Synthesis and characterization of highly soluble and oxygen permeable new polyimides bearing a noncoplanar twisted biphenyl unit containing tert-butylphenyl or trimethylsilyl phenyl groups

Article abstract of DOI:10.1021/ma0214557

The two new dianhydrides, 2,2′-bis(4″-tert-butylphenyl)-4,4′,5,5′-biphenyltetracar boxylic dianhydride (BBBPAn) and 2,2′-bis(4″-trimethylsilylphenyl)-4,4′,5,5′-biphenyltetr acarboxylic dianhydride (BTSBPAn), composed of a noncoplanar twisted biphenyl unit containing tert-butylphenyl or trimethylsilyphenyl groups were prepared. Two series of new organosoluble polyimides were prepared from the dianhydride and aromatic diamines by the conventional polyamide acid reaction followed by chemical imidization as well as high-temperature one-step polymerization. The structures of polymers were confirmed by various spectroscopic techniques. The weight-average molecular weight and polydispersities of resulting polymers were in the ranges 60 400-332 400 and 1.87-3.51, respectively. The polyimides showed good solubility in various organic solvents such as chloroform and THF. The polyimides exhibited 5% weight loss at 540-550°C in nitrogen as measured by thermogravimetric analysis. The polyimides showed high T_g's and good mechanical properties. The oxygen permeability coefficient (P_O2) and permselectivity of oxygen to nitrogen (P_O2/P_N2) of the films were in the ranges 31-110 barrer and 2.8-4.3, respectively.

Full text of DOI:10.1021/ma0214557

Macromolecules 2003, 36, 2327-2332
2327
Synthesis and Characterization of Highly Soluble and Oxygen
Permeable New Polyimides Bearing a Noncoplanar Twisted Biphenyl
Unit Containing tert-Butylphenyl or Trimethylsilyl Phenyl Groups†
Hyu n g-Su n Kim , Yu n -Hi Kim , Seu n g-Ku k Ah n , a n d Soon -Ki Kw on *
Department of Polymer Science & Engineering and Engineering Research Institute,
Gyeongsang National University, Chinju 660-701, Korea
Received September 9, 2002
ABSTRACT: The two new dianhydrides, 2,2′-bis(4′′-tert-butylphenyl)-4,4′,5,5′-biphenyltetracarboxylic
dianhydride (BBBPAn) and 2,2′-bis(4′′-trimethylsilylphenyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride
(BTSBPAn), composed of a noncoplanar twisted biphenyl unit containing tert-butylphenyl or trimethyl-
silylphenyl groups were prepared. Two series of new organosoluble polyimides were prepared from the
dianhydride and aromatic diamines by the conventional polyamide acid reaction followed by chemical
imidization as well as high-temperature one-step polymerization. The structures of polymers were
confirmed by various spectroscopic techniques. The weight-average molecular weight and polydispersities
of resulting polymers were in the ranges 60 400-332 400 and 1.87-3.51, respectively. The polyimides
showed good solubility in various organic solvents such as chloroform and THF. The polyimides exhibited
5% weight loss at 540-550 °C in nitrogen as measured by thermogravimetric analysis. The polyimides
showed high Tg’s and good mechanical properties. The oxygen permeability coefficient (PO2) and
permselectivity of oxygen to nitrogen (PO /PN2) of the films were in the ranges 31-110 barrer and 2.8-
2
4.3, respectively.
In tr od u ction
lylphenyl, which are expected to give good solubility,
high permeability, and permselectivity with good ther-
mal stability and mechanical property. In this article,
we report the synthesis and characterization of new
polyimides derived from s-BPDA containing a bulky
p-tert-butylphenyl group or p-trimethylsilylphenyl group
and conventional aromatic diamines, and oxygen per-
meability (PO ) and permselectivity of oxygen to nitrogen
(PO /PN ) are 2also studied.
Aromatic polyimides are widely used in the semicon-
ductor and electronic packaging industry because of
their outstanding thermal stability, good insulation
properties with low dielectric constant, good adhesion
to common substrates, and superior chemical stability.1,2
However, their applications were limited in many fields
because the early polyimides were insoluble and intrac-
table. Therefore, considerable research has been under-
taken to devise new ways to circumvent these restric-
tions.
2
2
Exp er im en ta l Section
Ma ter ia ls. N,N-Dimethylformamide (DMF) was dried over
CaH2 and distilled under nitrogen prior to use. m-Cresol was
dried over CaCl2 and then over 4 Å Linde type molecular
sieves, distilled under reduced pressure, and stored under
nitrogen in the dark. 4,4′-Oxydianiline (ODA, mp 192 °C), 4,4′-
methylenediamine (MDA, mp 90 °C), and 4,4′-(hexafluoroiso-
propylidene)dianiline (FDA, mp 196 °C; all diamines from
Aldrich) were used without additional purification. Other
chemicals were purchased from Aldrich Chemical Co. or Fluka.
and used as received.
Mea su r em en ts. A Genesis II FT-IR spectrometer was used
to record IR spectra. 1H NMR and 13C NMR spectra were
recorded with a DRX 500 MHz NMR spectrometer, and
chemical shifts are reported in ppm units with tetramethyl-
silane as internal standard. Thermogravimetric analysis (TGA)
was performed under nitrogen on a TA instruments 2050
thermogravimetric analyzer. The sample was heated using a
20 °C/min heating rate from 50 to 800 °C. Differential scanning
calorimetry (DSC) was conducted under nitrogen on a TA
instruments 2100 differential scanning calorimeter. The sample
was heated at 20 °C/min from 30 to 400 °C. Inherent viscosities
were measured with a Cannon Fenske viscometer in chloro-
form solution (0.5 g/dL, 25 °C). Tensile properties were
determined from stress-strain curves obtained with a uni-
versal testing machine (UTM) (LR-10K, Lloyd Instrument
Ltd.) with a load cell of 10 kgf. A gauge of 2 cm and a strain
rate of 0.5 cm/min were used for this study. Measurements
were performed at room temperature with film specimens (1
cm wide, 5 cm long, and 30-80 µm thick). Gas permeabilities
for the polymer membrane with about 30-60 µm of thickness
To circumvent these problems, solutions of polyimides
would be desirable instead of precursors. Flexible links
(e.g., -O-, -SO2-, -CH2-) or bulky groups (hexaflu-
oroisopropylidene, isopropylidene, etc.) are commonly
employed as solubilizing moieties, but it often costs in
thermal stability and chain stiffness of polyimides.
Alternatively, the solubilizing units may be derived from
substituted biphenyls, increasing entropy factors due to
rotational isomerism of benzene rings relative to an
aryl-aryl bond.3,4 The relevant compounds of this class
are biphenyltetracarboxylic dianhydrides. Among them,
3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA)
was commercialized in the early 1980s and is widely
used at present.5,6
It has been known that polymers containing a tert-
butyl group or trimethylsilyl groups have excellent gas
permeability.7-13 Soluble polyimides containing a bulky
tert-butylphenyl group or trimethylsilylphenyl groups
offer particular promise in gas separation membrane
application due to their superior mechanical property,
high permeability, and permselectivity. Thus, we syn-
thesized new s-BPDA-based polyimides containing a
bulky p-tert-butylphenyl group and/or p-trimethylsi-
† This article is dedicated to professor Won-J ei Cho on the
occasion of his retirement.
* To whom all correspondence should be addressed.
10.1021/ma0214557 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/05/2003

2328 Kim et al.
Macromolecules, Vol. 36, No. 7, 2003
were measured with a conventional permeability apparatus,
which consists of upstream and downstream parts separated
by a membrane.14-17 The upstream part was maintained to a
constant pressure of 3 kgf/cm2 of either pure O2 or N2 gas
during the experimental period, and the downstream part was
opened to the atmosphere. A bubble gas flowmeter was
employed to measure the steady-state permeation flux, from
which the gas permeability was calculated.
Mon om er Syn t h esis. 3,3′,4,4′-Tet r a m et h ylb ip h en yl
(TMBP ). NiCl2 (0.68 g, 0.05 mol), 2,2′-bipyridyl (0.84 g, 0.05
mol), triphenylphosphine (5.38 g, 0.19 mol), and Zn (16.24 g,
0.248 mol) were dissolved in 100 mL of DMF. The solution
was heated to 60 °C for 1 h. 4-Bromo-o-xylene (20 g, 0.108 mol)
was added with a syringe, and the flask was heated to 90 °C
for another 24 h. After being cooled to room temperature, the
product was poured into 500 mL of 2 N HCl solution, and then
the organic layer was extracted twice with 200 mL portions
of diethyl ether. The combined organic layer was washed twice
with NaHCO3 solution and dried over MgSO4. The solvent was
evaporated, and product was recrystallized from methanol to
give a white product. Yield after recrystallization: 18 g (80%);
mp: 75 °C. Anal. Calcd for C16H18 (210.31): C, 91.37%; H,
8.63%. Found: C, 91.22%; H, 8.54%. 1H NMR (CDCl3/TMS):
δ 2.35-2.38 (d, 12H, Ar-CH3), 7.22-7.24 (d, 1H, Ar-H), 7.36-
7.37 (d, 1H, Ar-H), 7.41 (s, 1H, Ar-H). FT-IR (KBr pellet):
3040 (aromatic C-H), 2912 cm-1 (aliphatic C-H).
2,2′-Bis(4′′-ter t-b u t ylp h en yl)-4,4′,5,5′-t et r a m et h ylb i-
p h en yl (BBTMBP ). The TBBBA (21.3 g 0.15 mol), DBTMBP
(18.4 g, 0.05 mol), and tetrakis(triphenylphosphine)palladium-
(0) (1 g, 0.87 mmol) were dissolved in dry toluene (300 mL). A
solution of 2 M Na2CO3 (75 mL) was added, and the mixture
was refluxed with stirring for 24 h in an atmosphere of
nitrogen. After being cooled, the organic phase was separated,
and the water phase was extracted twice with 200 mL portions
of chloroform. The combined organic layer was washed twice
with NaHCO3 solution and dried over MgSO4. The solvent was
evaporated, and product was recrystallized from methanol to
give a white product. Yield after recrystallization: 17 g (73%);
mp: 161 °C. Anal. Calcd for C36H42 (474.72): C, 91.08%; H,
8.92%. Found: C, 91.23%; H, 8.85%. 1H NMR (CDCl3/TMS):
δ 1.21 (s, 18H, Ar-tert-butyl), 2.21-2.25 (d, 12H, Ar-CH3),
6.29-6.30 (d, 2H, Ar-H), 6.81-6.83 (d, 2H, Ar-H), 6.84 (s,
1H, Ar-H), 7.13 (s, 1H, Ar-H). FT-IR (KBr pellet): 3018
(aromatic C-H), 2975 cm-1 (aliphatic C-H).
2,2′-Bis(4′′-tr im eth ylsilylp h en yl)-4,4′,5,5′-tetr a m eth yl-
bip h en yl (BTSTMBP ). The synthesis of BTSTMBP was
conducted as described above. Yield after recrystallization: 18
g (72%); mp: 160 °C. Anal. Calcd for C34H42Si2 (506.87): C,
80.57%; H, 8.35%. Found: C, 80.63%; H, 8.44%. 1H NMR
(CDCl3/TMS): δ 0.14 (s, 18H, Si-CH3), 2.18-2.22 (d, 12H,
Ar-CH3), 6.32-6.33 (d, 2H, Ar-H), 6.79 (s, 1H, Ar-H), 6.92-
6.93 (d, 2H, Ar-H), 7.10 (s, 1H, Ar-H). FT-IR (KBr pellet):
3018 (aromatic C-H), 2975 (aliphatic C-H), 1119 cm-1
(C-Si stretching).
2,2′-Dibr om o-4,4′,5,5′-tetr a m eth ylbip h en yl (DBTMBP ).
TMBP (50 g, 0.237 mol), FeCl3 (0.58 g, 15.3 mmol), and
methylene dichloride (300 mL) were charged into a three-
necked round-bottom flask, equipped with a magnetic stirrer,
addition funnel, condenser, drying tube, and thermometer. The
solution was cooled to below -5 °C with an ice/salt bath,
followed by adding bromine (79.41 g, 0.495 mol) dropwise over
a period of 3-4 h under vigorous stirring. After reacting
overnight, the product was poured into cooled 2 N NaOH
solution, and then the organic layer was extracted twice with
200 mL portions of diethyl ether. The combined organic layer
was washed twice with water and dried over MgSO4. The
solvent was evaporated, and product was recrystallized from
hexane to give a white product. Yield after recrystallization:
65.7 g (75%); mp: 115 °C. Anal. Calcd for C16H16Br2 (368.11):
2,2′-Bis(4′′-ter t-bu tylp h en yl)-4,4′,5,5′-bip h en yltetr a ca r -
boxylic Acid (BBBP TA). The oxidation was performed using
BBTMBP (5 g, 10.5 mmol), pyridine (240 mL), distilled water
(40 mL), and KMnO4 (16.59 g 0.105 mol). The reaction mixture
was refluxed for 24 h. For the second reaction step NaOH (8
g, 0.2 mol), distilled water (200 mL), and KMnO4 (8.29 g, 0.053
mol) were used. The residual manganese dioxide was filtered
and washed several times with hot water. After treatment of
the cooled solution with excess concentrated HCl a white solid
precipitated slowly, which was collected and dried in vacuo at
90 °C for 24 h. Yield: 5 g (80%); mp: 214 °C. Anal. Calcd for
C
36H34O8 (594.65): C, 72.71%; H, 5.76%. Found: C, 72.52%;
1
H, 5.61%. H NMR (DMSO-d6/TMS): δ 1.05 (s, 18H, Ar-tert-
butyl), 6.11-6.15 (d, 2H, Ar-H), 6.75-6.82 (d, 2H, Ar-H), 7.19
(s, 1H, Ar-H), 7.58 (s, 1H, Ar-H). FT-IR (KBr pellet): 3777
(carboxylic C-OH stretching), 3020 (aromatic C-H), 2980
(aliphatic C-H), 1721 cm-1 (CdO stretching).
1
C, 52.21%; H, 4.38%. Found: C, 52.35%; H, 4.45%. H NMR
(CDCl3/TMS): δ 2.27-2.32 (d, 12H, Ar-CH3), 7.02 (s, 1H,
Ar-H), 7.45 (s, 1H, Ar-H). FT-IR (KBr pellet): 3073 (aromatic
C-H), 2995 cm-1 (aliphatic C-H).
p-ter t-Bu tylben zen ebor on ic Acid (TBBBA) a n d p-Tr i-
m eth ylsilylben zen ebor on ic Acid (TSBBA). Magnesium
turnings (6.72 g, 0.28 mol) were weighed into 500 mL three-
necked round-bottom flask, equipped with a magnetic stirrer
and activated with iodine. The solution of 46.9 g of bromo
compound (0.22 mol) in dry THF (500 mL) was added at a rate
that kept the reaction mixture refluxing. Finally, the reaction
mixture was refluxed for 4-5 h until consumption of almost
all magnesium turnings. After being cooled to room temper-
ature, the trimethylborate (0.33 mol) was added dropwise to
a cooled (-78 °C) solution with a syringe. After complete
addition the mixture was allowed to warm to room tempera-
ture and stirred overnight. Subsequently, 2 N HCl solution
(300 mL) was added. The organic layer was separated, and
the aqueous layer was extracted twice with 200 mL of diethyl
ether. The combined organic layer was washed with water and
dried over MgSO4. The solvent was evaporated, and the white
powdered boronic acid derivative was washed several times
with hexane to remove any impurity. Yield of TBBBA: 31 g
(80%); mp: 209.8 °C. Anal. Calcd for C10H15BO2 (178.04): C,
67.46%; H, 8.49%. Found: C, 67.32%; H, 8.39%. 1H NMR
(CDCl3/TMS): δ 1.32 (s, 9H, Ar-tert-butyl), 7.42-7.47 (d, 2H,
Ar-H), 7.83-7.89 (d, 2H, Ar-H). FT-IR (KBr pellet): 3336
(B-OH stretching), 3000 (aromatic C-H), 2966 cm-1 (aliphatic
C-H). TSBBA: 32 g (76%); mp 179.40 °C. Anal. Calcd for
C9H15BO2Si (194.11): C, 55.69%; H, 7.79%. Found: C, 55.91%;
H, 7.70%. 1H NMR (CDCl3/TMS): δ 0.31 (s, 9H, Si-CH3),
7.72-7.77 (d, 2H, Ar-H), 8.21-8.26 (d, 2H, Ar-H). FT-IR
(KBr pellet): 3217 (B-OH stretching), 3050 (aromatic C-H),
2955 (aliphatic C-H), 1101 cm-1 (C-Si stretching).
2,2′-Bis(4′′-tr im eth ylsilylp h en yl)-4,4′,5,5′-bip h en yltet-
r a ca r boxylic Acid (BTSBP TA). The synthesis of BTSBPTA
was conducted as described above. Yield: 4.5 g (72%); mp: 210
°C. Anal. Calcd for C34H34O8Si2 (626.80): C, 65.15%; H, 5.47%.
Found: C, 65.32%; H, 5.55%. 1H NMR (DMSO-d6/TMS): δ 0.02
(s, 18H, Si-CH3), 6.18-6.19 (d, 2H, Ar-H), 6.88-6.92 (d, 2H,
Ar-H), 7.29 (s, 1H, Ar-H), 7.57 (s, 1H, Ar-H). FT-IR (KBr
pellet): 3500 (carboxylic C-OH stretching), 3020 (aromatic
C-H), 2980 (aliphatic C-H), 1720 cm-1 (CdO stretching).
Dia n h yd r id es. In 100 mL round-bottom flask, tetracar-
boxylic acid (12 mmol) was dissolved in acetic acid (35 mL)
and acetic anhydride (35 mL). The solution was refluxed for 4
h. After the solution was cooled to room temperature and
filtered, the product was recrystallized for 1 day. The product
was filtered and washed with toluene and dried at 120 °C
under vacuum. Yield of 2,2′-bis(4′′-tert-butylphenyl)-4,4′,5,5′-
biphenyltetracarboxylic dianhydride (BBBPAn): 4.5 g (67%);
mp: 310 °C. Anal. Calcd for C36H30O6 (558.62): C, 77.40%; H,
5.41%. Found: C, 77.25%; H, 5.36%. 1H NMR (DMSO-d6/
TMS): δ 1.27 (s, 18H, Ar-tert-butyl), 6.36-6.39 (d, 2H, Ar-
H), 6.97-7.03 (d, 2H, Ar-H), 7.81 (s, 1H, Ar-H), 8.33 (s, 1H,
Ar-H). FT-IR (KBr pellet): 3015 (aromatic C-H), 2980
(aliphatic C-H), 1770 cm-1 (CdO stretching). Yield of 2,2′-
bis(4′′-trimethylsilylphenyl)-4,4′,5,5′-biphenyltetracarboxylic di-
anhydride (BTSBPAn): 5 g (73%); mp: 300 °C. Anal. Calcd
for C34H30O6Si2 (590.77): C, 69.12%; H, 5.12%. Found: C,
69.02%; H, 5.06%. 1H NMR (DMSO-d6/TMS): δ 0.007 (s, 18H,
Si-CH3), 6.16-6.19 (d, 2H, Ar-H), 6.86-6.89 (d, 2H, Ar-H),
7.59 (s, 1H, Ar-H), 8.11 (s, 1H, Ar-H). FT-IR (KBr pellet):

Macromolecules, Vol. 36, No. 7, 2003
Polyimides 2329
Sch em e 2. P r ep a r a tion s of P olyim id es
Sch em e 1. Syn th etic Rou tes for Mon om er s
by Ni-coupling reaction of 4-bromo-o-xylene. The bro-
mination of 3,3′,4,4′-tetramethylbiphenyl gave 2,2′-
dibromo-4,4′,5,5′-tetramethylbiphenyl, which was re-
acted with p-tert-butylbenzene boronic acid or p-trimeth-
ylsilylphenyl boronic acid to generate 2,2′-bis(4′′-tert-
butylphenyl)-4,4′,5,5′-tetramethylbiphenyl and 2,2′-bis-
(4′′-trimethylsilylphenyl)-4,4′,5,5′-tetramethylbiphen-
yl, respectively. 2,2′-Bis(4′′-tert-butylphenyl)-4,4′,5,5′-bi-
phenyltetracarboxylic dianhydride (BBBPAn) and 2,2′-
bis(4′′-trimethylsilylphenyl)-4,4′,5,5′-biphenyltetracar-
boxylic dianhydride (BTSBPAn) were obtained by oxi-
dation and dehydration of 2,2′-bis(4′′-tert-butylphenyl)-
4,4′,5,5′-tetramethylbiphenyl and 2,2′-bis(4′′-trimeth-
ylsilylphenyl)-4,4′,5,5′-tetramethylbiphenyl. The yields
in each step were very high, and the obtained products
in each steps were confirmed by spectroscopic tech-
niques. The structures of obtained monomers, BBBPAn
3017 (aromatic C-H), 2982 (aliphatic C-H), 1770 cm-1 (CdO
stretching).
P olym er Syn th esis. Polymerization procedures were as
follows. Corresponding diamine (1 mmol) was dissolved in
m-cresol in a 25 mL flask. After the diamine was dissolved
completely, 1 mmol of dianhydride was added. The mixture
was stirred at room temperature for 2 h. The temperature of
the mixture was gradually elevated to 200 °C and then stirred
for 5 h. An excess m-cresol was added to the mixture, and the
mixture was cooled. The obtained polymer was precipitated
in methanol. The precipitation procedure was carried out
several times. Yield: 93-98%.
P r ep a r a tion of Den se F ilm . A 5-7 wt % solution of
polymer in chloroform was prepared and filtered through a
0.2 µm syringe filter to remove the nondissolved materials and
dust particles. The solution was then poured into a casting
ring on a leveled clean glass plate. The casting ring was
covered with a piece of filter paper. The casting process took
about 8 h at room temperature. The cast films were dried in
an oven at 40 °C for 6 h without vacuum and for another 6 h
with vacuum, and the film samples were dried at 80 °C for 6
h and then at 100 °C for 10 h. Dense films were obtained at
150 °C in a vacuum oven for an additional 24 h before the gas
permeability measurement.
1
and BTSBPAn, were also analyzed by H NMR, FT-IR,
and elemental analysis. For BBBPAn, the proton peaks
of tert-butyl and aromatic ring appeared at 1.2 and
6.3-8.2 ppm, respectively. In the case of BTSBPAn,
those of trimethylsilyl and aromatic ring appeared at
0.2 and 6.2-8.2 ppm, respectively. In the FT-IR spec-
trum, instead of the broad acid peak, a peak character-
istic of a carbonyl group of dianhydrides appeared at
1770 and 1860 cm-1
.
Resu lts a n d Discu ssion
The synthetic routes for monomers are shown in
Scheme 1. 3,3′,4,4′-Tetramethylbiphenyl was obtained
It is known that there are two general methods for
the preparation of polyimides. One procedure involves
Ta ble 1. Yield s a n d P r op er ties of th e P olyim id es
elemental analysis
e
yield
(%)
inh viscd
(dL/g)
Mw
PDI
(Mw/Mn)
moisture
polymer code
(GPC)
C
H
N
intake (%)f
polyimide 1
94
96
96
96
98
93
1.43
0.56
0.41
0.59
0.58
0.38
332 400
141 500
138 000
110 400
2.16
2.33
1.87
3.51
calcd
found
calcd
found
calcd
found
calcd
found
calcd
found
calcd
found
79.76
80.80
81.64
82.19
71.49
70.49
73.18
74.06
74.97
76.52
66.20
65.15
5.30
5.03
5.59
6.23
4.47
4.57
5.07
4.74
5.35
5.92
4.31
4.39
3.88
3.83
3.89
3.96
3.27
3.42
3.71
3.95
3.72
4.07
3.15
3.32
1.89
0.88
0.38
0.14
0.93
0.26
(BBBPAn-ODAa)
polyimide 2
(BBBPAn-MDAb)
polyimide3
(BBBPAn-FDAc)
polyimide 4
(BTSBPAn-ODA)
polyimide 5
BTSBPAn-MDA)
polyimide 6
60 400
1.92
(BTSBPAn-FDA)
a
d
4,4′-Oxydianiline. b 4,4′-Methylenediamine. c 4,4′-(Hexafluoroisopropylidene)dianiline. Inherent viscosity measured at a concentration
of 0.5 g/dL in DMAc at 25 °C. Relative to polystyrene. e Moisture intake (%) ) 100 × (W - W0); W ) weight of polymer sample after
standing at room temperature for 3 days; W0 ) weight of polymer sample after being dried in a vacuum at 100 °C for 10 h.

2330 Kim et al.
Macromolecules, Vol. 36, No. 7, 2003
F igu r e 1. FT-IR and 1H NMR spectra of polyimide 1 (above) and polyimide 4 (below).
two steps and proceeds via poly(amic acid)s intermedi-
ate. The other is one-step solution polymerization. The
one-step method has some advantages over the two-step
method. Polyimides in bulk are more easily produced,
and polyimides with higher crystallinity can be more
readily obtained.18 The one-step method is also useful
for unreactive diamines and dianhydrides which cannot
form high-molecular-weight poly(amic acid)s by the two-

Macromolecules, Vol. 36, No. 7, 2003
Polyimides 2331
Ta ble 2. Solu bility of P olyim id esa
solvent
polyimide
m-cresol
NMP
DMF
DMAc
DMSO
pyridine
1,2-dichlorobenzene
chloroform
THF
acetone
polyimide I
polyimide 2
polyimide 3
polyimide 4
polyimide 5
polyimide 6
+ -
+ +
+ +
+ -
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ -
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ -
+ +
+ -
- -
- -
- -
- -
- -
- -
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ -
+ +
+ +
+ -
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ -
+ +
- -
- -
- -
- -
- -
- -
a
+ + ) soluble, + - ) partially soluble, - - ) insoluble.
Ta ble 3. Th er m a l Beh a vior of P olyim id es
step method.19,20 The disadvantage of the one-step
method is that insoluble polyimides cannot form high-
molecular-weight polyimides because of premature pre-
cipitaion. We tried two general methods (Scheme 2).
When polyimides were synthesized by the conventional
two-step procedure involving a ring-opening polyaddi-
tion and subsequent thermal cyclodehydration, the
polymer molecular weights were not enough for prepar-
ing of free-standing films. It may be suggested that
dianhydrides are deactivated by steric hindrance due
to bulky tert-butylphenyl or trimethylsilylphenyl sub-
stituents in the 2,2′-position of the s-BPDA unit. Oth-
erwise, the one-step polymerization at high temperature
resulted in high viscosity. The yields of polymerization
were 93-98%. The results of the polymerization are
summarized in Table 1. The inherent viscosities of
polyimides are 0.38-1.43 dL/g. The gel permeation
chromatography (GPC) measurements demonstrated
that polymers exhibited Mw and polydispersities
(Mw/Mn) in the ranges 60 400-332 400 and 1.87-3.51,
respectively. The inherent viscosity decreased in the
following order: polyimide 1 > polyimide 2 > polyimide
3 for BBBPAn and polyimide 4 > polyimide 5 >
polyimide 6 for BTSBPAn. From the results, it appears
that a diamine with an electron-donating group is active
for imidization; thus, polyimides from diamines with an
electron-donating group have higher molecular weights.
The elemental analysis values agreed quite well with
calculated values for the proposed structures of poly-
imides. The structures of obtained polyimides were also
a
b
polyimide
Tg (°C)
Td (°C)
char yieldc (%)
polyimide 1
polyimide 2
polyimide 3
polyimide 4
polyimide 5
polyimide 6
370
360
381
348
348
365
550
540
549
552
542
543
65
58
57
62
65
57
a
From the second heating trace of DSC measurements con-
heating rate of 20 °C/min under nitrogen
ducted with
a
a
b
atmosphere. 5% weight loss temperature in TGA at 20 °C/min
heating rate. c Residual yield in TGA at 800 °C under a nitrogen
atmosphere.
Ta ble 4. Mech a n ica l P r op er ties of P olyim id es
tensile
strength (MPa)
elongation at
break (%)
tensile
modulus (MPa)
polyimide
polyimide 1
polyimide 2
polyimide 3
polyimide 4
polyimide 5
polyimide 6
58
54
44
94
63
38
9
9
6
11
10
7
805
630
780
1129
696
656
of the bulky substituents restricts the segmental motion.
The polyimides derived from BBBPAn showed higher
Tg’s than those derived from BTSBPAn. The result can
be explained in terms of the flexibility and low rotation
barrier of the trimethylsilyl group and (or) the rigidity
of the tert-butyl group. The results of the TGA and DSC
analyses showed an excellent thermal stability of the
polyimides even though the polyimides showed high
solubility.
The mechanical properties of the two series of poly-
imides prepared by solution casting from chloroform
solution are summarized in Table 4. The polyimide films
had tensile strengths of 38-94 MPa, elongation at break
of 6-11%, and tensile modulus of 0.63-1.13 GPa. The
polyimides derived from BBBPAn showed higher elon-
gation at break than those derived from BTSBPAn. It
may be due to the flexibility of the trimethylsilylphenyl
group. The polyimide 3 and polyimide 6 derived from
hexafluoroisopropylidenediamine (FDA) showed lower
elongation at break than those derived from 4,4′-
methylenedianiline (MDA) and 4,4′-oxydianiline (ODA).
The result can be explained by the rigidity of FDA
having trifluoromethyl groups. The results of mechan-
ical properties of films are relatively low for high-
molecular-weight polyimides. It may result from low
intermolecular interaction due to bulky and twisted
molecular structure.
1
confirmed by IR and H NMR spectroscopies. Figure 1
showed IR and 1H NMR spectra of polyimide 1 and
polyimide 4, respectively. The IR spectra supported the
formation of polyimides. The characteristic absorption
bands of the polyimides rings appeared near 1778 (asym
CdO str), 1722 (sym CdO str), 1371 (C-N str), and 742
cm-1 (imide ring deformation). The proton peaks are
assigned for the obtained polyimide 1 and polyimide 4,
respectively.
The solubility of polyimides was studied in various
solvents (Table 2). Regardless of the nature of substit-
uents introduced in dianhydride and diamine, the
synthesized polyimides showed good solubility in com-
mon organic solvents such as chloroform and THF. It
is supposed that this is due to the noncoplanar twisted
biphenyl structure and the bulkiness of introduced
substituents.
The thermal properties of the polyimides were inves-
tigated by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) in a nitrogen atmo-
sphere (Table 3). The polyimides exhibited 5% weight
loss at 540-550 °C in nitrogen as measured by ther-
mogravimetric analysis. The thermal transitions (Tg’s)
of polyimides were observed between 348 and 381 °C.
The high Tg’s of polyimides can be explained by the fact
that noncoplanar twisted biphenyl and steric hindrance
From such good solubility, good thermal stability, and
mechanical properties, it is suggested that the obtained
polyimides have adequate properties for gas perme-
ability. Therefore, the oxygen gas permeation of films
of obtained polyimides was studied (Table 5). The newly
synthesized polyimides showed a high oxygen perme-
ability coefficient [P; cm3 (STP) cm/(cm2 scmHg)] (PO
2

2332 Kim et al.
Macromolecules, Vol. 36, No. 7, 2003
Ta ble 5. Ga s Tr a n sp or t P r op er ties of P olyim id es
permeabilitya
O2 N2
polyimides showed the highest oxygen permeability ever
reported. Thus, these new polyimides could be consid-
ered as newly processable and for varied uses in high-
performance polymeric material.
permselectivity
polyimide
(PO /PN
)
2
2
polyimide 1
polyimide 2
polyimide 3
polyimide 4
polyimide 5
polyimide 6
43
31
110
61
52
105
12
8
35
18
12
37
3.58
3.9
3.14
3.4
4.3
2.84
Ack n ow led gm en t. This work was supported by
Korea Research Foundation Grant (KRF-2000-005-
E00005).
Refer en ces a n d Notes
a
1 barrer ) 10-10 cm3(STP) cm/(s cm2 cmHg).
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) 31-110 barrer) and the permselectivity of oxygen to
nitrogen ( P /P ) 2.8-4.3). The results showed that
O2 N2
the newly obtained polyimides had the highest oxygen
permeability ever reported.21 The results may be ex-
plained by the fact that the highly twisted main chain
with bulky p-tert-butylphenyl or p-trimethylsilylphenyl
inhibits intermolecular packing, and this provides high
free volume. The polyimides derived from BTSBPAn
containing trimethylsilylphenyl showed better oxygen
permeability than the polyimides derived from BBBPAn
containing a tert-butylphenyl group. It is possible that
BTSBPAn with a bulky and flexible trimethylsilyl group
has a high solubility toward oxygen, and (or) it can
afford the high free volume. Changing the diamine part
from FDA to MDA, the permeability for oxygen in-
creased in the order MDA, ODA containing flexible
oxygen linkage, FDA containing bulky and highly
oxygen soluble hexafluoroisopropylidene linkage.
Con clu sion s
Two new dianhydride monomers composed of a non-
coplanar twisted biphenyl unit containing tert-butyl-
phenyl or trimethylsilylphenyl groups were prepared by
various organic reactions. A series of polyimides with
high molecular weights were obtained from such dian-
hydride monomers and three commercially available
diamines. The obtained polymers showed high thermal
stability with high Tg’s, good mechanical properties, and
high solubility in common organic solvents such as
chloroform and THF. In addition, the newly obtained
MA0214557