Metal-Organic Polyhedral Core as a Versatile Scaffold for Divergent and Convergent Star Polymer Synthesis

Article abstract of DOI:10.1021/jacs.6b01758

We herein report the divergent and convergent synthesis of coordination star polymers (CSP) by using metal-organic polyhedrons (MOPs) as a multifunctional core. For the divergent route, copper-based great rhombicuboctahedral MOPs decorated with dithiobenz

Full text of DOI:10.1021/jacs.6b01758

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Article
Metal–Organic Polyhedral Core as a Versatile Scaffold
for Divergent and Convergent Star Polymer Synthesis
Nobuhiko Hosono, Mika Gochomori, Ryotaro Matsuda, Hiroshi Sato, and Susumu Kitagawa
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01758 • Publication Date (Web): 27 Apr 2016
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Metal–Organic Polyhedral Core as a Versatile Scaffold for Divergent
and Convergent Star Polymer Synthesis
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Nobuhiko Hosono,*,† Mika Gochomori,† Ryotaro Matsuda,†,# Hiroshi Sato,†,§ and Susumu Kitagawa*,†,‡
†Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
‡Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-
ku, Kyoto 615-8510, Japan.
ABSTRACT: We herein report the divergent and convergent synthesis of coordination star polymers (CSP) by using metal–organic polyhe-
dra (MOP) as a multifunctional core. For the divergent route, copper-based great rhombicuboctahedron MOPs decorated with dithiobenzo-
ate or trithioester chain transfer groups at the periphery were designed. Subsequent reversible addition–fragmentation transfer (RAFT)
polymerization of monomers mediated with the MOPs gave star polymers, in which 24 polymeric arms were grafted from the MOP core. On
the other hand, the convergent route provided identical CSP architectures by simple mixing of a macroligand and copper ions. Isophthalic
acid-terminated polymers (so-called macroligands) immediately formed the corresponding CSPs through a coordination reaction with cop-
per(II) ions. This convergent route enabled us to obtain miktoarm CSPs with tunable chain compositions through ligand mixing alone. This
powerful method allows instant access to a wide variety of multi-component star polymers that conventionally have required highly skilled
and multistep syntheses. MOP-core CSPs are a new class of star polymer that can offer a design strategy for highly processable porous soft
materials by using the coordination nanocages as a building component.
Miktoarm star polymers, in which two or more chemically different
INTRODUCTION
arms radiate from an identical core, are an important subclass of
star polymers.7b Due to their segmented block architectures, mik-
Tailoring macromolecular architecture is a grand challenge for
polymer chemistry.1 Following the discovery of living polymeriza-
toarm stars have been extensively studied with regard to self-
tion in the middle of the last century,2 advances in polymer chemis-
assembly in the bulk and in solution. However, the syntheses of
try have provided us with a variety of synthetic polymers with com-
plex architectures, including block copolymers, gradient copoly-
mers, cyclic polymers, graft (comb) polymers, dendrimers, and star
polymers.1 In particular, star polymers,3–6 which are a class of multi-
armed macromolecules with more than three “arm chains” con-
nected at an identical center, have attracted significant interest due
to not only their topological importance, but also their unique
physical properties originating from their compact macromolecular
shape. For instance, because of the radiating architecture, intermo-
lecular entanglements are suppressed, resulting in an extremely low
viscosity compared with their linear analogues with similar molecu-
lar weights.
these systems have required multiple protecting/deprotecting steps,
orthogonal coupling reactions, and different polymerization meth-
ods.7b
Recently, porous coordination polymers (PCPs) and metal–
organic frameworks (MOFs) have received considerable attention
as new classes of porous materials that are constructed through self-
assembly of organic ligands and metal ions.8 Metal–organic poly-
hedra (MOPs) or coordination nanocages are discrete cage-like
analogues of PCP/MOFs.9 MOPs have been used as building
blocks for MOF frameworks10 or molecular containers,11 and as
fillers of mixed-matrix membranes for gas separation applications.12
In the present work, we disclose a new class of star polymers,
termed coordination star polymers (CSPs), which has a MOP core
at the center. Coordination-driven self-assembly, which has been
utilized extensively for the synthesis of dendrimers and molecular
capsules,13 plays a crucial role in structuring the star polymer archi-
tecture. This approach facilitates the synthesis of star polymers,
including multi-component miktoarm stars, which has often been
constrained by the need for time-consuming procedures and highly
skilled polymerization techniques.
Conventional star polymers are synthesized via either divergent
or convergent approaches. The divergent approach entails conven-
tional living polymerization with multi-functional initiators.3 On
the other hand, the convergent approach involves covalent,4 su-
pramolecular,5 and coordination bond6 formation to couple end-
functionalized polymeric precursors with multi-functional cores.
Whereas the divergent route is advantageous for synthetic precision,
the convergent route is more accessible because of its facile syn-
thetic protocol, as well as the structural versatility of the resulting
macromolecular architectures. A combination of both approaches
is generally employed for the synthesis of miktoarm star polymers.7
The MOP-core CSPs were synthesized via both divergent (core-
first) and convergent (arm-first) routes. For the divergent route, we
employed great rhombicuboctahedron MOPs as a multifunctional
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core that consists of a total of 24 isophthalic acid ligands intercon-
methylpyrrolidone (NMP) instantly provided a MOP, MOP(1).
nected via 12 dicopper paddlewheel clusters (Figure 1). The organ-
ic ligands were substituted with dithiobenzoate or trithioester chain
transfer groups in order to graft polymer chains to the periphery of
the MOP. Reversible addition–fragmentation transfer (RAFT)
polymerization14 mediated with the MOP led to MOP-core star
polymers, in which a total of 24 arm chains were grafted to a single
core.
The formation of MOP(1) was monitored in situ by size-exclusion
chromatography (SEC) using THF as the eluent (Figure S1). After
mixing 1 with an equimolar amount of copper(II) acetate in NMP,
the peak of 1 in SEC disappeared within 5 min and a new higher-
molecular-weight peak of 4070 g/mol (relative to polystyrene
standards) with exceptionally narrow polydispersity (Đ = Mw/Mn =
1.02) appeared (Figure S1). This observation is a strong indication
of the formation of MOP(1). The product was readily isolated by
precipitation from methanol. MOP(2) (Mn,MOP (SEC) = 5090, Đ =
1.02) was prepared in the same way using 2, although additional
purification by preparative SEC was needed. During the reaction of
2 and copper(II) acetate, a small amount of unidentified byproduct
with a higher molecular weight was formed (Figure S2). This is
presumably due to a trace amount of a difunctional impurity of 2,
S,S′-di(5-methylisophthalic) trithiocarbonate, as detected by ESI-
MS (Figure S3).15
For comparison, a known MOP, MOP(tBipa),9e was measured
by SEC. This MOP is composed of tert-butylisophthalic acid (tBi-
pa) with copper(II) ions, and its structure has been determined by
single-crystal X-ray diffraction.9e As expected, SEC of MOP(tBipa)
gave a monodisperse peak at Mn,MOP (SEC) = 3490 (Đ = 1.01)
(Figure S1). It should be noted that SEC always gives a lower mo-
lecular weight (M) value than that expected for MOPs because of
their compact spherical shape. In fact, the actual M values, Mn,MOP
(calcd.), for MOP(1), MOP(2), and MOP(tBipa) are 9454, 9744,
and 6810 g/mol, respectively, based on the molecular formula.
Both MOP(1) and MOP(2) were slightly soluble in THF, which
allowed us to characterize them by electronic absorption spectros-
copy. The isolated MOPs showed an absorption band at ~690 nm
attributed to Band(I) of the dinuclear copper(II) paddlewheel
cluster (Figure S4), which is characteristic of typical copper-based
MOPs.9c Elemental analysis suggested reasonable formulae for the
MOPs that included coordinated solvents, as indicated by specific
weight losses in the thermogravimetric (TG) analysis (Figure S5).
However, all crystallization attempts gave amorphous products,
which could be due to the bulky and flexible CT moieties dangling
at the periphery of the MOPs.
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Figure 1. (a) Modeled structure of MOP(1); Z = Ph. (b) Chemical
structure of the building subunit of MOP(n) (n = 1 and 2), which
carries a chain transfer group.
Meanwhile, we discovered that a convergent approach was also
feasible for this system. Pre-synthesized linear polymers with
isophthalic acid end groups (so-called macroligands) instantly gave
the corresponding MOP-core CSP through a coordination reaction
with copper(II) ions. By using a mixture of ligands, this convergent
route provided a variety of miktoarm star polymers with the desired
arm compositions via one-step solution mixing. Furthermore, the
number of arm chains in the star polymer could be easily tuned in
the range of 5–24 by simply adjusting the mixing ratio between the
macroligand and low-molecular weight co-ligand.
In contrast to conventional covalent star polymers, the MOP-
core CSPs were easily dissociated into their free arm chains by the
addition of acid or a competing copper-chelating agent, which ena-
bled us to fully characterize the absolute molecular weight and
number of arm chains in the star polymers. The powerful approach
developed in this study offers access to a wide variety of star poly-
mers, including miktoarm stars with the desired number and com-
position of arm chains, without complex synthesis. Moreover, as
clearly demonstrated by the established use of PCPs/MOFs,8 the
cavity in the MOP core has obvious potential as a functional space
for reactions, catalysis, drug containment,11 and gas separation ap-
plications.12 Selection of the arm polymers, as well as the precise
control of the arm length, can be used to tune the physical proper-
ties, such as thermophysical properties, of the MOP-core CSPs on
demand. This would lead to highly processable porous soft materi-
als using MOPs as the building components, whereas the formation
of malleable materials from analogous PCP/MOFs is generally
difficult because of their crystalline nature.
Surface-initiated RAFT polymerization of tert-butyl acrylate
(tBA) mediated with MOP(2) was performed in THF in the pres-
ence of 2,2′-azobisisobutyronitrile (AIBN) (Figure 2a). The SEC
traces and kinetic plots of the polymerization are shown in Figure
2b and 2d, respectively. After an induction period of ~20 min, the
MOP(2) peak in SEC shifted to the higher-molecular-weight re-
gion with reaction time, which indicates the successfully elongation
of the polymeric arms from the MOP(2) surface under RAFT equi-
librium. The reaction was stopped at 135 min to obtain poly(tBA)-
grafted MOP(2) (termed MOP(2)-PtBA39 CSP, Mn,MOP (SEC) =
59200, Đ = 1.04) bearing PtBA arm chains with a degree of
polymerization (DP) of 39 (Table 1). As evidenced by the kinetic
plots (Figure 2d), the polymerization was well controlled. The SEC
peaks of MOP(2)-PtBA were quite narrow (Đ < 1.1) and uni-
modal for conversions up to 50% (Figure 2b and 2e), although the
molecular weights and dispersities determined by polystyrene-
calibrated SEC do not reliably indicate the actual values for MOP-
core CSPs. The number density of arm chains on the MOP-core
CSP is expected to be 24 per core. It should be noted that a small
amount of free polymer inevitably co-exists with the MOP-core
CSPs because of the RAFT equilibrium.4c
RESULTS AND DISCUSSION
Divergent Route. For the divergent approach, we employed a
RAFT14 polymerization method because it requires no metal cata-
lyst, which could disturb the coordination framework of the MOP
core. We designed isophthalic acid ligands substituted with dithio-
benzoate (1: Z = Ph) and trithioester (2: Z = SCH4H9), which are
typical chain transfer (CT) moieties for RAFT (Figure 1). The
coordination reaction between 1 and copper(II) acetate in N-
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The MOP core can be broken apart into its free arm chains by
the addition of a competitive copper chelator, N,N,N′,N′′,N′′-
pentamethyldiethylenetriamine (PMDETA), which enabled us to
characterize the arm chains in situ using SEC and NMR.15 After the
addition of PMDETA (ca. 20 µL of 0.1 M THF solution to ca. 5
mg/mL CSP solution), MOP(2)-PtBA39 disappeared immediate-
ly, as observed by SEC, to leave the free arm chains, PtBA39 (Mn,AC
(SEC) = 5780, Đ = 1.18, Figure 1b). The dependence of Mn,AC
(SEC), thus measured by in situ decomposition of MOP(2)-PtBA,
on the conversion (Figure 2f) indicated successful control over the
elongation reaction of the arm chains.
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Interestingly, the polydispersity of the free arm chains is some-
what wider (Đ > 1.1) than that of the original CSP, which probably
originates from the compact star architecture and statistical convo-
1
lution of the polydispersities (vide infra). SEC and H NMR meas-
urements after the decomposition showed no unreacted ligand, 2,
indicating that all 24 ligands of the MOP core were converted into
polymeric arms. The absorption band observed at 690 nm for
MOP(2)-PtBA39 does not change significantly compared with
that of MOP(2) (Figure S4), indicating that the MOP core is intact
after the polymerization. The total molecular weight of MOP(2)-
PtBA39 was calculated to be Mn,MOP (calcd.) = 127300, whereas
Mn,MOP (SEC) was 59200. Using SEC-multi-angle light scattering
(MALS) analysis, the absolute weight-averaged M, Mw,MOP (SEC-
MALS), of MOP(2)-PtBA39 was determined to be 157400, which
is quite consistent with the calculated value.
Figure 2. (a) Schematic diagram of the divergent route for polymer-
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grafted MOPs. The number of arm chains was reduced by half in the
illustration for clarity. (b,c) SEC traces of RAFT polymerization medi-
ated with MOP(2) (THF, 60 °C) for (b) tBA and (c) St. SEC traces of
(top) graft polymerizations at given reaction times and (bottom) free
arm chains prepared by addition of PMDETA to the reaction mixture.
(d,e,f) Results of the RAFT polymerization of tBA meditated with
MOP(2): (d) kinetic plots; dependence of (e) Mn,MOP (SEC) and (f)
Mn,AC (SEC) on the monomer conversion with respective Mw/Mn.
In the present acrylate system, radical combination termination
does not seem to be a major side reaction because no significant
star–star coupling product was observed by SEC (Figure 2b).
However, it should be noted that other side reactions, such as in-
tramolecular radical transfer resulting in mid-chain radical (MCR)
formation, might be operative.16 These reactions may affect the
inherent microstructures of CSPs by forming extra side-branches as
well as star–star coupling in situ. In fact, the small high-molecular-
weight shoulder observed for MOP(2)-PtBA39 could be indica-
tive of such larger by-products (Figure 2b) (vide infra).
By tuning the monomer feed and reaction time, we could easily
control the arm length and obtain MOP(2)-PtBA15 with a much
smaller size (Mn,MOP (SEC) = 23600, Đ = 1.06). The hydrodynamic
diameters of MOP(2), MOP(2)-PtBA15, and MOP(2)-PtBA39
were determined by dynamic light scattering (DLS) in THF to be
7.3, 9.9, and 16.8 nm, respectively (Figure S6).
Table 1. Results for the synthesis of MOP-core CSPs through divergent (core-first) and convergent (arm-first) approaches.
MOP
Arm Chain
Mn,MOP
(SEC)a
Mn,MOP
(calcd.)c
Mn,AC
(SEC)a
Mn,AC
(NMR)e
DP
(NMR)f
Sample
Đb
approachd
Đb
MOP(2)-PtBA15
MOP(2)-PtBA39
MOP(2)-PSt22
23600
59200
45300g
62900
138700
226300
21500
62600
162100
1.06
1.04
1.67g
1.03
1.04
1.10
1.04
1.03
1.07
56200
130400
65500
2340
5780
2570
6700
12700
21800
1880
6100
1.24
1.18
1.23
1.11
1.25
1.27
1.22
1.23
2280
15.1
39.2
22.3
53.7
147.5
271.5
13.7
56.0
à
à
à
ß
ß
ß
ß
ß
5370
2670
7230
19250
35140
2100
7520
MOP(PtBA54)
175000
521200
845000
51900
MOP(PtBA148)
MOP(PtBA272)
MOP(PnBA14)
MOP(PnBA56)
182100
434000
MOP(PnBA560.6/PtBA2720.4
)
Incorporation ratio: PnBA56/PtBA272 = 0.62/0.38
ß
b
c
aDetermined by SEC using THF as the eluent, calibrated with polystyrene standards. Mw/Mn. Molecular weight of MOP-core CSP calculated as
d
e
Mn,MOP (calcd.) = (Mn,AC (calcd.) + 63.5) × 24. Synthetic approach: divergent (à); convergent (ß). Molecular weight of free polymers calculated
as Mn,AC (calcd.) = DP × molecular weight of monomer + molecular weight of CT ligand. fDegree of polymerization (DP) determined by NMR. gMul-
timodal distribution.
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In contrast to tBA, the RAFT polymerization of styrene (St) The relationship between the Mn,MOP and Mn,AC values deter-
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mediated with MOP(2) appeared unsuccessful. The SEC traces of
MOP(2)-PSt22 were multimodal with a higher-molecular-weight
shoulder (Figure 2c). However, as evidenced by the kinetic plots
(Figure S7), the polystyrene arm chains underwent a controlled
elongation, leading to a narrower polydispersity (Đ ~ 1.2). These
contradictory observations could be due to star–star coupling4c
because the termination reactions of styrene polymerization are
dominated by combination reactions between living chain ends.17
As a large number of living chains (theoretically 24 per core) are
grafted from the single core, star–star coupling is very likely and
sensitive to the probability of radical combination reactions. Thus,
even a tiny rate of inter-star radical combination may result in a
considerable amount of star–star aggregates, leading to a multi-
modal peak with extremely high molecular weights.
mined by SEC are plotted in Figure 3d. The dashed line denotes a
linear fitting based on the RAFT polymerization results for
MOP(2)-PtBA54 (red markers), which shows that Mn,MOP (SEC)
is strictly proportional to Mn,AC (SEC). This trend in the SEC data
enabled us to prove the structural integrity of the star polymers
synthesized via the convergent approach. The data for
MOP(PtBA54), MOP(PtBA148), and MOP(PtBA272) are
plotted in Figure 3d. These data are perfectly matched with the fit
line, indicating that the MOP-core CSPs synthesized via the con-
vergent approach have a star architecture that is identical to that
obtained via the divergent approach. In fact, the PtBA39 arm chain
obtained by degrading MOP(2)-PtBA39, which was originally
synthesized via the divergent route, was used to successfully syn-
thesis a CSP, namely MOP(PtBA39), via the convergent route.
The SEC profile of MOP(PtBA39) is superimposable on that of
MOP(2)-PtBA39 (Figure S10), which strongly indicates the
equivalence of the divergent and convergent routes.15
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Unlike MOP(2), the solubility of MOP(1) was too low in any
solvent to give a sufficient concentration for the polymerization
reaction. However, MOP(1) was slightly dissolved in 1,4-dioxane
in the presence of St monomers (1.6 mg/mL),15 which allowed us
to examine the RAFT polymerization of St mediated with MOP(1).
Although MOP(1) similarly provided MOP(1)-PSt, a considera-
bly larger amount of free polymer was co-generated (Figure S8).
This free polymer formation is presumably caused by the low acti-
vation efficiency of the CT group in MOP(1) due to the poor sol-
ubility.
Mixed-Ligand Experiments. Using a mixed-ligand strategy, we
were able to obtain miktoarm star polymers in one step. When a
binary mixture of n-butyl acrylate, PnBA56 (DP = 56), and
PtBA272 with a mixing ratio of 0.6:0.4 (w/w) was reacted with
copper(II) acetate (1.5 equiv to total amount of macroligands), the
macroligands immediately converged into a single miktoarm CSP,
termed MOP(PnBA560.6/PtBA2720.4) (Figure 3a and 3c). The
Mn,MOP value of MOP(PnBA560.6/PtBA2720.4) determined by
SEC was between those of the homo star polymers of the respec-
tive macroligands, MOP(PnBA56) and MOP(PtBA272) (Table
1 and Figure S11), suggesting the statistical incorporation of the
two different arms. MOP(PnBA560.6/PtBA2720.4) was isolated
from any unreacted free polymer by repeated precipitation in
methanol/water (8:2, v/v) (Figure S12). To determine the incor-
poration ratio of the two arm chains, the isolated product was dis-
solved in deuterated acetone-d6; the addition of a drop of DCl
caused the star polymer to dissociate into the free arm chains. The
actual incorporation ratios of PnBA56 and PtBA272 were deter-
Convergent Route. Convergent synthesis is considered to be a
facile and conventional way for preparation of dendrimers,18 poly-
meric nanoparticles,19 and star polymers5–7 because it has the po-
tential to reduce synthetic efforts. We examined a convergent syn-
thesis (arm-first) approach (Figure 3a) using isophthalic acid-
terminated poly(tBA) with a DP of 54, termed macroligand
PtBA54 (Mn,AC (SEC) = 6700, Đ = 1.11). PtBA54 was prepared
via conventional RAFT polymerization mediated with 2.15 Interest-
ingly, upon mixing PtBA54 with copper(II) acetate in NMP, ano-
ther macromolecular compound that had a higher molecular
weight and quite narrow polydispersity was immediately formed
(Figure 3b). This macromolecular product was attributed to the
corresponding MOP-core CSP, termed MOP(PtBA54) (Mn,MOP
(SEC) = 62900, Đ = 1.03).
1
mined using H NMR measurements to be 0.62 and 0.38, respec-
tively (Figure S13). The incorporation ratio was quite similar to the
loading ratio, indicating that the coordination-driven self-assembly
of these different length macroligands occurred equally in this re-
gime. However, owing to the statistical nature of the convergent
reaction, we should note that MOP(PnBA560.6/PtBA2720.4) is
not a discrete product and should have a distribution of constitu-
ents.
To probe the limit of the macroligand length, we synthesized
long precursor chains with an isophthalic acid end group, PtBA148
(Mn,AC (SEC) = 12700, Đ =1.25, DP = 148) and PtBA272 (Mn,AC
(SEC) = 21800, Đ =1.27, DP = 272). In contrast to our assumption
that longer macroligands are incapable of MOP-core formation
because of steric congestion, even the longest PtBA272 provided
the corresponding MOP-core star polymer, termed
MOP(PtBA272), with an extremely high molecular weight and
narrow polydispersity (Mn,MOP (SEC) = 226300, Đ = 1.10) (Figure
S9). SEC-MALS analysis allowed us to determine the absolute
Mw,MOP (SEC-MALS) for MOP(PtBA148) and MOP(PtBA272)
to be 522000 and 836000, which showed good agreement with the
calculated values (Table 1). Unfortunately, the limit of the macro-
ligand length has not yet been determined because the size-
exclusion limit of SEC columns hampered the characterization of
MOP formation from longer macroligands. The results for the
convergent synthesis of MOP-core CSPs are summarized in Table
1.
It should be noted that a small shoulder is occasionally observed
by SEC in the higher-molecular-weight region for both the conver-
gent and divergent CSPs (e.g., 13.5 min for MOP(2)-PtBA39, 13
min
for
MOP(PtBA54),
and
12
min
for
MOP(PnBA560.6/PtBA2720.4)) (Figures 2b, 3b, and 3c, respec-
tively). The origin of this shoulder is at present unknown. One
explanation for this peak could be the formation of star–star cou-
pling aggregates, which are caused by trace contamination with
telechelic by-products formed via unfavorable radical polymeriza-
tion side reactions.
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Figure 3. (a) Schematic diagram of the convergent route to the PnBA56/PtBA272 binary miktoarm star polymer. The number of arm chains was
reduced by half in the illustrations for clarity. (b) SEC traces of PtBA54 (dashed line) and the reaction mixture of PtBA54 and copper(II) acetate
(1.5 equiv) in NMP after 60 min (solid line). (c) SEC traces of a binary solution of PnBA56 and PtBA272 with a mixing ratio of 0.6:0.4 (dashed
line) and the reaction mixture of MOP(PnBA560.6/PtBA2720.4) and copper(II) acetate (1.5 equiv) in NMP after 60 min (solid line). (d) Double
logarithm plot of the relationship between Mn,MOP and Mn,AC determined by THF-SEC. The results of the divergent synthesis of MOP(2)-PtBA15
and MOP(2)-PtBA39 are plotted as blue triangles and red squares, respectively. The results of the convergent synthesis of MOP(PtBA54),
MOP(PtBA148), and MOP(PtBA272) are plotted as black circles, black squares, and black rhombuses, respectively. The dashed line denotes the
linear fitting of the results for MOP(2)-PtBA39.
In conventional star polymer syntheses, using either the diver-
gent or convergent route, one has to modify the synthetic scheme
to control the number of arm chains. In contrast, on MOP-core
CSPs, the number of arm chains is easily tuned using the mixed-
ligand method. Mixtures of macroligand PtBA54 and co-ligand
tBipa at variable mixing ratios (PtBA54:tBipa = 3:21, 4:20, 6:18,
12:12, and 18:6) were reacted with copper(II) acetate in NMP,
which readily gave corresponding MOP-core CSPs
MOP(PtBA54n/tBipam) (n and m denote the molar loading ratios
of PtBA54 and tBipa, respectively; n + m = 1). After 60 min, the
reaction mixture of each NMP solution was subjected to THF-SEC
measurement. Figure 4a depicts the SEC results for
MOP(PtBA54n/tBipam) (n = 1, 0.75, 0.5, 0.25, 0.167, 0.125, and 0,
As shown in the SEC traces, the peaks of the MOP-core CSPs
shifted to lower molecular weights as the loading ratio of tBipa
increased. This observation clearly indicates that the tBipa co-
ligand was concomitantly incorporated onto the MOP core to “di-
lute” the arm chain density. Each star polymer was isolated from
any free polymer by repeated precipitation in methanol/water (8:2,
v/v). 1H NMR measurements of each star polymer were performed
in acetone-d6 with a small aliquot of DCl to determine the incorpo-
ration number of PtBA54 on the core, namely, the number of arm
chains (Figure S14). The actual numbers of arm chains were de-
termined to be 18.5, 12.8, 7.3, 6.0, and 4.9 for the respective
MOP(PtBA54n/tBipam) (n = 0.75, 0.5, 0.25, 0.167, and 0.125).
The actual numbers of arm chains were quite consistent with the
loading ratios, indicating that the macroligand and small co-ligand
have an equal chance of incorporation onto a single MOP core.
Thus, this self-assembly process is independent of the ligand size in
this composition range.
where n = 1 and 0 correspond to MOP(PtBA54) and
MOP(tBipa), respectively).
The relationship between Mn,MOP determined by SEC of
MOP(PtBA54n/tBipam) and the actual number of arm chains is
plotted in Figure 4b. An exponential-like trend was observed,
whereas a linear relationship with the number of arm chains is ex-
pected. The exponential-like trend is in contrast to the linear trend
observed between Mn,MOP and Mn,AC (Figure 3d). Moreover, the
polydispersity (Đ = Mw/Mn) determined by SEC becomes nar-
rower as the number of arm chains increases (Figure 4b). These
observations can be explained by the compact shape of the star
polymers, as well as the known deviation from the linear calibration
standards for the analysis of star polymers.20 Moreover, the statisti-
cal convolution of the polymer polydispersities may play an addi-
tional role. Previous experimental and theoretical studies have
demonstrated that the quantitative ligation of multiple polymers
with similar polydispersities may result in a product that has a much
lower polydispersity than those of the original polymers.5b,21
Figure 4. (a) THF-SEC traces for the reaction mixture of
MOP(PtBA54n/tBipam); n = 1, 0.75, 0.5, 0.25, 0.167, 0.125, and 0,
where n = 1 and 0 correspond to MOP(PtBA54) and MOP(tBipa),
respectively. (b) Relationship between Mn,MOP determined by SEC for
MOP(PtBA54n/tBipam) and the actual number of arm chains deter-
1
mined by H NMR (square markers) (Figure S14). The colors of the
markers correspond to the spectra in (a). The dashed line denotes an
exponential fit as a guide for the eyes. The theoretical relationship
between Mn,MOP (calcd.) for MOP(PtBA54n/tBipam) and the number
of arm chains is plotted as a dotted line.
AFM Micrographs of Individual MOP-Core CSPs. Atomic
force microscopy (AFM) was used to visualize the shape of indi-5
vidual CSPs (Figure 5a and 5b). We prepared a thin-film sample
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of MOP(PtBA272) by spin-casting a highly diluted chloroform curve of MOP(PnBA14), which has a Tg of –43.7 °C, is depicted in
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solution (0.01 µg/mL) on a mica substrate. The individual mole-
cules adopted a core–shell structure with so-called “sunny-side up
egg” configuration (Figure 5c). Every particle was clearly shown to
have an individual core that is covered by a polymeric corona (Fig-
ure 5b). The height of the inner core was ~2.5 nm, which is con-
sistent with the diameter of MOP(2) (Figure 5d). This fact indi-
cates that the cage-like structure of the MOP core is maintained in
the dry state, even after polymerization, thus retaining the internal
cavity. It should be emphasized that this sophisticated macromo-
lecular architecture is formed instantly in one step: mixing of
isophthalic acid-terminated polymers and copper(II) ions in NMP
at 25 °C.
Figure S13. MOP(PnBA14) and MOP(PnBA56) synthesized
through the convergent method showed single peaks at q = 0.149
and 0.098 Å–1, respectively, with d-spacings of 4.21 and 6.40 nm
(Figure 6b). This polymer grafting technique realized an organized
arrangement of coordination nanocages with a uniform inter-MOP
distance, even in the liquid phase at ambient temperature. This
approach could offer a new design principle for highly processable
porous soft materials. Film fabrication and gas sorption tests on the
MOP-core CSPs are now under way to validate their potential as a
gas separation material.
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At present, we have no rational way to determine the actual mo-
lecular weight distributions of CSPs, as discussed earlier. AFM
micrographs could provide us with a visual insight regarding this
issue. The individual CSP particles of MOP(PtBA272) have a
wider size distribution than that deduced from the narrow polydis-
persity in the SEC analysis.
Figure 6. (a) SAXS profiles of bulk MOP(2)-PtBA15 (solid line) and
MOP(2)-PtBA39 (dashed line) measured at 25 °C. (b) SAXS profiles
of bulk MOP(PnBA14) (solid line) and MOP(PnBA56) (dashed
line) measured at 25 °C. (c) Schematic diagram of the envisioned pack-
ing structure of MOP-core CSPs in the bulk state. The hexagonal-like
arrangement of molecules adopted in the illustration is not supported
by the SAXS data. (d) Visual appearance of MOP(PnBA14) at room
temperature.
Figure 5. (a) AFM height image of individual particles of
MOP(PtBA272) (scale bar: 1 µm). (b) Magnified view of a single
particle of MOP(PtBA272) (scale bar: 100 nm). (c) Schematic dia-
gram of so-called “sunny-side up egg” morphology. (d) Height profile
along the green line in (a), where the yellow star denotes the zero-
length point.
CONCLUSION
In conclusion, we have shown both divergent and convergent syn-
thesis of a new class of CSPs with metal–organic polyhedral cores.
While the divergent route is accomplished through surface-initiated
RAFT polymerization mediated with pre-formed multifunctional
MOP cores, the convergent route readily provides the MOP-core
star polymer via coordination-driven self-assembly of end-
functionalized polymeric precursors (macroligand) and copper(II)
ions. The latter method further enabled us to control the number of
arm chains in situ and access a wide variety of star polymers, includ-
ing miktoarm stars, with desired compositions in one step. Such
MOP-core macromolecules may not only accelerate fundamental
studies on polymer topologies, but also open up a new field of CSPs.
In addition, the MOP core, in which small molecules can be ac-
commodated, provides many possible applications, including catal-
ysis,22 drug delivery,11 molecular channels,23 and gas separation.12
The physical properties of MOP-core CSPs could be tuned by
careful selection and precise control of the polymeric arms, which
provides us with a design strategy for highly processable porous soft
materials by using the coordination nanocages as a building com-
ponent.
Bulk Structure of MOP-Core CSPs. Small-angle X-ray scatter-
ing (SAXS) measurements were employed to gain insight into the
microstructure of the MOP-core CSPs in the bulk state. The SAXS
profiles of MOP(2)-PtBA15 and MOP(2)-PtBA39 showed re-
spective peaks at q = 0.145 and 0.103 Å–1, which correspond to d-
spacings of 4.33 and 6.13 nm, respectively (Figure 6a). The meas-
urements were carried out at 25 °C, at which temperature, the CSPs
are in the glassy state according to the glass transition temperature
(Tg) of 41.8 °C determined by differential scanning calorimetry
(DSC). A representative DSC curve for MOP(2)-PtBA39 is
shown in Figure S15. The observed d-spacings depended on the
length of the polymer arms, indicating that each MOP core is spa-
tially separated by the grafted polymeric arms (Figure 6c). The d-
spacings, which can be interpreted as the diameter of the CSPs in
the bulk state, are quite small compared with the diameters deter-
mined by DLS. This result suggests a densely packed organization
of the CSPs with interdigitation of the arm chains in the bulk, alt-
hough the single peak observed in the SAXS pattern is insufficient
to determine the microstructures unambiguously.
ASSOCIATED CONTENT
Supporting Information. Synthesis and characterization, kinetic
plots of RAFT polymerization, SEC traces, DLS data, electronic
absorption spectra, 1H NMR spectra, DSC curves, and FT-IR spec-
tra. This material is available free of charge via the Internet at
Interestingly, the MOP-core CSPs with n-butyl acrylate arms,
MOP(PnBA14) and MOP(PnBA56), also showed similar peaks
in the SAXS profile, although these CSPs are viscous liquids at the
measurement temperature (25 °C) (Figure 6b and 6d). The DSC
http://pubs.acs.org.
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Noro, S. Angew. Chem. Int. Ed. 2004, 43, 2334–2375. (c) Uemura, T.;
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Corresponding Authors
nhosono@icems.kyoto-u.ac.jp
kitagawa@icems.kyoto-u.ac.jp
Notes
The authors declare no competing financial interest.
Present Address
#Department of Applied Chemistry, Graduate School of Engi-
neering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-
8603, Japan.
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§Department of Chemistry and Biotechnology, School of Engi-
neering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku,
Tokyo 113- 8656, Japan.
ACKNOWLEDGMENTS
This work was supported by KAKENHI Grant-in-Aid for Specially
Promoted Research (25000007) from the Japan Society of the
Promotion of Science (JSPS) and the Regional Innovation Strategy
Support Program (Next-generation Energy System Creation Strat-
egy for Kyoto) from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan. N.H. acknowledges JSPS
for KAKENHI Grant-in-Aid for Young Scientists (B) (16K17959).
R.M. thanks PRESTO program of the Japan Science and Technol-
ogy Agency (JST) for the financial support. This work is also sup-
ported in part by the ACCEL program of JST. The authors thank
Dr. T. Terashima and Mr. Y. Hirai for useful discussions and SEC-
MALS measurements.
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