N-Spirocyclic Quaternary Ammonium Ionenes for Anion-Exchange Membranes

Article abstract of DOI:10.1021/jacs.6b12944

The development of cationic polymers for anion-exchange membranes (AEMs) with high alkaline stability and conductivity is a considerable challenge in materials chemistry. In response, we here present the synthesis and properties of N-spirocyclic quaternary ammonium ionenes (spiro-ionenes) containing 5- and 6-membered rings fused by central nitrogen cations. High-molecular weight and film-forming spiro-ionenes are successfully synthesized in cyclo-polycondensations of tetrakis(bromomethyl)benzene and dipiperidines under mild conditions. These polyelectrolytes show excellent thermal and alkaline stability with no degradation detected by NMR spectroscopy after more than 1800 h in 1 M KOD/D₂O at 80 °C. Even at 120 °C, the spiro-ionenes display reasonable alkaline stability. Transparent and mechanically robust AEMs based on ionically cross-linked blends of spiro-ionene and polybenzimidazole reach OH^- conductivities up to 0.12 S cm^-1 at 90 °C. The current findings demonstrate that spiro-ionenes constitute a new class of alkali-stable anion-exchange polymers and membranes.

Full text of DOI:10.1021/jacs.6b12944

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N-Spirocyclic Quaternary Ammonium Ionenes for Anion-Exchange Membranes
Thanh Huong Pham, Joel S. Olsson, and Patric Jannasch
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12944 • Publication Date (Web): 13 Feb 2017
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N-Spirocyclic Quaternary Ammonium Ionenes for Anion-
Exchange Membranes
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Thanh Huong Pham, Joel S. Olsson, Patric Jannasch*
Polymer & Materials Chemistry, Department of Chemistry, Lund University, P.O. Box 124, SEꢀ221 00 Lund, Sweden
Supporting Information Placeholder
both ether linkages and strongly electron drawing groups. Altꢀ
hough Müllen et al. have reported on a similar polymer, only the
solubility and thermal properties were disclosed.8 In the current
work, we synthesized two novel spiroꢀionenes from commercially
available building blocks. Hence, tetrakis(bromomethyl)benzene
ABSTRACT: The development of cationic polymers for anionꢀ
exchange membranes (AEMs) with high alkaline stability and
conductivity is a considerable challenge in materials chemistry. In
response, we here present the synthesis and properties of Nꢀ
spirocyclic quaternary ammonium ionenes (spiroꢀionenes) conꢀ
taining 5ꢀ and 6ꢀmembered rings fused by central nitrogen catiꢀ
ons. Highꢀmolecular weight and filmꢀforming spiroꢀionenes are
successfully synthesized in cycloꢀpolycondensations of
tetrakis(bromomethyl)benzene and dipiperidines under mild
conditions. These polyelectrolytes show excellent thermal and
alkaline stability with no degradation detected by NMR spectrosꢀ
(4BMB)
was
employed
in
straightforward
cycloꢀ
polycondensations with two commercially available dipiperidines,
4,4’ꢀbipiperidine (BP) and 4,4’ꢀtrimethylenedipiperidine (TMDP),
to form spiroꢀionene 1 and 2, respectively (Figure 1). 4BMB was
synthesized inꢀhouse in order to obtain a highꢀpurity material (see
supplementary information). The polymerizations were carried
out in N,Nꢀdimethylformamide/water mixtures at 60 oC, using
N,Nꢀdiisopropylethylamine, DIPEA as catalyst.
o
copy after more than 1800 h in 1 M KOD/D2O at 80 C. Even at
o
120 C, the spiroꢀionenes display reasonable alkaline stability.
Transparent and mechanically robust AEMs based on ionically
crosslinked blends of spiroꢀionene and polybenzimidazole reach
o
OH– conductivities up to 0.12 S cm−1 at 90 C. The current findꢀ
ings demonstrate that spiroꢀionenes constitute a new class of
alkaliꢀstable anionꢀexchange polymers and membranes.
The development of anionꢀexchange membranes (AEMs) which
combine longꢀterm stability and high hydroxide ion conductivity
in alkaline electrochemical systems is currently an intensive area
of materials research1. In general, these membranes are based on
polymers tethered with quaternary ammonium (QA) cations.1
Because of its highly basic and nucleophilic nature, the hydroxide
ion is prone to attack and degrade archetypal QA and other cationꢀ
ic groups via, e.g., Hofmann eliminations and nucleophilic substiꢀ
tutions.2 In addition, polymer backbones may degrade via, e.g.,
cleavage of activated ether bridges.3 Consequently, the hydroxide
ion conductivity and mechanical properties of AEMs normally
deteriorate over time. This severely limits their applicability,
especially at temperatures above 60 oC.
Figure 1. Synthetic pathway to spiroꢀionenes 1 and 2 via cycloꢀ
polycondensation of 4BMB with BP and TMDP, respectively, and
photographs of their films cast from water solutions at 120 oC.
Successful strategies used to improve the alkaline stability of
4
cationic groups include the utilization of steric protection,2d,
In the twoꢀstep cyclization reaction, a deprotonated piperidine
nitrogen first makes an intermolecular nucleophilic attack on one
of the benzylic carbons of 4BMP, followed by a ringꢀforming
intramolecular quaternization involving the neighboring benzyl
bromide (Scheme S1). Despite potential side reactions, the desired
intramolecular cyclization reaction leading to chain growth was
found to completely dominate. The monomer concentration of the
reaction mixture was kept low to depress side reactions, resulting
in final polymer concentrations of 1.5ꢀ3 wt%. Still, the polyconꢀ
densations gave highꢀmolecular weight products in quantitative
yields within 1ꢀ2 h without any signs of sideꢀreactions. This clearꢀ
ly demonstrated the high specificity and efficiency of the
polymerization.
conformational restrictions,5 inductive effects6 and metal comꢀ
plexes.7 Recently, Marino and Kreuer reported on the exceptional
alkaline stability of certain Nꢀspirocyclic QA salts, especially 6ꢀ
azoniaꢀspiro[5.5]undecane.5a The high stability of the sixꢀ
membered spirane ring system was ascribed to the constrained
ring conformation which increases the transition state energy of
both substitution and elimination reactions.5a
Introducing Nꢀspirocyclic QA cations into polymers presents a
significant challenge and calls for new synthetic pathways.5b
Using a new approach, we have in the present work molecularly
designed and synthesized ionenes via cycloꢀpolycondensations in
which spiroꢀcentered QA cations are formed directly in the polyꢀ
mer backbone. As seen in Figure 1, these “spiroꢀionenes” lack
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The H NMR spectra of the spiroꢀionenes shown in Figure 2
explained by its lower ionic content. Spiroꢀionene films were
1
2
3
4
5
6
7
8
revealed that the equatorial and axial protons at α and β positions
in the piperidinium moiety displayed a large difference in chemiꢀ
cal shifts, leading to splitting of each position into two distinct
signals (c1 and c2, and d1 and d2, respectively). This demonstrated
the rigid structure and slow conformational changes of the spiroꢀ
cyclic system. All the shifts in the spectra confirmed the desired
ionene structures. By comparing the intensities of the signals
originating from the piperidinium end groups (c2’) and the Nꢀ
spirocyclic QA groups (c2), the number average molecular weight
(Mn) of spiroꢀionene 1 and 2 was calculated as 67 and 80 kg
mol−1, respectively (the intensity of c2’ was corrected by taking
into account the DMF signal at 2.8 ppm). Additionally, the intrinꢀ
sic viscosity ([η]) in H2O was measured to be 0.4 and 0.56 dL g−1,
respectively (Table S1). The ion exchange capacity (IEC, meq
g−1) of the spiroꢀionenes was determined by Mohr's titration. In
the case of 2, the titrated IEC was in complete agreement with the
theoretical IEC (4.0 meq g−1,Table S1). From the synthesis, 1
contained a mix of Br− and Cl− counter ions. Hence, the theoretiꢀ
cal IEC was calculated as a span, 4.4ꢀ5.4 meq g−1, to be compared
with the titrated IEC of 4.6 meq g−1, Table S1.
analyzed by small angle Xꢀray scattering (SAXS) in order to
study structure formation (Figure S7). After equilibration at 75%
RH, 2 showed a weak ionomer peak at qmax ~ 4.8 nm−1, correꢀ
sponding to a characteristic distance of d ~ 1.3 nm. This distance
was in the same range as the length between two neighboring
ionic centers via an extended trimethylene spacer (~1.1 nm). In
contrast, no ionomer peak was observed for 1, possibly because of
the much stiffer structure and shorter interꢀionic distance (~0.7
nm) prevented the formation of a distinct morphology on the nm
level.
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Thermogravimetrical analysis (TGA) revealed a high thermal
o
stability with decomposition only above 300 C under N2 (Figure
S4). Spiroꢀionene 1 showed a higher thermal decomposition
temperature (Td,95) than 2, presumably because of the stiffer strucꢀ
ture and higher degree of aromaticity of the former. Alkaline
stability is a critical property for applications of AEMs which
determines the lifeꢀtime of electrochemical devices such as fuel
cells and electrolyzers.1a Here, the spiroꢀionenes were dissolved in
1 M KOD in D2O and kept at 80 and 120 oC, respectively, before
1
analysis by H NMR spectroscopy. Figure 3 displays selected
spectra of the two samples before and after storage at 80 and 120
a)
c1+c2
b
d1+d2
oC (all spectra are provided in Figure S9ꢀS12).
a
D2O
DMF
c2’
~
a)
+
-
+N
N
~
e
n
-
X
-
X
-
-
336 h at 120 o
C
X
= mixed Cl and Br
d2+e
d1
c1
a
c2
b
~
~
672 h at 80 o
C
ppm
b)
7.0
c1+c2
6.0
5.0
D2O
4.0
3.0
2.0
~
d1+d2
f
DMF
a
b
Initial
~
+
-
+N
Br
N
c2’
f+g
d2+e
e
g
n
-
r
B
c1
b
ppm 7.0
6.0
5.0
4.0
3.0
2.0
1.0
a
c2
d1
~
b)
~
ppm
336 h at 120 o
C
7.0
6.0
5.0
4.0
2.0
3.0
~
~
~
1
Figure 2. H NMR spectra of (a) spiroꢀionene 1 and (b) spiroꢀ
ionene 2 (chain endꢀgroup signals are enlarged).
672 h at 80 o
C
~
~
In spiroꢀionene 1, the stiff segments with the five interconnected
rings were directly linked via single CꢀC bonds in the backbone.
In contrast, spiroꢀionene 2 contained flexible –(CH2)3– spacer
chains inꢀbetween the rigid ring moieties. As expected, the chain
flexibility had a direct impact on the properties of the resulting
ionenes. For example, the stiff chain structure of spiroꢀionene 1
reduced the solubility in relation to 2. Consequently, it was necesꢀ
sary to use a higher water content during the polymerization of 1.
While 2 was readily soluble in dimethyl sulfoxide at room temꢀ
Initial
ppm 7.0
6.0
5.0
4.0
3.0
2.0
1.0
1
Figure 3. H NMR spectra of (a) spiroꢀionene 1 and (b) 2 before
and after storage in 1 M KOD/D2O during 672 and 336 h at 80
o
and 120 C, respectively (black arrows indicate changes in the
spectra due to HꢀD exchange and red arrows indicate changes due
to degradation).
o
perature, 1 did not dissolve in this solvent even at 80 C. Both
ionenes were readily soluble in methanol and water, but did not
dissolve in the polar aprotic solvents Nꢀmethylꢀ2ꢀpyrrolidone and
dimethylacetamide.
The high pH and temperature of the solutions facilitated protonꢀ
deuterium (HꢀD) exchanges at benzylic and aromatic positions. Hꢀ
D exchange at the benzylic positions resulted in disappearance of
the corresponding signals within 168 h at 80 oC (Figure 3). At 120
oC, both the aromatic and benzylic signals disappeared within 72
h. As expected, these exchanges were reversible. After treatment
As seen in Figure 1, the spiroꢀionenes formed light yellow, transꢀ
parent and flexible films when cast from 5 wt% aq. solutions at
o
120 C. The water uptake of the spiroꢀionene 1 film reached 27
and 68 wt% at 55 and 93% relative humidity (RH), respectively,
after 48 h equilibration at room temperature (Figure S5). Under
the same conditions, spiroꢀionene 2 took up 17 and 44 wt% water,
respectively. The lower water uptake of the latter film may be
o
in 1 M KOH in H2O at 120 C during 72 h, the labile deuterium
atoms were exchanged back to protons, resulting in the increase of
the intensities of the corresponding 1H NMR signals (Figure S13).
2
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Besides effects of HꢀD exchange, the H NMR spectra of both
spiroꢀionenes showed no detectable changes after 672 h storage in
respectively, the degradation via 5ꢀmembered ring opening substiꢀ
tution was estimated to be ~15% after 336 h at 120 C. Furtherꢀ
o
1
2
3
4
5
6
7
8
o
1 M KOD/D2O at 80 C, indicating a very high alkaline stability
at this temperature (Figure 3). Spiroꢀionene 2 was even kept for
1896 h without any noticeable signs of structural degradation
(Figure S10). However, at 120 C the spectra of both samples
revealed distinct signs of degradation as new signals appeared and
increased in intensity over time (Figure S11 and S12).
more, the relatively weak multiplets at ~5.5 and 5.0 ppm correꢀ
sponded well with the formation alkene (=CH) and alkenyl (ꢀ
CH=CH) protons, clearly suggesting degradation via ringꢀopening
elimination (Scheme S2c). The degradation via route was estimatꢀ
ed to be ~10% from the intensity ratio of the signals at 5.5 ppm
and 3.7 ppm. Still, the total loss of cations was most probably
higher due to additional degradation reactions. A possible explaꢀ
nation for the markedly higher stability of spiroionene 2, in comꢀ
parison to 1, may be that the flexible trimethylene bridges in the
former greatly facilitate ring strain relaxation. This is likely to
mitigate the distortion of the 6ꢀmembred ring system, which is
necessary during the degradation reaction.4a
o
Each cation in the spiroꢀionenes is the center of a spirocyclic
system of one 5ꢀ and one 6ꢀmembered ring. Because of higher
ring strain, the former ring is more susceptible to alkaline attack.5a
In addition, the presence of the fused benzene ring can be exꢀ
pected to further destabilize the 2 and 4 (benzylic) positions of the
5ꢀmembered ring. This favors ringꢀopening substitution at these
positions as the dominating degradation pathway (Scheme S2a).
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In order to practically employ waterꢀsoluble polymers as AEMs
they need to be efficiently immobilized to prevent dissolution. In
the present case, we developed novel blend membranes containing
spiroꢀionene 2 and a commercially available polybenzimidazole
(PBIꢀOO) to demonstrate the possibility to prepare water insoluꢀ
ble and highly hydroxide ion conductive AEMs based on spiroꢀ
ionenes. Blend membranes containing 70, 75 and 80 wt% spiroꢀ
ionene 2, respectively, were cast at 65 or 80 oC from 5 wt% soluꢀ
tions of the polymers in dimethyl sulfoxide (Table S2). The resultꢀ
ing membranes, denoted S70P30, S75P25 and S80P20, respecꢀ
tively, were brownꢀyellow, transparent and flexible (Figure 4a),
1
The H NMR spectrum of spiroꢀionene 2 recorded after 336 h at
120 oC confirmed this. The new signals emerging at ~2.7 and 2.0
ppm consisted of a doublet and a triplet, respectively, with a
coupling constant of J ~ 12 Hz. This corresponded well with the
axial and equatorial αꢀprotons in the 1ꢀbenzylꢀ4ꢀ
methylpiperidine.9 Further signals appearing at ~ 1.1, 1.0 and 0.9
ppm may arise from additional aliphatic protons in the degraded
trimethylene bipiperidine moiety. Assuming that ringꢀopening
substitution was the only active degradation reaction, the degree
of degradation was estimated by comparing the intensity of the
new signal at ~ 2.7 ppm and the original spiroꢀionene signal at ~
3.6 ppm in the same spectrum. This suggested a loss of a mere
~10% of the cations of 2 after 336 h at 120 oC.
o
and decomposed only above 350 C (Figure S4). Treatment of
these membranes in 0.5 M aqueous KOH solution exchanged the
counter ion of the spiroꢀionene to OH−. In addition, at least a part
of the –NH– groups of PBIꢀOO were deprotonated to enable the
formation of ammoniumꢀimidazolate complexes between the
spiroꢀionene and PBIꢀOO (Figure 4b). This resulted in completely
waterꢀinsoluble and flexible AEMs, as long as kept under basic
conditions.
The SAXS profiles of the blend AEMs were very similar to that
of the neat spiroꢀionene 2 (Figure S7). The water uptake of the
AEMs in the OH− form was found to increase with temperature
and spiroꢀionene content (Figure S6). At 20 oC membrane S70P30
and S80P20 took up just above 100 and 450 wt% water, respecꢀ
tively. Despite the high water uptake, the membranes remained
intact and did not disintegrate. Figure 4c shows the OH− conducꢀ
tivity of the AEMs fully immersed in water. As seen, membrane
S80P20, S75P25 and S70P30 reached 27, 34 and 49 mS cm−1,
respectively, at 20 C. At 90 C, the conductivity of the same
membranes had risen to very high values: 70, 90 and 120 mS
cm−1, respectively. Notably, the conductivity increased with deꢀ
creasing spiroꢀionene content and water uptake. This is typically
observed for highly waterꢀswollen membranes where the dilution
of charge carries leads to limitations of the conductivity.10 It
implied that even higher conductivity values may be reached after
further blend optimization.
o
o
Figure 4. (a) Photographs of blend AEM S80P20 indicating
excellent transparency and foldability. (b) Scheme of the ammoꢀ
niumꢀimidazolate complexes formed by spiroꢀionene 2 and PBIꢀ
OO in the water insoluble blend AEMs. (c) Arrhenius OH− conꢀ
ductivity plots of blend membranes S80P20, S75P25 and S70P30
fully immersed in water (the sharply increase of conductivity up
to 20 oC was caused by ice melting).
In conclusion, highꢀmolecular weight ionenes with a spirane
structure were synthesized via cycloꢀpolycondensations under
mild conditions using commercially available precursors. The
restricted ring system induced an excellent thermal and alkaline
stability, and both spiroꢀionenes remained intact over at least 672
o
1
After storage at 120 C, multiple new signals emerged in the H
NMR spectra of spiroꢀionene 1 to indicate a much more complex
degradation process than for 2 (Figure 3). Signals emerging at
~2.8 and 1.9 ppm may imply ringꢀopening substitution at the 5ꢀ
membered ring (Scheme S2a). However, the appearance of at
least four broad signals, instead of the expected two, between 1.4ꢀ
1.0 ppm suggested that ringꢀopening substitution at 6ꢀmembered
ring may also have occurred (Scheme S2b). Based on the intensity
ratio between the signals at ~2.8 and 3.7 ppm, arising from αꢀ
protons in the original and the degraded piperidinium moiety,
o
h in 1 M KOD/D2O at 80 C, with a reasonable stability also at
120 oC. The applicability of these ionenes was shown by the
preparation of highly OH− conductive AEMs based on ionic
blends with polybenzimidazole. The results demonstrate that
spiroꢀionenes constitute a new class of unique alkaliꢀstable anionꢀ
exchange polymers and membranes. Our future work will focus
on the separation of the spiroꢀcentered QA cations along the
3
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