Synthesis of sulfonate ester and sulfonic acid-containing poly(arylene perfluorocyclobutane)s (PFCB) by direct copolymerization of a sulfonate ester-containing precursor

Article abstract of DOI:10.1016/j.polymer.2016.11.061

This paper reports the first synthesis of sulfonated poly(arylene perfluorocyclobutane)s by direct polycondensation of a new sulfonate ester-containing bis(trifluorovinylether) precursor and subsequent hydrolysis of the resulting polymers. The multi-step synthesis of a bis(trifluorovinylether) monomer bearing sulfonic acid groups protected as sulfonate esters (SE-TFVE) is described. Based on this precursor and 4,4′-bis(trifluorovinyloxy)biphenyl (4,4′-TFVE), sulfonate ester-containing poly(arylene perfluorocyclobutane) copolymers (SE-PFCB) were synthesized by 2π?+?2π step-growth cyclopolymerization. Their thermal and physico-chemical properties were characterized. Free acid polymers (SA-PFCB) were subsequently obtained by hydrolysis of sulfonate ester groups. A preliminary evaluation of the proton conductive properties of these polymers is reported.

Full text of DOI:10.1016/j.polymer.2016.11.061

Polymer 108 (2017) 179e192
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of sulfonate ester and sulfonic acid-containing poly(arylene
perfluorocyclobutane)s (PFCB) by direct copolymerization of a
sulfonate ester-containing precursor
*
Catherine Marestin , Xavier Thiry, Sergio Rojo, Edouard Chauveau, Regis Mercier
Universit ꢀe de Lyon, Univ Lyon 1, CNRS, Ing ꢀe nierie des Mat ꢀe riaux Polym ꢁe res (IMP-UMR 5223), 15 Boulevard Latarjet, 69622, Villeurbanne, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 October 2016
Received in revised form
This paper reports the first synthesis of sulfonated poly(arylene perfluorocyclobutane)s by direct poly-
condensation of a new sulfonate ester-containing bis(trifluorovinylether) precursor and subsequent
hydrolysis of the resulting polymers. The multi-step synthesis of a bis(trifluorovinylether) monomer
bearing sulfonic acid groups protected as sulfonate esters (SE-TFVE) is described. Based on this precursor
21 November 2016
Accepted 22 November 2016
Available online 23 November 2016
0
0
and 4,4 -bis(trifluorovinyloxy)biphenyl (4,4 -TFVE), sulfonate ester-containing poly(arylene per-
fluorocyclobutane) copolymers (SE-PFCB) were synthesized by step-growth cyclo-
polymerization. Their thermal and physico-chemical properties were characterized. Free acid polymers
SA-PFCB) were subsequently obtained by hydrolysis of sulfonate ester groups. A preliminary evaluation
of the proton conductive properties of these polymers is reported.
© 2016 Elsevier Ltd. All rights reserved.
2
p
þ
2p
Keywords:
(
Sulfonated poly(arylene
perfluorocyclobutane)
Sulfonate ester bistrifluorovinyl ether
1
. Introduction
bistrifluorovinyl precursor appears as a particularly attractive
alternative as it permits a better control of the sulfonic acid groups
incorporated in the polymer chains and gives access to a wide range
of sulfonated poly(arylene perfluorocyclobutane)s structures. Very
few examples have however been reported yet [15,16]. One prob-
able reason is associated to synthetic difficulties. The classical sul-
fonation reactions of bistrifluorovinyl ether monomers are known
to be unsuccessful [6], because of the significant formation of un-
desirable by-products. An alternative approach which consists in
the sulfonation of fluoroalkylated precursors and the subsequent
formation of trifluorovinyl groups has been addressed. In this case,
sulfonated fluoroalkylated compounds are isolated in good yields.
However, their transformations into trifluorovinyl ether com-
pounds were unfortunately hampered by the presence of sulfonic
acid groups and the excessive formation of hydrogenated com-
pounds was observed. When the elimination reaction is performed
on substrates bearing different sulfonic acid salts (in their sodium,
potassium or ammonium form), the elimination reaction was
ineffective, probably because of the fair solubility of the reactants in
the reaction medium. Such synthetic difficulties have also been
reported by other authors [6]. To the best of our knowledge, only 3
patents from LG [16], General Motor [15] and 3 M [17] claim the
synthesis of functionalized bistrifluorovinyl ether monomers,
either in their sulfonic acid potassium salt form or as fluorosulfonyl
analogues. However, no high molecular weight polymers
Due to their specific thermal and chemical properties, poly(-
arylene perfluorocyclobutane)s (PFCB) are considered as high per-
formance engineering materials for many applications [1e3]. The
incorporation of proton conducting groups in such macromolecular
structures has recently been investigated for the development of
fuel cell membranes [4e6]. Proton conducting PFCB are typically
obtained by post-modification reactions of readily available PFCB
structures. The incorporation of sulfonic acids as proton conducting
groups is most often achieved by direct sulfonation [7e9], but other
reactions have been used such as bromation and subsequent Ull-
man coupling reactions [10,11], acylation and nucleophile substi-
tution, or metallation and reaction with 1,3-propanesultone [12]. In
order to control the final polymer microstructure (the polymer
þ
ꢀ1
ionic exchange capacity (IEC, meq H g ) as well as the incorpo-
ration of the sulfonic groups along the macromolecular backbone),
most reported post-modification reactions are performed on block
PFCB precursor polymers bearing specific sequences prone to post-
modification [9,13,14].
The direct polycondensation of a sulfonic acid-containing
*
Corresponding author.
E-mail address: Catherine.marestin@univ-lyon1.fr (C. Marestin).
http://dx.doi.org/10.1016/j.polymer.2016.11.061
032-3861/© 2016 Elsevier Ltd. All rights reserved.
0

180
C. Marestin et al. / Polymer 108 (2017) 179e192
synthesized by direct 2
are reported.
Therefore, the strategy described in this article for the synthesis
of sulfonic acid-containing PFCB is different: we describe the multi-
step synthesis of a bis(trifluorovinyl ether) precursor containing
sulfonic acids groups protected as sulfonate esters (SE-TFVE). A
p
þ 2
p
cyclodimerisation of such monomers
100 mL of n-hexane. Insoluble impurities were filtered off and the
solvent was evaporated to dryness. The crude powder was purified by
column chromatography with petroleum ether as eluent, affording
27 g (70% yield) of a white crystalline powder.
H3
H2
3
2
2
series
of
sulfonate
ester-containing
poly(arylene
per-
F
O
4
1
O
F1
6
fluorocyclobutane)s (SE-PFCB) are synthesized by direct poly-
Br
Br
0
3
5
condensation of this monomer with 4,4 -bis(trifluorovinyloxy)
F
F2
0
F
H3
H2
F1
biphenyl (4,4 -TFVE). Subsequent hydrolysis of the resulting poly-
F
F
2
mers affords sulfonic acid containing PFCB (SA-PFCB).
(
FABP)
2
. Experimental part
19F NMR (DMSO-d6, 188 MHz):
d
ꢀ70.15 (t, F , JF1-
2
.1. Materials
1
1
F2 ¼ 5.6 Hz), ꢀ85.29 (t, F
2
, JF2-F1 ¼ 5.6 Hz). H NMR (DMSO-d
6
,
Dimethyl sulfoxide (DMSO) was distilled under reduced pres-
200 MHz):
d
7.82 (d, 4H , J
¼ 8.0 Hz), 7.43 (d, 4H , J
¼ 8.0 Hz).
2
H-H
3
H-H
13
sure. Acetonitrile was dried over NaH and distilled. Zinc was acti-
vated before use. (Zinc dust was rapidly washed with 0.1 M
hydrochloric acid, filtered, and successively washed with demin-
eralised water, acetone and diethyl ether). It was then dried under
6 4 1 2
C RMN (DMSO-d , 50 MHz): d 147.66 (C ), 137.92 (C ), 128.73 (C ),
121.92 (C3).
0
2.3.2. Synthesis of 4,4 -bis(2-bromo-tetrafluoroethoxy)-3,3'-
ꢁ
vacuum, at 140 C for 12 h. All other chemicals were used as
bis(chlorosulfonyl)-biphenyl (CSFABP)
received unless otherwise stated. Dibromotetrafluoroethane was
kindly provided by ERAS Labo.
A 250 mL reactor equipped with a condenser and a magnetic
stirrer was charged with 10 g (13.5 mmol) of 4,4 -bis(2-bromo-tet-
0
rafluoroethoxy) biphenyl dissolved in 30 mL of 1,2-dichloroethane.
2
.2. Characterization methods
40 mL (600 mmol) of chlorosulfonic acid were added dropwise.
ꢁ
The mixture was stirred at 90 C for 20 min. The evolution of the
1H, 13C and 19F Nuclear Magnetic Resonance (NMR) spectra were
19
reaction was monitored by F NMR. After completion of the reaction,
recorded on a Brucker Avance 200 operating at the following reso-
the mixture was added dropwise to 200 mL of ice-water and the
product was extracted with dichloromethane. The organic phases
were collected and concentrated. After crystallization in chloroform/
ethanol (50/50), 8.75 g (60% yield) of pure compound were isolated.
1
13
nance frequencies: 200 MHz for H, 50 MHz for C and 188 MHz for
1
9
F, and on a Brucker Avance 400 spectrometer operating at the
1
following resonance frequencies: 400.16 MHz for H, 100.63 MHz for
13
19
3
C and 376.48 MHz for F. Tetramethylsilane (TMS) and CFCl were
ClO S
H2
SO Cl
used as chemical shift references. Thermogravimetric analyses (TGA)
were performed with a TA Q50 Instrument, under nitrogen and at
2
2
3
2
6
ꢁ
ꢀ1
F
O
4
1
O
F
1
8
1
0 C.min . The polymer glass transition temperature of polymers
Br
Br
5
7
was determined by Differential Scanning Calorimetry (DSC), with a
F
F2
ꢁ
ꢀ1
F
H5
H6
F1
DSC 822e Mettler Toledo equipment under argon, at 5 C.min . The
reported values were determined after a second heat scan, by the
midpoint method. Molecular weights were determined by Size
Exclusion chromatography (SEC) on a system equipped with a Shi-
madzu LC-20AD pump and a differential refractive index detector.
THF was used as eluent. SEC analyses were achieved with Waters
Styragel columns (HR2þHR1þHR0.5). Reported values (Mn(PS) and
F
F
2
(
CSFABP)
19F NMR (DMSO-d
NMR (DMSO-d , 200 MHz):
2H
NMR (DMSO-d
128.17 (C ), 127.28 (C
, 188 MHz):
d
ꢀ68.09 (t, F
8.06 (d, 2H
), ꢀ82.55 (t, F
1
6
1
2
). H
6
d
2
, JH2-H6 ¼ 2.2 Hz), 7.69 (dd,
13
Mw(PS)) are determined from a polystyrene calibration curve.
6
, JH6-H5 ¼ 8.6 Hz, JH6-H2 ¼ 2.2 Hz), 7.43 (d, 2H
, 50 MHz): 144.72 (C ), 140.54 (C
), 121.27 (C ).
5
, JH-H ¼ 8.6 Hz).
C
),
6
d
4
1
), 136.09 (C
3
2.3. Monomer synthesis
2
6
5
0
0
2
(
.3.1. Synthesis of 4,4 -bis(2-bromo-tetrafluoroethoxy)biphenyl
2.3.3. Synthesis of 4,4 -bis(2-bromo-tetrafluoroethoxy)-3,3'-bis(4-
fluorobenzenesulfonate)-biphenyl (SE-FABP)
FABP)
2 g (64 mmol) of 4,4’-dihydroxybiphenyl were suspended in
1
In a 100 mL round-bottom flask equipped with a magnetic
stirrer and an addition funnel, were placed 3.49 g (30.8 mmol, 2.2
equivalents) of 4-fluorophenol, 0.32 g (2.8 mmol, 0.2 equivalents)
of diazabicyclooctane (DABCO) and 12 mL of dichloromethane. The
ꢀ
1
9
4.8 mL of 1.48 mol l potassium methoxide solution in methanol, in
a 250 mL round-bottom flask equipped with a condenser and a
magnetic stirrer. The mixture was refluxed for 4 h. The solvent was
evaporated to dryness and the crude powder was dried under vac-
uum at 120 C for 12 h. The resulting dried bisphenate was suspended
in 30 mL of dried DMSO in a three-necked reactor equipped with a
mechanical stirrer and a nitrogen inlet. A solution of 19 mL
140.8 mmol) of 1,2-dibromotetrafluoroethane (DBFE) dissolved in
0 mL of dried DMSO was added dropwise over 3 h to the bisphenate.
During the addition, the reaction mixture was slightly cooled by a
water bath, so that temperature did not exceed 20 C. The mixture
was then heated at 50 C for a night. After cooling, the mixture was
ꢁ
mixture was cooled at 0 C in an ice water bath and 3.8 mL
ꢁ
(22 mmol, 2 equivalents) of triethylamine were added. 10.39 g
(14 mmol, 1 equivalent) of 4,4'-bis(2-bromo-tetrafluoroethoxy)-
3,3'-bis(chlorosulfonyl)-biphenyl dissolved in 25 mL of dichloro-
methane were added dropwise. The evolution of the reaction was
monitored by F NMR. After completion of the reaction the organic
phase was washed with demineralised water, dried over sodium
sulfate, filtered and the solvent was evaporated. The yellow crude
solid was refluxed in petroleum ether. The product was collected by
filtration, rinsed with petroleum ether and dried under vacuum at
(
2
19
ꢁ
ꢁ
pouredinto 100 mL ofbrine and extractedwithdichloromethane. The
organic solvent was concentrated and the residue was dissolved in
ꢁ
130 C giving 11.46 g of a white powder (92% yield).

C. Marestin et al. / Polymer 108 (2017) 179e192
181
19F NMR (DMSO-d
F1-F2 ¼ 93.1 Hz, JF1-F3 ¼ 58.7 Hz), ꢀ126.10 (dd, F
F3 ¼ 109.2 Hz), ꢀ138.47 (dd, F
NMR (DMSO-d , 400 MHz):
H2 ¼ 2.4 Hz), 8.03 (d, 2H , JH2-H4 ¼ 2.4 Hz), 7.86 (dd, 2H
¼ 8.4 Hz, J
, 376 MHz):
d
ꢀ116.65 (s, F
), ꢀ118.16 (dd, F
6
4
1
,
F3
O
J
2
, JF2-F1 ¼ 93.1 Hz, JF2-
1
3
d
, JF3-F1 ¼ 58.7 Hz, JF3-F2 ¼ 109.2 Hz). H
6
8.17 (dd, 2H
4
, JH4-H5 ¼ 8.8 Hz, JH4-
, JH5-
2
5
13
H4
H5-H2
¼ 1.2 Hz), 7.27e7.16 (m, 8 H, H and H ).
8
9
C
O S
2
F1 Br
1
NMR (DMSO-d , 200 MHz):
6
d
160.84 (d, C , J ¼ 506 Hz), 151.44
10
C-F
O
(C
6
), 145.12 (C
7
), 136.50 (C
4
), 135.08 (C
3
), 130.26 (C
2
), 124.54 (C
1
),
).
H2
F1
3
2
F2 F2
123.89 (d, C
8
, JC-F ¼ 18 Hz), 117.22 (d, C
9
, JC-F ¼ 48 Hz), 117.22 (C
5
H3
F
F
0
0
F
2.3.5. Synthesis of 4,4 -bis(trifluorovinyloxy)biphenyl (4,4 -TFVE)
O
H1
To a 100 mL round-bottom flask equipped with a condenser and
a magnetic stirrer were added 8.24 g (15.15 mmol) of 4,4'-bis(2-
bromo-tetrafluoroethoxy) biphenyl, 3 g (45 mmol) of actived zinc
and 50 mL of acetonitrile. The mixture was refluxed and the reac-
tion was monitored by F NMR. After completion of the reaction,
the excess of zinc and salts formed were filtered over celite and
washed with diethyl ether. 200 mL of water were added to the
filtrate which was extracted several times with diethyl ether. The
organic phase was dried over sodium sulfate, filtered and the sol-
vent was removed. The crude product was purified by chroma-
tography on silica gel using petroleum ether as eluent to give 3.15 g
of a white solid (60% yield).
Br
F
SO2
O
F
H4
H5
H4
H5
19
(
SE-FABP)
1
9
F NMR (DMSO-d
6
, 188 MHz):
d
ꢀ70.13 (t, F
1
), ꢀ84.48 (t,
8.25 (dd, 2H
, JH3-H2 ¼ 8.8 Hz),
1
F
J
7
2
), ꢀ113.97 (s, F
3
). H NMR (DMSO-d
6
, 200 MHz):
d
2
,
H2-H3 ¼ 8.8 Hz), 8.105 (d, 2H
1
), 7.90 (d, 2H
3
F
F
F H3
H2
3
2
2
.31e7.11 (m, 8 H, H
4
and H
5
).
O
4
1
O
F2
F1
0
3
5
6
2
.3.4. Synthesis of 4,4 -bis(trifluorovinyloxy)-3,3'-bis(4-
fluorobenzensulfonate)-biphenyl (SE-TFVE)
1.46 g (12.84 mmol) of 4,4'-bis(2-bromo-tetrafluoroethoxy)-
,3'-bis(4-fluorobenzenesulfonate)-biphenyl, 3 g (44.96 mmol) of
actived zinc and 60 mL of dry acetonitrile were placed in a high-
pressure reactor equipped with a magnetic stirrer, a temperature
sensor and a pressure sensor. The reaction mixture was heated at
H3
H2
F3
1
3
4,4’-TFVE
ꢁ
19
1
10 C. After completion of the reaction (monitored by F NMR),
the excess of zinc and salts formed were filtered and washed with
diethyl ether. 100 mL of demineralised water were added to the
filtrate and the mixture was extracted several times with diethyl
ether. The organic phase was dried over sodium sulfate, filtered and
the solvent was evaporated. The crude yellow oil was purified by
chromatography on silica gel using a mixture of petroleum ether
and ethyl acetate (90:10). Several successive crystallizations in a
hexane/ethyl acetate (78:22) were necessary to obtain a polymer
grade monomer as a white solid (5.45 g - 61% yield).
19F NMR (DMSO-d
56.4 Hz), ꢀ126.36 (dd,
F3 ¼ 109.0 Hz), ꢀ134.20 (dd, F , JF3-F2 ¼ 109.0 Hz, JF3-F1 ¼ 56.4 Hz).
7.72 (d, 2H
, JH-H ¼ 8.0 Hz), 7.36 (d,
, JH-H ¼ 8.0 Hz). C NMR (DMSO-d , 50 MHz): 153.74 (C ),
36.11 (C ), 128.61 (C ), 116.12 (C ).
, 188 MHz):
d
ꢀ118.48 (dd, F
, JF1-F2 ¼ 97.8 Hz,
97.8 Hz,
6
1
¼
J
F1-F3
¼
F ,
2
J
F2-F1
J
F2-
3
1
H NMR (DMSO-d
6
, 200 MHz):
d
2
1
3
2
1
H
3
6
d
4
1
2
3
2.4. Synthesis of polymers
F4
2.4.1. Synthesis of sulfonate ester containing poly(arylene
perfluorocyclobutane)s (SE-PFCB)
9
00 mg (1.296 mmol) of sulfonate ester precursor, 953.3 mg
0
(
2.754 mmol) of 4,4 -TFVE and 1.85 g of diphenyl ether were
introduced in a Schlenk tube equipped with a mechanical stirrer
and a nitrogen inlet. The reaction mixture was heated at 250 C for
O
O S
2
F2
ꢁ
O
5 h, under nitrogen, in a tubular oven. After cooling, the viscous
solution was poured in petroleum ether. The polymer was isolated
by filtration, thoroughly rinsed with petroleum ether and dried
H4
4
F1
H5
O
F3
F
5
6
3
2
ꢁ
under vacuum at 100 C.
1
1
F
1
2
1
H2
F
SO2
2.4.2. Synthesis of sulfonic acid containing PFCB (SA-PFCB)
O
7
0.5 g of the SE-PFCB precursor were dissolved in 2 mL of NMP.
ꢀ1
H8
H9
H8
2 mL of a potassium methoxide solution in MeOH (1.5 mol L
)
8
9
8
9
were then added to the polymer solution. The basic hydrolysis of
the sulfonate esters (in sulfonic acids, potassium salt form) was
1
0
H9
19
monitored by F NMR, by direct analysis of the medium. After
completion of the reaction, the polymer was precipitated in
methanol, isolated by filtration, thoroughly rinsed with methanol
and dried under vacuum.
F
(
SE-TFVE)

182
C. Marestin et al. / Polymer 108 (2017) 179e192
2.5. Membrane preparation
high yields [19]. The thermal decomposition temperature of sul-
fonate esters depends on the nature of the alcohol involved (sul-
fonate esters of primary alcohols are more stable than those of
secondary or tertiary alcohols [20]. In addition, many sulfonate
esters are easily hydrolysed with various chemical agents (NaI,
Thin SA-PFCBK films were obtained by conventional solution
casting method, from a 20 wt% polymer solution in DMAC. The
solutions were cast onto clean glass plates and successively dried
ꢁ
ꢁ
ꢁ
ꢁ
overnight at 50 C, 1 h at 80 C, 1 h at 120 C, 1 h at 150 C and 1 h at
3 3
piperidine, NaN , HBr, BBr , NaOH) [21]. In this case, the depro-
ꢁ
1
80 C. The resulting films were peeled off by immersion in water.
tection reaction allows the regeneration of the protecting alcohol/
phenol and releases a sulfonic acid-containing product. This reac-
tion has therefore recently been considered as an elegant way to
protect sulfonic acids as sulfonate esters groups [22]. Different
advantages can be expected from this strategy. The use of sulfonate
esters-containing monomers can be required if the presence of
sulfonic acids is incompatible with the polymerization reaction. It
can also facilitate the synthesis and characterization of well-
defined sulfonate ester-containing polymer architectures, which
are finally hydrolysed in their sulfonic acid-containing analogues.
In this way, the synthesis of poly(vinylsulfonate)s has been suc-
cessfully performed by polymerizing sulfonate ester-containing
precursors and by a final hydrolysis of the polymer obtained
(LiBr, 2-butanone, 24 h @reflux) [23]. Other authors used a similar
method for the synthesis of poly(styrene-bloc styrene sulfonic acid
The membranes were re-acidified by immersion in a 2 M sulphuric
acid solution for 12 h and thoroughly rinsed with demineralised
water.
2
.6. Conductivity measurements
Proton conductivities of the membranes were measured in-
plane using the four point probe method, with a Fumatech MK3
conductivity measuring cell equipped with a Gamry 600 poten-
tiostat. A 4 cm by 1 cm membrane of SA-PFCBH was placed in a
thermostated cell equipped with a humidity controller and a
temperature sensor. Measurements were performed at 95% relative
ꢁ
6
humidity (RH), at 90 C and over a frequency range of 0.1e10 Hz. A
first analysis was done after 5 min equilibration and six other
measurements were recorded every 10 min.
[24,25].
In this work, such a strategy has been considered, with the
synthesis of sulfonate ester containing trifluorovinyl ether mono-
mers. Highly pure sulfonate ester-containing bisTFVE monomer
3
. Results and discussion
(
SE-TFVE) was prepared by a four steps procedure, according to the
The work described in this article reports the synthesis of a
synthetic pathway described in Scheme 1:
functionalized monomer from a multi-step synthesis, its polymer-
ization affording sulfonate ester-containing PFCB (SE-PFCB), the
elaboration of thin membranes from these polymers and a last
treatment leading to sulfonic acid-containing PFCB (SA-PFCB). The
physico-properties of the resulting polymers are presented.
The fluoroalkylated biphenol (FABP) was prepared from
0
commercially available 4,4 -biphenol (BP), by formation of a
phenoxide and subsequent fluoroalkylation with dibromotetra-
fluoroethane according to a well-known procedure [3]. FABP was
purified by flash chromatography in order to remove any undesir-
2 2
able tetrafluoroethyl ether (Ar-O-CF CF H) containing by-product
3
.1. Monomer synthesis and characterization
formed in this step. Sulfonyl chloride groups were incorporated
on this substate in the presence of a large excess of chlorosulfonic
acid, affording chlorosulfonated fluoroalkylated biphenol (CSFABP).
Because of the electrophilic character of the chlorosulfonation re-
action, the sulfonyl chloride groups were easily grafted in ortho
position to the ether groups. The expected structure was confirmed
Aromatic sulfonyl chlorides (such as tosyl chloride) are
commonly used in organic chemistry as convenient protecting
agents for alcohols and phenols [18]. It is well known that cyclo-
hexylarene sulfonate can be thermally cleaved, affording the for-
mation of sulfonic acids containing compounds and cyclohexene in
Scheme 1. Multi-step synthesis of sulfonate ester-containing bistrifluorovinyl ether monomer. 1) Fluoroalkylation 2) Chlorosulfonation 3) Protection step 4) Elimination.

C. Marestin et al. / Polymer 108 (2017) 179e192
183
Fig. 1. 1H NMR (DMSO-d
) of a) FABP, b) CSFABP.
6
Fig. 2. 1H NMR (DMSO-d
) of SE-FABP.
6

Fig. 3. 1H NMR spectrum (DMSO-d
) of SE-TFVE.
6
Fig. 4. 19F NMR spectra (DMSO-d
) a) FABP, b) CSFABP, c) SE-FABP, d) SE-TFVE.
6

C. Marestin et al. / Polymer 108 (2017) 179e192
185
Fig. 5. 19F NMR of a) SE-TVFE and b) SE-TVFE in MeOK.
1
by H NMR (Fig. 1). This precursor was found to be readily prone to
requires higher temperatures than in classical experimental con-
ditions (reflux in acetonitrile), as described for the synthesis of 4,4
0
hydrolysis. Therefore, it was freshly prepared before use as reactant
for the synthesis of sulfonate ester.
Sulfonate ester fluoroalkylated biphenol (SE-FABP) was syn-
thesized in quasi-quantitative yield (92%) by reaction of 4-
fluorophenol on the disulfonylchloride precursor, in the presence
of triethylamine and catalytical amounts of DABCO.
This fluoroalkylated compound (Fig. 2) was then dehalogenated
by a treatment with activated zinc. In the presence of sulfonate
ester protecting groups, the formation of trifluorovinyl ethers
bistrifluorovinylether biphenol. Therefore, the reaction was per-
ꢁ
formed under pressure, at 110 C. In the course of the reaction,
2
some undesirable mono and di-CF H containing side-products
were produced. The purification of the crude compound is of
utmost importance as any tetrafluoroethyl ether containing
monomer would behave as chain terminating agents in the course
of a polycondensation reaction, and subsequently preclude the
synthesis of high molecular weight polymers.

186
C. Marestin et al. / Polymer 108 (2017) 179e192
0
0
Fig. 6. DSC thermograms of 4,4 -TFVE (1), SE-TFVE (2), and of a mixture of monomers (84% of 4,4 -TFVE and 16% of SE-TFVE) (3).
Fig. 7. 19F NMR spectra (DMSO-d
) of a) SE-TFVE and b) 4,4 -TFVE.
0
6

C. Marestin et al. / Polymer 108 (2017) 179e192
187
A polymer grade monomer was obtained by successive crys-
tallisations in ethyl acetate/hexane. NMR ( H and F) were used to
studied in this work, all attempts (by varying the reaction tem-
perature, basic agent…) were unsuccessful: these experimental
conditions were found to affect the integrity of the TFVE groups,
1
19
assess for the efficiency of the purification method as well as to
1
ꢁ
evidence the purity of the final compound (see H spectrum in
even at very low temperature (MeOH, ꢀ75 C). The fluorine NMR
Fig. 3).
spectra of the reaction media (Fig. 5) suggest that the base readily
reacts with the trifluorovinyl groups, by nucleophilic addition.
These results are in agreement with the work described by Moody
et al. [27].
In the frame of this multi-step synthesis, 19F NMR proved to be a
very sensitive tool and was considered as a method of choice to
monitor the evolution of the different reactions as well as to
identify the different compounds formed (Fig. 4).
The peaks respectively at e 70.15 ppm and e 85.29 ppm are
The thermal properties of SE-TFVE were evaluated by TGA and
DSC. No obvious decomposition is observed until the main-chain
ꢁ
assigned to the fluorine atoms F
biphenol. The incorporation of electro-withdrawing SO
stituents induces an upfield shift from - 70.15 to - 68.09 ppm for the
peak corresponding to fluorine F and from - 85.29 to - 82.55 ppm
for the peak corresponding to fluorine F . Upon the protecting step,
1
and F
2
of the fluoroalkylated
decomposition A 5% weight loss is observed above 331
C
2
Cl sub-
(Supporting information, Figure S1). No significant weight loss was
ꢁ
recorded after a 24 h isotherm at 110 C. These results attest for the
1
high thermal stability of this monomer. As shown in Fig. 5, both
0
2
4,4 -TFVE and SE-TFVE are crystalline and exhibit melting transi-
ꢁ
ꢁ
the reaction with 4-fluorophenol leads to a slight downfield vari-
ation of chemical shifts of these two fluorine atoms, from ꢀ68.09
tions around 46 C and 133 C, respectively.
At higher temperatures exothermal phenomena are attributed
to the thermal cyclodimerisation of these monomers. This result is
in agreement with those typically reported for various bistri-
fluorovinyl ether monomers as TFVE groups thermally react above
to ꢀ70.13 ppm for F
1
and from ꢀ82.55 to ꢀ84.48 ppm for F
2
. The 4-
fluorophenol addition is also characterized by the apparition of a
new peak at - 126.42 ppm. A quantitative analysis confirms a
ꢁ
quantitative conversion of SO
Upon delalogenation reaction with zinc metal, the two singlets
corresponding to the fluoroalkyl fluorines (F and F ), initially
2
Cl functions.
150 C and provide the formation of hexafluorocyclobutane rings
[28,29].
1
2
This cycloaddition reaction involves the formation of a biradical
intermediate which is stabilised by the presence of electron-
donating groups [3]. Comparing the DSC behaviour of the two
monomers (Fig. 6), it clearly appears that the exotherm onset of the
non-fnunctionalized monomer occurs at lower temperature than
its sulfonate ester-containing analogue. This can be attributed to
the deactivating effect of the electron-withdrawing sulfonate ester
present at ꢀ70.13 ppm and ꢀ84.48 ppm disappear and 3 signals
characterising trifluorovinyl ether groups centered at ꢀ119, - 125
and e 134 ppm appear.
The basic hydrolysis of 4-[(a,b,b-trifluorovinyl)oxy]benzene
sulfonyl chloride in the presence of KOH and methanol was re-
ported by Am eꢀ duri et al. [26] to enable the formation of sulfonic
0
acid groups in the presence of trifluorovinyl ethers with an 88%
yield. Such experimental conditions were therefore expected to
enable the formation of sulfonic acid containing bistrifluorovinyl
ether monomer from SE-TVFE. Unfortunately, on the substrate
groups on the biphenyl. In a mixture composed of 84% of 4,4 -TFVE
0
and 16% of SE-TFVE, it seems that 4,4 -TFVE melts rapidly, behaving
as a solvent for the second monomer.
Comparing the 19F NMR spectra of the two bis TFVE monomers
Scheme 2. Synthesis of sulfonate ester-containing poly(arylene perfluorocyclobutane)s (SE-PFCB) by direct polycondensation of a functionalized precursor.

188
C. Marestin et al. / Polymer 108 (2017) 179e192
Fig. 8. 19F NMR spectra (diphenyl ether þ DMSO-d
coaxial) of TFVE monomers initial mixture a) and of the resulting polymer b).
6
(
Fig. 7), some differences in chemical shifts of the TFVE peaks
(Fig. 8a)). Then, the evolution of the polymerization was monitored
19
clearly appear. Such differences are attributed to the electron-
withdrawing effect of the sulfonate ester groups. Based on Spraul
by quantitative fluorine NMR, by relative integration of the F peak
signal of the TFVE groups compared with the signals assigned to the
fluorine atoms involved in hexafluorocyclobutane rings. After 5 h
reaction, a quantitative conversion of the aryl TFVE in per-
fluorocyclobutane rings was evidenced by the complete disap-
pearance of the characteristic peaks assigned to the TFVE groups as
well as by the presence of multiplets present in the
19
[30] correlation between the F characterization of trifluorovinyl
ethers and their reactivity towards a cyclodimerisation reaction,
the lower reactivity of the sulfonate ester-containing monomer is
confirmed.
[
-125 ppm; ꢀ135 ppm] range, which are ascribed to hexa-
3
.2. Sulfonate ester and sulfonic acid-containing PFCB synthesis
fluorocyclobutane rings (Fig. 8b)). In addition, the incorporation of
sulfonate ester groups was proven by the presence of peaks
around ꢀ114 ppm. The complexity of the spectrum part related to
the fluorine atoms belonging to the hexafluorocyclobutane rings is
attributed to the presence of different isomers and/or different
and characterization
When the polymerization was performed in solution in NMP,
the reaction was incomplete. This was confirmed by the presence of
residual TFVE, even after long reaction times. This is the reason why
in this work, polymerizations were performed in diphenylether,
which is known for its high boiling point and chemical inertness.
High molecular weight sulfonated ester-containing poly(arylene
0
types of sequences (condensation of 4,4 -TFVE on itself, SE-TFVE
condensation on itself or mixed condensation).
The proportion of sulfonate ester groups present in the final
polymer was determined by the initial ratio of 4,4'bisTFVE mono-
mer and of its functionalized analogue. A series of polymers bearing
different bis TFVE monomers feed ratios was synthesized. For the
sake of clarity, polymers will be further named SE-PFCB-X, in which
X represents the molar ratio of sulfonate ester precursor. The
perfluorocyclobutane)s were obtained by a thermal (2
cyclopolymerization of TFVE monomers, at 250 C, in diphenylether
pþ2
p)
ꢁ
(Scheme 2).
A first aliquot was directly taken from the homogeneous reac-
ꢁ
tion mixture, at 80 C, before the polymerization starts. The
experimental ratio of the two monomers can be determined by
0
comparing the integral of each TFVE system. (I4,4 -TFVE and ISE-TFVE)
19
structure of the polymers has been characterized by F NMR
Fig. 9).
(

C. Marestin et al. / Polymer 108 (2017) 179e192
189
Fig. 9. 19F NMR spectra (CDCl
) of SE-PFCB-X (X: 0, 16, 27, 32, 100).
3

190
C. Marestin et al. / Polymer 108 (2017) 179e192
Table 1
Physico-chemical characterization of SE-PFCB and SA-PFCB.
SE-PFCB (sulfonate ester form)
SA-PFCBK (potassium salt form) and SA-
PFCBH (sulfonic acid form)
b
c
d
Td5%b
( C)
c
Acronym
% Th
SE-TFVE
% Expa
SE-TFVE
Td5%
( C)
Tg
M
(g/mol)
nPS
M
wPS
Acronym
Tg
ꢁ
ꢁ
ꢁ
ꢁ
( C)
(g/mol)
( C)
SE-PFCB-0
SE-PFCB-16
0
16
0
16
> 450
365
151
146
22 500
10 600
17 800
nd
56 900
55 600
44 400
nd
SA-PFCBK-16
SA-PFCBH-16
SA-PFCBK-27
SA-PFCBH-27
SA-PFCBK-32
SA-PFCBH-32
SA-PFCBK-100
393
125
143
192
>250
SE-PFCB-27
SE-PFCB-32
27
27
355
356
344
143
140
115e
343
315
349
293
288
32
30
SE-PFCB-100
100
100
a
Experimental (19F NMR).
b
c
T
d5%: 5% weight los temperature (TGA).
(DSC).
T
g
d
e
(
SEC in THF, PS calibration).
This value was under-estimated, due to some residual diphenyl ether in the sample.
The molar fraction of SE-TFVE (y) effectively incorporated in the
polymers was determined by F NMR, by comparing the integral
Ip) of fluorine atoms present on the sulfonate ester groups (Fp) and
the integral Im of the fluorine assigned to the hexa-
fluorocyclobutane rings (Fm), according to the following equation
eq 1):
were dissolved in NMP and potassium methanolate was added to
the solution (Scheme 3).
19
(
The deprotection reaction was monitored by 19F NMR. This re-
action was quantitative in less than 10 min. The fluorine atom
present on the sulfonate ester group, initially present at ꢀ113 ppm,
completely disappears. In the meantime, the fluorophenate char-
acterized by the apparition of a new singlet at ꢀ137 ppm appears
(
(Fig. 10).
y ¼ 3
ꢂ I
p
(1)
Upon deprotection of the sulfonate ester groups, a drastic
I
m
change of polymer solubilities occur. Indeed, SA-PFCBK-X are no
longer soluble in chlorinated solvents, THF or even DMSO. They can
be however dissolved in DMAC, cyclopentanone or NMP. Because of
the high proportion of sulfonic acid groups incorporated in SA-
PFCBK-100, this polymer is even soluble in water.
As reported in Table 1, the thermal stability of sulfonic acid-
containing PFCB (in their potassium salt (SA-PFCBK)) has been
investigated (supporting information Figure S4). The results
The experimental values are in good agreement with the ex-
pected theoretical ones (Table 1).
The thermal properties of SE-PFCB were investigated by TGA,
under nitrogen. The resulting thermograms (supporting
information S2) suggest a high thermal stability of these struc-
ꢁ
tures up to 300 C. The incorporation of increasing proportions of
sulfonate results in a slight stability decrease at high temperatures.
However, this does not jeopardize the thermal stability of the
polymers. Indeed, the 5% weight loss temperatures are in all cases
ꢁ
higher than 343 C (Table 1). No additional thermo-oxidative
degradation was evidenced when experiments were performed
under air. The glass transition temperature of these polymers were
determined by DSC. No melting endotherm are observed, which
suggest an amorphous nature for the polymers. The glass transition
ꢁ
temperatures is lower than 151 C. (corresponding to the Tg of the
non functionalized PFCB). Not surprisingly, a slight plasticizing ef-
fect is observed with increasing proportions of bulky fluorophenol
sulfonate ester side-chains. The polymers are soluble in common
organic solvents (THF, Chloroform, NMP, DMAC…), as reported for
many various poly(arylene perfluorocyclobutane)s. Mean molecu-
lar weights were determined by size exclusion chromatography, in
THF. Analyses show monomodal distributions for all polymers
(Supporting information Figure S3). The reported values in Table 1
were assessed from a polystyrene calibration curve. High molecular
weight polymers were obtained, which was confirmed by the
elaboration of thin tough membranes from 20% weight polymer
solutions in DMAC, by a conventional solvent casting method.
It is worth noting that attempts to regenerate sulfonic acid
functions on the polymer preliminary cast into thin membranes
19
failed. F NMR spectra of a thin film refluxed in MeOK and of
pristine SE-PFCB are identical. One probable reason is related to the
extremely low swelling availability of these membranes in meth-
anolic potassium hydroxide. Sulfonic acid containing PFCB were
therefore produced by basic hydrolysis of their sulfonate ester
containing analogues before the elaboration of membranes (SA-
PFCBK) and subsequent re-acidification (SA-PFCBH). The precursors
Scheme 3. Sulfonate ester poly(arylene perfluorocyclobutane)s hydrolysis leading to
sulfonic acid-containing poly(arylene perfluorocyclobutane)s.

C. Marestin et al. / Polymer 108 (2017) 179e192
191
Fig. 10. Deprotection of sulfonate ester groups. 19F NMR spectra (NMP þ coaxial DMSO-d
) (1) SE-PFCB, (2) in-situ deprotection reaction, with MeOK, (3) isolated SA-PFCBK.
6
indicate a high thermal stability, with a 5% weight loss temperature
above 288 C. Concerning DSC analyses, increasing proportions of
polymers present ionic exchange capacities ranging from 0.86
meq Hþ/g of dry polymer (for SA-PFCBH-16) to 4.1 meq ion/g of dry
polymer, (for SA-PFCBH-100) as assessed by the following equation
(eq (2)):
ꢁ
sulfonic acid groups (in their potassium salt form) induce a Tg in-
crease. This result is not surprising as a similar behaviour has been
observed on different polymer backbones (sulfonated polyimides,
sulfonated polarylethers…). For the polymer based exclusively on
the functionalized monomer (SA-PFCBK-100), no Tg could be
determined in the temperature range investigated.
2y
IEC ¼
ꢂ 1000
y ꢂ Msulfonated þ x ꢂ Mnon sulfonated
The polymers (in their sulfonic acid, potassium salt form) were
cast into thin films, from 20% weight DMAC solutions. Surprisingly,
6
ꢂ I
p
IEC ¼ À
Á
ꢂ 1000 (2)
only thin 30
mm thick membranes were successfully elaborated,
I
m
ꢀ 3I
p
ꢂ M
þ 3I
p
ꢂ Msulfonated
non sulfonated
while thicker membranes had fair mechanical properties. Further
experiments dedicated to understand this phenomenon are
currently under investigations. The thin membranes were re-
ꢀ1
with
M
sulfonated
¼
506.364
.
g
mol
and
M
non
ꢀ
1
sulfonated ¼ 346.224 g mol
acidified in
2
M
H
2
SO
4
.
The deprotected and re-acidified
The stability of the materials in their SO
3
H acidic form (SA-

192
C. Marestin et al. / Polymer 108 (2017) 179e192
PFCBH-X) against desulfonation reactions was evaluated by ther-
mogravimetry analysis. The results obtained (supporting
information S5) are consistent with typical desulfonation re-
J.P. Ferraris, Perfluorocyclobutyl (PFCB)-based polymer blends for proton ex-
change membrane fuel cells (PEMFCs), J. Membr. Sci. 431 (2013) 86e95.
7] J. Park, N. Tomar, H. Colon-Mercado, D. Hobbs, D.W. Smith Jr., PFCB polymer
electrolytes: a promising material for hydrogen production, Polym. Prepr.
(Am. Chem. Soc., Div. Polym. Chem.) 50 (2009) 559e560.
8] D.-J. Kim, B.-J. Chang, J.-H. Kim, S.-B. Lee, H.-J. Joo, Sulfonated poly(fluorenyl
ether) membranes containing perfluorocyclobutane groups for fuel cell ap-
plications, J. Membr. Sci. 325 (2008) 217e222.
[9] B.-J. Chang, D.J. Kim, J.H. Kim, S.-B. Lee, H.J. Joo, Sulfonated poly(fluorene-co-
sulfone)ether membranes containing perfluorocyclobutane groups for fuel
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10] L. Zou, S.M. MacKinnon, T.J. Fuller, Combination of Main-chain and Side-chain
Sulfonation of PFCB-6F High-temperature Fuel Cell Membranes,
US20110257278A1, GM Global Technology Operations, Inc, USA, 2011, pp.
16pp.
[
[
ꢁ
actions of aromatic sulfonic groups (above 250 C).
Preliminary proton conductivity measurements were per-
þ
ꢀ1
formed on a membrane cast from a 1,4 meqH g polymer solution
ꢁ
(
SA-PFCBH-27). The value measured at 90 C and under 95% of
ꢀ
1
relative humidity reached 138 mS cm . In analogous conditions,
the well-known Nafion has a proton conductivity of 270 mS. cm
ꢀ
1
.
[
[
This result is coherent with the proton conductivity of sulfonated
ꢁ
block PFCB obtained by post-sulfonation (106,9 mS @80 C for a 2
meqH g sulfonated PFCB) reported by Park [7].
þ
ꢀ1
11] T.J. Fuller, S.M. Mackinnon, M.R. Schoeneweiss, Polyelectrolyte Membranes
Made of Poly(perfluorocyclobutanes) with Pendant Perfluorosulfonic Acid
Groups and Blends with Poly(vinylidene Fluoride), US20110053036A1, GM
Global Technology Operations, Inc, USA, 2011, pp. 12pp.
4
. Conclusion
[
[
12] S.M. Mackinnon, T.J. Fuller, F. Coms, M.R. Schoeneweiss, Proton Exchange
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Copolymers, their Precursors and Proton Conductive Polymer Membranes, in,
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22pp.
15] G. Maier, M. Gross, Fluorinated Polymer Blocks for PEM Applications,
US20080027152A1, Gm Global Technology Operations, Inc, USA, 2008, pp. 15
pp.
In this work, we have successfully prepared a sulfonate ester-
containing bis TFVE precursor by a multi-step synthesis. The
physico-chemical properties of this monomer were investigated
1
19
(
structural characterization by H and F NMR, thermal properties
by TGA and DSC). A series of sulfonate ester-containing per-
[
fluorocyclobutanes were then synthesized by direct copolymeri-
0
zation of the functionalized monomer with 4,4 -TFVE. Increasing
sulfonate ester-containing units were obtained by adjusting the
molar ratio of the two monomers. These polymers were soluble in
common organic solvents and were proven to be highly thermo-
stable. Upon basic treatment in the presence of KOH/MeOH, the
sulfonate ester protecting groups were hydrolysed, producing sul-
fonic acid-containing polymers, in their potassium salt form. After
the elaboration of thin membranes from solution casting method
[
[16] C.K. Shin, B.K. Lee, J.H. Kim, B.J. Chang, S.B. Lee, I.J. Park, D.K. Kim, K.W. Lee,
J.W. Ha, K.H. Kim, D.J. Kim, M.C. Yoo, Block Copolymers Containing Per-
fluorocyclobutane Rings for Electrolyte Membranes with Good Proton Con-
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Korea; Korea Research Institute of Chemical Technology, 2007, pp. 61pp.
[17] D.D. Desmarteau, C.W. Martin, L.A. Ford, Y. Xie, Sulfonated Perfluorovinyl
and re-acidification in 2 M H
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materials was evaluated. Promising results were obtained
2001, pp. 11 pp.
ꢀ1
ꢁ
(
138 mScm @90 Ce95% RH). To the best of our knowledge, this is
[18] Reaction, Mechanisms and structure, in: J. March (Ed.), Advanced Organic
Chemistry, fourth ed., 1991, p. 498.
the first example of the synthesis of sulfonated PFCB by the direct
cyclodimerisation of a functionalized monomer precursor. This
opens new perspectives for the synthesis of original sulfonated
[19] E.J. Corey, G.H. Posner, R.F. Atkinson, A.K. Wingard, D.J. Halloran, D.M. Radzik,
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copolymers).
architectures
(sulfonated
multiblock
[20] M. Shirai, A. Kawaue, H. Okamura, M. Tsunooka, Photo-cross-linkable poly-
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4075e4081.
[21] S.C. Miller, Profiling sulfonate ester stability: identification of complementary
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Acknowledgements
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22] K. Lienkamp, I. Schnell, F. Groehn, G. Wegner, Polymerization of styrene sul-
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for Suzuki polycondensation, Macromol. Chem. Phys. 207 (2006) 2066e2073.
Financial support for this work was provided by the Agence
Nationale de la Recherche (ANR- MAMEIRIP).
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Appendix A. Supplementary data
7021e7032.
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