Mitigating chain-transfer and enhancing the thermal stability of co-based olefin polymerization catalysts through sterically demanding ligands

Article abstract of DOI:10.1002/pola.28783

Sterically demanding Fe- and Co-based olefin polymerization catalysts 2-Fe and 2-Co bearing 2,6-bis(biphenylmethyl)-4-methylaniline substituted bis(imino)pyridine ligands were synthesized and evaluated for ethylene polymerization. The late-transition meta

Full text of DOI:10.1002/pola.28783

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Mitigating Chain-Transfer and Enhancing the Thermal Stability of Co-
Based Olefin Polymerization Catalysts through Sterically Demanding
Ligands
Nolan E. Mitchell, W. Curtis Anderson, Jr., Brian K. Long
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
Correspondence to: B. K. Long (E-mail: long@utk.edu)
Received 12 July 2017; accepted 4 September 2017; published online 00 Month 2017
DOI: 10.1002/pola.28783
ABSTRACT: Sterically demanding Fe- and Co-based olefin poly-
merization catalysts 2-Fe and 2-Co bearing 2,6-bis(biphenyl-
methyl)-4-methylaniline substituted bis(imino)pyridine ligands
were synthesized and evaluated for ethylene polymerization.
The late-transition metal complexes were characterized by X-
ray diffraction, NMR spectroscopy, and HRMS, while their
resultant polymers were characterized by size-exclusion chro-
matography and 1H NMR spectroscopy. While catalyst 2-Fe
was inactive, catalyst 2-Co was found to polymerize ethylene
and avoid any detectable chain-transfer to aluminum events
that are known to plague other Fe- and Co-based catalyst
systems and to limit molecular weight. Furthermore, 2-Co dis-
plays virtually perfect thermal stability up to 80 8C and shows
greatly enhanced thermal stability at 90 8C as compared to pre-
viously reported analogues. These observations are attributed
to the extreme steric demand imposed by the ligand which
mitigates catalyst transfer, deactivation, and decomposition
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KEYWORDS: bis(imino)pyridine; catalysis; chain-transfer; cobalt;
polyethylene (PE); thermal stability; transition metal chemistry
thermal stability, which is well documented for many Ni-
and Pd-based a-diimine catalysts and (2) a notable propen-
sity to undergo deleterious chain transfer events, which has
been well-documented for Fe- and Co-based bis(imino)pyri-
dine catalysts.13–18 Recent work by our group has addressed
a portion of these issues by demonstrating that Ni(II) a-
diimine catalysts bearing sterically demanding 2,6-dibenz-
hydryl-4-methylaniline moieties exhibit greatly enhanced
thermal stability (at temperatures as high as 90 8C) and
virtually no detectable chain transfer events.19,20 Further-
more, these moieties have also been used to enhance Pd(II)
a-diimine and Ni(II) salicylaldimine catalysts as well.21–24
INTRODUCTION The development of well-defined, homoge-
neous olefin polymerization catalysts remains a vital area of
research within industry and academia alike. In particular,
homogeneous single-site catalysts have enabled researchers
to gain tremendous insight into the mechanistic underpin-
nings of olefin polymerization and the identity of their cata-
lytically active species.1–5 This information has enabled
researchers to intimately examine how ligand sterics, ligand
electronics, and transition metal choice influence overall cat-
alytic activity, reactivity, and selectivity, which has in turn
lead to enhanced polymerization control.3,5–11
For many years, researchers focused heavily on single-site
catalysts that use early transition metals such as Ti, Zr, and
Hf.5 However, in the late 1990s, it was discovered that late
transition metal catalysts using metals such as Ni, Pd, Fe,
and Co were also capable of producing high-molecular-
weight polyolefins.12 This capability was enabled through
careful ligand design and use of sufficiently bulky substitu-
ents that effectively block the axial sites of the active metal
center and prevent undesirable associative displacement of
the growing polymer chain by monomer.3,12
The enhanced performance of these, Ni catalysts are believed
to arise due to restricted N-aryl rotations within the ligand,
which in turn mitigates deleterious catalyst decomposition
and deactivation reactions.14,18,25–31 We hypothesized that
such sterically demanding dibenzhydryl moieties may also
enhance the performance of related Fe and Co bis(imino)pyr-
idine catalysts 1-Co and 1-Fe (Fig. 1).32–34
Late transition metal precatalysts 1-Co and 1-Fe drew con-
siderable attention as they use earth abundant and inexpen-
sive metal precursors.35,36 They are known to produce
Despite their appeal, late transition metal-based catalysts
often suffer from many limitations that can include: (1) poor
Additional Supporting Information may be found in the online version of this article.
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viscometer, and two-angle light-scattering detectors. Polymer
melting transition temperatures (Tms) were measured on a
TA Instruments Q2000 differential scanning calorimeter on
the second heating cycle at a heating rate of 10 8C/min. High
resolution mass spectrometry was obtained using a direct
inject, positive ion mode with an electrospray ionization
onto
a Thermo Scientific Exactive Plus Orbitrap mass
spectrometer.
FIGURE 1 Comparison of previously reported Co and Fe
precatalysts 1 and the symmetrically substituted Co and Fe
precatalysts 2 bearing dibenzhydryl moieties.
Synthesis of 2,6-Bis[1-(2,6-bis(diphenylmethyl)-4-
methylaniline)] pyridine (3)
To a solution of 2,6-diacetylpyridine (185 mg, 1.14 mmol)
in absolute ethanol (20 mL) was added 2,6-bis(diphenyl-
methyl)-4-methylaniline (1.00 g, 2.28 mmol). A few drops of
glacial acetic acid were added and the solution was refluxed
overnight. Upon cooling to room temperature, the product
crystallized from the ethanol solution. The product was
washed with ethanol and dried under vacuum overnight to
give 3 as an off-white solid (787.2 mg) in 69.5% yield.
highly linear polyethylene at low ethylene feed pressures,
which is in stark contrast to their Ni- and Pd-based counter-
parts that often produce branched polyethylene via a chain-
walking mechanism.37 However, despite these advantages,
precatalysts 1-Co and 1-Fe are limited in that they often
produce lower molecular weight polyethylene due to their
propensity to undergo chain transfer to aluminum. The pres-
ence of these aluminum species stems from the use of meth-
ylaluminoxane (MAO), which is required for the activation of
1-Co and 1-Fe.32 Furthermore, although these catalysts have
been reported to be active at elevated temperatures, no sys-
tematic determination of their time-resolved thermal stabil-
ity has ever been reported, to the best of our knowledge.
1H NMR (CDCl3, 500 MHz): d 8.12 (d, 2 H), d 7.78 (t, 1 H), d
7.26–7.02 (m, 40 H), d 6.71 (d, 4 H), d 5.30 (s, 4 H), d 2.19
(s, 6 H), d 0.91 (s, 6 H). 13C NMR (CDCl3, 75 Hz) 170.1,
154.8, 146.0, 143.8, 142.6, 136.4, 132.2, 131.6, 129.8, 129.4,
128.6, 127.9, 126.0, 121.9, 121.9, 77.2, 76.7, 51.9, 21.3, 16.8.
HRMS C75H63N3 (H1 adduct) 5 1006.5056 m/z; HRMSexpt
C75H63N3 (H1 adduct) 5 1006.5076 m/z.
Toward this goal, herein, we describe the synthesis and poly-
merization behavior of symmetric, dibenzhydryl-substituted
precatalysts 2-Co and 2-Fe. We will show that precatalyst 2-
Co is highly active for ethylene polymerization, while 2-Fe is
virtually inactive. Furthermore, we will compare the suscep-
tibility of precatalysts 1-Co and 2-Co to chain-transfer to
aluminum as well as evaluate their time-resolved thermal
stability.
Synthesis of 2,6-Bis[1-(2,6-bis(diphenylmethyl)-4-
methylaniline)] pyridine cobalt(II)bromide (2-Co)
A solution of 3 (400 mg, 0.40 mmol) in tetrahydrofuran
(20 mL) was added to cobalt(II) dibromide hydrate
(83.1 mg, 0.38 mmol) in a Schlenk flask and stirred over-
night at room temperature. Diethyl ether (10 mL) was then
added to the reaction to precipitate the complex and the
resulting solids were filtered in air, washed using diethyl
ether and pentane, and dried in vacuo to yield complex 2-Co
as a brown powder (268.8 mg) in 67.2% yield.
EXPERIMENTAL
General Methods and Materials
All reactions were performed under an inert nitrogen atmo-
sphere using an MBraun UniLab glovebox or using standard
Schlenk techniques, unless otherwise noted. All solvents were
dried using an Innovative Technologies PureSolv Solvent Purifi-
cation System and degassed via three freeze–pump–thaw
cycles. CDCl3 was dried over activated molecular sieves (4 Å).
The compounds 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]-
pyridine, 2,6-bis[1-(2,6-diisopropylphenyl-imino)ethyl] pyridine
cobalt(II)bromide, 2,6-bis-(diphenylmethyl)-4-methylaniline, and
2-Fe were prepared as reported previously.18,20,32,38 MAO was
received as a gift from Albemarle Corp. and used as received.
All other reagents were purchased from commercial vendors
and used without further purification.
1HNMR (DMSO-d6, 500 MHz) d 8.18 (d, 2 H), d 7.98 (m,
1 H), d7.27–6.95 (m, 40 H), d 6.64 (s, 4 H), d 5.21 (s, 4 H), d
2.21 (s, 6 H), d 0.80 (s, 6 H). HRMScalc C75H63N3Br2Co (H1
adduct) 5 1224.2700 m/z; HRMSexpt C75H63N3Br2Co (H1
adduct) 5 1224.5700 m/z.
RESULTS AND DISCUSSION
Synthesis and Characterization of Complexes
The well-known cobalt precatalyst 1-Co and 1-Fe were synthe-
sized as reported previously.32 Precatalysts 2-Co and 2-Fe
were synthesized according to Scheme 1 in which 2,6-dibenz-
hydryl-4-methylaniline was synthesized as described previously
via electrophilic aromatic substitution of p-toluidine using
benzhydrol and catalytic ZnCl2.19,20 This substituted aniline
was condensed onto 2,6-diacetylpyridine to provide the target
ligand 2-[1-(2,6-dibenzhydryl-4-methyl-phenyl-imino)ethyl]-6-
[1-arylimino)eth-yl]-pyridine (3) in good overall yield (69.5%).
Ligand 3 was metallated using the appropriate Co or Fe pre-
cursor in THF to provide precatalysts 2-Co and 2-Fe.
1H and 13C spectra were recorded at ambient temperature
on a Varian VNMRS 500 MHz narrow-bore broadband sys-
tem. 1H and 13C NMR chemical shifts were referenced to
residual solvent. Gel permeation chromatography was per-
formed at 145 8C in 1,2,4-trichlorobenzene at 1.0 mL/min on
a Malvern Viscotek HTGPC equipped with refractive index,
2
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TABLE 1 Ethylene polymerization results for precatalyst 2-Coa
Yield Prod.b
Entry Trxn (8C) (mg)
Mw
c
c
d
-
D
(kg/mol) (kg/mol)
Tm (8C)
1
20
75
14.9
27.8
42.2
54.1
62.4
56.5
151
33
23
13
10
4.3
3.0
57
29
10
4.58 134.0
3.06 133.9
3.62 130.5
1.21 129.5
1.49 127.8
1.25 121.9
1.96 133.4
2.84 132.0
1.84 130.6
2
40
139
211
271
312
282
757
1240
707
3
60
4
80
5
90
6
100
40
7e
8e
9e
80
247
100
141
SCHEME 1 Synthesis of precatalysts 2-Co and 2-Fe.
a
Polymerization conditions: [2-Co] 5 5 mmol, 50 mL toluene, 30 psi eth-
ylene, 300 equiv MAO, 30 min.
b
NMR spectroscopy confirmed that both precatalysts 2-Co
and 2-Fe display paramagnetic behavior, which was expected
for high-spin Fe and Co complexes. Single crystals of precatalyst
2-Co suitable for X-ray diffraction analysis were obtained by
vapor diffusion of hexanes into a saturated dichloromethane
(DCM) solution (Fig. 2). Complex 2-Co displayed a distorted
square-pyramidal geometry in which the Co center is pushed
out of the chelating [N,N,N]-ligand plane. This observation is
believed to be due to ligand sterics, which has also been noted
for analogous complexes.17,18,32 As a note, we were unable to
obtain suitable crystals of 2-Fe for analysis by X-ray diffraction.
Productivity 5 kilogram of ethylene/mol cat.
Determined using gel permeation chromatography at 145 8C in 1,2,4-
c
trichlorobenzene.
d
Determined via differential scanning calorimetry, second heating
cycle.
e
Polymerizations performed at 100 psi of ethylene.
of the origins of inactivity of 2-Fe, it agrees strongly with
related studies by Sun and coworkers and may potentially
be due to the instabilities of the MAO activated Fe spe-
cies.17,18,35 Because of the inactivity of 2-Fe, the remainder
of this work will only focus on the cobalt containing precata-
lysts 1-Co and 2-Co so as to make a direct comparison of
the effects of ligand sterics on catalytic behavior.
Ethylene Polymerization Using 2-Co and 2-Fe
Both precatalysts 1-Co and 1-Fe are known to be highly
active for ethylene polymerization, and have been detailed in
the literature.32 In contrast, initial ethylene polymerizations
quickly revealed that while 2-Co is active for ethylene poly-
merization, precatalyst 2-Fe is essentially inactive, yielding
only trace amounts of polymer. Although we are not certain
To evaluate the catalytic behavior of 2-Co, ethylene polymer-
izations were conducted at temperatures ranging from 20 to
100 8C (Table 1). It was found that each polyethylene sample
produced was highly linear, as evidenced by their high Tm
(121.9–134 8C), and a decreasing trend in Tm was observed
as a function of increasing temperature (Table 1, entries 1–
6). For comparison, perfectly linear polyethylene is generally
regarded to have a Tm 5 137 8C.39 Similarly, molecular weight
was also found to decrease as a function of increasing tem-
perature (33 ! 3 kg/mol) which can be attributed to either
the decreased solubility of ethylene gas in the reaction sol-
vent (toluene) or increased rates of chain transfer at ele-
vated temperatures (Table 1, entries 1–6).13,40,41 However,
we will provide evidence in the following sections of this
work that ethylene solubility is the likely cause. Finally, poly-
merizations conducted at or below 60 8C yielded polymer
-
dispersity values (D) that are significantly broader than
those obtained at higher polymerization temperatures. This
is due to the presence of a high-molecular-weight shoulder
in their respective GPC chromatograms (Supporting Informa-
tion), but which is virtually eliminated for all polymeriza-
tions conducted at or above 80 8C.
Polymerizations conducted under an ethylene feed pressure of
30 psi (Table 1, entries 1–6) show a steady increase in polyeth-
ylene productivity as polymerization temperature is increased,
reaching a maximum of 62.4 kg/mol at 90 8C (Table 1, entry
FIGURE 2 An ORTEP representation of precatalyst 2-Co with
thermal ellipsoids drawn at 30% probability. Hydrogens are
omitted for clarity. [Color figure can be viewed at wileyonline-
library.com]
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TABLE 2 Evaluating the effects of [Al] for precatalysts 1-Co and
This is particularly true for precatalyst 1-Co and its Fe-based
analogue, which are known to produce lower molecular
weight polymers when using high Al:cat ratios. This effect
has been directly attributed to chain transfer to Al during
polymerization.32 Therefore, to evaluate the propensity of
precatalyst 2-Co toward chain transfer to Al, varying ratios
of Al:cat were screened and compared with analogous poly-
merizations using 1-Co (Table 2).
2-Coa
c
Yield
(mg)
Prod.b
Mw
c
-
D
Entry
Cat.
Al:cat
(kg/mol)
(kg/mol)
1
2
1-Co
1-Co
1-Co
1-Co
1-Co
2-Co
2-Co
2-Co
2-Co
2-Co
100:1
300:1
500:1
1000:1
1500:1
100:1
300:1
500:1
1000:1
1500:1
4383
5160
4868
5879
6263
295
876
1032
973
1176
1253
59
23
10
8.3
6.7
5.6
39
57
61
67
64
1.3
1.4
2.1
1.2
1.9
3.4
4.4
3.2
2.4
2.0
3
4
As expected, polymerizations using catalyst 1-Co showed a
slightly increasing trend in productivity as a function of
increasing Al:cat ratio. However, a corresponding notable
decrease in Mw was also observed (23 ! 5.6 kg/mol) (Table
2, entries 1–5) as the Al:cat ratio was increased, which sup-
ports previous reports that 1-Co undergoes deleterious chain
transfer from cobalt to aluminum. Furthermore, analysis of
these samples via 1H NMR spectroscopy also revealed the
presence of olefinic end groups (Supporting Information Figs.
S12 and S13) that is indicative of b-hydride transfer events,
which may result due to b-hydride elimination followed by
dissociation or chain-transfer to monomer (Table 2).
5
6
7
774
155
153
260
214
8
764
9
1302
1068
10
a
Polymerization conditions: [cat] 5 5 mmol, 50 mL toluene, 100 psi,
40 8C, 30 min.
b
Productivity 5 kilogram ethylene/mol cat.
Determined using gel permeation chromatography at 145 8C in 1,2,4-
c
trichlorobenzene.
In contrast to catalyst 1-Co, the productivity of catalyst 2-Co
was found to increase steadily up to an Al:cat ratio of
1000:1, reaching a maximum productivity of 260 kg/mol
(Table 2, entry 9). Further increasing the Al:cat ratio to
1500:1 resulted in a decrease in productivity; however, it
did not alter the molecular weight of the resultant polyethyl-
ene (Table 2, entry 10). The absence of associative chain
transfer events is further supported by the lack of olefinic
end groups via 1H NMR spectroscopy (Supporting Informa-
tion Figs. S14 and S15).
5). However, further increasing the polymerization temperature
to 100 8C resulted in a slight drop in productivity suggesting
that some degree of catalyst decomposition or deactivation
may be occurring at this temperature (Table 1, entry 6). This
drop in productivity at 100 8C was also observed for polymer-
izations conducted at higher ethylene feed pressure (100 psi)
(Table 1, entries 7–9), which partially suggests that catalyst 2-
Co may potentially be thermally unstable at this temperature.
Additionally, polymerizations conducted at 100 psi of ethylene
feed pressure produced polymers with significantly higher
molecular weights and increased melting temperatures relative
to those conducted at 30 psi. We attribute both of these obser-
vations to increased monomer concentration in solution, which
increases polymerization rates and decreases the propensity of
the catalyst to undergo any chain-walking events.37
These observations suggest that while activity of catalyst 2-Co
is directly dependent on Al:cat ratio, it is not readily suscepti-
ble to deleterious chain transfer to Al events, as is observed
for catalyst 1-Co. We hypothesize that chain transfer to Al is
avoided due to the extreme steric bulk of the dibenzhydryl
substituted ligand and effective steric congestion of the active
Co-center’s axial coordination sites.
It was noted that the productivity of catalyst 2-Co was con-
sistently lower than observed when using catalyst 1-Co
under identical conditions (62.4 kg/mol versus 264 kg/mol,
respectively) (Supporting Information). We attribute the
decreased productivity of 2-Co to the greatly increased
ligand sterics surrounding the open coordination site of the
activated Co center, which thereby hinders ethylene coordi-
nation and insertion. Polymerizations conducted at 100 psi
of ethylene feed pressure showed that productivity of 2-Co
can be greatly enhanced (54.1 ! 247 kg/mol) by increasing
ethylene feed pressure (Table 1, entries 4 and 8). This sug-
gests that rate of chain propagation of catalyst 2 is heavily
dependent on ethylene concentration.
Evaluating the Thermal Stability of 1-Co and 2-Co
To evaluate the thermal stability of precatalysts 1-Co and 2-
Co, ethylene polymerizations were conducted at 80, 90, and
100 8C under constant ethylene feed pressure (30 psi).
Although catalyst 2-Co displays optimal productivity when
using an Al:cat ratio of 1000:1, a lower ratio of 300:1 was
chosen for this study so that chain-transfer to aluminum is
minimized when studying comparative polymerizations using
precatalyst 1-Co. Furthermore, although the exact deactivation
pathways for Fe- and Co-based bis(imino)pyridine catalysts
are not fully known,42 we suspected that chain transfer to alu-
minum could be a potential source of catalyst deactivation
and ultimately complicate a direct comparison between the
temporal thermal stability profiles of catalyst 1-Co and 2-Co.
Comparing the Effects of [Al] for 1-Co and 2-Co
As mentioned previously, the ratio of MAO activator to cata-
lyst (Al:cat) can have a strong influence on the activity of the
propagating catalyst, particularly in regards to polymer yield,
To study the temporal thermal stability of precatalysts 1-Co
and 2-Co, polyethylene productivities were monitored as a
function of time. In these studies, a linear increase in
-
molecular weight (Mw), and molecular weight dispersity (D).
4
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In contrast, polymerizations using catalyst 2-Co, which bears
sterically demanding dibenzhydryl moieties, shows a per-
fectly linear increase in productivity as a function of time at
80 8C (Fig. 4). This suggests that catalyst 2-Co is thermally
stable at 80 8C for the entire time period studied (70 min).
Ethylene polymerizations at even higher temperatures
revealed that polyethylene productivity deviates from linear-
ity at 90 8C and shows significant deviation from linearity at
100 8C, which we attribute to decomposition or deactivation
of the active catalytic species. Although catalyst 2-Co is not
perfectly stable at 90 8C, it shows only slightly diminished
productivity after 1 h of polymerization and is far more sta-
ble at this temperature than catalyst 1-Co which is virtually
inactive after 50 min. In similarity to catalyst 1-Co, polymer
molecular weights generally decreased as temperature was
increased; however, no observable trends in polymer disper-
sity or molecular weight were observed as a function of
time, which again was expected for this non-living polymeri-
zation catalyst.
FIGURE 3 Plot of polyethylene productivity versus time for pre-
catalyst 1-Co. Polymerization conditions: [1-Co] 5 5 mmol, 30
psi ethylene, 50 mL toluene, 300 equiv MAO. [Color figure can
be viewed at wileyonlinelibrary.com]
productivity versus time is expected if the catalyst is stable
at a given temperature. Each reaction was equilibrated at the
desired temperature (80, 90, or 100 8C) and ethylene pres-
sure (30 psi) before being activated by injection of 300
equivalents of MAO. The polymerizations were stopped at
the prescribed time interval by injecting methanol into the
polymerization reactor, and the polymer samples were
stirred in acidic methanol for 24 h to dissolve any polymeric
MAO-based contaminants before being isolated and dried
before analysis.
Although the detailed decomposition pathways of bis(imino)-
pyridine-ligated Fe- and Co-based catalysts are not fully
understood, we attribute the enhanced thermal stability of
catalyst 2-Co (at 80 8C and 90 8C), relative to 1-Co, to the
bulky dibenzhydryl N-aryl moieties of the ligand backbone.
We hypothesize that the extreme steric demand of 2-Co
ligand severely inhibits N-aryl rotations and rigidifies the
catalyst structure. This rigid ligand effectively blocks the
axial coordination sites of the active Co metal center and
prevents deleterious reactions that lead to the premature
deactivation or decomposition of catalyst 2-Co.15,20
The results of this study are shown in Figure 3, where cata-
lyst 1-Co clearly shows little or no polyethylene productivity
after the first ꢀ20 min of polymerization, which clearly sug-
gests deactivation of the catalyst. This same result was
observed at each elevated temperature studied (80, 90, and
100 8C), and in each case catalyst 1-Co was virtually inactive
after just 30 min of polymerization time. Polymer molecular
weights decreased as temperature was increased; however,
no observable trends in polymer dispersity or molecular
weight were observed as a function of time, which was
expected for this non-living polymerization catalyst.
Finally, several structural parameters have been previously
proposed to influence catalyst thermal stability. These
include the presence of short metal to nitrogen bond lengths,
and the presence of a firm/rigid ligand framework.33 Catalyst
2-Co clearly represents a complex in which ligand rigidity is
enhanced due to steric constraints, and based upon the
trends presented herein support those hypotheses.
CONCLUSIONS
In sum, we synthesized and evaluated the ethylene polymeri-
zation behavior of catalysts 2-Fe and 2-Co. Upon activation
with MAO, catalyst 2-Fe was found to be essentially inactive
for ethylene polymerizations while catalyst 2-Co showed
good polymerization activity and was used for the funda-
mental studies presented herein. Specifically, the time-
resolved thermal stability of catalyst 2-Co, and its sensitivity
to Al:cat ratio were investigated relative to the extensively
studied catalyst 1-Co.
Through these investigations, it was found that catalyst 2-Co
displays virtually no deleterious chain transfer to Al events,
which is in stark contrast to the results obtained for 1-Co.
Furthermore, catalyst 2-Co demonstrates near perfect time-
resolved thermal stability at 80 8C and greatly enhanced ther-
mal stability at 90 8C as compared to catalyst 1-Co. We attri-
bute this enhancement in thermal stability and decrease in
FIGURE 4 Plot of productivity versus time for precatalyst 2-Co.
Polymerization conditions: [2-Co] 5 5 mmol, 30 psi ethylene,
50 mL toluene, 300 equiv MAO. [Color figure can be viewed at
wileyonlinelibrary.com]
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18 J. Yu, H. Liu, W. Zhang, X. Hao, W.-H. Sun, Chem. Commun.
2011, 47, 3257.
deleterious chain transfer to Al events to sterically demand-
ing dibenzhydryl substituted ligand framework of catalyst 2-
Co. These bulky substituents are proposed to restrict N-aryl
rotations and effectively block the axial coordination sites of
the active metal center. We hypothesize that this steric pro-
tection mitigates any unwanted reactions that lead to prema-
ture catalyst transfer, deactivation, and/or decomposition.
19 J. L. Rhinehart, N. E. Mitchell, B. K. Long, ACS Catal. 2014,
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20 J. L. Rhinehart, L. A. Brown, B. K. Long, J. Am. Chem. Soc.
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ACKNOWLEDGMENTS
This work was supported by the Army Research Office (con-
tract number: W911NF-1–0127). The authors thank Albemarle
Corp. for generously providing the MAO used in this study and
Dr. Gaya Elpitiya for her assistance with x-ray crystallography.
25 A. Berkefeld, S. Mecking, J. Am. Chem. Soc. 2009, 131,
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26 Y. Zhang, C. Huang, X. Hao, X. Hu, W.-H. Sun, RSC Adv.
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