Solvent effect on cis-1,4-specific polymerization of 1,3-butadiene with CoCl2(PRPh2)2-EASC catalytic systems

Article abstract of DOI:10.1039/c3cy00634d

A series of phosphine-containing cobalt complexes [CoCl ₂(PRPh₂)₂, R = Ph (1), Cy (2), ⁿPr (3), Et (4), Me (5), H (6)] has been used for the highly active and stereospecific polymerization of 1,3-butadiene upon activation with ethylaluminum sesquichloride (EASC). The high catalytic activities and polybutadienes with high cis-1,4 selectivity were obtained in the solution polymerization in toluene. The conversion of butadiene and the microstructure and molecular weight of the resulting polymers were affected by reaction parameters and the R group on the phosphine ligand. The dispersion medium was also a sensitive factor for the current catalytic systems, influencing the catalytic activity and properties of products. In comparison with the solution polymerization in toluene, the heterogeneous polymerization in isooctane yielded slightly lower catalytic activity, cis-1,4 content and molecular weight under similar reaction conditions.

Full text of DOI:10.1039/c3cy00634d

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Solvent effect on cis-1,4-specific polymerization
of 1,3-butadiene with CoCl2(PRPh2)2–EASC
catalytic systems
Cite this: Catal. Sci. Technol., 2014,
4, 773
*
Qinzhuo Zhou, Bo-Geng Li, Suyun Jie and Na Zheng
A series of phosphine-containing cobalt complexes [CoCl2(PRPh2)2, R = Ph (1), Cy (2), nPr (3), Et (4), Me
(5), H (6)] has been used for the highly active and stereospecific polymerization of 1,3-butadiene upon
activation with ethylaluminum sesquichloride (EASC). The high catalytic activities and polybutadienes
with high cis-1,4 selectivity were obtained in the solution polymerization in toluene. The conversion of
butadiene and the microstructure and molecular weight of the resulting polymers were affected by
reaction parameters and the R group on the phosphine ligand. The dispersion medium was also a sen-
sitive factor for the current catalytic systems, influencing the catalytic activity and properties of prod-
ucts. In comparison with the solution polymerization in toluene, the heterogeneous polymerization in
isooctane yielded slightly lower catalytic activity, cis-1,4 content and molecular weight under similar
reaction conditions.
Received 26th August 2013,
Accepted 16th December 2013
DOI: 10.1039/c3cy00634d
www.rsc.org/catalysis
the type and amount of cocatalysts also have a great influ-
ence on catalytic activity and stereoselectivity of the
Introduction
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.
In particular, selective cis-1,4-specific polymerization has
attracted much attention, because the resulting cis-1,4-PBDs
are the most important elastomers used for tires and
other elastic materials.1,2 The TiCl4 or TiCl3–AlR3 catalytic
systems were firstly used in butadiene polymerization,3
just after their application in the polymerization of ethylene
and propylene. For an industrial scale, cis-1,4-PBDs are
produced commercially via solution polymerization of
1,3-butadiene catalyzed by Ziegler–Natta catalysts, such as
TiCl4–I2–Al(i-Bu)3,4 CoCl2–AlEt2Cl,5,6 Ni(OOCR)2–BF3·OEt2–AlEt3,7,8
or Nd(OOCR)3–Et3Al2Cl3–Al(i-Bu)2H (ref. 9), in aromatic or
aliphatic hydrocarbon solvents. Among those catalysts, the
cobalt-based catalysts are of great importance in butadiene
polymerization and have been widely investigated probably
because they can produce PBDs with different microstruc-
tures, including cis-1,4-PBDs and syndiotactic 1,2-PBDs.
The halides and carboxylates of cobalt are stereoselective to
high cis-1,4-PBDs when activated with methylaluminoxane
(MAO) or modified MAO (MMAO).10–15 However, the addition
of phosphines to certain Co-based catalytic systems could
alter the regio- and/or stereospecificity of the active species
from cis-1,4-specific to syndiotactic 1,2-specific.15–28 Besides,
resulting polymers.29–32 In our previous study, the addition
of triphenylphosphine to the [Co]–EASC catalytic systems
didn't change the polymer microstructure.31
Heterogeneous polymerization is widely applied on an
industrial scale because of the low viscosity of the polymer
solution, easy dissipation of heat and temperature control,
milder reaction conditions, product homogeneity, etc.33,34 The
cobalt allyl complex [Co(C8H13)(C4H6)] (ref. 35, 36) in combina-
tion with CS2 for the catalytic polymerization of 1,3-butadiene
afforded syndiotactic 1,2-PBDs with high 1,2-vinyl selectivity in
aqueous emulsion.37–40 The suspension polymerization of
butadiene in the presence of titanium–magnesium nano-
catalysts combined with Al(i-Bu)3 yielded PBDs containing up
to 99% trans-1,4-units.41 The cationic nickel(II) complexes were
reported to catalyze butadiene polymerization in disperse
aqueous systems with nonaqueous prepolymerization and
special precautions to prevent the precatalyst from decom-
posing prematurely, producing PBDs with low monomer con-
version and cis-1,4 content.42,43 Furthermore, water is a toxic
agent that makes catalysts inactive and causes danger in the
process of olefin polymerization. Therefore, a nonaqueous sol-
vent as the continuous phase is important in the heteroge-
neous polymerization of butadiene. The hydrocarbon mixtures
(comprising a volume fraction greater than 80% of C4 hydro-
carbons) were reported to be used as a dispersion medium for
the production of cis-1,4-PBDs in suspension polymerization,
where the polymerization was initiated by a rare earth com-
pound;44 however, the catalytic activities were low and residual
State Key Laboratory of Chemical Engineering, Department of Chemical and
Biological Engineering, Zhejiang University, Hangzhou 310027, China.
E-mail: jiesy@zju.edu.cn; Fax: +86 (0)571 87951612; Tel: +86 (0)571 87951515
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Nd concentrations in the polymers were high (180–250 ppm).45
Thus, a highly active and stereospecific catalytic system
and a proper dispersion medium are necessary for the pro-
duction of cis-1,4-PBDs in the heterogeneous polymerization
of 1,3-butadiene.
butadiene catalyzed by the CoCl2(PPh3)2 (1)–EASC catalytic
system in toluene was investigated in detail under various
reaction conditions to optimize the polymerization parame-
ters, such as monomer concentration, Al/Co molar ratio, and
reaction temperature (Table 1).
In order to deeply explore the phosphine-containing
cobalt-based catalytic systems and obtain high cis-1,4-PBDs, a
series of phosphine-containing cobalt complexes [CoCl2(PRPh2)2]
has been investigated with EASC as the cocatalyst in the poly-
merization of 1,3-butadiene in different solvents. The effect of
solvents, one aromatic (toluene) hydrocarbon, one aliphatic
(isooctane) hydrocarbon and their mixture, on the catalytic
activity, the microstructure and molecular weight characteris-
tics of polybutadienes will be discussed in this paper.
As the butadiene concentration increased, the reaction
solution became more viscous; however, the conversion of
butadiene to polymers wasn't affected and the complete con-
versions of butadiene were obtained for all the cases studied
(entries 1–3 in Table 1). All the polymers obtained with differ-
ent monomer concentrations were predominantly cis-1,4
isomers and the higher butadiene concentration resulted
in the slight increase in cis-1,4 content from 96.2% to 97.1%.
A similar trend was also observed in the polymerization of
butadiene with the Co(acac)3–MAO system.10 The molecular
weight of PBDs increased gradually along with the increase
in the Bd/Co molar ratio, while the molecular weight distri-
bution didn't change greatly.
Results and discussion
Polymerization of butadiene in toluene
The CoCl2(PRPh2)2–MAO systems were reported to be
extremely active in the polymerization of butadiene, produc-
ing 1,2-PBDs (70–88%) essentially, and the stereoregularity of
products was affected by the bulkiness of the R group on the
phosphorous atom.46 In order to gain high cis-1,4-PBDs,
ethylaluminum sesquichloride (EASC) was employed as the
In the binary catalytic systems, it is known that the
amount of cocatalyst has a great influence on the catalytic
performance of precatalysts. However, the variation of the
Al/Co molar ratio slightly affected the conversion of butadiene
in the current systems and the high catalytic activities were
obtained under a broad range of Al/Co molar ratio (entries 1
and 4–6 in Table 1). The butadiene conversion reached up to
96.5% even in the small amount of EASC (Al/Co = 20) and the
100% conversions of butadiene were obtained under the
Al/Co molar ratio of both 40 and 80. The further increase in the
n
cocatalyst for CoCl2(PRPh2)2 [R = Ph (1), Cy (2), Pr (3), Et (4),
Me (5), H (6)] because it has been proven to be the more effec-
tive cocatalyst for the cobalt complexes in the polymerization
of butadiene.29–32,47–49 Hence, the solution polymerization of
Table 1 Polymerization of 1,3-butadiene with CoCl2(PRPh2)2 (1–6)–EASC in toluenea
Microstructurec (mol%)
b
b
Entry
Cat.
Bd/Co
Al/Co
T (°C)
t (min)
Conv. (%)
Mn (104 g mol−1
)
Mw/Mn
cis-1,4-
trans-1,4-
1,2-
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
2
3
4
5
6
1
2000
3000
4000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
80
80
80
20
40
160
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
25
25
25
25
25
25
0
50
70
90
25
25
25
25
25
25
25
25
25
25
25
25
25
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
10
10
10
10
10
10
60
60
100
100
99.4
96.5
100
97.8
22.4
98.8
96.1
96.3
100
9.80
17.6
26.3
12.3
9.45
2.61
2.97
2.82
2.75
2.39
3.10
96.2
96.9
97.1
96.8
95.8
97.0
97.0
88.3
83.5
78.8
95.9
96.1
95.7
96.0
94.9
97.2
96.2
96.7
96.7
97.0
96.1
—
1.8
1.6
1.4
1.5
2.1
1.3
1.3
6.8
10.4
15.6
1.9
2.0
2.1
1.8
1.5
1.2
1.6
1.4
1.6
1.4
0.9
—
2.0
1.5
1.5
1.7
2.1
1.7
1.7
4.9
6.1
5.6
2.2
1.9
2.2
2.2
3.6
1.6
2.2
1.9
1.7
1.6
3.0
—
11.5
30.1, 1.43d
4.28
1.40, 1.63d
2.73
9
3.80
3.46
11.3
8.69
7.82
7.83
5.25
14.8
14.3
13.6
15.3
14.0
2.70
3.27
2.74
2.72
2.63
2.71
2.70
1.57
1.55
1.53
1.49
1.52
10
11
12
13
14
15
16
17
18
19
20
21
22
23e
100
100
100
92.0
51.7
62.7
63.9
58.7
53.9
29.0
Trace
2.8
2
3
4
5
6
8.90
1.61
CoCl2
CoCl2
—
—
41.9, 2.28f
1.35, 1.80f
94.4
1.8
3.8
a
b
Polymerization conditions: cat.: 5 μmol; cocat.: EASC; solvent: toluene, total volume: 15 mL. Determined by GPC against polystyrene
standards and reported uncorrected. Determined by 1H and 13C NMR spectroscopy. Bimodal distribution: 30.1 × 104 g mol−1 section
c
d
occupied 81.2%, 1.43 × 104 g mol−1 section occupied 18.8%. 10 μmol of CoCl2. Bimodal distribution: 41.9 × 104 g mol−1 section occupied
e
f
45.6%, 2.28 × 104 g mol−1 section occupied 54.4%.
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Al/Co molar ratio to 160 led to a slight decrease in butadiene
conversion (97.8%). In the polymerization of olefins catalyzed
by transition metal catalysts, the chain transfer to aluminum
is usually an important factor to decrease the molecular
weight of products, whereas its influence on the polymeriza-
tion of butadiene didn't show good regularity. In addition,
the variation of Al/Co molar ratio led to a slight change of
the microstructure of PBDs.
under the same conditions indicated the importance of
the phosphine ligand to the active centers (entry 22 in
Table 1). Except for the 6 (R = H)–EASC catalytic system
(conv.% = 92%, entry 15 in Table 1), all the other complexes
(1–5)–EASC systems gave the complete conversions of butadi-
ene in 60 min. In order to compare the difference between
the R groups clearly, the reaction time was shortened to 10 min
(entries 16–21 in Table 1) and it was observed that the R
group on the phosphine ligand had an obvious influence on
the catalytic activity. When the substituent R was an alkyl
group, the less bulky R group led to lower conversion of buta-
diene (in the order of nPr > Et > Me > H) and a relatively
lower molecular weight. For the cyclic R group, the 2–EASC
system with the saturated cyclohexyl group showed higher
catalytic activity than the 1–EASC system with the conjugated
phenyl group due to the different electron-donating nature of
the R groups. The cis-1,4 contents in the polymers obtained
by all the complexes were in the range 94.9–97.2%. The
molecular weight of PBDs produced in 10 min ranged from
89 to 153 kg mol−1 with narrow molecular weight distribution
(1.49–1.61). When the polymerization time was lengthened to
60 min, PBDs with lower molecular weight (52.5–113 kg mol−1)
and broader molecular weight distribution (2.39–2.74) were
produced for each complex. It was proposed that PPh3 could be
a chain transfer agent when it was combined in the cobalt-
based catalytic systems.27,31 Speculatively, the PRPh2 ligand
might be an important factor to induce the chain transfer
reaction and the decrease in molecular weight.
The solution polymerization of butadiene was also carried
out over the temperature range 0 to 90 °C with the 1–EASC
catalytic system (entries 5 and 7–10 in Table 1). Although a
lower conversion of butadiene (22.4%) was gained at 0 °C,
some cobalt-based catalytic systems gave almost no activity
under similar conditions.30,31 The highest activity was
observed at 25 °C with a complete conversion of butadiene.
As the polymerization temperature was elevated, the conver-
sion of butadiene decreased slightly, which demonstrated
that the 1–EASC catalytic system has remarkable thermal
stability at higher temperatures. At the same time, a lower
molecular weight and a relatively broader molecular weight
distribution of PBDs were obtained at higher temperature.
The variation of reaction temperature had a significant influ-
ence on the microstructure of PBDs; that is, the cis-1,4 con-
tent of the resulting polymers decreased sharply and the
amount of the trans-1,4 isomer increased gradually along
with the elevation of reaction temperature (Fig. 1). As a
result, the PBDs obtained at 90 °C contained 78.8% cis-1,4
isomer and 15.6% trans-1,4 isomer, which were largely differ-
ent from the PBDs obtained at 25°C with 95.8% cis-1,4 con-
tent. Besides, the 1,2-vinyl isomer content slightly increased
from 1.7% to 6.1% from 0 to 90 °C.
Polymerization of butadiene in isooctane
In order to explore the influence of the R group on the
phosphine ligand, all the CoCl2(PRPh2)2 (1–6)–EASC catalytic
systems were investigated in the solution polymerization of
butadiene in toluene under the Al/Co molar ratio of 40 at 25 °C.
The negligible amount of polymer obtained by CoCl2–EASC
In a typical solution polymerization of 1,3-butadiene, cyclo-
hexane, hexane or toluene is usually used as the solvent. The
selection of a non-aqueous immiscible solvent as the disper-
sion medium for polybutadiene is very important to perform
heterogeneous polymerization of butadiene. The solubility
parameter is considered to be the important basis to judge
the solubility of the solute in a solvent.50 When the differ-
ence of solubility parameters between the polymer and the
solvent is larger than 1.5 (cal cm−3)1/2, the heterogeneous
polymerization system could be generated. Isooctane (δ =
6.85 (cal cm−3)1/2) was chosen as the dispersion medium for
the heterogeneous polymerization of butadiene herein and
the resultant polybutadienes (δ = 8.60 (cal cm−3)1/2) are
expected to become a powdery suspension in the medium
under stirring.50–53
All the cobalt complexes (1–6)–EASC catalytic systems were
used in the polymerization of butadiene with isooctane as the
dispersion medium (Table 2). It was observed that the current
catalytic systems were immiscible and suspended in isooctane
before the butadiene solution was added. First, the different
monomer concentration was used with the 1–EASC system
under the Al/Co molar ratio of 80 at 25 °C (entries 1–3 in
Table 3). Along with the increase in the Bd/Co molar ratio from
2000 to 4000, the conversion of butadiene declined from
95.0% to 86.9%, which was relatively lower than the
Fig.
1 Microstructure of PBDs produced by the 1–EASC catalytic
system at different temperatures (entries 5 and 7–10 in Table 1).
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Table 2 Polymerization of 1,3-butadiene with CoCl2(PRPh2)2 (1–6)–EASC in isooctanea
Microstructurec (mol%)
Mn (104 g mol−1
)
Mw/Mn
cis-1,4-
trans-1,4-
1,2-
b
b
Entry
Cat.
Bd/Co
Al/Co
Conv. (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
2
3
4
5
6
2000
3000
4000
2000
2000
2000
2000
2000
2000
80
80
80
40
40
40
40
40
40
95.0
91.5
86.9
92.6
100
0.98
1.16
1.37
0.85
0.93
0.73
0.72
0.70
0.88
3.22
3.73
3.92
3.29
4.56
3.31
3.15
3.24
2.71
87.3
86.9
87.3
87.0
85.8
86.0
86.2
85.8
81.3
7.2
6.6
5.6
7.1
7.4
7.3
7.4
5.9
5.1
5.5
6.5
7.1
5.9
6.8
6.7
6.4
8.3
13.6
100
90.9
83.9
49.0
a
Polymerization conditions: cat.: 5 μmol; cocat.: EASC; dispersion medium: isooctane, total volume: 15 mL; reaction temperature: 25 °C;
reaction time: 60 min. Determined by GPC against polystyrene standards and reported uncorrected. Determined by 1H and 13C NMR
b
c
spectroscopy.
corresponding conversion in toluene. A consistent regularity
was achieved when compared to the results in toluene under
similar reaction conditions (in the orders of Cy > Ph and
nPr > Et > Me > H). The CoCl2(PCyPh2)2 (2)– and CoCl2(PnPrPh2)2
(3)–EASC systems also yielded 100% conversions of butadiene
although the PBDs obtained had lower molecular weights and
cis-1,4 contents (entries 5 and 6 in Table 2). Different from
solution polymerization in toluene, a small amount of insolu-
ble gels formed during the polymerization in isooctane, which
probably can be attributed to the concentrated active species
adhering onto the reactor walls because of their poor solubil-
ity in isooctane. In comparison with solution polymerization
in toluene, it is clearly noted that the molecular weight of
PBDs (ca. 104 g mol−1) decreased in isooctane. This behavior
could be attributed to the fact of the chain transfer reaction is
more favorable than propagation in isooctane, which is supported
by the broader molecular weight distribution (2.71–4.56).
Moreover, the microstructure of PBDs has large differences:
the cis-1,4 content decreased to 81.3–87.3% and the amounts
of both trans-1,4 and 1,2 isomers increased to some extent.
In isooctane, the newly inserted monomer molecule is proba-
bly more favourable to tend to coordinate with only one dou-
ble bond (1,2-insertion), decreasing the cis-1,4 content,
because isooctane has a lower solubility parameter than tolu-
ene (δ = 8.90 (cal cm−3 1/2
). A similar regularity can be found
)
in a neodymium catalyst system in cyclohexane–hexane.54
Polymerization of butadiene in mixed solvents
In order to avoid the formation of gels, the ageing of catalysts
proceeded in the mixture of toluene and isooctane to gener-
ate the homogenous solutions (Table 3). An important point
should be emphasized that the volume ratio of toluene and
isooctane in the ageing stage of catalysts must be equal to or
larger than 1.0 to ensure the homogenization of catalytic sys-
tems. When the volume ratio of toluene and isooctane was
controlled strictly at 1.0 in the ageing stage and the Bd/Co
molar ratio was increased from 2000 to 4000, the conversion
of butadiene slightly declined from 100% to 96.8% and the
molecular weight distribution of the products increased grad-
ually from 2.77 to 3.75 (entries 1–3 in Table 3). Keeping the
monomer concentration constant (Bd/Co
= 4000), the
increase in toluene volume from 2.5 to 5 mL resulted in the
higher molecular weight of products, but didn't greatly
change the catalytic activity and the microstructure of PBDs
(entries 3 and 4 in Table 3). Under the same reaction
Table 3 Polymerization of 1,3-butadiene with CoCl2(PRPh2)2 (1–6)–EASC in mixed solventsa
Microstructured (mol%)
T–I–B–Ib (mL)
Conv. (%)
Mn (104 g mol−1
)
Mw/Mn
cis-1,4-
trans-1,4-
1,2-
c
c
Entry
Cat.
Bd/Co
Al/Co
1
2
3
4
5
6
7
8
9
1
1
1
1
1
2
3
4
5
6
2000
3000
4000
4000
2000
2000
2000
2000
2000
2000
80
80
80
80
40
40
40
40
40
40
5/5/5
4/4/7
2.5/2.5/10
5/0/10
5/5/5
5/5/5
5/5/5
5/5/5
5/5/5
100
98.6
96.8
97.9
100
100
100
100
96.5
56.1
3.40
4.98
4.05
5.32
3.12
4.08
3.55
3.20
4.58
1.07
2.77
3.50
3.75
3.79
2.70
2.92
2.67
2.76
2.87
2.23
92.4
94.2
92.1
92.3
91.8
92.7
92.4
92.3
94.0
84.3
4.1
3.0
3.6
3.5
4.4
3.9
4.1
3.9
3.4
2.9
3.5
2.8
4.3
4.2
3.8
3.4
3.5
3.8
2.6
12.8
10
5/5/5
a
Polymerization conditions: cat.: 5 μmol; cocat.: EASC; solvents: mixture of toluene and isooctane, total volume: 15 mL; reaction temperature:
25 °C; reaction time: 60 min. T–I–B–I = toluene–isooctane–butadiene solution in isooctane ([Bd] = 2.12 mol L−1). Determined by GPC against
b
c
polystyrene standards and reported uncorrected. d Determined by 1H and 13C NMR spectroscopy.
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conditions (entries 5–10 in Table 3), complexes 1–4–EASC sys-
tems obtained the complete conversions of butadiene in 60 min
and the 6–EASC system bearing the less bulky H group
on the phosphine ligand gained the lower catalytic activity
(conv.% = 56.1%). For all the cases of the mixed solvents, the
polymerization systems became muddy with a small amount
of powders and no gel was formed. The molecular weights
and cis-1,4 content of the resultant polymers fell in between
those in toluene and isooctane.
Comparison of butadiene polymerization in toluene,
isooctane and mixed solvents
Comparing the results obtained by the 1–EASC catalytic sys-
tem in different solvents under similar conditions, wherein
the higher concentrations of monomer and cocatalyst
(Bd/Co = 4000 and Al/Co = 80) were used, the conversion of
butadiene, molecular weight and cis-1,4 content of the
resulting polymers declined gradually as the volume fraction
of isooctane increased in the solvents (entries 3 in Table 1, 3
and 4 in Table 3, and 3 in Table 2). When the butadiene
concentration and the amount of cocatalyst were reduced by
half (Bd/Co = 2000 and Al/Co = 40) simultaneously, the con-
version of butadiene also reached 100% in the mixed sol-
vents and slightly decreased to 92.6% in isooctane, although
both the molecular weight and cis-1,4 content of PBDs
decreased gradually (entries 5 in Table 1, 5 in Table 3 and 4
Fig. 3 The 13C NMR spectra of PBDs obtained by the 1–EASC catalytic
system in toluene (a, entry 3 in Table 1), mixed solvents (b, entry 4 in
Table 3) and isooctane (c, entry 3 in Table 2).
1
in Table 2). The H and 13C NMR spectra of PBDs obtained
by the 1–EASC catalytic system in different solvents are
shown in Fig. 2 and 3, respectively.
For complexes 1–4 having higher catalytic activity, the
composition of solvents didn't largely affect the conversion
of butadiene; however, as for complexes 5 and 6 with less
bulky R groups (R = Me, H), the great influence of the isooc-
tane fraction on the conversion of butadiene was observed
(Fig. 4). The highest catalytic activity was obtained in solu-
tion polymerization; on the contrary, the heterogeneous
Fig. 4 The conversion of butadiene obtained by the CoCl2(PRPh2)2
(1–6)–EASC systems in different solvents. Polymerization conditions:
5 μmol of Co, Bd/Co = 2000, Al/Co = 40, T = 25 °C, t = 60 min,
Vtotal = 15 mL.
polymerization in isooctane gave the lowest catalytic activity
under similar conditions.
Comparing the properties of PBDs produced by com-
plexes 1–6–EASC systems in different solvents, it was found
that the cis-1,4 content and molecular weight of PBDs
decreased gradually when isooctane was introduced into the
polymerization systems (Fig. 5). The PBDs with higher
molecular weight (5.25–11.3 × 104 g mol−1) were obtained in
the homogeneous solution polymerization in toluene; while
the heterogeneous polymerization in isooctane produced the
PBDs with the lower molecular weight (0.70–0.93 × 104 g mol−1).
The molecular weight (1.07–4.58 × 104 g mol−1) of polymers
obtained in the mixed solvents fell in between those above.
The addition of isooctane into the reaction systems resulted
in the decrease in cis-1,4 content and the increase in both
trans-1,4 and 1,2-vinyl isomers for each complex. The PBDs
Fig. 2 The 1H NMR spectra of PBDs obtained by the 1–EASC catalytic
system in toluene (a, entry 3 in Table 1), mixed solvents (b, entry 4 in
Table 3) and isooctane (c, entry 3 in Table 2).
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Catalysis Science & Technology
CoCl2·6H2O was obtained from Beijing Chemical Regents
Co., Ltd. and dehydrated prior to use. All the phosphine com-
pounds [triphenylphosphine (PPh3), Sinopharm Chemical
Reagent Co., Ltd.; cyclohexyldiphenylphosphine (PCyPh2),
Aladdin Chemistry Co., Ltd.; propyldiphenylphosphine (PnPrPh2)
and ethyldiphenylphosphine (PEtPh2), Tokyo Chemical Indus-
try Co., Ltd.; and methyldiphenylphosphine (PMePh2) and
diphenylphosphine (HPPh2), J&K Scientific Ltd.) were used
without further purification. Ethylaluminum sesquichloride
(EASC, 0.4 M in hexane) was purchased from Acros Chemicals
and used directly without further purification. Toluene and
isooctane were refluxed over sodium-benzophenone and
distilled under nitrogen prior to use. Polymerization grade
butadiene was purified by passing it through columns of
KOH and molecular sieves. All other chemicals were obtained
commercially and used without further purification unless
otherwise stated.
Fig. 5 The cis-1,4 content and molecular weight of PBDs produced
by the CoCl2(PRPh2)2 (1–6)–EASC systems in different solvents.
Polymerization conditions: 5 μmol of Co, Bd/Co = 2000, Al/Co = 40,
T = 25 °C, t = 60 min, Vtotal = 15 mL.
The phosphine-containing cobalt dichloride complexes,
CoCl2(PRPh2)2 (R = Ph (1), Cy (2), nPr (3), Et (4), Me (5),
H (6)), were prepared from the corresponding phosphine
compound and CoCl2 in degassed ethanol in high yields
according to the literature.18–20
obtained by the 6 (R = H)–EASC system contained a larger
amount of 1,2-isomers than those by other complexes.
Conclusions
In summary, a series of phosphine-containing cobalt com-
plexes, CoCl2(PRPh2)2 (R = Ph, Cy, nPr, Et, Me, H), was
proven to be highly active and stereospecific in solution
polymerization of 1,3-butadiene in toluene on treatment
with ethylaluminum sesquichloride (EASC). The small
amount of cocatalyst (Al/Co = 40) could induce the com-
plete conversion of butadiene and the production of PBDs
with high cis-1,4 content. The bulkier R groups on the
phosphine ligands led to higher catalytic activities. The dis-
persion medium was an important factor influencing the
catalytic activity and the properties of the resulting poly-
mers. The heterogeneous polymerization in isooctane
yielded relatively lower catalytic activity, cis-1,4 content and
molecular weight of PBDs under similar reaction condi-
tions. The mixture of toluene and isooctane as a dispersion
medium could avoid the formation of insoluble gels and
the molecular weight and cis-1,4 content of PBDs fell in
between those in toluene and isooctane.
Polymerization of 1,3-butadiene
The solution polymerizations of 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 firstly charged with the desired amounts of precatalyst
and cocatalyst solutions. The mixture was stirred for 2 min at
various temperatures, followed by the addition of butadiene
solution in toluene to start the polymerization. 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.
The polymerizations of butadiene in isooctane were car-
ried out in the same way except that toluene was replaced by
isooctane.
The polymerizations of butadiene in the mixed solvents
were conducted in the same way except for the ageing stage
of catalysts. The reactor was firstly charged with the desired
amounts of precatalyst and cocatalyst in toluene and stirred
for 2 min, followed by the addition of the desired amount of
isooctane to form a homogenous active solution. Finally the
butadiene solution in isooctane was injected into the reactor
using a syringe to start the polymerization.
Experimental
Methods and materials
All manipulations of air- or moisture-sensitive compounds
were carried out under an atmosphere of nitrogen using
standard Schlenk techniques. The molecular weight and
molecular weight distribution of polybutadienes were mea-
sured by GPC using a Waters 2414 series system in THF at
25 °C calibrated with polystyrene standards. The micro-
structure of polybutadienes was determined from 1H NMR
and 13C NMR spectra recorded using a Bruker DMX-400
instrument in CDCl3 with TMS as the internal standard at
room temperature.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant no. 21006085) and the National
High Technology Research and Development Program of
China (grant no. 2012AA040305).
778 | Catal. Sci. Technol., 2014, 4, 773–779
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Paper
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