Triptycene-based polymers of intrinsic microporosity: Organic materials that can be tailored for gas adsorption

Article abstract of DOI:10.1021/ma100640m

We report the synthesis and properties of network polymers of intrinsic microporosity (network - PIMs) derived from triptycene monomers that possess alkyl groups attached to their bridgehead positions. Gas adsorption can be controlled by the length and branching of the alkyl chains so that the apparent BET surface area of the materials can be tuned within the range 618 - 1760 m² g^{- 1}. Shorter (e.g., methyl) or branched (e.g., isopropyl) alkyl chains provide the materials of greatest microporosity, whereas longer alkyl chains appear to block the microporosity created by the rigid organic framework. The enhanced microporosity, in comparison to other PIMs, originates from the macromolecular shape of the framework, as dictated by the triptycene units, which helps to reduce intermolecular contact between the extended planar struts of the rigid framework and thus reduces the efficiency of packing within the solid. The hydrogen adsorption capacities of the triptycene-based PIMs with either methyl or isopropyl substituents are among the highest for purely organic materials at low or moderate presures (1.83% by mass at 1 bar/77K; 3.4% by mass at 18 bar/77 K). The impressive hydrogen adsorption capacity of these materials is related to a high concentration of subnanometre micropores, as verified by Horvath - Kawazoe analysis of low-pressure nitrogen adsorption data.

Full text of DOI:10.1021/ma100640m

Macromolecules 2010, 43, 5287–5294 5287
DOI: 10.1021/ma100640m
Triptycene-Based Polymers of Intrinsic Microporosity: Organic Materials
That Can Be Tailored for Gas Adsorption
Bader S. Ghanem,†,^ Mohammed Hashem,† Kenneth D. M. Harris,† Kadhum J. Msayib,†
Mingcan Xu,† Peter M. Budd,‡ Nhamo Chaukura,‡ David Book,§ Steven Tedds,§ Allan Walton,§
and Neil B. McKeown*,†
†School of Chemistry, Cardiff University, Cardiff, CF10 3AT, U.K., ‡School of Chemistry, University of
Manchester, Manchester, M13 9PL,U.K., §School of Metallurgy and Materials, University of Birmingham,
Birmingham, B15 2TT, U.K., and ^Department of Chemistry, Taibah University, P.O. Box 344, Almadinah
Almonawarah, Saudi Arabia
Received March 23, 2010; Revised Manuscript Received May 1, 2010
ABSTRACT: We report the synthesis and properties of network polymers of intrinsic microporosity
(network-PIMs) derived from triptycene monomers that possess alkyl groups attached to their bridgehead
positions. Gas adsorption can be controlled by the length and branching of the alkyl chains so that the
apparent BET surface area of the materials can be tuned within the range 618-1760 m2 g-1. Shorter (e.g.,
methyl) or branched (e.g., isopropyl) alkyl chains provide the materials of greatest microporosity, whereas
longer alkyl chains appear to block the microporosity created by the rigid organic framework. The enhanced
microporosity, in comparison to other PIMs, originates from the macromolecular shape of the framework, as
dictated by the triptycene units, which helps to reduce intermolecular contact between the extended planar
struts of the rigid framework and thus reduces the efficiency of packing within the solid. The hydrogen
adsorption capacities of the triptycene-based PIMs with either methyl or isopropyl substituents are among
the highest for purely organic materials at low or moderate presures (1.83% by mass at 1 bar/77K; 3.4% by
mass at 18 bar/77 K). The impressive hydrogen adsorption capacity of these materials is related to a high
concentration of subnanometre micropores, as verified by Horvath-Kawazoe analysis of low-pressure
nitrogen adsorption data.
Introduction
chloromethylated polystyrene (e.g., Davankov resins),32 or by
polymerization of suitable aromatic monomers (e.g., p-xylylene-
Microporous materials are solids that contain interconnected
pores of less than 2 nm in size, and consequently, they possess
large and accessible internal surface areas that can be exploited
for heterogeneous catalysis, adsorption, separation and gas
storage.1 Conventional microporous materials consist of a crys-
talline inorganic framework (e.g., zeolites and related structures)
or an amorphous structure (e.g., silica, activated carbon).2,3
However, the impressive properties of crystalline organic-inor-
ganic hybrid materials, such as the metal-organic-frameworks
(MOFs),4-11 has inspired the recent synthesis of purely organic
porous materials such as the structurally related covalent-organic-
frameworks (COFs)12-17 and the study of microporous crystals
derived from low molecular mass organic compounds.18-22 Micro-
porosity can also be obtained within amorphous organic materials,
as demonstrated by hypercrosslinked polystyrenes,23-27 and the
recent extension of this concept to microporous polymer networks
derived from a number of different monomers and polymeriza-
tion reactions.28-31 The open microporous structure of hypercros-
slinked polymers arises from the formation of a solvent-swollen,
rigid polymer network, which on removal of the included solvent
does not collapse into a dense solid. Hypercrosslinked polymers
can be made from a range of precursors, but those that exhibit
the greatest microporosity (apparent BET surface area >2000 m2
g-1, pore volume >1.5 mL g-1) are those in which efficient
network formation is achieved using Friedel-Crafts alkylation of
dichloride) using similar reaction conditions.30
In recent years, we have developed a class of microporous
organic materials termed “polymers of intrinsic microporosity”
(PIMs), based upon the concept of designing highly rigid and
contorted macromolecules that cannot pack space efficiently
in the solid state, thus leaving interconnected molecular-sized
holes.33-35 PIMs are not necessarily network polymers and a
number of them (e.g., PIM-1; Scheme 1) are soluble in common
solvents facilitating the fabrication of microporous films and
coatings by solution processing.36-40 These materials are promis-
ing for use ingas separationmembranes as theydemonstrateboth
high permeability and good selectivity for several gas pairs of
commercial relevance (e.g., O2/N2 and CO2/CH4).41,42 It was
anticipated that combining the concept of hypercrosslinked net-
work polymers with that of PIMs, by preparing network poly-
mers using similar monomers and polymerization chemistry to
those used for the preparation of soluble PIMs, would provide
materials (i.e., network-PIMs) with greater microporosity.
However, most of these network-PIMs demonstrate an apparent
BET surface area significantly less than 1000 m2 g-1 43-48
.
The rigid fused-ring framework and 3-fold symmetry of tri-
ptycene make it an attractive component for the synthesis of
network PIMs with enhanced microporosity. Previously, tripty-
cenes have been exploited widely as components of functional
molecular systems including supramolecular receptors,49-58
ligands,59,60 molecular cages,61 components for molecular ma-
chines (e.g., rotors, gyroscopes,62-66 ratchets,67-69 brakes70,71 and
gears72,73), technomimetic molecules74,75 and materials such as liquid
*To whom correspondence should be addressed. E-mail: mckeownnb@
cardiff.ac.uk. Telephone: þ44 (0)2920-875851.
r 2010 American Chemical Society
Published on Web 05/25/2010
pubs.acs.org/Macromolecules

5288 Macromolecules, Vol. 43, No. 12, 2010
Ghanem et al.
Scheme 1. Synthesis of PIM-1a
a Reagents and conditions: K2CO3, DMF, 60 ꢀC, and 48 h.
Scheme 2. Synthesis of the Trip(R)-PIMsa
a Reagent and conditions: (i) cH2SO4, 5 ꢀC, 2 h; (ii) diazonium salt of 4,5-dimethoxy-2-aminobenzoic acid, 1,2-epoxypropane, CH2Cl2, reflux, 12 h;
(iii) Br2, Fe CH2Cl2; (iv) MeONa, toluene, CuBr. v. BBr3, CH2Cl2, room temperature, 3 h; (vi) 2,3,5,6-tetrafluoroterephthalonitrile, DMF, 80 ꢀC, 24 h.
crystals,76-78 clathrate crystals,79-83 coordination networks,84-87
and polymers.88-92 Of particular interest to the development of
novel microporous polymers is the concept of internal molecular free
volume (IMFV), devised by Swager,93 which suggests that the shape
and width of the triptycene unit delineates a significant amount of
unoccupied space.94 This concept has been used to design useful
functional materials by exploiting IMFV to obtain polymeric
materials with large free volumes for applications as low-κ dielectric
materials95 and as a permeable matrix for nanoparticle fabrication.96
Alternatively, liquid crystals or polymers can be obtained with
enhanced alignment due to the minimization of IMFV via inter-
locking of the triptycenes.78 It has also been demonstrated that the
introduction of triptycene into a polymer can enhance its mechanical
strength97-99 and this too has been attributed to greater polymer
cohesion due to the interlocking of the triptycene units. Structural
building units with greater IMFV can be prepared by extending the
triptycene system to give “iptycenes”, a term introduced by Hart, to
describe oligomeric systems that contain only fused arene and
bicyclo[2,2,2]octane rings.100-103 For example, the large amount
of IMFV of the H-shaped pentiptycene can be used to design highly
soluble, easily aligned, conjugated polymers with high free volumes
that are beneficial for adsorbing aromatic molecules in fluorescence-
based sensors.98,104-110 It was noted by Hart that larger iptycenes
(the term “polyiptycene” was coined)101 would possess spacious
cavities that may have applications as molecular hosts for ad-
sorption.102,111-113 In this context, recent work by Chong and
MacLachlan has prepared oligomeric triptycenes some of which
form amorphous states capable of significant nitrogen and hydrogen
adsorption.60,114
structure containing a high concentration of micropores of
subnanometre dimensions (i.e., ultramicropores).115 In addition,
the microporosity, and thus gas adsorption, of these materials
can be controlled by the length and branching of the two alkyl
chains attached to the polymer at the bridgehead positions of the
triptycene subunits, thus allowing the apparent BET surface area
of the polymers to be tuned over the range 600-1760 m2 g-1
.
Hence these triptycene-based network-PIMs represents one of
the few families of microporous material for which gas adsorp-
tion properties can be tailored exquisitely by subtle changes in
structure.116
Experimental Section
Materials. Examples of synthetic procedures for the prepara-
tion of the triptycene monomers and their anthracene precur-
sors are provided in the Supporting Information and are
summarized in Scheme 2.
Example of the Synthesis of a Network-Triptycene PIM.
Trip(Et)-PIM. To a stirred solution of 2,3,5,6-tetrafluoroter-
ephthalonitrile (0.30 g, 1.5 mmol) and 9,10-diethyl-2,3,6,7,12,
13-hexahydroxtriptycene (0.41 g, 1.0 mmol) in anhydrous DMF
(70 mL) was added anhydrous potassium carbonate (9 mmol) as
a fine powder. The mixture was then heated at 80 ꢀC for 24 h
under a nitrogen atmosphere. On cooling, the mixture was
poured into water (300 mL) and acidified with 2 M HCl. The
crude product was collected by filtration and washed with water
and MeOH. Purification was achieved by stirring in refluxing
MeOH and THF. The resulting orange solid was ground into a
fine powder and dried in a vacuum oven at 100 ꢀC for 16 h to give
0.63 g of Trip(Et)-PIMs (103% yield); IR (KBr cm-1): ν =
3060, 3010, 2956, 2870 (C-H), 2235 (CN), 1610 (C-C Ar),
1170, 1250 (C-O); high-resolution solid state 13C NMR (75.480
MHz): isotopic δ 8 (CH3), 20 (CH2), 53 (bridgehead C) 96
(C-CN), 112 (aromatic C-H), 135-155 (aromatic carbons).
Here we report the synthesis and properties of network-PIMs
derived from triptycene monomers and we show that they possess
enhanced microporosity, with surface areas that approach those
of the best hypercrosslinked polymers but with a more defined

Article
Macromolecules, Vol. 43, No. 12, 2010 5289
Table 1. Gas Adsorption Data for the Trip(R)-PIMs
H2 uptake at 77 K, mmol g-1 (% mass)
R
BET area (m2 g-1
)
N2 uptake; (mmol g-1
)
density (mL g-1
)
1 bar
10 bar
calcd max.a
H
Me
1318
1760
22.8
42.2
1.57
1.67
7.46 (1.50)
8.90 (1.79)b
6.30 (1.27)c
8.33 (1.68)b
7.69 (1.55)
9.08 (1.83)
6.55 (1.32)
7.34 (1.48)
5.61 (1.13)
3.67 (0.74)
6.00 (1.20)
11.8 (2.6)
15.6 (3.2)
12.9 (2.6)c
13.7 (2.8)
12.2 (2.5)
15.3 (3.1)
10.3 (2.1)
11.7 (2.4)
8.8 (1.8)
19.9 (4.0)
25.5 (5.2)
Et
1416d
1343
1601
978
1076
947
618
39.5
32.1
45.8
23.8
24.5
17.9
11.6
16.1
1.40
1.40
1.54
1.48
1.43
1.48
1.27
1.43
20.9 (4.2)
17.6 (3.5)
25.7 (5.2)
16.2 (3.2)
17.5 (3.5)
13.8 (2.8)
9.3 (1.9)
n-Pr
i-Pr
n-Bu
i-Bu
Pe
Oct
benzyl
6.0 (1.2)
8.7 (1.7)
880
12.0 (2.4)
a Calculated H2 saturation value using Sips equation.136 b Volumetric measurement for H2 uptake =1.83% for Trip(Me)-PIM and 1.65% for
Trip(Et)-PIM by mass at 1 bar (Micromeritics ASAP 2020). c Measurement at 87 K. d Mass measurement error resulted in this value being previously
quoted as 1065 m2 g-1 115
.
Anal. Found: C, 69.73; H, 3.02; N, 6.68; F, 0.88. Calcd for
C36H16N3O6: C, 73.72; H, 2.75; N, 7.16; F, 0.00. (Note that after
routine drying in a vacuum oven, TGA typically shows a loss of
∼4-6% mass below 150 ꢀC, probably due to adsorbed water or
solvent. TGA analysis of samples for which rigorous removal
of adsorbed species was achieved by prolonged exposed to high
vacuum in the outgas port of the surface area analyzer at 120 ꢀC
gives a mass loss of <0.5% mass below 100 ꢀC).
from the reaction between a 2,3,6,7,12,13-hexahydroxytrip-
tycene monomer and commercially available 2,3,5,6-tetra-
fluoroterephthalonitrile (Scheme 2). With one exception
(i.e., R = H), the triptycene monomers are readily prepared
from the Diels-Alder reaction between 9,10-dialkyl-2,3,6,7-
tetramethoxyanthracene and 4,5-dimethoxybenzyne (prepared
in situ from the diazonium salt of 4,5-dimethoxy-2-aminoben-
zoic acid),119 followed by demethylation of the 9,10-dialkyl-
2,3,6,7,12,13-hexamethoxytriptycene product using BBr3.50,54
A range of 9,10-dialkyl-2,3,6,7-tetramethoxyanthracenes (R =
Me, Et, Pr, i-Pr, Bu, i-Bu, t-Bu, Pe, Oct, Bz) was readily
accessible from the simple reaction between the appropriate
aldehyde and veratrole in cH2SO4, by slight modifications to
published procedures.120-122 Surprisingly, the triptycene-form-
ing Diels-Alder reaction proved relatively unaffected by the
presence of a branched alkyl chain or benzyl group at the 9,10-
positions of the anthracene, thus allowing the synthesis of
monomers with isopropyl or isobutyl groups attached to the
bridgehead positions, although bulkier tert-butyl groups did
prohibit the benzyne cycloaddition. In contrast, attempts to
prepare the novel precursor 2,3,6,7,12,13-hexamethoxytripty-
cene (i.e., R = H) were unsuccessful due to the failure of the
Diels-Alder reaction. Instead, it was prepared by the copper(I)
bromide mediated reaction of 2,3,6,7,12,13-hexabromotri-
ptycene94 with sodium methoxide (Scheme 2).123
Each of the Trip(R)-PIMs was obtained as an insoluble
yellow-orange powder in quantitative yield and was initially
characterized by elemental analysis, from which the lack of
residual fluorine (F < 1.0%) indicates an efficient network
formation. It should be noted that the network-forming
reaction (i.e., a double aromatic nucleophilic substitution
reaction) is the same as that used to prepare high molecular
mass PIM-1 (Mw > 1 ꢀ 105 g mol-1) suitable for robust film
formation (Scheme 1). High resolution solid state 13C NMR
(Figure 1) and IR spectra support the ideal structure of the
networks depicted in Scheme 2. Powder X-ray diffraction
confirms that the material is amorphous, as expected. ther-
mal gravimetric analysis (TGA) of the Trip(R)-PIMs shows
no decomposition below 400 ꢀC.
High-Resolution Solid-State 13C NMR Spectroscopy. Spectra
were recorded at 293 K on a Chemagnetics CMX-Infinity 300
spectrometer (13C frequency, 75.480 MHz) using a 4 mm triple-
channel probe. The spectra were recorded under conditions of
13C r 1H cross-polarization, magic angle sample spinning and
1
high power H TPPM decoupling. In each experiment, 10000
scans were accumulated, with spinning frequency 6 kHz, CP
contact time 5 μs and recycle delay 10 s. Dipolar dephasing 13
C
NMR spectra117 were recorded with dephasing delays of 40 and
60 μs.
Gas Adsorption Studies. Gas adsorption measurements were
performed on samples that had been heated previously under
ultra high vacuum (10-9 bar) at 100-120 ꢀC for 8-12 h to
remove residual solvent and other adsorbates. Basic volumetric
N2 sorption studies were undertaken at 77 K using a Beckman
Coulter 3100 Surface Area Analyzer (Fullerton, California,
USA). Low-pressure data, suitable for modeling micropore size
distribution, were obtained using a Micromeritics Instrument
Corporation (Norcross, GA) accelerated surface area and por-
osimetry (ASAP) 2020 system. Helium was used for the free
space determination, after nitrogen sorption analysis, both at
ambient temperature and at 77 K. Apparent surface areas were
calculated from N2 adsorption data by multipoint BET analysis.
Apparent micropore distributions were calculated from N2
adsorption data by the Horvath-Kawazoe method,118 assum-
ing a slit-pore geometry and the original H-K carbon-graphite
interaction potential. Gravimetric H2 sorption studies were
undertaken using a Hiden Isochema (Warrington, Cheshire,
U.K.) intelligent gravimetric analyzer (Hiden IGA-001), which
incorporates an electronic microbalance capable of measuring
mass change with a resolution of (0.2 μg. After degassing,
pressure composition isotherms were measured in ultrapure H2
(Air Products 99.99995%, additionally passed through a liquid
nitrogen trap) up to 18 bar at liquid nitrogen and liquid argon
temperatures (77 and 87 K). The measured masses were cor-
rected for buoyancy using the weights and densities of all the
components on both sides of the balance. The sample density
was measured using helium pycnometry using a Micromeritics
AccuPyc II 1340 System (Table 1) with ultrapure helium (Air
Products -99.99996%).
Gas Adsorption. The N2 adsorption isotherms for the
Trip(R)-PIMs obtained at 77 K (e.g., Figure 2) show signi-
ficant adsorption for each member of the series at low
relative pressures (p/po < 0.1), consistent with microporos-
ity. Apparent BET surface areas (Table 1), calculated from
the isotherms, range from 618 m2 g-1 (R = octyl) to 1760
m2 g-1 (R = methyl). The total nitrogen uptake at p/po = 1.0
also displays a smooth trend across the homologous series,
decreasing from 42.2 mmol g-1 to 11.6 mmol g-1 (Table 1
and Figure 3). Of interest is the effect of branched alkyl
chains on the microporosity, with a greater BET surface area
Results
Synthesis. The triptycene-based PIMs, denoted Trip-
(R)-PIMs (where R = bridgehead substituent), were prepared

5290 Macromolecules, Vol. 43, No. 12, 2010
Ghanem et al.
Figure 1. High-resolution solid-state 13C NMR spectra for Trip(Me)-PIM, Trip(Et)-PIM and Trip(i-Pr)-PIM with upper spectra being standard
13C NMR spectra (with no dipolar dephasing) and lower spectra obtained with dephasing delays (tdd as indicated). The arrows indicate the signals
due to the methyl, methylene and methine carbons, respectively, attached directly to the bridgehead positions of the triptycene. Complete suppression
of these signals occurs in Trip(Et)-PIM and Trip(i-Pr)-PIM but not in Trip(Me)-PIM, as discussed in the text.
Figure 2. Nitrogen adsorption (solid symbols) and desorption (open
Figure 3. Amount of gas adsorbed within the Trip(R)-PIMs at 77 K
versus the length of the alkyl chain (R; note that 0 corresponds to R = H)
attached to the bridgehead position of the triptycene subunits (( N2 at
p/po = 1.0; 2 H2 at 10 bar). The filled symbols are for n-alkyl side-chains
whereas empty symbols represent the data for Trip(R)-PIMs containing
isopropyl and isobutyl.
symbols) isotherms for Trip(Me)-PIM at 77 K, obtained in Manche-
ster (b) and Liverpool (9, data courtesy of Prof. Andrew I. Cooper and
Dr. Colin Wood) and for Trip(H)-PIM ((), with the values of the
X-axis displaced by p/po = 0.1 for clarity. Good agreement was
achieved between differentlaboratories; slight variations at highrelative
pressure may be attributed to textural differences between samples.
Note the very pronounced hysteresis between the adsorption and
desorption isotherms for Trip(Me)-PIM and the relatively small
hysteresis for Trip(H)-PIM.
Discussion
Gas adsorption demonstrates that the Trip(R)-PIMs possess
a greater degree of microporosity relative to any PIM prepared
previously (Table 1), with the exception of those possessing long
(R = Pe, Oct) or bulky side-chains (R = Bz). Consideration of a
molecular model of a fragment of Trip(Et)-PIM (Figure 5)
reveals a possible explanation for this enhanced microporosity.
The triptycene component constrains the growth of the polymer
within the same plane, with the faces of the ribbon-like “struts”
between the triptycenes being oriented perpendicular to the plane
of the macromolecular growth. This arrangement may help to
block face-to-face association between these planar struts and
may thus help to frustrate further the space-efficient packing of
the polymer network, leading to greater microporosity. The
reduction in gas adsorption of Trip(R)-PIMs as the length of
the n-alkyl groups is increased suggests that the flexible side-
chains occupy an increasingly larger proportion of the free
volume created by the rigid polymer framework. A similar
pore-blocking effect has been reported recently for mesoporous
alkyl-substituted COFs.13 In contrast, the isopropyl side-chains
do not diminish the microporosity. It is known that the archi-
tecture of triptycene imposes severe steric restrictions on the
and overall nitrogen uptake for Trip(i-Pr)-PIMs in compar-
ison with Trip(n-Pr)-PIM. A smaller enhancement of mi-
croporosity is obtained for Trip(i-Bu)-PIM over Trip-
(n-Bu)-Trip. A particularly distinct feature of the adsorption
isotherms, especially those of the Trip(R)-PIMs with short-
er alkyl chains, is the pronounced hysteresis that extends to
low relative pressures between the adsorption and desorp-
tion cycles (Figure 2). It is notable that the absence of a
bridgehead alkyl substituent in Trip(H)-PIM reduces signi-
ficantly the degree of hysteresis and the amount of N2
adsorbed at p/po = 1.0, although this material is still highly
microporous (apparent BET surface area =1318 m2 g-1).
The values for H2 adsorption are provided in Table 1 and
Figure 3. A typical H2 adsorption isotherm for Trip(i-Pr)-
PIM is given in Figure 4. The isotherms are completely
reversible and Type 1 in shape, as is typical for the physisorp-
tion of H2 on microporous materials. Significant hydrogen
adsorption was observed for each material, with Trip-
(Me)-PIM and Trip(i-Pr)-PIM displaying uptake in excess
of 3% by mass at 10 bar.

Article
Macromolecules, Vol. 43, No. 12, 2010 5291
whereas longer n-alkyl side-chains have sufficient conformational
freedom to fill the free volume.
The pronounced hysteresis between the adsorption and de-
sorption isotherms of the Trip(R)-PIMs (Figure 2) extends
to low relative pressure and is distinct from that associated
with mesoporosity, which closes at a high relative pressure
(p/po>0.4).129 For the Trip(R)-PIMs, the hysteresis may be
caused by activated adsorption arising either from a continuous
swelling of the material as nitrogen is adsorbed or from the
restricted access of nitrogen molecules at lower relative pressures
due to constricted pore openings or, most likely, a combination
of both of these mechanisms. Swelling may be a particular feature
of these networks because their ideal structure comprises two-
dimensional meshes (including many macrocyclic structures)
with no covalent attachment between the resulting macromole-
cular layers (Figure 5). In reality, defects and mechanical inter-
locking might provide a relatively small number of robust link-
ages between the layers, so that the material is likely to behave as
a lightly cross-linked network. The swelling of the network may
be facilitated by the relaxation of internal stress initially induced
from the removal of solvent from the original solvent-swollen
state as N2 fills the micropores. It is notable that the hysteresis of
the N2 isotherm of Trip(H)-PIM is relatively small (Figure 2),
which may be due to the strong dipolar interactions between
interdigitated nitrile groups in different layers reducing swelling.
In contrast, the bridgehead alkyl groups in the Trip(R)-PIMs
are likely to block nitrile interdigitation and allow greater
swelling of the network during N2 adsorption.
Figure 4. Hydrogen adsorption isotherm for Trip(i-Pr)-PIM obtained
at 77 K (empty squares represents the desorption isotherm). Note the
lack of hysteresis between the adsorption and desorption isotherms.
The amount of hydrogen that can be loaded within the Trip-
(R)-PIMs containing shorter alkyl chains at moderate pressure
(up to 3.4% by mass at 18 bar for Trip(Me)-PIMs) is signifi-
cantly greater than that obtained for any PIM to date, and is
comparable with the loadings obtained for other types of micro-
porous materials with similar BET surface areas (Table 2) such
as activated carbons (e.g., PICACTIF-SC),130 hypercrosslinked
polymers (e.g., CDX/BCMBP),30 COFs (e.g., COF-5),14 and
MOFs (e.g., MOF-505).131,132 However, activated carbons,
MOFs, COFs and the recently reported porous aromatic frame-
work (PAF-1)133 with higher surface areas adsorb greater
amounts of hydrogen (Table 2). It is notable that the loadings
of hydrogen at low pressure (1 bar) within Trip(i-Pr)-PIM and
Trip(Me)-PIMs are among the highest reported for purely
organic polymers134 and that these values are competitive with
the best performing examples of other classes of porous materials.
The impressive low pressure H2 adsorption of the Trip(R)-PIMs
can be attributed to a high concentration of pores with subnano-
metre dimensions, which are suitable for multiwall adsorption
(Figure 5). Analysis of the low-pressure N2 adsorption data of
Trip(Me)-PIM, based on the Horvath-Kawazoe method,118
confirms that the pore size distribution is strongly biased toward
pores that are less than a nanometre in diameter (Figure 6). It has
been noted previously that pores of diameter in the range 0.6 - 0.8
nm are optimal for hydrogen physisorption at low pressures for a
range of microporous materials132,135 including polymers.29,46
The heat of adsorption for Trip(Me)-PIM, calculated using a
Virial equation and isotherms at 77 and 87 K, is 6.5 kJ mol-1 at
zero coverage and 5.2 kJ mol-1 at 8 mmol g-1 loading. Using the
Virial analysis other microporous materials have given similar
values (e.g., 6.2 and 7.0 kJ mol-1 for COF-1 and COF-6
respectively at zero surface coverage - both of which have a pore
size of 0.9 nm).12 Applying the Clausius-Clapeyron equation
to the H2 adsorption data for Trip(Me)-PIM obtained at 77 and
87 K gives a slightly higher range for the heat of adsorption of 8.8-
6.0 kJ mol-1 for loadings of 0.1-1% mass H2. This is a similar
result (7.5-6.2 kJ mol-1) to that obtained for the BCMBP/
p-DCX hypercrosslinked polymer (median pore width =0.89 nm)
using the Clausius-Clapeyron equation.30
Figure 5. Representation of two ideal fragments of the network of
Trip(Et)-PIM showing how the shape of each macromolecule, as
dictated by the architecture of the triptycene units, prevents close
intermolecular interactions between the planar “struts”. The loose
network which arises from the ideal layered structure may account
for the tendency of the materials to swell in organic solvents or during
nitrogen adsorption, as indicated by the arrows, especially when bridge-
head alkyl chains block interpenetration of the nitrile groups.
motion of a short alkyl chain attached to the bridgehead carbon.
The dipolar dephasing solid state 13C NMR experiments, at
293 K, provide some direct qualitative insights regarding such
reorientational dynamics of these alkyl groups within the Trip-
(R)-PIMs. For rigid organic solids, a dipolar dephasing delay of
40 μs is generally sufficient to suppress the 13C NMR signal for a
1
13C nucleus directly bonded to H, but when a 13C-1H bond
undergoes reorientational dynamics (as often observed in the case
of methyl groups), a dipolar dephasing delay significantly longer
than 40 μs is typically required to suppress the 13C NMR signal.
As can be see from Figure 1, the methyl groups in Trip(Et)-PIM
and Trip(i-Pr)-PIM undergo significant reorientational dyna-
mics, whereas the 13C-1H bonds in the methylene group of
Trip(Et)-PIM and in the methine group of Trip(i-Pr)-PIM do
not undergo significant reorientation.124 For Trip(Me)-PIM,
our results suggest that the methyl group undergoes reorienta-
tion, but that the reorientation is significantly more hindered than
is typical for methyl groups in organic solids. Solutionstate NMR
studies on triptycenes substituted with an isopropyl group at the
bridgehead show that the activation energy for rotation about the
C-CH(CH3)2 bond is in excess of 75 kJ mol-1 125-128
Hence, it
.
can be concluded that ethyl and isopropyl side-chains are unable
to alter their conformation in order to occupy free volume and
can be considered as part of the rigid framework of the PIM,

5292 Macromolecules, Vol. 43, No. 12, 2010
Table 2. Comparison of Hydrogen Loadings of Microporous Materials at 77 K
Ghanem et al.
H2 uptake at 77 K (% mass)
10 bar
BET area, m2 g-1
1 bar
maximum
ref
PIMs
1.80
1.83
1.20
1.35
Trip(Me)-PIM
Trip(i-Pr)-PIM
Porph-PIM
CTC-PIM
1760
1601
960
770
750
3.2
3.1
1.9
1.7
1.5
3.9 [2.2]a
2.3
3.4 (18 bar)
3.3 (18 bar)
2.0 (18 bar)
1.7 (10 bar)
1.5 (18 bar)
3.9 (10 bar)
3.7 (60 bar)
46
137
137
138
31
PIM-1
0.95
1.56 [1.40]a
OFP-3
poly(spirobisfluorene)
1159
1043
Hypercrosslinked Polymers
polystyrene
polystyrene
DCX/BCMBP
BCMBP
1930
1466
1904
1366
630
1.40
1.27
1.50
1.70
0.80
3.1
2.8
3.3
2.4
1.7
5.2 (80 bar)
3.1 (15 bar)
3.7 (15 bar)
2.8 (15 bar)
2.2 (30 bar)
139, 140
141
30
134
28
polyaniline
COFs/PAFs
COF-5
COF-102
PAF-1
1590
3620
7100
0.9
1.20
2.4
5.8
∼4.5
3.6 (50 bar)
7.2 (30 bar)
7.0 (48 bar)
12, 14
12
133
∼1.5
Activated Carbons
PICACTIF-SC
AX-21
zeolite-templated
1700
2421
3200
1.90
2.40
2.60
3.0
4.0
6.9
130
130
142
MOFs
Cu2(bptc)/(MOF-505)
Cu2(qptc)
MIL-101
1670
2932
5500
5640
2.59
2.24
2.50
1.25
3.7
5.3
4.0 (20 bar)
7.0 (20 bar)
6.1 (40 bar)
7.5 (70 bar)
132, 143
132
144
MOF-177
131
a Figure in brackets represent hydrogen loadings obtained for OFP-3 using the same instrument (Hiden, IGA-1) and conditions as those used for the
Trip(R)-PIMs in this study. The unbracketed values were obtained previously using a Micromeritics ASAP 2050 surface area analyzer.138 We thank
Prof. Saad Makhseed, Kuwait University, for kindly supplying a sample of OFP-3.
example, for Trip(i-Pr)-PIM the amount of N2 adsorbed at 1 bar
(at 77 K) is three times the amount of H2 adsorbed at 10 bar. The
additional N2 adsorption at higher relative pressure is associated
with large hysteresis, which, as discussed above, is probably
related to the swelling of the polymer network. Unfortunately,
hydrogen adsorption does not give rise to a similar effect under
the conditions used in this study, although it is possible that
higher pressures or lower temperatures might enhance loadings
via this mechanism. Studies to explore this possibility are in
progress.
Acknowledgment. We are grateful to the EPSRC for funding
(EP/D074312/1) and we thank Prof. Andrew I. Cooper and Dr.
Colin Wood, University of Liverpool, U.K., for independent
verification of the N2 and H2 adsorption isotherms for Trip-
(Me)-PIM.
Supporting Information Available: Text giving additional
synthetic procedures and figures giving additional gas adsorp-
tion isoterms. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 6. Micropore size distribution for Trip(Me)-PIM based on the
Horvath-Kawazoe analysis of low-pressure nitrogen adsorption data
(combined data from Figure 1) showing that the major contribution to
porosity is from pores that are less than a nanometre in diameter.
References and Notes
For each Trip(R)-PIM, the amount of H2 adsorbed at 10 bar
is very similar to the amount of N2 adsorption at low relative
pressure (p/po = 0.01), as both values depend upon the number of
accessible micropores. However, the total amount of N2 ad-
sorbed by the Trip(R)-PIMs with shorter or branched alkyl
substituents is much greater due to the uptake in the higher
relative pressure range p/po = 0.1 - 1.0 (Table 1, Figure 2). For
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Singapore, 2007.
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