Step-Growth Copolymerization Between an Immobilized Monomer and a Mobile Monomer in Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.201901308

The A–A/B–B step-growth copolymerization between a monomer immobilized in the crystalline state and a monomer mobile in the solution state is demonstrated. One of the two monomers was immobilized as organic ligands of the metal–organic framework (MOF) and

Full text of DOI:10.1002/anie.201901308

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Accepted Article
Title: Step-growth copolymerization between immobilized monomer
and mobile monomer in metal-organic frameworks
Authors: Shizuka Anan, Yumi Mochizuki, Kenta Kokado, and Kazuki
Sada
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Step-growth copolymerization between immobilized monomer
and mobile monomer in metal-organic frameworks
[
a]
[b]
[a][b]
and Kazuki Sada*[a][b]
Shizuka Anan, Yumi Mochizuki, Kenta Kokado,*
Abstract: The A-A/B-B step-growth copolymerization between a
monomer immobilized in the crystalline state and a monomer mobile
in the solution state is demonstrated. One of the two monomers was
immobilized as organic ligands of the metal-organic framework (MOF)
and polymerized with the mobile guest monomer, resulting in the
formation of linear polymers. The polymerization behavior was
completely different from that of the solution polymerizations. In
particular, the degrees of polymerization (DP) converged to a specific
value depending on the MOF structures. The inevitable termination is
caused not by imperfectness of the polymerization reaction, but by the
selection of the two polymerization partners among the several
adjacent immobilized monomers. This is fully supported by the Monte
Carlo simulation on the basis of the polymerization mechanism.
Precise immobilization of monomers in the supramolecular
assemblies is a promising way for the controlled A-A/B-B step-growth
polymerization.
Kitagawa reported radical copolymerization of host monomers
immobilized as organic ligands in the metal-organic frameworks
(MOF) and guest monomers to synthesize sequence controlled
polymers.[20] However, the polymerization behaviors of the step-
growth polymerization between the immobilized monomer and the
mobile monomer remain unclear. In the present work, we
performed the A-A/B-B type step-growth copolymerization of
immobilized A-A monomer and mobile B-B monomer to form
linear polymer by using MOFs.
Our approach is based on the construction of the MOF from
an organic ligand containing two reactive functional groups as an
A-A monomer.[18] By treatment with metal ions, the A-A monomer
provides robust frameworks in which the reactive sites of the A-A
monomer are three-dimensionally arranged along the MOF. The
porosity of the MOF enables to include B-B monomers in the
nano-sized 3D channels from the outer solution by soaking MOF
in it. The chemical reaction between the A and B functional groups
should grow the polymer chain in the MOF. Consequently, the
step-growth polymerization occurs between the A-A monomer
immobilized in the framework of the MOF and the mobile B-B
monomer that is included in the 3D channels and supplied from
the outside solution. After digestion of the MOF, the obtained
polymers have the identical unit structure to those prepared by
the solution polymerizations.
During the polymerization of a monomer to form a linear
polymer, each monomer needs to form two new chemical bonds
for chain propagation by either the step-growth or chain-growth
mechanism. The control of environment of monomers has a
significant influence on the polymerization behavior. [3–6]
In the solution state, the polymerization randomly occurs due to
random encounters between the freely-mobile monomers and the
mobile propagating species. Thus, for controlled polymerizations,
control of the reactivity is needed by adding catalysts, dormant
additives and activators that interact with the propagating species.
[
7–11]
On the other hand, in the crystalline state, polymerization
occurs with preserving the regularity because monomers are
immobilized in crystalline state.[3,5,6,12–16] A typical example is the
topochemical polymerizations.[12–16] The immobilization and the
highly-restricted orientation of the monomers provides the
conformation or stereoregularity controlled polymer without any
additives.
Because polymerization behaviors of mobile monomers in
the solution state and immobilized monomers in the crystalline
state are different, copolymerization of mobile and immobilized
monomers is expected to show unique polymerization behavior.
Only a few approaches have been reported due to the difficulty of
the controlled immobilization of the monomer that can
copolymerize with the mobile monomers.[17–20] Uemura and
Figure 1. (a) Polymerization of Aztpdc and CL2 via azide-alkyne click reaction.
(b) Schematic illustration of polymerization of Aztpdc and CL2 in ZnAzMOF
and subsequent decomposition of ZnAzPMOF to obtain linear polymer. Optical
microscopy images show ZnAzMOF (left) and ZnAzPMOF (right) (Scale bars:
200 μm).
[
a]
b]
Dr. S. Anan, Dr. K. Kokado, Prof. Dr. K. Sada
Department of Chemistry, Faculty of Science
Hokkaido University
Kita 10, Nishi 8, Kita-Ku, Sapporo, Hokkaido 060-0810
E-mail: kokado@sci.hokudai.ac.jp, sadatcm@sci.hokudai.ac.jp
Ms. Y. Mochizuki, Dr. K. Kokado, Prof. Dr. K. Sada
Graduate School of Chemical Sciences and Engineering
Hokkaido University
According to our previous papers,[18] Aztpdc with two azide
groups and CL2 with two alkynyl groups were used for the A-A
and B-B monomers, respectively. The Huisgen Cu(I)-catalyzed
azide-alkyne cycloaddition to form triazole rings was used for the
polymerization. Aztpdc was treated with Zn2+ for the solvothermal
synthesis in N,N-diethylformamide (DEF) at 80 °C for two days to
[
Kita 10, Nishi 8, Kita-Ku, Sapporo, Hokkaido 060-0810
Supporting information for this article is given via a link at the end of
the document.
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obtain rectangular-shaped crystals (ZnAzMOF) (Figure 1). A
powder X-ray diffraction (XRD) measurement revealed that
ZnAzMOF was isomorphous to IRMOF-15 (Figure S1a),[21] and
the existence of the azide group was confirmed by the
indicated that the azide groups of Aztpdc were quantitatively
converted to the triazole rings, and the polymerization between
Aztpdc and CL2 occurred. CL2 could penetrate into the 3D
channels of ZnAzMOF and react with the immobilized Aztpdc.
CuBr existed in the 3D channel catalyze the click reaction.
After the isolation of polyAzCL2 by reprecipitation into
water followed by methyl esterification of the carboxylate groups
by diazomethane (polyAzCL2-me), the molecular weight of
polyAzCL2-me was evaluated by size exclusion chromatography
−
1
characteristic band at 2090 cm (νN3) in infrared (IR) spectrum
Figure S1b).
(
(
SEC) as shown in Figure 2. The chromatogram showed a
monomodal peak and the peaktop DP (X ) was ca. 13 (M = 8600,
= 4800, M = 9800). The observed poly dispersity index (Đ)
was 2.0, and this was nearly ideal for the step-growth
polymerization, but the of the resulting polymer was
p
p
M
n
w
M
n
unexpectedly high in spite of the stoichiometric the mismatch of
Aztpdc and CL2 (1/20). To compare the polymerization
behaviors, we carried out a model polymerization in the solution
state with 0.1 M Aztpdc-methyl ester (Aztpdc-me) and CL2 (1/1
and 1/20 stoichiometry) in DEF at 80 °C for 48 hours in the
presence of CuBr (Figure 3). The SEC chromatogram in the case
p
of 1/1 stoichiometry showed that the X become slightly higher
than that via ZnAzMOF and the multimodal peaks specifically in
the oligomer region were observed. Moreover, DP increased
versus time for the solution polymerization (Figure S3a). This
behavior is typical for a step-growth polymerization. The SEC
chromatogram in the case of 1/20 stoichiometry showed only
dimer, trimer, and tetramer, indicating little progress of solution
polymerization in this stoichiometrically mismatch condition
Figure 2. SEC chromatogram of polyAzCL2-me after methyl esterification
[
CHCl
3
, 40 °C, PS standard, detector: UV (254 nm)].
(
Figure S3b).
The polymerization of Aztpdc and CL2 was carried out as
follows. ZnAzMOF was soaked in 0.1 M DEF solution of CL2 and
incubated at 80 °C for two days in the presence of saturated CuBr
(
1.0 mM, determined by inductively coupled plasma atomic
emission spectrometer (ICP-AES)) as a catalyst for click reaction.
The ratio between Aztpdc in ZnAzMOF and CL2 in the solution
was about 1/20 (mol/mol), which usually provides the dimer at the
maximum due to the large excess amount of CL2 according to the
classical theory of the A-A/B-B step-growth polymerization in
solution.[
1,2]
After the polymerization, the shape of the crystal did
not change, furthermore, the XRD measurement indicated that
peak position and the crystallinity was unchanged judging from
the half maximum full-width of the peaks. Several new peaks
corresponding to simulated pattern of non-interpenetrated
IRMOF-16[21] were observed after two days reaction, which
indicates the location of the two-fold-interpenetrated framework
moved from the center when the amount of CL2 immobilized in
ZnAzMOF increased (Figure S1c). The IR spectra showed the
disappearance of the peak at 2090 cm−1 confirming the progress
of the click reaction. Subsequently, the polymerized ZnAzMOF
Figure 3. Polymerization of Aztpdc-me and CL2 in a solution and SEC
chromatogram of the product (orange) and polyAzCL2-me (blue, dashed line)
[CHCl , 40 °C, PS standard, detector: UV (254 nm)].
3
The time-course study for the polymerization inside MOFs
was next conducted. The FT-IR and 1H NMR measurements
revealed that the click reaction between Aztpdc in ZnAzMOF and
CL2 solution was completed in two days (Figure S4). The SEC
measurement showed that the peaks derived from the monomer,
dimer, tetramer, pentamer, and hexamer decreased with time,
which were observed at the early stage (Figure 4). Increasing the
peak derived from tridecamer was also observed within two days.
Additional heating for 8 days induced neither the shift of the
peaktop nor any apparent change in the molecular weight
distribution.
2 4 6
was immersed in 0.5 M D SO / DMSO-d for liberating the
polymer (polyAzCL2) and Zn2+ ion by the decomposition of the
coordination bonds. In contrast to the cases of the multivalent
alkynyl monomers,[18] the polymerized ZnAzMOF was completely
dissolved in the acidic conditions, suggesting the formation of a
1
linear polymer. In the H NMR spectra, the benzyl protons
adjacent to the azide group in Aztpdc at 4.5 ppm shifted to 5.6
ppm, and the characteristic resonance for the triazole ring at 7.8
ppm appeared after the reaction (Figure S2). These results
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For understanding the mechanism of the polymerization, the
inclusion behavior of CL2 from the outside solution was
1
investigated without the copper catalyst by H NMR. By soaking
ZnAzMOF in a 0.1 M solution of CL2 at 80 °C, the concentration
of CL2 in ZnAzMOF ([CL2]MOF) rapidly increased, and the molar
ratio of CL2/Aztpdc was saturated to 0.05 within 5 min (Figure
S5a). This result suggested that unless click reaction occurs, the
amount of CL2 in ZnAzMOF does not increase as time. The
penetration of CL2 is equilibrium relation of inflow and outflow of
CL2 into ZnAzMOF ([CL2]sol ⇌ [CL2]MOF). The rate constant of
1
inflow (k ) and outflow (k−1) of CL2 were calculated from the fitting
−
1 [22]
curve of [CL2]MOF versus time plot as 52.8 and 57.2 h . The
molar ratios of CL2 to Aztpdc inside the ZnAzMOF after the
saturation were found to be 0.05, 0.47, 0.90, and 1.67 for the 0.1
M, 1.0 M, 2.0 M and 5.5 M solutions of CL2, respectively,
revealing that the amount of CL2 in ZnAzMOF correlate positively
with the concentration of CL2 solution (Figure S5b). The [CL2]MOF
calculated from k
values under 2.0 M of CL2 indicating the k
1
and k−1 were corresponded to the experimental
and k−1 are probable.
Figure 4. Time-course SEC chromatogram of polyAzCL2-me. Polymerization
1
time was changed from 1 hour to 8 days [CHCl
UV (254 nm)].
3
, 40 °C, PS standard, detector:
Furthermore, we investigated the dependence of CL2
concentration in DEF solution outside of ZnAzMOF on the
polymerization degree of the polymers. The solutions with five
concentrations of 0.01 M, 0.1 M, 1.0 M, 2.0 M and 5.5 M CL2 were
used for this polymerization, i.e. 2.0, 20, 200, 400, 1100
equivalents corresponding to Aztpdc, respectively. Based on the
SEC measurement, CL2 was polymerized with Aztpdc in all
cases, and the X
decreased to 10, 9, 3 in the high concentrations at 1.0 M, 2.0
M and 5.5 M (Figure 5). However, the X was much higher than
p
was around 13 at 0.1 M and 0.01 M, whereas
X
p
p
those based on the theory of A-A/B-B type step-growth
polymerization in the solution state.[1,2] These results revealed that
the polymerization of Aztpdc in ZnAzMOF by CL2 was totally
different from that in the solution state, particularly the DP
converged to specific values.
Figure 6. (a) Schematic illustration of the two steps reaction of Aztpdc
immobilized in MOF and mobile CL2, penetration and click reaction. The
symbols v , v , k , k−1 and k represent the velocities of penetration of CL2 and
p c 1 2
click reaction and the rate constatnts of inflow and outflow of CL2 and click
reaction, respectively. (b–d) Time-course of the polymerization by using
ZnAzMOF in (b) 0.01 M, (c) 0.1 M and (d) 1.0 M of CL2 / DEF solution. The
graph shows the reaction ratio of CL2 (blue), reaction ratio of Aztpdc (green),
and the ratio of unreacted CL2 to Aztpdc (red) inside ZnAzMOFs.
The kinetic analysis of the polymerization in ZnAzMOF was
conducted by monitoring the time-course change in the amount of
the unreacted and reacted CL2 and Aztpdc inside ZnAzMOF by
1H NMR (Figures 6, S6 and S7).[23] In the 0.01 M solution of CL2,
the unreacted CL2 in ZnAzMOF was not detected, but in the 0.1
M and 1.0 M solution, CL2 remained unreacted in ZnAzMOF
(
0.02 and 0.20 eq. / Aztpdc) throughout the polymerization while
the reacted Aztpdc increased gradually. These results indicated
that the penetration and click reaction occurs simultaneously
(
Figure 6b). When the velocity of click reaction (v
proportional to [CL2]MOF and [Aztpdc], v is described as v
[CL2]MOF[Aztpdc], and k
The k and k−1 are much larger than k
Aztpdc] is the maximum (~1.0 M), providing that each rate of
c
) inside MOF is
c
c
=
Figure 5. SEC chromatograms of polyAzCL2-me. Each polymer was
synthesized in ZnAzMOF immersed in 0.01, 0.1, 1.0, 2.0 and 5.5 M of CL2 /
was calculated as 0.399 L mol−1 h .
−1
k
2
2
1
2
[Aztpdc] even when
DEF solution at the same volume 3.0 mL [CHCl
UV (254 nm)].
3
, 40 °C, PS standard, detector:
[
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inflow (vin) and outflow (vout) is faster than that of click reaction.
framework of MOF. The other unreacted alkynyl group of CL2
reacts with an azide group among the several surrounding
Aztpdc as the polymer growth reaction that links two Aztpdcs.
After reaction, another CL2 enters the 3D channel due to the
decrease of the CL2 concentration in ZnAzMOF. CL2 is small
enough to penetrate into the 3D channel during polymerization
(Figure S11). This cycle was repeated until complete
consumption of the azide groups of Aztpdc. As the termination of
the polymerization, the unreacted alkynyl groups in the framework
are left as dangling ends, when all the azide groups of Aztpdc
surrounding them are reacted. This termination process causes
the convergence of DP in the polymerizations between the
monomer (Aztpdc) immobilized in the framework as the
crystalline-state and the mobile monomer (CL2) in the 3D channel
and supplied from the solution phase. Therefore, this
polymerization is not explained by the classical theory[ of the A-
A/B-B type step-growth polymerization in the solution state.
Note that the velocity (v
the solution (Figure S8), therefore, v
saturated condition ([CuBr] = 1.0 mM). This difference between
indicates that the click reaction does not occur from
c
) was also proportional to the [CuBr] in
c
was the largest in this
v
in, vout and v
c
the surface of the ZnAzMOF.
To investigate the difference between polymerization inside
MOF and in a solution, we compared polymerization between
2
Aztpdc and a series of isomeric monomers, o-, m-, and p-CH C2
in an MOF and a solution. The SEC chromatograms (Figure S9)
from the polymerization by using ZnAzMOF revealed that the
bimodal peaks derived from only the monomer and the polymers
with the converged DP in all cases. The polymerization behavior
was quite similar to that of CL2. In particular, in the case of o-
2 2
CH C , the peak derived from cyclized oligomers were observed
1,2]
in the solution polymerization, however, no peak of cyclized
oligomers was observed in this polymerization. Therefore,
macrocyclization should be suppressed due to immobilization of
Aztpdc in ZnAzMOF.
To evaluate the effect of the arrangement of Aztpdc in the
frameworks of MOF for polymerization, we polymerized Aztpdc
and CL2 in ZrAzMOF[18,25] and CuAzMOF[26] with different crystal
1
structures under the same conditions (Figure S10). The H NMR
spectra revealed that all of the resulting polymers had identical
polymer chains after liberation by acid treatment. However, the
SEC chromatograms of the polymers from ZrAzMOF and
p
CuAzMOF showed monomodal peaks and the X of 5 and 13,
respectively. They also provided the converged DP similar to
ZnAzMOF, but the DP depended on the monomer arrangement
of MOF.
Figure 7. Schematic illustration of the mechanism of the polymerization
between immobilized and mobile monomers. Firstly, CL2 outside of the MOF
penetrates into MOF and one of the alkynyl group reacts with an azide. When
the nearest azide around the immobilized CL2 remained unreacted, the other
alkynyl group reacts with one of them. Otherwise, the other alkynyl group cannot
react with any azides and become a terminal of a polymer chain.
Figure 8. (a) Schematic illustration of the method of Monte Carlo simulation for
the polymerization by using MOF. (b) Crystal structure of ZnAzMOF and the
alignment of Aztpdc in ZnAzMOF. The blue balls represent Aztpdc. (c) The
geometry of polymers calculated in a simulation (5×5×5). (d) Weight fraction
distribution of the calculated polymer (100×100×100). Horizontal axis was
converted to molecular weight in the case of polyAzCL2. (e) Weight fraction
distribution of the linear and cyclic polymers calculated in NbO structure. Red
and white bars represent cyclic and linear polymers, respectively.
Consequently, the following mechanism can explain the
experimental results (Figure 7). Initially, CL2 in the solution phase
immediately penetrates into the 3D channel of ZnAzMOF
depending on the concentration of the solution. One of the two
alkynyl groups of CL2 reacts with an azide group of Aztpdc in the
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To elucidate the unique polymerization behaviors and to
validate the proposed polymerization mechanism, we attempted
a Monte-Carlo simulation for the polymerization of Aztpdc in
ZnAzMOF by CL2 using the following procedure (Figure 8a): (1)
Immobilized monomers with two reactive sites are arranged in the
NbO type lattice that is identical to the molecular arrangement of
Aztpdc in the IRMOF-15 structure of ZnAzMOF (Figure 8b). (2)
Among the four adjacent monomers, two are selected randomly.
In the NbO type lattice, all the monomers have four adjacent
monomers and each monomer (Aztpdc) can connect with two of
the nearest monomers around it. (3) When the adjacent two
monomers mutually select, "a bond" forms between them,
otherwise, the selections are removed and back to (2). When a
monomer has one or two bonds, the monomer can select one or
zero monomer around it. This cycle is repeated until all the
monomers cannot form any new bonds in the 100×100×100
lattice. The termination occurs under the following two conditions:
In conclusion, we demonstrated the A-A/B-B type step-
growth polymerizations between the immobilized monomer and
the mobile monomers in MOF to yield the polymers with
converged DP. Compared to solution polymerizations, this
polymerization was totally insensitive to the apparent
stoichiometry of the two monomers and the consumption of the
reactive sites of the monomers. However, the monomer
arrangements in the frameworks affected DP. Taking into account
of the selection probability of the polymerization partners among
the immobilized monomers, the Monte Carlo simulations explain
these distinct behaviors as well as the unfavorable formation of
p
cyclic oligomers and the crystallite size dependence of X .
Therefore, designing immobilized monomers and their monomer
arrangements provide polymers with predictable DP values. Our
approach is a new design of the controlled A-A/B-B type step-
growth polymerizations in polymer synthesis.
(
i) monomers have already connected with two adjacent
monomers, and (ii) all four surrounding monomers are connected
with other two monomers. The geometry of polymers calculated
in this simulation was visualized in Figure 8c. This Monte Carlo
Acknowledgements
We acknowledge financial support from a JSPS KAKENHI Grant
Numbers JP17H03062, JP18H04495, JP15K17861. We are very
grateful to Professor Dr. M. Kato and Professor Dr. A. Kobayashi
for the XRD measurement.
simulation of the polymerization in ZnAzMOF revealed that X
p
converged to 13 (Figure 8d). This value is close to the
experimental result as shown in Figure 2. This indicated that high
polymers are hardly formed, even if the chemical reaction
between the A-A monomers and B-B monomers perfectly occurs
theoretically. In other words, the inevitable termination is caused
not by the imperfect chemical reaction between the Aztpdc and
CL2, but by the selection of the two polymerization partners
among the four adjacent immobilized Aztpdc by the mobile CL2.
This is a good contrast to the ideal step-growth polymerizations in
the solution state, in which DP diverge to infinity with quantitative
chemical reaction yield, stoichiometry ideal and no cyclizations as
the side reaction. [1,2]
This simulation provided three additional significant insights
for the uniqueness of this polymerization. First, it predicted that
few cyclic polymers are generated (Figure 8e). The weight fraction
of the cyclic polymers was a mere 1.5 %. This simulation results
corresponds to the experimental results of suppressing the
generation of cyclic polymer in the polymerization by using MOF
compared to solution polymerization inevitably generating cyclic
polymers.[24] Secondly, we revealed that DP depends on the
monomer arrangement from the simulations of other lattices, i.e.
simple cubic (sc), body-centered cubic (bcc), and face-centered
Keywords: step-growth polymerization • metal-organic
framework • solid state polymerization • Monte Carlo simulation•
cyclic polymer
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p
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was 5 (Figures S10 and S12). The
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supporting information.
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Angewandte Chemie International Edition
10.1002/anie.201901308
COMMUNICATION
Entry for the Table of Contents
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COMMUNICATION
Two types of monomers in different
environment were copolymerized in a
metal-organic framework (MOF). One
of the two monomers was immobilized
in MOF as an organic linker and the
other is mobile in the 3D channels of
MOF. The polymerization degree
converged to specific value depending
on the crystal structure of MOFs.
Immobilization of monomers regularly
in supramolecular assemblies is a
promising way for controlling step-
growth copolymerization.
Shizuka Anan, Yumi Mochizuki, Kenta
Kokado*, Kazuki Sada**
Page No. – Page No.
Step-growth copolymerization
between immobilized monomer and
mobile monomer in metal-organic
frameworks
This article is protected by copyright. All rights reserved.