Alternating ring-opening metathesis copolymerization of amino acid derived norbornene monomers carrying nonprotected carboxy and amino groups based on acid-base interaction

Article abstract of DOI:10.1021/ja903248c

Amino acid derived norbornene monomers having carboxy (1) and amino groups (2) were synthesized and subjected to ring-opening metathesis copolymerization with various feed ratios using the Grubbs second generation ruthenium catalyst. The M_n's o

Full text of DOI:10.1021/ja903248c

Published on Web 07/02/2009
Alternating Ring-Opening Metathesis Copolymerization of
Amino Acid Derived Norbornene Monomers Carrying
Nonprotected Carboxy and Amino Groups Based on
Acid-Base Interaction
Sutthira Sutthasupa,† Masashi Shiotsuki,† Toshio Masuda,‡ and Fumio Sanda*,†
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura
Campus, Nishikyo-ku, Kyoto 615-8510, Japan, and Department of EnVironmental and Biological
Chemistry, Faculty of Engineering, Fukui UniVersity of Technology, 3-6-1 Gakuen,
Fukui 910-8505, Japan
Received April 22, 2009; E-mail: sanda@adv.polym.kyoto-u.ac.jp
Abstract: Amino acid derived norbornene monomers having carboxy (1) and amino groups (2) were
synthesized and subjected to ring-opening metathesis copolymerization with various feed ratios using the
Grubbs second generation ruthenium catalyst. The Mn’s of the copolymers ranged from 5300 to 9400 (Mw/
Mn ) 1.40-1.69). The monomer conversion and Mn of the copolymer were maximized when the monomer
feed ratio was 1:1. The monomer unit ratios in the copolymers were almost 1:1 at 10% conversion,
irrespective of the feed ratios. The monomer reactivity ratios r1 and r2 were estimated to be 0.08 and 0.02,
which confirmed that alternating copolymerization occurred. It is considered that alternating copolymerization
is brought about by the acid-base interaction between the monomers and/or between the propagating
polymer end and the incoming monomer.
Introduction
synthesis of block copolymers, some of which form micelles
and nanoparticles,21-30 which are applicable to emulsification,
Precise control of polymer structure is an issue of great
importance, not only in polymer chemistry but also in materials
science, because of the improvement in properties of the
resulting materials. Numerous studies have focused on the
development of precisely controlled polymer synthesis. Ring-
opening metathesis polymerization (ROMP) has attracted much
attention due to the remarkable development of well-defined
transition metal catalysts, including molybdenum and ruthenium
(Ru) complexes.1,2 In particular, ROMP of norbornene deriva-
tives achieves a high level of control over tacticity, backbone
configuration,molecularweight,andmolecularweightdistribution.3-6
Among a wide variety of ROMP catalysts, Ru complexes
developed by Grubbs and co-workers are highly tolerant toward
polar functional groups under ambient conditions.7 As a result,
ROMP of functionalized norbornene derivatives facilitates the
synthesis of functional polymers such as hydrogels,8 biologically
active polymers,9-17 and liquid crystalline polymers.18-20
Copolymerization with controlled unit sequences is another
approach to functionalize polymers. Living ROMP enables
drug delivery control, sensing and patterning materials, and
electroactive polymers. Thus, controlled ROMP raises interest,
not only from the mechanistic aspect of polymer chemistry but
also for the synthesis of functional materials. Alternating
copolymerization is another method of synthesizing sequence-
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† Kyoto University.
‡ Fukui University of Technology.
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10546 J. AM. CHEM. SOC. 2009, 131, 10546–10551
10.1021/ja903248c CCC: $40.75  2009 American Chemical Society

ROMP of Amino Acid Derived Norbornene Monomers
A R T I C L E S
controlled copolymers.31,32 This is commonly achieved by
employing a combination of electron-accepting monomers like
maleic anhydride with electron-donating monomers like alkyl
vinyl ethers. In the case of ROMP, however, it is difficult to
introduce electron-accepting or -withdrawing groups close to
the double bond of cyclic olefin monomers. Therefore, other
approaches have been used to attain alternating ring-opening
metathesis copolymerization. For example, racemic 1-methyl-
norbornene undergoes ROMP alternatingly between the two
enantiomeric monomers catalyzed with ReCl5, but no homo-
polymerization takes place due to steric effects.33 Alternating
ring-opening metathesis copolymerization is also achieved by
the combination of a small amount of highly polymerizable
norbornene and a large amount of less polymerizable cyclo-
pentene using RuCl3-phenol34 and Grubbs Ru complex-Lewis
acid35 as catalysts, wherein the “cage effect” plays an important
role. Appropriately designed dual-site Ru carbene complexes
catalyze the alternating copolymerization of norbornene and a
large excess of cyclooctene, wherein one site of the complex
shows chemoselectivity while the other site does not.36,37 The
combination of polar 2,3-difunctionalized 7-oxanorbornene
derivatives and nonpolar cyclic olefins, including cyclooctene,
also works satisfactorily.38 The alternating copolymers form
well-controlled micrometer-scale aggregates by complementary
noncovalent interactions when diaminopyridine and thymine side
chains are introduced. Very recently, cyclobutene 1-carboxylic
esters and cyclohexene derivatives have been found to undergo
alternating ring-opening metathesis copolymerization.39 This
success derives from the combination of two monomers, neither
of which forms a homopolymer under ROMP conditions.
In spite of those attempts at alternating ring-opening metath-
esis copolymerization, to the best of our knowledge, there is
Scheme 1. Alternating Copolymerization of 1 and 2
no successful example of the combination of two kinds of
norbornene monomers substituted with different functional
groups. This is because the methods mentioned above require
comonomers with largely different ROMP activity, except in
the case of enantioselective ROMP of racemic 1-methylnor-
bornene.
We have reported that amino acid bifunctionalized norbornene
derivatives efficiently undergo ROMP to give polymers with
fairly high molecular weights in good yields.40 The polymeri-
zation proceeds in a living fashion to some extent, and the
polymerizability of the monomers largely depends on the
substituents, stereostructure (endo- and exo-), solvents, and
catalysts. A norbornene monomer having amino acid derived
carboxy groups successfully undergoes ROMP with the Grubbs
second generation Ru catalyst. The carboxy groups need no
protection,41 presumably because the spacer between the nor-
bornene ring and the carboxy groups prevents the carboxy
groups from interacting with the Ru center of the catalyst, which
is coordinated at the double bond in the metathesis intermediate.
This is also operative in the ROMP of norbornene monomers
having amino acid derived nonprotected amino groups, wherein
N-methyl substitution is effective in enhancing the polymeriz-
ability.42 In the course of our study on the ROMP of such acidic
and basic norbornene monomers, we considered trying alternat-
ing copolymerization utilizing acid-base interactions. The
present article describes the alternating ring-opening metathesis
copolymerization of an amino acid derived monomer having
carboxy groups (1) with a monomer having amino groups (2)
using the Grubbs Ru catalyst as illustrated in Scheme 1.
(23) Carillo, A.; Kane, R. S. J. Polym. Sci., Part A: Polym. Chem. 2004,
42, 3352–3359.
(24) Ahmed, S. R.; Bullock, S. E.; Cresce, A. V.; Kofinas, P. Polymer
2003, 44, 4943–4948.
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(26) Saunders, R. S.; Cohen, R. E.; Wong, S. J.; Schrock, R. R.
Macromolecules 1992, 25, 2055–2057.
(27) Stubenrauch, K.; Moitzi, C.; Fritz, G.; Glatter, O.; Trimmel, G.; Stelzer,
F. Macromolecules 2006, 39, 5865–5874.
(28) Stubenrauch, K.; Fritz-Popovski, G.; Ingoli, E.; Grogger, W.; Glatter,
O.; Stelzer, F.; Trimmel, G. Macromolecules 2007, 40, 4592–4600.
(29) Chen, B. Z.; Sleiman, H. F. Macromolecules 2004, 37, 5866–5872.
(30) Gratt, J.; Cohen, R. E. Macromolecules 1997, 30, 3137–3140.
(31) Bianchini, C.; Meli, A. Coord. Chem. ReV. 2002, 225, 35–66.
(32) Rzaev, Z. M. O. Prog. Polym. Sci. 2000, 25, 163–217.
(33) Hamilton, J. G.; Ivin, K. J.; Rooney, J. J.; Waring, L. C. J. Chem.
Soc., Chem. Commun. 1983, 159–161.
(34) (a) Al Samak, B.; Carvill, A. G.; Hamilton, J. G.; Rooney, J. J.;
Thompson, J. M. Chem. Commun. 1997, 2057–2058. (b) Al Samak,
B.; Amir-Ebrahimi, V.; Corry, D. G.; Hamilton, J. G.; Rigby, S.;
Rooney, J. J.; Thompson, J. M. J. Mol. Catal. A.: Chem. 2000, 160,
13–21.
Results and Discussion
(35) Amir-Ebrahimi, V.; Rooney, J. J. J. Mol. Catal. A: Chem. 2004, 208,
11–121.
Monomer Synthesis. N-Methyl-L-phenylalanine derived ex-
o,exo-disubstituted novel norbornene monomer 2 having non-
protected amino groups was synthesized from the N-Boc-
protected precursor (2-Boc) by deprotection using TFA, followed
by neutralization with a base as illustrated in Scheme 2.
EDC·HCl was employed as a condensation agent because the
urea derivative can be easily removed from the reaction mixture
by washing with water.40 The monomer structure was deter-
(36) (a) Bornand, M.; Chen, P. Angew. Chem., Int. Ed. 2005, 44, 7909–
7911. (b) Bornand, M.; Torker, S.; Chen, P. Organometallics 2007,
26, 3585–3596.
(37) Vehlow, K.; Wang, D.; Buchmeiser, M. R.; Blechert, S. Angew. Chem.,
Int. Ed. 2008, 47, 2615–2618.
(38) (a) Nakade, H.; Ilker, M. F.; Jordan, B. J.; Uzun, O.; LaPointe, N. L.;
Coughlin, E. B.; Rotello, V. M. Chem. Commun. 2005, 3271–3273.
(b) Ilker, M. F.; Coughlin, E. B. Macromolecules 2002, 35, 54–58.
(39) Song, A.; Parker, K. A.; Sampson, N. S. J. Am. Chem. Soc. 2009,
131, 3444–3445.
(40) (a) Sutthasupa, S.; Terada, K.; Sanda, F.; Masuda, T. J. Polym. Sci.,
Part A: Polym. Chem. 2006, 44, 5337–5343. (b) Sutthasupa, S.; Terada,
K.; Sanda, F.; Masuda, T. Polymer 2007, 48, 3026–3032. (c)
Sutthasupa, S.; Sanda, F.; Masuda, T. Macromol. Chem. Phys. 2008,
209, 930–937.
(41) Sutthasupa, S.; Sanda, F.; Masuda, T. Macromolecules 2008, 41, 305–
311.
(42) Sutthasupa, S.; Sanda, F.; Masuda, T. Macromolecules 2009, 42, 1519–
1525.
(43) Fineman, M.; Ross, S. D. J. Polym. Sci. 1949, 5, 259–265.
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10547

A R T I C L E S
Sutthasupa et al.
Scheme 2. Synthesis of Monomer 2
1
mined by IR, H, and 13C NMR spectroscopies, as well as by
high resolution mass spectrometry.
Homopolymerization. The homopolymerizations of mono-
mers 1 and 2 were carried out with the Grubbs second generation
Ru catalyst (2.0 mol %) in DMF-d7 at 50 °C while monitoring
the conversions by 1H NMR spectroscopy. The conversions of
monomers 1 and 2 were calculated from the integration ratios
of the olefinic protons (6.21 and 6.03 ppm) with those of the
polymers (5.29-5.52 and 5.27 ppm), respectively. Monomer 2
polymerized faster than 1 as plotted in Figure 1a. The Mn’s of
isolated poly(1) and poly(2) were 4700 and 11 000, as listed in
Table 1.
Copolymerization. The copolymerization of 1 and 2 was
carried out at a feed ratio of 1:1 under the same conditions as
those for the homopolymerizations. Monomers 1 and 2 were
converted at the same rate [Figure 1b], which was intermediate
between the rates of the homopolymerizations [Figure 1a]. The
first-order kinetic plots (Figure S2) revealed that the rate
constants of homopolymerizations of 1 and 2 [Figure 1a] and
copolymerization of 1 and 2 at a ratio of 1:1 [Figure 1b] were
0.61 × 10-2, 3.75 × 10-2, and 2.29 × 10-2 s-1, respectively.
Copolymerizations of 1 and 2 were also carried out at feed
ratios of 1:2 and 2:1. In the former case, the conversion of
monomer 2 was half of that of 1 at all times, as plotted in Figure
2a. In the latter case, the conversion of monomer 1 was half of
that of 2 at all times, as plotted in Figure 2b. Thus, equimolar
amounts of monomers were consumed in both cases, suggesting
that 1:1 copolymerization took place.
Figure 3a demonstrates the relationship between the feed and
unit ratios of monomer 1 in the copolymers obtained at 10%
conversion in the copolymerizations. The monomer unit ratios
were calculated from the conversion of each monomer. The unit
ratio of 1:2 was 50:50 in the copolymer when the feed ratio
was 50:50. The unit ratios remained almost the same, irrespec-
tive of the feed ratios, which ranged from 17:83 to 83:17.
Alternating copolymerization was further verified by monomer
reactivity ratios (r1 ) 0.08, r2 ) 0.02), which were calculated
by the Fineman-Ross method.43 The values were close to zero,
indicating that the copolymerization proceeded alternatingly.36b,44
Figure 3b depicts the relationship between the feed and unit
Figure 1. (a) Time-conversion plots of the homopolymerizations of 1
(]) and 2 (b). (b) Time-conversion plots of the copolymerization of 1
(]) and 2 (b) at a ratio of 1:1. Conditions: [M]0,total ) 0.11 M, [M]0/[catalyst]
) 50 in DMF-d7 at 50 °C.
Table 1. Homo- and Copolymerizations of 1 and 2a
feed ratio (mol %)
unit ratiob (mol %)
total
conversionb
(mol %)
1
2
Mnc
Mw/Mnc
1
2
100
83
67
50
33
17
0
0
17
33
50
67
36
31
68
78
59
35
89
4700
5300
7100
9400
7000
1.26
1.67
1.41
1.51
1.69
1.40
1.50
100
76
57
50
45
25
0
0
24
43
50
55
83
100
5600
11 000
75
100
a Conditions: [M]0,
) 0.11 M, [M]0/[catalyst] ) 50 in DMF-d7 at
total
50 °C for 60-90 min. b Determined by 1H NMR. c No elution peaks
could be observed when the samples of poly(1) and poly(1-co-2)s were
analyzed by GPC (DMF, 10 mM LiBr). To facilitate elution, the
carboxy groups were transformed into the corresponding methyl esters
using TMSCHN2 prior to GPC separation.
ratios of 1 in the copolymers obtained after the copolymerization
for 15 min (20-45% conversion). Although the deviation of
the unit ratio became large from 50:50 compared to Figure 3a,
it still exhibited the alternating character.
The Mn’s of the copolymers ranged from 5300 to 9400
(Mw/Mn ) 1.40-1.69) as listed in Table 1. The yield and Mn
of the copolymer were maximum at a monomer feed ratio of
50:50, as shown in Figure 4. This result clearly indicates
that the acidic and basic monomers interact most efficiently
(44) Hahn, M.; Jacger, W.; Schmolke, R.; Behnisch, J. Acta Polym. 1990,
41, 107–111.
(45) Schmidt, C.; Merz, F.; Jiang, S.; Drache, M.; Schmidt-Naake, G.
Macromol. Mater. Eng. 2007, 292, 428–436.
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ROMP of Amino Acid Derived Norbornene Monomers
A R T I C L E S
Figure 2. Time-conversion plots of the copolymerizations of 1 (]) and
2 (b) at ratios of (a) 1:2 and (b) 2:1. Conditions: [M]0,total ) 0.11 M, [M]0/
[catalyst] ) 50 in DMF-d7 at 50 °C.
Figure 3. Relationships between the contents of 1 in copolymer and
monomer feed in the copolymerization of 1 and 2. (a) Conditions: [M]0,total
) 0.11 M, [M]0,total/[catalyst] ) 50 in DMF-d7 at 50 °C at ca. 10%
conversions. r1 ) 0.08, r2 ) 0.02. (b) For 15 min, conversions 20-45%.
r1 ) 0.08, r2 ) 0.14. The lines through the data points are theoretical, based
on the r1 and r2 values.
when the molar ratio is 1:1. Alternating character is also
observed in the radical copolymerization of acidic and basic
vinyl monomers, such as 2-acrylamido-2-methyl-1-propane-
sulfonic acid (APSA) and 1-vinylimidazole (1-VIm).45 But
in that case, it is concluded that acceptor-donor interaction
between APSA and 1-VIm seems more important than an
acid-base interaction, because the combination of styrene-
4-sulfonic acid (SSA) and 1-VIm does not undergo alternating
copolymerization. Thus, the large difference of electronic
character of the double bonds in APSA and 1-VIm plays an
important role, instead of acid-base interactions. On the other
hand, the difference of electronic character between the
present monomers 1 and 2 is small; the net atomic charges
of olefin carbon atoms of 1 and 2 were calculated to be
-0.124 and -0.136.46 It is therefore considered that acid-base
interaction is the key factor in alternation of the copolym-
erization of 1 and 2.
Figure 4. Total monomer conversion (b) and Mn (O) of the product
copolymers as a function of the content of monomer 1 in the feed in the
copolymerization of 1 and 2.
It is likely that the acid-base interaction between monomers
1 and 2 affects the polymerization rate. To confirm that
acid-base interaction is responsible for the alternating copo-
lymerization of 1 and 2, the copolymerization of amine
monomer 2 and ester monomer 3 (Chart 1) was carried out for
comparison under the same conditions as those of the copo-
lymerization of 1 and 2. Figure 5 depicts the relationship
between the monomer feed and content of 2 in the copolymer
at 11-13% conversions. The content of 2 in copolymer
increased almost linearly with increasing content of 2 in feed,
which demonstrates that the copolymerization of 2 and 3 did
not take place alternatingly but rather that random copolymer-
ization occurred. The monomer reactivity ratios of 2 and 3 were
(46) The calculation was carried out by the semiempirical molecular orbital
method using the AM1 Hamiltonian, running on Wavefunction Inc.
Spartan ’08 for Windows version 1.1.1.
(47) The 1H NMR spectra of mixtures of monomers 1 and 2 were measured
at various compositions to verify the existence of an interaction
between them. As depicted in Figure S1, the-CO2H signal of 1
gradually broadened and shifted from 12.3 ppm to higher field upon
raising the content of 2. On the other hand, the-NH-signal of 2
gradually broadened and shifted from 9.2 ppm to lower field upon
raising the content of 1. The-CO2H and-NH-signals coalesced when
1 and 2 were mixed. These results indicate that the carboxy and amino
groups of monomers 1 and 2 undergo an acid/base interaction.
9
J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10549

A R T I C L E S
Sutthasupa et al.
Chart 1. Structure of Monomer 3
calculated to be 0.60 and 1.02 at 11-13% conversions by the
Fineman-Ross method. These results confirm the randomness
of the copolymerization of 2 and 3, along with the importance
of the acid-base interaction for the alternating copolymerization
of 1 and 2.
Figure 6a illustrates the acid-base interaction between the
carboxy and amino groups of 1 and 247 which enhances the
Figure 5. Relationship between the contents of 2 in copolymer and
monomer feed in the copolymerization of 2 and 3. Conditions: [M]0,total
)
0.11 M, [M]0,total/[catalyst] ) 50 in DMF-d7 at 11-13% conversions, r1 )
0.60, r2 ) 1.02. The line through the data points is theoretical, based on
the r1 and r2 values.
1
Figure 7. Olefinic region of H-13C HSQC NMR spectra (700 MHz) of
poly(1), poly(2), and poly(1-co-2) measured in DMSO-d6 at 35 °C.
local monomer concentration to bring about the alternating
copolymerization of the monomers. Another possibility is an
acid-base interaction between the metal carbene propagating
species and the incoming monomer. As illustrated in Figure 6b,
it is likely that the monomer 1 derived metal carbene moiety
preferably interacts with the amino groups of monomer 2 rather
than with the carboxy groups of monomer 1 due to an acid-base
interaction, leading to the alternating copolymerization. In a
similar fashion, a monomer 2 derived propagating end preferably
reacts with monomer 1 rather than 2. By contrast, such
acid-base interaction does not exist between monomers 2 and
3. Consequently, it is considered that 2 does not copolymerize
with 3 alternatingly but that random copolymerization occurs.
One-dimensional 13C NMR spectra of the polymers did
not provide useful structural information, because the olefinic
carbon signals of the homopolymers and copolymer appeared
Figure 6. Probable mechanisms for alternating copolymerization. (a)
Acid-base interaction between monomers that enhances the local monomer
concentration. (b) Acid-base interaction between the metal carbene
propagating species and the incoming monomer.
1
at the same position. We therefore measured the H-13C
HSQC (Heteronuclear Single Quantum Coherence) spectra
of the polymers using a 700 MHz NMR spectrometer, to
9
10550 J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009

ROMP of Amino Acid Derived Norbornene Monomers
A R T I C L E S
Table 2. Solubility of the Polymersa
amino groups successfully undergo alternating copolymeri-
zation. The key factor favoring alternating copolymerization
seems to be acid-base interaction between the monomers,
leading to enhancement of local monomer concentration, and
acid-base interaction between the metal carbene propagating
species and the incoming monomer. This reasoning is
polymer
solvent
poly(1)
poly(2)
poly(1-co-2)
hexane
toluene
CHCl3
CH2Cl2
THF
acetone
MeOH
DMF
DMSO
H2O
HCl (1 M)
NaOH aq. (1 M)
-
-
-
-
+
-
+
+
+
-
-
+
-
-
-
-
+
+
+
+
+
+
+
-
-
-
-
-
-
-
+
+
+
-
-
+
1
supported by H NMR spectroscopic analysis and a control
experiment, in which 2 was copolymerized with monomer 3
having ester-protected carboxy groups. As far as we know,
the present study is the first successful example of an
alternating ring-opening metathesis copolymerization between
two kinds of norbornene monomers substituted with different
functional groups. We believe that our achievement contrib-
utes not only to the development of well-defined ROMP
chemistry but also to the enhancement of properties of
ROMP-based polymers.
a -: insoluble, +: soluble.
observe differences in the signal shapes of the olefinic regions
of the homopolymers compared to the copolymer. As shown
in Figure 7, the olefin signal of poly(1-co-2) is different from
those of poly(1) and poly(2), especially from that of poly(2).
Since the pattern of the copolymer signal was not a simple
sum of those of the homopolymers, it is considered that the
copolymer contains a unit sequence other than the homose-
quences of monomer units 1 and 2. The alternating dyads
could not be calculated due to the almost identical chemical
shifts of the homo- and copolymers.
Table 2 gives solubility data for the homopolymers and
alternating copolymer. Both of the homopolymers are insoluble
in low-polarity solvents like hexane, toluene, CH2Cl2, and
CHCl3, but they are soluble in polar solvents, including THF,
MeOH, DMF, and DMSO. Poly(1) having carboxy groups was
insoluble and soluble in HCl and NaOH aq., respectively, while
poly(2) having amino groups was soluble and insoluble in HCl
and NaOH aq., respectively, as predicted from the functional
groups. Poly(2) was also soluble in neutral H2O. The solubility
of poly(1-co-2) was the same as that of poly(1) except in THF.
The interaction of the carboxy groups with solvents seems to
be more pronounced compared to the amino-solvent interaction.
Acknowledgment. This research was partly supported by a
Grant-in-Aid from the Global COE Program, “International
Center for Integrated Research and Advanced Education in
Materials Science” from the Ministry of Education, Culture,
Sports, Science and Technology of Japan. Thanks are also due
to Materia, Inc. for the donation of the Grubbs second generation
Ru catalyst. We are grateful to Prof. Kenneth B. Wagener and
Dr. Kathryn R. Williams at University of Florida, and Prof.
Takashi Ishizone at Tokyo Institute of Technology for their
helpful suggestions and comments. We are indebted to Prof.
Masahiro Shirakawa, Prof. Hidehito Tochio, and Mr. Naotaka
Sekiyama at Kyoto University for measuring the 1H-13C HSQC
NMR spectra.
Supporting Information Available: Experimental procedures,
1H NMR spectra of mixtures of 1 and 2 with various composi-
tions (Figure S1), relationships between ln([M]0/[M]) and time
in the homopolymerizations of 1 and 2 and in the copolymer-
ization of 1 and 2 at a ratio of 1:1 (Figure S2). This material is
available free of charge via the Internet at http://pubs.acs.org.
Conclusions
In this paper, we have demonstrated that norbornene
monomer 1 having carboxy groups and monomer 2 having
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J. AM. CHEM. SOC. VOL. 131, NO. 30, 2009 10551