Alternating copolymerization of carbon dioxide and propylene oxide under bifunctional cobalt salen complexes: Role of lewis base substituent covalent bonded on salen ligand

Article abstract of DOI:10.1002/pola.23792

Alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO₂) was realized under mild conditions with a moderate turnover frequency (TOF), employing sole bifunctional cobalt salen complexes containing Lewis acid metal center and covalent bonded Lewis base on the ligand. Variation of the covalent bonded Lewis base substituents on the salen ligands could tailor the catalytic activity with TOF changing from 19.3 to 34.9 h¹, polymeric/cyclic carbonate selectivity from 95.3 to 72.8%, and the head-to-tail structure in the polymer from 72.2 to 86.0%. The IR analysis confirmed that the Lewis base moiety on one molecule could coordinate with cobalt center of adjacent molecule, playing similar role to the Salen metal complex/Lewis base binary catalytic system.

Full text of DOI:10.1002/pola.23792

Alternating Copolymerization of Carbon Dioxide and Propylene Oxide
Under Bifunctional Cobalt Salen Complexes: Role of Lewis Base
Substituent Covalent Bonded on Salen Ligand
BINYUAN LIU,1 YANHAO GAO,1 XIN ZHAO,1 WEIDONG YAN,1 XIANHONG WANG2
1Institute of Polymer Science and Engineering, Hebei University of Technology, Tianjin 300130, People’s Republic of China
2State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
Received 31 May 2009; accepted 18 October 2009
DOI: 10.1002/pola.23792
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Alternating copolymerization of propylene oxide
(PO) and carbon dioxide (CO2) was realized under mild condi-
tions with a moderate turnover frequency (TOF), employing
sole bifunctional cobalt salen complexes containing Lewis acid
metal center and covalent bonded Lewis base on the ligand.
Variation of the covalent bonded Lewis base substituents on
the salen ligands could tailor the catalytic activity with TOF
changing from 19.3 to 34.9 hꢀ1, polymeric/cyclic carbonate se-
lectivity from 95.3 to 72.8%, and the head-to-tail structure in
the polymer from 72.2 to 86.0%. The IR analysis confirmed that
the Lewis base moiety on one molecule could coordinate with
cobalt center of adjacent molecule, playing similar role to
the Salen metal complex/Lewis base binary catalytic system.
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2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
48: 359–365, 2010
KEYWORDS: carbon dioxide; catalysts; cobalt salen complex;
copolymerization; polycarbonate; propylene oxide
INTRODUCTION Copolymerization of CO2 and epoxides to
make polycarbonates has been regarded as one of the attrac-
tive pathways for the utilization of CO2, since it provides an
environmentally friendly route to convert CO2 into biode-
gradable polymers.1,2 Initiated by Inoue’s historic discovery
of alternating copolymerization of CO2 and propylene oxide
using a ZnEt2-H2O binary catalyst,3 a wide variety of catalytic
systems have been developed to promote this process,
especially using metal complexes with sterically bulky
ligands,2,4–10 such as b-diketoimine,4 salen,2e,5 phenoxide,6
porphyrin,7 salicylaldiminato (Schiff-base),8 and b-ketoi-
mine.9 As far as metal complex catalysts were considered,
metal salen complexes (SalenMX), (M¼¼Co, Cr, or Al) alone or
in conjunction with Lewis base, onium salt, or ionic liquid as
cocatalysts have been of significant interest,5a-e mainly due
to easy synthesis, good stability against moisture and air, un-
precedented opportunity for controlling the regio- and
stereo- chemistry of copolymerization, and high selectivity of
polycarbonate over cyclic carbonate.
workers took the strategy of incorporation of cationic piperi-
dinium and neutral piperidinyl group onto salen ligand of
salen cobalt complex to achieve a new unique cobalt salen
complexes-based catalyst for the PO/CO2 copolymerization,12
reporting high selectivity for polycarbonate over cyclic car-
bonate at high conversion and improved catalytic activity
with the turnover frequency (TOF) 602 hꢀ1. Recently, Lee
and coworkers5c developed a novel bifunctional salen cata-
lyst containing a Lewis acidic cobalt center and a quaternary
ammonium salt unit in one cobalt (III) salen complex mole-
cule, demonstrating very high catalytic activity (TOF up to
3500 hꢀ1) with H-T linkages from 83 to 94%, indicating that
incorporation of quaternary ammonium salt into cobalt salen
complexes was beneficial for enhancing the catalytic activity.
Furthermore, earlier studies have shown that the addition of
a Lewis basic cocatalysts into porphyrin or salen-type cata-
lysts could substantially enhance catalytic activities in epox-
ides/CO2 copolymerizations,5a,13–21 and bifunctional metal
salen complexes consisting of a metal center and Lewis base
moiety in one compound could catalyze organic reactions
such as alkylation of aldehydes, a-ketoesters, olefins epoxida-
tion, and cyclic carbonate formation,22 in which Lewis acid
center acted as electrophilic activator and basic functional
group as nucleophilic activator. Initiated by these results,
bifunctional cobalt salen complexes containing a Lewis acidic
cobalt center and a Lewis base unit in one molecule were
Compared with binary metal salen complexes, sole metal
salen complexes were also attractive catalysts for the copoly-
merization of epoxides and carbon dioxide, Coates and co-
workers first reported a well-defined cobalt complexes alone
as catalysts for the copolymerization of CO2 and PO,11 selec-
tively yielding poly(propylene carbonate) (PPC) with head-
to-tail (H-T) linkages approaching 80%. Nozaki and co-
Additional Supporting Information may be found in the online version of this article. Correspondence to: B. Liu (E-mail: byliu@hebut.edu.cn) or X.
Wang (E-mail: xhwang@ciac.jl.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 359–365 (2010) 2009 Wiley Periodicals, Inc.
ALTERNATING COPOLYMERIZATION OF CO2/PO, LIU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
ing vibration of nitrine anion. The electronic spectra of the
free ligand showed three absorption bands at 330, 252–265,
and 245 nm, whereas the electronic spectra of the complexes
1a–d and 2b–e displayed new weak transition in the range
of 400–450 nm, and complex 1e showed new weak transi-
tion at 492 nm, which were attributed to the ligand-to-metal
charge transfer transition.24 These observations suggested
that the dicompartmental ligand could coordinate with Co
(III) atom through charged phenolic oxygen and nitrogen
atoms of azomethine groups in the N2O2 compartment.
Copolymerization of PO and CO2 Under CoIII(salen)X
Complexes
SCHEME 1 Structure of SalenCoX complexes.
To understand the role of the Lewis base component on
copolymerization reaction, a series of SalenCoCl complexes
with amino salens (complexes 1a-d) or tert-butyl salen (com-
plexes 1e) ligands were used for the copolymerization of PO
and CO2 at 40 ꢁC for 30 h, and the data were listed in Table 1,
where a [PO]:[Co] ratio of 2000:1 was applied with CO2 pres-
sure at 3.0 MPa. As can be seen from Table 1, SalenCoCl com-
plexes containing amino group (runs 1–4, Table 1) were
much more efficient than that of containing tert-butyl group
(run 5, Table 1). Recently, studies on the role of Lewis base
cocatalyst in porphyrin or salen complex-based binary cata-
lytic systems for CO2/epoxides copolymerization have been
conducted, there is such consideration that the Lewis base
can coordinate with the active metal center, thereby activates
the metal alkoxide bond as well as the subsequent CO2 inser-
tion process.6c,13–21,25 Therefore, an amine substituent on one
molecule may function in analogous manner, which could
coordinate with the central metal ion of adjacent SalenCoX
molecule (Scheme 2), benefiting the insertion of epoxide and
CO2 into the CoAX bond of SalenCoX. If this conjecture was
right, replacement of the amine with nonbasic group should
change the catalytic activity, in fact, the absence of catalytic
reactivity of the complex 1e with salen free of Lewis basic
group (run 5, Table 1) did confirm this idea.
designed and shown in Scheme 1, where a Lewis base moi-
ety was incorporated into the cobalt salen complex, and the
catalytic behavior for the copolymerization of PO and CO2
was studied, especially the role of the Lewis base substitu-
ents in salen ligand of the cobalt salen complexes on cata-
lytic activity, selectivity of polycarbonate (PPC), over cyclic
carbonate (PC), and regioselectivity of PPC.
RESULTS AND DISCUSSION
Spectroscopic Studies of the CoIII(salen)X Complexes
The salen ligands were obtained as yellow solid by simple
condensation reaction of the corresponding derivatives of
salicylaldehyde with (1R, 2R)-diaminocyclohexane in high
yields, and corresponding SalenCoX complexes were pre-
pared according to analogous procedure in literature.23 Com-
pared with the stretching vibration m(C¼¼N) of the free ligand,
the m(C¼N) bands in the IR spectra of all the complexes were
split into two peaks, at around 1614 and 1636 cmꢀ1 for
complex 1a, 1b, and 2b, at 1638 and 1599 cmꢀ1 for complex
1c, at 1639 and 1601 cmꢀ1 for complex 1d, and at 1638
and 1611 cmꢀ1 for complex 2e, respectively, mainly due to
the coordination of the azomethine nitrogens with the metal
ion. Furthermore, the new band at 2014 and 2006 cmꢀ1 for
complex 2b and 2e, respectively, was assigned to the stretch-
However, there is no obvious rule in these bifunctional
cobalt salen complexes between Lewis basicity of amino
group on the ligand and catalytic activity. This is also true in
TABLE 1 CO2/PO Copolymerization Under Various Bifunctional Cobalt Salen Complexesa
Carbonate
Linkages[%]c
d
d
Entry
Complex
TOFb
Mn (ꢂ 10ꢀ3
)
Mw/Mn
H-T [%]e
1
2
3
4
5f
6
7
a
1a
1b
1c
1d
1e
2b
2e
34.8
34.9
19.3
22.2
NA
97.5
97.1
98.4
99.0
NA
18.3
28.4
17.2
16.9
NA
1.46
1.21
1.43
1.31
NA
79.8
86.0
73.8
72.2
NA
37.6
NA
99.4
NA
28.5
NA
1.24
NA
84.5
NA
d
Reaction conditions: PO/Co ¼ 2000; Time: 30 h; Temperature: 40 ꢁC;
Determined by gel permeation chromatography using polystyrene
Pressure: 3.0 MPa; PO: 5.0 mL (72.0 mmol).
standards.
e
b
Turnover number for PPC calculated as mole of PO consumed per
Head-to-Tail structure determined by 13C NMR spectroscopy.
f
mole of catalyst per hour.
Polymer not available.
c
Determined using 1H NMR spectroscopy.
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ARTICLE
dinate to Co center of adjacent molecules. In addition, the
complex 2b showed catalytic activity with TOF 37.6 hꢀ1(Table
1, entry 6), while using the complex 2e as catalyst, similar to
complex 1e, no PPC was obtained (Table 1, entry 7).
The selectivity of PPC over PC is also a great concern for the
catalyst design. The intensity of absorption peak in IR spec-
tra at 1800 cmꢀ1 is characteristics of cyclic carbonate, and
IR spectra of the product before isolation of PPC obtained
with complexes 1a-d were recorded in Figure 2, which were
normalized using absorption intensity of mC¼O of PPC as the
standard. The intensity of 1800 cmꢀ1 absorption peak was
weak, and varied insignificantly with the change of Lewis
base moiety on cobalt salen complex. Compared with the
major absorbance at 1748 cmꢀ1 corresponding to the mC¼O
in PPC, the weak intensity at 1800 cmꢀ1 did mean that the
cyclic carbonate was only produced in small quantity,3,26
indicating that the sole cobalt salen complexes provided a
chance of high selectivity of PPC over PC. As can be seen in
Figure 2, the PC content in the mixture was estimated by the
ratio of absorption intensity of the stretching vibration of
carbonyl group of PC and PPC, which is 4.7 for complex 1a,
6.8 for complex 1b, 8.6 for complex 1c, and 27.2% for com-
plex 1d, respectively. The result indicated that the less selec-
tivity for copolymer was obtained by complex 1d in compar-
ison with complexes 1a, 1b, and 1c.
SCHEME 2 Proposed model of interaction between two cobalt
salen complex molecules.
the metal salen complex/Lewis base binary catalytic system,
where the role of Lewis base dramatically relied on steric
effect, nucleophilicity, and coordination ability to the metal
species in addition to Lewis basicity6c,18 and the initiation of
CO2/epoxides copolymerization could proceed in different
pathway according to the steric bulk, nucleophilicity, and
coordination ability of Lewis base moiety.6c,19
To gain more information on coordination mode between
cocatalyst and metal center in the metal salen complex, IR
spectra of complex 2b in toluene solution, along with those of
complex 2e in THF and toluene solutions with or without trie-
thylamine, were recorded and shown in Figure 1. The change
of azide stretching vibration (m(N3)) provides important infor-
mation for both cocatalyst binding and anion ring-opening
steps. For complex 2e, m(N3) appeared at 1983 and 1981
cmꢀ1 in toluene and THF solution, respectively, indicating that
there was no interaction between the oxygen atom of THF
with Co center of the complex 2e. Once 3 equiv. of triethyl-
amine was added into toluene solution of complex 2e, it
showed a significant red shift to 2013 cmꢀ1, consistent with
the azide stretching band of complex 2b (at 2014 cmꢀ1). It
should be noted that no shift was observed when triethyl-
amine was added into toluene solution of complex 2b, sug-
gesting that the Lewis base group on one molecule could coor-
It is interesting to observe that incorporation of Lewis base
group on salen ligand has significant influence on the regior-
egularity of PPC. The carbonate regions of the 13C NMR spec-
tra of PPC obtained with complexes 1a-d are illustrated in
Figure 3. There are three peaks located at 154.7, 154.2
(split), and 153.9–153.6 (two peaks) ppm, corresponding to
tail-to-tail (T-T), head-to-head (H-H), and head-to-tail (H-T)
structure in the copolymer, respectively,27,28 where H-T struc-
ture is generally dominant. As summarized in Table 1, the
content of H-T linkage in PPC chain varied with complexes
1a-d, respectively. Among them Complex 1b is highly selec-
tive producing 86.0% H-T linkages. Therefore, the tethered
Lewis base group on the aromatic ring of salen ligand can
FIGURE 1 IR spectra of complex 2b in toluene solution, com-
plex 2e in THF and toluene solutions with or without addition
of triethylamine.
FIGURE 2 IR spectra of the product obtained with complexes
1a–d before isolation of PPC.
ALTERNATING COPOLYMERIZATION OF CO2/PO, LIU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 13C spectra (100 MHz) of the
carbonyl region for PPC obtained from
complex 1a (a), complex 1b (b), com-
plex 1c (c), and complex 1d (d),
respectively.
tailor the stereoregularity of PPC. As indicated by previous
works, the Lewis base at the vacant axial site of SalenMX/
Lewis base binary catalysts system could affect the nucleo-
philicity of the metal center, altering the regioselectivity
chain-growing center in the bifunctional SalenCoCl com-
plexes. Similar phenomenon was reported by Lee utilizing Co
salen complex containing a Lewis acidic metal center and a
quaternary ammonium salt unit in one molecule for the PO/
CO2 copolymerization, and also observable from other metal
salen complexes-based binary catalyst system.6d,7f,29
of epoxide ring-opening by nucleophilic axial
X
anion.13,15,16,19–21 This is also true for sole salen complex
catalyst with Lewis base on the salen ligand.
Considering that the pressure has a significant effect on the
CO2/epoxides copolymerization such as reaction rate, selec-
tivity for PPC over cyclic carbonate, and carbonate linkage
versus ether linkage in polymer chain,3b,30 theꢁcopolymeriza-
tions were performed with complex 1b at 45 C under pres-
sure from 2 to 5 MPa, the results were summarized in Table
2. Copolymerization of PO and CO2 could be realized at 2.0
MPa (Table 2, entry 1) with TOF of 27.7 hꢀ1, a sharp con-
trast to (1R, 2R)-SalcyCoX (X¼¼OAc, Cl, Br) complexes lacking
Lewis base substituent on the ligand where they were inef-
fective to produce the copolymer at low pressure,11,18,25 fur-
ther indicated that the aforementioned proposal of Lewis
base group played a positive role in the copolymerization
reaction for sole bifuncational cobalt salen complexes. As far
In addition, the peak at 154.4 ppm attributed to the ether
linkage of more than two propylene oxide units in the
copolymer disappeared,27 indicating that alternating copoly-
mer dominated the resultant PPC. This was consistent with
1H NMR spectra, where the polyether linkage in the PPC
chain was less than 3.0% assessed by integrated areas in the
1H NMR spectra (Table 1, entries 1–4).
The number-average molecular weight (Mn) of the alternat-
ing copolymer was obtained by gel permeation chromatogra-
phy (GPC), and the GPC curves exhibited a narrow bimodal
distribution for each copolymer prepared by complexes 1a-d,
which might be attributed to the presence of more than one
TABLE 2 Copolymerization of PO and CO2 Catalyzed by Complex 1ba
d
d
Entry
Pressure (MPa)
TONb
TOFc
Mn (ꢂ10ꢀ3
)
Mw/Mn
Conversione (%)
1
2
3
4
a
2.0
3.0
4.0
5.0
831.8
1,101.8
807.3
27.7
36.7
26.9
25.5
15.3
32.1
20.0
22.0
1.41
1.17
1.26
1.37
41.3
54.7
40.1
37.9
763.7
d
Reaction conditions: PO/Co ¼ 2000, Time: 30 h, T: 45 ꢁC, PO: 5.0 mL
Determined by gel permeation chromatography using polystyrene
(72.0 mmol).
b
standards.
e
Turnover number for PPC calculated as mole of PO consumed per
Based on the isolated PPC.
mole of catalyst.
c
Turnover frequency for PPC calculated as mole of PO consumed
per mole of catalyst per hour.
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ARTICLE
as the copolymerization pressure was concerned, as sum-
marized in Table 2, the yield increased with increasing pres-
sure before 3.0 MPa, higher pressure may reduce the yield.
One explanation was due to the major volumetric expansion
of the liquid phase.30a,31As a result, the solubility of the co-
polymer decreased under higher pressure, which was sup-
ported by the fact that the molecular weight of copolymer
slightly decreased with increasing pressure as P > 3.0 MPa.
However, it should be noted that a change in the reaction
pressure from 2.0 to 5.0 MPa had no apparent effect on the
carbonate linkage in resulting copolymer and selectivity of
PPC over PC (supporting information), indicating that com-
plex 1b showed a high selectivity for alternating copolymer.
Measurements
The 1H-NMR and 13C-NMR spectra were recorded on a
Unity-400 spectrometer using tetramethylsilane as internal
reference at room temperature. Infrared (IR) spectra were
recorded on a Bruker Vector 22 spectrometer from 400–
4000 cmꢀ1 at a resolution of 4 cmꢀ1 (16 scans collected).
The element analysis of monomers was made on EA 1112
Element Analyzer. The molecular weight and its distribution
(Mw/Mn) of copolymers were measured by gel permeation
chromatography (GPC) on PL-GPC 220 spectrometer (Poly-
mer Laboratories, UK) using polystyrene as standard, and
tetrahydrofuran (THF) was used as eluent. UV-visible spec-
troscopy was performed on a UV-CARY 300 spectrometer.
Synthesis of 3-Tert-butyl-2-hydroxy-5-pyrrolidin-1-
ylmethylbenzaldehyde
CONCLUSIONS
A mixture of 3-tert-butyl-5-chloromethyl-2-hydroxylbenzalde-
hyde (2.0 g, 8.0 mmol), pyrrolidine (0.63 g, 8.8 mmol), and
triethylamine (1.78 g, 17.6 mmol) in freshly distilled toluene
In summary, bifunctional cobalt salen complexes containing
a Lewis acidic metal center and a Lewis base moiety in one
molecule have been prepared, they were effective catalysts
for the copolymerization of CO2 and PO without assistance
of cocatalysts, providing high polymeric/cyclic carbonate se-
lectivity. The catalytic activity, molecular weight, and regio-
structure of the resultant copolymer can be tailored by the
Lewis base group on the ligand of cobalt salen complexes.
The carbonate unit content in the copolymer and selectivity
of PPC over PC were insensitive to CO2 pressure in our
research pressure region. Further work is underway to apply
the concepts to develop more selective catalysts. These
results might shed new lights on designing new metal salen
complex-based catalysts for the copolymerization of epoxide
and CO2.
ꢁ
(30.0 mL) was stirred and heated to 85–90 C for 12 h. After
the evaporation of the solvent under reduced pressure, water
(60.0 mL) was poured into the residue. The solution was then
extracted with dichloromethane (3 ꢂ 20 mL). The organic
layer was washed with brine (3 ꢂ 10 mL), dried over MgSO4,
concentrated and purified through flash chromatography
(SiO2, petroleum ether–EtOAc ¼ 50:1) to generate a pale yel-
low liquid, yield in 78%, 1H NMR (400 MHz, CDCl3) d ¼
11.71 (s, 1H, OH), 9.87 (s, 1H, CHO), 7.46 (s, 1H, ph-H), 7.40
(s, 1H, ph-H), 3.60 (s, 2H, CH2AN), 2.54 (m, 4H, N(CH2)2),
1.82 (m, 4H, CH2), 1.42 (s, 9H, t-Bu) ppm. 13C NMR (100
MHz, CDCl3) d ¼ 197.10, 158.92, 136.71, 134.67, 131.35,
131.04, 121.12, 60.31, 54.02, 34.76, 33.23, 24.31 ppm.
Synthesis of (R,R)-N,N0-[2,20-bis(nitrilomethylidyne)]
bis[4-(pyrrolidin-1-ylmethyl)-6-tert-butyl phenolato]-
1,2-cyclohexanediamine (Ligand 1c)
EXPERIMENTAL
General Procedure and Materials
All manipulations of air and/or moisture-sensitive com-
pounds were carried out under dry high purity nitrogen
using standard Schenk techniques. Toluene was refluxed and
distilled from Na-benzophenone under dry nitrogen. Triethyl-
amine was refluxed over KOH and then distilled under nitro-
gen before use. 1,2-diaminocyclohexane (isomer, B.P. 92–93
ꢁC/18 mm) was purchased from ABCR GmbH & Co. Propyl-
ene oxide was refluxed over CaH2 and distilled before
use. Carbon dioxide of 99.99% purity was used without fur-
ther purification. (R,R)-N,N0-Bis(3,5-di-tert-butylsalicylidene)-
1, 2-diaminocyclohexanecobalt ((R,R)-(salcy)CoII) were pur-
chased from Aldrich. 3-tert-butyl-5-(chloromethyl)-2-hydrox-
The synthetic procedure was analogous to methods reported.32
Yellow solid, yield in 63%, mp: 67–70 ꢁC. IR: 2957, 2862,
2783, 1629 (C¼¼N), 1596, 1440, 1378, 1347, 1315, 1265,
1208, 1162, 1138, 1045, 997, 866, 798, 774, 756. 1H
NMR(400 MHz, CDCl3) d ¼ 13.80 (2H, OH), 8.25 (2H, CH¼¼N),
7.15 (2H, ph-H), 6.96 (2H, ph-H), 3.44 (4H, ph-CH2AN), 3.30
(2H, CHAN), 2.41(8H, N(CH2)2), 1.46–1.97 (8H, (CH2)4),
1.40(18H, t-Bu) ppm. 13C NMR (100 MHz, CDCl3) d ¼ 165.53,
159.24, 136.93, 136.70, 130.27, 130.06, 118.21, 72.41, 60.25,
54.00, 34.72, 33.14, 29.40, 24.31, 23.37 ppm. Anal. Calcd for
C38H56N4O2: C, 75.96; H, 9.39; N, 9.32. Found: C, 75. 94, H,
9.40, N, 9. 29. Uv-Vis: 241, 262, and 331 nm.
ylbenzaldehyde,
bis[4-(piperidin-1-ylmethyl)-6-tert-butyl
(R,R)-N,N0-[2,20-bis(nitrilomethylidyne)]-
phenolato]-1,2-
General Synthesis of CoIII(salen)Cl Complexes
cyclohexanediamine (Ligand 1a),32 (R,R)-N,N0-[2,20-bis(nitri-
lomethylidyne)]bis[4-(morpholin-1-ylmethyl)-6-tert-butyl
phenolato]-1, 2-cyclohexanediamine (Ligand 1b),32 and
(R,R)-N,N0-[2,20-bis(nitrilomethylidyne)]bis[4-(methylene-N,N0-
dibutylamino)-6-tert-butyl phenolato]-1, 2-cyclohexanediamine
(Ligand 1d),23d were prepared according to the literature and
identified by IR and NMR spectroscopy. Other Chemical
reagents were used as received.
The salen ligands (1.0 equiv) and cobalt (II) acetate (1.1
equiv) were dissolved in ethanol and stirred under argon at
ambient temperature for 24 h.23 Solid LiCl was added, and
the resulting reaction mixture was exposed to air and stirred
for an additional 24 h at room temperature. The solvent was
removed under reduced pressure, and the residual was
extracted with CH2Cl2. The extract was washed with distilled
water, then brine, dried over anhydrous Na2SO4, and
ALTERNATING COPOLYMERIZATION OF CO2/PO, LIU ET AL.
363

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
filtrated. The filtrate was distilled under reduced pressure to
leave a brown powder.
Copolymerization of CO2 and PO
The copolymerization of CO2 with PO was carried out in a
100 mL autoclave equipped with a magnetic stirrer. The
clean autoclave was dried at 80 ꢁC for 8 h in a vacuum
before use. Cobalt salen complex and PO were introduced
into the dried autoclave under CO2 atmosphere, and the
entire assembly was connected to high pressure CO2 cylin-
der. The CO2 was continuously fed in and its pressure was
maintained at expected pressure by one-way electromagnetic
valve in whole reaction period. The copolymerization was
run in predetermined conditions, after evacuation of carbon
dioxide, a very small quantity of raw mixture was taken for
IR measurement to determine cyclic carbonate produced
during copolymerization, and then methanol was added to
terminate the copolymerization. The products were purified
by dissolved in methylene chloride, precipitated in dilute hy-
drochloric acid methanol solution for three turns, and dried
Complex 1a
Yield in 88 %. IR: 2942, 2862, 1636 and 1614(C¼¼N), 1536,
1443, 1386, 1324, 1243, 1208, 1168, 1104, 1034, 952, 896,
824, 787, 564. Anal. Calcd for C40H58ClCoN4O2: C, 66.61; H,
8.10; N, 7.77. Found: C, 66.64; H, 8.09; N, 7.73. Uv-Vis: 258,
331 (very weak), 410 nm.
Complex 1b
Yield in 85%. IR: 2949, 2861, 2804, 1636, and 1614 (C¼¼N),
1535, 1440, 1386, 1342, 1322, 1243, 1205, 1166, 1117, 1069,
1033, 1006, 947, 911, 865, 823, 785, 567. Anal. Calcd for
C38H54ClCoN4O4: C, 62.93; H, 7.50; N, 7.73. Found: C, 62.92; H,
7.53; N, 7.71. Uv-Vis: 259, 332 (weak), and 412 nm.
Complex 1c
Yield in 89 %. IR: 2946, 2865, 1638, and 1599 (C¼¼N), 1537,
1459, 1442, 1385, 1348, 1322, 1242, 1204, 1159, 1102,
1047, 868, 793, 566. Anal. Calcd for C38H54ClCoN4O2: C,
65.84; H, 7.85; N, 8.08. Found: C, 65.80; H, 7.88; N, 8.06. Uv-
Vis: 262, 331 (some strong), 410 nm.
ꢁ
at 30 C in vacuo for 24 h.
The authors are grateful for the financial support by the
National Natural Scientific Research Foundation of China (grant
No. 50973026) and the State Key laboratory of Polymer Physics
and Chemistry (200901, China). X. H. Wang appreciates the fi-
nancial support from Natural Science Foundation of China
(grant No. 20634040).
Complex 1d
Yield in 91 %. IR: 1639 and 1601 (C¼¼N), 1537, 1461, 1443,
1423, 1385, 1350, 1324, 1245, 1206, 1161, 1103, 1032, 949,
869, 793, 736, 707, 565. Anal. Calcd for C46H74ClCoN4O2: C,
68.25; H, 9.21; N, 6.92. Found: C, 68.23; H, 9.18; N, 6.89. Uv-
Vis: 254, 330 (medium), and 409 nm.
REFERENCES AND NOTES
Complex 1e
Complex 5 was prepared as previously described.23 Yield in
80%. Anal. Calcd for C36H52ClCoN2O2: C, 67.65; H, 8.20; N,
4.38. Found: C, 67.61; H, 8.19; N, 4.41. Uv-Vis: 244 (some
strong), 279, 366, and 492 nm.
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2
For reviews on the copolymerization of epoxide/CO2, see: (a)
Calculated amount of Co(salen)Cl complex was first dissolved
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tion mixture was filtered after overnight stirring to ensure
complete reaction, and then 2.0 equiv. of NaN3 was added
into filtrate, keeping exposure to air, the reaction was
allowed to stir for 24 h. The mixture was diluted with di-
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4
Zinc b-diketoiminato complexes such as: (a) Cheng, M.; Lobkovsky,
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1615 (C¼¼N), 1536, 1455, 1440, 1421, 1387, 1342, 1319,
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Uv-Vis: 242, 265, 331 (weak), and 410 nm.
Complex 2e
-
Yield in 79 %. IR: 2952, 2865, 2006 (N3 ), 1638 and 1611
(C¼¼N), 1528, 1458, 1436, 1408, 1386, 1360, 1321, 1256,
1169, 1102, 1043, 870, 833, 784, 745, 567, 541. Anal. Calcd
for C36H52CoN5O2: C, 66.96; H, 8.12; N,10.85. Found: C,
66.98; H, 8.10; N, 10.86. Uv-Vis: 241, 265, 352, and 423 nm.
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ALTERNATING COPOLYMERIZATION OF CO2/PO, LIU ET AL.
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