Convenient synthetic route to tetraarylphosphonium polyelectrolytes via palladium-catalyzed P–C coupling of aryl triflates and diphenylphosphine

Article abstract of DOI:10.1002/pola.28564

A series of eight tetraarylphosphonium polyelectrolytes (TPELs) has been successfully synthesized by polymerization of diphenylphosphine and bis(aryl triflate)s. The bis(aryl triflate)s are readily prepared from bisphenols, some of which are commodity fee

Full text of DOI:10.1002/pola.28564

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Convenient Synthetic Route to Tetraarylphosphonium Polyelectrolytes
via Palladium-Catalyzed P–C Coupling of Aryl Triflates and
Diphenylphosphine
Wang Wan,1 Xiaoyan Yang,1 Rhett C. Smith1,2
1Department of Chemistry, Clemson University, Clemson, South Carolina 29634
2Center for Optical Materials Science and Engineering Technologies, Clemson University, Anderson, South Carolina 29634
Correspondence to: R. C. Smith (E-mail: rhett@clemson.edu)
Received 5 January 2017; accepted 21 February 2017; published online 00 Month 2017
DOI: 10.1002/pola.28564
ABSTRACT: A series of eight tetraarylphosphonium polyelectro-
lytes (TPELs) has been successfully synthesized by polymeriza-
tion of diphenylphosphine and bis(aryl triflate)s. The bis(aryl
triflate)s are readily prepared from bisphenols, some of which
are commodity feedstocks such as bisphenol A. The polymeri-
zation via palladium catalyzed P–C bond formation produces
degrees of polymerization up to 65. All polymeric triflates have
reasonable thermal stability in the range of 350–450 8C. The
stability of the TPELs to alkaline solution is strongly depending
on the spacer between adjacent phosphonium sites. Polymers
with electron-releasing and bulky substituent para- to the phos-
phonium site have improved stability while those with
electron-withdrawing substituent para- to phosphonium site
have decreased alkaline stability due to decomposition via a
nucleophilic aromatic substitution pathway. These findings
have important ramifications for the design of ionomers for
alkaline exchange membrane fuel cells and related electro-
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chemical energy conversion devices. 2017 Wiley Periodicals,
Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: alkaline stability; ionomer; P–C coupling; phospho-
nium; polycondensation; polyelectrolytes; thermogravimetric
analysis (TGA)
two methods involving the much more efficient transition
metal-catalyzed P–C bond formation28,30 similar to the route
that is the focus of the current contribution.
INTRODUCTION Polyelectrolytes have been attracting tre-
mendous interest in recent decades due to the growing
range of applications available based on their intriguing and
unique properties.1 Phosphonium-derivatized polyelectro-
lytes in particular have been investigated for application in
diverse applications ranging from their use as antimicrobial
agents and biodelivery vectors,2–16 to materials and energy
applications such as alkaline fuel cell membranes,17–20
redox-active films,21,22 ordered optically active materials23–25
and optoelectronic devices.26
In 2015, we reported the first transition metal-catalyzed P–C
bond formation route to TPELs via coupling of bis(triaryl-
phosphine) and aryl dihalide monomers.28 The scope and
potential for commercialization of this methodology is limit-
ed by the expensive, multistep methodologies required to
prepare the requisite bis(triarylphosphine) monomers.31 We
recently reported a significantly simplified route to TPELs
via direct Ni- or Pd-catalyzed P–C coupling polymerization of
commercial aryl dihalides and diphenylphosphine [Scheme
1(C)].27 The use of aryl dihalides, however, is still relatively
expensive and the halide counteranions need to be
exchanged for other anions (i.e., triflate (–OTf) or ditriflamide
(NTf–2)) to obtain the most thermally robust materials. It is
thus of interest to develop direct routes to TPELs bearing
triflate couteranions in a single-step process.
Compared to polyelectrolytes bearing alkylphosphonium
sites, tetraarylphosphonium polyelectroplytes (TPELs) have
demonstrated significantly improved thermal and chemical
(particularly alkaline) stability.27,28
Because of the advantageous properties of TPELs and their
potential in practical applications, we were inspired to devel-
op simple and convenient polymerization routes to TPELs
based on transition metal-catalyzed P–C coupling.
Triflate is a common leaving group in well-established transi-
tion metal-catalyzed C–C coupling reaction,32,33 and more
recently has found use in small molecule P–C coupling as
There are at present three reported methods to prepare TPELs:
one requiring multistep postpolymerization modification,29 and
Additional Supporting Information may be found in the online version of this article.
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TABLE 1 General Properties of Triflate TPELs
Polymer
Mn (kDa)
Xn
Td ( 8C)
Tg ( 8C)
P1
P2
P3
P4
P5
P6
P7
P8
34.4
16.6
27.8
11.9
18.2
13.0
16.5
32.1
65
33
47
21
28
20
30
62
433
441
415
373
362
374
355
384
251
260
N/A
N/A
N/A
N/A
N/A
244
SCHEME 1 Established P-C coupling to triarylphosphine (A)
and tetraarylphosphonium salts (B). The reported coupling of
aryl bromides with HPPh2 (C) inspired the new route to poly-
meric triflates (D).
ditriflates via room-temperature esterification with triflic
anhydride is well established.37
Herein, we report the development of a new P–C coupling route
and its application to the preparation of eight structurally diverse
triflate TPELs. The relationship between structure and the ther-
mal and alkaline stability of the materials is also discussed.
well. In a 1996 report, Gilbertson et al. reported that aryl tri-
flates can be converted to aryldiphenylphosphine derivatives
in 80–90% yield employing a PdII-catalyst and diphenylphos-
phine [Scheme 1(A)].34 Furthermore, in 2007, Charette’s
group reported that functionalized aryl triflates can be cou-
pling efficiently with triphenylphosphine [Scheme 1(B)] to
yield tetraarylphosphonium salts via a catalytic cycle sum-
marized in Scheme 2.35,36
EXPERIMENTAL
General Considerations
All air-sensitive manipulations were performed in an MBraun
UNILab glovebox under nitrogen. Anhydrous solvents were
dried and degassed using an MBraun solvent purifier. NMR
spectra were collected on a Joel ECX-300 MHz spectrometer
operating at 300, 75.5, 282, and 121 MHz for 1H, 13C, 19F,
and 31P, respectively. Thermogravimetric analysis (TGA) was
performed on TA Instruments SDT Q600 from 25 to 800 8C
with a heating rate of 20 8C min21 under a N2 atmosphere.
Differential scanning calorimetry (DSC) was performed on
TA Instruments SDT Q600 with a heat/cool/heat cycle at 5 8C
min21, and the Tg data reported in Table 1 are for the second
scan (cooling and first/second heating curves are provided in
the Supporting Information Fig. S42). UV–vis absorption spectra
were acquired on a Cary 50 spectrophotometer. Phosphorus-31
NMR spectra were collected with a 10-s relaxation delay to
obtain accurate integration of main chain versus end group
Charette further revealed that these coupling reaction are
compatible with various functional groups including electron-
donating groups and electron-withdrawing groups at positions
para- and meta- to the coupling site. These promising small
molecule proof-of-principle studies inspired us to explore cou-
pling of aryl ditriflates with diphenylphosphine as a direct
route to triflate TPELs [Scheme 1(D)]. In addition to eliminat-
ing the anion exchange step, the triflate monomers can be
readily prepared from a wide variety of commercial bisphenol
compounds. Bisphenols (notable bisphenol A (BPA)) are
affordable and widely used in the industrial-scale production
of polymers, and their facile, quantitative conversion to
SCHEME 2 Proposed catalytic cycle for the P-C coupling reaction.
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resonances. The instrument was referenced to phosphoric acid
standard (0 ppm) and an additional PPh3 external standard
(24.70 ppm) was employed to account for instrument drift.
MHz, CDCl3, d): 148.3, 147.6, 129.1, 125.1, 121.3, 120.9,
116.7, 112.4, 46.1, 37.3, 26.1, 22.7
4,40-(9-Fluorenylidene)-Bis(Phenyl Triflate) (M5)
Yield: 6.04 g (98.4%). 1H NMR (300 MHz, CDCl3, d): 7.81–
7.84 (d, 2H), 7.26–7.47 (s, 10H); 7.15–7.18 (m, 4H); 13C
NMR (75.5 MHz, CDCl3, d): 149.6, 148.5, 145.8, 140.1, 128.3
(d), 125.9, 125.3, 121.3, 120.9, 120.6, 116.6, 112.2, 64.4
Materials
Anhydrous ethylene glycol (99.8%, Sigma Aldrich), anhydrous
pyridine (99.8%, Sigma Aldrich), 4,40-(9-fluorenylidene) diphe-
nol (97%, Sigma Aldrich), 4,40-(1-phenylethylidene) bisphenol
(99%, Sigma Aldrich), 4,40-cyclohexylidenebisphenol (98%, Sig-
ma Aldrich), 4,40-thiodiphenol (99%, Sigma Aldrich), tris(di-
benzylideneacetone)dipalladium(0) (97%, Sigma Aldrich), 4,40-
dihydroxydiphenyl ether (98%, TCI America), 4,4-dihydroxyte-
traphenylmethane (98%, TCI America), trimethylamine (99%,
TCI America), 4,40-sulfonyldiphenol (99.7%, ACROS), diphenyl-
phosphine (99%, STREM Chemicals), triflic anhydride (97%,
STREM Chemicals), lithium triflate (99%, STREM Chemicals),
and dichloromethane (99.5%, BDH Chemicals) were used with-
out further purification.
4,40-(Diphenylmethylene)-(bisphenyl triflate) (M6)
Yield: 6.13 g (99.4%). 1H NMR (300 MHz, CDCl3, d): 7.12–
7.36 (m, 18H); 13C NMR (75.5 MHz, CDCl3, d): 147.8, 146.6,
145.2, 132.8, 130.8, 128.0, 126.7, 120.8, 120.5, 116.6, 64.3
4,40-Sulfonyldiphenyl Bistriflate (M7)
Yield: 5.07 g (98.6%). 1H NMR (300 MHz, CDCl3, d): 7.45–
7.50 (m, 4H), 8.06–8.11 (m, 4H); 13C NMR (75.5 MHz, CDCl3,
d): 152.7, 141.0, 130.4, 125.0, 122.8, 120.7, 116.5, 112.2
4,40-Thiodipheyl Bistriflate (M8)
Yield: 4.79 g (99.2%). 1H NMR (300 MHz, CDCl3, d): 7.25–
7.30 (m, 4H), 7.40–7.45 (m, 4H); 13C NMR (75.5 MHz, CDCl3,
d): 148.8, 135.7, 132.6, 125.0, 122.5, 120.8, 116.6, 112.2
General Route for the Synthesis of Bis(triflate)
Monomers
The general procedure that follows is based on a reported
method.37 To a 100-mL round bottom flask were added the
requisite bisphenol (10.0 mmol) and anhydrous pyridine
(40.0 mmol) dissolved in 10 mL dichloromethane (DCM)
under a nitrogen atmosphere. The flask was cooled to 0 8C
in an ice bath for 10 min and then a 10-mL DCM solution of
triflic anhydride (40.0 mmol) was added slowly via cannula
under a positive pressure of N2. After stirring overnight, the
reaction mixture was cooled in an ice bath and 20 mL water
was added to quench any remaining triflic anhydride. The
mixture was extracted with 100 mL DCM. The DCM layer
was subsequently washed with 50 mL 2 M HCl(aq), 50 mL 1
M NaCl(aq) and 50 mL deionized water. The DCM phase was
concentrated to 10 mL under reduced pressure and further
purified by passage through a 5-cm high silica gel plug in a
30-mL fritted funnel to yield pure products. Yields and spec-
troscopic characterization data for each monomer follows.
General Procedure for Polymerization
The requisite bistriflate monomer (2.69 mmol), diphenyl-
phosphine (0.525 g, 2.82 mmol), diisopropylamine (0.274 g,
2.69 mmol) Pd2(dba)3 (0.0270 mmol, 0.0280 g), and 6 mL
anhydrous ethylene glycol were mixed together in a 15-mL
heavy-wall pressure tube sealed with a Teflon stopper and
Viton O-ring under a nitrogen atmosphere. The sealed vessel
was heated at 145 8C with stirring for 24 h. After cooling
down to room temperature, the reaction vessel was opened
to air and 50 mL DCM was added. The mixture was then
washed with 0.5 M LiOTf(aq) (2 3 50 mL). The addition of
triflate anions in the aqueous phase was undertaken as a
precaution against any adventitious anion exchange. The
organic layer was dried over anhydrous MgSO4 and concen-
trated to 10 mL under reduced pressure by rotary evapora-
tion. The crude solution was slowly poured into 200 mL
diethyl ether with stirring to yield a cloudy suspension. The
solid was separated by vacuum filtration and dried in a vac-
uum oven overnight at 60 8C to give the desired product.
The structure of each polymer is provided in Scheme 2 and
characterization for each member of the series follow.
4,40-(Propane-2,2-Diyl)-Bis(Phenyl Triflate) (M1)
1
Yield: 4.85 g (98.5%). H NMR (300 MHz, CDCl3, d): 7.16–7.30
(m, 8H), 1.69 (s, 6H); 13C NMR (75.5 MHz, CDCl3, d): 150.2,
147.8, 128.7, 125.2, 121.1, 120.9, 116.6, 112.4, 43.0, 30.8
4,40-Oxydiphenyl Bistriflate (M2)
Yield: 3.825 g (82.0%). 1H NMR (300 MHz, CDCl3, d): 7.05–
7.10 (m, 4H), 7.26–7.30 (m, 4H); 13C NMR (75.5 MHz, CDCl3,
d): 156.2, 145.3, 125.2, 123.2, 121.0, 120.4, 116.7, 112.5
P1
Yield: 0.947 g (69.4%). 1H NMR (300 MHz, (CD3)2CO, d):
8.15–7.42 (br m, 18H), 1.88–1.61 (br, 6H); 31P NMR (121
MHz, (CD3)2CO, d): 25.52 (end groups), 24.42–21.54 (back-
bone phosphonium moieties); 19F NMR (282 MHz, (CD3)2CO,
d): 278.44. Anal. calcd for monomer formula C28H24F3O3PS
(ignoring end groups): C, 63.63; H, 4.58; found: C, 63.39; H,
4.60%
4,40-(1-Phenylethylidene)-(Bisphenyl Triflate) (M3)
Yield: 4.89 g (88.5%). 1H NMR (300 MHz, CDCl3, d): 7.03–
7.35 (m, 13H), 2.20 (s, 3H); 13C NMR (75.5 MHz, CDCl3, d):
148.9, 148.0, 147.2, 130.5, 128.5 (d), 125.2, 121.0, 116.7,
112.4, 52.2, 30.7
P2
Yield: 0.680 g (51.1%). 1H NMR (300 MHz, (CD3)2CO, d):
8.03–7.69 (br m, 14H), 7.69–7.45 (br, 4H); 31P NMR (121
MHz, (CD3)2CO, d): 25.15 (ignoring end groups), 24.53–20.29
4,40-Cyclohexylidene-Bis(Phenyl Triflate) (M4)
Yield: 4.21 g (79.4%). 1H NMR (300 MHz, CDCl3, d): 7.16–
7.34 (m, 8H), 2.26 (d, 4H), 1,53 (br s, 6H); 13C NMR (75.5
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(backbone phosphonium moieties); 19F NMR (282 MHz,
(CD3)2CO, d): 278.52. Anal. calcd for monomer formula
C25H18F3O4PS (ignoring end groups): C, 59.76; H, 3.61;
found: C, 60.02; H, 3.70%.
P3
Yield: 1.50 g (92%). 1H NMR (300 MHz, (CD3)2CO, d): 8.05–
7.03 (br m, 23H), 2.36–2.13 (br, 3H); 31P NMR (121 MHz,
(CD3)2CO, d): 24.86 (end groups), 23.60–21.46 (backbone
phosphonium moieties); 19F NMR (282 MHz, (CD3)2CO, d):
278.47. Anal. calcd for monomer formula C33H26F3O3PS
(ignoring end groups): C, 67.11; H, 4.44; found: C, 66.88; H,
4.50%.
P4
Yield: 1.29 g (81.4%). 1H NMR (300 MHz, (CD3)2CO, d):
8.13–6.95 (br m, 18H), 2.54–2.19 (br, 4H), 1.65–1.24 (br,
6H); 31P NMR (121 MHz, (CD3)2CO, d): 25.35 (end groups),
23.34–22.76 (backbone phosphonium moieties); 19F NMR
(282 MHz, (CD3)2CO, d): 278.49. Anal. calcd for monomer
formula C31H28F3O3PS (ignoring end groups): C, 75.33; H,
7.71; found: C, 74.13; H, 7.89%.
P5
Yield: 1.120 g (64.1%). 1H NMR (300 MHz, (CD3)2CO, d):
8.23–6.82 (br m, 26H); 31P NMR (121 MHz, (CD3)2CO, d):
25.35 (end groups), 24.13–21.98 (backbone phosphonium
moieties); 19F NMR (282 MHz, (CD3)2CO, d): 278.42. Anal.
calcd for monomer formula C38H26F3O3PS (ignoring end
groups): C, 70.15; H, 4.03; found: C, 69.37; H, 4.27%.
P6
SCHEME 3 Synthesis of polymeric triflates.
1
Yield: 1.50 g (85.5%). H NMR (300 MHz, (CD3)2CO, d): 8.09–
7.42 (br, 18H), 7.37–7.02 (br, 10H); 31P NMR (121 MHz,
(CD3)2CO, d): 26.21 (end groups), 23.61–21.20 (backbone phos-
phonium moieties); 19F NMR (282 MHz, (CD3)2CO, d): 278.48.
Anal. calcd for monomer formula C38H28F3O3PS (ignoring end
groups): C, 69.93; H, 4.32; found: C, 69.38; H, 4.64%.
the lower part of the inner wall of the flask. The polymer film
was soaked in 1 M NaOH solution for 24 h. The organic mate-
rial was extracted into 0.3 mL CH2Cl2, and 0.2 mL (CD3)2CO
was added. This solution was characterized by NMR spec-
trometry. The protocol was repeated at 65 8C, with the only
difference being that the solution was cooled to room temper-
ature prior to extraction. Each trial of alkaline stability test
was done in duplicate and the percentage of decomposition
(% decomp.) data provided in Table 3 are the average values.
P7
Yield: 0.719 g (50.6%). 1H NMR (300 MHz, (CD3)2CO, d):
8.52–6.94 (br m, 18H); 31P NMR (121 MHz, (CD3)2CO, d):
26.69 (end groups), 25.32–22.59 (backbone phosphonium
moieties); 19F NMR (282 MHz, (CD3)2CO, d): 278.63. Anal.
calcd for monomer formula C25H18F3O5PS2 (ignoring end
groups): C, 54.54; H, 3.30; found: C, 57.02; H, 3.82%.
RESULTS AND DISCUSSION
Synthetic Design and Characterization
The goal of the current work was to develop a P–C bond-
forming route for the convenient preparation of tetraarylphos-
phonium polyelectrolytes (TPELs) using bis(aryl triflate)s and
diphenylphosphine as the monomers.
P8
Yield: 0.424 g (30.4%). 1H NMR (300 MHz, (CD3)2CO, d):
8.06–7.30 (br m, 18H); 31P NMR (121 MHz, (CD3)2CO, d):
25.51 (end groups), 24.62–22.42 (backbone phosphonium
moieties); 19F NMR (282 MHz, (CD3)2CO, d): 278.52. Anal.
calcd for monomer formula C25H18F3O3PS2 (ignoring end
groups): C, 57.91; H, 3.50; found: C, 59.47; H, 3.71%.
On the basis of prior work in which different catalysts were
evaluated for the preparation of TPELs,27 tris-(dibenzylide-
neacetone) dipalladium(0) (Pd2(dba)3) was selected for the
current work as well. Eight bisphenols bearing different
functional groups were selected as starting materials for the
bis(aryl triflate) monomers (Scheme 3). In order to evaluate
the scope of the synthetic route, monomers that contribute
electron-releasing substituents (as in P2 and P8), electron-
Procedures for Alkaline Stability Testing
To a 50-mL round bottom flask was added a solution of
20 mg polymer powder dissolved in 0.2 mL DCM. The solu-
tion was evaporated slowly to coat a film homogeneously on
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TABLE 3 Stability of Polymeric Triflates Upon Exposure to
Aqueous Alkaline Solutions for 24 h
Polymer
% Decomp. (20 8C)
% Decomp. (65 8C)
P1
P2
P3
P4
P5
P6
P7
P8
11
4
67
18
72
23
22
10
88
60
10
23
20
3
SCHEME 4 Features of TPELs showing the formation of the
phosphine oxide end groups used in calculation of degree of
polymerization.
55
17
withdrawing groups (as in P6), and a range of steric hin-
drance to conformational freedom (i.e., P1 vs. P3-5) were
targeted.
both aryl triflate end groups (evident in 19F NMR spectra)
and phosphine oxide (evident in 31P NMR spectra) were
observed. To simplify NMR endgroup analysis, a slight excess
of diphenylphosphine monomer was employed, thus insuring
that the end groups will be comprised almost entirely of
phosphine moieties (confirmed by NMR spectra provided in
the Supporting Information) that, upon aerobic workup, are
transformed into phosphine oxide units (Scheme 4). This end
group composition allowed for convenient determination of
Mn via 31P NMR end group analysis (Table 1). Accurate 31P
NMR integrations were obtained by using a 10-s delay
between scans following the reported procedure.30 As
expected, each polymer’s 31P NMR spectrum features a major
resonance consistent with main chain phosphonium salt and a
significantly smaller resonance consistent with a phosphine
oxide. The main chain phosphonium resonance for P1–P8 is
in the range of 20.29–25.32 ppm (Table 2) and the phosphine
oxide end group is in the range of 24.86–26.69 ppm. The
phosphonium and phosphine oxide resonances of P7 are both
slightly down field of those for other known TPELs due to the
presence of the electron-withdrawing sulfonyl substituent
para- to the phosphorus nuclei.
Polymerization reactions were carried out in anhydrous eth-
ylene glycol because of its success in high-yield P–C coupling
to form small molecular tetraarylphosphonium salts and its
ability to solubilize previously reported TPELs.2,36,38
Polymerization of diphenylphosphine (1.05 equiv) with M1-
M8 (1.00 equiv) catalyzed by Pd2(dba)3 at 145 8C in ethylene
glycol thus led to successful preparation of the target triflate
TPELs as summarized in Scheme 3.
1
The polymers were initially characterized via H, 19F, and 31P
NMR spectra. The absence of a 31P NMR resonance for diphe-
nylphosphine (a doublet at 241.00 ppm)39 in crude mixtures
confirmed the consumption of HPPh2. Likewise, 19F NMR
spectrometry confirms complete consumption of aryl-bound
triflate units (appearing at about 274.0 ppm) and that ionic
triflate groups (appearing at about 278.5 ppm)36,37 are the
only fluorine-containing species detectable by NMR analysis
(see Supporting Information Figs. S25–32).
Polyelectrolytes and related polymers can have very high
affinity to GPC columns, sometimes requiring highly polar
solvents such as formic acid for their elution.28,40,41 Follow-
ing the techniques previously reported,42–44 NMR endgroup
analysis was therefore selected as a convenient method to
determine the Mn of the TPELs.
The observed Mn values (Table 1) span a range from ꢀ12 to
34 kDa, similar to those for TPELs prepared by coupling of
HPPh2 with aryl bromides. The variability of Mn for the iso-
lated polymers appears to depend more on the relative solu-
bility of lower molecular weight fractions that are removed
in the precipitation step than on the efficiency of the poly-
merization process itself, as crude analysis suggests that pol-
ymers as-formed have Mn of about 20 prior to workup. An
Mn of about 20 is expected on the basis of the monomer
feed ratios employed, assuming an essentially quantitative
coupling efficiency. Polymer P2 was identified as a particu-
larly attractive candidate for scaleup, so a 10-g sample of P2
was prepared to assess the scalability of the process. The
synthetic efficiency and molecular weight of the larger batch
of P2 were essentially identical to those of the smaller batch
(see Supporting Information Fig. S42B).
When equimolar amount of the bis(aryl triflate) and diphenyl-
phosphine monomers were employed in the polymerization
TABLE 2 Phosphorus-31 NMR Chemical Shifts for P1-P8
Polymer
d of Phosphonium (ppm)
d of End Group (ppm)
P1
P2
P3
P4
P5
P6
P7
P8
22.87
23.21
23.08
22.76
23.02
22.64
24.41
23.55
25.52
25.15
25.32
25.35
25.35
26.21
26.69
25.51
Thermal Stability and Transitions
Davis and coworkers45 demonstrated that tetraarylphospho-
nium salts are the most thermally stable ionic liquids known,
so it was of interest to evaluate the thermal stability of P1-
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leaving group positioned either ortho- or para- to a strongly
electron-withdrawing substituent on an aryl ring. This is the
situation in P7, which consequently undergoes the greatest
decomposition, with 55% decomposition occurring after only
24 h at room temperature, which we attribute to via the path-
way shown in Scheme 5(B). Decomposition of related ammo-
nium polyelectrolytes has been misattributed to the inherent
instability of sulfone units to hydroxide.51 In contrast to P7,
P2 has electron-releasing units para- to the phosphonium
center and has significantly improved stability under alkaline
conditions.
UV–Vis Absorption
The maximum absorption peaks in the UV–vis absorption
spectra for P1-P8 are located in the 220–230 nm range,
attributable to the p-p* transition of the backbone aryl
units.38 These data agree with the fact that TPELs do not
exhibit extended p-conjugation along the backbone.27 In
addition to the p-p* transition, the spectra for P2 and P8
exhibit absorptions at 268 and 300 nm, respectively. These
can be attributed to n-p* transitions involving the oxygen-
(P2) or sulfur- (P8) centered lone pairs.
SCHEME 5 Mechanism of decomposition for some phosphonium
segments.
P8, and to study the effect of different repeat unit composi-
tion on thermal stability. Thermogravimetric analysis (TGA)
was employed to determine the decomposition temperature
(Td) for each polymer. These data are summarized in Table 1.
CONCLUSIONS
Our previous work revealed that the counteranion has a sig-
nificant effect on thermal stability of TPELs.27 In one case,
for example, the TPEL having ditriflamide counteranions
exhibited a Td of ꢀ460 8C while its bromide analogue had a
Td of 400 8C. In addition to the counteranions, thermal sta-
bility should also be affected by the backbone composition.
A P–C bond-forming route to TPELs from bis(aryl triflates)
and diphenylphosphine is reported. Eight triflate TPELs have
been prepared via this route and their thermal and alkaline
stability has been assessed. The polymerization route has
been proven compatible and effective for a range of functional
groups of interest in polymeric materials, including a sulfone,
ether, and thioether. The TPELs have good thermal stability
up to 441 8C. A relationship between the chemical composi-
tion of the backbone and the susceptibility of these materials
to decomposition under alkaline conditions has been revealed.
Tests of these materials as anion exchange membranes for
alkaline fuel cell applications are currently underway.
Neutral polymers bearing spacers identical to those between
phosphonium units in P1-P8 all have excellent thermal sta-
bility (Td > 400 8C)46–50 and so their presence was not
expected to diminish the inherent thermal stability of their
composite TPELs. As anticipated, the Td values for P1-P8 are
all relatively high, ranging from 355 to 441 8C. The lowest
thermal stability is for P7. The diminished stability is attrib-
utable to the presence of two strongly electron-withdrawing
groups on the same phenylene unit, an effect that has been
observed in similarly substituted TPELs previously.28
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