Highly cis-1,4 selective polymerization of isoprene promoted by α-diimine cobalt(II) chlorides

Article abstract of DOI:10.1002/pola.28247

A series of 1,2-bis(arylimino)acenaphthylenes (L1–L5) was synthesized and reacted with CoCl₂ to afford the corresponding cobalt complexes LCoCl₂ (C1–C5). All cobalt complexes have been fully characterized and in the case of C1 by sin

Full text of DOI:10.1002/pola.28247

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Highly cis-1,4 Selective Polymerization of Isoprene Promoted
by a-Diimine Cobalt(II) Chlorides
Xinxin Wang,1,2 Linlin Fan,1,2 Chuanbing Huang,1,2 Tongling Liang,2 Cun-Yue Guo,1
Wen-Hua Sun2
1School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China
2Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100190, China
Correspondence to: W.-H. Sun (E-mail: whsun@iccas.ac.cn) or C.-Y. Guo (E-mail: cyguo@ucas.ac.cn)
Received 28 June 2016; accepted 24 July 2016; published online 00 Month 2016
DOI: 10.1002/pola.28247
ABSTRACT: A
series of 1,2-bis(arylimino)acenaphthylenes
influence of reaction temperature and [Al]/[Co] molar ratio on
both catalytic performance and the microstructural properties of
(L1–L5) was synthesized and reacted with CoCl2 to afford the
corresponding cobalt complexes LCoCl2 (C1–C5). All cobalt com-
plexes have been fully characterized and in the case of C1 by
single crystal X-ray diffraction; its molecular structure reveals a
distorted tetrahedral geometry. On activation with AlEtCl2, C1–
C5 efficiently polymerize isoprene to give polyisoprenes (PIs)
with high contents of cis-1,4 units (between 90% and 94%). The
C
V
the PIs is investigated.
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KEYWORDS: cationic polymerization; stereospecific polymers;
rubber; cis-1,4-polyisoprenes; low molecular weight polymer
Herein a series of a-diiminocobalt complexes is reported and
their catalytic behavior toward isoprene polymerization
investigated. On activation with ethylaluminum dichloride
(AlEtCl2), high catalytic conversions to polyisoprene are
achieved. Moreover, the resultant polymers show high selec-
tivities for cis-1,4 polyisoprenes (up to 94%) with low
molecular weights and narrow polydispersities, materials
that have demand as additives in coatings and inks.
INTRODUCTION The stereospecific polymerization of iso-
prene using metal catalysts has been investigated since the
1950s and allows access to a variety of stereoregular materi-
als including isotactic and syndiotactic polyisoprenes. cis-1,4
Polyisoprene, the structure adopted in natural rubber, was
first synthetically prepared in 1956 using a system com-
posed of AlR3-TiCl4 in heptane.1 Subsequently many catalysts
for cis-1,4-polyisoprenes have been reported based on
lanthanides (e.g., neodymium2) and transition metals (e.g.,
chromium,3 iron,4 and cobalt5–8). Two decades ago, a-
diiminometal (Ni or Pd) complexes were found as highly
active catalysts for ethylene polymerization,9 this disclosure
triggering extensive subsequent investigations by both aca-
demic and industrial groups on using ethylene or a-olefin
monomers.10–12 Elsewhere cobalt complexes bearing NNN
tridentate ligands were reported as efficient precatalysts
toward isoprene polymerization.8 Although a-diiminocobalt
complexes were also synthesized and used to polymerize
ethylene,13 there are no reports of bidentate cobalt com-
plexes, to the knowledge of authors, being employed for iso-
prene polymerization. From our viewpoint, the less steric
hindrance imparted by a-diimine ligands6 when compared
with their tridentate counterparts should enhance the cata-
lytic properties of the cobalt complexes in isoprene
polymerization.8
EXPERIMENTAL
General Procedure
All manipulations were carried out using standard Schlenk
techniques under nitrogen. All organic ligands, L1–L5, were
prepared according to literature methods. All solvents were
refluxed and purified under a nitrogen atmosphere before
use. Methylaluminoxane (MAO) (1.46 mol/L in toluene) was
purchased from Akzo Nobel Corp. Ethylaluminum dichloride
(AlEtCl2) (1.44 mol/L in toluene) and ethylaluminum sesqui-
chloride (Et3Al2Cl3, EASC, 0.87 mol/L in toluene) were pur-
chased from Acros Chemical. High-purity isoprene was
purchased from Beijing Yansan Petrochemical Co. and puri-
fied by distillation over CaH2 under a nitrogen atmosphere
before use. NMR spectra were recorded on a Bruker Avance
III 400MHz HD spectrometer at ambient temperature using
tetramethylsilane (TMS) as an internal standard; d values are
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1294 (m), 1242 (m), 1224 (m), 1196 (m), 1127 (m), 1081
(m), 1030 (m), 993 (w), 921 (w), 892 (w), 854 (m), 832
(m), 775 (s), 738 (s). Anal. Calcd. For C28H24N2Cl2Co
(518.34): C, 64.88; H, 4.67; N, 5.40. Found: 64.68; H, 4.77; N,
5.20.
1,2-Bis(2,6-Diethylphenylimino)Acenaphethylyl Cobalt
Dichloride (C2)
Using the same procedure as described for C1, with L2 in
place of L1, C2 was obtained as a brown powder (0.148 g,
57%). FT-IR (cm21): 2969 (m), 2933 (w), 2874 (w), 1651 (m
(C@N), m), 1624 (m (C@N), m), 1580 (m), 1488 (w), 1455
(m), 1428 (m), 1369 (w), 1290 (m), 1256 (w), 1221 (w),
1191 (w), 1122 (w), 1048 (m), 951 (w), 903 (w), 861 (w),
828 (m), 808 (m), 773 (s). Anal. Calcd. For C32H32N2Cl2Co
(574.45): C, 66.91; H, 5.61; N, 4.88. Found: 66.78; H, 5.77; N,
4.60.
SCHEME 1 Synthetic procedures for L12L5 and C12C5.
TABLE 1 Crystal Data and Structure Refinements for C1
given in ppm and J values in Hz. Fourier transform infrared
(FT-IR) spectra were recorded with a PerkinElmer System
2000 FT-IR spectrometer. Elemental analyses were per-
formed with a Flash EA 1112 micro-analyzer. The glass tran-
sition temperature (Tg) of the polyisoprenes was measured
from the last scanning run of a PerkinElmer TA-Q2000 DSC
analyzer under a nitrogen atmosphere. In this procedure, a
sample of about 5.0 mg was heated from 275 8C to 100 8C
at a rate of 20 8C/min. After an isothermal period of 5 min
at 100 8C, the sample was cooled to 275 8C at the same
rate, and finally heated to 100 8C. The weight-average molec-
ular weight (Mw) and polydispersity index (PDI; Mw/Mn) of
the polymers were measured using gel permeation chroma-
tography (GPC) using a PL-GPC50 fitted with a refractive
index detector and mixed columns having 650 mm total
length and 7.5 mm internal diameter along with the samples
in tetrahydrofuran at 30 8C (the polyisoprenes dissolved in
THF to form homogeneous solutions after 6 h). THF was
eluted at a flow rate of 1.0 mL/min. The columns were cali-
brated against standard polystyrene samples.
Empirical formula
Formula weight
Temperature/K
C28H24N2CoCl2
518.32
173(2)
˚
Wavelength/A
0.71073
Monoclinic
P21/m
Crystal system
Space group
˚
a/A
7.6941(15)
18.215(4)
8.6608
˚
b/A
˚
c/A
Alpha/8
Beta/8
90
92.24(3)
90
Gamma/8
3
˚
Volume/A
1,212.9(4)
2
Z
D
calcd/(gꢀcm23
)
1.419
l/mm21
0.947
F(000)
534
Synthesis and Characterization
Crystal size/mm
h Range (ꢁ)
0.17 3 0.14 3 0.05
2.35–27.51
The syntheses of the 1,2-diaryliminoacenaphethylenes, L1–
L5, and complexes C3 and C4, were performed according to
literature procedures (Scheme 1).13,14
Limiting indices
29 ꢂ h ꢂ 9
223 ꢂ k ꢂ 23
211 ꢂ l ꢂ 11
1,2-Bis(2,6-Imethylphenylimino)Acenaphthylyl Cobalt
Dichloride (C1)
No. of rflns collected
No. unique rflns
R(int)
8,358
2,852
Compound L1 (0.194 g, 0.50 mmol) and CoCl2 (0.058 g, 0.45
mmol) were dissolved in a mixture of ethanol (2 mL) and
dichloromethane (8 mL), and stirred at room temperature
for 24 h. The solution was concentrated and diethyl ether
(20 mL) added to precipitate the complex, which was collect-
ed by filtration and further washed with diethyl ether to
give C1 as a green solid (0.175 g, 75%). FT-IR (cm21): 2976
(w), 2909 (w), 1654 (m (C@N), m), 1631 (m (C@N), m), 1603
(m), 1583 (m), 1469 (m), 1439 (m), 1419 (m), 1381 (w),
0.0336
No. of params
2,852/0/159
Completeness to h
Goodness of fit on F2
99.4%
1.137
P
Final R indices [I > 2 (I)]
R1 5 0.0409wR2 5 0.1002
R1 5 0.0437wR2 5 0.1021
0.436 and 20.471
R indices (all data)
23
˚
Largest diff. peak, and hole/(e A
)
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Ið5:10Þ
1,2-Bis(2,6-Diethyl-4-Methylphenylimino)
Acenaphethylylcobalt Dichloride (C5)
cis-1; 4 PI ð%Þ 5 100 3
3; 4 PI ð%Þ 5 100 3
(1)
(2)
Ið5:10Þ 1 Ið4:75Þ=2
Ið4:75Þ=2
Using the same procedure as described for C1, with L5 in
place of L1, C5 was obtained as a brown powder (0.106 g,
39%). FT-IR (cm21): 2966 (s), 2932 (m), 2874 (m), 1658 (m
(C@N), m), 1633 (m (C@N), s), 1585 (s), 1458 (s), 1419 (m),
1377 (m), 1291 (m), 1262 (m), 1226 (m), 1201 (m), 1158
(m), 1052 (m), 953 (w), 903 (w), 870 (s), 834 (s), 777 (s).
Anal. Calcd. For C34H36N2Cl2Co (602.50): C, 67.78; H, 6.02;
N, 4.65. Found: 67.58; H, 5.98; N, 4.76.
Ið5:10Þ 1 Ið4:75Þ=2
where, I(5.10) and I(4.75) represent the signal area at 5.10
and 4.75 ppm, respectively. The microstructures of the PI
based on FT-IR spectra can be calculated according to the
following eqs (3–6)8b
:
A
1375 5 24 CIp cis-1;4 L 1 32:6 CIp 3;4
L
(3)
(4)
X-Ray Crystallographic Studies
A
889 5 101 CIp 3;4
L
A single crystal of C1 was obtained by slow diffusion of
diethyl ether into a dichloromethane solution containing the
complex at room temperature. The single crystal X-ray dif-
fraction study was carried out on a Rigaku Saturn 7241
CCD with graphite-monochromatic Mo-Ka radiation
(k 5 0.710747 Å) at 173(2) K, the cell parameters were
obtained by global refinement of the positions of all collected
reflections. Intensities were corrected for Lorentz and polari-
zation effects and empirical absorption. The structure was
solved by direct methods and refined by full-matrix least
squares on F2. All hydrogen atoms were placed in calculated
positions. Structure solution and refinement were performed
by using the SHELXL–97 package.15 Details of the crystal
data and structure refinements for C1 are shown in Table 1.
CCDC 1487899 (C1) contains the supplementary crystallo-
graphic data for this article, which could be obtained free of
charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
CIp cis-1;4
ðCIp cis-1;4 1 CIp 3;4
cis-1; 4 PI ð%Þ 5 100 3
3; 4 PI ð%Þ 5 100 3
(5)
(6)
Þ
CIp 3;4
ðCIp cis-1;4 1 CIp 3;4
Þ
where, A1375 is the absorption intensity at 1375 cm21, which
was expressed by the peak height; CIpcis-1,4 represents the
molar content of isoprene cis-1,4 units, CIp3,4 represents the
molar content of isoprene 3,4 units, and L indicates the
thickness of the sample.
RESULTS AND DISCUSSION
Synthesis and Characterization of Acenaphthylene-1,2-
Diaryliminocobalt Complexes (C1–C5)
The 1,2-diaryliminoacenaphthylenes (L1–L5), prepared
according to literature procedures,9,14 were reacted with
CoCl2 in dichloromethane to form the acenaphthylene-1,2-
diaryliminocobalt complexes (C1–C5, Scheme 1) in good
yield; complexes C3 and C4 have been previously reported.13
The FT-IR spectra of C1–C5 show stretching vibrations for
the C@N group in the range 1658–1623 cm21 which com-
pares with values of around 1680–1645 cm21 for the free
organic ligands L1–L5, consistent with effective coordination
between the cobalt ion and Nimino atoms. Complex C1 has
also been the subject of a single crystal X-ray diffraction
study.
Isoprene Polymerization
In a typical polymerization, a 50 mL Schlenk tube equipped
with a magnetic stirrer bar was charged with the desired
precatalyst. Ethylaluminum dichloride (AlEtCl2) and toluene
were then added with a syringe. The mixture was magneti-
cally stirred for 1 min at room temperature before the addi-
tion of isoprene (2 mL, 0.02 mol). The polymerization was
conducted under an atmosphere of nitrogen, at a given moni-
tored temperature and fixed time. After the desired time had
been reached, a dilute HCl solution of ethanol, containing
2,6-di-tert-butyl-4-methylphenol (BHT, 1.0 wt %), was
injected into the system to quench the polymerization. The
mixture was poured into a large amount of ethanol to pre-
cipitate the polymer. After filtration and drying under vacu-
um at 40 8C for 24 h, the polymer obtained was weighed.
X-Ray Crystallography
Crystal of C1 suitable for the X-ray determination was
obtained by layering a dichloromethane solution of the com-
plex with diethyl ether. Its molecular structure is shown in
Figure 1; selected bond lengths and angles are collected in
Table 2. The structure of C1 comprises a symmetry generat-
ed species in which the cobalt dichloride unit is bound by a
neutral N,N-chelating 1,2-bis(2,6-dimethylphenylimino)ace-
naphthylene to form a four-coordinate geometry at cobalt.
The bite angle of the a-diimine ligand is 81.79(9)8 while the
ClACoACl angle is 115.38(5)8 leading to a distorted tetrahe-
dral geometry at cobalt. The cobalt-nitrogen bond distances
[Co1–N1 2.0801(16) Å] are typical as are the cobalt-chloride
distances [Co1–Cl1 2.2202(11), Co1–Cl2 2.2162(11) Å]. The
N1–C1 [1.280(2) Å] distances are also consistent with C@N
double-bond character. The N-aryl groups are inclined at
Calculation of the Microstructural Content of the PIs
FT-IR spectra were recorded on a PerkinElmer System
2000 FT-IR spectrometer at a resolution of 4 cm21 per 32
scans. 1H NMR (400 MHz) spectra were recorded at Bruker
Avance III 400MHz HD spectrometer at ambient temperature
with CDCl3 containing tetramethylsilane as a standard.
Based on the calculated area of the characteristic signals at
1
4.75 and 5.10 ppm in the H NMR spectrum, the molar con-
tent of 3, 4 PI and cis-1, 4 PI can be calculated,8a as shown
in the following eqs 1 and 2:
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TABLE 3 Co-Catalyst Screen Using C1 as the Precatalysta
Run
Co-cat.
Al/Co
T (8C)
t (min)
Conv.b (%)
1
2
3
EASC
MAO
100
40
40
40
30
30
30
4.8
1,000
100
Trace
41.9
EtAlCl2
a
Conditions: 0.002 mmol catalyst loading, 8 mL toluene, 2 mL isoprene.
Determined by the weight of polymer.
b
Isoprene Polymerization with C1–C5/AlEtCl2 Systems
A catalyst system composed of C1 and AlEtCl2 was used to
optimize three polymerization parameters namely, the Al/Co
molar ratio, the reaction temperature and the reaction dura-
tion (Table 4). With the temperature set at 40 8C, the catalyt-
ic activities gradually increased with increasing Al/Co molar
ratios from 100 to 200 (runs 1–4, Table 4); however, above
200 the activity slightly decreased (runs 5, 6, Table 4).
Therefore, the optimum Al/Co ratio was fixed at 200.
FIGURE 1 ORTEP drawing of C1 with thermal ellipsoids at a
30% probability level. Hydrogen atoms and molecule of
dichloromethane have been omitted for clarity.
angles close to orthogonal as shown by the dihedral angle
between the N1, N1i, and Co1 chelate plane and an N1-aryl
ring of 86.998.
Isoprene Polymerization
With the Al/Co ratio fixed at 200, the reaction temperatures
were increased from 20 8C to 110 8C (runs 4, 7–15, Table 4)
and the two highest conversions were observed with 78.4%
and 82.3% at 60 8C and 100 8C, respectively (runs 10, 14,
Table 4). Within the 20 8C–70 8C temperature range, the
molecular weights of the polyisoprenes increased as the
temperature was raised. By contrast, between 70 8C and
110 8C the molecular weights of the resultant polyisoprenes
lowered as the temperature increased. Given the observa-
tion of two optimal temperatures for the polymerization
runs, it would seem likely that there are two active species
that are temperature affected; its thermostability decreased
over 100 8C. Furthermore, the polydisperties (PDI) of the
polyisoprenes broaden as the temperature is raised (runs 4,
7–15, Table 4) with narrow PDIs apparent between 20 8C
and 40 8C. Above 40 8C, it would seem probable that activa-
tion is leading to the generation of an alternative catalytical-
ly active species. Using the conditions established for C1/
AlEtCl2, with the Al/Co ratio fixed at 200 and the tempera-
ture set at either 60 8C or 100 8C, the other cobalt com-
plexes were screened for isoprene polymerization (runs 15–
18 in Table 4). At both 60 8C and 100 8C, the activities
decrease in the order C1 [2,6-di(Me)] (runs 10 and 14,
Table 4) > C3 [2,6-di(i-Pr)] (runs 17 and 21, Table 4) > C2
[2,6-di(Et)] (runs 16 and 20, Table 4) > C4 [2,4,6-tri(Me)]
(runs 18 and 22, Table 4) > C5 [2,6-di(Et)24-Me] (runs 19
and 23, Table 4). This order would suggest that the combi-
nation of both steric and electronic effects is influential on
the catalytic activity. On comparing the data for C1 and C4
as well as C2 and C5, the additional para-methyl group
within C4 and C5 has a negative influence on the catalytic
activity. This negative effect played by an electron donating
group may also explain the lower activities of C2 and C3
compared with C1. However, the more bulky C3 showed a
higher activity than C2, highlighting the role played by ste-
ric properties, although the increased solubility of C3 over
C2 may also be influential.
Complex C1 was used as the test precatalyst to establish the
most suitable co-catalyst for isoprene polymerization. A vari-
ety of alkylaluminum co-catalysts were screened including
methylaluminoxane (MAO), dichloride ethylaluminum
(AlEtCl2) and ethylaluminum sesquichloride (Et3Al2Cl3,
EASC) at 40 8C with the cobalt/isoprene molar ratio fixed at
1:10,000; the results are summarized in Table 3. Inspection
of the results indicates that the highest conversion was
achieved using AlEtCl2 as the co-catalyst (run 3, Table 3).
Hence, the polymerizations for the remaining precatalysts
were evaluated using AlEtCl2.
TABLE 2 Selected Bond Lengths and Angles for C1
˚
Bond Lengths (A)
Co(1)–N(1)
Co(1)–N(1)i
2.0801(16)
2.0801(16)
2.2202(11)
2.2162(10)
1.280(2)
Co(1)–Cl(1)
Co(1)–Cl(2)
N(1)–C(1)
N(1)–C(8)
N(1)i–C(1)i
N(1)i–C(8)i
1.443(2)
1.280(2)
1.443(2)
Bond Angles (deg)
N(1)–Co(1)–N(1)i
N(1)–Co(1)–Cl(2)
N(1)i–Co(1)–Cl(2)
N(1)–Co(1)–Cl(1)
N(1)i–Co(1)–Cl(1)
Cl(2)–Co(1)–Cl(1)
81.79(9)
115.06(5)
115.06(5)
112.59(5)
112.59(5)
115.38(5)
The atoms labeled with i have been generated by symmetry.
4
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a
TABLE 4 Isoprene Polymerization by C1–C5/AlEtCl2
Conv.b
(%)
Mw
cis-1,4d
3,4d
cis-1,4e
3,4e
c
f
Run
Cat.
Al/Co
T (8C)
(103 g/mol)
PDIc
(mol %)
(mol %)
(mol %)
(mol %)
Tg (8C)
1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C2
C3
C4
C5
C2
C3
C4
C5
100
150
175
200
225
250
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
200
40
41.9
49.4
69.8
70.5
67.0
63.1
55.7
70.3
74.5
78.4
62.2
64.4
69.0
82.3
60.8
68.0
73.3
59.7
54.7
73.4
79.7
63.2
60.8
1.6
1.9
2.2
3.6
2.3
2.9
2.8
2.9
4.5
4.6
6.6
2.6
2.3
2.1
1.9
4.0
4.1
3.6
10.6
2.6
3.3
2.7
2.6
1.3
1.2
1.3
1.1
1.4
1.3
1.1
1.1
1.3
1.2
1.3
1.4
1.5
1.8
1.9
1.1
1.2
1.1
1.9
1.3
1.3
1.2
2.1
94.2
95.1
93.3
89.2
93.2
93.5
95.1
–
5.8
4.9
6.7
10.8
6.8
6.5
4.9
–
90.5
91.6
91.4
92.1
92.2
93.9
91.5
92.3
91.9
94.9
91.8
93.9
93.7
92.3
93.0
91.3
93.2
93.3
90.5
93.5
93.5
91.9
91.6
9.5
8.4
8.6
7.9
7.8
6.1
8.5
7.7
8.1
5.1
8.2
6.1
6.3
7.7
7.0
8.7
6.8
6.7
9.5
6.5
6.5
8.1
8.4
16
21
25
42
22
22
24
25
24
31
39
39
40
39
41
22
28
37
41
44
44
32
27
2
40
3
40
4
40
5
40
6
40
7
20
8
30
9
50
94.8
91.7
–
5.2
8.3
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
60
70
80
–
–
90
90.6
89.7
90.0
92.6
92.4
93.2
92.7
–
9.4
10.3
10.0
7.4
7.6
6.8
7.3
–
100
110
60
60
60
60
100
100
100
100
–
–
–
–
–
–
a
d
Conditions: 0.002 mmol catalyst loading, 8 mL of toluene, 2 mL iso-
prene, AlEtCl2 as co-catalyst, time 30 min.
Determined by 1H NMR spectroscopy.
Determined by FT–IR spectroscopy.
Determined by DSC.
e
f
b
Determined by the weight of polymer.
c
Determined by GPC.
With regard to the effect of time on conversion, the isoprene
polymerization for C1/AlEtCl2 was conducted over 5, 15, 30,
45, 60, 90, 150 and 180 min; the results are collected in
Table 5 and also illustrated in Figure 2. Within 5 min, almost
half of isoprene was polymerized (run 1, Table 5), indicating
the active species quickly formed upon the addition of
a
TABLE 5 Isoprene Polymerization by C1/AlEtCl2
Conv.b
(%)
Mw
cis-1,4d
3,4d
cis-1,4e
3,4e
Tg
c
f
Run
t (min)
(103 g/mol)
PDIc
(mol %)
(mol %)
(mol %)
(mol %)
(8C)
1
2
3
4
5
6
7
8
5
49.7
79.4
85.9
90.3
92.5
96.4
99.4
>99.9
3.6
3.7
4.5
4.8
5.2
5.5
5.9
6.2
1.1
1.2
1.2
1.3
1.3
1.3
1.4
1.4
91.7
–
8.3
–
90.8
92.8
94.9
92.1
92.4
88.4
92.9
94.2
9.2
7.2
5.1
7.9
7.6
11.6
7.1
5.8
4
15
30
45
60
90
150
180
19
31
28
31
32
46
52
94.2
–
5.8
–
92.0
–
8.0
–
90.1
–
9.9
–
a
d
Conditions: 0.002 mmol C1 loading,
catalyst with Al/Co 200, temp. 60 8C.
8
mL toluene, AlEtCl2 as co-
Determined by 1H NMR spectroscopy.
Determined by FT-IR spectroscopy.
Determined by DSC.
e
f
b
Determined by GC.
Determined by GPC.
c
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FIGURE 2 Plots of the conversions against polymerization time
for isoprene polymerization by using C1/AlEtCl2 catalyst in tolu-
ene at 60 8C, Co/Al/IP 5 1:200:10,000.
FIGURE 4 FT-IR spectra of PIs obtained with cobalt pre-
catalysts (runs 10, 16–19, Table 4).
toluene as the internal standard. However, the two
approaches showed some variation, for example, the conver-
sion yields were 78.4% and 85.9% using C1/AlEtCl2 under
the conditions 60 8C, Co/Al/IP 5 1:200:10,000 and 30 min,
respectively (run 10, Table 4 and run 3, Table 5). On using
the GC approach for run 3 (Table 5), it was found that in
addition to isoprene, new peaks corresponding to dimers,
trimmers, and other oligomers could also be detected. There-
fore, on the basis of the latter observations the conversions
could be more reliably determined by measuring the weights
of the polymers collected.
AlEtCl2. In addition, no isoprene was detectable in the solu-
tion after quenching the reaction run over 180 min, implying
full conversion occurred (run 8, Table 5). In general, the
molecular weights of the polyisoprenes increased with pro-
longing the reaction time (Table 5).
In general, the conversion of the isoprene to PI was detected
in two ways, either by measuring the polymer obtained
(runs 1–23, Table 4) or by measuring the unreacted isoprene
using standardized GC (runs 1–8, Table 5). To measure the
conversion efficiency of isoprene, the polymerization of
10 mL isoprene in 40 mL toluene with the presence of 0.01
mmol C1 were monitored by taking samples through a
syringe into a 5 mL ice water; the ratios of isoprene to tolu-
ene in the organic layer were measured by the GC using
Polymer Microstructure Characterization
The molecular weights and polydispersity indices of the pol-
yisoprenes obtained were measured by GPC, while the Tg
values of most of the polymers were determined by DSC and
were found to fall in the range of 16.5 8C–52.4 8C. The micro-
structures of the PIs were analyzed via 1H NMR, 13C NMR,
and FT-IR spectroscopies. The 1H NMR spectra of the PIs
obtained (runs 10, 16–19, Table 4) are shown in Figure 3.
The typical 1H NMR chemical shifts for the PI are assigned
as follows (CDCl3; d, ppm): 4.7–4.8 (@CH2 of 3,4 isoprene
units), 5.0–5.1 (@CH of 1,4 isoprene units), 1.74 (@CH3 of
cis-1,4 units), and 1.65 (@CH3 of trans21,4 units). There is
no evidence for signals at 5.9 ppm,8a,16 highlighting these PIs
do not contain 1,2-units.
With regard to the cis-trans geometric isomers, FT-IR meas-
urements were performed to characterize the microstructure
of the PIs (Fig. 4 and runs 10, 16–19, Table 4). These spectra
indicate distinct peaks attributable to cis-1,4 units at 840,
1130, 1325, and 1375 cm21 along with weak peaks around
889 cm21 for the 3,4 units. The characteristic bands at 845,
1152, 1325, and 1385 cm21 for trans21,4 units and
911 cm21 for 1,2-units are notably absent.8b,17 Indeed, the
FT-IR spectra are supportive of the observations seen in the
13C NMR spectra. In general, all the PIs obtained possessed
FIGURE 3 1H NMR spectra of the PIs with cobalt precatalysts
(runs 10, 16–19, Table 4).
6
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(c) J. Raynaud, J. Y. Wu, T. Ritter, Angew. Chem. Int. Ed. 2012,
51, 11805–11808.
high levels of cis-1,4 units (between 90% and 94%) along
with 6%–10% levels of 3,4 units. The catalytic stereoselectiv-
ities were only slightly affected by the nature of the pre-
catalyst employed.
5 (a) G. Ricci, A. Forni, A. Boglia, T. Motta, J. Mol. Catal. A
2005, 226, 235–241; (b) G. Ricci, A. Sommazzi, F. Masi, M. Ricci,
A. Boglia, G. Leone, Coordin. Chem. Rev. 2010, 254, 661–676.
6 X. Jia, H. Liu, Y. Hu, Q. Dai, J. Bi, C. Bai, X. Zhang, Chin. J.
Catal. 2013, 34, 1560–1569.
CONCLUSIONS
A series of acenaphthylene-1,2-bis(arylimino)cobalt chlorides,
C1–C5, was synthesized and characterized and, in the case
of C1, by single crystal X-ray diffraction revealing a tetrahe-
dral geometry. Upon activation with AlEtCl2, all cobalt pre-
catalysts exhibited high activities toward isoprene polymeri-
zation at both 60 8C and 100 8C. The molecular weights of
the PIs were in the range 2 3 103210 3 103 g/mol and dis-
played narrow molecular weight distributions in the range
1.1–2.1. Furthermore, the PIs showed high contents of cis-1,4
units.
7 G. Ricci, G. Leone, A. Boglia, A. C. Boccia, L. Zetta, Macromo-
lecules 2009, 42, 9263–9267.
8 (a) A. He, G. Wang, W. Zhao, X. Jiang, W. Yao, W. H. Sun,
Polym. Int. 2013, 62, 1758–1766; (b) G. Wang, X. Jiang, W.
Zhao, W. H. Sun, W. Yao, A. He, J. Appl. Polym. Sci. 2014, 131,
39703–39708; (c) F. S. Mair, M. N. Alnajrani, RSC Adv. 2015, 5,
46372–46385.
9 L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc.
1995, 117, 6414–6415.
10 C. M. Killian, D. J. Tempel, L. K. Johnson, M. Brookhart, J.
Am. Chem. Soc. 1996, 118, 11664–11665.
11 (a) D. H. Camacho, Z. Guan, Chem. Commun. 2010, 46,
7879–7893; (b) D. H. Camacho, E. V. Salo, J. W. Ziller, Z. Guan,
Angew. Chem. Int. Ed. 2004, 43, 1821–1825.
ACKNOWLEDGMENTS
12 (a) S. Du, S. Kong, Q. Shi, J. Mao, C. Y. Guo, J. Yi, T. Liang,
W. H. Sun, Organometallics 2015, 34, 582–590; (b) L. Fan, S.
Du, C. Y. Guo, X. Hao, W. H. Sun, J. Polym. Sci., Part a: Polym.
Chem. 2015, 53, 1369–1378; (c) S. Du, Q. Xing, Z. Flisak, E. Yue,
Y. Sun, W. H. Sun, Dalton Trans. 2015, 44, 12282–12291; (d) L.
Fan, E. Yue, S. Du, C. Y. Guo, X. Hao, W. H. Sun, RSC Adv.
2015, 5, 93274–93282.
This work is supported by the National Natural Science Foun-
dation of China (Nos. 51373176, 21374123 and U1362204).
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