Synthesis and characterization of phenol-based biaryl proton conducting polymers

Article abstract of DOI:10.1002/pola.25880

Biaryl hydroxy polymers with orthogonal disposition of proton transporting iOH moieties have been synthesized via conventional free radical polymerization. The polymers are characterized for their thermal stability and proton conductivity, and the results

Full text of DOI:10.1002/pola.25880

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Synthesis and Characterization of Phenol-Based Biaryl
Proton Conducting Polymers
Chikkannagari Nagamani, Jing Guo, S. Thayumanavan
Department of Chemistry, University of Massachusetts Amherst, Massachusetts 01003
Correspondence to: S. Thayumanavan (E-mail: thai@chem.umass.edu)
Received 19 October 2011; accepted 1 December 2011; published online 29 December 2011
DOI: 10.1002/pola.25880
ABSTRACT: Biaryl hydroxy polymers with orthogonal disposi-
tion of proton transporting AOH moieties have been synthe-
sized via conventional free radical polymerization. The
polymers are characterized for their thermal stability and pro-
ton conductivity, and the results are compared with the corre-
sponding styrenic hydroxy polymers. The orthogonal
disposition of AOH moieties in biaryl polymers does result in
lower Ea for proton transport. However, the lower Ea values in
biaryl polymers did not translate into a net increase in proton
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conductivity. 2011 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 50: 1187–1196, 2012
KEYWORDS: synthesis; proton transport; phenolic polymer; pol-
yhydroxy polymer
INTRODUCTION Proton transporting polymers that can selec-
tively and efficiently transfer protons play a pivotal role in
the overall operation of a hydrogen fuel cell. Polymers that
can conduct protons under anhydrous conditions are partic-
ularly interesting for applications as proton exchange mem-
branes in high temperature hydrogen fuel cells. Polymers
based on N-heterocycles such as imidazole, triazole and
benzimidazole have been widely studied to develop efficient
anhydrous proton conducting polymers.1–9 In the case of N-
heterocyclic polymers, the reorientation step in Grotthuss
proton transport (PT) is well recognized to be energetically
demanding, thereby adversely affecting the overall PT.10–12
We have recently introduced a new class of functional
groups, phenols, for anhydrous PT and have shown that poly-
mers based on phenols allow for facile reorientational dy-
namics.13 It has also been shown, with the help of theoretical
modeling, that the pentamers of polystyrene (tri OH) (Fig. 1)
can form extended intrachain hydrogen bond network.13
Both intrachain and interchain proton transfer events are im-
portant for long range PT. We hypothesized that a molecular
architecture that presents PT moieties in orthogonal planes
would be interesting as it would allow for greater interchain
hydrogen bonding interactions along with intrachain interac-
tions among PT moieties, thereby enhancing the overall PT
(Fig. 1).
lead to a twist in the phenyl rings,14,15 thereby inducing the
orthogonality at the molecular level, (ii) both the phenyl
rings of the biaryl scaffold can be functionalized with PT
moieties, thus providing an interesting spatial disposition of
the PT moieties in orthogonal planes, and (iii) a polymeriz-
able double bond can be easily introduced onto one of the
phenyl rings to generate a biaryl monomer, which could be
conveniently polymerized similar to that of styrene. We
chose to introduce phenols as the PT moieties on the biaryl
architecture. A space-filling model of a pentahydroxy biaryl
compound, energy minimized using MM2 calculations, sup-
ports the presumed orthogonal spatial disposition of the PT
AOH moieties (Fig. 1). The dihedral angle between the phe-
nyl rings was found to be 55ꢀ. In this work, we report the
synthesis and characterization, thermal properties, and pro-
ton conductivity of biaryl phenolic polymers (see Chart 1).
To further investigate if the biaryl architecture provides any
advantage over the linear styrenic architecture, the proton
conductivities of the biaryl and styrenic hydroxy polymers
shown in Chart 1 are compared.
EXPERIMENTAL
General Procedures for the Synthesis of Biaryl
Monomers
Procedure 1 for Deprotection of -OMe Group Using BBr3
A solution of the appropriate methoxy benzene (1.0 equiv)
in dichloromethane (DCM) at room temperature (RT) under
argon was cooled to ꢁ78 ꢀC for 30 min, and boron tribro-
mide (1.5 equiv per methoxy group) was added at ꢁ78 ꢀC.
The reaction mixture was stirred at ꢁ78 ꢀC for 30 min and
MOLECULAR DESIGN
We envisaged that biaryl would be an interesting scaffold for
the proposed molecular design because (i) the steric interac-
tions between the ortho hydrogen atoms is well known to
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 (a) Proton wire of polystyrene (tri OH) pentamer; (b) illustration of the spatial disposition of proton transporting AOH
moieties in orthogonal planes.
was slowly warmed to RT and continued to stir at RT for
overnight. The reaction mixture was then cooled to ꢁ78 ꢀC
and the excess BBr3 was quenched with saturated NH4Cl so-
lution. The precipitate was filtered and washed twice with
excess DCM. The DCM and aqueous layers were separated,
and the DCM layer was concentrated. The aqueous layer was
extracted twice with ethyl acetate and the combined organic
layers were dried over Na2SO4, concentrated under reduced
pressure and the crude was purified by column chromatog-
raphy (SiO2).
Procedure 2 for protection of phenolic hydroxyl
group as methoxymethyl ether (MOM)
A solution of the appropriate hydroxy benzene (1.0 equiv) in
dry tetrahydrofuran (THF) under argon was cooled to 0 ꢀC.
N,N-diisopropylethylamine (DIPEA) (1.5 equiv per hydroxyl
group) and chloromethyl methyl ether (MOMCl) (1.5 equiv
per hydroxyl group) were added and the reaction was slowly
warmed to RT and continued to stir at RT. The progress of
the reaction was monitored using thin layer chromatography
(TLC). The reaction is usually complete in about 12 h. After
CHART 1 Structures of biaryl and styrenic hydroxy polymers.
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complete disappearance of the starting material, the reaction
was cooled to 0 C and quenched with saturated NH4Cl. The
reduced pressure and purified by column chromatography
(SiO2).
ꢀ
organic and aqueous layers were separated and the organic
layer was concentrated. The aqueous layer was extracted
thrice with ethyl acetate and the combined organic layers
were dried over Na2SO4, concentrated under reduced
pressure and the crude mixture was purified by column
chromatography (SiO2).
Procedure 7 for the Deprotection of MOM Group Using
DOWEX Resin
The appropriate MOM-protected biaryl aldehyde (1.0 equiv)
was dissolved in a mixture of (MeOH: H2O: 1,4-dioxane)
(1.0:0.5:0.1 v/v) at RT under argon. DOWEX resin (4.0 equiv
with respect to each MOM group) was added and the solu-
tion was refluxed. The progress of the reaction was moni-
tored for every 10 min using TLC. If the reaction is not com-
plete within 30 min, then an additional amount of DOWEX
resin (1.0 equiv) was added each time until the starting ma-
terial disappeared completely. The mixture was filtered, and
the resin was washed with MeOH. The filtrate was concen-
trated under reduced pressure, and the crude mixture was
taken to the next step without further characterization.
Procedure 3 for Conversion of Bromobenzene to
the Aromatic Tributyltin
A solution of the appropriate bromobenzene (1.0 equiv) in
dry THF at RT under argon was cooled to ꢁ78 ꢀC for 30
min, and n-BuLi (2.0 equiv) was added. The mixture was
ꢀ
stirred at ꢁ78 C for 1 h and tributyltin chloride (1.5 equiv)
was added. The reaction was slowly warmed to RT and
stirred for overnight. The reaction mixture was then cooled
ꢀ
to 0 C and quenched with saturated NH4Cl solution. The or-
Procedure 8 for Boc-Protection
ganic and aqueous layers were separated and the organic
layer was concentrated. The aqueous layer was extracted
twice with ethyl acetate and the combined organic layers
were dried over Na2SO4, concentrated under reduced pres-
sure and the crude mixture was purified by column chroma-
tography (SiO2).
The appropriate biaryl hydroxy aldehyde (1.0 equiv) was dis-
solved in THF at RT under argon. The reaction mixture was
cooled to 0 ꢀC and DIPEA (1.5 equiv per hydroxyl group),
(Boc)2O (1.5 equiv per hydroxyl group), and 4-Dimethylami-
nopyridine (0.05 equiv) were added. The reaction mixture
was then warmed to RT and continued to stir at RT for over-
night. THF was evaporated and the crude was taken up in
ethyl acetate and washed twice with 1M NaOH and saturated
NaCl solutions. The combined ethyl acetate layers were dried
over Na2SO4, concentrated under reduced pressure and the
crude was purified by column chromatography (SiO2).
Procedure 4 for Stille Coupling Reaction
The appropriate tributyltin (1.0 equiv), bromobenzene (1.2
equiv), and PdCl2(PPh3)2 (0.05 equiv) were dissolved in tolu-
ene under argon at RT. The reaction mixture was degassed
for 1 h with an argon inlet/outlet and the reaction mixture
was then refluxed for 48 h. After evaporating the solvent, the
resultant mixture was extracted twice with ethyl acetate and
water. The combined organic layers were dried over Na2SO4,
concentrated under reduced pressure, and the crude mixture
was purified by column chromatography (SiO2).
Procedure 9 for Wittig Reaction
MePPh3Br (1.5 equiv) and KOtBu (1.5 equiv) were taken in
an oven-dried Schlenk flask at RT and dried under vacuum
for 30 min. The flask was cooled to 0 ꢀC using ice bath and
anhydrous THF (50 mL) was added under argon. The solu-
tion immediately turned yellow, indicating the formation of
Procedure 5 for the Reduction of Ester with LiAlH4
A solution of LiAlH4 (ꢀ2.0 equiv) in dry THF at RT under
argon was cooled to 0 C. The appropriate biaryl ester com-
pound (1.0 equiv) in dry THF was then added, and the reac-
tion mixture was slowly warmed to RT and stirred for about
12 h. The progress of the reaction was monitored using TLC.
After complete disappearance of the biaryl ester compound,
ꢀ
ylide. The reaction mixture was allowed to stir at 0 C for 30
min and was then warmed to RT. A solution of the appropri-
ate Boc-protected biaryl aldehyde (1.0 equiv) in anhydrous
THF was added using syringe and the reaction mixture was
continued to stir at RT for 12 h. The reaction was quenched
by the addition of water and extracted thrice with ethyl ace-
tate. The combined ethyl acetate layers were dried over
Na2SO4, concentrated under reduced pressure and the crude
was purified by column chromatography (SiO2).
ꢀ
the reaction was cooled to 0 C and quenched with saturated
NH4Cl solution. The precipitate was filtered and washed with
ethyl acetate. The filtrate was concentrated and extracted
thrice with ethyl acetate and water. The combined organic
layers were dried over Na2SO4, concentrated under reduced
pressure, and the crude mixture was purified by column
chromatography (SiO2).
Synthesis of Biaryl (penta Boc) Monomer 9
Compounds 116 and 217 were synthesized following reported
procedures (Scheme 1).
Procedure 6 for the Oxidation of Benzyl Alcohol
Synthesis of Compound 3
to Aldehyde Using Pyridiniumchlorochromate
According to general procedure 1, 5-bromo-1,2,3-trimethoxy-
benzene (10.0 g, 40.5 mmol) was treated with boron tribro-
mide (BBr3) (45.6 g, 182.1 mmol). The product was eluted
with ethyl acetate/hexane (80:20 v/v) to afford 5-bromo-
1,2,3-trimethoxybenzene (8.2 g, 100%) as yellow oil, which
was carried to the next step without further characterization.
According to general procedure 2, 5-bromo-1,2,3-trimethoxy-
benzene (8.2 g, 40.2 mmol) was treated with DIPEA (23.4 g,
To a stirring solution of the appropriate benzyl alcohol (1.0
equiv) in DCM at RT, was added pyridiniumchlorochromate
(PCC) (1.2 equiv). The mixture was exposed to air while stir-
ring, and the progress of the reaction was monitored using
TLC. The reaction is usually complete within 2–8 h depending
on the scale of the reaction. After complete disappearance of
the benzyl alcohol, the crude mixture was concentrated under
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SCHEME 1 Synthesis of Biaryl (penta Boc) monomer 9.
181.0 mmol) and MOMCl (14.6 g, 181.0 mmol). The product
was eluted with ethyl acetate/hexane (25:75 v/v) to afford
compound 3 (8.8 g, 65%) as colorless oil. 1H NMR (400
MHz, CDCl3) d: 7.01 (s, 2H), 5.17 (s, 4H), 5.10 (s, 2H), 3.60
(s, 3H), 3.49 (s, 6H).13C NMR (100 MHz, CO(CD3)2) d 152.81,
137.08, 116.23, 114.10, 98.95, 96.00, 57.13, 56.46. FAB/MS
m/z 337.024 [M þ H]þ (expected m/z ¼ 337.02).
Synthesis of Compound 5
According to general procedure 4, compound 2 (6.4 g, 18.3
mmol) were treated with compound 4 (8.36 g, 15.3 mmol)
and PdCl2(PPh3)2 (0.536 g, 0.8 mmol) to afford compound 5
(5.5 g, 55%) as white solid. The product was eluted with
1
ethyl acetate/hexane (20:80 v/v). H NMR (400 MHz, CDCl3)
d: 7.52 (s, 2H), 6.89 (s, 2H), 5.20 (s, 2H), 5.19 (s, 4H), 5.12
(s, 4H), 4.40–4.37 (q, J ¼ 7.10Hz, 2H), 3.66 (s, 3H), 3.48 (s,
6H), 3.37 (s, 6H), 1.40–1.38 (t, J ¼ 7.10Hz, 3H). 13C NMR
(100 MHz, CDCl3) d: 166.16, 155.09, 150.54, 135.88, 131.09,
129.39, 126.20, 113.24, 110.07, 98.68, 95.64, 94.83, 61.35,
57.22, 56.34, 56.26, 14.51. FAB/MS m/z 527.206 [M þ H]
(expected m/z ¼527.20).
Synthesis of Compound 4
According to general procedure 3, compound 3 (8.8 g 26.0
mmol) was treated with 1.6 M n-BuLi/hexane (32.5 mL, 52.0
mmol) and SnBu3Cl (12.7 g, 39.0 mmol) to afford compound
4 (10.0 g, 70%) as yellow oil. The product was eluted with
ethyl acetate/hexane (3:97 v/v). The tributylstannane is not
very stable and hence was used immediately in next steps.
1H NMR (400 MHz, CDCl3) d: 6.92 (s, 2H), 5.19 (s, 4H), 5.15
(s, 2H), 3.62 (s, 3H), 3.51 (s, 6H), 1.58–0.89 (m, 27H).
Synthesis of Compound 6
According to general procedure 5, compound 5 (5.4 g, 10.3
mmol) was treated with LiAlH4 (0.78 g, 20.6 mmol) to afford
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SCHEME 2
compound 6 (4.8 g, 96%) as white solid. The product was eluted
with ethyl acetate/hexane (25:75 v/v). 1H NMR (400 MHz,
CDCl3) d: 6.88 (s, 4H), 5.19 (s, 2H), 5.18(s, 4H), 5.08 (s, 4H),
4.69–4.67 (d, J ¼ 6.10 Hz, 2H), 3.66 (s, 3H), 3.48 (s, 6H), 3.38
(s, 6H), 1.77 (t, J ¼ 6.10 Hz, 1H). 13C NMR (100 MHz, CDCl3) d:
155.43, 150.45, 142.23, 135.56, 130.03, 120.85, 113.57, 107.47,
98.69, 95.65, 94.81, 65.45, 57.21, 56.24. FAB/MS m/z 485.192
[M þ H]þ (expected m/z ¼ 485.19).
Synthesis of Biaryl Polymers
General Procedures for Polymer Syntheses
All the Boc-protected biaryl monomers were polymerized via
free radical polymerization with AIBN as the initiator. The
Boc groups were then deprotected using trifluoroacetic acid
(TFA) to obtain the corresponding biaryl phenolic polymers.
The synthetic scheme for the synthesis of polybiaryl (penta
OH) is shown in Scheme 2.
Synthesis of Compound 7
Procedure 10 for Polymerization
According to general procedure 6, compound 6 (4.3 g, 9.0
mmol) were treated with PCC (2.3 g, 10.7 mmol) to afford
compound 7 (4.2 g, 98%) as light yellow solid. The product
was eluted with ethyl acetate/hexane (25:75 v/v). 1H NMR
(400 MHz, CDCl3) d: 9.94 (s, 1H) 7.38 (s, 2H), 6.89 (s, 2H),
5.21 (s, 2H), 5.19 (s, 4H), 5.14 (s, 4H), 3.67 (s, 3H), 3.49 (s,
6H), 3.40 (s, 6H).13C NMR (100 MHz, CDCl3) d: 191.59,
155.83, 150.62, 136.93, 136.00, 129.06, 127.60, 113.08,
109.86, 98.66, 95.63, 94.78, 57.23, 56.36, 56.26. FAB/MS m/
z 483.178 [M þ H]þ (expected m/z ¼ 483.17).
A solution of the appropriate Boc-protected biaryl monomer in
anhydrous THF was taken in a 10-mL oven-dried Schlenk flask
under argon at RT. AIBN was added and the Schlenk flask was
sealed and subjected to three freeze-pump-thaw cycles. The
reaction mixture was stirred at RT for 5 min and the Schlenk
flask was then transferred to an oil bath preheated to 70 ꢀC. Af-
ter the polymerization was complete, the polymer was diluted
with THF and precipitated twice into large excess of hexane.
The white precipitate was filtered, and was washed with either
isopropanol or methanol to get rid of the residual monomꢀer.
The white powder obtained was dried under vacuum at 40 C
for 24 h to obtain the final polymers as white solids.
Synthesis of Compound 8
According to general procedure 7, compound 7 (4.2 g, 8.7
mmol) was treated with DOWEX resin to afford 0.9 g (40%)
of the crude penta hydroxy biaryl aldehyde. The hydroxyl
groups were further protected with Boc according to general
procedure 8 to obtain compound 8 (2.5 g, 95%) as white
solid. The product was eluted with ethyl acetate/hexane
Procedure 11 for the Deprotection of Boc
Groups Using TFA
The Boc protecting groups were deprotected following a
reported procedure.13 The appropriate Boc-protected biaryl
polymer was taken in 5 mL DCM at RT under argon and 5
mL TFA was added to it. The clear solution obtained was
stirred at RT for 30 min, during which the solution initially
turned turbid and finally a precipitate was obtained. An
additional 10-mL DCM was added, and the solution was cen-
trifuged. The precipitate obtained was washed thrice with
excess DCM and was dried under vacuum at 40 ꢀC for 24 h
to obtain the corresponding biaryl phenolic polymer.
1
(10:90 v/v). H NMR (400 MHz, CDCl3) d: 9.98 (s, 1H), 7.65
(s, 2H), 7.18 (s, 2H), 1.54–1.53 (m, 27H), 1.35 (s, 18H).13
C
NMR (100 MHz, CDCl3) d: 189.79, 150.79, 149.98, 149.93,
149.03, 143.76, 137.20, 135.53, 133.65, 128.35, 122.09,
121.38, 84.81, 84.03, 83.95, 27.72, 27.37.
Synthesis of Biaryl (penta Boc) Monomer 9
According to general procedure 9, compound 8 (2.5 g, 3.3
mmol) was reacted with MePPh3Br (1.8 g, 5.0 mmol) and
KOtBu (0.6 g, 5.0 mmol) to afford compound 9 (1.7 g, 68%)
as white solid. The product was eluted with ethyl acetate/
hexane (8:92 v/v). 1H NMR (400 MHz, CDCl3) d: 7.17 (s,
2H), 7.15 (s, 2H), 6.70–6.62 (dd, 17.56, 10.92 Hz, 1H), 5.80–
5.75 (d, 17.56 Hz, 1H), 5.36–5.33 (d, 10.92 Hz, 1H), 1.54–
1.52 (m, 27H), 1.35–1.33 (s, 18H). 13C NMR (100 MHz,
CDCl3) d: 151.14, 150.03, 149.27, 149.11, 143.57, 139.45,
135.09, 135.00, 129.40, 126.73, 122.33, 118.13, 116.36,
84.18, 83.84, 83.75, 27.74, 27.41.
Polybiaryl (penta Boc)
According to general procedure 10, monomer 9 (1.35 g,
1.778 mmol) was reacted with AIBN (2.93 mg, 0.018 mmol)
in THF (4.0 mL) for 7 h. The white precipitate was washed
with excess isopropanol to obtain the polymer (0.85 g, 63%)
as a white solid. Gel permeation chromatography (GPC)
(THF) ***Mn: 37,000 g/mol; PDI: 1.56. 1H NMR (400 MHz,
CDCl3) d: 7.06 (s, 2H, ArH), 6.62 (br, 2H, ArH), 2.34 (br, 1H,
ACH of polymer backbone), 1.44–1.23 (m, 47H, OAC(CH3)3,
and ACH2 of polymer backbone).
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FIGURE 2 GPC (THF) chromatograms of Boc protected
FIGURE 3 FTIR spectra of biaryl polymers.
polymers.
via Stille coupling reaction, followed by subsequent reduc-
tion, oxidation, deprotection, and protection steps. The syn-
thetic scheme for the biaryl (penta Boc) monomer 9 is
shown in Scheme 1. The synthetic schemes for the other
monomers are shown in ESI. The monomers were polymer-
ized via free radical polymerization with AIBN as the initia-
tor (Scheme 2 and ESI). Monomer:AIBN ratio and the solvent
were optimized to obtain control over the polymerization
reaction. The synthesis and characterization of polystyrene
(tri OH) are described in our previous report.13
Polybiaryl (penta OH)
According to general procedure 11, polybiaryl (penta Boc)
(0.77 g, 1.013 mmol) was reacted with TFA/DCM (1:1 v/v)
to obtain the polybiaryl (penta OH) (0.25 g, 95%) as a dark
1
brown solid. H NMR (400 MHz, DMSO-d6) d: 8.53–7.93 (bd,
5H, AOH), 6.25–5.95 (bd, 4H, ArH), 2.2–0.8 (bs, 3H, ACH
and ACH2 of polymer backbone).
Characterization
Molecular weights and the molecular weight distributions of
the Boc protected biaryl polymers were determined by GPC
using THF as eluent and toluene as internal reference. The
molecular weights of biaryl hydroxy polymers (after Boc
deprotection) were estimated based on the complete depro-
GPC chromatograms of the Boc-protected biaryl and styrene
polymers are shown in Figure 2. All polymers exhibit unimo-
dal distribution. The Boc groups were further deprotected
using TFA/DCM to obtain the corresponding biaryl hydroxy
polymers. The polymers were obtained in good yield and the
complete deprotection of Boc groups was confirmed by 1H
NMR based on the disappearance of the signal corresponding
to the tert-butyl unit. The polymers were also characterized
by FTIR spectroscopy (Fig. 3). Polybiaryl (penta Boc) and
1
tection of Boc groups, which was confirmed by both H NMR
and FT-IR. Thermal properties of the biaryl hydroxy poly-
mers were determined by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC). Glass transition
temperature (Tg) of the polymers were obtained by DSC
using TA instruments Dupont DSC 2910. The proton conduc-
tivity of biaryl hydroxy polymers was measured using elec-
trochemical impedance spectroscopy under anhydrous condi-
tions (see supporting information for details).
polybiaryl (tri Boc) polymers exhibit a peak at ꢂ 1750 cmꢁ1
,
characteristic of the C¼¼O stretch of Boc protecting groups.
The peak at 1750 cmꢁ1 hascompletely disappeared in poly-
biaryl (penta OH) and polybiaryl (tri OH) polymers, confirm-
ing the complete deprotection of Boc protecting groups.
These polymers also exhibit a broad band at around 3000–
3600 cmꢁ1, with a maximum at ꢂ 3400 cmꢁ1, characteristic
of hydrogen bonded OAH stretch. The molecular weight
details of the polymers are summarized in Table 1.
RESULTS AND DISCUSSION
Synthesis and Characterization
Boc protected biaryl monomers were obtained by coupling
the appropriately functionalized top and bottom phenyl rings
TABLE 1 Polymer Details, Thermal Stability (Td,5%, Under Nitrogen), Thermo-Oxidative Stability (Td,5%, Under Air), and Glass
Transition Temperature (Tg) of Biaryl Polymers
a
b
c
d
e
Polymer
Mn (g/mol)
PDI
Polymer
Mn (g/mol)
Td,5% (ꢀC)
Td,5% (ꢀC)
Tg (ꢀC)
Polybiaryl (penta Boc)
Polybiaryl (tri Boc)
Polystyrene (tri Boc)
a
37,000
11,600
60,000
1.56
1.71
1.5
Polybiaryl (penta OH)
Polybiaryl (tri OH)
Polystyrene (tri OH)
c
12,500
5,000
347
318
265
223
248
267
309
217
233
20,000
Estimated by GPC (THF) using PS standards.
Temperature at 5% weight loss when heated under nitrogen at 10 ꢀC/
min.
b
Estimated based on the complete deprotection of Boc groups, which
was confirmed by both 1HNMR and FT-IR (Fig. 3).
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FIGURE 4 (a) Proton conductivity and (b) normalized proton conductivity of biaryl and styrenic polymers under anhydrous
conditions.
Thermal Analyses
(penta OH) with maximum number of AOH moieties exhibits
the highest Tg of 309 ꢀC. Although polybiaryl (tri OH) and
Polystyrene (tri OH) have same number of AOH groups, the
former exhibits slightly lower Tg compared with the later
one. This might be due to the lower molecular weight of
polybiaryl (tri OH) since Tg is known to be dependent on the
molecular weight of polymers.18 Also, since the polymers are
not crosslinked, the thin films of all these polymers are in
general brittle. The thin films of polybiaryl (penta OH) and
polybiaryl (tri OH) are comparatively more brittle than those
of polystyrene (tri OH). This could be, in part, due to the
low molecular weight of biaryl polymers.
Since proton conductivity measurements were carried out
under variable temperature conditions, and the conductivity
could be affected by the glass transition temperature (Tg) of
the polymers, it is necessary that we investigate the thermal
properties of these polymers. The thermal stability of poly-
mers was determined by TGA under nitrogen atmosphere.
Since phenol based polymers are susceptible to oxidation,
the polymers were also analyzed for thermo-oxidative stabil-
ity, that is, the thermal stability of polymers was examined
under air atmosphere. The thermal and thermo-oxidative
stabilities of polymers are shown in Table 1 and Supporting
Information Figure S4. The decomposition temperatures are
reported at 5% weight loss. All polymers were found to be
stable up to atleast 220 ꢀC both under nitrogen and air
atmosphere.
Proton Conductivity
The proton conductivity of biaryl hydroxy polymers is shown
in Figure 4(a). In general, bulk proton conductivity is known
to be influenced by several factors, especially the nature of
the PT moiety (for e.g., imidazole, triazole, phenol etc.), varia-
tions in the polymer architecture, Tg of polymers, and charge
carrier density (density of proton transporting moi-
eties).4,5,9,19,20 Since AOH is the proton-transporting moiety
in all the polymers, the effect of the nature of PT moiety can
be neglected. The weight fraction of AOH groups contained
in each polymer was calculated by dividing the product of
the equivalent weight of AOH unit (17 g/mol) and the
The glass transition temperature (Tg) of polymers was deter-
mined by DSC and is shown in Table 1 and Supporting Infor-
mation Figure S5. The glass transition temperature steadily
increases with increasing number of AOH groups. This is
expected because the hydrogen bonding interactions among
hydroxyl groups along the polymer chains would increase
with increasing number of AOH groups, leading to increased
polymer chain interactions and hence the Tg. Polybiaryl
TABLE 2 Charge Carrier Density, Apparent Activation Energy (Ea), and Glass Transition
Temperature (Tg) of Phenolic Polymers
Weight Fraction
of AOH Units
Log [r (Scmꢁ1)]
at 140 ꢀC
Tg
(ꢀC)
Polymer
Ea/kJ molꢁ1
Polybiaryl (penta OH)
Polybiaryl (tri OH)
Polystyrene (tri OH)
32
22
33
ꢁ6.4
ꢁ7.6
ꢁ4.5
77
74
94
309
217
233
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CHART 2 Structures of biaryl and styrenic hydroxy polymers.
number of AOH units per polymer repeat unit by the equiva-
lent weight of the polymer repeat unit. The charge carrier
density for each polymer is shown in Table 2.
rene (tri OH). Thus, the proton transfer in polybiaryl (penta
OH) is more facilitated compared with polystyrene (tri OH).
Although polybiaryl (penta OH) presents lower Ea for proton
transfer, it is surprising that its net proton conductivity is
almost two orders of magnitude lower than that of polysty-
rene (tri OH) over the entire temperature range. This might
be, in part, due to the variations in polymer architecture
and/or the differences in Tg values of polymers.
Unlike N-heterocycle-based proton conducting polymers, the
proton conductivity (r) in phenol-based polymers as a func-
tion of temperature follow an Arrhenius-type behavior over
the entire temperature range investigated here. The proton
conductivity can be described by equation r ¼ ro exp (-Ea/
RT) (eq 1), where ro is a pre-exponential factor, Ea is the
apparent activation energy, R is a universal gas constant, and
T is the temperature in Kelvin. The conductivity plots were
fitted by Arrhenius eq 1 and the activation energies (Ea)
were computed from the slope of the linear fit of log (r) ver-
sus 1/T and are shown in Table 2.
To eliminate the effect of Tg on proton conductivity, log (r)
is plotted against reduced temperature. Reduced tempera-
ture is defined as [(T-Tg)/Tg], where all temperatures are
considered in Kelvin, and the term is known to take into
account the differences in Tg values of polymers. The nor-
malized conductivity versus reduced temperature plot is
shown in Figure 4(b). Considering that polybiaryl (penta
OH) and polystyrene (triOH) have similar proton carrier den-
sities, the differences in their proton conductivities in log (r)
versus reduced temperature plots can now be solely ascribed
to the variations in biaryl versus styrenic polymer architec-
tures. The normalized proton conductivities of polybiaryl
(penta OH) and polystyrene (triOH) converge with each
other with the difference being less than an order magnitude
over the entire temperature range. Thus, the difference in
conductivity can be attributed to the difference in the Tg of
the polymers, and the biaryl architecture does not seem to
provide any inherent advantages for proton conductivity.
Polybiaryl (penta OH) and polybiaryl (tri OH) have similar
polymer architecture. In addition, both polymers also present
similar Ea values of 74–77 kJ/mol. Thus, the PT is equally
feasible in both these polymers. Nonetheless, polybiaryl (tri
OH) exhibits poor proton conducting ability, and its proton
conductivity is lower by an order of magnitude compared
with polybiaryl (penta OH). This might be due to the lack of
AOH groups in one of the biaryl rings of polybiaryl (tri OH),
resulting in lower weight fraction of the proton conducting
AOH moieties. The reduced proton carrier density might
impede AOH groups from forming continuous hydrogen
bond pathway, thereby adversely affecting the overall PT.
Polybiaryl (penta OH) and polystyrene (tri OH) have similar
AOH weight fractions. The Ea for proton transfer in poly-
biaryl (penta OH) is 17 kJ/mol lower than that in polysty-
To further confirm this, we also synthesized another set of
polymers shown in Chart 2 and investigated their thermal
and proton conducting properties. polybiaryl (tetra OH) and
TABLE 3 Polymer Details, Thermal Stability (Td,5%, Under Nitrogen), Thermo-Oxidative Stability (Td,5%, Under Air), and Glass
Transition Temperature (Tg) of Biaryl Polymers
a
b
c
d
e
Polymer
Mn (g/mol)
PDI
Polymer
Mn (g/mol)
Td,5% (ꢀC)
Td,5% (ꢀC)
Tg (ꢀC)
Polybiaryl (tetra Boc)
Polybiaryl (di Boc)
Polystyrene (di Boc)
a
37,000
15,000
64,000
1.6
1.8
1.4
Polybiaryl (tetra OH)
Polybiaryl (di OH)
Polystyrene (di OH)
c
14,000
7600
364
339
258
274
287
239
294
216
227
25,000
Estimated by GPC (THF) using PS standards.
Temperature at 5% weight loss when heated under nitrogen at 10 ꢀC/
min.
b
Estimated based on the complete deprotection of Boc groups, which
was confirmed by both 1HNMR and FT-IR (Fig. S3).
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FIGURE 5 (a) Proton conductivity and (b) normalized proton conductivity of biaryl and styrenic hydroxy polymers under anhy-
drous conditions.
polybiaryl (di OH) polymers were synthesized following sim-
ilar synthetic protocols described for polybiaryl (penta OH)
and polybiaryl (tri OH), respectively. The synthetic details
are described in detail in ESI. The synthesis, thermal analy-
sis, and proton conducting properties of polystyrene (di OH)
are described in our previous report.13 The polymer details
and thermal properties are shown in Table 3 and Supporting
Information Figures Sꢀ2, S3, S4, and S5. All polymers are sta-
ble up to atleast 230 C. Polybiaryl (tetra OH) with the maxi-
mum number of AOH moieties exhibits the highest Tg of
energies, but is complicated by the fact that the glass transi-
tion temperature of the polymers significantly increases.
CONCLUSIONS
A series of biaryl hydroxy polymers with varying number of
hydroxyl groups have been successfully synthesized and
characterized. The biaryl architecture is observed to
enhance the thermal stability of phenol-based polymers.
Incorporation of proton transporting AOH moieties on both
phenyl rings of the biaryl scaffold increases the Tg of biaryl
polymers. The proton conductivities of the biaryl and styr-
enic polymers are compared to elucidate the role of orthog-
onal disposition of PT AOH moieties in biaryl architecture
on net PT. The orthogonal disposition of AOH moieties in
biaryl polymers does provide low Ea for PT compared with
the linear styrenic hydroxy polymers. However, this did not
afford a net increase in the proton conductivity, compared
with the styrenic polymers over the temperature range
investigated here. This is attributed to the observed
increase in Tg, since the normalized proton conductivities of
polybiaryl (penta OH) and polybiaryl (tetra OH) are either
comparable or slightly higher than that of the correspond-
ing styrenic polymers. From a practical materials perspec-
tive, the proton conductivities of the phenol-based polymers
are lower than Nafion.21 However, phenols are indeed use-
ful model systems for pKa-independent design strategies for
PT polymers. We are encouraged by the fact that the func-
tional group placement strategy does provide lower activa-
tion energy. The remaining challenge, though, is to design
polymers where the glass transition temperature does not
change when incorporating such functional group place-
ments. Addressing this challenge should be the focus of fur-
ther work in this area. The two-dimensional disposition of
ꢀ
294 C.
The conductivity and normalized conductivity plots are
shown in Figure 5(a,b), respectively. It can be seen from Fig-
ure 5(a) that the proton conductivity of polybiaryl (tetra OH)
is slightly lower than that of polystyrene (di OH) in the
higher temperature range and the difference is less than an
order of magnitude over the entire temperature range. On
the other hand, the normalized proton conductivity of poly-
biaryl (tetra OH) is about 1–2 orders of magnitude higher
compared with polystyrene (di OH). Since polybiaryl (tetra
OH) and polybiaryl (di OH) have similar charge carrier den-
sities (27 and 25, respectively), the differences in their nor-
malized proton conductivities can be ascribed to the varia-
tions in polymer architecture. Thus, given a constant charge
carrier density and similar Tg values, the biaryl architecture
might prove advantageous over the linear styrenic polymers,
especially in the lower temperature range. Nevertheless, con-
sidering that the placement of hydroxyl groups in both rings
of the biaryl hydroxy polymers is always accompanied by a
corresponding increase in Tg, the advantage of the biaryl
architecture, if any, on the net proton conductivity is not
substantial. Thus, the orthogonal placement of proton trans-
porting functional groups that could promote both intra and
interchain hydrogen bonding seem to lower the activation
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8 Wang, J. T. W.; Hsu, S. L. C. Electrochim. Acta. 2011, 56,
2842–2846.
PT functionalities could also be achieved by using block
copolymers that provide hexagonal cylindrical self-
assembled nanostructures with the PT moieties constituting
the hexagonal cylinder. Moreover, block copolymers also
provide an opportunity to fine tune the glass transition
temperature by varying the nonconducting block without
adversely affecting the PT characteristics.
9 Zhou, Z; Li, S. W.; Zhang, Y. L.; Liu, M. L.; Li, W. J. Am.
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11 Daycock, J. T.; Jones, G. P.; Evans, J. R. N.; Thomas, J. M.
Nature 1968, 218, 672–673.
This work is supported by Army Research Office (54635CH)
and National Science Foundation (NSF) (CHE-0739227). We
also thank Craig Versek and A. Chandrasekaran for help with
the proton conductivity measurements.
12 Munch, W.; Kreuer, K. D.; Silvestri, W.; Maier, J.; Seifert, G.
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