Mechanistic Insight into the Photocontrolled Cationic Polymerization of Vinyl Ethers

Article abstract of DOI:10.1021/jacs.7b09539

The mechanism of the recently reported photocontrolled cationic polymerization of vinyl ethers was investigated using a variety of catalysts and chain-transfer agents (CTAs) as well as diverse spectroscopic and electrochemical analytical techniques. Our study revealed a complex activation step characterized by one-electron oxidation of the CTA. This oxidation is followed by mesolytic cleavage of the resulting radical cation species, which leads to the generation of a reactive cation - this species initiates the polymerization of the vinyl ether monomer - and a dithiocarbamate radical that is likely in equilibrium with the corresponding thiuram disulfide dimer. Reversible addition-fragmentation type degenerative chain transfer contributes to the narrow dispersities and control over chain growth observed under these conditions. Finally, the deactivation step is contingent upon the oxidation of the reduced photocatalyst by the dithiocarbamate radical concomitant with the production of a dithiocarbamate anion that caps the polymer chain end. The fine-tuning of the electronic properties and redox potentials of the photocatalyst in both the excited and the ground states is necessary to obtain a photocontrolled system rather than simply a photoinitiated system. The elucidation of the elementary steps of this process will aid the design of new catalytic systems and their real-world applications.

Full text of DOI:10.1021/jacs.7b09539

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX
Mechanistic Insight into the Photocontrolled Cationic Polymerization
of Vinyl Ethers
Quentin Michaudel, Timothee Chauvire, Veronika Kottisch, Michael J. Supej, Katherine J. Stawiasz,
́
́
Luxi Shen, Warren R. Zipfel, Hector D. Abruna, Jack H. Freed, and Brett P. Fors*
́
̃
Cornell University, Ithaca, New York 14853, United States
S
* Supporting Information
ABSTRACT: The mechanism of the recently reported
photocontrolled cationic polymerization of vinyl ethers was
investigated using a variety of catalysts and chain-transfer
agents (CTAs) as well as diverse spectroscopic and electro-
chemical analytical techniques. Our study revealed a complex
activation step characterized by one-electron oxidation of the
CTA. This oxidation is followed by mesolytic cleavage of the
resulting radical cation species, which leads to the generation
of a reactive cationthis species initiates the polymerization
of the vinyl ether monomerand a dithiocarbamate radical
that is likely in equilibrium with the corresponding thiuram
disulfide dimer. Reversible addition−fragmentation type
degenerative chain transfer contributes to the narrow
dispersities and control over chain growth observed under these conditions. Finally, the deactivation step is contingent upon
the oxidation of the reduced photocatalyst by the dithiocarbamate radical concomitant with the production of a dithiocarbamate
anion that caps the polymer chain end. The fine-tuning of the electronic properties and redox potentials of the photocatalyst in
both the excited and the ground states is necessary to obtain a photocontrolled system rather than simply a photoinitiated
system. The elucidation of the elementary steps of this process will aid the design of new catalytic systems and their real-world
applications.
INTRODUCTION
■
The wealth of photoredox reactions developed during the past
decade has offered a blank canvas on which to design “living”
polymerizations in which polymer chain growth can be
controlled at will by the intensity or wavelength of light.1 The
intrinsic resolution of light enables unparalleled spatial control
over these polymerizations, a factor that may prove desirable in a
wide array of settings. Photocontrolled variants of living radical
polymerizations, including atom-transfer radical-polymerization
(ATRP),2 organotellurium-mediated radical polymerizations
(TERP),3 and reversible addition−fragmentation chain transfer
(RAFT) polymerizations,4 have already demonstrated usefulness
Figure 1. Cationic polymerization of vinyl ethers regulated with blue
with a variety of monomers. This unprecedented command over
light.
polymeric architectures inspires numerous applications from the
complex patterning of surfaces5 to the synthesis of sequence-
controlled polymers.6
and narrow dispersity (Đ) values were obtained by modulating
the CTA-to-monomer ratio. These conditions reversibly
generate cationic chain ends that propagate in the presence of
light and are deactivated in the dark, thus enabling the temporal
regulation of chain growth through light irradiation. In contrast
to previously reported radical systems that rely on the chain end
reduction, this cationic process requires oxidation of the terminal
In an effort to expand the range of monomers capable of light-
regulated polymerization, we recently reported a photo-
controlled cationic polymerization of vinyl ethers.7 As shown
in Figure 1, our initial reaction conditions capitalized on the high
potency of the oxidative photocatalyst (PC) 2,4,6-tris(4-
methoxyphenyl)pyrylium tetrafluoroborate (1a) when com-
bined with chain-transfer agents (CTAs) 2a or 2b as a means to
control the cationic polymerization of various vinyl ethers (3a−
e). Polymers with predictable number-average molar mass (Mn)
Received: September 6, 2017
Published: October 6, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b09539
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Article
were kept at ambient temperature under a constant stream of air
and placed at a fixed distance from the light. Isobutyl vinyl ether
(IBVE, 3a) was used as a reference monomer.
Influence of PCs on Polymerization Rates. Our mechanistic
hypothesis states that the rates of activation (step I) and
deactivation (step IV) for a given CTA should be governed
primarily by the nature of the PC. In an attempt to pinpoint the
key features of an efficient PC for this system, we prepared an
array of pyrylium derivatives (1a−e; Figure 4; see Supporting
Figure 2. Photocontrolled radical polymerization vs cationic polymer-
ization: two mechanistically distinct pathways. PC = photocatalyst; Z =
Br, S2CR, S2CSR′, etc.
group (Figure 2), which suggests that the transformation
proceeds through a mechanistic pathway that is fundamentally
distinct from those of its radical counterparts. A deeper
understanding of the mechanism of this unique polymerization
is critical for applications to a broader range of monomers and,
more important, the design of more intricate systems.
Our current mechanistic hypothesis is depicted in Figure 3.
Single-electron transfer (SET) from the CTA (or polymer chain
Figure 4. Photocatalysts used for the study.
Information [SI] for details).8−11 The para substituent of each
aryl group as well as the heteroatom of the pyrylium core are
easily tuned through synthesis, which allows for simple
modulation of the redox and photophysical properties of the
catalyst (Table 1). Other strongly oxidizing photoredox catalysts
were used for this study as well, including acridinium 4 and
ruthenium or iridium complexes 5 and 6.12−17
Figure 3. Proposed mechanism for the light-regulated polymerization of
vinyl ethers via a photocatalyst (PC) and chain-transfer agent.
end) to the excited PC generates a radical cation species (step I)
that undergoes mesolytic cleavage, leading to the formation of an
active cationic chain end and a stable dithiocarbamate or
trithiocarbonate radical (step II). The resulting cationic species is
then engaged in a RAFT-type degenerative chain transfer (step
III). Finally, one-electron reduction of the stable radical by PC•
turns over the PC while producing a dithiocarbamate (or
trithiocarbonate) anion, which caps the polymer chain end and
deactivates chain growth (step IV).
To probe the elementary steps, we used a combination of
spectroscopiesnamely, time-resolved photoluminescence,
UV−visible absorption, and electron spin resonance (ESR)
and electrochemistry. The efficiencies of the polymerization were
also explored with a number of PCs and CTAs. The body of data
presented herein reveals much about the processes at play and
should serve as a platform for the development of related
transformations and translational technologies.
Table 1. Redox Potentials and Photophysical Properties of
Photocatalysts
+
+
EPC
*
EPC
/PC•
/PC•
PC (V vs SCE) (V vs SCE) ε450 (L·mol−1·cm−1
)
Φf
ΦISC
a
a
8
8
b
,
1a
1b
1c
1d
1e
4
+1.84
+2.55
−0.508
−0.328
−0.559
−0.0310
−0.198
−0.498
−0.8013
−1.3713
67 300
0.958
0.588
−
0.038
0.428
−
b
,
6350
+2.239
32 100
30 100
b
b
+2.4910
0.3311
0.038
0.0358
0.03415
0.6816
0.6711
0.948
0.388
0.6815
−
a
,
8
b
+2.45
+2.18
7330
a
,
8
403012
900014
176016
5
+1.4513
+1.2113
6
a
b
Potential of the singlet excited state. Experimentally determined in
dichloromethane (see Supporting Information). Reference numbers
for each reported physical data are indicated.
The results from the polymerization of 3a using CTA 2a, a PC,
and blue LEDs are summarized in Table 2. Notably, only
pyrylium derivatives afforded polymers (Table 2, entries 1−7),
all of which had Mn’s close to theoretical values and Đ’s of
approximately 1.2. In the dark, no polymerization was observed
(Table 2, entry 11).7 Phenyl and tolyl derivatives 1b and 1c
RESULTS AND DISCUSSION
■
Exploration of the Photocatalytic System. Building on
our initial report, we investigated several factors that could
influence the polymerization kinetics and control in these
systems, such as the choice of PC, CTA, and solvent. All reactions
B
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Table 2. Selected Results of Cationic Polymerization of
Isobutyl Vinyl Ether (3a) with Various Photocatalysts
time
(min)
Mn (exp)
(kg/mol)
Mn (theo)
f
a
entry catalyst
(kg/mol)
Đ
1
1a
1b
1b
1c
480
10
10.7
10.5
10.2
11.1
10.7
10.5
10.3
−
10.1
10.1
10.1
10.1
10.1
10.1
10.1
10.1
10.1
10.1
10.1
1.19
1.23
1.19
1.17
1.17
1.18
1.21
−
2
b
3
10
4
10
b
5
1c
30
6
1d
1e
4
480
300
7
e
8
−
−
−
−
e
9
5
−
−
−
−
−
−
e
10
11
6
d
e
1a
a
Reaction conditions: 3a (1 equiv), photocatalyst (PC; 0.02 mol %),
and 2a (0.01 equiv) at room temperature in dichloromethane with
b
Figure 5. Library of chain-transfer agents used for this study.
blue-light-emitting diode irradiation (9 W bulb). Using 0.01 mol % of
d
e
PC. No irradiation. No conversion was observed after 720 min.
f
Conversion was determined by 1H NMR using benzene as an internal
Table 3. Selected Results of the Cationic Polymerization of
Isobutyl Vinyl Ether (3a) and Ethyl Vinyl Ether (3b) with
Various Chain-Transfer Agents
standard.
(Table 2, entries 2−5) demonstrated the highest polymerization
rates, reaching full conversion after less than 10 min compared
with several hours for 1a. This outcome is likely due to the higher
Mn (exp)
Mn (theo) (kg/mol)
a
b
entry CTA monomer
(kg/mol)
Đ
1
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
3a
9.11
7.39
10.1
8.20
15.1
10.6
15.0
10.9
9.43
7.23
12.9
10.4
74.9
40.5
52.2
10.1
7.3
1.16
1.12
1.43
1.35
1.98
1.89
2.10
2.11
1.49
1.47
2.18
2.21
3.76
3.18
3.96
•
•
oxidation potentials of the PCs (E°1b*/1b = +2.55 V and E°1c*/1c
2
•
= +2.23 V, whereas E°1a*/1a = +1.84 V vs SCE). The relatively
slower rate observed with thiopyrylium 1e compared with that of
1b despite the similar redox potentials and molar attenuation
coefficients of these compounds may be linked to a low quantum
yield of fluorescence (Φf = 0.03), which implies that the singlet
excited state of the pyrylium contributes mainly to the electron
transfer step. However, additional experiments are required to
confirm the exact role of both the singlet and the triplet states in
this reaction.18 Finally, the lower reactivity of tribromo congener
1d compared with that of the other pyryliums is likely due to
poor solubility in dichloromethane (Table 2, entry 6).
3
10.1
7.3
4
5
10.1
7.3
6
7
10.1
7.3
8
9
10.1
7.3
10
11
12
13
14
15
10.1
7.3
2f
2g
2g
−
10.1
7.3
Influence of the CTA on Chain Growth Control. Because 2a
and 2b play the dual role of initiator and CTA, we hypothesized
that the structure of the CTA may profoundly affect the
photocontrol of the process.19 We therefore synthesized a library
of CTAs (2c−f) with various groups appended to the
dithiocarboxylic core. We also synthesized one CTA (2g)
prepared from 4-methoxystyrene rather than IBVE (Figure 5).
We anticipated that xanthate 2c, dithioesters 2d and 2e, and
pyrrolidinone dithiocarbamate 2f would show electronic proper-
ties distinct from those of 2a and 2b, a factor that should impact
the rate of each step in the mechanism.
−
a
Reaction conditions: 3a (1 equiv), 1a (0.01 mol %), and chain-
transfer agent (0.02 equiv) at room temperature in dichloromethane
with blue-light-emitting diode irradiation (ca. 12 W LED strip) for 360
min. Conversion was determined by H NMR using benzene as an
internal standard.
b
1
than predicted with both monomers (Table 3, entries 5−8). By
contrast, CTA 2e enabled control similar to that offered by 2b
(Table 3, entries 9 and 10). Subtle increase of the electron
donating character of the aryl group therefore has a crucial effect
on the control over the chain growth. The CTA derived from 4-
methoxystyrene (2g) resulted in uncontrolled polymerization
with both monomers (Table 3, entries 13 and 14), which
indicates that the monothioacetal structure is critical. Notably,
uncontrolled polymerization occurs in the absence of CTA
(Table 3, entry 15), which suggests that the direct activation of
the monomer is a possible background pathway. Based on the
aforementioned results, electron-rich X groups (see Figures 1
and 3 for the notation) such as nitrogen- and sulfur-containing
moieties should be favored in the design of future CTAs.
Moreover, only CTAs derived from vinyl ethers exhibited good
control over chain growth.
In our previous study,7 we found that, compared with 2a, CTA
2b was applicable to a wider monomer scope and delivered
polymers with only a slightly broader Đ. However, the
preparation of 2a with greater purity enabled the controlled
polymerization of ethyl vinyl ether (EVE; Table 3, entry 2), n-
propyl vinyl ether, and n-butyl vinyl ether (see SI). Polymer-
ization attempts with 2-chloroethyl vinyl ether and 2a resulted in
no conversion (see SI), and 2b is still required for this monomer.
Consequently, the purity of the CTA is paramount to the
polymerization of less reactive monomers.
The polymerization of IBVE (3a) or EVE (3b) using CTAs 2c,
2d, and 2f yielded macromolecules with Mn’s mostly in
agreement with theoretical calculations but with broader Đ’s
(1.9 < Đ < 2.2; Table 3, entries 5−8 and 11−12). The
experimental Mn’s of xanthate 2c and dithioester 2d were higher
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Suitable Solvents for the Polymerization.20 Pyrylium salts
have poor solubility in hydrocarbon solvents; therefore, benzene,
toluene, and small alkanes are unsuitable for this methodology.
Similarly, the high oxidizing strength of excited pyryliums
precludes the use of solvents such as tetrahydrofuran (THF).
Polymerizations in acetonitrile yielded well-controlled polymers,
but poly(IBVE) started to phase-separate at an Mn of ∼5 kg/mol.
A mixture of dichloromethane and acetonitrile prevented this
issue. Last, almost no polymerization was observed in nitro-
methane, an outcome that agrees with the reported low
propagation rates of cationic polymerization of vinyl ethers in
this solvent.21
Initial One-Electron Oxidation, a Critical Step for
Activation. The elucidation of the electron transfers among
species in solution is key to understanding the activation of the
cationic process (step I). Our previous studies revealed that the
fluorescence of 1a* in a steady-state experiment is quenched by
both CTA 2a and IBVE (3a).7 However, this result does not
distinguish between static and dynamic quenching (Figure 6),
Figure 7. Fluorescence decay of 1a after a 440 nm pulsed excitation: (A)
For various concentrations of chain-transfer agent 2a. (B) For various
concentrations of isobutyl vinyl ether (3a).
Figure 6. Static vs dynamic quenching of a photocatalyst (PC) by a
quencher (Q).
and the latter is the only quenching at play for an electron
transfer.22 Therefore, time-resolved fluorescence spectroscopy
was used in conjunction with ESR and electrochemical analysis
to analyze electron transfer in these reactions.
Time-Resolved Fluorescence Spectroscopy. The fluores-
cence decay for PC 1a using 440 nm pulsed excitation was
measured with various amounts of potential quenchers, CTA 2a,
and monomer 3a (Figures 7, 8, and S14). A clear decrease in the
PC* lifetime (τ) was observed with increasing concentrations of
both 2a and 3a. More precisely, the relationship between τ and
the concentration of both quenchers (Figure 8) follows eq 1,
which is directly derived from the Stern−Volmer equation,
where τ0 is the fluorescence lifetime of catalyst without quencher,
kq is the bimolecular quenching constant, and [Q] is the
concentration of quencher.22
Figure 8. Linear relationship between the fluorescence lifetime of
photocatalyst 1a and the concentrations of 2a (red) and 3a (blue).
to the CTA is the more favorable pathway (kq = 7.52 × 109 M−1·
s−1); however, in the early stages of polymerization, the direct
oxidation of the highly concentrated IBVE is feasible (Table 3,
entry 15).
τ0
τ
= 1 + kqτ0[Q]
Electropolymerization of IBVE. The spectroscopic evidence
(1)
discussed above agrees with the measured potentials correspond-
•+
This behavior is consistent with collisional quenching
occurring with both the CTA and the monomer. However,
calculations of bimolecular quenching constants (kq’s) suggest
that the former is a more efficient quencher (kq = 7.52 × 109 M−1·
s−1) than 3a (kq = 1.23 × 108 M−1·s−1) by nearly 2 orders of
magnitude. The quenching rate of CTA 2a is near 1010 M−1·s−1,
which is characteristic of a diffusion-controlled process.22 These
observations suggest that SET can indeed occur between the
singlet excited PC and either the CTA or the monomer. Transfer
ing to the onset of oxidation of both CTA 2a (E°2a/2a = +0.98 V
vs Ag/Ag+ [+1.19 V vs SCE]) and 3a (E°3a/3a = +1.25 V vs Ag/
•+
Ag+ [+1.46 V vs SCE]) (Figure 9), the latter being more difficult
to oxidize. Nevertheless, the oxidation of 3a could potentially
lead to uncontrolled polymerization, as seen when no CTA is
added (Table 3, entry 15).23 The absence of this background
reaction when a CTA is used and at low PC loading can be
rationalized by several electronic considerations. First, compared
to 3a, CTA 2a is oxidized more readily by catalysts 1. Second, if
D
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Figure 9. Cyclic voltammogram of 2a (red), 3a (blue), and a
combination of 2a and 3a (black) using a platinum microelectrode
(12.5 μm).
the oxidation of 3a still occurs, the 270 mV difference in
potentials should allow for a second electron transfer from
oxidized 3a to CTA 2a.
Figure 10. Electron spin resonance spectrum of 1a with 2a (red) and 1b
with 2a (black) under steady-state 450 nm irradiation.
To assess whether the direct oxidation of CTA 2a leads to
polymerization, we obtained a cyclic voltammogram of 3a with
and without 2a (Figure 9). The absence of a reduction wave in
both cases likely reflects the irreversibility of the reaction and,
potentially, the passivation at the electrode due to the growth of
poly(IBVE). Compared to IBVE alone, the onset of oxidation
with 2a lies ca. 300 mV lower, which suggests that the
polymerization of 3a can be effected at a lower potential (ca.
+0.8 V vs Ag/Ag+) when CTA 2a is present. Indeed, the
polymerization of 3a could be initiated at an applied voltage of
+0.8 V vs Ag/Ag+ in the presence of 2a (see SI), whereas no
polymer was isolated in the absence of 2a under the same
conditions. This result supports the conclusion that the direct
oxidation of CTA 2a is the major pathway for step I and that a
two-step oxidation via an oxidized monomer species is likely only
a minor concurrent pathway.
these results with the spectrum in Figure 10 corroborates the
suggested formation of pyranyl radical 2a in the reaction
conditions.
Having confirmed the one-electron oxidation of CTA 2a by
excited PCs 1a or 1b, we turned our attention to the putative
electron transfer between 1a (or 1b) and vinyl ether 3a. Steady-
state irradiation of a mixture of 1a and 3a or 1b and 3a revealed
the formation of a long-lived radical species (Figure 11). A
ESR and UV−visible Characterization of Pyranyl Radicals
1a• and 1b•. ESR spectroscopy has proven to be a powerful tool
for the observation of free radical species in similar polymer-
ization systems;18,24 moreover, both 1a• and 1b• have previously
been characterized using this technique,25,26 which prompted the
use of ESR as a means to track our postulated electron transfers.
Samples containing a mixture of CTA 2a and PCs 1a or 1b were
monitored during steady-state irradiation with blue LEDs. The
beginning of irradiation coincided with the appearance of an ESR
absorption signal with both PCs (Figure 10), an observation
characteristic of stable radical intermediates. With the combina-
tion of 1b and 2a, a hyperfine splitting structure was observed,
which was expected due to the numerous aromatic protons
adorning the pyrylium core. This hyperfine coupling (hfc)
structure could be reproduced through simulation (Figure S15A)
and was in complete agreement with previously reported hfc
constants for that compound.25 Moreover, the formation of 1b•
was confirmed with UV−visible absorption spectroscopy (Figure
S21A).25b By contrast, no hyperfine splitting structure was
observed for the analogous system containing 1a and 2a, even
when a low-amplitude modulation was used (M = 0.08 G).27
However, this absence of hyperfine structure was noted by
Kawata and colleagues26 and could be due to a broadening
induced by the Heisenberg exchange effect.28 To unambiguously
attribute the signal arising from the irradiation of 1a and CTA 2a
to pyranyl radical 1a•, we used THF, a known sacrificial electron
donor,25,26 in lieu of CTA 2a (Figure S16). A comparison of
Figure 11. Electron spin resonance spectrum of 1a with IBVE (3a; red)
and 1b with 3a (black) under steady-state 450 nm irradiation.
comparison of these results to the traces in Figure 10 revealed
striking differences. In particular, these new radical species have
broader signals and different hyperfine splitting structures.
Furthermore, the ESR spectra showed a broadening of the line
width over the course of the acquisition, which may be caused by
the increase in viscosity during the polymerization.29 Considered
together, these results suggest that radical species other than 1a•
and 1b• are created in the presence of 3a. However, the complex
hyperfine splitting structure of these new radical species indicates
that the adducts contain a pyrylium core structure.
Several hypotheses may explain the discrepancies between the
spectra in Figures 10 and 11. First, after the initial photoinduced
electron transfer, radicals 1a• or 1b• may react with 3a to
generate a new adduct. Second, the high concentration of IBVE
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may induce conformational change through noncovalent
bonding, thereby leading to a nonequivalent set of hfc’s via the
loss of C2 symmetry. To distinguish between these possibilities,
we added 3a to a sample containing 1b•, which was pregenerated
using THF as an electron donor. No change was observed in
either the hyperfine structure or the intensity of the signal after
3a addition (Figure S20), which indicates that no reaction
occurred between 1b• and 3a. Thus, we investigated the potential
formation of a donor−acceptor complex between ground state
1a (or 1b) and the electron-rich monomer (3a). Equilibrium 1
illustrates this hypothesis in the case of 1a (Figure 12).
polymerization mixture, no poly(methyl methacrylate), a
product expected from alkyl radical formation, was isolated.33
Significantly, when blue light was shone on the reaction of
CTA 2a and catalyst 1a in a ratio typical of the polymerization,
the only isolated products were thiuram disulfide 7 and starting
material 2a (see Figure 13A and SI). Compound 7 likely arises
Figure 13. (A) Isolation of thiuram disulfide 7 via putative mesolytic
cleavage. (B) Crossover experiment with disulfides 7 and 8.
from radical recombination after mesolytic cleavage. Indeed,
one-electron oxidation of sodium dithiocarbamate affords
thiuram disulfide 7 in excellent yields with various oxidants
such as iodine (see SI).34
Thiuram disulfides are commonly used as CTAs for radical
RAFT polymerization and as reagents for the vulcanization of
rubber.35 Notably, when substituted for CTA 2a, disulfide 7
affords no control over the polymerization (Table S7), a result
that agrees with the proposed cationic process. Disulfide 7 was
not detected during NMR analysis of aliquots obtained over the
course of the polymerization reactions, which may be attributed
to the detection limits of NMR spectroscopy or the rapidity of
the reduction of the radical or disulfide dimer (step IV, vide
infra). Moreover, a crossover experiment with disulfides 7 and 8
indicated that 7 is in equilibrium with the radical dithiocarbamate
in the reaction conditions (Figure 13B), which agrees with the
results of previous studies.12
Is a RAFT Equilibrium Occurring? In their initial report of a
cationic RAFT process, both Kamigaito19b,36 and Sugihara37
invoked a degenerative chain transfer to account for the observed
control over chain growth. However, from a mechanistic
standpoint, our photoredox process differs in that CTA 2a
plays the role of both initiator and CTA, as shown by steps I, II,
and IV. From this proposed catalytic cycle, it is unclear whether
the RAFT equilibrium (step III) is truly necessary to achieve
control over chain growth. Therefore, we devised experiments to
ascertain the presence or absence of such equilibrium in this
polymerization. Quantum yields of the polymerization were
estimated through actinometry, and temporal control was
investigated with PCs 1a and 1b.
Figure 12. Formation of a donor−acceptor complex between 1a and 3a:
UV−visible spectra of 1a with various concentrations of 3a.
The results of careful UV−visible absorption spectroscopy of
1a and 1b with various amounts of 3a revealed a slight shift of the
local absorption maximum above 450 nm (Figure 12). This new
band at 487 nm was attributed to the absorption of [1···3a]+, and
the equilibrium constants KDA were estimated to be 0.06 M (for
1a) and 0.19 M (for 1b) according to the Benesi−Hildebrand
method (Figure S22).30 Nicewicz and co-workers have
previously demonstrated the impact of similar donor−acceptor
complexes on the dynamics of alkene oxidation by acridinium
PCs,12 and we expect that, similarly, the formation of [1···3a]+
significantly influences the kinetics of electron transfer.
Consistent with this finding, a spectrum that has good correlation
with the experimental ESR signal in Figure 11 could be simulated
by adding two equivalent protons to the spin system of 1b•
(Figure S19).31 Finally, ESR analysis of the complete polymer-
ization systemnamely, 1a (or 1b), CTA 2a, and monomer
3aproduced spectra similar to those in Figure 11 (Figures S17
and S18).
Exploration of the Mesolytic Cleavage. The second key
step of our mechanism involves the cleavage of the radical cation
arising from CTA or chain end oxidation into a dithiocarbamate
radical and a propagating cationic chain. Vinyl ethers are known
to homopolymerize under cationic conditions, but the analogous
process has not been observed under radical conditions,32 which
substantiates the fragmentation pattern described above.
Furthermore, when methyl methacrylate was added to the
Quantum Yields of the Polymerization. Using the well-
studied potassium ferrioxalate actinometer (see SI, Figure S23,
for more details),38 the quantum yields of polymerization were
estimated to be approximately 6 monomer additions per photon
absorbed for PC 1a (0.02 mol %) and approximately 35
monomer additions/photon for 1b (0.01 mol %). This difference
was anticipated based on the higher polymerization rate
measured for 1b compared with that of 1a (Table 2). By
contrast, photoinitiated cationic polymerizations are character-
F
DOI: 10.1021/jacs.7b09539
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Journal of the American Chemical Society
Article
ized by higher quantum yields such as 200 monomer additions/
photon.39 However, the fact that both quantum yields are well
above unity suggests that a chain-degenerative mechanism, as
shown in step III, is likely occurring, because narrow Đ’s and
predictable Mn’s were obtained for all pyrylium PCs.
reduction of 1a with a stoichiometric amount of cobaltocene
(CoCp2)41 afforded radical 1a•, as shown in Figure 15 (see SI for
On/Off Experiments with PCs 1a and 1b. The stark
differences in polymerization rates and quantum yields between
PCs 1a and 1b led us to investigate whether temporal control
over chain growth is possible with the latter. A reaction mixture
containing monomer 3a, catalyst 1a (or 1b), and CTA 2a was
exposed to light and then stirred in the dark for the same time
period and re-exposed to light (Figure 14). Conversion and Mn
Figure 14. Monomer conversion vs time with intermittent light
exposure with photocatalysts (A) 1a and (B) 1b.
were monitored at each switching point with NMR spectroscopy
and size exclusion chromatography analyses of aliquots.
Although the plot of conversion versus time clearly illustrates
that polymerization proceeds only in the presence of light with
1a (Figure 14A), the corresponding plot for 1b shows that
polymerization is not halted in the dark (Figure 14B).
Consequently, pyrylium 1b should be considered a fine catalyst
for photoinitiated, rather than photocontrolled, cationic polymer-
ization,1f as it allows for predictable Mn and a Đ of ∼1.2 but offers
no temporal control of chain growth. This outcome also suggests
that the recapping step (step IV) is much slower for 1b than for
1a, which can be explained by the difference in ground state
Figure 15. Reduction of 1a to 1a• with CoCp2, followed by oxidation
back to 1a with disulfide 7. (A) Electron spin resonance spectra of 1a
(black underneath green curve), 1a• after the addition of CoCp2 (red),
and 1a after the addition of 7 (green). (B) UV−visible spectra of 1a
(black), 1a• after the addition of CoCp2 (red), and 1a after the addition
of 7 (green). Characteristic absorption bands are indicated.
details). The spin concentration of this stable radical (lifetime >6
h) was estimated to be 250 μM under these conditions. The
addition of an excess of thiuram disulfide 7 resulted in an
instantaneous depletion of the signal to a spin concentration of
approximately 0.8 μM (Figure 15A). This change indicated a
SET from 1a• to disulfide 7. The reduction of 7 concomitantly
regenerates PC 1a, as shown on the UV−visible spectra (Figure
15B) and produces the dithiocarbamate anion likely responsible
for capping the chain end and thus deactivating chain growth. In
the polymerization process itself, this SET event likely happens
with either 7 or the dithiocarbamate radical, which are postulated
to be in equilibrium (Figure 13B).
+
•
+
•
redox potentials (E°1a /1a = −0.50 V versus SCE, and E°1b /1b
=
−0.32 V versus SCE).8 Indeed, the redox potential for the
•−
reduction of disulfide 7 has been measured at E°7/7 = −0.302 V
versus SCE by Nichols and Grant,40 which supports our
conclusion that reduction of 7 is more likely with 1a• than
with 1b•, therefore resulting in a more efficient deactivation of
chain growth with the former. However, the fact that living
characteristics are observed with both photocatalysts is a strong
indicator that a RAFT equilibrium influences chain growth.
Catalyst Turnover and Chain End Capping. The last step
of our proposed mechanism was interrogated via an ESR
experiment coupled with UV−visible spectroscopy. The
Notably, the addition of CTA 2a to radical 1a• generated by
cobaltocene reduction resulted in no changes in the ESR and
UV−visible spectra (Figure S26). This result corroborates the
finding that the CTA does not interact with the reduced PC.
G
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Article
CONCLUSION
Hector D. Abruna: 0000-0002-3948-356X
Jack H. Freed: 0000-0003-4288-2585
Brett P. Fors: 0000-0002-2222-3825
́
̃
■
Using various spectroscopic techniques, we gained intimate
knowledge of each elementary step in a novel cationic
polymerization of vinyl ethers regulated by visible light. Fine-
tuning of the electronic structure of both the PC and the CTA
unveiled a number of factors that govern the rate of
polymerization as well as the prerequisites for a well-behaved
living process: while more oxidizing pyrylium salts generally
engender higher polymerization rates, quantum yields of
fluorescence and the solubility profile also play a role in the
activity of the PCs. Interestingly, no other family of PCs has
proven competent for the photocontrolled polymerization of
vinyl ethers yet. CTAs synthesized from a vinyl ether derivative
and containing an electron-rich heteroatom appended to the
dithiocarboxylic core delivered the best control over chain
growth.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Cornell University, made use of the
NMR facility at Cornell University, and was supported in part by
the National Science Foundation under Award CHE-1531632. It
also made use of the National Biomedical Research Center for
AdvanCed ESR Technology (ACERT) and was supported by the
National Institute of General Medical Sciences of the National
Institutes of Health under Award Number P41GM103521.
B.P.F. thanks 3M for a Nontenured Faculty Award.
Time-resolved photoluminescence and ESR spectroscopy
revealed single-electron transfers from excited PCs 1a* and
1b* to CTA 2a and monomer 3a. Oxidation of 3a by PCs is
responsible for the uncontrolled background polymerization.
Comparison of bimolecular quenching constants and behaviors
during cyclic voltammetry of 2a and 3a, however, suggests that
oxidation of 2a is more facile than that of 3a, and that a second
electron transfer from oxidized 3a to 2a might prevent the
background polymerization. Moreover, polymerization of 3a can
be mediated by 2a at a voltage lower than the onset of oxidation
of 3a, which indicates that the direct oxidation of 2a by 1* is the
major contributor to the activation step. Finally, the formation of
a donor−acceptor complex between monomer and PC in the
ground state was uncovered by meticulous ESR and UV−visible
spectroscopic analyses of a mixture of 3a and PCs 1a and 1b.
Such complexes presumably play a key role in the activation step.
Isolation of thiuram disulfide 7 supports a mesolytic cleavage
pathway following oxidation of the CTA. In the reaction
conditions, 7 is in equilibrium with the radical dithiocarbamate
species arising from homolytic breaking of the disulfide bond.
Determination of the quantum yields of polymerization for
various PCs, as well as on/off experiments, substantiate the
existence of a degenerative chain transfer mechanism. This
RAFT-type equilibrium is likely pivotal in the obtention of
polymers with predictable Mn’s and narrow Đ’s. Last, chain end
deactivation and PC regeneration via electron transfer between
1a• and 7 were corroborated by ESR and UV−visible
spectroscopies.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b09539.
General experimental considerations, experimental pro-
cedures, and additional supporting data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*bpf46@cornell.edu
ORCID
Timothee Chauvire: 0000-0002-9466-4785
́
́
H
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