Synthesis of poly(3-dodecyl-2,5-thienylene vinylene) by solid-state metathesis polycondensation

Article abstract of DOI:10.1021/ma2020529

The synthesis of poly(thienylene vinylenes) (PTV) has been attracting attention due to the low band gaps and high electrical conductivities of these materials, making them applicable for charge storage devices, transparent conductive coatings, and electrochromic devices. Unsubstituted PTV is an intractable polymer that is usually synthesized via a processable precursor. This article reports the synthesis of conjugated polymers using solid-state metathesis conditions, demonstrating the efficiency of this methodology for preparation of a processable polymer, such a 3-dodecyl PTV (P3DDTV). The 3-dodecyl-2,5-dipropenylthiophene was synthesized and subsequently polymerized using ADMET conditions with Grubbs' second-generation catalyst. The prepolymer film (M_n = 4000 g/mol) was further polymerized in the solid state to give a final product with M_n = 14 000 g/mol (a 3.5-fold increase while in the solid state). The polymer obtained by this methodology exhibited thermal (T_g = 43 °C and T_m = 115 °C) and electrochemical (optical band gap of 1.65 eV and HOMO energy level of 5.35 eV) properties similar to those of PTV polymers synthesized by ADMET polymerization using a high boiling solvent or by cross-coupling reactions.

Full text of DOI:10.1021/ma2020529

ARTICLE
pubs.acs.org/Macromolecules
Synthesis of Poly(3-dodecyl-2,5-thienylene vinylene) by Solid-State
Metathesis Polycondensation
Paula A. Delgado, David Y. Liu, Zachary Kean, and Kenneth B. Wagener*
George & Butler Polymer Research Laboratory: Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200,
United States
S Supporting Information
b
ABSTRACT: The synthesis of poly(thienylene vinylenes)
(PTV) has been attracting attention due to the low band gaps
and high electrical conductivities of these materials, making
them applicable for charge storage devices, transparent con-
ductive coatings, and electrochromic devices. Unsubstituted
PTV is an intractable polymer that is usually synthesized via a
processable precursor. This article reports the synthesis of
conjugated polymers using solid-state metathesis conditions,
demonstrating the efficiency of this methodology for prepara-
tion of a processable polymer, such a 3-dodecyl PTV (P3DDTV). The 3-dodecyl-2,5-dipropenylthiophene was synthesized and
subsequently polymerized using ADMET conditions with Grubbs’ second-generation catalyst. The prepolymer film (Mn = 4000 g/mol)
was further polymerized in the solid state to give a final product with Mn = 14 000 g/mol (a 3.5-fold increase while in the solid state). The
polymer obtained by this methodology exhibited thermal (Tg = 43 °C and Tm = 115 °C) and electrochemical (optical band gap of 1.65 eV
and HOMO energy level of 5.35 eV) properties similar to those of PTV polymers synthesized by ADMET polymerization using a high
boiling solvent or by cross-coupling reactions.
’ INTRODUCTION
release of a small molecule to achieve a low molecular weight
polymer (prepolymer), which is then cooled to obtain a powder
or flakes. Polymerization then continues in the solid-state, at a
temperature lower than the onset melting temperature, doubling
its molecular weight while in the solid state.22,23 Our group has
been investigating the influence of solid-state polymerization on
the molecular weights of polymers obtained by ADMET poly-
merization for a variety of functionalized polyolefins. Results for
the polymerization of 1,9-decadiene showed an ∼20-fold in-
crease in molecular weight using solid-state conditions.24
We have extended these investigations to the synthesis of a
processable PTV by solid-state ADMET polymerization to
characterize the polymer structure and compare its electronic
properties with those made by other methodologies. This would
be quite difficult for truly intractable material. Therefore, this
report presents results for poly(3-dodecyl-2,5-thienylene
vinylenes) (P3DDTV) to demonstrate the reliability of this
process in the eventual synthesis of intractable PTV.
The synthesis of poly(thienylene vinylene)s (PTVs) is an
active research area in the manufacture of electroactive materials
due to the relatively low band gaps and high conductivities
achieved upon doping of PTV materials.1À3
Unfunctionalized PTV is an intractable polymer, and its
polymerization is usually carried out by indirect synthesis of a
processable prepolymer, but this route produces defects in the
final polymer.4À7 In contrast, substituted PTVs are processable
and usually synthesized by the Wittig8 or WittigÀHorner12
methods, by cross-coupling reactions (e.g., the Stille,9,10 Heck,9
and Kumada11 reactions), by oxidative polymerization,11 or by
ROMP.13,14 However, functionalization may limit the applica-
tions of the resulting polymers.15
Acyclic diene metathesis (ADMET) polymerization has been
used in the synthesis of conjugated polymers, including
polyacetylene16 functionalized PPVs,17,18 polyfluorenes,18 sily-
lene/siloxane functionalized conjugated polymers,19 and
PTVs,20 in bulk or in the presence of high-boiling solvents.
The well-established mechanism, high selectivity for new trans-
olefins, mild reaction conditions, tolerance for different func-
tionalities, well-defined end-groups, absence of branching within
the polymer structure, and the possibility of bulk polymerization
make this methodology a useful tool for the synthesis of
conjugated polymers.21
’ DISCUSSION
The monomer 4 was synthesized according to a literature
method,20 as shown in Scheme 1. The 3-bromothiophene
was alkylated with 1-bromododecane by a Kumada coupling
reaction with 80% yield.25 Under Wittig reaction conditions,
Solid-state polymerization (SSP) has been widely practiced in
the polyester, polyamide, and polycarbonate industries. This
polymerization starts in bulk at high temperature with the
Received: September 9, 2011
Revised:
November 7, 2011
Published: November 30, 2011
r
2011 American Chemical Society
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3-dodecylthiophene (2) was then lithiated with n-butyllithium
and formylated using DMF in the presence of tetramethylethy-
lenediamine (TMEDA).26 A Wittig reaction using the synthe-
sized aldehyde and ethyltriphenylphosphonium bromide was
performed to form monomer 4, which was purified by distillation
after passing through a filtration column to remove the triphe-
nylphosphine oxide. The monomer proved to be unstable in air
and light and was therefore kept cold in the dark under inert
conditions.
Monomer 4 was then placed in a Teflon mold (Figure 1a) and
subjected to ADMET polymerization conditions using Grubbs’
second-generation catalyst at 70 °C for 3 h to yield a solid
prepolymer 5 with a number-average molecular weight of
4000 g/mol (Figure 1b). Since step growth polymerization can
continue in the solid state under the proper conditions (e.g.,
polyesters, nylon, etc.), the increment in the molecular weight of
solid prepolymer 5 was investigated. The key to success was
sprinkling of Grubbs’ second-generation catalyst over prepoly-
mer 5, which resulted in an increase of molecular weight under
vacuum at 70 °C. Catalyst was then sprinkled every 3 days
(0.2 mol % dose) under an inert atmosphere to ensure the catalyst
activity and a productive metathesis polymerization. Since the
polymer was designed to be soluble (the dodecyl chain left in
place), the molecular weight increase under solid-state conditions
was determined by gel permeation chromatography (GPC,
Table 1) and 1H NMR spectroscopy.
Hillmyer et al.20 have previously reported synthesis of an
analogous monomer yielding a mixture of stereoisomers. In the
present study, monomer 4 resulted in four diastereomers from
the Wittig reaction having the following percentages: 32% 2E-5E,
26% 2E-5Z, 15% 2Z-5Z, and 27% 2Z-5E.20 Referring to Figure 2,
spectrum 1, the percentage of each diastereomer was evaluated
from the aromatic protons (c, d, e, f). Because peak f overlaps
with the olefinic protons, its integration was obtained indirectly.
The total integration c + d + e + f should equal one-sixth of the
total integration for the methyl end groups (multiplets b). The
peak f integral was obtained by subtracting the sum of c + d + e
from one-sixth of the integration for b.
The 1H NMR spectrum of monomer 4 is shown at the top of
Figure 2, where the four possible diastereomers are observed.
Terminal methyl units give peaks between 1.90 and 1.82 ppm,
with methyl groups resulting from the combination of Z-stereo-
isomers at 1.90 ppm and from E-stereoisomers at 1.85 ppm.
Because the monomer is unstable in air and light, its decomposi-
tion is evidenced by the presence of aldehyde protons between
9.0 and 9.5 ppm. After monomer polymerization via ADMET
(middle spectrum in Figure 2), the progress of the polymeriza-
tion was analyzed by the peak area ratios of the terminal methyl
unit (b) and the methylene unit attached to the thiophene ring
(a). Monomer 4 presents a ratio of 3:1, while prepolymer 5
exhibits a 1:5 ratio, confirming the diminution of terminal methyl
units in the system and thereby corroborating the GPC results of
a number-average molecular weight of 4000 g/mol. Aromatic
protons were observed as three broad peaks, which became less
shielded as the degree of polymerization increased. After the
third week of polymerization under solid-state conditions
(Figure 2, bottom), the methyl group units in the polymer
backbone were almost undetectable (in a 1:16 ratio), confirming
the increase of the molecular weight to 14 000 g/mol indicated
by GPC.
Scheme 1. Synthesis of 3-Dodecyl-2,5-dipropenylthiophene
Monomer 4
Different conditions were attempted in order to obtain the
highest molecular weight by this methodology. Formation of
prepolymer 5 was studied at 40 and 70 °C with Grubbs’ second-
generation catalyst. At 40 °C, low molecular weight films were
obtained (Mn = 2300 g/mol, entry 2, Table 1) after a longer
period of time (10 h), while a Mn of 4000 g/mol was obtained
after 3 h of polymerization at 70 °C (entry 3, Table 1). After a
sprinkle of catalyst (0.2 mol %) was added to prepolymer 5, the
molecular weight was analyzed every week. During the first 2
Figure 1. Solid-state polymerization of the 3-dodecyl-2,5-diprope-
nylthiophene monomer 4. Figures represent the Teflon mold and the
different stages of the polymerization, with the catalyst represented as
brown dots. (a) Liquid monomer 4 before the polymerization.
(b) Prepolymer 5 represented as a red shiny film. (c) Final polymer 6.
Table 1. Molecular Weight Analysis for the Synthesis of P3DDTV
Mn (kg/mol)a
Mw (kg/mol)a
Mw/Mn
DP
yieldb
a
entry
conditions
catalyst addition (mol %)
1
2
3
4
5
6
7
40 °C, G1, 24 h
40 °C, G2, 10 h
70 °C, G2, 3 h
70 °C, G2, 2 weeks
70 °C, G2, 3 weeks
70 °C, G2, 4 weeks
60 °C, G2, 3 weeks-bulk
0.6
0.6
0.6
1.2
1.8
2.4
1.8
1.3
2.3
1.8
4.4
1.5
1.9
1.9
2.1
2.3
2.2
1.9
5
8
35
80
87
86
88
85
55
4.0
7.7
14
18
48
50
8
5.3
10.8
33.3
31.8
4.4
14.0
14.5
2.4
a Molecular weight determined using GPC analysis (PS standards) and corroborated by 1H NMR spectroscopy. G1 and G2 refer to Grubbs’ first- and
second-generation catalyst, respectively. b Isolated yield after quenching with ethyl vinyl ether and precipitation from methanol.
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Figure 2. 1H NMR of 3-dodecyl-2,5-dipropenylthiophene monomer 4 (spectrum 1), P3DDTV prepolymer 5 (spectrum 2), and P3DDTV 6
(spectrum 3).
Figure 4. Cyclic voltammogram of P3DDTV drop-cast from CHCl3
solution (2 mg mLÀ1) onto a platinum button electrode (0.02 cm2)
in 0.1 M TBAPF6/ACN supporting electrolyte using a scan rate of
50 mV/s.
Figure 3. Spectroelectrochemistry of P3DDTV. Film was spray-cast
onto ITO-coated glass from dichloromethane solutions (∼1 mg mLÀ1).
Electrochemical oxidation of the film was carried out in 0.1 M TBAPF6/
ACN supporting electrolyte using an Ag/AgNO3 reference electrode
(entry 5, Table 1), in agreement with a polycondensation
mechanism, where high monomer conversion is needed to
and a platinum counter electrode. The applied potential was increased in
obtain a high molecular weight polymer. After the third week,
100 mV increments from À0.1 to 0.2 V, in 25 mV steps from +0.225 to
polymerization growth decreased substantially.
+0.35 V, and in 100 mV steps from +0.4 to +0.8 V.
Bulk polymerization (rather than solid-state polymerization)
was also attempted in order to compare the two techniques. With
weeks only a slight increase was noticed (Mn = 5300 g/mol, entry
4, Table 1). The largest increase was observed after the third
week, when the molecular weight increased to 14 000 g/mol
this methodology, only low molecular weight polymers (Mn =
2400 g/mol) were obtained (entry 7, Table 1). This can be
explained by the decreased accessibility of the propenyl units to
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the active catalyst species as polymers solidify into clumps.
Hillmyer et al. also obtained molecular weights of 2500 g/mol
using the same bulk conditions. We also tried the polymerization
using Grubbs’ first-generation catalyst, but only pentamers were
obtained, possibly because of the catalyst’s lower reactivity (entry
1, Table 1).
Hillmyer et al. studied the synthesis of 3-hexyl PTV (P3HTV)
by ADMET polymerization using a variety of high-boiling
solvents, catalysts, and temperatures. They concluded that
P3HTV can be obtained by ADMET polymerization with a
molecular weight of 10 200 g/mol using second-generation
Grubbs’ catalyst at 90 °C for 48 h with trichlorobenzene as a
solvent.20 On the other hand, our results show that solid-state
polymerization in a Teflon mold can definitely increase the
accessibility of active propenyl units without the need for
solvents to obtain higher molecular weight polymers with the
desired final shape.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures with
b
complete spectral and thermal analysis data. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wagener@chem.ufl.edu.
’ ACKNOWLEDGMENT
The authors thank NSF for support and the Army Research
Office for their contribution in understanding catalyst activity.
We also thank Materia Inc. for their help.
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The TGA and DSC results showed that P3DDTV is thermally
stable to 353 °C, with a glass transition temperature of 43 °C and
a broad endotherm at 115 °C, which could correspond to the
melting transition of the polymer. The UVÀvis absorption
spectra shown in Figure 3 exhibit an onset of the low-energy
absorption edge (onset of the πÀπ* transition) for the neutral
spectrum at 750 nm, which corresponds to an optical band gap of
1.65 eV. The electrochromic behavior was observed by recording
the spectral changes upon oxidation of the polymer thin film. The
cyclic voltammogram (Figure 4) exhibits an onset of polymer
oxidation at +0.25 V vs Fc/Fc+, which corresponds to a HOMO
energy level of 5.35 eV. (The conversion of the HOMO energy
was accomplished by adding 5.1 eV to the onset of the oxidation
of the polymer, assuming that Fc/Fc+ is at 5.1 eV below the
vacuum level.1,20) These results all agree with the corresponding
data for previous synthesized 3-alkyl PTVs using ADMET
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’ CONCLUSIONS
There is no question that solid-state polymerization creates the
analogous polymer, poly(3-dodecyl-2,5-thienylene vinylene), as was
reported by Hillmyer using conventional polymerization procedures.
The approach is simple: sprinkle catalyst occasionally on the solid-
state polymerizing mass. The catalyst “reacts” its way into the
polymer and performs ADMET chemistry, allowing the release of
2-butene (in this case). Molecular weights are similar to (actually
slightly larger than) those reported by conventional techniques.
This polymerization was chosen to demonstrate the viability
of solid-state methodology. The real opportunity lies in using
solid-state polymerization techniques to create truly “intractable”
polymers, those that cannot be made by any other direct
polymerization method (for example, if the C12H25 alkyl branch
were omitted in the case of thienylene vinylenes). Removing the
branch would yield a “cleaner” polymer, one not diluted by
solubilizing branches. Pure conjugated polymers might be made
in this fashion. We are pursuing this objective now.
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