Synthesis of a novel macroinimer based on thiophene and poly(ε-caprolactone) and its use in electrochromic device application

Article abstract of DOI:10.1002/pola.24860

Synthesis of a novel macroinimer comprising poly(ε-caprolactone) (PCL) and thiophene (Th) and its use in electrochromic device (ECD) application have been reported. First, a novel Th monomer (5) with miktofuntional initiator groups (primary hydroxyl and tertiary bromide at the third position of the thiophene ring) was synthesized in a four-step reaction sequence. Density functional theory-predicted bond lengths, angles, and vibrations of 5 were in good agreement with available experimental vibrational spectra. Subsequently, ring-opening polymerization of ε-caprolactone (ε-CL) was carried out in bulk using 5 as the initiator and tin(II) 2-ethylhexanoate (Sn(Oct) ₂) as the catalyst at 115 °C, which led to α-thiophene end-capped PCL macroinimer (PCL-Th). Furthermore, PCL-Th macroinimer was used in electrochemical copolymerization with pyrrole (Py) and Th. PCL-Th/PTh copolymer film synthesized on indium tin oxide-coated glass slide showed electrochromic behavior. Optical analyses of the PCL-Th/PTh copolymer film indicated that the copolymer film was suitable to be used as an anodically coloring material for ECD applications.

Full text of DOI:10.1002/pola.24860

ARTICLE
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Synthesis of a Novel Macroinimer Based on Thiophene and
Poly(e-caprolactone) and Its Use in Electrochromic Device Application
Asuman Celik Kucuk,1 Faruk Yilmaz,1 Hatice Can,1 Hakan Durmaz,2 Arif Kosemen,3
Ali Ekrem Muftuoglu4
1Department of Chemistry, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey
2Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
3Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey
4Department of Chemistry, Fatih University, 34500 Buyukcekmece, Istanbul, Turkey
Correspondence to: F. Yilmaz (E-mail: fyilmaz@gyte.edu.tr)
Received 23 March 2011; accepted 27 June 2011; published online 22 July 2011
DOI: 10.1002/pola.24860
ABSTRACT: Synthesis of
a
novel macroinimer comprising
a-thiophene end-capped PCL macroinimer (PCL-Th). Further-
more, PCL-Th macroinimer was used in electrochemical
copolymerization with pyrrole (Py) and Th. PCL-Th/PTh copoly-
mer film synthesized on indium tin oxide-coated glass slide
showed electrochromic behavior. Optical analyses of the PCL-
Th/PTh copolymer film indicated that the copolymer film was
poly(e-caprolactone) (PCL) and thiophene (Th) and its use in
electrochromic device (ECD) application have been reported.
First, a novel Th monomer (5) with miktofuntional initiator
groups (primary hydroxyl and tertiary bromide at the third
position of the thiophene ring) was synthesized in a four-step
reaction sequence. Density functional theory-predicted bond
lengths, angles, and vibrations of 5 were in good agreement
with available experimental vibrational spectra. Subsequently,
ring-opening polymerization of e-caprolactone (e-CL) was car-
ried out in bulk using 5 as the initiator and tin(II) 2-ethylhexa-
noate (Sn(Oct)2) as the catalyst at 115 ꢀC, which led to
suitable to be used as an anodically coloring material for ECD
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applications. 2011 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 4180–4192, 2011
KEYWORDS: conductive polymers; copolymerization; electro-
chromic device; macroinimer; ring-opening polymerization
INTRODUCTION Conjugated polymers synthesized via poly-
merization of electroactive monomers have been the subject
of a great deal of research, because these polymers display
novel properties and have quite a number of applications in
rechargeable batteries,1,2 electrochromic devices,3,4 sensors,5
and enzyme immobilization.6 However, their limited proces-
sibility and poor mechanical properties set a barrier for their
utilization in such applications. To overcome this problem,
efforts have been directed toward obtaining polymeric struc-
tures comprising diverse polymer chains carrying electroac-
tive moieties.7 One way to access such architectures is to use
polymer telechelics,8,9 which bear one or more functional-
ities at chain ends. Depending on the choice of functionality,
telechelics can act as macromonomers,10 and macroinitia-
tors11–14 for different modes of polymerization, such as step-
growth,15 controlled radical polymerizations,16–21 and elec-
tropolymerization.22–24 It is even more interesting that a
macromolecule is able to behave as an initiator and a mono-
mer simultaneously, termed as a macroinimer. Use of such
telechelics in further chain growth facilitates a versatile com-
bination of chemically different monomer units, covalently
linked in a single macromolecule. The resulting polymer will
possibly display hybrid properties.
Polythiophenes are a class of conjugated polyheterocycles,
which have been extensively investigated due to the syn-
thetic versatility of the thiophene monomer in obtaining
electron conducting copolymers with side chain functional-
ities.25 Previously, a-thiophene end-capped poly(e-caprolac-
tone) (PCL)26 and poly(methyl methacrylate) (PMMA)27a
polymers were prepared by ring-opening polymerization
(ROP) and atom transfer radical polymerization (ATRP) tech-
niques, respectively. 3-Thiophenemethanol for ROP and tri-
chloro acetylcarbamate derivative of thiophene for ATRP
were used as the initiators to obtain the corresponding poly-
mers with thiophene end functionalities. Then, their electro-
polymerizations with Py and Th through the incorporated
a-thiophene units were carried out. In another study,27b a thio-
phene derivative bearing a tertiary bromide moiety at second
position of thiophene ring was reported and used as an ATRP
initiator for the synthesis of a-thiophene end-capped poly(tert-
butyl acrylate). Thiophene-terminated polyacrylate was then
copolymerized with 3-hexylthiophene by oxidative polymeriza-
tion (FeCl3 being the oxidant) to get rod-coil and coil-rod-coil
block copolymers. We believe that combining the aforemen-
tioned routes in the preparation of an electron conducting
terpolymer,
poly(thiophene-co-e-caprolactone-co-methyl
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthetic route of thiophene monomer with miktofunctional initiator groups.
methacrylate), might yield distinct mechanical properties
besides interesting electrochromic behavior.
thylamine (TEA, 99.5%) was obtained from Fluka, dried over
CaH2, and stored over 3-Å molecular sieves. Acetonitrile (ACN)
and other solvents were purified by conventional procedures.
e-CL (97%; Aldrich) was distilled over CaH2 and stored over
dry 4-Å molecular sieves in vacuo. Thiophene (Th, 98%;
Aldrich) and pyrrole (Py, 99%; Aldrich) were distilled before
use. 2,2-Bis(hydroxymethyl)propionic acid (98%; Aldrich), 2,2-
dimethoxypropane (98%; Aldrich), N,N0-dicyclohexylcarbodii-
mide (DCC, 99%; aldrich), stannous octanoate [Sn(Oct)2, 95%;
Aldrich], 3,4-ethylenedioxythiophene (EDOT, 97%; Aldrich), 3-
thiophenemethanol (98%; Aldrich), propylene carbonate (PC,
99.7%; Aldrich), tetrabutylammonium tetrafluoroborate
(TBAFB, 99%; Aldrich), and PMMA (Mw: 120,000; Aldrich)
were used without further purification. All other reagents were
purchased from Aldrich and used as received. 4-
(Dimethylamino)pyridinium 4-toluenesulfonate (DPTS) was
obtained through the reaction of anhydrous p-toluenesulfonic
acid (p-TSA; Aldrich) with 4-dimethylamino pyridine. ITO-
coated glass slide (Aldrich) has resistance of 8–12 X.
In this article, as part of a broader study, we report the syn-
thesis of a novel thiophene monomer with miktofunctional
initiator sites for ROP and ATRP at the third position of the
thiophene ring in a four-step reaction sequence (Scheme 1).
Detailed calculations based on density functional theory
(DFT) were also carried out to shed more light on the accu-
rate prediction of geometry and IR spectroscopic characteris-
tic for the thiophene monomer in the gas phase. The primary
hydroxyl initiating site of the thiophene monomer was first
used in the ROP of e-caprolactone (e-CL), which led to PCL
macroinimer with a-thiophene and tertiary bromide end
groups (PCL-Th). Furthermore, the copolymer of PCL-Th with
Th was electrochemically synthesized by using the former as
a macromonomer and electrochromic and spectroelectrochem-
ical behaviors of the copolymer film (PCL-Th/PTh) were stud-
ied. Finally, we constructed a dual-type electrochromic device
(ECD) based on PCL-Th/PTh copolymer film and poly(3,4-eth-
ylenedioxythiophene) (PEDOT). The device was assembled in
a sandwich configuration of electrochromic materials (PCL-
Th/PTh and PEDOT) deposited on indium tin oxide (ITO)
glass electrodes. In the construction of the device, PEDOT was
used as the cathodically coloring material, whereas PCL-Th/
PTh served as the anodically coloring agent. Further potential
of PCL-Th macroinimer bearing Br initiating site for ATRP of
methyl methacrylate (MMA) in the block copolymer formation,
consecutive electropolymerization with Th yielding poly(thio-
phene-co-e-caprolactone-co-methyl methacrylate), and ECD
characteristics of the terpolymer will be reported elsewhere.
Measurements
All the calculations were done using GAUSSIAN 03W pro-
gram on Windows. The geometry of 5 was optimized first
using semiempirical AM1 and then B3LYP (Becke, three-
parameter, Lee-Yang-Parr).28,29 Frequency/intensity calcula-
tions were carried out using B3LYP. In all cases, the 6-31G
1
(d,p) basis set was used. H NMR and 13C NMR spectra were
recorded at room temperature using Varian UNITY INOVA 500-
MHz NMR (125.7 MHz for 13C). The chemical shifts were rela-
tive to tetramethylsilane at d ¼ 0 ppm for protons. CDCl3 was
used as the deuterated solvent for polymers. ATR-FTIR spectra
were recorded on Perkin–Elmer Paragon 1000 spectrometer.
Molecular weights and molecular weight distributions of the
polymers were measured on an Agilent Instrument (Model
1100) consisting of a pump, a refractive index and a UV detec-
tor, and four Water Styragel columns (HR 5E, HR 4E, HR 3, and
EXPERIMENTAL
Materials
Tetrahydrofuran (THF, 99%; Riedel) was dried by refluxing
over Na and then distilled in a dry argon atmosphere. Trie-
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HR 2), THF being used as eluent at a flow rate of 0.3 mL/min at
30 ꢀC and toluene being used as an internal standard. Molecular
weights were calculated on the basis of a calibration curve,
drawn using monodisperse polystyrene standards.
The electrochemical polymerizations and characterizations
were performed with CHI 760C potentiostat, and the thick-
nesses of the coated polymer films onto ITO-coated glasses
were measured by using PHE101 optical ellipsometer. Spec-
troelectrochemical studies were carried out on Perkin–Elmer
Lambda 30 UV–vis spectrometer. Scanning electron micros-
copy (SEM) images were taken with Philips XL30 SFEG.
Synthesis of 2,2-Bis[(2,2-propyl)dioxymethyl]propionic
Acid (2)
2,2-Bis[(2,2-propyl)dioxymethyl]propionic acid (2) was syn-
thesized through the reaction of 2,2-bis(hydroxymethyl)pro-
pionic acid (1), 2,2-dimethoxypropane, and p-TSA in acetone,
according to a published procedure.30
SCHEME 2 Synthesis of PCL-Th.
Synthesis of 2,2,5-Trimethyl-(1,3)dioxane-5-carboxylic
Acid Thiophene-3-yl Methyl Ester (3)
30 min. The reaction mixture was stirred for 4 h at room tem-
perature. After dilution with 200 mL of CH2Cl2, the mixture was
extracted three times with saturated aqueous solution of so-
dium bicarbonate (150 mL). The organic phase was dried over
Na2SO4 and evaporated to dryness. The crude product was puri-
fied by column chromatography over silica gel eluting by 1:10
ethyl acetate/hexane and 5 was obtained as yellowish oil.
3-Thiophenemethanol (1.2 g, 10.52 mmol), 2 (2.17 g, 12.50
mmol), and DPTS (0.67 g, 2.13 mmol) were dissolved in
CH2Cl2 (50 mL). DCC (2.58 g, 12.50 mmol), dissolved in
CH2Cl2 (10 mL), was then immediately added, and the mix-
ture was stirred at room temperature for 24 h. The insoluble
urea (byproduct) was filtered out, and the filtrate was
evaporated to dryness. The remaining product was purified
by column chromatography, in which silica gel was used as
the stationary phase and n-hexane as the eluent. Compound
3 was obtained as a white solid.
Yield ¼ 1.6 g, 65%. Anal. calcd. for C13H17BrO5S: C, 42.75%;
H, 4.69%; S, 8.78%. Found: C, 42.95%; H, 4.80%; S, 8.55%.
1H NMR (500 MHz; d, ppm from TMS in CDC13 at ambient
temperature): 1.24 (3H, s, ACH3), 1.92 (6H, s, ACH3), 2.22,
(1H, broad, AOH), 3.85, (2H, m, ACH2A), 4.25–4.45, (2H, dd,
ACH2A), 5.20 (2H, s, ACH2A) and 7–7.4 (3H, m, 2, 4, and 5
positions in the thiophene ring). 13C NMR (125.7 MHz; d,
ppm from TMS in CDC13 at ambient temperature): 18.5, 30,
49.5, 55, 62, 65, 68, 120–140, 172, 174.
Yield ¼ 1.95 g, 68%. 1H NMR (500 MHz; d, ppm from TMS
in CDC13 at ambient temperature) 1.22 (3H, s ACH3), 1.38
(3H, s, ACH3), 1.42 (3H, s, ACH3), 3.65, (2H, d, ACH2), 4.15,
(2H, d, ACH2A), 5.18 (2H, s, ACH2A) and 7–7.4 (3H, m, 2, 4,
and 5 positions in the thiophene ring).
Synthesis of 3-Hydroxy-2-hydroxymethyl-2-methyl-
propionic Acid Thiophene-3-yl Ester (4)
Synthesis of PCL-Macroinimer (PCL-Th) by ROP of e-CL
The PCL macroinimer (PCL-Th) was prepared by ROP of e-CL in
bulk with Sn(Oct)2 as the catalyst (Scheme 2). Polymerization
was carried out at 115 ꢀC in an inert atmosphere for a given
time. A typical polymerization procedure was as follows: To a
previously flamed schlenk tube equipped with a magnetic stir-
ring bar, a certain amount of initiator (5), catalyst, and monomer
were added in that order. The mixture was deoxygenated by
flushing with O2-free argon for 10 min, after which the tube was
tightly sealed and immersed in a thermostatic oil bath at 115 6
1 ꢀC. After a certain polymerization time, the mixture was diluted
with CH2Cl2 and precipitated into 10-fold cold methanol under
vigorous stirring. The precipitate was then isolated by filtration
and dried at room temperature in vacuo. The polymer yield was
determined gravimetrically. Details of the experimental condi-
tions of the polymerization are given in Table 1.
Compound 3 (1.9 g, 7.03 mmol) was dissolved in a mixture
of 30 mL of THF and 15 mL of 1 M HCl (aqueous). The reac-
tion mixture was stirred for 4 h at room temperature. After
filtration of the insoluble salts, the reaction mixture was con-
centrated and extracted with 160 mL of CH2Cl2 and 40 mL
of water. The combined organic phases were dried over
Na2SO4, and filtered. The filtrate was evaporated to dryness,
and the product was obtained as a white solid.
1
Yield ¼ 1.55 g, 96%. H NMR (500 MHz; d, ppm from TMS in
CDC13 at ambient temperature) 1.15 (3H, s ACH3), 2.75 (2H,
broad, OH), 3.85, (4H, m, ACH2), 5.16 (2H, s, ACH2A) and 7–
7.4 (3H, m, 2, 4, and 5 positions in the thiophene ring).
Synthesis of 2-(2-Bromo-2-methyl-propioyloxymethyl)-3-
hydroxy-2-methyl-propionic Acid Thiophene-3-yl Ester (5)
Compound 4 (1.50 g, 6.52 mmol) was dissolved in dry TEA
Preparation of Electrochromic Layers (PCL-Th/PTh
and PEDOT) on ITO Surface
ꢀ
(1.38 mL, 10 mmol) and CH2Cl2 (20 mL) and cooled to 0 C.
a-Bromoisobutyryl bromide (0.81 mL, 6.52 mmol) in 5 mL of
CH2Cl2 was added dropwise to the reaction mixture within
A three-electrode cell containing ITO-coated glass slide
as the working electrode, a platinum foil as the counter
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TABLE 1 Synthesis of PCL-Th via ROP Technique
[M]o (mol Lꢂ-1
9.02
)
[M]o/[I]o
40
I
Time (h)
Con. (%)
87
Mn,Theo
Mn,NMR
Mn,GPC
Mw/Mn
a
Entry
M
b
PCL-Th
e-CL
5
46
4,337
4,132
4,800
1.11
b
M, monomer; I, initiator (inimer).
Polymerization was carried out at 110 ꢀC ( [Initiator]/[Sn(Oct)2] ¼ 300).
a
Calculated from GPC calibrated with linear polystyrene standards.
electrode, and a silver wire as the reference electrode were
used for electrodeposition of PCL-Th/PTh and PEDOT by
potentiostatic methods. A 10 mM solution of EDOT in 0.1 M
TBAFB/ACN was used to deposit PEDOT film onto ITO-
coated glass electrode at 1.5 V versus Ag/Agþ for 10 s. In
the electrochemical polymerization of PCL-Th/PTh, BFEE was
used as both solvent and supporting electrolyte. First, PCL-
Th was dissolved in dichloromethane (1%, w/v) and then
coated on one side of ITO by spin-casting. PCL-Th coated ITO
was used as the working electrode. After introducing 10 mM
Th to the reaction medium, the copolymerization was per-
formed at 1.9 V versus Ag/Agþ for 7 s. The electrochemistry
experiments were carried out at room temperature under
inert argon atmosphere. The electrochemical polymerizations
and characterizations were performed with a CHI 760C
potentiostat, and the thicknesses of the coated polymer films
onto ITO-coated glasses was measured around 100 nm by
Optical Elipsometry ‘‘Phe101.’’
solution. The mixture was stirred and heated until a highly
conducting transparent gel was formed.
Construction of Electrochromic Device
ECD was assembled with a configuration of ITO/PCL-Th/
PTh||gel electrolyte||PEDOT/ITO as shown in Figure 1. Cath-
odically coloring layer PEDOT and anodically coloring layer
PCL-Th/PTh were synthesized on ITO according to the men-
tioned procedures. The device was dried under atmospheric
conditions.
Electrochemical Synthesis of the Homopolymers
and the Copolymers
Electrochemical synthesis of homopolymers of Th and Py
and their copolymers with the PCL-Th were conducted by
constant potential electrolysis, using a CHI842AB galvani-
stat/potentiostat, under argon atmosphere at room tempera-
ture. Electrolyses were performed in a one compartment cell
consisting of a Ag/Agþ reference electrode, an auxiliary Pt
wire electrode, and a Pt foil (A ¼ 1 cm2) working electrode.
Homopolymerization of Th (40 lL) was performed in 10 mL
ACN solution at 2.0 V using TBAFB (0.03 M) as the support-
ing electrolyte. PCL-Th (2 mg, 4.8 ꢁ 10ꢂ4 mmol according to
Mn,NMR calculation) and Th (40 lL) were electrocopolymer-
ized separately in ACN (10 mL) in the presence of TBAFB
(0.03 M) at 2.0 V (Scheme 3).
Preparation of Gel Electrolyte
For the construction of dual type electrochromic device, gel
electrolyte was prepared in the following form containing
ACN þ PC þ PMMA þ TBAFB. The ratio of the composition
of ACN:PC:PMMA:TBAFB was 70:20:7:3 by weight. A high
vapor pressure solvent, ACN, was used to dissolve the PMMA
and to allow an easy mixing of the gel components. TBAFB
as the key supporting elements of the proposed gel and PC
as the plasticizer were added to the gel mixture. The proce-
dure was as follows: After dissolving TBAFB in ACN, PMMA
was added into the solution. The complete dissolution of
PMMA was achieved by vigorous stirring and heating. When
PMMA was completely dissolved, PC was introduced to the
Py (40 lL) was homopolymerized electrochemically in ACN
(10 mL) by using TBAFB (0.03 M) as the supporting electro-
lyte at a constant potential (1.0 V). The copolymer of PCL-Th
(2 mg, 4.8 ꢁ 10ꢂ4 mmol according to Mn,NMR calculation) with
Py (40 lL) was prepared electrochemically in ACN (10 mL) in
the presence of TBAFB (0.03 M) at 1.0 V (Scheme 3).
FIGURE 1 Schematic illustration of a dual ECD based on PCL-Th/PTh and PEDOT electrochromic polymers. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
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SCHEME 3 Electrochemical copolymerization of PCL-Th with Th and Py.
The obtained copolymer films were rinsed thoroughly with
ACN to remove TBAFB, unreacted monomer, and precursor
polymer.
with regard to the bond lengths, with a maximal deviation of
0.024 Å in the calculated data.
In addition, small distortions with a maximal deviation of
0.9ꢀ in calculated bond angles in the thiophene ring were
obtained. All the geometric data for the thiophene ring, cal-
culated at either 6-31g(d) or 6-311g(d, pd) basis set level
were very close to the experimental results.31
RESULTS AND DISCUSSION
Theoretical Results
Geometry Optimization
Vibrational Frequencies
The molecular structure of 5 is shown in Figure 2, and the
optimized geometric parameters obtained by B3LYP/ 6-31G
(d,p) level in vacuum are presented in Table S1 (See Sup-
porting Information). It should be emphasized that we were
able to compare the calculated and experimental geometric
data of the thiophene part of 5 only, due to unavailability of
the experimental data of the whole molecule. The DFT-pre-
dicted C¼¼C bonds in the thiophene ring, C(18)AC(19) and
C(20)AC(21), were close to experimental values, and they
were about 0.01 Å shorter than the corresponding experi-
mental ones. From the comparison of calculated and experi-
mental geometric data, it was concluded that the distortions
originating from the other part of 5 were small, especially
The calculated IR absorption spectrum (4000–200 cmꢂ1) of
5 in the gas-phase at the B3LYP/6-31G (d,p) level is shown
in the Figure S1 (Supporting Information). The calculated
values show a good correlation with the experimental data
obtained from the literature,28,29,32–34 most of the bands hav-
ing been correctly predicted (see Supporting Information
Table S2). To compare the calculated vibrations with the
experimental observations, scaling of the vibrational frequen-
cies was implemented. For the frequencies calculated with
B3LYP, a scaling factor of 0.963 is commonly accepted.35
The most pronounced vibrational features of 5 were the
intense peaks at 1744 and 1730 cmꢂ1 corresponding to
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intensity of hydroxyl peak in the spectrum of PCL-Th consider-
ably decreased after ROP as the molecular weight of the poly-
mer increased. It was also observed that ROP caused an
increase in the intensity of carbonyl group, futher indicating
the incorporation of e-CL monomer in ROP.
Figure 3(a,b) displays 1H and 13C NMR spectra of 5, respec-
1
tively. H NMR spectrum of 5 clearly showed the signals that
arose from ATRP initiator (ACBr(CH3)2, i, d ¼ 1.85), and
monofunctional hydroxyl group (ACH2OH, f, d ¼ 2.24). The
signals at around 7.01–7.38 ppm were assigned to the aro-
matic protons (a, b, and c) of thiophene ring. The proton-
decoupled 13C NMR of 5 provided further support of the
desired structure due to the presence of two different car-
bonyl peaks as expected. The resonance signals at 175 and
172 ppm were due to the presence of two different ester
carbonyl carbons (Cf and Cm) in 5. The signals of the carbon
atoms of the thiophene ring in 5 were also observed at 125
ppm (Ca), 127 ppm (Cb), ppm, and 128 ppm (C3). The obser-
vation of these peaks in 1H and 13C NMR spectra proved
that 5 was successfully synthesized.
FIGURE 2 Geometry optimized structure and atom numbering
of 5 at the B3LYP/6-31G (d,p) level. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
Compound 5 can be used as a core to prepare block copoly-
mer (PCL-b-PMMA-Th) possessing thiophene moiety at the
junction point. Synthetic strategy is based on a two-step reac-
tion procedure36 consisting of the synthesis of well-defined
linear a-tertiary bromide and thiophene PCL using a mikto-
funtional initiator (5) in ROP of e-CL, followed by the ATRP of
MMA using the obtained a-tertiary bromide functional PCL as
the macroinitiator. In this study, we report the a-thiophene-
capped PCL. In the ROP, the use of 5 as the initiator in the
presence of Sn(Oct)2 as the catalyst resulted in PCL macroi-
nimer, PCL-Th. Scheme 2 illustrates the synthesis of PCL-Th.
The synthesis of block copolymer possessing thiophene func-
tionality at the junction point of homopolymer segments, and
its ECD application will be reported in a further study.
C¼¼O stretching in the ester group. The second most intense
peak was observed at 1268 cmꢂ1 for CH2 wagging of the
whole molecule. Typical hydroxyl stretching was predicted at
3628 cmꢂ1. In-plane stretching of CAH at 3152–3126 cmꢂ1
in thiophene ring was very well established. One of the other
well predicted strong mode was C¼¼C in-plane stretching of
thiophene, located at 1424 cmꢂ1. Thiophene ring of the mol-
ecule showed the CAS ring deformation stretching at 618
and 609 cmꢂ1 for calculated and experimental values,
respectively. CABr stretching was precisely predicted to
emerge at 626 cmꢂ1
.
1
The structure of PCL-Th was confirmed by H NMR spectro-
Experimental Results
scopic method and GPC analysis. 1H NMR spectrum of PCL-
Th is shown in Figure 4. The signals of the methylene groups
of the repeating units (g and h) and the primary hydroxyl
end units (i) were clearly seen in the spectrum. The peak
assigned to the terminal methylene protons (CH2OH, i, d ¼
3.65 ppm, multiplet) indicated that linear PCL chains were
terminated by hydroxyl end groups. The measured and theo-
retical integral ratio of the terminal methylene protons (Hi)
to that of methylene protons next to the thiophene ring (Hd)
is nearly the same (Hi/Hdmeasured ¼ 1.01, the theoretical
value is 1.00). This observation confirms that e-CL monomers
have been inserted into the ‘‘ACH2CH2OAH’’ group of 5 via
selective acyl-oxygen cleavage of e-CL and that a-thiophene-
ended PCL was successfully and quantitavely prepared. The
theoretical number-average molecular weight (Mn,theo) of the
The novel thiophene monomer with two miktofuntional ini-
tiatitor groups (5) was prepared according to Scheme 1.
First, a diacetal of bis(hydroxymethyl)propionic acid (2) as a
core compound was obtained from 1. The reaction of 2 with
3-thiophenemethanol resulted in a thiophene functional com-
pound (3) with DCC and DPTS in high yields after purifica-
tion with column chromatography. The deprotection of the
acetonide group of 3 was easily achieved quantitatively in
the presence of 1 M HCl and afforded compound 4. The
ATRP initiator functionality was introduced into the core
through an esterification reaction of one hydroxyl group of 4
with 1 equiv of a-bromoisobutyryl bromide. The disubsti-
tuted product did not form, probably due to steric hindrance.
The expected structure was confirmed by FTIR, 1H and 13C
NMR spectroscopies.
PCL-Th macroinitiator was calculated according to Mn,theo
¼
([M]0/[I]0) ꢁ conversion ꢁ MWe-CL þ MWinitiator, where MWi-
FTIR spectra of 5 together with PCL-Th are shown in Figure
S2 (see Supporting information). There were significant
changes in the FTIR spectral properties of 5 and the polymeric
product. Compound 5 exhibited an intense broad absorption
corresponding to the OAH stretching at 3500 cmꢂ1. The
is the molecular weight of 5 and [M]0 and [I]0 are the
nitiator
initial concentrations of e-CL and 5, respectively. In addition,
the NMR number-average molecular weight (Mn, NMR), which
was determined from the ratio of the integral peak areas of
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FIGURE 3 1H (a) and 13C NMR (b) spectra of 5 in CDCl3 at ambient temperature.
the peak, Hd, to that of Hh, was consistent with the theoreti-
cal Mn value. The GPC trace of PCL-Th showed a symmetrical
and unimodal elution peak with a low polydispersity index.
The measurements were performed in a CV cell that was
equipped with Pt-foil (A ¼ 1 cm2) working and counter elec-
trodes and a Ag/Agþ reference electrode. Cyclic voltammo-
grams of Th and Py and their copolymers with PCL-Th were
studied in 10 mL ACN solutions containing 0.03 M TBAFB as
a supporting electrolyte by applying sequential linear poten-
tial scan rate of 400 mV/s between ꢂ1.2 and 1.2 V (for Py)
and between ꢂ0.8 and 2.2 V (for Th).
The CV of PCL-Th in an ACN-TBAFB system implied that the
precursor polymer is not electroactive. On addition of Py
into the reaction medium, there was an increase in the incre-
ments between the consecutive cycles, and the oxidation–
reduction potentials of the material were different from those
Cyclic Voltammetry
As PCL-Th has polymerizable a-thiophene group, it can be
called macromonomer and can be used in electrochemical
polymerization via its thiophene unit. Therefore, electro-
chemical copolymerization of PCL-Th with Th and Py, in situ
spectroelectrochemistry of PCL-Th/PTh thin film, and its ECD
application were also studied.
The oxidation/reduction behaviors of PCL-Th in the presence
of Th and Py were investigated by cyclic voltammetry (CV).
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FIGURE 4 1H NMR spectrum of PCL-Th.
of pure Py. Oxidation and reduction peaks of PCL-Th/Py were
observed at 0.42 and ꢂ0.55 V, respectively [Fig. 5(b)],
whereas for pristine Py in Figure 5(a), the redox peaks were
The possibility of the reaction between Th unit at the a-chain
end of PCL-Th and Th is also clearly shown in Figure 5(c).
When the CV of PCL-Th was performed in the presence of
Th, an oxidation peak at þ1.21 V and a reduction peak at
þ0.48 V were observed, which were quite different from
those of pristine thiophene observed at þ1.12 and þ0.41 V,
respectively. These results imply that the reaction of the
not at the same positions (pure PPy—Ep,a: 0.39 V and Ep,c
:
ꢂ0.42 V), and the peak height with the same scan number
was different. These shifts are known to be indications for the
reaction between Py and Th end groups of the PCL-Th.
FIGURE 5 Cyclic voltammograms of pure Py (a), PCL-Th in the presence of Py (b), pure Th (c), and PCL-Th in the presence of Th in
0.03 M TBAFB/ACN on a bare Pt electrode (d). Scan rate is 0.400 V sꢂ1, versus Ag/Agþ.
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FIGURE 6 SEM images of PCL-Th/PPy (electrode side) (a), PCL-Th/PPy (solution side) (b), PCL-Th/PTh (electrode side) (c), and PCL-
Th/PTh (solution side) (d).
PCL-Th with Th occurred through the thiophene moieties at
the chain end. The product (PCL-Th/PTh) did not behave
like pure polythiophene.
PCL-Th and Py or Th, which presents them a new identity
with a new topological appearance.
In situ Spectroelectrochemistry of PCL-Th/PTh Thin Film
The electronic structure and optical behavior of PCL-Th/PTh
can be explained by spectroelectrochemical measurements.
In situ spectroelectrochemistry studies were performed in
the monomer free three-electrode cell with ACN/TBAFB so-
lution by switching between 0.2 and þ1.5 V vs Ag-Agþ elec-
trode. At different potentials, a series of UV–vis spectra were
run as seen in Figure 7. The kmax value at 480 nm corre-
sponds to the p–p* transition and the color of the PCL-Th/
PTh film in the neutral state is red. During the scan from 0.2
to 1.5 V, the maximum absorbance peak decreased and the
electronic band gap estimated at the edge of p–p* transition
was found to be 1.88 eV (660 nm).
Surface Morphology Studies
Electrocopolymerization of PCL-Th was achieved in the pres-
ence of Py and Th by constant potential electrolysis. PCL-Th/
PTh copolymer film doped with TBAFB was washed in
CH2Cl2. The reaction product was black. The surface mor-
phologies of the electrode and solution sides of the copoly-
mer film were investigated by SEM (Fig. 6). The surface
appearance of PCL-Th/PPy copolymer film revealed that the
electrode [Fig. 6(a)] and solution [Fig. 6(b)] sides of PCL-Th/
PPy film were different. Although the electrode side was
rough, the solution side of the film showed homogeneous
compact structure in globular form. PCL-Th/PPy also exhib-
ited different morphologies than that of pristine PPy, which
had a smooth appearance on the electrode side and standard
cauliflower structure on the solution side. A similiar type of
behavior was also observed in the case of PCL-Th/PTh copol-
ymer film [Fig. 6(c,d)]. The structure of the electrode side of
PCL-Th/PTh was not as smooth as that of pure PTh, whereas
the morphology of the solution side of PCL-Th/PTh had cau-
liflower-like morphology with small globules. This may be
attributed to the presence of a covalent interaction between
As the applied potential was increased gradually, on doping,
the new maximum peak emerged at 713 nm, shifted to near-
IR region, and disappeared at higher potential values. The
appearance of the maximum and the shift to longer wave-
length indicate the occurrence of polarons (radical cation)
and bipolarons on PCL-Th/PTh film on the polymer back-
bone.26 These lower transitions lead to the production of a
second color, blue. This strong shift of the maximum absorp-
tion on applied voltage is related to the electrochromic
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Colorimetry Studies
The colors of the electrochromic materials are defined
clearly by colorimetry measurements. They were performed
by using Konica Minolta CS-200 chronometer with viewing
geometry as recommended by Commision Internatonale
I’Eclariage (CIE). This system is used as a quantitative scale
to define and compare colors. According to the CIE system,
color is made up of three attributes; luminance (L), hue (a),
and saturation (b). L, a, and b values of the PCL-Th/PTh film
were measured at the fully oxidized and reduced states on
the ITO/glass surface. The color of the copolymer film
switched from blue in the oxidized state to red in the
reduced (neutral) state. For comparison, the values for the
PTh and related studies from literature are also included in
Table 2. In these studies, Th end-capped polymers were syn-
thesized using different medhods such as anionic and cati-
onic living polymerizations and ‘‘click chemistry.’’ They were
all macromonomers due to the presence of polymerizable Th
units. The films obtained from the copolymerizations of
these macromonomers with Th were found to exhibit elec-
trochromic behavior.
FIGURE 7 In situ spectroelectrochemical graphs of PCL-Th/PTh
at 0.2 V (a), 0.4 V (b), 0.6 V (c), 0.8 V (d), 0.9 V (e), 1.0 V (f),
1.1 V (g), 1.2 V (h), 1.3 V (i), and 1.5 V (k).
Switching and Stability Properties of PCL-Th/PTh
Thin Film
The ability of electrochromic polymers to fast switching and
rapidly color chance is important for electrochromic aplica-
tion as display and monitoring. The experiments carried out
by spectroelectrochemistry showed the ability of PCL-Th/
PTh film to switch between its neutral and doped states
with a change in transmittance at a fixed wavelength. The
electrochromic parameters of PCL-Th/PTh were extracted by
analysis of transmittance change decrement or increment of
the absorption band at 500, 750, and 900 nm with respect
to time, whereas the potential was stepwise switched
between ꢂ0.5 and þ1.5 V for a residence time of 5 s. During
property of the polymer. When the experiment was
repeated by decreasing the applied potential gradually, no
change in the band gap or kmax values was observed. The
spectroelectrochemistry results clearly showed that PCL-
Th/PTh reversibly changes color between red and blue on
doping–dedoping processes, and the results obtained are
different from those of PTh, indicating the formation of
PCL-Th/PTh copolymer.38
TABLE 2 Electrochemical, Electronic, and Electrochromic Properties of PCL-Th/PTh and Related Studiesa
b
b
Material
PTh
Epa (V)
Epc (V)
kmax (nm)
L
a
b
Eg (eV)
Reference
37
0.63
1.10
0.58
0.60
495
57 (ox)
51 (red)
53 (ox)
63 (red)
ꢂ-7 (ox)
52 (red)
ꢂ-5 (ox)
47 (red)
ꢂ-2 (ox)
46 (red)
7 (ox)
1.92
PCL/PTh
473
1.83
26
25 (red)
(N3P3-(PCL-Th)6)/PTh
PTHF/PTh
1.30
1.40
0.45
0.60
477
495
1.85
1.88
7
54 (ox)
57 (red)
63 (ox)
70 (red)
52 (ox)
60 (red)
65 (ox)
79 (red)
b
ꢂ-10 (ox)
36 (red)
ꢂ-1 (ox)
40 (red)
ꢂ-10 (ox)
61 (red)
ꢂ-3 (ox)
34 (red)
ꢂ-7 (ox)
19 (red)
19 (ox)
38
PTh/PTHF/PTh
PEO/PTh
PCL-Th/PTh
a
1.30
0.50
470
515
480
1.91
1.95
1.88
23
29 (red)
ꢂ-4 (ox)
27 (red)
ꢂ-4 (ox)
27 (red)
39
0.91
0.39
This study
PCL: a-Thiophene end-capped poly(e-caprolactone) (N3P3-(PCL-Th)6):
Hexa-armed and thiophene end-capped poly(e-caprolactone) star poly-
mer containing cyclotriphosphazene core. PTHF: Polytetrahydrofuran
was terminated thiophene group at one or both end(s). PEO: a-Thio-
phene capped poly(ethylene oxide)
Volts versus Ag/Agþ.(ox), oxidized state; (red), reduced state; Epa, an-
odic potential; Epc, cathodic potential; Eg, band gap.
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FIGURE 8 Electrochromic switching, optical absorbance change monitored at 500, 750, and 900 nm for PCL-Th/PTh film between
ꢂ0.5 and 1.5 V.
this measurement, % transmittance at the wavelength of
maximum contrast was determined by using a UV–vis spec-
trophotometer. The optical contrast measured as the differ-
ence between %T in the reduced and oxidized states were
found to be 37, 39, and 47% T for 500, 750, and 900 nm,
respectively. The oxidation and reduction response times
were calculated as 0.25 s with a switch from ꢂ0.5 to 1.5 V
and 0.88 s with a switch from 1.5 to ꢂ0.5 V at 750 nm,
respectively (Fig. 8).
the amount of light passing through. A typical ECD is a sand-
wich-like structure with two glass plates and an electrolyte.
Each glass plate is coated on the inside with an optically
transparent and electrically conducting layer of indium-tin
oxide (ITO), which operates as an electrode. Optically trans-
parent ITO electrodes are coated with complementary elec-
trochromic materials, which may be organic or inorganic
substances. The operation of electrochromic devices relies
on their electrochromic materials that are able to intercon-
vert between two or more color states upon oxidation or
reduction, that is, on electrolytic loss or gain of electrons.
Redox stability is an important requirement for production
of reliable electrochromic devices with long lifetimes. The
stability of the PCL-Th/PTh film was investigated via CV. It
was accomplished by nonstop cycling of the applied poten-
tial between ꢂ0.5 and þ1.5 V versus Ag/ Agþ electrode with
400 mV/s scan rate. Even after 3000th run, the PCL-Th/PTh
film indicated only a slight decrease in electroactivity accom-
panied by unperturbed color change under atmospheric con-
ditions (Fig. 9).
PCL-Th/PTh film can be useful for electrochromic application
due of the reversible redox behavior, stability, reasonable
switching time, ease of fabrication, and low cost. For this
purpose, PCL-Th/PTh film was used as an anodically coloring
electrochromic layer in a dual-type ECD construction.
Spectroelectrochemistry of PCL-Th/PThkPEDOT
Electrochoromic Device
ECD is a two-electrode electrolytic cell, which changes light
transmission properties in response to externally applied
voltage across the two electrodes and thus allow to control
FIGURE 9 Cyclic Voltammogram of PCL-Th/PTh film as a func-
tion of repeated scans 400 mV/s.
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ECD was assembled with a configuration of ITO|PCL-Th/
PThkGel electrolytekPEDOT|ITO as shown in Figure 1. The
dual-type ECD was prepared as follows: first, anodically col-
oring polymer (PCL-Th/PTh) and cathodically coloring poly-
mer (PEDOT) were coated on ITO/glass surfaces potentiody-
namically. Then, the gel electrolyte (see Experimental part)
was spread over the faces of PCL-Th/PTh and PEDOT layers
on ITO. Finally, PCL-Th/PTh and PEDOT layers with gel elec-
trolyte were stacked face-to-face.
Optoelectrochemical spectra of the dual-type PCL-Th/PTh
and PEDOT ECD as a function of applied voltage from þ0.2
to þ1.7 V are shown in Figure 10. At 0.2 V, the device
revealed a maximum absorption at 480 nm and its color was
red. At this stage, the copolymer was in its neutral state
revealing red and PEDOT was in its oxidized form (highly
transparent blue) revealing on significant absorption at the
visible region and hence the color of the ECD was red. On
incremental increase in the applied potential, simultaneous
reduction of the PEDOT and oxidation of PCL-Th/PTh layers,
which were signified with the decrease in the intensity of the
peak at 450 nm and increase in the intensity of the peak at
622 nm, were observed. At 1.7 V as the spectral signatures of
both layers become dominant, the color of the device turned
into blue, indicating that the PEDOT and copolymer were in
their fully neutral and oxidized states, respectively.
FIGURE 11 Switching histogram between ꢂ0.2 V and þ1.7 V.
oxidation (switching from ꢂ0.2 to 1.7 V; red to blue) and
reduction (switching from 1.7 to ꢂ0.2 V; blue to red)
response time of the device were 0.39 and 0.95 s, respec-
tively. The device also retained its performance by 75% even
after 3000th cycle.
CONCLUSIONS
A novel thiophene monomer with miktofunctional initiator
groups (primary hydroxyl and tertiary bromide) at the third
position of the thiophene ring was successfully prepared in a
four-step reaction sequence. Investigation of the accurate
prediction of geometry and IR spectroscopic characteristic
for the thiophene monomer in the gas phase was carried out
by DFT. DFT calculated bond lengths, angles, and vibrations
were predicted in good agreement with available experimen-
tal data and experimental vibrational spectra. Primary
hydroxyl initiating site of the thiophene monomer for e-CL
gave a-Th end-capped PCL homopolymer via ROP. In addi-
tion, copolymerizations of PCL-Th with Py and Th via constant
potential electrolyses indicated the possibility of the reaction
between Py or Th and a-Th end group of PCL-Th. Spectroelec-
trochemical analysis of PCL-Th/PTh copolymer film supports
the idea that an electrochromic copolymer was obtained. The
copolymer obtained from the electropolymerization reaction
between PCL-Th and Th on ITO-coated glass electrode was
employed as an anodically coloring material in ECD application.
Reversible and distinctive color changes of PCL-Th/PTh film
showed that it can be used in the construction of electrochro-
mic devices. Further study related to the preparation of block
copolymer having thiophene functionality at the junction point
via the ATRP of MMA using PCL-Th as the macroinitiator and
its ECD application are now in progress.
Switching Properties of ECD
Color-switching times of ECDs are of primary importance in
display applications. It should have low response time (in
order of a few seconds) and resistance to applied potential
between its reduced and oxidized states. The graph demon-
strating the switching time for the device is given in Figure
11. The change in optical contrast of the device was
recorded as 630 nm, as the potential was switched between
ꢂ0.2 and þ1.7 V with a residence time of 5 s. The optical
contrast of the device at 630 nm was found to be 22%. The
The authors thank The Gebze Institute of Technology Research
Foundation for financial support (Project No: 2005-A-3).
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