New iron-based bis(imino)pyridine and acetyliminopyridine complexes as single-site catalysts for the oligomerization of ethylene

Article abstract of DOI:10.1016/j.molcata.2003.11.003

New Fe-based bis(imino)pyridine complexes 6,7 and the acetyliminopyridine complex 8a are active catalysts together with the co-catalyst MAO for the oligomerization of ethylene. The influence of electron donating methoxy- and electron withdrawing trifluoromethyl-groups attached to the imino phenyl substituents was tested in the catalysis. The bis(imino)pyridine and the acetyliminopyridine complexes 5-8a revealed differences in their behavior as precatalysts for the oligomerization of ethylene, which depended on the different electronic influence of the various substituents at the rings and on the reaction conditions. Olefins ranging from C₄-C₁₈ were produced in all experiments but the turnover numbers varied between 1000 and 43,000. Variation of the catalyst ligand, temperature, and pressure had a considerable effect on α-olefin selectivity and on the formation of the main olefin species in the products. The complexes 5-8a showed small Schulz-Flory α values, which proved a low chain propagation.

Full text of DOI:10.1016/j.molcata.2003.11.003

Journal of Molecular Catalysis A: Chemical 212 (2004) 13–18
New iron-based bis(imino)pyridine and acetyliminopyridine complexes
as single-site catalysts for the oligomerization of ethylene
Martin E. Bluhm∗, Cristina Folli, Manfred Döring
Forschungszentrum Karlsruhe, Institute for Technical Chemistry (ITC-CPV), P.O. Box 3640, 76021 Karlsruhe, Germany
Received 16 September 2003; received in revised form 5 November 2003; accepted 5 November 2003
Abstract
New iron-based bis(imino)pyridine complexes 6, 7 and the acetyliminopyridine complex 8a are active catalysts together with the co-catalyst
MAO for the oligomerization of ethylene. The ligands 1–4 with electron donating methoxy- and electron withdrawing trifluoromethyl-substi-
tuents react with FeCl2(thf)2 to the Fe(II) complexes 6–8a. Especially p-methoxy substituents in 6b increase the selectivity in the production of
␣-olefins. The distribution of oligomers obtained in the catalyzed ethylene oligomerization with 5–8a follows Schulz–Flory rules for oligomers
>C4. The probability of chain propagation α is increased by lowering the reaction temperature, while higher pressure of the ethylene gas in
the reaction has nearly no effect on the α-values.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Oligomerization; Single-site catalysts; Ethylene; Bis(imino)pyridines; Acetylimino-pyridines
1. Introduction
We investigated the influence of electron donating
methoxy- and electron withdrawing trifluoromethyl-groups
attached to the imino phenyl substituents. We thus describe
the synthesis of the bis(imino)pyridine ligands 1, 2 and
acetyliminopyridine ligand 3a which react with FeCl2(thf)2
[17] to the ferric complexes 6–8a. Additionally, the results
of the catalytic oligomerization of ethylene with the pre-
catalysts 6–8a together with MAO as co-catalyst will be
presented.
The oligomerization of ethylene is one of the major
industrial processes for the production of linear ␣-olefins
[1–3]. Those oligomers in the range C6–C20 are used as
comonomers in the polymerization of ethylene to give
linear low-density polyethylene (LLDPE) or for the prepa-
ration of detergents and synthetic lubricants. Catalysts
currently used in industry for the shell higher olefin process
(SHOP) [4–8] contain Ni(II) complexes bearing bidentate
monoanionic ligands [1–8]. Also cationic Ni(II) ␣-diimine
complexes were reported to be effective ethylene oligomer-
ization catalysts [9], while iron-based bis(imino)pyridine
catalysts were described as highly active compounds in
the oligomerization of ethylene in combination with the
co-catalyst MAO or MMAO [10–14]. Several modifi-
cations of the bis(imino)pyridine backbone are already
described in the literature [15] which mostly lead to a
decrease of catalytic activity. Recently fluoro-substituted
bis(imino)pyridine complexes and their catalytic reactivity
in ethylene oligomerizations were described [16].
2. Experimental
All manipulations were carried out under an atmosphere
of argon using standard Schlenk techniques. NMR spectra
were recorded on a Bruker spectrometer 250 MHz (1H) and
62.9 MHz (13C) at 293 K. Mass spectra were obtained us-
ing electron ionisation (EI), electron spray ionisation (ESI)
or field ionisation (FI). Oligomer products were analysed
by GC with a flame ionisation detector, using a 50 m DB1
column, injector temperature 40 ◦C and the following tem-
perature program: 40 ◦C/5 min, 40–300 ◦C, 5 ◦C/10 min−1
.
The individual products were integrated, using n-tridecane
as internal standard.
Materials: 2,6-bis[1-(phenylimino)ethyl]pyridine [18]
was prepared according to the literature. MAO (10%
∗
Corresponding author. Tel.: +49-7247-82-2383;
fax: +49-7247-82-2244.
E-mail address: martin.bluhm@itc-cpv.fzk.de (M.E. Bluhm).
1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.11.003

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M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 13–18
solution in toluene) and all other anilines were purchased
commercially and used as received.
at 5 ◦C over night. Colorless crystals of 2b formed which
were filtered and washed with n-hexane, yield: 0.08 g
(0.18 mmol, 8%). 1H NMR (CDCl3) δ 8.26 (d, 2H, Py-Hm),
7.82 (t, 1H, Py-Hp), 7.53 (d, 4H, Haryl), 6.82 (d, 4H, Haryl),
2.31 (s, 6H, N=CMe); 13C NMR (CDCl3) δ 166.5, 153.5,
152.9, 135.6, 125.4 (q), 125.1 (q), 124.9 (q), 121.3, 117.8,
15.0. EI+-MS m/z = 449 (M+); FI+-HR-MS: m/z 449.1327
(calcd.), found 449.1260 for C23H17N3F6, δ = 6.7 mDa;
mp = 141 ◦C.
2.1. Synthesis of ligands
2,6-Bis[1-(3-methoxyphenylimino)ethyl]pyridine (1a) [19]:
2,6-diacetylpyridine (0.74 g, 4.5 mmol) was stirred with
m-anisidine (1.83 g, 15 mmol, 3.3 eq) in 20 ml of dry ben-
zene at ambient temperature in the presence of 8.5 g of
molecular sieves. The reaction was performed under argon
in a closed flask for approximately 50 days. Then the reac-
tion mixture was filtered, the solvent was removed under
reduced pressure and a yellow oil was obtained. A few ml of
n-hexane were added to the oil and the mixture was stored
at 5 ◦C over night. A yellow precipitate formed which was
filtered and washed with n-hexane. Yield: 1.1 g (3.0 mmol,
66%). 1H NMR (CDCl3) δ 8.26 (d, 2H, Py-Hm), 7.80 (t, 1H,
Py-Hp), 7.22 (t, 2H, Haryl), 6.61 (d, 2H, Haryl), 6.38–6.34
(m, 4H, Haryl), 3.76 (s, 6H, OMe), 2.34 (s, 6H, N=CMe);
13C NMR (CDCl3) δ 167.9, 160.7, 155.8, 153.1, 137.2,
130.3, 122 7, 111.9, 109.6, 105.3, 55.6, 16.6. EI+-MS
m/z = 373 (M+); mp = 133 − 135 ◦C.
1-(6-{N-[3-(trifluoromethyl)phenyl]ethanimidoyl}pyridin-
2-yl)ethanone (3a): 2,6-diacetyl-pyridine (1.63 g, 10 mmol)
was stirred with 3-trifluoromethylaniline (1.61 g, 10 mmol)
in 20 ml of dry benzene at room temperature in the presence
of 11 g of molecular sieves. The reaction was performed un-
der argon in a closed flask for approximately 80 days. Then
the reaction mixture was filtered, the solvent was removed
under reduced pressure and a yellow oil was obtained.
The product was stored at 5 ◦C. Slowly a yellow precipi-
tate formed which was filtered and washed with methanol.
1
Yield: 2.1 g (7.0 mmol, 70%). H NMR (CDCl3) δ 8.38 (d,
1H, Py-Hm), 8.06 (d, 1H, Py-Hm), 7.88 (t, 1H, Py-Hp), 7.43
(t, 1H, Haryl), 7.32 (d, 1, Haryl), 7.04 (s, 1H, Haryl), 6.94 (d,
1H, Haryl), 2.71 (s, 3H, O=CMe), 2.35 (s, 3H, N=CMe);
13C NMR (CDCl3) δ 200.2, 168.3, 155.8, 152.8, 151.8,
137.8, 131.9 (q), 130.0, 125.1, 124.5 (q), 123.2, 122.9,
120.8 (q), 116.5 (q), 26.0, 16.6; EI+-MS m/z = 306 (M+);
FI+-HR-MS: m/z 306.0980 (calcd.), found 306.0917 for
C16H13N3OF3, δ = 6.3 mDa; mp = 116 − 118 ◦C.
2,6-Bis[1-(4-methoxyphenylimino)ethyl]pyridine (1b) [20–
22]: 2,6-diacetylpyridine (0.38 g, 2.3 mmol) and p-anisidine
(0.67 g, 5.4 mmol, 2.3 eq) were dissolved in 16 ml of
methanol under stirring. 0.3 ml of 97% formic acid were
added to this solution and a beige precipitate slowly ap-
peared from the brown solution. The reaction mixture was
stirred for 8 h at room temperature, then the precipitate was
collected by vacuum filtration and washed with methanol.
The dried solid is a fluorescent greenish-yellow powder,
yield: 0.78 g (2.1 mmol, 91%). 1H NMR (THF-d8) δ 8.35 (d,
2H, Py-Hm), 7.89 (t, 1H, Py-Hp), 6.85 (q, 8H, Haryl), 3.78
(s, 6H, OMe), 2,42 (s, 6H, N=CMe); 13C NMR (THF-d8)
δ 164.6, 154.6, 153.9, 141.3, 134.5, 119.8, 118.8, 112.0,
52.7, 13.1; EI+-MS m/z = 373 (M+); mp = 199 ◦C.
2.2. Synthesis of ferric complexes
General procedure for the synthesis of the ferric com-
plexes (5–8a): the appropriate ligand was added to a sus-
pension of FeCl2(thf)2 [17] in dry THF in a Schlenk-flask
under argon. The mixture was stirred for 3 h at room tem-
perature, then the solvent was removed and finally the
product was dried in vacuum.
2,6-Bis[1-(phenylimino)ethyl]pyridyliron(II) chloride (5):
2,6-bis[(phenylimino)ethyl] pyridine (200 mg, 0.64 mmol),
FeCl2(thf)2 (173 mg, 0.64 mmol), THF 30 ml. Compound
5 was obtained as a dark blue powder, yield 251 mg
(0.56 mmol, 89%). FI+-HR-MS: m/z 439.0305 (calcd.),
2,6-Bis[1-(3-trifluoromethylphenylimino)ethyl]pyridine
(2a) [19]: 2,6-diacetylpyridine (0.82 g, 5.0 mmol) was
stirred with 3-trifluoromethylaniline (1.62 g, 10 mmol, 2 eq)
in 25 ml of dry benzene at ambient temperature in the
presence of 2.8 g of molecular sieves. The reaction was per-
formed under argon in a closed flask for 48 h. The molecular
sieves were removed by filtration, the reaction mixture was
concentrated and a few ml of dichloromethane were added.
The obtained pale yellow crystals were filtered and dried,
yield: 0.36 g (0.8 mmol, 16%). 1H NMR (CDCl3) δ 8.08 (d,
2H, Py-Hm), 7.89 (t, 1H, Py-Hp), 7.48-6.94 (m, 8H, Haryl),
2.36 (s, 6H, N=CMe); EI+-MS m/z = 449 (M+).
found 439.0225 for C21H19N3Cl2Fe, δ
C21H19N3Cl2Fe: calcd. C 57.30, H 4.35, N 9.54, found C
57.0, H 4.52, N 9.28.
=
8.0 mDa;
2,6-Bis[1-(3-methoxyphenylimino)ethyl]pyridyliron(II)
chloride (6a): 2,6-bis[1-(3-methoxyphenylimino)ethyl]pyri-
dine 1a (200 mg, 0.54 mmol), FeCl2(thf)2 (145 mg,
0.54 mmol), THF 30 ml. Compound 6a was obtained as a
dark blue powder, yield 208 mg (0.42 mmol, 77%). C23H23-
N3O2Cl2Fe: calcd. C 55.22, H 4.63, N 8.40, found C 55.16,
H 4.65, N 8.15.
2,6-Bis[1-(4-methoxyphenylimino)ethyl]pyridyliron(II)
chloride (6b): 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyri-
dine 1b (150 mg, 0.42 mmol), FeCl2(thf)2 (109 mg,
0.42 mmol), THF 30 ml. Compound 6b was obtained as
2,6-Bis[1-(4-trifluoromethylphenylimino)ethyl]pyridine
(2b) [23]: 2,6-diacetylpyridine (0.38 g, 2.3 mmol), 4-trifluo-
romethylaniline (2.4 g, 15 mmol, 6.5 eq) and a few mil-
ligrams of p-toluenesulfonic acid were heated under reflux
in 50 ml of toluene for 15 h. Then the brown reaction
mixture was concentrated and dissolved in a mixture of
n-hexane/ethylacetate = 1:1. Filtration over silica-gel and
removal of the solvents gave a brown oil which was stored

M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 13–18
15
a dark blue powder, yield 189 mg (0.38 mmol, 90%).
were added under argon. For several minutes ethylene
was introduced while streaming through the reactor to
displace the argon. Then the reactor was closed and
pressurized to 3 bar with ethylene. The reactor pressure
maintained constant throughout the oligomerization run
by manually controlled addition of ethylene. Runs were
terminated by venting off volatiles and extracting the
solution with dilute hydrochloric acid and water. Quan-
titative GC analysis of the organic layer was performed
immediately after the extraction.
FI+-HR-MS: m/z 499.0517 (calcd.), found 499.0475 for
C23H23N3O2Cl2Fe, δ
calcd. C 55.22, H 4.63, N 8.40, found C 55.44, H 4.92, N
8.11.
=
0.8 mDa. C23H23N3O2Cl2Fe:
2,6-Bis[1-(3-trifluoromethylphenylimino)ethyl]pyridyli-
ron(II) chloride (7a): 2,6-bis[1-(3-trifluoromethylphenyli-
mino)ethyl]pyridine 2a (54 mg, 0.12 mmol), FeCl2(thf)2
(33 mg, 0.12 mmol), THF 15 ml. Compound 7a was ob-
tained as a dark blue powder, yield 44 mg (0.08 mmol,
64%). FI+-HR-MS: m/z 575.0053 (calcd.), found 575.0061
for C23H17N3F6Cl2Fe, δ = 0.8 mDa. C23H17N3F6Cl2Fe:
calcd. C 47.94, H 2.97, N 7.29, found C 47.50, H 3.02, N
7.03.
2,6-Bis[1-(4-trifluoromethylphenylimino)ethyl]pyridyli-
ron(II) chloride (7b): 2,6-bis[1-(4-trifluoromethylpheny-
limino)ethyl]pyridine 2b (100 mg, 0.22 mmol), FeCl2(thf)2
(60 mg, 0.22 mmol), THF 30 ml. Compound 7b was ob-
tained as a dark blue powder, yield 114 mg (0.2 mmol,
89%). FI+-HR-MS: m/z 575.0053 (calcd.), found 574.9968
for C23H17N3F6Cl2Fe, δ = 8.5 mDa. C23H17N3F6Cl2Fe:
calcd. C 47.94, H 2.97, N 7.29, found C 47.64, H 2.72, N
6.98.
1-{6-[N-(3-trifluoromethyl)ethanimidoyl]pyridin-2-yl}
ethanone iron(II) chloride (8a): 1-{6-[N-(3-trifluoromethyl)
ethanlimidoyl]pyridin-2-yl}ethanone 3a (150 mg, 0.49
mmol), FeCl2(thf)2 (133 mg, 0.49 mmol), THF 30 ml. Com-
pound 8a was obtained as a dark blue powder, yield 127 mg
(0.29 mmol, 60%). ESI+-MS: m/z = 446 (M+ + NH+4 ).
C16H13N2OF3Cl2Fe: calcd. C 44.37, H 3.02, N 6.46, found
C 44.67, H 3.28, N 6.11.
(b) High-pressure tests: Same procedure as described for
low-pressure tests. Instead of a glass reactor was used
a 150 ml stainless steel reactor with cooling mantle.
After the reactor was closed it was pressurized to 30 bar
with ethylene. The temperature of the reaction was
controlled by cooling the reactor vessel with water.
3. Results and discussion
Iron-based bis(imino)pyridine complexes 6 and 7 and
the acetyliminopyridine complex 8a were prepared to in-
vestigate the influence of methoxy- and trifluoromethyl-
substituted phenyl groups in the ligands (Fig. 1) [24].
It was previously reported that the length of the formed
macromolecule and the selectivity to ␣-olefins depends on
the steric bulk and on the electronic characteristics of the
substituents on each aryl ring of the bis(imino)pyridine
complexes [11,12]. We developed bis(imino)pyridine 1, 2
and the acetyliminopyridine ligand 3a with substituents
which especially influence the electronic properties in the
ferric precursor complexes 6–8a. 2,6-Bis(acetyl)pyridine
reacts with m,p-methoxy and m,p-trifluoromethyl substi-
tuted anilines under different reaction conditions to the
corresponding bis(imino)pyridine ligands 1, 2 and to the
acetyliminopyridine 3a. For the syntheses of 1a, 2a, and
3a both reactants have to be stirred in benzene for many
days whereas the reaction conditions are different for the
syntheses of 1b and 2b [24]. The yields were not optimized
and are fairly low for 2b (8%) and higher up to 90% for 1b.
The ligands 1–3a react with FeCl2(thf)2 [17] to the Fe(II)
complexes 6–8a (Fig. 2). It failed to prepare ligands with
mixed methoxy- and trifluoromethyl- substituents because
2.3. Oligomerization procedure
(a) Low-pressure tests: The precatalyst was dissolved in
30 ml of solvent in a Schlenk-flask under argon. A
complete solution was obtained by leaving the flask in
an ultrasonic bath for several minutes. A 150 ml glass
reactor was evacuated and then filled with argon. The
precatalyst solution was added under argon into the
reactor vessel. Under stirring the co-catalyst MAO
(0.6 ml, approximately 100 eq of a 10 mol% MAO so-
lution in toluene) and n-tridecane standard solution
Fig. 1. Structure of the precatalysts 5-8a.

16
M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 13–18
Fig. 2. Synthesis of the Fe(II) complexes 6–8a.
of decomposition of the reactants and the formation of
side-products.
Table 2
Yield (%) of oligomers in the experiments 1–6
The precatalysts 6–8a are active compounds in combi-
nation with the co-catalyst MAO in the oligomerization of
ethylene (Table 1). The turnover number for the unsubsti-
tuted precatalyst 5 is one of the highest among the tested
catalysts but it lacks a good selectivity in the production
of ␣-olefins. The p-methoxy substituted precatalyst 6b pro-
duces ␣-olefins in up to 77% yield at 3 bar ethylene pres-
sure, whereas when the pressure is increased to 30 bar it
gives ␣-olefins in 88% yield with a loss of catalytic activ-
ity. A positive effect for the production of ␣-olefins is how-
ever only observed for the p-substituted derivative 6b, not
for the m-methoxy compound 6a. This is in line with the re-
sults from catalytic experiments with electron withdrawing
trifluoromethyl-substituted derivatives 7a and 7b: in fact in
case of p-trifluoromethyl substituents in 7b the selectivity is
not improved. Only a low selectivity to ␣-olefins in 36 %
yield is obtained with the m-trifluoromethyl derivative 7a.
Fraction (%)
1
2
3
4
5
6
C4
C6
C8
C10
C12
C14
C16
C18
27.6
45.4
16.2
7.2
2.5
1.0
0.3
–
26.8
45.2
16.1
7.4
51.4
31.1
11.8
4.0
1.2
0.5
0.1
–
51.9
41.2
5.5
35.2
42.3
13.7
6.0
2.2
0.5
0.1
–
31.3
41.2
15.4
7.6
0.9
0.3
0.1
0.03
2.8
1.1
0.4
0.1
3.0
1.1
0.3
–
0.1
A different behavior of the precatalysts 5–8a is also
shown in the distribution of the formed oligomers (Tables 2
and 3 and Fig. 3). Mostly hexenes are formed with the
bis(imino)pyridine precatalysts 5, 6a and 7, while butenes
outweigh in case of the acetyliminopyridine precatalyst 8a.
On the other hand 6b mostly gives butenes both at 3 bar
and 30 bar ethylene pressure.
Table 1
Results of oligomerization of ethylene with catalysts 5–8a
Number Catalysta
Loading (␮mol) Time (h) p (bar) T (◦C) Yield (g)b
TON
TOF (×10−3/h) Solvent
α
␣-Olefinsb (%)
1
2
3
4
5
6
7
8
5
10
10
10
10
2
3
3
3
30
3
3
22–75
22–75
22–60
22–60
22–75
22–75
22–70
22–70
11.4
12.0
1.0
0.3
9.3
7.8
2.5
10.5
40813 20.4
43032 17.2
Toluene 0.36 28
Toluene 0.39 29
Toluene 0.39 77
Toluene 0.30 88
Toluene 0.30 36
Toluene 0.36 27
Toluene 0.39 77
Toluene 0.36 82
6a
6b
6b
7a
7b
8a
8a
2.5
2.5
1
2.5
2
3710
892
1.5
0.9
9.3
35750 14.3
27890 13.9
10
10
10
2.75
1.5
3
30
8989
3.3
37332 24.9
a
All precatalysts were first dissolved in 30 ml of solvent in an ultrasonic bath and then activated with approximately 100 eq MAO (0.6 ml of a
10 mol% MAO solution in toluene).
b
The yield and the 1-olefin content C4–C18 were determined by GC using calibration curves with standard solutions.

M.E. Bluhm et al. / Journal of Molecular Catalysis A: Chemical 212 (2004) 13–18
17
Table 3
Yield (%) of oligomers in the experiments 7–8
Fraction (%)
7
8
C4
C6
C8
C10
C12
C14
C16
C18
49.3
31.0
12.8
4.6
1.4
0.6
46.7
35.3
12.4
3.4
1.4
0.5
0.2
0.1
0.2
0.1
The reaction temperature rises in all catalytic experiments
with 5–8a. The catalytic activity decreases or completely
stops when cooling the reaction vessel and keeping it at room
temperature during the reaction. Higher ethylene pressure
leads to an increased formation of ␣-olefins [11] in up to
82% yield (runs 4, 8, Table 1).
Fig. 4. Schulz–Flory distribution: ln mol% of each fraction vs. carbon
number.
The distribution of oligomers obtained in the catalyzed
ethylene oligomerization with 5–8a follows Schulz–Flory
rules for oligomers >C4, which can be characterized by the
constant α representing the probability of chain propagation
(α = rate of propagation/[(rate of propagation) + (rate of
chain transfer)] = (moles of Cn+2)/(moles of Cn)) [25–28].
The α value can be determined in our case by the molar ratio
of C12 and C14 fractions (Fig. 4); the obtained α values range
between 0.30 and 0.39 for the catalytic oligomerization runs
with 5, 6, 7, and 8a. Pressure shows nearly no effect on
the ␣-values (runs 4, 8), which is consitent with previous
observations in bis(iminopyridine) complexes [11,12]. All α
numbers found in the catalytic runs 1–8 are much lower than
those described in the ethylene oligomerization catalyzed
by the monoalkyl-substituted iron complexes which reach
α values of 0.70–0.85 [11,12]. In contrast α between 0.33
and 0.34 are found for the reported 2,4-difluoro-substituted
bis(imino)pyridine complexes [16], which is in line with the
results for the precatalysts 5–8a.
4. Conclusions
In summary, the described bis(imino)pyridine and the
acetyliminopyridine complexes 5–8a reveal differences in
their behavior as precatalysts for the oligomerization of ethy-
lene, which depend on the different electronic influence of
the various substituents at the rings and on the reaction
conditions in these experiments, too. Olefins ranging from
C4–C18 are produced in all experiments but the turnover
numbers vary between 1000 and 43000. Additionally, vari-
ation of the catalyst ligand, temperature and pressure has a
significant influence on ␣-olefin selectivity and on the for-
mation of the main olefin species in the products. The com-
plexes 5–8a show small Schulz–Flory α values, which prove
a low chain propagation.
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Fig. 3. Oligomer distribution: yield of each oligomer fraction vs. carbon
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18
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