Transition-metal-free controlled polymerization of 2-perfluoroaryl-5- trimethylsilylthiophenes

Article abstract of DOI:10.1021/ja505282z

A catalytic amount of fluoride anion promoted the polymerization of 2-pentafluorophenyl-5-trimethylsilylthiophene, providing a high-molecular-weight conjugated polymer in a chain-growth-like manner. The resulting polymer with a controlled molecular weight and a low molecular weight distribution was obtained in a high yield. The mechanism involves a pentacoordinated fluorosilicate as a key intermediate. The active polymerization end also initiated the second polymerization to afford well-defined block copolymers.

Full text of DOI:10.1021/ja505282z

Communication
pubs.acs.org/JACS
Transition-Metal-Free Controlled Polymerization of 2‑Perfluoroaryl-
5-trimethylsilylthiophenes
Takanobu Sanji* and Tomokazu Iyoda
Iyoda Supra-Integrated Material Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology
Agency (JST), and Frontier Research Center, Tokyo Institute of Technology, 4259-S2-3 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan
S
* Supporting Information
because the fluoride anion catalyzed carbon−silicon bond
activation resulting in a nucleophilic pentacoordinate silicate5,6
ABSTRACT: A catalytic amount of fluoride anion
and the SNAr reaction of perfluoroaryl groups7,8 have been well
promoted the polymerization of 2-pentafluorophenyl-5-
studied.9 The polycondensation of hexafluorobenzene and
trimethylsilylthiophene, providing a high-molecular-weight
conjugated polymer in a chain-growth-like manner. The
resulting polymer with a controlled molecular weight and a
low molecular weight distribution was obtained in a high
yield. The mechanism involves a pentacoordinated
fluorosilicate as a key intermediate. The active polymer-
ization end also initiated the second polymerization to
afford well-defined block copolymers.
bis(trimethylsilyl)thiophenes was recently reported, but not
controlled.10
The addition of a catalytic amount of the fluoride anion to 1
smoothly promoted the polymerization. Table 1 summarizes
the polymerization results. For example, the reaction of 1 with
5 mol % tetrabutylammonium fluoride (TBAF) as a fluoride
anion source in tetrahydrofuran (THF) at room temperature
for 2 h led to poly(tetrafluorophenylene-thienylene) 2 with a
MW of 6700 and a PDI of 1.55 in 93% yield (entry 2).11,12
When the polymerization of 1 was examined with varying mol
% of TBAF to the monomer, the MW of the polymer was
increased with increasing monomer to TBAF ratio, while the
PDIs were relatively low (entries 1−3). The addition of 1 to a
solution of TBAF also gave the polymer (entry 4). Tetra-
methylammonium fluoride, however, was not a good initiator,
because it remained undissolved in THF. Metal fluorides, such
as potassium fluoride and cesium fluoride, in the presence of
18-crown-6 were not effective, but potassium fluoride/
cryptand[2.2.2] was effective (entries 6−8) for the polymer-
ization of 1. Among the fluorosilicates examined (entries 9,10),
tetrabutylammonium difluorotriphenylsilicate (TBAT)13 initi-
ated the polymerization to give 2 with a relatively low PDI.
The structure of the polymer is highly regulated, as
ver the past decades, the design and synthesis of π-
Oconjugated polymers have been active areas of research
because of their applications as optoelectronic materials, such as
organic field-effect transistors (FETs) and organic electro-
luminescent (EL) devices. Transition-metal catalyzed C−C
coupling polycondensations are one of the most important
synthetic methods for these polymers.1 For example, Kumada−
Tamao−Corriu coupling and Suzuki−Miyaura coupling are
employed for the synthesis of conjugated polymers. However,
the molecular weight (MW) and polydispersity of the polymers
synthesized in such a way are not well controlled. As an
alternative for these conventional step-growth polycondensa-
tions, chain-growth polymerization has been demonstrated
recently.2,3 The polymerization affords conjugated polymers
with a controlled MW and low polydispersity index (PDI), but
requires transition-metal catalysts.4
1
demonstrated by H, 13C, and 19F NMR analyses (Figure S1
1
in the Supporting Information). The H NMR spectrum of 2
showed three major signals around 0.8, 1.4, and 4.0 ppm from
the side chain on the thienylene units, along with a small signal
at 6.7 ppm arising from the thienyl group at the polymer end.
In the 19F NMR spectrum of the polymer, a strong signal and
three weak types of signals, which are assigned to the 1,4-
tetrafluorophenylene units in the main chain and the
pentafluorophenyl group at the polymer end, respectively,
were observed. Thus, the NMR spectra are consistent with a
highly regulated structure for the polymer main chain,
indicating that the polymerization process itself must be highly
regioselective. In addition, because the polymer ends are
designated as the pentafluorophenyl and the thienyl groups, the
integral ratio of the peaks from the side chain on the main chain
and the end group in the NMR spectra assessed the number-
Herein, we report an alternative controlled polymerization of
2-perfluoroaryl-5-trimethylsilylthiophenes catalyzed by the
fluoride anion. The polymerization requires no transition-
metal catalyst and also proceeds in a chain-growth-like manner
to afford the polymers with a controlled MW and low PDI.
We designed 2-pentafluorophenyl-5-trimethylsilylthiophene
1 as a monomer and examined the polymerization with a
catalytic amount of the fluoride anion (Scheme 1). This is
Scheme 1. Polymerization of 1
Received: May 27, 2014
Published: July 10, 2014
© 2014 American Chemical Society
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a
Table 1. Polymerization of 1 Catalyzed by Fluoride Anion
bc
,
bc
,
entry
F− (mol %)
reaction conditions
yield (%)
Mn
PDI
1.30
1
2
3
4
5
6
7
8
9
10
Bu4NF (TBAF) (10)
Bu4NF (TBAF) (5)
Bu4NF (TBAF) (0.5)
Bu4NF (TBAF) (5)
Me4NF (5)
THF, rt, 2 h
THF, rt, 2 h
THF, rt, 2 h
81
93
89
54
0
4900
6700 (7900)
1.55 (1.34)
13 800 (15 300)
1.71 (1.35)
d
THF, rt, 20 h
6300
−
−
2.01
−
−
THF, rt, 2 h
KF (5)/18-C-6 (10)
CsF (5)/18-C-6 (10)
KF (5)/cryptand[2.2.2] (10)
[(Me2N)3S]+[Me3SiF2]− (5)
Bu4N+[Ph3SiF2]− (5)
THF, 80 °C, 5 days
toluene, 80 °C, 5 days
THF, rt, 20 h
0
0
−
−
83
95
93
6900
20 500
7740
1.57
3.19
1.94
THF, rt, 2 h
THF, rt, 2 h
a
b
All reactions were run with [1] = 0.3 mmol in 5 mL of solvent. The number-average MW (Mn) and PDI were determined by SEC using
c
d
polystyrene standards. The figures in parentheses are the weight-average MW (Mw) and PDI determined by SEC-MALS. A THF solution of 1 was
added to a TBAF solution.
averaged MW (Mn) of ca. 5200, in reasonable agreement with
the MW estimated by size-exclusion chromatography (SEC)
(Table 1, entry 2). In addition, matrix-assisted laser desorption
ionization time-of-flight (MALDI−TOF) mass spectra also
show that the polymer has the pentafluorophenyl and the
thienyl groups at the ends (Figure S2 in the Supporting
Information).
Scheme 2. Reactions of 1 with Potassium Alkoxides
In view of the controlled MW and the relatively low PDIs
(<2) found for most of the entries in Table 1, the
polymerization proceeds in a chain-growth-like manner under
these conditions. To prove this feature, we investigated the
MW of the polymers formed as a function of monomer
conversion with 5 mol % TBAF. Figure 1 shows the Mn and
1
yield. Potassium 3-trimethylsilylpropoxide also gave 3b. H
NMR spectra of 3b showed peaks at around 0 and 6.7 ppm
arising from the trimethylsilyl and thienyl groups at the
polymer ends, respectively, where the observed integral ratio
was in good agreement with the calculated value (calcd for
Me3Si-/thienyl-H 9.0, found 8.0) within the experimental errors
(Figure S4 in the Supporting Information). To elucidate the
initial step of the polymerization for this case, the reaction of 1
with a 1 equiv amount of potassium tert-butoxide, followed by
the addition of ethanol, was examined. This reaction exclusively
gave 2-(4′-tert-butoxytetrafluorophenyl)-5-trimethylsilylthio-
phene 4, indicating that tert-butoxide attacks on the
pentafluorophenyl group of 1.
Scheme 3 shows a possible polymerization mechanism. In
the initial step of the reaction, a fluoride anion attacks the
trimethylsilyl group of 1 to form a pentacoordinate silicate. The
silicate is quite reactive and could regioselectively attack the 4-
position of the pentafluorophenyl group of 1 to reproduce a
fluoride anion. Then, the fluoride anion catalyzes the
polymerization to give polymer 2. In the polymerization, the
reactivity of the trimethylsilyl group and/or the silicate at the
polymer end is changed by replacing the 4-position of the
pentafluorophenyl group of 1. Watson et al. discussed the
change in the reactivity by substitution in the reaction of
bis(trimethylsilyl)thiophene and hexafluorobenzene.10 Further,
Figure 1. Mn and PDI of polymer 2 formed as a function of monomer
conversion in the polymerization of 1 with 5 mol % TBAF.
PDI as a function of monomer conversion (see also Table S1
and Figure S3 in the Supporting Information). The polymer-
ization of 1 was almost completed within a few minutes. The
linear relationship between the Mn and conversion, and the
nearly constant and relatively low PDI, support the chain-
growth process.
As a control experiment, the reaction of hexafluorobenzene
and bis(trimethylsilyl)thiophene with 5 or 10 mol % TBAF in
THF at room temperature was examined, but no polymer was
found (see Supporting Information).14
We further evaluated metal alkoxide/cryptand[2.2.2] as an
initiator for the polymerization (Scheme 2). The addition of 5
mol % potassium tert-butoxide in the presence of 10 mol %
cryptand[2.2.2] initiated the polymerization of 1 efficiently to
give polymer 3a with an Mn of 10 700 and a PDI of 1.75 in 82%
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Journal of the American Chemical Society
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Scheme 3. Possible Polymerization Mechanism of 1
Scheme 5. Synthesis of Block Copolymers
the fluoride anion would transfer intramolecularly to the
trimethylsilyl group at the polymer end in the polymerization of
1, where there may be an anion−π interaction15 that produces
an associated pair to allow the polymerization to proceed via
chain growth. This is because of the lack of polymerization of
bis(trimethylsilyl)thiophene and hexafluorobenzene under our
conditions as the control experiment. A further detailed
description of the polymerization mechanism requires further
studies. On the other hand, when using metal alkoxides as an
initiator, the initial step of the polymerization occurs by the
attack of the alkoxide on the pentafluorophenyl group of 1 to
give a fluoride anion, resulting in the formation of a
pentacoordinate silicate, and then the polymerization proceeds
in the same way as mentioned above.
Figure 2. SEC curves (RI) obtained by the first polymerization of 1
(polymer 2, black) and sequential addition of 1 (polymer 2′, blue) and
7 (polymer 8, red) after the first polymerization of 1.
Further, we extended our method to the polymerization of
nonafluorobiphenyl and heptafluoronaphthyl-substituted thio-
phenes (Scheme 6). The reaction of (nonafluorobiphenyl)-
Next, to exploit the chain-growth nature of this polymer-
ization process, we demonstrated end-capping (Scheme 4). The
Scheme 6. Polymerization of Perfluoroaryl-Substituted
Trimethylsilylthiophene 9a,b
Scheme 4. End-Capping with 5
addition of an excess amount 3-(pentafluorophenyl)propyl-
trimethylsilane 5 to the reaction mixture of 1 terminated the
polymerization to give end-capped polymer 6, as confirmed by
NMR spectroscopy (Figure S5 in the Supporting Information).
We also applied the “living” character of this growing end to the
synthesis of well-defined block copolymers (Scheme 5). After
polymerization of 1 with a catalytic amount of TBAF, a part of
the mixture was studied to analyze the MW of the first block,
and then a solution of 1 or 7 was added to the reaction mixture
to afford block copolymer 2′ or 8, respectively.16 As shown in
Figure 2, the SEC curves of the polymer obtained at the end of
the reaction shifted toward a higher molecular weight than
those of the first polymerization. The first/second block ratio
trimethylsilylthiophene 9a with 5 mol % TBAF gave polymer
10a with an Mn of 5500 and a PDI of 1.39 in a moderate yield.
(Heptafluoronaphthyl)trimethylsilylthiophene 9b was also
polymerized by the addition of 5 mol % TBAF to give polymer
10b with an Mn of 5600 and a PDI of 1.51 in a good yield. The
structures of these polymers were highly ordered, as evidenced
by NMR analyses (Figure S7 in the Supporting Information),
indicating that the polymerizations of perfluoroarylthiophene 9
also proceed with high regioselectivity on the perfluoroaryl
groups.
1
(n/m) of 8 was found to be 0.60 by H NMR analysis (Figure
S6 in the Supporting Information).
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Journal of the American Chemical Society
Communication
Bohme, U.; Roewer, G. Organometallics 2004, 23, 6066−6069.
̈
In summary, we have demonstrated the transition-metal-free
controlled polymerization of 2-perfluoroaryl-5-trimethyl-
silylthiophenes promoted by the fluoride anion. The polymer-
ization proceeds in a chain-growth-like manner to afford
polymers with a controlled MW and low PDI. In addition,
because the polymerization end is active, the block copolymers
are prepared by addition of a second monomer to the
polymerization mixture. The polymerization in this report
would be an alternative synthetic method for well-controlled π-
conjugated polymers. The present initiation- and termination-
defined controlled polycondensation will open a new avenue of
macromolecular 3-D modeling architecture with the stiffness as
well as conductivity of the π-conjugated main chain as the
growth-controlled building block. Further study along this line
is currently in progress.
(f) Lerebours, R.; Wolf, C. J. Am. Chem. Soc. 2006, 128, 13052−13053.
(7) (a) Brooke, G. M. J. Fluorine Chem. 1997, 86, 1−76. (b) Terrier,
F. Modern Nucleophilic Aromatic Substitution; Wiley-VCH: Weinheim,
2013. (c) Amii, H.; Uneyama, K. Chem. Rev. 2009, 109, 2119−2183.
(8) (a) Kim, J.-P.; Lee, W.-Y.; Kang, J.-W.; Kwon, S.-K.; Kim, J.-J.;
Lee, J.-S. Macromolecules 2001, 34, 7817−7821. (b) Woody, K. B.;
Bullock, J. E.; Parkin, S. R.; Watson, M. D. Macromolecules 2007, 40,
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(10) Wang, Y.; Watson, M. D. J. Am. Chem. Soc. 2006, 128, 2536−
2537.
(11) A THF solution of TBAF (1 M) containing 5 wt % H2O was
used in this study.
(12) The MWs measured by SEC-multiangle light scattering (SEC-
MALS) were almost the same as those measured by SEC.
(13) Pilcher, A. S.; Ammon, H. L.; DeShong, P. J. Am. Chem. Soc.
1995, 117, 5166−5167.
(14) Our polymerization conditions were quite different from the
reported conditions by Watson and Wang:10 ([bis(trimethylsilyl)thio-
phene] = 0.2 M in toluene, 10 mol % CsF, 20 mol % 18-crown-6, 80
°C, 5 days).
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and characterization of new compounds
and polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
sanji.t.aa@m.titech.ac.jp
■
(15) Quinonero, D.; Garau, C.; Rotger, C.; Frontera, A.; Ballester, P.;
̃
Costa, A.; Deya, P. M. Angew. Chem., Int. Ed. 2002, 41, 3389−3392.
̀
(16) The polymerization of 2-(pentafluorophenyl)-3,4-bis(3-
trimethylsilylpropoxy)-5-trimethylsilylthiophene 7 with 5 mol %
TBAF gave a polymer with an Mn of 7700 and a PDI of 1.40 in
78% yield.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Ms. Rie Ushikubo for assistance with the NMR and
SEC measurements.
■
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