Polymerization of 1,3-butadiene catalyzed by cobalt(II) and nickel(II) complexes bearing imino- or amino-pyridyl alcohol ligands in combination with ethylaluminum sesquichloride

Article abstract of DOI:10.1016/j.jorganchem.2012.01.013

A series of cobalt(II) (1a-6a) and nickel(II) (1b-6b) complexes supported by imino- or amino-pyridyl alcohol ligands (tridentate [NNO]), 2-(2,6-R ¹₂C₆H₃N=CMe)-6-{(HO)CR ₂}C₅H₃N (L1-L4) and 2-(2,6-R¹ ₂C₆H₃NHCHMe)-6-(CH₂OH)C ₅H₃N (L5 and L6), were synthesized and characterized by elemental and spectroscopic analysis along with X-ray diffraction analysis. The X-ray diffraction demonstrated that all the complexes adopted distorted trigonal bipyramidal configuration with the equatorial plane formed by the pyridyl nitrogen atom and two chlorine atoms. On activation with ethylaluminum sesquichloride (EASC), the cobalt complexes (1a-6a) displayed high catalytic activity for the polymerization of 1,3-butadiene to yield cis-1,4-polybutadiene with high selectivity (>96%) under the Al/Co molar ratio of 40 at 25 °C. The conversion of butadiene, microstructure and molecular weight of the resulting polymers were affected by the reaction parameters and ligand environment. However, in comparison with the corresponding cobalt complexes, the nickel complexes (1b-6b) obtained relatively lower catalytic activity, cis-1,4 content and molecular weight under the similar reaction conditions.

Full text of DOI:10.1016/j.jorganchem.2012.01.013

Journal of Organometallic Chemistry 705 (2012) 51e58
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Polymerization of 1,3-butadiene catalyzed by cobalt(II) and nickel(II) complexes
bearing imino- or amino-pyridyl alcohol ligands in combination with
ethylaluminum sesquichloride
*
*
Pengfei Ai, Lin Chen, Yintian Guo, Suyun Jie , Bo-Geng Li
State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of cobalt(II) (1ae6a) and nickel(II) (1be6b) complexes supported by imino- or amino-pyridyl
alcohol ligands (tridentate [NNO]), 2-(2,6-R12C6H3N¼CMe)-6-{(HO)CR2}C5H3N (L1eL4) and 2-(2,6-
R12C6H3NHCHMe)-6-(CH2OH)C5H3N (L5 and L6), were synthesized and characterized by elemental and
spectroscopic analysis along with X-ray diffraction analysis. The X-ray diffraction demonstrated that all
the complexes adopted distorted trigonal bipyramidal configuration with the equatorial plane formed by
the pyridyl nitrogen atom and two chlorine atoms. On activation with ethylaluminum sesquichloride
(EASC), the cobalt complexes (1ae6a) displayed high catalytic activity for the polymerization of 1,3-
butadiene to yield cis-1,4-polybutadiene with high selectivity (>96%) under the Al/Co molar ratio of
40 at 25 ꢀC. The conversion of butadiene, microstructure and molecular weight of the resulting polymers
were affected by the reaction parameters and ligand environment. However, in comparison with the
corresponding cobalt complexes, the nickel complexes (1be6b) obtained relatively lower catalytic
activity, cis-1,4 content and molecular weight under the similar reaction conditions.
Received 30 November 2011
Received in revised form
10 January 2012
Accepted 17 January 2012
Keywords:
Cobalt
Nickel
1,3-Butadiene
Polymerization
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
production of high cis-1,4-PBD since the resulting PBDs have good
processing properties and high tack due to broad molecular weight
Polybutadienes (PBDs) are formed via 1,4- or 1,2-insertions,
which lead to cis-1,4, trans-1,4 or 1,2-vinyl microstructures. Among
these different isomers, high cis-1,4-PBD has obtained much
industrial importance due to its natural-rubber-like properties [1],
which is produced commercially by the solution polymerization of
1,3-butadiene with the ZieglereNatta catalysts based on transition
metals or rare earth metals, such as TiC14/I2/Al(i-Bu)3 [2], CoCl2/
AlEt2Cl [1b,3], Ni(OOCR)2/BF3$OEt2/AlEt3 [4], or Nd(OOCR)3/
Et3Al2Cl3/Al(i-Bu)2H [5] in aromatic or aliphatic hydrocarbons at
50e70 ꢀC. The cobalt-based catalysts have been widely investigated
probably because they can produce PBDs with different micro-
structures, including cis-1,4-PBD and syndiotactic 1,2-PBD
depending on the polymerization parameters and catalyst struc-
ture. The halides and carboxylates of cobalt are stereoselective to
high cis-1,4-PBD when activated with methylaluminoxane (MAO)
[6]. However, the cobalt halides when combined with alkylphos-
phines or pyridyl adducts produced predominantly 1,2-PBD [7]. The
nickel-based catalysts are also of particular interest in the
distribution and many branches [8].
In the polymerization of butadiene with transition metal cata-
lysts, the ligand structures play an important role in determining
both the catalytic activity and the microstructures of PBDs. Cobalt
complexes bearing bi-, tri- or multi-dentate organic ligands have
been used as catalysts in the polymerization of butadiene. For
example, four-coordinated (salen)cobalt(II) ([ONNO]2ꢁ) [9] and
bis(salicylaldiminate)cobalt(II) complexes ([NO]ꢁ) [10], five-
coordinated cobalt dichloride complexes bearing neutral tri-
dentate ligands (especially such as bis(imino)pyridine ligands [11],
bis(benzimidazolyl)amine ligands [12], bis(benzimidazolyl)pyri-
dine ligands [13]) have achieved high activity and high cis-1,4
selectivity in the polymerization of butadiene in combination with
MAO or EASC. As for nickel catalysts, (salen)nickel(II) ([ONNO]2ꢁ
)
[14], nickel-tropolonoide and nickel-1,3-propanedionate ([OO]ꢁ)
[15] complexes have produced high conversions of butadiene with
high cis-1,4 content in the polymers. Whereas nickel dihalide
complexes bearing neutral
a-dimine ligands ([NN]) [14,16] or
bis(imino)pyridine ligands ([NNN]) [11b,15] were less active for the
polymerization of butadiene.
During the exploration of new effective chelate metal complexes
for the stereospecific polymerization of butadiene, we have found
* Corresponding authors. Tel.: þ86 571 87951515; fax: þ86 571 87951612.
E-mail addresses: jiesy@zju.edu.cn (S. Jie), bgli@zju.edu.cn (B.-G. Li).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.01.013

52
P. Ai et al. / Journal of Organometallic Chemistry 705 (2012) 51e58
that dinuclear cobalt(II) complexes bearing 3-aryliminomethyl-2-
hydroxybenzaldehyde ligands ([NOO]) were highly active and
produced high contents of cis-1,4-PBD by using low amount of EASC
as cocatalyst [17]. Thus it would be of interest to investigate the
tridentate [NNO] ligands; therefore 2-arylimino-6-(alcohol)pyri-
dines were chosen since their nickel and cationic palladium
complexes showed high catalytic activities for the vinyl polymeri-
zation of norbornene [18]. Herein we will report the synthesis and
characterization of cobalt and nickel complexes bearing 2-
arylimino- or 2-arylamino-6-(alcohol)pyridine ligands and their
application in the stereospecific polymerization of 1,3-butadiene to
produce mainly cis-1,4-PBD in combination with ethylaluminum
sesquichloride (EASC).
2. Results and discussion
2.1. Synthesis of imino- or amino-pyridyl alcohol ligands and their
complexes
The
starting
materials,
2-carboxylate-6-iminopyridine
compounds, 2-COOEt-6-(2,6-R12C6H3N¼CMe)C5H3N (R1 ¼ Me, 1;
Et, 2; i-Pr, 3) [19], were readily prepared by the condensation
reaction of 6-acetylpyridine-2-carboxylate and the corresponding
substituted anilines. The desired (imino)pyridyl alcohol ligand, 2-
(2,6-Et2C6H3N¼CMe)6-{(HO)CMe2}C5H3N (L4), was prepared by
the reaction of 2.0 equiv. of MeLi with compound 2 according to
the literature procedure (Scheme 1) [18,20,21]. The ligands, 6-
CH2OH-2-(2,6-R12C6H3N¼CMe)C5H3N (R1 ¼ Me, L1; Et, L2; i-Pr,
L3) were prepared via the reduction of compound 1e3 by 1.1 equiv.
of sodium borohydride with the assistance of calcium chloride in
methanol in good yield (Scheme 1) [18]. It is unexpected that the
C]N double bonds (imino groups) in compound 1 and 2 could be
also reduced to amino groups with the increased amount of sodium
borohydride, leading to the formation of ligands L5 and L6 (Scheme
1). However, this reaction didn’t occur for the 2,6-diisopropyl-
substituted compound 3 despite of the addition of more excess of
sodium borohydride, which was probably ascribed to more bulki-
ness of isopropyl groups than methyl and ethyl groups at the ortho-
positions of imino-N aryl ring. The ligands, L1, L2, L5 and L6, were
identified on the basis of FT-IR, 1H and 13C NMR spectra as well as
elemental analysis.
Scheme 2. Synthesis of cobalt complexes 1ae6a and nickel complexes 1be6b.
double bonds of cobalt complexes (1ae4a) (1618e1624 cmꢁ1) and
nickel complexes (1be4b) (1616e1621 cmꢁ1) apparently shifted to
lower wave number and the peak intensity greatly reduced, as
compared to the corresponding free ligands (1644e1645 cmꢁ1),
indicating the coordination interaction between the imino
nitrogen atom and the metal center. The stretching vibration bands
of all the hydroxyl groups and amino groups in complexes 5a, 6a, 5b
and 6b also shifted to lower wave number than those in the free
ligands. All the data of elemental analysis were in accordance with
the theoretical calculations. The unambiguous structures were
further confirmed by the single-crystal X-ray diffraction analysis.
2.2. Crystal structures
Single-crystals of complexes 2a (blue), 4a (dark green), 6a
(bright blue) and 6b (brown) suitable for X-ray diffraction analysis
were individually obtained by slow diffusion of diethyl ether into
their methanol solutions. According to their structures, the coor-
dination geometry around the cobalt or nickel center can be
described as a distorted trigonal bipyramidal, in which the nitrogen
atom of pyridyl group (N1) and two chlorides (Cl1 and Cl2)
compose an equatorial plane. Their crystal structures are shown in
Figs.1e4, respectively, and the selected bond lengths and angles are
collected in Table 1.
The 2-arylimino-6-(alcohol)pyridines (L1eL4) or 2-arylamino-
6-(alcohol)pyridines (L5 and L6) reacted with one equiv. of
CoCl2$6H2O or NiCl2$6H2O in absolute ethanol at room tempera-
ture to yield the corresponding cobalt complexes 1ae6a and nickel
complexes 1be6b as air-stable powders (Scheme 2). All the
complexes were characterized by FT-IR spectra and elemental
analysis. In the FT-IR spectra, the stretching vibration bands of C]N
R1
N
1.1eq NaBH4/CaCl2
4.0eq NaBH4/CaCl2
N
OH
L1: R1 = Me
L2: R1 = Et
L3: R1 = i-Pr
R1
OEt
R1
N
N
N
O
2.0eq MeLi
N
OH
R1
R1
N
1: R1 = Me
2: R1 = Et
3: R1 = i-Pr
L4
NH
R1
OH
L5: R1 = Me
L6: R1 = Et
Scheme 1. Synthesis of ligands L1eL6.

P. Ai et al. / Journal of Organometallic Chemistry 705 (2012) 51e58
53
Fig. 1. Molecular structure of complex 2a with thermal ellipsoids at the 30% proba-
bility level. Hydrogen atoms (apart from H1) have been omitted for clarity.
Fig. 3. Molecular structure of complex 6a with thermal ellipsoids at the 30% proba-
bility level. Hydrogen atoms (apart from H1, H2 and H6A) have been omitted for clarity.
In the structure of 2a (Fig. 1), the cobalt atom deviates by
0.1290 Å from the triangular plane of N1, Cl1 and Cl2 with the
equatorial angle ranging from 101.34(8)ꢀ to 142.88(9)ꢀ. The axial
CoeN2 and CoeO bonds subtend an angle of 145.93(12)ꢀ
(OeCoeN2). The equatorial plane is nearly perpendicular to the
pyridyl plane with a dihedral angle of 92.2ꢀ. The dihedral angle
between the phenyl ring and the pyridyl plane is 84.4ꢀ. The
CoeN1(pyridyl) bond (2.044(3) Å) in the equatorial plane is shorter
than the axial CoeN2(imino) (2.149(3) Å) and CoeO (2.219(3) Å)
bonds, which is similar to the corresponding iron and nickel
complexes [18,20]. The two CoeCl bond lengths show a slight
difference between CoeCl1 (2.3085(11) Å) and CoeCl2 (2.2383(13)
Å). The imino C6eN2 bond length is 1.285(5) Å with the typical
character of a C]N double bond. The structure of complex 4a (Fig. 2)
with two gem-methyl groups on the C18 atom is similar to that of 2a.
Therefore, only the structure of 2a was discussed in detail here.
The cobalt complex 6a and nickel complex 6b with the reduced
(amino)pyridinyl ligand show very similar geometries and
structural parameters (Table 1 and Figs. 3 and 4) and only the
structure of 6a will be discussed below, in order to compare with
the corresponding complex 2a with the(imino)pyridinyl ligand. The
pyridyl nitrogen atom (N1) and two chlorides form the equatorial
plane with the slight deviation of cobalt atom by 0.0323 Å from this
plane. The bond angles around the cobalt center are in the range of
76.08(7)ꢀ (OeCoeN1) to 154.65(6)ꢀ (OeCoeN2). The dihedral
angles between the equatorial plane and the pyridyl plane, and the
phenyl ring and the pyridyl plane, are 83.6ꢀ and 97.1ꢀ, respectively.
The CoeN2(amino) bond distance (2.2572(17) Å) in 6a is longer by
about 0.11 Å than that of CoeN2(imino) bond (2.149(3) Å) in 2a, and
longer by about 0.23 Å than that of the CoeN1 bond (2.0263(17) Å)
in the equatorial plane. The two CoeCl bond distances are similar.
The bond length of C6eN2 is 1.496(3) Å in the range of CeN single
bond, indicating the reduction of imino group to amino group.
Fig. 2. Molecular structure of complex 4a with thermal ellipsoids at the 30% proba-
Fig. 4. Molecular structure of complex 6b with thermal ellipsoids at the 30% proba-
bility level. All hydrogen atoms have been omitted for clarity.
bility level. Hydrogen atoms (apart from H1, H2 and H6A) have been omitted for clarity.

54
P. Ai et al. / Journal of Organometallic Chemistry 705 (2012) 51e58
Table 1
However, the Al/Co molar ratio slightly affected the molecular
weight distribution.
Selected bond lengths (Å) and angles (ꢀ) of complexes 2a, 4a, 6a and 6b.
The butadiene polymerizations were also carried out over the
temperature range from 0 to 90 ꢀC at an Al/Co molar ratio of 40 with
the 2a/EASC catalytic system (entries 5e9 in Table 2). The 39.3%
conversion of butadiene was gained at 0 ꢀC; however, other cobalt-
based catalytic systems gave almost no activity at 0 ꢀC [11b,13,17].
The highest activity was observed at 25 ꢀC with the complete
conversion of butadiene. The 2a/EASC catalytic system had
remarkable thermal stability at 25e70 ꢀC affording the 88.7%
conversion of butadiene at 70 ꢀC, but the conversion dropped
sharply to 56.6% when the temperature was increased up to 90 ꢀC
probably due to the deactivation of active species at higher
temperatures. The decrease in polymerization rate at the elevated
reaction temperature is very common in olefin polymerization
catalyzed by late-transition metal complexes [22]. The molecular
weights of PBDs obtained at 0e50 ꢀC showed only slight differ-
ences, but decreased as the temperature was increased to 70 and
90 ꢀC. The molecular weight distributions of PBDs didn’t change
greatly at the different temperatures. One of the most distinguished
features was the important change of polymer microstructure
induced by the variation of reaction temperature. The amount of
trans-1,4-PBDs in the polymers increased gradually along with the
elevation of reaction temperature (Fig. 5). As a result, the poly-
merization at 90 ꢀC produced the 12.5% content of trans-1,4 isomer
and the cis-1,4 content reduced to 82.9%, which was largely
different from the PBDs obtained at 25 ꢀC containing 2.4% of trans-
1,4 and 96.0% of cis-1,4 isomers. However the content of 1,2-
inserted isomer slightly increased from 1.6% to 4.6% over the
temperature range from 0 to 90 ꢀC.
In order to explore the influence of ligand environment,
including steric and electronic effects, on the catalytic activity and
properties of resulting polymers, the butadiene polymerizations
catalyzed by all the cobalt complexes (1ae6a) were carried out
under similar conditions with the Al/Co molar ratio of 40 over
30 min and at 25 ꢀC (entries 10e15 in Table 2). The substituents (R1)
on the imino- or amino-N aryl ring had an obvious influence on the
catalytic performances of the cobalt complexes. The bulkier
substituents led to lower conversion of butadiene and relatively
higher molecular weight. The more bulkiness of the R1 groups at
the ortho-positions of aryl ring may to some extent prevent the
access of monomer to the active center in the catalytic system,
therefore resulting in the decrease of catalytic activity. For instance,
2a (M ¼ Co)
4a (M ¼ Co)
6a (M ¼ Co)
6b (M ¼ Ni)
Bond lengths
MeN1
MeN2
MeCl1
MeCl2
MeO
2.044(3)
2.149(3)
2.3085(11)
2.2383(13)
2.219(3)
2.0810(15)
2.1459(14)
2.3111(7)
2.2781(8)
2.1939(13)
1.283(2)
2.0263(17)
2.2572(17)
2.2935(8)
2.2595(8)
2.1443(17)
1.496(3)
1.9877(19)
2.1686(18)
2.3055(8)
2.2535(9)
2.1035(18)
1.505(3)
C6eN2
1.285(5)
Bond angles
OeMeN1
OeMeN2
72.99(12)
145.93(12)
97.83(10)
92.53(9)
101.34(8)
142.88(9)
103.33(9)
102.30(9)
76.86(12)
114.62(5)
73.36(5)
140.61(5)
93.14(4)
103.05(4)
98.72(4)
150.68(4)
104.73(5)
103.00(4)
75.29(6)
76.08(7)
154.65(6)
99.85(6)
97.80(6)
109.64(5)
133.66(5)
89.04(5)
99.29(5)
78.58(7)
116.64(3)
76.93(7)
157.56(7)
98.43(7)
96.20(7)
102.55(6)
143.93(6)
88.63(5)
100.46(6)
80.76(7)
113.50(3)
OeMeCl1
OeMeCl2
N1eMeCl1
N1eMeCl2
N2eMeCl1
N2eMeCl2
N1eMeN2
Cl1eMeCl2
109.79(2)
2.3. Solution polymerization of 1,3-butadiene
Ethylaluminum sesquichloriide (EASC) was proved to be more
effective for cobalt complexes in the polymerization of butadiene
[10,12a,17]. Therefore, the cobalt complex 2a/EASC catalytic system
was investigated under various reaction parameters to optimize the
polymerization conditions, such as Al/Co molar ratio, reaction
temperature and reaction time. The Al/Co molar ratio had a great
influence on the catalytic activity albeit the microstructure of the
resulting polymers didn’t change apparently; the cis-1,4 content
was in the range of 94.1%e96.0% along with small amounts of trans-
1,4-PBD and 1,2-PBD. The lower concentration of EASC ([Al]/
[Co] ¼ 5) could initiate the polymerization of butadiene, affording
61.1% conversion of butadiene. The conversion of butadiene
increased considerably along with the increase of Al/Co molar ratio
from 5 to 40 (entries 1e5 in Table 2) and a complete conversion of
butadiene was obtained under the Al/Co molar ratio of 40 in
60 min. It was observed that the molecular weight initially
increased and then decreased and the highest molecular weight
was obtained under the Al/Co molar ratio of 20, which was different
from the case of olefin polymerization in that the chain transfer to
aluminum was usually an important factor for molecular weight.
Table 2
Polymerization of 1,3-butadiene with complexes 1ae6a/EASC.a
Mn (105 g/mol)
b
Mw/Mn
Microstructurec (mol%)
b
Entry
Cat.
Al/Co
t (min)
T (ꢀC)
Conv. (%)
cisꢁ1,4ꢁ
transꢁ1,4ꢁ
1,2-
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2a
1a
2a
3a
4a
5a
6a
CoCl2
5
10
20
30
40
40
40
40
40
40
40
40
40
40
40
40
60
60
60
60
60
60
60
60
60
30
30
30
30
30
30
30
25
25
25
25
25
0
50
70
90
25
25
25
25
25
25
25
61.1
81.9
94.5
97.6
100
39.3
97.7
88.7
56.6
93.3
82.9
73.2
62.6
90.8
77.1
5.6
1.98
2.18
2.30
1.95
1.99
2.01
2.03
1.64
1.54
2.31
2.57
3.08
2.53
2.53
2.61
e
1.48
1.47
1.51
1.94
1.44
1.31
1.62
1.43
1.31
1.57
1.45
1.50
1.81
1.59
1.46
e
94.1
95.3
96.0
95.7
96.0
95.6
91.5
88.1
82.9
96.6
96.6
96.3
96.8
96.9
96.7
e
4.0
2.8
2.3
2.1
2.4
1.8
5.9
8.6
12.5
1.7
1.9
1.8
1.7
1.5
1.6
e
1.9
1.9
1.7
2.2
1.6
2.6
2.6
3.3
4.6
1.7
1.5
1.9
1.5
1.6
1.7
e
9
10
11
12
13
14
15
16
a
Polymerization conditions: precatalyst: 10.0 mmol, cocat.: EASC, solvent: toluene, total volume: 20 mL, [BD]/[Co]: 2000.
b
c
Determined by GPC against polystyrene standards and reported uncorrected.
Determined by 1H and 13C NMR spectroscopy.

P. Ai et al. / Journal of Organometallic Chemistry 705 (2012) 51e58
55
Table 3
Polymerization of 1,3-butadiene with complexes 1be6b/EASC.a
Entry Cat. Conv. (%) Mnb (103 g/mol) Mw/Mn Microstructurec (mol%)
b
cisꢁ1,4ꢁ transꢁ1,4ꢁ 1,2-
1
2d
3
4
5
6
7
8
1b
2b
2b
3b
4b
5b
6b
67.9
27.2
61.1
59.2
76.2
75.3
68.6
7.15
9.36
6.74
6.06
6.17
6.34
6.00
e
3.42
3.18
3.74
4.25
3.76
3.69
4.08
e
91.7
91.4
91.6
91.8
91.5
91.6
91.7
e
5.5
5.0
5.3
5.0
6.6
6.0
5.4
e
2.8
3.6
3.1
3.2
1.9
2.4
2.9
e
NiCl2 16.4
a
Polymerization conditions: precatalyst: 10.0 mmol, cocat.: EASC, [Al]/[Ni]: 40,
solvent: toluene, total volume: 20 mL, [BD]/[Ni]: 2000, reaction temperature: 25 ꢀC;
reaction time: 2 h.
b
Determined by GPC against polystyrene standards and reported uncorrected.
c
Determined by 1H and 13C NMR spectroscopy.
d
Reaction time: 30 min.
Fig. 5. Microstructure of PBDs produced by 2a/EASC system at different temperatures
(entries 5e9 in Table 2).
groups in the ligands were more active than the corresponding
nickel complexes 1b and 2b with imino groups (conversion orders:
1b < 5b; 2b < 6b). The results in Table 3 demonstrated that the
ligand environment didn’t significantly influence molecular weight,
molecular weight distribution and microstructure of the obtained
polymers. Themicrostructureof PBDsproduced bynickel complexes
was also found to be mainly cis-1,4-PBD together with small
amounts of trans-1,4 and 1,2-vinyl isomers although the cis-1,4
contents were lower than those afforded by cobalt complexes
(Fig. 6).
complex 1a (R1 ¼ Me, 93.3% conv.) bearing two methyl groups on
the imino-N aryl ring achieved higher conversion than complexes
2a (R1 ¼ Et, 82.9% conv.) and 3a (R1 ¼ i-Pr, 73.2% conv.) with ethyl
and isopropyl groups on the imino-N aryl ring, respectively. Simi-
larly, when comparing complexes 5a and 6a with amino-pyridyl
alcohol ligands, 5a (R1 ¼ Me, 90.8% conv.) also had much higher
conversion of butadiene than 6a (R1 ¼ Et, 77.1% conv.). In contrast to
the catalytic activity, the molecular weights of PBDs obtained by
these cobalt complexes exhibited the converse regularity (in the
orders of 1a < 2a < 3a and 5a < 6a). In the same way, the
substituents (R) on the HOeC atom had an important influence on
the catalytic performance. On replacing two hydrogen atoms by
two methyl groups on the HOeC atom, complex 4a (62.6%) ob-
tained much lower conversion of butadiene than the corresponding
complex 2a (82.9%). After the imino groups were reduced to amino
groups in the ligands, slightly lower conversions of butadiene were
observed by comparison with 1a and 5a, 2a and 6a (conversion
orders: 1a > 5a; 2a > 6a). The PBDs produced by complexes 1ae6a
had very similar microstructure with high cis-1,4 contents (96.3%e
96.9%) and narrow molecular weight distributions (1.45e1.81) in
spite of their different molecular weights.
3. Conclusions
A series of cobalt(II) and nickel(II) complexes ligated by imino- or
amino-pyridyl alcohol ligands (L1eL6) were synthesized and char-
acterized. All the complexes adopted distorted trigonal bipyramidal
configuration with the equatorial plane formed by the pyridyl
nitrogen atoms and two chlorine atoms conformed by single-crystal
X-ray analysis. Upon activation with EASC, all the cobalt complexes
1ae6a obtained cis-1,4-polybutadiene with high activity and
selectivity (>96%) under the reaction conditions employed here. The
increase of Al/Co molar ratio enhanced the conversion of butadiene
and the catalytic system kept active over the wide temperature
`The nature of the metal center had a large influence on the
catalytic performance. In general, cobalt-based catalysts were more
active than the corresponding nickel-based analogs; under the
similar conditions employed here, nickel complex 2b/EASC system
gained much lower conversion of butadiene (27.2%) than cobalt
complex 2a with the same ligand (82.9%), although the conversion
increased to 61.1% after 2 h (entry 11 in Table 2 and entry 2e3 in
Table 3). The metal center also greatly influenced the molecular
weight, molecular weight distribution and stereoregularity of the
resulting polymers. All the nickel complexes 1be6b yielded much
lower molecular weights (6000e7150 g/mol), broader molecular
weight distributions (3.42e4.25), and lowercis-1,4 contents (91.5%e
91.8%) than the cobalt complexes 1ae6a sharing the same corre-
sponding ligand environments. Similarly, as for the nickel
complexes, the more bulkiness of the R1 groups at the ortho-posi-
tions of the imino- or amino-N aryl ring also resulted in the lower
conversion of butadiene (in the orders of 1b > 2b > 3b and 5b > 6b);
whereas the influence was relatively smaller than that on the cor-
responding cobalt complexes. The substituents (R) on the HOeC
atom had more important influence on the catalytic activity.
Nickel complex 4b with two methyl groups produced higher
conversion of butadiene than 2b with CH2OH group on the HOeC
atom. On the contrary, nickel complexes 5b and 6b with amino
Fig. 6. The 13C NMR of PBDs obtained by 2a/EASC (a, entry 11 in Table 2) and 2b/EASC
(b, entry 3 in Table 3). *, CDCl3.

56
P. Ai et al. / Journal of Organometallic Chemistry 705 (2012) 51e58
range of 0e90 ꢀC. However the reaction temperature importantly
influenced the microstructure of the resulting polymers. The
difference of ligand environment led to the variation of catalytic
activity although it didn’t affect the cis-1,4 contentof PBDs greatly. In
contrast to cobalt complexes, the corresponding nickel complexes
1be6b bearing the same ligands yielded lower conversion of buta-
diene, cis-1,4 content and molecular weight, and broader molecular
weight distribution under the similar reaction conditions. The
polymerization reactions with CoCl2 or NiCl2/EASC systems
produced remarkably lower conversions, elucidating the impor-
tance of ligand influence to the active centers.
3.99 (b, 1 H, OH), 2.20 (s, 3 H, CH3C]N), 2.03 (s, 6 H, AreCH3). 13
NMR (125 MHz, CDCl3), (ppm): 166.7 (C]N), 157.6, 155.0, 148.4,
137.2, 127.9, 125.4, 123.1, 121.5, and 119.9 (pyeC and aromatic-C),
63.7 (CH2OH), 17.9 (AreCH3), 16.6 (CH3C]N). Anal. Calcd. for
C16H18N2O (254.33): C, 75.56; H, 7.13; N, 11.01. Found: C, 74.55; H,
7.41; N, 10.55.
C
d
4.2.2. 2-(2,6-Et2C6H3N¼CMe)-6-(CH2OH)C5H3N (L2)
By using the same procedure described for L1, ligand L2 was
obtained from compound 2 (0.9258 g, 2.85 mmol) and NaBH4
(0.1175 g, 3.11 mmol) as yellow sticky oil (yield: 0.6403 g, 79.5%). FT-
IR (liquid film, cmꢁ1): 3411 (nOeH), 2965, 2931, 2873, 1644 (
nC]N),
4. Experimental
1582, 1451, 1365, 1309, 1260, 1196, 1110, 1059, 1024, 801, 771. FT-IR
(KBr disk, cmꢁ1): 3430 (nOeH), 3063, 2965, 2931, 2873, 1644
4.1. General considerations and materials
(
n
C]N),1582,1451,1365,1309,1260,1226,1196,1151,1110,1059, 801,
771. 1H NMR (400 MHz, CDCl3),
d
(ppm): 8.29 (d, 1 H, J ¼ 7.6 Hz,
All manipulations of air- or moisture-sensitive compounds were
carried out under an atmosphere of nitrogen using standard Schlenk
techniques. FT-IR spectra were recorded on a PerkineElmer FT-IR
2000 spectrometer by using KBr disks in the range
4000e400 cmꢁ1 1H NMR and 13C NMR spectra were recorded on
a Bruker DMX-400 instrument in CDCl3 with TMS as the internal
standard. Splitting patterns are designated as follows: s, singlet; d,
doublet; dd, double doublet; t, triplet; q, quadruplet; sept, septet; m,
multiplet. Elemental analysis was performed on a Flash EA1112
microanalyzer. The molecular weight and molecular weight distri-
bution of polybutadienes were measured by GPC using Waters 2414
series system in THF at 25 ꢀC calibrated with polystyrene standards.
Toluene and tetrahydrofuran were refluxed over sodium-
benzophenone and distilled under nitrogen prior to use. Ethyl-
aluminum sesquichloride (EASC, 0.4 M in hexane) and all the
anilines were purchased from Acros Chemicals. Methyl lithium
(1.0 M inTHF) was purchased from Jingyan Chemicals (Shanghai) co.,
Ltd. NiC12$6H2O, CoCl2$6H2O and sodium borohydride were ob-
tained from Beijing Chemical Regents Co. and directly used without
further purification. Polymerization grade butadiene was purified
bypassing it through columns of KOH and molecular sieves. All other
chemicals were obtained commercially and used without further
purification unless otherwise stated. The compounds, 2-COOEt-6-
(2,6-R12C6H3N¼CMe)C5H3N (R1 ¼ Me, 1; Et, 2; i-Pr, 3) [19] and the
ligands, 2-(2,6-i-Pr2C6H3N¼CMe)-6-(CH2OH)C5H3N (L3) [20,21]
and 2-(2,6-Et2C6H3N¼CMe)-6-{(HO)CMe2}C5H3N (L4) [18] were
prepared according to the literature.
pyeH), 7.82 (t, 1 H, J ¼ 7.6 Hz, pyeH), 7.32 (d, 1 H, J ¼ 7.6 Hz, pyeH),
7.11 (d, 2 H, J ¼ 7.6 Hz, AreH), 7.06e7.02 (m, 1 H, AreH), 4.84 (s, 2 H,
CH2OH), 3.99 (b, 1 H, OH), 2.43e2.31 (m, 4 H, CH2CH3), 2.22 (s, 3 H,
CH3C]N),1.14 (t, 6 H, J ¼ 7.6 Hz, CH2CH3).13C NMR (125 MHz, CDCl3),
d
(ppm): 166.4 (C]N), 157.6, 155.0, 147.5, 137.2, 131.1, 125.9, 123.4,
121.5, and 119.9 (pyeC and aromatic-C), 63.7 (CH2OH), 24.5
(CH2CH3), 17.0 (CH3C]N), 13.7 (CH2CH3). Anal. Calcd for C18H22N2O
(282.38): C, 76.56; H, 7.85; N, 9.92. Found: C, 75.32; H, 8.00; N,10.05.
4.2.3. 2-(2,6-Me2C6H3NHCHMe)-6-(CH2OH)C5H3N (L5)
To a methanol solution (50 mL) containing compound 1
(0.8285 g, 2.80 mmol) and anhydrous calcium chloride (1.2430 g,
11.20 mmol), was slowly added 4.0 equivalents of sodium borohy-
dride (0.4297 g,11.36 mmol) at 0 ꢀC. Thereaction solutionwas slowly
warmed to room temperature and then refluxed for 24 h. After the
solution was cooled to room temperature, distilled water was added
and methanol was removed under reduced pressure. The resulting
solution was then extracted with chloroform (30 mL ꢂ 3), and the
combined organic extracts were dried over anhydrous Na2SO4 and
filtered. After all the volatiles were removed under reduced pres-
sure, the resultant crude product was purified on a silica column
(petroleum ether/ethyl acetate ¼ 5:1) to give L1 (the first eluting
part; yield: 0.2460 g, 34.6%) and L5 as an off-white solid (the second
eluting part; yield: 0.4686 g, 65.4%). FT-IR (KBr disk, cmꢁ1): 3360
(
nOeH, NeH), 2968, 2924, 2858, 1593, 1574, 1469, 1449, 1370, 1261,
1219, 1154, 1121, 1091, 1030, 993, 800, 764, 669. 1H NMR (400 MHz,
CDCl3),
d
(ppm): 7.57 (t,1 H, J ¼ 7.6 Hz, pyeH), 7.08 (d,1 H, J ¼ 7.6 Hz,
pyeH), 7.02 (d, 1 H, J ¼ 7.6 Hz, pyeH), 6.94 (d, 2 H, J ¼ 7.2 Hz, AreH),
6.77 (t, 1 H, J ¼ 7.4 Hz, AreH), 4.76 (s, 2 H, CH2OH), 4.45 (q, 1 H,
J ¼ 6.8 Hz, CH3CHNH), 3.99 (s, 1 H, NH), 3.94 (s, 1 H, OH), 2.23 (s, 6 H,
AreCH3),1.49 (d, 3 H, J ¼ 6.8 Hz, CH3CH). 13C NMR (125 MHz, CDCl3),
4.2. Synthesis of ligands (L1, L2, L5, L6)
4.2.1. 2-(2,6-Me2C6H3N¼CMe)-6-(CH2OH)C5H3N (L1)
To a methanol solution (50 mL) containing compound 1
(0.7352 g, 2.48 mmol) and anhydrous calcium chloride (0.5658 g,
5.10 mmol), was slowly added 1.1 equivalent of sodium borohydride
(0.1034 g, 2.73 mmol) at 0 ꢀC. The reaction solution was slowly
warmed to room temperature and then refluxed for 8 h. After the
solution was cooled to room temperature, distilled water was
added and methanol was removed under reduced pressuere. The
resulting solution was then extracted with chloroform (30 mL ꢂ 3),
and the combined organic extracts were dried over anhydrous
Na2SO4 and filtered. After all the volatiles were removed under
reduced pressure, the resultant crude product was purified on
a silica column (petroleum ether/ethyl acetate ¼ 5:1) to give L1 as
yellow sticky oil (yield: 0.4764 g, 75.5%). FT-IR (liquid film, cmꢁ1):
d (ppm): 162.3,158.3,144.6,137.2,129.1,128.8,121.5,120.0, and 118.7
(pyeC and aromatic-C), 63.8 (CH2OH), 57.4 (CHNH), 22.3 (CH3CH),
18.9 (AreCH3). Anal. Calcd for C16H20N2O (256.34): C, 74.97; H, 7.86;
N, 10.93. Found: C, 74.47; H, 7.74; N, 11.20.
4.2.4. 2-(2,6-Et2C6H3NHCHMe)-6-(CH2OH)C5H3N (L6)
By using the same procedure described for L5, ligand L6 was
obtained from compound 2 (0.7960 g, 2.45 mmol) and NaBH4
(0.4655 g, 12.30 mmol) as an off-white solid (yield: 0.4096 g,
58.7%). FT-IR (KBr disk, cmꢁ1): 3364 (nOeH, NeH), 2965, 2930, 2873,
1593, 1576, 1455, 1368, 1262, 1202, 1154, 1121, 1087, 797, 754. 1H
NMR (400 MHz, CDCl3),
d
(ppm): 7.57 (t, 1 H, J ¼ 7.6 Hz, pyeH), 7.09
(d, 1 H, J ¼ 7.6 Hz, pyeH), 7.03e7.00 (m, 3 H, 1 pyeH and 2 AreH),
6.91 (t, 1 H, J ¼ 7.6 Hz, AreH), 4.77 (d, 2 H, J ¼ 4.0 Hz, CH2OH), 4.39
(q,1 H, J ¼ 6.8 Hz, CH3CHNH), 4.14 (s,1 H, NH), 3.94 (t,1 H, J ¼ 4.4 Hz,
OH), 2.61 (q, 4 H, J ¼ 7.6 Hz, CH2CH3),1.47 (d, 3 H, J ¼ 6.8 Hz, CH3CH),
3356 (nOeH), 2962, 2920, 2852, 1645 (
1308, 1260, 1203, 1112, 1090, 1059, 1025, 801, 769. 1H NMR
(400 MHz, CDCl3),
nC]N), 1587, 1464, 1441, 1364,
d
(ppm): 8.29 (d, 1 H, J ¼ 7.6 Hz, pyeH), 7.81 (t,
1 H, J ¼ 7.6 Hz, pyeH), 7.32 (d, 1 H, J ¼ 7.6 Hz, pyeH), 7.07 (d, 2 H,
1.21 (t, 6 H, J ¼ 7.6 Hz, CH2CH3). 13C NMR (125 MHz, CDCl3),
d (ppm):
J ¼ 7.6 Hz, AreH), 6.94 (t,1 H, J ¼ 7.6 Hz, AreH), 4.83 (s, 2 H, CH2OH),
162.2, 158.3, 143.4, 137.1, 135.7, 126.4, 122.1, 120.0, and 118.7 (pyeC

P. Ai et al. / Journal of Organometallic Chemistry 705 (2012) 51e58
57
and aromatic-C), 63.8(CH2OH), 58.5 (CHNH), 24.5 (CH2CH3), 22.1
(CH3CH), 14.4 (CH2CH3). Anal. Calcd for C18H24N2O (284.40): C,
76.02; H, 8.51; N, 9.85. Found: C, 76.37; H, 8.51; N, 9.85.
yield. FT-IR (KBr disk, cmꢁ1): 3324, 3298, 2974, 2926, 1606, 1578,
1471, 1453, 1417, 1371, 1283, 1192, 1162, 1129, 1101, 1044, 974, 882,
823, 801, 788. Anal. Calcd for C16H20Cl2CoN2O (386.18): C, 49.76; H,
5.22; N, 7.25. Found: C, 49.31; H, 5.33; N, 7.14. 6a: Deep blue powder
in 81.1% yield. FT-IR (KBr disk, cmꢁ1): 3312, 3221, 2968, 2932, 2871,
1607, 1579, 1474, 1454, 1421, 1377, 1293, 1170, 1130, 1044, 964, 925,
855, 825, 795. Anal. Calcd for C18H24Cl2CoN2O (414.24): C, 52.19; H,
5.84; N, 6.76. Found: C, 51.79; H, 5.72; N, 6.71.
4.3. Preparation of cobalt and nickel complexes
4.3.1. General procedure
To a stirred solution of the corresponding ligand (0.50 mmol) in
anhydrous ethanol (20 mL), one equivalent of CoCl2$6H2O or
NiCl2$6H2O (0.50 mmol) was added, and the reaction mixture was
stirred at room temperature overnight. The resultant solution was
concentrated to ca. 3 mL. Diethyl ether was added to precipitate the
product, which was filtered, washed repeatedly with diethyl ether
and dried under vacuum. All of the complexes were prepared in
high yield in this manner.
4.3.3. Synthesis of nickel complexes 1be6b
1b: Brick-red powder in 91.1% yield. FT-IR (KBr disk, cmꢁ1):
3366, 3227, 2917, 1617 (nC]N), 1590, 1471, 1440, 1374, 1299, 1267,
1207, 1097, 1041, 841, 792. Anal. Calcd for C16H18Cl2N2NiO (383.93):
C, 50.05; H, 4.73; N, 7.30. Found: C, 49.75; H, 4.43; N, 7.08. 2b: Brick-
red powder in 94.3% yield. FT-IR (KBr disk, cmꢁ1): 3386, 3223, 3076,
2969, 2932, 2876, 1621 (nC]N), 1596, 1447, 1371, 1297, 1201, 1102,
1039, 801, 773. Anal. Calcd for C18H22Cl2N2NiO (411.98): C, 52.48; H,
5.38; N, 6.80. Found: C, 52.39; H, 5.11; N, 6.72. 5b: Yellowish brown
powder in 85.2% yield. FT-IR (KBr disk, cmꢁ1): 3320, 3163, 2977,
2931, 1604, 1579, 1473, 1375, 1283, 1191, 1162, 1129, 1098, 1036, 982,
937, 901, 806. Anal. Calcd for C16H20Cl2N2NiO (385.94): C, 49.79; H,
5.22; N, 7.26. Found: C, 49.42; H, 4.98; N, 7.07. 6b: Yellowish brown
powder in 72.6% yield. FT-IR (KBr disk, cmꢁ1): 3288, 3219, 2968,
2932, 2872, 1609, 1583, 1453, 1418, 1379, 1290, 1249, 1169, 1129,
1041, 973, 933, 796. Anal. Calcd for C18H24Cl2N2NiO (414.00): C,
52.22; H, 5.84; N, 6.77. Found: C, 51.89; H, 5.70; N, 6.58.
4.3.2. Synthesis of cobalt complexes 1ae6a
1a: Green powder in 62.1% yield. FT-IR (KBr disk, cmꢁ1): 3379,
3090, 2976, 2915, 1618 (nC]N), 1593, 1473, 1455, 1441, 1371, 1291,
1204, 1094, 1049, 798, 781. Anal. Calcd for C16H18Cl2CoN2O (384.17):
C, 50.02; H, 4.72; N, 7.29. Found: C, 49.98; H, 4.54; N, 6.90. 2a: Green
powder in 86.0% yield. FT-IR (KBr disk, cmꢁ1): 3267, 3073, 2966,
2930, 2873, 1624 (nC]N), 1598, 1471, 1451, 1370, 1295, 1199, 1044,
1027, 802, 771. Anal. Calcd for C18H22Cl2CoN2O (412.22): C, 52.45; H,
5.38; N, 6.80. Found: C, 52.66; H, 5.23; N, 6.51. 3a: Green powder in
94.5% yield. FT-IR (KBr disk, cmꢁ1): 3252, 2964, 2923, 2868, 1618
(nC]N), 1592, 1468, 1458, 1439, 1385, 1372, 1322, 1296, 1195, 1057,
1028, 811, 797, 781. Anal. Calcd for C20H26Cl2CoN2O (440.27): C,
54.56; H, 5.95; N, 6.36. Found: C, 54.58; H, 5.71; N, 6.36. 4a: Light
green powder in 94.5% yield. FT-IR (KBr disk, cmꢁ1): 3409, 3256,
4.4. Polymerization of 1,3-butadiene
Solution polymerizations of 1,3-butadiene in toluene were
carried out in a sealed glass reactor (100 mL) with a rubber septum
and a connection to a vacuum system. The reactor was charged
with the desired amounts of precatalyst and cocatalyst solutions.
The mixture was stirred for 2 min at the desired temperature and
3066, 2975, 2936, 2875, 1619 (nC]N), 1590, 1467, 1458, 1450, 1375,
1351, 1325, 1288, 1258, 1210, 1171, 1136, 1095, 1023, 820, 802. Anal.
Calcd for C20H26Cl2CoN2O (440.27): C, 54.56; H, 5.95; N, 6.36.
Found: C, 54.54; H, 5.84; N, 6.32. 5a: Deep blue powder in 96.8%
Table 4
Crystallographic data and refinement parameters for complexes 2a, 4a, 6a and 6b.
2a
4a
6a
6b
Formula
C18H22Cl2CoN2O
412.21
293(2)
0.71073
Triclinic
C20H26Cl2CoN2O
440.26
293(2)
0.71073
Monoclinic
P2(1)/n
8.3666(17)
12.386(3)
20.052(4)
90.00
91.32(3)
90.00
C18H24Cl2CoN2O
414.22
293(2)
0.71073
Monoclinic
P2(1)/c
8.2399(16)
14.804(3)
15.856(3)
90.00
97.59(3)
90.00
C18H24Cl2N2NiO
414.00
293(2)
0.71073
Monoclinic
P2(1)/c
8.4036(17)
14.531(3)
15.764(3)
90.00
98.53(3)
90.00
Formula weight
Temp. (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
Pe1
7.2885(15)
9.0459(18)
16.274(3)
75.73(3)
82.62(3)
69.89(3)
975.3(3)
2
1.404
1.160
b (Å)
c (Å)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
Volume (Å3)
Z
2077.5(7)
4
1.408
1917.2(7)
4
1.435
1903.7(7)
4
1.444
Dcalc (Mg mꢁ3
)
m
(mmꢁ1
)
1.094
1.181
1.307
F(000)
Crystal size(mm)
range (ꢀ)
426
916
860
864
0.39 ꢂ 0.24 ꢂ 0.10
3.01e27.42
ꢁ9ꢃh ꢃ 9,ꢁ11 ꢃ k ꢃ 11,
ꢁ21 ꢃ l ꢃ 21
0.40 ꢂ 0.28 ꢂ 0.20
3.09e27.42
ꢁ10 ꢃ h ꢃ 9,ꢁ16 ꢃ k ꢃ 16,
ꢁ25 ꢃ l ꢃ 25
0.39 ꢂ 0.28 ꢂ 0.24
2.99e27.41
ꢁ9 ꢃ h ꢃ 10,ꢁ19 ꢃ k ꢃ 18,
ꢁ20 ꢃ l ꢃ 20
0.48 ꢂ 0.39 ꢂ 0.33
3.09e27.40
ꢁ10 ꢃ h ꢃ 10,ꢁ18 ꢃ k ꢃ 18,
ꢁ20 ꢃ l ꢃ 18
q
Limiting indices
No. of rflns collected
No. of unique rflns
No. of obsd rflns(I > 2
9668
4423
2834
0.992 (
208
19645
4715
3736
0.998 (
236
18150
4358
3233
0.998 (
219
17224
4281
3539
s
(I))
Completeness to
No. of parameters
Goodness-of-fit on F2
q
(%)
q
¼ 27.42ꢀ)
q
¼ 27.42ꢀ)
q
¼ 27.41ꢀ)
98.8 (
219
q
¼ 27.40ꢀ)
1.121
1.081
1.084
1.083
Final R indices [I > 2
R indices(all data)
s
(I)]
R1 ¼ 0.0413, wR2 ¼ 0.1058
R1 ¼ 0.0766, wR2 ¼ 0.1406
0.754, ꢁ0.593
R1 ¼ 0.0283, wR2 ¼ 0.0689
R1 ¼ 0.0413, wR2 ¼ 0.0776
0.305, ꢁ0.247
R1 ¼ 0.0348, wR2 ¼ 0.0810
R1 ¼ 0.0523, wR2 ¼ 0.0898
0.357, ꢁ0.366
R1 ¼ 0.0378, wR2 ¼ 0.0977
R1 ¼ 0.0468, wR2 ¼ 0.1027
0.544, ꢁ0.514
largest diff peak, hole (e Åꢁ3
)

58
P. Ai et al. / Journal of Organometallic Chemistry 705 (2012) 51e58
(d) S.K.-H. Thiele, D.R. Wilson, J. Macromol. Sci. Part C: Polym. Rev. C43 (2003)
581e628.
[2] G.J. Van Amerongen, Adv. Chem. Ser. 52 (1966) 136e152.
[3] W. Cooper, Ind. Eng. Chem. Prod. Res. Develop. 9 (1970) 457e466.
[4] (a) J. Furukawa, Pure Appl. Chem. 42 (1975) 495e508;
(b) J. Furukawa, Acc. Chem. Res. 13 (1980) 1e6.
[5] A. Oehme, U. Gebauer, K. Gehrke, M.D. Lechner, Angew. Makromol. Chem. 235
(1996) 121e130.
[6] (a) K. Endo, N. Hatakeyama, J. Polym. Sci. Part A: Polym. Chem. 39 (2001)
2793e2798;
followed by the addition of a solution of 1,3-butadiene in toluene.
The polymerization reaction was carried out by vigorous stirring of
the reaction mixture at the various temperatures. After the poly-
merization, the resulting solution was poured into a large amount
of acidified ethanol (5% v/v solution of HCl) containing 2,6-di-tert-
butyl-4-methylphenol as a stabilizer. The precipitated polymers
were filtered, washed with ethanol and dried under vacuum at
50 ꢀC overnight.
(b) D.C.D. Nath, T. Shiono, T. Ikeda, Macromol. Chem. Phys. 203 (2002)
756e760;
(c) D.C.D. Nath, T. Shiono, T. Ikeda, Appl. Catal. A: Gen. 238 (2003) 193e199;
(d) G. Kwag, C. Bae, S. Kim, J. Appl. Polym. Sci. 113 (2009) 2186e2190;
(e) Z. Cai, M. Shinzawa, Y. Nakayama, T. Shiono, Macromolecules 42 (2009)
7642e7643.
4.5. X-ray crystallography measurements
Single-crystal X-ray diffraction studies for 2a, 4a, 6a and 6b were
[7] (a) G. Leone, A. Boglia, F. Bertini, M. Canetti, G. Ricci, J. Polym. Sci. Part A:
Polym. Chem. 48 (2010) 4473e4483;
carried out on a Rigaku RAXIS Rapid IP diffractometer with graphite
monochromated MoeKa radiation (
l
¼ 0.71073 Å). Cell parameters
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were obtained by global refinement of the positions of all collected
reflections. Intensities were corrected for Lorentz and polarization
effects and empirical absorption. The structures were solved by
direct methods and refined by full-matrix least-squares on F2. All
non-hydrogen atoms were refined anisotropically. All hydrogen
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Crystallographic data and processing parameters for complexes 2a,
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Grant No. 21006085) and the State Key
Laboratory of Chemical Engineering (Grant No. SKL-ChE-11D03).
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Appendix A. Supplementary material
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CCDC 855903(2a), 855904(4a), 855905(6a) and 855906(6b)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_
request/cif.
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