Novel aromatic polyimides derived from 5′-t-Butyl-2′- pivaloylimino3,4,3″,4″-m-terphenyltetracarboxylic dianhydride with potential application on gas separation processes

Article abstract of DOI:10.1021/ma901943j

A new family of polyimides having two bulky groups in the central ring of a m-terphenyl moiety has been obtained. These polyimides have been synthesized by reaction of four commercial diamines with the electrophilic monomer, 5′-t-butyl-2′-pivaloylimino-3,

Full text of DOI:10.1021/ma901943j

2268 Macromolecules 2010, 43, 2268–2275
DOI: 10.1021/ma901943j
Novel Aromatic Polyimides Derived from 50-t-Butyl-20-pivaloylimino-
3,4,300,400-m-terphenyltetracarboxylic Dianhydride with Potential
Application on Gas Separation Processes
ꢀ
Mariola Calle, Angel E. Lozano, Jose G. de La Campa, and Javier de Abajo*
ꢀ
´
´
Instituto de Ciencia y Tecnologıa de Polımeros, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
Received September 1, 2009; Revised Manuscript Received January 19, 2010
ABSTRACT: A new family of polyimides having two bulky groups in the central ring of a m-terphenyl
moiety has been obtained. These polyimides have been synthesized by reaction of four commercial diamines
with the electrophilic monomer, 50-t-butyl-20-pivaloylimino-3,4,300,400-m-terphenyltetracarboxylic acid
dianhydride (PTPDA). This monomer has been obtained by a modern C-H-activated arylation method
with good yield and purity. The attained polymers showed Tg values ranging from 300 to 430 °C, thermal
stability >500 °C, and excellent solubility in a broad range of solvents, including tetrahydrofuran. Despite
having an amide group, uptake of water was low and comparable to that of classical aromatic polyimides.
Mechanical properties of polymer films were good enough to permit the use of these polyimides on gas
separation applications at high pressures. Films obtained by casting offered a good balance of permeability
and permselectivity with values close to the Robeson upper-bound, in particular for the O2/N2 gas pair.
Introduction
In view of that, the major goal of this ongoing research has
consisted of further improving the gas separation capacity of
Aromatic polyimides are considered tobe relevant materialsfor
advanced technologies because of their outstanding thermal,
mechanical, and chemical stability. However, it is known that
fully aromatic polyimides are very often insoluble in most organic
solvents, which highly limits their application in many fields.1-6
To overcome this drawback, different approaches have been
outlined, most of them based on the modification of their chemical
structure. Flexible links such as -O-, -SO2-, or -CH2-, or
bulky groups (hexafluoroisopropylidene, isopropylidene, etc.) are
commonly employed as solubilizing moieties,7-9 but it often
brings about a loss of thermal stability and chain stiffness of
polyimides. Alternatively, tractable aromatic polyimides can be
obtained from nonplanar monomers where the nonplanarity
provides the conformational freedom to lower chain regularity
and rigidity.7,10 With the purpose of improving the tractability,
polyimides derived from m-terphenyl dianhydride have been
synthesized, yet they still showed limited solubility in organic
solvents.11 Furthermore, it has been observed that the incorpora-
tion of a t-butyl pendent group in the 50 position of the
m-terphenyl moiety improves their solubility. Therefore, poly-
imides from 50-t-butyl-m-terphenyl-3,4,300,400-tetracarboxylic acid
dianhydride (BTPDA) were soluble in m-cresol, N-methylpyrro-
lidinone (NMP), and N,N-dimethylacetamide (DMAc).12 These
soluble m-terphenyl polyimides are promising materials in differ-
ent fields, including gas separation applications, because they
exhibit a combination of desirable properties such as high thermal
resistance, high glass-transition temperatures, and good mechan-
ical properties. In addition, the effective chain separation effect
caused by the presence of the bulky t-butyl pendant group, results
in a hindered molecular packing affording high free volume, and
hence, they have an excellent permeability to low Mw molecules.13
However, these polyimides exhibit a moderated permselectivity
and a moderated gas separation capacity owing to the trade-off
between permeability and selectivity.14-16
these versatile aromatic polyimides from BTPDA as well as their
processability from different organic solvents without impairing
their whole outstanding properties by means of an adequate,
tailormade modification.
This article reports the synthesis of a novel dianhydride derived
from BTPDA, which in addition contains a bulky substituent in
position 20 such as the pivalamide group (pivalamide or pivaloyl-
imino: C(CH3)3CONH-). It has been prepared by a suitable,
economical, and original route involving C-H activation metho-
dology.17 The incorporation of the pivaloyl substituent in position
20 has simultaneously brought about a noteworthy improvement
in polymer chain rigidity and solubility relative to those polyimides
derived from BTPDA. Additionally, the improvement in polymer
chain rigidity has led to a better gas separation capacity. Poly-
condensation of this dianhydride with four aromatic diamines,
characterization of the resulting polyimides, and a preliminary
evaluation as gas separation membranes are described and com-
pared with some polyimides derived from BTPDA.
Experimental Section
Materials. Solvents and reactants were of reagent-grade qual-
ity and used without further purification. Pivaloyl chloride, 4-t-
butylaniline, and palladium acetate were purchased from Aldrich
and silver acetate to Fluka. 2,2-Bis-(4-aminophenyl)hexafluoro-
propane (Criskev Co., Kansas) (6FpDA), 2,2-bis-(3-amino-4-
methylphenyl)hexafluoro-propane (Apollo, U.K.) (m6FMe),
2,4,6-trimethyl-m-phenylendiamine (Aldrich) (3MemPD), and
2,3,5,6-tetramethyl-p-phenylendiamine (Aldrich) (4MepPD) were
sublimed just before being used. Dimethyl 4-iodophthalate was
synthesized by oxidation of 4-iodo-ortho-xylene using potassium
permanganate in a mixture of pyridine/water,18 followed by
Fisher esterification of the obtained diacid with methanol.19
Monomer Synthesis. Elemental analyses and spectroscopic
characterization (1H and 13C NMR) data of intermediates and
monomers is included in the Supporting Information
*Corresponding author.
pubs.acs.org/Macromolecules
Published on Web 02/03/2010
r 2010 American Chemical Society

Article
4-t-Butylpivanilide. Pivaloyl chloride (78 mmol, 9.6 mL)
was added dropwise to a cooled solution (0 °C) of chloroform
(150 mL), 4-t-butylaniline (67 mmol, 10.7 mL), and pyridine
(67 mmol, 5.4 mL). After stirring for 15 min at 0 °C, the mixture
was allowed to warm up to room temperature and stirred for 2 h.
The solvent was removed, and the solid residue was washed with
water and recrystallized from EtOH/H2O (1/1) to give a white
product. Yield: 95%. mp 189 °C.
Macromolecules, Vol. 43, No. 5, 2010 2269
Afterward, benzoic acid (2.0 mmol) was added, and the mixture
was stirred at 180 °C for 8 h. After cooling, the solution was
poured slowly into a 1/2 (v/v) mixture of water/ethanol. The
obtained polymer was filtered off and washed thoroughly with a
1/1 (v/v) mixture of water/ethanol, extracted with ethanol in a
Soxhlet apparatus, and finally dried in a vacuum oven at 120 °C
overnight. Yields were virtually quantitative in every case.
Measurements. Elemental analyses were performed with a
Carlo Erba EA1108 elemental analyzer. 1H and 13C NMR
spectra were recorded on a Varian Gemini spectrometer tuned
at 299,95 and 75,43 MHz, respectively. FT-IR spectra were
recorded on a Perkin-Elmer RX-1 device on powder for mono-
mers and intermediates and on films (50-60 μm) for polymers.
An attenuated total reflection accessory (ATR) was used in all
cases. Differential scanning calorimetry (DSC) analyses were
performed on a TA instruments Q-2000 calorimeter at a heating
rate of 20 °C/min under nitrogen. Thermogravimetric analyses
(TGAs) were performed on a TA Q-500 thermobalance, heating
under controlled flux of nitrogen at 10 °C/min. Inherent visco-
sities were measured at 25 °C with an Ubbelohde viscometer
with NMP as a solvent on polymer solutions with a concentra-
tion of 0.3 g/dL. Melting points of intermediates and monomer
were measured by DSC. Qualitative solubility was determined
with 10 mg of polymer in 1 mL of solvent at room temperature
after 24 h or heating until dissolution for samples soluble on
heating. Measurements of mechanical properties were con-
ducted on a MTS Synergie 200 Universal Testing dynamometer
of vertical extension on 30 mm length, 5 mm width film strips
using mechanical clamps with an initial separation of 12 mm and
5 mm/min extension rate.
Weight-average molecular weights (Mw) and number-average
molecular weights (Mn) were determined by gel permeation
chromatography (GPC) with Perkin-Elmer Series 200 pump,
oven, and UV-vis detector. Wavelength was 271 nm, oven
temperature was 70 °C, and solvent flow was 0.3 mL/min.
Polyethyleneglycol standards were used as references. The
samples were prepared by dissolving 1 mg of polymer in 1 mL
of N,N-dimethylformamide. The software TotalChrom from
Perkin-Elmer was used for data acquisition and analysis. Two
Polymer Laboratories Resipore columns (3 μm, 300 ꢀ 4.6 mm2,
molecular weights range from 200 to 400 000) were used. Eluent
was N,N-dimethylformamide containing 0.1% (w/v) LiBr.
Permeability to pure gases was studied on polymer films,
made by casting 10% (w/v) DMAc solutions of polymers onto a
leveled glass plate and by heating at 80 °C for 12 h and at 180 °C
under vacuum overnight. A barometric method was used to
determine steady-state pure gas permeability at 30 °C, applying
a pressure of 3 bar and an initial pressure in the expansion
chamber <0.1 mbar.
50-t-Butyl-20-pivaloylimino-tetramethyl-3,4,300,400-m-terphenyl-
carboxilate. A solution of 4-t-butylpivanilide (2.92 g, 12.5 mmol),
dimethyl 4-iodophthalate (20 g, 62.4 mmol), palladium acetate
(0.14 g, 0.62 mmol), and silver acetate (5.8 g, 50.0 mmol) in
trifluoroacetic acid (40.0 mL) was heated under nitrogen at
120 °C. The reaction was followed by TLC until itwas completed.
During this time, a dark-yellow precipitate was formed, and the
supernatant became clear dark red. The reaction mixture was
diluted with toluene (60 mL) and filtered through Celite, and the
solvent was evaporated under reduced pressure. The residue was
purified by flash chromatography on silica gel (CH2Cl2/AcOEt
(10/1)) to give the product after solvent evaporation as a white
solid. Yield (5 runs): 80%. mp 167 °C.
50-t-Butyl-20-pivaloylimino-3,4,300,400-m-terphenyltetracarboxylic
Acid. The tetramethyl-m-terphenylcarboxilate (20.0 g, 32.4 mmol)
and a solution of 10.4 g of sodium hydroxide in 150 mL of water
were charged into a 500 mL round-bottomed flask equipped with a
reflux condenser. The mixture was boiled until the ester dis-
appeared, as checked by TLC. The reaction solution was diluted
with an equal volume of water, and after cooling, it was poured
with vigorous stirring onto 120 mL of hydrochloric acid 1 N. The
crude acid precipitated as a clear solid, which was filtered, dis-
solved in a NaOH solution, and poured again into concentrated
HCl for further purification. The tetracarboxylic acid was then
obtained as a white solid, which was thoroughly washed with water
and dried under vacuum at 90 °C for 24 h. Yield: 18 g (90%). mp
283 °C (cyclization).
50-t-Butyl-20-pivaloylimino-3,4,300,400-m-terphenyltetracarboxylic
Dianhydride. The dianhydride PTPDA was prepared by refluxing
the tetraacid precursor (20 mmol) in acetic anhydride (50 mL) and
a small amount of AcONa for 3 h. The solution was filtered and
cooled to room temperature, and the monomer crystallized off as a
white solid, which was filtered, washed with toluene, and dried at
120 °C under vacuum. Yield: 95%. mp 292 °C.
Polymer Synthesis. Two Steps Polymerization Method. A
three-necked flask, equipped with a mechanical stirrer and gas
inlet and outlet, was charged with 3.0 mmol of diamine and
3.0 mL of NMP. The mixture was stirred at room temperature
under a blanket of nitrogen until the solid was entirely dissolved.
Then, the solution was cooled to 0 °C, and pyridine (6.0 mmol)
and trimethylchlorosilane (6.0 mmol) were slowly added. The
solution was allowed to warm up to room temperature and
stirred for 15 min to ensure the formation of the silylated
diamine. After this time, the solution was cooled again to
0 °C, and pivaloyl dianhydride (3.0 mmol) was added, followed
by 3.0 mL of NMP. The reaction mixture was stirred for 15 min
at 0 °C; then, the temperature was raised up to room tempera-
ture and left overnight. Acetic anhydride (30 mmol) and pyri-
dine (30 mmol) were then added, and the viscous solution was
stirred at room temperature for 6 h, followed by heating for a
further 1 h at 60 °C to promote imidation. The flask was cooled
down to room temperature, and the polymer solution was
poured dropwise on 500 mL of water, washed several times
with water, and extracted in a Soxhlet with ethanol to remove
traces of solvent and oligomers. The polymer was dried over-
night under vacuum at 120 °C. Yield was quantitative.
Water sorption measurements were determined gravimetrically
at 25 °C in a 65% relative humidity atmosphere. Films of 100 mg,
previously dried at 120 °C for 24 h over P2O5, were placed in a
closed desiccator containing a saturated aqueous solution of
NaNO2. The films were weighed periodically during 12 h and then
were allowed to humidify for 3 more days until they had equili-
brated with their surroundings, as denoted by no weight change.
Results and Discussion
Monomer Synthesis. The synthetic route for dianhydride
PTPDA is shown in Scheme 1. Intermediates were not
commercially available, so they had to be prepared. 4-t-
Butylpivalanilide 1 was obtained by facile acylation of 4-t-
butylaniline with pivaloyl chloride. Dimethyl 4-iodophtha-
late 2 was synthesized by oxidation of 4-iodo-ortho-xylene
using potassium permanganate12,18 in a mixture of pyridine/
water, followed by Fisher esterification of the obtained
diacid with methanol.19 The overall yield was ∼85%.
One-Step Polymerization Method. m-Cresol (12 mL) and 3.0
mmol of diamine were mixed in a 50 mL three-necked flask with
mechanical stirring and blanketed by dry nitrogen. After the
diamine was dissolved, 3.0 mmol of dianhydride was added, and
the mixture was heated to 80 °C. Then, pyridine (2.0 mmol) was
added, and the solution was kept at that temperature for 1 h.
The key step of this synthetic route was the coupling
reaction to give the m-terphenyl moiety. We have previously

2270 Macromolecules, Vol. 43, No. 5, 2010
Calle et al.
Scheme 1. Synthesis of Dianhydride PTPDA
described the preparation of m-terphenyl dianhydrides by
means of a Suzuki catalyzed coupling reaction of arylbro-
mides with arylboronic acid.11,12 This highly efficient and
selective methodology involves the use of a dibromo deriva-
tive and an ulterior oxidation of four methyl groups to give
finally a tetraacid compound. This step can represent very
often a drawback in the synthetic process because achieving a
whole oxidation is cumbersome and, moreover, the yield in
terms of pure tetracarboxylic compound can be not very
high. In view of that, we have now developed a new proce-
dure based on a double arylation by C-H activation meth-
odology.17 In this method, the presence of a directing group,
such as pivalamide, allows the direct ortho-arylation of the
benzene ring. Besides, doing the reaction on an acid deriva-
tive is an additional advantage because no oxidation is
needed. Therefore, the reaction of the 4-t-butylpivalanilide
1 with dimethyl 4-iodophthalate 2, in combination with
silver acetate and palladium acetate as catalysts, yielded
the m-terphenyltetraester derivative 3 in a clean and selective
mode. The next steps, saponification and dehydratation,
afforded the dianhydride PTPDA in high yield and purity.
Saponification of the tetraester precursor was carried out by
refluxing with NaOH solution,20 and its completion was
determined by TLC. The tetracarboxylic acid 4 was cyclo-
dehydrated by means of acetic anhydride and NaOAc to give
PTPDA in quantitative yield and pure enough for preparing
polymers of high molecular weight. The obtained intermedi-
ates in each step as well as the structure of the novel monomer
were confirmed by spectroscopic techniques. As an example,
polymer solutions. However, when this two-step procedure
was tried out with diamine 2,2-bis-(3-amino-4-methyl-
phenyl) hexafluoro-propane (m6FMe), very low viscosities
were achieved because of the premature precipitation of the
poly(amic acid) precursor. In contrast, it was found out that
the use of the one-pot method in m-cresol solution at high
temperature22-24 led to viscosities up to 1.26 dL/g for
PTPDA-m6FMe polyimide and up to 0.65 dL/g when
the sterically hindered 2,4,6-trimethyl-m-phenylendiamine
(3MemPD) was employed. These results confirmed that the
one-step procedure seems to be a more suitable and con-
venient method for this dianhydride when the polyimide is
soluble in the reaction solvent. Only when the 2,3,5,6-tetra-
methyl-p-phenylendiamine (4MepPD) was employed, a
modest value of viscosity was achieved (η = 0.41 dL/g),
presumably because of the higher steric hindrance and rigid
character of this tetramethylated diamine.
The results of polymerization are compiled in Table 1.
From GPC measurements, it was confirmed that polymers
had high molecular weights in the range of 46 800-301 000 g/mol
for Mw and 14 300-77 500 g/mol for Mn. The elemental
analyses agreed quite well with calculated values for the
proposed structures of polyimides. The chemical composi-
tion of the polymers was also confirmed by IR and 1H NMR
1
spectroscopies. As an example, the H NMR spectrum of
PTPDA-6FpDA is showed in Figure 3. The IR spectra
supported the complete imidation; characteristic absorption
bands of polyimides appeared at 1780 (asym CdO str), 1718
(sym CdO str), 1364 (C-N str), and 750 cm-1 (imide ring
deformation), whereas no signals associated with polyamic
acid (higher than 3000 cm-1) were observed.
1
FT-IR and H NMR spectra of dianhydride PTPDA have
been reproduced in Figure 1.
The solubility of polyimides was studied in different
solvents (Table 2). Satisfactorily, all polymers were soluble
in aprotic polar solvents and even in common organic
solvents such as THF and chloroform. Taking into account
the limited solubility showed by classical aromatic poly-
imides and the related polyimides from 50-t-butyl-m-terphe-
nyl-3,4,300,400-tetracarboxylic acid dianhydride,12 it is clear
that the incorporation of the pivalamide group on the
dianhydride moiety significantly enhances the solubility,
and hence the processability. This improvement is most
likely due to the combination of the steric and polar nature
of the pivaloyl substituent and the t-butyl placed in
50 position, which greatly favors the solution in polar organic
media. Also, the steric hindrance of this bulky group in the
20 position of the m-terphenyl moiety twists the rings dra-
matically out of plane, resulting in a noncoplanar, rigid, and
The melting point for PTPDA crystallized from Ac2O was
determined by DSC (Figure 2). On heating, PTPDA showed
a melting endotherm at 292 °C. However, when the sample
was quenched from the melt, it was possible to detect the
glass transition of the amorphous phase at 134 °C, followed
by a cold crystallization at 186 °C, and finally the melting
endotherm at 292 °C.
Polymer Synthesis and Characterization. Four commercial
diamines were combined with PTPDA to attain polyimides
bearing the m-terphenyl unit (Scheme 2). The polyimide
from diamine 2,2-bis-(4-aminophenyl) hexafluoro-propane,
6FpDA, was prepared by the two-step low-temperature
polycondensation method, using trimethylsilyl chloride
and pyridine as polycondensation promoters and NMP as
solvent.21 The molecular weight achieved for this polyimide
was high enough to prepare creasable films by casting from

Article
Macromolecules, Vol. 43, No. 5, 2010 2271
Figure 1. FT-IR and 1H NMR spectra of dianhydride PTPDA.
contorted conformation capable of efficiently hindering the
chain packing. This highly contorted conformation can be
seen in the simulated structure of the PTPDA diimide, as
calculated by the semiempirical AM1 method (Figure 4). In
addition, it can be noticed from this model that the amide group
is not very accessible because it is shielded by the t-butyl and the
side phenyl groups of the m-terphenyl moiety. This may
explain the good chemical stability of this group, which does
not easily undergo hydrolysis reaction as well as the moderate
moisture absorption showed by these polyimides, as commen-
ted below, despite having an amide function.
Furthermore, the introduction of this substituent should
bring about an improvement in the polymer chain stiffness,
as predicted by molecular modeling. Therefore, a theoretical
study using the semiempirical AM1 method was carried out
to calculate the torsional mobility and rotational energy
barrier around the bonds that join the aromatic rings of
the m-terphenyl unit. The calculations were performed on
the repeating unit of PTPDA-6FpDA and also on BTPDA-
6FpDA for comparative purposes. Figure 5 presents the
AM1 enthalpy barriers calculated by rotating the dihedral
angle, Φ, in 10° increments for X = H and NHCOtBu,
respectively. The maximum value of the rotational barrier
was ∼9.8 kcal/mol for X = NHCOtBu, whereas for X = H,
it was only 2.4 kcal/mol. Therefore, the presence of the
pivalamide group did greatly heighten the rotational barrier
of the aryl moieties, resulting in a much more restricted
torsional mobility and consequently in a much higher
chain rigidity. The effect of this group on the mobility of
the chains could also be observed upon looking at the glass-
transition temperatures (Table 3), which were higher for
PTPDA polymers than those reported for polyimides from

2272 Macromolecules, Vol. 43, No. 5, 2010
Calle et al.
BTPDA dianhydride. For instance, the Tg of polyimide
PTPDA-6FpDA is 340 °C, whereas for the homologous
polyimide without pivalamide substituent (BTPDA-6FpDA)
is 325 °C.12 It is worth remarking the very high glass-
transition temperatures of PTPDA-3MemPD and PTPDA-
4MepPD, 420 and 430 °C, respectively, which are among the
highest values ever reported for lineal, soluble polyimides.
The results of TGA showed an excellent thermal stability
of the new polyimides, even though they bear a pivalamide
side group. They exhibited values of decomposition tem-
peratures (Td, taken as the temperature at initial weight loss
onset in the TGA curves) >500 °C. Therefore, the introduc-
tion of the amide substituent did not seem to impair largely
the thermal resistance, and thus PTPDA-3MemPD showed a
Td of ∼510 °C, whereas its homologous without the pivaloyl
group has a Td of 525 °C.12
The mechanical properties of polymers films by solution
casting from DMAc are summarized in Table 3. Polyimide
PTPDA-4MepPD could not be cast into flexible films be-
cause of their moderate molecular weight and elevated chain
stiffness (Tg = 430 °C), so we were not able to evaluate its
mechanical properties. It was not the case for PTPDA-
6FpDA, which even having a low inherent viscosity could
be processed into good quality films. Polyimide films had
tensile strengths of 50-80 MPa, elongations at break of
5-8%, and tensile moduli of 1.5 to 1.8 GPa. These values of
tensile strengths and tensile modulus are acceptable for high-
molecular-weight polyimides, although they did not match
the mechanical resistance of classical wholly, unmodified
polyimides. It may result from low intermolecular interac-
tion because of the presence of the bulky pendant t-butyl
group and the highly twisted molecular structure. Further-
more, these results do confirm that the pivalamide group, as
could be presumed from molecular modeling of the diimide
of PTPDA (Figure 4), is hindered by the t-butyl and imide
Figure 2. DSC curve of PTPDA dianhydride after being quenched
from the melt state.
Scheme 2. Synthesis of Polyimides
Table 1. Properties and Elemental Analyses of Polyimides
elemental analysis (%)
C
H
N
polymer
yield (%)
95
η
inh (dL/g)a
0.39
Mn (GPC)b
Mw (GPC)b
PDI (Mw/Mn)
PTPDA-6FpDA
14 300
46 800
3.3
calcd
found
calcd
found
calcd
found
calcd
found
67.07
66.85
67.67
67.59
75.10
74.89
75.32
75.08
4.28
4.41
4.61
4.71
5.83
6.08
6.01
6.22
5.10
5.22
4.93
4.80
6.57
6.36
6.43
6.70
PTPDA-m6FMe
PTPDA-3MemPD
PTPDA-4MepPD
96
96
93
1.26
0.65
0.41
77 500
62 600
16 200
301 000
136 000
76 300
3.9
2.2
4.7
a Inherent viscosity measured at a concentration of 0.3 g/dL in NMP at 25 °C. b Relative to polyethylenglycol.

Article
Macromolecules, Vol. 43, No. 5, 2010 2273
Figure 3. 1H NMR spectrum of PTPDA-6FpDA polyimide.
Table 2. Solubility of Polyimides
Figure 5. Dihedral angle rotation enthalpy by AM1 molecular modeling.
polyimide
m-cresol NMP DMAc DMSO CHCl3 THF toluene
polyimides without polar groups (Table 4). This fact is again
explained by the protection of the amide group for the t-butyl
and the edge phenyl groups of the m-terphenyl unit. As
expected, PTPDA-6FpDA and PTPDA-m6FMe had signifi-
cantly lower moisture uptake than polyimides from 3MemPD
and 4MepPD diamines because of the well-known hydro-
phobic effect of the trifluoromethyl groups. Nevertheless, the
water uptake values were slightly higher than those not having
the pivaloyl imide moiety and used as references in this article.
Gas Transport Properties of Polyimides. The permeability
coefficients for five gases and ideal separation factors for
some interesting gas pairs are given in Table 5. For com-
parative purposes, the permeability and selectivity data for
related polyimides from 50-t-butyl-m-terphenyl-3,4,300,400-
tetracarboxylic acid dianhydride, BTPDA-6FpDA and
BTPDA-3MemPD, without the pivalamide group have also
been included. As it was previously reported,13 these latter
exhibit outstanding permeabilities with moderate selectivity
values for gas pairs O2/N2 and CO2/CH4. Therefore, because
of the convenient synthesis route and high molecular volume
of the pivalamide group, we rationalized that this new
monomer could bring about materials with enhanced prop-
erties for gas separation. Accordingly, the newly synthesized
membranes derived from the PTPDA dianhydride showed
higher ideal selectivity values based on ratios of pure gas
permeability coefficients when compared with polymers
from BTPDA dianhydride. Therefore, polyimide PTPDA-
6FpDA showed an enhancement of 9 and 12% in CO2/CH4
and O2/N2 selectivities, respectively, regarding BTPDA-
6FpDA. The improvement was even greater for the more
permeable PTPDA-3MemPD polyimide, namely, the 24%
increase for O2/N2 and 26% enhancement for CO2/CH4 ideal
selectivity values when compared with BTPDA-3MemPD.
Because the well-established trade-off between permeabi-
lity and selectivity14-16 in size-sieving polymers such as those
considered in this study, this enlarged selectivity is certainly
accompanied by a decrease in the permeability coefficients
for every gases studied.25,26 Therefore, from the permeabi-
lity/selectivity map presented in Figure 6 for O2/N2 separa-
tion, it can be noticed the improved separation performance
of polyimides from PTPDA dianhydride relative to the
performance of BTPDA polymers. On this map, the perfor-
mance of the best materials for this separation is character-
ized by the so-called upper bound line established by
Robeson.14,15 Although all materials in this study fall below
the recently established upper bound, PTPDA polyimides
PTPDA-6FpDA
PTPDA- m6FMe þ þ
PTPDA-3MemPD 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 -
PTPDA-4MepPD þ þ
a
þ þ: soluble, þ -: partially soluble, - -: insoluble.
Figure 4. Molecular modeling of 50-t-butyl-3,4,300,400-m-terphenyl di-
methylimide (top) and 50-t-butyl-20-pivaloylimino-3,4,300,400-m-terphe-
nyl dimethylimide (bottom) resulting of the reaction of methyl amine
with BTPDA and PTPDA, respectively.
groups, preventing interchain hydrogen bonding and hence
avoiding an enhancement of the intermolecular attractions
between polymer chains.
Interestingly, even having an amide function, these poly-
imides showed low water uptake, similar to other commercial

2274 Macromolecules, Vol. 43, No. 5, 2010
Table 3. Thermal and Mechanical Properties of Polyimides
Calle et al.
polyimide
Tg (°C)a
Td (°C)b
char yield (%)c
tensile strength (MPa)
elongation at break (%)
tensile modulus (MPa)
PTPDA-6FpDA
PTPDA-m6FMe
PTPDA-3MemPD
PTPDA-4MepPD
340
300
420
430
500
526
510
500
54
57
58
56
55
64
5.0
4.7
1.8
1.8
80
8.0
1.5
d
d
d
a From the second trace of DSC measurements conducted with a heating rate of 20 °C/min under a nitrogen atmosphere. b Onset weight loss
temperature in TGA at 20 °C/min heating rate. c Residual yield in TGA at 800 °C under a nitrogen atmosphere. d Brittle polymer.
Table 4. Water Uptake of Polyimidesa
The new dianhydride monomer with a m-terphenyl group,
50-t-butyl-20-pivaloylimino-3,4,300,400-m-terphenyltetracarboxylic
moisture uptake
(%)
dianhydride (PTPDA), has been obtained by a regioselective,
efficient, and cost-effective synthesis using a modern C-H aryl
coupling methodology. In addition, this route of synthesis
involves the use of reactants bearing acid groups, which over-
comes the necessity and drawback of oxidizing tetramethyl
derivatives. The monomer was attained with good yield and high
purity. Upon using the one- and two-step methods of polyimida-
tion, a new set of polyimides was obtained, which could be
processed into flexible and creasable films (except for the polymer
derived from 4MepPD). These films were thoroughly character-
ized, showing good solubility in polar aprotic solvents and also in
less common solvents for aromatic polyimides as chloroform and
THF in some cases. Thermal stability was only slightly lower than
reference polyimides from BTPDA, and Tg values were quite high
with values higher than most of the literature-referenced poly-
imides. Polymer films exhibited a low water uptake and average
mechanical properties good enough to allow their use on gas
separation processes at high pressures. The increase in rigidity
brought about a decrease in permeability with a concurrent
augment of permselectivity, which is translated into a better
balance of gas separation properties than those of polymers from
BTPDA because these novel materials are closer to the Robeson
limit, and thus they could be used on specific gas applications as
air or natural gas purification.29
PTPDA-6FpDA
PTPDA-m6FMe
PTPDA-3MemPD
PTPDA-4MepPD
BTPDA-6FpDA
BTPDA-3MemPD
2,3
1,5
4,5
3.1
0.3
3.2
a Moisture uptake (%) = 100(W - W0)/W0; W = weight of polymer
sample after standing in a 65% relative humidity atmosphere. W0
=
weight of polymer sample after being dried at 120 °C for 24 h over P2O5.
Table 5. Gas Transport Properties of Polyimides
permeability
(barrers)a
ideal separation
factors
He CO2 O2
N2 CH4 O2/N2 CO2/CH4
polyimide
PTPDA-6FpDA
PTPDA-m6FMe
104.3 86.2 18.3 3.9 3.5 4.7
89.1 58.5 14.2 2.7 2.3 5.2
24.9
25.2
15.9
22.9
12.6
PTPDA-3MemPD 187 276
BTPDA-6FpDA 115 114
58.5 12.7 17.4 4.6
24.2 5.8 5.0 4.2
BTPDA-3MemPD 270 600 130 35.1 47.6 3.7
a 1 barrer = 10-10 cm3 (STP) cm/(s cm2 cmHg).
Acknowledgment. We gratefully acknowledge the financial
support provided by the Spanish Ministerio de Ciencia e
ꢀ
Innovacion, project MAT2007-62392. M. Calle would like to
thank the Spanish Consejo Superior de Investigaciones Cientıficas,
CSIC, for a research contract.
Supporting Information Available: More detailed experi-
mental section with elemental analyses and spectroscopic char-
acterization (1H and 13C NMR) data of intermediates and
monomers, along with the description and bibliography of the
computational methods employed in this article. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 6. Permeability/selectivity plot for the polyimides of this study.
The upper bound calculated by Robeson (1991) and Robeson (2008)5,6
are also included.
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