Co(III)/Alkali-Metal(I) heterodinuclear catalysts for the ring-opening copolymerization of CO2and propylene oxide

Article abstract of DOI:10.1021/jacs.0c07980

The ring-opening copolymerization of carbon dioxide and propene oxide is a useful means to valorize waste into commercially attractive poly(propylene carbonate) (PPC) polyols. The reaction is limited by low catalytic activities, poor tolerance to a large excess of chain transfer agent, and tendency to form byproducts. Here, a series of new catalysts are reported that comprise heterodinuclear Co(III)/M(I) macrocyclic complexes (where M(I) = Group 1 metal). These catalysts show highly efficient production of PPC polyols, outstanding yields (turnover numbers), quantitative carbon dioxide uptake (>99%), and high selectivity for polyol formation (>95%). The most active, a Co(III)/K(I) complex, shows a turnover frequency of 800 h-1 at low catalyst loading (0.025 mol %, 70 °C, 30 bar CO2). The copolymerizations are well controlled and produce hydroxyl telechelic PPC with predictable molar masses and narrow dispersity (D < 1.15). The polymerization kinetics show a second order rate law, first order in both propylene oxide and catalyst concentrations, and zeroth order in CO2 pressure. An Eyring analysis, examining the effect of temperature on the propagation rate coefficient (kp), reveals the transition state barrier for polycarbonate formation: ΔG? = +92.6 ± 2.5 kJ mol-1. The Co(III)/K(I) catalyst is also highly active and selective in copolymerizations of other epoxides with carbon dioxide.

Full text of DOI:10.1021/jacs.0c07980

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Co(III)/Alkali-Metal(I) Heterodinuclear Catalysts for the Ring-
Opening Copolymerization of CO2 and Propylene Oxide
Arron C. Deacy, Emma Moreby, Andreas Phanopoulos, and Charlotte K. Williams*
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ABSTRACT: The ring-opening copolymerization of carbon dioxide and
propene oxide is a useful means to valorize waste into commercially
attractive poly(propylene carbonate) (PPC) polyols. The reaction is
limited by low catalytic activities, poor tolerance to a large excess of chain
transfer agent, and tendency to form byproducts. Here, a series of new
catalysts are reported that comprise heterodinuclear Co(III)/M(I)
macrocyclic complexes (where M(I) = Group 1 metal). These catalysts
show highly efficient production of PPC polyols, outstanding yields
(turnover numbers), quantitative carbon dioxide uptake (>99%), and high
selectivity for polyol formation (>95%). The most active, a Co(III)/K(I)
complex, shows a turnover frequency of 800 h−1 at low catalyst loading
(0.025 mol %, 70 °C, 30 bar CO2). The copolymerizations are well controlled and produce hydroxyl telechelic PPC with predictable
molar masses and narrow dispersity (Đ < 1.15). The polymerization kinetics show a second order rate law, first order in both
propylene oxide and catalyst concentrations, and zeroth order in CO2 pressure. An Eyring analysis, examining the effect of
temperature on the propagation rate coefficient (kp), reveals the transition state barrier for polycarbonate formation: ΔG‡ = +92.6
2.5 kJ mol−1. The Co(III)/K(I) catalyst is also highly active and selective in copolymerizations of other epoxides with carbon
dioxide.
used petrochemicals.9 High molar mass polycarbonates are also
INTRODUCTION
Carbon dioxide utilization is a grand challenge for contempo-
rary chemistry, and although several efficient reactions are
■
explored as toughened plastics, elastomers, and adhesives.10 In
terms of polyols, the use of propylene oxide as the epoxide is
particularly important because it is an inexpensive commodity
chemical already used at large-scale in polyol manufacture,11
and it delivers attractive material properties.4,6−8 While
tremendous advances have been made in high molar mass
PPC catalysis,12−24 the production of the desired low molar
mass polyols from propylene oxide and carbon dioxide is
challenging.4,25,26 One difficulty is that any catalyst must
tolerate a large excess of the protic starter or chain transfer
agent (CTA), typically a diol or diacid, essential to control the
chain end-group chemistry and ensure that polyols achieve the
correct molar mass, even at high epoxide conversions (Figure
1).4
known, few are viable or true utilizations when considered
more broadly.1,2 Many require esoteric, greenhouse gas
emitting, and expensive stoichiometric reagents, while others
yield products with no current large-scale application, or need
unacceptably high catalyst loadings or operate under
conditions, including carbon dioxide purity, incompatible
with scale-up.3 In contrast, epoxide and carbon dioxide ring-
opening copolymerization (ROCOP) is well suited for large-
scale deployment since it applies already commercial
monomers, enables significant carbon dioxide sequestration,
and produces valuable polymers whose properties allow for the
replacement of existing petrochemicals.3−5 This process can
sequester up to 50 wt % carbon dioxide into the polymer and it
is truly catalytic allowing for multiple turnovers and high
converson of epoxide.3−5 At the cutting-edge are carbon
dioxide and propylene oxide derived polyols which are low
molar mass, hydroxyl end-capped polypropylene carbonate
(PPC) or polyethercarbonates. These polyols are applied to
make polyurethanes to construct mattresses and furniture
foams, insulation sheet foam, coatings, sealants, and
elastomers.4,6−8 Life-cycle assessment shows that polyols with
just 20 wt % CO2 content (i.e., imperfectly alternating
polymers) display a ∼20% reduction in green-house gas
emissions and fossil fuel consumption compared to currently
Among the most active high molar mass PPC catalysts are
cobalt(III) salen complexes, [(salen)Co(III)X] where X =
carboxylate or halide, and these compounds are best applied
with an equimolar quantity of an ionic cocatalyst, typically
Received: August 3, 2020
https://dx.doi.org/10.1021/jacs.0c07980
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© XXXX American Chemical Society
A

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Figure 1. (a) PO/CO2 ROCOP to make poly(propylene carbonate) (PPC) polyols. (b) Illustration of one of the heterodinuclear catalysts used in
this investigation (2). (c) Catalytic cycle for PO/CO2 ROCOP illustrating both propagation and chain transfer reactions (epoxide ring-opening k1,
CO2-insertion k2 and chain-transfer k3).
bis(triphenyl phosphine)iminium chloride (PPNCl).12−21
Although this type of catalyst can be used to prepare polyols,
they usually show significantly compromised activity. For
example, [(salen)CoX]/PPNX shows an activity, TOF, of just
10 h−1 when applied with the added water (as CTA) necessary
to selectively produce polyols.26 Another problem is the use of
PPNX salts which are toxic, expensive, poorly soluble in many
epoxides, and may be corrosive to steel. As the active catalyst
comprises an ion pair, rapid deactivation occurs at the
necessary low catalyst loading, which is another detraction.14,15
To address these limitations, Nozaki and co-workers, and
deliver PCHC polyols.34−46 The most active are hetero-
dinuclear catalysts which show synergy in CHO/CO2
ROCOP.43−49 Nonetheless, so far these dinuclear catalysts
underperform in propylene oxide (PO)/carbon dioxide
ROCOP: they either scarcely turnover to PPC or produce
cyclic carbonate.34,38,43 Motivated by the heterodinuclear
synergy concept, this study targets heterodinuclear PO/CO2
ROCOP catalysts. The catalyst selection is underpinned by the
following guidelines: (1) The optimum internuclear separation
should be ∼3−4 Å to provide for intermetallic “cooperation” in
the ring-opening transition state and intermediate.35,40,45,50 (2)
The chain-shuttling mechanism, operative for other dinuclear
catalysts, is accelerated by trans- disposed coligands and chain
growth alternating between the two metals.34−36,38,43−45 (3)
For CHO/CO2 ROCOP, the most effective metal combina-
tions feature M(II)M′(II), where M = Co(II) and M′ =
Mg(II).43 Here, PO/CO2 ROCOP, catalysts of the form
M(III)/M′(I), where M = Co(III) and M′ = Na, K, Cs, Rb(I),
are proposed to exploit the high nucleophilicity of Co(III)-
propylene carbonate intermediates and the oxophilicity of
Group 1 metals toward epoxide coordination. Additionally,
Group 1 metals are attractive due to their abundance, low cost,
lack of color, and low toxicity. Heterodinuclear complex
formation is essential since prior work has established that
ROCOP using mixtures of [(salen)Co(III)X] complexes and
Group 1 crown ether additives yielded cyclic carbonates.13 To
make the heterodinuclear M(III)/M(I) complex, a macrocyclic
ligand featuring differentiated binding cavities was selected,
with a tetradentate Schiff base to coordinate the Co(III) and a
“crown-ether” moiety for M(I) (Figure 2). The ligand features
two bridging phenolate sites which could enhance electronic
communication between Co(III) and M(I), which may
increase electronic synergy in catalysis (Figure 1a).34,36,43
Related transition metal/M(I) heterodinuclear complexes are
successful magnetic resonances probes (Co(II)/Na(I)), CH
oxidation catalysts (Fe(III)/K(I)), and epoxidation catalysts
(Mn(III)/Na(I)).51−56
+
subsequently many others, reported modified [(salen[NR3 ])-
Co(III)X]X′ complexes, i.e., single component catalysts, where
the salen ligand is modified to include a covalently bonded
organic salt cocatalyst, most commonly an ammonium
salt.12,14,15,18 These catalysts show better turnover frequencies
for low molar mass PPC, for example, achieving a TOF of 48
h
−1 with excess methanol.18 Detailed optimization resulted in a
+
catalyst featuring four ammonium “arms”, [(salen[NBu3 ]4)-
Co(III)(OAc)](NO2 )4, that achieved an impressive TOF of
−
10 300 h−1 (0.001 mol % catalyst, 80 °C, 20 bar CO2) and
importantly is stable to copious quantities of CTA (400
equiv).27 Unfortunately, this catalyst is somewhat complex to
prepare, requiring >8 synthetic steps, and has an ill-defined
composition and unclear mechanism.28,29 Very recently, the
tethered cocatalyst strategy also significantly improved an
Al(III)-porphyrin catalyst, but for CO2/CHO ROCOP.30
Lewis acid−base pair catalysts, of the form Et3B/tetra-butyl-
ammonium carbonate (TBAC), also successfully yielded PPC
polyols, although with low TOF = 3 h−1 and requiring high
catalyst loading (7.5 mol % BEt3, 2.5 mol % TBAC, 40 °C, 10
bar).31 Recently, recycling of this catalyst system has been
reported with >95% catalyst recovery.32 The stratagem of
tethering the Lewis acid−base pair has significantly enhanced
rates for CO2/CHO ROCOP, but there are not yet reports of
these catalysts for PPC polyol synthesis.33
Homogeneous dinuclear metal ROCOP catalysts operate
without cocatalyst, show high activity and selectivity at low
carbon dioxide pressures for CHO/CO2 ROCOP, and retain
performances when using chain transfer agents to selectively
B
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Figure 2. (a) Synthesis of the heterodinuclear complexes 1−4. Reagents and conditions: (i) M(OAc) [M = Na, K, Rb, or Cs], Co(OAc)2, MeCN,
25 °C, 30 min, N2. (ii) Ethylenediamine, MeCN, 25 °C, 16 h, N2. (iii) AcOH (2 equiv), MeCN, air, 72 h, >50% yield. (b) ORTEP representation
of the molecular structures of complexes 1−3, with hydrogen atoms and residual solvents omitted for clarity, and thermal ellipsoids are represented
at 50% probability (see SI for experimental details).
hydrodynamic radius (6.66 Å) falls in the expected range for a
dimer. The change in complex nuclearity on descending Group
1 can be rationalized by increasing larger radii, e.g., of Rb(I)
and Cs(I), enabling access to higher coordination numbers and
facilitating complex aggregation.58
RESULTS AND DISCUSSION
■
Complexes 1−4 were synthesized from the macrocycle ligand
(SI for ligand synthesis) by the addition of an equivalent of the
appropriate alkali-metal(I) acetate, followed shortly after by
the addition of an equivalent of cobalt(II) acetate. The
reaction mixture was stirred for 30 min, at 25 °C, and then
ethylenediamine was added and the solution stirred for a
further 16 h. The resulting solution was exposed to air and two
equivalents of acetic acid added to oxidize the Co(II) center.
The resulting Co(III)/M(I) complexes were isolated in >50%
overall yield as pale brown solids. The complexes were
characterized by NMR spectroscopy (Figure S2−S10), mass
spectrometry (Figure S11−S13), IR spectroscopy (Figure
S14), elemental analysis, and where possible, by single crystal
X-ray diffraction (SI for details). For example, the NMR
spectrum of 1 shows the disappearance of the phenolic
protons, at ∼10.86 ppm in the free ligand, and the appearance
after complexation of both ethylene (1H: 4.31; 13C: 59.4 ppm)
and acetate (1H: 1.45; 13C: 179.5, 24.8 ppm) resonances,
respectively. The transformation of the aldehyde to imine
functional groups results in shifting of a singlet resonance from
The MALDI-ToF mass spectra display molecular ions, at
495, 511, and 604 amu, for complexes 1, 2, and 4, respectively,
and values correspond to the molecular cation, [LCo(II)M]+
(Figure S11−S13). The isotope distribution patterns match
those expected. Crystals suitable for single crystal X-ray
diffraction were obtained via vapor diffusion of pentane (1)
or diethyl ether (2) into a saturated solution of each complex
in dichloromethane. Structural elucidation confirmed the
desired heterodinuclear complexes (Figure 2b). Catalysts 1
and 2 are isostructural, as both are monomeric with an
octahedral cobalt(III) coordinated in the bis(iminophenol)
ligand cavity, and the alkali metal(I) coordinated in the crown
ether cavity. There is a measurable increase in Co−M
separation (M = Na; 3.388 Å, K; 3.698 Å) consistent with
the increase in radii. In each structure, one acetate ligand
bridges (κ2) between the Co(III) and M(I) and the other is
coordinated only to cobalt (κ1). The structures of complexes 1
and 2 are formally cobalt “ate” species; i.e., the Co(III) is
anionic while the M(I) is cationic.
Catalysts 1−4 were each tested in CO2/PO ROCOP, at 50
°C, using neat PO (14 M, 6 mL), 0.025 mol % catalyst, 20 bar
CO2 pressure and 0.5 mol % 1,2-cyclohexane diol (i.e., 20
equiv CTA vs catalyst) to deliver PPC polyols (Table 1).
Under these polyol synthesis conditions, all catalysts showed
very good activity and resulted in near quantitative CO2
uptake. The reactions were also highly selective with catalysts
2 and 3, in particular, showing very little cyclic carbonate
byproduct. In all cases, the poly(propylene carbonate) formed
without detectable ether linkages and with molar mass values
consistent with theoretical values and within the desired polyol
range. The PPC polyols show monomodal, narrow dispersity
distributions (Đ < 1.10) with excellent selectivity for hydroxyl
end-groups (vide infra).
1
9.94 to 7.70 ppm in the H NMR spectrum. During the
synthesis, both metals are added in short succession and yet
there is selective formation of the heterodinuclear complex.
This arises because the ligand binding “pockets” discriminate
between the metals’ coordination chemistry and ionic radii
(Co(II) = 0.75 Å, Na(I) = 1.1 Å, K(I) = 1.5 Å, Rb(I) = 1.6 Å
and Cs(I) = 1.7 Å vs binding sites N2O2 = 1.9 Å, 18C6 = 2.7
Å).57
The 2D DOSY NMR spectra show a single diffusion
coefficient for complexes 1−4, indicative of a single structure
in solution (Figure S10). The solution hydrodynamic radii are
5.14, 5.17, 5.65, and 6.66 Å for 1−4, respectively. For
complexes 1 and 2, values correlate well with those calculated
from the solid state structures (vide infra) and indicate
monomeric species. Complex 3 shows a smaller value (5.65 Å)
than that calculated for the dimeric solid (6.45 Å) and
correlates better to a “half-dimeric” structure (5.23 Å)
suggesting it is monomeric in solution. Although the solid
state structure of complex 4 was not obtained, its solution
Catalyst 2, i.e., Co(III)K(I), is the most active and has a
turnover frequency (TOF) of 340 h−1 at 50 °C (kp = 11.20
C
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a
Table 1. ROCOP of CO2/PO of Catalysts 1−4
time
(h)
conv.
b
CO2
c
polym.
TOF
kp
Mn [Đ]
h
d
e
f
g
#
Co(III)/M(I)
(%)
(%)
(%)
TON
600
(h−1
120
)
(mM−1 s−1
)
(g mol−1
)
1
2
3
4
5
6
7
Na = 1
K = 2
K = 2
K = 2
Rb = 3
Cs = 4
5.0
4.0
1.4
19.8
23
23
3.0
48
15
34
28
90
31
27
27
95
>99
>99
>99
>99
>99
>99
>99
>99
79
98
93
98
91
84
41
2.09
11.20
24.0
10.70
1.77
1.76
−
2300 [1.08]
5900 [1.10]
5800 [1.07]
8800 [1.04]
6500 [1.07]
5600 [1.08]
4700 [1.43]
7800 [1.06]
1360
1120
1800
1240
1080
540
340
800
91
54
47
i
j
k
13
,
[(salen)Co(2,4-DNP)]/18C6/KI
182
10
l
8 ,
26
[(salcy)Co(O2CCF3)]PPN(O2CCF3) (20 equiv
H2O)
[(salen[Pip+]2)Co(OAc)2] (20 equiv MeOH)
>99
475
−
m
18
,
9
20
1
95
10
>99
>99
96
>99
960
10 300
48
10 300
−
−
5100 [1.06]
2600 [1.05]
n
27
,
+
10
[(salen[NBu3 ]4)Co(OAc)](NO2)4 (400 equiv
adipic acid)
o
31
,
+
11
12
Et3B:[NBu4 ]2[O3C2−
]
14
30
95
64
91
75
95
98
37
1280
g/g
3
−
−
4100 [1.10]
1500 [1.10]
p
59
,
Zn-Co-DMCC (15 equiv sebacic acid)
43 g/g/h
a
b
Reaction conditions: Catalyst (0.025 mol %, 3.5 mM), PO (6 mL, 14 M), 1,2-cyclohexanediol (0.5 mol %, 70 mM), 20 bar CO2, 50 °C. PO
1
conversion determined from the relative integrals in the H NMR spectrum of PPC (4.92 ppm, 1H), PC (4.77 ppm, 1H), and PPO (3.46−3.64
c
ppm, 3H) using mesitylene as an internal standard (6.70 ppm). CO2 selectivity determined by the relative integrals in the 1H NMR spectrum of
d
PPC (4.92 ppm, 1H) and PC (4.77 ppm, 1H) compared with PPO (3.46−3.64 ppm, 3H). Polymer selectivity determined by the relative integrals
e
1
in the H NMR spectra of PPC (4.92 ppm, 1H) against PC (4.77 ppm, 1H). Turnover number (TON) = number of moles of PO consumed/
f
g
number of moles catalyst. Turnover frequency (TOF) = TON/time (h). kp = kobs/[cat]1; kobs determined as the gradient of the plot of ln[PO]t/
h
i
[PO]0 vs time. Determined by GPC analysis, in THF, calibrated with narrow-Mn polystyrene standards; dispersity values in parentheses. Catalyst
j
(0.025 mol %, 3.5 mM), PO (6 mL, 14 M), 1,2-cyclohexanediol (0.5 mol %, 70 mM), 30 bar CO2, 70 °C. Catalyst (0.05 mol %, 3.5 mM), PO (3
k
mL, 7 M), diethyl carbonate (3 mL), 1,2-cyclohexanediol (0.5 mol %, 70 mM), 20 bar CO2, 50 °C. Catalyst (0.05 mol %, 7.1 mM), PO (14 mL,
l
14 M), KI (0.05 mol %, 7.1 mM), 15 bar CO2, 25 °C. Catalyst (0.2 mol %, 10.0 mM), PO (0.5 mL, 4.6 M), toluene/chloroform (1 mL), PPNX
m
(0.2 mol %, 10.0 mM), H2O (2.0 mol %, 1 M), 15 bar CO2, 25 °C. Catalyst (0.1 mol %, 7.2 mM), PO (1 mL, 7 M), 1,2-dimethoxyethane (1
n
mL), methanol (1.0 mol %, 0.14 M), 14 bar CO2, 25 °C. Catalyst (0.001 mol %, 1.7 μM), PO (12 mL, 14 M), adipic acid (0.4 mol %, 0.68 M), 25
o
bar CO2, 75 °C. Catalyst (7.5 mol %, 0.25 M, 1 mL from a 1 M THF solution), tetra-butyl ammonium carbonate (TBAC) (2.5 mol %, 0.09 M),
p
PO (2 mL, 7 M), THF (1 mL), 10 bar CO2, 40 °C. Catalyst (50 mg), PO (100 mL, 14 M), sebacic acid (95 mmol, 0.95 M), 40 bar CO2, 50 °C.
For illustrations of the literature catalyst structures, see Figure S15.
mM−1 s−1, 98% PPC, 20 bar CO2) which is increased to an
impressive 800 h−1 at 70 °C (kp = 24.0 mM−1 s−1, 93% PPC,
30 bar CO2). Catalysts 1, 3, and 4 are all considerably less
active (kp ∼ 2.00 mM−1 s−1) and selective than 2.
is less effective in polyol synthesis, with 2 showing 8× higher
activity at 2× lower catalyst loading and delivering better
selectivity.18 Catalyst 2 is less active than the highly optimized
+
tetra-ammonium substituted catalyst [(salen[NBu3 ]4)Co-
The outstanding performance of the potassium hetero-
dinuclear complex, compared to the other Group 1 metals,
likely arises from a combination of carefully balanced metal
sizes and binding affinities. When the alkali metal is too small,
e.g., Na(I) in complex 1, it may be coordinatively saturated by
the macrocycle crown ether which hinders propylene oxide
coordination. When the alkali metal is too large, e.g., Rb(I) or
Cs(I) in complexes 3 or 4, coplanar metal coordination within
the macrocycle is no longer possible and aggregates form, as
indicated by the DOSY NMR data. Catalyst 2 also produced
PPC polyols with excellent productivity, reaching >90%
conversion of PO while maintaining high activity and
selectivity (Table 1, Entry 2). This data is significant since it
demonstrates the ability to use these catalysts to fully convert
the epoxide into the target PPC polyols.
Compared against other literature catalysts, the performance
of 2 stands out. For example, Lu and co-workers reported a
catalyst mixture of [(salen)Co(III)X] (X = 2,4-dinitropheno-
late) with an equimolar amount of 18-crown-6/KI (1:1) which
formed mostly cyclic carbonate (41% selectivity for PPC) with
only half the activity of 2 and without any polyol formation
(Table 1, Entry 7).13 Compared against the [(salen)Co(III)-
X]/PPNX system, applied under polyol formation conditions
where X = trifluoroactetate, catalyst 2 shows >30× higher
activity, at 10× lower loading.26 As mentioned in the
Introduction, the tethered catalyst [(salen[Pip+]2)Co(OAc)2]
(OAc)](NO2)4, but is substantially higher yielding compared
to the 10% PPC conversion reported.27 Beneficially, 2 is fully
characterized and does not contain any salts or anion mixtures.
Heterogeneous double metal cyanide catalysts are used
industrially to produce poly(propylene oxide-ran-propylene
carbonate) polyolsthey show excellent activity but much
lower carbon dioxide uptake (carbonate linkages <20%).
Unfortunately these heterogeneous catalysts also produce
significant quantities of cyclic carbonate byproducts (Table
1, Entry 12).60,61 Wang and co-workers reported conditions,
specifically at lower temperatures and using specific acidic
cocatalysts and starters, that increase carbon dioxide uptake (%
carbonate <75%). Nonetheless, these conditions impact
negatively upon catalyst activity (TOF = 43 g/g/h, 15 equiv
sebacic acid, 50 °C, 40 bar CO2).59 In comparison, 2 displays
an equivalent activity (35 g/g/h at 50 °C), but, unlike the
DMC catalysts, it may be applied at higher temperature
without loss of selectivity. Thus, at 70 °C, its activity increases
to 83 g/g/h without significant compromise in polymer
selectivity.
Using low loadings of catalyst 2, the molar mass of the
resulting PPC was easily controlled by varying the amount of
chain-transfer agent used (0−250 equiv, Table 2). When the
catalyst was applied without CTA, the resulting PPC showed
bimodal molar mass distributions attributed to chains which
are catalyst initiated (lower MW series) and chains initiated
D
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a
Table 2. Molecular Weight Control with Addition of Chain-Transfer Agent Using Catalyst 2
conv.
b
CO2
c
polym.
d
e
f
g
h
i
#
CTA
(%)
(%)
(%)
TON Mn exp CHD [OAc] (kg mol−1
)
Mn theoCHD + OAc (kg mol−1
)
CHD:OAc
Đ
j
1
2
3
4
5
6
7
0
5
21
58
46
52
63
69
37
>99
>99
>99
>99
>99
>99
>99
93
94
96
97
97
94
97
840
33.7 [14.4]
79.6 [38.9]
36.8 [15.4]
23.1 [10.7]
12.3
42.8
84.5
39.1
26.5
12.9
7.0
50:50
76:24
83:17
95:05
99
1.04 [1.10]
1.06 [1.05]
1.06 [1.08]
1.03 [1.03]
1.06
5800
4600
5200
6300
6900
3700
10
20
50
100
250
7.2
1.3
99
99
1.07
1.16
1.5
a
b
Reaction conditions: Catalyst (0.01 mol %, 1.9 mM), PO (15 mL, 14 M, 290 mmol), CTA = 1,2-cyclohexanediol, 20 bar CO2, 50 °C. PO
1
conversion determined from the relative integrals in the H NMR spectrum of PPC (4.92 ppm, 1H), PC (4.77 ppm, 1H), and PPO (3.46−3.64
c
ppm, 3H) using mesitylene as an internal standard (6.70 ppm). CO2 selectivity determined by the relative integrals in the 1H NMR spectrum of
d
PPC (4.92 ppm, 1H) and PC (4.77 ppm, 1H) compared with PPO (3.46−3.64 ppm, 3H). Polymer selectivity determined by the relative integrals
in the H NMR spectra of PPC (4.92 ppm, 1H) against PC (4.77 ppm, 1H). Turnover number (TON) = number of moles of PO consumed/
e
1
f
g
number of moles catalyst. Determined by GPC analysis, in THF, calibrated with narrow Mn polystyrene standards. Theoretical molar mass
h
determined by (TON × 102 g mol−1)/number of initiators (CHD + OAc). Determined by the relative ratios of the lower (OAc initiated) and
i
j
upper (CHD initiated) molar mass distributions observed in GPC analysis. Dispersity calculated by Mw/Mn. 0.025 mol % catalyst.
■
Figure 3. Polymerization data using catalyst 2 (Co(III)/K(I) for PO/CO2 ROCOP (Table S1). (a) Plot of PPC molar mass (Mn: ) and
−1
▲
dispersity (Đ: ) versus turnover number (TON). (b) Evolution of the PPC molar masses showing an increase in molar mass (g mol ) with
turnover number (TON) (note the low molar mass shoulder present in some cases arises from chains initiated from catalyst acetate groups). (c)
●
MALDI-ToF spectrum (1000−6000 m/z) of poly(propylene carbonate) initiated from cyclohexanediol ( ) and cyclohexane diol + one ether
■
linkage ( ). (d) Expanded region of the MALDI-ToF spectrum (4000−5000 m/z) showing both polymer distributions having a repeat unit of 102
g mol−1 consistent with the value expected for poly(propylene carbonate).
from 1,2-propane diol (upper series, note the diol evolves by
ring opening of epoxide and trace water as discussed
previously25). For polymerizations conducted with progres-
sively greater numbers of equivalents of CTA, the quantity of
catalyst initiated polymer chains decreases resulting in the
desired monomodal GPC traces when >50 equiv of CTA are
used. The molar mass of the resulting polymers are
progressively reduced as the quantity of CTA used increases,
allowing access to PPC with controllable Mn values spanning
79.6−1.3 kg mol−1 correlating well with theoretical values.
Catalyst 2 shows excellent absolute performance and so a
more detailed examination of polymerization control is
warranted. The PPC is regiorandom as indicated by
quantitative 13C{1H} NMR spectroscopy with a head-to-tail
proportion (HT) of 63% (Figure S16). A series of polymer-
izations conducted at systematically increasing catalyst
loadings, from 0.05 to 0.01 mol % (Table S1), showed fast
and highly selective catalysis in all cases and, as expected,
allowed for good control over PPC molar mass. Plots of PPC
molar mass vs turnover number (TON) are linear and there is
close agreement between experimental and theoretical values.
E
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Figure 4. Kinetic data and pathway for catalyst 2 in PO/CO2 ROCOP. (a) Semilogarithmic plot of ln[PO]t/[PO]0 versus time (Table 1, Entry 4).
(b) Plot of ln[kobs] vs ln[2], where [2] = 1.56−7.13 mM (Table S1). (c) Plot of kobs vs PCO from 5 to 30 bar. (d) Illustration of polymerization
2
pathway and rate-determining step. All errors are calculated from duplicate runs and there is an average error of 5% on all data.
Both findings are consistent with controlled polymerizations,
as are the observation that the molar mass distributions are
monomodal, with consistently narrow dispersity values (Figure
3b). MALDI-ToF data show two distributions, both
corresponding to PPC polyols and differing only in terms of
one ether linkage. End group analysis, using 31P{1H} NMR
spectroscopy, also confirms polyol formation (Figure 3c and d,
Figure S17).
One puzzle for the previously reported [(salen)Co(III)-
(X)]/PPNX catalysts is the complexity of the polymerization
rate law. Reaction orders in [Co(III)] are typically between 1
and 2 and the optimal [PPNCl] loading lies from 0.5 to 2 (vs
[Co]).13−15,29,62 Authors have speculated regarding mono-,
dinuclear, and ionic (i.e., off-metal) intermediates and
mechanisms to rationalize this data.13,14,16,17,22,29,45,62−65
Because the ionic cocatalyst performs multiple functions,
including nucleophilic attack, coordination, and stabilizing free
ionic polymer chains, it is very difficult to design better
catalysts.66
Given the impressive performance of 2, particularly as its
structure is not yet optimized, it is important to establish its
rate law and mechanism. To determine the order in propylene
oxide concentration, catalyst 2 was dissolved in a 50:50
mixture of PO:diethyl carbonate (total volume 6 mL) with a
resulting PO concentration of 7 M and heated to 50 °C, under
20 bar CO2 pressure. Using a ReactIR instrument the
concentration of PO vs time was evaluated and the data
plotted. In the early stages of the reaction (0−30% PO
conversion), the data shows a sigmoidal decrease in PO
concentration, consistent with slow initiation. The semi-
logarithmic plot shows a linear relationship (kobs = 3.82 ×
10−5 s−1, R2 = 0.9992) indicative of a first order in epoxide
concentration (Figure 4a). To determine the order in catalyst
concentration, a series of reactions were carried out in neat PO
(14 M), 20 bar CO2, at 50 °C using a range of concentrations
of catalyst 2 (1.56−7.13 mM) (Table S1). All reactions
afforded perfectly alternating PPC, without ether linkages, and
without any significant cyclic carbonate (<5%). Plotting the
logarithm of the observed rate coefficient vs the logarithm of
catalyst concentration shows a linear fit, with a gradient of 0.96
(R2 = 0.9526), indicating a first order in catalyst concentration.
The rate dependence on CO2 pressure was determined by
measuring the observed rate coefficient (kobs) over a range of
CO2 pressures from 5 to 30 bar, using 3.57 mM catalyst, neat
PO (14 M) and at 50 °C (Table S2). The plot of the rate
coefficient versus pressure (kobs vs PCO ) resulted in a slightly
2
curved fit to the data, but without significant rate changes over
the range 10−25 bar. Slightly lower rates were observed at 5
bar and this is attributed to the increased formation of cyclic
carbonate, perhaps resulting from the relatively low CO2
concentration in solution. At higher CO2 pressures (>20
bar) activity again slightly decreases likely due to gas
expansion, prior to the critical point, reducing the overall
catalyst and epoxide concentrations.67 Overall, the reaction is
proposed to operate via a second order rate law: first order in
both catalyst and epoxide concentrations.
Rate = [Cat]1[PO]1[CO2]0
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The kinetic data could be interpreted by a rate-determining
step (RDS) involving the ring-opening of the potassium-
coordinated epoxide by a cobalt(III)−carbonate intermediate
which would be in accordance with a dinuclear mechanism
(Figure. 4d, Figure S18). In the solid state complex 2 exhibits a
cobaltate structure, i.e., both acetate ligands are coordinated to
cobalt (Figure 2b). Accordingly it is hypothesized that
propylene oxide (PO) coordination occurs at the oxophilic
cationic potassium (Figure S18). In related Ni(II)/Na(I)
complexes an internal electric dipole is proposed to localize a
positive electric field at Na(I).55 By analogy, potassium may be
electrostatically biased toward epoxide coordination. The
mechanistic hypothesis involves propylene oxide coordination
being followed by cobaltate acetate nucleophilic attack, during
initiation, and by a cobaltate-carbonate intermediate, during
propagation. The PO ring-opening results in the formation of a
potassium coordinated alkoxide. Carbon dioxide insertion
(re)generates the putative cobaltate carbonate intermediate
and is proposed as the catalyst resting state. Chain propagation
involves both metals and is proposed to occur by a “chain-
shuttling” mechanism, akin to CHO/CO2 ROCOP by
dinuclear catalysts.34,36,38,44,45,47,48,65
Figure 5. Eyring plot, ln(kp/T) versus 1/T, for complex 2 over the
temperature range 40−70 °C, 3.57 mM catalyst, neat PO (6 mL), 1,2-
cyclohexene diol (71 mmol), under 20 bar CO2 (Table S3).
cyclohexene diol (71.4 mM), 50 °C, and 20 bar CO2. Catalyst
2 shows impressive activity for most other epoxides and in all
cases ensures high carbon dioxide uptake (>99%) without
ether linkage formation. In most cases, the selectivity for
polymer is very high (>95%) with only trace quantities of
cyclic carbonate (<5%). All the polycarbonate polyols show
molar mass values consistent with theoretical values and
monomodal distributions, with narrow dispersity (>1.30 Đ),
consistent with well-controlled polymerizations (Figure S21).
For example, ROCOP using 2-vinyl oxirane (vPO) is
important for postfunctionalization and cross-linking of the
resulting polycarbonate.72−75 Using catalyst 2, vPO/CO2
ROCOP was slightly slower than for PO, perhaps due to
oxirane deactivation by the allyl substituent. There are only a
few vPO/CO2 ROCOP catalysts, and compared with these, 2
shows outstanding performances. It shows equivalent activity
to the highly optimized quaternary ammonium [(salen)Co-
(III)(NR3)X′)] system (TOF = 40 h−1, 0.1 mol % cat, 30 bar
CO2, 40 °C).76,77 This finding demonstrates the significant
potential for future improvement to this new class of catalyst,
for example, by ligand structure−activity studies. ROCOP
using allyl glycidyl ether (AGE) was also successful and
illustrates the importance of comparing reactions according to
their polymerization rate coefficient (kp). Although the TOF
for PO ROCOP appears greater, the kp for AGE ROCOP is in
fact significantly greater (15.4 vs 11.2 mM−1 s−1). The
difference arises because the intrinsic molarity of PO is higher
than that of AGE (14.3 M vs 8.4 M). Therefore, after taking
into consideration the absolute epoxide concentrations, the
ROCOP of CO2/AGE is faster than CO2/PO. This result is
important since it demonstrates the potential to produce
unsaturated polycarbonates with even greater activity than
using PO, thereby providing an efficient future route to
polycarbonate amphiphiles or functionalized materials. Co-
polymerization of tert-butyl glycidyl ether (tBGE)/CO2 was
about half the activity when using AGE (219 h−1 vs 116 h−1)
with a slight reduction in polymer selectivity (cyclic carbonate
= 8 vs >1%). The differences may arise from different steric
profiles for the two monomers. Catalysts 2 displays comparable
To better understand the catalysis, the temperature
dependence of rate and selectivity were investigated (Table
S3). Polymerizations were conducted from 40 to 70 °C, using
[2] of 3.57 mM, in neat PO (14 M), at 20 bar CO2 pressure,
and using 20 equiv of 1,2-cyclohexane diol. All polymerizations
resulted in quantitative carbon dioxide uptake (>99%).
Increasing the temperature increased the catalytic activity,
from 134 to 834 h−1, but the polymer selectivity only slightly
decreased. The cyclic carbonate is proposed to form by chain
backbiting reactions and these may occur from the alkoxide
intermediate. The concentration of the alkoxide intermediate
likely increases with temperature due to decreased carbon
dioxide solubility. To test this hypothesis, a reaction was
conducted at elevated temperature (and otherwise identical
conditions) but with a greater CO2 pressure (30 bar): these
reaction conditions restore polymer selectivity to >90% while
maintaining very high catalytic activity (TOF = 800 h−1). All
polymerizations resulted in molar mass values consistent with
theoretical expectations and in monomodal distributions (Đ <
1.1), i.e., catalyst 2 showed very high control and selectivity for
polyol even at higher temperatures. Eyring analysis, i.e., a plot
of ln(kp/T) versus 1/T, determined the transition state
enthalpy, ΔH‡ = 56.0
1.8 kJ mol−1, and entropy, ΔS‡ =
−112.7 5.5 J mol−1 K−1 (Figure 5). Overall, the transition
state Gibbs free energy was determined as ΔG‡ = 92.6 2.5 kJ
mol−1 (50 °C). To compare with ROCOP catalysts, which are
often only analyzed by Arrhenius methods (Figure S19),
simple activation energy values (Ea) were also determined. For
PPC formation, 2 shows Ea = 58.9
1.8 kJ mol−1. Its
activation energy is lower than [(salen)Cr(III)(Cl)]/N-MeIm
(Ea = 67.6 kJ mol−1),68 but higher than [(salen)Co(X)]/
+
PPNCl or tethered [(salen[NR3 ])Co(X)2], where Ea = 29−35
kJ mol−1.69−71
Compared to M(II)M′(II) dinuclear catalysts, used in
CHO/CO2 ROCOP, it shows significantly lower Gibbs free
energy, ΔG = 95−110 kJ mol−1.40,43 Given the outstanding
performance of complex 2 in PO/CO2 ROCOP, it was useful
to observe that it was also an excellent catalyst for a range of
other epoxides in ROCOP with CO2 (Table 3, Figure S20). In
all cases, the same conditions were applied as for PO/CO2
ROCOP: 3.57 mM catalyst, neat epoxide (6 mL), 1,2-
activity to a [(salen)Co(III)(NR3 )(DNP)2] catalyst (119 h−1,
+
0.05 mol %, 40 °C, 15 bar CO2)78 Reaction of styrene oxide
SO/CO2 formed mostly styrene carbonate (91%), presumably
because the electron withdrawing phenyl substituent increases
the methine carbon’s electrophilicity and favors backbiting by
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a
Table 3. Monomer Scope Data for CO2/Epoxide ROCOP Using Catalyst 2
b
c
d
e
f
g
h
entry
epoxide
PO
time (h)
conv. (%)
CO2 (%)
polym. (%)
TON
TOF (h−1
)
kp (mM−1 s−1
)
Mn [Đ] (g mol−1
)
1
2
3
4
5
6
7
8
Acyclic
4.2
23.5
5.6
34
60
51
46
21
52
60
32
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
92
1352
2091
1220
908
340
89
219
116
23
631
408
162
11.2
5.9
15.4
7.4
5900 [1.10]
4100 [1.30]
3900 [1.28]
5100 [1.15]
n.a.
5900 [1.10]
9500 [1.08]
4200 [1.07]
vPO
AGE
tBGE
SO
7.9
22.3
2.3
3.2
9
511
n.a.
31.7
24.6
5.1
Cyclic
CHO
vCHO
CPO
>99
>99
95
1430
1285
1062
6.3
a
b
Reaction conditions: Catalyst 2 (3.57 mM), neat epoxide (6 mL), 1,2-cyclohexane diol (20 equiv vs 2), 20 bar CO2 and 50 °C. Epoxide
1
conversion determined by comparison of the relative integrals in the H NMR spectrum of PC, CC, and PE against mesitylene (6.59 ppm, 10
c
d
1
equiv). CO2 selectivity determined by comparison of the relative integrals in the H NMR spectrum for PC and CC against PE. Polymer
e
selectivity determined by comparison of the relative integrals in the 1H NMR spectrum for PC against CC. Turnover number (TON) = number of
f
g
moles of epoxide consumed/number of moles catalyst. Turnover frequency (TOF) = TON/time (h). kp = kobs/[cat]1; kobs determined as the
h
gradient of the plot of ln([Epoxide]t/[Epoxide]0) vs time, [2] = 3.57 mM. GPC analysis, in THF, calibrated using narrow-Mn polystyrene
standards, dispersity values in parentheses. Note that Figure S22 illustrates the leading literature catalysts for each epoxide.
the nucleophilic carbonate/alkoxide intermediates. Dare-
nsbourg and co-workers applied [(salen)Co(III)(X)]/PPNCl
catalysts for CO2/SO ROCOP and noted that only X= DNP
(2,4-dinitrophenolate) performed effectively (X = DNP 90%;
Cl = 58%; and Br = 5% PPC formation).79
(95%) (Table 2, Entry 8). The catalytic activity is high, TOF =
162 h−1, particularly given the low catalyst loading applied
(0.025 mol % cat, 50 °C, 20 bar CO2, neat CPO). Indeed the
activity of 2 is four times higher than the most active cobalt-
salen derivative which is applied at four times higher loading
(TOF = 42 h−1, 0.1 mol % cat, 50 °C, 20 bar CO2).88 It is also
double the activity of a chromium-salen derivative, again
applied at four times higher loading than 2, (TOF = 77 h−1, 0.1
mol % cat, 70 °C, 20 bar CO2).88 Overall, the heterodinuclear
Co(III)/K(I) catalyst shows outstanding performance using a
range of other epoxides in alternating copolymerizations with
carbon dioxide. We plan to continue to optimize the ligand and
metal selection so as to further increase performances.
In the future it may be possible to improve the selectivity of
catalyst 2 for polycarbonate by changing the acetate ligand or
by substituting 1,2-cyclohexene diol for 1-phenyl-1,2-ethane-
diol as the chain transfer agent. Cyclic epoxides generally
proceeded with a higher rate constant (kp) than acyclic
epoxides (Table 2). The release of ring-strain provides an
additional driving force, and 6-membered ring epoxides, e.g.,
cyclohexene oxide (CHO) or vinyl-cyclohexene oxide
(vCHO), are usually polymerized significantly faster than 5-
membered ring epoxides, such as cyclopentene oxide (CPO).
Catalyst 2 was highly active for CO2/CHO ROCOP and
showed a polymerization rate constant more than double that
of a previous high performance [Mg(II)Co(II)] heterodinu-
clear catalyst (2, kp = 31.7 mM−1 s−1 (50 °C) versus
[Co(II)Mg(II)], kp = 15.1 mM−1 s−1(60 °C))43 and achieves
comparable activity to the quaternary ammonium cobalt
catalyst systems (TOF = 1018 h−1, catalyst = 0.02 mol %, 50
°C, 20 bar CO2).80 Its overall performance in CHO/CO2
ROCOP is better than the widely investigated β-diiminate di-
Zn-catalyst (TOF = 2290 h−1, 50 °C, 0.1 mol % cat, 69 bar
CO2, 90% PCHC formation), as it is applied at 4× lower
catalyst loading.81 It is significantly less active than the tethered
β-diiminate di-Zn catalysts, reported by Rieger and co-workers,
which delivers poly(cyclohexene carbonate) with ∼80%
carbonate linkages only (155 000 h−1, 0.0125 mol %, 100
°C, 30 bar CO2) although 2 is tested at significantly lower
temperature preventing easy comparison.82 It is also important
to note that these optimized di-Zn catalysts show negligible
activity for propylene oxide/carbon dioxide copolymerization
(1 h−1, 0.025 mol %, 60 °C, 23% polymer).83
CONCLUSION
■
In conclusion, a new class of heterodinuclear Co(III)/M(I)
complexes were highly active, selective, and controlled catalysts
for propylene oxide/carbon dioxide ring-opening copolymer-
izations. The complexes demonstrate the power of asym-
metrical macrocycle ligands to bias the coordination chemistry
and reactivity to deliver highly effective catalysis. These
complexes have significant potential for future optimization,
including through modifications of the substituents, metal
combinations, and coligand chemistries. Since this is the first
demonstration of how to exploit Group 1 metals in this
catalysis, further investigation into the power of these
inexpensive, colorless, and abundant metals for carbon dioxide
utilization catalysis is certainly warranted. Group 1 metals offer
tremendous potential in terms of operability and scale-up, and
the Co(III)/K(I) catalyst challenged the performance of the
status-quo tethered catalyst systems which rely on expensive
salts, like PPNCl. The best catalyst, Co(III)/K(I), delivered
high polymerization conversions, excellent activity, and an
impressive operating temperature window (25−70 °C). The
activity reached values of TOF = 800 h−1, and it was applied at
useful, low loading (0.025 mol %). It was also highly active for
a range of other epoxides, including those substituted with
alkene groups. Kinetic analyses showed an overall second order
rate law and implicate a chain shuttling dinuclear mechanism.
The catalysts warrant future investigation for related polymer-
izations, such as lactone ring-opening polymerizations, switch
polymerization catalysis, and epoxide/anhydride ROCOP. The
beneficial combination of Co(III) and Group 1 metals could
also accelerate other carbon dioxide, and heterocumulene,
ROCOP using functionalized vinyl-cyclohexene oxide
(vCHO)/CO2 is also highly active and selective; the product
polycarbonate undergoes postfunctionalization, cross-linking
and network formation.84,85 The CO2/cyclopentene oxide
(CPO) ROCOP forms a polycarbonate which under certain
conditions can be depolymerized to epoxide, rather than to
cyclic carbonate, thus providing a future chemical recycling
route.86,87 Using catalyst 2 for the ROCOP of CPO/CO2
resulted in polycarbonate formation with excellent selectivity
H
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of Thermoplastic Polyurethane for Corrosion-Resistant Metal Coat-
ing. ACS Sustainable Chem. Eng. 2017, 5 (5), 3871−3881.
(7) Langanke, J.; Wolf, A.; Hofmann, J.; Bohm, K.; Subhani, M. A.;
Muller, T. E.; Leitner, W.; Gurtler, C. Carbon dioxide (CO2) as
sustainable feedstock for polyurethane production. Green Chem. 2014,
16 (4), 1865−1870.
transformations, including production of heterocycles, N-
containing polymers, polyesters, and even carbon dioxide
reduction products.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c07980.
■
sı
(8) Lee, S. H.; Cyriac, A.; Jeon, J. Y.; Lee, B. Y. Preparation of
thermoplastic polyurethanes using in situ generated poly(propylene
carbonate)-diols. Polym. Chem. 2012, 3 (5), 1215−1220.
(9) von der Assen, N.; Bardow, A. Life cycle assessment of polyols
for polyurethane production using CO2 as feedstock: insights from an
industrial case study. Green Chem. 2014, 16 (6), 3272−3280.
Characterization data of catalysts (1D and 2D NMR
spectroscopy, mass spectrometry, infrared spectroscopy,
and X-ray crystallography); Polymerization kinetics and
characterization (NMR spectroscopy and gel permeation
chromatograms) (PDF)
(10) Sulley, G. S.; Gregory, G. L.; Chen, T. T. D.; Pena
̃
Carrodeguas, L.; Trott, G.; Santmarti, A.; Lee, K.-Y.; Terrill, N. J.;
Williams, C. K. Switchable Catalysis Improves the Properties of CO2-
Derived Polymers: Poly(cyclohexene carbonate-b-ε-decalactone-b-
cyclohexene carbonate) Adhesives, Elastomers, and Toughened
Plastics. J. Am. Chem. Soc. 2020, 142 (9), 4367−4378.
(11) Leow, W. R.; Lum, Y.; Ozden, A.; Wang, Y.; Nam, D.-H.; Chen,
B.; Wicks, J.; Zhuang, T.-T.; Li, F.; Sinton, D.; Sargent, E. H.
Chloride-mediated selective electrosynthesis of ethylene and
propylene oxides at high current density. Science 2020, 368 (6496),
1228−1233.
Crystal data (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Charlotte K. Williams − Department of Chemistry, Chemistry
Research Laboratory, University of Oxford, Oxford OX1 3TA,
U.K.; orcid.org/0000-0002-0734-1575;
(12) Lu, X. B.; Ren, W. M.; Wu, G. P. CO2 Copolymers from
Epoxides: Catalyst Activity, Product Selectivity, and Stereochemistry
Control. Acc. Chem. Res. 2012, 45 (10), 1721−1735.
(13) Lu, X. B.; Shi, L.; Wang, Y. M.; Zhang, R.; Zhang, Y. J.; Peng, X.
J.; Zhang, Z. C.; Li, B. Design of highly active binary catalyst systems
for CO2/epoxide copolymerization: Polymer selectivity, enantiose-
lectivity, and stereochemistry control. J. Am. Chem. Soc. 2006, 128
(5), 1664−1674.
(14) Liu, J.; Ren, W. M.; Liu, Y.; Lu, X. B. Kinetic Study on the
Coupling of CO2 and Epoxides Catalyzed by Co(III) Complex with
an Inter- or Intramolecular Nucleophilic Cocatalyst. Macromolecules
2013, 46 (4), 1343−1349.
Email: charlotte.williams@chem.ox.ac.uk
Authors
Arron C. Deacy − Department of Chemistry, Chemistry
Research Laboratory, University of Oxford, Oxford OX1 3TA,
U.K.
Emma Moreby − Department of Chemistry, Chemistry Research
Laboratory, University of Oxford, Oxford OX1 3TA, U.K.
Andreas Phanopoulos − Department of Chemistry, Chemistry
Research Laboratory, University of Oxford, Oxford OX1 3TA,
U.K.
(15) Noh, E. K.; Na, S. J.; Sujith, S.; Kim, S. W.; Lee, B. Y. Two
components in a molecule: Highly efficient and thermally robust
catalytic system for CO2/Epoxide copolymerization. J. Am. Chem. Soc.
2007, 129 (26), 8082−8083.
(16) Nakano, K.; Kobayashi, K.; Nozaki, K. Tetravalent Metal
Complexes as a New Family of Catalysts for Copolymerization of
Epoxides with Carbon Dioxide. J. Am. Chem. Soc. 2011, 133 (28),
10720−10723.
(17) Nakano, K.; Hashimoto, S.; Nozaki, K. Bimetallic mechanism
operating in the copolymerization of propylene oxide with carbon
dioxide catalyzed by cobalt-salen complexes. Chem. Sci. 2010, 1 (3),
369−373.
(18) Nakano, K.; Kamada, T.; Nozaki, K. Selective formation of
polycarbonate over cyclic carbonate: Copolymerization of epoxides
with carbon dioxide catalyzed by a cobalt(III) complex with a
piperidinium end-capping arm. Angew. Chem., Int. Ed. 2006, 45 (43),
7274−7277.
(19) Cohen, C. T.; Chu, T.; Coates, G. W. Cobalt catalysts for the
alternating copolymerization of propylene oxide and carbon dioxide:
Combining high activity and selectivity. J. Am. Chem. Soc. 2005, 127
(31), 10869−10878.
(20) Qin, Z. Q.; Thomas, C. M.; Lee, S.; Coates, G. W. Cobalt-based
complexes for the copolymerization of propylene oxide and CO2:
Active and selective catalysts for polycarbonate synthesis. Angew.
Chem., Int. Ed. 2003, 42 (44), 5484−5487.
(21) Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W.
High-activity, single-site catalysts for the alternating copolymerization
of CO2 and propylene oxide. J. Am. Chem. Soc. 2002, 124 (48),
14284−14285.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c07980
Notes
The authors declare the following competing financial
interest(s): CKW is a director of Econic Technologies, Ltd.
ACKNOWLEDGMENTS
■
The Engineering and Physical Sciences Research Council (EP/
S018603/1; EP/R027129/1) and the Oxford Martin School
(Future of Plastics) are acknowledged for research funding.
REFERENCES
■
(1) Burkart, M. D.; Hazari, N.; Tway, C. L.; Zeitler, E. L.
Opportunities and Challenges for Catalysis in Carbon Dioxide
Utilization. ACS Catal. 2019, 9 (9), 7937−7956.
(2) Artz, J.; Muller, T. E.; Thenert, K.; Kleinekorte, J.; Meys, R.;
̈
Sternberg, A.; Bardow, A.; Leitner, W. Sustainable Conversion of
Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle
Assessment. Chem. Rev. 2018, 118 (2), 434−504.
(3) Hepburn, C.; Adlen, E.; Beddington, J.; Carter, E. A.; Fuss, S.;
Mac Dowell, N.; Minx, J. C.; Smith, P.; Williams, C. K. The
technological and economic prospects for CO2 utilization and
removal. Nature 2019, 575 (7781), 87−97.
(4) Darensbourg, D. J. Chain transfer agents utilized in epoxide and
CO2 copolymerization processes. Green Chem. 2019, 21 (9), 2214−
2223.
(5) Wang, Y.; Darensbourg, D. J. Carbon dioxide-based functional
polycarbonates: Metal catalyzed copolymerization of CO2 and
epoxides. Coord. Chem. Rev. 2018, 372, 85−100.
(6) Alagi, P.; Ghorpade, R.; Choi, Y. J.; Patil, U.; Kim, I.; Baik, J. H.;
Hong, S. C. Carbon Dioxide-Based Polyols as Sustainable Feedstock
(22) Vagin, S. I.; Reichardt, R.; Klaus, S.; Rieger, B. Conformation-
ally Flexible Dimeric Salphen Complexes for Bifunctional Catalysis. J.
Am. Chem. Soc. 2010, 132 (41), 14367−14369.
(23) Zhang, D. Y.; Boopathi, S. K.; Hadjichristidis, N.; Gnanou, Y.;
Feng, X. S. Metal-Free Alternating Copolymerization of CO2 with
I
https://dx.doi.org/10.1021/jacs.0c07980
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Epoxides: Fulfilling ″Green″ Synthesis and Activity. J. Am. Chem. Soc.
2016, 138 (35), 11117−11120.
(40) Jutz, F.; Buchard, A.; Kember, M. R.; Fredriksen, S. B.;
Williams, C. K. Mechanistic Investigation and Reaction Kinetics of
the Low-Pressure Copolymerization of Cyclohexene Oxide and
Carbon Dioxide Catalyzed by a Dizinc Complex. J. Am. Chem. Soc.
2011, 133 (43), 17395−17405.
(24) Jia, M. C.; Hadjichristidis, N.; Gnanou, Y.; Feng, X. S.
Monomodal Ultrahigh-Molar-Mass Polycarbonate Homopolymers
and Diblock Copolymers by Anionic Copolymerization of Epoxides
with CO2. ACS Macro Lett. 2019, 8 (12), 1594−1598.
(25) Wu, G.-P.; Darensbourg, D. J. Mechanistic Insights into Water-
Mediated Tandem Catalysis of Metal-Coordination CO2/Epoxide
Copolymerization and Organocatalytic Ring-Opening Polymerization:
One-Pot, Two Steps, and Three Catalysis Cycles for Triblock
Copolymers Synthesis. Macromolecules 2016, 49 (3), 807−814.
(26) Darensbourg, D. J.; Wu, G. P. A One-Pot Synthesis of a
Triblock Copolymer from Propylene Oxide/Carbon Dioxide and
Lactide: Intermediacy of Polyol Initiators. Angew. Chem., Int. Ed.
2013, 52 (40), 10602−10606.
(41) Buchard, A.; Kember, M. R.; Sandeman, K. G.; Williams, C. K.
A bimetallic iron(III) catalyst for CO2/epoxide coupling. Chem.
Commun. 2011, 47 (1), 212−214.
(42) Kember, M. R.; Knight, P. D.; Reung, P. T. R.; Williams, C. K.
Highly Active Dizinc Catalyst for the Copolymerization of Carbon
Dioxide and Cyclohexene Oxide at One Atmosphere Pressure. Angew.
Chem., Int. Ed. 2009, 48 (5), 931−933.
(43) Deacy, A. C.; Kilpatrick, A. F. R.; Regoutz, A.; Williams, C. K.
Understanding metal synergy in heterodinuclear catalysts for the
copolymerization of CO2 and epoxides. Nat. Chem. 2020, 12 (4),
372−380.
(44) Trott, G.; Saini, P. K.; Williams, C. K. Catalysts for CO2/
Epoxide Ring-Opening Copolymerization. Philos. Trans. R. Soc., A
2016, 374 (2061), 20150085.
(45) Buchard, A.; Jutz, F.; Kember, M. R.; White, A. J. P.; Rzepa, H.
S.; Williams, C. K. Experimental and Computational Investigation of
the Mechanism of Carbon Dioxide/Cyclohexene Oxide Copolymer-
ization Using a Dizinc Catalyst. Macromolecules 2012, 45 (17), 6781−
6795.
(46) Kember, M. R.; White, A. J. P.; Williams, C. K. Highly Active
Di- and Trimetallic Cobalt Catalysts for the Copolymerization of
CHO and CO2 at Atmospheric Pressure. Macromolecules 2010, 43
(5), 2291−2298.
(47) Asaba, H.; Iwasaki, T.; Hatazawa, M.; Deng, J.; Nagae, H.;
Mashima, K.; Nozaki, K. Alternating Copolymerization of CO2 and
Cyclohexene Oxide Catalyzed by Cobalt−Lanthanide Mixed Multi-
nuclear Complexes. Inorg. Chem. 2020, 59 (12), 7928−7933.
(48) Nagae, H.; Aoki, R.; Akutagawa, S.; Kleemann, J.; Tagawa, R.;
Schindler, T.; Choi, G.; Spaniol, T. P.; Tsurugi, H.; Okuda, J.;
Mashima, K. Lanthanide Complexes Supported by a Trizinc Crown
Ether as Catalysts for Alternating Copolymerization of Epoxide and
CO2: Telomerization Controlled by Carboxylate Anions. Angew.
Chem., Int. Ed. 2018, 57 (9), 2492−2496.
(49) Deacy, A. C.; Durr, C. B.; Williams, C. K. Heterodinuclear
complexes featuring Zn(ii) and M = Al(iii), Ga(iii) or In(iii) for
cyclohexene oxide and CO2 copolymerisation. Dalton Trans. 2020, 49
(1), 223−231.
(50) Klaus, S.; Lehenmeier, M. W.; Herdtweck, E.; Deglmann, P.;
Ott, A. K.; Rieger, B. Mechanistic Insights into Heterogeneous Zinc
Dicarboxylates and Theoretical Considerations for CO2−Epoxide
Copolymerization. J. Am. Chem. Soc. 2011, 133 (33), 13151−13161.
(51) Du, K.; Thorarinsdottir, A. E.; Harris, T. D. Selective Binding
and Quantitation of Calcium with a Cobalt-Based Magnetic
Resonance Probe. J. Am. Chem. Soc. 2019, 141 (17), 7163−7172.
(52) Chantarojsiri, T.; Ziller, J. W.; Yang, J. Y. Incorporation of
redox-inactive cations promotes iron catalyzed aerobic C−H
oxidation at mild potentials. Chem. Sci. 2018, 9 (9), 2567−2574.
(53) Martinez, A.; Hemmert, C.; Gornitzka, H.; Meunier, B.
Synthesis and activity of macrocyclized chiral Mn(III)−Schiff-base
epoxidation catalysts. J. Organomet. Chem. 2005, 690 (9), 2163−2171.
(54) Reath, A. H.; Ziller, J. W.; Tsay, C.; Ryan, A. J.; Yang, J. Y.
Redox Potential and Electronic Structure Effects of Proximal
Nonredox Active Cations in Cobalt Schiff Base Complexes. Inorg.
Chem. 2017, 56 (6), 3713−3718.
(55) Kang, K.; Fuller, J.; Reath, A. H.; Ziller, J. W.; Alexandrova, A.
N.; Yang, J. Y. Installation of internal electric fields by non-redox
active cations in transition metal complexes. Chem. Sci. 2019, 10 (43),
10135−10142.
(56) Chantarojsiri, T.; Reath, A. H.; Yang, J. Y. Cationic Charges
Leading to an Inverse Free-Energy Relationship for N−N Bond
Formation by MnVI Nitrides. Angew. Chem., Int. Ed. 2018, 57 (43),
14037−14042.
(57) Rollinson, H.; Adetunji, J. Ionic Radii. In Encyclopedia of
Geochemistry: A Comprehensive Reference Source on the Chemistry of the
(27) Cyriac, A.; Lee, S. H.; Varghese, J. K.; Park, E. S.; Park, J. H.;
Lee, B. Y. Immortal CO2/Propylene Oxide Copolymerization:
Precise Control of Molecular Weight and Architecture of Various
Block Copolymers. Macromolecules 2010, 43 (18), 7398−7401.
(28) Yoo, J.; Na, S. J.; Park, H. C.; Cyriac, A.; Lee, B. Y. Anion
variation on a cobalt(iii) complex of salen-type ligand tethered by four
quaternary ammonium salts for CO2/epoxide copolymerization.
Dalton Trans. 2010, 39 (10), 2622−2630.
(29) Na, S. J.; Sujith, S.; Cyriac, A.; Kim, B. E.; Yoo, J.; Kang, Y. K.;
Han, S. J.; Lee, C.; Lee, B. Y. Elucidation of the Structure of a Highly
Active Catalytic System for CO2/Epoxide Copolymerization: A salen-
Cobaltate Complex of an Unusual Binding Mode. Inorg. Chem. 2009,
48 (21), 10455−10465.
(30) Deng, J.; Ratanasak, M.; Sako, Y.; Tokuda, H.; Maeda, C.;
Hasegawa, J.-y.; Nozaki, K.; Ema, T. Aluminum porphyrins with
quaternary ammonium halides as catalysts for copolymerization of
cyclohexene oxide and CO2: metal−ligand cooperative catalysis.
Chem. Sci. 2020, 11 (22), 5669−5675.
(31) Patil, N. G.; Boopathi, S. K.; Alagi, P.; Hadjichristidis, N.;
Gnanou, Y.; Feng, X. Carboxylate Salts as Ideal Initiators for the
Metal-Free Copolymerization of CO2 with Epoxides: Synthesis of
Well-Defined Polycarbonates Diols and Polyols. Macromolecules 2019,
52 (6), 2431−2438.
(32) Patil, N.; Bhoopathi, S.; Chidara, V.; Hadjichristidis, N.;
Gnanou, Y.; Feng, X. Recycling a Borate Complex for Synthesis of
Polycarbonate Polyols: Towards an Environmentally Friendly and
Cost-Effective Process. ChemSusChem 2020, 13 (18), 5080−5087.
(33) Yang, G.-W.; Zhang, Y.-Y.; Xie, R.; Wu, G.-P. Scalable
Bifunctional Organoboron Catalysts for Copolymerization of CO2
and Epoxides with Unprecedented Efficiency. J. Am. Chem. Soc. 2020,
142, 12245.
(34) Trott, G.; Garden, J. A.; Williams, C. K. Heterodinuclear zinc
and magnesium catalysts for epoxide/CO2 ring opening copoly-
merizations. Chem. Sci. 2019, 10 (17), 4618−4627.
(35) Deacy, A. C.; Durr, C. B.; Garden, J. A.; White, A. J. P.;
Williams, C. K. Groups 1, 2 and Zn(II) Heterodinuclear Catalysts for
Epoxide/CO2 Ring-Opening Copolymerization. Inorg. Chem. 2018,
57 (24), 15575−15583.
(36) Garden, J. A.; Saini, P. K.; Williams, C. K. Greater than the Sum
of Its Parts: A Heterodinuclear Polymerization Catalyst. J. Am. Chem.
Soc. 2015, 137 (48), 15078−15081.
(37) Chapman, A. M.; Keyworth, C.; Kember, M. R.; Lennox, A. J.
J.; Williams, C. K. Adding Value to Power Station Captured CO2:
Tolerant Zn and Mg Homogeneous Catalysts for Polycarbonate
Polyol Production. ACS Catal. 2015, 5 (3), 1581−1588.
(38) Saini, P. K.; Romain, C.; Williams, C. K. Dinuclear metal
catalysts: improved performance of heterodinuclear mixed catalysts
for CO2-epoxide copolymerization. Chem. Commun. 2014, 50 (32),
4164−4167.
(39) Kember, M. R.; Williams, C. K. Efficient Magnesium Catalysts
for the Copolymerization of Epoxides and CO2; Using Water to
Synthesize Polycarbonate Polyols. J. Am. Chem. Soc. 2012, 134 (38),
15676−15679.
J
https://dx.doi.org/10.1021/jacs.0c07980
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Earth; White, W. M., Ed.; Springer International Publishing: Cham,
2018; pp 738−743.
(75) Darensbourg, D. J.; Chung, W.-C.; Arp, C. J.; Tsai, F.-T.;
Kyran, S. J. Copolymerization and Cycloaddition Products Derived
from Coupling Reactions of 1,2-Epoxy-4-cyclohexene and Carbon
Dioxide. Postpolymerization Functionalization via Thiol−Ene Click
Reactions. Macromolecules 2014, 47 (21), 7347−7353.
(76) Darensbourg, D. J.; Tsai, F.-T. Postpolymerization Function-
alization of Copolymers Produced from Carbon Dioxide and 2-
Vinyloxirane: Amphiphilic/Water-Soluble CO2-Based Polycarbon-
ates. Macromolecules 2014, 47 (12), 3806−3813.
(77) Chen, K.; Shi, G.; Li, H.; Wang, C.; Darensbourg, D. J. Design
of Betaine Functional Catalyst for Efficient Copolymerization of
Oxirane and CO2. Macromolecules 2018, 51 (15), 6057−6062.
(78) Konieczynska, M. D.; Lin, X.; Zhang, H.; Grinstaff, M. W.
Synthesis of Aliphatic Poly(ether 1,2-glycerol carbonate)s via
Copolymerization of CO2 with Glycidyl Ethers Using a Cobalt
Salen Catalyst and Study of a Thermally Stable Solid Polymer
Electrolyte. ACS Macro Lett. 2015, 4 (5), 533−537.
(79) Wu, G.-P.; Wei, S.-H.; Lu, X.-B.; Ren, W.-M.; Darensbourg, D.
J. Highly Selective Synthesis of CO2 Copolymer from Styrene Oxide.
Macromolecules 2010, 43 (21), 9202−9204.
(80) Ren, W.-M.; Zhang, X.; Liu, Y.; Li, J.-F.; Wang, H.; Lu, X.-B.
Highly Active, Bifunctional Co(III)-Salen Catalyst for Alternating
Copolymerization of CO2 with Cyclohexene Oxide and Terpolyme-
rization with Aliphatic Epoxides. Macromolecules 2010, 43 (3), 1396−
1402.
(81) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W.
Electronic and Steric Effects on Catalysts for CO2/Epoxide
Polymerization: Subtle Modifications Resulting in Superior Activities.
Angew. Chem., Int. Ed. 2002, 41 (14), 2599−2602.
(82) Kissling, S.; Lehenmeier, M. W.; Altenbuchner, P. T.; Kronast,
A.; Reiter, M.; Deglmann, P.; Seemann, U. B.; Rieger, B. Dinuclear
zinc catalysts with unprecedented activities for the copolymerization
of cyclohexene oxide and CO2. Chem. Commun. 2015, 51 (22),
4579−4582.
(83) Kissling, S.; Altenbuchner, P. T.; Lehenmeier, M. W.;
Herdtweck, E.; Deglmann, P.; Seemann, U. B.; Rieger, B. Mechanistic
Aspects of a Highly Active Dinuclear Zinc Catalyst for the Co-
polymerization of Epoxides and CO2. Chem. - Eur. J. 2015, 21 (22),
8148−8157.
(84) Wang, Y. Y.; Darensbourg, D. J. Carbon dioxide-based
functional polycarbonates: Metal catalyzed copolymerization of
CO2 and epoxides. Coord. Chem. Rev. 2018, 372, 85−100.
(85) Chen, T. T. D.; Zhu, Y.; Williams, C. K. Pentablock Copolymer
from Tetracomponent Monomer Mixture Using a Switchable Dizinc
Catalyst. Macromolecules 2018, 51 (14), 5346−5351.
(86) Darensbourg, D. J.; Wei, S.-H.; Yeung, A. D.; Ellis, W. C. An
Efficient Method of Depolymerization of Poly(cyclopentene carbo-
nate) to Its Comonomers: Cyclopentene Oxide and Carbon Dioxide.
Macromolecules 2013, 46 (15), 5850−5855.
(87) Darensbourg, D. J.; Yeung, A. D.; Wei, S.-H. Base initiated
depolymerization of polycarbonates to epoxide and carbon dioxide
co-monomers: a computational study. Green Chem. 2013, 15 (6),
1578−1583.
(58) Akine, S.; Utsuno, F.; Piao, S.; Orita, H.; Tsuzuki, S.;
Nabeshima, T. Synthesis, Ion Recognition Ability, and Metal-Assisted
Aggregation Behavior of Dinuclear Metallohosts Having a Bis-
(Saloph) Macrocyclic Ligand. Inorg. Chem. 2016, 55 (2), 810−821.
(59) Gao, Y.; Gu, L.; Qin, Y.; Wang, X.; Wang, F. Dicarboxylic acid
promoted immortal copolymerization for controllable synthesis of
low-molecular weight oligo(carbonate-ether) diols with tunable
carbonate unit content. J. Polym. Sci., Part A: Polym. Chem. 2012,
50 (24), 5177−5184.
(60) Hinz, W.; Wildeson, J.; Dexheimer, E. M.; Neff, R. Formation
of polymer polyols with a narrow polydispersity using double metal
cyanide (DMC) catalysts. US6713599B1, 2006.
(61) Gurtler, C.; Hofmann, J.; Wolf, A.; Grasser, S. Method for
̈
producing polyether carbonate polyols. US20130123532A1, 2013.
(62) Ohkawara, T.; Suzuki, K.; Nakano, K.; Mori, S.; Nozaki, K.
Facile Estimation of Catalytic Activity and Selectivities in Copoly-
merization of Propylene Oxide with Carbon Dioxide Mediated by
Metal Complexes with Planar Tetradentate Ligand. J. Am. Chem. Soc.
2014, 136 (30), 10728−10735.
(63) Darensbourg, D. J.; Fitch, S. B. Copolymerization of Epoxides
and Carbon Dioxide. Evidence Supporting the Lack of Dual Catalysis
at a Single Metal Site. Inorg. Chem. 2009, 48 (18), 8668−8677.
(64) Darensbourg, D. J. Making Plastics from Carbon Dioxide: Salen
Metal Complexes as Catalysts for the Production of Polycarbonates
from Epoxides and CO2. Chem. Rev. 2007, 107 (6), 2388−2410.
(65) Liu, Y.; Ren, W.-M.; Liu, C.; Fu, S.; Wang, M.; He, K.-K.; Li,
R.-R.; Zhang, R.; Lu, X.-B. Mechanistic Understanding of Dinuclear
Cobalt(III) Complex Mediated Highly Enantioselective Copolymer-
ization of meso-Epoxides with CO2. Macromolecules 2014, 47 (22),
7775−7788.
(66) Romain, C.; Thevenon, A.; Saini, P. K.; Williams, C. K.,
Dinuclear Metal Complex-Mediated Formation of CO2-Based
Polycarbonates. In Carbon Dioxide and Organometallics; Lu, X. B.,
Ed.; Springer, 2016; Vol. 53, pp 101−141.
(67) Mang, S.; Cooper, A. I.; Colclough, M. E.; Chauhan, N.;
Holmes, A. B. Copolymerization of CO2 and 1,2-Cyclohexene Oxide
Using a CO2-Soluble Chromium Porphyrin Catalyst. Macromolecules
2000, 33 (2), 303−308.
(68) Darensbourg, D. J.; Yarbrough, J. C.; Ortiz, C.; Fang, C. C.
Comparative Kinetic Studies of the Copolymerization of Cyclohexene
Oxide and Propylene Oxide with Carbon Dioxide in the Presence of
Chromium Salen Derivatives. In Situ FTIR Measurements of
Copolymer vs Cyclic Carbonate Production. J. Am. Chem. Soc.
2003, 125 (25), 7586−7591.
(69) Wu, G.-P.; Wei, S.-H.; Ren, W.-M.; Lu, X.-B.; Xu, T.-Q.;
Darensbourg, D. J. Perfectly Alternating Copolymerization of CO2
and Epichlorohydrin Using Cobalt(III)-Based Catalyst Systems. J.
Am. Chem. Soc. 2011, 133 (38), 15191−15199.
(70) Liu, J.; Ren, W.-M.; Liu, Y.; Lu, X.-B. Kinetic Study on the
Coupling of CO2 and Epoxides Catalyzed by Co(III) Complex with
an Inter- or Intramolecular Nucleophilic Cocatalyst. Macromolecules
2013, 46 (4), 1343−1349.
(71) Chukanova, O. M.; Bukhovets, E. V.; Perepelitsina, E. O.;
Belov, G. P. The kinetics of carbon dioxide and propylene oxide
copolymerization catalyzed by binary catalyst system. Polym. Sci., Ser.
B 2015, 57 (3), 224−227.
(88) Darensbourg, D. J.; Chung, W.-C.; Wilson, S. J. Catalytic
Coupling of Cyclopentene Oxide and CO2 Utilizing Bifunctional
(salen)Co(III) and (salen)Cr(III) Catalysts: Comparative Processes
Involving Binary (salen)Cr(III) Analogs. ACS Catal. 2013, 3 (12),
3050−3057.
(72) Darensbourg, D. J.; Wang, Y. Terpolymerization of propylene
oxide and vinyl oxides with CO2: copolymer cross-linking and surface
modification via thiol−ene click chemistry. Polym. Chem. 2015, 6
(10), 1768−1776.
(73) Scharfenberg, M.; Hilf, J.; Frey, H. Functional Polycarbonates
from Carbon Dioxide and Tailored Epoxide Monomers: Degradable
Materials and Their Application Potential. Adv. Funct. Mater. 2018, 28
(10), 16.
(74) Yi, N.; Chen, T. T. D.; Unruangsri, J.; Zhu, Y.; Williams, C. K.
Orthogonal functionalization of alternating polyesters: selective
patterning of (AB)n sequences. Chem. Sci. 2019, 10 (43), 9974−9980.
K
https://dx.doi.org/10.1021/jacs.0c07980
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX