Synthesis of branched ultrahigh-molecular-weight polyethylene using highly active neutral, single-component Ni(II) catalysts

Article abstract of DOI:10.1021/cs501948d

Neutral nickel methyl complexes incorporating 2,8-diarylnaphthyl groups have been synthesized and characterized. Salicylaldiminato nickel systems 1a,b are exceptionally active neutral nickel single component catalysts for the polymerization of ethylene capable of producing lightly branched ultrahigh-molecular-weight polyethylene (UHMWPE). In addition, complex 1a shows a "quasi-living" polymerization behavior. (Chemical Equation Presented).

Full text of DOI:10.1021/cs501948d

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Article
Synthesis of Branched Ultra-High-Molecular-Weight Polyethylene
Using Highly Active Neutral, Single-Component Ni(II) Catalysts
Zhou Chen, Milad Mesgar, Peter S. White, Olafs Daugulis, and Maurice Brookhart
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs501948d • Publication Date (Web): 11 Dec 2014
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Page 1 of 6
ACS Catalysis
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Synthesis
of
Branched
Ultra-High-Molecular-Weight
8
9
Polyethylene Using Highly Active Neutral, Single-Component
Ni(II) Catalysts
Zhou Chen,1 Milad Mesgar,2 Peter S. White,1 Olafs Daugulis,2*and Maurice Brookhart1*
1 Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599ꢀ3290
2 Department of Chemistry, University of Houston, Houston, TX 77204ꢀ5003
Supporting Information Placeholder
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ranging from linear to highly branched can be produced. Hyperꢀ
branched structures can also be observed with proper choice of
catalyst, temperature and ethylene pressure.3 Moreover, these
catalysts have proved capable of copolymerizing of ethylene and
αꢀolefins with polar vinyl monomers such as acrylates, methyl
vinyl ketones, and silyl vinyl ethers to yield highly branched
copolymers.2,4
ABSTRACT: Neutral nickel methyl complexes incorporating
2,8ꢀdiarylnaphthyl groups have been synthesized and
characterized. Salicylaldiminato nickel systems 1a,b are
exceptionally active neutral nickel single component catalysts for
the polymerization of ethylene capable of producing lightly
branched ultraꢀhighꢀmolecularꢀweight polyethylene (UHMWPE).
In addition, complex 1a shows a “quasiꢀliving” polymerization
behavior.
Typically, cationic nickel catalysts are more sensitive to polar
solvents and impurities relative to the neutral analogues.
Therefore there has been strong interest in developing less
electrophilic neutral nickel catalysts that polymerize ethylene to
high molecular weights. Early examples of neutral nickel catalysts
bearing phosphinoꢀenolate ligands normally provided lowꢀ
molecularꢀweight polyethylene.5
A significant advance in
development of neutral nickel polymerization catalysts, discovery
of the salicylaldiminatoꢀbased nickel systems by the groups from
DuPont6 and Caltech7 was reported in 1998. Neutral Ni(II)
complexes based on salicylaldiminato ligands which incorporate a
bulky orthoꢀdisubstituted aryl group could produce high
molecular weight polyethylene. Catalyst 2 bearing a bulky 9ꢀ
anthracenyl group, believed to promote phosphine dissociation,
was found to be particularly productive and was shown to
copolymerize ethylene and functionalized norbornenes.7b
Mecking’s group reported a series of neutral Ni(II) methyl
catalysts bearing terphenyl substituted N,O ligands which showed
higher activity than the similar isopropylꢀsubstituted complexes.8
Significant effects of remote substituents (R, Figure 1) on
polymer branching and molecular weights were observed.
Complex 3a bearing methyl groups at the terphenyl 3,5ꢀpositions
produced hyperbranched oligomers which could be further
functionalized.8m However, the complex 3b bearing CF3 groups at
the terphenyl 3,5ꢀpositions produced high molecular weight (Mn
ca. 104 g/mol) semicrystalline PE. Moreover, complex 3c could
produce strictly linear polyethylene with high molecular weight
(Mn up to 5.8 × 105 g/mol) in supercritical CO2.9
KEYWORDS: nickel catalysts, polymerization, UHMWPE,
salicyaldimine, branched polyethylene
INTRODUCTION
The development of late transition metal catalysts for olefin
polymerization has advanced substantially1 since the pioneering
discovery of cationic Ni(II) and Pd(II) arylꢀsubstituted diimine
catalysts in 1995.2 The key to generating high molecular weight
polymers using these catalysts is the presence of bulky orthoꢀalkyl
substituents on the aryl rings which provide steric bulk in the two
axial sites of the square coordination plane. The positioning of
these groups retards the rate of chain transfer relative to the rate of
propagation. βꢀElimination/readdition reactions move the metal
down the alkyl chain (“chainꢀwalking”) and can occur without
chain transfer. As a result, polyethylenes with microstructures
In addition to the salicylaldiminato systems, complexes based
on other ligand backbones have been reported by several
groups.10ꢀ13
Bazan’s
group
investigated
various
αꢀ
iminocarboxamide nickel complexes which exhibited excellent
performance in promoting
Figure 1. Salicylaldiminato Nickel Catalysts
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Page 2 of 6
Scheme 1. Synthesis of Complexes 1
1
2
3
4
Me
R
R
Me
Me
Me
NaOH/EtOH
/H2O
Me
Me
Me
Me
Me
O
I
O
B(OH)2
R
N
N
HN
NBS/ZrCl4
CH2Cl2
Me
HN
NH2
NH2
NH2
Br
Pd(PPh3)4/Na2CO3
EtOH/Toluene
Refulx
Pd(OAc)2/AgOAc
140o
R
5
C
6
7
8
9
9a, R = Me, 81%;
9b
8
, 56 %
6, 88 %
4
5, 91 %
, R = CF3, 72%.
O
O
TsOH/toluene
MeOH/H+
OH
OH
Ant
R
R
R
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11
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R
NiMe2Py2
OH
Me
NiMe2Py2
N
N
O
Py
Me
Ni
N
OH
Me
N
O
Py
H3C
Ni
Me
Me
Me
Me
Me
H3C
unstable
7, 92 %
1a, R = Me, 62%;
1b
10a, R = Me, 50%;
10b
, R = CF3, 98%.
, R = CF3, 66%.
ethylene polymerization and showed “quasiꢀliving” behavior
when combined with Ni(COD)2.10 Brookhart and coworkers
reported a series of neutral nickel catalysts containing fiveꢀ
membered nickel chelates that showed high activities toward
ethylene polymerization.11 Li’s group reported a series of highly
active βꢀketiminato neutral nickel(II) catalysts.12
yield. Unfortunately, the corresponding nickel complex derived
from the salicylaldimine 7 could not be obtained. Attempts to
prepare salicylaldiminato nickel complexes derived from less
sterically hindered amines afford the bisꢀligated complexes.7c The
2,8ꢀ(diaryl)naphthalenꢀ1ꢀamines 9a,b derived from
6 were
synthesized in a twoꢀstep procedure. Selective bromination of
amine 6 with Nꢀbromosuccinimide (NBS) gave 2ꢀbromoꢀ8ꢀ(3,5ꢀ
dimethylphenyl)naphthaleneꢀ1ꢀamine 8 in moderate yield.18 We
then employed Suzuki coupling to synthesize 2,8ꢀ
(diaryl)naphthalenꢀ1ꢀamines 9a,b in good yields by treating
bromo amine 8 with aryl boronic acids in the presence of a Pd(0)
catalyst. Finally, the amines 9a,b were used in the preparation of
salicylaldimines 10a,b by the acidꢀcatalyzed condensation with 3ꢀ
(9ꢀanthryl)salicyaldehyde. Salicylaldiminato Ni(II)ꢀMe complexes
Most of these systems produce PEs with Mn values on the order
of 105 g/mol or less. Polyethylenes with molecular weights on the
order of 106 g/mol are difficult to obtain even for the early metal
catalyst systems.14 PEs with Mn values in the range of 106ꢀ107
g/mol are termed ultraꢀhighꢀmolecularꢀweight polyethylenes
(UHMWPEs). Catalysts which produce UHMWPEs must exhibit
rate ratios of chain propagation:chain transfer (or chain
termination) of greater than ca. 3.5 × 104. The synthesis of
UHMWPEs from late transition metal catalysts is rare.15,16a Rieger
reported that arylꢀsubstituted αꢀdiimine nickel complexes bearing
bulky 2,6ꢀdiphenyl substituents could produce linear ultraꢀhigh
molecular weight polyethylenes.15b
19
1a,b were prepared in good yields by adding (pyridine)2NiMe2
to toluene solutions of
ligands 10a,b.8g Notably, no
decomposition of the nickel methyl complex 1a was observed
over two weeks in C6D6. Single crystals of complex 1b were
grown by slowly evaporation of a hexane solution under argon. In
the solid state, Ni in 1b adopts a nearly square planar coordination
geometry; the methyl group is trans to oxygen as expected on
electronic grounds (Figure 3).8b As can be seen in Figure 3, the
aryl rings effectively shield the axial sites above and below the
square coordination plane.
Recently, we reported that “sandwich” nickel(II) αꢀdiimine
complexes incorporating the 8ꢀtolylnapthylimino group yield
highly branched PE (Figure 2).16a The 8ꢀtolyl groups are
positioned over the axial sites and greatly retard chain transfer.
The nickel system was shown to yield highly branched PE with
Mn values up to 1.8 × 106 g/mol.16a Coates and Daugulis reported
that such “sandwich” nickel(II) αꢀdiimine complexes polymerize
αꢀolefins to produce “chainꢀstraightened” polymers (via 2,1
insertion and chainꢀwalking) to produce polymers which resemble
semicrystalline PE.17
R1
R
R
N
Br
N
Ni
Br
R1
Figure 2. “Sandwich” Nickel Diimine Catalysts
Herein, we report the synthesis of neutral nickel complexes
containing 8ꢀarylnaphthylimino groups and their remarkable
behavior as catalysts for ethylene polymerization.
RESULTS AND DICUSSION
Synthesis of Catalysts
Figure 3. Solid state molecular structure of complex 1b (ORTEP
view, 30 % probability ellipsoids). Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å) and angles (deg). Ni1ꢀO1
1.908 (2), Ni1ꢀN1 1.905 (3), Ni1ꢀN2 1.922 (3), Ni1ꢀC27 1.931 (3),
N2ꢀNi1ꢀC27 89.1 (1).
8ꢀ(3,5ꢀDimethylphenyl)naphthalenꢀ1ꢀamine 6 was synthesized
using a known procedure (Scheme 1).16 Yields for each step are
high and the amine 6 is readily purified. Condensation of amine 6
with salicylaldehyde gives the desired salicylaldimine 7 in 92%
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ACS Catalysis
Table 1. Results of Ethylene Polymerization Reactions Using 1a
1
2
3
4
5
6
7
8
9
TOF
c
Mn
Ethylene
(psig)
Time
(min)
Yield
(g)
Tm
c
entrya
T(°C )
Branchesb
Mw/Mn
(×10ꢀ4 hꢀ
(×10ꢀ4 g/mol)
(°C )
1
)
1
2
3
4
5
6
7d
50
50
50
50
60
40
50
200
200
400
600
600
600
600
60
30
30
30
30
30
30
6.3
3.3
5.8
8.4
3.0
3.1
5.5
4.5
4.7
8.3
11.9
4.3
4.4
7.8
31
28
20
17
24
10
16
94.3
2.5
2.8
2.8
2.3
2.4
2.3
2.4
99
55.0
101
111
118
103
123
116
100.8
128.7
62.9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
31.7
124.1
aConditions: 1a = 5 ꢀmol, V(toluene) = 200 mL, ethylene: Matheson purity. b Branching numbers per 1000 carbons were determined
by using 1H NMR spectroscopy. c Molecular weight was determined by GPC in trichlorobenzene at 150 oC. d 5 equiv. pyridine.
Ethylene Polymerization
Table 2. Polymerization Reactions Using 1a/Ni(COD)2
Neutral nickel complex 1a was investigated as a single
component catalyst for ethylene polymerization in toluene. The
results are summarized in Table 1. The catalyst is quite stable at
50 °C. Complex 1a exhibits constant polymerization activities
over 1 hour (TOF: 4.5 × 104 hꢀ1 for 60 min and 4.7 × 104 hꢀ1 for 30
min; entries 1 and 2). Polymerization activities are strongly
dependent on ethylene pressure and reaction temperature. As seen
in entries 2ꢀ4, activities increase nearly linearly with ethylene
pressure with a TOF of up to 1.2 × 105 hꢀ1 at 600 psig ethylene
pressure. From these observations it appears that the (major or
exclusive) resting state is either the nickel alkyl pyridine species
or a nickel βꢀagostic alkyl complex.20 The activity decreases as
the temperature is increased to 60 °C, perhaps due to catalyst
degradation. At 40 °C the activity is reduced, but no evidence of
catalyst decomposition was found.21 When the polymerization is
carried out in the presence of 5 equivalents of pyridine, a decrease
in TOF is observed under the same reaction conditions (entries 7
vs 4). Supression by added pyridine suggests under these
conditions a fraction of the catalyst rests as the pyridine adduct.
entrya
Time
(min)
TOF
Mn
b
Yield
(g)
Mw/M
(×10ꢀ4 hꢀ (×10ꢀ4
b
n
1
)
g/mol)
1
10
20
30
60
120
30
0.75
1.61
2.26
4.21
9.69
0.64
3.2
3.5
3.2
3.0
3.5
0.9
22.7
1.36
1.38
1.31
1.47
2.11
1.64
2
49.1
3
77.7
4
122.8
160.0
26.2
5
6 c
aConditions: 1a = 5 mol, V(toluene) = 200 mL, 10
equivNi(COD)2, 28 C, 100 psig ethylene; ethylene: Matheson
ꢀ
o
b
purity.
Molecular weight was determined by GPC in
trichlorobenzene at 150 °C. c without Ni(COD)2.
Compared with typical UHMWPE (Tm = 130ꢀ136 °C),14 lower
melting points (e.g. 99 °C, entry 1; 118 °C, entry 4) of the
UHMWPE produced by 1a were observed.
The most notable feature of this catalyst system is the
production of lightly branched ultraꢀhighꢀmolecularꢀweight
polyethylene. The molecular weight Mn is 5.5 × 105 g/mol in a 30
min run at 200 psi ethylene and increases to 9.4 × 105 g/mol in a
60 min run. The PE produced exhibits ca. 30 branches per 1000
carbon atoms and a Tm of 99 °C. In an ideal coordination/insertion
polymerization where chain transfer via βꢀhydride elimination
limits the Mn, the maximum value of Mn is attained when the
Mw/Mn reaches 2. The fact that Mn nearly doubles when the
polymerization time doubles from 30 min to 60 min (see entries 1
and 2, Table 1) yet the Mw/Mn is 2.5ꢀ2.8, implies that the Mn
under these conditions is not chainꢀtransfer limited. This aspect
will be discussed in more detail below.
To gain further insight into the increase in Mn with the reaction
time, Ni(COD)2 was used as a cocatalyst10a to sequester pyridine.
At low temperature (28 °C) and low ethylene pressure (100 psig),
constant turnover frequencies were found over 2 hours (Table 2).
The molecular weight, Mn, increased nearly linearly as a function
of reaction time over 60 min (e.g. Mn = 2.3 × 105 g/mol, 10 min;
4.9 × 105 g/mol, 20 min; 7.8 × 105 g/mol, 30 min; 1.2 × 106
g/mol, 60 min ). This feature is a characteristic of living
polymerization, yet the Mw/Mn values (1.3ꢀ1.4) are considerably
greater than expected for a living polymerization. This behavior is
likely due to rapid precipitation of the high molecular weight
polymer resulting in inhomogeneous reaction conditions with a
distribution of sites possessing varying activities. This
phenomenon has been previously encountered by Bazan who has
termed this behavior as “quasiꢀliving” polymerization.10a Notably,
the molecular weight Mw reaches 3.4 × 106 g/mol (Mn = 1.6 ×
106 g/mol) after 2 hours accompanied by a broadening molecular
weight distribution (entry 5).
The degree of branching of the polyethylene produced is
sensitive to ethylene pressure and reaction temperature. For
example, catalyst 1a at 50 °C produces PE with a branching
density of 28 at 200 psig, 20 at 400 psi and 17 branches per 1000
carbon atoms at 600 psig. The number of branches increases from
10 to 24 branches per 1000 carbon atoms upon raising the reaction
temperature from 40 °C to 60 °C. The molecular weight Mw
reaches 3.0 × 106 g/mol (Mn = 1.3 × 106 g/mol) with 17 branches
per 1000 carbon atoms at 600 psig ethylene over 30 min (entry 4).
13C NMR spectroscopy indicates that mainly methyl branches are
present in the polymer. Similar molecular weights and branching
numbers are observed with and without added pyridine (entries 4
vs 8). The presence of the branches in the polymer backbone is
reflected by the melting temperatures determined by DSC.
Complex 1b bearing 3,5ꢀCF3 groups in place of 3,5ꢀCH3 groups
was also studied for ethylene polymerization (Table 3). At 50 °C,
catalyst 1b showed higher activity of 6.2 × 104 hꢀ1 over 30 min
(entry 2) compared with that of catalyst 1a (entry 2, Table 1).
However, the TOF for 1b drops slightly to 5.0× 104 hꢀ1 over 60
min (entry 1). Similar to catalyst 1a, the TOF of catalyst 1b is
nearly proportional to ethylene pressure (entries 2ꢀ5). The highest
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Table 3.Results of Ethylene Polymerization Reactions Using 1b
Page 4 of 6
1
2
3
4
5
6
7
8
c
TOF
(×10ꢀ4 hꢀ1)
Mn
Ethylene
(psig)
Time
(min)
Yield
(g)
Tm
c
entrya
T(°C )
Branchesb
Mw/Mn
(×10ꢀ4 g/mol)
113.2
86.5
(°C )
1
2
3
4
5
6
7
50
50
50
50
60
60
40
200
200
400
600
600
200
200
60
30
30
30
30
30
30
7.1
4.3
7.8
11.7
6.7
3.1
4.4
5.0
9
2.5
2.5
2.4
2.4
2.8
2.4
1.9
126
125
126
130
122
120
129
6.2
9
11.1
16.7
9.6
7
109.6
125.7
105.3
90.5
6
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4.4
11
4
6.2
91.5
aConditions: 1b = 5 ꢀmol, V(toluene) = 200 mL, ethylene: Matheson purity. b Branching numbers per 1000 carbons were determined by
using 1H NMR spectroscopy. c Molecular weight was determined by GPC in trichlorobenzene at 150 °C.
TOF was reached at 1.7 × 105 hꢀ1 as ethylene pressure was
Table 4. Kinetics of Displacement of Pyridine by 4-Picoline
in 1a at – 80 °C.
increased to 600 psig. To the best of our knowledge, this is among
the very highest activities reported for salicylaldiminato nickel
systems.8 Similar to catalyst 1a, a drop in activity was observed at
elevated reaction temperatures (entries 2 vs 6, entries 4 vs 5).
However, polymerizations at 40 °C exhibited a similar TOF, 6.2 ×
104 hꢀ1, compared with the TOF at 50 °C (entries 2 vs 7).
Entry
[Ni](mM) [4ꢀpicoline]
(mM)
k1[4ꢀ
picoline]
(sꢀ1)
6.19 × 10ꢀ4
10.6 × 10ꢀ4
18.8 × 10ꢀ4
k1(Mꢀ1sꢀ1)
Similar
to
catalyst
1a,
ultraꢀhighꢀmolecularꢀweight
polyethylene was also obtained with catalyst 1b. For example,
polymerization at 50 °C, 200 psig ethylene for 60 min yields PE
with Mw = 2.8 × 106 g/mol (Mn = 1.1 × 106 g/mol) (entry 1). In
comparison with catalyst 1a, the polyethylenes produced by 1b
are more linear22 and exhibit higher melting temperatures (120ꢀ
130 °C). For example, the branching density was only 6 branches
per 1000 carbon atoms at 600 psi (entry 4). The linear ultraꢀhighꢀ
molecularꢀweight polyethylene demonstrates thermal properties
similar to those of UHMWPE produced with early metal systems.
In the first heating cycle, high peak melting temperatures (137 °C)
and high crystallinity (64%)23 are observed for the nascent
polymer. In the second heating cycle, the crystallinity (45%) is
much lower due to entanglement hindering recrystallization from
the melt.
1
2
3
7.1
7.1
7.1
121
199
355
5.1 × 10ꢀ3
5.3 × 10ꢀ3
5.3 × 10ꢀ3
CONCLUSION
Neutral nickel methyl complexes incorporating 2,8ꢀ
diarylnaphthyl groups have been synthesized and characterized.
These singleꢀcomponent catalysts are quite stable and are highly
active for ethylene polymerization at 50oC or below. Turnover
frequencies increase linearly with the ethylene pressure implying
that alkyl ethylene complexes are not the resting states. Compared
with complex 1a, the –CF3ꢀsubstituted analog 1b has a slightly
higher polymerization activity (e.g, a TOF of 1.7 × 105 hꢀ1 vs 1.2
× 105 hꢀ1 at 600 psig C2H4, 50 °C). For complex 1a, the
molecular weights of polyethylene produced increase nearly
linearly as a function of reaction time and conform to a “quasiꢀ
living” behavior.10a
The reaction of ethylene (4.0 equivalents) with nickel methyl
complex 1a in tolueneꢀd8 was monitored by in situ NMR
spectroscopic studies at 40 °C. The rate of ethylene insertion into
the nickel methyl bond of complex 1a is much slower than
subsequent insertions and the only species detected during
consumption of ethylene was pyridine complex 1a. No defined
resonances for ethylene complexes, βꢀagostic species, olefin
hydrides or free pyridine could be observed. This is consistent
with results presented earlier suggesting that alkyl pyridine
complexes are the dominant resting states under polymerization
conditions at high ethylene pressures. If we designate the first
insertion as initiation and subsequent insertions as propagation,
then the rate of initiation is much slower than the rate of
propagation.
These singleꢀsite catalysts produce ultraꢀhigh molecular weight
polyethylenes (Mw up to 3.0 × 106g/mol) with low to moderate
branching densities in the range of 4ꢀ31 branches per 1000
carbon atoms which can be controlled by variations in
temperature and ethylene pressure. Such lightly branched
UHMWPEs appear to be unique and bridge the gap between the
normal14,15b highly linear UHMWPE and the extremely highly
branched UHMWPEs reported with nickel diimine catalysts.16a
Since the ethylene complexes couldn’t be detected, the
mechanism for ligand exchange at the metal center was examined
using pyridine complex 1a. We investigated displacement of
pyridine by 4ꢀpicoline at ꢀ80 °C in CD2Cl2 under pseudoꢀfirst
order conditions in which the concentrations of 4ꢀpicoline used
greatly exceed the concentration of 1a. The data are summarized
in Table 4. The pseudo firstꢀorder rate constants are linearly
dependent on the concentration of 4ꢀpicoline implying the
displacement is clearly associative even in this highly hindered
system. The secondꢀorder rate constant is 5.3 × 10ꢀ3 Mꢀ1sꢀ1 at ꢀ
80 °C. It is reasonable to assume that ethylene exchanges with
pyridine through an analogous associative mechanism.
ASSOCIATED CONTENT
Supporting Information
Synthetic procedures for preparation of new ligands and
complexes the method used for polymerization of ethylene using
1a and 1b. 1H and 13C NMR spectra of new compounds, H
1
and13C NMR spectra of polymers, DSC data of polymers and a
CIF file for 1b. This material is available free of charge via the
Internet at http://pubs.acs.org.
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2362. (h) Bastero, A.; GöttkerꢀSchnetmann, I.; Röhr, C.; Mecking, S. Adv.
AUTHOR INFORMATION
Synth. Catal. 2007. 349, 2307ꢀ2320. (i) Berkefeld, A.; Möller, H. M.;
Mecking, S. Organometallics 2009, 28, 4048ꢀ4055. (j) Berkefeld, A.;
Mecking, S. J. Am. Chem. Soc. 2009, 131, 1565ꢀ1574. (k) Osichow, A.;
Rabe, C.; Vogtt, K.; Narayanan, T.; Harnau, L.; Drechsler, M.; Ballauff,
M.; Mecking, S. J. Am. Chem. Soc. 2013, 135, 11645ꢀ11650. (l) Osichow,
A.; GöttkerꢀSchnetmann, I.; Mecking, S. Organometallics 2013, 32, 5239ꢀ
5242. (m) Wiedemann, T.; Voit, G.; Tchernook, A.; Roesle, P.; Göttkerꢀ
Schnetmann, I.; Mecking, S. J. Am. Chem. Soc. 2014, 136, 2078ꢀ2085. (n)
Weberski, M. P.; Chen, C.; Delferro, M.; Zuccaccia, C.; Macchioni, A.;
Marks, T. J. Organometallics 2012, 31, 3773ꢀ3789. (o) Wehrmann, P.;
Mecking, S. Organometallics 2008, 27, 1399ꢀ1408. (p) Guironnet, D.;
Friedberger, T.; Mecking, S. Dalton Trans., 2009, 8929ꢀ8934. (q) S. S.;
Joe, D.; Na, S.; Park, Y.; Choi, C.; Lee, B. Macromolecules, 2005, 38,
10027ꢀ10033.
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Corresponding Author
Email: mbrookhart@unc.edu, olafs@uh.edu
Notes
The authors declare no competing financial interests.
9
ACKNOWLEDGMENT
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This research was supported by the National Science Foundation
(CHEꢀ1010170 to M.B.) and the Welch Foundation (Grant Eꢀ
1571 to O.D.). Z.C. acknowledges financial support from
Shanghai Institute of Organic Chemistry, Zhejiang Medicine, and
Pharmaron for a joint postdoctoral fellowship. We thank Anne
LaPointe, Cornell University, for obtaining gpc data.
(9) Guironnet, D.; GöttkerꢀSchnetmann, I.; Mecking S.
Macromolecules 2009, 42, 8157ꢀ8164.
(10) (a) Diamanti, S. J.; Ghosh, P.; Shimizu, F.; Bazan, G. C.
Macromolecules 2003, 36, 9731ꢀ9735. (b) Diamanti, S. J.; Khanna, V.;
Hotta, A.; Yamakawa, D.; Shimizu, F.; Kramer, E. J.; Fredrickson, G. H.;
Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 10528ꢀ10529. (c) Diamanti, S.
J.; Khanna, V.; Hotta, A.; Coffin, R. C.; Yamakawa, D.; Kramer, E. J.;
Fredrickson, G. H.; Bazan, G. C. Macromolecules 2006, 39, 3270ꢀ3274. (d)
Coffin, R. C.; Diamanti, S. J.; Hotta, A.; Khanna, V.; Kramer, E. J.;
Fredrickson, G. H.; Bazan, G. C. Chem. Commun. 2007, 3550ꢀ3552.
(11) (a) Hicks, F. A.; Brookhart, M. Organometallics 2001, 20, 3217ꢀ
3219. (b) Jenkins, J. C.; Brookhart, M. Organometallics 2003, 22, 250ꢀ
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(12) (a) Song, D. P.; Ye, W. P.; Wang, Y. X.; Liu, J. Y.; Li, Y. S.
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(22) Effects on branching through substitution of an aromatic –CH3
group by a CF3 group has been noted before.8b The reasons for the
decrease in branching densities of polymers produced by 1a relative to 1b
are not clear.
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R
1
R
Branched UHMWPE
Ethylene
Toluene
6
7
8
9
5
-1
x
1.
2.
to 1.7 10 h
N
Me
TOF
;
up
to
O
High activity,
Ni
.mol-1
6
x
Py
M
3.0 10
,
w up
g
UHMWPEs,
4-31 branches
H
3C
1000 carbon
per
"Quasi-living" polymerization.
atoms;
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H
3C
3.
1
:
:
=
=
1a
1b
R
R
CH3
CF3
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