Photocontrolled Synthesis of n-Type Conjugated Polymers

Article abstract of DOI:10.1002/anie.201915819

Current approaches to synthesize π-conjugated polymers (CPs) are dominated by thermally driven, transition-metal-mediated reactions. Herein we show that electron-deficient Grignard monomers readily polymerize under visible-light irradiation at room temperature in the absence of a catalyst. The product distribution can be tuned by the wavelength of irradiation based on the absorption of the polymer. Conversion studies are consistent with an uncontrolled chain-growth process; correspondingly, chain extension produces all-conjugated n-type block copolymers. Preliminary results demonstrate that the polymerization can be expanded to donor–acceptor alternating copolymers. We anticipate that this method can serve as a platform to access new architectures of n-type CPs without the need for transition-metal catalysis.

Full text of DOI:10.1002/anie.201915819

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Photocontrolled synthesis of n-type conjugated polymers
Authors: Eliot F. Woods, Alexandra J. Berl, and Julia Ann Kalow
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915819
Angew. Chem. 10.1002/ange.201915819
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10.1002/anie.201915819
Angewandte Chemie International Edition
RESEARCH ARTICLE
Photocontrolled synthesis of n-type conjugated polymers
Eliot F. Woods, Alexandra J. Berl, Julia A. Kalow*[a]
[a]
E. F. Woods, A. J. Berl, Prof. Dr. J. A. Kalow
Department of Chemistry
Northwestern University
2145 Sheridan Rd., Evanston, IL 60208
E-mail: jkalow@northwestern.edu
Supporting information for this article is given via a link at the end of the document.
Abstract: Current approaches to synthesize π-conjugated polymers
(CPs) are dominated by thermally driven, transition metal-mediated
reactions. Herein we show that electron-deficient Grignard monomers
readily polymerize under visible light irradiation at room temperature
in the absence of a catalyst. The product distribution can be tuned by
the wavelength of irradiation based on the absorption of the polymer.
Conversion studies are consistent with an uncontrolled chain-growth
process; correspondingly, chain extension produces all-conjugated n-
type block copolymers. Preliminary results demonstrate that the
polymerization can be expanded to donor-acceptor alternating
copolymers. We anticipate that this method can serve as a platform
to access new architectures of n-type CPs without the need for
transition metal catalysis.
conditions to polymerize phenylene and naphthalene monomers
(Figure 1b).15 Promisingly, this polymerization was hypothesized
to proceed via a chain-growth mechanism.
(a) Cross coupling via thermal SRN1 (Hayashi, 2012)
Ar1
Ar1
Ar2
+
110 °C
Ar2
MgBr
I
- SRN1 mechanism
- High temperature required
(b) Polymerization via SRN1 (Studer, 2012)
I
MgCl
TEMPO
C8H17
C8H17
n
Introduction
- Chain-growth mechanism
- Electron-neutral monomers
π-Conjugated polymers (CPs) are a viable alternative to inorganic
semiconductors in optoelectronic and energy storage applications
where weight or flexibility are predominant concerns.1,2 The
synthesis of these polymers relies on transition metal-mediated
polycondensation, in which a bis-organoelement (Sn, Mg, Zr, B)
monomer undergoes cross-coupling with a complementary bis-
halogen monomer.3 Recently, Pd-catalyzed C–H activation
protocols have been adapted to the synthesis of high-
performance donor-acceptor CPs, eliminating the need for
organoelement pre-activation.4 To gain control over molecular
weight and architecture, researchers have developed catalyst
transfer polycondensation (CTP), a living polymerization relying
on Ni and Pd catalysts that associate strongly with the growing
polymer chain. CTP has been primarily optimized for p-type
systems.5,6 Nevertheless, in all these examples, synthetic
methods for CPs rely on mechanisms and transition metal
catalysts first developed for small-molecule sp2–sp2 cross-
coupling reactions. Unfortunately, even ppm quantities of residual
catalyst can compromise device performance and stability, driving
the need for polymerization conditions that do not rely on
transition-metal catalysts.7-10
(c) Polymerization via oxidative PET (Yagci, 2018)
Et
N
Et
N
–
Ph2I+ PF6
350 nm
n
- Step-growth mechanism
- Electron-rich monomers
- Solubility limits characterization
(d) This work: polymerization via reductive PET
visible light
Ar
Ar
Br
MgBr
LiCl, THF
n
- Photocontrolled
- Electron-deficient monomers
- Chain extension possible
Figure 1. (a) Transition metal-free coupling of aryl Grignard reagents and aryl
halides under thermal conditions. (b) Transition metal-free synthesis of
phenylene CPs through
a
proposed chain-growth mechanism. (c)
Photopolymerization to prepare p-type CPs via oxidative photoinduced electron
transfer (PET). (d) This work: photocontrolled polymerization to prepare n-type
CPs.
Herein we describe a photocontrolled, transition metal-free
synthesis of n-type conjugated polymers that takes advantage of
an inherent property of CPs: their ability to absorb visible light. We
initially sought to develop polymerization conditions for CPs
based on catalyst-free conditions for coupling aryl Grignards and
aryl halides reported by Hayashi and coworkers in 2012 (Figure
1a).11,12 These reactions are proposed to proceed through a
thermally initiated SRN1 pathway via an aryl halide radical
anion.13,14 Shortly thereafter, Studer and coworkers used similar
Given the SRN
1
mechanism proposed by Hayashi, we
hypothesized that electron-deficient monomers might be able to
stabilize the radical anion intermediates and facilitate
polymerization to form n-type CPs. In contrast to the thermal
reactions described by Hayashi and Studer, we have found that
the synthesis of n-type CPs requires photochemical conditions.
Photopolymerizations offer the opportunity for spatiotemporal
control16-18 and selective catalyst activation by different
wavelengths of light.19-21 For non-conjugated polymers,
1
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10.1002/anie.201915819
Angewandte Chemie International Edition
RESEARCH ARTICLE
researchers have developed
a
range of photocontrolled
p-type (carbazole and thiophene) polymers (Figure 1c). Our
method offers complementary access to n-type and donor-
acceptor CPs through a photocontrolled process, which exhibits
chain-growth behavior analogous to free radical polymerization.
polymerizations.22-25 To date, there have been few examples of
photopolymerizations to produce CPs.26-33 The existing methods
rely on strongly oxidizing conditions and produce largely insoluble
Table 1. Optimization and control reactions for photomediated polymerization of 1-MgBr.[a]
C10H21 C10H21
C10H21
C8H17
C8H17
C8H17
1) n-BuLi (0.9 equiv.)
N
N
N
2) MgBr2.Et2O (1.0 equiv.)
N
N
N
N
N
N
Conditions
THF, -78°C, 1 h
Br
Br
Br
MgBr
n
1
1-MgBr
P1
Entry 1[a]
Irradiation
Solvent
Additive
Temperature
Mn (kg/mol)
Ð
Yield
1[b]
None
2:1 PhMe/THF
4 equiv. LiCl
90 ºC
2.8
1.44
20%[c]
2
3
White
White
THF
THF
None
23 ºC
23 ºC
4.1
2.81
1.61
<5%[c]
39%
15 equiv. LiCl
13.7
4
5[d]
6
White
None
None
White
2:1 PhMe/THF
THF
15 equiv. LiCl
15 equiv. LiCl
15 equiv. LiCl
15 equiv. LiCl
23 ºC
23 ºC
60 ºC
60 ºC
12.3
3.1
1.80
1.57
n.d.
37%
<5%[c]
5%[c]
19%
THF
n.d.
6.0
7
THF
1.37
[a]Reactions were performed with 0.045 mmol 1 in sealed vials under static N2 for 24 hours. Irradiated samples were cooled with a fan; dark reactions were wrapped
in foil. P1 was isolated by precipitation into MeOH (2x) and acetone (1x). Mn and Ð were determined by gel permeation chromatography (GPC) in THF at 40 ºC
relative to polystyrene standards, using RI detection. Each entry was performed twice; see SI for details. [b] Conditions in analogy to ref. 12. [c] Polymer could not
be precipitated into MeOH; yield is based on chloroform extraction of crude material after removing solvent and Mn is based on the 480 nm GPC trace. [d]Average
of two separate trials. n.d. = not determined (outside the calibration limit).
polymerization.35, 36 The need for superstoichiometric addition of
LiCl suggests a further contribution to the reaction medium. The
mixed solvent system (toluene/THF) described by Hayashi and
Results and Discussion
coworkers provided similar results to THF alone (entry 4).
Our exploratory studies employed a mono-Grignard reagent, 1-
MgBr, derived from benzotriazole 1 by lithiation and trapping with
magnesium bromide diethyletherate. This monomer was
previously reported to provide soluble n-type homopolymers
When light was excluded from the reaction, trace oligomer was
obtained (entry 5); this suggests that either a thermal background
reaction is terminated at the oligomer stage, or sufficient light is
introduced during Grignard preparation to enable oligomerization.
McNeil and coworkers have previously observed oligomerization
of another electron-deficient Grignard monomer, a thiazole
derivative, in the absence of any transition-metal catalyst, but the
effect of light was not discussed.37 Heating the reaction in the
presence or absence of light proved detrimental (entries 6-7).
under
Ni-catalyzed
catalyst-transfer
polycondensation
conditions.34 Our initial attempt using Hayashi’s thermal
conditions for small-molecule coupling provided only oligomer
(Table 1, entry 1).11 We anticipated that single electron transfer
could be promoted under milder conditions by photoexcitation.
While 1 only absorbs light below 400 nm, 1-MgBr exhibits an
absorption tail in the visible region (see Supporting Information
(SI), Figure S45). Gratifyingly, when the reaction was irradiated
with white LEDs, we observed oligomers of benzotriazole (Mp 2.8
kg/mol; predominantly tetramer by MALDI-TOF, Figure S58)
along with small amounts of higher-molecular-weight P1 (Mp 25.5
kg/mol; Figure S12) (entry 2).
Literature reports of “transition metal-free” cross-coupling
reactions inevitably must account for the high catalytic activity of
Ni and Pd, which can promote reactions even at ppm levels.38-42
ICP-MS of crude reaction mixtures showed <2 ppm Ni and Pd.
Dark control reactions doped with ppm levels of nickel showed
that >50 ppm of nickel was required to achieve comparable
reactivity to the photoinduced polymerization (Table S13).
The addition of lithium chloride (LiCl) as a saturated solution in
THF provided a significant increase in molecular weight (Table 1,
entry 3). After precipitation, P1 was obtained in 39% yield (Mn 13.7,
Ð 1.61 by THF GPC at 40 °C). “Turbo Grignard” reagents of the
stoichiometry RMgCl•LiCl have been shown to benefit from
enhanced reactivity for both small molecule coupling and
We next explored the scope of the polymerization for monomers
with
varying
electronic
properties.
Pleasingly,
the
photopolymerization also proceeds efficiently for thienopyrazine 2
to produce the low-bandgap polymer P2 in 75% yield (Table 2).
2
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10.1002/anie.201915819
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RESEARCH ARTICLE
P2 is a solution-processable near-IR-absorbing CP (Eg 0.93 eV).43
While the measured molecular weight is modest (7.9 kg/mol), this
can be ascribed to the low solubility of P2, which was previously
observed by Rasmussen and coworkers.43 Optimal yields were
observed when we generated the Grignard thienopyrazine 2-
by precipitation into MeOH (2x) and acetone (1x). [b] The Grignard reagent was
prepared by metathesis with iPrMgBr (1 equiv). P2 isolated by precipitation into
MeOH (3x). See SI for details. [c] Polymer could not be precipitated into MeOH;
GPC indicated a bimodal distribution. [d] See SI for replicate experiments.
To learn more about the mechanism of polymerization, we
followed the conversion 1-MgBr and formation of P1 over time by
1H NMR and GPC, respectively (Figure 2a). Aliquots were
quenched with CD3OD; in addition to the quenched monomer 1-
D, we observe mono-debrominated (1-H) and bis-debrominated
(1-HH) monomer. Detailed analysis of the mass balance of the
polymerization reveals that the modest yield of P1 can be
attributed to incomplete conversion; formation of unreactive
monomers by protonation, metathesis, or over-metallation; and
formation of oligomers that are removed during precipitation
(Table S2). The formation of P1 over 7 hours can be tracked by
in-situ UV-vis measurements, which showed a red-shift in the
absorption during the early stages of the reaction, followed by a
continued increase in absorption intensity at 550 nm (Figure 2b,
S26). This behavior is consistent with the increase in conjugation
length observed in the early stages of polymerization. Surprisingly,
even though the concentration of P1 increases over 7 hours, the
molecular weight of P1 rapidly increases during the first 30
minutes of the reaction, followed by a plateau (Figure 2c).47 The
rapid increase and plateau in molecular weight may be indicative
of an uncontrolled chain-growth polymerization, analogous to
free-radical polymerization.48
MgBr by
a milder metathesis protocol. In contrast, the
photopolymerization of electron-neutral fluorene 3-MgBr and
electron-rich thiophene 4-MgBr provided only trace oligomer.44
Thus, the photopolymerization has complementary reactivity to
the most well-developed CTP systems.
C10H21
S
C8H17
n
C8H17
C8H17
N
S
N
N
N
N
n
n
C2H5
C4H9
C2H5
C4H9
C6H13
n
P1
P2
P3
P4
Table 2. Polymerization results and LUMO levels of the conjugated polymers
Polymer
Yield (%)
Mn
(kg/mol)
Ð
LUMO
(eV)
Poly(benzotriazole)
39
75
<5
13.7
7.9
1.61
1.99
1.69
-3.534
-4.143
-2.245
(P1) [a],[d]
Poly(thienopyrazine)
(P2) [b],[d]
To determine whether polymerization is photoinitiated or
photocontrolled, we conducted an “on-off” experiment. While
unproductive conversion of the monomer does occur in the dark
(Figure S27), when we track the formation of P1 by UV-vis, the
red shift consistent with polymer growth only occurs when the light
is on (Figure 2d). We confirmed that the trace oligomers arising
from the dark background reaction do not absorb at 550 nm; thus,
the 550 nm absorption solely corresponds to the photoinduced
reaction. This behavior suggests that the polymerization is
photocontrolled.
Poly(8,8-
5.1
dioctylfluorene) (P3)
[b]
Poly(3-
hexylthiophene)
(P4) [b],[c]
<5
0.5
2.63
-2.746
[a] The Grignard reagent was prepared via lithiation/magnesiation. Reactions
were performed with 0.045 mmol monomer in sealed vials under static N2.
Samples were irradiated with white LEDs and cooled with a fan. P1 was isolated
Figure 2. (a) Concentration of benzotriazole species (BTz) monitored over the course of the polymerization (R = 2-octyldodecyl). (b) Absorbance of reaction mixture
from (a) monitored over the course of the polymerization at 550 nm. (c) Molecular weight and dispersity of P1 measured with conversion of 1. (d) UV-Vis of
polymerization reaction solution over time. Blue to red traces indicate increasing time. Irradiation cycles were 1 hour long. Dashed traces indicate the reaction had
been protected from light for the previous hour.
For a chain-growth process involving a photoinduced electron
transfer step, the photoexcited species could be the monomer or
the chain. If the chain is photoexcited, the polymerization
represents an unusual scenario wherein the optical properties of
the substrate are changing during the reaction. Thus, changing
the wavelength of light used for the polymerization could change
3
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10.1002/anie.201915819
Angewandte Chemie International Edition
RESEARCH ARTICLE
the product composition (that is, for a homopolymer, molecular
weight or dispersity). Indeed, when 1-MgBr was polymerized
using LEDs with different emission wavelengths, there were clear
differences in peak molecular weights of P1 (Figure 3). LEDs
centered at both 400 nm and 470 nm produced lower molecular
weight polymers than white LEDs, with peak molecular weights
(Mp) of 10 kg/mol. The highest molecular weight was seen with
525 nm LEDs, a wavelength corresponding to the absorption tail
of P1, albeit at the expense of yield. When the
homopolymer is not observed by MALDI-TOF or GPC. As a
control, performing an analogous chain-extension experiment
with white light produced only homopolymer P2 and low-
molecular-weight P1 (Figure S34-35). These experiments
suggest that excitation of growing chains, rather than monomer,
is involved in polymerization.
(a)
photopolymerization was attempted with 626 nm light,
wavelength P1 does not absorb, no reaction was observed.
a
S
S
Br
MgBr
n
67% yield
Mn :7.9 kg/mol
Ð: 1.77
THF, LiCl, 24h
626 nm LEDs
N
R
N
R
N
R
N
R
2-MgBr
P2
(b)
S
S
X
X
1-MgBr
m
n
n
2-MgBr
THF, LiCl, 19 h
626 nm LEDs
THF, LiCl, 24 h
626 nm LEDs
N
R
N
N
R
N
N
N
N
R'
R
R
P2 (X = Br, MgBr)
P5
Scheme 1. (a) Polymerization of 2-MgBr using 626 nm LEDs. (b) In situ chain
extension from P2 with 1-MgBr. R = 2-ethylhexyl; R’ = 2-octyldodecyl.
Figure 3. GPC traces of the products obtained from the polymerization of 1-
MgBr using LEDs with different emission wavelengths.
The polymerization of 2-MgBr also shows wavelength
dependence, but with more subtle differences due to the solubility
limitations (Figure S38). Interestingly, 2-MgBr can be
polymerized using 626 nm light (Scheme 1a). P2 absorbs light
across the visible spectrum and into the NIR (SI, Figure S47). This
absorption of longer wavelength light is likely the reason that 2-
MgBr can be polymerized using 626 nm LEDs while 1-MgBr
cannot.
Figure 4. GPC trace of polymer P5 before and after addition of 1-MgBr with RI
detection (left) and 890 nm detection (right).
We envisioned that the differing reactivity of 1-MgBr and 2-MgBr
under 626-nm irradiation could be exploited to prepare novel all-
conjugated block copolymers. If the photopolymerization does
indeed proceed by a chain-growth mechanism, it will be possible
to extend a homopolymer chain by adding additional monomer.
Moreover, if the reactivity indeed depends on excitation of the
growing polymer chain, we will be able to extend a low-bandgap
chain with higher HOMO-LUMO gap monomer using the longer
wavelength of light.
This result, in combination with the molecular weight vs.
conversion studies, suggests that the polymerization proceeds
through a chain-growth mechanism. Unfortunately, termination
pathways, which are not yet understood, render the
polymerization uncontrolled, in analogy to free radical
polymerization. Chain-growth polymerizations are rare for n-type
conjugated polymers.34, 37, 43, 49-54 If the control can be improved
by accelerating initiation relative to propagation and eliminating
termination reactions, this reaction manifold affords the
opportunity for sequence and architecture control.55, 56
To test this hypothesis, we designed
a chain extension
experiment (Scheme 1b). The polymerization of 2-MgBr was
initiated with 626 nm LEDs. When 2-MgBr was consumed, after
nineteen hours, 1-MgBr was added and irradiation with 626 nm
was continued. As 1-MgBr does not homopolymerize under these
conditions, any polymerization of 1-MgBr is chain extension of P2.
GPC traces before and after the addition of 1-MgBr exhibit
monomodal peaks, providing evidence of successful chain
extension (Figure 4). 1H NMR (Figure S33) confirms incorporation
Finally, we have preliminary evidence that the scope of this
photopolymerization can extend to donor-acceptor alternating
copolymers. We prepared several ABA-type monomers and
subjected them to the reaction conditions, using either the
lithiation/magnesiation or Grignard metathesis protocol (Figure 5).
Donor-acceptor alternating polymers were obtained, albeit with
modest molecular weights for the soluble fractions. MALDI-TOF
MS analysis of the insoluble fractions reveals higher molecular
weight species, suggesting that the insolubility of these polymers
in room-temperature THF limits their molecular weight.
1
of both 1 and 2, while H-DOSY confirms that these monomers
reside in the same polymer (Table S5). The absorption spectrum
of copolymer P5 provides additional evidence of a block structure.
P2 is a NIR absorber with λmax 1010 nm (Figure S47), but
statistical copolymers of 1 and 2 lose this NIR absorption (Figures
S47-S48). P5 retains this absorption, suggesting that the
copolymer contains an uninterrupted block of P2. P1
4
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10.1002/anie.201915819
Angewandte Chemie International Edition
RESEARCH ARTICLE
C10H21
[3] Xu, S.; Kim, E. H.; Wei, A.; Negishi, E.-i. Pd- and Ni-catalyzed cross-coupling
reactions in the synthesis of organic electronic materials. Sci. Technol.
Adv. Mat. 2014, 15, 044201.
C8H17
O
C10H21
S
C8H17
N
N
N
N
S
S
S
S
n
N
N
[4] Bura, T.; Blaskovits, J. T.;Leclerc, M. Direct (Hetero)arylation Polymerization:
Trends and Perspectives. J. Am. Chem. Soc. 2016, 138, 10056-10071.
[5] Leone, A. K.;McNeil, A. J. Matchmaking in Catalyst-Transfer
Polycondensation: Optimizing Catalysts based on Mechanistic Insight.
Acc. Chem. Res. 2016, 49, 2822-2831.
n
N
S
S
C12H25
C12H25
n
O
C8H17
C10H21
P6
P7
P8
37% yield[a]
Mn: 1.6 kg/mol[c]
Ð: 1.19
46% yield[b]
Mn: 3.3 kg/mol[c]
Ð: 1.43
19% yield[a]
Mn: 2.4 kg/mol
Ð: 1.16
[6] Leone, A. K.; Mueller, E. A.; McNeil, A. J. The History of Palladium-Catalyzed
Cross-Couplings Should Inspire the Future of Catalyst-Transfer
Polymerization. J. Am. Chem. Soc. 2018, 140, 15126-15139.
[7] Krebs, F. C.; Nyberg, R. B.; Jørgensen, M. Influence of Residual Catalyst
on the Properties of Conjugated Polyphenylenevinylene Materials:ꢀ
Palladium Nanoparticles and Poor Electrical Performance. Chem. Mater.
2004, 16, 1313-1318.
Figure 5. Donor-acceptor-donor monomers compatible with the polymerization.
See SI for replicate and control experiments. [a] Generation of Grignard through
lithium–halogen exchange followed by quench with magnesium bromide
diethyletherate. [b] Generation of Grignard via metathesis with
isopropylmagnesium bromide. [c] THF-insoluble material obtained from the
reaction mixture was analyzed by MALDI-TOF MS (Figure S61-S63).
[8] Usluer, Ö.; Abbas, M.; Wantz, G.; Vignau, L.; Hirsch, L.; Grana, E.; Brochon,
C.; Cloutet, E.; Hadziioannou, G. Metal Residues in Semiconducting
Polymers: Impact on the Performance of Organic Electronic Devices.
ACS. Macro Lett. 2014, 3, 1134-1138.
Conclusion
[9] Kuwabara, J.; Yasuda, T.; Takase, N.; Kanbara, T. Effects of the Terminal
Structure, Purity, and Molecular Weight of an Amorphous Conjugated
Polymer on Its Photovoltaic Characteristics. ACS Appl. Mater. Interfaces
2016, 8, 1752-1758.
In summary, we have developed a catalyst-free, visible-light-
mediated photopolymerization to prepare n-type and donor-
acceptor CPs. Conversion and chain extension studies of the
polymerization are consistent with an uncontrolled chain-growth
process. The wavelength dependence of the polymerization
represents a new opportunity to exploit the inherent properties of
CPs, tailoring the reaction conditions to the target material. Future
work will focus on elucidation of the reaction mechanism and
preparation of new n-type CP architectures accessible through
this method.
[10] Bannock, J. H.; Treat, N. D.; Chabinyc, M.; Stingelin, N.; Heeney, M.; de
Mello, J. C. The influence of polymer purification on the efficiency of
poly(3-hexylthiophene):fullerene organic solar cells. Sci. Rep. 2016, 6,
23651.
[11] Shirakawa, E.; Hayashi, Y.; Itoh, K.-i.; Watabe, R.; Uchiyama, N.;
Konagaya, W.; Masui, S.; Hayashi, T. Cross-Coupling of Aryl Grignard
Reagents with Aryl Iodides and Bromides through SRN1 Pathway. Angew.
Chem. Int. Ed. 2012, 51, 218-221.
[12] Shirakawa, E.; Okura, K.; Uchiyama, N.; Murakami, T.; Hayashi, T.
Improved Procedure for Single-electron-transfer-induced Grignard
Cross-coupling Reaction. Chem. Lett. 2014, 43, 922-924.
[13] Uchiyama, N.; Shirakawa, E.; Hayashi, T. Single electron transfer-induced
Grignard cross-coupling involving ion radicals as exclusive intermediates.
Chem. Commun. 2013, 49, 364-366.
Acknowledgements
This work was supported by funding from the Air Force Office of
Scientific Research Young Investigator Program (FA9550-18-1-
0159), a 3M Non-Tenured Faculty Award, and the National
Science Foundation Graduate Research Fellowship Program
(A.J.B., DGE-1842165). The authors thank the Harris laboratory
for use of their UV-vis spectrophotometer, the Chen laboratory for
the use of their CCD array detector, and Drs. Jacob Ishibashi and
Pradipta Das for helpful discussions. This work made use of NMR
and MS instrumentation at the Integrated Molecular Structure
Education and Research Center (IMSERC) at Northwestern,
which has received support from the NSF (NSF CHE-9871268);
Soft and Hybrid Nanotechnology Experimental (SHyNE)
Resource (NSF ECCS-1542205); the State of Illinois; and the
International Institute for Nanotechnology. Metal analysis was
performed at the Northwestern University Quantitative Bio-
element Imaging Center generously supported by NASA Ames
Research Center NNA06CB93G.
[14] Haines, B. E.; Wiest, O. SET-Induced Biaryl Cross-Coupling: An SRN
1
Reaction. J. Org. Chem. 2014, 79, 2771-2774.
[15] Murarka, S.; Studer, A. Radical/Anionic SRN1‐Type Polymerization for
Preparation of Oligoarenes. Angew. Chem. Int. Ed. 2012, 51, 12362-
12366.
[16] Discekici, E. H.; Pester, C. W.; Treat, N. J.; Lawrence, J.; Mattson, K. M.;
Narupai, B.; Toumayan, E. P.; Luo, Y.; McGrath, A. J.; Clark, P. G.; Read
de Alaniz, J.; Hawker, C. J. Simple Benchtop Approach to Polymer Brush
Nanostructures Using Visible-Light-Mediated Metal-Free Atom Transfer
Radical Polymerization. ACS. Macro Lett. 2016, 5, 258-262.
[17] Pester, C. W.; Narupai, B.; Mattson, K. M.; Bothman, D. P.; Klinger, D.; Lee,
K. W.; Discekici, E. H.; Hawker, C. J. Engineering Surfaces through
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Entry for the Table of Contents
§
§
§
n-type polymers
catalyst free
one-pot chain
extension
Electron-poor Grignard monomers can be photopolymerized to form n-type conjugated polymers. Our investigations show that the
polymerization is chain-growth in nature, molecular weight can be tuned through the wavelength of irradiation, and all-conjugated
block copolymers can be synthesized in one pot by selective irradiation.
Institute and/or researcher Twitter usernames: juliakalow
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