Ligand steric effects on α-diimine nickel catalyzed ethylene and 1-hexene polymerization

Article abstract of DOI:10.1039/c7ra11783c

A series of ortho-dibenzhydryl or ortho-sec-phenethyl substituted α-diimine nickel complexes with systematically varied ligand steric effects were used as precatalysts for the polymerization of ethylene and 1-hexene upon activation with Et₂AlCl. The effects of ligand sterics and polymerization temperature on the catalytic activity, molecular weight and polymer microstructure were evaluated in detail. In ethylene polymerization, it is possible to tune the catalytic activities [(0.98-2.41) × 10⁶ g (mol Ni h)^-1], polymer molecular weights [M_n: (1.8-13.1) × 10⁵ g mol^-1] and branching densities (55-108/1000C) over a very wide range. The molecular weights and branching structure depended on the nickel complexes as well as the polymerization temperature, and the polymer branching densities were decreased with increasing ligand steric effects and decreasing polymerization temperature. Polymerization of 1-hexene with ortho-dibenzhydryl substituted nickel complexes resulted in branched polymers (106-140/1000C) with high molecular weights [M_n: (0.64-3.88) × 10⁴ g mol^-1] and narrow molecular weight distribution (M_w/M_n = 1.16-1.52, 40-80 °C). The increasing steric hindrance of the catalyst leads to enhanced 2,1-insertion of 1-hexene and the chain-walking reaction.

Full text of DOI:10.1039/c7ra11783c

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Ligand steric effects on a-diimine nickel catalyzed
ethylene and 1-hexene polymerization†
Cite this: RSC Adv., 2017, 7, 55051
ab
a
a
Jinlong Sun,a Fuzhou Wang,
*
*
Weimin Li and Min Chen
A series of ortho-dibenzhydryl or ortho-sec-phenethyl substituted a-diimine nickel complexes with
systematically varied ligand steric effects were used as precatalysts for the polymerization of ethylene
and 1-hexene upon activation with Et2AlCl. The effects of ligand sterics and polymerization temperature
on the catalytic activity, molecular weight and polymer microstructure were evaluated in detail. In
ethylene polymerization, it is possible to tune the catalytic activities [(0.98–2.41) ꢀ 106 g (mol Ni h)ꢁ1],
polymer molecular weights [Mn: (1.8–13.1) ꢀ 105 g molꢁ1] and branching densities (55–108/1000C) over
a very wide range. The molecular weights and branching structure depended on the nickel complexes as
well as the polymerization temperature, and the polymer branching densities were decreased with
increasing ligand steric effects and decreasing polymerization temperature. Polymerization of 1-hexene
with ortho-dibenzhydryl substituted nickel complexes resulted in branched polymers (106–140/1000C)
with high molecular weights [Mn: (0.64–3.88) ꢀ 104 g molꢁ1] and narrow molecular weight distribution
(Mw/Mn ¼ 1.16–1.52, 40–80 ꢂC). The increasing steric hindrance of the catalyst leads to enhanced 2,1-
insertion of 1-hexene and the chain-walking reaction.
Received 25th October 2017
Accepted 28th November 2017
DOI: 10.1039/c7ra11783c
rsc.li/rsc-advances
a long methylene sequence in a u,1-enchainment (iv). If the
metal fails to chain walk aer 1,2-insertion, the nal polymer
Introduction
contains n-alkyl branches (ii), which is found in common
poly(a-olen)s. The highly-branched polymers with high
amounts of methyl and alkyl branches are amorphous, while
the chain-straightened polymers will be semi-crystalline.6
Previous studies indicated that sterically bulky ligands are
usually required to afford nickel and palladium catalysts
capable of generating high molecular weight polymers.2c Chain
straightening polymerization of a-olens can generate highly
linear polyethylene, which is the most abundantly produced
plastic owing to its inexpensive monomer, thermoplastic
properties, and semi-crystalline nature.4d,8 The branching
degree of the polymer can be controlled via catalyst structures
(Chart 1) and polymerization conditions such as ethylene
pressures or polymerization temperatures.9,10 For example, Wu
et al. synthesized a camphyl-based a-diimine nickel catalyst C1
that can polymerize ethylene, propylene and a-olens in a living
fashion under the optimized conditions.11 This catalyst exhibi-
ted 1,3-enchainment fraction of 45% in propylene polymeriza-
tion, and produced poly(a-olen)s with an obvious Tm.
Brookhart et al. reported a “sandwich” (8-p-tolyl naphthyl)
substituted a-diimine nickel catalyst C2 that yielded highly-
branched (up to 152 branches/1000C) polyethylene.12 Coates
et al. reported that the living/controlled polymerization of
propylene and higher a-olens using C2-symmetric a-diimine
nickel catalyst C3a bearing chiral sec-phenethyl moiety.13
Recently, the same group synthesized a dibenzobarrelene-
bridged a-diimine nickel catalyst C3b bearing chiral sec-
The research on ethylene and a-olen polymerizations cata-
lyzed by late-transition metal catalysts has been a great high-
light for the development of polyolen materials in recent
years.1,2 The nickel and palladium complexes ligated by a-dii-
mine3,4 showed good activity for ethylene and a-olen poly-
merizations to produce a variety of branched polyolens,
because the metal-alkyl species can move on the polymer
backbone before the next monomer insertion, i.e., chain-
walking,5 via rapid b-hydride elimination and re-insertion
(Scheme 1). Chain-walking polymerization can give polymers
with unique structures which cannot be obtained by common
vinyl polymerization.6,7 The mechanism of chain-walking for a-
olen polymerization involves 1,2- and 2,1-insertion followed by
chain-walking (Scheme 1(ii–iv)), in which the active metal
center undergoes chain-walking to the terminal carbon fol-
lowed by monomer insertion.8 1,2-Insertion gives the methyl
branch (iii) in a u,2-enchainment, while 2,1-insertion gives
aAdvanced Catalysis and Green Manufacturing Collaborative Innovation Center,
School of Petrochemical Engineering, Changzhou University, Changzhou 213164,
China. E-mail: wangfuzhou1718@126.com; misschen@ustc.edu.cn
bGraduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-
Hiroshima 739-8527, Japan
† Electronic supplementary information (ESI) available: NMR spectra of ligands
and polymers; GPC curves of polymers. CCDC 1576125, 1576349, 1468457
contain the supplementary X-ray crystallographic data for complexes 2, 3 and 5.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra11783c
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Scheme 1 Chain-walking mechanism for ethylene and a-olefin polymerization catalyzed by a-diimine nickel and palladium complexes.
Chart 1 Selected examples of a-diimine nickel and palladium catalysts.
phenethyl moiety.4d The catalyst afforded high molecular weight molecular weight and low branching density. They also
polyethylenes with narrow dispersities and low degrees of designed a series of a-diimine palladium catalysts C6 bearing
branching, and demonstrated living behavior at room temper- both the dibenzhydryl moiety and with systematically varied
ature and produced linear polyethylene (Tm ¼ 135 ꢂC) at ꢁ20 ꢂC. ligand sterics for ethylene and 1-hexene polymerization,17 the
Guan et al. designed a a-diimine nickel catalyst C4 bearing molecular weights and branching densities could be tuned over
a
macrocyclic ligand, which showed signicantly higher a very wide range. In this work, we wish to report the synthesis
thermal stability than the acyclic analogs, and promoted living of a series of ortho-dibenzhydryl and ortho-sec-phenethyl
polymerization of propylene even at 50–75 ꢂC.14 This type of substituted a-diimine nickel complexes with systematically
catalyst enhances both the 2,1-insertion of propylene and the varied ligand sterics, and the investigation of the inuence of
chain-walking reaction. Long et al. examined a-diimine nickel ligand structure and polymerization conditions on ethylene and
catalysts C5a containing ortho-dibenzhydryl moiety for high- 1-hexene polymerizations.
temperature ethylene polymerization,15 where the catalyst
ꢂ
maintained high activities at temperatures as high as 100 C.
Results and discussion
Synthesis and characterization of the nickel complexes
Chen et al. recently reported that a-diimine palladium
catalysts C5b bearing the dibenzhydryl moiety displayed high
thermal stability and high activity in slow chain-walking
ethylene (co)polymerization,16 producing semicrystalline poly-
ethylene and ethylene/methyl acrylate copolymer with high
The desired ligands were prepared using the literature proce-
dure in high yields without using column chromatography.17,18
a-Diimine nickel precatalysts used in this study are summarized
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Scheme 2 Synthesis of a-diimine nickel complexes 1–6.
in Scheme 2. Complexes 2–4 were synthesized from the reac- groups are rotated about 180ꢂ from the position they must
tions of the corresponding ligand with (DME)NiBr2 in high occupy to chelate metal Ni. The rotation has been conrmed by
yields. Chen et al. previously reported ortho-dibenzhydryl and the crystal structure of its complex. The X-ray structures of
ortho-sec-phenethyl substituted nickel complexes 14f and 518 that ligand18 and complex exhibit trans and cis conformation about
exhibited high activities in ethylene polymerization and the central C–C bond of the backbone, respectively. Both aryl
produced different branched polyethylene with high molecular
weight. Complexes 1 and 5 were prepared and studied in order
to obtain accurate comparisons in ethylene polymerization.
This way, a series of a-diimine nickel complexes bearing ortho-
dibenzhydryl and ortho-sec-phenethyl groups and with system-
atically varied ligand sterics could be studied in the polymeri-
zation of ethylene and 1-hexene. In addition, the methyl
substituted complex 6 was also used for comparison in this
study.
X-ray crystallographic studies
Suitable crystals of complexes 2, 3 and 5 were obtained by
layering n-hexane onto their dichloromethane solution at room
temperature, respectively. The molecular structures of 2, 3 and 5
was conrmed by single-crystal X-ray diffraction and the cor-
responding ORTEP diagram with selected bond distances and
angles are shown in Fig. 1, 2 and 3, respectively. Crystal data,
data collection and renement parameters are listed in Table S1
(see ESI†).
Both unsymmetrical a-diimine nickel complexes 2 (Fig. 1)
and 3 (Fig. 2) bearing ortho-dibenzhydryl moiety exhibited
similar geometries, and a distorted tetrahedron geometry was Fig. 1 Molecular structure of complex 2 with thermal ellipsoids drawn
at 30% probability, and H atoms have been omitted for clarity. Selected
observed for the Ni center. However, complex 5 (Fig. 3) bearing
ortho-sec-phenethyl groups exhibited symmetrical chiral pocket
bond lengths (A˚) and angles (ꢂ): Ni1–N1 ¼ 2.018(13), Ni1–N ¼ 2.026(13),
Ni1–Br1 ¼ 2.331(3), Ni1–Br2 ¼ 2.334(3), N1–C2 ¼ 1.27(2), N1–C5 ¼
about the Ni center, and exhibited near C2-symmetry. In the
1.43(2); N1–Ni1–N2 ¼ 80.4(5), N1–Ni1–Br1 ¼ 114.3(4), N2–Ni1–Br1,
solid state, the most interesting feature of ligand is the
110.0(5), N1–Ni1–Br2 ¼ 111.8(4), N2–Ni1–Br2 ¼ 118.2(4), Br1–Ni1–Br2
conformation of the substituents attached to N1 and N2. These ¼ 116.90(13), C2–N1–Ni1 ¼ 111.6(11), C5–N1–Ni1 ¼ 128.7(10).
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Fig. 3 Molecular structure of complex 5 with thermal ellipsoids drawn
at 30% probability, and H atoms have been omitted for clarity. Selected
bond lengths (A˚) and angles (ꢂ): Br1–Ni1 ¼ 2.3308(12), Br1i–Ni1 ¼
2.3309(12), Ni1–N1 ¼ 2.034(5), N1–C4 ¼ 1.421(7), N1–C23 ¼ 1.297(8),
C23–C23i ¼ 1.514(11); Br1–Ni1–Br1i ¼ 117.30(9), N1i–Ni1–N1 ¼
81.2(3), N1i–Ni1–Br1 ¼ 112.14(14), N1–Ni1–Br1 ¼ 114.40(14), N1–Ni1–
Br1i ¼ 112.14(14), N1i–Ni1–Br1i, 114.40(14), C4–N1–Ni1, 124.5(4).
Fig. 2 Molecular structure of complex 3 with thermal ellipsoids drawn
at 30% probability, and H atoms have been omitted for clarity. Selected
bond lengths (A˚) and angles (ꢂ): Ni1–N1 ¼ 2.018(13), Ni1–N2 ¼
2.026(13), Ni1–Br1 ¼ 2.331(3), Ni1–Br2 ¼ 2.334(3), N1–C2 ¼ 1.27(2),
N1–C5 ¼ 1.43(2), N2–C3 ¼ 1.17(2), N2–C38 ¼ 1.48(2), C2–C3 ¼
1.53(2); N1–Ni1–N2 ¼ 80.4(5), N1–Ni1–Br1 ¼ 114.3(4), N2–Ni1–Br1 ¼
110.0(5), N1–Ni1–Br2 ¼ 111.8(4), N2–Ni1–Br2 ¼ 118.2(4), Br1–Ni1–Br2
¼ 116.90(13), C2–N1–C5 ¼ 119.4(14), C2–N1–Ni1 ¼ 111.6(11), C5–N1–
Ni1 ¼ 128.7(10), C3–N2–C38 ¼ 121.4(16), C3–N2–Ni1 ¼ 115.4(13),
C38–N2–Ni1 ¼ 123.2(11).
On the other hand, the polymer molecular weight was
strongly depended on the polymerization temperature with
narrow molecular weight distributions of Mw/Mn # 2.50. The
maximum molecular weight was observed at 50 ꢂC (entries 2, 5,
8, 11 and 14, Table 1). Further increase of temperature caused
a decrease in the molecular weight accompanied by broadening
of the molecular weight distribution (entries 3, 6, 9, 12 and 15,
Table 1). This suggests that fast chain transfer takes place at
higher temperatures.1a
rings bonded to the imine nitrogen of the a-diimine lie nearly
perpendicular to the plane formed by the nickel and coordi-
nated nitrogen atoms. The bond angles and bond distances are
consistent with those previously reported for a-diimine nickel
complexes.4 In fact, the Ni1–N1 bond distances in complex 2
Then, the effect of catalyst precursors 1–6 were investigated
with [Al]/[Ni] ratio of 600 (entries 1–18, Table 1). Complexes 1–5
produced polymers in high yields with high molecular weights
(Mn $ 1.8 ꢀ 105 g molꢁ1) and narrow molecular weight distri-
butions (Mw/Mn ¼ 1.50–2.50). Steric effect of ortho-position in
the anilinic moiety can be evaluated by comparing 1 to 4 and 5,
˚
˚
˚
(2.026 A) and 3 (2.018 A) are similar to these in 5 (2.034 A), as
˚
˚
well as the Ni1–Br1 bond distances (2.331 A for 2; 2.331 A for 3;
ꢂ
˚
2.3308 A for 5). In addition, the Br1–Ni1–Br2 angles are 116.90
for complex 2 and 116.90ꢂ for complex 3 are similar to these in
complex 5 (117.30ꢂ).
ꢂ
6. At 50 C and 9 atm of ethylene pressure, the polymerization
activity decreased in the following order, 2 $ 3 $ 4 $ 5 $ 1 > 6
(entries 2, 5, 8, 11, 14 and 17, Table 1).
Complex 2 with isopropyl groups showed the highest activity
among these complexes, but gave the lower molecular weight
than that of complex 1. At 50 C, the molecular weight of the
polymers decreased in the following order, 1 $ 5 $ 2 $ 3 $ 4 $
6. These results indicated that the rate of chain propagation was
greatly promoted by the bulky ortho-dibenzhydryl and ortho-sec-
phenethyl substituents of the ligand's aryl rings. The highest
molecular weight of the polymer obtained by complex 1
suggests that the bulky ortho-dibenzhydryl could efficiently
suppress chain-transfer reactions.
Ethylene polymerization studies
All the dibromo nickel complexes 1–6 were applied to ethylene
polymerization activated by Et2AlCl at the [Al]/[Ni] molar ratio of
600 for 30 min under 9 atm of ethylene (Table 1). First, the
inuence of the polymerization temperature was studied by
varying the temperature from 20 to 80 ꢂC. The catalytic activities
of complex 1 were increased with polymerization temperatures
and the highest activity was observed at 80 ꢂC (entry 3, Table 1).
The maximum catalytic activities for complexes 2–5 were
observed at 50 ꢂC (entries 5, 8, 11 and 14, Table 1), and the
polymerization at 80 ꢂC still gave high catalytic activities on the
level of 106 g of PE per (mol Ni h) (entries 6, 9, 12 and 15, Table
1). Therefore, these a-diimine nickel complexes 1–5 were highly
active in ethylene polymerization, and showed much higher
thermal stability than the corresponding methyl substituted
complex 6.
ꢂ
The branching densities of the obtained polyethylenes were
1
determined by H NMR spectroscopy,19 and the results are
shown in Table 1. The branching densities were increased with
polymerization temperature from 20 to 80 ꢂC (1, 55–64/1000C; 2,
81–90/1000C; 3, 86–98/1000C; 4, 95–108/1000C; 5, 87–103/
1000C; 6, 90–95/1000C). The branching densities were
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Table
1 Effect of catalyst and temperature on ethylene
polymerizationa
Entry Precat. Temp (ꢂC) Yield (g) Activityb Mn
Mw/Mn Bd
c
c
1
2
3
4
5
6
7
8
1
1
1
2
2
2
3
3
3
4
4
4
5
5
5
6
6
6
20
50
80
20
50
80
20
50
80
20
50
80
20
50
80
20
50
80
1.18
1.69
1.89
2.21
2.89
2.59
1.75
2.85
2.09
2.11
2.76
1.78
2.13
2.65
1.84
0.76
0.53
Trace
0.98
1.41
1.58
1.84
2.41
2.16
1.46
2.38
1.74
1.76
2.30
1.48
1.78
2.21
1.53
0.63
0.44
—
9.9
13.1 1.99
1.80
55
58
64
81
84
90
86
91
98
95
101
108
87
93
103
90
95
—
8.0
7.2
8.1
5.2
4.0
6.5
2.9
2.2
2.9
1.8
7.3
8.7
5.4
0.8
0.6
—
1.69
1.62
1.50
2.13
2.09
2.06
2.19
1.96
2.32
2.50
1.52
1.80
2.09
1.67
1.98
—
9
10
11
12
13
14
15
16
17
18
Fig. 4 13C NMR spectra of polyethylenes obtained by complex 1 at
20 ꢂC (ii) and complex 4 at 80 ꢂC (i) (A and B refer to methyl carbon of
sec-butyl branches, entries 1 and 12, Table 1).
a
Polymerization conditions: Ni ¼ 2.4 mmol in CH2Cl2 (2 mL); cocatalyst
Et2AlCl, Al/Ni ¼ 600; toluene ¼ 20 mL; 9 atm of ethylene; time ¼ 30 min.
b
c
106 g of PE (mol of Ni)ꢁ1 hꢁ1
.
Mn and Mw/Mn determined by GPC,
105 g molꢁ1
NMR.
.
Branching numbers per 1000C were determined by 1H
d
and with systematically varied ligand sterics.17 In the above-
mentioned ethylene polymerization, a series of a-diimine
dibenzhydryl substituted nickel complexes 1–4 with systemati-
cally varied ligand sterics showed high activities and high
thermal stability, and produced polyethylene with high molec-
ular weight. The tuning in ligand sterics enables the tuning of
the polymer microstructures such as molecular weight and
branching density. Therefore, polymerization of 1-hexene using
these complexes 1–4 activated by Et2AlCl were carried out at
various polymerization temperatures (20–80 ꢂC) with the [Al]/
[Ni] ratio of 500, and the results are listed in Table 2. In
comparison with polymerization of ethylene, a decrease in
catalytic activity was observed, which is due to the steric bulk-
iness of the 1-hexene monomer. The lowest catalytic activity was
observed for 1 among these complexes, because bulky steric
hindrance of complex 1 is unfavorable for 1-hexene monomer
insertion, especially at lower temperature (no polymer was ob-
decreased by increasing the steric bulkiness of the ortho
substituents onꢂthe a-diimine ligands by comparing 1 to 4. For
example, at 50 C, the number of chain branching was depen-
ded on the precatalysts and decreased in the following order, 4
[2,6-di(H), 101/1000C] > 3 [2,6-di(Me), 91/1000C] > 2 [2,6-di(iPr),
84/1000C] > 1 [2,6-di(CHPh2), 58/1000C] (entries 2, 5, 8 and 11,
Table 1). These results indicated that the introduction of
dibenzhydryl substituent into the ortho-position of the imines
ligands can decrease the branching degree of the poly-
ethylenes.15,16 In addition, complex 5 [2,6-di(CHMePh), 50 ꢂC,
93/1000C] led to slightly lower branching density than complex
ꢂ
6 [2,6-di(Me), 50 C, 95/1000C].
The branching structures analysis based on 13C NMR
showed that 74 methyl, 7 ethyl, 5n-propyl, 4n-butyl, 1-sec-butyl
(branch on branch structure) and 17 longer chains (>C4
branches) exist for the polyethylene produced by complex 4 at
ꢂ
tained at 20 C, entry 1, Table 2).
As shown in Table 2, both the catalytic activities of
complexes 1, 3 and the molecular weight of the obtained poly-
mers were slightly increased with polymerization temperatures
(entries 1–4 and 9–12), but the opposite trend was observed for
complexes 2 and 4 (entries 6–8 and 13–16). Complex 2 showed
ꢂ
80 C (Fig. 4(i), entry 12, Table 1). In contrast, only 55 methyl
branches were observed for complex 1 at 20 ꢂC (Fig. 4(ii), entry 1,
Table 1). The microstructure difference and the lower degree of
branching density for the sterically bulkier catalysts are prob-
ably due to the relatively greater preference of insertion of
ethylene into a primary metal alkyl species versus insertion into
a secondary metal-alkyl species.16 The rationale for the forma-
tion of the major types of branches (methyl, ethyl, propyl, butyl
and longer chains) in the polyethylenes obtained in this work is
shown in Scheme 1(i).
the highest polymer molecular weight (Mn ¼ 3.88 ꢀ 105
g
molꢁ1, entry 6, Table 2) at 40 ꢂC among these catalysts.
Complexes 2 and 3 showed good activity for 1-hexene poly-
merization to produce polymers with high molecular weight
and narrow molecular weight distribution (Mw/Mn ¼ 1.16–1.32,
entries 6–12, Table 2), indicating the living characteristics of the
polymerizations.
1-Hexene polymerization studies
Generally, polymer microstructure is strongly dependent on
catalyst structure and polymerization temperature. The
branching structures of the poly(1-hexene)s produced by a-dii-
mine nickel complexes 1–4 were determined and analyzed to
Chen et al. recently reported the tuning of polyethylene and
poly(1-hexene) microstructures using a series of a-diimine
palladium complexes C6 bearing both the dibenzhydryl moiety
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Table 2 Polymerization of 1-hexene by a-diimine nickel complexes 1–4a
Mol fraction of C6 unit (c)d
Linear
(cL)
Methyl-branched
(cM)
Alkyl-branched
(cA)
Entry
Precat.
Temp (ꢂC)
Yield (g)
Act.b
Mn
Mw/Mn
Bd
c
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
20
40
60
80
20
40
60
80
20
40
60
80
20
40
60
80
Trace
0.03
0.11
0.16
0.84
0.79
0.65
0.40
0.46
0.75
0.82
0.83
1.10
0.64
0.54
0.25
—
—
—
—
—
—
—
0.10
0.37
0.53
2.80
2.63
2.17
1.33
1.53
2.50
2.73
2.77
3.67
2.13
1.80
0.83
0.66
1.22
1.63
3.16
3.88
1.17
0.80
1.67
2.06
3.39
3.51
—
1.16
1.20
1.28
2.14
1.29
1.32
1.32
1.16
1.25
1.31
1.23
—
111.3
106.7
105.9
118.6
119.0
121.6
123.0
124.5
118.1
118.7
119.0
—
0.332
0.360
0.365
0.289
0.286
0.271
0.262
0.253
0.292
0.288
0.286
—
0.526
0.521
0.519
0.428
0.492
0.565
0.590
0.427
0.460
0.523
0.558
—
0.142
0.119
0.116
0.283
0.222
0.164
0.148
0.320
0.248
0.189
0.156
—
9
10
11
12
13
14
15
16
1.83
1.12
0.64
1.51
1.52
1.33
134.6
138.9
140.0
0.193
0.167
0.160
0.585
0.613
0.631
0.222
0.220
0.209
a
Polymerization conditions: Ni ¼ 10 mmol in CH2Cl2 (2 mL); cocatalyst Et2AlCl, Al/Ni ¼ 500; monomer concentration [1-hexene] ¼ 1 M; time: t ¼ 3 h;
solvent: toluene, total volume: 20 mL. b Activity in 104 g (mol Ni)ꢁ1 hꢁ1
NMR spectroscopy.8
.
Mn and Mw/Mn determined by GPC, 105 g molꢁ1
.
Determined using 1H
c
d
study the relationship between polymer microstructure and (cL), methyl-branched (cM), and alkyl-branched (cA) (see ESI†
catalyst structure.
for details).8 The highly branched polymers with methyl and
1H NMR spectroscopy analyses have shown that the branching alkyl branches will be amorphous, while the chain-straightened
degree of the obtained poly(1-hexene)s are lower than the theo- (cL) polymers will be semi-crystalline. As shown in the polymer
retical value (166.7/1000C) due to the 2,1-insertion of 1-hexene microstructures by complexes 1 and 2, the mole fractions of the
and the 1,6-enchainment (Scheme 1(iv)). The branching degree three major C6 units decreased in the following order, cM > cL $
could be only tuned over a narrow range (106–140/1000C, Table 2) cA. The increasing steric hindrance of catalyst leads to
comparing with that in ethylene polymerization. For example, the enhanced insertion for 2,1-insertion of 1-hexene (cL: 1 > 2 z 3 >
branching densities (106–111/1000C, entries 2–4, Table 2) of the 4) and the chain-walking reaction. This indicated that the steric
polymers obtained by complex 1 decreased with increasing hindrance of ortho-dibenzhydryl substituent on catalysts could
temperature, but complex 4 increased slightly (135–140/1000C, suppress chain-transfer reactions (Fig. 5). The predominance of
entries 14–16, Table 2). However, the branching densities of the methyl branch (cM ¼ 0.43–0.63) means that the predominate
obtained polymers by complexes 2 and 3 are very close to 120 1,2-insertion followed by chain-walking forms a 2,u-enchain-
branches per 1000C, and are not depend on the polymerization ment (Scheme 1(iii)).
temperature (entries 5–12, Table 2).
In addition, the polymerization temperature also inuences
Polymers were analyzed by quantitative NMR spectroscopy to the branch-type distribution. The mole fraction of linear
obtain the mole fractions of the three major C6 units: linear enchainment (cL) of the obtained amorphous polymers (no Tm
observed) by complexes 1 and 3 slightly increased from 0.33 to
0.37 and 0.25 to 0.29 with increasing temperatures, while cA
slightly decreased to 0.12 and 0.16 (entries 2–4 and 9–12, Table
2). The use of the complexes 2 and 4 result in a considerable
decline in cL with increasing temperature (entries 5–8 and 14–
16, Table 2), and the obtained polymers were amorphous.
Conclusion
In summary, polymerizations of ethylene and 1-hexene were
catalyzed using a series of a-diimine nickel complexes bearing
ortho-dibenzhydryl or ortho-sec-phenethyl groups activated by
Et2AlCl. The ligands were modied in an attempt to change the
coordination environment, steric effects and the electronic
density of the metal center, eventually to modulate the activity
in the polymerization of ethylene/1-hexene and to control the
Fig. 5 Ligand steric effects of the 1-hexene insertion for a-diimine
nickel complexes 1–4.
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microstructure of polymers. The polymerization results indi- 3.45. MALDI-TOF-MS (m/z): calcd for C49H50BrN2Ni: 803.2511,
cated the possibility of precise microstructure control, found: 803.1885 [M ꢁ Br]+.
depending on the catalyst structure and polymerization
(
MeN^N)NiBr2 (3). (0.71 g, 95%): anal. calcd for (C45H42Br2-
temperature, which in turn strongly affects the physical prop- N2Ni): C, 65.17; H, 5.10; N, 3.38. Found: C, 65.49; H, 5.42; N,
erties. The sterically bulky complex showed higher catalytic 3.41. MALDI-TOF-MS (m/z): calcd for C45H42BrN2Ni: 747.1885,
activities and thermal stability in ethylene polymerization and found: 747.1255 [M ꢁ Br]+.
produced polyethylene with high molecular weight and much
(HN^N)NiBr2 (4). (0.66 g, 92%): anal. calcd for (C43H38Br2-
lower polyethylene branching density. The polyethylene N2Ni): C, 64.45; H, 4.78; N, 3.50. Found: C, 64.49; H, 4.52; N,
branching densities (55–108/1000C) were decreased with 3.45. MALDI-TOF-MS (m/z): calcd for C43H38BrN2Ni: 719.1572,
increasing the bulkiness of the ligand and decreasing the found: 719.1244 [M ꢁ Br]+.
polymerization temperature. However, sterically bulky complex
is unfavorable for 1-hexene monomer insertion, and leads to X-ray structure determinations
low catalytic activity in 1-hexene polymerization.
Single crystals of complexes 2, 3 and 5 for X-ray analysis were
obtained by dissolving the nickel complex in CH2Cl2, followed
by slow layering of the resulting solution with at room
temperature. Data collections were performed at 296(2) K on
Experimental section
a Bruker SMART APEX diffractometer with a CCD area detector,
using graphite monochromated MoKa radiation (l ¼ 0.71073
General considerations
All experiments were carried out under a dry N2 atmosphere by
using standard Schlenk techniques or a glovebox. Research
grade ethylene was puried by passing it through an Agilent
˚
A). The determination of crystal class and unit cell parameters
was carried out by the SMART program package. The raw frame
data were processed using SAINT and SADABS to yield the
reection data le. The structures were solved by using the
SHELXTL program. Renement was performed on F2 aniso-
tropically for all non-hydrogen atoms by the full-matrix least-
squares method. The hydrogen atoms were placed at the
calculated positions and were included in the structure calcu-
lation without further renement of the parameters. Details of
the crystal data and structure renements for complex 2 are
listed in Table S1.†
1
oxygen/moisture trap. H and 13C NMR spectra were recorded
with a Bruker Ascend 400 spectrometer at ambient temperature
unless otherwise stated. The chemical shis of the H and 13C
1
NMR spectra were referenced to tetramethylsilane (TMS). The
molecular weight and the molecular weight distribution of the
polymers were determined by gel permeation chromatography
(GPC, Tosh, Tokyo, Japan) equipped with two linear Styragel
ꢂ
columns at 40 C using THF as a solvent and calibrated with
polystyrene standards, and THF was employed as the eluent at
a ow rate of 0.35 mL minꢁ1. Elemental analysis was performed
by the Analytical Center of the University of Science and Tech-
nology of China. Mass spectra were recorded on a P-SIMS-Gly
from Bruker Daltonics Inc (EI+). 1-Hexene was purchased
from Kanto Chemical Co on Aldrich Chemical Company were
dried over CaH2, and distilled before use. Dichloromethane,
toluene, THF, and hexane were puried by solvent purication
systems. a-Diimine ligands L1–617,18 and complexes 1,4f,15a 5,18
618 were prepared according to reported procedures. Other
chemicals were commercially obtained and puried with
common procedures.
Procedure for ethylene polymerization
In a typical experiment, a 350 mL glass thick-walled pressure
vessel was charged with required amount of AlEt2Cl, 20 mL
toluene and a magnetic stir bar in the glovebox. The pressure
vessel was connected to a high pressure polymerization line and
the solution was degassed. The vessel was warmed to the
desired temperature using an oil bath and allowed to equili-
brate for 5 min. Then 2.4 mmol of nickel complex in 2 mL
CH2Cl2 was injected into the vessel via syringe. With rapid
stirring, the reactor was pressurized and maintained at 9.0 atm
of ethylene. Aer the desired amount of polymerization time,
the vessel was vented and terminated in acidied methanol
(methanol/HCl ¼ 50/1). The polymers obtained were adequately
washed with methanol and dried under vacuum at 50 ꢂC for
Synthesis of complexes 2–4
All complexes were prepared in a similar manner by the reaction
of (DME)NiBr2 (DME ¼ 1,2-dimethoxyethane) with the corre-
sponding ligands in dichloromethane. A typical synthetic
procedure of 2–4 is as follows: (DME)NiBr2 (0.31 g, 1.0 mmol)
and ligand17 (1.0 mmol) were combined in a Schlenk ask under
a N2 atmosphere. CH2Cl2 (20 mL) was added, and the reaction
mixture was stirred at room temperature for 12 h. The resulting
suspension was ltered. The solvent was removed under
vacuum, and the resulting powder was washed with diethyl
1
24 h. Analysis of the polyethylene branching by H NMR spec-
troscopy: branching density, branches/1000C ¼ (CH3/3)/[(CH +
CH2 + CH3)/2] ꢀ 1000. CH3 (alkyl methyl, alk-CH3, m, 0.70–0.95
ppm), CH2 and CH (alk-CH and alk-CH2, m, ca. 1.00–1.45 ppm)
refer to the intensities of the methyl, methylene and methine
resonances in 1H NMR spectra.19
Procedure for 1-hexene polymerization
ether (2 ꢀ 10 mL), and then dried under vacuum at room In a typical procedure, a round-bottom Schlenk ask with stir-
temperature to obtain a brown solid powder.
ring bar was heated 1 h at 150 ꢂC under vacuum and cooled to
i-PrN^N)NiBr2 (2). (0.75 g, 93%): anal. calcd for (C49H50Br2- room temperature. Aer drying the reactor under N2 atmo-
N2Ni): C, 66.47; H, 5.69; N, 3.16. Found: C, 66.79; H, 5.60; N, sphere, toluene was added to the reactor. 1-Hexene was added
(
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to the toluene kept at polymerization temperature via a syringe.
Then the co-catalyst AlEt2Cl was added to the toluene and the
mixture was stirred for 10 min. Polymerization was started by
the addition of the catalyst solution (10 mmol, 2 mL CH2Cl2) into
the reactor via syringe, and the total volume of the solution was
kept at 20 mL. Aer the desired amount of time, the polymer-
ization was terminated by adding 50 mL of the acidied
methanol (methanol/HCl ¼ 50/1). The polymers obtained were
adequately washed with methanol and dried in vacuum at 40 ꢂC
to a constant weight. Poly(1-hexene)s show long methylene
sequences [linear (cL)], methyl branch (cM) and alkyl branches
[i.e., butyl and longer than hexyl branches (cA)].4e,17 Analysis of
the poly(1-hexene) branching by 1H NMR spectroscopy:8
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branching density, branches/1000C ¼ (CH3/3)/[(CH + CH2
+
CH3)/2] ꢀ 1000. CH3 (alk-CH3 (cM), d, 0.70–0.87 ppm; alkyl
methyl (cA), t, 0.87–0.95 ppm), CH2 and CH (alk-CH and alk-
CH2, m, ca. 1.0–1.45 ppm) refer to the intensities of the methyl,
methylene and methine resonances in 1H NMR spectra. 2,1-
Insertion was calculated by the following equation: 2,1-Ins.% ¼
cL ¼ 2,1-insertion ¼ (166.7 ꢁ B)/166.7,11b,12 B ¼ branches per
1000C; cM + cA ¼ 1 ꢁ cL.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (NSFC, 21704094), the Chinese Postdoctoral
Science Foundation (2017M612076), Advanced Catalysis and
Green Manufacturing Collaborative Innovation Center
(ACGM2016-06-01) and Yixing Taodu Ying Cai Program.
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