Oxygen-Triggered Switchable Polymerization for the One-Pot Synthesis of CO2-Based Block Copolymers from Monomer Mixtures

Article abstract of DOI:10.1002/anie.201906140

Switchable polymerization provides the opportunity to regulate polymer sequence and structure in a one-pot process from mixtures of monomers. Herein we report the use of O₂ as an external stimulus to switch the polymerization mechanism from the

Full text of DOI:10.1002/anie.201906140

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
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Accepted Article
Title: Oxygen Triggered Switchable Polymerization for One-Pot
Synthesis of CO2-Based Block Copolymers from Monomer
Mixtures
Authors: Yajun Zhao, Yong Wang, Xingping Zhou, Zhigang Xue,
Xianhong Wang, Xiaolin Xie, and Rinaldo Poli
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906140
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Oxygen Triggered Switchable Polymerization for One-Pot
Synthesis of CO
Mixtures
2
-Based Block Copolymers from Monomer
Yajun Zhao, [a] Yong Wang,* [a] Xingping Zhou, [a] Zhigang Xue, [a] Xianhong Wang, [b] Xiaolin Xie and
[a]
Rinaldo Poli[c]
Abstract: Switchable polymerization affords a unique opportunity to
stimulus-responsive functional group into the ligand as well as by
direct alternation of the oxidation state of the metal center.[ For
example, Diaconescu pioneered a redox switch that allowed the
one-pot synthesis of polylactide-b-polycaprolactone.[ Byers
demonstrated that controlling the oxidation state of Fe catalysts
by redox or electrochemical triggers enabled the facile
discrimination of lactide and epoxide in the ring-opening
4]
regulate polymer sequence and structure in a one-pot process from
2
mixtures of monomers. Here we report a strategy that uses O as an
5]
external stimulus to switch the polymerization mechanism from the
organometallic mediated radical polymerization (OMRP) of vinyl
III
monomers mediated by (Salen)Co -R [Salen=N,N’-bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanediamine; R = alkyl] to the ring-
[6]
opening copolymerization (ROCOP) of CO
2
/epoxides. A critical issue
polymerization (ROP). Long and coworkers have successfully
tailored the polyethylene branching via a redox switch.[ Recently,
Williams and Rieger achieved switchable ROCOP of
7]
is the unprecedented mono oxygen insertion into the Co-C bond, as
demonstrated by experimental results and rationalized by DFT
III
calculations, leading to the formation of (Salen)Co -O-R as an active
2
CO /anhydrides/epoxides and ROP of cyclic esters by controlling
species to initiate ROCOP. Diblock poly(vinyl acetate)-b-
the chemistry of metal-chain end-group, demonstrating the
possibility to switch the reactivity of a single-site catalyst between
different polymerization mechanisms [ However, the limitation is
that ROP and ROCOP share the metal-alkoxide active species.
polycarbonate could be obtained from ROCOP of CO
preactivation of (Salen)Co end-capped poly(vinyl acetate).
Furthermore, poly(vinyl acetate)-b-poly(methyl acrylate)-b-
polycarbonate triblock copolymer has been successfully synthesized
2
/epoxides with
8]
a
via a (Salen)cobalt-mediated sequential polymerization with an O
triggered switch in a one-pot process.
2
-
Introduction
In nature, feedback loops and a variety of trigger-induced effects
endow enzymes with necessary temporal and spatial control to
ensure highly-ordered biological activities through extremely
[1]
complex catalytic processes. Such smart activities have evolved
into an intriguing design principle for catalysis systems to simplify
the procedures for obtaining a high degree of structural
[
2]
complexities. Specifically, switchable catalysts that display
different polymerization activities in response to stimuli provide a
unique opportunity to regulate polymer sequence and architecture
from mixtures of monomers for fine tuning the material properties
[3]
at the molecular scale. Typically, modulating the polymerization
activity of a known catalyst is achieved by incorporation of
Scheme 1. Mechanistic aspects for ROCOP and OMRP mediated by (Salen)Co
complex. a Chain propagation process for cobalt(III) complex mediated ROCOP
[
a]
Y.J. Zhao, Dr. Y. Wang, Prof. Dr. X. P. Zhou, Prof. Dr. Z.G. Xue, X.
L. Xie.
2
of CO /epoxides. b Reversible termination (RT) and degenerative transfer (DT)
School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan 430074, P. R. China
E-mail: tcwy@mail.hust.edu.cn
Prof. Dr. X. H. Wang.
Key Laboratory of Polymer Ecomaterials
Changchun Institute of Applied Chemistry, CAS
Changchun 130022, P. R. China
mechanisms occurring in cobalt(II)/cobalt(III) complex mediated OMRP.
2 2
Derived from the ROCOP of CO /epoxides, CO -based
polycarbonates have been drawing increasing attention owing to
their biodegradable and biocompatible properties as well as to the
[
[
b]
c]
utilization and sustainable chemistry.[9]
stringent demand for CO
Although CO
2
2
-based polycarbonates may suffer from poor
Prof. Dr. R. Poli.
Laboratoire de Chimie de Coordination (LCC-CNRS)
Université de Toulouse, UPS, INPT
mechanical/thermal properties and limited functionalities, the
living nature of ROCOP in the presence of various metal
205, route de Narbonne, 31077 Toulouse (France)
2
complexes enables the facile synthesis of CO -based block
copolymers as a possible solution.[10] Current achievements have
Supporting information for this article is given via a link at the end of
the document.
been mainly focused on the sequential addition of epoxide
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Angewandte Chemie International Edition
10.1002/anie.201906140
RESEARCH ARTICLE
monomers to afford polycarbonate-b-polycarbonate or on the
preparation of polyester-b-polycarbonate by combining the
The first step of the polymer synthesis consisted of the OMRP of
II
VAc mediated by (Salen)Co , with initiation by AIBN at 60 °C in
bulk, following the protocol of Peng et al.[ The only difference
consisted in running the polymerization in an autoclave rather
than in regular glassware, in order to facilitate the one-pot switch
20]
/epoxides and the ROP of cyclic esters.[8]
ROCOP of CO
2
However, the possibility to switch polymerization mechanism
gives access to block copolymers with mechanistically
incompatible monomers, therefore providing facile access to a
wider array of high-value-added nanomaterials with attractive
phase separation behavior.[11] To address this, we have recently
2
to the ROCOP step, which requires work under CO pressure.
[
21]
This also allowed the reaction to occur under exclusion of light.
III
Various (Salen)Co -PVAc products were obtained under different
conditions (Table 1), confirming the results reported by Peng et al.
and also demonstrating the purely thermal nature of this OMRP
system. Indeed, comparative experiments conducted in ampules
without protection from light displayed faster polymerization rates
2
reported a one-step route to CO -based block copolymers with
tailorable functionalities and compositions by concurrent RAFT
polymerization of conjugated vinyl monomers and ROCOP of
/epoxides using a bifunctional chain transfer approach.[12] Yet, but inferior control (Table 1, entries 3-6). Moreover, higher AIBN
CO
new methods need to be developed to further expand the
structural diversity of CO -based block copolymers.
2
loading significantly accelerated the polymerization but gave
products with broader molecular weight distributions. An
2
II
[
AIBN]/[Co ] ratio of 7/1 could well balance the efficiency and
polymerization control (Figures S1-S2). The kinetic plots for the
OMRP displayed good linearity and an initiation period of 310 min,
which is typical for this system.[ Also, repeat experiments
20]
showed excellent reproducibility. Electrospray ionization (ESI)
III
mass analysis of low molecular weight (Salen)Co -PVAc (M
n
=
2.1 kDa, Đ = 1.05) was performed in methanol, wherein two main
series of signals were observed in accordance with
]Na+ and [CN(CH
)
2
C(VAc)
]H , respectively
+
[
CN(CH
3
)
2
C(VAc)
n
3
n
(
Figure. 1a). This suggests that the cyanoisopropyl radical
•
generated from AIBN [ C(CH
and generates (Salen)Co -(VAc) -C(CH CN dormant species
3
2
) CN] predominantly adds to VAc
III
n
3 2
)
(
Figure 2). In this respect, this controlling system differs from
II
(TMP)Co (TMP = tetramesityl porphyrin), which was shown by
•
Wayland et al. to abstract a β-H atom from C(CH
ultimately generate the (TMP)Co -(VAc) -H dormant species.
n
3
)
2
CN and
Scheme 2. O
synthesis of CO
2
-triggerd switch from cobalt mediated OMRP to ROCOP for the
-based diblock copolymers.
III
[22]
2
III
Among all the effective catalysts for ROCOP, (Salen)Co X (X =
Cl, OAc etc.) complexes, first employed by Coates, have
displayed the most ideal activities, wherein the reversible
formation and dissociation of polar Co-O bonds are crucial for the
[13]
chain propagation process (Scheme 1a).
Relative to Co-O
bonds, Co-C bonds are less polar and tend to break homolytically,
which is key to various enzymatic reactions mediated by vitamin
B
12.[14] Recently, cobalt complexes have allowed the well-
controlled organometallic mediated radical polymerization
OMRP) of challenging non-conjugated monomers such as vinyl
(
[15]
[16]
[17]
acetate (VAc),
vinyl chloride,
N-vinyl amides
and
[18]
vinylidene fluoride (VDF) (Scheme 1b). In particular, the radical
polymerization of VAc was shown to be well-controlled also when
mediated by the (Salen)Co system through a degenerative
II
transfer mechanism with AIBN initiation at 60 °C, leading to the
III
[19]
generation of (Salen)Co -PVAc as dormant species.
we report the switch of polymerization mechanism from OMRP to
ROCOP using O as a trigger and cobalt Salen complexes as
Herein,
2
mediators (Scheme 2). Key to this protocol is the unprecedented
III
mono oxygen insertion into Co -C bonds, which generates
III
(
Salen)Co -O-R as an active species for the ROCOP of
2
CO /epoxides.
III
Figure 1. Polymer chain end group analysis using ESI spectra. a (Salen)Co -
III
PVAc (Mn,GPC=2.1 kDa, Đ=1.05). b The acidolysis product of (Salen)Co -PVAc
2
(Mn,GPC=1.8 kDa, Đ=1.07) in the presence of 3.0 MPa O and acetic acid.
Results and Discussion
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Table 1. Polymerization of VAc mediated by (Salen)Co in bulk under different conditions.a
II
Entry
[AIBN]/[Co]
Time (min)
Conv.b (%)b
M
n,GPC (kDa)c
M
n,NMR (kDa)d
Đc
1
2
3e
4e
5
5/1
10/1
7/1
7/1
7/1
7/1
7/1
7/1
7/1
600
120
300
360
360
365
395
430
500
34
20
16
29
4
15.3
9.9
14.7
8.6
1.06
1.35
1.13
1.11
1.07
1.05
1.06
1.07
1.14
9.5
6.9
17.5
1.8
12.3
1.9
6
5
2.1
2.3
7f
8f
9
15
25
38
5.6
5.5
10.5
16.1
10.2
15.7
aReaction conditions: 60 oC in 5 mL VAc with [Co]/[VAc]=1/500. Conversion values detected by 1H NMR. cDetermined by gel permeation chromatography
b
GPC) with polystyrene as standard. Number-average molar mass calculated from H NMR spectra. eComparative reactions carried out in ampule, which
d
1
(
f
was exposed to natural light. Conducted in 20 mL VAc with [Co]/[VAc]=1/500.
To switch from the VAc OMRP to the CO
2
/epoxide ROCOP, a
component at 14.9 kDa and a minor one at 31.2 kDa (Figure 3a).
III
III
selective transformation of (Salen)Co -PVAc into (Salen)Co -O-
PVAc is needed. Although the ROCOP process catalyzed by an
alkoxide derivative of cobalt(III) appears unprecedented, metal
alkoxide complexes (L-M-OR) are generally regarded as the
active species when the reaction is initiated by related L-M-X
complexes (X = halides or carboxylic groups),[13b, 23] which are
preferred precatalysts given their lower tendency to decompose
by hydrolysis. During the initiation periods, nucleophilic attack by
X facilitates the ring-opening of an epoxide and results in the L-
M-OR active species.
Given the monomodal distribution and low Đ of the original
III
(Salen)Co -PVAc, we reasoned that a side process involving
some kind of irreversible radical coupling might have occurred. A
comparative reaction, carried out in the autoclave under 3.0 MPa
2
O , yielded a chemically similar polymer, except that the GPC
curve displayed this time a monomodal distribution (Đ = 1.15)
(Figure 3b). The radical nature of this reaction is indicated by an
experiment run in the presence of TEMPO [2,2,6,6-
tetramethylpiperidine-1-oxyl], leading only to the generation of
II
(Salen)Co (Figure S10) and TEMPO-capped PVAc (Figure S11).
III
Thus, the reaction appears to take place by initial Co -C bond
II
cleavage to produce (Salen)Co and PVAc• intermediates.
Figure 2. Plausible pathway for PVAc with cyanoisopropyl chain end group.
II
Reactions of the cyanoisopropyl radical with (Salen)Co and VAc.
Therefore, we have investigated the reaction of the (Salen)CoIII-
II
PVAc product of the (Salen)Co -mediated VAc polymerization (M
n
=
16.1 kDa, Đ = 1.09) with O
2
under different conditions. An initial
study with O
2
bubbling at atmospheric pressure in the presence
of acetic acid (HOAc) yielded a colorless polymer by precipitation
in hexane, confirmed to be PVAc by 1H NMR spectroscopy
(
Figure S4). The FT-IR investigation of this polymer revealed a
-1
broad absorption band around 3400 cm , indicative of the
substitution of the cobalt chain-end by an OH group (Figure S5).
This conclusion was confirmed by the ESI-MS spectrum (Figure
1
b), which exhibited two main series of signals in accordance with
OH]Na+ and [CN(CH
)
2
C(VAc)
OH]H . This
+
[
CN(CH
3
)
2
C(VAc)
n
3
n
result suggests the predominance of oxidation over acidolysis,
because the latter should yield PVAc-H. On the basis of the
III
III
Figure 3. GPC curves of (Salen)Co -PVAc (M
n,GPC
= 16.1 kDa, Đ = 1.14) (black)
known aptitude of (Salen)Co -OR to be readily hydrolyzed by
and of the corresponding oxidation/acidolysis product (red). a Comparison of
protonic reagents to yield ROH,[24] we propose that the observed
III
(
Salen)Co -PVA and the acidolysis product obtained with O
2
bubbling at
III
atmospheric pressure. b Comparison of (Salen)CoIII-PVA and the acidolysis
PVAc-OH results from the acidolysis of a (Salen)Co -O-PVAc
product obtained under a high O pressure (3.0 MPa).
2
intermediate by HOAc, with the concomitant formation of
III
(
Salen)Co -OAc (Figures S6-S7). Moreover, the GPC curve of
the polymer exhibited a bimodal distribution with a main
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Furthermore, an isotopic labeling experiment was conducted by
sphere. This was computationally verified for other similar metal-
alkyl systems.[34] Therefore, the thermodynamic bond strength
can be considered a reasonable lower limit approximation of the
kinetic activation barrier. This bond strength is much greater than
reaction of (Salen)Co -PVAc with 18O
III
in the presence of HOAc.
2
18
In this case, the hydrolysis product was confirmed as PVAc- OH
according to the ESI-MS spectrum (Figure S12), suggesting that
the single O inserted into the Co-C bonds comes from O
of other reagents.
2
instead
2
that of (acac) Co-CH(Me)OCOMe (recalculated as 9.2 kcal/mol at
25 °C at the same level of theory), in agreement with a more
difficult polymerization by reversible termination for the (Salen)Co
system. Using the Eyring equation and the bond strength at 60 °C
as an approximation of the activation barrier yields a rate of
homolytic bond cleavage of 3.4 h-1 and a half-life of 0.2 h (12 min).
On the basis of the above results, the two following points are
worthy of note: (i) the unexpected insertion of a single O atom
between Co and the polymer chain upon oxidation with O
increase of selectivity, relative to the chain-chain coupling side
products, upon increasing the pressure. In previous
2
; (ii) the
The
O
2
addition to the alkyl radical to generate
•OOCH(Me)OCOMe (Figure 4 Eq. (2)) is exoergic as expected
O
2
investigations of alkylcobalt(III) oxidation, using a variety of
ligands such as porphyrins,[25] dimethylglyoximates,[26] and other
substituted glyoximates,[27] alkylperoxocobalt(III) derivatives were
generated via a radical mechanism that consists of three steps
and can be considered quantitative in the presence of a high O
2
II
concentration. The O
2
addition to (Salen)Co , on the other hand,
is slightly endoergic, whether the adduct is calculated as a doublet
or as a quartet. Trapping of the resulting alkylperoxyl radical by
II
(Scheme 3): homolytic cleavage Eq. (1), dioxygen addition to the
(Salen)Co (Figure 4 Eq. (3)) is also exoergic, but only by 10.8
alkyl radical to generate a peroxyl radical Eq. (2) and radical
recombination Eq. (3). A few of these alkylperoxo derivatives,[25b,
kcal/mol (9.0 at 60 °C). This O-based radical trapping is another
example of bond formation by antiparallel combination of two S =
½ products with no electronic barrier. Thus, the formation of the
2
7b]
as well as others made by other methods,[28] have been
III
isolated and structurally characterized. To the best of our
(Salen)Co -OOCH(Me)-OCOMe derivative is not an irreversible
III
knowledge, the insertion of a single O atom from O
2
into a Co -C
process under the experimental conditions used for the oxidation.
Note that an alternative process for the alkyl radicals generated
by Eq (1) is dimerization (Figure 4 Eq. (4)), which is a model for
III
bond or the rearrangement of a Co -OOR derivative into the
III
corresponding Co -OR system is unprecedented.
•
PVAc termination by the combination. This process is calculated
as thermodynamically very favorable (irreversible), more than the
O
2
addition. Hence, this process may occur at low [O
Eq. (2) prevails over Eq. (4) at high [O ]. Dimerization by Eq. (4)
Figure S12) may explain at least in part the double molar mass
2
], whereas
2
(
III
Scheme 3. Mechanistic aspects referring to the formation of LCo -OOR via
dioxygen insertion into Co-C bonds.
distribution observed in the GPC trace of Figure 3a.
In order to throw light onto the above two points, a DFT
investigation was carried out using the short alkyl group -
CH(Me)OCOMe as model for the PVAc chain (see the
computational details in the SI). The results are shown in Figure
4
with the changes of each individual steps shown as ΔG° at 60 °C
in kcal/mol. Energy profiles for the entire transformation at 25 and
6
0 °C are available in the SI. The starting point of the investigation
III
is the structure of (Salen)Co -CH(Me)OCOMe and the sequence
of Eq (1), (2) and (3) in Scheme 3. The starting complex is
diamagnetic with a square pyramidal geometry, as experimentally
III
found in several (Salen)Co X derivatives, e.g. with X = Cl, Br or
I,[29] O-3,5-C
CONHR (R = CH(Me)Cy, CH
by the acetate carbonyl group, to yield a 6-coordinated isomer is
H
3
F
2
,[30] COOMe,[19b] C(=CH
)CH=CH
4 3
H
O).[32] Chelation
,[31] and
6
2
2
2
C≡CH, CH -2-C
2
endoergic, contrary to
what
was
found for the
III
[33]
bis(acetylacetonate) analogue, (acac)
2
Co -CH(Me)-OCOMe.
This is probably related to the cost of rearranging the Salen ligand
III
to a non-planar coordination environment. The homolytic Co -C
bond dissociation is 26.2 kcal/mol at 25 °C and 24.2 kcal/mol at
III
Figure 4. Mechanism for the oxidation of (Salen)Co -CH(Me)OCOMe to
(
60 °C. Processes resulting in bond cleavage without spin state
III
2
Salen)Co -OCH(Me)-OCOMe under O . The bold blue numbers represent
individual elementary steps. The bold black numbers are calculated standard
Gibbs energy changes (ΔG°) at 60 °C in kcal/mol. In the geometry-optimized
structures shown for starting and ending complexes, only the donor atoms of
the Salen ligand are shown for clarity.
change (the antiparallel combination of the two S = ½ products
yields an overall S = 0 system) have no electronic barrier for the
reverse recombination process and may only have a small steric
barrier related to the need to reorganize the metal coordination
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Angewandte Chemie International Edition
10.1002/anie.201906140
RESEARCH ARTICLE
/epoxides mediated by (Salen)Co -PVAc with pre-activation by O
2
.a
III
Table 2. ROCOP of CO
2
III
PPC yield/% e,f
PC yield/% e,g
TOF/h-1 h
n,GPC/kDa i
M
n,NMR/kDa j
Đ f
--
Entry
1b
Epoxide
PO
(Salen)Co -PVAc M
n
/kDa
Time/h
48
M
10.5
10.5
10.5
10.5
5.6
--
--
2
6
5
2
1
1
1
0
0
0
5
9
3
2
--
--
--
2c
PO
48
--
--
3d
PO
48
--
--
--
--
4
PO
24
10
10
12
10
19.5
9.4
12.5
12.2
20.4
10.6
14.2
13.8
1.11
1.09
1.16
1.15
5k
PO
12
6k
CHO
SO
5.6
24
7k
5.6
24
a
III
Reaction conditions: (Salen)Co -PVAc (from Table 1, entry 8) = 0.05 mmol, PO=3.5 mL, [Co]/[MTBD]/[epoxide] = 1/0.5/1000; pre-activated by 3.0 MPa O
2
at
o
at 30 oC. Comparative experiment by (Salen)Co -PVAc without pre-activation.
b
III
6
0
C for 12 h before the ROCOP was conducted under 3.0 MPa CO
Comparative experiment in the presence of AIBN, [Co]/[AIBN]=1/2. Comparative experiment in the presence of TEMPO, [Co]/[TEMPO]=1/2. Determined by
H NMR spectra. PPC = poly(propylene carbonate). GPC = propylene carbonate. Turnover frequency (TOF) = moles of product (PPC) per mole of catalyst
per hour. Number-average molar mass calculated from the H NMR spectra. Determined by GPC in THF, calibrated by polystyrene standards. (Salen)Co -
PVAc (from Table 1, entry 7) = 0.1 mmol, PO=3.5 mL, [Co]/[MTBD]/[epoxide] = 1/0.5/500.
2
c
d
e
1
f
g
h
i
1
j
k
III
Since the alkylperoxyl radical is not irreversibly trapped by Eq. (3),
it accumulates in the medium. At a certain point, reaction 5, which
is thermodynamically quite favorable, becomes possible. Its rate
processes. However, the competition between Eq. (5) and (3),
which is much in favor of the former at 60 °C, leads us to prefer
the scheme involving the sequence of reactions 1-2-5-6-8. In fact,
there is also a third possible pathway, which involves a
•
•
law is k
over reaction 2 (rate law: k
presumably lower activation barrier (as expected from
5
[ CH(Me)OCOMe][ OOCH(Me)-OCOMe]. It can prevail
[•CH(Me)OCOMe][O
]) because of a
bimolecular reaction between two PVAc-OO radicals to produce
•
2
2
•
two PVAc-O radicals and O
2
. This is equivalent to the sequence
Hammond’s
principle)
and
also
over
reaction
4
of Eq. (5) and (6) and would still be followed by trapping of the
[ CH(Me)OCOMe] ) because [•OOCH(Me)OCOMe] >>
•
2
product PVAc-O radical by the (salen)Co complex (Figure 4
reaction 8). However, the second order rate law and especially
the much lower calculated exergonicity of this reaction (ΔG = -
•
II
(k
4
[•
CH(Me)OCOMe]. Therefore, the peroxo product of Eq. (4) may
•
form quantitatively before all CH(Me)OCOMe is consumed by O
2
•
in reaction 2. Note, however, that reaction 2 has a slight degree
18.70 kcal/mol) vs. trapping by PVAc (Figure 4 reaction 5) makes
of reversibility at 60 °C (ΔG = -20.6 kcal/mol) and can therefore
us believe that the latter will be much faster (by application of
•
serve as a form of moderating equilibrium for the active CH(Me)-
Hammond’s postulate) and prevail for the transformation of 2
•
to 2 PVAc-O•.
OCOMe radical. In turn, the peroxide generated by Eq. (5) can
homolytically break in Eq. (6) to generate two alkoxyl radicals,
•OCH(Me)OCOMe, with a relatively low energy cost (ΔG = 7.4
kcal/mol at 60 °C). The latter can now be trapped either by an
PVAc + O
2
The most important contribution of the computational study is to
showthat the putative alkylperoxo complex, which is not observed
in this work but is amply documented in the literature for other
ligand systems, is thermally fragile and not thermodynamically
stable at high temperatures. It evolves to the more stable alkoxo
derivative in an environment where additional alkyl radicals are
present. In all the previous work dealing with alkylperoxo
•
additional CH(Me)OCOMe radical (Figure 4 Eq. (7)), to yield an
ether product, MeCOOCH(Me)-O-CH(Me)-OCOMe, in a very
favorable (irreversible) step, or by (Salen)CoII to yield the
III
observed (Salen)Co -O-CH(Me)-OCOMe by Eq. (8). Eq. (7) may
also be responsible for the generation of the double molar mass
III
polymer at low O
2
pressure.
derivatives of Co , these complexes were generated by either
ligand exchange using hydroperoxides, ROOH (no O atom
acceptors were present to produce alkoxo derivatives) or by
autoxidation of alkylcobalt(III) derivatives as in the present case,
but the processes were carried out at low temperatures where
trapping by Eq. (3) becomes favorable and allows completions of
the primary alkyl radical oxidation by Eq. (2).
It is also possible to envisage an alternative pathway leading to
III
the final (Salen)Co -O-CH(Me)OCOMe product, which involves a
homolytic O-O bond cleavage for the intermediate alkylperoxo
III
•
complex (reaction 9). This generates the (Salen)Co -O and
•OCH(Me)OCOMe radicals, and the process is completed by
reactions 10 and 8 as indicated in Figure 4. Indeed, there is
precedent for the O-O bond cleavage in alkylperoxo-cobalt(III)
compounds.[28b, 35] The calculations suggest that the metal
complex is a CoIII complex with an oxyl radical, rather than a
III
Finally, the (Salen)Co -O-PVAc product obtained by oxidation, in
III
the absence of acetic acid, of (Salen)Co -PVAc was tested as the
2
initiator for the ROCOP of CO /propylene oxide, targeting the
IV
Co =O complex: the system has lower energy as S = 3/2 with two
synthesis of PVAc-b-PPC diblock copolymers. The results are
III
unpaired electrons on Co and one on the O atom (spin densities
of 1.893 and 0.910, respectively). There is also an excited S = ½
state at 8.9 kcal/mol higher in G that can also be described as an
oxyl radical (spin densities of 0.161 on Co and 0.816 on O). The
O-O bond cleavage of Eq. (9) is quite facile at 60 °C (ΔG° = 15.4
kcal/mol) and is followed by thermodynamically favorable
collected in Table 2. When (Salen)Co -PVAc (Mn,GPC = 10.5 kDa,
Table 1, entry 8) and 0.5 equivalent of 7-methyl-1,5,7-
triazabicyclo[4.4.0]dec-5-ene (MTBD) (cocatalyst for ROCOP),
dissolved in propylene oxide (PO), were placed under 3.0 MPa of
CO
2
and allowed to stir at room temperature for 48 h, no
1
poly(propylene carbonate) (PPC) was detected by H NMR (Table
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Angewandte Chemie International Edition
10.1002/anie.201906140
RESEARCH ARTICLE
III
Figure 5. Synthesis of triblock copolymer. a Procedures for O
2
-triggered switchable polymerization from mixture of (Salen)Co -PVAc macro-initiator,
III
o
MA and PO under condition: [(Salen)Co -PVAc]/[MA]/[PO] =1/220/500, OMRP of MA within 10 h at 60 C, conv.=30%; O
2
-Activation within 12 h at 60
o
o
III
C under 3.0 MPa O
n
=5.5 kDa, Đ=1.09; (Salen)Co - PMA-b-PVAc, M
2
, ROCOP of PO/CO
2
within 8 h at 25 C under 3.0 MPa CO
2
, conv.=7.5%. b GPC traces of the (Salen)Co -PVAc macro-initiator,
III
M
n
=11.2 kDa, Đ=1.13; PPC-b-PMA-b-PVAc, M
n
=15.0 kDa, Đ=1.18.
2
, entry 1). Also, no polymerization was observed when, either
5b). In fact, this polymerization can be carried out in the presence
of propylene oxide (but absence of CO ) and only MA was
AIBN (Table 2, entry 2) or TEMPO (Table 2, entry 3) was
2
III
deliberately added to the (salen)Co -PVAc macroinitiator before
incorporated in the polymer. Starting from the mixture of
III
the O
2
-triggered activation phase. It is important to note that the
(Salen)Co -PVAc (Mn,GPC = 5.3 kDa, Table S1, entry 2), MA and
III
(Salen)Co -PVAc used as macroinitiator was isolated by
PO (Figure 5a), the first step of MA polymerization by OMRP gave
III
precipitation before the O
2
-triggered switch and subsequent
(salen)Co -PMA-b-PVAc. Subsequent
O
2
activation and
allowed the chain extension by ROCOP
with incorporation of CO /PO, while the residual MA from the
ROCOP, thus eliminating any residual AIBN. When a switch was
pressurization with CO
2
directly attempted starting from
a
one-pot mixture of
2
II
(Salen)Co /VAc/PO/AIBN/MTBD, the reaction only produced
previous step was no longer incorporated. This constitutes the
synthesis of a PPC-b-PMA-b-PVAc triblock copolymer with
monomodal and narrow molar mass distribution in a one-pot
1
PVAc and cyclic propylene carbonate (PC) according to the H
NMR spectrum of the reaction mixture (Figure S 34). Obviously,
the presence of these reagents somehow blocks the
1
procedure (Figure 5b) and the H DOSY NMR spectrum gave a
III
III
transformation of (salen)Co -PVAc to (salen)Co -O-PVAc.
single diffusion coefficient for all the PMA, PVAc and PPC
resonances, whereas three diffusion coefficients were observed
for a PMA/PVAc/PPC blend (Figures S32-33). This result
demonstrates that an efficient one-pot switch from OMRP to
III
However, a comparative reaction with (Salen)Co -PVAc pre-
2
activated by 3.0 MPa O led to a slight pressure drop (Table 2,
entry 4). The formation of PPC with a perfectly alternating
1
III
structure could be confirmed by H NMR spectrum of the reaction
ROCOP also occurs from a (salen)Co -capped PMA chain.
mixture, while the GPC curve exhibited a monomodal distribution
1
with a low Đ (Figure S15). The H DOSY NMR spectrum gave a
single diffusion coefficient for all the PVAc and PPC resonances, Conclusion
whereas two diffusion coefficients were observed for a PVAc/PPC
In summary, the first example of a switch from an organometallic
mediated radical polymerization of vinyl monomers to ring-
opening copolymerization of CO /epoxides has been described,
using (Salen)Co system as common moderating
agent/catalyst and O as a trigger for the mechanistic switch.
blend (Figures S16-17). We then evaluated the generality of the
ROCOP step using other epoxides including cyclohexene oxide
2
(CHO) and styrene oxide (SO) (Table 2, entries 6-7). In all cases,
a
a
block copolymers with a completely alternating structure for the
polycarbonate segments were obtained. It is notable that all the
produced polymers displayed low Đ. Typically, salen catalysts
2
Mechanistic aspects concerning the activation of the OMRP
III
dormant species (Salen)Co -R by O
2
have been reasonably
2
have been shown to be highly active for ROCOP of epoxides/CO ,
especially in the presence of active co-catalysts, like MTBD.[36]
proposed, as confirmed by experimental results and supported by
DFT calculations. The key to the switch lies in an unprecedented
single oxygen atom insertion into Co-C bonds, resulting in a
However, significant decreases of catalytic activity were observed
-1
in our experiments (TOF ranging from 2 to 10 h ), which is
attributed to the use of a macroinitiatior.[37]
III
ROCOP-active (Salen)Co -O-R species. Temporal control over
the polymerization processes has been demonstrated, allowing
for the facile synthesis of well-defined CO
copolymers in a one-pot procedure.
2
-based block
Synthesis of a PVAc-b-PMA-b-PPC triblock copolymer.
III
(Salen)Co -PVAc proved to be a viable macroinitiator for
thermally induced chain extension with methyl acrylate (MA) in
the absence of additional radical source (RT pathway), yielding a Experimental Section
III
(
salen)Co -capped PVAc-b-PMA copolymer with increased
Typical Procedure for OMRP of VAc in autoclave. In an argon filled
glove box, (Salen)Co (260 mg, 0.432 mmol), AIBN (124 mg, 0.756 mmol)
molecular weight and narrow molecular wright distribution (Fig.
II
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
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(
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Entry for the Table of Contents (Please choose one layout)
RESEARCH ARTICLE
[
a]
[a]
Yajun Zhao, Yong Wang,* Xingping
Zhou, Zhigang Xue,[a] Xianhong Wang,
Xiaolin Xie[a] and Rinaldo Poli[c]
[a]
[
b]
Page No. – Page No.
Oxygen Triggered
Switchable
Polymerization for One-Pot Synthesis
of CO -Based Block Copolymers from
Monomer Mixtures
2
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