Zirconium complexes bearing bis(phenoxy-imine) ligands with bulky o-bis(aryl)methyl-substituted aniline groups: Synthesis, characterization and ethylene polymerization behavior

Article abstract of DOI:10.1002/aoc.2984

A series of bis(phenoxy-imine) zirconium complexes bearing bulky o-bis(aryl)methyl-substituted aryl groups on the aniline moiety have been synthesized, characterized and tested as catalyst precursors for ethylene polymerization. ¹H NMR spectroscopy suggests that these complexes exist as a single chiral C₂-symmetric isomer in the solution. X-ray crystallographic analysis of the resulting biszwitterionic-type adduct complex C1 · 2HCl reveals that the phenoxy-imine groups function as a monodentate phenoxy ligand and the oxygen atoms are oriented trans to each other at the central metal atom. Using modified methylaluminoxane (MMAO) as co-catalyst, C1 · 2HCl, C2-C6 exclusively produce linear aluminium-terminated polyethylenes (Al-PEs) with high activity (up to 16.89 × 10⁶ g PE (mol Zr h)^-1, suggesting that chain transfer to aluminum is the predominant termination mechanism. It is noteworthy that the introduction of an excessively bulky o-bis(aryl)methyl substituent adjacent to the imine-N produces low molecular-weight Al-PEs (M_v 1.6-10.1 × 10³) due to the enhanced rate of chain transfer to alkylaluminium groups during polymerization. Copyright

Full text of DOI:10.1002/aoc.2984

Full Paper
Received: 21 November 2012
Revised: 9 February 2013
Accepted: 28 February 2013
Published online in Wiley Online Library: 8 May 2013
(wileyonlinelibrary.com) DOI 10.1002/aoc.2984
Zirconium complexes bearing bis(phenoxy-imine)
ligands with bulky o-bis(aryl)methyl-substituted
aniline groups: synthesis, characterization and
ethylene polymerization behavior
Aike Li, Haiyan Ma and Jiling Huang*
A series of bis(phenoxy-imine) zirconium complexes bearing bulky o-bis(aryl)methyl-substituted aryl groups on the aniline
moiety have been synthesized, characterized and tested as catalyst precursors for ethylene polymerization. 1H NMR spectros-
copy suggests that these complexes exist as a single chiral C2-symmetric isomer in the solution. X-ray crystallographic analysis
of the resulting biszwitterionic-type adduct complex C1 ꢀ 2HCl reveals that the phenoxy-imine groups function as a
monodentate phenoxy ligand and the oxygen atoms are oriented trans to each other at the central metal atom. Using mod-
ified methylaluminoxane (MMAO) as co-catalyst, C1 ꢀ 2HCl, C2–C6 exclusively produce linear aluminium-terminated polyethyl-
enes (Al-PEs) with high activity (up to 16.89 ꢁ 106 g PE (mol Zr h)ꢂ1, suggesting that chain transfer to aluminum is the
predominant termination mechanism. It is noteworthy that the introduction of an excessively bulky o-bis(aryl)methyl substit-
uent adjacent to the imine-N produces low molecular-weight Al-PEs (Mv 1.6–10.1 ꢁ 103) due to the enhanced rate of chain
transfer to alkylaluminium groups during polymerization. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found on the online version of this article.
Keywords: zirconium; phenoxy-imine; polymerization; ethylene; polyethylene
development of a catalyst which favors chain transfer to aluminium
and selectively produces Al-PEs with high activity and great
Introduction
During the last two decades, non-metallocene metal catalysts have
attracted considerable attention in academic and industrial fields
because they can provide high catalyst efficiency and greater control
of molecular weight and polymer microstructure.[1–4] In particular,
the discovery of homogeneous well-defined, single-site transition
metal catalysts has provided an excellent opportunity to study
polymerization reaction mechanisms, and the relationship between
polymer microstructure and the architecture of the catalyst. As a
result of extensive studies, group IV non-metallocene catalysts bear-
ing phenoxy-imine ligands developed by Fujita and coworkers are an
excellent example.[2–17] This type of catalyst can be used to produce
many important polyolefins, such as selective vinyl-terminated
polyethylenes (vinyl-PEs)[5] and aluminium-terminated polyethylenes
(Al-PEs),[6] ultrahigh-molecular-weight linear polyethylenes,[7–9]
isotactic[10–13] and syndiotactic polypropylene,[14,15] regio- and
stereo-irregular high-molecular-weight poly(higher a-olefin)s[16] and
a wide variety of ethylene/a-olefin copolymers.[4] Importantly, the
catalytic behavior of these pre-catalysts and the microstructure of
the resultant polyolefins could be fine-tuned by modifying the
substituents of the ligands.
control of molecular weight is a challenging subject and quite
desirable.[6,17,22–24] Recently, Kempe reported that hafnium
aminopyridinate complexes were capable of producing lower-
molecular-weight Al-PEs (Mw 0.4–22 ꢁ 103, Mw/Mn 1.2–3.2) with
reasonable activity in the presence of triethylaluminium (TEA) as a
chain transfer agent; it was also revealed that molecular weight can
be controlled by varying the alkylaluminium concentration.[24] On
the basis of a ligand-oriented catalyst design concept, Fujita et al.
developed a bis(phenoxy-imine) zirconium complex (e) incorporat-
ing a 2-isopropylphenyl group on the imine-N,[6] which favors
chain transfer to aluminium and selectively yields Al-PEs with high
efficiency (Scheme 1). Upon activation with methylaluminoxane
(MAO), very-high-molecular-weight PEs (Mv 1130 ꢁ 103)
were obtained.[26] Conversely, bis(phenoxy-imine) zirconium
catalysts (a, b and c) containing sterically less encumbered sub-
stituents (e.g. methyl, cyclopentyl, phenyl) on the imine-Ns
selectively formed vinyl-terminated PEs (vinyl-PEs).[17]
Considering the fact that variation of the ligand structure may
lead to profound changes in catalytic activity and polymer
Al-PEs are highly potential precursors for end-functionalized PEs,
block copolymers and graft copolymers.[18–22] Generally speaking, a
chain transfer reaction involves b-H transfer to a metal or monomer
and chain transfer to aluminium in ethylene polymerization. How-
ever, the selective production of Al-PEs is generally limited because
homogeneous catalysts usually have a marked preference for b-H
elimination as the chain termination fashion.[22] Therefore, the
*
Correspondence to: Jiling Huang, Laboratory of Organometallic Chemistry,
East China University of Science and Technology, 130 Meilong Road, Shanghai
200237, People’s Republic of China. E-mail: jlhuang@ecust.edu.cn
Laboratory of Organometallic Chemistry, East China University of Science
Technology, Shanghai 200237, People’s Republic of China
Appl. Organometal. Chem. 2013, 27, 341–347
Copyright © 2013 John Wiley & Sons, Ltd.

A. Li et al.
Results and Discussion
Ligand and Complex Synthesis
A general synthetic route for phenoxy-imine ligands is shown
in Scheme 2. Ligands L1–L6 were synthesized by Schiff base
condensation of the corresponding salicylaldehyde with equiva-
lent o-bis(aryl)methyl-substituted anilines A1–A4 in ethanol using
p-toluenesulfonic acid as catalyst. They have been well character-
ized by elemental analysis, FT-IR, 1H NMR and 13C NMR.
Scheme 1. Bis(phenoxy-imine) zirconium dichloride complexes used for
The desired zirconium complexes C2–C6 possessing two
phenoxy-imine chelate ligands were synthesized according to
literature procedures[5,26,27] as shown in Scheme 3. Complexes
C2–C6 were obtained in moderate to good yield (29.6–70.3%)
by the reaction of ZrCl4 ꢀ 2THF with 2 equiv. of the lithium salt
of the corresponding ligand in tetrahydrofuran (THF) at ꢂ78ꢃC.
Purification was performed by recrystallization from a mixture
of CH2Cl2–petroleum ether or Et2O–petroleum ether at ambient
temperature. These complexes were characterized by elemental
the selective synthesis of vinyl- and Al-terminated PEs.
microstructure, we herein report the syntheses and ethylene poly-
merization behavior of new bis(phenoxy-imine) zirconium com-
plexes C1 ꢀ 2HCl, C2–C6, which incorporate sterically larger o-bis
(aryl)methyl-substituted anilines groups on the ligand backbone.
When activated with modified methylaluminoxane (MMAO), these
zirconium complexes were highly active toward ethylene polymer-
ization and produced low-molecular-weight Al-PEs. The effects of
changing reaction parameters such as [Al]/[Zr] ratio, polymerization
temperature and polymerization time are discussed in detail. We
also report an X-ray diffraction study for the molecular structure
of a biszwitterionic-type adduct complex C1ꢀ 2HCl.
1
analysis, 1H NMR and 13C NMR. H and 13C NMR spectra showed
that complexes C2–C6 have a single chiral C2-symmetric isomer
in chloroform-d solution at room temperature. This is in agree-
ment with the observed structures of previously reported analo-
1
gous zirconium complexes.[27] Moreover, the H NMR spectra of
complexes C2–C6 revealed the chemical shift of CH¼N protons
Scheme 2. General synthetic route of ligands L1–L6.
Scheme 3. Synthesis of complexes C2–C6.
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 341–347

Bis(phenoxy-imine) zirconium complexes for ethylene polymerization
to high field at approximately 0.21–0.46 ppm relative to free li-
gands. These results indicate the obvious coordination of the im-
ino nitrogen atom to the metal center.
In contrast, similar reaction of the lithium salt of L1 with ZrCl4
2THF did not lead to the desired zirconium complex and gave a
precipitate that would not dissolve in ordinary organic solvents. Un-
expectedly, during the attempt to treat ZrCl4 ꢀ 2THF with silylated
iminophenol ((3,5-Me2ꢂ2-OSiMe3)C6H2CH¼N((2,6- (CHPh2)2ꢂ4-
MeC6H2)) via trimethylsilyl chloride (Me3SiCl) elimination,[28]
a
biszwitterionic adduct complex C1ꢀ 2HCl was obtained (Scheme 4).
So far, rare examples related to biszwitterionic-type adduct with
an iminium structure have been observed by Erker and co-
workers[29,30] for a bis(salicylaldiminato)titanium complex and a series
of bis(‘ferrocene-salaldiminato’)zirconium complexes with ferrocene-
derived salicylaldimine-type ligands, respectively. The adduct C1 ꢀ 2
HCl was characterized by elemental analysis, 1H NMR and X-ray
1
diffraction. The H NMR of C1 ꢀ 2HCl revealed three singlets at 3.25,
1.80 and 1.71 ppm for the Me groups, which is consistent with the
proposed C2-symmetric configuration. A doublet at 13.71 ppm
(J= 16.2 Hz) for ¼NHꢂ resonance was also detected.
Figure 1. Molecular structure of the biszwitterionic complex C1 ꢀ 2
HCl, with thermal ellipsoids at the 50% probability level. The
disordered dichloromethane solvate molecule was removed by
PLATON/SQUEEZE.[46]
Single crystals suitable for the X-ray crystal structure anal-
ysis of the adduct complex C1 ꢀ 2HCl were obtained from
dichloromethane. The molecular structure of C1 ꢀ 2HCl is
shown in Fig. 1. Crystallographic data and structure refine-
ment details are listed in Table 1.
was carried out in toluene under an ethylene pressure of 1 Mpa
and the results are summarized in Table 2.
These zirconium complexes showed high catalytic activity (up to
16.89 ꢁ 106 g PE (mol Zr h)ꢂ1) toward ethylene polymerization
and exclusively produce low-molecular-weight linear Al-PEs
(Mv 1.6–10.1 ꢁ 103). End group analysis of the polymers obtained
(polymerization were terminated by the addition of ethanol) by
1H NMR as well as infrared spectroscopy showed peaks corre-
sponding to methyl end groups, and there were no peaks of
unsaturated double-bond end groups formed by b-H elimina-
tion due to the marked preference for chain transfer as the chain
termination mechanism.
X-ray crystallographic analysis of the adduct C1 ꢀ 2HCl revealed
that it exists as a biszwitterionic complex in which H and Cl are
situated in close proximity to the imine nitrogen atoms and the
central metal (Fig. 1 and Table 1). The X-ray molecular structure
also indicated that in the solid state it adopts a six-coordinate,
distorted octahedral geometry around the zirconium center. The
molecule contains a central ZrCl4 core that is near to square-
planar (Cl1ꢂZr1ꢂCl2 88.83(3)ꢃ, Cl1ꢂZr1ꢂCl1A 180.0ꢃ). In the crys-
tal, the two phenoxy-imine groups function as a monodentate
phenoxy ligand and the salicyl oxygen atoms are oriented trans
to each other at the central zirconium atom. It is noted that the
bond lengths of ZrꢂO (Zr1ꢂO1 2.041(2) Å) and the bond lengths
of ZrꢂCl (Zr1ꢂCl1 2.477(8) Å, Zr1ꢂCl2 2.458(9) Å) are slightly longer
than that of the reported bis(phenoxy-imine) zirconium dichloride
complexes.[5,26,31] The 2,6-dibenzhydryl-4-methylphenyl substitu-
ent at nitrogen is significantly rotated relative to the plane of the
salicylaldiminium core (θ C7ꢂN1ꢂC8ꢂC9 71.4(4)ꢃ). In addition, in
the complex C1ꢀ 2HCl the imino C¼N bond has a distinctive
double-bond character (N1ꢂC7 1.282(4) Å, C7ꢂN1ꢂC8 123.8(3)ꢃ).
Effect of Catalyst Structure on Ethylene Polymerization
Fujita et al. reported that bis(phenoxy-imine) zirconium catalysts
(c) bearing a phenyl group on the imine-Ns selectively formed
vinyl-PEs of low molecular weight (Scheme 1).[26] In sharp
contrast, catalysts containing sterically encumbered substituents
(e.g. 2-methylphenyl (d) and 2-isopropylphenyl (e)) at the same
position selectively yielded Al-PEs and dramatically increased
the molecular weight of the resultant polymers in the order H
(Mv 9 ꢁ 103) < Me (Mv 320 ꢁ 103) < iPr (Mv 1130 ꢁ 103). In our
studies, all zirconium complexes C1 ꢀ 2HCl, C2–C6 containing a
sterically larger o-bis(aryl)methyl-substituted aryl group attached
to the imine-N produced polyethylenes of low molecular weight
(Mv 1.6–10.1 ꢁ 103). The polymer samples produced by C1 ꢀ 2HCl,
C2–C6 were found to be exclusively Al-PEs, suggesting that chain
Ethylene Polymerization
The catalytic properties of zirconium complexes C1 ꢀ 2HCl, C2–C6
for ethylene polymerization were evaluated using modified
methylaluminoxanes (MMAO) as co-catalyst. The polymerization
Scheme 4. Synthesis of complex C1 2HCl.
ꢀ
Appl. Organometal. Chem. 2013, 27, 341–347
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. Li et al.
it is probably due to excessive steric congestion exerted by a
sterically larger bis(aryl)methyl group on the aniline moiety in
close proximity to the active site,[32] which increases the
rate of chain termination more significantly relative to that of
chain propagation. In particular, the adduct C1 ꢀ 2HCl introducing
a 2,6-dibenzhydryl-4-methylphenyl group afforded very-low-
molecular-weight PEs (Mv 2.9 ꢁ 103).
Table 1. Crystal data and structure refinement for complex C1 2HCl
ꢀ
Empirical formula
C84H74Cl4N2O2Zr
Formula weight
1376.47
P-1
Space group
Crystal system
Triclinic
a (Å)
b (Å)
c, Å
11.9879(11)
The results discussed above suggest that the effects of the
substituents attached to the imine-N of these complexes might
lead to unexpected changes in activity and in molecular weight
and polymer microstructure.[5,6,17,26]
12.2846(11)
16.1005(15)
a (ꢃ)
109.892(2)
b (ꢃ)
98.741(2)
It was found that changing the alkyl substituent R1 significantly
affected polymerization activity. Complex C2 bearing a methyl
g (ꢃ)
Volume (Å3)
96.928(2)
2165.5(3)
substituent gave polyethylene with an activity of 0.52ꢁ 106
g
Z
1
PE (mol Zr h)ꢂ1 (Mv = 8.6ꢁ 103). The catalytic activity of ethylene
polymerization was greatly enhanced by replacement of the
methyl group (R1) in complex C2 with a sterically larger tert-butyl
group (C5 and C6). Complexes C5 and C6 displayed activities of
Dcalcd (Mg mꢂ3
)
1.056
Absorption coefficient (mmꢂ1
)
0.291
F(000)
716
Crystal size (mm)
θ range (ꢃ)
0.297 ꢁ 0.213 ꢁ 0.101
1.82–26.00
8.28 ꢁ 106 g PE (mol Zr h)ꢂ1 and 8.41 ꢁ 106 g PE (mol Zr h)ꢂ1
,
respectively. The enhancement in catalytic activity may be ascribed
to the effective separation between the cationic active species and
the anionic co-catalyst as a result of introducing bulkier R1 on the
ortho-position of the oxygen atom in the phenoxy-imine ligand.
This therefore favors the coordination of ethylene to the metal
and prevents deactivation of the metal site.[3,26,33] Fujita also
reported that this type of group IV metal bis(phenoxy-imine)
catalyst bearing a bulkier tert-butyl group at the R1 position
displayed higher ethylene polymerization activity than correspond-
ing catalysts with a methyl substituent at the same position.[26]
The steric bulk of the R2 substituent (para to the phenolic
oxygen) had very little influence on catalytic activity. The activity
value obtained with complex C5 (R2 = tBu) is slightly lower than in
complex C6 (R2 = Me). Although there are differences in
electronic structure between the complexes, the R2 position is
located far from the active site. This case has been reported in
the literature.[26]
Index ranges
ꢂ14 < ¼ h < ¼ 14
ꢂ15 < ¼ k < ¼ 14
0 < ¼ l < ¼ 19
8354/8354 [R(int) = 0.0000]
98.1%
Reflections collected/unique
Completeness to θ (%)
Data/restraints/parameters
Goodness-of-fit on F^2
Final R indices [I > 2 sigma(I)]
R indices (all data)
8354/0/416
0.937
R1 = 0.0555, wR2 = 0.1320
R1 = 0.0829, wR2 = 0.1446
0.360 and ꢂ0.293
Largest diff. peaka and hole (e/Åꢂ3
)
aLargest peak (hole) in difference Fourier map, R1 = Σ(| |Fo| ꢂ |Fc| |)/
Σ|Fo|, wR2 = [Σ(|Fo|2 – |Fc|3)2/Σ(F2o)]1/2
.
transfer to aluminium is the predominant termination mecha-
nism. Although the reason for the production of the low-
molecular-weight PEs with C1 ꢀ 2HCl, C2–C6 is unclear at present,
Table 2. Ethylene polymerization results catalyzed by zirconium complexes C1ꢀ2HCl, C2–C6/MMAO catalytic systemsa
Entry
Complex (mmol)
[Al]/[Zr]
Time (min)
Temp. (ꢃC)
Yield (mg)
Activityb
Mvc (103 g molꢂ1
)
1
C1 ꢀ 2HCl (1.0)
C2 (1.0)
C3 (1.0)
C4 (1.0)
C5 (1.0)
C6 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
C5 (1.0)
2500
2500
2500
2500
2500
2500
500
5
5
30
30
30
30
30
30
30
30
30
30
0
198
43
2.38
0.52
1.54
0.13
8.28
8.41
2.04
3.12
9.10
9.78
11.92
6.66
2.35
16.89
3.17
1.65
2.9
8.6
9.6
n.d.
4.4d
5.0
10.1
7.0
4.1
3.4
4.8
3.8
1.6
2.9
7.6
9.0
2
3
5
128
11
4
5
5
5
690
701
170
260
758
815
993
555
196
563
792
825
6
5
7
5
8
1250
5000
7500
2500
2500
2500
2500
2500
2500
5
9
5
10
11
12
13
14
15
16
5
5
5
60
90
30
30
30
5
2
15
30
aPolymerization conditions: in toluene, V = 25 ml, ethylene pressure 1.0 MPa.
b106 g PE (mol Zr h)ꢂ1
.
cIntrinsic viscosity was determined in decahydronaphthalene at 135ꢃC and molecular weight was calculated using the relation: [ꢀ]=6.77ꢁ 10ꢂ4 M0v.67
.
dMw/Mn were determined by GPC; for entry 5, Mw/Mn = 2.36.
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Appl. Organometal. Chem. 2013, 27, 341–347

Bis(phenoxy-imine) zirconium complexes for ethylene polymerization
13
12
The effect of the R3 substituent at the substituted bis(aryl)
methyl group of the aniline moiety on catalytic activity has also
been investigated. Compared with C2, the introduction of a
methyl group (C3) resulted in an increase of catalytic activity to
1.54 ꢁ 106 g PE (mol Zr h)ꢂ1, while the introduction of a fluorine
group (C4) led to a decrease of catalytic activity to 0.13 ꢁ 106 g PE
(mol Zr h)ꢂ1. Catalytic activity followed the order C4 (F) < C2 (H)
C3 (Me). The results indicate that the electron-donating effect of
the R3 group plays a key role in the enhanced catalytic activity,
probably due to the increased positive charge which favored
the stability of the catalytic active species.
The above results indicate that the introduction of a bulkier
alkyl substituent at the R1 position significantly enhanced the
catalytic activity for ethylene polymerization, while the steric bulk
of the R2 substituent has little effect on catalytic activity. The
introduction of an electron-donating group (R3) led to an
increase in catalytic activity of the complexes.
10
9
11
10
9
v
8
7
8
7
6
6
5
5
4
4
3
2
3
2
1
0
-10
1
100
0
10 20 30 40 50 60 70 80 90
Figure 3. Plots of polymerization temperature versus catalytic activity
and viscosity average molecular weight (Mv) of the resultant PEs obtained
by C5/MMAO catalytic system (Table 2, entries 5, 11–13).
The ethylene polymerization behavior of the biszwitterionic-type
adduct C1 ꢀ 2HCl was also investigated (Table 2, entry 1). Upon
activation with MMAO, C1 ꢀ 2HCl provided lower-molecular-weight
PEs (Mv 2.9 ꢁ 103) with high activity (2.38 ꢁ 106 g PE (mol Zr h)ꢂ1).
highest activity of the catalyst was obtained at 0 ꢃC (11.92 ꢁ 106
g PE (mol Zr h)ꢂ1) and gradually decreased to 2.35 ꢁ 106 g PE
(mol Zr h)ꢂ1 at 90ꢃC, which may be due to instability of the
catalytic active species at high temperature. In addition, the
Mv value of polymers slightly decreases as the polymerization
temperature increases from 0 to 60ꢃC. Further increasing the
temperature to 90ꢃC resulted in a significant decrease of the
Mv value (Mv = 1.6 ꢁ 103). This can be attributed to a facilitated
chain transfer reaction at elevated temperatures.[33] It is worth
noting that the decrease in Mv value by changing the polymer-
ization temperature is smaller than that induced by variation of
MMAO concentration. Consistent with previous work,[6] these
observations demonstrate that chain transfer to aluminium is
a major termination reaction and a viable route for the forma-
tion of low-molecular-weight polymers.
To investigate the catalytic lifetime of complex C5, ethylene
polymerizations were conducted for 2, 5, 15, and 30 min at
30ꢃC (Fig. 4). The relationship between the polymerization time
and the catalytic activity indicates that the active species could
be formed rapidly during the initial stage of the polymerization.
The highest activity was obtained in a period of 2 min
(16.89 ꢁ 106 g PE (mol Zr h)ꢂ1). Although the catalytic activity
decreases rapidly with prolonged reaction time, C5 displayed
Effect of Polymerization Conditions on Catalytic Behavior
Because of the better catalytic performance observed by the
complex containing a tert-butyl at the R1 position, complex C5
was investigated in detail by changing reaction parameters such
as [Al]/[Zr] ratio, polymerization temperature and polymerization
time (Table 2, entries 5, 7–16).
Variation of the MMAO/catalyst molar ratios, expressed here as the
[Al]/[Zr] molar ratio, displayed significant effects on catalyst activity
and the molecular weight of the resultant polymers (Fig. 2). With
the increase of the [Al]/[Zr] molar ratio from 500 to 2500, polymeriza-
tion activity rapidly increased from 2.04ꢁ 106 to 8.28ꢁ 106 g PE (mol
Zr h)ꢂ1 may be due to an increase in the number of active sites.
Meanwhile, the Mv value of the polymers was significantly
decreased from 10.1 ꢁ 103 to 4.4 ꢁ 103, possibly due to the
enhanced rate of chain transfer to aluminium for the termination.[6]
After further increasing the [Al]/[Zr] molar ratio to 7500, a slight
increase in the catalytic activity and decrease of the Mv value of the
polymer were observed.
The influence of polymerization temperature on activity was
studied at temperatures between 0 and 90ꢃC (Fig. 3). The
11
18
10
9
10
9
17
10
9
16
15
14
13
12
11
10
9
v
8
7
6
5
4
3
2
1
8
7
6
5
4
3
2
1
8
7
6
5
4
3
2
1
0
v
8
7
6
5
4
3
2
1
0
0
1000 2000 3000 4000 5000 6000 7000 8000
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
[Al]/[Zr]
Time / min
Figure 2. Plots of [Al]/[Zr] molar ratio versus catalytic activity and viscosity
average molecular weight (Mv) of the resultant PEs obtained by C5/MMAO
catalytic system (Table 2, entries 5, 7–10).
Figure 4. Plots of polymerization time versus catalytic activity and
viscosity average molecular weight (Mv) of the resultant PEs obtained
by C5/MMAO catalytic system (Table 2, entries 5, 14–16).
Appl. Organometal. Chem. 2013, 27, 341–347
Copyright © 2013 John Wiley & Sons, Ltd.
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A. Li et al.
bis(aryl)methanol[38–41] (i.e. bis(4-methylphenyl)methanol, bis(4-
fluorophenyl)methanol, diphenyl methanol) – were prepared
according to literature procedures. 2,6-Dibenzhydryl-4-methylaniline
(A1) and 2-benzhydryl-4,6-dimethylaniline (A2) were prepared
according to literature procedures.[42,43] ZrCl4 ꢀ 2THF[44] were synthe-
sized according to published procedures. All other chemicals were
commercially available and used as received.
high activity for ethylene polymerization within 30 min
(1.65 ꢁ 106 g PE (mol Zr h)ꢂ1). As expected, the molecular
weight of the resultant polymers increased with increasing
reaction time.
Polymer analysis
1H NMR analysis of the polymer obtained by complex C5
revealed that no unsaturated double bonds (no signals for
olefinic protons from 4.0 to 6.0 ppm) exited within the polymer,
which suggests that chain transfer to aluminium plays a dominant
role in chain termination, leading to the exclusive formation of Al-
PEs.[6,17] Infrared spectroscopy also confirmed that the resulting PEs
are Al-PEs. The lack of absorption bands between 1680 and 1620
cmꢂ1 demonstrates that the end groups of the PEs are saturated.[34]
Typical absorption for linear PEs at 1468 and 720 cmꢂ1 can be
observed. The double peaks at 720 and 731 cmꢂ1 prove that the
resulting PEs are crystalline.[34] Gel permeation chromatography
(GPC) of the polymer generated from complex C5 (Table 2, entry 5)
revealed a moderate molecular weight distribution (Mw/Mn) of 2.36,
suggesting that the polymer is formed by a single-site catalytic
species mechanism. According to the results of differential scanning
calorimetry (DSC), polymer samples showed sharp single melting
peaks at 129ꢃC and very high crystallinity, indicating that the
polymer is a linear polyethylene.[35]
Measurements
NMR spectra of complexes were recorded on a Bruker Avance-
400 spectrometer with CDCl3 or C6D6 as the solvent at ambient
temperature unless otherwise indicated. H NMR and 13C NMR
1
chemical shifts are reported in ppm using the residual solvent
resonances or reported relative to tetramethylsilane. Elemental
analyses (C, H, N) were carried out on an EA–1106 type analyzer.
IR spectra were recorded on a PerkinElmer System 2000 FT-IR
1
spectrometer. H NMR spectra of PEs were recorded on a Bruker
Avance-500 spectrometer using o-dichlorobenzene with 20%
benzene-d6 as a solvent at 120ꢃC.[5] The intrinsic viscosities (ꢀ)
of PEs were measured with an Ubbelohde viscometer in
decahydronaphthalene at 135ꢃC and viscosity average molecular
weights (Mv) were calculated as follows: [ꢀ] = 6.77 ꢁ 10ꢂ4 M0v.67 [45]
.
Molecular weights (Mn and Mw) and polydispersities (Mw/Mn) of
polyethylene were determined by high-temperature GPC using
a Waters 150 ALC/GPC system with 1,2-dichlorobenzene as
solvent at 135ꢃC. DSC trace and melting points of polyethylene
were obtained from the second scanning run oꢂn1 a Universal
Conclusion
V2.3C TA instrument at a heating rate of 10ꢃC min
.
In summary, six new bis(phenoxy-imine) zirconium complexes bear-
ing bulky o-bis(aryl)methyl-substituted aryl groups on the aniline
moiety have been successfully synthesized and characterized. All
zirconium complexes exhibit only one chiral C2-symmetric isomer
in the solution. In addition, the biszwitterionic-type adduct C1 ꢀ 2HCl
was obtained during attempts to synthesize the corresponding bis
(phenoxy-imine) zirconium dichloride complex. In the presence of
modified methylaluminoxane (MMAO), these zirconium complexes
showed high activity toward ethylene polymerization and produce
linear Al-PEs with low molecular weights (Mv 1.6–10.1 ꢁ 103). More-
over, it was revealed that the introduction of bulky o-bis(aryl)methyl
substituents on the aniline moiety of the phenoxy-based ligands
significantly increased the rate of chain transfer to aluminium as
the chain termination mechanism.
Synthesis and Characterization of Complexes C1 ꢀ 2HCl, C2–C6
See the supporting information.
Crystal Structure Determination
Single-crystal X-ray diffraction studies for complex C1 ꢀ 2HCl was
carried out on a Bruker AXSD8 diffractometer with graphite-
monochromated Mo-Ka radiation (l = 0.71073 Å) at 293 K. Unit
cell dimensions were obtained with least-square refinements.
Intensities were corrected for Lorentz and polarization effects
and empirical absorption. The structures were solved by direct
methods[47] and refined by full-matrix least squares on F2.[48] All
non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were included in idealized position. Details of
the crystal data and structure refinements are summarized in
Table 1. Crystallographic data have been deposited with the
Cambridge Crystallographic Data Centre under the number CCDC
896278 (for C1 ꢀ 2HCl). Copies of the data can be obtained free
of charge from the Cambridge Crystallographic Data Centre
(www.ccdc.cam.ac.uk/data_request/cif).
Experimental
General Considerations
All manipulations of air- and/or moisture-sensitive compounds
were performed under a dry argon atmosphere using standard
Schlenk techniques or a glove-box unless otherwise indicated.
Methylene dichloride and chloroform-d were dried over calcium
hydride under argon prior to use. Toluene, diethyl ether and THF
were refluxed with sodium/benzophenone ketyl and were distilled
under nitrogen prior to use. Benzene-d6 was dried over sodium
under argon. ZrCl4 was purchased from Aldrich. Polymerization-
grade ethylene was directly used for polymerization. Modified
methylaluminoxane (MMAO) (2.5 M in toluene) was purchased
from Sigma-Aldrich. n-BuLi (2.4 M in n-hexane) was purchased
from J&K Chemical Ltd. The starting materials – salicylaldehydes[36,37]
(i.e., 3-tert-butyl-5-methyl-2-hydroxybenzaldehyde, 3,5-dimethyl-2-
hydroxybenzaldehyde, 3,5-di-tert-butyl-2-hydroxybenzaldehyde) and
Polymerization of Ethylene
Ethylene polymerization was carried out in a 100 ml autoclave
equipped with a magnetic stirrer. The autoclave was heated at
100ꢃC under vacuum for 30 min, cooled to the desired temperature
by immersing in a thermostatically heated bath, then filled with
ethylene. Appropriate amounts of modified methylaluminoxane
(MMAO) solution and toluene were added to the autoclave, which
was filled with ethylene for 15 min at the reaction temperature. After
the appropriate amount of toluene solution of zirconium complex
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Appl. Organometal. Chem. 2013, 27, 341–347

Bis(phenoxy-imine) zirconium complexes for ethylene polymerization
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SUPPORTING INFORMATION
Supporting information may be found in the online version of
this article. Text giving experimental details for the synthesis
and characterization of complexes C1 2HCl and C2-C6, figures
ꢀ
giving representative NMR spectra of the polymer sample, a table giving
selected bond lengths and angles of C1 2HCl and a CIF file giving X-ray
ꢀ
crystallographic data of C1 2HCl are provided.
ꢀ
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Acknowledgments
[31] S. Chen, X. Zhang, H. Ma, Y. Lu, Z. Zhang, H. Li, Z. Lu, N. Cui, Y. Hu,
The authors are grateful for financial support from the key project of the
Chinese Ministry of Education (No. 109064) and the National Natural
Science Foundation of China (NSFC) (No. 21274041, 20774027).
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Appl. Organometal. Chem. 2013, 27, 341–347
Copyright © 2013 John Wiley & Sons, Ltd.
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