Enol Ethers Are Effective Monomers for Ring-Opening Metathesis Polymerization: Synthesis of Degradable and Depolymerizable Poly(2,3-dihydrofuran)

Article abstract of DOI:10.1021/jacs.9b11834

Enol ethers are widely used as quenching reagents for Grubbs catalysts. However, we report the surprisingly effective ring-opening metathesis polymerization (ROMP) of cyclic enol ethers, because the resulting electron-rich ruthenium alkylidene complex remains active toward metathesis of electron-rich olefins, despite its deactivation toward hydrocarbon olefins. We demonstrate the first example of ROMP of cyclic enol ethers, using 2,3-dihydrofuran as the monomer, producing a new type of degradable and depolymerizable poly(enol ether). The polymers exhibited perfect regioregularity, and their molecular weights can be regulated by the loading of Grubbs initiators or by the use of a linear vinyl ether as the chain transfer agent. We also developed protocols to deactivate the catalyst following metathesis of enol ethers and cleave the catalyst off the resulting polymers using H₂O₂ oxidation. The resulting poly(dihydrofuran) can be recycled to monomer via depolymerization with Grubbs catalyst or degraded to small molecules by hydrolysis under acidic conditions. This work opens exciting opportunities for a new class of ROMP monomers that lead to degradable polymers.

Full text of DOI:10.1021/jacs.9b11834

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Communication
Enol Ethers Are Effective Monomers for Ring-
Opening Metathesis Polymerization: Synthesis of
Degradable and Depolymerizable Poly(2,3-dihydrofuran)
John D Feist, and Yan Xia
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b11834 • Publication Date (Web): 27 Dec 2019
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Enol Ethers Are Effective Monomers for Ring-Opening Metath-
esis Polymerization: Synthesis of Degradable and Depolymer-
izable Poly(2,3-dihydrofuran)
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John D. Feist and Yan Xia*
Department of Chemistry, Stanford University, Stanford, California 94305, United States
Supporting Information Placeholder
To test our hypothesis, we selected 2,3-dihydrofuran (DHF), an
inexpensive, commercial cyclic enol ether as a potential monomer,
considering that cyclopentene has similarly low strain but under-
goes equilibrium ROMP.16-20 To probe the ROMP reactivity of
DHF, neat monomer was subjected to first and second generations
of Grubbs catalysts, G1 and G2, at room temperature. Both of these
catalysts polymerized DHF to produce a rubbery polymer with Tg
= -50.5 °C and thermal stability up to 320 °C (Figures S1 and S2).
At [DHF]0/[Ru]0 = 4000, ROMP of DHF using G1 under nitrogen
with degassed monomer became too viscous to stir after 24 h at
55% conversion, yielding PDHF with Mn = 80 kDa, and Mn de-
creased to 56 kDa with Ɖ = 1.3 after 3 days (Figure S3a). With
more robust G2, ROMP of DHF was simply performed under air
using non-degassed commercial DHF. The ROMP solution solidi-
fied in 15 min at 15% monomer conversion, yielding a polymer
with Mn = 309 kDa and Ɖ = 1.4, and the ROMP reached equilibrium
after 4 h to form PDHF with Mn = 47 kDa and Ɖ = 1.6 and 80%
DHF conversion (Figure S3b).
ABSTRACT: Enol ethers are widely used as quenching reagents
for Grubbs catalysts. However, we report the surprisingly effective
ring-opening metathesis polymerization (ROMP) of cyclic enol
ethers, because the resulting electron-rich ruthenium alkylidene
complex remains active towards metathesis of electron-rich olefins,
despite its deactivation toward hydrocarbon olefins. We demon-
strate the first example of ROMP of cyclic enol ethers, using 2,3-
dihydrofuran as the monomer, producing a new type of degradable
and depolymerizable poly(enol ether). The polymers exhibited per-
fect regioregularity and their molecular weights can be regulated
using the loading of Grubbs initiators or the use of a linear vinyl
ether as the chain transfer agent. We also developed protocols to
deactivate the catalyst following metathesis of enol ethers and
cleave the catalyst off the resulting polymers using H2O2 oxidation.
The resulting poly(dihydrofuran) can be recycled to monomer via
depolymerization with Grubbs catalyst or degraded to small mole-
cules by hydrolysis under acidic conditions. This work opens ex-
citing opportunities for a new class of ROMP monomers that lead
to degradable polymers.
To quantify conversions and better understand the polymeriza-
tion thermodynamics of DHF, we needed a method to stop the
ROMP of enol ether and prevent re-equilibration when the neat
samples were diluted for NMR spectroscopy. Tricyclohexyl phos-
phine (PCy3) is a strongly coordinating ligand,21 and we found that
the dissolution of the polymerization mixture into a solution con-
taining 40 mM PCy3 was sufficient to inhibit metathesis activity
(Figure S7). Using this protocol, we measured the conversion using
NMR analysis of the crude polymerization after ROMP reached
equilibrium at 6, 22, and 40 C with 0.05 mol% G2. At 22 C, the
reaction reached 76% conversion, corresponding to a residual mon-
omer concentration of 3.2 M. Similarly, we determined the equilib-
rium DHF conversion at 6 and 40 C to be 83% and 56%, respec-
tively, corresponding to equilibrium monomer concentrations of
2.2 and 5.9 M. Using these values, we calculated the thermody-
namic parameters for ROMP of neat DHF to be ΔH (neat) = -5.0
kcal mol-1 and ΔS (neat) = -19.4 cal mol-1 K-1 (Figure S9-S10, Table
S1). These values are similar to those reported by Grubbs for
ROMP of cyclopentene (in toluene-d8): ΔH = -5.6 kcal mol-1 and
ΔS = -18.5 cal mol-1 K-1.17
Ring-opening metathesis polymerization (ROMP) has emerged as
a powerful method to synthesize a wide variety of polymers from
cyclic olefinic monomers.1 In a typical ROMP procedure using
Grubbs catalysts, excess vinyl ether (a terminal enol ether) is added
at the end of the polymerization to quench the catalyst by forming
an electron-rich Ru carbene.2-9 This so-called Fischer carbene is
more thermodynamically stable than the propagating Grubbs al-
kylidene and has significantly reduced activity towards further me-
tathesis reaction with olefins.10 Therefore, vinyl ethers have been
widely conceived as terminating agents for ROMP, and ROMP of
cyclic enol ethers would seem impossible and has never been in-
vestigated. However, the Ru Fischer carbene has been shown to be
not completely deactivated and may still act as a competent, albeit
slow, metathesis catalyst for ROMP or ring-opening cross metath-
esis of cyclic olefins, particularly under elevated temperature or
photochemical conditions.10-14 Grubbs and coworkers further
showed that Ru Fischer carbenes formed from terminal vinyl
ethers, vinyl sulfides, and enamines can undergo further metathesis
with these electron rich olefins.10, 15 Intrigued by these observa-
tions, we became curious if in situ formed Ru Fischer carbene can
indeed catalyze ROMP of cyclic enol ethers, despite the common
belief that they are quenching reagents for Grubbs catalysts. If suc-
cessful, this would enable a new family of monomers for ROMP,
leading to a novel class of degradable polymers, poly(enol ether)
1H NMR spectroscopy of isolated PDHF revealed that the
polymerization proceeded with complete head-to-tail regioselectiv-
ity regardless of the catalyst used (Figure 1), which is consistent
with the high regioselectivity of vinyl ether addition to Grubbs cat-
alysts.4, 8, 10 Throughout the polymerizations, an E:Z ratio around
4:6 to 1:1 was determined for the backbone olefins based on the
integrations of olefin signals.
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MW >100 kDa could be obtained. In ROMP of low-strain hydro-
Table 1. Romp of 2,3-Dihydrofuran.a
carbon cyclic olefins, a linear internal olefin is often used as a chain
transfer agent (CTA) to regulate MW.22-25 We hypothesized that a
linear enol ether, such as simple ethyl vinyl ether, could be used as
a CTA in ROMP of cyclic enol ethers. Indeed, the MWs of the re-
sulting polymers were directly proportional to the [DHF]0/[CTA]0
ratio, ranging from 5700 to 290 Da as [DHF]0/[CTA]0 was varied
from 5 to 100 (Table 1, entry 11-15 and Figure S5).
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Entry [DHF]0: [I]0: [CTA]0 Cat.
Temp. Mn,MALLS
(°C)
Ðd
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(kDa)c
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Despite the extensive studies of cross metathesis between differ-
ent hydrocarbon olefins, we are not aware of any previous study on
cross metathesis between enol ethers. In order to better probe the
secondary metathesis evident in the polymerization of DHF, we
monitored the cross-metathesis between 1-(hexyloxy)hex-1-ene
(2:1 mixture of Z and E isomers, totaling 90 mM) and ethyl vinyl
ether (180 mM) catalyzed by G2 (2.5 mM) at room temperature
using GC (Figure S13). The observed equilibration time is similar
to that of cross metathesis of common hydrocarbon olefins with G1
but is roughly an order of magnitude slower than that with G2.26-27
This finding provides useful guidelines to design ROMP of other
cyclic enol ethers in the future.
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Since ethyl vinyl ether is an effective CTA, we cannot use excess
vinyl ether to remove the chain end Ru via exchange reactions. It
was thus necessary to develop a new strategy to quench and remove
Ru for ROMP of cyclic enol ethers. Hydrogen peroxide has been
shown to rapidly oxidize Grubbs catalyst to ruthenium(IV) oxide
and allow removal of Ru via filtration through silica,28 so we hy-
pothesized that this oxidation may be used to terminate the
polymerization and cleave Ru off the polymer chain end. Since
H2O2 solution is slightly acidic, to prevent any acid-catalyzed hy-
drolysis of poly(enol ether), an alkaline solution of 30% H2O2 in
water was added dropwise to rapidly stirring polymer solution in
THF with added PCy3 (> 10 equiv. relative to catalyst). During this
addition, the polymer precipitated as a white solid or a clear, color-
less viscous material depending on the polymer MW. Low MW (Mn
< 5 kDa) polymers that do not precipitate were purified by extrac-
tion into chloroform, filtration through silica, and concentration in
vacuo.
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In order to identify the chain-end functionality of PDHF termi-
nated by H2O2 oxidation, tetraethylene glycol monomethyl vinyl
ether was selected as a model compound. Grubbs catalyst was re-
acted with tetraethylene glycol methyl vinyl ether in THF, and the
catalyst was subsequently reacted with alkaline 30% H2O2 (See SI
for details). We identified the isolated product as tetraethylene gly-
aROMP was performed using neat DHF under ambient atmosphere for
4 h (using G2) or N2 atmosphere for 72 h (using G1), and quenched
with basified H2O2. bInitial equivalents of DHF:Grubbs initiator:ethyl
vinyl ether. cDetermined by GPC MALLS analysis in THF. dMw/Mn.
1
col monomethyl ether formate by H and 13C NMR spectroscopy
and mass spectrometry (Figure S20-21). 1H NMR spectroscopy of
an isolated PDHF quenched with H2O2 also showed the character-
istic formate signal at 8.1 ppm. Further, the intensity of this formate
end group signal matched that of the phenyl chain end from the
benzylidene of the Grubbs initiator at equal stoichiometry, suggest-
ing complete oxidative termination of all active chain ends (Figure
S12).
The ability of PDHF to depolymerize and degrade is attractive
for a circular economy. We examined the metathesis depolymeri-
zation of PDHF by adjusting conditions to shift the ROMP equilib-
rium toward monomer formation. A PDHF sample was prepared
with 0.01 mol% G2 and allowed to equilibrate for 3 h at room tem-
perature. Without removal of the catalyst, the resulting solid poly-
mer was warmed to 60 °C under flowing N2 gas to remove mono-
mer. After only 2.5 h, > 90% of pure monomer was recovered by
condensation (Figure 2). This demonstration shows the facile and
clean depolymerization of PDHF under mild conditions.
Figure 1. 1H NMR spectrum of PDHF synthesized using G2 at 22
°C.
We regulated the MWs of PDHF by varying the catalyst loading
(Table 1, entry 1-8). Mn scaled linearly with [DHF]0/[G2]0, ranging
from 6 to 125 kDa as [DHF]0/[G2]0 was varied from 500 to 10000
(Figure S4). Notably, even at 0.01 mol% catalyst loading, PDHF of
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Following degradation of PDHF, no residual enol ether was ob-
served by 1H NMR spectroscopy. Degradation gave a mixture dom-
inated by 4-hydroxybutanal and its cyclic hemiacetal isomer, 2-hy-
droxytetrahydrofuran, identical to that from the hydrolysis of DHF
under the same conditions (Figure S16). Because hydrolyzed
poly(enol ether)s revert to hydrolyzed monomer, the identity of
degradation products can be controlled via design of monomer
structures.
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Interestingly, despite its rapid degradation in solution, solid
PDHF has relative stability in an acidic environment. Following
submersion of 54 kDa PDHF in aqueous HCl (0.1 M) for 24 h, Mn
only decreased to 12 kDa (Figure S15). The slow solid-state degra-
dation is favorable for extending the shelf life of PDHF during us-
age but allowing complete degradation eventually. We attribute the
slow solid-state degradation to the hydrophobicity of PDHF: while
the polymer backbone is highly susceptible to hydrolysis, slow pen-
etration of water into solid samples becomes the rate limiting step
in its degradation. Similar behavior has been recently reported for
solid state degradation of polyacetals.36
Figure 2. Facile depolymerization of PDHF to DHF.
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Degradable ROMP polymers are desired but rare.29-31 The abundant
backbone enol ether linkage in PDHF makes it degradable under
acidic conditions. PDHF after H2O2 quenching was dissolved in
THF in the presence of 135 mM water and 25 mM acid and de-
graded at different rates depending on the strength of the acid.
Acid-dependent degradation rate is expected because olefin proto-
nation is the rate limiting step of enol ether hydrolysis.32 When HCl
was used, complete degradation to small molecules occurred in <
10 min as observed by GPC (Figure 3). When trifluoroacetic acid
(TFA) was used, the MW of PDHF decreased over several hours
(Figures 4, S14). Because the polymer hydrolysis follows 1st order
reaction kinetics in both proton concentration and enol ether con-
centration, the inverse of Mn is expected to increase linearly with
time.33-35 Indeed, this trend was observed (Figure 4), indicating that
polymer hydrolysis can be tuned by acid identity, acid concentra-
tion, and water concentration.
In summary, we report the first example of effective ROMP of a
cyclic enol ether under ambient conditions, despite the common be-
lief that enol ethers are deactivating/quenching agents for Grubbs
catalysts. We demonstrated that the in situ formed Ru Fischer car-
benes are effective to catalyze ring-opening and cross metathesis of
enol ethers and produce poly(enol ether)s in variable MWs. These
results enable a new class of monomers for ROMP and generate a
new type of depolymerizable and degradable polymers.
Pristine
ASSOCIATED CONTENT
10 min
40 min
1
The Supporting Information is available free of charge on the ACS
Publications website at DOI:
70 min
0.8
1
Synthetic procedures and characterizations, H and 13C
NMR spectra and GPC traces, determination of thermo-
dynamic parameters, Fischer carbene quenching and in-
hibition study, cross metathesis study of internal enol
ethers.
0.6
0.4
0.2
0
AUTHOR INFORMATION
Corresponding Author
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24
Elution Time (min)
*E-mail: yanx@stanford.edu
Figure 3. GPC analysis of PDHF hydrolysis in THF with 135 mM
water and 25 mM HCl.
Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
This work was supported by the National Science Foundation
(CAREER, CHE-1553780). Y.X. thanks the Alfred Sloan Foundation
for the Sloan fellowship. We thank Jessica Su for helpful discussion.
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