Junction-Controlled Topological Polymerization

Article abstract of DOI:10.1002/anie.201713026

Methodology that enables the controlled synthesis of linear and branched polymers from an identical monomer will be a novel pathway for polymer synthesis and processing. Herein we first describe the control of one or both of the C(3)-C(3′) and C(6)-C(6′) coupling reactions of carbazolyl. In a second approach, an identical monomer containing two carbazolyls is polymerized using chemical and electrochemical oxidizers, leading to topologically controllable growth of linear polymers in weak oxidizer or of cross-linked polymer chains in strong oxidizer, with satisfactory long chain propagation of step growth polymerization (M_n=6.0×10⁴ g mol^?1, M_w/M_n=2.3). This very simple polymerization with cheap reagents and low levels of waste has provided a flexible pathway for synthesis and processing of polymers.

Full text of DOI:10.1002/anie.201713026

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Accepted Article
Title: Junction-Controlled Topological Polymerization
Authors: Mao Li, Shusen Kang, Jia Du, Jian Zhang, Jinxin Wang, and
Katsuhiko Ariga
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201713026
Angew. Chem. 10.1002/ange.201713026
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10.1002/anie.201713026
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Junction-Controlled Topological Polymerization
Mao Li,*[a] Shusen Kang,[a] Jia Du,[a] Jian Zhang,[a] Jinxin Wang,[a] Katsuhiko Ariga[b]
Abstract: Methodology that enables controlled synthesis of linear
and branched polymers from an identical monomer will present a
novel pathway for polymer synthesis and processing. In this paper,
firstly, controllable one or both of C(3)-C(3') and C(6)-C(6') coupling
reactions of carbazolyl are described. In the second approach, an
identical monomer containing two carbazolyls is polymerized using
chemical and electrochemical oxidizers, leading to topologically
controllable growth of linear in weak oxidizer or cross-linked polymer
chain in strong oxidizer, with satisfactory long chain propagation of
step growth polymerization (Mn = 6.0×104 g mol-1, Mw/Mn = 2.3). This
very simple polymerization with cheap reagents and low levels of
waste has provided a flexible pathway for synthesis and processing
of polymers.
product if its oxidative reaction is carefully carried out in weak
oxidizer medium (FeCl3) or at low oxidative potential (c.a. 1.0 V
vs. Ag/Ag+) (Figure S1-3), probably due to trapping of 9,9'-
dihexyl-3,3'-bicarbazolyls radicals could appear in N-C-C-N
delocalization region that effectively disable coupling at the 6,6'-
positions because it could be most stable among other
resonance structures (Figure S4). Multiple oligomers and
polymers appeared in strong oxidizer medium ((NH4)2S2O8) or at
high oxidative potential (c.a. > 1.2 V vs. Ag/Ag+) (Figure S2-3).
Though oxidative products of carbazolyls have been studied
before,3 structural definition has been remaining difficult because
their polymers are generally insoluble with inadequate
characterizations3g,h
and
unknown
dependence
of
electrochemical potential.3i,j,k,4 Herein, we have first provided the
effect of oxidative strength on N-alkylcarbazolyl reactions.
The topology of macromolecules is an essential parameter
to govern their properties and functions.1 Driven by the need of
novel polymeric materials with sophisticated properties and
performances, tremendous efforts on the design and synthesis
of well-defined polymers with controllable architectures and
topologies have been a central theme in the research area of
polymer science in the past three decedes.2 For step growth
polymerization, a linear polymer is prepared from monomers
with two reactive sites, and a branched polymer is prepared from
monomers with more than two reactive sites. Methodology that
enables controlled synthesis of linear and branched topological
polymers from an identical monomer will offer a flexible pathway
for polymer synthesis and processing. However, starting an
identical monomer strategy, for controlling polymeric topology by
simple reaction conditions to afford linear and cross-linked
polymeric materials with controllable molecular weight, has been
an enduring ongoing synthetic challenge in polymer science and
technology. In this paper, by marrying the controllable radical C-
C coupling reactions (Figure 1A), starting from an identical
monomer, both chemical and electrochemical radical initiated
step growth polymerization is developed. This polymerization
can prepare the polymers with different primary structures with
linear or cross-linked backbone via one or both of C(3)-C(3') and
C(6)-C(6') coupling reactions of carbazolyl in presence of weak
or strong chemical or electrochemical oxidizer (Figure 1B).
C-C Coupling Control of Carbazolyl
Figure 1. (A) C-C coupling mechanism of carbazolyl; (B) radical initiated
topological polymerizations of monomer containing two carbazolyl.
Topologically Controlled Polymerizations
The reaction conditions for general step growth
polymerization can be drastic (high temperatures and without
oxygen and water), which could affect the properties of desirable
functional building blocks. To marry mild radical C-C coupling
reactions of carbazolyls with chain propagation of step growth
polymerization will be interesting for synthesis of polymers.
Based on sole C(3)-C(3') coupling reaction, α,ω-bis(N-
carbazolyl)hexane (Figure 2a) has two carbazolyls, whose
radicals can react with other carbazolyls of monomers and any
size oligomers, and form 3,3'-bicarbazolyl inter-connected
polymers (Figure 2b). Via chemical radical initiated step growth
polymerization in FeCl3/CHCl3 medium, rapid loss of monomer
α,ω-bis(N-carbazolyl)hexane early in the reaction can be
confirmed by absorption peak intensity of 295 nm in reaction
system (Figure S5), which is one of typical features of step
growth polymerization. In the reaction system of FeCl3/MeCN,
only insoluble dimer of α,ω-bis(N-carbazolyl)hexane (Figure 2c)
can be found as precipitated phase, most probably due to
difficult coupling dynamics of carbazolyl in solid state.
We firstly demonstrate that the sole formation of 9,9'-
dihexyl-3,3'-dicarbazolyls can be considered to be the only
[a]
[b]
Prof. Dr. M. Li, Mr. S. Kang, Miss. J. Du, Mr. J. Zhang, Mr. J. Wang
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry (CIAC)
Changchun 130022, China
E-mail: limao@ciac.ac.cn
Prof. Dr. K. Ariga
World Premier International Research Center for Materials
Nanoarchitectonics
National Institute for Materials Science (NIMS)
Tsukuba 305-0044, Japan
Graduate School of Froniter Science
The University of Tokyo
Kashica 277-0827, Japan
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201713026
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Interestingly, this dimer in FeCl3/CHCl3 with good solubility can
further react via C(3)-C(3') coupling reactions of solitary
carbazolyls at both ends of dimer and produce precipitate
poly(α,ω-bis(N-carbazolyl)hexane) as well as polymerization of
polymers, Đ of polymers in precipitated phase is suggested to
be narrow with the assistance of solubility limitation of polymers.
α,ω-bis(N-carbazolyl)hexane
in
FeCl3/CHCl3.
Via
electrochemical radical initiated step growth polymerization of
α,ω-bis(N-carbazolyl)hexane and its dimer at low oxidative
potential of 1.0 V, poly(α,ω-bis(N-carbazolyl)hexane) film as
product can be formed on indium tin oxide (ITO) (Figure S6). In
principle, dimer or other any size oligomer of α,ω-bis(N-
carbazolyl)hexane has two unreactive carbazolyls at ends of
chain with further coupling possibility.
Figure 3. (A) Structure of monomer, and percent conversions of monomer and
molecular weight of polymers measured by GPC as a function of reaction time
via one (a,b) or both (c,d) of C(3)-C(3') and C(6)-C(6') couplings of
carbazolyls; (B) Topological controllable synthesis: short linear polymer (a)
and its further polymerization with additional monomer, cross-linked
polymerizations from initial monomer (c) and short linear polymer (d).
Figure 2. Chemical and electrochemical initiated radical step growth
polymerizations of α,ω-bis(N-carbazolyl)hexane via C(3)-C(3') coupling of
carbazolyl.
As for reaction shown in Figure 3B-a,b, the linear
polymerization was terminated after molecular weight reached
Mn = 7300 g mol-1. This polymer purified can further react with
monomer again and provide the polymer with high molecular
weight (Mn = 33400 g mol-1), indicating that this polymerization of
polymer can be further activated. In strong oxidizer medium (c.a.
(NH4)2S2O8 and >1.2 V), the identical monomer can have four
reactive sites on C(3) and C(6) positions of two carbazolyls.
Thus, there is no surprising that the cross-linked polymer can be
obtained from identical monomer or linear polymer (Figure 3B-
c,d). Similarly, the linear polymers synthesized via chemical
C(3)-C(3') coupling can react with each other via electrochemical
C(6)-C(6') coupling on surface of ITO electrode at 1.5 V. After
washing any unreactive linear polymer with CHCl3 solvent, the
insoluble (cross-linked) polymeric film can be observed in AFM
images on ITO substrate (Figure 4A-C) and TEM images as a
presence of free-standing film (Figure 4D,E), which could have a
great potential for surface active film applications.
In order to synthesize the soluble polymer and define the
structures of polymers, two solutions were taken into account:
(1) N-hexylcarbazolyl as end-terminating reagent was
added into reaction in order to terminate the long term
polymerization and reduce the molecular weight (m<n, Figure
2d). This synthesis of soluble polymer can also imply that C(3)-
C(3') coupling of carbazolyl should be only reaction, otherwise,
no soluble polymer could be obtained.
(2) Long alkyl chain was introduced into monomer (Figure
3A). As reaction time lasting, rapid loss of monomer early in the
reaction can be observed (Figure 3A-a), and linear molecular
weight (Mn, measured by GPC) slowly increases and reaches
54700 g mol-1 after 40 h and 59800 g mol-1 after 1752 h (73
days), respectively (Figure 3A-b), with dispersity (Đ) of 2.1 ~ 2.3.
There is relatively sharp increase in molecular weight at 3 ~ 5 h,
probably due to rapid consumption of short polymers, and the
slow increase in molecular weight after 5 h probably because of
lack presence of short polymers. For more reactive time, the
molecular weight of polymer slowly increases, and more
precipitated phase can be observed, demonstrating that the
molecular weight of polymer has reached the limitation of
solubility. In the presence of (NH4)2S2O8, the polymerization via
both C(3)-C(3') and C(6)-C(6') couplings show more rapid loss of
the monomer and rapid increase of molecular weight within 2 h
(Figure 3A-c,d). This reaction does not run for long time because
of solubility limitation. In this paper, all polymerizations did run in
non-purified CHCl3 and they are indeed insensitive to oxygen
and water.5 Though Đ of 2.1~2.3 was obtained from soluble
Synthesis and Process of Electrochromic Polymers
3,3'-bicarbazolyl unit is better transparent in the visible and
infrared regions (Figure S7), and does not impair the optical
properties of additional functional building blocks. Thus, N-
alkylcarbazolyl could be a universal reactive site to incorporate
various building blocks with good oxidative stability into
polymers. As shown in Figure 5, linear PDI containing polymer
has similar absorption to monomer in solution, and the
absorption of dipped polymer film show a broad absorption peak
due to strong π-π* interaction of PDI in solid state.6 Similarly to
experiment in Figure 4, the cross-linked polymer film can be
prepared via electrochemical C(6)-C(6') coupling of spin-coated
linear polymers on surface of ITO electrode at 1.5 V. The cross-
linked polymer as 2D free-standing film can be observed in TEM
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10.1002/anie.201713026
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images (Figure 5C). Electrochromic function can be
demonstrated by in situ spectroelectrochemistry, that the
absorption of linear and cross-linked polymers can be enhanced
through negatively charged state of PDI. Though there is clear
absorption shoulder peak at 350 nm (Figure 5E) owning to
polycarbazole species after electrochemical C(6)-C(6') coupling,
the electrochromic behavior at visible absorption region remains.
Beyond the one-step synthesis of functional polymers, this
polymerization can also take place at nanoscale and enable to
preprogram the dimension of 1D gold nanorods (Figure S8).
Figure 5. (A) Synthesis of linear PDI containing polymer and its cross-linked
polymer; (B) Absorption spectra of monomer (a, 4 mM) and linear polymer (b,
2 mM) in solution, and dipped linear polymer film (c); (C) TEM images of
cross-linked polymer film (scale bar: 0.5 µm (left) and 5 µm (right)); In situ
spectroelectrochemistry of dipped linear polymer before (D) and after (E)
electrochemical C(6)-C(6') coupling on ITO in 0.1 M TBAPF6 acetonitrile.
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Figure 4. AFM images of ITO electrode (A), spin-coated linear polymer film of
150 nm (B), and cross-linked film (C) after further electrochemical oxidation at
1.5 V and CHCl3 rinsing; TEM images of cross-linked polymer nanofilms (D,E).
Scale bar: 2 µm.
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In conclusion, this paper has showed a junction controlled
topological
polymerization
via
both
chemically
and
electrochemically initiated C(3)-C(3') and C(6)-C(6') couplings of
carbazolyls. Taking advantages of versatile modification at N-
position of carbazole, this very simple and general
polymerization with cheap reagents and low levels of waste as
novel conceptual methodology has produced a revival of interest
in oxidative C-C coupling reaction, and also offered a novel
pathway for synthesis and processing of functional polymers.
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The authors are grateful to Prof. Christopher W. Bielawski
(Ulsan National Institute of Science and Technology), Prof.
Dongmei Cui and Prof. Jun Liu (CIAC), Dr. Shinsuke Ishihara
(NIMS) for useful discussions. This work was supported by the
National Natural Science Foundation of China (21374115,
51573181, 21774121), the Hundred Talents Program, CAS,
China.
Keywords: topological control, step polymerization, C-C
coupling, C-H bond activation, radical coupling
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[4]
N-(4-bromophenyl)carbazole after electrolysis at both 1.0 and 1.5 V
gives only dimer probably due to good delocalization of dimer dication,
which are difficult for further coupling of bicarbazole.
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Mao Li,* Shusen Kang, Jia Du, Jian
Zhang, Jinxin Wang, Katsuhiko Ariga
Page No. – Page No.
Junction-Controlled Topological
Polymerization
Text for Table of Contents
By marrying radical C-C coupling reactions of carbazoles, junction-controlled topological
polymerization was developed. This very simple and general polymerization with cheap reagents and
low levels of waste has produced a novel pathway for synthesis and processing of polymers.
This article is protected by copyright. All rights reserved.