Synthesis, characterization and 1,3-butadiene polymerization studies of cobalt dichloride complexes bearing pyridine bisoxazoline ligands

Article abstract of DOI:10.1016/j.polymer.2015.01.006

A series of ion-pair cobalt complexes bearing pyridine bisoxazoline ligands were successfully synthesized and characterized by IR spectroscopy and elemental analysis. Determined by X-ray crystallographic analysis, complexes 4a, 4b, 4e, and 4f existed as i

Full text of DOI:10.1016/j.polymer.2015.01.006

Accepted Manuscript
Synthesis, Characterization and 1,3-Butadiene Polymerization Studies of Cobalt
Dichloride Complexes Bearing Pyridine Bisoxazoline Ligands
Jun Guo, Baolin Wang, Jifu Bi, Chunyu Zhang, Hexin Zhang, Chenxi Bai, Yanming
Hu, Xuequan Zhang
PII:
S0032-3861(15)00034-8
10.1016/j.polymer.2015.01.006
JPOL 17537
DOI:
Reference:
To appear in: Polymer
Received Date: 11 October 2014
Revised Date: 8 December 2014
Accepted Date: 3 January 2015
Please cite this article as: Guo J, Wang B, Bi J, Zhang, C, Zhang, H, Bai C, Hu Y, Zhang X, Synthesis,
Characterization and 1,3-Butadiene Polymerization Studies of Cobalt Dichloride Complexes Bearing
Pyridine Bisoxazoline Ligands, Polymer (2015), doi: 10.1016/j.polymer.2015.01.006.
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ACCEPTED MANUSCRIPT
Synthesis, Characterization and 1,3-Butadiene Polymerization
Studies of Cobalt Dichloride Complexes Bearing Pyridine
Bisoxazoline Ligands
Jun Guo,1,2 Baolin Wang,1,2 Jifu Bi,1 Chunyu Zhang,1 Hexin Zhang,1 Chenxi Bai,1
Yanming Hu, 1* Xuequan Zhang1*
1
Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, P. R. China
2 University of Chinese Academy of Sciences, 100049, P. R. China
*Correspondence to: Xuequan Zhang (E-mail: xqzhang@ciac.ac.cn); Yanming Hu (E-
mail: ymhu@ciac.ac.cn).

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Graphical abstract

ACCEPTED MANUSCRIPT
Synthesis, Characterization and 1,3-Butadiene Polymerization
Studies of Cobalt Dichloride Complexes Bearing Pyridine
Bisoxazoline Ligands
Jun Guo,1,2 Baolin Wang,1,2 Jifu Bi,1 Chunyu Zhang,1 Hexin Zhang,1 Chenxi Bai,1
Yanming Hu,1* Xuequan Zhang1*
1 Key Laboratory of Synthetic Rubber, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, P. R. China
2 University of Chinese Academy of Sciences, 100049, P. R. China
*Correspondence to: Xuequan Zhang (E-mail: xqzhang@ciac.ac.cn); Yanming Hu
(E-mail: ymhu@ciac.ac.cn).
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Abstract
A series of ion-pair cobalt complexes bearing pyridine bisoxazoline ligands were
successfully synthesized and characterized by IR spectroscopy and elemental analysis.
Determined by X-ray crystallographic analysis, complexes 4a, 4b, 4e, and 4f existed
as ion pairs comprised of [CoL2]2+ and [CoCl4]2-. In the complex, the cobalt atom was
coordinated by six nitrogen atoms from two ligands, and its coordination geometry at
the cobalt center of [CoL2]2+ can be described as distorted octahedron. In combination
with ethylaluminum sesquichloride, these cobalt complexes displayed high catalytic
activity and cis-1,4-selectivity towards 1,3-butadiene polymerization. The selectivity
of the catalytic system can be tuned to predominantly 1,2-fashion via addition of PPh3.
The effects of reaction parameters and the ligand environment on the polymerization
were also investigated.
Keywords: pyridine bisoxazoline, cobalt complexes, polybutadiene.
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1. Introduction
The last decades have witnessed a surge in interest in the stereoselective
polymerization of 1,3-dienes, namely butadiene and isoprene, due to the diverse
applications of the resulting polymers[1-4]. The properties of polydienes mainly
depend
on
their
microstructures
generated
during
polymerization.
cis-1,4-Polybutadiene is one of the most important elastomers widely used for tires
and many other elastic materials[5]. Therefore, a variety of catalysts based on
titanium[6-10], nickel[11-13], cobalt[14-16], and neodymium[17-23] have been
developed for the polymerization of 1,3-butadiene in a cis-1,4 fashion.
In order to get better control over catalytic activity, molecular weight, and
polymer compositions, considerable attention has been devoted to the design and
synthesis of well-defined single-site catalysts. Since the discovery of
Brookhart-Gibson-type Fe and Co complexes ligated by 2,6-bis(arylimino)pyridine,
numerous transition metal complexes bearing different ligands have been developed
and explored in the field of olefin polymerization and oligomerization[24-30].
Seminal advances are also realized in the aspects of 1,3-diene polymerization[31-35].
Cobalt-based catalysts have aroused special attention because polymers with
versatile microstructures can be produced depending on catalyst formulation. For
example, cobalt halides and carboxylates activated with methylaluminoxane (MAO)
produce high cis-1,4-polybutadienes[14,36,37]; the addition of alkylphosphines or
pyridyl adducts into the polymerization systems can switch the selectivity of catalyst
to form 1,2-polybutadiene[38-40]. Recently, considerable progress has been made in
the syntheses of well-defined cobalt complexes containing various ligands. For
example, (salen)cobalt(II) [(−ONNO−)Co][41] and bis(salicylaldiminate)cobalt(II)
complexes [(NO−)2Co][42] activated with MAO and ethylaluminum sesquichloride
(EASC) exhibited both high catalytic activity and cis-1,4 selectivity for 1,3-butadiene
polymerization. Cobalt dichloride complexes bearing tridentate ligands such as
2,6-bis(imino)pyridine[43],
bis(benzimidazolyl)amine[44],
2,6-bis(pyrazol)pyridine[45], 2-arylimino-6-(alcohol) pyridine/2-arylamino-6-(alcohol)
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pyridine[46], and 3-aryliminomethyl-2-hydroxybenzaldehyde[47] in combination
with MAO, MMAO or EASC are efficient systems to afford polybutadienes with high
cis-1,4 contents. .
Pyridine bis(oxazoline) (Pybox) and its derivatives are well-known ligands in
scientific research because they can be easily synthesized by the reaction of
pyridine-2,6-dicarboxylic acid with a variety of 1,2-aminol and form complexes with
various metal ions. Transition metal complexes bearing Pybox ligand have been
widely applied in asymmetric catalysis reactions, such as adol addition[48,49],
cyclopropanation[50-52], and aziridination reactions[53]. However, only a few
transition metal complexes with Pybox ligands have been reported concerning olefin
and 1,3-diene polymerizations so far[54-57]. Iron(II), ruthenium(II), and
vanadium(III)-based complexes have been used as catalyst precursors for ethylene
and propylene polymerizations[54-56], but exhibited very low activities. To our
knowledge, the only example reported for 1,3-butadiene polymerization is
chromium(III)-based complex which gave polybutadiene in moderate yield of
50%[57]. On the other hand, the polymerization behaviors of 1,3-dienes catalyzed by
metal complex are remarkably affected by the steric and electronic properties of
substituent of the lignad and the central metal. Thus, it is expected that the catalytic
activity and selectivity of complex bearing Pybox ligand could be improved by
fine-tuning the ligand structure.
In the present study, a series of cobalt dichloride complexes bearing Pybox
ligands was synthesized and fully characterized. The catalytic properties of these
complexes for 1,3-butadiene polymerization were examined. It further disclosed the
influences of the substituent of the ligand and polymerization conditions on the
polymerization of 1,3-butadiene.
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2. Experimental Part
2.1 General Consideration
All manipulations were performed under an atmosphere of dry argon by using
standard Schlenk techniques or in a glove box. Toluene, tetrahydrofuran (THF), and
diethyl ether were freshly obtained by distillation under nitrogen from
sodium-benzophenone ketyl. CHCl3 and triethylamine were refluxed over CaH2 and
distilled prior to use. N,N-dimethylformamide (DMF) was dried over CaH2 and then
distilled under reduced pressure before use. 1,3-Butadiene was supplied by Jinzhou
Petrochemical Corporation and purified by passing through four columns packed with
4 Å molecular sieves and KOH prior to use. MAO (10 wt%) was purchased from
Akzo Noble and used as recieved. Ethylaluminum sesquichloride (EASC) was
purchased from Sigma-Aldrich Co. and diluted into 1.0 mol/L solution by toluene
before use. Other chemicals were commercially available and used as received unless
otherwise mentioned. The ligands were synthesized referring to the modified
literature methods (see Scheme 1) [62-64].
Scheme 1. Synthesis of complexes 4a-h and 6a.
The molecular weights (Mn) and molecular weight distributions (Mw/Mn) of
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polymer were measured at 30 °C by gel permeation chromatography (GPC) equipped
with a Waters 515 HPLC pump, four columns (HMW 7 THF, HMW 6E THF × 2,
HMW 2 THF), and a Waters 2414 refractive index detector. THF was used as eluent at
a flow rate of 1.0 mL/min. The molecular weight of polymer was determined using
1
the polystyrene calibration. H-NMR (400 MHz) and 13C-NMR (100 MHz) were
recorded on a Varian Unity spectrometer in CDCl3 or DMSO-d6 at room temperature.
IR spectra were performed on a BRUKE Vertex-70 FIR spectrophotometer. Elemental
analyses were recorded on an elemental Vario EL spectrometer. The proportion of 1,2,
cis-1,4, and trans-1,4 units of polymer were determined by IR spectra, 1H-NMR, and
13C-NMR[59-61].
2.2 X-ray Structure Determinations
Crystals for X-ray analysis were obtained as described in the experimental section.
°
Data collections were performed at -88.5 C on a Bruker SMART APEX
diffractometer with a CCD area detector, using graphite monochromated Mo K
radiation (λ = 0.71073 Å). The determination of crystal class and unit cell parameters
was carried out by the SMART program package. The raw frame data were processed
using SAINT and SADABS to yield the reflection data file. The structures were
solved by using SHELXTL program. Refinement was performed on F2 anisotropically
for all non-hydrogen atoms by the full-matrix least-squares method. The hydrogen
atoms were placed at the calculated positions and were included in the structure
calculation without further refinement of the parameters. CCDC 979274 to 979277.
2.3 Syntheses and Characterization of Ligands and Complexes
2.3.1 2,6-bis[oxazolin-2'-yl]pyridine, Pybox (3a).
Pyridine-2,6-dicarboxylic acid (1.68 g, 10 mmol) was treated with SOCl2 (15 mL)
at reflux temperature under a nitrogen atmosphere. Excess SOCl2 was then removed
under reduced pressure to give the acid chloride as a white solid (100 %). The
chloroform solution of crude 2, 6-pyridine carbonyl dichloride was slowly added to an
aqueous solution of chloroethylamine hydrochloride (2.55 g, 22 mmol) and KOH
(1.69 g, 30 mmol) at 0 °C. After stirring for 1 h, the aqueous layer was discarded. The
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organic layer was filtered and washed with water. After evaporating the solvent, the
crude carboxamide was dissolved in THF and then added to a suspension of NaH (1.3
g) in THF at 0 °C. The reaction mixture was stirred at room temperature for 24 h, and
then quenched with 10% HCl aq. After removing THF, the residue was dissolved in
dichloromethane. The resulting organic layer was dried over anhydrous magnesium
sulfate followed by evaporation of dichloromethane to give the product as a white
solid. Yield, 55 %. 1H NMR (CDCl3, ppm): 8.15 (d, J = 7.9 Hz, 1H), 7.89 (t, J = 7.8
13
Hz, 1H), 4.54 (t, J = 9.7 Hz, 2H), 4.13 (t, J = 9.7 Hz, 2H). C NMR (CDCl3, ppm):
163.78, 147.04, 137.49, 125.70, 68.53, 55.29. FT-IR (KBr, cm−1): 3049, 1659, 1600,
1579, 1569, 1451, 1357, 1272, 1240, 1123, 1068, 959, 838, 742, 661, 688. Anal. Calc.
for C11H11N3O2: C, 60.82; H, 5.11; N, 19.35. Found: C, 60.78; H, 5.13; N, 19.30.
2.3.2 2,6-Bis[4'-(S)-methyloxazolin-2'-yl]pyridine, Me-Pybox (3b).
Pyridine-2,6-dicarboxylic acid (1.68 g, 10 mmol) was treated with SOCl2 (15 mL)
at reflux temperature under a nitrogen atmosphere. Excess SOCl2 was then removed
under reduced pressure to give the acid chloride as a white solid (ca. 100 %). A
solution of crude 2,6-pyridine carbonyl dichloride in chloroform was slowly added to
a solution of (S)-2-Amino-1-propanol (1.65 g, 22 mmol) and triethylamine (8.4 mL,
°
60 mmol) in chloroform (40 mL) at 0 C. After stirring for 24 h at room temperature,
SOCl2 (7.4 mL, 0.50 mol) was added. The mixture was heated to reflux for 2 h and
then poured into ice water. The organic layer was washed with brine (3 × 20 mL) and
aqueous K2CO3 (0.1 M, 30 mL) and then dried over anhydrous magnesium sulfate.
After evaporating the solvent, the crude product was purified by silica gel column
chromatography (eluent: dichloromethane/ether = 3/2) to give a white solid. The solid
was treated with a solution of NaOH (2.4 g) in H2O/MeOH (1/2) at room temperature
for 3 days. The mixture was extracted with CH2Cl2 (3 × 100 mL), and the organic
layer was dried over sodium sulfate. After evaporating the solvent, the crude product
was purified by recrystallization in hexane/ethyl acetate to give 3b as white solid.
(1.72 g, 70% yield). 1H NMR (CDCl3, ppm): 8.18 (d, J = 7.9 Hz, 2H), 7.87 (t, J = 7.9
Hz, 1H), 4.65–4.58 (m, 2H), 4.50–4.39 (m, 2H), 4.08 (t, J = 8.0 Hz, 2H), 1.38 (d, J =
6.7 Hz, 6H). 13C NMR (CDCl3, ppm): 162.49, 147.13, 137.49, 125.84, 74.96, 62.50,
21.57. FT-IR (KBr, cm−1): 2966, 1635, 1566, 1389, 1371, 1260, 1113, 1069, 1058,
960, 943, 883, 839, 742, 675, 645, 524, 465. Anal. Calc. for C13H15N3O2: C, 63.65; H,
6.18; N, 17.16. Found: C, 63.57; H, 6.21; N, 17.20.
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2.3.3 2,6-Bis[4'-(S)-ethyloxazolin-2'-yl]pyridine, Et-Pybox (3c).
This compound was prepared by the same method as for 3b. Yield, 0.72 g (78%).
1H NMR (CDCl3, ppm): 8.16 (d, J = 7.8 Hz, 2H), 7.87 (t, J = 7.8 Hz, 1H), 4.65 – 4.54
(m, 2H), 4.31 (dt, J = 15.9, 8.1 Hz, 2H), 4.16 (t, J = 8.2 Hz, 2H), 1.86 – 1.72 (m, 2H),
1.71 – 1.57 (m, 2H), 1.03 (t, J = 7.4 Hz, 6H). 13C NMR (CDCl3, ppm): 162.43, 146.92,
137.52, 125.75, 119.06, 73.11, 68.38, 28.74, 10.31. FT-IR (KBr; cm−1): 2967, 2931,
1645, 1575, 1458, 1381, 1357, 1261, 1242, 1170, 1076, 965, 831, 746. Anal. Calc. for
C15H19N3O2 (273.33): C, 65.91; H, 7.02; N, 15.38. Found: C, 65.82; H, 7.08; N,
15.40.
2.3.4 2,6-Bis[4'-(S)-isopropyloxazolin-2'-yl]pyridine, i-Pr-Pybox (3d).
This compound was prepared by the same method as for 3b. Yield, 1.22 g (81%).
1H NMR (CDCl3, ppm): 8.20 (d, J = 7.8 Hz, 2H), 7.86 (t, J = 7.8 Hz, 1H), 4.59–4.48
(m, 2H), 4.23 (t, J = 8.4 Hz, 2H), 4.19–4.10 (m, 2H), 1.93–1.87 (m, 2H), 1.06 (d, J =
6.7 Hz, 6H), 0.95 (d, J = 6.7 Hz, 6H). 13C NMR (CDCl3, ppm): 162.41, 147.09,
137.33, 125.88, 73.08, 71.15, 33.02, 19.21, 18.47. FT-IR (KBr, cm−1): 3063, 2960,
1633, 1566, 1470, 1460, 1377, 1359, 1331, 1264, 1103, 1069, 1038, 963, 940, 904,
839, 738, 647. Anal. Calc. for C17H23N3O2: C, 67.75; H, 7.71; N, 13.95. Found: C,
67.69; H, 7.78; N, 13.91.
2.3.5 2,6-Bis[4'-(R)-phenyloxazolin-2'-yl]pyridine, Ph-Pybox (3e).
This compound was prepared by the same method as for 3b. Yield, 1.57 g (85%).
1H NMR (CDCl3, ppm): 8.35 (d, J = 7.9 Hz, 2H), 7.92 (t, J = 7.8 Hz, 1H), 7.41–7.27
13
(m, 10H), 5.51–5.42 (m, 2H), 4.97–4.89 (m, 2H), 4.43 (t, J = 8.6 Hz, 2H). C NMR
(CDCl3, ppm): 163.69, 146.98, 141.91, 137.64, 129.04, 128.01, 127.05, 126.54, 75.73,
70.57. FT-IR (KBr, cm−1): 3082, 1652, 1565, 1439, 1355, 1307, 1262, 1231, 1164,
979, 752, 699. Anal. Calc. for C23H19N3O2: C, 74.77; H, 5.19; N, 11.38. Found: C,
74.75; H, 5.25; N, 11.35.
2.3.6 4-Chloro-2,6-bis[oxazolin-2'-yl]pyridine, Cl-Pybox (3f).
This compound was prepared by the same method as for 3b. Yield, 1.26 g (50%).
1H NMR (CDCl3, ppm): 8.17 (s, 2H), 4.55 (t, J = 9.7 Hz, 4H), 4.13 (t, J = 9.7 Hz, 4H).
13C NMR (CDCl3, ppm): 162.75, 148.19, 145.82, 125.91, 68.50, 55.30. FT-IR (KBr;
cm−1): 3084, 1638, 1560, 1386, 1311, 1278, 1123, 1092, 978, 940, 917, 879, 790, 532.
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Anal. Calc. for C11H10ClN3O2 (251.67): C, 52.43; H, 4.01; N, 16.70. Found: C, 52.50;
H, 4.05; N, 16.65.
2.3.7 4-Methoxy-2,6-bis[oxazolin-2'-yl]pyridine, MeO-Pybox (3g).
Chelidamic acid (1.83 g, 10 mmol) was treated with SOCl2 (20 mL, 2 drops of
DMF as a catalyst) to yield the acid chloride, which undergoes coupling with
chloroethylamine hydrochloride (2.55 g, 22 mmol) in the presence of KOH (1.69 g,
30 mmol) to form the carboxamide. The carboxamide was purified by separation of
the organic layer, washing with brine, drying the organic layer with anhydrous
magnesium sulfate, and removal of the organic solvent under reduced pressure. The
carboxamide was then treated with an excess of KOH in methanol and refluxed
overnight. Addition of water to the cooled reaction mixture led to the formation of the
product as a white precipitate. After filtration and drying under reduced pressure, the
desired product was obtained in 30% yield (0.74 g). 1H NMR (CDCl3, ppm): 7.69 (s,
2H), 4.53 (t, J = 9.7 Hz, 4H), 4.11 (t, J = 9.7 Hz, 4H), 3.94 (s, 3H). 13C NMR (CDCl3,
ppm): 166.74, 163.72, 148.41, 111.91, 68.48, 55.97, 55.14. FT-IR (KBr; cm−1): 1642,
1591, 1563, 1459, 1395, 1214, 1089, 1031, 980, 916, 861, 585. Anal. Calc. for
C12H13N3O3 (247.25): C, 58.29; H, 5.31; N, 17.00. Found: C, 58.25; H, 5.28; N,
17.05.
2.3.8 2,6-Bis[(4R)-4-isopropyl-4,5-dihydro-1,3-thiazol-2-yl]pyridine, i-Pr-Thio-Pybox
(3h).
Pyridine-2,6-dicarboxylic acid (1.68 g, 10 mmol) was treated with SOCl2 (15 mL)
to give the acid chloride. The acid chloride was then reacted with (S)-valinol (2.27 g,
22 mmol) and triethylamine (8.4 mL, 60 mmol) to give the intermediate salt (3.8 g,
8.5 mmol). Triethylamine (42.8 g, 423.6 mmol) was added to a mixture of
carboxamide intermediate (3.8 g, 8.5 mmol) and phosphorus pentasulfide (5.74 g,
25.4 mmol) in dry toluene (80 ml). The mixture was heated to reflux temperature and
stirred for 4 h. After the mixture cooled, water was added (20 mL) and the organic
layer separated and dried over sodium sulfate. Removal of the solvent and
recrystallisation from methanol gave a yellow solid. Yield, 1.07 g (32%).1H NMR
(CDCl3, ppm): 8.16 (d, J = 7.8 Hz, 2H), 7.82 (t, J = 7.7 Hz, 1H), 4.53 (dd, J = 15.6,
9.3 Hz, 2H), 3.37 (t, J = 10.0 Hz, 2H), 3.10 (t, J = 10.2 Hz, 2H), 2.11 (m, 2H), 1.12 (d,
13
J = 6.7 Hz, 6H), 1.03 (d, J = 6.7 Hz, 6H). C NMR (CDCl3, ppm): 150.87, 137.11,
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123.02, 84.97, 34.34, 33.64, 19.93, 19.20. FT-IR (KBr; cm−1): 2976, 1657, 1635,
1604, 1263, 1045, 1021, 941, 649. Anal. Calc. for C17H23N3S2 (333.51): C, 61.22; H,
6.97; N, 12.60. Found: C, 61.25; H, 7.01; N, 12.58.
2.3.9 2,6-Bis[(1-phenylimino) ethyl]pyridine (5a).
2,6-Diacetylpyridine (0.49 g, 3 mmol) and aniline (0.61 g, 6.5 mmol) were added
to a 50 mL flask containing methanol (15 mL) and formic acid (0.2 mL). The reaction
mixture was refluxed for 72 h. After evaporating methanol, the solution was kept at
°
-30 C for 72 h, and yellow crystals were obtained. The crystals were washed with
°
1
cold methanol and dried under vacuum at 40 C. Yield, 0.658 g (70 %). H NMR
(CDCl3, ppm): 8.35 (d, J = 7.8 Hz, 2H), 7.87 (t, J = 7.8 Hz, 1H), 7.38 (t, J = 7.7 Hz,
4H), 7.12 (t, J = 7.3 Hz, 2H), 6.85 (d, J = 7.9 Hz, 4H), 2.40 (s, 6H). 13C NMR (CDCl3,
ppm): 167.55, 155.70, 151.51, 137.04, 129.22, 123.82, 122.55, 119.47, 16.41. FT-IR
(KBr, cm−1): 3453, 1636, 1590, 1571, 1480, 1446, 1359, 1319, 1216, 1093, 1075, 819,
773, 705, 518. Anal. Calc. for C21H19N3: C, 80.47; H, 6.12; N, 13.41. Found: C, 80.45;
H, 6.13; N, 13.40.
Synthesis of Cobalt Complexes 4a-h and 6a
All the complexes were prepared by mixing CoCl2 with 1.0 equivalent of ligand
in THF. The solution turned green immediately, and a green suspension was obtained
after several minutes. The mixture was stirred at room temperature overnight. The
precipitate was filtered and washed with dry diethyl ether and dried under vacuum at
40 °C for 24 h.
2.3.10 [Co(2,6-bis[oxazolin-2'-yl] pyridine)2]2+[CoCl4]2-(4a).
Yield, 0.260 g (75%). FT-IR (KBr, cm−1): 1649, 1636, 1581, 1381, 1264, 1193, 1073,
1015, 959, 917, 837, 755, 656. Anal. Calc. for C22H22Co2Cl4N6O4: C, 38.07; H, 3.20;
N, 12.11. Found: C, 38.12; H, 3.24; N, 12.17.
2.3.11 [Co(2,6-Bis[4'-(S)-methyloxazolin-2'-yl]pyridine)2]2+[CoCl4]2- (4b).
Yield, 0.285 g (76%). FT-IR (KBr, cm−1): 3073, 2973, 1642, 1624, 1577, 1484, 1447,
1397, 1271, 1202, 1075, 945, 833, 755. Anal. Calc. for C26H30Co2Cl4N6O4: C, 41.62;
H, 4.04; N, 11.20. Found: C, 41.55; H, 4.11; N, 11.13.
2.3.12 [Co(2,6-Bis[4'-(S)-ethyloxazolin-2'-yl]pyridine)2]2+[CoCl4]2- (4c).
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Yield, 0.271 g (67%). FT-IR (KBr, cm−1): 2964, 2937, 1643, 1622, 1575, 1399, 1274,
1203, 1077, 959, 920, 833, 741. Anal. Calc. for C30H38Co2Cl4N6O4: C, 44.68; H, 4.76;
N, 10.42. Found: C, 44.65; H, 4.80; N, 15.33.
2.3.13 [Co(2,6-Bis[4'-(S)-isopropyloxazolin-2'-yl]pyridine)2]2+[CoCl4]2- (4d).
Yield, 0.315 g (73%). FT-IR (KBr, cm−1): 3092, 2960, 1652, 1620, 1583, 1481, 1382,
1278, 1249, 1196, 1079, 965, 925, 827, 747, 673. Anal. Calc. for C34H46Co2Cl4N6O4:
C, 47.35; H, 5.39; N, 9.75. Found: C, 47.31; H, 5.42; N, 9.70.
2.3.14 [Co(2,6-Bis[4'-(R)-phenyloxazolin-2'-yl]pyridine)2]2+[CoCl4]2- (4e).
Yield, 0.409 g (82%). FT-IR (KBr, cm−1): 1642, 1574, 1393, 1269, 1202, 976, 930,
748, 700. Anal. Calc. for C46H38Co2Cl4N6O4: C, 55.33; H, 3.84; N, 8.42. Found: C,
55.20; H, 3.86; N, 8.45.
2.3.15 [Co(4-Chloro-2,6-bis[oxazolin-2'-yl]pyridine)2]2+[CoCl4]2- (4f).
Yield, 0.286 g (75%). FT-IR (KBr, cm−1): 1630, 1576, 1369, 1264, 1023, 913, 796,
537. Anal. Calc. for C22H20Co2Cl6N6O4: C, 34.63; H, 2.65; N, 11.02. Found: C, 34.60;
H, 2.61; N, 11.01.
2.3.16 [Co(4-Methoxy-2,6-bis[oxazolin-2'-yl]pyridine)2]2+[CoCl4]2- (4g).
Yield, 0.298 g (79%). FT-IR (KBr, cm−1):1624, 1590, 1477, 1411, 1377, 1265, 1244,
1058, 913. Anal. Calc. for C24H26N6O6: C, 38.22; H, 3.48; N, 11.15. Found: C, 38.16;
H, 3.52; N, 11.10.
2.3.17 2,6-Bis[(4R)-4-isopropyl-4,5-dihydro-1,3-thiazol-2-yl]pyridine Cobalt(II)
chloride (4h).
Yield, 0.315 g (68%). FT-IR (KBr, cm−1): 1654, 1636, 1558, 1459, 1233, 1177, 1119,
1024, 808. Anal. Calc. for C17H23CoCl2N3S2: C, 44.07; H, 5.01; N, 9.07. Found: C,
44.05; H, 5.03; N, 9.01.
2.3.18 2,6-Bis[(1-phenylimino)ethyl]pyridine Cobalt(II) chloride (6a).
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Yield, 0.403g (91%). FT-IR (KBr, cm−1): 3448, 1628, 1589, 1487, 1371, 1260, 1228,
1027, 816, 778, 694. Anal. Calc. for C21H19CoCl2N3: C, 56.90; H, 4.33; N, 9.48.
Found: C, 56.85; H, 4.30; N, 9.45.
2.4 General Butadiene Polymerization Procedures
A typical procedure for the polymerization is as follows (entry 2 in Table 2): a
toluene solution (10 mL) of 1,3-butadiene (1 g, 1.85 × 10-2 mol) was added to a
oxygen- and moisture-free ampoule preloaded with complex 4b (3.4 mg, 9.26 × 10-6
mol). Then, EASC (1.0 × 10-3 mol/ml, 0.185 ml) was injected to initiate the
polymerization at 20 °C. After 15 min, acidified ethanol containing
2,6-di-tert-butyl-4-methylphenol (1.0 wt%) as a stabilizer was added to quench the
polymerization. The polymer was washed with ethanol repeatedly, and then dried
°
under vacuum at 40 C to constant weight. The polymer yield was determined by
gravimetry.
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3. Results and Discussion
3.1 Synthesis and Characterization of Complexes.
Complexes 4a-h and 6a were synthesized in good yields (68%-91%) by the
reaction of anhydrous CoCl2 with 1.0 equivalent of the corresponding ligand at room
temperature (Scheme 1). Complexes 4a-h are obtained as green powders except
complex 6a as dark yellow one, and they are soluble in THF and DMF. These
complexes were characterized by FT-IR, elemental analysis, and single crystal X-ray
analysis. In the IR spectra, absorption bands of the imine C=N in the complexes
appeared at 1659–1633 cm−1, which were obviously red-shifted about 8–23 cm−1
compared to the corresponding ligands, indicating that the nitrogen atoms of imines
coordinated to the central metal. Single crystals of complexes 4a, 4b, 4e, and 4f
suitable for X-ray diffraction analysis were obtained by slow diffusion of Et2O into
their saturated DMF solutions, and the data collection, crystal, and refinement
parameters are listed in Table S1. Quite surprisingly, X-ray crystallography analysis
revealed that these four complexes are ion-paired complexed [L2Co]2+[CoCl4]2− (Fig.
1-4). Similar results were observed in the cases of cobalt(II) and iron(II) complexes
ligated by terpyridine, bis(benzimidazolyl)pyridine or bis(arylimino)pyridine[65-68].
In the present study, even the bulky phenyl substituent did not provide enough steric
hindrance to avoid bis(chelate) formation. Because these four cobalt complexes had
very similar geometries and structural parameters, only complex 4e is described in
detail and the selected bond lengths and angles of compounds 4a, 4b, 4e and 4f are
collected in Table 1.
Table 1. Selected bond length (Å) and angles (o) for complexes 4a, 4b, 4e and 4f.
Complex
4a
4b
4e
4f
Bond lengths (Å)
Co(1)–N(1)
2.151 (2)
2.103 (2)
2.137 (2)
2.156 (3)
2.075 (3)
2.147 (3)
2.180 (5)
2.071 (4)
2.176 (5)
2.162 (5)
2.081 (4)
2.200 (4)
Co(1)–N(2)
Co(1)–N(3)
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Co(2)–Cl(1)
Co(2)–Cl(2)
2.2856 (9)
2.2862 (9)
2.2705 (10)
2.2769 (10)
2.2933 (9)
2.2520 (11)
2.2961 (9)
2.2809 (10)
2.2916 (12)
2.2798 (16)
2.2766 (19)
2.2596 (18)
2.2862 (16)
2.2835 (17)
2.2804 (17)
2.2983 (17)
Co(2)–Cl(3)
Co(2)–Cl(4)
Bond angles (°)
N(1)–Co(1)–N(2)
N(2)–Co(1)–N(3)
N(1)–Co(1)–N(3)
Cl(1)–Co(2)–Cl(2)
Cl(1)–Co(2)–Cl(3)
Cl(1)–Co(2)–Cl(4)
Cl(2)–Co(2)–Cl(3)
Cl(2)–Co(2)–Cl(4)
Cl(3)–Co(2)–Cl(4)
74.74 (9)
74.68 (9)
75.99 (10)
76.24 (10)
152.18 (10)
113.15 (4)
106.11 (4)
107.02 (4)
107.25 (4)
110.84 (5)
112.47 (4)
75.47 (19)
75.52 (19)
150.99 (17)
111.27 (6)
107.00 (7)
108.33 (7)
106.86 (7)
106.92 (7)
116.49 (8)
75.82 (17)
74.55 (17)
150.30 (18)
105.52 (6)
107.60 (7)
111.83 (6)
111.93 (7)
111.56 (7)
108.36 (7)
147.46 (9)
106.09 (3)
109.32 (4)
110.49 (4)
112.38 (4)
111.00 (4)
107.58 (5)
The cobalt center in the cationic complexes of the [Co(PhPybox)2]2+ dication is
six-coordinate and chelated by two Ph-Pybox ligands with Co-N (imino nitrogen and
pyridine nitrogen) bonds. The bond lengths (Co-N) ranged from 2.069 Å to 2.199 Å.
The length of Co1−N2 (pyridine) bond (2.071 Å) is shorter than those of Co1−N1
(oxazoline) bond (2.164 Å) and Co1−N3 (oxazoline) bond (2.164 Å and 2.190 Å,
respectively). The dihedral angle between the pyridine (defined by C5, N1, and C3)
and the oxazoline (defined by N2, C6, and O1) plane is 0.05o, indicating the highly
coplanar nature of this rigid ligand framework. The cobalt atom (Co1) slightly lies
0.0089 (0.0047) Å out of one Pybox plane (defined by N1, N2, and N3) and 0.0133
(0.0047) Å for the other one (defined by N4, N5, and N6). The [CoCl4]2− anion
displays a lightly distorted tetrahedral geometry with nearly uniform Co−Cl bond
lengths of 2.2596–2.2916 Å and Cl−Co−Cl angles ranging from 106.86(7)° to
116.49(8)°. The imino N(2)=C(6) and N(3)=C(15) bond lengths are 1.269 Å and
1.267 Å, respectively, indicating the typical character of C=N bond.
Further inspection of the [Co(Ph-Pybox)2]2+[CoCl4]2− X-ray structure revealed
14

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that the phenyl group (C32-C37, Fig. 3) of one Pybox ligand located under the
pyridine (N1, C1, and C5) ring of another Pybox ligand in a parallel fashion, and the
dihedral angle is 4.69°. This geometrical arrangement and angle are consistent with a
parallel offset face-to-face π-π interaction between the phenyl and pyridyl moieties.
The inter-planar angle for the two Pybox ligands of [Co(Ph-Pybox)2]2+ (defined by N1,
N2, N3 and N4, N5, N6) is 88.32°, indicating that they are almost orthogonal to each
other. The corresponding angles for [Co(Me-Pybox)2]2+ (4b) and [Co(Pybox)2]2+ (4a)
are 87.83° and 87.51°, respectively. The larger dihedral angle between the Ph-Pybox
ligands in complex 4e might be due to the larger bulkiness of phenyl group compared
with that of methyl group and hydrogen atom, leading to more steric crowding in the
Co complex.
The unit cell of complex 4a and 4b contain two ion-pairs (Fig. 1 and Fig. 2). The
geometrical parameters are comparable to those of 4e. The lengths of imino C=N
bond (1.281, 1.273, 1.272, and 1.276 Å for 4a; 1.280, 1.267, 1.269, and 1.280 Å for
4b) indicate the character of C=N double bond. As for complex 4f, the distance of
bond between the cobalt atom and the pyridyl nitrogen is 0.022 Å shorter than that of
complex 4a, indicating that the introduction of chlorine atom enhances the
electropositivity of central metal.
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Fig. 1. ORTEP drawing of cationic complex 4a with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Fig. 2. ORTEP drawing of cationic complex 4b with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Fig. 3. ORTEP drawing of cationic complex 4e with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
16

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Fig. 4. ORTEP drawing of cationic complex 4f with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
3.2 1,3-Butadiene Polymerization
3.2.1 Effect of substituent of the ligand of the complex on 1,3-butadiene
polymerization
The complexes activated by EASC were examined for 1,3-butadiene
polymerization, and the results are summarized in Table 2. Neither the cobalt
complex nor EASC alone can initiate the polymerization of 1,3-butadiene. Moreover,
CoCl2 activated by EASC only produced trace polymers. The catalytic activities of
the complexes depended on the structure of substituent on oxazoline ring of the ligand.
The polymer yield decreased as the bulkiness of the substituent increased. Complex
4a without substituent on oxazoline of the ligand gave polybutadiene with 95.1%
cis-1,4 content in a high yield of 91.1%. It is worthy to mention that the activity of 4a
is much higher than that of classical Brookhart/Gibson-type complex 6a, although its
cis-1,4 selectivity is somewhat lower. As the bulkiness of substituent increased, from
methyl, ethyl to phenyl group, the polymer yield gradually decreased from 86.5% to
17.4%. Meanwhile, the Mn of the polymer increased from 5.9 × 104 to 11.1 × 104 and
the Mw/Mn decreased from 3.7 to 2.0. This might be due to steric reason, that is, the
space around the central metal is too crowded for the incoming monomer to
coordinate to active species[35,43,45,46,69]. On the other hand, the cis-1,4 selectivity
17

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of the complex displayed an opposite trend; the cis-1,4 content of the obtained
polymer increased when the bulkiness of R1 increased.
Table 2. Polymerization of 1,3-butadiene with 4a-h and 6a activated by EASC.a
Microstructureb(%)
c
c
Entry
Cat.
Yield(%)
91.1
Cis-1,4
95.1
95.8
96.3
97.1
97.7
96.5
96.5
92.3
97.6
−
Trans-1,4 1,2 Mn × 10-4
Mw/Mn
3.7
3.4
3.0
2.6
2.0
2.3
1.8
4.5
2.5
−
1
2
3.0
2.5
2.4
1.7
1.5
2.3
2.4
4.5
1.6
−
1.9
1.7
1.3
1.2
0.8
1.2
1.1
3.2
0.8
−
5.9
6.9
7.6
7.6
11.1
11.9
9.7
4.9
13.1
−
4a
4b
86.5
3
85.4
4c
4
84.4
4d
5
17.4
4e
6
93.1
4f
7
46.4
4g
8
79.0
4h
9
20.4
6a
10
CoCl2
trace
a
Polymerization conditions: precatalyst: 9.26 ꢀmol, [BD]/[Co] = 2000,
[Al2Et3Cl3 ]/[Co] =10, in toluene, 20 °C, 15min.
b Determined by 1H NMR and FT-IR.
c Determined by GPC against polystyrene standards.
The effect of the electronic property of substituent on the catalytic activity of the
complex was also assessed. As expected, the incorporation of chlorine atom into the
para-position of the pyridine of the ligand enhanced the catalytic activity. Complex 4f
exhibited slightly higher activity than 4a (93.1% vs 91.1%), whereas complex 4g with
the electron-donating methoxy group displayed much lower activity (46.4%). This
result might be due to that electron-withdrawing group could enhance the
electropositivity of the center metal, thus facilitated the coordination of the incoming
monomer, and in turn, improved the activity[46, 70-72]. The poor activity of 4g might
also be ascribed to decomposition during the polymerization process. Furthermore,
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the sulfur-containing complex 4h exhibited lower catalytic activity and cis-1,4
selectivity than its oxygen-containing counterpart 4d. Since the electronegativity of
oxygen is greater than that of sulfur, thus the electropositivity of the central metal
might be lower in the case of 4h, and as aforementioned, the catalytic activity would
decrease. Since complex 4b exhibited high activity, high cis-1,4 selectivity, and better
control over molecular weight and molecular weight distribution of the resultant
polymer, it was used in further experiments.
3.2.2 Effect of cocatalyst type on 1,3-butadiene polymerization
The polymerization of 1,3-butadiene was examined by using complex 4b upon
activation by various cocatalysts (Table 3). When alkylaluminum, namely AlEt3 and
Al(i-Bu)3, were used as cocatalysts at [Al]/[Co] ratio of 40, only trace polymers were
obtained. The MAO-based catalyst afforded polybutadiene with high cis-1,4 content
(94.1%) in a high yield (84.5%). In the case of EASC, the highest yield (86.5%, in 15
min) and cis-1,4 selectivity (95.8%) was obtained even at a low [Al]/[Co] ratio of 20,
demonstrating that EASC is the most efficient cocatalysts.
Table 3. Effect of cocatalyst type on 1,3-butadiene polymerization.a
Microstructureb(%)
c
Yield
(%)
Mn ×
10-4
−
c
Entry
Cocat. [Al]/[Co]
Cis-1,4 Trans-1,4 1,2
Mw/Mn
11
12
13
14
AlEt3
Al(i-Bu)3
MAOd
40
40
50
20
trace
trace
84.5
86.5
−
−
−
−
−
−
−
−
−
94.3
95.8
3.7
2.5
2.0
1.7
1.3
6.9
4.5
3.4
EASC
a Polymerization conditions: precatalyst: 9.26 ꢀmol, [BD]/[Co] = 2000, in toluene,
20 °C, 15 min.
b Determined by 1H NMR and FT-IR.
c Determined by GPC against polystyrene standards.
d Polymerization time: 3 h.
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3.2.3 Effects of [Al]/[Co] ratio and polymerization temperature on 1,3-butadiene
polymerization
The effects of reaction parameters, [Al]/[Co] ratio, and polymerization
temperature on the polymerization of 1,3-butadiene were examined (Table 4). The
catalytic activity slightly increased from 86.5 to 89.5% with an increment of [Al]/[Co]
ratio from 20 to 100. Meanwhile, the Mw/Mn decreased from 3.7 to 2.0, and the Mn
value was almost constant, approximately 7.0 × 104. These results might be due to the
active species generation procedure, that is, the active species are not formed
simultaneously at low [Al]/[Co] ratios, affording polymers with broad Mw/Mn; and the
high [Al]/[Co] ratio leads to the faster activation, giving rise to narrow Mw/Mn due to
the almost simultaneously generated active species. When the [Al]/[Co] ratio further
increased to 150, the lower yield is probably ascribed to the over-alkylation, thus,
leading to the deactivation of the active species.
Table 4. Effects of [Al]/[Co] ratio and temperature on the polymerization of
1,3-butadiene with 4b/EASC.a
Microstructureb(%)
c
Yield
(%)
Mn ×
10-4
6.9
7.0
7.0
6.1
5.6
8.8
4.5
3.7
3.4
Entry [Al]/[Co] T (°C)
Cis-1,4 Trans-1,4 1,2
Mw/Mn
c
15
16
17
18
19
20
21
22
23
20
60
20
20
20
20
20
0
86.5
88.1
89.0
89.5
82.0
11.1
93.9
91.9
85.9
95.8
96.2
96.5
96.5
96.8
97.9
89.1
84.5
82.5
2.5
2.6
1.7
1.2
1.2
1.2
1.0
0.9
4.0
5.1
5.7
3.4
2.5
2.1
2.0
2.6
1.8
5.7
7.1
5.9
80
2.4
100
150
20
2.5
2.2
1.2
20
40
60
80
6.8
20
10.4
11.8
20
a Polymerization conditions: precatalyst: 9.26 ꢀmol, [BD]/[Co] = 2000, in toluene, 15
20

ACCEPTED MANUSCRIPT
min.
b Determined by 1H NMR and FT-IR.
c Determined by GPC against polystyrene standards.
With the [Al]/[Co] ratio fixed at 20, the effect of temperature on 1,3-butadiene
polymerization with 4b/EASC catalytic system was investigated. The polymerization
was accelerated initially by raising the polymerization temperature from 0 °C to 40 °C.
When the temperature further increased to 80 °C, the polymer yield slightly decreased,
presumably due to the deactivation of the active species. In general, late transition
metal-based catalysts suffered from the decay of catalytic activity as polymerization
temperature increased. It is noticeable that the present catalyst exhibited a quite high
°
activity affording a high yield of 85.9% even at 80 C, indicating the high thermal
stability of the active species. This result is different from the previously reported
Co-based catalyst, which significantly suffered from the decay of catalytic activity as
polymerization temperature increased.
Meanwhile, the higher polymerization temperature led to a decrease in the Mn of
the polymer and broader molecular distribution (entries 15 and 20-23, see Fig S1 for
GPC curves), suggesting the facilitated chain transfer reaction at the elevated
temperature. The cis-1,4 content of the resulting polymer decreased with the
increment of temperature. For example, the cis-1,4 selectivity decreased from 97.9%
at 0 °C to 82.5% at 80 °C. In principle, the metal center can coordinate to the π-allylic
end of the polymer chain in either anti- or syn- conformation. The anti conformer
gives rise to cis-1,4 and 1,2-polymerization, while the syn conformer leads to
trans-1,4- and 1,2-polybutadiene formation[1,4]. At lower temperature, most of the
monomers formed cis-1,4 polybutadiene through path A (Scheme 2) based on the
hypothesis that the newly formed double bonds of the molecular chains stabilize the
active sites and solidify the architecture around the active sites through
π-back-bonding and it is kinetically preferred. With the temperature increasing,
anti-syn isomerization through the π-σ-rearrangement is gradually facilitated due to
the greater thermodynamic stability of the syn-isomer, which lead to an increase of
21

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trans-1,4 and 1,2- contents (paths C and D).
Scheme 2. Proposed mechanism of 1,3-butadiene polymerization.
3.2.4 1,2-enriched polymerization of 1,3-butadiene via addition of PPh3
The addition of Lewis bases plays an important role in the control of selectivity
of cobalt-based catalytic systems[38,73,74]. As seen from Table 5, the addition of
PPh3 into the EASC-activated systems resulted in an increase of polymer yield from
86.5% to 90.2%. Although the polymer yield slightly increased at a low level of PPh3
([P]/[Co] = 1.0), the addition of 5.0 equivalents of PPh3 to the EASC-activated system
resulted in an obvious decrease of polymer yield. Further increase of EASC dosage
returned the polymer yield to a high value. It may be speculated that the PPh3 interacts
with EASC. Surprisingly, in the case of MAO-based systems, although the
unmodified system gave cis-1,4 polybutadiene, whereas polybutadienes having high
1,2-contents (57.5% to 86.8%) were obtained upon addition of PPh3. Similar result
was observed in previous report[68]. This result is presumably attributed to PPh3
coordination to the central metal, which provides the π-η3-allyl intermediate capable
of insertion of the coordinated monomer to C3 position, leading to the formation of
1,2-units[1,4,73,74].
Table 5. Effect of PPh3 on polymerization behaviors with 4b catalyst system.a
Microstructureb (%)
c
c
Entry Cocat. PPh3/Co
Yield
Cis-1,4 Trans-1,4 1,2
Mn
Mw/Mn
22

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(%)
×10-4
6.9
7.1
0.26
2.8
1.3
2.8
3.4
3.9
4.5
4.5
24
25
26
27
28
29
30
31
32
33
EASC
EASC
EASC
EASCd
MAOe
MAOe
MAOe
MAOe
MAOe
MAOe
−
1
86.5
90.2
48.9
99.0
84.5
85.5
86.2
87.0
87.4
91.2
95.8
95.9
58.9
90.5
94.3
40.2
33.6
18.9
15.6
12.4
2.5
2.5
6.1
5.0
3. 7
2.3
2.4
1.5
1.3
0.8
1.7
1.6
3.4
3.0
1.9
5.7
4.5
4.9
4.3
3.7
3.2
3.2
5
35.0
4.5
5
−
2.0
0.1
0.2
0.5
1
57.5
64.0
79.6
83.1
86.8
2
a
Polymerization conditions: precatalyst: 9.26 ꢀmol, [BD]/[Co] = 2000,
[Al2Et3Cl3]/[Co] = 10, MAO/Co = 50, in toluene, 20 °C, 15 min.
b Determined by 1H NMR and FT-IR.
c Determined by GPC against polystyrene standards.
d [Al2Et3Cl3]/[Co] = 30.
e Polymerization time: 3 h.
4. Conclusions
A series of cobalt (II) complexes supported by pyridine(bisoxazoline) 4a-g were
synthesized and fully characterized. X-ray crystallographic analysis revealed that the
complexes existed as ion pairs comprised of [CoL2]2+ and [CoCl4]2-. The cationic
moiety of the complexes adopted distorted octahedron configuration, and the anionic
part can be described as distorted tetrahedron. Upon activation with EASC, all the
complexes gave polybutadienes with cis-1,4-content up to 97.7% under mild and
convenient conditions. The substituents on oxazoline of the ligand remarkably
influenced the catalytic activity of the complex. The activity decreased as the
bulkiness of the substituent of oxazoline of the ligand increased due to the steric
reason. Incorporation of chlorine atoms into the para position of the pyridine ring
23

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further increased the activity probably via promoting the electronpositivity of the
central metal, whereas the complex having electron-donating methoxy substituent
exhibited much lower activity. The catalytic activity of the complex decreased with
the increasing polymerization temperature, while the polymer yield remained 85.9%
even at 80 °C, indicating the high thermal stability of the active species. The addition
of PPh3 into catalytic systems enhanced polymer yield and tuned the selectivity from
cis-1,4 to 1,2 fashion.
Acknowledgments
The authors appreciate financial supports from The CAS Hundred Talents
Program (Y220011001), National Natural Scientific Foundation of China (No.
51203147) and Jilin Provincial Research Fund for Basic Research, China (No.
20130102007JC).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.xxx.
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