Multiple Olefin Metathesis Polymerization That Combines All Three Olefin Metathesis Transformations: Ring-Opening, Ring-Closing, and Cross Metathesis

Article abstract of DOI:10.1021/jacs.5b06033

We demonstrated tandem ring-opening/ring-closing metathesis (RO/RCM) polymerization of monomers containing two cyclopentene moieties and postmodification via insertion polymerization. In this system, well-defined polymers were efficiently formed by tandem cascade RO/RCM reaction pathway. Furthermore, these polymers could be transformed to new A,B-alternating copolymers via a sequential cross metathesis reaction with a diacrylate. Additionally, we demonstrated the concept of multiple olefin metathesis polymerization in which the dicyclopentene and diacrylate monomers underwent all three olefin metathesis transformations (ring-opening, ring-closing, and cross metathesis) in one shot to produce A,B-alternating copolymer.

Full text of DOI:10.1021/jacs.5b06033

Communication
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Multiple Olefin Metathesis Polymerization That Combines All Three
Olefin Metathesis Transformations: Ring-Opening, Ring-Closing, and
Cross Metathesis
Ho-Keun Lee,† Ki-Taek Bang,† Andreas Hess,† Robert H. Grubbs,‡ and Tae-Lim Choi*,†
†Department of Chemistry, Seoul National University, Seoul 151-747, Korea
‡Department of Chemistry, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
of control. As a result, almost all polymers prepared by olefin
ABSTRACT: We demonstrated tandem ring-opening/
ring-closing metathesis (RO/RCM) polymerization of
monomers containing two cyclopentene moieties and
postmodification via insertion polymerization. In this
system, well-defined polymers were efficiently formed by
tandem cascade RO/RCM reaction pathway. Furthermore,
these polymers could be transformed to new A,B-
alternating copolymers via a sequential cross metathesis
reaction with a diacrylate. Additionally, we demonstrated
the concept of multiple olefin metathesis polymerization in
which the dicyclopentene and diacrylate monomers
underwent all three olefin metathesis transformations
(ring-opening, ring-closing, and cross metathesis) in one
shot to produce A,B-alternating copolymer.
metathesis polymerization have been produced by a single type
of olefin metathesis transformation, yielding polymers with
simple repeat units.2−4
Over the past decade, a handful of examples have been
reported detailing successful polymerizations produced by two
different olefin metathesis transformations. The first example was
the ring-opening insertion metathesis polymerization (ROIMP),
a general method for synthesizing A,B-alternating copolymers by
combining ROMP and highly selective CM in a one-pot
synthesis.9 Another example was the two-pot polymerization
using ROMP and ADMET, wherein two metathesis reactions
sequentially occurred (first ROMP and then ADMET) and
produced branched polymers in an independent manner.10 The
most recent result was a simultaneous tandem ring-opening/
ring-closing metathesis (RO/RCM) polymerization, reported by
our group. This polymerization of monomers containing
cycloalkenes and terminal alkynes proceeded via a relay-type
mechanism in a living manner.6 These attempts increased the
complexity in the polymer microstructures accessible via olefin
metathesis polymerization.
In our search to broaden the overall scope of polymerization,
herein, we report a new tandem olefin metathesis polymerization
methodology via a cascade RO/RCM polymerization of
monomers containing two cyclopentenes.5 The concept of
multiple olefin metathesis polymerization (MOMP) entails
combining a CM reaction to produce A,B-alternating copoly-
mers in either two-step or one-shot methods. This is the first
example of polymerization in which the combination of all three
olefin metathesis transformations produced well-defined poly-
mers via precise reaction pathways.
To achieve tandem RO/RCM polymerization, we designed
novel monomers containing cyclopentene moieties, because
monomers should be able to undergo both ring-opening and
ring-closing metathesis reactions simultaneously. Consequently,
we synthesized three monomers M1−3; in each, two cyclo-
pentene moieties were connected by oxygen, nitrogen, or carbon
linkers, respectively (Scheme 1a).
However, two issues need to be first overcome in order to
achieve selective tandem RO/RCM polymerization from these
new monomers containing two polymerizable cyclopentene
moieties. The first issue was to determine whether these
uring the last two decades, the olefin metathesis reaction
Dhas been widely used as an efficient method to synthesize
various molecules by forming new carbon−carbon double
bonds.1 Olefin metathesis transformation can be carried out by
three main types metathesis: ring-opening metathesis (ROM),
ring-closing metathesis (RCM), and cross metathesis (CM);
these metathesis have become versatile tools in organic synthesis.
Furthermore, the olefin metathesis reaction has been applied to
various polymerizations such as ring-opening metathesis
polymerization (ROMP),2 cyclopolymerization3 derived from
RCM, and acyclic diene metathesis (ADMET) polymerization4
derived from CM. These polymerizations have produced various
polymers, including both conjugated and nonconjugated
polymers, and in some cases, living polymerization is also
possible.3f,h,j,5,6
The utility of the olefin metathesis reaction was further
broadened when organic chemists discovered tandem olefin
metathesis reactions in which two or more olefin metathesis
transformations occur simultaneously in a single step. This
cascade reaction allowed chemists to efficiently synthesize
various complex organic molecules.7 Various examples of natural
product total synthesis using this cascade reaction method as a
key strategy for the formation of the core skeletons have been
reported, demonstrating the versatility of these tandem olefin
metathesis reactions.8
Similar methods could be introduced to polymer synthesis, but
combining two or more transformations tends to produce ill-
defined polymers with a random microstructure due to the lack
Received: June 10, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b06033
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Journal of the American Chemical Society
Communication
Scheme 1. (a) Structure of the Dicyclopentene Monomers and
(b) Reaction Scheme of the Tandem RO/RCM
Polymerization and Proposed Polymerization Pathway for the
Selective Formation of Five-Membered Ring Repeat Units
Table 1. Tandem RO/RCM Polymerizations of M1−3
a
b
conc.
(M)
time
(h)
Mn
conv
a
entry monomer
M:C
(k)
PDI
(%)
1
2
3
4
5
6
7
8
9
M1
M1
M1
M2
M2
M2
M3
M3
M3
50:1
150:1
250:1
50:1
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
12
12
12
24
24
48
24
48
48
13.8
38.0
52.3
25.9
66.2
114.5
29.9
62.9
114.4
1.79
1.91
1.83
1.53
1.91
2.14
1.48
1.70
1.90
100
100
100
100
97
150:1
250:1
50:1
98
100
97
150:1
250:1
95
a
Determined by THF SEC calibrated using polystyrene standards.
Conversion was determined by crude H NMR analysis
b
1
The structure of purified P1 was characterized with NMR
(Figure 1, Figure S2), which clearly confirmed the formation of a
monomers having 3-substituted cyclopentene moieties would
polymerize at all. The Grubbs group reported that the ROMP of
3-substituted cyclopentene derivatives always failed,11 because
the substituent at the 3-position increased the steric hindrance
and decreased the ring strain, thereby preventing the ROMP.
The second issue was to determine how to control the competing
ROM and RCM equilibrium of the cyclopentene moieties in the
monomer so that the desired, structurally well-defined polymers
would be produced selectively (Scheme 1b). For example, if the
ROMP became dominant, a cross-linked gel was produced. Even
if the cascade RO/RCM reaction did occur, there were two
possible pathways depending on the orientation of the
approached catalyst. Only Pathway A was a productive pathway
for the tandem polymerization, because the second RCM step
was favored due to the proximity effect. On the other hand, in
Pathway B, a new carbene underwent RCM back to the
monomer (Scheme 1b). Thus, optimizing the polymerization
condition to control the competing equilibrium between ROM
and RCM was the key to the successful tandem polymerization.
Initially, to search for the most suitable catalyst, we screened
various catalysts for the tandem RO/RCM polymerization of
M1. Generally, a first-generation Grubbs catalyst (Catalyst 1)
has lower activity than that of second- or third-generation
Grubbs catalysts. In our system, however, Catalyst 1 showed the
best performance when comparing the monomer consumptions
Figure 1. 1H NMR spectrum of P1.
low ring-strained, five-membered, 2,5-dihydrofuran backbone.
Also, an internal olefin with an E:Z ratio of 5:1 for the acyclic
olefin was observed. Interestingly, M1 was polymerized at
concentrations <0.1 M despite possessing the low-strained
cyclopentene and the substitution at the 3-position. This finding
was in sharp contrast to the case of cyclopentene, which was
seemingly a more reactive monomer but did not undergo ROMP
at concentrations <0.8 M.12 Instead, at concentrations below the
critical monomer concentration for the ROMP, the cascade RO/
RCM occurred to produce the polymer with a rearranged
backbone with the 2,5-dihydrofuran moiety, implying that the
2,5-dihydrofuran backbone moiety in P1 was thermodynamically
more stable than the cyclopentene in the monomer. Also, due to
the substitutions at the 2- and 5-positions on dihydrofuran, the
reverse reaction or depolymerizion would be slower than chain
propagation.
1
via H NMR (Figure S1). Next, we screened the reaction
concentration to determine the effect on the polymerization. At
0.1 M, M1 with a monomer:catalyst ratio (M:C) of 50:1 was
successfully polymerized into poly(2,5-disubstituted-2,5-dihy-
drofuran) with 87% conversion after 24 h. To shorten the
reaction time, we increased the concentration up to 1 M, but an
insoluble cross-linked gel formed within 10 min, meaning that
the concentration reached its critical concentration at which the
ROMP of the cyclopentene moiety became dominant. This
finding was somewhat expected because the critical monomer
concentration for the ROMP of the cyclopentene is 0.8 M at 25
°C.12 On the other hand, at 0.5 M, full conversion was obtained
after 12 h without any cross-linking, and simple precipitation in
methanol produced a rubbery polymer with moderate yields.
When decreasing the catalyst loading or increasing the M:C ratio
to 150:1 and even 250:1, we achieved quantitative conversion
under optimized conditions, producing P1 with Mn of 13.8−52.3
kDa in proportion to the M:C ratio (Table 1, entries 1−3).
To broaden the monomer scope, we investigated the tandem
polymerization of M2 and M3 in which two cyclopentenes were
connected by nitrogen and carbon, respectively. Unlike M1, the
analogous polymerizations of M2 and M3 at 0.5 M did not
achieve full conversions, but qualitative tandem polymerization
without any cross-linking occurred when the concentration
increased to 1.5 M. It seems that the ring-strain and the
competing equilibrium between ROM and RCM for M2 and M3
were slightly different than those for M1. Regardless, both
monomers efficiently underwent the tandem polymerization
even with low catalyst loading with M:C ratios of 250:1, and the
Mn value of the resulting P2 and P3 were roughly controlled and
proportional to the M/C ratios (Table 1, entries 4−9), and
structures of purified P2 and P3 were also characterized by NMR
(Figures S3, S4, S9, and S10) However, in all three cases, the
polydispersity index (PDI) values were broad due to chain-
transfer reactions.
B
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As we previously observed, the polymerization of M1 at
concentrations over that of the critical monomer concentration
produced cross-linked gel due to the dominant intermolecular
ROMP process. However, by diluting the solution containing
this cross-linked gel to 0.5 M, the gel completely disappeared
after 6 h, mainly yielding a soluble polymer containing the same
dihydrofuran backbone. This result was obtained because the
dilution led to the reversible decross-linking via the intra-
molecular RO/RCM process to produce P1 (Scheme 2).
Scheme 4. MOMP Reactions with P1−3
Scheme 2. Concentration-Dependent Polymerization
Table 2. Analysis of MOMP Products
a
b
b
b
polymer
A,B-alt. (%)
Mn (k)
Mw (k)
PDI
P4
P5
P6
P4′
97.5
94.3
94.2
95.5
10.6
7.3
22.0
13.5
30.7
2.09
1.82
1.87
2.11
16.4
9.3
19.8
b
a
Determined by1H NMR spectroscopic analysis. Determined by
Since the entire polymerization was in thermodynamic
equilibrium, the isolated polymers could also be depolymerized
to the corresponding monomers via the reverse tandem
intramolecular RO/RCM process. Each purified P1−3 was
redissolved into a 0.04 M CH2Cl2 solution, and fresh Catalyst 1
was added. After 12 h, 13% of M1, 26% of M2, and 34% of M3
were observed with 1H NMR, implying that even depolymeriza-
tion indeed occurred in dilute conditions (Scheme 3). These
results demonstrated that by understanding the equilibrium, the
direction of the ROMP or RO/RCM processes could be
predicted depending on the concentration.
THF SEC calibrated using polystyrene standards.
Figure 2. 1H NMR spectrum of P4.
Scheme 3. Depolymerization of P1−3
new peaks corresponding to the α,β-unsaturated carbonyl olefins
appeared at 7.0 and 5.8 ppm (Figure 2, P4 Ha and Hb,
respectively). In particular, the newly formed olefin was 100% E
isomer with a coupling constant of 16 Hz (Figure S13−1).
Furthermore, the signal from the cyclic olefin (Figure1, P1 Hd)
still remained at 5.8 ppm (Figure 2, P4 Hc). A,B-alternations of
the polymers were calculated by comparing the ratio of newly
formed α,β-carbonyl olefins and the remaining internal olefins on
the original homopolymers; in all cases, significant alternations
were observed (97.5% for P4, 94.3% for P5, and 94.2% for P6,
Table 2). These structural analyses demonstrated the successful
preparation of well-defined A,B-alternating copolymers via the
sequential CM reaction.
To broaden the utility of this tandem polymerization, we
experimented to determine if the resulting polymers could
undergo postmodification with a second-generation Grubbs
catalyst. If the coupling between internal olefins on P1−3 and
α,β-unsaturated carbonyl olefins on diacrylate monomers
proceeded with high conversion and high selectivity, as in the
case of CM,13 the diacrylates could be selectively inserted into
the polymers to yield A,B-alternating copolymer containing α,β-
unsaturated carbonyl olefins. To test this idea, the purified
polymers (P1−3) were treated with 1,4-butandiol-diacrylate and
Catalyst 2 under optimized conditions (Scheme 4).
After the backbiting of the catalyst into the acyclic olefins on
the polymers, a series of exclusive CMs occurred selectively with
the diacrylate,13 and this process converted P1−3 into
thermodynamically more stable P4−6, respectively, containing
α,β-unsaturated carbonyl olefin with moderate molecular
weights (Table 2).
We expected that it would also be possible to obtain the
identical A,B-alternating copolymer via the one-shot polymer-
ization method instead of the two-step sequential method
described above. M1 with the diacrylate was polymerized with
Catalyst 2 under the same conditions as those of the previous
sequential method (Scheme 5). After 12 h of polymerization, the
identical A,B-alternating copolymer P4′ was obtained and
1
confirmed with H NMR (Table 2, Figure S13−2). This result
provides the first example of MOMP, wherein all three types of
olefin metathesis transformations (ring-opening, ring-closing,
and cross metathesis) were combined in an orderly manner to
produce just one uniform polymer microstructure via precisely
controlled pathways.
The structural features of these well-defined A,B-alternating
copolymers (P4−6) were explicitly confirmed with H NMR
analysis (Figure 2 and Figures S11 and S12). First, the internal
1
In conclusion, we investigated the tandem RO/RCM
polymerization of monomers containing two polymerizable
olefins near 5.5 ppm on P1 disappeared (Figure 1, P1 Ha), and
C
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cyclopentene moieties with a first-generation Grubbs catalyst.
This tandem RO/RCM polymerization was carried out
efficiently without side reactions such as cross-linking or
depolymerization, because with the appropriate concentration,
the precisely controlled cascade reaction occurred exclusively to
produce a well-defined polymer microstructure. Furthermore,
these resulting polymers successfully transformed to new A,B-
alternating copolymers by insertion polymerization via sequen-
tial cross metathesis with a diacrylate using a second-generation
Hoveyda−Grubbs catalyst. Finally, the one-shot polymerization
of M1 and diacrylate successfully produced the identical A,B-
alternating copolymer. This report covers the first example of
multiple olefin metathesis polymerization in which all three types
of olefin metathesis transformations were used simultaneously
with precise control to yield well-defined polymers.
ASSOCIATED CONTENT
* Supporting Information
■
S
(10) Ding, L.; Xie, M.; Yang, D.; Song, C. Macromolecules 2010, 43,
10336.
(11) Hejl, A.; Scherman, O. A.; Grubbs, R. H. Macromolecules 2005, 38,
Experimental procedures, NMR data for polymers, and SEC
traces for polymers. The Supporting Information is available free
of charge on the ACS Publications website at DOI: 10.1021/
jacs.5b06033.
7214.
(12) (a) Monfette, S.; Fogg, D. E. Chem. Rev. 2009, 109, 3783.
(b) Ofstead, E. A.; Calderon, N. Makromol. Chem. 1972, 154, 21.
(c) Tuba, R.; Grubbs, R. H. Polym. Chem. 2013, 4, 3959.
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AUTHOR INFORMATION
Corresponding Author
*tlc@snu.ac.kr
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Authors dedicate this paper to Prof. Kwan Kim on his lifelong
achievements in research and education at SNU and his
retirement. We are grateful for financial support from Basic
Science Research and Nano-Material Technology Development
through NRF of Korea.
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D
DOI: 10.1021/jacs.5b06033
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