Film polymerization - A new route to the synthesis of insoluble polyimides containing functional nickel(II) Schiff base units in the main chain

Article abstract of DOI:10.1002/ejic.200300959

The synthesis and characterization of a new, amino-derivatized functional bis(salicylaldiminato)Ni^II complex is reported. It is a useful and reactive precursor in the synthesis of polyimide thermosets. Thus, the first synthesis of a polyimide c

Full text of DOI:10.1002/ejic.200300959

FULL PAPER
Film Polymerization ؊ A New Route to the Synthesis of Insoluble Polyimides
Containing Functional Nickel(II) Schiff Base Units in the Main Chain
Santo Di Bella,*[a] Giuseppe Consiglio,[b] Nicoletta Leonardi,[a] Salvatore Failla,*[b]
[b]
[a]
`
Paolo Finocchiaro, and Ignazio Fragala
Keywords: Coordination polymer / Nickel / Film polymerization / Nonlinear optics
The synthesis and characterization of a new, amino-derivat- characteristics analogous to those of the insoluble polymer
ized functional bis(salicylaldiminato)NiII complex is reported.
It is a useful and reactive precursor in the synthesis of poly-
imide thermosets. Thus, the first synthesis of a polyimide con-
taining a nonlinear optical NiII Schiff base complex in the
main chain is achieved by thermally controlled film poly-
merization. Polymerization was monitored on silicon sub-
obtained by polymerization in solution, as evidenced by FT-
IR spectroscopy and TGA analysis. The present approach
might represent a general strategy for achieving thin films of
insoluble, functional polymers.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
strates by FT-IR spectroscopy. The polymer film possesses Germany, 2004)
Introduction
We report here on the synthesis and characterization of
novel polyimide thermosets by thermally controlled, film
polymerization, which might represent a general method for
accomplishing films of insoluble, functional polymers.
Moreover this contribution represents the first example of
a co-polyimide containing a nonlinear optical NiII metal
complex unit[6] in the main chain.[7,8] To this end, the am-
ino-derivatized Schiff base bis(salicylaldiminato)NiII com-
plex (1) and the bis(maleimide) (2) were chosen as starting
monomers (Scheme 1).
The development of easily processable, functional molec-
ular materials is of central interest in the emerging field of
molecular electronics.[1] In this regard, functional polymers
are amongst the most promising materials, by virtue of their
facile processability into composites,[2] as well as in thin
films.[3] The exploration of new polymers with functional
transition metal complexes in the main chain is intriguing
and the unique electronic properties associated with the me-
tal center certainly deserve to be investigated, especially the
distinct or combined, optical (linear and nonlinear), electri-
cal, and magnetic properties.[4]
Results and Discussion
Synthesis of the Derivatized Schiff Base Complex 1
The very high thermal stability and high glass transition
temperature of polyimides makes them well suited for plas-
tic electronics,[2,3,5] although in some cases their low solu-
bility prevents the application as thin-film materials. An
alternative synthetic strategy to polyimides is the in situ film
polymerization, with the potential to obtaining better ther-
mal and mechanical characteristics of thin films. This strat-
egy involves the choice of suitable and reactive monomers,
in order to achieve a thermally controlled polymerization.
The appropriate derivatization of starting monomers is thus
of relevance for the design and synthesis of new functional
thermoset materials.
The synthesis of bis(salicylaldiminato), amino-derivat-
ized compounds represents a difficult task. In fact, since
these Schiff bases necessarily involve salicylaldehydes as
starting compounds, the latter are chemically incompatible
with the concurrent existence of free amines. On the other
hand, nitrosalicylaldehydes, which could be used as starting
materials, give rise to very low soluble Schiff bases. Actu-
ally, both the free ligand and the NiII complex of the related
5-nitro Schiff base, synthesized by standard pro-
cedures,[6b,6c] have a very low solubility in most solvents.
Consequently, all attempts to obtain the diamine by re-
duction failed. We have thus developed an alternative route
to the synthesis of amino-derivatized, Schiff base NiII com-
plexes. The adopted synthetic approach is sketched in
Scheme 2. It involves protection of the aldehyde function
of the starting 5-nitrosalicylaldehyde by acetic anhydride to
afford 3; H2-catalyzed reduction of the 5-nitro group to the
free amine 4; protection of the amine group with Boc2O,
followed by hydrolysis of the phenol and aldehyde func-
[a]
`
Dipartimento di Scienze Chimiche, Universita di Catania and
INSTM UdR di Catania,
95125 Catania, Italy
Fax: (internat.) ϩ39-095-580138
E-mail: sdibella@unict.it
[b]
Dipartimento di Metodologie Fisiche
e
Chimiche per
`
l’Ingegneria (sede Enna), Universita di Catania,
95125 Catania, Italy
E-mail: sfailla@dmfci.unict.it
Eur. J. Inorg. Chem. 2004, 2701Ϫ2705
DOI: 10.1002/ejic.200300959
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Di Bella, G. Consiglio, N. Leonardi, S. Failla, P. Finocchiaro, I. Fragala
FULL PAPER
Scheme 1
tionalities with NaOH to get the salicylaldehyde derivative (βµ ϭ Ϫ22.8 ϫ 10Ϫ30 cm5·esuϪ1), which can be compared
5; template reaction with the appropriate 1,2-diamine and with the calculated (βµ ϭ Ϫ17.3 ϫ 10Ϫ30 cm5·esuϪ1 [6c] and
NiII ion affords the 5-amino protected, Schiff base complex experimental (βµ ϭ Ϫ20.5 ϫ 10Ϫ30 cm5·esuϪ1 [6c]
values for
)
)
6; deprotection with trifluoroacetic acid of the amine the unsubstituted Ni(salophen) derivative. The derivatized
groups of 6 leads to the 5-amino derivatized Schiff base NiII functional complex 1 is thus a potential candidate for non-
complex 1. All these intermediate precursors 3Ϫ6, as well linear optical applications.
as the amino-derivatized, Schiff base NiII complex 1, were
obtained in high yields (Ͼ 82%), except compound 5 (65%),
and fully characterized by mass spectrometry and 1H NMR
spectroscopy, and gave satisfactory microanalyses.
Polymerization in Solution
Polymerization was first carried out in solution. This al-
lows for verification of the chemistry involved, by means of
the synthesis and analysis of the prepolymer. Partial poly-
merization was performed by heating a solution of 1 and
2 (0.034  in each component) in 1-methyl-2-pyrrolidone
(NMP) for 1 h, at 90 °C. The prepolymer, that precipitated
1
from chloroform, was analyzed by H NMR and FT-IR
spectroscopy. The 500 MHz 1H NMR of the crude prepoly-
mer in [D6]DMSO solution shows a partial disappearing of
the sharp resonance at δ ϭ 7.15 ppm (olefinic protons of
2)[10] and the broad resonance (D2O exchangeable) at δ ϭ
4.57 ppm, assigned to the primary amine protons of 1.
Moreover, the spectrum reveals the presence of new signals,
with respect to the starting monomers. In particular, the
peaks at δ ϭ 2.80, 3.85, 4.95, and 5.95 ppm, are evidence
for the formation of the succinimide ring[11] and the second-
ary (ϪNHϪ) proton of the nitrogen bridge,[10] respectively
(see Exp. Sect.). Along with the typical absorption bands,[12]
the FT-IR spectrum of 2 also shows a strong absorption at
688 cmϪ1, from the maleimide ring deformation, and the
1150 cmϪ1 absorption due to asymmetric CϪNϪC stretch-
ing. They represent the reference absorptions used to moni-
tor the reaction between 1 and 2. The comparison of the
FT-IR spectrum of a 1:1 mixture of 1 and 2 with that of
the prepolymer indicates an increase of the transmittance
Scheme 2. Reagents and conditions: (i) (CH3CO)2O, H2SO4 cat.,
room temp.; (ii) H2 (45 psi), 10% Pd/C, EtOH; (iii) Boc2O, CH2Cl2,
room temp.; (iv) MeOH/H2O, NaOH, reflux; (v) 4,5-dichloro-1,2-
phenylenediamine, NiOAc2·4H2O, MeOH; (vi) CF3COOH, DCM,
PhOMe, room temp.; then NaOH, H2O
INDO/SCI-SOS calculations[9] on 1, upon geometry opti- of bands at 688 and 1150 cmϪ1, and the formation of a
mization, indicate a molecular quadratic hyperpolarizability new band at 1175 cmϪ1 due to the asymmetric CϪNϪC
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A New Route to the Synthesis of Insoluble Polyimides
FULL PAPER
stretching of the succinimide ring.[13] In summary, both the
1H NMR and FT-IR spectroscopic experiments on the pre-
polymer enable us to establish that the reaction between
1 and 2 involves a Michael addition reaction, as sketched
in Scheme 1.
Polymerization was performed by curing a solution of the
prepolymer in NMP for 2 h, at 140 °C. The resulting brown
material has a low solubility in most solvents, typical of the
polyimides, thus preventing, in principle, its application as
a functional material. Longer reaction times and higher re-
action temperatures did not result in appreciable changes in
polymer properties (see infra). Thermogravimetric analysis
of the polymer indicates a high thermal stability with a de-
composition onset of 380 °C. In comparison with mono-
mers (2 melts at 157 °C, while 1 starts to decompose, with-
out melting, at 330 °C) the higher thermal stability of the
polymer is in agreement with its rigid structure. Accord-
ingly, differential scanning calorimetry analysis indicates
the absence of a transition glass temperature in the 30Ϫ400
°C range.
Figure 1. FT-IR spectra of a polyimide thermoset cast film on a
silicon substrate at different polymerization stages, using as a refer-
ence sample the same substrate before casting. The arrows at 1176,
and 1150, 688 cmϪ1 indicate the emerging succinimide function and
the disappearing maleimide function, respectively
Film Polymerization
Casting of filtered chloroform solutions of 1 and 2 mono-
mers (0.034  in each component) on silicon substrates
[Czochralsky-grown Si(100), optically transparent in the
600Ϫ4000 cmϪ1 IR region of interest, transmittance Ն
0.90][14] followed by heating, allows the degree of film poly-
merization to be monitored by FT-IR analysis (Figure 1).
When a film of cast monomers is heated at 140 °C for 1 h,
the peaks at 688 and 1150 cmϪ1 associated with the maleim-
ide ring decrease, while the peak at 1176 cmϪ1 associated
with the succinimide function appears. After the polymer
film is further heated at 140 °C for 2 h, the peak at 688
cmϪ1 almost completely disappears, and a band broadening
in the range 1100Ϫ1600 cmϪ1 is observed, thus indicating
an almost complete polymerization. This is also evident
from the comparison of this spectrum with the almost
identical one of the polymer obtained by polymerization in
solution. The further heating of the film does not produce
further changes in the FT-IR spectra. The polymer film
possesses almost indistinguishable characteristics with re-
spect those of the polymer obtained by polymerization in
solution, as confirmed by the TGA analysis.
The polymer films were also characterized by optical
spectroscopy. The optical absorption spectrum of the cast
film, before heating, shows a band at 380 nm, and two
shoulders at about 450 and about 560 nm (Figure 2). After
polymerization is complete, the optical spectrum remains
almost unchanged, except for a red-shift (ഠ 10 nm) of the
band at 380 nm along with a better defined structure. This
is consistent with a more homogeneous film upon polymer-
ization and good optical quality. These spectral features are
Figure 2. Optical absorption spectra (bottom) of a polyimide ther-
moset cast film on a glass substrate before heating (- - -) and after
3 h of heating at 140 °C (Ϫ). The absorption spectrum of complex
1 (top) in chloroform solution is reported for comparison
Conclusion
In this contribution we have achieved a convenient syn-
similar to those of the optical spectrum of 1 in chloroform thesis for novel functional, amino-derivatized NiII metal
solution, thus indicating that the electronic structure of complexes. These complexes can be easily incorporated in
monomer 1 remains essentially unchanged upon polymeriz- the main chain of polyimides and thermally controlled film
ation.
polymerization can be obtained by heating the monomers,
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S. Di Bella, G. Consiglio, N. Leonardi, S. Failla, P. Finocchiaro, I. Fragala
FULL PAPER
with water, and dried under vacuum. The solid was purified by
column chromatography (SiO2, CHCl3, Rf ϭ 0.22) and crystallized
from cyclohexane, to yield 1.23 g (65%) of pale yellow needles (m.p.
122Ϫ123 °C). 1H NMR (500 MHz, CDCl3): δ ϭ 10.75 (s, 1 H,
OH), 9.84 (s, 1 H, CHO), 7.82 (d, Jmeta ϭ 2.8 Hz, 1 H, ArH), 7.24
(dd, Jortho ϭ 9.0, Jmeta ϭ 2.8 Hz, 1 H, ArH), 6.90 (d, Jortho ϭ 9.0 Hz,
1 H, ArH), 6.39 (br. s, 1 H, NH), 1.49 (s, 9 H, CH3C) ppm. ESI
MS: m/z (%) ϭ 238 (34) [MH]ϩ. C12H15NO4 (237.25): calcd. C
60.75, H 6.37, N 5.90; found C 60.90, H 6.45; N 5.97.
as monitored by FT-IR spectroscopy. Such an approach
might represent a general method for accomplishing films
of insoluble, otherwise rather intractable, functional poly-
mers.
Further work is currently in progress with the aim of
identifying more flexible monomers, in order to obtain co-
polymers with definite transition glass temperatures.
{N,NЈ-Bis[5-(tert-butoxycarbonylamino)-2-hydroxybenzylidene]-4,5-
dichloro-1,2-phenylene-diaminato}NiII (6): A solution of compound
5 (0.475 g, 2.00 mmol) in 4 mL of methanol was slowly added to a
warm solution of 4,5-dichloro-1,2-phenylenediamine (0.177 g,
1.00 mmol) and nickel() acetate tetrahydrate (0.250 g, 1.00 mmol)
in 10 mL of methanol. The brown suspension obtained was re-
fluxed for 1 h, and then cooled to room temperature. The solid was
filtered off, washed with methanol, and dried under vacuum, to
yield 0.64 g (95%) of product as red-brown needles. 1H NMR
(500 MHz, [D6]DMSO): δ ϭ 9.11 (br. s, 2 H, NH), 8.87 (s, 2 H,
CHϭN), 8.57 (s, 2 H, ArH), 7.86 (br. s, 2 H, ArH), 7.22 (dd,
Experimental Section
Materials: Nickel() acetate tetrahydrate, 2-hydroxy-5-nitrobenzal-
dehyde, and all the other reagents (Aldrich) were used without puri-
fication. 4,5-Dichloro-1,2-phenylenediamine (Aldrich) was purified
by sublimation in vacuo. 1,1Ј-(Methylenedi-4,1-phenylene)bis-
(maleimide) (2) (Aldrich), was purified by recrystallization from
toluene. Dichloromethane (DMC) was distilled from CaH2.
Physical Measurements
Elemental analyses were performed with a Carlo Erba 1106 el-
emental analyzer. 1H NMR spectra were recorded with a Varian
Unity Inova 500 MHz spectrometer, using TMS as internal stand-
ard. ESI MS were recorded with an Agilent 1100 Series ESI/MSD
spectrometer. Experimental conditions were as follows: Capillary
voltage, 3.5 kV; fragmentor, 100 V; source temperature, 350 °C; dry-
ing gas, N2 (10 L/min), carrier solvent, methanol (0.4 mL/min).
Samples were dissolved in methanol. For complex 1 the methanol
solution was acidified with trifluoroacetic acid. Infrared spectra
were recorded with a JASCO FT-IR spectrometer. Thermal meas-
urements were performed with a Mettler Toledo TGA/SDTA85e
system. Thermal analyses were made both in air and under pre-
purified nitrogen (30 mL/min) using a 10 °C/min heating rate. DSC
measurements were obtained from a Mettler DSC 20 calorimeter,
under nitrogen atmosphere using a 20 °C/min heating rate. UV-vis
spectra were recorded with a Beckman DU 650 spectrophotometer.
Jortho ϭ 9.2, Jmeta ϭ 2.8 Hz, 2 H, ArH), 6.80 (d, Jortho ϭ 9.2 Hz, 2 H,
ArH), 1.48 (s, 18 H, CH3C) ppm. ESI MS: m/z (%) ϭ 671 (12), 673
(13) [MH]ϩ. C30H30Cl2N4NiO6 (672.18): calcd. C 53.61, H 4.50, N
8.33; found C 53.93, H 4.66, N 8.27.
[N,NЈ-Bis(5-amino-2-hydroxybenzylidene)-4,5-dichloro-1,2-phen-
ylenediaminato]NiII (1): Trifluoroacetic acid (0.81 mL) was added
to a suspension of compound 6 (0.540 g, 0.803 mmol) and anisole
(0.4 mL) in DCM. The solution obtained was stirred under nitro-
gen for 24 h, at room temperature. The volatile material was re-
moved under vacuum, and the red solid was collected and washed
with diethyl ether. The solid was dissolved in 100 mL of water and
filtered. Solid NaOH was added to the aqueous solution (pH ഠ
11), and the dark-brown powder that formed was collected by fil-
tration, washed with water, and dried under high vacuum over
P2O5, to yield 0.36 g (95%) of 1. 1H NMR (500 MHz, [D6]DMSO):
δ ϭ 8.64 (s, 2 H, CHϭN), 8.47 (s, 2 H, ArH), 6.87 (d, Jortho
ϭ
Syntheses
8.5 Hz, 2 H, ArH), 6.72 (d, Jortho ϭ 8.5 Hz, 2 H, ArH), 6.65 (s, 2
H, ArH), 4.57 (br. s, 4 H, NH2) ppm. ESI MS: m/z (%) ϭ 471 (95),
473 (100) [MH]ϩ. UV/vis (CHCl3): λmax. ϭ 392, 440 (sh), 544 nm.
TGA analysis: Tdec. ϭ 330 °C (onset). C20H14Cl2N4NiO2 (471.95):
calcd. C 50.90, H 2.99, N 11.87; found C 51.03, H 3.02, N 11.79.
Acetyloxy(2-acetyloxy-5-nitrophenyl)methyl Acetate (3): A solution
of 2-hydroxy-5-nitrobenzaldehyde (1.67 g, 10.0 mmol), acetic anhy-
dride (50 mL), and H2SO4 (3 drops) was stirred under nitrogen for
1 h at room temperature. The solution obtained was poured into
ice cold water (500 mL), and stirred for 30 minutes in order to
hydrolyze the anhydride. The white solid was collected by filtration,
air dried, and crystallized from an EtOAc/cyclohexane mixture to
yield 2.55 g (82%) of a white crystalline solid (m.p. 113Ϫ115 °C).
ESI MS: m/z (%) ϭ 312 (23) [MH]ϩ. C13H13NO8 (311.24): calcd.
C 50.17, H 4.21, N 4.50; found C 50.23, H 4.19, N 4.53.
Prepolymer: In a two-necked flask (100 mL), equipped with a reflux
condenser, magnetic bar and a nitrogen inlet, a solution of 1
(0.25 g, 0.53 mmol) and 2 (0.19 g, 0.53 mmol) in NMP (16 mL) was
stirred under nitrogen at 90 °C for 1 h. The mixture was poured
into chloroform to give a brown solid, which was collected by fil-
tration, washed with chloroform, several times, and then dried un-
der vacuum (0.40 g). FT-IR (KBr pellet): ν˜ ϭ 3330 (br. m), 1710
(vs), 1598 (m), 1529 (s), 1460 (s), 1358 (m), 1175 (m), 1136 (m),
825 (s), 688 cmϪ1 (w). 1H NMR (500 MHz, [D6]DMSO), in the
spectrum are present new peaks, with respect to the starting mono-
mers: δ ϭ 5.95 (s, D2O exchangeable, ArϪNH succinimide), 4.95,
3.85 and 2.80 (m, succinimide H) ppm.
Acetyloxy(2-acetyloxy-5-aminophenyl)methyl Acetate (4): A solu-
tion of compound 3 (2.49 g, 8.00 mmol) in EtOH (40 mL) and
THF (10 mL), was hydrogenated (H2, 45 psi) using 10% Pd/C as
catalyst (0.5 g). After 4 h, the filtrate was quickly concentrated to
quantitatively give compound 4 (2.25 g) as light brown oil, which
was used without any further purification.
tert-Butyl (3-Formyl-4-hydroxyphenyl)carbamate (5): A solution of
compound 4 (2.25 g, 8.00 mmol), and di(tert-butyl) dicarbonate
(1.75 g, 8.00 mmol) in anhydrous DCM (15 mL), was stirred under
nitrogen for 8 h. The solvent was evaporated under vacuum, and
the oil obtained was dissolved in MeOH (10 mL). Sodium hydrox-
ide (1.6 g, 40 mmol) and water (5 mL) were then added, and the
resulting solution refluxed for 5 h. After cooling, the solution was
neutralized, and the brown solid was collected by filtration, washed
Polymer: A solution of 1 (0.25 g, 0.53 mmol) and 2 (0.19 g,
0.53 nmol) in NMP (16 mL) was placed into a two-necked flask
(100 mL) equipped with a reflux condenser and a nitrogen inlet.
The solution was stirred under nitrogen at 90 °C for 1 h. The tem-
perature was then raised to 140 °C and stirring continued for 2 h.
The dark mixture was cooled to room temperature, and then
poured into chloroform to give a brown solid. The solid was col-
lected by filtration, washed several times with chloroform, and then
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A New Route to the Synthesis of Insoluble Polyimides
FULL PAPER
1995, 601; T. J. Marks, M. A. Ratner, Angew. Chem. Int. Ed.
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For a recent review, see for example: H. Saadeh, D. Yu, L. M.
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dried under vacuum (0.42 g). FT-IR (KBr pellet): ν˜ ϭ 3335 (br. m),
1710 (vs), 1598 (m), 1529 (s), 1460 (s), 1358 (m), 1179 (m), 1137
(m), 825 cmϪ1 (s). TGA analysis: Tdec. ϭ 380 °C (onset). Anal.
calcd. for a polymer structure as in Scheme 1, C41H28Cl2N6NiO6:
calcd. C 59.31, H 3.40, N 10.12; found C 59.95, H 3.55; N 10.80.
[4]
[5]
[6] [6a]
[6b]
`
S. Di Bella, I. Fragala, New J. Chem. 2002, 26, 285Ϫ290.
Film Polymerization: A filtered (0.2-µm syringe filter) chloroform
solution of the monomers (0.034 , 1:1 molar ratio) was cast on 2
ϫ 2 cm silicon substrates. Polymerisation was achieved by heating
the film under nitrogen by curing steps of 1 h at 140 °C. Polymeris-
ation was monitored by FT-IR measurements after each step. FT-
IR (silicon substrate, after 3 h heating): ν˜ ഠ 3350 (br. w), 1710 (vs),
1593 (m), 1514 (s), 1452 (s), 1340 (m), 1180 (m), 1142 (m), 826 (s),
689 cmϪ1 (vw). TGA analysis: Tdec. ϭ 370 °C (onset).
`
S. Di Bella, I. Fragala, I. Ledoux, J. Zyss, Chem. Eur. J.
[6c]
`
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[8]
Acknowledgments
We thank the MIUR (PRIN 2001 and FIRB 2002 research pro-
grams) for financial support and Prof. E. Scamporrino for helpful
discussions.
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´
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`
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Received December 22, 2003
[3]
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Early View Article
(Eds.: G. A. Lindsay, K. D. Singer), ACS Symposium Series
Published Online May 5, 2004
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