Novel soluble and optically active polyimides containing axially asymmetric 9,9′-spirobifluorene units: Synthesis, thermal, optical and chiral properties

Article abstract of DOI:10.1016/j.polymer.2012.10.024

A new optically active spirobifluorene-based diamine monomer, (R)-2,2′-bis(4-amino-2-trifluorom-ethylphenoxy)-9,9′- spirobifluorene, was synthesized using (R)-2,2′-dihydroxy-9,9′- spirobifluorene as starting material. The absolute stereochemistry of the chiral diamine was determined by single crystal X-ray diffraction. A series of novel chiral polyimides were prepared from the diamine with various aromatic dianhydrides via a two-stage process with chemical imidization method. All these optically active polyimides were readily soluble in many organic solvents and could afford flexible, tough and transparent films with an UV-vis absorption cutoff wavelength at 351-375 nm. The glass transition temperatures of these polyimides were recorded between 262 and 281°C by differential scanning calorimetry, and the 5% weight loss occurred at temperatures above 510°C. The specific rotations of these chiral polyimides ranged from -48.5°to +73.0°depending on the structures of the aromatic dianhydrides. All the optically active polyimides exhibited high chiral stability in solid state or in DMAc solvent at high temperature due to introduction of chiral spirobifluorone group into polymer backbones. The circular dichroism (CD) spectroscopic properties of these chiral polyimides in tetrahydrofuran solutions were also studied.

Full text of DOI:10.1016/j.polymer.2012.10.024

Polymer 53 (2012) 5706e5716
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Novel soluble and optically active polyimides containing axially asymmetric
9,90-spirobifluorene units: Synthesis, thermal, optical and chiral properties
*
Zuolin Wu, Baochun Han, Chunhua Zhang, Danyang Zhu, Lianxun Gao, Mengxian Ding, Zhenghua Yang
Polymer Composites Engineering Laboratory, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
A new optically active spirobifluorene-based diamine monomer, (R)-2,20-bis(4-amino-2-trifluorom-
ethylphenoxy)-9,90-spirobifluorene, was synthesized using (R)-2,20-dihydroxy-9,90-spirobifluorene as
starting material. The absolute stereochemistry of the chiral diamine was determined by single crystal X-
ray diffraction. A series of novel chiral polyimides were prepared from the diamine with various aromatic
dianhydrides via a two-stage process with chemical imidization method. All these optically active pol-
yimides were readily soluble in many organic solvents and could afford flexible, tough and transparent
films with an UVevis absorption cutoff wavelength at 351e375 nm. The glass transition temperatures of
these polyimides were recorded between 262 and 281 ꢀC by differential scanning calorimetry, and the 5%
weight loss occurred at temperatures above 510 ꢀC. The specific rotations of these chiral polyimides
ranged from ꢁ48.5ꢀ to þ73.0ꢀ depending on the structures of the aromatic dianhydrides. All the optically
active polyimides exhibited high chiral stability in solid state or in DMAc solvent at high temperature due
to introduction of chiral spirobifluorone group into polymer backbones. The circular dichroism (CD)
spectroscopic properties of these chiral polyimides in tetrahydrofuran solutions were also studied.
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 10 July 2012
Received in revised form
6 October 2012
Accepted 12 October 2012
Available online 22 October 2012
Keywords:
Polyimides
Optically active
Spirobifluorene
1. Introduction
have been investigated [12e16]. However, most of aromatic poly-
imides have high melting or glass transition temperatures (Tg) and
Chiral phenomena play important roles in nature. Many
important naturally occurring polymers, such as proteins, DNA, and
polysaccharides are optically active. The design, synthesis and
characterization of chiral polymers are of particular interest from
the viewpoint of material science. Chiral polymers have been
successfully used in chiral chromatographic separation of enan-
tiomers [1e3], chiral liquid crystals [4,5], non-liner optical devices
[6], optical switches [7], biodegradable materials [8], etc. A direct
and effective way for synthesizing optically active polymers is to
introduce chiral elements into the polymer backbone or the side
chains.
Aromatic polyimides are well known as high performance
polymer materials because of their good chemical resistance, high
mechanical strength, and excellent thermal stability [9e11]. The
introduction of chiral structure units into polyimide backbone is
expected to affect the packing of polymer chains and impose
noncentrosymmetric structures and may, therefore, lead to
different physical properties. Recently, several optically active
polyimides have been synthesized, and their chiroptical properties
limited solubility in most organic solvents because of their rigid
backbones and strong intermolecular interactions, which may
restrict their applications in some fields. Therefore, many studies
have been made to enhance their processability and solubility
while other advantageous polymer properties are retained either
by introducing flexible linkages [17e21], bulky lateral groups [22e
26], noncoplannar structures [27e29] and spiroskeletons [30,31]
into the polymer backbones. Recent studies demonstrated that
polyimides derived from ether bridged aromatic diamines with
trifluoromethyl (CF3) group were generally soluble high tempera-
ture polymer materials with low dielectric constant and high
optical transparency [32e36].
It has been reported that the spirobifluorene monomer consists
of two identical fluorene moieties, connected through a common
tetra coordinate carbon atom. In the spiro segment, the rings of the
connected bifluorene entities are orthogonally arranged. The
resulting polyimides would be expected to have a polymer back-
bone periodically twisted by an angle of 90ꢀ at each spiro center.
This structure would restrict the close packing of the polymer
chains, thereby reducing the probability of interchain interactions,
resulting in higher polymer solubility [37e41]. Additionally, for the
spiroannulated segment, the rigidity of the polymer backbone
would be preserved. More importantly, in spiro segment, two rings
* Corresponding author. Tel.: þ86 431 85262272; fax: þ86 431 85685653.
E-mail address: zhhyang@ciac.jl.cn (Z. Yang).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.10.024

Z. Wu et al. / Polymer 53 (2012) 5706e5716
5707
connected at a quaternary center through
s
-bonds, which make
with an Ubbelodhe viscometer at 30 ꢀC. Molecular weights were
obtained via gel permeation chromatography (GPC) on the basis of
polystyrene calibration using Waters 2410 as an apparatus and N,N-
dimethylformamide (DMF) as the eluent. Differential scanning
calorimetry (DSC) was performed on a PerkineElmer DSC-7 system
at a heating rate of 20 ꢀC minꢁ1 under nitrogen flow. Glass transi-
tion temperature (Tg) was read as the midpoint temperature of the
heat capacity jump and was taken from the second heating scan
after a quick cooling down from 360 ꢀC to room temperature.
Thermogravimetric analysis (TGA) was performed under nitrogen
and air atmosphere on a PerkineElmer TGA-6 thermobalance at
a heating rate of 10 ꢀC minꢁ1. Dynamic mechanical thermal analysis
(DMA) was performed on thin film specimens (length: 30 mm,
racemization of chiral spiro compounds virtually impossible [42].
Recently, a large number of chiral small molecule derived from
optically active 1,10- or 2,20- dihydroxyspirobifluorene have been
synthesized and used in asymmetric catalysis with excellent
enantioselectivities [42e47]. However, up to now, the literature on
the structure and properties of optically active polymers with chiral
spirobifluorene unit are few [48,49].
In this article, we reported the synthesis of a new optically active
diamine monomer, (R)-2,20-bis(4-amino-2-trifluoromethylpheno-
xy)-9,90-spirobifluorene, and its use in the preparation of soluble
and chiral polyimides by the reaction of the diamine with aromatic
dianhydrides. To the best of our knowledge, this is the first case of
producing opticallyactivepolyimides containing axiallyasymmetric
9,90-spirobifluorene skeleton at the polymer backbones. The solu-
bility, crystallinity, thermal, optical and chiral properties of these
polyimides were investigated. Optically inactive polyimides based
on (ꢂ)-2,20-bis(4-amino-2-trifluoromethylphenoxy)-9,90-spirobi-
fluorene have also been prepared for comparison.
width: 6.5 mm, and thickness: 30e60
DMA RSA II at a heating of 5 ꢀC minꢁ1, with a load frequency of 1 Hz
in air. The peak on the tan as a function of temperature curves was
mm) by using a TA instrument
d
regarded as the Tg of the films. The tensile properties were per-
formed on an Instron 3365 Tensile Apparatus with a load cell of 5 kg
at a drawing speed of 5 mm minꢁ1 on strips approximately 50e
80
mm thick and 0.5 cm wide with a 4 cm gauge length. An
2. Experimental
average of at least five individual determinations was used. Water
uptakes were determined by weighing the changes of polyimide
films before and after immersion in deionized water at 25 ꢀC for 3
days. Wide-angle X-ray diffraction (WAXD) measurements were
performed at room temperature with a Siemens Kristalloflex
2.1. Materials
(R,R)-(þ)-2,3-dimethoxy-N,N,N0,N0-tetracyclohexylsuccinamide
was synthesized in five steps from (R,R)-(þ)-tartaric acid [50,51].
Optically active ((R)-(þ)) and inactive 2,20-dihydroxy-9,90-spirobi-
fluorene were prepared according to the literature [44]. Commer-
cially available 2-fluoro-5-nitrobenzotrifluoride (from Aldrich),
potassium carbonate (from Acros), hydrazine monohydrate (from
Acros) and 10% palladium on activated charcoal (Pd/C) (from Fluka)
were used as received. 4,40-oxydiphthalic anhydride (ODPA, from
Acros) and 3,30,4,40-benzophenonetetracarboxylic dianhydride
(BTDA, from Acros) were recrystallized from acetic anhydride. 2,20-
bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA,
from TCI) and 3,30,4,40-biphenyltetracarboxylic dianhydride (BPDA,
from TCI) were sublimated before use. N,N0-dimethylacetamide
(DMAc, from FLuka) was purified by distillation under reduced
D5000 X-ray diffractionmeter (Cu K
a, 40 kV, 30 mA), and samples
were scanned from 2ꢀ to 60ꢀ at 2ꢀ minꢁ1
.
2.3. Monomer synthesis
2.3.1. Optical resolution of (ꢂ)-2,20-dihydroxy-9,90-spirobifluorene
(ꢂ)-2,20-dihydroxy-9,90-spirobifluorene(DHSBF, 2.0g, 5.74mmol)
and (R,R)-(þ)-2,3-dimethoxy-N,N,N0,N0-tetracyclohexylsuccinamide
((R,R)-(þ)-DMTCHS, 2.90 g, 5.74 mmol) were dissolved in ethanol
(25 mL). The mixture was stirred at room temperature for 12 h. The
precipitated white solid was collected by filtration. The crude product
was recrystallized from ethanol three times to give a complex [(1:1
clathrate of (R)-DHSBF and (R,R)-(þ)-DMTCHS)] as colorless prisms
(2.08 g, 85%). Mp: 237e239 ꢀC (literature [44] 236e238 ꢀC). The
complex was dissolved in 50 mL of benzene and washed with 1 N
NaOH. The aqueous layer was acidified by concentrated HCl and
extracted with ether. The ether phase was dried with Na2SO4 and
ꢀ
pressure over calcium hydride and stored over 4 A molecular sieves.
Other commercially available materials were used without further
purification.
2.2. Measurement
concentrated to give (R)-4 (0.83 g, 83%) in 99% ee. ½a D20
ꢃ
¼ þ27.2ꢀ
(c ¼ 0.50 g/dL, methanol), literature [44] þ27.1ꢀ.
1H NMR and 13C NMR spectra were recorded on a Brucker AMX
600 spectrometer with chloroform-d (CDCl3) or dimethylsulfoxide-
d6 (DMSO-d6) as the solvents, and tetramethylsilane (TMS) as the
internal reference. IR spectra were taken with a Nicolet 360 FTIR
spectrometer. Elemental analysis was run on an Elementar Vario EL
instrument. Ultravioletevisible (UVevis) spectra were recorded on
a Beckman DU-600 UVevis spectrophotometer. Melting points
were measured by WBR-IB melting point digital apparatus. MALDI-
FTICR MS spectra were taken on a Bruker Apex II FTICR mass
spectrometer. Chromatographic separation of the racemates was
accomplished on a JASCO HPLC system equipped with a UV
(254 nm) detector using a DAICEL eAD-H column. The X-ray
structure measurement was performed on a Rigaku Satum 724 CCD
The mother liquid of the resolution was concentrated under
reduced pressure and the residue was purified by column chro-
matography over silica gel (n-hexane/ethyl acetate 2:1) to yield (S)-
4 (0.92 g, 92%) in 93% ee. ½a D20
ꢃ
¼ ꢁ25.3ꢀ(c ¼ 0.50 g/dL, methanol).
The ee values of the resolution products were determined by
HPLC analysis on DAICEL-AD-H column. Hexane/i-PrOH ¼ 8:2, flow
rate ¼ 1 mL minꢁ1 tR ¼ 7.0 min, tS ¼ 17.0 min.
2.3.2. (R)-2,20-bis(4-nitro-2-trifluoromethylphenoxy)-9,90-
spirobifluorene
(R)-2,20-dihydroxy-9,90-spirobifluorene (3.48 g, 10.0 mmol) and
2-fluoro-5-nitrobenzotrifluoride (4.39 g, 21.0 mmol) were dis-
solved in 35 mL of N,N0-dimethylacetamide (DMAc) in an l00-mL
round-bottomed flask with stirring. Then, potassium carbonate
(2.76 g, 20.0 mmol) was added, and the mixture was heated to 60 ꢀC
for 6 h. After cooled to room temperature, the solution was poured
into 120 mL of water. The precipitate was collected by filtration and
dried under vacuum. The crude product was recrystallized from
DMF/methanol to give pale-yellow crystal (6.61 g, 91.1%). mp: 120e
single crystal X-ray diffractmeter. Detector equipped with graphite
monochromatic Mo K
ꢀ
a
radiation (
l
¼ 0.7107 A). Structure analysis
was performed utilizing the SHELXTL/PC program. Specific optical
rotations were measured on a JASCO P-1020 digital polarimeter at
20 ꢀC with a sodium lamp as the light source. Circular dichroism
(CD) spectra were carried out at 25 ꢀC on a JASCO J-700 apparatus
fitted with a cell of an optical path length of 1 mm. Inherent
viscosities were measured at a concentration of 0.5 g/dL in DMAc
122 ꢀC ½a 2D0
ꢃ
¼ ꢁ26.9ꢀ (c ¼ 0.50 g/dL, DMAc). FT-IR (kBr, cmꢁ1): 1530,
1351 (eNO2), 1259 (AreOeAr), 1143 (eCF3). 1H NMR (600 MHz,

5708
Z. Wu et al. / Polymer 53 (2012) 5706e5716
CDCl3,
d
, ppm): 8.53 (d, J ¼ 2.4 Hz, 2H, Hj), 8.21 (dd, J ¼ 9.6, 2.4 Hz,
washed thoroughly with ethanol, and dried under vacuum at
100 ꢀC to produce the corresponding polyimide (R)-7a. The poly-
mer was further purified by reprecipitation from DMAc into
ethanol twice.
2H, Hi), 7.89 (d, J ¼ 8.4 Hz, 2H, He), 7.83 (d, J ¼ 7.8 Hz, 2H, Hd), 7.42
(dd, J ¼ 7.8, 7.2 Hz, 2H, Hc), 7.19 (dd, J ¼ 7.8, 7.2 Hz, 2H, Hb), 7.11 (dd,
J ¼ 8.4, 2.4 Hz, 2H, Hf), 6.83 (d, J ¼ 9.6 Hz, 2H, Hh), 6.79 (d, J ¼ 7.8 Hz,
2H, Ha), 6.57 (d, J ¼ 2.4 Hz, 2H, Hg). 13C NMR (600 MHz, CDCl3,
d
,
(R)-7a. FT-IR (KBr, cmꢁ1): 1788 (asymmetric C]O stretch), 1734
(symmetric C]O stretch), 1377 (CeN stretch), 1258 (AreOeAr),
1142 (CF3), 1053, 744 (imide ring deformation). 1H NMR
ppm): 160.94 (C14), 153.72 (C11), 150.88 (C13), 148.09 (C2), 141.70
(C17), 140.45 (C7), 139.97 (C8), 128.75 (C18), 128.45 (C5), 128.39 (C4),
124.04 (C3),123.81 (C16),122.11 (q, 1JCeF ¼ 271.5 Hz, C20),121.86 (C9),
(600 MHz, CDCl3,
d
, ppm): 7.97 (d, J ¼ 7.8 Hz, 2H), 7.89 (s, 2H), 7.84
120.60 (C10), 120.49 (q, JCeF ¼ 32.3 Hz, C15), 120.32 (C6), 116.75
(d, J ¼ 8.4 Hz, 4H), 7.80 (d, J ¼ 7.8 Hz, 2H), 7.66 (s, 2H), 7.42 (d,
J ¼ 9.0 Hz, 2H), 7.37 (dd, J ¼ 7.2, 7.2 Hz, 2H), 7.13 (dd, J ¼ 7.2, 7.2 Hz,
2H), 7.08 (d, J ¼ 8.4 Hz, 2H), 6.90 (d, J ¼ 9.0 Hz, 2H), 6.77 (d,
2
(C19), 116.70 (C12), 66.00 (C1). MALDI-FTICR MS (m/z): [M]þ calcd for
C39H20F6N2O6, 726.12; found, 726.12. Elemental analysis: Calcd for
C39 H20F6N2O6: C, 64.47%; H, 2.77%; N, 3.86%. Found: C, 64.56%; H,
2.85%, N, 3.81%.
J ¼ 7.2 Hz, 2H), 6.60 (s, 2H). 13C NMR (600 MHz, CDCl3,
d, ppm):
165.77, 165.60, 155.73, 155.00, 150.73, 148.25, 140.77, 139.29, 138.91,
136.03, 132.47, 132.19, 131.23, 128.19, 127.92, 125.62, 125.44, 125.18,
124.25, 124.02, 122.68, 122.36, 121.49, 121.10, 120.06, 118.13, 116.48,
65.95, 65.24.
Optically inactive 2,20-bis(4-nitro-2-trifluoromethylphenoxy)-
9,90-spirobifluorene was synthesized by the same procedure. The
yield was 93%. The melting point was 267e269 ꢀC with pronounced
effervescence at 115 ꢀC and resolidification around 160e170 ꢀC.
(R)-7b. FT-IR (KBr, cmꢁ1): 1783 (asymmetric C]O stretch), 1726
(symmetric C]O stretch), 1376 (CeN stretch), 1260 (AreOeAr),
1138 (CF3), 1053, 743 (imide ring deformation). 1H NMR
2.3.3. (R)-2,20-bis(4-amino-2-trifluoromethylphenoxy)-9,90-
spirobifluorene
(600 MHz, CDCl3,
d
, ppm): 7.92 (d, J ¼ 7.8 Hz, 2H), 7.83 (d, J ¼ 8.4 Hz,
Optically active dinitro compound (R)-5 (5.09 g, 7.0 mmol) and
10% Pd/C (0.10 g) were suspended in ethanol (80 mL) in a 250-mL
flask. The suspension was heated to reflux, and 80% hydrazine
monohydrate (3 mL) was added dropwise to the stirred mixture.
After a further 6 h reflux, the resulting clear, darkened solution was
filtered hot to remove Pd/C, and the filtrate was dried by rotary
evaporation. The crude product was purified by recrystallization
from 1,2-dichloroethane to afford pale-yellow needle-like crystals
2H), 7.79 (d, J ¼ 7.8 Hz, 2H), 7.66 (s, 2H), 7.46 (s, 2H), 7.44(d,
J ¼ 7.8 Hz, 2H), 7.42 (d, J ¼ 7.8 Hz, 2H), 7.37 (dd, J ¼ 7.2, 7.2 Hz, 2H).
6.77 (d, J ¼ 7.8 Hz, 2H), 6.60 (s, 2H). 13C NMR (600 MHz, CDCl3,
d,
ppm): 165.90, 165.80, 161.16, 155.60, 155.02, 150.75, 148.26, 140.78,
138.90, 134.45, 131.27, 128.19, 127.93, 127.15, 126.37, 125.61, 125.45,
124.88, 124.04, 122.74, 121.50, 120.97, 120.13, 120.06, 117.99, 116.49,
114.04, 65.94.
(R)-7c. FT-IR (KBr, cmꢁ1): 1784 (asymmetric C]O stretch), 1730
(symmetric C]O stretch), 1376 (CeN stretch), 1259 (AreOeAr),
1139 (CF3), 1053, 746 (imide ring deformation). 1H NMR
(4.01 g, 86%). mp: 262e264 ꢀC ½a D20
ꢃ
¼ þ67.0ꢀ (c ¼ 0.50 g/dL, DMAc).
FT-IR (KBr, cmꢁ1): 3471, 3386 (eNH2), 1228 (AreOeAr), 1121 (e
CF3). 1H NMR (600 MHz, DMSO-d6,
d, ppm): 7.93 (d, J ¼ 8.4 Hz,
(600 MHz, CDCl3,
d
, ppm): 8.14 (d, J ¼ 7.2 Hz, 2H), 8.10 (s, 2H), 8.00
2H, He), 7.90 (d, J ¼ 7.2 Hz, 2H, Hd), 7.37 (dd, J ¼ 7.8, 7.2 Hz, 2H, Hc),
7.09 (dd, J ¼ 7.8, 7.2 Hz, 2H, Hb), 6.91 (dd, J ¼ 8.4, 2.4 Hz, 2H, Hf), 6.84
(s, 2H, Hj), 6.71 (s, 4H, Hh and Hi), 6.59 (d, J ¼ 7.2 Hz, 2H, Ha), 6.05 (d,
(d, J ¼ 7.2 Hz, 2H), 7.85 (d, J ¼ 7.8 Hz, 2H), 7.80 (d, J ¼ 6.6 Hz, 2H),
7.67 (s, 2H), 7.47 (d, J ¼ 9.0 Hz, 2H), 7.38 (dd, J ¼ 7.2, 7.2 Hz, 2H), 7.14
(dd, J ¼ 6.6, 7.2 Hz, 2H), 7.09 (d, J ¼ 7.8 Hz, 2H), 6.85 (d, J ¼ 9.0 Hz,
J ¼ 2.4 Hz, 2H, Hg), 5.42 (s, 4H, Hk). 13C NMR (600 MHz, DMSO-d6,
d
,
2H), 6.78 (d, J ¼ 7.2 Hz, 2H) 6.61(s, 2H). 13C NMR (600 MHz, CDCl3,
d,
ppm): 158.49 (C11), 150.01 (C13), 147.81 (C2), 145.75 (C14), 142.12
(C17), 140.64 (C7), 135.80 (C8), 128.02 (C5), 127.28 (C4), 123.30 (C3),
123.34 (q, 1JCeF ¼ 271.5 Hz, C20), 122.62 (C18), 121.71 (C9), 121.30 (q,
2JCeF ¼ 30.0 Hz, C15), 120.04 (C6), 118.34 (C19), 116.65 (C10), 116.62
(C12), 110.73 (C18). MALDI-FTICR MS (m/z): [M]þ for C39H24F6N2O2,
666.17; found, 666.17. Elemental analysis: Calcd for C39H24F6N2O2:
C, 70.27%; H, 3.63%; N, 4.20%. Found: C, 70.35%; H, 3.60%; N, 4.16%.
Crystal data: pale-yellow crystal grown during slow crystalli-
zation in ethanol/ethyl acetate (3:1 v/v), 0.34 ꢄ 0.21 ꢄ 0.12 mm3,
ppm): 192.54, 165.73, 155.98, 154.75, 150.81, 148.20, 141.81, 140.74,
139.08, 135.91, 134.80, 131.99, 131.31, 128.23, 127.99, 125.67, 125.05,
124.67, 124.32, 124.07, 122.70, 121.58, 120.84, 120.41, 120.09, 117.65,
116.64, 65.93.
(R)-7d. FT-IR (KBr, cmꢁ1): 1779 (asymmetric C]O stretch), 1726
(symmetric C]O stretch), 1374 (CeN stretch), 1250 (AreOeAr
stretch), 1135 (CF3), 1052, 736 (imide ring deformation). 1H NMR
(600 MHz, CDCl3,
d
, ppm): 7.88 (d, J ¼ 7.8 Hz, 2H), 7.84 (d, J ¼ 8.4 Hz,
2H), 7.80 (d, J ¼ 7.2 Hz, 2H), 7.77 (s, 2H), 7.76 (d, J ¼ 7.8 Hz, 2H), 7.65
(s, 2H), 7.46 (d, J ¼ 8.4 Hz, 2H), 7.37 (dd, J ¼ 6.6, 7.2 Hz, 2H), 7.13 (dd,
J ¼ 6.6, 6.6 Hz, 2H), 7.07 (d, J ¼ 7.8 Hz, 2H), 6.83 (d, J ¼ 8.4 Hz, 2H),
ꢀ
ꢀ
tetragonal P43212 with
a
¼
16.198(2) A,
b
¼
16.198(2) A,
¼ 90ꢀ,
b
¼ 90ꢀ and
g
¼ 90ꢀ, where Dc ¼ 1.374 Mg/
ꢀ
c ¼ 12.285(4) A,
a
m3 for Z ¼ 4 and V ¼ 3223.2 Å3. For crystallographic data for the
6.78 (d, J ¼ 7.2 Hz, 2H), 6.61 (s, 2H). 13C NMR (600 MHz, CDCl3,
d,
structure, see the Supporting Information (Table S1).
ppm): 166.24, 166.16, 155.89, 154.71, 150.81, 148.20, 144.81, 140.78,
139.06, 133.17, 132.72, 131.43, 128.22, 127.96, 125.78, 125.15, 124.59,
124.07, 122.73, 122.09, 121.59, 120.69, 120.45, 120.09, 117.55, 117.49,
116.64, 65.93.
Optically inactive diamine compound 6 was synthesized by the
same procedure, except that it was purified by recrystallization
from benzene. The yield was 89%, mp: 274e276 ꢀC.
2.4. Synthesis of polyimides
3. Results and discussion
A typical polymerization procedure was as follows. To a solution
of 0.9954 g (1.49 mmol) of diamine (R)-6 in 10 mL DMAc in a 50-mL
flask, 0.6634 g (1.49 mmol) of 6FDA was added in one portion. The
solid content of the solution is approximately 10%. The mixture was
stirred at room temperature for 10 h under nitrogen atmosphere to
yield a viscous poly (amic acid) solution. Chemical imidization was
carried out by adding a mixture of acetic anhydride (0.60 mL) and
triethylamine (0.30 mL) into the poly (amic acid) solution. This
mixture was stirred at room temperature for 6 h and then heated at
80 ꢀC for 2 h. After cooling, the viscous solution was added slowly
into ethanol (100 mL). The precipitate was collected by filtration,
3.1. Monomer synthesis
The synthetic route to new optically diamine monomer (R)-6 is
outlined in Scheme 1. The precursor, 9,90-spirobifluorene (1) was
prepared according to literature procedures [44]. On acylation and
oxidation, followed by alkaline hydrolysis, 1 gave (ꢂ)-2,20-dihy-
droxy-9,90-spirobifluorene (4). Tartaric acid derivative [(R,R)-
(þ)-2,3-dimethoxy-N,N,N0,N0-tetracyclohexylsuccinamide ((R,R)-
(þ)-DMTCHS, chiral host)] could selectively recognize (R)-4 (chiral
guest) in the solution of ethanol and formed crystalline complex.
The absolute stereochemistry of the complex was confirmed by

Z. Wu et al. / Polymer 53 (2012) 5706e5716
5709
X-ray crystal analysis. In the crystal, the complex was constructed
of one (R)-4 and one (R,R)-(þ)-DMTCHS through a hydrogen band
between the hydroxyl group of the diol and the carbonyl group of
the amide [44]. The inclusion complex was decomposed in aqueous
sodium hydroxide and subsequently acidified with hydrochloric
acid to give (R)-4 in 99% ee. The aromatic nucleophilic substitution
reaction of (R)-4 with 2-fluoro-5-nitrobenzotrifluoride in DMAc/
K2CO3 medium yielded (R)-5 in high yields, which on catalytic
reduction over hydrazine hydrate and Pd/C afforded the target
diamine monomer (R)-6. The structures of the dinitro (R)-5 and
diamine (R)-6 were confirmed by elemental analysis, IR, NMR and
mass spectroscopy. Figure S1 shows the IR spectra of the dinitro
intermediate and the diamine. The nitro groups of compound (R)-5
gave two characteristic bands at 1530 and 1351 cmꢁ1 (NO2 asym-
metric and symmetric stretching, respectively). After reduction, the
characteristic absorptions of the nitro group disappeared, and the
amino group showed the typical NeH stretching bands in the
region of 3300e3500 cmꢁ1. The 1H NMR and 13C NMR spectra of
diamine monomer (R)-6 in DMSO-d6 are illustrated in Fig. 1.
Assignments of each proton and carbon are assisted by two-
dimensional NMR spectra shown in Fig. 2 and Figure S2, and
these spectra agree well with the proposed molecular structure of
(R)-6. The 1H NMR spectrum of diamine (R)-6 confirmed that the
nitro groups have been completely converted into amino groups by
the high field shift of the aromatic protons and by the signal at
5.42 ppm corresponding to the primary aromatic amine protons. In
the 13C NMR spectrum of diamine (R)-6, C15 and C20 exhibited clear
quartet absorptions at 121.00e121.60 and 120.63e126.05 ppm,
respectively, due to JCeF and JCeF coupling of the carbons with
fluorines. The resonance associated with the central spiro carbon
(C1) appeared in 65.33 ppm. The other spectra of compounds (R)-5
and (R)-6 were displayed in Figures S3eS6. All the spectroscopic
data obtained were in good agreement with the expected struc-
tures. The molecular structure of (R)-6 was also confirmed by X-ray
crystal analysis. Single crystal of (R)-6 was obtained from ethanol/
ethyl acetate solution by slow evaporation of solvent. Fig. 3 displays
the Oak Ridge thermal ellipsoid plot (ORTEP) of (R)-6 accomplished
by X-ray diffraction at 295 K. The spiro molecule consists of two
2
1
Scheme 1. Synthesis of diamine monomer (ꢂ)- and (R)-6.

5710
Z. Wu et al. / Polymer 53 (2012) 5706e5716
Fig. 1. 1H NMR and 13C NMR spectra of (R)-6 in DMSO-d6.
identical 4-amino-2-trifluoromethylphenoxyfluorene moieties
connected through a common tetra coordinate carbon atom (the
spiro center). In the spiro segment, the arrangement of the con-
nected bifluorene entities is nearly orthogonal (dihedral angle ¼
88.1ꢀ). The structure agrees with the proposed one.
The optical purity of the monomers, (R)-4 and (R)-6, was
determined by HPLC analysis on DAICEL-AD-H column (Figure S8
and Fig. 4). The ee value of (R)-6 is 99%, which showed that there
was almost no rotational loss during two-step reactions. The
specific rotation and melting points of (R)-5 and (R)-6 are shown in
the experimental section. Different melting points [(ꢂ)-5, 267e
269 ꢀC; (R)-5, 120e122 ꢀC; (ꢂ)-6, 274e276 ꢀC; (R)-6, 262e264 ꢀC]
between the racemic and chiral compounds were noticed, and
this was attributed to the different crystal structures. Optically
active diamine (R)-6 could be easily crystallized from chloroform,
while the corresponding racemate was almost insoluble in chlo-
roform on heating at boiling temperature.
anhydride and triethylamine (Scheme 2). For comparison, a series
of corresponding polyimides based on racemic diamine 6 were also
prepared and characterized. The results of the polymerization are
summarized in Table 1. These optically active polyimides had
inherent viscosities ranging from 0.38 to 0.72 dL/g in DMAc. Their
numbereaverage (Mn) and weigheaverage (Mw) molecular weights
were 1.8e4.0 ꢄ 104 and 3.9e9.3 ꢄ 104, respectively, and the poly-
dispersity index (PDI ¼ Mw/Mn) was in the range of 2.12e2.33, as
measured by gel permeation chromatography (GPC) using poly-
styrene standards. The viscosities and molecular weights of opti-
cally active polyimides were somewhat lower than the
corresponding racemic polyimides prepared from diamine 6. The
possible reason was that the relatively regular structures of chiral
polyimides (R)-7aed, which originating from optically active
spirodiamine, would inactivate the end-groups of the propagating
polymers by the steric hindrance, while the random comply-
merization of racemic 6 with dianhydrides could overcome this
problem [52].
3.2. Preparation of polymides
The chemical structures of the polyimides were characterized by
the elemental analysis, IR and NMR spectra. The elemental analysis
values of the polyimides were in good agreement with their
respective structures. The FT-IR spectra (Fig. 5) of the polyimides
exhibited characteristic imide absorptions at around 1783 and
1730 cmꢁ1 (typical of imide carbonyl asymmetric and symmetric
Optically active polyimides (R)-7aed were prepared by the
traditional two-step method, that is, the synthesis of polyamic acid
through the reaction of (R)-6 with aromatic dianhydride in DMAc at
room temperature, followed by chemical imidization with acetic

Z. Wu et al. / Polymer 53 (2012) 5706e5716
5711
Fig. 2. 1H-1H COSY NMR spectrum of (R)-6 in DMSO-d6.
stretches), 1376 (CeN stretch), and 1053 and 743 (imide ring
deformation), together with strong absorption bands in the region
of 1100e1300 cmꢁ1 due to the CeO and CeF stretching. The typical
1H NMR spectrum of optically active polyimide (R)-7a is shown in
Fig. S9. All the protons resonated in the region of 6.60e7.98 ppm.
The protons Hm close to the imide appeared at the farthest
Fig. 4. (a) HPLC resolution of (ꢂ)-6 and (b) the determination of optical purity of (R)-6
on DAICEL-AD-H column. Eluent: Hexane: i-PrOH ¼ 7:3; flow rate: 1 mL minꢁ1 at room
temperature.
Fig. 3. ORTEP diagram of diamine (R)-6.

5712
Z. Wu et al. / Polymer 53 (2012) 5706e5716
Scheme 2. Synthesis of polyimides (ꢂ)- and (R)-7aed.
downfield region of the spectrum because of the resonance. The
protons Hg shifted to higher field because of the electron-donating
property of aromatic ether. The other NMR spectra of these chiral
polyimides (R)-7aed are shown in Supporting Information
(Figures S10eS16). All the spectroscopic data obtained were in
good agreement with the desired polymer structures.
as chlorinated solvents like chloroform and dichloromethane. The
good solubility of these polyimides might be attributed to the
presence of kinked spirobifluorene units, with bulky tri-
fluoromethyl pendent groups and flexible aryl ether linkages along
the polymer backbone. This kind of macromolecular architecture
would restrict the close packing of the polymer chains and lessen
the probability of interchain interactions, so that it improved the
solubility of resultant polymers. Optically active polyimides (R)-7c
and (R)-7d exhibited slightly better solubility than corresponding
racemic polymers in tetrahydrafuran and 1,4-dioxane solvents.
The crystallinity of the polyimides was characterized by wide
angle X-ray diffraction (WAXD) and the results are shown in
Figures S17 and S18. All these polymides showed amorphous
patters. The amorphous nature of these polyimides was attributed
3.3. Polymer solubility and crystallinity
The solubility of these polyimides was tested in various organic
solvents, and the results are listed in Table 2. All these polyimides
exhibited good solubility in dipolar amide-type solvents, such as
NMP, DMAc and DMF at room temperature or upon heating. They
were also soluble in pyridine and phenol solvent m-cresol, as well
Table 1
Inherent viscosity, molecular weight and elemental analysis of polyimides.
a
Polymer hinh
GPC data
Elemental analysis (%)
(dL/g)
b
Mw
Mw/Mn Formula (formula weight)
C
H
N
(ꢄ104)
(ꢂ)-7a 0.51 5.3
2.21
2.14
2.34
2.28
2.15
2.33
2.16
2.12
(C58H26F12N2O6)n Calcd. 64.81 2.44 2.61
(1074.82) Fould 64.76 2.53 2.68
(C58H26F12N2O6)n Calcd. 64.81 2.44 2.61
(R)-7a 0.47 4.8
(1074.82)
Fould 64.56 2.51 2.49
Calcd. 70.22 2.79 2.98
Fould 70.12 2.97 2.88
Calcd. 70.22 2.79 2.98
Fould 69.97 2.92 2.95
Calcd. 70.59 2.75 2.94
Fould 70.68 2.79 2.83
Calcd. 70.59 2.75 2.94
Fould 70.65 2.81 2.76
Calcd. 71.43 2.83 3.03
Fould 71.26 2.88 2.95
Calcd. 71.43 2.83 3.03
Fould 71.10 2.89 2.92
(ꢂ)-7b 0.75 8.9
(C55H26F6N2O7)n
(940.79)
(R)-7b
(ꢂ)-7c
(R)-7c
0.73 8.5
0.78 9.9
0.76 9.3
(C55H26F6N2O7)n
(940.79)
(C56H26F6N2O7)n
(952.81)
(C56H26F6N2O7)n
(952.81)
(ꢂ)-7d 0.39 4.1
(C55H26F6N2O6)n
(924.80)
(R)-7d
0.38 3.9
(C55H26F6N2O6)n
(924.80)
a
Measured at a polymer concentration of 0.5 g/dL in DMAc at 30 ꢀC.
Molecular weight was determined by GPC in DMF based on polystyrene
b
standards.
Fig. 5. FT-IR spectra of polyimides (R)-7aed.

Z. Wu et al. / Polymer 53 (2012) 5706e5716
5713
Table 2
Solubility of the polyimides.a
Solventb
Polyimide
(R)-7a (ꢂ)-7a (R)-7b (ꢂ)-7b (R)-7c (ꢂ)-7c (R)-7d (ꢂ)-7d
NMP
DMAc
DMF
DMSO
m-Cresol
Chloroform
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ
þ þ
þ þ
þ þ
þ
þ þ
þ þ
þ þ
þ
þ þ
þ þ
þ þ
þ
þ þ
þ þ
þ
þ þ
þ þ
þ
þ ꢁ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
ꢁ ꢁ
þ ꢁ
þ þ
þ þ
þ þ
þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ ꢁ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ ꢁ
þ þ
þ þ
þ þ
þ þ
þ þ
þ þ
þ ꢁ
þ þ
þ þ
þ þ
þ þ
þ
Dichloromethane þ þ
Pyridine
1,4-Dioxane
THF
þ þ
þ þ
þ þ
þ þ
þ
þ
þ
Acetone
þ ꢁ
ꢁ ꢁ
The symbol þ þ: soluble at room temperature; þ: soluble on heating at 100 ꢀC or
boiling temperature; þ ꢁ: partially soluble or slightly swollen on heating; ꢁ ꢁ:
insoluble even on heating.
a
Solubility was tested with a 5 mg sample in 1 mL of stirred solvent.
NMP: N-methyl-2-pyrolidone; DMF: N,N0-dimethylformamide; DMAc: N,N0-
b
dimethylacetamide; DMSO: dimethylsulfoxide; THF: tetrahydrofuran.
Fig. 6. DSC traces of polyimides (R)-7aed and (ꢂ)-7b.
not only to the bulkiness of the CF3 substituent but also to the
spiroskeletons structure coming from the diamine monomer.
The thermal stability of these polyimides was evaluated by TGA
measurements in both air and nitrogen atmospheres. Fig. 7 shows
the typical TGA curves for optically active polymers (R)-7a and (R)-
7d. Almost all the polyimides did not show remarkable weight loss
below 500 ꢀC in nitrogen or air atmosphere. The 5% weight loss
temperatures of these polyimides in nitrogen and air were in the
range of 527e569 and 512e543 ꢀC, respectively. They left more
than 58% char yield at 800 ꢀC in nitrogen. Especially, the chiral
polyimide (R)-7d derived from BPDA had the highest residue yield
up to 68%. The TGA data indicated that these polyimides containing
spirobifluorene moieties in the backbone had fairly high thermal
stability even with the introduction of bulky trifluoromethyl
pendent groups.
3.4. Thermal properties
The thermal properties of these polyimides were evaluated by
DSC, DMA and TGA, and the results are summarized in Table 3. The
incorporation of rigid spirobifluorene units in the polymer back-
bone resulted in polyimides with high glass transition temperature
(Tg). DSC curves for the polyimides (R)-7aed and (ꢂ)-7b are dis-
played in Fig. 6. The Tgs of the optically active polyimides (R)-7ae
d were in the range of 260e281 ꢀC depending on the chemical
structure of the aromatic dianhydride component. As expected,
polymer (R)-7d showed the highest Tg of 281 ꢀC, which can be
attributed to the rigid biphenyl group in the polymer chain. On the
contrary, polyimide (R)-7b showed the lowest Tg of 260 ꢀC due to
the relative flexible ether linkage. Figures S19 and S20 show the
DMA curves of polyimides (R)-7aed and (ꢂ)-7aec, respectively.
Regarding the peak temperature in the tan curves as the Tg, the Tg
values obtained by DMA were comparable to that measured by DSC
method. The optically active polyimides (R)-7 and optically inactive
polyimides 7 exhibited almost identical Tgs, though the melting
point of diamine (R)-4 differed from that of (ꢂ)-4. This result
seemed to imply that there was no stereocomplex formed as for
other optically active polymers [53].
3.5. Optical and mechanical properties
Except for polyimide (ꢂ)-7d, all these polyimides could be
processed to flexible and tough films conveniently by casting from
polymer solutions. The optical properties of these polyimide films
are given in Table 4 and the UVevis spectra of polymer films (R)-
7aed about 20
mm thicknesses are shown in Fig. 8. The cut-off
wavelength (l0) values of the optically active polyimide films
ranged from 351 to 375 nm, and the optical transmittance values at
a wavelength of 500 nm were all high than 78%. Because of the
highly conjugated aromatic structures and intermolecular charge
transfer complex (CTC) formation, most polyimides showed strong
absorption in UV and the visible region. However, these polyimides
which had bulky spirobiflurene structure and trifluoromethyl
groups in the center of diamine broke the conjugation along the
polymer backbone and reduced the intermolecular CTC between
alternating electron-donor (diamine) and electron-acceptor (dia-
nhydride) moieties. The polyimides (R)-7a and (R)-7b derived from
6FDA and ODPA showed the lower l0 values together with higher
optical transparency in comparison to polyimides based on other
dianhydrides, which could be explained from the decreased inter-
molecular interactions [31]. The optically inactive polyimides films
7aed showed similar UVevis spectra to the corresponding optically
active polymers.
Table 3
Thermal properties and specific rotations of polyimides.
a
b
b
Polymer
Tg (ꢀC)
T5% (ꢀC)
T10% (ꢀC)
Rwc (%)
½
a 2D0
d (ꢀ)
ꢃ
DSC
DMA
N2
Air
N2
Air
(R)-7a
(ꢂ)-7a
(R)-7b
(ꢂ)-7b
(R)-7c
(ꢂ)-7c
(R)-7d
(ꢂ)-7d
275
276
260
258
269
270
280
281
279
280
262
262
272
274
286
de
527
530
556
562
549
555
569
567
512
517
535
539
528
533
543
541
546
550
584
586
576
579
592
591
531
537
562
568
557
559
565
562
58
60
65
64
63
62
68
67
þ73.0
þ16.5
þ12.9
ꢁ48.5
a
Glass transition temperature (Tg) was measured by DSC and DMA at a heating
rate of 20 ꢀC minꢁ1 and 10 ꢀC minꢁ1, respectively.
b
Temperaꢁtu1re at 5% and 10% weight loss were recorded by TGA at a heating rate
Residual weight percentage at 800 ꢀC in nitrogen.
Specific rotations of polymers were measured at 20 ꢀC in DMAc, c ¼ 0.10 g/dL.
The polymer film was too brittle to be measured.
The polyimide films were subjected to tensile tests, and the
results are summarized in Table 4. The optically active polyimide
films had tensile strengths of 72.0e84.8 MPa, elongations at break
of 3.6e4.3% and tensile moduli of 3.11e3.67 GPa, which indicate
of 10 ꢀC min
.
c
d
e

5714
Z. Wu et al. / Polymer 53 (2012) 5706e5716
Fig. 7. TGA curves of polyimides (R)-7a and (R)-7d at a heating rate of 10 ꢀC minꢁ1
.
Fig. 8. UVevis spectra of polyimide films (R)-7aed.
that they are strong and tough polymeric materials. These chiral
polymers have the potentials to be used as the chiral membrane
separation materials for the resolution of racemates.
These polyimides showed low water uptakes in the range of
0.20e0.54% (Table 4). Obviously, the low water absorption could
mainly be attributed to the polymer hydrophobicity derived from
the concentration and distribution of trifluoromethyl groups in the
polyimide backbones. Among these optically active polyimides,
polymer (R)-7a exhibited the lowest water absorption (0.23%) due
to its highest fluorine content.
of the resulting polyimides show optical rotation and therefore are
optically active. We have also studied the optical stability of (R)-
7aed in solid state or in DMAc solvent. We found that the optical
rotation of (R)-7aed remained unchanged after heating at 250 ꢀC in
solid state for 48 h or after fluxing in DMAc for 24 h. The high
optical stability of polyimides (R)-7aed at high temperatures might
be attributed to incorporation spirobifluorene units into the poly-
mer structures. In the spiro segment, the two fluorene rings of the
spiro compound lie in perpendicular planes. This structure feature
not only restricts the rotation of the two rings and gives rises to an
axial chirality in spiro compounds having substituents on rings but
also increases molecular rigidity [42].
In order to obtain further information on the conformation, CD
and UVevis spectra of the monomers (R)-5, (R)-6 and the polyimides
(R)-7aed in THF were measured (Figure S21 and Fig. 9) and the CD
spectra data are listed in Supporting Information (Table S2). In the
UVevis spectra of polyimide (R)-7a solution (Fig. 9), an absorption
3.6. Chiro-optical properties
The specific rotations of these polyimides (R)-7aed were
measured in DMAc at a concentration of 0.10 g/dL at 20 ꢀC and the
data are listed in Table 3. The value of the polyimide (R)-7a
(½a 2D0
ꢃ
¼ þ73.0ꢀ) derived from 6FDA was slightly higher than that of
band at around 279 nm was observed due to
transition of aromatic rings. The UVevis spectra of polyimide (R)-7a
also showed shoulder peak at around 313 nm due to the n/
p/p* electronic
diamine (R)-6 (½a D20
ꢃ
¼ þ67.0ꢀ) building up into the polymer. Poly-
imides (R)-7b and (R)-7c exhibited specific rotation values
of þ16.5ꢀ and þ12.9ꢀ, respectively. It is noteworthy that the poly-
p*
transition of non-bonding electrons present in oxygen or nitrogen
atoms in the polymer backbone. Other optically active polyimides
(R)-7bed exhibited similar UVevis spectra. The CD spectra of
imide (R)-7d was levorotatory and showed an ½a D20
ꢃ
value of ꢁ48.5ꢀ.
Similar phenomena were reported that the optical rotations for the
chiral polymers were opposite to those of the corresponding
monomers [54]. The reversal in optical rotations indicates that the
optical activity of the polymer (R)-7d does not only arise from the
chiral spirodiamine moiety and suggests that a higher ordered
structure, likely a secondary helical structure, has been formed. All
Table 4
Water absorption, optical and mechanical properties of the polyimide films.
Polymer lcut off T500 Water
Tensile
Elongation Initial
(nm)
(%)
absorption strength
to break
(%)
modulus
(GPa)
(%)
(MPa)
(R)-7a
(ꢂ)-7a
(R)-7b
(ꢂ)-7b
(R)-7c
(ꢂ)-7c
(R)-7d
(ꢂ)-7d
351
348
357
354
372
370
375
377
86
85
83
82
77
78
79
80
0.23
0.20
0.43
0.46
0.54
0.52
0.31
0.35
84.8 ꢂ 2.5a 3.9 ꢂ 0.3
3.11 ꢂ 0.10
2.97 ꢂ 0.12
3.67 ꢂ 0.14
3.30 ꢂ 0.09
3.63 ꢂ 0.13
3.52 ꢂ 0.15
3.34 ꢂ 0.11
e
78.7 ꢂ 2.3
72.0 ꢂ 1.9
76.2 ꢂ 2.6
77.1 ꢂ 2.4
80.1 ꢂ 2.7
73.1 ꢂ 1.8
eb
3.5 ꢂ 0.2
3.7 ꢂ 0.2
3.3 ꢂ 0.1
4.3 ꢂ 0.3
4.1 ꢂ 0.2
3.6 ꢂ 0.1
e
a
The former value (84.8) was the mean of the five determinations, while the
latter (2.5) was the sample standard deviation.
Fig. 9. UVevis absorption and CD spectra of polyimides (R)-7aed in THF at 25 ꢀC.
Concentration: 3 ꢄ 10ꢁ5 (repeating unit) mol/L.
b
The polymer film was too brittle to be measured.

Z. Wu et al. / Polymer 53 (2012) 5706e5716
5715
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and two medium positive bands at 239 and 294 nm, respectively. If
a comparison is made between the CD spectra of polyimides (R)-7b
and (R)-7a, we can see clearly that the intensity and position of the
peaks are different. Inpolyimide (R)-7b, the negative peak at 271 nm
showed an increase in intensity while the positive peaks at 245 and
298 nm displayed decrease in their ellipicity. Polyimide (R)-7d
(levorotatory) also exhibited a significantly different CD pattern
from polymers (R)-7aec (dextrorotatory). (R)-7d showed a strong
negative peak at 274 nm and only one positive Cotton effect at
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formed well-defined secondary structure such as helix. In the opti-
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rodiimide is interlinked by the rigid biphenyl unit, and this results in
a persistence of the asymmetric structure of 9,90-spirobifluorene
along the backbone. The addition of flexible ether linkage, carbonyl
or hexafluoroisopropyl groups to the dianhydride moieties allows
variations of polymer chain conformation and, therefore, breaks the
communication between neighboring chiral spirodiimides, and this
results in relatively low structures ((R)-7aec).
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A
novel optically active diamine monomer, (R)-2,20-bis(4-
amino-2-trifluoromethylphenoxy)-9,90-spirobifluorene,
was
(d) Ree M. Macromol Res 2006;14(1):1e33;
successfully synthesized and characterized in the present work,
which was employed in polycondensation with various aromatic
dianhydrides to prepare a series of chiral polyimides. The resulting
polyimides exhibited good solubility in many organic solvents such
as chloroform, NMP and m-cresol. For THF and 1,4-dioxane, opti-
cally active polyimides (R)-7c and (R)-7d derived from BTDA and
BPDA showed slightly better solubility than corresponding racemic
polymers. The specific rotations of these chiral polyimides were in
the range of ꢁ48.5ꢀ to þ73.0ꢀ. Attributed to the spiro structure, all
the chiral polyimides exhibited high optical stability in solid state
or in DMAc solvent at high temperatures. In the case of the spiro
segment, the two mutually perpendicular fluorene rings were
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a quaternary center through s-bonds, which
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restricted the rotation of the two rings and increased the rigidity of
the polymers. These optically active polyimides also displayed high
thermal stability and outstanding mechanical property. The
synthetic chiral polyimides are expected to have the potential as
chiral catalysts for asymmetric synthesis, chiral membrane sepa-
ration materials and chiral stationary phases in chromatographic
technique for the resolution of racemates.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (gs1) (No. 20674082) and Science &
Technology Development Project Agreement, Jilin Province (gs2)
(20106022). The authors thank Doctor Zhen Bian (Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences) for
his help with the measurements of CD spectra.
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
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