Structurally-Defined, Sulfo-Phenylated, Oligophenylenes and Polyphenylenes

Article abstract of DOI:10.1021/jacs.5b07865

We report the synthesis and molecular characterization of structurally defined, sulfo-phenylated, oligo- and polyphenylenes that incorporate a novel tetra-sulfonic acid bistetracyclone monomer. The utility of this monomer in the [4 + 2] Diels-Alder cycloaddition to produce well-defined, sulfonated oligophenylenes and pre-functionalized polyphenylene homopolymers is demonstrated. Characterization of the oligophenylenes indicates formation of the meta-meta and para-para adducts in a ～ 1:1 ratio. These functionalized monomers and their subsequent coupling provide a route to prepare novel, sterically encumbered, sulfonated polyphenylenes possessing unprecedented structural control.

Full text of DOI:10.1021/jacs.5b07865

Communication
pubs.acs.org/JACS
Structurally-Defined, Sulfo-Phenylated, Oligophenylenes and
Polyphenylenes
Thomas J. G. Skalski, Benjamin Britton, Timothy J. Peckham, and Steven Holdcroft*
Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
S
* Supporting Information
is primarily achieved by spatially controlling the placement of
ABSTRACT: We report the synthesis and molecular
characterization of structurally defined, sulfo-phenylated,
oligo- and polyphenylenes that incorporate a novel tetra-
sulfonic acid bistetracyclone monomer. The utility of this
monomer in the [4 + 2] Diels−Alder cycloaddition to
produce well-defined, sulfonated oligophenylenes and pre-
functionalized polyphenylene homopolymers is demon-
strated. Characterization of the oligophenylenes indicates
formation of the meta−meta and para−para adducts in a ∼
1:1 ratio. These functionalized monomers and their
subsequent coupling provide a route to prepare novel,
sterically encumbered, sulfonated polyphenylenes possess-
ing unprecedented structural control.
sulfonic acid groups on the polymer, but such control is
difficult, if not impossible, to achieve by post-sulfonation of
polyphenylenes. To this end, we have explored strategies
(Scheme 1) leading to the controlled synthesis of novel
Scheme 1. Synthesis of Sulfo-Phenylated Dienes and
a
Polyphenylene Homopolymer
n the past, methods to synthesize polyphenylenes,1 such as
1f,k−m
and Mullen,1e,i,j have drawn
Ithose reported by Stille
̈
attention due to the polymer’s inherent chemical stability and
mechanical strength. Of more recent interest are routes to
branched polyphenylenes bearing ionic functionality. Sulfo-
nated versions of branched polyphenylenes are currently
prepared by post-sulfonation of polyphenylenes, for the
purpose of preparing polymers for electrochemical mem-
branes.2 Such membranes are reported to be mechanically
robust and to possess high ionic (protonic) conductivity.
Recently, they have been examined for use in proton exchange
membrane fuel cells (PEMFCs), and post-quaternized
ammonium derivatives have been examined in anionic exchange
membrane fuel cells (AEMFCs).2c,3
Nonetheless, reports of sulfonated polyphenylenes are
comparatively sparse because of the difficulty of forming
rigid, sterically encumbered, aryl−aryl linkages and the need to
manipulate near-intractable polymers in polar media for the
purpose of later introducing ionic functionality. Moreover,
current examples of sulfonated polyphenylenes are structurally
ill-defined and relatively disorganized due to the uncertainty of
meta- vs para-coupling of the phenyl linkages as well as the
multitude of positions available, on multiple phenyl rings, for
post-sulfonation.2b As consistently demonstrated for acid-
bearing polymers, precise control of the polymer structure
and accurate placement of ionic functionality along the polymer
backbone enhances short and long-range order of ionic
channels and thus ionic conductivity.1c,4 Ding et al.
demonstrated this with block and graft copolymers of
sulfonated polystyrene.5 Numerous examples have since been
reported for many, primarily aromatic-based classes of ion-
containing polymers.1c,4b,d,6 A high degree of molecular control
a
(i) KOH/EtOH, reflux; (ii) Me3SiOSO2Cl, 1,2-C2H4Cl2; (iii) NEt3,
n-BuOH; (iv) PhNO2: sand bath (180 °C, 12 h) or microwave reactor
(195 °C, 2 h) (v) 2 M KOH; (vi) 0.5 M H2SO4.
sulfonated monomers and demonstrated their utility in
synthesizing sulfonated, branched oligophenylenes as well as
the homopolymer (sPPP-H+), with precise control of the
position and number of sulfonic acid groups.
Sulfonated dienes were prepared as shown in Scheme 1. The
tetracyclone 31g,7 was sulfonated using trimethylsilyl chlorosul-
fonate to produce the novel disulfonic acid tetracyclone 4. The
1H NMR spectrum of 4 (Figure S5) reveals a symmetrical
structure, containing one doublet (integration 4 H) at 7.46
ppm. Using COSY (Figure S7), this doublet correlates with the
Received: August 4, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b07865
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
doublet at 7.11 ppm (4 H). These two doublets indicate
disulfonation of 3 at the para position of the two phenylene
rings juxtaposed to the ketone.8 Sulfonation occurs at these
positions due to delocalization of the electronic charge
introduced by the ketone.9 The remaining 10 protons are
observed as doublets at 6.95 and 7.22−7.29 ppm for the
unsulfonated phenylenes as well as the multiplet between 7.22
and 7.29 ppm and 6.95 ppm for the unsulfonated phenylene.10
The acidic protons in 4 were exchanged for triethylammonium
cations by treatment with triethylamine to produce 5.
are observed at 7.00, 6.68, and 6.62 ppm (Figure S32).
According to the literature,1j,k,m this reaction does not afford a
pure isomer as both meta and para additions can occur as
shown in Scheme S10. As a result, three regio-isomers for 16
are evidenced by three main NMR signals at 6.41, 6.31, and
6.14 ppm for Hcore, which correspond to the para−para (p−p),
meta−meta (m−m), and meta−para (m−p) isomers, respec-
tively. Signals at 7.28 ppm are attributed to HDA protons,
according to the lack of correlation on the COSY with other
protons of the molecule (Figure S33). The spectrum for
compound 16 also shows signals at 7.23 and 7.11 ppm due to
Hα1, 6.57 and 6.77 ppm due to Hβ1, 7.55 and 7.46 ppm due to
Hα2, and 6.93 and 6.85 ppm due Hβ2. COSY was used to
establish the pairing of Hα1 with Hβ1, as well as Hα2 with Hβ2,
wherein the downfield shift of the Hα proton correlates with
the upfield shift of the Hβ proton (e.g., the peaks at 7.23 and
6.57 correlate) with a similar situation for the downfield pairs
(e.g., the peaks at 7.46 and 6.85 ppm correlate). We have
interpreted this to mean that 16 consists of a mixture of “H”
and “A” conformers, in contrast to the almost exclusive
formation of the “A” conformer for 14. For each conformer, a
set of three regio-isomers can be formed, and thus, a total of six
peaks, corresponding to the Hcore protons of the three regio-
isomers of each conformer, are observed between 6.00 and 6.50
ppm (Figure S32).
The symmetrical, tetrasulfonated monomer 8 was synthe-
1
sized in a similar fashion to 4. H NMR analysis of 8 (Figure
S15) reveals two doublets at low field (7.55 and 7.50 ppm).
Using COSY (Figure S17), these protons correlate with
doublets at 7.10 and 7.16 ppm, which is consistent with p-
substitution of the phenyl ring adjacent to the ketone. The
1
signal for the acidic proton appears at 7.57 ppm. H NMR
analysis indicates that 8 is symmetrical, with Hcore represented
by a singlet peak at 6.87 ppm. COSY and the 1D NOE analysis
(Figure S19) were used to distinguish whether all four of the
sulfonic acid groups are distant to one another (“H”
conformation) or whether two are in close proximity (“A”
conformation) (Figure S18). The Hα1 and Hβ1 protons (7.55
and 7.10 ppm, respectively) exhibit no through space
correlation with protons (Ho, Hm, and Hp) on the unsulfonated
phenyl rings, whereas Hα2 and Hβ2 (7.51 and 7.17 ppm
respectively) do, indicating 8 exclusively adopts the “A”
conformation in solution.
+
The protected sulfonated poly(phenylene), sPPP-NHEt3
(Scheme S12), was synthesized via the [4 + 2] D-A
cycloaddition of comonomers 9 and 13. GPC analysis indicated
Compound 8 was converted to the ammonium derivative 9
1
a Mn of 186 000 Da and a polydispersity index of 1.44. H
for greater thermal stability, prior to Diels−Alder (D-A)
+
NMR analysis of sPPP-NHEt3 (Figure S35) revealed methyl
1
coupling. H NMR analysis of 9 revealed additional signals
+
groups of NHEt3 at 1.12 ppm (36 H), which were
resulting from the Et3NH+:HN+ (8.88 ppm); −CH2− (two
overlapping quadruplets at 3.09 and 3.10 ppm); and −CH3
(1.16 ppm).
subsequently used as an internal reference for quantification
of the remaining protons. The methylene protons (24 H) are
represented as two overlapping quadruplets at 3.05 ppm, and
the ammonium protons (4 H) are found at 8.92 ppm. Signals
for the protons of the polymer backbone are observed in the
region between 5.90 and 7.60 ppm. The integration ratio
between the methyl from the NHEt3+ salt for one unit and the
polymer backbone is observed to be 1:1, proving that the
sulfonate group in the salt form remains intact during the D-A
reaction. As with the model compounds, the polymer shows
evidence for regio-isomers: signals for Hcore are found for the
m−m (6.32 ppm), p−p (6.17 ppm), and m−p (5.98 ppm)
isomers. Integration of these peaks yields an isomeric
composition of 42%, 40%, and 18%, respectively (Figure S36).
The effect of regio-isomerization is also observed for the HDA
protons, whereby they can be either para (HDA1) or meta
(HDA2) to the central phenyl ring (i.e., Hcore), as shown in
Figure S35 (between 7.60 and 7.45 ppm). However, the HDA
protons are observed as a broad peak in the vicinity of 7.23
ppm due to their low intensity as well as being partially
obscured by Hα1 situated at 7.43 ppm (assigned by COSY
analysis and from 14 and 16). Model compound 16 shows a
signal for Hα1 at 7.43 ppm, according to a COSY analysis
(Figure S33), but according to a COSY analysis of sPPP-
NHEt3+ (Figure S37), the peak at 7.22 ppm does not correlate
with any of the other peaks, hence our assignment of this signal
to HDA of the polymer. Protons on the sulfonated phenyl rings
meta and para to the core phenyl ring appear at 7.43 (Hα2) and
7.18 (Hα1) ppm which, by COSY analysis, are shown to
correlate with 6.82 (Hβ2) and 6.64 (Hβ1) ppm, respectively.
The protons on the unsulfonated, outer phenyl rings, namely
The synthesis of bis-dienophile 13 is described in the
Supporting Information. Oligomers 14 and 16 were synthe-
sized (Scheme S1) to investigate the mode of D-A coupling of
comonomers 9 and 13. Compound 14 was obtained by D-A
cycloaddition of dienophile 13 and 2 molar equiv of 5. As
shown in Figure S30, the D-A proton (HDA) originates from
the terminal alkyne of 13, Hα1 and Hβ1 are, respectively,
protons ortho and meta to the sulfonic acid group of the phenyl
ring adjacent to HDA, while Hα2 and Hβ2 are the analogous
1
protons located on the other sulfonated phenyl ring. The H
NMR spectrum of 14 (Figure S28) reveals the presence of HDA
at 7.38 ppm and Hcore of the central phenyl ring at 6.93 ppm. As
for the peripheral phenyl rings, the protons on the sulfonated
rings reflect the different chemical environments with signals at
7.36 ppm (Hα1) and 7.23 ppm (Hα2) correlated (Figure S30)
with the peaks at 7.14 (Hβ1) and 6.77 (Hβ2) ppm, respectively.
The signals corresponding to the unsulfonated phenyl rings are
observed between 6.96 and 6.83 ppm. The possibility of
conformational isomers was investigated using 1D NOE
(Figure S31). Irradiation of HDA at 7.38 ppm reveals the
proximity of Hcore at 6.88 ppm (single peak) as well as Hβ1 at
7.11 ppm. By irradiating Hβ1 at 7.11 ppm, the proximity of
Hcore and HDA is also confirmed. Irradiation of Hβ1, however,
does not indicate the relative proximity of HDA, but a
correlation does exist with the unsulfonated phenylene (6.88
ppm, one peak) and Hcore (two peaks). This suggests that
compound 14 also adopts the “A” conformation in solution.
Compound 16 was synthesized by D-A cycloaddition of 9
with 15. Protons in 16 originating from the phenyl ring of 15
B
DOI: 10.1021/jacs.5b07865
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Ho, Hm, and Hp, have signals at 7.34, 6.53, and 7.02 ppm,
respectively.
tigation of copolymer derivatives, with a view to controlling
polymer morphology, limiting water sorption, enhancing
proton conductivity, and strengthening the mechanical proper-
ties of thin films, are warranted.
Following conversion of sPPP-NHEt3 to sPPP-H+, films
+
were cast from DMSO. The ion exchange capacity (IEC) was
determined by acid−base titration to be 3.47 mequiv g−1, close
to the theoretical value of 3.70 mequiv g−1. This is a very high
IEC value for an aromatic polymer, and yet the polymer was
found to be insoluble and free-standing in water at room
temperature (Figure S41) (water content, 85 wt %). For
comparison, a previously reported, postsulfonated polypheny-
lene, possessing an average of four sulfonic acid groups per
repeat unit and an IEC of 2.2 mequiv g−1, formed a hydrogel in
water.2b sPPP-H+ membranes dissolved when placed into
Fenton’s reagent,11 but a subsequent 1H NMR analysis (Figure
S42) revealed no changes in chemical structure, suggesting an
extraordinarily high oxidative stability.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b07865.
■
S
Experimental procedures and characterization details
(PDF)
AUTHOR INFORMATION
Corresponding Author
*holdcrof@sfu.ca
■
The proton conductivity of sPPP-H+ (Figure S43) was
studied at 30 °C on water-saturated samples and partially
hydrated (30−95% RH) membranes. As is commonly observed
for aromatic membranes, proton conductivity increases as a
function of RH from a low of 8.65 mS cm−1 at 40% RH to 106
mS cm−1 at 95% RH. In contrast to most aromatic membranes,
however, sPPP-H+ exhibits conductivity competitive to NR211
at low RH. The conductivity of sPPP-H+ is reduced when
water-saturated (77 vs 106 mS cm−1 at 95% RH), which reflects
the high water uptake of sPPP-H+ in contact with liquid water
and a reduction of the analytical acid concentration, [−SO3H],
0.92 M for sPPP-H+ vs 1.55 M for N211.4c,12
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the APC Catalysis Research for
Polymer Electrolyte Fuel Cells (CaRPE-FC) Network and
NSERC for financial support. We thank Dr. Andrew Lewis
(SFU) for his assistance in running the 1D NOE spectra and
Prof. Robert Young for use of his microwave reactor. We also
thank 4D LABS for the use of their fuel cell testing facilities.
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based CCL (at 90% RH) compared to those in which Nafion
D520 was used in the CCL (Figure S44) (in both cases, N211
was used as the membrane). However, when the cathode inlet
was reduced to 0% RH, not only does the sPPP-H+-based CCL
perform better than at 90% RH, it also outperforms Nafion-
based CCLs by a significant margin, e.g., a current density of
3000 mA cm−2 can be extracted for a sPPP-H+-based CCL,
whereas only 800 mA cm−2 can be achieved for Nafion-based
CCLs. Calculation of in situ membrane conductivity (using eq
S6 and the iR drop in the Ohmic region) (Figure S45) reveals
that sPPP-H+ increases the in situ conductivity of the
membrane by 4−6 times.
■
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DOI: 10.1021/jacs.5b07865
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Journal of the American Chemical Society
Communication
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D
DOI: 10.1021/jacs.5b07865
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX