Binuclear Titanium Catalysts Based on Methylene-Bridged Tridentate Salicylaldiminato Ligands for Ethylene Homo- and Copolymerization

Article abstract of DOI:10.1007/s10562-017-1985-9

Abstract: A series of methylene-bridged salicylaldiminato tridentate [ONS] ligands bearing different alkylthio sidearms and the corresponding binuclear titanium complexes (Ti₂^a, Ti₂^b, Ti₂^c) were synthesized and characterized by elemental analysis, ESI-MS, FT IR, ¹H and ¹³C NMR. To the best of our knowledge, these were the first non-metallocene tridentate binuclear Ti complexes reported. When activated by modified methylaluminoxane (MMAO), these binuclear Ti complexes displayed extremely high activities in the range of 10⁶?g?mol^?1?h^?1?atm^?1 for ethylene polymerization and ethylene/α-olefins copolymerization at atmospheric pressure, producing high molecular weight polymers with narrow polydispersity. Both the catalytic behavior and the spectroscopic characterization indicated that the catalysts assumed symmetric structure and contained single-site active species. Most importantly, the catalytic activities towards ethylene (co)polymerization as well as comonomer incorporation ratios were much higher than that of the corresponding mononuclear complexes. Meanwhile, the properties of polymers and the comonomer incorporation ratios can be effectively tuned by the reaction conditions and the alkylthio side group on ligands. Complex Ti₂^c bearing the long octylthio sidearm showed significantly higher activity for ethylene polymerization than the methylthio functionalized Ti₂^a, presumably due to the increased solubility of the catalyst in the reaction medium. While for copolymerization, complex Ti₂^a demonstrated the highest catalytic activity and comonomer incorporation ratio, due the small steric hindrance of the methylthio side group. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-017-1985-9

Catal Lett
DOI 10.1007/s10562-017-1985-9
Binuclear Titanium Catalysts Based on Methylene-Bridged
Tridentate Salicylaldiminato Ligands for Ethylene Homo-
and Copolymerization
Tingcheng Li1 · Zhi Lan1 · Guangyong Xie1 · Derong Luo1 · Long Li1 ·
Shaofeng Xiong2 · Li Zhang1 · Lipeng Ouyang1 · Aiqing Zhang1
Received: 14 November 2016 / Accepted: 21 January 2017
© Springer Science+Business Media New York 2017
Abstract A series of methylene-bridged salicylaldimi-
nato tridentate [ONS] ligands bearing different alkylthio
sidearms and the corresponding binuclear titanium com-
active species. Most importantly, the catalytic activities
towards ethylene (co)polymerization as well as comonomer
incorporation ratios were much higher than that of the cor-
responding mononuclear complexes. Meanwhile, the prop-
erties of polymers and the comonomer incorporation ratios
can be effectively tuned by the reaction conditions and the
a
b
c
plexes (Ti2 , Ti2 , Ti2 ) were synthesized and characterized
1
by elemental analysis, ESI-MS, FT IR, H and 13C NMR.
To the best of our knowledge, these were the first non-met-
allocene tridentate binuclear Ti complexes reported. When
activated by modified methylaluminoxane (MMAO), these
binuclear Ti complexes displayed extremely high activities
in the range of 106 g mol−1 h−1 atm−1 for ethylene polym-
erization and ethylene/α-olefins copolymerization at atmos-
pheric pressure, producing high molecular weight polymers
with narrow polydispersity. Both the catalytic behavior and
the spectroscopic characterization indicated that the cata-
lysts assumed symmetric structure and contained single-site
c
alkylthio side group on ligands. Complex Ti2 bearing the
long octylthio sidearm showed significantly higher activity
for ethylene polymerization than the methylthio functional-
a
ized Ti2 , presumably due to the increased solubility of the
catalyst in the reaction medium. While for copolymeriza-
tion, complex Ti2a demonstrated the highest catalytic activ-
ity and comonomer incorporation ratio, due the small steric
hindrance of the methylthio side group.
* Guangyong Xie
xiegy@scuec.edu.cn
1
Key Laboratory of Catalysis and Materials Science
of the State Ethnic Affairs Commission and Ministry
of Education, Hubei Province, South-Central University
for Nationalities, Wuhan 430074, China
2
College of Chemical Engineering, Xiangtan University,
Xiangtan 411105, China
Vol.:(0123456789)
1 3

T. Li et al.
Graphical Abstract
OHC
HO
OHC
HO
CHO
OH
(CH2O)n, H+
(CH2O)n
SnCl4
HO
tBu
But
tBu
tBu
2
1
3
NH2
S
R
N
TiCl4
N
S
N
N
S
Cl
S
S
O
O
Ti
HO
OH
R
R
Ti
R
R
tBu
tBu Cl
tBu
tBu
Cl
Cl
Cl
Cl
c
R=CH3(Ti2a), C3H7(Ti2b), C8H17(Ti2
)
R=CH3(La), C3H7(Lb), C8H17(Lc)
CH2=CH2 + 1-hexene or Octene
MMAO
CH2=CH2
MMAO
Ethylene (co)polymerization activities > 106g.mol-1.h-1.atm-1
Higher α -olefin insertion ratio compared with the mononuclear analogue
Polyolefin
and selectivity, and the explosion of research advances
have led to deeper mechanistic understanding of the cata-
lytic system and production of polyolefin materials with
well-defined microstructure and high performance.
Keywords Binuclear titanium complex · Ethylene
polymerization · α-Olefin · Copolymerization ·
Salicylaldiminato tridentate ligand · Non-metallocene
catalysts
Among the non-metallocene candidates, phenoxy-
imine group 4 transition metal complexes developed by
Fujita and co-workers, the so called FI catalyst, exhib-
ited high performances in both catalytic activity and ste-
reoregularity control in olefin polymerization [12–17].
Nevertheless, these FI catalysts required the use of large
amount of cocatalyst and showed weak copolymerization
capacities for ethylene and other olefins bearing steric
hindrance groups due to the crowded coordination envi-
ronment of bis-ligation. Tang et al. have introduced some
sidearms with weak coordination capacity and developed
a series of mono-ligated tridentate [ONX] TiCl3 com-
plexes, where X=O, S, Se, and P [25–30], and found
that the catalytic performances were highly dependent
on the type and the steric effect of the ancillary side-
arm. The complexes bearing the S–Ar/R sidearm were
highly active for ethylene polymerization and copoly-
merization, and showed good capability to incorporate
comonomers due partly to the open coordination envi-
ronment. In addition, the activity is not much diminished
1 Introduction
Polyolefins are presently the most important and the most
produced synthetic polymers and are used in everything
from packaging to automotive and electrical components,
to lubricants and artificial limbs. Historically, polyolefins
were produced using ill-defined but highly active Zie-
gler–Natta catalyst composed of supported metal halides
(typically MgCl2-supported TiCl4, which still dominates
the market) activated by aluminum alkyls [1]. Begin-
ning in the 1990s, following the great success of Kamin-
sky’s metallocene catalysts [2], significant efforts have
been directed toward the development and application of
catalysts with various non-metallocene ligands such as
diamide, β-diketiminates, iminopyrrolides, amidinates,
diimine, alkoxide, aryl oxide, and phenoxyimine [3–24].
These single site molecular catalysts were far better
defined and more tunable in terms of structure, activity
1 3

Binuclear Titanium Catalysts Based on Methylene-Bridged Tridentate Salicylaldiminato Ligands…
even when the amount of MMAO is fairly low (Al/Ti
ratio = 100) [27]. Compared with the bis-ligated biden-
tate complexes, these mono-ligated tridentate complexes
were highly stable and could be prepared in one step by
simply mixing the tridentate ligands and titanium tetra-
chloride, without the need to deprotonate the ligands in
advance.
Since 2000s, there have also been growing interests in
binuclear and multinuclear complexes for olefin polymeriza-
tion [31–54]. As compared to the mononuclear analogues,
it was shown that introduction of a proximate metal center
could significantly enhance catalytic activity, modify polyole-
fin molecular weight, branch structure, and increase comono-
mer enchainment selectivity as a result of the cooperative
effects between adjacent catalytic centers. Heterobimetallic
catalysts are also capable of producing LLDPE with ethyl-
ene as the sole feed [39, 41, 53]. However, compared with
the mononuclear complexes, the research on the design and
synthesis, and the catalytic performance and mechanism for
Thus, in order to take the advantage of the open coordi-
binuclear complexes is far from enough. Most of these works
were focused on metallocene and late transition metal com-
plexes, while non-metallocene early transition metal binu-
clear complexes have rarely been reported, due probably to
the difficulty in synthesis.
nation environment of tridentate ligands and the coopera-
tive effect of binuclear complexes, we tethered a series of
tridentate [ONS] salicylaldiminato ligands bearing various
S–R (R=methyl, propyl, octyl) sidearms with a methyl-
ene group and prepared the corresponding Ti complexes.
To the best of our knowledge, these are the first non-met-
allocene tridentate binuclear Ti complexes reported. The
structures of the ligands and complexes were character-
ized by spectroscopic techniques and elemental analysis,
and the catalytic performances for ethylene homo- and
co-polymerization were investigated. High activities over
106 g mol−1 h−1 atm−1 were obtained. Meanwhile, the binu-
clear catalysts showed 28% increase of activity for ethylene
polymerization and significantly increased enchainment
of α-olefins such as 1-hexene and 1-octene as compared
with the mononuclear analogues, clearly demonstrating the
cooperativity between the two metal centers.
Marks’ group systematically studied a series of binu-
clear catalysts, including metallocene and late transition
metal complexes, and found that the binuclear catalysts were
more efficient in terms of activity and commoner inser-
tion than their mononuclear analogues [31–41]. Recently
they also reported a class of early transition bimetallic cata-
lysts (as shown below, I) [35]. However, the salicylaldimine
ligated mononuclear complexes demonstrated low activity
(~103 g mol−1 h−1 atm−1), whilst the binuclear analogue dem-
onstrated moderate activity (~104 g mol−1 h−1 atm−1). Ma
Yuguo et al. lately reported a bidentate salicylaldimine het-
eroligated binuclear Ti(IV) catalyst (shown below, II) with
high activity (over 106 g mol−1 h−1 atm−1), yet the synthesis
was very tedious [54].
2 Experimental Section
2.1 Materials and Characterization
All manipulations involving air- and/or moisture-sensi-
tive compounds were performed under dry nitrogen using
standard Schlenk-line and glovebox. Toluene and hex-
ane were purified by distillation over sodium/benzophe-
none ketyl, while CH2Cl2 was refluxed over CaH2. Gases
and other solvents were purified by standard techniques.
Modified methylaluminoxane (MMAO) was purchased
from Akzo Chemical as a 7 wt.% solution in heptane.
1 3

T. Li et al.
All other chemical reagents were used as received unless
2.2 Ethylene Polymerization and Copolymerization
noted otherwise. Compounds 2, 3, alkylthio anilines and
c
mononuclear Ti complex Ti1 (Scheme 1) were synthe-
A flame-dried Schlenk flask purged with N2 was filled
with ethylene gas. 30 mL of freshly distilled toluene was
added and raised to the reaction temperature for 10 min.
MMAO was then injected using a syringe and the mixture
was stirred for 5 min. The polymerization was initiated by
adding a solution of the titanium complex in toluene with
a syringe. After a desired time, the polymerization was
quenched with acidified ethanol (100 mL, 8 vol.% HCl in
ethanol). The precipitated polymer was filtered off, washed
with ethanol, then dried under vacuum overnight at 60°C
till a constant weight. For copolymerization, α-olefins
(1-hexene, 1-octene) and MMAO were injected in sequence
via a syringe.
sized according to the literature [29].
1H and 13C NMR spectra of ligands and complexes
were recorded on a Bruker Avance III 400 MHz spectrom-
eter with tetramethylsilane as an internal standard. Ele-
mental analyses were carried out using Vario EL 111. 13
C
NMR spectra of polymers were obtained on a Varian XL
300 MHz spectrometer at 120°C with o-C6D4Cl2 as the
solvent. IR spectra were collected with a Nicolet Nexus
470 Fourier transform infrared (FTIR) spectrometer. Mass
spectra were measured by an Agilent ESI-Q-TOF MS
6520 instrument. DSC measurements were performed
on a Netzsch DSC200 F3 instrument at a heating rate
of 10°C/min from 20 to 160°C, with the melting points
obtained from the endothermic peak of the second heating
scan. The Mn and Mw/Mn of the polymers were determined
at 150°C with a Viscotek 350A HT-GPC System using
a polystyrene calibration. 1,2,4-Trichlorobenzene was
employed as the solvent at a flow rate of 1.0 mL min−1.
2.3 Synthesis of the Ligands and Binuclear Titanium
Complexes
The synthesis of the methylene-bridged salicylaldiminato
tridentate [ONS] ligands La–Lc and the corresponding
a
binuclear Ti complexes Ti2 –Ti2c is shown in Scheme 2.
2.3.1 Synthesis of the Methylene‑Bridged
Salicylaldiminato Tridentate [ONS] Ligands (La, Lb
and Lc)
tBu
N
S
O
Ti
(CH2)7CH3
Cl
Cl
tBu Cl
To a 100 mL 3-neck flask were added 0.92 g (2.5 mmol) of
3, 0.695 g (5 mmol) of methylthio aniline, 30 mL of etha-
nol, and 0.31 mL of formic acid, and heated to reflux for
12 h. The mixture was then vacuum dried, and recrystal-
lized with ethanol to afford La as a yellow solid (1.2 g, 80%
c
Ti1
Scheme 1 Structure of mononuclear Ti complex Ti1
c
OHC
OHC
HO
CHO
(CH2O)n, H+
(CH2O)n
SnCl4
HO
tBu
HO
But
OH
tBu
tBu
2
1
3
NH2
S
R
N
TiCl4
N
N
N
S
S
S
S
O
Cl
O
Ti
HO
OH
R
R
Ti
R
R
Cl
tBu
tBu Cl
tBu
tBu
Cl
Cl
Cl
a
c
R=CH3(Ti2 ), C3H7(Ti2b), C8H17(Ti2
)
R=CH3(La), C3H7(Lb), C8H17(Lc)
Scheme 2 Synthesis of the methylene-bridged salicylaldiminato tridentate [ONS] ligands and the corresponding Ti complexes
1 3

Binuclear Titanium Catalysts Based on Methylene-Bridged Tridentate Salicylaldiminato Ligands…
1
yield). H NMR (400 MHz, ppm, CDCl3): δ13.46 (s, 2H,
OH), 8.56 (s, 2H, N=CH), 7.0–7.27 (m, 12H, ArH), 3.91
(s, 2H, CH2), 2.47 (s, 6H, CH3), 1.47 (s, 18H, C(CH3)3).
13C NMR (101 MHz, CDCl3): δ 162.8 (CH=N), 159.0,
145.8, 137.9, 134.7, 131.5, 130.6, 130.4, 127.2, 125.3,
124.9, 119.0, 117.4 (Ar–C), 40.4, 35.0, 29.4, 14.8. ESI-MS
[m/z (%)]: 611 (100) [M+]. Anal. Calcd for C37H42N2O2S2:
C, 72.75; H, 6.93; N, 4.59%. Found: C, 72.88; H, 6.46; N,
4.25%. IR (KBr, cm−1): 2957, 2917, 1614, 1566, 1437,
1266, 1203, 1160, 805, 755.
22.7, 14.1. ESI-MS [m/z (%)]: 915 (100), 917 (15) [M+].
Anal. Calcd for C37H40Cl6N2O2S2Ti2: C, 48.45; H, 4.40;
N, 3.05%. Found: C, 48.87; H, 4.12; N, 3.43%. IR (KBr,
cm−1): 2959, 1632, 1548, 1387, 1266, 856, 759, 608.
a
Ti2b was prepared using the same procedure as for Ti2 .
Yield: 98.2%. 1H NMR (400MHz,CDCl3): 8.7 (s, 2H,
CHO); 7.1–7.7 (m, 12H, ArH); 4.1 (s, 2H, CH2); 2.48 (tr,
4H, CH2); 2.05 (m, 4H, CH2); 1.55 (s, 18H, C(CH3)3);
1.13 (m, 6H, CH3). 13C NMR (101 MHz, CDCl3): δ 162.9
(CH=N), 159.4, 152.6, 145.0, 142.6, 136.3, 136.2, 134.0,
131.4, 131.3, 130.3, 128.0, 119.3 (Ar–C), 53.4, 48.0, 35.4,
29.8, 21.8, 13.4. ESI-MS [m/z (%)]: 971 (100), 973 (86)
[M+]. Anal. Calcd for C41H48Cl6N2O2S2Ti2: C, 50.59; H,
4.97; N, 2.88%. Found: C, 50.06; H, 4.25; N, 3.12%. IR
(KBr, cm−1): 2963, 2869, 1626, 1548, 1386, 1271, 1109,
857, 758, 612.
Ligand Lb was synthesized with a similar procedure as
1
for La. Yield: 85%. H NMR (400 MHz, CDCl3): δ13.5
(s, 2H, OH); 8.5 (s, 2H, CHO); 6.9–7.4 (m, 12H, ArH);
4.0 (s, 2H, CH2); 2.9 (tr, 4H, CH2); 1.8 (m, 4H, CH2);
1.4 (s, 18H, C(CH3)3); 1.05 (m, 6H, CH3). 13C NMR
(101 MHz, CDCl3): δ 163.0 (CH=N), 159.1, 147.0, 137.9,
133.1, 131.5, 130.6, 130.4, 127.5, 127.0, 126.0, 119.1,
118.0 (Ar–C), 40.4, 35.0, 34.3, 29.5, 22.4, 13.7. ESI-
MS [m/z (%)]: 663 (100), 666 (33) [M+]. Anal. Calcd for
C41H50N2O2S2: C, 73.83; H, 7.56; N, 4.20%. Found: C,
73.29; H, 7.31; N, 4.38%. IR (KBr, cm−1): 2959, 2872,
1614, 1567, 1438, 1269, 1202, 1159, 802, 755.
c
a
Ti2 was prepared using the same procedure as for Ti2 .
1
Yield: 70%. H NMR (400 MHz, ppm, CDCl3): δ 8.72 (s,
2H, N=CH), 7.65–7.25 (m, 12H, ArH), 4.09 (s, 2H, CH2),
3.50 (s, 4H, SCH2), 2.00 (m, 4H, CH2), 1.55 (m, 10H,
CH2); 1.27 (s, 18H, C(CH3)3), 0.87 (t, 6H, CH3). 13C NMR
(101 MHz, CDCl3): δ 163.0 (CH=N), 161.7, 150.5, 137.5,
136.3, 136.1, 134.3, 134.2, 131.5, 130.3, 128.8, 128.0,
119.4 (Ar–C), 46.3, 35.3, 31.8, 29.9, 29.8, 29.1, 29.0, 28.8,
28.1, 22.6, 14.1. ESI-MS [m/z (%)]: 1113 (100) [M+].
Anal. Calcd for C51H68Cl6N2O2S2Ti2: C, 55.00; H, 6.15;
N, 2.52%; Found: C, 55.22; H, 6.34; N, 2.29%. IR (KBr,
cm−1): 2923, 2850, 1602, 1545, 1391, 1263, 864, 752, 623.
Ligand Lc was synthesized with a similar procedure
1
as for La. Yield: 75%. HNMR (400 MHz, ppm, CDCl3):
δ13.53 (s, 2H, OH), 8.54 (s, 2H, N=CH), 7.32–7.00 (m,
12H, ArH), 3.92 (s, 2H, CH2), 2.91 (s, 4H, SCH2), 1.69
(m, 4H, CH2), 1.47 (m, 10H, CH2), 1.27 (s, 18H, C(CH3)3),
0.87 (t, 6H, CH3). 13C NMR (101 MHz, CDCl3): δ 163.2
(CH=N), 159.5, 138.4, 135.0, 132.9, 132.7, 131.7, 131.4,
130.0, 127.6, 126.5, 120.5, 118.9 (Ar–C), 40.6, 35.0,
33.1, 31.8, 29.5, 29.4, 29.3, 29.2, 29.0, 22.6, 14.1. ESI-
MS [m/z (%)]: 804 (100), 806 (16) [M+]. Anal. Calcd for
C51H70N2O2S2: C, 75.88; H, 8.74; N, 3.47%. Found: C,
75.32; H, 8.96; N, 3.75%. IR (KBr, cm−1): 2950, 2925,
2857, 1613, 1565, 1467, 1437, 1364, 1268, 1202, 1158,
803, 756.
3 Results and Discussion
3.1 Synthesis of Binuclear Ti Complexes
Methylene-bridged bis(salicylaldehyde) 3 was synthesized
through two steps with o-tbutyl phenol as starting material
according to the reference [55], which then reacted with
alkylthio aniline to obtain the methylene-bridged bis-salic-
ylaldiminato tridentate [ONS] ligands La–Lc. The ligands
reacted directly with excess TiCl4 to form the binuclear Ti
2.3.2 Synthesis of the Binuclear Ti Complexes
a
Synthesis of the binuclear Ti complexes Ti2 : 0.61 g
(1 mmol) of ligand La was added to a 100 mL Schlenk
flask, dissolved in 15 mL of CH2Cl2. A solution of TiCl4
(0.28 mL, 2.6 mmol) in CH2Cl2 (15 mL) was added to
another 100 mL Schlenk flask. The ligand solution was
then slowly added to the TiCl4 solution at −78°C. After
4 h, the mixture was slowly warmed to RT, then heated to
35°C for 24 h under stirring. The solvent was vacuum dried
a
c
complexes Ti2 –Ti2 , with no need to deprotonate with a
base. This procedure is much simpler than that for the syn-
thesis of bidentate binuclear Ti(IV) complexes [34], and the
products were also more stable. The structures of the free
ligands and the corresponding binuclear Ti complexes were
1
confirmed by H NMR, 13C NMR, ESI-MS, FT IR and
a
1
to afford Ti2 as a red brown solid (0.6 g, 65%). H NMR
(400 MHz, CD2Cl2): 8.76 (s, 2H, N=CH), 7.69–7.19 (m,
12H, ArH), 4.17 (s, 2H, CH2), 3.05 (s, 6H, CH3), 1.28 (s,
18H, C(CH3)3). 13C NMR (101 MHz, CD2Cl2): δ 162.4
(CH=N), 159.5, 154.5, 148.2, 145.3, 138.4, 136.1, 134.0,
129.7, 129.0, 128.2, 125.1, 122.7 (Ar–C), 53.4, 31.6,
elemental analysis. The 1H NMR of La and the responding
a
complex Ti2a was shown in Fig. 1. Notable changes for Ti2
were the disappearance of OH resonance at δ13.46 ppm,
and the shift of the thiomethyl from 2.47 to 3.05 ppm. The
1H NMR spectra also proved that the molecule adopted a
symmetric geometry.
1 3

T. Li et al.
5
optimum activity was achieved with Al:Ti=1500 at 55°C
under ethylene pressure of 1.0 atm (Table 1, entry 8).
As the temperature was raised from 25 to 65°C, the
catalytic activity remained at high level, and a maximum
activity of 2.43×106 g mol−1 h−1 atm−1 was achieved at
55°C (Table 1, entry 4). The activity slightly decreased as
the polymerization temperature further increased to 65°C
(Table 1, entry 5). The temperature also had great influence
on the molecular weight of the products. With the tem-
perature increase, the Mw of the polymer decreased from
8.74×104 g mol−1 at 25°C to 1.16×104 g mol−1 at 65°C
(Fig. 2), indicating that the high polymerization tempera-
ture might promote chain transfer reaction.
2
3
H O
2
N
N
S
S
HO
OH
4
tBu
tBu
4
1
CD2Cl2
5
Ph
1
2
3
5
La
a
4
Ph
2
3
Ti
2
The influence of Al/Ti molar ratio was investigated
by maintaining the temperature at 55°C, and the maxi-
mum activity was achieved with an Al/Ti ratio of 1500
(Table 1, entry 8). The catalyst maintained high activity
of over 106 g mol−1 h−1 atm−1 even at a low Al/Ti ratio of
500. The molecular weight of the polyethylene was sen-
sitive to the Al/Ti ratio and decreased from 5.09×104 to
1.48×104 g mol−1 with the increase of the Al/Ti ratio from
500 to 2500, suggesting that chain transfer to aluminum
compounds occurred in the process.
14
12
10
8
6
4
2
0
ppm
a
Fig. 1 1H NMR spectra of ligand La and complex Ti2
3.2 Ethylene Polymerization
a
c
The binuclear Ti complexes Ti2 –Ti2 were examined
for ethylene polymerization under MMAO activation,
with the results summarized in Table 1. Overall, these
binuclear complexes showed very high activity (over
106 g mol−1 h−1 atm−1) under suitable conditions. The melt-
ing points (Tm) of the products were about 133°C, which
were typical of high-density polyethylene. This was in full
accordance with the 13C NMR spectra (not shown), which
showed no branches on the polymer backbone.
The influences of the alkylthio sidearm on catalytic per-
formance were also investigated. Under the same condition,
the activity of the three binuclear complexes increased in
c
b
a
the order of Ti2 >Ti2 >Ti2 (Table 1, entry 8, 10, 11),
it appeared that the longer the alkyl group, the higher the
homopolymerization activity. This is somehow different
from the corresponding mononuclear complexes observed
by Tang et al. [29], where the activities were similar. For
ethylene homopolymerization, the steric hindrance of the
substituents in the mononuclear Ti complexes appeared to
The influences of different reaction conditions such as
the reaction temperature and Al:Ti molar ratio upon the
c
catalytic behaviors of Ti2 /MMAO were also studied. The
Table 1 The results of ethylene
polymerization catalyzed by
binuclear Ti complexes
Entry
Cat. (μmol)
Al/Ti
Temp (oC)
PE (g)
Acta
Tmb (oC)
Mwc (104g/mol)
PDIc
1
2
3
4
5
6
7
8
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2a (3)
Ti2b (2)
Ti1c (4)
2000:1
2000:1
2000:1
2000:1
2000:1
500:1
1000:1
1500:1
2500:1
1500:1
1500:1
1500:1
25
35
45
55
65
55
55
55
55
55
55
55
0.3922
0.3941
0.5057
0.8105
0.5425
0.4213
0.5645
0.8537
0.5705
0.7479
0.7425
0.6682
1.17
1.18
1.52
2.43
1.63
1.26
1.69
2.56
1.71
1.50
2.24
2.00
141.6
–
136.9
–
133.7
136.4
–
135.4
134.9
130.8
133.4
130.8
8.74
–
3.24
–
1.16
5.09
–
1.57
1.48
0.79
1.45
0.95
3.56
–
2.52
–
2.35
2.75
–
2.48
2.27
2.71
2.78
2.79
9
10
11
12
Toluene 30 mL, 1 atm ethylene pressure, reaction time 5 min
aActivity, 106 g mol−1 h−1 atm−1
bMelting temperature determined by DSC
cDetermined by GPC using polystyrene standard
1 3

Binuclear Titanium Catalysts Based on Methylene-Bridged Tridentate Salicylaldiminato Ligands…
c
1.2
Ti
1
65oC
c
45oC
Ti
1.0
0.8
0.6
0.4
0.2
0.0
2
25 oC
1.0
0.8
0.6
0.4
0.2
0.0
a
Ti
2
b
Ti
2
2
3
4
5
6
2
3
4
5
logMw
logMw
Fig. 2 GPC traces of PE samples from entries 2, 3, and 6 in Table 1
Fig. 3 GPC traces of PE samples from entries 8, 10–12 in Table 1
have little effect on activity, due to the wide open coordina-
tion environment of the tridentate complexes and the small
size of ethylene monomer. Therefore, the difference in
activity here may be due to their different solubilities. The
The products were apparently branched polyethylene, as
revealed by the much diminished melting points and the
high temperature 13C NMR spectra. The 1-hexene incorpo-
ration ratio in the copolymer was calculated from the 13C
NMR spectra [33] and found tunable from 1.4 to 7.3% by
varying the initial feed of α-olefin commoners from 5 to
15 mmol (Fig. 4). The melting points of the copolymers
also varied from 106.7 to 89.5°C (Fig. 5).
a
complex Ti2 bearing a methylthiol group had the lowest
solubility and consequently lowest catalytic activity. Tang
also found that the increase of solubility could increase the
catalytic activity of phenoxy-imine [ONS] trident Ti com-
plexes. The molecular weight of the polyethylene catalyzed
by the binuclear complexes increased in the same order
For ethylene copolymerization, the steric hinderance of
the binuclear Ti complexes played important role in cata-
lysts performance. Under the same condition, the binu-
c
(Fig. 3), with the products catalyzed by Ti2 of the highest
a
Mw, which was attributed to the larger steric hindrance of
clear complex Ti2 with methylthio sidearm demonstrated
octylthio group that inhibited the chain transfer reaction.
the highest catalytic activity and comonomer incorpora-
tion ratio, as a result of the small steric hindrance of the
side group. Furthermore, higher activity was observed as
compared with the mononuclear complex (Table 2, entries
2 vs. 6), and the incorporation of 1-hexene was 6.9, 5.7,
c
Compared with the mononuclear analogue (Ti1 ), the
catalytic activity of the binuclear Ti complex Ti2c increased
modestly by 28%, and the molecular weight produced by
c
c
Ti2 was higher than that by Ti1 . However, the molecular
weight distribution of the polyethylenes catalyzed by binu-
clear Ti complexes were within 2–3, similar to that of the
mononuclear analogue (Table 1, entry 8 vs. 12).
c
b
a
and 9.3% respectively for Ti2 , Ti2 , and Ti2 , all of which
were significantly higher than the 4.5% ratio obtained by
Ti1c (Table 2, entries 2, 5, 4, 6, and Fig. 6). It is speculated
that the higher catalytic activity and comonomer incorpora-
tion ratio are caused by the larger steric hindrance of these
binuclear Ti complexes which would reduce the interac-
tion between the catalyst and the cocatalyst molecules and
therefore improve the catalytic activity and comonomer
incorporation ratio.
3.3 Ethylene Copolymerization with α-Olefins
These binuclear Ti complexes were also excellent cata-
lysts for copolymerization of ethylene and α-olefins under
MMAO activation. The copolymerization of ethylene with
1-hexene or 1-octene was investigated under similar con-
dition with various feeds of α-olefins and the results were
summarized in Table 2.
Marks and Ma also found that the agostic interaction
between the comonomer and metal center could stabilize
the coordinated bimetallic olefin complex and enchain ole-
fin comonomers more easily than mononuclear complex
[54]. The molecular weights of the copolymers were found
in the range 1.33–1.93× 104 g mol-1, and a narrower poly-
dispersity (2.08–2.39) as compared with the homopolym-
erization was obtained (Fig. 7).
Comparable high activity (over 2× 106 g mol−1 h−1 atm−1)
with the copolymerzation was observed for all the reac-
tion system. As the feed of 1-hexene was increased from 5
to 15 mmol, the copolymerization activity increases from
2.07 to 2.42×106 g mol−1 h−1 atm−1 (Table 2, entry 1–3).
1 3

T. Li et al.
Table 2 Copolymerization of
ethylene and α-olefins catalyzed
by binuclear Ti complexes
b
Entry
Cat. (μmol)
α-Olefins (mmol)
Poly (g)
Acta
Tm (oC)
Mw
–
PDIb
Incorpb
(mol%)
1
2
3
4
5
6
7
8
9
10
Ti2c (2)
Ti2c (2)
Ti2c (2)
Ti2a (2)
Ti2b (2)
Ti1c (4)
Ti2a (2)
Ti2b (2)
Ti2c (2)
Ti1c (4)
C6 (5)
0.6887
0.7065
0.8079
0.8401
0.8055
0.6859
0.9794
0.9073
0.7912
0.7570
2.07
2.12
2.42
2.52
2.42
2.05
2.94
2.72
2.37
2.27
106.7
95.4
89.5
85.5
99.1
102.8
92.1
98.2
94.8
100.7
–
2.08
–
2.14
2.39
2.29
2.20
2.42
2.16
2.36
1.4
6.9
7.3
9.3
5.7
4.5
9.8
8.0
7.9
5.9
C6 (10)
C6 (15)
C6 (10)
C6 (10)
C6 (10)
C8 (10)
C8 (10)
C8 (10)
C8 (10)
1.93
–
1.33
1.36
1.40
1.61
1.29
1.42
1.44
Toluene 30 ml, 1 atm ethylene pressure, reaction time 5 min, temp 55°C, Al:Ti 1500 equiv. MMAO
aActivity, 106 g mol−1 h−1 atm−1
bDetermined by GPC using polystyrene standard
a
b
Ti
5mmol
2
Ti
Ti
2
10mmol
15mmol
c
c
2
Ti
1
40
35
30
25
20
15
10
ppm
ppm 40
35
30
25
20
15
10
Fig. 4 13C NMR spectra of PE samples from entries 1–3 in Table 2
Fig. 6 NMR spectra of PE samples from entries 2, 4–6 in Table 2
Similarly, for copolymerization with 1-octene, higher
activities (2.37–2.94×106 g mol−1 h−1 atm−1) and comono-
mer incorporation ratios (7.9–9.8%) are obtained (Table 2,
entries 7–10) as compared with the mononuclear complex
(2.27×106 g mol−1 h−1 atm−1 of activity, 5.9% of incorpo-
ration ratio). Both these results were higher than that for
1-hexene copolymerization, although 1-octene was a bulk-
ier comonomer. It is likely that 1-octene produced longer
branches and rendered the product easier to dissolve, thus
facilitated the polymerization reaction. Similar phenome-
non has also been observed for the mononuclear complexes
[29, 56]. These tridentate binuclear Ti complexes showed
better catalytic properties for ethylene-α-olefin copolym-
erization than the bis-ligated binuclear catalyst FI2–Ti2
reported by Ma, which generated similar activities and
1-hexene or 1-octene incorporation ratios as compared to
the mononuclear FI–Ti1.
5mmol
106.7 oC
10mmol
95.4oC
15mmol
89.5oC
40
60
80
100
120
140
Temperature (oC)
Fig. 5 DSC curves of PE samples from entries 1–3 in Table 2
1 3

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Fig. 7 GPC traces of PE samples from entries 2, 4–6 in Table 2
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4 Conclusions
21. Xie G, Liu G, Li L, Li T, Zhang A, Feng J (2014) Catal Com-
mun 45:7–10
a
c
Binuclear Ti complexes (Ti2 –Ti2 ) based on methylene-
bridged salicylaldiminato tridentate [ONS] ligands bearing
different alkylthio sidearms were prepared and examined
for ethylene (co)polymerization under MMAO activation.
High activity of over 106 g mol−1 h−1 atm−1 was obtained
for both homo- and co-polymerization. Compared with the
corresponding mononuclear complex, the binuclear cata-
lysts showed increased activity and comonomer insertion
ratio. The alkylthio sidearm also significantly influenced
22. Xie G, Zhang X, Li T, Li L, Liu G, Zhang A (2014) J Mol
Catal A Chem 383–384:121–127
23. Xie G, Song W, Li T, Xu X, Lan Z, Li Y, Zhang A (2014) J
Appl Polym Sci 131:41178
24. Li T, Song W, Ai H, You Q, Zhang A, Xie G (2015) J Polym
Res 22:631
25. Redshaw C, Tang Y (2012) Chem Soc Rev 41:4484–4510
26. Hu WQ, Sun XL, Wang C, Tang Y, Shi LP, Xia W, Sun J,
Dai HL, Li XX, Yao XL, Wang XR (2004) Organometallics
23:1684–1688
27. Wang C, Sun, XL, Guo YH, Gao Y, Liu B, Ma Z, Xia W, Shi
LP, Tang Y (2005) Macromol Rapid Commun 26:1609–1614
28. Wang C, Ma Z, Sun XL, Gao Y, Guo YH, Tang Y, Shi LP
(2006) Organometallics 25:3259–3266
c
the polymerization behavior. Complex Ti2 with the long
octylthio side group showed higher activity for ethylene
a
polymerization than the methylthio functionalized Ti2 , as
29. Gao ML, Gu YF, Wang C, Yao XL, Sun XL, Li CF, Qian CT,
Liu B, Ma Z, Tang Y, Xie ZW, Bu SZ, Gao Y (2008) J Mol
Catal A Chem 292:62
a result of the increased solubility of the catalyst in toluene.
a
While for copolymerization, complex Ti2 demonstrated
the highest catalytic activity and comonomer incorporation
ratio, due the small steric hindrance of the methylthio side
group.
30. Gao ML, Sun XL, Gu YF, Yao XL, Li CF, Bai JY, Wang C,
Ma Z, Tang Y, Xie ZW, Bu SZ, Qian CT (2008) J Poly Sci Part
A 46:2807–2819
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32. Guo N, Li L, Marks TJ (2004) J Am Chem Soc 126:6542–6543
33. Li H, Li L, Marks TJ (2004) Angew Chem Int Ed
43:4937–4940
Acknowledgements The authors are grateful for the financial sup-
port from the Natural Sciences Foundation of China (21172269).
34. Li H, Marks TJ (2006) PNAS 103:15295–15302
35. Salata MR, Marks TJ (2008) J Am Chem Soc 130:12–13
36. Salata MR, Marks TJ (2009) Macromolecules 42:1920–1933
37. Rodriguez BA, Delferro M, Marks TJ (2009) J Am Chem Soc
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