Synthesis and characterization of fully aliphatic polyimides from an aliphatic dianhydride with piperazine spacer for enhanced solubility, transparency, and low dielectric constant

Article abstract of DOI:10.1002/pola.27242

A new series of fully aliphatic polyimide (API) based on a novel aliphatic dianhydride monomer-2,2-(1,4-piperazinediyl)-disuccinic anhydride (PDA), in which two units of succinic anhydride have been connected by an aliphatic heterocyclic piperazine spacer that possesses aminomethylene (-NCH₂) moiety in the aliphatic/alicylic backbone capable of inducing charge transfer (CT) interactions in the polyimide network, was successfully synthesized. The APIs were soluble in common polar organic solvents. The polyimide films of PDA with alicyclic diamines were almost colorless. T₁₀ (temperature of 10% weight loss) of APIs were ranged from 299-418 C and T_g of API3-API6 were in the temperature range of 170 to 237 C. The light-colored polyimide films of API3-API6 possessed good mechanical properties with tensile strength of 54-72 Mpa, tensile modulus of 1.6-2.3 Gpa and elongation at break of 4-9%. The polyimide films of API3-API6 were highly flexible and free-standing which is quite rare in fully APIs. The dielectric constant of one of the synthesized API (API4) was as low as 2.14.

Full text of DOI:10.1002/pola.27242

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Synthesis and Characterization of Fully Aliphatic Polyimides from an
Aliphatic Dianhydride with Piperazine Spacer for Enhanced Solubility,
Transparency, and Low Dielectric Constant
Pradip Kumar Tapaswi, Myeon-Cheon Choi, Young Sik Jung, Hun Jeong Cho, Deok Jin Seo,
Chang-Sik Ha
Department of Polymer Science and Engineering, Pusan National University, Busan 609–735, Republic of Korea
Correspondence to: C.-S. Ha (E-mail: csha@pnu.edu)
Received 27 March 2014; accepted 3 May 2014; published online 00 Month 2014
DOI: 10.1002/pola.27242
ABSTRACT: A new series of fully aliphatic polyimide (API) based
on a novel aliphatic dianhydride monomer-2,20-(1,4-piperazine-
diyl)-disuccinic anhydride (PDA), in which two units of succinic
anhydride have been connected by an aliphatic heterocyclic
piperazine spacer that possesses aminomethylene (-NCH2) moi-
ety in the aliphatic/alicylic backbone capable of inducing
charge transfer (CT) interactions in the polyimide network, was
successfully synthesized. The APIs were soluble in common
polar organic solvents. The polyimide films of PDA with alicy-
clic diamines were almost colorless. T10 (temperature of 10%
weight loss) of APIs were ranged from 299–418 ꢀC and Tg of
API3-API6 were in the temperature range of 170 to 237 ꢀC. The
light-colored polyimide films of API3-API6 possessed good
mechanical properties with tensile strength of 54–72 Mpa, ten-
sile modulus of 1.6–2.3 Gpa and elongation at break of 4–9%.
The polyimide films of API3-API6 were highly flexible and free-
standing which is quite rare in fully APIs. The dielectric con-
stant of one of the synthesized API (API4) was as low as 2.14.
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2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2014, 00, 000–000
KEYWORDS: aliphatic polyimides; dielectric properties; pipera-
zine spacer; free-standing polyimide film; good mechanical
properties; high-temperature materials; low dielectric constant;
polyimides; transparency
low molecular density, low polarity, and low probability of
undergoing inter- or intramolecular charge transfer.14
Accordingly, great effort has been made to synthesize new
aliphatic/alicyclic dianhydride or diamine monomers. Schenk
et al. reported the synthesis of cis-cyclobutane-l,2,3,4-tetra-
carboxylic dianhydride by irradiating a solution of maleic
anhydride in dioxane with a high-pressure mercury lamp.15
Suzuki et al. synthesized a substituted 1,2,3,4-cyclobutanete-
tracarboxylic dianhydride by photodimerization of maleic
anhydrides.16 The 1,2,4-tricarboxyl-3-methylcarboxyl cyclo-
pentane dianhydride was obtained by oxidizing a Diels–Alder
adduct of cyclopentadiene and maleic anhydride with nitric
acid, followed by neutralization and subsequent dehydra-
tion.17–19 The procedure for the synthesis of 1,3-diaminoada-
mantane was first reported by Smith et al. and was later
modified by Chern et al.20,21 Polyimides synthesized from
these monomers showed excellent transparency and good
solubility and moderate to good thermal properties. How-
ever, in most of the cases, the mechanical properties of fully
alicyclic/API films were not good as the films were too much
brittle.22 Recently, Bin et al. reported that trimethyleneamino
INTRODUCTION Polyimides, a class of high-performance mac-
romolecules characterized by outstanding thermal stability,
mechanical, electrical, and solvent resistance properties orig-
inating from their rigid backbones, often find extensive appli-
cations in photoresists, passivation and dielectric films, soft
print circuit boards, and alignment films within displays.1,2
The main obstacles that seriously limit the applications of
aromatic polyimides in optoelectric materials and high-speed
multilayer printed wiring boards are their insolubility in
common organic solvents in the fully imidized form, the light
or dark-yellow color of their films caused by intra- and inter-
molecular charge transfer (CT) interactions, and their high
dielectric constants.3–5.
In recent years, polyimides with high optical transparencies
and low dielectric constants have been in demand for optoe-
lectronic and microelectronic applications. Alicyclic and ali-
phatic polyimides (APIs) are candidates for applications in
optoelectronics and interlayer dielectrics due to their higher
transparency and low dielectric constants when compared to
aromatic polyimides.6–13 These properties result from their
Additional Supporting Information may be found in the online version of this article.
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(-(CH2)3N) group can induce significant CT interaction even
in fully API network that could lead to the synthesis of high
modulus API.23 Therefore, incorporating rigid alicyclic struc-
tures with some electron donor/acceptor atoms or group
that can induce some sort of moderate to weak CT interac-
tion in the polymer network, may lead to the synthesis of
transparent, soluble aliphatic/alicyclic polyimides with good
thermal and mechanical properties. We were driven by this
fact in designing the aliphatic dianhydride monomer. Here,
we report the successful synthesis of novel fully APIs based
on a novel aliphatic dianhydride monomer-2,20-(1,4-piperazi-
nediyl)-disuccinic anhydride (PDA), in which two units of
succinic anhydride have been connected by an aliphatic het-
erocyclic piperazine spacer that possesses aminomethylene
(-NCH2) moiety in the aliphatic/alicylic backbone capable of
inducing CT interactions in the polyimide network. The opti-
cal, dielectric, thermal, physical, and mechanical properties
of those polymers were also thoroughly investigated and are
discussed in this paper.
FIGURE 1 FTIR spectra of PDSTE, PDSTA, and PDA. [Color fig-
ure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
EXPERIMENTAL
Materials
The alicyclic diamine, 4,40-methylene bis(2-methylcyclohexyl-
amine) (MMCA), was distilled under reduced pressure and
stored in the dark before use while 4,4’-methylene bis(cyclo-
hexylamine) (MCA) was recrystallized from hexane and dried
at 50 ꢀC. Ethylenediamine (EN) and 1,3-diaminopropane
(13DAP) were also distilled under reduced pressure before
use. The solvent m-cresol was dried over CaCl2, then over 4
Å Linde-type molecular sieves, distilled under reduced pres-
sure, and stored under nitrogen in the dark. All other
reagents and chemicals were purchased from Aldrich Chemi-
cal Company and were used as received.
and the filtrate was concentrated and cooled to give more
precipitate which was again filtered. Both of these precipi-
tates were collected and dried in vacuum for 12 h to give a
highly crystalline colorless solid.
Yield: 82%. mp: 158 ꢀC (EtOAc). 1H NMR (300 MHz, CDCl3,
d, ppm): 2.38–2.50 (m, 4H, Het-CH2CH2), 2.54–2.70 (m, 6H,
b-CH2, Het-CH2CH2), 2.81 (dd, J 5 16.0 and 9.2 Hz, 2H,
b-CH2), 3.64 (s, 6H, 2OCH3), 3.68 (dd, 2H, overlapped signals,
a-CH), 3.70 (s, 6H, 2OCH3) (Supporting Information Fig. S1).
13C NMR (75 MHz, CDCl3, d, ppm): 171.7 and 170.9 (ester
C), 63.4 (a-CH), 51.8 (OCH3), 51.5 (OCH3), 49.9 (Het-
CH2CH2), 34.0 (b-CH2) (Supporting Information Fig. S2).
Anal. calcd. for C16H26N2O8: C, 51.33; H, 7.00; N, 7.48. Found:
C, 51.19; H, 7.09; N, 7.53; FTIR (KBr, cm21): 1734 (C5O)
(Fig. 1).
Synthesis of Dimethyl Fumarate
This compound was synthesized following an already
reported procedure24 with some modifications. To fumaric
acid (FA) (14.5 g, 125 mmol) suspended in MeOH (200 mL)
was added concentrated H2SO4 (5 mL), and the mixture was
heated to reflux for 1 h. The mixture was then cooled in an
ice bath and neutralized with 10% Na2CO3 which resulted in
some white precipitate to come. The precipitate was filtered
and washed several times with water and finally was dried
Synthesis of 2,20-(1,4-Piperazinediyl)-disuccinic acid
About 0.03 mol of 2,20-(1,4-piperazinediyl)-di-[succinic acid]-
tetramethyl ester (PDSTE) (11.2 g) was taken in a 500-mL
round bottomed flux containing 120 mL of 2 (N) NaOH and
120 mL of MeOH. The mixture was heated at 60 ꢀC to dis-
solve the solid and was heated further for 3 h at the same
temperature. Finally, the reaction mixture was cooled to
room temperature which resulted in the precipitation of the
tetra-acid salt. The salt was collected by filtration followed
by repeated washing with MeOH. It was then dissolved in
200 mL of water and the pH of the resulting solution was
adjusted carefully to 3.8 by concentrated HCl and was stirred
for 0.5 h at room temperature. The precipitated white com-
pound was filtered and washed several times with cold
water. The white residue was dried in vacuum at 80 ꢀC for
overnight. 2,20-(1,4-Piperazinediyl)-disuccinic acid (PDSTA)
was not soluble in most common organic solvents as well as
ꢀ
in vacuum for 12 h at 50 C.
This compound was known and thus was characterized by
melting point determination after crystallizing from hot eth-
ylacetate (EtOAc). Yield: 91%. mp: 102 ꢀC (EtOAc) (lit. mp
ꢀ
102.4 C).
Synthesis of 2,20-(1,4-Piperazinediyl)-di-[succinic acid]-
tetramethyl ester
This compound is already reported25 but we adapted a dif-
ferent synthetic procedure to simplify the synthesis. This
synthesis was affected by refluxing 0.05 mol of piperazine
(4.3 g) and 0.1 mol of the ester (dimethyl fumarate, DMF)
(14.4 g) in 100 cc of 1,4-dioxane for 16 h. Upon cooling, the
pure compound crystallizes out. The precipitate was filtered
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in water. So, in order to make NMR sample, minimum
amount of solid NaOH was used to dissolve the compound in
D2O.
respectively, and then subjected to imidization following the
heating cycle described above.
Characterization
Yield: 92%. mp: 218–219 ꢀC (H2O1MeOH). 1H NMR
(300 MHz, D2O, d, ppm): 2.46–2.54 (bd, 4H, Het-CH2CH2),
2.56–3.40 (bm, 6H, b-CH2, Het-CH2CH2), 3.44–3.54 (bm, 2H,
b-CH2), 4.80 (dd, 2H, overlapped signals, a-CH) (Supporting
Information Fig. S3). 13C NMR (75 MHz, D2O, d, ppm): 178.8
(COO2), 67.7 (a-CH), 49.1 (Het-CH2CH2), 37.4 (b-CH2) (Sup-
porting Information Fig. S4). Anal. calcd. for C12H18N2O8: C,
45.28; H, 5.70; N, 8.80. Found: C, 45.16; H, 5.79; N, 8.83.
FTIR (KBr, cm21): 1723(C5O for carboxylic acid) and 1623
(C5O for carboxylate) (Fig. 1).
1H and 13C NMR spectra were recorded on a Varian Unity
Plus-300 (300 MHz) NMR spectrometer. Infrared spectra
(KBr disks) were recorded on a Shimadzu IR Prestige-
21 spm using a Ge-KBr beam splitter. Average molecular
weight (Mn) and polydispersity index (PDI) of the soluble
APIs were estimated by gel permeation chromatography
(GPC) using a Waters 515 differential refractometer with
Waters 410 HPLC Pump and two Styrogel HR 5E columns in
DMF (0.1 mg L21) at 42 ꢀC, calibrated with polystyrene
standards. The solubility test was performed using equal
amounts of APIs in matched quantities of commonly used
solvents. The level of theory with the three-parameter
Becke-style hybrid functional (B3LYP) was adopted for the
density functional theory (DFT) calculations in conjunction
with the Pople-type 6–311G(d,p) basis set for elucidating the
molecular structures of 2,20-(1,4-piperazinediyl)-disuccinic
anhydride (PDA) and two APIs. The geometry optimization
under no constraints was performed using the Gaussian-03
(Rev. D02) software installed on the TSUBAME grid cluster
system in Tokyo Tech. The DFT functional and the basis set
used were B3LYP/6–31111G(d,p). To investigate the ther-
mal stability of the APIs, thermogravimetric analyses were
performed under nitrogen using a TGA Q50 Q Series thermal
Synthesis of 2,20-(1,4-Piperazinediyl)-disuccinic
anhydride
PDSTA (5.4 g, 17 mmol), pyridine (8 mL), and acetic anhy-
dride (10 mL) were placed in a 50 mL flask equipped with a
condenser and a magnetic stirrer. The reaction was carried
out by stirring the reaction mixture at 40 ꢀC for 24 h in
nitrogen atmosphere. The resulting anhydride after cooling
at room temperature was filtered off and washed thoroughly
with acetic anhydride and dry diethyl etꢀher. The white pow-
der was then dried under vacuum at 40 C and recrystallized
from acetic anhydride.
Yield: 60%. mp: 156 ꢀC (decompose); 1H NMR (300 MHz,
d6-DMSO, d, ppm): 2.39 (bd, J 5 6.8 Hz, 4H, Het-CH2CH2),
2.76 (bd, J 5 6.8 Hz, 4H, Het-CH2CH2), 3.04 (d, J 5 8.4 Hz, 4H,
b-CH2), 4.21 (t, J 5 16.4 Hz, 2H, a-CH). 13C NMR (75 MHz, d6-
DMSO, d, ppm): 171.6 (COOCO), 170.6 (COOCO), 63.6 (a-CH),
48.9 (Het-CH2CH2), 31.9 (b-CH2) (Supporting Information
Fig. S5). Anal. calcd. for C12H14N2O6: C, 51.06; H, 5.00; N,
9.93. Found: C, 49.93; H, 5.10; N, 9.96. IR (KBr, cm21): 1860,
1781 (C5O), 1210, 1127 (C-O-C).
21
ꢀ
analyzer at a heating rate of 10 C min from room temper-
ature to 800 ꢀC. Measurements of glass transition tempera-
ture were performed using a DSC Q 100 TA instrument at a
heating rate of 10 ꢀC min21 under a nitrogen atmosphere.
Dynamic mechanical analysis (DMA) was conducted with
DMA2980 (TA instron) instrument. The storage modulus E,
and tan d were determined as the sample was subjected to a
temperature scan mode at a programmed heating rate of 10
ꢀC min21 at a frequency of 1Hz. The transparencies of the
polyimide films were measured from ultraviolet–visible spec-
tra recorded from one accumulation on a SHIMADZU UV-
1650 PC spectrometer optimized with a spectral width of
200–800 nm, a resolution of 0.5 nm, and a scanning rate of
200 nm min21; the thickness of each film was ꢁ15 lm. The
excitation and fluorescent-emission spectra of the PI films
were characterized using a Hitachi F-4500 fluorescence spec-
trophotometer with a resolution of 0.2 nm and a scanning
rate of 240 nm min21.Wide-angle X-ray diffraction (WXAD)
measurements of the pulverized samples were conducted at
room temperature in the reflection mode using a Rigaku dif-
fractometer (Model Rigaku Miniflex). The Cu Ka radiation
(k 5 1.54 Å) source was operated at 50 kV and 40 mA. The
2h scan data were collected at 0.01ꢀ intervals over the range
1.5–80ꢀ and at a scan speed of 0.58(2h)/min. Tensile proper-
ties were determined from stress–strain curves obtained
with Kyungsang Universal Testing Measurement (UTM) Mate-
rials Testing System (Model KSU.05) at a strain rate of
10 mm min21 at room temperature. Measurements were
performed with film specimens (50 mm in gauge length,
10 mm wide, and 0.1 mm thick). Each reported value is the
average of five different measurements. The dielectric
General Polymerization Procedure and Polyimide Film
Preparation
In a 30-mL three-necked flask equipped with a mechanical
stirrer were placed the dianhydride (2.0 mmol) and 4 mL of
m-cresol. Under slow stream of nitrogen gas, the mixture
was stirred until the dianhydride was entirely dissolved. Dia-
mines (2.0 mmol) and an additional 2 mL of m-cresol were
then added into the clear solution. The flask was heated at
ꢀ
60 C, and the solution was stirred for 2 days. An aliquot of
the polycondensation solution containing poly(amic acid)
(PAA) was cast on a glass plate. The polyimide film was pre-
pared by heating the glass plate at 80 ꢀC for 3 h, at 200 ꢀC
ꢀ
for 1 h, and then at 250 C for 1 h under vacuum. After cur-
ing, the glass plate was immersed in boiling water to facili-
tate removal of the flexible, free-standing polyimide film.
API1 and API2 were too brittle and the polyimide films
come off the glass surface immediately after imidization and
therefore no need to immerse in water further to remove
the films from the glass surface. To perform the dielectric
constant and transparency measurements, the PAA solutions
were spin-coated onto clean ITO glass and quartz plates,
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SCHEME 1 Synthesis of 2,20-(1,4-piperazinediyl)-disuccinic anhydride (PDA).
constant (e) was obtained at 1 MHz using an impedance-gain
phase analyzer (HP4194A) and the formula e 5 C 3 d/Ae0,
where C is the observed capacitance, d is the film thickness,
A is the area, and e0 is the free permittivity. The thickness of
each film was 1.0 6 0.05 lm. Each reported value is the
average of three different measurements.
amounts of reagents, at different temperatures and reaction
times it revealed that best yield can be obtained by stirring
the reaction mixture at 40 ꢀC for 24 h. At relatively higher
temperatures, a black pasty mass appeared which made the
purification of the dianhydride extremely difficult. The opti-
mized condition for the synthesis of novel piperazine con-
taining dianhydride monomer was to stir the reaction
mixture at 40 ꢀC for 24 h at N2 atmosphere. The detailed
characterizations on the dianhydride monomer (PDA) were
RESULTS AND DISCUSSION
1
done by FTIR (Fig. 1), H NMR (Fig. 2), 13C NMR (Supporting
Monomer Synthesis
Information Fig. S5) spectroscopy, and elemental analyses,
As shown in Scheme 1, the novel piperazine containing ali-
phatic dianhydride monomer (PDA) was obtained through a
four-step synthetic route. The first step involved the synthe-
sis of DMF which functioned as Michael acceptor and under-
went Michael addition reaction in the second step with
Michael donor piperazine to yield 1,4-piperazinediyl-tetrame-
thyldisuccinate (PDSTE). This is the key step for the synthe-
sis of dianhydride monomer (PDA). The corresponding
disuccinic acid derivative PDSTA can be obtained by the base
(NaOH)-catalyzed hydrolysis of the PDSTE followed by acidi-
fication of the sodium salt of the PDSTA. Acetic anhydride in
presence of pyridine had been chosen as the anhydrating
agent and only after thorough optimization with various
FIGURE 2 1H NMR spectrum of PDA in d6-DMSO.
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SCHEME 2 Synthesis of aliphatic polyimides.
which support unambiguously the structure shown in
Scheme 1. The analytical data of PDA and the intermediate
compounds DMF, PDSTE, and PDSTA were given in the
experimental section (scanned spectra are provided in the
Supporting Information). The results indicate the successful
synthesis of novel piperazine containing dianhydride mono-
mer (PDA) in this work.
monomers 4,40-methylene bis(cyclohexylamine) (MCA), and
4,40-methylene bis(2-methylcyclohexylamine) (MMCA) were
selected with the objective to study how the basic traits of
the polyimides vary with respect to the effects of the
methyl substituent, and steric hindrance of their diamine
subunits.
The APIs were prepared by a conventional two-step proce-
dure through a ring-opening polyaddition to give PAAs and
the subsequent thermal imidization (Scheme 2) at higher
temperature. Carboxylic acid salts generally result from the
reactions of dianhydrides with highly basic diamines, such as
aliphatic and alicyclic diamines, together with polyamic acids.
This salt formation prevents the isolation of high molecular-
weight polyamic acids. Salt formation can be reduced consid-
erably by using higher reaction temperature (ꢂ60 ꢀC) for
Polyimides Synthesis and Characterization
For the synthesis of APIs, six different aliphatic diamines
were used (Scheme 2). Ethylene diamine (EN), 1,3-diamino-
propane (13DAP), 1,6-diaminohexane (16DAH), and 1,12-
diaminododecane (112DAD) were selected as flexible linear
aliphatic diamines with the intention of studying the impact
that increasing chain length and flexibility has upon the
inherent characteristics of the APIs. Two alicyclic
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TABLE 1 Elemental Analysis and GPC Molar Mass of APIs
Elemental Analysis (%)
GPC Molar Mass
Polyimide
Diamine
16DAH
Code
C
H
N
104 Mn
PDI
1.6
API3
Calcd.
Found
Calcd.
Found
Calcd.
Found
Calcd.
Found
59.49
58.79
64.40
63.72
65.62
64.51
66.78
66.02
7.49
7.02
8.78
8.27
8.15
7.92
8.51
8.62
15.42
15.58
12.52
13.04
12.24
12.01
11.54
10.95
2.19
112DAD
MCA
API4
API5
API6
2.23
3.2
1.8
2.3
2.3
MMCA
2.9
PAA preparation. Therefore, we prepared our APIs by two
step procedure: first formation of PAAs at 60 C in m-cresol
In the FTIR spectra (Fig. 4) of all the polyimides showed
characteristic imide absorption bands at 1771–1775 cm21
ꢀ
,
and then thermal imidization of the PAAs under programmed
curing temperatures. The reaction of PDA with most basic
amines, namely, EN and 13DAP, yielded lower molecular
weight (Mn) (3800 and 4700) polyimides (Supporting Infor-
mation Table S1) while relatively higher molecular weights
(32,000 and 29,000) were resulted when alicyclic diamines
(MCA and MMCA) were used (Table 1). Extensive salt forma-
tion by more basic EN and 13DAP prevents the formation of
high molecular weight PAAs which led to the isolation of
APIs with lower molecular weights. PDI of these APIs was in
the range 1.6–2.3. Nevertheless, these results demonstrate
that the dianhydride monomer PDA has good polymerization
activity for forming APIs. Table 1 summarizes the GPC
results of these APIs.
which were attributed to the asymmetrical carbonyl stretch-
ing vibrations, and at 1691–1697 cm21, which were attrib-
uted to the symmetrical carbonyl stretching vibrations.
Nonconjugations of the imide carbonyl group due to the
absence of an aromatic ring cause the absorption shift in the
APIs. Apart from these two carbonyl stretching frequencies,
additional peaks at 1625 and at 1622 cm21 appeared in
case of API1 and API2, respectively. This may be interpreted
as the C5O stretching of amide of their polyamic acids
(PAA). Salt formation by these two extremely basic diamines
(EN and DAP) with PDA prevents them from complete imid-
ization. Incomplete imidization of these two diamines can
also be understood from the appearance of relatively
broader IR peaks centred around 3400 cm21 (O-H and N-H
stretching frequencies of PAA) in the FTIR spectra of API1
and API2. In case of the remaining four APIs, there were no
characteristic absorption bands of the amide group around
1
The chemical structures of APIs were confirmed by H NMR
and FTIR spectroscopy and by Elemental Analysis. In
1
Figure 3, a typical H NMR spectrum of the API4 is reported
in which all the peaks were clearly assigned: the down field
protons are of PDA while two most up field peaks can be
attributed to the protons of 112 DAD. The protons of PDA
residue appear at 3.82 ppm [-CH2, 2,2’], 3.33 ppm [-CH2CO,
1,1’], 2.74 ppm [-NCH2, 3] and 2.40 ppm [-NCH2, 4] while
that of 112 DAD at 1.44 ppm [-NCH2, 5] and at 1.22 ppm [-
CH2, 6]. The absence of any peaks above 10 ppm (10–14
ppm) confirms complete imidization.
FIGURE 4 FTIR spectra of the aliphatic polyimides: (a) API1, (b)
API2, (c) API3, (d) API4, (e) API5, and (f) API6. [Color figure can
be viewed in the online issue, which is available at wileyonline-
library.com.]
FIGURE 3 1H NMR spectrum of API4 in d6-DMSO.
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FIGURE 5 Molecular structures of (a) PDA; (b) API4; (c) API5. Gray, white, red, and blue each molecular structure display C, H, O,
and N, respectively. The geometries were optimized by the DFT (B3LYP/6–311G(d,p)) calculations. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
3300 cm21 (O-H and N-H stretching frequencies of PAA) or
1630–1640 cm21 (C5O stretching), indicating that the poly-
mers had been fully imidized. In all cases, we observed
peaks at around 1360 cm21 due to the C-N stretching.
Because of the presence of so many different types of C-N
bonds in the API backbone, however, it was rather difficult
to assign them all. The elemental analysis values of the poly-
mers generally agreed well with the calculated values for the
proposed structures. The results of the elemental analyses of
the polyimides are listed in Table 1 and Supporting Informa-
tion Table S1. The observed values were in good agreement
with the calculated ones.
to some extent distorted as well. API4 contain long, linear,
alkyl chain diamine (1,12-diaminododecane) which can
arrange in a zigzag fashion to give it a linear shape while the
presence of rigid, bulky alicyclic diamine (MCA) ensure it to
be distorted and nonlinear in shape. These subtle changes in
the chain geometry of these two kinds of polyimides,
resulted in enormous differences in their properties as we
are going to witness in the following discussions. The nonlin-
ear, distorted conformation of the polyimide chains in API5
and API6, effectively reduces the interchain charge transfer
interaction and creates free-space in the polyimide matrix
and thereby increases the solubility and decreases the color
of these polyimides significantly.
Molecular Structures
Solubility
Figure 5 shows the three-dimensional molecular structures
of PDA and its two representative polyimides (API4 and
API5). The geometries were optimized by the DFT calcula-
tions using a large basis-set function.26–31 From the opti-
mized structure, it is revealed that PDA has (R,S)
configuration at the two stereo centers present in it. The
two dianhydride units are not completely trans (parallel) to
each other. The two planes containing the two dianhydride
units make an angle of 94ꢀ, which means that they are
almost perpendicular. On the other hand, in API4 and API5,
two dianhydride planes (now imide plane) are almost paral-
lel (dihedral angle is 179ꢀ). Therefore, after imidization, the
strain in the polymer chain forced these planes to be nearly
parallel in order to avoid the steric repulsion.
The solubility of the synthesized APIs was investigated in
different organic solvents. All the APIs showed excellent sol-
ubility in m-cresol and in H2SO4. Apart from API1 and API2,
other APIs exhibited complete solubility in polar solvents
ꢀ
like NMP, DMAc, DMF, and DMSO only upon heating at 60 C.
Now this is quite interesting, because the presence of flexible
aliphatic and alicyclic-backbone in the APIs should have
resulted in APIs with higher solubility in both polar and non-
polar solvents. The relatively moderate solubility of these
APIs may be due to the presence of the nucleophilic (elec-
tron-donating) aminomethylene group (-NCH2) in the pipera-
zine unit of the dianhydride segment of the APIs which
could have electrostatic interaction with electrophilic (elec-
tron-withdrawing) carbonyl groups in imide and thereby
strengthen the intermolecular interactions that are used to
be very less in so called APIs. Polyimides resulted from EN
and 1,3-diaminopropane were quite soluble in water which
It is very interesting to observe that even though the imide
planes are parallel in both API4 and API5, the polyimide
chain in API4 is almost linear while in API5 is nonlinear and
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TABLE 2 Solubility Data of the Polyimides Synthesized in This Work
Solventsa
API
NMP
DMAc
DMF
DMSO
m-Cresol
CHCl3
H2O
Pyridine
H2SO4
API3
API4
API5
API6
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
11
11
11
11
–
–
–
–
6
6
–
1
1
1
1
11
11
11
11
–
a
Solubility: (11) soluble at room temperature; (1) soluble upon heating at 60 ꢀC; (6) partially soluble or swells; (–) insoluble.
may be due to the extensive salt formation by these rela-
tively basic diamines with PDA. The solubility behavior of
these polymers in different solvents is presented in Table 2
and Supporting Information Table S2.
absorption band of each polyimide film were estimated by
the point where the absorption curve intersects a bisected
line drawn through the intersection of the extrapolations of
the two slopes. The absorption edges (kE) for API3, API4,
API5, and API6 were 369, 349, 339, and 333 nm, respec-
tively, while the same for shoulder absorption band were
454, 447, 429, and 427 nm, respectively. Because of the tail-
ing of these shoulder absorption bands in the visible region,
none of these polyimide films were completely colorless. The
shoulder absorption bands of API5 and API6 (with MCA and
MMCA) were less intense and also appeared at shorter
wavelengths. Hence, these polyimide films were very less
colored. The occurrences of the shoulder absorption bands
in addition to main absorption bands certainly indicate that
there may be two energy band gaps: lower energy band gap
may be from dianhydride while the higher energy band gap
from diamines.
Optical Properties
Fully APIs normally exhibit high transparency because of
their low molecular density, polarity, and probabilities of
mediating inter- and intramolecular CT. Figure 6 shows the
UV–visible absorption spectra of the polyimide films with a
thickness of 15 lm. Three distinct types of polyimides were
obtained after the thermal imidization of the polyamic acid
solutions of PDA with six different diamines. The polyimide
films of EN and 13DAP with PDA were highly brittle and
reddish-black in color while flexible, free-standing yellow
colored films were obtained with 16-DAH and 112-DAD. But,
by far, the most transparent polyimide films (API5 and API6)
were obtained by alicyclic diamines (MCA and MMCA). These
films were almost colorless with a very little yellowish tint.
As shown in Figure 6, all the polyimides showed a shoulder
absorption band at higher wavelength region in addition to
the main absorption band that occurred at the shorter wave-
length region. The absorption edges (kE) of these two
Figure 7 shows the optical transmission spectra of the API
films. The spectra are principally the same as those shown
in Figure 6. However, it is important to understand the dif-
ference in the optical transparency at the longer
wavelengths. As expected, APIs with alicyclic diamines (API5
and API6) exhibited highest transmittance of 88% above
450 nm wavelength region while APIs with longer linear
aliphatic diamines (API3 and API4) showed around 86%
FIGURE 6 UV–visible absorption spectra of API films. [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 7 Optical transmission spectra of API films.
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API5 which registered highest d-spacing. In API1, because of
the presence of short alkyl chain diamine, the internal order
in the polymer matrix is very high which is revealed from its
lower d spacing value (4.1 Å). Now as the chain length of
the aliphatic diamine part increases (API4), the flexibility of
the polyimides also increases and that to some extent dis-
rupts the internal order of the polymer chains and eventu-
ally leads to the polyimide (API4) with higher d-spacing than
that of short alkyl chain aliphatic diamine-based polyimide
(API1). Normally, as the d-spacing value increases, the free
volume also increases and so as the solubility of polymer.33
API5 with higher d-spacing value showed higher solubility in
common organic solvents than API1 with lower d-spacing
value.
Mechanical and Dielectric Properties
The mechanical properties of the fully APIs (API6, API5 and
API3, and API4) were investigated using the stress–strain
curves (Fig. 9) obtained from UTM and the average values of
five different measurements for each APIs are listed in Table
3. All these four polyimides showed appreciable mechanical
stiffness, tensile strength varying from 54 to 72 MPa, elonga-
tion at break from 4 to 9%, and tensile modulus in the range
1.6–2.3 GPa (Table 3). API4 and API3 with flexible, linear,
long alkyl chain aliphatic diamines exhibited the lowest ten-
sile strength and highest percentage of elongation at break
while API6 and API5 with more rigid, bulky alicyclic dia-
mines, demonstrated highest tensile strength and lowest per-
centage of elongation at break. Usually, the linear aliphatic
polymers have low rigidity and tensile strength because of
the flexible and compliant chain structures, thus these
enhanced mechanical strength may be due to appreciable
charge transfer interaction between the nucleophilic (elec-
tron donating) aminomethylene group (-NCH2) in the pipera-
zine unit of the dianhydride and electrophilic (electron
withdrawing) carbonyl groups in imide. The polyimide films
of API1 and API2 were too brittle to measure their mechani-
cal properties by UTM.
FIGURE 8 XRD patterns of APIs. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
transmittance above 500 nm. APIs with short linear aliphatic
diamines (EN and 13-DAP) exhibited lowest transmittance.
These films were reddish-black in color. Linear APIs have
more densely packed chain structures compared to APIs
with bulky, nonlinear alicyclic diamines (MCA and MMCA).
Again, APIs with linear, short alkyl chain diamines (EN and
13-DAP) possess higher density of electrophilic amide group
and nucleophilic aminomethylene groups (-NCH2). These two
factors lead to the considerable degree of interchain charge
transfer interaction in the linear alkyl chain polyimides and
API1 and API2 with very short alkyl chain diamines experi-
enced maximum CT interaction leading to highly colored pol-
yimide films much like aromatic polyimides. APIs with bulky
and nonlinear alicyclic diamines effectively reduce the inter-
chain CT interaction and thereby yielded almost colorless
polyimide films. The extreme broadening of the optical trans-
mission spectra of API1 and API2 is also an indication of the
existence of severe charge transfer interaction between the
polymer chains.
Crystallinity
Figure 8 shows WAXD patterns of three representative APIs
(API1, API4, and API5). For all these three polyimides, the
reflection patterns are featureless, showing only broad amor-
phous halos that appeared in the region 2h 5 5–38ꢀ and cen-
tered around 2h 5 20ꢀ. This is due to the diffraction of a
poor intermolecular packing combined with amorphous
halo.32 The broad peaks gradually shifted toward higher 2h
values as we moved from alicyclic diamine to short chain ali-
phatic diamine through long chain aliphatic diamine-based
polyimides (API5, 2h 5 17.2ꢀ, d-space 5 5.2 Å; API4,
2h 5 19.4ꢀ, d-space 5 4.6 Å; API1, 2h 5 21.8ꢀ, d-space 5 4.1
Å). The d-spacing is usually considered to represent the dis-
tance between segments of different chains and related to
the free volume of a polymer. Introduction of the bulky, non-
linear alicyclic moieties in the polymer backbone may be to
some extent contributed to the disruption of the internal
order of the polymer chains in the alicyclic diamine-based
FIGURE 9 Stress–strain curves of APIs. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 3 Thermal, Mechanical Properties, and Dielectric Constants of API Films
Tensile
Strength
(Mpa)
Elongation
at Break
(%)
Tensile
Modulus
(Gpa)
Dielectric
APIs
Constant (e)
kex (nm)
kex (nm)
Tg (ꢀC)a
Tg (ꢀC)b
5
Td (ꢀC)
10
Td (ꢀC)
API3
API4
API5
API6
5568
5467
66610
7269
661
961
461
461
1.860.2
1.660.2
2.160.5
2.360.4
2.67
2.14
3.30
2.87
355
364
381
386
505
498
488
491
323
286
382
376
360
307
418
413
212
170
224
237
196
172
218
233
a
b
Measured by DSC.
Peak temperature of tan(d) in the DMA thermogram.
The dielectric constant (e) was varied according to the type
of diamine monomers used: the lowest value was measured
for API4 (e 5 2.14) (Table 3), whereas the highest value was
obtained for API2 (e 5 5.09) (Supporting Information
Table S3).
excitation and emission spectra of the API films. All API films
showed an apparent fluorescent emission in the visible
region. Table 3 lists the wavelengths of the excitation and
emission peaks. The Stokes shifts were estimated from the
difference in wavelength between the excitation and emis-
sion peaks. The API5 and API6 based on alicyclic diamines
(MCA and MMCA) show excitation peaks at 381and 386 nm
and emission peaks at 488 and 491 nm, and the calculated
Stokes shifts were 0.71 and 0.69 eV, respectively. The API3
and API4 based on long aliphatic chain diamines (16DAH
and 112DAD) show excitation peaks at 355 and 364 nm and
emission peaks at 505 and 498 nm, and the calculated
Stokes shifts were 1.04 and 0.92 eV, respectively.
The lowest dielectric constant of API4 (with 112DAD) is con-
sidered due to the long alkyl chain that reduces the number
of polarizable molecular units in a given volume. Generally,
APIs possessed low dielectric constant due to the diminution
in the degrees of chain packing and interchain interactions.34
The dielectric constant of API4 was even lower than polyeth-
ylene (extensively used in various dielectric applications).
Polyethylene (plastic) is a tough inexpensive polymer made
of long chains of carbons attached with hydrogens. It has
fine dielectric properties with its low dielectric constant
(2.3) and low moisture absorption; however, as the tempera-
ture increases, it tends to soften as its crystallites melt and
its dielectric strength will decrease. As we move on to end
of this discussion, we will find that API4 is thermally much
The Stokes shifts of API3 and API4 are much larger than
that of the API5 and API6. The large Stokes shifts values in
API3 and API4 and along with their relatively broaden
nature of fluorescent emission spectra clearly indicate the
existence of significant CT interactions in the polyimide net-
work that is quite typical with aromatic polyimides.37 These
CT interactions are weakened considerably in API5 and API6
due to the presence of bulky and rigid alicyclic diamines.
The excitation and emission spectra of API2 are too broad to
measure the corresponding kmax values. This extreme broad
nature of the fluorescent emission spectra once again dem-
onstrates the existence of enormous CT interaction in API2
originating from the very high density of nucleophilic (elec-
tron donating) aminomethylene group (-NCH2) in the pipera-
zine unit of the dianhydride and electrophilic (electron
withdrawing) carbonyl groups in imide.
5
more stable (Td 286 ꢀC and Tg 170 ꢀC) than polyethylene
(Tg of low-density polyethylene is 2120 ꢀC, while melting
temperature (Tm) is 135 ꢀC) and this make API4, a very
promising materials for high temperature dielectric applica-
tions. The abnormally high dielectric constant of API1 and
API2 (Supporting Information Table S3) may be due to the
presence of extensive interchain CT interaction between the
nucleophilic (electron donating) aminomethylene group
(-NCH2) in the piperazine unit of the dianhydride and elec-
trophilic (electron withdrawing) carbonyl groups in the
imide. The increased density of polar imide groups in these
two APIs may also contributed to their much enhanced
dielectric constant values. APIs with alicyclic diamines (API5
and API6) showed higher dielectric constants than expected
as the presence of the highly rigid and bulky alicyclic moi-
eties, which sterically hinder the packing and enlarge the
free volume in the polyimides, inducing low dielectric prop-
erties.35 Interchain CT interactions might have contributed to
some extent in these cases as well.
Thermal Stability and Phase Transitions
The thermal properties of all APIs were evaluated by TGA,
DSC, and DMA measurements. Figure 11 displays representa-
tive TGA curves; Table 3 and Supporting Information Table
S3 summarize the results. The polymers underwent their
5% weight losses within the temperature range from 286 to
382 ꢀC. The temperatures for 10% gravimetric loss (T10),
which is an important criterionꢀfor evaluating thermal stabil-
ity, were in the range 299–418 C. The polyimides containing
linear aliphatic chains on their backbones were the least
resistant to temperature. It already has been reported that
the thermal stability of aliphatic linear diamine-based polyi-
mides decreased upon increasing the length of their ther-
mally fragile alkyl chains (mainly because of the dilution
Fluorescent Properties
Fluorescent spectroscopy is used widely to examine the elec-
tronic structures and aggregation states of polymers on
account of its high sensitivity to microenvironmental changes
in polymer matrices.36 Figure 10 shows the one-dimensional
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FIGURE 10 Excitation and fluorescent emission spectra of APIs.
effect of aliphatic imide moieties).9 The synthesized APIs
with linear alkyl chain exhibited some confusing results.
API1 and API4 with shortest and longest alkyl chain show
API3-API6 were in the range of 170–237 C. The results are
summarized in Table 3. For, pure APIs (API3 and API4), the
glass transition temperatures decreased with increasing the
aliphatic chain length of the diamines. The greater flexibility
of 1,12-diaminododecane in API4 brought down its Tg to 170
ꢀC in comparison to 1,6-diaminohexane in API3 which
ꢀ
5
the lowest Td (286 ꢀC). API3 with longer alkyl chain was
more stable than short alkyl chain API2. On the other hand,
as predicted earlier, API3 with six carbon alkyl chain diamine
was thermally more stable than its 12 carbon counterpart
API4. This apparently anomalous behavior of API1 and
API2 may be due to their incomplete imidization that left
considerable number of thermally labile amic acid groups in
the polymer backbone and thereby result polyimides with
lower thermal stability. API2 was thermally more stable than
API1 due to its higher degree of imidization as evident from
their FTIR spectra (Fig. 4). The thermal stability of the APIs
was improved significantly when aliphatic diamines were
replaced by alicyclic diamines (MCA and MMCA). API5 and
ꢀ
showed the glass transition at 212 C. Polyimides with alicy-
clic diamines (API5 and API6) showed higher Tg in compari-
son to the polyimides with linear aliphatic diamines (API3
and API4). The increased chain rigidity due to the incorpora-
tion of rigid alicyclic diamines restricted the free rotation
about the polymer and hence produced polyimides with
higher Tg values. The enhanced values of Tg of API6
API6 with rigid alicyclic diamines were found to be stable
10
ꢀ
ꢀ
even up to 400 C (Td 418 and 413 C, respectively). Apart
from the rigid nature of these two alicyclic diamines, the
charge transfer interaction in API5 and API6 (as is evident
from large Stokes shifts in the excitation and fluorescent
emission spectra) may have played a crucial role in improv-
ing the thermal stability of these two polyimides. The resid-
ual weight of API1 and API2 were 30 and 33% at 800 ꢀC
that are much higher than the rest of the polyimides. The
densities of the thermally stable imide units in API1 and
API2 (with short alkyl chain diamines) are higher than the
remaining polyimides and this may have contriꢀbuted to the
higher residual weight of API1 and API2 at 800 C.
The glass transition temperatures (Tg) of APIs were deter-
mined by DSC (Fig. 12) and DMA (Fig. 13). No significant
changes were observed in the DSC analysis of API1 and API2
up to 300 ꢀC. The glass transition temperatures (Tg) of
FIGURE 11 TGA curves of the APIs. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 14 The intermolecular charge transfer/electrostatic
interaction in APIs of PDA.
the infletion point accurately. It can be seen from Figure 13
that the storage modulus of APIs remains almost constant
on heating below their glass transition temperatures. The
storage modulus at room temperature of API3, API4, API5,
and API6 were 1.6, 1.5, 2.4, and 2.6 GPa, respectively, which
were comparable to their tensile modulus estimated from
the stress–strain curves (Fig. 9) obtained from UTM. API5
and API6 with rigid alicyclic diamines exhibited higher stor-
age modulus than flexible, long alkyl chained APIs (API3 and
API4). Unfortunately, the polymer films of API1 and API2
were too brittle to analyze by DMA.
FIGURE 12 DSC thermograms of the APIs: (a) API4; (b) API1;
(c) API2; (d) API3; (e) API5 and (f) API6. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
possessing MMCA backbones, relative to those possessing
MCA units, suggest that the glass transition temperatures of
these types of polyimides increase gradually upon increasing
the chain rigidity induced by the methyl substitution. Figure
13 shows the variations in the storage modulus (E0) and loss
factor (tan d) at different temperatures. The thermograms
show the a-relaxation (main peaks) between 160 and
Presence of electron withdrawing groups in the dianhydride
structure and electron donating groups in the diamine struc-
ture lead to strong charge transfer complex (CTC) formation
which consequently results more colored polyimides.38,39
The lack of color in aliphatic and alicyclic polyimides is due
to the absence or inhibition of intra and/or intermolecular
charge-transfer interactions. On the other hand, in case of
PDA, the nucleophilic (electron-giving) trimethyleneamino
group can have electrostatic/intermolecular charge transfer
interaction with electrophilic (electron-withdrawing) car-
bonyl groups in imide (Fig. 14), although the N atoms in
imide groups also have lone electron pairs, they are delocal-
ized by the resonance of electron-withdrawing carbonyl
groups next to it and are unavailable for such kind of inter-
action. This kind of interchain interaction is different from
the so-called charge transfer interaction in aromatic polyi-
mides where the major part of polymer chain is in conjuga-
tion. Since the nucleophilic trimethyleneamino group in PDA
is not in conjugation with the electrophilic carbonyl group, it
has a very little influence on the electron accepting property
(electron affinity) of the carbonyl group. The absence of con-
jugation in PDA inhibits all kinds of possible intramolecular
charge transfer interactions (interactions are mainly through
space not through bonds). Intermolecular charge transfer/
electrostatic interactions between electron giving trimethyle-
neamino groups and electron-accepting carbonyl groups are
the only possible interactions between the polymer chains of
APIs of PDA and this kind of interchain interactions in PDA
containing APIs were mainly responsible for their higher
thermal mechanical stabilities and lower transparencies than
the traditional aliphatic/alicyclic polyimides.
ꢀ
250 C. In the DMA thermograms, the Tg was estimated from
the peak temperature of tan d curve. The Tgs for API3, APꢀI4,
API5, and API6 were observed at 196, 172, 218, and 233 C,
respectively. The glass transitions were increased consider-
ably by replacing long chain aliphatic diamines (16DAH and
112DAD) with comparatively rigid alicyclic diamines (MCA
and MMCA). The Tgs estimated from DSC and DMA were
almost similar except API3. The estimated Tg of API3 by
DMA was substantially lower than that obtained by DSC. The
DSC curve of API3 was in fact too flat (Fig. 12) to estimate
FIGURE 13 Temperature dependence of storage modulus and
tan d for APIs. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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11 D.-J. Liaw, B.-Y. Liaw, Macromol. Chem. Phys. 1999, 200,
CONCLUSIONS
1326–1332.
Six new fully APIs based on a novel aliphatic dianhydride
monomer-2,20-(1,4-piperazinediyl)-disuccinic anhydride (PDA),
in which two units of succinic anhydride have been connected
by an aliphatic heterocyclic piperazine spacer that possesses
aminomethylene (-NCH2) moiety in the aliphatic/alicylic back-
bone capable of inducing charge transfer (CT) interactions in
the polyimide network, were successfully synthesized by a two-
step polycondensation method using four aliphatic and two ali-
cyclc diamines. The polyimides (API3-API6) produced organo-
soluble, free-standing, less-colored films with reasonable ther-
mal, mechanical properties. T10 (temperature of 10% weight
loss) of APIs were ranged from 299 to 418 ꢀC, Tg of API3-API6
12 D.-J. Liaw, B.-Y. Liaw, C.-Y. Chung, Macromol. Chem. Phys.
1999, 200, 1023–1027.
13 D.-J. Liaw, B.-Y. Liaw, Polym. J. 1999, 31, 1270–1273.
14 D.-J. Liaw, C. C. Huang, W. H. Chen. Macromol. Chem.
Phys. 2006, 207, 434–443.
15 G. O. Schenck, W. Hartmann, S. P. Mannsfeld, W. Metzner,
C. H. Krauch. Chem. Ber. 1962, 95, 1642–1647.
16 H. Suzuki, T. Abe, K. Takaishi, M. Narita, F. Hamada,
J. Polym. Sci. Part A: Polym. Chem. 2000, 38, 108–116.
17 T. Matsumoto, Yuki Gosei Kagaku Kyokaishi 2000, 58, 776–
786.
18 M. I. Fremery, E. K. Fields, J. Org. Chem. 1963, 28, 2537–
2541.
ꢀ
were in the temperature range of 170–237 C, tensile strength
of 54–72 Mpa, tensile modulus of 1.6–2.3 Gpa, and elongation
at break of 4–9%. The dielectric constant of one of the synthe-
sized API (API4) was as low as 2.14. This thermally stable
ultra-low dielectric polyimide could be a very promising mate-
rial for high-temperature dielectric applications. Charge trans-
fer/electrostatic interaction in APIs induced by nucleophilic
aminomethylene group (-NCH2) was mainly responsible for
showing intermediate behavior (between aliphatic and aro-
matic polyimides) by these APIs.
19 A. Maggiolo, A. L. Tumolo, U.S. Patent 3023233, 1962.
20 G. W. Smith, H. D. Williams, J. Org. Chem. 1961, 26, 2207–
2212.
21 Y. T. Chern, W. H. Chung, J. Polym. Sci. Part A: Polym.
Chem. 1996, 34, 117–124.
22 W. Volksen, H. J. Cha, M. I. Sanchez, D. Y. Yoon, React.
Funct. Polym. 1996, 30, 61–69.
23 B. Li, T. Liu, W.-H. Zhong, Polymer 2011, 52, 5186–5192.
ꢀ
24 G. Carr, D. E. Williams, A. R. Dıaz-Marrero, B. O. Patrick, H.
Bottriell, A. D. Balgi, E. Donohue, M. Roberge, R. J. Andersen,
J. Nat. Prod. 2010, 73, 422–427.
ACKNOWLEDGMENTS
25 T. Makoto, Y. Kazuhiro, D. Matsumi, W. Masaaki, M.
Hiroyuki, K. Toshimitsu, Y. Naohisa, K. Yoshitane, Bull. Chem.
Soc. Jpn. 2001, 74, 707–715.
The work was supported by the National Research Foundation
of Korea (NRF). Grant funded by the Ministry of Science, ICT,
Future planning, Korea (Acceleration Research Program (No.
2009-0078791); Pioneer Research Center Program (No. 2010-
0019308/2010-0019482); and the Brain Korea 21 Plus
Program(21A2013800002)).
26 S. Ando, T. Fujigawa, M. Ueda, Jpn. J. Appl. Phys. 2002, 41,
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27 Gaussian 03, Revision D.02; Gaussian, Inc.: Wallingford, CT,
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28 G. Slonimskii, A. Askadskii, A. Kitaigodorodskii, Polym. Sci.
USSR 1970, A12, 556.
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