Supramolecular Polymerization Controlled through Kinetic Trapping

Article abstract of DOI:10.1002/anie.201709797

A method of controllable supramolecular polymerization through kinetic trapping is developed. To this end, two bifunctional monomers with cucurbit[7]uril (CB[7]) and adamantane end groups were synthesized. The CB[7]-containing monomer was presaturated with a pH-responsive competitive guest for kinetic control. Then, the kinetics of supramolecular polymerization of the two monomers was easily controlled through the modulation of pH. As a result, supramolecular polymerization was kinetically trapped at certain stages, and supramolecular polymers with different molecular weights were obtained. It is anticipated that this research will enrich the methods of controllable supramolecular polymerization.

Full text of DOI:10.1002/anie.201709797

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Accepted Article
Title: Supramolecular Polymerization Controlled through Kinetical
Trapping
Authors: Hao Chen, Zehuan Huang, Han Wu, Jiang-Fei Xu, and Xi
Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709797
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http://dx.doi.org/10.1002/ange.201709797

Angewandte Chemie International Edition
10.1002/anie.201709797
COMMUNICATION
Supramolecular Polymerization Controlled through Kinetic
Trapping
Hao Chen, Zehuan Huang, Han Wu, Jiang-Fei Xu, and Xi Zhang*
Abstract:
A
new method of controllable supramolecular
6
bifunctional monomers, namely C CB[6]-bis-CB[7] and PEG-bis-
polymerization through kinetic trapping is developed. To this end,
two bifunctional monomers with cucurbit[7]uril (CB[7]) and
adamantane end groups were synthesized. The CB[7]-containing
monomer was presaturated with a pH-responsive competitive guest
for kinetic control. Then, the kinetics of supramolecular
polymerization of the two monomers was easily controlled through
the modulation of pH. As a result, supramolecular polymerization
was kinetically trapped at certain stages, and supramolecular
polymers with different molecular weights were obtained. It is
anticipated that this research will enrich the methodology of
controllable supramolecular polymerization.
DMAD, with cucurbit[7]uril (CB[7]) and N,N-dimethyl
adamantanyl ammonium (DMAD) end groups (Scheme 1).
Strong host-guest interaction between CB[7] and DMAD motif
guarantees
the
thermodynamic
stability
of
desired
supramolecular polymers even under competitive binding
[10]
condition. To realize controlling over polymerization kinetics, a
-
[11]
pH-responsive competitive guest, V-bis-C
5
COO ,
was
introduced into supramolecular polymerization process through
[
12]
its complexation with C
6
CB[6]-bis-CB[7]. By increasing pH, the
-
decreased proportion of protonated V-bis-C
5
COO could result in
the deceleration of polymerization kinetics. Through control over
the kinetics of polymerization, supramolecular polymerization
could be kinetically trapped at certain stages, and
supramolecular polymers with different molecular weights could
be obtained.
Supramolecular polymers are a kind of polymers that monomeric
[1]
units are connected by directional non-covalent interactions.
Dynamic nature of non-covalent interactions endows
supramolecular polymers with attractive attributions including
The instant supramolecular polymerization of C
CB[7] and PEG-bis-DMAD was characterized. As shown in
Figure 1, by mixing C CB[6]-bis-CB[7] and PEG-bis-DMAD 1:1
6
CB[6]-bis-
[2]
self-healing ability, stimuli responsiveness and processability.
Rational control over supramolecular polymerization is critical for
advancing the methodologies for the preparation of
supramolecular polymers with controllable properties. For this
purpose, the methods of supramolecular polymerization from
controllable fabrication to living polymerization have been
6
in molar ratio, the proton peaks of PEG-bis-DMAD ascribed to
the DMAD groups at 2.39, 2.23, 1.79 and 1.72 ppm exhibited
upfield shifts to 1.60, 1.43, 1.29 and 0.95 ppm. This data
indicates the successful encapsulation of DMAD motif into CB[7]
[3]
developed, such as chain-growth polymerization, seeded living
6
cavity of C CB[6]-bis-CB[7]. In addition, the thermodynamics
[4]
[5]
polymerization,
polymerization of supramonomers,
self-
behind the host-guest complexation was further investigated by
employing isothermal titration calorimetry (ITC) and multistep
competition experiments (Figure s11 and Table s1). The binding
constant between CB[7] and DMAD motifs was determined as
[6]
[7]
sorting strategies and interfacial polymerization . It should be
pointed out that the kinetic control over supramolecular
polymerization is playing an increasingly important role for
fabricating supramolecular polymers.
1
4
-1
1.4 × 10 M , which is sufficient to drive the supramolecular
polymerization. Furthermore, diffusion-ordered NMR
Herein, we introduce
a new method of controllable
supramolecular polymerization through kinetic trapping. For a
given supramolecular system, different kinetically trapped states
can provide a variety of supramolecular architectures and
spectroscopy (DOSY) was also utilized to characterize the
obtained supramolecular polymers (Figure s12). Compared with
[8]
functions. Many kinetically trapped supramolecular polymers
produced through pathway selection have been reported, which
[9]
greatly enrich the diversity of supramolecular polymers. During
the process of supramolecular polymerization, the structure of
supramolecular polymers keeps evolving before the system
reaches its minimum of energy landscape. Therefore we
anticipate that it can provide a new controllable method of
supramolecular
polymerization
through
trapping
a
supramolecular polymerization system en route to the
thermodynamic equilibrium by controlling the polymerization
kinetics.
As a proof of concept, we designed and prepared two
[*]
H. Chen, Z. Huang, H. Wu, Dr. J.-F. Xu, Prof. X. Zhang
Key Lab of Organic Optoelectronics & Molecular Engineering,
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
Scheme 1. Schematic illustration of supramolecular polymerization controlled
E-mail: xi@mail.tsinghua.edu.cn
through kinetic trapping. The polymerization kinetics of C
PEG-bis-DMAD can be well tuned by introducing V-bis-C COO as a pH-
responsive competitive guest. Supramolecular polymers could be kinetically
trapped at certain states through the modulation of pH.
6
CB[6]-bis-CB[7] and
-
Supporting information for this article is given via a link at the end of
the document.
5
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Angewandte Chemie International Edition
10.1002/anie.201709797
COMMUNICATION
between CB[7] and DMAD motif, competitive replacement of V-
-
-
5 6 5
bis-C COO from its complex C CB[6]-bis-CB[7]/2V-bis-C COO
by PEG-bis-DMAD should be feasible in terms of
thermodynamics.
We tried to monitor the process of supramolecular
polymerization of C
presence of V-bis-C
polymerization, PEG-bis-DMAD was mixed with the complex of
6
CB[6]-bis-CB[7] and PEG-bis-DMAD in the
-
5
COO . In
a
typical procedure of
-
6 5 2
C CB[6]-bis-CB[7]/2V-bis-C COO in D O at pD 9.7. The
competitive replacement and the process of supramolecular
1
polymerization were firstly monitored by employing H NMR. As
shown in Figure 2a, upon the evolution of time, the proton peaks
ascribed to bonded DMAD groups at 2.76, 1.43 and 0.95 ppm
gradually showed up, accompanied by the diminishing of free
-
DMAD proton signals. Meanwhile, free V-bis-C
5
COO peaks
appeared at 9.13 ppm and 8.55 ppm, followed by the
1
Figure 1. H NMR spectra of a) C
6
CB[6]-bis-CB[7] (blue squares denote the
-
diminishing of bonded V-bis-C
5
COO viologen peaks at 8.96 and
signals of CB[7] motif and red cycles denote the signals of CB[6]), b) PEG-bis-
DMAD and c) 1:1 mixture of PEG-bis-DMAD and C CB[6]-bis-CB[7].
7
.03 ppm. These results indicated that the competitive
6
-
replacement of V-bis-C
CB[7]/2V-bis-C
5
COO from the complex C
COO occurred successfully followed by the
6
CB[6]-bis-
-
5
desired supramolecular polymerization.
6
the diffusion coefficients of C CB[6]-bis-CB[7] and PEG-bis-
-
10
-11
2
-1
DMAD of 1.6 × 10 and 9.0 × 10 m ·s , the self-assembled
species demonstrated a much lower diffusion coefficient of 3.4 ×
It is worth noting that the above polymerization process
lasted for more than 100 h, which is far slower than the time
required for direct polymerization of C CB[6]-bis-CB[7] and PEG-
6
-
11
2
-1
10
m ·s , suggesting the formation of supramolecular
polymers.
bis-DMAD. Through the integration of the corresponding proton
peaks, the relationship between the conversion and time was
investigated in detail as shown in Figure 2b (black line). This plot
was fitted with first-order kinetics to obtain a rate constant of 9.7
To control the kinetics of the above supramolecular
polymerization, pre-saturation of C CB[6]-bis-CB[7] by V-bis-
COO is an essential step. The host-guest interaction between
6
-
C
C
5
-
6
-1
-
× 10 s . According to the reference and the model experiment
6
5
CB[6]-bis-CB[7] and V-bis-C COO was confirmed by
[11]
-
1
in Figure s15,
the slow dissociation of V-bis-C COO from
5
employing H NMR (Figure s14). Considering that the binding
constant between viologen motif and CB[7] ranges from 10 to
5
CB[7] cavity of C CB[6]-bis-CB[7] should be the rate-determining
6
7
-1 [13]
step of the modulated supramolecular polymerization process.
10 M , which is significantly lower than the binding constant
1
Figure 2. a) H NMR spectra of the supramolecular polymerization system recorded at varied time. b) Time dependent end group conversion and diffusion
coefficient evolution of the supramolecular polymers. c) AsF-FFF elution curves of the supramolecular polymers obtained at varied time. d) Time dependent
increase in molecular weight of the supramolecular polymers determined by employing AsF-FFF. e) Kinetic trapping of the supramolecular polymerization system
by tuning pD.
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Angewandte Chemie International Edition
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COMMUNICATION
To further characterize the propagation of the
supramolecular polymers, DOSY and asymmetric flow field flow
fractionation (AsF-FFF) were employed (Figure 2b, 2c and 2d).
The diffusion coefficient of the supramolecular polymers
-
11
-11
2
-1
gradually decreased from 7.1 × 10 to 3.2 × 10 m ·s , which
is corresponding to the increase of molecular weight. More
quantitative data of the supramolecular polymers was obtained
from AsF-FFF. As shown in Figure 2c and 2d, the retention time
of supramolecular polymers increased from 8.8 to 15.4 min, and
the molecular weights of supramolecular polymers could be as
4
high as 9.5 × 10 Da. The molecular weight was also plotted with
the conversion of end groups (Figure s16). It turns out that this
polymerization process exhibits a step-growth manner, which is
similar to most of supramolecular polymerizations.
We then tried to realize kinetic trapping of the polymerization.
As shown in Figure 2e, we chose two time points at 22 and 48 h
of the polymerization process as demonstration. By tuning pD
from 9.7 to 10.8, the processes of supramolecular
polymerization were both paused as indicated by retarded
growth of conversion, and such kinetically trapped
supramolecular polymers were stable for days. Considering the
above established relationship between time and molecular
weight, the molecular weights of the supramolecular polymers
can be controlled in this way.
To investigate the mechanism behind the kinetic trapping,
supramolecular polymerization of C
bis-DMAD with the existence of V-bis-C
under a series of pD conditions. As shown in Figure 3a, the rate
6
CB[6]-bis-CB[7] and PEG-
-
Figure 3. a) Supramolecular polymerization process of C
6
CB[6]-bis-CB[7] and
5
COO was performed
-
PEG-bis-DMAD with the presence of V-bis-C
5
COO at different pD, and b) plot
of logarithmic rate constant of polymerization with pD.
-
3
constant of polymerization decreased drastically from 1.2 × 10
-
6
-1
to 8.6 × 10
s
as pD increased from 7.6 to 9.8. Further
changing pD may lead to further variation of the polymerization
kinetics, but such polymerization processes would be either too
fast or too slow to be traced with NMR.
competitive binding is widely available in many supramolecular
systems, this strategy may be of general applicability for various
systems of supramolecular polymerization. It is anticipated that
this line of research may enrich the controllable methodologies
for fabricating supramolecular polymers with tailored structures
and functions.
To elucidate the reason behind the change of rate constant,
-
5
kinetic calculations of the dissociation process of V-bis-C COO
were carried out (see supporting information). The dissociation
-
rate of V-bis-C
5
COO is an apparent rate, which is contributed by
-
direct dissociation of V-bis-C
protonated V-bis-C
assumption that the whole dissociation process could be
5
COO and the dissociation through
COO intermediates. We made an Acknowledgements
-
5
dominated by the dissociation through mono-protonated V-bis-
This work was financially supported by the National Natural
Science Foundation of China (21434004) and Postdoctoral
Innovation Talents Support Program (BX201600083). We also
thank Dr. Yuetong Kang, Yang Jiao and Dr. Bin Yuan for helpful
discussion.
-
C
5
COO intermediate. If so, apparent dissociation rate constant
would follow a relationship of log(k
is the apparent dissociation rate constant and k
absolute dissociation rate constant of mono-protonated V-bis-
a
) = -pH + pK
a
+ log(k
1
), where
k
a
1
is the
-
5 a
C COO . Through plotting log(k ) with pD as shown in Figure 3b,
a straight line with a slope of -0.97 was obtained, which supports
Keywords: supramolecular polymerization • self-assembly •
the above hypothesis. This result also indicates that the energy
-
cucurbituril • supramolecular chemistry
barrier for direct dissociation of V-bis-C
5
COO is very high, and
the high energy barrier is critical for realizing the kinetic trapping
of the supramolecular polymerization.
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supramolecular polymerization through kinetic trapping.
Supramolecular polymers can be trapped in different states en
route to the thermodynamic equilibrium, and their molecular
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COMMUNICATION
COMMUNICATION
Hao Chen, Zehuan Huang, Han Wu,
Jiang-Fei Xu, Xi Zhang*
Page No. – Page No.
Supramolecular Polymerization
Controlled through Kinetic Trapping
Controllable supramolecular polymerization: The kinetics of supramolecular
polymerization is controlled through the introduction of a pH-responsive competitive
guest. The supramolecular polymerization system can be kinetically trapped at
different stages, and supramolecular polymers with different molecular weights can
be obtained.
This article is protected by copyright. All rights reserved.