Chain-Walking Polymerization of Linear Internal Octenes Catalyzed by α-Diimine Nickel Complexes

Article abstract of DOI:10.1021/acs.organomet.8b00042

The chain-walking polymerization of linear internal alkenes (i.e., trans-2-, 3-, and 4-octenes) using α-diimine nickel catalysts activated with modified methylaluminoxane (MMAO) was studied in comparison with the corresponding terminal alkene polymerization. The rates of polymerization were found to decrease in the following order: 1-octene > 4-octene ≥ 2-octene ≥ 3-octene. The obtained branched poly(2-octene)s and poly(4-octene)s with high molecular weight and M_w/M_n less than 2 were amorphous polymers with low glass transition temperature (T_g) of approximately -66 °C. At 0 °C, 4-octene polymerized in a living/controlled manner. The NMR analyses of the polymers showed that the chain-walking polymerization of 4-octene gave periodically branched polymers with the constant branching density, while that of 2-octene gave the polymer possessing fewer branches than the expected value due to monomer-isomerization. The (n+2),(n+3)- and (n+3),(n+2)-insertions of the internal (n+2)-alkene [CH₃(CH₂)_nCH=CH(CH₂)_mCH₃] followed by chain-walking were confirmed by the ¹³C NMR analysis of the produced polymers.

Full text of DOI:10.1021/acs.organomet.8b00042

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Chain-Walking Polymerization of Linear Internal Octenes Catalyzed
by α-Diimine Nickel Complexes
Fuzhou Wang,*,†,‡ Ryo Tanaka,‡ Qingshan Li,§ Yuushou Nakayama,‡ and Takeshi Shiono*,‡
†Institute of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, China
‡Graduate School of Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan
§Department of Chemistry, Nankai University, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: The chain-walking polymerization of linear
internal alkenes (i.e., trans-2-, 3-, and 4-octenes) using α-
diimine nickel catalysts activated with modified methylalumi-
noxane (MMAO) was studied in comparison with the
corresponding terminal alkene polymerization. The rates of
polymerization were found to decrease in the following order:
1-octene > 4-octene ≥ 2-octene ≫ 3-octene. The obtained
branched poly(2-octene)s and poly(4-octene)s with high
molecular weight and Mw/Mn less than 2 were amorphous
polymers with low glass transition temperature (Tg) of
approximately −66 °C. At 0 °C, 4-octene polymerized in a living/controlled manner. The NMR analyses of the polymers
showed that the chain-walking polymerization of 4-octene gave periodically branched polymers with the constant branching
density, while that of 2-octene gave the polymer possessing fewer branches than the expected value due to monomer-
isomerization. The (n+2),(n+3)- and (n+3),(n+2)-insertions of the internal (n+2)-alkene [CH3(CH2)nCHCH(CH2)mCH3]
followed by chain-walking were confirmed by the 13C NMR analysis of the produced polymers.
moieties polymerized ethylene with high activity and high
thermal stability,10 and they generated semicrystalline poly(1-
alkene)s with high melting temperatures. Coates et al. reported
precision chain-walking polymerization of linear 1-alkenes using
the “sandwich” α-diimine nickel catalysts, which afforded
semicrystalline “polyethylene”.7b α-Diimine nickel complexes
were reported to catalyze polymerization of 2-butene11 and 2-
pentene12 to afford high-molecular-weight polymers. However,
the chain-walking polymerization of higher 2-, 3-, and 4-alkenes
with nickel and palladium catalysts have been reported only in a
limited number of cases.
We have previously reported a series of phenyl substituted α-
diimine nickel complexes13 activated by Et2AlCl or MMAO
exhibited high activities toward ethylene polymerization to
afford highly branched polyethylene, and we conducted 1-
alkene polymerization14 to afford branched polymers with high
molecular weight. Recently, we synthesized and characterized a
series of high molecular weight poly(2-alkene) by a phenyl
substituted α-diimine nickel complex,15 and these 2-alkenes
were polymerized in a living manner to produce amorphous
polymers at 0 °C. We also reported the first polymerization of
trans-4-octene with α-diimine nickel catalysts bearing ortho-sec-
phenethyl groups at low temperature to obtain periodically
propyl-branched polymethylene (i.e., poly(1-propylpentan-1,5-
diyl)).16 In this context, we became very interested in the
INTRODUCTION
■
The research of ethylene and 1-alkene polymerizations
catalyzed by late-transition-metal complexes1 is a great highlight
for the development of novel polyolefin materials in recent
years.2 Nickel and palladium catalysts3,4 ligated by α-diimine
showed good activities for ethylene and 1-alkene polymer-
izations to produce a variety of branched polyolefins because of
their chain-walking feature.5 The branching degree of the
polymer can be controlled by polymerization conditions and
the catalyst structures.6,7
Chain-branching formation in linear 1-alkene polymer-
ization7 with α-diimine nickel and palladium catalysts is
shown in Scheme 1. The mechanism involves 1,2- and 2,1-
insertion followed by chain-walking,7 in which the active metal
center undergoes chain-walking to the terminal carbon followed
by monomer insertion. 1,2-Insertion gives methyl branch (ii) in
a 2,ω-enchainment, while 2,1-insertion gives a long methylene
sequence (iii) in a 1,ω-enchainment (chain-straightening).8 If
the metal fails to chain walk after 1,2-insertion, the final
polymer contains n-alkyl (Cω‑2) branches (i) found in common
poly(1-alkene)s. The highly branched polymers with methyl
and n-alkyl groups are amorphous, while the chain-straightened
polymers are semicrystalline.7b
The amount and type of branching are dependent on the
catalyst structure and polymerization conditions,9 which in turn
strongly influence the polymer crystallinity.1d For example,
Chen et al. reported that a series of iminopyridyl nickel
catalysts containing both the dibenzhydryl and the naphthyl
Received: January 22, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00042
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Organometallics
Article
Scheme 1. Chain-walking mechanism for linear 1-alkene polymerization by α-diimine nickel and palladium catalysts
polymerization of linear internal octenes (i.e., 2-, 3-, and 4-
octenes). This work extends our previous studies on 2-octene15
and 4-octene16 polymerizations and gives a whole mechanism
on the chain-walking polymerization of linear internal alkenes.
The effects of catalyst structure, polymerization temperature,
[Al]/[Ni] molar ratio, and linear octene isomers type on
productivity, selectivity of monomer insertion, and polymer
structure/properties (i.e., total branching, branch-type and
thermal behavior) are investigated.
Figure 1. Molecular structure of the catalyst precursor 2 with 50%
probability level, and H atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (deg) for complexes: Br1−Ni1, 2.333(3);
Br2−Ni1, 2.299(3); Ni1−N1, 2.041(11); Ni1−N2, 1.992(11); N1−
C10, 1.436(17); N1−C19, 1.248(16); Br1−Ni1−Br2, 125.83(12);
N1−Ni1−N2, 81.0(5); N1−Ni1−Br2, 115.1(3); N2−Ni1−Br2,
116.7(4); C10−N1−C19, 119.6(12).
RESULTS AND DISCUSSION
■
Synthesis and Characterization. α-Diimine nickel
precatalysts used in this study are summarized in Scheme 2.
Complexes 1, 3, 5, and 6 were previously reported.14,17
Complexes 2 and 4 were synthesized in this study from the
18
corresponding ligand and (DME)NiBr2 in high yields.
above and below the Ni1 coordinative geometry plane (Figure
1). The two aryl rings of the α-diimines lie nearly perpendicular
to the plane formed by the nickel and coordinated nitrogen
atoms. The N1−Ni1−N2 bite angle in complex 2 is 81.0(5)°
and highlights the distortion imposed by the chelating ligand on
the geometry. The Br1−Ni1−Br2 angle is 125.83(12)°. The
two imino CN bonds have typical double bond character
with C19N1 bond lengths of 1.248 (16) Å. The molecular
structures of complexes 1 and 2 are very similar (Figure S9).14
In fact, the Ni−N bond distances in 2 (2.041(11) and
1.992(11) Å) are similar to those determined for complex 1
(2.041(5) Å), and the Ni−Br bond distances in complexes 1
and 2 are almost identical (2.333(3) and 2.299(3) Å for 2 vs
2.3343(12) Å for 1). In addition, the N−Ni−Br angles of
complex 2 (115.1(3) and 116.7(4)°) also approximate to those
of complex 1 (112.96(6)°). These para-phenyl substituted α-
diimine nickel complexes may induce a conjugation effect in
olefin polymerization.14
Scheme 2. Synthesis of α-Diimine Nickel Complexes 1−6
Polymerization of Linear Octene isomers. All the
catalyst precursors 1−6 were applied to trans-4-octene (4O)
polymerization at room temperature (20 °C) using MMAO as
a cocatalyst. The initial results are summarized in Table 1.
First, polymerization of 4O was investigated by 1-MMAO at
20 °C with the [Al]/[Ni] molar ratio from 300 to 700 (entries
1−3, Table 1). The catalytic activity of complex 1 did not
significantly depend on the [Al]/[Ni] ratio and the maximum
catalytic activity was observed with the [Al]/[Ni] ratio of 500
(entry 3). Besides, the highest molecular weight (Mn = 46.7 kg
mol−1) of the product was also observed with the [Al]/[Ni]
ratio of 500 (entry 2).
Suitable crystal of complex 2 for X-ray diffraction were
obtained at room temperature by double layering a CH2Cl2
solution of the complex with n-hexane. The molecular structure
of 2 was confirmed by single-crystal X-ray diffraction and the
corresponding ORTEP diagram is shown in Figure 1. Crystal
data, data collection, and refinement parameters are listed in
Table S1 (see the Supporting Information). The molecular
structure of complex 114 was reported in our recent publication
(Figure S9).
The X-ray structure analysis indicates that complex 2 has a
pseudo-tetrahedral geometry about the Ni center, showing C2v
molecular symmetry. In solid state, complex 2 clearly shows the
N,N-chelating bis(imino)acenaphthene structure with a phenyl
group connected in the para-aryl position. The two isopropyl in
the 2,6-position of the aniline fragments in complex 2 located
Then, the effect of catalyst precursors 1−6 was investigated
with [Al]/[Ni] ratio of 500 at 20 °C (entries 2 and 4−8, Table
1). All complexes 1−6 produced polymers in moderate yields
with high molecular weight (Mn ≥ 18.7 kg mol−1) and narrow
B
DOI: 10.1021/acs.organomet.8b00042
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a
Table 1. Polymerization of trans-4-Octene (4O) Catalyzed with 1−6 Activated by MMAO
complex
b
c
c
d
e
entry
R1
R2
R3
[Al]/[Ni]
yield (mg)
activity
Mn (kg mol−1
)
Mw/Mn
N
(μmol)
branches /1000C
1
2
3
4
5
6
7
8
1
1
1
2
3
4
5
6
Me
Me
Ph
300
500
700
500
500
500
500
500
268
346
297
302
205
199
181
195
13.4
17.3
14.8
15.1
10.3
10.0
9.1
31.9
46.7
44.1
49.0
20.7
22.9
27.9
33.2
1.49
1.55
1.94
1.52
1.46
1.89
1.49
1.85
8.4
7.4
6.7
6.2
9.9
8.7
6.5
5.9
125
125
125
125
125
125
125
125
iPr
Me
Me
iPr
iPr
Me
iPr
iPr
iPr
Ph
H
H
H
iPr
Br
9.8
a
Polymerization conditions: Ni = 10 μmol; cocatalyst MMAO; 0.5 g of trans-4-octene; polymerization time, 2 h; temperature, RT ≈ 20 °C; solvent,
b
c
d
toluene; total volume, 5 mL. In unit of kg (mol Ni h)−1. Determined by GPC relative to polystyrene standards. Number of polymer chains
calculated by yield and Mn. Branching numbers per 1000 carbon atoms were determined by H NMR spectroscopy.
e
1
Table 2. Polymerization of 1-Octene (1O), trans-2-Octene (2O), trans-3-Octene (3O), and trans-4-Octene (4O) with 1-
a
MMAO
b
c
c
d
e
entry
monomer
temp (°C)
time (h)
yield (mg)
activity
Mn (kg mol−1
)
Mw/Mn
N
(μmol)
branches /1000 C
1
2
4O
4O
4O
4O
4O
4O
4O
4O
4O
3O
3O
2O
2O
1O
1O
−25
0
2
2
106
207
338
253
90
5.3
10.4
16.9
12.6
4.5
30.3
53.8
45.1
34.7
13.3
16.1
28.5
83.4
73.4
1.25
1.19
1.33
1.64
1.72
1.14
1.15
1.20
1.42
3.5
3.8
7.5
7.3
6.8
3.5
3.9
3.9
5.9
125
125
125
125
125
125
125
125
125
3
25
50
75
0
2
4
2
5
2
6
0.5
1
57
11.4
11.0
8.1
7
0
110
325
434
trace
trace
182
235
324
456
8
0
4
9
25
0
4
10.9
10
11
12
13
14
15
2
25
0
2
2
9.1
25.5
46.8
1.11
1.63
1.08
1.31
7.1
5.0
3.6
3.4
121
119
100
25
0
2
11.8
16.2
22.8
2
89.0
25
2
132.6
86
a
b
Polymerization conditions: Ni = 10 μmol; cocatalyst MMAO, [Al]/[Ni] = 500; 0.5 g of trans-octene; solvent, toluene; total volume, 5 mL. In unit
c
d
e
of kg (mol Ni·h)−1. Determined by GPC relative to polystyrene standards. Number of polymer chains calculated by yield and Mn. Branching
numbers per 1000 carbon atoms were determined by H NMR spectroscopy.
1
molecular weight distribution (Mw/Mn = 1.40−1.94). The
activities decreased in the following order, 1 ≥ 2 ≥ 3 ≥ 4 ≥ 6 ≥
5. The molecular weight of the polymers decreased in the
following order, 2 ≥ 1 ≥ 6 ≥ 5 ≥ 4 ≥ 3.
Electronic effect of the ligands can be discussed by
comparing nickel complexes 2, 5, and 6 (entries 4, 7, and 8).
The results indicate that the nickel complexes with electron-
withdrawing groups [2(Ph) > 6(Br) > 5(H)] in the para-aryl
position showed higher catalytic activity for 4O polymerization
and gave the polymer with higher molecular weight.14,15
Complex 1 bearing a phenyl group in the para-aryl position also
exhibited higher activity than the corresponding nonsubstituted
complex 3 (entries 4 vs 7, Table 1)
distribution using a traditional nickel α-dimmine catalyst
activated with Et2AlCl. Recently, we reported that the
polymerization of 2-octene15 and 4-octene16 with α-diimine
nickel catalysts to afford branched polymers with high
molecular weight. However, comparison of the effect of linear
octene isomers type or polymerization temperature on
productivity and polymer structure has not been reported.
Herein, we report extension of our studies about the chain-
walking polymerization of isomers of linear octane using
screened 1-MMAO under different polymerization conditions.
Polymerization of 1-octene (1O), trans-2-octene (2O), 3-
octene (3O), and 4O with 1-MMAO were carried out at
various polymerization temperature and time with the [Al]/
[Ni] ratio of 500, and the results are listed in Table 2. The
catalytic system produced polymers with high molecular weight
and narrow molecular weight distribution (Mw/Mn = 1.11−
1.72) in moderate yields except 3-octene (entries 10 and 11).
We also examined the trans-3-hexene using 1-MMAO under
the same conditions, but no polymer was obtained. This result
showed that the chain-walking polymerization of 3-octene
would not proceed by these α-diimine nickel catalysts. The
reason may be attributed to the monomer isomerization of 3-
alkene to suppress polymerization. In addition, no polymeric
product was obtained from the polymerization of correspond-
Steric effect of ortho-position in anilinic moiety can be
evaluated by comparing 1 to 2 and 3 to 5 (entries 2, 4, 5, and
7). The complexes with methyl groups (1 and 3) showed
higher activity than those with the corresponding isopropyl
groups (2 and 5) but gave lower molecular weight. The higher
molecular weight of the polymers and the lower number of
polymer chains (N) obtained by 2 and 5 suggest that the ortho-
isopropyl should suppress chain-transfer reactions.
Leone7f et al. reported the polymerization of 1-octene to
afford semicrystalline, branched poly(ethylene)-like materials
with high molecular weight and narrow molecular weight
C
DOI: 10.1021/acs.organomet.8b00042
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ing cis-isomers of internal octenes using 1-MMAO under the
similar conditions because these cis-isomers suppressed the
propagation rate.15
ization of 4O with 1-MMAO proceeded in a living manner at 0
°C within a certain period of time.
Polymerization of 2O was also investigated by 1-MMAO
with the [Al]/[Ni] ratio of 500 at 0 and 25 °C for 2 h (entries
12 and 13, Table 2). These results demonstrated that 2O
polymerized slower than 4O. This result may be attributed to
the isomerization reaction of 2O to 1O indicated by the lower
branching numbers, 121 and 119 per 1000C.13 The number of
branches in the poly(4O), 125/1000C implies that monomer
isomerization did not occur in the 4O polymerization, which is
discussed in the subsequent section. For comparison, 1O
polymerization was performed (entries 14 and 15, Table 2).
The catalytic activities decreased in the following order: 1O >
4O ≥ 2O ≫ 3O (0 °C, entries 2, 10, 12, and 14, Table 2). The
molecular weight also follows the same trend. The catalytic
performances could not be explained with the sole factor of
either monomer structure or monomer-isomerization15 influ-
ences, but likely were caused by a combination of both
monomer structure and monomer-isomerization effects.
Copolymerization of Linear Octene Isomers. We
continued our investigation on the copolymerization of 1O
with 2O, 3O, and 4O catalyzed by para-phenyl-substituted
complexes 1 and 2 activated by MMAO at 0 and 25 °C with the
[Al]/[Ni] ratio of 500 and an equal comonomer feed ratio. The
copolymerization results are listed in Table 3.
4O polymerization was conducted at temperatures ranging
from −25 to 75 °C (entries 1−5, Table 2), and the results
indicated that the highest activity was observed at 25 °C (entry
3). Besides, the increased temperature also affected the
molecular weight and its distribution of the produced polymers.
Both the highest molecular weight and the narrowest molecular
weight distribution (entry 2, Mn = 53.8 kg mol−1, Mw/Mn =
1.19, Table 2) were observed at 0 °C. Further increase of
temperature caused a decrease in the molecular weight
accompanied by broadening the molecular weight distribution.
The time dependence of 4O polymerization was also
investigated (entries 2, 3 and 6−9, Table 2). The catalytic
activities were well maintained along with prolonged the
reaction time, and the Mn value increased with keeping the
narrow molecular weight distribution (0 °C, entries 2 and 6−8,
Table 2). Thus, the time-dependences of molecular weight and
polydispersity indexes in the polymerization of 4-octene were
investigated in more detail under the same polymerization
conditions. (Table S2, Supporting Information).
The plot of Mn against the monomer conversion for 4O
polymerization catalyzed by 1-MMAO at 0 °C is plotted in
Figure 2. The Mn value increased linearly with increasing 4O
In comparison with homopolymerization of 2O, 3O, and 4O
under the same conditions (Table 2), the copolymerization
proceeded in high activity. The yields and the molecular weight
of the copolymers show the similar trend to those of the
homopolymers. For example, the 1O-4O copolymer obtained
with 1-MMAO at 0 °C showed narrow molecular weight
distribution (Mw/Mn = 1.13, entry 1, Table 3). Complex 1
bearing two ortho-methyl groups showed higher activity than
the corresponding isopropyl groups 2 (entries 2 vs 3, Table 3),
but gave the copolymer with lower molecular weight. The rates
of copolymerization by 1-MMAO decreased in the following
order: 1O-4O > 1O-2O > 1O-3O (entries 2, 4, and 5, Table 3).
The molecular weights of the copolymers also follow the same
trend.
Structure of the Resulting Polymers. A series of linear
octene isomers were successfully polymerized with the α-
diimine nickel catalysts activated by MMAO. The total
■
▲
Figure 2. Plots of Mn ( ) and Mw/Mn ( ) as a function of monomer
1
branching degrees of the polymers was determined by H
conversion for 4-octene polymerization catalyzed by 1-MMAO at 0
°C.
NMR spectroscopy, and the results are shown in Tables 1 and
2. The total branching degree of the obtained poly(4O)s was
equal to the expected value (125/1000 C) and independent of
the [Al]/[Ni] ratio, polymerization temperature, and the
catalyst structure. It is known that the total branching degree
of the obtained poly(1O)s is lower than the expected value
conversion keeping the narrow molecular weight distribution
(Mw/Mn = 1.13−1.20, Figure 2) and the constant number of
polymer chains (N). These results verify that the polymer-
a
Table 3. Copolymerization of 1O with 2O, 3O, 4O by Nickel Complexes 1 and 2 Activated by MMAO
monomers
b
c
c
d
e
entry
1O
M
complex
temp (°C)
yield (mg)
activity
Mn (kg mol−1
)
Mw/Mn
N
(μmol)
branches /1000 C
1
2
3
4
5
1O
1O
1O
1O
1O
4O
4O
4O
3O
2O
1
1
2
1
1
0
308
419
401
235
302
15.4
21.0
20.0
11.8
15.1
85.9
92.7
107.5
34.7
64.0
1.13
1.20
1.21
1.64
1.08
3.58
4.52
3.73
6.77
4.72
98
110
106
99
25
25
25
25
103
a
Polymerization conditions: Ni = 10 μmol; cocatalyst MMAO, [Al]/[Ni] = 500; 0.25 g of 1O, 0.25 g of M; polymerization time, 2 h; solvent,
b
c
d
toluene; total volume, 5 mL. In unit of kg (mol Ni·h)−1. Determined by GPC relative to polystyrene standards. Number of polymer chains
calculated by yield and Mn. Branching numbers per 1000 carbon atoms were determined by H NMR spectroscopy.
e
1
D
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Article
Figure 3. Possible inserted monomer units and their peak assignments of 13CNMR spectra shown in Figure 4. Note on labels: In the denotation of
aBn, the subscript n indicates the branch type (n = 1, 2, 5 or 6). The Arabic number “a” indicates the position from the methyl group of the side
chain. The methylene carbons in main chain are labeled by S with two Greek letters followed by an Arabic number which indicate the position from
the neighboring methine carbon: for example, Sαmβn indicates the methylene carbon at α position from Bm branch and β position from Bn branch
and δ implies the branched carbon at or beyond δ position. The methine carbons are labeled with Tn where n indicates the number of carbons in the
branch.
Table 4. Branching Distributions of Poly(4O), Poly(2O), and Poly(1O)
a
CH3/1000 C
a
d
b
c
entry/table
complex nO in poly(nO) temp (°C) total CH3
Me
Et
Pr
Bu
Amy
He
(n+2),8- enchain. (%) Tg (°C)
4/Table 1
7/Table 1
1/Table 2
2/Table 2
3/Table 2
4/Table 2
5/Table 2
12/Table 2
13/Table 2
14/Table 2
15/Table 2
2
5
1
1
1
1
1
1
1
1
1
4O
4O
4O
4O
4O
4O
4O
2O
2O
1O
1O
RT
RT
−25
0
125
125
125
125
125
125
125
117
116
95
87
90
38
35
16
23
36
57
61
69.6 (4,8-)
72.0 (4,8-)
87.2 (4,8-)
81.3 (4,8-)
70.9 (4,8-)
54.1 (4,8-)
51.2 (4,8-)
43.2 (2,8-)
46.4 (2,8-)
36.1 (1,8-)
44.8 (1,8-)
-
-
109
102
89
−64.4
−64.7
−65.5
−68.3
−68.9
−66.5
−70.0
−38.9
−51.5
25
50
75
0
68
64
54
58
20
39
20
23
37
26
6
9
25
0
1
74
25
80
41
a
b
Branching numbers per 1000 carbon atoms were determined by 13C NMR spectroscopy. (n+2),8-Enchainment (n = −1, 0 and 2) was calculated
by the following equation: (n+2),8-% = [Cn+1] × 100, such as 4,8- = 1,5- = [C3] = [Pr];16 2,8- = [C1] = [Me];15 1,8- = [C0] = [1000−6B]/(1000 +
2B),14 B = is the total branching. Determined by DSC. Amy is the abbreviation for amyl.
c
d
because the 2,1-insertion of 1O always results in 1,8-
enchainment.13 Actually, the total branching degree of the
poly(1O)s obtained at 0 °C was 100/1000 C and decreased to
86/1000 C by raising the polymerization temperature to 25 °C.
However, the total branching degree of the obtained poly(2O)s
(119−121/1000 C) were slightly less than the expected value
(entries 12 and 13, Table 2). The isomerization of 2-octene to
1-octene which forms 1,8-enchainment via 2,1-insertion should
be the reason for the lower branch number.
Generally, polymerization temperature has a significant
influence on insertion fashion of monomer and chain-walking,
thus leading to the change of polymer microstructure.1d,7 The
13C NMR spectra of the poly(4O)s obtained by 1-MMAO at
polymerization temperatures from −25 to 75 °C are shown in
Figure 5. The results showed that the branch structures
significantly depended on the polymerization temperature:
butyl branch gradually increased as the temperature raised
(entries 1−5/Table 2, Table 4) accompanied by the decrease of
propyl branch keeping the total branching density constant
(125/1000C) as shown in Figure 6. This is consistent with the
ortho-sec-phenethyl substituted α-diimine nickel initiator.16
Interestingly, the predominance of propyl branch at low
temperature disappeared at 75 °C.
The branching structures of the homopolyoctenes were
determined by 13C NMR spectroscopy on the basis of previous
work.7e,15,16,19 The 13CNMR spectra displayed in Figure 3
showed various peaks depending on the monomer, which can
be assigned to each monomer-inserted unit as described below,
and their contents are summarized in Table 4.
The 13C NMR spectrum of the poly(2O)15 (Figure 4 (ii))
confirms the presence of methyl (19.92 ppm, 1B1), ethyl (11.08
ppm, 1B2), amyl (32.60 ppm, 3B5), hexyl (31.99 ppm, 3B6)
branches and a small amount of the long methylene sequence
(29.95 ppm, CH2) derived from 1O formed by monomer-
isomerization, whereas that of poly(1O) (Figure 4 (iii))
showed the resonances of methyl (19.76 ppm, 1B1) and
Figure 4 (i) shows the 13C NMR spectrum of the poly(4O)16
obtained by 2-MMAO at 20 °C (entry 4, Table 1), which
exhibits only propyl (14.76 ppm, 1B3; 20.01 ppm, 2B3; 36.29
ppm, 3B3 and 37.55 ppm, T3) and butyl (14.38 ppm, 1B4; 23.37
ppm, 2B4; 29.14 ppm, 3B4; 33.55 ppm, 4B4 and 37.56 ppm, T4)
branches.
E
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Figure 4. 13C NMR spectra of (i) poly(4O) (entry 4, Table 1), (ii) poly(2O) (entry 13, Table 2), and (iii) poly(1O) (entry 15, Table 2).
■
●
▲
)
Figure 6. Total branching densities ( ) and the branch (Pr: /Bu:
structures of the poly(4O)s versus polymerization temperature at −25,
0, 25, 50, and 75 °C (entries 1−5/Table 2, Table 4).
The 13C NMR spectrum of poly(1O-co-4O) obtained at 25
°C is shown in Figure 7 (i), which indicates the presence of
methyl, propyl, butyl, and hexyl branches.7e,19 On the
assumption that 4O gives either propyl or butyl branch, the
comonomer contents and the ratio of 1,8-insertion of 1O can
be determined by the following equation (see Table 5).
The branched structure of the poly(1O-co-4O) can be safely
identified by analyzing Figure 7 (i). The branched 1-octene
sequences resulting from catalyst chain-walking, in the presence
of methyl (T1) and hexyl (2B6) branches, are easily detected. At
25 °C, the 4O content in the poly(1O-co-4O) produced by 1 is
52 mol % higher than that of 14 mol % at 0 °C (entries 1 and 2,
Table 5). The comonomer content in the copolymer depended
on the catalyst used, indicating the ligand structure affected the
monomer reactivity ratios. For example, the 4O content in the
poly(1O-co-4O) obtained by complex 1 was slightly higher than
that of complex 2 (entries 2 and 3, Table 5) due to the ortho-
methyl substituted complex 1 shows relatively fast insertion and
chain-walking.
Figure 5. 13C NMR spectra of the poly(4O)s prepared by 1-MMAO at
different polymerization temperatures (entries 1−5, Table 2).
hexyl (14.18 ppm, 1B6) and the long methylene sequence
(29.80 ppm, CH2) due to chain-straightening.
The branching structure of the octene copolymers produced
by 1-MMAO is singled out and analyzed to study the
relationship between polymer microstructure and chain-walking
mechanism. The branching numbers of the copolymers were
decreased in the following order: poly(1O-co-4O) > poly(1O-
co-2O) > poly(1O-co-3O) (entries 2, 4 and 5, Table 3). The
results can be explained by the relative reactivity of the
comonomer to 1O and the probability of comonomer
isomerization.
F
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Figure 7. 13C NMR spectra of (i) poly(1O-co-4O) (entry 2, Table 3), (ii) poly(1O-co-3O) (entry 4, Table 3), and (iii) poly(1O-co-2O) (entry 5,
Table 3) obtained by 1-MMAO at 25 °C.
Table 5. Branching Distributions of Octene Copolymers
a
CH3/1000C
a
b
c
d
entry complex
copolymer
temp (°C) total CH3
Me
Et
Pr
Bu
Amy
He
1O/M (%) 1,8-En. (%) Tg (°C)
1
2
3
4
5
1
1
2
1
1
poly(1O-co-4O)
poly(1O-co-4O)
poly(1O-co-4O)
poly(1O-co-3O)
poly(1O-co-2O)
0
100
108
105
98
22
21
33
45
46
12
41
25
1
5
24
30
2
61
22
17
86/14
48/52
56/44
20.0
13.6
16.0
21.6
18.4
−60.6
−61.6
−64.6
−47.9
−54.2
25
25
25
25
e
3
47
n.d.
e
102
10
4
42
n.d.
a
b
Branching numbers per 1000 carbon atoms were determined by 13C NMR spectroscopy. [1O]/[4O]-Contents of copolymer was calculated by the
following equation: [4O] = [Pr] + [Bu]; [1O] = 1 − [4O]; 1,8-Enchainment was calculated by the following equation: 1,8- = [C0] = [1000−6B]/
(1000 + 2B), B = is the total branching. Determined by DSC. Not determined.
c
d
e
The 13C NMR spectra of poly(1O-co-3O) and poly(1O-co-
2O) are also shown in Figure 7 (ii) and (iii), respectively. A
small amount of the ethyl branches from 3O and 2O branched
units can be detected in the 13C NMR spectra of the poly(1O-
co-3O) and poly(1O-co-2O) (entries 4 and 5, Table 3).19
However, the 3O content of the poly(1O-co-3O) could not be
determined because the peak of the amyl branch that was
overlapping with that of the hexyl branch (Figure 7 (ii)). The
2O content of the poly(1O-co-2O) could also not be
determined because of the methyl and hexyl branches from
1O and 2O branched units.
The poly(2O) and poly(4O) obtained had low Tg of
approximately −70 ∼ −64 °C (Table 4) and no melting points
(Tm), indicating that the polymers were amorphous.7b The Tg
values were slightly decreased with increasing the polymer-
ization temperature from −25 to 75 °C. The Tg values of the
poly(2O) and poly(4O) were almost the same, which were
lower than those of the poly(1O)s. The Tg values of the 1O-
2O, 1O-3O, and 1O-4O copolymers (Table 5) are higher than
that of the 2O, 3O, and 4O polymers, which were also lower
than those of the poly(1O).
than the rate of alkene insertion.1e The poly(4O)s obtained
possessed the propyl and butyl branches, of which contents
were equal to the amount of monomer units (125/1000 C),
while the total branching degree of the poly(2O)s obtained was
lower than 125/1000C due to the monomer-isomerization to
1O. The number and the distribution of branches in the
poly(octene)s obtained in this study can be explained by the
mechanism outlined in Scheme 3.
Linear internal octenes, [CH3(CH2)nCHCH(CH2)mCH3,
n = 0, 1, and 2], are inserted into Ni−polymer bond via (n+2),
(n+3)- or (n+3),(n+2)-insertion. The successive alkene cannot
be inserted into the propagation chain of secondary carbons.
After the propagation chain-end becomes the methylene carbon
via chain-walking, next monomer insertion occurs. If the chain
walks to the C1 carbon, the Cm+1 and Cm+2 branches are formed
in 1,(n+3)- and 1,(n+2)-enchainment ((1) and (4)),
respectively. If the chain walks to the opposite direction (C8
carbon), the Cn+1 and Cn+2 branches are formed in (n+2), 8-
and (n+3), 8-enchainment ((3) and (2)), respectively.
Thus, the linear octenes give the polymers with Cn+1, Cn+2
,
C
m+1, and Cm+2 branches. For example, the methyl (C1), hexyl
Polymerization Mechanism of Linear Octenes. Chain-
walking alkene polymerization with α-diimine nickel and
palladium catalysts affords polymers with unique micro-
structures.8 The rate of chain-walking is generally much greater
(C6) groups and the long methylene sequence (C0 branch) are
detected in the poly(1-octene) (n = −1, m = 5), where 2,1-
insertion always gives a long methylene sequence (C0 branch)
in a 1,8-enchainment (3) because of no 1,1-enchainment (4).
G
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octene. At 0 °C, 4-octene polymerized in a living/controlled
manner. The NMR analyses of the polymers showed that 4-
octene was achieved by chain-walking polymerization to give
periodically branched polymers with the constant branching
density, while the total branching degree of the obtained
poly(2-octene)s are less than the expected value due to
monomer isomerization.
Scheme 3. Chain-Branching Formation in Linear Octenes
Polymerization with α-Diimine Nickel Catalysts
Based on chain-walking polymerization mechanism studies of
linear internal octenes, the general formula of chain-branching
formation of linear alkene was provided to indicate the
direction for the design of new catalysts. The (n+2),(n+3)-
and (n+3),(n+2)-insertion of the internal (n+2)-alkene
[CH3(CH2)nCHCH(CH2)mCH3], chain-walking, and inser-
tion at certain points along the chain was confirmed by the 13C
NMR analysis of the produced polymers with various types of
branches including Cn+1, Cn+2, Cm+1, and Cm+2 branches. The
obtained internal octene polymers are amorphous elastomers
with low Tg of approximately −66 °C. The branching structure
depended on the polymerization temperature. Interestingly, 1
was found to polymerize trans-4-octene with high levels of 1,5-
regioselectivity at −25 °C (up to 87.2%). At 75 °C, the number
of propyl and butyl branches became almost equal. Addition-
ally, copolymerization of 1-octene and these linear octene
isomers were studied by para-phenyl-substituted complexes
with equal comonomer feed ratio. These copolymerization
showed higher activities than the 2-, 3-, and 4-octene
homopolymerization, and the molecular weight of the
copolymers showed the similar trend to those of the
homopolymers.
The 2,3- and 3,2-insertion of 2O give four types of branches
(i.e., methyl, ethyl, amyl, and hexyl groups). The 4,5- or 5,4-
insertion of 4O always give two types of the propyl and butyl
groups because of its symmetrical structure. The intensity of
the peaks indicated that the propyl sequence ((1) and (3)) is
favorable in the poly(4O), because the chain-walking through
tert-carbon should be harder than that through the secondary
carbon.
Microstructure analysis of the linear octenes polymers (Table
4) indicated that the prioritization of structure units was
decreased in the following order, (3) > (1) ≈ (2) > (4) ≫ (5).
For example, the structure units of the poly(2O) at 25 °C
decreased in the following order, Me (3) > Amy (1) ≈ Et (2) >
He (4) ≥ C0 (5), and those of the poly(4O), Pr (3)/(1) > Bu
(2)/(4). In addition, the intensity of the peaks also indicated
that the structure unit of (4) should be suppressed at low
temperature.
On the other hand, we have confirmed that the monomer-
isomerization20 did not occur in the 4O and higher 1-alkene
polymerization by α-diimine nickel catalysts. The former may
be suppressed at the polymerization temperature (T ≤ 80 °C),
and the relatively fast insertion and chain-walking, resulting in
the production of the no isomerized 4-octene polymer. The
microstructures of the produced poly(2O)s indicated the
presence of a small amount of the long methylene sequence as
shown in Figure 4 (ii), and the number of total branches (116−
117) is close to the theoretical value (125). Thus, the rate of 2-
octene isomerization is much lower than the rate of its
insertion.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under nitrogen gas using standard Schlenk techniques. Modified
methylaluminoxane (MMAO, 6.5 wt % Al, 2.17 M in toluene) was
donated by Tosoh-Finechem Co. Dichloromethane was distilled from
CaH2 under dry nitrogen. Toluene and 1,2-dimethoxyethane (DME)
were distilled from sodium/benzophenone under nitrogen atmosphere
and distilled before use. 1-, trans-2-, trans-3-, and trans-4-octenes were
purchased from TCI Chemical Co., and these monomers were dried
over 4 Å molecular sieves overnight and degassed under nitrogen flow
before use. Nickel complexes 1,13 3,13 5,16 and 616 were synthesized
according to the literature. Other chemicals were commercially
obtained and purified with common procedures.
1H and 13C NMR spectra of compounds and polymers were
recorded on a Varian 400 MHz NMR instrument at room temperature
and 50 °C, respectively, using CDCl3 as a solvent and
tetramethylsilane (TMS) as a reference for the compounds. The
peak of the solvent (13C: 77.16 ppm) was used as an internal reference
in 13C NMR spectra of the polymers. X-ray diffraction data were
collected at 298(2) K on a Bruker Smart CCD area detector with
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). IR
spectra were recorded on a JASCO FT/IR-300E spectrophotometer
on KBr pellets. Elemental analysis was carried out using a PerkinElmer
2400 Series II. Number-average molecular weight (Mn) and molecular
weight distribution (Mw/Mn) of the polymers were determined by a
Tosoh HLC-8320GPC chromatograph at 40 °C using tetrahydrofuran
(THF) as an eluent and calibrated with polystyrene standard. DSC
analysis of the polymers was performed by a SII EXSTAR6000 system.
Synthesis of Complexes 2 and 4. Synthesis of Bis[N,N′-(2,6-
diisopropyl-4-phenylphenyl)imino]acenaphthene (L2). Formic acid
(0.2 mL) was added to a stirred solution of 1,2-acenaphthylenedione
(0.091 g, 0.50 mmol) and 2,6-diisopropyl-4-phenylaniline (0.27 g, 1.05
mmol) in MeOH (30 mL). The mixture was refluxed for 12 h, then
cooled and the precipitate was separated by filtration. The solid was
recrystallized from MeOH/CH2Cl2 (v/v = 15:1), washed with cold
CONCLUSIONS
■
In summary, we reported the polymerization of internal
alkenes, i.e., trans-2-, trans-3-, and trans-4-octene using the
screened α-diimine nickel catalysts containing phenyl groups
upon activation with MMAO in comparison with the 1-octene
polymerization. The results indicated that 2- and 4-octenes
underwent chain-walking polymerization to give new materials
of up to 83.4 kg mol−1 and narrow Mw/Mn values (<2.0).
However, 3-octene was not polymerized by this nickel catalytic
system. The rates of polymerization were found to decrease in
the following order: 1-octene > 4-octene ≥ 2-octene ≫ 3-
1
ethanol, and dried under vacuum (0.24 g, 74% yield). H NMR (400
H
DOI: 10.1021/acs.organomet.8b00042
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Article
MHz, CDCl3): δ 7.89 (d, J = 8.0 Hz, 2H, Nap−H), 7.71−7.76 (m, 4H,
Nap−H and Ar−H), 7.48−7.54 (m, 8H, Ar−H), 7.35 (dd, J = 7.6 Hz,
4H, Ar−H), 6.78 (d, J = 7.6 Hz, 2H, Nap−H), 3.07−3.13 (m, 4H,
−CH(CH3)2), 1.29 (d, J = 7.8 Hz, 12H, −CH3), 1.04 (d, J = 7.8 Hz,
12H, −CH3). 13C NMR (100 MHz, CDCl3): δ 161.09 (CN),
146.95, 141.70, 140.82 (Nap−C), 136.87, 135.84, 131.11,
129.45(Nap−C), 128.99, 128.74 (Nap−C), 127.93, 126.87, 126.80,
123.43, 122.30 (Nap−C), 28.77 (−CH(CH3)3), 23.19 (−CH3). Anal.
Calcd for C48H48N2: C, 88.30; H, 7.41; N, 4.29. Found: C, 88.19; H,
7.38; N, 4.23. FT-IR (KBr): 1652 cm−1 (vC=N).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00042.
■
S
NMR spectra of compounds (Figures S1−S8) and
(co)polymers (Figures S12−S23); GPC and DSC curves
of polymers samples (Figures S11 and S24−S37);
figures, tables, and CIF files giving crystallographic data
(PDF)
Synthesis of Bis[N,N′-(2-methy-6-isopropylphenyl)imino]-
acenaphthene (L4). Using the same synthetic procedure for L2, L4
Accession Codes
1
was obtained as an orange-red powder (0.19 g, 89% yield). H NMR
CCDC 979238 contains the supplementary crystallographic
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.
(400 MHz, CDCl3): δ 7.88 (d, J = 8.0 Hz, 2H, Nap−H), 7.36 (t, J =
7.8 Hz, 2H, Nap−H), 7.26 (dd, J = 7.2 Hz, 2H, Ar−H), 7.14−7.17 (m,
4H, Ar−H), 6.67 (d, J = 7.2 Hz, 2H, Nap−H), 3.03−3.07 (m, 2H,
−CH(CH3)2), 2.11 (s, 12H, −CH3), 1.24 (d, J = 6.8 Hz, 6H,
−CH(CH3)2), 0.95 (d, J = 6.8 Hz, 6H, −CH(CH3)2). 13C NMR (100
MHz, CDCl3): δ 161.10, 148.09, 140.72, 136.00, 132.61, 131.03,
129.40, 128.97, 128.12, 124.15, 123.68, 122.89, 28.63 (−CH(CH3)3),
23.71 (−CH(CH3)3, 22.69 (−CH(CH3)3, 18.85 (−CH3). Anal. Calcd
for C32H32N2: C, 86.44; H, 7.25; N, 6.30. Found: C, 86.37; H, 7.31; N,
6.21. FT-IR (KBr): 1633 cm−1 (vCN).
AUTHOR INFORMATION
Corresponding Authors
*E-mail for F.W: wangfuzhou1718@126.com.
*E-mail for T.S: tshiono@hiroshima-u.ac.jp.
■
Synthesis of {Bis[N,N′-(2,6-diisopropyl-4-phenylphenyl)imino]-
acenaphthene}dibromonickel (2). [(DME)NiBr2] (0.16 g, 0.50
mmol) and L2 (0.37 g, 0.50 mmol) were combined in a Schlenk
flask under a N2 atmosphere. CH2Cl2 (20 mL) was added, and the
reaction mixture was stirred at room temperature for 24 h. The
resulting suspension was filtered. The solvent was removed under
vacuum and the residue was washed with diethyl ether (3 × 15 mL),
and then dried under vacuum at room temperature to give nickel
complex 2 (0.38 g, 87% yield). Anal. Calcd for C48H48Br2N2Ni: C,
66.16; H, 5.55; N, 3.21. Found: C, 66.23; H, 5.50; N, 3.17. FT-IR
(KBr) 1650 cm−1 (vCN).
ORCID
Fuzhou Wang: 0000-0003-0470-7110
Ryo Tanaka: 0000-0002-6085-074X
Takeshi Shiono: 0000-0002-1118-9991
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to Tosoh-Finechem Co., Ltd. for the
generous donation of MMAO. The NMR measurements were
made at the Natural Science Center for Basic Research and
Development (N-BARD), Hiroshima University.
Synthesis of {Bis[N,N′-(2-methy-6-isopropylphenyl)imino]-
acenaphthene}dibromonickel (4). Using the same synthetic
procedure for 2, 4 was obtained as a dark-red powder (0.31 g, 93%
yield). Anal. Calcd for C32H32Br2N2Ni: C, 57.96; H, 4.86; N, 4.22.
Found: C, 57.91; H, 4.79; N, 4.28. FT-IR (KBr) 1642 cm−1 (vCN).
X-ray Structure Determinations. A single crystal of complex 2
suitable for X-ray analysis was obtained by dissolving the nickel
complex in CH2Cl2, followed by slow layering of the resulting solution
with n-hexane at room temperature. Data collections were performed
at 296(2) K on a Bruker SMART APEX diffractometer with a CCD
area detector, using graphite monochromated Mo Kα radiation (λ =
0.71073 Å). 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 reflection data
file. The structures were solved by using the SHELXTL program.
Refinement was performed on F2 anisotropically 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 calculation without further refinement of the parameters.
Details of the crystal data and structure refinements for complex 2 are
listed in Table S1.
Procedure for Polymerization of Octenes. Polymerization of
octene was carried out in a 10-mL glass reactor equipped with a
magnetic stirrer. After drying the reactor under N2 atmosphere,
toluene was added to the reactor. Octene was added to the toluene
kept at polymerization temperature via a syringe. Then the co-catalyst
MMAO was added to the toluene and the mixture was stirred for 10
min. Polymerization was started by the addition of the catalyst solution
in toluene. Polymerization was terminated with 100 mL of a 3% HCl−
MeOH solution. The polymers obtained were adequately washed with
methanol and dried under vacuum at 60 °C for 3 h.
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DOI: 10.1021/acs.organomet.8b00042
Organometallics XXXX, XXX, XXX−XXX