Fine-Tuning Nickel Phenoxyimine Olefin Polymerization Catalysts: Performance Boosting by Alkali Cations

Article abstract of DOI:10.1021/jacs.5b10351

To gain a better understanding of the influence of cationic additives on coordination-insertion polymerization and to leverage this knowledge in the construction of enhanced olefin polymerization catalysts, we have synthesized a new family of nickel pheno

Full text of DOI:10.1021/jacs.5b10351

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Article
Fine-Tuning Nickel Phenoxyimine Olefin Polymerization
Catalysts: Performance Boosting by Alkali Cations
Zhongzheng Cai, Dawei Xiao, and Loi H. Do
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b10351 • Publication Date (Web): 12 Nov 2015
Downloaded from http://pubs.acs.org on December 1, 2015
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FineꢀTuning Nickel Phenoxyimine Olefin Polymerization Catalysts:
Performance Boosting by Alkali Cations
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Zhongzheng Cai, Dawei Xiao, Loi H. Do*
Department of Chemistry, University of Houston, 4800 Calhoun Rd., Houston, TX 77004, United States
ABSTRACT: To gain a better understanding of the influence of cationic additives on coordinationꢀinsertion polymerization and to
leverage this knowledge in the construction of enhanced olefin polymerization catalysts, we have synthesized a new family of nickꢀ
el phenoxyimineꢀpolyethylene glycol complexes (NiL0, NiL2–NiL4) that form discrete molecular species with alkali metal ions
(M+ = Li+, Na+, K+). Metal binding titration studies and structural characterization by Xꢀray crystallography provide evidence for
the selfꢀassembly of both 1:1 and 2:1 NiL:M+ species in solution, except for NiL4/Na+ which form only the 1:1 complex. It was
found that upon treatment with a phosphine scavenger, these NiL complexes are active catalysts for ethylene polymerization. We
demonstrate that the addition of M+ to NiL can result in up to a 20ꢀfold increase in catalytic efficiency as well as enhancement in
polymer molecular weight and branching frequency compared to the use of NiL without coꢀadditives. To the best of our
knowledge, this work provides the first systematic study of the effect of secondary metal ions on metalꢀcatalyzed polymerization
processes and offers a new general design strategy for developing the next generation of high performance olefin polymerization
catalysts.
INTRODUCTION
In our goal to develop high performance catalysts for the
controlled polymerization of olefins, our laboratory is interestꢀ
ed in the application of dual metal catalysis.31 There is comꢀ
pelling evidence that bimetallic complexes, such as those
based on the doubleꢀdecker27,28 or areneꢀbridged structures,25,26
allow for better incorporation of polar comonomers compared
to mononuclear catalysts due to the presence of metalꢀmetal
cooperativity.12,32 Most of the bimetallic catalysts reported in
the literature, however, contain metal centers that are both
The discovery that homogeneous late transition metal catalysts
can exhibit olefin polymerization activity similar to that of
early transition metal catalysts led to a major paradigm shift in
olefin polymerization catalysis.1ꢀ12 Because late transition metꢀ
al catalysts (e.g. Ni, Pd) are far less susceptible to inhibition
by heteroatom donors compared to their early transition metal
counterparts (e.g. Ti, Hf, Zr), the former typically exhibit
greater tolerance of polar monomers, solvents, and impurities
compared to the latter. Although recent developments in nickꢀ
el and palladium catalysis have led to the creation of systems
that can copolymerize ethylene and polar vinyl monomers
through a coordinationꢀinsertion mechanism,13ꢀ15 the resulting
polymers tend to have low molecular weight and the catalyst
activity tends to be poor. To have utility in commercial polyꢀ
mer synthesis,9 the ideal catalyst should have high catalytic
efficiency, be thermally robust, yield polymers with high moꢀ
lecular weight and narrow polydispersity, and display good
control over polymer microstructure.
active
in
olefin
polymerization
(or
trimerizaꢀ
tion/oligomerization in some cases). In our research, we wish
to explore the olefin polymerization behavior of complexes
that comprise two functionally distinct metal centers,33ꢀ35
In an effort to engineer catalysts that satisfy the stringent
requirements above, a variety of design strategies have been
explored. Some of the most notable examples are shown in
Chart 1, which include the use of structural constraints,16ꢀ21
fluorine bonding,22,23 hemilabile ligands,24 and bimetallic acꢀ
tive sites.25ꢀ29 One of the key findings from these studies is that
sterically bulky ligands that protect the axial sites of square
planar nickel and palladium complexes tend to promote polyꢀ
mer chain elongation over chain transfer, which can lead to the
formation of ultrahigh molecular weight polymers (e.g. Mn up
to 3 x 106 g/mol)20 and give catalysts that show “quasiꢀliving”
behavior.30 It has also been suggested that weak C(ligand)–
FꢁꢁꢁH–C(polymer) interactions through fluorine bonding of a
catalyst with a growing polymer chain can help suppress βꢀ
hydride elimination to furnish linear polyethylene.22
Chart 1. Examples of design strategies explored in the developꢀ
ment of improved nickel phenoxyimine olefin polymerization
catalysts.
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Scheme 1. Synthesis of NiL0, NiL2–NiL4.a
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Figure 1. The Xꢀray crystal structure of Ni(Mes)(PPh3)(L2)
aReaction conditions: i) 2,6ꢀdiisopropylaniline, acetic acid,
MeOH; ii) sodium hydride, THF; iii) NiBr(Ph)(PPh3)2, THF. The
phenoxyimine ligands are denoted as L, followed by a number to
indicate the length of the PEG chain attached to the phenol unit of
the ligand.
(NiL2Mes, ORTEP view, displacement ellipsoids drawn at 50%
probability level). Hydrogen atoms and solvent have been omitted
for clarity.
as group I and II metal ions might exhibit metalꢀmetal cooperꢀ
ativity in olefin polymerization when paired with a convenꢀ
tional nickel catalyst. We postulate that having two functionalꢀ
ly distinct metal centers within a single catalyst scaffold would
impart new reactivity patterns that are not accessible using
homobimetallic catalysts. Furthermore, we favor using alkali
and alkaline cations as the secondary metal because they do
not engage in redox reactions and form relatively stable metalꢀ
ligand interactions with hard Lewis bases such as the carbonyl
groups of polar monomers (e.g. acrylate, acrylamide, etc.).
where one metal ion carries out olefin polymerization and the
other serves as an activator and binding site for polar functionꢀ
alities. We hypothesize that such siteꢀdifferentiated heterobiꢀ
metallic species can enhance the coordinationꢀinsertion of
olefins compared to homobimetallic species because the two
metal centers do not compete with each other for monomer
binding and there is no steric interference from two growing
polymer chains within the same catalyst structure.
To obtain discrete heterobimetallic complexes, we preꢀ
pared a new family of dinucleating ligands based on the pheꢀ
noxyimine platform (Scheme 1).14 Polyethylene glycol (PEG)
moieties containing 0–4 ethylene glycol units were attached to
the phenol ring of Nꢀ(2,6ꢀdiisopropylphenyl)phenoxyimine to
yield a series of ligands HL (the ligand number specifies the
number of ethylene glycol units in the PEG chain). The O,Nꢀ
chelate of the phenoxyimine unit will be ligated to nickel,
whereas the PEG/phenolate groups will be ligated to either a
group I or II cation. Having different HL variants will allow
us to determine the optimal PEG chain length required to acꢀ
commodate cations with different ionic radii.37,38,40 The HL
ligands are modular and simple to prepare starting from comꢀ
mercially available precursors.
As proof of concept, we have prepared a new class of
nickel complexes supported by phenoxyimine ligands having
pendant polyethylene glycol (PEG) side chains.36ꢀ38 We show
that the spontaneous selfꢀassembly of dinuclear nickelꢀalkali
metal complexes generates highly active catalysts for ethylene
polymerization, which displays a remarkable increase in polꢀ
ymer branching, molecular weight, and turnover frequency
compared to polymerizations performed in the absence of alꢀ
kali metal ions. These findings demonstrate the beneficial
effects of cationic Lewis acids on olefin polymerization and
provide a new conceptual framework with which to guide
future catalyst design efforts.
RESULTS AND DISCUSSION
The HL ligands were synthesized according to the proceꢀ
dure depicted in Scheme 1. The aldehydes 1A/1C–1E were
obtained from alkylation of 2,3ꢀdihydroxybenzaldehye by
treatment with sodium hydride, followed by reaction with the
appropriate tosylꢀPEG or bromoꢀPEG reagent.36 Reaction of
the 3ꢀalkylated compound 1 with 2,6ꢀdiisopropylaniline and
acetic acid afforded ligands HL in moderate to excellent
yields (70–100%).
Catalyst Design Rationale and Synthesis. We were inspired
by a literature report demonstrating that nickel phosphineꢀ
alkoxide complexes were more productive in the copolymeriꢀ
zation of ethylene and hexyl acrylate when excess LiB(C6F5)4
salts were used as coꢀadditives.39 Although the authors reportꢀ
ed that the precatalysts used in the study have dinuclear nickꢀ
elꢀlithium structures, the precise role of the lithium cations in
polymerization was not further elaborated. In our work, we
were intrigued by the possibility that “hard” Lewis acids such
2
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Table 1. Association Constants Ka1 and Ka2 Determined from
Metal Titration Studiesa
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2
3
4
5
6
7
8
complex
NiL2
Li+
Na+
K+
1.09 (Ka1)
0.63 (Ka2)
0.77 (Ka1)
0.76 (Ka2)
1.65 (Ka1)
0.92 (Ka2)
0.23 (Ka1)
2.73 (Ka2)
5.74 (Ka1)
0.58 (Ka2)
26.66 (Ka1)
–
0.75 (Ka1)
0.60 (Ka2)
4.50 (Ka1)
0.55 (Ka2)
1.83 (Ka1)
0.71 (Ka2)
NiL3
NiL4
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aThe association constants have units ×10ꢀ2 ꢂM–1.
Metal Binding Studies. With the NiL complexes in
hand, we performed metal ion titration studies by UVꢀvis abꢀ
sorption spectroscopy to examine their metal binding behavꢀ
ior. For these experiments, solutions containing 100 ꢂM NiL
(i.e. NiL2, NiL3, or NiL4) in Et2O were treated with aliquots
of 0.1 equiv. of MBArF4 salts (M = Li+, Na+, and K+; BArF –
=
4
tetrakis(3,5ꢀtrifluoromethylphenyl)borate) and then allowed to
equilibrate for ~20ꢀ30 min before recording the spectral
changes. Upon addition of the alkali metal salts, the absorption
bands centered at ~340 and ~420 nm decreased, whereas the
absorbance at ~375 nm increased (Figure 2 and S1). Titration
studies could not be performed with NiL0 or using alkaline
salts (e.g. Mg(SO3CF3)2, Ca(SO3CF3)2) due to their poor soluꢀ
bility in Et2O. The titration plots show that the complexation
of M+ to NiL does not follow a simple Aꢀ B binding model
due to the lack of any clear isosbestic points, except for the
reaction of Na+ with NiL4. The introduction of NaBArF to a
4
solution of NiL4 led to the development of clean isosbestic
points at 360 and 405 nm (Figure 2B). The spectral data obꢀ
tained from these titration studies were fit using nonꢀlinear
least square regression by the program DynaFit.41 The followꢀ
ing chemical equilibria were used in the data fitting:
[ꢀꢁꢄꢂ]
ꢀꢁꢂ + Mꢃ ⇆ ꢀꢁꢄꢂ
Ka1
Ka2
=
=
(1)
(2)
ꢆ
[
]
ꢀꢁꢂ [ꢅ ]
Figure 2. Metal titration plots showing the spectral changes due
[ꢀꢁ ꢄꢂ ]
ꢇ
ꢇ
to the addition of NaBArF4 to A) NiL2 and B) NiL4 in Et2O (100
ꢂM). The black traces are the starting spectra of NiL and the colꢀ
ored traces are the spectra obtained after the addition of 0.1 equiv.
of Na+, relative to Ni. The insets show the absorbance changes at
340 nm as black dots and the DynaFit nonꢀlinear regression fit as
black solid lines.
ꢀꢁꢄꢂ + ꢀꢁꢂ ⇆ ꢀꢁꢇꢄꢂꢇ
[
]
ꢀꢁꢄꢂ [ꢀꢁꢂ]
where
NiML
is
the
1:1
NiL:M+
complex
[NiM(Ph)(PPh3)(L)]+, Ni2ML2 is the 2:1 NiL:M+ complex
[Ni2M(Ph)2(PPh3)2(L)2]+, and the Ka values are their correꢀ
sponding association constants. In almost all cases, the abꢀ
sorbance changes at 340 nm fit better to a model involving the
formation of both 1:1 and 2:1 species compared to one involvꢀ
ing just the formation of the 1:1 species (Figure 2A inset and
S2). Only the titration data for NiL4/Na+ fit well to a simple
1:1 binding model (Figure 2B inset). As shown in Table 1, Ka1
values range from 0.23–26.66 × 10–2 ꢂM–1 whereas Ka2 values
range from 0.55–2.73 × 10–2 ꢂM–1. These data are consistent
with the observed trend that the most stable alkaliꢀPEG comꢀ
plexes are formed when the PEG chain length matches the
ionic radius of the metal ion.37,38,40 For example, the Ka1 values
for NiL3 are 0.77, 5.76, and 4.50 ×10–2 ꢂM–1 with Li+, Na+,
and K+, respectively, which indicate that NiL3 containing a
Metallation of HL was accomplished by first treatment of
the ligands with sodium hydride, which yielded NaL as yellow
solids (Scheme 1). The phenolate salt was then combined with
the nickel precursor Ni(Br)(Ph)(PPh3)2 to give the
Ni(Ph)(PPh3)(L) complexes NiL in good yields (80–90%). Xꢀ
ray crystallographic characterization of Ni(Mes)(PPh3)(L2)
(NiL2Mes, where Mes = 2,4,6ꢀtrimethylphenyl), which was
prepared from the reaction of Ni(Br)(Mes)(PPh3)2 with NaL2,
shows that the nickel center adopts a square planar geometry,
in which the aryl group is coordinated trans to the phenolate
donor (Figure 1).
3
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Figure
4.
The
Xꢀray
crystal
structure
of
[Ni2Na(Ph)2(PPh3)2(L2)2](BArF ) (Ni2NaL22) shown in ORTEP
4
view with displacement ellipsoids drawn at 50% probability level.
The hydrogen atoms, BArF – anions, and phenyl rings have been
4
omitted for clarity.
ray structure of NiNaL3 is shown in Figure 3A. As expected,
the nickel center adopts a square planar geometry with the
phenyl group coordinated trans to the phenolate donor. The
sodium cation is ligated by the phenolate group (Na(1)–O(1) =
2.52 Å) and four oxygen donors from the PEG chain (Na(1)–
Oave = ~2.43 Å)42,43 and has a metalꢀπ interaction with one of
the phenyl rings of triphenylphosphine (Na(1)–Ph(centroid) =
2.64 Å).44 Crystals of the [NiK(Ph)(PPh3)(L4)](BArF ) comꢀ
4
plex (NiKL4) were grown by mixing NiL4 and KBArF4 (1:1)
in Et2O and layering with pentane. Xꢀray diffraction analysis
reveals that the nickel center in NiKL4 has a fourꢀcoordinate
geometry (Figure 3B), similar to that in NiNaL3. The potassiꢀ
um ion is coordinated to the phenolate group (K(1)–O(1) =
2.84 Å) and six ether oxygen donors (K(1)–Oave = 2.79 Å)45 as
well as a phenyl ring from triphenylphosphine (K(1)–
Ph(centroid) = 2.99 Å).46
Figure 3. The Xꢀray crystal structures of A) [Niꢀ
Na(Ph)(PPh3)(L3)](BArF )
(NiNaL3)
and
B)
4
[NiK(Ph)(PPh3)(L4)](BArF ) (NiKL4) shown in ORTEP view
4
with displacement ellipsoids drawn at 50% probability level. Hyꢀ
To grow crystals of the 2:1 complex, NiL2 and NaBArF
(2:1) were dissolved in benzene and the mixture was slowly difꢀ
fused with pentane. The orange crystals obtained were analyzed
by Xꢀray crystallography, which shows a compound with the
4
drogen atoms and the BArF – anions have been omitted for clarity.
4
triethylene glycol unit binds to Na+ better than to either Li+ or
K+. The most stable 1:1 complex is formed between NiL4 and
Na+, with a Ka1 value of 26.66 × 10–2 ꢂM–1, which is a signifiꢀ
cantly higher association constant compared to other NiL
complexes with M+. These metal binding studies suggest that
the speciation of the NiL complexes can differ in solution due
to the specific alkali ions used, which has important implicaꢀ
tions in their olefin polymerization activity as described beꢀ
low.
molecular
composition
[Ni2Na(Ph)2(PPh3)2(L2)2]BArF
4
(Ni2NaL22, Figure 4). Unlike the 1:1 NiL:M+ structures, the alkaꢀ
li ion in Ni2NaL22 links two NiL2 units together by binding to
two separate diethylene glycol chains, resulting in a sixꢀ
coordinate sodium center (Na(1)–Oave = 2.44 Å).47
A
structural comparison between the mononuclear
(NiL2Mes) and dinuclear (NiNaL3 and NiKL4) species shows
some slight variations in their bond metrics (Table 2). For
example, binding of Na+ or K+ to the phenolate group of NiL
leads to elongation of both their Ni–O and Ni–N bond distancꢀ
es (i.e. ~0.03 Å for NiNaL3 and ~0.01 Å for NiKL4) comꢀ
pared to those in NiL2Mes, suggesting that the phenoxyimine
ligand donates less electron density to the nickel center when a
Lewis acid is bound. In contrast, the nickel primary coordinaꢀ
tion spheres in NiL2Mes and Ni2NaL2, which are not interactꢀ
ing with an alkali metal ion, have nearly identical metal–
ligand bond lengths.
Structural Characterization. The metal binding studies
above strongly suggest that both 1:1 NiML and 2:1 Ni2ML2
complexes are formed in solution. To obtain evidence for such
species and to determine their molecular structures, single
crystals of the nickelꢀalkali complexes were prepared and anaꢀ
lyzed by Xꢀray crystallography. To obtain crystals of [Niꢀ
Na(Ph)(PPh3)(L3)](BArF ) (NiNaL3), NiL3 and NaBArF (1:1)
4
4
were combined in Et2O and then layered with pentane to give
orangeꢀcolored blocks upon standing for several days. The Xꢀ
4
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Table 2. Comparison of the Bond Distances (Å) and Angles (°)
Between the Xꢀray Structures of the Nickel Complexes
1
2
3
4
5
Bond
Lengths (Å)/
Angles (°)
NiL2Mes
NiNaL3
NiKL4
Ni2NaL22
6
7
8
9
Ni–O
Ni–N
Ni–C
Ni–P
1.9195(7) 1.952(2)
1.9310(8) 1.960(3)
1.929(2)
1.909(4)
1.895(4)
1.920(5)
1.929(5)
1.898(7)
1.890(6)
2.176(2)
1.938(2)
1.889(3)
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1.912(1)
1.899(3)
Figure 5. Representative 13C NMR spectra (DCEꢀd2, 150 MHz,
120°C) of A) amorphous and B) semiꢀcrystalline polyethylene
obtained in this work. Peak assignments were made according to
ref. 49. Branches are given the label xBy, where y is the branch
length and x is the carbon number starting from the methyl group
as 1. Greek letters and “br” are used instead of x for the methꢀ
ylene carbons in the polymer backbone and a branch point, reꢀ
spectively. The (+) sign indicates overlapping signals.
2.1794(3) 2.194(1) 2.1656(9)
2.176(2)
N–Ni–C
O–Ni–P
95.13(4)
85.65(2)
91.9(1)
85.8(1)
92.8(1)
94.5(2)
94.2(3)
89.9(1)
89.1(2)
performed without Na+ (entry 15). When reactions were carꢀ
86.48(6)
ried out using NiL0, NaBArF , and tetraethylene glycol dimeꢀ
4
thyl ether (1:1.1:2) instead of NiL4/NaBArF , no increase in
4
productivity was observed, indicating that the sodiumꢀPEG
group must be attached to NiL in order to interact with the
catalyst in a synergistic manner. The different effects of M+ on
different NiL variants seem to correlate well with the stabiliꢀ
Ethylene Polymerization. The NiL complexes were inꢀ
vestigated as singleꢀcomponent catalysts in olefin polymerizaꢀ
tion (Table 3). Upon treatment with the phosphine scavenger
Ni(COD)2 (COD = 1,5ꢀcyclooctadiene) in toluene under 100
psi of ethylene, all of the NiL complexes produced semiꢀ
crystalline polyethylene with a turnover frequency (TOF) of
~2.7 × 103 g/mol Niꢁh (entries 1, 5, 9, and 15), which is similar
to other nickel phenoxyimine catalysts reported in the literaꢀ
ture.14,21 Characterization by quantitative 13C NMR spectrosꢀ
copy48ꢀ50 indicates that the polyethylene obtained contains ~20
branches per 1000 carbon atoms and comprises mostly methyl
branches (~75–100%, Figure 5B). Gel permeation chromatogꢀ
raphy (GPC) analysis indicates that their molecular weights
(Mn) range from 2.32–4.36 × 103 g/mol with polydispersities
(Mw/Mn) between 1.4–1.8. These data suggest that the NiL
complexes are nonꢀliving singleꢀsite catalysts. Because NiL2–
NiL4 (entries 5, 9, and 15, respectively) exhibit nearly the
same activity as the parent NiL0 compound (entry 1), it apꢀ
pears that having additional PEG chains in the NiL structures
neither promote nor inhibit polymerization.
Next, the influence of salt additives on ethylene polymerꢀ
ization by the NiL catalysts was examined. The nickelꢀalkali
complexes were preꢀassembled by combining NiL and
MBArF4 (1:1.1) in toluene and then stirred for 30 min to give a
clear yellowꢀorange solution. The mixture was then treated
with Ni(COD)2 and then charged with ethylene in a highꢀ
pressure glass reactor. The polymerization data are shown in
Table 3. Addition of Li+, Na+, or K+ salts to NiL0 or NiL2 led
to a decrease (entries 2, 3, 4, 6, 7, 8), whereas the addition of
Na+ or K+ to NiL3 or NiL4 (entries 11, 12, 17) led to an inꢀ
crease in TOF compared to polymerizations performed in the
absence of salt additives. The highest activity was achieved
using NiL4 with Na+ (TOF = 47 × 103 g/ mol Niꢁh, entry 17),
which is a ~20ꢀfold enhancement compared to polymerizations
ties of their bimetallic [NiM(Ph)(PPh3)(L)](BArF ) (NiML)
4
species
and
their
propensities
(Ni2ML2) complexes (vide
to
form
[Ni2M(Ph)2(PPh3)2(L)2](BArF )
4
infra). Polymerizations were also attempted using dicationic
salts, such as Mg(SO3CF3)2 and Ca(SO3CF3)2 (entries 13 and
14, respectively); unfortunately, the alkaline salts have poor
solubility in toluene and could not form discrete nickelꢀ
alkaline complexes in this solvent.
Interestingly, polymerizations by NiL/M+ that show an
increase in TOF yielded amorphous rather than semiꢀ
crystalline polyethylene (entries 11, 12 17). Analysis by NMR
spectroscopy reveals that the amorphous polymer is highly
branched, with ~80–110 branches per 1000 carbon atoms
(Figure 5A).51 The polymer branches vary in length, with an
appreciable amount of C4+ chains (~10% of all branches). The
amorphous polyethylenes have Mn values between 3.02–7.69 ×
103 g/mol and Mw/Mn between 2.3–2.8. The significantly difꢀ
ferent polymer morphologies afforded by NiL with and withꢀ
out M+ clearly indicate that the cationic additives have a direct
influence on the coordinationꢀinsertion process during catalyꢀ
sis.
To evaluate the stability of the NiL catalysts, polymerizaꢀ
tion studies were conducted in increments of 0.5, 1, 2, and 3 h
(Table 4). In the absence of alkali salts, NiL3 produced semiꢀ
crystalline polyethylene with an average Mn of ~2.6 × 103
g/mol and Mw/Mn of ~1.4. These values remained relatively
constant over the course of 3 h (entries 1–4). The gradual deꢀ
cline in TOF during this same time period suggests that NiL3
decomposes slowly, possibly due to the formation of inactive
nickelꢀbis(phenoxyimine) species.52 In the presence of Na+,
NiL3 consistently yielded amorphous polyethylene with an Mn
5
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Journal of the American Chemical Society
Table 3. Polymerization Data for NiL0, NiL2–NiL4a
Page 6 of 12
1
2
3
4
polymer
yield
(mg)
TOF
(×103
g/mol·h)
c
c
c
d
polymer
type
branchesb
(/1000 C)
C1
C2
C3
C4+ c
(%)
Mn
d
entry
cat.
salt
Mw/Mn
(%)
(%)
(%)
(× 103)
5
6
7
8
1
2
NiL0 none
67
10
0.5
53
60
5
2.8
0.40
0.02
2.2
2.5
0.2
0.3
0.2
2.8
1.4
6.7
3.1
2.1
2.5
2.8
1.2
47
solid
solid
26
-
100
-
0
-
0
-
0
-
4.36
1.8
NiL0
NiL0
NiL0
Li+
Na+
K+
-
-
-
3
solid
-
-
-
-
-
-
9
4
solid
-
-
-
-
-
3.24
1.5
1.4
-
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
5
NiL2 none
solid
16
-
100
-
0
-
0
-
0
2.32
6
NiL2
NiL2
NiL2
Li+
Na+
K+
solid
-
-
7
6
solid
-
-
-
-
-
-
-
8
4
solid
-
-
-
-
-
-
-
9
NiL3 none
67
32
160
75
50
59
67
28
1,130
5
solid
20
-
75
-
13
-
2
-
10
-
2.65
1.4
-
10
11
12
13e
14e
15
16
17
18
NiL3
NiL3
NiL3
Li+
Na+
K+
solid
-
amorphous
amorphous
solid
107
106
17
24
19
-
81
73
90
84
100
-
7
11
0
0
0
-
1
3
0
0
0
-
11
13
10
16
0
7.69
2.3
2.8
-
3.02
NiL3 Mg2+
NiL3 Ca2+
NiL4 none
-
solid
-
3.01
-
-
solid
1.5
-
NiL4
NiL4
NiL4
Li+
Na+
K+
amorphous
amorphous
solid
-
82
79
7
1
13
4.66
2.3
0.2
-
-
-
-
-
-
-
aPolymerization conditions: nickel precatalyst (24 ꢂmol), Ni(COD)2 (48 ꢂmol), MBArF4 (26 ꢂmol, if any), ethylene (100 psi), 5 mL toluꢀ
b
c
ene, 1 h at RT. The total number of branches per 1000 carbons was determined by 1H NMR spectroscopy. Branching ratio was deterꢀ
mined by 13C NMR spectroscopy. dDetermined by GPC in trichlorobenzene at 140°C. eThe salt additive is poorly soluble in toluene.
Table 4. Polymerization Time Study for NiL3a
Time
polymer
yield
(mg)
TOF
(×103
c
c
c
d
branchesb
(/1000 C)
C1
C2
C3
C4+c
(%)
Mn
d
entry
cat.
salt
Mw/Mn
(%)
(%)
(%)
(× 103)
g/mol·h)
(h)
1
2
3
4
5
6
7
8
NiL3 none
NiL3 none
NiL3 none
NiL3 none
0.5
1
53
67
4.4
2.8
1.8
1.2
10.0
6.7
6.8
6.1
20
25
75
69
66
44
79
81
80
82
13
13
21
26
7
2
4
2
6
3
1
3
2
10
14
11
24
11
11
11
11
2.48
2.65
2.95
2.51
9.87
7.69
9.58
9.93
1.2
1.4
1.4
1.4
2.0
2.3
2.1
2.3
2
84
24
3
83
31
NiL3
NiL3
NiL3
NiL3
Na+
Na+
Na+
Na+
0.5
1
120
160
330
440
115
107
100
105
7
2
6
3
5
aPolymerization conditions: NiL3 (24 ꢂmol), Ni(COD)2 (48 ꢂmol), NaBArF (26 ꢂmol, if any), ethylene (100 psi), 5 mL toluene, at RT.
4
bThe total number of branches per 1000 carbons was determined by 1H NMR spectroscopy. cBranching ratio was determined by 13C NMR
spectroscopy. dDetermined by GPC in trichlorobenzene at 140°C.
of ~9.3 × 103 g/mol and Mw/Mn of ~2.2, which suggests that
the NiNaL3 catalyst is nonꢀliving and that Mn is limited by the
rate of chain transfer. Analysis by NMR spectroscopy shows
that the polymer branching structures are unaffected by the
polymerization time (entries 5–8). The reaction of
of inactive nickelꢀbis(phenoxyimine) species compared to the
mononickel complex but further studies are needed to clarify.
It should be possible to improve the catalyst stability by inꢀ
creasing the steric bulk of the phenoxyimine ligand,2,11 which
we aim to do in future work.
StructureꢀActivity Correlation. A plot of the TOF of
the NiL catalysts (Table 3) versus their association constants
Ka1 with various alkali cations (Table 1) suggests that there is
a strong correlation between one another (Figure 6). For exꢀ
NiL3/NaBArF /Ni(COD)2 with ethylene also shows a slight
4
decrease in TOF over a 3 h period, but to a lesser extent than
in the absence of added Na+. It is possible that the heterobimeꢀ
tallic nickelꢀsodium complex is less susceptible to formation
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Journal of the American Chemical Society
Scheme 2. Proposed Model for the Reaction of NiL with Ethylene
1
2
in the Presence and Absence of Alkali Metal Ions.
3
4
5
6
7
8
9
10
11
12
13
Figure 6. Structureꢀactivity correlation plot showing the effect of
different cations (Li+, Na+, and K+) on the ethylene polymerizaꢀ
tion activity of the NiL variants. The association constants Ka1 are
shown as blue dots whereas the TOFs are shown as red diamonds.
Entries on the xꢀaxis denoted with (–) indicate that no salt addiꢀ
tives were present.
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
ample, NiL3 and Li+ has a Ka1 value of 0.77 × 10ꢀ2 ꢂMꢀ1 and
TOF of 1.4 × 103 g/mol Niꢁh, whereas NiL3 and Na+ has a Ka1
value of 5.74 × 10ꢀ2 ꢂMꢀ1 and TOF of 6.7 × 103 g/mol Niꢁh.
The ~7ꢀfold increase in stability of NiNaL3 compared to NiLꢀ
iL3 also shows ~5ꢀfold increase in polymerization activity.
This trend is most apparent for NiL4, which exhibits the
strongest binding to sodium (Ka1 = 26.66 × 10ꢀ2 ꢂMꢀ1) and
gave the highest polymerization activity when Na+ was used as
an additive (TOF = 47 × 103 g/mol Niꢁh). In contrast, the lowꢀ
er affinity of NiL4 for lithium (Ka1 = 1.65 × 10ꢀ2 ꢂMꢀ1) and
potassium (Ka1 = 1.83 × 10ꢀ2 ꢂMꢀ1) compared to for sodium,
yielded significantly less active polymerization catalysts (i.e.
~39ꢀfold and ~235ꢀfold decrease in TOF, respectively, comꢀ
pared to Na+). We hypothesize that the dinuclear NiML speꢀ
cies are responsible for the enhancement in polymerization
activity and changes in polymer microstructure (Scheme 2). It
should be noted that the NiL species can dimerize in the presꢀ
ence of M+ to furnish trinuclear Ni2ML2 complexes (Ka2, Taꢀ
ble 1), which may also be involved in polymerization.
A possible reaction model for the polymerization of ethꢀ
ylene by the NiL complexes is depicted in Scheme 2. We have
demonstrated that when NiL is treated with Ni(COD)2 ethꢀ
ylene polymerization can take place, presumably through the
formation of nickelꢀethylene intermediates (NiL′). When an
alkali salt is added to NiL prior to catalyst activation, either
dinuclear NiML (e.g. NiNaL3 and NiKL4 in Figure 3) or
trinuclear Ni2ML2 (e.g. Ni2NaL22 in Figure 4) species are
generated. The relative ratio of NiML:Ni2ML2 is determined
by their equilibrium distribution (i.e. Ka1 and Ka2). Abstraction
of triphenylphosphine from Ni2ML2 under ethylene might
ylene polymerization catalysts. We hypothesize that the alkali
metal ion enhances the electrophilicity of the nickel center,
which appears to result in more efficient olefin binding and
insertion as well as faster rates of chain walking. It is also
possible that the increased steric bulk of the alkali–PEG unit
of the NiML complex, compared to NiL, may also play a role
in modulating its catalytic behavior. At present, we are uncerꢀ
tain to what extent electronic versus steric effects have on
tuning the nickel catalyst’s properties. Future studies will seek
to determine the identities of the active NiL′ and NiML′ speꢀ
cies in solution.
CONCLUSIONS
We have synthesized a new siteꢀdifferentiated phenoxyimine
ligand platform containing polyethylene glycol chains for the
preparation of heterobimetallic nickelꢀalkali metal complexes.
We showed through metal titration studies that the addition of
alkali salts to mononuclear NiL complexes resulted in the
formation of 1:1 and 2:1 NiL:M+ species in solution. Structurꢀ
al characterization of the 1:1 complexes by Xꢀray crystallogꢀ
raphy demonstrates that the phenoxyimine ligands in both
NiNaL3 and NiKL4 are metallated to nickel and the PEG
chains encapsulate the alkali cation to form discrete molecular
structures. Crystals of the 2:1 complex Ni2NaL22 were also
analyzed by Xꢀray diffraction, which reveals that two NiL2
units are linked via binding to a single sodium cation. Ethꢀ
ylene polymerization studies show that NiL/Ni(COD)2 yield
slightlyꢀbranched semiꢀcrystalline polyethylene, whereas
afford
the
corresponding
bis(ethylene)
adduct
Ni2M(Ph)2(C2H4)2(L)2 (Ni2ML2′). We expect that a species
such as Ni2ML2′ would behave similarly to NiML′ in ethylene
polymerization, except that the former is expected to be less
catalytically active due to the increased steric environment
around its nickel centers. This model might account for the
observation that certain combinations of NiL/M+ yield cataꢀ
lysts that exhibit a lower TOF in ethylene polymerization
compared to the mononuclear nickel catalysts. On the other
hand, we have also shown that when the ionic radius of M+ is
a suitable match for the PEG chain in NiL, stable dinuclear
NiML species are obtained. Activation by Ni(COD)2 would
yield NiML′, which our studies suggest are highly active ethꢀ
NiL/MBArF /Ni(COD)2 yield highlyꢀbranched amorphous
4
polyethylene in some cases. The polymerization efficiency of
various NiL/M+ combinations was high when the association
constants Ka1 for the corresponding NiML complexes were
large, suggesting that they are the catalytically active species.
Remarkably, the NiML complexes show significant increases
7
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Journal of the American Chemical Society
Page 8 of 12
in polymerization activity, molecular weight, and branching 1.90. Found: C, 73.29; H, 6.37; N, 1.76. Trace amounts of
1
2
3
4
5
6
7
8
frequency compared to the mononuclear NiL catalysts. These
results provide compelling evidence that alkali cations can
have a beneficial effect on coordinationꢀinsertion polymerizaꢀ
tion and provide a new design strategy for developing imꢀ
proved catalysts for the copolymerization of ethylene with
functional monomers in future work. Further studies will be
conducted to obtain a better understanding of the precise role
of alkali ions in coordinationꢀinsertion polymerization and to
explore the generality of this effect on other catalyst systems.
pentane and dichloromethane, which were used in recrystalliꢀ
zation of the material and confirmed by 1H NMR spectroscopy,
could not be removed completely by vacuum drying overnight.
Preparation of NiL2. The same procedure was used as
described for NiL0, except that NaL2 (71 mg, 0.17 mmol, 1
equiv.) was used instead of NaL0. The ligand was combined
with 1 equiv. of NiBr(Ph)(PPh3)2 (125 mg, 0.17 mmol). The
product was isolated as a yellow solid (105 mg, 0.13 mmol,
1
77%). H NMR (CDCl3, 400 MHz): δ (ppm) = 7.96 (d, JHP=
9
9.2 Hz, 1H), 7.56 (t, JHH= 8.8 Hz, 6H), 7.33 (t, JHH= 8.4 Hz,
3H), 7.23 (m, 6H), 6.94 (t, JHH= 7.6 Hz, 1H), 6.88–6.83 (m,
4H), 6.55 (d, JHH= 7.2 Hz, 2H), 6.42 (t, JHH= 7.2 Hz, 1H), 6.21
(t, JHH= 7.2 Hz, 1H), 6.07 (t, JHH= 7.2 Hz, 2H), 3.86 (m, 2H),
3.32–3.27 (m, 9H), 2.92 (t, JHH= 5.2 Hz, 2H), 1.12 (d, JHH= 7.2
Hz, 6H), 1.09 (d, JHH= 6.8 Hz, 6H). 13C NMR (CDCl3, 100
MHz): δ (ppm) = 165.47, 158.57, 151.26, 149.66, 145.79 (d,
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
EXPERIMENTAL SECTION
General. Commercial reagents were used as received. All
airꢀ and waterꢀsensitive manipulations were performed using
standard Schlenk techniques or under a nitrogen atmosphere
using a glovebox. Anhydrous solvents were obtained from an
Innovative Technology solvent drying system saturated with
Argon. Highꢀpurity polymer grade ethylene was obtained from
Matheson TriGas without further purification. Compound
J
CP = 50 Hz), 140.64, 136.89, 134.38 (d, JCP= 9.7 Hz), 132.03,
131.60, 129.60, 127.79 (d, JCP= 9.7 Hz), 127.48, 125.72,
124.82, 122.56, 120.86, 119.97, 112.89, 71.92, 70.24, 69.86,
69.22, 59.09, 28.66, 25.78, 22.64. 31P NMR (CDCl3, 162
MHz): δ (ppm) = 22.22. UVꢀvis (Et2O): λmax/nm (ε/cm–1M–1) =
340 (4870), 416 (3250). FTꢀIR: 2958 (vCNH), 1603 (vCN), 1445,
1436, 1222, 1108, 1093, 744, 729, 529 cmꢀ1. Mp (decomp.) =
~135°C. Anal. Calc. for C48H52NNiO4Pꢁ(C4H8O): C, 71.90; H,
6.96; N, 1.61. Found: C, 71.82; H, 6.56; N, 1.80. Trace
1A,53 HL0,54 NaBArF ,55 KBArF ,56 and NiBr(Ph)(PPh3)2
57
4
4
were prepared according to literature procedures. The syntheꢀ
ses of the HL/NaL ligands and LiBArF are given in the Supꢀ
4
porting Information.
Physical Methods. NMR spectra were acquired using
JOEL spectrometers (ECAꢀ400, 500, and 600) and referenced
using residual solvent peaks. 31P NMR spectra were referenced
to phosphoric acid. IR spectra were measured using a Thermo
Nicolet Avatar FTꢀIR spectrometer. Highꢀresolution mass
spectra were obtained from the mass spectral facility at the
University of Texas at Austin. Gas chromatographyꢀmass
spectrometry was performed using an Agilent 7890 GC/
5977A MSD instrument equipped with an HPꢀ5MS capillary
column. Solution samples for UVꢀvis absorption measureꢀ
ments were contained in 1 cm septum sealed quartz cuvettes
and recorded using an Agilent Cary 60 spectrophotometer.
Elemental analyses were performed by Atlantic Microlab.
1
amounts of diethyl ether, which was confirmed by H NMR
spectroscopy, could not be removed completely by vacuum
drying overnight.
Preparation of NiL3. The same procedure was used as
described for NiL0, except that NaL3 (123 mg, 0.27 mmol, 1
equiv.) was used instead of NaL0. The ligand was combined
with 1 equiv. of NiBr(Ph)(PPh3)2 (196 mg, 0.27 mmol). The
product was isolated as a yellow solid (208 mg, 0.25 mmol,
1
93%). H NMR (CDCl3, 600 MHz): δ (ppm) = 8.01 (d, JHP=
9.0 Hz, 1H), 7.61 (t, JHH= 9.0 Hz, 6H), 7.35 (t, JHH= 6.6 Hz,
3H), 7.25 (t, JHH= 7.2 Hz, 6H), 6.98 (t, JHH= 7.8 Hz, 1H), 6.93
(d, JHH= 7.2 Hz, 1H), 6.89 (m, 3H), 6.62 (d, JHH= 7.8 Hz, 2H),
6.46 (t, JHH= 7.8 Hz, 1H), 6.25 (t, JHH= 6.6 Hz, 1H), 6.10 (t,
JHH= 7.2 Hz, 2H), 3.90 (m, 2H), 3.63 (m, 2H), 3.57 (m, 2H),
3.50 (t, JHH= 4.2 Hz, 2H), 3.42 (s, 3H), 3.35 (m, 4H), 2.97 (t,
JHH= 6.0 Hz, 2H), 1.17 (d, JHH= 7.2 Hz, 6H), 1.14 (d, JHH= 6.6
Hz, 6H). 13C NMR (CDCl3, 150 MHz): δ (ppm) = 165.53,
158.68, 151.32, 149.72, 145.81 (d, JCP = 48 Hz), 140.68,
136.94, 134.43 (d, JCP= 10.35 Hz), 131.86 (d, JCP = 44 Hz),
129.67, 128.49, 127.86 (d, JCP= 8.85 Hz), 127.61, 125.80,
124.90, 122.62, 121.06 (d, JCP = 32 Hz), 120.07, 112.98,
72.09, 70.66, 70.62, 70.40, 69.92, 69.27, 59.21, 28.72, 25.84,
22.71. 31P NMR (CDCl3, 243 MHz): δ (ppm) = 22.23. UVꢀvis
(Et2O): λmax/nm (ε/cm–1M–1) = 340 (4400), 416 (2950). FTꢀIR:
2957(vCHN), 1602 (vCN), 1462, 1435, 1243, 1223, 1095, 742,
729, 692, 531 cmꢀ1. Mp (decomp.) = ~102°C. Anal. Calc. for
C50H56NNiO5P: C, 71.44; H, 6.71; N, 1.67. Found: C, 71.16;
H, 6.63; N, 1.62.
Synthesis
Preparation of NiL0. Inside the glovebox, NaL0 (91 mg,
0.27 mmol, 1.0 equiv.) and NiBr(Ph)(PPh3)2 (201 mg, 0.27
mmol, 1.0 equiv.) were combined in 15 mL of THF. The mixꢀ
ture was stirred at room temperature for 4 h. The resulting red
solution was filtered through a pipet plug and then dried under
vacuum to give a dark red oil. Upon the addition of pentane
and after stirring for ~5 min, a yellow solid formed. The solid
was isolated by filtration and then washed with fresh pentane.
The product was dried to yield a yellow solid (181 mg, 0.26
mmol, 94%). 1H NMR (CDCl3, 400 MHz): δ (ppm) = 7.97 (d,
JHP= 8.8 Hz, 1H), 7.53 (t, JHH= 8.4 Hz, 6H), 7.36 (t, JHH= 7.2
Hz, 3H), 7.24 (m, 6H), 6.95 (t, JHH= 7.6 Hz, 1H), 6.86–6.75
(m, 4H), 6.68 (d, JHH= 7.2 Hz, 2H), 6.44 (t, JHH= 7.6 Hz, 1H),
6.23 (t, JHH= 7.6 Hz, 1H), 6.11 (t, JHH= 7.6 Hz, 2H), 3.85 (m,
2H), 3.12 (s, 3H), 1.19 (d, JHH= 6.8 Hz, 6H), 1.10 (d, JHH= 6.4
Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ (ppm) = 165.67,
158.22, 152.80, 149.86, 146.09 (d, JCP = 49 Hz), 140.65,
137.56, 134.50 (d, JCP= 9.7 Hz), 131.59 (d, JCP = 44 Hz),
129.47, 127.79 (d, JCP= 9.7 Hz), 126.35, 125.67, 124.84,
122.52, 120.95, 119.53, 117.03, 112.83, 56.82, 28.65, 25.86,
22.63. 31P NMR (CDCl3, 162 MHz): δ (ppm) = 23.06. UVꢀvis
Preparation of NiL4. Inside the glovebox, NaL4 (68 mg,
0.13 mmol, 1.0 equiv.) and NiBr(Ph)(PPh3)2 (99 mg, 0.13
mmol, 1.0 equiv.) were combined in 10 mL of THF. The mixꢀ
ture was stirred at room temperature for 4 h. The resulting red
solution was filtered through a pipet plug and then dried under
vacuum to give a dark red oil. The product was washed with a
small amount of pentane to remove triphenylphosphine; howꢀ
ever, NiL4 is also somewhat soluble in pentane and trace
(Toluene): λmax/nm (ε/cm–1M–1)
2961(vCNH), 1604 (vCN), 1463, 1446, 1240, 1226, 1172, 746,
731, 692, 531cmꢀ1. Mp (decomp.) = ~140°C. Anal. Calc. for
C44H44NNiO2Pꢁ(C4H8O)0.15(CH2Cl2)0.2: C, 73.32; H, 6.32; N,
= 359 (3743). FTꢀIR:
8
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Journal of the American Chemical Society
amounts of triphenylphosphine (<5%) could not be removed standard acquisition parameters (120°C). The Mn values were
1
2
3
4
5
6
7
8
completely. The product was isolated as a red viscous material
and used without further purification (89 mg, 0.10 mmol,
~75%). H NMR (CDCl3, 400MHz): δ (ppm) = 7.96 (d, JHP=
determined using the method described by Daugulis and
Brookhart,58 and the total number of branches per 1000 carꢀ
bons (Nbranches) were determined by the method described by
Mecking and coworkers.59
1
8.8 Hz, 1H), 7.55 (t, JHH= 9.2 Hz, 6H), 7.33(m, 3H), 7.21 (m,
6H), 6.94 (t, JHH= 8.4 Hz, 1H), 6.88–6.83 (m, 4H), 6.57 (d,
JHH= 7.6 Hz, 2H), 6.42 (t, JHH= 7.6 Hz, 1H), 6.20 (t, JHH= 6.8
Hz, 1H), 6.05 (t, JHH= 7.6 Hz, 2H), 3.85 (m, 2H), 3.67ꢀ3.60
(m, 6H), 3.55 (m, 2H), 3.44 (t, JHH= 4.5 Hz, 2H), 3.38 (s, 3H),
3.29 (m, 4H), 2.91 (t, JHH= 5.6 Hz, 2H), 1.12 (d, JHH= 7.2 Hz,
6H), 1.09 (d, JHH= 6.8 Hz, 6H). 13C NMR (CDCl3, 100 MHz):
δ (ppm) = 165.47, 158.62, 151.25, 149.66, 145.78 (d, JCP = 49
Hz), 140.64, 136.87, 134.47 (d, JCP= 9.7 Hz), 132.01, 131.58,
129.61, 128.82, 127.76 (d, JCP= 9.8 Hz), 125.73, 124.83,
122.56, 120.98 (d, JCP = 21 Hz), 120.01, 112.89, 72.04, 70.69,
70.64, 70.61, 70.56, 70.34, 69.84, 69.22, 59.18, 28.67, 25.78,
22.64. 31P NMR (CDCl3, 162 MHz): δ (ppm) = 22.21. UVꢀvis
(Et2O): λmax/nm (ε/cm–1M–1) = 340 (4500), 416 (2930). FTꢀIR:
2857(vCHN), 1603(vCN), 1461, 1434, 1244, 1223, 1093, 741,
692, 530 cmꢀ1.
Analysis of branching ratio by quantitative 13C NMR
spectroscopy. The NMR samples contained ~10–20 wt% of
polymer and 50 mM chromium acetylacetonate Cr(acac)3 in
1,1,2,2ꢀtetrachlorethaneꢀd2 and were recorded at 150 MHz
(120°C). For solid polymers, the samples were acquired using
a 70° pulse of 9.25 ꢂs, a relaxation delay of 0 s, an acquisition
time of 0.67 s, and inverse gated decoupling. The T1 values of
the carbon atoms were measured to be 0.7 s. For amorphous
polymers, the samples were acquired using an 80° pulse of
10.58 ꢂs, a relaxation delay of 0 s, an acquisition time of 0.67
s, and inverse gated decoupling. The T1 values of the carbon
atoms were measured to be 0.4 s. The samples were preheated
for 15 min prior to data acquisition. The carbon spectra were
assigned based on the chemical shift values reported in the
literature.49 The branch ratios were determined by dividing the
integrated value for a type of branch end over the total number
of branches.
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Metal Titration Studies. Stock solutions of NiL and
MBArF salts (M = Li+, Na+, K+) were prepared inside of the
4
glovebox. The 500 ꢂM stock solutions of NiL were obtained
by dissolving 25 ꢂmol of NiL in 50 mL of Et2O. A 10 mL
aliquot of this 500 ꢂM solution was diluted to 50 mL using a
volumetric flask to give a final concentration of 100 ꢂM. The
Analysis of molecular weight (Mn, Mw) and polydispersity
(Mn/Mw) by gel permeation chromatography (GPC). GPC
analyses were performed using a Malvern high temperature
GPC instrument equipped with refractive index, viscometer,
and light scattering detectors. Polyethylene samples were preꢀ
pared with a concentration of ~30 mg of polymer in 10 mL of
solvent. The solid polymers were preꢀdissolved in decalin at
135°C for at least 1 h before injection, whereas the amorphous
polymers were preꢀdissolved in 1,2,4ꢀtrichlorobenzene (TCB)
at 135°C for at least 1 h before injection. Samples were acꢀ
quired at 140°C using TCB as the mobile phase. A calibration
curve was established with polystyrene standards. All the
samples measured yielded refractive index increments (dn/dc)
of 0.08–0.10, which are consistent with the reported value of
0.1 for polyethylene.60
3.0 mM stock solutions of MBArF were obtained by dissolvꢀ
4
ing 30 ꢂmol of MBArF in 10 mL of Et2O using a volumetric
4
flask. A 3.0 mL solution of NiL was transferred to a 1 cm
quartz cuvette and then sealed with a septum screw cap. A 100
ꢂL airtight syringe was loaded with the 3.0 mM solution of
MBArF . The cuvette was placed inside a UVꢀvis spectrophoꢀ
4
tometer and the spectrum of the NiL solution was recorded.
Aliquots containing 0.1 equiv. of MBArF (10 ꢂL), relative to
4
NiL, were added and the solution was allowed to reach equiꢀ
librium before the spectra were measured (~20–30 min). The
titration studies were stopped after the addition of up to 4.0
equiv. of MBArF . The spectral data were corrected for diluꢀ
4
Xꢀray Crystallography. Single crystals of NiL2Mes were
grown from vapor diffusion of pentane into a solution of the
complex in benzene, whereas single crystals of NiNaL3,
NiKL4, and Ni2NaL22 were grown from saturated solutions of
the complexes in Et2O/pentane. The crystals were mounted at
123 K on a Bruker diffractometer equipped with a CCD APEX
II detector using MoꢀKα radiation. Data reduction was perꢀ
formed within the APEX II software and empirical absorption
corrections were applied using SADABS. The structures were
solved by Direct methods in SHELXS and refined by fullꢀ
matrix least squares based on F2 using SHELXL. All nonꢀ
hydrogen atoms were located and refined anisotropically. Hyꢀ
drogen atoms were fixed using a riding model and refined
isotropically. Additional crystallographic details are provided
in the supporting information, including a summary of the
crystallographic data in Table S1.
tion and the binding model and binding constants were deterꢀ
mined using the program DynaFit.41 The data analysis proceꢀ
dure using DynaFit is provided in the Supporting Information.
Ethylene Polymerization. Inside the glovebox, solid NiL
(24 ꢂmol) and MBArF (26 ꢂmol) were dissolved in 5 mL of
4
toluene and stirred for 30 min. Solid Ni(COD)2 (48 ꢂmol) was
added and the solution was transferred to a FischerꢀPorter
glass vessel along with a magnetic stir bar and then the reactor
was sealed. The highꢀpressure apparatus was removed from
the glovebox and then securely fastened on top of a stir plate.
The ethylene line was attached and the reactor was purged
with ethylene three times by pressurizing with ethylene and
then releasing the pressure. The reactor was then pressurized
to 100 psi of ethylene and stirred at RT for a specified amount
of time. The ethylene line was closed and the vessel was slowꢀ
ly vented. About 1 mL of HCl(aq) was added, followed by the
addition of 2 mL of MeOH. The aqueous layer was removed
by pipetting and the organic layer was evaporated to dryness
under vacuum. The resulting material was washed with MeOH
and CH2Cl2 and then dried under vacuum.
ASSOCIATED CONTENT
Supporting Information. Synthesis procedures and characterizaꢀ
tion, metal titration data and fitting plots, Xꢀray crystallographic
summary. The Supporting Information is available free of charge
on the ACS Publication website at DOI: XXX.
Polymer Characterization. Analysis of molecular weight
(Mn) and total branching by 1H NMR spectroscopy. The NMR
samples contained ~10–20 wt% of polymer in 1,1,2,2ꢀ
tetrachlorethaneꢀd2 and were recorded at 600 MHz using
9
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(27) Takano, S.; Takeuchi, D.; Osakada, K.; Akamatsu, N.;
AUTHOR INFORMATION
Corresponding Author
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* loido@uh.edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors acknowledge the American Chemical Society–
Petroleum Research Fund (ACSꢀPRF #54834ꢀDNI3) and the Uniꢀ
versity of Houston new faculty startꢀup for funding this work. The
authors also thank Prof. Olafs Daugulis (UH) for helpful suggesꢀ
tions.
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