Synthesis and characterization of titanium(IV) complexes bearing end functionalized biphenyl: Efficient catalysts for synthesizing high molecular weight polyethylene

Article abstract of DOI:10.1016/j.inoche.2014.01.005

Two phenoxy-imine complexes 1d and 2d bearing bulky substituents were synthesized by introducing ethyl or vinyl substituted phenyl to the para position of N-aryl group and characterized by ¹H NMR, ¹³C NMR, and elemental analysis. The X-ray crystallographic analysis revealed a distorted octahedral geometry of complex 2d. When activated by dried MAO, two titanium complexes exhibited good to high activity (up to 4.5 × 10 ⁶ g mol^{- 1}(Ti) h^{- 1}) for ethylene polymerization. Using these two complexes, ultra-high molecular weight polyethylene (UHMWPE, M_w > 1,000,000) can be prepared only by changing polymerization time in mild conditions.

Full text of DOI:10.1016/j.inoche.2014.01.005

Inorganic Chemistry Communications 41 (2014) 68–71
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterization of titanium(IV) complexes bearing end
functionalized biphenyl: Efficient catalysts for synthesizing high
molecular weight polyethylene
⁎
Yi Wang, Hong Fan , Suyun Jie, Bo-Geng Li
Department of Chemical & Biological Engineering, State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two phenoxy-imine complexes 1d and 2d bearing bulky substituents were synthesized by introducing ethyl or vinyl
substituted phenyl to the para position of N-aryl group and characterized by 1H NMR, 13C NMR, and elemental anal-
ysis. The X-ray crystallographic analysis revealed a distorted octahedral geometry of complex 2d. When activated
by dried MAO, two titanium complexes exhibited good to high activity (up to 4.5 × 106 g mol−1(Ti) h−1) for eth-
ylene polymerization. Using these two complexes, ultra-high molecular weight polyethylene (UHMWPE,
Mw N 1,000,000) can be prepared only by changing polymerization time in mild conditions.
© 2014 Elsevier B.V. All rights reserved.
Received 14 October 2013
Accepted 5 January 2014
Available online 11 January 2014
Keywords:
Phenoxy-imine titanium(IV) complex
UHMWPE
Bulky substituents
Ethylene polymerization
Ultra-high molecular weight polyethylene (UHMWPE), the polyeth-
ylene with molecular weight higher than 1,000,000 g/mol, shows an
optimized mechanical properties, such as tensile strength, modulus,
and abrasion resistance, which reach an extent that the polymer can
be applied for special applications, including prostheses, additives for
foaming resins, ropes for replacement of steel cables and bullet-
resistance armor [1]. Now, the industrial UHMWPE production is dom-
inated by heterogeneous Ziegler-Natta and Philips catalysts and the
resulted polymer exhibits broad molecular weight distribution
(MWD) and chemical composition distribution (CCD), which directly
result in inferior mechanical property of polymer [2]. However, with
the advent of single site catalysts (SSC), the optimized control over
molecular weight, specific tacticity and molecular weight distribution
of UHMWPE becomes possible; thus, the research for industrial
UHMWPE production by SSC attracts great attention from both acade-
mia and industry. Furthermore, the ultimate structural control over
UHMWPE microstructure needs “living” catalysts, which can fine tune
the molecular weight and keep a very narrow molecular weight distri-
bution of polymer. This is just their uniqueness rather than common
SSC [3].
pentafluoroaniline]TiCl2 to the support of MgCl2·0.24AlEt2.30(OEt)0.70
and applied this heterogeneous catalyst for ethylene polymerization to
prepare UHMWPE (Mw = 2152000, PDI = 3.1) under a slightly harsh
condition (10 bar of ethylene, 50 °C, 2 h). Romano and Reyes [6] modi-
fied MAO solution with phenol to activate bis[N-(3-t-butylsalicylidene)
pentafluoroaniline]TiCl2 in ethylene polymerization and easily obtained
UHMWPE (Mn = 1,800,000) in mild conditions (1.1 bar, 30 °C). From
the research of ethylene polymerization catalyzed by some FI-Zr
complexes [7–11], a rule was disclosed that the molecular weight of re-
sulted PE would increase by increasing the steric hindrance of ortho-
substituents on the N-aryl group. However, increasing the bulk size of
meta or para substituents on the N-aryl group would not result in the
same change for these FI-Zr complexes. In this research, we introduce
large substituents (4-ethylphenyl and 4-vinylphenyl) to para position
of N-aryl group on titanium complexes and implement ethylene poly-
merization to initially explore the feasibility of synthesizing UHMWPE
by using these two complexes in mild polymerization conditions. We re-
port here the synthesis and structural characterization of complexes 1d
and 2d bearing bulky substituents.
The synthesis of phenoxyimine ligands and complexes was summa-
rized in Scheme 1. The starting material a (substituted aniline) for li-
gand synthesis was prepared by Suzuki coupling reaction of 4-bromo-
2,3,5,6-tetrafluoroaniline and 4-vinylphenylboronic acid at 85 °C and
purified by column chromatography prior to use. Ligand b was prepared
by the condensation reaction of 3-tertbutylsalicylaldehhyde and the
bulky aniline a in ethanol at 80 °C, then purified by recrystallization.
The hydrogenation of ligand b using HSi(OEt)3 as H donor and Pt cata-
lyst afforded the formation of ligand c. The addition of ether solution
of lithium salts of ligand b or c to 0.5 equiv. of TiCl4 in ether gave
bis(phenoxyimine) titanium complexes 1d and 2d in moderate to
Since 1997, the ortho-fluorinated FI catalysts were invented and gave
a breakthrough to living coordination polymerization, which made con-
trolled synthesis of UHMWPE possible. Pärssinen and Luhtanen [4]
synthesized complex bis[N-(salicylidene)-2,6-difluoroanilinato]TiCl2
and applied it for preparing UHMWPE (Mw = 1,540,000,PDI = 1.5) in
suitable conditions (10 bar of ethylene, 60 °C, 50 min). Severn and
Chadwick [5] immobilized the complex bis[N-(3-t-butylsalic-ylidene)
⁎
Corresponding author. Tel.: +86 13819191707.
E-mail address: hfan@zju.edu.cn (H. Fan).
1387-7003/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2014.01.005

Y. Wang et al. / Inorganic Chemistry Communications 41 (2014) 68–71
69
HO
OH
B
O
OH
TiCl2
N
2
NH2
N
NH2
1.n-BuLi
F
F
F
F
OH
F
F
F
F
F
F
F
F
F
F
F
F
2.1/2TiCl4
O
Pd(PPh3)4, Na2CO3
EtOH, PSTA, refulx
DME, EtOH,H2O,reflux
Br
1d
b
a
H2PtCl6
HSi(OEt)3
toluene
reflux
O
OH
TiCl2
N
2
N
1.n-BuLi
F
F
F
F
F
F
F
F
2.1/2TiCl4
c
2d
Scheme 1. Synthesis of complex 1d and 2d.
high yield. The 1H NMR spectra of complex 1d reveal that the chemical
shift of protons on end group (CH_CH2) appear at 5.36 ppm (_CH2),
5.84 ppm (_CH2), and 6.76 ppm (CH_), respectively, while the chem-
ical shifts of CH2CH3 on complex 2d shift to 1.30 ppm (CH3), and
2.72 ppm (CH2). The carbon chemical shifts of both benzene rings and
side groups also show up in the 13C NMR spectra of complexes 1d and
2d. The elemental analysis reveals a high purity of the complexes.
The structure of complex 2d was determined by single-crystal X-ray
diffraction analysis (Fig. 1), which shows that the geometry around
the titanium center can be described as distorted octahedral with
cis-N/trans-O/cis-Cl and two ligands take a C2 symmetry. The two cis
components N1\Ti1\N2 and Cl2\Ti1\Cl1 exhibit the angle values
of 96.45(6)° and 90.25(12)°, respectively, and the trans component
O2\Ti1\O1 exhibits angle values of 164.02(13)°. Both of Ti1\N1
(2.256(3) Å) and Ti1\N2 (2.246(3) Å) bonds are longer than those of
pentafluorinated FI titanium complex (2.217–2.234 Å), [1,12] indicating
a decreased interaction between ligand and metal center when replacing
the F atom with 4-ethylphenyl group. The Ti1\O2 (1.843(3) Å), Ti1\O1
(1.849(3) Å) and Ti1\Cl2 (2.2708(14) Å), Ti1\Cl1 (2.2775(14) Å)
bonds are nearly the same as those of pentafluorinated FI titanium
Fig. 1. Molecular structure of complex 2d. Thermal ellipsoids are shown at 50% probability level. All hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles
(°): Ti1\O2 1.843(3), Ti1\O1 1.849(3), Ti1\N1 2.256(3), Ti1\N2 2.246(3), Ti1\Cl2 2.2708(14), Ti1\Cl1 2.2775(14) and O2\Ti1\O1 164.02(13), O2\Ti1\N1 87.27(12),
O1\Ti1\N1 80.22(12), O2\Ti1\N2 80.92(11), O1\Ti1\N2 89.16(12), N1\Ti1\N2 90.25(12), O2\Ti1\Cl2 96.34(10), O1\Ti1\Cl2 95.40(10), N1\Ti1\Cl2 174.16(10),
N2\Ti1\Cl2 85.81(9), O2\Ti1\Cl1 94.11(9), O1\Ti1\Cl1 95.32(10), N1\Ti1\Cl1 87.85(9), N2\Ti1\Cl1 174.76(10), Cl2\Ti1\Cl1 96.45(6).

70
Y. Wang et al. / Inorganic Chemistry Communications 41 (2014) 68–71
Table 1
Result of ethylene polymerization of complex 1d and 2d with dried MAO.
c
Entrya Cat Time/min Ab
Mnc/104 Mwc/104 Mw/Mn Tmd/C ΔHe
χf/%
1
2
3
4
5
6
7
8
1d
2d
2
2
1.299 27.14
4.348 21.28
1.211 49.79
3.380 52.13
0.962 53.17
2.839 88.02
0.773 121.68
4.498 69.9
33.63
25.15
85.85
1.24
1.18
1.74
1.50
2.08
1.71
1.58
1.87
139.6 234.0 80.4
139.2 237.5 82.7
140.8 238.2 82.9
142.8 238.1 82.9
141.1 243.3 84.7
142.0 238.7 83.1
141.1 238.5 83.0
143.3 199.1 69.3
1d 10
2d 10
1d 30
2d 30
1d 60
2d 60
78.33
110.54
150.79
191.88
130.73
a
b
c
d
e
f
Conditions: 100 ml toluene, 1 atm ethylene pressure, 30 °C, 5 μmol complex, 10 mmol Al.
Activity, 106 g PE/(mol Ti·h).
Determined by GPC using polystyrene calibration.
Measured by DSC.
Unit, J/g.
On the basis of polyethylene fully crystallization ΔH0f = 287.3 J/g.
complex (1.841–1.845 Å for Ti1\O and 2.2578–2.2876 Å for Ti1\O)
[1,12]. For complex 2d, the phenol plane (Ph1) and the phenyl (Ph2)
on para position of the aniline orient themselves tilt to the phenyl ring
of the aniline moiety (Ph3) with dihedral angle as 66.1° and 55.3°, re-
spectively. The Ti1\O1\Ph1\C15\N1 combination of complex 2d is
almost coplanar with an average deviation from the mean plane of
0.12 Å. The Cl1\Ti1\Cl2 angle is 96.45° and the cis-location of chlorine
atoms around the metal center in the complex indicates its potential as
catalyst precursor for olefin polymerization [12–14].
Upon activation with excess of dried MAO (Al/Ti = 2000), com-
plexes 1d and 2d turned to be active catalysts for ethylene polymeriza-
tion. The complexes 1d and 2d possess high activity and ability to
synthesize UHMWPE in specified time and the results are summarized
in Table 1.
Complex 2d exhibits much higher catalytic activity than that of com-
plex 1d under the same polymerization conditions (Fig. 2). The great
difference in catalytic activity can be only attributed to the subtle struc-
tural difference of the two complexes. The catalytic activity was
undermined when the end group changes from ethyl to vinyl group,
which was probably ascribed that the vinyl group in 1d was possibly
inclined to coordinate to the metal center and occupied the active site
for ethylene insertion, resulting in a decreased activity [15–17].
Complex 1d possessed a higher potential in synthesizing UHMWPE
when prolonging the polymerization time in mild conditions (Fig. 3).
Complex 1d exhibited quasi living characteristic that the molecular
weight of resultant PEs increased from 33.6 × 104 at 2 min (PDI =
1.24) period to 191.88 × 104 at 60 min (PDI = 1.58). In the whole
Fig. 3. Variation of Mw with polymerization time for complexes 1d and 2d.
process, the polymer chain is consistently propagating with a sup-
pressed chain transfer. However, complex 2d is inferior to 1d in terms
of controlling the molecular weight of PEs. For complex 2d, chain trans-
fer has an advantage over chain propagation in longer polymerization
time, which could be observed by the decreased Mw of PEs from
150.79 × 104 (30 min) to 130.73 × 104 (60 min). It is probable that
the inactive species (caused by the coordination of Ti center with double
bond) in 1d would impose a larger steric hindrance around Ti center
than that in 2d, which would suppress both the chain transfer and
chain propagation. All the PEs synthesized by these two complexes ex-
hibit high melting points (139–144 °C) and high crystallinity (70–85%),
which are consistent with the typical HDPE.
In summary, two phenoxy-imine titanium complexes bearing end
functionalized phenyl on para position of N-aryl group were synthesized
and characterized. The complexes exhibit good to high activity upon
activation with dried MAO. Only by changing polymerization time,
UHMWPE (Mw N 1,000,000) can be synthesized in mild conditions.
But for complex 2d, with polymerization time increasing, there is a ten-
dency to decrease the molecular weight of the resulted UHMWPE when
chain transfer turned to be serious.
Acknowledgment
The work is supported by the National Natural Science Foundation of
China (No. 20936006 and No.20976152) and National Basic Research
Program of China (2011CB606001). The work is also supported by the
Special Research Fund for the Doctoral Program of Higher Education of
China (No. 20100101110041) and Program for Changjiang Scholars
and Innovative Research Team in Zhejiang University.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2014.01.005.
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