Synthesis of Polyethylenes with Controlled Branching with α-Diimine Nickel Catalysts and Revisiting Formation of Long-Chain Branching

Article abstract of DOI:10.1021/acscatal.7b03282

The synthesis of polyethylenes with precise branching, especially long-chain branching (LCB), using ethylene monomer as a single feedstock is of a significant academic and industrial interest. On the basis of the ortho-aryl effect, a series of α-diimine n

Full text of DOI:10.1021/acscatal.7b03282

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Article
Synthesis of Polyethylenes with Controlled Branching with #-Diimine
Nickel Catalysts and Revisiting Formation of Long-Chain Branching
Lixia Pei, Feng-Shou Liu, Heng Liao, Jie Gao, Liu Zhong, Haiyang Gao, and Qing Wu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03282 • Publication Date (Web): 26 Dec 2017
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Synthesis of Polyethylenes with Controlled Branching
with αꢀDiimine Nickel Catalysts and Revisiting
Formation of LongꢀChain Branching
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Lixia Pei1,2, Fengshou Liu3, Heng Liao1, Jie Gao1, Liu Zhong, Haiyang Gao1*, and
Qing Wu1
1
School of Materials Science and Engineering, PCFM Lab, GD HPPC Lab, Sun
Yatꢀsen University, Guangzhou 510275, China.
2
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510641, China
3
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical
University, Zhongshan, 528458, China.
* Corresponding author. Fax: +86ꢀ20ꢀ84114033. Tel.: +86ꢀ20ꢀ84113250. Email: Gao
H.: gaohy@mail.sysu.edu.cn
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ABSTRACT:
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The synthesis of polyethylenes with precise branching, especially longꢀchain
branching (LCB), using ethylene monomer as a single feedstock is of a significant
academic and industrial interest. On the basis of the orthoꢀaryl effect, a series of
αꢀdiimine nickel complexes with monoꢀarylꢀsubstituted anilines has been designed
and prepared for the synthesis of the polyethylenes with controlled branching. The
introduction of the orthoꢀaryl on aniline moieties enhanced the branching control
ability of the αꢀdiimine nickel catalysts. A different mechanistic model was proposed
to interpret the presence of methyl and LCB but absence of other short branches in the
obtained polyethylenes. LCB was formed by ethylene insertion into the primary
Niꢀalkyl species originating from nickel migration to methyl terminal of the growing
chain because of restricted ethylene insertion into secondary Niꢀalkyl species with an
αꢀethyl or a bulkier alkyl group.
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Keywords: nickel catalyst, chain walking, ethylene, polyethylene, longꢀchain
branching, controlled branching
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INTRODUCTION
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The discovery of late transition metal nickel and palladium catalysts has initiated
a new olefin polymerization field because it leads to the formation of new polymeric
materials, especially αꢀdiimine type catalysts initially reported by Brookhart.1 A
distinguishing feature of αꢀdiimine nickel and palladium catalysts for ethylene
polymerization is chain walking, which involves a series of sequential βꢀhydride
eliminations, alkene rotations, and reinsertions that relocates the active metal center
along the growing polymer chain.2 The polyethylenes (PEs) with various types of
branching can be synthesized with αꢀdiimine nickel and palladium catalysts by tuning
steric and electronic effects and polymerization conditions.3
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Generally, chain walking produces statistical amounts of the expected branches
for αꢀdiimine nickel catalyst systems. The branched PEs obtained by standard
αꢀdiimine nickel catalysts contain most of methyl branch (C1), small amount of ethyl,
propyl, and butyl branches (C2ꢀC4), trace of amyl branch (C5), and large amounts of
longꢀchain branching (LCB) (≥C6).1a,4 As described mechanistic model by Brookhart,
LCB is produced by chain walking involving successive βꢀH elimination (at least 6
steps) and reinsertion process.2 MonteꢀCarlo simulation of branching distribution in
αꢀdiimine nickelꢀcatalyzed polyethylenes has showed that branching frequencies
decrease steadily from methyl (C1) to decyl branches (C12), and large amounts of LCB
(≥C6) is a result of sum of hexyl and longer branches.5 The synthesis of polyethylenes
with controlled branching by chain walking mechanism therefore remains a great
challenge although precise branching can be used to tune material properties over a
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wide range, making polyolefins extremely versatile materials.6 LCB in particular, is
nevertheless important because it has a dramatic effect on polymer properties and
material processing properties, thereby the synthesis of the longꢀchain branched
polyethylene using ethylene monomer as a single feedstock is of a significant
academic and industrial interest.7
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Numerous studies on ethylene polymerizations using nickel and palladium
catalysts have established that the branching density and distribution strongly depend
on the nature of the ligand.1a,4 An effective approach to controlling branching
distribution of polyethylene products is the modification of ligand frameworks. For
instance, αꢀdiimine nickel catalysts with modified ortho substituents afforded
methylꢀbranched polyethylenes by controlling chain walking reaction.8 The
polyethylenes containing high content of LCB were also obtained by nickel catalysts,
and the level of LCBs was hardly interpreted by common chain walking mechanism.9
The alternative mechanisms of LCB formation such as incorporation of longꢀchain
αꢀolefins or macromonomers and intraꢀ or intermolecular C‒H activation were
presumably presented.9a,b,f Recently, methyl and LCB were merely observed in
semicrystalline polyethylenes and ethylene based copolymers by αꢀdiimine palladium
catalysts, but no mechanistic model of LCB formation was provided.10 These
contributions shed light on these issues that controlling branching density and
distribution of polymeric products in Ni/Pdꢀcatalyzed ethylene polymerization was
feasible and the LCB might be formed by various mechanistic models.
Rieger groups have previously reported the “orthoꢀaryl effects” of αꢀdiimine
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nickel and palladium catalysts, which provide a viable access to controlling
polyethylene microstructure by tailorꢀmade coordination geometry.11 αꢀDiimine nickel
catalysts with orthoꢀaryl substituted anilines are therefore ideal candidates for
microstructure control of polyethylenes. In this paper, a series of novel αꢀdiimine
nickel complexes with monoꢀorthoꢀarylꢀsubstituted anilines has been designed for
precise synthesis of polyethylenes with methyl and LCB. An attractive mechanistic
model of controlled branching is also proposed, which enables researchers to gain
greater mechanistic understanding on the LCB formation in Ni/Pdꢀcatalyzed ethylene
polymerization.
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RESULTS AND DISCUSSION
Synthesis and crystal structure of nickel complexes
αꢀDiimine ligands with 2,6ꢀunsymmetrically substituted anilines were
synthesized by the Schiff base condensation reaction using trimethylaluminum (TMA)
as an activator of the corresponding anilines according to our previously reported
methods.12 The αꢀdiimine ligands were obtained as nonꢀstatistical stereochemical
mixtures because of the existence of geometric (E/Eꢀ, E/Zꢀ, and Z/Zꢀ) and
conformational (synꢀ and antiꢀ) isomers, which was similar to previous observation
with related αꢀdiimine compounds.4a,9d,e,13 αꢀDiimine nickel complexes with different
substituents were obtained by addition of the corresponding ligands to a stirring
suspension of (DME)NiBr2 (DME: dimethoxyethane) in CH2Cl2 (Figure 1). An
attempt to synthesize the diphenyl αꢀdiimine nickel complex with
2,4ꢀdimethylꢀ6ꢀnaphthyl anilines was unsuccessful under similar conditions. Nickel
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complex 7 containing unsymmetric aniline moieties was also prepared by twoꢀstep
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condensation reactions and used for comparison.
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N
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N
N
Ni
Br2
Ni
Br2
Ni
Br2
3
2
1
Ph
N
Ph
Ph
N
Ph
N
Ph
N
Ph
N
N
Ni
Br2
Ni
Br2
Ni
Br2
6
4
5
Ph
N
Ph
N
N
Ni
Br2
N
Ni
Br2
standard Ni
7
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Figure 1. Molecular structures of αꢀdiimine nickel complexes.
Figure 2. Crystal structure of nickel complex 2 with thermal ellipsoids of 30 %
probability. The hydrogen atoms and four CH2Cl2 molecules are omitted for clarity.
Figure 3. Crystal structure of nickel complex 5 with thermal ellipsoids of 30 %
probability. The hydrogen atoms and two CH2Cl2 are omitted for clarity.
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Figure 4. Crystal structure of nickel complex 7 with thermal ellipsoids of 30 %
probability. The hydrogen atoms are omitted for clarity.
Single crystals of nickel complexes 2, 5, and 7 suitable for Xꢀray diffraction
analysis were obtained by slow evaporation of the nickel complex solutions in CH2Cl2
(Figures 2ꢀ4). Nickel complexes 2 and 7 adopt a fourꢀcoordinate geometry for the
nickel center (Figures 2 and 4), whilst the dimerization of complex 5 gives a
fiveꢀcoordinate geometry (Figure 3). Single crystal Xꢀray diffraction analysis shows
that nickel complex 2 is present as a racꢀisomer and exhibits pseudo C2ꢀsymmetry in
the solid state. Both of twist angles between the aniline plane and the naphthyl ring
are 64.5o, suggesting that two naphthyl rings provide the unique surrounding space
around the nickel center, a feature linked to chain walking.14
Ethylene polymerization
Preliminary ethylene polymerizations were carried out using nickel complexes
1ꢀ8 after activation with methylaluminoxane (MAO) under 0.5 atm ethylene pressure
and 0 °C. Comparisons of polymerization results in Table 1 clearly demonstrated the
effects of ortho substituent and ligand backbone on ethylene polymerization. For
acenaphthyl αꢀdiimine nickel catalysts, increasing the steric bulks of the ortho
substituent (entries 1 vs 3 vs 2) led to an increase in polymer molecular weight and
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polymerization yield, which was well consistent with previous observation.4a,12a When
two phenyl groups were introduced on the ligand backbone instead of acenaphthyl
group, the polymer molecular weight increased (entries 1 vs 5 and 3 vs 6).
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Table 1. Ethylene polymerization results with nickel catalysts 1ꢀ8/MAO at 0 °C. a
b
c
yield
(g)
Mn
Tm
BD d
(/1000C)
Me content e
%
entry cat.
PDI b
(kg/mol)
(°C)
1
0.82 f
2.87
1.61
2.03
2.10 f
0.94
1.93
2.40
1.2
1.8
2.5
2.4
3.3
3.1
2.0
2.2
2.9
120
0
0
1
2
3
4
5
6
7
8
127.1
99.5
7.7
128
126
4.8
100
100
100
100
100
92.0
74.6
2
3
4
5
6
7
8
16.0
14.5
11.2
8.5
119,125
114, 121
127
3.1
180.7
151.0
224.9
117
24.5
81
48.1
a Polymerization conditions: 5 ꢁmol of nickel, Al(MAO)/Ni = 600, 30 min, 0.5 atm
b
ethylene pressure, 20 mL toluene. Mn and PDI were determined by gel permeation
chromatography (GPC) in 1,2,4ꢀtrichlorobenzene at 150 °C using narrow polystyrene
c
standards as calibration. Determined by differential scanning calorimetry (DSC),
second heating. d Branching density, branches per 1000 carbon atoms determined by 1
e
H NMR spectroscopy. Me content is the mole ratio of the methyl branching in the
various branching distributions, determined by 13 C NMR spectroscopy. f polyethylene
wax.
Herein, more attentions were focused on the effects of ortho substituent and
ligand backbone on the branching structure of the obtained polyethylene including
branching density and branching distribution. It was interesting to note that catalysts
1ꢀ6 with monoꢀorthoꢀaryl substituted anilines afforded polyethylenes with much
lower branching density than the standard αꢀdiimine nickel catalyst 8 with
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2,6ꢀdiisopropyl substituents. Similar observations have been reported with an
αꢀdiimine palladium catalyst bearing dibenzhydryl (CH(Ph)2) groups and αꢀdiimine
nickel catalysts with 2,6ꢀdiphenyl substituents for ethylene polymerization.10,11
Reducing branching density by introducing oꢀaryl substituted aniline could be
reasonably attributed to the “orthoꢀaryl effect” proposed by Rieger.11
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Figure 5. 13C NMR spectra of polyethylenes produced by catalysts 3, 4, and 8 (doubt
bond singles are omitted for 4) (entries 3, 4, 8 in Table 1).
To gain deep insight into definitive microstructure of the polyethylenes,
13
investigations using C NMR spectroscopy were further undertaken (Figure 5). The
branching distributions were quantitatively calculated on the basis of previous
resonance assignments.15 Surprisingly, only methyl (19.99 ppm, 1B1) was present in
the polyethylenes produced by catalysts 2ꢀ6 and other branching such as ethyl, propyl,
butyl, amyl, and LCB were not detected with the exception of the fully linear
polyethylene obtained by catalyst 1 (Table 1). For lowꢀmolecularꢀweight
polyethylenes obtained by catalysts 1, 4, and 5, the resonances at 32.04, 22.75, 13.99
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ppm were assigned to the end group of the polyethylene chain rather than LCB
(Figure 5).16 This assignment was further supported by the observation of the double
bond of the chain end at 139.05, 114.12 ppm (Figures S1 in Supporting
Information).15 This result strongly indicated that orthoꢀaryl substituents also affected
branching distribution, besides branching density of polyethylenes.
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Table 2. Characterization of polyethylenes obtained by 2, 3, 6ꢀ8/MAO. a
branching distribution (mol%)e
c
P
T
yield
(g)
Tm
b
entry cat.
Mn
PDI b
BD d
(atm) (°C)
(°C)
Me Et Pr Bu Am LCB
2
9
2
2
2
0.5
0.5
0.5
0
2.87 127.1
2.5
2.5
3.0
128
118
112
4.8
6.0
100
100
0
0
0
0
0
0
0
0
0
0
0
0
0
0
20
40
1.37
1.29
78.1
30.8
10
20.5 81.5
18.5
97,
11
0.5
60
1.20
21.1
2.1
31.9 60.7 4.6
0
0
0
34.8
2
109
12
13
14
15
3
10
10
0
6.65 174.3
7.12 125.7
2.9
2.5
2.7
2.4
2.4
2.3
4.2
4.5
2.0
1.9
1.7
1.8
2.2
2.3
2.4
2.9
137
136
120
114
126
99
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2
2
2
2
3
3
3
3
6
6
6
6
7
7
7
8
20
40
60
0
10
7.78
60.3
8.3
93.9
6.1
34.2
0
10
10.35 36.2
22.9 65.8
16.0 100
41.3 89.9
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1.61
1.77
1.60
1.12
99.5
50.5
9.7
16
17
18
6
20
40
60
0
10.1
19.7
f
‒
93.7 77.1 2.4 0.8
f
4.9
‒
125.3 65.4 6.1 2.1 4.0 0.9 21.5
0.94 180.7
1.41 126.1
127
92
8.5
100
0
0
0
0
0
0
0
0
0
0
0
0
0
0
19
20
21
7
20
40
60
0
43.0 88.1
11.9
14.0
16.3
0
f
1.02
0.87
87.8
73.4
‒
100.3 84.2 1.8
f
‒
123.9 75.8 4.8 1.6 1.5
24.5 92.0 8.0
1.93 151.0
117
87
0
0
22
23
8
20
40
0
2.46
1.21
98.2
61.8
48.7 75.9 2.3 2.3 1.7 0.7 17.1
105.7 72.8 2.9 2.3 2.2 0.8 19.0
48.1 74.6 5.9 4.1 2.8 2.8 9.8
d
‒
2.40 224.9
81
a
Polymerization conditions: 5 ꢁmol of nickel, Al(MAO)/Ni = 600, 30 min, 20 mL
toluene; 100 mL toluene for high pressure polymerization of 10 atm (entries 12ꢀ15). b
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in unit of kg/mol, Mn and PDI were determined by gel permeation chromatography
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(GPC) in 1,2,4ꢀtrichlorobenzene at 150 °C using narrow polystyrene standards as
c
calibration. Determined by differential scanning calorimetry (DSC), second heating.
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d
1
Branching density, branches per 1000 carbon atoms determined by H NMR
spectroscopy. e Branching distribution is the mole ratio of the desired branching in the
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13
f
various branching distributions, determined by
determined.
C NMR spectroscopy. Not
Commonly, the branching structure of the polyethylene obtained by αꢀdiimine
nickel catalysts is sensitive to polymerization temperature and ethylene pressure.
Nickel catalysts 2, 3, and 6 were selected to investigate the influences of reaction
temperature and ethylene pressure on the polymer branching structure (Table 2),
considering that the obtained highꢀmolecularꢀweight polyethylenes could eliminate
the effect of the chain end group on LCB calculation. As previously reported,
increasing temperature also led to a remarkable increase in branching density in a
range of temperature from 0 to 60 °C for catalysts 2, 3, and 6.4a
Notably, 13C NMR analysis showed that the order of branching presence could
not follow branching length order with an increase in temperature. For polyethylenes
obtained by catalysts 3 and 6 with 2,4ꢀdimethylꢀ6ꢀphenyl anilines, only methyl was
observed at 0 °C. When the temperature was increased to 20 °C, only methyl and
LCB appeared and other short branches including ethyl, propyl, butyl, and amyl
branches were not present (Table 2 and Figures S4 and S6 in Supporting Information).
The intensive CH3 peak at 14.09 ppm was typical for butyl or longer branches (1B4+).
The existence of LCB and the exclusion of butyl branch were further supported by the
absence of a signal at 23.39 ppm, which was characteristic for butyl branch (2B4).
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9
Additionally, the methine (CH) signal at 38.15 ppm of LCB on the main chain (brB4+)
was also detected.15 Additional ethyl branch appeared in the polyethylene at
temperatures up to 40 °C. Further increasing temperature to 60 °C, all branches were
clearly detected in the 13C NMR spectroscopy (Figures S4 and S6 in supporting
information). Compared with catalysts 3 and 6, catalyst 2 with the installation of
6ꢀnaphthyl groups instead of 6ꢀphenyl groups also followed the same trend but
showed better controlled behavior. Propyl, butyl, and amyl branches were not present
in the polyethylene obtained by catalyst 2 at 60 °C (entries 11). When the ethylene
pressure was increased from 0.5 to 10 atm, the branching density decreased and
branching distribution was more facilely controlled. Only methyl and LCB were
observed for the polyethylenes obtained by catalyst 2 in a range of temperature from 0
to 60 °C (entries 12ꢀ15).
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In comparison, αꢀdiimine nickel catalysts with monoꢀorthoꢀaryl substituents
showed better control behavior for polyethylene branching structure than standard
αꢀdiimine nickel catalyst 8 with 2,6ꢀalkyl substituents (entry 8 in Table 2). Catalyst 2
also showed comparable branching selectivity to previously reported αꢀdiimine nickel
catalysts. For example, αꢀdiimine nickel catalysts bearing 2,6ꢀdiaryl aniline moieties
reported by Rieger produced polyethylenes with relatively low branching density of
3ꢀ48/1000C, and predominant methyl, LCB and trace of ethyl were observed in the
formed polyethylenes.11 The cyclophane derived αꢀdiimine catalyst reported by Guan
produced the polyethylenes with higher branching density (66ꢀ96/1000C) as
compared to the Rieger’s catalyst. The obtained PEs contained short chain branches
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with most being simple methyl branches.3b The “sandwich” αꢀdiimine nickel catalyst
reported by Brookhart and Daugulis produced highly branched polyethylenes (up to
152/1000C) containing most of methyl branching.17 αꢀDiimine nickel catalysts
bearing dibenzhydryl (CH(Ph)2) anilines developed by Long afforded moderately
branched polyethylenes (63ꢀ75/1000C), and branching contents consisted of both
methyl branching and LCB even at high temperatures.18 Dibenzobarrelene derived
αꢀdiimine nickel catalyst with 4ꢀmethylꢀ2ꢀ(1ꢀ(2,4,6ꢀtrimethylphenyl) ethyl)aniline
moieties reported by Coates catalyzed ethylene polymerization to form lowly
branched polyethylenes (1ꢀ63/1000C), and branching density decreased with
increasing ethylene pressure and reducing temperature.14 Precise control of the
polyethylene branching structure, especially branching distribution, became possible
by the introduction of monoꢀorthoꢀaryl on aniline moieties on αꢀdiimine ligands. This
behavior could be attributed to the unique coordination geometry of αꢀdiimine nickel
complexes with monoꢀorthoꢀarylꢀanilines. Our claim was further supported by
experimental results of ethylene polymerization using an unsymmetrical αꢀdiimine
nickel catalyst 7 with 2,6ꢀdiisopropyl and 2ꢀphenyl anilines. For the polyethylenes
obtained by catalyst 7, the order of branching presence followed branching length
order with an increase in temperature and all of branches were clearly detected in the
13C NMR spectroscopies above 20 °C (entries 22, 23), which was similar to the
observation using standard αꢀdiimine nickel catalyst 8 (entry 8).15a
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Mechanistic studies of LCB formation
In principle, branches on the polyethylenes produced by αꢀdiimine nickel and
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palladium catalysts are believed to form by the migration of metal center along the
growing chain. This chain walking process occurs via βꢀhydrogen elimination
generating a metalꢀhydride olefin species, followed by 2,1ꢀreinsetion with opposite
regiochemistry. Coordination and insertion of an ethylene monomer result in a methyl
branch. An additional chainꢀwalking step followed by ethylene insertion produces an
ethyl branch. Further chain walking results in the formation of longer branches
(pathway II in Scheme 1).4a Therefore, the order of branching presence follows
branching length order.
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For the polyethylenes with controlled branching structure obtained by αꢀdiimine
nickel catalysts, a crucial question, the branching formation mechanism, remains
unanswered. Methyl branch placed in the polyethylene samples in the present work is
reasonably interpreted by the chain walking mechanism. However, LCB is unlikely
attributed to the previously reported chain walking process because of absence of
other shortꢀchain branches in the polyethylene samples produced by our nickel
catalyst systems. Some researchers have noted similar observations using other nickel
catalysts, and an alternative mechanism of LCB formation by incorporation of
longꢀchain αꢀolefins or macromonomers has been speculated.9a,b This incorporation
mechanism has precedent in early transition metalꢀcatalyzed olefin polymerization (Ti,
Zr, and Hf based catalysts), but is hardly operated for nickel and palladium catalysts
because insertion of αꢀolefins into nickel and palladium active center generally leads
to 1,ωꢀ or 2,ωꢀ enchainment by chain straightening.19,20 Intraꢀ or intermolecular CꢀH
activation
has
been
presumably
presented
in
iminophosphonamide
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nickelꢀcatalyzedꢀethylene polymerization,9f but authors also note that it is not clear
why the formation of longꢀchain branches (> C6) is favored over other intramolecular
CꢀH activation reactions. As a matter of fact, the most common and presumably
strongest interaction between nickel and palladium metal centers and the alkyl group
is the βꢀagostic interaction.4a
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Scheme 1. Proposed mechanistic model for the formation of LCB.
We herein provide an attractive mechanism for LCB formation in these
polyethylene materials. As shown in Scheme 1, methyl branch is formed by chain
walking process involving oneꢀstep βꢀH elimination followed by ethylene insertion.
Twoꢀstep chain walking involving successive twice βꢀhydrogen elimination followed
by 2,1ꢀreinsetion leads to the generation of a secondary Niꢀalkyl species with an
αꢀethyl, but ethyl branch cannot form because of no occurrence of ethylene insertion.
As a result, nickel active center further migrates on the polyethylene chain.
Analogously, propyl, butyl, amyl, and LCB cannot also be formed because the
secondary Niꢀalkyl species possesses bulkier substituents. Therefore, the methyl
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branched polyethylene growing chain is formed in the initial time. When nickel metal
migrates to methyl terminal by successive multipleꢀstep chain walking on the methyl
branched polyethylene growing chain, a primary Niꢀalkyl species is produced. LCB is
formed when ethylene inserts into the primary Niꢀalkyl species (pathway I in Scheme
1).
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In the proposed mechanism, it is a crucial point that ethylene insertion into a
secondary Niꢀalkyl species with an αꢀethyl or a bulkier alkyl cannot occur at lower
temperatures (pathway II in in Scheme 1). Therefore, a mechanistic question remains.
Why does ethylene merely insert into the primary Niꢀalkyl species or the secondary
Niꢀalkyl species with an αꢀmethyl? The reasons could be attributed to steric
interactions because orthoꢀaryl substituted anilines provided unique enclosed
surrounding space around nickel metal center. Brookhart has previously reported that
1ꢀhexene can merely insert into secondary nickelꢀalkyl species with an αꢀmethyl for
the αꢀdiimine nickel catalyst with monoꢀtertꢀbutyl substituted anilines, and insertion
of 1ꢀhexene into secondary nickelꢀalkyl species with bulkier groups such as ethyl and
propyl groups cannot occur.17 The restricted insertion of αꢀolefin into primary
Niꢀalkyl species or secondary nickelꢀalkyl species with an αꢀmethyl has been
commonly observed using nickel catalysts with two 2,6ꢀdiisopropylaniline moieties
under the adopted conditions.20 Regioselective 2,1ꢀinsertion and precision chain
walking have also been observed in “sandwich” αꢀdiimine nickel catalyzedꢀαꢀolefins
polymerizations.17 Coates has recently performed a detailed study on the insertion
pathways and chain straightening mechanisms of “sandwich” αꢀdiimine nickel
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catalysts for αꢀolefin polymerizations using 13Cꢀlabeling technique.21 The positions of
the nickel species prior to the next monomer insertion are mostly located at the
primary chain end position, the previously installed methyl terminal, and the
penultimate chain end position. Osakada also reported that the αꢀdiimine palladium
catalyst with 2,4ꢀdimethylꢀ6ꢀphenylanilines polymerized 4ꢀalkylcyclopentenes to
produce the polymers with the higher isotacticity than the standard palladium catalyst
with 2,6ꢀdiisopropylanilines because of steric repulsion.22 Therefore, the restricted
insertion of ethylene into primary Niꢀalkyl species or secondary nickelꢀalkyl species
with an αꢀmethyl is presumably reasonable by design of coordination geometry.
Analogously, ethylene insertion into a secondary Niꢀalkyl species does not occur
in the pathway I. Probably, no ethylene insertions take place until nickel center
migrates the final destination at the previously installed methyl terminal to form a
primary Niꢀalkyl species. According to the proposed mechanism, LCB should be
always accompanied with methyl branching on the branched polyethylene chain. The
experimental results of branching distribution in Table 2 further support this claim.
For the obtained polyethylenes by catalyst 2 under 10 atm ethylene pressure and low
temperatures (entries 12 and 13 in Table 2), LCB is also not formed when methyl
branching is not produced.
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The proposed mechanistic model can also reasonably explain the effect of
temperature on branching structure. Increasing temperature leads to more strong
walking ability of nickel species, thereby nickel species migrates to methyl terminal
more readily and LCB forms with higher frequency at elevated temperature. Further
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increasing temperature leads to the presence of other short branching, which is a result
of twoꢀstep or multipleꢀstep chain walking followed by insertion of ethylene (pathway
II in Scheme 1) because the ethylene insertion into the secondary nickelꢀalkyl species
with an αꢀethyl or a bulkier alkyl is favored energetically at high temperature.19 When
temperature is increased to a critical value, LCB is formed by two pathways including
ethylene insertion into the primary Niꢀalkyl species arising from nickel migration to
methyl terminal (pathway I) and ethylene insertion into the secondary Niꢀalkyl
species (> C6) by multipleꢀstep chain walking (pathway II).
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In contrast to previous work on the formation of LCB, our study provides a novel
mechanistic model. We believe that the proposed mechanistic model can reasonably
interpret high content of LCB in the polyethylenes obtained by other nickel
catalysts.9,17,18 Besides, LCB formation by the proposed mechanism (pathway I) may
be not fully excluded for polyethylenes obtained by previously reported αꢀdiimine
nickel catalysts with ortho alkyl substituents. 4a
CONCLUSIONS
In summary, we have reported αꢀdiimine nickel catalysts with the
monoꢀarylꢀsubstituted anilines that afford the polyethylenes with precise branching.
The introduction of the orthoꢀaryl on aniline moieties remarkably enhanced the
branching control ability of the αꢀdiimine nickel catalysts. The polyethylenes with
precise branching including methyl and LCB were synthesized and polymer
molecular weight could be tuned by varying ligand substituents. Mechanistic study
showed that methyl was produced by oneꢀstep chain walking whilst LCB was formed
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by ethylene insertion into the primary Niꢀalkyl species originating from nickel
migration to methyl terminal because of restricted ethylene insertion into the
secondary Niꢀalkyl species with an αꢀethyl or a bulkier alkyl. The novel mechanistic
model (pathway I) is helpful in revisiting the LCB formation obtained by nickel
catalysts and enables researchers to gain greater mechanistic understanding on
branching formation in Ni/Pdꢀcatalyzed ethylene polymerization.
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EXPERIMENTAL SECTION
General Procedures
All manipulations involving airꢀ and moisture sensitive compounds were
performed under dried and purified nitrogen (99.999%) using standard vacuumꢀline,
Schlenk, or dry glovebox techniques.
Materials
Toluene and hexane were distilled from Na/K alloy before being used.
Dichloromethane was distilled from CaH2 under nitrogen atmosphere.
2ꢀPhenylaniline,
2ꢀbromotoluene,
1ꢀbromonaphthalene,
(Pd(PPh3)4),
trimethyl
borate,
and
tetrakis(triphenylphosphine)palladium
2ꢀbromoaniline
2ꢀbromoꢀ4,6ꢀdimethylanilne, acenaphthenequinone, benzil, phenylboronic acid, and
1ꢀnaphthylboronic acid were purchased from Alfa Aesar Chemical and used as
received. 2,6ꢀDiisopropylaniline were purchased from Aldrich Chemical and were
distilled under reduced pressure before use. Trimethylaluminium (1M, hexane) were
purchased from Aldrich Chemical and used as received. Methylaluminoxane (MAO)
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solution (10 wt. % in toluene) was purchased from Acros. Ethylene (99.99%) was
purified by passing through Agilent moisture and oxygen traps. Other commercially
available reagents were purchased and used without purification. A standard
acenaphthyl αꢀdiimine nickel complex with 2,6ꢀdiisopropylaniline was synthesized
according to the literature.
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Measurements
Elemental analyses were performed on a Vario EL microanalyzer. Mass spectra
were obtained using electro spray ionization (ESI) LCMSꢀ2010A for organic
compounds. Matrix assisted laser desorption ionization time of flight mass
spectrometry (MALDIꢀTOFꢀMS) were performed on Bruker ultrafleXtreme for nickel
complexes. NMR spectra of organic compounds were carried out on a Varian
13
MercuryꢀPlus 300 MHz spectrometer in CDCl3 using TMS as a reference. C NMR
spectra of polymers were carried out on a Bruker 500 MHz at 120 °C. Sample
solutions of the polymer were prepared in oꢀC6H4Cl2/oꢀC6D4Cl2 (50% v/v) in a 10
mm sample tube. The spectra of the quantitative 13C NMR were taken with a 74° flip
angle, an acquisition time of 1.5 s, and a delay of 4.0 s. DSC analyses were conducted
with a Perkin Elmer DCSꢀ7 system. The DSC curves were recorded at second heating
curves at a heating rate of 10 °C/min and a cooling rate of 10 °C/min. GPC analysis of
the molecular weights and molecular weight distributions (PDI = Mw/Mn) of the
polymers at 150 °C were performed on a PLꢀGPC 220 highꢀtemperature
chromatograph equipped with
a
differential refractiveꢀindex detector.
1,2,4ꢀTrichlorobenzene (TCB) was used as the eluent at a flow rate of 1.0 mL/min.
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Crystal structure determination
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The crystal of nickel complex was mounted on a glass fiber and transferred to a
Bruker CCD platform diffractometer. Data obtained with the ωꢀ2θ scan mode was
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collected on
a
Bruker SMART 1000 CCD diffractometer with
graphiteꢀmonochromated Cu Kα radiation (λ= 1.54178 Å). The structure of nickel
complexes was solved by direct methods using the program SHELXS97, while further
refinement with fullꢀmatrix least squares against F2 was obtained with the SHELXL97
program package. All nonꢀhydrogen atoms were refined anisotropically, and all
hydrogen atoms were introduced in calculated positions with the displacement factors
of the host carbon atoms.
Ethylene polymerization at atmosphere pressure
A roundꢀbottom Schlenk flask with stirring bar was heated for 3 h at 150 °C under
vacuum and then cooled to room temperature. The Schlenk flask was pressurized to
0.5 atm of ethylene pressure (gage pressure) and vented three times. The appropriate
MAO solution and toluene were added into the glass reactor under 0.5 atm of ethylene.
Toluene and 2 mL of a solution of nickel complex were added sequentially by syringe
to the wellꢀstirred solution, and the total reaction volume was kept at 20 mL. The
ethylene pressure was kept constant at 0.5 atm by continuous feeding of ethylene
throughout the polymerization reaction. The reaction temperatures were controlled
with an external oil bath or a cooler in polymerization experiments. The
polymerizations were terminated by the addition of 200 mL of acidic methanol (95:5
methanol/HCl) after continuously stirring for an appropriate period. The resulting
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precipitated polymers were collected and treated by filtration, washed with methanol
several times, and dried in vacuum at 60 °C to a constant weight.
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Ethylene polymerization at high pressure
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A mechanically stirred 300 mL Parr reactor was heated to 150 °C for 2 h under
vacuum and then cooled to room temperature. The autoclave was pressurized to 2 atm
of ethylene and vented three times. The autoclave was then charged with solution of
MAO in toluene under 0.5 atm of ethylene at initialization temperature. After
continuously stirring for an appropriate period, 2 mL solution of nickel complex and
toluene solvent were charged into the autoclave under 0.5 atm of ethylene and the
total reaction volume was kept at 100 mL. The ethylene pressure was raised to 10 atm.
The reaction temperature was controlled by means of a heater or cooler and found to
be ± 2 °C as monitored by an internal thermocouple. The reaction was carried out for
30 min. Polymerization was terminated by addition of acidic methanol after releasing
ethylene pressure. The resulting precipitated polymers were collected and treated by
filtering, washed with methanol several times, and dried under vacuum at 60 °C to a
constant weight.
Synthesis of substituted aniline
Synthesis of 2ꢀnaphthylꢀaniline (1a)
In a 250 mL Schlenk flask, Pd(PPh3)4 (0.3 mmol, 340 mg) and 2ꢀbromoaniline
(23.5 mmol, 4.04 g) were dissolved in 80 mL toluene. Ethanol solution (30 mL) of
1ꢀnaphthylboronic acid (40 mmol, 6.88 g) and an aqueous solution K2CO3 (100 mmol,
13.8 g) were added under nitrogen atmosphere. The reaction mixture was refluxed for
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3 days, then the organic layer was separated and the aqueous phase was extracted with
ether. After dried over MgSO4 and evaporated solvents, the mixtures were purified by
column chromatography on silica gel using petroleum ether/ethyl acetate (4:1) as
eluent. The final product was crystallized from petroleum ether as white solid in 87.4%
yield. 1H NMR (300 MHz, CDCl3): 7.87ꢀ6.83 (m, 11H, phenyl and naphthyl), 2.90 (s,
2H, NH2). EIꢀMS (m/z): 220 [M]+.
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Synthesis of 2,4ꢀdimethylꢀ6ꢀnaphthylꢀaniline (2a)
The synthesis of compound 2a was carried out as described for 1a, the white solid
was in 82.3% yield. 1H NMR (300MHz, CDCl3): 7.90 (t, 2H, Ar), 7.66 (d, 1H, Ar),
7.58ꢀ7.41(m, 3H, Ar), 7.02 (s, 2H, Ar), 6.89 (s, 1H, Ar), 3.13 (s, 2H, NH2), 2.32 (s, 3H,
CH3), 2.27 (s, 3H, CH3). EIꢀMS (m/z): 248 [M]+.
Synthesis of 2,4ꢀdimethylꢀ6ꢀphenylꢀaniline (3a)
The synthesis of compound 3a was carried out as described for 1a, phenylboronic
1
acid was used instead of 1ꢀnaphthylboronic acid. The white solid was in 74.2%. H
NMR (300 MHz, CDCl3): 7.57ꢀ6.90 (m, 7H, ArꢀH), 3.10 (s, 2H, NH2) ), 2.25 (s, 3H,
CH3), 2.17 (s, 3H, CH3). EIꢀMS (m/z): 198 [M]+.
Synthesis of αꢀdiimine ligands
Synthesis of L1 Ar–N=C(An)–(An)C=N–Ar (An = acenaphthyl, Ar =
2ꢀnaphthylꢀphenyl)
Under a nitrogen atmosphere, trimethylaluminum (8 mL, 1.0 M in hexane) was
slowly injected into 2ꢀnaphthylꢀaniline 1.752 g (8 mmol) in toluene (20 mL) solution
at room temperature, and then the reaction system was heated to reflux for 2 h. After
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the solution was cooled to room temperature, acenaphthenequinone (0.612 g, 3.36
mmol) was added into the reaction Schlenk flask. The mixture was continuously
stirred for 6 h at refluxing temperature. When the solution was cooled to room
temperature, the reaction mixture was hydrolyzed with 5% aqueous NaOH solution.
The organic product was extracted with ethyl acetate and dried over MgSO4, and the
solvent was evaporated off. The desired product was obtained by recrystallization
from ethanol as orange crystal in 75.3% yield. Three isomers were detected in a
solution according to 13C NMR spectrum. 1H NMR (300 MHz, CDCl3): 7.80ꢀ6.87 (m,
28H, ArꢀH). 13C NMR (75 MHz, CDCl3): 160.19 (C=N), 157.24 (C=N), 156.34
(C=N), 150.20 (C–N), 149.46 (C–N), 148.78 (C–N), 140.81, 140.05, 138.21, 136.90,
136.20, 133.89, 133.45, 133.33, 133.17, 133.04, 132.09, 131.95, 131.74, 131.19,
130.63, 130.35, 130.22, 129.39, 129.23, 129.00, 128.63, 128.57, 128.36, 128.23,
127.79, 127.74, 127.70, 127.55, 127.49, 127.46, 127.30, 127.16, 126.95, 126.90,
126.02, 125.27, 125.12, 125.00, 124.74, 124.68, 124.39, 123.69, 123.26, 122.90,
122.48, 122.36, 119.07, 118.27, 117.37. Anal. Calcd for C44H28N2: C: 90.38, H: 4.83,
N: 4.79%. Found: C: 89.91, H: 4.74, N: 4.77%.
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Synthesis of L2 Ar–N=C(An)–(An)C=N–Ar (An = acenaphthyl, Ar =
2,4ꢀdimethylꢀ6ꢀnaphthylꢀphenyl)
A similar synthesis method as L1, the orange crystal was obtained in 86.4% yield.
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Three isomers were detected with a ratio of 5.0 : 2.5 : 1. 1H NMR (300 MHz, CDCl3):
7.95ꢀ6.70 (m, 24H, ArꢀH), 2.42 (s, 6H, CH3, major + middle isomers), 2.35 (m, 6H,
CH3, minor isomer), 2.25 (s, 3H, CH3, minor isomer), 2.14 (s, 3H, CH3, minor
isomer), 2.09 (s, 3H, CH3, major isomer), 1.94 (s, 3H, CH3, major isomer), 1.62 (s, 3H,
CH3, middle isomer), 1.47 (s, 3H, CH3, middle isomer). 13C NMR (75 MHz, CDCl3):
160.65 (C=N), 146.34, 139.92, 137.20, 136.54, 133.57, 133.15, 132.02, 132.39,
133.22, 131.51, 131.26, 130.61, 130.53, 130.31, 130.17, 129.87, 129.75, 128.67,
128.54, 128.32, 127.96, 127.71, 127.36, 127.03, 126.87, 126.64, 126.60, 125.98,
125.74, 125.65, 125.52, 125.41, 125.35, 124.71, 124.51, 124.38, 123.89, 122.82,
122.76, 122.66, 122.59, 122.47, 122.33, 20.96 (CH3ꢀAr), 18.16 (CH3ꢀAr), 17.84
(CH3ꢀAr), 17.52 (CH3ꢀAr). Anal. Calcd for C48H36N2: C: 89.97, H: 5.66, N: 4.37%.
Found: C: 89.94, H: 5.68, N: 4.31%.
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Synthesis of L3 Ar–N=C(An)–(An)C=N–Ar (An = acenaphthyl, Ar =
2,4ꢀdimethylꢀ6ꢀphenylꢀphenyl)
A similar synthesis method as L1, the orange crystal was obtained in 84.1% yield.
Two isomers were detected with a ratio of 0.35 : 1. 1H NMR (300 MHz, CDCl3): 7.78
(s, 1H, ArꢀH, minor isomer), 7.76 (s, 1H, ArꢀH, minor isomer), 7.74 (s, 1H, ArꢀH,
major isomer), 7.71 (s, 1H, ArꢀH, major isomer), 7.49ꢀ7.46 (m, 4H, ArꢀH, minor +
major isomer), 7.37ꢀ7.27 (m, 2H, ArꢀH, minor + major isomers), 7.14ꢀ7.01 (m, 8H,
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ArꢀH, minor + major isomers), 6.80 (s, 1H, ArꢀH, minor isomer), 6.77 (s, 1H, ArꢀH,
minor isomer), 6.73 (s, 1H, ArꢀH, major isomer), 6.70 (s, 1H, ArꢀH, major isomer),
2.46 (s, 6H, CH3, major isomer), 2.45 (s, 6H, CH3, minor isomer), 2.11 (s, 6H, CH3,
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major + minor isomers). C NMR (75 MHz, CDCl3): 160.91 (C=N, minor isomer),
160.42 (C=N, major isomer), 145.63 (major isomer), 142.69 (minor isomer), 140.48
(major isomer), 140.41 (minor isomer), 139.66 (minor isomer), 139.59 (major
isomer), 133.44 (major isomer), 133.28 (minor isomer), 131.75 (minor isomer),
130.73 (minor isomer), 130.62 (major isomer), 130.49 (minor isomer), 130.33 (minor
isomer), 129.82 (major isomer), 129.57 (minor isomer), 129.46 (major isomer),
129.30 (major isomer), 129.13 (minor isomer), 129.05 (minor isomer), 128.99 (minor
isomer), 128.82 (major isomer), 128.62 (major isomer), 128.55 (minor isomer),
128.46 (major isomer), 127.84 (minor isomer), 127.83 (major isomer), 127.65 (minor
isomer), 127.64 (major isomer), 127.37 (major isomer), 126.22 (major isomer),
125.91 (major isomer), 125.23 (minor isomer), 122.54 (minor isomer), 122.43 (major
isomer), 20.91 (CH3, major isomer), 20.86 (CH3, minor isomer), 18.03 (CH3, minor
isomer), 17.95 (CH3, major isomer). Anal. Calcd for C40H32N2: C: 88.85, H: 5.97, N:
5.18%. Found: C: 88.72, H: 5.84, N: 5.10%.
Synthesis of ligand L4 Ar–N=C(An)–(An)C=N–Ar (An = phenyl, Ar =
2ꢀphenylꢀphenyl)
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A similar synthesis method as L1, the orange crystal was obtained in 65.1% yield.
1H NMR (300 MHz, CDCl3): 7.57 (d, 4H, ArꢀH), 7.34 (m, 3H, ArꢀH), 7.22ꢀ7.20 (m,
5H, ArꢀH), 7.16ꢀ7.11 (m, 6H, ArꢀH), 7.05ꢀ7.00 (m, 3H, ArꢀH), 6.87ꢀ6.82 (m, 5H,
ArꢀH), 6.44 (d, 2H, ArꢀH). 13C NMR (75 MHz, CDCl3): 162.70 (C=N), 145.70 (C–N),
139.24, 137.51, 134.70, 130.49, 130.19, 129.56, 128.51, 128.16, 127.30, 127.24,
126.21, 125.66, 118.23. Anal. Calcd for C38H28N2: C: 89.03, H: 5.51, N: 5.46%.
Found: C: 88.78, H: 5.46, N: 5.34%.
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Synthesis of L5 Ar–N=C(An)–(An)C=N–Ar (An
2ꢀnaphthylꢀphenyl)
=
phenyl, Ar
=
A similar synthesis method as L1, the orange crystal was obtained in 71.4% yield.
1H and 13C NMR spectroscopies of ligand L5 were very complex because of
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appearance of geometric and conformational isomers. H NMR (300 MHz, CDCl3):
7.96ꢀ6.27 (m, 32H, ArꢀH). 13C NMR (75 MHz, CDCl3): 164.16 (C=N), 161.75 (C=N),
148.14, 147.74, 146.75, 146.75, 144.16, 137.59, 137.18, 136.84, 136.0ꢀ124.72 (br, m),
123.94ꢀ123.35 (br), 120.9, 120.12, 118.35, 118.16, 115.18. Anal. Calcd for C46H32N2:
C: 90.16, H: 5.26, N: 4.57%. Found: C: 89.73, H: 5.13, N: 4.46%.
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Synthesis of L6 Ar–N=C(An)–(An)C=N–Ar (An
2,4ꢀdimethylꢀ6ꢀphenylꢀphenyl)
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A similar synthesis method as L1, the orange crystal was obtained in 58.1% yield.
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Two isomers were detected with a ratio of 4.3:1. H NMR (300MHz, CDCl3):
7.49ꢀ6.12 (m, 24H, ArꢀH, major + minor isomers), 2.33ꢀ2.16 (m, 9H, CH3, major
isomer), 2.06 (s, 6H, CH3, minor isomer), 1.30 (s, 3H, CH3, minor isomer), 1.06 (s,
3H, CH3, major isomer), 0.90 (s, 3H, CH3, minor isomer). 13C NMR (75 MHz,
CDCl3): 164.07 (C=N), 161.86 (C=N), 144.18, 143.88, 140.50, 140.04, 137.00,
132.86, 132.47, 130.97, 130.51, 129.89, 129.69, 129.60, 129.43, 129.16, 128.54,
128.22, 128.08, 127.94, 127.68, 127.55, 127.05, 126.93, 126.61, 126.07, 125.97,
124.06, 20.75 (CH3), 19.35 (CH3), 18.94 (CH3), 18.72 (CH3), 16.73 (CH3). Anal.
Calcd for C42H36N2: C: 88.69, H: 6.38, N: 4.93%. Found: C: 88.51, H: 6.32, N:
4.76%.
Synthesis of L7 Ar1–N=C(R)–(R)C=N–Ar2 (R = phenyl, Ar1 = 2ꢀphenylꢀphenyl,
Ar1 = 2,6ꢀdiisopropylphenyl)
Ligand L7 was synthesized by two step condensation reactions of αꢀdione
compounds with various anilines.
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Benzil (1.05 g, 5.0 mmol) and 2,6ꢀ diisopropylaniline (0.95 mL, 5.0 mmol) were
charged in a round bottom flak with acetic acid (100 mL), and then the reaction
system was heated to reflux. After 8 h, the reaction mixture was cooled to room
temperature overnight. The yellow crystal was collected and treated by filtration,
washed with ethanol several times, and dried under vacuum. The monoꢀcondensation
product was produced in 89.6% yield. 1H NMR (300 MHz, CDCl3): 8.15ꢀ7.91 (m, 2H,
ArꢀH), 7.59ꢀ7.10 (m, 9H, ArꢀH), 6.88 (s, 2H, ArꢀH), 2.87 (hept, 2H, CH(CH3)2), 1.06
(d, 6H, CH(CH3)2), 0.96 (d, 6H, CH(CH3)2). 13C NMR (75 MHz, CDCl3): 196.57
(C=O), 165.22 (C=N), 144.60 (C–N), 136.16, 135.02, 133.54, 131.44, 130.05, 128.75,
128.43, 127.82, 124.03, 123.31, 122.17, 28.45 (CH(CH3)2), 24.10 (CH3), 21.22 (CH3).
Elemental analysis calculated for C26H27NO: C, 84.51; H, 7.37; N, 3.79%. Found: C,
84.38; H, 7.26; N, 3.61%.
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The second step condensation reaction was carried out according to a similar
synthesis method as L1. The monoꢀcondensation product was used instead of αꢀdione
compounds, and 1 equiv. 2ꢀphenylꢀaniline was used. The desired product was
obtained as orange crystal in 68.7 % yield. Three isomers were detected with a ratio
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of 1 : 2.3 : 3.4. H NMR (300 MHz, CDCl3): 7.93ꢀ6.38 (m, 22H, ArꢀH, minor +
middle + major isomers), 2.88 (m, 2H, CH(CH3)2, minor isomer), 2.64 (m, 2H,
CH(CH3)2, middle isomer), 2.51 (m, 2H, CH(CH3)2, major isomer). 1.26ꢀ1.21(m, CH3,
12H, minor isomer), 0.96ꢀ0.69 (m, CH3, 12H, middle + major isomers). 13C NMR (75
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