Systematic Investigations of Ligand Steric Effects on α-Diimine Nickel Catalyzed Olefin Polymerization and Copolymerization

Article abstract of DOI:10.1021/acs.organomet.9b00267

So far, ligand steric effects of the α-diimine nickel catalysts on the polyolefin branching densities are not systematically investigated. Generally, in contrast to the α-diimine palladium systems, the branching densities of the polyethylene obtained by the α-diimine nickel catalysts increased when the more sterically encumbering substituent was employed. In this contribution, we described the synthesis and characterization of a series of α-diimine ligands and the corresponding nickel catalysts bearing the diarylmethyl moiety and varied steric ligands. In ethylene polymerization, the catalytic activities [(2.82-15.68) × 10⁶ g/(mol Ni·h)], polymer molecular weights [M_n: (0.37-131.51) × 10⁴ g mol^-1], branching densities [(28-81)/1000 C], and polymer melting temperatures (-4.7-122.9 °C) can be tuned over a very wide range. To our surprise, the polymer branching density first rose and then fell when we systematically increased the steric bulk of α-diimine nickel catalysts, like a downward parabola, not in line with previous conclusions. In ethylene-methyl 10-undecenoate (E-UA) copolymerization, the catalytic activities [(1.0 × 10³) - (104.8 × 10⁴) g/(mol Ni·h)], copolymer molecular weights [(1.2 × 10³) - (242.4 × 10³) g mol^-1], branching densities [(42-70)/1000 C], and UA incorporation ratio (0.17-2.12%) can also be controlled over a very wide range. The tuning in steric ligands enables the tuning of the polymer microstructures such as molecular weight and branching density. In this way, the best polyethylene elastomer catalysts are screened out.

Full text of DOI:10.1021/acs.organomet.9b00267

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Systematic Investigations of Ligand Steric Effects on α‑Diimine
Nickel Catalyzed Olefin Polymerization and Copolymerization
Yanfeng Gong,†,‡ Shuaikang Li,‡ Qi Gong,‡ Shaojie Zhang,*,‡ Binyuan Liu,*,† and Shengyu Dai*,‡
†National-Local Joint Engineering Laboratory for Energy Conservation of Chemical Process Integration and Resources Utilization,
School of Chemical Engineering and Technology, Hebei University of Technology, No 8 Guangrong Road, 300130 Tianjin, China
‡Institutes of Physical Science and Information Technology, School of Computer Science and Technology, Anhui University, Hefei,
Anhui 230601, China
S
* Supporting Information
ABSTRACT: So far, ligand steric effects of the α-diimine nickel
catalysts on the polyolefin branching densities are not systemati-
cally investigated. Generally, in contrast to the α-diimine palladium
systems, the branching densities of the polyethylene obtained by
the α-diimine nickel catalysts increased when the more sterically
encumbering substituent was employed. In this contribution, we
described the synthesis and characterization of a series of α-diimine
ligands and the corresponding nickel catalysts bearing the
diarylmethyl moiety and varied steric ligands. In ethylene
polymerization, the catalytic activities [(2.82−15.68) × 106 g/
(mol Ni·h)], polymer molecular weights [Mn: (0.37−131.51) × 104
g mol−1], branching densities [(28−81)/1000 C], and polymer
melting temperatures (−4.7−122.9 °C) can be tuned over a very
wide range. To our surprise, the polymer branching density first rose and then fell when we systematically increased the steric
bulk of α-diimine nickel catalysts, like a downward parabola, not in line with previous conclusions. In ethylene-methyl 10-
undecenoate (E-UA) copolymerization, the catalytic activities [(1.0 × 103) − (104.8 × 104) g/(mol Ni·h)], copolymer
molecular weights [(1.2 × 103) − (242.4 × 103) g mol−1], branching densities [(42−70)/1000 C], and UA incorporation ratio
(0.17−2.12%) can also be controlled over a very wide range. The tuning in steric ligands enables the tuning of the polymer
microstructures such as molecular weight and branching density. In this way, the best polyethylene elastomer catalysts are
screened out.
Scheme 1. Two Examples of Systematic Investigations of
Ligand Steric Effects on Late Transition Metal Catalyzed
Ethylene Polymerization and Copolymerization
INTRODUCTION
■
Ligand steric and electronic effects are two of the most
important factors in late transition metal catalyzed ethylene
polymerization and copolymerization.1 Systematic investiga-
tions of ligand electronic effects in late transition metal
catalyzed ethylene polymerization and copolymerization have
been widely explored in different catalytic systems.2−8
However, systematic investigations of ligand steric effects are
rarely reported since the substituents’ change is tedious work.
One of the famous examples is the quantification of the steric
influence of palladium/alkylphosphine−sulfonate catalysts on
ethylene polymerization9 (Scheme 1, 1). Good correlation
between the steric bulk of the ligands and the molecular weight
of the (co)polymers was observed. Consequently, bulky
alkylphosphine−sulfonate Pd complexes bearing menthyl
groups (Scheme 1, 1f) was used to obtain high-molecular-
weight polymers and copolymers. Notably, in the α-diimine
Pd(II) and Ni(II) catalyst system,10−16 steric effects on the
ortho-aryl substituents in the ligand could slow the chain
transfer process and thus is the key factor to produce high
polymer molecular weight polyethylene. The slow rate of chain
transfer was ascribed to a slow associative displacement of the
unsaturated polymer chain since the bulky ortho substituents
can block the axial approach of the monomer.17 Chen’s group
reported a series of systematically varied steric ligands of α-
diimine palladium catalysts (Scheme 1, 2) and further
Received: April 25, 2019
© XXXX American Chemical Society
A
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Scheme 2. Previously Reported α-Diimine Nickel Catalysts
Scheme 3. Synthesis of Nickel Complexes
confirmed the conclusions above.18 It is normal that the more
sterically encumbering substituents lead to higher catalytic
activities, higher polymer molecular weights, and lower
branching densities. In ethylene polymerization, the catalytic
activities [(0.77−8.85) × 105 g/(mol Pd·h)], polymer
molecular weights [Mn: (0.2−164.7) × 104], branching
densities [(25−116)/1000 C], and polymer melting temper-
atures (amorphous to 98 °C) of this system can be tuned over
a very wide range when we systematically improved the steric
bulk effects. Similar results were found in ethylene-methyl
acrylate (E-MA) copolymerization.
The α-diimine nickel catalysts with simply steric ligands
increased are reported extensively (Scheme 2, I−V). A series of
α-diimine Ni(II) with various backbone structures such as
butane (Scheme 2, I),19 acenaphthene (Scheme 2, II),19
camphyl (Scheme 2, III),20 and dibenzobarrelene (Scheme 2,
IV)21 were investigated previously; the polyethylene obtained
by these catalysts showed similar regularity, i.e., the molecular
weights of the polyethylene increased when the more sterically
encumbering substituent was added. This is consistent with the
α-diimine palladium systems. However, in contrast to the α-
diimine palladium systems, the branching densities of the
polyethylene obtained by the α-diimine nickel catalysts
increased when the more sterically encumbering substituent
was employed. The xanthene bridged dinuclear α-diimine
Ni(II) system (Scheme 2, V) also showed the same trend.19
The branching densities of the polyethylene or copolymers are
among the most important factors in determining the
macroscopic physical and chemical properties of a polymer.14
Therefore, it is very meaningful to systematically investigate
the ligand steric effects of the α-diimine nickel catalysts.
In this contribution, a series of acenaphthene-based α-
diimine nickel complexes bearing both diarylmethyl moiety
and systematically varied ligand steric environments were
designed to address this issue (Scheme 2, D). Catalysts with
similar structures and polymerization results have previously
been reported (Scheme 2, A−C).22−24 Very interestingly, the
ligand steric effect was found to be different from that of the
corresponding α-diimine palladium systems. The correlation
between the branching densities of the obtained polyethylene
and the steric hindrance seems to be like a parabola. In
addition, the mechanical properties of the generated poly-
ethylene also followed the same trend.
RESULTS AND DISCUSSIONS
■
Catalyst Synthesis. The monoimine ligand 2-((2,6-bis(di-
p-tolylmethyl)-4-methylphenyl)imino) acenaphthylen-1-one
was prepared from the reaction of 2,6-bis(diphenylmethyl)-4-
methylaniline with 1 equiv of acenaphthylen-1,2-dione at 70%
yield on the multigram scale (Scheme 3). Subsequently, the
reaction with 2 equiv of the corresponding anilines catalyzed
B
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Figure 1. Molecular structures of the nickel complexes 2 and 3 with 30% probability level, and H atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (deg) for (a) complex 2: Ni(1)−N(1), 2.009(6); Ni(1)−N(2), 2.044(5); Ni(1)−Br(1), 2.3466(15); Ni(1)−Br(2),
2.3363(15); N(1)−Ni(1)−N(2), 82.5(3); Br(2)−Ni(1)−Br(1), 120.89(6); (b) complex 3: Ni(1)−N(1), 2.059(4); Ni(1)−N(2), 2.017(5);
Ni(1)−Br(1), 2.3283(7); N(1)−Ni(1)−N(2), 82.65(19); Br(i1)−Ni(1)−Br(1), 116.60(4).
a
Table 1. Effect of Catalyst and Temperature on Ethylene Polymerization
b
c
c
d
a
Entry
Precat.
T (°C)
Yield/g
Actual
Mn
Mw/Mn
B
Tm/(°C)
1
2
3
4
5
6
7
8
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
20
40
60
80
20
40
60
80
20
40
60
80
20
40
60
80
20
40
60
80
7.84
5.77
4.26
2.05
2.98
2.66
2.69
2.64
2.50
3.91
3.14
2.48
3.24
3.42
3.12
2.18
1.41
1.52
1.86
2.01
15.68
11.54
8.52
4.10
5.96
5.32
5.38
5.28
5.00
7.82
6.28
4.96
6.48
6.84
6.24
4.36
2.82
3.04
3.72
4.02
0.65
0.86
0.58
2.20
2.49
2.27
1.92
2.19
2.37
2.70
2.67
1.79
2.07
2.39
2.30
1.97
1.86
2.05
2.35
1.46
1.58
1.74
1.95
39
55
62
74
51
74
76
80
64
64
81
81
35
48
52
61
28
38
45
115.1, 122.9
122.1
119.8
wax
73.6
46.0
31.0
10.1
47.2
40.2
−3.2
−4.7
81.7
73.4
58.5
51.6
107.3
81.6
72. 0
71.0
0.37
51.24
31.28
21.78
12.02
71.55
68.53
43.46
38.44
100.25
103.76
67.55
54.56
131.51
104.84
95.22
86.34
9
10
11
12
13
14
15
16
17
18
19
20
46
a
b
General conditions: Ni = 1.0 μmol, Al/Ni = 600, CH2Cl2 = 2 mL, toluene = 40 mL, ethylene = 8 atm, time = 30 min. Activity = 106 g of PE (mol
c
d
of Ni)−1 h−1. Mn: 104 g mol−1, Mn and Mw/Mn determined by GPC. Branching numbers per 1000 C were determined by H NMR. Branching
numbers for low molecular weight samples (Mn < 10 000) were corrected for end groups. Melting temperature determined by DSC.
1
e
by p-Ts (p-toluene sulfonic acid) led to the formation of the
acenaphthene-based α-diimine ligands L1−L4 at 31−53%
yields. Symmetric acenaphthene-based α-diimine bearing the
bulky diarylmethyl moiety L5 was prepared with no
chromatography involved in 48.2% yields by an efficient
method we previously reported.25
The nickel complexes 1, 2, 3, 4, and 5 were readily prepared
in 86−93% yields from the reaction of the corresponding
ligands with 1 equiv of (DME)NiBr2 (DME = 1,2-dimethoxy-
ethane). These nickel complexes were characterized by mass
spectrometric and elemental analysis as well as X-ray
crystallography. The molecular structures of complexes 2 and
3 are shown in Figure 1. The bond distances and bond angles
are typical, compared with some previously reported α-diimine
nickel complexes.22,23,26 The effective blockage of the axial
positions of the metal center from the N-aryl moiety was
observed. The steric properties of nickel complexes 1−5
bearing a diarylmethyl moiety and varied steric ligands were
evaluated using the percentage of buried volume (%VBur) as a
molecular descriptor.27,28 The steric maps of the five
complexes are shown in Figure S2. The %VBur of 1−5 was
calculated to be 77.9%, 82.3%, 83.7%, 86.4%, and 87.6%,
respectively, indicating that the steric bulk of the α-diimine
nickel catalysts increased from 1 to 5.
Ethylene Polymerization. Nickel complexes 1−5 were
screened for ethylene polymerization with MAO (methyl-
aluminoxane) as cocatalyst under 8 atm of ethylene pressure at
different temperatures, and the polymerization results are
summarized in Table 1. These nickel complexes are all highly
active (well above 106 g of PE (mol of Ni)−1 h−1) in ethylene
C
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Figure 2. (a) Molecular weight (Mn) of the polyethylene obtained by catalysts 1−5 (Table 1, entries 1, 5, 9, 13, 17). (b) Branching density of the
polyethylene generated using catalysts 1−5 (Table 1, entries 1, 5, 9, 13, 17).
polymerization (Table 1). Complex 1 showed the highest
activity at 1.6 × 107 g·mol−1·h−1 at 20 °C and 8 atm of
ethylene, which was comparable to those of metallocene
catalysts.29 In view of the mass transfer-limited rates caused by
the high polymerization activities and the highly branched PEs
produced by complexes 2−4, the polymerizations were
conducted at short times to minimize the effects from mass
transport (Table S2, Supporting Information). It was indicated
that the polymerization activity decreased with steric ligands
increasing from complex 1 to complex 5. In the α-diimine
nickel catalysts, the rate-determining step in ethylene polymer-
ization is the coordination of ethylene monomers to metal
centers; therefore, the higher the steric resistance of catalyst,
the lower the activity of ethylene polymerization. At the same
time, the molecular weight (Mn) of polyethylene were
increased gradually from 6.5 × 103 to 1.31 × 106 g/mol
(Table 1, Figure 2a) due to the bulkier substituents slowing the
chain transfer efficiency. This is consistent with previous
reports and theories.9,17,18
Most interestingly, the polymer branching density increased
from 39 to 64/1000 C and decreased later from 64 to 28/1000
C (Table 1, Figure 2b) along with the increasing steric bulk of
α-diimine nickel catalysts, indicating a different relationship
between the polymer branching density and the steric
bulk.17,19−21 The branching degree of polyethylene is
determined by two factors: the chain walking speed and the
chain growth rate. When the ortho-aryl substituents in the
ligand, or the axial substituents of the metal center, become
bulkier, the chain walking speed and the chain growth rate
both become slower, but the ratio of the two factors changes
with the increase of the steric hindrance. From complexes 1 to
3, the bulky substituents can slow chain growth rate more than
the rate of chain walking, thus more chain walking and
branching can occur in the catalysts from complexes 1 to 3. In
contrast, from 3 to 5, the highly bulky substituents could make
the rate of chain walking slower than the rate of chain growth.
The previously reported α-diimine palladium systems showed
that the insertion of ethylene into a primary metal alkyl species
is easier than the insertion into a secondary metal alkyl
species.18 Similarly, the highly bulky α-diimine nickel systems
might have similar behavior. Consequently, the relatively
greater preference of insertion of ethylene into a primary metal
alkyl species versus insertion into a secondary metal alkyl
species may also contribute; thus less chain walking and
branching can occur in the catalysts from complexes 3 to 5.
Correspondingly, the melting points of polyethylene products
follow the contrasting trend, first falling from 123 to 47 °C and
then rising from 47 to 107 °C, like an upward parabola.
The polymerization activity of complex 1 with the lowest
steric hindrance decreases rapidly from 1.6 × 107 g·mol−1·h−1
at 20 °C to 4.1 × 106 g·mol−1·h−1 at 80 °C. In contrast,
complexes 2−4 with moderate steric hindrance exhibited
similar activities at 20−80 °C. Furthermore, complex 5 with
the bulkiest substituents diarylmethyl displayed higher activity
at higher temperatures. Complex 5 has a good thermal stability
at 100 °C (Table 2, Figure 3). At 80 °C, almost constant
Table 2. Effect of Reaction Time on Ethylene
Polymerizations Using Catalyst 5 at Elevated
a
Temperatures
b
b
c
c
Entry T (°C) Time (min) Yield/g
Act.
Mn
Mw/Mn
1
2
3
4
5
6
7
8
9
10
80
80
80
80
80
100
100
100
100
100
5
10
15
30
60
5
10
15
30
60
0.31
0.61
0.95
2.01
3.04
0.39
0.78
1.12
1.57
2.52
3.72
3.66
3.80
4.02
3.04
4.68
4.68
4.48
3.14
2.52
35.15
53.98
60.33
86.34
120.94
52.15
50.78
57.30
54.50
30.28
1.61
1.71
1.88
1.95
1.52
1.53
1.83
1.87
2.02
3.51
a
General conditions: Ni = 1.0 μmol, Al/Ni = 600, CH2Cl2 = 2 mL,
b
toluene = 40 mL, ethylene = 8 atm. Activity = 106 g of PE (mol of
c
Ni)−1 h−1. Mn: 104 g mol−1, Mn and Mw/Mn determined by GPC.
activity [(3.04−4.02) × 106 g·mol−1·h−1] was found over 60
min (Figure 3a). Moreover, the molecular weight increased
with polymerization time and reached over 1.21 × 106 g·mol−1
(Figure 3b). Further, polymerizations conducted at 100 °C
showed a slight decrease of activity (from 4.68 × 106 g·mol−1·
h−1 to 2.52 × 106 g·mol−1·h−1) (Figure 3a) as well as molecular
weight (from 57.30 × 104 g·mol−1 to 30.28 × 104 g·mol−1)
(Figure 3b) with time. These results indicated that the chain
transfer and catalyst deactivation of complex 5 were suppressed
by the bulkiest ligand.
Ethylene-Methyl 10-Undecenoate (UA) Copolymer-
ization. Recently, the nickel-catalyzed ethylene copolymeriza-
tion with polar functionalized comonomers have been widely
studied;30−39 the investigations of ligand steric effects on the
nickel-catalyzed ethylene copolymerization with polar func-
tionalized comonomers are rarely reported. Here we use the
nickel complexes 1−5 to catalyze the copolymerization of
ethylene with methyl 10-undecenoate in the presence of 1000
equiv of Et2AlCl (Table 3). The activities and polymer
molecular weights were largely reduced in ethylene copoly-
merization, except complex 3 with moderate steric hindrance
showed the highest activities at 1.0 × 106 g·mol−1·h−1 at 20 °C
(Table 3, entry 3), and complex 4 showed the highest
molecular weight at 24.2 × 104 g/mol at 20 °C (Table 3, entry
D
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Figure 3. Plots of (a) activity versus time and (b) Mn versus time for 5 at 80 °C (black) and 100 °C (red).
a
Table 3. Ethylene-Methyl 10-Undecenoate (UA) Copolymerization
b
c
d
e
Entry
Catalyst
T (°C)
[UA](M)
Yield (g)
Actual
XUA (%)
Mn (×10−3
)
PDI
B
1
2
3
4
5
6
7
8
1
2
3
4
5
5
5
5
5
5
2
3
4
2
3
4
20
20
20
20
20
40
60
80
80
80
20
20
20
20
20
20
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.16
0.32
0.16
0.16
0.16
0.32
0.32
0.32
0.05
2.86
5.24
1.62
1.46
1.36
1.52
1.03
1.58
0.13
2.51
1.15
1.30
0.98
0.58
1.02
1.0
57.2
104.8
32.4
29.2
27.2
30.4
20.6
31.6
2.6
50.2
23.0
26.0
19.6
11.6
20.4
0.32
0.53
0.43
0.23
0.17
0.23
0.51
0.59
1.03
1.54
1.04
1.02
0.59
2.03
2.12
0.99
1.2
22.1
85.8
242.4
10.4
3.3
2.7
3.0
2.2
1.6
18.9
40.3
65.7
7.8
18.8
17.7
3.05
3.01
2.99
4.21
1.80
3.53
2.36
5.10
2.36
2.45
2.83
2.85
6.95
3.40
2.78
3.31
59
61
70
45
42
51
54
57
64
58
61
59
45
55
66
47
9
10
11
12
13
14
15
16
a
General conditions: Ni = 5.0 μmol, Al/Ni = 1000, CH2Cl2 = 2 mL, total volume of toluene and UA: 20 mL, ethylene = 2 atm, time = 60 min.
b
c
d
e
Activity = 104 g of PE (mol of Ni)−1 h−1. XUA = UA incorp. (mol %). Mn: 104 g mol−1, Mn and Mw/Mn determined by GPC. B = branches per
1000 carbons, branching numbers were determined using 1H NMR spectroscopy, the branches ending with functional groups are added to the total
branches.
Figure 4. Stress−strain curves for polyethylene generated by (a) 2, (b) 3, (c) 4, and (d) 5 at 20, 40, 60, and 80 °C.
4). Complex 1 with the lowest steric hindrance showed the
lowest activity and molecular weight, revealing that it is easily
poisoned by the polar comonomer. The comonomer
incorporation ratio decreased with steric ligand increasing
from complex 2 to complex 5 (Table 3, entries 2−5). This is
consistent with previous reports and theories.9,18 For
E
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Figure 5. Plots of hysteresis experiments of ten cycles at a strain of 300% for samples generated by 2 at 60 °C (a), 3 at 60 °C (b), 4 at 60 °C (c),
and 5 at 60 °C (d). Stress−strain curves during step-cycle tensile deformation at different strains of the polyethylene samples generated using
catalysts 2 at 20 °C (e), 40 °C (f), 60 °C (g), and 80 °C (h).
complexes 2−5, an increase in comonomer concentration led
to decreases in both activity and copolymer molecular weight
and an increase in the comonomer incorporation ratio (Table
3, entries 8−16). The copolymers with molecular weights of
between 1.2 × 103 and 24.2 × 104 g/mol and comonomer
incorporation ratios 0.17−2.12% were generated (Table 3). As
complex 5 has great thermal stability at high temperatures, the
copolymerization using complex 5 was conducted at 40, 60,
and 80 °C, the comonomer incorporation ratios went up when
the temperature rose, and the activity almost remained the
same (Table 3, entries 5−8). This shows that complex 5 is also
thermally stable in the copolymerization. However, the
copolymer molecular weights obtained by complex 5 at high
temperatures are very low. The probable reason is that the
chain transfer may accelerate with the comonomer at high
temperatures. It should be pointed out that the amount of
Et2AlCl is much larger than that of polar comonomers. As
such, the protecting effect of the aluminum cocatalyst on the
polar comonomers helps in avoiding their poisoning toward
the nickel catalysts.
Properties of the Polymer Products. The steric bulk of
the ligand has a significant influence on the degree of
branching density of the generated PEs (Table 1, Figure 2b).
This translates into tuning of the properties of the resulting
polymeric products such as melting temperatures and
mechanical properties. The polyethylene products generated
using catalyst 1 are low molecular weight wax which cannot be
tested in the tensile tests. To examine the mechanical
properties of these branched PEs, tensile tests were carried
out for all the polyethylene samples prepared by using 2, 3, 4,
and 5 catalysts at different polymerization temperatures
(Figure 4 and Table S1, Supporting Information). These
samples showed stress at break values in the range from 0.3 to
22.1 MPa and exhibited typical features of elastomers with high
strain at break values ranging from 208% to 1558%. In general,
the higher the reaction temperature, the higher the branching
density of the obtained polyethylene, the lower the melting
point of the obtained polyethylene, and the greater the strain at
break of the obtained polyethylene. Furthermore, polyethylene
obtained by the catalysts 2 and 3 with moderate steric
hindrance displayed lower stress at break values (0.3−9.6
MPa) than those obtained by catalysts 4 and 5 with larger
steric hindrance (7.8−22.1 MPa) at the same polymerization
conditions. Clearly, the polyethylene microstructures including
molecular weights and branching densities, which can be
modulated by catalyst steric bulk and polymerization
conditions, have a significant influence on the mechanical
properties of these polyethylene.
In order to study the elastic recovery of the obtained
polyethylene, that is, the ability to return to the initial state
once the force is removed, the polyethylene samples were also
used in a hysteresis experiment in which each sample was
extended to 300% strain in 10 cycles. (Figure 5 and Table S1,
Supporting Information). The strain recovery values (SR) can
be calculated by SR = 100 (εa − εr)/εa (εa = the applied strain,
εr = the strain in the cycle at zero load after the 10th cycle).
These polyethylene samples exhibit a certain amount of
unrecovered strain after the first cycle, followed by minimal
deformation in each subsequent cycle. A permanent structural
change occurs during the first cycle, after which a better
elastomeric property is produced. Overall, these samples
exhibit SR values ranging from 22% to 80%, which are
comparable with previously reported elastic polyolefin
materials obtained by late transition metal catalysts.40−43 The
high molecular weight and highly branched microstructure of
the polyethylene appear to contribute to their unique elastic
properties. Therefore, the current catalytic system uses only
ethylene as the feedstock to synthesize thermoplastic
elastomers in only one step, which has been reported
recently.26,41−43 The steric bulk of the catalyst plays an
important role in the elastic properties of these polyethylene
samples. The strain recovery values (SR) for the polyethylene
sample produced using 2, 3, 4, and 5 at 60 °C are 77%, 80%,
46%, and 22%, respectively (Figure 5a−d). Catalysts 2 and 3
F
DOI: 10.1021/acs.organomet.9b00267
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Organometallics
Article
Accession Codes
with moderate steric hindrance clearly stand out, displaying
excellent elastic properties, maintaining SR values of >75%.
This result can be attributed to the highest branching density
of polyethylene samples produced using catalysts 2 and 3 with
moderate steric hindrance. In addition, the samples generated
by 2 at different temperatures (20, 40, 60, and 80 °C) were
tested during step-cycle tensile deformation at different strains
(Figure 5e−h). The sample generated by 2 at 60 °C has the
best elastic properties, maintaining SR values of >80%
throughout the cyclic tensile deformation. Curiously, the
polymer sample generated by 2 at 80 °C showed the much
lower SR value (SR = 45%) and stress values (0.3 MPa) at
300% (Figure S1, Supporting Information) than that generated
at 60 °C. Since these two samples have similar branching
densities, the higher molecular weight of the polyethylene
sample generated by 2 at 60 °C (21.78 × 104 g·mol−1) than
that at 80 °C (12.02 × 104 g·mol−1) seems to enhance the
elastic property. 13C NMR analysis showed that the poly-
ethylene prepared by complexes 2 composed of various alkyl
branches from C1 to C ≥ 5, even sec-butyl (branch on branch
structure), the highly branched microstructures, and the
presence of various alkyl branches in these samples probably
contribute to their unique properties (Figure S3, Supporting
Information).
CCDC 1886683−1886685 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhangshaojie@ahu.edu.cn.
*E-mail: byliu@hebut.edu.cn.
*E-mail: daiyu@ustc.edu.cn.
■
ORCID
Binyuan Liu: 0000-0003-2204-8489
Shengyu Dai: 0000-0003-4110-7691
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by National Natural Science
Foundation of China (Grant No. 51703215, 51873055), the
Fundamental Research Funds for the Central Universities
(WK2060200024), China Postdoctoral Science Foundation
(2017M612077). Natural Science Foundation of Hebei
Province (Grant No. B2018202112).
CONCLUSIONS
■
In summary, a series of acenaphthene-based α-diimine nickel
complexes bearing bulky diarylmethyl moiety and with
systematically varied steric ligands have been synthesized and
characterized. Activated by MAO, these nickel catalysts
showed high activities (up to 1.6 × 107 g·mol−1·h−1) and
great thermal stability (quite stable at up to 100 °C) and
generated branched polyethylene (Mn up to 1.31 × 106 g·
mol−1) with branching density in a wide range of (28−81)/
1000 C in ethylene polymerization. The systematically varied
steric ligands in these complexes and polymerization temper-
ature exert great influence on the catalytic properties of these
nickel catalysts as well as the mechanical properties of the
generated polyethylene. Specifically, the moderate steric
hindrance catalysts 2 and 3 with much higher branching
density and elastic strain recovery value than other catalysts at
the same polymerization temperature. These branched poly-
ethylene materials displayed properties characteristic of
thermoplastic elastomers. As a result, this work may provide
an alternative and effective strategy to screen out the best
polyethylene elastomer catalyst. Activated by Et2AlCl, the
investigations of the steric ligand effects on the nickel-catalyzed
ethylene copolymerization with methyl 10-undecenoate have
been studied; the moderate steric hindrance catalysts 2 and 3
also showed the best copolymerization properties by both the
comonomer incorporation ratio and copolymer molecular
weight.
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