A robust Ni(II) α-diimine catalyst for high temperature ethylene polymerization

Article abstract of DOI:10.1021/ja408905t

Sterically demanding Ni^II α-diimine precatalysts were synthesized utilizing 2,6-bis(diphenylmethyl)-4-methyl aniline. When activated with methylaluminoxane, the catalyst NiBr₂(ArN=C(Me)-C(Me)=NAr) (Ar = 2,6 bis(diphenylmethyl)-4-methylbenzene) was highly active, produced well-defined polyethylene at temperatures up to 100 C (M_w/M _n = 1.09-1.46), and demonstrated remarkable thermal stability at temperatures appropriate for industrially used gas-phase polymerizations (80-100 C).

Full text of DOI:10.1021/ja408905t

Communication
pubs.acs.org/JACS
A Robust Ni(II) α‑Diimine Catalyst for High Temperature Ethylene
Polymerization
Jennifer L. Rhinehart, Lauren A. Brown, and Brian K. Long*
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States
S
* Supporting Information
propagation and to the decreased solubility of ethylene in
ABSTRACT: Sterically demanding NiII α-diimine pre-
catalysts were synthesized utilizing 2,6-bis(diphenyl-
methyl)-4-methyl aniline. When activated with methyl-
aluminoxane, the catalyst NiBr2(ArNC(Me)−C(Me)
NAr) (Ar = 2,6 bis(diphenylmethyl)-4-methylbenzene)
was highly active, produced well-defined polyethylene at
temperatures up to 100 °C (Mw/Mn = 1.09−1.46), and
demonstrated remarkable thermal stability at temperatures
appropriate for industrially used gas-phase polymerizations
(80−100 °C).
toluene at elevated temperatures, further compounds the need
for thermally robust catalysts, as extended reaction times are
required to produce polymers of sufficient molecular weight.13
Consequently, the development of late transition-metal-based
α-diimine catalysts with enhanced temperature stability is of
tremendous importance to the field of olefin polymer-
ization.13,24,25
To date, few examples of α-diimine-based catalysts with
improved thermal stability have been reported.22,26−29 To
address these temperature sensitivities, researchers have
examined the effects that ligand backbone modifications and
N-aryl moiety substitutions have on the thermal stability of α-
diimine-based catalysts. For example, catalysts bearing α-
diimine ligands with camphor-derived backbones have
displayed modest activity at temperatures as high as 80 °C,
yet their turnover frequency (TOF) was shown to decay as a
function of time and the polymers produced had broad
molecular weight distributions (Mw/Mn ≥ 2.0) and were highly
branched (no Tm was observed).22 In contrast, NiII-based
catalysts developed by Guan and co-workers, which featured
cyclophane-derived N-aryl substituents, have demonstrated
remarkable activities for ethylene polymerization (TOFs =
1 000 000−1 400 000 h−1), produced well-defined polymers
(Mw/Mn = 1.2−1.7), and showed constant TOFs at 70 and 90
°C for 10 min; however, the catalysts did show decomposition
upon extended reaction times.27 In addition, while an elegant
example of synthetic chemistry, cyclophane-based ligands
require an elaborate multistep synthesis involving three
separate transition metal catalyzed reactions to construct the
ligand alone.
ver the past three decades, there has been an explosion of
Oresearch dedicated to the design and development of
homogeneous, single-site olefin polymerization catalysts.
Single-site catalysts provide tremendous flexibility in ligand
design and have revolutionized the field of polyolefin research
by establishing new opportunities for mechanistic under-
standing, catalyst control, and tailored polyolefin synthesis
that were impossible using heterogeneous catalyst systems.1−6
Perhaps one of the most significant advances during this time
period was the development of highly active, late transition-
metal-based olefin polymerization catalysts by Brookhart and
co-workers.7,8 These Ni- and Pd-based α-diimine catalysts were
capable of producing high molecular weight polyethylenes
(PEs), would tolerate and incorporate numerous polar
comonomers,9−12 and could produce multiple polymer top-
ologies ranging from linear to hyperbranched by simply varying
ethylene pressure.13−20
Despite these advantages, Ni- and Pd-based catalysts
generally exhibit poor thermal stability at elevated temperatures
that are required for industrially used gas-phase polymerizations
(70−110 °C).13,21 As a result, the commercialization and
overall industrial appeal of α-diimine-derived catalyst systems
has been greatly hindered. For example, typical PdII- and NiII-
based α-diimine catalyst systems have been shown to undergo
rapid catalyst decomposition at temperatures ≤60 °C. This
observation has been attributed to increased N-aryl rotations
from perpendicular relative to the ligand backbone leading to
events such as C−H activation of the ligand itself and increased
associative chain transfer, as well as potential decomposition
pathways arising from in situ generated Ni-hydride spe-
cies.13,18,22,23 In addition, ethylene polymerizations that utilize
Ni- and Pd-based catalysts often produce PE with decreased
molecular weights as reaction temperatures are elevated. This
phenomenon, which is attributed to increased rates of chain
transfer (via β-hydride elimination) relative to chain
In an effort to develop a complementary, yet readily
accessible alternative to cyclophane-derived catalysts, we sought
to design and synthesize α-diimine catalysts featuring sterically
demanding iPr* moieties (iPr* = 1,3-bis(2,6-bis(diphenyl-
methyl)-4-methylphenyl)imidazo-2-ylidene). The iPr* func-
tionality has recently gained popularity in the field of bulky
N-heterocyclic carbenes and is structurally related to several
reported olefin polymerization catalysts.30−32 We envisioned
that the steric bulk of the iPr* moieties would greatly inhibit N-
aryl rotations of the α-diimine ligand and thereby dramatically
enhance their thermal stability. Herein, we report the successful
synthesis and high temperature polymerization of ethylene by a
bulky, thermally robust NiII α-diimine catalyst bearing sym-
metrical iPr* moieties. To the best of the authors’ knowledge,
Received: August 28, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja408905t | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
this represents the most thermally stable NiII α-diimine catalyst
reported to date.
(PMAO-IP) as an activator.33 Catalyst 2a displayed good
activity toward ethylene polymerization at 20 °C producing
high molecular weight PE (Mn = 165 000 g/mol) with a Tm =
109 °C; however, the polymer displayed broad molecular
weight distribution (Mw/Mn = 2.64). In contrast, catalyst 2b
exhibited slightly lower activity at 20 °C, but the molecular
weight of the PE produced was in perfect agreement with those
theoretically calculated (Mnexp. = 44 000 g/mol, Mntheo = 44 000
g/mol) (Table 1, entry 5), and the resultant PE displayed a
The synthesis of NiII precatalysts 2a and 2b (Scheme 1) were
completed in three steps from commercially available starting
Scheme 1. Synthesis of α-Diimine Precatalysts 2a and 2b
Table 1. Ethylene Polymerization Results for Temperature
a
Screening of Catalyst 2b
b
c
d
Trxn
entry (°C)
time
(min)
yield
(mg)
TOF
(h−1
Mn
(kg/mol) Mw/Mn
Tm
c
)
(°C)
1
2
3
4
5
6
7
8
9
−60
−40
−20
0
1050
110
60
213
190
187
120
69
277
2363
4258
3652
6297
4959
4076
2029
2578
118
115
102
91
3.25
2.00
1.55
1.15
1.09
1.12
1.06
1.22
1.21
132
e
127
111
77
60
39
39
32
30
45
20
15
44
materials. p-Toluidine was alkylated using 2 equiv of
biphenylmethanol in the presence of stoichiometric HCl and
ZnCl2 to yield 2,6-dibenzhydryl-4-methylaniline, which was
subsequently condensed onto glyoxal to form ligand 1a.32 In an
analogous reaction, 2,6-dibenzhydryl-4-methylaniline was con-
densed onto 2,3-butadione using magnesium sulfate as a
desiccant and purified via column chromatography to afford α-
diimine ligand 1b. Ligand 1a was metalated in methylene
chloride at room temperature using the nickel(II) dibromide
dimethoxyethane adduct to form precatalyst 2a. In contrast,
ligand 1b required gentle heating at 30 °C for 5 h to form
precatalyst 2b. Both complexes 2a and 2b displayed para-
40
15
55
47
60
15
45
63
80
15
22
82
100
20
38
94
a
Polymerization conditions: [2b] = 1.57 μmol, 100 mL of toluene, 15
psi, and 100 equiv of methyl aluminiumoxane (PMAO-IP). Turnover
frequency (TOF) = mol of ethylene/(mol of cat.*h). Determined
using gel permeation chromatography at 145 °C in 1,2,4-
trichlorobenzene. Determined by differential scanning calorimetry,
second heating. Second heating showed a bimodal melting temper-
ature of 127 and 130 °C.
b
c
d
e
1
remarkably narrow molecular weight distribution (Mw/Mn =
1.09) and a Tm = 60 °C. These observations suggested that
catalyst 2b underwent rapid initiation, could produce branched
PE and that little to no chain termination or chain transfer
events were detected during the polymerization. Encouraged by
this result, all subsequent polymerizations were conducted
using catalyst 2b.
magnetic H NMR spectra, which was indicative of tetrahedral
coordination geometries about their NiII centers.
X-ray quality crystals of precatalyst 2b were grown by
layering a saturated methylene chloride solution with pentane
(Figure 1). The observed bond lengths are typical for α-diimine
A temperature-dependent study was performed at 15 psi of
ethylene using catalyst 2b (Table 1). Polymerizations
conducted at temperatures ≤ −20 °C (entries 1−3) exhibited
low activities (ie. TOFs), broad molecular weight distributions
(Mw/Mn ≥ 1.55), and produced highly linear polymer with Tm
= 132, 127, and 111 °C respectively (entries 1−3). In contrast,
polymerizations performed at temperatures ≥0 °C (entries 4−
7) produced lower molecular weight PEs; however, all PEs
were well-defined with narrow molecular weight distributions
(Mw/Mn ≤ 1.22). It should be noted that decreased PE yields
and molecular weights were observed at elevated temperatures,
which is an observation that is attributed to increased rates of
chain transfer (via β-hydride elimination) relative to chain
propagation and to the decreased solubility of ethylene in
toluene at elevated temperatures.13,24,25
To evaluate the thermal stability of catalyst 2b and to
overcome the limitations associated with elevated reaction
temperatures, additional polymerizations were performed at
100 psi of ethylene at 80, 90, and 100 °C (Table 2).
Polymerizations were run for durations up to 20 min in 5 min
intervals, and the polymer obtained was thoroughly charac-
terized. As can be seen in Figure 2, polymerizations conducted
at 80 and 90 °C displayed nearly linear growth of molecular
weight with time, reached molecular weights of over 600 000 g/
mol, and maintained narrow molecular weight distributions
Figure 1. ORTEP drawing of precatalyst 2b with thermal ellipsoids
drawn at 50% probability.
NiII complexes with N1−Ni = 1.9978(15) Å and N2−Ni =
2.0020(15) Å. The Ni−Br bond lengths were 2.3286(3) and
2.3351(3) Å for Ni−Br1 and Ni−Br2, respectively. Complex 2b
displayed a distorted tetrahedral geometry with a N1−Ni−N2
angle of 80.76(6)° and a Br1−Ni−Br2 angle of 125.767(12)°.
Ethylene polymerizations using precatalysts 2a and 2b were
evaluated using polymethylaluminoxane-improved performance
B
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Journal of the American Chemical Society
Communication
a
suggesting that the PE was moderately branched. This was
confirmed by NMR spectroscopy in which branching contents
of 63−75 branches per 1000 carbons were observed (see
Supporting Information) and consisted of both short- and long-
chain branches.
Additionally, TOFs and productivities of catalyst 2b were
plotted as a function of polymerization time (Figure 3). At 80
Table 2. Ethylene Polymerization Results at 100 psi for 2b
b
c
d
Trxn
time
yield
TOF
Mn
Tm
c
entry (°C) (min) (mg) ( × 103 h−1
)
(kg/mol) Mw/Mn (°C)
1
2
3
4
80
80
80
80
5
10
15
20
241
538
66
74
76
79
208
376
508
625
1.13
1.18
1.25
1.27
46
45
44
43
831
1144
5
6
7
8
90
90
90
90
5
10
15
20
303
652
83
89
88
88
260
377
516
605
1.10
1.26
1.31
1.31
43
42
40
42
972
1273
9
100
100
100
100
5
10
15
20
346
746
95
102
100
79
235
422
459
564
1.19
1.24
1.42
1.46
40
39
38
37
10
11
12
1100
1139
a
Polymerization conditions: [2b] = 1.57 μmol, 100 mL of toluene, 100
b
psi of ethylene, and 300 equiv of PMAO-IP. TOF = mol of ethylene/
(mol of cat.*h). Determined using gel permeation chromatography at
145 °C in 1,2,4-trichlorobenzene. Determined by differential
scanning calorimetry, second heating.
c
d
Figure 3. Plots of (a) turnover frequency (TOF) versus time and (b)
■
productivity versus time for catalyst 2b at 80 °C (blue ), 90 °C (red
⧫
●
), and 100 °C (green ).
°C, the turnover frequency of catalyst 2b was found to slightly
increase with polymerization time, which could be attributed to
the slight exothermicity of the polymerization, and TOFs at 90
°C remained almost perfectly constant for the entire polymer-
ization supporting their stability at these temperatures. At 100
°C, TOFs remained relatively constant up to 15 min; however,
a dramatic decrease was observed upon extended polymer-
ization times. An identical trend was also observed in plots of
productivity versus time (Figure 3b) in which catalyst 2b
remained highly productive throughout the entire 20 min
polymerization at 80 and 90 °C, while productivities at 100 °C
do not increase upon polymerization times longer than 15 min,
thereby signifying significant catalyst decomposition at this
temperature.34
The molecular weight, molecular weight distribution, TOF,
and productivity data all strongly support that catalyst 2b is
remarkably stable at both 80 and 90 °C and that it is
moderately stable at 100 °C for polymerizations up to 15 min.
This enhanced stability represents a dramatic improvement
over other reported α-diimine-based systems, and we attribute
this behavior to the unique nature of the iPr* N-aryl moieties.
The steric demand of the iPr* moieties greatly inhibits N-aryl
rotation preventing premature catalyst decomposition while
effectively blocking the axial sites of the active NiII center,
thereby preventing associative chain transfer that can drastically
limit polymer molecular weight.
■
○
Figure 2. Plot of Mn (blue ) and Mw/Mn (red ) versus yield for
catalyst 2b at (a) 80 °C, 100 psi, (b) 90 °C, 100 psi, and (c) 100 °C,
100 psi.
(Mw/Mn ≤ 1.31, Table 2, entries 1−8). In contrast,
polymerizations conducted at 100 °C clearly deviate from
this behavior displaying nonlinear growth of molecular weight
with time and noticeable increases in molecular weight
distribution after polymerization times of just 10 min (Table
2, entries 9−12). Each of these PE samples displayed melting
transitions just above room temperature (37−46 °C)
In summary, we report a synthetically simple, NiII α-diimine
precatalyst that demonstrates remarkable thermal stability for
ethylene polymerization. Precatalyst 2b was activated with
C
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
Synthesis and characterization of 1b, 2
■
S
̧
a, and 2b, and
polymerization data for 2a and 2b. This material is available
free of charge via the Internet at http://pubs.acs.org.
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J. N.; Marko, I. E. Dalton Trans. 2010, 39, 1444.
(33) See Supporting Information for complete ethylene polymer-
ization data for catalysts 2a.
(34) Polymerization times extending beyond 20 min were
performed; however, polymer precipitation from solution became
problematic making TOF and productivity calculations unreliable.
AUTHOR INFORMATION
Corresponding Author
Long@utk.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to acknowledge the Army Research Office
(Contract No. W911NF-1-0127) and the University of
Tennessee for their financial support of this work.
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