The influences of electronic effect and isomerization of salalen titanium(iv) complexes on ethylene polymerization in the presence of methylaluminoxane

Article abstract of DOI:10.1039/c9ra08899g

Herein, two salalen titanium(iv) complexes were synthesized and characterized. These complexes coexisted as two isomers in certain conditions and underwent isomerization, as evidenced by ¹H NMR spectroscopy. Furthermore, the molar ratio of the

Full text of DOI:10.1039/c9ra08899g

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PAPER
The influences of electronic effect and
isomerization of salalen titanium(IV) complexes on
ethylene polymerization in the presence of
methylaluminoxane†
Cite this: RSC Adv., 2019, 9, 41824
a
Sihan Li,a Yuqiong Zhu,a Huaqing Liang,a Xiuli Xie,a Yipeng Zhan,a Guodong Liang
ab
*
and Fangming Zhu
Herein, two salalen titanium(IV) complexes were synthesized and characterized. These complexes coexisted
as two isomers in certain conditions and underwent isomerization, as evidenced by 1H NMR spectroscopy.
Furthermore, the molar ratio of the two isomers ranged from 100 : 15 at 30 ꢀC to 100 : 34 at 120 ꢀC, driven
by thermal energy, based on variable temperature 1H NMR characterization. Both complexes were
employed as catalysts for ethylene polymerization in the presence of methylaluminoxane (MAO). The
influence of the electronic effects of different substituent groups at the ortho position of the phenolate
on ethylene polymerization behaviors, molecular weight and molecular weight distributions of the
resulting polyethylene was investigated. The fluorinated salalen titanium(IV) complex revealed relatively
high catalytic activity and thermal stability owing to the electron-withdrawing inductive effect. Moreover,
disentangled linear polyethylene with ultrahigh molecular weight (Mw up to 3000 kDa) and narrow
molecular weight distribution (Mw/Mn ꢁ 2) was obtained in the polymerization temperature range of
30 ꢀC to 50 ꢀC.
Received 29th October 2019
Accepted 5th December 2019
DOI: 10.1039/c9ra08899g
rsc.li/rsc-advances
coordination polymerization is the most widespread tech-
nology, which includes the heterogeneous Ziegler–Natta,14,15
homogeneous metallocene16–18 and post-metallocene19–22
Introduction
Polyethylene is the most widely produced plastic (by volume)
in the world, with 80 million metric tons produced annually.1,2
It has attracted considerable attention owing to a combination
of its relatively low cost of production, large-scale production
in factories, and extremely wide range of applications, as well
as the variableness of its accomplishable physical perfor-
mance.3 The physical properties of polyethylene are controlled
by nearly limitless combinations of branched structures,4–6
molecular weights and molecular weight distributions.2,7,8
Polyethylene originates from ethylene addition polymeriza-
tion, including radical polymerization3,9–11 and coordination
polymerization.12–14 The strategy of radical polymerization is
limited to high-pressure apparatus and forms low-density
polyethylene as a result of the high degree of short and long
chain branching, as well as hyperbranching.9 Ethylene
catalysts. Advances in catalyst research with homogeneous
catalysts have given rise to new classes of polyethylene,23–25 as
these catalysts can provide control over branch structures,26–28
molecular weight and molecular weight distributions. Inves-
tigations of homogeneous catalysts have provided useful
insights into the inuences of the nature of the transition
metal29,30 and ligand geometry31,32 on polymerization behav-
iour, branch structure and molecular weight of the polymeri-
zation product. In the last 30 years, considerable efforts have
focused on post-metallocene coordination catalysts for
ethylene polymerization in light of the vast array of different
ligands and metal combinations for these catalysts.33 Our
understanding of the inuences of the metal and ligands on
ethylene polymerization behaviour and its structures is still
gradually developing. [OABO]-type bridged bis(phenolate)
group 4 metal complexes have attracted considerable attention
for the controlled coordination polymerization of olens30,34–44
because of the abundant modiable positions at the skeleton
ligands and the formation of various stereoisomers as well as
their interactive conversion.45,46 Such complexes typically
include two labile monodentate groups of cis geometrical
relationship and a chelating ligand that controls the electronic
and steric environments, thereby affecting the activity of the
aPCFM and GDHPPC Lab, School of Chemistry, Sun Yat-Sen University, 510275,
China. E-mail: ceszfm@mail.sysu.edu.cn; Fax: +86-20-84114033; Tel: +86-20-
84113250
bPCFM and GDHPPC Lab, School of Materials Science and Engineering, Sun Yat-Sen
University, Guangzhou, 510275, China
† Electronic supplementary information (ESI) available: CCDC 1921379 for
[LigHTiCl2] and 1921380 for [LigFTiCl2]. NMR analyses of titanium complex,
characterization of polymers. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c9ra08899g
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catalyst and the molecular weights and molecular weight
distributions of resulting polymers.42,47,48
As reported by Kol and co-workers,45 titanium(IV) complexes
bearing salalen-type ligands with bulky alkyl substituents at the
ortho position of the imine-side phenol and electron-
withdrawing halo groups at the ortho and para positions of
the amine-side phenol were employed for propylene polymeri-
zation. The electronic effect of the halo phenolate substituent
was found to play a less signicant role not only in polymeri-
zation behaviour but also in the molecular weight of the
resulting polypropylene. In order to investigate the inuence of
the electronic effects on ethylene polymerization behaviours,
molecular weight, and the molecular weight distribution of
polyethylene, the hydrogen atom at the ortho position of the
imine-side phenol was replaced by a uorine group. We pre-
dicted that a conjugated system consisting of a phenol ring and
an imine donor could contribute to the strong electron-
withdrawing inductive effect of the uorine group on
decreasing the electron cloud density on the titanium(IV) centre.
It is obvious that the electronic effect of the substituent on the
phenol ring at the amine side makes it difficult to modify the
electron cloud density of the titanium(IV) centre due to the non-
conjugated amine moiety.
Herein, we demonstrated the synthesis of two salalen tita-
nium(IV) complexes and their performance in ethylene poly-
merization in the presence of methylaluminoxane (MAO).
Complex [LigFTiCl2] showed exceptional thermal stability and
catalytic activity in ethylene polymerization, yielding linear
polyethylene with ultrahigh molecular weight. Most impor-
tantly, both complexes were found to reveal thermally-driven
isomerization behaviour in toluene and promoted the forma-
tion of polyethylene with broad molecular weight distribution at
higher polymerization temperature.
Fig. 1 (a) Molecular structure of [LigHTiCl2]. Thermal ellipsoids are
shown at the 50% probability level. Hydrogen atoms and one toluene
molecule have been omitted for clarity. Selected bond lengths (A˚) and
angles (deg): Ti(1)–O(1) 1.8246(48), Ti(1)–O(2) 1.8397(39), Ti(1)–N(1)
2.2926(46), Ti(1)–N(2) 2.1559(44), Ti(1)–Cl(1) 2.3246(17), Ti(1)–Cl(2)
2.3624(21); Cl(1)-Ti(1)-Cl(2) 89.69(6), Cl(2)–Ti(1)–N(2) 87.18(13), N(2)–
Ti(1)–O(1) 89.56(18), O(1)–Ti(1)–Cl(1) 92.43(14), N(1)–Ti(1)–Cl(2)
91.88(13), Cl(2)–Ti(1)–O(2) 92.97(13), O(2)–Ti(1)–O(1) 92.25(17), O(1)–
Ti(1)–N(1) 81.91(17), Cl(1)–Ti(1)–O(2) 107.16(13), O(2)–Ti(1)–N(2)
83.96(18), N(2)–Ti(1)–N(1) 76.66(18), N(1)–Ti(1)–Cl(1) 92.50(13), Cl(1)–
Ti(1)–N(2) 168.60(14), Cl(2)–Ti(1)–O(1) 173.52(14), N(1)–Ti(1)–O(2)
159.76(17). (b) Molecular structure of [LigFTiCl2]. Thermal ellipsoids are
shown at the 50% probability level. Hydrogen atoms have been
omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ti(1)–
O(1) 1.8141(12), Ti(1)–O(2) 1.8422(13), Ti(1)–N(1) 2.2975(16), Ti(1)–N(2)
2.1509(17), Ti(1)–Cl(1) 2.3032(6), Ti(1)–Cl(2) 2.3934(5); Cl(1)-Ti(1)-Cl(2)
90.09(2), Cl(2)–Ti(1)–N(2) 83.32(4), N(2)–Ti(1)–O(1) 90.95(6), O(1)–
Ti(1)–Cl(1) 93.99(5), N(1)–Ti(1)–Cl(2) 90.74(4), Cl(2)–Ti(1)–O(2)
92.22(4), O(2)–Ti(1)–O(1) 93.53(6), O(1)–Ti(1)–N(1) 81.71(6), Cl(1)–
Ti(1)–O(2) 109.14(4), O(2)–Ti(1)–N(2) 84.40(6), N(2)–Ti(1)–N(1)
76.18(6), N(1)–Ti(1)–Cl(1) 90.75(4), Cl(1)–Ti(1)–N(2) 165.23(5), Cl(2)–
Ti(1)–O(1) 171.45(5), N(1)–Ti(1)–O(2) 159.88(6).
Results and discussion
The salalen ligand precursors (L1 & L2) were synthesized by
Schiff-base condensation of N-methylethylenediamine with
salicylaldehyde and uoro-substituted salicylaldehyde, followed
by nucleophilic attack of the N-methyl secondary amine with
the bromomethyl derivative of the corresponding phenol.
Subsequently, L1 and L2 were reacted with 2 equivalents of n-
BuLi to remove protons from the hydroxyl moieties, resulting in
the formation of oxygen anions. Finally, the salalen titanium(IV)
complexes were synthesized with tetradentate dianionic
Scheme 1 Synthesis route for salalen ligand precursors and their titanium(IV) complexes.
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Fig. 2 1H NMR of complex [LigHTiCl2]. Asterisks indicate some resonances of the minor component.
chelating ligands, including an imine and an amine internal
The typical 1H NMR spectrum of [LigHTiCl2] strongly
donor bound to phenol arms, in high to quantitative yields, as suggests that there are two isomers that coexist in toluene
ꢀ
described in Scheme 1.
solution at 30 C, [LigHTiCl2]-A and [LigHTiCl2]-B (Fig. 2). The
The resulting complexes were labelled [LigHTiCl2] and major resonance signals of the corresponding hydrogen atoms
[LigFTiCl2], and their single crystals were grown from toluene/n- in [LigHTiCl2] were attributed to [LigHTiCl2]-A, in accordance
hexane and dichloromethane/n-hexane at room temperature, with the single-crystal structure. Meanwhile, the minor reso-
respectively. As shown in Fig. 1, single-crystal X-ray diffraction nance signals were attributed to [LigHTiCl2]-B. The chemical
analysis indicated that the titanium atom adopts distorted shi of the hydrogen atoms of N-methyl displays an upeld
octahedral coordination, with one tetradentate ligand moiety shi from 3.21 to 2.70 ppm, indicating that the hydrogen atoms
and two chlorine atoms. The distortion is mainly in terms of the of the methyl group are far from the hydrogen atoms on the
N1–Ti1–O2 angles of 159.76(17)ꢀ and 159.88(6)ꢀ deviating by carbon bridge and on the methylene group, according to the van
180ꢀ. The tetradentate salalen ligands form a fac (around the der Waals effect.
amine) – mer (around the imine) mode, wrapping around the
As shown in Fig. 3, the content of the [LigHTiCl2]-B isomer
titanium(IV) center for both complexes and leading to a cis- increases from 15% to 34%, as calculated from variable
1
ꢀ
geometrical relationship between the two labile monodentate temperature H NMR, with increasing temperature from 30 C
ꢀ
chlorine ligands. The structure parameters, including selected to 120 C. Moreover, the corresponding chemical shis of the
bond lengths and angles, are displayed in Fig. 1.
Note that the introduction of a uorine group at the ortho
position of the imine-side phenol leads the Ti–Cl bond length in
˚
[LigFTiCl2] of 2.3934(5) A to be much longer than that in
˚
[LigHTiCl2] of 2.3624(21) A. Furthermore, the longer bond
length corresponding to weaker binding suggests that the
[LigFTiCl2] is more easily alkylated by MAO than [LigHTiCl2]
and more in favour of the formation of active species as well as
the insertion of ethylene into the titanium–carbon bond.
Therefore, the [LigFTiCl2]/MAO could afford higher catalytic
activity for ethylene polymerization than [LigHTiCl2]/MAO.
Interestingly, the Ti–N1 bond lengths in both complexes
were longer than Ti–N2, indicating that the bond energy
between the titanium atom and imine N-donor is stronger than
that between the titanium atom and amine N-donor. In addi-
˚
tion, the Ti–N1 bond length of 2.2926(46) A in [LigHTiCl2] is
˚
slightly shorter than that in [LigFTiCl2], 2.2975(16) A, implying
the electron-withdrawing effect of the uorine group at the
Fig. 3 The 1H NMR spectra of [LigHTiCl2] at different temperatures.
ortho position of the imine side phenol.
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Table 1 Polymerization of ethylene catalyzed by salalen titanium(IV) complex activated by MAOa
Activityb
Mw
Mw/Mn
c
c
d
Tm (ꢀC)
Entry
Precat.
T (ꢀC)
Al/Ti
Yield (g)
1
2
3
4
5
6
7
8
LigHTiCl2
LigHTiCl2
LigHTiCl2
LigHTiCl2
LigHTiCl2
LigHTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
LigFTiCl2
30
50
70
100
120
140
30
50
70
100
120
140
50
500
500
500
500
500
500
500
500
500
500
500
500
300
700
900
500
500
500
0.074
0.158
0.521
0.820
0.482
0.140
0.520
0.565
0.630
1.382
0.969
0.896
0.414
0.365
0.312
0.376
0.783
0.941
7.4
15.8
52.1
82.0
48.2
12.0
52.0
56.5
63.0
138.2
96.9
89.6
41.4
36.5
31.2
75.2
52.2
47.0
295
201
56.2
19.8
6.7
2.3
2.7
4.8
8.5
6.2
6.2
2.1
2.4
3.7
4.5
4.0
4.0
2.4
5.7
4.5
2.5
3.0
3.2
137.4
134.7
133.7
131.7
129.7
129.0
137.0
134.6
133.4
131.5
130.5
130.0
133.8
135.2
134.6
132.9
134.5
135.2
3.4
298
264
163
51.1
32.5
12.9
270
258
237
111
349
436
9
10
11
12
13
14
15
16e
17f
18g
50
50
50
50
50
a
b
Polymerization conditions: Ti ¼ 20 mmol; Al/Ti ¼ 500; ethylene pressure ¼ 220 psi, toluene ¼ 70 mL; reaction time ¼ 30 min. In g[polymer]
mmolꢂ1 [Ti] hꢂ1
.
In 104 g molꢂ1, obtained from HT-GPC at 150 ꢀC in 1,2,3-trichlorobenzene (TCB) against polystyrene standards.
f g
c
d
Determined by differential scanning calorimetry, second heating cycle. e Reaction time ¼ 15 min. Reaction time ¼ 45 min. Reaction time ¼
60 min.
hydrogen atoms in both conformational isomers can still be narrow molecular weight distribution (Mw/Mn ꢁ 2), as expected
1
assigned though H NMR.
for an approximate single active species catalyst. As the poly-
Upon activation by MAO, both complexes were tested for merization temperature increases, the resulting polyethylene
ethylene polymerization in toluene at temperatures ranging shows a gradually broadening molecular weight distribution,
from 30 ꢀC to 140 ꢀC (Table 1). [LigHTiCl2] and [LigFTiCl2] suggesting the presence of double or multiple active species as
showed moderate catalytic activities and produced linear poly-
a
result of the conformational isomerization of the
ethylene with ultrahigh molecular weight (Mw ¼ 2950 and 2980 complexes.42,49
kDa, respectively) at room temperature (30–50 ꢀC). However,
It should be noted that the polymerization temperature had
[LigFTiCl2]/MAO displayed much higher catalytic activity than a signicant inuence on the catalytic activities of the salalen
[LigHTiCl2]/MAO, supporting the structure analysis of both titanium(IV) complexes/MAO and on the molecular weight as
complexes. The formation of ultrahigh-molecular-weight poly- well as the molecular weight distribution of the resulting
ethylene is mainly due to the combination of the steric bulk of polyethylene.
the octahedral clathrate-like structure and the strong electron-
Upon activation by MAO, [LigHTiCl2] and [LigFTiCl2] dis-
donating ability of the two nitrogen-coordinated atoms rela- played their highest catalytic activities at 100 ꢀC (Fig. 4(b)). The
tive to the two sulfur atoms in [OSSO]-type titanium complexes, extraordinary thermal stability can be explained by the structure
which usually give rise to moderate-molecular-weight poly- of these complexes, which displayed an octahedral spatial
ethylene (Mw ꢁ 10 kDa).30,35,43 As shown in Fig. 4(a), the GPC conguration with tetradentate dianionic chelating ligands.
traces for polyethylene obtained at 30 ꢀC revealed a relatively
Fig. 4 (a) GPC traces of polyethylene formed using LigHTiCl2/MAO at various temperatures. (b) Polymerization activity versus temperature. (c)
Plots of Mn and Mw/Mn as a function of ethylene polymerization time.
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Furthermore, the ratio of the area under the low and high
melting temperature peaks was found to change with the
ꢀ
annealing time (at 160 C), which also coincides with previous
reports.52,53 Although this change in our experiments is not
obvious as in previous reports due to the insufficient annealing
time in our experiments, it still can prove the transformation of
the heterogeneous distribution of entanglement density to
a homogeneous state.52,53
Conclusions
In summary, two titanium(IV) complexes bearing salalen ligands
were synthesized and characterized by single-crystal X-ray
diffraction analysis and NMR. All the ligands bind to the tita-
nium(IV) in a fac–mer mode with an approximate octahedral
spatial conguration. It is worth noting that this type of
complexation in solution yielded two coexisting isomers. It was
discovered that [LigHTiCl2] and [LigFTiCl2] contain about 15%
isomers at 30 ꢀC, and the content of the isomers also increased
with the increase of temperature. The result implies that the
resulting polyethylene shows broader molecular weight distri-
bution at relatively high polymerization temperatures. Both
complexes were found to have the ability to catalyze the poly-
merization of ethylene in the presence of MAO. In addition, the
complex containing a uorine group at the ortho position of the
imine-side phenol ([LigFTiCl2]) revealed higher catalytic activity
and afforded disentangled linear polyethylene with ultrahigh
molecular weight (Mw up to 3000 kDa) and narrow molecular
Fig. 5 DSC plots of dis-UHMWPE sample (Table 1, Entry 7) obtained
from the second heating cycle (Scheme S1† ramp G-H) with annealing
time of 5, 15, 30, 60 and 360 min.
Furthermore, the titanium(IV) atom was located in the centre
of the octahedron, inhibiting the decomposition and associa-
tive chain transfer pathways.50 When the polymerization
temperature reached 140 ꢀC, the catalytic activity of [LigHTiCl2]/
MAO decreased drastically, while [LigFTiCl2]/MAO retained
a relatively high catalytic activity, which was 6-fold higher than
that of [LigHTiCl2]/MAO. It is theorized that the uorine atom
may interact with the b-H of the growing polyethylene chain to
reduce b-elimination reactions.51
ꢀ
weight distribution (Mw/Mn ꢁ 2) at 30–50 C. At high tempera-
As shown in Table 1, the inuence of different Al/Ti molar
ratios on ethylene polymerization was also investigated. The
maximum catalytic activity of 5.7 ꢃ 104 g of PE (mol Ti)ꢂ1 ꢃ hꢂ1
is observed at an Al/Ti molar ratio of 500 (Entry 8, Table 1). The
lower Al/Ti molar ratios could not promote all the catalyst
precursors to form active species. In contrast, the TMA coex-
isting in MAO increased with the increase of Al/Ti molar ratio.
Therefore, high Al/Ti molar ratio would poison some catalyst
precursors, causing the catalytic activity to decrease. In addi-
tion, the molecular weight of polyethylene remained at a rela-
tively stable value, Mw ꢁ 2500 kDa, indicating that the chain
transfer to co-catalyst was not the main chain transfer mode.
The polymerization was carried out from 15 to 60 min, and
the molecular weight increased with increasing polymerization
time (Fig. 4(c)). However, the catalytic activity decreased
signicantly with increased reaction time, which can be
attributed to the embedding of active species by polyethylene.
In order to investigate the disentangled state of the poly-
ethylene, DSC experiments were performed using a thermal
analysis protocol developed by Rastogi and co-workers (Scheme
S1†).52 The presence of disentangled UHMWPE was conrmed
by the appearance of two separate melting peaks in the DSC
curve (Fig. 5), aer annealing and isothermal crystallization for
stipulated time, and our result is in good agreement with
ture (70–120 ꢀC), these salalen titanium(IV) complexes gave rise
to polyethylene with broad molecular weight distribution (Mw/
Mn ꢁ 6), resulting from the multiple active species present due
to the isomerization of the complexes. Future investigations will
focus on designing novel salalen titanium(IV) complexes with
high catalytic activities for ethylene polymerization, affording
ultrahigh-molecular-weight polyethylene with narrow molec-
ular weight distribution.
Experimental section
General considerations
All operations involving air- or moisture-sensitive compounds
were performed under an atmosphere of dried and puried
nitrogen using standard vacuum-line, Schlenk, or glovebox
techniques. The salalen ligand precursors L1, L2 and their
complexes [LigHTiCl2], [LigFTiCl2] were synthesized according
to published literature procedures.48
Materials
Toluene and n-hexane were distilled from Na/K alloy, and
dichloromethane was distilled from CaH2. Methylaluminoxane
(MAO) was purchased from Akzo-Nobel as 1.5 M toluene solu-
tion and used as received. Commercial ethylene was used
directly for polymerization without further purication. All the
other reagents were purchased and used as received.
previous studies.52,53
A low-temperature melting peak at
130.7 ꢀC and a higher-temperature melting peak at 139.2 ꢀC
indicate the crystallization from entangled and disentangled
domains of the heterogeneous polymer melt, respectively.52
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Physical and analytical measurements
Synthesis of 2-(bromomethyl)-4,6-di-tert-butylphenol
NMR spectroscopy of the organic compounds was carried out To a clear solution of 2,4-di-tert-butylphenol (17.4 g, 84.4 mmol)
on a Bruker 400 MHz instrument in CDCl3 using TMS as in glacial acetic acid (40 mL), paraformaldehyde (2.8 g,
a reference, and characterization of the complexes was carried 93.24 mmol, 1.1 equiv.) was added at room temperature and
out on a Bruker 500 MHz in C2D2Cl4 (1,1,2,2-tetrachloroethane- stirred for 2 h. A HBr solution in acetic acid (90 mL, mmol,
d2). Chemical shis were reported in ppm on the scale relative equiv.) was then added dropwise into the resulting solution.
1
to CDCl3 (d ¼ 7.26 for H NMR, d ¼ 77.00 for 13C NMR) and Aer 40 min of stirring at room temperature, the acetic acid was
1
C2D2Cl4 (d ¼ 6.00 for H NMR, d ¼ 75.00 for 13C NMR). NMR removed in vacuo, and an orange oil was obtained. Using n-
analysis of polymers was carried out on a Bruker 500 MHz at hexane as a recrystallization solvent, a pale orange crystalline
120 ꢀC. Sample solutions of the polymer were prepared in o- solid was obtained (20.7 g, 82%). 1H NMR (500 MHz, CDCl3)
C6H6Cl2/o-C6D6Cl2 (50% v/v) in a 10 mm sample tube. DSC d 1.29 (s, 9H, C(CH3)3), 1.43 (s, 9H, C(CH3)3), 4.58 (s, 2H, CH2Br),
analyses were conducted with a PerkinElmer DSC-4000 system. 5.28 (s, 1H, OH), 7.10 (d, 1H, ArH), 7.33 (d, 1H, ArH). 13C NMR
The DSC curves were recorded as second heating curves from (125 MHz, CDCl3) d 32.23, 34.14, 35.46, 37.00, 37.59, 125.93,
ꢂ1
ꢀ
ꢀ
ꢀ
30 C to 200 C at a heating rate of 10 C min and a cooling 127.55, 128.48, 139.20, 145.91, 154.48.
rate of 10 ꢀC minꢂ1. High-temperature gel permeation chro-
matography (GPC) was performed on a Polymer Laboratories
Ltd. PL-220 equipped with a refractive index (RI) detector. GPC
Synthesis of 2-[((methylamino)ethylimino)methyl]-phenol
N-Methylethylenediamine (1.1 g, 15 mmol) was added to
a solution of salicylaldehyde (1.8 g, 15 mmol) in toluene and
reuxed for 2 h. The solvent was removed under vacuum,
columns were eluted with 1,2,4-triꢂc1hlorobenzene (TCB) at
ꢀ
150 C with a run rate of 1.0 mL min
.
1
yielding a yellow oil (2.20 g, 82%). H NMR (500 MHz, CDCl3)
d 2.36 (s, 3H, CH3), 2.83 (t, 2H, CH2), 3.62 (t, 2H, CH2), 6.75–6.89
(m, 2H, ArH), 7.13–7.22 (m, 2H, ArH), 8.28 (s, 1H, N]CH). 13C
NMR (125 MHz, CDCl3) d 36.3 (CH3), 51.9, 59.6 (CH2), 117.2,
118.5, 131.4, 132.4, (Ar–CH), 138.5 (Ar–C), 161.4 (Ar–O), 166.1
(CH).
Crystal structure determination
The crystals of the titanium complex were mounted on a glass
ber and transferred to a Bruker CCD platform diffractometer.
Data obtained under u–2q scan mode were collected on
a Bruker SMART 1000 CCD diffractometer with graphite-
˚
monochromated Cu Ka radiation (l ¼ 1.54184 A) or Mo Ka
˚
radiation (l ¼ 0.71073 A). The structures were solved using
Synthesis of 2-[((methylamino)ethylimino)methyl]-6-uorinephenol
direct methods, while further renement with full-matrix least-
squares on F2 was obtained with the SHELXTL program
package. All the non-hydrogen atoms were rened anisotropi-
cally. Hydrogen atoms were introduced in calculated positions
with the displacement factors of the host carbon atoms.
N-Methylethylenediamine (1.1 g, 15 mmol) was added to
a solution of 3-uoro-2-hydroxybenzaldehyde (2.1 g, 15 mmol)
in toluene and reuxed for 2 h. The solvent was removed under
vacuum, yielding a yellow oil (2.75 g, 93%). 1H NMR (500 MHz,
CDCl3) d 2.46 (s, 3H, CH3), 2.93 (t, 2H, CH2), 3.74 (t, 2H, CH2),
6.86 (t, 1H, ArH), 6.94 (d, 1H, ArH), 7.28 (d, 1H, ArH), 8.39 (s, 1H,
N]CH). 13C NMR (125 MHz, CDCl3) d 36.3 (CH3), 48.9, 51.5
(CH2), 117.5, 125.6, 128.8, 130.2 (Ar–C), 151.1 (Ar–O), 154.3 (Ar–
F), 161.2 (CH).
Ethylene polymerization
A mechanically stirred 200 mL Parr reactor was heated to 150 ꢀC
for 3 h under vacuum and then cooled to reaction temperature.
The autoclave was pressurized to 15 psi of ethylene and vented
three times. The autoclave was then charged with toluene under
Synthesis of L1
15 psi of ethylene at the initialization temperature. The system 2-[((Methylamino)ethylimino)methyl]-phenol (0.87 g, 4.9 mmol)
was maintained by continuously stirring for 5 min, and then was dissolved in THF (40 mL), to which a solution of 2-(bro-
methylaluminoxane (MAO) and 5 mL of a solution of the tita- momethyl)-4,6-di-tert-butylphenol (1.4 g, 4.9 mmol) in THF (40
nium complex in toluene were sequentially charged into the mL) was added. Triethylamine (5 mL) was then added, and the
autoclave under 15 psi ethylene. The ethylene pressure was mixture was stirred at room temperature for 2 h. The white
raised to the specied value. The polymerization temperature precipitate was then ltered and the solvent removed under
was controlled by means of a heater and error was found to be vacuum. Using methanol as a recrystallization solvent, a yellow
1
ꢄ2 ꢀC, as monitored by an internal thermocouple. The poly- crystalline solid was obtained (1.1 g, 57%). H NMR (500 MHz,
merization was carried out for a specied time before being CDCl3) d 1.27 (s, 9H, C(CH3)3), 1.36 (s, 9H, C(CH3)3), 2.39 (s, 3H,
terminated by the addition of 5 mL methanol into the autoclave, CH3), 2.84 (t, 2H, CH2), 3.74 (s, 2H, CH2), 3.78 (t, 2H, CH2), 6.82
aer releasing the ethylene pressure. The resulting solution was (s, 1H, ArH), 6.88 (t, 1H, ArH), 6.96 (d, 1H, ArH), 7.17–7.35 (m,
then transferred into 500 mL of acidic methanol (10 : 1 3H, ArH), 8.37 (s, 1H, N]CH). 13C NMR (125 MHz, CDCl3)
methanol/HCl) with stirring overnight. The resulting precipi- d 29.6, 31.7, 34.2, 34.8, 42.1, 57.0, 57.6, 62.3, 117.0, 118.7, 118.8,
tated polymer was collected and treated by ltration, washed 121.0, 123.0, 131.4, 132.4, 135.9, 140.6, 154.3, 161.0, 166.3.
with methanol several times, and dried in vacuum at 40 C to HRMS (ESI) m/z: [M + H]+ calcd for C25H37N2O2, 397.2855,
ꢀ
a constant weight.
found, 397.2842.
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equiv.) within 5 min. The solution was stirred for 1 h at this
temperature. TiCl4 (1 M in toluene, 1.2 mL, 1.2 mmol) and
toluene (20 mL) were added to the resulting solution of L2-Li2 at
ꢂ78 C, and the mixture was stirred for 1 h. The solution was
Synthesis of L2
2-[((Methylamino)ethylimino)methyl]-6-uorophenol (1.96 g, 10
mmol) was dissolved in THF (40 mL), to which a solution of 2-
(bromomethyl)-4,6-di-tert-butylphenol (3.29 g, 11 mmol) in THF
(40 mL) was added. Triethylamine (5 mL) was then added and
the mixture stirred at room temperature for 2 h. The white
precipitate was then ltered and the solvent removed under
vacuum. Using methanol as a recrystallization solvent, a yellow
crystalline solid was obtained (1.82 g, 44%).1H NMR (500 MHz,
CDCl3) d 1.27 (s, 9H, C(CH3)3), 1.35 (s, 9H, C(CH3)3), 2.39 (s, 3H,
CH3), 2.85 (t, 2H, CH2), 3.74 (s, 2H, CH2), 3.80 (t, 2H, CH2), 6.74–
6.84 (m, 2H), 7.03 (dd, 1H, ArH), 7.13 (td, 1H, ArH), 7.20 (dd, 1H,
ArH), 8.37 (s, 1H, N]CH). 13C NMR (125 MHz, CDCl3) d 29.5,
31.7, 34.1, 34.8, 42.1, 56.8, 57.2, 62.1, 117.7, 118.5, 120.8, 123.0,
126.4, 135.6, 140.6, 149.9, 150.4, 152.3, 154.1, 167.0. 19F NMR
(470 MHz, CDCl3) d ꢂ138.1. HRMS (ESI) m/z: [M + H]+ calcd for
ꢀ
then gradually allowed to warm to room temperature. The
insoluble inorganic salts were then ltered under nitrogen
atmosphere. The ltration solvent was removed in vacuo, and
the residual solid was washed three times with n-hexane and
dried in vacuo to give a dark red powder. Recrystallization in
cold toluene gave deep red crystals (443.1 mg, 71.2%). The
complex [LigFTiCl2] is composed of a mixture of two isomers, A
and B, in the ratio of 75 : 25 at 30 ꢀC, respectively (from the 1H
NMR spectra). For the major isomer, the amine atom is not
detached from the titanium(IV) center and is represented by A,
whereas B represents the amine atom detached from the tita-
nium(IV) center in the minor isomer. Owing to the low content
of the minor isomer, only the major isomer (A) was observed in
the 13C NMR. 1H NMR (500 MHz, C2D2Cl4) d 1.08 (s, 9H,
C(CH3)A3), 1.29 (s, 9H, C(CH3)3A), 1.33 (s, 9H, C(CH3)B3), 1.55 (s, 9H,
C(CH3)B3), 2.69 (d, 1H, CHA2), 2.71 (s, 3H, CHB3), 2.80 (dd, 1H,
CHB3), 3.00 (t, 1H, CHA2), 3.23 (s, 3H, CH3A), 3.51 (d, 1H, CHB2), 3.57
(d, 1H, CHA2), 3.65 (d, 1H, CH2A), 3.85–4.01 (m, 2H, CHB2), 4.43 (t,
1H, CHA2), 4.47 (t, 1H, CH2B), 5.02 (d, 1H, CHA2), 5.34 (d, 1H,
CHB2), 6.95–7.04 (m, 2H, Ar-HA), 7.06 (s, 1H, Ar-HB), 7.16–7.23 (m,
2H, Ar-HA), 7.24–7.36 (m, 4H, Ar-HB), 7.39 (t, 1H, Ar-HA), 8.21 (s,
1H, CHA), 8.35 (s, 1H, CHB). 13C NMR (125 MHz, C2D2Cl4) d 30.6,
31.6, 32.6, 35.6, 35.7, 54.3, 57.8, 59.7, 67.7, 122.5, 123.3, 123.5,
124.7, 124.7, 125.3, 126.2, 127.4, 129.8, 136.9, 147.3, 148.6,
160.1, 162.7, 162.8. 19F NMR (470 MHz, C2D2Cl4) d ꢂ129.9 (Ar-
FA), ꢂ129.0 (Ar-FB).
C
25H36FN2O2, 415.2761, found, 415.2755.
Synthesis of [LigHTiCl2]
To a solution of L1 (0.597 g, 1.51 mmol) in Et2O (30 mL) at 0 ꢀC
was added n-BuLi (2.5 M in hexane, 1.28 mL, 3.2 mmol, 2.1
equiv.) within 5 min. The solution was stirred for 1 h at this
temperature. Then, TiCl4 (1 M in toluene, 1.2 mL, 1.2 mmol)
and toluene (20 mL) was added to the resulting solution of L1-
Li2 at ꢂ78 ꢀC, and the mixture was stirred for 1 h. The solution
was then gradually allowed to warm to room temperature. The
insoluble inorganic salts were then ltered under nitrogen
atmosphere. The ltration solvent was removed in vacuo, and
the residual solid was washed with hexane three times and
dried in vacuo to give a dark red powder. Recrystallization in
cold toluene gave deep red crystals (537.5 mg, 70%). The
complex [LigHTiCl2] is composed of a mixture of two isomers, A
and B, in the ratio of 85 : 15 at 30 ꢀC, respectively (from the 1H
NMR spectra). For the major isomer, the amine atom is not
detached from the titanium(IV) center and is represented by A,
whereas B represents the amine atom detached from the tita-
nium(IV) center in the minor isomer. Owing to the low content
of the minor isomer, only the major isomer (A) was observed in
the 13C NMR. 1H NMR (500 MHz, C2D2Cl4) d 1.09 (s, 9H,
C(CH3)A3), 1.29 (s, 9H, C(CH3)3A), 1.33 (s, 9H, C(CH3)B3), 1.56 (s, 9H,
C(CH3)B3), 2.67 (d, 1H, CH2A), 2.70 (s, 3H, CHB3), 2.75 (d, 1H,
CHB2), 3.00 (t, 1H, CHA2), 3.05 (s, 3H, CH3A), 3.50 (d, 1H, CHB2), 3.58
(d, 1H, CHA2), 3.62 (d, 1H, CH2A), 3.86–3.98 (m, 2H, CHB2), 4.42 (t,
1H, CHA2), 4.45 (t, 1H, CH2B), 5.04 (d, 1H, CHA2), 5.36 (d, 1H,
CHB2), 6.91 (d, 1H, Ar-HA), 7.00 (s, 1H, Ar-HA), 7.06 (t, 1H, Ar-HA),
7.19 (s, 1H, Ar-HA), 7.27 (t, 1H, Ar-HB), 7.33 (m, 1H, Ar-HB), 7.41
(d, 1H, Ar-HA), 7.48–7.53 (m, 1H, Ar-HB), 7.56 (t, 1H, Ar-HB), 7.61
(t, 1H, Ar-HA), 8.18 (s, 1H, CHA), 8.32 (s, 1H, CHB). 13C NMR (125
MHz, C2D2Cl4) d 30.6, 31.6, 32.7, 35.6, 35.7, 54.0, 57.7, 59.6, 67.5,
116.9, 123.1, 124.4, 124.7, 125.2, 127.3, 129.4, 130.2, 134.9,
136.9, 137.6, 136.9, 160.1, 163.2, 163.4.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21174167, 51573212) and the NSF of
Guangdong Province (S2013030013474, 2014A030313178).
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41830 | RSC Adv., 2019, 9, 41824–41831
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