Scalable Bifunctional Organoboron Catalysts for Copolymerization of CO2and Epoxides with Unprecedented Efficiency

Article abstract of DOI:10.1021/jacs.0c03651

The metallic catalyst-dominated alternating copolymerization of CO2 and epoxides has flourished for 50 years; however, the involved multistep preparation of the catalysts and the necessity to remove the colored metal residue in the final product present significant challenges in scalability. Herein, we report a series of highly active metal-free catalysts featured with an electrophilic boron center and a nucleophilic quaternary ammonium halide in one molecule for copolymerization of epoxides and CO2. The organocatalysts are easily scaled up to kilogram scale with nearly quantitative yield via two steps using commercially available stocks. The organocatalyst-mediated copolymerization of cyclohexane oxide and CO2 displays high activity (turnover frequency up to 4900 h-1) and >99percent polycarbonate selectivity in a broad temperature range (25-150 °C) at mild CO2 pressure (15 bar). At a feed ratio of cyclohexane oxide/catalyst = 20 ?000/1, an efficiency of 5.0 kg of product/g of catalyst was achieved, which is the highest record achieved to date. The unprecedented activity toward CO2/epoxide copolymerization for our catalyst is a consequence of an intramolecular synergistic effect between the electrophilic boron center and the quaternary ammonium salt, which was experimentally ascertained by reaction kinetics studies, multiple control experiments, 11B NMR investigation, and the crystal structure of the catalyst. Density functional theory calculations further corroborated experimental conclusions and provided a deeper understanding of the catalysis process. The metal-free characteristic, scalable preparation, outstanding catalytic performances along with long-term thermostability demonstrate that the catalyst could be a promising candidate for large-scale production of CO2-based polymer.

Full text of DOI:10.1021/jacs.0c03651

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Article
Scalable bifunctional organoboron catalysts for copolymerization
of CO2 and epoxides with unprecedented efficiency
Guan-Wen Yang, Yao-Yao Zhang, Rui Xie, and Guang-Peng Wu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03651 • Publication Date (Web): 15 Jun 2020
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Scalable bifunctional organoboron catalysts for copolymerization of
CO2 and epoxides with unprecedented efficiency
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Guan-Wen Yang, Yao-Yao Zhang, Rui Xie and Guang-Peng Wu*
MOE Laboratory of Macromolecular Synthesis and Functionalization, Adsorption and
Separation Materials and Technologies of Zhejiang Province, Department of Polymer Science
and Engineering, Zhejiang University, Hangzhou 310027, China
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ABSTRACT
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The metallic catalyst-dominated alternating copolymerization of CO2 and epoxides has
flourished for 50 years, however, the involved multistep preparation of the catalysts and the
necessary to remove the colored metal residue in the final product present significant
challenges in scalability. Herein, we report a series of highly active metal-free catalysts
featured with an electrophilic boron center and a nucleophilic quaternary ammonium halide in
one molecule for copolymerization of epoxides and CO2. The organocatalysts are easily
scaled up to kilogram scale with nearly quantitative yield via two steps using commercially
available stocks. The organocatalyst-mediated copolymerization of cyclohexane oxide and
CO2 displays high activity (turnover frequency up to 4,900 h-1) and > 99% polycarbonate
selectivity in a broad temperature range (25 ~ 150 oC) at mild CO2 pressure (15 bar). At a feed
ratio of cyclohexane oxide/catalyst = 20,000/1, an efficiency of 5.0 kg product/g catalyst was
achieved, which is the highest record achieved to date. The unprecedented activity toward
CO2/epoxides copolymerization for our catalyst is a consequence of an intramolecular
synergistic effect between the electrophilic boron center and the quaternary ammonium salt,
which was experimentally ascertained by reaction kinetics studies, multiple control
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experiments, B NMR investigation, and the crystal structure of catalyst. Density functional
theory calculations further corroborated experimental conclusion and provided deeper
understanding on the catalysis process. The metal-free characteristic, scalable preparation,
outstanding catalytic performances along with long-term thermostability demonstrates the
catalyst could be a promising candidate for large-scale production of CO2-based polymer.
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INTRODUCTION
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Anthropogenic carbon dioxide (CO2) emission and non-degradable plastics pollution are
two major challenges confronting sustainable society.1, 2 In terms of the two issues, alternating
copolymerization of CO2 and epoxides into degradable aliphatic polycarbonate (CO2-PC) is
an ideal stone that can kill the two birds.3 Since Inoue and colleagues demonstrated the
effectiveness of a mixture of diethylzinc and water for the coupling reaction in 1969,4, 5
tremendous efforts have been devoted to developing efficient catalyst systems for this green
chemical transformation,6-12 and numerous metal-based catalysts including Zn,13-20 Mg,20-23
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28−35
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Al,24-27 Co,23,
Cr,36−38 Fe,39−41 Cu,42 Ti,43−45 In,46 Zr,47,
Ni49,
and Lanthanide
complexes51−53 have been well explored. In general, the metal complexes-mediated
CO2/epoxide copolymerization process is initiated by coordination of the epoxide to the metal
center followed by ring-opening of the monomer by a nucleophile with concomitant
formation of a metal bound alkoxide, then CO2 inserts into the metal-alkoxide bond and
provides a metal-carbonate. The successive alternating incorporation of epoxide and CO2 at
the metal center produces linear polycarbonates.10 Owing to the thermo-stability and chemical
inertness of CO2, the catalysts that can considerably reduce the activation energy of the
coupling reaction are essential to increase the catalytic efficiency. In this regard, the seeking
of appropriate organic ligand plays a crucial role in the design of organometallic catalysts for
the coupling reaction of CO2 and epoxides, since the circumjacent ligand can efficiently tune
the steric and electronic properties of the metal centers, thereby altering the activity of the
resultant organometallic catalysts. Excellent examples have been demonstrated by
metal-based catalysts that embraced a well-defined ligand, such as, salen, salan, porphyrin,
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β‑diiminate, etc. These elaborate compounds are really active even under mild reaction
conditions and in certain circumstance afford regio- and/or stereoselective copolymer
synthesis. Despite the fact that significant advances were achieved, the time-consuming
multistep synthesis of the ligands and the need of removing the chromatic metal residue in the
product remain key challenges for the dominant organometallic catalysts, especially for those
toxic cobalt and chromium systems.54−56
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Over the last two decades, organocatalysis has evolved into one of the most powerful
methodologies in organic and polymeric chemistry as its potential for savings in cost,
reductions in chemical waste and toxicity concerns.57-62 Vast amount of organocatalysts have
been used for the coupling of CO2 and epoxides, but products were finally obtained only
cyclic carbonates during the coupling reaction, since the five-membered cyclic carbonates are
thermodynamically more stable than the corresponding linear polycarbonates.63 In 2016,
Feng, Gnanou, and co-workers reported the first example of metal-free CO2-PC synthesis
using Lewis pairs triethyl borane (TEB)/onium salts [e.g., bis(triphenylphosphine)-iminium
chloride (PPNCl)] as binary catalyst system (Figure 1a).64 The experimental results illustrated
that the application of two equivalents of TEB to the PPNCl is necessary to provide a high
activity during the copolymerization of CO2 and cyclohexane oxide (CHO). Under this
circumstance, poly(cyclohexane carbonate) (PCHC) was obtained with perfectly alternating
carbonate content and over 94% polymer selectivity. The subsequent density functional
theory (DFT) calculations elucidated each role of the two equivalents of TEB during the
copolymerization process. That is, one TEB molecule acted as the epoxide activator to
decrease the energy barrier of the polymerization process, and the other one played a role of
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stabilizer for the growing carbonate anion to eliminate the backbiting reaction (Figure 1a).65
The joint effect of the two TEB molecules afforded a high turnover frequency (TOF) value of
600 h-1 for the coupling of CHO and CO2.64 Although the TEB/onium salts Lewis pairs
system breaks the proprietary of the metal complexes-mediated copolymerization of CO2 and
epoxides,66-68 the catalytic performance is still lower than many of the state-of-the-art
metal-based catalysts.
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a Binary triethyl borane/onium salt catalyst system
B
Cl
O
B
B
CHO + CO2
O
O
P
N P
B
O
O
O
O
O
Cl
O
n(CHO +CO2)
O
B
O
n
TEB
PPNCl
PPN
Cl
PPN
b Bifunctional organoboron catalysts in this work
R
R
R
R
R3N
9-BBN
THF, 60 oC
X
n
N
n
X
N
B
X
ethanol, 95 oC
n
R
R
1
2
: n = 1; R = Et; X = Br
: n = 2; R = Et; X = Br
Catalyst preparation
Catalytic performance
3a
3b
3c
: n = 3; R = Et; X = Cl
: n = 3; R = Et; X = Br
: n = 3; R = Et; X = I
: n = 4; R = Et; X = Br
: n = 5; R = Et; X = Br
: n = 3; R = Pr; X = Br
: n = 3; R = Bu; X = Br



~100% yield in two steps
cost effective



broad working temperature
(25–150 oC)
low catalyst loading
(1/20000)
unprecedented efficiency
(5.0 kg PCHC/g catalyst)
4
5
6
7
cat.3b
0.6 kg
kilogram scale
Figure. 1. Organic catalysts for CO2/CHO copolymerization. (a) The binary TEB/PPNCl catalyst
system reported previously. (b) The bifunctional organoboron catalysts in this work.
In this study, we present a series of highly active organoboron catalysts for alternating
copolymerization of epoxides and CO2 (Figure 1b). In sharp contrast with the
above-mentioned binary catalyst system involved using essential two equivalents of boron
centers, our organocatalysts integrate one boron center and a quaternary ammonium salt in
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one molecule, while display unprecedented reactivity towards copolymerization of CHO and
CO2. The organocatalyst-mediated polymerization can endure a reaction temperature as high
as 150 oC without the observation of any byproduct and provide a turnover number (TON) of
13,000 at a feed ratio of CHO/catalyst = 20,000/1 at 80 oC and 15 bar CO2. This TON value is
equivalent to an efficiency of 5.0 kg PCHC/g catalyst, to the best of our knowledge, which is
the highest record achieved to date in all of homogeneous organic and metallic
catalysts-mediated coupling reaction. The unprecedented reactivity of our organocatalysts is
ascribed to a novel intramolecular cooperative catalysis mechanism, which is verified on the
basis of the structure−performance relationships, 11B NMR investigation, the crystal structure
of catalyst, and density functional theory (DFT) calculations. Besides the outstanding
catalytic performance, the catalysts have combined advantages of using wide raw material
sources and simple synthetic procedure, and can be produced in kilogram-scale in laboratory,
enabling the catalysts provided here to be a rare candidate for industrial application.
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RESULTS AND DISCUSSION
Organoboron catalysts synthesis
The synthetic procedures for our organoboron catalysts are provided in Figure 1b,
wherein two-step reaction with nearly quantitative yield is applied (for detailed synthetic
procedures, see Supporting Information). In the first step, the stoichiometric treatment of
trialkylamines with alkenyl halides (i.e., Menschutkin reaction) in ethanol produced
quaternary ammonium salts. In the second step, the target bifunctional organocatalysts were
obtained through the hydroboration reaction of the double bond on quaternary ammonium
salts using a widely used hydroboration reagent, 9-borabicyclo[3.3.1]nonane (9-BBN). All the
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catalysts were collected as white solids by simply washing with n-hexane and subsequent
drying in vacuum with negligible losses. The catalysts were well defined by means of nuclear
magnetic resonance (NMR) (see Supporting Information) and X-ray diffraction (SC-XRD)
(vide infra). Since the process is extremely simple and efficient while all the involved
chemicals are inexpensive and commercially available, scalable preparation of the catalyst is
practicable as exemplified by an image of 0.6 kg 3b afforded in our laboratory (Figure 1b).
Relationship between structure and performance
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The comprehensive study of the effects of catalyst structure, catalyst loading, reaction
temperature, reaction time and CO2 pressure on coupling of CHO and CO2 were collected in
Table 1. As an overview, > 99% polymer selectivity for the coupling reaction and > 99%
carbonate linkages for the resultant PCHC were observed for all the organocatalysts mediated
CO2/CHO copolymerization as detected by 1H NMR and matrix-assisted laser
desorption/ionization time-of-flight mass (MALDI-TOF MS) spectroscopy (Figures S1–S3).
The low polydispersity index (Đ < 1.25) for all the obtained PCHCs, in combination with the
deviation between the theoretical molecular weight and measured number-average molar
mass (Mn) determined by gel permeation chromatography (GPC) with the bimodal traces
(Figure S4), indicated the immortal polymerization characteristics.69, 70 This phenomenon
could be attributed to the stubbornly residue water in epoxides and CO2. The MAIDL-TOF
MS spectrum (Figure S3) provided visible evidence, wherein the dihydroxy-end-capped
PCHC can be clearly observed. Based on the corresponding bimodal GPC trace, the fraction
31, 34, 71−73
of the dihydroxy-end-capped PCHC was determined to 29.4%.25,
It is worth
mentioning that all the produced PCHC are white (Figure S5) and odorless and can be easily
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obtained by precipitating from ethanol for only once without post-purifications as manifested
by elemental analysis in Supporting Information.
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Table 1. Epoxides/CO2 copolymerization results.a
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O
O
Catalyst
O
O
O
O
n
+
CO2
+
O
PCHC
CHC
c
entry
Cat.
Mon. /Cat.
Time
Temp.
Conv.b
TONb
Efficiencyd
poly(g)/cat(g)
150
PCHC/CHCb
(%)
ether linkageb
Mn
Ðc
(h)
3
(oC)
80
(%)
7
(%)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
(kg/mol)
15.6
19.2
27.2
29.8
22.7
26.6
23.4
22.3
21.2
27.8
28.3
7.8
1
2
1
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
5000
10000
10000
20000
2000
350
1300
1100
1550
950
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
1.11
1.14
1.16
1.14
1.23
1.13
1.13
1.11
1.12
1.16
1.16
1.13
1.16
1.20
1.18
1.21
1.25
1.15
1.23
1.18
1.17
2
3
80
26
22
31
19
25
26
30
29
32
30
2
520
3
3a
3b
3c
4
3
80
480
4
3
80
590
5
3
80
320
6
3
80
1250
1300
1500
1450
1600
1500
100
460
7
5
3
80
460
8
6
3
80
510
9
7
3
80
450
10e
11f
12
13
14
15
16
17
18
19g
20
21h
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3
80
610
3
80
570
12
3
25
40
60
14
58
68
49
49
74
95
65
46
700
270
27.9
30.3
28.1
20.0
18.7
27.1
22.5
24.0
23.7
3
100
120
140
150
80
2900
3400
2450
2450
7400
9500
13000
920
1110
1300
940
2
1
0.5
26
26;6
48
3
940
2830
3630
4960
420
80;120
80
80
aAll the polymerizations were carried out in 50 mL autoclaves under 15 bar CO2 or otherwise mentioned. Cat.,
catalyst; Temp., temperature; Conv., conversion. Calculated by 1H NMR. Determined by GPC in THF with
b
c
d
polystyrene standard. The efficiency of catalyst was calculated by: efficiency = TON×142.1/molecular
e
f
g
weight of catalyst. Polymerization was carried out under 20 bar CO2. 30 bar CO2 was applied. The
polymerization was conducted at 80 oC for 26 h, and then moved to 120 oC for 6 h. hVinylcyclohexene oxide
was tested.
Our research interest starts with the relationship between structure and performance of
the catalysts. Catalysts 1, 2, 3b, 4 and 5 were initially investigated since they have the same
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bromide ion (Br–), and the only difference between them is the spacer length between boron
atom (B) and ammonium cation (N+) (Table 1, entries 1, 2, 4, 6 and 7). As shown, the
variation in the spacer length exerts a tremendous influence on the catalytic performance. In
the presence of 1 (with a spacer length of trimethylene, –(CH2)3–), a TON value of 330 was
achieved in 3 h and a PCHC was obtained with a Mn of 15.6 kg/mol and low polydispersity
index (Đ = 1.11) on GPC trace (Table 1, entry 1). When the spacer length increased from
trimethylene –(CH2)3– (1) to tetramethylene –(CH2)4– (2), pentamethylene –(CH2)5– (3b),
hexamethylene –(CH2)6– (4), and heptamethylene –(CH2)7– (5), great increase in catalytic
activity were observed with TONs of 1300 (2), 1550 (3b), 1250 (4) and 1300 (5), respectively
(Table 1, entries 2, 4, 6, and 7). The comparison of catalytic performance under the same
conditions indicates that 3b is the most active catalyst and gives an efficiency of 595 g
PCHC/g 3b (Table 1, entry 4), revealing that the spacer length of pentamethylene is the
optimal spatial effect between B and N+ for the bifunctional catalysts. We therefore selected
this skeleton for further catalyst optimization in the following studies.
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We then focused on the influence of the structure of the quaternary ammonium salt, and
found that both of the substitute groups on N+ and the counterions affect the catalytic
performance. As shown in Table 1, entries 3–5, the anions of chloride (Cl–), Br– and iodide
(I–) made obvious difference in the polymerization activity in the following order: Br– (3b) >
Cl– (3a) > I– (3c), corresponding to TOFs of 517 h-1, 367 h-1 and 317 h-1, respectively, under
o
the same conditions (CHO/cat. = 5000/1, 80 C, 3 h, 15 bar CO2). The influence of the
substitute groups on N+ on the reaction rate could be observed through a comparative research
on catalysts 3b, 6 and 7. Compared to 3b equipped with triethyl groups on N+, catalysts 6 and
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7 were prepared with sterically larger tri-n-propyl groups and tri-n-butyl groups on N+
respectively. It is found that the increase of steric hindrance on N+ resulted in a tiny negative
effect on the CHO/CO2 copolymerization activity, as presented by the resultant TON values
of 1550 (3b), 1500 (6), and 1450 (7) under the same conditions (Table 1, entries 4, 8, and 9).
With the optimized catalyst 3b in hand, the influences of CO2 pressure, temperature, and
catalyst loading on the activity and selectivity were explored. It is found that the alteration of
CO2 pressure made minor difference in the polymerization activity, as disclosed by the slight
variations of TONs (1500–1600) over the pressure range of 15–30 bar (Table 1, entries 4, 10
and 11). The tiny influence of pressure suggests that the CO2 insertion may not the
rate-limiting step in the copolymerization process. Temperature impacts huge influence on the
coupling reaction. To our delight, 3b could efficiently catalyze the copolymerization in a
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o
broad temperature range from 25 to 150 C (Table 1, entries 12–17). At room temperature,
even with a low catalyst loading of 0.02 mol%, the copolymerization proceeds smoothly with
o
a TOF of 8 h-1. An increase in the reaction temperature from 60 to 80, 100, 120 and 140 C
resulted in a significant increase in the catalytic activity and the TOF values increased from
233 h-1 to 517 h-1, 967 h-1, 1700 h-1, and 2450 h-1, respectively (Table 1, entries 4 and 13–16).
o
It was delight to find that the maximum working temperature of 3b can reach 150 C, and a
high TOF of 4,900 h-1 was accordingly obtained (Table 1, entry 17). It should be noted that,
even at such high temperature there was no evidence for cyclic carbonate byproduct (CHC)
formation, meanwhile >99% carbonate linkages were still retained for the resultant PCHC
(Figure S6). Since copolymerization of CO2 and epoxides is intrinsic exothermic, the
development of a thermally robust catalyst is industrially desirable. To the best of our
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knowledge, a catalyst can endure such a high temperature while maintaining high activity and
selectivity has not been reported yet.
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With the goal of exploring the catalytic limits of 3b, the catalyst loading for CHO/CO2
copolymerization decreased to 0.01 mol%, a high reactivity with a TON of 7,400 in 26 h was
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observed at 80 C (Table 1, entry 18). Keeping the catalyst loading at 0.01 mol %, 95%
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conversion of CHO was achieved when the reaction was conducted at 80 C for 26 h,
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followed by reacting at 120 C for another 6 h (Table 1, entry 19). Given the polymerization
system usually solidifies at high monomer conversion, thereby the addition of solvent is
required to ensure sufficient reaction of the monomers. In our case, the heat tolerance of
catalyst 3b circumvents this embarrassment since the reduced viscosity of the polymerization
system allows the smooth progress of the copolymerization at this temperature (higher than
the glass transition temperature of PCHC). Of importance, under such extreme
conditions, >99% polymer selectivity with perfect carbonate linkages are still preserved.
Further reducing the catalyst loading to 0.005 mol % gave a TON of 13,000 in 48 h and an
efficiency of 5.0 kg PCHC/g catalyst (Table 1, entry 20). To the best of our knowledge, a 5.0
kg PCHC/g catalyst efficiency has not been achieved yet by any previously reported
homogeneous metallic or organic catalyst for the coupling reaction of CHO and CO2, in spite
of the fact that diversely optimal reaction conditions were performed under these highly
active catalyst systems. The unprecedented efficiency in combination with the long-time
thermostability enables the catalysts provided here to be a rare candidate for industrial
application.
Catalyst 3b was also used to copolymerizing CO2 with a range of other common epoxides
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with a catalyst loading of 0.05 mol % at 80 C and 15 bar CO2 pressure. For cyclopentene
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oxide and propylene oxide, the thermodynamically five-membered cyclic carbonates are the
only product under 3b (results not shown). The copolymerization of CO2 and
vinylcyclohexene oxide produced poly(vinylcyclohexene carbonate) (PVCHC) with a TOF of
307 h-1 without evidence of ether linkages and cyclic carbonate formation (Table 1, entry 21
and Figure S5). The generated PVCHC shows moderate Mn (23.7 kg/mol) and narrow
dispersity (Đ = 1.17), thus may be used to construct functional materials by
post-polymerization modifications.23
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To highlight the prominent advantages of our organoboron catalysts, we intend to make
a comprehensive comparison of 3b with the state-of-the-art metallic and organic catalysts that
have been reported (Figure S7). However, it is hard to directly compare the TONs, TOFs, and
catalytic efficiency of different catalysts because of the wide variation in reaction conditions.
Under the circumstances, we only compared our catalyst 3b with these catalysts using
evaluation criterions of the reported upper temperature of polymerization and PCHC
selectivity. It is clearly seen from Figure S7 that 3b is competitive with these once-dominant
catalysts.
Reaction kinetics study
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a
b
Catalyst order
Order in monomer
-1.8
-2.0
-2.2
-2.4
-2.6
-0.4
-0.8
-1.2
-1.6
-2.0
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y = 1.0307x - 2.0607
R2 = 0.99
y = 1.0251x - 3.9823
R2 = 0.98
0.0
0.4
0.8
1.2
1.6
1.4
1.6
1.8
2.0
ln [Cat. (mM)]
ln [CHO0 (M)]
c
d
Ea = 57.4 kJ mol-1
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
Order in CO2
-7.5
-8.0
-8.5
-9.0
-9.5
y = -6.91x + 10.20
-10.0
-10.5
R2 = 0.99
0
5
10 15 20 25 30 35 40 45
CO2 pressure (bar)
2.5
2.6
2.7
2.8
2.9
3.0
1000 T -1 (K-1)
Figure 2. Kinetic measurements. (a) First order in catalyst 3b. (b) First order in epoxide, CHO. (c)
Zero order in CO2 pressure. (d) Arrhenius plot for the formation of PCHC.
To gain mechanistic understanding of the copolymerization process, kinetic
measurements were performed by In-situ IR spectroscopy using 3b as the model catalyst
(Figure 2). The individual influence of monomers (CHO and CO2) and catalyst were explored
by implementing a series of experiments. The order in catalyst was determined by applying
o
five 3b concentrations from 1 mM to 5 mM at 80 C in neat CHO and 15 bar CO2. In all
cases, low CHO conversions (<5 %) and the C=O stretching vibration (at ~1750 cm-1) of the
carbonate group were used to determine the initial rate coefficient (kobs, was determined as the
slopes of fits to absorbance versus time). The plot of ln(kobs) versus ln([3b]) was obtained and
showed a linear fit to the data with a slope of 1.03, indicating a first order dependence on 3b
(Figure 2a). The reaction order in CHO was determined by conducting four independent
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experiments with varying CHO concentration (4, 5, 6, and 8 M) in diethyl carbonate (DEC).
The using DEC as the solvent owning to its similar density to CHO, meanwhile a reaction
mixture could be homogeneous formed in DEC, analogous to that in neat CHO.74 As depicted
in Figure 2b, the plot of ln(kobs) presents a linear dependence on ln([CHO0]) with a slope of
1.02, suggesting the copolymerization is first-order with respect to CHO. The reaction order
in CO2 was investigated by varying the initial CO2 pressure from 5 bar to 40 bar under
otherwise identical reaction conditions. By plotting kobs against CO2 pressure, a zero-order
dependence in CO2 pressure is determined (at least over the range of 5–40 bar, Figure 2c).
This experimental phenomenon was well consistent with the investigation of the effect of CO2
pressure on the reactivity (as shown in Table 1, entries 4, 10 and 11), which reveals that the
CO2 insertion is not rate determining step during the coupling process. It is notable that the
reaction rate was weakly dropped at high CO2 pressure (40 bar) which might due to
CO2-induced diluting effect under the solvent-free condition.74 At last, an apparent activation
energy of 57.4 kJ/mol for the formation of PCHC was obtained from low-conversion
experiments of CHO/CO2 copolymerization at various temperatures (60, 80, 100, and 120 oC)
(Figure 2d). The activation energy is consistent with values previously reported for CHO/CO2
coupling reaction under metal-based catalysts.21, 75 In short, the first-order rate dependence on
catalyst 3b and CHO while immune to the CO2 pressure (zero-order in CO2) indicates that
only one catalyst molecule is involved in the rate-determining step during the coupling
reaction. In other words, an intramolecular collaborative catalysis dominated the
copolymerization process, and this result prompts us to systematically investigate
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structure-performance relationships in the following paragraphs.
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Mechanistic investigation
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To probe the intramolecular cooperative effect between boron center and the quaternary
ammonium salt, a tricoordinate organoborane 8 without intramolecular quaternary ammonium
salt was prepared (Figure 3a). The boron center in 8 is embraced with a BBN structure that is
the same with 3b to ensure their identical Lewis acidity. Control experiments were conducted
using 8 in combination with one equivalent of (1-butyl)triethylammonium bromide (BTEAB)
(Figure 3a). As expect, the binary control catalyst system of 8/BTEAB provided poor
reactivity, irrelevant with the catalyst loading (Figure 3b and Table S1), whereas high
copolymerization rates were clearly observed using 3b. For example, 8/BTEAB demonstrated
a negligible activity (TOF = 5 h-1) for the copolymerization of CHO and CO2 with a catalyst
loading of 0.033 mol% (Figure 3b and Table S1), only one hundredth of that of 3b under the
same conditions. Even increasing the catalyst loading to 0.2 mol% while prolonging the
reaction time from 6 h to 12 h, there was little improvement on copolymerization activity
(TOF = 12 h-1, Figure 3b and Table S1). Once two equivalents of 8 was utilized in the binary
8/BTEAB catalyst system (8/BTEAB/CHO = 2/1/3000), a TOF of 330 h-1 was reached at 80
oC and 15 bar CO2 pressure (Table S1), which follows well with Feng and co-workers’
diboric pathway that is shown in Figure 1a. Even though, all the control copolymerizations
using the binary catalyst system are far inferior to our bifunctional catalyst 3b under the same
conditions. These comparison experiments manifest that an intramolecular synergistic effect
between the electrophilic boron center and the quaternary ammonium salt in 3b exerts a key
role on its high activity, revealing that a different copolymerization mechanism occurs in our
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bifunctional catalysts-mediated CO2/CHO coupling reaction.
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N
Br
Br
B
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BTEAB
N
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B
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b
8/BTEAB
800
600
400
200
0
3b
710
680
600
12
10
5
500
1000
3000
CHO/catalyst (mole ratio)
c
8
8/BTEAB (1:1)
3b
11
Figure 3. Control experiments and B NMR studies. (a) The chemical structures of control catalyst
system 8/BTEAB and the high-activity catalyst 3b. (b) The catalytic performances of 8/BTEAB and 3b
with different catalyst loadings. All the polymerizations were carried out in 50 mL autoclaves under 15
bar CO2 in neat CHO at 80 C. (c) The 11B NMR spectra of 8, 8/BTEAB (1/1, mole ratio) and 3b in
o
CDCl3 (192 MHz, 25 oC).
To further explore the intramolecular synergistic effect between the electrophilic boron
center and quaternary ammonium salt, 11B NMR studies of 3b, 8, and 8/BTEAB (1/1 in mole
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ratio) were performed and the results were shown in Figure 3c. A sharp resonance at δ =
88.11 ppm with a full width at half max (FWHM) of 296 Hz was observed for 8 on the
spectrum, indicating a classical signal assigned to tricoordinate organoborane. Negligible
variation was observed for the binary control catalyst system 8/BTEAB, which suggests that
the intermolecular interaction between 8 and BTEAB is imperceptible. In sharp contrast, a
significant difference was clearly observed for the bifunctional catalyst 3b, wherein a broad
resonance (FWHM = 1371 Hz) with a slight upfield chemical shift (Δδ = -0.34 ppm) was
observed. This phenomenon indicates the existence of an interaction between the
intramolecular quaternary ammonium salt and the boron center.76
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In an effort to find direct evidence to support the intramolecular cooperative effect
between the electrophilic boron center and quaternary ammonium salt, we attempted to get
the crystal of 3b. Despite the fact that many trials have been made, we were unable to grow a
crystal suitable for X-ray diffraction in non-coordinate solvent. Revisiting its 11B NMR
spectrum (Figure 3c), we proposed that the fail to get a crystal of 3b might be due to the
existence of a dynamic interaction between the quaternary ammonium salt and the boron
center. Based on this consideration, we attempted to break this intramolecular dynamic
equilibrium and cultivate the crystal in a solution that has good coordination ability to the
boron center. As expected, we were pleased to find a good single crystal of 3b grown by
diffusing n-hexane into a N,N-dimethylformamide (DMF) solution (mixed with 95%
chloroform ν/ν) of 3b at room temperature. The as-cultured colorless crystals were picked up
directly for SC-XRD measurement and its crystal structure was provided in Figure 4a (for
detailed crystal data, see Table S2). As shown in Figure 4a, the boron center of catalyst 3b is
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strongly coordinated with a DMF molecule with a B–O distance of 1.639(2) Å. The
tetracoordinate boron center is in distorted tetrahedral coordination sphere and linking to N+
by the flexibility spacer with a B–N distance of 7.025 Å. The nucleophilic Br– is confined by
N+ via Coulombic interaction with a Br–…N distance of 4.769 Å. On the basis of the crystal
structure, one can reason that the electron-deficient B and electropositive N+ should
cooperatively interact with Br– in non-coordinate solvent (i.e., without the coordinated DMF
molecule on the boron atom). This assumption is well consistent with the 11B NMR spectrum
of 3b in CDCl3 shown in Figure 3c, wherein a broadened and upfield shifted boron nucleus
signal appear due to the weak coordination interaction with Br–.
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a
Br
B
N
O
b
+0.261(αH1)
-0.508(Br)
-0.434(N)
+0.253(αH2)
+0.431(B)
Figure 4. (a) ORTEP drawing of the X-ray crystal structure of catalyst 3b coordinated with a DMF
molecule on the boron center. Thermal ellipsoids projected at 50% probability, all the hydrogen atoms
were omitted for clarity. (b) The optimized structure of catalyst 3b determined by DFT computational
study.
To verify the postulated cooperative interaction between Br– and the boron center and
N+, we turned to DFT investigations (Figure 4b, computational details are included in
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Supporting Information). Conformation search and the geometry optimization process were
initially carried out on the basis of the crystal structure of 3b. An unexpected phenomenon
was observed that the electropositive N+ distributes its positive charge on the hydrogen atoms
of the α carbons (-βCH2-αCH2-N+-αCH2-βCH2-), rather than carries the positive charge on itself
(Figure 4b). As a consequence, the Br– is synergistically stabilized in the intramolecular
cavity via Coulombic interaction with these positively charged hydrogen atoms (αH1…Br–
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α
distance, 2.71 Å; H2…Br– distance 2.48 Å) and the coordination interaction with the boron
center (B…Br– distance, 2.28 Å). The flexible spacer between B and N+ further facilitates the
synergistic stabilization of the Br–, as manifested by the attraction between αH2 and Br– with a
distance of 2.48 Å. The synergistic stabilization of Br– by B and the electropositive hydrogen
atoms can also be clearly observed in the corresponding noncovalent interactions analysis plot
(Figure S8).
O
O
Pn
O
Pn
IV
B
O
O
P
n : Br or polymer chain
O
CO2
B
III
O
O
Pn
Br
V
Br
ring-opening
II
O
O
B
I
B
B
Pn
O
B
Br
Br
O
Pn
B
B
N
B
Figure 5. Proposed mechanism based on 3b-mediated CO2/CHO copolymerization. The coulombic
interaction of propagating anion (alkoxide or carbonate anion) with specific hydrogen atoms was
simplified.
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On the basis of the control experimental results, the crystal structure and 11B NMR
analysis, an intramolecular B/N+ interaction mechanism was proposed and depicted in Figure
5. In the initial state, the Br– was synergistically confined between the Lewis acidic boron
center and the electropositive N+ through coordination to the boron center and Coulombic
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α
interaction with the H atoms. In the presence of CHO, the Lewis-base CHO monomer tends
to coordinate to the electron-deficient boron center (I), followed by the nucleophilic attack of
Br– to the methine carbon of the activated CHO, generating an alkoxide-participated
tetracoordinate boron center (II). Subsequently, CO2 molecule inserts into the boron-alkoxide
bond and gives a carbonate anion (III). The above-observed zero reaction order in CO2
indicates the formation of the carbonate anion is not the rate-limiting step. Given the
carbonate anion was less nucleophilic than the alkoxyl anion, the generated carbonate anion
would dissociate from the boron center and interact with the electropositive hydrogen atoms
through Coulombic interaction (IV).32 Other possible factors such as steric hindrance of the
boron center and the flexibility of the spacer between B and N+ may also facilitate the
dissociation of the carbonate anion from the boron center. Upon the carbonate anion
transferring from the boron center to the electropositive part, the vacated boron center could
be able to activate the next CHO monomer (V). In this context, the dissociated carbonate
anion hangs around the boron center and stands prepared to ring-open the next activated
CHO, leading to a high activity even at a high feeding ratio of CHO/catalyst. The alternating
coupling of CHO and CO2 process repeats between the boron center and the electropositive
part to afford the PCHC product.
DFT calculations
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O
Br
∆G (kcal/mol)
O
O
O
O
O
Br
O
B
O
Br
B
O
B
C
Br
O
O
O
23.38
TS3
23.08
TS2
O
9
Br
O
18.38
TS1
B
B
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11.01
Br
B
10.55
IN3
7.94
IN5
5.77
IN2
CHO IN6
CO2
1.92
0
O
CHO IN1
-3.28
IN4
O
-5.25
IN7
3b
O
Br
Br
O
B
O
O
Br
O
O
B
O
O
B
O
Br
Br
O
O
B
B
Figure 6. DFT calculated free energy profiles with the involved key intermediates and transition states
for 3b-catalyzed CO2/CHO copolymerization. The coulombic interaction of propagating anion (alkoxide
or carbonate anion) with specific hydrogen atoms of the α or/and β carbons was simplified, because the
α
β
propagating chain end in each step was stabilized by different electropositive H or/and H atoms
(-βCH2-αCH2-N+-αCH2-βCH2-). The detailed three-dimensional structures of optimized intermediates and
transition states were provided in Figure S9 in Supporting Information.
To gain a better mechanistic insight into the proposed reaction pathway, DFT
calculations were performed (Figure 6, for the three-dimensional structures of optimized
intermediates and transition states, see Figure S9). At initial period, the coordination of CHO
to the boron center causes a slight increase of Gibbs free energy compared with the initial
state, which is attributed to a comprehensive result of both entropy and enthalpy, because the
formation of coordination bond is enthalpy favorable but the combination of CHO and
catalyst is entropy unfavorable. Meanwhile the coordination of CHO to the boron center
isolates Br– from the boron center, which leads the Br– to merely interact with the H and H
atoms and be in an active state (IN1). As a consequence, the Gibbs free energy of the
ring-opening of CHO by Br– is as low as 16.46 kcal/mol (TS1). Upon ring-opening, the O-B
bond is shortened from 1.68 Å to 1.52 Å (IN2) (Figure S9) and CO2 inserts quickly with a
α
β
4.78 kcal/mol-1 increase in Gibbs free energy (IN3). Transition state TS2 describes the
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transformation of the coordination status of the boron center during the CO2 insertion process,
wherein the carbonate anion forms and coordinates to the boron center after overcoming an
energy barrier of 12.53 kcal/mol. After then, the carbonate anion dissociates from B and is
stabilized by Coulombic interaction with the αH and βH atoms with 11.22 kcal/mol increase in
Gibbs free energy. Then, the vacated boron center could be able to activate the next CHO
monomer (IN4–IN6) accordingly. The nucleophilic attack of the CHO by the carbonate anion
leads to the formation of IN7 via transition state TS3 with an energy barrier of 12.37
kcal/mol. The computationally estimated activation energy for the entire process is 26.66
kcal/mol, which corresponds close to the experimental value,18 revealing that the
computational studies provide a deeper understanding on our proposed mechanism in Figure
5.
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CONCLUSION
In summary, a series of highly active metal-free catalysts incorporating an electrophilic
boron center and a nucleophilic quaternary ammonium halide for copolymerization of
CHO/CO2 were prepared via an extremely simple and efficient way. The
structure−performance relationships of the bifunctional catalysts have been thoroughly
studied through the systematic variation of the substitute groups on the nitrogen atom, the
corresponding anion, as well as the spacer length between the boron center and the
ammonium cation. The catalysts demonstrated excellent catalytic performance in a broad
o
temperature range from 25 to 150 C with low catalyst loading (0.02–0.005 mol%) under
moderate pressure (5–30 bar) without the generation of cyclic carbonate byproducts, and
provided the corresponding polycarbonates in a controlled manner with more than 99%
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carbonate linkages. The highest turnover number of CHO reached up to 13000, equivalent to
an efficiency of 5.0 kg PCHC/g catalyst, which is the highest record achieved to date in all of
homogeneous organic and metallic catalysts for the CHO/CO2 coupling reaction. On the basis
of multiple control experiments, 11B NMR investigation and the crystal structure of catalyst, a
novel intramolecular cooperative catalysis mechanism is proposed and verified by DFT
calculations. We believe that the combined advantages of scalability synthetic process,
moisture and thermo stability, and the outstanding catalytic performance would enable the
catalysts to be an expectant candidate for producing CO2-based polymeric materials.
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ASSOCIATED CONTENT
Experimental procedures, characterization of all compounds and DFT computational data
Crystallographic data for 3b
AUTHOR INFORMATION
Corresponding Author
*gpwu@zju.edu.cn
Notes
The authors declare no competing interests.
Crystallographic data are available free of charge from the Cambridge Crystallographic Data
Centre under reference CCDC No. 1990910.
ACKNOWLEDGMENTS
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This work was supported by the National Natural Science Foundation of China (Grants
21674090, 91956123 and 51973186). The authors thank Dr. Shangfu Yuan in Tsinghua
University for assistance in X-ray structure measurement.
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O
O
O
CO2
+
n
R1
R2
R1
R2
O
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Catalyst preparation
Catalytic performance



cost effective



low catalyst loading (1/20000)
broad working temperature (25–150 oC)
cat.3b
kilogram scale
quantitative yield
0.6 kg
unprecedented efficiency (5.0 kg PCHC/g cat.)
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