Functionalization of coordination nanochannels for controlling tacticity in radical vinyl polymerization

Article abstract of DOI:10.1021/ja100406k

Systematic functionalization of porous coordination polymers (PCPs), [Cu₂(L)₂(ted)]_n (where L = dicarboxylates and ted = triethylenediamine), by introducing various substituents onto the component organic ligand, L, was performed to regulate the radical polymerization of methyl methacrylate (MMA) in the nanochannels. The effect of the substituent groups on stereoregularity of the resulting poly(methyl methacrylate) (PMMA) was observed, where the tacticity of the PMMA strongly depended on the number and position of the substituent. In particular, polymerization of MMA in [Cu₂(2,5-dimethoxyterephthalate) ₂(ted)]_n gave PMMA with high isotactic and heterotactic triad fractions, which is one of the most effective systems for changing the tacticity of PMMA in radical polymerization. To understand the mechanism of this drastic stereoregularity change, a variety of experimental and theoretical analyses, such as IR, N₂ adsorption, a statics study, and molecular dynamics (MD) calculations, were performed. Accurate MD calculations were helpful to determine the most plausible structures of [Cu₂(L) ₂(ted)]_n and revealed that the specific channel shape of [Cu₂(2,5-dimethoxyterephthalate)₂(ted)]_n induces the large tacticity change of the resulting PMMA.

Full text of DOI:10.1021/ja100406k

Published on Web 03/12/2010
Functionalization of Coordination Nanochannels for
Controlling Tacticity in Radical Vinyl Polymerization
,†,‡
†
†
,†,‡,§
Takashi Uemura,*
Yukari Ono, Yuh Hijikata, and Susumu Kitagawa*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, PRESTO, Japan Science and
Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan, and Institute for Integrated
Cell-Material Sciences (iCeMS), Kyoto UniVersity, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Received January 15, 2010; E-mail: uemura@sbchem.kyoto-u.ac.jp; kitagawa@sbchem.kyoto-u.ac.jp
2 2 n
Abstract: Systematic functionalization of porous coordination polymers (PCPs), [Cu (L) (ted)] (where
L ) dicarboxylates and ted ) triethylenediamine), by introducing various substituents onto the component
organic ligand, L, was performed to regulate the radical polymerization of methyl methacrylate (MMA) in
the nanochannels. The effect of the substituent groups on stereoregularity of the resulting poly(methyl
methacrylate) (PMMA) was observed, where the tacticity of the PMMA strongly depended on the number
and position of the substituent. In particular, polymerization of MMA in [Cu
2
(2,5-
dimethoxyterephthalate) (ted)] gave PMMA with high isotactic and heterotactic triad fractions, which is
2
n
one of the most effective systems for changing the tacticity of PMMA in radical polymerization. To understand
the mechanism of this drastic stereoregularity change, a variety of experimental and theoretical analyses,
such as IR, N
2
adsorption, a statics study, and molecular dynamics (MD) calculations, were performed.
(L) (ted)] and
induces the large tacticity
Accurate MD calculations were helpful to determine the most plausible structures of [Cu
revealed that the specific channel shape of [Cu
2
2
n
2 2 n
(2,5-dimethoxyterephthalate) (ted)]
change of the resulting PMMA.
2
,3
Introduction
feature of PCP catalysts. Larger, or bulkier, substrates cannot
react in the nanochannels of PCPs, which is accounted for by
the channel size and uniformity of the pores. Recently, several
In recent years, there has been considerable interest in porous
coordination polymers (PCPs) or metal-organic frameworks
groups have successfully performed asymmetric catalytic reac-
tions using chiral porous frameworks,
channels have demonstrated their utility in heterogeneous
asymmetricreactions,suchastransesterification, hydrogenation,
(
MOFs) composed of transition metal ions and bridging organic
2c,4
and the chiral PCP
1
ligands. In this research area, the structural versatility of
molecular chemistry has allowed the rational design and
assembly of materials with novel topologies and exceptional
host-guest properties. Thus, PCPs have emerged as a particular
class of functional solid materials for applications in many areas,
4a
4b,c
4d
and epoxidation. In this regard, the ability to incorporate a
variety of organic moieties into the nanochannels makes PCPs
excellent candidates as nanosized reactors, which contrasts
strongly with zeolites or other porous metal-oxide materials.
That is to say, the capability for modifying the environment of
the PCP pores using organic ligands allows for the tuning of
the guest molecular arrangement and serves as a route to tailor
the reactions in the channels. The important key to attain this
chemistry is the systematic functionalization of the pore surface
by introducing various substituents into the organic ligands,
where the substituent groups can be preinstalled within precursor
1
including storage, separation, and exchange. An especially
notable characteristic of PCPs is their regular nanochannels that
can be utilized for catalysts and molecular reactions. Well-
defined nanochannel structures based on PCPs have potential
advantages for inducing highly selective reactions. For example,
substrate size selectivity has been reported as a noteworthy
2
†
Department of Synthetic Chemistry and Biological Chemistry, Kyoto
University.
‡
PRESTO, JST.
iCeMS, Kyoto University.
§
(3) (a) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem.
Soc. 1994, 116, 1151. (b) Horike, S.; Dinca, M.; Tamaki, K.; Long,
J. R. J. Am. Chem. Soc. 2008, 130, 5854. (c) Dybtsev, D. N.; Nuzhdin,
A. L.; Chun, H.; Bryliakov, K. P.; Talsi, E. P.; Fedin, V. P.; Kim, K.
Angew. Chem., Int. Ed. 2006, 45, 916. (d) Xamena, F. X. L. I.; Abad,
A.; Corma, A.; Garcia, H. J. Catal. 2007, 250, 294. (e) Hasegawa, S.;
Horike, S.; Matsuda, R.; Furukawa, S.; Kinoshita, Y.; Kitagawa, S.
J. Am. Chem. Soc. 2007, 129, 2607.
(
1) For selected reviews on PCPs and MOFs, see: (a) F e´ rey, G. Chem.
Soc. ReV. 2008, 37, 191. (b) Yaghi, O. M.; O’Keeffe, M.; Ockwig,
N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705.
(
c) Kitagawa, S.; Kitaura, R.; Noro, S.-i. Angew. Chem., Int. Ed. 2004,
43, 2334. (d) Fischer, R. A.; W o¨ ll, C. Angew. Chem., Int. Ed 2008,
7, 8164. (e) Bradshaw, D.; Claridge, J. B.; Cussen, E. J.; Prior, T. J.;
4
Rosseinsky, M. J. Acc. Chem. Res. 2005, 38, 273.
(
2) For recent reviews on PCP catalysis and nanosized reactors, see: (a)
Forster, P. M.; Cheetham, A. K. Top. Catal. 2003, 24, 79. (b) Lee,
J. Y.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S.-B. T.;
Hupp, J. T. Chem. Soc. ReV. 2009, 38, 1450. (c) Ma, L.; Abney, C.;
Lin, W. Chem. Soc. ReV. 2009, 38, 1248. (d) Wang, Z.; Chen, G.;
Ding, K. Chem. ReV. 2009, 109, 322.
(4) (a) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim,
K. Nature 2000, 404, 982. (b) Wu, C.-D.; Hu, A.; Zhang, L.; Lin, W.
J. Am. Chem. Soc. 2005, 127, 8940. (c) Wu, C. D.; Lin, W. Angew.
Chem., Int. Ed. 2007, 46, 1075. (d) Cho, S.-H.; Ma, B.; Nguyen,
S. B. T.; Hupp, J. T.; Albrecht-Schmitt, T. E. Chem. Commun 2006,
2563.
10.1021/ja100406k  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 4917–4924 9 4917

A R T I C L E S
Uemura et al.
ligands before PCP synthesis or be postmodified after the
formation of the PCPs. However, alternation of substituents
the pores on the stereoregularity of the resulting polymers is
still unclear because of the limitation of pore design and
versatility. Thus, tailor-made porous matrix design would
contribute to the rational control of tacticity in a radical
polymerization system.
5
in organic ligands has been studied mainly for tuning the
5a-e
adsorption properties of PCPs,
and few studies have focused
5
h
on their application to nanoreactors. The design of PCPs by
tuning the ligands and substituent groups is strongly related to
the control of pore characteristics, and such pore engineering
will lead to the successful performance of regulating reactions
in PCP nanochannels.
Tunable microporous PCP channels can be utilized for this
1
0
strategy. In a previous work, the radical polymerization of
vinyl monomers was performed in PCP channels without any
1
1
substituent groups on the aromatic ligands. However, the
stereoregularity of the obtained polymers was found to be almost
unchanged, and the effect of the PCP nanochannels on the
tacticity seemed relatively small. Thus, significant improvement
is a prerequisite for regulating the tacticity of a vinyl polymer.
Here, we demonstrate a remarkable effect of PCPs on the
tacticity of poly(methyl methacrylate) (PMMA), where the
introduction of substituents on the organic ligands of the PCPs
had a strong influence on the tacticity of PMMA prepared in
The advent of efficient methods for taming radical polymer-
izations into controlled polymerizations has long been required
for the further development of functional polymer materials by
6
tuning primary polymer structures. However, in conventional
processes, the highly reactive free-radical species induce an
uncontrolled rapid chain growth without stereoselectivity, and
the reaction undergoes inevitable termination via a radical-radical
coupling and disproportion. In particular, control of the tacticity
in radical polymerization of vinyl monomers is very difficult
because of the lack of efficient methods for providing a
stereospecific environment around the propagating radical
2 2 n
the channels of [Cu L (ted)] (where ted ) triethylenediamine
and L ) monosubstituted terephthalate, 1; L ) 2,5-disubstituted
terephthalate, 2; and L ) 2,3-disubstituted terephthalate, 3). The
control of PMMA stereoregularity was accomplished by local
and also global design of the PCP channel structure. The
appropriate positioning of substituents on the terephthalate
moiety resulted in the formation of PCP with a highly regular,
very narrow, and helical channel structure, which turned out to
be effective for increasing isotacticity of PMMA.
7
species. To overcome this problem, many attempts at control-
ling the tacticity have been carried out by the addition of polar
8
molecules or Lewis acids to the reaction medium. In such a
system, these additives are supposed to interact with vinyl
monomers and/or around the propagating radical species to
induce a stereospecific chain growth via coordination. A more
precise method to control the tacticity is solid-state radical
Experimental Section
9
polymerization utilizing nanoporous materials. Although me-
sopores (2-50 nm) are too large to regulate vinyl polymeriza-
tion, microporous hosts (<2 nm) can allow the production of a
few stereospecific polymers. Isotactic poly(acrylonitrile) has
Materials. All the reagents and chemicals used were obtained
from commercial sources, unless otherwise noted. The 2,2′-
azobis(isobutyronitrile) (AIBN) was recrystallized from MeOH
solution, and the methyl methacrylate (MMA) was purified by
vacuum distillation prior to use. Several bridging ligands, such as
9
d
been prepared in microporous channels composed of urea. It
should be also noted that polymerization within macromolecu-
larly stereoregular porous thin films provides highly stereoreg-
12
13
2
-fluoroterephthalic acid, 2-chloroterephthalic acid, 2-meth-
14 15
oxyterephthalic acid, 2,3-difluoroterephthalic acid, 2,3-dimethox-
yterephthalic acid, 2,5-difluoroterephthalic acid, and 2,5-
dimethoxyterephthalic acid, were prepared using methods described
previously.
9
f
ular vinyl polymers. However, in these systems, the effect of
16
17
18
(
5) (a) Devic, T; Horcajada, P.; Serre, C.; Salles, F.; Maurin, G.; Moulin,
B.; Heurtaux, D.; Clet, G.; Vimont, A.; Greneche, J.-M.; Le Ouay,
B.; Moreau, F.; Magnier, E.; Filinchuk, Y.; Marrot, J.; Lavalley, J.-
C.; Daturi, M.; F e´ rey, G. J. Am. Chem. Soc. 2010, 132, 1127. (b)
Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Watcher, J.; O’Keefe,
M.; Yaghi, O. M. Science 2002, 295, 469. (c) Rowsell, J. L. C.; Yaghi,
O. M. J. Am. Chem. Soc. 2006, 128, 1315. (d) Babarao, R.; Jiang, J.
Langmuir 2008, 24, 6270. (e) Tanabe, K. K.; Wang, Z.; Cohen, S. M.
J. Am. Chem. Soc. 2008, 130, 8508. (f) Wang, Z.; Cohen, S. M. Chem.
Soc. ReV. 2009, 38, 1315. (g) Goto, Y.; Sato, H.; Shinkai, S.; Sada,
K. J. Am. Chem. Soc. 2008, 130, 14354. (h) Ingleson, M. J.; Barrio,
J. P.; Guilbaud, J. B.; Khimyak, Y. Z.; Rosseinsky, M. J. Chem.
Commun. 2008, 2680.
Preparation of 2-Methylterephthalic Acid. Anhydrous THF
50 mL) was added to magnesium turnings (0.96 g, 40 mmol) in a
(
(9) (a) Farina, M.; Di Silvestro, G.; Sozzani, P. In ComprehensiVe
Supramolecular Chemistry; Reinhoudt, D., Ed.; Pergamon: Oxford,
1996; Vol. 10, p 371. (b) Miyata, M. In ComprehensiVe Supramo-
lecular Chemistry; Reinhoudt, D. Ed.; Pergamon: Oxford, 1996; Vol.
10, p 557. (c) Allcok, H. R.; Silverberg, E. N.; Dudley, G. K.
Macromolecules 1994, 27, 1033. (d) Minagawa, M.; Yamada, H.;
Yamaguchi, K.; Yoshii, F. Macromolecules 1992, 25, 503. (e)
Blumstein, A.; Malhotra, A. L.; Watterson, A. C. J. Polym. Sci. Part
A-2 1970, 8, 1599. (f) Serizawa, T.; Hamada, K.-i.; Akashi, M. Nature
2004, 429, 52.
(
6) For representative reviews on controlled or living radical polymeri-
zation, see: (a) Moad, G.; Solomon, D. H. The Chemistry of Radical
Polymerization, 2nd ed.; Elsevier Science: Oxford, UK, 2006. (b)
Matyjaszewski, K., Davis, T. P. Handbook of Radical Polymerization;
Wiley-Interscience: New York, 2002. (c) Braunecker, W. A.; Maty-
jaszewski, K. Prog. Polym. Sci. 2007, 32, 93. (d) Ouchi, M.;
Terashima, T.; Sawamoto, M. Chem. ReV. 2009, 109, 4963. (e)
Yamago, S. Chem. ReV. 2009, 109, 5051. (f) Iha, R. K.; Wooley, K. L.;
Nystr o¨ m, A. M.; Burke, D. J.; Kade, M. J.; Hawker, C. J. Chem. ReV.
(10) (a) Uemura, T.; Yanai, N.; Kitagawa, S. Chem. Soc. ReV. 2009, 38,
1228. (b) Uemura, T.; Horike, S.; Kitagawa, S. Chem. Asian J. 2006,
1, 36–44. (c) Vitorino, M. J.; Devic, T.; Tromp, M.; Ferey, G.;
Visseaux, M. Macromol. Chem. Phys. 2009, 210, 1923.
(11) Uemura, T.; Ono, Y.; Kitagawa, K.; Kitagawa, S. Macromolecules
2008, 41, 87.
(12) Bernhard, H. G.; Markus, C. G.; Peter, N.; Ulrich, W. S. Macromol.
Chem. Phys. 2000, 201, 1476.
2
009, 109, 5620.
(
(
7) For recent reviews on stereoregulated radical polymerization, see: (a)
Satoh, K.; Kamigaito, M. Chem. ReV. 2009, 109, 5120. (b) Kamigaito,
M.; Satoh, K. Macromolecules 2008, 41, 269. (c) Habaue, S.;
Okamoto, Y. Chem. Rec. 2001, 1, 46.
(13) Shen, H. C.; McDowell, C. C.; Sankar, S. S.; Freeman, B. D.; Kumpf,
R. J.; Wicks, D. A.; Lantman, C. W.; Noel, C. J. Polym. Sci. Part B
1996, 34, 1347.
(14) Higashi, F.; Nakajima, K.; Zhang, X. J. Polym. Sci: Part A 1994, 32,
747.
8) (a) Yamada, K.; Nakano, T.; Okamoto, Y. Macromolecules 1998, 31,
7
598. (b) Isobe, Y.; Fujioka, D.; Habaue, S.; Okamoto, Y. J. Am.
(15) Sugawara, S.; Ishikawa, N. Kogyokagakuzasshi 1970, 74, 972.
(16) Crowther, G. P.; Sundberg, R. J.; Sarpeshkar, A. M. J. Org. Chem.
1984, 49, 4657.
Chem. Soc. 2001, 123, 7180. (c) Lutz, J.-F.; Neugebauer, D.;
Matyjaszewski, K. J. Am. Chem. Soc. 2003, 125, 6986. (d) Lutz, J.-
F.; Jakubowski, W.; D.; Matyjaszewski, K. Macromol. Rapid Commun.
(17) Huang, X.-S.; Qing, F.-L. J. Fluorine Chem. 2008, 129, 1076.
(18) Passaniti, P.; Browne, W. R.; Lynch, F. C.; Hughes, D.; Nieuwen-
huyzen, M.; James, P.; Maestri, M.; Vos, J. G. J. Chem. Soc., Dalton
Trans. 2002, 1740.
2
004, 25, 486. (e) Wan, D.; Satoh, K.; Kamigaito, M. Macromolecules
006, 39, 6882. (f) Hirano, T.; Miyazaki, T.; Ute, K. J. Polym. Sci.,
2
Part A: Polym. Chem. 2008, 46, 5698.
4
918 J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010

Control of Tacticity in Radical Vinyl Polymerization
A R T I C L E S
Table 1. Radical Polymerization of MMA in Nanochannels of [Cu
2
(L)
2
(ted)]
n
(1-3) at 70 °C for 9 h
a
b
c
1
Determined by TGA. Obtained by GPC calibrated by PMMA standards. Determined by H NMR measurement in nitrobenzene-d
5
at 110 °C.
d
e
f
ΣP ) Pm/r + Pr/m, where Pm/r ) [mr]/(2[mm] + [mr]) and Pr/m ) [mr]/(2[rr] + [mr]). F ) 2[m][r]/[mr]. Bulk polymerization was carried out under
a comparable condition to those employed in PCPs.
100 mL three-neck flask under a nitrogen atmosphere. After addition
Table 2. Temperature Effects on Polymerization of MMA in 2OMe
of 2,5-dibromotoluene (1.99 g, 8 mmol), the mixture was heated
to 60 °C, and then 1,2-dibromoethane (3.30 g, 18 mmol) was added
to activate the magnesium metal. The resulting mixture was stirred
for a period of 15 h at 60 °C and cooled to -78 °C. An excess
amount of dry ice was added to the reaction mixture, and the
mixture was allowed to reach room temperature and then stirred
for 3 h. The reaction was quenched with a 3 N aqueous solution of
HCl (20 mL) and extracted with diethyl ether (20 mL). The organic
a
b, c
conv (%)
d
tacticity mm:mr:rr
reaction temp (°C)
reaction time (h)
9
7
6
5
2
0
0
0
0
0
3
9
9
9
48
42
65
63
n.d.
n.d.
23
21
0
26:59:16
28:53:19
23:55:22
24:54:22
24:46:30
24:47:29
25:50:25
23:56:21
9
48
96
96
96
168
-
-
20
40
-78
layer was washed with water several times and dried over Na
2 4
SO .
The solvent was removed, and the white powder product was dried
1
in a vacuum (1.20 g, 83%). H NMR (DMSO-d
H), δ ) 7.77-7.87 (m, 3H).
6
): δ ) 2.55 (s,
a
3
Low-temperature polymerizations below 20 °C were initiated with
b c
tributylborane-oxygen system. Determined by TGA. n.d. ) not
Preparation of 1-3. These porous compounds were prepared
according to reported preparation methods. Solutions of tereph-
thalic acids (6.4 mmol) in DMF or MeOH (300 mL) were slowly
d
1
1
9
determined. Determined by H NMR measurement in nitrobenzene-d5
at 110 °C.
added to a methanol solution (50 mL) containing CuSO
4
2
·5H O
the PMMA inside the channels, the composite was stirred in a 0.05
M aqueous solution (15 mL) of sodium ethylenediaminetetraacetate
(
1.6 g, 6.4 mmol). After being stirred for 3 days at room
temperature, the resultant blue precipitate was collected by cen-
trifugation and transferred to a stainless sealed Teflon reaction
vessel. After the addition of DMF (30 mL), a toluene solution (20
mL) of ted (0.36 g, 3.2 mmol) was then added to the vessel. The
mixture was allowed to react at 140 °C for 6 h, and then the resultant
precipitates was filtered, washed with MeOH, and dried under
reduced pressure.
(
Na-EDTA) for 3 h for the complete dissolution of the host porous
framework. The collected polymer product was washed with water
(
2 mL × 3) and dried under a reduced pressure at room temperature.
The amount of monomer MMA adsorbed in the PCPs without
AIBN was calculated from the weight loss up to 200 °C by
thermogravimetric analysis (TGA), as shown in Table 1. We
compared the value of the weight loss corresponding to the adsorbed
MMA in the PCPs before and after polymerization to determine
the monomer conversion rate (Table 1 and 2). Quantitative recovery
of the resulting PMMA was confirmed after the dissolution of PCP
frameworks by comparison with the conversion ratio.
Polymerization of MMA in 1-3. The dried host compound
50 mg) was prepared by evacuation (<0.1 kPa) at 130 °C for 5 h
(
in a Pyrex reaction tube. Subsequently, the host powder was
immersed in MMA (0.13 mL) with AIBN (1 mg) at room
temperature for 0.5 h to incorporate MMA and the radical initiator
into the nanochannels. After excess monomer external to the host
crystals was removed completely by evacuation (2.0 kPa) at room
temperature for 1 h, the reaction tube was filled with nitrogen and
heated to 70 °C to perform the polymerization over a period of
Polymerization of MMA below 20 °C. The dried powder of 2
composed of 2,5-dimethoxyterephthalate (2OMe; 50 mg) was
immersed in a mixture of Et
M Et O solution of tributylborane (10 µL) for 20 min at room
temperature in a nitrogen atmosphere. The Et O was evacuated at
0 kPa for 10 min, followed by removal of any excess MMA at
.0 kPa for 0.5 h. Then, the reaction vessel was filled with nitrogen
2
O (170 µL), MMA (80 µL), and a 1.0
2
9 h. After the polymerization was complete, the composite was
washed with MeOH to remove any unreacted MMA. To isolate
2
2
2
and cooled to the appropriate temperature. The polymerization of
the MMA was initiated using oxygen by inletting air using a
microsyringe. To quench the polymerization, a 10 wt % MeOH
solution of di-tert-butylcresol (3 mL) was added, and the resulting
(
19) (a) Seki, K.; Mori, W. J. Phys. Chem. B 2002, 106, 1380. (b) Kitaura,
R.; Iwahori, F.; Matsuda, R.; Kitagawa, S.; Kubota, Y.; Takata, M.;
Kobayashi, T. C. Inorg. Chem. 2004, 43, 6522. (c) Matsuda, R.;
Kitaura, R.; Kitagawa, S.; Kubota, Y.; Takata, M. To be submitted.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010 4919

A R T I C L E S
Uemura et al.
composite was washed with MeOH (3 mL × 2). Isolation of the
encapsulated PMMA was performed in an aqueous Na-EDTA
solution using the method discussed above.
Molecular Dynamics Simulation. Molecular dynamics (MD)
simulations of the PCPs were performed using the Materials Studio
Modeling v4.4 software package (Accelrys Inc., San Diego, CA)
2
0
using the Universal Force Field, as implemented in the Forcite
module. The charges were dealt with by charge equilibration method
in this system. The initial structures of the PCPs were generated
on the basis of the X-ray crystal structures. The quench dynamics
with the optimized structures were conducted at 3000 K, and then
MD calculations were carried out at 300 K for 200 ps under NVT
conditions.
Measurements. The X-ray powder diffraction (XRPD) data were
collected using a Rigaku RINT 2000 Ultima diffractometer employ-
ing Cu KR radiation. The TGA was carried out from room
-
1
temperature to 500 °C at a heating rate of 10 °C min using a
Rigaku Instrument Thermo plus TG 8120 in a nitrogen atmosphere.
Nitrogen adsorption measurements at 77 K were carried out using
a Quantachrome Autosorb-1. The samples were dried under high
2 2 n
Figure 1. Schematic illustration of [Cu (L) (ted)] (1-3).
encapsulating MMA in the nanochannels exhibited a weight loss
corresponding to the adsorbed MMA up to 200 °C, also
suggesting a nanoporous structure was present in 1-3. From
these analyses, a correlation between the amount of adsorbed
guest molecules and the pore size of 1-3 was clearly seen,
where the adsorption amount decreased with increasing number
or size of the subsutituent on the terephthalate (Table 1).
Radical Polymerization of MMA in PCPs. Considering the
size of MMA (molecular size ) 5.9 × 4.1 Å), only single chains
of PMMA can propagate in the channels of the PCPs, and thus,
the pore walls of the PCPs will strongly influence the polym-
erization. Radical polymerization of MMA in the channels of
-2
vacuum (<10 Pa) at 120 °C for 5 h before the measurements
were performed. The Dubinin-Radushkevich (DR) equation was
2
1
used to characterize the detailed porous properties of the PCPs.
The DR equation is given by
2
ln W ) ln W + (A/ꢀE )
0
0
where W and W
pressure (P/P ) and the saturated amount of nitrogen adsorbed,
respectively, E is a characteristic adsorption energy, and the
parameter A is Polanyi’s adsorption potential, defined as A ) RT
ln(P /P). The parameter ꢀ is the affinity coefficient and is related
to the adsorbate-adsorbent interaction. The DR plots showed a
linear relationship in the lower P /P region, from which the
micropore volume and the value of ꢀE were obtained. The
micropore volume, V , was calculated from the value of W and
the density of liquid nitrogen at 77 K (0.807 g cm ). The H and
0
are the amount of nitrogen adsorbed at a relative
0
0
0
1
with monosubstituted terephthalates was carried out as follows.
The MMA monomer was adsorbed in the channels by immer-
sion of 1 in MMA, followed by removal of any excess MMA
external to the host crystals under reduced pressure. The
host-monomer inclusion compounds were then heated with
AIBN to induce polymerization at 70 °C for 9 h. No change in
the peak patterns was observed in the XRPD profiles of 1 during
the polymerization, indicating that the channel structures of 1
were maintained on the inclusion and polymerization of MMA.
The resulting PMMA was quantitatively recovered from the
composites by dissolution of the frameworks of 1 in an aqueous
Na-EDTA solution. Table 1 summarizes the polymerization
results (conversion, molecular weight, and tacticity) of MMA
in the nanochannels of 1. The polymerization proceeded in good
yields, except for the case of using 1 with a nitro-substituted
ligand. This is because the generated radical was quenched by
0
0
m
0
-3
1
13
C NMR spectra were obtained using a JEOL A-500 spectrometer
operating at 500 MHz. The GPC measurements on the PMMA were
performed in CHCl at 40 °C on three linear-type polystyrene gel
columns (Shodex K-805 L) that were connected to a JASCO PU-
80 precision pump, a JASCO RI-930 refractive index detector,
3
9
and a JASCO UV-970 UV/vis detector set at 256 nm. The columns
were calibrated against standard PMMA samples. The infrared
spectra were measured using KBr disks employing a Perkin-Elmer
Spectrum BX FT-IR system.
Results and Discussion
Preparation of Host PCPs. Most of the host compounds 1-3
22
the nitro group on the pore walls. From the viewpoint of
2
+
were newly prepared by heating a mixture of Cu , ted, and
the various types (mono-, 2,5-di-, and 2,3-disubstitution) of
1
tacticity, the H NMR spectrum of the resulting PMMA was
measured to determine the triad fractions corresponding to the
1
9
terephthalate ligands in sealed vessels (Figure 1). XRPD
23
methyl groups on the main chain. In these measurements, the
measurements on 1-3 gave diffraction patterns characteristic
nanochannels of 1 were found to produce PMMA with more
isotactic and heterotactic triads compared with conventional bulk
and solution polymerizations (Table 1). Radical polymerization
2 2 n
of the previously reported [Cu (terephthalate) (ted)] (Figure
1
1,19a
2+
2
),
which reveals that layered structures based on Cu
and terephthalates are linked with ted as a pillar ligand and one-
dimensional microporous channels are formed along the pillar
ligand (Figure 1). The microporous structure of 1-3 was
confirmed by nitrogen adsorption measurement, which showed
Type I isotherms for all the samples, and the size of the pores
was ca. 4-7 Å. TGA of the host compounds 1-3 fully
2 2 n
of MMA using the channels of [Cu (terephthalate) ted] (a
nonsubstituted terephthalate) provided PMMA with a triad
11
tacticity mm:mr:rr ) 8:40:52. Thus, a measurable effect of
the substituent group on the stereoregularity of PMMA was
confirmed; in particular, the use of terephthalate ligands with
amino and methoxy substituents showed relatively large influ-
ences on tacticity (Table 1 and Figure 3).
(
20) (a) Rapp e´ , A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III;
Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (b) Casewit, C. J.;
Colwell, K. S.; Rapp e´ , A. K. J. Am. Chem. Soc. 1992, 114, 10035.
Next, we performed polymerization of MMA utilizing the
channels of 2 consisting of 2,5-disubstituted terephthalates for
(
c) Casewit, C. J.; Colwell, K. S.; Rapp e´ , A. K. J. Am. Chem. Soc.
992, 114, 10046.
21) Dubinin, M. M. Chem. ReV. 1960, 60, 235.
1
(22) Bovey, F. A.; Kolthoff, I. M. Chem. ReV. 1948, 42, 491.
(
(23) Hatada, K. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 245.
4
920 J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010

Control of Tacticity in Radical Vinyl Polymerization
A R T I C L E S
2 2 n
Figure 2. XRPD patterns of [Cu (L) (ted)] (1-3 and L ) terephthalate).
To study the details of the effect of substituent, several
PCPs with 2,3-disubstituted groups (3) were prepared and
used in the polymerization of MMA (Table 1). In contrast
to the polymerization in 2, syndiotacticity of the PMMA
obtained in 3 increased in comparison with those prepared
in the corresponding channels of 1. Note that the triad
tacticities of PMMA obtained in 3 with 2,3-dimethoxytereph-
thalate (3OMe) (mm:mr:rr ) 8:39:53) were almost the same
to those obtained for nonsubstituted [Cu
2
(terephthalate)
2 n
ted]
1
1
(
Figure 3). This result clearly suggests that the tacticity of
PMMA synthesized in PCPs strongly depends on the position
of substituents on the ligands.
Detailed Analysis of the Polymerization. The interaction
between MMA and the pore walls of the PCPs was examined
using IR spectroscopy. The IR spectrum of neat MMA shows
a strong band corresponding to the CdO stretching occurring
Figure 3. 1H NMR spectra of PMMA in nitrobenzene-d
in the bulk condition and in the nanochannels of [Cu
methoxy-substituted ligands). The polymerizations were carried out at
0 °C unless otherwise noted.
at 110 °C prepared
(L) ted] (L )
5
-1
at 1726 cm . Similarly, the peak from the CdO moiety of
2
2
n
-
1
MMA appears at 1720-1729 cm in all the spectra of
PCP-MMA inclusion compounds, and the peak position is
independent of the host structures of 1-3. In addition, the peaks
of the MMA unit do not shift after the polymerization of MMA
in the channels of 1-3. Thus, these results rule out the
possibility that the stereoregularity of PMMA is affected by a
strong interaction between MMA and the channel walls of the
PCPs.
7
further change of the tacticity of PMMA (Table 1). Although
PMMA could be prepared in all the channels, the yield and
molecular weight of the obtained polymer were relatively low
in the case of using 2 with a bromo-substituted ligand, which
was probably because of the low crystallinity of the host (Figure
In vinyl polymer systems, when stereoregulation in the
addition of monomers to propagating radicals is not influ-
enced by the stereostructure of all the other placements, then
the distribution of tacticity obeys the theory of Bernoulli
2
). A marked effect of the substitution groups on the stereo-
regularity of the PMMA was observed in this polymerization
system, revealing that additional attachments of the substituents
lead to an increase in the mm and mr fractions of PMMA
compared with those prepared in 1 composed of corresponding
monosubstituted terephthalates (Table 1). Interestingly, this
effect became more obvious as the substituent group on the
terephthalate in 2 was larger. The use of a larger substituent
fills the pore space, resulting in a narrower channel structure.
Such narrow channels induce polymerization with less sterically
2
5
statistics. Previous NMR analysis has indicated that Ber-
noulli statistics can be applied to the radical polymerization
2
6
of MMA at 70 °C. To examine the effect of PMMA tacticity
on the resulting stereosequence during the polymerization
in PCPs, we calculated the important parameters using the
following relationships:
1
1,24
bulky isotactic units rather than syndiotactic units.
we polymerized MMA in 2 with 2,5-dimethoxyterephthalate
2OMe), PMMA with high mm and mr contents (mm:mr:rr )
8:53:19) was obtained (Table 1 and Figure 3). This change of
When
Pm/r ) 1 - Pm/m ) [mr]/(2[mm] + [mr])
(
2
Pr/m ) 1 - Pr/r ) [mr]/(2[rr] + [mr])
PMMA tacticity is much larger than those previously reported
in controlled radical polymerization systems, and thus, the
utilization of this porous framework is one of the most effective
methods for changing tacticity in radical polymerization of
where Pm/r is the probability that a racemo addition follows
a meso addition, Pr/m is the probability that a meso addition
follows a racemo addition, and [mm], [mr], and [rr] are the
9
e,f
MMA.
(25) (a) Coleman, B. D.; Fox, T. G. J. Polym. Sci: Part A 1963, 1, 3183.
(
1
b) Hatada, K.; Kitayama, T.; Ute, K. Prog. Polym. Sci. 1988, 13,
89. (c) Tanaka, Y.; Yamaguchi, F.; Shiraki, M.; Okada, A. J. Polym.
(
24) Hunt, M. A.; Shamsheer, J. M.; Uyar, T.; Tonelli, A. E. Polymer 2004,
Sci.: Polym. Chem. Ed. 1978, 16, 1027.
4
5, 1345.
(26) Bovey, F. A.; Tires, G. V. D. J. Polym. Sci. 1960, 44, 173.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010 4921

A R T I C L E S
Uemura et al.
Table 3. Micropore and Thermodynamic Parameters of 2OMe and
3OMe
3
3
W
0
(cm /g)
V
m
(cm /g)
0
ꢀE (kJ/mol)
2
3
OMe
OMe
297
294
0.46
0.46
10.0
9.7
Figure 4. Effect of polymerization temperature on syndiotacticity of
2
5b,27
PMMA prepared in 2OMe (filled circle) and under bulk conditions
(
open circle).
observed triad fractions. If stereoregulation follows Bernoul-
lian statistics, then
Pm/m ) Pr/m ) Pm
Pr/r ) Pm/r ) Pr
Figure 5. Pore-size distributions of (a) 2OMe and (b) 3OMe based on
HK model.
and thus
fectively control the arrangement of encapsulated MMA,
resulting in the large tacticity change of PMMA.
ΣP ) Pm/r + Pr/m ) 1
Structural Analysis of 2 and 3 with Dimethoxyterephthalates.
To understand the cause of the different tacticity values of
PMMA obtained from the channels of 2 and 3, we carried
out nitrogen adsorption measurements on 2OMe and 3OMe
at 77 K to elucidate the pore characteristics. As expected
from the crystal structures, these PCP compounds exhibited
Type I adsorption isotherms, showing a microporous struc-
ture. Dubinin-Radushkevich (DR) analysis of the obtained
m r
where P and P are the fractions of meso and racemo diads,
respectively. The value of ΣP obtained in the case of bulk
polymerization is almost equal to unity, supporting a Ber-
2
6
noullian pathway. However, the results of the analysis of
our polymerization studies show that the tacticity of PMMA
prepared in PCP channels slightly deviates from Bernoullian
theory (Table 1). In particular, it seems that PMMA with
lower syndiotacticity shows a larger deviation, suggesting
that the addition of the monomer was influenced by the
isotherms afforded the saturated amount of adsorption (W
micropore volume (V ), and isosteric heat of adsorption
). Table 3 summarizes the micropore and thermody-
0
),
m
2
5
stereochemistry of the growing chain ends in those systems.
To study the details of the polymerization mechanism, the
21
(
ꢀE
0
namic parameters of 2OMe and 3OMe, in which no obvious
difference in the parameters between 2OMe and 3OMe
suggests similar micropore dimensions and surface interac-
tions. However, the pore-size distributions of 2OMe and
2
5a
Coleman-Fox parameter, which is defined in the equation
below, was calculated from the tacticity values of the PMMA.
F ) 2[m][r]/[mr]
3
OMe, calculated using the Horvath-Kawazoe (HK)
28
model, showed a critical difference in the pore regularity
(Figure 5). The profile of 2OMe was almost unimodal, with
a peak occurring at 5.2 Å; however, several peaks and a broad
distribution from 4 to 11 Å were clearly observed in the
profile of 3OMe.
The value of F for the polymers obtained in our experi-
ments was found to be equal to (ΣP)-1 (Table 1), which
means that the polymerization of MMA in the PCP channels
2
5
can be described using first-order Markov statistics. Thus,
this fact clearly demonstrates that there is a penultimate effect
in the stereoregulation of the polymerization in the PCP
nanochannels.
Usually, X-ray analysis is the most powerful methods for
determining the crystal structures of PCPs. However, in the case
of 2OMe and 3OMe, the experimental X-ray structures showed
a disordered form of the ligand-rotational isomers, and no
distinct structural difference between 2OMe and 3OMe was
observed. To overcome this problem, we performed MD
simulations to determine the global minimum structures of
2OMe and 3OMe. Molecular simulation is a useful tool to
Using the nanochannels of 2OMe, the effect of the
polymerization temperature on the tacticity of PMMA was
studied from -78 to 90 °C (Table 2 and Figure 4). In the
usual polymerization of MMA in solution or in the bulk state,
syndiotacticity of PMMA continuously increases with de-
2
5b,27
creasing reaction temperature (Figure 4).
In contrast,
2
9
screen PCP structures for specific applications. In particular,
accurate MD calculations are helpful in interpreting the ambigu-
ous results from the X-ray analysis and to determine plausible
the syndiotacticity of PMMA prepared in 2OMe once
increased slightly, and then became lower, as the reaction
temperature decreased. Thus, PMMA obtained at lower
temperatures still contained high mm and mr triad fractions.
This unique phenomenon is ascribable to the suppressed
mobility (vibration and rotation) of the PCP framework at
lower temperatures. Such quiescent channels would ef-
3
0
structures of the PCPs. Figure 6 shows typical MD structures
for 2OMe and 3OMe. From our MD simulations, we could
(
(
28) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470.
29) (a) D u¨ ren, T.; Bae, Y.-S.; Snurr, R. Q. Chem. Soc. ReV. 2009, 38,
1
237. (b) Keskin, S.; Liu, J.; Rankin, R. B.; Johnson, J. K.; Sholl,
(
27) Reinm o¨ ller, M.; Fox, T. G. Polym. Prepr., Am. Chem. Soc. DiV. Polym.
D. S. Int. Eng. Chem. Res. 2009, 48, 2355.
Chem. 1966, 7, 999.
(30) Amirjalayer, S.; Schmid, R. J. Phys. Chem. C 2008, 112, 14980.
4
922 J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010

Control of Tacticity in Radical Vinyl Polymerization
A R T I C L E S
Figure 6. Typical MD structures of (a and b) 2OMe and (c and d) 3OMe (Cu, green; O, red; N, blue; C, gray). Hydrogen atoms are omitted for clarity.
The series of calculations was performed three times, and the nearly identical structures of 2OMe and 3OMe were reproducibly obtained.
Figure 7. Schematic images of possible rotational isomers of (a-d) 2OMe and (e-h) 3OMe.
3
2
understand how the global structures of the PCP materials were
arranged as a consequence of the local interactions among the
substituents on ligands. 2OMe and 3OMe have four possible
isomers because of the ability of the ligand rotation (Figure 7).
Our MD simulations suggest that more than 80% of the unit
structures of 2OMe form the highly regular and symmetrical
structure shown in Figure 7a. In contrast, the four rotational
isomers (Figure 7e-h) randomly and equivalently exist in the
MD structure of 3OMe, resulting in an irregular channel
structure. This different regularity of the PCP pores for 2OMe
and 3OMe is also supported by the pore size distribution data
rigid helices. This helical conformation forces the monomer
addition to be more and more favorable for isotactic chain
propagation, resulting in a deviation from Bernoullian statistics
2
5b,32a
and satisfying the first-order Markov model.
Because
polymerization in the channels of 2OMe is explained using first-
order Markov statistics, the increase of isotacticity of PMMA
obtained in the channels would be caused by the regular, narrow,
and helical natures of the 2OMe pores. A previous report has
shown that the ligand of [Zn L (ted)] (L ) 1,4-naphthalene-
2 2 n
dicarboxylate) does not freely rotate in the framework, and the
rotational motion is considerably decelerated by guest adsorp-
tion. Thus, our results here strongly suggest that the appropri-
ate positioning of the substituent groups in the PCP is of utmost
importance for determining and tuning PCP systems for practical
applications.
33
2
based on the N adsorption measurements. In this conformational
study, it should be also noted that the most plausible rotational
isomer of 2OMe (Figure 7a) forms a helically twisted channel
in the framework. Usually, isotactic vinyl polymers prefer helical
3
1
Conclusions
conformations owing to the steric effects of the side chains.
For example, radical polymerization of methacrylates with large
substituent groups produces highly isotactic polymers that form
In this work, we have prepared a variety of [Cu
compounds (1-3) by introducing substituent groups into the
2 2 n
L (ted)]
(
32) (a) Yuki, H.; Hatada, K.; Nishimori, T.; Kikuchi, Y. Polym. J. 1970,
1, 36. (b) Nakano, T.; Mori, M.; Okamoto, Y. Macromolecules 1993,
26, 867.
(
31) (a) Natta, G.; Danusso, F.; Moraglio, G. Makromol. Chem. 1958, 28,
66. (b) Ashby, G. E.; Hoeg, D. F. J. Polym. Sci. 1959, 39, 535. (c)
Cojazzi, G.; Malta, V.; Celotti, G.; Zannetti, R. Makromol. Chem.
976, 177, 915.
1
(33) Horike, S.; Matsuda, R.; Tanaka, D.; Matsubara, S.; Mizuno, M.; Endo,
1
K.; Kitagawa, S. Angew. Chem., Int. Ed. 2006, 45, 7226.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010 4923

A R T I C L E S
Uemura et al.
organic ligand L. The ability of surface functionalization by
the substituents is one of the most advantageous features of
PCPs, and thus, the control of tacticity in radical polymerization
of MMA could be attained using the nanochannels of 1-3. We
found that the tacticity of the resulting PMMA strongly
depended on the number and position of the substituent, and
we have demonstrated the remarkable effect of the PCP pores
on the tacticity of PMMA by experimental and theoretical
analysis. Note that tacticity of PMMA drastically changed using
Acknowledgment. We thank Prof. R. Schmid (Ruhr-Universit a¨ t
Bochum) for his helpful advice and discussions on MD simulations.
This work was supported by PRESTO-JST and a Grant-in-Aid from
the Ministry of Education, Culture, Sports, Science and Technology,
Government of Japan.
Note Added after ASAP Publication. An incorrect reference
citation appeared in the first paragraph in the version published
ASAP March 12, 2010. The corrected version was published March
2
OMe because of the specific channel shape (regular, narrow,
1
7, 2010.
and helically twisted channel) of 2OMe. Application of PCPs
to nanocatalysts or nanoreactors for inducing highly selective
reactions is currently attracting much attention but is still in its
infancy. We believe that our method presented here will
contribute to the tailor-made synthesis of polymer materials
owing to the ability to design the pores.
Supporting Information Available: X-ray crystal structures,
IR, and NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA100406K
4
924 J. AM. CHEM. SOC. 9 VOL. 132, NO. 13, 2010