Bis-(salicylaldehyde-benzhydrylimino)nickel complexes with different electron groups: Crystal structure and their catalytic properties toward (co)polymerization of norbornene and 1-hexene

Article abstract of DOI:10.1039/c8ra06561f

Eight bis-(salicylaldehyde-benzhydrylimino)nickel complexes with different electron groups (Ni1-Ni8), Ni{(3-R₁)(5-R₂)C₆H₂(O)CHNCH(C₆H₅)₂}₂, (R₁ = H, R₂ = H, Ni1; R₁ = H, R₂ = CH₃, Ni2; R₁ = H, R₂ = OCH₃, Ni3; R₁ = H, R₂ = Br, Ni4; R₁ = CH₃, R₂ = H, Ni5; R₁ = OCH₃, R₂ = H, Ni6; R₁ = Br, R₂ = Br, Ni7; R₁ = Cl, R₂ = Cl, Ni8), were synthesized and their crystal structures were characterized using single crystal X-ray diffraction. The results revealed that Ni1-Ni6 belong to the monoclinic system (space group P2(1)/n), Ni7 belongs to the monoclinic system (space group C2/c) and Ni8 belongs to the triclinic system (space group P1). All nickel complexes exhibited high activities (0.46-2.07 × 10⁶ g_polymer mol_Ni^-1 h^-1) toward norbornene homopolymerization, and a strong electron-withdrawing group on the salicylaldimino aromatic ring can enhance the catalytic activity and favor polymerization. Ni1 and Ni2 exhibited high activities (0.55-2.40 × 10⁵ g_polymer mol_Ni^-1 h^-1) toward copolymerization of norbornene and 1-hexene in the presence of B(C₆F₅)₃. The 1-hexene content in the copolymers could be controlled up to 7.98-12.50% by varying the comonomer feed ratio of 1-hexene from 10 to 50%. It is observed that when the 5-position of the salicylaldimino aromatic ring has a substituent (-CH₃), the 1-hexene insertion rate is lower than that without a substituent. In addition, the polymers showed high molecular weights (1.5-2.4 × 10⁵ g mol^-1) and narrow molecular weight distributions (1.62-1.89). The obtained polymers were also verified to be amorphous copolymers and had high thermal stability, good solubility and optical transparency.

Full text of DOI:10.1039/c8ra06561f

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Bis-(salicylaldehyde-benzhydrylimino)nickel
complexes with different electron groups: crystal
structure and their catalytic properties toward (co)
polymerization of norbornene and 1-hexene†
Cite this: RSC Adv., 2018, 8, 36298
a
Guangshui Tu,a Feng Zhang,a Shengmei Huang,ac
*
Xiaohui He,
Changwen Cheng,a Chuanyi Zhu,a Yapeng Duan,a Suli Wanga and Defu Chenb
Eight bis-(salicylaldehyde-benzhydrylimino)nickel complexes with different electron groups (Ni1–Ni8), Ni
{(3-R1)(5-R2)C6H2(O)CHNCH(C6H5)2}2, (R1 ¼ H, R2 ¼ H, Ni1; R1 ¼ H, R2 ¼ CH3, Ni2; R1 ¼ H, R2 ¼ OCH3,
Ni3; R1 ¼ H, R2 ¼ Br, Ni4; R1 ¼ CH3, R2 ¼ H, Ni5; R1 ¼ OCH3, R2 ¼ H, Ni6; R1 ¼ Br, R2 ¼ Br, Ni7; R1 ¼ Cl,
R2 ¼ Cl, Ni8), were synthesized and their crystal structures were characterized using single crystal X-ray
diffraction. The results revealed that Ni1–Ni6 belong to the monoclinic system (space group P2(1)/n), Ni7
belongs to the monoclinic system (space group C2/c) and Ni8 belongs to the triclinic system (space
6
ꢁ1
ꢁ1) toward
ꢀ
group P1). All nickel complexes exhibited high activities (0.46–2.07 ꢀ 10 gpolymer molNi
h
norbornene homopolymerization, and a strong electron-withdrawing group on the salicylaldimino
aromatic ring can enhance the catalytic activity and favor polymerization. Ni1 and Ni2 exhibited high
ꢁ1
activities (0.55–2.40 ꢀ 105 gpolymer molNi
h
ꢁ1) toward copolymerization of norbornene and 1-hexene in
the presence of B(C6F5)3. The 1-hexene content in the copolymers could be controlled up to 7.98–
12.50% by varying the comonomer feed ratio of 1-hexene from 10 to 50%. It is observed that when the
5-position of the salicylaldimino aromatic ring has a substituent (–CH3), the 1-hexene insertion rate is
lower than that without a substituent. In addition, the polymers showed high molecular weights (1.5–2.4
ꢀ 105 g molꢁ1) and narrow molecular weight distributions (1.62–1.89). The obtained polymers were also
verified to be amorphous copolymers and had high thermal stability, good solubility and optical
transparency.
Received 4th August 2018
Accepted 18th October 2018
DOI: 10.1039/c8ra06561f
rsc.li/rsc-advances
polymerization method had high thermal stability, high
optical permeability, a low dielectric constant, low moisture
1. Introduction
absorption and other excellent properties and were widely
used in micro electronics and optics. However, the further
development of norbornene is hampered by its poor bond
properties, low solubility and poor mechanical properties. At
present, the introduction of polar groups18–20 or exible
chains21–25 on the norbornene backbone is the most effective
means to solve this problem. In order to achieve this goal, it
is important to synthesize some efficient catalysts. It is well-
known that conventional Ziegler–Natta catalysts26,27 and
metallocene catalysts28–30 have many disadvantages due to
their polar centers as early transition metals, for instance,
poor tolerance to polar groups and large amounts of cocata-
lyst. However, owing to the lower oxophilicity, single active
site and resistance toward deactivation by polar functional-
ities, the design of late-transition-metal catalysts has attrac-
ted widespread interest.31–35 The usual late-transition-metal
catalysts are Ni(II) and Pd(II) complexes. Comprehensive
investigations had approved Ni(II) complexes because of their
Over the last few decades, polyolen materials as a good
substitute for engineering plastics and optical device appa-
ratus attracted much attention in the scientic community
and industry.1–3 Norbornene and its derivatives can be very
easy to polymerize because of the presence of their bicyclic
structures. There are three polymerization methods: ring-
opening metathesis polymerization (ROMP),4–8 cationic or
radical polymerization,9 and vinyl-addition polymeriza-
tion.10–17 The polymers obtained by the vinyl addition
aSchool of Materials Science and Engineering, Nanchang University, 999 Xuefu Avenue,
Nanchang 330031, China. E-mail: hexiaohui@ncu.edu.cn
bSchool of Civil Engineering and Architecture, Nanchang University, 999 Xuefu Avenue,
Nanchang 330031, China
cSchool of Materials Science and Engineering, Nanchang Hangkong University,
Nanchang 330063, China
† Electronic supplementary information (ESI) available. CCDC 1575905, 1575909,
1587113, 1817084, 1830324, 1843964, 1843971 and 1851039. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c8ra06561f
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superior catalytic performance, polymer properties and their
expounded reaction mechanisms.
2.2 Characterization
The single crystal intensity data for the catalysts were
collected with a Bruker Smart APEX II CCD system. The 1H
NMR and 13C NMR spectra of the nickel complexes and
copolymers were gained by a Bruker ARX 600 NMR spec-
trometer at ambient temperature with CDCl3 as solvent and
tetramethylsilane (TMS) as an internal reference. The Fourier
transform infrared (FT-IR) spectra were recorded by a Shi-
madzu IR Prestige-21 FTIR spectrophotometer. The molec-
ular weight and molecular weight distribution of the
polymers samples were gained by gel permeation chroma-
tography (GPC, PL-GPC220), with a refractive index detector,
chloroform as a solvent with a ow rate of 0.5 mL minꢁ1 and
detector at 40 ꢂC. Thermal gravimetric analysis (TGA) was
carried out with a TAQ600 SDT for thermogravimetry from
Norbornene is a typical cycloolen with many advantages of
cycloolens, but due to the presence of bicyclic skeletons in the
polymer, it is brittle and has a small elongation at break. In
order to improve its defects, the copolymer obtained by
copolymerizing norbornene with a-olen22,23,36 not only can
reserve the heat resistance and dimensional stability of poly-
norbornene, but also has excellent optical properties, good
processing and solubility, which has attracted scholars's
attention at both home and abroad. For example, Kaminsky
group37 achieved the copolymerization of norbornene with
ethylene using a metallocene catalyst. The results showed that
the glass transition temperature decreases with the increase of
ethylene content in the copolymer, and the exibility and pro-
cessing performance of the polymer can be improved. Shiono
group38 achieved the copolymerization of norbornene with long-
chain alpha-olens (1-hexene, 1-octene and 1-decene) through
a titanium metal catalyst. And the catalyst afforded low molec-
ular weight distribution, high light transmittance (up to 90%)
and excellent processing performance.
ꢁ1
ꢂ
ꢂ
room temperature to 650 C at a rate of 20 C min under
nitrogen atmosphere. The wide-angle X-ray diffraction (XRD)
curves of the polymers were gained with a Bruker D8 Focus X-
ray diffractometer, with monochromatic radiation at a wave-
length of 1.54 A at a scanning rate of 2 minꢁ1. Scanning was
ꢂ
˚
carried out with 2q ranging from 3ꢂ to 50ꢂ.
In this study, a series of salicylaldimine ligands, (3-R1)(5-
R2)C6H2(OH)CHNCH(C6H5)2 (R1 ¼ H, R2 ¼ H, L1; R1 ¼ H, R2
¼ CH3, L2; R1 ¼ H, R2 ¼ OCH3, L3; R1 ¼ H, R2 ¼ Br, L4; R1 ¼
CH3, R2 ¼ H, L5; R1 ¼ OCH3, R2 ¼ H, L6; R1 ¼ Br, R2 ¼ Br, L7;
R1 ¼ Cl, R2 ¼ Cl, L8) (L1–L8) and the corresponding bis-
(salicylaldehyde-benzhydrylimino)nickel complexes (Ni1–
Ni8) were synthesized, and the catalytic activities of
complexes Ni1–Ni8 with only B(C6F5)3 (ref. 39–42) as cocata-
lyst in norbornene homopolymerization were investigated. In
addition, the Ni1 and Ni2 complexes for norbornene and 1-
hexene copolymerization were also investigated. Finally, the
properties of polynorbornene were improved by introducing
exible chain, and the structure, thermal stability and optical
properties of the copolymers were also characterized.
2.3 Crystal structure determination
The X-ray diffraction data of single crystals of the complexes
were obtained with the uꢁ2q scan mode of a Bruker Smart
APEX II CCD diffractometer with graphite-monochromated
Mo Ka radiation (l ¼ 0.71073 nm). The structures were
worked out using direct methods, and further renements
with full-matrix least-squares on F2 were gained using the
SHELXTL program package. All non-hydrogen atoms were
rened anisotropically.
2.4 Syntheses of pro-ligands (L1–L8) and nickel complexes
(Ni1–Ni8)
The syntheses and structures of the pro-ligands and nickel
complexes were outlined in Scheme 1.
2. Experimental
All the operations involving air- and moisture-sensitive
chemical compounds were performed under an atmosphere
of puried and dried nitrogen using standard vacuum-line,
Schlenk or glovebox techniques.
The typical synthetic means of the C6H4(OH)CHNCH(C6H5)2
(L1) was described as follows: under a nitrogen atmosphere,
salicylaldehyde 1.22 g (0.01 mol), aminodiphenylmethane
1.83 g (0.01 mol) and a catalytic amount of p-toluenesulfonic
acid (p-TsOH) were successively added to a 250 mL reaction
ask equipped with a magnetic stirrer, then added to 150 mL
toluene. The mixture was reuxed for 24 h. The resulting water
2.1 Materials
The required tetrahydrofuran (THF) and toluene were dried over during the reaction was removed by using a Dean–Stark appa-
sodium/benzophenone for 48 h and distilled under dry nitrogen ratus in the form of a water–toluene azeotrope. The crude
atmosphere. Norbornene (NB) was purchased from Alfa Aesar product were further puried by silica column chromatography
and puried by drying over metallic sodium and distilling with n-hexane and ethyl acetate (20/1 in v/v) and were crystal-
under dry nitrogen atmosphere, then used as a solution (0.4 g lized from a mixture of the n-hexane and ethyl acetate to get L1
mL) in toluene. 1-Hexene was purchased from Aladdin and the as yellow crystals. Yield: 1.96 g (68.3%). 1H NMR (CDCl3, d,
inhibitor was removed by washing three times with aqueous ppm): 13.51 (s, 1H, OH); 8.47 (s, 1H, –C]N–H); 7.24–7.58 (m,
sodium hydroxide solution (5.0 wt%), and then distillation over 10H, 2C6H5); 6.86–7.24 (m, 3H, C6H4); 5.62 (s, 1H, C13H11). FT-
CaH2 under dry nitrogen atmosphere at a reduced pressure. IR (KBr): 3343.22(w), 2928.01(w), 2762.30(w), 1725.69(w),
Other chemical reagents available on the market are purchased 1661.36 (vs., nC]N), 1592.06(s), 1516.49(w), 1433.45(w),
and used without purication.
1300.56(w).
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Scheme 1 The synthetic routes of salicylaldehyde-benzhydrylimine ligands and bis-(salicylaldehyde-benzhydrylimino)nickel complexes.
The other salicylaldimine ligands (L2–L8) were synthesized deprotonation. When the solution changed from yellow to
and characterized according to the method for L1 synthesizing, orange, the solvent was removed in vacuo. Then, (DME)NiBr2 (1.0
and yellow crystals were also obtained.
mmol) and freshly distilled dichloromethane (20 mL) were added
The typical synthetic procedure for Ni{(3-H)(5-H)C6H2(O) and stirred for 12 hours at room temperature. The dark-green
CHNCH(C6H5)2}2 (Ni1) was described as follow: ligand L1 crystal Ni1 complex was crystallized from dichloromethane in
(0.574 g, 0.002 mol) was added to a 100 mL Schlenk ask 70% yield. 1H NMR (CDCl3, d, ppm): 8.45 (s, 2H, 2C–N–H); 0.97–
equipped with a magnetic stirrer, 0.06 mol of 60% sodium 1.49 (m, 20H, 4C6H5); 7.28–7.69 (m, 8H, 2C6H4); 5.62 (s, 2H,
hydride was added to the reaction ask, and nally 20 mL of 2C13H11). 13C NMR (CDCl3, d, ppm): 164.90, 132.61, 131.19,
freshly distilled tetrahydrofuran solution was added. The mixed 129.48, 129.04, 127.43, 126.90, 118.85, 29.59. Analysis calculated
solution was stirred at room temperature for 8 hours for for Ni1: C, 75.94; H, 5.75; N, 4.32; found: C, 76.01; H, 5.78; N, 4.33.
Scheme 2 Norbornene homopolymerization catalysed by Ni1–Ni8/B(C6F5)3 and copolymerization of norbornene and 1-hexene catalyzed by
Ni1 and Ni2/B(C6F5)3.
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Fig. 1 ORTEP plots of Ni1 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
The other Ni{(3-R1)(5-R2)C6H2(O)CHNCH(C6H5)2}2 (Ni2–Ni8) appropriate B(C6F5)3 standard solution and a certain amount of
were synthesized and characterized according to the method for toluene were added to the two-necked round bottom ask, then
synthesizing Ni1, and dark-green crystals were also obtained.
the appropriate amount toluene solution of norbornene (NB)
(0.4
g
mLꢁ1
)
and the catalyst standard solution (5
ꢀ
10ꢁ3 mol Lꢁ1) were injected into the stirred solution in order.
The reaction mixture was continuously stirred at the set
temperature. The reaction was terminated by the addition of
2.5 Polymerization of norbornene
All manipulations were carried out under a dry nitrogen atmo-
sphere. The polymerization process was as follows: the
Fig. 2 ORTEP plots of Ni2 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
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Fig. 3 ORTEP plots of Ni3 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
a mixed solution of hydrochloric acid/methanol (v/v ¼ 1/9). added to a 100 mL two-necked round bottom ask under
Finally, the resulting polymers were collected by ltration at nitrogen atmosphere, and then a standard solution of B(C6F5)3
room temperature and were dried in a vacuum at 50 C until (5 ꢀ 10ꢁ2 mol Lꢁ1) and complexes (5 ꢀ 10ꢁ3 mol Lꢁ1) were
ꢂ
constant weight. The procedure was shown in Scheme 2.
syringed into the well-stirred solution. The reaction mixture was
continuously stirred for 1 h at 60 C. The reaction was termi-
ꢂ
nated by the addition of a mixed solution of hydrochloric acid/
methanol (v/v ¼ 1/9). The resulting polymers were collected by
ltration at room temperature and were puried by
2.6 Copolymerization of norbornene and 1-hexene
The copolymerization process was as follows: norbornene
standard solution (0.4 g molꢁ1), 1-hexene and toluene were
Fig. 4 ORTEP plots of Ni4 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
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Fig. 5 ORTEP plots of Ni5 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
Fig. 6 ORTEP plots of Ni6 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
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Fig. 7 ORTEP plots of Ni7 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
Fig. 8 ORTEP plots of Ni8 with thermal ellipsoids at the 30% probability level showing the atom-labeling scheme. Hydrogen atoms and solvent
have been omitted for clarity.
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a
Table 3 Homopolymerization of norbornene catalyzed by Ni1–Ni8 combined with B(C6F5)3
Ni complexes
NB/Ni (mol molꢁ1
)
B/Ni (mol molꢁ1
)
Tp, ꢂ
C
Reaction time (min)
PNB Yield (%)
Activityb
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni2
Ni2
Ni2
Ni3
Ni3
Ni3
Ni4
Ni4
Ni4
Ni5
Ni5
Ni5
Ni6
Ni6
Ni6
Ni7
Ni7
Ni7
Ni8
Ni8
Ni8
1000
2000
3000
4000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
10
10
10
10
5
20
10
10
10
5
10
20
5
10
20
5
10
20
5
10
20
5
10
20
5
10
20
5
60
60
60
60
60
60
20
40
80
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
5
5
5
5
5
5
60.42
70.25
42.61
27.67
29.63
58.35
40.66
62.52
66.37
34.67
73.35
61.85
32.34
71.25
60.73
37.31
78.45
67.72
33.42
72.21
61.25
30.48
69.86
58.67
38.42
85.67
70.26
40.45
91.78
73.65
0.68
1.62
1.44
1.25
0.67
1.32
0.46
0.71
1.50
0.78
1.65
1.39
0.73
1.61
1.37
0.84
1.77
1.53
0.75
1.63
1.38
0.68
1.58
1.32
0.87
1.93
1.59
0.91
2.07
1.66
10
10
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
10
20
a
b
ꢁ1
Reaction conditions: n[Ni] ¼ 5.0 ꢀ 10ꢁ6 mol; total volume toluene 10 mL. In units of 106 gpolymer molNi hꢁ1
.
trichloromethane/methanol reverse precipitation. Finally, the advantageous for stabilizing the activity of the catalyst. In
polymers were dried in a vacuum at 50 ꢂC until constant weight. addition, the ligand electronic effect generate different charge
The procedure was shown in Scheme 2.
distribution on the nickel metal atom, and the catalytic activity
markedly increased with an increase in the electrophilicity of
the nickel metal center.
3. Results and discussion
3.1 Crystal structure
3.2 Polymerization of norbornene
Dark-green single crystals of complexes suitable for single A series of nickel complexes were tested with B(C6F5)3 (ref. 43
crystal X-ray diffraction were gained by slow diffusion toluene and 44) as cocatalyst in toluene and the detailed data were
into dichloromethane solutions. The crystallographic data were summarized in Table 3. In order to determine the most suitable
summarized in Tables 1 and 2 and Table S1 and S2.† The norbornene homopolymerization conditions, the catalytic
ORTEP plots of Ni1–Ni8 were shown in Fig. 1–8, respectively. performance of Ni1 was investigated by changing the NB/Ni
CIF data of Ni1– Ni8 are available as ESI.† In the solid state, all molar ratios, the B/Ni molar ratios, and the polymerization
the complexes are both mononuclear and four coordinate and temperature. From Table 3 we found that the polymer yields
adopt geometries best described as square planar about each and catalytic activities depended on the reaction conditions,
nickel center, having slight distortions from idealized geometry and the NB/Ni molar ratio at 2000/1, B/Ni molar ratio at 10/1
and are quiet similar to some known nickel(II) complexes.19 In and the polymerization at 60 C were the best polymerization
ꢂ
fact, the Ni–O bond lengths of Ni1 (1.816(4) A), Ni2 (1.8295(12) conditions. All catalysts exhibited very high activity (up to 106
˚
ꢁ1
A), Ni3 (1.831(2) A), Ni4 (1.8480(14) A), Ni5 (1.826(8) A), Ni6 gpolymer molNi hꢁ1), but there are also certain differences
˚
˚
˚
˚
˚
˚
˚
(1.8298(16) A), Ni7 (1.840(2) A) and Ni8 (1.846(3) A) are shorter between the different catalysts. It can be seen that when the R2
˚
than the Ni–N bond lengths of Ni1 (1.927(4) A), Ni2 (1.9300(13) substituent at the 5-position of the salicylaldimino aromatic
˚
˚
˚
˚
A), Ni3 (1.916(2) A), Ni4 (1.9261(16) A), Ni5 (1.921(7) A), Ni6 ring is a strong electron-withdrawing group (–Br), the catalytic
˚
˚
˚
(1.919(2) A), Ni7 (1.899(3) A) and Ni8 (1.931(3) A). The pro- activity is higher than that at the corresponding position for the
ligands are very stable structure and the catalysts structure are electron-donating group (–CH3 or –OCH3) (Ni4 > Ni1, Ni2, Ni3),
also stable and symmetrical, therefore there are very indicating that the electron-withdrawing group on the
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a
Table 4 Copolymerization of norbornene and 1-hexene catalyzed by Ni1–Ni2 combined with B(C6F5)3
Activity (gpolymer molNi hꢁ1
)
Mw
Mw/Mn
1-Hexene incorpc (mol%)
ꢁ1
b
b
Ni complexes
NB/1-hexene (n/n)
Yield (%)
Ni1
Ni1
Ni1
Ni1
Ni2
Ni2
Ni2
Ni2
100/0
90/10
70/30
50/50
100/0
90/10
70/30
50/50
70.25
54.40
36.21
15.52
73.35
64.50
34.61
22.30
1.62 ꢀ 106
2.03 ꢀ 105
1.32 ꢀ 105
0.55 ꢀ 105
1.65 ꢀ 106
2.40 ꢀ 105
1.25 ꢀ 105
0.79 ꢀ 105
Insoluble
2.13
1.99
1.45
Insoluble
2.39
—
0
1.66
1.68
1.66
—
1.89
1.64
1.62
8.36
10.10
12.50
0
7.98
9.45
11.40
2.04
1.48
Reaction conditions: n[Ni] ¼ 5.0 ꢀ 10ꢁ6 mol; n[NB] + n[1-hexene] ¼ 0.01 mol; solvent: toluene; reaction time: 30 min; temperature: 60 ꢂC; n[B]/n[Ni]
a
¼ 10/1. Determined by GPC vs. polystyrene standards in CHCl3. In unites of 105 g molꢁ1
.
Determined by 1HNMR spectroscopy in CDCl3.
b
c
ꢁ1
salicylaldimino aromatic ring enhances the activity of the gpolymer molNi hꢁ1) did. The main reason may be that the 5-
catalyst. If the salicylaldimino aromatic ring contains the same position substituent (–CH3) of the salicylaldimino aromatic ring
electron-donating group (–CH3 or –OCH3), the 5-position has a certain steric hindrance effect to shield the active central
substituent (R2) has a higher activity than the 3-position axis surface, on the one hand, it can reduce chain transfer and
substituent (R1) (Ni2 > Ni5, Ni3 > Ni6), which most important prevent chain termination, on the other hand, it can also inhibit
reason may be that the 3-position substituent (R1) of the sali- the occurrence of ligand rearrangement and disproportionation
cylaldimino aromatic ring increased the steric hindrance effect reaction.47,50 Besides, with the 1-hexene exible chain increases,
of the catalyst, and this had little effect on the chain transfer the polymerization activity and the polymer yield decrease. The
reaction of polynorbornene. If the 3-position (R1) and 5-posi- obtained polymers had good solubility and were soluble in
tion (R2) both contain strong electron-withdrawing groups (–Br common organic solvents (such as CHCl3, CH2Cl2, cyclohexane,
or –Cl), the catalytic activity is higher than those of the 3-posi- THF) as well as chlorobenzene at ambient temperature, which
tion (R1) or the 5-position (R2) (Ni7 > Ni4), which indicated that exhibited better solubility than PNB did.
the activity of the catalyst may be the result of the synergistic
effect of the electronic and steric effects of the substituent.39,45
These results indicate that the strong electron-withdrawing
3.4 GPC curves of copolymers
group facilitates the increase of the activity of the catalyst,
and the reactivity and the yield of the polymer increase with the
electron-withdrawing ability of the salicylaldimino aromatic
ring substituent.
Gel permeation chromatography (GPC) curves of the poly(NB-
co-1-hexene)s with different 1-hexene exible chain incorpora-
tion ratios achieved by the Ni1 and Ni2/B(C6F5)3 systems are
shown in Fig. 9. GPC measurements indicated that all the
polymers had high molecular weight (Mw) ranging from 1.45 ꢀ
3.3 Copolymerization of norbornene and 1-hexene
To further investigate the effect of the catalytic activity, Ni1 and
Ni2 were applied to the copolymerization of norbornene (NB)
and 1-hexene. The detailed data were summarized in Table 4
and were collected under similar conditions. It is apparent that
the activities of Ni1 and Ni2 are signicantly lower than those
observed with norbornene homopolymerization. This may be
related to the stronger binding of 1-hexene to the Ni center and
the longer chain structure of 1-hexene relative to norbornene,
delaying the rate of chain growth compared to norbornene
homopolymerization. The incorporation levels of 1-hexene are
also different, Ni1 displayed higher incorporation ability for 1-
hexene and provided copolymers having higher 1-hexene
contents (8.6–12.50 mol%) than Ni2 (7.9–11.40 mol%) under
the same conditions. In addition, we learned that the substit-
uents on salicylaldimino aromatic ring have a profound effect
on the copolymerization reactivity of the catalyst system.46 It is
well known that the methyl (–CH3) belongs to the electron
donating group and can cause an increase in the electron cloud
density on the salicylaldimino aromatic ring, but the results in
Table 4 showed that the Ni2/B(C6F5)3 had higher activities
Fig. 9 The GPC curves of poly(NB-co-1-hexene)s with (a) 8.36, (b)
10.10, (c) 12.50, (d) 7.98, (e) 9.45 (f) 11.40 mol% 1-hexene. (a), (b) and (c)
obtained byNi1/B(C6F5)3 system, (d), (e) and (f) obtained by Ni2/
(0.79–2.40 ꢀ 105 gpolymer molꢁ1 hꢁ1) than Ni1 (0.55–2.03 ꢀ 105 B(C6F5)3 system respectively.
36308 | RSC Adv., 2018, 8, 36298–36312
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105 to 2.39 ꢀ 105 g molꢁ1 and decreased with the increase of 1-
hexene insertion rate. The distribution of molecular weight
(PDI, Mw/Mn) is very narrow in the range of 1.62–1.89. In addi-
tion, the GPC curves for all polymers are all unimodal, indi-
cating that the polymerization was initiated by a single active
site, which established that the resulting polymer is copolymers
rather than blends of homopolymers.
3.5 1H and 13C NMR spectra of copolymers
The poly(NB-co-1-hexene)s were characterized by 1H NMR
spectroscopy and the results were shown in Fig. 10, which
demonstrated that the polymers were vinyl-addition type by the
absence of the resonance of the proton hydrogen connected to
the double bond at 5.3–6.0 ppm.48 From the 1HNMR spectra of
Fig. 10, the characteristic peaks at 0.8–0.9 ppm could be
assigned to the hydrogen corresponding to H60, the character-
istic peaks at 0.9–1.6 ppm could be attributed to the hydrogen
corresponding to H5/H6/H7/H10/H30/H40/H50, the characteristic
peaks at 1.6–2.0 ppm could be attributed to the hydrogen cor-
responding to H1/H4 and those at 2.0–2.3 ppm could be
attributed to the hydrogen corresponding to H2/H3/H20. The
Fig. 11 13C NMR spectra of poly(NB-co-1-hexene)s with (a) 10.10, (b)
12.50, (c) 9.45, (d) 11.40 mol% 1-hexene. (a) and (b) obtained by Ni1/
B(C6F5)3 system, (c) and (d) obtained by Ni2/B(C6F5)3 system
respectively.
Ni1 and Ni2/B(C6F5)3 system was performed via vinyl addition
mechanism.
1
molar fractions of 1-hexene could be calculated23 by H NMR
analyses and were 24.9, 22.4 and 11.3 mol%.
The 13C NMR spectra of the poly(NB-co-1-hexene)s were
shown in Fig. 11. The absorption peaks at 14.0 ppm and
22.9 ppm were attributed to the methyl C60 and methylene C50
on the alkyl carbon chain. The absorption peaks between 45.8–
48.1 ppm and 50.3–54.3 ppm were attributed to methine C1/C4
and C2/C3 on norbornene, respectively. And the absorption
peak of the copolymer between 28.8–42.8 ppm could be attrib-
uted to C5/C6/C7/C10/C20/C30/C40. In addition, the characteristic
peak of ring-opening metathesis polymerization of norbornene
was not found at 120 ppm, which further indicated that the
copolymerization of norbornene and 1-hexene catalyzed by the
3.6 FTIR spectra of copolymers
The structures of obtained copolymers were characterized by
FTIR spectroscopy and shown in Fig. 12. All of the copolymers
obtained from the FTIR spectrum show an absorption peak at
941 cmꢁ1 could be assigned to the ring of bicyclo[2.2.1] heptane,
as Kennedy and Makowski reported.49 There were no charac-
teristic absorption peak at about 1620–1680 cmꢁ1 and 960 cmꢁ1
of the carbon–carbon double bond (C]C) of the structure of
ROMP type PNB. In addition, there were also no absorptions
peak at about 3050 cmꢁ1 and 2100 cmꢁ1 to the 1-hexene
Fig. 10 1H NMR spectra of poly(NB-co-1-hexene)s with (a) 10.10, (b) Fig. 12 FT-IR spectra of poly(NB-co-1-hexene)s with (a) 10.10, (b)
12.50, (c) 9.45, (d) 11.40 mol% 1-hexene. (a) and (b) obtained by Ni1/ 12.50, (c) 9.45, (d) 11.40 mol% 1-hexene. (a) and (b) obtained by Ni1/
B(C6F5)3 system, (c) and (d) obtained by Ni2/B(C6F5)3 system B(C6F5)3 system, (c) and (d) obtained by Ni2/B(C6F5)3 system,
respectively.
respectively.
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chain increased, the thermal decomposition temperature
decreased, but it still had good thermal stability.
3.8 WXRD analyses of the copolymers
Some supporting information about the conformation of the
poly(NB-co-1-hexene)s were gained with wide-angle X-ray
diffraction (WAXD), as shown in Fig. 14. No Bragg diffraction
appears in the spectral region of the crystal, so the polymers
were noncrystalline. Two broad halos at 2 values of 10.42–10.58ꢂ
and 18.04–18.28ꢂ were characteristic peaks for polynorbornene.
When the catalytic system is changed, the insertion rate of 1-
hexene is different, but the change of the diffraction peak is less
obvious and the chain spacing of all the polymers is not much
different. These all results shown that the packing density of the
copolymers changed little when the catalytic system changed.
The interchain distances were computed according to eqn (1)
Fig. 13 TGA curves of poly(NB-co-1-hexene)s with (a) 10.10, (b) 12.50,
(c) 9.45, (d) 11.40 mol% 1-hexene. (a) and (b) obtained by Ni1/B(C6F5)3
system, (c) and (d) obtained by Ni2/B(C6F5)3 system, respectively.
˚
and amounted to be 8.50 and 4.77 A, separately.
l
dinerchain ¼ 1:22dBragg ¼ 1:22
(1)
characteristic trans form of C]C and C–H bonds stretching. All
these results shown that norbornene and 1-hexene were
successfully copolymerized and further indicated 1-hexene had
been inserted into the copolymers chains rather than simple
blends.
2 sin q
3.9 The transparency properties of the copolymer lms
The UV-Vis curves of poly(NB-co-1-hexene)s lms with different
1-hexene insertion rate were shown in Fig. 15. The spectrum
shown that the transmittance of the copolymer lm in the
visible light range of 400–800 nm were all over 80%. Although
the transmittance of the copolymers were lower than the poly-
norbornene (92%), the copolymer still had good light trans-
mittance. In addition, as the insertion rate of 1-hexene in the
copolymer was increased, the light transmittance of the poly-
mer lms were decreased, which indicated that the addition of
1-hexene had a negative effect on the light transparency of the
copolymer lms.
3.7 TGA analyses of copolymers
The TGA curves of poly(NB-co-1-hexene) with different 1-hexene
insertion rates gained with the Ni1 and Ni2/B(C6F5)3 catalytic
system were shown in Fig. 13. The thermal properties of all the
copolymers obtained from the TGA curve were similar and all
began to decompose within the range of 350–450 C. In addi-
tion, as the insertion of the 1-hexene content in the polymer
ꢂ
Fig. 14 WXRD spectra of poly(NB-co-1-hexene)s with (a) 8.36%, (b) Fig. 15 The UV-Vis curves of poly(NB-co-1-hexene)s membranes
10.10, (c) 12.50, (d) 7.98, (e) 9.45, (f) 11.40 mol% 1-hexene. (a), (b) and (c) with (a) 10.10, (b) 12.50, (c) 9.45, (d) 11.40 mol% 1-hexene. (a) and (b)
obtained by Ni1/B(C6F5)3 system, (d), (e) and (f) obtained by Ni2/ obtained by Ni1/B(C6F5)3 system, (c) and (d) obtained by Ni2/B(C6F5)3
B(C6F5)3 system, respectively.
system, respectively.
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4. Conclusions
Eight bis-(salicylaldehyde-benzhydrylimino)nickel complexes
with different electron groups (Ni1–Ni8) were synthesized and
characterized by single crystal X-ray diffraction, and these
catalysts achieved homopolymerization of norbornene wiꢁth1
B(C6F5)3 as cocatalyst, and the activity reached 106 gPNB molNi
hꢁ1. The Ni1 and Ni2 two catalysts also displayed high activities
(up to 105 gpolymer molNiꢁ1 hꢁ1) towards the copolymerization of
norbornene and 1-hexene, and Ni2 exhibited higher activity
than Ni1 did, and the catalytic activity decreased with the
increased of the insertion rate of 1-hexene. The poly(NB-co-1-
hexene)s showed better solubility and high molecular weights
(up to 105 g molꢁ1) than that of the PNB and were conrmed to
be noncrystalline. The analyses of the structure and properties
of the copolymers showed that the polymers of norbornene and
1-hexene were copolymers rather than blends of the two. In
addition, the copolymerization of norbornene with 1-hexene
was carried out via a vinyl-addition copolymerization mode and
the poly(NB-co-1-hexene) copolymer had good thermal stability.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21674045 and 51463014).
27 E. S. M. Blaakmeer, G. Antinucci, A. Correa, V. Busico,
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