Identification and quantification of defect structures in poly(2,5-thienylene vinylene) derivatives prepared via the dithiocarbamate precursor route by means of NMR spectroscopy on 13C-labeled polymers

Article abstract of DOI:10.1021/ma200228g

During the past decades several synthetic routes toward the low band gap polymer poly(2,5-thienylene vinylene) (PTV) and derivatives have been studied. This study describes an extensive NMR characterization of ¹³C-labeled 3-octyl-PTV and its precursor polymer prepared via the dithiocarbamate route which is, since stable monomers are available, a promising route toward PTV derivatives. By introducing ¹³C-labeled vinylene carbons, we were able to characterize these polymers in a quantitative way, taking the end groups and structural polymerization defects, which disturb the conjugated system, into account. Several NMR techniques and the synthesis of model compounds were used to fully assign the proton and carbon chemical shifts. Moreover, the classically used thermal conversion of the precursor toward the conjugated polymer has been compared to a smoother, acid-induced elimination procedure.

Full text of DOI:10.1021/ma200228g

ARTICLE
pubs.acs.org/Macromolecules
Identification and Quantification of Defect Structures in
Poly(2,5-thienylene vinylene) Derivatives Prepared via the
Dithiocarbamate Precursor Route by Means of NMR Spectroscopy
on 13C-Labeled Polymers
H. Dili€en,† S. Chambon,† T. J. Cleij,† L. Lutsen,‡ D. Vanderzande,†,‡ and P. J. Adriaensens*,†
†Institute for Materials Research (IMO), Chemistry Division, Hasselt University, Universitaire Campus, Agoralaan Building D,
B-3590 Diepenbeek, Belgium
‡Division IMOMEC, IMEC, Universitaire Campus, Agoralaan Building D, B-3590 Diepenbeek, Belgium
ABSTRACT: During the past decades several synthetic routes toward the low band
gap polymer poly(2,5-thienylene vinylene) (PTV) and derivatives have been studied.
This study describes an extensive NMR characterization of 13C-labeled 3-octyl-PTV
and its precursor polymer prepared via the dithiocarbamate route which is, since stable
monomers are available, a promising route toward PTV derivatives. By introducing
13C-labeled vinylene carbons, we were able to characterize these polymers in a
quantitative way, taking the end groups and structural polymerization defects, which
disturb the conjugated system, into account. Several NMR techniques and the
synthesis of model compounds were used to fully assign the proton and carbon chemical shifts. Moreover, the classically used
thermal conversion of the precursor toward the conjugated polymer has been compared to a smoother, acid-induced elimination
procedure.
’ INTRODUCTION
therefore allows the polymerization of better defined precursor
polymers.
During the past two decades rapid strides have been made in
the field of conjugated polymers. Their semiconductor proper-
ties allow their use in all kinds of electronic devices, such as
organic field effect transistors,1,2 biosensors,3 light-emitting
diodes,4,5 and photovoltaic cells.6ꢀ9 The main advantage of such
organic semiconductors is that their structure and thus their
electro-optical properties can be tuned virtually to any combina-
tion of specifications needed for a dedicated application. For
example, in polymeric bulk heterojunction solar cells the mis-
match between the absorption spectrum of the active layer and
the solar emission spectrum can be decreased by focusing on
the design and synthesis of different classes of low band gap
materials.10ꢀ16 In this context poly(2,5-thienylene vinylene)
(PTV) derivatives have been considered as a promising class of
low band gap polymer.17ꢀ22 The most efficient synthetic routes
toward PTVs make use of the polymerization behavior of
quinodimethane systems. They can be obtained via a base
induced elimination in a premonomer of type 1g (Schemes 1
and 2). On formation of the quinodimethane system, a fast
polymerization reaction yields high molecular weight pre-
cursor polymers which can be converted in situ or ex situ to
the conjugated structure. Substantial research efforts have been
devoted to the optimization of these synthetic pathways to-
ward PTV derivatives, e.g. the Wessling,23ꢀ25 xanthate,26,27
sulfinyl,28,29 and dithiocarbamate30,31 routes. The dithiocarba-
mate (DTC) route toward PTV, developed in our lab, has as
main advantage that the premonomer is much more stable as
compared to the premonomers of the other precursor routes and
In this paper a thorough investigation is presented into the
detection and identification of defect structures and end groups
in 3-octyl-PTV (O-PTV) precursor polymers and their con-
verted, still soluble, conjugated O-PTV polymers. Hereto, 13C-
labeled polymers were prepared by introducing 13C labels in the
premonomers. Quantitative NMR techniques and model com-
pounds allowed for the identification of said defects. Moreover,
the precursor polymer was converted to the conjugated polymer
by a thermal as well as acid-induced conversion process. Com-
parison of the results obtained indicates that the acid-induced
conversion proceeds smoother than the thermal-induced one.
’ EXPERIMENTAL SECTION
Materials. All the commercially available chemicals were purchased
from Acros or Aldrich and were used without further purification unless
stated otherwise. Tetrahydrofuran (THF) and diethyl ether used in the
synthesis were dried by distillation from sodium/benzophenone.
Analytical. 1H and 13C NMR spectra were taken on a Varian Inova
300 or 400 spectrometer from solutions in deuterated chloroform
(D, 99.8%). The chemical shifts were calibrated by means of the
(remaining) proton and carbon resonance signals of CDCl3 (7.24 and
77.7 ppm for 1H and 13C, respectively). The T1C relaxation decay times
were determined via the inversionꢀrecovery method. To quantify the
Received: February 1, 2011
Revised:
May 3, 2011
Published: May 19, 2011
r
2011 American Chemical Society
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amount of structural defects and end groups from the 13C NMR spectra,
the summed integration of three signals of the side chain (carbon atoms
17, 18, and 19) was taken as an internal reference to which the other
resonances were normalized. Molecular weights and molecular weight
distributions were determined relative to polystyrene standards
(Polymer Laboratories) by size exclusion chromatography (SEC).
Chromatograms were recorded on a Spectra series P100 (Spectra
Physics) equipped with two MIXED-B columns (10 μm, 0.75 ꢁ
30 cm, Polymer Laboratories) and a relative index (RI) detector
(Shodex). GC/MS analyses were carried out on a TSQ-70 and Voyager
mass spectrometer (Thermoquest); capillary column: Chrompack
Cpsil5CB or Cpsil8CB.
diethyl ether and dried over MgSO4. The solvent was evaporated, and
the highly reactive dichloride 1f was obtained as an orange oil. Because of
the high reactivity of 1f, the dichloride was used in the next reaction step
without purification or characterization.
Synthesis of 3-Octyl-2,5-diylbismethylene N,N-Diethyldithiocar-
bamate (1g). To a solution of 1f (2.08 g, 4.0 mmol) in ethanol (50 mL),
diethyldithiocarbamic acid sodium salt trihydrate (3.61 g, 16 mmol) was
added as solid. The mixture was stirred at ambient temperature over-
night. Then, water was added and the desired monomer was extracted
with diethyl ether and dried over MgSO4. The monomer 1g was
obtained after column chromatography (eluent: chloroform/hexane
1
1/1) as a yellow oil (2.5 g, 67%). H NMR (CDCl3, δ in ppm, J in
Fourier transform-infrared spectroscopy (FT-IR) was performed on a
Perkin-Elmer Spectrum One FT-IR spectrometer (nominal resolution
4 cmꢀ1, summation of 16 scans).
Ultravioletꢀvisible spectroscopy (UVꢀvis) was performed on a
Varian cary 500 UVꢀvisꢀNIR spectrophotometer (interval: 1 nm; scan
rate: 600 nm/min, continuous run from 200 to 700 nm).
Hz): 6.75 (d, J = 3.3, 1H), 4.65 (d, J = 146, 2H), 4.58 (d, J = 146, 2H),
4.01 (q, J = 6.8, 4H), 3.68 (q, J = 6.8, 4H), 2.48 (t, J = 8.0, 2H), 1.59ꢀ1.49
(m, 2H), 1.33ꢀ1.22 (m, 16H), 0.85 (t, J = 6.8, 3H). 13C NMR (CDCl3,
δ in ppm): 195.3, 195.0, 141.9, 137.8, 131.4, 129.6, 49.9, 47.3, 32.5, 31.2,
30.1, 29.8, 29.1, 23.3, 14.7, 13.1, 12.2. MS (CI, m/e): 517 (Mþ).
B. Synthesis of the 13C-Labeled Precursor Polymer 1h. The mono-
mer 1g (0.93 g, 1.8 mmol) was previously freeze-dried. A solution with a
monomer concentration of 0.4 M in dry THF was degassed by passing
through a continuous nitrogen flow. The solution was cooled to 0 °C.
Sodium bis(trimethylsilyl)amide (NaHMDS) (3.6 mL of a 1.0 M
solution in THF) was added in one go to the stirring monomer solution.
The resulting mixture was stirred for 90 min under continuous nitrogen
flow at 0 °C. The polymer was precipitated in ice water, and the water
layer was neutralized with diluted HCl before extraction with chloro-
form. The solvent of the combined organic layers was evaporated under
reduced pressure, and a second precipitation was performed in pure cold
methanol. The polymer 1h was collected and dried in vacuo (0.45 g,
yield 67%). The polymer has been fractionated by multiple reversed
precipitations (the polymer was solved in CHCl3 and MeOH was added
dropwise until the high molecular weight fraction precipitated) to collect
the high molecular weight fraction (180 mg = precursor polymer 1D
(large batch)).
TLC analyses were made on Merck aluminum sheets, 20 ꢁ 20 cm,
covered with silica gel 60 F254
.
Synthesis and Characterization. A. Synthesis of the 13C-La-
beled Monomer (1g). The synthesis of 3-octylthiophene 1b and 3-octyl-
2,5-dibromothiophene 1c has been described elsewhere.32
Synthesis of 3-Octyl-2,5-thiophenedicarboxaldehyde (1d). In a
three-necked round-bottom flask a solution of 2,5-dibromo-3-octylthio-
phene, 1c (4.3 g, 12 mmol), in THF (100 mL) was stirred under a
nitrogen atmosphere at ꢀ78 °C. A solution of n-butyllithium (16.7 mL,
27 mmol of a 1.6 M solution in hexane) was slowly added with a cannula,
the mixture was stirred for 30 min, and afterward 13C enriched DMF
(2.0 g, 0.27 mol), previously distilled, was slowly added. The resulting
mixture was stirred for 12 h at room temperature. HCl (2 M, 50 mL) was
added to quench the excess of n-BuLi followed by extraction with
chloroform. The organic layers were dried over MgSO4, and the
obtained compound was purified by column chromatography on silica
gel with chloroform/hexane (1/1) as a solvent. The dialdehyde 1d was
obtained as orange oil (1.8 g, yield 58%). 1H NMR (CDCl3, δ in ppm,
J in Hz): 10.12 (d, 1JHꢀ13C = 180, 1H), 9.95 (d, 1JHꢀ13C = 180, 1H), 7.63
(d, J = 3.5, 1H), 2.97 (t, J = 8.1, 2H), 1.73ꢀ1.62 (m, 2H), 1.41ꢀ1.20 (m,
10H), 0.86 (t, J = 7.3, 3H). 13C NMR (CDCl3, δ in ppm): 184.1, 183.7,
152.7, 148.9, 144.3, 138.2, 32.5, 31.9, 30.0, 29.9, 29.8, 29.2, 23.3, 14.8.
MS (EI, m/e): 254 (Mþ).
Polymers 1A and 1B are the low and high molecular weight fractions
of a small polymerization batch. Starting from 280 mg (5.4 ꢁ 10ꢀ4 mol)
of monomer 1g, 0.13 g (yield 65%) of precursor polymer 1h was
obtained. After multiple reversed precipitations, 45 mg of high molecular
weight and 54 mg of low molecular weight precursor polymer were
obtained. Precursor polymer 1C is the isolated high molecular weight
fraction of another small batch. Here 210 mg of precursor polymer 1h
was obtained from 442 mg (8.5 ꢁ 10ꢀ4 mol) monomer 1g (yield 67%).
After multiple reversed precipitations, 61 mg of high molecular weight
polymer was collected. IR (in cmꢀ1): 2931, 2846, 1486, 1415, 1268,
1206. SEC: Mw = 4.6 ꢁ 103; PD = 6.2 (batch 1A); Mw = 6.8 ꢁ 103; PD =
5.0 (batch 1B); Mw = 8.3 ꢁ 103; PD = 3.5 (batch 1C); Mw = 18.8 ꢁ 103;
PD = 3.4 (batch 1D). 1H NMR (CDCl3, δ in ppm, J in Hz): 6.49 (1H),
5.35 (1H), 3.96 (2H), 3.66 (2H), 3.17 (2H), 2.28 (2H), 1.40ꢀ1.05
(18H), 0.85 (3H). 13C NMR (CDCl3, δ in ppm): 194.7, 140.2, 139.3,
134.4, 128.4, 53.3, 49.8, 47.4, 37.2, 32.7, 31.6, 30.4, 30.1, 29.0, 23.4, 14.9,
13.4, 12.3.
Synthesis of 3-Octyl-2,5-bis(hydroxymethyl)thiophene (1e). In a
three-necked round-bottom flask a mixture of LiAlH4 (0.54 g, 14 mmol)
in dry THF (50 mL) was made under an argon atmosphere. This slurry
was cooled to 0 °C, and the dialdehyde 1d (1.8 g, 7.1 mmol) in THF
(50 mL) was slowly added with a dropping funnel. When the addition
was completed, the slurry was heated at reflux temperature for 5 h.
Afterward, the mixture was placed in an ice bath and quenched very
careful with water and an aqueous 15% NaOH solution. The solution
was extracted with diethyl ether and dried over MgSO4, and the solvent
was evaporated under reduced pressure. The diol 1e was obtained as a
yellow oil in a yield of 96% (1.7 g). 1H NMR (CDCl3, δ in ppm, J in Hz):
6.76 (d, 3JHꢀ13C = 3.2, 1H), 4.73 (d, J = 144, 2H), 4.70 (d, J = 144, 2H),
2.52 (t, J = 8.0, 2H), 1.57ꢀ1.48 (m, 2H), 1.34ꢀ1.19 (m, 10H), 0.85 (t,
J = 6.4, 3H). 13C NMR (CDCl3, δ in ppm): 142.6, 140.9, 137.8, 128.3,
60.9, 58.4, 32.5, 32.3, 31.7, 30.1, 29.9, 28.9, 23.3, 14.8. MS (EI, m/e): 224
(Mþ ꢀ hydroxyl functions, Mþ is not stable).
C. Synthesis of the 13C-Labeled Polymer 1i. Thermal Conversion.
The precursor polymer 1h (batch 1D: 60 mg, 0.16 mmol) was dissolved
in o-dichlorobenzene (5 mL) and refluxed for 1 h. After being cooled, the
obtained slurry was precipitated in methanol. The precipitate was
filtered off, washed several times with methanol, and dried in vacuo. A
purple/black solid was obtained (25 mg, yield 70%). UVꢀvis: λmax
=
574 nm (shoulder: 611 nm). SEC: Mw = 48 ꢁ 103; PD = 2.2. 1H NMR
(CDCl3): see chemical shifts of 1h except for the signals of the
dithiocarbamate group which disappear during elimination and the
13C-labeled positions which form a double bond around 6.8ꢀ
6.9 ppm. 13C NMR (CDCl3): see chemical shifts of 1h except for
the dithiocarbamate signals which disappear during elimination and the
13C-labeled positions which form a double bond around 121 ppm.
Synthesis of 3-Octyl-2,5-bis(chloromethyl)thiophene (1f). To a
cooled (0 °C), stirred solution of diol 1e (1.7 g, 6.8 mmol) in THF
(30 mL) was slowly added a solution of SOCl2 (2.16 g, 18 mmol) in
THF (40 mL). The temperature of the reaction mixture was allowed to
increase to RT under continuous stirring for 1 h. Then, the mixture was
cooled down again to 0 °C, and a saturated sodium carbonate solution
was added dropwise until neutral. The mixture was extracted with
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Acid-Induced Conversion. The precursor polymer 1h (batch 1D:
60 mg, 0.16 mmol) was dissolved in o-dichlorobenzene (5 mL) and
heated until 70 °C before trifluoroacetic acid (0.018 mL, 0.24 mmol)
was added. The solution was stirred for 10 min at 70 °C. After being
cooled, the solution was poured into H2O and extracted with diethyl
ether. The solvent was evaporated under reduced pressure, and the
obtained slurry was precipitated in MeOH, filtered off, and dried in
vacuo. A purple/black solid was obtained (34 mg, yield 97%). UVꢀvis:
λmax = 574 nm (shoulder: 614 nm). SEC: Mw = 109 ꢁ 103; PD = 2.8. 1H
NMR (CDCl3): see chemical shifts of 1h except for the signals of the
dithiocarbamate group which disappear during elimination and the 13C-
labeled positions which form a double bond around 6.8ꢀ6.9 ppm. 13C
NMR (CDCl3): see chemical shifts of 1h except for the dithiocarbamate
signals which disappear during elimination and the 13C-labeled positions
which form a double bond around 121 ppm.
2d was obtained as an oil (0.14 g, 0.7 mmol) after column chromatog-
raphy (eluent: hexane) in a yield of 20%. 1H NMR (CDCl3, δ in ppm,
J in Hz): 7.14 (dd, J1 = 5.1, J2 = 1.2, 2H), 6.93 (dd, J1 = 5.1, J2 = 3.6, 2H),
6.82 (dd, J1 = 3.6, J2 = 1.2, 2H), 3.21 (s, 4H). 13C NMR (CDCl3, δ in
ppm): 144.3, 127.4, 125.3, 124.0, 32.8. MS (EI, m/e): 194 (Mþ).
Synthesis of 2j. A solution of thiophene, 2i (10 g, 0.12 mol), and
2-thiophene acetyl chloride, 2h (23.9 g, 0.15 mol), in toluene was stirred
at room temperature. AlCl3 (19.8 g, 0.15 mol) was added as a solid in a
time frame of 10 min, and the reaction mixture was refluxed for 30 min.
The solution was cooled to room temperature and quenched carefully
with a 2 M HCl solution. The resulting reaction mixture was extracted
with toluene, washed with HCl, NaOH, and H2O, and dried over
MgSO4. The crude product was purified by distillation to give a colorless
oil (6.6 g, 32 mmol, 29%). 1H NMR (CDCl3, δ in ppm, J in Hz): 7.79
(dd, J1 = 3.7, J2 = 1.1, 1H), 7.65 (dd, J1 = 5.0, J2 = 1.1, 1H), 7.21 (dd, J1 =
3.7, J2 = 2.9, 1H), 7.13 (dd, J1 = 3.8, J2 = 5.0, 1H), 6.95ꢀ6.94 (m, 2H),
4.38 (s, 2H). 13C NMR (CDCl3, δ in ppm): 189.5, 143.8, 135.9, 135.1,
133.4, 128.9, 127.57, 127.55, 125.8, 40.8. MS (EI, m/e): 208 (Mþ).
Synthesis of 2k. A solution of 2j (1 g, 4.8 mmol) was made in a
mixture of CH2Cl2 and MeOH (1/1). The mixture was cooled to 0 °C,
and NaBH4 (0.36 g, 9.6 mmol) was added as a solid. The resulting
solution was stirred for 12 h at room temperature. The reaction was
quenched with NH4Cl and extraction has been carried out by Et2O and
dried over MgSO4. The crude product was purified by flash column
D. Synthesis of Model Compounds for End Groups. The chemical
shifts of the model compounds are assigned using the numbering system
in the following figure:
1
Model compound 2a: 2-thiophene-carbaldehyde (commercially
available). 1H NMR (CDCl3, δ in ppm, J in Hz): 9.91 (d, J = 1.2, 1H, 6),
7.77 (dd, J1 = 3.9, J2 = 1.2, 1H, 5), 7.75 (dt, J1 = 4.9, J2 = 1.2, 1H, 3), 7.20
(dd, J1 = 4.9, J2 = 3.9, 1H, 4). 13C NMR (CDCl3, δ in ppm): 183.7 (6),
144.6 (2), 137.0 (3), 135.8 (5), 129.0 (4).
chromatography to give a colorless oil (0.95 g, 4.5 mmol, 94%). H
NMR (CDCl3, δ in ppm, J in Hz): 7.25 (dd, J1 = 4.7, J2 = 1.7, 1H), 7.17
(dd, J1 = 5.1, J2 = 1.2, 1H), 6.99ꢀ6.92 (m, 3H), 6.86 (dd, J1 = 3.4, J2 =
1.0, 1H), 5.14 (dd, J1 = 6.9, J2 = 5.6, 1H), 3.32 (dd, J1 = 2.4, J2 = 0.8, 2H).
13C NMR (CDCl3, δ in ppm): 147.6, 140.0, 127.4, 127.2, 127.1, 125.3,
125.1, 124.6, 71.4, 40.5. MS (EI, m/e): 209 (Mþ).
Synthesis of 2l. In a three-necked round-bottom flask, the alcohol 2k
(0.95 g, 4.5 mmol) was added dropwise to a stirred solution of SOCl2
(1.18 g, 9.9 mmol) in THF (60 mL). The reaction mixture was stirred
under a nitrogen atmosphere for 30 min. The solution was cooled to
0 °C and quenched carefully with a NaHCO3 solution. The resulting
reaction mixture was extracted with Et2O and dried over MgSO4. The
solvent was removed under reduced pressure. Because of the unstable
nature of the product 2l (1.0 g, 4.4 mmol), the crude product has been
used without further purification or characterization.
Model Compound 2e. To a solution of the chloride 2l (1.0 g,
4.4 mmol) in ethanol (25 mL), diethyldithiocarbamic acid sodium salt
trihydrate (3.9 g, 18 mmol) was added as a solid. The mixture was stirred
at ambient temperature overnight. Then, water was added and the
desired monomer was extracted with diethyl ether and dried over
MgSO4. The model compound 2e was obtained after column chroma-
tography (eluent: chloroform/hexane 1/1) as a yellow oil (1.24 g,
3.6 mmol) in a yield of 59%. 1H NMR (CDCl3, δ in ppm, J in Hz): 7.19
(dd, J1 = 5.2, J2 = 1.3, 1H), 7.06 (dd, J1 = 5.0, J2 = 1.3, 1H), 6.99 (dd, J1 =
3.5, J2 = 1.3, 1H), 6.90 (dd, J1 = 5.2, J2 = 3.5, 1H), 6.84 (dd, J1 = 5.0, J2 =
3.5, 1H), 6.81 (dd, J1 = 3.5, J2 = 1.3, 1H), 5.62 (dd, J1 = 9.9, J2 = 4.7, 1H),
4.01 (q, J = 6.9, 2H), 3.84 (dd, J1 = 14.8, J2 = 4.7, 1H), 3.69 (q, J = 6.9,
2H), 3.43 (dd, J1 = 14.8, J2 = 9.9, 2H), 1.26 (t, J = 6.9, 3H), 1.25 (t, J = 6.9,
3H). 13C NMR (CDCl3, δ in ppm): 194.2, 143.1, 141.3, 127.4, 127.2,
127.1, 126.9, 125.7, 124.7, 52.9, 50.0, 47.4, 38.8, 13.2, 12.3.
F. Synthesis of Model Compounds for Structural Defects in the
Conjugated Polymer. Synthesis of 2m. A suspension of Zn powder
(6.9 g, 0.1 mol) was made in anhydrous THF (200 mL). This suspension
was stirred at ꢀ10 °C under nitrogen atmosphere. TiCl4 (10 g, 0.05
mol) was added dropwise to this suspension. The resulted mixture was
heated at reflux temperature for 1 h. Afterward the mixture was cooled to
room temperature, the aldehyde 2a was added slowly and the reflux was
continued for 4 h. The mixture was poured into a K2CO3 (10%)
solution, extracted with Et2O and the organic layers were dried over
MgSO4. The product was purified by means of column chromatography
Model compound 2b: 2-thiophene methanol. A solution of 2a (3 g,
27 mmol) in a 1:1 mixture of methanol and CH2Cl2 was cooled to 0 °C,
and NaBH4 (1.5 g, 40 mmol) was added as a solid. The mixture was
stirred overnight and afterward quenched by the addition of H2O
(50 mL), followed by an extraction with Et2O. The organic layers were
dried over MgSO4, and the obtained compound was purified by column
chromatography on silica gel with hexane as a solvent. The alcohol 2b
1
was obtained as a colorless oil in a yield of 93% (2.9 g). H NMR
(CDCl3, δ in ppm, J in Hz): 7.27 (dd, J1 = 5.0, J2 = 1.3, 1H, 5), 7.00 (m,
1H, 3), 6.96 (dd, J1 = 5.0, J2 = 3.5, 1H, 4), 4.82 (d, J = 3.5, 2H, 6), 1.79 (s,
1H, 60). 13C NMR (CDCl3, δ in ppm): 144.2 (2), 127.0 (3), 125.6 (4),
125.5 (5), 59.3 (6).
Model compound 2c: 2-thiophenecarboxylic acid (commercially
1
available). H NMR (CDCl3, δ in ppm, J in Hz): 12.50 (s, 1H, 6),
7.89 (dd, J1 = 3.8, J2 = 1.2, 1H, 3), 7.64 (dd, J1 = 5.0, J2 = 1.2, 1H, 5), 7.13
(dd, J1 = 5.0, J2 = 3.8, 1H, 4). 13C NMR (CDCl3, δ in ppm): 168.8 (6),
135.8 (3), 134.8 (5), 133.5 (2), 128.8 (4).
E. Synthesis of Model Compounds for Structural Defects in the
Precursor Polymer. Synthesis of 2-(Chloromethyl)thiophene (2g). A
solution of SOCl2 (6.9 g, 58 mmol) in dry CH2Cl2 (40 mL) was made in
a three-necked round-bottom flask. A solution of the alcohol 2b (3 g,
26 mmol) in dry THF (20 mL) was added dropwise at 0 °C, and the
reaction mixture has been stirred for 30 min at room temperature. The
resulting reaction mixture was quenched with NaHCO3 and extracted
with CH2Cl2. The crude product was distilled by vacuum distillation to
1
get a colorless oil (3.0 g, 23 mmol) in a good yield (88%). H NMR
(CDCl3, δ in ppm, J in Hz): 7.30 (dd, J1 = 5.1, J2 = 1.2, 1H), 7.07 (dd,
J1 = 3.5, J2 = 1.2, 1H), 6.94 (dd, J1 = 5.1, J2 = 3.5, 1H), 4.80 (s, 2H). 13
NMR (CDCl3, δ in ppm): 140.2, 127.8, 127.0 (2C), 40.5.
C
Model Compound 2d. A three-necked round-bottom flask was filled
with dry THF and sodium (0.9 g, 40 mmol). 2-(Chloromethyl)thio-
phene, 2g (0.5 g, 3.8 mmol), was added dropwise to this dispersion.
The reaction mixture was stirred overnight. Afterward, the reaction was
quenched by the addition of ethanol (50 mL) and H2O (50 mL). The
mixture was extracted with CH2Cl2 and dried over MgSO4. The product
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Scheme 1. Synthetic Route toward 13C-Labeled O-PTVa
a Reagents and conditions: (i) BrMgC8H17, NiCl2(dppp); (ii) NBS, DMF; (iii) 1, BuLi; 2, 13C-enriched DMF; (iv) LiAlH4, THF; (v) SOCl2, THF;
(vi) NaSC(S)NEt2.3H2O, EtOH; (vii) NaHMDS, THF; (viii) ΔT or CF3COOH.
Scheme 2. Polymerization Mechanism for the DTC Precursor Route
with petroleum ether as eluent. The product 2m was obtained as a
colorless powder (1.37 g, 13%). 1H NMR (CDCl3, δ in ppm, J in Hz):
7.18 (dd, J1 = 5.0, J2 = 1.1, 2H), 7.06 (s, 2H), 7.04 (dd, J1 = 3.6, J2 = 1.1,
2H), 6.99 (dd, J1 = 5.0, J2 = 3.6, 2H). 13C NMR (CDCl3, δ in ppm):
143.0, 128.3, 126.7, 125.0, 122.1. MS (EI, m/e): 192 (Mþ).
’ RESULTS AND DISCUSSION
Synthesis of the 13C-Labeled Monomer (1g). The first two
steps in the synthesis of the 13C-labeled monomer for octyl-PTV
(O-PTV) are exactly the same as for the nonlabeled variant
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Table 1. Average Number of Repetition Units in the Precursor and Conjugated O-PTV Polymers As Calculated by GPC
and NMRa
GPC
PD
NMR
polymer
Mn ꢁ 103 (g molꢀ1
)
av no. of repetition units
183.5 ppm
162.3 ppm
59.7 and 61.5 ppm
av no. of repetition units
precursor O-PTV
1A
7.4
13.7
24
6.2
5.0
3.5
3.4
20
37
2.2%
1.2%
1.4%
0.3%
9.3%
4.4%
2.0%
1.1%
9.0%
4.9%
2.5%
1.3%
10
19
34
74
1B
1C
65
1D
55
149
O-PTV
thermal
acid
22
39
2.2
2.8
100
177
0.4%
0.4%
1.0%
0.7%
1.0%
0.8%
83
105
a Molecular weight of the repetition units is 369 and 220 Da for the precursor and conjugated polymer, respectively. The conjugated O-PTVs were
obtained by conversion of the precursor polymer 1D.
Table 2. Chemical Shift Assignment of the Proton and
Carbon NMR Signals of Precursor O-PTV
chemical shift
13C (ppm)
chemical shift
1H (ppm)
carbon atom
T1C (s)
C1/C2
C3/C4
C5
12.3/13.4
1.05ꢀ1.40
3.66/3.96
47.4/49.8
194.7
C6
53.3
5.35
3.17
C7
37.2
C8/C11/C9
C10
134.4/139.3/140.2
Figure 1. Chemical structure of precursor O-PTV including a carbon
numbering scheme for the assignment of the resonances in the 13C
NMR spectra.
128.4
29.0
31.6
30.4
30.1
32.7
23.4
14.9
6.49
C12
0.21
0.41
0.52
0.76
1.32
1.85
2.80
2.28
C13
1.05ꢀ1.40
1.05ꢀ1.40
1.05ꢀ1.40
1.05ꢀ1.40
1.05ꢀ1.40
0.85
(Scheme 1).32 First, the substitution of 3-bromothiophene (1a)
by means of a Kumada coupling33 has been performed, followed
by bromination of the R-positions of the thiophene (1b).34 In the
third step of the reaction pathway, the 13C label was incorpo-
rated. This was done by a metalꢀhalogen exchange under the
influence of n-BuLi, followed by addition of 13C-enriched DMF,
which results into the 13C-labeled dialdehyde 1d.35 Thereafter,
the dialdehyde 1d was reduced with lithium aluminum hydride
(LiAlH4), followed by chlorination of the diol 1e with thionyl
choride (SOCl2).36 Because of the instability of the product 1f, it
must be converted in situ toward the bis(dithiocarbamate)
monomer 1g.17 The monomer 1g was purified using column
chromatography with hexane/chloroform (50/50) as an eluent.
Synthesis of the 13C-Labeled O-PTV Precursor Polymer (1h).
The optimized polymerization route toward the precursor poly-
mer has been reported earlier.32,37 The polymerization of the
precursor polymers has been accomplished under influence of
sodium bis(trimethylsilyl)amide (NaHMDS) (Scheme 1). Sev-
eral precursor polymer batches were prepared and fractionated
by means of reversed precipitation in order to separate the low
from the high molecular weight fractions toward characterization
by GPC and quantitative NMR (see later in the characterization
part). After fractionation the amounts of precursor polymers 1A,
1B, and 1C were too low to allow a comparative study of the acid
and thermally induced conversion to the conjugated state.
Therefore, a larger batch of precursor polymer has been prepared
of which the high molecular weight fraction (1D) was divided
into two parts. It can be noted that the higher the amount of
C14/C15
C16
C17
C18
C19
starting monomer used for the polymerization, the higher the
molecular weight of the high molecular weight fraction (see also
Experimental Section) appears to be.
Synthesis of the 13C-Labeled O-PTV (1i). To investigate and
compare two different conversion mechanisms toward the con-
jugated state in more detail, the large batch of precursor polymer
(1D) has been divided in two equal parts of which one part was
converted thermally while a smoother acid induced method was
used for the other part (Scheme 1).38 The resulting O-PTV
polymers were studied by quantitative NMR spectroscopy.
Characterization of the 13C-Labeled Precursor Polymer.
The first step in the polymerization pathway is the formation of
the actual monomer, a quinodimethane system (Scheme 2), by a
base-induced 1,6-elimination from the premonomer. Then, a
self-initiating radical polymerization reaction is triggered by the
recombination of two quinodimethane systems yielding the
initiating species, a diradical.39,40 In this way intrinsic defects
can be formed in the central part of the precursor polymer.
Consequently, the propagation occurs by addition of quinodi-
methane systems on both sides of the initiator. Because of the
nonsymmetric nature of the monomer, three different ways of
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monomer addition can take place for the growing polymer chain.
More specific, next to the normal head-to-tail addition which will
lead to the formation of double bonds upon conversion to the
conjugated state, head-to-head and tail-to-tail additions can take
place, which are considered as structural defects.41ꢀ44
In order to identify and quantify the amount of structural
defects present in the main chain, four different 13C-enriched
O-PTV precursors were studied by 13C NMR (1A, 1B, 1C, 1D)
of which the molecular weights are listed in Table 1.
relaxation decay times. For mobile side chains it can be stated
that the closer the carbon is situated toward the side chain end,
the longer its relaxation decay time will be due to less restricted
conformational motions.
However, next to the signals described in Table 2, some
remaining carbon signals were detected in the 13C spectra
(Figure 2). On the basis of their intensities, these signals have
to arise from 13C-labeled carbons situated in end groups or in
structural defects in the main chain. The carbonyl signals around
183.5 and 162.3 ppm do not show a 1JCꢀC coupling and therefore
have to represent end groups. Table 1 shows a decrease in their
intensity with increasing Mn, confirming that it concerns end-
group signals. DEPT spectra indicate that the 162.3 ppm signal
originates from a quaternary carbon while the 183.5 ppm signal
represents a methine carbon. The resonances at 59.7 and 61.5 ppm
are methylene carbons (DEPT) and show the same trend; i.e.,
their intensity increases with decreasing molecular weight (see
Figure 1 presents the chemical structure and carbon number-
ing for precursor O-PTV.
Analysis of the proton-decoupled 13C spectrum, DEPT and
INADEQUATE experiments allowed us to assign the NMR
signals as presented in Table 2. The chemical shift assignment of
the carbon nuclei of the octyl side chain is based on the T1C
1
insets of Figure 2), and no JCꢀC coupling is observed. The
assignment of these end-group signals to aldehyde (183.5 ppm),
carboxylic acid (162.3 ppm), and methylol (59.7 and 61.5 ppm)
functional groups will be discussed later on. By means of
quantitative 13C NMR spectroscopy, the amount of all end
groups was determined, and the number of repetition units
was calculated and compared to that obtained by GPC as
presented in Table 1. As the GPC measurements are calibrated
against polystyrene standards, it is not unusual that the GPC
results show an overestimation of the molecular weights.
Characterization of the 13C-Labeled Conjugated O-PTV.
The thermal conversion of the precursor polymer to the con-
jugated state can be accomplished by heating a solution of
precursor O-PTV in dichlorobenzene at a typical temperature
of 180 °C. The mechanism is presented below as a concerted
mechanism although a stepwise (radical) mechanism cannot be
excluded (Scheme 3). However, since these high temperatures
may lead to thermal degradation, an acid-induced conversion
method has been developed that allows to convert the precursor
O-PTV at much lower temperatures (typically 70 °C).38
At first glance the NMR spectra of the converted polymers
seem to be very similar (Figures 3 and 4). The 13C spectra
indicate that the polymer is fully converted; i.e., no trace of
the signal of the C6 carbon bearing the dithiocarbamate group
(53.3 ppm) is detected anymore, and the resonance character-
istic of the double bond appears around 120 ppm. The reso-
nances at 183.5, 162.3, 61.5, and 59.7 ppm are still present in the
13C NMR spectrum which confirms their assignment to end
groups. The amount of end groups is presented in Table 1. Two
independent characterization techniques, i.e., GPC and NMR,
Figure 2. 13C NMR spectrum of precursor polymer 1D (top inset =
polymer 1A, bottom inset = polymer 1D).
Scheme 3. Thermal- and Acid-Induced Conversion of Precursor O-PTV
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Figure 5. Model compounds for end groups.
Figure 6. Model compounds for possible structural units in the
precursor polymer.
Figure 3. 13C NMR spectrum of the acid converted O-PTV.
Scheme 4. Synthesis of Model Compound 2d
the conjugated polymer is accompanied by a chain scission
process which does not occur or to a lesser extent for the acid-
induced conversion process. Most probably the high temperature
(180 °C) induces chain scission at the weakest points in the
polymers structure, i.e., at the head-to-head defects.
Identification of End Groups and Structural Defects. End
Groups. While the 13C chemical shifts of the signals at 183.5 and
162.3 ppm are pointing to aldehyde and carboxylic end groups,
the chemical shifts at 61.5 and 59.7 ppm point to methylol end
groups. In order to confirm this assignment, model compounds
2a, 2b, and 2c (Figure 5) have been fully characterized by NMR
(see Experimental Section).
The assignment of the resonance at 183.5 ppm to an aldehyde
group is confirmed by means of the model compound 2a. Further
oxidation toward a carboxylic acid would not be unexpected. Model
compound 2c shows a resonance at 168.8 ppm which confirms
the assignment of the 162.3 ppm resonance to a carboxylic acid
function. The methylol model compound 2b was synthesized and
presents a 13C resonance at 59.3 ppm, which allows to assign the
signals at 59.7 and 61.5 ppm to methylol end groups.
Structural Defects. With the end groups and regular head-to-tail
polymer signals determined, only some very small signals remain
undefined, i.e., a signal for the precursor polymer at 58.2
ppm and two for the conjugated polymer around 36ꢀ38 ppm
(remark that the chemical shift difference is too large to assign
them to a doublet as a result of J13Cꢀ13C coupling). Since the
intensity of these undefined signals, arising from 13C-labeled
carbons, is very small, it demonstrates that the amount of
structural head-to-head and tail-to-tail polymerization defects
has to be very small.
Figure 4. 13C NMR spectrum of the thermally converted O-PTV.
demonstrate that the average number of repetition units is higher
in acid converted O-PTV as compared to thermally converted
O-PTV. Note that the higher Mn found for acid converted
O-PTV, as compared to the O-PTV precursor, can be explained
by a difference in hydrodynamic volume for GPC and an error
margin of around 0.3% for the NMR quantification. It can be
concluded from these molecular weight data that starting from
the same batch of precursor polymer the thermal conversion to
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Scheme 5. Synthesis of Model Compound 2e
Scheme 6. Synthesis of Model Compound 2m
Figure 7. Additional structural units possible in O-PTV (a, b, c) and
model compound for a head-to-tail unit after conversion (2m).
To get chemical shift information regarding the possible struc-
tural units in the precursor and conjugated polymers, several
model compounds have been made. Toward the identification of
structural defects at the precursor polymer level, two different
model compounds 2d (for head-to-head coupling) and 2e (for
normal head-to-tail) have been synthesized (Figure 6). The
synthesis of model compound 2f for the tail-to-tail coupling
has been explored extensively, but without success. The first
model compound 2d has been synthesized by the well-known
Wurtz coupling of 2-(chloromethyl)thiophene, 2g (Scheme 4).45
This chloride 2g has been made by a two step synthesis from
2-thiophene carbaldehyde, 2a. After a reduction with NaBH4, the
alcohol 2b has been converted into the chloride 2g by SOCl2.46
The second model compound 2e has been synthesized by a
four-step pathway (Scheme 5). After a FriedelꢀCrafts reaction
between 2-thiopheneacetyl chloride (2h) and thiophene (2i,) the
resulting keton 2j has been reduced to the alcohol 2k by NaBH4 in
an almost quantitative yield.47 In a next step the alcohol 2k was
transformed into a rather unstable chloride 2l which was imme-
diately converted into model compound 2e via a substitution
reaction with diethyldithiocarbamic acid sodium salt.17,46
Three additional structural units can appear at the level of the
conjugated O-PTV (Figure 7). The model compound 2m contains
the double bond present in the structural unit obtained upon
elimination of a normal head-to-tail unit. This model compound
has been synthesized by a McMurry coupling of 2-thiophene
carbaldehyde 2a under influence of Zn and TiCl4 (Scheme 6).47
The elimination of dithiocarbamate groups out of a tail-to-tail
structural unit can lead to defects as presented in Figure 7 for a
partialelimination(b) and double elimination (c). Thelatter would
show a triple-bond resonance signal that would be easily observed
in the 13C NMR spectra around 90 ppm and which is not detected.
Regarding the undefined signal at 58.2 ppm in the 13C
spectrum of the precursor polymer, its relative intensity is not
Figure 8. Chemical structure of tail-to-tail (left) and head-to-head
(right) defects in precursor O-PTV.
influenced by the molecular weight. This suggests that the corres-
ponding carbon atom resides in the main chain. However, since
the signal appears as a singlet, the corresponding 13C atom
should be linked to a chemically identical 13C atom, explaining
why no JCꢀC coupling is observed. It can be expected that C6/C7
of a tail-to-tail addition would have a resonance at this position
(Figure 8). Moreover, the observations that (a) DEPT spectra
indicate that this signal corresponds to a CH group and (b) this
signal disappears in the conjugated polymer are strong indica-
tions to assign the resonance at 58.2 ppm to a tail-to-tail defect.
The model compound 2d shows a methylene signal at 32.8
ppm. Since no signal was detected at this position in the 13C
spectrum of the precursor polymer, no direct evidence for head-
to-head defects was found at the precursor level. It can however
not be excluded that the position of this methylene signal is
slightly shifted downfield in the precursor polymer and so would
end up in the foot of the huge C7 signal (37.2 ppm) of the normal
head-to-tail units. The 13C spectrum of converted O-PTV (see
below) should make this clear.
The chemical shifts of the bridge methylene and methine
carbon resonances of model compound 2e (38.8 and 52.9 ppm)
are in good agreement with those of C7 and C6 in normal head-
to-tail units of the precursor O-PTV (37.2 and 53.3 ppm).
Regarding the two undefined signals in the 13C spectrum of
the conjugated polymer at 36ꢀ38 ppm, DEPT spectra indicate
that these resonances correspond to CH2 groups. These methy-
lene signals might arise from units of incomplete elimination
(represented by model compound 2e) or from head-to-head
defects (represented by model compound 2d). However, in-
complete elimination can be excluded since no signal appears
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from the dithiocarbamate group bearing methine carbon around
53.3 ppm. Moreover, since no J13Cꢀ13C coupling is observed,
these resonances are assigned to head-to-head defects. As
discussed above, these resonances were not observed for the
precursor O-PTV as they were masked by the huge C7 signal of
normal head-to-tail structural units.
Regarding the 58.2 ppm signal in the precursor spectrum,
assigned to tail-to-tail defects, no characteristic signal of a triple
bond is detected in the spectra of the converted polymers
(around 90 ppm). This indicates that no full elimination of the
tail-to-tail defects occurs, leaving a structural unit as indicated in
Figure 7 (b) of which the carbon signals will end up in the huge
signal of the 13C-labeled olefinic carbons of the normal head-to-
tail units.
’ ACKNOWLEDGMENT
We gratefully acknowledge Belspo in the frame of network IAP
P6/27. We also thank the EU for the FP6-Marie Curie-RTN
“SolarNtype” (MRTN-CT-2006-035533) and the IWT (Institute
for the Promotion of Innovation by Science and Technology in
Flanders) for the financial support via the SBO-project 060843
“PolySpec”. Last but not least, the support of the Fund for
Scientific Research-Flanders (FWO) via the projects G.0252.04N
and G.0091.07N is acknowledged.
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This study describes an extensive structural characterization of
13C-labeled 3-octyl-PTV and its precursor polymer, prepared via
the dithiocarbamate route, by means of several NMR techniques
and model compounds. Aldehydes, carboxylic acids, and methylol
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’ AUTHOR INFORMATION
Corresponding Author
*Phone þ32 (0) 11 268396; e-mail peter.adriaensens@uhasselt.be.
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