Nanometer-sized shell molecules that confine endohedral polymerizing units

Article abstract of DOI:10.1002/anie.200603561

Polymerization in a nutshell: A discrete spherical Pd₁₂L ₂₄ complex was prepared that confines 24 tethered methyl methacrylate (MMA) units internally (see picture; shell blue, Pd yellow, O red, C gray; MMA shown as space-filling representation). Endohedral radical polymerization of the monomer proceeds efficiently as a result of the high concentration of MMA units within the restricted space of the core (inner diameter: 3.7 nm). (Figure Presented).

Full text of DOI:10.1002/anie.200603561

Angewandte
Chemie
DOI: 10.1002/anie.200603561
Molecular Nanoparticles
Nanometer-Sized Shell Molecules That Confine Endohedral
Polymerizing Units**
Takashi Murase, Sota Sato, and Makoto Fujita*
Polymerization that proceeds in a restricted nanosized region
under external stimuli, without any volumetric shrinkage, is a
fascinating task in materials science because the physical and
optical properties of the polymerized region dramatically
change with nanoscale resolution, which has relevance to
high-density data-storage materials, for example.[1] Polymer-
izations in zeolites,[2] porous organic crystalline hosts,[2] and
porous coordination polymers[3] are currently active research
fields. These host materials offer well-defined nanosized
spaces or channels, but the nanometric regions are grouped
together in a bundle. Therefore, it is impossible to handle
them as soluble discrete scaffolds with high processability.
Emulsion polymerization in a micelle is another approach to
polymerization in spatially confined regions.[4] However,
micelles do not have well-defined structures and shapes, and
the number of monomers accommodated and polymerized in
micelles is not precisely restricted.
Scheme 1. Self-assembly of M12L24 molecular nanoparticles 2 with 24
Recently, we succeeded in the endohedral functionaliza-
tion of a self-assembled spherical complex that consists of 12
metals and 24 ligands.[5–7] These complexes are regarded as a
well-defined “molecular nanoparticle” with a precise chem-
ical structure and uniform diameter (up to 4.6 nm). It is
possible to position 24 functional groups precisely inward
through suitable linkers in the spherical complex.[6,7] Herein,
we report the preparation of molecular spheres that confine
24 methyl methacrylate (MMA) units at the interior
(Scheme 1). We show the endohedral radical polymerization
of MMA in the spheres, providing a new nanomolecular
material that is potentially applicable to high-density data-
storage materials.
MMA units.
1H NMR spectroscopy. Spheres 2b–d were prepared in a
À
similar way from 1b–d. After anion exchange from NO3 to
CF3SO3À, cold-spray ionization mass spectroscopy (CSI-
MS)[8] of the complexes 2a–d clearly confirmed an M12L24
composition with molecular weights of 14658, 15715, 16772,
and 17830 Da, respectively.
A comparison of the 1H NMR spectra of spherical
complexes 2a–d revealed their endohedrally functionalized
structures. While the signals derived from the aromatic
protons of the shell are almost identical, those from the
MMAunits are shifted downfield with an increase in length of
the oligo(ethylene oxide) linker, suggesting the aggregation
of MMA units at the core of the sphere. Diffusion-ordered
NMR spectroscopy (DOSY) experiments in [D6]DMSO
revealed the same diffusion coefficient (logD = À10.45) for
complexes 2a–d, indicating that all MMA units tethered to
the shell are confined in the spherical shell and do not come
out of the shell.[9] Molecular modeling of complexes 2a–d
using the Cerius2 program clearly shows that the effective
concentration of MMA units in the sphere is strictly
controlled by the linker length (Figure 1).
MMA-anchored ligands 1a–d were prepared in high
yields
in
three
steps
from
2,6-dibromophenol
=
(pMeC6H4SO2O(CH2CH2O)nH/K2CO3 (n = 1–4), CH2 C-
(CH3)COCl/NEt3; then 4-ethynylpyridine/[Pd(PhCN)2Cl2]/
tBu3P/(iPr)2NH; see Supporting Information). When ligand
1a (0.014 mmol) was treated with Pd(NO3)2 (0.007 mmol) in
deuterated dimethyl sulfoxide ([D6]DMSO, 0.7 mL) for 4 h at
708C, the formation of 2a as a single product was observed by
[*] T. Murase, Dr. S. Sato, Prof. Dr. M. Fujita
Department of Applied Chemistry
School of Engineering
Radical polymerization was carried out in the spherical
complex 2c (0.788 mm) in DMSO at 708C for 17 h using 2,2’-
azobis(isobutyronitrile) (AIBN, 8.3 mol% per MMA unit) as
radical initiator.[10] Figure 2shows the 1H NMR spectra of the
complex before and after polymerization. The intensity of the
signals of the vinyl protons (d = 5.7 and 5.4 ppm) decreased
and the signals from protons on sp3 carbon atoms appeared
after the polymerization. The conversion of MMA units into
PMMA segments was calculated from the ratio of the area
under the peaks corresponding to MMA units and those
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7257
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
[**] This research was supported by the CREST project of the Japan
Science and Technology Agency (JST), for which M.F. is the principal
investigator.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Communications
It is particularly interesting that the spherical complex can
“concentrate” 24 MMA monomer units. When radical
polymerization was carried out using ligand 1c at the same
concentration of MMA units (18.9 mm) as in the case of
complex 2c, the reaction hardly proceeded. With an internal
diameter of 3.7 nm for the spherical complex 2c and a
capacity of 24 MMAunits, the concentration of MMA units in
the complex is approximately 1.5m. Therefore, polymeri-
zation is efficiently promoted inside the spherical complex
even at very low concentrations of monomer.
When radical polymerization was carried out in the other
MMA-containing complexes, 2a, 2b, and 2d, under the same
polymerization conditions, 22, 29, and 62% conversion,
respectively, of MMA into PMMA was observed. Therefore,
complex 2c with a tri(ethylene oxide) linker afforded the best
efficiency (73% conversion). These results are in good
agreement with the molecular models of the complexes
(Figure 1). The 24 MMA units are the most closely packed at
the core of complex 2c, thus providing the most desirable
arrangement of the MMAunits for polymerization. The mono
and di(ethylene oxide) linkers seem to be slightly too short for
polymerization in the spherical complex, preventing the
frequent close approach of MMA units. When the tetra-
(ethylene oxide) linker is used (2d), MMA units repel each
other and lie away from the center of the complex because the
linker is too long.
To examine the degree of polymerization of the polymer-
ized sample, the PMMA segments tethered to the shell were
cleaved by hydrolysis and converted into the methyl ester.
From the sample obtained in 2c with 81% conversion, the
resulting oligomer was revealed by gel permeation chroma-
tography (GPC) analysis to have a number-average molecular
weight (Mn) of 1315 relative to PMMA standards and a
polydispersity index (Mw/Mn) of 1.60.
Figure 1. Molecular models of a) 2a, b) 2b, c) 2c, and d) 2d optimized
using a force-field calculation with Cerius2 3.5. MMA units are shown
in space-filling representation (aromatic shell blue, Pd yellow, O red,
C gray, H white).
In summary, we have prepared a spherical complex
containing 24 MMA units in which radical polymerization
can be effectively promoted. The complex has the ability to
concentrate monomer units at the core of the complex. The
present method is widely applicable to the preparation of
other complexes that contain reactive monomers. For exam-
ple, by attaching photochemically polymerizing functional-
ities at the interior surfaces of complexes, a photopolymer-
incarcerated sphere can be obtained. Studies on such a
photoresponsive sphere are in progress in our laboratory.
Figure 2. 1H NMR spectra of complex 2c a) before and b) after poly-
merization (500 MHz, [D6]DMSO, 300 K, tetramethylsilane).
Experimental Section
PMMA cleavage[12] and GPC analysis: The polymerized complex 2c
(81% conversion) was precipitated by adding ethyl acetate and dried
under reduced pressure. The PMMA segments tethered to the shell
were cleaved by hydrolysis using concentrated H2SO4 (0.5 mL) at
room temperature for 5 days, then the resulting solution was poured
into ice water to precipitate the product. The precipitated polymeth-
acrylic acid was esterified with trimethylsilyldiazomethane (0.1 mL,
2.0m in hexane) in CH3OH/THF (0.2and 0.7 mL, respectively) at
room temperature for 12h to yield PMMA oligomer. The number-
average molecular weight (Mn) and polydispersity index (Mw/Mn) of
the obtained and standard PMMA were measured with GPC (Japan
Analytical Industry LC-918) with refractive index detection. The
corresponding to the outer aromatic shell. It was found that
73% of the MMA units were converted into PMMA
oligomers. No new peaks appeared around the aromatic
region after the polymerization, indicating that the structure
of the outer shell was maintained during the polymeri-
zation.[11] With prolonged reaction time (31 h), the conversion
was increased to 85% but the shell was partially decomposed.
The conversion was also increased to 81% by increasing the
amount of AIBN used (42mol% per MMA unit).
columns were the connection of two Tosoh TSK-GEL G2000HxL
.
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Angewandte
Chemie
THF was used as eluent, with a flow rate of 1.0 mLminÀ1 at 358C. See
Supporting Information for a list of the physical properties of
complexes 2a–d.
Kitagawa, Angew. Chem. 2006, 118, 4218 – 4222; Angew. Chem.
Int. Ed. 2006, 45, 4112– 4116; c) T. Uemura, S. Horike, S.
Kitagawa, Chem. Asian J. 2006, 1, 36 – 44.
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Ruckenstein, Adv. Polym. Sci. 1997, 127, 1 – 58; c) I. Capek, Adv.
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[7] S. Sato, J. Iida, K. Suzuki, M. Kawano, T. Ozeki, M. Fujita,
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[8] CSI-MS is quite effective for analyzing the solution structures of
metal complexes: a) S. Sakamoto, M. Fujita, K. Kim, K.
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[9] The non-tethered (hollow) spherical complex also showed the
same diffusion coefficient. Therefore, the sizes of the spherical
complexes are uniform and do not depend on the contents.
[10] Although MMA monomers are concentrated in the complex,
AIBN is not. Therefore, a relatively large amount of AIBN is
required in the present study. Presumably, AIBN enters through
the portals of the shell framework under equilibrium.
[11] The diffusion coefficient of the complex did not change before
and after polymerization, indicating that the polymerization
proceeded only in the complex. See Supporting Information.
[12] a) A. Katchalsky, H. Eisenberg, J. Polym. Sci. 1951, 6, 145 – 154;
b) E. M. Loebl, J. J. OꢀNeill, J. Polym. Sci. 1960, 45, 538 – 540.
Received: August 31, 2006
Revised: October 5, 2006
Published online: January 5, 2007
Keywords: cage compounds · nanostructures · palladium ·
.
polymerization · self-assembly
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