Ethylene polymerization by the chromium catalysts based on bidentate [O, P=O] or [S, P] ligands

Article abstract of DOI:10.1002/pola.23785

Novel chromium catalysts based on bidentate phenoxy- phosphinoyl (HO-2R₁-4R₂-6(Ph₂P=O)C6H2: R1 = R2 = H, 3a; R₁ = tBu, R₂ = H, 3b; R₁ = R₂ = tBu, 3c; R₁ = R₂

Full text of DOI:10.1002/pola.23785

Ethylene Polymerization by the Chromium Catalysts
Based on Bidentate [O, P¼¼O] or [S, P] Ligands
LI-PENG HE,1,2 HONG-LIANG MU,1,2 BAI-XIANG LI,1 YUE-SHENG LI1
1State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
2Graduate School of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
Received 30 August 2009; accepted 11 October 2009
DOI: 10.1002/pola.23785
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Novel chromium catalysts based on bidentate phe-
noxy-phosphinoyl (HO-2R1-4R2-6(Ph2P¼O)C6H2: R1 ¼ R2 ¼ H,
3a; R1 ¼ tBu, R2 ¼ H, 3b; R1 ¼ R2 ¼ tBu, 3c; R1 ¼ R2 ¼ cumyl,
3d; R1 ¼ anthracenyl, R2 ¼ H, 3e) and thiophenol-phosphine
(HS-2R1-4R2-6(Ph2P)C6H2: R1 ¼ R2 ¼ H, 4a; R1 ¼ SiMe3, R2 ¼ H,
4b) were prepared and characterized. Treatment with modified
methyaluminoxane, these catalysts displayed moderate to
high-catalytic activities for ethylene polymerization. The activ-
ities of them were higher than those of the corresponding cata-
lysts based on bidentate phenoxy-phosphine ligands. Both the
coordinated donors and the ortho-substituent of the ligands
played an important role in improving catalytic activity. The
effects of reaction parameters, such as cocatalyst and Al/Cr
molar ratio as well as reaction temperature, on ethylene poly-
merization behaviors were investigated in detail for two favor-
able catalytic systems, 3b/CrCl3(thf)3 and 4b/CrCl3(thf)3. Catalyst
4b/CrCl3(thf)3 displayed higher catalytic activity and better tem-
perature tolerance for ethylene polymerization than 3b/
C
V
CrCl3(thf)3.
2009 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 48: 311–319, 2010
KEYWORDS: catalysis; polyolefins; Ziegler-Natta polymerization
INTRODUCTION The design and synthesis of novel transition
metal catalysts for olefin polymerization has been one of the
most attractive subjects in the field of polymer chemistry.1–3
Chromium catalysts have played a crucial role due to the wide-
spread commercial application of silica-supported Phillips cata-
lysts.4–9 In more recent years, a significant amount of attention
has been given to the development of homogeneous nonmetallo-
cene chromium catalysts. A number of neutral and anionic
ligands were used to stabilize chromium complexes, and some
of them have already been shown to afford highly active olefin
polymerization catalysts.10–19 The ligands containing relatively
hard nitrogen and oxygen donors have featured in the reported
chromium catalysts. For example, chromium catalysts based on
monoanionic bidentate phenoxy-amide [O, N] ligands displayed
high activities toward ethylene polymerization, and the triden-
tate phenoxy-amide chromium catalysts containing additional
coordinated nitrogen donor [O, N, N] generated low-molecular
weight linear polyethylene.12(a,b) A family of air-stable chromium
catalysts bearing bis(imino)pyridine [N, N, N] displayed high ac-
tivity and promising temperature tolerance toward ethylene po-
lymerization, and afforded linear polyethylene.13 Bis(benzimida-
zolyl)pyridine [N, N, N] chromium catalysts also showed high
activities toward ethylene oligomerization and polymeriza-
tion.14(a) Recent researches have suggested that there may be a
surprising advantage using the ligands bearing soft phosphorus
or sulfur donor. For example, the phosphorus-bridged
bisphenoxy chromium complexes were found to be highly active
toward ethylene polymerization,16 and aminebis(phosphine)
chromium complexes selectively trimerized ethylene to 1-hexe-
ne.17(a) Our previous research also indicated that introducing an
additional soft donor pendant in the amino-pyrrolide ligands
can stabilize the chromium catalysts and increase the activity to-
ward ethylene polymerization.18(b)
Monoanionic phenoxy-phosphine containing hard oxygen and
soft phosphorus donors has been one of the most attracted
ligands to have emerged in olefin polymerization catalysis over
the past years. Although encouraging olefin polymerization
results have been obtained by bis(phenoxy-phosphine), Group
4 catalysts,20 the chromium catalysts based on phenoxy-
phosphine ligands only exhibited low-catalytic activity.12(b) Pre-
viously, we reported a new class of bisphenoxy ligands [O, P¼¼O,
O] based titanium complexes.21 We found the neutral P¼¼O do-
nor can improve the catalytic activity toward ethylene polymer-
ization. This result prompted us to introduce the neutral P¼¼O
donor into the monoanionic phenoxide ligand anchored chro-
mium catalysts. We hope that the neutral P¼¼O donor could be
beneficial for stabilizing chromium catalysts. In addition, previ-
ous researches also showed that introducing an extra soft sulfur
donor into amino-pyrrolide ligand can improve the performance
of the chromium catalyst toward ethylene polymerization.18(b)
Although favorable results have been obtained using chromium
complexes that contain soft phosphorus or sulfur donor, catalyst
Additional Supporting Information may be found in the online version of this article. Correspondence to: Y.-S. Li (E-mail: ysli@ciac.jl.cn)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 311–319 (2010) 2009 Wiley Periodicals, Inc.
ETHYLENE POLYMERIZATION BY CHROMIUM CATALYSTS, HE ET AL.
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system containing both phosphorus and sulfur donors in the
same ligand has yet to be reported. In this article, we reported
two new classes of effective bidentate phenoxy-phosphinoyl and
thiophenol-phosphine ligands for chromium catalysts for ethyl-
ene polymerization. The catalyst based on bidentate [S, P] ligand
displayed higher catalytic activity and better temperature toler-
ance for ethylene polymerization than the catalyst based on
bidentate [O, P¼¼O] ligand.
was chromatographed in a silica gel column rapidly to give
2a as a white solid (3.4 g, 59% yield).
1H NMR (CDCl3): d 6.26 (br, 1H, Ar-OH), 6.81–6.98 (m, 3H,
Ar-H), 7.28–7.37 (m, 11H, Ar-H). 13C NMR (CDCl3): d 114.5,
120.15, 127.6, 127.7, 127.9, 130.3, 132.2, 133.6, 134.0, 158.2
(d, J ¼ 18 Hz). Anal. Calc. for C18H15OP: C, 77.69; H, 5.43.
Found: C, 77.76; H, 5.39.
2-Tert-butyl-6-diphenylphosphanyl Phenol (2b)
Compound 2b was prepared via the same procedure in 63%
yield.
EXPERIMENTAL
General Procedure and Materials
All manipulation of air- and/or moisture-sensitive com-
pounds were carried out under a dry argon atmosphere by
using standard Schlenk techniques or under a dry argon
atmosphere in an MBraun glovebox unless otherwise noted.
All solvents were purified from an MBraun SPS system. The
NMR data of the ligands were obtained on a Bruker
300 MHz spectrometer (75 MHz for 13C) at ambient temper-
ature, with CDCl3 as a solvent. The 13C NMR data of the poly-
mers were obtained on a Varian Unity-400 MHz spectrome-
ter at 135 ꢀC, with O-C6D4Cl2 as a solvent. Elemental
analyses were recorded on an elemental Vario EL spectrome-
ter. The DSC measurements were performed on a Perkin-
Elmer Pyris 1 differential scanning calorimeter at a rate of
10 ꢀC/min. The weight-average molecular weight (Mw) and
the polydispersity index of polymer samples were deter-
mined at 150 ꢀC by a PL-GPC 220 type high-temperature
chromatograph equipped with three Plgel 10 lm Mixed-B LS
type columns. 1,2,4-Trichlorobenzene was employed as the
solvent at a flow rate of 1.0 mL/min. The calibration was
made by polystyrene standard EasiCal PS-1 (PL). The 2.5 M
n-butyllithium solution in hexane was purchased from Acros.
CrCl3(thf)3 was purchased from Aldrich. Modified methylalu-
minoxane (MMAO, 7% aluminum in heptane solution, 2 mol/
L) was purchased from Akzo Nobel Chemical. Commercial
ethylene was directly used for polymerization without fur-
ther purification. Tetrahydropyranyl (THP) ether 1a-e and
ligands 5a-b, 6, and 7 were synthesized according to the
reported methods.21,22 The other reagents and solvents were
commercially available.
1H NMR (CDCl3): d 1.41 (s, 9H, t-Bu), 6.82–6.86 (m, 2H, Ar-
H), 7.25–7.26 (m, 1H, Ar-H), 7.34–7.38 (m, 10H, Ar-H). 13C
NMR (CDCl3): d 30.1, 35.4, 120.9, 121.5, 129.0, 129.3, 129.5,
132.5, 133.7, 135.0, 136.5, 158.6 (d, J ¼ 20.3 Hz). Anal. Calc.
for C22H23OP: C, 79.02; H, 6.93. Found: C, 79.09; H, 6.98.
2,4-Di-tert-butyl-6-diphenylphosphanyl Phenol (2c)
Compound 2c was prepared via the same procedure in 66%
yield.
1H NMR (CDCl3): d 1.15 (s, 9H, t-Bu), 1.41 (s, 9H, t-Bu),
6.78–6.81 (m, 1H, Ar-H), 6.86–6.88 (m, 1H, Ar-H), 7.32–7.36
(m, 10H, Ar-H). 13C NMR (CDCl3): d 30.1, 31.9, 34.8, 35.6,
120.4, 126.7, 129.0, 129.3, 129.5, 129.7, 133.9, 135.8, 142.7,
156.6 (d, J ¼ 19.5 Hz). Anal. Calc. for C26H31OP: C, 79.97; H,
8.00. Found: C, 79.87; H, 8.05.
2,4-Di-cumyl-6-diphenylphosphanyl Phenol (2d)
Compound 2d was prepared via the same procedure in 61%
yield.
1H NMR (CDCl3): d 1.53 (s, 6H, cumyl-Me), 1.60 (s, 6H,
cumyl-Me), 6.54–6.57 (m, 1H, Ar-H), 7.13–7.33 (m, 21H, Ar-
H). 13C NMR (CDCl3): d 30.1, 31.3, 42.6, 43.1, 123.6, 126.2,
127.0, 128.4, 128.9, 129.2, 131.5, 134.1, 135.1, 136.8, 143.0,
149.4, 151.1, 154.6 (d, J ¼ 17.3 Hz). Anal. Calc. for
C36H35OP: C, 84.02; H, 6.85. Found: C, 79.97; H, 6.93.
2-Anthracenyl-6-diphenylphosphanyl Phenol (2e)
Compound 2e was prepared via the same procedure in 60%
yield.
1
1H NMR (CDCl3): H NMR (CDCl3): d 6.96–7.66 (m, 19H, Ar-H),
8.06 (d, J ¼ 8.1 Hz, 2H, Ar-H), 8.56 (s, 1H, Ar-H). 13C NMR
(CDCl3): d 121.4, 123.9, 124.6, 125.8, 126.5, 126.6, 128.3, 128.9,
129.0, 129.3, 130.4, 131.1, 131.9, 133.9, 134.1, 134.4, 136. 4,
156.2 (d, J ¼ 15.6 Hz). Anal. Calc. for C32H23OP: C, 84.56; H, 5.10.
Found: C, 84.48; H, 5.05.
Synthesis of Compounds 2a-e
2-Diphenylphosphanyl Phenol (2a)
To a solution of 1a (3.7 g, 21 mmol) in hexane (30 mL) was
added TMEDA (2.9 g, 25 mmol), n-butyllithium (2.5 M in
hexane, 10 mL, 25 mmol) at ꢁ10 ꢀC. The mixture was
allowed to warm to room temperature and stirred for 12 h,
giving a yellow precipitate. The appropriate PPh2Cl (5.5ꢀ g,
25 mmol) was added to the suspension dropwise at ꢁ10 C.
The mixture was stirred at room temperature for additional
12 h. To the mixture was added water, and extracted with
ether. The extract was evaporated under reduced pressure,
and then successively dissolved in thf (30 mL) and 5 M HCl
(3 mL) added. The mixture was stirred at room temperature
for 1 h. The product was neutralized with NaHCO3 to pH ¼
7, and then extracted with ether. The organic phase was
washed with water (2 ꢂ 50 mL), dried with Na2SO4, and
concentrated to give the crude product. The crude product
Synthesis of Compounds 3a-e
2-Diphenylphosphinoyl Phenol (3a)
Compound 3a (2.5 g, 9 mmol) was dissolved in thf (20 mL),
and two drops of H2O2 were added to the solution, then the
solution was stirred at room temperature for 3 h. The evapo-
ration of the solvent under reduced pressure yielded a crude
product, recrystallization from thf/hexane to give 3a as a
white solid (2.4 g, 91% yield).
1H NMR (CDCl3): d 6.80–6.86 (m, 1H, Ar-H), 6.95–7.05 (m,
2H, Ar-H), 7.38–7.45 (m, 1H, Ar-H), 7.47–7.53 (m, 4H, Ar-H),
7.54–7.62 (m, 2H, Ar-H), 7.65–7.73 (m, 4H, Ar-H). 13C NMR
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ARTICLE
(CDCl3): d 110.5, 112.0, 118.6, 119.1, 128.6, 131.1, 131.9,
132.1, 134.4, 163.9. Anal. Calc. for C18H15O2P: C, 73.46; H,
5.14; Found: C, 73.55; H, 5.09.
stirred for 2.5 h. The mixture was added dropwise to
CrCl3(thf)3 (0.63 g, 1.7 mmol) in dried thf (30 mL) at
ꢀ
ꢁ78 C with stirring over 30 min. The mixture was allowed
to warm to room temperature and stirred overnight. The
evaporation of the solvent in vacuum yielded a crude prod-
uct. The crude product was added 30 mL dried hexane and
the mixture was stirred for 10 min at 60 ꢀC and then fil-
2-Tert-butyl-6-diphenylphosphinoyl Phenol (3b)
Compound 3b was prepared via the same procedure in 93%
yield.
1H NMR (CDCl3): d 1.41 (s, 9H, t-Bu), 6.75–6.86 (m, 2H, Ar-
H), 7.42–7.72 (m, 11H, Ar-H). 13C NMR (CDCl3): d 29.8, 35.6,
110.4, 111.8, 118.8, 129.1, 130.0, 131.7, 132.5, 133.2, 139.2,
163.0. Anal. Calc. for C22H23O2P: C, 75.41; H, 6.62; Found: C,
75.36; H, 6.65.
ꢀ
tered. The deep red solution was cooled to ꢁ35 C slowly to
give the crystals of complex A (0.72 g, 78% yield).
2,4-Di-cumyl-6-diphenylphosphinoyl-phenoxy bis(tetrahy-
drofuran) chromiumdichloride (B) and 2-anthracenyl-6-
diphenylphosphinoyl-phenoxy bis(tetrahydrofuran) chro-
miumdichloride (C) were prepared by the same procedure in
75 and 69% yield, respectively.
2,4-Di-tert-butyl-6-diphenylphosphanyl Phenol (3c)
Compound 3c was prepared via the same procedure in 91%
yield.
Ethylene Polymerization
1H NMR (CDCl3): d 1.16 (s, 9H, t-Bu), 1.43 (s, 9H, t-Bu), 6.78–
6.83 (m, 1H, Ar-H), 7.45–7.50 (m, 5H, Ar-H), 7.51–7.54 (m, 2H,
Ar-H), 7.65–7.72 (m, 4H, Ar-H). 13C NMR (CDCl3): d 29.6, 31.4,
34.1, 35.4, 108.9, 110.3, 126.1, 128. 6, 131.9, 132.4, 133.1, 138.0,
140.5, 160.9. Anal. Calc. for C26H31O2P: C, 76.82; H, 7.69. Found:
C, 76.90; H, 7.73.
The polymerization was carried out in a 150 mL Schlenk
flask equipped with a mechanical stirrer. The flask was
repeatedly evacuated and refilled with nitrogen, and finally
filled with ethylene gas (ambient pressure) from a 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, and eth-
ylene consumption was noted as a function of time from the
gas burette. The polymerization was initiated by the addition
of a heptane solution of MMAO into the reactor with vigor-
ous stirring. After the prescribed reaction time, the reaction
was stopped, and then the resulted mixture was poured into
a solution of hydrochloric acid/ethanol (10 vol %) to precipi-
tate the polymer. The resulted polyethylene was recovered
by filtration, washed with ethanol, and dried in a vacuum
2,4-Di-cumyl-6-diphenylphosphinoyl Phenol (3d)
Compound 3d was prepared via the same procedure in 90%
yield.
1H NMR (CDCl3): d 1.57 (s, 6H, cumyl-Me), 1.67 (s, 6H,
cumyl-Me), 6.65 (dd, J ¼ 2.4 Hz, 14.1 Hz, 1H, Ar-H), 7.10–
7.23 (m, 10H, Ar-H), 7.39–7.53 (m, 11H, Ar-H). 13C NMR
(CDCl3): d 29.5, 30.8, 42.5, 109.5, 110.9, 125.3, 125.8, 126.7,
128.0, 128.6, 130.5, 132.0, 132.4, 137.5, 140.4, 150.3, 160.3.
Anal. Calc. for C36H35O2P: C, 81.48; H, 6.65; Found: C, 81.53;
H, 6.70.
ꢀ
oven at 60 C.
High-pressure polymerization experiments were carried out
in a mechanically stirred 200 mL stainless steel reactor,
equipped 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 sub-
sequently cooled to the temperature of polymerization. The
reagents were transferred via a gastight syringe to the evac-
uated 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 the prescribed reaction time, the stir-
ring motor was stopped and the reactor was vented, then
the resulting mixture was poured into a solution of hydro-
chloric acid/ethanol (10 vol %) to precipitate the polymer.
The polyethylene thus obtained was recovered by filtration,
2-Anthracenyl-6-diphenylphosphinoyl Phenol (3e)
Compound 3e was prepared by the same procedure in 93%
yield.
1H NMR (CDCl3): d 7.06–7.84 (m, 19H, Ar-H), 8.04 (d, J ¼ 9
Hz, 2H, Ar-H), 8.51 (s, 1H, Ar-H), 11.17 (s, 1H, Ar-OH). 13C
NMR (CDCl3): d 111.5, 112.8, 119.4, 123.6, 124.8, 125.5,
126.0, 126.8, 127.6, 128.1, 129.0, 129.3, 130.7, 131.9, 132.5,
133.0, 138.0, 162.4. Anal. Calc. for C32H23O2P: C, 81.69; H,
4.93; Found: C, 81.72; H, 4.98.
Synthesis of In-Situ Chromium Catalysts
(Ligand/CrCl3(thf)3)
In-situ chromium catalysts were generated by mixing the
ligands with CrCl3(thf)3 (1 equiv) in toluene, followed by re-
moval of the solvent in reduced pressure. The resulting com-
plexes were dissolved in toluene and tested as the catalyst
for ethylene polymerization.12(a),16,18(b)
ꢀ
washed with ethanol, and dried in a vacuum oven at 60 C.
RESULTS AND DISCUSSION
Synthesis of Complexes A–C
Bis(2-tert-butyl-6-diphenylphosphinoyl-phenoxy
Tetrahydrofuran Chromiumdichloride) (A)
To a stirred solution of 3b (0.60 g, 1.7 mmol) in dried thf
(20 mL) at ꢁ78 ꢀC, a 2.5 M n-butyllithium hexane solution
(0.68 mL, 1.7 mmol) was added dropwise over 5 min. The
mixture was allowed to warm to room temperature and
Ethylene Polymerization by Chromium Catalysts Based
on Bidentate [O, P¼¼O] Ligands
The synthetic route for phenoxy-phosphinoyl [O, P¼¼O]
ligands 3a–e used in this study is depicted in Scheme 1.
THP ether 1a–e were lithiated with n-BuLi/TMEDA and
quenched with PPh2Cl. The protective group could be easily
removed to give 2a–e by the addition of hydrochloric acid.
ETHYLENE POLYMERIZATION BY CHROMIUM CATALYSTS, HE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
tral P¼¼O donor and the ortho-substituent of the ligand play
an important role in improving catalytic activity. Catalyst 3c/
CrCl3(thf)3 displayed an activity of 236 kg PE/molCr ꢃ h for
ethylene polymerization. The activity is comparable with the
value of 3b/CrCl3(thf)3, indicating that the para-substituent
in the phenoxide donor had little influence on the catalytic
activity. Catalyst 3d–e/CrCl3(thf)3 with bulky ortho-substi-
tute cumyl or anthracenyl displayed lower activities than
3b/CrCl3(thf)3. This result is different from the phenoxy-am-
ide chromium catalysts, in which introducing a bulky ortho-
substitute can dramatically increase the catalytic activity.12(b)
In all experiments, the catalytic activity was valued based on
the solid polymer obtained, while GC analysis of the soluble
fraction indicated a small amount of a-olefins by oligomeri-
zation of ethylene was present in the reaction mixtures.
SCHEME 1 Synthesis of ligands 3a–e.
To further elucidate the structure of the catalyst and the steric
hindrance influence on the catalytic activity, we tried to isolate
and characterize the catalysts. Chromium complexes based on
phenoxy-phosphinoyl ligands were prepared by the reaction of
CrCl3(thf)3 with one equivalent of the lithium salts of the corre-
sponding ligands, as shown in Scheme 2. Attempted characteriza-
tion of these complexes by NMR resulted in failure because of its
severe line broadening in the spectra, which indicates that the
complexes are paramagnetic species. Fortunately, the crystals
(A–C) suitable for X-ray structure determination were obtained
by diffusion of hexane into a dichloromethane solution. Their
molecular structures were shown in Figures 2–4. Crystal B con-
sists of two crystallographically independent molecules in the
unit cell and there is only few difference between each other.
One molecule is omitted for clarity in Figure 3.
Compounds 2a–e were further oxidated to ligands 3a–e
using H2O2. All the ligands were identified by elemental anal-
ysis and NMR spectra.
Phenoxy-phosphinoyl in-situ chromium catalysts 3a–e/
CrCl3(thf)3 were investigated as the catalysts for ethylene
polymerization in the presence of MMAO under the mild
conditions (25 ꢀC, Al/Cr ¼ 2000/1, 1 atm ethylene pres-
sure). Phenoxy-phosphine catalysts 2a–e/CrCl3(thf)3 were
also studied for comparison. The polymerization results are
collected in Figure 1. The metal precursor alone did not sup-
port significant catalysis (80 kg PE/molCr ꢃ h), while 2a–e/
CrCl3(thf)3 also displayed low-catalytic activities for ethylene
polymerization (ꢄ106 kg PE/molCr ꢃ h). Catalyst 3a/
CrCl3(thf)3 was not active for ethylene polymerization,
whereas, 3b/CrCl3(thf)3 exhibited an activity of 264 kg PE/
molCr ꢃ h which was not only higher than the activity
obtained by 3a/CrCl3(thf)3, but also the value of correspond-
ing 2b/CrCl3(thf)3 (106 kg PE/molCr ꢃ h). These results sug-
gest that monoanionic phenoxy-phosphinoyl is effective
ligand for chromium catalysts and both the additional neu-
FIGURE 1 Ethylene polymerization results using the in-situ cat-
alysts (5 lmol ligand, 5 lmol CrCl3(thf)3; Al/Cl ¼ 2000/1; 25 ꢀC;
10 min; Vtotal ¼ 50 mL; 1 atm ethylene pressure).
SCHEME 2 Synthesis of chromium complexes A–C.
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ARTICLE
FIGURE 2 Molecular structure of com-
plex A with thermal ellipsoids at 30%
probability level. Hydrogen atoms are
omitted for clarity. Selected bond
lengths (A) and bond angles (ꢀ):
˚
Cr(1)AO(1), 1.9611(18); Cr(1)AO(2),
1.9078(18); Cr(1)AO(3), 2.0620(19);
Cr(1)ACl(1), 2.3969(8); Cr(1)ACl(1A),
2.3998(8);
Cr(1)ACl(2),
2.3003(8);
O(1)ACr(1)AO(2), 92.51(8); O(1)ACr(1)A
O(3), 86.68(8); O(2)ACr(1)AO(3), 87.41(8);
O(1)ACr(1)ACl(1), 173.78(6); O(2)ACr(1)A
Cl(1), 90.88(6); O(3)ACr(1)ACl(1), 88.27(6);
O(1)ACr(1)ACl(2), 90.88(6); O(2)ACr(1)A
Cl(2), 93.08(6); O(3)ACr(1)ACl(2), 177.53(6);
Cl(1)ACr(1)ACl(2), 94.15(3); Cl(1)ACr(1)A
Cl(1A), 95.33(3).
As shown in Figure 2, complex A reveals chloro-bridged
binuclear structure in which each monomer moiety is related
by a crystallographic center of symmetry. Each chromium
center in complex A displays a six-coordinated distorted-
octahedral geometry with one additional thf molecule. The
two bridging groups are located at the position trans to the
oxygen atoms of the phenoxy-phosphinoyl ligands and the
chlorine atoms occupies a position trans to the thf molecules.
The nonbonded CrꢃꢃꢃCr distance (3.546 Å) is in the range
found in a number of other chloride-bridged chromium com-
plexes (3.64–3.23 Å).23 Different from A, complexes B and C
with bulky ortho-phenoxy substituents show octahedral
coordination at the chromium center with a single bidentate
[O, P¼¼O] ligand and two cis-coordinated thf molecules, as
shown in Figure 3 (O(1)ACrAO(2) 91.70ꢀ; Cl(1)ACrACl(2)
95.35ꢀ) and Figure 4 (O(1)ACrAO(2) 92.22ꢀ; Cl(1)ACrA
Cl(2) 94.66ꢀ). The different coordination mode of complexes
A–C are probably associated with the steric crowding of the
bulky group around the ortho-position, which may hinder
FIGURE 3 Molecular structure of complex B with thermal ellipsoids
FIGURE 4 Molecular structure of complex C with thermal ellip-
at 30% probability level. Hydrogen atoms are omitted for clarity.
soids at 30% probability level. Hydrogen atoms are omitted for
ꢀ
ꢀ
˚
˚
Selected bond lengths (A) and bond angles ( ): CrAO(1), 1.9905(19);
clarity. Selected bond lengths (A) and bond angles ( ): CrAO(1),
CrAO(2), 1.9243(19); CrAO(3), 2.054(2); CrAO(4), 2.082(2); CrACl(1),
1.956(3); CrAO(2), 1.917(3); CrAO(3), 2.038(3); CrAO(4), 2.077(3);
CrACl(1), 2.2889(14); CrACl(2), 2.3247(14); O(1)ACrAO(2),
92.22(13); O(1)ACrAO(3), 175.00(14); O(1)ACrAO(4), 89.42(15);
O(2)ACrAO(3), 86.96(13); O(2)ACrAO(4), 85.66(14); O(3)ACrAO(4),
85.60(15); O(1)ACrACl(1), 92.74(10); O(2)ACrACl(1), 90.75(10);
O(3)ACrACl(1), 92.20(11); O(4)ACrACl(1), 175.89(11); O(1)ACrA
Cl(2), 90.51(10); O(2)ACrACl(2), 173.81(10); O(3)ACrACl(2),
89.84(10); O(4)ACrACl(2), 88.82(11); Cl(1)ACrACl(2), 94.66(6).
2.2776(9);
CrACl(2),
2.3214(9);
O(1)ACrAO(2),
91.70(8);
O(1)ACrAO(3), 85.63(9); O(1)ACrAO(4), 85.38(8); O(2)ACrAO(3),
89.33(8); O(2)ACrAO(4), 175.74(8); O(3)ACrAO(4), 87.35(9);
O(1)ACrACl(1), 90. 10(6); O (2)ACrACl(1), 93.76(6); O(3)ACrACl(1),
174. 81(7); O(4)ACrACl(1), 89.36(7); O(1)ACrACl(2), 173. 70(6);
O(2)ACrACl(2), 91.08(6); O(3)ACrACl(2), 88.75(7); O(4)ACrACl(2),
91.52(6); Cl(1)ACrACl(2), 95.35(4).
ETHYLENE POLYMERIZATION BY CHROMIUM CATALYSTS, HE ET AL.
315

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
just like 3a/CrCl3(thf)3. It is noteworthy that 4b/CrCl3(thf)3
displayed a high-catalytic activity of 540 kg PE/molCr ꢃ h for
ethylene polymerization, which is higher than both the val-
ues of corresponding 2b/CrCl3(thf)3 (106 kg PE/molCr ꢃ h)
and 3b/CrCl3(thf)3 (264 kg PE/molCr ꢃ h). The catalytic activ-
ity of 4b/CrCl3(thf)3 was much higher than the value of cor-
responding ortho-tert-butyl phenoxy-amide chromium cata-
lysts reported by Gibson and coworkers under the similar
conditions, although it was still lower than the activity of the
ortho-anthracene one.12(b) This result reveals that bulky thio-
phenol-phosphine is more effective ligand for chromium cat-
alysts and both the soft sulfur and phosphorus donors in the
ligand were essential for the high-catalytic activity. Based on
the encouraging results of 3b and 4b, bisanionic ligands 5
and 6 (Scheme 3) were also prepared and tested via the in-
situ catalyst polymerization. Compared with 3b/CrCl3(thf)3
and 4b/CrCl3(thf)3 systems, however, 5/CrCl3(thf)3 and 6/
CrCl3(thf)3 only possessed low-catalytic activity as shown in
Figure 1, which indicates that bisanionic [O, P¼¼O, O] and [S,
P, S] were not suitable ligands for efficient chromium
catalysts.
SCHEME 3 Ligands 4a-b, 5, and 6 used in this study.
the adjacence of two molecules and restrain the formation of
the chloro-bridge. Although B and C exhibit similar coordina-
tion structure, there are also some differences between
them. For example, the CrAO(1) and CrAO(2) bond distan-
ces in B (1.990 and 1.924 Å) are slightly longer than those
in C (1.956 and 1.917 Å, respectively). The phosphor atom
lying 0.539 Å out of the phenoxy ring plane in B, whereas
they are coplanar in C. In complex C, the anthracenyl moiety
is oriented approximately orthogonally to the phenoxy ring
(ꢅ77.68ꢀ), which is slightly shorter than that in correspond-
ing phenoxy-amide chromium complex (ꢅ85ꢀ).12(b)
Complexes A–C displayed catalytic activity of 278, 111, and
76 kg PE/molCrꢃh, respectively, for ethylene polymerization
under the mild conditions. The catalytic activities are con-
sistent with the values obtained by in-situ catalysts under
the same conditions. Complex A displayed much higher cata-
lytic activity than B and C. The effect of the ligand environ-
ments on the productivity is similar to the results obtained
by in-situ process. These results suggest that the identical
catalytic species were generated in these two systems. We
assumed the low-catalytic activities of complexes B and C
were probably due to bulky ortho-substitute in complexes.
Considering the steric protection to the active site afforded
by diphenylphosphinoyl in phenoxy-phosphinoyl complexes,
a further bulky ortho-anthracenyl substituent in close prox-
imity to the active site in B and C may hinder the access of
ethylene to the active site and restrain the growth of the
polymer chain, and thus lead to the low-catalytic activity.
Influence of Reaction Parameters on Ethylene
Polymerization by 3b/CrCl3(thf)3 and
4b/CrCl3(thf)3 Systems
We focused on in-situ chromium catalytic systems 3b/
CrCl3(thf)3 and 4b/CrCl3(thf)3, which displayed the highest
catalytic activity for ethylene polymerization among the
tested ones. Ethylene polymerization behaviors of 3b/
CrCl3(thf)3 and 4b/CrCl3(thf)3 were investigated in detail
and the typical results are summarized in Table 1. The
effects of various cocatalysts on the ethylene polymerization
were first studied. In addition to MMAO, diethyl aluminum
chloride (AlEt2Cl) and triethylaluminum (TEA) were also
used as the cocatalyst. The catalytic activity was strongly
influenced by the chosen cocatalysts. The systems activated
with MMAO (entries 1–2) showed higher activities than the
ones activated with AlEt2Cl (entries 3–4), whereas little eth-
ylene consumption was observed and trace amount of poly-
mer was obtained by the systems activated with TEA
(entries 5–6).
The influence of reaction conditions such as Al/Cr molar ra-
tio and reaction temperature on polymerization was further
investigated in detail in the presence of MMAO. For the two
systems, the increasing Al/Cr molar ratio resulted in a grad-
ual increase of the catalyst activities, as shown in Figure 5,
whereas the molecular weight of the resultant polyethylenes
decreased with increasing cocatalyst dosage via chain trans-
fer to aluminum (entries 1–2 and 7–10). The effects of reac-
tion temperature on the catalytic activities were studied with
an Al/Cr molar ratio of 2000/1 and the temperature depend-
ence can be seen from Figure 6. The catalytic activity of 3b/
CrCl3(thf)3 decreased above room temperature, revealing
that the catalyst active species decay at elevated tempera-
ture. Contrarily, 4b/CrCl3(thf)3 displayed the promising tem-
perature tolerance toward ethylene polymerization. The cata-
lytic activity first increased with temperature and reached a
maximum value at ꢅ55 ꢀC, and then gradually decreased,
Ethylene Polymerization by Chromium Catalysts Based
on Bidentate [S, P] Ligands
The aforementioned results revealed that the catalytic activ-
ities of the phenoxy-phosphine based chromium catalysts
could be readily tuned via modification of the coordination
donors. Therefore, thiophenol-phosphine ligands 4a–b
(Scheme 3) were designed and synthesized by changing the
hard oxygen donor to soft sulfur donor to further explore
the potential of soft sulfur donor-based ligand systems. The
polymerization results are also shown in Figure 1. Catalyst
4a/CrCl3(thf)3 was not active for ethylene polymerization
316
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
a
TABLE 1 Ethylene Polymerization Results by 3b/CrCl3(thf)3 and 4b/CrCl3(thf)3
c
d
Al/Cr
Temperature
(ꢀC)
P
Time
(min)
Polymer
Yield (g)
Mw
Tm
c
Entry
Ligand
Cocatalyst
(molar ratio)
(atm)
Activityb
(103)
Mw/Mn
(ꢀC)
1
2
3b
4b
3b
4b
3b
4b
3b
4b
3b
4b
3b
4b
4b
3b
4b
3b
4b
MMAO
MMAO
AlEt2Cl
AlEt2Cl
TMA
2,000
2,000
2,000
2,000
2,000
2,000
1,000
1,000
3,000
3,000
2,000
2,000
2,000
2,000
2,000
2,000
2,000
25
25
25
25
25
25
25
25
25
25
55
55
75
25
25
25
25
1
1
10
10
10
10
10
10
10
10
10
10
10
10
10
30
30
30
30
0.22e
0.45
0.04
0.11
Trace
Trace
0.19
0.29
0.26
0.49
0.06
0.61
0.28
0.51
1.18
2.13
5.80
264
540
48
4.0
20.1
6.3
2.9
3.5
5.6
13.1
–
125
126
125
131
–
3
1
4
1
132
–
25.3
–
5
1
6
TMA
1
–
–
–
–
7
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
1
228
348
312
588
72
12.7
45.3
2.2
7.0
7.5
2.1
2.5
2.0
2.3
5.2
2.0
5.0
1.9
4.6
126
128
123
127
122
126
127
126
130
127
130
8
1
9
1
10
11
12
13
14
15
16f
17f
a
1
12.5
1.7
1
1
732
336
204
472
426
1,160
10.2
7.2
1
1
6.7
1
50.5
8.6
10
10
d
66.2
Conditions: 5 lmol ligand, 5 lmol CrCl3(thf)3; Vtotal ¼ 50 mL.
Activity in kg PE/molCrꢃh.
Melting temperature determined by DSC.
b
c
e
f
2.6 methyl branches per 1000 C, 1.7 vinyls per 1000 C.
10 lmol ligand, 10 lmol CrCl3(thf)3; Vtotal ¼ 100 mL.
GPC data in trichlorobenzene versus polystyrene standards.
which indicates that ligand 4b can restrain the chromium
center from deactivation. The molecular weights of the re-
sultant polymers decreased with the reaction temperature
for the both systems (entries 11–13). Although the activities
of the two catalytic systems decreased with prolonged reac-
tion time (entries 14–15 vs. 1–2), the catalytic activity of
4b/CrCl3(thf)3 for 30 min (472 kg PE/molCr ꢃ h) was still
comparable with that for 10 min (540 kg PE/molCr ꢃ h),
revealing that 4b/CrCl3(thf)3 possess long lifetime for
ethylene polymerization. Increasing the ethylene pressure
can obviously enhance productivity (entries 16–17). Catalyst
4b/CrCl3(thf)3 shows a higher catalytic activity of 1160 kg
PE/molCr ꢃ h than 3b/CrCl3(thf)3 (426 kg PE/molCr ꢃ h) at 10
atm ethylene pressure, which is similar to the results at
atmosphere pressure.
All the polymers obtained have been analyzed by GPC. As
shown in Table 1, the molecular weight of the resultant
FIGURE 5 Plots of the activity of 3b/CrCl3(thf)3 and 4b/
CrCl3(thf)3 versus Al/Cr molar ratio (5 lmol ligand, 5 lmol
CrCl3(thf)3; 25 ꢀC; 10 min; Vtotal ¼ 50 mL; 1 atm ethylene
pressure).
FIGURE 6 Plots of the activity of 3b/CrCl3(thf)3 and 4b/
CrCl3(thf)3 versus the polymerization reaction temperature
(5 lmol ligand, 5 lmol CrCl3(thf)3; Al/Cr ¼ 2000/1; 10 min; Vtotal
¼ 50 mL; 1 atm ethylene pressure).
ETHYLENE POLYMERIZATION BY CHROMIUM CATALYSTS, HE ET AL.
317

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
polyethylenes obtained by 4b/CrCl3(thf)3 were higher than
the ones produced by 3b/CrCl3(thf)3 under the same condi-
tions. The results may be ascribed to the fact that soft
donors [S, P] chelate 4b could stabilize the chromium com-
plex and diminish the rate of chain termination compared
with the hard donors [O, P¼¼O] chelate 3b. On examination
of the GPC data, most of the traces showed unimodal behav-
ior and some of them displayed bimodal or even polymodal
behavior. The melting temperatures (Tms) of the resulting
polyethylene were determined by DSC. For different catalyst
systems and various reaction conditions, the Tms of the poly-
The crystal date and structure refinement of complexes A–C;
13C NMR spectra and GPC traces of the typical polyethylene
samples are obtained. This material is available in the Sup-
porting Information.
The authors are grateful for subsidy provided by the National
Natural Science Foundation of China (Nos. 20734002 and
50525312), and by the Special Funds for Major State Basis
Research Projects (No. 2005CB623800) from the Ministry of
Science and Technology of China.
REFERENCES AND NOTES
ꢀ
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1 Gibson, V. C.; Spitzmesser, S. K. Chem Rev 2003, 103, 283–315.
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