A new iron catalyst for ethylene polymerization

Article abstract of DOI:10.1016/j.molcata.2003.10.005

The iron-based catalyst 2,6-bis[1-(4-bromo-2,6-dimethylphenylimino)ethyl]- pyridineiron(II) chloride, 1, has been successfully synthesized and characterized. Its catalytic activity was evaluated for the polymerization of ethylene at different temperatures and Al/Fe ratios and compared with that of 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridineiron(II) chloride, 2. The results show that the catalytic activity of catalyst 1 is highly dependent on the amount of MAO used in the polymerization of ethylene and presents a maximum value at about 20°C.

Full text of DOI:10.1016/j.molcata.2003.10.005

Journal of Molecular Catalysis A: Chemical 211 (2004) 55–58
A new iron catalyst for ethylene polymerization
Icaro S. Paulino, Ulf Schuchardt∗
Instituto de Qu´ımica, Universidade Estadual de Campinas, P.O. Box 6154, 13084-971 Campinas, SP, Brazil
Received 10 June 2003; received in revised form 6 October 2003; accepted 6 October 2003
Abstract
The iron-based catalyst 2,6-bis[1-(4-bromo-2,6-dimethylphenylimino)ethyl]-pyridineiron(II) chloride, 1, has been successfully synthesized
and characterized. Its catalytic activity was evaluated for the polymerization of ethylene at different temperatures and Al/Fe ratios and compared
with that of 2,6-bis[1-(2,6-dimethylphenylimino)ethyl]pyridineiron(II) chloride, 2. The results show that the catalytic activity of catalyst 1 is
highly dependent on the amount of MAO used in the polymerization of ethylene and presents a maximum value at about 20 ◦C.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Ethylene polymerization; Iron(II) complex; 2,6-Bis(imino)pyridyl ligand
1. Introduction
2. Experimental
It has been shown that in the presence of methylalumi-
noxane (MAO) 2,6-bis(imino)pyridyliron(II) complexes cat-
alyze the polymerization of ethylene with high activity [1,2].
These catalysts present great advantages since their dichlo-
ride derivatives can be easily prepared and isolated as air
stable blue solids. Other advantages of these systems are
their low cost and the ease with which they can be tai-
lored. Indeed, the substitution pattern of the tridentade lig-
and is crucial in controlling the molar mass of the polymer,
since the size of the aryl group is the main factor affect-
ing olefin insertions, chain transfer reactions and catalytic
activity.
The synthesis of the catalyst and the polymerization
of ethylene were carried out under an argon atmosphere.
Solvents (THF and toluene) were refluxed over an appro-
priate drying agent and distilled prior to use. Elementary
analyses were performed with a Perkin-Elmer 2401 anal-
yser. The mass spectra were obtained by electron impact
at 70 eV and 10−5 bar, using a Shimadzu QP-5000 spec-
trometer, coupled to a direct inlet for solids, Shimadzu
DI-50. 2,6-Diacetylpyridine, 4-bromo-2,6-dimethylaniline,
2,6-dimethylaniline and FeCl2·4H2O were purchased from
Aldrich; MAO was obtained from Witco and ethylene was
´
obtained from Petroquımica União.
For example, decreasing the size of the ortho-aryl sub-
stituents from isopropyl to methyl groups reduces the
polyethylene molar mass [3]. In this way a range of linear
polyethylene products can be synthesized by simple modi-
fication of the catalyst structure, as well as by changing the
temperature and pressure of the polymerization.
Here we wish to report the synthesis of a new member
of this family bearing a bulky para-bromo-aryl substituent.
This catalyst is highly active in the polymerization of ethy-
lene at low temperatures.
2.1. Synthesis of 2,6-bis[1-(4-bromo-2,6-
dimethylphenylimino)ethyl]pyridine
2,6-Diacetylpyridine (9.2 mmol, 1.5 g) and 25 ml an-
hydrous ethanol were placed in a 50 ml flask fitted with
a condenser. Under stirring, 4-bromo-2,6-dimethylaniline
(18.6 mmol, 3.7 g) and a few drops of formic acid were
added consecutively. The mixture was heated to 80 ◦C and
kept at this temperature for 48 h. The yellow solid formed
was then filtered and washed with cold methanol and dried
under vacuum; the yield was 60%. Mass spectrum, m/z 525
[M+]. Elemental analysis: Calculated: C, 56.9; H, 4.8; N,
8.0. Found: C, 56.3; H, 4.8; N, 8.2.
∗
Corresponding author. Tel.: +55-19-3788-3071/3099;
fax: +55-19-3788-3023.
E-mail address: ulf@iqm.unicamp.br (U. Schuchardt).
1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.10.005

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I.S. Paulino, U. Schuchardt / Journal of Molecular Catalysis A: Chemical 211 (2004) 55–58
2.2. Synthesis of 2,6-bis[1-(2,6-
dimethylphenylimino)ethyl]pyridine
2.6. Determination of the melting point
The thermal characteristics of the polymers were exam-
ined using a DSC 4 instrument (Perkin-Elmer) with a heating
rate of 10 ◦C/min, in the range from 50 to 200 ◦C. Melting
points (m.p.) are related to the second heating cycle.
Using a procedure similar to that described above,
this ligand was obtained from 2,6-diacetyl pyridine and
2,6-dimethylaniline as a yellow powder in 75% yield.
Mass spectrum, m/z 369 [M+]. Elemental analysis: Calcu-
lated: C, 81.6; H, 7.4; N, 11.4. Found: C, 80.2; H, 7.6; N,
11.2.
2.7. Determination of the molar masses
The intrinsic viscosity [η] of a 0.1 g/dl polymer solu-
tion in decalin was determined using a Canon–Fenks vis-
cosimeter at 135 ◦C and the one-point intrinsic viscosity
method [4]. The viscosity–average molar masses (Mv) of
the polyethylenes were calculated using the Mark–Houwink
equation [5]:
2.3. Synthesis of 2,6-bis[1-(4-bromo-2,6-
dimethylphenylimino)ethyl]pyridineiron(II)
chloride (1)
¯
The bromo ligand (0.24 mmol, 0.12 g) and FeCl2·4H2O
(0.23 mmol, 0.046 g) were added to a Schlenk flask under
argon. Then, 10 ml of n-butanol were added with a syringe.
The mixture was magnetically stirred at 50 ◦C for 30 min
and then filtered. The blue powder was collected, washed
with ethyl ether and pentane and dried under vacuum; the
yield was 95%. Mass spectrum, m/z 651 [M+]. Elemental
analysis: Calculated: C, 45.9; H, 3.9; N, 6.4. Found: C, 44.2;
H, 4.0; N, 6.6.
[η] = 62 × 10−3 Mv
0.7
¯
3. Results and discussion
Complexes 1 and 2 were used in the homogeneous ethy-
lene polymerization in order to investigate the effect of a
para-bromo-aryl substituent on the catalytic properties of
this compound, since it is very difficult to compare results
obtained by different authors, as reaction conditions are usu-
ally different. The active catalysts were generated in situ in
toluene by the addition of MAO. Typical results obtained in
the polymerization reactions are shown in Table 1.
2.4. Synthesis of 2,6-bis[1-(2,6-
dimethylphenylimino)ethyl]pyridineiron(II)
chloride (2)
Using the procedure described above, this catalyst was ob-
tained from the unbrominated ligand as a dark blue power in
85% yield. Mass spectrum, m/z 495 [M+]. Elemental anal-
ysis: Calculated: C, 60.5; H, 5.5; N, 8.5. Found: C, 60.6; H,
5.8; N, 8.2.
The results show that the variation of the para substituent
at the aryl ring has a pronounced effect both on the catalytic
activity and on the average molar mass of the resulting poly-
mer. We observe that the replacement of the para-aryl hydro-
gen by a bromine results in an increase of the activity from
3750 to 5900 kg PE (mol Fe)−1 bar−1 h−1 and a decrease in
the average molar mass Mv from 65.4 to 49.6 kg/mol.
These results are interesting, since polymerization mech-
anisms of homogeneous Ziegler–Natta catalysts involve
cationic intermediate species. In this way, steric and elec-
tronic ligand effects that make cationic species more stable
or unstable can lead to the increase or decrease of cat-
alytic activity. For catalysts 1 and 2 we can consider that
the electronic effect is more important since a substitution
in the para position is not expected to have a significant
steric effect. The electronic effect of the para-bromo-aryl
substituint probably accelerates the fundamental steps of
the polymerization process (olefin insertion and chain ter-
mination), thus resulting in an increase in catalytic activity
and a decrease of the polymer molar mass. This theory is
plausible since the electron-withdrawing bromine increases
the electrophilic character of the metallic center, thus accel-
erating ethylene coordination and chain transfer.
2.5. Polymerization
The polymerization experiments were carried out in a
1 l Büchi reactor varying the temperature from −5 ◦C up
to 70 ◦C, the Al/Fe ratio from 1000 to 4000, and the pres-
sure of ethylene from 2 to 5 bar, using 50 ml of toluene as
solvent. After 30 min, the polymerization reactions were
interrupted by the addition of ethanol. The polymers were
filtered, washed with ethanol and dried in an oven at 60 ◦C
for 4 h.
In Table 1 we also observe that an increase of the poly-
merization pressure (entries 1 and 2) results in an increased
yield, but in a decrease in catalytic activity. Furthermore,
the polymerization pressure does not have an influence on

I.S. Paulino, U. Schuchardt / Journal of Molecular Catalysis A: Chemical 211 (2004) 55–58
57
Table 1
Results obtained in the ethylene polymerization using catalysts 1 and 2
Entry
Catalyst
Al/Fea
Polymerization Conditions
Activity
(kg PE (mol Fe)−1 bar−1 h−1
Yield (g)
m.p. (◦C)
¯
Mv (kg/mol)
)
1
2
3
4
5
6
1
1
1
1
2
2
1000
1000
1000
2000
1000
2000
Fe = 2.0 ␮mol, 50 ◦C, 5 bar
Fe = 2.0 ␮mol, 50 ◦C, 2 bar
Fe = 2.0 ␮mol, 30 ◦C, 2 bar
Fe = 2.0 ␮mol, 30 ◦C, 2 bar
Fe = 2.0 ␮mol, 30 ◦C, 2 bar
Fe = 2.0 ␮mol, 30 ◦C, 2 bar
1300
2350
3580
5900
2150
3750
6.5
4.7
6.0
11.8
4.3
8.0
16.7
16.7
25.4
49.6
38.5
65.4
129.6
129.7
131.4
134.8
134.1
135.1
a
Al/Fe ratio during polymerization.
the average molar mass or melting point of the polymers
obtained.
The strong temperature dependence of the catalytic ac-
tivity is a common characteristic of Ziegler–Natta catalysts
due to the high activation energy required for the polymer-
ization process. However, for different kinds of catalysts,
we can observe maximum values at different temperatures.
When the temperature is increased, the catalytic activity is
gradually increased up to a maximum value and then be-
gins to decrease. This behavior is intrinsically related to the
stability of the catalyst at higher polymerization tempera-
tures. For example, for zirconocene catalysts the catalytic
activity increases with temperature up to 80 ◦C and then
decreases [6]. The same behavior was observed for the
bis(salicylaldiminato)titanium catalysts, which shows max-
imum activity at 40 ◦C [7].We observed the same tendency
for the 2,6-bis[1-(4-bromo-2,6-dimethylphenylimino)ethyl]
pyridineiron(II) chloride catalyst 1, since the catalytic ac-
tivity increases up to 20 ◦C and then decreases quickly, due
to deactivation of the catalyst. For catalyst 2 the highest
activity was observed at 10 ◦C, however, the activity was
always lower than that observed for catalyst 1 at similar
temperatures.
In order to investigate the effects of the Al/Fe ratio on
the catalytic activity, polymerizations of ethylene were per-
formed with 1 and 2 at 30 ◦C, varying the Al/Fe ratio from
1000 to 4000. Fig. 1 shows the results for the polymeriza-
tions of ethylene at different concentrations of MAO. When
we used catalyst 1 at an Al/Fe ratio of 1000, the catalytic
activity was 3580 kg PE (mol Fe)−1 bar−1 h−1, while using
an Al/Fe ratio of 4000, the catalytic activity was 9550 kg PE
(mol Fe)−1 bar−1 h−1. These results show that the catalytic
activity of catalyst 1 is highly dependent on the amount of
MAO used in the polymerization process. Additionally, we
observed an increase of the average molar mass with an in-
crease of the Al/Fe ratio, indicating that the MAO concen-
tration has little influence on the chain termination process,
independent of the Al/Fe ratio used.
A similar behavior was observed for catalyst 2 for an
increase of the Al/Fe ratio from 1000 to 4000, however, the
catalyst activity was smaller for all Al/Fe ratio used and
the molar mass of the polyethylene always higher for the
reasons already explained before.
Several studies were carried out to determine the effect
of the temperature on the catalytic activity. Fig. 2 shows the
results of these experiments in a temperature range of −5 to
70 ◦C, using catalysts 1 and 2.
The temperature also shows a significant influence on the
average molar mass of the polyethylene obtained with cata-
lyst 1, which decreases quickly as the temperature increases.
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
90
80
70
60
50
40
30
20
10
Activity Cat. 1
Activity Cat. 2
Molar Mass Cat. 1
14000
12000
10000
8000
6000
4000
2000
0
90
75
60
45
30
15
0
Activity Cat. 1
Activity Cat. 2
Molar Mass Cat. 1
Molar Mass Cat. 2
-10
0
10
20
30
40
50
60
70
80
Temperature (oC)
0
1000
2000
3000
4000
5000
Al/Fe ratio
Fig. 2. Effect of temperature on the performance of catalyst 1 and 2.
Pressure of ethylene, 2 bar; time of polymerization, 0.5 h; Al/Fe ratio,
1000.
Fig. 1. Effect of the Al/Fe ratio on the catalytic activity of catalyst 1 and 2.
Ethylene pressure, 2 bar; polymerization time, 0.5 h; temperature, 30 ◦C.

58
I.S. Paulino, U. Schuchardt / Journal of Molecular Catalysis A: Chemical 211 (2004) 55–58
This can be attributed to an increase of chain transfer
processes with increasing polymerization temperatures,
since chain length is determined by the relative rates of prop-
agation (insertion) and chain termination (by chain transfer)
[8]. At lower temperatures the chain transfer rates are lower
and polymers with higher molar mass can be obtained.
Acknowledgements
The authors would like to thank FAPESP and CNPq for
financial support and Petroqu´ımica União and Witco for sup-
plying the starting materials.
References
4. Conclusions
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We have shown that 2,6-bis(iminoethyl)pyridine iron
complexes, bearing bulky para-bromo substituted aryl
groups, present high catalytic activity for the polymeriza-
tion of ethylene, mainly when the reactions are carried out
around 20 ◦C. The electronic effect of the para-bromo-aryl
groups results in an increase of activity and a decrease
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mass while the variation of the ethylene pressure does not
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