Propylene polymerization with a bisiminepyridine iron complex: Activation with Ph3C [B(C6F5)4] and AlR3; iron hydride species in the catalytic cycle

Article abstract of DOI:10.1016/S1381-1169(02)00340-0

The complex 2,6-bis [1-(2-isopropyl-6-methylphenylimino)ethyl]-pyridineiron(II)di chloride was used for propylene polymerization. Activation with Ph₃C [B(C₆F₅)₄] and subsequent treatment with triisobutylaluminum or triethylaluminum generated a very active polymerization catalyst for propylene. The activity was much higher than that obtained with methylaluminoxan (MAO) and even higher than using modified MAO. Detection of the propylene consumption with mass-flowmeters allowed the first view of the kinetic profile of propylene polymerization with this type of catalysts. Hydrogen addition to the polymerization led to higher activities and allowed formulation of a complete catalytic reaction cycle containing an iron hydride species. This cycle comprised one starting cycle and two propagation cycles, which could be controlled with the aluminumtrialkyl-propylene ratio.

Full text of DOI:10.1016/S1381-1169(02)00340-0

Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
Propylene polymerization with a bisiminepyridine iron complex:
activation with Ph3C [B(C6F5)4] and AlR3; iron hydride
species in the catalytic cycle
Sebastian Thomas Babik, Gerhard Fink∗
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
Received 10 December 2001; accepted 6 May 2002
Abstract
In this work, the complex 2,6-bis [1-(2-isopropyl-6-methylphenylimino)ethyl]-pyridineiron(II)dichloride was used for
propylene polymerization. Activation with Ph3C [B(C6F5)4] and subsequent treatment with triisobutylaluminium or triethy-
laluminium generated a very active polymerization catalyst for propylene. The kinetics of this polymerization reaction were
investigated with mass-flowmeters. Further we show propagation-time-profiles of this reaction. Depending on the nature of
the aluminiumalkyl we are able to determine different aliphatic endgroups in the polymer using 13C NMR spectroscopy.
Addition of hydrogen to the polymerization leads to higher activities and allows us to formulate a complete catalytic reaction
cycle containing an iron hydride species. This cycle comprises one starting cycle and two propagation cycles which can be
controlled with the aluminiumtrialkyl-propylene ratio.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Bisiminepyridine iron complex; Propylene polymerization; Propagation-time-profiles; Aluminiumtrialkyls; Iron hydride species
1. Introduction
the catalyst activity for the propylene polymerization,
because activation with MAO (methylaluminoxan)
used by Brookhart yields an unsatisfactory amount
of polymer. Activation of the bisiminepyridine iron
complex with Ph3C [B(C6F5)4] and subsequent treat-
ment with TIBA produces very high maximum activ-
ities exceeding 1100 kg PP/mol Fe h bar [C3H6]. With
different aluminiumtrialkyls the nature of the active
iron alkyl species that starts the polymerization can
be varied and the relevant aliphatic endgroups can
be detected by 13C NMR spectroscopy. Addition of
8 vol.% hydrogen to the polymerization leads once
more to higher activities and implies the presence of
an iron hydride species in the catalytic cycle. With
our results we can propose a complete catalytic cycle
consisting of two propagation cycles and one starting
cycle.
For some years now, there has been a great acade-
mic [1–9] and industrial [10–13] interest in polymer-
ization catalysts based on several imine complexes
of late transition metals such as cobalt, iron, nickel
or palladium. These kinds of catalysts are easier to
synthesise and more tolerant to polar groups than
metallocenes. Based on the works of Small and
Brookhart [5], in which propylene was polymerized
with bisiminepyridine iron complexes, we investi-
gated the kinetics and mechanism of propylene poly-
merization. First of all, it was necessary to increase
∗
Corresponding author. Tel.: +49-208-306-2240;
fax: +49-208-306-2980.
E-mail address: fink@mpi-muelheim.mpg.de (G. Fink).
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(02)00340-0

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S.T. Babik, G. Fink / Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
2. Experimental part
flame-dried two-neck-flask. A solution of ligand L
(2.4 g; 5.7 mmol), in 25 ml dry THF was added slowly
at room temperature via a dropping funnel. The brown
suspension of FeCl2 turned into a immediately to a
blue colour. The mixture was then stirred under argon
for one additional hour. Then pentane was added to the
blue suspension and the solid was filtered and dried
under argon. The light-blue complex was isolated in
near quantitative yield.
2.1. General considerations
The handling of water- and air-sensitive compo-
unds was performed under an argon atmosphere using
Schlenk techniques.
2.2. Materials
C29H35N3FeCl2 (552.37): calcd. C, 63.06; H, 6.39;
N, 7.61. Found C, 62.89; H, 6.45; N, 7.56. MS (70 eV):
m/z = 551 (M+), 516 (M+–Cl), 425 (M+–FeCl2 =
Ligand+), 410 (100%, L+–Me).
Methanol was dried over CaH2/Mg and dis-
tilled. Toluene was distilled from sodium. THF
was distilled from MgH2. 2,6-Diacetylpyridine,
2-isopropyl-6-methylaniline, 97% formic acid and
FeCl2 were purchased from Aldrich and used with-
out further purification. MAO (10 wt.% solution in
toluene) and Ph3C [B(C6F5)4] were purchased from
Witco. Triisobutylaluminium (TIBA) and triethyla-
luminium (TEA) were produced in our institute’s
facility. Propylene (99.5%) was purchased from
Messer-Griesheim and purified by passage through
columns of molecular sieves (3 Å) and NaAlEt4.
2.5. Polymerization procedure
The polymerizations were carried out in a 250 ml
glass autoclave (Büchi AG, Uster/CH) with a mechan-
ical stirrer (1200 rpm) under constant propylene pres-
sure of 2 bar and constant temperature. To guarantee
inert reaction conditions the autoclave was evacuated
and back-filled with argon three times. Then the reac-
tor was filled with toluene and the aluminiumtrialkyl,
thermostated and saturated with propylene (total vol-
ume of the liquid phase: 120 ml). After saturation, the
iron complex and the borate were added in the reac-
tor via an injection system with an excess pressure of
argon or, in case of the experiments with hydrogen,
with an excess pressure of hydrogen. The consumption
of propylene was continuously detected with Brooks
mass-flowmeters (Brooks Instruments B.V.). To stop
the polymerization the excess pressure of propylene
was vented and methanol was added to the reactor.
The reaction mixture was poured into 600 ml of a di-
luted solution of hydrochloric acid in methanol and
stirred overnight. The precipitated polymer was fil-
tered, washed with fresh methanol and dried at 50 ◦C
in a vacuum oven.
2.3. Synthesis of 2,6-bis [1-(2-isopropy-6-
methylphenylimino)ethyl]-pyridine (L)
2,6-Diacetylpyridine (1.14 g, 7 mmol) was dissol-
ved in 15 ml of dry methanol in a 50 ml round-bottom
flask equipped with a condenser. 2-Isopropyl-6-
methylaniline (6.27 g, 42 mmol) was added at 45 ◦C
via a dropping funnel. Four drops of 97% formic acid
were added and the clear, brown solution was allowed
to stir in the sealed flask at 60 ◦C for 1 h. After stir-
ring overnight at room temperature the resultant pale
yellow solid precipitate, was collected by filtration,
washed with cold methanol and dried. The yield was
2.44 g (82%) of pure ligand.
1H NMR (CDCl3): δ = 8.43–8.41 (d, 2, py-Hm);
7.89–7.84 (t, 1, py-Hp); 7.13–7.10 (m, 2, Haryl);
7.02–6.96 (m, 4, Haryl); 2.79–2.74 (septet, 2, CHMe3);
2.19 (s, 6, N = CCH3); 1.97 (s, 6, ArCH3); 1.15–1.13
2.6. Polymer characterization
(d, 6, CHCH3); 1.09–1.07 ppm (d, 6, CHCH3,
3
The microstructure of the polymers was analysed
with 13C NMR spectroscopy. The polymer (159 mg)
was dissolved in 3 ml of 1,2,4-trichlorobezene/1,1,2,2-
tetrachloroethane–d2 (volume ratio, 2.5/1) in a 10 mm
NMR tube and measured with proton broad-band
decoupling at 120 ◦C on a Bruker AMX 300 spec-
trometer at 75.5 MHz. The molar mass experiments
=
JH,H(i-pr) = 6.93 Hz). IR (KBr): 3068 (m, H–C ),
2963–2868 (s, H–C), 1640 cm−1 (vs, C N).
=
2.4. Synthesis of the iron complex
Dry FeCl2 (700 mg; 5.5 mmol) in 20 ml dry THF
was stirred under an argon atmosphere in a 100 ml

S.T. Babik, G. Fink / Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
247
were carried out in 1,2,4-trichlorobezene at 155 ◦C
Table 2
Pentadanalysis of polypropylene produced by MAO activationa
using gel permeation chromatography (Waters) 150-C
with a viscosity detector (Viscotek) and a LC 300
Transform (Lab Connections). GPC data were ob-
tained using a calibration file which was performed
with narrow polystyrene and polyethylene standards.
Pentad mmmm mmmr mmrr mrmm/rrmr mrmr mrrm
65 14.3 3.9 12.9 2.8 1.1
%
a Conditions: [Fe] = 2.6 × 10−5 mol; p = 1.9 bar propylene
pressure; TP = 25 ◦C, duration of polymerization: 1 h; solvent:
toluene; [Al]/[Fe]-ratio = 80 : 1.
3. Results and discussion
spectroscopy. In the region between 45 and 10 ppm
significant signals arising from endgroups are found.
With the MAO activated polymerization only de-
tectable aliphatic endgroups are n-butyl groups. We
explain later how n-butyl endgroups are formed dur-
ing the polymerization.
3.1. Activation with MAO
Using the methodology described in Section 2,
propylene was polymerized with the iron complex
(Fig. 1) using MAO as co-catalyst. Under the given
conditions (Table 1) we always observed low activi-
ties and obtained wax-like polymers. The isotacticity
index was about 65% in all cases and independent of
the Al/Fe ratio.
The tacticity of the obtained polymers was deter-
mined with 13C NMR spectra focusing on the region
of the methylpentads between 22.0 and 19.5 ppm. The
pentad distribution is shown in Table 2.
3.2. Activation with Ph3C [B(C6F5)4] and
subsequent treatment with aluminiumtrialkyls[14,15]
Due to the low activity of MAO as co-catalyst we in-
vestigated alternative methods to activate the iron com-
plex. It is well known, that metallocenedialkyls can
be activated with boranes or borates. Because dialkyl
complexes of iron are not known, we chose a different
route. Treatment of the dichloride of the iron complex
(Scheme 1) with Ph3C [B(C6F5)4] followed by TIBA
generates a high concentration of active species for
the propylene polymerization.
It was possible to determine the nature of the
aliphatic endgroups in the polymer using the 13C NMR
Step 1. The formation of the predicted triphenyl-
methylchloride was confirmed by coupled HPLC/
UV–Vis spectroscopy. With the addition of 80 equiv.
TIBA (Scheme 2) the second chloride is substituted
by an isobutyl group and an iron–carbon bond that
can insert propylene is generated.
Step 2. With this activation procedure we achieved
activities of about 1100 kg PP/mol Fe h bar [C3H6]
which is more than 200 times higher than with MAO
activation. In Fig. 2 we present the polymerization
rate–time profiles of both activation methods for
comparison.
Fig. 1. Structure of the used iron complex: 2,6-bis [1-(2-isopropyl-
6-methylphenylimino)ethyl]-pyridineiron(II)dichloride.
Table 1
Propylene produced by MAO activationa
Al/Fe-ratio
Activity
(kg PP/mol Fe h)
Isotacticity
%-mmmm
pentad
Mn
Mw/Mn
We also used TEA instead of TIBA and mea-
sured the polymerization rate–time profile (Fig. 3).
In comparison with TIBA activation, the maximum
rate appears much faster, but it is seven times less
active. After reaching the maximum very quickly, the
polymerization rate declines rapidly until almost no
consumption of propylene is registered.
60:1
80:1
100:1
150:1
14
19
11
7
65
65
62
64
3500
3300
3700
3800
1.30
1.35
1.30
1.26
a Conditions: [Fe] = 2.6 × 10−5 mol; p = 1.9 bar propylene
pressure; TP = 25 ◦C; duration of polymerization: 1 h; solvent:
toluene.

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S.T. Babik, G. Fink / Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
Scheme 1. Reaction of the iron complex with borate.
Scheme 2. Reaction of the cationic iron species with TIBA.
Fig. 2. Polymerization rate–time profiles. (a) [Fe] = 2.45 × 10−5 mol; Ph3C [B(C6F5)4] = 2.42 × 10-5 mol; p = 1.9 bar propylene; 0.5 ml
TIBA; TP = 25 ◦C; solvent: toluene. (b) [Fe] = 2.6 × 10−5 mol; co-catalyst: MAO 80:1; p = 1.9 bar propylene; TP = 25 ◦C; solvent:
toluene.
Fig. 3. Polymerization rate–time profile: [Fe] = 2.45 × 10−5 mol; Ph3C [B(C6F5)4] = 2.42 × 10−5 mol; p = 1.9 bar propylene; 0.5 ml
TEA; TP = 15 ◦C; solvent: toluene.

S.T. Babik, G. Fink / Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
249
Table 3
Iron species and polymer endgroups that occur with TIBA and TEA activation
3.3. Endgroup determination
For clarity we generated different iron alkyls in
situ to obtain defined starting species for the propy-
lene polymerization. Corresponding to the second
activation step (Scheme 2), only iron isobutyl species
exist as the starting species after activation with
TIBA. With a subsequent 2,1 propylene insertion the
3-methyl-n-butyl endgroup is generated. The termi-
nation step via the ␤-H elimination leads to the iron
hydride. To generate the n-butyl endgroup it is neces-
sary to start with the iron hydride. Propylene adds to
the iron hydride bond via a 1,2 insertion step forming
iron n-propyl species. Now propylene inserts via a 2,1
step into the previously formed iron n-propyl species
and generates the n-butyl endgroup in the growing
polymer.
The same sequence of reaction steps should be ob-
served using TEA in the second activation step. Only
iron ethyl groups exist after the second activation
step. Via a 2,1 propylene insertion into the iron ethyl
bond the n-propyl endgroup in the growing poly-
mer is generated. After the ␤-H elimination step the
iron hydride is formed and inserts propylene via a
1,2 mechanism to generate the known iron n-propyl
bond. This bond is expected to insert propylene via
a 2,1 mechanism to generate n-butyl endgroups in
the growing polymer as is described with TIBA acti-
vation. However, only n-propyl endgroups are found
in the polymer. In the case of TEA there must be a
reaction that excludes the iron n-propyl species from
the 2,1 propylene insertion step. There is only one
Small and Brookhart [5] investigated this in an
earlier work with activation with MMAO. The low
molecular weights (∼3500) and high solubilities of
the polymer enabled detection of aliphatic endgroups
by 13C NMR due to good resolution and high inten-
sity of the signals. The iron species and structures of
polymer endgroups are presented in Table 3.
Using TIBA leads to two different aliphatic end-
groups. We found both n-butyl endgroups and 3-
methyl-n-butyl endgroups. This shows that propylene
is inserted into two different iron alkyl species.
In the case of TEA we found only one aliphatic end-
group: the n-propyl endgroup. Small and Brookhart
[5] assumed without experimental proof that the
n-butyl endgroup can only be generated when start-
ing with an iron hydride bond in which propylene
inserts via a 1,2 mechanism to form an iron n-propyl
species. Subsequent 2,1 propylene insertion into the
iron n-propyl species forms the n-butyl endgroup in
the polymer. In this report we will demonstrate the
experimental evidence for the suggested iron hydride
species and show that it plays an important role in
the catalytic cycle. The 3-methyl-n-butyl endgroup
is explained by Brookhart et al. by a 2,1 propy-
lene insertion into an iron isobutyl species, which is
formed by activating the iron complex with MMAO
(modified MAO, contains 25% isobutyl groups on
aluminium).

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S.T. Babik, G. Fink / Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
Fig. 4. Polymerization rate–time profiles: (a) polymerization with hydrogen; (b) polymerization without hydrogen. [Fe] = 2.1 × 10−5 mol;
Ph3C [B(C6F5)4] = 2.05 × 10−5 mol; p = 1.9 bar propylene; 0.1 ml TIBA; TP = 16 ◦C; solvent: toluene.
plausible explanation: the iron n-propyl species is
rapidly transformed by exchange with TEA into an
iron ethyl bond which generates the n-propyl endgroup
in the polymer after a 2,1 propylene insertion step.
the highly active iron hydride. The polypropylene pro-
ducts from reactions containing hydrogen were anal-
ysed by 13C NMR spectroscopy. As seen in the
spectrum (Fig. 5), only the n-butyl endgroup can
be detected in the polymer. Although TIBA was used
for the second activation step no 3-methyl-n-butyl
groups can be detected in the polymer obtained.
Scheme 3 implies that all iron isobutyl species
are transformed into iron hydride species, because
the characteristic 3-methyl-n-butyl endgroup for 2,1
propylene insertion in the iron isobutyl bond is not
detectable. As an effect of hydrogen addition the
molecular mass goes down to 1300 and the Mw/Mn
value rises up slightly to 1.8. The reason for the
3.4. Influence of hydrogen on the polymerization[16]
The previous discussion provides further evidence
for the statement, that an iron hydride species is a
central part in the catalytic cycle of propylene poly-
merization. In Fig. 4 and Table 4 we show the in-
fluence of hydrogen on the polymerization rate–time
profiles. Adding 8 vol.% hydrogen to the polymer-
ization as described in Section 2 leads to an almost
100% increased activity.
This implies that addition of hydrogen must lead to
a higher concentration of active species. In Scheme 3
the reaction that generates the more active iron hydride
is shown.
The iron isobutyl species, formed in the second ac-
tivation step, reacts with added hydrogen to generate
Scheme 3. Reaction of the iron isobutyl species with hydrogen
generating a high active species for propylene polymerization.
Table 4
Comparison of polymerizations without and with hydrogen 16 ◦C (solvent: toluene)
Conditions
FeLCl2 (mol/l) Ph3C [B(C6F5)4] (mol/l) TIBA (ml) Max. activity (1/s mol[C3H6]) Yield (g) Mn
Mw/Mn
Without H2
With H2
2.1 × 10−5
2.1 × 10−5
2.05 × 10−5
2.05 × 10−5
0.1
0.1
1.8 × 10−4
3.6 × 10−4
3.68
6.76
3500 1.40
2000 1.80

S.T. Babik, G. Fink / Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
251
Fig. 5. 13C NMR spectrum of a polypropylene made with hydrogen addition conditions: [Fe] = Ph3C [B(C6F5)4] = 2.5 × 10−5 mol;
p = 1.9 bar propylene; TP = 5 ◦C; 0.5 ml TIBA; solvent: toluene; 8 vol.% hydrogen.
lower molecular mass is, according to the reaction in
Scheme 3, the reaction of hydrogen with the growing
polymer chain. This yields a new iron hydride species
and the chain reaction can continue.
From the iron n-propyl species 3, the reaction can
follow two different competing courses. Through re-
alkylation with TIBA the iron isobutyl species 1 can
be generated, which continuously inserts propylene in
a 2,1 step to form species 6 in propagation cycle A.
On the other hand 2,1 propylene insertion in species
3 in propagation cycle B leads to species 4. Hence,
realkylation and 2,1 propylene insertion are running
in competition. After ␤-H elimination from a grown
polymer species 4 or 6 the iron hydride 2 is generated
and the polymers 5 and 7 are formed. The very active
iron hydride 2 inserts propylene in a unique 1,2 step
to form the iron n-propyl species 3 and the catalytic
cycle is closed.
With regards to the 55 to 45% polymer endgroup
ratio we studied the influence of the TIBA concentra-
tion. Under the polymerization conditions described
in Section 2 the propylene concentration is 130 times
higher than the TIBA concentration in toluene. De-
spite the great excess of propylene, the 2,1 propylene
insertion and the realkylation have the same rates,
3.5. Catalytic cycle
With the knowledge of the polymer endgroup for-
mation and consideration of the experimentally proven
iron hydride species we can formulate for the first time
the complete catalytic reaction cycle in Scheme 4 and
upgrade the earlier published cycle from Small and
Brookhart [5].
It might be helpful to have a look back at Table 4,
where all occurred iron alkyl species and polymer end-
groups are mentioned. In the polypropylenes made by
the TIBA activation we found 45% 3-methyl-n-butyl
endgroups and 55% n-butyl endgroups. Due to this
product distribution we can say that chain propagation
cycles A and B contribute about 50% to the polymer
growth in each case.

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S.T. Babik, G. Fink / Journal of Molecular Catalysis A: Chemical 188 (2002) 245–253
Scheme 4. Catalytic cycle for start, chain growth and chain termination for propylene polymerization with bisiminepyridine iron complexes
and the described activation.
indicated by the polymer endgroup ratio of almost
50–50. Consequently the realkylation from the iron
n-propyl species 3 to the iron isobutyl species 1 is
about 130 times faster than the 2,1 propylene inser-
tion into the iron n-propyl bond. Reducing the TIBA
amount to 0.1 ml the propylene concentration is now
650 times higher and amount of the 3-methyl-n-butyl
endgroups in the polymer declines from 45 to 10%.
Because of lower TIBA concentration the realkyla-
tion is decreased and propagation cycle B is favoured.
It is of course conceivable that the growing chain
species 4 and 6 could be realkylated as well as spe-
cies 3, but this does not take place because these
species are secondary iron alkyls. The steric bulk-
iness prevents realkylation of species 4 and 6.
Addition of hydrogen to the reaction containing TIBA
yields a polymer with n-butyl endgroups (species
5). According to Scheme 3 the iron isobutyl species
is transformed by hydrogen to an iron hydride. In
Scheme 4 this reaction is the step from species 1
to 2. In this case we only run through propagation
cycle B.

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253
Using TEA instead of TIBA leads only to n-propyl
endgroups in the polymer. These groups are formed
by 2,1 propylene insertion into an iron ethyl species.
According to Scheme 4 it means that we only run
through propagation cycle A.
Increased activity on addition of hydrogen to the re-
action mixture confirms the presence of the highly
active iron hydride in the catalytic cycle (Scheme 4).
Using different aluminiumtrialkyls (TIBA, TEA) and
varying their amount in the polymerization allows
control of the mechanism in the propagation cycle.
We showed that if the realkylation is faster and
more complete, the smaller is the alkyl of the alu-
miniumtrialkyl. The isobutyl group in TIBA is bigger
than the n-propyl in species 3 and cannot substitute it
completely. So there are still iron n-propyl bonds in
which propylene can insert and generate the n-butyl
endgroups in the polymer. With pure TIBA activation
we run through propagation cycle A and B, but using
TEA gives rise to a different situation. The ethyl group
in TEA is smaller than the n-propyl group in species
3 and substitutes it completely to form an iron ethyl
bond which leads to n-propyl aliphatic endgroups in
the polymer.
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Activating 2,6-bis [1-(2-isopropy-6-methylphenyl-
imino)ethyl]-pyridineiron(II)dichloride with Ph3C
[B(C6F5)4] and subsequent treatment with TIBA
generates highly active catalysts for propylene poly-
merization. The activity is much higher than that
obtained with MAO and even higher than using
MMAO. Detection of the propylene consumption with
mass-flowmeters allows the first view of the kinetic
profile of propylene polymerization with this type of
catalysts. We assume an iron hydride species to be
a key species in the catalytic cycle, because the de-
tected aliphatic endgroups in the polypropylenes can
only be explained when an iron hydride is involved.