Cationic para-benzhydryl substituted α-diimine nickel catalyzed ethylene and 1-decene polymerizations via controllable chain-walking

Article abstract of DOI:10.1016/j.ica.2018.11.004

A series of α-diimine nickel complexes bearing para-benzhydryl groups, {[(2,6-R₂-4-CHPh₂C₆H₂N = C)₂Nap]NiBr₂, Nap: 1,8-naphthdiyl, R = Me (C1); R = Et (C2); R = ⁱPr (C3)}, were s

Full text of DOI:10.1016/j.ica.2018.11.004

Accepted Manuscript
Research paper
Cationic para-benzhydryl substituted α-diimine nickel catalyzed ethylene and
1-decene polymerizations via controllable chain-walking
Susu Tian, Yang Zhang, Ruiping Li, Fuzhou Wang, Weimin Li
PII:
S0020-1693(18)31510-X
https://doi.org/10.1016/j.ica.2018.11.004
ICA 18619
DOI:
Reference:
Inorganica Chimica Acta
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25 September 2018
5 November 2018
5 November 2018
Please cite this article as: S. Tian, Y. Zhang, R. Li, F. Wang, W. Li, Cationic para-benzhydryl substituted α-diimine
nickel catalyzed ethylene and 1-decene polymerizations via controllable chain-walking, Inorganica Chimica Acta
(2018), doi: https://doi.org/10.1016/j.ica.2018.11.004
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Cationic para-benzhydryl substituted α-diimine nickel catalyzed
ethylene and 1-decene polymerizations via controllable chain-walking
Susu Tian,† Yang Zhang,† Ruiping Li, Fuzhou Wang* and Weimin Li*
Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, School of
Pertrochemical Engineering, Changzhou University, Changzhou 213164, China.
*Correspondence: Fuzhou Wang, E-mail: wangfuzhou1718@126.com
Weimin Li, E-mail: liweimin@cczu.edu.cn
†Susu Tian and Yang Zhang contribute equally to the article.
ABSTRACT: A series of α-diimine nickel complexes bearing para-benzhydryl
groups, {[(2,6-R2-4-CHPh2C6H2N C)2Nap]NiBr2, Nap: 1,8-naphthdiyl, R = Me (C1);
R = Et (C2); R = iPr (C3)}, were synthesized and characterized. Upon activation with
AlEt2Cl, these para-benzhydryl substituted catalysts conducted the ethylene and
1-decene polymerizations via controlled chain-walking process. These nickel
complexes exhibited good thermal stability and high activities (up to 106 g PE (mol Ni
h)−1) towards ethylene polymerization, producing high molecular weight polyethylenes
with a relatively low degree of branching and narrow polydispersities (Mw/Mn < 2.01).
The catalytic activities [(0.81−2.42) × 106 g PE (mol Ni·h)−1], polyethylene molecular
weights [Mn: (7.9−29.3) × 104 g mol−1] and total branching densities (57−102/1000 C)
can be tuned over a wide range. These controlled branching degrees of the obtained
polyethylenes were decreased with increasing the bulkiness of the ligand and
decreasing the polymerization temperature. Polymerization of 1-decene using these
cationic nickel complexes generated branched poly(1-decene)s (69−87/1000C) with
high molecular weights and narrow polydispersities (Mw/Mn = 1.18–1.55).
Keywords: α-Diimine nickel catalysts; Chain-walking polymerization; Olefin;
Branched polyolefin; para-Benzhydryl
1

1. Introduction
Over the past two decades, cationic nickel and palladium catalyst systems [1–7] for
olefin polymerization have attracted considerable attention because of their low
oxophilicity, and correspondingly the potentials in copolymerizing olefins with polar
comonomers [8–22]. In particular, the discovery of α-diimine-based Brookhart-type
nickel and palladium catalysts in the 1990s are one of the most classic examined
systems in olefin polymerization studies [23,24]. This unique mechanistic feature of
these catalysts for olefin polymerization is featured with the controlled chain-walking
process [25–31] and allow the unprecedented one-step preparation of branched
polyolefins with variable branch-type distribution [32–38]. Consequently, this
Brookhart type α-diimine catalysts [39–41] showed high catalytic activity for ethylene
and α-olefin polymerization to produce high molecular weight polyethylenes with
different polymer topologies and branched poly(α-olefin)s [42–48]. The inserted
monomeric units containing the methyl, alkyl branches and the long methylene
sequence were assembled onto the polyolefin backbone before the second monomer
insertion, generating different branched polymers.
Scheme 1. Selected examples of benzhydryl-based α-diimine nickel and palladium catalysts.
2

The backbone structures and N-aryl substituents on the α-diimine ligands
(Scheme 1) provide different coordination environment, steric and electronic
influences over the electronic density of the metal center, which correspondingly
modulate the polymerization process and control the microstructures of the resulting
polymers [49–52]. These sterically bulky ligands containing ortho-dibenzhydryl
groups are usually required to afford nickel and palladium catalysts capable of
generating high activity, stability and high molecular weight polymers [53–55], giving
elastomeric polyolefin with moderate or even low branching degree. For example,
Long et al. discovered that some cationic ortho-dibenzhydryl substituted α-diimine
nickel A catalyzed ethylene polymerization at high temperature [56,57], and these
catalysts displayed high activity up to 2.12 × 106 g PE (mol Ni h)−1 at 100 °C. Chen
et al. designed a series of α-diimine palladium catalysts B bearing a range of
electron-donating and -withdrawing groups (OMe, Me, Cl and CF3) [58,59]. These
catalysts displayed high thermal stability, high activity (up to 3.2 ×106 g PE (mol
Pd·h)−1) and high polymer molecular weights (Mn up to 538000) in ethylene
polymerization. More interestingly, semi-crystalline polyethylenes with very low
o
branching densities (23–29/1000C) and high melting temperatures (up to 99 C)
were generated. Based on these works, a series of dibenzhydryl-based unsymmetrical
α-diimine nickel and palladium catalysts (C [60] and D1 [61]) bearing different
substituents for ethylene polymerization. These nickel complexes exhibited high
thermal stability and high activities (up to 106 g PE (mol Ni h)−1) towards ethylene
polymerization, generating high molecular weight polyethylenes with a relatively low
branching degree. Our group also reported that a class of bulky ortho-dibenzhydryl
substituted α-diimine nickel D2 catalyzed ethylene and 1-hexene polymerizations
[62], the branching degrees and Mn values could be controlled over a wide range.
However, only a limited number of α-diimine catalysts containing para-benzhydryl
groups have been reported for olefin polymerization [63].
In the field of olefin polymerization, cationic α-diimine nickel catalyzed α-olefin
polymerization via precise controlled over both the regioselective insertion and
chain-walking pathways represent a primary challenge. For example, Coates et al.
3

showed that cationic α-diimine nickel catalyzed 1-butene polymerization via
stereoretentive chain-walking to produce semi-crystalline isotactic 4,2-poly(1-butene)
[64]. Interestingly, they also reported that precision chain-walking polymerization of
higher α-olefins using the “sandwich” α-diimine nickel catalysts to obtain
semi-crystalline “polyethylene” [65].
Recently we have reported synthesis of cationic phenyl-substituted α-diimine
nickel complexes that yielded highly branched polyethylenes (up to 143
branches/1000 C) [66,67]. In this work, we report applications of a class of α-diimine
nickel complexes containing para-benzhydryl groups promote efficient
polymerization of ethylene and 1-decene, and investigated the effects of the ligand
structures and polymerization temperatures in detail.
2 Experimental
2.1 General
All experiments, unless otherwise mentioned, were carried out under an atmosphere
of dry nitrogen in glove box. Nuclear magnetic resonance (1H, 13C NMR) spectra
were recorded on a Bruker 400 MHz instrument at ambient temperature. Molecular
weights and molecular weight distributions were determined by gel permeation
chromatography (GPC) at 150 °C using a PL-GPC 220 high-temperature GPC system.
Elemental analysis was performed by the Analytical Center of the Changzhou
University. Single crystal of the nickel complex was got by slow diffusion of
n-hexane into CH2Cl2 at room temperature. X-ray crystallography analysis was
performed on an Oxford Diffraction Gemini S Ultra CCD diffraction meter equipped
with mirro Cu Kα (λ = 1.54184 Å) radiation at room temperature. Toluene, CH2Cl2,
THF, and n-hexane were purified over 4 Å molecular sieves. 1-Decene was dried over
CaH2, and distilled before use. Other chemicals were purchased and used as recieved.
All para-benzhydrylaniline derivatives [63] and C4 [68] were synthesized according
to literature procedures.
4

2.2. Synthesis of the para-benzhydryl substituted α-diimine ligands L1–L3
Synthesis of bis [N,N’-(4-benzhydryl-2,6-dimethylphenyl)imino]acenaphthene L1
In a 100 mL flask, formic acid (0.2 mL) was added in to the stirred solution of
4-benzhydryl-2,6-dimethylaniline (1.21 g, 4.2 mmol) and acenaphthenequinone (0.36
g, 2.0 mmol) in ethanol (35 mL). The mixture stirred for 12 h at 50 °C, then cooled
and the precipitate was separated by filtration to afford orange-red solid. This obtained
solid was purified by recrystallization with a mixture of EtOH/CH2Cl2, washed with
cold ethanol and dried under vacuum to obtain orange-red powder (1.23 g, 85% yield).
1H NMR (400 MHz, CDCl3, ppm): δ 8.20 (d, J = 8.0 Hz, 2H, C13), 7.94–7.84 (m, 2H,
C14), 7.39–7.36 (m, 8H, C8), 7.30–7.25 (m, 12H, C7 and C9), 6.94 (s, 4H, C3), 6.69
13
(d, J = 7.6 Hz, 2H, C15), 5.62 (s, 2H, C5), 2.09–2.01 (m, 12H, C10). C NMR (100
MHz, CDCl3, ppm): δ 160.33 (C11), 147.00 (C1), 144.24 (C6), 142.93 (C17), 139.84
(C4), 139.04 (C16), 132.14 (C15), 130.99 (C12), 129.35 (C15), 129.50 (C8), 129.41
(C3), 128.32 (C7), 127.82 (C14), 126.29 (C2), 124.79 (C9), 122.58 (C13), 56.48 (C5),
17.93 (C10). Anal. Calcd. for C54H44N2: C, 89.96; H, 6.15; N, 3.89. Found: C, 89.49;
H, 6.37; N, 3.95. FT-IR (KBr): 1650 cm-1 (vC N).
＝
Synthesis of bis [N,N’-(4-benzhydryl-2,6-diethylphenyl)imino]acenaphthene L2
5

L2 was prepared in a similar synthetic procedure to that for L1 as an orange-red
powder (1.34 g, 86% yield). 1H NMR (400 MHz, CDCl3, ppm): δ 7.87 (d, J = 8.0 Hz,
2H, C14), 7.39–7.33 (m, 12H, C15 and C8), 7.28–7.22 (m, 12H, C15 and C8), 6.94 (s,
4H, C3), 6.69 (d, J = 7.2 Hz, 2H, C16), 5.62 (s, 2H, C5), 2.56–2.30 (m, 8H, C10),
1.24–0.74 (m, 12H, C11). 13C NMR (100 MHz, CDCl3, ppm): δ 160.94 (C12), 147.05
(C1), 144.58 (C6), 140.63 (C18), 139.24 (C4), 131.05 (C2), 130.72 (C17), 129.70
(C16), 129.56 (C8), 128.92 (C13), 128.30 (C7), 128.01 (C15), 127.64 (C3), 126.22
(C9), 122.99 (C14), 56.68 (C5), 24.92 (C10), 13.97 (C11). Anal. Calcd. for C58H52N2:
C, 89.65; H, 6.75; N, 3.61. Found: C, 89.37; H, 6.92; N, 3.46. FT-IR (KBr): 1642 cm-1
(vC N).
＝
Synthesis of bis [N,N’-(4-benzhydryl-2,6-diisopropylphenyl)imino]acenaphthene L3
L3 was prepared in a similar synthetic procedure to that for L1 as an orange-red
powder (1.38 g, 83% yield). 1H NMR (400 MHz, CDCl3, ppm): δ 7.91 (d, J = 8.0 Hz,
2H, C14), 7.39–7.36 (m, 10H, C15 and C8), 7.30–7.26 (m, 12H, C7 and C9), 7.04 (s,
4H, C3), 6.67 (d, J = 7.2 Hz, 2H, C16), 5.62 (s, 2H, C5), 5.67 (s, 2H, C5), 3.02–2.97
(m, 4H, C10), 1.15–0.88 (m, 24H, C11). 13C NMR (100 MHz, CDCl3, ppm): δ 161.22
6

(C12), 146.97 (C1), 144.71 (C6), 140.76 (C18), 139.40 (C4), 135.24 (C2), 131.14
(C17), 129.57 (C16), 129.51 (C8), 128.83 (C13), 128.24 (C7), 127.72 (C15), 126.18
(C9), 124.79 (C3), 123.36 (C14), 56.94 (C5), 28.70 (C10), 23.21(C11). Anal. Calcd.
for C62H60N2 (832.48): C, 89.38; H, 7.26; N, 3.36. Found: C, 88.95; H, 7.57; N, 3.29.
FT-IR (KBr): 1645 cm-1 (vC N).
＝
2.3. Synthesis of complexes C1–C3
All nickel complexes were obtained in a similar synthetic procedure by the reaction of
the corresponding ligands with (DME)NiBr2 (DME = 1,2-dimethoxyethane) in
CH2Cl2. A typical synthetic way of C1–C3 is as follows: ligand (0.5 mmol),
(DME)NiBr2 (0.15 g, 0.5 mmol) and CH2Cl2 (20 mL) were combined in a 100 mL
Schlenk flask under an atmosphere of nitrogen. After stirred for 12 h at room
temperature, and the resulting suspension was filtered. The reaction solution was
removed under vacuum and the residue was washed with diethyl ether (3 × 14 mL),
and then dried under vacuum to obtain nickel complex as a brown solid powder.
(MeNˆN)NiBr2 (C1) (0.44 g, 94%): Anal. Calcd for C54H44Br2N2Ni: C, 69.04; H, 4.72;
N, 2.98. Found: C, 69.01; H, 4.69; N, 3.02. FT-IR (KBr): 1630 cm-1 (vC N).
＝
(EtNˆN)NiBr2 (C2) (0.47 g, 94%): Anal. Calcd for C58H52Br2N2Ni: C, 69.97; H, 5.26;
N, 2.81. Found: C, 70.51; H, 5.63; N, 2.73. FT-IR (KBr): 1631 cm-1 (vC N).
＝
(
i-PrNˆN)NiBr2 (C3) (0.48 g, 91%): Anal. Calcd for C62H60Br2N2Ni: C, 70.81; H, 5.75;
N, 2.66. Found: C, 70.78; H, 5.71; N, 2.59. FT-IR (KBr): 1628 cm-1 (vC N).
＝
2.4. Ethylene polymerization
Polymerization of ethylene was conducted in a 100 mL glass thick-walled pressure
reactor equipped with a magnetic stirrer. A certain amount of toluene (50 mL) and
7

AlEt2Cl were charged into the reactor. Stirring the mixture until the desired
temperature was established, the 2.8 μmol catalyst solution in 2 mL CH2Cl2 injected
into the reactor. Then ethylene was purged into the reactor and maintained at 6.0 atm.
The polymerization was carried out for a certain time and terminated in acidified
methanol. The obtained polyethylenes were adequately washed with methanol and
dried under vacuum at 50 °C for 6 h until a constant weight was reached.
2.5. 1-Decene polymerization
Polymerization of 1-decene was performed in a 10 mL glass reactor equipped with a
magnetic stirrer. A certain amount of toluene, 1-decene and AlEt2Cl were added into
the drying reactor. Stirring the mixture until the desired polymerization temperature
was established, and started by the addition of the catalyst solution in toluene. The
polymerization was carried out for a certain time and terminated in acidified methanol.
The obtained polymers were adequately washed with methanol and dried under
vacuum at 50 °C for 5 h.
3. Results and discussion
3.1. Synthesis and characterization of the α-diimine ligands L1–L3 and their
complexes C1–C3
Scheme 2 showed the synthesis route for the benzhydryl substituted α-diimine
ligands and their nickel complexes. Ligands L1–L3 were easily synthesized by the
condensation reaction of 1 equiv. of acenaphthequinone with 2 equiv. of the
appropriate aniline, and formic acid as a catalyst in 83–86% yields. The reaction of
the corresponding ligands L1–L3 and equimolar amounts of (DME)NiBr2 in CH2Cl2
led to displace 1,2-dimethoxyethane to afford the catalyst precursors C1–C3 as brown
solids in almost quantitative yields (91–94%), respectively (Scheme 2). In addition,
8

the methyl substituted complex {[(2,4,6-Me3C6H2N C)2Nap]NiBr2 (C4)} [68] was
also as catalyst precursor for comparison in this study.
Scheme 2. Synthesis of α-diimine ligands L1–L4 and their nickel complexes C1–C4.
Fig. 1. Molecular structure of complex C1 with thermal ellipsoids drawn at 30% probability,
and H atoms have been omitted for clarity. Selected bond lengths (Å) and angles (o): Ni1−N1
= 2.024 (4), Ni1−N2 = 2.035 (4), Ni1−Br1 = 2.3432 (10), Ni1−Br2 = 2.3248 (9), N1−C33 =
1.286 (5), N1−C22 = 1.280 (5); Br1−Ni1−Br2 = 124.26 (4), N1−Ni1−N2 = 83.05 (15),
N1−Ni1−Br1 = 104.88 (11), N1−Ni1−Br2 = 115.34 (11), N2−Ni1−Br1 = 114.82 (11),
N2−Ni1−Br2 = 107.14 (11), C22−N2−Ni1 = 111.1 (3), C33−N1−Ni1 = 111.0 (3).
The crystal structure of C1 was determined by X-ray diffraction studies and the
corresponding ORTEP diagram is shown in Fig. 1. This complex has
pseudo-tetrahedral geometry about Ni, and exhibit pseudo-C2v-symmetry. The bond
9

distances and angles are typical and similar to those reported complex C4 [68]. For
example, the Ni N bond distances of C1 (2.024 and 2.035 Å) are similar to those
determined for C4 (2.021 Å), and the Ni Br bond distances in C1 and C4 are almost
identical (2.3432 and 2.3248 Å for C1 vs. 2.323 Å for C4). Besides, the N Ni Br
angles for C1 (115.34 and 114.82) are also approximate to those for C4 (114.4).
The introduction of para-benzhydryl group of these catalysts may induce a
coordination effect in olefin polymerization.
3.2. Ethylene polymerization studies
Sun et al. studied a series of corresponding 2,3-diiminobutylnickel complexes
containing para-benzhydryl moiety for ethylene polymerization [63], where these
catalysts was only activated by Et2AlCl and exhibited high activities in the order of
106 g PE (mol Ni h)−1 at 10 atm of ethylene. Therefore, polymerization of ethylene
using complexes C1–C4 activated by Et2AlCl was carried out at various
polymerization temperatures and steric effects at 6 atm of ethylene, and Table 1 listed
the results of ethylene polymerization.
First, the effect of the [Al]/[Ni] ratio with Et2AlCl was studied by C1 at room
o
temperature (RT = 25 C). The catalytic activity of C1 and the molecular weights
have been no significant changes with increasing the [Al]/[Ni] ratio (Table 1, entries
1−4). The highest activity and molecular weights (entry 2) was observed with the
[Al]/[Ni] ratio of 600.
Then, the effect of the reaction temperature was examined by increasing the
temperature from 0 °C to 60 °C (Table 1, entries 5−20) at the [Al]/[Ni] molar ratio of
600. The maximum catalytic activities of complexes C1–C3 were observed at 40 °C
(Table 1, entries 2, 11 and 15). The catalytic activities of C4 were decreased with
polymerization temperatures (Table 1, entries 17–20) and the highest activity was
observed at 0 °C (entry 17). This polymerization at 60 °C, the catalytic activities of
the para-benzhydryl substituted C1–C3 showed much higher thermal stability and
higher catalytic activities (up to 0.81 × 106 g PE (mol Ni h)−1) than the corresponding
10

methyl substituted C4 (Table 1, entries 8, 12, 16 and 20). The partial deactivation of
the active species led to reduce catalytic activity at elevated temperature [11]. Besides,
the raise of reaction temperature also affected the polymer molecular weight and
polydispersity. The maximum molecular weights of the polymers obtained by C1–C3
were observed at 20 °C (Table 1, entries 6, 10 and 14), while the molecular weights of
the polymers obtained by complex C4 were decreased with polymerization
temperature from 0 to 60 °C (Table 1, entries 17–20). The obtained polymer
molecular weights decreased by increasing the temperature, and accompanied by
broadening their polydispersity.
Table 1. Results of ethylene polymerization with nickel complexes C1–C4.a
c
Temp.
(oC)
25
25
25
25
0
Yield
(g)
Mn
(×104)
22.1
24.6
23.2
23.4
21.1
25.6
18.9
7.9
c
Entry
Cat.
[Al]/[Ni]
Activityb
Mw/Mn
Bd
1
2
C1
C1
C1
C1
C1
C1
C1
C1
C2
C2
C2
C2
C3
C3
C3
C3
C4
C4
C4
C4
400
600
800
1000
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
600
1.81
2.03
1.92
1.38
1.79
1.98
2.26
0.91
1.66
1.89
2.06
0.83
1.29
1.53
1.74
0.76
0.97
0.63
0.34
trace
1.94
2.18
2.06
1.48
1.92
2.12
2.42
0.98
1.78
2.03
2.21
0.89
1.38
1.64
1.86
0.81
1.04
0.68
0.36
-
1.61
1.81
1.59
1.73
1.63
1.79
1.82
1.97
1.80
1.82
1.95
2.09
1.73
1.84
1.89
2.01
1.75
1.86
1.99
-
-
-
3
-
4
-
5
72
80
86
102
64
76
83
95
57
72
80
86
81
83
89
6
20
40
60
0
7
8
9
23.4
26.3
19.6
8.7
10
11
12
13
14
15
16
17
18
19
20
20
40
60
0
25.8
29.3
23.6
10.2
8.4
20
40
60
0
20
40
60
4.0
2.4
-
a Polymerization conditions: Ni = 2.8 μmol in CH2Cl2 (2 mL); cocatalyst Et2AlCl; solvent = toluene (50
b
c
mL); 6 atm of ethylene; time = 20 min. 106 g of PE (mol of Ni)-1 h-1. Mn and Mw/Mn determined by
d
GPC, 104 g mol-1. Branching density, branches/1000 C = (CH3/3)/[(CH + CH2 + CH3)/2]×1000,
determined by 1H NMR analysis [69].
11

The effect of catalyst structures with different steric effects were also examined at
different temperatures (Table 1, entries 2 and 5−20). Complexes C1–C3 produced
polymers in high yields with molecular weights over a very wide range ((7.9–29.3) ×
104 g mol−1, entries 2 and 5−16) and narrow molecular weight distributions (Mw/Mn =
1.63–2.09). The rates of polymerization decreased in the following order, C1 ≥ C2 ≥
C3 > C4. However, the molecular weights of the produced polymers follow the
opposite trend. These nickel complexes exhibited good thermal stability and high
activities (up to 106 g PE (mol Ni h)−1) towards ethylene polymerization, producing high
molecular weight polyethylenes with a relatively low degree of branching and narrow
polydispersity. Complex C1 bearing a benzhydryl group in the para-aryl position
exhibited higher thermal stability and high activities than the corresponding C4 (Table
1, entries 5–8 vs 17−20), respectively, which should be attributed to the electronic
effect of the 4-benzhydryl substituent.
Steric effect of ortho-position in anilinic moiety can be evaluated by comparing C1
to C3. The polymer molecular weights were increased with increasing the bulkiness of
the ortho substituents on the ligand N-aryl moiety [C3 (iPr) > C2 (Et) > C1 (Me),
Table 1, entries 5–16]. The complexes with methyl groups (C1) showed higher
activity than those with the corresponding isopropyl groups (C3), but gave the lower
molecular weight. The highest molecular weight of the polymers obtained by complex
C3 suggest that the ortho-isopropyl should suppress chain-transfer reactions [11].
1
The H NMR spectroscopy analysis (Table 1) showed that the polymer branching
structures was strong dependent on the ligand structures and polymerization
temperature [69]. For example, the branching degrees of resulting polyethylenes were
increased with the raise of reaction temperature from 0 °C to 60 °C (C1, 72−102/1000
C; C2, 64−95/1000 C; C3, 57−86/1000 C; C4, 81−89/1000 C). Furthermore, the
number of chain branching was depended on the precatalysts at 0 °C and decreased in
the following order, C1 [Me, 72/1000 C] > C2 [Et, 64/1000 C] > C3 [iPr, 57/1000 C]
(Table 1, Fig. 2, entries 5, 9 and 13). The results also indicated that the
para-benzhydryl substituted complex C1 afford polyethylenes with a considerably
lower branching degree than the corresponding C4 (Fig. 2, entries 5 and 17, Table 1),
12

which may was attributed to the coordination ability of the bulky benzhydryl
substituent in the para-position of the ligand N-aryl moiety [11]. Therefore, the
introduction of para-benzhydryl substituent was effective in reducing the polymer
branching degree.
Fig. 2. 1H NMR spectra of polyethylenes obtained by complex C1−C4 at 0 °C (Table 1,
entries 5, 9 and 13).
Fig. 3. 13C NMR spectra of polyethylenes generated by complex C1 at 60 °C (i) and complex
C3 at 0 °C (ii) (A and B refer to methyl carbon of sec-butyl branch, Table 1, entries 8 and 13).
13

Besides the difference branching degrees, the chain branching distribution was
determined by 13C NMR spectroscopy analysis [66]. Fig. 3 (i) showed that the
obtained highly-branched polyethylene using complex C1 at 60 °C possessed 70
methyl, 6 ethyl, 2 n-propyl, 3 n-butyl, 1 sec-butyl and 18 longer chains (> C4
branches) (Table 1, entry 13), among branches was predominately methyl. However,
only 57 short methyl branch was obtained by complex C3 at 0 °C (Fig. 3 (ii), Table 1,
entry 8). These branching distribution in the obtained polyethylenes can be explained
by the ethylene chain-walking mechanism [11,66].
3.3. 1-Decene polymerization studies
Table 2. Results of 1-decene polymerization with complexes C1–C3.a
c
Temp
(oC)
Yield
(g)
Mn
10,1-e
(%)
Entry
Cat.
Activityb
Mw/Mn
Bd
c
(×104)
1
2
3
4
5
6
0
20
40
60
20
20
0.38
1.06
0.74
0.54
0.71
0.58
1.9
5.3
3.7
2.7
3.5
2.9
5.7
10.8
13.7
8.9
1.18
1.23
1.49
1.55
1.41
1.37
87
84
80
75
76
69
26
28
31
35
34
39
C1
C1
C1
C1
C2
C3
11.7
14.7
a Polymerization conditions: Ni = 10 μmol; cocatalyst Et2AlCl, [Al]/[Ni] = 600; monomer
b
concentration [1-decene] = 1.0 M; time: t = 2 h; solvent: toluene, total volume: 20 mL.
Activity in 104 g (mol Ni)-1 h-1. c Mn and Mw/Mn determined by GPC, 104 g mol-1. B =
d
1
e
branches per 1000 carbon atoms, determined by H NMR analysis. 10,1-Enchainment was
calculated by the following equation: 10,1% = [500−4B]/(500 + B) [65].
Polymerization of 1-decene was conducted using complex C1 activated by Et2AlCl
at temperatures from 0 °C to 60 °C (Table 2, entries 1–4). The highest activity was
observed at 20 °C (entry 2), and the maximum Mn was observed at 40 °C (entry 3).
The narrow polydispersities (Mw/Mn = 1.18–1.55) was observed for the generated
poly(1-decene)s, and the Mw/Mn values increased by raising the polymerization
temperatures. At 20 °C, complexes C2 and C3 also exhibited good activity for
1-decene polymerization to obtain high molecular weight polymers with narrow
14

polydispersities (Table 2, entries 5 and 6), indicating the Mn values were increased
with increasing the bulkiness of the ortho substituents on the ligand N-aryl moiety.
1
Analysis of the generated polymers by H NMR spectroscopy revealed the total
branching degrees were strongly depended on the polymerization temperatures and
catalyst structures (Table 2) [66]. The poly(1-decene) branching degrees are lower
than the theoretical value (100/1000 C) because of the 2,1-insertion in
10,1-enchainment [43]. Over a narrow range of the poly(1-decene) branching degrees
(69−87/1000 C, Table 2) could be tuned by comparing with those of these obtained
polyethylenes (Table 1). For example, the polymer branching degrees (75−87/1000 C,
Table 2, entries 1−4) using C1 were slightly decreased with increasing polymerization
temperatures, and also decreased with increasing the bulkiness of the ligand (Table 2,
entries 2, 5 and 6).
Table 2 also indicated that the chain-transfer reaction has been suppressed with
increasing catalyst steric hindrance, and leaded to enhanced insertion for 2,1-insertion
of 1-decene (C3 > C2 > C1) and the chain-walking process. In addition, the mole
fraction of linear enchainment of the obtained amorphous polymers (no Tm observed)
slightly increased from 0.26 to 0.35 and 0.28 to 0.39 with increasing polymerization
temperatures and increasing the bulkiness of the catalyst, respectively.
4. Conclusions
In summary, chain-walking polymerization of ethylene was catalyzed by a series of
para-benzhydryl substituted α-diimine nickel complexes containing different steric
i
effects (Me, Et and Pr) activated by AlEt2Cl. The chain-walking process of these
cationic nickel systems were efficiently modulated by the introduction of
para-benzhydryl group of the N-aryl substituents, which results in the high thermal
stability, high catalytic activity and yielded high molecular weight polyethylene with
a relatively low branching degree. Specifically, complex C1 containing ortho-methyl
groups exhibited highest activities and generated highly branched polyethylene at
15

60 °C (up to 102 branches/1000 C). These cationic α-diimine nickel catalyzed
1-decene polymerization to obtain high molecular weight polymers with narrow
polydispersities (Mw/Mn = 1.18–1.55). The branching densities (57−102/1000 C) of
the generated polyethylene could be tuned over a wide range comparing with those
of the generated poly(1-decene)s (69−87/1000 C).
Acknowledgements
This work was supported by National Natural Science Foundation of China (Grant
No. 21801002), Advanced Catalysis and Green Manufacturing Collaborative
Innovation Center (ACGM2016-06-01), Fundamental Research Funds for the Central
Universities (WK2060200025) and Yixing Taodu Ying Cai Program.
Appendix A. Supplementary data
CCDC 1850020 contains the supplementary crystallographic data for complex C1.
These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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18

Graphical abstract
19

Highlights:
 Three para-benzhydryl substituted α-diimine nickel complexes were
synthesized.
 These nickel catalytic systems conducted the ethylene and 1-decene
polymerizations via controlled chain-walking process.
 These catalytic systems exhibited good thermal stability and high activities,
producing high molecular weight branched polymers with narrow polydispersity.
 The polyethylene branching densities (57−102/1000 C) could be tuned o er a
wide range comparing with those of the obtained poly(1-decene)s.
20