Homo- and heteroligated salicylaldiminato titanium complexes with different substituents ortho to the phenoxy oxygens for ethylene and ethylene/1-hexene (Co)polymerization

Article abstract of DOI:10.1021/ma5017677

A series of homoligated (1c-1e) and heteroligated (2a-2e) salicylaldiminato titanium dichloride complexes with different substituents ortho to the phenoxy oxygens were efficiently prepared. X-ray diffraction studies on these new dichloride complexes 2b, 2d, and 2e reveal a distorted octahedral coordination of the central metal. In the presence of dried methylaluminoxane, all the complexes exhibit high ethylene polymerization productivities. Surprisingly, complex 1d incorporating an o-(trimethylsilyl)ethynyl group displays the highest activity [5.26 × 10³ kg of polyethylenes (mol Ti)^-1 h^-1]. In ethylene/1-hexene copolymerization, the heteroligated complexes 2a-2e display improved activities and intermediate incorporation ability compared with their homoligated counterparts 1a-1f. The activity and incorporation ability for 1-hexene are highly dependent on the nature of the ortho-substituents. Among them, (trimethylsilyl)ethynyl-substituted precatalyst (1d) achieves the highest incorporation ratio (27.3 mol %), while ethynyl-substituted precatalyst (2c) achieves the highest copolymerization activity [2.89 × 10³ kg of copolymers (mol Ti)^-1 h^-1].

Full text of DOI:10.1021/ma5017677

Article
pubs.acs.org/Macromolecules
Homo- and Heteroligated Salicylaldiminato Titanium Complexes
with Different Substituents Ortho to the Phenoxy Oxygens for
Ethylene and Ethylene/1-Hexene (Co)polymerization
Erdong Yao, Jianchun Wang, Zhongtao Chen, and Yuguo Ma*
Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Soft Matter Science and Engineering, Key Lab of Polymer
Chemistry & Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China
S
* Supporting Information
ABSTRACT: A series of homoligated (1c−1e) and heteroligated (2a−2e) salicylaldiminato titanium dichloride complexes with
different substituents ortho to the phenoxy oxygens were efficiently prepared. X-ray diffraction studies on these new dichloride
complexes 2b, 2d, and 2e reveal a distorted octahedral coordination of the central metal. In the presence of dried
methylaluminoxane, all the complexes exhibit high ethylene polymerization productivities. Surprisingly, complex 1d
incorporating an o-(trimethylsilyl)ethynyl group displays the highest activity [5.26 × 103 kg of polyethylenes (mol Ti)−1
h−1]. In ethylene/1-hexene copolymerization, the heteroligated complexes 2a−2e display improved activities and intermediate
incorporation ability compared with their homoligated counterparts 1a−1f. The activity and incorporation ability for 1-hexene
are highly dependent on the nature of the ortho-substituents. Among them, (trimethylsilyl)ethynyl-substituted precatalyst (1d)
achieves the highest incorporation ratio (27.3 mol %), while ethynyl-substituted precatalyst (2c) achieves the highest
copolymerization activity [2.89 × 103 kg of copolymers (mol Ti)−1 h−1].
high comonomer incorporation ability are highly desirable,
since they can offer the polyolefin products with an extensive
wide range of physical and mechanical properties, such as
polyolefin elastomers.5
INTRODUCTION
■
Remarkable progresses have been made in the design of non-
metallocene catalysts for olefin polymerization since the mid-
1990s.1 Among these candidates, the salicylaldiminato titanium
complex represents a well-studied catalyst family,1d,2 which is
highly active for ethylene polymerization. In particular, the
ortho-fluorinated N-aryl bis(salicylaldiminato)titanium deriva-
tives can catalyze the polymerization of ethylene and/or
propylene in a “living” manner, facilitating the synthesis of a
series of block copolymers with controlled structures.3
However, very few successful phenoxyimine catalysts have
been reported to effectively catalyze the copolymerization of
ethylene and other olefins with high activity.4 There is generally
a trade-off. For example, while sterically hindered systems such
as the o-tert-butyl-substituted bis(salicylaldiminato)titanium
complexes are extremely productive in ethylene polymerization,
they are relatively poor at incorporating bulky monomers. On
the other hand, when less bulky ligand is employed,
comonomer incorporation ratio improves at the expense of
productivity.3c,4 Nevertheless, catalysts with high activity and
It is generally known that catalyst structure has a pronounced
impact on the catalytic properties. In the area of bis-
(salicylaldiminato) group 4 catalysts, Pellecchia disclosed that
the presence of additional halogen substituents on the
phenolate rings of the ligand results in an inversion of the
sterospecificity of the corresponding titanium catalyst.6
Recently, Fujita found out that the phenoxyimine titanium
catalysts with the o-phenyl group display remarkable catalytic
properties.4,7 They can efficiently produce high-molecular-
weight polymers with high α-olefin incorporation, which is an
exceptional behavior considering the steric bulk of the phenyl
group. DFT calculations show that they can evade the steric
Received: August 26, 2014
Revised: November 1, 2014
© XXXX American Chemical Society
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Scheme 1. Synthetic Route of the Homoligated Complexes 1a−1f and Heteroligated Complexes 2a−2e
congestion by rotation of the o-phenyl group.7b These studies
provide a new strategy to modify the properties of catalysts by
introducing functional groups. So far, however, only a few
functional groups have been investigated in salicylaldiminato
titanium polymerization systems.7 We believe that functional
groups can provide new opportunities for development of
catalysts with higher catalytic efficiency and more precise
control over polymer microstructures.
behaviors of these catalysts were investigated. The results
provide a strategy for the development of phenoxyimine
catalysts that have high ethylene insertion ability and high α-
olefin uptake.
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations with air- and/or
moisture-sensitive compounds were performed under dry nitrogen
using standard Schlenk techniques or in an mBraun Labstar nitrogen
Over the past decade, several classes of hybrid titanium
catalysts have also been documented.8 For instance, the
combination of pyrrolylaldiminato with salicylaldiminato
ligands, which affords hybrid octahedral complexes containing
two different classes of bidentate monoanionic ligands, has
recently been studied. These hybrid catalysts can successfully
combine the high activity of the bis(salicylaldiminato) system
with the enhanced comonomer incorporation of the bis-
(pyrrolylaldiminato) catalysts.8a In another study, Coates used
a combinatorial screening method to have successfully
identified novel high activity catalysts bearing two salicylaldi-
minato ligands with different N-aryl groups, which can be a
class of unusually active phenoxyimine catalysts for syndiospe-
cific propylene polymerization.8f These studies have demon-
strated the potential of the hybrid approach. Yet up until now,
none of the heteroligated phenoxyimine complexes with
different substituents ortho to the phenoxy oxygen have been
reported. As substituents ortho to the phenoxy oxygen have a
significant impact on both productivity and comonomer
incorporation of the obtained polymers,3c,d,9 we speculate
that the combination of two ancillary ligands with different
substituents ortho to the phenoxy oxygen in one complex will
provide catalysts with unique steric environment as well as
electronic properties.
Given intense investigations regarding catalyst structure and
performance relationships within the family of homoligated
salicylaldiminato catalysts, it is necessary to explore the catalytic
properties of heteroligated catalysts. With fine-tuning one
substituent in one ligand, we designed and synthesized a series
of heteroligated and homoligated salicylaldiminato titanium
dichloride complexes bearing different substituents ortho to the
phenoxy oxygens, including some less explored substitutes,
such as (trimethylsilyl)ethynyl or acetenyl group. To access a
family of high activity heteroligated complexes, all these hybrid
precatalysts are equipped with a sterically bulky ligand bearing
tBu ortho to the phenoxy oxygen.2 Next, the ethylene
polymerization and ethylene/1-henxene copolymerization
glovebox with a high capacity recirculator (<1 ppm of O2). 1H and 13
C
NMR spectra for small molecules were recorded on a Varian 300 (300
MHz) and a Bruker 400 (400 MHz) spectrometer and referenced
versus internal tetramethylsilane. 19F NMR was recorded on a Varian
Inova (289 MHz) and referenced versus external CF3COOH. X-ray
crystallographic data were collected using an MM007-HF CCD
(Saturn 724+) detector system (Mo Kα, λ = 0.710 73 Å), and
structures were solved and refined using software contained in
SHELXS-97 programs. All solvents (CH2Cl2, diethyl ether, THF, n-
hexane, and toluene) for ligand and metal complex synthesis were
purified through the mBraun solvent purification system and stored
over activated 4 Å molecular sieves in resealable flasks. Methyl-
aluminoxane (MAO, 10 wt % Al in toluene) was concentrated in vacuo
to dryness (30 Pa) at 30 °C for 24 h. Commercial ethylene was used
directly for polymerization without further purification. 1-Hexene for
polymerization was purchased from the Alfa Aesar Co. and distilled
from CaH2 before use. All the other reagents and solvents were
purchased and used as received.
General Procedure for Ethylene Homopolymerization.
Ethylene polymerizations were carried out in a 250 mL three-neck
flask equipped with a large magnetic stirring bar. For this experiment, a
preassembled, degassed reactor containing toluene (50 mL) and MAO
(250 equiv relative to Ti) was presaturated under ethylene (1.0 atm)
and equilibrated at the desired reaction temperature using an external
bath. The required amount of the titanium catalyst was dissolved in
toluene (2 mL) at room temperature under nitrogen, and the solution
was added to the reactor via a gastight syringe to initiate the
polymerization. The temperature of the toluene solution was
monitored using a thermocouple probe. The temperature rise
observed for the exothermic reaction was invariably less than 2 °C
during the polymerizations. After a given period of time, the
polymerization was quenched by the addition of 2% acidified methanol
(150 mL). The polymer was collected by filtration, washed with
methanol, and dried in the vacuum oven overnight until a constant
weight was reached.
General Procedure for Copolymerization of Ethylene and 1-
Hexene. Copolymerizations were carried out in a 250 mL three-neck
flask equipped with a large magnetic stirring bar. In a typical
experiment, the comonomer was injected into a preassembled,
degassed reactor containing toluene (50 mL) and MAO (250 equiv
relative to Ti). The solution was presaturated with ethylene (1.0 atm)
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1
Figure 1. H NMR spectra of ligand, 1d, 2d, and 1f. Asterisks mark peaks assignable to residual solvents.
and equilibrated to the desired reaction temperature using an external
5-(R′)C6H2CHN(C6F5)}Cl3(THF) (L1Ti(THFCl3) were
conveniently obtained from the reaction between the free
ligands and TiCl4(THF)2 in dichloromethane (DCM)
solution.8f,12 Deprotonation of salicylaldiminato ligands with
KH in a mixture solution of DCM and THF gave the potassium
salts of the corresponding salicylaldimines K[2-O-3-R-5-(R′)
C6H2CHN(C6F5)]. Treatment of red DCM solutions of
monoligated L1Ti(THF)Cl3 with 1 equiv of the potassium
phenoxides afforded deep red solutions of hybrid complexes
2a−2e. In all cases, solvent was removed after 3 h, and the
crude products were recrystallized from DCM/n-hexane,
yielding 2a−2e as dark red crystalline compounds with
expected composition and high yields (70−97%). This
approach proved to be highly efficient without need of low
temperature and liquid reactants (such as TiCl4 and LinBu).
From 1H NMR of the crude product, only a negligible amount
of homoligated titanium complex was found (Supporting
Information, Figure S3, complex 2d as an example).
bath. The required amount of the titanium catalyst was dissolved in
toluene (2 mL) at room temperature under a nitrogen atmosphere,
and the solution was then injected into the reactor via a gastight
syringe to initiate the polymerization. The temperature of the toluene
solution was monitored by a thermocouple probe. The temperature
rise observed for the exothermic reaction was invariably less than 2 °C
during the polymerizations. After a given period of time, the
polymerization was quenched by the addition of 2% acidified methanol
(150 mL). The polymer was collected by filtration, washed with
methanol, and dried in the vacuum oven overnight until a constant
weight was reached.
Polymer Characterization. 1H and 13C NMR spectra of the
polymers were recorded on a Bruker VXR-400 (400 MHz)
spectrometer, and resonance assignments were made according to
the literature for copolymers of ethylene and 1-hexene.10 Gel
permeation chromatography (GPC) was carried out at 150 °C for
the polymers, with 1,2,4-trichlorobenzene as the eluent at a flow rate
of 1.0 mL min−1. The experiments were performed on a Waters
GPC2000CV instrument coupled with a RI detector and a viscometer
detector equipped with four 5 μm PL gel columns (Polymer
Laboratories). Chromatograms were processed with Waters Empower
software (version 2002); number-average molecular weight (Mn) and
polydispersity index (Mw/Mn) of the polymers were given relative to
polystyrene standards. Melting temperatures of the polymers were
measured by DSC (DSC 822e, Mettler Toledo) from the second scan
with a heating rate of 10 °C min−1.
Formation of 2a−2e as heteroligated salicylaldiminato
titanium dichloride complexes was supported by the 1H
NMR spectra, which showed two sets of resonances for
titanium-bonded salicylaldiminato ligands in a 1:1 ratio. For
example, the spectrum of heteroligated 2d was similar to the
superposed spectra of corresponding homoligated 1d and 1f
except for some shifts of the peaks (Figure 1). The 19F NMR
spectra of 2a−2e indicated sterically hindered rotation of the
N-pentafluorophenyl groups at room temperature which
rendered the ortho- and meta-fluorine inequivalent (Supporting
Information, Figure S6). If the rotation of the N-pentafluor-
ophenyl groups is fast on the NMR time scale, there are
expected to be only three inequivalent fluorine signals.
However, all fluorines are differentiated in these homoligated
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Complexes. The
general routine for the synthesis of new titanium complexes is
shown in Scheme 1. Three previous reported homoligated
precatalysts 1a, 1b, and 1f are also synthesized as compar-
isons.3d In addition, we modified literature procedures by
employing KH as deprotonation agent and TiCl4(THF)2 as the
metal source to obtain a series of homoligated complexes 1a−
1f.11 Obviously, the syntheses of heteroligated complexes
require a stepwise approach through a monoligated inter-
mediate. The monosalicylaldiminato complexes Ti{3-tBu-2-O-
1
and heteroligated complexes. The H NMR spectra remained
unchanged after 8 days in d1-chloroform solutions under inert
atmosphere, indicating that the isolated compounds were
thermodynamically stable isomers (Supporting Information,
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Figure S4). Ligand redistribution reactions, which would result
in the homoligated complexes 1a−1f, did not occur. At elevated
temperature, the ligand redistribution reaction proceeded
slowly, which was negligible for the period of time during
which polymerization was carried out (Supporting Information,
Figure S5).
Single crystals of complexes 2b, 2d, and 2e suitable for X-ray
structure determination were grown from a CH2Cl2/n-hexane
solution at room temperature in a glovebox. The molecular
structures of 2b, 2d, and 2e are depicted in Figures 2−4, and
Figure 4. Crystal structure of complex 2e. Thermal ellipsoids are
drawn at the 50% probability level; H atoms and solvent molecule are
omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of 2b,
2d, and 2e
a
complex
2b
2d
2e
1f
bond distance
2.2815(8)
2.2676(8)
1.8468(17)
1.8623(17)
2.245(2)
Ti−Cl(1)
2.299(2)
2.274(2)
1.847(5)
1.872(5)
2.250(6)
2.216(6)
2.3021(15)
2.2744(18)
1.827(4)
1.870(3)
2.238(5)
2.224(4)
2.2876 (7)
2.2578 (8)
1.841 (1)
1.845 (1)
2.234 (2)
2.217 (2)
Ti−Cl(2)
Ti−O(1)
Ti−O(2)
Ti−N(1)
Ti−N(2)
Figure 2. Crystal structure of complex 2b. Thermal ellipsoids are
drawn at the 50% probability level; H atoms and enantiomer are
omitted for clarity.
2.247(2)
bond angles
98.02(3)
Cl(1)−Ti−Cl(2)
O(1)−Ti−O(2)
N(1)−Ti−N(2)
95.94(9)
163.1(2)
87.3(2)
95.01(6)
96.42 (3)
163.61 (6)
86.94 (7)
163.67(8)
86.14(8)
160.15(16)
90.78(16)
a
From ref 3b.
one side of metal center. As for heteroligated complex 2d (R =
(trimethylsilyl)ethynyl), the linear (trimethylsilyl)ethynyl group
provides a relatively remote steric hindrance, while leaving a
“locally” open coordination environment of the active site.
Besides, in the phenyl-substituted heteroligated complex 2e,
planes of the ortho-phenyl and N-pentafluorophenyl groups are
about 3.62 Å apart, consistent with the distance of a π−π
interaction.13
A quantitative comparison of the key bond lengths and
angles of the heteroligated complexes 2b, 2d, and 2e and
homoligated complex 1f3b is shown in Table 1. The asymmetric
structure of heteroligated complexes is particularly reflected in
the different bond lengths of Ti−O(1) and Ti−O(2), while
homoligated complex 1f has nearly equal Ti−O bond length. In
heteroligated complex 2b (R = Me), the Ti−Cl, Ti−O, and
Ti−N bond lengths are roughly longer than those in the
Figure 3. Crystal structure of complex 2d. Thermal ellipsoids are
drawn at the 50% probability level; H atoms are omitted for clarity.
selected bond lengths and angles are given in Table 1. The
crystallographic data together with the collection and refine-
ment parameters are summarized in Tables S1−S9 (see
Supporting Information). In the solid state, these complexes
adopt a slightly distorted octahedral geometry around the
titanium centers. The two oxygen atoms are situated in the
trans position, while both the two nitrogen and chlorine atoms
are cis to each other, similar to those found in previous
bis(phenoxyimine) catalysts.3a,b Apparently, all complexes are
C1 symmetric in crystal structures. In heteroligated complex 2b,
methyl substituent provides a partly open environment around
t
homoligated complex 1f (R = Bu), and the bond angles
between the ligands (O−Ti−O, N−Ti−N, and Cl−Ti−Cl) are
actually close to 1f. In the case of 2d (R = (trimethylsilyl)-
ethynyl), larger O−Ti−O and Cl−Ti−Cl angles but narrower
N−Ti−N angle are found, suggesting a more open active
site.8d,g On the contrary, the phenyl-substituted heteroligated
complex 2e (R = phenyl) has a wider N−Ti−N but narrower
Cl−Ti−Cl and O−Ti−O angles, and the two Ti−O bond
lengths are obviously different (1.83 Å vs 1.87 Å, Table 1),
which may be caused by the phenyl−pentafluorophenyl
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interaction (see Figure 4).13 In short, different substituents in
heteroligated catalysts give these complexes different steric and
electronic environment, while the o-tert-butyl substituent
provides suitable steric congestion for inhibiting deactivation.
Ethylene Polymerization by the Titanium Complexes
1a−1f and 2a−2e. When activated with 250 equiv of
methylaluminoxane (MAO), all complexes formed highly active
catalysts for the polymerization of ethylene to high molecular
weight (Table 2). The melting points of the obtained polymers
were determined by DSC to be 135−139 °C, which indicates
linear structure.
In the ethylene polymerization, heteroligated complexes 2a−
2e (entries 7−11, Table 2) show generally improved activity
compared with corresponding homoligated complexes 1a−1e
(entries 1−6, Table 2). For example, complex 2a is 4.6 times as
productive as 1a, and 2e is 5.7 times as productive as 1e. This
result demonstrates that introducing one sterically bulky ligand
is effective in increasing the activity of catalysts by slowing the
deactivation rate.2 In addition, similar to previous reports, we
found that the catalytic activity increases with increasing steric
hindrance in these heteroligated catalysts (1f > 2b > 2a, entries
7, 8, and 11, Table 2).3c,d,9,14
Ethylene/1-Hexene Copolymerization. The ability of
complexes 1a−1f and 2a−2e to mediate ethylene/1-hexene
copolymerization upon MAO activation was studied under
atmospheric pressure with 1-hexene concentration at 0.7 M.
Results are compiled in Table 3. All the complexes afford
a
Table 2. Ethylene Polymerization Results
b
c
c
d
entry complex yield (g) activity
Mn
Mw/Mn
Tm (°C)
1
1a
1b
1c
1d
1e
1f
0.121
0.438
0.505
0.876
0.116
0.846
0.557
0.604
0.669
0.782
0.662
7.3
26.3
30.3
52.6
7.0
8.0
23.7
37.0
36.7
3.9
3.02
2.00
2.39
2.87
2.36
2.52
3.15
3.50
2.70
2.61
1.87
137
140
140
139
135
139
138
139
138
138
140
2
a
3
Table 3. Ethylene and 1-Hexene Copolymerization Results
4
d
e
yield
(g)
Tm
H
5
b
c
c
entry complex
activity
Mn
Mw/Mn
(°C)
(mol %)
6
50.8
33.4
36.2
40.1
46.9
39.7
39.7
34.7
27.6
30.6
50.0
25.3
1
1a
1b
1c
1d
1e
1f
0.127
0.260
0.265
0.180
0.069
0.198
0.353
0.395
0.481
0.329
0.200
7.6
15.6
15.9
10.8
4.1
8.0
11.8
10.8
5.0
1.66
1.44
2.45
3.47
1.47
1.45
2.21
2.17
2.05
1.25
1.29
52
15.8
15.1
22.3
27.3
9.1
7
2a
2b
2c
2d
2e
2
55
8
3
n/a
n/a
92
9
4
10
11
5
3.6
6
11.9
21.2
23.7
28.9
19.7
12.0
9.6
116
87
3.1
a
Polymerizations carried out for 5 min at 40 °C with 2 μmol of Ti and
7
2a
2b
2c
2d
2e
17.2
18.0
15.9
15.1
8.1
6.8
MAO as cocatalysts (Al:Ti = 250:1) in 50 mL of toluene under an
8
96
5.2
b
c
ethylene pressure of 1.0 atm. In 105 g (mol Ti)−1 h−1. From GPC
d
9
78
8.3
calibrated with polystyrene standards. Data determined by DSC from
10
11
64
14.9
5.3
the second melting curve.
97
a
Copolymerizations were carried out for 5 min at 40 °C with 2 μmol
First, we investigated the ethylene polymerization properties
of homoligated complexes 1a−1f. The polymerization results of
the known complexes 1a, 1b, and 1f were similar to the
reference (entries 1, 2, and 6, Table 2),3c and the catalytic
activity increases with growing steric hindrance in these
homoligated catalysts. The o-ethynyl-substituted complex 1c
obtains similar modest productivity as the o-methyl-substituted
complex 1b (entries 2 and 3, Table 2), which we also attributes
to the similar size of groups. Surprisingly, complex 1d
containing (trimethylsilyl)ethynyl ortho to the phenoxy oxygen
shows higher activity than the o-tert-butyl complex 1f (entries 4
and 6, Table 2). As a general rule, increasing the steric
hindrance around the metal center will slow the deactivation
rate, resulting in higher activity.1d Since (trimethylsilyl)ethynyl
only provides a relatively remote steric hindrance (Si−Ti
distance: 6.25 Å) with respect to titanium, we reason that such
a remote steric hindrance might be sufficient to inhibit
deactivation. Furthermore, considering the relatively lower
activity of complex 1c, the high activity of 1d is actually caused
by the remote steric trimethylsilyl group. Catalysts bearing an
ortho-substituted phenyl group usually display enhanced
activity,4,7 but in our case, phenyl-substituted 1e shows lower
activity (entry 5, Table 2). Although, the reason is unclear at
the current time, we think that the π−π interaction between the
o-phenyl and N-pentafluorophenyl groups may influence
catalyst conformation or racemization.15 The gel permeation
chromatography (GPC) curves of obtained PEs give relatively
narrow polydispersities (1.8−3.5) (Table 2) and support that
these precatalysts give rise to single site catalysts. The molecular
weight of polymers follows a similar trend as polymerization
activity.
of Ti and MAO as cocatalysts (Al:Ti = 250:1) in 50 mL of toluene
under an ethylene pressure of 1.0 atm. 1-Hexene concentration is 0.7
b
c
M. In 105 g (mol Ti)−1 h−1. From GPC calibrated with polystyrene
d
standards. Data determined by DSC from the second melting curve.
e
Comonomer incorporation based on 1H NMR or 13C NMR
spectra;3c,10 n/a = not applicable.
polymers, which range from semicrystalline to amorphous
materials, with high productivity [106 kg (mol Ti)−1 h−1] under
the given conditions. These results show that the substituent
ortho to the phenoxy oxygen has a profound influence on the
copolymerization reactivity of the catalyst systems. The
incorporation levels are in the range of 3.1−27.3 mol %.
Their 13C NMR analyses indicate that butyl branches mostly
exist in separate form in the polymers with only very few 1-
hexene/1-hexene dyads (Tables S9−S11).
Under identical conditions, homoligated complex 1d displays
the highest 1-hexene incorporation ratio (27.3 mol %),
followed by complexes 1c (22.3 mol %) (entries 3 and 4,
Table 3). A similar sequence is also found among heteroligated
catalysts (2d, 14.9 mol %; 2c, 8.3 mol %; entries 9 and 10,
Table 3). The high incorporation ratio of 1-hexene clearly
demonstrates that the remote bulky trimethylsilyl group does
not inhibit the coordination of 1-hexene to the metal center.
These results once again testify to the superiority of the remote
steric hindrance. But steric effect alone is not sufficient to
explain the outstanding performance of (trimethylsilyl)ethynyl-
substituted precatalysts because the incorporation levels
achieved by 1c and 1d are even higher than that of complex
1a with the smallest steric hindrance. The electronically flexible
properties of the ligand of 1c and 1d, due to the more
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conjugated nature caused by the presence of the o-ethynyl
group, may also be responsible for this behavior.1d,4 Again,
similar to ethylene polymerization, complexes 1e and 2e with
ortho-substituted phenyl shows only modest 1-hexene incorpo-
ration ratio with low activity.
National Basic Research Program (2013CB933500) of the
Ministry of Science and Technology of China.
REFERENCES
■
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In all these experiments, nearly 2-fold activity is observed for
heteroligated complexes 2a−2e compared with corresponding
homoligated complexes 1a−1e (entries 1−6 and 7−11, Table
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followed by complex 2b. The ortho-substituents in an
appropriate size will allow a bulky comonomer to coordinate
to the metal and meanwhile avoid the deactivation pathways by
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Information, Table S10) suggest that they can be used as
polyolefin elastomers or LLDPE.16
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CONCLUSIONS
■
In this work, we reported a series of heteroligated
salicylaldiminato titanium dichloride complexes with different
substituents ortho to the phenoxy oxygens. They can be
conveniently synthesized through treatment of mono-
(salicylaldiminato)titanium complexes with another potassium
salicylaldiminates at room temperature. These complexes are
stable with respect to isomerization and ligand redistribution
reactions. Moreover, hybrid precatalysts 2b, 2d, and 2e are
structurally characterized by single crystal X-ray diffraction
analysis. The hybrid titanium complexes effectively combine the
high incorporation capability of corresponding bis-
(salicylaldiminato)titanium catalysts with high activity. On the
other hand, functional groups have pronounced effects on the
properties of catalysts. Complex 1d containing the
(trimethylsilyl)ethynyl group not only gives the highest
ethylene polymerization productivity but also shows the highest
1-hexene incorporation ratio (27.3 mol %). The exceptional
polymerization property can be attributed to the remote steric
hindrance induced by (trimethylsilyl)ethynyl group and the
flexible electronic effect of alkynyl group. We hope our studies
offer a new strategy for designing high productivity and high
comonomer uptake catalysts in olefin polymerization by
introducing new functional groups and using hybrid methods.
(9) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Matsukawa,
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(12) In order to conveniently purify targeted complexes, tert-butyl at
the para-position (R′) was introduced in some cases. But from
previous studies, tert-butyl at the para-position of the oxygen did not
shown remarkable influence on the polymerization behaviors of
bis(salicylaldiminato) catalysts. See ref 3d.
ASSOCIATED CONTENT
* Supporting Information
Synthesis and characterization of ligands and catalysts, tables of
crystal structure data, ethylene polymerization results, and
polymer analysis data. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: ygma@pku.edu.cn (Y.M.).
Notes
■
(13) Tsuzuki, S.; Uchimaru, T.; Mikami, M. J. Phys. Chem. A 2006,
110, 2027.
The authors declare no competing financial interest.
(14) Matsui, S.; Fujita, T. Catal. Today 2001, 66, 63.
ACKNOWLEDGMENTS
This research was financially supported by National Natural
Science Foundation (No. 21074004 and 91227202) and the
■
(15) (a) Talarico, G.; Busico, V.; Cavallo, L. J. Am. Chem. Soc. 2003,
125, 7172. (b) Milano, G.; Cavallo, L.; Guerra, G. J. Am. Chem. Soc.
2002, 124, 13368.
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dx.doi.org/10.1021/ma5017677 | Macromolecules XXXX, XXX, XXX−XXX

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dx.doi.org/10.1021/ma5017677 | Macromolecules XXXX, XXX, XXX−XXX