Organo-soluble phosphinated polyimides from asymmetric diamines

Article abstract of DOI:10.1016/j.polymer.2010.05.059

Two new asymmetric diamines (1-2) were prepared via a facile, one-pot procedure. Based on diamine (1-2), a series of asymmetric polyimides (3-4) were prepared in NMP/xylene by high-temperature solution polymerization. The resulting polyimides are readily

Full text of DOI:10.1016/j.polymer.2010.05.059

Polymer 51 (2010) 3899e3906
Contents lists available at ScienceDirect
Polymer
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Organo-soluble phosphinated polyimides from asymmetric diamines
*
Ching Hsuan Lin , Sheng Lung Chang, Li An Peng, Siao Ping Peng, Yu Hao Chuang
Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new asymmetric diamines (1e2) were prepared via a facile, one-pot procedure. Based on diamine
(1e2), a series of asymmetric polyimides (3e4) were prepared in NMP/xylene by high-temperature
solution polymerization. The resulting polyimides are readily soluble in some organic solvents, and can
be solution casted into flexible and creasable films. An intramolecular charge complex mechanism was
proposed to the structureeoptical transparency relationship. Polyimides 3e4 display high-Tg
(319e401 ꢀC), high moduli (2.40e7.20 GPa), moderate coefficient of thermal expansion (38e53 ppm/ꢀC),
and excellent flame retardancy. These results show that the introduction of the asymmetric structure is
an effective way to improve organo-solubility while maintaining thermal properties. Because of these
properties, polyimides 3e4 can be considered as excellent high-Tg and flame-retardant materials for
microelectronic applications.
Received 3 November 2009
Received in revised form
17 May 2010
Accepted 29 May 2010
Available online 19 June 2010
Keywords:
Asymmetric
One-pot
Phosphinate
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
not soluble in the solvent, precipitate will form during polymeri-
zation, leading to a polyimide with low molecular weight. As
Aromatic polyimides are well-known as heat-resistant material
and have been widely applied in electronics, optics, and composites
[1]. However, aromatic polyimides are difficult to process because
of their insolubility and infusibility. As a result, the processing of
polyimides is generally carried out by a two-step procedure. The
first step is the reaction of dianhydride and diamine in polar aprotic
solvent, forming poly(amic acid). The second step is the thermal
imidization of poly(amic acid) to polyimide. However, from
a industrial viewpoint, the strict moisture limitation in the first
step, and the storage instability of poly(amic acid) are drawbacks of
the two-step procedure. This prompts the development of one-step
high-temperature solution polymerization procedure, in which m-
cresol or NMP/xylene is employed as solvent [2e5]. In the m-cresol
system, the condensed water is blown away or repelled from m-
cresol at elevated temperature since it has much lower boiling
point than m-cresol. In the NMP/xylene system, condensed water is
distillated via azeotrope distillation of water/xylene during poly-
merization. One feature of high-temperature solution polymeriza-
tion is that it is suitable for diamines with low reactivity at low
temperatures. The other feature of the one-step procedure is that
the limitation of water content in solvent is not as strict as that in
the two-step procedure because the excess water will be distillated
during azeotrope distillation. However, if the resulting polyimide is
a result, the prerequisite of solution polymerization is that the
resulting polyimide must be soluble in the solvent.
Studies have reported that incorporating bulky pendant [6e12]
into polymer chains can impart better organo-solubility because of
the decreased packing density and crystallinity. Other studies have
reported that phosphorus-functionalized polymers, whether in the
phosphine oxide form [13e17], in the phosphonate [18,19] or in the
phosphinate form [20e28], can improve organo-solubility, adhesion
to metal, flame retardancy and atomic oxygen resistance. As a result,
polyimides with bulky phosphorus pendant seem to be a good
strategy for better organo-solubility and flame retardancy. Recently,
we reported a phosphinate-functionalized polyimides [10] and poly
(ether imides) [25]. However, although a bulky pendant [10,25] or
two ether linkages were present [25], the organo-solubility of the
resulting polyimides was not good. Studies have shown that incor-
porating asymmetric structure into polymer chains can impart
better organo-solubility because of the disturbed sequence distri-
bution [29e36]. Herein, with our continuing effort to prepare
organo-soluble polyimides with excellent flame retardancy, two
asymmetric diamines, 1-(4-aminophenyl)-1-(3,5-dimethyl-4-ami-
nophenyl)-1-(6-oxido-6H-dibenz <c,e> <1,2> oxaphosphorin-6-yl)
ethane (1) and 1-(4-aminophenyl)-1-(3,5-diethyl-4-aminophenyl)-
1-(6-oxido-6H-dibenz <c,e> <1,2> oxaphosphorin-6-yl)ethane (2),
were developed. Based on diamines (1e2), a series of polyimides
(3e4) were prepared from the condensation of (1e2) with various
aromatic dianhydrides in NMP/xylene by high-temperature solution
polymerization. We then investigated the chemical structures,
* Corresponding author. Tel.: þ886 4 22850180; fax: þ886 4 22854734.
E-mail address: linch@nchu.edu.tw (C.H. Lin).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.05.059

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C.H. Lin et al. / Polymer 51 (2010) 3899e3906
solubility, crystallinity, and thermal properties of polyimides 3e4.
This study also discusses the electron-withdrawing/donating effect
of 2,6-disubstituted aniline on the preparation of asymmetric
diamines (1e2) and (5). In addition, properties such as organo-
solubility, optical transparency, and thermal properties of asym-
metric polyimides were discussed and the structureeproperty
relationship was discussed.
rigid. The two flame applications were 3 s instead of 10 s. After the
first ignition, the flame was removed and the time for the polymer
to self-extinguish (t1) was recorded. Cotton ignition was noted if
polymer dripping occurred during the test. After cooling, the
second ignition was performed on the same sample and the self-
extinguishing time (t2) and dripping characteristics were recorded.
If t1 plus t2 was less than 10 s without any dripping, the polymer
was considered to be a VTM-0 material. If t1 plus t2 was in the range
of 10e30 s without any dripping, the polymer was considered to be
a VTM-1 material.
2. Experimental
2.1. Materials
2.3. Synthesis of (1)
9,10-Dihydro-oxa-10-phosphaphenanthrene-10-oxide (DOPO,
TCI), 4-aminoacetophenone (Acros), 2,6-dimethylaniline (Acros),
2,6-diethylaniline (Acros), 2,6-difluoroaniline (Acros), p-toluene-
sulfonic acid monohydrate (p-TSA, Showa) were used as received.
Pyromellitic dianhydride (PMDA, Acros) was dried at 170 ꢀC over-
night before use. 3,30,4,40-Benzophenonetetracarboxylic dianhy-
dride (BTDA, Acros), 4,40-oxydiphthalic anhydride (ODPA,
Chriskev), and 3,30,4,40-biphenyltetracarboxylic dianhydride
(BPDA, Chriskev) were recrystallized from acetic anhydride. The
solvents used are commercial products (HPLC grade) and were used
without further purification.
DOPO 25.94 g (0.12 mol), 2,6-dimethylaniline 46.14 g (0.36 mol),
4-aminoacetophenone 16.46 g (0.12 mol), and p-TSA 1.038 g (4 wt%
of DOPO) were introduced into a 250 mL round-bottom glass flask
equipped with a nitrogen inlet and a magnetic stirrer. The reaction
mixture was stirred at 130 ꢀC for 24 h. After that, the precipitate
was filtered and recrystallized from methanol, and then dried in
a vacuum oven at 150 ꢀC for 8 h. Light white crystal (75% yield) with
a melting point of 280 ꢀC (DSC) was obtained. HR-MS(FABþ) m/z:
calcd. for C28H27O2N2P 454.1810; anal., 455.1898 (M
þ
1)þ,
C28H28O2N2P. Elemental analysis of C28H27O2N2P: calculated C,
73.99%; H, 5.99%; N, 6.16%. Found: C, 73.77%; H, 5.69%; N, 5.93%. 1H
2.2. Characterization
NMR (DMSO-d6),
d
¼ 1.51 (3H,H6), 1.94 (6H,H22), 4.43 (2H,NH2),
5.04 (2H,NH2), 6.43 (2H,H2), 6.81 (2H,H20), 7.01 (2H,H3), 7.11 (1H,
NMR measurements were performed using a Varian Inova 600
NMR in DMSO-d6, and the chemical shift was calibrated by
setting the chemical shift of DMSO-d6 as 2.49 ppm. IR spectra
were obtained in the standard wavenumber range of
400e4000 cmꢁ1 by PerkineElmer RX1 infrared spectrophotom-
eter. High-resolution mass (hr-ms) measurements were per-
H
11), 7.14 (1H,H17), 7.16 (1H,H15), 7.32 (1H,H10), 7.34 (1H,H16), 7.64
(1H,H9), 7.95 (1H,H14), 8.04 (1H,H8). 13C NMR (DMSO-d6),
d
¼ 18.11
(C22), 24.21 (C6), 51.45 (C5), 113.09 (C2), 118.85 (C17), 119.80 (C12),
120.93 (C13), 122.99 (C7), 123.12 (C8), 123.65 (C15), 125.23 (C14),
126.40 (C16), 127.62 (C10), 128.05 (C20), 129.43 (C3), 130.05 (C21),
130.45 (C19), 131.89 (C11), 132.97 (C9), 135.96 (C4), 142.56 (C23),
formed by
a
Finnigan/Thermo Quest MAT 95XL mass
147.29 (C1), 150.81 (C18). 31P NMR, (DMSO-d6),
synthesis of diamine (1) is shown in Scheme 1.
d
¼ 42.4(s). The
spectrometer by fast atom bombardment. Elemental analysis was
performed on an Elementar Vario EL III. Wide-angle X-ray
diffraction measurements were performed at room temperature
14 15
1817
by a MAC Science DMAX2000 X-ray diffractometer with mono-
11
12
10
8
ꢀ
chromatic Cu Ka radiation (
l
¼ 1.5418 A, operating at 40 kV and
16
23
9
13
30 mA). The scanning rate was 3ꢀ/min over a range of 2
q
¼ 5e40ꢀ.
7
Differential scanning calorimeter (DSC) scans were obtained by
a PerkineElmer DSC 7 in a nitrogen atmosphere at a heating rate
of 20 ꢀC/min. Tg was taken as the midpoint of the heat capacity
transition between the upper and lower points of deviation from
the extrapolated liquids and glass lines. Dynamic mechanical
analysis (DMA) was performed with a PerkineElmer Pyris Dia-
mond DMA with a sample size of 5.0 ꢂ 1.0 ꢂ 0.002 cm. The
O
P O
1
5
19
H2N
NH2
4
21
22
3
20
2
6
2.4. Synthesis of (2)
storage modulus E0 and tan
d were determined as the sample was
subjected to the temperature scan mode at a programmed heat-
Diamine (2) was synthesized with a procedure similar to (1)
using 2,6-diethylaniline as the starting material. White crystal (70%
yield) with a melting point of 246 ꢀC (DSC) was obtained. HR-MS
(FABþ) m/z: calcd. for C30H31O2N2P 482.2123; anal., 483.2199
(M þ 1)þ, C30H32O2N2P. Elemental analysis of C30H31O2N2P: calcu-
lated C, 74.67%; H, 6.48%; N, 5.81%. C, 74.21%; H, 6.66%; N, 5.79%. 1H
ing rate of 5 ꢀC/min at a frequency of 1 Hz. The test was per-
formed by tension mode with an amplitude of 25 mm. Thermo
mechanical analysis (TMA) was performed by a SII TMA/SS6100 at
a heating rate of 5 ꢀC/min. The coefficient of thermal expansion
(CTE) was measured in the range from 100 ꢀC to 250 ꢀC. Thermal
gravimetric analysis (TGA) was performed by a Seiko Exstar 600
at a heating rate of 20 ꢀC/min in a nitrogen atmosphere from
105 ꢀC to 800 ꢀC. Gel permeation chromatography (GPC) was
carried out on a Hitachi L2130 with a UV detector (L2400) using
N,N-dimethylformamide (DMF) as the eluent at 60 ꢀC with a flow
rate of 1.0 mL/min. The data were calibrated with polystyrene
standard.
Optical rotation is detected by a PerkineElmer 241 polarimeter.
Flame retardancy of polyimides was performed by a UL-94VTM
vertical thin test. In that test, an 8” ꢂ 2” sample was wrapped
around a 1/2” mandrel, and then taped on one end. The mandrel
was removed, leaving a cone-shaped sample that was relatively
NMR (DMSO-d6),
d
¼ 0.99 (3H,H23), 1.57 (3H,H6), 2.30 (2H,H22), 1.94
(6H,H22), 4.41 (2H,NH2), 5.03 (2H,NH2), 6.43 (2H,H2), 6.80 (2H,H20),
7.04 (2H,H3), 7.07 (1H,H11), 7.12 (1H,H15), 7.22 (1H,H17), 7.32 (1H,
H
10), 7.34 (1H,H16), 7.63 (1H,H9), 7.91 (1H,H14), 8.02 (1H,H8). 13C
NMR (DMSO-d6),
d
¼ 13.18 (C23), 23.88 (C22), 24.42 (C6), 51.35 (C5),
113.04 (C2), 118.73 (C11), 120.90 (C12), 122.72 (C7), 123.18 (C8), 123.57
(C15), 125.23 (C14), 125.64 (C20), 126.05 (C13), 126.54 (C21), 127.63
(C10), 129.45 (C19), 130.05 (C3), 130.41 (C16), 131.84 (C17), 132.93 (C9),
135.95 (C4), 141.19 (C24), 147.24 (C1), 150.75 (C18). 31P NMR (DMSO-
d6),
d
¼ 38.5(s). The synthesis of diamine (2) is shown in Scheme 1.
Data from single crystal diffractogram of (2) were listed in Sup-
porting information.

C.H. Lin et al. / Polymer 51 (2010) 3899e3906
3901
substitution. The disappearance of methyl signal (H6) suggests that
an elimination reaction between the methyl and hydroxy groups
occurred in acidic conditions. Detailed researches for the elimina-
tion reaction in acidic conditions and synthesis of (5) in basic
condition [40] are under investigation. From above discussion, the
electron-donating or electron-withdrawing characteristic of 2,6-
disubstituents is crucial for the proceeding of electrophilic substi-
tution. 2,6-Disubstituted aniline with the electron-donating methyl
or ethyl groups can carry out the electrophilic substitution. In
contrast, 2,6-disubstituted aniline with the electron-withdrawing
fluoro group cannot accomplish the electrophilic substitution.
14 15
1817
11
12
10
8
16
24
9
13
7
O
P O
1
5
19
H2N
NH2
4
21
3
20
2
6
23
22
2.5. Preparation of polyimides
Polyimides syntheses are exemplified by the synthesis of 3a
from the condensation of (1) and PMDA. To a 100-mL three-neck
round-bottom flask equipped with a magnetic stirrer, a DeaneStark
trap, and a nitrogen inlet, (1) 1.3536 g (3 mmol), PMDA 0.6544 g
(3 mmol) and NMP 7.0 g were added. After the monomer had dis-
solved completely, xylene 7 g was added. The reaction was carried
out at reflux temperature for 12 h. Then, the viscous polyimides
solution was casted on glass by an automatic film applicator, and
dried at 80 ꢀC overnight and 100 ꢀC (1 h), 200 ꢀC (2 h), respectively.
The other polyimides (3be3d, 4ae4d) were similarly prepared. The
synthetic equation of polyimides 3e4 is shown in Scheme 2.
3.2. Characterization of (1e2)
Fig. 1 shows the 1H NMR spectra of (1). Two amino peaks at 5.0
and 4.4 ppm confirm the asymmetric structure of diamine (1). The
signals of methyl group at 1.5 ppm were split into two peaks with
3
a coupling constant of 17.4 Hz because of a JPeH coupling. The
assignment of AreH, assisted by the correlations in the 1He1H
COSY NMR spectrum (Fig. 2) (Fig. S1) confirms the structure of (1).
In the 13C NMR spectrum, due to the PeC 1J coupling, the signals
of C5 were split into two peaks at 51.1 and 51.8 ppm with
a coupling constant of 105 Hz. The signals of C7 were also split
into two peaks at 122.6 and 123.4 ppm with a coupling constant
of 120 Hz for the same reason. The assignment of AreC, assisted
by the correlations in the 1He13C HETCOR NMR spectrum (Fig. S2)
also confirms the structure of (1). In the 31P NMR spectrum, only
one 31P NMR peak was observed at 42.4 ppm, indicating both the
purity and the integrity (no ring opening) of the biphenylene
phosphinate pendant. 1H, 13C, 1He1H COSY and 1He13C HETCOR
NMR spectra of (2) were reported in the supplementary infor-
mation (Figs. S3eS6), and the spectroscopic data supported the
structure of (2).
3. Results and discussion
3.1. Synthesis of (1e2)
Asymmetric diamine (1e2) were prepared by a one-pot reaction
of DOPO, 4-aminoacetophenone in excess 2,6-dimethylaniline (or
2,6-diethylaniline) using p-TSA as the catalyst. The excess 2,6-
dimethylaniline (or 2,6-diethylaniline) played the roles of both
reactant and reaction medium. Generally, aromatic diamines are
prepared from reduction of their nitro-containing precursors,
except for those prepared acid catalyzed condensation of ketone or
aldehyde compounds with excess aniline [37e39]. However, cata-
lytic hydrogenation requires high pressure, an expensive palla-
dium-on-carbon catalyst, and large investment in facilities for
industrial-scale production. Thus, this non-reduction, one-pot
synthesis makes diamines (1e2) attractive for to industrial appli-
cations. Although some aromatic diamines can also be prepared by
a one-pot procedure [37e39], however, only symmetric diamines
can be obtained. In this work, asymmetric diamines can easily be
prepared by using 2,6-disubstituted aniline as reactant. To the best
of our knowledge, this is the first case of producing asymmetric
diamines in a one-pot procedure. In addition to (1e2), we also
attempted to prepare asymmetric diamine (5) using 2,6-difluoro-
aniline as a starting material (Scheme 3). However, the synthesis
was unsuccessful under various acidic conditions. According to 1H
NMR analysis of the reaction product, the methyl signal (H6) was
disappeared. Obviously, the electron-withdrawing fluoro groups
deactivated the electrophilicity of aniline for the electrophilic
Since the phosphorus and aliphatic carbon in diamines (1e2)
are both chiral centers, four stereoisomers should result. Theoret-
ically, each stereocenter can be either R or S configuration, and
hence, the possible combinations are RR, RS, SR, and SS configu-
rations. The four stereoisomers can be grouped into two pairs of
enantiomers, resulting in two pairs of diastereomers: RR þ SS and
RS þ SR. The RR stereoisomer is the enantiomer of the SS stereo-
isomer, and is diastereomerically related to the RS and SR stereo-
isomers. In contrast with enantiomers, diastereomers are distinct
molecules with different physical and chemical properties because
they are not mirror images of each other. However, according to the
1H and 13C NMR spectra of (1e2), there is no evidence shown that
two pairs of diastereomers were obtained in this synthesis. In
addition, no optical rotation is observed in a polarimeter. That is,
only racemic mixture with equal RR and SS configurations, or equal
RS and SR configurations were obtained. At this stage, no reason-
able explanation is provided. A more detailed research is required
to explain this phenomenon.
R
NH2
excess
R
R
O
P O
O
R
NH2
+ H-O-H
H2N
C
H2N
O
P O
H
p-TSA
R = CH3 for (1))
R = C2H5 for (2)
Scheme 1. One-pot synthesis of diamine (1e2).

3902
C.H. Lin et al. / Polymer 51 (2010) 3899e3906
O
O
R
O
P O
H2N
NH2
O
O
Ar
+
(1),(2)
R
R
O
O
O
N
O
O
P
O
R = CH3 for (1),(3)
NMP / Xylene
N
Ar
R = C2H5 for (2),(4)
n
R
O
O
R = H for ref PI
(3a-3d),(4a-4d)
Ar :
O
O
C
(a)
(c)
(b)
(d)
Scheme 2. Synthesis of polyimides 3e4.
F
excess
NH2
F
O
P O
O
F
NH2
+ H-O-H
H2N
C
H2N
H+
O
P O
H
(1)
F
Scheme 3. Synthesis of diamine (5). However, the synthesis is unsuccessful due to poor reactivity of 2,6-difluoroaniline.
Fig. 1. 1H NMR spectrum of diamine (1).

C.H. Lin et al. / Polymer 51 (2010) 3899e3906
3903
Fig. 2. 13C NMR spectrum of diamine (1).
3.3. Polyimide synthesis
the steric hindrance. However, to prevent precipitate during
polymerization, the polyimides should be soluble in the solvent.
In this case, the bulky pendant, together with the asymmetric
linkage, makes polyimides 3e4 soluble in the reaction solvent. In
the IR spectrum of 3a, the characteristic peaks of imide at
1776 cmꢁ1 (C]O asymmetric stretch) and 1723 cmꢁ1 (C]O
symmetric stretch), 1379 cmꢁ1 (CeN stretch), 1113 and 721 cmꢁ1
(imide ring deformation) confirm the complete imidization.
Polyimides 3e4 were prepared by reacting (1e2) with equal
mole of dianhydrides (aed) in NMP/xylene by high-temperature
solution polymerization. According to the principle of NMR,
a larger chemical shift of an amino proton means a lower elec-
tron density of amino group, and corresponds to a lower reac-
tivity. The chemical shifts of the amino group of (1e2) are
around 5.0 and 4.4 ppm, which is similar to that (5.0 ppm) dia-
minodiphenylmethane. This suggests that, from the viewpoint of
electron density, the reactivity of (1e2) is high. Initially, we
prepared polyimides 3e4 via the standard two-step procedure.
However, the viscosity of poly(amic acid) in the first step was not
very high. We speculated that the steric hindrance resulting from
the dimethyl groups reduced the reactivity. Therefore, a high-
temperature solution polymerization was applied to overcome
The peaks of P]O absorption at 1202 cmꢁ1
, and PeOePh
absorption at 923 cmꢁ1 indicate that the biphenylene phosphi-
nate pendant did not cleave. As listed in Table 1, the number-
average molecular weights range from 1.7 to 5.9 ꢂ 104 g/mol, and
the weight-average molecular weights range from 3.2 to
8.9 ꢂ 104 g/mol, demonstrating moderate-to-high molecular
weight, and reflecting high-purity and reactivity of (1e2) at high
temperature.
Table 1
Molecular weight and solubility data of polyimides 3e4.
Polyimides
GPC dataa
Solubility datac
Mn ꢂ 10ꢁ4
Mw ꢂ 10ꢁ4
NMP
DMAc
DMF
DMSO
m-Cresol
CHCl3
CH2Cl2
THF
b
3a
3b
3c
3d
4a
4b
4c
4d
1.7
2.9
3.8
3.4
1.7
3.1
4.9
5.9
3.5
5.2
8.5
6.7
3.2
5.1
8.5
8.9
þ(ꢃ)d
þ(ꢃ)
þ(ꢃ)
þ(ꢃ)
þ
þ(ꢃ)
þ(ꢃ)
þ(ꢃ)
þ(ꢃ)
þ
þ(ꢃ)
þ(ꢃ)
þ(ꢃ)
þh(ꢃ)
þ
þh(ꢁ)
þ(ꢁ)
þh(ꢁ)
þh(ꢁ)
þ
þ(þ)
þ(ꢃ)
þ(ꢃ)
þ(þ)
þ
þh(ꢁ)
þ(þ)
þh(ꢁ)
þ(ꢃ)
þ
þ(ꢁ)
þ(þ)
ꢃ(ꢁ)
þ(ꢃ)
ꢃ
ꢁ(ꢁ)
ꢁ(ꢁ)
ꢁ(ꢁ)
ꢁ(ꢁ)
ꢃ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
þ
ꢃ
þ
þ
þ
þh
þ
þ
þ
ꢃ
a
Relative to polystyrene standard, using DMF as eluent.
Mn: number-average molecular weight, Mw: weight-average molecular weight.
Solubility was tested with a 5 mg sample in 0.5 g of solvent at room temperature. þ, soluble; þh, soluble on heating; ꢃ, partially soluble on heating; ꢁ, insoluble on
b
c
heating.
d
Data in parentheses are those of reference polyimides.

3904
C.H. Lin et al. / Polymer 51 (2010) 3899e3906
Fig. 5. DMA thermograms of polyimides 3.
Fig. 3. Pictures of ref PI, 3a and 4a.
yellow to amber in color, and the intermolecular charge-transfer
complex (CTC) formation is responsible for the color. It has been
reported that the intermolecular CTC formation can be reduced by
introducing a bulky pendant group to prevent the polyimide chains
from being well-stacked [41,42]. However, as presented in the
picture, although a bulky phosphinate and two alkyl pendants were
presented, polyimides 3a and 4a are not transparent enough, and
are even darker than the ref PI, in which no alkyl groups were
presented. Obviously, the dialkyl groups have negative effect on the
transparency of phosphinate polyimides. The reason might be
explained by the unusual orientation of phosphinate pendant
shown in the single crystal diffractogram of (2) (Fig. 4). Instead of
being distant from the diethyl groups to reduce the steric and
electronic repulsion, the phosphinate inclined to the diethyl
groups. As a result, it is speculated that the unusual orientation is
related to the interaction between phosphorus and diethyl groups.
Judging from the structure of (2), the phosphorus element is
electron-deficient due to the electron-withdrawing effect of two
adjacent oxygen atoms. In contrast, the diethyl groups are electron-
donating groups, which can stabilize the electron-deficient
phosphorus via a intramolecular interaction. As a result, the
intramolecular interaction explained the unusual orientation of
phosphinate pendant. At present, it is speculated that the charge-
transfer complex should be responsible for the darker colors of
polyimides 3e4. However, a detailed investigation is needed to
prove the speculation.
3.4. Properties of the polyimides
3.4.1. Organo-solubility and crystallinity
The solubility data of polyimides 3e4 are listed in Table 1. As
shown in Table 1, asymmetric polyimides 3e4 display better
organo-solubility than the ref PIs. The asymmetric structure, which
leads to the formation of configuration isomers of the repeat unit
and resulted in the disruption of packing sequence, should be
responsible for the improved solubility. Among them, polyimides 4
show better solubility than polyimides 3 due to the increased free
volume caused by ortho-diethyl groups. According to the patterns
of wide-angle X-ray diffraction of polyimides 3e4 (Fig. S7), all
polyimides are amorphous. The bulky biphenylene phosphinate,
together with the asymmetric linkage, should be responsible for
the amorphous characteristic and improved solubility.
3.4.2. Film quality and color
Fig. 3 shows pictures of ref PI, 3a and 4a. All polyimide films are
creasable and tough although no flexible ether linkages were
present in their structure. Generally, polyimide films are typically
3.4.3. Dynamic and thermal mechanical properties
Since polyimides 3e4 are flexible and creasable, DMA and
TMA were applied to evaluate their thermal mechanical prop-
erties and dimensional stability. Fig. 5 shows the DMA thermo-
grams of polyimides 3. The Tg obtained from the peak
temperature of tan(d
) is as high as 401, 327, 341 and 371 ꢀC for
3ae3d, respectively. Compared with Tgs of the ref PIs (Table 2),
one can find Tgs of polyimide 3 are enhanced. This result indi-
cates that the ortho-dimethyl groups hinder the rotation of
polyimide chains, and thus explains the higher Tg phenomenon.
However, as listed in Table 2, Tgs were reduced when ortho-
diethyl groups instead of ortho-methyl groups were incorpo-
rated. The increased free volume and flexibility of ethyl group
may be responsible for the reduced Tg. Fig. 6 shows a typical
TMA curve of 3b. A CTE of 38 ppm/ꢀC was obtained, which is
relatively low for an organo-soluble polyimide without bi-axial
orientation. Table 2 lists TMA data of other polyimides. The CTEs
of polyimides 3 ranged from 38 to 48 ppm/ꢀC, and Tgs from TMA
Fig. 4. Single crystal diffractogram of (2).

C.H. Lin et al. / Polymer 51 (2010) 3899e3906
3905
Table 2
Thermal and mechanical properties of polyimides 3e4.
E0 (GPa)a
Tan
d
(ꢀC)a
Tg (ꢀC) (TMA)b
CTEc (ppm/ꢀC)
Td5 (ꢀC)
Char yield (%)e
d
Polymer
Film quality
N2
Air
N2
Air
3a
3b
3c
3d
4a
4b
4c
4d
Creasable
Creasable
Creasable
Creasable
Creasable
Creasable
Creasable
Creasable
7.20
4.64
4.72
4.46
4.63
2.76
4.25
2.40
401 (392)f
327 (318)
341 (326)
371 (343)
365
390 (388)f
43 (42)f
460
449
448
453
454
441
461
459
443
444
445
438
433
434
445
433
57
56
57
59
59
56
58
58
52
47
51
46
51
45
51
45
317 (304)
328 (312)
351 (330)
e
48 (48)
44 (50)
38 (43)
51
319
329
347
307
317
332
53
42
42
a
Measured by DMA; heating rate, 5 ꢀC/min; storage modulus (E0) are recorded at 50 ꢀC.
Measured by TMA; heating rate, 10 ꢀC/min.
b
c
d
e
f
Coefficient of thermal expansion is recorded at 100e250 ꢀC.
The 5% decomposition temperature.
Residual wt% at 800 ꢀC in nitrogen.
Data in parentheses are those of reference polyimides.
contrast to the electro-donating dialkyl groups that activate the
electrophilic substitution, the electron-withdrawing difluoro
groups deactivate the electrophilic substitution. This explains the
difficulty in preparing diamine (5). The asymmetric polyimides
3e4 display better organo-soluble property than the ref PIs. The
asymmetric structure, which leads to the formation of configu-
ration isomers of the repeat unit and results in the disruption of
packing sequence, is responsible for the improved solubility.
Although a bulky phosphinate and two alkyl pendant were pre-
sented, polyimides 3e4 are not transparent enough, and are even
darker than the ref PIs. From the unusual orientation of phos-
phinate pendant shown in the single crystal diffractogram of (2), it
is thought that the darker color of polyimides 3e4 is due to the
intramolecular charge-transfer complex. Even though with darker
color, the combination of flexibility, high glass transition
temperature, improved organo-solubility, and excellent flame
retardancy makes polyimides 3e4 promising polymers electronic
applications, such as in halogen-free flexible copper clad laminate.
1.000
0.500
==Av.Expansion(DMMA-BPDA0404100)==
Cel Cel
(1/cel)
100.1 - 250.0 3.754510E-05
350.5Cel
0.198mm
0.000
0.0
100.0
200.0
Temp Cel
300.0
400.0
Fig. 6. TMA curve of 3b.
ranged from 317 to 390 ꢀC (Table 2). Compared with those of
reference polyimides, Tg and CTE of polyimides 3 are both
enhanced, displaying the high-Tg characteristic and good
dimensional stability of ortho-dimethyl substituted polyimides.
In contrast, the CTE of polyimides 4 was slightly higher than
those of polyimide 3. The result may also be attributed to the
increased free volume caused by ortho-diethyl group (Fig. 6).
Appendix. Supplementary data
Supplementary data associated with this article can be found in
online version at doi:10.1016/j.polymer.2010.05.059.
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