Interconvertible living radical and cationic polymerization through reversible activation of dormant species with dual activity

Article abstract of DOI:10.1002/anie.201406590

The polymerization of vinyl monomers generally requires the selection of an appropriate single intermediate whereas in copolymerization the selection of the comonomer is limited by the intermediate. Herein we propose interconvertible dual active species that can connect comonomers through different mechanisms to produce specific comonomer sequences in a single polymer chain. More specifically two different stimuli that is a radical initiator and a Lewis acid are used to activate the common dormant CSC(S)Z group into radical and cationic species thereby inducing interconvertible radical and cationic copolymerization of acrylate and vinyl ether to produce a copolymer chain that consists of radically and cationically polymerized segments. The dual reversible activation provides control over molecular weights and multiblock copolymers with tunable segment lengths.

Full text of DOI:10.1002/anie.201406590

Angewandte
Chemie
DOI: 10.1002/anie.201406590
Copolymerization
Interconvertible Living Radical and Cationic Polymerization through
Reversible Activation of Dormant Species with Dual Activity**
Hiroshi Aoshima, Mineto Uchiyama, Kotaro Satoh,* and Masami Kamigaito*
Abstract: The polymerization of vinyl monomers generally
requires the selection of an appropriate single intermediate,
whereas in copolymerization, the selection of the comonomer
is limited by the intermediate. Herein, we propose interconver-
tible dual active species that can connect comonomers through
different mechanisms to produce specific comonomer sequen-
ces in a single polymer chain. More specifically, two different
stimuli, that is, a radical initiator and a Lewis acid, are used to
activate the common dormant CÀSC(S)Z group into radical
intermediate. One plausible strategy to ease this restriction is
the conversion of a reactive intermediate into another one
during the polymerization process. This alteration would
enable the synthesis of novel copolymers with unprecedented
comonomer sequences that have not been accessible using
a single intermediate. However, the direct interconversion of
intermediates during polymerization has not been fully
achieved because of the rapid rate of propagation. In
conventional chain-growth polymerization, the reactive inter-
mediate propagates into a growing polymer chain immedi-
ately after its formation, and then undergoes termination and/
or chain-transfer reactions, which result in a “dead” polymer.
Thus, the propagating species has almost no chance of
interconversion into another species because of its generally
short lifetime and unstable nature. Although one-way con-
versions during copolymerization have been reported in the
and cationic species, thereby inducing interconvertible radical
and cationic copolymerization of acrylate and vinyl ether to
produce a copolymer chain that consists of radically and
cationically polymerized segments. The dual reversible activa-
tion provides control over molecular weights and multiblock
copolymers with tunable segment lengths.
[2–13]
A
chemical reaction often proceeds via a specific intermedi-
synthesis of block copolymers,
most remain incomplete as
ate, such as a cation, anion, and radical, thereby converting
a starting material into a product, preferably with high
selectivity and yield. Different intermediates are believed to
be fundamentally incompatible, because they are supposed to
interfere with each other and are usually unnecessary in the
formation of a desired product based on a single step. This
observation is also true for chain-growth polymerization, in
which an appropriate intermediate, initiator, or catalyst, is
chosen depending on the monomer type in order to produce
the desired polymer chain through repetitive reactions
a result of low efficiency and/or are limited to one occurrence
because of an irreversible transformation.
Recent decades have seen significant developments in
[14,15]
“living” or controlled polymerizations,
which proceed via
various single intermediates, such as anionic, cationic, radical,
and coordinating species. In most of these modern “living”
polymerizations, that is, reversible deactivation polymeri-
zation according to the terminology recommended by the
[16]
IUPAC,
the reactive intermediates are transiently con-
verted or reversibly deactivated into the dormant covalent
species. That is, the short-lived and unstable intermediates are
reversibly deactivated for some time, resulting in dormant
covalent species, which can propagate again after reversible
activation. Such a reversible interconversion between the
dormant and active species affords an almost equal oppor-
tunity of chain growth for all of the initiator or polymer
molecules, resulting in polymers with approximately equiv-
alent chain lengths. Thus, the dormant species play an
important role in controlling the chain length of the resulting
polymers by extending the lifetime of the chain, including its
dormant period. Contributions from the dormant species are
necessary for the precise control of the molecular weight,
[
1]
between the reactive intermediate and the monomers. In
order to obtain polymers that possess the desired properties,
different monomers may be copolymerized, thereby changing
the properties of the resulting polymer; accordingly, copoly-
merizations are widely employed for many common poly-
mers. However, the selection of comonomers is limited,
because not every monomer can be polymerized with every
[*] Dr. H. Aoshima, M. Uchiyama, Prof. Dr. K. Satoh,
Prof. Dr. M. Kamigaito
Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan)
E-mail: satoh@apchem.nagoya-u.ac.jp
kamigait@apchem.nagoya-u.ac.jp
[17–20]
[21–26]
especially in the cationic
and radical
polymerization
of vinyl ether monomers, in which both of the propagating
species, that is, carbocationic and radical intermediates, are
short-lived and unstable.
Here, to copolymerize different types of monomers that
cannot be copolymerized via a single intermediate, we
propose the activation of the dormant species into two
different intermediates using dual reversible activation
(Scheme 1). Specifically, one intermediate is deactivated
into the dormant state and then activated into another one
using a different route. To realize this strategy, the method
[
**] This work was supported in part by JSPS Research Fellowships for
Young Scientists for H.A. (No. 24-10127), a Grant-in-Aid for Young
Scientists (A) (No. 23685023) and Scientific Research (A) (No.
26248032) for K.S. by the Japan Society for the Promotion of
Science, and Program for Leading Graduate Schools “Integrative
Graduate Education and Research Program in Green Natural
Sciences”.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406590.
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Figure 1. Vinyl ether derived RAFT agents used in this study.
Scheme 1. Concept for concurrent copolymerizations proceeding
through the reversible activation of common dormant species by two
different stimuli.
(
2
IBVE) and methyl acrylate (MA) monomers in toluene at
08C.
As a control experiment for the copolymerization with
required at least one common dormant species and two
different stimuli, which activate the common dormant state
into different intermediates. Additionally, both intermediates
both stimuli, a single stimulus, that is, the Lewis acid or the
radical initiator, was first used with a mixture of IBVE and
[
27]
and stimuli have to be compatible.
MA in the presence of 1. When EtAlCl was combined with 1,
2
To demonstrate the feasibility of this strategy, we report
the interconvertible, living cationic and radical polymeri-
zation of vinyl ether and acrylate monomers using a thioester
as the common dormant species and a Lewis acid catalyst and
an azo initiator as the stimuli to generate the carbocation and
carboradical, respectively, from the dormant thioester
IBVE was consumed quantitatively without the consumption
of MA, indicating that only the cationic homopolymerization
of IBVE occurred (Supporting Information, Figure S1a). In
contrast, when V-70 was used, MA was consumed quantita-
tively along with the slower consumption of IBVE, the
conversion of which finally reached approximately 30%
(Figure S1b), thus indicating that radical copolymerization of
MA occurred with a small amount of IBVE. The polymers
obtained in both control experiments showed unimodal size-
exclusion chromatography (SEC) curves and controlled
molecular weights, which increased in direct proportion to
the conversion of the monomers and agreed well with the
calculated values, assuming that one molecule of 1 generates
one polymer chain (Figure S2a). Thus, the dormant CÀS bond
(
Scheme 2). The selection of components was based on the
following facts: the vinyl ether monomer can be homopoly-
merized only via a cationic intermediate, which cannot
of trithiocarbonate is suitable for both cationic and radical
reversible deactivation polymerization reactions when cata-
lyzed by a Lewis acid and a radical species, respectively; both
reactions proceed at similar rates under the optimized
conditions.
Next, both catalysts, EtAlCl2 and V-70, were used to
induce the simultaneous and quantitative consumption of
both monomers, with a slightly faster rate for IBVE than MA
Scheme 2. Mechanism of concurrent radical and cationic polymeri-
zation of acrylate and vinyl ether monomers through reversible
activation of the dormant thioester by radical species (RC) and Lewis
(Figure S1c). The consumption rates of IBVE and MA in the
acids (MtX ).
n
presence of both catalysts were almost the same as those of
IBVE with EtAlCl and MA with V-70. These kinetic results
2
polymerize acrylate monomers at all, while a radical inter-
mediate can efficiently homopolymerize acrylate monomers
and also copolymerize vinyl ether monomers to an extent.
Thioester compounds were extensively employed as rever-
sible addition–fragmentation chain-transfer (RAFT) agents
for controlling the radical polymerization of various vinyl
ether monomers, including acrylate monomers, through the
reversible activation of the dormant thioester by a radical
suggest that both the cationic and radical polymerization
reactions proceeded in the presence of both catalysts without
interference. Furthermore, the resulting polymers possessed
unimodal SEC curves and controlled molecular weights that
agreed well with the calculated values (Figure 2). Thus, the
dormant trithiocarbonate group is simultaneously activated
by both a Lewis acid and a radical species to induce
simultaneous living cationic and radical polymerizations
through the reversible deactivation process.
[
24]
species that originates from the radical initiator. Addition-
ally, a similar thioester can be reversibly activated into the
carbocationic species in the presence of a Lewis acid catalyst
to induce the living cationic homopolymerization of vinyl
1
13
The H and C NMR analyses of these products further
support the hypothesis that the concurrent living cationic and
radical polymerizations proceeded via the dual active inter-
[
11]
1
ether monomers, as we reported recently.
mediate. The H NMR spectrum of the polymers obtained by
We examined this strategy by using a trithiocarbonate (1
in Figure 1) capable of generating cationic and radical
using both catalysts (Figure S3c) is essentially a superposition
of the two spectra of the homopolymer of cationically
prepared IBVE (Figure S3a) and the copolymer of radically
prepared MA and IBVE (Figure S3b), thus indicating that the
copolymers obtained using both catalysts possess sequences
intermediates in conjunction with EtAlCl and 2,2’-azobis(4-
2
methoxy-2,4-dimethylvaleronitrile) (V-70), respectively, to
copolymerize an equimolar mixture of isobutyl vinyl ether
2
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Chemie
radically copolymerized IBVE units was lower with both
catalysts because of the simultaneous consumption of IBVE
by the cationic intermediate. As for the carbon atoms of the
vinyl ether units, the chemical shifts of the main-chain
methine carbon atoms of the IBVE units are distinctly
different for III (74 ppm) and MIM (76 ppm), which were
obtained for the cationic (Figure 3a) and radical propagations
(Figure 3b), respectively. Between the two absorptions, which
can also be observed for the polymers obtained using both
catalysts (Figure 3c), a new resonance appears at 75 ppm.
This signal can be assigned to another triad, either the IIM or
MII sequence, thus indicating that the intermediate is
converted from a cation to a radical or from a radical to
a cation in this triad because the II and IM (or MI) sequences
only form through cationic and radical propagations, respec-
tively.
Figure 2. Interconvertible polymerization of an equimolar mixture of
IBVE and MA in toluene at 208C. a) Number-average molecular weight
(
M ) and M /M (where M is the weight-average molecular weight)
n w n w
with respect to the total conversion of the monomers. b) Size-
exclusion chromatography curves of polymers obtained with EtAlCl2
and V-70 at varying monomer conversions in the polymerization.
Figure 4a shows the cumulative contents of the triad
comonomer sequences (MMM, IMM (MMI), IMI, III, IIM
[
IBVE] =[MA] =2.0m; [1] /[EtAlCl ] /[V-70] =40/2.5/10 mm;
0
0
0
2 0
0
[
EtOAc]
0
=1.0m.
(MII), and MIM), which were obtained by the integration of
these resonances against the monomer conversions. These
values were then employed to calculate the cumulative
contents of the cationic and radical propagations. As shown
in Figure 4b, the contents of the cationic propagations were
higher than the radical propagations in the early to middle
stages of polymerization, which is also supported by the
slightly faster consumption of IBVE than MA, as shown in
Figure S1c. However, the radical propagation finally became
larger because IBVE was radically copolymerized with MA.
More importantly, the number of interconversions per chain
that were polymerized via both cationic and radical inter-
mediates.
1
3
The C NMR spectra (Figures 3 and S4) give detailed
information regarding the monomer sequences. The carbonyl
carbon atoms of the MA units split into three resonances
because of the possible triad monomer sequences, that is,
MMM, IMM (MMI), and IMI (M = MA, I = IBVE), in which
MMM is higher with both catalysts (Figure 3c) than with V-70
alone (Figure 3b). This result indicates that the number of
(Nconv) was calculated using the content of IIM (MII) and the
number-average degree of polymerization (DP ) at each
n
conversion. As shown in Figure 3b, Nconv increased with the
monomer conversion, finally reaching 4.66 when DPn
increased to 96.9. This result indicates that interconversion
occurred approximately five times during 100 monomer
propagations per chain under typical conditions. As discussed
later, Nconv depends on many factors, including the dormant
species, Lewis acid, and radical initiator, as well as the
concentrations, temperature, and solvents. The number-aver-
1
3
Figure 3. C NMR spectra (in CDCl at 558C) of polymers obtained in
Figure 4. Interconvertible cationic and radical polymerization of IBVE and
MA. a) Triad monomer sequences of copolymers obtained in the intercon-
vertible radical and cationic polymerization in Figure 1 with respect to total
monomer conversion (where I and M mean IBVE and MA monomer units).
b) Cumulative contents of radical and cationic propagations and number of
interconversions per chain (Nconv) in the copolymerizations with respect to
total monomer conversion. [IBVE] =[MA] =2.0m; [1] /[EtAlCl ] /[V-
3
a) cationic, b) radical, and c) interconvertible polymerization of IBVE
and MA. [IBVE] =[MA] =2.0m; [1] =40 mm; [EtAlCl ] /[V-70] =2.5/0
a), 0/10 (b), 2.5/10 mm (c); [EtOAc] =1.0m; in toluene at 208C.
0
a) Conversion(IBVE/MA)=99%:2%, M (SEC)=5100, M /M =1.42.
b) Conversion(IBVE/MA)=31%:94%, M (SEC)=7300, M /M =1.29.
c) Conversion(IBVE/MA)= >99%:94%, M (SEC)=9600, M /
0
0
0
2 0
0
(
n
w
n
n
w
n
n
w
0
0
0
2 0
M =1.31.
n
70] =40/2.5/10 mm; [EtOAc] =1.0m; in toluene at 208C.
0 0
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age degree of propagation during one cycle of interconver-
sion, that is, the average segment length (n = 20.8), was also
A series of RAFT (R-SC(S)Z) agents consisting of the
same R (R = 2-isobutoxyethyl) and different Z (Z = 2-pyrro-
lidonyl (2), phenyl (3), ethoxy (4)) groups (Figure S11 and
S12) were also examined in the presence of both V-70 and
ZnCl2 under the same conditions. With 2 and 4, both
monomers were consumed at almost the same rate as with
calculated by DP /Nconv, and became slightly shorter as the
n
polymerization proceeded or as the monomer concentration
decreased (Figure S5).
The result of interconvertible copolymerization was
further supported by matrix-assisted laser desorption-ioniza-
tion time-of-flight mass spectrometry (MALDI-TOF-MS).
The signals of the copolymers obtained using both catalysts
1 (Figure S12). The M of the products increased in direct
n
proportion to the monomer conversion and agreed well with
the calculated values (Figure S13a). However, the MWDs
were slightly broader with 2 (M /M = 1.43) and were
(
Figure S6c) are separated by 14 Da, that is, the molar mass
w
n
difference between IBVE (100) and MA (86), thus indicating
that most of the polymer chains are copolymers. Additionally,
the spectral shape varies slightly for the copolymers obtained
only via the radical intermediate (Figure S6b), which differs
significantly from the result obtained for the homopolymer of
IBVE (Figure S6a). In the radical RAFT copolymerization
bimodal with 4 (Figure S13b), suggesting that the intercon-
version was slightly slower with 2 and significantly slower with
1
3
4. C NMR analyses also indicate that the IIM (MII) signal is
smaller with 2 and almost nonexistent with 4 (Figure S14a)
and that Nconv is smaller with 2 (3.06) and significantly smaller
with 4 (ꢀ 0.83) (Figure S14b). This result is due to the
electron-donating Z groups, which retard the addition of
radical species to the RAFT moiety, thereby resulting in slow
activation or interconversion. More specifically with 4, the
cationic and radical polymerization processes proceeded
separately, almost without transformation, resulting in bimo-
dal MWDs. Using another RAFTagent, 3, no consumption of
MA occurred (Figure S12c). With 3, no radical homopolyme-
rization of MA occurred, most likely because of the stable
intermediate derived from the dithiobenzoate-type RAFT
(
Figure S6b), a series of polymer chains with the same total
number of monomer units but a different number of each unit
type regularly appears in a mountain-like shape because of
[
28]
the nature of the statistical distribution, which is governed
by the reactivity ratios of the monomers in radical copoly-
merization. However, this statistical distribution is hampered
by the concurrent cationic propagation reaction, which results
in a flattened spectral pattern (Figure S6c).
The obtained polymers were also analyzed by high-
performance liquid chromatography (HPLC), in which the
solvent polarity was gradually increased by increasing the
ratio of THF to n-hexane, which were used as the eluent. The
homopolymer of IBVE obtained through cationic polymer-
ization (Figure S7a) eluted faster than the copolymer of MA
and IBVE obtained through radical copolymerization (Fig-
ure S7b) because of the lower polarity of IBVE relative to
that of MA. The copolymers obtained using both catalysts
eluted at almost the same or a slightly faster rate than the
copolymers obtained through radical copolymerization and
showed broader peaks (Figure S7c). Thus, most of the
products obtained when using both catalysts are copolymers
that contain a larger number of IBVE units than the products
obtained using only radical copolymerization because of the
concurrent cationic homopolymerization of IBVE. However,
a very small peak at a shorter elution time suggests a slight
amount of remaining homopolymer of IBVE, which was not
converted into radical polymerization.
[
24]
agent. Thus, the choices of RAFT agent and Lewis acid are
crucial for achieving successful interconvertible polymeri-
zation.
The interconversion number, that is, the average segment
length in the copolymers affects the thermal properties of the
resulting polymer (Figure 5). The copolymers with low Nconv
(3.06 and ꢀ 0.83) or high average segment lengths (n ꢁ 30)
showed two glass transition temperatures (T ), below À158C
g
and at approximately 58C, originating from T of poly(IBVE)
g
and poly(MA-stat-IBVE), respectively. However, with a high
Nconv (7.24) or low n (12.8), only one T was observed (À148C)
g
because of the presence of miscible short segments. Thus,
novel copolymers with tunable properties can be obtained
through interconvertible cationic and radical polymerizations.
The frequency of interconversion as well as the rate of
polymerization is dependent on the Lewis acid catalyst. When
the copolymerization was carried out with a weaker Lewis
[20]
acid, ZnCl2, instead of EtAlCl , the consumption of IBVE
2
became slower and closer to that of MA (Figure S8b). The
resulting products showed narrower molecular weight distri-
butions (MWDs) (M /M = 1.25) and controlled M , which
w
n
n
agreed well with the calculated values. Nconv increased to 7.24
(
(
Figure S9b), which is larger than that obtained with EtAlCl2
4.66, see above). The average segment length (n) was thus
calculated to be n = 12.8. With ZnCl , the interconversion
2
became more frequent, partially because of the reduced rate
of cationic polymerization. In contrast, another Lewis acid,
Figure 5. Differential scanning calorimetry (DSC) curves of polymers
obtained in the interconvertible cationic and radical polymerization of
IBVE and MA with various RAFT agents in the presence of both ZnCl2
and V-70 in toluene at 208C. [IBVE] =[MA] =2.0m; [RAFT
FeCl , inhibited the radical polymerization of MA, which
3
resulted in only a homopolymer of IBVE as a result of redox
activity (Figure S8c).
0
0
Agents] =40 mm; [ZnCl ] =2.5 mm, [V-70] =10 mm, [EtOAc] =1.0m.
0
2 0
0
0
4
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Another combination of monomers, IBVE and methyl
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a result of the lower radical copolymerizability of IBVE with
MMA. However, unimodal MWDs were attained at a higher
IBVE feed ratio (IBVE/MA = 3:1), which enhances the
relative formation of an IBVE chain end possessing both
radical and cationic reactivity. Thus, the selection of mono-
mers and conditions is important for the interconvertible
polymerization, and more detailed studies including other
combinations are now under way in our laboratory.
Herein, we demonstrated the first concurrent radical and
cationic polymerizations that proceed via interconvertible
dual active species for the production of multiblock copoly-
mers with controlled chain and segment lengths (Figure S16).
This method provides a new strategy for producing copoly-
mers comprised of different types of monomers that cannot
be copolymerized via a single intermediate.
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3
Received: June 25, 2014
Published online: && &&, &&&&
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Angewandte
Communications
Communications
Copolymerization
H. Aoshima, M. Uchiyama, K. Satoh,*
M. Kamigaito*
&&&& — &&&&
Interconvertible Living Radical and
Cationic Polymerization through
Reversible Activation of Dormant Species
with Dual Activity
Dr. Jekyll and Mr. Hyde: A dormant
the controlled synthesis of multiblock
copolymers comprised of different types
of segments that cannot be prepared with
a single intermediate.
thioester bond with dual activity is rever-
sibly activated by radicals and Lewis acids
to induce concurrent radical and cationic
polymerizations. This method enables
6
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