Copolymerization of carbon dioxide and cyclohexene oxide catalyzed by chromium complexes bearing semirigid [ONSO]-type ligands

Article abstract of DOI:10.1002/pola.28052

Chromium complexes supported by tetradentate dianionic imine-thioether-bridged bis(phenol) ligands were prepared and employed in the synthesis of poly(cyclohexene carbonate) via the copolymerization of CO₂ and cyclohexene oxide. The catalytic a

Full text of DOI:10.1002/pola.28052

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Copolymerization of Carbon Dioxide and Cyclohexene Oxide Catalyzed
by Chromium Complexes Bearing Semirigid [ONSO]-Type Ligands
Bing Han,1 Li Zhang,1 Samuel J. Kyran,2 Binyuan Liu,1 Zhongyu Duan,1
Donald J. Darensbourg2
1School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China
2Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, TX 77843
Correspondence to: B. Liu (E-mail: Byliu@Hebut.Edu.Cn) or D. J. Darensbourg (E-mail: Djdarens@Chem.Tamu.Edu)
Received 1 December 2015; accepted 9 January 2016; published online 8 February 2016
DOI: 10.1002/pola.28052
ABSTRACT: Chromium complexes supported by tetradentate dia-
nionic imine-thioether-bridged bis(phenol) ligands were prepared
and employed in the synthesis of poly(cyclohexene carbonate) via
the copolymerization of CO2 and cyclohexene oxide. The catalytic
activity, product selectivity, and kinetic behaviors of these
[ONSO]CrIII complexes have been systematically investigated.
Results indicate the presence of electron-withdrawing substitu-
ents on the ligands to enhance catalytic activity and polymer
selectivity. A turnover frequency of 100 h21 is observed at a tem-
perature of 110 8C, producing polycarbonate with >60% selectiv-
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KEYWORDS: carbon dioxide; catalysts; copolymerization; poly-
carbonates; semirigid ligands
(Mn < 1800 Da) containing detectable ether linkages
(ꢀ6%).13(a) Capacchione and coworkers introduced a dinu-
clear FeIII complex coordinated by a dithioether–triphenolate
ligand for the coupling reaction of CO2 with epoxides, but this
complex produces cyclic carbonates exclusively.13(b)
INTRODUCTION The alternative and greener approach to
produce polycarbonates by the copolymerization of CO2 with
epoxides is of considerable interest as CO2 utilization as a
chemical feedstock is complementary to carbon capture and
storage.1 Since the pioneering work of Inoue et al. with
ZnEt2/H2O for CO2 and propylene oxide copolymerization in
1969,2 numerous homogeneous catalysts with magnesium,
titanium, chromium, iron, cobalt, and zinc metal centers stabi-
lized by various ancillary ligands such as porphyrin,3 phenox-
ides,4 b-diiminates,5 b-ketoiminates,6 salen/salalen/salan,7–9
anilido-aldiminates,10 and many others11 have been explored
to promote this process. These single-site metal catalysts
with well-defined structures are active under mild reaction
conditions and can also afford regio- and/or stereoselective
copolymers with select ligand scaffolds.3(c),5(e),7(b),7(c),12 Promi-
nently employed are the [ONNO]-type tetradentate dianionic
ligands with CrIII and CoIII centers as they provide remarkable
activity for CO2/epoxide copolymerization.7–9 But upon
screening all homogeneous metal catalysts to date, very few
examples of ligands bearing soft Lewis base donors are found
for this copolymerization process.13 Recently, Duchateau and
coworkers employed an aminophosphine chelate on a CrIII
complex to catalyze the production of poly(cyclohexene car-
bonate) (PCHC) from CO2 and cyclohexene oxide (CHO). The
catalyst, however, showed low activity (turnover frequency
(TOF) < 30 h21) and provided low molecular weight polymers
In this work, we describe the preparation of CrIII catalysts
bearing tetradentate imine-thioether-bridged bis(phenolate)
ligands (Fig. 1) and study their use in CO2/CHO copolymeriza-
tion. Within the literature, there are only a couple instances
where similar [ONSO]-type ligands are used in catalysis,
namely their TiIV and ZrIV complexes in olefin polymerization
and the ring-opening polymerization of lactides, respec-
tively.14,15 These [ONSO]-type ligands can be considered to be
analogous to a salalen ligand where the saturated amine has
been replaced with a softer sulfur donor atom. It is known
via comparative studies that the (salalen)CrIII and (salan)CrIII
complexes exhibit higher catalytic activities for the copolymer-
ization process than their unsaturated (salen)CrIII counter-
part.8(a),9 This is attributed to the cis-coordination modes
adopted by the more flexible [ONNO] ligands as noted
through single-crystal X-ray crystallography and in part to the
lower electrophilicity at the metal center. The semirigidity of
[ONSO] ligands also provides a cis-coordination mode as was
observed in the structure of an [ONSO]TiCl2 catalyst.14 It is
then reasonable to anticipate [ONSO]CrIII complexes to prefer
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 [ONSO]CrIII catalysts employed in this study (1–4) and a (salalen)CrCl catalyst, 5 shown for comparison. Note: coordina-
tion geometries are not defined.
a cis-configuration as well and show favorable catalytic activ-
ity for CO2/epoxide copolymerization. Further, the ease in
preparation of various substituted derivatives of [ONSO]
ligands, including those asymmetrical in nature will help mod-
ify the catalyst as desired. Thus, the catalytic behavior of
[ONSO]CrIII complexes is explored below with regards to its
structure, activity, and the resulting copolymer selectivity.
formed in CH2Cl2 on a UV-CARY300 spectrometer. Molecular
weight determinations were performed using a PL-GPC 220
instrument with a refractive index detector. The columns
used were MIXED-B 300 3 7.5 mm columns held at 35 8C,
using THF as eluent at a flow rate of 1.0 mL/min. ESI-MS
were performed on a Thermo Finnigan LCQ Advantage spec-
trometer in ESI mode with a spray voltage of 4.8 kV and a
spray current of 0.24 lA, capillary temperature 230 8C.
EXPERIMENTAL
Synthesis of Ligand 1
A similar procedure to ligand 2 (see below) but using 3,5-di-
tert-butyl-2-hydroxybenzaldehyde was carried out.
Materials and Methods
All manipulations of air- and moisture-sensitive compounds
were carried out using standard Schlenk and vacuum line tech-
niques under an argon atmosphere. Chromium(III) chloride
tetrahydrofuran adduct (CrCl3Á3THF) was purchased from
Strem Chemicals (98%). 3,5-Dichloro-2-hydroxybenzaldehyde
(98%) was purchased from Tianjin Heowns Biochem Technolo-
gies. 3,5-Di-tert-butyl-2-hydroxybenzaldehyde was purchased
from Shanghai Haiqu Chemical industrial (98%). Bis(triphenyl-
phosphine)iminium chloride ([PPN]15[Ph3P-N5PPh3]1)
(PPNCl) (90%, Tianjin Heowns Biochem Technologies) was
purified thrice by dissolving in acetone and precipitating by an
excess amount of ether. The precipitate was dried under vac-
uum. Tetrabutylammonium chloride (TBACl) was recrystallized
from ethanol prior to use. Cyclohexene oxide was refluxed over
calcium hydride and distilled under a nitrogen atmosphere.
Carbon dioxide (99.99% purity) was used without further puri-
fication. Toluene and tetrahydrofuran (THF) were refluxed and
distilled from Na-benzophenone under dry nitrogen. Triethyl-
amine was distilled over CaH2. 2-Aminoethanethiol hydrochlor-
ide (98%), 2-aminothiophenol (99%), and n-butyllithium (n-
BuLi) (2.5 M/L in hexane) were used without further purifica-
tion. Bis(triphenylphosphine) iminium azide (PPNN3) and
2-(bromomethyl)-4,6-di-tert-butylphenol were synthesized
according to reported procedures.16,17
Yield: 78%. IR: 1627 cm21(mC5N), 1483 cm21 (Ph ring mC5C),
1272 cm21 (mAr-O). UV–vis: 231, 263, 338 nm. 1H NMR matches
previously reported data:15 (400 MHz, CDCl3), d 5 8.31 (s, 1H,
NCH), 7.42 (d, 1H, ArH), 7.27 (d, 1H, ArH), 7.15 (d, 1H, ArH),
6.95 (d, 1H, ArH), 3.84 (s, 2H, ArCH2S), 3.64 (t, 2H, CH2), 2.72
(t, 2H, CH2), and 1.27–1.43 ppm (s, 36H, C(CH3)3).
Synthesis of Ligand 2
A solution of 3,5-dichloro-2-hydroxybenzaldehyde (4.77 g, 25
mmol) in THF (20 mL) was added to a solution of triethyl-
amine (2.51 g, 25 mmol) and cysteamine hydrochloride
(2.27 g, 20 mmol) in THF (20 mL) and stirred 2 h at 20 8C.
The reaction mixture was filtered under argon atmosphere
and the filtrate was used for the next step without further
purification. Triethylamine (2.51 g, 25 mmol) was added to
the above filtrate, then a solution of 2-(bromomethyl)-4,6-di-
tert-butylphenol (5.94 g, 20 mmol) in THF (10 mL) was
added dropwise over a period of 30 min. After the reaction
was stirred for an additional 2 h, it was filtered and the filtrate
was concentrated under reduced pressure. The residue was
purified by silica gel chromatography (hexanes:EtOAc 5 10:1)
to afford the ligand 1 in yield of 65%.
IR: 1636 cm21 (mC5N), 1454 cm21 (Ph ring mC5C), 1287 cm21
(mAr-O). UV–vis: 233, 264, and 338 nm. 1H NMR (400 MHz,
CDCl3), d 5 8.19 (s, 1H, NCH), 7.42 (d, 1H, ArH), 7.28 (d, 1H,
ArH), 7.16 (d, 1H, ArH), 6.93 (d, 1H, ArH), 3.83 (s, 2H, ArCH2S),
3.69 (t, 2H, CH2), 2.75 (t, 2H, CH2), and 1.28–1.41 (s, 18H,
C(CH3)3), which is consistent with the reported data.15
1H and 13C NMR spectra were recorded on a Bruker-400
spectrometer at frequencies of 400 MHz (1H) and 100 MHz
(
13C), respectively. All chemical shifts are provided in parts
per million using tetramethylsilane as an internal reference
at ambient temperature. IR spectra were obtained on a
Bruker Vector 22 spectrometer at a resolution of 4 cm21 (16
scans collected). High-pressure reaction kinetic measure-
ments were performed using an ASI React IR 1000 reaction
analysis system with a stainless steel Parr autoclave modi-
fied with a permanently mounted ATR crystal (SiComp) at
the bottom of the reactor. UV–visible spectroscopy was per-
Synthesis of Ligand 3
To
a solution of 3,5-di-tert-butyl-2-hydroxybenzaldehyde
(5.58 g, 25 mmol) in THF (20 mL) was added 2-
aminothiophenol (2.50 g, 20 mmol), and the reaction mixture
was refluxed for 2 h. Following this, triethylamine (2.51 g, 25
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SCHEME 1 Preparation of ONSO donor ligands and their respective chromium(III) complexes. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
mmol) was added. After stirring for 15 min, a solution of 2-(bro-
momethyl)-4,6-di-tert-butylphenol (5.94 g, 20 mmol) in dry
THF (10 mL) was added dropwise over 0.5 h. The reaction mix-
ture was refluxed for 4 h and filtered. The filtrate was evapo-
rated under reduced pressure. Chromatography over silica gel
eluting with hexanes-EtOAc (10:1) gave the target product as a
yellow solid in a final yield of 74%.
Complex 2
Yield: 92%. IR: 1623 cm21 (mC5N), 1437 cm21 (Ph ring
m
C5C), 1262 cm21 (mAr-O). UV–vis: 232, 281, and 405 nm
(weak). ESI-MS (m/z): [M1K1THF]1 663.15, Found 665.87
Complex 3
Yield: 95%. IR: 1611 cm21 (mC5N), 1470 cm21 (Ph ring
m
C5C), 1273 cm21 (mAr-O). UV–vis: 233, 333, and 475 nm
IR: 1614 cm21 (mC5N). UV–vis: 234, 276, 363 nm. 1H NMR
(400MHz, CDCl3), d 5 8.58 (s, 1H, NCH), 7.48 (d, 1H, ArH), 7.36–
7.45 (d, 1H, ArH), 7.28 (d, 1H, ArH), 7.22 (d, 1H, ArH), 7.18 (d,
1H, ArH), 7.14 (d, 1H, ArH), 7.11 (d, 1H, ArH), 6.82 (d, 1H, ArH),
4.13 (s, 2H, ArCH2S), and 1.20–1.49 ppm (s, 36H, C(CH3)3).
(weak). ESI-MS (m/z): [M1K1THF]1: 755.35, Found 757.13
Preparation of Complex-49(c)
AgClO4 (0.07 g, 3.3 mmol) was added to an acetonitrile solu-
tion of Complex 3 (0.22 g, 3.3 mmol) and stirred for 24 h.
The reaction mixture was then filtered and NaN3 (0.64 g, 9.9
mmol) was added under argon. The reaction was allowed to
stir for an additional 24 h. The solvent was removed in
vacuo, and the resulting solid was redissolved in diethyl
ether. The mixture was filtered and solvent was removed in
vacuo yielding a dark brown powder in 60% yield. IR:
1611 cm21 (mC5N), 2094 cm21 (mN3), 1477 cm21 (Ph ring
Preparation of Complexes 1–3
All complexes were synthesized using a modified procedure
to that previously reported.9(c) n-BuLi (1.7 mL, 4.2 mmol)
was added dropwise to a stirred solution of ligand (2.0
mmol) in THF (20 mL) at 278 8C. A color change from yel-
low to colorless is observed. The reaction mixture was
allowed to warm to room temperature. After 0.5 h, the mix-
ture was added to a flask containing CrCl3Á3THF (0.82 g, 2.2
mmol) and THF (10 mL) over a period of 30 min. Following
an immediate color change of the chromium starting mate-
rial from purple to dark green, the reaction mixture was
stirred 24 h at 20 8C. The solvent was removed in vacuo, and
the resulting solid was redissolved in dichloromethane. The
reaction mixture was filtered to remove LiCl, and the solvent
removed in vacuo to afford dark green complexes 1–3.
m
C5C), 1273 cm21 (mAr-O). UV–vis: 229, 317, and 467 nm
(weak). ESI-MS (m/z): [M1K1THF]1: 762.39, Found 758.13.
General Method for Copolymerization of CO2 and CHO
The copolymerization reactions were carried out in a 100-
mL stainless-steel autoclave equipped with a magnetic stirrer
which had been previously dried at 60 8C for 2 h under vac-
uum. In a typical experiment, the reactor is charged with the
appropriate amount of catalyst and epoxide and pressurized
with CO2 to the desired pressure. The mixture is then heated
to the temperature of interest while stirring at about
200 rpm for an expected time. The reactor is then cooled to
ambient temperature, and the pressure slowly released. The
crude mixture is treated with CH2Cl2 and stirred to give a
Complex 1
Yield: 90%. IR: 1616 cm21 (mC5N),1458 cm21 (Ph ring mC5C),
1264 cm21 (mAr-O). UV–vis: 230, 261, and 421 nm (weak).
ESI-MS (m/z): [M1K1THF]1: 708.36, Found 710.20
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TABLE 1 Copolymerization of CO2 and CHO with Complex 3
and Various Cocatalystsa
TABLE 2 Effect of [ONSO]CrX Complexes on CO2/CHO
Copolymerizationa
Cocatalyst
TOF
Conv.
(%)c
TOF
Entry Cat./Cocat. (h21 b
Conv.
(%)c
Mn
% PCHCd (g/mol)e PDIe
Entry
(Equiv. to Cr)
(h21 b
)
% PCHCd
)
1
2
3
4
5
PPNN3 (2)
PPNCl (2)
PPNCl (1)
TBACl (2)
TBACl (1)
69
67
58
34
30
54
57
40
29
20
77
72
87
71
90
1
2
3
4
5
6
1/PPNCl
2/PPNCl
3/PPNCl
3/PPNN3
4/PPNCl
4/PPNN3
49
54
67
69
68
71
43
44
57
54
55
53
69
74
72
77
75
81
7,200
8,300
10,900
–
1.25
1.23
1.28
–
9,900
–
1.25
–
a
Reaction conditions: CHO/catalyst 5 1000/1, Complex 3: 0.065 g (0.1
mmol), 3.0 MPa, 80 8C, 6 h.
a
Reaction conditions: CHO/catalyst/cocatalyst 5 1000/1/2; 3.0 MPa, 80
b
Turnover frequency as moles of CHO consumed per mole of catalyst
8C, 6 h; CHO: 9.8 g (100 mmol).
b
per hour.
c
Turnover frequency as moles of CHO consumed per mole of catalyst
Based on isolated yield of polymer and in conjunction with IR data.
per hour.
c
d
Selectivity of poly(cyclohexene carbonate), PCHC determined by IR
Based on isolated yield of polymer and in conjunction with IR data.
spectroscopy: A1745/(A17451A1802).
d
Selectivity of PCHC determined by IR spectroscopy: A1745/(A17451A1802).
Polydispersity index (PDI) was determined by GPC.
e
homogeneous solution. A small sample of the CH2Cl2 solution
is used for IR measurement to determine selectivity of PCHC
and CHC. The residual mixture is condensed by rotary evapo-
ration and treated with methanol. The resulting precipitate
is filtered and purified twice more by the CH2Cl2/methanol
workup. The final isolated solid is dried at 45 8C under vac-
uum overnight. The polymer was subjected to NMR analysis.
400-500 nm range which are assigned to the ligand-to-metal
charge transfers (Supporting Information Fig. S1).
With the complexes at hand, we first looked at the effect of
three onium salts as cocatalyst for CO2/CHO copolymeriza-
tion activity with Complex 3 at 80 8C (Table 1). It was read-
ily apparent that the bis(triphenylphosphine)iminium salts
(PPNX) performed better, showing twice the catalytic activity
than n-TBACl as seen in Entries 1–3 versus 4–5. This is due
to the highly hygroscopic nature of TBACl which renders it
difficult to isolate in a rigorously dry form unlike the PPNX
salts. Thus, the presence of water lowers the activity via its
competitive binding at the coordination site. Additionally,
alkylammonium cations are more interactive with the anions
than their bulky and delocalized PPN counterpart.7(g),8(a)
Increasing the cocatalyst loading to two equivalents
enhanced the TOFs (>60 h21 with PPN salts) but dropped
the copolymer selectivity below 80% (Entries 2 vs. 3; 4 vs.
5). These data indicate that, at higher loadings of the nucleo-
philic cocatalysts, the growing polycarbonate chain is more
susceptible to being displaced from the metal center which
can then backbite to produce the cyclic carbonate byproduct.
While the use of azide versus chloride gave comparable
activity, the selectivity for copolymer is slightly favored with
the former as it is a poorer leaving group (Entries 1 vs. 2).
On the other hand, a control reaction in the absence of a
cocatalyst showed negligible activity with Catalyst 3 confirm-
ing that the active species must be a six-coordinate metal
center derived from [ONSO]CrCl and a cocatalyst as has been
RESULTS AND DISCUSSION
Condensation of various 2-hydroxybenzaldehydes with ami-
nothiols provides the respective imine-thiol phenol com-
pounds. Substitution of bromine in 2-bromomethylphenols
by the thiol group in the presence of a base yields the
desired bis(phenol) [ONSO] ligands (Scheme 1). Subsequent
treatment with n-BuLi and an equivalence of CrCl3(THF)3
then generates the desired dark green [ONSO]CrCl complexes
1-3 in high yields. Complex 4 can be obtained from the reac-
tions of Complex 3 with AgClO4 and NaN3 and is similar to
the synthetic route employed for (salan)CrN3.9(c) The para-
magnetism of CrIII precludes reliable NMR data but the com-
plexes have been characterized via ESI-MS, IR, and UV–vis
spectroscopies. The mass spectral data show the appropriate
mass peaks of each complexes but with an added THF sol-
vent molecule, that is, [ONSO]CrXÁTHF. Infrared analyses
reveal the stretching frequency of C5N bond to have shifted
to lower wavenumbers upon coordination of the ligand to the
metal. In the electronic spectra, while the free ligands exhibit
three absorption bands between 200 and 400 nm, the com-
plexes display newer and weaker transitions including in the
TABLE 3 Activity and Selectivity of PCHC Production with [ONSO]CrCl, 1 versus (salalen)CrCl, 5
Entry
Cat./Cocat.
Loadinga
P (MPa)
Time (h)
Temperature (8C)
TOF (h21
)
% PCHC
1
2b
1/PPNCl
5/PPNCl
1,000/1/1
1,000/1/1
1.3
1.3
3
3
70
70
31
51
87
100
a
b
Loading ratio depicts CHO/catalyst/cocatalyst.
Data obtained from Ref. 8(a).
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where the effect of chlorides from at least three bonds away
are presumably less felt by the phenolate O donor. Catalyst 4
bearing an azide X ligand, versus X 5Cl of 3, displays nearly
identical activity when it is employed with PPNCl to the reac-
tion observed with 3/PPNN3 (Entries 4 and 5). This is as
expected for they both essentially result in the same mixture
of active species, that is, {[ONSO]Cr(Cl)(N3)}PPN and the
scrambled complexes, {[ONSO]CrX2}PPN where X 5Cl or N3.
Polymer selectivity is slightly enhanced with the use of azides
(Entries 3–6) for reasons previously noted above in Table 1.
A comparison of Catalyst 1 with its salalen analog 5 shows
the [ONSO]CrCl complex to be less active (TOF of 31 h21 vs
51 h21) and produce more cyclic carbonate (>10%) when
hardly any is observed with the (salalen)CrCl under identical
conditions shown in Table 3. The poorer polymer selectivity
can be attributed to a weaker interaction of the growing ani-
onic polymer chain with a more electron-rich CrIII center
arising from the more electron-releasing sulfur donor. This
then leads to a higher propensity for the polycarbonate to
backbite on itself and yield the cyclic carbonate. It would
also explain why electron-withdrawing substituents on the
[ONSO] ligands help enhance the activity and selectivity for
producing PCHC.
FIGURE 2 Absorbance versus time profiles of poly(cyclohexene
carbonate) (PCHC) and cyclohexene carbonate (CHC) with
[ONSO]CrCl complexes (1–3). Reaction conditions: [CHO]/catalyst/
cocatalyst 5 1400/1/2, 3.4 MPa, 80 8C. Dashed and continuous lines
represent PCHC and CHC, respectively. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
established before.7(g,h),8(a) This can also be noted through
To further assess the influence of the structural features of
[ONSO]CrCl Complexes (1–3) on the copolymerization process,
reactions were monitored via in situ infrared spectroscopy by
following the carbonate absorption intensities of polycarbon-
ate (1750 cm21) and cyclic carbonate (1804 cm21) products.
The reaction profiles are shown in Figure 2. Little to no induc-
tion periods and rapid formation of cyclic carbonate in the ini-
tial stages of the reaction (0–1 h) after which their growth is
nearly suppressed is noted for all catalysts. Copolymer forma-
tion is only observed close to 50 min into the reaction with
Catalysts 1 and 2; about the same time, they display a signifi-
cant rate drop in cyclic carbonate production. On the other
hand, the least electron-rich CrIII catalyst, 3, favors polycar-
bonate production right from the start. Over a prolonged reac-
tion time, the rate of copolymerization appears to be fastest
infrared analysis on a mixture of PPNN3 and Complex 3. The
metal-coordinated azide stretching frequency (2054 cm21
)
rather than the free azide band (2003 cm21) is observed
(Supporting Information Fig. S2).18,19
When the catalytic activity was assessed for the [ONSO]CrIII
complexes (1–3), it was found that the better catalysts bore
electron-withdrawing substituents on their ligands (Table 2).
Thus, Catalyst 2 bearing two chlorides on a phenolate rather
than tert-butyls and Catalyst 3 bearing a phenylene back-
bone instead of an ethylene linker display higher TOFs in
comparison to Catalyst 1 (Entries 1–3). With the inductive
withdrawing effect of phenylene felt directly by the N and S
donor atoms, Catalyst 3 outperforms 2 (TOF of 67 vs. 54 h21
)
SCHEME 2 Proposed mechanism for the production of cyclohexene carbonate, Path A and poly(cyclohexene carbonate), Path B.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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len)CrIII complexes by bearing a more electron-releasing S
donor versus a saturated N donor, these [ONSO]CrIII catalysts
show moderate activity and polycarbonate selectivity in com-
parison. The lower electrophilicity at the metal center disfa-
vors the growth of anionic polymer chain in the initial stages
of the reaction and tends to produce cyclic carbonates. The
severity of the backbiting process to produce the byproduct
can be lessened by introducing electron-withdrawing groups
on to the ligands, which additionally increases the catalytic
activity as well.
TABLE 4 Copolymerization of CO2 and CHO with Catalyst 3
under Various Conditionsa
CHO/3/
Temperature
TOF
P (MPa) (h21 b
Entry PPNCl
(8C)
)
% PCHCc
1
1,000/1/2 70
1,000/1/2 80
3.0
3.0
3.0
3.0
3.0
3.5
1.0
0.5
3.0
3.0
31
67
95
96
100
67
56
42
54
33
76
72
70
66
64
71
75
73
71
70
2
3
1,000/1/2 90
1,000/1/2 100
1,000/1/2 110
1,000/1/2 80
1,000/1/2 80
1,000/1/2 80
1,500/1/2 80
2,000/1/2 80
4
5
6
7
ACKNOWLEDGMENTS
8
This work was supported by National Natural Science Founda-
tion of China (51373046 and 51473045), Program of Study
Abroad for Young Scholar sponsored by China Scholarship
Council (2010670505), Natural Science Foundation of Hebei
Province (B2014202013), High-level Excellent Talents in Uni-
versity of Hebei Province, Changjiang Scholars and Innovative
Research Team in University (IRT13060), and New Century
Excellent Talents in University (NCET-10-0125). Z. Duan
expresses thanks to the Open Foundation of the State Key Labo-
ratory of Polymer Ecomaterials and Chinese Academy of Scien-
ces (No.20130101). D. J. Darensbourg and S. J. Kyran
acknowledge their financial support from Robert A. Welch
Foundation (A-0923). The authors declare no competing finan-
cial interest.
9
10
a
Reactions were carried out over a period of 6 h.
b
Turnover frequency as moles of CHO consumed per mole of catalyst
per hour.
c
Selectivity of PCHC determined by IR spectroscopy: A1745/(A17451A1802).
for 2. From these observations, we have summarized a possi-
ble mechanism in Scheme 2. The reactions with [ONSO]CrIII
complexes are initially dominated by the backbiting process of
a one-mer unit which leads to the cyclohexene carbonate
(CHC) product, Path A. However, once enchainment of the
monomer unit begins, backbiting to displace an alkyl carbon-
ate is a lot less favored and the propagation of polymer chain
is preferred (Path B). Similar findings are also seen with (sale-
n)AlCl catalysts and porphyrin chromium complexes.3,20(e)
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