Highly active iron and cobalt catalysts for the polymerization of ethylene [18]

Article abstract of DOI:10.1021/ja9802100

Using the previously examined Ni(II) and Pd(II) α-diimine catalysts as a guide, new iron(II) and cobalt(II) catalysts based on tridentate pyridine bis-mine ligands were prepared. The resulting catalysts are robust and extremely active for polymerization of ethylene to linear, high-density polyethylene.

Full text of DOI:10.1021/ja9802100

J. Am. Chem. Soc. 1998, 120, 4049-4050
4049
Highly Active Iron and Cobalt Catalysts for the
Polymerization of Ethylene
Brooke L. Small,† Maurice Brookhart,*,† and
Alison M. A. Bennett‡
Department of Chemistry, UniVersity of North Carolina at
Chapel Hill, Chapel Hill, North Carolina 27599-3290
DuPont Central Research and DeVelopment
Experimental Station, Wilmington, Delaware 19880-0328
ReceiVed January 20, 1998
Study of the polymerization of olefins by soluble, well-defined
transition metal complexes is an ever-growing area. While most
attention has been focused on early transition metal d0 and
lanthanide d0fn systems,1 recently late metal Pd(II)- and Ni(II)-
catalyst systems incorporating R-diimine ligands have been
reported which convert both ethylene and R-olefins to high molar
mass polymers.2 Unique features of these systems include the
ability to produce highly branched polymers from ethylene and
to copolymerize ethylene with certain polar monomers using the
Pd(II) catalysts. Most late metal systems produce low molecular
weight oligomers from ethylene and particularly R-olefins. The
key to high polymer production using the aryl-substituted R-di-
imine systems is the incorporation on the aryl rings of bulky ortho
substituents that greatly retard the rate of chain transfer. We
report here the synthesis of tridentate Fe(II) and Co(II) complexes
incorporating bulky substituted arylimine moieties and demon-
strate that these are extremely active and long-lived catalysts for
the polymerization of ethylene.
Figure 1. X-ray crystal structure of 1a. Selected bond distances (Å)
and angles (deg): Fe(1)-N(1), 2.222(4); Fe(1)-N(2), 2.091(4); Fe(1)-
N(3), 2.225(5); Fe(1)-Cl(1), 2.3173(19); Fe(1)-Cl(2), 2.2627(17);
Cl(1)-Fe(1)-Cl(2), 117.58(7); N(1)-Fe(1)-Cl(1), 100.57(12); N(3)-
Fe(1)-Cl(1), 102.47(12); N(2)-Fe(1)-Cl(1), 94.52(13); N(1)-Fe(1)-
N(3), 140.23(16); N(1)-Fe(1)-N(2), 73.67(16).
Scheme 1
The tridentate ligands used in this study are pyridine diimine
ligands of general structures 1-3 prepared by the Schiff-base
condensation of 2 equiv of the desired aniline with 2,6-
diacetylpyridine. The precatalysts, formed by addition of the
ligand to the appropriate hydrated or anhydrous metal salt
(Scheme 1), are neutral Fe(II) and Co(II) complexes {[(2,6-
ArNdC(Me))2C5H3N]MX2} (Ar ) 2,6-C6H3(i-Pr)2, 1; 2,6-C6H3-
Me2, 2; 2-C6H4(t-Bu), 3; M ) Fe, a; Co, b; X ) Cl-, Br-, NO3-).
Figure 1 shows the X-ray crystal structure for 2,6-bis[1-(2,6-
diisopropylphenylimino)ethyl]pyridineiron(II) chloride (1a).3 The
structure for 2,6-bis[1-(2-tert-butylphenylimino)ethyl]pyridinecobalt-
(II) chloride (3b) is shown in the Supporting Information.4 Both
complexes are pentacoordinate with pseudo-square-pyramidal
geometry, the most notable features being the nearly perpendicular
arrangement in both complexes of the aryl rings relative to the
square plane as well as the syn conformation of the tert-butyl
groups in complex 3b. Complexes 1a-b, 2a-b, and 3a-b are
paramagnetic, high-spin species, as indicated by magnetic sus-
ceptibility measurements.5 Both the crystallographic data and the
magnetic susceptibility measurements are consistent with the
results reported for similar complexes lacking ortho substituents
on the aryl rings.6
The active catalysts are generated in situ in toluene by the
addition of modified methylalumoxane (MMAO, g 300 equiv)
to the precursors in the presence of ethylene (Scheme 1). Data
for the polymerizations of ethylene are summarized in Table 1.7
All of the catalysts reported convert ethylene to highly linear
polyethylene (PE) as determined by differential scanning calo-
rimetry (Tm values 133-139 °C).8 In contrast to the Ni(II)- and
Pd(II)-diimine catalysts, no branching is observed, even with the
bulkiest ligands at high temperatures and low ethylene pressures.
However, the polymer molecular weights vary dramatically with
modifications in ligand, metal, and concentration of activator. Like
the Ni(II) and Pd(II) systems, increasing the steric bulk of the
ortho aryl substituents increases molecular weight. For example,
the tetraisopropyl-substituted Fe system (1a/MMAO, Table 1,
entry 1) yields a polymer with a peak MW of 71K, while the
† University of North Carolina at Chapel Hill.
‡ DuPont Central Research and Development.
(1) (a) Brintzinger, H. H.; Fischer, D.; Muelhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (b) Ziegler
Catalysts: Recent Scientific InnoVations and Technological ImproVement;
Fink, G., Muelhaupt, R., Brintzinger, H. H., Eds.; Springer-Verlag: Berlin,
1995. (c) Bockman, M. J. Chem. Soc., Dalton Trans. 1996, 255. (d) Coates,
G. W.; Waymouth, R. M. In ComprehensiVe Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Hegedus, L., Vol. Ed.;
Pergamon Press: 1995; Vol. 12; pp 1193-1208. (e) Yang, X.; Stern, C. L.;
Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015. (f) Crowther, D. J.;
Baenzinger, N. C.; Jordan, R. F. J. Am. Chem. Soc. 1991, 113, 1455.
(2) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. S. J. Am. Chem. Soc.
1995, 117, 6414. (b) Killian, C. M.; Tempel, D. J.; Johnson, L. K.; Brookhart,
M. S. J. Am. Chem. Soc. 1996, 118, 11664.
(5) Magnetic susceptibilities (µeff, BM) were determined for the following
complexes: 1a, 5.54; 2a, 5.22; 3a, 5.00; 1b, 4.55; 2b, 4.67; 3b, 4.65. See
Supporting Information for experimental details.
(6) (a) Edwards, D. A.; Edwards, S. D.; Martin, W. R.; Pringle, T. J.;
Thornton, P. Polyhedron 1992, 11, 1569. (b) Goldschmied, E.; Stephenson,
N. C. Acta Crystallogr. 1970, B26, 1867. (c) Reiff, W. M.; Erickson, N. E.;
Baker, W. A., Jr. Inorg. Chem. 1969, 8, 2019.
(7) Nearly equivalent activities are observed when the corresponding
Fe(III) complexes (prepared from ligands 1-3 and the corresponding FeX3
hydrates) are activated with MMAO.
(3) Crystal data of 1a: triclinic, P 1h, blue; a ) 8.7953(6) Å, b ) 9.8587-
(6) Å, c ) 20.9583(13) Å; V ) 1646.45(18) Å3; Z ) 2; R ) 0.060; GOF )
2.52.
(8) All methyl groups visible by 13C NMR are attributable to end groups;
there are less than 0.4 methyl branches per 1000 carbons. Heat of fusion data
from the DSC traces indicates very high crystallinity (226 J/g vs 170 J/g for
commercial HDPE).
(4) Crystal data of 3b: triclinic, P 1h, gold; a ) 12.7329(7) Å, b ) 15.7633-
(8) Å, c ) 15.8220(8) Å; V ) 3138.5(3) Å3; Z ) 4; R ) 0.084; GOF ) 2.90.
S0002-7863(98)00210-8 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/14/1998

4050 J. Am. Chem. Soc., Vol. 120, No. 16, 1998
Communications to the Editor
Table 1. Results of Ethylene Polymerization by 1a-b, 2a-b, and 3a-b in Toluene
entry
catalysta
loading (µmol)
pressure (psig)
temp (°C)
reaction length (min)
yield (g)
peakb MW
TOFc(× 10-6/h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
1a
2a
3a
1a
2a
3a
1b
2b
3b
1b
2b
3b
1b
1b
1b
1a
1a
1a
1a
2a
1.1
1.2
1.3
0.6
0.6
0.7
1.4
1.6
1.2
0.8
0.9
1.0
8.0
7.9
8.0
0.6
0.5
0.5
1.3
4.2
15
15
15
15
15
15
15
15
15
25
25
25
0
0
0
25
25
25
0
0
0
50
50
50
60
60
60
90
125
50
50
50
50
50
50
50
50
50
50
50
50
12
12
12
10
10
10
10
10
3.09
2.78
2.88
1.35
1.17
1.36
1.20
2.99
1.00
0.45
0.59
1.36
17.5
17.5
21.6
12.8
17.3
28.5
46.6
107.6
71 000
33 000
81 000
19 000
15 000
56 000
24 000
1 400
0.12
0.10
0.10
0.09
0.08
0.09
0.04
0.08
0.04
0.02
0.03
0.06
0.39
0.40
0.48
4.8
183 000
43 000
3 700
15
15
15
v. highd
11 300
9 700
200
400
600
200
400
600
600
600
8 900
26 600
27 600
31 100
17 400
9 000
7.0
11.8
7.6
5.5
a All of the precursor complexes were activated with MMAO. b Entries 1-6 exhibit bimodal molecular weight distributions. c Entries 1-6 and
19-20 are likely mass transport limited. d The sample was not soluble.
(entries 16-18) where TOFs are 4.8 × 106/h at 200 psig, 7.0 ×
106/h at 400 psig, and 11.8 × 106/h at 600 psig, thus establishing
that the rate of chain growth is dependent on ethylene concentra-
tion. In contrast, the TOFs of the cobalt analogues at 50 °C show
little dependence on ethylene pressure under these conditionss3.9
× 105/h at 200 psig, 4.0 × 105/h at 400 psig, and 4.8 × 105/h at
600 psig (entries 13-15). The activities of the iron catalysts are
remarkably high; TOFs of greater than 107/h can be achieved at
60 °C and 600 psig C2H4 (entry 18) which corresponds to 3.3 ×
105 kg PE/mol Fe‚h! These activities are thus comparable to the
most active Ziegler-Natta systems.12 The cobalt complexes
generally exhibit activities that are an order of magnitude lower
than their iron analogues. The catalysts are stable at elevated
temperatures as demonstrated by runs 19 and 20. In these cases
activation with an increased catalyst loading resulted in rapid
exotherms to 90 and 130 °C, respectively. These temperatures
remained constant for the duration of each run.
Figure 2. Activator effect on molecular weight (catalyst 1a/MMAO, 25
°C, 15 min, 1 atm ethylene): (a) 300 equiv of Al, MN ) 20 900, MW
)
135 000; (b) 1500 equiv of Al, MN ) 709, MW ) 52 400; (c) 4500 equiv
of Al, MN ) 390, MW ) 9570.
In summary, using the previously examined Ni(II) and Pd(II)
R-diimine catalysts as a guide, we have prepared new iron(II)
and cobalt(II) catalysts based on tridentate pyridine bis-imine
ligands in which the imine moieties are bulky ortho-substituted
aryl imines. These catalysts are robust and extremely active for
polymerization of ethylene to linear, high-density polyethylene.
To the best of our knowledge, these are the first reported iron-
based homogeneous catalysts for ethylene polymerization. Mecha-
nistic studies are in progress.
tetramethyl system (2a/MMAO, entry 2) shows the peak MW at
33K.9 For comparison of the cobalt analogues, see entries 7 and
8. Generally, the iron systems produce higher molecular weight
materials relative to their cobalt analogues (entries 1 vs 7 and 2
vs 8), but the tert-butyl-substituted systems do not exhibit this
trend (entry 3 vs 9).
For the iron systems in particular, increasing the amount of
activator leads to broadened polydispersities and bimodal behavior
(Figure 2). This observation is consistent with the proposal that
chain transfer to aluminum is a viable route for the formation of
low molecular weight materials early in the polymerization.10,11
Thus, by using large amounts of activator or short reaction times,
predominantly low molecular weight polymers are made (see GPC
trace c, Figure 2).
Acknowledgment is made to DuPont and the National
Science Foundation for funding.
Supporting Information Available: Crystal structure data for 1a and
3b, syntheses of ligands and complexes, magnetic susceptibility data,
polymerization procedures, and polymer characterization data (selected
GPC and DSC traces; 13C NMR data) (30 pages, print/PDF). See any
current masthead page for ordering information and Web access
instructions.
In the case of the iron catalysts, the turnover frequencies (TOFs)
increase with ethylene pressure as shown by the 60 °C runs
(9) Bimodal behavior is sometimes observed with a low MW fraction due
to chain transfer (see text). If a distinct low MW fraction is not observed,
then low molecular weight shoulders or tails are seen. Thus, the best measure
of the molecular weight of the high MW fraction is the peak MW. See
Supporting Information for sample gpc traces.
(10) As an alternative explanation for bimodal behavior, a referee suggested
the existence of two catalytic species whose ratios vary with changing Al:Fe
ratios. However, we have shown by 1H NMR that the end groups of the low
MN materials are saturated, which supports chain transfer via transmetalation.
JA9802100
(11) (a) Marques, M. V.; Nunes, P. C.; Tait, P. J. T.; Dias, A. R. J. Polym.
Sci.-A. 1993, 31, 219. (b) Komiya, S.; Katoh, M.; Ikariya, T.; Grubbs, R. H.;
Yamamoto, T.; Yamamoto, A. J. Organomet. Chem. 1984, 260, 115.
(12) (a) Spaleck, W.; Ku¨ber, F.; Winter, A.; Rohrmann, J.; Bachmann, B.;
Antberg, M.; Dolle, V.; Paulus, E. F. Organometallics 1994, 13, 954. (b) Alt,
H. G.; Milius, W.; Palackal, S. J. J. Organomet. Chem. 1994, 472, 113. (c)
Gianetti, E.; Nicoletti, G. M.; Mazzocchi, R. J. Polym. Sci.-A. 1985, 23, 2117.