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Nickel(II) complexes bearing pyrazolylimine ligand synthesis structure and catalytic properties for vinyl-type polymerization of norbornene.

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Full Paper
Received: 19 September 2009
Revised: 5 November 2009
Accepted: 10 November 2009
Published online in Wiley Interscience: 29 December 2009
(www.interscience.com) DOI 10.1002/aoc.1603
Nickel(II) complexes bearing pyrazolylimine
ligand: synthesis, structure, and catalytic
properties for vinyl-type polymerization
of norbornene
Yuan-yuan Wang∗ , Ben-xia Li and Yu-zhang Zhu
Two nickel(II) complexes of {2-[C3 HN2 (R1 )2 -3,5]}[C(R2 ) N(C6 H3 i Pr2 -2,6)]NiBr2 (complex 1: R1 = CH3 , R2 = 2,4,6trimethylphenyl; complex 2: R1 = R2 = Ph) were synthesized and characterized. The solid-state structure of complex
1 has been confirmed by X-ray single-crystal analysis. Activated by methylaluminoxane (MAO), complexes 1 and 2 are
capable of catalyzing the polymerization of norbornene with moderate activities [up to 10.56 × 105 gPNBE (mol Ni h)−1 ] with
high molecular weights (Mw ≤ 13.56 × 105 g mol−1 ) and molecular weight distributions were around 2. The influences of
polymerization parameters such as reaction temperature and Al–Ni molar ratio on catalytic activity and molecular weight
of the polynorbornene were investigated in detail. The obtained polynorbornenes were characterized by means of 1 H-NMR
and FTIR techniques. The analytical results of polymer structures indicated that the norbornene polymerization is vinyl-type
c 2009 John Wiley & Sons, Ltd.
polymerization rather than ROMP. Copyright Supporting information may be found in the online version of this article.
Keywords: nickel(II) complexes; X-ray single-crystal analysis; norbornene; vinyl-type polymerization
Introduction
308
In recent years, there has been considerable interest in the
development of the polymerization of olefins based on the late
transition metal catalysts.[1 – 3] Since Brookhart and co-workers
reported that nickel complexes bearing diimine ligands were
effective for olefin oligomerization or polymerization, the interest
in designing new catalysts of late transition metal complexes with
various nitrogen ligands has increased.[4] These ligands include
diimine,[5 – 9] bipyridine,[10] unsymmetric pyridinylimine,[11 – 13] and
alkylnitrile.[14] There were also a few reports about late metal
complexes bearing pyrazole and pyrazolyl ligands for olefin
polymerization.[15 – 18]
It is well known that norbornene (bicycle[2,2,1] hept-2-ene) can be
polymerized via three different modes: ring-opening metathesis
polymerization (ROMP),[19] cationic or radical polymerization[20]
and vinyl (or addition) polymerization.[21] Each polymerization
mechanism leads to its own polymer type which is different in
structure and properties from the other two. For vinyl addition
polymerization, the bicyclic structure unit remains intact, and
only the double bond of the π component is opened. Therefore,
vinyl-type polynorbornene, a special polymer with constrained
rings in each unit, possesses interesting and unique properties
such as high chemical resistance, good UV resistance, low
dielectric constant, high glass transition temperature, excellent
transparency, large refractive index and low birefringence.[22] The
vinyl-type polynorbornene is also more attractive to promote
homopolymerization and copolymerization with ethylene. Many
transition metal complexes including titanium,[23,24] zirconium,[25]
iron,[26] nickel,[27 – 33] palladium[34,35] and cobalt[36 – 38] have been
used as precursors for vinyl polymerization of norbornene. The
Appl. Organometal. Chem. 2010, 24, 308–313
resulting norbornene polymers may be crystalline or amorphous,
depending on the catalysts used.
This research addressed the synthesis of nickel(II) complexes {2[C3 HN2 (R1 )2 -3,5]}[C(R2 ) N(C6 H3 i Pr2 -2,6)]NiBr2 (complex 1: R1 =
CH3 , R2 = 2,4,6-trimethylphenyl; complex 2: R1 = R2 = Ph)
bearing new nitrogen ligands and investigated their behavior
for vinyl-type polymerization of norbornene after activation
by methylaluminoxane (MAO). Influences of polymerization
parameters such as reaction temperature and Al–Ni molar ratio
on catalytic activity and molecular weight of the polynorbornene
were studied.
Experimental
All manipulations involving air- and moisture-sensitive compounds were performed under nitrogen atmosphere using a
glove box and Schlenk techniques.
Materials
Extra-pure-grade nitrogen was further purified before feeding into
the reactor by passing them through a DC-IB gas purification
instrument. Toluene and hexane were refluxed over metallic
∗
Correspondence to: Yuan-yuan Wang, College of Material Science and
Engineering, Anhui University of Science and Technology, Huai-nan, 232001,
China. E-mail: wyy zsu@126.com
College of Material Science and Engineering, Anhui University of Science and
Technology, Huainan, 232001, China
c 2009 John Wiley & Sons, Ltd.
Copyright Nickel(II) complexes bearing pyrazolylimine ligand
sodium for 24 h, CH2 Cl2 was refluxed over P2 O5 for 8 h, and then
they were distilled under nitrogen atmosphere before use. 2,6Dipropylaniline and 3,5-dimethylpyrazole were purchased from
Aldrich; 3,5-diphenylpyrazole and 2,4,6-trimethylbenzoyl chloride
were purchased from Alfa Aesar. Norbornene (NBE) was purchased
from Aldrich, and was purified and dried using potassium at 60 ◦ C
for 12 h and distilled under nitrogen atmosphere, then dissolved
in toluene to make a 0.4 g ml−1 solution. The other reagents
were purchased and used as received. Methylaluminoxane was
prepared by partial hydrolysis trimethylaluminum (TMA) in toluene
at 0–60 ◦ C with H2 O from Al2 (SO4 )3 · 18H2 O.
Characterization
Elemental analyses were determined with a Vario EL Series
Elemental Analyzer from Elementar. MS-FAB spectra were obtained
with a VG ZAB-HS scan instrument, using m-nitrobenzylalcohol as
matrix. 1 H-NMR spectra were recorded on a Varian Mercury-Plus
300 NMR spectrometer at room temperature using CDCl3 for
ligands and o-dichlorobenzene-d4 for polymers using TMS as
internal standard. FTIR spectra were recorded as KBr pellets on a
Perkin-Elmer 1600 spectromer. Gel permeation chromatography
(GPC) analyses of molecular weight and molecular weight
distribution of the polymers were performed on a Waters 2000
instrument using 1,2,4-trichlorobenzene as eluent at 135 ◦ C and
standard polystyrene as reference. Thermogravimetric analyses
(TGA) were preformed under nitrogen atmosphere at a heating
rate of 20 ◦ C min−1 with a Netzsch TG 209 thermogravimetric
analyzer.
and 3,5-diphenylpyrazole (2.20 g, 10.0 mmol). The products were
recrystallized from ethanol to afford L2 as yellow crystals in
60.2% yield (3.00 g, 6.0 mmol). FAB+ -MS: m/z 483, 484, 485 [M+ ];
264, 265, 266 [M+ − (C3 HN2 Ph2 -3,5)]. Anal. calcd for C34 H33 N3 :
C, 84.43; H, 6.88; N, 8.69; found: C, 84.18; H, 7.12; N, 8.59%. 1 H-NMR
(300 MHz, CDCl3 , ppm): δ: 7.84 (s, 2H, Ar–H); 7.49–7.36 (m, 8H,
Ar–H); 7.14–6.95 (m, 6H, Ar–H); 6.96–6.94 (d, 2H, Ar–H); 6.80 (s,
1H, Pz–H); 2.87 (s, 2H, CH); 0.98–0.88 (d, 12H, CH3 ).
2-(C3 HN2 Me2 -3,5)(C(C6 H2 Me3 -2,4,6) N(C6 H3 i Pr2 -2,6)NiBr2
(complex 1)
A 2.0 mmol aliquot of (DME)NiBr2 was added under nitrogen
atmosphere to 2.0 mmol of L1, which was dissolved in 40 ml dry
CH2 Cl2 . The mixture was stirred at room temperature for 18 h. The
resulting solution was concentrated and then hexane was added
to precipitate the product which was washed with 20 ml of hexane
and dried in vacuum to obtain a purple red powder in 75.6% yield.
FAB+ -MS: m/z 619, 620, 621 [M+ ]; 540, 541, 542 [M+ − Br]; 459,
460, 461 [M+ − 2Br]. Anal. calcd for C27 H35 N3 NiBr2 : C, 52.30; H,
5.69; N, 6.78; found: C, 52.02; H, 5.38; N, 6.51%.
2-(C3 HN2 Ph2 -3,5)(C(Ph) N(C6 H3 i Pr2 -2,6)NiBr2 (complex 2)
In a similar method described for complex 1, complex 2 was
obtained from (DME)NiBr2 and L2 as a purple red powder in 70.8%
yield. Anal. calcd for C34 H33 N3 NiBr2 : C, 58.16; H, 4.74; N, 5.98; found:
C, 57.85; H, 4.54; N, 6.18%. FAB+ -MS: m/z 701, 702, 703 [M+ ]; 622,
623, 624 [M+ − Br]; 542, 543, 544 [M+ − 2Br].
Crystal Structure Determination
In a typical procedure, the appropriate MAO solid was added into
a 50 ml flask, and then freshly distilled toluene and the solution
of norbornene dissolved in toluene (0.4 g ml−1 ) was added via a
syringe at the desired polymerization temperature. The resulting
mixture was stirred for a further 10 min, and the precursor catalyst
solution in toluene was injected via a syringe. The polymerization
was carried out for the desired time and then quenched with
concentrated HCl in ethanol (150 ml, HCl–ethanol, 5 : 95, v/v). The
precipitated polymer was collected and washed with ethanol, and
then dried overnight in a vacuum at 50 ◦ C.
The crystals were mounted on a glass fiber using the oil drop
scan method. Data obtained with the ω − 2θ scan mode were
collected on a Bruker Smart 1000 CCD diffractometer with graphitemonochromated Cu Kα radiation (λ = 1.54178 Å). The structures
were solved by direct methods, while further refinement with fullmatrix least squares on F2 was obtained with the SHELXTL program
package. All non-hydrogen atoms were refined anistotropically.
Hydrogen atoms were introduced in calculated positions with the
displacement factor of the host carbon atoms.
Preparation
Results and Discussion
2-(C3 HN2 Me2 -3,5)(C(C6 H2 Me3 -2,4,6) N(C6 H3 i Pr2 -2,6) (L1)
Syntheses of Pyrazolylimine Ligands and Nickel(II) Complexes
L1 was prepared according to our previous work[39] using
(C6 H3 i Pr2 -2,6) NH[(C6 H2 Me3 -2,4,6)C O] (3.24 g, 10.0 mmol) and
3,5-dimethylpyrazole (0.96 g, 10.0 mmol). The resulting brown
mixture was purified by column chromatography on silica
gel using petroleum ether–ethyl acetate (5 : 1) as eluent, and
recrystallization was attempted from ethanol to afford L1 as
yellow crystals in 52.4% yield (2.10 g, 5.2 mmol). FAB+ -MS: m/z
401, 402, 403 [M+ ]; 306, 307, 308 [M+ − (C3 HN2 Me2 -3,5)]. Anal.
calcd for C27 H35 N3 : C, 80.75; H, 8.78; N, 10.46; found: C, 80.72;
H, 8.63; N, 10.10%. 1 H-NMR (300 MHz, CDCl3 , ppm): δ: 6.99–6.89
(m, 3H, Ar–H); 6.75 (s, 2H, Ar–H); 6.01 (s, 1H, Pz–H); 2.48 (s, 3H,
Pz–CH3 ); 2.23 (s, 3H, Pz–CH3 ); 2.96 (s, 2H, CH); 2.14 (s, 6H, Ar–CH3 );
1.59 (s, 3H, Ar–CH3 ); 1.25–0.87 (m, 12H, CH3 ).
Two phenyl-substituted pyrazolylimine ligands L1–L2 and
their corresponding nickel(II) complexes 1–2 were synthesized
(Scheme 1). For the synthesis of pyrazolylimines, in the first step,
the amide formation occurred rapidly at room temperature via
the reaction of corresponding 2,6-diisopropylaniline and benzoyl
chloride (or 2,4,6-trimethylbenzoyl chloride). In the second step,
the intermediate benzimidolyl chlorides were prepared from the
corresponding benzamides (or 2,4,6-trimethylbenzamaides) by
treatment with thionyl chloride. In the last step, the syntheses of
pyrazolylimines were based on the reaction of benzimidoyl (or
2,4,6-trimethylbenzimidoyl) chlorides with 3,5-dimethylpyrazole
(or 3,5-diphenylpyrazole) in the presence of triethylamine. The
pure pyrazolylimines could be obtained after purification in
yields for L1 of 52.4% and for L2 of 60.2%, respectively.
At first, L3 [2-(C3 HN2 Ph2 -3,5)][C(C6 H2 Me3 -2,4,6) N(C6 H3 i Pr2 -2,6)]
was designed to be prepared with (C6 H3 i Pr2 -2,6) NH[(C6 H2 Me3 2,4,6)C O] and 3,5-diphenylpyrazole, but it failed, possible
2-(C3 HN2 Ph2 -3,5)(C(Ph) N(C6 H3 i Pr2 -2,6) (L2)
L2 was prepared according to the similar method describes
for L1 using (C6 H3 i Pr2 -2,6) NH[(C6 H5 )C O] (2.81 g, 10.0 mmol)
Appl. Organometal. Chem. 2010, 24, 308–313
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
309
Norbornene Polymerization
Y. Wang, B.-x. Li and Y.-z. Zhu
R1
R2COCl
NH2
Et3N, THF, RT
R1
R1
N
N
O
N
H
R2
R2
SOCl2
Reflux
Cl
N
R1
NH
N
Et3N, Toluene, reflux
R1
R2
N
(DME)NiBr2
R1
CH2Cl2, RT
L1 R1=CH3, R2=2,4,6-Trimethylphenyl
L2 R1=Ph, R2=Ph
R2
N
N
N
Ni
Br
Br
Complex 1 R1=CH3, R2=2,4,6-Trimethylphenyl
Complex 2 R1=Ph, R2=Ph
Scheme 1. Synthesis of pyrazoylimine ligands L1–L2 and nickel(II) complexes 1–2.
not to be significantly strained. The narrow N1&sbond Ni&sbond
N3 angle of 80.25(11)◦ results from the relatively small bite
size of the ligand. The 2,6-diisopropyl-phenyl ring of the imine
side arm adopts a position almost perpendicular to the metal
coordination plane with the 2,6-diisopropyl-phenyl-cooridination
plane angle 71.91(14)◦ , and thus shields the metal center from one
side. Moreover, the 2,4,6-trimethylphenyl group on the C6 atom
swings out of the coordination plane forming a dihedral angle of
57.78(19)◦ . In contrast with the phenyl group on the C6 atom,[39]
the dihedrals of 2,6-diisopropyl-phenyl-cooridination plane and
2,4,6-trimethylphenyl-cooridination plane were smaller because
of the greater bulkiness of the 2,4,6-trimethylphenyl group.
Norbornene Polymerization
Figure 1. ORTEP diagram for complex 1. Hydrogen atoms were omitted
for clarity.
because of the bulky substituents of 2,4,6-trimethyphenyl, 2,6diisopropyl and diphenyl groups.
Nickel complexes bearing pyrazolylimine ligand were obtained
under mild conditions by the reaction of pyrazolylimine ligands
with 1.0 equivalent of (DME)NiBr2 in dry CH2 Cl2 at room
temperature in good yields (complex 1, 75.6%, complex 2, 70.8%).
They have been characterized by elemental analysis and mass
spectrometry, and the two nickel complexes turned out to be
paramagnetic in solution.
The nickel(II) complexes 1 and 2 can be activated for norbornene
polymerization with MAO as cocatalyst. The two nickel complexes
activated by MAO exhibit moderate catalytic activity for the
vinyl polymerization of norbornene. The polynorbornene was
white solid and all polymers were soluble in chlorobenzene at
room temperature, implying that the polynorbornene was low
steroregularity.[40] The molecular weight of all PNBE was between
105 and 106 g mol−1 , and the molecular weight distribution (MWD)
was around 2.0. The resulting PNBEs were very stable up to
about 400 ◦ C as determined by TGA under nitrogen. Moreover,
according to the polymerization results, the yield, catalytic
activity, molecular weight and molecular weight distribution
depended significantly on the polymerization parameters, such as
polymerization temperature and Al–Ni molar ratio.
The polymerization temperature had a remarkable effect on
catalytic activity and molecular weight of the obtained polymers.
Single-crystal X-ray structure of complex 1
310
Single crystal of nickel complex 1 suitable for single-crystal X-ray
diffraction study were grown from CH2 Cl2 –hexane solutions under
nitrogen atmosphere at room temperature. The single-crystal X-ray
structure of nickel complex 1 was obtained, and the ORTEP diagram
for complex 1 is included in Fig. 1. The crystal data, together
with the data collection and structure refinement parameters, are
presented in Table 1. Selected bond lengths and angles for the
nickel complex 1 are given in Table 2.
Complex 1 adopts a distorted tetrahedral geometry structure
in which the central Ni atom binds to two bromine atoms and
one bidentate pyrazolylimine ligand. In the predicted structure,
the five-membered chelate ring is substantially flat and appears
www.interscience.wiley.com/journal/aoc
Table 1. Selected bond lengths and bond angles for complex 1
Complex 1
Bond lengths (Å)
Ni1&sbond Br1
Ni1&sbond Br2
Ni1&sbond N1
Ni1&sbond N3
c 2009 John Wiley & Sons, Ltd.
Copyright Bond angles (deg)
2.3481(6)
2.3460(6)
1.976(3)
2.055(2)
N1&sbond Ni1&sbond N3
N1&sbond Ni1&sbond Br2
N3&sbond Ni1&sbond Br2
N1&sbond Ni1&sbond Br1
N3&sbond Ni1&sbond Br1
Br2&sbond Ni1&sbond Br1
80.25(11)
98.67(8)
117.38(7)
111.87(8)
111.70(7)
125.48(2)
Appl. Organometal. Chem. 2010, 24, 308–313
Nickel(II) complexes bearing pyrazolylimine ligand
Table 2. Crystallographic data for complex 1
Complex 1
Empirical formula
Fw
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å 3 )
Z
Dcalc (mg m−3 )
Absorption coefficient (mm−1 )
F(000)
Crystal size (mm)
θ range (deg)
Reflections collected
Unique reflections
Completeness to θ (%)
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices[I > 2σ (I)]
R indices (all data)
Largest difference in peak and hole (e Å −3 )
C27 H35 N3 NiBr2
620.11
293(2)
Monoclinic
P21/c
10.6297(2)
14.1955(6)
18.6518(3)
90
105.530(2)
90
2711.69(8)
4
Table 3. Influences of temperature on norbornene polymerization
with complex 1 and complex 2–MAO catalytic systemsa
Entry
1
2
3
4
5
6
7
8
Complex
Tp (◦ C)
Activityb
Yield (%)
Mw c
Mw /Mn
1
1
1
1
2
2
2
2
20
40
60
80
20
40
60
80
4.35
7.10
5.15
3.19
3.58
6.50
4.74
2.69
13.6
22.2
16.1
10.0
11.2
20.3
14.8
8.4
12.22
9.65
8.05
4.93
13.56
10.86
8.58
5.12
1.95
2.12
2.05
2.25
2.09
2.15
2.20
2.28
1.519
4.621
1264
0.34 × 0.25 × 0.15
3.97–60.00
9550
3723
92.6
3723/0/298
1.007
R1 = 0.0278, wR2 = 0.0672
R1 = 0.0377, wR2 = 0.0693
0.424 and −0.368
Table 4. Influences of Al/Ni molar ratio on nobornene polymerization
with with complex 1 and complex 2–MAO catalytic systemsa
Entry
1
2
3
4
5
6
7
8
Complex
Al : Ni
Activityb
Yield (%)
Mw c
Mw /Mn
1
1
1
1
2
2
2
2
600
800
1000
1200
600
800
1000
1200
7.10
8.58
10.56
6.56
6.50
6.98
8.64
6.02
22.2
26.8
33.0
20.5
20.3
21.8
27.0
18.8
9.65
8.82
7.44
6.15
10.86
9.46
8.17
6.58
2.12
2.35
2.57
2.86
2.15
2.08
2.49
2.62
a Polymerization conditions: solvent, toluene; total volume, 16 ml;
3.0 µmol complex; 3.2 g NBE; Tp = 40 ◦ C; time = 20 min.
b In units of 105 g PNBE (mol Ni h)−1 .
c In units of 105 g mol.
As shown in Table 3, these catalytic systems showed moderate
activity over a wide temperature range. Increasing temperature is
helpful to enhance the yield and activity, but the activity decreases
with further temperature increase. This is probably because that
enhancing temperature can accelerate the formation of active
species and the rate of chain insertion and chain propagation. It
should be noted that further increase of temperature could cause
the instability or decomposition of the active species. However,
the molecular weight decreased significantly with the increase in
temperature all the while. In general, the rate of chain transfer is
more sensitive to temperature relative to that of chain propagation,
and, at higher temperature, chain transfer is in the predominant
state. Therefore, the molecular weight of PNBEs will fall as the
polymerization temperature increases. A molecular weight of
13.56 × 105 g mol−1 could be obtained at a reaction temperature
of 20 ◦ C (entry 5 in Table 3).
MAO was essential for the polymerization of norbornene
catalyzed by these pyrazolylimine nickel complexes. As shown
in Table 4, the two catalytic systems showed similar tendencies.
The polymer yield and the catalyst activity increased with the
increase in MAO amount until the Al–Ni molar ratio was up to
1000 for complex 1– MAO and complex 2– MAO systems, and
then decreased respectively with further increase in MAO ratio.
The highest catalytic activity of up to 10.56 × 105 g PNBE (mol Ni
h)−1 could also be observed in a complex 1–MAO catalytic system
(entry 3 in Table 4). A suitable value of Al–Ni is required because the
MAO is necessary for reaction of MAO with pyrazolylimine nickel
complexes to produce sufficient active species for norbornene
polymerization and to scavenge impurities. However, MAO has
been well known to act as a chain transfer agent, so the higher
Al–Ni ratio makes the catalyst activity and molecular weight of
PNBE decrease.
Complex 3 is represented by {2-[C3 HN2 (R1 )2 -3,5]}[C(R2 )
N(C6 H3 i Pr2 -2,6)]NiBr2 , when R1 = CH3 and R2 = Ph, which
has been reported in our previous work.[39,41] In the same
polymerization conditions, such as Tp = 40 ◦ C and Al–Ni = 600,
the catalytic activities of complex 1 [7.10 × 105 g PNBE (mol Ni
h)−1 ], complex 2 [6.50 × 105 g PNBE (mol Ni h)−1 ] and complex 3
[6.59×105 g PNBE(mol Ni h)−1 ][41] suggest that the activities of the
three nickel complexes have no obvious difference, which indicates
that the change of substituents of R1 and R2 has no significant
influence on the steric hindrance of the three complexes.
The microstructure of the polynorbornene was characterized by
FTIR and 1 H-NMR. FTIR spectrum (Fig. 2) revealed the characteristic
signal at about 941 cm−1 , which can be assigned to the ring
system of bicycol[2,2,1] heptane.[42] The absence of absorption
at 1620–1680 cm−1 , especially 960 cm−1 in the FTIR spectrum,
also supports vinyl polymerization of norbornene.[43] The 1 H-NMR
Appl. Organometal. Chem. 2010, 24, 308–313
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
311
a Polymerization conditions: solvent, toluene; total volume, 16 ml;
3.0 µmol complex; 3.2 g NBE; time = 20 min; Al : Ni = 600.
b In units of 105 g PNBE (mol Ni h)−1 .
c
In units of 105 g mol−1 .
1.0
0
80
-5
-10
TG (%)
Transmittance
0.8
100
0.6
60
-15
40
-20
20
-25
0.4
0.2
-30
0
0.0
3500
3000
2500
2000
1500
1000
-35
500
100
Wavenumber cm-1
Figure 2. FTIR spectrum of polynorbornene obtained by complex 1–MAO
catalytic system (entry 2 in Table 3).
7
1
5
6
200
300
400
Temperature (°C)
500
600
Figure 4. TG–DTG curves of polynorbornene obtained by complex
1–MAO catalytic system (entry 2 in Table 3).
two nickel complexes exhibited moderate activity for the vinyl
polymerization of norbornene when activated by MAO. Under the
optium polymerization conditions (complex 1–MAO, Tp = 40 ◦ C
and Al–Ni = 1000), the highest catalytic activity of 10.56 × 105
gPNBE (mol Ni h)−1 was obtained. The molecular weight of the
polymer decreased with an increase in the reaction temperature.
The structure characterization of polynorbornene indicated that
the norbornene polymerization is vinyl-type polymerization rather
than ROMP.
3
2
DTG (%min)
Y. Wang, B.-x. Li and Y.-z. Zhu
4
n
C2/C3
C1/C4 C7
C5/C6
Supporting information
Supporting information may be found in the online version of this
article.
10
9
8
7
6
5
4
3
2
1
0
Figure 3. 1 H-NMR spectrum of polynorbornene in o-dichlorobenzene-d4
obtained by complex 1–MAO catalytic system (entry 2 in Table 3).
spectrum (Fig. 3) of the resulting polynorbornene obtained by the
complex 1–MAO catalytic system (entry 2 in Table 3) is shown
in Fig. 3. The 1 H-NMR spectrum indicates that there are four
group resonance peaks appearing in δ = 0–3 ppm, which can be
assigned to the methene hydrogen corresponding to C5/C6 and
C7, and methine hydrogen corresponding to C1/C4 and C2/C3,
respectively. In addition, no proton signals were observed from
3.0 to 6.0 ppm, which usually indicates ROMP of norbornene.[25]
The TG/DTG curve of polynorbornene shown in Figure 4
revealed that the loss of polymer weight was about 5%
when the temperature increased to 400 ◦ C. The decomposition
of polynorbornene was accelerated above 400 ◦ C and the
highest decomposition rate up to 46.2% could be obtained
at 460 ◦ C; however, complete decomposition occurred when
the temperature was above 600 ◦ C. Thermogravimetric analysis
demonstrated that the polynorbornenes obtained by complexes
1/2–MAO catalytic systems exhibit good thermostability under
nitrogen.
Conclusion
312
Two pyrazolylimine ligands and their corresponding nickel(II)
complexes were successfully synthesized and characterized. The
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nickell, properties, typed, complexes, ligand, structure, synthesis, norbornene, catalytic, vinyl, pyrazolylimine, bearing, polymerization
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