Synthesis of nickel and palladium complexes with diarylamido-based unsymmetrical pincer ligands and application for norbornene polymerization

Article abstract of DOI:10.1039/c8dt04413a

A set of diarylamido-based unsymmetrical [PNN^ox] pincer ligands containing a chiral oxazoline ring have been synthesized and their nickel and palladium complexes [(2-PPh₂(R¹)ArN(R¹)Ar-2-(R)oxazoline)MCl] (R¹ = 4-H, R = (S)-4-ⁱPr, M = Pd (Pd1); R¹ = 4-H, R = (S)-4-Bn, M = Pd (Pd2); R¹ = 4-H, R = (S)-4-Ph, M = Pd (Pd3); R¹ = 4-Me, R = (S)-4-Bn, M = Pd (Pd4); R¹ = 4-Me, R = (S)-4-Ph, M = Pd (Pd5); R¹ = 4-H, R = 4-Me₂, M = Pd (Pd6); R¹ = 4-H, R = Benzo[d]-, M = Pd (Pd7); R¹ = 4-H, R = (S)-4-Bn, M = Ni (Ni1); R¹ = 4-H, R = (S)-4-Ph, M = Ni (Ni2); R¹ = 4-Me, R = (S)-4-Bn, M = Ni (Ni3); R¹ = 4-Me, R = (S)-4-Ph, M = Pd (Ni4)) were tested to show high catalytic activities for polymerization of norbornene. After activation of methylaluminoxane (MAO), all the nickel and palladium complexes could catalyze the polymerization of norbornene to yield vinyl-type polymers with activities up to 40.3 × 10⁵ g of PNB (mol of Pd)^?1 h^?1. The copolymerization of norbornene with functional norbornene comonomers was also investigated by catalyst Pd2, accompanied by decreased catalytic activity and low incorporation of functional comonomers.

Full text of DOI:10.1039/c8dt04413a

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Synthesis of nickel and palladium complexes with
diarylamido-based unsymmetrical pincer ligands
and application for norbornene polymerization†
Cite this: Dalton Trans., 2019, 48,
609
Hui Liu,‡ Haibin Yuan‡ and Xiaochao Shi
*
A set of diarylamido-based unsymmetrical [PNNox] pincer ligands containing a chiral oxazoline ring have
been synthesized and their nickel and palladium complexes [(2-PPh2(R1)ArN(R1)Ar-2-(R)oxazoline)MCl]
(R1 = 4-H, R = (S)-4-iPr, M = Pd (Pd1); R1 = 4-H, R = (S)-4-Bn, M = Pd (Pd2); R1 = 4-H, R = (S)-4-Ph, M =
Pd (Pd3); R1 = 4-Me, R = (S)-4-Bn, M = Pd (Pd4); R1 = 4-Me, R = (S)-4-Ph, M = Pd (Pd5); R1 = 4-H, R =
4-Me2, M = Pd (Pd6); R1 = 4-H, R = Benzo[d]-, M = Pd (Pd7); R1 = 4-H, R = (S)-4-Bn, M = Ni (Ni1); R1 =
4-H, R = (S)-4-Ph, M = Ni (Ni2); R1 = 4-Me, R = (S)-4-Bn, M = Ni (Ni3); R1 = 4-Me, R = (S)-4-Ph, M = Pd
(Ni4)) were tested to show high catalytic activities for polymerization of norbornene. After activation of
methylaluminoxane (MAO), all the nickel and palladium complexes could catalyze the polymerization of
norbornene to yield vinyl-type polymers with activities up to 40.3 × 105 g of PNB (mol of Pd)−1 h−1. The
copolymerization of norbornene with functional norbornene comonomers was also investigated by cata-
lyst Pd2, accompanied by decreased catalytic activity and low incorporation of functional comonomers.
Received 6th November 2018,
Accepted 30th November 2018
DOI: 10.1039/c8dt04413a
rsc.li/dalton
ligands and their chelated pre-catalysts have rarely been repor-
Introduction
ted.5b,j,k Wu et al. reported a series of unsymmetrical tridentate
Homogeneous late-transition metal catalysts for coordination– pincer [NNS] ligands, and the corresponding nickel and palla-
insertion polymerization of olefins have attracted extensive dium complexes have proved efficient catalysts for norbornene
investigation over the last few decades.1 As representative pre- polymerization.5b Recently, Li et al. described Ni and Pd com-
catalysts, nickel and palladium complexes bearing bulky plexes bearing chiral C2-symmetrical bis(oxazoline) [NoxNNox]
α-diimine,2 salicylaldiminato3 and phosphine-sulfonate1g pincer ligands as exhibiting unique catalytic performance in
ligands have been applied to the catalysis of ethylene polymer- norbornene polymerization, even with extremely high activities
ization and copolymerization with polar monomers to obtain in the presence of air and water.5j,k
polymers with diverse micro-architectures and tailored physi-
Generally, ligands chelated to metal centres in homo-
cal properties. Some nickel and palladium complexes have geneous catalysis systems have a remarkable influence on the
also proved to be amongst the most efficient catalysts for the catalytic properties and the microstructures of the resultant
vinyl-type polymerization of norbornene with high activities.4,5 products, thus intensive efforts have been paid to the ligands
Of the nickel and palladium pre-catalysts reported for norbor- with new architectures. Tridentate pincer ligands represent a
nene polymerization, most are involved in bidentate neutral or ligand architecture in transition-metal chemistry, particularly
monoanionic ligand chelating complexes.4 Nickel and palla- from the point of view of their wide applications in catalysis.6
dium complexes based on unsymmetrical and symmetrical tri- Due to the convenient modularity of the diarylamido scaffold,
dentate [NNX] (X = N, P, S), [ONX] (X = C, N, P) ligands have derived pincer ligands, including [PNP],7 [NNN]8 and [PNN]9
also been designed and extensively tested for the polymeriz- ligands, have been reported. As a very useful supporting ligand
ation of norbornene,5 however, the tridentate “pincer” type for metals across the periodic table, the diarylamino-based
[PNP] ligand class (Fig. 1A) has revealed species with unusual
structures or reactivity.10,11 The [PNP] ligands have also proved
versatile in olefin polymerization, and their rare-earth com-
plexes could serve as excellent catalysts for the polymerization
of both isoprene and butadiene.12 The C2-symmetric tridentate
bis-(oxazolinylphenyl)amine [NoxNNox] ligand class (Fig. 1B) is
another unique scaffold which has been extensively studied in
diverse applications, of which the field of asymmetric reac-
Department of Polymer Materials, College of Materials Science and Engineering,
Shanghai University, Materials Building, Nanchen Street 333, Shanghai 200444,
China. E-mail: xcshi@shu.edu.cn
†Electronic supplementary information (ESI) available. CCDC 1854346–1854349.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt04413a
‡These authors contributed equally to this work.
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special catalytic performance and promote catalytic stereo-
selectivity of the ligated complexes in some circumstances.
We report a set of new diarylamido-based unsymmetrical
[PNNox] pincer ligands containing a chiral oxazoline-ring and
their corresponding nickel and palladium complexes. Upon
activation with MAO, these palladium complexes catalyse the
vinyl-type polymerization of norbornene to yield insoluble
polymers with high activities up to 40.3 × 105 g of PNB
(mol of Pd)−1 h−1. Meanwhile, the analogous nickel complexes
exhibited relatively lower activities, and soluble high-mole-
cular-weight polymers with moderate molecular weight distri-
butions are obtained. The copolymerization of norbornene
with functional norbornene comonomers is also studied.
Fig. 1 Typical diarylamido pincer ligands for transition-metal
complexes.
Results and discussion
tions has highlighted their versatility and tunability.13 The
polymerization of isoprene and norbornene catalysed by chiral
bis-(oxazolinylphenyl)amine ligated metal complexes has
recently been reported.5j,k,14 As we are interested in designing
new pincer ligands, we combined these two special skeletons
to obtain a series of new unsymmetrical [PNNox] pincer
ligands (Fig. 1D), which can be seen as half the bis(phosphi-
nophenyl)amido [PNP] ligand and half the bis-(oxazolinyl-
phenyl)amine [NoxNNox] ligand. Previously, Ozerov et al.
reported a similar diarylamido-based unsymmetrical [PNN]
pincer ligand (Fig. 1C) and tested the electronic properties of
their Ni, Pd, Pt and Rh complexes.9b Herein, the tunable oxazo-
line-ring skeleton can provide different electronic effects and
novel chiral surroundings to metal centres which may impart
The unsymmetrical [PNNox] pincer ligands with different
chiral oxazoline rings were synthesized as described in
Scheme 1. The one-pot condensation of 2-fluorobenzoic acid
and substituted amino alcohols afforded the fluorophenyloxa-
zolines (M) with good yield.15 The subsequent nucleophilic
aromatic substitution of fluorine in M by salt elimination with
the corresponding 2-phosphinoarylamino lithium salts in THF
gave the desired tridentate [PNNox] pincer ligands. The achiral
ligands L6 and L7 were also synthesized for comparison, and
the benzoxazole-containing ligand L7 was synthesized in a
different way according to Scheme 2. After purification by
column chromatography on silica gel, all the tridentate
[PNNox] pincer ligands were obtained as yellow solids except
L1 which was a yellow sticky oil. Various analysis methods,
Scheme 1 Synthesis of unsymmetrical pincer ligands and their palladium and nickel complexes.
Scheme 2 Synthesis of achiral ligands L7 and its palladium complex Pd7.
610 | Dalton Trans., 2019, 48, 609–617
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including 1H and 13C NMR spectra and elemental analysis,
proved the formation and purity of the ligands L1–L7, such as
the new –NH signal at 10.50 ppm in the proton spectrum reso-
nance spectroscopy of L1 indicating the coupling of the reac-
tants. Previous studies concerning the synthesis of anilido-
oxazolines always involved the Pd-catalysed aryl amination,16
and the approach involving lithium salt-elimination described
here provides an alternative route to prepare these analogous
compounds.
Having the unsymmetrical tridentate [PNNox] pincer
ligands in our hands, we tried to synthesize their corres-
ponding nickel and palladium complexes. According to pre-
vious reports, the [PNNox] pincer ligands in this study are
ideally suitable to support various square-planer complexes of
the general formula (PNNox)MCl (M = Ni, Pd).5 Firstly, the
treatments of the lithium salts of the ligands with (COD)PdCl2
in THF at RT or 70 °C gave dark red powders, however, impuri-
ties were observed in both the cases determined by 1H NMR
spectroscopy. However, the side reactions could be suppressed
using toluene as solvent enabling pure palladium complexes
to be obtained. In addition, the direct reactions of the neutral
[PNNox] pincer ligands with (COD)PdCl2 in toluene at 70 °C
could also afford the desired palladium complexes in pure
form, and the addition of a base such as NEt3 was necessary to
remove the HCl by-product during the formation of (PNNox)
PdCl, providing an optimal pathway to synthesize these analo-
gous complexes. Similarly, the reaction of selected chiral
[PNNox] pincer ligands with anhydrous NiCl2 in the presence
of NEt3 produced the corresponding pure (PNNox)NiCl
(Scheme 1). All the nickel and palladium complexes were fully
characterized by 1H, 13C NMR spectroscopy. However, due to
difficulties in complete solvent removal from the complexes,
we could not obtain satisfactory elemental analysis data. The
disappearance of the –NH signals at around 10–11 ppm in the
1H NMR spectra indicated the formation of the coordinated
complexes. All the resulting nickel and palladium complexes
are soluble in common organic solvents such as toluene and
dichloromethane, and stable in oxygen and moisture-contain-
ing environments.
Fig. 2 Molecular structure of Pd1 (thermal ellipsoids at the 30% prob-
ability level). Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (°): Pd(1)–N(1) 2.0166(19), Pd(1)–N(2)
2.0885(19), Pd(1)–P(1) 2.2134(11), Pd(1)–Cl(1) 2.3121(12); N(1)–Pd(1)–N(2)
87.68(7), N(1)–Pd(1)–P(1) 84.18(6), N(2)–Pd(1)–P(1) 170.35(5), N(1)–Pd(1)–Cl(1)
177.50(5), N(2)–Pd(1)–Cl(1) 94.08(6), P(1)–Pd(1)–Cl(1) 94.21(3).
Fig. 3 Molecular structure of Pd2 (thermal ellipsoids at the 30% prob-
ability level). Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å) and angles (°): Pd(1)–N(1) 2.022(3), Pd(1)–N(2)
2.063(3), Pd(1)–P(1) 2.213(2), Pd(1)–Cl(1) 2.312(2); N(1)–Pd(1)–N(2)
89.28(11), N(1)–Pd(1)–P(1) 84.84(8), N(2)–Pd(1)–P(1) 171.72(8), N(1)–Pd(1)–
Cl(1) 178.14(8), N(2)–Pd(1)–Cl(1) 92.36(9), P(1)–Pd(1)–Cl(1) 93.61(4).
Single crystals of the complexes Pd1, Pd2, Pd6 and Pd7 suit-
able for X-ray structure determination were obtained from slow
evaporation of their CH2Cl2/hexane solutions at room tempera-
ture. The corresponding ORTEP diagrams are presented in
Fig. 2–5 along with selected bond lengths and bond angles,
respectively. Molecular structure analyses revealed that all the
palladium complexes adopt an approximately square-planar
geometry around the metal centre with coordinated P, N, N
and a chloride atom. Due to the similar structures and coordi-
nation environments, the palladium complexes resemble each
other in bond lengths and bond angles, nevertheless, the role
Fig. 4 Molecular structure of Pd6 (thermal ellipsoids at the 30% prob-
ability level). Hydrogen atoms have been omitted for clarity. Selected bond
distances (Å) and angles (°): Pd(1)–N(1) 2.010(2), Pd(1)–N(2) 2.095(2), Pd(1)–
P(1) 2.1977(10), Pd(1)–Cl(1) 2.3123(10); N(1)–Pd(1)–N(2) 88.73(9), N(1)–
Pd(1)–P(1) 84.61(7), N(1)–Pd(1)–P(1) 166.98(6), N(1)–Pd(1)–Cl(1) 173.56(6),
N(2)–Pd(1)–Cl(1) 96.68(7), P(1)–Pd(1)–Cl(1) 90.74(5).
of the chiral substituents cannot be ignored. The bond lengths 92.36(9)°, smaller than the corresponding angles in Pd6 and
of Pd(1)–N(2) are 2.0885(19) Å (Pd1) and 2.063(3) Å (Pd2), are Pd7 (96.68(7)° and 97.62(11)°, respectively). The N(1)–Pd(1)–
not as much as those in achiral Pd6 (2.095(2) Å) and Pd7 Cl(1) angles are 177.50(5)° (Pd1) and 178.14(8)° (Pd2) are
(2.109(3) Å). The chiral substituents have a significant influ- notably larger than the corresponding angles in Pd6
ence on the bond angles while involving the chloride atom. (173.56(6)°) and Pd7 (173.16(9)°), indicating that the metal
The N(2)–Pd(1)–Cl(1) angles in Pd1 and Pd2 are 94.08(6)° and centre and the coordinated atoms in chiral palladium com-
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species, to maintain active catalyst precursors as well as to
form catalytically active cation–anion ion pairs.18 In the
absence of MAO, it was unsurprising that the chloride complex
of Pd2 was inactive for the polymerization of norbornene due
to the lack of a cationic M+–C centre for initiation and propa-
gation.18a The catalytic activity of Pd2 increased sharply from
2.2 to 13.0 × 105 g of PNB (mol of Pd)−1 h−1 along with the [Al]/
[Pd] from 3000 increasing to 5000 (entries 2 and 3 in Table 1).
When the ratio of [Al]/[Pd] reached 7000, the highest catalytic
ability, 26.5 × 105 g of PNB (mol of Pd)−1 h−1, was obtained
Fig. 5 Molecular structure of Pd7 (thermal ellipsoids at the 30% prob-
ability level). Hydrogen atoms have been omitted for clarity. Selected (entry 4 in Table 1). The extreme excess of MAO needed in the
bond distances (Å) and angles (°): Pd(1)–N(1) 2.032(3), Pd(1)–N(2)
2.109(3), Pd(1)–P(1) 2.1919(15), Pd(1)–Cl(1) 2.3118(17); N(1)–Pd(1)–N(2)
89.06(14), N(1)–Pd(1)–P(1) 83.56(11), N(1)–Pd(1)–P(1) 172.12(10), N(1)–
Pd(1)–Cl(1) 173.16(9), N(2)–Pd(1)–Cl(1) 97.62(11), P(1)–Pd(1)–Cl(1)
polymerization might be mainly attributed to scavenging
impurities and regenerating the active species deactivated by
transformation/elimination in the system.18,19 According to
previous reports, the polymerization of norbornene possibly
proceeded through a bimetallic mechanism (Scheme S1†).5j,18
When MAO was replaced by a borate ca. ([Ph3C][B(C6F5)4] or
[PhMe2NH][B(C6F5)4] and a borane B(C6F5)3) as a co-catalyst,
no polymers were obtained,5j even at a prolonged polymeriz-
ation time of 600 minutes (entry 11 in Table 1). We reason that
the formation of active cationic species for polymerization
does not operate due to the absence of an initial M–C bond in
Pd2.18a,20 The reaction temperature is another significant para-
meter to control the polymerization process besides the usage
of MAO. When the norbornene polymerization was conducted
by the Pd2/MAO system at 0 °C, only a 12.3% yield of PNB was
obtained in 10 minutes with a catalytic activity of 4.9 × 105 g of
PNB (mol of Pd)−1 h−1 (entry 6 in Table 1). Notable increases
in polymer yield and catalytic activity were observed when the
temperature increased to 25 °C, and the activity soared to 26.5
× 105 g of PNB (mol of Pd)−1 h−1 (entry 4 in Table 1). However,
when the temperature exceeded 80 °C, a significant decrease
ca. 50% in catalytic activity compared to that at 25 °C was
found, implying the importance of an appropriate polymeriz-
ation temperature for this Pd2/MAO system. It is not difficult
to understand that the yields of PNB increased along with the
polymerization time, while the catalytic activities of Pd2
89.84(6).
plexes are more approaching one perfect square-plane. Due to
the different coordination effects of P and N atoms in the
unsymmetrical palladium complexes,9b longer Pd–N (oxazo-
line) (2.063(3)–2.109(3) Å) and shorter Pd–P (2.1919(15)–
2.2134(11) Å) bond lengths are found compared to those in
related symmetrical [NoxNNox]PdCl (around 1.996(6) Å) and
[PNP]PdCl (2.3010(18) Å).5j,17 The remaining parameters are
unexceptional with typical ligand–metal distances and angles
as previously reported.
Norbornene polymerization by unsymmetrical palladium and
nickel complexes
Under the co-activation of MAO, all the palladium complexes
could catalyse the polymerization of norbornene efficiently to
afford insoluble vinyl-type polymers, and the corresponding
nickel complexes exhibited poor catalytic performances. To
clarify the optimal reaction parameters in the polymerization,
Pd2 was studied in detail as catalytic precursor, and the repre-
sentative results are collected in Table 1. As an essential part
for coordination polymerization, the cocatalyst ca. methyl-
aluminoxane (MAO) plays an important role to form an active
decreased (from 40.3 to 14.3 × 105 g of PNB (mol of Pd)−1 h−1
)
due to the viscosity of the converted PNB slowing down the
catalytic rate. The obtained polymers were insoluble in ordin-
ary solvents, even in boiling 1,2-dichlorobenzene. The in-
soluble polymers prevented us from obtaining detailed infor-
mation of the molecular weight by high temperature gel-per-
meation chromatography (HT-GPC) and chemical structures by
NMR spectroscopy. Characterization of all the obtained in-
soluble PNBs by IR spectroscopy showed very similar signals,
and no signals at 1620–1680, 966 and 755 cm−1 indicating the
polymers were of the vinyl-addition type (Fig. S37†).
The nickel and palladium complexes bearing different
unsymmetrical [PNNox] pincer ligands with various chiral and
achiral substituents and skeletons have been applied in nor-
bornene polymerization to study the structure–reactivity
relationships. The norbornene polymerization results of Pd1–
Pd7 and Ni1–Ni4 are presented in Table 2. Under the optimal
conditions established by the Pd2/MAO system, all the palla-
dium complexes showed high catalytic activity for the polymer-
Table 1 Results of norbornene polymerization with Pd2/cocatalysts
systemsa
Entry
T (°C)
co-Cat.
[Al]/[Pd]
t (min)
Yield (%)
Activityc
1
2
3
4
5
6
7
8
25
25
25
25
25
0
50
80
25
25
25
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
A/B/Cb
1000
3000
5000
7000
10 000
7000
7000
7000
7000
7000
10
10
10
10
10
10
10
10
10
5
Trace
5.4
—
2.2
32.4
66.2
56.5
12.3
65.8
35.2
50.4
71.5
—
13.0
26.5
22.6
4.9
26.3
14.1
40.3
14.3
—
9
10
11
20
600
a Polymerization conditions: Complex, 1.5 μmol, toluene, Vtotal 10 mL,
norbornene 1 g. b Activator: [Ph3C][B(C6F5)4] (A), [PhMe2NH][B(C6F5)4]
(B), borane B(C6F5)3 (C). c In units of 105 g of PNB (mol of Pd)−1 h−1
.
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Table 2 Results of norbornene polymerization with palladium and nickel complexes/cocatalysts systemsa
R1–R
co-Cat.
[Al]/[Pd]
t/min
Yield/%
Activityb
Mnc/ × 105
Mw/Mn
c
Entry
Cat.
1
2
3
4
5
6
7
8
Pd1
Pd2
Pd3
Pd4
Pd5
Pd6
Pd7
Ni1
Ni2
Ni3
Ni4
H–iPr
H–Bn
H–Ph
Me–Bn
Me–Ph
H–Me2
H–benzoxazole
H–Bn
H–Ph
Me–Bn
Me-Ph
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
7000
7000
7000
7000
7000
7000
7000
7000
7000
7000
7000
10
10
10
10
10
10
10
60
86.2
66.2
43.1
28.6
6.8
11.2
22.6
22.2
21.8
9.2
34.5
26.5
17.2
11.4
2.7
4.5
9.0
1.5
0.82
1.44
1.24
1.56
1.84
2.16
1.62
1.56
9
10
11
720
270
300
0.12
0.14
0.19
14.2
a Polymerization conditions: Complex, 1.5 μmol, toluene, Vtotal 10 mL, norbornene 1 g, 25 °C. b In units of 105 g of PNB (mol of Pd)−1 h−1
.
c Determined by GPC against polystyrene standard at 150 °C in 1,2,4-trichlorobenzene.
ization of norbornene to give insoluble polymers. When the groups.21 Unfortunately, when polar monomers 2-acetyl-5-nor-
substituent R1 was H, the complex Pd1 showed the highest bornene (ANB) and 5-norbornene-2-methanol (NBM) were
activity of 34.5 × 105 g of PNB (mol of Pd)−1 h−1 (entry 1 in used, no copolymers could be isolated. The copolymerization
Table 2), and the catalytic activity decreased in the order of of norbornene with an alkenyl monomer, such as dicyclopen-
Pd1 > Pd2 > Pd3 > Pd7 > Pd6. The steric hindrance and elec- tadiene (DCPD) or 5-vinyl-2-norbornene (VNB) have been con-
tronic effect of the ligand have a crucial influence on the cata- ducted with low catalytic activity. When the ratio of VNB or
lytic performance. The relatively lower activity of achiral Pd6 DCPD to NB was 1 : 10, the activities of Pd2 were 0.26 and 0.19
with two methyl groups substituted on the oxazoline ring × 105 g of PNB (mol of Pd)−1 h−1, respectively (Table S1†).
showed the negative effect of a bulky hindrance. The catalytic However, the incorporation of comonomers in the obtained
activity order of Pd2 > Pd3 > Pd7 clearly proves that the conju- copolymers were rather low (ca. 1%). Increasing the ratio of
gated structures or electron-donating substituents were VNB/NB to 1 : 5, the catalytic activity of Pd2 dropped to 0.11 ×
unfavourable to high catalytic activity. When the R1 position 105 g of PNB (mol of Pd)−1 h−1, and the incorporation of VNB
was substituted by a methyl group, the palladium complex Pd4 was around 11.6% (see Fig. S38†) in the copolymer (Mn 2.28 ×
exhibited a relatively lower activity of 11.4 × 105 g of PNB 104 g mol−1, Mw/Mn 1.89). Further increasing the feed of VNB/
(mol of Pd)−1 h−1 (entry 5 in Table 2) compared to Pd2, and NB to 1 : 1 gave only trace copolymer.
the same trend was also found in Pd6 with its counterpart
Pd3. These results further proved the disadvantageous
influences of electron-donating groups.
Conclusions
The nickel complexes bearing selected chiral ligands were
also synthesized and tested for polymerization of norbornene A series of new diarylamido-based unsymmetrical [PNNox]
under similar conditions. However, all the chiral nickel com- pincer ligands were synthesized incorporating various substi-
plexes showed much lower catalytic activity compared to their tuents to adjust the steric and electronic properties. The
corresponding palladium complexes (entries 8–11 in Table 2), corresponding nickel and palladium complexes could be
but soluble polymers were obtained and characterized by obtained easily in high yield by the reaction of (COD)PdCl2 or
HT-GPC. The complex Ni1 exhibited an activity of 1.5 × 105 g anhydrous NiCl2 with the unsymmetrical [PNNox] pincer
of PNB (mol of Ni)−1 h−1 with a molecular weight Mn 0.82 × ligands in the presence of NEt3 at high temperature. Upon acti-
105 g mol−1 (entry 8 in Table 2), while the corresponding palla- vation with MAO, the palladium complexes served as promis-
dium complex Pd2 possessed nearly 20 times higher catalytic ing catalysts for vinyl-type polymerization of norbornene with
activity. When Ni3, the R1 = methyl counterpart of Ni1, was an activity up to 40.3 × 105 g of PNB (mol of Pd)−1 h−1, while
used as a catalytic precursor, only 9.2% norbornene was finally the corresponding chiral nickel complexes exhibited relatively
converted to polymer after 270 minutes, that was 0.14 × 105 g lower activities. It has been demonstrated that the steric
of PNB (mol of Ni)−1 h−1 in activity (entry 10 in Table 2). The hindrance and electronic effect of the ligands have a signifi-
influence of steric hindrance and electronic effect were also cant influence on the complexes and the consequent catalytic
undoubtedly important in nickel-catalysed norbornene properties. The Pd2/MAO system could also promote the
polymerization.
copolymerization of alkenyl NB derivatives (DCPD and VNB)
Direct copolymerization of olefin with a special functiona- with norbornene to give copolymers with low comonomer
lized monomer represented a simple but challenging approach incorporation. The study of these unsymmetrical [PNNox]
to improve the properties of the original polyolefin. Based on pincer ligand coordinated complexes with different metal
the Pd2/MAO catalytic system, we turned our focus on prepar- centres and their application to olefin polymerization is
ing copolymers of norbornene possessing valuable functional currently in progress.
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CH3). 13C NMR (CDCl3, 101 MHz, ppm): δ 163.2, 146.4, 145.3,
145.1, 137.3, 137.2, 137.1, 137.0, 134.4, 134.1, 133.9, 132.8,
Experimental
All the operations were carried out under a pure nitrogen 132.7, 131.6, 129.7, 129.6, 128.7, 128.6, 128.5, 124.4, 124.3,
atmosphere using standard Schlenk techniques. 116.9, 113.7, 110.7, 72.6, 68.2, 32.6, 19.1, 18.0. Anal. Calc. for
Tetrahydrofuran (THF), hexane, and toluene were distilled C30H29N2OP: C 77.57, N 6.03, H 6.29; Found: C 77.71, N 6.92,
from sodium-benzophenone. Dichloromethane was distilled H 5.41.
1
from calcium hydride. Commercial reagents, namely nBuLi,
L2 White solids (3.03 g, 61%). H NMR (CDCl3, 400 MHz,
methylaluminoxane (MAO), 2-fluorobenzoic acid, (S)-2-amino- ppm): δ 10.36 (s, 1H, Ph–NH–), 7.72–6.72 (m, 23H, Ph–H), 4.25
3-methylbutan-1-ol, PPh3, benzoxazole, and 2-fluorobenz- (dd, 1H, –N–CH–), 4.10 (m, 1H, –O–CH2–), 3.95(dd, 1H,
aldehyde were purchased from TCI and used without further –O–CH2–), 2.75 (m, 1H, Ph–CH2–), 2.39 (m, 1H, Ph–CH2–).
purification. The (COD)PdCl2 complexes were synthesized 13C NMR (CDCl3, 101 MHz, ppm): δ 163.8, 146.4, 145.1, 144.9,
according to the literature. Other commercially available 138.1, 137.4, 137.3, 137.2, 137.1, 134.4, 134.3, 134.2, 134.1,
reagents were purchased and used without purification. 134.0, 131.9, 129.8, 129.7, 129.4, 128.8, 128.7, 128.6, 126.5,
1H (400 MHz) and 13C NMR (101 MHz) measurements were 124.6, 124.5, 124.4, 117.1, 113.9, 110.6, 69.8, 67.6, 41.4. Anal.
obtained on a JNM-ECZ400S/L 400 MR spectrometer in CDCl3 Calc. for C34H29N2OP: C 79.67, N 5.47, H 5.70; Found: C 79.11,
solution (25 °C) or in o-dichlorobenzene-d4 (120 °C). The FT-IR N 5.41, H 5.77.
1
spectra were recorded on a NICOLET iS50 FT-IR spectrometer
L3 White solids (2.59 g, 52%). H NMR (CDCl3, 400 MHz,
in the range of 4000–400 cm−1 by using KBr pellets. The ppm): δ 10.53 (s, 1H, Ph–NH–), 7.95–6.83 (m, 23H, Ph–H), 5.21
molecular weight and molecular weight distribution of the (m, 1H, –N–CH–), 4.65 (dd, 1H, –O–CH2–), 4.12 (m, 1H,
polymers were measured by means of gel-permeation chrom- –O–CH2–). 13C NMR (CDCl3, 101 MHz, ppm): δ 165.0, 146.7,
atography (GPC) on a PL-GPC 220 type high-temperature 145.0, 144.8, 142.9, 137.2, 137.1, 136.9, 136.8, 134.5, 134.4,
chromatograph equipped with three PL-gel 10 μm Mixed-B LS 134.2, 134.0, 133.8, 133.0, 132.9, 132.2, 130.2, 129.7, 128.8,
type columns at 150 °C.
128.7, 128.6, 128.5, 128.4, 127.4, 126.8, 124.6, 117.2, 114.1,
110.5, 73.3, 69.9. Anal. Calc. for C33H27N2OP: C 79.50, N 5.62,
H 5.46; Found: C 79.08, N 5.41, H 5.64.
Synthesis of the ligands
1
To
a
solution of (S)-2-amino-3-methylbutan-1-ol (2.67 g,
L4 White solids (3.13 g, 58%). H NMR (CDCl3, 400 MHz,
30 mmol) in acetonitrile–pyridine mixture containing 2-fluoro- ppm): δ 9.95 (s, 1H, Ph–NH–), 7.46–6.62 (m, 21H, Ph–H), 4.19
benzoic acid (4.20 g, 30 mmol) was successively added (m, 1H, –N–CH–), 4.04 (m, 1H, –O–CH2–), 3.89 (m, 1H,
perchloromethane (8.7 mL, 90 mmol) and triethylamine –O–CH2–), 2.66 (d, 1H, Ph–CH2–), 2.32 (m, 1H, Ph–CH2–), 2.21
(12.5 mL, 90 mmol) at room temperature. After stirring for (s, 3H, CH3–Ph), 2.19 (s, 3H, CH3–Ph). 13C NMR (CDCl3,
30 min, the PPh3 dissolved in acetonitrile–pyridine mixture 101 MHz, ppm): δ 163.8, 144.6, 142.9, 142.6, 138.2, 137.6,
was added to the reaction dropwise. The reaction mixture was 137.5, 137.4, 137.3, 134.5, 134.2, 134.1, 134.0, 133.9, 133.7,
stirred at room temperature for at least 24 h. The solvent was 132.8, 132.2, 132.1, 130.5, 129.7, 129.4, 128.7, 128.6, 128.5,
evaporated and extracted with ether, after being filtered and 126.4, 125.8, 124.5, 124.4, 114.0, 110.2, 69.8, 67.6, 41.4, 21.2,
washed with saturated copper sulfate (2 × 100 mL) and satu- 20.5. Anal. Calc. for C36H33N2OP: C 79.98, N 5.18, H 6.15;
rated salt water (2 × 100 mL), a yellow ether solution was Found: C 79.48, N 5.06, H 6.25.
obtained. The ether solution was dried over anhydrous Na2SO4
L5 Yellow oil (2.79 g, 53%). 1H NMR (CDCl3, 400 MHz,
and evaporated to yield a yellow oil. The residue was purified ppm): δ 9.93 (s, 1H, Ph–NH–), 7.56–6.81 (m, 21H, Ph–H), 5.05
by passing through a silica gel column with n-hexane/EtOAc (s, 1H, –N–CH–), 4.54 (s, 1H, –O–CH2–), 3.98 (d, 1H, –O–CH2–),
(10 : 1) as eluent to give the corresponding M.
2.23 (s, 3H, CH3–Ph), 2.17 (s, 3H, CH3–Ph). 13C NMR (CDCl3,
A solution of nBuLi (6.87 mL, 11 mmol, 1.6 M in hexane) 101 MHz, ppm): δ 164.9, 144.9, 142.9, 137.4, 137.3, 137.0, 136.9,
was added dropwise over 10 min to a solution of 2-(diphenyl- 134.7, 134.5, 134.3, 134.1, 133.9, 133.1, 130.5, 129.9, 129.0, 128.8,
phosphino)benzenamine (2.77 g, 10 mmol) in 10 mL dry THF 128.6, 128.5, 128.4, 128.1, 127.3, 126.0, 125.8, 124.7, 124.6, 114.2,
at −78 °C. After stirring for 4 h at room temperature, the 110.1, 73.2, 69.9, 21.3, 20.6. Anal. Calc. for C35H33N2OP: C 79.83,
lithium solution was transferred to the solution of
(12 mmol) in dry THF at −78 °C. The final reacted system was
M
N 5.32, H 5.93; Found: C 79.80, N 5.18, H 6.05.
1
L6 White solids (2.16 g, 48%). H NMR (CDCl3, 400 MHz,
slowly warmed to room temperature and then stirred for 48 h ppm): δ 10.43 (s, 1H, Ph–NH–), 7.72–6.70 (m, 18H, Ph–H), 3.92
at 70 °C. The solvent was removed in vacuum and a large quan- (m, 2H, –O–CH2–), 1.14–0.99 (m, 6H, –C–CH3). 13C NMR (CDCl3,
tity of deionized water was added. The aqueous phases were 101 MHz, ppm): δ 161.7, 146.2, 145.4, 145.1, 137.4, 137.3, 134.6,
extracted with EtOAc (4 × 100 mL), and the organic phases 134.0, 133.9, 133.8, 132.0, 131.9, 131.6, 129.6, 129.5, 128.6, 128.5,
were dried over anhydrous Na2SO4. The residue was purified 124.2, 117.0, 113.8, 110.9, 67.8, 28.3. Anal. Calc. for C29H27N2OP:
by passing through a silica gel column with hexane/EtOAc as C 77.31, N 6.22, H 6.04; Found: C 77.13, N 5.93, H 6.35.
eluent to give the desired L1 as a yellow oil (1.86 g, 40%).
Synthesis of L7
1H NMR (CDCl3, 400 MHz, ppm): δ 10.50 (s, 1H, Ph–NH–),
7.73–6.70 (m, 18H, Ph–H), 4.23 (m, 1H, –N–CH–), 3.98 (m, 2H, To a solution of benzoxazole (7.14 g, 60 mmol) and 2-fluoro-
–O–CH2–), 1.61 (m, 1H, –CH–CH3), 0.93–0.82 (dd, 6H, –CH– benzaldehyde (3.72 g, 30 mmol) in PhCl (30 mL) was added
614 | Dalton Trans., 2019, 48, 609–617
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dry DMF (3 mL) at room temperature. The crude reaction
Pd4 Red powder. 1H NMR (CDCl3, 400 MHz, ppm): δ
mixture was filtered under nitrogen, followed by the addition 7.95–6.79 (m, 21H, Ph–H), 5.19 (s, 1H, –N–CH–), 4.39 (d, 2H,
of I2 (15.24 g, 60 mmol), and then stirred for 48 h at 130 °C. –O–CH2–), 3.72 (d, 1H, Ph–H), 2.90 (d, 1H, Ph–H), 2.20 (d, 3H,
The solution was cooled to room temperature and 60 mL CH3–Ph), 2.17 (d, 3H, CH3–Ph). 13C NMR (CDCl3, 101 MHz,
saturated Na2S2O3 added. After extraction with EtOAc ppm): δ 163.5, 163.4, 162.0, 161.8, 151.8, 137.2, 134.3, 134.1,
(4 × 100 mL) and drying over anhydrous Na2SO4, the solution 133.7, 133.3, 133.0, 132.9, 131.5, 131.3, 130.6, 130.3, 129.9,
was evaporated to yield a brown oil. The residue was purified 129.1, 129.0, 128.9, 128.7, 128.6, 128.3, 126.8, 126.2, 122.4,
by passing through a silica gel column with hexane/EtOAc 121.9, 121.0, 120.8, 119.6, 114.4, 72.5, 64.3, 42.0, 20.4, 20.3.
as eluent, giving the corresponding 2-(2-fluorophenyl)
benzoxazole.
Pd5 Red powder. 1H NMR (CDCl3, 400 MHz, ppm):
δ 7.85–6.72 (m, 21H, Ph–H), 6.03 (dd, 1H, –N–CH–), 4.86 (t, 1H,
The ligand L7 was synthesized in a similar way as for the –O–CH2–), 4.57 (dd, 1H, –O–CH2–), 2.20 (m, 3H, CH3–Ph), 2.16
synthesis of ligand L1 with the usage of 2-(2-fluorophenyl)ben- (m, 3H, CH3–Ph). 13C NMR (CDCl3, 101 MHz, ppm): δ 163.9,
zoxazole. Pure products were obtained as white solids by 163.8, 161.8, 161.6, 152.2, 142.1, 134.3, 134.2, 133.9, 133.2,
column chromatography with hexane/EtOAc as eluent (1.55 g, 133.1, 132.8, 132.7, 131.4, 131.3, 131.1, 131.0, 130.7, 130.3,
33%) 1H NMR (CDCl3, 400 MHz, ppm): δ 10.01 (s, 1H, 129.8, 129.1, 128.9, 128.6, 128.5, 128.4, 127.8, 127.3, 126.1,
Ph–NH–), 8.08–6.79 (m, 22H, Ph–H). 13C NMR (CDCl3, 121.0, 120.8, 119.6, 114.2, 66.1, 20.4, 20.3.
101 MHz, ppm): δ 162.7, 149.2, 145.9, 144.5, 144.3, 137.0,
Pd6 Red powder. 1H NMR (CDCl3, 400 MHz, ppm):
136.9, 134.4, 134.2, 134.0, 133.7, 133.6, 132.2, 129.8, 128.9, δ 7.90–6.61 (m, 18H, Ph–H), 4.32 (d, 1H, –O–CH2–), 4.13 (d,
128.8, 128.7, 128.6, 125.6, 125.5, 125.1, 124.7, 124.1, 119.8, 1H, –O–CH2–), 1.80 (d, 3H, –C–CH3), 1.70 (d, 3H, –C–CH3). 13C
117.7, 114.3, 110.2, 110.0. Anal. Calc. for C31H23N2OP: C 79.14, NMR (CDCl3, 101 MHz, ppm): δ 163.7, 163.5, 153.3, 134.3,
N 5.95, H 4.93; Found: C 78.89, N 5.69, H 5.13.
134.2, 133.7, 132.9, 132.8, 132.1, 131.5, 131.4, 131.3, 131.2,
130.1, 129.6, 129.1, 128.9, 128.6, 128.5, 123.2, 122.6, 121.4,
121.2, 119.8, 119.7, 117.5, 117.4, 29.0, 28.1.
Synthesis of the metal complexes
To a solution of (COD)PdCl2 (57.1 mg, 0.2 mmol) or NiCl2
Pd7 Red powder. 1H NMR (CDCl3, 400 MHz, ppm):
(25.9 mg, 0.2 mmol) in dry toluene containing the ligand δ 8.79–6.69 (m, 22H, Ph–H). 13C NMR (CDCl3, 101 MHz, ppm):
(0.2 mmol) was added triethylamine (0.03 mL, 0.22 mmol) at δ 163.4, 153.5, 134.2, 134.1, 133.7, 133.0, 132.9, 132.6, 132.4,
room temperature, and the mixture was stirred for 5 h at 131.8, 131.7, 131.6, 131.5, 130.3, 129.2, 129.1, 128.8, 128.6,
70 °C. The reaction mixture was filtered under nitrogen and 125.5, 125.2, 124.5, 122.0, 121.8, 121.2, 120.5, 120.4, 120.2,
the filtrate was concentrated under reduced pressure to ca. 118.2, 115.9, 110.3.
2 mL, and then dry hexane (15 mL) was added. Solvent was
Ni1 Green powder. 1H NMR (CDCl3, 400 MHz, ppm):
removed from the precipitate via cannula filtration, and the δ 7.91–6.53 (m, 23H, Ph–H), 4.86 (m, 1H, –N–CH–), 4.30 (m,
residue was washed with n-hexane. Recrystallization of the 2H, –O–CH2–), 3.83 (m, 1H, Ph–H), 3.05 (dd, 1H, Ph–H).
crude product from CH2Cl2/hexane gave the desired complex.
13C NMR (CDCl3, 101 MHz, ppm): δ 163.8, 146.3, 145.0, 144.8,
Pd1 Red powder. 1H NMR (CDCl3, 400 MHz, ppm): δ 138.1, 137.3, 137.2, 137.1, 137.0, 134.3, 134.2, 134.1, 134.0,
7.87–6.56 (m, 18H, Ph–H), 4.84 (s, 1H, –N–CH–), 4.43 (d, 2H, 133.9, 132.4, 132.3, 131.8, 131.1, 131.0, 129.8, 129.7, 129.4,
–O–CH2–), 2.60 (s, 1H, –CH–CH3), 1.28 (d, 3H, –CH–CH3), 1.24 128.7, 128.6, 128.5, 126.4, 124.4, 117.0, 113.9, 110.6, 69.8, 67.6,
(d, 3H, –CH–CH3). 13C NMR (CDCl3, 101 MHz, ppm): δ 163.2, 41.3.
146.4, 145.3, 145.1, 137.3, 137.2, 137.1, 137.0, 134.4, 134.1,
Ni2 Green powder.1H NMR (CDCl3, 400 MHz, ppm):
133.9, 133.7, 132.8, 132.7, 131.6, 129.7, 129.6, 128.8, 128.6, δ 7.76–6.47 (m, 23H, Ph–H), 5.73 (dd, 1H, –N–CH–), 4.82 (dd,
128.5, 128.4, 124.4, 116.9, 113.6, 110.7, 72.6, 68.2, 32.6, 19.1, 1H, –O–CH2–), 4.47 (dd, 1H, –O–CH2–). 13C NMR (CDCl3,
18.0. Due to the residual solvent, we were unable to obtain sat- 101 MHz, ppm): δ 164.5, 144.7, 145.5, 142.6, 134.9, 134.8,
isfactory elemental analysis data.
133.2, 132.0, 131.9, 131.8, 131.7, 131.6, 131.5, 131.4, 131.3,
Pd2 Red powder. 1H NMR (CDCl3, 400 MHz, ppm): δ 131.2, 129.6, 128.6, 128.5, 128.4, 128.3, 128.2, 128.0, 127.4,
7.92–6.56 (m, 23H, Ph–H), 5.16 (m, 1H, –N–CH–), 4.40 (m, 2H, 126.7, 124.2, 124.0, 117.6, 114.0, 110.9, 73.4, 69.8.
–O–CH2–), 3.72 (d, 1H, Ph–H), 2.85 (d, 1H, Ph–H). 13C NMR
Ni3 Green powder. 1H NMR (CDCl3, 400 MHz, ppm):
(CDCl3, 101 MHz, ppm): δ 163.9, 163.7, 163.5, 153.6, 137.1, δ 7.94–6.72 (m, 21H, Ph–H), 4.88 (dd, 1H, –N–CH–), 4.33 (m,
134.2, 134.1, 133.9, 132.9, 132.8, 132.5, 132.1, 131.5, 131.3, 2H, –O–CH2–), 3.86 (dd, 1H, Ph–H), 3.08 (dd, 1H, Ph–H), 2.16
131.1, 129.8, 129.2, 129.1, 129.0, 128.8, 128.6, 128.3, 126.8, (m, 6H, CH3–Ph). 13C NMR (CDCl3, 101 MHz, ppm): δ 162.7,
121.2, 121.1, 120.0, 119.9, 119.8, 117.4, 72.5, 64.3, 42.0.
161.2, 160.9, 151.6, 137.3, 134.1, 134.0, 133.1, 132.9, 132.5,
Pd3 Red powder. 1H NMR (CDCl3, 400 MHz, ppm): 132.4, 130.9, 129.9, 129.5, 129.4, 129.2, 129.1, 129.0, 128.8,
δ 7.85–6.59 (m, 23H, Ph–H), 6.01 (m, 1H, –N–CH–), 4.87 (m, 128.6, 128.5, 128.3, 126.8, 125.5, 125.4, 124.6, 124.1, 120.8,
1H, –O–CH2–), 4.58 (dd, 1H, –O–CH2–). 13C NMR (CDCl3, 120.7, 120.6, 114.2, 71.9, 62.7, 42.3, 20.4, 20.3.
101 MHz, ppm): δ 164.0, 163.7, 163.5, 154.1, 142.0, 134.3,
Ni4 Green powder. 1H NMR (CDCl3, 400 MHz, ppm):
134.2, 133.8, 132.8, 132.7, 131.9, 131.5, 131.4, 131.2, 129.9, δ 7.76–6.65 (m, 21H, Ph–H), 5.73 (dd, 1H, –N–CH–), 4.80 (t, 1H,
129.1, 129.0, 128.9, 128.6, 128.5, 127.9, 127.3, 123.0, 122.4, –O–CH2–), 4.45 (dd, 1H, –O–CH2–), 2.20 (m, 3H, CH3–Ph), 2.11
121.3, 121.1, 119.9, 119.8, 119.7, 117.3, 114.7, 76.4, 66.2.
(m, 3H, CH3–Ph). 13C NMR (CDCl3, 101 MHz, ppm): δ 163.1,
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Dalton Transactions
160.9, 160.7, 151.8, 142.9, 134.2, 134.1, 133.2, 132.7, 132.6, gration of the signals at 4.87–4.65 ppm (vinyl proton of the nor-
132.5, 132.4, 132.3, 130.7, 130.6, 129.9, 129.4, 129.3, 129.2, bornene derivative unit), and IH3 is the integration of the signals
129.1, 129.0, 128.9, 128.4, 128.3, 128.2, 127.8, 127.5, 125.6, at 2.13–0.67 ppm (proton of the norbornene unit) in the 1H NMR
120.8, 120.5, 120.3, 113.9, 75.8, 65.0, 20.3.
spectrum.
X-ray crystallography
Single crystals suitable for X-ray analysis were obtained from Conflicts of interest
CH2Cl2/n-hexane solutions. The intensity data of the single
crystals were collected on the CCD-Bruker Smart APEX system.
There are no conflicts to declare.
All determinations of the unit cell and intensity data were per-
formed with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å). All data were collected at room temperature using
Acknowledgements
the ω scan technique. These structures were solved by direct
This work was supported by the National Natural Science
methods using Fourier techniques and refined on F2 by a full-
matrix least-squares method. All the non-hydrogen atoms were
refined anisotropically, and all the hydrogen atoms were
included but not refined. Crystallographic data are summar-
ized in Table S2.†
Foundation of China (21704060) and The Program for
Professor of Special Appointment (Eastern Scholar) at
Shanghai Institutions of Higher Learning.
Crystallographic data (excluding structure factors) for the
structure analysis have been deposited with the Cambridge
Crystallographic Data Centre, CCDC 1854348 (Pd1), 1854347
(Pd2), 1854349 (Pd6) and 1854346 (Pd7).†
Notes and references
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Polymerization of norbornene
In a typical synthesis, 1.5 μmol of complex and 1.0 g of norbor-
nene in toluene (1.5 mL) were placed in a 25 mL special
polymerization bottle with a strong stirrer under a nitrogen
atmosphere. After the mixture was kept at room temperature
for 10 min, MAO (10 wt% in toluene) was charged into the
polymerization system via a syringe and the reaction was
initiated. Several minutes later, acidic ethanol (Vethanol/Vconcd.
= 20/1) was added to terminate the reaction. The PNB was
HCl
isolated by filtration, washed with ethanol, and dried at 80 °C
for 24 h under vacuum. For all polymerization procedures, the
total reaction volume was 10.0 mL, which can be achieved by
varying the amount of toluene when necessary.
Copolymerization of norbornene with 5-vinyl-2-norbornene
Copolymerization was carried out under a nitrogen atmo-
sphere in toluene with a mechanical stirrer. The norbornene
and 5-vinyl-2-norbornene in 1.0 mL of toluene, 1.5 μmol of
complex in 0.5 mL of toluene were placed in the bottle, and
another 1.5 mL of fresh toluene was added to adjust the total
volume to 10 mL. After the mixture was stirred at room temp-
erature for 10 min, 7.0 mL of MAO was charged into the
polymerization system via syringe and the reaction
was initiated. Several minutes later, acidic ethanol
(Vethanol/Vconcd.HCl = 20/1) was added to terminate the reaction.
The copolymer was isolated by filtration, washed with ethanol,
and dried at 80 °C for 12 h under vacuum.
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The incorporation of 5-vinyl-2-norbornene were calculated
from the 1H NMR spectrum according to the following formula.
Incorp: ð%Þ ¼ fð10IH1 þ 10IH2Þ=ðIH1 þ IH2 þ 3IH3Þg ꢀ 100%
in which IH1 is the integration of the signals at 5.81–5.60 ppm
(vinyl proton of the norbornene derivative unit), IH2 is the inte-
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