Novel non-symmetric nickel-diimine complexes for the homopolymerization of ethene: Control of branching by catalyst design

Article abstract of DOI:10.1515/znb-2002-1011

Non-symmetric diimine ligands (Ar-N=C(CH₃)-(CH₃)C=NAr*; Ar: 2,6-diisopropyl-phenyl; Ar*: 2,6-di(4-tert-butyl-phenyl)phenyl (4b), 2,6-di(4-OCH₃)-phenyl)phenyl (4c)) were synthesized and converted in-situ into the corresponding nickel dibromo complexes (5b, c) by reaction with (DME)NiBr₂. The complexes were activated for ethene polymerization by treatment with MAO at ambient temperature. The resulting high molecular weight polymer products (M_w > 4.0 × 10⁶ g mol^-1) have a branched microstructure (predominantly methyl groups), as indicated by ¹³C NMR spectroscopy. The degree of branching can be controlled by a proper choice of the 2,6-diphenyl modified aniline moieties resulting in melting transitions ranging from 92 - 130°C.

Full text of DOI:10.1515/znb-2002-1011

Novel Non-Symmetric Nickel-Diimine Complexes for the Homopoly-
merization of Ethene: Control of Branching by Catalyst Design
Markus Schmid, Robert Eberhardt, Ju¨rgen Kukral, and Bernhard Rieger
Division for Materials and Catalysis, University of Ulm, D-89069 Ulm, Germany
Reprint requests to Prof. Dr. B. Rieger.
E-mail: bernhard.rieger@chemie.uni-ulm.de
Z. Naturforsch. 57 b, 1141–1146 (2002); received June 28, 2002
Non-Symmetric Diimine Ni(II) Catalysts, Branched Polyethenes, C2/C3-Copolymers
Non-symmetric diimine ligands (Ar-N=C(CH3)-(CH3)C=NAr*; Ar: 2,6-diisopropyl-phenyl;
Ar*: 2,6-di(4-tert-butyl-phenyl)phenyl (4b), 2,6-di(4-OCH3)-phenyl)phenyl (4c)) were synthe-
sized and converted in-situ into the corresponding nickel dibromo complexes (5b, c) by reaction
with (DME)NiBr2. The complexes were activated for ethene polymerization by treatment with
MAO at ambient temperature. The resulting high molecular weight polymer products (MW > 4.0
106 g mol 1) have a branched microstructure (predominantly methyl groups), as indicated
by 13C NMR spectroscopy. The degree of branching can be controlled by a proper choice of
the 2,6-diphenyl modified aniline moieties resulting in melting transitions ranging from 92 -
130 C.
1. Introduction
We reported recently on new palladium(II) and
nickel(II) terphenyl diimine complexes (4d), based
on sterically demanding 2,6-diphenyl-modified ani-
line moieties as highly active catalysts for the poly-
merization of ethene [9, 10]. The steric demand of
the 2,6-diphenyl substituents was exalted by intro-
ducing para-substituents leading to basically linear
polyethenes. Our subsequent investigations were di-
rected towards the synthesis of non-symmetric di-
imine ligands bearing the 2,6-diisopropyl aniline
group on one side and 2,6-diphenyl modified ani-
line fragments on the other side of the diacetyl back-
bone (4b,c). Thereby, it is intended to combine the
properties of the symmetric catalysts leading to the
formation of primarily linear polyethene bearing a
controllable degree of short-chain branches. Such
During the last years intensive research has fo-
cused on late transition metal complexes as catalysts
for the polymerization of olefins [1 - 3]. Following
the pioneering studies of Keim [2] and Fink [3]
in the 1970s and 1980s, mainly Brookhart et al.
have reported on the development of highly active
palladium(II) and nickel(II) diimine complexes that
polymerize ethene to high molecular weight poly-
mers with a branched microstructure [4 - 5]. As pos-
tulated by Brookhart et al., the steric hindrance of
bulky isopropyl substituents in the 2,6-positions of
the aryl groups retards the rate of associative olefin
displacement, thus affording polymers with higher
molecular weights [6 - 8, 19, 20].
Scheme 1. Different architectures of diimine complexes for the polymerization of ethene.
c
¨
¨
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M. Schmid et al. · Nickel-Diimine Complexes for the Homopolymerization of Ethene
Scheme 2. Synthesis of the terphenylamines.
products can be regarded as ethene/propene copoly- B uses the terphenyl amino compounds 2b, c to give
mers (CH3-side groups) or as LLDPE-analogues the solid products 3b, c.
(longer alkyl branches) based on ethene as a single
component.
Following these approaches, the p-tert-butyl sub-
stituted (4b) and the p-methoxy substituted ligand
(4c) can be easily synthesized in yields up to 85 %.
The corresponding nickel dibromo complexes re-
sulted from in situ reactions of the ligands with
(DME)NiBr2 in toluene solution [14].
2. Results and Discussion
2.1. Ligand and complex synthesis
Diimine ligands can be obtained by a simple con-
densation reaction of , -diketones with suitable
aniline derivatives [12, 13]. While the diisopropyl
amino compound 2a is commercially available, the
terphenyl anilines 2b,c had to be synthesized via
a Suzuki cross-coupling protocol based on 2,6-di-
bromoaniline [15, 16]. The substituted arylboronic
acids 1b,c are easily accessible by treatment of
the corresponding aryl Grignard compounds with
trimetyl borate [17].
The preparation of the symmetric diimine ligands
had to be performed by refluxing a solution of 2,3-
butanedione and the amino compound in benzene
under the catalytic influence of p-toluenesulfonic
acid, due to the steric demand of the substituted
terphenyl building blocks. The new non-symmetric
diimine ligands are accessible by a two-step pro-
2.2. Polymerization reactions
The nickel dibromo complexes could be acti-
vated for the polymerization of ethene by addition
of MAO at ambient temperature. Due to the high
catalyst activities and the precipitating polymers,
the polymerization experiments were performed in
a 2000 ml steel autoclave with a strong mechanical
stirrer that guaranteed controlled conditions during
polymerization reaction (Table 1).
The polyethenes obtained with Brookhart’s cata-
lyst (5a) had a molecular weight of 4 106 g mol
1
and showed a branched microstructure resulting in a
broad melting range around 90 C. NMR measure-
ments indicated about 55 methyl groups per 1000
C-atoms [18]. In contrast, the symmetric terphenyl
cedure. In a first reaction the mono condensation complex 5d produced high molecular weight, crys-
talline polyethene which exhibited a sharp melting
transition at around 130 C. The molecular weights
exceeded 4.5 106 g mol 1. For these products,
high temperature NMR experiments gave no in-
dication for methyl branching. The polymers ob-
product is synthesized by combination of equimolar
amounts of diketone and the corresponding amino
compound at ambient temperature. Subsequently,
condensation of the resulting ketimine with the cor-
responding amino component is performed.
Two routes are conceivable for the preparation tained with the non-symmetric catalysts 5b,c give
of the mono-condensation product. Route A starts melting transitions of 105 and 112 C, respectively.
from the diisopropyl aniline 2a, resulting in the liq- The degree of branching is 38 CH3 / 1000 C (5b)
uid ketimine 3a which can easily be purified by and 22 CH3 / 1000 C (5c). 13C NMR experiments
vacuum distillation of the reaction mixture. Route give decisive evidence that predominantly methyl
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M. Schmid et al. · Nickel-Diimine Complexes for the Homopolymerization of Ethene
1143
Table 1. Polymerization experiments with nickel dibromo complexes (p(ethene) = 10 bar)
a
Run
Catalyst
N(Cat.)
[ mol]
Time
[min]
Activity
[Kg(PE) (mol(Nixh))
MW
[g mol
D
N b
Tm c [ C]
[ C]
1
1
]
]
1
2
3
4
5a
5b
5c
5d
20
40
40
20
15
60
60
15
4.000
400
1.200
6.500
3.98
> 4.0
> 4.0
> 4.0
2.3
–
55
38
22
0
92
105
112
130
–
–
a Measured in 1,2,4-trichlorobenzene at 135 C; all molecular weights measured against narrow polystyrene standards;
b
molecular weights of more than 4.0 106 are outside of the range of the used gpc setup; methyl groups / 1000
carbon atoms (predominantly methyl branches); c determined by DSC measurements.
Scheme 3. Synthesis of mono-condensation products.
branches are produced during the polymerization weight homopolymers. This shows clearly that the
reaction. Only minor concentration of longer alkyl degree of branching can be controlled by catalyst
side chains occur. Under these conditions 5a-c / design combining aniline moieties bearing differ-
MAO produce ethene / propene copolymers from ent substituents in 2,6 position. Our first empiri-
ethene monomer, while the symmetric substituted cal approach to achieve such a control might find
complex 5d / MAO leads to linear high molecular an explanation in an elegant theoretical work of
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M. Schmid et al. · Nickel-Diimine Complexes for the Homopolymerization of Ethene
Scheme 3. Synthesis
of mono-condensation
products.
Ziegler et al. [19 - 21]. As already discussed above, ture and are referenced to TMS. Mass spectra were ac-
quired with Finnigan (SSQ 7000) and Varian (MAT-711)
instruments. Elemental analyses were determined in the
Microanalytical Laboratory of the University of Ulm.
they assume the degree of branching to be a conse-
quence of the steric hindrance around the active
center. Thereby, a precise control of the rate of
the associative olefin exchange and of the rate of
2,1-insertion of the vinyl chain end after -hydride
elimination becomes accessible by combining the
isopropyl motif with the more sterically demand-
ing para-phenyl substituted anilines. A fine tuning
of 2,1-relative to 1,2-insertion seems to take place
allowing to control the degree of methyl branching
along the linear main chain which leads to ethene /
propene copolymers just from ethene monomer.
3.2. Ligands and complexes
3.2.1. General procedure for the synthesis of
arylboronic acids 1
In a 500 ml Schlenk flask, a solution of 1 equiv of the
aryl bromide was added dropwise to 1.02 equiv of mag-
nesium turnings. The resulting Grignard solution was fil-
tered and added slowly dropwise with stirring to a calcu-
lated amount of trimethyl borate in ether at such a rate that
the temperature did not rise above –70 C. The suspension
was allowed to come to room temperature overnight. The
mixture was added under stirring to ice (100 g) and con-
centrated sulfuric acid (5 ml). After stirring for 30 min,
the reaction product was extracted repeatedly with ether.
Evaporation of the dried (Na2SO4) ether extract gave a
solid which was dried in vacuo.
3. Experimental Section
3.1. Materials
The synthesis of the complexes was performed un-
der dry argon atmosphere using conventional Schlenk
techniques. All Ni(II) complexes were generated by in
situ reactions of the ligands with (DME)NiBr2 in toluene
and were activated using MAO as cocatalyst. Hydrocar-
bons and ether solvents were dried by distillation from
LiAlH4, methylene chloride was distilled from CaH2.
Aryl bromides, magnesium turnings, trimethyl borate,
2,6-dibromoaniline, 2,6-diisoproylaniline, 2,3-butanedi-
one, and (DME)NiBr2 were used as received from Fluka,
Merck and Aldrich. (Ph3P)4Pd, 2,3-bis((2,6-diisopropyl-
phenyl)imino)butane and the more highly substituted di-
imine ligands (4d) were prepared according to literature
procedures [9, 11 - 13].
4-tert-Butyl-phenylboronic acid (1b)
Synthesis from 1-bromo-4-tert-butylbenzene (117.3
mmol, 25.00 g), magnesium (119.6 mmol, 2.92 g) and
trimethyl borate (142.3 mmol, 14.78 g) gave 1b as a white
1
solid; yield: 21.1 g (83.4 %). – H NMR (200 MHz,
methanol-d4): = 1.34 (s, 9H, tBu), 7.42 (d, 2Harom),
7.68 (d, 2Harom). – 13C NMR (50 MHz, methanol-d4):
= 31.95, 35.79, 125.71, 135.02, 154.51. – C10H15BO2
(178.0): calcd. C 67.46, H 8.49; found C 66.98, H 8.39.
Methylalumoxane was purchased from Witco and
toluene as polymerization medium from Merck. All com-
4-Methoxy-phenylboronic acid (1c)
1
pounds were analyzed by H and 13C NMR on Bruker
Synthesis from 4-bromoanisole (20.0 mmol, 37.4 g),
AC200 or AMX 500 spectrometers at ambient tempera- magnesium (20.4 mmol, 4.95 g) and trimethyl borate
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M. Schmid et al. · Nickel-Diimine Complexes for the Homopolymerization of Ethene
1145
(26.0 mmol, 2.7 g) gave 1c as a white solid; yield: 28.8 g 3-((2,6-Diisopropylphenyl)imino) 2-butanone (3a)
1
(95%). – H NMR (200 MHz, methanol-d4): = 3.71
To a solution of 2 ml of 2,3-butanedione (22.9 mmol)
and 4.3 ml of 2,6-diisopropylaniline (22.9 mmol) in 5 ml
of benzene a catalytic amount of p-toluenesulfonic acid
monohydrate was added and the reaction mixture was
stirred overnight at room temperature. Than the mixture
was diluted with 20 ml of benzene and dried over Na2SO4.
The crude product could be isolated as a brownish oil, af-
ter evaporation of the solvent. Distillation of the ketimine
in vacuo gave a yellow oil, b. p. 79 - 81 C (0.7 mbar).
(s, 3H, OCH3), 6.82 (d, 2Harom), 7.66 (d, 2Harom). – 13C
NMR (50 MHz, methanol-d4): = 55.65, 114.27, 136.75,
137.69, 163.06. – C7H9O3B (152.0): calcd. C 55.63, H
5.97; found C 54.98, H 6.08.
3.2.2. General procedure for the synthesis of the
terphenylamines 2
1 Equiv of 2,6-dibromobenzene and [Pd(PPh3)4]
(12 mol%) was dissolved in 200 - 240 ml of benzene.
A solution of 3 equiv of the boronic acid in ethanol and
an aqueous 2M Na2CO3 solution (6 equiv) were added
to the flask, which than was connected to a reflux con-
denser and flushed with argon. The reaction mixture was
refluxed for the given reaction time. After that, the or-
ganic layer was separated and the amine precipitated as
hydrochloride or hydrobromide salt by addition of 20 ml
of HBr (2b) or HCl (2c). The precipitate was filtered and
washed with pentane.
In a 1 l flask, the salt was suspended in ether, and
2M Na2CO3 solution was added slowly under vigorous
stirring until the precipitate vanished. The organic layer
was dried over Na2SO4 and the solvent evaporated until
the amine began to crystallize. The product was filtered
and dried in vacuo.
1
Yield: 4.67 g (83 %). – H NMR (200 MHz, C2D2Cl4):
= 1.18 (d, 12H, CH3), 1.85 (s, 3H, CH3), 2.61 (s, 3H,
CH3) 2.52-2.67 (m, 2H, CH), 7.01 - 7.23 (m, 3Harom). –
13C NMR (50 MHz, C2D2Cl4): = 15.16, 22.75, 23.22,
25.10, 28.44, 122.31, 124.49, 134.51, 145.17, 166.58,
168.14, 200.33. – C41H23NO (561.6): calcd. C 78.32, H,
9.45, N, 5.71; found C 78.15, H, 9.35, N, 5.64.
3.2.3 General procedure for the synthesis of the
non-symmetric diimine ligands
In a 250 ml flask 1 equiv of the monocondensation
compound (3a) and 1.1 equiv of the terphenylamine were
dissolved in benzene, and a catalytic amount of p-toluene-
sulfonic acid monohydrate was added. The flask was con-
nected to a water separator and a reflux condenser. After
refluxing the mixture for the needed reaction time, the
solvent was evaporated. The crude product was dissolved
in methylene chloride, and the ligand was precipitated by
addition of methanol, filtered and dried in vacuo.
4,4''-Di(tert-butyl)-m-terphenyl-2-amine (2b)
2,6-Dibromoaniline (27.4 mmol, 6.87 g), Pd(PPh3)4
(3.28 mmol, 3.80 g), p-tert-butyl-phenylboronic acid
(82.2 mmol, 14.6 g), Na2CO3 (164.4 mmol), reaction
time: 72 h. Amine 2b was obtained as a white solid;
yield: 5.5 g (56%). – 1H NMR (200 MHz, acetone-d6): =
1.40 (s, 18H, tBu), 6.83 (t, Harom), 7.05 (d, 2Harom), 7.48
(d, 4Harom), 7.53 (d, 4Harom). - 13C NMR (50 MHz, ace-
ton-d6): = 31.95, 35.33, 118.72, 126.72, 128.54, 129.88,
130.54, 138.30, 142.51, 150.82. – C26H31N (357.5):calcd.
C 87.34, H 8.74, N 3.92; found C 87.12, H 8.71, N 3.89.
2,3-Butanedione-((4,4''-di(tert-butyl)-m-terphenyl-2'-yl)-
imine)(2,6-diisopropylphenylimine) (4b)
Terphenylamine 2b (4.95 mmol, 1.77 g), ketimine 3a
(4.5 mmol, 1.09 g), 75 ml benzene, reaction time: 48 h.
Compound 4b was obtained as a yellow powder; yield:
2.14 g (81 %). – 1H NMR (200 MHz, C2D2Cl4): = 0.97
- 1.19 (m, 12H, CH3), 1.39 (s, 18H, CH3), 1.76 (s, 3H,
CH3), 1.82 (s, 3H, CH3), 2.40 – 2.59 (m, 2H, -CH), 7.00 -
7.21 (m, 3Harom), 7.26 - 7.62 (m, 11Harom). – 13C NMR (50
MHz, C2D2Cl4): = 16.81, 22.89, 23.12, 28.22, 31.51,
34.47, 122.97, 123.60, 124.31, 124.70, 129.05, 129.33,
130.91, 134.89, 136.94, 146.43, 149.68, 167.59, 167.69.
– C42H52N2 (584.9): calcd C 86.25, H 8.96, N 4.79; found
C 85.93, H 8.97, N 4.77.
4,4''-Di(methoxy)-m-terphenyl-2-amine (2c)
2,6-Dibromoaniline (28.0 mmol, 7.15 g), Pd(PPh3)4
(3.4 mmol, 3.91 g), p-methoxy-phenylboronic acid
(84.0 mmol, 12.9 g), Na2CO3 (168.0 mmol), reaction
time: 50 h. Aniline 2c was obtained as a white solid; yield:
8.2g(95%). –1H NMR(200MHz, CDCl3/TMS): =3.73
(s, 12 H, OCH3, 12H), 6.74 (t, 2Harom), 6.87 (d, 8Harom),
2,3-Butanedione-((4,4''-dimethoxy-m-terphenyl-2'-yl)-
imine)(2,6-diisoropylphenylimine) (4c)
4-Methoxy-terphenylamine (2c) (4.95 mmol, 1.51 g),
6.97 (d, 4Harom), 7.32 (d, 8Harom). – 13C NMR (50 MHz, ketimine 3a (4.5 mmol, 1.09 g), 75 ml benzene, reaction
CDCl3/TMS): = 55.30, 114.25, 118.09, 127.60, 129.52, time: 48 h. Compound 4c was obtained as a yellow pow-
130.43, 132.07, 141.19, 158.82. – C20H19NO2 (305.4): der; yield: 2.0 g (85 %). – 1H NMR (200 MHz, C2D2Cl4):
calcd. C 78.70, H 6.22, N 4.59; found C 78.58, H 6.24,
N 4.59.
= 0.88 - 1.24 (m, 12 H, CH3), 1.70 (s, 3H, CH3), 1.81 (s,
3H, CH3), 2.23 - 2.49 (m, 2H, CH), 3,82 (s, 6H, OCH3),
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M. Schmid et al. · Nickel-Diimine Complexes for the Homopolymerization of Ethene
6.90 (d, 4Harom), 6.98 - 7.21 (m, 3Harom), 7.29 - 7.42 3.4. Polymerization procedure
(m, 7Harom). – 13C NMR (50 MHz, C2D2Cl4): = 16.61,
All polymerization reactions were performed in a
17.07, 22.72, 22.97, 28.25, 55.33, 123.28, 122.94, 123.55,
124.32, 129.06, 130.50, 130.68, 132.48, 134.87, 146.36,
146.57, 158.43, 167.40, 167.61. – C36H40N2O2 (532.7):
calcd. C 81.17, H 7.57, N, 5.26; found C 81.40, H 7.49,
N 5.27.
2000 ml steel autoclave which was charged with 800 ml
of toluene and than brought to the desired ethene pres-
sure. After the solution was saturated with monomer, the
solution of the activated catalyst was injected into the
autoclave via a pressure burette. The monomer pressure
was kept constant during the entire polymerization period.
The polymerization reactions were quenched by vent-
ing off excess ethene and the product was isolated after
precipitation from acidified methanol. The polymer was
washed several times with methanol and dried in vacuo.
The experimental conditions and results are summarized
in Table 1.
3.3. Synthesis of the nickel catalysts
The nickel complexes resulted from an in situ reaction
of the ligand with (DME)NiBr2 in toluene for 12 h [14].
The solutions were directly activated by addition of MAO
and the homogeneous catalyst solutions were immedi-
ately used for the polymerization experiments.
[13] R. Van Asselt, E. E. C. G. Gielens, R. E. Ru¨lke,
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[14] A series of experiments with different reaction times
showed that the conversion was complete after stir-
ring the reaction mixture overnight.
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[16] We modified the literature procedure of Miura by
substituting the laborious workup by column chro-
matography by precipitating the amines as hy-
drochloride or hydrobromide salts (cf. also ref. 9).
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[18] The high temperature NMR experiments were per-
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xylene at 100 C.
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119, 1094 (1997).
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