Pinwheel-Shaped Tetranuclear Organoboron Catalysts for Perfectly Alternating Copolymerization of CO2and Epichlorohydrin

Article abstract of DOI:10.1021/jacs.0c12425

The copolymerization of carbon dioxide (CO2) and epoxides to produce aliphatic polycarbonates is a burgeoning technology for the large-scale utilization of CO2 and degradable polymeric materials. Even with the wealth of advancements achieved over the past 50 years on this green technology, many challenges remain, including the use of metal-containing catalysts for polymerization, the removal of the chromatic metal residue after polymerization, and the limited practicable epoxides, especially for those containing electron-withdrawing groups. Herein, we provide kinds of pinwheel-shaped tetranuclear organoboron catalysts for epichlorohydrin/CO2 copolymerization with >99% polymer selectivity and quantitative CO2 uptake (>99% carbonate linkages) under mild conditions (25-40 °C, 25 bar of CO2). The produced poly(chloropropylene carbonate) has the highest molecular weight of 36.5 kg/mol and glass transition temperature of 45.4 °C reported to date. The energy difference (ΔEa = 60.7 kJ/mol) between the cyclic carbonate and polycarbonate sheds light on the robust performance of our metal-free catalyst. Control experiments and density functional theory (DFT) calculations revealed a cyclically sequential copolymerization mechanism. The metal-free feature, high catalytic performance under mild conditions, and no trouble with chromaticity for the produced polymers imply that our catalysts are practical candidates to advance the CO2-based polycarbonates.

Full text of DOI:10.1021/jacs.0c12425

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Article
Pinwheel-Shaped Tetranuclear Organoboron Catalysts for Perfectly
Alternating Copolymerization of CO2 and Epichlorohydrin
Guan-Wen Yang, Cheng-Kai Xu, Rui Xie, Yao-Yao Zhang, Xiao-Feng Zhu, and Guang-Peng Wu*
Cite This: J. Am. Chem. Soc. 2021, 143, 3455−3465
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ABSTRACT: The copolymerization of carbon dioxide (CO2) and
epoxides to produce aliphatic polycarbonates is a burgeoning
technology for the large-scale utilization of CO2 and degradable
polymeric materials. Even with the wealth of advancements
achieved over the past 50 years on this green technology, many
challenges remain, including the use of metal-containing catalysts
for polymerization, the removal of the chromatic metal residue
after polymerization, and the limited practicable epoxides,
especially for those containing electron-withdrawing groups.
Herein, we provide kinds of pinwheel-shaped tetranuclear
organoboron catalysts for epichlorohydrin/CO2 copolymerization
with >99% polymer selectivity and quantitative CO2 uptake (>99%
carbonate linkages) under mild conditions (25−40 °C, 25 bar of CO2). The produced poly(chloropropylene carbonate) has the
highest molecular weight of 36.5 kg/mol and glass transition temperature of 45.4 °C reported to date. The energy difference (ΔEa =
60.7 kJ/mol) between the cyclic carbonate and polycarbonate sheds light on the robust performance of our metal-free catalyst.
Control experiments and density functional theory (DFT) calculations revealed a cyclically sequential copolymerization mechanism.
The metal-free feature, high catalytic performance under mild conditions, and no trouble with chromaticity for the produced
polymers imply that our catalysts are practical candidates to advance the CO2-based polycarbonates.
Currently, the vast majority of the CO2-PCs have been
produced from aliphatic epoxides that have electron-donating
substituents, as exemplified by propylene oxide (PO),
cyclohexene oxide (CHO), and their derivatives.32,33 In
contrast, very limited successful examples have been shown
concerning the synthesis of CO2-PCs using epoxides with
electron-withdrawing groups, such as epichlorohydrin (ECH)
(Figure 1a). As an central C3 chemical intermediate, ECH has
a wide range of applications in rubber, adhesives, ion-exchange
resins, and pharmaceutical intermediates with a production of
up to ∼2 million tonnes per year.34 Notably, a fourth of the
current global production of ECH is now produced from
vegetable glycerin by Solvay and Dow using bio-based
industrial processes.35 From the perspective of polymerization,
it is also intriguing to use ECH as a comonomer to construct
CO2-PC, since the introduction of chlorine into polymer
backbones can impart rigidity to improve its physical
properties, to equip the materials with the fire-retardant ability,
INTRODUCTION
■
The development of efficient and green techniques for the
chemical transformation of CO2, the major greenhouse gas,
into industrially viable products has awakened a global
awareness.1−6 Among various strategies for the chemical
utilization of CO2 being investigated, the construction of
degradable CO2-based polycarbonates (CO2-PCs) via alter-
nating copolymerization of CO2 with energy-rich epoxides is
an intriguing, practical service platform,3,7−9 which was
pioneered by Inoue and co-workers in 1969.10,11 Driven by
the deep research on a mechanistic understanding of the
copolymerization process, especially by developing a highly
active catalyst,12−27 attempts at the industrialization of CO2/
epoxide-derived copolymers are visible in several places in the
world.28,29 Encouragingly, the life cycle assessment of CO2-PC-
derived polymeric materials demonstrated that the incorpo-
ration of 1 kg of CO2 into the polymer could reduce 3 kg of
CO2 greenhouse gas emissions, further sparking the interest of
academic and industrial circles in this CO2-derived copoly-
mers.30,31 Even with the significant advancements achieved
over the past 50 years, many challenges remain in the
copolymerization of CO2 with epoxides: e.g., the sophisticated
synthesis of the metallic catalysts involved, the cumbersome
removal of the metal residue in the products, and the restricted
practicable epoxides.
Received: November 29, 2020
Published: February 16, 2021
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Figure 1. Copolymerization and the high-selectivity catalysts involved. (a) Coupling of ECH and CO2 to yield PCPC and cPC. (b) The state-of-
the-art Salen-Co complex for ECH/CO2 copolymerization. (c) The alkoxy backbiting pathways (A and B) and carbonate backbiting pathways (C
and D) leading to the formation of undesired cyclic carbonates. Pn and Pm present alkyl and carbonate substituents, respectively. (d) Photographs
of the representative crude polymer solution (in 5 mL of CH2Cl2) and the purified polymer after three dissolution−precipitation cycles using the
catalyst Salen-Co. (e) The tetranuclear organoboron catalysts in this work. (f) The crystal structure of TetraB2·4DMF, where all the H atoms and
solvent molecules are omitted and the coordinated DMF is truncated for clarity. (g) Photographs of the representative crude polymer solution (in 5
mL of CH2Cl2) and the purified ECH/CO2 copolymer using the catalyst TetraB2.
and to provide postpolymerization sites for new material
design with the desired functionalities.36,37 In fact, the
exploration of copolymerization of ECH/CO2 to produce
poly(chloropropylene carbonate) (PCPC) has never stopped
since the date of CO2-PC birth. The first attempt of the ECH/
CO2 copolymerization was also disclosed by Inoue using a
heterogeneous catalyst system of diethyl zinc/H2O that is
effective for the copolymerization of PO/CO2; unfortunately,
only <1% PCPC was obtained even when the the reaction time
was prolonged to 48 h for ECH/CO2 copolymerization.10 In
1994, Shen and co-workers presented a kind of heterogeneous
rare-earth-metal catalyst system for the copolymerization of
ECH and CO2, but only <30% carbonate linkages was
produced in the obtained poly(carbonate-co-ether) materials.38
In 2013, Zhang et al. reported a Zn-Co(III) double-metal
cyanide complex for the copolymerization, affording polymers
with a carbonate content of up to 70.7%.36 Three years later,
the carbonate content was increased to 83.1% by the Yoon
group using zinc glutarate as the catalyst.39 A landmark
improvement in this sector was made in 2011 by Darensbourg
and Lu, who used the homogeneous bifunctional salen-Co(III)
catalysts (Salen-Co, Figure 1b) and produced a completely
alternating copolymer of CO2 and ECH for the first time.40 It
is worth noting that a low temperature (0 °C) was necessary to
suppress the production of the thermally stable byproduct,
cyclic carbonate (cPC); otherwise, a considerable amount of
cPC (28%) was generated if the polymerization was conducted
at 25 °C. The quite small difference in activation energy (ΔEa
= 45.4 kJ/mol) between PCPC and cPC formation rationally
explains the fact that the selective synthesis of a copolymer of
ECH/CO2 is more challenging in comparison with the
conventional copolymerization processes of PO/CO2 (ΔEa =
53.5 kJ/mol).40 This small energy difference arises from the
electron-withdrawing chloromethyl group and leads to a strong
tendency for cPC byproduct formation rather than the PCPC,
because the alkoxy backbiting (routes A and B in Figure 1c)
and carbonate backbiting (routes C and D in Figure 1c)
pathways are susceptible to occur during the copolymerization
of ECH/CO2. This explanation clarifies why cPC is usually the
sole product for the coupling of ECH and CO2 using the
extensively studied metal compounds (such as Zn, Cr, etc.),
which have high polymer selectivity in the conventional
copolymerization of CHO/CO2 or/and PO/CO2 within broad
temperature and pressure windows.41−43 Despite the fact that
Salen-Co has enabled the successful synthesis of PCPC with a
perfect alternating structure, the time-consuming synthesis of
the catalyst (11-step synthetic procedure with ∼22% total
yield), the need to remove the chromatic, toxic cobalt residue
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a
Table 1. ECH/CO2 Copolymerization Results
b
b
mon./
cat.
time
(h)
temp
(°C)
conversn
(%)
selectivity (polymer
carbonate linkages
b
b
b
c
c
entry
cat.
TON TOF
%)
(%)
Mn (kg/mol)
Đ
1
2
3
4
5
6
TetraB1
TetraB2
TetraB3
TetraB2
TetraB2
TetraB2
TetraB2
TetraB2
TetraB2
TetraB2
500
500
24
24
24
24
8
25
25
25
40
50
60
25
25
25
25
36.1
39.6
45.6
61.3
33.5
26.3
41.4
27.2
43.9
37.0
180
198
228
307
168
132
207
136
439
740
7.5
>99
>99
>99
>99
93
>99
>99
>99
>99
95
16.3
20.6
23.5
15.4
10.8
3.2
1.19
1.13
1.20
1.28
1.36
1.69
1.30
1.21
1.22
1.26
8.3
9.5
500
500
12.8
21.0
32.9
8.6
500
500
4
90
92
d
7
500
24
24
60
120
>99
>99
>99
97
>99
>99
>99
>99
19.7
13.0
36.5
27.3
e
8
500
5.7
9
1,000
2,000
7.3
10
6.2
a
All of the polymerizations were carried out in 50 mL autoclaves under 25 bar of CO2 unless otherwise mentioned. Abbreviations: cat., catalyst;
b
c
1
mon., monomer; temp, temperature; conversn, conversion. Calculated by H NMR. Determined by gel permeation chromatography (GPC) in
THF with a polystyrene standard. Polymerization was carried out under 15 bar of CO2. 40 bar of CO2 was applied.
d
e
Figure 2. Characterization of ECH/CO2 copolymerization catalyzed by TetraB2. (a) In situ FTIR spectra monitoring the formation of polycarbonate
(∼1760 cm−1) and cyclic carbonate (∼1810 cm−1) at different reaction temperatures. Reaction conditions: ECH/TetraB2 = 500/1, solvent free, 25
bar of CO2. (b) Representative GPC traces of PCPC samples from Table 1 (blue curve, entry 4; red curve, entry 9). (c) 1H NMR (400 M, 25 °C,
CDCl3) and 13C NMR (126 M, 25 °C, CDCl3) spectra of a representative PCPC sample (entry 2, Table 1). (d) MALDI-TOF MS spectrum of the
PCPC oligomer produced by TetraB2.
in the product (more than 3 cycles of precipitation−
dissolution; Figure 1d), and the involved harsh copolymeriza-
tion conditions (0 °C) greatly limited its practical utilization.40
Inspired by the enzyme-mimetic multimetallic catalysis,44−46
herein, we provide simple, practicable organoboron catalysts
for the highly selective copolymerization of ECH and CO2
under mild conditions. The organoboron catalysts composed
of four Lewis acidic 9-borabicyclo[3.3.1]nonane (9-BBN)
centers and a central quaternary ammonium halide (Figure 1e)
can be facilely synthesized using low-cost and commercially
available chemicals in nearly quantitative yield via a two-step
procedure. The catalysts show >99% polymer selectivity and
>99% carbonate linkages in ECH/CO2 copolymerization
under mild conditions (25−40 °C, 25 bar of CO2), affording
PCPCs with the highest molecular weight of 36.5 kg/mol and
glass transition temperature of 45.4 °C reported so far. A
thermodynamic study indicated that the difference in the
energy of activation for cPC vs PCPC is 60.7 kJ/mol (ΔEa) for
our metal-free catalyst, 15.3 kJ/mol higher than the value
obtained using the salen-Co(III) complex (ΔEa = 45.4 kJ/
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mol).40 A cyclically sequential copolymerization mechanism
was rationally proposed and verified by multiple experimental
methods as well as density functional theory (DFT)
calculations. The metal-free feature, outstanding catalytic
performance under mild conditions, and no trouble with
chromaticity for the produced polymers imply that the
catalysts are practical candidates to advance the challenging
ECH/CO2 copolymerization.
ECH/CO2 copolymerization at 25 °C is provided in Figure 2a,
which clearly confirms the exclusive polymer selectivity during
the coupling reaction, wherein the characteristic absorption
peak (CO stretching vibration) of the polymer carbonate
linkage at ∼1760 cm−1 was gradually intensified without the
observation of an absorption at ∼1810 cm−1 assigned to cyclic
carbonate. We then used TetraB2 as a model catalyst to
investigate the effect of reaction temperature on the catalytic
performance, and a trend for increasing reactivity with an
increase of temperature was clearly observed. It was gratifying
to find that, even when the copolymerization was performed at
40 °C, there was no evidence of cPC formation and a
quantitative carbon dioxide uptake (>99% carbonate linkages)
was retained for the resultant PCPC (entry 4 in Table 1, Figure
2a, and Figure S5). Notably, a GPC trace of the resultant
PCPC exhibits a narrow and monomodal distribution of molar
masses, as shown in Figure 2b. When the reactions were
performed at 50 and 60 °C, the formation of cyclic carbonate
became thermodynamically unavoidable. Nonetheless, >90%
polymer selectivity and >90% carbonate linkages could still be
maintained (entries 5 and 6 in Table 1, Figure 2a, and Figure
S6).
RESULTS AND DISCUSSION
■
Catalyst Synthesis and Characterization. The chemical
structures of our catalysts TetraB1, TetraB2, and TetraB3 featuring
four Lewis acidic BBN centers and a central quaternary
ammonium halide (Cl−, Br−, and I−, respectively) are provided
in Figure 1e. The synthetic methodology is simple and efficient
with nearly quantitative yields in two steps (Figure S1).47 The
first step is the synthesis of tetraallylammonium halides via
quaternization of triallylamine with allyl halides (or treatment
by ion-exchange resin). Next, hydroboration of the tetraally-
lammonium halides with 4.2 equiv of 9-BBN afforded the
targeted metal-free catalysts as white solid powders. Given the
extreme simplicity of the methodology and ease of handing, all
of the catalysts could be prepared readily on a 100 g scale from
commercially available, inexpensive starting materials. All of
these catalysts were well-characterized by high-resolution mass
spectra and NMR spectra (see the Supporting Information). In
the presence of N,N-dimethylformamide (DMF), single
crystals of suitable quality could be obtained for single-crystal
X-ray diffraction tests. The crystal structure of TetraB2 is shown
in Figure 1f, wherein the molecule of TetraB2·4DMF adopts a
pinwheel-shaped geometry, and each boron center is strongly
coordinated by a DMF molecule. The four boron centers are
linked to the central N+ by the soft trimethylene arms with B···
N+ distances and B−N+−B angles in the ranges of 5.047−
5.282 Å and 85.2−99.4°, respectively. Given that each boron
center is occupied by a DMF molecule, the nucleophilic Br−
hangs adjacent to the central N+ via a Coulombic interaction
with a Br−···N+ distance of 4.557 Å (for detailed information,
see Table S1).
The perfect alternating structure of the polymer was
1
substantiated by the H and 13C NMR and matrix-assisted
laser desorption/ionization time-of-flight mass (MALDI-TOF
1
MS) spectra (Figure 2c,d). Figure 2c presents the H and 13C
NMR spectra of the resultant polymer, wherein the integrals of
Ha (CH2), Hb (CH), and Hc (CH2Cl) at 4.5, 5.1, and 3.7 ppm,
respectively, were conformed well to 2/1/2. In the
corresponding 13C NMR spectrum, the peaks at 152, 76, 65,
and 42 ppm could be clearly assigned to Cd (CO), Cb (CH),
Ca (CH2), and Cc (CH2Cl) of the alternating PCPC. The full
carbonate content of PCPC in the MALDI-TOF MS in Figure
2d exhibited three series of mass populations individually
separated by 136 mass units (the molar masses of the repeating
unit of ECH/CO2). The primary population (red circles) is
assigned to the molecular ions with mass corresponding to the
sodium adduct of the alternating copolymer with α-bromide/
ω-proton end groups. The secondary population (blue
triangles) reveals the occurrence of the substitution of the
bromide group on the chain end by an active alkoxy anion
during the polymerization process. The third population
(green diamonds) is 80 mass units less than the primary
population, which corresponds to the elimination of a HBr
molecule that may occur during the MALDI-TOF MS analysis.
Notably, the cyclization side reaction that has led to the
formation of a cyclic carbonate chain end in the salen-Co(III)-
mediated CO2/ECH copolymerization was not observed in
our system,40 which confirms the efficient suppression of the
occurrence of a cyclization reaction using our metal-free
catalyst. All three populations validated the alternating nature
of the resultant PCPC (>99% carbon dioxide uptake) and
further manifested that the tetranuclear catalyst TetraB2 is solely
selective (>99%) for alternating enchainment over ether
linkage formation and cyclization.
ECH/CO2 Copolymerization Studies. We began our
investigation with an evaluation of the influence of the initiator
anions Cl−, Br−, and I− on the catalytic performance for ECH/
CO2 copolymerization, and the results are collected in Table 1.
As is shown, the utilization of TetraB1 featuring the Cl− gave a
36.1% conversion (turnover frequency, TOF = 7.5 h−1) in 24 h
at 25 °C and 25 bar of CO2 pressure (ECH/TetraB1 = 500/1
mole ratio), affording PCPC with a Mn value of 16.3 kg/mol
and a narrow polydispersity (Đ = 1.19) (Table 1, entry 1). In
lieu of TetraB1 with Cl−, catalysts TetraB2 with Br− and TetraB3
with I− resulted in gradually increased reaction rates, and
conversions of 39.6% (TOF = 8.3 h−1, Table 1, entry 2) and
45.6% (TOF = 9.5 h−1, Table 1, entry 3) were achieved,
respectively. It should be noted that, under these conditions,
>99% polymer selectivity and >99% carbonate linkages were
achieved (Figures S2−S4). Moreover, the uncolored PCPC
sample could be easily obtained by only one precipitation in
methanol (Figure 1g), and the efficient removal of the catalyst
residue was manifested by elemental analysis (see the
Supporting Information), which is in marked contrast to the
chromatic sample catalyzed by Salen-Co even after three
cycles of dissolution−precipitation (Figure 1d).40
A variation in CO2 pressure had no influence on the polymer
selectivity and alternating enchainment, and >99% polymer
selectivity and quantitative carbon dioxide uptake were
observed over the pressure range of 15−40 bar at room
temperature. Decreasing the CO2 pressure from 25 to 15 bar
made a tiny change in activity (turnover number, TON of 198
vs 207, Table 1, entries 2 and 7), while increasing the CO2
pressure to 40 bar resulted in a reduced activity with a TON
A representative in situ fourier transform infrared spectros-
copy (FTIR) three-dimensional stack plot of TetraB2-mediated
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Figure 3. Kinetic measurements: (a) first order in concentration of catalyst TetraB2; (b) zero order in CO2 pressure; (c) first order in ECH; (d)
experimental activation energy (Ea) for the formation of PCPC.
value of 136 (Table 1, entry 8), probably arising from a
reduced solubility of the reagents in a supersaturated CO2
solution, a common experimental phenomenon in the coupling
reaction of epoxides/CO2.48,49
conversions, and the CO stretching vibration at ∼1760 cm−1
of the carbonate group was monitored to calculate the initial
rate coefficient, kobs. The copolymerization is first order with
respect to the concentration of TetraB2, as was ascertained by
the linear fit of ln kobs to ln [TetraB2] from five concentrations
of TetraB2 (8.52, 10.23, 12.80, 17.05, and 25.57 mM; Figure
3a). A zero-order dependence on CO2 pressure was confirmed
by varying the initial CO2 pressure from 5 to 35 bar without
varying other reaction conditions (ECH/TetraB2 = 500/1, 25
°C; Figure 3b), which reveals that CO2 incorporation is not
the rate-determining step during the copolymerization process.
The order in ECH concentration was determined by
conducting a copolymerization with an ECH/TetraB2 ratio of
500/1 at 25 °C and 25 bar of CO2 pressure. The absorbance vs
time data were well fitted by an exponential fit with a
coefficient of determination of 0.9999, demonstrating that the
copolymerization is first order with respect to ECH monomer
(Figure 3c).51 On the basis of the reaction kinetics studies, the
overall rate law dependent upon all reagents can be determined
as reaction rate = kp[TetraB2]1[ECH]1p(CO2)0, where kp is the
propagation rate constant. The equation clearly manifests that
the ring opening of ECH is the rate-determining step during
the copolymerization, while highlighting that only one TetraB2
molecule (i.e., an intramolecular synergistic catalysis) domi-
nated the copolymerization process. Finally, by implementa-
tion of five experiments at different reaction temperatures (25,
37, 45, 53, and 65 °C), an apparent activation energy of 46.1
kJ/mol for PCPC formation was calculated from an Arrhenius
plot (Figure 3d), which is 7.0 kJ/mol lower than that of the
salen-Co(III)-mediated copolymerization (53.1 kJ/mol).40 It is
important to find that the difference in the energies of
activation for cPC vs PCPC was determined as ΔEa = 60.7 kJ/
mol under TetraB2 (Figure S10), which is 15.3 kJ/mol higher
than the value obtained by Lu and Darensbourg using the
The catalytic performance of TetraB2 under dilution
conditions was also conducted. It was found that, at a low
catalyst loading of 0.1 mol % (Table 1, entry 9), the catalytic
activity (TOF = 7.3 h−1) of TetraB2 was observed without any
loss in polymer selectivity and carbonate content, and the
afforded PCPC gave a relatively high Mn value of 36.5 kg/mol
with a unimodal distribution as shown by the GPC curve
(Figure 2b, red curve; for other GPC traces of PCPCs in Table
1, see Figure S7). To the best of our knowledge, this is the
highest molecular weight obtained so far. The basic thermal
performance of the produced PCPC examined by differential
scanning calorimetry (DSC) also substantiated its status, in
which a glass transition temperature value (Tg) of 45.4 °C
(Figure S8), the highest value achieved to date, was observed
for the copolymer of ECH and CO2. However, no melt
endothermic peak at ∼108 °C on the DSC curve was detected,
indicating the atactic property of the produced PCPC.50
Furthermore, the amorphous nature of PCPC was also
demonstrated by using wide-angle X-ray diffraction, where
no diffraction was observed (Figure S9). When the catalyst
loading was further reduced to 0.05 mol %, the copolymeriza-
tion could also proceed with a moderate rate (TOF = 6.2 h−1)
and >97% polymer selectivity (Table 1, entry 10).
Reaction Kinetics Study. To obtain a mechanistic insight
into the copolymerization process catalyzed by our tetranuclear
catalysts, we next turned our attention to the reaction kinetics
(Figure 3). The in situ FT-IR technique was performed to
explore the individual influences of TetraB2 loading, CO2
pressure, and ECH concentration. For the sake of accuracy,
all of the kinetic experiments were conducted with <5% ECH
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Figure 4. Control catalyst systems, crystal structures, and catalytic performances. (a) Chemical and crystal structures of control catalysts TriB, DiB,
and MoB in comparison with TetraB2 for ECH/CO2 copolymerization. The crystal structures of DiB and MoB were taken from our previous
studies.52,53 All of the H atoms and solvent molecules are omitted, and the coordinated DMF are truncated for clarity. (b) Average B···B distances
and B−N+−B angles of catalysts TetraB2, TriB, DiB, and MoB. N.A. denotes not applied. (c) Influence of nuclearity on catalytic activity, polymer
selectivity, and carbonate content of the copolymer of ECH/CO2. (d) Catalytic performances of the experiments using four kinds of binary control
catalyst systems (TriB/C1, DiB/C1, MoB/C1, and C1/TBAB), where each binary system has an intermolecular B/N+ ratio of 4 to verify the
necessity of intramolecular tetranuclear incorporation for TetraB2. All control experiments were conducted in neat ECH with a catalyst loading of
0.2 mol % at 25 °C and 25 bar of CO2 for 24 h.
salen-Co(III) complex (ΔEa = 45.4 kJ/mol).40 This com-
parative results shed light on the high polymer selectivity at
enhanced temperature using our organoboron catalysts, and
the rare tolerance of temperature encouraged us to probe the
structure−property relationships of our catalysts, as detailed in
the next section.
With the aim of demonstrating the validity of the TetraB2
design that incorporates four boron centers in the molecule, we
carried out a set of control experiments using tetranuclear
TetraB2, trinuclear catalyst TriB, dinuclear catalyst DiB, and
mononuclear catalyst MoB, respectively, with a catalyst loading
of 0.2 mol % at 25 °C and 25 bar of CO2 pressure. The
comparative results indicated that when the number of boron
centers in the organoboron catalysts were reduced from four
Control Catalyst Systems and Polymerizations. With
an aim to clarifying the structure−performance relationships of
our intramolecular tetranuclear catalyst (taking TetraB2 as a
model), we synthesized a series of control catalysts that have
the same boron centers, trimethylene arms between B and N+,
and initiator Br− to TetraB2. The chemical structures of these
control catalysts, including the intramolecular trinuclear
catalyst TriB, dinuclear catalyst DiB, and mononuclear catalyst
MoB, are shown in Figure 4a. For unambiguous comparison, the
crystal structures of these catalysts were also measured and are
provided in Figure 4a (the crystal structures of DiB and MoB
were taken from our previous studies;52,53 for detailed crystal
information on TriB, see Table S2). The key information on
average B···B distances and B−N+−B angles of TetraB2·4DMF,
TriB·3DMF, DiB·2DMF, and MoB·DMF are given in Figure 4b.
It is clearly found that the average B···B distance and B−N+−B
angle increased from 7.476 Å/91.9° for TetraB2·4DMF to 8.380
Å/107.7° for TriB·3DMF and 8.839 Å/114.3° for DiB·2DMF
with a decrease in boron centers.
(
TetraB2) to three (TriB), two (DiB), and one (MoB), a dramatic
reduction in catalytic activity, polymer selectivity, and
carbonate content were observed (Figure 4c and Table S3).
For example, catalyst TriB with one boron center less than
TetraB2 exhibited a much lower activity of 5.4 h−1, affording
PCPC with 76% polymer selectivity and 95% carbonate
linkages. On a further decrease in the intramolecular boron
centers to two (DiB), the dinuclear catalyst presented an
inferior activity of 2.5 h−1 and a poorer control of ECH/CO2
copolymerization with only 40% polymer selectivity and 77%
carbonate content. In sharp contrast, the mononuclear catalyst
MoB showed activity comparative to that of TetraB2, but cPC
was the sole product.52 These control experiments manifest
that the proximity of the boron centers could provide higher
reactivity, selectivity, and carbonate content for the copoly-
merization, thus substantiating the necessity of our tetranuclear
organoboron catalyst design. In addition, the validity of the
TetraB2 design was also manifested by the copolymerization of
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Figure 5. Proposed cyclically sequential copolymerization mechanism of ECH and CO2 in the presence of TetraB2.
CO2 with the benchmark epoxides PO (Table S4) and CHO
(Table S5), respectively. For PO, >99% polymer selectivity
with an increase in the number of boron centers in the
organoboron catalysts was observed, even though a small
quantity of polyether content appeared. For CHO, TetraB2,
TriB, DiB, and MoB all exhibited >99% polymer selectivity and
carbonate linkages. These comparative experiments explained
the fact that the selective synthesis of copolymer of ECH/CO2
is more challenging in comparison with the conventional
copolymerization processes of PO/CO2 and CHO/CO2,
highlighting the design of our tetranuclear organoboron
catalysts.
We then probed the necessity of intramolecular cooperative
effects between the four boron centers and ammonium
bromide. Four kinds of binary catalyst systems with an
intermolecular ratio of B/N+ = 4 were used as control catalysts:
that is, TriB/C1 (1/1 mole ratio), DiB/C1 (1/2 mole ratio),
MoB/C1 (1/3 mole ratio), and C1/tetrabutylammonium
bromide (TBAB) (4/1 mole ratio). The catalytic performances
of these binary catalysts were systematically compared with
that of the intramolecular tetranuclear catalyst TetraB2 (B/N+ =
4) that displayed high selectivity and activity for ECH/CO2
copolymerization (Figure 4d and Table S3). The results
indicated that a negligible synergistic effect occurred in the
four intermolecular boron centers for these four kinds of
control catalysts, as manifested by the following two
experimental phenomena. First, the combination of the
intermolecular components caused no promotion of the
copolymerization behavior, including the TOF values, polymer
selectivities, and carbonate contents. For example, the binary
control catalyst TriB/C1 displayed a TOF value, polymer
selectivity, and carbonate content of 5.4 h−1, 76%, and 96%,
respectively; this performance is no better than that of using
TriB without the addition of C1 (TOF value 5.4 h−1, polymer
selectivity 76%, and carbonate content 95%, respectively).
Similar trends were also found in the binary DiB/C1- and MoB/
C1-mediated ECH/CO2 coupling reactions (Figure 4d and
Table S3). The second fact is that the catalytic performances of
these binary systems (TriB/C1, DiB/C1, MoB/C1, and C1/
TBAB, with an intermolecular ratio of B/N+ = 4) were far
inferior to that of TetraB2 which has an intramolecular B/N+
ratio of 4. In comparison with TetraB2 (TOF = 8.3 h−1, >99%
polymer selectivity, and >99% carbonate content), a 35%
decrease in TOF value (5.4 h−1) and 23% decrease in polymer
selectivity (76%) were obtained by using the TriB/C1 catalyst
system, affording a PCPC with 96% carbonate content. When
the binary DiB/C1 catalyst system was used, the catalytic
activity was reduced to 2.9 h−1, only a third of that of TetraB2;
meanwhile, the polymer selectivity and carbonate content were
sharply reduced to 39% and 80%, respectively. Notably, when
the MoB/C1 and C1/TBAB catalyst systems were used, only
the cyclic carbonate was obtained, though high TOFs of 6.3
and 12.5 h−1 were obtained, respectively (Figure 4d and Table
S3). The above control experiments indicated that the
integration of four boron centers and ammonium halide into
one molecule was indispensable to suppress the backbiting side
reactions and continuous insertion of ECH (polyether linkage)
during the ECH/CO2 copolymerization process.
Proposed Copolymerization Mechanism. On the basis
of the aforementioned findings, we tentatively propose a
cyclically sequential copolymerization mechanism based on
catalyst TetraB2 in Figure 5. At the beginning, the nucleophilic
Br− was randomly interacted with the four Lewis acidic boron
centers and the electropositive ammonium cation under the
action of Coulomb forces.53 Upon the coordination of Lewis
basic ECH monomers to the Lewis acidic BBN centers (I), the
nucleophilic attack of Br− to the activated ECH on B1, leading
to the formation of an alkoxide-participated tetracoordinate B1
center (chain initiation, rate-determining step; II−III). Due to
the stabilization of the active alkoxy anion by a strong
interaction with the boron center, the alkoxy backbiting
reaction leading to the production of cPC was effectively
suppressed. This proposal was demonstrated by the ring-
opening experiment of ECH in the absence of CO2, as the
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Figure 6. Gibbs free energy profile with the involved intermediates (IN1−IN7) and transition states (TS1−TS3) for TetraB2-catalyzed CO2/ECH
copolymerization. The insert is the crystal structure of TetraB2·2THF.
formation of an “ate” complex (a boron center was strongly
coordinated with the alkoxy anion) was clearly observed by 1H
NMR and 11B NMR spectra (Figures S11 and S12). Given that
the copolymerization is zero order in CO2 pressure, a CO2
molecule then quickly inserts into the boron−alkoxide bond
and generates the boron−carbonate bond (IV). The CO2
insertion process was experimentally manifested and spec-
troscopically observed (Figure S13). Assisted by the
Coulombic interaction with the central N+, the weakly
coordinated carbonate anion subsequently dissociates from
B1 to ring-open the already activated monomer on B2; in the
meantime, the unoccupied B1 is coordinated by a fresh ECH
molecule (V−VI). Note that the high density of intramolecular
tetranuclear boron centers provides a high local concentration
of Lewis acidic centers to efficiently trap the dissociated
carbonate anion, thus preventing the carbonate backbiting
reaction for the formation of cPC. Following this propagation
manner, the repeatedly alternating insertion of ECH and CO2
propagates from B2 to B3 to B4 to B1 in the cycle, producing
the perfectly alternating ECH/CO2 copolymer (VII−IX). It is
important to note that the random chain enchainment between
two proximate boron centers (e.g., from B2 back to B1) could
be not exclusively ruled out during the copolymerization.
Computational Study. To gain a better mechanistic
insight into the proposed copolymerization mechanism,
computational investigations were conducted (for detailed
information, see the Supporting Information), and the
resultant Gibbs free energy profile with the intermediates
(IN1−IN7) and transition states (TS1−TS3) involved for
TetraB2-catalyzed CO2/ECH copolymerization is provided in
Figure 6 (for the three-dimensional structures of IN1−IN7
and TS1−TS3, see Figure S14). From the initial state to IN1,
it was surprising to find that a favorable conformation of IN1
only accommodates two ECH monomers on TetraB2, and the
coordination of the two ECH monomer to the initial state is
computed to be endergonic by 4.9 kcal/mol. To demonstrate
the validity of the optimized IN1, we cultivated the crystal of
catalyst TetraB2 in THF solution, since THF is a non-
polymerizable cyclic ether and has a coordination ability
similar to that of ECH. The crystal structure of the catalyst
TetraB2·2THF is provided in the bottom left corner of Figure 6
(for detailed crystal data, see Table S6). As expected, the
coordination of two THF molecules was clearly observed,
which manifests the rationality for the optimized structure of
IN1. Subsequent ECH ring opening by the Br− on B4 proceeds
with an activation free energy of 19.35 kcal/mol (TS1). Then
the alkoxy anion on B1 of IN2 activates a CO2 molecule with a
4.59 kcal/mol increase in energy (IN3), followed by the quick
insertion of the activated CO2 into the B1−alkoxy bond under
the synergistic effect of B1 and B4 to afford IN4 by overcoming
an energy barrier of 10.99 kcal/mol (TS2 to IN4). The
following chain propagation involves a repositioning of the
carbonate anion, leading to the formation of IN5, where the
carbonate-bridged B1 and B4 could cooperatively stabilize the
active carbonate chain end (Figure S14). These two steps are
calculated to be exergonic by 0.85 kcal/mol, implying that the
joint stabilization of the carbonate anion by B4 and B1 is more
favored. After the coordination of an additional ECH
monomer on B3 (IN5 to IN6), the ring opening of the
activated ECH monomer on B2 by the carbonated anion
(which has a weak electrostatic interaction with B1) leads to
the formation of intermediate IN7 with alkoxy-bonded B2,
releasing 7.78 kcal/mol free energy in comparison with the
initial state. The aforementioned alternating enchainment
ECH/CO2 takes place cyclically to give the final PCPC
product.
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Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China
CONCLUSIONS
■
In summary, kinds of bifunctional tetranuclear catalysts
composed of four intramolecular Lewis acid centers and one
ammonium salt for the highly selective copolymerization of
ECH and CO2 were synthesized in ∼100% yields by
employing a simple two-step synthetic procedure. The well-
defined tetranuclear compounds achieve >99% polymer
selectivity in ECH/CO2 copolymerization within a broad
temperature window of 25−40 °C, affording a perfectly
alternating polycarbonate (>99%) with the highest molecular
weight of 36.5 kg/mol and glass transition temperature of 45.4
°C reported to date. A combination of multiple experimental
methods and density functional theory calculations provided
mechanistic insights into the copolymerization, wherein the
synergy among the intramolecular four boron centers plays a
crucial role in suppressing the formation of a cyclic carbonate
byproduct and ether linkage. The cyclically sequential
polymerization manner along with a detailed understanding
of the synergistic effect among the multinuclear borane centers
should be beneficial for the design of novel multicentric
catalyst systems.
Rui Xie − MOE Laboratory of Macromolecular Synthesis and
Functionalization, Key Laboratory of Adsorption and
Separation Materials & Technologies of Zhejiang Province,
Department of Polymer Science and Engineering, Zhejiang
University, Hangzhou 310027, People’s Republic of China
Yao-Yao Zhang − MOE Laboratory of Macromolecular
Synthesis and Functionalization, Key Laboratory of
Adsorption and Separation Materials & Technologies of
Zhejiang Province, Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China; orcid.org/0000-0002-8031-
9139
Xiao-Feng Zhu − MOE Laboratory of Macromolecular
Synthesis and Functionalization, Key Laboratory of
Adsorption and Separation Materials & Technologies of
Zhejiang Province, Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c12425
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c12425.
■
Notes
sı
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Experimental procedures, characterization of all com-
pounds, control polymerizations, DFT computational
data, and crystallographic data for TetraB2·4DMF
(CCDC 1974842), TriB·3DMF (CCDC 1974839), and
TetraB2·2THF (CCDC 2041735) (PDF)
This work was supported by the Zhejiang Provincial Natural
Science Foundation of China (R21B040004) and the National
Natural Science Foundation of China (Grants 91956123 and
51973186). The authors thank Prof. Xiaochao Shi from
Shanghai University for helpful assistance in NMR measure-
ments.
Accession Codes
CCDC 1974839, 1974842, and 2041735 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
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AUTHOR INFORMATION
Corresponding Author
■
Guang-Peng Wu − MOE Laboratory of Macromolecular
Synthesis and Functionalization, Key Laboratory of
Adsorption and Separation Materials & Technologies of
Zhejiang Province, Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China; orcid.org/0000-0001-8935-
964X; Email: gpwu@zju.edu.cn
Authors
Guan-Wen Yang − MOE Laboratory of Macromolecular
Synthesis and Functionalization, Key Laboratory of
Adsorption and Separation Materials & Technologies of
Zhejiang Province, Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027,
People’s Republic of China; orcid.org/0000-0003-2471-
2953
Cheng-Kai Xu − MOE Laboratory of Macromolecular
Synthesis and Functionalization, Key Laboratory of
Adsorption and Separation Materials & Technologies of
Zhejiang Province, Department of Polymer Science and
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J. Am. Chem. Soc. 2021, 143, 3455−3465