Cycloaddition approach to the curing of polyimides via precursor containing thiophene-S,S-dioxide

Article abstract of DOI:10.1002/hc.20249

A new method for linear polymerization of maleimides via the Diels-Alder reaction has been developed. This method involves use of a new cross-linking agent, benzene-3,4-dimethylenesuccinimide, which can be generated in situ from its thiophene precursor, benzene-2,5-dihydrothiophene-3,4dicarboximide-S,S- dioxide. This new cross-linking agent is reasonably reactive, readily prepared, and stable at room temperature. A controlled molecular weight oligomer has been synthesized and applied to the polymerization to yield a highly thermal stable polyimide.

Full text of DOI:10.1002/hc.20249

Heteroatom Chemistry
Volume 17, Number 7, 2006
Cycloaddition Approach to the Curing
of Polyimides via Precursor Containing
Thiophene-S,S -dioxide
Andrew Magyarosy,1 Rafat M. Mohareb,2 and Jonathan Z. Ho3
1Department of Chemical Engineering and Chemistry, University of California,
Berkeley, CA 94720, USA
2Department of Chemistry, University of California, Berkeley, CA 94720, USA
3Merck & Co., Inc., P.O. Box 2000, RY80R-164, 126 East Lincoln Avenue, Rahway, NJ 07065, USA
Received 28 July 2005; revised 2 December 2005
an arene diamine, e.g., p-phenylenediamine, and a
ABSTRACT: A new method for linear polymerization
tetracarboxylic dianhydride. Processing generally
of maleimides via the Diels–Alder reaction has been
takes place at the intermediate polyamic acid stage,
developed. This method involves use of a new cross-
and curing involves thermolysis to expel water and
linking agent, benzene-3,4-dimethylenesuccinimide,
produce the imide linkage.
which can be generated in situ from its thio-
Processing of polyimides presents a conundrum.
phene precursor, benzene-2,5-dihydrothiophene-3,4-
That is, their desirable characteristics are a combina-
dicarboximide-S,S-dioxide. This new cross-linking
tion of strength and impermeability, two properties
agent is reasonably reactive, readily prepared, and
which are antithetical to conventional processing
stable at room temperature. A controlled molecular
via melt spinning or solution-casting techniques.
weight oligomer has been synthesized and applied to
One solution to this conundrum is the use of
the polymerization to yield a highly thermal stable poly-
polyamide resins which are terminated with a cross-
ꢀ
C
imide.
2006 Wiley Periodicals, Inc. Heteroatom Chem
linking group, e.g., a maleimide, which can un-
dergo addition polymerization at elevated temper-
atures or cross-linking with an added “enophile,”
e.g., an alkane dithiol, via the Michael addition [2].
Polyimide resins with terminal maleimide groups
are readily available [3]. Because there are a va-
riety of bismaleimide derivatives and commercially
available dithiols, the polymer properties of the re-
sulting materials range from those rubber-like elas-
tomers to those of high-melting thermoplastics [4].
Thermogravimetric analysis (TGA) indicates that
most of those polyimides decompose at 325–360◦C
or below [5]. Moreover, the polymer produced
through such cross-linking exhibits significantly de-
teriorated tensile strength [6,7],presumably because
car-sulfur bonds are weaker than carbon–carbon
bonds. Ottenbrite et al. [8] and others [9] have
17:648–652, 2006; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20249
INTRODUCTION
The special requirements of the aerospace and elec-
tronics industry for thermally stable, lightweight,
and high-strength materials has resulted in an
explosion of efforts toward the so-called high-
performance polymers. Premier among these are
the polyimides [1]. Polyimides are commonly syn-
thesized through a condensation process involving
Correspondence to: Jonathan Z. Ho; e-mail: jonathan ho@
merck.com.
2006 Wiley Periodicals, Inc.
c
ꢀ
648

Cycloaddition Approach to the Curing of Polyimides via Precursor Containing Thiophene-S,S-dioxide 649
introduced another approach to this problem, i.e.,
rangement to give 4 in a polar solvent (DMPU). It is
crucial to heat 3 in the presence of excess amount of
dimethyl acetylenedicarboxylate (5) so that in situ
formed 4 would be immediately reacted with 5 to
form the desired product 6. Otherwise, sulfoxide
3 would be further rearranged to its byproduct,
Me3Si-O-CH2SCH2SiMe3, which will not react with
5. Hydrolysis of 6 followed by dehydration gave rise
to 1 in good yield (71%).
linear polymerization by Diels–Alder reactions be-
tween carbon–carbon double bonds of terminal
maleimide groups and dienes, although no informa-
tion on mechanical properties is given, and simi-
lar considerations may apply here as to the sulfur
cross-linked polymers. Thus, the ultimate solution
to this conundrum requires use of a cross-linking
technique, which maintains the functional integrity
of the rod polymer.
We now report the development of an alterna-
tive Diels–Alder method for consolidation of poly-
imides using 2,5-dihydrothiophene-3,4-dicarboxylic
anhydride (1) as an end-capping agent, then oxida-
tion to its S,S-dioxide which serves as a source of 1,4-
butyldiene in the Diels–Alder reaction to react with
maleimides as a dienophile during chain-extension.
Polymers produced with this method not only have
thermal properties better than that from the conven-
tional polyamide, but also hold promise as candi-
dates for synthesizing block polyimide copolymers.
The feasibility of using thiophene 1 to build a
building block as a precursor of diene in the Diels–
Alders reaction, and hence the value of developing
a synthesis of polyimide, was first explored with a
simple system (Scheme 2), in which all compounds
were characterized by full spectroscopic data.
The synthesis was initiated by reacting 1 with
phenyl amine followed by imidization with acetic
anhydride to give 7. Oxidation of 7 with mcpba
afforded the sulfone 8 as a stable intermediate.
By thermogravimetric analysis (TGA), we observed
facile loss of SO2 from 8 to form diene 9 at about
100◦C. After well-mixing 8 with 10, the mixture was
heated at 210◦C without any solvent. Upon releas-
ing sulfur dioxide, newly formed diene 9 efficiently
reacted with dienophile 10 to give the desired Diels–
Alder adduct 11 (88%), which was confirmed by
RESULTS AND DISCUSSION
2,5-Dihydrothiophene-3,4-dicarboxylic anhydride
(1) could be synthesized in six steps using a lit-
erature route [10] from commercially available
carbomethoxymethyl 2-carbomethoxyethylsulfide
with utilizing liquid HCN in a key step [10]. Because
it is very dangerous to use liquid HCN in large
quantity (e.g., 20 mL), we have felt that we needed
an alternative approach to avoid that. A much more
efficient three-step sequence has been developed
(Scheme 1). Bis(trimethylsilylmethyl)sulfide (2) is
a commercial available regent, and it was oxidized
with m-chloroperoxybenzoic acid (mcpba) in CH2Cl2
at −78◦C to afford 3. Sulfoxide 3 was a very labile
compound, which underwent the Pummerer rear-
1
both H an 13C NMR analyses. Due to the electron-
withdrawing carbonyl groups attached to the 2 and
3 positions of diene 9, vigorous conditions (210◦C
and 48 h) for the Diels–Alder reaction were required.
To prevent retro Diels–Alder reaction for 11 and en-
hance its thermostability, there is a need to trans-
form the newly formed double bond in 11 either into
a part of aromatic ring (as in 12) by oxidation, or
into an alkane (as in 13) by reduction. A number
of oxidation reagents [12] were used to aromatized
11 to 12, but the yields were generally poor (15–
27%) with numerous byproducts. On the other hand,
SCHEME 1 Reagents and conditions: (a) mcpba, CH2Cl2, −78◦C; (b) 5, DMPU, 100◦C; (c) (1) NaOH, (2) SOCl2, PhH,
reflux.
Heteroatom Chemistry DOI 10.1002/hc

650 Magyarosy, Mohareb, and Ho
SCHEME 2 Reagents and conditions: (a) (1) aniline, CHCl3, (2) AcONa, Ac2O; (b) mcpba, CH2Cl2, rt; (c) 10, xylene, 210◦C;
(d) DDQ, dioxane, reflux; (e) diimide, rt.
reduction of 11 with diimide [13] afforded 13 in ex-
cellent yield (91%). Because the purpose of develop-
ing this method was for polymer synthesis, the cat-
alytic hydrogenation was ruled out, although some
of them resulted in good yields (85–98%) for product
13.
The overall sequence is straightforward, which
includes incorporation 1 with an amine to form a
sulfide then a sulfone. Upon heating, a diene was
formed; the Diels–Alder reaction followed by reduc-
tion yields desired imide. At this point, we envi-
sioned that if we applied one polyimide in the place
of 7 whereas another in the place of 10, we could
obtain an even larger polyimide.
using an excess of oxydianiline (ODA) to react
with pyromellitic dianhydride (PMDA), whereas the
oligomer containing terminal maleimide groups
(18) was obtained by using an excess of oxydiani-
line (ODA), pyromellitic dianhydride (PMDA), and
meleic anhydride (Scheme 3). Using a procedure
similar to that used for preparation of 7, compound
14 reacted with 1 to give sulfide 15 then to 16
with the treatment with hydrogen peroxide. This
oligomer 17 was allowed to react with 18 at 210◦C to
yield the polymer 19, in which the double bond was
subsequently reduced, and a new polyamide 20 was
formed.
TGA studies revealed a remarkable increase in
decomposition temperature from 350◦C (for 14) to
ca. 550◦C (for 20). This clearly indicates that a poly-
mer with longer chain has been obtained.
Finally, synthesis of about 2000 molecular
weight oligomer containing terminal amine groups
(14) was achieved by the known methods [11]
SCHEME 3 Reagents and conditions: (a) 14, NMP, 1, Ac2O, AcONa; (b) mcpba, CH2Cl2, rt; (c) 18, xylene, 210◦C; (d) maleic
anhydride, AcONa; (e) diimide, rt.
Heteroatom Chemistry DOI 10.1002/hc

Cycloaddition Approach to the Curing of Polyimides via Precursor Containing Thiophene-S,S-dioxide 651
CONCLUSION
glacial acetic acid (30 mL) and concentrated HCl
(30 mL) was stirred at its reflux temperature for 5 h.
Use of the previously unreported 2,3-
dimethylenesuccinimide moiety as a Diels–Alder
cross-link agent provides a new versatile method for
consolidation of polyimides. Use of this moiety on
one type of oligoimide and maleimide end groups on
another oligoimide provides ready entry into a num-
ber of polyamide block copolymers. Further studies
on mechanical properties and block copolymer for-
mation are underway and will be reported in due
course.
The mixture was poured into ice water, and 2,2,4,4-
tetrahydrothiophene-3,4-dicarboxylic acid was col-
lected by vacuum filtration. After drying, freshly dis-
tilled thionyl chloride (SOCl2, 5 mL) and benzene
(40 mL) were added. The resulting mixture was
heated at its reflux temperature for 30 min then sol-
vent was removed by distillation. The product was
recrystallized from benzene to give 1 (3.5 g, 71%).
1
mp 165–166◦C; H NMR δ 4.05 (s, 4H); 13C NMR
δ 37.3, 139.4, 165.2. Anal. Calcd for C6H4O3S: C,
46.15; H, 2.58; S, 20.53. Found: C, 46.27; H, 2.73; S,
20.23.
EXPERIMENTAL
General
Compound 7. A solution of aniline (930 mg) in
CHCl3 (4 mL) was added to a solution of 1 (1.56 g)
and was stirred at 45◦C for 0.5 h, then solvent was
removed to give a solid. At this point, acetic anhy-
dride (6 mL) and NaOAc (1 g) was added, and the
mixture was heated at 100◦C for 0.5 h. After cooling
to rt, the mixture was poured to ice-water. The crys-
talline product 7 (1.86 g, 81%) was formed, collected
and dried under a high vacuum for 18 h. 1H NMR
δ 4.08 (s, 4H), 7.24 (t, 1H, J = 7.2), 7.41 (m, 2H), 7.66
(d, 2H, J = 7.2); 13C NMR δ 38.2, 121.6, 124.4, 129.0,
132.8, 138.6, 169.8. Anal. Calcd for C12H9NO2S: C,
62.32; H, 2.92; N, 6.06, S, 13.86. Found: C, 62.44; H,
2.87; N, 6.13, S, 14.01.
All reactions were conducted under a nitrogen atmo-
sphere. All NMR data were obtained at 400 MHz for
proton and 100 MHz for carbon spectra, in CDCl3
unless otherwise noted, using TMS as an internal
standard. Melting points are uncorrected. Col-
umn chromatography was performed using 230–400
mesh silica gel. Thermogravimetric analysis (TGA)
was performed on a Perkin–Elmer TGA series 7.
Oxydianiline (ODA) and pyromellitic dianhydride
(PMDA) were either purchased as Gold Label zone-
refined quality or were sublimed prior to use.
Compound 3. A solution of 3-chloroper-
oxybenzoic acid (mcpba, 35 g, 55%, 123 mmol in
CH2Cl2 (250 mL) was added to a solution of sulfide
2 (20.6 g, 100 mmol) in CH2Cl2 (250 mL) at −78◦C.
After stirring at −78◦C for 1 h, the mixture was
warmed to 0◦C, washed with ice water, NaHCO3,
dried over MgSO4, then concentrated under vacuum
(at 10◦C) to give 3 as an oil, which was used in next
without purification. 1H NMR 0.22 (s, 18 H), 2.15
(d, 2H, J = 13.6), 2.43 (d, 2H, J = 13.6).
Compound 8. A solution of mcpba (2 g, 55%,
6 mmol) and compound 7 (1.16 g, 5 mmol) in CH2Cl2
was stirred at rt for 3 h. The mixture was washed
NaHCO3 then dried over MgSO4. Solvent was re-
moved under vacuum, and residue was recrystal-
lized from water to give 8 (0.88 g, 67%). 1H NMR
(DMSO-d6) δ 4.51 (s, 4H), 7.34 (d, 1H, J = 7.2), 7.42
(t, 2H, J = 7.2), 7.51 (t, 2H, J = 7.2). 13C NMR δ 58.1,
121.6, 124.4, 129.0, 132.8, 138.6, 169.8. Anal. Calcd
for C12H9NO4S: C, 54.75; H, 3.45; N, 5.32, S, 12.18.
Found: C, 55.01; H, 3.58; N, 5.46, S, 12.24.
Dimethyl 2,5-Dihydrothiophene-3,4-dicarboxylate
(6). A solution of freshly prepared sulfoxide 3
(100 mmol) and dimethyl acetylenedicarboxylate (5,
7.1 g, 50 mmol) in 1,3-dimethyl-3,4,5,6-tetrahydro-
2(1H)-pyrimidinone (DMPU, 10 mL) was added to a
heated DMPU (100◦C) in 3 min. The resulting mix-
ture was stirred at 100◦C for 10 min then poured
into ice water. CH2Cl2 (100 mL × 5) was washed to
extract, and combined organic layer was dried
(MgSO4). After the solvent was removed under the
vacuum, the product was purified by distillation to
Compound 11. A mixture of sulfone 8 (190 mg,
1 mmol) and 10 (123 mg, 1 mmol) in xylene (3 mL)
was heated at 210◦C in a resealable reaction vial for
48 h to afford 11 (151 mg, 59%) after crystallization
1
from CHCl3. H NMR δ 2.82 (m, 4H), 3.65 (m, 2H),
7.32 (d, 4H, J = 8.4), 7.40 (t, 2H, J = 8.4), 7.49 (t, 4H,
J = 7.8); 13C NMR δ 26.8, 34.2, 121.6, 124.4, 129.0
135.4, 174.4. Anal. Calcd for C22H16N2O4: C, 70.96;
H, 4.33; N, 7.52. Found: C, 70.88; H, 4.56; N, 7.38.
1
give 6 (5.6 g, 59%). bp 93–95◦C (0.3 mm); H NMR
3.81 (s, 6H), 4.05 (s, 4H).
Compound 12. The compound 11 (26 mg) was
oxidized to 12 (6.8 mg, 27%) based on a literature
condition [12]. For 12: 1H NMR δ 7.05 (t, 2H,
2,2,4,4-Tetrahydrothiophene-3,4-dicarboxylic An-
hydride (1). A solution of 6 (5.6 g, 30 mmol) in
Heteroatom Chemistry DOI 10.1002/hc

652 Magyarosy, Mohareb, and Ho
J = 7.2), 7.35 (m, 4H), 7.60 (d, 4H, J = 7.2), 9.05 (s,
2H); 13C NMR δ 121.6, 124.4, 125.3, 129.0, 132.8,
135.6, 167.1. Anal. Calcd for C22H12N2O4: C, 71.74;
H, 3.28; N, 7.61. Found: C, 71.65; H, 3.50; N, 7.78.
at 60◦C for 1 h. AcONa (1 g) was added, and the mix-
ture was heated to 90◦C for 4 h. The mixture was
poured into water, and solid was collected by vac-
uum filtration to give 18 (1.11 g, 95%) after drying.
Compound 13. The compound 11 (26 mg) was
oxidized to 13 (23.7 mg, 91%) based on a literature
condition [13]. For 13: 1H NMR ꢀ 2.21 (m, 4H),
2.65 (m, 4H), 7.05 (t, 2H, J = 7.2), 7.30 (m, 4H), 7.60
(d, 4H, J = 7.2); 13C NMR ꢀ 26.8, 34.2, 121.6, 124.4,
129.0 135.4, 174.4. Anal. Calcd for C22H18N2O4: C,
70.58; H, 4.85; N, 7.48. Found: C, 70.66; H, 4.77; N,
7.42.
Synthesis of Cross-Linked Copolymer 20
A suspension of disulfone 16 (0.1 g) and oligomer 18
(1.0 g) in xylene was sealed and heated at 210◦C for
2 days to form 19, which was further treated with
diimide (5 mL) to reduce the double bond resulting
from the Diels–Alder reaction. The final product was
purified by Soxhlet extraction (acetone, 100 cycles)
to remove small molecules. The residue was dried
under a high vacuum for 24 h to give the desired
polymer 20 (0.9 g, 89%).
Compound 14. A suspension of oxydianiline
(ODA, 714 mg, 3.37 mmol), PMDA (650 mg, 2 mmol)
in N-methyl-2-pyrrolidone (NMP) was stirred at 60◦C
for 1 h. Acetic anhydride (2 mL) and AcONa (1 g) was
added, and the mixture was heated to 90◦C for 4 h.
The mixture was poured into water, and solid was
collected by vacuum filtration to give 14 (1.1 g, 96%)
after drying.
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Compound
15. A
solution
of
2,2,4,4-
tetrahydrothiophene-3,4-dicarboxylic
anhydride
(1, 0.5 g, 3.5 mmol) in CHCl3 (8 mL) was added to a
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1
yield 15 (0.9 g, 86%). mp: >300◦C; H NMR (solid
state) δ 4.09 (s), 7.10 (m), 7.46 (m), 8.88 (s).
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Compound 18. A suspension of 14 (1 g) and
maleic anhydride (0.5 g) in NMP (10 mL) was stirred
Heteroatom Chemistry DOI 10.1002/hc