Access to highly active and thermally stable iron procatalysts using bulky 2-[1-(2,6-dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl] pyridine ligands

Article abstract of DOI:10.1039/c0cc05373b

Ethylene polymerization was performed using a series of 2-[1-(2,6-dibenzhydrylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridyliron(ii) chlorides with the activity in the range of 10⁷ g PE mol^-1 (Fe) h^-1, which is the highe

Full text of DOI:10.1039/c0cc05373b

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COMMUNICATION
Access to highly active and thermally stable iron procatalysts using bulky
2-[1-(2,6-dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine
ligandsw
Jiangang Yu,a Hao Liu,a Wenjuan Zhang,a Xiang Haoa and Wen-Hua Sun*ab
Received 5th December 2010, Accepted 24th December 2010
DOI: 10.1039/c0cc05373b
Ethylene polymerization was performed using a series of
2-[1-(2,6-dibenzhydrylphenylimino)ethyl]-6-[1-(arylimino)ethyl]-
pyridyliron(II) chlorides with the activity in the range of
107 g PE molꢀ1 (Fe) hꢀ1, which is the highest observed in iron
procatalysts at elevated reaction temperatures such as 80 1C in
the presence of MMAO and 60 1C in the presence of MAO,
without any trace of ethylene oligomerization.
of oligomers at elevated reaction temperatures.1a,6,14 Though
impressive bis(imino)pyridyliron procatalysts employing
bulky anilines enhanced the thermostability,15 exploring more
bulky anilines is still an attractive subject. In addition, the
design of new bis(imino)pyridyliron procatalysts which will
perform solely ethylene polymerization with high activity at
elevated reaction temperature (suitable for industrial consid-
eration between 60 to 80 1C) is highly beneficial.
In the classical bis(imino)pyridyliron(II) system,1 the aniline
moiety bearing bulky substituents on the 2,6-positions resulted
in a lower activity. However, the new iron(II) procatalysts
bearing 2-[1-(2,6-dibenzhydryl-4-methylphenylimino)ethyl]-6-
[1-(aryl-imino)ethyl]pyridines show high activity up to the
range of 2.2 ꢁ 107 g PE molꢀ1 (Fe) hꢀ1 for solely ethylene
polymerization and a unique thermal behavior with best
performance in the presence of modified methylaluminoxane
(MMAO) at 80 1C, or methylaluminoxane (MAO) at 60 1C.
The stoichiometric condensation reaction of 2,6-diacetylpyr-
idine and 2,6-dibenzhydryl-4-methylphenylamine produced
2,6-bis[1-(2,6-dibenzhydryl-4-methylphenylimino)ethyl]pyridine
(L0) as a minor product and 2-[1-(2,6-dibenzhydryl-4-methyl-
phenylimino)ethyl]-6-acetylpyridine as a major product, which
further reacted with various anilines to form nonsymmetrical
2-[1-(2,6-dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)
ethyl]pyridines (L1–L5) (Scheme 1). The bis(imino)pyridines
reacted with iron(II) dichloride to form the corresponding
bis(imino)pyridyliron(II) dichlorides in good yields (Scheme 1).
All organic and iron complexes were fully characterized, and the
molecular structures of representative iron complexes were
confirmed as a pseudo-square-pyramidal geometry by the single
crystal X-ray diffraction. According to the molecular structure of
Fe1 (Fig. 1), the chlorine Cl(2) occupied its apical position, and the
square plane is formed with three nitrogen atoms N(1), N(2), N(3)
and one chlorine Cl(1) atom. The iron lies 0.513 A out of the plane
of chelating nitrogen atoms, N(1), N(2) and N(3) (crystallographic
data of Fe1, Fe3 and Fe5 and molecular structures of Fe3 and Fe5
are available in ESIw).
The emergence of bis(imino)pyridyliron(II) chlorides marked a
milestone in iron-based procatalysts for ethylene polymeriza-
tion and oligomerization.1 Research papers on bis(imino)-
pyridyliron(II) chlorides have mushroomed and were reviewed
in a number of articles.2 In addition, there has also been much
work focusing on understanding the reactive species3 as well as
self-activating catalysts4 of bis(imino)pyridyliron derivatives.
Previously we reported iron procatalysts using nonsymmetrical
bis(imino)pyridines5 and designed bulky anilines, however,
our ongoing project was halted because of concerns about
any overlapping research and similar works reported by
Gibson et al.6 Consequently we have turned our attention
to iron(II) procatalysts employing new ligands such
as 2-imino-1,10-phenanthrolines,7 2-(2-benzimidazolyl)-1,
10-phenanthrolines,8 2-(benzoxazolyl)-1,10-phenanthrolines,9
2-benzimidazolyl-6-(1-(arylimino)ethyl)-pyridines,10 6-(quino-
xalin-3-yl)-2-iminopyridines,11 N-((pyridin-2-yl)methylene)-8-
aminoquinolines12 and 2,8-bis(1-aryliminoethyl)quinolines.13
These catalytic systems are useful for either oligomerization
or polymerization of ethylene.2f Impeding the progress of
bis(imino)pyridyliron(II) procatalysts in ethylene polymeriza-
tion, the critical problems are their deactivation and production
a Key laboratory of Engineering Plastics and Beijing Natural
Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing, 100190, China.
E-mail: whsun@iccas.ac.cn; Fax: +86 10-62618239;
Tel: +86 10-62557955
b State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou, 730000, China
The catalytic behaviour of complex Fe3 was routinely
evaluated for the optimum condition in the presence of either
MAO or MMAO (Table 1). Under the ambient pressure of
ethylene at room temperature, good activity was observed for
ethylene polymerization in the presence of MMAO (entries
1–8, Table 1), whereas moderate activity was shown with
w Electronic supplementary information (ESI) available: Synthesis and
characterization of organic and complex compounds, detail data of
ethylene polymerization under 1 atm ethylene, and crystallographic data
and ORTEP drawings for complexes Fe1, Fe3 and Fe5. CCDC 798843
(Fe1), 798844 (Fe3) and 798845 (Fe5). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0cc05373b
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3257–3259 3257

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Table 1 The catalytic behaviour of complex Fe3a
c
Pres.
Mw
/
Entry Cocat. atm T/1C Al/Fe Activityb kg molꢀ1 Mw/Mn Tmd/1C
c
1
2
3
4
5
6
7
8
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
1
1
1
1
1
1
1
1
20 1000 0.67
20 1500 1.04
20 2000 1.64
20 2500 1.29
39.0
47.0
5.8
42.2
47.8
2.1
23.6
31.6
7.5
31.5
17.4
3.6
129.3
125.1
123.0
124.7
130.0
118.0
98.0
0
2000 1.44
40 2000 1.12
60 2000 0.37
80 2000 0.17
1.1
1.0
2.3
2.0
67.3
9
MMAO 10 20 1500 2.45
MMAO 10 20 2000 3.20
MMAO 10 20 2500 2.85
MMAO 10 40 2000 5.07
MMAO 10 60 2000 7.80
MMAO 10 70 2000 9.51
MMAO 10 80 2000 11.9
MMAO 10 90 2000 6.67
374.7
365.0
131.3
39.7
21.8
15.7
17.7
11.0
93.2
299.0
355.5
338.0
187.3
166.5
53.1
88.7
49.5
15.6
13.5
6.3
3.8
4.3
2.9
70.2
19.3
34.8
44.3
19.2
10.9
130.0
127.5
129.1
124.6
130.0
128.3
129.3
134.0
127.0
135.7
134.0
134.7
134.3
136.7
133.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
MAO
MAO
MAO
MAO
MAO
MAO
MAO
1
20 2000 0.26
10 20 1000 2.00
10 20 1500 3.44
10 20 2000 2.43
10 40 1500 3.07
10 60 1500 9.95
10 80 1500 4.89
4.93
a
General conditions: 1.5 mmol Fe; 30 min; 30 mL toluene for 1 atm
ethylene, and 100 mL toluene for 10 atm ethylene. 106 g molꢀ1 (Fe) hꢀ1
c Determined by GPC. d Determined by DSC.
.
b
Scheme 1 Synthesis of iron complexes Fe0, Fe1–Fe5.
at elevated temperature, but the bimodal or multimodal poly-
ethylenes were produced at 40 1C and lower reaction tempera-
ture due to multi-active species formed in the catalytic system.
Subsequently, polymerization behaviors of all iron pro-
catalysts were exploited using MAO at 60 1C and Al/Fe molar
ratio of 1500 under 10 atm ethylene (Table 2). The activities
decreased in the order of Fe1 > Fe2 > Fe3 consistent with
increase in the bulk of the ortho-substituent in the imino
groups, whilst random effects of the additional para-methyl
in the imino groups were observed within Fe1 and Fe4, Fe2
and Fe5. However, the procatalyst Fe0 displayed moderate
activity due to steric bulk. As a reference, the 2,6-bis
[1-(2,6-diisopropylphenylimino)ethyl]pyridyliron(II) chloride
(FeR) displayed good activity in polymerization, but with a
little (likely ignorable) oligomers. Fe1–Fe5 showed sole poly-
merization without any trace of oligomers being observed, and
higher activities were observed for Fe1, Fe2 and Fe5 than FeR.
The molecular weights and distributions of resultant poly-
ethylenes controllably relied on the nature of ligands used.
Fig. 1 ORTEP drawing of complex Fe1 with thermal ellipsoids at
30% probability level. Hydrogen atoms have been omitted for clarity.
MAO (entry 17, Table 1). The increasing temperature resulted
in deactivation (entries 3, 5–8, Table 1) due to unstable active
species and lower ethylene solubility.6,11c,12
Considering industrial application of highly exothermal poly-
merization, the critical factor is to maintain good activity within
industrial operation temperature13 such as around 60 1C for
UHMPE (ultra high molecular weight polyethylene) or 80 1C
even higher for common polyolefins. Then the evaluation was
carried out under 10 atm ethylene (entries 9–16 and 18–23,
Table 1), and the best performance was obtained with Fe3/
MMAO at 80 1C and Al/Fe molar ratio of 2000 (entry 15,
Table 1), and Fe3/MAO at 60 1C and Al/Fe molar ratio of 1500
(entry 22, Table 1), respectively. At the Al/Fe molar ratio of
1500, polyethylenes with high molecular weights and narrow
distributions were obtained as observed in our work16 and by
Brookhart,1a Gibson et al.14b A unique feature is that poly-
ethylenes with narrow molecular distributions were noticeable
Table 2 Ethylene polymerization using MAOa
Activity 106 g molꢀ1
Entry Procat. (Fe) hꢀ1
Mwb/kg molꢀ1 Mw/Mn Tmc/1C
b
1
2
3
4
5
6
7
Fe1
Fe2
Fe3
Fe4
Fe5
Fe0
13.1
12.4
9.95
4.89
22.3
313.7
260.0
166.5
443.1
99.8
16.4
8.8
10.9
23.4
8.3
134.0
132.3
136.7
133.7
132.7
134.4
133.3
0.76
5.71
285.2
101.6
7.2
14.1
FeR
a
b
Conditions: 1.5 mmol of Fe; 100 mL toluene; 30 min. Determined
by GPC. Determined by DSC.
c
c
3258 Chem. Commun., 2011, 47, 3257–3259
This journal is The Royal Society of Chemistry 2011

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Table 3 Ethylene polymerization using MMAOa
Notes and references
Activity 106 g molꢀ1
1 (a) B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am. Chem.
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Entry Procat. (Fe) hꢀ1
Mwb/kg molꢀ1 Mw/Mn Tmc/1C
b
1
2
3
4
5
6
7
Fe1
Fe2
Fe3
Fe4
Fe5
Fe0
21.5
19.4
11.9
22.4
5.10
Trace
4.80
7.6
6.5
17.7
25.5
294.9
—
1.6
1.5
4.3
5.1
12.2
—
129.3
131.0
129.3
129.7
133.7
—
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130.0
a
Condition: 1.5 mmol of Fe; Al/Fe: 2000; total volume: 100 mL;
reaction time: 30 min; reaction temperature: 80 1C. Determined by
GPC. Determined by DSC.
b
c
Extensively, all procatalysts were investigated under 10 atm
ethylene in the presence of MMAO with Al/Fe molar ratio of
2000 at 80 1C (Table 3). A similar catalytic behaviour was
observed for these iron complexes. Comparison with systems
employing MAO, the optimum temperature with MMAO
illustrated 20 1C higher up to 80 1C, the commonly used
industrial temperature. Fe1–Fe5 performed sole polymerization
without any trace of oligomers being observed, but Fe0 showed
a negative response to ethylene polymerization while FeR
showed good activity in ethylene polymerization. Generally
Fe1–Fe4 showed higher activities than FeR. In comparison with
MAO, different influences by the para-methyl were within Fe1
and Fe4, Fe2 and Fe5 due to plausible electronic effects.
Considering the industrial application of 2,6-bis(imino)-
pyridyliron(II) procatalysts in ethylene polymerization, with
the best of our knowledge, the current iron complexes bearing
the nonsymmetric 2,6-bis(imino)pyridines have the high poten-
tial relying on not only the high activity maintained at elevated
reaction temperature but also obtaining polyethylenes with
controllable molecular weights and distributions. In addition,
there is an important advantage of obtaining polyethylenes
without any trace of ethylene oligomerization.
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Within iron procatalysts bearing the nonsymmetric 2,6-
bis(imino)pyridines, the bulk of one 2,6-dibenzhydrylaniline
was helpful in stabilizing the complexes. The nonsymmetri-
cally spherical environment, besides the active species
commonly obtained within 2,6-bis(imino)pyridyliron procata-
lysts, oriented the orderly spaces for ethylene coordination and
growing stretch polymer chain, therefore, the combined influ-
ences resulted in active species favorable for ethylene coordi-
nation and chain propagation. This represents a success of
fine-tuning 2,6-bis(imino)pyridyliron dichlorides, and the
catalytic activities were improved from analogues bearing
symmetrical ligands such as Fe0 and FeR (represents common
2,6-bis(imino)pyridyliron dichlorides).1,15,17
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In summary, the 2-[1-(2,6-dibenzhydrylphenylimino)ethyl]-6-
[1-(arylimino)ethyl]pyridyliron(II) procatalysts showed high
activities in ethylene polymerization at 80 1C with MMAO and
60 1C with MAO, without any trace of oligomers. Beyond the
high activity and industrially suitable temperature, the molecular
weights and distributions of resultant polyethylenes could be
controllable. The iron procatalysts are worthy of further investiga-
tion, and are potentially applicable for industrial consideration.
This work is supported by NSFC No. 20874105 and MOST
863 program No. 2009AA033601.
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c
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Chem. Commun., 2011, 47, 3257–3259 3259