Highly active half-metallocene chromium(III) catalysts for ethylene polymerization activated by trialkylaluminum

Article abstract of DOI:10.1021/ja0671363

Highly active half-metallocene type chromium(III) catalysts for ethylene polymerization have been developed. High molecular weight linear polyethylene (M_n > 1000000 g/mol) was produced by these catalysts in the presence of a relatively small amount of AlR₃ (25 equiv). Copyright

Full text of DOI:10.1021/ja0671363

Published on Web 02/03/2007
Highly Active Half-Metallocene Chromium(III) Catalysts for Ethylene
Polymerization Activated by Trialkylaluminum
Tieqi Xu, Ying Mu,* Wei Gao, Jianguo Ni, Ling Ye, and Yanchun Tao
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, School of Chemistry,
Jilin UniVersity, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
Received October 5, 2006; E-mail: ymu@mail.jlu.edu.cn
Polyolefins are the largest volume synthetic materials used in
the world and have been almost applied in every aspect of our daily
lives and productions. The growing demand for high performance
polyolefin products in recent years has inspired extensive industrial
and academic interest to develop high performance olefin polym-
erization catalysts.1 Metallocene type catalysts can produce a variety
of high performance polyolefin products, including isotactic
polypropylene,2 syndiotactic polypropylene,3 atactic polypropylene,4
high-density polyethylene,5 linear low-density polyethylene,6 syn-
diotactic polystyrene,7 and cyclo-olefin copolymer.8 Despite the
success of metallocene-based catalysts in catalyzing the above-
mentioned olefin polymerization reactions, they also exhibit
disadvantageous features. A large amount of methylaluminoxane
-
(MAO) or expensive fluorinated borate such as Ph3C+B(C6F5)4
is generally required as cocatalyst or activator for obtaining
acceptable polymerization activity,9 which raises issues of the high
Figure 1. Molecular structure of complex 1. Selected bond lengths (Å)
cost of the metallocene type catalyst systems and the high ash
and angles (deg): Cr-O 1.927(3), Cr-N 2.071(4), Cr-Cl 2.296(4), C24-N
1.291(5), C11-C24 1.438(6), C10-O 1.306(5), O-Cr-N 87.44(14), O-Cr-
Cl 92.40(10), N-Cr-Cl 100.60(12).
content (Al2O3) of the product polymers. Consequently, there is a
great need to develop new catalyst systems which can provide
equivalent catalytic activity with no need for large amounts of expen-
sive cocatalyst.10,11 Recent development of chromium-based olefin
polymerization catalysts has attracted intensive research interest.12-16
So far, except that some of nonmetallocene type chromium
complexes have been known to show reasonable catalytic activity
for ethylene polymerization when activated with Et2AlCl, a few
half-metallocene type chromium catalyst systems have also been
reported. Complexes [Cp*CrL2R]+A- (L ) py, 1/2dppe, MeCN,
THF; R ) Me, Et; A ) PF6, BPh4) were first reported to show
catalytic activity for ethylene polymerization.14 A Cp*Cr(acac)Cl/
Et3Al (Al/Cr ) 300) system was found to show a catalytic activity
of 4.2 × 104 g PE mol Cr-1 h-1 under 50 atm of ethylene at room
temperature.15 Another similar system, Cp*Cr(C6F5)(η3-Bz)/Et3Al
(Bz ) benzyl, Al/Cr ) 90), produced oligomers with a catalytic
activity of 2.1 × 105 g PE mol Cr-1 h-1 under 1.1 atm of ethylene
at room temperature.16a We herein report a novel half-metallocene
type chromium(III) catalyst system, Cp*Cr[2,4-tBu2-6-(CHdNR)-
some salicylaldiminato chromium complexes12a and cyclopentadi-
enyl chromium complexes.4a The Cr-N bond lengths are obviously
affected by the R group on N and are 2.071(4) Å for 1, 2.049(4) Å
for 2, and 2.097(2) Å for 3. The order of the dihedral angles between
the Cp* ring and the phenoxide ring is 83.7° (3) > 34.0° (2) >
5.5° (1), while the dihedral angles between the Cp* ring and the
plane through Cr, O, and N atoms are changed in a similar sequence
of 56.5° (3) > 48.4° (2) > 48.2° (1). It is obvious that these dihedral
angles are directly related to the sterically opening degree around
the chromium atom in these complexes. The latter might play a
significant role in allowing the ethylene molecule to coordinate to
the chromium atom and to insert into the growing polyethylene
chain, as well as in determining the rate of the polymer chain
termination.17 In other words, these dihedral angles might be related
to the catalytic activity of these catalysts for ethylene polymerization
and the molecular weight of the produced polyethylene.
Some experimental results of ethylene polymerization with
complexes 1-3 as catalysts are summarized in Table 1. Upon
activation with a small amount of AlMe3, complexes 1-3 show
reasonable to good catalytic activity for ethylene polymerization
under 5 bar, producing polyethylene with high molecular weight
(1.00-1.45 × 106 g/mol). The melt transition temperatures (Tm)
of the obtained polyethylenes are in the range of 134-140 °C, and
the crystallinity of these polymers is in the range of 56.7-77.1%.
13C NMR analysis on the polymer samples indicates that the
polyethylenes produced by these catalysts are linear and have no
branch. The catalytic activity of these complexes for ethylene
polymerization under similar conditions (see runs 1, 8, and 11 in
Table 1) is in the order of 1 > 2 > 3, which indicates that the
catalytic activity of these catalysts is remarkably affected by the
nature of the R group on the nitrogen atom of the salicylaldiminato
t
C6H2O]Cl/AlR′3 [R ) Bu (1), Ph (2), 2,6-iPr2C6H3 (3); R′ ) Me,
i
Et, Bu], that catalyzes ethylene polymerization to produce high
molecular weight polyethylene in high catalytic activity when
activated with only a small amount of AlR3 (Al/Cr ≈ 25).
The green color Cr(III) complexes 1-3 were synthesized in 50-
60% yields via the reaction of [Cp*CrCl2]2 with the lithium salt of
the corresponding salicylaldiminato ligand in tetrahydrofuran at
-78 °C. Their molecular structures have been determined by single-
crystal X-ray diffraction, and the representative structure of complex
1 along with selected bond distances and angles is depicted in Figure
1. All three complexes adopt a pseudo-octahedral coordination
environment of a three-legged piano stool. Most of the metal-
ligand bond lengths in these complexes, including the Cp*-Cr,
Cr-Cl, and Cr-O bond lengths, are close to the ones observed in
9
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J. AM. CHEM. SOC. 2007, 129, 2236-2237
10.1021/ja0671363 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Results of Ethylene Polymerization Using Procatalysts
1-3a
molecular weight polyethylene in high catalytic activity when
activated with only a small amount of AlR3 (Al/Cr ≈ 25). These
new catalysts represent a remarkable addition to the limited list of
half-metallocene type chromium ethylene polymerization catalysts.
At present, further investigations exploring more high performance
catalysts of this type and possibilities applying these types of
catalysts to produce copolymers of ethylene with R-olefins are
underway.
c
d
e
procatalyst
mol)
activator
mol/equiv)
yield
(g)
Mn
Tm
C)
Xc
(%)
4
run
(
µ
(
µ
activityb
(
×
10-
)
(
°
1
2
3
4
5
6
7
8
9
10
11
12
13
1 (5)
1 (5)
1 (5)
1 (5)
1 (5)
1 (10)
1 (10)
2 (10)
2 (10)
2 (10)
3 (10)
3 (10)
3 (10)
AlMe3 (125/25)
AlMe3 (250/50)
AlMe3 (500/100)
AlEt3 (125/25)
AlEt3 (500/100)
AliBu3 (125/25)
AliBu3 (500/100)
AlMe3 (125/25)
AlMe3 (250/50)
AlMe3 (500/100)
AlMe3 (125/25)
AlMe3 (250/50)
AlMe3 (500/100)
10.11 4044
9.44 3776
5.12 2048
4.26 1704
2.52 1008
101
106
110
120
100
109
101
126
120
110
145
137
135
136.8 60.3
136.7 64.3
136.2 56.7
139.9 57.6
139.3 57.3
138.0 62.7
137.9 64.9
134.5 70.0
134.9 74.2
134.0 65.1
138.4 77.1
137.8 56.8
136.7 64.4
1.57
0.83
314
166
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (Nos. 20674024 and 20374023).
10.54 2108
9.52 1904
4.25
1.62
1.35
0.78
850
324
270
156
Supporting Information Available: Experimental procedures,
crystallographic data in CIF format, and selected bond lengths and
angles of all compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
a Polymerization conditions: solvent 60 mL of toluene, temperature
20 °C, ethylene pressure 5 bar, time 30 min. b Units of kg PE (mol Cr)-1
h-1 c Measured in decahydronaphthalene at 135 °C. d Determined by DSC
.
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In summary, we have successfully developed a new type of half-
metallocene chromium(III) catalyst system for ethylene polymer-
ization that catalyzes ethylene polymerization to produce high
JA0671363
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J. AM. CHEM. SOC. VOL. 129, NO. 8, 2007 2237