Synthesis and structural characterization of a nickel(II) precatalyst bearing a β-triketimine ligand and study of its ethylene polymerization performance using response surface methods

Article abstract of DOI:10.1002/pola.26522

The reaction of N-(4-(mesitylamino)pent-3-en-2-ylidene)-2,4,6- trimethylbenzenamine (1) with n-butyl lithium and then with N-(2,4,6-trimethyl- phenyl)-acetimidoyl chloride yields a new β-triketimine ligand, N-(4-(mesitylamino)-3-(1-(mesitylimino)ethyl)pen

Full text of DOI:10.1002/pola.26522

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Synthesis and Structural Characterization of a Nickel(II) Precatalyst
Bearing a b-Triketimine Ligand and Study of Its Ethylene Polymerization
Performance Using Response Surface Methods
Nona Ghasemi Hamedani,1 Hassan Arabi,1 Gholam Hossein Zohuri,2
Francis S. Mair,3 Andrew Jolleys3
1Department of Polymerization Engineering, Iran Polymer and Petrochemical Institute, P.O. Box: 14965/115, Tehran, Iran
2Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, P.O. Box: 91775, Mashhad, Iran
3School of Chemistry, University of Manchester, Brunswick Street, Manchester M13 9PL, United Kingdom
Correspondence to: H. Arabi (E-mail: h.arabi@ippi.ac.ir) or F. S. Mair (E-mail: francis.s.mair@manchester.ac.uk)
Received 26 August 2012; accepted 30 November 2012; published online
DOI: 10.1002/pola.26522
ABSTRACT: The reaction of N-(4-(mesitylamino)pent-3-en-2-yli-
dene)-2,4,6-trimethylbenzenamine (1) with n-butyl lithium and
then with N-(2,4,6-trimethyl-phenyl)-acetimidoyl chloride yields
a new b-triketimine ligand, N-(4-(mesitylamino)-3-(1-(mesitylimi-
no)ethyl)pent-3-en-2-ylidene)-2,4,6-trimethylbenzenamine, 2. The
of responses (catalyst activity, crystallinity, and weight-average
molecular weight of polymer (Mw)) and visualized via the
response surface method (RSM). Activity and Mw responses
show a second-order variation with temperature and vary line-
arly with pressure. Conversely, crystallinity follows a second-
order model while varying temperature, pressure, and CC. Fur-
thermore, a set of polymerization conditions for reaching desira-
ble responses was predicted and then experimentally verified.
The activities achieved challenge the best reported activities for
Ni(II) catalysts with b-connected imine ligand supports, but fall
addition of
2 to nickel (II) dibromide 1,2-dimethoxyethane
(NiBr2(DME)) in the presence of [Na]þ[3,5-(CF3)4C6H3]4B]ꢀ
(NaBAr’4) gives a five-coordinate dimeric complex [(2.NiBr)2].2
[(BAr’4)], 3. The structure of 3 has been determined by single
crystal X-ray diffraction. This complex generates catalytically
active species for the homopolymerization of ethylene in combi-
nation with methylaluminoxane to produce elastomeric,
branched polyethylene. The effect of factors (temperature, pres-
sure, and cocatalyst to catalyst molar ratio (CC)) on the polymer-
ization process has been investigated using regression models
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short of those for a-diimines.
2013 Wiley Periodicals, Inc. J
Polym Sci Part A: Polym Chem 000: 000–000, 2013
KEYWORDS: b-triketimine ligand; Ni polymerization catalysts;
polyethylene (PE); response surface method
INTRODUCTION Polyolefins are a range of commodity mate-
rials with major economic implications. The range of polyole-
fin products will grow steadily to meet the increasingly so-
phisticated needs of consumers.1 Therefore, the requirement
of designing new catalysts for tailoring of bulk polyolefin
properties is a major focus of many industrial and academic
research groups. The design of new catalysts initially focused
on early transition metal systems. Recently, however there
has been a shift to an increased emphasis on the develop-
ment of late transition metal complexes. This shift is based
largely on reduced oxophilicity, high polar functional group
tolerance, and also the ease with which large ligand libraries
and catalyst of late transition metal systems can be gener-
ated.2 With the late transition metal system, it was not only
feasible to prepare known polyolefinic materials under gen-
tle conditions (i.e., low pressure and temperature) but also
new materials with well-defined molecular characteristics,
including a variety of functionalities.3–9 Due to the influence
of the ligand framework on the reactivity of the metal center,
the design of the ligand is a key step in the development of
new catalyst systems. A large number and range of ligands
(bi, tri, and tetra dentate organic molecules) for use in late
transition metal complexes have been reported.10 But the
field of designing ligand structure was revitalized by the
seminal work reported by Brookhart and Gibson on these
systems. Brookhart and coworkers’11 ortho-bulky a-diimine
nickel catalysts offer a relatively unconfined five-membered
ring structure, 4 (Scheme 1). Gibson’s iron-based catalysts
used a tridentate ligand, sharing the bis-imino structure of
the Brookhart systems but containing an additional interac-
tion through a backbone integrated pyridine, 5 (Scheme 1).
This led to a more planar and rigid ligand geometry in addi-
tion to substantial changes to the electronics of the metal
center; the N,N,N ligand set was presented to the metal in a
meridional arrangement.12 The simplicity and symmetry of
these structures was striking and led to great flexibility in
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Related ligand and complex structures.
ligand design, with much work focused on introducing new
ligand families that share some structural similarities to the
Brookhart and Gibson systems. More recent attention has
focused on provision of additional locations from which to
incorporate steric and electronic modifications.13
makes it susceptible to O-coordination by Lewis acids, which
can be used to activate the metal center through remote loca-
tions.21,22 This generated catalysts with much-improved activ-
ities of several hundred kg [(mol Ni)ꢀ1 hꢀ1 barꢀ1],21 associ-
ated with much greater catalyst stability, including claims of
living character at high temperatures.22 In a similar vein,
nickel precatalysts bearing (N-imidoylamidine) ligands have
recently been reported.23 The variation of the electron with-
drawing/donating capacity of the substituted phenyl ring on
the central nitrogen of the ligand was found to influence po-
lymerization activity of the complexes, with optimal values
reaching over 100 kg [(mol Ni)ꢀ1 hꢀ1 barꢀ1].23
The b-diketiminate class of ligands, generally denoted as
‘‘nacnac,’’ or {ArNC(R)}2CH (where Ar ¼aryl and R¼CH3 or
bulkier group) was originally introduced in the 1968,14 but
attracted sustained interest only with the introduction of
diaryl diketiminates possessing extreme ortho-bulk,15
a
requirement of applications in polymerization, as it hinders
chain-transfer and b-hydride elimination processes, while still
allowing the b-hydride transfer and readdition sequences
that result in chain walking. The first demonstration of their
use as N,N bidentate, neutral ligands for NiBr2 was accompa-
nied by reports of very modest polymerization activity, of
less than 1 kg [(mol Ni)ꢀ1 hꢀ1 barꢀ1].15 The same ligand, but
in deprotonated, monoanionic form, was almost contempora-
neously reported, in an article which highlighted the remark-
able degree of steric control the ligands offered.16 They sub-
sequently generated wide employment through their ability
to stabilize unique coordination environments and to support
reactive organometallic reagents, especially those in low oxi-
dation states and with low co-ordination numbers, as is
required for polymerization catalysis.17,18 When ligands such
as 1 were used in anionic form as diketiminates, modest
improvement in activity, to 18 kg [(mol Ni)ꢀ1 hꢀ1 barꢀ1],
was found.19 Fluorination of the backbone produced further
improvement, to 38.6 kg [(mol Ni)ꢀ1 hꢀ1 barꢀ1].20 Only most
recently, through the incorporation of further functional
groups which confer the ability to remotely modify the elec-
tronic properties of the metal center without significantly
altering the steric environment, have activities competitive
with a-diimine complexes such as 4 been achieved. In the
case of 6, usage of an a-keto-b-diketimine ligand framework
We recently reported on a new class of ligands, derived from
Nacnac 1, termed the b-triketimines, 2 (Scheme 1), neutral
and tridentate, as for the mer ligand in 5, but fac capping, as
shown in the products of their reaction with M(CO)3 [M ¼
Cr, Mo, W].24 This family of ligands with finely tunable bulk
have wide potential in coordination chemistry; a number of
their nickel complexes have been prepared, as neutral com-
plexes and as cations, such as 3a (Scheme 1), which when
paired with large anions and activated with methylaluminox-
ane (MAO), show activity as catalysts for polyethylene (PE)
polymerization.25
The systems share the tendency of other nickel catalysts to
show complex behavior in polymerization, with short-chain
branching accompanying linear chain propagation,17 in ratios
dependent on temperature, cocatalyst to catalyst ratio, and
pressure. This complex interdependent behavior is appropri-
ate for study by experimental design methods such as
response surface method (RSM). This method identifies any
synergistic effect of the reaction parameters on polymeriza-
tion performance and polymer properties; it provides the
best information regarding the effects of independent varia-
bles (termed ‘‘factors,’’ e.g., temperature, pressure, etc.) and
2
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their interactions on observable outputs (termed ‘‘model pa-
rameters,’’ e.g., activity, Mw, etc.) with the minimal number of
experimental runs.26 Although there has been use of RSM to
evaluate the effects of polymerization controlling factors in
Ziegler Natta and metallocene systems,26–29 the work
reported herein represents its first application to the field of
late transition metal polymerizations.
All reagents were used as received from Aldrich and Merck
unless otherwise specified. Hexane was distilled immediately
before use from Na/K alloy, Tetrahydrofuran (THF) and Et2O
from sodium/benzophenone and dichloromethane (DCM)
from calcium hydride. Triethylamine and 2, 4, 6-trimethylani-
line was dried over calcium hydride. Polymerization grade
ethylene with high purity was obtained from Arak Petro-
chemical Co. (Arak, Iran) and was further purified by pas-
sage through an oxygen/moisture trap. Industrial toluene for
polymerization was obtained from Arak Petrochemical Co.
(Arak, Iran) and was distilled over sodium wire. The starting
compounds N-(2, 4, 6-trimethyl phenyl acetimidoyl chloride
and NaBAr’4 ([Na]þ[3,5-(CF3)4C6H3]4B]ꢀ) were prepared
according to literature procedures.24,31
The main objective of this work is to demonstrate the usage
of RSM in charting the complex behavior of nickel-catalyzed
polymerizations, while investigating the effect of three
critical factors [temperature, pressure, and cocatalyst to
catalyst molar ratio (CC)] on polymerization behavior of one
representative example of an MAO-activated nickel(II) b-tri-
ketimine complex, 3. This is the first report in the open liter-
ature of nickel b-triketimine complexes and their potential
as polymerization catalysts. Furthermore, we report the
development of catalyst activity, polymer molecular weight,
and polymer crystallinity regression models using RSM.
Synthesis
2,4,6-Trimethyl-N-[4-(2,4,6-trimethylphenyl)
iminopent-2-en-2-yl]aniline, 1
1 was produced by a modification of a literature method:32
2,4,6-trimethylaniline(30 mL,0.21 mol) and acetyl acetone
(10 mL, 0.1 mol) were stirred in toluene (100 mL) and p-tol-
uene sulfonic acid (ꢁ 0.05 g) was added. The solution was
refluxed for 5 h using a Dean-Stark azeotropic distillation
trap to remove the fractions of amine and toluene (55–61
ꢂC) and yellow oil (enamineone) at 103–126 ꢂC. The dark
brown solid obtained was recrystallized with methanol and
DCM. The Product 1 was isolated as white crystals. Yield:
RSM
RSM is a collection of mathematical and statistical techni-
ques that can be used for studying the effect of independent
variables (factors) and their influence on each other. Further-
more, it models a relationship between the factors and the
‘‘response’’ (the measurable outputs). The models can be
used to generate surface and contour plots that provide effi-
cient visualization of the parameter interaction. The objective
of RSM is to allow a user to find the optimum values of the
input ‘‘factors,’’ which will deliver the desired outputs
(‘‘response’’).30 In this experiment, the response surface
design developed was based on Box–Behnken design; accord-
ingly, a quadratic regression model was used to develop sec-
ond-order response surface models. The general form is as
shown in eq 1:
1
ꢂ
30.74%, mp: 60 C, H NMR (400 MHz, CDCl3, d, ppm): 1.88
(s, 12H), 2.10 (s, 6H), 2.20 (s, 6H), 4.76 (s, 1H); 6.85(s, 2H),
7.40 (s, 2H), 12.05 (br s, 1H). Anal. calcd for C23H30N2: C,
82.58; H, 9.05; N, 8.37. Found: C, 82.41; H, 8.80; N, 8.38.
Data concurred with those reported.32
N-(4-(Mesitylamino)-3-(1-(mesitylimino)ethyl)
pent-3-en-2-ylidene)-2,4,6-rimethylbenzenamine, 2
To a stirred solution of 1 (3.42 g, 0.0102 mol) in hexane (20
mL) in a Schlenk tube was added N-butyl lithium (6.4 mL of
a 1.6 M solution in hexanes, 0.0102 mol) under a nitrogen
n
n
n
X
X
X
R ¼ b0 þ
bixi þ
biixi2 þ
bijxixj
(1)
i¼1
i¼1
i<j
ꢂ
atmosphere. The solution was stirred at 50 C for 5 min and
then N-(2,4,6-trimethyl phenyl acetimidoyl chloride (2 g,
0.0102 mol) was added, and the reaction stirred at room
temperature for 48 h. The solution was washed three times
with deionized water, and the organic layer was separated,
dried over MgSO4, and the solvent removed. The residue was
recrystallized from methanol and DCM, and the final product
was isolated as pale yellow crystals of 2. Yield: 34%, mp:
132 C. H NMR (400 MHz, CDCl3, d, ppm): (solution compo-
sition immediately after dissolution: enamine-diimine tauto-
mer (E-isomer) 87%, b-triketimine tautomer 13%; solution
composition ꢁ 24 h after dissolution: enamine-diimine tauto-
mer (E-isomer) 66%, b-triketimine tautomer 34%; peaks
due to both isomers (unless otherwise specified): 1.61, 1.85
(2 s, 18H), 1.91, 2.15 (2 s, 9H), 2.18 (s, 9H), 6.80, 7.20 (2 s,
6H), 4.70, 13.20 (2s, 1H, a-CH of b-triketimine tautomer, NH
of enamine-diimine tautomer). 13C NMR (100 MHz, CDCl3, d,
ppm) (peaks due to both isomers, unless otherwise speci-
fied): 18.47, 18.51, 20.72, 20.81, 20.87, 24.91 (CH3CN, 4 ꢃ
Ar-CH3), 72.18 (a-CH, b-triketimine tautomer), 108.42 (alke-
nyl a-C, enamine-diimine tautomer), 125.63, 128.56, 128.66,
where R denotes the predicted response of the process, xi
refers to the coded factors (temperature, xT, pressure, xP, and
cocatalyst to catalyst ratio, xC) and b0, bi, bii, bij are regres-
sion coefficients. From this general form, three different
equations were derived, one for each of the three observable
responses, RA (activity), RMw (weight-average Molar mass),
and RXtl (percent crystallinity). After performing the experi-
mental test runs and determining the coefficients of the
models, insignificant coefficients for terms which did not
influence the response variation were removed from each of
the fitted models. Such reduction simplified the regression
model while maintaining its high accuracy. The statistical
basis for this process is detailed in Supporting Information.
1
ꢂ
EXPERIMENTAL
Materials
All manipulations involving air or water sensitive com-
pounds were performed under an inert atmosphere using
standard high vacuum Schlenk line techniques and glovebox.
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TABLE 1 Experimental Design of Polymerization Process
Co-catalyst to
129.12, 131.78, 132.10, 146.17, (7 ꢃ aromatic carbon),
159.74 (conjugated C¼¼N, enamine diimine tautomer), 169.20
(C¼¼N, b-triketimine tautomer), 172.19 (C¼¼N, enamine dii-
mine tautomer). IR (KBr, powder, cmꢀ1): 1640, 1662 (C¼¼N),
1537 ((C¼¼C) aromatic). HRMS (m/z): calcd for C34H43N3,
493.72; Found, 494.4[MþH]þ. Anal. calcd for C34H43N3: C,
82.71; H, 8.77; N, 8.52. Found: C, 82.79; H, 8.63; N, 8.18.
Run
Temperature
(ꢂC)
Pressure
(bar)
Catalyst Molar
Ratio (CC)
Number
1
10
50
10
50
10
50
10
50
30
30
30
30
30
30
30
1
1
5
5
3
3
3
3
1
5
1
5
3
3
3
2000
2000
2000
2000
1000
1000
3000
3000
1000
1000
3000
3000
2000
2000
2000
2
3
Nickle(II) b-Triketimine Complex, 3
4
NaBAr’4 (0.89 g, 0.001 mol), 2 (0.49 g, 0.001 mol), and
NiBr2(DME) (0.309 g, 0.001 mol) were added to a Schlenk
tube under a nitrogen atmosphere. THF (10 mL) was added
and the reaction stirred for 24 h. The solvent was removed
in vacuo and DCM (20 mL) was added to the reaction mix-
ture. The dark obtained solution was filtered through celite
under a nitrogen atmosphere. The celite plug was washed
twice with DCM (10 mL). The combined DCM extracts were
then concentrated by 40% of their volume in vacuo, and
then layered with distilled hexane (60 mL) and left to pre-
cipitate for 72 h. Dark green crystals formed which were iso-
lated by filtration, washed with hexane, and dried in vacuo.
Yield: 47%. IR (KBr, powder, cmꢀ1): 1630, 1658 (C¼¼N),
5
6
7
8
9
10
11
12
13a
14a
15a
a
1610 ((C¼¼C) aromatic). Anal. calcd for
C132H110N6
Replication of center point-related run in Box–Behnken design space.
Ni2Br2F48B2 : C, 53.02; H, 3.67; N, 2.80; Br, 5.34. Found: C,
52.82; H, 3.63; N, 2.80; Br, 5.31.
tors were chosen based on preliminary results with this cat-
alyst.25 The experimental plan generated using the Minitab
Ethylene Polymerization Procedure
R
V
15 software involved 13 runs. Also, to provide an estimate
of the experimental error in the process and achieve more
precise estimates of the factor effects, the center point, that
is, the midpoint between the high and low levels, was repli-
cated twice. Therefore, in total, the software suggested 15
test runs which are shown in Table 1.
Ethylene polymerization was performed in a 300-mL stain-
less steel reactor containing systems for full control of tem-
perature and reaction pressure. Before starting, the reactor
ꢂ
was warmed up to 100 C, purged with argon and vacuumed
sequentially. It was repeated several times to remove oxygen
and humidity. For start-up, the reactor was cooled down
while kept purging with argon. The reactor was then
charged with toluene (150 mL) and saturated with ethylene.
The cocatalyst was added (in an amount as recorded in
Table 1), and the ethylene pressure increased to 1 bar. The
appropriate amount of catalyst was suspended in 3 mL of
toluene and rapidly added to the stirring solution in the re-
actor. It was then sealed and pressurized to the software
suggested ethylene pressure, and the solution was stirred for
1 h. The polymerization was terminated by venting the reac-
tor followed by quenching the mixture with 100 mL of acidi-
fied methanol (HCl, 1 vol %). The precipitated polymer was
filtered, washed twice with 100 mL of methanol, and dried
in vacuo at 40 ꢂC for several hours. If the polymer did not
precipitate from the quenched reaction mixture, it was iso-
lated by solvent evaporation on a rotary evaporator, followed
by washing with acidified methanol and drying in vacuo.
The responses measured (i.e., the model parameters) were
RA (catalyst activity), RMw (weight-average Molar mass), and
RXtl (percent crystallinity).
Characterization
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance III 400 MHz or Bruker Avance II 500 MHz spectrom-
eter. IR spectra were recorded on a Perkin-Elmer Spectrum
RX1 Fourier transform infrared (FTIR) spectrometer using
Nujol mulls between KBr plates or on a Perkin-Elmer Spec-
trum BX FTIR spectrometer using neat solids. Elemental
analysis measurements were performed by CHNSO Elemen-
tar Analyzer (Vario EL ???). Melting points of molecular com-
pounds was determined using an Electrothermal melting
point apparatus in open capillary tubes. Differential scanning
calorimetry (DSC) of polymers was performed using a Met-
tler-Toledo model 822e instrument, interfaced to a digital
computer equipped with Star E 9.01 software (SencorꢂFRS5).
Samples were heated from room temperature to 170 C at a
rate of 10 ꢂC minꢀ1 and held there for 2 min, followed by
cooling to ꢀ120 ꢂC at a rate of 10 ꢂC minꢀ1. Finally, the
samples were reheated to 170 ꢂC using the same heating
rate. The melting point and crystallinity were determined
according to the results obtained from the final step. Crystal-
linity was calculated by integration of the second heat
Response Surface Experimental Design
To create a response surface design, the type of design, the
number of independent variables (or ‘‘factors’’), the name of
the factors, their upper and lower levels, and replication
points must be entered into the software. In this Box–
Behnken design, the three factors were temperature (xT:
lower level 10, midpoint 30, higher level 50 ꢂC), pressure
(xP: levels 3, 5, and 7 Bar), and cocatalyst to catalyst ratio,
CC (xC: levels 1000, 2000, and 3000). The levels of these fac-
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SCHEME 2 Synthesis of 2.
melting endotherm and reported as a percentage based on
the value 293 J gꢀ1 for 100% crystalline PE. High tempera-
ture gel permeation chromatography (GPC) was performed
by Varian (PL-GPC220) at 145 ꢂC in trichlorobenzene with
polystyrene standards. Crystals suitable for X-ray diffraction
were obtained by careful layering of hexane onto a saturated
DCM solution of complex. Single crystals were mounted in
perfluoropolyether oil into an Oxford Instruments Cryo-
stream 700. Diffraction measurements were performed on
an Oxford Diffraction X-Calibur 2 diffractometer using graph-
ite-monochromated Mo-Ka radiation, and the data were col-
lected and processed by the programs CrysAlis PRO and Cry-
sAlis RED.33 The structure was solved using SHELXS34 and
refined with SHELXL.34 Full experimental details, bond
lengths and angles, and atomic co-ordinates are available as
Supporting Information.
forms, as well as E/Z-isomerism in the pendant imine of the
enamine-diimine tautomer.24 The substituent pattern on
three aryl groups has a significant effect on the position of
the equilibria: for 2 the major component in solution imme-
diately after dissolution was found to be the E-isomer of the
enamine-diimine tautomer, with the relative amount of the
b-triketimine tautomer increasing over time. Due to steric
repulsion, the Z-isomer is very unlikely to occur.24 This flux-
ional behavior of compounds in solution has also been
observed for some a-diimine and b-diimine ligands.15,36
The synthesis of 3 by addition of 2 to NiBr2(DME) in the
presence of NaBAr’4 in THF is depicted in Scheme 3. Crystal-
lization from DCM and hexane allowed isolation of the prod-
uct as dark green crystals in 47% yield. Single crystals of 3
suitable for X-ray diffraction studies were obtained by vapor
diffusion of hexane into a DCM solution under nonanhydrous
conditions. The resulting molecular structure (cation only) is
shown in Figure 1. The crystallographic parameters and
crystal data and refinement are summarized in Tables 2 and
3, respectively. The molecular structure features a centro-
symmetric dimeric bromide bridged dication, in which the
nickel centers are in a slightly distorted square-pyramidal
geometry. The bond distances in the X-ray structure of 3
reveal that the NiAN bond distances are noticeably longer
than corresponding distances seen in other four-coordinate
nickel complexes of tridentate b-triketimines which is simply
as a result of the increased coordination number at nickel.25
The increase in coordination number is also responsible for
the fact that the NAN separation of 2.887(4) Å is somewhat
larger than the corresponding distances seen in the four-
coordinate nickel complexes. The basal Ni(1)-N(1) and Ni(1)-
RESULTS AND DISCUSSION
Synthesis and Structure of the Catalyst
The b-triketimine ligand 2 was synthesized by reaction of b-
diketimine 1 with n-BuLi giving a lithium diketiminate which
was then reacted with acetimidoyl chloride, giving pale yel-
low crystals of 2 in good yield after recrystallization from
methanol and DCM (Scheme 2).24 This reaction appears to
proceed with good chemoselectivity for CAC bond formation
as was previously found for other electrophiles.35 The ligand
was fully characterized by 1H and 13C NMR spectroscopy as
well as elemental analysis. The solution-phase behavior of b-
triketimines is complicated by the presence of equilibria
between numerous isomeric species, as a result of tautomer-
ism between the true b-triketimine and enamine-diimine
SCHEME 3 Synthesis of 3.
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The color in solutions of 3 is not the same as that of the crys-
tal. It is likely that the dimeric structure is broken down to
monomers in solution, as has been observed for diketiminate
analogs.19 This is even more probable after reaction with MAO,
during which the bromides are replaced with the less-effec-
tively bridging ligand, methyl. Whether the active form of the
complex remains tridentate on co-ordination by ethylene is
not known. There are reports of tridentate scorpionate com-
plexes of nickel with some polymerization activity;36 however,
in our case, we suspect that there may be decomplexation of
one imine arm, to provide sufficient space for attachment of
ethylene; the detached arm then becomes a point for reaction
with or co-ordination by MAO, in a manner analogous to 6.
We will present further evidence in support of this hypothesis
in a following article. For the moment, we will focus on the
use of RSM for studying the catalyst performance on ethylene
polymerization. Specific operational conditions can either favor
FIGURE 1 ORTEP representation of the crystal and molecular
structure of the dicationic unit in 3 at 60% probability. Hydro-
gen atoms are omitted.
˚
N(2) distances of 2.083(3) Å and 2.126(3) Å, respectively,
are somewhat longer than the axial Ni(1)-N(3) distance
(2.038(3) Å), which is likely due to the trans-influence of the
two bridging bromide ligands. Additionally, The C¼¼NACAr
bond angles all lie close to the sp2 ideal of 120ꢂ, as would
be expected for an all-methyl backbone substituted ligand.17
The structure of 3 reported here represents the first struc-
turally characterized dicationic [{LNi(l-X)}2]2þ complex for
any halide X and any tridentate neutral ligand L.
TABLE 3 Selected Bond Distances (A) and Angles (8) for the
Dicationic Units in 3
Bond Distances
Br(1)-Ni(1)
Br(1)0-Ni(1)
2.4969(6)
2.5072(5)
2.083(3)
2.126(3)
2.038(3)
1.283(4)
1.276(4)
1.282(4)
1.455(4)
1.454(4)
1.451(4)
1.536(4)
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
N(1)-C(2)
TABLE 2 Crystallographic Data Collection and Refinement
Details in 3
N(2)-C(13)
N(3)-C(24)
Formula
C132H110Br2N6Ni2B2F48
2991.06
Triclinic
P-1
N(1)-C(4)
Formula weight
Crystal system
Space group
N(2)-C(15)
N(3)-C(26)
C(1)-C(2)
˚
a [A]
14.5663(5)
14.8071(6)
16.3965(6)
82.985(3)
85.599(3)
69.118(4)
3277.4(2)
1
Bond Angles
Ni(1)-Br(1)-Ni(1)0
Br(1)-Ni(1)-Br(1)0
Br(1)-Ni(1)-N(1)
Br(1)-Ni(1)-N(2)
Br(1)-Ni(1)-N(3)
Br(1)0-Ni(1)-N(1)
Br(1)0-Ni(1)-N(2)
Br(1)0-Ni(1)-N(3)
N(1)-Ni(1)-N(2)
N(1)-Ni(1)-N(3)
N(2)-Ni(1)-N(3)
Ni(1)-N(1)-C(4)
Ni(1)-N(2)-C(15)
Ni(1)-N(3)-C(26)
C(2)-N(1)-C(4)
C(13)-N(2)-C(15)
C(24)-N(3)-C(26)
˚
b [A]
98.70(2)
81.30(2)
˚
c [A]
a [ꢂ]
b [ꢂ]
c [ꢂ]
159.16(7)
93.69(8)
113.59(8)
94.64(7)
3
˚
Volume [A ]
Z
166.39(7)
103.00(7)
85.55(10)
87.25(11)
90.61(10)
121.70(19)
123.90(19)
123.69(19)
121.1(3)
Dcalc [g cmꢀ3
]
1.515
l [mmꢀ1
T [K]
]
1.017
100
y Range [ꢂ]
2.9–26.4
19,471
Reflections measured
Unique reflections (Rint
Obs. data [I > 2.0 r(I)]
R (observed)
)
13,225 (0.033)
7889
0.0408
wR2a
0.0859
119.3(3)
S
0.86
118.8(3)
a
wR2 ¼ {r[w(Fo2 – Fc2)2]/r[w(Fo2)2]}1/2.
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TABLE 4 Experimental Values for Activity, Mw, and Crystallinity
Independent Variables (Factors)
Experimental
Run
Activity (kg
[(mol Ni)ꢀ1
Crystallinity
(%)
xT (ꢂC)
xP (bar)
xC
h
ꢀ1])
Mw (g molꢀ1
)
PDI
1
10
50
10
50
10
50
10
50
30
30
30
30
30
30
30
1
1
5
5
3
3
3
3
1
5
1
5
3
3
3
2000
2000
2000
2000
1000
1000
3000
3000
1000
1000
3000
3000
2000
2000
2000
40
30
1,20,000
1,92,815
2,71,600
5,78,984
2,30,823
4,49,778
1,57,870
4,11,656
3,56,399
4,54,516
4,24,259
5,87,144
4,50,861
4,53,689
4,07,616
1.58
2.18
1.68
2.60
1.70
2.39
1.77
2.75
2.55
2.98
2.16
2.79
2.25
2.29
2.27
28.24
1.90
2
3
100
137
65
29.79
7.60
4
5
31.1
6
50
5.07
7
70
32.08
5.61
8
50
9
130
250
50
9.84
10
11
12
13
14
15
13.05
12.70
13.60
12
370
240
220
212.5
11.07
10.55
propagation over chain-branching, leading to moderately crys-
talline plastomeric PE, or vice versa, giving more highly
branched, low-crystallinity elastomer.37 The prospect of having
sufficient knowledge of the dependence of polymer properties
on experimental factors to be able to select desired values of
branching, hence crystallinity, is attractive.
The response contour and surface plots were generated
R
V
using Minitab 15 software to reveal the effect of tempera-
ture, pressure and CC on the activity, Mw, and crystallinity.
Figure 2 depicts the relations of activity with pressure, tem-
perature and CC. In Figure 2(a), the cocatalyst ratio is held
constant at 3000. The U-shaped curve indicates two compet-
ꢂ
ꢂ
ing processes. From 10 C to ꢁ 25 C, there is normal Arrhe-
nius behavior, the rate of propagation steeply increasing
with temperature. However, activity passes through a maxi-
Estimated Regression Models
The performance of the nickel (II) b-triketimine complex 3
was evaluated in the ethylene polymerization using MAO as
cocatalyst. Table 4 indicates the experimental values of
responses namely, activity, weight-average molecular weight
(Mw), and crystallinity, according to the experimental plan.
Stepwise regression was applied using backward elimination
methods to exclude insignificant coefficients from the initial
response surface models.
ꢂ
mum at 30 C. Figure 2(a) shows that this maximum is inde-
pendent of pressure, so that the maximum activity {291.86
kg [(mol Ni)ꢀ1 hꢀ1]} appears at 30 C after raising pressure
ꢂ
and CC to 5 bar and 3000, respectively [Fig. 2(a,b)]. The rea-
son for the drop in activity beyond 30 ꢂC must be catalyst
de-activation through a thermally activated process which is
zero-order in monomer. This reaction must have a higher
activation energy than propagation. Decreasing ethylene sol-
ubility due to the rising temperature could also play a role,
but catalyst decay clearly has the major effect on decreasing
the activity. In this respect, Catalyst 3 is similar to previously
reported nickel-based catalysts, though the particular value
of temperature which is optimum must be determined for
each catalyst.37
By reducing eq 1, three partial quadratic models with
adequate goodness-of-fit were obtained for the responses of
activity (RA), weight-average molecular weight (RMw), and
crystallinity (%) (RXtl) as shown in eqs 2–4, respectively.
RA ¼ 222:3 ꢀ xT þ 75:9xP þ 5:6xC ꢀ 144:1xT2
ꢀ 20:8xP2 þ 50xPxC
ð2Þ
Figure 2(b) also shows the U-shaped dependence of activity
on temperature versus CC, with no effect of CC on the opti-
mum temperature. This does not mean that co-catalyst is not
involved in the de-activation; it is most-probably an MAO-
mediated reductive process, but over the short range of CC
studied, the pre-equilibrium is saturated, that is, MAO is in
such large excess that the process will appear pseudo-first
order in catalyst. It is also apparent from this figure that the
activity showed a rigorously linear increase with reaction
pressure. This is also clear in Figure 2(c). This shows that
propagation is first-order in monomer, at all pressures. This
RMW ¼ 447783 þ 106618xT þ 99846xP þ 11177xC
ꢀ 146093xT2 þ 58642xTxP
ð3Þ
ð4Þ
Rxtl ¼ 11:2 ꢀ 12:6xT þ 1:4xP þ 0:6xC þ 5:9xT2 ꢀ 0:2xP2
þ 1:3xC2 þ xTxp ꢀ 0:5xPxC
Details of the parameter reduction, goodness of fit, analysis
of variance (ANOVA table), and probability plot of residuals
are appended in Supporting Information.
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FIGURE 2 Contour and surface plots of activity-related regression model at constant level of: (a) CC; (b) pressure; (c) temperature.
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FIGURE 3 Contour and surface plots of Mw-related regression model at constant level of: (a) CC; (b) pressure; (c) temperature.
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SCHEME 4 Possible chain-transfer processes limiting Mw.
is in contrast to kinetic studies on catalysts similar to 4,
with bidentate a-diimine ligands, which found first-order
behavior only at low pressures. At higher pressures, rate
was inhibited, a fact ascribed to co-ordination of a further
monomer molecule.38 It is possible that the better encapsula-
tion of the active site offered by the b-connected imines (see
Scheme 1), or perhaps the additional co-ordinating imine of
the tridentate ligand in 3, is responsible for this difference.
Given that this is the first quantitative kinetic study of any
b-connected diimine-supported nickel-centered catalyst with
pressure as a variable, it is not yet possible to distinguish
these two hypotheses. Given the linear pressure-dependence
of activity found here, for effective comparisons of activities
across other catalysts, it is instructive to consider values cor-
rected for such effects by reporting in units of activity per
unit pressure (vide infra). To conclude discussion of activity
response, it is important to note that CC has only a minor
effect on activity so that increasing this ratio leads to a small
increase in catalyst activity, over the limited range studied. A
wider range may well produce more significant variation.
come the higher activation energy of chain transfer, thus
explaining the dip in Mw beyond approximately 30 ꢂC.37
Interestingly, careful examination of Figure 3(a–c) allows us
to comment on the three main types of chain transfer, as
shown in Scheme 4: The Mw varies linearly with the pres-
sure at all temperatures. This suggests that transfer to
monomer, either by hydride transfer in a Ni-polymeryl-ethyl-
ene-co-ordinated resting state, or by associative exchange of
ethene on a Ni-hydride-alkene-terminated-polymeryl inter-
mediate, are both slow; either route should be accelerated at
higher pressure, leading to drops in Mw. The opposite is
seen. For example, Mw changes from less than 4,00,000 to
above 5,00,000ꢂ g molꢀ1 when the pressure increases from 1
to 5 bar at 30 C [Fig. 3(c)]. Thus, chain transfer is predomi-
nantly to aluminum, and not to monomer. No attempt was
made to remove residual Me3Al from the MAO used, and it
is known that chain transfer to Al is favored in the presence
of Me3Al.21,22 Yet. the pressure effect dominates; molecular
weight is not yet fully limited by chain transfer; the higher
propagation rate at higher pressures contributes to longer
chains. It may be that this effect is enhanced by high pres-
sures distorting the equilibrium at the head of Scheme 4 to
the right, hence suppressing chain transfer to aluminum.
Moreover, the increase in Mw with pressure is greater at
higher temperature than at lower temperature. It is sus-
pected that at the higher temperatures, catalyst de-activation
reduces the number of active centers, hence increases Mw by
that mechanism. The fact that 13C NMR evidence shows very
little alkene end groups also suggests that chainꢂ transfer to
aluminum is the main mediator of Mw above 30 C, whereas
below this level, the polymerization time and number of
active centers dominate in a pseudo-living way.
Figure 3 shows the contour and response plots of the Mw
regression model. Like activity, Mw increases with tempera-
ture to a maximum, then decreases, but unlike activity, the
optimum Mw depends on both temperature and pressure.
The most surprising aspect of the data is in the range from
10 ꢂC to 30 ꢂC, where the molecular weight increases with
temperature. We have previously seen that catalyst de-activa-
tion is not important in this temperature range. If there was
fast initiation, very slow chain transfer, and constant active
site concentration, then the Mw would indeed be expected to
scale with temperature, according to Arrhenius, as seen. In
this region, then, the polymerization seems to have some ‘‘liv-
ing’’ character. Consistent with this, the lowest polydispersity
index (PDI) values recorded (1.6–1.8) are in the low tempera-
ture region. However, these values are not low enough to jus-
tify a claim of fully ‘‘living’’ behavior. As temperature rises
Similarly, there is a linear variation of Mw with CC, but it is
negligibly small over the short range studied. Maximum Mw
(i.e., 5,57,000) is achieved at 5 bar, 30 ꢂC and CC equal to
3000. Reducing the amount of Me3Al in the MAO may have a
ꢂ
beyond 30 C, a greater proportion of active sites may over-
10
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FIGURE 4 Contour and surface plots of crystallinity-related regression model at constant level of: (a) CC; (b) pressure; (c)
temperature.

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sand carbon atoms.41 At 22% crystallinity, as in the tested
response, a PE would be likely to have a branching rate
closer to 30 branches per 1000 carbons.41 Hence, with the
help of the models, operating conditions can be chosen to
deliver the polymer properties required.
TABLE 5 A Typical Polymerization Condition and its Predicted
Responses
Polymerization Condition
Predicted Response
Factor
Amounts Response Amounts
Temperature (ꢂC)
15.65
5
Activity
(kg [(mol Ni)ꢀ1
211
Given the linear pressure-dependence of activity shown in
Figure 2, for effective comparisons of activities across other
catalysts, it is instructive to consider values corrected for
such effects by reporting in units of activity per unit pres-
sure. The predicted value for activity, when reported in pres-
h
ꢀ1])
Pressure (bar)
CC
Mw (g/mol)
3,58,901
25
3000
Crystallinity (%)
sure-corrected units, of 42.2 kg [(mol Ni)ꢀ1 barꢀ1 hꢀ1
]
more significant effect on Mw than its variation over a three-
related to the defined desirable values of high Mw and mod-
erate crystallinity. If an elastomer with lower crystallinity is
desired, and a lower Mw could be tolerated, then the highest
activity was obtained during Run 9, which achieved 130 kg
fold range.21
Figure 4 depicts crystallinity responses to variation in tem-
perature, pressure, and CC. It is clear that a higher level of
crystallinity is obtained at lower temperatures. This mirrors
behavior in other nickel-catalyzed polymerizations; the
chain-walking process which results in short-chain branches,
which reduce crystallite size and increase amorphous con-
tent is suppressed relative to linear propagation at lower
temperatures because it has a higher activation energy, that
is, polymer with greater branching numbers (and hence
lower crystallinity) is produced by increasing the tempera-
ture.39 Although the crystallinity increases with increasing
the pressure, as linear propagation is first-order in ethylene,
but branching is zero order, the temperature is the most
effective factor in controlling the crystallinity of the polymer.
Therefore, the maximum crystallinity level (above 30%)
would be achieved at 5 bar, 10 ꢂC and CC equal to 1000 or
3000 [Fig. 4(b)]. We have no explanation for the minor dip
in crystallinity at intermediate levels of CC.
[(mol Ni)ꢀ1 barꢀ1
0.712 to 38.620 kg [(mol Ni)ꢀ1 barꢀ1
complexes, and 255 kg [(mol Ni)ꢀ1 barꢀ1
h
ꢀ1]. This value compares with values of
ꢀ1] for diketiminate
ꢀ1] for keto-dii-
h
h
mine complex 6 when activated, as here, with MAO. How-
ever, that value was extrapolated from a 10-min run and is
therefore not directly compatible with the full 1-h run data
reported here. The values reported here may in fact be com-
parable with those obtained with 6; indeed, the active site
may be similar, in a putative bidentate active form of Cation
3a, with a pendant, unco-ordinated enamine taking the role
of the keto function in 6. All these systems derived from b-
connected imines fall well-short of the activities achievable
with the best a-diimine nickel complexes (3000 kg [(mol
Ni)ꢀ1 barꢀ1
h
ꢀ1], 15 min run).42 Notwithstanding these
details, it is clear that activity alone cannot be optimized
with no effect on Mw and crystallinity, both of which are crit-
ical to the ultimate usefulness of the polymer produced. Fur-
ther studies on nickel-catalyzed polymerizations may benefit
from similar treatment of all relevant variables using RSMs.
In addition to visualization of the variation of the responses,
R
V
Minitab 15 provides the means to determine a set of pro-
cess conditions to meet a polymer user’s desired specifica-
tions. For this purpose, the desirable range of responses
must be introduced to the software; in return, it provides
the experimental conditions which give the most optimal
compromise, along with predicted values for the responses.40
In this investigation, the desired range of the responses were
CONCLUSIONS
In summary, the synthesis and characterization of a bulky
nickel (II) b-triketimine complex 3 through reaction of 2 and
NiBr2(DME) was demonstrated. In the solid state, 3 exists as
a dimeric bromide bridged dication, in which the nickel cen-
ters are in a slightly distorted square-pyramidal geometry. It
was used as catalyst precursor for ethylene polymerization
in the presence of MAO; under these conditions, it is
assumed that the active form is monomeric nickel alkyl.
Forthcoming papers will report evidence that this active
form may contain a bidentate form of the triketimine ligand.
RSM was applied to study the effect of polymerization condi-
tions (temperature, pressure, and CC) on catalyst perform-
ance. This provided predictive models for activity, weight-av-
erage molecular weight (Mw), and crystallinity with adequate
goodness-of-fit. The activity, Mw, and crystallinity regression
models illustrated that temperature and pressure played an
important role in controlling the measured outputs (activity,
Mw, and crystallinity): activity showed first-order depend-
ence on pressure at all pressures but was mediated by a
pressure-independent thermally activated catalyst decay pro-
cess, while CC had only minor effects over the limited
defined as activity higher than 200 kg [(mol Ni)ꢀ1 ꢀ1], Mw
h
between 3,00,000 and 5,00,000, and crystallinity higher than
20%. Accordingly, the software suggested a set of polymer-
ization conditions as shown in Table 5. This set of conditions
was experimentally checked to evaluate the predicted
responses. The experimental values from the input factors
shown in Table 5 were 200 kg [(mol Ni)ꢀ1
h
ꢀ1] activity
(5.2% error), 3,94,791 g molꢀ1 Mw (10% error), and 22.3%
crystallinity (10.3% error). This indicates that all three
regression models have predicted the experimental
responses from set input factors reasonably well. The largest
error relates to crystallinity, which, given the broad melting
endotherms of these branched PEs, is the least-precisely
known parameter. It would be equally valid to define a crys-
tallinity of 0% as a desirable outcome for a highly flexible
elastomer. Such a value would correspond to a branching
rate of 50–60 or more methyl-terminated branches per thou-
12
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21 J. D. Azoulay, R. S. Rojas, A. V. Serrano, H. Ohtaki, G. B.
Galland, G. Wu, G. C. Bazan, Angew. Chem. Int. Ed. Engl. 2009,
48, 1089–1092.
threefold range studied. Additionally, the software accurately
predicted a set of polymerization conditions in which the de-
sirable responses (i.e., activity > 200 kg [(mol Ni)ꢀ1
h
ꢀ1],
22 J. D. Azoulay, Z. A. Koretz, G. Wu, G. C. Bazan, Angew.
Chem. Int. Ed. Engl. 2010, 49, 7890–7894.
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