Syntheses of chromium(III) complexes with Schiff-base ligands and their catalytic behaviors for ethylene polymerization

Article abstract of DOI:10.1016/j.molcata.2005.08.060

A series of chromium(III) complexes LCrCl₃ (4a-c) bearing chelating 2,2′-iminodiphenylsulfide ligands [L = (2-ArMeCNAr) ₂S] was synthesized in good yields from the corresponding ligands and CrCl₃·(THF). Using modified methylaluminoxane (MMAO) as a cocatalyst, these complexes display moderate activities towards ethylene polymerization, and produce highly linear polyethylenes with broad molecular weight distribution. Polymer yields, catalyst activities and the molecular weights, as well as the molecular weight distributions of the polymers can be controlled over a wide range by the variation of the structures of the chromium(III) complexes and the polymerization parameters, such as Al/Cr molar ratio, reaction temperature and ethylene pressure.

Full text of DOI:10.1016/j.molcata.2005.08.060

Journal of Molecular Catalysis A: Chemical 244 (2006) 99–104
Syntheses of chromium(III) complexes with Schiff-base ligands
and their catalytic behaviors for ethylene polymerization
Jing-Yu Liua, Yue-Sheng Lia,∗, Jing-Yao Liub, Ze-Sheng Lib
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
b Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and Computational Chemistry,
Jilin University, Changchun 130023, China
Received 1 October 2004; received in revised form 20 July 2005; accepted 30 August 2005
Available online 10 October 2005
Abstract
A series of chromium(III) complexes LCrCl3 (4a–c) bearing chelating 2,2ꢀ-iminodiphenylsulfide ligands [L = (2-ArMeC NAr)2S] was syn-
thesized in good yields from the corresponding ligands and CrCl3·(THF). Using modified methylaluminoxane (MMAO) as a cocatalyst, these
complexes display moderate activities towards ethylene polymerization, and produce highly linear polyethylenes with broad molecular weight
distribution. Polymer yields, catalyst activities and the molecular weights, as well as the molecular weight distributions of the polymers can be
controlled over a wide range by the variation of the structures of the chromium(III) complexes and the polymerization parameters, such as Al/Cr
molar ratio, reaction temperature and ethylene pressure.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Chromium complex; Catalysts; Ziegler–Natta polymerization; Ethylene
1. Introduction
merization since they may allow an even greater control over
the properties of the resultant polymers. Several new chromium
catalyst families have been described in the recent literatures
[21–30]. For example, the di(organoimido)chromium(VI) com-
plexes (RN)2CrX2 (X Cl, Me, CH2Ph) have been proved to
be excellent precatalysts for ethylene polymerization [21–23].
This prompts us to search new chromium catalysts for olefin
polymerization. Here, we describe the synthesis, characteri-
zation and ethylene polymerization behavior of a series of
novel chromium(III) complexes [LCrCl3] bearing chelating
2,2ꢀ-iminodiphenylsulfide ligands. We also report the effects of
the reaction conditions, such as cocatalyst, Al/Cr molar ratio, the
temperature and pressure of the polymerization on the catalytic
behaviors of the chromium(III) precatalysts towards ethylene
polymerization.
Numerous catalysts have been developed for the production
of polyolefins since the late 1950s. A large fraction of the cata-
lysts, especially the industrially relevant ones, are heterogeneous
systems prepared by the deposition of soluble Ti-, Zr- or Cr-
containing precursors on high surface area supports, such as
silica, alumina or magnesium chloride. Among these transition
metals that catalyze the polymerization of olefins, chromium
occupies a prominent position [1–17].
In general, two classes of chromium-based catalysts are used
commercially, namely the so-called Phillips catalyst [18] and the
Union Carbide catalyst [19,20]. The Phillips catalyst is essen-
tially composed of a chromium oxide and an inorganic carrier
like SiO2, SiO2 Al2O3, etc., while the Union Carbide catalyst
is formed by treatment of silica with low-valent organometallic
compound, most notably chromocene (Cp2Cr).
2. Experimental
There is an increasing interest in the development of the
novel non-metallocene chromium catalysts for ethylene poly-
All manipulations of water- and/or moisture-sensitive com-
pounds were performed by means of standard high vacuum
Schlenk and cannula techniques under a N2 atmosphere. Toluene
was refluxed and distilled from sodium/benzophenone under
∗
Corresponding author. Tel.: +86 431 5262124; fax: +86 431 5685653.
E-mail address: ysli@ciac.jl.cn (Y.-S. Li).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.08.060

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J.-Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 99–104
6H, Ar H), 1.99 (s, 6H, CH3). EI–MS: m/z = 420 [M+]. Anal.
Calcd for C28H24N2S: C, 79.96; H, 5.75; N, 6.66%. Found: C,
80.06; H, 5.71; N, 6.60%.
dry nitrogen. Modified methylaluminoxane (MMAO) (7% alu-
minum in heptane solution), diethylaluminum chloride, ethy-
laluminum dichloride, triisobutylaluminium, triethylaluminum
and trimethylaluminum were purchased from Akzo Nobel
Chemical Inc.
2.1.4. 2,2ꢀ-Iminodiphenylsulfide ligand 3b
The NMR data of the ligands were obtained on a Bruker
300 MHz spectrometer at ambient temperature, CDCl3 or
DMSO as solvent. The NMR data of the polymers were obtained
on a Varian Unity-400 MHz spectrometer at 110 ◦C with o-
C6D4Cl2 as the solvent. Mass spectra were obtained using
electron impact (EI–MS). Elemental analyses were obtained
using Carlo Erba 1106 and ST02 apparatus. The IR spectra were
recorded on a Bio-Rad FTS-135 spectrophotometer. The DSC
measurements were performed on a Perkin-Elmer Pyris 1 differ-
ential scanning calorimeter at a rate of 10 ◦C/min. The molecular
weights and the molecular weight distributions of the polymer
samples were determined at 150 ◦C by a PL-GPC 220 type high-
temperature chromatograph equipped with three PLgel 10 ␮m
Mixed-B LS type columns. 1,2,4-Trichlorobenzene (TCB) was
employed as the solvent at a flow rate of 1.0 mL/min. The cal-
ibration was made by polystyrene standard EasiCal PS-1 (PL
Ltd.).
Using the procedure described above, we obtained ligand 3b
in 70% yield. H NMR (CDCl3): δ 7.83 (d, 2H, Ar H), 7.49
(d, 6H, Ar H), 7.04 (m, 8H, Ar H), 2.35 (s, 6H, Ar CH3),
2.23 (s, 6H, CH3). EI–MS: m/z = 448 [M+]. Anal. Calcd for
C28H24N2S: C, 80.32; H, 6.29; N, 6.24%. Found: C, 80.38; H,
6.34; N, 6.19%.
1
2.1.5. 2,2ꢀ-Iminodiphenylsulfide ligand 3c
Using the procedure described above, we obtained ligand 3c
1
in 62% yield. H NMR (CDCl3): δ 7.82 (d, 2H, Ar H), 7.46
(d, 6H, Ar H), 7.02 (m, 8H, Ar H), 2.33 (s, 6H, Ar CH3),
2.20 (s, 6H, CH3). EI–MS: m/z = 448 [M+]. Anal. Calcd for
C28H24N2S: C, 80.32; H, 6.29; N, 6.24%. Found: C, 80.26; H,
6.27; N, 6.26%.
2.1.6. 2,2ꢀ-Iminodiphenylsulfide Cr(III) complex 4a
Ligand 3a 0.72 g (1.72 mmol) and CrCl3 (THF)3 0.27 g
(1.72 mmol) were combined in a Schlenk flask under an Argon
atmosphere. Then 50 mL of THF was added with a syringe.
The mixture was stirred at room temperature for 10 h. 4a was
isolated as a green powder in 84% yield. The chromium com-
plexes 4b and 4c were prepared by the same procedure with
similar yields. EI–MS (70 eV): m/z = 578 [M+]. Anal. Calcd for
C28H24Cl3CrN2S:C, 58.09;H, 4.18;N, 4.84%. Found:C, 58.16;
H, 4.11; N, 4.89%.
2.1. Synthesis of ligands and complexes
2.1.1. 2,2ꢀ-Dinitro diphenyl thioether 1
To a solution of 42 mmol of o-chloronitrobenzene in toluene,
10 mL of aqueous Na2S (2 M) and a catalytic amount of tetra-
butylammonium bromide were given. The mixture was refluxed
overnight. Upon cooling to room temperature, the product crys-
tallized from solution. The solid was recrystallized from ethanol
to yield 3.48 g of 1 as yellow crystals (63%). 1H NMR (DMSO-
d6): δ 8.12 (dd, 2H, Ar H), 7.51 (m, 4H, Ar H), 7.30 (dd, 2H,
Ar H).
2.1.7. 2,2ꢀ-Iminodiphenylsulfide Cr(III) complex 4b
Yield, 87%. EI–MS (70 eV): m/z = 606 [M+]. Anal. Calcd
for C30H28Cl3CrN2S: C, 59.36; H, 4.65; N, 4.62%. Found: C,
59.31; H, 4.62; N, 4.65%.
2.1.2. 2,2ꢀ-Diamino diphenyl thioether 2
A 250 mL Schlenk flask containing 150 mL of degassed
ethanol was charged with 1.38 g (5 mmol) of 1, 1 g of Pd C
catalyst (5% Pd), and hydrogen. After the mixture was stirred at
35 ◦C for 2 days, Pd C was removed by filtration, and the sol-
vent was evaporated. The residue was recrystallized in ethanol
to give 2,2ꢀ-diamino diphenyl thioether 2 as a white crystal
2.1.8. 2,2ꢀ-Iminodiphenylsulfide Cr(III) complex 4c
Yield, 89%. EI–MS (70 eV): m/z = 606 [M+]. Anal. Calcd
for C30H28Cl3CrN2S: C, 59.36; H, 4.65; N, 4.62%. Found: C,
59.40; H, 4.60; N, 4.58%.
2.2. General procedures for ethylene polymerization
1
in 92% yield (9.96 g). H NMR (DMSO-d6): δ 7.20 (dd, 2H,
Ar H), 7.11 (m, 2H, Ar H), 6.70 (m, 4H, Ar H), 4.19 (s, 2H,
NH2).
The polymerization was carried out in a 200 mL Schlenk flask
equipped with a magnetic stirrer. The flask was repeatedly evac-
uated and refilled with nitrogen, and finally filled with ethylene
gas (ambient pressure) from Schlenk line. MMAO and toluene
were added via a gastight syringe. The catalyst, dissolved in
toluene under a dry nitrogen atmosphere, was transferred into
the Schlenk flask to initiate the polymerization. After polymer-
ization for 30 min at given temperature, all the polymerization
experiments were stopped by a large excess of methanol con-
taining a small amount of hydrochloric acid. The coagulated
polymer was washed with methanol, filtered and dried under
vacuum.
2.1.3. 2,2ꢀ-Iminodiphenylsulfide ligand 3a
Two equivalents of phenyl methyl ketone (6 g, 0.05 mol) with
one equivalent of 2 (5.4 g, 0.025 mol) were dissolved in 50 mL
methyl alcohol. After the addition of a few drops of glacial
acetic acid, the solution was refluxed overnight. Upon cooling
to room temperature, the mixture was poured into petroleum
ether. The crude product was recrystallized in ethanol to give
2,2ꢀ-imindodiphenylsulfide ligand 3a as a yellow crystal in 75%
yield (7.9 g). The other ligands 3b and 3c were prepared by the
same procedure with similar yields. 1H NMR (CDCl3): δ 7.82
(d, 2H, Ar H), 7.40 (m, 6H, Ar H), 6.96 (t, 4H, Ar H), 6.68 (m,
High-pressure polymerization experiments were carried out
inamechanicallystirred200 mLstainlesssteelreactor, equipped

J.-Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 99–104
101
with an electric heating mantle controlled by a thermocouple
dipping into the reaction mixture. The reactor was baked under
nitrogen flow for 24 h at 150 ◦C and subsequently cooled to
the temperature of polymerization. The reagents were trans-
ferred via a gastight syringe to the evacuated reactor. Ethylene
was introduced into the reactor, and the reactor pressure was
maintained at given pressure throughout the polymerization run
by continuously feeding the ethylene gas. After proceeding for
30 min, the polymerization was stopped by turning the ethy-
lene off and relieving the pressure. The reaction mixture was
poured into a solution of HCl/ethanol (10 vol%) to precipitate
the polymer. The polymer was isolated by filtration, washed with
ethanol, and dried under vacuum.
Fig. 1. Molecular structure of complex 4a.
(DFT) calculations are performed on Origin 3900 workstation
by DMol3 module of the Material Studio Modeling software
package, which prove to obtain high accuracy while keeping the
computational cost fairly low for an ab initio method. The struc-
ture optimized at the BLYP/DND level is shown in Fig. 1. The
geometryofthesix-memberaroundthechromiumatomcouldbe
described as a distorted octahedron. In order to characterize the
nature of the optimum structure, we also calculate the harmonic
vibrational frequencies by normal-mode analysis at the same
level. The optimum complex corresponds to all real frequen-
cies, indicative of a stationary point. The total energy, binding
energy, and zero-point energy for the optimum complex 4a are
−3056.8182408, −10.5471239 and 0.435 au, respectively.
3. Results and discussion
3.1. Synthesis and characterization of complexes
A general synthetic route for chromium complexes 4a–c
bearing chelating 2,2ꢀ-iminodiphenylsulfide ligands used in this
study is shown in Scheme 1. The tridentate ligands 3a–c were
prepared via the condensation reaction of two equivalents of
the corresponding methyl aryl ketone with one equivalent of
2,2ꢀ-iminodiphenylsulfide (yields: 3a, 75%; 3b, 70%; 3c, 62%).
The chromium complexes 4a–c were synthesized in good yield
(yields: 4a, 84%; 4b, 87%; 4c, 89%) via the complexation
reaction of CrCl3·(THF) with corresponding ligands 3a–c in
tetrahydrofuran (THF) at room temperature. All the chromium
complexes are green in color.
3.2. Catalysis
The characterization of these complexes was difficult because
of their poor solubility in organic solvents and the fact that
they are paramagnetic, thus high-resolution NMR spectroscopic
analysis is impossible. We predict theoretically the molecular
model of 4a using Becke’s nonlocal-exchange functional with
the nonlocal correlation functional of Lee–Yang–Parr (BLYP)
functional with DND basis set. The density functional theory
The initial attempts to polymerize ethylene with chromium
complexes 4a–c were unsuccessful. These complexes were not
active as a single component catalyst, and only low catalytic
activities (less than 10 kg PE/molCr·h) were observed when
diethylaluminum chloride or ethylaluminum dichloride was
used as a cocatalyst. However, there were remarkable increases
Scheme 1. Synthesis of chromium(III) 2,2ꢀ-iminodiphenylsulfide complexes.

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J.-Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 99–104
Fig. 2. 13C NMR spectrum of polyethylene prepared with complex 4a.
in catalytic activities (in the range 30–50 kg PE/molCr·h) while
triisobutylaluminium, triethylaluminum or trimethylaluminum
was used as a cocatalyst. It is interesting that chromium com-
plexes 4a–c displayed moderate activities towards ethylene
polymerization (more than 70 kg PE/molCr·h) upon activa-
tion with MMAO, and converted ethylene into highly linear
polyethylene (Fig. 2).
Fig. 4. Effect of temperature on the catalytic activity of complexes 4a (᭹), 4b
(ꢀ) and 4c (ꢁ). Pressure of ethylene, 1 atm; time of polymerization, 30 min;
Al/Cr = 1200 (molar ratio); cocatalyst, MMAO.
The conditions of the polymerizations will influence the cat-
alyst activities and the properties of the polymers. To examine
these effects, we undertook a series of polymerization experi-
ments under various conditions. The effect of the Al/Cr molar
ratio on the polymerization reaction was firstly investigated. The
polymerizations of ethylene were performed with complexes
4a–cat25 ◦C, varyingtheAl/Crmolarratiofrom800to2400. As
shown in Fig. 3, the catalyst activities increase with an increase
in the Al/Cr molar ratio from 800 to 1200, nevertheless, further
increase of the Al/Cr molar ratio leads to only a slight improve-
ment of catalyst activities.
The strong temperature dependence of activity is a common
characteristic of Ziegler–Natta catalysts due to the high activa-
tionenergyrequiredforpolymerizationprocess. Forthedifferent
kinds of catalysts, however, we would observe maximum val-
ues at different temperatures. For chromium complexes 4a–c,
the catalytic activities towards ethylene polymerization increase
gradually to maximum values and then decrease gradually with
the increase of reaction temperature. Under atmospheric pres-
sure of ethylene, the optimum reaction temperature is ca. 25 ◦C,
as shown in Fig. 4. This behavior is intrinsically related to the
unstability of the catalyst at higher polymerization temperatures.
The results of polymerizations performed under different
pressures (1, 5 and 10 atm) employing complexes 4a–c are sum-
marized in Table 1. The data listed in Table 1 indicate that
the ethylene pressure considerably influences the yields and the
molecular weights (MWs) as well as the molecular weight dis-
tributions (MWDs) of the resultant polymers. First, significant
increases in the productivities of the catalysts were observed
(4a: from 72 kg PE/molCr·h at 1 atm to 146 kg PE/molCr·h at
5 atm;4b:from100 kgPE/molCr·hat1 atmto188 kgPE/molCr·h
at 5 atm; 4c: and from 88 kg PE/molCr·h at 1 atm to 162 kg
PE/molCr·h at 5 atm). Second, changes in MWs and MWDs of
the resulted polymers could be observed in Fig. 5, which shows
two GPC traces for the polymerization tests with 4a/MMAO
catalytic system under different pressures. Employing precat-
alyst 4a at 1 atm ethylene with 1200 equiv MMAO (Entry 1),
the MWD of PE displays clearly bimodal distribution and the
low molecular weight fraction is dominant. On increasing the
pressure to 5 atm (Entry 4), a unimodal distribution is observed.
The similar trends can be observed using complexes 4b and 4c.
This indicates that increasing ethylene pressure could control
␤-H elimination reaction during the polymerization of ethylene
with 4a–c/MMAO systems.
It is noteworthy that the 4a–c/MMAO systems also exhibit
high catalytic activities towards ethylene polymerization at
high temperature (50 ◦C) in the case of high pressure (5 atm).
Moreover, high molecular weight polyethylenes can be easily
obtained under this condition (Entries 10–12).
The data listed in Table 1 show that the structure of the lig-
ands considerably affects the performances of the precatalysts
and the properties of the polymer produced. For example, the
replacement of the proton at R1 position (4a) with methyl (4b)
Fig. 3. Effect of Al/Cr molar ratio on the catalytic activity of complexes 4a (᭹),
4b (ꢀ) and 4c (ꢁ). Pressure of ethylene, 1 atm; time of polymerization, 30 min;
temperature, 25 ◦C; cocatalyst, MMAO.

J.-Y. Liu et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 99–104
103
Table 1
Results of ethylene polymerization by complexes 4a–c activated with MMAOa
Temperature (◦C)
Pressure (atm)
Yield (g)
Activity (kg PE/molCr·h atm)
Mw (kg/mol)
Mw/Mn
b
b
c
Tm (◦C)
Entry
Catalyst
1
2
3
4
5
6
7
8
9
4a
4b
4c
4a
4b
4c
4a
4b
4c
4a
4b
4c
25
25
25
25
25
25
25
25
25
50
50
50
1
1
1
5
5
0.18
0.25
0.22
1.83
2.35
2.03
2.52
3.58
3.24
2.54
3.51
3.11
72
100
88
60.5
72.7
70.1
234
268
261
339
389
378
288
346
317
61
67
66
127.3
128.6
128.1
135.6
136.9
136.2
138.3
139.8
139.5
137.3
138.6
138.1
146
188
162
101
143
130
102
140
124
12.8
13.5
13.0
9.6
10.1
9.8
9.3
9.8
9.6
5
10
10
10
10
10
10
10
11
12
a
Polymerization conditions: 5 ␮mol catalyst, Al/Cr = 1200 (molar ratio), polymerization for 30 min.
Mw and Mw/Mn were determined by GPC vs. polystyrene standards, uncorrected.
Tm, melting temperatures were determined by means of DSC with a heating rate of 10 ◦C/min in nitrogen.
b
c
increase of the steric hindrance of the ligand can result in the
enhancement of catalyst activity and the molecular weight of the
polymer prepared. The polyethylenes obtained under an atmo-
spheric pressure display relatively low Tm (less than 129 ◦C)
and bimodal distributions. The increase of ethylene pressure not
only increases the molecular weight and Tm (up to 140 ◦C), but
also decreases the molecular weight distribution of the polymers
by controlling chain transfer reaction.
Acknowledgements
The authors are grateful for the financial support by the
National Natural Science Foundation of China and SINOPEC
(No. 20334030).
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