Cis/Cis-2,5-dipropenylthiophene monomers for high-molecular-weight poly(2,5-thienylene vinylene)s through acyclic diene metathesis polymerization

Article abstract of DOI:10.1002/pola.27042

The synthesis of the 2,5-dipropenylthiophene monomer possessing exclusively cis-double bond configurations through the reduction of a dipropynylthiophene precursor is reported. Lindlar's catalyst is found to be completely ineffective, while a dissolving metal method involving activated zinc is successful in alkyne reduction to cis-alkenes with high yields and excellent selectivity. The acyclic diene metathesis polymerization of the monomer affords the poly(2,5-thienylene vinylene) polymer of a high molecular weight. Copyright

Full text of DOI:10.1002/pola.27042

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Cis/Cis-2,5-Dipropenylthiophene Monomers for High-Molecular-Weight
Poly(2,5-thienylene vinylene)s through Acyclic Diene Metathesis
Polymerization
Guoshun Yang, Keda Hu, Yang Qin
Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131
Correspondence to: Y. Qin (E-mail: yangqin@unm.edu)
Received 12 August 2013; accepted 24 November 2013; published online 7 December 2013
DOI: 10.1002/pola.27042
KEYWORDS: acyclic diene metathesis (ADMET); conjugated polymers; metathesis; hydrogenation; kinetics (polym.); poly(thienylene
vinylene)
INTRODUCTION Conjugated polymers1 have found wide-
spread applications in solution-processed thin film electronic
high charge mobility and have found applications in FETs, nonlin-
ear optics, and OPVs.9 A variety of synthetic methods have been
devices including field effect transistors (FETs),2 organic
developed over the years for PTV preparation, including earlier
light emitting diodes (OLEDs),3 and organic photovoltaics
examples involving Gilch-type reactions and elimination from
soluble precursor polymers,10 McMurry coupling, and Wittig-type
reactions of aldehyde-containing monomers,11 transition metal
catalyzed coupling reactions,12 and more recently, ADMET poly-
merization of thiophene monomers bearing double bonds at 2,5-
positions.13 In these existing examples of ADMET synthesis of
PTVs, dipropenylthiophene monomers were exclusively applied
instead of the more reactive divinylthiophene analogs,13(a) as the
later are far more susceptible to free-radical-induced polymeriza-
tion and side-reactions.13(b) Propenyl groups, on the other hand,
can have both trans and cis configurations, which result in hard-
to-separate stereo-isomers. Dipropenylthiophene monomers in
existing ADMET examples were all prepared from Wittig-type
reactions and monomers containing up to four trans/cis stereo-
isomers of varying ratios were unavoidably obtained. Speros et al.
have systematically studied the effects of monomer stereochemis-
try on ADMET polymerization and found a steady increase in PTV
MWs with increasing contents of cis-double bonds.13(d) Less steric
hindrance and higher energy in cis double bonds are likely the
reason for such observations. Thus, to prepare high-MW PTVs,
and more generally poly(arylene vinylene)s, from dipropenyl
monomers, high contents of cis double bonds, ideally 100%, are
desirable. However, a general method for the preparation of
dipropenyl monomers of all-cis configuration is still lacking.
(OPVs).4 Materials solution processability and good film
forming ability are prerequisites in these devices, for which
soluble conjugated polymers of high molecular weight (MW)
are preferable over those of low MW due to higher degree of
chain-entanglement and thus improved connectivity among
ordered domains within the film. Besides a few quasi-living
polymerization techniques,5 typical conjugated polymers are
synthesized through AA 1 BB-type condensation polymeriza-
tions of two different monomers using transition metal cata-
lyzed cross-coupling reactions.6 MWs of the resulting
polymers are theoretically limited by not only monomer con-
versions but also strict monomer stoichiometric balance that
is however sometimes difficult to achieve. Acyclic diene
metathesis (ADMET), on the other hand, has proved to be a
highly versatile method for the preparation of various polyo-
lefins, conjugated polymers as well as metal-containing poly-
mers.7 Well-defined transition metal catalysts used in
ADMET reactions, especially ruthenium-based Grubbs-type
catalysts, can tolerate a wide range of functional groups.8
Although a condensation polymerization in nature, ADMET
involves only one diene monomer, thus avoiding limitations
from stoichiometric imbalance typically encountered in two-
component AA 1 BB-type condensation polymerizations.
These attributes make ADMET an intriguing and emerging
methodology in the field of conjugated polymer synthesis.
In this report, we describe the synthesis of cis/cis 2,5-dipro-
penylthiophene monomers (cis23DTV) by selective reduc-
tion in alkyne precursors. ADMET polymerization of
cis23DTV using Grubbs second generation catalyst led
smoothly to P3DTV of high MW.
Poly(2,5-thienylene vinylene)s (PTVs) are a unique class of conju-
gated polymers under extensive investigation for several decades.
PTVs possess high crystallinity, broad absorption spectra, and
Additional Supporting Information may be found in the online version of this article.
C
V
2013 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of monomer and polymer.
The synthesis of monomers and polymers is summarized in
Scheme 1 and detailed synthetic procedures and characteriza-
tion data can be found in the Supporting Information. Negishi-
type coupling reaction between 3DTBr and propynylzinc bro-
mide, which was prepared in situ from 1,2-dibromopropane,
gave 3-decyl-2,5-dipropynylthiophene (3DTE) in high yield.
Reduction in the propynyl groups to cis double bonds was first
attempted by hydrogenation in the presence of Lindlar’s cata-
lysts. However, no reactions were observed and starting mate-
rials were quantitatively recovered using commercially
available Lindlar’s catalysts under various hydrogenation con-
ditions. The lack of reactivity was possibly due to higher sta-
bility of conjugated alkynes in 3DTE than that of isolated
triple bonds. We thus turned to dissolving metal methods for
generation of the target cis23DTV monomer.
1H NMR spectrum (Fig. 1, bottom) of cis23DTV closely
matches those of cis-stereoisomers of 3-alkyl-2,5-
dipropenylthiophenes prepared from Wittig reactions.13(b–d)
One-dimensional nuclear overhauser enhancement spectrum
(Fig. 1, top), excited at 6.82 ppm (Th-H) giving a strong cross-
peak at 1.99 ppm (ThCH@CHCH3), confirms cis nature of the
double bonds. Signals due to trans isomers, normally seen at
6.68 and 6.63 ppm (Th-H), 5.9–6.1 ppm (trans-ThCH@CHCH3),
and 1.65–1.90 ppm (trans-ThCH@CHCH3) are barely observed
1
in the H NMR spectrum of cis23DTV. Integration gave a cis
content of ꢀ95%, corresponding to a cis/trans selectivity over
19/1, which is significantly higher than those obtained using
salt-free Wittig reaction conditions.13(b,d)
To investigate the ADMET polymerization behavior of
cis23DTV in detail, we have performed kinetic studies by moni-
toring monomer conversions with time. For comparison,
another analogous monomer, trans/cis23DTV, having low cis
Textbook dissolving metal methods involving alkali metals in
liquid ammonia normally lead to exclusively trans alkenes,
the thermodynamically more stable products. An alternative
method that selectively reduces alkynes, including conju-
gated alkynes, to cis-alkenes has existed for several decades.
These methods used activated zinc metals, such as Rieke
zinc, in the presence of proton donors including water, acids,
and alcohols.14 Cis-reduction in these reactions is achieved
through double electron transfer from Zinc to triple bond,
forming three-membered cyclometallated alkene–zinc
adducts, which force the following double-proton addition to
happen in the same face opposite to zinc, resulting in cis-
alkenes.14(a) We adopted this method in efforts toward
cis23DTV. Using ethanol as the solvent and proton donor,
commercial zinc powder was first activated by refluxing with
1,2-dibromoethane, followed by refluxing with CuBr and
LiBr. 3DTE was then added and the reaction mixture was
brought to reflux and monitored by thin layer chromatogra-
phy and nuclear magnetic resonance (NMR). The reaction
was completed within approximately 12 h, and cis23DTV
was purified by column chromatography as a slightly green
liquid in approximately 90% yield.15
FIGURE 1 1H NMR (bottom) and 1D-NOE (top, excited at 6.82
ppm) spectra of cis23DTV (300.13 MHz, CDCl3). Insert: magni-
fied olefinic region.
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contents of approximately 40% was prepared through typical
Wittig type reactions (Supporting Information). Both monomers
were subjected to identical ADMET polymerization conditions,
that is, 0.2 M monomer concentration in 1,2,4-trichlorobenzene,
1 mol % Grubbs second-generation catalyst ([Ru], Scheme 1)
ꢁ
and at 90 C under dynamic vacuum. Samples were withdrawn
at predetermined time intervals and subjected to 1H NMR analy-
sis. Conversion versus time plots of both monomers are shown
in Supporting Information Figure S1, and selected 1H NMR spec-
tra of both monomers during polymerization are included in
Supporting Information Figures S2 and S3. As seen in Support-
ing Information Figure S1, cis23DTV has a much faster initial
polymerization rate with approximately 21% monomer conver-
sion within the first 2 h. During the same time period, ADMET
polymerization of trans/cis23DTV led only to approximately
3% monomer conversion. Polymerization rates for both mono-
mers significantly reduced and started to level off after 10–24 h,
possibly due to catalyst decomposition. After 48 h reaction
times, cis23DTV reached a monomer conversion of approxi-
mately 57% and trans/cis23DTV gave a monomer conversion
of approximately 44%. These observations confirm that mono-
mers possessing higher cis-contents do polymerize faster under
ADMET conditions, especially during initial stages of the reac-
tion. Interestingly, isomerization of propenyl double bonds was
observed during ADMET polymerization of cis23DTV as shown
in Supporting Information Figure S2. Almost all signals due to
cis-propenyl double bonds disappeared after only 1 h, accompa-
nied by the appearance of signals corresponding to oligomers
and trans-propenyl groups. The ADMET reaction temperature
was identical to that of the zinc reduction reaction for the syn-
thesis of cis23DTV during which no isomerization was
observed. As a result, the appearance of trans signals can only
be explained by catalyst-induced cis to trans isomerization. Such
initial-stage isomerization events would compete with ADEMT
polymerization and likely explain the rapid decrease in
cis23DTV polymerization rate after the first few hours of reac-
tion. Reduction in isomerization rates, while keeping relatively
high enough ADMET polymerization rates, will require system-
atic optimization of reaction conditions including concentra-
tions, temperatures, solvents, and different catalysts, which is
currently under investigation.
FIGURE 2 (A) 1H NMR (300.13 MHz, CDCl3) spectrum of
P3DTV. (B) 13C NMR (75.48 MHz, CDCl3) spectrum of P3DTV.
(C) UV-Vis absorption spectra of P3DTV in chlorobenzene (1025
M repeating units, solid line) and as thin film spin-cast from
chlorobenzene solution (dashed line). (D) X-ray scattering pro-
files of P3DTV as powder (solid line) and as thin film spin-cast
from chlorobenzene solution (out-of-plane, dashed line).
Stille coupling reactions but are significantly broader than
those of regio-regular P3DTVs prepared from Wittig type
condensation reactions.9(h),11(f) Regio-irregularity is expected
in ADMET polymerization of cis23DTV as propenyl groups
at both 2- and 5-positions of the monomer can cross-
metathesize with each other. Signals due to propenyl and
aldehyde13(b,d) end-groups could not be observed in the 1H
NMR spectrum (Fig. 2A, insert), indicating high MW of the
polymer. PTVs have attracted significant amount of interest
due to the low optical energy gaps as indicated by UV-Vis
absorption spectra in Figure 2C. In dilute chlorobenzene
solution, an optical energy gap of approximately 1.82 eV is
estimated for P3DTV from the absorption edge (680 nm).
Red shifted absorption and more structured lineshape are
observed for the thin film of P3DTV, indicating semicrystal-
line nature of the polymer and an optical energy gap of
approximately 1.65 eV is calculated. Semicrystalline nature is
also reflected by X-ray scattering measurements as shown in
Figure 2D. Sharp signals at 2h values of 5.4ꢁ (100), 10.9ꢁ
(200), and 16.4ꢁ(300) are observed in both powder and out-
of-plane thin film scattering profiles, corresponding to a
lamella distance of 16.3 Å. The crystallite size of P3DTV in
the (100) direction is estimated from Scherrer’s equation
(t 5 0.9 k/B cosh; k 5 1.54 Å, B: full width at half-maximum
As suggested by the kinetic studies, ADMET polymerization
of cis23DTV slows down significantly after approximately
10–24 h. We have thus carried out the polymerization by
adding the [Ru] catalyst in three equal portions every 16 h,
for a total of 2 mol % catalyst and 48 h reaction time. The
reaction mixture turned from yellow to red and then to pur-
ple and eventually to blue. After precipitation and Soxhlet
extraction, P3DTV was isolated as a black powder. MW of
P3DTV was estimated from size exclusion chromatography
(Supporting Information Figure S4) to be 1.42 3 104 (Mn)
with a polydispersity (PDI) of 2.0 before Soxhlet extraction
and 2.13 3 104 (Mn) with a PDI of 2.1 after extraction.
Figure 2 summarizes characterization data of P3DTV. Both
1H and 13C NMR spectra of P3DTV [Fig. 2(A,B)] match
closely with those of regio-random P3DTVs obtained from
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In summary, we have developed a facile methodology for the
preparation of a 2,5-dipropenylthiophene derivative possess-
ing predominantly cis configuration. These cis double bonds
facilitate ADMET polymerization of the monomer, leading to
PTVs of high MW. Double bond isomerization was observed
during early stages of ADEMT polymerizationwhich competes
with ADMET processes and decreases polymerization rates.
Suppressing such isomerization reactions through systematic
reaction condition optimization is currently underway. Our
method opens up doors for other cis-dipropenylarylenes and
possible preparation of novel high MW poly(arylene vinyl-
ene)s using the versatile ADMET polymerization techniques.
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ACKNOWLEDGMENTS
The authors would like to acknowledge University of New Mex-
ico (UNM) for financial support for this research. National Sci-
ence Foundation (NSF) is acknowledged for supporting the NMR
facility at UNM through grants CHE-0840523 and 0946690.
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