An α-Diimine-Nickel(II) catalyst bearing an electron-withdrawing substituent for olefin polymerization

Article abstract of DOI:10.1007/s11243-011-9487-x

A Br-substituted α-diimine ligand, bis[N,N′-(4-bromo-2,6- dimethylphenyl)imino]-2,3-butadiene L1, and its corresponding Ni(II) complex, {bis[N,N′-(4-bromo-2,6-dimethylphenyl)imino]-2,3-butadiene} dibromonicke [NiBr ₂ (L1)], have been synthesize

Full text of DOI:10.1007/s11243-011-9487-x

Transition Met Chem (2011) 36:433–439
DOI 10.1007/s11243-011-9487-x
An a-Diimine-Nickel(II) catalyst bearing an electron-withdrawing
substituent for olefin polymerization
•
Jianchao Yuan Xuehu Wang Tongjian Mei
•
•
•
Yufeng Liu Chengping Miao Xiaoli Xie
•
Received: 24 January 2011 / Accepted: 21 March 2011 / Published online: 6 April 2011
ꢀ Springer Science+Business Media B.V. 2011
Abstract A Br-substituted a-diimine ligand, bis[N,N0-
(4-bromo-2,6-dimethylphenyl)imino]-2,3-butadiene L1, and
its corresponding Ni(II) complex, {bis[N,N0-(4-bromo-2,6-
potential to yield polymers with different microstructures
and also the need for catalysts that are more tolerant of
functionalized monomers [2–12].
dimethylphenyl)imino]-2,3-butadiene}
dibromonicke
The catalyst activity and properties of the resulting
polymers are greatly dependent on the reaction conditions
[13–15] and ligand structure [16–18]. For a given catalyst,
increasing the polymerization temperature increases the
branching content of the polymer [19–23]. This effect is
usually related to the competition between the chain
walking and chain propagation processes. To gain more
insight into the factors controlling the activity of such
catalysts and the effect of catalyst structure on polymer
structure and properties, the steric effects of the ortho
substituent of the 2,6-dialkylaryl imine group on the cata-
lytic activity of a-diimine-Ni(II) complexes has been
studied in detail by Brookhart [6]. However, the effects of
para substituents of the 2,6-dialkylaryl imine group [24,
25], especially of electron-withdrawing substituents, on the
catalytic behavior of a-diimine-Ni(II) complexes and the
properties of the resulting polymers has only been reported
in limited cases [3].
[NiBr2(L1)], have been synthesized and characterized. The
crystal structure of the free ligand (L1) was determined
by X-ray crystallography. Two a-diimine-Ni(II) catalysts,
{bis[N,N0-(2,4,6-dimethylphenyl)imino]-2,3-butanedione}
dibromonickel [NiBr2(L2)] and {bis[N,N0-(2,6-dimethyl-
phenyl)imino]-2,3-butanedione} dibromonickel [NiBr2(L3)],
were also synthesized and characterized for comparison.
The complex [NiBr2(L1)], when activated by diethylalu-
minum chloride, produces the most active catalytic system
for the polymerization of ethylene among the three com-
plexes. NMR analysis shows that the degree of branching
of polyethylene increases in the presence of electron-
withdrawing groups under the same reaction conditions.
Introduction
A major focus of our work has involved the development
of olefin polymerization catalysts based upon a-diimine
complexes of Ni(II) or Pd(II), the first report of which
appeared in 1995 by Brookhart and coworkers [1]. The
development of late-transition-metal catalysts for the
polymerization of olefins has been spurred by their
In this work, we carried out an investigation into the
catalytic behavior of three Ni(II) a-diimine catalysts acti-
vated by diethylaluminum chloride (DEAC). In this paper,
we describe the effects of different substituents at the para-
position of the 2,6-dialkylaryl imine group on the catalyst
activity and polymer properties.
Experimental
J. Yuan (&) ꢀ X. Wang ꢀ T. Mei ꢀ Y. Liu ꢀ C. Miao ꢀ X. Xie
Key Laboratory of Eco-Environment-Related Polymer Materials,
Ministry of Education, Key Laboratory of Polymer Materials
of Gansu Province, College of Chemistry and Chemical
Engineering, Northwest Normal University,
All manipulations involving air- and/or water-sensitive
compounds were carried out with standard Schlenk tech-
niques using magnetic stirring under argon or nitrogen.
Toluene and methylene chloride were distilled from
Lanzhou 730070, China
e-mail: jianchaoyuan@nwnu.edu.cn
123

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Transition Met Chem (2011) 36:433–439
1
under vacuum. Yield: 1.22 g (82%). H NMR (400 MHz,
sodium/benzophenone under dry nitrogen. O-dichloroben-
zene was dried over phosphorus pentoxide for 24 h, and
then distilled under nitrogen. DEAC (0.9 mol/L solution in
toluene) was purchased from Acros Organics. DME (1,2-
dimethoxyethane) was distilled under N2 from sodium.
Anhydrous NiBr2 (99%) was obtained from Aldrich. 2,3-
Butanedione, 2,6-dimethylaniline (98%), 2,4,6-trimethyl-
aniline (98%), and 4-bromo-2,6-dimethylaniline (98%)
were purchased from Alfa Aesar and used without further
purification. [NiBr2(DME)] was synthesized according to
the literature [26].
CDCl3): d 2.02 (s, 12H, Ar–CH3), 2.35 (s, 6H, CCH3), 7.26
(s, 4H, Ar–H). 13C NMR (100 MHz, CDCl3): d 15.91 (C–
CH3), 17.61 (Ar–CH3), 115.85 (Ar–C–Br), 126.88 (Ar–C),
130.56 (Ar–C–CH3), 147.22 (Ar–C–N), and 168.50 (C=N).
FTIR (KBr) 1,642 cm-1 (C=N). Anal. Calc. for C20H22N2Br2:
C, 53.36; H, 4.93; N, 6.22. Found: C, 53.38; H, 4.92;
N, 6.21.
Synthesis of bis [N, N0-(2, 4, 6-trimethylphenyl) imino]-
2,3-butadiene (L2)
NMR spectra were recorded at 400 (13C) and 100 (1H)
MHz, respectively, on a Varian Mercury plus-400 instru-
ment, using TMS as internal standard. FTIR spectra were
recorded on a Digilab Merlin FTS 3000 FTIR spectro-
photometer on KBr pellets. X-ray photoelectron spectra
(XPS) were obtained from a PHI-5702 multifunction
instrument. The molecular weights and polydispersities
(PDI) of the polymers were determined via an Alliance
GPCV2000 (Waters) gel permeation chromatograph (GPC)
using 1,2,4-trichlorobenzene as the eluent at a flow rate of
1.0 mL/min and operated at 140 ꢁC.
Formic acid (0.5 ml) was added to a stirred solution of 2,3-
butanedione (0.260 g, 3 mmol) and 2,4,6-trimethylaniline
(0.812 g, 6 mmol) in ethanol (20 mL). The mixture was
stirred at 45 ꢁC for 24 h, then cooled, and the precipitate
was filtered off. The solid was recrystallized from EtOH/
CH2Cl2 (v/v = 10:1), washed with cold ethanol, and dried
1
under vacuum. Yield: 0.91 g (85%). H NMR (400 MHz,
CDCl3): d 2.00 (s, 12H, Ar–o–CH3), 2.03 (s, 6H, Ar–p–
CH3), 2.29 (s, 6H, CCH3), 6.89 (s, 4H, Ar–H). 13C NMR
(100 MHz, CDCl3): d 15.77 (C–CH3), 17.70 (Ar–o–CH3),
20.70 (Ar–p–CH3), 124.52 (Ar–C), 128.55 (Ar–C–o–CH3),
132.37 (Ar–C–p–CH3), 145.86 (Ar–C–N), 168.32 (C=N).
FT-IR (KBr) 1,637 cm-1 (C=N). Anal. Calc. for C22H28N2:
C, 82.45; H, 8.81; N, 8.74. Found: C, 82.50; H, 8.85;
N, 8.64.
Polymerization procedure
The polymerization was carried out in a 250-mL Schlenk
flask equipped with a magnetic stirrer. The flask was
repeatedly evacuated and refilled with nitrogen and finally
filled with ethylene gas (ambient pressure). DEAC and
toluene (50 mL) were added into the reactor. Ethylene was
introduced under 0.02 Mpa until the solution was saturated
and the pressure was maintained throughout each run. The
reactor was evacuated, pressurized with ethylene, and then
placed in an oil bath set at the operating temperature, and
allowed to equilibrate for 10–15 min. The catalyst was
dissolved in o-dichlorobenzene under dry nitrogen, and the
solution was transferred into the Schlenk flask to initiate
the polymerization. After 30 min at the desired tempera-
ture, the polymerization was stopped by turning the eth-
ylene off and relieving the pressure. The reaction mixture
was poured into a 2% HCl–MeOH solution to precipitate
the polymer. The polymer was isolated by filtration,
washed with methanol, and dried under vacuum.
Synthesis of bis [N, N0-(2, 6-dimethylphenyl) imino]-2,
3-butadiene (L3)
Formic acid (0.5 ml) was added to a stirred solution of 2,3-
butanedione (0.346 g, 4 mmol) and 2,6-dimethylaniline
(0.970 g, 8 mmol) in ethanol (30 ml). The mixture was
stirred at 45 ꢁC for 24 h, then cooled, and the precipitate
was filtered off. The solid was recrystallized from EtOH/
CH2Cl2 (v/v = 10:1), washed with cold ethanol, and dried
1
under vacuum. Yield: 1.16 g (88%). H NMR (400 MHz,
CDCl3): d 2.00 (s, 12H, Ar–CH3), 2.35 (s, 6H, CCH3), 6.96
(s, 4H, Ar–CH3–p–H), 7.08 (s, 2H, Ar–CH3–m–H). 13C
NMR (100 MHz, CDCl3): d 15.80 (C–CH3), 17.78 (Ar–
CH3), 123.22 (Ar–C–m–CH3), 124.63 (Ar–C–o–CH3),
127.90 (Ar–C–CH3), 148.30 (Ar–C–N), 168.03 (C=N). FT-
IR (KBr) 1,637 cm-1 (C=N). Anal. Calc. for C20H24N2: C,
82.15; H, 8.27; N, 9.58. Found: C, 82.20; H, 8.23; N, 8.63.
Synthesis of bis [N, N0-(4-bromo-2,6-
dimethylphenyl) imino]-2,3-butadiene (L1)
Synthesis of [NiBr2(L1)]
Formic acid (0.5 ml) was added to a stirred solution of 2,3-
butanedione (0.260 g, 3 mmol) and 4-bromo-2,6-dimeth-
ylaniline (1.224 g, 6 mmol) in ethanol (20 ml). The mix-
ture was refluxed for 24 h, then cooled, and the precipitate
was filtered off. The solid was recrystallized from EtOH/
CH2Cl2 (v/v = 8:1), washed with cold ethanol, and dried
[NiBr2(DME)] (0.615 g, 2 mmol) was added to a stirred
solution of L1 (0.894 g, 2 mmol) in dichloromethane
(30 ml). The mixture was stirred at room temperature for
18 h. The resulting suspension was filtered. The solvent
was removed under vacuum and the residue was washed
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Transition Met Chem (2011) 36:433–439
435
with diethyl ether (3ꢀ16 ml) and dried under vacuum. Yield
1.21 g (80%). Anal. Calc. (%) for C20H22Br4NiN2: C,
35.92; H, 3.32; N, 4.19. Found: C, 38.05; H, 3.84; N, 3.89.
FT-IR (KBr) 1,639 cm-1 (C=N).
Table 1 Crystal data and structure refinement for a-diimine ligand
L1
Empirical formula
Formula weight
Crystal system
Space group
C20 H22 Br2 N2
450.22
Monoclinic
I2/a
Synthesis of [NiBr2(L2)]
Temperature
296(2) K
16.335(16)
7.376(7)
˚
a(A)
[NiBr2(DME)] (0.615 g, 2 mmol) was added to a stirred
solution of L2 (0.643 g, 2 mmol) in dichloromethane
(30 ml). The mixed was stirred for 18 h under nitrogen.
The solution was filtered, and the filtrate was evaporated
to dryness. The residue was washed with diethyl ether
(3ꢀ16 mL) and dried under vacuum. Yield 1.07 g (85%).
Anal. Calc. (%) for C22H28Br2NiN2: C, 49.03; H, 5.24; N,
5.20. Found: C, 49.11; H, 5.40; N, 5.25. FT-IR (KBr)
1,628 cm-1 (C=N).
˚
b(A)
˚
c(A)
a(o)
b(o)
c(o)
16.89(3)
90 deg
90.977(7) deg
90 deg
3
˚
2,035(4) A
Volume
Z
4
Calculated density
Crystal size (mm)
F(000)
1.470 mg/m3
0.30 9 0.28 9 0.21
904
Synthesis of [NiBr2(L3)]
Theta range for data
Reflections collected
Unique
2.49–25.50 deg.
4,582
[NiBr2(DME)] (0.615 g, 2 mmol) was added to a stirred
solution of L3 (0.584 g, 2 mmol) in dichloromethane
(30 ml). The mixer was stirred for 18 h under nitrogen.
The solution was filtered, and the solvent was evaporated
to dryness. The residue was washed with diethyl ether
(3ꢀ16 mL) and dried under vacuum. Yield 0.99 g (83%).
Anal. Calc. (%) for C20H24Br2NiN2: C, 47.02; H, 4.73; N,
5.48. Found: C, 49.20; H, 4.80; N, 5.50. FT-IR (KBr)
1,628 cm-1 (C=N).
1,829 [R(int) = 0.0428]
0.4881 and 0.3809
Max and min transmission
Limiting indices
-19 \= h \= 19, -8 \= k \= 6,
-19 \= l \= 20
1,829/0/112
Data/restraints/parameters
Refinement method
Goodness-of-fit on F2
Full-matrix least-squares on F2
1.094
Final R indices [I [ 2sigma(I)] R1 = 0.0531, wR2 = 0.1503
R indices (all data)
R1 = 0.1163, wR2 = 0.1766
0.311 and -0.517 e.A-3
Largest diff. peak and hole
X-ray crystallography
Crystallographic date for the structure has been deposited at the
Cambridge Crystallographic Data Centre, CCDC 800153, deposition
@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk/deposit, Telephone:
(44) 01223 762910, Facsimile: (44) 01223 336033, Postal Address:
CCDC,12 Union Road, CAMBRIDGE CB2 1EZ, UK
Crystals of L1 suitable for X-ray analysis were obtained at
room temperature by dissolving the ligand in CH2Cl2,
following by slow layering of the solution with pentane.
X-ray diffraction data were collected on a Bruker APEX
diffractometer equipped with a normal focus, 2.4 kW sealed
ligands in dichloromethane at room temperature. All three
complexes are red–brown powders.
˚
tube X-ray source (MoKa radiation, k = 0.71073 A) oper-
ating at50 kV and 30 mA. The structure was solved by direct
methods using SHELXS-97 and refined by full-matrix least-
squares techniques with SHELXL-97. Crystal data, data
collection, and refinement parameters are listed in Table 1.
In order to define the coordination mode of these a-
diimine ligands, small area X-ray photoelectron spectros-
copy of NiBr2, free ligand [(4-Br-2,6-Me)2DABMe2], and
its corresponding complex have been studied (Fig. 1). The
binding energy of Ni2p3/2 of the complex is 0.01 eV lower
than with that of NiBr2. This clearly shows an increase in
the election density at Ni and therefore coordination of Ni
with N in [NiBr2(L1)]. Compared with L1, the increase
(1.06 eV) in binding energy of N1s1/2 in the complex pro-
vides further evidence for the coordination of Ni to N.
Suitable crystals of L1 for X-ray diffraction were
obtained by layering a solution of the ligand in CH2Cl2
with pentane. The molecular structure of L1 was deter-
mined and the corresponding diagram is shown in Fig. 2,
while selected bond distances and angles are summarized
Results and discussion
The general synthetic route to these nickel(II) complexes is
shown in Scheme 1. The free ligands L1–L3 were prepared
by the condensation of two equivalents of the appropriate
aniline with one equivalent of 2,3-butanedione, usually in
the presence of an acid catalyst. The corresponding a-dii-
mine Ni(II) complexes were synthesized in good yields by
treating [NiBr2(DME)] with the corresponding Schiff-base
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Transition Met Chem (2011) 36:433–439
C2H5OH
HCOOH
NH2
R
R
N
N
R
O
O
NiBr2
N
R
N
R
Ni
Br Br
Fig. 2 ORTEP drawing of a-diimine ligand L1 with 30% probability
displacement ellipsoid (the hydrogen atoms are omitted for clarity)
Scheme 1 Synthetic routes of the a-diimine ligands (L1: R=Br, L2:
R=CH3; L3: R=H) and their nickel (II) complexes ([L1NiBr2],
[L2NiBr2], [L3NiBr2])
the N=C–C angles, which are very close to 120o. The
˚
shortest C–H…p contact has a H …p distance of 0.9300 A
and C–H …p angle of 120ꢁ.
in Table 2. The X-ray structure of L1 exhibits trans-con-
formation about the central C–C bond of the ligand back-
bone. In the solid state, the most interesting feature of this
asymmetric ligand is the conformation of the substituents
attached to N(1) and N(1A). These groups are rotated about
180ꢁ from the position they must occupy to chelate the
metal. Bond lengths and angles are within the expected
range for a-diimines; for example, the bond distances for
the C(8)=N(1) double bond and the central C(8)–C(8A)
Effect of ligand structure on catalyst activity
and polymer properties
On treatment with DEAC, all the nickel complexes are
highly active toward ethylene polymerization. The results
of the polymerization experiments are shown in Table 3. In
order to investigate the effect of ligand structure on the
performance of the nickel catalysts, three catalysts (para-
Br [NiBr2(L1)], para-methyl [NiBr2(L2)], and para-H
[NiBr2(L3)]) were screened. Table 3 shows that the replace-
ment of the para-aryl proton in L3 by an electron-withdrawing
˚
˚
single bond are 1.282(7) A and 1.517(11) A, which are
very close to the values for other structurally characterized
free a-diimines [27]. Both C(4) and C(8) possess essen-
tially rather planar geometry (sp2 character), as shown by
2350
2400
398.6655
N1s1/2 L1
399.7485
N1s1/2 L1NiBr
2
2300
2250
2200
2150
2100
2050
2000
2300
2200
2100
2000
1900
1800
1700
410
405
400
395
390
410
405
400
395
390
binding energy(eV)
bind energy(eV)
3200
3150
3100
3050
3000
2950
2900
2850
855.7795
Ni2p2/3 of NiBr
6000
5500
5000
4500
4000
Ni2p2/3 L1NiBr
2
2
855.7485
890
880
870
860
850
840
890
880
870
860
850
840
bind energy(eV)
binding energy(eV)
Fig. 1 XPS spectra of NiBr2, L1 and [L1NiBr2]
123

Transition Met Chem (2011) 36:433–439
437
˚
donating groups could decrease the favorability of C–H
activation by perturbing the ligand geometry to limit the
close approach of the metal to the ligand axial alkyl
moieties.
Table 2 Selected bond lengths [A] and angles [deg] for a-diimine
ligand L1
Bond (lengths)
Bond (angles)
(o)
˚
(A)
Br(1)–C(1)
C(1)–C(2)
C(4)–N(1)
C(8)–N(1)
C(1)–C(6)
C(2)–C(3)
C(5)–C(10)
C(8)–C(8A)
1.897(6)
1.384(9)
1.428(7)
1.282(7)
1.368(9)
1.389(8)
1.519(10)
1.517(11)
C(6)–C(1)–C(2)
C(2)–C(1)–Br(1)
C(6)–C(1)–Br(1)
C(5)–C(4)–N(1)
N(1)–C(8)–C(8A)
N(1)–C(8)–C(9)
C(3)–C(4)–N(1)
C(8)–N(1)–C(4)
120.9(6)
119.6(5)
119.5(5)
117.3(6)
115.2(7)
126.3(5)
121.0(5)
120.4(5)
Influence of polymerization temperature
Polymerization temperature has a great influence on the
catalyst activity, as well as on polymer branching and
molecular weights. Table 3 shows that the catalytic activity
and molecular weight of the polyethylene produced by all
three catalysts decreased as the temperature was increased.
This reveals that the chain transfer increases at high tem-
perature, although catalyst decomposition may also affect
catalytic activity and molecular weight. The distribution
of molecular weights was also observed to increase as
the reaction temperature was increased. In addition, the
branching numbers increased dramatically with increasing
temperature (Table 3), indicating that the chain walking
increases dramatically at high reaction temperature. The
type and amount of branches formed during ethylene
polymerization depended mainly on the reaction tempera-
ture but also varied with the ethylene pressure and catalyst
structure. Ni(II)-a-diimine catalysts are known to produce
more branches at high temperature and low ethylene
pressure. As show in Table 3, the [NiBr2(L1)]/DEAC
catalytic system containing an electron-withdrawing Br
group generated the highest degree of branching of poly-
ethylene at 40 ꢁC. The branching degree of polyethylene
prepared with catalyst was 125 branches per 1,000, while
the values for [NiBr2(L1)], [NiBr2(L2)], and [NiBr2(L3)]
were 107.3 and 113.2 branches per 1,000 ꢁC, respectively.
Thus, the catalyst containing an electron-withdrawing group
favors chain walking.
Br in L1 results in an increase in both catalyst activity and
the average molecular weight (Mw) of the polymers
obtained (Entries 1 and 2 in Table 3). However, the activity
of the catalyst with an electron-donating methyl group on
the para-position (L2) is decreased (enteries 1 and 3). We
conclude that the electron-withdrawing group strengthens
the electrophilic character of the nickel, which increases
the rate of chain propagation relative to chain transfer,
leading to the increase in the molecular weight of the
polyethylenes obtained. From these observations [3], the
polymer Mw is the result of complex interplay between
monomer insertion, chain transfer, and potential catalyst
decomposition. Interestingly, the variation in the para-
substituents of these a-diimine ligands has a pronounced
effect not only on catalyst activity and the molecular
weight but also on the degree of branching of the resulting
polymer. Thus, when an electron-withdrawing substituent
is introduced into the para-position of the 2,6-dialkylaryl
imine group, the branching degree of the polyethylenes
increases. It is possible that the substitution of electron-
Table 3 The results of polymerization with the nickel complexes
Entry
Precatalyst
Yield (g)
Temp (ꢁC)
Activityb
Mwc (kg/mol)
Mw/Mcn
Branchesd
(per 1,000 ꢁC)
1
2
3
4
5
6
7
8
9
a
[L1NiBr2]
[L2NiBr2]
[L3NiBr2]
[L1NiBr2]
[L2NiBr2]
[L3NiBr2]
[L1NiBr2]
[L2NiBr2]
[L3NiBr2]
1.4936
1.2826
1.3551
1.0653
0.7652
0.8080
0.6544
0.4823
0.5568
0
0
7.47
6.41
6.78
5.33
3.83
4.04
3.27
2.41
2.78
220.9
207.5
218.0
187.2
136.0
155.4
83.9
1.79
1.69
1.61
1.84
1.82
1.65
2.06
1.93
1.69
58.3
39.2
47.6
89.2
70.7
76.9
125
0
20
20
20
40
40
40
50.5
107.3
113.2
66.5
Polymerization condition: 2 lmol precatalyst, Al/Ni = 600 (molar ratio), polymerization reaction under 0.02 Mpa for 30 min
{9106 g PE/(molNi.h.Bar)}
b
c
d
Weight-average molecular weight and polydispersity index determined by GPC
1
Degree of branching determined by H NMR
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Transition Met Chem (2011) 36:433–439
Figure 3 summarizes the 13C NMR spectra of the poly-
methyl branch. However, the small shoulder resonances at
0.98 ppm suggest the presence of other longer branches.
This is consistent with 13C NMR analysis.
mers prepared with the three catalysts at 40 ꢁC. The 13C
NMR spectra (Fig. 3) of the polyethylenes prepared at
40 ꢁC exhibited resonances at 19.60, 27.41, and 33.32 ppm
due to the methyl branch carbon. Ethyl branch was identi-
fied by the presence of resonances at 11.20 and 37.10 ppm,
while the resonance at 22.71 ppm corresponds to the butyl
branch carbon [28]. From the resonance intensities of
Fig. 3, the main branch is methyl.
Conclusion
A new a-diimine ligand L1 bearing an electron-with-
drawing Br group and its Ni(II) complex have been pre-
pared and characterized. L1 was also modified in an
attempt to change the electronic density of the metal center
and so to improve the activity of the catalyst, the molecular
weight, and the branching degree of polyethylene pro-
duced. In comparison with the non-brominated a-diimine
nickel complexes, the complex [NiBr2(L1)], activated by
DEAC, showed high activity and produced high molecular
weight and branching of the polymer. Investigations into
the copolymerization catalysis of the a-diimine nickel
complexes are now in progress.
1
Figure 4 shows the H NMR spectra of the polymers
prepared with the three catalysts at 40 ꢁC. The doublets
at 0.86 ppm were assigned to the methyl group, those at
1.15 ppm to the methylene group, and the broad reso-
nances at 1.28 ppm to the methine group. The resonances
of the methyl group are split into doublets due to the
Acknowledgments The research was financially supported by NSFC
(20964003). We also thank Key Laboratory of Eco-Environment-
Related Polymer Materials (Northwest Normal University), Ministry of
Education, for financial support.
L1NiBr2
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L3NiBr2
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