Electrochemically Controlled Cationic Polymerization of Vinyl Ethers

Article abstract of DOI:10.1021/jacs.8b00173

Control of polymer initiation, propagation and termination is important in the development of complex polymer structures and advanced materials. Typically, this has been achieved chemically, electrochemically, photochemically or mechanochemically. Electrochemical control has been demonstrated in radical polymerizations; however, regulation of a cationic polymerization has yet to be achieved. Through the reversible oxidation of a polymer chain end with an electrochemical mediator, temporal control over polymer chain growth in cationic polymerizations was realized. By subjecting a stable organic nitroxyl radical mediator and chain transfer agent to an oxidizing current, control over polymer molecular weight and dispersity is demonstrated and excellent chain end fidelity allows for the synthesis of block copolymers.

Full text of DOI:10.1021/jacs.8b00173

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Communication
Electrochemically Controlled Cationic Polymerization of Vinyl Ethers
Brian M. Peterson, Song Lin, and Brett P. Fors
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00173 • Publication Date (Web): 31 Jan 2018
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Electrochemically Controlled Cationic Polymerization of Vinyl Ethers
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Brian M. Peterson, Song Lin,* and Brett P. Fors*
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Cornell University, Ithaca, New York 14853, United States
Supporting Information Placeholder
ABSTRACT: Control of polymer initiation, propagation and
termination is important in the development of complex
polymer structures and advanced materials. Typically, this
has been achieved chemically, electrochemically, photo-
chemically, or mechanochemically. Electrochemical control
has been demonstrated in radical polymerizations; however,
regulation of a cationic polymerization has yet to be
achieved. Through the reversible oxidation of a polymer
chain end with an electrochemical mediator, temporal con-
trol over polymer chain growth in cationic polymerizations
was realized. By subjecting a stable organic nitroxyl radical
mediator and chain transfer agent to an oxidizing current,
control over polymer molecular weight and dispersity is
Figure 1: Proposed electrochemically mediated cationic
polymerization of vinyl ethers.
light, which would give a system where chain growth would
be regulated electrochemically (Figure 1).
demonstrated and excellent chain end fidelity allows for the
synthesis of block copolymers.
Our group has previously shown that upon photochemical
oxidation of a dithiocarbamate chain transfer agent (CTA), 1,
mesolytic cleavage occurs to generate an oxocarbenium ion
and a stabilized dithiocarbamate radical.11c Cyclic voltamme-
try (CV) of 1 showed that an irreversible oxidation was occur-
ring, suggesting that a similar oxidation followed by meso-
lytic cleavage process could be occurring at the electrode
surface (Figure 2a). To test our hypothesis that this oxida-
tion/mesolytic cleavage at the anode would give rise to cati-
onic polymerization, we looked at the polymerization of iso-
butyl vinyl ether (IBVE) in a divided electrochemical cell
with reticulated vitreous carbon (RVC) electrodes. Encourag-
ingly, when a constant potential of 325 mV (vs Fc+/Fc) was
Recently there has been a significant push to develop
polymerizations where polymer chain growth is controlled
by a chemical,1 electrochemical,2 photochemical,3 or mecha-
nochemical4 stimulus. The spatiotemporal control offered by
these stimuli give an additional means to precisely regulate
polymer structure and, hence, function.5 Of these stimuli,
electrochemistry offers unique advantages because both ap-
plied voltage and current can be modified and monitored
throughout the polymerization process.6 Taking advantage of
this, Matyjaszewski developed an electrochemically mediated
atom transfer radical polymerization (eATRP),7 which has
proved powerful in a number of applications.8 More recently,
Matyjaszewski9 and Yan10 independently reported electro-
chemically mediated reversible addition fragmentation chain
transfer processes (eRAFT). However, to date, electrochemi-
cal mediation of controlled polymerizations has been re-
stricted to radical mechanisms and its implementation in
other polymerization types remains a challenge.1,2 In this
study, we address this challenge and disclose a controlled
cationic polymerization where chain growth is electrochemi-
cally regulated.
Recently, our group developed a cationic polymerization
of vinyl ethers that was controlled by light.11 By selectively
oxidizing a dithiocarbamate polymer chain end with an ap-
propriate photoredox catalyst, we were able to reversibly
form a propagating carbocation that participated in a con-
trolled cationic RAFT process.11,12 In an analogous strategy,
we hypothesized that we could reversibly form the carbo-
cation in this process with electrical potential instead of
Figure 2: CV of (a) 1 X 10–3 M isobutoxy-N,N-diethyl dithi-
ocarbamate, (b) 5 X 10–4 M (2,2,6,6-tetramethyylpiperidin-
1-yl)oxyl (TEMPO), and (c)1 X 10–3 M isobutyl vinyl ether
in 0.1 M tetrabutylammonium perchlorate in dichloro-
methane at 20 mVs–1.
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applied to a solution of IBVE, 1, and tetrabutylammonium
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perchlorate (Bu4NClO4) in dichloromethane, cationic
polymerization did occur (Table 1, entry 1). However, the
polymers resulting from this direct electrolysis of the CTA
exhibited broad dispersities (Đ) with bimodal molecular
weight distributions (see supporting information, Figure S7).
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Table 1: Development of IBVE polymerization mediated by the
electrooxidation TEMPO.
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b
Entrya Stimuli
[M]:[1]:
[TEMPO]
100:1:0
Mn,Theo
Mn,Exp
(kg/mol)
Đ
(kg/mol)
1c
2c
3c
325 mV
325 mV
325 mV
10.3
8.3
10.9
8.4
7.1
1.97
1.23
1.50
100:1:1
100:1:0.1
9.9
4c
5c
6
325 mV
325 mV
1 mA
100:1:50
100:0:1
9.9
–
6.6
27.9
6.8
1.39
2.93
1.07
1.09
1.15
Figure 3: (a) Conversion of isobutyl vinyl ether with time.
(b) Relationship between Mn and conversion.
100:2:1
5.0
6.9
10.4
28.8
9.5
9.9
resulted in controlled polymerizations that could be run to
full conversion (Table 1, entries 6–9).16 Interestingly, switch-
ing the electrolyte to tetrabutylammonium hexafluorophos-
phate or tetrabutylammonium tetrafluoroborate resulted in
slightly broader Đs (Table 1, entries 10,11).17
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1 mA
100:1.5:1.5
100:1:1
8.8
8
1 mA
10.1
22.5
8.1
9
0.1 mA 100:0.25:0.25
1.33
1.45
1.20
10d
11e
1 mA
1 mA
100:1:1
100:1:1
Under optimal galvanostatic conditions, monitoring the
reaction conversion as a function of time revealed a short
induction period followed by fast polymerization with full
conversion being reached within 3 hours (Figure 3a). Addi-
tionally, Mn increased linearly and Đ decreased with conver-
sion, characteristic of a controlled chain growth mechanism
(Figure 3b).
10.7
a[IBVE] = 3.84 M (in DCM), Vtot= 5 mL, [Bu4NClO4] = 0.1
M, RVC Anode, Ag Reference, RVC Cathode (Divided cell).
bMn,Theo = [M]/[CTA] ☓ MWM ☓ Conversion + MWCTA.
cPotential vs Fc+/Fc. dElectrolyte = [Bu4NPF6] = 0.1 M.
eElectrolyte = [Bu4NBF4] = 0.1 M
To demonstrate temporal control over polymer chain
growth, a reaction mixture of IBVE, CTA, and TEMPO was
subjected to 1 mA anodic current for twenty minutes. Subse-
quently, the electrode was set to (-)875 mV vs Fc/Fc+ for 30
minutes; we reasoned that this potential should reduce the
dithiocarbamate radical (or disulfide) to the corresponding
anion, which would cap propagating oxocarbenium ions.18
Indeed, switching to a reducing potential stopped polymer
chain growth with little to no background reaction observed.
We reasoned that the observed uncontrolled polymeriza-
tion could be attributed to polymer plating on the electrode
as a result of the irreversible oxidation of the CTA.9 To cir-
cumvent this issue, we envisaged that the use of a mediator
capable of reversible electron transfer at the electrode and
homogenous oxidation of the CTA would provide controlled
polymerization.13 Indeed, the addition of TEMPO (Figure 2b),
which fit the criteria for the mediator, to the reaction mix-
ture resulted in a 8.4 kg/mol polymer with a Đ of 1.23 (Table
1, entry 2).14 Importantly, excellent agreement between theo-
retical and experimental molar masses was observed, demon-
strating that we have a controlled cationic polymerization
that is electrochemically mediated. Approximately 1 equiva-
lent of TEMPO with respect to CTA was found to be optimal
for these polymerizations, with deviations to higher or lower
concentrations leading to a loss in control over the molecular
weight (Table 1, entries 2 vs 3-4). As a control experiment and
in further support of our mechanistic hypothesis, elimination
of the RAFT equilibrium by removal of the CTA led to un-
controlled polymerization (Table 1, entry 5).
Upon further investigation, we found that higher conver-
sion of the monomer could be obtained under galvanostatic
conditions, wherein the potential is allowed to drift in unison
with E1/2 of the TEMPO redox couple as the concentration of
TEMPO+ increases.15 Delivering an anodic current (1 mA)
Figure 4: Temporal control of polymer chain growth
with intermittent oxidizing current and reducing poten-
tial.
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Reinitiation was then achieved by again applying the oxidiz-
polymerization.
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ing current for 15 minutes. This process was successfully re-
peated multiple times, with polymerization only occurring
when an oxidizing current was applied (Figure 4). These data
demonstrated that we have excellent electrochemical control
over polymer chain growth and the reactions can be reversi-
bly initiated or terminated by switching the direction of cur-
rent flow in the cell.
To further probe chain end fidelity delivered by this meth-
od we successfully chain extended a macroinitiator. A 5.1
kg/mol poly(EVE) was first synthesized under the optimized
conditions, to which, IBVE was added to the anodic chamber
and an oxidizing current was resumed. A clear shift to higher
molar masses was observed to give a 8.0 kg/mol poly(EVE-b-
IBVE) with narrow dispersity (Figure 5).
To further understand the role of TEMPO in these reac-
tions, we found that when an oxidizing potential (325 mV vs
Fc+/Fc) was applied to a solution of the CTA and TEMPO,
tetraethylthiuram disulfide was observed as a result of the
Table 2: Polymerization of vinyl ether and stryl monomers.
b
Entrya
Monomer
Mn,Theo
(kg/mol)
9.9
Mn,Exp
(kg/mol)
9.7
Đ
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1
nBuVE
nPrVE
1.32
1.11
2
10.2
8.1
3
4
5c
EVE
10.4
10.3
10.2
9.7
11.3
9.2
1.14
1.12
1.33
Cl-EVE
p-OMe-Styrene
a[M] = 42% in DCM v/v, Vtot = 6 mL, [Bu4NClO4] = 0.1 M,
RVC Anode, Ag Reference, RVC Cathode (Divided), Reac-
tion time = 4 h. bMn,Theo = [M]/[CTA] ☓ MWM ☓ Conver-
sion + MWCTA. cApplied current = 2 mA.
To explore the scope of our electrochemical polymeriza-
tion protocol, we surveyed an array of vinyl ethers. n-Butyl
vinyl ether (nBuVE), n-propyl vinyl ether (nPrVE), ethyl vinyl
ether (EVE), and 2-chloroethyl vinyl ether (Cl-EVE) all un-
derwent polymerization under the optimized galvonostatic
conditions (Table 2). The resulting polymers showed good
agreement between theoretical and experimental molecular
weights, along with narrow Đs. Notably, 4-methoxystyrene
was polymerized under slightly more demanding conditions
(2 mA), constituting an improvement upon the photocon-
trolled polymerizations we previously reported, which did
not promote polymerizations of styryl monomers.11a,19 Alt-
Figure 6: Proposed catalytic cycle of the TEMPO mediat-
ed polymerization of vinyl ethers.
dimerization of II, a byproduct of oxidatively cleaved CTA.
Based on this result and our polymerization data above, we
propose that the oxidized TEMPO cation undergoes an at-
tack by the CTA to form a stabilized cationic intermediate I
(Figure 6).20 Fragmentation of the cation gives the dithiocar-
bamate radical II and the oxocarbenium ion that participates
in the RAFT process, as well as regenerates TEMPO; togeth-
er, this two step process completes an innersphere electron
transfer. We hypothesize that when the current in the elec-
trochemical cell is reversed, dithiocarbamate radical II or its
dimeric form gets reduced to the anion and caps the propa-
gating cation.18 This recapping step gives efficient electro-
chemical control over polymer chain growth.
In summary, we have developed an electrochemically me-
diated cationic polymerization, which offers excellent tem-
poral control over polymer growth. A variety of poly(vinyl
ethers) with narrow dispersities and predictable Mn values
were obtained and the ability to polymerize less activated
monomers was demonstrated. High chain end fidelity allows
for the synthesis of block copolymers, while reversible elec-
trochemical activation/deactivation of chain ends allows
precise temporal control of chain growth. This new electro-
chemical handle for controlling cationic polymer growth
Figure 5: Synthesis and GPC traces of poly(ethyl vinyl
ether) and poly(ethyl vinyl ether-block-isobutyl vinyl
ether.
hough the resulting polymer has slightly broader dispersities
when compared to the vinyl ethers, this preliminary finding
highlights the versatility of this electrochemically mediated
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Hawker, C. J. Angew. Chem. Int. Ed. 2013, 52, 199.
should allow for the synthesis of complex polymer architec-
tures, and provide a template for future electrochemically
controlled polymerizations.
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(7) (a) Magenau, A. J. D.; Strandwitz, N. C.; Gennaro, A.;
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the
ACS Publications website at DOI:
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H.-J.; Jurga, S.; Matyjaszewski, K. Macromolecules, 2013, 46, 5856.
(9) Rinsing the electrode after the reaction revealed bound poly-
mer. For other examples of electrode passivation, see: Wang, Y.;
Fantin, M.; Park, S.; Gottlieb, E.; Liye, F.; Matysjaszewski, K. Macro-
molecules 2017, 50, 7872.
(10) Sang, W.; Xu, M.; Yan, Q. ACS Macro Lett. 2017, 6, 1337.
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General experimental considerations, experimental
procedures, and additional supporting data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*bpf46@cornell.edu
*songlin@cornell.edu
Notes
The authors declare no competing financial interest.
(12) For references on cationic RAFT see: (a) Uchiyama, M.;
Satoh, K.; Kamigaito, M. Angew. Chem., Int. Ed. 2015, 54, 1924. (b)
Sugihara, S.; Konegawa, N.; Maeda, Y. Macromolecules 2015, 48, 5120.
(c) Uchiyama, M.; Satoh, K.; Kamigaito, M. Macromolecules 2015, 48,
5533.
ACKNOWLEDGMENTs
This work made use of the NMR Facility at Cornell Universi-
ty that was supported in part by the NSF under Award CHE-
1531632. This work was supported by the Cornell Center for
Materials Research with funding from the NSF MRSEC pro-
gram (DMR-1719875). BPF thanks 3M for a Non-Tenured
Faculty Award.
(13) For a review on mediators in organic electrosynthesis see:
Francke, R.; Little, R. D. Chem. Soc. Rev. 2014, 43, 2492.
(14) (a) Hickey, D. P.; McCammant, M. S.; Giroud, F.; Sigman, M.
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(15) This also circumvents the need for a reference electrode and
allows for polymerization using a DC power source, greatly simplify-
ing polymerization procedure (Figure S23), see: (a) Fu, N.; Sauer, G.
S.; Saha, A.; Loo, A.; Sin, S. Science 2017, 357, 575. (b) Magenau, A. J.
D.; Bortolamei, N.; Frick, E.; Park, S.; Gennaro, A.; Matyjaszewski, K.
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Chmielarz, P.; Sobkowiak, A.; Matyjaszewski, K. Polymer, 2015, 77,
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(16) Currents were selected so that initial voltage was near 325
mV(vs Fc+/Fc) and at lower loadings of 1 and TEMPO lower currents
were necessary to achieve higher molar masses.
(17) For an example of tetrafluoroborate reacting with an oxo-
carbenium ion see: Suzuki, S.; Matsumodo, K.; Kawamura, K.; Suga,
S.; Yoshida, J. Org. Lett. 2004, 6, 3755.
(18) In the absence of any stimuli (I=0), polymerization rates are
retarded and observable background reaction occurs. For the reduc-
tion potential of the disulfide see: Nichols, P. J.; Grant, M. W. Aust. J.
Chem. 1982, 35, 2455.
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Am. Chem. Soc. 2015, 137, 7580. (b) Messina, M. S.; Axtell, J. C.;
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Alexandrova, A. N.; Maynard, H. D.; Spokoyny, A. M. J. Am. Chem.
Soc. 2016, 138, 6952.
(20) Alternatively, attack could also occur at the nitrogen, yielding
the same intermediates after fragmentation. (a)Yoshida, E.; Nakamu-
ra, K.; Takata, T.; Endo, T. J. Polym. Sci. A Polym. Chem. 1993, 31,
1505. (b) Bailey, W. F.; Bobbitt, J. M.; Wiberg, K. B. J. Org. Chem.
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Oxidizing Current (On)
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Mn
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Time
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