Dehydration Polymerization for Poly(hetero)arene Conjugated Polymers

Article abstract of DOI:10.1002/chem.201800642

The lack of scalable and sustainable methods to prepare conjugated polymers belies their importance in many enabling technologies. Accessing high-performance poly(hetero)arene conjugated polymers by dehydration has remained an unsolved problem in syntheti

Full text of DOI:10.1002/chem.201800642

A Journal of
Accepted Article
Title: Dehydration Polymerization for Poly(hetero)arene Conjugated
Polymers
Authors: Derek Schipper, Rafael Alberto Mirabal, Luke Vanderzwet,
Sara Abuadas, and Michael Emmett
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201800642
Link to VoR: http://dx.doi.org/10.1002/chem.201800642
Supported by

Chemistry - A European Journal
10.1002/chem.201800642
COMMUNICATION
Dehydration Polymerization for Poly(hetero)arene Conjugated
Polymers
†
†
1*
Rafael A. Mirabal, Luke Vanderzwet, Sara Abuadas, Michael R. Emmett, Derek Schipper .
Dedication ((optional))
Abstract: The lack of scalable and sustainable methods to prepare
conjugated polymers belies their importance in many enabling
technologies. Accessing high-performance poly(hetero)arene
conjugated polymers via dehydration has remained an unsolved
problem in synthetic chemistry and has historically required
transitional-metal coupling reactions. Here we report a dehydration
method that allows access to conjugated heterocyclic materials.
Using the technique, we have prepared a series of small molecules
and polymers. The reaction avoids using transition metals, proceeds
at room temperature, the only required reactant is a simple base and
water is the sole by-product. The dehydration reaction is technically
simple and provides a sustainable and straightforward method to
prepare conjugated heteroarene motifs.
toxic waste streams.
In stark contrast, other classes of polymers have several
highly efficient methods for their construction, including
dehydration polymerization which, in terms of scale, is
the most widely deployed method for the preparation of
organic polymers (Figure 1). Glycosidic and peptide
bonds are the result of a formal dehydration and Nature
uses these reactions for the preparation of biopolymers
such as polysaccharides and polypeptides (Fig. 1B).
Indeed, cellulose is the most abundant organic polymer
[
19]
on earth.
Moreover, the so-called “Age of Plastics”
was ushered in by the 1907 discovery of Bakelite via the
dehydration polymerization of phenol and formaldehyde
[
20,21]
by Leo Baekeland.
Since then, similar processes
The immense scientific and commercial value of
conjugated polymers is illustrated by their use as active
materials in flexible, lightweight, low-cost electronic
have been deployed for manufacturing some of the
largest scale and most industrially important polymers.
[
22]
[
1,2]
For example, polyamides, such as nylon 6,6,
and
are
devices[3,4]such as photovoltaics,
light emitting
chemical and biochemical sensors, and field-
[
23]
[5]
polyesters, such as polyethylene terephthalate,
diodes,
[
6,7]
prepared on a scale of many millions of tons annually
(Fig. 1C). Despite the vast utility and history of the
dehydration polymerization method for polymer
preparation, and the marked value of poly(hetero)arene
conjugated polymers, no progress has yet been reported
in the development of a dehydration polymerization for
the synthesis of poly(hetero)arenes.
effect transistors.
Arguably, the most studied class of
[
1-4,6-
materials are poly(hetero)arene conjugated polymers.
8
]
The high interest in these materials is owed to their
outstanding properties such as, high absorbance
coefficients, good charge carrier mobilities, high
fluorescence quantum yields and ease of processability,
all while having tunable properties through structural
variation.
However,
methods
developed
for
Here, we describe the discovery and development of a
poly(hetero)arene preparation are chiefly limited to reaction that meets this challenge and validates this long-
[
9]
transition metal-catalyzed coupling methods such as the sought synthetic strategy (Fig. 1D). The dehydration
[
10]
[11]
Stille coupling,
Suzuki [13c] oupling,
Negishi coupling
and direct arylation under ambient atmosphere, proceeds at room
While the available methods have proven temperature, gives rapid and complete conversion and the
Kumada reaction exploits readily available substrates, takes place
[
12]
coupling,
[
14,15]
(
Fig. 1A).
indispensable for exploring of the applications of resulting products require only simple purification
poly(hetero)arene conjugated polymers, one or more procedures. The method uses only a simple alkoxide base
drawbacks can hamper their synthetic utility such as: the and can yield either small molecules or high molecular
use of relatively expensive catalysts, residual transition weight conjugated polymers without the addition of
[
16,17,18]
metal
impurities,
synthetically
complex precious transition metal catalysts. Given the value of
monomers, hazardous organometallic intermediates and conjugated poly- and oligo-heteroarene products and the
efficiency with which they can be produced by the use of
this method, our observations should find wide use both
in the preparation of this important class of materials as
[a]
R. A. Mirabal, L. Vanderzwet, S. Abuadas, M. R. Emmett, Prof D. J.
Schipper
Department of Chemistry and Waterloo Institute for Nanotechnology
University of Waterloo
well as in the development of related transformations.
During the exploration of various building blocks for
the synthesis of conjugated organic materials, we became
interested in the thiozole N-oxide motif. Our curiosity
2
00 University Ave W, Waterloo, ON, N2L 3G1
E-mail: derek.schipper@uwaterloo,ca
†These authors contributed equally.
was spurred by traditional prevalence of thiozoles in
Supporting information for this article is given via a link at the end of
the document.
[24,25,36]
conjugated organic materials.
and the ease of
which thiozole N-oxide can be selectively functionalized
with conjugated units via palladium-catalyzed C-H bond
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Chemistry - A European Journal
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COMMUNICATION
A
Traditional Poly(hetero)arene Synthesis
D Poly(hetero)arene Via Dehydration (described herein)
Z
Z
Y
Y
Z
X
X
Pd, Ni
catalyst
Y
X
Y
X
H
nH2O
M
M
Y
M
H
Y
Y
Z
X
O
Z
n
Z
n
Y
B
Biopolymers Via Dehydration:
C
Industrial Polymers Via Dehydration:
OH
O
OH
OH
O
OH
O
O
HO
O
O
HO
H
O
HO
HO
nH2O
N
OH
O
N
H
2
nH O
HO
OH
n
OH
NH2
O
OH
H2N
n
HO
O
O
O
R
H
N
O
O
O
H2N
nH2O
O
OH
N
H
O
OH
nH2O
O
R
R
O
OH
n
HO
n
Figure.1..Polymer.Synthesis. (A) Tradional methods for the preparation of poly(hetero)arene conjugated polymers. M is an organometallic reagent, or H in the
case of direct arylation. Y is a heteroatom. Z is a carbon or heteroatom group. X is a halide. (B) Biopolymers generated through a formal dehydration reaction. (C)
Large scale industrial polymers prepared with dehydration polymerization. (D) Outline of strategy to produce poly(hetero)arene polymers with dehydration
polymerization
[
27,28]
arylation reactions.
During these studies, we found ammonium chloride to give the deoxygenated product in
[
30]
that compound 1 dimerized to give 2 under the action of 97% yield.
only alkoxide base without the addition of any transition
metal catalyst (Fig. 2A). Formally, the formation of 2
from 1 constitutes a dehydration reaction as two
hydrogens and one oxygen are lost in the process (Fig.
A
O
O
N
N
Base
S
S
Solvent, Temp
Time
S
N
2
B).
1
2
Entry Solvent
Base
Temp
Time Yield
Given this reactivity could verify of a highly-sought
1
2
3
4
5
6
7
PhMe
PhMe
PhMe
PhMe
Et2O
THF
NaOtBu
NaOtBu
NaOtBu
LiOtBu
90
30
30
30
30
30
30
5
25
63
25
83
70
77
89
synthetic strategy to access conjugated materials, we
proceeded to optimize the reaction conditions (Fig. 2A).
Initial reactions used sodium tert-butoxide and were
carried out in toluene at 90 °C for 30 minutes giving a
50
rt
rt
NaOtBu
LiOtBu
rt
0
2
5% yield of desired product. Lowering the reaction
THF
LiOtBu
0 to rt
temperature to 60 °C improved the yield to 60% but a
further reduction in temperature gave diminished yields.
However, increased product formation was observed
upon switching the base to lithium tert-butoxide or
employing an ether solvent. The optimal conditions of
B
O
S
O
H
N
N
Base
S
N
H2O
S
H
O
S
N
Dehydration Reaction
No Transition Metals
Only Byproduct is Water
1
.5 equivalents of lithium tert-butoxide added at 0 °C, in
THF and allowed to warm to room temperature and stir
for 5 minutes gave 2 in 89% yield. The product was pure
after only simple extraction and required no further
purification efforts.
New Heterocyclic Biaryl C-C Bond
Figure 2. Dehydration of Thiazole N-oxides. (A) Reaction development. 1
and solvent were added to a round-bottom flask. Base (1.5 equiv.) was then
added and the resulting solution was allowed to stir at the indicated
temperature (°C) for the indicated time (minutes). Products were purified by
water/dichloromethane extraction. Values in the yield column denote percent
isolated yield. Single crystal X-ray depicted. Hydrogens omitted for clarity. (B)
Demonstration and attributes of the dehydration process.
We found that a variety of substituted thiazole N-
oxides are compatible with the observed reactivity (Table
1
4
). The dehydration reaction proceeds well with alkyl (2-
), aryl (5-12) and heteroaryl (13,14) substitution on the
thiazole substrate. Given that (hetero)aryl group-bearing
substrates are most relevant for conjugated material
synthesis, they were explored in greater detail. Electron-
rich (6), electron-poor (10,11) and electron neutral (5,7)
substituents are all well- tolerated. Additional
heterocycle-containing substrates (13,14) are also
competent participants in the reaction as are substrates
with extended conjugated motifs (9). We can also
successfully access cross-dehydration adducts (15,16),
Condensation Polymerizations reactions are inherently
more challenging, than small molecule reactions, given
the requirement for high conversion to achieve high
[
31]
molecular weight.
We endeavored to further validate
our dehydration strategy by applying it to the synthesis of
conjugated polymers. Difunctional monomers 17, 19 and
2
1 were prepared. Exposure of monomer 17 to base,
under similar reaction conditions employed previously
reaction time was extended to 3 hours to ensure
[
29]
yielding all possible isomers.
It may be important to
(
note the remaining N-oxide functionality can be easily
removed under reducing conditions. For example, 9 was
reduced with zinc dust in biphasic THF/aqueous
maximum conversion), resulted in a rapid polymerization
to polymer 18. The resulting polymer had a number
[
32]
weight average molecular weight of 80 kg/mol.
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Chemistry - A European Journal
10.1002/chem.201800642
COMMUNICATION
O
O
LiOtBu
R
N
R
N
H2O
THF
0 °C-RT
S
S
S
5
min
N
R
O
O
O
Non
O
O
N
S
S
N
N
N
N
N
OMe
S
S
S
S
S
S
S
MeO
S
N
N
N
O
N
2
89%
4
57%
Non
6
81%
3
72%
5
81%
O
O
O
N
N
N
S
N
OMe
CF3
S
S
S
S
S
MeO
S
F3C
S
N
N
N
N
9
98%
10 88%
7
92%
8
92%
O
Hex
O
O
N
S
Hex
O
N
N
CN
N
N
S
S
S
Hex Hex
S
S
S
S
S
S
S
S
S
Hex
Hex
NC
N
S
N
N
N
Hex
Hex
N
13 56%
11
57%
12 86%
1
4
90%
O
N
S
O
N
N
N
S
S
S
S
S
S
N
S
N
N
32% (5:1)
O
15b
N
21% (2:1)
O
16b
1
5a
16a
Table 1. Scope of the base-mediated dehydrative coupling of thiazole N-oxides. Thiazole N-oxide and THF were added to a round-bottom flask and the mixture
cooled in an ice bath. Lithium tert-butoxide (1.0 M in THF) solution was then added dropwise and the resulting solution was allowed to warm to room temperature
and stir for 5 minutes. Products were purified by water/dichloromethane extraction. Values in the percent denote isolated yield. Non, n-nonyl; Hex, n-hexyl.
S
N
S
S
N
Zn dust (5.0 eq)
THF:NH4Cl (0.3M)
exploration of this reaction will enable the transformation
of broader range of heterocycles, ultimately providing a
powerful tool for the synthesis of conjugated materials.
N
N
S
RT
O
17 89%
S
N
N
Zn dust (5.0 eq)
THF:NH4Cl (0.3M)
S
N
N
S
O
EH
EH
O
EH EH
O
N
N
LiOtBu
N
N
S
RT
2
H O
THF
0 °C-RT
quant.
S
S
O
H
S
S
H
n
Mn = 37 kg/mol
Mw = 80 kg/mol
1
8
97%
19
20
O
Figure 3. Deoxygenation of dehydration dimers. N-Oxide dimers were
O
N
N
ODec
S
ODec
S
LiOtBu
dissolved in a 1:1 mixture of THF:NH
reduced dimers.
4
Cl and reacted with Zn dust to yield the
H
THF
S
2
H O
S
H
0
°C-RT
DecO
DecO
quant.
N
n
N
2
1
O
Mn = 43 kg/mol
Mw = 110 kg/mol
2
2
Monomer 19 was polymerized to give 20 with a
number average molecular weight of 43 kg/mol and
weight average molecular weight of 110 kg/mol.
Monomer 21 was polymerized to give 22 with a number
average molecular weight of 17 kg/mol and weight
average molecular weight of 46 kg/mol. Practically, it is
worth noting that without the aid of transition metals, the
O
O
O
N
N
S
N
N
LiOtBu
S
2
H O
S
H
H
THF
S
S
S
0
°C-RT
n
Hex
Hex
quant.
Hex
Hex
Mn = 17 kg/mol
Mw = 46 kg/mol
2
3
24
Figure 4. Dehydration Polymerization. Monomers and THF were added to a
round-bottom flask and the mixture cooled in an ice bath. Lithium tert-butoxide
1.0 M in THF) solution was then added dropwise and the resulting solution
[
33]
polymerization initiates instantly upon addition of base
(
at 0 °C and is complete within 3 hours under ambient
conditions after which polymer products are recovered in
quantitative yields.
was allowed to warm to room temperature and stir for 3 hours. Molecular
weights were determined by gel permeation chromatography relative to
[
34]
polystyrene standards. EH, 2- ethylhexyl; Dec, n-decyl; Hex, n-hexyl.
Coupling reactions that are traditionally employed for
the synthesis of high-performance conjugated materials Acknowledgements
often require transition-metal catalysts, high
We thank NSERC, the University of Waterloo, the Canada
Foundation for Innovation and the Canada Research Chairs
Program (CRC-Tier II, D.J.S.) for financial support. L.V. thanks
NSERC for a postgraduate Scholarship. Jalil Assoud (University
of Waterloo) is gratefully acknowledged for assistance in
obtaining and solving the crystal structure 2 and 5.
temperatures, inert atmosphere and long reaction times to
achieve useful molecular weight polymers. In contrast,
the C–C bond forming dehydration process we describe
requires no transition metals while proceeding rapidly
under ambient conditions. By providing straightforward,
sustainable access to poly(hetero)arene conjugated motifs
our dehydration method may have a substantial impact on Conflict of interest
the many fields that benefit from ready access to
conjugated polymers. We anticipate that further The authors declare no conflict of interest
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Chemistry - A European Journal
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COMMUNICATION
Keywords: dehydration, conjugated polymer, conjugated
[
[
[
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1
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[
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COMMUNICATION
Rafael A. Mirabal, Luke Vanderzwet,
Sara Abuadas, Michael R Emmett,
Derek J. Schipper*
Page No. – Page No.
Dehydration Polymerization for
Poly(hetero)arene Conjugated
Polymers
Parched Polymers: A dehydration reaction allows simple access to conjugated
heterocyclic materials. The reaction exploits readily available substrates, takes
place under ambient conditions, gives rapid and complete conversion and the
resulting products require only simple purification procedures.
This article is protected by copyright. All rights reserved.