A General and Air-tolerant Strategy to Conjugated Polymers within Seconds under Palladium(I) Dimer Catalysis

Article abstract of DOI:10.1002/anie.201903765

While current M⁰/M^II based polymerization strategies largely focus on fine-tuning the catalyst, reagents and conditions for each and every monomer, this report discloses a single method that allows access to a variety of different conjugated polymers within seconds at room temperature. Key to this privileged reactivity is an air- and moisture stable dinuclear Pd^I catalyst. The method is operationally simple, robust and tolerant to air.

Full text of DOI:10.1002/anie.201903765

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Accepted Article
Title: A General and Air-tolerant Strategy to Conjugated Polymers
within Seconds under Pd(I) Dimer Catalysis
Authors: Guillaume Magnin, Jamie Clifton, and Franziska
Schoenebeck
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903765
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Angewandte Chemie International Edition
10.1002/anie.201903765
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Polymerization Methods
A General and Air-tolerant Strategy to Conjugated Polymers within
Seconds under Pd(I) Dimer Catalysis
Guillaume Magnin, Jamie Clifton and Franziska Schoenebeck*
(
0)
(II)
Abstract: While current M /M based polymerization strategies
largely focus on fine-tuning the catalyst, reagents and conditions
for each and every monomer, this report discloses a single method
that allows access to a variety of different conjugated polymers
within seconds at room temperature. Key to this privileged
reactivity is an air- and moisture stable dinuclear Pd(I) catalyst.
The method is operationally simple, robust and tolerant to air.
polyhalogenated arenes are typically not fully predictable with regard
[
11]
to the favored coupling site and are highly substrate-specific. By
contrast, our group recently established that the use of the air-and
(
I)
moisture stable Pd dimer 1 allows for extremely rapid and highly
selective carbon-carbon bond formation of C-Br in the presence of C-
[
12]
Cl and C-OTf.
Most importantly, the coupling was completely
independent of the steric or electronic characteristics of the particular
substrate.
A
lthough polymers are already omnipresent in nowadays
modern society, there is an ever increasing need for innovative
materials to meet the next frontier of organic electronics, solar cells
[
1]
or fuel cell developments. Ultimately, any advance in this arena is
intertwined and dependent on the ability to synthesize the desired
materials with precision properties. With regard to conjugated
(0)
(II)
polymers, which are largely assembled via M /M -based (M = Ni
[
2]
or Pd) polymerization, the consensus in the field is that “instead of
searching for a universal catalyst, one should tune the catalyst’s
electronic and steric properties for each monomer.”[3] Indeed, for the
selection of the various widely employed conjugated polymers in
Figure 1 (i.e. polyfluorene, polythiophene, poly-p-phenylenes or
polycarbazoles that are featured in transistors , photodetectors and
solar cells ), there are also several different, specialized protocols
for the best polymerization results reported, featuring Suzuki,
Kumada or Negishi type transformations with Ni or Pd catalysts and
reaction times anywhere between 30 minutes and 24 hours and
temperatures ranging from 0°C to 80°C.[8]
[
4]
[
5]
[6]
[
7]
However, the discovery of innovative materials would be greatly
accelerated if there was a single powerful polymerization strategy that
allowed access to a library of conjugated polymers via high
throughput polymerizations of diverse monomers. Ideally, this
strategy is coupled with high practicability, such as a robust catalyst
that needs no specialized equipment and is characterized by high
[
9]
speed to maximize throughput.
(
0)
(II)
Figure 1. Important polymer classes for materials applications (top), current
approaches in transition metal-catalyzed polymerizations, vision and outline of
this work (bottom).
In the small molecule world, Pd /Pd based cross coupling
strategies are highly dependent on the electronic and steric bias of the
substrate and as such frequently require specialized catalysts and
[
10]
conditions.
For example, chemoselective coupling strategies of
We envisioned that this substrate-independence paired with a priori
(
I)
predictable reactivity of Pd for small molecules might be an ideal
feature also for the next dimension of control to assemble
macromolecules, therefore potentially allowing the polymerization of
a wide range of monomers of diverse steric and electronic
characteristics. To this end, we embarked on exploring the potential
[
]
M.Sc. G. Magnin, Dr. J. Clifton, Prof. Dr. F. Schoenebeck
Institute of Organic Chemistry, RWTH Aachen University,
Landoltweg 1, 52074 Aachen (Germany).
E-mail: franziska.schoenebeck@rwth-aachen.de
Homepage: http://www.schoenebeck.oc.rwth-aachen.de
(
I)
[13]
of the air- and moisture stable Pd dimer 1 in polymerizations of
di-brominated monomers. More specifically, building on our
previous findings of extremely rapid C-C couplings with organozinc
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201xxxxxx.
[
14]
(I)
reagents, we investigated the Pd enabled polymerization of ‘Br-
1
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Angewandte Chemie International Edition
10.1002/anie.201903765
a
aryl-ZnX’ units and targeted the widely employed polymers shown in
Figure 1.
Conditions: [Pd(μ-I)(PtBu
equiv.), ZnCl
3
)]
2
(1), nBuLi (0.98 equiv.), monomer (0.5 mmol, 1.0
[
18b]
2
(1.1 equiv.). *open-flask.
We started our investigations with the all-carbon fluorene system
see Table 1). Owing to their blue-light emitting optical and electronic
(
I)
With these promising Pd -triggered polymerization results in
hand, we subsequently explored the generality of the method. The
alkyl substituents of the polymer units strongly influence the physical
properties of the resulting polymers, such as solubility, but also the
overall self-assembly and consequently impact the ultimate device
(
(
hole-transport) properties, polyfluorene polymers find widespread
[
15]
applications ranging from organic devices to nanoscience.
To
attempt polymerization, we metalated one C-Br site in dibromo-
fluorene to the corresponding organozinc species in THF at room
[
19]
(
I)
performance.
Variability in the latter side chain will hence be
temperature and subsequently added a solution of Pd dimer 1 (0.5
advantageous and we next tested whether alkyl chains other than
branched ethyl butyl chains are also tolerated. To this end, we
subjected the dioctylfluorene derivative 3 to the same polymerization
conditions with Pd (0.5 mol%) at room temperature (Table 2). After
just 30 seconds reaction time, we obtained a polymer with M = 14.3
kg/mol, M = 20.8 kg/mol and PDI =1.45. Again, performing the
polymerization open to the laboratory atmosphere had no impact (see
Previously, the same polymer was synthesized under
Suzuki-type polymerization by Yokozawa and co-workers, requiring
0 min reaction time and 5 mol% of sensitive Pd to obtain a similar
mol%) in THF to the mixture in one portion. To our delight, within
3
0 seconds at room temperature polymerization had already taken
place to yield polyfluorene 2a with a molecular (M = 24.5 kg/mol)
as well as average molecular weight (M of 53.4 kg/mol) and
n
(
I)
w
n
polydispersity (PDI) of 2.18 (see Table 1). When we repeated the
reaction and left it stirring for a total of 2 minutes at room
temperature, a polymer with greater molecular weight was generated
and isolated in 87% yield (entry 2). Considering the rapid speed and
operational simplicity of the method, the polymerization capability
seemed remarkable. As such, we challenged the method further and
systematically lowered the catalyst loading. We found that using as
w
[
18b]
entry 6).
3
[
20]
polymer (M
n
= 19.2 kg/mol and PDI = 1.34), and under Ni(cod)
= 99.5
The need for relatively high palladium
loading in the former case might stem from the fact that an oxidative
addition complex, i.e. [(PtBu )Pd(Ph)Br], was utilized as catalyst.
2
-
(
I)
catalyzed Yamamoto conditions over 3 days (to give M
n
little as 0.005 mol% of Pd dimer still gave excellent polymerization
within 10 min reaction time (entry 4). These results were fully
competitive with previous polymerization methods to make
-1
[15d]
kg.mol and PDI of 2.36).
3
polyfluorenes, which range from Pd-catalyzed Suzuki or Ni(dppp)Cl
2
Although this pre-formed species circumvents potential weighing
inaccuracies in achieving the correct Pd-to-ligand ratios, these
complexes were previously found to suffer from inconsistent
catalyzed Kumada couplings to oxidative strategies (e.g. Ni(cod)
2
-
based Yamamoto catalysis at 80°C).[
8c]
A
Negishi-type
polymerization was previously realized by Kiriy’s laboratory, having
prepared 2a with M = 53.1 kg/mol, M = 123.1 kg/mol and PDI =
.32 after 20 h reaction time and using a combination of 1:1 mixture
[
21]
[22]
initiation and instabilities.
n
w
2
Table 2: Comparison of catalyst performance in the polymerization to afford 2ba
(
II)
of Pd
source with the air-sensitive, free phosphine, tri-tert-
[16]
butylphosphine. Since precise metal-to-ligand-ratios are crucial for
reproducibility and performance in metal catalyzed coupling
reactions,
[
17]
(I)
the use of the bench-stable Pd dimer 1 presents a
convenient advantage, as it circumvents potential reproducibility
issues due to inconsistent weighing and metal/ligand ratios.
Entry
Catalyst
[mol%]
1.0
Time [s]
M
n
M
w
PDI
-derived Pd(0) or Pd
(II)
Moreover, as opposed to alternative PtBu
3
[kg/mol]
21.8
[kg/mol]
54.6
catalysts, dimer 1 is completely stable to oxygen. Thus, we were
intrigued whether polymerization would also be possible in the
presence of oxygen. To this end, we added a solution of dimer 1 in
THF to the prepared zincated monomer under open flask
1
2
30
30
2.50
1.45
1.40
1.66
1.71
1.82*
0.5
14.3
20.8
3
0.05
30
13.2
18.5
4
0.005
0.005
0.5
30
15.4
25.8
[
18]
conditions. Entry 6 presents the results (Table 1). There was no
impact of external oxygen; the same polymer was obtained as under
inert conditions, showing the practicability and robustness of our
polymerization method.
5
120
30
15.3
26.1
6
(in O
2
)
13.1*
23.8*
a
Conditions: [Pd(μ-I)(PtBu
equiv.), ZnCl
3
)]
2
(1), nBuLi (0.98 equiv.), monomer (0.5 mmol, 1.0
[18b]
2
(1.1 equiv.). *open-flask.
Table 1: Comparison of catalyst performance in the polymerization to afford 2aa
As such, the Pd(I) protocol offers much faster polymerization (30
seconds vs. 30 min) using a robust and air-stable catalyst and
requiring lower loadings in Pd. To gain greater insight on the Pd-
loading, we studied various different catalyst loadings ranging from
1
n
.0 mol % to 0.005 mol % (Entries 1 to 4). In all cases, good M and
Entry
Catalyst
Time
[min]
0.5
2
M
n
M
w
PDI
PDIs were obtained after only 30 seconds reaction time, and the
polymerization was found to be largely independent of the actual
catalyst loading. Similarly, the reaction time had no marked impact
[
mol%]
0.5
[kg/mol]
24.5
[kg/mol]
53.4
1
2
2.18
2.02
2.03
2.06
1.94
2.15*
0.5
59.7
120.9
34.2
n
on the polymerization; there was no significant change in M whether
3
0.05
0.005
0.005
0.5
0.5
10
16.9
the reaction time was 30 seconds or 2 minutes. These data suggest
that the polymerization is completed within 30 seconds at 0.005
4
43.3
89.1
(
I)
5
30
51.0
98.9
mol% Pd dimer loading, giving the polymeric material in 82% yield
[
23]
upon isolation.
6
(in O
2
)
0.5
24.3*
52.3*
2
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Angewandte Chemie International Edition
10.1002/anie.201903765
With independence from the nature of the side chain
the monomer or the presence of nitrogen heteroatoms, suggesting that
this method could indeed have potential for syntheses of a range of
polymers. A frequent motif in materials for electronic applications are
also arenes or heterocycles. We therefore next assessed the
compatibility of the polymerization protocol to make π-rich materials.
(
I)
demonstrated, we next explored the compatibility of the Pd -based
polymerization with electronically varied motifs, more specifically
heteroatoms. In this context, a representative materials class is
polycarbazole (3). Owing to their greater chemical stability compared
to polyfluorenes, polycarbazoles are of interest for applications in
[
27]
We succeeded in the generation of poly(3-hexylthiophene)
5
[
24]
(I)
solar cells.
Once again, we utilized the dibrominated monomer,
within 1 min at r.t. under Pd dimer catalysis (0.5 mol%). In addition,
we assessed having an arene in the side chain as spacer (2c) or as
actual monomer to generate polyarene 4. In both cases, ether bonds
are present as linkers which could potentially sterically and
electronically impact the catalyst performance. Once again, we saw
very rapid and efficient polymerizations (table 3) in less than 1 min
converted it to the mono-zincated derivative via convenient metal-
halogen exchange and subjected it to the analogous polymerization
(
I)
conditions for 1 min (0.5 mol% Pd dimer in THF at room
temperature). Pleasingly, this resulted in polycarbazole 3 in 74%
isolated yield (with M
respectively), which is in the realm of previous polymer data with
different methods: under Kumada cross coupling using Ni(dppp)Cl
0.9 mol%) a polymer with M = 17.4 kg/mol and a PDI of 1.27 was
obtained after 24 h reaction time. Pd-based Suzuki coupling [with
a catalyst system of Pd (dba) and P(o-tol) ] and Stille-based
polymerization were also previously used.
n
and PDI of 21.3 kg/mol and 2.14,
(
I)
in these cases (using 0.5 mol% of Pd dimer 1). This gave rise to 4
(with M of 9.9 kg/mol and PDI of 1.45) and poly(2,7-[9,9-bis(4-
hexyloxyphenyl)fluorene) polymer 2c with a molecular weight of M
2
n
(
n
n
[25]
of 44.3 kg/mol and a PDI of 1.80 (in 74% isolated yield). For
comparison, while 2c was not previously made under Negishi-type
coupling, under Pd(PPh ) (0.5 mol%) catalyzed Suzuki-type
3 4
2
3
3
[
24b]
While organotin
is associated with
[
24b]
reagents are known for their toxicity,
Pd
2
(dba)
3
polymerization over 48 h at reflux, polymer 2c was previously made
[26]
[28]
inconsistent quality and stability. Consequently, there is the danger
of imprecise Pd-to-ligand-ratios (and hence inconsistent
polymerization performance), as the amount of homogeneous Pd
n
with M of 12 kg/mol and PDI of 2.40 (in 71%).
Table 3: Performance of Pd(I) iodo dimer 1 in the polymerization of different
a
[
26]
polymer classes.
(
relative to nanoparticles) in commercial sources tends to vary. An
organozinc-based polymerization has not previously been
demonstrated for 3.
Given the rapid polymerization rates under Pd(I) conditions, we
set out to have a closer look at the consumption of monomer over time
and set up 13 parallel polymerization runs to make 3 with 0.5 mol%
(
I)
Pd dimer 1, which were quenched with methanol at different time
intervals and subjected to GC-MS analyses (Figure 2). Our data
indicate that within 15-20 seconds, roughly 90% of the monomer had
already been consumed. Moreover, a GPC analysis of an aliquot taken
Entry
Poly-
mer
Catalyst
[mol%]
Time [s]
M
n
M
w
PDI
[kg/mol]
[kg/mol]
at 20 seconds showed that a polymer with a molecular weight of M
15.3 kg/mol and PDI of 2.14 had formed. Additional GPC analyses
after 1 and 2 min reaction times (when also no more consumption of
monomer was seen), gave analogous polymers with M of 21.9 and
PDI of 2.14, indicating that polymerization appears to be completed
within 1 min reaction time at room temperature under Pd(I) dimer
conditions.
n
1
2
3
3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
20
60
15.3
21.3
23.1
9.9
32.9
45.7
43.2
15.6
18.0*
80.1
7.2
2.14
2.14
1.86
1.45
1.94*
1.80
1.20
=
3
3
120
35
n
4
4
5
(in O
6
2
)
4
30
9.3*
44.3
5.9
2c
5
60
7
60
a
Conditions: [Pd(μ-I)(PtBu
mmol, 1.0 equiv.), ZnCl
3
)]
2
(1, 0.5 mol%), nBuLi (0.98 equiv.), monomer (0.5
100
[18b]
2
(1.1 equiv.). *open-flask.
90
80
70
60
50
40
30
20
10
0
In summary, using a single air-and moisture stable Pd(I) dimer
catalyst, we achieved extremely rapid polymerization to a variety of
widely employed conjugated polymer classes, i.e. polyfluorenes,
polycarbazoles, polyphenylenes and polythiophene. The method is
operationally simple, highly robust, tolerates air as well as catalyst
loadings as low as 0.005 mol%. We anticipate that these findings will
result in widespread applications in academic and industrial materials
research.
0
10
20
30
40
50
60 t [s]
Acknowledgements
Figure 2. Monomer consumption versus time in the polymerization to form
polycarbazole 3. Conditions: [Pd(μ-I)(PtBu )] (1), nBuLi (0.98 equiv.), monomer
0.5 mmol, 1.0 equiv.), ZnCl (1.1 equiv.).
We thank the RWTH Aachen University and the European Research
Council (ERC-637993) for funding. We thank Dr. Jens Kohler and
Mr. Rainer Haas from the DWI Leibniz Institute for Interactive
Materials for undertaking the gel-permeations-chromatography
3
2
(
2
Clearly, the polymerizations under Pd(I) dimer conditions proceed
similarly rapidly and efficiently regardless of the alkyl side chain in
3
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Angewandte Chemie International Edition
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4
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Angewandte Chemie International Edition
10.1002/anie.201903765
Polymerization Methods
G. Magnin, J. Clifton, F. Schoenebeck*
Page – Page
A General and Air-tolerant Strategy to
Conjugated Polymers within Seconds
under Pd(I) Dimer Catalysis
5
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