Highly resilient polyethylene elastomers prepared using α-diimine nickel catalyst with highly conjugated backbone

Article abstract of DOI:10.1002/aoc.4566

An α-diimine nickel catalyst (Cat-1) with highly conjugated acenaphthylene backbone was synthesized and used in ethylene polymerization. Compared with typical Brookhart catalyst B-Cat, Cat-1 could produce polyethylene with both higher molecular weight (up to 5.1?×?10⁵?g?mol^?1) and branching density (up to 135 branches per 1000 carbons) in good polymerization activity. The polyethylene prepared using Cat-1 with higher molecular weight and branching density possessed exceptional mechanical properties (ultimate tensile strength and elongation at break could reach 12.08?MPa and 1148%) and outstanding elastomeric recovery as compared with the polymer prepared using B-Cat. Cat-1 was synthesized using a simple reaction route and the polymerization conditions were suitable for the industrial polymerization process. The novel nickel catalyst Cat-1 is promising for producing highly resilient polyethylene elastomers industrially.

Full text of DOI:10.1002/aoc.4566

Received: 12 July 2018
DOI: 10.1002/aoc.4566
Revised: 24 July 2018
Accepted: 31 July 2018
F U L L P A P E R
Highly resilient polyethylene elastomers prepared using
α‐diimine nickel catalyst with highly conjugated backbone
Feng He
| Dan Wang | Anyang Wu | Baiyu Jiang | Zhen Zhang | Zhenmei Cheng |
Zhisheng Fu | Qisheng Zhang | Zhiqiang Fan
Department of Polymer Science and
An α‐diimine nickel catalyst (Cat‐1) with highly conjugated acenaphthylene
Engineering, MOE Key Laboratory of
Macromolecular Synthesis and
Functionalization, 38 Zheda Road,
Hangzhou 310027, China
backbone was synthesized and used in ethylene polymerization. Compared
with typical Brookhart catalyst B‐Cat, Cat‐1 could produce polyethylene with
5
−1
both higher molecular weight (up to 5.1 × 10 g mol ) and branching density
up to 135 branches per 1000 carbons) in good polymerization activity. The
Correspondence
(
Zhisheng Fu, MOE Key Laboratory of
Macromolecular Synthesis and
Functionalization, Department of Polymer
Science and Engineering, Zhejiang
University, Hangzhou 310027, China.
Email: fuzs@zju.edu.cn
polyethylene prepared using Cat‐1 with higher molecular weight and
branching density possessed exceptional mechanical properties (ultimate
tensile strength and elongation at break could reach 12.08 MPa and 1148%)
and outstanding elastomeric recovery as compared with the polymer prepared
using B‐Cat. Cat‐1 was synthesized using a simple reaction route and the
polymerization conditions were suitable for the industrial polymerization pro-
cess. The novel nickel catalyst Cat‐1 is promising for producing highly resilient
polyethylene elastomers industrially.
Funding information
Open Research Project of the State Key
Laboratory of Industrial Control Technol-
ogy, Zhejiang University, Grant/Award
Number: No. ICT1800411; National Key
Research and Development Program of
China, Grant/Award Number:
KEYWORDS
α‐diimine nickel catalyst, elastomeric recoverymechanical propertiespolyethylene elastomer
2017YFE0106700; State Key Laboratory of
Industrial Control Technology, Zhejiang
University, Grant/Award Number:
ICT1800411
1
| INTRODUCTION
the sterically bulky ortho substituents on the aryls of the
ligand that are positioned in axial sites of the central
α‐Diimine Ni(II) and Pd(II) catalysts developed by
Brookhart and co‐workers bearing bulky aryl substituents
can catalyze, with high catalytic activity, ethylene poly-
merization to form high‐molecular‐weight polyethyl-
metal, the chain transfer reaction is greatly suppressed,
and high‐molecular‐weight hyperbranched polyethylene
(with ca 100 branches per 1000 carbons) can be prepared
using typical α‐diimine Ni(II) catalysts (Figure 1, B‐Cat).
The high‐molecular‐weight hyperbranched polyethylene
chains are prone to form physical crosslinks in high
density, endowing the material with excellent mechanical
[1–3]
enes.
Due to the superior properties of tolerance
towards polar functional groups and the development of
[4]
some innovative catalyst design strategies, α‐diimine
Ni(II) and Pd(II) catalysts can catalyze the copolymeriza-
tion of ethylene with polar‐functionalized co‐mono-
[
24]
and elastomeric recovery properties.
It is feasible that
by tuning of the steric and electronic properties of
ligands, specific α‐diimine Ni(II) or Pd(II) catalysts can
be designed for the preparation of high‐molecular‐weight
[
5,6]
mers.
ranging from linear to hyperbranched structures) can
be controlled by simply modifying the catalyst's ligand
Moreover, the topologies of polymer chains
(
[
25]
polyethylenes with controlled branching
which can
[
7–19]
structure
temperature
and polymerization conditions such as
act as alternative materials to thermoplastic elasto-
[20]
[21–23]
[26–28]
and ethylene pressure.
Owing to
mers.
Appl Organometal Chem. 2018;e4566.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.4566

2
of 12
HE ET AL.
FIGURE 1 Selected examples of previously reported α‐diimine Ni(II) catalysts
In recent years there have been interesting research
works concerning copolymerization of ethylene and α‐ole-
fins (1‐hexene, 1‐octene, 1‐decene, 1‐dodecene) catalyzed
by α‐diimine Ni(II) catalyst for the preparation of
benzofuran‐substituted α‐diimine nickel catalysts could
afford polyethylene with ultrahigh molecular weights
[
35]
(Figure 1, I).
Guan et al. designed rigid cyclophane
ligand and the corresponding α‐diimine nickel catalyst,
and the cyclophane‐based catalyst was found to form
high‐molecular‐weight polyethylene even at high
[29–32]
thermoplastic
elastomers.
Simultaneously,
copolymerization of ethylene and some polar‐functional-
ized co‐monomers catalyzed by α‐diimine palladium and
phosphine–sulfonate–palladium catalysts can also gener-
[
36,37]
polymerization temperature (Figure 1, II).
The
benzhydryl‐derived α‐diimine nickel catalysts developed
by Long et al. showed outstanding thermal stability
and could form high‐molecular‐weight polyethylene
[
33,34]
ate copolymers with good mechanical properties.
Yet, it is more convenient and economical if ethylene
can be used as the only monomer to prepare polymers
with good mechanical properties.
−
1
[38,39]
(M > 600 000 g mol ) (Figure 1, III).
The groups
n
of Brookhart, Daugulis and Coates reported sandwich
structured α‐diimine and half‐sandwich structured
pyridine–imine nickel catalysts that could produce
polyethylene with higher molecular weight and narrower
In order to simultaneously improve the mechanical
and elastomeric recovery properties of hyperbranched
polyethylene, both the polymer's molecular weight and
branch density should be high enough. Introducing
sterically bulky ortho substituents on aniline moieties of
α‐diimine Ni(II) catalysts has been found to be an
effective way of enhancing the polyethylene molecular
weight. For example, Ionkin et al. found ortho‐
[40,41]
molecular weight distribution (Figure 1, IV).
Recently, Sun et al. used a group of unsymmetric α‐
diimine nickel catalysts to prepare elastomeric polyethyl-
enes that showed good elastomeric recovery and high
[
42,43]
elongation at break (Figure 1, V).
According to the

HE ET AL.
3 of 12
literature reports, hyperbranched polyethylene with high
molecular weight can be prepared using α‐diimine Ni(II)
catalysts with bulky substituents on the aniline moieties.
However, such bulky substituents were found to reduce
the branching density of polyethylenes compared with
the synthesized polymer were determined and compared
with those of the polymer synthesized with B‐Cat. Inter-
estingly, compared with B‐Cat, Cat‐1 was found to lead
to polyethylene with higher molecular weight, while the
polymerization activity and branch density of
polyethylene were not decreased, which could hardly be
achieved using α‐diimine Ni(II) catalysts with very bulky
aniline moieties. Moreover, the polyethylene prepared
using Cat‐1 possessed exceptional mechanical properties
and elastomeric recovery.
[38,39]
B‐Cat.
In order to prepare polyethylenes with con-
trolled branching, Wu et al. prepared a series of α‐
diimine nickel complexes with monoaryl‐substituted ani-
[25]
lines (Figure 1, VI).
Modification of the backbone part of an α‐diimine
ligand is another potential strategy for improving the
catalyst's molecular‐weight ability. Bulky camphyl‐
substituted and dibenzobarrelene‐derived α‐diimine
nickel catalysts designed by Wu et al. have been found
to produce high‐molecular‐weight polyethylene even at
2
| EXPERIMENTAL
.1 | Materials
2
[18,44–46]
8
0°C (Figure 1, VII and VIII).
Chen et al. synthe-
sized a series of α‐diimine Ni(II) complexes with various
substituents on the backbone, and the α‐diimine Ni(II)
complexes with two methoxy substituents on the ortho
position of the acenaphthyl backbone could produce
polyethylene with three times higher molecular weight
Toluene, n‐heptane and diethyl ether were refluxed over
sodium and benzophenoneketyl before being used.
Methylene chloride was dried over activated 4 Å
molecular sieves and refluxed over CaH2 before use.
Acenaphthene, aluminium bromide, oxalyl bromide,
trichlorobenzene, 1,2‐dichlorobenzene, potassium iodide,
N‐bromosuccinimide, benzoyl peroxide, acetic acid, 2,6‐
diisopropylaniline, petroleum ether, dimethoxyethane
[47]
than that using B‐Cat (Figure 1, IX).
Our group has
carried out research work on the effects of backbone
modification on the catalytic properties of α‐diimine
[48,49]
nickel catalysts.
Yet, studies of the effects of back-
nickel(II) bromide ((DME)NiBr ), dichloroethyl alumin-
2
bone modification of α‐diimine nickel catalysts on the
molecular weight of the synthesized polyethylene are still
limited.
ium (EADC), diethylaluminium chloride (DEAC) and
methylaluminoxane (MAO) were purchased from J&K.
Isopropyl ether was purchased from Aladdin. Acetonitrile
was purchased from Aldrich Chemical and used as
received. B‐Cat and pyracyloquinone (C3) were prepared
In the work reported here, a vinylidene substituent
was introduced to the backbone of Cat‐1 to modify its
catalytic properties. The synthetic route of Cat‐1 is shown
in Scheme 1. The electronic and steric effects of the
modified acenaphthylene backbone on the catalytic
properties such as polymerization activity, and on the
molecular weight and branch density of the resulting
polyethylene were investigated. Mechanical properties of
[
1,50]
according to the literature.
The high‐purity nitrogen
and ethylene used in catalyst synthesis and ethylene
polymerization were further purified through a gas
purification column. All of the catalyst synthesis and
polymerization reactions were performed under a nitro-
gen atmosphere.
SCHEME 1 Synthetic routes for backbone‐modified α‐diimine nickel complex Cat‐1

4
of 12
HE ET AL.
2
.2 | Synthesis of N,N‐
in the ethanol and filtered directly. The obtained polymer
was washed with ethanol three times. Transparent or
white viscoelastic polymer was obtained after drying in
vacuum at 50°C for 48 h.
Bis(2,6‐diisopropylaniline)
cyclopentaacenaphthylene‐1,2‐diimine (L1)
C3 (0.25 g, 1.21 mmol) and acetic acid (6 ml) were added
into acetonitrile (50 ml) successively at room tempera-
ture. The solution was heated under reflux for 1 h. Then,
2.5 | Ethylene Polymerization in 2 l
2
,6‐diisopropylaniline (0.60 ml, 3.15 ml) was added to the
Autoclave
solution, and the solution was heated at 90°C for an
additional 24 h. After the reaction was completed, the
solution was cooled to room temperature and the solvent
was evaporated off. The designed ligand was purified by
column chromatography on silica gel (petroleum ether–
methylene chloride = 1:1, 0.75% triethylamine). An
amount of 0.499 g of L1 (chrome yellow powders)
High‐pressure ethylene polymerization was carried out in
a 2 l Büchi autoclave using standard Schlenk techniques.
n‐Heptane (1 l) was added into the reactor and ethylene
was introduced at 1 atm until the n‐heptane solution
was saturated. The co‐catalyst (DEAC) and nickel catalyst
solution (2 μmol, 10 ml of toluene) were added to the
reactor. Then, ethylene pressure was raised to the
designed value to start ethylene polymerization. The
polymerization was terminated by releasing the mono-
mer and adding excess ethanol. The polymer was precip-
itated in ethanol and filtered directly. Transparent or
white elastomer‐like polymer was obtained after drying
in vacuum at 60°C to a constant weight.
1
was obtained in 78.5% yield. H NMR (400 MHz,
CD OD, δ, ppm): 6.6–7.8 (m, 12H, Ar‐H), 2.95–3.07
3
(
sept, 4H, CH(CH ) ), 0.97–1.29 (dd, 24H, CH(CH ) ).
3
2
3 2
1
3
C
NMR (100 MHz, CDCl3, δ, ppm): 161.97
(
C═N), 147.81 (C ─N), 140.57 (C ─C ─C═N),
Ph
Na
Na
1
40.36 (C ─CH═CH─C ), 135.92 (C ─CH), 132.55
Na Na Ph
(
(
(
(
7
C ─C ─CH), 129.88 (C ─CH═CH─C ), 126.62
Na
Na
Na
Na
CNa─C═N),
124.78,
124.37
(C ─H),
Ph
123.60
C ─C H─C H─C ), 28.70 (CH(CH ) ), 23.57, 23.25
2.6 | Characterization
Na
Na
Na
N
3 2
CH(CH ) ). Anal. Calcd for C H N (%): C, 86.98; H,
1
3
2
38 44 2
H NMR spectra were recorded at 400 MHz with a Bruker
.68; N, 5.34. Found (%): C, 85.54; H, 7.67; N, 4.97. ESI‐
Advance 2B‐400 spectrometer using CDCl as solvent and
3
+
MS: m/z 525.0 ([M + H] ).
13
tetramethylsilane as standard. The C NMR data for L1
were obtained with a Bruker Advance 2B‐400 spectrome-
13
ter using CDCl as solvent, and the C NMR data for
3
2
.3 | Synthesis of Cat‐1
polyethylenes prepared under high ethylene pressure
were obtained at 600 MHz with an Agilent Technologies
DD2‐600 spectrometer using CDCl3 as solvent. The
molecular weight of L1 was measured by electrospray
ionization (ESI) mass spectrometry, with chloroform used
as solvent. Element analysis was performed with a Vario
Micro cube element analyzer (Elemental Analysensystem
GmbH). Infrared (IR) spectra were recorded with a
Thermo Nicolet 6700 spectrometer using the potassium
bromide disc technique. The molecular weight and
molecular weight distribution of polyethylenes were
characterized using a Polymer Laboratories PL‐220 gel
permeation chromatography (GPC) instrument at 40°C
with tetrahydrofuran as the eluent at a flow rate of
A solution of L1 (0.40 g, 0.76 mmol) in methylene
chloride (20 ml) was added into (DME)NiBr (0.24 g,
2
0
.78 mmol). The solution was stirred at 30°C for 24 h
and then methylene chloride was removed under reduced
pressure. The resultant brick red powders were washed
with diethyl ether (4 × 15 ml) and dried in vacuum. Yield:
−
1
8
8.7% (0.501 g). IR (KBr; cm ) C═N: 1610 (s), 1644 (s).
Anal. Calcd for C H N NiBr (%): C, 61.41; H, 5.42; N,
38
44
2
2
3
.77. Found (%): C, 59.35; H, 5.20; N, 3.24.
2.4 | Ethylene Polymerization at 1 atm
Ethylene Pressure
−
1
Ethylene polymerization under atmospheric pressure was
performed in a 100 ml Schlenk flask equipped with a
magnetic stirrer using standard Schlenk techniques.
Toluene and co‐catalyst (EADC, DEAC or MAO) were
added successively to the flask. Ethylene was introduced
under 1 atm until the toluene solution was saturated.
Then, a toluene solution of the nickel catalyst was
injected to the flask to start the ethylene polymerization.
The polymerization was terminated with excess etha-
nol–HCl mixture (95:5). The polymer was precipitated
1.0 ml min . The molecular weight of the polyethylenes
prepared under high ethylene pressure was characterized
using GPC at 150°C with 1,2,4‐trichlorobenzene as the
−
1
eluent at a flow rate of 1.0 ml min . Differential
scanning calorimetry analysis was performed with a TA
Instruments Q200 differential scanning calorimeter and
was recorded on the second heating cycle at a heating
−
1
rate of 10°C min . The stress–strain and stress–strain
recovery tests were carried out with a GT‐AI‐7000S
−
1
universal tester at a test speed of 500 mm min .

HE ET AL.
5 of 12
The stress–strain test for each polyethylene sample
The Ni(II) complex Cat‐1 was synthesized by the
complexation reaction of L1 and Ni(II) dibromide
dimethoxyethane adduct in high yields. Cat‐1 was charac-
terized using IR spectroscopy and elemental analysis
(supporting information, Figure S6). A single crystal of
Cat‐1 suitable for X‐ray diffraction analysis was obtained
by laying diethyl ether on a saturated methylene chloride
solution of Cat‐1 at room temperature. As shown in
Figure 2, the geometry at the nickel center of Cat‐1 was
distorted tetrahedral. Thus, the conformation of C1–N1–
Ni–N2–C2 five‐membered ring (supporting information,
Figure S7 and Table S1) in Cat‐1 with an acenaphthylene
showed no obvious difference from that of B‐Cat with a
(25 mm × 4 mm × 1 mm) was performed with five paral-
lel specimens, and the intermediate value was used to
represent the mechanical properties. The single‐crystal
structure of Cat‐1 was determined using an X‐ray diffrac-
tometer (CrysAlisPro) with graphite monochromated Mo
Kα radiation (λ = 0.71073 Å) at 173 K. The cell parame-
ters of Cat‐1 were obtained by global refinement of the
positions of all collected reflections. The structures were
solved by direct methods and refined by full‐matrix
2
least‐squares on F . All hydrogen atoms were placed in
the calculated positions.
[
20]
naphthalene backbone.
Interestingly, the dihedral
angles between two aniline phenyl planes were 3.03° for
B‐Cat and 13.20° for Cat‐1. The difference in conforma-
tion of the aniline moieties would change their steric
effects on the central metal, which could lead to different
catalytic behavior of the catalyst.
3
3
| RESULTS AND DISCUSSION
.1 | Synthesis of α‐Diimine Ligand and
Nickel(II) Complex
The designed diketone C3 for synthesizing the Ni(II) com-
plex Cat‐1 was synthesized from acenaphthene via three
[50]
simple reactions. Firstly, C1 was prepared by the acyla-
tion reaction of acenaphthene and oxalyl bromide. Further
bromination reaction of C1 using N‐bromosuccinimide
produced C2 in satisfactory yield. C2 was then reacted
with potassium iodide to form the diketone C3. The
3
.2 | Ethylene Polymerization at 1 atm
Ethylene Pressure
Ethylene polymerizations using precatalysts B‐Cat and
Cat‐1 activated by MAO, DEAC or EADC were per-
formed in the range 30 to 80°C. Both B‐Cat and Cat‐1
showed high activity and good chain walking ability in
ethylene polymerization at moderate temperature. To
evaluate the influence of catalyst backbone structure on
the catalytic behavior in ethylene polymerization, the
polymerizations catalyzed by B‐Cat or Cat‐1 were con-
ducted under similar conditions. The effects of co‐catalyst
1
diketones C1, C2 and C3 were characterized using H
NMR analysis (supporting information, Figures S1–S3).
Ligand L1 was prepared by template acid‐catalyzed
condensation reaction of C3 and 2,6‐diisopropylaniline in
1
a yield of around 78%. L1 was characterized using H
13
NMR,
C NMR, ESI‐MS and elemental analysis
(supporting information, Figures S4 and S5).
FIGURE 2 Molecular structure of the
nickel complex Cat‐1. Hydrogen atoms
have been omitted for clarity. Atoms are
drawn at a 50% probability level. Selected
bond distances (Å) and angles (°): Ni(1)–
N(1) 2.042(2), Ni(1)–N(2) 2.027(3), Ni(1)–
Br(1) 2.3288(8), Ni(1)–Br(2) 2.3359(7),
N(1)–Ni(1)–N(2) 82.8(1), N(1)–Ni(1)–Br(1)
1
08.33(8), N(1)–Ni(1)–Br(2) 116.84(8),
N(2)–Ni(1)–Br(1) 116.37(9), N(2)–Ni(1)–
Br(2) 103.46(9), Br(1)–Ni(1)–Br(2)
1
22.34(3)

6
of 12
HE ET AL.
and polymerization temperature on ethylene polymeriza-
tion were also investigated in this work.
Compared with B‐Cat, Cat‐1 catalyzed ethylene
polymerization to form polyethylene with higher Mw
under different conditions investigated in this work. Fur-
thermore, when MAO was used as co‐catalyst or when
the polymerization temperature was elevated (especially
at 80°C), the polyethylenes prepared using Cat‐1 showed
As evident from Table 1, Cat‐1 showed catalytic
behavior in ethylene polymerization similar to that of
typical α‐diimine nickel catalyst B‐Cat. With an increase
of polymerization temperature, the polymerization
activity and the molecular weight of polyethylene
decreased gradually due to increased chain transfer reac-
tions and potential decomposition of the Ni(II) complexes
at elevated temperature. In the case of Cat‐1/MAO system
even much higher M than when using B‐Cat. In the case
w
of entry 9 and entry 18, when EADC was used as co‐cat-
alyst and the polymerization temperature was 80°C, M
w
1
−
of polyethylene prepared using Cat‐1 (50.1 kg mol )
was almost twice that of polyethylene prepared using B‐
(entries 10–12), the polymerization activity and weight‐
−
1
average molecular weight (M ) decreased from 580.9 to
Cat (27.4 kg mol ). Yet, the polymerization activity,
branch density and Tg of the polyethylenes prepared
using Cat‐1 were almost the same as those prepared using
B‐Cat. Hence, with the introduction of vinylidene group
on the naphthalene backbone of α‐diimine Ni(II) catalyst,
polyethylene with higher molecular weight can be
synthesized at the same activity as the typical Brookhart
Ni(II) catalysts.
w
1
−
−1
1
57.9 kg PE (mol of Ni)
h
and from 486.8 to
−
1
1
71.9 kg mol as the temperature was raised from 30
to 80°C. Because the solubility of ethylene in toluene
decreases with increasing temperature, the ‘chain walk-
ing ability’ (β‐H elimination reaction and reinsertion) of
α‐diimine nickel catalyst will be enhanced with increas-
ing temperature, which would lead to slight increase in
branch density and decrease in glass transition tempera-
The changes of number‐average molecular weight
ture (T ) of the polyethylene. In this work, MAO, DEAC
(M ) of the polymer and the number of polymer chains
g
n
and EADC were separately used as co‐catalyst to activate
the nickel catalysts. When MAO was used as co‐catalyst
(Npol/N , N
= mpol/M ) with polymerization time
pol n
Ni
ranging from 3 to 20 min were investigated at 60°C and
using DEAC as co‐catalyst. As shown in Figure 3, Mn
and Npol/NNi both increased with increasing polymeriza-
tion time. Mn of polyethylenes produced using the
Cat‐1/DEAC system was higher than that using yhe B‐
Cat/DEAC system throughout the whole polymerization
(entries 1–3 or entries 10–12), the polyethylene exhibited
the highest M and branch density accompanied by the
w
lowest polymerization activity, which was probably due
to larger distance between the cationic active center and
the counter anion in the MAO‐activated catalysis system.
TABLE 1 Ethylene polymerizations with nickel complexes B‐Cat and Cat‐1
a
b
c
d
e
Entry
Catalyst
Co‐catalyst
Temp. (°C)
Activity
M
w
M
w
/M
n
B
g
T (°C)
1
2
3
4
5
6
7
8
9
B‐Cat
MAO
30
60
80
30
60
80
30
60
80
649.2
450.1
121.6
869.5
791.4
395.8
987.3
1178.8
616.3
38.2
19.3
11.8
34.1
14.6
8.1
18.4
8.9
2.7
1.66
1.69
1.63
1.94
2.15
1.87
2.11
1.68
1.75
116
132
133
99
108
115
104
118
121
−67.1
−71.2
−71.9
−61.9
−65.8
−69.3
−62.8
−67.1
−70.9
DEAC
EADC
10
11
12
13
14
15
16
17
18
Cat‐1
MAO
30
60
80
30
60
80
30
60
80
580.9
481.3
157.9
848.2
867.5
392.7
1038.1
981.2
594.9
48.7
32.8
17.2
35.6
19.6
12.2
19.6
9.2
1.60
1.68
1.67
2.01
2.16
1.95
2.37
1.83
1.87
111
131
135
103
112
117
103
119
121
−65.0
−70.3
−73.0
−63.6
−67.2
−68.7
−62.5
−68.3
−70.2
DEAC
EADC
5.0
a
p
Polymerization conditions: [Ni] = 100 μM, Al/Ni = 200, t = 30 min, 1 atm of ethylene, solvent: toluene.
b
−1 −1
Activity: kg PE (mol Ni)
h .
c
4
−1
×
10 g mol
.
d
1
Branch density per 1000 carbons, determined by H NMR.
e
Determined by differential scanning calorimetry, second heating scan.

HE ET AL.
7 of 12
the molecular weight and chain structure of polyethyl-
ene produced using an α‐diimine Ni(II) catalyst. In this
work, it has been proved that B‐Cat and Cat‐1 showed
different catalytic behavior in ethylene polymerization,
and Cat‐1 produced polyethylene with higher molecular
weight. To understand the reasons for such differences,
the conformation and steric effect of Cat‐1 were
investigated.
According to the single‐crystal molecular structure,
the bond lengths and bond angles around the nickel
center in B‐Cat and Cat‐1 showed no significant differ-
ences, and the conformation of C1–N1–Ni–N2–C2 five‐
membered ring in Cat‐1 was similar to that in B‐Cat.
The analogous conformation around the nickel center
could be the reason for the similar catalytic activity
and thermal stability of B‐Cat and Cat‐1 in ethylene
polymerization.
n
FIGURE 3 Changes of M and Npol/NNi of polyethylenes prepared
B‐Cat/DEAC and Cat‐1/DEAC systems with polymerization time.
Conditions: nNi = 5 μmol, T = 60°C, co‐catalyst DEAC, AlDEAC/Ni =
2
00, toluene = 50 ml, Pethylene = 1 atm, Npol = mpol/M
n
Yet, as shown in Figure 4, the introduction of vinyli-
dene group on the naphthalene backbone in Cat‐1
resulted in a more symmetric naphthalene backbone than
that of B‐Cat. The dihedral angle between two benzene
rings in the naphthalene moiety of Cat‐1 was 178.76°,
while the corresponding dihedral angle in B‐Cat was
process ranging from 3 to 20 min. The number of polyeth-
ylene chains prepared using B‐Cat and Cat‐1 showed no
significant difference during the 20 min ethylene
polymerization process.
1
76.70°. The two benzene rings in naphthalene backbone
3
.3 | Correlation with Conformation of
in Cat‐1 were more coplanar than in B‐Cat. Further, as
shown in Figure 5 and Table 2, the dihedral angles of
plane I (naphthalene), plane II (five‐membered carbon
ring) and plane III (N═C─C═N ring) between B‐Cat
Nickel Complexes
It has been supposed that a different steric environment
of the nickel metal center would lead to a difference in
FIGURE 4 Different backbone conformation between B‐Cat and Cat‐1

8
of 12
HE ET AL.
FIGURE 5 Different aniline moiety conformation between B‐Cat and Cat‐1
TABLE 2 Dihedral angles (°) of plane I, plane II and plane III
between B‐Cat and Cat‐1
3.4 | Ethylene Polymerization under
Higher Monomer Pressure
Catalyst
I and II
I and III
II and III
The ability of α‐diimine Ni(II) catalyst Cat‐1 with modi-
fied backbone to produce polyethylene with both high
molecular weight and high branch density may find prac-
tical applications. To evaluate the mechanical properties
B‐Cat
Cat‐1
176.65
179.36
169.79
176.60
173.11
177.05
(
e.g. stress–strain and stress–strain recovery tests) of
and Cat‐1 showed a significant difference. The backbone
conformation of Cat‐1 was highly conjugated and more
coplanar than that of B‐Cat.
highly branched polyethylene synthesized with Cat‐1, a
2 l autoclave was used to prepare larger amounts of poly-
mer via ethylene polymerization under higher monomer
pressure. The effects of ethylene pressure and polymeriza-
tion temperature on the mechanical properties of poly-
ethylene were investigated, and the differences between
B‐Cat and Cat‐1 were compared. The polymerization
temperature was set at 50 or 60°C considering the
requirements of the industrial polymerization process.
As evident from Table 3, Cat‐1 could produce polyeth-
ylene with higher molecular weight (as high as 510.16 kg
We suppose that rotation of the substituted phenyl
around the C ─N bond would be facilitated to some
Ar
extent in Cat‐1 as a feature of the more symmetric and
coplanar backbone. As shown in Figure 5, the dihedral
angles of two benzene ring planes in the aniline moieties
of B‐Cat and Cat‐1 were 3.03° and 13.20°, respectively,
displaying a notable difference. This was quite different
from their similar C1–N1–Ni–N2–C2 five‐membered ring
conformation. The more twisted benzene rings in the
aniline moieties of Cat‐1 may create a more sterically
congested environment around the nickel center,
because the isopropyl groups become closer to nickel.
Hence, the steric effects of the aniline moieties of Cat‐1
could change the relative rate of chain transfer reaction
to produce polyethylene with higher molecular weight,
which was consistent with our previous work
−
1
−1
PE (mol Ni) h , larger branch density and lower T ,
g
melting point (T ) and crystallinity than B‐Cat, but its
m
catalytic activity was lower than that of B‐Cat when the
ethylene pressure was 1.0 MPa. The polymerization
activity, molecular weight, T and T of polyethylene
g
m
increased with an increase in ethylene pressure and
decrease in polymerization temperature. On the contrary,
the branch density of polyethylene increased with a
decrease of ethylene pressure and increase of polymeriza-
tion temperature. And the branched chain distributions
of the polyethylene prepared using B‐Cat and Cat‐1
showed obvious differences (supporting information, Fig-
ure S8 and Table S3).
As discussed in the previous section, the improvement
in polyethylene molecular weight using Cat‐1 could be
attributed to the enhanced steric congestion of the aniline
moieties. The higher branch density of polyethylene pre-
pared using Cat‐1 could be attributed to the introduction
of vinylidene group on the naphthalene backbone. In this
[48]
(supporting information, Table S2).
Interestingly, the
branch density of polyethylene prepared using Cat‐1
was very close to that prepared using B‐Cat, which could
be correlated with the similar electronic effects and
conformation of the C1–N1–Ni–N2–C2 five‐membered
ring between B‐Cat and Cat‐1. According to previous
[38,39]
studies,
sterically congested environment around
the nickel center would lead to a decrease in the branch
density of polyethylene. It seemed that the steric bulk
aniline moieties in Cat‐1 had little influence on the chain
walking process.

HE ET AL.
9 of 12
TABLE 3 Ethylene polymerizations with B‐Cat and Cat‐1 activated by DEAC under high monomer pressure
a
Entry
19
20
21
22
23
24
Catalyst
B‐Cat
Cat‐1
B‐Cat
Cat‐1
B‐Cat
Cat‐1
Temp. (°C)
50
0.7
50
50
1.0
50
60
1.0
60
Pethylene (MPa)
0.7
1.0
1.0
b
Activity
1898
36.5
1.86
62
2011
38.5
3068
41.2
1.86
55
2411
51.0
2115
26.8
1.87
73
1596
37.0
c
w
M
w
M /M
w
1.87
1.88
1.91
d
B
T
T
75
−54.9
23.0
7.9
65
−48.2
42.5
8.3
79
−54.9
16.1
5.7
e
g
(°C)
−48.6
39.8
11.4
17.41
1404
45.3
−46.7
49.7
10.8
15.88
1137
56.0
−52.6
32.9
10.2
7.80
1578
66.7
e
m
(°C)
e
X
c
(%)
f
Stress (MPa)
9.44
12.08
1148
62.7
5.12
f
Strain (%)
1146
58.0
1292
69.9
g
ER (%)
a
p
Polymerization conditions: [Ni] = 2 μM, Al/Ni = 400, t = 30 min, solvent: n‐heptane, co‐catalyst: DEAC.
b
4
−1 −1
Activity: 10 g PE (mol Ni)
h .
c
4
−1
×
10 g mol
.
d
1
Branch density per 1000 carbons, determined by H NMR.
e
Determined by differential scanning calorimetry, second heating scan.
Determined using a universal tester.
f
g
Elastic recovery, determined using a universal tester after 10 cycles stretched to strain of 300%.
FIGURE 6 Mulliken charges of the
atoms in B‐Cat and Cat‐1 with hydrogens
summed into heavy atoms

10 of 12
HE ET AL.
work, the Mulliken charges of every atom in B‐Cat and
Cat‐1 were calculated using the Gaussian 09 program with
Perdew–Burke–Ernzerhof (PBE) functional. The basis set
of C, H, N was 6‐311G(d,p) and that of Br and Ni was
LanL2Dz and the charges of hydrogens were summed into
heavy atoms. Due to the stronger conjugation effects of
the more coplanar acenaphthylene backbone in Cat‐1,
its π‐electrons could be more firmly limited in the ace-
naphthylene backbone to make an electron‐deficient
nickel metal center (Figure 6; Mulliken charges of nickel
in B‐Cat and Cat‐1 were 0.433 versus 0.440), which would
contribute to the chain walking reaction during the
ethylene polymerization process. Moreover, the bulky
aniline moieties in Cat‐1 would suppress chain propaga-
tion rather than chain walking reaction, leading to higher
branch density. It is interesting that this effect is not so
obvious when the polymerizations were carried out at
0.1 MPa. The higher steric hindrance around nickel metal
center of Cat‐1 could have caused the lower polymeriza-
tion activity in ethylene polymerization to some extent
at relatively high ethylene pressure.
The polyethylenes prepared using both B‐Cat and
Cat‐1 showed excellent mechanical properties. The
ultimate tensile strength and elongation at break of the
polyethylenes could reach as high as 17.41 MPa and
1578%. B‐Cat produced polyethylenes with higher tensile
FIGURE 7 Comparative stress–strain hysteresis loops for polyethylenes prepared by B‐Cat/DEAC and Cat‐1/DEAC systems

HE ET AL.
11 of 12
strength than when using Cat‐1, in spite of their lower
molecular weight. It seemed that the polymer molecular
weight is not the critical parameter for influencing the
mechanical properties. The mechanical properties of
highly branched polyethylenes would be widely affected
by branch density, structure of branched chains and poly-
Cat‐1 showed excellent mechanical properties and elastic
resilience, therefore presenting good promise of novel
thermoplastic elastomers.
4
| CONCLUSIONS
[26]
ethylene crystallinity.
More linear polyethylenes with
A novel α‐diimine nickel catalyst, Cat‐1, bearing a highly
conjugated acenaphthylene backbone was synthesized by
a simple reaction route. Catalytic behaviors in ethylene
polymerization of Cat‐1 and typical Brookhart catalyst B‐
Cat were compared. As compared with B‐Cat, Cat‐1
produced polyethylene with both higher molecular weight
and higher branch density under various polymerization
conditions. Comparisons of molecular structures between
Cat‐1 and B‐Cat revealed that the dihedral angles of two
benzene ring planes in the aniline moieties of B‐Cat and
Cat‐1 were 3.03° and 13.20°, respectively, displaying a
notable difference. This difference could be the main
factor that gives Cat‐1 the ability to produce polyethylene
with both high molecular weight and high branch density.
Moreover, by the use of computational methods, we found
that the highly conjugated backbone in Cat‐1 would limit
the π‐electrons of acenaphthylene in the backbone,
making an electron‐deficient nickel metal center, which
would contribute to the chain walking reaction and lead
to highly branched polyethylene. The polyethylene pre-
pared using Cat‐1 showed excellent mechanical properties
and evidently improved elastic resilience as compared
with the polymer prepared with B‐Cat.
higher crystallinity could have larger tensile strength
because of the presence of more crystalline phases.
The stress–strain recovery tests of polyethylenes
prepared using B‐Cat and Cat‐1 were performed with a
universal tester for 10 cycles. Every polyethylene sample
was stretched to strains of 100, 300 and 500%. As shown
in Figure 7, we found that the polyethylenes could main-
tain their elasticity after 10 cycles, and there was only a
small decrease in the elastic recovery. With an increase
of polymerization temperature, the obtained polyethyl-
enes showed better elastic resilience due to their higher
branch density. So, the polyethylenes prepared using
Cat‐1 with higher branch density also showed better elas-
tic resilience than those prepared using B‐Cat. Especially,
polyethylene prepared using Cat‐1 at 50°C and 0.7 MPa
ethylene pressure showed nearly complete recovery when
it was stretched to 100% strain. When this sample was
stretched to strain of 300%, the elastic recovery was
5
8.0%, which was much better than that of the sample
prepared using B‐Cat (45.3%), revealing the better elastic
[
26]
resilience of the former. According to previous studies,
the branch density and crystallinity would play an
important role in the elastic properties of polyethylene
materials. The polyethylenes prepared using Cat‐1 had
higher branch density (Table 3; 75 versus 62 branches
per 1000 carbons) and lower crystallinity (Table 3; 7.9
versus 11.4%) than those prepared using B‐Cat. So higher
branch density and lower crystallinity might be the main
reasons for the better elastic resilience of polyethylenes
prepared using Cat‐1. Meanwhile, the ultimate tensile
strength and elongation at break of polyethylene pre-
pared with Cat‐1 were as high as 9.44 MPa and 1146%,
respectively. Such mechanical performances could be
related to the formation of physical crosslinks and chain
entanglements in the highly branched polyethylenes
ACKNOWLEDGMENTS
Support by the National Key Research and Develop-
ment Program of China (no. 2017YFE0106700) and
the Open Research Project of the State Key Laboratory
of Industrial Control Technology, Zhejiang University
(
no. ICT1800411) is gratefully acknowledged.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
[24,51]
during stretching.
Overall, introducing a vinylidene group in the ligand
backbone evidently modified the catalytic performances
of α‐diimine Ni(II) catalyst by changing the stereochemi-
cal effects of the aniline moieties and electronic effect on
the central metal, and the novel catalyst (Cat‐1) showed
an ability to produce polyethylenes with both high
molecular weight and high branch density. This kind of
chain structure combination can hardly be achieved
using α‐diimine Ni(II) catalysts with very bulky aniline
moieties. The resultant polyethylene prepared using
ORCID
Feng He http://orcid.org/0000-0002-7217-8006
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SUPPORTING INFORMATION
https://doi.org/10.1039/c8py00725j
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