Metal-free ring-opening metathesis polymerization

Article abstract of DOI:10.1021/ja512073m

We have developed a method to achieve ring-opening metathesis polymerization (ROMP) mediated by oxidation of organic initiators in the absence of any transition metals. Radical cations, generated via one-electron oxidation of vinyl ethers, were found to react with norbornene to give polymeric species with microstructures essentially identical to those traditionally obtained via metal-mediated ROMP. We found that vinyl ether oxidation could be accomplished under mild conditions using an organic photoredox mediator. This led to high yields of polymer and generally good correlation between M_n values and initial monomer to catalyst loadings. Moreover, temporal control over reinitiation of polymer growth was achieved during on/off cycles of light exposure. This method demonstrates the first metal-free method for controlled ROMP.

Full text of DOI:10.1021/ja512073m

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Communication
Metal-Free Ring-Opening Metathesis Polymerization
Kelli A. Ogawa, Adam E Goetz, and Andrew J Boydston
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja512073m • Publication Date (Web): 08 Jan 2015
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Metal-Free Ring-Opening Metathesis Polymerization
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Kelli A. Ogawa, Adam E. Goetz, and Andrew J. Boydston*
Department of Chemistry, University of Washington, Seattle, WA 98195
Supporting Information Placeholder
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These attributes are afforded by the highly optimized reactivi-
ABSTRACT: We have developed a method to achieve ring-
opening metathesis polymerization (ROMP) mediated by oxida-
tion of organic initiators in the absence of any transition-metals.
Radical cations, generated via one-electron oxidation of vinyl
ethers, were found to react with norbornene to give polymeric
species with microstructures essentially identical to those tradi-
tionally obtained via metal-mediated ROMP. We found that
vinyl ether oxidation could be accomplished under mild condi-
tions using an organic photoredox mediator. This led to high
ties of primarily Ru- and Mo-based alkylidene initiators, as well
as other transition metal complexes (Scheme 1, bottom). With
such tremendous breakthroughs coming from organometallic
developments, we became curious if a metal-free variant for
ROMP could ever be achieved. Motivation for a departure from
transition metal catalysts and initiators comes in part from
recognition that metal-based byproducts can be difficult to re-
move from the polymeric materials, and also by the promise of
new synthetic capabilities stemming from unique polymeriza-
tion strategies.
4
yields of polymer and generally good correlation between M
n
values and initial monomer to catalyst loadings. Moreover,
temporal control over reinitiation of polymer growth was
achieved during on/off cycles of light exposure. This method
demonstrates the first metal-free method for controlled ROMP.
In general, metallic byproducts or residual complexes can
lead to complications with biological studies, electronic proper-
ties measurements, or optical properties. Moreover, down-
stream reactivity of residual metallic species can be problemat-
5
ic, often leading to untoward oxidative processes in the final
material. At a minimum, the potential for metal contaminants
often warrants quantitation by advanced techniques such as
inductively-coupled plasma mass spectrometry. These issues
Ring-opening metathesis polymerization (ROMP, Scheme 1) is
one of the most prevalent technologies that has emerged from
the development of transition metal-based olefin metathesis.
6
1,2
have motivated a number of protocols for removing metal-based
4
The widespread implementation of ROMP in both academic
and industrial settings has led to breakthroughs in areas such as
drug delivery, biomedical engineering, photovoltaics, and pro-
duction of structural and engineering materials. In general,
ROMP can be used to achieve living polymerizations, provide
polymers of narrow dispersity, enable excellent control over end
group functionality, and incorporate a broad range of function-
al groups into polymer scaffolds and network materials.
components, which even when successful, add additional pro-
cessing steps for material production. Beyond circumventing
issues directly related to metal species, moving toward organic
initiators and potentially different polymerization mechanisms
could offer unique control over polymer end group functionali-
ty, main chain microstructure, and methods for spatiotemporal
control over polymer production.
1,3
Scheme 2. (top) Electrochemical olefin metathesis and cyclo-
butane formation, and (bottom) photoredox mediated cyclo-
butane synthesis
Scheme 1. (top) Generalized depiction of ROMP using transi-
tion metal alkylidene initiators, and (bottom) representative
metal alkylidene complexes used in ROMP
We envisioned a method by which ROMP polymers could be
obtained in a metal-free manner, using “all organic” initiators.
A key to how this might be accomplished was first reported by
Chiba and coworkers, in which an electrochemical intermolecu-
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lar olefin cross metathesis reaction was achieved with an excess polymer structures consistent with a ROMP mechanism was
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of terminal alkene in the presence of vinyl ethers (Scheme 2,
top). To our knowledge, however, this has only been demon-
encouraging.
7
strated for a limited set of olefins, and without a catalytic role of
the vinyl ether. More commonly, oxidation of the vinyl ethers
leads to formation of a [2 + 2] complex that is reduced to give
8
cyclobutanes. Similar overall reactivity has also been reported
using photoredox mediators, as opposed to bulk electrolysis, to
Figure 1. Monomers and initiators used in this study.
9
The main challenges with further development of the elec-
trochemical approach appeared to be poor solubility of 1 and
PNB in nitromethane (a requisite solvent in Chiba’s work), and
heterogeneous oxidation of the initiator (2) at the electrode
surface. To circumvent these issues, we pursued similar redox
processes that were amenable to broader solvent scope and
homogeneous oxidations. We were inspired by recent break-
throughs in photoredox polymerizations, which have emerged
as a powerful method for achieving metal-free protocols and
spatiotemporal control over polymerizations. Notably, photo-
redox polymerization strategies have focused almost exclusively
on controlled radical addition polymerizations in which a redox
process is inherent in the activation/deactivation of the poly-
mer chain end. Traditional metal-mediated ROMP (Scheme 1),
on the other hand, is redox-neutral at all stages, and the metal
complex is covalently attached to each chain end until chemi-
cally cleaved at the end of the polymerization.
accomplish olefin dimerization (Scheme 2, bottom). Outcom-
peting the reduction to form cyclobutanes would be key to ex-
panding the scope of this method, and could unlock access to a
new mechanism for ROMP.
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As depicted in Scheme 3, we envisioned an approach to
ROMP that would be initiated by one-electron oxidation of a
vinyl ether initiator (A) to produce the activated radical cation
B. Subsequent reaction with a cycloalkene monomer would
form a [2 + 2] complex (C). We hypothesized that reduction of
C could be out-competed by rapid ring-opening to alleviate ring-
strain in an appropriate cycloalkene. This would complete the
ROMP initiation event arriving at D. Continued propagation
with additional cycloalkene monomers would ultimately yield
ROMP polymers, potentially bearing a reactive radical cation
chain end (E). Reversible reduction of D and E are also likely,
which would result in iterative termination-reinitiation cycles to
arrive eventually at F. Encouraged by the propensity for ring
strain in cycloalkene monomers to facilitate ROMP, we were
inspired to develop an electro-organic approach to this widely
used polymerization method. Herein, we report the initial de-
velopment of a metal-free ROMP method.
1
0
Pyrylium and acridinium salts (3a – c, Figure 2) have been
9
b
identified as good candidates for facilitating photo-oxidation.
These mediators are capable of facilitating electron transfer
when in the photo-excited state, and were expected to be good
oxidizers for the vinyl ether initiators (2). Whereas the initiators
Scheme 3. Idealized mechanism of redox-initiated, metal-free
ROMP
2
a – c display oxidation potentials in the range of 1.43 to 1.30
6
V vs SCE, the oxidizing power of excited state pyrylium and
acridinium cations have been calculated to be 1.74 and 2.06 V
vs SCE, respectively.
ROMP, we used an initial monomer concentration of ca. 1.9
M in CH Cl , with a monomer to initiator (1:2a) molar ratio of
1
1,12
To explore the photoredox initiation of
2
2
ca. 100:1. Using a blue LED light source (ꢀ = 450 – 480 nm) in
the presence of 3, the best yields were obtained from the
pyrylium tetrafluoroborate salt 3a. In general, 3b gave lower
yields than 3a, and 3c failed to produce any detectable PNB.
We focused on norbornene (1) as a monomer, as this scaffold
exhibits relatively high ring strain among common ROMP
monomers (Figure 1). To investigate the direct oxidation of
vinyl ethers and propensity for the ensuing radical cation to
initiate ROMP, we conducted bulk electrolysis on solutions of 1
containing readily available vinyl ether initiators (2a – c). After
electrolysis of 1 and 2a for 3 h, the reaction solution and elec-
trodes were analyzed for any presence of polynorbornene
(PNB). A small amount of material was obtained (3 % yield),
and analysis by NMR spectroscopy revealed signals consistent
with PNB. Moreover, end group signals consistent with the
vinyl ether were observed even after precipitation of the poly-
mer to remove any residual small molecule initiator. Gel per-
meation chromatography (GPC) analysis revealed a number-
Figure 2. Photoredox mediators used in this study.
1
The structure of the PNB was confirmed by H NMR analysis
in comparison with an authentic sample prepared via tradition-
st
al ROMP using the Grubbs 1 -generation initiator (see Sup-
porting Information). The glass transition temperature (T ) of
g
samples prepared by either traditional or photoredox mediated
ROMP were also found to be consistent with one another. Spe-
average molecular weight (M ) of 11.8 kDa (Ð = 2.2). Similar
n
results were obtained using initiators 2b and 2c. Although the
yields of PNB were consistently low, the confirmation that an
anodic oxidation of 2 could initiate polymerization of 1 to give
cifically, the T of PNB prepared by Ru-mediated ROMP (M =
g
n
49.5 kDa) was found to be 53.3 °C, versus 49.5 °C for a sample
prepared by photoredox initiation (M = 43.9 kDa).
n
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Table 1. Polymerization results and GPC data for photoredox mediated ROMP
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
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3
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1 : 2 : 3a
[1]
(M)b
conversion (%)c time (min)
M
,
theo [kDa]
8.0
M
n, exp [kDa]
15.1
14.9
15.8
13.5
19.2
8.1
entry initiator
0
n
Ð
1
2
3
4
5
6
7
8
9
2a
2b
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
97 : 1 : 0.03
97 : 1 : 0.03
106 : 1 : 0.03
104 : 1 : 0.10
104 : 1 : 0.25
48 : 1 : 0.03
57 : 1 : 0.03
491 : 1 : 0.03
494 : 1 : 0.03
1000 : 1 : 0.03
103 : 1 : 0.03
1.9
1.9
2.0
1.9
1.9
1.8
1.2
5.3
1.8
1.9
1.9
1.9
88 (73)
92 (80)
87 (67)
80 (76)
90 (73)
95 (78)
93 (58)
51 (25)
72 (50)
61 (47)
53 (29)
88 (53)
30
30
1.7
1.6
1.6
1.4
1.6
1.4
1.4
1.5
1.5
1.6
1.3
1.4
8.4
30
9.0
0
1
2
3
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0
1
2
3
4
5
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0
1
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0
1
2
3
4
5
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9
0
1
2
3
4
5
6
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8
9
0
150
120
60
7.8
8.9
4.3
120
120
60
5.0
11.5
22.2
43.9
60.2
7.2
23.6
33.4
57.4
5.0
1
0
120
2580
90
1
1
1d
2e
102: 1: 0.03
8.5
15.5
a
b
c
1
Initial molar ratio of 1, 2, and 3. Initial concentration of 1. Conversion of 1, as determined by H NMR analysis; isolated yields after pre-
d
e
cipitation given in parentheses. Reaction mixture exposed only to ambient light from fume hood. Reaction mixture exposed to blue LED
light with half of the bulbs blocked out. Mn, theo is theoretical number-average molecular weight calculated from initial 1:2 ratio and % conver-
sion of 1. Mn, exp is experimental number-average molecular weight, calculated from a weight-average molecular weight determined by GPC
using multi-angle laser light scattering (MALLS). Dispersities (Ð) determined by GPC analysis.
Each initiator (2) gave PNB in good yield via the photoredox
method (Table 1, entries 1-3). Whereas 2a provides a distin-
guishable NMR handle (see Supporting Information), the ma-
jority of the polymerizations were conducted with 2c simply due
to commercial availability of this initiator. The amount of pho-
toredox mediator 3a that was required for successful polymeri-
zation was found to be quite low. Specifically, we observed con-
blocked out also gave PNB at an expected longer reaction time
(entry 12). It is noteworthy that the reduced light intensity in
each case gave lower Ð in comparison with other experiments,
although the extent and origin of this trend are not yet clear. In
complete absence of light, no PNB was observed.
During the course of the polymerization, we observed a grad-
ual increase in M with increasing conversion of monomer,
n
sistent M values and % conversions when using mediator to
n
consistent with the chain growth nature of ROMP (Figure3).
Although there was a positive correlation, the linearity was not
as precise as traditional “living” ROMP using, for example,
initiator ratios (3a:2) of 0.03 to 0.25 (Table 1, entries 3 – 5).
Higher loading of mediator did manifest some bimodality in
the GPC traces, with high MW shoulders appearing with in-
creasing amounts of mediator. We speculate that this may be
due to increased concentration of active chain ends and there-
fore greater extent of chain-chain coupling. Additionally, the
complexity of reversible chain end deactivation/re-activation is
likely influenced by the amount of mediator.
rd
1c,13
Grubbs 3 -generation initiator.
This could be ascribed to the
relative rates of initiation and propagation in the photoredox
method, or any number of early termination events.
1
5
2
9
6
3
0
2.4
2
.2
.0
1
2
Varying the initial monomer to initiator ratio provided some
1.8
1.6
1.4
1.2
degree of control over the final M
served a consistent correlation between the theoretical and
experimental M values, with experimental values generally
n
(entries 3, 6 – 10). We ob-
n
0
20
40 60
Conversion (%)
80
100
being greater than expected for the given monomer to initiator
ratios and % conversions. Dispersities were found to vary be-
tween 1.3 and 1.7 across different experiments, and remained
fairly consistent during the course of each polymerization. We
also found that the initial monomer concentration could be
varied, with even very high (5.3 M) concentrations giving suc-
cessful polymerizations (entry 8). This suggests that bulk
polymerization using liquid monomers may be possible in the
future with this approach.
60
50
40
30
20
10
0
2.4
2.2
2.0
1.8
1.6
1.4
1.2
Importantly, the initiator, mediator, and light source were
each found to be required for the polymerization. In absence of
blue LED light, but with exposure to ambient lighting from the
fume hood, we observed slow conversion to give PNB (Table 1,
entry 11). Similarly, using blue LED light with half of the bulbs
0
20
40
60
80
100
Conversion (%)
Figure 3. Plot of M
monomer using initial 1:2c of (top) 100:1 and (bottom) 500:1.
n
(circles) and Ð (triangles) vs % conversion of
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Mechanistically, we envision oxidation of the vinyl ether via
Page 4 of 5
pounds. This materials is available free of charge via the Internet at
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http://pubs.acs.org
electron transfer to the excited pyrylium cation to give the vinyl
ether radical cation (e.g., A  B, Scheme 3). Notably, the
propagating radical cation chain end likely forms a dynamic
redox couple with the reduced pyrylium species. The reversibil-
ity would manifest an ability for the radical cation chain end to
be reduced to terminate polymerization, and then oxidize and
reinitiate upon exposure to light. We investigated this temporal
control by monitoring the polymerization with intermittent
exposure to blue LED light. As shown in Figure 4, polymeriza-
tion ceased in the dark and was reinitiated upon exposure to
blue light. Specifically, we observed little to no further conver-
AUTHOR INFORMATION
Corresponding Author
boydston@chem.washington.edu
ACKNOWLEDGMENT
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Financial support of this research by the University of Washington.
We are grateful to Professor Robert H. Grubbs (California Institute
of Technology) for helpful discussions and encouragement, and to
Professors Dean Waldow (Pacific Lutheran University) and Chris-
tine Luscombe (University of Washington) for assistance with GPC
analyses.
1
sion of monomer in the dark as determined by H NMR spec-
troscopy, and no significant changes in M as judged by GPC
n
analysis. This suggested to us that the pyrylium cation and vinyl
ether form a dynamic redox couple and that the radical cation
chain end is undergoes oxidation-reduction cycles during the
polymerization. Furthermore, the correlation between % con-
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In summary, we have developed the first protocol for achiev-
ing metal-free ROMP. The approach utilizes one-electron oxida-
tion of electron-rich vinyl ethers to initiate the process, which
can be achieved either electrochemically or via photoredox pro-
cesses. A photoredox approach enabled high yields of polymeri-
zation in short reaction times under mild conditions. This
demonstration may lead to a complementary approach to syn-
thesizing ROMP polymers via a metal-free protocol, with poten-
tial to offer unique synthetic control over end group functional-
ity. The success of the photoredox mediation may also provide
new opportunities for spatiotemporal control over production
of ROMP-based polymers and materials.
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ASSOCIATED CONTENT
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13) Choi, T.-L.; Grubbs, R. H. Angew. Chem. Int. Ed. 2003, 42,
Detailed experimental procedures, representative cyclic voltammo-
grams and current-time plots, and characterization of all new com-
1743-1746.
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