Supramolecularly Catalyzed Polymerization: From Consecutive Dimerization to Polymerization

Article abstract of DOI:10.1002/anie.201803749

Herein, we propose a new method for promoting covalent polymerization by supramolecular catalysts. To this end, we employed cucurbit[8]uril (CB[8]) as a supramolecular catalyst, and successfully prepared polyelectrolytes in an aqueous solution by taking a

Full text of DOI:10.1002/anie.201803749

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Accepted Article
Title: Supramolecular Catalyzed Polymerization: From Consecutive
Dimerization to Polymerization
Authors: Xiaoyan Tang, Zehuan Huang, Hao Chen, Yuetong Kang,
Jiang-Fei Xu, and Xi Zhang
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10.1002/anie.201803749
Angewandte Chemie International Edition
COMMUNICATION
Supramolecular Catalyzed Polymerization: From Consecutive
Dimerization to Polymerization
Xiaoyan Tang, Zehuan Huang, Hao Chen, Yuetong Kang*, Jiang-Fei Xu, and Xi Zhang*
Abstract: Herein, we propose a new method of supramolecular
catalyzed polymerization for promoting covalent polymerization by
supramolecular catalyst. To this end, we employed cucurbit[8]uril
(CB[8]) as a supramolecular catalyst, and successfully prepared
polyelectrolytes in an aqueous solution by taking advantage of the
CB[8]-enhanced photodimerization of Brooker’s merocyanine
moieties carried by monomer. Interestingly, 10 mol% CB[8] is
enough to effectively catalyze this polymerization, because CB[8]
can be spontaneously replaced by terminal groups from
photodimerized products. In addition, molecular weights of the
obtained polyelectrolytes can be varied by irradiation time or
recruiting monomers. By marrying supramolecular catalysis to
polymer chemistry, this line of research may enrich the methodology
of polymerization and bring a new horizon of supramolecular
polymer chemistry.
Supramolecular catalysis aims to control direction and rate of
covalent reactions by utilizing non-covalent interactions[1-11]
.
Inspired by natural enzymes, the original concept of
supramolecular catalysis was firstly proposed a long time
ago[1,12,13]. Along with the development of artificial enzyme,
various macrocyclic hosts have been widely used as a large
class of supramolecular catalysts, such as cyclodextrins,
cucurbit[n]urils, molecular cages and so forth[14-22]. The substrate
can be captured into a certain conformation to promote more
Scheme 1. Schematic illustration of
catalyzed polymerization based on cucurbit[8]uril.
a new method of supramolecular
Herein, taking advantage of the self-driven supramolecular
catalyzed polymerization, which the unreacted reactant will
spontaneously replace the photodimeric moiety from the CB[8]
cavity to start the next cycle. A bifunctional monomer (MPM)
was designed and prepared, bearing two BM end groups and
one propyl group as the spacer, as shown in Scheme 1.
According to our previous work, CB[8] is able to effectively
encapsulate two BM end groups into its hydrophobic cavity, thus
pre-organizing the monomers to form a homo-ternary host-guest
complex[46]. The propyl group of MPM was used as the short
spacer to suppress noncovalent dimerization and cyclization of
MPM, the two unfavourable factors for linear polymerization.
Next, after UV irradiation by 365 nm light source,
photodimerization of BM end groups can be achieved to form
the photodimerized products. Crucially, these products possess
weaker affinity with CB[8] than BM end groups. Hence, the
product moieties would be spontaneously replaced from CB[8]’s
cavity by BM end groups, thus further propagating polymer
chains through consecutive photodimerization. Therefore, it is
anticipated that a catalytic amount of CB[8] should be enough to
promote covalent polymerization through photodimerization of
MPM monomers.
effective
intermolecular
collision
or
intramolecular
rearrangement, thus improving regio- or stereo-selectivity of
products[23-32]. Taking these unique advantages, macrocyclic
hosts have been employed as supramolecular catalysts in many
interdisciplinary fields, such as organic synthesis, polyrotaxanes
assemblies, and dye degradation[33-39]
.
To overcome the effect of product inhibition has been a
key issue for the research of supramolecular catalyst, which
determines their feasibility for practical applications, especially
for polymerization[40-43]. One practical solution is to employ
competitive species, named as ‘chemical fuel’ in some cases, to
promote the catalytic cycle[44,45]. In this case, the chemical fuel
usually has to be added stoichiometrically and consecutively in
order to reactivate the supramolecular catalyst. Furthermore, if
the supramolecular catalyst combines with the reactant stronger
than the product, the reactant itself can act as the competitive
species, thus the catalytic cycle could be self-driven, which may
facilitate the supramolecular catalyst to be applied for
polymerization[46,47]
.
At the beginning, 1H NMR was employed to confirm pre-
organization of MPM monomers and CB[8]. As shown in Figure
1a, MPM has six sets of proton signals from 6.5 ppm to 9.0 ppm
(peak a-f), which are ascribed to the BM end groups. After
adding one equivalent of CB[8], these signals of BM end groups
exhibited significant upfield shifts, suggesting that BM end
groups can be encapsulated into CB[8]’s cavity to form a
[*]
X. Tang, Z. Huang, Dr. H. Chen, Dr. Y. Kang, Dr. J.-F. Xu,
Prof. X. Zhang
Key Lab of Organic Optoelectronics & Molecular Engineering,
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
E-mail: xi@mail.tsinghua.edu.cn; kyt2017@mail.tsinghua.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201803749
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COMMUNICATION
Table 1. Diffusion coefficients of MPM, MPM monomer and MPM + 10% CB[8]
with irradiation.
Species
UV irradiation time
[h]
Diffusion coefficients
[10-10 m2 s-1]
MPM
0
4
4
3.35
2.32
1.00
MPM
MPM+10%CB[8]
photodimerization of MPM in the presence of CB[8] with UV
irradiation of 365 nm light. We firstly prepared a series of
solutions of MPM containing different molar amount of CB[8]
catalyst from 0% to 25%, and then these solutions were
exposed under 365 nm UV light for photodimerization. Typically
in the presence of 10% CB[8] shown in Figure 2a, the
absorbance around 381 nm, corresponding to π-π* transition of
BM end groups, gradually decreased by increasing irradiation
time, along with the increase on the absorbance at 226 nm. As
CB[8] has no absorption in the range of 250 nm to 600 nm, this
phenomenon means that the BM end groups of MPM were
converted to smaller conjugated structures after UV irradiation
because of their photodimerization.
Figure 1. a) 1H NMR spectra of MPM : CB[8] = 1 : 0 and MPM : CB[8] = 1 : 1;
b) ITC titration curve and fitted data of MPM to CB[8] in the 50 mM NaAc/HAc
buffer of pH 4.75.
homo-ternary host-guest complex. To further measure their
binding affinity, isothermal titration calorimetry (ITC) was used to
collect detailed thermodynamic information behind their
complexation. 1 mM solution of MPM, i.e. 2 mM of BM end
group was titrated into the 0.1 mM CB[8] solution. As shown in
Figure 1b, there existed an abrupt transition in 2:1 molar ratio of
BM:CB[8], which means that the binding stoichiometry of MPM
monomer and CB[8] is 1:1. By fitting this curve with two sets of
sites binding model, the overall binding constant Ka was
calculated to be as high as 9.65 × 1011 M-2, which can guarantee
the effective pre-organization of MPM monomer and CB[8]
catalyst.
To quantitatively evaluate the catalytic activity of CB[8], we
further calculated the conversion rates of photodimerization with
different irradiation time and contents of CB[8]. The conversion
rate was monitored by the absorption at 381 nm, as its decrease
can directly reflect the consumption of BM end groups. As
shown in Figure 2b, we found that after irradiation for 4.0 h, 5%
or 10% CB[8] was enough to accelerate the photodimerization of
MPM monomers, and their conversion rates were calculated to
be about 92.8% and 93.5%, respectively. By contrast, the
conversion rate without the presence of CB[8] could only reach
62.0%. With higher content of CB[8] (25%), it took only 1.0 h to
complete the photodimerization. Therefore, CB[8] remained its
catalytic activity on photodimerization of MPM monomers, and
10% CB[8] catalyst was chosen for further investigations as an
optimized factor, considering both the low-content addition of
catalyst and the relatively high molecular weights of yielded
polymers.
We wondered whether CB[8] could still function as a
supramolecular catalyst when confronting MPM. To answer this
question, UV-Vis spectroscopy was utilized to monitor the
To verify if the photodimerization of BM end group is aligned
with the formation of polymers, we employed 1H NMR and
1
diffusion-ordered NMR (DOSY). As shown in Figure S2, the H
NMR spectrum proved the occurrence of photodimerization by
the disappearance of two ethylene signals and the remain of
four aromatic signals, which matched with UV-Vis results and
the previous literature[46]. Moreover, as shown in Table 1, the
diffusion coefficient of MPM itself was estimated to be 3.35 × 10-
10 m2/s. After photodimerization of MPM-10% CB[8], the diffusion
coefficient of the products declined to 1.00 × 10-10 m2/s, while it
only decreased to 2.32 × 10-10 m2/s without CB[8]. Thus, these
results suggest that high-molecular-weight species were formed.
Figure 2. a) UV-Vis spectra of the 2.0 mM MPM + 10% CB[8] solution under
UV irradiation. b) Diagram of conversion rates of 2.0 mM MPM with 0%, 1%,
5%, 10%, 25% CB[8] versus irradiation time calculated by the absorbance at
381 nm.
To obtain further detailed information about molecular
weight of these polymers, asymmetrical flow field flow
fractionation (AsF-FFF) was utilized, which was integrated with
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Figure 3. a) AsF-FFF differential refraction responses of the polymers obtained at varied irradiation times; b) AsF-FFF UV responses of the polymers obtained at
varied irradiation times; c) Diagram of molecular weight of polymer species with various irradiation time via AsF-FFF.
UV-Vis, differential refraction (RI), and multi-angle light
scattering (MALS) detectors. Typical elution curves of the MPM-
10% CB[8] solution after irradiation lasting from 30 min to 4.0 h
were recorded by UV-Vis and RI detectors (Figure 3a and 3b)
respectively. According to the separation mechanism of AsF-
FFF, the low-molecular-weight species would elute out earlier,
and the high-molecular-weight species would stay longer in the
separation channel. As shown in Figure 3, at the beginning of
the irradiation, the main signal distributed mainly at around 6.7
min, with a “shoulder peak” positioned at 7.5 min, which meant
the high-molecular-weight species existed while the low-
molecular-weight species dominated. With irradiation lasting, the
signals gradually distributed mainly to around 7.5 min, indicating
high-molecular-weight species became dominated.
Moreover, we also calculated molecular weights in different
irradiation time based on the above results as shown in Figure
3c. After irradiation for 4.0 h, the molecular weight of the
polymers was calculated around 5004 Da, and the degree of
polymerization was around 11. Therefore, these results
elucidate that covalent polymers can be successfully prepared
through supramolecular catalyzed polymerization.
To test if CB[8] could still function as catalyst after
polymerization, we refueled the above solution with equivalent
amount of MPM monomers after each 4-hour irradiation. Such
refueling-irradiating experiments were repeated for 4 times, and
the obtained polymers were characterized by AsF-FFF. As
shown in Figure 4a, elution curves of these polymers were
recorded by RI detector. The RI response of polymers gradually
increased, which meant that the amounts of the polymers were
increased. It may be ascribed that CB[8] can catalyze the
formation of new polymers from monomers. More importantly,
the molecular weights of these polymers continued to grow from
5004 Da (1st cycle) to 6707 Da (5th cycle) as shown in Figure 4b.
These data indicate that the BM end groups are still reactive,
thus enabling further elongation of the polymers. Therefore,
these results confirm that CB[8] still has catalytic activity after 5-
times polymerization.
The plausible mechanism behind this catalytic process can
be described as follows. At the beginning, BM end groups of
MPM can be encapsulated by CB[8] to form a homo-ternary
host-guest complex, which provide a certain conformation to
promote more effective intermolecular collision or intramolecular
rearrangement[46]. Then, the photodimerization of BM end groups
can be realized and accelerated by CB[8] under UV irradiation.
Next, because of relatively lower binding affinity between CB[8]
and the photodimerized product and the dynamic nature of host-
guest complexation, the product can be spontaneously replaced
by BM end groups, thus regenerating CB[8]’s catalytic ability.
Therefore,
CB[8]
can
continuously
catalyze
the
photodimerization by repeating the above cycle.
As the two step binding constants of CB[8] and Brooker’s
merocyanine are Ka1 of 2.95 × 105 M-1 and Ka2 of 2.88 × 106 M-1,
with the cooperativity factor (α) as 39.0, the complexation of
CB[8] and Brooker’s merocyanine is a positively cooperative
process[46]. Therefore, when CB[8] has been on the end of
Figure 4. a) Differential refraction responses versus irradiation time and b)
Molecular weight of polymer determined by employing AsF-FFF with
continuous addition MPM for 5 cycles.
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COMMUNICATION
COMMUNICATION
Xiaoyan Tang, Zehuan Huang, Hao
Chen, Yuetong Kang*, Jiang-Fei Xu and
Xi Zhang*
Page No. – Page No.
Supramolecular Catalyzed
Polymerization: From Consecutive
Dimerization to Polymerization
A new method of supramolecular catalyzed polymerization for promoting covalent
polymerization by supramolecular catalyst is reported. We prepared polyelectrolytes
by taking advantage of the CB[8]-enhanced photodimerization of Brooker’s
merocyanine moieties carried by monomers. Interestingly, 10 mol% CB[8] is
enough to effectively catalyze this polymerization. This line of research may enrich
the methodology of polymerization and bring a new horizon of supramolecular
This article is protected by copyright. All rights reserved.