Soluble polyacetylene derivatives by chain-growth polymerization of dienes

Article abstract of DOI:10.1021/ma2004713

A new method for the fabrication of soluble polyacetylene derivatives was developed based on bromination-dehydrobromination of bicyclic diene polymers. High molecular weight polymer precursors were synthesized by radical 1,4-polymerization of the corresponding dienes, which contained a bicyclo[2.2.2]octane skeleton. Polymer precursors with narrow molecular distributions were prepared by nitroxide-mediated polymerization of the bicyclic diene monomers. Regioselective elimination from the brominated polymer afforded a polyacetylene derivative contaning bicyclic substituents, which was readily soluble in common organic solvents. The polymer electronic bandgap, obtained by optical and electrochemical measurements, was in the range 1.4-1.7 eV. Low bandgap values were attributed to the conformational inflexibility of the bicyclic substituent forcing coplanar orientation of the backbone double bonds. Solid-state conductivity of the produced polymer in the undoped form was measured to be 1.5 × 10^-5 S/m. This new synthetic method allows for the chain-growth production of polyacetylene derivatives that possess favorable electronic properties and superior solubility characteristics to pristine polyacetylene.

Full text of DOI:10.1021/ma2004713

ARTICLE
pubs.acs.org/Macromolecules
Soluble Polyacetylene Derivatives by Chain-Growth Polymerization
of Dienes
Kai Luo,† Sung Jin Kim,‡ Alexander N. Cartwright,§ and Javid Rzayev*,†
†Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States
‡Department of Electrical and Computer Engineering, University of Miami, Coral Gables, Florida 33146, United States
§Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260-1900,
United States
S Supporting Information
b
ABSTRACT: A new method for the fabrication of soluble
polyacetylene derivatives was developed based on bromina-
tionꢀdehydrobromination of bicyclic diene polymers. High
molecular weight polymer precursors were synthesized by
radical 1,4-polymerization of the corresponding dienes, which
contained a bicyclo[2.2.2]octane skeleton. Polymer precursors
with narrow molecular distributions were prepared by nitrox-
ide-mediated polymerization of the bicyclic diene monomers. Regioselective elimination from the brominated polymer afforded a
polyacetylene derivative contaning bicyclic substituents, which was readily soluble in common organic solvents. The polymer
electronic bandgap, obtained by optical and electrochemical measurements, was in the range 1.4ꢀ1.7 eV. Low bandgap values were
attributed to the conformational inflexibility of the bicyclic substituent forcing coplanar orientation of the backbone double bonds.
Solid-state conductivity of the produced polymer in the undoped form was measured to be 1.5 ꢁ 10ꢀ5 S/m. This new synthetic
method allows for the chain-growth production of polyacetylene derivatives that possess favorable electronic properties and
superior solubility characteristics to pristine polyacetylene.
’ INTRODUCTION
Recently, there has been tremendous progress in developing chain-
growth polymerization strategies for the preparation of conjugated
polyaromatics.14ꢀ18
Polymers with π-conjugated backbones have been a subject of
intensive research for use in electronic devices, including light-
emitting diodes,1,2 solid-state lasers,3 field-effect transistors4,5
and photovoltaics,6,7 as well as in sensors,8,9 actuators,10 and
corrosion-resistant coatings.11 Controlling multiple factors af-
fecting the performance of conjugated polymers, such as the
electronic bandgap, charge-carrier mobility, and environmental
stability, is an ongoing challenge in this field. Each application
puts its own stringent demands on the performance of conju-
gated macromolecules, and scientists are striving to address these
challenges by manipulating the intrinsic chemical structure and
the aggregation behavior of the conjugated polymer chains. A
vital part of this multidisciplinary effort is the preparation of
tailored semiconducting polymeric materials and understanding
their complex structureꢀproperty relationships. Currently, most
conjugated polymers are synthesized via step-growth polymeriza-
tion protocols, often involving metal-mediated aryl couplings.12
Fundamental limitations of the step polymerization kinetics13
significantly limit the attainable molecular weights of the conjugated
polymers and also prohibit the use of controlled polymerizations to
produce well-defined conjugated polymers or block copolymers.
There is an increasing interest in developing new synthetic methods
to generate conjugated polymers via controlled chain-growth
polymerization mechanisms, which will allow for a superior chain
length control and the synthesis of complex nanostructured systems.
Polyacetylene (PA) is the simplest conjugated polymer with a
linear polyene backbone structure. Since its initial discovery, PA
has been a subject of intensive research due to its unique structure
and extremely high conductivities in the doped form.19ꢀ21 How-
ever, the processing of PA is hindered by its poor solubility in
organic solvents. There are two distinct approaches that can be used
to circumvent this limitation and improve the processability of PA.
The first approach takes advantage of soluble polyacetylene pre-
cursors that can be converted to PA post processing.22ꢀ25 The
second approach uses PA derivatives with regularly spaced substit-
uents to improve solubility characteristics of PA.26ꢀ28 The latter
method can also be used to alter or fine-tune the electronic
properties of polyacetylene.
There are two general strategies that have been successfully
utilized for the preparation of substituted polyacetylene deriva-
tives via a chain-growth polymerization mechanism: metal-
catalyzed alkyne polymerization28ꢀ30 and ring-opening metath-
esis polymerization (ROMP) of cyclooctatetraenes.31ꢀ35 The
outcome of these two methods is different in the relative
Received: March 1, 2011
Revised:
May 13, 2011
Published: May 26, 2011
r
2011 American Chemical Society
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Scheme 1
Scheme 2
positions of the substituents, which has a dramatic effect on the
polymer properties. The direct alkyne polymerization method
provides PA substituted at every second carbon. These polymers
have excellent solubility characteristics, but having substituents
so close to each other results in twisted backbone conformations,
which leads to bandgaps much larger than pristine PA.29 On the
other hand, the ROMP method generates polyacetylene deriva-
tives with a substituent at every eighth carbon. Such PA
derivatives have electronic properties very close to pristine PA,
but their solubilities are only marginally better than polyacety-
lene itself.34
In this contribution, we report a new chain-growth polymer-
ization strategy to synthesize PA derivatives with bicyclic sub-
stituents. We demonstrate that polymer precursors prepared
from bicyclic dienes can be converted to soluble polyacetylene
derivatives by brominationꢀdehydrobromination sequence in
an efficient manner. The bicyclic substituents not only improve
the solubilities of the produced PA derivatives but also force the
coplanar orientation of the backbone, resulting in favorable
electronic properties.
’ RESULTS AND DISCUSSION
Synthetic Strategy. The new approach to the synthesis of
polyacetylene derivatives is illustrated in Scheme 1. A bicyclic
diene monomer is polymerized in a 1,4-addition mode to
produce the corresponding polydiene polymers. We have shown
previously that similar dienes polymerize predominantly (>95%)
by 1,4-addition under free-radical polymerization conditions.36
Backbone unsaturations in the polymer are then brominated
by a bromine addition reaction. Finally, dehydrobromination
reaction introduces double bonds in the backbone and converts it
to a fully conjugated structure. A unique aspect of this elimina-
tion reaction is that it is completely regioselective: only methy-
lene protons in the backbone can be eliminated with the
corresponding bromide, while the bridgehead CH cannot parti-
cipate in the reaction due to violation of the Bredt’s rule. Such
regioselectivity cannot be achieved by using traditional poly-
dienes, such as polybutadiene or polyisoprene, or even during
HCl elimination from poly(vinyl chloride), another strategy
employed for the synthesis of a conjugated polyene structure.37
The brominationꢀdehydrobromination sequence can proceed
in an efficient manner and has been used before to prepare
conjugated polymers, for example, from poly(cyclopentadiene).38
However, in this case, polymers with a multitude of defects and
isomeric repeat units were produced due to the presence of multiple
elimination sites.
Figure 1. 13C NMR spectra of monomer 1 (A), poly(1) (B), and
poly(10) before (C) and after (D) thermal treatment at 150 °C.
fabrication of well-defined polyacetylene derivatives with con-
trolled molecular weights and low polydispersities.
Monomer Synthesis. Bicyclic diene monomers were synthe-
sized according to a literature procedure (Scheme 2).39
DielsꢀAlder reaction between maleic anhydride and cyclohex-
adiene produced the bicyclic skeleton in high yields. The
anhydride group was converted to a diol by LiAlH4-mediated
reduction and subsequently esterified by reacting with p-tolue-
nesulfonyl chloride. The hydroborationꢀoxidation sequence
was then used for indirect hydration of the double bond. The
diene structure was obtained by elimination of p-toluenesulfonic
acid in the presence of potassium tert-butoxide. Finally, the
hydroxy substituent was acetylated to produce monomer 1
(Figure 1A).
Another advantage of our synthetic strategy is that free-radical
polymerization, utilized for the synthesis of polydiene precursors,
can produce high molecular weight polymers and can be adapted
to proceed in a controlled/living fashion, thus allowing for the
Polyacetylene Synthesis. The bicylic diene 1 as well as the
hydroxy-substituted derivative 1a and the unsubstituted diene
(2,3-bis(methylene)bicyclo[2.2.2]octane) were all successfully
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polymerized under free-radical polymerization conditions.
Semicrystalline polymer produced from the unsubstituted diene
was only soluble in organic solvents at high temperatures, while
the polymer produced from the OH-substituted diene 1a was
only soluble in polar solvents, such as DMF and DMSO. There-
fore, we chose to work with poly(1), which exhibited good
solubility characteristics in most organic solvents.
Polymerization of monomer 1 in toluene provided a white
powdery polymer after precipitation in methanol. Even without
optimization, polymers with molecular weights as high as 33 kg/mol
(relative to polystyrene calibration) could be obtained. This is
not surprising given the fast polymerization kinetics that we
previously reported for similar bicyclic diene monomers.36 Nitr-
oxide-mediated polymerization, one of the few methods to
achieve controlled synthesis of polydienes,40,41 was used to
prepare poly(1) with narrow molecular weight distributions.
Thus, in the presence of a nitroxide radical, 2,2,5-trimethyl-4-
phenyl-3-azahexane-3-nitroxide, and benzoyl peroxide, poly(1)
with a polydispersity index as low as 1.20 was obtained.
Figure 2. Thermogravimetric analysis of poly(10) as prepared (A) and
after thermotreatment at 150 °C (B).
1H and 13C NMR spectra of poly(1) exhibited well-resolved
signals that were consistent with the expected polymer structure.
In the 1H NMR spectrum of poly(1), no signals in the olefinic
region were detected, indicating that the polymerization pro-
ceeded predominantly (>95%) by 1,4-addition (Figure S1,
Supporting Information). In the 13C NMR spectrum, two signals
at 136 and 140 ppm were assigned to the olefinic carbons in the
backbone (Figure 1B). The backbone methylene carbon signals
moved from 104 and 107 ppm to 30 ppm after polymerization
(Figure 1A,B). The remaining signals of the bicyclic group and
the acetate substituent in the polymer were in close agreement to
the corresponding signals in the monomer, indicating that no
rearrangements or other major side reactions were taking place
during polymerization. Since the acetoxy substituent was so far
away from the backbone, it was impossible to deduce stereo-
regularity of the polymer from NMR analysis. Poly(1) had a glass
transition temperature of 78 °C, as measured by differential
scanning calorimetry (DSC, Figure S2 in the Supporting In-
formation). Despite the presence of bulky bicyclic substituents,
two methylene groups in the backbone afforded enough flex-
ibility for the polymer to exhibit a relatively low Tg.
Bromination of the backbone unsaturations in poly(1) was
achieved by a simple bromine addition. When an equimolar
amount of bromine was used, the completion of the reaction was
evident by the disappearance of yellow/brownish color to form a
colorless solution, suggesting a complete consumption of bro-
mine. After a short delay (∼5 min), spontaneous dehydrobro-
mination commenced, changing the color of the solution to
darker brown and ultimately black. When a slight excess of
bromine was used, no spontaneous dehydrobromination was
observed, presumably due to the slightly acidic nature of the
remaining solution. Subsequently, when aqueous sodium bisul-
fite was added to remove the excess bromine, the solution color
slowly turned black, suggesting that dehydrobromination was
taking place. Upon completion of transformations, the polymer
was recovered as a black powder by precipitation from methanol.
The produced polymer was readily soluble in common organic
solvents, such as tetrahydrofuran, chloroform, and toluene. We
hypothesize that dehydrobromination is driven by the formation
of the conjugated backbone, which is also responsible for the dark
color of the forming polymer. Similar behavior has been observed
during the synthesis of conjugated polymers by bromina-
tionꢀdehydrobromination of polycyclopentadiene.38
Scheme 3
NMR spectra of the newly formed polymer (poly(10)) ex-
hibited broad signals, suggesting the formation of a rigid struc-
ture. This was also corroborated by DSC analysis, where poly(10)
did not exhibit any thermal transitions (glass transition or
melting) up to 200 °C (Figure S2, Supporting Information). In
1
the H NMR spectrum, a new peak at 6.8 ppm was observed,
which was attributed to the backbone CH protons. The broad-
ness of the peaks and the overlap with other signals prohibited
any quantitative analysis. In the 13C NMR spectrum, while
significantly broadened, the peaks were well-resolved and pro-
vided additional evidence for the formation of an unsaturated
backbone. A new signal at 125 ppm in the olefinic region was
attributed to the CH carbon in the backbone (Figure 1C). DEPT
analysis (Figure S3, Supporting Information) confirmed that the
signal at 125 ppm was originating from a CH carbon, while the
signal centered around 140 ppm was coming from a quaternary
carbon, thus accounting for all carbons of the conjugated back-
bone repeat unit. A new peak at 50 ppm was also observed. We
assigned this signal to the carbon attached to a bromine atom, a
structural fragment remaining after incomplete dehydrobromi-
nation. Thermogravimetric analysis (TGA) of the polymer
showed a marked weight loss at 140 °C, corresponding to about
20 wt % (Figure 2A), which was attributed to the elimination of
the remaining HBr from the sample. Thus, dehydrobromination was
facilitated by thermal treatment at 150 °C, after which the polymer
exhibited improved thermal stability up to 270 °C (Figure 2B). Inthe
13C NMR spectrum of the thermally treated polymer, the signal at 50
ppm was significantly diminished (Figure 1D). Elemental analysis
indicated that there were only 1.1 mol % bromine atoms left in the
sample after thermal treatment, some of which may be originating
from brominated 1,2-addition repeat units.
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Figure 5. Cyclic voltammogram (20 mV/s) of poly(10) on a platinum
electrode in CH3CN (0.1 M Bu4NPF6).
peaks remained mostly unchanged after backbone modifications.
When a UV detector (set to detect at 600 nm) was used, no signal
was observed for poly(1), while the conjugated poly(10) showed
a GPC trace matching the one observed by the refractive index
detector. The results suggest that the color of the polymer sample
is originating from the polymer itself and not from small
molecular weight impurities in the sample.
Figure 3. GPC traces of poly(1) (A) and poly(10) before (B) and after
(C) thermotreatment: RI detector (solid lines), UVꢀvis detector at
600 nm (dashed lines).
The presence of the bicyclo[2.2.2]octane skeleton in poly(1)
was crucial for the successful outcome of backbone transforma-
tions. When polymers containing a norbornane substituent were
used, such as those prepared from 2,3-dimethylenenorbornane,36
bromination led to complete polymer degradation, as evidenced
by GPC and NMR analyses. This might be a result of the
susceptibility of norbornyl cations to undergo rearrangements42
leading to undesired transformations. Therefore, the bicyclo-
[2.2.2]octane substituent not only provided directionality for
clean dehydrobromination but also afforded stability necessary to
complete the transformations with a minimum amount of side
reactions.
Electronic Properties. The UVꢀvis spectrum of the prepared
polyacetylene derivative dissolved in chloroform showed a strong
absorption band with a maximum at 460 nm (Figure 4A),
indicating the presence of an extended conjugation. There was
also a significant band appearing in the near-IR region. Such a
band is often observed in polyacetylene films doped to semi-
conductor levels.43ꢀ45 It is not clear whether this band is
indicative of doping in our case as well. No metal catalyst, which
can cause adventitious doping, was used during either polymer-
ization or backbone modification reactions. Bromine (which can
act as a dopant) was removed by reaction with sodium bisulfite
before the formation of a conjugated polymer structure. In
addition, the polymer sample was thermotreated at 150 °C prior
to UVꢀvis analysis and was handled with minimal exposure to
air. When deposited as a film, the polymer exhibited an absorp-
tion maximum of 480 nm with a tail stretching beyond 700 nm,
while no near-IR band was observed (Figure 4B). The bandgap
measured from the optical absorption edge was 1.7 eV. A slight
red shift in the absorption maximum when going from solution to
film suggests reorganization of polymer chains in the solid state.
Prolonged exposure to air deteriorated the conjugated polymer
structure, as evidenced by disappearance of absorption bands
from the UVꢀvis spectrum.
Figure 4. UVꢀvis spectra of poly(10) in a chloroform solution (A) and
as a thin film (B).
It must be noted that thermal degradation of poly(10)
proceeded in two steps, where the first step commenced at
270 °C and led to about 45% weight loss (Figure 2). We propose
that this decrease in weight corresponds to the elimination of
acetic acid and subsequent retro-DielsꢀAlder reaction,36 form-
ing poly(o-phenylenevinylene) (Scheme 3). These transforma-
tions should lead to about 50% weight loss, which is in agreement
with the observed TGA results.
Molecular weight characterization of the polymers by GPC
post brominationꢀdehydrobromination and thermal treatment
revealed that there were no major side reactions, such as cross-
linking or polymer chain degradation, during these transforma-
tions (Figure 3). The shape and the elution time of the GPC
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Cyclic voltammetry (CV) analysis of the synthesized poly-
acetylene derivative revealed a presence of broad oxidation and
reduction waves, consistent with an electroactive organic struc-
ture (Figure 5). The electrochemical bandgap was found to be
1.4 eV, calculated based on the onset of oxidation at 0.25 V and
the onset of reduction at ꢀ1.15 V. The obtained bandgaps are
considerably lower than those reported for other polyacetylene
derivatives with substituents at every second carbon.46,47 In fact,
the bandgap measured for poly(10) is surprisingly close to that of
a pristine polyacetylene.21,43 Larger bandgaps of substituted
polyacetylene derivatives have often been attributed to the steric
repulsion between the substituents resulting in a conformational
twist of the backbone.29 We hypothesize that a low bandgap of
poly(10) is a result of conformational inflexibility of the bicyclic
substituent, which forces coplanar orientation between backbone
unsaturations. A similar effect has been observed during poly-
merizations of bicyclic dienes, where high polymerization rates
have been attributed to coplanar orientation of the double bonds in
the monomer.36 It appears that the bicyclic substituent allows for
markedly improved solubility, and thus processability, of polyace-
tylene, while preserving its desirable electronic properties.
(1.2 mg, 5.4 μmol), and benzoyl peroxide (0.4 mg, 1.6 μmol) were
dissolved in toluene (0.1 mL), and the mixture was degassed by three
freezeꢀpumpꢀthaw cycles, sealed under nitrogen, and placed in an oil
bath at 100 °C. After 120 h, the contents were diluted with dichloro-
methane and precipitated in methanol. The polymer was filtered and
dried in a vacuum oven overnight. Yield = 65 mg. GPC (polystyrene
standards): Mn = 12.5 kg/mol, Mw/Mn = 1.20.
Radical Polymerization of 1. A mixture of 1(0.2 g), AIBN (2 mg),
and 0.2 mL of toluene was degassed by three freezeꢀpumpꢀthaw cycles,
sealed under nitrogen, and placed in an oil bath at 65 °C. After 12 h, the
contents were diluted with dichloromethane and precipitated in methanol.
The polymer was filtered and dried in a vacuum oven overnight. Yield = 0.15 g.
GPC (polystyrene standards): Mn = 32.7 kg/mol, Mw/Mn = 2.45.
BrominationꢀDehydrobromination. Bromine (1.5 mmol) in
5 mL of dichloromethane was added dropwise to poly(1) (1 mmol)
dissolved in 5 mL of dichloromethane. After stirring for an additional
hour, 5 mL of 1 M NaHSO3(aq) was added to the reaction mixture. The
organic layer became colorless first and then got darker with time,
eventually turning dark brown. After 1 h, the organic phase was
separated and the solvent was evaporated. The residue was diluted with
dichloromethane and precipitated in methanol three times. Subse-
quently, the dried polymer was thermally treated at 150 °C for 16 h in
a vacuum oven.
Conductivity Measurements. A metal semiconductor metal
(MSM) structure was used to measure the conductivity of poly(10).
A glass substrate was cleaned using acetone, methanol, and deionized
water. For submicrometer patterning, a primer and a photoresist
(Shipley 1818) were spin-coated on the glass substrate at 5000 rpm
for 30 s. Then, it was baked on the hot plate at 120 °C for 90 s. The
substrate was exposed under UV by using a photomask and a mask
aligner (Karl Suss MJB3) for 10 s and developed by using MF319
developer for 45 s. Subsequently, metal deposition was performed by
using an electron beam evaporator. A titanium layer (10 nm) for good
adhesion and a gold layer (50 nm) were deposited at a deposition rate of
1 Å/s under 5 ꢁ 10ꢀ8 Torr. Finally, the patterned electrodes were
obtained after a lift-off process. The resulting metal electrodes have an
interdigitated structure to increase the effective device area, with a 4 or
5 μm gap. The device was diced to a smaller piece in order to fit into a
ceramic lead chip carrier. The device was fixed in the chip carrier and two
electrodes were connected to the leads using a standard wire bonding
process. This prepared device structure was then placed into a nitrogen
purged glovebox system and poly(10) (2 wt %) in a 1,2-dichlorobenzene
solution was spin-coated on the device surface at 1800 rpm for 30 s. The
final metalꢀpolymerꢀmetal device was dried using a hot plate at 80 °C
for 10 min. The prepared sample was kept in a sealed box with a feed
through to prevent any oxygen exposure that causes device degradation.
The conductivity measurement was performed using a computer-
controlled source meter (Keithley 2400) by applying an electric field
(sweeping from 0 to 200 kV/cm) across the two electrodes with a
4 μm gap.
The conductivity of poly(10) was obtained by measuring current
as a function of applied electric field on a metalꢀpolymerꢀmetal
structure (Figure S4, Supporting Information). The undoped
polymer sample used for conductivity studies was thermotreated
at 150 °C and was handled under air-free conditions during
synthesis, processing, and measurements. An interdigitated metalꢀ
polymerꢀmetal device was fabricated and measured to obtain
current values corresponding to applied electric fields across the
device. To calculate the conductivity of the polymer film, an effective
device area of 4.2 ꢁ 10ꢀ12 m2 was used to obtain the current
density. Then, the conductivity was calculated using the equation J =
σE, where J is the current density, σ is the conductivity, and E is the
electric field. The conductivity of poly(10) obtained from the slope
of the linear fit between the electric field and the current density was
1.5 ꢁ 10ꢀ5 S/m. The obtained conductivity value is consistent with
those of other undoped conjugated polymers,48 suggesting that
bicyclic substituents do not have a detrimental impact on charge
transport in the solid state. Detailed characterization of the electro-
nic properties of poly(10) is under way in our laboratories.
’ EXPERIMENTAL SECTION
Materials. Azobis(isobutyronitrile) (AIBN) was recrystallized
from methanol; all other commercially available reagents and solvents
were used without purification. Monomer 1a39 and 2,2,5-trimethyl-
4-phenyl-3-azahexane-3-nitroxide40 were prepared according to literature
procedures.
Synthesis of 1. Monomer 1a (0.7 g) was dissolved in a mixture of
pyridine (3 mL) and acetic anhydride (2.4 mL) and stirred at room
temperature overnight. Afterward, the reaction mixture was poured into
ice water and extracted with dichloromethane (3 ꢁ 10 mL). The organic
layer was dried over MgSO4, and the solvent was evaporated. The
residue was purified by flash chromatography (hexanes/ethyl acetate =
Measurements. NMR measurements were performed on a Varian
Inova-500 (500 MHz) spectrometer by using CDCl3 as a solvent. Gel
permeation chromatography (GPC) analysis was carried out on Visco-
tek’s GPCMax with three Olexis columns (Polymer Laboratories, Varian
Inc.) and TDA302 tetradetector array system, which contained a
refractive index, UV, viscosity, and low (7°) and right angle light
scattering modules. Tetrahydrofuran was used as a carrier solvent at
30 °C. The system was calibrated with 10 linear polystyrene standards
from 1.2 ꢁ 106 to 500 g/mol. UVꢀvis spectra were recorded on Agilent
8453 at room temperature by using dichloromethane as a solvent.
Thermogravimetric analysis was performed on a Perkin-Elmer TGA7
under nitrogen flow at a heating rate of 10 °C/min.
1
10:3, Rf = 0.48) to obtain monomer 1 (0.82 g, 82% yield). H NMR
(CDCl3): δ 5.40 (s, 1H), 5.28 (s, 1H), 4.92 (s, 1H), 4.90 (s, 1H), 4.77
(s, 1H), 2.61 (s, 1H), 2.42 (s, 1H), 1.98ꢀ2.13 (m, 5H), 1.77 (m, 1H),
1.54 (m, 1H), 1.51 (m, 1H), 1.27 (m, 1H). 13C NMR (CDCl3): δ
170.64, 147.59, 145.35, 109.98, 104.03, 71.74, 40.79, 36.56, 34.95, 25.53,
21.31, 19.27.
Nitroxide-Mediated Polymerization of 1. Monomer 1
(100 mg, 0.52 mmol), 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide
Cyclic voltammetry was carried out in a three-electrode compartment
cell with a volume of electrolyte solution (0.1 M Bu4NPF6 in acetonitrile),
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using the saturated calomel electrode (SCE) as reference, a platinum wire as
counter electrode, and a platinum disk (effective area 0.5 cm2) as a working
electrode, on which a thin polymer film was produced by dip-coating. The
experiment was performed using a Model 263A potentiostatꢀgalvanostat
(EG&G Princeton Applied Research) under a nitrogen atmosphere.
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’ CONCLUSIONS
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We developed a novel strategy for the synthesis of polyacety-
lene derivatives by using radical polymerization of bicyclic dienes
and subsequent polymer transformations. The polymer precur-
sor was synthesized via 1,4-polymerization of monomer 1. High
molecular weight polymers were obtained by a traditional radical
polymerization, while a nitroxide-mediated polymerization of 1
afforded polymers with narrow molecular weight distributions. The
conjugated polymer was synthesized by bromination/dehydrobro-
mination of poly(1). The presence of a bicyclic substituent allowed
for regioselective elimination to produce a well-defined conjugated
backbone. The obtained polyacetylene derivative was soluble in
common organic solvents. The bandgap of poly(10) was obtained
by optical and electrochemical measurements to be in the range
1.4ꢀ1.7 eV, which was notably lower than bandgaps reported for
polyacetylene derivatives with substituents on every second
carbon. We hypothesize that conformational inflexibility of the
bicyclic substituent was responsible for the coplanar orientation
of backbone double bonds, thus leading to lower bandgaps. The
conductivity of undoped poly(10) in the solid sate was measured
to be 1.5 ꢁ 10ꢀ5 S/m. The described methodology allows for the
preparation of polyacetylene derivatives with favorable electronic
properties and superior solubility characteristics than pristine
polyacetylene.
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Supporting Information. 1H NMR, 13C NMR, DSC,
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