jjc jordan journal of chemistry vol. 2 no.3, 2007, pp. 219-233jjc.yu.edu.jo/issues/vol2no3pdf/02...
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Jordan Journal of Chemistry Vol. 2 No.3, 2007, pp. 219-233
JJC Diimine Chromium Complexes as Catalyst Precursors for
Homogeneous Ethylene Polymerization 1
Alexandra Kestel-Jakob, Helmut G. Alt*
Laboratorium für Anorganische Chemie, Universität Bayreuth, D-95440 Bayreuth, Germany
Received on July 8, 2007 Accepted on Nov. 15, 2007
Abstract The synthesis and characterization of 19 new α- and β-diimine chromium complexes are
reported. After activation with methyl aluminoxane (MAO), these complexes are catalysts for
homogeneous ethylene polymerization. The influences of various substituents and the ligand
backbone on the catalyst activities and the polymer properties are discussed. Oligomers with
odd and even numbers of carbon atoms can be obtained. This is an indication that these
catalysts cannot only oligomerize and polymerize but also isomerize and metathesize olefins
simultaneously.
Keywords: Diimine chromium complexes; Catalysis; Ethylene oligomers with odd
and even numbers of carbon atoms.
Introduction Various types of chromium catalysts for the homogeneous polymerization of
olefins are still of big interest [1-5]. One of the latest research activities describes
salicylaldimines bearing bulky ortho-phenoxy substituents and small imine substituents
to give very active chromium catalysts for ethylene polymerization [6]. Chromium
complexes with tridentate pyridine-based ligands as highly active precatalysts for the
oligomerization of ethylene are already known [7,8]. Also complexes with tridentate
ligands such as triazacyclohexane are highly active polymerization catalysts [9,10].
Chromium complexes with bidentate „nacnac“ ligands seem to be very
interesting.
The ligand structure was developed in the sixties of the last century [11-13].
The first chromium complex of this kind was described in 1989 [14]. These
complexes were used in the catalytic polymerization of ethylene several years
later [15-20].
In this report, chromium complexes with α- and β-dimimine ligands were
synthesized and used as catalysts for the polymerization and oligomerization of
ethylene. One highlight is the catalytic production of oligomers with odd numbers of
carbon atoms.
1 Dedicated to Professor Dr.Drs.h.c.mult.Wolfgang A. Herrmann on the occasion of his 60th birthday (April
18, 2008) * Corresponding author. Tel.:+49-921-55-2555; fax: +49-921-55-2044.
E-mail: [email protected] (H.G.Alt)
220
Experimental NMR spectroscopic investigations were performed with a Bruker ARX 250
instrument. All samples were measured at 25 °C. CDCl3 served as solvent. The
chemical shifts (δ) in the 1H NMR spectra are referenced to the residual proton signal
of the solvent (δ = 7.24 ppm for chloroform) and in 13C NMR spectra to the solvent
signal (δ = 77.0 ppm for chloroform-d1).
MS spectra were recorded with a VARIAN MAT CH7 mass spectrometer (direct
inlet system, electron impact ionization, 70 eV). In addition, a Hewlett Packard 5917A
mass spectrometer was routinely used to record MS spectra and in combination with a
Hewlett Packard Series II 5890 gas chromatograph to record GC/MS spectra.
Molecular weight determinations of the polyethylene samples were performed
using a HT-GPC equipment.
Gas chromatograms were recorded using a Perkin Elmer Auto System gas
chromatograph with flame ionization detector (FID) and helium as carrier gas (5.7
mL/min).
The temperature program was as follows.
Starting phase: 3 min at 50 °C
Heating phase: 4 °C/min (15 min)
Plateau phase: 250 °C (37 min)
Methylaluminoxane was supplied by Witco GmbH, Bergkamen, as 30% solution
in toluene (average molecular weight 1100 g/mol, aluminum content: 13.1%, 3.5% as
trimethylaluminum).
General synthesis procedure for the diimine compounds 1 - 8
To a solution of 40 mmol of the respective aniline derivative in dichloromethane,
15 mmol of the corresponding diketo compound and a catalytic amount of p-toluene
sulfonic acid were added. The mixture was heated under reflux for 3-36 h. The
progress of the reaction was observed by GC. The reaction mixture was cooled to
room temperature and filtered over silica. After removing the solvent in vacuo, the
product was washed with cold methanol. For purification, the products were
recrystallized from a methanol/ethanol mixture. The products were obtained as yellow
crystals. Yields: 25-80%.
All compounds were characterized by NMR spectroscopy (Table 1).
General synthesis procedure for the diimine compounds 9 - 21 To a solution of 40 mmol of the respective aniline derivative in toluene, 15 mmol
of the corresponding diketo compound and a catalytic amount of p-toluene sulfonic
acid were added. The mixture was heated under reflux in a Dean Stark apparature for
3-48 h. The produced water was permanently removed. The progress of the reaction
221
was observed by GC. The reaction mixture was cooled to room temperature and
filtered over silica. After removing the solvent in vacuo, the product was washed with
cold methanol. For purification, the products were recrystallized from a
methanol/ethanol mixture. The ligands were obtained as yellow crystals. Yields: 25-
80%.
All compounds were characterized by NMR spectroscopy (Table 1).
General synthesis procedure for the diimine chromium complexes 22 – 29 and 38 - 42
To 0.7 mmol of the respective diimine compound dissolved in 50 mL THF, 0.7
mmol of trichlorotris(tetrahydrofuran)chromium(III) were added under argon
atmosphere. The mixture was stirred 8-15 h at room temperature. For purification, the
volume of the solvent was reduced in vacuo and the complexes were precipitated by
adding pentane. After washing several times with pentane until the solvent stayed
colorless, the products were dried in vacuo. The complexes were obtained as green
powders. Yield: 30-60 %.
The complexes were identified by mass spectrometry (Table 2).
General synthesis procedure for the diimine chromium complexes 30 - 37
To 0.7 mmol of the respective diimine compound dissolved in 50 mL THF at -30
°C, 0.7 mmol methyllithium (1.6 M in ether) were added. After stirring over night, 0.7
mmol of trichlorotris(tetrahydrofuran)chromium(III) were added under argon
atmosphere. The mixture was stirred for 8-15 h at room temperature. For purification,
the volume of the solvent was reduced in vacuo and the complexes were precipitated
by adding pentane. After washing several times with pentane until the solvent stayed
colorless, the products were dried in vacuo. The complexes were obtained as green
powders. Yield: 60-80 %.
The complexes were identified by mass spectrometry (Table 2).
General procedure for the activation of the complexes
An amount of 5–10 mg of the corresponding complex was suspended in toluene
and activated with an excess of MAO (Al/Cr = 2500). The solvent was removed in
vacuo and the activated catalyst was suspended in 50 mL n-pentane. The catalyst
suspension was used for ethylene polymerization.
Homogenous polymerization of ethylene
n-Pentane, 250 mL, was placed in a 1 L Büchi laboratory autoclave, mixed with
the catalyst solution and the autoclave thermostated at 60 °C. An ethylene pressure
(99.98 % ethylene) of 10 bars was applied after an inside temperature of 50 °C was
reached. The mixture was stirred for 1 h at 60(± 2) °C, and subsequently the reaction
was terminated by releasing the pressure in the reactor. For the separation of the
oligomers and the polymers, the polymerization mixture was filtered and the remaining
222
polymer was washed with half concentrated hydrochloric acid in order to remove MAO.
After that it was dried in vacuo and weighed. The pentane of the oligomer solution was
removed by destillation over a Vigreux column and the oligomers were analyzed by
GC. The obtained polymer was dried in vacuo.
Results and discussion Synthesis of the diimine chromium complexes Synthesis of the diimine compounds 1 - 21
The diimine compounds can be synthesized by a condensation reaction of two
equivalents of an aniline derivative and one equivalent of a diketone according to
Scheme 1. They can be divided in different classes according to the ligand backbone.
R'
R'
R'
R
R N
NR
R O
O
+ 2
NH2
[H+]
- 2H2O
R = substituents at the backbone
R' = substituents at the phenyl rings Scheme 1. Synthesis of the diimine compounds.
All compounds are listed in Figure 1 and were characterized by NMR
spectroscopy (Table 1). Compounds 1 [21], 2 [22], 3 [23], 4 [24], 5 [25], 8 [26], 9 [17-19], 10 [27], 13 [17-19] and 20 [28] are already known in the literature.
223
R =
N
N
R
R
R =
Bu NMe2
Ph
5 6 7 8
1 2 3 4
Cl Br
NR
NR
H
NMe2
R =
R =
13 14 15 16
9 10 11 12
butanediimines pentanediimines
N
NR
R
NMe2Cl
R =
R =
20 21
18 1917
hexanediimines
Figure 1. Synthesized diimine compounds.
Table 1. NMR data of compounds 1-21.
1H-NMRa) 13C-NMRb)
1 7.37 (vt, 2H, 3JHH = 7.5 Hz); 7.12 (vt, 4H, 3JHH = 7.5 Hz); 6.79 (d, 4H, 3JHH = 7.5 Hz);
2.15 (s, 6H)
Cq: 168.3, 150.9
CH: 129.0, 123.8, 118.7
CH3: 15.4
2 7.12 (m, 8H), 6.93 (dt, 2H, 3JHH = 7.5 Hz, 4JHH = 1.2 Hz), 6.55 (dd, 2H, 3JHH = 7.8 Hz, 4JHH = 0.9 Hz),2.03 (s, 6H)
Cq: 167.7, 149.5, 126.7
CH: 130.4, 126.4, 123.9, 117.6
CH3: 17.8, 15.6
3 7.04 (s, 2H), 7.01 (d, 2H, 3JHH = 7.5), 6.55
(d, 2H, 3JHH = 7.5), 2.32 (s, 6H), 2.12 (s,
6H), 2.09 (s, 6H)
Cq: 167.8, 146.9, 133.2, 133.1
CH: 131.1, 126.8, 117.6
CH3: 20.8, 17.7, 15.5
4 6.92 (s, 4H), 2.32 (s, 6H), 2.07 (s, 6H), 2.03
(s, 12H)
Cq: 168.3, 145.9, 132.4, 124.5
CH: 128.6
CH3: 20.7, 17.7, 15.8
224
1H-NMRa) 13C-NMRb)
5 7.14 (d, 4H, 3JHH = 7.5), 6.68 (d, 4H, 3JHH =
7.5), 2.62 (t, 4H, 3JHH = 7.5), 2.13 (s, 6H),
1.59 (tt, 4H, 2J = 11, 3J = 7.5? ), 1.33 (qt,
4H, 2J = 11, 3J = 7.5), 0.92 (t, 6H, 3JHH =
7.5)
Cq: 168.2, 148.5, 138.3
CH: 128.8, 118.8
CH2: 35.0, 33.7, 22.3
CH3: 15.4, 14.0
6 7.23 (m, 16H), 6.63 (m, 2H), 3.89 (s, 4H),
1.84 (s, 6H)
Cq: 167.9, 149.3, 140.6, 130.2
CH: 130.6, 130.4, 128.8, 128.7, 128.3, 127.0,
125.9, 124.0, 118.0
CH2: 38.1
CH3: 15.4
7 7.88 (dd, 2H, 3JHH = 6.4 Hz, 4JHH = 2.1 Hz),
7.82 (dd, 2H, 3JHH = 7.3 Hz, 4JHH = 2.1 Hz),
7.66 (d, br, 2H, 3JHH = 8.3 Hz), 7.51 (m,
6H), 6.84 (dd, 2H, 3JHH = 7.3 Hz, 4JHH = 0.9
Hz), 2.03 (s, 6H)
Cq: 169.1, 147.0, 134.1
CH: 128.1, 126.3, 125.8, 125.7, 124.1, 123.4,
113.0
CH3: 15.9
8 7.26 (d, 4H, 3JHH = 7.5), 6.81 (d, 4H, 3JHH =
7.5), 3.43 (s, 12H), 2.71 (s, 6H).
Cq: 167.6, 147.8, 140.7
CH: 121.2, 113.06
CH3: 41.0, 15.6
9 7.23 (t, 4H, 3JHH = 7.5 Hz), 6.99 (t, 2H, 3JHH
= 7.5 Hz), 6.91 (d, 2H, 3JHH = 7.5 Hz), 4.87
(s, 1H), 3.40 (br, 1H), 1.95 (s, 6H)
Cq: 159.7, 145.9
CH: 129.0, 123.4, 122.8, 97.5
CH3: 21.1
10 7.14 (m, 4H), 6.98 (vt, 2H, 3JHH = 7.5), 6.91
(d, 2H, 3JHH = 7.5), 4.90 (s, 1H), 2.19 (s,
6H), 1.90 (s, 6H)
Cq: 159.3, 144.3, 130.4
CH: 130.0, 125.8, 123.3, 122.7, 96.2
CH3: 20.5, 18.0
11 6.92 (d, 2H, 3JHH = 7.5), 6.91 (s, 2H), 6.80
(d, 2H, 3JHH = 7.5), 4.86 (s, 1H), 2.29 (s,
6H), 2.15 (s, 6H), 1.88 (s, 6H)
Cq: 159.5, 141.7, 132.7, 130.3
CH: 130.7, 126.3, 122.7, 95.8
CH3: 20.5, 20.4, 17.9
12 7.02 (vt, 2H, 3JHH = 7.5), 6.89 (d, 2H, 3JHH =
7.5), 6.77 (d, 2H, 3JHH = 7.5), 4.88 (s, 1H),
2.27 (s, 6H), 2.11 (s, 6H), 1.86 (s, 6H)
Cq: 159.6, 144.2, 137.0, 129.3
CH: 125.1, 125.0, 120.9, 95.5
CH3: 20.4, 20.2, 13.8
13 7.25 (d, 4H, 3JHH = 7.5), 7.23 (vt, 2H, 3JHH =
7.5), 4.96 (s, 1H), 3.39 (qq, 1H, 3JHH = 7.5),
1.73 (s, 6H), 1.25 (d, 24H, 3JHH = 7.5)
Cq: 160.9, 142.2, 141.8
CH: 125.3, 123.0, 93.7, 28.0
CH3: 23.9, 22.8
14 7.09 (d, 2H, 4JHH = 2.4 Hz), 7.80 (dd, 2H, 3JHH = 8.8 Hz, 4JHH = 2.4 Hz), 6.76 (d, 2H, 3JHH = 7.5 Hz), 4.84 (s, 1H), 3.00 (br, 1H),
2.08 (s, 6H), 1.82 (s, 6H)
Cq: 160.2, 143.3, 132.7, 128.8
CH: 130.3, 126.4, 124.2, 97.3
CH3: 21.0, 18.4
15 7.37 (d, 4H, 3JHH = 8.6), 6.80 (d, 4H, 3JHH =
8.6), 4.88 (s, 1H), 1.97 (s, 6H)
Cq: 159.3, 144.2, 115.9
CH: 131.5, 123.8, 97.7
CH3: 20.5
16 6.89 (d, 4H; 3JHH = 8.8), 6.79 (d, 4H, 3JHH =
8.8), 4.71 (s, 1H), 2.82 (s, 12H), 1.98 (s,
6H)
Cq: 159.7, 147.1, 135.5
CH: 123.9, 112.9, 95.5
CH3: 40.8, 20.3
225
1H-NMRa) 13C-NMRb)
17 6.28 (vt, 2H, 3JHH = 7.5), 7.04 (vt, 4H, 3JHH =
7.5), 6.70 (d, 4H, 3JHH = 7.5), 2.56 (q, 4H, 3JHH = 7.5), 1.00 (t, 6H, 3JHH = 7.5)
Cq: 171.5, 150.7
CH: 128.9, 123.4, 118.3
CH2: 21.9
CH3: 12.6
18 7.31 (d, 2H, 3JHH = 7.5 Hz); 7.19-7.1 (m,
4H); 6.62 (d, 2H, 3JHH = 7.5 Hz); 2.95 (m,
2H); 2.64 (q, 4H, 3JHH = 7.5 Hz); 1.22 (d,
12H, 3JHH = 7.5 Hz); 1.11 (t, 6H, 3JHH = 7.5
Hz)
Cq: 171.0; 148.1; 137.3
CH: 126.0; 125.5; 124.0; 117.8; 28.4
CH2: 22.2
CH3: 22.7; 12.2
19 7.8 (t, 2H, 3JHH = 7.5 Hz); 7.06 (t, 2H, 3JHH =
7.5 Hz); 6.66 (d; 4H, 3JHH = 7.5 Hz); 2.61 (q,
4H, 3JHH = 7.5 Hz); 2.45 (t, 4H, 3JHH = 7.5
Hz); 1.63-1.54 (m, 4H); 1.1 (t, 6H, 3JHH =
7.5 Hz); 0.94 (t, 6H, 3JHH = 7.5 Hz)
Cq: 171.1; 149.0; 131.3
CH: 129.6; 126.3; 123.7; 117.8
CH2: 34.0; 22.8; 22.1
CH3: 14.0; 12.2
20 7.11 (t, 2H, 3JHH = 2.9 Hz), 6.77 (d, 2H, 3JHH
= 2.8 Hz), 6.56 (d, 2H, 3JHH = 2.8 Hz), 2.58
(q, 4H, 3JHH = 7.5 Hz), 2.15 (s, 6H), 1.04 (t,
6H, 3JHH = 7.5 Hz)
Cq: 171.7, 150.6, 135.3, 124.6
CH: 126.8, 124.4, 116.0
CH2: 22.1
CH3: 14.9, 12.0
21 7.26 (m, 8H), 3.44 (s, 12H), 3.20 (q, 4H, 3JHH = 7.5), 1.59 (t, 6H, 3JHH = 7.5)
Cq: 153.3, 147.6, 141.0
CH: 120.2, 113.4
CH2: 21.7
CH3: 41.1, 12.7 a) 25°C, in Chloroform-d1, δ [ppm] rel. Chloroform (7.24). b) 25°C, in Chloroform-d1, δ [ppm] rel. Chloroform (77.0).
Synthesis of the (α-diimine) chromium(III) complexes 22 – 29 and 38 - 42
The (α-diimine) chromium(III) complexes were prepared by the reaction of the
corresponding diimine with an equimolar amount of trichlorotris(tetrahydrofuran)
chromium(III) in THF.
+
- 2THFCrCl3 3THF. CrCl3thf
N
NR'
R'
R
RR'
R' N
N
R
R Scheme 2. Synthesis of (α-diimine) chromium(III) complexes.
Synthesis of the (β-diimine) chromium(III) complexes 30 - 37
For the synthesis of the (β-diimine) chromium(III) complexes the ligand precursor
was first deprotonated with BuLi. The lithiumsalt was formed. The reaction with
trichlorotris(tetrahydrofuran)chromium(III) in THF gave the product. This procedure has
been described in the literature [15-20].
226
NR
NR
H+BuLi
-BuH NR
NR
Li+-+ CrCl3 3THF.
- THF- LiCl N
R
NR
CrCl2(thf)2
Scheme 3. Synthesis of (β-diimine) chromium(III) complexes.
The synthesized complexes are summarized in Figure 2.
NMe2
R =
R =
Bu
Ph
N
N
R
R
CrCl3thf
22 23 24 25
26 27 28 29
NR
NR
CrCl2(thf)2
Cl Br NMe2
R =
R =
33
34[17-19] 3635
30[17-19] 31 32
37
38 39 40
41
NMe2Cl
R =
R =
N
N
R
R
CrCl3thf
42
Figure 2. Synthesized chromium complexes.
Due to the paramagnetic nature of these complexes, it is not very informative to
characterize them by NMR spectroscopy. The mass spectrometric analyses did not
reveal the molecular ion in every case. The fragments deriving from the loss of THF
molecules or the chlorine are observed (see Table 2).
227
Table 2. Mass spectrometric data of complexes 22 - 42.
complex fragment [m/z] (intensity [%]) 22 M+(-THF) = 393 (10), 236 (10), 118 (20), 93 (100) 23 M+(-THF) = 421 (5), 316 (10), 264 (10), 249 (50), 132 (40), 106 (100), 77(20) 24 M+(-THF, 3Cl) = 344 (15), 318 (20), 277 (20), 146 (50), 121 (100), 120 (85), 106
(65), 77 (20) 25 M+(-THF) = 477 (10), 371 (10), 305 (55), 160 (100), 119 (25), 91 (20) 26 M+(-THF, 3Cl) = 400 (15), 374 (15), 174 (100), 106 (55), 91 (30) 27 M+(-THF, 3Cl) = 440 (5), 388 (15), 194 (100), 169 (40), 72 (45) 28 M+(-THF, 3Cl) = 388 (10), 336 (70), 168 (100), 127 (65) 29 M+(-THF, 3Cl) = 374 (10), 322 (20), 161 (100), 136 (20), 121 (20) 30 M+ = n. d., 133 (10), 118 (20), 77 (100), 31 M+(-2THF)=399 (80), 364 (20), 329 (10), 277 (20), 173( 50), 132 (100), 106 (20), 91
(100) 32 M+(-2THF) = 427 (5), 305 (5), 291 (10), 187 (100), 146 (70), 106 (20), 105 (80), 91
(35), 77 (100) 33 M+(-2THF, 2Cl) =357 (5), 306 (30), 291 (25), 187 (85), 146 (100), 106 (40), 105
(40), 91 (15), 77 (35) 34 M+ = n. d., 417 (10), 403 (100), 375 (28), 202 (100), 187 (60), 160 (30), 91(20) 35 M+ = n. d., 346 (20), 207 (100), 166 (70), 125 (30), 77 (10) 36 M+ = n. d., 407 (5), 237 (40), 197 (40), 171 (70), 157 (50), 91 (50), 65 (70), 36 (100) 37 M+(-2THF, Cl) = 422 (10), 387 (5), 335 (20), 321 (20), 258 (20), 202 (70), 161 (50),
136 (20) 38 M+(-THF, 3Cl) = 316 (10), 264 (20), 132 (100) 39 M+(-THF, 2Cl) = 436 (10), 348 (10), 174 (80), 135 (50), 120 (100), 40 M+ = n. d., 348 (10), 305 (30), 174 (100), 91 (10) 41 M+ = n. d., 360 (10), 180 (100), 125 (20), 106 (60) 42 M+(-THF, 3Cl) = 402 (10), 350 (20), 175 (100), 136 (65)
Polymerization of ethylene
All catalyst precursors are chromium chloride complexes. They were activated
with MAO (Al:Cr = 2500) and they were used for the homogeneous polymerization of
ethylene.
In the activation step an electronically and sterically unsaturated cationic
chromium methyl cation is formed that represents the actual catalyst.
isomerisation
β-H-eliminination
metathesis
N
N
Cr R
N
N
CrH
R
- R N
N
CrH
N
N
Cr
H
R
N
N
Cr
H
R
N
N
Cr
H
R
R-
-
The obtained products were in some cases mixtures of oligomers and polymers.
The polymerization results are summarized in Table 3.
228
Table 3. Results of the homogeneous ethylene polymerization reactions with 22 – 42 activated with MAO.
complex activity [kg(Prod)/mol(Cr)⋅h]
polymer share [wt. %]
HT-GPC (polymer share)
Mn [g/mol] Mw [g/mol] Mz [g/mol]
Mz+1 [g/mol] MP [g/mol]
D
α
22 868 78.5 n. b. n. d. 23 487 85.1 10 800
217 900 1 041 400 1 660 900
65 500 20.2
0.97
24 636 84.6 10 300 1 037 700 6 172 800 9 576 700
73 200 100.6
0.93
25 594 100 18 700 904 600
5 591 100 9 127 700
68 800 48.3
-
26 923 87.5 9 700 274 500
1 131 200 1 683 200
75 700 28.3
n. d.
27 223 100 12 900 278 100
1 106 600 1 688 300
49 500 21.6
-
28 638 48.2 n. b. 0.93 29 142 100 109 600
303 300 668 000
1 133 700 200 000
2.8
-
30 140 100 5 600 536 600
3 662 500 6 763 500
54 900 95.2
-
31 152 100 6 500 622 600
7 675 000 13 660 400
46 200 96.4
-
229
complex activity [kg(Prod)/mol(Cr)⋅h]
polymer share [wt. %]
HT-GPC (polymer share)
Mn [g/mol] Mw [g/mol] Mz [g/mol]
Mz+1 [g/mol] MP [g/mol]
D
α
32 85 100 12 800 857 300
4 241 600 6 613 700
95 500 66.8
-
33 1 820 54.8 5 900 176 600
1 066 900 1 726 700
33 900 30.0
0.95
34 102 100 2 000 12 400
106 200 245 000
1 400 6.6
-
35 211 100 7 300 805 900
7 970 200 14 239 200
54 500 109.7
-
36 514 100 12 100 362 100
1 304 300 1 855 400
71 500 29.9
-
37 54 100 62 000 378 200
1 199 100 1 806 200 102 700
6.1
-
38 57 100 19 900 171 900 603 400 918 200 51 700
8.7
-
39 440a 78.9 n. b. n. d. 40 388 38.8 n. b. 0.98 41 541 78.6 n. b. n. d. 42 137 100 5 800
443 300 4 569 700 8 848 200
57 200 76.4
-
Polymerization conditions: Al/Cr = 2500/1; polymerization in 250 ml pentane, 60 °C, 1 L autoclave, 10 bar ethylene pressure, 60 min.
constant-Flory-Schulz )(
=+
=+
=transferchainofratenpropagatioofrate
npropagatioofrate
kkk
CB
Bα
D = polydispersity Mw/Mn of the polymer share n.d. = not determined
230
Discussion of the polymerization results
(α-Diimine)butane chromium(III) complexes
The highest activities were obtained when the active center of the catalyst was
easily accessible. This is the case for 22 and 26. There are no substituents or only a
butyl group in para-position which is too far away from the active center to block it.
Catalyst 27 has a bulky benzyl group. Therefore the activity decreases.
Figure 3. Polymerization activities of complexes 22 – 29/MAO.
(β-Diimine)pentane chromium(III) complexes
All (β-diimine)pentane chromium(III) complexes show a similar polymerization
behavior. A highlight was the pentanediimine chromium complex 33 which had a very
high activity of 1820 [kg(prod)/mol(Cr)⋅h]. A polymer/oligomer mixture was produced
with an oligomer share of 45%. The GC/MS-spectra revealed oligomers with odd
(25%) and even (75%) numbers of carbon atoms (Figure 7). The formation of
oligomers with odd numbered carbon atoms is indicative for the following reaction
cascade at one and the same active center (Scheme 4): The originally formed even
numbered 1-olefin is isomerized to give the 2-olefin. The 2-olefin and ethylene undergo
an olefin metathesis reaction to yield oligomers with odd numbers of carbon atoms.
This behavior can be explained by the substituents at the phenyl rings. The two methyl
groups are in the ortho and meta position. The positions of the two groups influence
the polymerization behavior in a way that the catalytic active center is blocked for
further insertion reactions. At the same time termination and metathesis reactions are
preferred.
231
Figure 4. Polymerization activities of complexes 30 – 37/MAO.
(α-Diimine)hexane chromium(III) complexes
Compound 38 has the least activity (Figure 5). It is not substituted. The presence
of substituents at the phenyl rings influences the activities due to electronic effects.
Complexes 39, 40 and 41 have electron pushing groups and the activities increase in
contrast to 38. The dimethylamino group in 42 has a negative inductive effect and the
activity decreases.
Figure 5. Polymerization activities of complexes 38 – 42/MAO.
232
1.1.1 Discussion of the oligomers
Figure 6. GC/MS-spectra of the oligomers produced with 28/MAO.
Some complexes produced a mixture of polymers and oligomers. The oligomer
mixtures were studied by GC to determine the Schulz-Flory coefficient which gives
informations about the molecular weight distributions of the oligomers (Table 3).
The various fractions were characterized by GC/MS.
Figure 6 shows the different oligomers from C-6 to C-22. Each fraction consists
of several isomers which are different in terms of the position of the double bond and
branching. The isomers were formed by the isomerizations of the primarily formed 1-
olefins. In the spectra there is also a peak for toluene, the solvent for MAO, which
could not be fully removed. An early termination of the chain growth by β-H elimination
produces the oligomers. The oligomers show a broad molecular weight distribution.
Figure 7. GC/MS-spectra of the oligomers produced with 33/MAO.
233
isomerisation
β-H-eliminination
metathesis
N
N
Cr R
N
N
CrH
R
- R N
N
CrH
N
N
Cr
H
R
N
N
Cr
H
R
N
N
Cr
H
R
R-
-
Scheme 4. Metathesis reaction to form oligomers with odd and even numbers of
carbon atoms.
Acknowledgements We thank SABIC, Riyadh, Saudi Arabia, for the financial support of the project.
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