Systematic study on chemical oxidative and solid-state polymerization of poly(3,4-ethylenedithiathiophene)

Article abstract of DOI:10.1002/pola.25971

Systematic research on the synthesis, chemical oxidative polymerization of 3,4-ethylenedithiathiophene (EDTT) in the presence of surfactants or not, and solid-state polymerization of 2,5-dibromo-3,4-ethylenedithiathiophene (DBEDTT) and 2,5-diiodo-3,4-ethylenedithiathiophene (DIEDTT) under solventless and oxidant-free conditions has been investigated. Effects of oxidants (Fe ³⁺ salts, persulfate salts, peroxides, and Ce⁴⁺ salts), solvents (H₂O, CH₃CN/H₂O, and CH ₃CN), surfactants, and so forth on polymerization reactions and properties of poly(3,4-ethylenedithiathiophene) (PEDTT) were discussed. Characterizations indicated that FeCl₃ was more suitable oxidant for oxidative polymerization of EDTT, while CH₃CN was a better solvent to form PEDTT powders with higher yields and electrical conductivities. Dispersing these powders in aqueous polystyrene sulfonic acid (PSSH) solution showed better stability and film-forming property than sodium dodecylsulfate and sodium dodecyl benzene sulfonate. Oxidative polymerization of EDTT in aqueous PSSH solutions formed the solution processable PEDTT dispersions with good storing stability and film-forming performance. Solvent treatment showed indistinctive effect on electrical conductivity of free-standing PEDTT films. As-formed PEDTT synthesized from solid-state polymerization showed similar electrical conductivity, poorer stability, but better thermoelectric property than oxidative polymerization. Contrastingly, PEDTT synthesized from DIEDTT showed higher electrical conductivity (0.18 S cm^-1) than DBEDTT which showed better thermoelectric property with higher power factor value (6.7 × 10^-9 W m^-1 K^-2).

Full text of DOI:10.1002/pola.25971

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Systematic Study on Chemical Oxidative and Solid-State Polymerization
of Poly(3,4-ethylenedithiathiophene)
1
1,2
1
1,2
Shuai Chen, Baoyang Lu, Xuemin Duan, Jingkun Xu
1
School of Pharmacy, Jiangxi Science & Technology Normal University, Nanchang 330013, China
School of Materials Science & Engineering, Shandong University, Jinan 250061, China
2
Correspondence to: X. Duan (E-mail: duanxuemin@126.com) or J. Xu (E-mail: xujingkun@tsinghua.org.cn)
Received 16 September 2011; accepted 30 January 2012; published online 23 February 2012
DOI: 10.1002/pola.25971
ABSTRACT: Systematic research on the synthesis, chemical oxi-
dative polymerization of 3,4-ethylenedithiathiophene (EDTT) in
the presence of surfactants or not, and solid-state polymerization
of 2,5-dibromo-3,4-ethylenedithiathiophene (DBEDTT) and 2,5-
diiodo-3,4-ethylenedithiathiophene (DIEDTT) under solventless
and oxidant-free conditions has been investigated. Effects of oxi-
Oxidative polymerization of EDTT in aqueous PSSH solutions
formed the solution processable PEDTT dispersions with good
storing stability and film-forming performance. Solvent treatment
showed indistinctive effect on electrical conductivity of free-stand-
ing PEDTT films. As-formed PEDTT synthesized from solid-state
polymerization showed similar electrical conductivity, poorer sta-
bility, but better thermoelectric property than oxidative polymer-
ization. Contrastingly, PEDTT synthesized from DIEDTT showed
3þ
4þ
dants (Fe salts, persulfate salts, peroxides, and Ce salts), sol-
vents (H O, CH CN/H O, and CH CN), surfactants, and so forth on
2
3
2
3
ꢀ
1
polymerization reactions and properties of poly(3,4-ethylenedithia-
thiophene) (PEDTT) were discussed. Characterizations indicated
higher electrical conductivity (0.18 S cm ) than DBEDTT which
showed better thermoelectric property with higher power factor
value (6.7 ꢁ 10ꢀ W m
9
ꢀ1
ꢀ2
K
). VC 2012 Wiley Periodicals, Inc.
that FeCl
3
was more suitable oxidant for oxidative polymerization
CN was a better solvent to form PEDTT pow-
of EDTT, while CH
3
J Polym Sci Part A: Polym Chem 50: 1967–1978, 2012
ders with higher yields and electrical conductivities. Dispersing
these powders in aqueous polystyrene sulfonic acid (PSSH) solu-
tion showed better stability and film-forming property than so-
dium dodecylsulfate and sodium dodecyl benzene sulfonate.
KEYWORDS: analogs of PEDOT; chemical oxidative polymeriza-
tion; conducting polymers; poly(3,4-ethylenedithiathiophene);
solid-state polymerization
INTRODUCTION Conducting polymers, since their initial dis-
covery in 1977, have won fast-growing attention for their
applications in transparent conductive coatings, solid electro-
lyte capacitors, antistatic coatings, electroluminescent lamps,
organic light emitting diodes (OLEDs), organic solar cells
processable PEDOT product, poly(3,4-ethylenedioxythio-
phene)/poly(styrene sulfonate) (PEDOT/PSS, Scheme 1), a
dark-blue aqueous dispersion, was marked by Bayer AG.
2
–4
By coating or printing this dispersion, highly stable, conduct-
ing, transparent, mechanically durable, and insoluble films
can be formed and their properties can be adjusted by add-
ing various formulation components or processing condi-
(
OSCs), electrochromic materials, organic field-effect transis-
1
,2
tors (OFETs), printed wiring boards, and so on. However,
in the early stage, typical conducting polymers such as poly-
anilines, polypyrroles, polythiophenes exhibited low solubil-
ity and poor processability. Therefore, a great deal of efforts
have been focused on the modification of their chemical
structures, in order to alter their electronic structures and
improve their electrical properties, environmental stability,
2
,3,5
tions.
To date, PEDOT/PSS has been the most successfully
commercialized polythiophene. Based on it, conducting poly-
mers enclosed PEDOT attract more and more attention
6
–11
because of their wide commercial applications,
for exam-
ple, as antistatic coating materials, as transparent anodes
and buffer layers in OSCs, as hole-injection layers in OLEDs,
2
2
and processability. A breakthrough progress has been
as electrodes, and active layers in OFETs. In the past twenty
achieved in the preparation of poly(3,4-ethylenedioxythio-
years, vast amount of work has been carried out on the
design and polymerization of EDOT analogs with novel struc-
ture and the properties of the resulting polymers.
phene) (PEDOT), which was developed from polythiophene
2
2,3
(
as shown in Scheme 1). It not only benefits smaller steric
hindrance, lower oxidation potential, and higher stability, but
also overcomes lower electrical conductivities of polyalkylth-
Up to now, the family of EDOT analogs includes (1) ring-size
variations, 3,4-methylenedioxythiophene (MDOT), 3,4-propyle-
nedioxythiophene (PDOT), and 3,4-butylenedioxythiophene
2
,3
iophenes and polyalkoxythiophenes.
In 1990, the first
Shuai Chen and Baoyang Lu contributed equally to this work.
2012 Wiley Periodicals, Inc.
V
C
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SCHEME 1 Structural evolution from polyacetylene to PEDOT and PEDOT/PSS.
1
2,13
(
(
BDOT);
MDTT), 3,4-ethylenedithiathiophene (EDTT), EDTT-R (R ¼ Br,
CH OH etc.), 3,4-propylenedithiathiophene (PDTT), and
(2) sulfur analogs, 3,4-methylenedithiathiophene
Therefore, systematical investigation of chemical oxidative
polymerization of EDTT is very necessary.
2
In this article, chemical oxidative polymerization of EDTT by
different oxidants, with different kinds of sulfonate surfac-
tants or no, was discussed in detail. Solid-state polymeriza-
tion of 2,5-dibromo-/diiodo-EDTT under solventless and oxi-
dant-free conditions was also studied carefully for
comparison. The effect of factors such as oxidants, solvents,
surfactants, additives, reaction temperatures, etc. on electri-
cal conductivity, processability, stability, and other properties
of corresponding polymers were investigated.
14–18
thieno[3,4-b]-1,4-oxathiane (EOTT);
,4-ethylenedioxyselenophene (EDOS), 3,4-propylenedioxysele-
nophene (PDOS), PEDOS-R (R ¼ C H , n ¼ 4, 6, 10), 3,4-
(3) selenium analogs,
3
2
2
n
2nþ1
ethylenedithioselenophene (EDTS), seleno[3,4-b]-1,4-oxathiane
19–25
(
EOTS), and 3,4-ethylenediselenathiophene (EDST);
nitrous analogs, 3,4-methylenedioxypyrrole (MDOPy), 3,4-ethyl-
enedioxypyrrole (EDOPy), 3,4-propylenedioxypyrrole (PDOPy),
(4)
3
(
,4-butylenedioxypyrrole (BDOPy), 3,4-methylenedithiopyrrole
2
6–29
MDTPy), and 3,4-ethylenedithiopyrrole (EDTPy);
phosphorous analog, 3,4-ethylenedithiaphosphole (EDPP);
6) oxygenic analogs, 3,4-ethylendithiafurane (EDTF), 3,4-eth-
ylenedioxyfuran (EDOF), and 3,4-dihydro-2H-thieno[3,4-b]py-
ran (EDTP);
phene (EDOTe) as a predicted structure based on 3,4-
(5)
3
0
(
EXPERIMENTAL
Materials
3
1–33
(7) telluric analog, 3,4-ethylenedioxytelluro-
3,4-Dibromothiophene (Shanghai Bangcheng Chemical Co.,
China), 1,2-ethanedithiol (Shanghai Zhuorui Chemical Co.,
China), and ethylene glycol (EG; Beijing East Longshun
Chemical Plant, China) were all analytical grade (98þ%) and
distilled under reduced pressure prior to use. Chloroform
(99þ%; Beijing East Longshun Chemical Plant) and acetoni-
trile (99þ%; Tianjin Bodi Chemicals Co., China) were puri-
fied by distillation with calcium hydride under a nitrogen
atmosphere before use. Toluene (98þ%; Beijing East Long-
shun Chemical Plant) was washed with concentrated sulfuric
acid and saturated sodium bicarbonate in sequence, and
then distilled under reduced pressure before use. Methanol
(99þ%; Tianjin Bodi Chemicals Co.) was distilled from mag-
nesium powder in the presence of catalytic amount of iodine
under a dry argon flow. N-bromosuccinimide (NBS, 99%),
N-iodosuccinimide (NIS, 99%), polystyrene sulfonic acid
3
4
dimethoxytellurophene (DMOTe). Structures of the above
are summarized in Scheme 2.
Among them, EDTT, as the all-sulfur analog of EDOT, shows
some unique properties.
lower oxidation potential compared to EDOT, and there is
a better electronic interaction between polymer chains
1
5,35–39
For example, EDTT has a
…
through intermolecular S S attractions in PEDTT than
1
5,35,39,40
PEDOT.
Nowadays, PEDTT and its derivatives have
found possible applications as the electron donor materials
in photovoltaic devices,
materials for rechargeable lithium batteries with better cath-
ode capacity than PEDOT. More recently, electrosynthesized
PEDTT materials were introduced into organic electronic dis-
play field and sensors owing to their electrochromic and elec-
3
5,37,38,41–44
and as the cathode active
4
5
2
1,46–48
trochemical catalytic properties.
However, since firstly
(PSSH, M
w
¼ 75,000; 30 wt % aq.), and polystyrene sulfo-
1
5
reported by Kanitzidis and coworkers in 1995, much of the
research associated with EDTT has entailed the incorporation
of it with other conjugated structures or introduced other
nate sodium salt (PSSNa, M ¼ 70,000) were products of
w
Alfa Aesar and used as received. Strongly basic anion-
exchange resin (201 ꢁ 7, batch number F20080403) and
strongly acid cation-exchange resin (001 ꢁ 7, batch number
F20090303) were purchased from China National Medicine
Group Shanghai Chemical Reagent Company. Other chemicals
and reagents were all purchased commercially in analytical
grade (98þ%) from Beijing East Longshun Chemical Plant
(China) and used directly without any further treatment.
1
6,17,35,37,38,41,42,46
functional groups into its structure,
and
most polymers relating to PEDTT were prepared by electro-
chemical polymerization. Inconsistent with its significance to
actual applications, chemical oxidative polymerization of
EDTT and its derivatives was barely reported and characteri-
zation of corresponding polymers has been not adequate.
1
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SCHEME 2 Analogs derived from the structure of EDOT.
Synthesis of 3,4-Dimethoxythiophene
umn chromatography using petroleum ether as the eluent to
give about 17.50 g (121.5 mmol) of product as colorless oil.
Sodium (14.220 g, 618.3 mmol) was added into a 500-mL
three-necked round bottom flask which fitted with a reflux con-
denser under an atmosphere of nitrogen. Methanol (150 mL)
was added dropwise and stirred until sodium dissolves
completely. 3,4-Dibromothiophene (30.630 g, 126.6 mmol),
KI (0.205 g, 1.237 mmol), and CuO (9.840 g, 123.7 mmol)
Yield: 96%. R : 0.05 (silca gel, petroleum ether; the by-prod-
f
uct 3-bromo-4-methoxythiophene: 0.28, 3,4-dibromothio-
1
phene: 0.71). H NMR (400 MHz, CDCl , d, ppm): 3.86
3
(s, 6H), 6.20 (s, 2H).
ꢂ
were added and then the mixture was refluxed at 97 C for
Synthesis of EDTT
4
1
8 h. After that, another 0.205 g (1.237 mmol) of KI and
0 mL of 30% sodium methoxide/methanol solution was
A mixture of 3,4-dimethoxythiophene (16.400 g, 113.9 mmol),
p-toluenesulfonic acid (0.824 g, 4.784 mmol), 1,2-ethanedithiol
(43.100 g, 457.5 mmol), and toluene (100 mL) was stirred
added. The mixture was refluxed for 12 h and another
0 mL of 30% sodium methoxide/methanol solution was
ꢂ
1
under nitrogen atmosphere for 48 h at 90 C. The reaction
added and stirred for another 12 h. Thin-layer chromatogra-
phy (TLC) was used to detect the completion of the reaction.
After most of the solvent was distilled out at the same tem-
perature, the mixture was cooled, filtered, diluted with water,
and extracted with ether. The collected organic layer was
was monitored by TLC. Later, another 43.100
g
(457.5 mmol) 1,2-ethanedithiol and 0.824 g (4.784 mmol)
p-toluenesulfonic acid was added and stirred for another
24 h at the same temperature. After that, the mixture was
allowed to cool to room temperature, removed toluene under
reduced pressure, extracted with ether, and then washed
with 5% NaOH solution and water. The collected organic
dried over anhydrous MgSO , filtered, and the solvent was
4
removed on a rotary evaporator to afford a yellow-brown oil
liquid. The crude compound was purified by silica gel col-
phase was dried over anhydrous MgSO , filtered, and
4
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Dispersion of Oxidatively Polymerized PEDTT in PSSH
Above PEDTT (doped) powder (0.500 g) was added into
1
00.455 g PSSH aqueous solution (1.5 wt %), underwent a
ꢂ
high speed shear process at 25 C for 1 h and then treated
with homogenizer. Dispersion (Sample 1) was obtained with
pH of 0.91 and viscosity of 1.9 mPa s. Following the same
treatment, neutral PEDTT powder was transferred to a dis-
persion (Sample 2) with pH of 0.93 and viscosity of 0.7 mPa
s. Other anionic surfactants such as sodium dodecylsulfate
(
SDS), sodium dodecyl benzene sulfonate (SDBS), and PSSNa
were also used to replace PSSH.
SCHEME 3 Synthesis of EDTT and EDOT. pTSA ¼ p-toluene-
sulfonic acid.
Oxidative Polymerization of EDTT in Aqueous
PSSH Solutions
concentrated in vacuum. The crude product was purified by
means of silica gel column chromatography using petroleum
EDTT (0.614 g, 3.523 mmol) was added into 86.169 g PSSH
aqueous solution (1.5 wt%) and the mixture was stirred vig-
ether as the eluent to afford a colorless liquid (19.050 g).
1
Yield: 90%. R : 0.31 (silca gel, petroleum ether). H NMR
f
ꢂ
orously for 20 min in 25 C water-bath under nitrogen.
(
400 MHz, CDCl , d, ppm): 3.23 (s, 4H), 6.98 (s, 2H).
3
Upon addition of FeCl3 (0.742 g, 4.574 mmol), EDTT poly-
merized immediately, leading to a cream mixture. This mix-
Synthesis of 2,5-Dibromo-3,4-ethylenedithiathiophene
,5-Dibromo-3,4-ethylenedithiathiophene (DBEDTT) crystal
ꢂ
2
ture was stirred for 24 h at 25 C and subjected to desalting
was prepared by straightforward bromination of EDTT with
NBS in 58% yield as described in Scheme 3 according to the
literature.
as follows. The as-prepared mixture was diluted with twice
its volume of deionized water, and stirred for 7 h at 25 C in
ꢂ
2
1
the presence of granulated strongly basic anion-exchange
resins (201 ꢁ 7). Then, the ion-exchange resins were filtered
off, and the filtrate was treated with strong acid cation-
exchange resin (001 ꢁ 7) in the same way. The final filtrate
was condensed to the solid content of 2.1 wt %. Following
treatment with homogenizer, it finally gave an aqueous
PEDTT/PSS dispersion (Sample 3) with pH of 0.55 and vis-
cosity of 1.7 mPa s.
Synthesis of 2,5-Diiodo-3,4-ethylenedithiathiophene
To a solution of EDTT (1.743 g, 10 mmol) in CHCl (40 mL)
3
ꢂ
and glacial acetic acid (AcOH, 20 mL) at 0 C, NIS (4.837 g,
2
1.50 mmol) was added slowly under nitrogen atmosphere.
ꢂ
After stirring at 0 C for 15 min, the solution was further
stirred at room temperature for 24 h. The resulting mixture
was diluted with 100 mL deionized water and extracted
2
þ
Other oxidants such as Fe(NO ) ꢃ9H O, H O /Fe , Na S O ,
3
3
2
2
2
2 2 8
with CHCl (50 mL ꢁ 3). The organic phase was neutralized
3
3þ
and Na S O /Fe were also used to replace FeCl .
2
2
8
3
with 10% NaHCO solution, washed with deionized water, dried
3
with anhydrous MgSO , and concentrated. The crude product
4
was isolated by column chromatography (silica gel; hexane/
CHCl : 5/1, v/v) to give a light brown solid. It was recrystal-
3
lized from ethanol to produce 1.747 g of light tawny crystals.
Treat PEDTT Dispersions with Organic Solvents
PEDTT dispersions (Sample 3) were mixed with various
amount of dimethyl sulfoxide (DMSO) (0 wt %, 5 wt %,
Yield: 41%. R : 0.31 (silca gel; hexanes/CHCl : 5/1, v/v). m.p.:
f
3
1
0 wt %, 15wt %, 20 wt %, and 25 wt %). These mixtures
ꢂ
30.0 C. H NMR (400 MHz, CDCl , d, ppm): 3.22 (s, 4 H).
3
1
1
ꢂ
were stirred for 24 h at 25 C, treated by using ultrasonic
vibration for 5 min, and then cast onto polypropylene (PP)
film substrate. After drying at 80 C in vacuum (38 mmHg)
Chemical Oxidative Polymerization of EDTT
ꢂ
EDTT (4.673 g, 26.811 mmol) was dropped into 154 mL
CH CN solution of FeCl (9.992 g, 61.600 mmol), and the
until constant weight, free-standing PEDTT films were formed.
3
3
ꢂ
mixture was stirred vigorously for 72 h in 25 C water-bath.
A dark-green powder was filtered, washed successively with
ꢂ
CH
3
CN and acetone. After drying at 60 C for 18 h, an
amount of 2.845 g PEDTT product was obtained in the
doped form. Moreover, a brown neutral PEDTT powder
[
0.821 g, Fig. 1(right)] was obtained by treating the dark-
green powder [1.005 g, Fig. 1(left)] with hydrazine hydrate
(
5 mL) and H O (100 mL) for 48 h, and was filtered, washed
2
ꢂ
with water, and dried at 60 C for 24 h.
Following above oxidative polymerization route, other oxi-
2
þ
dants such as (NH
4 2 2 8 2 2 4 2
) S O , H O /Fe , Ce(SO ) , and
(
NH ) Ce(NO ) were used to replace FeCl3 and other sol-
FIGURE 1 Photographs of as formed (doped) and neutral
4
2
3 6
vents such as H O and CH CN/H O (1/1, v/v) were used to
PEDTT powders formed by chemical oxidative polymerization
2
3
2
replace CH CN.
3 3
of EDTT with FeCl in CH CN.
3
1
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SCHEME 4 Chemical oxidative and solid-state polymerization of EDTT. AcOH ¼ glacial acetic acid.
Other organic solvents such as EG, dimethyl formamide
DMF), N-methyl-2-pyrrolidone (NMP), and acetone were
also used to replace DMSO.
(CH-1-S/ST, Shanghai Liuling Meter Co) was used to measure
the thickness of pellets and films. Melting points were
recorded on a hot-coil stage melting point apparatus (XSZ-
(
4
G, Beijing Fuka Keyi Science and Technology Co.). X-ray dif-
Solid-State Polymerization of DBEDTT and DIEDTT
DBEDTT (2.494 g, 7.508 mmol) and 2,5-diiodo-3,4-ethylene-
dithiathiophene (DIEDTT) (3.194 g, 7.496 mmol) were added
into two 50-mL round flasks, respectively. After heating at
fraction (XRD) investigations were performed on a Bruker
D8 ADVANCE X-ray diffractometer using Cu Ka radiation
method (40 kV, 40 mA). A Cold Field Emission Scanning
Electron Microscope (S-4300 N, Hitachi) was used to analyze
the surface morphologies of the polymer films.
ꢂ
ꢂ
40
C (for DBEDTT) and 110 C (for DIEDTT) for 7 days
ꢂ
under room conditions (T ¼ 30 C, RH ¼ 36%), correspond-
ing PEDTT solids (SSP-DBEDTT and SSP-DIEDTT) were
formed, as shown in Scheme 4.
RESULTS AND DISCUSSION
Synthesis of EDTT
Following the synthetic strategy depicted in Scheme 3, EDTT
was prepared in one step by acid-catalyzed nucleophilic sub-
stitution of 3,4-dimethoxythiophene with 1,2-ethanedithiol
Characterization
H NMR spectra were recorded on a Bruker AV 400 NMR
spectrometer and CDCl
spectra were measured with a Perkin-Elmer Lambda 900
UV–vis-near-infrared spectrophotometer. The pH values of
PEDTT dispersions were tested by a pen-like pH meter
1
3
was used as the solvent. UV–vis
2
1
49
with a yield of 90% (lit. 77%, lit. 40%). It should be
noted here that the precursor 3,4-dimethoxythiophene was
synthesized from a commercial chemical, 3,4-dibromothio-
4
9
50
phene, with a yield of 96% (lit. 81%, lit. 70%). In com-
parison with other routes,
(
CT-6022, Shanghai Rentong Meter Co.). The viscosities were
1
5,16,22,35,51–54
this transetherifica-
measured by a digital viscometer (NDJ-5S, Shanghai Hengp-
ing Meter Co.). The temperature dependence of both electri-
cal conductivity (r) and Seebeck coefficient (S) were deter-
mined by using the standard four-probe technique by
measurement unit of thermoelectric properties coupled with
a liquid nitrogen container for pressed pellets or free-stand-
ing films at temperatures from room temperature to
tion reaction can avoid complex operating procedures and
strict reaction conditions. More importantly, by modifying
TABLE 1 Yields and Electrical Conductivities (Measured at
2
98 K and 60% RH) of PEDTT from Chemical Oxidative
3
Polymerization of EDTT by FeCl in Different Solutions
ꢀ
1
100–150 K. Cu (99.9%) wires as electrodes and the thermo-
Solvent
Yield
r (S cm )
couples were bound to a sample with conductive carbon
paint (colloidal graphite in isopropanol-20% solids) at room
temperature. PEDTT pellets were compressed for the mea-
surements under a pressure of 15 MPa. Thickness Gauge
H
2
O
55%
58%
61%
0.11
0.14
0.19
CH
3
CN/H
2
O (1/1, v/v)
CH
3
CN
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thiol as shown in Scheme 3, but also much higher than pre-
2
1,35–37,49,50
vious reports.
Effect of Oxidants and Solvents on Chemical
Oxidative Polymerization of EDTT
5
5
Although electrochemical polymerization and many other
2
polymerization methods, such as in-situ polymerization,
5
6,57
Stille coupling reaction,
transition metal-mediated oxida-
tive dispersion polymerization, Kumada coupling polymer-
5
8
5
9
60
ization, ring-opening metathesis polymerization, and so
forth, have been developed to prepare polythiophenes
including PEDOT and its analogs, chemical oxidative poly-
merization is still the most important and irreplaceable way
for large-scale industrial applications until now. Therefore,
herein, chemical oxidative polymerization of EDTT with oxi-
dants was performed in different solvents. The main atten-
tion was paid to achieve high conducting polymers with
good yields by optimizing oxidation conditions, including sol-
vents and oxidants. In the viewpoint of the cost, processing,
and safety, water was the best choice as the solvent for oxi-
dation polymerization. However, EDTT owns extremely low
2
water-solubility in H O, which partially offsets its advan-
tages. A possible improving strategy is adding CH CN, a
3
much suitable highly polar solvent media allowing complete
dissolving of EDTT. Finally, in this study, H O, CH CN, and
2
3
CH CN/H O were investigated for the chemical polymeriza-
3
2
2
þ
tion of EDTT. For oxidants, FeCl , (NH ) S O , H O /Fe
,
3
4 2
2
8
2 2
4 2 4 2 3 6
Ce(SO ) , and (NH ) Ce(NO ) were used. Among them,
FeCl3 seems the most convenient, because it is dissolved
completely both in CH CN and in H O. Characterization of
3
2
UV–vis, solubility, yields, electrical conductivity, and so forth,
of the final polymers was carried out. The primary results of
6
1
them have been enclosed in another report. In conclusion,
EDTT oxidized with FeCl3 led to PEDTT with the highest
ꢀ
1
15
electrical conductivity (0.1–0.2 S cm
at 298 K, lit.
ꢀ
1
FIGURE 2 UV–vis absorption (a) and fluorescence emission
0.1 S cm ) and better yields (55–61%). Moreover, as listed
in Table 1, a dark-green PEDTT powder with the highest con-
ductivity of 0.19 S cm at 298 K and the best yield of 61%
spectra (b) of doped and neutral PEDTT formed by chemical
ꢀ
1
oxidative polymerization of EDTT with FeCl
3
in CH
3
CN. Solvent:
ꢀ1
NMP (0.1 mg mL ).
was achieved in CH CN. Brown neutral PEDTT powder was
3
obtained in 81.69% yield after treating this as-prepared
powder with hydrazine hydrate for 48 h (Fig. 1). Both poly-
mers can partly dissolve in NMP (0.1 mg mL ) at room
temperature. Electronic spectra and photoluminescence spec-
tra of them in NMP display two absorption bands (410 nm
the raw material quantity, reaction time and post-treatment
as described in the Experimental part, the total yield of
EDTT can rise up to 86.40%, not only a little better than
ꢀ
1
5
2
that of EDOT (80.64%, lit. 39%) when EG replaced the
FIGURE 3 Comparison of the storing stability and dispersive performances of PEDTT dispersions Sample 1 (a), Sample 2 (b), and
Sample 3 (c).
1
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FIGURE 4 Images of the free-standing films (the later was folded from the former) casting from PEDTT dispersion (Sample 3) on
PP substrate.
and 333 nm for doped, 418 nm and 341nm for neutral) in
the UV–visible region [Fig. 2(a)], and broad band lumines-
cence with a peak at 569 nm [Fig. 2(b)]. These values are
Chemical Oxidative Polymerization of EDTT
in Aqueous PSSH Solutions
In comparison to the aforementioned method to prepare
PEDTT dispersions, chemical oxidative polymerization of
EDTT with oxidants in aqueous PSSH solutions (as shown in
Scheme 4) has obvious advantages. It combined the polymer-
ization of EDTT with the dispersion of PEDTT, and finally
formed PEDTT/PSS complex rather than just blends of
PEDTT and PSSH. Three kinds of oxidants, including iron
salts, persulfates and peroxides, were chosen. Differently
from chemical oxidative polymerization forming PEDTT pow-
1
5
quite similar to Kanatzidis and coworkers’ results.
Effect of Surfactants on Dispersion of PEDTT
In order to improve the solubility of oxidatively polymerized
PEDTT, a simple processing method was first introduced to
prepare processable PEDTT dispersions in water, as shown
in Scheme 4. Generally, PEDTT powders were dispersed into
aqueous surfactant solutions through a high-speed shear
treatment. For this, aforementioned PEDTT powders (doped
and neutral) synthesized from oxidative polymerization of
3
þ
ders above, Fe(NO ) ꢃ9H O, Na S O , and Na S O /Fe
3
3
2
2
2
8
2 2 8
6
1
were used to replace unpleasant (NH ) S O , and more
4
2 2 8
6
1,62
EDTT with FeCl
3
in CH
3
CN, and several common sulfonate
aggressive ceric-based oxidants.
Following desalting with
surfactants, including SDS, SDBS, PSSH, and PSSNa were
used. It has been found that PEDTT dispersed in SDS and
SDBS solutions was such unstable that visibly precipitated
within 6 h, and no continuous and compact films can be
coated on substrates from the final dispersions. Contrast-
ingly, dispersing PEDTT in aqueous PSSH and PSSNa solu-
tions led to relatively stable dispersions from which durable
and free-standing films can be obtained on PP substrate.
Therefore, PSSH and PSSNa were the better choice among
surfactants. Simultaneously, in view of the unfavorable influ-
ence of inorganic metal ions in organic films, PSSH was the
final choice and state of corresponding PEDTT dispersions
ion-exchange resins and homogenizing, five aqueous colloidal
PEDTT/PSS dispersions with orange color (Samples 3–7 in
Table 2) were obtained. These samples can maintain their
steady colloidal state more than one month, while visible
PEDTT deposits can be seen in Sample 1 and Sample 2 after
storing for one month at room temperature. In Figure 3,
images gave a vivid illustration of the better storing stability
of Sample 3 in comparison with Sample 1 and Sample 2.
Here, it should be pointed out that more or less PEDTT
deposits as shown in Fig. 3 (a2,b2,c2), appearing after stor-
ing PEDTT dispersions [Fig. 3 (a1,b1,c1)] for one month at
(
Sample 1 for doped and Sample 2 for neutral) are shown in
Figure 3 (a1,b1). Electrical conductivity measurement of
pristine PEDTT films indicated value of 1.26
S cm at 298 K for Sample 1, much higher than that
TABLE 2 Comparison of Oxidants on Electrical Conductivities
of PEDTT Films (Measured at 298 K and 60% RH)
a
ꢁ
ꢀ
1
ꢀ
2
ꢀ1
Sample
Oxidants
r (S cm )
1
0
ꢀ
6
ꢀ1
3
4
5
6
7
FeCl
Fe(NO
2þ
H O /Fe
2 2
3
2.4 ꢁ 10ꢀ
3
of Sample 2 (2.35 ꢁ 10 S cm ). The electrical conductive
property of neutral PEDTT after dispersion can be attributed
to PSSH. As a water-soluble polyelectrolyte, it can not only
serve as good dispersant for PEDTT in water, but also
function as the charge balancing counter ion or dopant to
ꢀ4
ꢀ3
3
)
3
ꢃ9H
2
O
1.3 ꢁ 10
1.0 ꢁ 10
4.1 ꢁ 10
8.0 ꢁ 10
ꢀ
4
2 2 8
Na S O
3
þ
ꢀ5
4
Na S O /Fe
2
2
8
the PEDTT.
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FIGURE 5 Temperature dependence of electrical conductivity
of the film casting from PEDTT dispersion (Sample 3) on PP
substrate.
FIGURE 6 Electrical conductivity (measured at 298 K and 75%
RH) of the films versus the amount of DMSO (wt %) in the
Sample 3 (a), Sample 5 (b), and Sample 7 (c, inset).
room temperature, can be dispersed again into water by
shaking. This phenomenon is different to PEDOT/PSS, which
may result in the formation of non-redispersible deposits.
One reason for this is steric interaction between adjacent
EDTT moieties would decrease the extent of p-conjugation
through the polymer system and, thus, PEDTT presented rel-
ces. Therefore, only one example is given in Figure 6. It
indicated that DMSO treatment on PEDTT/PSS dispersions
tended to slightly decrease the electrical conductivities of
corresponding films within the same order of magnitude.
4
Such results were quite different to PEDOT/PSS. Although it
3
5,39
atively short conjugated chains in PEDTT/PSS complex.
is still unclear what factors govern this phenomenon, the
short conjugated chain of PEDTT, and the tight attachment of
Coating Samples 3–7 onto PP film formed free-standing thin
films, and can be cut into various shapes. A picture of cutting
films (in Fig. 4) from Sample 3 was taken as an example.
þ
ꢀ
…
PEDTT chains and PSS through S S interaction may be
one reason for it.
Effect of Solvent Treatment on Electrical Conductivity
of PEDTT/PSS films
Synthesis and Solid-State Polymerization
of DBEDTT and DIEDTT
Electrical conductivity measurement of PEDTT/PSS films
was taken under room conditions (298 K, 60% RH) by using
the standard four-probe method. A range of values within
Recently, solid-state polymerization of 2,5-dihalothiophene
derivatives under solventless and catalyst-free conditions has
attracted significant attention for the synthesis of highly
ordered polymers and macroscopic polymer single crys-
ꢀ
5
ꢀ3
ꢀ1
the order of magnitude between 10 and 10 S cm was
tested. The results (as listed in Table 2) indicated that the
highest electrical conductivity value was present when FeCl3
was used as the oxidant in polymerization reaction. The elec-
trical conductivity also showed temperature dependence as
1
4,63–65
tals.
In 2003, Skabara and coworkers demonstrated the
applicability of this method for the preparation of bromine-
1
4
doped PEDTT by heating the crystal of DBEDTT. However,
no detailed synthetic information and electrical conductivity
of PEDTT were given. Therefore, in this work, DBEDTT was
prepared by straightforward bromination of EDTT with NBS
under very mild conditions with EDTT as a starting material in
ꢀ
3
ꢀ1
ꢀ1
depicted in Figure 5. It was 2.38 ꢁ 10 S cm at ambient
ꢀ
3
temperature and decreased to 2.09 ꢁ 10 S cm at 100 K.
To make an attempt to increase the electrical conductivity of
PEDTT/PSS films, a range of organic solvents, including ace-
tone, DMSO, EG, DMF, and NMP, were used as additives into
the PEDTT/PSS dispersions. After casting on PP substrate
and dried, free-standing PEDTT/PSS films were subsequently
formed. Electrical conductivity measurement of these films
indicated that values of PEDTT/PSS decreased with the addi-
tive concentration of organic solvents out of proportion. Val-
ues between different solvents did not show many differen-
2
1
5
8% yield (lit. 55.8%). And, a new diiodo compound,
DIEDTT, was first synthesized by iodinization with NIS in chlo-
roform/glacial acetic acid (CHCl /AcOH) under ice-water bath
3
with a yield of 41%, as illustrated in Scheme 4.
Solid-state polymerization of DBEDTT and DIEDTT was per-
formed under room conditions by heating them at certain
temperatures. Corresponding polymers, SSP-DBEDTT and
TABLE 3 Properties of DBEDTT and DIEDTT, and their Polymers After Solid-State Polymerization
Polymerization
temperature
Reaction
time
Monomer
Color
Melting point
Polymer
Color
Appearance
ꢂ
ꢂ
DBEDTT
DIEDTT
White
55 C
40 C
7 days
7 days
SSP-DBEDTT
SSP-DIEDTT
Greenish black
Cyan
Slice
ꢂ
ꢂ
Tawny
130 C
110 C
Powder
1
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FIGURE 7 The release of an orange iodine vapor for SSP-
DIEDTT (a) and a brown bromine vapor for SSP-DBEDTT (b) af-
ter stored at room conditions for 3 weeks.
SSP-DIEDTT, were formed and can be used for characteriza-
tions without further treatment. During the polymerization,
2,5-dibromo- and 2,5-diiodo-EDTT crystals showed distinct
color change accompanied with release of brown or orange
vapor due to comitant release of dibromine or diiodine
which doped the polymer as they were formed. These results
are listed in Table 3. Here, it should be noted that the visible
color change and the presence of chromatic gas can be seen
for DBEDTT after standing over one day at room tempera-
ture, while for DIEDTT over three days. By detecting the rate
of color change and degassing, it indicated that solid-state
polymerization can be accelerated by heating, that is, a
higher polymerization temperature leads to a more rapid
polymerization rate. However, if the temperatures were set
to rapidly melt DBEDTT and DIEDTT as done during the
melting points test, no observable change in their color is
FIGURE 9 Temperature (T) dependence of electrical conductiv-
ity r and Seebeck coefficient S (a) for SSP-DBEDTT and SSP-
2
DIEDTT, power factor P (c) estimated as P ¼ rS from the corre-
sponding r and S at a certain temperature, and electrical con-
ductivity r (b) after stored at room temperature for 5 months.
1
seen. H NMR and IR characterization indicated that no poly-
merization reaction occurred. Therefore, the polymerization
temperatures highly depend on the melting points of
DBEDTT and DIEDTT. In this work, as shown in Table 3,
ꢂ
heating temperatures about 15–20 C below their melting
points for 7 days were chosen.
5
3,64
As-formed polymers SSP-
DBEDTT and SSP-DIEDTT were completely insoluble in com-
mon organic solvents such as DMF, DMSO, NMP, and THF at
FIGURE 8 XRD spectra of SSP-DBEDTT and SSP-DIEDTT.
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FIGURE 10 SEM images (50kꢁ) of the surface of SSP-DBEDTT (left) and SSP-DIEDTT (right).
ꢀ
1
room temperature. If shortening the heating time to one day,
solubility of the polymers can be improved due to relatively
lower degree of polymerization.
than SSP-DIEDTT (from 9.5 to 18.9 lV K ), and has the
ꢀ
9
ꢀ1 ꢀ2
highest power factor value up to 6.7 ꢁ 10 W m
K
at
285 K. These results are pleasant findings because they gave
the first valuable, regular, and positive thermoelectrical data
1
5
Electrical Conductivity and Thermoelectric Property of
SSP-DBEDTT and SSP-DIEDTT
for PEDTT. Moreover, although these values within a order
ꢀ
9
ꢀ1 ꢀ2
of 10 W m
K
are too low to meet the requirement for
solid-state polymerization still proved to
be a feasible way to synthesize thermoelectric materials.
6
6–68
According to the image in Figure 7, it can be seen that the
appearance of orange iodine vapor for SSP-DIEDTT and faint
brown bromine vapor for SSP-DBEDTT can be detected after
stored for 3 weeks at room temperature. A reasonable expla-
nation is, to solid-state polymerization of DBEDTT and
DIEDTT, further polymerization of higher oligomers may still
take place after complete transformation of all starting
applications,
CONCLUSIONS
A series of PEDTT powders or free-standing films have been
achieved via chemical oxidative polymerization of EDTT with
different oxidants in H O, CH CN, CH CN/H O, and aqueous
6
4
monomers. This can be supported by the existence of crys-
tal structures in SSP-DBEDTT and SSP-DIEDTT, as indicated
by XRD spectra (Fig. 8), which may be responsible for the
2
3
3
2
sulfonate surfactant solutions, and spontaneous polymeriza-
tion of dibromo- and diiodo- EDTT in their solid form. System-
atic discussions of the effect of oxidants, solvent, surfactants,
and so forth, on reactions and properties of polymers were
investigated. Although there are still many challenges ahead to
improve electrical conductivity and thermoelectric perform-
ance of PEDTT, results of this study can be regarded as an im-
portant reference toward the synthesis of other analogs of
PEDOT, including PEDTT derivatives. Research directed to
achieve higher conducting PEDTT materials using chemical
oxidative polymerization, solid-state polymerization, and
copolymerization is in progress in our group. Related theo-
retical research and applications of resulting polymers as
chief compositions, especially in antistatic coatings, organic
electrochromic materials, anticorrosive coatings, thermoelec-
trical materials, and sensor materials are also ongoing.
6
3
origin of solid-state polymerization reaction. Electrical con-
ductivity measurement [Fig. 9(a,b)] also confirmed the prop-
erty change of PEDTT. After stored at room temperature for
5
months, electrical conductivity of these samples decreased
by two orders of magnitude. The electrical conductivity of
SSP-DBEDTT and SSP-DIEDTT decreased gradually with a
decrease in the temperature (from 300 K to 100 K), showing
a typical behavior of semiconductors. SSP-DIEDTT exhibited
much higher electrical conductivities in the range between
ꢀ
ꢀ
1
0
0
.2 and 0.08 S cm
than that of SSP-DBEDTT (between
4
ꢀ1
.013 and 1.85 ꢁ 10 S cm ). This may be in virtue of a
more flattened surface as pictured by scanning electron
microscope (Fig. 10).
Specifically, temperature dependence of both the values of
Seebeck coefficient [S, Fig. 9(a)] and power factor P [P ¼
2
rS , Fig. 9(c)] of SSP-DIEDTT and SSP-DBEDTT was
The National Natural Science Foundation of China (Nos.
50963002 and 51073074) and Jiangxi Provincial Department
of Education (GJJ10678, GJJ11590 and GJJ12595) are acknowl-
edged for their financial supports.
described. Comparably, SSP-DBEDTT achieved much better
values of Seebeck coefficient in the range from 63 to
ꢀ
1
1
22 lV K as the temperature range from 300 to 175 K
1
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