Phthalimide based polymers of intrinsic microporosity

Article abstract of DOI:10.1016/j.polymer.2012.05.001

A series of phthalimide based microporous polymers were successfully prepared by conventional nucleophilic substitution reaction of several newly synthesized fluoro-monomers with commercially available 5,5′,6,6′- tetrahydroxy-3,3,3′,3′-tetramethylspirobisindane. FTIR, ¹H NMR, and elemental analyses were used to identify the proposed structures of the polymers. The synthesized polymers are of high molecular weight as demonstrated by Gel Permeation Chromatography (GPC). Thermogravimetric analysis shows that the prepared polymers were stable up to 300°C. From the porosity analysis it is clear that the prepared polymers are analogous to polymers of intrinsic microporosity (PIMs) with high surface area (500-900 m²/g). The t-plot analysis shown that the major contribution to the specific surface area is arising from the micropore surface area with narrow size distribution of ultramicropores as confirmed by the Horvath-Kawazoe (H-K) analysis. The hydrogen storage capacity of the prepared PIM-R(1-7) and CO-PIM(3,4,6,7) were promising (up to 1.26 wt%, 77 K, at 1.13 bar) with high isoteric heats of H₂ adsorption (8.5 kJ/mol).The results of this study demonstrate that controlling the appropriate monomer content via the three-dimensional structure can provide a uniform microporous morphology in the target polymers.

Full text of DOI:10.1016/j.polymer.2012.05.001

Polymer 53 (2012) 2964e2972
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Phthalimide based polymers of intrinsic microporosity
*
Saad Makhseed , Fadi Ibrahim, Jacob Samuel
Department of Chemistry, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of phthalimide based microporous polymers were successfully prepared by conventional
nucleophilic substitution reaction of several newly synthesized fluoro-monomers with commercially
available 5,50,6,60-tetrahydroxy-3,3,30,30-tetramethylspirobisindane. FTIR, 1H NMR, and elemental anal-
yses were used to identify the proposed structures of the polymers. The synthesized polymers are of high
molecular weight as demonstrated by Gel Permeation Chromatography (GPC). Thermogravimetric
analysis shows that the prepared polymers were stable up to 300 ꢀC. From the porosity analysis it is clear
that the prepared polymers are analogous to polymers of intrinsic microporosity (PIMs) with high
surface area (500e900 m2/g). The t-plot analysis shown that the major contribution to the specific
surface area is arising from the micropore surface area with narrow size distribution of ultramicropores
as confirmed by the Horvath-Kawazoe (H-K) analysis. The hydrogen storage capacity of the prepared
PIM-R(1e7) and CO-PIM(3,4,6,7) were promising (up to 1.26 wt%, 77 K, at 1.13 bar) with high isoteric
heats of H2 adsorption (8.5 kJ/mol).The results of this study demonstrate that controlling the appropriate
monomer content via the three-dimensional structure can provide a uniform microporous morphology
in the target polymers.
Received 29 December 2011
Received in revised form
18 April 2012
Accepted 2 May 2012
Available online 10 May 2012
Keywords:
Microporous polymers
Membrane
Molecular weight distribution
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
nonporous material. It is important to note that if there is a certain
amount of free volume, voids would be interconnected, and the
The preparation of purely organic microporous polymers
without the assistance of molecular template is becoming a fast
developing area in nanomaterial’s research and many approaches
have been used to develop various insoluble network organic
microporous polymers such as hyper cross-linked polymers (HCP)
[1,2], triptycene-based PIM (Trip-PIM) [3], organic framework
polymer (OFP) [4], covalent triazine frameworks (CTFs) [5] and
microporous polyimide networks [6] with high specific surface.
Conjugated microporous polymers (CMPs) and their analogues are
another important class of insoluble microporous organic polymers
which can be considered as cross-linked polymers with extended
conjugated network [7]. On the other hand, the inclusion of certain
functionalities into CMPs can affect their surface areas in
comparison with their non-functionalized analogues [8]. Recently
many reviews published on nanoporous polymers and their
potential applications with a focus on microporosity [9e12]. The
microporosity is maintained by a robust network of covalent bonds,
further complemented by rigid framework structure. Generally
non-network polymers can pack space efficiently leading to
polymer will therefore behave as a microporous material even
without a network structure. This kind of microporous polymer
could be soluble to facilitate easy solution-based processing which
can’t be achieved in other microporous materials. As a result they
have attracted good acceptance as outstanding membrane in gas
separation, gas storage, adsorption of small molecules and
heterogeneous catalysis [13]. So it is of great interest to synthesize
highly soluble porous polymers using the established dioxane-
forming polymerization reaction between appropriate fluorine-
containing monomers and the commercially available aromatic
tetrol. The porous properties mainly due to the architectural
features of the used monomers which defines the microporosity.
The combined features: high rigidity arising from dioxane linkages
and the randomly contorted structure due to either a spiro-centre
or a non-planar architecture, represent a prerequisite to induce
space inefficient packing producing a large amount of inter-
connected free volume. The most outstanding representation for
this kind of materials are the polymers of intrinsic microporosity
(PIMs) [14] developed by McKeown et al with high surface
areas in the range of 500e900 m2/g. Typical example is PIM-1,
prepared as a soluble polymer by the dioxane-forming reaction
between 5,50,6,60-tetrahydroxy-3,3,30,30-tetramethylspirobisindane
and tetrafluoroterephthalonitrile. The same concept of using rigid
* Corresponding author. Tel.: þ965 24985538; fax: þ965 24816482.
E-mail address: saad.makhseed@ku.edu.kw (S. Makhseed).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.05.001

S. Makhseed et al. / Polymer 53 (2012) 2964e2972
2965
and space inefficient packing subunits was expanded by Guiver
et al. in developing a high molecular weight PIM-1; synthesized by
polycondensation under high-intensity mixing conditions at about
155 ꢀC to yield cyclic-free products in a shorter time (8 min) [15].
McKeown et al, additionally synthesized a highly soluble micro-
porous polymer derived from Bis(phenazyl) monomers with
reasonable internal surface area and good film-forming character-
istics (PIM-7) [16]. Studies on evaluating the influence of inter-
molecular interactions in microporous polymers shown that such
interactions (e.g. hydrogen bonding) can lead to hindered or
reduced microporosity [17,18]. The structural features of PIM-1
further tailored by post- polymerization modifications through
the conversion of nitrile moieties into different functional groups
such as tetra azole and thioamide with enhanced CO2 capture and
gas transport properties [19,20]. To date, several soluble ladder
polymers such as polybenzodioxanes, polyimides, and PIMs con-
taining dinaphtyl or tianthrene segments have been reported and
the influence of their modified chemical structure on the physical
properties, sorption behaviour and gas permeability has been
explored [21e23]. Although the structural alteration of PIM-1 led to
low surface areas, these materials showed promising results for
certain applications [24].
Controlling the pore diameter at the micro scale (IUPAC pores of
dimension < 2 nm) make it possible to use such materials in
different commercial applications such as hydrogen adsorption
[13]. The lack of fundamental understanding of interaction between
the polymer surfaces and the gas molecules initiate many
approaches with the aim of designing polymer materials to estab-
lish the structure-porous property relationship. This may be ach-
ieved by selecting appropriate monomer precursors with certain
structural features. For example, monomers decorated by larger
substituents can frustrate the resulting polymer chain packing
which may create higher surface area along with tunable porosity.
In the present study we described a series of solution processable
phthalimide based organic polymers prepared from inexpensive
monomers with sufficient surface area and narrow pore diameter
distribution to store H2 in the confined space through enhanced
interaction. While considering hydrogen storage application the
prepared microporous polymers (PIM-Rs and CO-PIM-Rs) offer
many attractive features such as low intrinsic density, structural
homogeneity, hydrothermal stability and synthetic reproducibility.
The chemical features of the surfaces and nature of the pores have
thus tuned to achieve a high hydrogen adsorption.
Electron Microscopy (SEM: JEOL Model 6300) and High Resolution
Transmission Electron Microscopy (HRTEM: JEOL Model JEM-
3010). Nitrogen adsorption measurements (77 K) and volumetric
hydrogen adsorptions analysis (77 K and 87 K at 1.13 bar) were
performed on a Micromeritics ASAP 2020 Sorptometer equipped
with an out gassing platform, an online data acquisition and
handling system. Before analysis the samples were degassed for
12 h with a heating rate of 1 ꢀC/min in two stages (80 ꢀC for 1 h and
120 ꢀC for 11 h) under high vacuum (<10ꢁ4 mbar). The specific
surface area was calculated using the Brunauer-Emmet-Teller (BET)
equation (See supporting Information). The micropore area was
calculated using t-plot method (Harkins and Jura). The pore size
distributions were calculated from the adsorption isotherm using
the Horvath-Kawazoe (H-K) calculations (See supporting Informa-
tion). The heats of adsorption were calculated from the hydrogen
adsorption isotherms obtained at 77.3 K and 87 K using the ASAP
2020 software (Micromeritics, Norcross, GA).
2.2. Materials
The anhydrous solvents DMF and DMAc were purchased from
Aldrich co, and the potassium carbonate obtained from Merck was
finely grounded and dried at 200 ꢀC. Tetrafluoroterephthalonitrile
obtained from aldrich was used directly. 5,50,6,60-tetrahydroxy-
3,3,30,30-tetramethyl-1,10-spirobisindane obtained from alfa was
purified from methanol. Starting from tetrafluorophthalic acid,
tetrafluorophthalic anhydride was prepared in our laboratory. The
other commercially available materials and solvents were used
without further purification.
2.3. Film preparation
Polymer (550 mg) was dissolved in CHCl3 (15 mL) and filtered
(2 mm glass microfiber). The filtered solution was pouring into a flat
Petri dish (12 cm dia.) and then placed in a vacuum desiccator with
the gas inlet open to the atmosphere. The solution was allowed to
evaporate for 4 days. The dish was removed and the clear yellow
film released from the glass.
2.4. Synthesis of fluoro-monomers
2.4.1. 4,5,6,7-Tetrafluoro-2-phenyl-isoindole-1,3-dione. MR1
Tetrafluorophthalic anhydride (8.5g, 38.7 mmol) was added to
a stirred solution of aniline (3.00 g, 32.26 mmol) in glacial acetic
acid and refluxed for 12 h. The white solid obtained was further
refluxed in acetic anhydride for 12 h. After cooling to room
temperature, the precipitated product was filtered off and washed
with petroleum ether to give a light yellow solid. Yield: 84%; Mp
2. Experimental
2.1. Characterization
1H- NMR spectrum (400 MHz) of monomers and polymers were
recorded on a Bruker DPX 400 spectrometer using CDCl3 as the
solvent and tetramethylsilane as the internal standard. FTIR spectra
were recorded on a JASCO FT/IR-6300. Elemental analyses were
carried out using Elementar Vario Micro Cube. Mass analyses were
done on a Thermo DFS Mass spectrometer. The molecular weight
and its distribution were measured by Knauer Gel Permeation
Chromatograph fitted with a refractive index detector (flow rate
1 mL/min) according to polystyrene standards using CHCl3 as the
eluent. Thermogravimetric analysis (TGA) was performed with
a Shimadzu TGA-50 instrument at a heating rate of 10 ꢀC/min under
nitrogen atmosphere and DSC analysis was done on Shimadzu DSC-
60 instrument at a heating rate of 10 ꢀC/min under nitrogen.
Melting points were measured with a Griffin melting point appa-
ratus and further confirmed by DSC analysis. Wide-angle X-ray
diffraction (WAXD) of films was measured by a Siemens D5000
diffractometer. Microscopic techniques employed are Scanning
206 ꢀC. 1H NMR (CDCl3,
d ppm): 7.26-7.54 (m, 4H, Ar). FTIR
(KBr pellet, cmꢁ1): 3068, 1783 (asymmetry C]O), 1726 (symmetry
C]O), 1460 (C]C), 1371 (C-N), 945 (C-F). Anal. Calcd for
C14H5NO2F4 (295.08): C, 56.94; H, 4.74; N, 4.74. Found: C, 56,88; H,
1.80; N, 5.03. MS (EI): m/z (%) 295 (100%).
The following monomers were prepared from tetra-
fluorophthalic anhydride and corresponding amines using similar
procedures adopted for MR1.
2.4.2. 4,5,6,7-Tetrafluoro-2-hexyl-isoindole-1,3-dione. M-R2
Yield: 88%; Mp 118 ꢀC. 1H NMR (CDCl3,
d ppm): 3.68 (t, 2H, CH2),
1.63-1.70 (m, 2H, CH2), 1.29-1.35 (m, 6H, CH2), 0.9 (t, 3H, CH3). FTIR
(KBr pellet, cmꢁ1): 3078, 1780 (asymmetry C]O), 1720 (symmetry
C]O), 1465 (C]C), 1370 (C-N), 944 (C-F). Anal. Calcd for
C14H13NO2F4 (303.26): C, 55.45; H, 4.32; N, 4.62. Found: C, 54.70; H,
4.01; N, 4.84. MS (EI): m/z (%) 303 (100%).

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S. Makhseed et al. / Polymer 53 (2012) 2964e2972
2.4.3. 4,5,6,7-Tetrafluoro-2-(4-methoxy-phenyl)-isoindole-1,3-
2.18 (d, br, 2H, CH2), 1.58-1.30 (d, br, 12H, CH3). FTIR (KBr pellet,
cmꢁ1): 3080, 2940, 1774 and 1709 (imide). Anal. Calcd for
C35H33NO6 (563.01): C, 74.60; H, 5.86; N, 2.04. Found: C, 74.28; H,
5.77; N, 2.01.
dione. M-R3
Yield: 85%; Mp 250 ꢀC. 1H NMR (CDCl3,
d ppm): 7.28- 7.24 (dd,
2H, Ar), 7.06-7.02 (dd, 2H, Ar), 3.85 (s, 3H, CH3). FTIR (KBr pellet,
cmꢁ1): 3070, 2930,1775 (asymmetry C]O),1730 (symmetry C]O),
1468 (C]C), 1338 (C-N), 952 (C-F). Anal. Calcd for C15H7 NO3F4
(325.03): C, 55.40; H, 2.17; N, 4.31. Found: C, 55.50; H, 2.07; N, 4.43.
MS (EI): m/z (%) 325 (100%).
2.5.3. PIM-R3
Yield: 84%. Mp > 300 ꢀC. Mw 178,843. Mn 62,276. 1H NMR (CDCl3,
298 K,
d ppm): 6.95-6.77 (d br, Ar), 6.5 (s br, Ar), 3.85 (t br, OCH3),
2.30-2.17 (d, br, CH2), 2.17 (d, 2H, CH2), 1.31 (s, 12H, CH3). FTIR (KBr
pellet, cmꢁ1): 3090, 1768 and 1717 (imide). Anal. Calcd for
C36NO7H27 (585.07): C, 73.84; H, 4.61; N, 2.39. Found: C, 73.48; H,
4.52; N, 2.26.
2.4.4. 4,5,6,7-Tetrafluoro-2-trimethoxybenzene-isoindole-1,3-
dione. M-R4
Yield: 87%; Mp 217 ꢀC. 1H NMR (CDCl3,
d ppm): 6.61 (s, 2H, Ar),
3.91 (s, 3H, CH3), 3.88 (s, 6H, CH3). FTIR (KBr pellet, cmꢁ1): 3087,
2935, 1760 (asymmetry C]O), 1728 (symmetry C]O), 1462 (C]C),
1350 (C-N), 960 (C-F). Anal. Calcd for C17H11NO5F4 (385.10):
C, 52.98; H, 2.89; N, 3.64. Found: C, 52.58; H, 2.88; N, 3.85. MS (EI):
m/z (%) 385 (100%).
2.5.4. PIM-R4
Yield: 84%. Mp > 300 ꢀC. Mw 214,278. Mn 69,348. 1H NMR (CDCl3,
298 K, d ppm): 6.89 (s, 2H, Ar), 6.91-6.53 (m br, Ar), 3.90-3.79 (m br,
OCH3), 2.33-2.18 (d, CH2), 1.72 (s, 2H, CH2), 1.51(s, 12H, CH3). FTIR
(KBr pellet, cmꢁ1): 1779 and 1712 (imide). Anal. Calcd for
C38H31NO9 (645.03): C, 70.69; H, 4.80; N, 2.17. Found: C, 70.57; H,
4.75; N, 2.13.). Anal. Calcd for C38H31NO9 (645.03): C, 70.69; H, 4.80;
N, 2.17. Found: C, 70.37; H, 4.71; N, 2.13.
2.4.5. 4,5,6,7-Tetrafluoro-2- 4-tert -butylbenzene e isoindole-1,3-
dione. M-R5
Yield: 84%; Mp 221 ꢀC. 1H NMR (CDCl3,
d ppm): 7.54e7.51 (dd,
2H, Ar), 7.30e7.28 (dd, 2H, Ar), 1.35 (s, 9H, CH3). FTIR (KBr pellet,
cmꢁ1): 3095, 2950,1772 (asymmetry C]O),1735 (symmetry C]O),
1458 (C]C), 1350 (C-N), 960 (C-F). Anal. Calcd for C18H13NO2F4
(351.1): C, 61.52; H, 3.73; N, 3.98. Found: C, 61.70; H, 3.49; N, 4.17.
MS (EI): m/z (%) 351 (45%).
2.5.5. PIM-R5
Yield: 86%; Mp > 300 ꢀC. Mw 37,200. Mn 31,000. 1H NMR (CDCl3,
298 K,
d ppm): 7.44-7.31 (d br, Ar), 6.88-6.51 (d br, Ar), 2.30-2.17
(d br, 4H, CH2), 1.29-1.25 (d, 21H, CH3). 3086, 2954, 1771 and 1723
(imide). Anal. Calcd for C39H33NO6 (611.24): C, 76.59; H, 5.40; N,
2.29. Found: C, 76.28; H, 5.32; N, 2.21.
2.4.6. 4,5,6,7-Tetrafluoro-2-(2,6-diisopropyl-phenyl)-isoindole-1,3-
dione. M-R6
Yield: 92%; Mp 171 ꢀC. 1H NMR (CDCl3,
d
ppm): 7.48 (t, H, Ar),
2.5.6. PIM-R6
7.30-7.28 (d, 2H, Ar), 2.63 (sept, 2H, CH), 1.17-1.16 (d, 12H, CH3). FTIR
(KBr pellet, cmꢁ1): 3090, 2940, 1770 (asymmetry C]O), 1733
(symmetry C]O), 1450 (C]C), 1350 (C-N) and 970 (C-F). Anal.
Calcd for C20H17NO2F4 (379.10): C, 63.30; H, 4.48; N, 3.69. Found: C,
63.12; H, 4.26; N, 3.85. MS (EI): m/z (%) 379 (50%).
Yield: 87%. Mp > 300 ꢀC. Mw 162,594. Mn 67,748. 1H NMR (CDCl3,
298 K,
d ppm): 7.43 (d, 2H, Ar), 6.85 (t br, 2H, Ar), 6.53 (s br, Ar)
2.70(s br, 2H, CH) 2.34-2.09 (d br, 4H, CH2) 1.32-1.17 (d, 24H, CH3).
FTIR (KBr pellet, cmꢁ1): 3088, 2945, 1781 and 1727 (imide). Anal.
Calcd for C41H37NO6 (641.05): C, 76.90; H, 5.75; N, 2.21. Found: C,
76.79; H, 5.68; N, 2.17.
2.4.7. 4,5,6,7-Tetrafluoro -1-adamantylamine. M-R7
Yield: 73%; Mp > 300 ꢀC. 1H NMR (CDCl3,
d
ppm): 2.44 (s, 6H,
2.5.7. PIM-R7
CH2), 2.15 (s, 3H, CH), 1.77-1.74 (d, 3H, CH2), 1.71-1.68 (d, 3H, CH2).
FTIR (KBr pellet, cmꢁ1): 3103, 2911, 1779 (asymmetry C]O), and
1732 (symmetry C]O), 943 (C-F). Anal. Calcd for C18H15NO2F4
(353.10): C, 61.17; H, 4.24; N, 3.96. Found C, 60.86; H, 4.40; N, 4.08.
MS (EI): m/z (%) 353 (100%).
Yield: 80%. Mp > 300 ꢀC. Mw 152,916. Mn 27,803. 1H NMR (CDCl3,
298 K, d ppm): 6.84-6.72 (d br, 2H, Ar), 6.46 (s br, 2H, Ar), 2.51-2.39
(t br, CH), 2.17-210(d br, 4H CH2), 1.71-1.67(d br,12H, CH2),1.29 (s br,
12H, CH3). FTIR (KBr pellet, cmꢁ1): 3098, 2960, 1781 and 1709
(imide). Anal. Calcd for C39H35NO6 (613.08): C, 76.30; H, 5.70; N,
2.28. Found: C, 76.08; H, 5.61; N, 2.19.
2.5. Synthesis of PIM-R(1e7)
2.5.1. PIM-R1
2.6. Synthesis of CO-PIM-R(3,4,6,7)
A mixture of monomer MR1 (0.50 g, 1.69 mmol) and spi-
robiscatechol (0.57 g, 1.69 mmol) in dry DMF (50 ml) and K2CO3
(0.50g, 3.51 mmol) was stirred at 120 ꢀC for 4 h. After cooling, the
reaction mixture was poured into deionised water and the solid
product collected by filtration washed with methanol. The insoluble
polymer was then purified by refluxing in methanol and acetone.
The obtained yellow fluorescent powder was dried in vacuum oven
at 100 ꢀC for 12 h. Yield: 75%. Mp > 300 ꢀC. Mw 42,403. Mn 40,152. 1H
2.6.1. CO- PIM-R3
A mixture of monomer MR3 (0.20g, 0.52 mmol), spirobiscatechol
(0.35g, 1.04 mmol) and 2,3,5,6-tetrafluorophthalonitrile (0.10g,
0.52 mmol) in dry DMAc (40 ml) and K2CO3 (0.36g, 2.62 mmol) was
stirred at 120 ꢀC for 4 h. After cooling, the reaction mixture was
poured into deionised water and the solid product collected by
filtration and washed with methanol. The insoluble polymer was
then purified by refluxing in methanol, acetone and THF. The
obtained yellow fluorescent powder was dried in vacuum oven at
100 ꢀC for 12 h. Yield: 85%. Mp > 300 ꢀC. Mw 155,908. Mn 125,753. 1H
NMR (CDCl3, 298 K d ppm): 7.68-7.38 (s, br, 3H, Ar), 6.87-6.51 (d, br,
4H, Ar), 2.30 (d, br, 4H, CH2), 1.30 (s, br, 12H, CH3). FTIR (KBr pellet,
cmꢁ1): 3088, 1774 and 1723 (imide). Anal. Calcd for C35H25NO6
(555.45): C, 75.61; H, 4.50; N, 2.52. Found: C, 75.19; H, 4.43; N, 2.45.
The following polymers were prepared from corresponding
fluoro-monomers using similar procedure adopted for PIM-R1.
NMR (CDCl3, 298 K, d ppm): 6.95-6.89 (d br, 2H, Ar), 6.51 (s, 2H, Ar),
3.82 (t br, 3H, OCH3), 2.30-2.17 (d, 4H, CH2), 1.31 (s, 12H, CH3). FTIR
(KBr pellet, cmꢁ1): 3070, 2236 (C^N), 1768 and 1720 imide). Anal.
Calcd for C44H37N3O9 (741.19): C, 71.25; H, 3.67; N, 5.67. Found: C,
70.93; H, 3.52; N, 5.55.
Following copolymers were prepared from corresponding
fluoro-monomers using similar procedure adopted for CO- PIM-R3.
2.5.2. PIM-R2
Yield: 82%. Mp > 300 ꢀC. Mw 27,624. Mn 15,741. 1H NMR (CDCl3,
298 K,
d ppm): 6.91(s, br, Ar), 6.50 (s, br, Ar), 3.45 (m, br, 2H, CH2),

S. Makhseed et al. / Polymer 53 (2012) 2964e2972
2967
2.6.2. CO- PIM-R4
water, methanol and ethanol respectively followed by reprecipi-
tation in a chloroform/methanol mixture to give yellow fluorescent
powder in good yield (>85%) attributed to the efficiency of the
polycondensation reaction. The solution processable PIM-Rs can be
cast into membrane which can be used as a separation membrane,
selectively removing one component either from liquid or gas
mixture. According to our previous experiments, we note that the
synthesis of PIMs having high molecular weight could be carried
out at revised conditions especially elevated temperature and high
concentration [25]. After a number of trials, we found that the
solvent DMF is significantly compatible with the reaction mixture
to a great extent and could achieve high molecular weight linear
polymers while compared to DMAc and NMP. We optimized the
reaction conditions to get high molecular weight polymers using
DMF as a solvent that is significantly well-suited with the monomer
salts and the polymers dissolved in the reaction mixture at high
concentration conditions. The average molecular masses of
prepared polymers were estimated by gel permeation chromatog-
raphy (GPC) relative to polystyrene standard in CHCl3 (Fig. 1). The
data obtained from GPC is listed in Table 1. PIM-R3, PIM-R4 and
PIM-R6 with relatively high average molecular weight were
obtained. The high molecular weight can be attributed to the good
solubility of randomly forming polymers in the reaction mixture.
On the other hand, GPC curves (See supporting Information) show
shoulder peaks in the low molecular weight region while PIM-R5
with reasonable molecular weight was obtained in a quantitative
yield without any shoulder peak. This behaviour can be attributed
to the presence of cyclic oligomers as a result of the side reactions. It
has been reported that in classic polycondensation reactions of
bisphenols and bifluoro aromatics for the production of poly(aryl
ethers) that a large amount of macrocyclic oligomers were formed
[26,27]. During the first step of the base catalysed reaction, the tetra
functional spirobiscatechol reacted with a base to give a number of
salt which have low solubility and resulted in the formation of
cyclic compounds. The low molecular weight, in the case of PIM-R2,
probably caused by either less soluble salts of propagating chains or
cyclic oligomer formation. However the reaction rate which
depends on the structural features and the reactivity of each
monomer also represent another key factor in determining the
molecular weight. This clearly indicates that the rate-controlling
step in the reaction is the dissolution of the salt. Using the
similar procedure adopted for PIM-Rs a series of new copolymers
(CO-PIM-R3, CO-PIM-R4, CO-PIM-R6 and CO-PIM-R7) by employ-
ing tetrafluoroterephthalonitrile were prepared. After many trial
procedures by using different feed ratios of monomers, the
Yield: 80%. Mp > 300 ꢀC. Mw 186,930. Mn 148,563. 1H NMR
(CDCl3, 298 K, d ppm): 7.31e7.27 (dd, 2H, Ar), 7.10-7.06 (dd, 2H, Ar),
3.85 (s, 3H, CH3). FTIR (KBr pellet, cmꢁ1): 3088, 2239(C^N), 1767
and 1721 (imide). Anal. Calcd for C46H31N3O11 (801.23): C, 68.91; H,
3.90; N, 5.24. Found: C, 68.48; H, 3.74; N, 5.16.
2.6.3. CO-PIM-R6
Yield: 83%. Mp > 300 ꢀC. Mw 162,000. Mn 142,000. 1H NMR
(CDCl3, 298 K,
d ppm): 7.43e7.28 (d br, 2H, Ar) 6.94-6.90 (d br, 2H,
Ar), 6.52-6.44(d br, 2H, Ar), 3.96 (s, 3H, CH3), 3.80 (s, 6H, CH3). FTIR
(KBr pellet, cmꢁ1): 3090, 2236 (C^N), 1768, 1721 (imide). Anal.
Calcd for C49H37N3O8 (795.21): C, 73.95; H, 4.69; N, 5.28. Found: C,
73.55; H, 4.41; N, 5.19.
2.6.4. CO- PIM-R7
Yield: 83%. Mp > 300 ꢀC. Mw 178,808. Mn 139,788. 1H NMR
(CDCl3, 298 K,
d ppm): 6.87 (d br, 2H, Ar), 6.48 (s br, 2H, Ar), 2.54-
2.42 (t br, CH), 2.25-2.17(d br, 4H CH2), 1.73-1.69(d br, 12H, CH2),
1.32 (s br, 12H, CH3). FTIR (KBr pellet, cmꢁ1): 3111, 2965, 2236
(C^N), 1777 and 1731 (imide), Anal. Calcd for C47H35N3O8 (769.82):
C, 73.33; H, 4.58; N, 5.46. Found C, 73.15; H, 4.42; N, 5.34.
3. Results and discussion
The phthalimide based fluorinated monomers (MR [1-7]) were
prepared in good yield by the straight forward one step imidisation
reaction between tetrafluorophthalic anhydride and different
amines in refluxing acetic acid. The proposed structure and purity of
the obtained fluorinated monomers were confirmed by routinely
spectroscopic techniques as well as elemental analysis. The possi-
bility of the preparation of ladder polymer was confirmed by making
model compounds directly from a reaction between 2 M equivalents
of di-tertiarybutyl catechol with the different tetraflurophthalimide
(See Supporting Information). The reaction product was character-
ized by mass, 1H NMR, FTIR spectroscopy and elemental analysis
confirming that all the four F atoms were replaced by the two ben-
zodioxane units. The highly efficient reaction (yield >90%) promotes
the preparation of microporous organic polymers. The polymers
(PIM-R[1e7]) were synthesized by the dibenzodioxane formation
reaction between the 5,50,6,60-tetrahydroxy-3,3,30,30-tetramethyl-
spirobisindane (spirobiscatechol), and various fluorine-containing
monomers (MR[1e7]) in dry DMF as illustrated in Scheme 1. The
polymers were isolated by precipitation in acidified water and the
resulting powder samples were purified by refluxing in deionised
R
(1-7)
O
O
R(1-7)
N
OH
N
O
O
O
O
+
OH
HO
HO
O
O
O
F
F
O
n
F
F
PIM-R(1-7)
R2 =
R3 =
R7 =
R1 =
R4 =
MeO
OMe
OMe
OMe
R6 =
R5 =
Scheme 1. Synthesis of microporous polymers PIM-R(1e7)s. Reagents and conditions: Dry DMF, Anhydrous K2CO3, 120 ꢀC, 4 h.

2968
S. Makhseed et al. / Polymer 53 (2012) 2964e2972
(3,4,6,7)
O
R
N
CN
CN
OH
OH
O
F
F
F
F
+
+
HO
HO
F
F
F
F
(3,4,6,7)
O
R
O
N
O
O
CN
O
O
O
n
O
CO-PIM-R(3,4,6,7)
CN
Scheme 2. Synthesis of microporous polymers CO-PIM-R(3,4,6,7). Reagents and
conditions: Dry DMF, Anhydrous K2CO3, 120 ꢀC, 4 h.
Fig. 1. GPC curves of PIM-R5 and CO-PIM-R6 with narrow molecular weight
distribution.
precisely with the help of 1H NMR spectra of monomers and
precursor molecules. The 1H NMR spectra of the PIMs were fully
consistent with their proposed structures (See Supporting
Information). The signals originating from the hydroxyl groups of
spirobiscatechol disappeared, suggesting that the reaction was
carried out completely through dibenzodioxane formation. The
broad aromatic and aliphatic signals of PIMs were enough to
confirm the formation of high molecular weight polymers. The
thermal properties were evaluated by TGA and DSC. The decom-
position temperatures were also very high; where a 5e10% initial
loss around 300 ꢀC, were observed, corresponding to the vapor-
ation of the residual solvents which has been identified as the
entrapped solvents in the micropores used for processing the
sample (See Supporting Information). The good thermal stability
can be attributed to its double stranded structures. In DSC analysis
no melting (Tm) and glass transition (Tg) was observed. Wide Angle
X-ray Diffraction (WAXD) analysis of the films were conducted to
display no crystalline peaks and revealed that all the prepared
materials were amorphous. The solubility of the PIM-Rs and CO-
PIM-Rs in various organic solvents were tested and found to be
highly soluble in CHCl3, and partially in THF, and DMF. The PIMs
showed good flexibility, transparency, and the strong thin films
could be obtained readily by casting from CHCl3 solutions in all
cases (Fig. 2). The mechanical properties of these samples were
tested using dumb-bell shaped specimens, except PIM-R2, which
did not form a good membrane. As shown in Table 1, tensile
strength of all the prepared polymers was comparable irrespective
optimum ratio of fluorine-containing monomers and 2,3,5,6-tet-
rafluorophthalonitrile, which has been applied to produce the
maximum molecular weight copolymer is 0.5:0.5. with high
molecular weights (Mn; 125,753e148,563). Although the reaction is
complicated due to the monomers multiple reactive groups CO-
PIM-Rs show GPC curves equivalent to a linear unbranched chain
free of macrocyclic species and crosslinking (See Supporting
Information). Thus, under the same reaction conditions molecular
weight broadening and crosslinking are efficiently reduced by
introducing a fixed ratio of tetrafluorophthalonitrile into the reac-
tion mixture, and high-molecular-weight copolymers can be
obtained. It is understood that in the reaction mixture tetra-
fluorophthalonitrile has a higher reactivity, and its polar groups
create them active in the reaction system, by the formation of
randomly forming the copolymer to a great extent which results in
less crosslinking and formation of cyclic oligomers. On the other
hand, by using chlorine containing structurally equivalent mono-
mers at different conditions, the high-molecular-weight copoly-
mers still could not be obtained (Scheme 2).
The structures of all PIM-Rs and CO-PIM-Rs were characterized
by FTIR, 1H NMR spectroscopy and elemental analysis. All PIMs
retained its characteristic stretching bands related to the imide
groups (C]O symmetric and asymmetric stretching in the range of
1722e1792). Moreover it is clear from the spectrum that the base
catalysed polymerization condition does not make any destruction
in the imide units (See Supporting Information). In the 1H NMR
spectrum signal positions of PIM-Rs and CO-PIM-Rs were located
Table 1
GPC results and mechanical properties of PIM-Rs and CO-PIM-Rs and their
comparison with PIM-1.
PIMs
Mn
Mw
Mw/Mn
Tensile
strength (MPa)
PIM-1 [14b,23]
PIM-R1
PIM-R2
PIM-R3
PIM-R4
PIM-R5
PIM-R6
PIM-R7
CO-PIM-R3
CO-PIM-R4
CO-PIM-R6
CO-PIM-R7
96,428
270,000
42,403
27,624
2.8
47.1
48.1
e
40,152
15,741
62,276
69,348
31,000
67,748
27,803
125,753
148,563
142,000
139,788
1.27
1.75
2.87
3.09
1.20
2.38
5.50
1.24
1.25
1.1
178,843
214,278
37,200
162,594
152,916
155,908
186,930
162,000
178,808
45.0
46.5
39.4
48.6
41.0
49.0
48.4
48.8
44.0
Fig. 2. Transparent free standing PIM-R6 film (Mn ¼ 67748, PDI ¼ 2.38, thickness
1.27
70 mm).

S. Makhseed et al. / Polymer 53 (2012) 2964e2972
2969
Fig. 3. SEM images of CO-PIM-R6 a) powder and b) thin film.
of their molecular weight. This is attributed to the good structural
rigidity due to the presence of strong double stranded backbone
(ladder-like sequences). As usually the mechanical properties of
linear polymers depend on its molecular weight.
equation to the data obtained from the nitrogen adsorption/
desorption isotherms at 77 K. Table 2 lists several relevant
parameters obtained from the isotherm analysis including surface
areas, micropore area, micropore volume and pore diameter based
on Horvath-Kawazoe (H-K). The BET surface areas of PIM-R(1e7)s
and CO-PIM-Rs ranged from 500 to 900 m2/g, with adequate
micropore area (438e638 m2/g). The micropore pore volumes
ranging from 0.22 to 0.44 cm3/g. The difference in specific surface
area might be due to the structural geometry of the monomers
employed as well as the efficient polycondensation by the incor-
poration of highly reactive fluorinated monomers into the reaction
mixture, resulting in high-molecular-weight copolymers. When
comparing the porous properties PIM-Rs and CO-PIM-Rs which are
built up from different monomers and comonomers, a trend linking
porosity and the comonomer unit can be seen. Typical example of
nitrogen adsorption/desorption isotherm is shown in Fig. 5. The
adsorption isotherms show high uptake at very low relative pres-
sure, which is characteristic of a microporous material and
isotherms of the prepared polymers exhibit a combination of I and
II N2 sorption isotherm with a continuous increase after the
adsorption at low relative pressure and a broad hysteresis upon
desorption. Suggesting that, a broad hysteresis down to low pres-
sure is indicative of trapping effect at cryogenic temperature. The
extremely low relative pressure portion of the adsorption isotherm
is related to the apparent micropore distribution, which in turn is
expected to have a profound influence on the transport properties
of the polymer. From the t-plot analysis it was found that all the
prepared polymers showed significant amount of microporosity.
Surface area for CO-PIM-Rs found to be higher than its related
The particle size and morphology of microporous polymers were
evaluated by scanning electron microscopy (SEM). Fig. 3 depicts SEM
images of the powder and film samples of CO-PIM-R6. The phase
homogeneity and uniform size distribution of particles is observed in
most of the precipitated samples with considerable differences in
surface morphologies of the prepared polymers. The SEM images of
the precipitated samples also revealed an aggregation of loosely
packed spherically as well as non-spherically shaped micro and
nanoparticles (See Supporting Information). The formation of less
porous structure in the film can be ascribed to the more densely
packed of polymer chains after the solvent evaporation. However the
difference in the polymer microstructure due to processing is so
significant while considering the sorption ability. Using transmission
microscope at high resolution (HRTEM), polymers are seen to have
porous structure (Fig. 4). Overall, this is a fairly unique nanoporous
structure that dramatically enhances adsorption of small molecules.
Like other porous materials (e.g. activated carbons) our prepared
polymers were found to be stable under the HRTEM analysis
(300 kV) as an additional indication of the high structural stability.
3.1. N2 sorption analysis
The porous nature of PIMs was analysed by means of nitrogen
sorption at 77 K. The most common technique used for screening
the porosity involves the fitting of Brunauer-Emmett-Teller (BET)
Fig. 4. HRTEM images of PIM-R4 powder at different magnifications.

2970
S. Makhseed et al. / Polymer 53 (2012) 2964e2972
Table 2
Porous properties of PIM-Rs and CO-PIM-Rs and their comparison with PIM-1 and OFP-3.
PIMs
SABET (m2/g)a
PVmicro (cm3/g)
HK median pore
width (Å)
SALAN (m2/g)b
(77 K/87 K)
H
2 (wt.%)/1.13 bar,
Qst (kJ/mole)
(77 K/87 K)
PIM-1 [14b]
OFP-3 [4]
PIM-R1
PIM-R2
PIM-R3
PIM-R4
PIM-R5
PIM-R6
PIM-R7
CO-PIM-R3
CO-PIM-R4
CO-PIM-R6
CO-PIM-R7
860
e
6e7
5e6
7.5
6.6
8.2
7.6
8.3
6.9
6.8
6.6
6.5
6.7
7.8
e
1.04
e
1159 (1089)
702 (575)
595 (548)
628 (467)
889 (717)
636 (457)
714 (632)
680 (638)
843 (813)
880 (840)
855 (789)
857 (549)
0.69
0.28
0.25
0.23
0.37
0.23
0.32
0.33
0.41
0.42
0.44
0.37
934/801
432/323
343/275
467/353
531/436
422/315
453/326
440/346
502/406
545/451
541/422
553/443
1.56/1.36
1.01/0.76
0.79/0.63
0.97/0.74
1.18/0.92
0.86/0.63
1.03/0.87
0.92/0.81
1.08/0.92
1.26/0.95
1.21/0.96
1.17/0.91
7.45
7.24
7.35
7.14
6.85
6.66
7.24
7.37
6.60
7.54
8.55
7.37
a
BET surface area calculated from nitrogen adsorption isotherm. The number in the parenthesis is the micropore surface area calculated using the t-plot analysis. PVmicro is
the micropore volume.
b
Surface area calculated from the H2 adsorption isotherm using Langmuir equation at 77 K and 87 K.
polymer containing the common repeating unit. Analysis of low
relative pressure region of the N2 adsorption isotherm by the
Horvath-Kawazoe (H-K) method assuming a slit-pore geometry
gives the apparent micropore distributions is given in the inset of
Fig. 5. To induce microporosity in a prepared polymer at least one of
the monomer contain a site of contortion [28], which in many cases
represented by tetrahedral carbon of a spiral core and moreover
pore distribution with the majority of micropores populated at
width less than 10 Å. Such narrow pore size distribution is advan-
tageous for better interaction with H2 molecules and there by
enhancing the storage capacity. The high surface areas of intrinsi-
cally porous polymers can be illustrated by its rigid nonlinear
architecture. It is believed that nonlinear polymers can possess
large amount of interconnected void space. The presence of these
interconnected voids enhances the intrinsic microporosity of the
polymer even without the network structure. The spiro centre
present in the 5,50,6,60-tetrahydroxy-3,3,3,30-tetramethyl-1,10-spi-
robisindane creates the nonlinearity by propagating the polymer
chain in three-dimensional irregular fashion producing a polymer
chain that can also prevent the dense packing. The dioxane linkage
in conjunction with spiro centre forcing perpendicularly arranged
aromatic rings creates the conditions of supramolecular control
over the molecular structure.
The porous properties of some selected film samples were
tested by means of nitrogen adsorption at 77 K. As expected the
measurement were very slow and the film samples showed lower
surface area as compared to its powder form, which may be related
to the more densely packed structure as confirmed by SEM. Fig. 6
shows the N2 adsorption/desorption isotherm of CO-PIM-R4 film
with a remarkable BET surface area (467 m2/g). The isotherm
displays a considerable uptake at relative low pressure with larger
hysteresis due to the swelling/trapping effect [18]. In addition,
some film samples show a sudden jump at certain points which can
be explained by the getting pressure effect (see supporting infor-
mation). In comparison to powder samples, all the tested film
samples shows same behaviour in the adsorption isotherm which is
typical for finely microporous media.
hydrogen adsorption capacities up to 1.24 and 0.96 wt% respec-
tively at 1.12 bar. The isotherms are fully reversible and exhibit
a sharp rise at low pressure region which is consistent with the
physisorption of hydrogen on a microporous material. All the
prepared PIM-Rs and CO-PIM-Rs show similar behaviour in their
isotherm with a significant uptake at two different temperatures
(Table 2). From the isotherms it was found that there was no kinetic
trapping of hydrogen in small pores upon desorption. The
Hydrogen adsorption capacities by various samples with different
surface areas usually reveal a linear relationship between BET
surface area and H2 storage capacity at low pressure. For example
the H2 adsorption capacity of PIM-R2 (0.79 wt.%) and PIM-R4
(1.18 wt.%) at 1 bar and 77 K with their BET surface areas 595 and
889 m2/g respectively, is consistent with the above hypothesis.
Thus from the comparison of various synthesised PIM-Rs and
CO-PIM-Rs, the H2 adsorption capacities are largely influenced by
BET surface area. Several studies showed that a clear relationship
exists between total surface area and hydrogen adsorption density
of a porous material [29ꢁ31]. The hydrogen adsorption at 77 K,
physisorption mechanism is dominant and the H2 uptake is
controlled by the structural features of the adsorbent material.
Moreover small micropores can effectively adsorb hydrogen,
probably due to its much smaller kinetic diameter compared to
bigger gas molecules such as N2 [2b,32]. These ultramicropores
allow the dihydrogen molecule to interact with multiple portions of
3.2. Hydrogen adsorption
The volumetric hydrogen adsorption capacities of all PIMs were
measured using a ASAP 2020 instrument with 99.999% pure H2
over the pressure range 0e1.13 bar. The sorption of hydrogen at
cryogenic temperature is also an important tool to identify the
microporosity in polymers. Fig. 7 shows the adsorption/desorption
isotherms at 77 and 87 K for the CO-PIM-R4 with a maximum
Fig. 5. Nitrogen sorption isotherm of CO-PIM-R4 at 77 K and the inset picture is
micropore size distribution as calculated using HK method.

S. Makhseed et al. / Polymer 53 (2012) 2964e2972
2971
Fig. 6. Nitrogen sorption isotherm of CO-PIM-R4 film at 77 K.
Fig. 8. Isosteric heats of adsorption for H2 on PIM-R6 and CO-PIM-R6.
the double stranded framework and thereby increasing adsorption
potential and a stronger van der Waals interaction with the
hydrogen molecules [33,34]. It is therefore clear that the hydrogen
storage capacity is generally relative to their respective specific
surface areas as well the presence of ultramicroporosity. The
langmuir model is frequently used to measure specific surface area
of microporous polymers from hydrogen sorption isotherms and
the values ranging upto 550 m2/g as depicted in Table 2. The
Langmuir isotherm theory assumes monolayer coverage of adsor-
bate over a homogenous adsorbent surface of equal energy. The
isosteric heat of adsorption, (Qst), for dihydrogen molecules on all
samples was calculated from the adsorption isotherms at 77 and
87 K as shown in Fig. 7. The calculated Qst values (6.6e8.5 kJ/mol)
are comparable with other porous organic materials such as COFs,
PAFs and HCPs. The highest heat of sorption (8.5 kJ/mol) was
In a recent study based on FTIR spectroscopy revealed that the
dihydrogen molecules adsorb perpendicular to the surface, the
preferential adsorption site is benzene rings [35]. So materials
containing high electron rich sites were found to increase the
hydrogen adsorption enthalpy. The combination of microporosity
and chemical functionality of these PIMs are very promising for
applications in the air purification, or separation. As compared to
MOFs and similar metal containing porous structures, the concept
of preparing hydro-thermally stable PIMs having organic spacers
along with predefined functionality allows them to extend in many
adsorption applications.
4. Conclusions
observed for CO-PIM-R6indicating
a favourable interactions
between dihydrogen and polymer surface. As shown in typical
example in Fig. 8 the values of Qst decreases rapidly as the function
of degree of coverage. This is attributed to the heterogeneous
nature of polymer surface (sorption places are energetically
different) which is available for adsorption [2]. The chemical nature
of the accessable surfaces and morphology of the pores have thus to
be tuned to enable a high adsorption enthalpy. It has been already
known that microstructure had a deep impact on hydrogen uptake
which is further influenced by the geometry of the building units.
In conclusion, a new series of soluble phthalimide based organic
polymers exhibiting intrinsic microporosity were synthesized by
the nucleophilic substitution reaction using various fluoro-
monomers and 5,50,6, 60-Tetrahydroxy-3,3,30,30-tetramethylspir-
obisindane. The resulting ladder polymers were obtained in high
yield. The FTIR, 1H NMR, and elemental analyses indicated that the
polymers appear to have the same proposed structure. Reaction
conditions led to high molecular weight polymers which provided
surface areas up to 900 m2 ꢁ1. The pore size distribution analysis
g
shown that pores are narrowly distributed and fine-tuned with
a subnanometer dimension, close to the van der Waals diameters of
various small gas molecules. The intrinsic microporosity of these
polymers is supported by good hydrogen uptake at 77 K and 87 K.
Isosteric heat of H2 adsorption (Qst) measurement revealed that Qst
is not highly sensitive to the structural features and exhibit high
isoteric heats of adsorption up to 8.5 kJ/mol.
Acknowledgements
The authors thank Kuwait University for funding (SC03/08) and
the technical support of GFS (GS01/01, GS01/05 and GS01/03) and
EMU.
Appendix A. Supporting information
Supplementary data related to this article can be found online at
doi:10.1016/j.polymer.2012.05.001.
Fig. 7. Hydrogen sorption isotherms of CO-PIM-R4 at 77 K and 87 K.

2972
S. Makhseed et al. / Polymer 53 (2012) 2964e2972
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