A direct route to polythiophenes displaying lateral substituents: Easy one-step synthesis and polymerization of thiophene monomers substituted by a dimethylenecarboxylate (CH2CH2COOR) appendage on the 3-position

Article abstract of DOI:10.1002/pola.25842

Thiophene monomers displaying a dimethylenecarboxylate (CH ₂CH₂COOR) substituent on the 3-position of the aromatic ring can be easily obtained and in one step from the electrochemically induced reaction of 3-bromothiophene with the corresponding acrylate (CH ₂=CHCOOR). The synthesis of the ethyl ester monomer, of related 2,5-dihalogenothiophenes, and their polymerization are reported. Despite the surprisingly low solubilities displayed by the polymers, a full spectroscopic characterization could be performed and the data fully analyzed. Oxidative polymerizations (FeCl₃ or electropolymerization) yield a regioirregular polythiophene, with 60-70% of head-to-tail diads. Both experimental and theoretical results suggest that the nonconjugated ester plays a very minor role-if any-in the polymerizations under oxidative conditions, but has a significant influence on the polymer properties. Preliminary attempts to polymerize the dihalogenothiophenes under reductive conditions were hampered by the even lower solubilities exhibited by the regioregular oligomers.

Full text of DOI:10.1002/pola.25842

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A Direct Route to Polythiophenes Displaying Lateral Substituents: Easy
One-Step Synthesis and Polymerization of Thiophene Monomers
Substituted by a Dimethylenecarboxylate (CH2CH2COOR) Appendage
on the 3-Position
ꢀ
Stephane Sengmany, Claire Ceballos, Romain Belhadj, Christine Cachet-Vivier, Erwan Le Gall,
ꢀ
Blandine Brissault, Jacques Penelle, Eric Leonel
ꢀ
ꢀ
Institut de Chimie et des Materiaux Paris Est (ICMPE), CNRS—Universite Paris-Est, 2-8 rue H. Dunant, 94320 Thiais, France
ꢀ
Correspondence to: J. Penelle (E-mail: penelle@icmpe.cnrs.fr) or E. Leonel (E-mail: leonel@icmpe.cnrs.fr)
Received 23 June 2011; accepted 3 November 2011; published online 28 November 2011
DOI: 10.1002/pola.25842
ABSTRACT: Thiophene monomers displaying a dimethylenecar-
boxylate (CH2CH2COOR) substituent on the 3-position of the
aromatic ring can be easily obtained and in one step from the
electrochemically induced reaction of 3-bromothiophene with
the corresponding acrylate (CH2¼CHCOOR). The synthesis of
the ethyl ester monomer, of related 2,5-dihalogenothiophenes,
and their polymerization are reported. Despite the surprisingly
low solubilities displayed by the polymers, a full spectroscopic
characterization could be performed and the data fully ana-
lyzed. Oxidative polymerizations (FeCl3 or electropolymeriza-
suggest that the nonconjugated ester plays a very minor role—
if any—in the polymerizations under oxidative conditions, but
has a significant influence on the polymer properties. Prelimi-
nary attempts to polymerize the dihalogenothiophenes under
reductive conditions were hampered by the even lower solubil-
C
V
ities exhibited by the regioregular oligomers.
2011 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 900–911,
2012
KEYWORDS: conducting polymers; electrochemistry; monomer
tion) yield
a
regioirregular polythiophene, with 60–70% of
synthesis;
step-growth
polymerizations;
substituted
head-to-tail diads. Both experimental and theoretical results
polythiophenes
INTRODUCTION Because of their excellent optoelectronic and
processing properties as well as their reasonable costs when
compared with other organic-based semiconducting materials,
polythiophenes currently constitutes one of the most impor-
tant classes of conjugated polymers, with applications in place
or envisioned—among others—in photovoltaic devices, field-
effect transistors, and light-emitting diodes. Several recent
reviews are available that detail the synthesis, properties,
processing, scope, and limitations of these polymers.1–8
tation of the substituents in the solid state,9 and (c) added
substituents can be designed to fine-tune the optoelectronic
properties of the materials, either by direct interaction of
electron-rich or electron-poor substituents with the pi-conju-
gated backbone or by disrupting the planarity of the poly-
mer. In addition, specific substituents are occasionally added
to introduce a particular property such as chirality for enan-
tioselective recognition10 or a better dispersion/interaction
in hybrid organic–inorganic systems.11 As an illustration of
this trends, use of purposely substituted polythiophenes as
polyelectrolytes,12 thermochromic materials,13 or field-effect
transistors14 has been demonstrated recently.
The structures of polythiophenes usually imply CAC links
between neighboring thiophene units, joining together their
2- and 5-positions. In addition, lateral substituents are com-
monly placed on the 3-position of the heteroaromatic unit,
typically with three main motivations in mind: (a) first, lat-
eral substituents such as long alkyl chains or hydrophilic
groups can impart solubility in common organic solvents or
in water, facilitating characterization and enabling solution-
based processes to obtain the final materials in the desired
shapes and morphologies (fibers, particles, thin films, etc.),
(b) if properly chosen, the lateral substituents can provide
an added impetus for self-assembly, notably via the interdigi-
One of the main drawbacks of currently available approaches
to substituted polythiophenes is their lack of generality,
which usually requires that individual multistep monomer
synthesis be designed for each targeted new monomer even
if they are structurally close. We aim at partly solving this
difficulty by designing a direct, scalable synthetic route to
thiophene monomers such as 1–3. These monomers display
indeed two structural features of great interest in the afore-
mentioned context: (a) a first-rate flexibility with respect to
Additional Supporting Information may be found in the online version of this article.
C
V
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films cast from the above CHCl3 solution on a glass micro-
scope slide. The prepared thin films were also used for the
X-ray experiment on a Model X’Pert PRO MPD PANalytical
instrument (Almelo, The Netherlands), using a Co-Ka1-a2 radi-
ation source. Thermogravimetric experiments (TGA) were
carried out on a SETARAM SETSIS Evolution instrument
using argon as an inert gas. Size exclusion chromatography
(SEC) experiments were performed in chloroform (1 mL
minꢀ1) at room temperature using a Spectra Physics P100
pump, and two PLgel Polymer Laboratories linear columns
(5 mm Mix-C, separation range: 200 to 2 ꢁ 106). A Wyatt
Technology Optilab Rex interferometric refractometer (690
nm laser) and a Wyatt Technology Minidawn light scattering
photometer [three-angle detection, 20 mW semiconductor
laser (690 nm)] were used as detectors. The quantitative
mass recovery assumption was used when analyzing the
results. Given the large variation in the (dn/dc) parameter
with the degree of polymerization for this class of polymers,
the obtained number-average molecular weights should be
considered as indicative rather than absolute.
SCHEME 1 Structure of the investigated monomers.
the nature of the R substituent via the placement of an ester
functionality, and (b) the presence of an electronically
insulating short alkyl bridge (CH2ACH2) to prevent a direct
interaction of the ester group and its substituents with the
polymer chain. Although two specific examples have been
summarily investigated by others15,16 as will be examined fur-
ther in the ‘‘Discussion’’ section, no systematic study is cur-
rently available on this class of monomers. In this communica-
tion, results will be presented on the synthesis and both
oxidative and reductive polymerizations of model monomers
1a–3a that contain an ethyl group on the ester (Scheme 1).
Synthesis of Ethyl (E)-3-(Thiophen-3-yl)acrylate).
Into a three-necked flask, triethyl phosphonoacetate (2.77
mL, 14 mmol) was added to a stirred suspension of sodium
hydride (498 mg of 60% dispersion in mineral oil, 13 mmol)
in THF (20 mL) at 0 ꢂC. The resulting mixture was stirred
for 30 min at room temperature, and 3-thiophenecarboxalde-
hyde (875 lL, 10 mmol) in THF (20 mL) was added. The
reaction mixture was stirred for 1 h, and then quenched
with a saturated ammonium chloride solution (30 mL). The
organic layer was washed with water, dried over MgSO4, fil-
trated, and concentrated under reduced pressure. The crude
oil was purified by flash chromatography (elution with pen-
tane/Et2O: 95/5) to give 1.82 g (quantitative yield) of (E)-
ethyl 3-(thiophen-3-yl)acrylate as a yellow oil.17
EXPERIMENTAL
Materials and Methods
Solvents and reagents were purchased from commercial sup-
pliers and used, unless mentioned, without further purifica-
tion. All reactions on small molecules (monomer and organo-
metallic precursor synthesis) were monitored by gas
chromatography (GC) using a Varian 3300 chromatograph
equipped with a 5 m DB1 column (diameter: 0.32 mm, df:
0.4 lm). Infrared spectra were recorded on a Bruker Tensor
27 DTGS Fourier transform infra-red (FTIR) spectrometer in
ATR mode. 1H and 13C nuclear magnetic resonance (NMR)
spectra were recorded in CDCl3 or deuterated tetrahydrofu-
ran (THF-d8) on a Bruker Avance II 400, at 400 and 100
MHz, respectively, and on a Bruker DRX-600 MHz spectrome-
ter (1H NMR only). Chemical shifts (d) are reported in parts
per million (ppm) relative to the nondeuterated solvent sig-
nal. Coupling constant values (J) are indicated in Hertz (Hz)
and refer to multiplicities, indicated as follows: s (singlet), d
(doublet), t (triplet), q (quartet), and m (multiplet). Mass
spectra were obtained on a Thermo ITQ700 GC-MS spec-
trometer. Elemental analyses were performed at the ‘‘Institut
de Chimie des Substances Naturelles’’ in Gif-sur-Yvette
(France). Flash chromatography on silica (granulometry 70–
200 lm) was used to purify small organic molecules. Theo-
retical calculations were performed with the Spartan ‘08
software (Wavefunction, Irvine, CA). The conformation of the
investigated monomers was optimized by molecular mechan-
ics before the density functional calculations (basic optimiza-
tion after molecule buildup, followed by an merck molecular
force field (MMFF) optimization of the equilibrium geometry
at ground state). A frequency scale factor of 0.951 provided
the best fit with the experimental wavenumber of the C¼¼O
vibration band. UV–visible spectra were recorded with a Var-
ian CARY 50 Bio on polymer solutions in CHCl3 and thin
1H NMR (CDCl3): d 7.70 (d, J ¼ 15.9 Hz, 1H), 7.48 (s, 1H),
7.32 (m, 2H), 6.26 (d, J ¼ 15.8 Hz, 1H), 4.25 (q, J ¼ 7.2 Hz,
2H), 1.34 (t, J ¼ 7.1 Hz, 3H). 13C NMR (CDCl3): d 167.2,
138.1, 137.6, 128.0, 127.0, 125.2, 117.9, 60.5, 14.4. MS, m/z
(relative intensity): 183 (7), 182 (49), 154 (43), 139 (6),
138 (16), 137 (100), 112 (5), 111 (10), 110 (23), 109 (45),
108 (5), 97 (5), 69 (5).
Synthesis of Ethyl 3-(Thiophene-3-yl)propanoate (1a)
(E)-Ethyl 3-(thiophen-3-yl)acrylate (3.64 g, 20 mmol) and
NiCl2 (2.60 g, 20 mmol) in MeOH (200 mL) were introduced
in a three-necked flask. The solution was cooled down to 0
ꢂC, and NaBH4 (5.32 g, 140 mmol) was added portionwise
(CAUTION: vigorous, exothermic reaction). The resulting mix-
ture was stirred at room temperature for 3 h and then
quenched with a mixture made of a saturated NaHCO3 solu-
tion (70 mL) and ethyl acetate (70 mL). The resulting solu-
tion was extracted with ethyl acetate (3 ꢁ 30 mL), then
CH2Cl2 (3 ꢁ 30 mL). The combined organic layers were
washed with water, dried over MgSO4, filtrated, and concen-
trated under reduced pressure. The crude product was puri-
fied by flash chromatography (elution with pentane/Et2O:
95/5) to give 2.76 g (75%) of 1a as a yellow oil.
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Direct Electrochemical Synthesis of Ethyl
3-(Thiophene-3-yl)propanoate (1a)
263 (100), 261 (97), 257 (20), 255 (39), 253 (18), 235 (46),
233 (45), 221 (64), 219 (60), 193 (16), 193 (16), 191 (16),
190 (11), 110 (12), 109 (12). Anal. Calcd. for C9H10Br2O2S:
C, 31.60; H, 2.95; S, 9.37. Found: C, 31.96; H, 3.05; S, 9.34.
A mixture of dimethyl formamide (DMF)/pyridine (50 mL, 90/
10, v/v) as solvent, of tetrabutylammonium bromide (42 mg,
0.42 mmol) and tetrabutylammonium iodide (155 mg, 0.42
mmol) as supporting electrolytes, and 1,2-dibromoethane
(129 lL, 1.5 mmol) were added to an undivided cell fitted
with an iron/nickel (64/36) rod as the anode and a nickel
foam (Ni 99%, area 50 cm2) as the cathode. A pre-electroly-
sis was conducted at constant current (0.2 A) at room tem-
perature under nitrogen for 30 min. The current was turned
off, and NiBr2ꢃ3H2O (330 mg, 1.5 mmol), 3-bromothiophene
(2.44 g, 15 mmol), and ethyl acrylate (10 mL, 91.9 mmol)
Synthesis of Ethyl 3-(2-Bromothiophen-3-yl)propanoate
Ethyl 3-(thiophen-3-yl)propanoate 1a (2.52 g, 15 mmol) and
DMF (30 mL) were introduced at 0 ꢂC into a three-necked
flask fully covered by an aluminum sheet. N-Bromosuccini-
mide (NBS) (2.67 g, 15 mmol) in DMF (30 mL) was added
dropwise to the solution. The mixture was stirred at room
temperature overnight. The solvent was evaporated, water
was added, and the solution extracted with diethyl ether (2 ꢁ
25 mL). The combined organic layers were washed with a sat-
urated solution of sodium chloride (2ꢁ 20 mL), dried over
MgSO4, filtered, and concentrated under reduced pressure.
The crude product was purified by flash chromatography (elu-
tion with pentane/Et2O (95/5, v/v) to give 3.15 g (80%) of
ethyl 3-(2-bromothiophen-3-yl)propanoate as a yellow oil,
which was used as such in the next step (see below).
ꢂ
were added into the cell. The mixture was heated at 100 C,
and the electrolysis was run at constant current (0.2 A). The
reaction was monitored by GC and stopped after 3-bromo-
thiophene had been totally consumed. A 1 mol Lꢀ1 solution
of HCl (200 mL) was added, and the mixture extracted with
diethyl ether (3ꢁ 50 mL). The combined organic layers were
washed with a saturated NaHCO3 solution, then with a satu-
rated NaCl solution, dried over MgSO4, and evaporated. The
crude product was purified by flash chromatography [elution
with pentane/Et2O (95/5, v/v)] to give 2.43 g (88%) of ethyl
3-(thiophene-3-yl)propanoate (1a) as a yellow oil.
FTIR (neat, cmꢀ1): m 2,980, 1,729, 1,181, 1,154, 716. 1H
NMR (CDCl3): d 7.19 (d, J ¼ 5.7 Hz, 1H), 6.78 (d, J ¼ 5.7 Hz,
1H), 4.14 (q, J ¼ 7.1 Hz, 2H), 2.89 (q, J ¼ 8.0 Hz, H2), 2.52
(q, J ¼ 7.5 Hz, 2H), 1.28 (t, J ¼ 7.4 Hz, 3H). 13C NMR
(CDCl3): d 173.6, 141.2, 128.9, 126.3, 108.2, 61.2, 36.5, 25.9,
14.2. MS, m/z (relative intensity): 219.0 (6), 217 (6), 193
(12), 191 (24), 189 (13), 188 (8), 184 (10), 183 (100), 177
(30), 175 (28), 155 (44), 141 (58), 113 (28), 111 (11), 110
(13), 109 (10), 85 (10), 84 (10), 69 (6).
FTIR (neat, cmꢀ1): m 2,980, 1,729, 1,179, 1,154, 1,034, 776.
1H NMR (CDCl3): d 7.27 (aromatic H5, dd, J ¼ 4.9 and 2.9
Hz, 1H), 6.99 (aromatic H2, ddt, J ¼ 2.9, 0.9, and 0.9 Hz, 1H),
6.96 (aromatic H4, dd, J ¼ 4.9 and 0.9 Hz, 1H), 4.15 (q, J ¼
7.3 Hz, 2H), 2.99 (t, J ¼ 7.6 Hz, 2H), 2.64 (t, J ¼ 7.8 Hz, 2H),
1.28 (t, J ¼ 7.1 Hz, 3H). 13C NMR (CDCl3): d 173.1, 141.4,
138.0, 137.7, 121.8, 60.3, 35.9, 26.8, 14.0. MS, m/z (relative
intensity): 281 (15), 209 (30), 207 (100), 191 (15), 179
(15), 133 (19), 103 (22), 97 (15), 91 (22), 79 (41), 78 (19),
77 (22), 75 (15), 74 (15), 73 (30), 67 (19), 65 (19), 61 (37),
55 (37), 50 (26).
Synthesis of Ethyl 3-(2-Bromo-5-iodothiophen-3-yl)
propanoate (3a)
N-Iodosuccinimide (NIS) (782 mg 3.49 mmol) was added
portionwise at 0 ꢂC to a solution of ethyl 3-(2-bromothio-
phen-3-yl)propanoate (936 mg, 5 mmol) dissolved in a mix-
ture of CHCl3/CH3CO2H (30 mL, 2/1, v/v) in a three-necked
flask. The resulting solution was stirred in the dark for
Synthesis of Ethyl 3-(2,5-Dibromothiophen-3-yl)
propanoate (2a)
ꢂ
30 min at 0 C, then at room temperature for 24 h. The red
mixture was washed with H2O (4ꢁ 100 mL), then extracted
with CH2Cl2 (4ꢁ 50 mL). The combined organic layers were
dried over MgSO4, concentrated under reduce pressure, and
purified by flash chromatography [elution with pentane, then
with pentane/Et2O (95/5, v/v)] to give 1.57 g (81%) of 3a
as a yellow oil.
Ethyl 3-(thiophene-3-yl)propanoate 1a (920 mg, 5 mmol),
tetrabutylammonium bromide (49 mg, 5 mmol), and metha-
nol (80 lL, 2.5 mmol) were added to a freshly distilled
dichloromethane (40 mL). The solution was stirred at room
temperature, and Br2 (80 lL, 5 mmol) dissolved in dichloro-
methane (40 mL) was added dropwise. The mixture was
stirred for 10 min. After addition of water (50 mL) and trie-
thylamine (50 mL, in excess to neutralize the generated
HBr), the mixture was extracted with dichloromethane. The
combined organic layers were dried over MgSO4 and evapo-
rated. The crude product was purified by flash chromatogra-
phy (elution with pentane/ether: 95/5) to give 1.70 g (quan-
titative) of 2a as a yellow oil.
FTIR (neat, cmꢀ1): m 2,979, 1,730, 1,180, 1,028, 783. 1H
NMR (CDCl3): d 6.81 (s, 1H), 4.13 (q, J ¼ 7.5 Hz, 2H), 2.81
(t, J ¼ 3.0 Hz, 2H), 2.54 (t, J ¼ 8.0 Hz, 2H), 1.25 (t, J ¼ 8.0
Hz, 3H). 13C NMR (CDCl3): d 172.2, 140.7, 130.9, 110.7,
108.9, 60.7, 33.9, 24.8, 14.2. MS, m/z (relative intensity):
343 (13), 341 (20), 270 (25), 268 (26), 268 (16), 264 (11),
FTIR (neat, cmꢀ1): m 2,978, 1,727, 1,184, 1,022, 824. 1H
NMR (CDCl3): d 6.96 (s, 1H), 4.14 (q, J ¼ 7.2 Hz, 2H), 2.83
(q, J ¼ 7.5 Hz, 2H), 2.52 (q, J ¼ 7.5 Hz, 2H), 1.24 (t, J ¼ 7.2
Hz, 3H). 13C NMR (CDCl3): d 172.6, 141.9, 137.9, 112.7, 77.1,
60.7, 34.1, 27.1, 14.2. MS, m/z (relative intensity): 390 (21),
388 (20), 319 (6), 317 (11), 316 (6), 315 (5), 314 (6), 312
(6), 311 (11), 310 (100), 304 (13), 302 (12), 282 (28), 269
(7), 268 (53), 240 (6), 110 (5). Anal. Calcd. for C9H10BrIO2S:
C, 27.78; H, 2.29; S, 8.24. Found: C, 27.68; H, 2.59; S, 8.43.
Oxidative Polymerization of Monomer 1a with
Ferric Chloride
A solution of 3-substituted thiophene 1a (368 mg, 2 mmol)
dissolved in dry CHCl3 (10 mL) was added dropwise under
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nitrogen gas prior to electrolysis. The polymer was partly
recovered after the third scanning cycle (see Fig. 1 for the
conditions), by dissolving it in hot chloroform. The dissolu-
tion process could not be made complete.
FTIR (neat, cmꢀ1): m 2,964, 1,725, 1,180, 1,034, 852. 1H
NMR (CDCl3): d 7.06 and 7.03 (2s, 1H), 4.21–4.14 (m, 2H),
3.17–3.13 and 2.97–2.89 (2m, 2H), 2.73–2.69 and 2.62–2.60
(2m, 2H), 1.30–1.24 (m, 3H).
Attempted Reductive Polymerization of Monomer
2a via Organozinc Intermediates
Acetonitrile (20 mL), zinc dust (1.24 g, 19 mmol), cobalt (II)
bromide (166 mg, 0.76 mmol), 1,2-dibromoethane (50 lL,
0.57 mmol), and trifluoroacetic acid (50 lL) were added suc-
cessively to a round-bottomed flask under nitrogen. The mix-
ture was stirred at room temperature until the reaction
becomes exothermic (5 min), and then monomer 2a (1.3 g,
3.8 mmol) was added dropwise via a syringe (exothermic
reaction). The solution was stirred at room temperature for
30 min. Copper (II) triflate (1.65 g, 4.56 mmol) was added
(very exothermic reaction for 5 min), and the mixture stirred
at room temperature for 24 h. The reaction was quenched
with a 0.1 mol Lꢀ1 HCl solution, then filtered through a pad
of celite to remove the copper salt. The addition of methanol
did not precipitate a polymer, even after triturating on an ice-
bath. The resulting mixture was concentrated under vacuum.
The obtained aqueous solution was extracted four times with
a mixture of ethyl acetate and THF (10 vol %). The combined
organic layers were dried over MgSO4 and evaporated to pro-
vide a dark brown oil. This crude oil was washed with pen-
tane/Et2O (10 vol %) to get rid of the dehalogenated mono-
mer (reduction product) and the monomer that had not
reacted. A total of 150 mg of a dark thick oil was recovered.
FIGURE 1 Voltammograms (3 cycles) obtained on a platinum
electrode (2 ꢁ 2 cm2) for a solution of 1a at 0.1 mol Lꢀ1 in ace-
tonitrile containing 0.1 mol Lꢀ1 NBu4BF4 (scan rate: 20 mV sꢀ1).
FTIR (neat, cmꢀ1): m 2,981, 1,727, 1,251, 1,181, 1,042. 1H
NMR (CDCl3): d 6.90 and 6.77 (2s, 1H), 4.10–4.02 (m, 2H),
3.02–2.98 and 2.91–2.80 (2m, 2H), 2.61–2.46 (m, 2H), 1.20–
1.14 (m, 3H).
nitrogen to a FeCl3 suspension (1.30 g, 8 mmol) in 40 mL of
dry CHCl3. The mixture was stirred for 24 h at room temper-
ature. The polymer was then precipitated by the addition of
a large excess of MeOH, and the suspension filtered on a sin-
tered glass funnel. The polymer was recovered as a reddish
brown film (290 mg, 79%).
Attempted Reductive Polymerization of Monomers
3a via Organozinc Intermediates
1
FTIR (neat, cmꢀ1): m 2,978, 1,729, 1,178, 1,036, 827. H NMR
Acetonitrile (15 mL), zinc dust (490 mg, 7.5 mmol), cobalt
(II) bromide (219 mg, 1.0 mmol), 1,2-dibromoethane (65 lL,
0.75 mmol), and trifluoroacetic acid (50 lL) were added suc-
cessively to a round-bottomed flask under nitrogen. The mix-
ture was stirred at room temperature until the reaction
becomes exothermic (5 min), then monomer 3a (1.94 g,
5.0 mmol) was added dropwise via a syringe (reaction
slightly exothermic). The solution was stirred for 20 min at
room temperature, then dichloro-bis(triphenylphosphine)pal-
ladium (175 mg, 0.25 mmol) was added, and the mixture
(CDCl3): d 7.06 and 7.02 (2s, 1H), 4.20–4.44 (m, 2H), 3.14–
3.12 and 2.92–2.90 (2 broad signals, 2H), 2.72–2.62 and
2.63–2.59 (2 broad signals, 2H), 1.29–1.21 (broad signal, 3H).
Electropolymerization of Monomer 1a
The electrochemical polymerization of 1a was carried out in
a three-electrode cell using an AUTOLAB PT30 potentiostat
(ECOCHEMIE). Acetonitrile was used as the solvent and dis-
tilled before use. The supporting electrolyte, tetrabutylam-
monium tetrafluoroborate (NBu4BF4), was dried at 100 ꢂC,
and dissolved in the solvent to obtain a final 0.1 mol Lꢀ1
solution. The working electrode was a platinum plate (2 ꢁ
2 cm) carefully polished with an abrasive paper, rinsed with
ꢂ
stirred at 60 C for 24 h. After having been cooled down to
room temperature, the solution was filtered through a pad of
celite, and the filtrate concentrated under vacuum. Attempts
to precipitate a polymer with methanol failed. The obtained
crude oil was washed with pentane/Et2O (10 vol %) to get
rid of the dehalogenated monomer (reduction product) and
the unreacted monomer. A total of 771 mg of a dark thick
oil was recovered.
ꢂ
acetone, and dried at 100 C. The platinum counter-electrode
was placed in a separated compartment ended by a sinter
glass. The reference electrode was a KCl-saturated calomel
electrode (SCE). The solution was deoxygenated by bubbling
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SCHEME 2 Preparation of monomer 1a.
FTIR (neat, cmꢀ1): m 2,929, 1,730, 1,436, 782. 1H NMR
(CDCl3): d 7.82–6.83 (m), 4.20–4.15 (m, 2H), 2.96–2.85
(m, 2H), 2.64–2.58 (m, 2H), 1.32–1.27 (m, 3H).
the nature of the anode rod [aluminum, iron, iron/nickel (64/
36)] and the relative amount of acrylate, two parameters that
had previously been identified as being critical.22–24
The best conditions were obtained by using an iron/nickel
(64/36) anode rod and operating with a significant excess of
acrylate (5.7 equivalents with respect to the initial amount
of 3-bromothiophene; entry 4 in Table 1), to afford mono-
mer 1a in 85% yield. Lowering the amount of acrylate to 3
equivalents dropped the yield to 50%. This high amount of
unproductive acrylate is currently the key weakness of the
above process as it restricts the reaction to cheap acrylates
such as ethyl acrylate whose use in large excess is not
overly disadvantageous. A complete picture of the reaction
mechanism is not available at this time, making further opti-
mization a complex task, but a large excess of electrophilic
olefin appears to be needed in most of the previously
reported examples.22–24 The possible role played by some
excess acrylate as a ligand to stabilize/activate the nickel
catalyst has notably been postulated.21
RESULTS AND DISCUSSION
Monomer Synthesis
Few substituted thiophene monomers are commercially
available, probably due to the well-known difficulty of acti-
vating the 3-position of a thiophene ring. The primary objec-
tive in this study was to overcome this limitation and to
significantly expand the toolbox of easily obtainable substi-
tuted thiophene monomers by exploiting the direct, one-step
electrochemical reaction of commercially available 3-bromo-
thiophene with acrylates (route 2 in Scheme 2). As a large
number of acrylates are commercial or simple to synthesize,
this route has the potential to provide an interesting alterna-
tive for accessing laterally substituted polythiophenes.
Although this direct approach was the main objective of the
presently reported work, an alternative multistep approach
was also investigated (route 1 in Scheme 2), mostly to
ensure that the structure of the monomer obtained by the
electrochemical process and its purity were satisfying.
3-Substituted thiophene 1a was used as a key intermediate
for the preparation of the corresponding dihalothiophenes
2a and 3a, two monomers that could both be possibly used
in a reductive polymerization.4 Organozinc intermediates in
the polymerization were selected because commonly used
organomagnesium species4 would react intramolecularly or
intermolecularly with the ester moiety. It was initially postu-
lated that faster metallation of the CAI bond compared with
the CABr bond would occur on 2-bromo-5-iodothiophene
3a, providing access to the 5-substituted organozinc and
favoring a higher regioselectivity in the polymerization com-
pared with its 2,5-dibrominated analog 2a.
This latter approach, based on a traditional Horner-Wads-
worth-Emmons olefination,18,19 allowed for the introduction
of the carbonyl side-chain on the 3-position of the thiophene
ring via an initial C¼¼C bond formation. According to this
strategy, monomer 1a can be synthesized in two steps by first
reacting commercially available 3-thiophenecarboxaldehyde
with triethylphosphoacetate in THF, followed by a reduction
of the C¼¼C double bond with a mixture of nickel chloride and
sodium borohydride in ethanol (Scheme 2).20,21 This method
proved to be convenient and provided monomer 1a with a
good overall yield of 75% and excellent purity. However, its
extension is restricted by the limited commercial availability
and high costs of prospective phosphonates.
The 2- and 5-positions of thiophene 1a were efficiently bro-
minated to give the dibromo derivative 2a in 90% yield
TABLE 1 Influence of the Nature of the Anode Rod and the
Amount of Ethyl Acrylate in the Nickel(0)-Catalyzed Conjugated
Addition of 3-Bromothiophene (HetArX) on Ethyl Acrylate
The alternative, one-step electrochemical approach (route 2
in Scheme 2) was inspired by previous work on the
nickel(0)-catalyzed Michael addition of aryl halides on elec-
trophilic olefins, in particular the reaction of aryl halides—
such as substituted phenyl and naphthyl bromides—with
activated olefins, including acrylates, as described by Condon
et al.22–24 These reactions are based on a sacrificial anode pro-
cess, and uses cheap and commonly available reagents.25 Using
this approach, monomer 1a was indeed obtained with ethyl ac-
rylate as the electrophilic olefin (Table 1). A preliminary opti-
mization of the coupling conditions was realized by assessing
Ethyl Acrylate/
HetArX Ratio
Isolated
Entry
Anode
Yield (%)
1
2
3
4
5
Iron
2.5
5.7
5.7
5.7
3.0
20
36
38
85
50
Iron
Aluminum
Iron/nickel
Iron/nickel
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Blanchetie`re et al. (ꢄ2 V/SCE),16 the difference arising from
the use of a large size electrode in our case, bringing an
additional ohmic drop term to potential and voltammograms
distortion. The first oxidation cycle of 1a presents a crossing
effect, also called nucleation loop (indicated by arrows on
Fig. 1), this effect being usually observed on freshly polished
electrodes and interpreted as the initiation of the polymer
nucleation process.30 Polymer formation is evidenced by the
presence of a new reversible electrochemical system charac-
terized by a broad cathodic peak centered at 0.64 V/SCE and
a broad anodic wave extending from 0.4 to 1.5 V/SCE,
including three ill-defined components at 0.67, 0.95, and
1.31 V/SCE, respectively. As for the monomer oxidation peak,
the shape and the redox properties of the electrodeposited
polymer are close to the results obtained by Blanchetie`re
et al.,16 with some slight differences due to a change in elec-
tropolymerization conditions (electrode size and anodic
potential limit).
SCHEME 3 Halogenation of 3-substituted thiophene 1a.
ꢀ
(Scheme 3), using a method developed by Leonel and co-
workers that makes use of bromine as the reagent in the
presence of catalytic amounts of n-tetrabutylammonium bro-
mide and methanol.26 The 2-bromo-5-iodothiophene analog
3a was prepared in good yield (65%) in two subsequent hal-
ogenation steps, using one equivalent of NBS27,28 and NIS,
respectively (Scheme 3).
The electrodeposited polymer is green-gray colored (doped
oxidized form); its color shifts back to orange-red when solu-
bilized in hot chloroform. The electroactive mass was found
equal to 0.73 mg as calculated from the cathodic charge Qc3
determined in the third cycle at the end of the electrodeposi-
tion process according to Faraday’s law:
Polymer Synthesis
Polymerization conditions are summarized in Table 2. A sur-
prising result was observed during our preliminary attempts
at polymerizing 1a oxidatively, using FeCl3 as the oxidizing
agent. Although it was possible to dissolve small amounts in
chloroform after sonication at 40–60 ꢂC and an ordinary
polythiophene structure could be evidenced by NMR (see
‘‘Polymer Characterization’’ section), all investigated organic
solvents resulted in no or very little solubilization of the
polymers. The soluble fraction, as characterized by SEC, had
a relative number-average molecular weight of 26.7 ꢁ 103
and a very broad molecular weight distribution extending
over the experimental limit of the used SEC equipment (Mw/
Mn > 6). Given the good solubility exhibited by poly(3-hex-
ylthiophene)s under the same conditions, this results came
as a surprise as the replacement of two CH2s in the n-hexyl
moiety of the repeat unit by a (C¼¼O)AO is expected to
improve the solubility of the polymer in organic solvents, in
particular those solvents capable of acting as hydrogen-bond
donors. As many examples are available in the literature that
support the view of improved solvent-solute interactions in
the above case, including group method approaches such as
Van Krevelen’s group contribution method for the determina-
tion of solubility parameters,29 it seems reasonable to expect
that the observed poor solubility results mostly from specific
interactions of the ester groups in the solid phase, a behav-
ior that could have significant implications on the morpholo-
gies of the obtained polymeric materials, when compared
with those exhibited by poly(3-alkylthiophene)s.
Qc3:M
m ¼
F:e
where Qc3 is the cathodic charge determined in the third
cycle at the end of the electrodeposition process, M is the
molecular weight of the thiophene unit, F the Faraday con-
stant (96,500 C molꢀ1), and e the number of electron
exchanged per thiophene unit in the polymer.
Two preliminary attempts were made to obtain poly(1a) via
the reductive polymerization of halogenated monomers 2a
and 3a (entries 3–4 in Table 2). These endeavors proved
challenging, mostly for the following two reasons: (a) sam-
ples obtained were often less soluble than those obtained
under oxidative conditions, making characterization of the
TABLE 2 Polymerization Conditions
Entry
Monomer
Polymerization Conditions
Voltammograms corresponding to the electropolymerization
(entry 1, Table 2) of monomer 1a from a 0.1 mol Lꢀ1 solu-
tion between 0.25 and 2.5 V/SCE at 20 mV sꢀ1 during
3 cycles are shown in Figure 1. The oxidation starts at
ꢄ1.75 V/SCE on the first cycle and is characterized by a
peak at ꢄ2.15 V/SCE on further scans. This peak potential
value is close to the one found for the same monomer by
1
2
3
1a
1a
2a
Electropolymerization, CH3CN, RT
FeCl3, CHCl3, RT
1) Zn (5 eq), CoBr2 cat, CH3CN, RT
2) Cu(OTf)2 (1 eq), RT
4
3a
1) Zn (1.5 eq), CoBr2 cat, CH3CN, RT
2) PdCl2(PPh3)2 (5%), 60 ꢂC
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SCHEME 4 Distribution of thienylzinc species obtained after treatment of dibromothiophene 2a with zinc dust under Co (II)
catalysis.
full materials impossible (a SEC analysis of the tiny soluble
fraction indicated that it contained mostly the monomer con-
taminated by a few very short oligomers), and (b) efficient
monometallation of the 2,5-dihalogenothiophenes was found
to be very challenging.
cipitation of the growing polymer being unavoidable above a
certain chain length.
An attempt to use the above procedure to generate selectively
monothienylzinc at the CAI bond in 3-bromo-5-iodothiophene
monomer 3a using 1.5 equivalents of zinc dust (entry 4,
Table 2) was unsuccessful and yielded reduced intermediates
as major products. Not surprisingly, a subsequent attempt at
polymerizing the mixture via a Negishi polycondensation with
palladium did not yield any polymer/oligomer.
For example, an organozinc synthesis from 2,5-dibromothio-
phene and zinc dust, using an excess of zinc (5 equivalents),
led to numerous products—including some unreacted start-
ing dihalide. This poor selectivity could be evidenced by the
formation of (di)iodothiophenes of various structures after
iodination of the mixture of organometallics. After 15 min of
reaction for instance, a mixture of seven products could be
observed by GC coupled to mass spectrometry (Scheme 4):
monothienylzinc and dithienylzinc I and II (18%), reduced
products III, IV, and 1a (50%), thienylzinc V and VI from
reduced products (17%) and unreacted 2,5-dibromothio-
phene 2a (14%). An effort at minimizing the formation of
reduced products by using distilled acetonitrile led to the
same mixture. The above observations are not too surprising
as similar complexities have been observed with Grignard
reagents as reported by Holdcroft and coworkers, and can
usually be worked out by improving the synthetic conditions
and/or changing the metallation strategy.31 Surprisingly
though, an attempt to couple the above mixture by adding
copper (I) triflate (entry 3 in Table 2) yielded some insoluble
materials, whose soluble fraction could be identified by 1H
NMR in CDCl3 as a mixture of regioregular short oligomers
(mostly trimers and tetramers). The very poor solubility of
these oligomers in organic solvents, even worse than those
exhibited by the regioirregular polymers obtained previously,
convinced us that it would be very difficult to identify exper-
imental conditions to synthesize regioregular poly(1a), pre-
Polymer Characterization
As mentioned in the above section, oxidative polymerization
of 1a, either chemically (FeCl3) or electrochemically (Table 2,
entries 1–2), yielded reddish brown polymeric materials that
could be characterized by several techniques (NMR, IR, UV–
vis, TGA, and wide angle X-ray scattering (WAXS)), both in
solution and in the solid state.
1
The H NMR spectrum includes relatively simple signals from
the ethyl ester moiety, with a signal at 1.27 ppm for the
methyl group and at 4.18 ppm for the methylene directly
attached on the ester oxygen. The signals for the dimethylene
bridge connecting the esters to the polythiophene backbone
resulted in two separate sets for each methylene, correspond-
ing to the two possible diad configurations: head-to-tail (HT)
and head-to-head or tail-to-tail (HH or TT) (Scheme 5 and
Fig. 2). One set, at 3.07 and 2.84 ppm [B signals in Fig. 2(a,b)],
was assigned to the protons alpha to the thienyl ring (thio-
phene-CH2ACH2AC(¼¼O)O), whereas the second set, at 2.62
and 2.53 ppm [A signals in Fig. 2(a,b)], was attributed to the
protons on the methylene alpha to the ester carbonyl (thio-
phene-CH2ACH2AC(¼¼O)O). In each case, the upfield signals
in each series, at 3.07 and 2.62 ppm, respectively, were
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increase in HT diads described above and the simultaneous
increase in peak intensity at 6.96 ppm. It is interesting to
note that the relative position for the HT-HT triad configura-
tion among the four possible signals differs from those
observed for poly(3-alkylthiophene)s where the HT-HT signal
is always at the most upfield position among the four
possibilities. However, it correlates with the spectrum
reported for a regioirregular polythiophene displaying a
ACH2ACH2AOOCA (CH2)4CH3 lateral substituent, which also
had the HT-HT signal in the second position when reading
the spectrum according to rising d values.35 This similarity
suggests again that the effect arises from the long-distance
influence exerted by the ester group on the resonance of
the thienyl protons in the polymer.32 The exact assignment
of the other triad configurations is a risky business as
SCHEME 5 Structures of the four triad configurations theoreti-
cally possible for polythiophenes monosubstituted on the 3-
position of the aromatic ring (the substituent is represented by
a black circle).
attributed to the HT diads. This assignment is primarily based
on previous work on poly(3-alkylthiophene)s, whose NMR
spectra also display the above splittings, in addition to a com-
parison between the spectra obtained for the two polymer
samples [Fig. 2(a,b)], which obviously correspond to slightly
different amounts of HT links in the polymer.
The aromatic region of the spectrum contains, as expected,
four signals at 7.02, 7.03, 7.06, and 7.07 ppm for the only
proton left on the thienyl units after the polymerization
(position 4), corresponding to the four expected triads: HT-
HH, TT-HT, TT-HH, and HT-HT (Scheme 5). The peak resolu-
tion was not as good as the one we could obtain on an
authentic regioirregular poly(3-n-hexylthiophene). Attempts
to improve it by using a 600 MHz instrument or changing
the observation conditions did not significantly modify the
spectrum (see Supporting Information Fig. S1). A small
deshielding effect of about 0.1–0.2 ppm was observed with
respect to the four triad signals of poly(3-n-hexylthiophene)
whose positions are indicated by triangles in Figure 2. This
is expected, given the presence of the carboxylate unit in the
middle of the pendant group and the known long-distance
influence of ester carboxylates on the chemical shifts via
their magnetic anisotropies, electric fields, and steric
effects.32 An effect of slightly smaller magnitude (0.05 ppm)
could also be found in the monomer, with a chemical shift
for the proton on the 4-position of 2a at 6.96 ppm in CDCl3,
to be compared with literature values of 6.91–6.92 ppm for
3-(n-butyl)thiophene or 3-(n-hexyl)thiophene in the same
solvent.33 The signal at 7.03 ppm (second from right) was
attributed to the HT-HT triad in agreement with the signal
observed in the literature for the fully regioregular poly(n-
butyl thiophenepropionate) poly(1) (R ¼ n-butyl) whose
synthesis had been obtained by McCullough et al. via a post-
polymerization modification of regioregular poly(3-(2-oxazo-
linylethyl)thiophene (reaction with n-butanol).34 In addition,
the above assignment is compatible with the observed
FIGURE 2 Sections of the 1H NMR spectra sensitive to the diad
configuration [thiophene-CH2ACH2AC(¼O)O (B) and thiophene-
CH2ACH2AC(¼O)O (A) signals, see insets] and to the triad con-
figuration (aromatic region) for a poly(1a) sample obtained by
(a) FeCl3 oxidation and (b) electropolymerization, respectively.
The triangles in (a) indicate the position of the four triad sig-
nals for poly(3-n-hexylthiophene). All spectra provided in the q
scale in ppm and using the undeuterated chloroform peak as
the reference. The peaks identified by the SB acronym corre-
spond to the 13C satellite of the CHCl3 present in the solvent
(see text).
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the chemically (FeCl3) and electrochemically obtained sam-
ples (entries 1–2 in Table 2). In agreement with observations
already made for numerous polythiophenes, the absorption
maxima in solution were significantly blue shifted with
respect to values obtained for thin films.4 Peak maxima
(kmax) were indistinguishable for the two samples (electro-
polymerization and FeCl3 oxidation), at 444–446 nm in thin
films and 423 nm in CHCl3 dilute solutions. These numbers
are slightly blue-shifted compared with data obtained for
regioirregular poly(3-hexylthiophene), at 480 and 436 nm in
thin films and CHCl3 dilute solutions, respectively.39 The
CHCl3 solution value at 423 nm agrees very well, however,
with kmax values obtained in chloroform for other regioirre-
gular polythiophenes substituted on their 3-positions by
ester-containing substituents such as ACH2ACH2ACOOA
(CH2)2C6F13 (420 nm)15 or ACH2ACH2AOOCAR (402–428
nm (R ¼ alkyl)).40,41 This shift toward lower absorption
wavelengths might be attributed either to a local ‘‘solvato-
chromic’’ effect, the polar surrounding lateral substituents
interacting with the pi-conjugated backbone in the same way
a solvent molecule such as ethyl acetate would do, or to
lower conjugation lengths arising for example from strong
dipole–dipole interactions between the esters in head-to-
head linkages. It is noteworthy, in this context, to note that
the opposite effect (i.e., a red shift of the absorption maxima)
is observed for polythiophenes containing aliphatic ethers as
lateral susbtituents (kmax in the 450–517 nm range).42
FIGURE 3 Section of the IR spectrum containing vibrations
potentially sensitive to the regioregularity of the polymer: (a)
regiorandom poly(3-hexylthiophene),38 (b) poly(1a) obtained by
FeCl3 oxidation (containing ꢄ60% of HT diads), and (c) poly(1a)
obtained by electropolymerization (containing ꢄ70% of HT diads).
evidenced by the historical development of peak assignments
for poly(3-alkylthiophene)s, for which both correct and
wrong assignments have been proposed over time,31,36 until
the synthesis and full NMR characterization of all four possi-
ble model trimers allowed to unambiguously decide.37
The fact that, in the most regioregular sample, all three non-
(HT-HT) configurations provide signals of approximately the
same intensities makes the assignment even more difficult.
In this regard, it must be noted that a sample of poly(n-
dodecylthiophene) obtained by electropolymerization and
containing 71–72% of HT diad, that is, approximately the
same amount as our electropolymerized poly(1a), is charac-
terized by relative ratios of 16:13:16:54 for the four triad
signals,37 in agreement with the ratio observed in this study
for the triad signals of electropolymerized poly(1a).
X-ray scattering experiments on a thin film deposited on a
glass slide did not show any Braggs peak, suggesting that
side-chain crystallization does not happen, as expected in a
regioirregular sample.43
The thermal stability of the polymer was evaluated by ther-
mogravimetry analysis (TGA). The thermogram included in
Figure 5 is typical of results obtained under anaerobic condi-
tion (10 K minꢀ1 heꢂating rate, argon), with a small weight
loss in the 200–350 C range accounting to ꢄ5% of the ini-
ꢂ
tial amount, followed by an abrupt decomposition at 400 C
The infrared spectra exhibit more complex features than those
characteristic of poly(3-alkylthiophene)s [for instance, compare
Fig. 3(a) (regioirregular poly(3-n-hexylthiophene) and Fig. 3(b,c)
poly(1a)]. In particular, the ratio of signal intensities for bands at
ꢄ1460 and ꢄ1510 cmꢀ1 [identified by black triangles below Fig.
3(a)] could not be used, as it is usually the case for poly(3-
alkylthiophene)s to estimate the amount of regioselectivity, due
to the presence of several additional vibration peaks in the
region of interest. A comparison of spectra obtained for two sam-
ples displaying slightly different levels of regioselectivity (spectra
b and c in Fig. 3) suggests that bands at ꢄ830 and ꢄ860 cmꢀ1
(identified by black lozenges below spectrum c Fig. 3) are the
only ones whose ratio is affected by the regioselectivity. A theo-
retical simulation of the monomer IR spectrum (density func-
tional theory (DFT), EDF2/6-31G*, Supporting Information Fig.
S2) suggests that these bands correspond to CAS vibrations.
FIGURE 4 UV–visible spectra of (a,b) poly(1a) obtained by
FeCl3 oxidation (containing ꢄ60% of HT diads), and (c,d)
poly(1a) obtained by electropolymerization (containing ꢄ70%
of HT diads) (a,b: CHCl3 solutions; b,c: thin films on a glass
slide).
Figure 4 includes the UV–visible absorption spectrum of
poly(1a) in CHCl3 solutions and as solid thin films, for both
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alkylthiophenes, in particular the intra- or intermolecular
nucleophilic attack of the ester moiety by an organometallic
species during a reductive polymerization.
Based on our results on the polymerizability of monomers
1a–3a, it can be tentatively concluded that monomer 1a poly-
merized under oxidative conditions exhibits indeed a very
close similarity in reactivity with 3-n-alkylthiophenes.4,30 In
particular, the almost identical regioselectivities observed in
the polymers suggest that the unconjugated ester group has
little, if any, influence on the fate of the oxidatively generated
radical-cations. Preliminary investigations on the reductive po-
lymerization of 2,5-dihalogenated monomers 2a and 3a sug-
gest that regioregular poly(2a) is too insoluble in organic sol-
vents to allow for the synthesis of polymers of decent
molecular weights. Modifying the nature of the alkyl group on
the ester to make the polymer more lipophilic and easier to
dissolve in common organic solvents should help in circum-
venting this problem. Preliminary experiments to test this
hypothesis are currently under way in our group.
FIGURE 5 Thermogram corresponding to the thermal anaero-
bic decomposition of poly(1a) (10 K minꢀ1, Ar).
corresponding to a loss of about 35%. A very similar degra-
dation behavior had previously been reported for poly(3-n-
hexylthiophene), with a weight loss of about 40% occurring
rapidly at 400–450 ꢂC followed by an extremely slow extra
loss (ꢅ15%) in the 500–1200 ꢂC range.44 The initial loss at
400 ꢂC was attributed by the authors to the decomposition
of the pendant n-hexyl groups. The presence of an ester
functionality introduces a weaker link in the chain and is
expected to slightly lower the degradation temperature—as
observed experimentally. Based on the measured weight loss
The original goal of this work—as explained in more details
in the Introduction—was to design a general alternate strat-
egy to obtain functionalized polythiophenes displaying a
large variety of side substituents. This challenge lies by defi-
nition at the interface of organic and polymer synthesis as
both some easy access to targeted families of monomers and
an efficient route to polymerize them have to be proposed in
tandem. In addition, a fair assessment of the scope and limi-
tations should include the several criteria in the analysis:
versatility (i.e., ability to easily modify the nature of the side
groups), generality (i.e., number of side groups that can theo-
retically be inserted), efficiency (i.e., overall yield of the syn-
thesis), complexity (as defined by the number of synthetic
steps and their technical difficulties (use of dry or rare
reagents, involvement of complicated synthetic or purifica-
tion protocols)), and last but not least, cost.
ꢂ
at 400 C (ꢅ35%), it can be hypothesized that this decompo-
sition step corresponds to the loss of the CO2Et subunit (the-
oretical loss ¼ 40.1%).
DISCUSSION
Very little information is available in the literature on the po-
lymerization of 3-(2-(thienyl) propionate monomers, in con-
trast to 3-alkyl- or 3-arylthiophenes, ether-containing thio-
phenes or thiophenes directly substituted on the 3-position
by an ester group, whose polymerizations have been
described extensively by many authors.4,5,7 The only paper
dealing with the polymerization of 1a describes its assumed
electropolymerization, although the obtained materials was
not isolated, nor characterized.16 A more recent paper
describes the oxidative polymerization of perfluroalkyl
2-thienyl-3-propionates, but provides little details on the
structure of the obtained polymers and does not examine
the polymerizability extensively.42
Before offering some preliminary conclusions as to whether
the approach proposed in this contribution might help in
meeting the above goal, it might be worth to rapidly review
the competing approaches currently available to the syn-
thetic chemists. The reader interested in a more comprehen-
sive overview is referred to the several reviews available on
the topics,5,7 with a special emphasis on a recent contribu-
tion detailing the uses and preparation of chiral-conjugated
polymers, as the same basic culprit applies, that is, having to
decide how to efficiently place (in this case) chiral side sub-
stituents on a polymer such as a polythiophene.10
From a structural viewpoint, the insulating character of the
dimethylene bridge located between the ester and the aro-
matic ring in monomer 1a suggests that the reactivity of the
thiophene ring should not be affected greatly in comparison
with traditional 3-alkyl-substituted thiophene monomers.
This assumption was confirmed by a DFT computational
analysis (EDF2/6-31G* level) of 1a and 3-n-hexylthiophene,
which resulted in an identical HOMO-LUMO gap of 5.86 eV
for both monomers (HOMO: highest occupied molecular or-
bital, LUMO: lowest unoccupied molecular orbital). The key
difference in polymerizability is thus expected to arise from
possible side reactions for 1a that are not possible for 3-
The most common route to functionalized polythiophenes is
unquestionably the one using alkyl or aryl groups as side sub-
stituents. A few monomers of this type are even commercially
available, and many useful polymerization methods have been
proposed for this family of monomers. In addition, the attach-
ment of the side group to the backbone is very robust. The
main weaknesses lie in the price tag of the organometallic pro-
cedures needed to obtain the monomers, which is indirectly
reflected in the high costs of the commercial ones. Ether and
thioether groups have been proposed as well. They offer the
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same hydrolytic stability as the alkyl groups, but introducing
them on the 3-position of the monomer is very demanding
synthetically, so they are not considered very often.
able production cost. Further investigations are currently
under way to expand the scope and determine the limitations
of the developed process, in particular to examine whether
other monomers of interest could be obtained and polymer-
ized, including under controlled (reductive) conditions.
Another line of attack, considerably more versatile, consists
in using an ester group (in some cases an amide) as a multi-
purpose linker to connect the residue whose attachment is
required to the conjugated polymer. The monomers investi-
gated in this study belong to this class of precursors. The
related methodology is potentially compatible with many
substituents, and the insertion on the side substituents can be
designed in a convergent rather than in a sequential way, this
last feature being particularly attractive when a family of sub-
stituents rather than a single one has to be considered. Easy,
direct access to one of these key intermediates (2a) is the
main benefit brought by the electrochemical synthetic
approach designed in this study. In comparison, other synthe-
sis proposed to date is generally characterized by low overall
yields, multiple steps, and/or the use of expensive reagents. As
an example, the only two examples of thiophene monomers
with a ACH2ACH2ACOOR (R ¼ ethyl or CH2CH2C6F13) require
four or five consecutive steps in their synthesis,15,16 instead of
just one here (or two if the monomer is to be obtained from
2a by transesterification of the ethyl ester group).
The authors are very grateful to Nicole Thuaud [size exclusion
ꢀ
chromatography, Institut de Chimie et des Materiaux Paris Est
(ICMPE)], Sandrine Tussaud-Nenez (X-ray, ICMPE) and Oleg
Maciejak (NMR, Evry University), as well as Christine Gaillet
(NMR, ICMPE) and Nelly Lacoudre (thermogravimetry analysis,
ICMPE) for their help in the characterization of the polymers.
They also thank the ICMPE institute for an initial investigation
grant, and the IUT (East Paris University) for granting an ATER
fellowship to C. Ceballos.
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