at a heating rate of 128C/min.
RESULTS AND DISCUSSION
Design, synthesis, and characterization of
bisdiene monomers
Two bisdiene monomers (1) and (2), containing an azomethine linkage as a spacer between anthracene nuclei,
were synthesized by coupling reactions between 9anthrylcarboxaldehyde and 1,2-diaminoethane, or 1,3diaminopropane, respectively, (Scheme 1).
To elucidate the structure of the polymers, two
model compounds (15) and (16) were synthesized.
Compounds were obtained by thermal cycloaddition
of citraconic anhydride to anthracene (1 : 1M ratio)
and biscitraconimide (4) cycloaddition to anthracene
(1 : 2M ratio), respectively. The reactions were performed in bulk at 1108C for 24 h and in the presence
of traces of hydroquinone. After recrystallization
from methanol, pure products were obtained.
(15): Yield 79%; mp 158–1598C. (16): Yield 82%;
mp 181–1828C.
MEASUREMENTS
FTIR spectra were recorded in KBr pellets on a Digilab-FTS 2000 spectrometer while UV–vis absorption
Figure 3 The 1H NMR (CDCl3) spectrum of the model 16.
Figure 4 TGA and DTA thermograms of model compounds 15 and 16.
Journal of Applied Polymer Science DOI 10.1002/app
850
GRIGORAS ET AL.
Scheme 6
In the 1H NMR spectra of both monomers, the
multiplet situated in the range of 8.7–7.3 ppm is
associated with anthracene protons while the singlet
appearing at 9.5 ppm is assigned to CH¼
¼N protons. The peak signal that appears in the 4.52 and
4.23 ppm is attributed to the N CH2 groups in the
monomer (1) and (2), respectively, while the aliphatic protons in the middle (CH2) group of monomer (2) are the singlet at 2.59 ppm. Also, UV spectra
show strong and characteristic absorptions of 9anthryl substituted rings (260–390 nm).
Synthesis and characterization of the
bismaleimide and biscitraconimide monomers
Bismaleimide and biscitraconimide monomers were
synthesized according to the methods reported by
Dix et al.42 and Hoyt et al.43 and are presented in
Schemes 2 and 3.
The elemental analysis and 1H NMR spectra of
bismaleimides and biscitraconimides confirm the
expected structures.
Synthesis of the Diels–Alder model compounds
To obtain more information about the structure of
the polymers, two model compounds, 15 and 16,
were synthesized by cycloaddition reaction between
Journal of Applied Polymer Science DOI 10.1002/app
a monofunctional diene monomer (anthracene) and
citraconic anhydride, or biscitraconimide 4 as dienophile (Schemes 4 and 5).
The IR spectra of 15 and 16 show peaks at
3410 cm21 and 2990 cm21 due to C H from aromatic
rings, at 1850 and 1790 cm21 (the absorption characteristic of C¼
¼O stretching from imide) while the
O linkage was observed at 1455 and 1220 cm21.
The peaks at 1005, 950, 905, 750, 700, 550, and
505 cm21are relevant for the formation of cycloadducts, they being present only in 9,10-dyhydroanthracene structures, while bisanthracene monomers 1 and
2 exhibit characteristic absorptions due to the monosubstituted anthracene rings, mC H aromatic at 730,
840, and 875 cm21. Also, UV spectroscopy can very
well differentiate anthracene and dihydroanthracene
structures. Thus, monomers 1 and 2 show characteristic absorptions of anthracene nucleus, 250–260 nm and
320–395 nm, while cycloadducts 15 and 16 showed
absorption maxima characteristic to 9,10-dihydroanthracene structures (ortho-disubstituted benzenes)
(kmax at 270 and 295 nm), the absorptions situated
between 320 and 390 nm have disappeared due to
destruction of anthracene by cycloaddition) (Fig. 1).
The 1H NMR spectrum of 15 and 16 are presented
in Figures 2 and 3. Model 15 shows signals at 7.52–
7.17 ppm (8 aromatic protons from dihydroanthracene
structure), 4.76 ppm (dihydroanthracene proton),
ANTHRACENE-BASED COPOLYMERS
851
sented in Scheme 6. The reactions were performed in
DMF and polymers were precipitated in methanol.
The polymers are soluble in polar organic solvents
such as DMF, THF, DMSO, and NMP, and Table I
presents the yields and some characteristics of
obtained polymers.
The first observation that can be made is related
to low molecular weight, all products being oligomers due to the negative influence of the sterical hindrances and reversibility of the Diels–Alder reaction.
There are only few examples where Diels–Alder
polymerization has yielded to high-molecular-weight
polymers, and in all cases, the irreversibility of the
reaction was assured via subsequent elimination of a
small molecule.
According to the UV, FTIR, and NMR spectra,
polymers have resulted by addition of the dienophile to the mesopositions of anthracene nucleus
and formation of 9.10-dihydroanthracene structures.
Thus, the intensities of absorption bands at 260 and
320–390 nm, attributed to the p-p* transition in the
anthracene nucleus, are very diminished and are
assigned to the anthracene-end groups. In the same
time, new absorption bands appear at 258, 266, 275,
Figure 5
The UV spectra of the synthesized polymers.
4.40 ppm (dihydroanthracene proton), 3.0 ppm (CH
from anhydride), and 1.26 ppm (protons from CH3).
Model 16 shows signals at 7.43–7.18 ppm assigned
to aromatic protons, 6.90 (aromatic protons from
dienophile, ortho to imide), 6.45 ppm (aromatic protons from dienophile, ortho to
O ), 4.82 ppm
(dihydroanthracene proton), 4.46 ppm (dihydroanthracene proton), and 1.24 ppm ( CH3 protons).
The thermal stability of the model compounds
was evaluated by TGA, and Figure 4 presents typical
TGA and DTA traces of these compounds performed
in air atmosphere. The endotherm peak at 1488C is
assigned to melting for model 15 followed by a retro
Diels–Alder reaction with formation and evaporation
of anthracene and citraconic anhydride. The retro
Diels–Alder reaction in the case of model 16 is less
evident, probably because the dienophile monomer
is stable until 2008C. Both model compounds 15 and
16 afforded char yields 0% at 6008C.
Synthesis of polymers by
Diels–Alder polycycloaddition
The reactions used for synthesis of copolymers are
based on a Diels–Alder cycloaddition and are pre-
Figure 6 The 1H NMR spectra of polymers 11, 12, and 14
in DMSO-d6.
Journal of Applied Polymer Science DOI 10.1002/app
852
and 295 nm due to the formation of dihydroanthracene structure (Fig. 5).
The 1H NMR spectra of three polymers are presented in Figure 6 and show a distinct signal above
9.0 ppm attributed to the azomethine proton and a
signal at 4.5–5.0 ppm to the 10-dihydroanthracene
protons. Also, the presence of signals attributed to
anthracene and maleimide and citraconimide groups,
respectively, as ends indicate that the products are
oligomers, and molecular weight measurements confirm this conclusion.
The thermal stability of the polymers was evaluated by TGA. Figure 7 presents typical TGA traces
for five polymers (8, 11–14) in air atmosphere. The
polymer initial decomposition temperature was
determined by the intersection of the tangent to the
steepest portion of the TGA curve with its strain-line
before the polymer degradation. All polymers were
stable up to 130–1408C in air atmosphere and the
weight loss (2–4%) observed in this interval is associated with evaporation of humidity and solvent
traces. They afforded a char yield 1–21% at 6008C.
The decomposition of the polymers takes place in
many stages. This fact can be explained based on
structural peculiarities of each polymer. The DTA
curves indicate, for all polymers, moderate endothermal effects in the first studies (until 2508C) and
then a strong exothermal effect. Based on thermal
behavior of cycloadduct models 15 and 16, one may
GRIGORAS ET AL.
TABLE II
Thermogravimetric Characteristics of the Polymers
Polymers
7
8
11
12
13
14
IDT
(8C)
T10%
(8C)
Tdmax
(8C)
Yc
(%)
112
136
116
100
120
99
224
188
240
232
256
228
316
408
552
548
544
548
21
18
2
2
6
1
IDT, onset degradation temperature; T10%, temperature
at weight loss 10%; Tdmax, temperature of the maximum
decomposition; Yc, char yields at 6008C.
assume that the degradation includes a retro Diels–
Alder reaction and the rupture of CH¼
¼N-linkages
(endothermic processes) followed by oxidation of
polymer chain fragments. Some thermogravimetric
characteristics are showed in Table II.
CONCLUSIONS
The present work shows the feasibility of repetitive
Diels–Alder addition as an alternate approach to
obtain polymers with cycloadduct structure using
bisanthracene monomers with bismaleimides and
biscitraconimides. All polymerization products are
oligomers due to limited solubility of the growing
chains and the reversibility of the Diels–Alder reaction. Polymers have a limited solubility in strong polar solvents and a good thermal stability.
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Journal of Applied Polymer Science DOI 10.1002/app
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Journal of Applied Polymer Science DOI 10.1002/app