Visible-Light-Enabled Organocatalyzed Controlled Alternating Terpolymerization of Perfluorinated Vinyl Ethers

Article abstract of DOI:10.1002/anie.202107066

Polymerizations of perfluorinated vinyl ethers (PFVEs) provide an important category of fluoropolymers that have received considerable interests in applications. In this work, we report the development of an organocatalyzed controlled radical alternating terpolymerization of PFVEs and vinyl ethers (VEs) under visible-light irradiation. This method not only enables the synthesis of a broad scope of fluorinated terpolymers of low dispersities and high chain-end fidelity, facilitating tuning the chemical compositions by rationally choosing the type and/or ratio of comonomers, but also allows temporal control of chain-growth, as well as the preparation of a variety of novel fluorinated block copolymers. To showcase the versatility of this method, fluorinated alternating terpolymers have been synthesized and customized to simultaneously display a variety of desirable properties for solid polymer electrolyte design, creating new opportunities in high-performance energy storage devices.

Full text of DOI:10.1002/anie.202107066

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Visible-Light Enabled Organocatalyzed Controlled Alternating
Terpolymerization of Perfluorinated Vinyl Ethers
Authors: Mao Chen, Qinzhi Quan, Mingyu Ma, Zongtao Wang, and Yu
Gu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202107066
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10.1002/anie.202107066
Angewandte Chemie International Edition
RESEARCH ARTICLE
Visible-Light Enabled Organocatalyzed Controlled Alternating
Terpolymerization of Perfluorinated Vinyl Ethers
Qinzhi Quan, Mingyu Ma, Zongtao Wang, Yu Gu and Mao Chen*
Abstract: Polymerizations of perfluorinated vinyl ethers (PFVEs)
provide an important category of fluoropolymers that have received
considerable interests in applications. In this work, we report the
development of an organocatalyzed controlled radical alternating
terpolymerization of PFVEs and vinyl ethers (VEs) under visible-light
irradiation. This method not only enables the synthesis of a broad
scope of fluorinated terpolymers of low dispersities and high chain-
end fidelity, facilitating tuning the chemical compositions by rationally
choosing the type and/or ratio of comonomers, but also allows
temporal control of chain-growth, as well as the preparation of a
variety of novel fluorinated block copolymers. To showcase the
versatility of this method, fluorinated alternating terpolymers have
been synthesized and customized to simultaneously display a variety
of desirable properties for solid polymer electrolyte design, creating
new opportunities in high-performance energy storage devices.
Recent combinations of photoredox catalysis and
controlled/living polymerization have stimulated advances of
synthetic approaches that allow precise polymer engineering with
spatiotemporal control.[9] Elegant systems include photo-
controlled radical polymerization of acrylates and acrylamides,[10]
photo-controlled cationic polymerization of vinyl ethers[11] and
photo-controlled ring-opening metathesis polymerization of
cycloalkenes,[12] and so forth. However, the vast majority of photo-
controlled polymerizations developed to date are focused on non-
fluorinated monomers.[9] The controlled polymerization of
fluoroalkenes remains a major challenge.[13] In particular, the
controlled co- and terpolymerization of perfluorinated vinyl ethers
(PFVEs), which represent an important category of industrially
relevant fluoroalkenes (e.g., Viton,[6] Kalrez[7]), have been rarely
achieved. Conventional methods not only typically require
elevated temperature and pressure, but also suffer from
uncontrolled molecular weight, broad dispersity, poor chain-end
fidelity and incomplete conversion,[14] restricting the broader
accessibility of well-defined fluoropolymers and applications that
are facilitated by mild operation conditions.[5, 9a]
Introduction
Fluoropolymers are finding continuously expanding
applications because of outstanding thermal and chemical
stability, low dielectric constant and surface energy, etc.[1]
However, many high-end utilizations of fluorinated homopolymers
have been limited by their high crystallinity, incompatibility with
non-fluorinated composites and/or lack of functional
substituents.[2] Therefore, novel synthetic methods are being
actively pursued to promote the facile regulation of fluoropolymers
with well-defined architectures and desirable substituents for
applications such as electrolytes in lithium-ion batteries,[3]
drug/gene carriers,[4] photolithography,[5] etc. Terpolymerization is
an important approach to effectively extend the utility of
fluoropolymers by means of covalently introducing three
functional segments into the macromolecular structure, and
thereby alleviating crystallinity of fluoropolymers and minimizing
phase separation between fluorophilic and non-fluorophilic
moieties.[2] Additionally, it enables delicate regulation of
properties that are dictated jointly by multiple distinct domains on
the fluoropolymers without compromising their intrinsic attributes.
Commercialized products such as Viton,[6] Kalrez,[7] Lumiflon[8]
and others,[1a] all include terpolymers of fluoroalkenes.
In controlled terpolymerization, a growing chain would end up
with three different terminal units, which lead to dormant species
of varied reactivities for re-initiation and monomer addition, thus
raising difficulties to realize good livingness. We envisioned that
the photo-controlled polymerization conducted under mild
conditions could offer opportunities to mitigate increased
disturbance from propagating radicals in a complex reaction
15]
system.[9b,
Meanwhile, fluoropolymers have recently been
discovered as a particularly interesting class of solid polymer
electrolytes (SPEs) in the development of all-solid-state batteries
(ASSBs).[16] Nevertheless, fluorinated SPEs of preferential
properties are lacking as limited by exisiting synthetic methods to
access on-demand fluoropolymers. The controlled synthesis of
alternating terpolymers of PFVEs under operationally simple
conditions could facilitate access towards a series of fluorinated
polymeric electrolytes and allow the investigation of fluorinated
materials of potential.
Herein, we report the development of an organocatalyzed
controlled/living radical alternating terpolymerization of PFVEs
driven by visible light for the first time (Scheme 1), enabling the
facile transformation of various PFVEs (even for gaseous PFVE)
and VEs under ambient conditions. The obtained alternating
terpolymers exhibit low dispersity (Đ) and high chain-end fidelity
at high conversions of VEs, allowing chain-end extension with
different monomers (e.g., PFVEs, VEs, vinyl esters, styrene) to
[*]
Q. Quan, M. Ma, Z. Wang, Y. Gu, Prof. Dr. M. Chen
State Key Laboratory of Molecular Engineering of Polymers,
Department of Macromolecular Science
Fudan University
furnish
a
variety of unprecedented block copolymers.
Furthermore, taking advantage of this method, fluorinated
terpolymers that simultaneously provide a variety of desirable
properties for SPEs (e.g., low glass-transition temperature (Tg),
high storage modulus (G′), good ionic conductivity, non-
flammability) were obtained, which resulted in outstanding
Shanghai 200433, China
E-mail: chenmao@fudan.edu.cn
Supporting information for this article is given via a link at the end
of the document.
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10.1002/anie.202107066
Angewandte Chemie International Edition
RESEARCH ARTICLE
performance compared with conventional polyethylene oxide
(PEO) and mixtures of copolymers during the long-term cycling in
lithium metal-based ASSBs, affording a promising bottom-up
platform to access high-performance fluoropolymeric materials.
Condition Optimization
In order to optimize the terpolymerization conditions, we
employed perfluoro(propyl vinyl ether) (FM1), tert-
butyldimethylsilyl (TBS) protected ethylene glycol vinyl ether (M1)
and 2-(ethenyloxy)ethyl ethyl ester (M2) as comonomers (Figure
1A, Figure S1) with N-biphenylphenothiazine derivative[17] (biPh-
PTZ, Figure 1B) as a photoredox catalyst (PC) and diethyl
carbonate (DEC) as a low cost, non-fluorinated solvent under
visible-light irradiation (13 W white LED light, Figure S2). We first
attempted the reaction with cyanomethyl substituted
thiocarbonylthio compounds as chain transfer agents (CTAs) (1a-
1e, Figure 1C, Figures S3-S4 and Scheme S1). When CTAs from
1a to 1d were employed, polymers of low molecular weights (Mn,
entries 1-4 in Table 1) were obtained at moderate to high
conversions of M1 and M2, which could be attributed to the
oligomerization of VEs.[18] Although CTA 1e has shown good
control in the copolymerization of chlorotrifluoroethylene,[19] the
employment of 1e in the terpolymerization of FM1 resulted in poor
VE conversion (entry 5). The dithiocarbamate fragment generated
by the C-S bond cleavage[9a] has precipitated in the reaction
mixture upon light irradiaion (Figure S5). Additionally, PFVEs
have higher steric hinderance and lower solubility in conventional
solvents than many fluoroalkenes, imposing difficulties on the
chain growth.
Scheme 1. Organocatalyzed controlled alternating terpolymerization of PFVEs
and VEs driven by light.
Results and Discussion
Table 1. Condition optimization of CTAs for the organocatalyzed
terpolymerization under visible-light irradiation.[a]
M1 M2
Conv. of
(%/%)[b]
/
Mn,calc.
(kDa)[c]
Mn,SEC
(kDa)[d]
Entry
CTA
Ð[d]
1
2
1a
1b
1c
1d
1e
2a
2b
2c
2d
3a
3b
3c
97/83
96/71
65/43
86/71
16/6
12.9
12.1
8.0
6.6
3.9
3.7
8.9
4.7
4.1
7.1
4.6
9.6
8.2
10.8
12.3
1.37
1.42
1.29
1.35
1.30
1.31
1.52
1.37
1.29
1.32
1.30
1.23
3
4
11.3
1.8
5
6
76/46
75/38
44/17
81/70
79/66
84/72
94/81
8.9
7
8.4
8
4.8
9
10.9
10.5
11.3
13.0
10
11
12
[a] [FM1]/[M1]/[M2]/[CTA]/[PC] = 45/15/15/1/0.025, white LED light, 25 °C, 18 h,
ambient pressure. [b] Determined by GC analysis. [c] Calculated based on VE
conversions. [d] Determined by SEC.
Then, we turned our attentions to benzyl substituted CTAs
(2a-2d, entries 6-9, Scheme S2 and Figures S6-S7). We
observed that CTA 2d gave higher conversions of M1 and M2,
and the molecular weight detected by size-exclusion
chromatography (SEC) was close to the theoretical value
calculated based on monomer conversions (Mn,SEC = 9.6 kDa,
Mn,calc = 10.9 kDa). When CTAs from 2a to 2d were characterized
by cyclic voltammetry (CV), compound 2d showed the most
Figure 1. A) Chemical structures of PFVEs (FM1-FM4) and VEs (M1-M7). B)
Chemical structures of CTAs 1-3. C) Chemical structure of organic photocatalyst.
D) Cyclic voltammetry (CV) of CTAs conducted using 0.1 M nBu4NBF4 as
electrolyte in acetonitrile at 25 °C.
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10.1002/anie.202107066
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RESEARCH ARTICLE
positive potential of reduction (Figure 1D), indicating that the
pyrryl substituent could effectively promote the electron
acceptance from photo-activated catalyst (PC*) during the photo-
induced electron transfer process.[9b]
Investigation of the Terpolymerization Process
We next investigated the terpolymerization process of PFVE
and VEs at three initial ratios of [M1]/[M2] (8/32, 20/20, 32/8)
under optimized reaction conditions. As shown in Figures 2A-2C,
for both VEs, the degrees of polymerization (DPs, Equations S1-
S3, Tables S4-S6) increase with exposure time at different ratios.
DPs of FM1 and VEs (M1 + M2) are very close to each other
throughout the reaction processes, confirming the chemical
structures of alternating terpolymers.[14b] Plots of ln([M]0/[M]t) as a
function of exposure time display first order kinetics (Figure 2D).
Increasing the ratio of [M1]/[M2] afforded higher apparent
polymerization constants (e.g., [M1]/[M2] = 8/32, kp = 0.09 h-1;
[M1]/[M2] = 32/8, kp = 0.17 h-1), which follows the trend that FM1
has a faster copolymerization rate with M1 (Figure S13). During
the chain-growth, molecular weights of terpolymers increase with
monomer conversions (Figure 2E), and MWDs keep within a
narrow range (Ɖ = 1.17−1.28). The linear reaction kinetics and
low dispersities demonstrate that the terpolymerization could
provide a high degree of control for different VE ratios. Although
Mn,SEC of terpolymers were slightly lower than Mn,calc, when a
multiangle laser light scattering (MALLS) detector was employed,
the observed absolute molar masses (Mn,MALLS) were in good
agreement with Mn,calc as shown by the empty squares in Figure
2E. SEC profiles measured during the kinetics studies are
symmetrical and unimodal, which are in accordance with
characteristics of controlled/living terpolymerization (Figure 2F
and Figures S15-S16).
Next, we focused on tuning the chemical structure of leaving
group to regulate the reactivity of pyrryl substituted CTAs (3a-3c,
entries 10-12, Scheme S3 and Figures S8-S9). We observed that
compound 3b with an ester group on the phenyl ring gave slightly
improved conversions (84% vs 81% for M1, 72% vs 70% for M2)
and molecular weight (Mn,SEC = 10.8 vs 9.6 kDa) comparing to 2d,
while the molecular weight distribution (MWD) were kept close (Ɖ
= 1.30 vs 1.29).
Because FM1 has limited solubility in the reaction mixture,
which could influence the control in terpolymerization, a new
fluorinated ester substituted CTA 3c was synthesized (Scheme
S4 and Figures S10-S12). CTA 3c exhibited a more positive
reductive potential than other CTAs, suggesting that it could be
more easily reduced by PC* to generate a benzyl radical. The
employment of CTA 3c provided further increased conversions of
M1 (94%) and M2 (81%), and low dispersity (Ɖ = 1.23). The higher
conversions of M1 are due to its faster reaction rate with FM1
(Figures S13-S14). Further variations of the optimal
terpolymerization conditions including catalysts, solvents and light
sources resulted in decreased VE conversions and broad
dispersities (Tables S1-S3).
Figure 2. Investigations on the organocatalyzed alternating terpolymerization process at different initial ratios of [M1]/[M2]. Conditions: [FM1]/[VEs]/[CTA 3c]/[PC]
= 60/40/1/0.025 ([VEs] = [M1] + [M2], [M1]/[M2] = 8/32, 20/20, 32/8), 25 °C, white light. A) to C) Degrees of polymerization (DPs) of M1 (blue circle), M2 (red circle),
M1 + M2 (black empty rhombus) and FM1 (green circle). In D) and E), points of different colors represent results collected at different [M1]/[M2] ratios: 8/32 (blue),
20/20 (red), 32/8 (green). D) ln([M]0/[M]t) vs exposure time. [M]0 and [M]t are the concentrations of VEs at time points 0 and t, respectively. E) Mn and Đ vs %
conversion of VEs, filled squares and triangles represent Mn,SEC and Đ measured by SEC, respectively, empty squares represent Mn,MALLS measured by MALLS. F)
SEC profiles of terpolymers obtained at [M1]/[M2] = 8/32.
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Tuning the Chemical Composition by Changing the VE Ratio
Expanding the Synthetic Scope and Characterization Results
The controlled alternating terpolymerization could facilitate
With the established terpolymerization method, we turned
our attention to the synthesis of fluoropolymers with a variety of
PFVEs and VEs. As shown in Table 2 and Table S8, good to
excellent VE conversions (up to >99%) were realized for a broad
combination scope of PFVEs and VEs at low catalyst loadings
(0.025-0.25 mol% of VEs) by exposing to white light irradiation.
Although different comonomers were examined, the ratios of VEs
in terpolymers were determined by their initial ratios and
conversions as analyzed by 1H NMR (Figures S20-S54),
suggesting that VEs were efficiently transformed into target
terpolymers. The various combinations of PFVEs and VEs yielded
a series of main-chain and side-chain fluorinated terpolymers of
narrow MWDs (Ɖ = 1.04-1.28), which are still difficult to access
with other methods. All SEC traces were unimodal without
discernible shoulder peak (Figures S55-S61).
the regulation of chemical compositions at
a
constant
[PFVE]/[VEs]/[CTA] ratio by changing the relative amounts of two
VEs. As shown in Figure 3, a molar ratio of [FM1]/[VEs]/[CTA] =
30/20/1 ([VEs] = [M1] + [M2]) was maintained during the synthesis
of fluoropolymers under visible-light irradiation. When five
[M1]/[M2] ratios including 0/20, 4/16, 10/10, 16/4 and 20/0 were
used, the polymerization yielded fluoropolymers of varied
amounts of M1 and M2 as analyzed by the characteristic methyl
groups (protons a and b) in proton nuclear magnetic resonance
(1H NMR). Since almost quantitative VE conversions have been
achieved by extending the exposure time, the ratios of M1 and M2
incorporated in five polymers are very close to the corresponding
initial ratios, which are 0/20, 4.1/15.9, 9.8/10.2, 16.1/3.9 and 20/0,
respectively, showcasing the capability of incorporating desirable
ratios of VEs into polymers by simply adjusting their feeding
amounts (Figure S17, Table S7). The obtained co- and
terpolymers exhibited clearly observed terminal groups from CTA
3c in 1H NMR and narrow MWDs (Ɖ = 1.13-1.23, Figure S18). To
analyze the chain-end fidelity, terpolymer was synthesized at
[FM1]/[VEs]/[CTA 3c] = 24/16/1 ratio. The NMR result suggests
that the chain-end group (pyrryl(CS)S‒) is connected with vinyl
ether in the terpolymer chain, and the molecular weight calculated
based on 1H NMR (Mn,NMR) is close to Mn,calc (Figure S19),
illustrating that good chain-end fidelity was achieved with this
method.
When FM1 was used to react with VEs of diversified
substitutents including TBS protecting hydroxyl (M1), carbonate
(M2 and M3), ester (M4), alkyl (M5) and polyether (M6 and M7),
the terpolymerizations afforded fluoropolymers of Ɖ = 1.16-1.28
(entries 1-10 in Table 2, entries S1-S2 in Table S8). For
terpolymers exhibiting higher deviations of molecular weights
from the calculated values, absolute molar masses were
measured, providing Mn,MALLS similar to Mn,calc (e.g., Mn,MALLS
=
13.9 kDa, Mn,calc = 14.3 kDa, entry 8). The terpolymerization of
perfluoro(2-propoxypropyl vinyl ether) (FM2) allows the
incorporation of 16 fluorine atoms per PFVE molecule. As
attributed to the higher boiling point of FM2 (b.p. = 90 °C, entries
11-13, entries S3-S4 in Table S8), a declined initial ratio of
[PFVE]/[VEs] = 1/1 could be employed in this method.
Because the photo-controlled termpolymerization were
conducted under ambient temperature and pressure,
conventional glass tubes could be adopted as reaction vessels to
facilitate the operation of gaseous PFVE of perfluoro(methyl vinyl
ether) (FM3, b.p. = -22 °C) without handling high-pressure
metallic reactors. As shown in entries 14-17, when the [VEs]/[CTA]
ratio increased from 10/1 to 100/1, the terpolymerization
generated fluoropolymers of Ɖ = 1.10-1.19 and Mn,SEC = 3.6-24.1
kDa. Although Mn,SEC deviates from Mn,calc at a high [VEs]/[CTA]
ratio (entry 17), Mn,MALLS (32.8 kDa) is in a good agreement with
Mn,calc (34.9 kDa). Moreover, the facile operation with gaseous
FM3 allows the preparation of terpolymers containing different
fractions of VEs by simply varying the initial ratio (entries 18 and
19). When other VEs were used to react with FM3 instead of M1
and M2, the terpolymerization also delivered fluoropolymers of
low dispersities (entries S5-S6 in Table S8).
Beyond linear PFVEs, perfluoro-2,2-dimethyl-1,3-dioxole
(FM4), which has been used to synthesize materials of low index
of refraction in the terahertz range and amorphous copolymers,[20]
was investigated as an O-heterocyclic olefin to react with VEs
under visible-light irradiation. As shown in entries 20-23 in Table
2 and entries S7-S8 in Table S8, when the terpolymerization
partners were selected arbitrarily, reactions formed terpolymers
of narrow MWDs (Ɖ = 1.04-1.24) at high VE conversions, which
has not been demonstrated for perfluorinated cyclic monomer yet.
Figure 3. 1H NMR spectra of fluoropolymers synthesized via the
organocatalyzed photopolymerization.
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RESEARCH ARTICLE
Table 2. Organocatalyzed controlled alternating terpolymerization of various PFVEs and VEs under visible-light irradiation.[a]
Conv. of VEs
(%/%)[b]
Ratio of VEs
in terpolymer[c]
Mn,calc.
(kDa)[d]
Mn,SEC
(kDa)[e]
Initial feeding
ratio of VEs
Entry
PFVE
VEs
[PFVE]/[VEs]/[CTA]
Ð[e]
FM1
FM1
FM1
M1 M2
M1 M3
M1 M4
1
2
45/30/1
45/30/1
45/30/1
45/30/1
45/30/1
45/30/1
45/30/1
45/30/1
45/30/1
45/30/1
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
50/50
99/99
99/99
99/99
99/99
99/96
96/99
94/98
95/93
99/97
99/96
47/53
49/51
53/47
49/51
53/47
47/53
53/47
53/47
53/47
52/48
14.6
15.2
15.7
13.2
14.2
11.8
12.7
14.3
13.6
14.1
13.5
14.1
14.3
11.9
12.1
11.4
12.9
1.26
1.28
1.26
1.24
1.25
1.25
1.26
3
4
FM1
FM1
FM1
FM1
FM1
FM1
FM1
M1 M5
M1 M6
M2 M5
M2 M6
M2 M7
M3 M6
M4 M6
5
6
7
8
17.8(13.9)[f] 1.18
9
12.1
10.1
1.23
1.16
10
FM2:
16 fluorine atoms per monomer
11[g]
12[g]
13[g]
15/15/1
15/15/1
15/15/1
50/50
50/50
50/50
FM3:
94/90
92/91
90/91
51/49
52/48
53/47
9.4
9.2
9.5
8.9
7.8
1.22
1.21
1.15
FM2
FM2
FM2
M1 M2
M1 M6
M2 M7
11.5
Gaseous PFVE (b.p. = -22 °C)
FM3
FM3
FM3
FM3
FM3
FM3
M1 M2
M1 M2
M1 M2
M1 M2
M1 M2
M1 M2
30/10/1
75/25/1
50/50
50/50
97/95
99/99
99/99
95/90
99/98
99/96
52/48
49/51
49/51
52/48
78/22
18/82
4.2
9.8
3.6
9.2
1.10
1.14
1.19
14
15
16
17
18
19
150/50/1
300/100/1
120/40/1
120/40/1
50/50
50/50
80/20
20/80
18.9
34.9
16.2
13.9
16.8
24.1(32.8)[f] 1.18
14.6
12.5
1.21
1.16
FM4: Perfluorinated dioxole
FM4
FM4
FM4
FM4
M1 M2
M1 M4
M2 M5
M2 M6
45/30/1
45/30/1
45/30/1
45/30/1
50/50
50/50
50/50
50/50
99/96
99/97
95/99
95/98
48/52
51/49
52/48
52/48
13.7
14.8
11.7
12.1
11.2
12.8
10.9
10.6
1.16
1.22
1.24
1.04
20
21
22
23
[a] [CTA 3c]/[PC] = 1/0.025, [VEs] = 0.667 M in DEC, 13 W white LEDs, 25 °C, ambient pressure, 24 h. [b] Determined by GC analysis. [c] Calculated based on 1H
NMR. [d] Calculated based on VE conversions. [e] Determined by SEC. [f] Measured by a MALLS detector. [g] Conducted with a cosolvent (VDEC/Vn-pentane = 2/1)
instead of DEC.
“ON/OFF” Switch of the Controlled Terpolymerization
Photo-controlled polymerizations have an attractive
advantage to provide spatiotemporal control over the chain-
growth process at will, which highly relies on the reversible photo-
activation. While the vast majority of the photo-controlled
polymerizations
reported
to
date
are
based
on
homopolymerization,[9a, 9b, 21] we next investigated the ability of
switching the terpolymerization between “ON” and “OFF” states
using a reaction mixture of FM1, M1, M3, catalyst and CTA
([FM1]/[M1]/[M3]/[CTA 3c]/[PC] = 45/15/15/1/0.025) with white
LED light as an external trigger. As shown in Figure 4, when the
reaction mixture was kept in dark or the light irradiation was
switched “OFF”, there’s no monomer conversion or the
polymerization was temporarily ceased. Although different
lengths of dark periods were employed, the terpolymerization
could be efficiently re-initiated by exposing the mixture to light.
Figure 4. "ON/OFF" Temporal control of the terpolymerization of FM1, M1 and
M3.
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Similar conversions of two VEs were observed at different
exposure times as caused by similar reactivities. The SEC results
further demonstrate that the molecular weights grow only during
periods of light irradiation (Table S9 and Figure S62), supporting
that the organocatalyzed terpolymerization could provide high
degree of photo-control during chain growth, which is analogous
to metal-free homopolymerizations driven by light.[22]
P(FM1-alt-M4), P(FM1-alt-M6), P(FM4-alt-M6)) or homopolymers
(e.g., poly(vinyl propionate) (PVP), poly(vinyl benzoate) (PVBz),
polystyrene (PS)) as the second blocks, were both synthesized
with narrow MWDs (Ɖ = 1.21-1.34). For all block copolymers, the
SEC profiles displayed clear shifts to lower retention times in
comparison to the macroinitiators (Figures 5C and 5D), and no
discernible low-molecular-weight tailing. In contrast to P1 and P2,
block copolymers of P1a and P2a-P2c exhibited complete
inversions of the dRI signals, attributing to the connection of
fluorinated macroinitiators (negative dn/dc) with second blocks of
positive dn/dc values.[10b] Taken together, the successful chain-
extension with PFVEs, VEs, vinyl esters and styrene manifests
the good flexibility and compatibility of this system, indicating that
a large scope of fluoropolymers could be prepared via the photo-
controlled polymerization.
Preliminary Studies on Terpolymer Electrolyte
SPEs are one of the key materials for the development of
23]
advanced lithium-ion batteries.[3,
Replacing traditional liquid
electrolytes with polymers could overcome the safety issues (e.g.,
electrolyte leakage) and bring good processability.[3] Fluorinated
materials are a class of emerging solid electrolytes for their
chemical, physical properties as well as unique interfacial
electrochemical properties as ion conducting media.[16a-c]
However, many fluoropolymers can hardly be used as SPEs
because it is rather difficult to achieve low Tg, good Li ions (Li+)
dissolving capability, and sufficient mechanical strength at the
same time, all of which are required for practical SPEs. Inspired
by recent examples of fluoropolymer electrolytes[16] and
innovations enabled by the bottom-up engineering of copolymers
as SPEs,[24] we probed the potential of our alternating terpolymers
in electrochemical energy storage devices.
Firstly, terpolymers of P(FM1-alt-M2)-co-P(FM1-alt-M7)
(P3a-P3d, Mn = 16.8-19.6 kDa, Ɖ = 1.14-1.23)[25] composed of
fluorinated, ether and carbonate segments on the polymer chains,
which are by design responsible for different desired
functionalities for SPEs (Figure 6), were synthesized with the
terpolymerization (Table S10, Figure S71). Different ratios of
[M2]/([M2]+[M7]) in terpolymers were employed to allow the
regulation of properties. To investigate the necessity of
terpolymerization, copolymers of P(FM1-alt-M2) (P4, Mn = 16.1
kDa, Ɖ = 1.21) and P(FM1-alt-M7) (P5, Mn = 20.4 Da, Ɖ = 1.22)
with either only ether or carbonate segment besides PFVE were
prepared with the same method. As shown in Figure 6A, the Tg
values of polymers increase from -39.4 to 6.3 °C as the ratios of
[M2]/([M2]+[M7]) raised from zero to 1.0 based on differential
scanning calorimetry (DSC). In sharp contrast, when P4 and P5
were blended together at different ratios without covalent
connections as terpolymers, all blended materials gave two
distinct Tg (Figure 6B, Figure S72), which are the same as Tg of
P4 (6.3 °C) and P5 (-39.4 °C). Moreover, while the terpolymers
exhibited transparent appearance, the physical blend rendered
cloudy mixtures (e.g., P3a vs mixture of P4 and P5, Figures 6C
and 6D), suggesting that the phase separation problem could not
be avoided by simply mixing two alternating copolymers together.
Importantly, it is evident that terpolymers (e.g., P3a, Figure 6E
Figures S73-S74) delivered notably higher ionic conductivity (σ)
than the blended materials of P4 and P5, attributed to covalent
Figure 5. Synthesis and characterization of block fluoropolymers.
Photo-Controlled Chain-Extension from Terpolymers
To validate the living characteristics of this method and the
chain-end fidelity of fluoropolymers, we probed the synthesis of
block copolymers using terpolymers as macroinitiators in chain-
extension reactions. Two alternating terpolymers (P1 and P2)
were first prepared at complete conversions of VEs under
optimized conditions. Terpolymer P1 (Mn,SEC = 7.2 kDa, Ɖ = 1.21,
Figure 5A) is composed of FM1, M1 and M2, terpolymer P2
(Mn,SEC = 7.3 kDa, Ɖ = 1.26, Figure 5B) is composed of FM1, M2
and M5. After terpolymerization, comonomers for the formation of
second blocks were added into the reaction mixtures, and the
mixtures were directly exposed to white light irradiation without
additional catalyst (e.g., P1a and P1b). When the second blocks
were formed with a different PFVE (e.g., P1c) or homopolymers
(e.g., P2a-P2c), reaction mixtures were concentrated under
vacuum to remove excess FM1 before adding comonomers. The
1
obtained fluoropolymers were isolated and characterized by H
and 19F NMR (Figures S63-S70), which clearly manifest the
incorporation of new blocks. Of note, two types of block
copolymers, including using either alternating copolymers (e.g.,
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10.1002/anie.202107066
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RESEARCH ARTICLE
connectivity between the three otherwise incompatible functional
domains.
the leakage problem for a high-temperature application. Besides,
different from P3a, PEO, a conventionally employed polymer
electrolyte,[27] displayed a dramatic decline of the mechanical
strength above its melting point (Tm = 61 °C, Figure S76), further
highlighting the outstanding mechanical performance of a
fluorinated terpolymer.
The combined properties improvements achieved by
terpolymerization resulted in significantly enhanced interfacial
stability and electrodeposition behaviors in lithium metal-based
ASSBs. Investigations on galvanostatic polarization and the
reversibility of Li plating and stripping were conducted for P3a, P5
and PEO. Figure 6G shows the voltage profiles over time of three
SPEs. It is clear that the ASSBs assembled with P3a displayed
superior resistance to premature cell failure, and exhibited stable
cycling for more than 1100 h. In sharp contrast, ASSBs with both
P5 and PEO showed quick decline in voltage as caused by the
progressive penetration of dendrites,[16c, 28] and were completely
short-circuited within 180 h, suggesting unstable flux of Li ions in
the polymer matrix.[29]
Moreover, the fluorinated terpolymer exhibited excellent
thermal stability within the operating temperature range of Li
batteries (Figure S77), and exhibited nonflammable property
(Figure S78). Taken together, we expect that this synthetic
method furnishes
a useful system to engineering high-
performance SPEs by optimizing the ternary compositions of
fluoropolymers that could be difficult to realize by either
copolymerization of two components or physical mixing of
copolymers.
Conclusion
We have developed an organocatalyzed controlled alternating
terpolymerization of PFVEs and VEs driven by visible-light
irradiation, enabling the synthesis of a broad scope of fluorinated
terpolymers of narrow MWDs and good chain-end fidelity at high
monomer conversions. The synthetic advantages of this method
allow on-demand tuning the chemical compositions of
fluoropolymers via rational selection of types and/or ratios of
comonomers, smooth transformation of gaseous PFVE under
ambient conditions using conventional low-pressure glass tubes,
as well as temporal control of the chain-growth between “ON” and
“OFF” states. Based on the chain-extension from alternating
terpolymers, a variety of fluorinated block copolymers, which
contain either alternating copolymers or homopolymers as
second blocks, have been successfully synthesized. Furthermore,
as demonstrated with P(FM1-alt-M2)-co-P(FM1-alt-M7), this
terpolymerization provides a versatile and promising system to
effectively integrate diverse properties (e.g., compatibility of
functional domains, mechanical strength, ionic conductivity,
interfacial stability and non-flammability) for SPEs, which should
facilitate the design of advanced lithium-ion batteries. We believe
that this approach will offer new opportunities to create
fluoropolymers of complex structures and compositions,
facilitating the integration of desirable properties into polymers to
fulfill diverse requirements in practical applications.
Figure 6. Properties of fluorinated co- and terpolymers. A) Tg plots of P3-P5.
[M2]/([M2]+[M7]) indicates ratios of M2/VEs in co- and terpolymers. B) Tg of P4,
P5 and mixtures of P4 and P5. [M2]/([M2]+[M7]) reveals ratios of P4/P5 used to
prepare copolymer mixtures. C) and D) Optical images of P3a, mixture of P4
and P5.[26] E) Ionic conductivity (σ) of P3-P5 and mixtures of P4 and P5 at 70 °C.
P4/P5 = 4/1 (a), 1/4 (b). F) Storage modulus (G′, filled cycles) and loss modulus
(G′′, empty cycles) as functions of temperature measured by rheometer. G)
Voltage profiles comparison of P3a vs P5, PEO SPEs as a function of time,
whereby each half-cycle lasted for 3 h at a current density of 0.05 mA cm-2.
An important feature of SPEs is the capability to endow the
energy storage devices with flexible shapes without leakage
concern thanks to their dramatically improved mechanical
strength comparing to liquid electrolytes.[3, 23] The storage and
loss moduli (G′ and G″) of P3a and P5 were measured by
rheometer (Figure 6F). It is worthy noting that P3a maintained
good conductivity as P5, while affording much higher mechanical
strength than P5 from 25 to 100 °C, attributed to the incorporation
of carbonates in terpolymer. For example, P3a delivered G′ of
about 70 kPa at 60 °C, but P5 behaved like viscous liquid at
elevated temperatures (Figure S75), indicating that P5 could have
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10.1002/anie.202107066
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RESEARCH ARTICLE
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Acknowledgements
This research was financially supported by the NSFC (no.
21971044), funding from Fudan University and State Key
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Keywords: fluorine • terpolymer • controlled radical
polymerization • photocatalysis • synthetic methods
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RESEARCH ARTICLE
Entry for the Table of Contents
extension
F
F
F
F
F
F
F
X
Y
Y
+
F
+
xco
OR1
F
y
OR1
OR2
X
PC
OR2
ORf
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Rf
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Rf
Synthetic advantages
various PFVEs & VEs
room temperature
visible light
ambient pressure
Integrate diverse properties
into one chain
metal-free
temporal control
good chain-end fidelity
low Ð
A metal-free photoredox catalyzed controlled alternating terpolymerization of perfluorinated vinyl ethers has been established,
enabling the on-demand synthesis of various fluorinated terpolymers driven by light. This method offers a versatile and convenient
platform to tailor fluoropolymers, which facilitates the preparation of novel block copolymers and engineering of high-performance
solid terpolymer electrolytes.
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