Alternating copolymerization of CO2 and propylene oxide catalyzed by C2v-porphyrin cobalt: Selectivity control and a kinetic study

Article abstract of DOI:10.1016/j.jcat.2014.03.008

Some C_2v (C_s)-porphyrin cobalt complexes have been devised, synthesized and utilized as efficient catalysts in combination with dimethylaminopyridine for the alternating copolymerization of propylene oxide and carbon dioxide. The C_2v-catalyst [5,15-diphenylporphyrin]cobalt(III) chloride (1d) has demonstrated excellent reactivity and selectivity under optimized conditions. The axial counterions and the substituents of these metalloporphyrins exhibited distinct infection on the catalytic activity and selectivity. Synchronously, preliminary kinetic investigations of this reaction were achieved by using two catalyst systems illustrating the activation energies of copolymerization and coupling reaction, respectively. A competition mechanism between the polymerization and cycloaddition of propylene oxide and CO₂ has been carefully proposed according to our experimental results.

Full text of DOI:10.1016/j.jcat.2014.03.008

Journal of Catalysis 313 (2014) 159–167
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Alternating copolymerization of CO2 and propylene oxide catalyzed
by C2v-porphyrin cobalt: Selectivity control and a kinetic study
⇑
Xu Jiang, Faliang Gou, Huanwang Jing
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222 South Tianshui Road, Lanzhou, Gansu 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Some C2v (Cs)-porphyrin cobalt complexes have been devised, synthesized and utilized as efficient cata-
lysts in combination with dimethylaminopyridine for the alternating copolymerization of propylene
oxide and carbon dioxide. The C2v-catalyst [5,15-diphenylporphyrin]cobalt(III) chloride (1d) has demon-
strated excellent reactivity and selectivity under optimized conditions. The axial counterions and the
substituents of these metalloporphyrins exhibited distinct infection on the catalytic activity and selectiv-
ity. Synchronously, preliminary kinetic investigations of this reaction were achieved by using two cata-
lyst systems illustrating the activation energies of copolymerization and coupling reaction, respectively.
A competition mechanism between the polymerization and cycloaddition of propylene oxide and CO2 has
been carefully proposed according to our experimental results.
Received 21 December 2013
Revised 14 March 2014
Accepted 19 March 2014
Available online 16 April 2014
Keywords:
Carbon dioxide
C2v-Porphyrin cobalt
Propylene oxide
Alternating copolymerization
Polycarbonate
Ó 2014 Elsevier Inc. All rights reserved.
Kinetic study
1. Introduction
[19–22] and metalloporphyrins [23–27] have been commonly uti-
lized in this reaction as homogeneous catalysts and showed high
Carbon dioxide, a renewable, abundant, non-toxic, inexpensive
C1 resource, which is emitted as a waste product from a myriad
of metabolism of living organisms and industrial processes, is
increasingly viewed as an important feedstock for chemical pro-
cesses [1–3]. Thus, chemical fixation of CO2 is a very attractive sub-
ject in view of resource utilization and greenhouse gas reduction.
Developing new methods to activate and convert CO2 to usable
chemicals is a fascinating research area [4–7]. Some progresses
have been acquired over the last decades through a limited range
of reactions [8,9]. One of the promising reactions is the synthesis
of polycarbonates via metal-catalyzed copolymerization of CO2
with epoxides [10,11] (Fig. 1), which has great potential to replace
highly toxic phosgene in the synthesis of such important polycar-
bonates [12,13].
Polycarbonates are highly valuable polymeric materials, which
possess outstanding properties, including impact resistance, and
optical properties, especially biodegradation [14,15]. The first
example of polycarbonate synthesis via copolymerization of CO2
and epoxide using a mixture of ZnEt2 and H2O was discovered by
Inoue et al. [16,17]. In the last decade, more heterogeneous and
homogeneous catalyst systems were developed for this alternating
copolymerization [18], in which, the Salen-metal complexes
activity and selectivity [28,29].
After the first single-site homogeneous catalyst based on Al-
porphyrin complex was reported by Takeda and Inoue [30], many
studies on the metalloporphyrin systems [31–34], have been sub-
sequently reported, in which, a number of interesting cobaltopor-
phyrins were found to be more effective catalysts. Notably,
Sugimoto and Wang have employed [5,10,15,20-tetraphenylpor-
phyrin] cobalt (TPP-Co) to efficiently catalyze the alternating copo-
lymerization of CO2 with epoxide [33,34]. Rieger and co-workers
reported a series of functionalized cobaltoporphyrins (TXPP-Co)
in the meso-position to catalyze the copolymerization of propylene
oxide (PO) and CO2 yielding the polypropylene carbonate (PPC)
[35]. However, these catalysts with higher symmetry of C4v present
larger steric hindrance. Besides, this reaction often makes ether
linkages and more thermodynamically stable cyclic propylene car-
bonate (CPC) as byproducts which negatively impact on the prop-
erties of the resulting polymer.
On the base of previous literature [35], the selectivity of copoly-
mer is strongly dependent on ligand frameworks and electronic
features of the central metal ion. The steric barriers at the periph-
ery of the rigidly planar metalloporphyrin may also play an impor-
tant role for the activity and selectivity. To verify this hypothesis,
our attention was paid to synthesis of cobaltoporphyrins with
varied steric barriers and electronic environment, which can be
⇑
Corresponding author. Fax: +86 931 891 2582.
E-mail address: hwjing@lzu.edu.cn (H. Jing).
http://dx.doi.org/10.1016/j.jcat.2014.03.008
0021-9517/Ó 2014 Elsevier Inc. All rights reserved.

160
X. Jiang et al. / Journal of Catalysis 313 (2014) 159–167
employed in the alternating copolymerization of PO and CO2
(Fig. 2).
condenser and a magnetic stir bar under Ar, and merged into an
oil bath. When the mixture was heated to 70 °C, trifluoroacetic acid
(TFA) (1.2 mL, 16.16 mmol) was added immediately in one portion.
The mixture was stirred for about 2 h until the mixture became
clear and dark. After cooled down, it was quenched with 0.4 M
NaOH (60 mL) and extracted with dichloromethane (3 ꢀ 30 mL)
subsequently. The combined organic phase was washed with
water for several times and dried by Na2SO4. The dichloromethane
was removed via rotary evaporation. The excess of pyrrole was
recovered by distilling under reduced pressure to afford darkly
viscid oil. Then, it was purified by column chromatography using
eluent of CH2Cl2/hexane (2/1, v/v). The crude product was
recrystallized from ethanol–water to gain pure dipyrromethane
as a crystalline white solid in 52% yield [36]. 1H NMR (300 MHz,
CDCl3) d 3.96 (s, 2H), 6.16 (s, 2H), 6.30 (m, 2H), 6.64 (m, 2H),
7.55 (br, 2H).
2. Experimental
2.1. Materials and facilities
Propylene oxide was distilled from CaH2. N,N-dimethylamino-
pyridine (DMAP), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),
4-formylpyridine (PyCHO), tetrabutylammonium iodide (TBAI),
tetrabutylammonium bromide (TBAB), tetrabutylammonium chlo-
ride (TBAC), tetrabutylammonium fluoride (TBAF) were purchased
from Sinopharm Chemical Reagent Co., Ltd. Bis(triphenylphosphor-
anylidene) ammonium chloride (PPNCl) and phenyltrimethylam-
monium tribromide (PTAT) were supplied by Aladdin Chemistry
Co., Ltd. Other chemicals and reagents were commercial products
and used to the reaction without further purification unless other-
wise indicated.
2.2.2. Synthesis of 5,15-diphenylporphyrin (1b)
1H and 13C NMR spectra were recorded on a Bruker AM-300
spectrometer using CDCl3, d6-DMSO or CD3OD as the solvent and
TMS as an internal standard. MS spectra were obtained with a Bru-
ker ESQUIRE 6000 spectrometer. GPC was performed by using a
Waters1515 chromatograph at 35 °C with THF as eluent at a flow
rate of 1.0 mL/min referencing polystyrene standards.
Compound a (392 mg, 2.68 mmol) and benzaldehyde (0.28 mL,
2.75 mmol) was dissolved in 500 mL of freshly distilled dichloro-
methane in a dried 1000 mL three-neck round-bottom flask that
was shielded from light and equipped a magnetic stir bar. After
degased the solution with Ar for 10 min, TFA (0.12 mL, 1.62 mmol)
was introduced into the flask via syringe. The clear solution was
stirred for 4 h at room temperature. After addition of DDQ
(735 mg, 3.24 mmol), it was stirred for an additional hour. Then
4 mL of triethylamine was added to neutralize the mixture. When
the solvent was distilled off, the dark residue was purified by
column chromatography using CH2Cl2/hexane (5/1, v/v) as an
eluent. The target product was collected as purple crystal in 46%
yield [37]. 1H NMR (300 MHz, CDCl3) d ꢁ3.10 (s, 2H), 7.82 (m,
6H), 8.28 (m, 4H), 9.09 (d, J = 4.5 Hz, 4H), 9.40 (d, J = 4.5 Hz, 4H),
10.32 (s, 2H).
2.2. Synthesis of catalysts
The synthetic route of catalysts is shown in Fig. 3.
2.2.1. Synthesis of dipyrromethane (a)
A suspension of paraformaldehyde (1.86 g, 62.0 mmol) in pyr-
role (100 mL, 1.44 mol) was added to a 250 mL two-neck round-
bottom flask that was shielded from light, equipped with a water
2.2.3. Synthesis of 5-bromo-10,20-diphenylporphyrin (2b)
To a 150 mL round-bottom flask, a solution of compound 1b
(116 mg, 0.25 mmol) dissolved in 50 mL of CH2Cl2/MeOH (9/1,
v/v) and N-bromosuccinimide (NBS) (44.4 mg, 0.25 mmol) were
added. The reaction mixture was stirred at room temperature for
5 min and quenched with 5 mL of acetone. The resulting residue
was purified by column chromatography (hexanes/CH2Cl2, gradi-
ent from 10/1 to 4/1, v/v) leaving purple solid (2b) in 52% yield
[38]. 1H NMR (300 MHz, CDCl3) d ꢁ2.99 (s, 2H), 7.80 (m, 6H),
8.20 (m, 4H), 8.96 (d, J = 4.8 Hz, 4H), 9.29 (d, J = 4.5 Hz, 2H), 9.74
(d, J = 4.8 Hz, 2H), 10.18 (s, 1H).
Fig. 1. The reaction of epoxides and CO2.
2.2.4. Synthesis of 5,15-Dibromo-10,20-Diphenylporphyrin (3b)
To a 150 mL round-bottom flask, a solution of compound 1b
(116 mg, 0.25 mmol) dissolved in 50 mL of CH2Cl2/MeOH (9/1,
v/v) and NBS (91 mg, 0.51 mmol) were added. After 5 min, the
reaction completed and was quenched with 5 mL of acetone. The
volatile solvents were evaporated, and the product was washed
with several portions of methanol and dried in vacuum to yield
149 mg (0.24 mmol, 96%) of reddish purple solid [37]. 1H NMR
(300 MHz, CDCl3) d ꢁ2.72 (s, 2H), 7.77 (m, 6H), 8.17 (m, 4H),
8.84 (d, J = 4.8 Hz, 4H), 9.62 (d, J = 4.8 Hz, 4H).
2.2.5. General procedure for porphyrin-Co(II)
A 150 mL three-neck round-bottom flask equipped with a water
condenser and a magnetic stir bar under Ar was charged with DMF
(70 mL), porphyrin (1 mmol) and Co(OAc)2ꢂ4H2O (3 mmol). The
mixture was allowed to be stirring at 130 °C for 3 h. After cooled
down to room temperature, it was quenched by the addition of
ice-water. The amaranth precipitate was gotten by filtration,
washed with water for several times and dried in vacuo [35,39].
Fig. 2. Alternating copolymerization of CO2 and PO under cobaltoporphyrin
catalysis.

X. Jiang et al. / Journal of Catalysis 313 (2014) 159–167
161
Fig. 3. Synthetic route of catalysts.
[5,15-Diphenylporphyrin]cobalt(II) (1c): Yield: 99%. MS (ESI):
m/z = 519.3 [M]+.
2.2.7. General procedure for [5,15-diphenylporphyrin]cobalt(III)X
The procedure is similar to the previous description for porphy-
rin-Co(III)Cl.
[5-Bromo-10,20-Diphenylporphyrin]cobalt(II) (2c): Yield: 81%.
MS (ESI): m/z = 597.2 [M]+, 519.3 [M ꢁ Br]+.
X = Br (1e): Concentrated hydrobromic acid, 48%, 0.1 mL
(0.88 mmol); solvent: MeOH/CH2Cl2 (1/1, v/v), 100 mL; room tem-
perature. Yield: 94%. 1H NMR (300 MHz, DMSO-d6) d 7.92 (m, 6H),
8.25 (m, 4H), 9.31 (d, J = 4.5 Hz, 4H), 9.93 (d, J = 4.5 Hz, 4H), 10.99
(s, 2H). 13C NMR (101 MHz, DMSO-d6) d 143.2, 142.9, 140.3, 135.5,
134.4, 134.0, 128.3, 127.4, 118.1, 106.0. MS (ESI): m/z = 599.2 [M]+,
519.5 [M ꢁ Br]+. Elemental Anal. Calcd for C32H20N4CoBrꢂCH2Cl2: C,
57.92; H, 3.24; N, 8.19. Found: C, 58.39; H, 3.43; N, 8.02.
X = OAc (1f): Glacial acetic acid, 0.24 mL (4 mmol); solvent: tol-
uene, 10 mL; 70 °C. Yield: 91 mg, 78%. 1H NMR (300 MHz, CDCl3) d
2.35 (s, 3H), 9.52 (m, 4H), 9.70 (m, 6H), 12.40 (br, 4H), 15.21 (br,
4H), 17.11 (br, 2H). MS (ESI): m/z = 579.1 [M]+, 519.3 [M ꢁ OAc]+.
Elemental Anal. Calcd for C34H23N4O2Coꢂ1/4CH2Cl2: C, 68.63; H,
4.09; N, 9.28. Found: C, 69.07; H, 4.15; N, 9.08.
X = OTs (1g): p-Toluenesulfonic acid, 79 mg (0.4 mmol); solvent:
MeOH/CH2Cl2 (1/1, v/v), 100 mL; room temperature. Yield: 97%. 1H
NMR (300 MHz, DMSO-d6) d 2.27 (s, 3H), 7.09 (d, J = 7.5 Hz, 2H),
7.45 (d, J = 7.5 Hz, 2H), 7.92 (m, 6H), 8.25 (m, 4H), 9.31 (d,
J = 4.8 Hz, 4H), 9.92 (d, J = 4.8 Hz, 4H), 10.99 (s, 2H). 13C NMR
(101 MHz, DMSO-d6) d 146.0, 143.0, 142.7, 140.1, 137.3, 135.3,
134.2, 133.8, 128.1, 127.8, 127.2, 125.3, 117.9, 105.8, 20.5. MS
(ESI): m/z = 691.2 [M + H]+, 519.6 [M ꢁ OTs]+. Elemental Anal.
Calcd for C39H27N4O3CoSꢂ4H2O: C, 61.41; H, 4.63; N, 7.35. Found:
C, 61.18; H, 4.24; N, 6.93.
[5,15-Dibromo-10,20-Diphenylporphyrin]cobalt(II) (3c): Yield:
93%. MS (ESI): m/z = 677.0 [M]+, 599.2 [M ꢁ Br]+.
2.2.6. General procedure for porphyrin-Co(III)Cl
A suspension of porphyrin-Co(II) (0.2 mmol) in 100 mL of meth-
anol was added concentrated hydrochloric acid (0.5 mL, 5 mmol)
and then stirred at room temperature under oxygen. The reaction
mixture became clear within several minutes and was stirred for
additional 2 h. After removed the solvent under reduced pressure,
the residue was washed with several portions of water until the
filtrate was neutral and dried in vacuum to get hold of the purple
solid [40].
[5,15-Diphenylporphyrin]cobalt(III) Chloride (1d): Yield: 95%. 1H
NMR (300 MHz, CD3OD) d 7.85 (m, 6H), 8.24 (m, 4H), 9.42 (d,
J = 4.8 Hz, 4H), 9.87 (d, J = 4.8 Hz, 4H), 11.00 (s, 2H). 13C NMR
(101 MHz, CD3OD) d 145.7, 145.6, 142.4, 136.6, 136.4, 135.5,
129.6, 128.4, 120.6, 107.3. MS (ESI): m/z = 553.2 [M ꢁ H]+, 519.4
[M ꢁ Cl]+. Elemental Anal. Calcd for C32H20N4CoClꢂ2H2O: C, 65.04;
H, 4.09; N, 9.48. Found: C, 64.97; H, 3.96; N, 9.21.
[5-Bromo-10,20-Diphenylporphyrin]cobalt(III) Chloride (2d):
Yield: 97%. 1H NMR (300 MHz, DMSO-d6) d 7.90 (m, 6H), 8.20 (m,
4H), 9.04 (m, 4H), 9.64 (d, J = 4.8 Hz, 2H), 9.86 (d, J = 4.8 Hz, 2H),
10.62 (s, 1H). 13C NMR (101 MHz, DMSO-d6) d 144.2, 143.7,
143.2, 142.3, 140.6, 135.3, 134.9, 134.6, 134.2, 134.0, 133.9,
128.1, 127.3, 119.0, 106.5. MS (ESI): m/z = 633.0 [M]+, 599.1
[M ꢁ Cl]+. Elemental Anal. Calcd for C32H19N4CoBrClꢂ2H2O: C,
57.38; H, 3.46; N, 8.36. Found: C, 57.25; H, 3.24; N, 8.24.
[5,15-Dibromo-10,20-Diphenylporphyrin]cobalt(III) Chloride (3d):
Yield: 96%. 1H NMR (300 MHz, DMSO-d6) d 7.85 (6H), 8.04 (4H),
8.94 (4H), 9.74 (4H). 13C NMR (101 MHz, DMSO-d6) d 143.5,
143.2, 139.5, 136.2, 135.6, 133.6, 128.6, 127.5, 115.1, 107.1. MS
(ESI): m/z = 711.0 [M + H]+. Elemental Anal. Calcd for C32H18N4-
CoBr2Clꢂ2CH2Cl2ꢂCH3OH: C, 45.96; H, 2.87; N, 6.13. Found: C,
46.31; H, 2.33; N, 6.44.
X = OOCCF3 (1h): Trifluoracetic acid, 70 mg (0.9 mmol); solvent:
MeOH/CH2Cl2 (1/1, v/v), 100 mL; room temperature. Yield: 94%. 1H
NMR (300 MHz, DMSO-d6) d 7.90 (m, 6H), 8.23 (m, 4H), 9.29 (d,
J = 4.8 Hz, 4H), 9.90 (d, J = 4.8 Hz, 4H), 10.97 (s, 2H). 13C NMR
(101 MHz, DMSO-d6) d 154.2, 142.8, 142.1, 135.4, 134.6, 133.9,
129.3, 128.3, 127.3, 121.9, 114.1, 105.9. MS (ESI): m/z = 536.4
[M ꢁ OCCF3]+, 563.4 [M ꢁ CF3]+.
X = 2,4-Dinitrophenoxy
(1i):
2,4-Dinitrophenol,
37 mg
(0.2 mmol); solvent: toluene, 10 mL; 70 °C. Yield: 129 mg, 92%. 1
H
NMR (300 MHz, DMSO-d6) d 6.57 (s, 1H), 7.91 (s, 6H), 8.19 (s, 1H),
8.24 (s, 4H), 8.57 (s, 1H), 9.30 (s, 4H), 9.92 (s, 4H), 10.98 (s, 2H).

162
X. Jiang et al. / Journal of Catalysis 313 (2014) 159–167
13C NMR (101 MHz, DMSO-d6) d 143.1, 142.9, 141.3 140.3, 135.5,
134.4, 134.0, 132.8, 129.3, 128.3, 127.3, 125.1, 118.5, 118.0, 114.8,
106.0. MS (ESI): m/z = 519.3 [M ꢁ 2,4-Dinitrophenoxy + H]+, 559.4
[M ꢁ NO2]+. Elemental Anal. Calcd for C39H27N4O3CoSꢂphMeꢂH2O:
C, 66.50; H, 4.09; N, 10.34. Found: C, 66.11; H, 3.67; N, 10.45.
3. Results and discussion
3.1. Copolymerization of CO2 and PO by the metalloporphyrins-DMAP
system
In the metal-catalyzed copolymerization of CO2 and epoxide,
the steric and electronic features of the substituted ligands often
vision significant influences [35]. For screening out good catalysts,
some specific meso-substituted cobaltoporphyrins (1d–3d) with
C2v/Cs-symmetry were designed and synthesized, which reveal
less steric barriers than the metalloporphyrins with C4v-symme-
try. Their catalytic activities were explored toward the copoly-
merization of PO and CO2 (Table 1). It can be seen that
1d-DMAP system displayed the best reactivity (94.9% yield) and
PPC selectivity (>99%) owing to its minimized steric hindrance
(Table 1, entry 1). The activity and selectivity of 4d was lower
than 1d due to its higher steric hindrance (Table 1, entry 9).
When the meso-position of the porphyrin was substituted by bro-
mine to give Cs-porphyrin (2d), the electron withdrawing substi-
tuent led a slight reduction of total yield and PPC selectivity
(Table 1, entry 7). Furthermore, the more electron deficient 3d
threw out the worst activity and selectivity, and the only product
was cyclic carbonate (Table 1, entry 8). It is clear to see that the
reactivity and selectivity of this copolymerization do depend on
the framework of catalyst.
The activation of epoxides caused by the metal center of cata-
lyst is an important step in the copolymerization [28,29]. The
activity of catalyst relies on the Lewis acidity of the metal center
which can be also affected by the counterion. To select an optimal
counterion for the catalyst in this reaction, cobalt(III) porphyrins
with different counterions were prepared and tested in the pres-
ence of DMAP for the PO/CO2 copolymerization. The results
showed that the activity order is about 1e > 1d > 1f > 1g > 1h = 1i
(Table 1, entries 1–6). Considering the activity and selectivity,
1d-DMAP is the best catalytic system.
2.2.8. [5,10,15,20-Tetraphenylporphyrin]cobalt(III) chloride
[(TPP)CoCl] (4d)
Compound 4d was prepared by following the literature meth-
ods [35].
2.3. General procedure for the copolymerization of PO and CO2
All reactions were carried out in a dry 100 mL stainless steel
autoclave equipped with a magnetic stir bar and merged into an
oil bath. The required catalyst, co-catalyst and PO were added to
the reactor in turn. The reactor was charged with CO2 and vented
for three times, and then pressurized with CO2 to needed pressure,
and stirred at the requested temperature. After a proper time, the
autoclave was cooled down to the room temperature and then
vented slowly. The resulting mixture was analyzed by 1H NMR to
give the selectivity of PPC versus CPC as well as carbonate linkages
quantitatively. The desired polymer was precipitated by addition
of excess methanol into a solution of crude product in CH2Cl2. It
was then filtered and washed with methanol for several times to
receive a white polymer that was dried in vacuo [40,41].
2.4. The kinetic investigations
To get the kinetic data, the reactions were performed under var-
ious temperatures. Several autoclaves were parallelized in oil
baths. Then the reactions were commenced simultaneously and
ceased after a predicted time, respectively. To make sure the cred-
ibility of data, each reaction was repeated at least 3 times. The
selectivity of PPC to CPC was measured by 1H NMR spectroscopy
analysis immediately [42]. The yields of copolymer and cyclic car-
bonate were calculated in terms of the average selectivity and total
yield and used as their concentration data for analysis. All kinetic
data at various temperatures were listed in Table S1–S7 respec-
tively (Supplementary material). The copolymerization and cyclo-
addition reactions are supposed as pseudo first order reaction
depending on PO concentration respectively. Therefore, the reac-
tion rate constant k of PPC and CPC can be computed by Eq. (1):
3.2. The effect of co-catalyst for 1d on copolymerization of PO and CO2
The co-catalyst, such as amine, phosphine or quaternary onium
salt, was found quite important to the reaction of PO and CO2
[33,43]. Using the best catalyst 1d, the effect of co-catalyst on
copolymerization was investigated and listed in Table 2. The 1d-
DMAP system demonstrated superior PPC selectivity and Mn value
in comparison with other co-catalyst (Table 2, entry 1). When
DMAP was replaced by PTAT, the catalyst systems gave an excel-
lent yield (98.6%), but a lower PPC selectivity (64.1%) (Table 2,
entry 2). When quaternary ammonium salts (TBAF, TBAC, TBAB
or TBAI) were employed as co-catalyst, the catalytic activity was
increased in turn, but the PPC selectivity was decreased orderly
lnð1 ꢁ xÞ ¼ ꢁkt þ Cðx ¼ ½PPCꢃ or ½CPCꢃÞ
The activation energy of PPC and CPC can be accounted in the
light of the relationship between the rate constant and the temper-
ature using Arrhenius equation.
ð1Þ
Table 1
Copolymerization of CO2 and PO by the cobaltoporphyrins-DMAP system.
Entrya
Cat.
X
Total yieldb (%)
Selectivityc (PPC%)
Carbonate linkaged (%)
Mn
Mz/Mw
e
e
1
2
3
4
5
6
7
8
9
1d
1e
1f
1g
1h
1i
2d
3d
4d
Cl
Br
OAc
OTs
94.9
97.4
7.3
4.9
1.6
>99
32.7
0
0
0
0
93.9
0
>99
>99
/
/
/
/
>99
/
>99
16,984
14,303
/
/
/
/
7551
/
14,383
1.13
1.13
/
/
/
/
1.07
/
1.14
OOCCF3
2,4-Dinitrophenoxy
Cl
Cl
Cl
1.6
86.4
30.8
88.2
93.0
Reaction conditions:
a
Catalyst: 0.05 mmol, [cat.]0/[DMAP]0/[PO]0 = 1/1/500, 72 h reaction time, no solvent, 1.5 MPa, T = 25 °C.
Total yield (%) = isolated yield.
b
c
Yield ratio of the PPC to the CPC determined by 1H NMR spectrum of the reaction mixture.
Estimated from the 1H NMR spectrum of the reaction mixture.
Estimated by GPC on the basis of polystyrene standards.
d
e

X. Jiang et al. / Journal of Catalysis 313 (2014) 159–167
163
Table 2
Copolymerization of CO2 and PO by the 1d-Co-catalyst System.
e
e
Entrya
Co-cat.
Total yieldb (%)
Selectivityc (PPC%)
Carbonate linkaged (%)
Mn
Mz/Mw
1
2
3
4
5
6
7
8
9
DMAP
PTAT
TBAF
TBAC
TBAB
TBAI
PPNCl
pyridine
DBU
94.9
98.6
51.5
69.1
88.0
>99
26.7
43.7
6.6
>99
64.1
95.5
94.9
47.5
0
0
96.5
0
>99
97.4
95
>99
>99
/
16,984
6268
5208
10,293
5529
/
1.13
1.17
1.05
1.13
1.12
/
/
/
/
>99
/
7001
/
1.07
/
Reaction conditions:
a
Catalyst: 0.05 mmol, [cat.]0/[co-cat.]0/[PO]0 = 1/1/500, no solvent, 72 h reaction time, 1.5 MPa, T = 25 °C.
Total yield (%) = isolated yield.
b
c
Yield ratio of the PPC to the CPC determined by 1H NMR spectrum of the reaction mixture.
Estimated from the 1H NMR spectrum of the reaction mixture.
Estimated by GPC on the basis of polystyrene standards.
d
e
Table 3
Effects of Reaction Temperature and CO2 Pressure on Copolymerization of CO2 and PO Catalyzed by 1d-DMAP System.
Entrya
T (°C)
Pressure (MPa)
Total yieldb (%)
Selectivityc (PPC%)
Carbonate linkaged (%)
Mn
Mz/Mw
e
e
1
2
3
4
5
6f
7
8
9
0
1.5
1.5
1.5
1.5
1.5
1.5
2.0
1.0
0.5
22.0
83.7
94.9
95.6
>99
96.7
97.2
>99
96.2
76.0
95.2
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
5137
1.09
1.12
1.13
1.22
1.34
1.09
1.13
1.15
1.23
15
25
40
60
25
25
25
25
13,195
16,984
14,281
12,167
11,258
15,404
9032
>99
95.4
91.5
90.0
92.5
92.1
7795
Reaction conditions:
a
Catalyst: 0.05 mmol, [cat.]0/[DMAP]0/[PO]0 = 1/1/500, 72 h reaction time, no solvent.
Total yield (%) = isolated yield.
b
c
d
e
f
Yield ratio of the PPC to the CPC determined by 1H NMR spectrum of the reaction mixture.
Estimated from the 1H NMR spectrum of the reaction mixture.
Estimated by GPC on the basis of polystyrene standards.
Solvent = CH2Cl2 (3.5 mL).
(Table 2, entries 3–6). This phenomenon can be explained by the
different nucleophilicity and leaving property of X [43]. Surpris-
ingly, the co-catalyst of PPNCl combining with 1d only afforded
CPC in low yield (Table 2, entry 7), which is noteworthy with
C4v-porphyrins [34].
composed of 1d and an equimolar amount of DMAP provided the
production of the polycarbonate with considerable yield, PPC
selectivity, and polymer molecular weight, which was superior to
other mole ratios (Table 4, entry 3). Lowering the mole ratio of
DMAP to 1d from 1 to 0.25, the activity of catalyst system de-
creased orderly leading to the reduction of the molecular weight,
even if the PPC selectivity kept the same of quantity (Table 4,
entries 3–6). Strangely, when two equimolar amount of DMAP
was applied in the reaction, the catalyst of 1d was poisoned as
the absence of DMAP (Table 4, entries 1 and 8).
ꢀ
3.3. The effect of temperature and CO2 pressure on copolymerization of
PO and CO2
Temperature and CO2 pressure are key factors on copolymeriza-
tion of PO and CO2. As shown in Table 3, a rise of temperature fa-
vors PPC yield from 0 to 25 °C, however, further increase of
temperature causes decreases in PPC selectivity and polymer
molecular weight (Table 3, entries 1–5, also see Fig. 4). In addition,
an increase of reaction temperature led to a broader molecular
weight distribution.
When increasing the CO2 pressure from 0.5 to 1.5 MPa (Table 3,
entries 3, 8 and 9), the Mn value shot up from 7795 to 16,984, even
though both activity and selectivity only experienced a slight in-
crease. The highest yield occurred at 2.0 MPa (Table 3, entry 7)
with a trivial decrease of Mn value. Eventually, 1.5 MPa CO2 pres-
sure was chosen as the optimal reaction condition in the next
investigations.
3.5. The effect of the mole ratio of PO to 1d on copolymerization
A further set of experiments were performed to probe the effect
of the mole ratio of PO to 1d on copolymerization. As can be seen in
Table 5, when the mole ratio of [PO]/[catalyst] was 1000, the sys-
tem showed superior Mn value in comparison with other mole ratio
(Table 5, entry 4). A rise of the mole ratio of PO to 1d resulted in the
reduction of the molecular weight distribution (Table 5, entries
2–5, 7–8). The Mn value and yield can be improved by extending
the reaction time (Table 5, entry 6). For further understanding
the structure of polymers, 13C NMR spectra of the obtained PPC
were recorded and showed that the head-to-tail (H–T) connectivity
was 93.5% that was consistent with the literature results [35]
(Supplementary material, Fig. S1).
3.4. The effect of the mole ratio of DMAP to 1d on copolymerization
3.6. Kinetic studies of the PO/CO2 copolymerization
To tune the activity and selectivity of catalyst, the impact of the
mole ratio of DMAP to 1d on copolymerization was accomplished,
and the results were listed in Table 4. The catalyst system
Some kinetic studies on the copolymerization of epoxides and
CO2 have been fulfilled by in situ IR in Ding and Lu groups

164
X. Jiang et al. / Journal of Catalysis 313 (2014) 159–167
Fig. 5. Simple sketch for the PO/CO2 coupling reaction.
a competitive relation between the formation of PPC (path a) and
CPC (path b), which implied the PPC depolymerization (path c)
might not accrue during the progress of reactions. To investigate
the condition of path c, the resulting PPC was mixed with the cat-
alyst of 1d and DMAP in the autoclave charged with 1.5 MPa CO2
under stirring. After 12 h, the mixture was detected by 1H NMR
immediately (Fig. 6). As expected, no CPC was detected at 25 °C
(Fig. 6(a)). Trace CPC was produced at 40 °C (Fig. 6(b)). Only 2%
CPC was measured in 1H NMR spectrum at 60 °C (Fig. 6(c)), which
could not affect our kinetic data. However, when the temperature
was increased up to 100 °C, the PPC depolymerized to CPC in 13%
(Fig. 6(d)), which cannot be neglected.
Fig. 4. Total yield and selectivity of PPC with various temperatures for 1d-DMAP
under 1.5 MPa CO2.
[44,45]. The catalytic procedures would occur via three pathways
(Fig. 5). To clarify the mechanism for the copolymerization
(Fig. 5, path a) and the cycloaddition reaction (Fig. 5, path b) of
PO and CO2 catalyzed by our catalyst systems, some preliminary
kinetic studies were carried out by using 1d-DMAP system (see
the Supplementary material, Table S1–S4, Fig. S2–S5), in which,
the pseudo first order reactions depending on the PO concentration
were fitted well with Eq. (1) in this event, which was consistent
with our previous report using the catalyst system of metallopor-
phyrin/PTAT [42]. These two pseudo first order reactions exhibited
According to Arrhenius Law, the activation energies of EðaaÞ to
PPC and EðabÞ to CPC can be computed from the detailed kinetic data
Table 4
Effects of the Mole Ratio of DMAP to 1d on Copolymerization of CO2 and PO.
Entrya
[DMAP]0/[1d]0
Total yieldb (%)
Selectivityc (PPC%)
Carbonate linkaged (%)
Mn
Mz/Mw
e
e
1
2
3
4
5
6
7
8
2
1.5
1
0.75
0.5
0.25
0.1
0
0.5
82.4
94.9
93.3
79.3
66.4
35.8
0.7
0
/
/
/
88.8
>99
>99
>99
>99
97.4
0
>99
>99
>99
>99
>99
96.7
/
6245
16,984
13,481
13,977
10,998
4930
/
1.16
1.13
1.16
1.15
1.15
1.02
/
Reaction conditions:
a
Catalyst: 0.05 mmol, [cat.]0/[PO]0 = 1/500, 72 h reaction time, no solvent, 1.5 MPa, T = 25 °C.
Total yield (%) = isolated yield.
b
c
Yield ratio of the PPC to the CPC determined by 1H NMR spectrum of the reaction mixture.
Estimated from the 1H NMR spectrum of the reaction mixture.
Estimated by GPC on the basis of polystyrene standards.
d
e
Table 5
Effects of the Mole Ratio of PO to 1d on Copolymerization of CO2 and PO.
Entrya
[PO]0/[1d]0
Total yieldb (%)
Selectivityc (PPC%)
Carbonate linkaged (%)
Mn
Mz/Mw
e
e
1f
2
3
100
250
500
1000
2000
2000
5000
10,000
97.2
>99
97.8
>99
>99
98.0
98.8
98.9
94.5
96.1
>99
>99
>99
>99
>99
>99
>99
>99
7639
1.15
1.17
1.13
1.12
1.08
1.08
1.06
1.01
12,087
16,984
21,249
14,939
17,130
8540
94.9
77.0
51.9
94.6
20.5
13.8
4
5
6g
7
8h
4453
Reaction conditions:
a
Catalyst: 0.025 mmol, [cat.]0/[DMAP]0 = 1/1, 72 h reaction time, no solvent, 1.5 MPa, T = 25 °C.
Total yield (%) = isolated yield.
b
c
d
e
f
Yield ratio of the PPC to the CPC determined by 1H NMR spectrum of the reaction mixture.
Estimated from the 1H NMR spectrum of the reaction mixture.
Estimated by GPC on the basis of polystyrene standards.
Catalyst: 0.25 mmol.
g
h
Time: 7d.
Catalyst: 0.0125 mmol.

X. Jiang et al. / Journal of Catalysis 313 (2014) 159–167
165
Fig. 6. 1H NMR spectra of the mixture after the PPC deploymerization.
Fig. 7. Arrhenius plots for the formation of PPC and CPC from the coupling of CO2 and PO using different catalysts: (a) 1d-DMAP catalyst and (b) 2d-DMAP catalyst.
at various temperatures illustrated in Fig. 7. The EðaaÞ for PPC forma-
tion is only 11.7 kJ/mol, while the EðabÞ for CPC formation is up to
51.6 kJ/mol. It can be concluded that the PPC is more facile to
produce than the CPC catalyzed by 1d-DMAP system owing to
the lower barrier (Fig. 8a).
Furthermore, the kinetic data of another catalyst system con-
sisting of the Cs-porphyrin cobalt 2d and DMAP was got by the
same way (see the Supplementary material, Table S5–S7, Fig. S6–
S8). In this system, the activation energies E0aðaÞ (PPC) and E0aðbÞ
(CPC) are 15.7 and 39.3 kJ/mol, severally. It is clearly to see the
activation energy E0aðaÞ is higher than that of C2v-catalyst 1d attrib-
uting to its steric and electron effect. The difference of activation
the PO is activated by the cobaltoporphyrin catalyst to form
intermediate 1. Then, the nucleophile X (Cl or Br) released from
ꢀ
catalyst attacks the activated epoxide at the less hindered carbon
leading to the requisite metal alkoxide intermediate 2. And it at-
tacks CO2 at the Lewis acidic carbon to generate a new interme-
diate 3. Subsequently, it is another activated epoxide molecular
that is attacked by 3 to produce the intermediate 4, which is
the main reason why the activation barrier for PPC formation
is readily accessible. And then CO2 and 1 inserted to 4 alter-
nately forming copolymerization carbonate linkage. It can be
clearly seen that when the oxygen anion of intermediate 3
ꢀ
competitively attacks its inner carbon to release the X, the by-
energy
D
Ea is calculated to 23.6 kJ/mol (Fig. 8b), which is lower
product CPC was yielded, that is so-called back-biting mecha-
nism. Thereby, the better leaving ability of Br enables 1e
ꢀ
than the 39.9 kJ/mol in the 1d-DMAP system and results in lower
PPC selectivity. These provide a reasonable explanation for the high
PPC selectivity of copolymer formation by the 1d-DMAP system.
(X = Br) to behave worse than 1d (X = Cl) in path a, but better
in path b. In addition, when the mole ratio of PO to 1d was
augmented from 1000 to 5000, the PPC selectivity acquired at
the initial stage decreased in sequence (Supplementary material,
Table S8), which can be attributed to the reduction of intermo-
lecular collision probability and the augmentation of intramolec-
ular collision probability. This issue further identified the
proposed mechanism.
3.7. The proposed mechanism for the reaction of PO and CO2
Based on the above phenomena,
mechanism between the polymerization and cycloaddition of
PO and CO2 was carefully proposed and drawn in Fig. 9. Initially,
a plausible competition

166
X. Jiang et al. / Journal of Catalysis 313 (2014) 159–167
Fig. 8. Reaction coordinate diagrams of the coupling of CO2 and PO using different catalysts: (a) 1d-DMAP catalyst and (b) 2d-DMAP catalyst.
modification in the framework and the substituents could be a
potential methodology, which is waiting for us to further explore.
Acknowledgment
We gratefully acknowledge the financial support of the National
Natural Science Foundation of China (NSFC20973086, 21173106)
and the Research Fund for the Doctoral Program of Higher
Education of China (20100211110025).
Appendix A. Supplementary material
The 13C NMR Spectrum of obtained PPC (Figure S1). The
experimental procedure of PPC depolymerization. Kinetic data of
1d-DMAP catalyst system and 2d-DMAP catalyst system on copo-
lymerization of PO and CO2 (Table S1–S7). Logarithmic plots of
(1 ꢁ x) versus time by 1d/2d-DMAP system (x = [PPC] or [CPC])
(Fig. S2–S8). The data of PPC selectivity under the mole ratio of
PO to 1d from 1000 to 5000 on PO/CO2 copolymerization at the
initial stage (Table S8).
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2014.03.008.
Fig. 9. Proposed mechanism for alternating copolymerization of PO and CO2.
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