Tandem ring-opening/ring-closing metathesis polymerization: Relationship between monomer structure and reactivity

Article abstract of DOI:10.1021/ja4039278

Monomers containing either cycloalkenes with low ring strain or 1-alkynes are poor monomers for olefin metathesis polymerization. Ironically, keeping two inactive functional groups in proximity within one molecule can make it an excellent monomer for metathesis polymerization. Recently, we demonstrated that monomer 1 having cyclohexene and propargyl moieties underwent rapid tandem ring-opening/ring-closing metathesis (RO/RCM) polymerization via relay-type mechanism. Furthermore, living polymerization was achieved when a third-generation Grubbs catalyst was used. Here, we present a full account on this tandem polymerization by investigating how various structural modifications of the monomers affected the reactivity of the tandem polymerization. We observed that changing the ring size of the cycloalkene moieties, the length of the alkynes, and linker units influenced not only the polymerization rates but also the reactivities of Diels-Alder reaction, which is a post-modification reaction of the resulting polymers. Also, the mechanism of tandem polymerization was studied by conducting end-group analysis using ¹H NMR analysis, thereby concluding that the polymerization occurred by the alkyne-first pathway. With this mechanistic conclusion, factors responsible for the dramatic structure-reactivity relationship were proposed. Lastly, tandem RO/RCM polymerization of monomers containing sterically challenging trisubstituted cycloalkenes was successfully carried out to give polymer repeat units having tetrasubstituted cycloalkenes.

Full text of DOI:10.1021/ja4039278

Article
pubs.acs.org/JACS
Tandem Ring-Opening/Ring-Closing Metathesis Polymerization:
Relationship between Monomer Structure and Reactivity
Hyeon Park, Ho-Keun Lee, and Tae-Lim Choi*
Department of Chemistry, Seoul National University, Seoul 151-747, Korea
S
* Supporting Information
ABSTRACT: Monomers containing either cycloalkenes with
low ring strain or 1-alkynes are poor monomers for olefin
metathesis polymerization. Ironically, keeping two inactive
functional groups in proximity within one molecule can make
it an excellent monomer for metathesis polymerization.
Recently, we demonstrated that monomer 1 having cyclohexene
and propargyl moieties underwent rapid tandem ring-opening/
ring-closing metathesis (RO/RCM) polymerization via relay-
type mechanism. Furthermore, living polymerization was
achieved when a third-generation Grubbs catalyst was used.
Here, we present a full account on this tandem polymerization by investigating how various structural modifications of the
monomers affected the reactivity of the tandem polymerization. We observed that changing the ring size of the cycloalkene
moieties, the length of the alkynes, and linker units influenced not only the polymerization rates but also the reactivities of Diels−
Alder reaction, which is a post-modification reaction of the resulting polymers. Also, the mechanism of tandem polymerization
1
was studied by conducting end-group analysis using H NMR analysis, thereby concluding that the polymerization occurred by
the alkyne-first pathway. With this mechanistic conclusion, factors responsible for the dramatic structure−reactivity relationship
were proposed. Lastly, tandem RO/RCM polymerization of monomers containing sterically challenging trisubstituted
cycloalkenes was successfully carried out to give polymer repeat units having tetrasubstituted cycloalkenes.
by performing relay of two or more metathesis reactions in a
single step. This method provided a route for the efficient
synthesis of complex organic molecules9 and natural
products.10 One particular example is tandem enyne RO/
RCM reaction, which undergoes relay rearrangement reaction
from substrates comprising alkynes and cycloalkenes.11 The
products or intermediates of the enyne metathesis reaction
could be further functionalized by consecutive CM or Diels−
Alder reactions, thereby increasing the complexity of the
molecules.12,13
By adopting the concept of tandem metathesis reactions
from organic synthesis, we recently reported living tandem
RO/RCM polymerization of monomer 1 using the third-
generation Grubbs catalyst A (Scheme 1).14 Although 1
contained cyclohexene and propargyl moieties, which were
unreactive toward the conventional metathesis polymerization
INTRODUCTION
■
During the last two decades, olefin metathesis reaction has been
widely used as an efficient method for synthesizing new
molecules by forming new carbon−carbon double bonds.1
Three major types of the olefin metathesis, ring-opening
metathesis (ROM), cross metathesis (CM), and ring-closing
metathesis (RCM), were developed as versatile tools in organic
synthesis. Similarly, ring-opening metathesis polymerization
(ROMP)2 from ROM, acyclic diene metathesis (ADMET)
polymerization3 from CM, and cyclopolymerization4 from
RCM were developed; these reactions greatly broadened the
scope of polymerization methodology and produced a wide
variety of polymers from various monomers. However, some
monomers hardly undergo polymerization. For instance,
cycloalkenes with low ring strain, such as cyclohexenes, hardly
polymerize because the equilibrium between ROM and RCM is
far more favorable to the monomeric state as cyclohexene.5,6
Also, the polymerization of 1-alkynes is challenging, especially
for ruthenium-based catalysts, because the catalysts undergo α-
addition with 1-alkynes, generating sterically shielded 1,1-
disubstituted ruthenium carbenes. Therefore, the propagation is
retarded, and obtaining high-molecular-weight polymers in high
yield becomes difficult.7,8 Thus, conventional metathesis
polymerization of monomers containing these unreactive
functional groups is difficult.
Scheme 1. Example of Tandem RO/RCM Polymerization
In the organic chemistry community, researchers have
developed various elegant tandem olefin metathesis reactions
Received: April 20, 2013
Published: July 11, 2013
© 2013 American Chemical Society
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on their own, placing two functional groups in proximity made
1 an excellent monomer that underwent extremely fast
polymerization in 1 min at room temperature via regioselective
relay tandem RO/RCM reaction. We also achieved controlled
polymerization at −30 °C to inhibit the chain-transfer reaction
so that polydispersity index (PDI) was narrower than 1.2.
Notably, the newly generated alkene showed E/Z ratio of 6/4,
indicating kinetic control of polymerization. The reaction scope
of this tandem RO/RCM polymerization was further expanded
to include the synthesis of various diblock copolymers by
combining the tandem polymerization with either ROMP or
cyclopolymerization. In addition, post-functionalization on the
newly generated dienes of the polymer backbone was possible
by Diels−Alder reaction. Based on this basic skeleton of 1, one
can expand the monomer scope by modifying the structures of
1 so that various new polymers can be prepared by altering the
ring sizes, alkyne moieties, and linker groups. Herein, we
present a full report on the regioselective tandem RO/RCM
polymerization for various monomers and their post-
modification by Diels−Alder reaction. In addition, we system-
atically investigated how each modification of the monomer
structures affected reactivities and revealed the details of the
mechanism of the tandem polymerization. Notably, all these
monomers contained substituted cycloalkenes having low ring
strain so their ROMP was impossible,15 but now we present a
new strategy to polymerize these highly challenging monomers
via the powerful tandem RO/RCM polymerization.
Table 1. Tandem Metathesis Polymerization of Various
Monomers with Different Ring Sizes and Linker Alkynes
a
b
entry monomer (M/I) temp. (°C)
time
conv.
Mn/PDI
c
1
2
3
4
2 (15)
2 (30)
2 (50)
3 (100)
3 (50)
4 (50)
5 (50)
6 (50)
6 (50)
7 (50)
−30
−30
−30
rt
3 min
5 min
10 min
12 h
100%
100%
100%
30%
5.0 k/1.28
9.1 k/1.28
12.5 k/1.25
11.4 k/1.70
13.5 k/1.62
−
c
c
c
d
5
50
2 h
98%
c
,d
RESULTS AND DISCUSSIONS
6
50
12 h
<20%
0%
■
d
d
d
7
8
9
50
12 h
−
The core skeleton of monomer 1 contains cycloalkene and 1-
alkyne as polymerizable groups connected by toluenesulfonyl
amide group. To broaden the monomer scope, our initial trial
consisted of modifying the ring size of cycloalkenes. As in our
previous study, we used the monomer containing cyclohexene
that has the lowest ring strain among cycloalkenes. Therefore,
other cycloalkenes with relatively higher ring strain, such as
cycloheptene and cyclopentene, should undergo tandem RO/
RCM polymerization as well, although three-substituted
cyclopentenes did not undergo ROMP at all.15 Cycloheptene
derivative (2) showed rapid tandem RO/RCM polymerization
at room temperature with reactivity comparable to that of 1.
Furthermore, in order to suppress the chain-transfer reaction,
the polymerization temperature was lowered to −30 °C and
similar to 1, polymerization of 2 also showed fast propagation
and controlled polymerization as PDIs of the polymers were
narrower than 1.3, and a linear relationship between monomer-
to-initiator ratio (M/I) and molecular weight was observed
(Table 1, entries 1−3, Figure 1). Interestingly, when the
tandem RO/RCM polymerization of a monomer containing
cyclopentene moiety (3) was attempted, very sluggish
polymerization was observed achieving only 30% conversion
even after 12 h of polymerization at room temperature (entry
4). This was puzzling at first because the ring strain of
cyclopentene (3) was similar to that of cycloheptene (2) but
was larger than that of cyclohexene (1), and therefore, efficient
tandem RO/RCM polymerization was expected. To enhance
the propagation rate, a second-generation Grubbs−Hoveyda
catalyst (B) that was thermally more stable was used, and
almost complete conversion of 3 was achieved at 50 °C (entry
5). Since the much less reactive monomer 3 was polymerized at
a higher temperature, PDI broadening because of the chain-
transfer reaction inevitably occurred. Similar to P1, the polymer
50
12 h
82%
12.0 k/1.54
15.0 k/1.79
13.1 k/1.23
65
2 h
100%
90%
c
10
−10
5 min
a
b
1
Conversion determined by crude H NMR. Determined by THF
SEC calibrated using polystyrene (PS) standards. A was used as
catalyst. B was used as catalyst.
c
d
Figure 1. Plot of Mn versus M/I for P2. Numbers on the line indicate
PDI values.
microstructure showed excellent regiochemistry and the E/Z
ratio on P2 and P3 was 6/4.
Next, a structural change was made by changing the length of
alkynes. If 1-butynyl group was used instead of propargyl, the
polymer unit structure changed from a five-membered ring to a
six-membered ring. We investigated how this simple change
affected the propagation of the tandem RO/RCM polymer-
ization. Initial trial was done on a monomer 4 containing
cyclohexene. However, the polymerization hardly occurred
even at 50 or 65 °C (entry 6). No polymer was obtained
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because of low conversion. Even a monomer 5 containing
cycloheptene was unreactive and did not undergo polymer-
ization at all (entry 7). Surprisingly, a monomer 6 with
cyclopentene underwent tandem RO/RCM polymerization
with 82% conversion at 50 °C and, finally, full conversion at 65
°C in 2 h (entries 8 and 9). This result was unexpected, because
we initially anticipated that the monomers 4 and 5 would be
more reactive than 6 because their propargyl analogs 1 and 2
were more reactive than 3. However, the reactivity was
completely reversed by changing the propargyl group to the
homopropargyl group. Overall, the ring closure to make
polymers containing six-membered rings was much slower than
the five-membered ring cases.16
Table 2. Post-Functionalization of Polymers by Diels-Alder
Reaction
Another key structural factor for the tandem RO/RCM
polymerization was the linker unit between cycloalkenes and
alkynes. Modifying this linker unit would also change the ring
structure of the final polymer unit, and the monomer scope
would be expanded. First, sulfonamide was changed to the
amide group to give 7, and it also showed high reactivity and
controlled polymerization, thereby affording narrow PDI (entry
10). Nevertheless, the reactivity of 7 was slightly lower than
those of 1 and 2, as polymerization required relatively higher
temperature (−10 °C vs −30 °C for 1 and 2) to achieve high
conversion. However, when the linker atom was changed to
carbon (8) or oxygen (9), no polymer was obtained at all even
when they were subjected to the forcing conditions by using
the catalyst B at 50 °C (Scheme 2). This suggested that small
change in the linker unit on the monomers also led to huge
changes in the tandem polymerization reactivity.
prepolymer
a
final polymer
a
b
entry
1
(Mn/PDI)
time
48 h
conv.
(Mn/PDI)
P2
12.3 k/1.21
P3
P2a
14.3 k/1.28
P3a
d
60%
c
d
2
16 h
8 h
60%
12.0 k/1.56
P6
14.0 k/1.64
P6a
3
100%
8.9 k/1.65
11.0 k/1.69
b
a
Determined by THF SEC calibrated using PS standards. Conversion
c
1
determined by crude H NMR. Reaction was done at 60 °C in 1,2-
d
dichloroethane as a solvent due to low reactivity. Only the trans diene
underwent Diels−Alder reaction while all the cis diene remained.
Scheme 2. Unsuccessful Tandem Metathesis Polymerization
of Monomers Containing Carbon or Oxygen Linker
(entry 2). However, P6 with the six-membered ring structure
showed a much higher reactivity toward the Diels−Alder post-
modification with tetracyanoethylene, as all of the dienes
regardless of E or Z isomers were converted to Diels−Alder
adducts within just 8 h (entry 3). After all three Diels−Alder
post-modifications, the SEC traces of polymers were shifted left
to give new traces corresponding to higher-molecular-weight
polymers while maintaining the PDIs constant. These results
suggest that the post-modification did not alter the integrity of
the polymers, and a small modification of the polymer
structures significantly affected the reactivity of the polymers
toward the Diels−Alder post-modification.
Since this relay-type tandem RO/RCM is unique and
unprecedented, it would be worthwhile to investigate its
mechanism in detail. There are three possible pathways: first,
catalyst initiating from the alkyne selectively (pathway A:
alkyne first); second, catalyst initiating from the cycloalkene
selectively (pathway B: cycloalkene first); and last, random
initiation of the catalyst on either alkyne or cycloalkene
nonselectively (pathway C: alkyne−cycloalkene mixed)
(Scheme 3). If the catalyst selectively initiated and propagated
on a single functional group, such as in pathways A or B, the
polymer unit structure would always have head-to-tail structure,
and as a result, the polymer would have a regular micro-
structure. On the other hand, nonselective pathway C would
produce polymers comprising mixture junctions of head-to-tail,
head-to-head, and tail-to-tail, and the polymer structure would
be completely random. In our previous study, we intentionally
prepared P1a by ADMET polymerization of monomer 1a
(Scheme 3). By comparing 1H NMR spectra of P1 and P1a, we
could easily rule out the pathway C, because NMR spectrum of
P1 vividly showed highly regular repeat units, while that of P1a
showed random repeat units.14 Therefore, the pathways A and
Polymers synthesized by the tandem RO/RCM polymer-
ization contain 1,3-diene moiety. A new family of polymers can
be prepared by further modifying these functional groups via
Diels−Alder reaction.17 As previously reported, when P1 with a
pyrrolidine unit was reacted with a dienophile, tetracyano-
ethylene, dienes with trans-alkene were fully converted to a
Diels−Alder adduct after 48 h, whereas those with cis-alkene
remained unaltered.14 Similar post-functionalizations were
attempted on these new polymersP2, P3, and P6. Although
the lengths of the tethers on P2 and P3 were different (n = 1
for P3, n = 2 for P1, and n = 3 for P2), both contained the
same pyrrolidine structure and olefins with the E/Z ratio of 6/
4. These features were similar to those of P1, and not
surprisingly, Diels−Alder reaction of P2 and P3 with
tetracyanoethylene resulted in 60% conversion with the full
conversion of the diene with trans-isomers only, while the
remaining 40% of the dienes with cis-isomer did not react at all,
just like in the case of P1 (Table 2, entries 1 and 2). A minor
difference in reactivity between P2 and P3 was that Diels−
Alder reaction on P3 was relatively slower than P2, presumably
because of fewer number of methylene in the repeat unit. To
resolve this congestion issue, the Diels−Alder reaction was
conducted at 60 °C to achieve the same conversion as in P2
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Scheme 3. Possible Mechanisms of Tandem Metathesis
Polymerization
B were considered to be the possible pathways of the relay-type
tandem RO/RCM. We have previously proposed the pathway
A as the potentially correct mechanism for the tandem RO/
RCM polymerization, because this pathway was more suitable
for explaining the extremely fast polymerization.14
In order to confirm which mechanism was correct for the
tandem polymerization, we performed mechanistic studies on
monomers 1 and 2. If the polymerization followed pathway A,
the styryl group on the catalyst would be transferred onto the
conjugated diene group and the chain-end group would be the
terminal nonconjugated alkene obtained after quenching with
ethyl vinyl ether. On the other hand, if the catalyst initiated on
the cycloalkene first (pathway B), the styryl group would be
transferred to the nonconjugated alkene and the chain-end
group would be conjugated diene. Therefore, we could
determine the actual mechanism for the tandem RO/RCM
polymerization by conducting end-group analysis using 1H
NMR analysis. First, we prepared oligomeric P1 by treating 1
with 20 mol % A and quenching the polymerization by adding
ethyl vinyl ether so that its end-groups could be analyzed in
detail. For a comparison study, 1b was independently prepared
by selective CM between styrene and the more reactive
nonconjugated terminal alkene on 1a (Scheme 4). When the
1H NMR spectra of three substrates (1a, 1b, and oligo-P1)
were compared, peaks for all the terminal olefins could be
unambiguously assigned (Figure 2a−c). From these data, we
observed that oligo-P1 vividly showed nonconjugated terminal
alkene proton signals as HA, HB, and HC, whereas the chemical
Figure 2. NMR spectra of (a) 1a, (b) 1b, (c) oligo-P1, and (d) oligo-
P2.
shifts corresponding to H1−5 of 1b were totally absent. This
confirmed that the tandem RO/RCM polymerization of 1
followed the pathway A exclusively. We also conducted a
similar mechanistic study on 2, because the monomer
containing cycloalkenes with higher ring strains might follow
different pathway. Similar chemical shifts, HA*, HB*, and HC*
without H1−5, were observed for the oligomeric P2 as well,
suggesting that the polymerization pathway was not altered by
the ring strain of cycloalkenes (Figure 2d). All these
observations proved that the mechanism of the tandem RO/
RCM polymerization follows the pathway A exclusively.
One important question was what determined the vastly
different reactivities of the various monomers (Scheme 5). As
the tandem polymerization proceeded via pathway A, several
structure−reactivity relationships were postulated. First, there
was no clear relationship between the size of cycloalkenes and
the reactivity, because a cyclopentene with a propargyl group
(3) showed the lowest reactivity among the analogous
monomers containing different cycloalkenes (1 and 2), while
a cyclopentene with the homopropargyl group (6) was the only
monomer promoting successful polymerization (no polymer
obtained from 4 and 5). Second, the reactivity dependence on
the length of the alkynes (propargyl vs homopropargyl) was
not direct, because 3 and 6, which differed only by one carbon
on the alkyne, showed very similar reactivities. However, other
analogous monomers (1 vs 4 and 2 vs 5) showed completely
Scheme 4. Comparison between P1 and CM Product 1b
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Scheme 5. Possible Intermediates of 1−9 during RO/RCM
Reaction
Table 3. Tandem Metathesis Polymerization of Monomers
with Trisubstituted Cycloalkenes
a
b
entry mono. temp. (°C) conc. (M) time
conv.
0%
Mn (PDI)
1
2
3
4
5
10
10
10
11
11
rt
0.4
0.4
0.8
0.4
0.6
3 h
6 h
−
50
60
50
60
37%
50%
4.4 k (1.61)
3.9 k (1.25)
8.0 k (1.93)
6.0 k (1.57)
12 h
12 h
12 h
65%
100%
a
b
1
Conversion determined by crude H NMR. Determined by THF
SEC calibrated using PS standards.
temperature was increased to 50 °C to yield P10 containing
tetrasubstituted cyclopentene moiety in 37% conversion (entry
2) and further to 60 °C to achieve 50% conversion, with Mn of
3.9 k (entry 3). To our delight, monomer 11 containing
trisubstituted cyclopentene showed 65% conversion at 50 °C
and 100% conversion at 60 °C, implying that 11 was more
reactive monomer than 10 at the same reaction condition
(entries 4 and 5). These results were contrast to the previous
results which showed that the monomer containing propargyl
group and cyclohexene (1) was more reactive than its
cyclopentene derivative (3). In both cases, E/Z ratio on the
newly generated olefin was 1/1, similar to the previous results.
On the other hand, monomers containing trisubstituted
cycloalkenes and 1-butynyl moieties, 12 and 13, were totally
inactive for the tandem polymerization (Scheme 6). At least,
different reactivities. Finally, we postulated that both the ring
size and the alkyne length affected the reactivity. With the
mechanism of pathway A, various metallocyclobutane inter-
mediates from monomers having different cycloalkenes and
linker alkynes (Scheme 5, 1**−9**) should be produced. The
stability of these intermediates would be governed by the size of
both the cycloalkenes and the linker alkynes; even changing the
linker atoms from nitrogen (1) to carbon (8) or oxygen (9)
greatly affected the polymerization because of different bond
lengths and bond angles. In other words, the kinetics of RCM
from the resulting alkylidenes, which were obtained by α-
insertion, into the alkynes (Scheme 5, 1*−9*) would be
dramatically different because of the slight changes either in the
distance or in the bond angle.
Scheme 6. Unsuccessful Tandem Metathesis Polymerization
of Monomers Containing Carbon or Oxygen Linker
Up to now, sterically hindered trisubstituted cycloalkenes
with low ring strain, such as 1-methylcyclopentene or 1-
methylcyclohexene, have not been polymerized by ROMP.18
However, we envisioned that utilizing the relay sequence of this
efficient RO/RCM process, monomers containing extremely
challenging trisubstituted cycloalkenes might undergo the
tandem polymerization just as three-substituted cycloalkenes
underwent efficient tadem RO/RCM polymerization. Initially,
monomer 10 containing a trisubstituted cyclohexene and
propargyl group was subjected to 2 mol % of catalyst B at
room temperature, but no polymer was obtained because of
severe steric hindrance of the trisubstituted olefin (Table 3,
entry 1). In order to enhance the reactivity, reaction
polymerization result of 13 was rather disappointing because its
three-substituted cyclopentene derivative, 6, showed good
reactivity toward the tandem polymerization to give the
polymer having a six-membered ring repeat unit (Table 1,
entries 8 and 9). Also, 14 with carbon linker failed to give any
polymer which was consistent with its three-substituted
cyclohexene analogue 8 (Scheme 6). This suggested that the
monomers with sterically hindered trisubstituted cycloalkenes
were much more challenging to undergo the tandem
polymerization compared to the disubstituted cycloalkene
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H. Handbook of Metathesis; Wiley-VCH: Weinheim, 2003; Vols. 1, 2.
(d) Grubbs, R. H. Tetrahedron 2004, 60, 7117.
(2) (a) Novak, B. M.; Grubbs, R. H. J. Am. Chem. Soc. 1988, 110, 960.
(b) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158. (c) Bielawski, C. W.;
Grubbs, R. H. Angew. Chem., Int. Ed. 2000, 39, 2903.
derivatives, and their reactivities were also sensitive to the
monomer structures presumably because those slight structural
changes had large influence on the stability of intermediates as
postulated in Scheme 5.
(3) (a) Wagener, K. B.; Boncella, J. M.; Nel, J. G. Macromolecules
1991, 24, 2649. (b) Patton, J. T.; Boncella, J. M.; Wagener, K. B.
Macromolecules 1992, 25, 3862. (c) Brzezinska, K.; Wolfe, P. S.;
Watson, M. D.; Wagener, K. B. Macromol. Chem. Phys. 1996, 197,
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Soc. Rev. 2011, 40, 1404.
(4) For a review on olefin metathesis cyclopolymerization, see:
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1645. For examples of olefin metathesis cyclopolymerization, see:
(b) Fox, H. H.; Wolf, M. O.; Odell, R.; Lin, B. L.; Schrock, R. R.;
Wrington, M. S. J. Am. Chem. Soc. 1994, 116, 2827. (c) Anders, U.;
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CONCLUSION
■
In conclusion, we investigated the tandem RO/RCM polymer-
ization for various monomers containing cycloalkenes and
terminal alkynes. We observed that the reactivity was heavily
influenced by not only the ring size of the cycloalkenes but also
the length of the alkynes and the linker moieties. Monomers 1
and 2 were the most reactive so that even controlled
polymerization was possible. However, small modifications of
the monomer structure were subtle enough to totally shut
down the catalysis completely. The post-modification reaction
of the resulting polymers was successfully carried out by the
addition of a dienophile that underwent Diels−Alder reaction
with the diene moieties on the polymer backbone. This
reaction was also influenced by the ring structure in the repeat
unit of the polymers. Then, details of the mechanism for the
tandem polymerization were studied by conducting end-group
analysis using 1H NMR, and it was concluded that the
polymerization occurred by the alkyne-first pathway exclusively.
With this mechanistic conclusion, we proposed that the stability
of the metallocyclobutane intermediates or the accessibility of
the newly generated alkylidenes toward the cycloalkenes caused
the dramatic structure−reactivity relationship of the monomers
for the tandem polymerization. Lastly, we successfully
performed rather challenging RO/RCM polymerization of
monomers containing trisubstituted cycloalkenes, even though
they showed decreased reactivity because of the steric
hindrance. In short, we demonstrated a powerful tandem
polymerization of monomers containing functional groups that
were otherwise sterically and thermodynamically inactive for
the conventional ROMP.
(5) Irvin, K. J.; Mol, J. C. Olefin Metathesis and Metathesis
Polymerization; Academic Press: San Diego, 1997.
(6) For a rare examples of ring-opening cross metathesis reaction of
cyclohexene and its alternating copolymerization via enoic carbene
intermediates, see: (a) Choi, T.-L.; Lee, C. W.; Chatterjee, A. K.;
Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 10417. (b) Song, A.;
Parker, K. A.; Sampson, N. S. J. Am. Chem. Soc. 2009, 131, 3444.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, synthesis characterization data (1H and
13C NMR, MS, SEC traces, etc.), and spectra of the compounds
are available free of charge via the Internet at http://pubs.acs.
org.
AUTHOR INFORMATION
■
Corresponding Author
tlc@snu.ac.kr
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Authors dedicate this paper to Prof. Junghun Suh on his
retirement and 65th birthday. We are grateful for the financial
support from the Basic Science Research Program, the Nano-
Material Technology Development Program, BRL, and Mid-
Career Research Program through the National Research
Foundation of Korea. We also thank NCIRF at SNU for
supporting GC-MS experiments.
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