Regioregular poly(thiophene-3-alkanoic acid)s: Water soluble conducting polymers suitable for chromatic chemosensing in solution and solid state

Article abstract of DOI:10.1016/j.tet.2004.08.064

Regioregular polythiophenes with pendant carboxylic acid functionality, poly(thiophene-3-propionic acid) (PTPA, 3) and poly(thiophene-3-octanoic acid) (PTOA, 4), were prepared as water soluble conducting polymers and their chemosensory response was studied. Treatment with aqueous base generated intensely colored water soluble conducting polymer salts, with the color varying both as a function of counter ion size and length of the carboxyalkyl substituent. PTPA showed a different colorimetric response to each of the alkali earth metals whereas the longer chain PTOA was only sensitive to ions larger than Et₄N⁺. Distinct color changes were also noted in studies of divalent cations known to selectively bind carboxylates, such as Zn²⁺, Mn²⁺, and Cd²⁺. Cast films of PTPA have been found to act as sensors to acid vapor (Δλ_max of up to 132 nm). IR shows that the polymer self-assembles upon HCl vapor exposure, and that the process is completely reversible, leading to a good solid-state sensor for acids. This work demonstrates that regioregular polythiophenes containing acid side-chains have tremendous potential in the development of new sensors. The self-assembly and disassembly of regioregular poly(thiophene-3- alkanoic acid)s were driven by the analytes. The conformational changes resulted in clear visual color changes. These polymers show great potential in the development of sensors.

Full text of DOI:10.1016/j.tet.2004.08.064

Tetrahedron 60 (2004) 11269–11275
Regioregular poly(thiophene-3-alkanoic acid)s: water soluble
conducting polymers suitable for chromatic chemosensing in
solution and solid state
Paul C. Ewbank, Robert S. Loewe, Lei Zhai, Jerry Reddinger, Genevi e` ve Sauv e´
*
and Richard D. McCullough
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave., Pittsburgh, PA 15213, USA
Received 26 March 2004; revised 2 July 2004; accepted 20 August 2004
Available online 23 September 2004
Abstract—Regioregular polythiophenes with pendant carboxylic acid functionality, poly(thiophene-3-propionic acid) (PTPA, 3) and
poly(thiophene-3-octanoic acid) (PTOA, 4), were prepared as water soluble conducting polymers and their chemosensory response was
studied. Treatment with aqueous base generated intensely colored water soluble conducting polymer salts, with the color varying both as a
function of counter ion size and length of the carboxyalkyl substituent. PTPA showed a different colorimetric response to each of the alkali
C
4
earth metals whereas the longer chain PTOA was only sensitive to ions larger than Et N . Distinct color changes were also noted in studies
2
C
2C
2C
of divalent cations known to selectively bind carboxylates, such as Zn , Mn , and Cd . Cast films of PTPA have been found to act as
sensors to acid vapor (Dlmax of up to 132 nm). IR shows that the polymer self-assembles upon HCl vapor exposure, and that the process is
completely reversible, leading to a good solid-state sensor for acids. This work demonstrates that regioregular polythiophenes containing acid
side-chains have tremendous potential in the development of new sensors.
q 2004 Elsevier Ltd. All rights reserved.
1
8–20
1
. Introduction
electrical potential),
temperature),
moieties),
thermochromism (changes in
ionochromism (changes in chemical
6
,21–25
1
2,26–30
6,31–35
Due to their excellent electrical properties, regioregular
poly(3-alkylthiophene)s are rapidly being developed into
new products with applications such as field effect
transistors, sensors, solar cells and new electrostatic
dissipative coatings and materials.
ductivities and low band gaps are a consequence of their
biochromism,
and affinity
3
6
chromism. Chromatic responses are particularly useful
since discernable changes are generally apparent to the
human eye, obviating the need for expensive instrumenta-
tion and really represent an opportunity for chemists to
1
–8
Their high con-
6
,37,38
develop new sensors. We are particularly interested in
developing sensors based on head-to-tail regioregular
ability to self-assemble into p-stacked, planar aggre-
Previous work in our lab has shown that both
9
–11
10
polythiophenes (rr-PTs). These polymers do not have
gates.
conformational order and solid state organization in
regioregular polythiophene derivatives are remarkably
sensitive to the placement and nature of the substituent
coupling defects, have a tendency to form elongated
supermolecular structures with optimal orbital overlap and
3
9–41
small HOMO–LUMO gaps,
and should exhibit a very
1
2,13
chains.
design new conducting polymers in which self-assembly
and hence the color) is controlled by environmental factors.
This sensitivity provides the opportunity to
wide response range between the minimally and maximally
perturbed states, providing for excellent sensitivity.
Of rr-PTs, the water soluble salts of carboxylic acid
functionalized regioregular polythiophenes demonstrate one
(
Changes in conformation of conjugated systems are readily
detected not only by conductivity changes, but also by
optical absorption changes. Various polythiophene deriva-
tives are known to show striking chromatic responses, such
2
6
of the largest optical shifts in response to ions. Here, we
study the ionochromism of these polymers with the intent to
understand the underlying mechanism of chromatic
changes. Two representative polymers with differing
substituent lengths were selected: head-to-tail coupled
poly(thiophene-3-propionic acid) (PTPA) and head-to-tail
coupled poly(thiophene-3-octanic acid) (PTOA). The
synthesis, characterization, and studies of ion binding by
UV–vis and IR spectroscopy are reported.
6
,14,15
as piezochromism (changes in pressure),
mism (changes in light),
photochro-
electrochromism (changes in
6
,16,17
Keywords: Conducting polymers; Polythiophenes; Sensors.
*
Corresponding author. Tel.: C1-4122685124; fax: C1-4122686897;
e-mail: rm5g@andrew.cmu.edu
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.08.064

11270
P. C. Ewbank et al. / Tetrahedron 60 (2004) 11269–11275
Scheme 1. Synthesis of the water soluble regioregular polythiophenes.
C K
Table 1. UV/vis lmax (nm) of PTPA and PTOA in water with 1 equiv of base (R OH ) per carboxylic acid residue
C
4
C
N
C
C
C
C
C
Na
C
K
C
Cs
Polymer
NH
Me
4
Et
4
N
Pr
4
N
Bu
4
N
Li
PTPA (3)
PTOA (4)
550
—
486
556
476
554
446
534
426
513
546
556
532
554
512
554
506
—
2
. Results and discussion
longer chains can more easily accommodate the steric bulk
of the substituent without compromising the p-stacked
structure of the polymer.
2
.1. Synthesis
The following results further support the role of steric bulk
in the observed ionochromism. (1) All the solutions (in
Table 1) became yellow when treated with a vast excess of
base. (2) In a separate experiment, the yellow phase was also
obtained by titrating a solution of PTPA and 1 equiv KOH
(per carboxylic acid residue) with 18-Crown-6. This
effectively changes the steric bulk of the cation (by
complexation) without changing the ionic strength of the
solution. A twisted, yellow phase became more pronounced
as the concentration of crown ether increased, while the
absorption at longer wavelength decreased. A clear
isosbestic point was observed around 420 nm.
Regioregular (rr) head-to-tail (HT) poly(thiophene-3-pro-
pionic acid) (PTPA, 3) was synthesized by Stille/CuO
polymerization of an oxazoline protected monomer
followed by acid hydrolysis to afford a violet solid in
2
6
yield of greater than 80%. Regioregular head-to-tail
poly(thiophene-3-octanoic acid) (PTOA, 4), was prepared
4
2
as previously described. Although the polymers were not
very soluble in common organic solvents, they were both
soluble in aqueous base (species 3a and 4a) (Scheme 1).
2
.2. Spectroscopic behavior in solution
Intensely colored solutions were formed when PTPA (3) or
PTOA (4) reacted with 1 equiv alkali metal hydroxide or
tetraalkylammonium hydroxide per carboxylic acid group,
with colors ranging from violet to yellow depending on the
size of the counter ion (Table 1). For PTPA, the observed
lmax varied over a range of w124 nm when the size of the
All the solutions described in Table 1 were stable, except for
C
NH . For solutions of PTPA with 1 equiv of NH OH, the
4
4
polymer tended to precipitate when the solutions were left
undisturbed for several hours. This indicates that the
polymer forms large aggregates. We believe that the
C
C
ammonium counterion increased from NH4 to Bu N , and
over a range of w40 nm when the alkali metal size
increased from Li to Cs . For PTOA, on the other hand,
4
C
C
the observed lmax did not deviate much from 556 nm (violet
color) until the ion size exceeded Et N . Even with the
C
4
C
larger Bu N , the l
blue shifted by only 43 nm.
max
4
These results are explained as follows. Varying the size of
the counterion in the carboxylate polymer changes the
effective steric bulk of the substituent. Small cations favor a
planar, p-stacked purple phase while large cations can
prevent planarity and self-assembly to give a twisted,
isolated yellow phase (Fig. 1). When a longer alkyl side
chain was used, such as in PTOA, the chemoselectivity was
Figure 1. Polythiophene zipper sensors: analyte driven disassembly and
self-assembly.
C
C
limited to the larger cations (Pr N and Bu N ) because
4
4

P. C. Ewbank et al. / Tetrahedron 60 (2004) 11269–11275
11271
quite strong and could be used as a qualitative sensor to
detect the presence of divalent cations.
In an effort to better understand the self-assembly by
divalent cations, we attempted to get X-ray spectra of the
filtered solids (for the samples in Table 2). While the X-ray
study revealed several peaks in all cases, we could not easily
discern structural order in any of the powder samples.
Alternatively, we also tried to study the aggregation by
UV–vis in solution, but this tended to be difficult due to the
formation and precipitation of coordination polymers at
very low cation concentrations. We were able to titrate a
solution of PTOA in DMF with small quantities of aqueous
Figure 2. UV/vis of PTPA in water as a function of added NH
equiv; (B) 10 equiv; (C) 15 equiv; (D) 25 equiv of base per carboxylate
group.
4
OH. (A)
CdCl without precipitation (up to 0.4 equiv). The UV–vis
2
spectra are shown in Figure 3. Aggregation was promoted,
as indicated by the increasing absorption at 610 nm, while
1
4
Table 2. Summary of results obtained when we added divalent metals to a red solution of PTPA and Et NOH
Divalent metal added
Observations
2
Mn , Mg , Fe
C
2C
2C
Solutions turned purple, formation of a purple precipitate
Solutions were red to purple, formation of magenta to brown solids
2
C
2C
2C
2C
2C
Cd , Cu , Zn , Hg , Co , Ni
2C
ammonium cation is involved in the assembly of this
ordered, aggregated phase, for example, by hydrogen
bonding or salt pairing, as indicated by IR experiments
the proportion of disordered phase decreased. Polymer
PTOA could therefore be a potential candidate for
semiquantitative sensing of, for example, dangerous
divalent cations in drinking water.
C
(
not large enough to disrupt the ordered phase. However,
discussed later). One could argue that the NH4 cation is
C
solutions of PTPA with Li , a smaller cation than NH , are
C
4
2.3. Spectroscopic behavior in the solid state
quite stable.
Thin films of PTPA and PTOA with ammonium hydroxide
and with tetraalkylammonium hydroxide were prepared on
quartz and the UV–vis spectra were examined (Table 3).
Colors were similar to the parent solution, with larger ions
yielding a larger hypsochromic shift. We can therefore
conclude that the polymer structure in the films is similar to
that in solution.
C
The aggregates of PTPA with NH4 can disassemble when
large excess of base are added to the solution. For example,
solutions that contain a large excess of NH OH (w10 equiv
4
or greater) did not precipitate and solutions remained
homogeneous indefinitely. This suggests that at higher
concentrations of NH OH, steric or ionic repulsive
4
interactions overcame some of the attractive inter-chain
interactions and allowed for the rapid solvation in water.
This is quite similar to deaggregation of proteins in water.
Figure 2 shows the titration of an aqueous solution of PTPA
In order to gain more information about the polymer
structure and the nature of chromatic changes discussed
earlier, we analyzed films of PTPA with the ammonium
salts by IR spectroscopy. In the case of tetraalkylammonium
salts of PTPA, the IR spectra (Fig. 4) showed the presence of
with NH OH. Upon addition of excess ammonium
4
hydroxide, the ordered purple phase is depleted and a
higher energy non-aggregated and/or twisted (coiled) phase
is produced. The conformational change from planar to
twisted appears to occur via a cooperative mechanism, as
shown by the presence of a clear isosbestic point. The UV–
K1
a carboxylate anion (1575 cm ) and the absence of
vis spectra of PTPA with NH OH also showed a significant
4
absorption past 800 nm, which slowly decreased with
increasing NH OH concentration. The origin of this
4
absorption band was not studied, but suggests the presence
of self-doping in the aggregated phase.
We found that divalent metal cations can ‘zip up’ the
disordered solvated polymers back into an ordered phase
(
Fig. 1). When divalent cations were added to an aqueous
basic solution of PTPA (or PTOA), a precipitate was
formed, indicating aggregation (Table 2). Divalent metal
cations can form coordination complexes with the polymers
by binding to two carboxylate groups either intra- or inter-
molecularly, and the latter is expected to drive self-
assembly. The inter-chain attractive force is therefore
Figure 3. Titration of a 10 mM solution of PTOA in DMF with a 0.5 mM
aqueous solution of CdCl
- - -) to 0.4 equiv of Cd
formed.
2
. The amounts of added Cd Cl
2
2 2
Cl . At 0.45 equiv of CdCl , a precipitate was
2
went from 0.05
(
2

11272
P. C. Ewbank et al. / Tetrahedron 60 (2004) 11269–11275
a
Table 3. UV/vis lmax (nm) of PTPA and PTOA thin films
C
C
C
C
C
Polymer
H
4
N
Me
4
N
Et
4
N
Pr
4
N
4
Bu N
PTPA (3)
PTOA (4)
531
b
504
550
489
550
450
530
428
499
a
C
K
Cast from solutions in water with 1 equiv of base (R OH ) per carboxylic acid group.
Not measured. We expect a value around 550 nm.
b
through this color change (Dlmax up to 130 nm) without
noticeable degradation of the spectrum. This reversible self-
assembly and disassembly could provide a reliable, cheap
method for optical detection of acidic vapors.
In contrast, films made from a solution of PTPA with
1
the IR spectra revealed the presence of both hydrogen
equiv NH OH were purple (ordered phase). Analysis of
4
K1
bonded carboxylic acid (1709 cm ) and carboxylate anion
K1
1550 cm ) (Fig. 7A). Significant hydrogen bonding is
(
therefore present at equilibrium and influences the
molecular ordering.
To see if we could disassemble the ordered state with excess
base, dried films cast from the 1:1 solution of PTPA/NH OH
were exposed to ammonia vapor. Exposing the films to NH
Figure 4. IR spectra of films of PTPA with 1 equiv of (A) Me
Et NOH; (C) Bu NOH; and (D) Pr NOH.
4
NOH; (B)
4
4
4
4
3
C
vapor essentially temporarily adds more NH4 into the film.
K1
hydrogen bonded carboxylic acid (1710 cm ). These
results indicate that these films are completely deprotonated
and that hydrogen bonding does not play a significant role in
the structure.
The IR spectra obtained showed that the carboxylate stretch
K1
at 1550 cm
K1
intensified while the carbonyl stretch at
709 cm was suppressed (Fig. 7B). These films reverted
1
We then exposed these disordered films to HCl vapor to see
if carboxylic acid groups would form hydrogen bonds and
cause the polymers to self-assemble. To our amazement, all
four films quickly changed from red, orange or yellow
(
depending on the cation) to purple upon exposure to HCl
vapor, even though the polymers were in the solid state
Fig. 5). Furthermore, IR showed that the carboxylate
(
K1
stretch (1565 cm ) disappeared and that the hydrogen-
K1
bonded carbonyl stretch (w1725 cm ) appeared upon
exposure to HCl (Fig. 6). Protonation of the carboxylate
groups therefore allowed the polymer chains to relax from a
twisted conformation to a violet p-stacked conformation
with H-bonding, essentially zipping the polymer up into an
ordered phase (Fig. 1). When the films were left in air, they
reverted back to their original color and IR characteristics
due to HCl desorption. Films could be cycled several times
4
Figure 6. IR spectra of 3 with Bu NOH (A) before and (B) after exposure to
HCl.
Figure 5. UV–vis spectra of films of alkylammonium hydroxides after
exposure to HCl vapor. (A) Bu NOH; (B) Pr NOH; (C) Et NOH.
Figure 7. IR spectra of PTPA with NH
exposure to NH
4
OH (A) before and (B) after
4
4
4
3
.

P. C. Ewbank et al. / Tetrahedron 60 (2004) 11269–11275
11273
to their original IR characteristics upon standing, due to
NH desorption. Exposure to ammonia therefore removed
polythiophene containing acid side-chains therefore show
tremendous potential for the development of new sensors.
3
the H-bonds. However, the film color did not change,
suggesting that the ordered phase was not affected. We
surmise that salt-pairing through the ammonium cation
might play a role in the ordered phase. The absence of a
chromatic change in this experiment is quite unlike the
4. Experimental
4.1. General
behavior of aqueous solutions treated with excess NH OH
4
(
Fig. 2). This suggests that a change in solvation of the
All reactions were performed under prepurified nitrogen,
using dry glassware. Glassware was either dried in an oven
overnight or flame dried and then cooled under a stream of
nitrogen. Tetrahydrofuran was dried over Na benzophenone
ketyl radical and freshly distilled prior to use. Diisopropyl-
polymer system is important in the disassembly process in
solution.
2
.4. X-ray of PTPA and PTOA
amine was dried over CaH and distilled prior to use.
2
Bao and Lovinger have previously reported PTPA to be
amorphous, though slight crystallinity could be obtained by
slow evaporation of a DMSO solution. They observed a
All UV–vis spectra were recorded using a Perkin–Elmer
Lamda 900 UV/vis/near-IR spectrometer. NMR spectra
were recorded on an IBM Bruker FT300 spectrometer.
Infrared spectra were obtained using a Nicolet 7199 FTIR.
The thin films of PTPA salts (3a) were prepared by casting a
solution of PTPA with 1 equiv of NH OH per carboxylic
4
acid group onto 3M Teflon IR cards and dried overnight in
air. The GPC measurements were carried out on a Waters
4
3
˚
weak peak at 11.5 A, attributed to the lateral separation of
polymer chains by substituents. We have found that
crystalline X-ray can be observed upon careful preparation
of the films, however most samples do not form good films
as the acid polymer. The X-ray spectra of a thin film of
PTOA (4) drop cast from DMF is shown in Figure 8. Three
peaks were observed in the 2q scan of the polymer. The
2
690 separation module with two 5 mm Phenogel columns
˚
˚
connected in series (guard, 1000 and 100 A), and a Waters
487 dual l absorbance UV detector. Analyses were
broad reflection, corresponding to 3.7 A distance, is
assigned to the p–p stacking distance between the
2
performed at 30 8C using chloroform as the eluent and the
flow rate was 1.0 mL/min. The calibration was based on
polystyrene standards (Polymer Standards Service). GC–
MS analysis was performed on a Hewlett–Packard 59970
GC–MS workstation with a HP fused silica capillary
column cross-linked with 5% phenyl-methylsilicone as the
stationary phase. The X-ray diffraction spectra were
obtained with a Rigaku q/q X-ray diffractometer utilizing
Cu Ka source operating at 35 kV and 25 mA, a curved
graphite diffracted beam monochromator and a NaI
scintillation detector. Data was gathered from 3 to 608,
with steps of 0.058 and 2 s. Elemental Analysis was
performed by Midwest Microlab, Indianapolis, IN.
polythiophene chains. Although this is smaller than the
3
4
4
˚
.8 A reported for HT-PAT, the resolution is too poor to
draw any definitive conclusions. The two sharp reflections
correspond to the lamellar interlayer spacing of the
˚
polymer chains. The observed distance of 20.5G0.2 A
˚
compares favorably with 20.8 A previously reported for
HT-poly(3-octylthiophene).
4
4
4.2. Synthesis
The synthesis of HT-poly(3-(7-(4,5-dihydro-4,4-dimethyl-
2-oxazolyl)heptyl)thiophene) (1), and HT-poly(thiophene-
4
2
3-octanoic acid) (3) are reported in the literature.
4
.2.1. 2-Bromo-3-(bromomethyl)thiophene. Caution!
Repugnant, lachrymatory compound. This compound
4
has been described previously. This procedure has been
5
Figure 8. X-ray spectrum of PTOA drop cast from DMF.
adapted to utilize benzene rather than CCl as the solvent.
4
NBS (75.85 g, 427 mmol) was suspended in anhydrous
benzene (250 mL) and heated to reflux. 2-Bromo-3-
methylthiophene (75.0 g, 424 mmol) and AIBN (0.97 g,
5.9 mmol) were added, and the suspension was heated to
reflux for 45 min. The crude reaction mixture was cooled to
K40 8C to precipitate some of the succinimide, and this was
filtered off. The solvent was removed from the filtrate,
causing more succinimide to precipitate. This crude oil was
redissolved in excess hexanes and filtered through a pad of
silica using hexanes as the eluent to remove remaining
succinimide. The solvent was removed and the crude oil was
3
. Conclusions
In this work, we have explored the sensing properties of
PTPA and PTOA in both basic solutions and thin films. In
addition to ionochromism in basic solution with monovalent
cations, these polymers are also quite sensitive to the
presence of divalent metal cations. Furthermore, films cast
from a solution of PTPA and 1 equiv Bu NOH underwent a
4
dramatic visual color change from yellow to purple upon
HCl vapor exposure. We expect that other detection
methods such as fluorescence, conductivity and FET
sensing could be used with these materials. Regioregular
distilled (64 8C, 0.5 T) to recover the product in 80% yield
(86.8 g, 339 mmol). H NMR (300 MHz, CDCl ): d 4.46
1
3

11274
P. C. Ewbank et al. / Tetrahedron 60 (2004) 11269–11275
1
s, 2H), 7.00 (d, 1H) 7.26 (d, 1H). C NMR (75 MHz,
3
(
CDCl ): d 25.7, 113.0, 126.4, 128.2, 136.9.
acetone)dipalladium(0) (336 mg, 0.05 equiv) was added,
followed quickly by copper(II)oxide (367 mg, 1.0 equiv),
and triphenylphosphine (570 mg, 0.20 equiv). The solution
was warmed to 100 8C and allowed to stir for 2 days. The
reaction was quenched by pouring into methanol. The
resulting solid was filtered and transferred into a Soxhlet
thimble. The sample was extracted with methanol until the
eluent was colorless, then was recovered by extraction with
chloroform. The solvent was removed and the resulting
purple solid was dried under vacuum overnight to recover
3
4
4
.2.2. 2-(2-(2-Bromothiophen-3-yl)ethyl)-4,5-dihydro-
,4-dimethyloxazole. 2,4,4-Trimethyl-2-oxazoline
(
7.0 mL, 54.7 mmol) was dissolved in THF (50 mL) and
cooled to K70 8C. Butyllithium (2.62 M, 20 mL,
2.4 mmol) was added dropwise over a 10 min period
5
while maintaining a temperature below K50 8C. After the
mixture was stirred for 15 min, 2-bromo-3-(bromo-
methyl)thiophene (14.37 g, 56.11 mmol) was added pre-
cipitously. The reaction was extremely exothermic, heating
to 25 8C despite the cooling bath. The color varied from red
to yellow. The cooling bath was removed and the reaction
mixture was stirred for 20 min. The reaction was quenched
1
the polymer (1.075 g, 5.19 mmol) in 76% yield. H NMR
(300 MHz, CDCl
3.94 (s, 2H), 7.11 (s, 1H). C NMR (75 MHz, CDCl
26.4; 29.2; 29.6; 68.0; 80.0; 129.8; 132.4; 134.4; 138.4;
166.7. M Z8K, PDIZ1.2 (by GPC).
): d 1.31 (s, 6H), 2.75 (t, 2H), 3.19 (t, 2H),
3
1
3
): d
3
n
with H O and the solvent was removed. The crude oil was
2
poured into hexanes, the precipitated solids were filtered off,
and the filtrate was filtered through silica using hexanes as
the eluent. When TLC indicated no further elution of
precursor 2-bromo-3-(bromomethyl)thiophene, the column
was flushed with EtOAc to recover the product. (Note:
unreacted 2-bromo-3-(bromomethyl)thiophene catalyses
ring opening of the oxazoline upon heating.) The solvent
was removed and the remaining oil was distilled (80 8C,
4.2.5. HT-2,5-poly(thiophene-3-propionic acid) (3). A
sample of 1 (404.4 mg) was dissolved in 3 N HCl (90 mL)
and heated to reflux for 12 h. The solution consisted of a
dark purple suspension at this point. The solid was filtered,
rinsed with H O, and dried to recover the product in 82%
2
yield (248.4 mg). As prepared, most polymer samples
submitted for elemental analysis contained large amounts
of ash (10–15%). This suggests the presence of copper (II)
salts and other impurities, though this has not been
confirmed. A drastic reduction in the ash content was
achieved by dissolving polymer PTPA (3) in aqueous base,
re-precipitating it upon acidification, and recovering it by
filtration. Re-precipitating two to three times eliminated the
ash, giving elemental analysis results in reasonable agree-
0
4
2
2
.01 T) to recover the product in 77% yield (12.15 g,
1
2.20 mmol). H NMR (300 MHz, CDCl ): d 1.24 (s, 6H),
3
.52 (t, 2H, JZ8.2 Hz), 2.89 (t, 2H, JZ8.1 Hz), 3.91 (s,
H), 6.84 (d, 1H, J4,5Z5.6 Hz), 7.19 (d, 1H, J4,5Z5.6 Hz).
1
3
C NMR (75 MHz, CDCl ): d 25.9, 28.1, 28.3, 66.9, 79.0,
09.6, 125.4, 128.1, 139.7, 164.5. Anal. Calcd: C, 45.83; H,
.86; N, 4.86. Found: C, 45.80; H, 4.84; N, 4.79.
3
1
4
1
ment with the theoretical composition of this polymer. H
NMR (300 MHz, CD OD) (characterized as its cesium salt
3
4
.2.3. 2-(2-(2-Bromo-5-(trimethylstannyl)thiophen-3-
by adding CsOH): d 7.05 (s, 1H); 3.92 (t, 2H); 2.35 (t, 2H).
Anal. Calcd: C, 54.54; H, 4.08. Found: C, 52.64; H, 4.08.
yl)ethyl)-4,5-dihydro-4,4-dimethyloxazole. 2-(2-(2-
Bromothiophen-3-yl)ethyl)-4,5-dihydro-4,4-dimethyloxa-
zole (4.885 g, 16.96 mmol) was dissolved in THF (40 mL)
and cooled to K70 8C. LDA was prepared in a separate flask
from diisopropylamine (2.50 mL, 17.8 mmol) and butyl-
lithium (2.79 M, 6.10 mL, 17.0 mmol) in THF (10 mL).
This was added while maintaining a solution temperature
below K60 8C, and the reaction mixture was stirred at
K70 8C for 1 h. Trimethylstannyl chloride (1.0 M,
Acknowledgements
We are grateful to the NSF (CHE-0107178) for support. We
thank Jason Wolf of CMU for doing the X-ray measurement
of PTOA.
26.74 mL, 26.7 mmol) was added dropwise, maintaining
the temperature below K62 8C. The solution was stirred for
1
2
h, poured into 75 mL of aqueous KF and stirred for
0 min, then was partitioned between H O and ether. The
References and notes
2
organic layer was dried over MgSO and the solvent was
4
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