Binuclear heteroligated titanium catalyst based on phenoxyimine ligands: Synthesis, characterization, and ethylene (co)polymerization

Article abstract of DOI:10.1021/ma300384w

A binuclear heteroligated titanium(IV) catalyst based on phenoxyimine ligands (FI²-Ti₂) with its crystal structure elucidated has been developed for the first time for olefin (co)polymerization. In ethylene polymerization, the activity of the binuclear catalyst FI²-Ti ₂ can reach 3.0 A - 10⁶ g mol^-1 h ^-1, and the polydispersity of the resulting polymers is narrow. In copolymerization of ethylene and other olefins, similar incorporation ratios for monoenes are obtained for FI²-Ti₂ and FI-Ti₁. However, the incorporation ratio of 1,5-hexadiene increases from 3.2% using FI-Ti₁ to 8.8% with FI²-Ti₂. In comparison, the binuclear monophenoxyimine catalyst, [FI²-Ti₂(THF) ₂], exhibits higher catalytic activity and incorporates more α-olefins than its mononuclear analogue [FI-Ti₁(THF)] in the copolymerization. These results are interpreted as consequences of two effects. First, the cooperativity between the two metal centers facilitates the coordination of olefin. Second, the substitutions near the active sites exert a steric effect by blocking and suppressing the binding of comonomers.

Full text of DOI:10.1021/ma300384w

Article
pubs.acs.org/Macromolecules
Binuclear Heteroligated Titanium Catalyst Based on Phenoxyimine
Ligands: Synthesis, Characterization, and Ethylene
(Co)polymerization
Shuliang Han, Erdong Yao, Wei Qin, Shaofei Zhang, and Yuguo Ma*
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Lab of Polymer Chemistry & Physics of the Ministry of
Education, College of Chemistry, Peking University, Beijing 100871, China
S
* Supporting Information
ABSTRACT: A binuclear heteroligated titanium(IV) catalyst based on
phenoxyimine ligands (FI2-Ti2) with its crystal structure elucidated has been
developed for the first time for olefin (co)polymerization. In ethylene
polymerization, the activity of the binuclear catalyst FI2-Ti2 can reach 3.0 ×
106 g mol−1 h−1, and the polydispersity of the resulting polymers is narrow. In
copolymerization of ethylene and other olefins, similar incorporation ratios for
monoenes are obtained for FI2-Ti2 and FI-Ti1. However, the incorporation
ratio of 1,5-hexadiene increases from 3.2% using FI-Ti1 to 8.8% with FI2-Ti2.
In comparison, the binuclear monophenoxyimine catalyst, [FI2-Ti2(THF)2], exhibits higher catalytic activity and incorporates
more α-olefins than its mononuclear analogue [FI-Ti1(THF)] in the copolymerization. These results are interpreted as
consequences of two effects. First, the cooperativity between the two metal centers facilitates the coordination of olefin. Second,
the substitutions near the active sites exert a steric effect by blocking and suppressing the binding of comonomers.
more efficient than their mononuclear analogues in olefin
polymerization.9 It was proposed that when the olefinic unit of
an alkene comonomer was bound to one of the metal centers,
the second highly electrophilic d0 metal center could stabilize
the associated alkene, possibly by the agostic effect, thereby
modifying relative enchainment and chain transfer rates. It was
therefore revealed that the nuclearity and cooperative effects
between the two active centers were important. On the basis of
these facts, we are particularly interested in bringing two
bis(phenoxyimine)titanium catalysts to a close distance by
covalently linking them into one molecular framework. This
strategy may help develop catalysts that not only possess high
catalytic activity but also enchain more comonomers. In this
contribution, we report a new binuclear heteroligated titanium
catalyst possessing phenoxyimine ligands thus designed and its
catalytic properties in the olefin (co)polymerization.
INTRODUCTION
■
In order to gain control over the properties of polymers by
devising and adjusting the structures of organometallic catalysts,
non-metallocene catalysts with diamide, β-diketiminates,
iminopyrrolides, amidinates, diimine, alkoxide, and aryl
oxide1a−c as ligands have been developed since the mid-
1990s. Among these candidates, phenoxyimine catalysts
demonstrate high performance in both catalytic activity and
stereoregularity control in olefin polymerization.1d,e,2 In
particular, the ortho-fluorinated N-arylbis(phenoxyimine)tita-
nium derivatives catalyze the polymerization of ethylene and/or
propylene in a “living” manner,3 leading to the synthesis of a
series of block copolymers with controlled structures.3b,4
However, very few successful phenoxyimine catalysts have
been reported that effectively catalyze random copolymeriza-
tion of ethylene and other olefins due to the low comonomer
incorporation ratio. Polymers with high comonomer incorpo-
ration ratio are nonetheless highly desirable, since they can be
used as polyolefin elastmers,5 which have superior properties
when compared to common thermoplastic elastomers.
Recently, several examples have been reported suggesting the
importance of cooperativity among multiple metallic sites in the
development of new catalysts.6 Coates et al. employed a
binuclear cobalt catalyst in the polymerization of racemic
terminal epoxides, yielding isotactic polyethers.7 Guan’s group
introduced a pyridine group to an α-diimine ligand at the axial
position. In combination with AlMe2Cl, this catalyst can
produce high molecular weight polyethylenes, most likely due
to the formation of bimetallic Ni−Al or Pd−Al complexes via a
chloride bridge.8 Marks’ group systematically studied a series of
binuclear catalysts and found that the binuclear catalysts were
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 a mBraun Labstar nitrogen
glovebox with a high capacity recirculator (<1 ppm O2). H and 13C
1
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.
Elemental analyses were performed using a German Vario EL III
Received: February 23, 2012
Revised: April 19, 2012
© XXXX American Chemical Society
A
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Ti2, FI-Ti1,3b FI2-Ti2(THF)2, and FI-Ti1(THF) are shown in
Chart 1. The xanthene was used as a rigid linker to connect two
elemental analyzer. Electron spray ionization (ESI) mass spectra were
obtained on a Bruker APEX IV mass spectrometer. X-ray crystallo-
graphic 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 by passing
through the mBraun solvent purification system and stored over
activated 4 Å molecular sieves in resealable flasks. Methylaluminoxane
(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, 1-octene, 1,5-
hexadiene, and isoprene for polymerization were purchased from the
Alfa Aesar Co. and distilled from CaH2 before use. All other reagents
and solvents were purchased and used as received.
Chart 1
General Procedure for Ethylene Homopolymerization by Ti
Complexes. Ethylene polymerizations were carried out in 250 mL
three-neck flasks 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 exotherm 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
(15 mL). The polymer was collected by filtration, washed with
methanol, and dried in the vacuum line overnight until a constant
weight was reached.
General Procedure for Copolymerization of Ethylene and α-
Olefins by Ti Complexes. Copolymerizations were carried out in
250 mL three-neck flasks 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) and equilibrated to 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 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 exotherm 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.The polymer was collected by filtration, washed
with methanol, and dried in the vacuum line overnight until a constant
weight was reached.
phenoxyimine catalysts in order to maintain a constant distance
between the two active metal centers during the catalytic
process.
Initially, FI2-Ti2(THF)2 was synthesized (see Supporting
Information) as precursor to react with the lithium salt of
phenoxyimine ligand 5 for generating the binuclear hetero-
ligated catalyst. However, the presence of multiple tert-butyl
groups in the structure made the final heteroligated metal
complex difficult to crystallize and separate from the solution,
hindering further purification and characterizations. In
subsequent studies, xanthene scaffold without tert-butyl
substitution was used instead, and catalyst FI2-Ti2 was
synthesized successfully (Scheme 1).
9,9-Dimethyl-9H-xanthene-4,5-diyldiboronic acid was syn-
thesized following the literature procedures (see Supporting
Information). The product then underwent a Pd(PPh3)4-
catalyzed Suzuki coupling with 5-bromo-3-tert-butyl-2-hydrox-
ybenzaldehyde, generating a dialdehyde derivative which was
subsequently condensed with pentafluoroanilines to produce
ligand 3. Complex 4 was obtained by the metalation of 3 with
tetrachlorobis(tetrahydrofuran)titanium(IV) in dichlorome-
thane, which then reacted with the lithium salt of another
phenoxyimine ligand 5 to give the binuclear heteroligated
catalyst FI2-Ti2.
Polymer Characterization. 13C NMR spectra of the polymers
were recorded on a Bruker VXR-400 (100 MHz) spectrometer, and
resonance assignments were made according to the literature for
copolymers of ethylene−α-olefin and ethylene−diene.11 Gel perme-
ation 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. The experiments were performed on a Waters GPC2000CV
instrument coupled with a Waters RI detector and a Waters 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
PS 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, unless otherwise noted.
1H NMR spectroscopy and elemental analysis both
confirmed the structures of FI2-Ti2(THF)2 and FI2-Ti2.
Moreover, single crystals of FI2-Ti2 suitable for X-ray analysis
were successfully obtained via a slow diffusion of hexane into a
solution of mixed solvents of dichloromethane and ethyl ether.
The structure determined by X-ray diffraction crystallography is
shown in Figure 1. To the best of our knowledge, this is the
first single crystal structure reported for a binuclear
heteroligated catalyst based on phenoxyimine ligands. The
bimetallic molecule shows a tweezer-like structure of C2
symmetry; four molecules were packed in one unit cell. The
distance between the two titanium atoms is 7.88 Å, while the
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Binuclear Hetero-
ligated Catalysts. The chemical structures of catalysts FI2-
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Scheme 1. Synthetic Route to the Binuclear Complex FI2-Ti2 (PTSA = p-Toluenesulfonic Acid)
presence of methylaluminoxane (MAO) in toluene at 40 °C.
The results are summarized in Table 1. The activity of catalyst
FI2-Ti2 in homopolymerization reached 2946 kg mol−1 h−1,
which is similar to that of the mononuclear analogue at the
same concentration of the active titanium site. The molecular
weight of the produced polymer is 1.7 × 105 g/mol and the
polydispersity index is 1.71, both of which are comparable to
those obtained from catalyst FI-Ti1 (Table 1, entry 1 vs entry
2). The melting point of the product was determined by DSC
to be 139 °C, indicating that the polyethylene has a linear
1
structure. The H and 13C NMR spectra recorded at a high
temperature show no signals of branching points or vinylic end
groups were detected, suggesting that a chain transfer to Al is
the main termination pathway.10 These data indicate that
catalyst FI2-Ti2 retains the general polymerization features of
phenoxyimine-type catalysts. Additional results of ethylene
homopolymerization are summarized in the Supporting
Information (Table S6).
In the copolymerization of ethylene and monoenes, FI2-Ti2
and FI-Ti1 show similar catalytic activities, and the molecular
weights of corresponding copolymers exhibit little difference.
Nevertheless, the melting points are much lower than those of
ethylene homopolymers. The 13C NMR spectroscopy con-
firmed the occurrence of a branched structure and provided the
incorporation ratio of the comonomer in the polymer chain.11
Catalysts FI2-Ti2 and FI-Ti1 manifested similar efficiency at
incorporating 1-hexene or 1-octene into the copolymer chain
(Table 1, entry 3 vs entry 4; entry 5 vs 6). The incorporation
ratio of the comonomer is 2.3% for 1-hexene and 2.2% for 1-
octene. These data are comparable to those reported by Fujita’s
group, using the analogous type of catalysts.13a However,
catalyst FI2-Ti2 shows much better catalytic properties than the
mononuclear catalyst in the copolymerization of ethylene and
dienes. When 1,5-hexadiene was employed as the comonomer
at a concentration of 1.0 M, catalyst FI2-Ti2 introduced 8.8%
diene comonomer in the polymer chains, which is more than
twice the value obtained from FI-Ti1 (Table 1, entry 7 vs 8).
The percentages of cis- and trans-1,3-cyclopentyl fragments in
the polymer chains generated by FI2-Ti2 were 48% and 52%,
respectively, while the corresponding numbers are 30% and
Figure 1. Molecular structure of complex FI2-Ti2 with thermal
ellipsoids at 30% probability level.
oxygen−oxygen distance between two phenoxyimine con-
nected by xanthene (O2 in Figure 1) is only 6.35 Å. The
elongated distance between Ti atoms suggests significant
repulsive interaction between the two catalytic sites. The
length of the Ti1−O2 bond is 1.87 Å, which is longer than the
Ti−O bond length in catalyst FI−Ti1.3b The oxygen atoms are
still situated in the trans configuration in both of the
phenoxyimine catalysts. Meanwhile, the chlorine atoms are
oriented in the cis positions, near the other Ti center, which
indicates the two catalytic sites would possibly face each other
in the olefin polymerization after removing chloride atoms in
the activation process. This arrangement may facilitate the
cooperative effect of the two catalytic sites (vide infra).
Polymerization Properties of FI2-Ti2 Complex Acti-
vated by MAO. The ethylene (co)polymerization at ambient
pressure was examined using catalysts FI2-Ti2 and FI-Ti1 in the
C
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a
Table 1. Ethylene (Co)polymerization Results for FI2-Ti2 and FI-Ti1 Complexes
b
b
c
d
entry
catalyst [μmol]
comonomer
yield [g]
activity [kg/(mol h)]
Mw [×105]
Mw/Mn
Tm [°C]
incorporation (%)
1
FI-Ti1(2)
FI2-Ti2(1)
FI-Ti1(2)
FI2-Ti2(1)
FI-Ti1(2)
FI2-Ti2(1)
FI-Ti1(2)
FI2-Ti2(1)
FI-Ti1(2)
FI2-Ti2(1)
FI-Ti1(2)
FI2-Ti2(1)
n/a
0.520
0.491
0.156
0.147
0.207
0.189
0.187
0.212
0.178
0.194
trace
3120
2946
936
1.9
1.7
1.0
1.8
1.4
1.5
2.0
2.2
0.9
1.4
n/a
1.9
1.66
1.71
1.27
2.65
1.44
1.73
1.64
1.93
1.81
1.79
n/a
140
139
106
120
112
118
108
100
108
106
n/a
136
n/a
n/a
3.2
2.3
2.4
2.2
3.2
8.8
1.1
2.6
n/a
2
n/a
3
1-hexene
1-hexene
1-octene
1-octene
1,5-HD
1,5-HD
4
882
5
1242
1134
1122
1272
1068
1164
n/a
6
7
8
e
9
1,5-HD
e
10
11
12
1,5-HD
isoprene
isoprene
f
0.101
606
2.04
2.3
a
Copolymerizations carried out for 5 min at 40 °C on a high-vacuum line with 2 μmol of Ti and MAO as cocatalysts (Al:Ti = 250:1) in 50 mL of
b
c
toluene under an ethylene pressure of 1.0 atm. Comonomer concentration is 1.0 M. From GPC calibrated with polystyrene standards. Data
determined by DSC. Comonomer incorporation based on 13C NMR spectra.11a,b Comonomer concentration is 0.5 M. Comonomer incorporation
d
e
f
11c
1
based on H NMR spectra (n/a: not applicable).
a
Table 2. Ethylene (Co)polymerization Results for FI2-Ti2(THF)2 and FI−Ti1(THF) Catalysts
b
b
c
d
entry
catalyst [μmol]
comonomer
n/a
yield [g] activity [104 g/(mol h)] Mw [×105] Mw/Mn Tm [°C] comonomer incorporation (%)
1
2
3
4
5
6
7
8
FI-Ti1(THF) (10)
FI2-Ti2(THF)2 (5)
FI−Ti1(THF) (10)
FI2-Ti2(THF)2 (5)
FI-Ti1(THF) (10)
FI2-Ti2(THF)2 (5)
FI-Ti1(THF) (10)
FI2-Ti2(THF)2 (5)
0.057
0.161
0.053
0.126
0.031
0.068
trace
0.6
1.6
0.5
1.3
0.3
0.9
n/a
1.3
5.1
6.4
3.5
3.8
4.0
6.4
n/a
11
49.3
3.26
18.1
10.8
27.4
16.1
n/a
133
138
113
114
116
112
n/a
108
n/a
1-hexene (0.5 M)
1-hexene (0.5 M)
1-octene (0.7 M)
1-octene (0.7 M)
1,5-HD (0.7 M)
1,5-HD (0.7 M)
3.0
6.1
3.7
13.7
n/a
5.6
0.134
4.95
a
(Co)polymerizations carried out for 1 h at 40 °C in a high-vacuum line with 10 μmol of Ti and MAO as the cocatalysts (Al:Ti = 1000: 1) in 50 mL
b
c
d
of toluene under an ethylene pressure of 1.0 atm. From GPC calibrated by polystyrene standards. Data were determined by DSC. Comonomer
incorporations based on 13C NMR spectra (n/a: not applicable).11a,b
70% for FI-Ti1 produced polymers. When the concentration of
1,5-hexadiene is lowered to 0.5 M, catalyst FI2-Ti2 still
incorporates more 1,5-hexadiene into the polymer chain than
FI-Ti1. The incorporation ratio is 2.6%, which is more than
doubled compared to that in the polymer obtained from FI-Ti1
(Table 1, entry 9 vs 10). When the comonomer was isoprene,
only a trace amount of polymer was observed within the
monitored reaction time using catalyst FI-Ti1. In contrast, FI2-
Ti2 exhibits an activity of 606 kg mol−1 h−1 and an
incorporation ratio of 2.3% for 1,4-isoprene units under similar
conditions (Table 1, entry 11 vs 12).
catalyst under the same conditions. Similar results are observed
in a previous report.9f The large value of the polydispersity
index (3.26) suggests there are multiple active sites present in
the polymerization process.9f,12 DSC and 13C NMR measure-
ments indicate that the polyethylenes had linear structures.
In the copolymerization of ethylene with other olefins, the
activity of FI2-Ti2(THF)2 and the incorporation ratio of
comonomers are both found to be higher than those of FI-
Ti1(THF). Specifically, in the copolymerization of ethylene
with monoenes (Table 2, entry 4 vs entry 3, entry 6 vs entry 5),
the activity of the bimetallic catalyst is more than twice the
value of the monometallic catalyst. The comonomer incorpo-
ration ratios of FI2-Ti2(THF)2 are 6.1% and 13.7% for 1-
hexene and 1-octene, respectively, which are roughly double the
values of corresponding monometallic catalyst. The melting
points of the resulting copolymers are less than 120 °C,
indicating the presence of alkyl branches on the linear chains.
The molecular weights of the two copolymers from FI2-
Ti2(THF)2 are both higher than those generated by FI-
Ti1(THF), with narrower polydispersity than the former. The
improvement is more pronounced in the copolymerization of
ethylene with 1,5-hexadiene. The bimetallic catalyst produces a
copolymer of the diene at an activity of 1.3 × 104 g mol−1 h−1
and incorporates 5.6% of the comonomers in the polymer
chain. The Mw is 1.1 × 106 g/mol and PDI is 4.95. In contrast,
the monometallic catalyst produces only a trace amount of
polymer, unsuitable for characterizations.
Polymerization Properties of FI2-Ti2(THF)2 Complexes
Activated by MAO. Overall, the binuclear catalyst exhibits
superior catalytic properties to its mononuclear analogue,
especially in the copolymerization of ethylene with dienes. In
order to investigate the correlation between the catalyst’s
structure and properties, we used the FI2-Ti2(THF)2 as the
initiator, which features a less steric environment around the
active center.9f The polymerization results are compared with
those of its mononuclear analogue.
The polymerization of ethylene using FI2-Ti2(THF)2 and FI-
Ti1(THF) as catalysts is also performed at an atmosphere
pressure in the presence of methylaluminoxanue (MAO) in
toluene at 40 °C. The activity of the binuclear catalyst in
homopolymerization is found to be 1.6 × 104 g mol−1 h−1, also
more than twice that of the mononuclear analogue under
similar conditions (Table 2, entry 2 vs entry 1). Molecular
weight of the polymer obtained from the bimetallic catalyst is
6.4 × 105 g/mol, higher than that derived from mononuclear
In conclusion, the results from the present investigation
indicate that a binuclear catalyst is more efficient in
D
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Figure 2. Side view (left) and top view (right) of molecular structure of FI2-Ti2.
(co)polymerization than its mononuclear analogue, which is
probably due to the cooperative effect of the two metal
centers.9f
1,5-hexandiene, the two double bonds can coordinate with the
two metal sites properly, which enhances the binding capability
of 1,5-hexadiene, stabilizes the coordination complexes, and
therefore facilitates the diene insertion. Consequently, the
binuclear catalyst introduced more dienes in the polymer
chains. The copolymerization of ethylene and isoprene also
shows that the binuclear catalyst is more efficient than its
mononuclear analogue. But the methyl group in C2 position of
isoprene causes greater steric hindrance and thus lowers the
catalytic activity and comonomer incorporation.
Comparison of FI2-Ti2(THF)2 and FI2-Ti2 and Discus-
sion of Cooperative and Steric Effect. Comparing the
properties of catalysts FI2-Ti2(THF)2 to FI2-Ti2, the activity of
ethylene polymerization increased dramatically from 104 to 106
g mol−1 h−1 upon the introduction of the second phenoxyimine
ligand. The smaller molecular weights and the narrower
polydispersity of the polymers suggests that FI2-Ti2 are unlikely
to possess multiple catalytic sites, which is exhibited by FI2-
Ti2(THF)2.
CONCLUSION
■
We have synthesized and characterized a new heteroligated
titanium catalyst, FI2-Ti2. To our best knowledge, it is the first
reported binuclear heteroligated catalyst based on phenox-
yimine ligands with its crystal structure elucidated. For
comparison, a monophenoxyimine catalyst FI2-Ti2(THF)2
was also synthesized. Their performances in ethylene polymer-
ization and copolymerization of ethylene with monoenes or
dienes were investigated. FI2-Ti2(THF)2 exhibits a higher
catalytic activity and generally enchains more comonomers than
FI-Ti1(THF). However, the overall activity of FI2-Ti2(THF)2
is relatively low, and the polydispersity of the generated
polymer is relatively broad. In contrast, FI2-Ti2 exhibits a much
higher activity and produces polymers with a narrower
polydispersity than FI2-Ti2(THF)2. In addition, although
catalyst FI2-Ti2 generates similar polymerization results to FI-
Ti1 possibly due to the steric effect, it can enchain dienes more
efficiently than its mononuclear analogue. Therefore, the FI2-
Ti2 catalyst has the potential to be applied in the (co)-
polymerization of bifunctional monomers, offering new
polymer materials.
In the copolymerization of ethylene and α-olefins, in
comparison with their mononuclear analogue, the results are
rather different between FI2-Ti2 and FI2-Ti2(THF)2. The
catalyst FI2-Ti2(THF)2 shows a higher activity and incorpo-
rates a greater percentage of comonomer into the polymer
chains than FI-Ti1(THF) in all polymerization experiments.
According to the previous report,9a,f the agostic interaction
between the comonomer and metal center can stabilize the
coordinated bimetallic olefin complex and enchains olefin
comonomers more easily than mononuclear analogues. In
addition, in polymerization of the bifunctional comonomer 1,5-
hexadiene, the activity is even similar to that of ethylene
homopolymerization. We thus reasonably propose that the
coordination effect between the two double bonds and two
metal centers has a positive impact on this phenomenon.
The heteroligated catalyst FI2-Ti2 incorporated a similar
percentage of monoenes but a greater amount of dienes into
copolymers compared to FI-Ti1. The unexpected difference of
the polymerization capability prompts us to investigate the
catalyst’s structure in greater detail (Figure 2). The chlorine
atoms that become active sites in polymerization are close to
each other in FI2-Ti2, surrounded by tert-butyl groups and
pentafluorophenyl groups. The distance between the methyl
carbon atom in tert-butyl groups and the chlorine atom in the
titanium center is only 3.93 Å (Figure 2, side view), while the
nearest fluorine atom to the chlorine atom is 3.35 Å (Figure 2,
top view). Meanwhile, the chlorine atoms in different titanium
centers are close to each other, with a distance of 4.25 Å
(Figure 2, top view). Therefore, the steric hindrance in active
sites in FI2-Ti2 is more pronounced than that of FI-Ti1, which
is unfavorable to the access of the α-olefin monomer to the
active sites.9a,13 As a result, there are fewer monoene
comonomers incorporated into the polymer chains. As for
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and characterization of ligands and catalysts, tables of
crystal structure data, ethylene polymerization results of FI2-
Ti2, and polymer analysis data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ygma@pku.edu.cn.
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Notes
The authors declare no competing financial interest.
E
dx.doi.org/10.1021/ma300384w | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
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ACKNOWLEDGMENTS
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We thank the National Basic Research Program
(2007CB808000) from the Ministry of Science and Technol-
ogy and National Natural Science Foundation of China
(20604001) for support.
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dx.doi.org/10.1021/ma300384w | Macromolecules XXXX, XXX, XXX−XXX