Ethylene polymerization and oligomerization catalyzed by bulky β-diketiminato Ni(II) and β-diimine Ni(II) complexes/methylaluminoxane systems

Article abstract of DOI:10.1016/j.molcata.2005.12.027

β-Diketiminato complexes Ni{(N(Ar)C(Me))₂CH}Br (9), Ni{(N(Ar)C(Me))₂CH} Pph₃Br (10) and β-diimine complexes Ni{(N(Ar)C(Me))₂CH₂}Br₂ (5) (Ar = 2,6-iPr₂C₆H₃ (a), 2,6-Me₂C₆H₃ (b)) were used as catalyst precursors for ethylene polymerization in the presence of methylaluminoxane (MAO). High molecular weight ethylene polymers as well as short chain oligomers (C4-C8) were simultaneously produced from the catalysis reactions. Ethylene polymers obtained by using these β-N-N Ni(II)/MAO catalyst systems are mainly methyl branched. Small amounts of even number branches were also observed in the ¹³C NMR spectra of the obtained ethylene polymers, which are believed generating from the incorporation of simultaneously produced α-olefins. Except methyl branch and the branches derived from the incorporation of α-olefin oligomers, the formation of other branch types via the chain walking process is not favored in β-diketiminato Ni(II) systems.

Full text of DOI:10.1016/j.molcata.2005.12.027

Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
Ethylene polymerization and oligomerization catalyzed by bulky
␤-diketiminato Ni(II) and ␤-diimine Ni(II)
complexes/methylaluminoxane systems
Junkai Zhang, Zhuofeng Ke, Feng Bao, Jieming Long, Haiyang Gao, Fangming Zhu, Qing Wu∗
Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China
Received 13 November 2005; received in revised form 11 December 2005; accepted 21 December 2005
Available online 3 February 2006
Abstract
␤-Diketiminato complexes Ni{(N(Ar)C(Me))2CH}Br (9), Ni{(N(Ar)C(Me))2CH} Pph3Br (10) and ␤-diimine complexes Ni{(N(Ar)C(Me))2
CH2}Br2 (5) (Ar = 2,6-iPr2C6H3 (a), 2,6-Me2C6H3 (b)) were used as catalyst precursors for ethylene polymerization in the presence of methyla-
luminoxane (MAO). High molecular weight ethylene polymers as well as short chain oligomers (C4–C8) were simultaneously produced from the
catalysis reactions. Ethylene polymers obtained by using these ␤-N–N Ni(II)/MAO catalyst systems are mainly methyl branched. Small amounts
of even number branches were also observed in the 13C NMR spectra of the obtained ethylene polymers, which are believed generating from the
incorporation of simultaneously produced ␣-olefins. Except methyl branch and the branches derived from the incorporation of ␣-olefin oligomers,
the formation of other branch types via the chain walking process is not favored in ␤-diketiminato Ni(II) systems.
© 2006 Elsevier B.V. All rights reserved.
Keywords: ␤-Diketiminato Ni(II) complexes; Ethylene; Polymerization; Oligomerization; Triphenylphosphine
1. Introduction
higher molecular weight polymers. This axial steric bulk con-
cept had been validated with and extended to many kinds of
fine contrived LTM catalyst systems, such as pyridine bis(imine)
(PBI) Fe and Co catalyst systems, which produce high molecular
weight liner ethylene polymers [8].
The early report of the late transitional metal (LTM) cat-
alyst can be traced back to the polymerization of butadiene
catalyzed with diethyl bis(2,2ꢀ-bipyridyl)iron complexes devel-
oped by Yamamoto et al. in 1965 [1]. Later, Keim’s researches
founded base of the shell higher olefin process (SHOP) [2]. Due
to the strong ␤-H elimination tendency of the LTM systems, rare
LTM catalysts were found to produce solid ethylene polymers
efficiently [3]. Until that, Brookhart and co-workers reported the
cationic bulky ␣-diimine catalysts in 1995 [4]. Then, in 1998,
Grubbs and co-workers reported the neutral salicylaldimine cat-
alysts, which were capable of co-polymerizing the polar olefin
monomers even in the polar solvent [5]. In the past decade, many
researches on these two kinds of catalyst systems were carried
out on topics including ligand structure [4,6], polymerization
and branching mechanism [7]. The axial bulky aryl groups of
the ␣-diimine ligands retard the chain transfer, and thus lead to
The researches on ␤-diketiminato metal complexes went
afresh hot in the middle of 1990s [9]. The chemistry of the unsat-
urated three coordinate ␤-diketiminato LTM complexes were
studied effectively by Holland and co-workers [10] and Warren
and co-workers [11]. The works on the olefin polymerization
with ␤-diketiminato transition metal catalysts, were recently
reviewed in details by Gibson [12]. However, the researches on
the ethylene polymerization by using ␤-diketiminato and anal-
ogous six-member-chelate-ring LTM catalyst systems are rare
in the past decade [11a,13–15] (Fig. 1). The activities of these
reported catalyst systems and corresponding characterization
were listed in Table 1. Complexes of type 2 showed no activity
either with methylaluminoxane (MAO) or with ethylaluminum
sesquichloride (EASC) as cocatalyst. The amidoiminomalonate
complex 4 shows high activities in the presence of MAO as
cocatalyst, yielding oligomers with prevailing dimers (88.7%).
Neither 3a/BPh3 nor 3a/PMMAO are productive in catalyzing
∗
Corresponding author. Tel.: +86 20 84113250; fax: +86 20 84112245.
E-mail address: ceswuq@zsu.edu.cn (Q. Wu).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.12.027

32
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
Fig. 1. Reported Ni(II) complexes of bidentate nitrogen ligand with six-membered chelate ring (1–8) and ␤-diketiminato Ni(II) bromide complexes (9 and 10)
discussed.
Table 1
Ethylene oligomerization and polymerization catalyzed by compounds 2–8 in literature [11a,13–15]
Cat.
Cat. mol (×10−6
)
Co-cat. (ratio/type)
PE (atm)
t (h)
TON
Mn
Refs.
1
2
4
3a
3a
5a
6a
8b
∼5.3
∼55/MAO
MAO or EASC
300/MAO
2/BPh3
0.375
29.6
23.7
6.9
0.3
19
1
–
–
18
12
3.5
0.5
10
1120
Inactive
20530
81
–
[15b]
[14]
[14]
[15a]
[15a]
[13a]
[13b]
[11a]
10
10
60
110
80
2000
–
(C4–C16)
–
–
–
PMAO
88
60/MAO
200/MAO
None
1800
0.06 (g)
–
1
6.9
–
(C8–C18)
ethylene polymerization to form solid polyethylene. Without
the present of MAO, only ethylene oligomers (C8–C14) were
obtained with complex 8a at 6.9 atm ethylene pressure.
Ni{(N(C6H3R2-2,6)C(Me))2CH}(Pph3)Br (R = isopropyl 10a,
methyl 10b) were readily obtained from reaction of lithium
␤-diketiminato salts with NiBr2(Pph3)2. Hexane used to pre-
cipitate the target complex 10a and 10b should be enough to
dissolve the Pph3 released. These complexes are noteworthy
thermal-stable, but sensitive to air and moisture.
Bulky␤-diketiminatocomplexesarecatalystprecursorschar-
acterized by their axial steric bulk as ␣-diimine catalysts [4]
and the neutral active center as salicylaldimine catalysts [5].
The ligand of the ␤-diketiminato complexes adopts a wedge
shape configuration as compared with the open ligand config-
uration of the ␣-diimine complexes [6d]. In previous report,
1-hexene isomerization and dimmerization were investigated by
using 9/MAO catalyst systems [16]. Result shows that the migra-
tion (chain-walking) of Ni(II) center on the alkyl chain were
restrained by the coordination wedge of the ␤-diketiminato lig-
ands. Here, ␤-diketiminato Ni(II) bromide complexes 9 and 10
were used as catalyst precursors in the presence of MAO for
ethylene polymerization. Neutral ligand ␤-diimine Ni(II) com-
plexes 5 were synthesized following the literature method [13a]
and also used as catalyst precursors for comparison.
1
The H chemical shifts of these ␤-diketiminato complexes
are paramagnetically induced by the unpaired spin of the
Ni atom [17]. 1H NMR spectra of 10a and 10b show that
the equilibrium [10b,16] which shifts between monomer and
dimer in the solution of 9a and 9b is interrupted by the
1
introduction of Pph3. 31P{ H}NMR spectra of 10a and 10b
show a single peak value around 135–136 ppm, and the sig-
2. Result and discussion
2.1. Synthesis of β-diketiminato nickel complexes (9 and
10)
The synthetic routes of ␤-diketiminato Ni(II) complexes
9 and 10 are shown in Scheme 1. Complex 9 was synthe-
sized according to the method in our previous report [16].
Scheme 1. Synthesis of ligands and ␤-diketiminato Ni(II) complexes.

J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
33
Table 2
Ethylene oligomersa obtained with 9a/MAO at 150 psig
Temperature (◦C)
C4 (%)
C6 (%)
C8 (%)
0
84.3
12.6
3.1
2.6
15.4
9.4
20
40
60
80
78.8
41.3
60.6
72.5
18.6
43.3
30.0
21.1
6.4
Cat. 7 × 10−6 mol, MAO 275 mg, in 20 ml toluene.
a
Measured by GC–MS.
steric bulk of the complexes. The results suggest that bigger
ortho substituents are more effective in stabilizing the Ni-alkyl
active species and retarding the ␤-H elimination.
Thedataoftheoligomerpartproductsobtainedwith 9a/MAO
catalyst was listed in Table 2. The generation of these short
chain olefins can be ascribed to the formation of highly elec-
trophilic cationic species [Ni(II)-R]+ after the abstraction of the
␤-diketiminato ligand by MAO. As reported by Dubois et al.,
Groux et al. and Bryliakov et al., LM(halide) complexes com-
bined with MAO give both cationic species as well as neutral
species [18]. At 0 ◦C, oligomers are mainly butylene. Below
40 ◦C, the increased temperature leads to an increase in the
proportions of longer oligomers (hexenes and octenes). Above
40 ◦C, the proportions of longer oligomers fall with the increased
temperature, and butylene proportions increase. This changing
trend of oligomer proportions suggests that the increased tem-
perature have a bigger impact on the ␤-H elimination than on the
chain propagation in the oligomerization process. In addition,
temperature also has impact on the distribution of Cn oligomer
isomers (n = 4, 6, or 8). The C8 oligomers are mainly 1-octene
at 0 ◦C and the proportions of other C8 isomers were observed
increasing with the increased reaction temperature (Fig. 4).
GC–MS confirmed that these C8 olefins include branched iso-
mers and position isomers. In following discussion, activity
refers merely to the polymer part, otherwise specified.
Fig. 2. Molecular structure of complex Ni{(N(C6H3Me2-2,6)C(Me))2CH}
◦
˚
(Pph3)Br (10b). Selected bond distances (A) and angles ( ): Ni(1)-N(1) 1.949
(2), Ni(1)-N(2) 1.939 (3), Ni(1)-Br(1) 2.3743 (8), Ni(1)-P(1) 2.3483 (11), N(2)-
Ni(1)-N(1) 93.54 (11), P(1)-Ni(1)-Br(1) 95.52 (3), C(21)-N(1)-Ni(1) 115.57
(18), C(31)-N(2)-Ni(1) 114.9 (2). Hydrogen atoms are omitted for clarity.
nal corresponding to free Pph3 at 6.0 ppm was not detected.
The crystal structure of 10b adopts a tetrahedral structure
(Fig. 2).
2.2. Ethylene oligomerization and polymerization
The ␤-diketiminato Ni(II) systems catalyzed ethylene poly-
merization are characterized by simultaneous oligomerization.
Considerable amounts of short-chain olefins as well as high
molecular weight ethylene polymers were obtained (Fig. 3),
indicating that there are at least two kinds of catalytically
active species respectively for ethylene oligomerization and
polymerization in the reaction systems. This character of the
␤-diketiminato Ni(II) systems differs from that of ␣-diimine cat-
alyst system, especially the ␣-diimine Pd(II) system in which
unreacted short-chain oligomer residuals were not observed
[7a]. Total activities for producing oligomer and polymer and
the ratios of polymer to oligomer increase with the increased
Table 3 lists polymerization data obtained by using different
catalyst precursors at 30 ◦C. Bulky catalyst systems (9a, 10a
Fig. 3. Observed activities and polymer/oligomer ratios. Cond.: cat.,
7 × 10−6 mol; MAO, ∼250 mg; PE, 150 psig, 30 ◦C, 1.5 h, in 30 ml toluene.
Fig. 4. C8 oligomers distribution obtained with 9a/MAO at 150 psig of ethylene.

34
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
Table 3
Ethylene polymerization data for experiments at 30 ◦C
Entry
Cat.
Cat. mol (×10−6
)
PE (psig)
PE (g)
Time (h)
TOF (/h)
Mn (×10−3
)
PDI
Branch
(/1000 C)c
Tm (◦C)
1a
2b
3a
9a
9a
9b
10a
10a
10b
5a
5a
5b
5b
7
10
7
7
10
7
7
10
7
150
20
150
150
20
150
150
20
150
20
1.732
0.656
0.237
2.175
0.415
0.188
0.574
0.651
0.303
0.274
1.5
8
1.5
1.5
8
1.5
1.5
8
1.5
8
5900
300
800
7400
190
640
1950
290
1030
120
275
223
133
225
179
153
690/7
87/4.8
517/1.9
165/0.9
2.24
2.78
5.69
3.02
2.54
6.67
2.43/1.49
3.90/1.51
2.79/2.59e
3.48/2.12
19
–
21
25
–
30
20
–
29
–
103.6
105
99.3d/110.7
81.6
4a
5b
6a
102
93.2d/111.5
121.6
114
7a
8b
9a
117.6
112
10b
10
a
MAO 250 mg, toluene 30 ml.
b
c
d
e
MAO ∼135 mg, toluene 20 ml.
[branch methyl carbon (total)/total carbon] × 1000.
Main peak.
Value evaluated on Gauss distribution modal.
and 5a/MAO) are observed more active for ethylene polymer-
ization than less bulky catalyst systems (9b, 10b and 5b/MAO).
The ethylene polymers obtained with big bulk catalysts (9a, 10a
and 5a/MAO) have higher molecular weight and narrower dis-
tribution than that obtained with small bulk catalysts (9b, 10b
and 5b/MAO). The branch rates of the polyethylenes obtained
with less bulky catalyst systems are slightly higher than that of
corresponding bulky ones (Table 3). The result suggests that the
branching process is determined more by the free space between
the two aromatic arms in ␤-diketiminatos Ni(II) systems, dif-
fering from the ethylene access factor by the size of the ortho
substituents in ␣-diimine system [4a,7a]. The ␤-diketiminatos
Ni(II)/MAO systems exhibit relatively lower activities than ␣-
diimine Ni(II)/MAO analogues [4]. This activity decline should
be ascribed to the weaker binding affinity of ethylene to the neu-
tral Ni(II) center and the bigger binding restriction of ethylene to
the metal center by the wedge shape ligand. Moreover, the activ-
ities of ␤-diketiminatos Ni(II)/MAO systems are sensitive to
polymerization temperature. The highest activities are observed
around 40 ◦C for 9a/MAO and around 20 ◦C for 10a/MAO
(Table 4).
2.3. Polyethylene microstructure and branching mechanism
The ethylene polymers obtained with ␣-diimine Ni(II) and
Pi(II) catalysts exhibit a branching rich structure, and multiple
branching types including short and long, even and odd were
observed [4,6,7]. In ␣-diimine systems, especially in the ␣-
diimine Pd(II) system, these branches are believed to be formed
via chain walking mechanism rather than by generating from the
incorporation of short-chain oligomers [7a,19]. In following dis-
cussion, the used nomenclatures refer to Usami and Takayama
for isolated branches [20a] and to Galland et al. [20b] for paired
branches prefixes.
The ethylene polymers obtained with 9a/MAO systems are
basicallymethyl-branched, evenifthepolymerizationswerecar-
ried out at higher temperature (Fig. 5). The branching structures
of the ethylene polymers obtained with 10a/MAO are similar to
that obtained with 9a/MAO (Fig. 6). The methyl-branched struc-
tureconsistswellwiththeideasthattheisomerizationfrom1-Ni-
alkyl to 2-Ni-alkyl is favored in ␤-diketiminato Ni(II)/MAO sys-
tem (Scheme 2) [16]. Aside from methyl branch, a small quan-
tity of ethyl related carbon signals (1B2 11.190, 2B2 26.702,
Table 4
Influence of temperature on ethylene polymerization
Entry Cat.
Temperature (◦C) PE (g) TOF (/h) Mn (×103)
Mw/Mn, PDI Me (/1000 C)a Et (/1000 C)a ≥C4 (/1000 C)a Tm (◦C)
11
12
13
14
15
16
17
18
19
20
9a
9a
9a
9a
9a
10a
10a
10a
10a
10a
0
20
40
60
80
0
20
40
60
80
0.598
1.867
2.883
0.332
0.030
0.442
2.249
0.974
0.836
0.102
2030
6320
9810
1130
100
1500
7650
3310
2840
350
813
781
224
107
115
278
552
237
89
2.41
2.55
2.64
13.04
7.93
4.30
2.89
2.89
2.88
7.73
7.6
13.7
20.2
33.7
–
11.2
18.7
27.4
45.1
–
–
–
–
–
–
–
–
–
2.2
6.1
–
110.4/118.7b
99.5b/113.5
78.6
1.5
1.7
2.2
–
94.4b/110.5
96.2b/116
–
108.2/116.5b
97.8b/112.3
105.5b/109.6
93.3/112.5b
93.7/113.4b
0.9
3.9
3.8
–
84
PE, 150 psig; Cat., 7 × 10−6 mol; MAO, 275 mg, 1.5 h, in 20 ml toluene.
a
Density of branches in chain = [methyl carbon of R/total chain carbon] × 1000.
b
Main peak.

J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
35
Fig. 5. 13C NMR spectrum of polyethylene obtained with 9a/MAO.
Fig. 6. 13C NMR spectrum of polyethylene obtained with 10a/MAO.
␣B2 34.059, brB2 39.669) and butyl signals (1B4 14.058, 2B4
22.857, 3B4 29.527, brB4 38.197, ␣B4 34.063) were also iden-
tified. Moreover, no specific resonance corresponding to odd
number branches (propyl, amyl) was detected.
Theopold and co-workers [21] and should be ascribed to the
wedge-shape configuration of the ␤-diketiminato ligand. More-
over, no minimal resonance corresponding to amyl and propyl
was observed. This result suggests that the reinsertion of the
coordinated inner olefin of the (olefin)Ni–H species, which
formed after the ␤-H elimination of the 5-Ni-alkyl intermediate,
is difficult in ␤-diketiminato Ni(II)/MAO catalyst systems. Here,
these small quantities of even number branches observed in the
ethylene homo-polymers can be safely ascribed to the incorpora-
tion of the simultaneously produced short chain ␣-olefins rather
than chain walking process.
To understand of the origin of these ethyl and butyl branches,
additional polymerization experiments were carried out with
extra 6 ml 1-hexene added. Enhanced butyl related carbon res-
onance were observed in the 13C NMR spectra of co-polymers
(1B4 14.058, 2B4 22.857, 3B4 29.527, brB4 38.197, ␣B4
34.063) (Fig. 7). As considering the highly initial 1-hexene
concentration (6 ml), the incorporation rates by using 9a and
10a/MAO are very low with butyl branch number 6–8 per 1000
carbon. The low incorporation rate of ␣-olefins co-monomers
were also reported in ␤-diketiminato Cr(III)/MAO system by
The incorporations of the ␣-olefin oligomers were also
observed when complexes 9b, 10b and 5 were used as cata-
lyst precursors (Fig. 8). Aside from methyl branch, polyethylene
Scheme 2. Presumed formation pathway of methyl-branched polyethylene.

36
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
Fig. 7. 13C NMR spectrum of only butyl branched poly(ethylene-co-1-hexene). Cond.: cat., 7 × 10−6 mol; 1-hexene 6 ml, PE, 150 psig, 35 ◦C, 2 h, in 14 ml toluene.
Fig. 8. 13C NMR spectrum of polyethylene obtained with different catalyst precursors.
obtained with 5a/MAO system obviously bears chain end struc-
ture (or longer branches) (1Bn 14.061, 2Bn 22.857, 3Bn 32.165,
4Bn 29.566, (n − 2)Bn 30.384, (n − 1)Bn 27.325). The chain
end structure of low molecular weight products should have
contribution to the bigger integral intensity of the signal at
14.06 ppm. DSC analysis demonstrates that ethylene polymers
obtained with 5/MAO systems are crystalline (see supporting
information).
The influence of Pph3 trapping in 10/MAO system can be
traced by detecting the methyl branch numbers of the obtained
polymers (Table 4). At same polymerization temperature, the
single and 1,6-paired methyl branch signals of the polyethylene
Scheme 3. Presumed mechanism of ethylene polymerization by using
obtained with 10a/MAO are slightly higher than that obtained
10a/MAO.

J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
37
Fig. 9. GPC chromatograms of polyethylene obtained with 9a/MAO and 10a/MAO at different temperature (entry).
with 9a/MAO (Figs. 6–8). This observation suggests that Pph3
4. Experimental
shares the vacant coordination site of Ni(II)-alkyl intermedi-
ates with ␣-olefins and ␤-H and is not involved in the chain-
propagation status (Scheme 3). ␤-H atom is the inter-molecular
partofPph3 coordinatedintermediates, andthismightofferextra
opportunity for the formation of methyl branches [16]. This pre-
sumed scheme consists well with the coordination competition
among 2,4-lutidine, ␤-H and ethylene as reported by Warren
and co-workers [11]. At low temperature (entry 16), Pph3 trap-
ping results in relatively higher PDI value and lower molecular
weight.
Bimodal GPC chromatograms corresponding to the high
polymerization temperature in entries 14, 15 and 20 were
observed (Fig. 9). This bimodal molecular distribution might
be arisen from the possible coupling of the alkyl(polymeric)
radicals due to the decomposition of Ni(II)-alkyl species [10a].
The portion of the higher molecular part obtained with 9a/MAO
is bigger than that obtained with 10a/MAO. Moreover, this
bimodal distribution result occurred at higher polymerization
temperature in 10a/MAO system, indicating that the active
species might be stabilized in some degree by the introduction
of Pph3.
4.1. General remarks
All necessary manipulations were performed by stan-
dard Schlenk techniques under nitrogen atmosphere or in
a M. Braun glovebox. All the containers were thoroughly
dried by heating under vacuum. Hexane and toluene were
thoroughly dried over phosphorus pentoxide and sodium,
respectively. MAO solid was prepared by the controlled
hydrolyzation of the trimethylaluminum (TMA) at 0–60 ◦C
by using Al2(SO4)3·18H2O (H2O/Al = 1.3) that was dis-
persed in toluene. 2,6-diisopropylaniline, 2,6-dimethylaniline,
n-butyllithium were purchased from Aldrich. Other commer-
cially available reagents were purchased and used as received.
(DME)NiBr2 was prepared according to the reported litera-
ture [22]. Ligands {N(C6H3R2-2,6)C(Me)}2CH2 {R = iPr (a),
Me (b)} and complexes 9 (Ni{(N(C6H3R2-2,6)C(Me))2CH}Br)
were synthesized as the method in our previous report [16]. ␤-
diimine Ni(II)Br2 complexes 5 were synthesized as the method
in literature [13a].
NiBr2(Pph3)2 was prepared by refluxing NiBr2 (0.1 mol) and
Pph3 (0.22 mol) in anhydrous ethanol, then the dark green solu-
tion was hot filtrated, condensed and cooled to crystallization.
The NiBr2(Pph3)2 was collected by filtration and dried in vac-
uum, obtained as bright green solid (>90%). Anal. Calcd. for
C32H30Br2NiP2: C 58.19, H 4.07. Found: C 58.16, H 4.08.
3. Conclusion
In the presence of MAO, bulky ␤-diketiminato Ni(II) com-
plexes and ␤-diimine Ni(II) complexes were used as catalyst
precursors for ethylene polymerization. Ethylene polymeriza-
tion carried out by using these six-membered chelate ring sys-
tems are characterized by simultaneous oligomerization. Due
to the wedge-shaped ligand configuration of these catalyst pre-
cursors, the obtained ethylene polymers are basically methyl-
branded. These small quantities of even number branches,
observed in the ethylene homo-polymers, were ascribed to
the incorporation of the simultaneously produced short chain
␣-olefins rather than chain walking process. The fourth lig-
and Pph3 can stablize the active species in some degree and
result in ethylene polymers with slightly higher methyl-branch
rate.
4.2. Ni{(N(C6H3iPr2-2,6)C(Me))2 CH}(Pph3)Br (10a)
La 1.20 g (0.003 mol) was dissolved in 50 ml toluene and
the solution was cooled to −78 ◦C and stirred. n-butyllithium
1.1 ml (hexane solution 2.6 M) was injected and the solution
was allowed warm up to room temperature overnight. After that,
2.38 g NiBr2(Pph3)2 (0.0032 mol) were added, and the slurry
were stirred for 12 h at 80 ◦C. After hot filtration, the filtrate
was condensed by vacuum to 10–15 ml, and the complexes
were precipitated, washed twice by hexane and dried under
1
vacuum. Green solid were obtained, 1.47 g (62.8%). H NMR

38
J. Zhang et al. / Journal of Molecular Catalysis A: Chemical 249 (2006) 31–39
(CD2Cl2) (ppm): 56.55 (4H, m-Ar), 36.85 (4H, CH(CH3)2),
4.6. Measurement
9.58 (24H, CH(CH3)2), −27.08 (2H, p-Ar), −87.87 (6H, CH3).
31P{ H}NMR(C6D6): 136.08 ppm (s).
1H NMR spectra were recorded on a Varian INOVA 500NB
NMR spectrometer. Elemental analysis were determined with
a Vario EL Series Elemental Analyzer from Elementar. The
GC–MS data were recorded with a Finnigan Voyager GC-8000
Top Series GC–MS System with DB-5MS GC column. The GC
spectrums were recorded with Varian CP3800 Series GC System
with a HP-5MS GC column.
1
4.3. Ni{(N(C6H3Me2-2,6)C(Me))2 CH}(Pph3)Br (10b)
The synthesis of complex 10b was preformed as in the
case of complex 10a, by using Lb 0.92 g (0.003 mol), n-
butyllithium 1.1 ml (hexane solution 2.6 M) and 50 ml toluene,
NiBr2(Pph3)2 2.38 g (0.0032 mol) and a stirring time of 12 h
at 80 ◦C. Product was obtained as black green solid, 1.54 g
(76.4%). 1H NMR (CDCl3) (ppm): 42.58 (6H, CH3-Ar), 40.68
(6H, CH3-Ar), 39.41 (2H, m-Ar), 33.85 (2H, m-Ar), −33.34
(2H, p-Ar), −66.31 (6H, CH3), −177.60 (1H, H-backbone).
Acknowledgments
The supports by NSFC and SINOPEC (joint-project
20334030), the Science Foundation of Guangdong Province
(projects 039184) and the Ministry of Education of China (the
Foundation for PHD Training) are gratefully acknowledged.
31P{ H}NMR(C6D6): 135.39 ppm (s).
1
4.4. Ethylene polymerization
Appendix A. Supplementary data
A mechanically stirred 100 ml Parr reactor under an ethy-
lene atmosphere was charged with MAO toluene solution 13 ml
(MAO 275 mg). The system temperature was allowed to equi-
librate at reaction temperature under a positive ethylene pres-
sure (∼200 mmHg). Catalyst precursor (7 × 10−6 mol) in 7 ml
toluene was transferred into the reactor via cannula, and then
the autoclave was immediately pressurized to desired pressure
with ethylene and the polymerization begun. Usually, exother-
mic phenomena were observed at start. The system temperature
might rush 5–10 ◦C higher than the desired polymerization tem-
perature, and then was taken back by an inner cooler within
20 min. After 1.5 h polymerization, the autoclave was sat into a
cooling bath and carefully vented at −10 ◦C (to reduce oligomer
loss). Then the polymerization solution (∼5 ml) was quickly col-
lected and mixed with 15 ml cool ethanol in a 50 ml glass tube
at −20 ◦C. Additional water was added into the tube to separate
toluene phase from ethanol–water phase. The tube was sealed
after the CH4 bubble over and stored at −20 ◦C in refrigerator.
The upper colorless toluene solution was used for GC or GC–MS
analysis. The swelled polymer was excavated out and refluxed
with HCl acidified ethanol in a flask for 8 h. The polymer was
isolated by removal of the solvents, washed with ethanol and
dried overnight in the vacuum oven.
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.molcata.2005.12.027.
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